Silicon-carbon negative electrode material, preparation method thereof and lithium ion battery

Abstract

The application discloses a kind of silicon-carbon negative electrode material and its preparation method, lithium ion battery, belong to lithium ion battery technical field.The preparation method of the present application is as follows: biomass, calcium ion, sodium alginate are contacted in liquid phase for the first time, the reaction product is freeze-dried, carbonized, in the solution containing potassium ferrate, the preliminary carbonization product is obtained by adsorption treatment to obtain carbon precursor;Warm up to set temperature, in turn through second contact reaction, third contact reaction, the product after reaction is carried out water vapor activation, and porous carbon is obtained;The porous carbon is sequentially treated by silicon source gas deposition and gas-phase carbon source carbon coating, and the silicon-carbon negative electrode material obtained by the present application has high capacity, high initial efficiency, super-fast charging capacity and good cycle stability.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and in particular to a silicon-carbon anode material, its preparation method, and a lithium-ion battery. Background Technology

[0002] The information disclosed in the background section of this invention is intended only to enhance the understanding of the overall background of the invention and is not necessarily to be construed as an admission or in any way implying that such information constitutes prior art known to those skilled in the art.

[0003] In the field of lithium-ion batteries, positive and negative electrode materials are the core carriers of cell energy density, cycle performance, and safety performance, and their performance optimization is crucial to the development of power batteries. The actual specific capacity of current mainstream graphite anodes is approaching the theoretical limit of 372 mAh / g, which is insufficient to meet the continuously increasing energy density requirements under the application of high-nickel ternary cathode materials. Therefore, the development of new high-capacity anode materials has become a key research focus in the industry. Silicon-based materials, with their extremely high theoretical specific capacity of 4200 mAh / g, are considered an ideal alternative to graphite. However, their inherent defects severely restrict practical applications: on the one hand, silicon materials experience volume expansion of up to 300% during lithium insertion / extraction, leading to electrode structure pulverization and continuous rupture and regeneration of the solid electrolyte interphase (SEI), resulting in irreversible consumption of lithium ions and electrolyte. This not only causes the initial coulombic efficiency (first efficiency) to generally fall below 85%, but also leads to a rapid increase in impedance and accelerated capacity decay during cycling. On the other hand, silicon has extremely low intrinsic electronic conductivity, significantly limiting charge transfer dynamics and directly affecting the rate performance of the material.

[0004] To alleviate the aforementioned problems, the industry generally adopts a strategy of combining amorphous / nanosized silicon with porous carbon matrices (i.e., silicon-carbon materials). While this has improved cycle stability to some extent, it has failed to fundamentally solve the key bottleneck of poor rate performance. In existing silicon-carbon composite materials, the low graphitization degree and insufficient electronic conductivity of biomass or resin-derived hard carbon matrices further increase the charge transfer impedance inside the electrode, limiting lithium-ion insertion / extraction kinetics and resulting in severe capacity decay during high-rate charge and discharge, failing to meet the ultra-fast charging requirements of new energy vehicles. In addition, the contradiction between high capacity and high stability remains prominent: increasing silicon loading to pursue high capacity is often accompanied by more severe volume effects and more serious SEI film damage, while measures to enhance the stability of the carbon matrix (such as increasing graphitization) may lead to difficulty in controlling the porous structure, resulting in problems such as "difficulty in expanding pores, difficulty in depositing silicon, and difficulty in delithiating lithium." At the same time, high internal resistance can also cause excessive temperature rise during battery charging and discharging, affecting safety performance.

[0005] Therefore, developing silicon-carbon anode materials that combine high capacity, high initial efficiency, excellent rate performance, and cycle stability has become an urgent need to promote the application of lithium-ion batteries in the field of high-performance fast charging. Summary of the Invention

[0006] In view of this, the present invention provides a silicon-carbon anode material and its preparation method, and a lithium-ion battery. The silicon-carbon anode material provided by the present invention achieves high capacity characteristics of first discharge capacity >1800 mAh / g and first efficiency higher than 94% through the synergistic effect of high conductivity, high specific surface area and high pore volume, while also having ultra-fast charging capability, excellent rate performance and cycle stability.

[0007] In a first aspect, the present invention provides a method for preparing a silicon-carbon anode material, comprising the following steps: Biomass, calcium ions, and sodium alginate were subjected to a first contact reaction in the liquid phase. The reaction product was freeze-dried and carbonized to obtain a preliminary carbonized product. The preliminary carbonization product is adsorbed in a solution containing potassium ferrate to obtain a carbon precursor. The carbon precursor is heated to a set temperature and then passed through a second contact reaction and a third contact reaction in sequence. The product after the reaction is activated with water vapor to obtain porous carbon. The porous carbon is sequentially subjected to silicon source vapor deposition and vapor phase carbon source carbon coating treatment to obtain silicon-carbon anode material; The second contact reaction is carried out in a mixed atmosphere of hydrogen and inert gas, and the third contact reaction is carried out in a mixed atmosphere of CO2 and inert gas.

[0008] Preferably, in the step of carrying out the first contact reaction of biomass, calcium ions, and sodium alginate in the liquid phase, the concentration of sodium alginate is 1-3 wt%, the concentration of calcium ions is 0.5-2 mol / L, and the mass ratio of biomass to sodium alginate is 1:(0.5-2).

[0009] Preferably, in the step of obtaining preliminary carbonized products by carbonization, the carbonization temperature is 500~700℃ and the carbonization time is 1~3 h.

[0010] Preferably, the solution containing potassium ferrate further includes a dispersant, and the mass ratio of the preliminary carbonization product, potassium ferrate and dispersant is 1 : (0.2~0.5) : (0.05~0.2); the dispersant is polyvinylpyrrolidone or polyethylene glycol.

[0011] Preferably, in the solution containing potassium ferrate, the solvent includes a first alcohol solvent and a second alcohol solvent, wherein the first alcohol solvent is methanol or ethanol, and the second alcohol solvent is ethylene glycol or propylene glycol; the volume ratio of the first alcohol solvent to the second alcohol solvent is (7~9):(1~3).

