Modified graphite material, method for producing same, use thereof, and battery

Abstract

The application discloses a modified graphite material and a preparation method, application and battery thereof. The modified graphite material comprises a graphite material and a coating layer; the coating layer comprises aluminum oxide and carbon. The preparation method of the modified graphite material comprises the following steps: S1, stirring a mixture containing an aluminum salt, liquid pitch and a solvent to obtain a composite coating agent; the mass ratio of the aluminum salt to the liquid pitch is (3-7):(5-15); the solubility of the aluminum salt at 25 DEG C is 50 g / 100 mL or more, for example, 70 g / 100 mL; S2, uniformly mixing the composite coating agent with the graphite material to obtain a graphite material coated with the composite coating agent; S3, heat-treating the graphite material coated with the composite coating agent, and carbonizing the graphite material to obtain the modified graphite material; the heat-treating temperature is 350-500 DEG C. The finally obtained modified graphite material has good cycle stability and superior electrochemical performance, and is suitable for high-performance lithium ion batteries.

Description

Technical Field

[0001] This invention relates to a modified graphite material, its preparation method, applications, and batteries. Background Technology

[0002] In the field of lithium-ion batteries, the performance of the anode material directly affects the overall performance and lifespan of the battery. Graphite, as a traditional anode material, is widely used due to its excellent conductivity and stable structure. However, graphite materials are prone to problems such as electrolyte penetration, structural expansion, and capacity decay during high-rate charge-discharge and cycling processes, which limits its further application.

[0003] To address these issues, surface coating technology has been proposed as an effective improvement method. By coating the surface of graphite materials with a protective film, their electrochemical performance can be enhanced, and their cycle stability improved. Among these materials, nano-alumina films have become an ideal coating material due to their excellent mechanical strength and chemical stability. Liquid-phase pitch, as an organic material with good adhesion, can effectively improve the conductivity and interfacial stability of the negative electrode.

[0004] Patent CN113363445A discloses a method for coating modified graphite materials using a network of γ-alumina. The method first mixes a graphite dispersion solution with an aluminum salt solution in a specific ratio, then adds an appropriate amount of alkaline substance to adjust the pH value, thereby forming a sol-gel slurry. Next, an aging reaction is carried out at 50–120°C for 2–6 hours, followed by drying and pulverizing the slurry. Finally, the precursor is calcined at high temperature in an inert atmosphere, and after pulverization and sieving, the modified graphite material is obtained. While this modified graphite material can enhance the mechanical strength and chemical stability of the negative electrode, its conductivity is relatively low, and it is difficult to control the uniformity of the coating.

[0005] Patent CN115911304A discloses a liquid-phase pitch-coated graphite material and its preparation method. This method involves uniformly mixing graphite with liquid-phase pitch, followed by high-temperature carbonization of the surface coating layer, and finally obtaining the liquid-phase-coated graphite material through sieving and demagnetization. The advantage of this method is the good fluidity of the liquid-phase coating agent, enabling uniform coating. While this negative electrode material exhibits good conductivity and interfacial contact, improving the conductivity of the negative electrode, it may degrade and trigger side reactions under high-temperature conditions, exhibiting poor thermal stability. Summary of the Invention

[0006] To address the shortcomings of existing lithium-ion batteries using graphite as the anode material, such as poor thermal stability and initial efficiency, this invention provides a composite graphite material, its preparation method, applications, and a lithium-ion battery thereof. When this composite graphite material is used as the anode material in a lithium-ion battery, the resulting battery not only exhibits excellent initial efficiency and cycle performance, but also good discharge capacity, rate performance, and kinetic performance.

[0007] The present invention solves the above-mentioned technical problems by adopting the following technical solutions:

[0008] The present invention provides a modified graphite material comprising a graphite material and a coating layer; the coating layer comprises alumina and carbon.

[0009] The content of magnetic substances in the modified graphite material can be 0.1-0.6 ppm, for example 0.347 ppm, 0.481 ppm, 0.545 ppm or 0.532 ppm.

[0010] The D10 of the modified graphite material can be 4-6 μm, for example 5.31 μm, 5.22 μm, 5.16 μm or 5.07 μm.

[0011] The D50 of the modified graphite material can be 10-12 μm, for example 11.51 μm, 11.38 μm, 11.21 μm or 11.19 μm.

[0012] The D90 of the modified graphite material can be 31-33 μm, for example 32.10 μm, 31.46 μm, 31.13 μm or 31.01 μm.

[0013] The specific surface area of ​​the modified graphite material can be 1.5-2.5 m². 2 / g, for example 2.04m 2 / g, 1.98m 2 / g, 1.89m 2 / g or 1.87m 2 / g.

