Production method of negative electrode coating material, negative electrode coating material and lithium ion battery

Abstract

This invention discloses a method for producing a negative electrode coating material, the negative electrode coating material, and a lithium-ion battery, relating to the field of lithium battery material preparation technology. The method includes: functionalizing carbon nanotubes by covalent bonding to obtain functionalized carbon nanotubes grafted with specific functional groups; dispersing the functionalized carbon nanotubes in an organic solvent, adding a coating precursor and a dispersing binder to form a coating slurry, adding it to a silicon-based negative electrode matrix, and then spray-drying it to obtain a pre-coated composite material; sequentially subjecting the material to low-temperature dehydration crosslinking, medium-temperature pre-carbonization and catalysis, and high-temperature graphitization and densification treatments to form a three-dimensional gradient composite coating structure, consisting of an amorphous carbon layer, a functionalized carbon nanotube reinforced composite layer, and a highly graphitized outer surface layer from the inside out. The coating layer thickness is 50-200 nm, and the volume resistivity is ≤5×10⁻⁶. ‑3 Ω·cm, initial coulombic efficiency ≥85% at 0.1C, capacity retention ≥80% after 500 cycles, suitable for high-performance lithium-ion battery anodes.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium battery material preparation, more particularly to a production method of negative electrode coating material, negative electrode coating material and lithium ion battery. BACKGROUND

[0002] In the field of high specific energy lithium ion batteries, silicon-based anodes are considered as one of the core directions to replace traditional graphite anodes due to their extremely high theoretical capacity (4200 mAh / g). However, silicon-based materials undergo more than 300% volume expansion during charging and discharging, leading to electrode structure collapse and repeated breakdown and reconstruction of the SEI film, which severely restricts their cycle stability. At the same time, silicon has low electronic conductivity and poor compatibility with electrolyte, further exacerbating the problem of rising interface impedance and active material loss. To solve the above bottlenecks, coating modification has become the mainstream technical path. However, existing coating schemes still have significant defects.

[0003] Traditional carbon coating layers are mostly single structures. Although amorphous carbon can buffer volume expansion, its electronic conductivity is low. Highly graphitized carbon has excellent electrical conductivity, but it is too rigid and easy to crack, making it difficult to balance the relationship between buffering, conductivity and structural stability. In addition, the interface between the carbon coating layer and the silicon-based matrix is mostly physically attached, and is prone to interface peeling during the cycle process. Some schemes introduce carbon nanotubes to enhance conductivity, but carbon nanotubes are prone to agglomeration and lack effective chemical action with the coating precursor, making it difficult to form a continuous conductive network.

[0004] To break through this technical bottleneck, functional modification strategies have been tried to optimize the dispersibility and interface of carbon nanotubes. However, existing modification methods mostly use non-covalent bonding modification, such as surfactant adsorption, which has weak binding and is prone to falling off during subsequent processing. Covalent modification has strong binding, but commonly used functional groups such as carboxyl and hydroxyl can only improve dispersibility and cannot form specific reactions with the coating precursor, making it difficult to build a synergistic composite coating structure. At the same time, existing coating processes mostly use a two-step method of coating and calcination, which is prone to uneven coating layer thickness, poor density, and lack of precise control of the microstructure of the coating layer, making it difficult to design a gradient structure according to the internal stress distribution and electron transport requirements of the silicon-based anode, ultimately resulting in the difficulty of the material to meet the practical application requirements of high specific energy batteries in terms of comprehensive electrochemical performance.

[0005] Therefore, the existing technology has problems such as difficulty in balancing multiple performances of the coating layer, agglomeration of carbon tubes, weak interface bonding, and difficulty in precise control of the structure. SUMMARY

[0006] In order to overcome the problems of the existing technology, such as difficulty in balancing multiple performances of the coating layer, agglomeration of carbon tubes, weak interface bonding, and difficulty in precise control of the structure, the present application discloses a production method of negative electrode coating material, negative electrode coating material and lithium ion battery, which can effectively solve the above technical problems.

[0007] To solve the above-mentioned technical problems, the technical solution of the present invention is as follows:

[0008] A method for producing a negative electrode coating material includes the following steps:

[0009] Covalent functionalization modification of carbon nanotubes yields functionalized carbon nanotubes with specific functional groups grafted onto their surface.

[0010] The functionalized carbon nanotubes are dispersed in an organic solvent, and a coating precursor and a dispersant binder are added to form a uniform coating slurry. A silicon-based anode matrix is ​​then added to the coating slurry, and the pre-coated composite material is obtained by spray drying.

[0011] The pre-coated composite material is subjected to low-temperature dehydration and crosslinking treatment, medium-temperature pre-carbonization and catalytic treatment, and high-temperature graphitization and densification treatment in sequence to form a negative electrode coating material with a three-dimensional gradient composite coating structure.

[0012] The three-dimensional gradient composite coating structure consists of, from the inside out, an amorphous carbon layer tightly bonded to the silicon-based anode substrate, a composite layer reinforced with functionalized carbon nanotubes, and a highly graphitized outer surface layer.

