Preparation method of composite graphite graphite crucible for sintering of lithium iron manganese phosphate positive electrode material

Abstract

The present application relates to the technical field of carbon composite materials, and particularly relates to a preparation method of a composite graphite crucible for sintering of a lithium iron manganese phosphate positive electrode material. The present application aims to solve the problems of poor thermal conductivity, easy thermal shock cracking and high-temperature deformation of traditional refractory castable crucibles. In the method, silica anchor points are constructed on the surface of dopamine site-modified single-walled carbon nanotubes and terminated with silane, and then the silica anchor points are compounded with phenolic resin and boric acid. Finally, the mixture is kneaded, hot-pressed and carbonized in batches with flake graphite of a specific particle size. The prepared composite graphite crucible significantly improves the thermal conductivity, structural stability and thermal shock resistance, optimizes the microstructure uniformity, thereby effectively reducing the sintering energy consumption and greatly prolonging the service life of the crucible in continuous production.

Description

Technical Field

[0001] This invention relates to the field of carbonaceous composite materials technology, and in particular to a method for preparing a composite graphite sagger for sintering lithium manganese iron phosphate cathode materials. Background Technology

[0002] In the production of cathode materials for lithium-ion batteries, lithium manganese iron phosphate (LMFP) has become one of the key material systems under development due to its advantages of high voltage, high safety, and relatively low cost. Its preparation typically involves a high-temperature solid-state sintering process, during which the sagger containing the materials and subjected to heating is a crucial supporting vessel. Currently, the industry primarily uses refractory castable systems with silicon oxynitride, silicon carbide, and calcium aluminate as the main components for the integral saggers used in the sintering of this type of cathode material. The design intent of this type of material is to utilize its excellent refractoriness, chemical inertness, and high-temperature structural stability to withstand prolonged high-temperature environments.

[0003] However, with the lithium battery industry's continued pursuit of cost reduction and efficiency improvement, the limitations of traditional refractory castable saggers in actual large-scale continuous production have become increasingly apparent. The primary problem lies in their inherent low thermal conductivity. Materials primarily composed of oxides and nitrides generally have low thermal conductivity, resulting in a slow transfer of heat from the furnace to the material inside the sagger during sintering. This leads to a significant temperature gradient between the furnace set temperature and the actual material temperature. To achieve the required material core temperature, it is often necessary to extend the holding time or increase the furnace temperature. This not only directly leads to a significant increase in energy consumption per unit product but also prolongs the production cycle and reduces the utilization efficiency of the kiln.

[0004] Secondly, the thermal shock resistance of these sagger materials faces severe challenges. During continuous charging, heating, holding, and cooling cycles, the sagger undergoes frequent rapid heating and cooling processes. Traditional refractory castables, due to their high coefficient of thermal expansion and low thermal conductivity, generate significant internal thermal stress during drastic temperature changes. When this thermal stress exceeds the material's inherent strength limit, microcracks initiate and propagate. These microcracks accumulate and connect through repeated thermal cycles, ultimately leading to macroscopic cracking, spalling, and even structural collapse of the sagger, severely shortening its service life and increasing the frequency of replacement and the risk of breakage during production.

[0005] More importantly, in the pursuit of high loading capacity to increase single-furnace production, the insufficient structural load-bearing capacity of the sagger has been further amplified. To accommodate more material, saggers are typically designed with larger planar dimensions and thinner walls. At high temperatures, especially when the internal material undergoes phase changes and volume changes, the bottom and sidewalls of the sagger need to withstand greater mechanical and thermal stress coupling effects. Although traditional refractory castables have acceptable strength at room temperature, their high-temperature flexural strength and creep resistance are limited, making them prone to plastic deformation under long-term cyclic loading. This directly manifests as irreversible sinking and bulging in the central area of ​​the sagger bottom after repeated use, as well as brittle chipping at the edges and corners under mechanical impact and thermal stress. Bottom deformation affects the uniformity of material spreading, thus affecting sintering consistency; edge defects accelerate the initiation and propagation of hot cracks, creating a vicious cycle that ultimately significantly reduces the reliable service life of the sagger and increases unplanned downtime and equipment wear costs in cathode material production. Summary of the Invention

[0006] In view of this, the purpose of this invention is to propose a method for preparing a composite graphite sagger for sintering lithium manganese iron phosphate cathode materials, so as to solve the technical problems of high sintering energy consumption, easy cracking and damage, deformation and short service life of existing lithium manganese iron phosphate sintering saggers made of refractory castable systems combined with silicon oxynitride, silicon carbide and calcium aluminate, due to insufficient thermal conductivity, poor thermal shock stability and low high temperature structural strength.

