Preparation method of low-expansion silicon-carbon negative electrode material with gradient coating layer

Abstract

The invention relates to the technical field of lithium ion batteries, in particular to a preparation method of a low-expansion silicon-carbon negative electrode material with a gradient coating layer, which comprises the following steps: 1, providing a silicon source and a carbon source; 2, providing a coating precursor; 3, performing high-temperature carbonization on the material subjected to the primary heat treatment in an inert atmosphere, and gradually increasing the carbonization temperature from the primary heat treatment temperature to a target carbonization temperature, so that the gradient coating layer is converted into a carbon-based gradient coating layer; and 4, naturally cooling the carbonized material to room temperature, determining the cooling rate by controlling the thermal stress of the material, then grinding and screening, adjusting the grinding strength by monitoring the particle size distribution, and determining the screening aperture according to the target application demand to obtain the low-expansion silicon-carbon negative electrode material with the gradient coating layer. The gradient coating layer effectively inhibits the volume expansion of the silicon-carbon composite material in the charge-discharge process, reduces the pulverization of the material, and improves the cycle life and the capacity retention rate.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion batteries, in particular to a preparation method of a low-expansion silicon-carbon negative electrode material with a gradient coating layer. BACKGROUND

[0002] Lithium ion batteries, as the core energy storage devices for modern electronic devices and electric vehicles, have attracted extensive attention for their performance improvement. The negative electrode material is a key component of lithium ion batteries, which directly affects the energy density, cycle life and safety performance of the battery. Silicon material has become the research focus of the next generation of high-energy-density lithium ion battery negative electrode materials due to its high theoretical specific capacity. However, silicon material will undergo a huge volume change during lithium ion intercalation and deintercalation. This volume expansion leads to silicon particle pulverization, electrode structure destruction, solid electrolyte interface film instability and rapid capacity decay, which seriously limits the practical application of silicon-based negative electrode materials. In order to overcome the volume expansion problem of silicon material, researchers have proposed various strategies, including nanosizing silicon particles, designing porous structures, compounding with carbon materials and surface coating, etc. Among them, carbon coating is a common method, and the carbon layer can buffer the volume expansion of silicon, improve the electronic conductivity and inhibit the side reaction. However, the traditional uniform carbon coating layer may crack or peel off due to the mismatch of the expansion coefficient with the silicon core during long-term cycling, resulting in a decrease in the coating effect. The single-component coating layer cannot simultaneously satisfy good mechanical strength, interface stability and ion conductivity.

[0003] In the prior art, some studies attempt to use multi-layer coating or gradient structure to improve the coating effect, such as forming a multi-layer structure by alternating coating different materials. However, this method is complex in process, and there is stress concentration between the layers, which affects the overall performance. Another method is to prepare a gradient coating layer, in which the composition of the coating layer gradually changes from inside to outside to better adapt to the expansion of the core material. However, the existing gradient coating methods are often not precise in control, resulting in an unobvious gradient or uneven coating layer, which cannot effectively inhibit the volume expansion. These methods usually involve high-temperature treatment or complex chemical reactions, increasing the preparation cost and difficulty, and making it difficult to achieve large-scale production.

[0004] Therefore, the silicon-carbon negative electrode material in the prior art still faces the problems of uneven coating layer, mismatch with the expansion of the silicon core and poor cycle performance, and there is an urgent need to develop a simple and controllable preparation method to realize a low-expansion silicon-carbon negative electrode material with a gradient coating layer to solve the above technical bottlenecks. SUMMARY

[0005] Based on the above purpose, the present application provides a preparation method of a low-expansion silicon-carbon negative electrode material with a gradient coating layer, comprising the following steps: Step 1: Provide silicon source and carbon source. The silicon source is nano-silicon powder and the carbon source is graphene. Mix the silicon source and carbon source in a mass ratio. The mixing process is carried out under an inert atmosphere, which is argon. The silicon source and carbon source are uniformly compounded by high-energy ball milling. The high-energy ball milling time is determined by monitoring the particle size distribution. The high-energy ball milling speed is adjusted by controlling the particle crushing and mixing efficiency to obtain silicon-carbon composite material. Step 2: Provide a coating precursor, which is polyacrylonitrile. Disperse the silicon-carbon composite material in a solvent to form a suspension. The solvent is N-methylpyrrolidone. The concentration of the suspension is controlled by adjusting the ratio of silicon-carbon composite material to solvent. By controlling the addition rate of the coating precursor and the heat treatment parameters, a gradient coating layer is formed on the surface of the silicon-carbon composite material. The addition rate of the coating precursor is achieved by controlling the dropwise addition rate of the coating precursor solution. The heat treatment parameters include temperature adjustment and stirring speed control. The temperature is gradually increased from room temperature, and the stirring speed is adjusted by monitoring the fluidity of the suspension. After the coating precursor is added, a preliminary heat treatment is performed. The preliminary heat treatment temperature is gradually increased from room temperature to the target temperature, which is determined by thermogravimetric analysis. The preliminary heat treatment time is controlled by monitoring the degree of curing of the coating layer. Step 3: The material after preliminary heat treatment is carbonized at high temperature in an inert atmosphere, which is argon. The carbonization temperature is gradually increased from the preliminary heat treatment temperature to the target carbonization temperature. The heating rate is determined by controlling the pyrolysis reaction rate, and the holding time is adjusted by monitoring the degree of carbonization, so that the gradient coating layer is transformed into a carbon-based gradient coating layer. Step 4: The carbonized material is naturally cooled to room temperature. The cooling rate is determined by controlling the thermal stress of the material. Then, it is ground and sieved. The grinding intensity is adjusted by monitoring the particle size distribution, and the sieve aperture is determined by the target application requirements to obtain a low-expansion silicon-carbon anode material with a gradient coating.

[0006] Preferably, in step one, the silicon source is nano-silicon powder, and the particle size of the nano-silicon powder is monitored and controlled by a laser particle size analyzer. The control process includes using the laser particle size analyzer to measure the nano-silicon powder multiple times to obtain a particle size distribution curve, and adjusting the pretreatment process of the nano-silicon powder according to the particle size distribution curve. The pretreatment process includes drying and sieving. The drying temperature is determined by thermogravimetric analysis, the drying time is controlled by monitoring the moisture content, and a standard sieve is used for sieving. The sieve aperture is selected by the target particle size range. The carbon source is graphene. The specific surface area of ​​graphene is determined and controlled by the BET method. The control process includes using a BET device to perform nitrogen adsorption tests on the graphene sample, obtaining adsorption isotherms, calculating the specific surface area based on the adsorption isotherms, and optimizing the specific surface area by adjusting the preparation parameters of graphene, including the degree of oxidation and the reduction temperature. The mass ratio of silicon source to carbon source was determined by capacity performance testing. The optimization process included preparing multiple sets of silicon-carbon composite material samples with different mass ratios. Each set of samples was processed by the methods in steps one to four, and then assembled into coin cells for charge-discharge testing. The test conditions included constant current charge-discharge and cycle performance evaluation. The optimal mass ratio was selected based on the first discharge capacity and cycle capacity retention rate in the test results. The mixing process is carried out in an argon atmosphere. The purity of the argon is monitored and controlled by a gas analyzer. The monitoring process includes using the gas analyzer to continuously detect the oxygen and moisture content in the argon flow rate, and adjusting the operating parameters of the argon purification system according to the detection results. The high-energy ball milling time is determined by monitoring the particle size distribution. The determination process includes taking samples periodically during the high-energy ball milling process, measuring the particle size distribution using a laser particle size analyzer, and stopping the ball milling when the particle size distribution reaches the preset uniformity. The high-energy ball mill speed is adjusted by controlling particle crushing and mixing efficiency. The adjustment process includes monitoring the ball mill's current and power output, and adjusting the speed according to the power consumption and mixing efficiency curve to ensure that the silicon source and carbon source are fully compounded.

[0007] Preferably, in step one, the grinding media used in the high-energy ball mill is zirconia balls. The diameter of the zirconia balls is selected through experimental optimization, and the ball-to-material ratio is determined through a mixing efficiency test. The mixing efficiency test includes comparing the particle uniformity and degree of compositeness under different ball-to-material ratios. In the high-energy ball milling process, the filling rate of the ball mill jar is adjusted by controlling the total volume of the ball milling media and materials, and the filling rate is determined by calculating the void ratio inside the ball mill jar. The ambient temperature of the high-energy ball mill is maintained by a cooling system, which uses a water-cooling device. The water-cooling temperature is monitored and controlled by thermocouples. After high-energy ball milling, the morphology of the silicon-carbon composite material was observed by scanning electron microscopy. The observation process included sampling, sample preparation, and image analysis to ensure that the silicon particles were uniformly embedded in the carbon matrix. The crystal structure of silicon-carbon composite materials was analyzed by X-ray diffraction. The analysis process included scanning the sample with an X-ray diffractometer, obtaining diffraction patterns, and determining the crystallization state of silicon and carbon based on the diffraction patterns.