[0014] This invention provides a method for preparing modified graphite materials, comprising the following steps:

[0015] S1. A composite coating agent is obtained by stirring a mixture containing aluminum salt, liquid phase asphalt and solvent; the mass ratio of aluminum salt to liquid phase asphalt is (3-7):(5-15); the solubility of aluminum salt at 25°C is above 50g / 100mL, for example 70g / 100mL.

[0016] S2. Mix the composite coating agent described in step S1 with the graphite material evenly to obtain the graphite material coated with the composite coating agent.

[0017] S3. The graphite material coated with the composite coating agent is heat-treated and carbonized to obtain the modified graphite material; the heat treatment temperature is 350-500℃.

[0018] In this invention, the aluminum salt may be anhydrous aluminum nitrate and / or aluminum nitrate nonahydrate, for example, aluminum nitrate nonahydrate.

[0019] In a preferred embodiment, the aluminum salt is aluminum nitrate nonahydrate. Aluminum nitrate nonahydrate, also known as aluminum nitrate nonahydrate, has the chemical formula H₂O. 18 AlN3O 18 It can be represented by Al(NO3)3·9H2O.

[0020] For ease of storage and use, aluminum nitrate nonahydrate should be stored in a cool, dry, and well-ventilated warehouse. Keep away from fire and heat sources. The packaging must be sealed and protected from moisture.

[0021] In step S1, the solvent can be a solvent conventionally used in the art. Preferably, the solvent can be deionized water and / or ethanol. More preferably, the solvent can be deionized water and ethanol, and the volume ratio of the deionized water and ethanol can be (1-10):1, for example 1:1, 1.5:1, 2.33:1 or 4:1.

[0022] In step S1, the mass ratio of the aluminum salt to the solvent can be (0.2-0.8):1, for example 0.6:1, 0.5:1, 0.4:1 or 0.3:1.

[0023] In specific implementations, if there is too little solvent, the coating agent will have poor fluidity, resulting in uneven coating and affecting the material properties; if there is too much solvent, the material will have high moisture content after being mixed with graphite, which will cause agglomeration in the subsequent heat treatment stage.

[0024] In step S1, the mass ratio of the aluminum salt to the liquid phase asphalt can be 6:10, 5:10, 4:10 or 3:10.

[0025] In step S1, the density of the liquid phase asphalt can be 0.5-1.5 g / cm³. 3 For example, 1.10 g / cm³ 3 .

[0026] In step S1, the sulfur content of the liquid phase asphalt can be less than 0.15 wt%, for example, 0.10 wt%.

[0027] In step S1, the ash content of the liquid phase asphalt can be less than 0.03 wt%, for example, 0.01 wt%.

[0028] The ash content is defined as the weight ratio of the residue of the original sample after the dry liquid phase asphalt has been burned to constant weight at 800±20℃ under air conditions.

[0029] In step S1, the coking value of the liquid phase pitch can be 5-20 wt%, for example, 5 wt%, 10 wt%, 15 wt%, or 20 wt%. The coking value means the percentage of the weight of coke formed after the pitch undergoes thermal decomposition and condensation reaction during heating, compared to the weight of the raw coal pitch.

[0030] Different coking values ​​require different dilution ratios. To ensure the residual carbon content of the asphalt coating, it is necessary to determine the coking value at a given dilution ratio. This ensures both the fluidity of the coating agent and the residual carbon content.

[0031] In step S1, the residual carbon content of the composite coating agent can be 0.5wt%-2.0wt%, for example, 0.5wt%, 1wt%, 1.5wt%, or 2wt%. An excessively high residual carbon value will affect the specific surface area of ​​the material and reduce its performance.

[0032] The residual carbon value refers to the mass fraction of the charred black residue formed after heating, evaporation, pyrolysis, and combustion.

[0033] Excessively high residual carbon values ​​can easily lead to side reactions and cause performance degradation. Conversely, excessively low residual carbon values ​​do not significantly improve fast charging performance.

[0034] In a specific implementation plan, condensate oil can be used to dilute the liquid phase asphalt, resulting in a coking value of 5-20 wt%. The condensate oil is a carbonization product from the production line of Shanshan New Materials Co., Ltd. The main components of the condensate oil are a mixture of C5 to C11+ hydrocarbons, containing small amounts of hydrocarbons larger than C8, as well as impurities such as sulfur dioxide, thiophenes, thiols, thioethers, and polysulfides.

[0035] In step S1, the stirring time can be 10 to 20 minutes, for example, 10 minutes.

[0036] In step S1, the stirring temperature is a temperature conventionally used in the art. Preferably, the stirring temperature is room temperature, which is 15-30°C.

[0037] In step S2, the graphite material is a graphite material conventionally used in the art. The graphite material may be artificial graphite and / or natural graphite, for example, artificial graphite.

[0038] In step S2, the mass ratio of the composite coating agent to the graphite material can be (1-3):10, for example 1.6:10, 1.5:10, 1.4:10 or 1.3:10.