[0007] To achieve the above objectives, the present invention provides a method for preparing a composite graphite sagger for sintering lithium manganese iron phosphate cathode materials, comprising the following steps: (1) Disperse single-walled carbon nanotubes in anhydrous ethanol and buffer solution, and add dopamine hydrochloride to form a dopamine site layer on the surface of the single-walled carbon nanotubes; (2) Add ammonia to the liquid obtained in step (1) and add tetraethyl silicate dropwise to react. After separating the solid obtained, redisperse it in anhydrous ethanol and water, and add 3-aminopropyltriethoxysilane to react to obtain modified single-walled carbon nanotubes. (3) Dissolve phenolic resin in anhydrous ethanol, add the modified single-walled carbon nanotubes obtained in step (2) for dispersion, and then add boric acid to obtain a composite dispersion. (4) The composite dispersion obtained in step (3) is kneaded with flake graphite. First, some flake graphite is added and kneaded, then the remaining flake graphite is added and kneaded again to obtain composite granules. (5) The composite granules obtained in step (4) are hot-pressed to form a sagger green blank; (6) The green sagger obtained in step (5) is carbonized under nitrogen protection to obtain a composite graphite sagger for sintering lithium manganese iron phosphate cathode material.

[0008] Preferably, the single-walled carbon nanotube has a diameter of 1-2 nm and a length greater than 98 μm.

[0009] Preferably, the phenolic resin is a linear phenolic resin system with a softening point of 110°C, free phenol content of no more than 3.5%, and volatile matter content of no more than 2%.

[0010] Preferably, in step (1), the buffer solution is obtained by dissolving tris(hydroxymethyl)aminomethane in deionized water and adjusting the pH to 8.4-8.6 with hydrochloric acid.

[0011] Preferably, in steps (1) and (2), the mass ratio of the single-walled carbon nanotubes, dopamine hydrochloride, tetraethyl silicate and 3-aminopropyltriethoxysilane is 80-120:3.5-6.5:14-24:3-7.

[0012] Preferably, in steps (3) and (4), the mass ratio of the phenolic resin, modified single-walled carbon nanotubes, boric acid and flake graphite is 900-1100:88-132:20-40:8000-9000.

[0013] Preferably, in step (4), the flake graphite includes flake graphite with a particle size of 300 mesh and flake graphite with a particle size of 500 mesh, and the mass ratio of flake graphite with a particle size of 300 mesh to flake graphite with a particle size of 500 mesh is 4700-5200:3300-3800.

[0014] Preferably, in step (4), the first batch of flake graphite includes 300-mesh flake graphite accounting for 15wt%-30wt% of the total 300-mesh flake graphite; and 500-mesh flake graphite accounting for 15wt%-30wt% of the total 500-mesh flake graphite.

[0015] Preferably, in step (5), the hot pressing is performed as follows: press at 6-10 MPa for 6-10 min, then completely depressurize for 1 min; then heat up to 120-130℃ and press at 12-18 MPa for 10-15 min, then completely depressurize for 1 min; finally heat up to 160-170℃ and press at 20-24 MPa for 35-50 min, wherein in the 160-170℃ stage, after depressurizing for 10 s in the 12-18 min and 24-36 min respectively, immediately restore to 20-24 MPa, and after heat preservation, cool to 80℃ for demolding.

[0016] Preferably, in step (6), the nitrogen flow rate for carbonization is 8-12 L / min, and the heating regime is as follows: from room temperature to 120℃, the temperature is increased at 1℃ / min and held for 2 hours; from 120℃ to 240℃, the temperature is increased at 0.5℃ / min and held for 1 hour; from 240℃ to (400-430)℃, the temperature is increased at 0.3℃ / min and held for 2 hours; from (400-430)℃ to 650℃, the temperature is increased at 0.5℃ / min and held for 1 hour; from 650℃ to (930-960)℃, the temperature is increased at 1℃ / min and held for 2 hours; from (930-960)℃ to (1300-1400)℃, the temperature is increased at 1℃ / min and held for 2.5-3.5 hours, and then the temperature is lowered to below 200℃ before being removed from the furnace.

[0017] The beneficial effects of this invention are: This invention significantly improves the thermal conductivity and structural stability of the crucible: It uses flake graphite to construct the main thermally conductive framework and innovatively modifies the surface of single-walled carbon nanotubes through dopamine site-specific pretreatment, in-situ construction of silica anchors, and silane secondary end-capping. These nanotubes are then combined with phenolic resin to form a highly wettable dispersion. This composite dispersion effectively encapsulates and bridges the flake graphite particles in subsequent processes. During carbonization, the glassy carbon formed by the pyrolysis of phenolic resin forms a strong chemical and physical bond with the modified carbon nanotubes and flake graphite through anchors, constructing a low-interfacial thermal resistance three-dimensional network spanning the gaps between graphite particles. This network not only provides an efficient path for heat transfer, significantly improving the overall thermal conductivity, but also acts as a reinforcing phase, significantly enhancing the material's mechanical strength at both room and high temperatures, achieving a synergistic improvement in thermal conductivity and toughness.