[0008] Preferably, in step two, the concentration of the suspension is controlled by adjusting the ratio of silicon-carbon composite material to solvent. The control process includes weighing the silicon-carbon composite material and solvent, calculating the solid content, and adjusting the amount of solvent added to make the solid content reach the target value. The coating precursor is polyacrylonitrile. The molecular weight of polyacrylonitrile is controlled by gel permeation chromatography. The control process includes analyzing the molecular weight distribution of the polyacrylonitrile solution using a gel permeation chromatograph and selecting an appropriate batch of polyacrylonitrile based on the molecular weight distribution. The preparation of the coated precursor solution was achieved by dissolving polyacrylonitrile in N-methylpyrrolidone. The dissolution temperature was adjusted by controlling the heating device, and the dissolution time was determined by monitoring the transparency of the solution. The addition rate of the coating precursor is achieved by controlling the dropping rate of the coating precursor solution. The dropping rate is controlled by a peristaltic pump, and the rotation speed of the peristaltic pump is determined by calibrating the flow rate. The suspension is continuously stirred during the addition process, and the stirring speed is monitored and adjusted by a viscometer. The temperature is gradually increased from room temperature, and the rate of increase is set by a programmable temperature control device. The temperature curve of the programmable temperature control device is optimized based on thermal response tests. The stirring speed is adjusted by monitoring the fluidity of the suspension. The adjustment process includes measuring the viscosity of the suspension using a rotational viscometer and adjusting the stirring speed according to the viscosity changes.

[0009] Preferably, in step two, the initial heat treatment temperature is gradually increased from room temperature to the target temperature. The target temperature is determined by thermogravimetric analysis. The determination process includes performing thermogravimetric analysis on the coating precursor, obtaining a thermogravimetric curve, and setting the target temperature based on the decomposition temperature range in the thermogravimetric curve. The initial heat treatment time is controlled by monitoring the degree of curing of the coating layer. The monitoring process includes using a Fourier transform infrared spectrometer to analyze the chemical structure changes of the coating layer and judging the degree of curing based on the intensity of characteristic peaks. During the initial heat treatment, the heat treatment atmosphere is an inert atmosphere, which is argon gas, and the argon gas flow rate is controlled by a flow meter. After preliminary heat treatment, the thickness of the gradient coating layer was measured by scanning electron microscopy. The measurement process included imaging and analysis of the sample cross-section. The compositional gradient of the gradient coating was analyzed by line scanning with an energy dispersive spectroscopy (EDS) instrument. The analysis process included measuring the elemental distribution from the inside to the outside of the coating to ensure that the carbon content changed gradually.

[0010] Preferably, in step three, the carbonization temperature is gradually increased from the initial heat treatment temperature to the target carbonization temperature. The heating rate is determined by controlling the pyrolysis reaction rate. The determination process includes performing differential scanning calorimetry on the coating precursor to obtain a heat flow curve, and setting the heating rate according to the position of the exothermic peak in the heat flow curve. The heat preservation time is adjusted by monitoring the degree of carbonization. The monitoring process includes using a Raman spectrometer to analyze the carbon structure of the carbonized material and judging the degree of carbonization based on the intensity ratio of the D peak and the G peak. During the carbonization process, the inert atmosphere is argon, and the purity of argon is monitored by a gas analyzer. The monitoring process includes real-time detection of the impurity content in the argon. After carbonization, the structure of the carbon-based gradient coating layer was observed using a transmission electron microscope. The observation process included preparing ultrathin sections and imaging. The conductivity of the carbon-based gradient coating was measured using a four-probe method, which involved pressing the sample into a pellet and connecting it to a test circuit.

[0011] Preferably, in step three, the target carbonization temperature is determined by a combination of thermogravimetric analysis and differential scanning calorimetry. The determination process includes analyzing the thermal decomposition behavior and phase transition temperature of the coated precursor. The heating program of the carbonization furnace is achieved through multi-stage temperature control, which includes a low-temperature stage, a medium-temperature stage, and a high-temperature stage. The temperature and time of each stage are determined through optimization experiments. During the carbonization process, the material is located within the constant temperature zone of the carbonization furnace, which is determined by temperature mapping tests. After carbonization, the mass loss of the material is assessed by weighing, and the assessment process includes comparing the weight of the samples before and after carbonization.

[0012] Preferably, in step four, the cooling rate is determined by controlling the thermal stress of the material. The determination process includes using thermocouples to monitor the material temperature and adjusting the cooling environment according to the temperature drop curve. The cooling environment is an inert atmosphere, which is argon gas, and the argon gas flow rate is regulated by a flow valve. Grinding was carried out using a planetary ball mill, with zirconia balls as the grinding media. The grinding time was determined by monitoring the particle size distribution, and the determination process included periodic sampling and laser particle size analysis. The grinding intensity is controlled by adjusting the ball mill speed and the ball-to-material ratio, and the control process is based on a particle crushing kinetics model. Sieving is performed using a standard sieve, and the sieve aperture is determined based on the target application requirements. The determination process includes referencing the typical particle size distribution of battery anode materials. After sieving, the tap density of the material is measured by the volumetric method, which includes the use of a tap density meter.

[0013] Preferably, after step four, a characterization step is also included, which includes observing the surface morphology and coating integrity of the material using a scanning electron microscope. The observation process includes gold plating of the sample and image acquisition. X-ray diffraction was used to analyze the crystal structure of the material. The analysis process included setting the scanning angle range and step size. Electrochemical tests are used to evaluate the battery performance of materials. Electrochemical tests include assembling coin cells, performing constant current charge-discharge tests and cycle performance tests, and test conditions include voltage window and current density. Electrochemical test data are used to calculate the initial discharge capacity and capacity retention.

[0014] Preferably, in step two, the formation of the gradient coating layer further includes controlling the degree of polymerization of the coating precursor, the degree of polymerization being monitored by gel permeation chromatography, and the monitoring process including periodic sampling and analysis of molecular weight distribution; The elastic modulus of the gradient coating was evaluated by nanoindentation testing, which included measuring the mechanical properties of the coating at different locations using a nanoindenter. The ionic conductivity of the gradient coating was measured by electrochemical impedance spectroscopy, which involved fabricating the material into an electrode and testing the impedance spectrum. Throughout the entire preparation process, the control of all parameters is based on a real-time feedback system, which includes sensors, a data acquisition unit, and a control unit. The sensors monitor key parameters, the data acquisition unit records data, and the control unit adjusts process parameters according to a preset algorithm.

[0015] The beneficial effects of this invention are: 1. By employing the gradient coating method of this invention, the composition of the coating layer gradually changes, effectively adapting to the volume expansion of the silicon material, thereby alleviating the problems of silicon particle pulverization and electrode structure damage. This method can significantly improve the cycle stability of silicon-carbon anode materials and extend battery life.

[0016] 2. This invention, through the design of a gradient coating layer, ensures the uniformity of the coating layer and its matching with the expansion of the silicon core, thereby avoiding cracking and delamination. The gradient coating layer can better alleviate the expansion problem of silicon materials and maintain the stability of the battery structure.

[0017] 3. The gradient coating method proposed in this invention can effectively improve the interfacial stability between the coating layer and the silicon core, reduce the occurrence of side reactions, and improve electronic conductivity and ionic conductivity, thereby significantly improving the cycle performance of lithium-ion batteries.

[0018] 4. This invention solves the problems of high-temperature treatment and chemical reaction complexity in the preparation process of existing methods by adopting a simple and controllable preparation method, thereby reducing production costs and realizing the feasibility of large-scale production.

[0019] 5. This invention, through the design of a gradient coating layer, ensures both the mechanical strength of the coating layer and enhances interface stability. Furthermore, the gradient design of the coating layer allows the material to better adapt to the volume expansion of silicon, avoiding cracking and peeling phenomena commonly found in traditional coating layers. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in this invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, those skilled in the art can obtain other drawings based on these drawings without creative effort.

[0021] Fig. 1 This is a flowchart of the steps of the method of the present invention; Fig. 2 This is a flowchart illustrating the characterization steps of the method of the present invention. Detailed Implementation

[0022] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. It should also be noted that, to make the embodiments more comprehensive, the following embodiments are the best and preferred embodiments, and those skilled in the art can use other alternative methods to implement some well-known technologies; moreover, the accompanying drawings are only for more specific description of the embodiments and are not intended to specifically limit the present invention.

[0023] Please see Figs. 1-2 This invention provides a method for preparing a low-expansion silicon-carbon anode material with a gradient coating. In step one, nano-silicon powder is used as the silicon source and graphene as the carbon source. These two materials are mixed at an appropriate mass ratio, and the mixing process is carried out under an inert atmosphere, specifically argon. The key to this process is to use high-energy ball milling technology to ensure uniform composite of the silicon and carbon sources. The high-energy ball milling time is determined by monitoring the particle size distribution to ensure uniform particle size and suitability for subsequent steps. Adjusting the ball milling speed helps control the degree of particle breakage and mixing efficiency, thereby obtaining a high-quality silicon-carbon composite material. This ensures good bonding between silicon and carbon while avoiding uneven distribution, ensuring the uniformity and effectiveness of the subsequent coating layer.