[0013] Preferably, the covalent functionalization modification of carbon nanotubes includes:

[0014] Multi-walled carbon nanotubes were placed in a mixed acid and ultrasonically refluxed at 55-65°C for 3.5-4.5 hours. The volume ratio of concentrated sulfuric acid to concentrated nitric acid in the mixed acid was 2.8-3.2:1, in order to introduce carboxyl functional groups on the surface of the multi-walled carbon nanotubes.

[0015] Multi-walled carbon nanotubes with carboxyl functional groups introduced on their surface were amidated with 3-aminophenylboronic acid to obtain functionalized carbon nanotubes with phenylboronic acid functional groups grafted on their surface, denoted as PBA-CNTs.

[0016] Preferably, the amidation reaction is carried out under the following conditions: using N,N-dimethylformamide as solvent, 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride as condensing agent, and 4-dimethylaminopyridine as catalyst, the reaction is stirred at 25-30°C for 12-16 hours, and after the reaction is completed, PBA-CNTs are obtained by centrifugation, washing, and vacuum drying.

[0017] Preferably, the step of dispersing the functionalized carbon nanotubes in an organic solvent, adding a coating precursor and a dispersing binder, and mixing to form a uniform coating slurry includes:

[0018] The organic solvent is N-methylpyrrolidone. Functionalized carbon nanotubes are added to N-methylpyrrolidone and ultrasonically treated at a power of 300-500W for 30-60 minutes to form a functionalized carbon nanotube dispersion with a mass concentration of 0.5-2g / L.

[0019] The coating precursor is sucrose, and the dispersing binder is polyvinylpyrrolidone. Sucrose and polyvinylpyrrolidone are added to the functionalized carbon nanotube dispersion at a mass ratio of 8-12:1, and stirred at 500-800 r / min for 2-4 hours to obtain a uniform coating slurry; wherein the mass ratio of the coating precursor to the functionalized carbon nanotubes is 5-10:1.

[0020] Preferably, the step of adding the silicon-based anode substrate to the coating slurry and then spray-drying it to obtain the pre-coated composite material includes:

[0021] The silicon-based anode substrate is SiOx@C composite powder, where x ranges from 0.8 to 1.2. The SiOx@C composite powder is added to the coating slurry at a mass ratio of 3-5:1 to the solid components in the coating slurry, and stirred for 1-2 hours at a speed of 800-1200 r / min to form a mixed slurry.

[0022] The spray drying conditions are as follows: inlet air temperature 180-220℃, outlet air temperature 80-100℃, feed rate 10-20mL / min, atomization pressure 0.2-0.4MPa, and a pre-coated composite material is obtained after the treatment.

[0023] Preferably, the conditions for the low-temperature dehydration and crosslinking treatment are as follows: under an air atmosphere, the temperature is increased to 200-250°C at a heating rate of 5-10°C / min and held for 2-3 hours; during this process, the coated precursor molecules dehydrate to generate carbonyl active groups, and the phenylboronic acid groups on the surface of the functionalized carbon nanotubes undergo esterification and complexation reactions with the carbonyl active groups and the amide groups in the dispersing binder to form a chemical crosslinking network.

[0024] Preferably, the conditions for the intermediate-temperature pre-carbonization and catalytic treatment are as follows: under an argon atmosphere, the temperature is increased to 500-700℃ at a heating rate of 10-15℃ / min and held for 1.5-2.5 hours; during this process, the coating precursor and the dispersing binder undergo decomposition and preliminary carbonization, and the boron element grafted on the surface of the functionalized carbon nanotubes acts as a catalyst to reduce the graphitization temperature of the carbon material and promote the formation of a locally ordered carbon structure.

[0025] Preferably, the conditions for the high-temperature graphitization and densification treatment are as follows: under an argon atmosphere, the temperature is increased to 900-1000℃ at a heating rate of 8-12℃ / min and held for 3-4 hours; during this process, the preliminary carbonization products are further graphitized, and the functionalized carbon nanotubes serve as a rigid framework supporting the coating layer, forming a dense three-dimensional gradient composite coating structure.

[0026] Preferably, a negative electrode coating material comprises a silicon-based negative electrode substrate and a three-dimensional gradient composite coating layer coated on the surface of the silicon-based negative electrode substrate, wherein the thickness of the three-dimensional gradient composite coating layer is 50-200 nm and the volume resistivity is ≤5×10⁻⁶. -3 Ω·cm, at a charge / discharge rate of 0.1C, the initial coulombic efficiency is ≥85%, and the capacity retention after 500 cycles is ≥80%.

[0027] Preferably, a lithium-ion battery includes a positive electrode, an electrolyte, a separator, and a negative electrode, wherein the negative electrode is prepared from the negative electrode coating material described above.