[0039] In this invention, too little coating agent makes it difficult to form a complete membrane structure, while too much coating agent will cause aggregation.

[0040] In step S2, the mass percentage of liquid phase asphalt in the composite coating agent can be 5wt% to 15wt% to improve the conductivity and chemical stability of the negative electrode.

[0041] In this invention, the alumina coating ratio is determined by calculation based on the conversion ratio between aluminum salt and alumina.

[0042] In a preferred embodiment, the alumina coating ratio should be determined by calculating the conversion ratio between aluminum nitrate nonahydrate and alumina, and the aluminum nitrate nonahydrate, after dilution, will first decompose into aluminum ions (Al). 3+ ) and nitrate ions (NO 3- Simultaneously, water molecules hydrate with aluminum ions. Due to the introduction of the solute, the volume of the solution increases as the solute dissolves. This is because during the dissolution process in water, water molecules are occupied and form a new solution structure.

[0043] In step S2, the mixing adopts a mixing method conventionally used in the art.

[0044] In a preferred embodiment, the mixing in step S2 is carried out in a fusion machine. The mixing speed can be 400–900 rad / min, for example, 900 rad / min. The mixing time in step S2 can be 5–15 min, for example, 10 min.

[0045] In a preferred embodiment, the heat treatment in step S3 employs a segmented heating method, which includes the following steps:

[0046] ① Heating to 250-400℃, for example 400℃, 350℃, 300℃ or 250℃; the heating rate can be 1-5℃ / min, for example 1.5℃ / min; the heating isothermal time can be 2-5h, for example 4h, 3.5h, 3h or 2.5h;

[0047] ② Continue heating to 400-600℃, for example 600℃, 550℃, 500℃ or 450℃; the heating rate can be 1-5℃ / min, for example 1.5℃ / min; the heating isothermal time can be 5-10h, for example 7h, 6.5h, 6h or 5.5h.

[0048] In a preferred embodiment, the composite coating agent described in step S3 is generated in situ on the surface of the graphite material, that is, aluminum oxide is generated in situ at high temperature on the particle surface.

[0049] In a preferred embodiment, an inert gas should be introduced during the heat treatment process in step S3. The inert gas is one or more of nitrogen, argon, and helium, for example, argon.

[0050] In a specific implementation plan, after the heat treatment described in step S3, the material needs to be cooled to room temperature before the carbonization step is carried out.

[0051] In step S3, the carbonization temperature can be 950-1350℃, for example 1300℃, 850℃, 1000℃ or 1150℃.

[0052] In step S3, the carbonization time can be 4-10 hours, for example, 5 hours.

[0053] In a preferred embodiment, step S3, following carbonization, further includes a sieving step. The particle size range of the sieving step can be 100-400 mesh, for example, 300 mesh.

[0054] This invention provides a modified graphite material, which is prepared by the modified graphite material preparation method described above.

[0055] This invention provides an application of the modified graphite material described above as a negative electrode material in lithium-ion batteries.

[0056] The present invention also provides a lithium-ion battery comprising the modified graphite material as described above.

[0057] Based on common knowledge in the field, the above-mentioned preferred conditions can be combined arbitrarily to obtain various preferred embodiments of the present invention.

[0058] The reagents and raw materials used in this invention are all commercially available.

[0059] The positive and progressive effects of this invention are as follows:

[0060] (1) This invention incorporates liquid-phase asphalt while coating alumina. The coating agent has good fluidity, effectively controlling the heat generated during the reaction and ensuring the uniformity of the reaction, thus guaranteeing sufficient contact and reaction between the liquid and solid phases. Compared to coating any material alone, this not only achieves superior structural and performance control technically, but also significantly improves the overall performance of graphite materials in terms of electrochemical performance, conductivity, and chemical stability. The resulting modified graphite material exhibits a charge specific capacity of over 355.42 mAh / g, an initial efficiency of over 94.75%, and a capacity retention rate of over 90.1% after 7 days at high temperature (60°C). This method for preparing modified graphite material demonstrates its potential for widespread application in the field of high-performance lithium-ion batteries.

[0061] (2) The high specific surface area protective layer formed by nano-alumina effectively reduces the direct contact between the electrolyte and graphite, reduces the occurrence of side reactions, and thus extends the service life of the electrode. The use of nano-alumina film improves the electrochemical performance of the electrode, enhances its conductivity and stability, and due to its high specific surface area and excellent conductivity, it can effectively enhance the transport rate of lithium ions in the electrode material.

[0062] (3) Liquid-phase asphalt, as a binder, enhances the adhesion of the alumina film and prevents peeling during charging and discharging. The slow heating during the heat treatment process effectively avoids stress damage to the coating layer, resulting in a stronger bond between the alumina film and the graphite material, further improving the mechanical strength and electrochemical performance of the material. Attached Figure Description

[0063] Figure 1 This is an elemental scan of the modified graphite material obtained in Example 1. Detailed Implementation

[0064] The present invention is further illustrated below by way of embodiments, but the invention is not limited to the scope of the embodiments described herein. Experimental methods in the following embodiments that do not specify specific conditions were performed according to conventional methods and conditions, or as selected according to the product instructions.

