Preparation method of lithium battery silicon-carbon composite negative electrode material with high cycle stability

Abstract

The invention relates to the technical field of lithium battery negative electrode materials, and discloses a preparation method of a lithium battery silicon-carbon composite negative electrode material with high cycle stability, and the preparation method comprises the following steps: carrying out magnesiothermic reduction and acid etching on micron silicon powder and magnesium powder to prepare porous silicon particles; growing carbon nanotubes on the surfaces of the porous silicon particles and in pore channels in situ to construct a three-dimensional conductive network; styrene butadiene rubber and glucose are adopted as a composite carbon source to coat the product, and a composite carbon layer is formed through drying and gradient carbonization; and finally grinding and sieving the carbonized product. According to the preparation method disclosed by the invention, pores are constructed in micron silicon, and the micron silicon is externally coated with the composite carbon layer with flexibility and rigidity, so that the volume expansion of silicon is effectively buffered; the in-situ grown carbon nanotube network provides a stable electron conduction path; the micron-sized substrate reduces the specific surface area of the material and improves the first coulombic efficiency. Therefore, the prepared negative electrode material has high cycling stability and excellent rate capability.

Description

Technical Field

[0001] This invention relates to the field of lithium battery anode material technology, specifically to a method for preparing a high-cycle-stability lithium battery silicon-carbon composite anode material. Background Technology

[0002] With the rapid development of new energy vehicles and the energy storage industry, the market demand for the energy density of lithium-ion batteries is increasing. Currently, the most widely used commercial graphite anode material has a theoretical specific capacity of only 372 mAh / g, which is gradually approaching its performance limit and cannot meet the research and development needs of high-energy-density batteries. In contrast, silicon-based materials, due to their extremely high theoretical specific capacity (approximately 4200 mAh / g) and low lithium insertion / extraction potential, are considered the most promising next-generation high-energy-density anode materials.

[0003] However, silicon-based anode materials face a series of key technical bottlenecks in practical applications due to their inherent physicochemical properties. First, silicon undergoes a dramatic volume effect during lithium alloying, with a volume expansion rate exceeding 300%. This massive volume change generates strong internal stress, leading to the pulverization and fragmentation of active particles and the shedding of active material from the current collector, ultimately causing the electrode structure to disintegrate and drastically reduce cycle life. Second, silicon, being a semiconductor material, has poor intrinsic conductivity, resulting in significant resistance to lithium-ion and electron transport, severely limiting the battery's high-current charge / discharge capability, i.e., rate performance. Furthermore, during repeated expansion and contraction of silicon particles, the solid electrolyte interphase (SEI) film on its surface continuously ruptures and regenerates. This not only continuously consumes electrolyte and a limited source of active lithium but also leads to low initial coulombic efficiency, further impacting the battery's overall performance.

[0004] To overcome these shortcomings, existing technologies have explored various modification methods, but all have certain limitations. For example, while nano-sizing of silicon particles can alleviate the stress caused by volume expansion to some extent, nanoparticles have extremely high surface energy and specific surface area. This not only exacerbates interfacial side reactions with the electrolyte but also easily leads to agglomeration during slurry preparation, affecting the coating quality of the electrode. Carbon coating modification using a single carbon source (such as pitch or resin) can improve conductivity, but the resulting carbon layer is often brittle and cannot withstand the large deformation of silicon particles during cycling, easily cracking and losing its protective function. Simple silicon-carbon mechanical mixing processes, due to the weak van der Waals forces between silicon and carbon, have low interfacial bonding strength and are prone to phase separation during long cycles, failing to maintain a stable conductive network. Furthermore, while some high-end modification technologies offer better performance, they often rely on expensive equipment such as chemical vapor deposition or plasma treatment, resulting in complex processes and high costs, making it difficult to meet the cost control requirements of large-scale industrial production. Therefore, developing a method for preparing silicon-carbon composite anode materials that can balance structural stability, excellent conductivity, and interface stability, and with an economical and feasible process route, is a technical challenge that urgently needs to be solved in this field. Summary of the Invention

[0005] To address the problems of poor cycle performance of silicon anode materials due to huge volume expansion, low initial coulombic efficiency caused by nano-sizing strategies, and instability of traditional conductive networks and carbon coating structures in existing technologies, this invention provides a method for preparing a silicon-carbon composite anode material for lithium batteries with high cycle stability, aiming to solve the problems of structural stability and electrochemical performance degradation of silicon anode materials in practical applications.