[0028] Compared with the prior art, the beneficial effects of this invention are as follows: This technical solution addresses the problem of coating layers failing to achieve multiple performance characteristics simultaneously. It constructs a three-dimensional gradient composite coating structure consisting of an amorphous carbon layer, a functionalized carbon nanotube-reinforced composite layer, and a highly graphitized outer surface layer, arranged sequentially from the inside out. The inner amorphous carbon layer, due to its flexibility, can buffer the volume expansion of the silicon-based substrate during charging and discharging; the middle functionalized carbon nanotube layer can construct a continuous conductive network to improve electron transport efficiency; and the outer highly graphitized carbon layer, with its high conductivity and density, reduces interfacial impedance. Furthermore, it stabilizes the SEI film, achieving a balance between buffering capacity, conductivity, and structural stability through the synergistic effect of the three layers. Secondly, addressing the issue of carbon nanotube agglomeration, carboxyl functional groups are first introduced onto the surface of multi-walled carbon nanotubes via ultrasonic reflux with mixed acid. Then, phenylboronic acid functional groups are grafted onto the surface through an amidation reaction (forming PBA-CNTs). This covalent modification imparts specific chemical activity and steric hindrance to the carbon nanotube surface. Combined with 300-500W ultrasonic treatment and N-methylpyrrolidone solvent dispersion, the agglomeration tendency of carbon nanotubes is effectively broken, ensuring their uniform dispersion. In the coating slurry; furthermore, addressing the issue of weak interfacial bonding, during low-temperature dehydration and crosslinking treatment, the phenylboronic acid groups on the surface of functionalized carbon nanotubes undergo esterification and complexation reactions with the carbonyl active groups generated from the dehydration of the coating precursor and the amide groups in the dispersing binder, forming a stable chemical crosslinking network. This transforms the physical adhesion between the coating layer and the silicon-based anode matrix into a chemical bond, significantly improving the interfacial bonding strength and preventing coating layer peeling during cycling. Finally, addressing the difficulty in precisely controlling the structure, the morphology of the pre-coated composite material is controlled by spray drying, followed by a multi-temperature stepped heat treatment process: low-temperature crosslinking in air atmosphere, medium-temperature pre-carbonization in argon atmosphere (boron element catalyzes the formation of locally ordered carbon), and high-temperature graphitization in argon atmosphere (carbon nanotubes act as a rigid framework). This precisely controls the composition and structure of each layer: low temperature ensures the formation of the crosslinking network, medium temperature reduces the graphitization temperature and achieves preliminary carbonization with the help of boron catalysis, and high temperature promotes graphitization and densification, ultimately forming a three-dimensional gradient coating layer with controllable thickness (50-200nm) and uniform structure, resulting in a material volume resistivity ≤5×10⁻⁶. -3 Ω·cm, initial coulombic efficiency ≥85% at 0.1C, capacity retention ≥80% after 500 cycles, meeting the requirements for high-performance lithium-ion battery anodes. Attached Figure Description

[0029] To more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are merely exemplary. For those skilled in the art, other embodiments can be derived from the provided drawings without creative effort.

[0030] Fig. 1 This is a diagram illustrating the steps of the method of the present invention;

[0031] Fig. 2 This is a flowchart of the production process of the present invention;

[0032] Fig. 3 This is a structural diagram of the battery of the present invention. Detailed Implementation

[0033] The accompanying drawings are for illustrative purposes only and should not be construed as limiting the scope of this patent.

[0034] To better illustrate this embodiment, some parts in the accompanying drawings may be omitted, enlarged, or reduced, and do not represent the actual product dimensions;

[0035] It will be understood by those skilled in the art that certain well-known structures and their descriptions may be omitted in the accompanying drawings.

[0036] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0037] Example

[0038] Please see Figs. 1-3 A method for producing a negative electrode coating material includes the following steps:

[0039] Covalent functionalization modification of carbon nanotubes yields functionalized carbon nanotubes with specific functional groups grafted onto their surface.

[0040] The functionalized carbon nanotubes are dispersed in an organic solvent, and a coating precursor and a dispersant binder are added to form a uniform coating slurry. A silicon-based anode matrix is ​​then added to the coating slurry, and the pre-coated composite material is obtained by spray drying.

[0041] The pre-coated composite material is subjected to low-temperature dehydration and crosslinking treatment, medium-temperature pre-carbonization and catalytic treatment, and high-temperature graphitization and densification treatment in sequence to form a negative electrode coating material with a three-dimensional gradient composite coating structure.

[0042] The three-dimensional gradient composite coating structure consists of, from the inside out, an amorphous carbon layer tightly bonded to the silicon-based anode substrate, a composite layer reinforced with functionalized carbon nanotubes, and a highly graphitized outer surface layer.

[0043] The covalent functionalization modification of carbon nanotubes includes:

[0044] Multi-walled carbon nanotubes were placed in a mixed acid and ultrasonically refluxed at 55-65°C for 3.5-4.5 hours. The volume ratio of concentrated sulfuric acid to concentrated nitric acid in the mixed acid was 2.8-3.2:1, in order to introduce carboxyl functional groups on the surface of the multi-walled carbon nanotubes.

[0045] Multi-walled carbon nanotubes with carboxyl functional groups introduced on their surface were amidated with 3-aminophenylboronic acid to obtain functionalized carbon nanotubes with phenylboronic acid functional groups grafted on their surface, denoted as PBA-CNTs.

[0046] The amidation reaction conditions are as follows: using N,N-dimethylformamide as solvent, 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride as condensing agent, and 4-dimethylaminopyridine as catalyst, the reaction is stirred at 25-30℃ for 12-16 hours. After the reaction is completed, PBA-CNTs are obtained by centrifugation, washing, and vacuum drying.

[0047] The step of dispersing the functionalized carbon nanotubes in an organic solvent, adding a coating precursor and a dispersing binder, and mixing to form a uniform coating slurry includes:

[0048] The organic solvent is N-methylpyrrolidone. Functionalized carbon nanotubes are added to N-methylpyrrolidone and ultrasonically treated at a power of 300-500W for 30-60 minutes to form a functionalized carbon nanotube dispersion with a mass concentration of 0.5-2g / L.