[0065] Liquid phase asphalt was purchased from Liaoning Xinde New Material Technology (Group) Co., Ltd.

[0066] Artificial graphite, purchased from Shanshan New Materials Co., Ltd.

[0067] Aluminum nitrate nonahydrate, with a solubility of 70 mg / mL at 25°C, was purchased from Aladdin Biochemical Technology Co., Ltd.

[0068] Aluminum chloride, with a solubility of 45 mg / mL at 25°C, was purchased from Aladdin Biochemical Technology Co., Ltd.

[0069] Example 1

[0070] S1. Dilute 600g of aluminum nitrate nonahydrate with 500mL of anhydrous ethanol and 500mL of deionized water, and stir at 25℃ for 10min. Mix with 1kg of liquid asphalt (density 1.10g / cm³). 3 Mix (coking value of 20wt%, sulfur content of 0.10wt%, ash content of 0.01wt%) and stir evenly at 25℃ for 10min to form a composite coating agent (residual carbon value of 2wt%).

[0071] S2. Fully fuse with 10kg of artificial graphite material (D50 is 10.0±0.5μm) in a fusion machine (rotation speed is 1000rad / min, time is 10min) to make the coating agent uniformly cover and form a graphite material coated with composite coating agent.

[0072] S3. The graphite material coated with the composite coating agent was heat-treated using a thermal VC-5 mixer under an argon atmosphere. The heat treatment equipment operated at 12 Hz, heating to 400℃ at a rate of 1.5℃ / min, holding at that temperature for 4 hours, and then continuing to heat to 600℃ at a rate of 1.5℃ / min for 7 hours. After cooling to room temperature, the coated graphite material was carbonized at 1300℃ (heating rate of 2.5℃ / min) and held at that temperature for 5 hours. After mixing and sieving through a 300-mesh sieve, the modified graphite material was obtained.

[0073] Example 2

[0074] S1. Dilute 500g of aluminum nitrate nonahydrate with 400mL of anhydrous ethanol and 600mL of deionized water, and stir for 10min at room temperature (25℃). Mix with 1kg of liquid pitch (density 1.10g / cm³). 3 Mix (coking value of 15wt%, sulfur content of 0.10wt%, ash content of 0.01wt%) and stir evenly at 25℃ for 10min to form a composite coating agent (residual carbon value of 1.5wt%).

[0075] S2. Fully fuse with 10kg of artificial graphite material (D50 is 10.0±0.5μm) in a fusion machine (rotation speed is 950rad / min, time is 10min) to make the coating agent uniformly cover and form a graphite material coated with composite coating agent.

[0076] S3. The graphite material coated with the composite coating agent was heat-treated using a hot VC-5 mixer under an argon atmosphere. The heat treatment equipment operated at 10 Hz, heating to 350℃ at a rate of 1.5℃ / min, holding at that temperature for 3.5 hours, and then continuing to heat to 550℃ at a rate of 1.5℃ / min for 6.5 hours. After cooling to room temperature, the coated graphite material was carbonized at 1150℃ (heating rate of 2.5℃ / min) and held at that temperature for 5 hours. After mixing and sieving through a 300-mesh sieve, the modified graphite material was obtained.

[0077] Example 3

[0078] S1. Dilute 400g of aluminum nitrate nonahydrate with 300mL of anhydrous ethanol and 700mL of deionized water, and stir at 25℃ for 10min. Mix with 1kg of liquid asphalt (density 1.10g / cm³). 3 Mix (coking value 10wt%, sulfur content 0.10wt%, ash content 0.01wt%) and stir evenly at 25℃ for 10min to form a composite coating agent (residual carbon value 1%).

[0079] S2. Fully fuse with 10kg of artificial graphite material (D50 is 10.0±0.5μm) in a fusion machine (rotation speed is 900rad / min, time is 10min) to make the coating agent uniformly cover and form a graphite material coated with composite coating agent;

[0080] S3. The graphite material coated with the composite coating agent was heat-treated using a hot VC-5 mixer under an argon atmosphere. The heat treatment equipment operated at 8 Hz, heating to 300℃ at a rate of 1.5℃ / min, holding at that temperature for 3.0 h, and then continuing to heat to 500℃ at a rate of 1.5℃ / min for 6 h. After cooling to room temperature, the coated graphite material was carbonized at 1000℃ (heating rate of 2.5℃ / min) and held at that temperature for 5 h. After mixing and sieving through a 300-mesh sieve, the final composite-coated graphite material was obtained.