[0006] To achieve the above objectives, the present invention provides a method for preparing a silicon-carbon composite anode material for lithium batteries with high cycle stability, comprising the following steps:

[0007] S1. Preparation of porous silicon particles: Micron-sized silicon powder and magnesium powder are mixed and ball-milled, and a magnesium thermal reduction reaction is carried out under an inert atmosphere. Then, the product of the magnesium thermal reduction reaction is etched with an acid solution, and after washing and drying, porous silicon particles are obtained.

[0008] S2. Growth of carbon nanotubes: The porous silicon particles obtained in step S1 are mixed with iron acetate catalyst, and acetylene gas is introduced under high temperature and inert atmosphere to carry out the reaction, so as to obtain porous silicon composite particles with carbon nanotubes grown on the surface.

[0009] S3. Coating and carbonization: Styrene-butadiene rubber and glucose are dissolved in deionized water to prepare a carbon source solution. The porous silicon composite particles obtained in step S2 are mixed with the carbon source solution and dried to obtain a precursor. The precursor is then subjected to gradient carbonization in an inert atmosphere to obtain a carbonized product.

[0010] S4. Post-processing: Grind and sieve the carbonized product obtained in step S3 to obtain the composite anode material.

[0011] By adopting the above technical solution, the composite anode material prepared by this invention has a structural design that can simultaneously address the problems of volume expansion, poor conductivity, and interface instability inherent in silicon anodes. Its mechanism of action is as follows:

[0012] 1. Construction of the internal buffer structure (corresponding to step S1): through the magnesothermic reduction reaction (Si + 2Mg → Mg2Si) and the subsequent acid etching reaction (Mg2Si + 4H). + →Si+2Mg 2+ The addition of 2H₂↑ creates nanoscale pores in situ within micron-sized silicon particles. These pores provide an internal buffer for the volume changes of silicon during lithium insertion / extraction, allowing most of the volume expansion to be absorbed within the particles, thus preventing overall particle pulverization and fragmentation. Simultaneously, because this structure is built on micron-sized particles, it controls the overall external surface area of ​​the material, reducing the direct contact area with the electrolyte. This reduces excessive formation of the solid electrolyte interphase (SEI) film during the first cycle, improving the first coulombic efficiency.

[0013] 2. Construction of a Three-Dimensional Conductive Network (corresponding to step S2): Carbon nanotubes (CNTs) are grown in situ on the surface and within the pores of porous silicon particles using chemical vapor deposition. Since the catalyst particles are loaded onto the silicon substrate, the grown carbon nanotubes are rooted in the silicon surface, forming a tightly bound three-dimensional electron transport network that runs through the entire particle. Compared to simple physical mixing, this in-situ growth method results in a conductive pathway with lower contact resistance and a more stable structure. Even after the silicon particles undergo a certain degree of cracking, it can still ensure effective electrical connection between the internal and external current collectors of the particles, thereby improving the rate performance and conductivity stability during cycling.

[0014] 3. Construction of the composite functional carbon layer (corresponding to step S3): Styrene-butadiene rubber (SBR) and glucose are used as composite carbon sources. During the gradient carbonization process, glucose is mainly converted into high-hardness amorphous hard carbon, providing rigid support and basic conductivity for the material; while the polymer SBR is converted into graphitized soft carbon, which has both flexibility and conductivity. The composite carbon layer formed by the combination of the two has both the structural support capability of rigid carbon and the deformation adaptability of flexible carbon. This composite carbon layer can not only further confine the volume expansion of silicon particles and prevent them from expanding excessively outward, but also adapt to the deformation of silicon during cycling without breaking itself. In addition, this dense composite carbon layer also acts as a physical barrier, further stabilizing the SEI film.

[0015] In summary, the silicon-carbon composite anode material prepared by combining a porous silicon core, an in-situ carbon nanotube network, and a composite carbon shell in this invention improves capacity, cycle stability, and rate performance.

[0016] Preferably, in step S1, the mass ratio of the micron-sized silicon powder to the magnesium powder is 2.5:1 to 3.5:1; the temperature of the magnesiac reduction reaction is 600 to 700°C, and the holding time is 2 to 4 hours. By adopting the above technical solution, this range of proportions and reaction conditions can ensure the full progress of the magnesiac reduction reaction to form a sufficient amount of Mg2Si phase, while avoiding excessively high temperatures that would cause silicon particles to melt or excessively low temperatures that would cause incomplete reactions, thus providing a foundation for the subsequent formation of suitable porosity and pore structure.

[0017] Preferably, in step S1, the heating rate during the magnesian reduction reaction is 3–10 °C / min; the acid solution is a hydrochloric acid solution with a mass fraction of 5 wt%–15 wt%. By adopting the above technical solution, controlling the heating rate helps to ensure the uniformity of the reaction; controlling the acid concentration can ensure the effective removal of the Mg2Si phase while avoiding unnecessary corrosion of the silicon substrate due to excessive acidity.