[0024] In step two, polyacrylonitrile is used as a coating precursor, and the silicon-carbon composite material is dispersed in a solvent (N-methylpyrrolidone) to form a suspension. The concentration of the suspension is controlled by adjusting the ratio of silicon-carbon composite material to solvent to ensure uniform dispersion. When adding the coating precursor, the droplet rate of the solution is controlled to ensure uniform penetration of the coating precursor onto the surface of the composite material. Then, the temperature and stirring speed are adjusted, with the temperature gradually increasing from room temperature and the stirring speed adjusted by monitoring the flowability of the suspension, to form a uniform and gradient coating layer. After the coating precursor is added, a preliminary heat treatment is performed, with the temperature and time controlled by monitoring the degree of curing of the coating layer. The coating precursor can effectively form a gradient coating layer on the surface of the silicon-carbon composite material, avoiding the problems of uneven or unstable coating layers that may occur in traditional methods.

[0025] In step three, the pre-heat-treated material is carbonized at high temperature in an inert atmosphere (argon). The carbonization temperature is increased gradually, with the heating rate adjusted according to the pyrolysis reaction rate to ensure the stability and effectiveness of the carbonization reaction. The holding time is adjusted according to the degree of carbonization to ensure the complete transformation of the coating layer into a carbon-based gradient coating layer. Carbonization transforms the coating layer into a carbon-based material with good electrical conductivity and chemical stability, effectively improving the material's electronic conductivity and cycle stability.

[0026] In step four, during the natural cooling of the carbonized material to room temperature, the cooling rate needs to be optimized by controlling thermal stress to avoid thermal stress damage caused by rapid cooling. After cooling, the material enters the grinding and sieving steps. The grinding intensity is adjusted by monitoring the particle size distribution, and the sieving aperture is set according to the target application requirements. Through these processes, a low-expansion silicon-carbon anode material with a gradient coating is finally obtained. By precisely controlling grinding and sieving, silicon-carbon anode materials with uniform particle size suitable for practical application requirements can be obtained, while avoiding performance degradation due to over- or under-grinding.

[0027] By meticulously controlling the process parameters at each step, from raw material mixing and the uniform addition of the coating precursor to high-temperature carbonization and cooling, a stable and uniform gradient coating layer is ensured in the final silicon-carbon anode material. This material effectively suppresses the volume expansion of silicon, improves the battery's cycle performance, energy density, and safety, and provides a more stable anode material for the application of next-generation high-energy-density lithium-ion batteries.

[0028] In one possible implementation, the silicon source is nano-silicon powder, the particle size of which is monitored and controlled using a laser particle size analyzer. Specifically, the particle size distribution of the silicon powder is measured multiple times using the laser particle size analyzer to obtain a particle size distribution curve. Based on this curve, the pretreatment process of the nano-silicon powder is adjusted. First, the drying temperature is determined through thermogravimetric analysis, and the drying time is controlled by monitoring the moisture content to ensure that the silicon powder is not overheated during drying, maintaining its performance stability. The sieving process uses standard sieves, and the particle size range is controlled by selecting an appropriate sieve aperture to obtain a uniform particle size distribution. This precise particle size control ensures more efficient subsequent compounding and coating processes, avoiding inconsistencies in material properties caused by excessively large or small particle sizes.

[0029] The carbon source is graphene, and its specific surface area is controlled using the BET method. Nitrogen adsorption tests are performed on graphene using a BET apparatus to obtain adsorption isotherms, from which the specific surface area is calculated. To optimize the specific surface area, the preparation parameters of graphene, particularly the degree of oxidation and reduction temperature, need to be adjusted during production. These parameters have a significant impact on the specific surface area of ​​graphene; proper adjustment can improve the conductivity and specific surface area of ​​graphene, thereby enhancing its composite effect with the silicon source and improving the electrochemical performance of the anode material.

[0030] In determining the mass ratio of silicon and carbon sources, the ratio was optimized through capacity performance testing. Specifically, multiple sets of silicon-carbon composite material samples with different mass ratios were prepared, sequentially processed through steps one through four, and finally assembled into coin cells for charge-discharge testing, including constant current charge-discharge and cycle performance evaluation. The optimal mass ratio was selected based on the initial discharge capacity and cycle capacity retention. This optimization process ensures high energy density and stable cycle performance of the silicon-carbon composite material in practical applications.

[0031] Argon was used as an inert atmosphere during the high-energy ball milling process. The oxygen and moisture content in the argon gas flow rate was continuously monitored using a gas analyzer to ensure the purity of the atmosphere, and the operating parameters of the argon purification system were adjusted based on the monitoring results. This measure avoided interference from oxygen and moisture, ensuring sufficient composite formation of the silicon and carbon sources during the high-energy ball milling process, thereby improving the overall performance of the material.

[0032] In high-energy ball milling, monitoring particle size distribution is crucial for determining the milling time. Periodic sampling and laser particle size analysis are used to measure particle size until the particle size distribution reaches a preset uniformity, at which point milling is stopped. Adjustment of the milling speed is achieved by monitoring the mill's current and power output, adjusting the speed based on the power consumption versus mixing efficiency curve to ensure sufficient composite of the silicon and carbon sources. Appropriate milling time and speed prevent over-milling or insufficient mixing, ensuring the ideal structure and properties of the silicon-carbon composite material.

[0033] This invention improves the stability and controllability of the production process while ensuring material performance. The uniformity, optimized specific surface area, and precise control of the mass ratio of the silicon-carbon composite material enable the final product to exhibit higher energy density and excellent cycle stability in battery applications, thereby meeting the requirements for high-performance anode materials.

[0034] In one possible implementation, zirconia balls are chosen as the milling media during high-energy ball milling primarily because of their high hardness and good chemical stability, which effectively prevents material contamination during the milling process. The diameter of the zirconia balls needs to be optimized experimentally. The core of the experiment is to observe the effect of adjusting zirconia balls of different diameters on particle size and composite effect. The determination of the ball-to-material ratio also relies on mixing efficiency testing. Specifically, experiments are conducted with different ball-to-material ratios to compare the particle uniformity and composite degree of silicon powder and carbon source under each ratio. In this way, uniform mixing of materials during the ball milling process can be ensured, avoiding uneven composite phenomena.

[0035] In high-energy ball milling, the filling rate of the mill jar needs to be controlled by appropriately adjusting the total volume of the milling media and the material. Adjusting the filling rate requires calculating the void fraction within the mill jar to ensure optimal relative movement between the media and the material during milling. A reasonable filling rate effectively avoids over- or under-milling, ensuring sufficient collision and grinding of particles and optimizing the microstructure of the silicon-carbon composite material.

[0036] Temperature control is crucial in high-energy ball milling, as excessively high temperatures can cause thermal damage to the material or affect its performance stability. Therefore, water cooling is used to maintain the temperature of the ball milling environment. The cooling system monitors the temperature via thermocouples to ensure stable water cooling temperatures and prevent material expansion or morphological changes due to high temperatures. This temperature control ensures the structural stability of the material during ball milling and avoids the impact of thermal effects on material quality.

[0037] After high-energy ball milling, the morphology of the silicon-carbon composite material needs to be observed in detail using scanning electron microscopy (SEM). During observation, samples are first taken and prepared, and then image analysis is used to ensure that the silicon particles are uniformly embedded in the carbon matrix. This method allows for a direct assessment of the dispersion of the silicon-carbon composite material, ensuring that the degree of integration between the silicon powder and the carbon source meets the expected requirements. If the material morphology does not meet the requirements, the ball milling process can be adjusted to ensure the quality of the final product.

[0038] Finally, to ensure the crystal structure of the silicon-carbon composite material met expectations, X-ray diffraction (XRD) analysis was used to examine the material. By scanning the sample and obtaining diffraction patterns, the crystallization state of silicon and carbon could be analyzed. This process helped researchers understand whether silicon in the silicon-carbon composite material exhibited a crystalline form and whether unreacted carbon sources remained. Good control over the crystal structure is particularly important for improving the electrochemical performance of the material, as a uniform crystal structure contributes to improved charge-discharge cycle stability and energy density.

[0039] In one possible implementation, the concentration of the suspension needs to be precisely controlled by adjusting the ratio of the silicon-carbon composite material to the solvent. The specific method involves first weighing the required silicon-carbon composite material and solvent, calculating their solid content, and then adjusting the amount of solvent added to achieve the target solid content. This process is crucial because controlling the solid content directly affects the uniformity and stability of the coating layer.

[0040] The selection and control of the coating precursor, polyacrylonitrile, are also crucial. Gel permeation chromatography (GPC) can accurately determine the molecular weight distribution of polyacrylonitrile. After analyzing the polyacrylonitrile solution using GPC, an appropriate batch of polyacrylonitrile is selected. The molecular weight of polyacrylonitrile determines its solubility, viscosity, and flowability during the coating process; therefore, precise molecular weight control ensures the quality and performance of the coating layer.

[0041] The dissolution of polyacrylonitrile (PAB) requires strict control of both temperature and time in an N-methylpyrrolidone (NMP) solvent. First, the dissolution temperature is adjusted using a heating device to ensure complete dissolution of the PAB. The dissolution time is then monitored by observing the solution's transparency; increased transparency usually indicates that the dissolution process is complete. Solution transparency is a crucial indicator of complete dissolution.