[0081] Example 4

[0082] S1. Dilute 300g of aluminum nitrate nonahydrate with 200mL of anhydrous ethanol and 800mL of deionized water, and stir for 10min at room temperature (25℃). Mix with 1kg of liquid asphalt (density 1.10g / cm³). 3 Mix (coking value 5wt%, sulfur content 0.10wt%, ash content 0.01wt%) and stir evenly at 25℃ for 10min to form a composite coating agent (residual carbon value 0.5wt%).

[0083] S2. Fully fuse with 10kg of artificial graphite material (D50 is 10.0±0.5μm) in a fusion machine (rotation speed is 850rad / min, time is 10min) to make the coating agent uniformly cover and form a graphite material coated with composite coating agent;

[0084] S3. The graphite material coated with the composite coating agent was heat-treated using a hot VC-5 mixer under an argon atmosphere. The heat treatment equipment operated at 6 Hz, heating to 250℃ at a rate of 1.5℃ / min, holding at that temperature for 2.5 hours, and then continuing to heat to 450℃ at a rate of 1.5℃ / min for 5.5 hours. After cooling to room temperature, the coated graphite material was carbonized at 850℃ (heating rate of 2.5℃ / min) and held at that temperature for 5 hours. After mixing and sieving through a 300-mesh sieve, the modified graphite material was obtained.

[0085] Comparative Example 1

[0086] This comparative example involves coating a nano-alumina film separately using conventional methods, including the following steps:

[0087] (1) Mix 400g of aluminum nitrate nonahydrate with 300mL of anhydrous ethanol and 700mL of deionized water using a stirrer for 10min, keeping the speed at 300-400rad / min, to form an alumina precursor solution.

[0088] (2) Stir the alumina precursor solution obtained in step (1) uniformly to form an alumina coating agent, and fully fuse it with the graphite material in a fusion machine so that the coating agent uniformly covers the material and forms a nano alumina film coating layer; the fusion parameters are: 900 rad / min fusion for 10 min;

[0089] (3) The coated graphite material was heat-treated in an argon atmosphere using a hot VC-5 mixer. The heat treatment equipment was operated at 8 Hz, heated to 300 ℃ at a heating rate of 1.5 ℃ / min, held at that temperature for 3.0 h, and then heated to 500 ℃ for 6 h. After cooling to room temperature at a cooling rate of 3.0 ℃ / min, the coated graphite material was carbonized at 1000 ℃ and held for 5 h at a heating rate of 2.5 ℃ / min. After mixing and sieving through a 300-mesh sieve, the final alumina-coated graphite material was obtained.

[0090] Comparative Example 2

[0091] This comparative example involves coating liquid phase bitumen separately using traditional methods, including the following steps:

[0092] (1) 1 kg of material has a density of 1.10 g / cm³. 3 Liquid phase asphalt with a coking value of 10 wt% and graphite material are fully fused in a fusion machine to ensure uniform coverage by the coating agent and form a liquid phase asphalt coating layer; the fusion parameters are: 900 rad / min for 10 min.

[0093] (2) The coated graphite material was subjected to VC heat treatment in an argon atmosphere using a hot VC-5 mixer. The heat treatment equipment was operated at 8 Hz, heated to 300 ℃ at a heating rate of 1.5 ℃ / min, held at that temperature for 3.0 h, and then heated to 500 ℃ for 6 h. After cooling to room temperature at a cooling rate of 3.0 ℃ / min, the coated graphite material was subjected to high-temperature carbonization at 1000 ℃ and held at that temperature for 5 h. The heating rate was 2.5 ℃ / min. After mixing and sieving through a 300-mesh sieve, the final liquid-phase asphalt-coated graphite material was obtained.

[0094] Comparative Example 3

[0095] The difference between this comparative example and Example 1 is that in step S1, aluminum chloride is used instead of aluminum nitrate nonahydrate.

[0096] Comparative Example 4

[0097] The difference between this comparative example and the embodiment is that the heat treatment step S3 is as follows: the temperature is increased to 100°C at a heating rate of 1.5°C / min, held at the temperature for 4 hours, and then increased to 200°C at a heating rate of 1.5°C / min for 7 hours; the remaining steps are the same as in embodiment 1.

[0098] Comparative Example 5

[0099] The difference between this comparative example and Example 1 is that the mass of aluminum nitrate nonahydrate in step S1 is 1 kg and the mass of liquid phase pitch in step S2 is 0.5 kg. The other steps are the same as in Example 1.

[0100] Example 1

[0101] D10, D50 and D90 testing: The particle size range (D10, D50 and D90) and particle size distribution of the material were tested using a Malvern MS3000 laser particle size analyzer.

[0102] Specific surface area testing: The specific surface area of ​​the material was tested using the NOVAtouch™ fully automated specific surface area and pore size analyzer from CANTA Instruments, Inc.