[0018] Preferably, in step S1, the etching temperature is 60–90°C, and the etching time is 2–6 hours. By adopting the above technical solution, this temperature and time range can ensure the efficiency of the etching reaction and complete the hole-forming process within a reasonable time.

[0019] Preferably, in step S2, the mass ratio of the porous silicon particles to the iron acetate catalyst is 8:1 to 15:1; the reaction temperature is 650 to 800°C, and the reaction time is 1 to 3 hours. By adopting the above technical solution, the catalyst dosage and reaction conditions range can form a carbon nanotube network with moderate density and suitable length on the surface of the silicon particles, which not only ensures the effectiveness of the conductive network but also avoids the reduction in material specific capacity or pore blockage caused by excessive growth.

[0020] Preferably, in step S2, the heating rate to the reaction temperature is 5–15 °C / min; the acetylene gas flow rate is 30–100 sccm. By adopting the above technical solution, controlling the heating rate and the carbon source gas flow rate are key parameters for controlling the growth quality and morphology of carbon nanotubes, which helps to obtain a uniform conductive network.

[0021] Preferably, in step S3, the mass ratio of styrene-butadiene rubber to glucose in the carbon source solution is 2:1 to 4:1; the solid-liquid mass ratio when the porous silicon composite particles are mixed with the carbon source solution is 1.5:1 to 2.5:1. By adopting the above technical solution and adjusting the ratio of styrene-butadiene rubber to glucose, the ratio of flexible carbon to rigid carbon in the final composite carbon layer can be controlled, thereby balancing the mechanical properties and conductivity of the carbon layer; adjusting the solid-liquid ratio affects the thickness and uniformity of the coating layer.

[0022] Preferably, in step S3, the gradient carbonization treatment includes a first-stage carbonization and a second-stage carbonization; the temperature of the first-stage carbonization is 550–650°C, and the holding time is 1–3 hours; the temperature of the second-stage carbonization is 850–1050°C, and the holding time is 2–5 hours. By adopting the above technical solution, the low-temperature carbonization in the first stage causes the organic precursor to decompose slowly, forming a preliminary carbon skeleton and preventing structural collapse; the high-temperature carbonization in the second stage promotes further densification and graphitization of the carbon layer, improving its conductivity and structural strength.

[0023] Preferably, in step S3, the rate of heating to the first stage carbonization temperature is 3–10 °C / min; the rate of heating from the first stage carbonization temperature to the second stage carbonization temperature is 1–5 °C / min. By adopting the above technical solution, controlling the heating rate in stages is an important step in achieving the desired gradient carbonization effect, which helps to ultimately form a structurally stable composite carbon layer.

[0024] Preferably, in step S4, the sieve used for sieving has a mesh size of 150-300. By adopting the above technical solution, the sieving operation can remove agglomerated large particles, obtaining a final product with a more uniform particle size distribution, which is beneficial to subsequent electrode slurry preparation and coating processes.

[0025] The above solution achieves the following beneficial technical effects:

[0026] This invention improves the structural stability and cycle life of the anode material. It constructs nanopores within micron-sized silicon particles through magnesiothermal reduction and acid etching, and then coats them with a composite carbon layer formed from styrene-butadiene rubber and glucose. This internal pore and external coating structure effectively buffers the volume changes of silicon during charge and discharge. This structure limits excessive particle expansion, prevents the pulverization and peeling of active materials, and thus ensures the integrity of the electrode structure during long-term cycling.

[0027] This invention constructs a highly efficient electron transport network and improves rate performance. By using in-situ growth technology, a carbon nanotube conductive network is constructed on the surface and within the pores of porous silicon particles, enabling the carbon nanotubes to form a tight bond with the silicon matrix and constructing a three-dimensional electron transport channel that runs through the inside and outside of the particles. This structure reduces the contact resistance inside the particles and between the particles and the current collector, effectively improving the conductive contact failure problem caused by the silicon volume effect, thereby enhancing the performance of the material under high current charge and discharge conditions.