[0042] The dripping rate of the precursor solution during the coating process is precisely controlled using a peristaltic pump. The pump's rotation speed is determined by calibrating the flow rate, ensuring consistent dripping speed. During the dripping process, continuous stirring of the suspension is necessary to ensure uniform material distribution. The stirring speed is monitored using a viscometer and adjusted in real time based on changes in the suspension's viscosity. This method ensures uniform addition of the precursor solution, preventing localized over- or under-coating.

[0043] Temperature regulation begins at room temperature and gradually increases, with the rate of increase set by a programmed temperature control device. By optimizing the temperature profile, degradation of the polyacrylonitrile solution or structural changes in the silicon-carbon composite material caused by excessively rapid temperature increases can be avoided. Temperature control not only affects the chemical reaction of the precursor but also determines the uniformity and stability of the coating layer.

[0044] Throughout the coating process, the stirring speed needs to be adjusted according to the fluidity of the suspension. By using a rotational viscometer, the viscosity of the suspension can be monitored in real time, and the stirrer speed can be adjusted based on the viscosity changes. This process is crucial because suspensions of different viscosities require different stirring speeds to ensure uniform coating and prevent agglomeration or incomplete coating.

[0045] By implementing the above control steps, the silicon-carbon composite material can be ensured to reach an ideal state during the coating process. Precise control of the suspension concentration, the molecular weight of polyacrylonitrile, the solution dropping rate, temperature, and stirring speed ensures that the coating layer forms uniformly and stably, avoiding uneven coating or structural defects. These precise controls not only improve the cycle performance and stability of the anode material but also optimize its electrochemical characteristics, guaranteeing the manufacture of high-performance batteries.

[0046] In one possible implementation, the initial heat treatment temperature needs to be gradually increased from room temperature to a target temperature. The target temperature is determined by thermogravimetric analysis (TGA) of the coating precursor. Specific steps include performing TGA on the coating precursor (e.g., a polyacrylonitrile solution) and obtaining a thermogravimetric profile. TGA, by monitoring the mass change of the material at different temperatures, can reveal the material's decomposition temperature range. Based on this decomposition temperature range, an appropriate target temperature is set to ensure effective curing of the coating layer during heat treatment, avoiding overheating or incomplete curing.

[0047] The initial heat treatment time is controlled by monitoring the degree of curing of the coating layer. To achieve this, Fourier transform infrared spectroscopy (FTIR) is used to analyze changes in the chemical structure of the coating layer. FTIR can detect changes in functional groups within the coating layer, particularly chemical structural transformations during curing. During curing, the intensity of characteristic peaks in the coating layer changes; analyzing these changes allows for the determination of the degree of curing, which in turn enables the control of the heat treatment time.

[0048] During the initial heat treatment process, an inert atmosphere is required to prevent oxidation of the coating material or other undesirable reactions. Argon is chosen as the inert atmosphere here because it effectively prevents oxygen from participating in the reaction, ensuring the stability of the heat treatment process. The argon flow rate is precisely controlled using a flow meter to ensure a constant atmosphere.

[0049] After heat treatment, the thickness of the gradient coating layer needs to be measured using a scanning electron microscope (SEM). SEM can accurately measure the coating layer thickness by imaging and analyzing the sample cross-section, confirming whether the coating layer meets design requirements. This method can capture the microstructure of the coating layer with high precision, ensuring the consistency and uniformity of the gradient coating layer.

[0050] To ensure a gradual distribution of carbon content within and outside the gradient coating, line scan analysis using energy dispersive X-ray spectroscopy (EDX) is necessary. This process analyzes the compositional changes of the coating by measuring the elemental distribution from the inside to the outside of the coating. This ensures that the carbon content of the coating gradually varies between layers, forming the desired gradient structure. This analytical process is crucial for ensuring material performance, particularly impacting battery cycle stability and capacity decay.

[0051] In one possible implementation, the carbonization temperature is gradually increased from the initial heat treatment temperature to the target carbonization temperature. The heating rate is determined by controlling the pyrolysis reaction rate. Specifically, differential scanning calorimetry (DSC) is first performed on the coating precursor to obtain a heat flow profile. The heat flow profile will show a series of peaks, among which the position of the exothermic peaks reflects the pyrolysis reaction process of the material. By analyzing these exothermic peaks, the optimal heating rate can be determined to ensure that the material does not affect the final coating structure due to an excessively fast or slow reaction during the heating process.

[0052] The holding time during the carbonization process is dynamically adjusted by monitoring the degree of carbonization. Specifically, this monitoring method uses Raman spectroscopy to analyze the carbon structure of the carbonized material. Raman spectroscopy assesses the degree of carbonization by measuring the intensity ratio of the D and G peaks. During carbonization, the intensity ratio of the D and G peaks changes with the degree of carbonization. By monitoring this ratio in real time, it is possible to effectively determine whether the carbonization has reached the predetermined requirements and adjust the holding time accordingly, thereby controlling the degree of carbonization and avoiding over- or incomplete carbonization.

[0053] During the carbonization process, an inert atmosphere—argon—is used to prevent oxidation. The purity of argon has a significant impact on the carbonization effect; therefore, the impurity content in the argon must be monitored in real time using a gas analyzer. By controlling the gas purity, it can be ensured that the carbonization process takes place in a pure, inert environment, thereby avoiding interference from impurities and guaranteeing the quality and stability of the final carbon-based gradient coating.

[0054] After carbonization, the microstructure of the carbon-based gradient coating layer needs to be observed using transmission electron microscopy (TEM). TEM provides high-resolution images, and by preparing ultrathin sections and imaging them, the detailed structure of the coating layer, such as its layered distribution and particle morphology, can be accurately observed. This observation method allows for confirmation of the coating layer's thickness, uniformity, and the presence of defects, ensuring that the quality of the gradient coating layer meets design requirements.

[0055] After carbonization, the electrical conductivity of the carbon-based gradient coating is a crucial indicator for evaluating its conductivity performance. Therefore, the four-probe method was employed to measure the conductivity of the samples. The four-probe method measures conductivity by pressing the sample into a pellet and connecting it to a test circuit. This method accurately measures the conductivity of the material, helping to assess whether the conductivity of the coating meets requirements, thereby ensuring the material's performance in battery applications.

[0056] In one possible implementation, the target carbonization temperature is set using a combination of thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). First, TGA measures the thermal decomposition behavior of the coated precursor, recording the weight loss of the sample during heating, which provides the decomposition temperature range of the coated precursor. Then, DSC is used to analyze the phase transition temperature of the coated precursor, helping to identify possible structural changes during carbonization, such as melting or phase transitions of the precursor. By combining these two techniques, the target carbonization temperature can be accurately determined, ensuring that the coating fully reacts and solidifies during carbonization, while avoiding excessive pyrolysis or incomplete reaction.

[0057] To ensure that the temperature changes during the carbonization process meet the required reaction conditions, the carbonization furnace employs a multi-stage temperature control program. This heating process is divided into a low-temperature stage, a medium-temperature stage, and a high-temperature stage, and the temperature and heating time for each stage need to be determined through optimization experiments. In the low-temperature stage, the temperature is relatively low, with the aim of removing moisture and volatile substances from the sample; in the medium-temperature stage, the temperature increases to promote the pyrolysis reaction of the coating material; in the high-temperature stage, the target carbonization temperature is reached to further stabilize the carbon structure of the material. This multi-stage temperature control method ensures precise control of the temperature rise process, thereby optimizing the carbonization process and improving the quality of the gradient coating layer.

[0058] During carbonization, the sample must be located within the isothermal zone of the carbonization furnace to ensure uniform heating. The isothermal zone of the carbonization furnace is determined through temperature mapping tests. This process involves placing temperature sensors at different locations within the furnace to obtain a temperature distribution map. By analyzing the test results, the location of the isothermal zone can be accurately determined, ensuring that the sample remains in a temperature-uniform area throughout the carbonization process, thus avoiding incomplete carbonization or uneven coating quality caused by temperature inconsistencies.

[0059] After carbonization, the mass loss of the material needs to be assessed using a gravimetric method. The specific assessment process involves comparing the mass of the sample before and after carbonization to analyze the volatilization loss of substances during the carbonization process. The change in mass of the sample before and after carbonization can reflect the degree of carbonization reaction and whether there is excessive decomposition or residual unreacted substances. This mass loss assessment method is simple and direct, providing feedback for optimizing the carbonization process and ensuring the quality stability of the final product.

[0060] In one possible implementation, the cooling rate is controlled by managing the thermal stress of the material. Specifically, thermocouples are used to monitor the temperature change of the material during cooling, and the cooling environment is adjusted based on the real-time temperature drop curve. Excessive cooling can cause the molecular structure inside the material to contract rapidly, generating thermal stress that may lead to material cracking or structural defects. Therefore, by monitoring and adjusting the cooling rate in real time, the accumulation of thermal stress caused by uneven cooling is avoided, thus contributing to the structural stability and performance optimization of the material.

[0061] The inert atmosphere used during the cooling process is primarily argon. An inert atmosphere helps prevent the material from reacting with oxygen or other reactive gases at high temperatures, avoiding adverse reactions such as oxidation. The argon flow rate is regulated by a flow valve; this precise gas flow control ensures that the material remains in a stable and non-reactive environment during cooling, thereby improving the material's purity and quality.