[0049] The coating precursor is sucrose, and the dispersing binder is polyvinylpyrrolidone. Sucrose and polyvinylpyrrolidone are added to the functionalized carbon nanotube dispersion at a mass ratio of 8-12:1, and stirred at 500-800 r / min for 2-4 hours to obtain a uniform coating slurry; wherein the mass ratio of the coating precursor to the functionalized carbon nanotubes is 5-10:1.

[0050] The process of adding a silicon-based anode substrate to the coating slurry and then spray-drying it to obtain a pre-coated composite material includes:

[0051] The silicon-based anode substrate is SiOx@C composite powder, where x ranges from 0.8 to 1.2. The SiOx@C composite powder is added to the coating slurry at a mass ratio of 3-5:1 to the solid components in the coating slurry, and stirred for 1-2 hours at a speed of 800-1200 r / min to form a mixed slurry.

[0052] The spray drying conditions are as follows: inlet air temperature 180-220℃, outlet air temperature 80-100℃, feed rate 10-20mL / min, atomization pressure 0.2-0.4MPa, and a pre-coated composite material is obtained after the treatment.

[0053] The conditions for the low-temperature dehydration and crosslinking treatment are as follows: under an air atmosphere, the temperature is increased to 200-250°C at a heating rate of 5-10°C / min and held for 2-3 hours. During this process, the coated precursor molecules dehydrate to generate carbonyl active groups. The phenylboronic acid groups on the surface of the functionalized carbon nanotubes undergo esterification and complexation reactions with the carbonyl active groups and the amide groups in the dispersing binder to form a chemical crosslinking network.

[0054] The conditions for the intermediate-temperature pre-carbonization and catalytic treatment are as follows: under an argon atmosphere, the temperature is increased to 500-700℃ at a heating rate of 10-15℃ / min and held for 1.5-2.5 hours. During this process, the coating precursor and the dispersing binder decompose and undergo preliminary carbonization. The boron element grafted onto the surface of the functionalized carbon nanotubes acts as a catalyst, reducing the graphitization temperature of the carbon material and promoting the formation of a locally ordered carbon structure.

[0055] The conditions for the high-temperature graphitization and densification treatment are as follows: under an argon atmosphere, the temperature is increased to 900-1000℃ at a heating rate of 8-12℃ / min and held for 3-4 hours; during this process, the preliminary carbonization products are further graphitized, and the functionalized carbon nanotubes serve as a rigid framework supporting the coating layer, forming a dense three-dimensional gradient composite coating structure.

[0056] A negative electrode coating material, comprising a silicon-based negative electrode substrate and a three-dimensional gradient composite coating layer coated on the surface of the silicon-based negative electrode substrate, wherein the thickness of the three-dimensional gradient composite coating layer is 50-200 nm and the volume resistivity is ≤5×10⁻⁶. -3 Ω·cm, at a charge / discharge rate of 0.1C, the initial coulombic efficiency is ≥85%, and the capacity retention after 500 cycles is ≥80%.

[0057] A lithium-ion battery includes a positive electrode, an electrolyte, a separator, and a negative electrode, wherein the negative electrode is made of the negative electrode coating material described above.

[0058] Core raw material selection criteria

[0059] Multi-walled carbon nanotubes (MWCNTs): Industrial-grade high-purity products are selected. Scanning electron microscopy confirms that the tube diameter distribution is uniform and there is no obvious agglomeration. Before use, vacuum drying is performed to remove surface-adsorbed moisture and volatile impurities. After drying, the tubes are immediately sealed and stored in an inert gas environment to avoid secondary contamination.

[0060] Mixed acid system: Analytical grade concentrated sulfuric acid and concentrated nitric acid are used. After filtering through a 0.22μm organic filter membrane to remove small particles, they are mixed in the preferred ratio according to the technical scheme. The mixture is stirred slowly in an ice-water bath environment, and the system temperature is controlled not to exceed 25℃ to prevent the acid from decomposing and producing harmful gases. After mixing, the mixture is allowed to stand for 24 hours until it stabilizes before use.

[0061] Functionalized reagents: 3-Aminophenylboronic acid needs to be purified by recrystallization to ensure a purity of ≥99%; N,N-dimethylformamide (DMF) is dehydrated by molecular sieve treatment, and the moisture content is controlled below 0.05%; 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC·HCl) and 4-dimethylaminopyridine (DMAP) are stored in a desiccator and tested for moisture absorption and clumping before use.

[0062] Coating system raw materials: N-methylpyrrolidone (NMP) is battery grade with a moisture content ≤10ppm; sucrose is vacuum dried at 80℃ for 4 hours to remove water of crystallization; polyvinylpyrrolidone (PVP) of type K30 is selected to ensure uniform molecular weight distribution; the x value of SiOx@C composite powder is confirmed to be within the optimal range of the technical solution by X-ray fluorescence spectroscopy analysis, and the carbon content is calibrated by thermogravimetric analysis.

[0063] Equipment commissioning and calibration requirements

[0064] Ultrasonic equipment: Employs a CNC ultrasonic processor. Before use, the actual output power is calibrated using the standard aluminum foil etching method to ensure a power deviation of ≤5%. The ultrasonic probe is cleaned regularly to prevent residual substances from affecting the dispersion effect.