[0028] This invention optimizes the interfacial properties of the material and possesses good process economics. Through the dense coating effect of the composite carbon layer, direct contact between the internal silicon substrate and the electrolyte is reduced, thereby suppressing excessive formation of the solid electrolyte interfacial film during the first charge-discharge process and reducing irreversible lithium-ion consumption. Furthermore, this preparation process uses micron-sized silicon powder as raw material, combining ball milling and conventional heat treatment processes. The route is simple, raw material costs are controllable, and expensive equipment is not required, making it suitable for large-scale industrial production. Detailed Implementation

[0029] The technical solution of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0030] Preparation Example 1: Preparation of Porous Silicon Particles

[0031] This preparation example provides a general method for preparing porous silicon particles using a combination of magnesiothermal reduction and acid etching processes. The product is used as the silicon source core in subsequent embodiments. The method includes the following steps:

[0032] Mixing and ball milling: Micron-sized silicon powder and magnesium powder of predetermined particle size are placed in a ball milling jar at a preset mass ratio. Under the protection of an inert gas (such as argon), high-energy ball milling is performed at a specific speed and time to obtain a mixture in which the two powders are uniformly mixed at the microscale.

[0033] Magnesium reduction: The mixed powder obtained in step 1 is transferred to a reaction vessel in a tube furnace. Under an inert gas atmosphere (such as argon), it is heated to the target reduction temperature at a set heating rate and held at this temperature for a preset time to allow the silicon powder and magnesium powder to fully react and form an alloy phase of magnesium silicide (Mg2Si) and the remaining silicon. After the reaction is complete, the furnace is allowed to cool naturally to room temperature.

[0034] Acid etching to create pores: The alloy product obtained in step 2 is slowly added to an acid solution of a predetermined concentration (such as hydrochloric acid solution). At a set reaction temperature, mechanical or magnetic stirring is performed at a specific rate for a predetermined time. During this process, the acid solution selectively reacts with magnesium silicide, removing the magnesium element and thereby forming a nanoscale porous structure in situ inside the silicon particles.

[0035] Washing and Drying: After the reaction, the solid product is collected by centrifugation or filtration. The product is repeatedly washed and centrifuged / filtered with deionized water until the pH of the supernatant or filtrate is neutral (pH 6-8). Finally, the obtained wet product is placed in a vacuum oven and dried at a preset temperature for a specified time to obtain the final porous silica powder.

[0036] Examples 1-3:

[0037] Example 1:

[0038] This embodiment provides a method for preparing a silicon-carbon composite anode material for lithium batteries with high cycle stability, specifically including the following steps:

[0039] Preparation of porous silicon particles: The method of Preparation Example 1 was adopted, with the following parameters: micron-sized silicon powder and magnesium powder were mixed and ball-milled at a mass ratio of 2.5:1; the mixture was heated to 600°C at a heating rate of 3°C / min in an argon atmosphere and kept at the temperature for 4 hours for magnesium thermal reduction; subsequently, a 5 wt% hydrochloric acid solution was used to etch the particles by stirring at 60°C for 6 hours; finally, the particles were washed and dried to obtain porous silicon particles.

[0040] In-situ growth of carbon nanotubes: The porous silicon obtained in step 1 was mixed with iron acetate (catalyst) at a mass ratio of 15:1, dispersed in ethanol, sonicated, and then dried. The powder was placed in a tube furnace and heated to 650°C at a rate of 5°C / min under an argon atmosphere. Then, acetylene gas with a flow rate of 30 sccm was introduced, and the reaction was carried out under these conditions for 3 hours. After the reaction was completed, the mixture was allowed to cool naturally, thus forming in-situ grown carbon nanotubes on the surface of the porous silicon particles.

[0041] Composite carbon layer coating and carbonization: Styrene-butadiene rubber (SBR) and glucose were dissolved in deionized water at a mass ratio of 2:1 to prepare a carbon source solution. The product from step 2 was mixed with this carbon source solution at a solid-liquid mass ratio of 1.5:1 to form a slurry, which was then spray-dried to obtain precursor microspheres. The microspheres were subjected to gradient carbonization in an argon atmosphere: first, the temperature was increased to 550℃ at a rate of 3℃ / min and held for 3 hours; then, the temperature was further increased to 850℃ at a rate of 1℃ / min and held for 5 hours.

[0042] Post-processing: The cooled carbonized products are ground and sieved through a 150-mesh sieve to obtain the final composite anode material.

[0043] Example 2:

[0044] This embodiment provides a method for preparing a silicon-carbon composite anode material for lithium batteries with high cycle stability, specifically including the following steps:

[0045] Preparation of porous silicon particles: The method of Preparation Example 1 was adopted. The specific parameters were as follows: micron-sized silicon powder and magnesium powder were mixed at a mass ratio of 3:1 and ball-milled; under an argon atmosphere, the mixture was heated to 650°C at a heating rate of 5°C / min and held at that temperature for 3 hours for magnesium thermal reduction; subsequently, a 10 wt% hydrochloric acid solution was used to etch the particles by stirring at 80°C for 4 hours; finally, the particles were washed and dried to obtain porous silicon particles.