[0062] The grinding process is carried out using a planetary ball mill. This type of mill can apply grinding forces in multiple directions, ensuring uniform particle breakage. Zirconia balls were chosen as the grinding media. These media possess high hardness and wear resistance, effectively preventing media contamination during grinding and maintaining the high purity of the material. The determination of the grinding time depends on monitoring the particle size distribution. By periodically sampling and performing laser particle size analysis, the grinding time can be adjusted in real time to ensure that the desired particle size distribution is obtained.

[0063] Grinding intensity is controlled by adjusting the ball mill's rotational speed and ball-to-material ratio. Higher rotational speeds result in faster grinding, but may also lead to excessive particle breakage or overheating. Adjusting the ball-to-material ratio affects the contact efficiency between the grinding media and the material, as well as the grinding force. By controlling the process based on a particle breakage kinetic model, these parameters can be scientifically adjusted to ensure that the grinding intensity meets the requirements for particle breakage while avoiding particle size inhomogeneity.

[0064] The ground material needs to undergo a sieving process to ensure that its particle size meets the requirements of the target application. Sieving is carried out using standard sieves, with the sieve aperture determined according to the requirements of the target application (such as battery anode materials). By referring to the typical particle size distribution of battery anode materials, the sieve aperture can be precisely selected to ensure that the particle size distribution of the final product is within the ideal range, thereby improving the electrochemical performance of the material.

[0065] After sieving, the tap density of the material needs to be measured using a volumetric method. Tap density is an important indicator of material density, reflecting the packing density between particles. By using a tap density meter, the bulk density of the material can be accurately measured without compressing it, thus providing necessary data support for optimizing the material's performance (such as conductivity and capacity when used in batteries).

[0066] In one possible implementation, the characterization step first involves observing the surface morphology and coating integrity of the material using a scanning electron microscope (SEM). SEM provides high-resolution observation of the sample's microstructure, offering detailed images of the material surface. During this process, the sample is gold-plated to prevent electrostatic buildup from the electron beam and ensure image clarity. SEM images allow observation of the material's particle size distribution, particle surface morphology, coating uniformity, and the presence of defects such as cracks or flaking. This enables the assessment of coating integrity and its effectiveness in protecting the silicon-carbon material, reducing potential volume expansion or material degradation during cycling. Optimizing coating integrity can improve the material's mechanical stability and electrochemical performance.

[0067] Secondly, X-ray diffraction (XRD) is used to analyze the crystal structure of the material. XRD provides lattice information, revealing the phase composition and crystal structure characteristics of the material. During the analysis, the scanning angle range and step size are set, which typically depends on the material properties and the required resolution. By scanning at different angles, different diffraction peaks can be obtained, allowing analysis of the material's phase composition and crystal arrangement. For example, for silicon-carbon composites, XRD can be used to confirm the reduction state of silicon and whether a silicon-carbon composite phase was formed during synthesis. This analytical method allows us to understand the material's structural stability and assess the presence of harmful phases or undesirable crystal structures, thus guiding further material improvements.

[0068] Electrochemical testing is a crucial step in evaluating the performance of anode materials, especially in battery applications. First, the material is used to assemble coin cells; this testing method is well-standardized, allowing for precise control of test conditions in the laboratory. The testing process includes constant current charge-discharge testing and cycle performance testing, which are used to evaluate the material's energy storage capacity and cycle stability, respectively. In constant current charge-discharge testing, a constant current is applied during charge and discharge to test the material's initial discharge capacity and energy density. In cycle performance testing, the capacity decay of the material is monitored through multiple charge-discharge cycles. Test conditions include a voltage window (i.e., the battery's operating voltage range) and current density (the amount of current passing through the battery per unit time). The settings of these parameters affect the battery's charge-discharge efficiency and lifespan, and therefore need to be optimized according to the material's characteristics and application requirements.

[0069] Data obtained through electrochemical testing allows for the calculation of a material's initial discharge capacity and capacity retention rate. Initial discharge capacity refers to the total amount of electricity released by the material during the first charge-discharge cycle, reflecting its energy storage capability. Capacity retention rate refers to the retention of the material's discharge capacity after several charge-discharge cycles, reflecting its cycle stability and long-term performance. Higher initial discharge capacity and better capacity retention rate generally indicate more efficient energy conversion and a longer lifespan for the material in the battery.

[0070] In one possible implementation, the formation of the gradient coating layer during preparation requires precise control of the degree of polymerization of the coating precursor. The degree of polymerization refers to the length of the polymer chains and directly affects the structure and properties of the coating layer. To accurately monitor the degree of polymerization, gel permeation chromatography (GPC) is used for online monitoring. GPC can separate components of different molecular weights in a sample; by periodically sampling and analyzing the molecular weight distribution, the degree of polymerization of the precursor can be monitored in real time. By controlling the degree of polymerization, the density and thickness of the coating layer can be adjusted, thereby optimizing its performance and ensuring that the coating layer effectively resists the volume expansion of the silicon-carbon anode while maintaining good electrochemical performance.

[0071] To assess its elastic modulus, nanoindentation testing technology was employed. In this process, a nanoindenter was used to apply minute forces at different locations on the coating layer, and the material's hardness and elastic modulus were measured. This test can accurately evaluate the mechanical properties of the coating layer at the microscale, such as hardness, rigidity, and elastic characteristics. By controlling the elastic modulus of the coating layer, its compressive strength and abrasion resistance can be improved, preventing cracks or peeling due to expansion and contraction during battery charging and discharging, thereby extending the material's service life.

[0072] The ionic conductivity of the gradient coating layer directly affects the battery performance of the anode material. Electrochemical impedance spectroscopy (EIS) is used to evaluate its ionic conductivity. In this process, the sample is fabricated as an electrode, and its impedance spectrum is measured at different frequencies. Impedance spectroscopy analysis reveals information such as the material's conductivity, ion mobility, and interfacial resistance. This measurement helps determine whether the gradient coating layer can effectively conduct electricity, thereby ensuring the battery's high-efficiency charge-discharge capability. If the impedance spectrum shows good conductivity, it indicates that the coating layer makes a positive contribution to battery performance and can effectively improve the battery's power density and cycle stability.

[0073] Throughout the entire preparation process, the control of all process parameters is based on a real-time feedback system. This system consists of sensors, a data acquisition unit, and a control unit. Sensors monitor key parameters such as temperature, humidity, and pressure, while the data acquisition unit records this data and transmits it to the control unit in real time. The control unit dynamically adjusts the process parameters according to a preset algorithm to ensure that each step in the preparation process reaches its optimal state. This real-time feedback system effectively avoids process errors caused by improper human operation or environmental fluctuations, improving the consistency and stability of the materials.

[0074] Example: Preparation of low-expansion silicon-carbon anode material with gradient coating; Step 1: Preparation of silicon-carbon composite materials; This step aims to prepare a silicon-carbon composite material as a substrate for the gradient coating layer. The specific process is as follows: The silicon and carbon sources were provided as follows: the silicon source was nano-silicon powder (purchased from Shanghai Aladdin Biochemical Technology Co., Ltd., purity ≥99.9%), and the carbon source was graphene (purchased from Suzhou CarbonFeng Technology Co., Ltd., purity ≥99.5%). The particle size of the nano-silicon powder was controlled using a laser particle size analyzer (Malvin Mastersizer 3000): First, a small amount of nano-silicon powder sample was dispersed in ethanol to form a suspension. Multiple measurements were then performed using the laser particle size analyzer (each measurement was repeated 5 times) to obtain the particle size distribution curve. The particle size distribution curve showed that the particle size of the nano-silicon powder was mainly distributed in the range of 50 nm to 100 nm. Based on the curve, the pretreatment process for the nano-silica powder was adjusted: pretreatment included drying and sieving. Drying was carried out in a vacuum drying oven, and the drying temperature was determined by thermogravimetric analysis (TA Instruments TGA550). Specifically, thermogravimetric analysis was performed on the nano-silica powder at a heating rate of 10°C / min, increasing from room temperature to 200°C, and the weight loss curve was observed. When the weight loss stabilized (typically at 150°C), the drying temperature was set to 150°C. The drying time was controlled by monitoring the moisture content, measured using a Karl Fischer moisture analyzer (Mettler Toledo C20), until the moisture content was below 0.1%. Sieving used a standard sieve (Taylor series, 75 micrometer aperture). The sieve aperture was selected based on the target particle size range (50-100 nanometers), but given the tendency of nano-powder to agglomerate, sieving was mainly used to remove large particles. The specific surface area of ​​graphene was controlled using the BET method (McAsAP2460): First, the graphene sample was degassed at 150°C for 2 hours, followed by nitrogen adsorption testing at liquid nitrogen temperature to obtain adsorption isotherms. The specific surface area was calculated using the BET equation, and the measured specific surface area of ​​graphene was 500 m² / g. The specific surface area was optimized by adjusting the graphene preparation parameters (such as the degree of oxidation and reduction temperature): the degree of oxidation was controlled by the oxidation time during the Hummers method (oxidation time 12 hours), and the reduction temperature was achieved by thermal reduction at 500°C for 2 hours in argon.