[0103] Testing of magnetic materials: The content of magnetic materials in the materials was tested using a Thermo ICAP 7400 ICP analyzer.

[0104] Elemental scanning: The modified graphite material obtained in Example 1 was subjected to elemental scanning using a Thermo Fisher Scientific Prisma scanning electron microscope. The results are as follows: Figure 1 As shown.

[0105] DC internal resistance (DCIR): The DCIR of the material was tested using an Arbin test cabinet.

[0106] Example 2

[0107] The charge and discharge performance of button batteries was tested using the LAND battery testing system from Wuhan Landian Electronics Co., Ltd.

[0108] The modified graphite materials prepared in the various examples and comparative examples were coated onto copper foil, dried under vacuum at 105°C for 4 hours, and rolled to prepare negative electrode sheets. A coin cell battery was assembled in an argon-filled inert gas glove box system using a 1 mol / L LiPF6 three-component mixed solvent with a volume ratio of ethyl carbonate:dimethyl carbonate:ethyl methyl carbonate = 1:1:1 as the electrolyte, a polypropylene microporous membrane as the separator, and a lithium metal sheet as the positive electrode. Charge-discharge tests were conducted on the coin cell battery using a battery testing system. Under normal temperature conditions, constant current charge-discharge was performed at 0.1C, and the charge-discharge voltage was limited to 0.005–1.5V.

[0109] (1) Initial discharge specific capacity and initial coulombic efficiency

[0110] The test conditions were: electrode compaction density was 1.65 g / cm³. 3 The initial charge and discharge cycle is performed at a rate of 0.1C, followed by rate charging and discharging at current densities ranging from 0.1C to 2C, with the charging voltage limited to 0.005-2V. The initial coulombic efficiency is calculated as (initial discharge capacity / initial charge capacity) * 100%.

[0111] (2) Capacity retention rate

[0112] The compacted density of the electrode sheet is 1.65 g / cm³. 3 After being stored at 60℃ for 7 days, a charge-discharge test was conducted at a 0.2C rate. Capacity retention rate = (charge capacity after high-temperature storage / charge capacity before high-temperature storage) * 100%.

[0113] The modified graphite materials obtained in Examples 1-4 and Comparative Examples 1-5 were tested using the above testing methods. The test results are shown in Table 1.

[0114] Table 1

[0115]

[0116] As shown in Table 1, the electrochemical performance of the modified graphite materials obtained in Examples 1-4 is better than that of Comparative Examples 1-5. The DCIR of Examples 1-4 is below 4.82Ω, the specific charge capacity is above 355.42mAh / g, the first efficiency is above 94.75%, and the capacity retention rate at 60℃ for 7 days is above 90.1%.

[0117] Depend on Figure 1 As can be seen, the prepared modified graphite material has an inner layer of graphite particles and an outer layer of a composite coating of pitch and alumina. The successful coating of alumina onto the graphite particle surface indicates that aluminum nitrate nonahydrate reacts in situ to produce alumina under high-temperature carbonization. Furthermore, due to the good fluidity of the coating agent, the overall coating effect is uniform. Therefore, compared to Comparative Examples 1 and 2, which separately coat either material, this invention not only achieves superior structural and performance control technically, but also significantly improves the overall performance of the graphite material in terms of electrochemical properties, conductivity, and chemical stability.

[0118] As shown in Examples 1-4, the specific capacity of the modified graphite material increases with increasing doping amount of aluminum nitrate nonahydrate and increasing coking value of liquid-phase pitch. This is because after aluminum nitrate nonahydrate enters the graphite material, aluminum ions introduce defect sites into the graphite lattice. These defect sites provide more active sites for lithium ion adsorption and insertion, making it easier for lithium ions to interact with the graphite material, thereby increasing the specific capacity. A higher coking value helps to increase the graphitization degree of the graphite material. Increased graphitization degree makes the interlayer spacing of graphite more regular and enhances the interlayer force, which is beneficial for the insertion and extraction of lithium ions between graphite layers. It also improves the conductivity and structural stability of the material, thereby increasing the specific capacity. The particle size and specific surface area of ​​Examples 1-4 and Comparative Examples 1-5 are similar, indicating that different coating amounts do not have a significant impact on the morphology and particle size of the negative electrode material.

[0119] The capacity retention rates of Examples 1-4 after 7 days at 60°C were significantly better than those of Comparative Examples 1-5. This is because aluminum nitrate nonahydrate decomposes and transforms into alumina (Al2O3) during heat treatment. The thermal stability and high-temperature resistance of alumina effectively prevent the oxidation reaction of graphite at high temperatures, thus enhancing its heat resistance.