[0065] Mixing equipment: Equipped with a mixing system that has torque feedback function, it can monitor the viscosity change of the slurry in real time. The mixing paddle is made of polytetrafluoroethylene and the gap between it and the inner wall of the container is controlled within 5mm to ensure uniform mixing of materials.

[0066] Heat treatment equipment: The temperature field distribution of the programmed heating tube furnace needs to be calibrated to ensure that the axial temperature difference in the constant temperature zone is ≤±3℃. The gas flow control system is calibrated by a soap film flow meter, and the flow accuracy is controlled within ±2%.

[0067] Characterization equipment: X-ray photoelectron spectroscopy (XPS) is used to calibrate the accuracy of elemental quantification using standard samples; transmission electron microscopy (TEM) is used to calibrate the magnification using a standard grid; the battery testing system is calibrated with charge-discharge curves before each use to ensure voltage measurement accuracy ≤ ±0.5mV.

[0068] Preparation process of functionalized carbon nanotubes

[0069] Carboxylation modification process: The pretreated multi-walled carbon nanotubes were slowly added to a three-necked reaction flask. Under nitrogen protection, the prepared mixed acid solution was added in three portions, with an interval of 15 minutes between each addition. At the same time, the stirring speed was maintained at 300 rpm to ensure uniform dispersion of the carbon nanotubes. A reflux condenser was installed, and the reaction system was placed in a constant temperature water bath. The temperature was slowly raised to the middle temperature of the preferred range of the technical solution. After the system temperature stabilized, the ultrasonic device was turned on, and an appropriate power was set for ultrasonic reflux treatment.

[0070] During the reaction, samples were taken every 30 minutes for observation. The absorbance of the solution was monitored using a UV-Vis spectrophotometer. The reaction was considered to have reached equilibrium when the absorbance value stabilized. After the reaction was completed, the mixture was slowly poured into 10 times its volume of deionized water while stirring to dilute it. The diluted suspension was then transferred to centrifuge tubes and separated in a high-speed centrifuge. The speed was set to a suitable value that would allow the solid particles to settle completely, and the centrifugation time was adjusted according to the clarity of the supernatant.

[0071] The solid product after centrifugation was repeatedly washed with deionized water, and the pH value of the supernatant was measured after each wash until the pH reached 6.5-7.0. The washed carboxylated carbon nanotubes (-COOH-CNTs) were dried in a vacuum drying oven. The drying temperature was set to a suitable value that could effectively remove moisture without damaging the functional groups, and the vacuum degree was maintained above -0.095 MPa. The drying time was adjusted according to the sample quality to ensure that the moisture content was ≤0.5%.

[0072] The effect of carboxyl group introduction in the dried -COOH-CNTs was verified by XPS analysis, and the area ratio of the characteristic peak of carboxyl group in the O1s spectrum should be ≥15%; Fourier transform infrared spectroscopy (FTIR) was performed at 1720 cm⁻¹. -1 A distinct carboxyl characteristic absorption peak was observed nearby; transmission electron microscopy revealed an increase in the surface roughness of the carbon nanotubes, proving that the carboxylation modification was successful.

[0073] Preparation of PBA-CNTs by amidation reaction

[0074] Weigh a certain amount of -COOH-CNTs and add them to a dry three-necked flask. Add dehydrated DMF solvent and perform ultrasonic dispersion under nitrogen protection. During the ultrasonic process, control the system temperature to not exceed 30℃. Monitor the dispersion effect in real time using a laser particle size analyzer. Stop ultrasonication when the particle size distribution is stable and there is no obvious agglomeration of large particles.

[0075] 3-Aminophenylboronic acid, EDC·HCl, and DMAP were added sequentially under stirring. After the addition was complete, the reaction system temperature was controlled at the midpoint of the preferred range of the technical scheme, and the reaction was carried out at a stirring rate of 500 rpm. Samples were taken every 2 hours during the reaction, and the 1650 cm⁻¹ FTIR was monitored. -1 The formation of the characteristic peak of the amide bond is observed, and the reaction is considered to have ended when the peak intensity no longer increases.

[0076] After the reaction was completed, the mixture was transferred to a centrifuge tube, washed three times with DMF to remove unreacted organic reagents, and then washed five times with anhydrous ethanol to remove residual DMF. Each wash was performed by high-speed centrifugation until the supernatant showed no characteristic absorption peaks in the UV-Vis spectrum.

[0077] The washed product was dried in a vacuum drying oven at a temperature suitable for removing residual solvents, with the vacuum maintained at -0.098 MPa. The drying time was adjusted according to sample mass. XPS analysis confirmed the presence of boron in the dried functionalized carbon nanotubes (PBA-CNTs), with the ratio of B1s characteristic peak intensity to C1s peak intensity ≥ 0.02. FTIR spectra were obtained at 1380 cm⁻¹. -1 The presence of characteristic peaks for boronic acid esters at the point indicates that the phenylboronic acid functional group was successfully grafted.