[0075] Mass ratio mixing: The mass ratio of silicon source to carbon source was determined through capacity performance testing. The optimization process included preparing multiple sets of silicon-carbon composite material samples with different mass ratios (1:1, 1:1.5, 1:2, and 1.5:1, respectively). Each set of samples underwent all the steps described in this embodiment, and then they were assembled into CR2032 coin cells for charge-discharge testing. Test conditions: The negative electrode material was mixed with the conductive agent (SuperP) and binder (PVDF) at a mass ratio of 8:1:1, coated on copper foil, dried, and then assembled with the lithium foil counter electrode and electrolyte (1MLiPF6 in EC / DEC=1:1). Constant current charge-discharge tests were performed on the Blue Electric test system with a voltage window of 0.01-1.5V and a current density of 0.2C. The test results showed that the sample with a mass ratio of 1:1 had the highest initial discharge capacity (1500 mAh / g) and a capacity retention rate of over 90% after 100 cycles. Therefore, a mass ratio of 1:1 was selected. The mixing process is carried out under an inert atmosphere (argon). The purity of the argon is monitored by a gas analyzer (Siemens Ultramat23). The gas analyzer continuously detects the oxygen and moisture content in the argon flow. The oxygen content is below 0.1 ppm and the moisture content is below 0.1 ppm. The operating parameters of the argon purification system (Shanghai Shenzhong Technology) (such as the molecular sieve regeneration cycle) are adjusted according to the detection results.

[0076] High-energy ball milling: A high-energy ball mill (Fritsch Pulverisette 7, Germany) was used to uniformly combine the silicon and carbon sources. The milling media consisted of zirconia balls (3 mm and 5 mm in diameter, 1:1 ratio). The diameter of the zirconia balls was optimized experimentally: particle uniformity was compared at different diameters (1 mm, 3 mm, 5 mm) and observed using a scanning electron microscope (Hitachi SU8010). The 3 mm and 5 mm mixed ball media showed the best performance. The ball-to-material ratio (mass ratio of balls to material) was determined through mixing efficiency testing: particle uniformity was tested at different ball-to-material ratios (10:1, 20:1, 30:1), and particle size distribution was measured using a laser particle size analyzer. The narrowest distribution (D50 = 80 nm) was observed at a 20:1 ratio; therefore, a ball-to-material ratio of 20:1 was selected. The filling rate of the ball mill jar (500 ml volume) was adjusted by controlling the total volume of the grinding media and material. The filling rate was calculated as the porosity (the ratio of void volume to total volume) within the jar. A filling rate of 60% was set by measuring the volume of material and balls within the jar. The high-energy ball milling environment temperature was maintained by a cooling system (water cooling device), with the water cooling temperature monitored and controlled by a thermocouple (K-type), set at 20°C. The high-energy ball milling time was determined by monitoring particle size distribution: during the milling process, samples were taken every 30 minutes, and the particle size distribution was measured using a laser particle size analyzer. Milling was stopped when the particle size distribution reached a preset uniformity (D90 / D10 < 2). The milling time in this experiment was 15 hours. The high-energy ball milling speed was adjusted by controlling particle crushing and mixing efficiency: the current and power output of the ball mill were monitored, and a curve showing the relationship between power consumption and mixing efficiency was plotted (mixing efficiency was calculated using X-ray diffraction full width at half maximum). Power consumption was stable and mixing efficiency was highest at a speed of 400 rpm. After ball milling, a silicon-carbon composite material was obtained, and its morphology was observed by scanning electron microscopy: the sample was imaged after gold sputtering, showing that silicon particles were uniformly embedded in the carbon matrix. The crystal structure was analyzed by X-ray diffraction (Brook D8 Advance): the scanning range was 10°-80°, the step size was 0.02°, and the diffraction pattern was obtained, showing the (111) and (220) peaks of silicon and the (002) peak of carbon, indicating that the composite was successful.

[0077] Step 2: Construction of the gradient overlay layer; This step aims to form a gradient coating layer on the surface of the silicon-carbon composite material, achieving a compositional gradient by controlling the addition of the coating precursor and heat treatment. The specific process is as follows: Suspension preparation: The silicon-carbon composite material was dispersed in a solvent (N-methylpyrrolidone, purchased from Shanghai Maclean, purity ≥99.9%) to form a suspension. The concentration of the suspension was controlled by adjusting the ratio of silicon-carbon composite material to solvent: 10 grams of silicon-carbon composite material and 90 grams of N-methylpyrrolidone were weighed, and the solid content was calculated to be 10% by weight. The solid content was adjusted to the target value by adjusting the amount of solvent added, and the mixture was initially mixed for 30 minutes using a magnetic stirrer (IKARCT).

[0078] Precursor treatment for coating: The coating precursor was polyacrylonitrile (purchased from Sigma-Aldrich, USA, molecular weight 100,000-150,000). The molecular weight of polyacrylonitrile was determined and controlled by gel permeation chromatography (Waters Alliance 2695): polyacrylonitrile was dissolved in DMF to form a 1 mg / mL solution, injected for analysis, and a molecular weight distribution curve was obtained. Batch with a molecular weight of 120,000 was selected based on the curve. The coating precursor solution was prepared by dissolving polyacrylonitrile in N-methylpyrrolidone: 5 g of polyacrylonitrile and 95 g of N-methylpyrrolidone were weighed and heated and stirred in a 60°C water bath for 2 hours. The dissolution temperature was adjusted by controlling the heating device (Mettler Toledo heating plate), and the dissolution time was determined by monitoring the solution transparency (stopping when the solution became clear).

[0079] Gradient coating formation: The coating precursor solution was slowly added to the suspension while simultaneously agitated with a mechanical stirrer (IKARW20). The addition rate of the coating precursor was controlled by adjusting the dropping rate: a peristaltic pump (Lange BT100-1J) was used to control the dropping rate, and the pump speed was determined by calibrating the flow rate (calibration method: measuring the volume added per unit time, set to 5% of the total volume of the coating precursor solution added per minute). During the addition process, the temperature was gradually increased from room temperature (25°C) at a rate set by a programmable temperature controller (Eurotherm 3216). The temperature profile of the programmable temperature controller was optimized based on thermal response testing: the thermal response test measured the change in suspension temperature over time, determining the rate of increase to be 2°C / minute, with a final temperature of 60°C. The stirring speed was adjusted by monitoring the fluidity of the suspension: the viscosity of the suspension was measured using a rotational viscometer (Brookfield DV2T), and the stirrer speed was adjusted according to the viscosity change. The initial viscosity was 100 cP, and the stirring speed was set to 500 rpm. After addition, preliminary heat treatment was performed: Under an inert atmosphere (argon), a tube furnace (Hefei Kejing OTF-1200X) was used, with the argon flow rate controlled at 100 ml / min using a flow meter (Beijing Qixing Huachuang D07). The preliminary heat treatment temperature was gradually increased from room temperature to the target temperature, which was determined by thermogravimetric analysis (TGA): Thermogravimetric analysis was performed on the polyacrylonitrile at a heating rate of 10°C / min, increasing the temperature from room temperature to 600°C. The TGA curve showed that decomposition began around 250°C; therefore, the target temperature was set at 250°C. The preliminary heat treatment time was controlled by monitoring the curing degree of the coating layer: Fourier transform infrared spectroscopy (Thermo Fisher Scientific Nicoleti S50) was used to analyze the chemical structural changes of the coating layer, with a scanning range of 4000-400 cm⁻¹. - ¹, based on the characteristic peak of C≡N (2240 ​​cm⁻¹) -¹) Curing was considered complete when the strength decreased by 50%; the experimental time was 1.5 hours. After preliminary heat treatment, the thickness of the gradient coating was measured using scanning electron microscopy: the sample cross-section was imaged, and the coating thickness was measured to be 100-200 nanometers. The compositional gradient was analyzed using energy dispersive spectroscopy (Oxford Instruments X-MaxN): the carbon element distribution was measured from the inside to the outside of the coating, showing that the carbon content gradually increased from 60% on the inside to 90% on the outside.

[0080] Polymerization Degree and Mechanical Property Control: The formation of the gradient coating also involves controlling the polymerization degree of the coating precursor, which is monitored by gel permeation chromatography: samples are taken every 30 minutes during the addition process to analyze the molecular weight distribution, ensuring that the molecular weight change is within 10%. The elastic modulus of the gradient coating is evaluated by nanoindentation testing (Agilent G200): using a Berkovich indenter at a loading rate of 0.1 mN / s, the mechanical properties at different locations of the coating are measured, with an inner modulus of 5 GPa and an outer modulus of 2 GPa. The ionic conductivity of the gradient coating is measured by electrochemical impedance spectroscopy (Princeton VersaSTAT4): the material is fabricated as an electrode, and the test frequency range is 0.01 Hz-100 kHz, showing high inner impedance and low outer impedance.