[0046] In-situ growth of carbon nanotubes: The porous silicon obtained in step 1 was mixed with ferric acetate (catalyst) at a mass ratio of 10:1, dispersed in ethanol, sonicated, and then dried. The powder was placed in a tube furnace and heated to 700°C at a rate of 10°C / min under an argon atmosphere. Then, acetylene gas with a flow rate of 50 sccm was introduced, and the reaction was carried out under these conditions for 2 hours. After the reaction was completed, the mixture was allowed to cool naturally, thus forming in-situ grown carbon nanotubes on the surface of the porous silicon particles.

[0047] Composite carbon layer coating and carbonization: Styrene-butadiene rubber (SBR) and glucose were dissolved in deionized water at a mass ratio of 3:1 to prepare a carbon source solution. The product from step 2 was mixed with this carbon source solution at a solid-liquid mass ratio of 2:1 to form a slurry, which was then spray-dried to obtain precursor microspheres. The microspheres were subjected to gradient carbonization in an argon atmosphere: first, the temperature was increased to 600℃ at a rate of 5℃ / min and held for 2 hours; then, the temperature was further increased to 950℃ at a rate of 3℃ / min and held for 3 hours.

[0048] Post-processing: The cooled carbonized products are ground and sieved through a 200-mesh sieve to obtain the final composite anode material.

[0049] Example 3:

[0050] This embodiment provides a method for preparing a silicon-carbon composite anode material for lithium batteries with high cycle stability, specifically including the following steps:

[0051] Preparation of porous silicon particles: The method of Preparation Example 1 was adopted. The specific parameters were as follows: micron-sized silicon powder and magnesium powder were mixed and ball-milled at a mass ratio of 3.5:1; under an argon atmosphere, the mixture was heated to 700°C at a heating rate of 10°C / min and held at that temperature for 2 hours for magnesothermic reduction; subsequently, a 15wt% hydrochloric acid solution was used to etch the particles by stirring at 90°C for 2 hours; finally, the particles were washed and dried to obtain porous silicon particles.

[0052] In-situ growth of carbon nanotubes: The porous silicon obtained in step 1 was mixed with ferric acetate (catalyst) at a mass ratio of 8:1, dispersed in ethanol, sonicated, and then dried. The powder was placed in a tube furnace and heated to 800°C at a rate of 15°C / min under an argon atmosphere. Then, acetylene gas with a flow rate of 100 sccm was introduced, and the reaction was carried out under these conditions for 1 hour. After the reaction was completed, the mixture was allowed to cool naturally, thus forming in-situ grown carbon nanotubes on the surface of the porous silicon particles.

[0053] Composite carbon layer coating and carbonization: Styrene-butadiene rubber (SBR) and glucose were dissolved in deionized water at a mass ratio of 4:1 to prepare a carbon source solution. The product from step 2 was mixed with this carbon source solution at a solid-liquid mass ratio of 2.5:1 to form a slurry, which was then spray-dried to obtain precursor microspheres. The microspheres were subjected to gradient carbonization in an argon atmosphere: first, the temperature was increased to 650℃ at a rate of 10℃ / min and held for 1 hour; then, the temperature was further increased to 1050℃ at a rate of 5℃ / min and held for 2 hours.

[0054] Post-processing: The cooled carbonized products are ground and sieved through a 300-mesh sieve to obtain the final composite anode material.

[0055] Comparative Examples 1-6:

[0056] Comparative Example 1: Using dense micron-sized silicon instead of porous silicon

[0057] Compared to Example 2, the difference is that in step 1, no porosification treatment is performed; instead, commercially available dense micron-sized silicon powder with the same particle size as the raw material for preparing porous silicon is used directly as the core material for subsequent steps. All other steps and parameters are the same as in Example 2.

[0058] Comparative Example 2: Using nano-silicon to replace porous silicon

[0059] The difference from Example 2 is that commercially available nano-silicon powder (e.g., with an average particle size of 50-100 nm) is used instead of the porous silicon prepared in step 1 as the core material. All other steps and parameters are the same as in Example 2.

[0060] Comparative Example 3: Conductive Network Without Carbon Nanotubes

[0061] Compared to Example 2, the difference is that step 2, the in-situ growth of carbon nanotubes, is omitted, and the porous silicon prepared in step 1 is directly used for the composite carbon layer coating in step 3. All other steps and parameters are the same as in Example 2.