[0012] Preferably, the adsorption treatment is vacuum impregnation, the vacuum impregnation temperature is -5~5℃, the vacuum impregnation pressure is -0.08~-0.1 MPa, and the vacuum impregnation time is 0.5~3h.

[0013] Preferably, in the step of heating the carbon precursor to a set temperature, the set temperature is 800~900℃; in the mixed atmosphere of hydrogen and inert gas, the volume fraction of hydrogen is 3~10%; the second contact reaction time is 0.5~2h; in the mixed atmosphere of CO2 and inert gas, the volume fraction of CO2 is 15~30%; the third contact reaction time is 1~3h; and the water vapor activation time is 15~40min.

[0014] Preferably, the temperature of silicon source vapor deposition is 400~550℃, and the temperature of carbon coating is 500~900℃.

[0015] Preferably, the silicon mass fraction in the silicon-carbon anode material is 50-70 wt%.

[0016] Secondly, the present invention provides a silicon-carbon anode material prepared by the above-described preparation method.

[0017] Thirdly, the present invention provides a lithium-ion battery comprising the aforementioned silicon-carbon anode material.

[0018] Compared with the prior art, the present invention has achieved the following beneficial effects: (1) This invention constructs a porous carbon matrix with high specific surface area, suitable pore size distribution, and gradient graphitization structure by synergistic gelation and freeze-drying pretreatment of biomass and sodium alginate-calcium ion system, combined with a two-step carbonization and activation strategy. This matrix provides ample space for high silicon loading (50~70wt%), ensuring the high capacity characteristics of the material (initial discharge capacity >1850 mAh / g); its internal gradient graphitization domains form efficient electron transport channels, significantly reducing electrode internal resistance, improving the rate performance and fast charging capability of the material, while suppressing the thermal effect during fast charging.

[0019] (2) This invention uses potassium ferrate as the iron and potassium source. Through precise atmosphere control of reduction followed by oxidation, the in-situ generation of Fe3O4 nanocrystals, KOH activation for pore formation, and Fe3O4 catalytic graphitization are achieved simultaneously. This process forms a gradient conductive network and multi-level pore structure of "hard carbon-graphite microcrystals" inside the material. The mechanical strength of the carbon skeleton effectively constrains the volume expansion of silicon during cycling, inhibits particle pulverization and continuous rupture and regeneration of the SEI film, and improves the cycling stability of the material. The excellent conductive network ensures charge transport efficiency, laying the foundation for achieving high initial efficiency (>94%) and good rate performance.

[0020] (3) This invention achieves uniform loading and complete coating of nano-silicon on a porous carbon matrix through a continuous process of vapor deposition and carbon coating. This structural design makes full use of the high theoretical specific capacity of silicon. The carbon coating layer isolates silicon from direct contact with the electrolyte, reducing side reactions to ensure high first-time efficiency. The carbon skeleton and the external coating layer together maintain the integrity of the electrode structure.

[0021] (4) The silicon-carbon anode material prepared by the present invention solves the technical problem that it is difficult to balance high capacity, high first efficiency, fast charging performance and long cycle life, and is suitable for the next generation of high energy density, fast charging lithium-ion batteries. Attached Figure Description

[0022] The accompanying drawings, which form part of this specification, are used to provide a further understanding of the invention. The illustrative embodiments and descriptions of the invention are used to explain the invention and do not constitute an undue limitation thereof. Obviously, those skilled in the art can obtain other drawings based on these drawings without any inventive effort.

[0023] Figure 1 These are scanning electron microscope images of the silicon-carbon anode material of Embodiment 1 of the present invention; Figure 2 These are the X-ray diffraction patterns of porous carbon in Comparative Example 4 and Example 1 of this invention. Detailed Implementation

[0024] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0025] This invention provides a method for preparing a silicon-carbon anode material, comprising the following steps: Biomass, calcium ions, and sodium alginate were subjected to a first contact reaction in the liquid phase. The reaction product was freeze-dried and carbonized to obtain a preliminary carbonized product. The preliminary carbonization product is adsorbed in a solution containing potassium ferrate to obtain a carbon precursor. The carbon precursor is heated to a set temperature and then passed through a second contact reaction and a third contact reaction in sequence. The product after the reaction is activated with water vapor to obtain porous carbon. The porous carbon is sequentially subjected to silicon source vapor deposition and vapor phase carbon source carbon coating treatment to obtain silicon-carbon anode material; The second contact reaction is carried out in a mixed atmosphere of hydrogen and inert gas, and the third contact reaction is carried out in a mixed atmosphere of CO2 and inert gas.

[0026] This invention constructs a porous carbon matrix with a gradient structure through a multi-step synergistic process, achieving efficient loading and carbon coating of nano-silicon, thereby improving the overall performance of silicon-carbon anode materials. First, sodium alginate crosslinks with calcium ions to form an "egg-box" shaped hydrogel network structure, which can uniformly coat and fix biomass particles. Freeze-drying removes moisture from the gel network, effectively preserving the porous framework synergistically constructed by the natural pores of biomass and the sodium alginate gel network. This provides a structural basis for the subsequent formation of a high specific surface area, high pore volume carbon matrix. The high pore volume provides space for the large loading of silicon, a prerequisite for achieving high capacity. The initial carbonization process completes the transformation from an organic framework to an inorganic carbon framework, determining the initial conductivity and mechanical strength of the carbon matrix. The co-carbonization of biomass and sodium alginate forms an interwoven carbon network structure, providing structural support for subsequent functionalization modifications.

[0027] Potassium ferrate is loaded into the pores of the initial carbonization product through adsorption treatment, and then a gradient structure is constructed through a two-step heat treatment process (H2 / inert gas reduction - CO2 / inert gas oxidation activation). During the drying and heat treatment processes, the unstable potassium ferrate (K2FeO4) decomposes, releasing active iron species (ultimately converted into Fe2O3, etc.), providing an iron source for the subsequent generation of nanocatalytic sites; on the other hand, the potassium compound (KOH) produced by the decomposition serves as a highly efficient catalytic activator precursor, uniformly dispersed on the carbon framework.