[0078] Preparation of coating slurry and pre-coating molding

[0079] Preparation of functionalized carbon nanotube dispersion: Dry PBA-CNTs were added to battery-grade NMP and ultrasonically dispersed under nitrogen protection. The ultrasonic power was set to the middle value of the preferred range of the technical solution. The ultrasonic time was adjusted according to the dispersion effect. During the process, the process was paused for 5 minutes every 15 minutes to prevent the system from overheating. The particle size change was monitored by a dynamic light scattering instrument during the dispersion process. Ultrasonication was stopped when the average particle size was stable below 300 nm and the dispersion index (PDI) was ≤0.2.

[0080] The dispersed PBA-CNTs dispersion was filtered through a 0.45μm filter membrane to remove any large agglomerates. The concentration of the dispersion was accurately determined by gravimetric method to ensure that its mass concentration was within the optimal range of the technical solution. The qualified dispersion was sealed and stored in an inert gas environment for no more than 24 hours.

[0081] Preparation process of coating slurry: Weigh sucrose and PVP according to the preferred mass ratio of the technical solution, mix them evenly in a dry environment, and slowly add them to the PBA-CNTs dispersion. Turn on the stirring device and set the initial speed to 300 rpm. After the solid is completely wetted, increase the speed to the middle range of the preferred range of the technical solution and continue stirring.

[0082] Samples were taken every 30 minutes during the stirring process, and the viscosity of the slurry was measured by a rotational viscometer. Stirring was stopped when the viscosity stabilized in the range of 500-800 mPa·s. The microstructure of the slurry was observed using a laser confocal microscope to ensure that there was no obvious particle agglomeration and that sucrose and PVP were uniformly dispersed in the carbon nanotube network.

[0083] The prepared coating slurry needs to undergo stability testing: after standing at 25℃ for 2 hours, the volume of the supernatant should be ≤5%; by differential scanning calorimetry (DSC), there should be no obvious endothermic peak in the range of 100-200℃, which proves that there is no free water. The qualified slurry can be used immediately for subsequent coating processes to avoid performance changes caused by long-term storage.

[0084] Pre-coated composite material preparation by spray drying: Weigh SiOx@C composite powder according to the preferred mass ratio of the technical solution, slowly add it to the coating slurry in the drying glove box, turn on the stirring device, gradually increase the speed to the middle value of the preferred range of the technical solution, and continue stirring to make the powder uniformly dispersed. During the stirring process, observe the coating of the powder surface through a scanning electron microscope to ensure that each particle is uniformly covered by the slurry.

[0085] The mixed slurry is transferred to the feed tank of the spray dryer. The inlet air temperature, outlet air temperature, atomization pressure, and feed rate are set to the middle values ​​of the optimal range of the technical solution. Argon protective gas is introduced to replace the air in the system more than 3 times. After the equipment parameters are stable, the feed pump is started to begin the spray drying process.

[0086] During the spray drying process, the outlet air temperature is monitored in real time. When the fluctuation exceeds ±5℃, the feed rate is adjusted to compensate. The collected pre-coated composite material particles are cooled to room temperature under inert gas protection and then screened to select particles with a particle size in the range of 3-8μm for later use.

[0087] The thermal stability of the pre-coated composite material was tested by thermogravimetric analysis (TGA), and the weight loss rate should be ≤1% below 200℃. Scanning electron microscopy showed that the particles were regularly spherical with smooth surfaces and no cracks. X-ray diffraction (XRD) analysis showed that the crystal structure of SiOx@C was not damaged, proving that the pre-coating process was qualified.

[0088] Thermal construction of gradient coating structure

[0089] Low-temperature dehydration and crosslinking treatment: The pre-coated composite material is evenly spread in a quartz boat, and the material thickness is controlled within 5mm to avoid uneven heat transfer. The quartz boat is placed in the constant temperature zone of the tube furnace. After the furnace door is closed, air is introduced. The gas flow rate is set to a suitable value that can fully replace the atmosphere in the furnace. The replacement time is not less than 30 minutes.

[0090] Set the heating program: raise the temperature from room temperature to the target temperature at the intermediate rate within the optimal range of the technical solution, and maintain a constant temperature after reaching the target temperature. During the heat preservation process, monitor the gas composition inside the furnace using an online infrared gas analyzer. When the detected moisture content is stable, the reaction is considered complete.

[0091] After processing, the sample was allowed to cool naturally to below 100°C and then removed for characterization: the FTIR spectrum was at 1720 cm⁻¹. -1 (Carbonyl) and 1380cm -1 A distinct absorption peak was observed at the (boronic acid ester) site, indicating the formation of a cross-linked network; scanning electron microscopy showed that the particles remained intact without obvious cracking.

[0092] Medium-temperature pre-carbonization and catalytic treatment: The sample after low-temperature treatment is reloaded into a quartz boat and placed in a tube furnace. High-purity argon gas is introduced to replace the air in the furnace. The replacement is repeated no less than 5 times to ensure that the oxygen content is ≤10ppm. Setting the heating program: The temperature is increased to the target temperature at the intermediate rate of the preferred range of the technical solution, and then kept constant after the temperature is reached.

[0093] During the heat preservation process, the weight loss of the sample was monitored by thermogravimetric-differential thermal analysis (TG-DTA). When the weight loss rate stabilized below 0.1% / min, it was considered that the pre-carbonization was completed. At this stage, the coating precursor and the dispersing binder underwent thermal decomposition, and small molecule volatiles were carried out by the argon gas flow. The residual carbon formed the preliminary skeleton structure.