[0062] Comparative Example 4: Introducing carbon nanotubes using a mechanical mixing method

[0063] Compared to Example 2, the difference is that the in-situ growth of carbon nanotubes in step 2 is not performed. Instead, before step 3, the porous silicon prepared in step 1 is mechanically mixed with commercially available carbon nanotube powder (the amount added is equivalent to the theoretical growth amount in Example 2) using a high-energy ball mill. All other steps and parameters are the same as in Example 2.

[0064] Comparative Example 5: Coating with a single rigid carbon source

[0065] Compared to Example 2, the difference lies in that: in step 3, the composite carbon layer coating and carbonization, only glucose is used as the carbon source, and styrene-butadiene rubber (SBR) is not added. The amount of glucose is adjusted to be the same as the total mass of the carbon source in Example 2. All other steps and parameters are the same as in Example 2.

[0066] Comparative Example 6: Coating with a single flexible carbon source

[0067] Compared to Example 2, the difference lies in that: in step 3, the composite carbon layer coating and carbonization, only styrene-butadiene rubber (SBR) is used as the carbon source, without the addition of glucose, and the amount of SBR is adjusted to be the same as the total mass of the carbon source in Example 2. All other steps and parameters are the same as in Example 2.

[0068] Test Example 1-2:

[0069] Test Example 1: Characterization of the Physicochemical Properties of Composite Anode Materials

[0070] To confirm the physicochemical characteristics of the prepared materials, the products of each example and comparative example were characterized as follows:

[0071] Experimental steps:

[0072] The carbon content of the material was determined using a thermogravimetric analyzer. 5-10 mg of powder sample was weighed and placed in an alumina crucible. The temperature was increased from room temperature to 800 °C at a rate of 10 °C / min under air atmosphere (flow rate 50 mL / min). The total carbon content was calculated based on the mass loss between 300 °C and 700 °C.

[0073] The pore structure was tested using a fully automated specific surface area and pore size analyzer. 100 mg of sample was weighed and degassed under vacuum at 200 °C for 6 hours, followed by nitrogen adsorption-desorption isotherms at liquid nitrogen temperature (77 K). Specific surface area, pore size distribution, and total pore volume were calculated using the BET model, BJH model, and t-plot method, respectively.

[0074] Residual elements were analyzed using inductively coupled plasma optical emission spectrometry (ICP-OES). 20 mg of sample was weighed and placed in a polytetrafluoroethylene container. A mixture of hydrofluoric acid and nitric acid was added for microwave digestion. After cooling, the digest was diluted to 50 mL with deionized water. The concentrations of magnesium and iron were determined and converted to mass content.

[0075] The powder conductivity was tested using the four-probe method. 200 mg of sample was weighed, pressed into a circular sheet under a pressure of 10 MPa, and the conductivity was measured at room temperature. The average value of five different locations was taken.

[0076] Table 1. Physicochemical property characterization data of materials from the examples and comparative examples

[0077] Sample number Total carbon content (wt%) Specific surface area (m² / g) Total pore volume (cm³ / g) Residual Mg content (ppm) Residual Fe content (ppm) Powder conductivity (S / m) Example 1 18.2 43.5 0.39 95 112 89.4 Example 2 21.8 48.2 0.46 78 91 115.7 Example 3 25.1 53.7 0.51 65 74 128.2 Comparative Example 1 21.5 3.1 0.04 N / A 88 95.3 Comparative Example 2 22.1 95.6 0.55 N / A 94 102.1 Comparative Example 3 6.5 47.8 0.45 81 N / A 1.3 Comparative Example 4 21.6 46.9 0.44 79 N / A 25.6 Comparative Example 5 21.9 48.5 0.47 75 93 145.8 Comparative Example 6 21.7 48.0 0.45 77 89 55.1

[0078] Note: N / A indicates that the test is not applicable or was not performed.

[0079] in conclusion:

[0080] Physicochemical characterization data confirmed the structural and compositional characteristics of the materials prepared by this technical solution.

[0081] Specific surface area and pore volume tests showed that the specific surface area of ​​the samples in Examples 1-3 was 43.5-53.7 m². 2 The total pore volume (0.39-0.51 cm³ / g) and total pore volume (3.1 m² / g) of the micron-sized silicon sample were significantly higher than those of Comparative Example 1 (3.1 m² / g), indicating that the magnesiothermal reduction and acid etching process constructed a porous structure within the micron-sized silicon, providing a buffer space for volume expansion. Meanwhile, the specific surface area of ​​the sample in this example was lower than that of Comparative Example 2 (95.6 m² / g) of the nano-sized silicon structure. 2 The structure ( / g) indicates that it can control the contact area with the electrolyte, which is beneficial to reducing side reactions.