[0028] Under a H2 reducing atmosphere, highly dispersed iron species are transformed into nanoscale Fe / FeO particles and uniformly adhered to the carbon framework; simultaneously, K... +At high temperatures, it transforms into catalytically active potassium compounds (such as K₂O). In the subsequent CO₂ atmosphere, Fe / FeO undergoes an oxidation transformation to catalytically active Fe₃O₄ nanocrystals. The surface lattice oxygen of the Fe₃O₄ nanocrystals promotes the catalytic graphitization of surrounding carbon atoms, forming locally highly conductive graphitic microcrystalline regions. Regions far from the nanocrystals retain a hard carbon amorphous structure, thus spontaneously forming a gradient graphitization structure from the graphitic microcrystalline region to the amorphous carbon region. Simultaneously, K₂O acts as an activator, synergistically working with CO₂ to create deep pores in the carbon matrix through a gasification reaction (C + K₂O → K₂CO₃ + CO↑), constructing a multi-level porous structure. In this structure, the graphitized regions provide efficient electron transport channels, significantly reducing electrode internal resistance to improve rate performance and fast charging capability, while the abundant nanopores in the amorphous regions optimize electrolyte wettability and lithium-ion transport kinetics.

[0029] Water vapor achieves precise pore formation at high temperatures through the vaporization reaction of H2O with the carbon skeleton (C + H2O → CO + H2): On the one hand, water vapor acts as a mild oxidant to selectively etch disordered regions of carbon materials, expanding the micropore volume and generating mesopore structures, thus constructing a multi-level pore network of "micropore-mesopore"; on the other hand, the activation process is accompanied by the reconstruction of surface functional groups, introducing oxygen-containing groups such as hydroxyl (-OH) and carboxyl (-COOH) groups, which enhance the surface polarity and wettability of porous carbon materials.

[0030] The high specific surface area and suitable pore size distribution of the porous carbon matrix provide an ideal environment for silane infiltration and in-situ decomposition, achieving uniform dispersion of nano-silicon and effectively inhibiting silicon particle aggregation. The outer layer, formed by gas-phase carbon coating, creates a "core-shell" structure, which not only strengthens the structural stability of the composite material but also constructs a continuous conductive network, further improving electron conduction efficiency. This composite structure can buffer the volume expansion of silicon, inhibit the continuous growth of the SEI film, and ensure the material's high initial coulombic efficiency and cycling stability.

[0031] This invention achieves synergistic optimization of high silicon loading and high structural stability through precise control of the above-mentioned process steps, solving the technical bottleneck of traditional silicon-carbon anode materials that are difficult to balance capacity and cycle performance. At the same time, the renewability of biomass and the scalability of the process reduce production costs, providing a feasible path for the industrial application of high energy density lithium-ion batteries.

[0032] In an optional embodiment of the present invention, in the step of carrying out the first contact reaction of biomass, calcium ions, and sodium alginate in a liquid phase, the concentration of sodium alginate is 1-3 wt%, the concentration of calcium ions is 0.5-2 mol / L, and the solvent in the liquid phase is water; Ca 2+ Through the -COO of sodium alginate -Cross-linking is achieved through the formation of coordination bonds, thereby initiating a gelation reaction. The mass ratio of biomass to sodium alginate is 1:(0.5~2), ensuring that sodium alginate completely coats the biomass. This invention utilizes a biomass-sodium alginate composite system to overcome the performance bottleneck of single raw materials in the preparation of carbon-based materials. While biomass-derived carbon alone possesses natural pore template effects and cost advantages, it is prone to carbon skeleton collapse due to pyrolysis shrinkage and ash blockage of active sites, making it difficult to meet the requirements of high-capacity silicon-based anodes for nanoscale buffer space. Pure sodium alginate aerogel carbon is not only expensive but also has low carbon yield and a single pore size distribution. In the composite system of this invention, sodium alginate forms a rigid network through calcium ion cross-linking, inhibiting agglomeration and shrinkage during biomass carbonization. The natural porous structure of biomass serves as a template to introduce mesopores (2~50nm), synergistically constructing a multi-level "micropore-mesopore" pore system with the micropores of sodium alginate.

[0033] This invention does not impose special restrictions on the source of biomass; for example, coconut shells, straw, rice husks, wood, bamboo, sugarcane bagasse, nut shells, corn cobs, and other biomass can be used. Before use, the biomass needs to be dried and pulverized to ensure more uniform dispersion in the sodium alginate solution. This invention does not impose special restrictions on the time of the first contact reaction, as long as the gelation reaction can be completed; this reaction can be carried out at room temperature.

[0034] In an optional embodiment of the present invention, in the step of obtaining preliminary carbonized products by carbonization, the carbonization temperature is 500~700℃, more preferably 550~650℃, and the carbonization time is 1~3 h; ensuring that the recalcitrant components (such as lignin) in biomass are completely converted into amorphous carbon; in the present invention, the carbonization process is preferably carried out under an inert atmosphere, such as nitrogen or argon or other inert gases.

[0035] In this invention, the initial carbonization product is acid-washed to remove residual inorganic impurities from the carbonization process, thus preventing them from adversely affecting subsequent electrochemical performance. Simultaneously, moderate etching of the carbon material surface facilitates the entry of potassium ferrate into the microporous channels of the carbon material. Subsequent steps are then performed after water washing and drying.

[0036] In an optional embodiment of the present invention, the solution containing potassium ferrate further includes a dispersant, wherein the mass ratio of the preliminary carbonization product, potassium ferrate, and dispersant is 1:(0.2~0.5):(0.05~0.2); the dispersant is polyvinylpyrrolidone (PVP) or polyethylene glycol. The dispersant adsorbs onto the surface of the Fe species via molecular chains, utilizing steric hindrance to prevent agglomeration during impregnation and drying. The pyrrolidone groups of PVP can also react with Fe... 3+ Coordination bonds are formed, further enhancing dispersion stability; during subsequent heat treatment, the dispersant is completely decomposed and carbonized, eventually forming carbonaceous gas or leaving a very small amount of amorphous carbon.