[0120] Compared to Examples 1-4, Comparative Example 3 exhibits poorer electrochemical performance because nitrate itself has strong oxidizing properties. Under high-temperature conditions, nitrate ions decompose into oxygen (O2) and nitrogen oxides (such as NO2 and NO). These oxidizing substances have a certain oxidizing effect on the graphite surface, increasing the interlayer spacing of the graphite material, thus benefiting its electrochemical performance. However, aluminum chloride does not possess this effect, and the modified graphite material has a higher impurity content, resulting in poorer electrochemical performance.

[0121] Compared to Examples 1-4, Comparative Example 4 exhibits poorer electrochemical performance. This is because the formation of the alumina layer requires a higher temperature to promote the decomposition of aluminum nitrate, releasing nitrogen oxides which then deposit to form alumina. If the heat treatment temperature is too low, aluminum nitrate nonahydrate may not completely decompose or transform into alumina, resulting in an incomplete or uneven alumina coating layer and exposed areas on the graphite surface, affecting the effectiveness of subsequent carbonization processes. Liquid-phase bitumen is typically used to form a protective layer or enhance the adhesion between graphite and alumina. During heat treatment, bitumen undergoes pyrolysis, releasing volatile components and transforming into a stable carbon layer. If the heat treatment temperature is too low, the bitumen may not pyrolyze sufficiently, leading to incomplete carbonization and an uneven or insufficient carbon layer, potentially affecting the bonding strength between graphite and the coating layer.

[0122] Compared to Examples 1-4, Comparative Example 5 exhibits poorer electrochemical performance. This is because Comparative Example 5 has a high content of aluminum nitrate nonahydrate. During subsequent processing or use, the thermal decomposition of aluminum nitrate may lead to significant stress within the graphite material. When this stress exceeds the graphite's tolerance limit, it can easily cause cracks or even breakage, severely damaging the overall structural integrity of the graphite and thus affecting its performance as an electrode material. Furthermore, insufficient liquid-phase asphalt results in poor coating fluidity and uneven coating.

[0123] The above description is merely a partial preferred embodiment of the present invention. Any person skilled in the art can modify the described technical solutions or modify them into equivalent technical solutions. Although the present invention provides specific embodiments, it should be understood that further improvements can be made to the present invention. In summary, according to the principles of the present invention, this application is intended to include any changes, uses, or improvements to the present invention, including changes made using conventional techniques known in the art that depart from the scope disclosed in this application. Without departing from the concept of the present invention, all other implementation methods that can be conceived by those skilled in the art without creative effort, as well as other simple substitutions and various changes to the technical solutions of the present invention, are within the protection scope of the present invention.

Claims

1. A method for preparing a modified graphite material, characterized in that, It includes the following steps: S1. A composite coating agent is obtained by stirring a mixture containing aluminum salt, liquid asphalt and solvent; the mass ratio of aluminum salt to liquid asphalt is (3-6):10; the aluminum salt is aluminum nitrate nonahydrate; the mass ratio of aluminum salt to solvent is (0.2-0.8):1; S2. Mix the composite coating agent described in step S1 with the graphite material evenly to obtain the graphite material coated with the composite coating agent. S3. Graphite materials coated with composite coating agents are heat-treated and carbonized to obtain modified graphite materials. The heat treatment adopts a segmented heating method, which includes the following steps: ① heating to 250-400℃; ② continuing to heat to 400-600℃; the carbonization temperature is 950-1350℃.

2. The method for preparing the modified graphite material as described in claim 1, characterized in that, Step S1 satisfies one or more of the following conditions af: a. The mass ratio of the aluminum salt to the liquid phase pitch is 6:10, 5:10, 4:10 or 3:10; c. The solvent is deionized water and / or ethanol; d. The mass ratio of the aluminum salt to the solvent is 0.6:1, 0.5:1, 0.4:1, or 0.3:1; e. The stirring time is 10-20 minutes; f. The solubility of the aluminum salt at 25°C is 70 g / 100 mL.

3. The method for preparing the modified graphite material as described in claim 2, characterized in that, Step S1 satisfies the following conditions a and / or b: a. The solvent is deionized water and ethanol, and the volume ratio of deionized water to ethanol is (1-10):1; b. The stirring time is 10 minutes.

4. The method for preparing the modified graphite material as described in claim 3, characterized in that, In step S1, the volume ratio of deionized water to ethanol is 1:1, 1.5:1, 2.33:1, or 4:

1.

5. The method for preparing the modified graphite material as described in claim 1, characterized in that, The density of the liquid phase asphalt is 0.5-1.5 g / cm³. 3 ; And / or, the sulfur content of the liquid phase asphalt is less than 0.15 wt%; And / or, the ash content of the liquid phase asphalt is less than 0.03 wt%; And / or, the coking value of the liquid phase pitch is 5-20 wt%; And / or, the residual carbon of the composite coating agent is 0.5 wt%-2.0 wt%.