[0082] Powder conductivity tests showed that the conductivity of the sample in Example 2 (115.7 S / m) was higher than that of Comparative Example 3 (1.3 S / m) without carbon nanotubes and Comparative Example 4 (25.6 S / m) with carbon nanotubes introduced by mechanical mixing. The data indicate that the in-situ growth method resulted in a more tightly bonded conductive network between the carbon nanotubes and the silicon substrate, with superior electron transport efficiency compared to simple physical mixing.

[0083] Furthermore, while Comparative Example 5, with a single rigid carbon coating, exhibited high conductivity (145.8 S / m), it lacked flexibility; Comparative Example 6, with a single elastic carbon coating, had low conductivity (55.1 S / m). The conductivity of the example samples fell between the two, balancing conductivity with the structural and functional design of the composite carbon layer. Residual elemental analysis showed that the Mg and Fe contents in all examples were below 120 ppm, indicating that the etching and subsequent processing effectively removed impurities.

[0084] Test Example 2: Evaluation of the Electrochemical Performance of Composite Anode Materials

[0085] To evaluate the electrochemical performance of the prepared materials, coin cells were assembled and tested on the products of each example and comparative example.

[0086] Experimental steps:

[0087] A slurry was prepared by mixing the active material to be tested, the conductive agent SuperP, and the binder PVDF in NMP solvent at a mass ratio of 8:1:1. The slurry was coated onto copper foil and dried under vacuum at 80°C for 12 hours. The dried electrode was then rolled and punched into circular electrode sheets with a diameter of 12 mm, with the areal density of the active material controlled at 1.0-1.2 mg / cm³. 2 .

[0088] CR2032 coin cells were assembled in an argon-filled glove box, using the prepared electrode sheet as the working electrode, a lithium metal sheet as the counter electrode, and a Celgard 2400 membrane as the separator. The electrolyte was a solution of 1M LiPF6 dissolved in EC, DEC, and DMC (volume ratio 1:1:1).

[0089] Electrochemical performance was tested using a battery testing system within a voltage window of 0.01–1.5V. The initial charge-discharge test was conducted at a current density of 0.1C (1C = 2000 mA / g), and the initial discharge specific capacity and initial coulombic efficiency were calculated. Cycle performance testing was performed 500 times at a current density of 1C, and the capacity retention rate was calculated. In rate performance testing, the battery was charged at 0.1C and discharged at current densities of 0.1C, 0.2C, 0.5C, 1C, 2C, and 5C. The electrode thickness after cycling was measured using a thickness gauge, and the thickness expansion rate was calculated.

[0090] Table 2. Electrochemical performance data of the materials in the examples and comparative examples

[0091] Sample number Initial discharge specific capacity (mAh / g) First Coulomb Efficiency (ICE, %) Capacity retention (%) after 500 cycles at 1C 5C / 0.1C rate capacity retention (%) Electrode thickness expansion rate (%) Example 1 1853 91.2 86.4 72.1 75 Example 2 2085 92.8 89.1 75.3 69 Example 3 2157 91.9 85.3 73.8 78 Comparative Example 1 1950 85.5 28.7 (200 times) 65.2 >250 Comparative Example 2 1620 78.4 51.5 70.8 115 Comparative Example 3 1780 90.5 75.2 18.6 72 Comparative Example 4 1810 90.1 80.3 46.5 74 Comparative Example 5 1995 92.1 60.8 (300 times) 78.1 121 Comparative Example 6 1880 91.5 82.5 58.4 85

[0092] in conclusion:

[0093] Electrochemical performance test results show that the material prepared by this technical solution has improvements in cycle stability, rate performance and initial coulombic efficiency.

[0094] Cyclic performance and electrode expansion data show that Examples 1-3 exhibited capacity retention of over 85% and electrode thickness expansion of less than 80% after 500 cycles at 1C. In contrast, Comparative Example 1, using dense silicon, showed electrode expansion exceeding 250% and poor cycle stability. This indicates that the pre-constructed porous structure can buffer the volume changes of silicon. Meanwhile, Comparative Example 5, with only rigid carbon coating, saw its capacity retention drop to 60.8% after 300 cycles, indicating that its carbon layer fractured during cycling. The composite carbon layer design in this scheme helps maintain the integrity of the electrode structure.

[0095] Rate performance data show that Example 2 retains 75.3% capacity at 5C, which is higher than Comparative Example 3 (18.6%) without carbon nanotubes and Comparative Example 4 (46.5%) with carbon nanotubes introduced by mechanical mixing. This data indicates that the in-situ grown carbon nanotubes form an effective conductive network with the silicon matrix, reducing charge transport resistance and improving the rate performance of the material.