[0037] In an optional embodiment of the present invention, the solution containing potassium ferrate includes a first alcohol solvent and a second alcohol solvent, wherein the first alcohol solvent is methanol or ethanol, and the second alcohol solvent is ethylene glycol or propylene glycol; the volume ratio of the first alcohol solvent to the second alcohol solvent is (7~9):(1~3).

[0038] In an optional embodiment of the present invention, the adsorption treatment is vacuum impregnation, wherein the vacuum impregnation temperature is -5~5℃, the vacuum impregnation pressure is -0.08~-0.1 MPa, and the vacuum impregnation time is 0.5~3h, more preferably 1~2h.

[0039] Because potassium ferrate decomposes rapidly in aqueous solution, this invention significantly reduces its decomposition rate by using a mixed alcohol solvent and low-temperature vacuum impregnation. The first alcohol solvent provides a low-viscosity, low-surface-tension bulk solvent, ensuring good penetration and rapid drying characteristics, while also being cost-effective. The second alcohol solvent improves the solubility and dispersion stability of potassium ferrate, helping to prevent its aggregation within the pores.

[0040] In an optional embodiment of the present invention, in the step of heating the carbon precursor to a set temperature, the set temperature is 800~900℃; in the mixed atmosphere of hydrogen and inert gas, the volume fraction of hydrogen is 3~10%; the second contact reaction time is 0.5~2h; in the mixed atmosphere of CO2 and inert gas, the volume fraction of CO2 is 15~30%; and the third contact reaction time is 1~3h.

[0041] In an optional embodiment of the present invention, the steam activation time is 15-40 minutes. The present invention does not impose any special restrictions on the steam activation steps, and the commonly used steam activation method in the art can be used. The preferred temperature is 800-1000℃.

[0042] After the step of obtaining porous carbon through steam activation, the present invention further includes an airflow pulverization step to make the median particle size of the obtained porous carbon 5~8μm; and then silicon source vapor deposition is performed.

[0043] In an optional embodiment of the present invention, the temperature of the silicon source vapor deposition is 400-550°C, and the silicon source is selected from one or more of silane, silane, or halosilane; the gaseous carbon source is selected from at least one of C1-4 alkanes, C2-4 olefins, or C2-4 alkynes; and the carbon coating temperature is 500-900°C. In the silicon-carbon anode material of the present invention, the mass fraction of silicon is 50-70 wt%. The present invention does not impose special limitations on the specific conditions of silicon source vapor deposition and carbon coating; commonly used methods in the art can be adopted. However, the silicon source vapor deposition must ensure that the final silicon mass fraction is 50-70 wt% to ensure high capacity.

[0044] The present invention also provides silicon-carbon anode materials prepared by the above preparation method.

[0045] The present invention also provides a lithium-ion battery comprising the aforementioned silicon-carbon anode material.

[0046] In this invention, the lithium-ion battery includes a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode comprises the aforementioned silicon-carbon negative electrode material. This invention does not impose any special limitations on the preparation method of the lithium-ion battery; commonly used preparation methods in the art can be employed.

[0047] The technical solution of the present invention will be further described below with reference to specific embodiments. The present invention does not impose any special restrictions on the source of reagents used in the following embodiments; commercially available products well known to those skilled in the art can be used.

[0048] Example 1 This embodiment provides a silicon-carbon anode material and its preparation method.

[0049] (1) Preparation of preliminary carbonization products: 10g of dried, pulverized corn cob powder that has passed through a 200-mesh sieve was added to a 2 wt% sodium alginate aqueous solution, maintaining a 1:1 mass ratio of corn cob powder to sodium alginate, and stirred until homogeneous. Subsequently, a 0.5 mol / L calcium chloride solution was slowly added dropwise to the suspension until the mixture completely gelled. The resulting gel was frozen at -50°C for 24 hours, then transferred to a freeze dryer and dried for 48 hours to obtain a fluffy dry gel. The dry gel was placed in a tube furnace and heated to 600°C at a rate of 5°C / min under a nitrogen atmosphere, and held at this temperature for 2 hours for carbonization. After natural cooling to room temperature, a preliminary carbonized product was obtained.

[0050] (2) Preparation of carbon precursors: The preliminary carbonized product obtained in step (1) was soaked and stirred in 0.5M dilute hydrochloric acid solution for 6 hours to remove CaO impurities. It was then repeatedly washed with deionized water until neutral and dried overnight at 110°C. 5g of the acid-washed and dried carbon material was weighed and soaked in 50mL of a mixed solvent of ethanol and propylene glycol (volume ratio 8:2), which contained 1.5g potassium ferrate and 0.5g dispersant polyvinylpyrrolidone (PVP). The mixture was transferred to a vacuum impregnation apparatus and impregnated under vacuum at 0°C and -0.09 MPa for 2 hours. After impregnation, it was vacuum dried at 50°C for 12 hours to obtain the carbon precursor.

[0051] (3) Preparation of porous carbon: The carbon precursor obtained in step (2) was placed in a tube furnace and heated to 800°C at a rate of 3°C / min under an argon atmosphere. After reaching the target temperature, the atmosphere was switched to a 5% H2 / Ar mixed gas and held for 1 hour. Subsequently, the atmosphere was switched to a 20% CO2 / Ar mixed gas and the reaction continued for 2 hours under this atmosphere. After the reaction was completed, the material was naturally cooled to room temperature under argon protection. The reacted material was removed and washed again with a 0.5M dilute hydrochloric acid solution for 12 hours to thoroughly remove the metal catalyst particles and potassium salt residue. The material was washed with deionized water until neutral and then dried. The washed and dried material was activated at 900°C under a steam atmosphere for 20 minutes, ultrasonically cleaned, dried, and then pulverized by airflow to obtain a porous carbon material with a gradient pore structure and a gradient graphitization structure.