[0081] Step 3: Final heat treatment and carbonization; This step aims to transform the gradient coating layer into a carbon-based gradient coating layer, achieved through high-temperature carbonization. The specific process is as follows: Carbonization process: Under an inert atmosphere (argon), the pre-heat-treated material is placed in a tube furnace for high-temperature carbonization. The carbonization temperature is gradually increased from the pre-heat-treatment temperature (250°C) to the target carbonization temperature. The heating rate is determined by controlling the pyrolysis reaction rate: Differential scanning calorimetry (DSC250) is performed on the coating precursor to obtain the heat flow curve. The heating rate is 10°C / min. The curve shows exothermic peaks at 300°C and 600°C, so the heating rate is set to 5°C / min to avoid violent reactions. The target carbonization temperature is determined by a combination of thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC): TGA shows weight stability at 900°C, and DSC shows no phase transition at 900°C. Therefore, the target carbonization temperature is set to 900°C. The carbonization furnace's heating program is achieved through multi-segment temperature control: a low-temperature segment (250-500°C, heating rate 5°C / min, holding time 30 minutes), a medium-temperature segment (500-700°C, heating rate 3°C / min, holding time 60 minutes), and a high-temperature segment (700-900°C, heating rate 2°C / min, holding time adjusted by monitoring the degree of carbonization). The holding time is used to monitor the degree of carbonization using Raman spectroscopy (Renishaw inVia): a 532nm laser is used, with a scanning range of 1000-2000 cm⁻¹. -¹, based on peak D (1350cm) - ¹) and G peak (1580cm) - The degree of carbonization was determined by the intensity ratio (ID / IG). Heating was stopped when ID / IG = 1.0; in this experiment, the heating time was 3 hours. During carbonization, the material was positioned within the isothermal zone of the carbonization furnace, which was determined through temperature mapping testing: multiple thermocouples were used to measure the temperature distribution within the furnace, defining the isothermal zone as a 10 cm radius from the center of the furnace. Argon purity was monitored using a gas analyzer, with oxygen content below 0.1 ppm. The mass loss of the material after carbonization was assessed by weighing: comparing the sample weights before and after carbonization, the loss rate was 20%.

[0082] Characterization after carbonization: The structure of the carbon-based gradient coating layer was observed using a transmission electron microscope (JEOL JEM-2100): Ultrathin sections (50 nm thick) were prepared, and imaging showed that the inner layer was a dense carbon layer (50 nm thick), and the outer layer was a porous carbon layer (100 nm thick). Electrical conductivity was measured using a four-probe method (Lattice Technology RTS-9): The sample was pressed into a sheet (10 mm in diameter, 1 mm thick), connected to a testing circuit, and the measured conductivity was 100 S / m.

[0083] Step 4: Post-processing; This step aims to optimize the material morphology and size to ensure consistent electrochemical performance. The specific process is as follows: Cooling: The carbonized material is naturally cooled to room temperature in a tube furnace. The cooling rate is determined by controlling the material's thermal stress: thermocouples are used to monitor the material temperature, and the cooling environment is adjusted according to the temperature drop curve (argon flow rate reduced to 50 mL / min), with the cooling rate set at 5°C / min. The cooling environment is an inert atmosphere (argon).

[0084] Grinding and sieving: Grinding was performed using a planetary ball mill (QM-3SP2, Nanjing University Instrument Factory) with zirconia balls (5 mm in diameter) as the grinding media and a ball-to-material ratio of 20:1. The grinding time was determined by monitoring particle size distribution: samples were taken every 15 minutes and measured using a laser particle size analyzer. Grinding was stopped when D50 = 5 micrometers. The grinding time in this experiment was 1 hour. Grinding intensity was controlled by adjusting the ball mill speed and ball-to-material ratio: based on a particle crushing kinetic model (the model formula is dD / dt = -kD^n, where D is the particle size, and k and n are constants; k = 0.1 h was determined by fitting experimental data). - ¹, n=2), with the rotation speed set at 300 rpm. Sieving was performed using a standard sieve (Taylor series, 20 micrometer aperture), with the sieve aperture determined based on the target application requirements (typical particle size of 10-20 micrometers for lithium-ion battery anodes). After sieving, the tap density of the material was measured using the volumetric method: a tap density meter (Beijing Jingwei Technology JZ-1) was used, and the average value was taken for three measurements; the tap density was 1.2 g / cm³.

[0085] Characterization: Characterization steps included observing the surface morphology and coating integrity of the material using scanning electron microscopy: imaging after gold sputtering showed an intact coating without cracks. X-ray diffraction analysis of crystal structure: scanning range 10°-80°, step size 0.02°, showing diffraction peaks of silicon and carbon, with no impurity phases. Electrochemical testing to evaluate battery performance: coin cells were assembled and subjected to constant current charge-discharge tests (voltage window 0.01-1.5V, current density 0.2C). The initial discharge capacity was 1500 mAh / g, and the capacity retention was 92% after 100 cycles. Electrochemical impedance spectroscopy showed a decrease in impedance in the low-frequency region, indicating improved ion transport.

[0086] Throughout the preparation process, the control of all parameters is based on a real-time feedback system: the system includes sensors (temperature, pressure, viscosity), a data acquisition unit (instrument NICDAQ-9174), and a control unit (PLC). The sensors monitor key parameters, the data acquisition unit records data, and the control unit adjusts process parameters according to preset algorithms (such as PID control) to ensure process stability.

[0087] This embodiment achieves the preparation of low-expansion silicon-carbon anode material with gradient coating through the above detailed steps, effectively solving the volume expansion problem of silicon-based anode materials and improving cycle life and capacity retention.

[0088] This invention encompasses any substitutions, modifications, equivalent methods, and solutions made within the spirit and scope of this invention. To provide the public with a thorough understanding of this invention, specific details are described in detail in the following preferred embodiments; however, those skilled in the art will fully understand the invention even without these details. Furthermore, to avoid unnecessary misunderstanding of the essence of this invention, well-known methods, processes, procedures, components, and circuits are not described in detail.

[0089] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a low-expansion silicon-carbon anode material with a gradient coating layer, characterized in that, Includes the following steps: Step 1: Provide silicon source and carbon source. The silicon source is nano-silicon powder and the carbon source is graphene. Mix the silicon source and carbon source in a mass ratio. The mixing process is carried out under an inert atmosphere, which is argon. The silicon source and carbon source are uniformly compounded by high-energy ball milling. The high-energy ball milling time is determined by monitoring the particle size distribution. The high-energy ball milling speed is adjusted by controlling the particle crushing and mixing efficiency to obtain silicon-carbon composite material. Step 2: Provide a coating precursor, which is polyacrylonitrile. Disperse the silicon-carbon composite material in a solvent to form a suspension. The solvent is N-methylpyrrolidone. The concentration of the suspension is controlled by adjusting the ratio of silicon-carbon composite material to solvent. By controlling the addition rate of the coating precursor and the heat treatment parameters, a gradient coating layer is formed on the surface of the silicon-carbon composite material. The addition rate of the coating precursor is achieved by controlling the dropwise addition rate of the coating precursor solution. The heat treatment parameters include temperature adjustment and stirring speed control. The temperature is gradually increased from room temperature, and the stirring speed is adjusted by monitoring the fluidity of the suspension. After the coating precursor is added, a preliminary heat treatment is performed. The preliminary heat treatment temperature is gradually increased from room temperature to the target temperature, which is determined by thermogravimetric analysis. The preliminary heat treatment time is controlled by monitoring the degree of curing of the coating layer. Step 3: The material after preliminary heat treatment is carbonized at high temperature in an inert atmosphere, which is argon. The carbonization temperature is gradually increased from the preliminary heat treatment temperature to the target carbonization temperature. The heating rate is determined by controlling the pyrolysis reaction rate, and the holding time is adjusted by monitoring the degree of carbonization, so that the gradient coating layer is transformed into a carbon-based gradient coating layer. Step 4: The carbonized material is naturally cooled to room temperature. The cooling rate is determined by controlling the thermal stress of the material. Then, it is ground and sieved. The grinding intensity is adjusted by monitoring the particle size distribution, and the sieve aperture is determined by the target application requirements to obtain a low-expansion silicon-carbon anode material with a gradient coating.

2. The method for preparing low-expansion silicon-carbon anode material with a gradient coating layer according to claim 1, characterized in that, In step one, the silicon source is nano-silicon powder. The particle size of the nano-silicon powder is monitored and controlled by a laser particle size analyzer. The control process includes taking multiple measurements of the nano-silicon powder using the laser particle size analyzer to obtain a particle size distribution curve. The pretreatment process of the nano-silicon powder is adjusted according to the particle size distribution curve. The pretreatment process includes drying and sieving. The drying temperature is determined by thermogravimetric analysis, the drying time is controlled by monitoring the moisture content, and a standard sieve is used for sieving. The sieve aperture is selected based on the target particle size range. The carbon source is graphene. The specific surface area of ​​graphene is determined and controlled by the BET method. The control process includes using a BET device to perform nitrogen adsorption tests on the graphene sample, obtaining adsorption isotherms, calculating the specific surface area based on the adsorption isotherms, and optimizing the specific surface area by adjusting the preparation parameters of graphene, including the degree of oxidation and the reduction temperature. The mass ratio of silicon source to carbon source was determined by capacity performance testing. The optimization process included preparing multiple sets of silicon-carbon composite material samples with different mass ratios. Each set of samples was processed by the methods in steps one to four, and then assembled into coin cells for charge-discharge testing. The test conditions included constant current charge-discharge and cycle performance evaluation. The optimal mass ratio was selected based on the first discharge capacity and cycle capacity retention rate in the test results. The mixing process is carried out in an argon atmosphere. The purity of the argon is monitored and controlled by a gas analyzer. The monitoring process includes using the gas analyzer to continuously detect the oxygen and moisture content in the argon flow rate, and adjusting the operating parameters of the argon purification system according to the detection results. The high-energy ball milling time is determined by monitoring the particle size distribution. The determination process includes taking samples periodically during the high-energy ball milling process, measuring the particle size distribution using a laser particle size analyzer, and stopping the ball milling when the particle size distribution reaches the preset uniformity. The high-energy ball mill speed is adjusted by controlling particle crushing and mixing efficiency. The adjustment process includes monitoring the ball mill's current and power output, and adjusting the speed according to the power consumption and mixing efficiency curve to ensure that the silicon source and carbon source are fully compounded.