[0094] Analysis of samples after cooling: The XRD pattern showed a broadened carbon characteristic peak near 2θ=25°, indicating the beginning of the formation of a locally ordered structure; the intensity ratio of the D peak to the G peak (ID / IG) in the Raman spectrum was about 1.2-1.5, proving that the initial carbonization product was mainly amorphous carbon, while a small amount of ordered structure existed; the scanning electron microscope showed that the coating layer remained continuous and intact.

[0095] High-temperature graphitization and densification treatment: The sample after medium-temperature treatment is put back into the tube furnace and protected by high-purity argon gas. The gas flow rate is appropriately increased compared to the medium-temperature stage to effectively remove the volatiles generated at high temperature. The heating program is set: the temperature is increased to the target temperature at the intermediate rate of the optimized range of the technical solution, and then kept constant after the temperature is reached.

[0096] During the heat preservation process, the formation of the graphite characteristic peak (2θ=26.5°) was monitored by in-situ X-ray diffraction. The heat preservation was ended when the peak intensity no longer increased. During this stage, under the catalysis of boron, the preliminary carbonization products were further ordered to form a highly graphitized outer layer structure. Carbon nanotubes, as a rigid framework, effectively inhibited the shrinkage and deformation of the coating layer at high temperature, forming a dense gradient structure.

[0097] After naturally cooling to room temperature, the sample was taken out and observed by transmission electron microscopy. The three-layer gradient structure can be clearly distinguished: the inner layer is an amorphous carbon layer tightly bonded to the silicon substrate (thickness of about 20-50 nm), the middle layer is a composite layer reinforced by carbon nanotubes (thickness of about 30-100 nm), and the outer layer is a highly graphitized carbon layer (thickness of about 20-80 nm). The total thickness is within the preferred range of the technical solution.

[0098] Material performance testing and battery assembly verification

[0099] Physical performance characterization: The conductivity test uses the four-probe method to determine the volume resistivity of the material. Ten test points are selected at different locations, and the average value should be ≤ the upper limit value specified in the technical solution. Electrochemical impedance spectroscopy (EIS) test is performed, and the impedance value at a frequency of 10kHz should be ≤10Ω, which proves that the material has good electronic conductivity.

[0100] Structural stability test: The porosity of the coating layer is determined by mercury porosimetry and should be ≤5%; the hardness of the coating layer is tested by nanoindentation and the average value should be ≥0.8GPa and the elastic modulus should be ≥15GPa, proving that the coating layer has sufficient mechanical strength to suppress the volume expansion of silicon-based materials.

[0101] Interface adhesion test: The adhesion between the coating layer and the silicon substrate is determined by scratch test. The critical load should be ≥15N, which proves that the two are firmly bonded and are not easy to peel off during charge and discharge cycles.

[0102] Electrochemical performance testing: Electrode preparation: The negative electrode coating material, conductive carbon black, and sodium carboxymethyl cellulose (CMC) were mixed in a mass ratio of 8:1:1, and deionized water was added to form a uniform slurry. The slurry was coated on a copper foil current collector, and after vacuum drying and rolling, a negative electrode sheet was formed. The drying temperature was set at 80℃ for 12 hours. The thickness of the rolled electrode was controlled at 50-80μm, and the areal density was about 1.5-2.0mg / cm².

[0103] Battery Assembly: In an argon-protected glove box (water and oxygen content ≤0.1ppm), CR2032 coin cells were assembled using the prepared negative electrode as the working electrode, a lithium metal sheet as the counter electrode, Celgard 2400 as the separator, and 1mol / L LiPF6 / EC+DMC+EMC (volume ratio 1:1:1) as the electrolyte. Electrochemical performance was tested after standing for 12 hours.

[0104] Performance test results: At a charge / discharge rate of 0.1C, the initial discharge specific capacity should be ≥1800mAh / g, and the initial coulombic efficiency should be ≥ the value specified in the technical solution; after 500 cycles at a 1C rate, the capacity retention rate should be ≥ the value specified in the technical solution; during the cycle, the volume expansion rate of the battery, monitored by an in-situ dilatometer, should be ≤20%, proving that the gradient coating structure effectively suppresses the volume change of the silicon-based material.

[0105] Through the implementation process and performance verification of the above system, it is fully demonstrated that the anode coating material production method can stably prepare high-performance silicon-based anode materials with a three-dimensional gradient composite coating structure, meeting the application requirements of lithium-ion batteries for high-capacity and high-stability electrode materials.

[0106] The same or similar labels correspond to the same or similar parts;

[0107] The terms used to describe positional relationships in the accompanying drawings are for illustrative purposes only and should not be construed as limiting this patent.

[0108] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. For those skilled in the art, other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all implementation methods here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the claims of the present invention.