[0052] (4) Preparation of silicon-carbon anode materials: The porous carbon material obtained in step (3) was placed in a fluidized bed and heated to 500°C under argon protection. Then, a mixture of silane and argon (silane volume fraction 20%) was introduced to carry out a vapor-phase deposition reaction, allowing nano-silicon to be uniformly deposited into the internal pores of the porous carbon. After silicon deposition was completed, the silane supply was stopped, the temperature was raised to 580°C, and a mixture of acetylene and argon (acetylene volume fraction 30%) was introduced. The reaction was carried out for 1 hour, resulting in a uniform amorphous carbon layer coating the surface of the silicon-carbon composite material. After the reaction was completed, the material was cooled to room temperature under an argon atmosphere to obtain the final silicon-carbon anode material. The mass fraction of silicon in the silicon-carbon anode material of this embodiment was determined to be 58.8 wt%.

[0053] The scanning electron microscope image of the silicon-carbon anode material in this embodiment is as follows: Figure 1 As shown, the particle size distribution of the obtained silicon-carbon anode material is between 1 and 15 μm.

[0054] Example 2 This embodiment provides a silicon-carbon anode material and its preparation method.

[0055] (1) Preparation of preliminary carbonization products: 10g of dried, pulverized bamboo powder that has passed through a 200-mesh sieve was added to a 2 wt% sodium alginate aqueous solution, maintaining a bamboo powder to sodium alginate mass ratio of 1:1.5, and stirred until homogeneous. Subsequently, a 0.5 mol / L calcium chloride solution was slowly added dropwise to the suspension until the mixture completely gelled. The resulting gel was frozen at -50°C for 24 hours, then transferred to a freeze dryer and dried for 48 hours to obtain a fluffy dry gel. The dry gel was placed in a tube furnace and heated to 600°C at a rate of 5°C / min under a nitrogen atmosphere, and held at this temperature for 2 hours for carbonization. After naturally cooling to room temperature, a preliminary carbonized product was obtained.

[0056] (2) Preparation of carbon precursors: The preliminary carbonized product obtained in step (1) was soaked and stirred in a 0.5M dilute hydrochloric acid solution at 60°C for 6 hours at a speed of 400 rpm to remove CaO impurities. It was then repeatedly washed with deionized water until neutral and dried overnight at 110°C. 5g of the acid-washed and dried carbon material was weighed and soaked in a 50mL mixture of ethanol and propylene glycol (volume ratio 8:2), which contained 2g potassium ferrate and 1g dispersant polyethylene glycol 2000 (PEG2000). The mixture was transferred to a vacuum impregnation apparatus and impregnated under vacuum at 0°C and -0.09 MPa for 2 hours. After impregnation, it was vacuum dried at 50°C for 12 hours to obtain the carbon precursor.

[0057] (3) Preparation of porous carbon: The carbon precursor obtained in step (2) was placed in a tube furnace and heated to 800°C at a rate of 5°C / min under an argon atmosphere. After reaching the target temperature, the atmosphere was switched to a 5% H2 / Ar mixture and held for 1 hour. Subsequently, the temperature was raised to 850°C, the atmosphere was switched to a 20% CO2 / Ar mixture, and the reaction continued for 2 hours under this atmosphere. After the reaction was completed, the material was naturally cooled to room temperature under argon protection. The reacted material was removed and immersed and stirred again in a 0.5M dilute hydrochloric acid solution at 60°C for 6 hours at a speed of 400 rpm to thoroughly remove the metal catalyst particles and potassium salt residue. The material was washed with deionized water until neutral and then dried. The washed and dried material was activated at 900°C under a steam atmosphere for 25 minutes, ultrasonically cleaned, dried, and then pulverized by airflow to obtain a porous carbon material with a gradient pore structure and a gradient graphitization structure.

[0058] (4) Preparation of silicon-carbon anode materials: The porous carbon material obtained in step (3) was placed in a fluidized bed and heated to 500°C under argon protection. Then, a mixture of silane and argon (silane volume fraction 20%) was introduced to carry out a vapor-phase deposition reaction, allowing nano-silicon to be uniformly deposited into the internal pores of the porous carbon. After silicon deposition was completed, the silane supply was stopped, the temperature was raised to 580°C, and a mixture of acetylene and argon (acetylene volume fraction 30%) was introduced. The reaction was carried out for 1 hour, resulting in a uniform amorphous carbon layer coating the surface of the silicon-carbon composite material. After the reaction was completed, the material was cooled to room temperature under an argon atmosphere to obtain the final silicon-carbon anode material. The mass fraction of silicon in the silicon-carbon anode material of this embodiment was determined to be 61.7 wt%.

[0059] Example 3 This embodiment provides a silicon-carbon anode material and its preparation method.

[0060] (1) Preparation of preliminary carbonization products: 10g of dried, pulverized corn stalk powder that has passed through a 200-mesh sieve was added to a 2 wt% sodium alginate aqueous solution, maintaining a mass ratio of corn stalk powder to sodium alginate of 1:1.5, and stirred until homogeneous. Subsequently, a 0.5 mol / L calcium chloride solution was slowly added dropwise to the suspension until the mixture completely gelled. The resulting gel was frozen at -50°C for 24 hours, then transferred to a freeze dryer and dried for 48 hours to obtain a fluffy dry gel. The dry gel was placed in a tube furnace and heated to 600°C at a rate of 5°C / min under a nitrogen atmosphere, and held at this temperature for 2 hours for carbonization. After naturally cooling to room temperature, a preliminary carbonized product was obtained.