6. The method for preparing the modified graphite material as described in claim 5, characterized in that, The density of the liquid phase asphalt is 1.10 g / cm³. 3 ; And / or, the sulfur content of the liquid phase asphalt is 0.10 wt%; And / or, the ash content of the liquid phase asphalt is 0.01 wt%; And / or, the coking value of the liquid phase pitch is 5 wt%, 10 wt%, 15 wt%, or 20 wt%; And / or, the residual carbon content of the composite coating agent is 0.5 wt%, 1 wt%, 1.5 wt%, or 2 wt%.

7. The method for preparing the modified graphite material as described in claim 1, characterized in that, Step S2 satisfies one or more of the following conditions ad: a. The graphite material is artificial graphite and / or natural graphite; b. The mass ratio of the composite coating agent to the graphite material is (1-3):10; c. The mixing described in step S2 is carried out in a fusion machine; d. The mixing time is 5~15 min.

8. The method for preparing the modified graphite material as described in claim 7, characterized in that, Step S2 satisfies one or more of the following conditions ad: a. The graphite material is artificial graphite; b. The mass ratio of the composite coating agent to the graphite material is 1.6:10, 1.5:10, 1.4:10, or 1.3:10; c. The mixing speed is 400~900 rad / min; d. The mixing time is 10 min.

9. The method for preparing the modified graphite material as described in claim 8, characterized in that, In step S2, the mixing speed is 900 rad / min.

10. The method for preparing the modified graphite material as described in claim 1, characterized in that, Step S3 satisfies one or more of the following conditions af: a. The heat treatment employs a segmented heating method, which includes the following steps: ①Heat to 400℃, 350℃, 300℃ or 250℃; ② Continue heating to 600℃, 550℃, 500℃ or 450℃; b. The carbonization temperature is 1300℃, 850℃, 1000℃ or 1150℃; c. The carbonization time is 4-10 hours; d. The carbonization process also includes a sieving step.

11. The method for preparing the modified graphite material as described in claim 10, characterized in that, Step S3 satisfies one or more of the following conditions af: a. The heat treatment adopts a segmented heating method, and in ①, the heating rate is 1-5℃ / min; b. The heat treatment adopts a segmented heating method. In ①, the constant temperature time for heating is 2-5 hours. c. The heat treatment adopts a segmented heating method, and in ②, the heating rate is 1-5℃ / min; d. The heat treatment adopts a segmented heating method, and in ②, the constant temperature time for heating is 5-10 hours; e. The carbonization time is 5 hours; f. The particle size range of the sieve is 100-400 mesh.

12. The method for preparing the modified graphite material as described in claim 11, characterized in that, Step S3 satisfies one or more of the following conditions ae: a. The heat treatment adopts a segmented heating method, and in ①, the heating rate is 1.5℃ / min; b. The heat treatment adopts a segmented heating method. In ①, the constant temperature time for heating is 4h, 3.5h, 3h or 2.5h. c. The heat treatment adopts a segmented heating method, and in ②, the heating rate is 1.5℃ / min; d. The heat treatment adopts a segmented heating method. In ②, the isothermal time of the heating is 7 h, 6.5 h, 6 h or 5.5 h; e. The particle size range of the sieve is 300 mesh.

13. A modified graphite material, characterized in that, It is prepared by the method for preparing modified graphite materials as described in any one of claims 1-12.

14. The modified graphite material as described in claim 13, characterized in that, It includes graphite material and a coating layer; the coating layer contains aluminum oxide and carbon.

15. The modified graphite material as described in claim 13, characterized in that, The modified graphite material satisfies one or more of the following conditions: a. The content of magnetic substances in the modified graphite material is 0.1-0.6 ppm; b. The D10 of the modified graphite material is 4-6 μm; c. The D50 of the modified graphite material is 10-12 μm; d. The D90 of the modified graphite material is 31-33 μm; e. The specific surface area of ​​the modified graphite material is 1.5-2.5 m². 2 / g.

16. The modified graphite material as described in claim 15, characterized in that, The modified graphite material satisfies one or more of the following conditions: a. The content of magnetic substances in the modified graphite material is 0.347 ppm, 0.481 ppm, 0.545 ppm or 0.532 ppm; b. The D10 of the modified graphite material is 5.31 μm, 5.22 μm, 5.16 μm or 5.07 μm; c. The D50 of the modified graphite material is 11.51 μm, 11.38 μm, 11.21 μm or 11.19 μm; d. The D90 of the modified graphite material is 32.10 μm, 31.46 μm, 31.13 μm or 31.01 μm; e. The specific surface area of ​​the modified graphite material is 2.04 m². 2 / g, 1.98 m 2 / g, 1.89 m 2 / g or 1.87 m 2 / g.

17. The application of a modified graphite material as a negative electrode material in lithium-ion batteries as described in any one of claims 13-16.

18. A lithium-ion battery, characterized in that, It includes the modified graphite material as described in any one of claims 13-16.

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