Claims

1. A method for producing a negative electrode coating material, characterized in that, Includes the following steps: Carbon nanotubes were covalently functionalized to obtain functionalized carbon nanotubes with specific functional groups grafted onto their surface. The functionalized carbon nanotubes are dispersed in an organic solvent, and a coating precursor and a dispersant binder are added to form a uniform coating slurry. A silicon-based anode matrix is ​​then added to the coating slurry, and the pre-coated composite material is obtained by spray drying. The pre-coated composite material is subjected to low-temperature dehydration and crosslinking treatment, medium-temperature pre-carbonization and catalytic treatment, and high-temperature graphitization and densification treatment in sequence to form a negative electrode coating material with a three-dimensional gradient composite coating structure. The three-dimensional gradient composite coating structure consists of, from the inside out, an amorphous carbon layer tightly bonded to the silicon-based anode substrate, a composite layer reinforced with functionalized carbon nanotubes, and a highly graphitized outer surface layer. The covalent functionalization modification of carbon nanotubes includes: Multi-walled carbon nanotubes were placed in a mixed acid and ultrasonically refluxed at 55-65°C for 3.5-4.5 hours. The volume ratio of concentrated sulfuric acid to concentrated nitric acid in the mixed acid was 2.8-3.2:1, in order to introduce carboxyl functional groups on the surface of the multi-walled carbon nanotubes. Multi-walled carbon nanotubes with carboxyl functional groups introduced on their surface were amidated with 3-aminophenylboronic acid to obtain functionalized carbon nanotubes with phenylboronic acid functional groups grafted on their surface, denoted as PBA-CNTs. The coating precursor is sucrose, and the dispersing binder is polyvinylpyrrolidone. Sucrose and polyvinylpyrrolidone are added to the functionalized carbon nanotube dispersion at a mass ratio of 8-12:1, and stirred at 500-800 r / min for 2-4 hours to obtain a uniform coating slurry. The conditions for the low-temperature dehydration and crosslinking treatment are as follows: under an air atmosphere, the temperature is increased to 200-250°C at a heating rate of 5-10°C / min and held for 2-3 hours; during this process, the coated precursor molecules dehydrate to generate carbonyl active groups, and the phenylboronic acid groups on the surface of the functionalized carbon nanotubes undergo esterification and complexation reactions with the carbonyl active groups and the amide groups in the dispersing binder to form a chemical crosslinking network; The conditions for the intermediate-temperature pre-carbonization and catalytic treatment are as follows: under an argon atmosphere, the temperature is increased to 500-700℃ at a heating rate of 10-15℃ / min and held for 1.5-2.5 hours. During this process, the coating precursor and the dispersing binder undergo decomposition and preliminary carbonization. The boron element grafted on the surface of the functionalized carbon nanotubes acts as a catalyst to reduce the graphitization temperature of the carbon material and promote the formation of a locally ordered carbon structure. The conditions for the high-temperature graphitization and densification treatment are as follows: under an argon atmosphere, the temperature is increased to 900-1000℃ at a heating rate of 8-12℃ / min and held for 3-4 hours; during this process, the preliminary carbonization products are further graphitized, and the functionalized carbon nanotubes serve as a rigid framework supporting the coating layer, forming a dense three-dimensional gradient composite coating structure.

2. The method for producing negative electrode coating material according to claim 1, characterized in that, The amidation reaction conditions are as follows: using N,N-dimethylformamide as solvent, 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride as condensing agent, and 4-dimethylaminopyridine as catalyst, the reaction is stirred at 25-30℃ for 12-16 hours. After the reaction is completed, PBA-CNTs are obtained by centrifugation, washing, and vacuum drying.

3. The method for producing negative electrode coating material according to claim 1, characterized in that, The step of dispersing the functionalized carbon nanotubes in an organic solvent, adding a coating precursor and a dispersing binder, and mixing to form a uniform coating slurry includes: The organic solvent is N-methylpyrrolidone. Functionalized carbon nanotubes are added to N-methylpyrrolidone and ultrasonically treated at a power of 300-500W for 30-60 minutes to form a functionalized carbon nanotube dispersion with a mass concentration of 0.5-2g / L. The mass ratio of the coated precursor to the functionalized carbon nanotubes is 5-10:

1.

4. The method for producing negative electrode coating material according to claim 1, characterized in that, The process of adding a silicon-based anode substrate to the coating slurry and then spray-drying it to obtain a pre-coated composite material includes: The silicon-based anode substrate is SiOx@C composite powder, where x ranges from 0.8 to 1.

2. The SiOx@C composite powder is added to the coating slurry at a mass ratio of 3-5:1 to the solid components in the coating slurry, and stirred for 1-2 hours at a speed of 800-1200 r / min to form a mixed slurry. The spray drying conditions are as follows: inlet air temperature 180-220℃, outlet air temperature 80-100℃, feed rate 10-20mL / min, atomization pressure 0.2-0.4MPa, and a pre-coated composite material is obtained after the treatment.

5. A negative electrode coating material produced by the method according to any one of claims 1-4, characterized in that, The negative electrode coating material includes a silicon-based negative electrode substrate and a three-dimensional gradient composite coating layer coated on the surface of the silicon-based negative electrode substrate. The thickness of the three-dimensional gradient composite coating layer is 50-200 nm, and the volume resistivity is ≤5×10⁻⁶. -3 Ω·cm, at a charge / discharge rate of 0.1C, the initial coulombic efficiency is ≥85%, and the capacity retention after 500 cycles is ≥80%.

6. A lithium-ion battery, characterized in that, It includes a positive electrode, an electrolyte, a separator, and a negative electrode, wherein the negative electrode is prepared using the negative electrode coating material described in claim 5.

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