[0061] (2) Preparation of carbon precursors: The preliminary carbonized product obtained in step (1) was soaked and stirred in a 0.5M dilute hydrochloric acid solution at 60°C for 6 hours at a speed of 400 rpm to remove CaO impurities. It was then repeatedly washed with deionized water until neutral and dried overnight at 110°C. 5g of the acid-washed and dried carbon material was weighed and soaked in a 50mL mixture of ethanol and propylene glycol (volume ratio 8:2), which contained 2g potassium ferrate and 1g polyvinylpyrrolidone (PVP) dispersant. The mixture was transferred to a vacuum impregnation apparatus and impregnated under vacuum at 0°C and -0.09 MPa for 2 hours. After impregnation, it was vacuum dried at 50°C for 12 hours to obtain the carbon precursor.

[0062] (3) Preparation of porous carbon: The carbon precursor obtained in step (2) was placed in a tube furnace and heated to 900°C at a rate of 5°C / min under an argon atmosphere. After reaching the target temperature, the atmosphere was switched to a 5% H2 / Ar mixture and held for 1 hour. Subsequently, the atmosphere was switched to a 20% CO2 / Ar mixture and the reaction continued for 2 hours under this atmosphere. After the reaction was completed, the material was naturally cooled to room temperature under argon protection. The reacted material was removed and immersed and stirred again in a 0.5M dilute hydrochloric acid solution at 60°C for 6 hours at a speed of 400 rpm to thoroughly remove the metal catalyst particles and potassium salt residue. The material was washed with deionized water until neutral and then dried. The washed and dried material was activated at 900°C under a steam atmosphere for 20 minutes, ultrasonically cleaned, dried, and then pulverized by airflow to obtain a porous carbon material with a gradient pore structure and a gradient graphitization structure.

[0063] (4) Preparation of silicon-carbon anode materials: The porous carbon material obtained in step (3) was placed in a fluidized bed and heated to 500°C under argon protection. Then, a mixture of silane and argon (silane volume fraction 20%) was introduced to carry out a vapor-phase deposition reaction, allowing nano-silicon to be uniformly deposited into the internal pores of the porous carbon. After silicon deposition was completed, the silane supply was stopped, the temperature was raised to 580°C, and a mixture of acetylene and argon (acetylene volume fraction 30%) was introduced. The reaction was carried out for 1 hour, resulting in a uniform amorphous carbon layer coating the surface of the silicon-carbon composite material. After the reaction was completed, the material was cooled to room temperature under an argon atmosphere to obtain the final silicon-carbon anode material. The mass fraction of silicon in the silicon-carbon anode material of this embodiment was determined to be 61.3 wt%.

[0064] Comparative Example 1 The difference between this comparative example and Example 1 is that this comparative example does not add sodium alginate and directly carbonizes the corn cob powder; the remaining steps and conditions are the same as in Example 1. Step (1) of this comparative example is as follows: 10g of dried, pulverized corn cob powder that has passed through a 200-mesh sieve was placed in a tube furnace and heated to 600℃ at a rate of 5℃ / min under a nitrogen atmosphere. The temperature was then maintained at this level for 2 hours for carbonization. After natural cooling to room temperature, the preliminary carbonized product was obtained.

[0065] Comparative Example 2 The difference between this comparative example and Example 1 is that the catalyst used in this comparative example is different from that in Example 1.

[0066] The preparation method for this comparative example is as follows: (1) Preparation of preliminary carbonization products: 10g of dried, pulverized corn cob powder that has passed through a 200-mesh sieve was added to a 2 wt% sodium alginate aqueous solution, maintaining a 1:1 mass ratio of corn cob powder to sodium alginate, and stirred until homogeneous. Subsequently, a 0.5 mol / L calcium chloride solution was slowly added dropwise to the suspension until the mixture completely gelled. The resulting gel was frozen at -50°C for 24 hours, then transferred to a freeze dryer and dried for 48 hours to obtain a fluffy dry gel. The dry gel was placed in a tube furnace and heated to 600°C at a rate of 5°C / min under a nitrogen atmosphere, and held at this temperature for 2 hours for carbonization. After natural cooling to room temperature, a preliminary carbonized product was obtained.

[0067] (2) Preparation of porous carbon: The preliminary carbonization product obtained in step (1) was soaked and stirred in 0.5M dilute hydrochloric acid solution for 6 hours to remove CaO impurities. It was then repeatedly washed with deionized water until neutral and dried overnight at 110°C. 5g of the acid-washed and dried carbon material was weighed and soaked in 50mL of 1M KOH aqueous solution for 2 hours. After drying, it was placed in a tube furnace and heated to 800°C at a heating rate of 2°C / min under an Ar atmosphere, and held for 2 hours before being removed from the furnace. The resulting sample was soaked and stirred in 0.5M dilute hydrochloric acid solution at 60°C for 6 hours at a speed of 400 rpm. Subsequently, the sample was dispersed in 50mL of 1M Fe(NO3)3 aqueous solution and soaked for 2 hours. After drying, it was placed in a tube furnace and heated to 800°C at a heating rate of 2°C / min under an Ar atmosphere, and held for 2 hours for graphitization. The reacted material was removed and immersed again in a 0.5M dilute hydrochloric acid solution at 60℃ with stirring for 6 hours at 400 rpm. After washing with deionized water until neutral, the material was dried. The washed and dried material was then activated at 900℃ in a steam atmosphere for 20 minutes, ultrasonically cleaned, dried, and then pulverized by airflow to obtain porous carbon material.

[0068] (3) Preparation of silicon-carbon anode materials: The porous carbon material obtained in step (2) was placed in a fluidized bed and heated to 500°C under argon protection. Then, a mixture of silane and argon (silane volume fraction 20%) was introduced to carry out a vapor-phase deposition reaction, allowing nano-silicon to be uniformly deposited into the internal pores of the porous carbon. After silicon deposition was completed, the silane supply was stopped, the temperature was raised to 580°C, and a mixture of acetylene and argon (acetylene volume fraction 30%) was introduced. The reaction was carried out for 1 hour, resulting in a uniform amorphous carbon layer coating the surface of the silicon-carbon composite material. After the reaction was completed, the material was cooled to room temperature under an argon atmosphere to obtain the final silicon-carbon anode material. The mass fraction of silicon in this comparative silicon-carbon anode material was determined to be 51.2 wt%.