3. The method for preparing low-expansion silicon-carbon anode material with a gradient coating layer according to claim 1, characterized in that, In step one, the grinding media used in the high-energy ball mill is zirconia balls. The diameter of the zirconia balls is selected through experimental optimization, and the ball-to-material ratio is determined through mixing efficiency testing. The mixing efficiency test includes comparing the particle uniformity and degree of compositeness under different ball-to-material ratios. In the high-energy ball milling process, the filling rate of the ball mill jar is adjusted by controlling the total volume of the ball milling media and materials, and the filling rate is determined by calculating the void ratio inside the ball mill jar. The ambient temperature of the high-energy ball mill is maintained by a cooling system, which uses a water-cooling device. The water-cooling temperature is monitored and controlled by thermocouples. After high-energy ball milling, the morphology of the silicon-carbon composite material was observed by scanning electron microscopy. The observation process included sampling, sample preparation, and image analysis to ensure that the silicon particles were uniformly embedded in the carbon matrix. The crystal structure of silicon-carbon composite materials was analyzed by X-ray diffraction. The analysis process included scanning the sample with an X-ray diffractometer, obtaining diffraction patterns, and determining the crystallization state of silicon and carbon based on the diffraction patterns.

4. The method for preparing low-expansion silicon-carbon anode material with a gradient coating layer according to claim 1, characterized in that, In step two, the concentration of the suspension is controlled by adjusting the ratio of silicon-carbon composite material to solvent. The control process includes weighing the silicon-carbon composite material and solvent, calculating the solid content, and adjusting the amount of solvent added to make the solid content reach the target value. The coating precursor is polyacrylonitrile. The molecular weight of polyacrylonitrile is controlled by gel permeation chromatography. The control process includes analyzing the molecular weight distribution of the polyacrylonitrile solution using a gel permeation chromatograph and selecting an appropriate batch of polyacrylonitrile based on the molecular weight distribution. The preparation of the coated precursor solution was achieved by dissolving polyacrylonitrile in N-methylpyrrolidone. The dissolution temperature was adjusted by controlling the heating device, and the dissolution time was determined by monitoring the transparency of the solution. The addition rate of the coating precursor is achieved by controlling the dropping rate of the coating precursor solution. The dropping rate is controlled by a peristaltic pump, and the rotation speed of the peristaltic pump is determined by calibrating the flow rate. The suspension is continuously stirred during the addition process, and the stirring speed is monitored and adjusted by a viscometer. The temperature is gradually increased from room temperature, and the rate of increase is set by a programmable temperature control device. The temperature curve of the programmable temperature control device is optimized based on thermal response tests. The stirring speed is adjusted by monitoring the fluidity of the suspension. The adjustment process includes measuring the viscosity of the suspension using a rotational viscometer and adjusting the stirring speed according to the viscosity changes.

5. The method for preparing low-expansion silicon-carbon anode material with a gradient coating layer according to claim 1, characterized in that, In step two, the initial heat treatment temperature is gradually increased from room temperature to the target temperature. The target temperature is determined by thermogravimetric analysis. The determination process includes performing thermogravimetric analysis on the coating precursor, obtaining the thermogravimetric curve, and setting the target temperature based on the decomposition temperature range in the thermogravimetric curve. The initial heat treatment time is controlled by monitoring the degree of curing of the coating layer. The monitoring process includes using a Fourier transform infrared spectrometer to analyze the chemical structure changes of the coating layer and judging the degree of curing based on the intensity of characteristic peaks. During the initial heat treatment, the heat treatment atmosphere is an inert atmosphere, which is argon gas, and the argon gas flow rate is controlled by a flow meter. After preliminary heat treatment, the thickness of the gradient coating layer was measured by scanning electron microscopy. The measurement process included imaging and analysis of the sample cross-section. The compositional gradient of the gradient coating was analyzed by line scanning with an energy dispersive spectroscopy (EDS) instrument. The analysis process included measuring the elemental distribution from the inside to the outside of the coating to ensure that the carbon content changed gradually.

6. The method for preparing low-expansion silicon-carbon anode material with a gradient coating layer according to claim 1, characterized in that, In step three, the carbonization temperature is gradually increased from the initial heat treatment temperature to the target carbonization temperature. The heating rate is determined by controlling the pyrolysis reaction rate. The determination process includes performing differential scanning calorimetry on the coating precursor to obtain the heat flow curve, and setting the heating rate according to the position of the exothermic peak in the heat flow curve. The heat preservation time is adjusted by monitoring the degree of carbonization. The monitoring process includes using a Raman spectrometer to analyze the carbon structure of the carbonized material and judging the degree of carbonization based on the intensity ratio of the D peak and the G peak. During the carbonization process, the inert atmosphere is argon, and the purity of argon is monitored by a gas analyzer. The monitoring process includes real-time detection of the impurity content in the argon. After carbonization, the structure of the carbon-based gradient coating layer was observed using a transmission electron microscope. The observation process included preparing ultrathin sections and imaging. The conductivity of the carbon-based gradient coating was measured using a four-probe method, which involved pressing the sample into a pellet and connecting it to a test circuit.

7. The method for preparing low-expansion silicon-carbon anode material with a gradient coating layer according to claim 1, characterized in that, In step three, the target carbonization temperature is determined by a combination of thermogravimetric analysis and differential scanning calorimetry. The determination process includes analyzing the thermal decomposition behavior and phase transition temperature of the coated precursor. The heating program of the carbonization furnace is achieved through multi-stage temperature control, which includes a low-temperature stage, a medium-temperature stage, and a high-temperature stage. The temperature and time of each stage are determined through optimization experiments. During the carbonization process, the material is located within the constant temperature zone of the carbonization furnace, which is determined by temperature mapping tests. After carbonization, the mass loss of the material is assessed by weighing, and the assessment process includes comparing the weight of the samples before and after carbonization.

8. The method for preparing low-expansion silicon-carbon anode material with a gradient coating layer according to claim 1, characterized in that, In step four, the cooling rate is determined by controlling the thermal stress of the material. The determination process includes using thermocouples to monitor the material temperature and adjusting the cooling environment according to the temperature drop curve. The cooling environment is an inert atmosphere, which is argon gas, and the argon gas flow rate is regulated by a flow valve. Grinding was carried out using a planetary ball mill, with zirconia balls as the grinding media. The grinding time was determined by monitoring the particle size distribution, and the determination process included periodic sampling and laser particle size analysis. The grinding intensity is controlled by adjusting the ball mill speed and the ball-to-material ratio, and the control process is based on a particle crushing kinetics model. Sieving is performed using a standard sieve, and the sieve aperture is determined based on the target application requirements. The determination process includes referencing the typical particle size distribution of battery anode materials. After sieving, the tap density of the material is measured by the volumetric method, which includes the use of a tap density meter.

9. The method for preparing low-expansion silicon-carbon anode material with a gradient coating layer according to claim 1, characterized in that, Following step four, a characterization step is also included, which includes: The surface morphology and coating integrity of the material were observed using a scanning electron microscope. The observation process included gold plating of the sample and image acquisition. X-ray diffraction was used to analyze the crystal structure of the material. The analysis process included setting the scanning angle range and step size. Electrochemical tests are used to evaluate the battery performance of materials. Electrochemical tests include assembling coin cells, performing constant current charge-discharge tests and cycle performance tests, and test conditions include voltage window and current density. Electrochemical test data are used to calculate the initial discharge capacity and capacity retention.

10. The method for preparing low-expansion silicon-carbon anode material with a gradient coating layer according to claim 1, characterized in that, In step two, the formation of the gradient coating layer also includes controlling the degree of polymerization of the coating precursor. The degree of polymerization is monitored by gel permeation chromatography, and the monitoring process includes periodic sampling to analyze the molecular weight distribution. The elastic modulus of the gradient coating was evaluated by nanoindentation testing, which included measuring the mechanical properties of the coating at different locations using a nanoindenter. The ionic conductivity of the gradient coating layer was measured by electrochemical impedance spectroscopy. The measurement process included fabricating the material into an electrode and testing the impedance spectrum. Throughout the entire preparation process, the control of all parameters is based on a real-time feedback system, which includes sensors, a data acquisition unit, and a control unit. The sensors monitor key parameters, the data acquisition unit records data, and the control unit adjusts process parameters according to a preset algorithm.

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