[0096] The initial coulombic efficiency data showed that Example 2 had an ICE of 92.8%, which was higher than that of Comparative Example 2 (78.4%), which used high specific surface area nano-silicon. This indicates that the proposed method controlled the external specific surface area of ​​the material by creating pores inside the micron-sized particles, reducing contact with the electrolyte and thus suppressing excessive SEI film formation during the first cycle.

[0097] In summary, the electrochemical performance data and physicochemical characterization results are consistent, jointly demonstrating that the technical solution of this invention, which involves preparing porous silicon, growing carbon nanotubes in situ on its surface, and coating it with a composite carbon layer, can synergistically improve the volume expansion, conductivity, and interface stability of silicon anode materials.

[0098] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a silicon-carbon composite anode material for lithium batteries with high cycle stability, characterized in that, Includes the following steps: S1. Preparation of porous silicon particles: Micron-sized silicon powder and magnesium powder are mixed and ball-milled, and a magnesium thermal reduction reaction is carried out under an inert atmosphere. Then, the product of the magnesium thermal reduction reaction is etched with an acid solution, and after washing and drying, porous silicon particles are obtained. S2. Growth of carbon nanotubes: The porous silicon particles obtained in step S1 are mixed with iron acetate catalyst, and acetylene gas is introduced under high temperature and inert atmosphere to carry out the reaction, so as to obtain porous silicon composite particles with carbon nanotubes grown on the surface. S3. Coating and carbonization: Styrene-butadiene rubber and glucose are dissolved in deionized water to prepare a carbon source solution. The porous silicon composite particles obtained in step S2 are mixed with the carbon source solution and dried to obtain a precursor. The precursor is then subjected to gradient carbonization in an inert atmosphere to obtain a carbonized product. S4. Post-processing: Grind and sieve the carbonized product obtained in step S3 to obtain the composite anode material.

2. The method for preparing a high-cycle-stability lithium-ion battery silicon-carbon composite anode material according to claim 1, characterized in that, In step S1, the mass ratio of the micron-sized silicon powder to the magnesium powder is 2.5:1 to 3.5:1; the temperature of the magnesium thermal reduction reaction is 600 to 700°C, and the holding time is 2 to 4 hours.

3. The method for preparing a high-cycle-stability lithium-ion battery silicon-carbon composite anode material according to claim 2, characterized in that, In step S1, the heating rate during the magnesium thermal reduction reaction is 3-10 °C / min, and the acid solution is a hydrochloric acid solution with a mass fraction of 5 wt%-15 wt%.

4. The method for preparing a high-cycle-stability lithium-ion battery silicon-carbon composite anode material according to claim 3, characterized in that, In step S1, the etching temperature is 60–90°C and the etching time is 2–6 hours.

5. The method for preparing a high-cycle-stability lithium-ion battery silicon-carbon composite anode material according to claim 1, characterized in that, In step S2, the mass ratio of the porous silicon particles to the iron acetate catalyst is 8:1 to 15:1; the reaction temperature is 650 to 800°C; and the reaction time is 1 to 3 hours.

6. The method for preparing a high-cycle-stability lithium-ion battery silicon-carbon composite anode material according to claim 5, characterized in that, In step S2, the rate of heating to the reaction temperature is 5–15 °C / min, and the flow rate of the acetylene gas is 30–100 sccm.

7. The method for preparing a high-cycle-stability lithium-ion battery silicon-carbon composite anode material according to claim 1, characterized in that, In step S3, the mass ratio of styrene-butadiene rubber to glucose in the carbon source solution is 2:1 to 4:1, and the solid-liquid mass ratio when the porous silicon composite particles are mixed with the carbon source solution is 1.5:1 to 2.5:

1.

8. The method for preparing a high-cycle-stability lithium-ion battery silicon-carbon composite anode material according to claim 1, characterized in that, In step S3, the gradient carbonization process includes a first stage of carbonization and a second stage of carbonization. The temperature for the first stage of carbonization is 550–650℃, and the holding time is 1–3 hours. The second stage of carbonization is carried out at a temperature of 850–1050°C for 2–5 hours.

9. The method for preparing a high-cycle-stability lithium-ion battery silicon-carbon composite anode material according to claim 8, characterized in that, In step S3, the rate of heating to the first stage carbonization temperature is 3-10℃ / min; the rate of heating from the first stage carbonization temperature to the second stage carbonization temperature is 1-5℃ / min.

10. The method for preparing a high-cycle-stability lithium-ion battery silicon-carbon composite anode material according to claim 1, characterized in that, In step S4, the sieve used for sieving has a mesh size of 150 to 300.