[0069] Comparative Example 3 The difference between this comparative example and Example 1 is that this comparative example does not perform the steam activation step. Step (3) of this comparative example is as follows: The carbon precursor obtained in step (2) was placed in a tube furnace and heated to 800°C at a rate of 3°C / min under an argon atmosphere. After reaching the target temperature, the atmosphere was switched to a 5% H2 / Ar mixture and held for 1 hour. Subsequently, the atmosphere was switched to a 20% CO2 / Ar mixture and the reaction continued for 2 hours under this atmosphere. After the reaction was completed, the material was naturally cooled to room temperature under argon protection. The reacted material was removed and immersed again in a 0.5M dilute hydrochloric acid solution at 60°C with stirring for 6 hours at a speed of 400 rpm to thoroughly remove the metal catalyst particles and potassium salt residue. After washing with deionized water until neutral, the material was dried and then pulverized by air jet milling to obtain porous carbon material.

[0070] Comparative Example 4 Compared with Comparative Example 2, this comparative example does not undergo graphitization treatment. Step (2) of this comparative example is as follows: The preliminary carbonization product obtained in step (1) was soaked and stirred in 0.5M dilute hydrochloric acid solution for 6 hours to remove CaO impurities. It was then repeatedly washed with deionized water until neutral and dried overnight at 110°C. 5g of the acid-washed and dried carbon material was weighed and soaked in 50mL of 1M KOH aqueous solution for 2 hours. After drying, it was placed in a tube furnace and heated to 800°C at a heating rate of 2°C / min under an Ar atmosphere, and held for 2 hours before being removed from the furnace. The resulting sample was soaked and stirred in 0.5M dilute hydrochloric acid solution at 60°C for 6 hours at a speed of 400rpm; then washed with deionized water until neutral. The washed and dried material was activated at 900°C under a steam atmosphere for 20 minutes, ultrasonically cleaned, dried, and then pulverized by airflow to obtain porous carbon material.

[0071] Figure 2 The X-ray diffraction patterns of porous carbon in the comparative example and Example 1 show that Example 1 has peaks corresponding to graphite crystal planes, indicating that the material has a graphitized structure, while the comparative example has no obvious peaks, indicating that the carbon skeleton in the material is still amorphous.

[0072] Test case 1. Determination of porous carbon parameters The pore volume and average pore size of the porous carbon materials prepared in the examples and comparative examples were determined by nitrogen adsorption-desorption (BET) method. The test results are summarized in Table 1.

[0073] Table 1. Parameters of porous carbon materials in the examples and comparative examples.

[0074] As shown in Table 1, the porous carbon in Examples 1-3 exhibited high pore volume and suitable pore size distribution. This is attributed to the abundant initial pores provided by the biomass-sodium alginate composite gel, as well as the combined effects of simultaneous catalytic activation and subsequent steam activation. Comparative Example 1, without the addition of sodium alginate, relied solely on the biomass' own pores, resulting in the smallest pore volume and pore size. Comparative Example 2 used the traditional KOH+Fe(NO3)3 process, which had lower activation and pore-forming efficiency, and uneven iron species dispersion, leading to inferior pore volume and pore size development compared to the examples. Comparative Example 3 lacked a steam activation step, resulting in insufficient pore structure expansion and a significantly lower pore volume than the examples. Comparative Example 4 lacked a catalytic graphitization step but underwent KOH and steam activation, resulting in a larger pore volume, but the pore size distribution was biased towards large pores, leading to a poor structure.

[0075] 2. Measurement of parameters of silicon-carbon anode material The particle size (D50) of the silicon-carbon anode materials prepared in the examples and comparative examples was measured using a laser particle size analyzer. The specific surface area of ​​the materials was measured using the nitrogen adsorption-desorption (BET) method. The resistivity of the silicon-carbon anode material powder under 50 MPa pressure was measured using the four-probe method, and the resistivity was calculated. The test results are summarized in Table 2.

[0076] Table 2. Parameters of silicon-carbon anode materials in the examples and comparative examples.

[0077] Examples 1-3 exhibit low specific surface area and extremely low powder resistivity. The low specific surface area indicates that silicon was successfully loaded into the internal channels rather than being exposed; the extremely low resistivity directly proves that the gradient graphitization structure forms a highly efficient conductive network. Comparative Example 1 showed poor silicon loading in its porous carbon matrix, with a high specific surface area but poor conductivity. Comparative Example 2 showed poor catalytic effect due to low graphitization, resulting in high resistivity. Insufficient pore volume in Comparative Example 3 limited silicon loading, but the conductive framework was acceptable, and the resistivity was moderate. Comparative Example 4 exhibited abnormally high resistivity due to its lack of graphitization, leading to poor conductivity.

[0078] 3. Half-cell performance test The silicon-carbon anode material used in the examples and comparative examples was dispersed in deionized water along with commercial graphite anode material, conductive agent Super P, single-walled carbon nanotubes, binder styrene-butadiene rubber (SBR), sodium carboxymethyl cellulose (CMC), and polyacrylic acid (PAA) in a mass ratio of 6:90:0.95:0.1:0.5:0.3:2.15. The mixture was stirred until homogeneous to obtain an electrode slurry. This electrode slurry was coated onto a copper foil surface and dried at 85°C to obtain the working electrode. Using lithium metal sheets as the counter and reference electrodes, a CR2032 button half-cell was assembled in an argon-protected glove box for electrochemical testing. The initial coulombic efficiency (first efficiency) and capacity at 1.5V were obtained. The results are summarized in Table 3.

[0079] Table 3. Electrochemical performance data of half-cells in the examples and comparative examples.

[0080] First-efficiency: Examples 1-3 exhibit an ultra-high first-efficiency of over 94%, thanks to a stable SEI film (with well-coated silicon) and low irreversible capacity loss (excellent conductivity reduces side reactions). Capacity is positively correlated with silicon loading; Example 2 has the highest silicon loading and therefore the highest capacity.

[0081] 4. Full battery performance test Preparation of Soft-Pack Lithium-ion Batteries: The silicon-carbon anode materials prepared in the examples and comparative examples were dispersed in deionized water at a mass ratio of 6:90:0.95:0.1:0.5:0.3:2.15 with commercial graphite anode materials, conductive agent Super P, single-walled carbon nanotubes, binder styrene-butadiene rubber (SBR), sodium carboxymethyl cellulose (CMC), and polyacrylic acid (PAA) to obtain an electrode slurry. The electrode slurry was coated on the surface of copper foil and dried at 85°C to obtain a negative electrode sheet. The negative electrode sheet was combined with a commercial ternary cathode, an electrolyte of 1 M LiPF6 / EC+FEC+DEC+EMC (volume ratio 1:0.3:1:1), and a ceramic-coated PE membrane with a thickness of 13 μm to fabricate a soft-pack battery with a capacity of approximately 3 Ah. Full-cell performance was tested in the range of 2.75V-4.25V.

[0082] DC internal resistance (DCR): Charge the soft-pack battery to 4.25V at a constant current of 0.5C, let it stand for 1 hour, and discharge it at a constant current of 1C (3A) for 10 seconds. Record the voltage before discharge and the voltage after 10 seconds of discharge, and calculate the DCR.

[0083] DC charging rate (SOC up to 4.25V): The pouch battery was discharged at a constant current of 1C to 2.75V (fully discharged), and the discharge capacity (i.e., rated capacity 3Ah) was recorded. The battery was then charged at constant currents of 0.5C, 1C, 2C, and 3C to 4.25V, and the actual charging capacity (SOC%) at each rate was recorded.

[0084] Temperature rise: The soft-pack battery was placed in a 25℃ constant temperature chamber for 2 hours, and the initial temperature was recorded as T0. It was then charged at a constant current of 3C to 4.25V (cutoff current 0.05C). The surface temperature of the battery was monitored in real time using an infrared thermometer, and the highest temperature T was recorded. max Temperature rise = T max - T0.

[0085] Cycling performance: Calculate the discharge capacity retention rate after 500 cycles under 1C charge-discharge conditions.

[0086] The test results are summarized in Table 4.

[0087] Table 4. Electrochemical performance data of full cells in the examples and comparative examples.

[0088] The low internal resistance of the silicon-carbon anode materials in Examples 1-3 resulted in lower temperature rise during fast charging, which is crucial for battery safety. Comparative Example 4 (without graphitization) exhibited the highest internal resistance and temperature rise, and the worst fast-charging performance. The full batteries in Examples 1-3 could still be charged to over 60% capacity at a 3C high-rate charging, demonstrating excellent fast-charging capabilities. In contrast, the comparative examples generally had lower SOC at 3C, resulting in weaker fast-charging capabilities. The excellent structural buffering capacity and conductivity of Examples 1-3 significantly extended their cycle life (capacity retention >95% after 500 cycles). All comparative examples suffered from poor structural stability or conductivity due to various reasons, leading to faster cycle degradation.

[0089] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a silicon-carbon anode material, characterized in that, The steps include the following: Biomass, calcium ions, and sodium alginate were subjected to a first contact reaction in the liquid phase. The reaction product was freeze-dried and carbonized to obtain a preliminary carbonized product. The preliminary carbonization product is adsorbed in a solution containing potassium ferrate to obtain a carbon precursor. The carbon precursor is heated to 800~900℃ and then passed through a second contact reaction and a third contact reaction in sequence. The product after the reaction is activated by water vapor to obtain porous carbon. The porous carbon is sequentially subjected to silicon source vapor deposition and vapor phase carbon source carbon coating treatment to obtain silicon-carbon anode material; The second contact reaction is carried out in a mixed atmosphere of hydrogen and inert gas, and the third contact reaction is carried out in a mixed atmosphere of CO2 and inert gas.

2. The preparation method according to claim 1, characterized in that, In the step of carrying out the first contact reaction of biomass, calcium ions, and sodium alginate in the liquid phase, the concentration of sodium alginate is 1-3 wt%, and the concentration of calcium ions is 0.5-2 mol / L; the mass ratio of biomass to sodium alginate is 1:(0.5-2).

3. The preparation method according to claim 1, characterized in that, The carbonization temperature is 500~700℃, and the carbonization time is 1~3 h.

4. The preparation method according to claim 1, characterized in that, The solution containing potassium ferrate also includes a dispersant, and the mass ratio of the preliminary carbonization product, potassium ferrate and dispersant is 1:(0.2~0.5):(0.05~0.2); the dispersant is polyvinylpyrrolidone or polyethylene glycol.

5. The preparation method according to claim 1, characterized in that, The solution containing potassium ferrate includes a first alcohol solvent and a second alcohol solvent. The first alcohol solvent is methanol or ethanol, and the second alcohol solvent is ethylene glycol or propylene glycol. The volume ratio of the first alcohol solvent to the second alcohol solvent is (7~9):(1~3).

6. The preparation method according to claim 1, characterized in that, The adsorption treatment is vacuum impregnation, with a vacuum impregnation temperature of -5~5℃, a vacuum impregnation pressure of -0.08~-0.1 MPa, and a vacuum impregnation time of 0.5~3h.

7. The preparation method according to claim 1, characterized in that, In the mixed atmosphere of hydrogen and inert gas, the volume fraction of hydrogen is 3-10%; the second contact reaction time is 0.5-2h; in the mixed atmosphere of CO2 and inert gas, the volume fraction of CO2 is 15-30%; the third contact reaction time is 1-3h; and the water vapor activation time is 15-40min.

8. The preparation method according to claim 1, characterized in that, The temperature of silicon source vapor deposition is 400~550℃, and the temperature of carbon coating of vapor carbon source is 500~900℃; in the silicon-carbon anode material, the mass fraction of silicon is 50~70wt%.

9. The silicon-carbon anode material prepared by the preparation method according to any one of claims 1 to 8.

10. A lithium-ion battery, characterized in that, Including the silicon-carbon anode material as described in claim 9.

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