[0018] Effectively suppressing the initiation and propagation of thermal shock cracks and enhancing cyclic service reliability: By generating silica through in-situ hydrolysis at the dopamine site layer, shear slip resistance is improved while minimizing the negative impact of interfacial thermal resistance of continuous brittle inorganic phases and internal stress concentration caused by thermal expansion mismatch. Furthermore, the addition of boric acid after the modified carbon nanotubes are fully dispersed in the phenolic resin phase can more effectively promote the ordered graphitization of pyrolytic carbon near the bridging region, further optimizing interfacial bonding and stress buffering capacity. These designs enable the final product to effectively dissipate internal thermal stress during drastic temperature changes, making crack initiation and propagation less likely, thus exhibiting excellent thermal shock resistance and dimensional stability after cyclic use.

[0019] The process optimized microstructure uniformity, improving process repeatability and product performance consistency. A two-stage feeding method was employed: first, a small amount of graphite was introduced into the composite dispersion to form a high-viscosity trapping phase, followed by batches of main graphite for extrusion and kneading. This process ensures that the composite dispersion, with bridging components on its surface, can be forcibly and uniformly introduced into the gaps between the stacked flake graphite, avoiding random agglomeration or uneven distribution of the bridging components. This uniform microstructure is the foundation for the material to achieve high and stable thermal conductivity, mechanical strength, and excellent thermal shock resistance. This results in minimal batch-to-batch performance variation in the prepared composite graphite crucibles. Under long-term high-temperature cyclic loading, the bottom deformation resistance and edge integrity are significantly superior to products produced using traditional processes, thus extending their service life under continuous sintering conditions in lithium manganese iron phosphate. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0021] The raw materials used in the specific implementation are sourced as follows: 300-mesh flake graphite, Jiangsu Xianfeng Nanomaterials Technology Co., Ltd., item number 100037, part number XF053, purity 99.0wt%; 500-mesh flake graphite, Jiangsu Xianfeng Nanomaterials Technology Co., Ltd., item number 100038, part number XF054, purity 99.0wt%; single-walled carbon nanotubes, Jiangsu Xianfeng Nanomaterials Technology Co., Ltd., item number 105874, part number XFS33, single tube diameter 1-2nm, tube length greater than 98μm, purity 99±0.5wt%, specific surface area 1000±200m². 2 / g; Phenolic resin, Hangmo New Material Group Co., Ltd., linear phenolic resin system, softening point 110℃, free phenol not more than 3.5%, volatile matter not more than 2%.

[0022] Example 1: S1: Place 5000g of 300-mesh flake graphite and 3500g of 500-mesh flake graphite in a 120℃ oven and dry for 2 hours. Remove and seal for later use. Place 1000g of phenolic resin in a 50℃ vacuum drying oven and dry for 2 hours. Place 100g of single-walled carbon nanotubes in a 80℃ vacuum environment and dry for 1 hour. Sieve 30g of boric acid through a 300-mesh sieve for later use. S2: Dissolve 1g of tris(hydroxymethyl)aminomethane in 600g of deionized water, adjust the pH to 8.5 with hydrochloric acid to obtain a buffer solution; add 2400g of anhydrous ethanol, the above 600g buffer solution and 100g of single-walled carbon nanotubes to a dispersion container, disperse under ultrasonic conditions of 28kHz and 600W for 20min, and control the temperature of the solution below 30℃; then add 5g of dopamine hydrochloride, stir at 400rpm for 3h at 25℃ to form a dopamine site layer on the surface of the single-walled carbon nanotubes; S3: Add 4g of 25wt% ammonia solution to the liquid obtained in step S2, and add 20g of tetraethyl silicate dropwise over 20min. Continue stirring at 25℃ for 2h. After the reaction is complete, filter and wash the filter cake twice with 600g of anhydrous ethanol each time. Redisperse the filter cake in 1200g of anhydrous ethanol and 50g of deionized water, add 5g of 3-aminopropyltriethoxysilane, and stir at 25℃ for 20min. Filter again and wash with 800g of anhydrous ethanol. Then, vacuum dry at 45℃ and -0.08MPa for 8h to obtain modified single-walled carbon nanotubes. S4: Add 2200g of anhydrous ethanol to a stirred tank, heat to 50℃ and slowly add 1000g of phenolic resin, keep warm and stir for 50min to obtain phenolic resin ethanol binder; cool to 35℃ and add 110g of modified single-walled carbon nanotubes obtained in step S3, disperse at 3000rpm high shear for 20min; then add 30g of boric acid and continue stirring for 20min to obtain modified single-walled carbon nanotube / phenolic resin composite dispersion. S5: Transfer the composite dispersion obtained in step S4 into a kneader. First, add 1100g of 300-mesh flake graphite and 800g of 500-mesh flake graphite, and knead at 40℃ for 20 minutes. Then, add 3900g of 300-mesh flake graphite and 2700g of 500-mesh flake graphite in three portions, and continue kneading at 45℃ for 30 minutes. Then, raise the temperature to 55℃ and exhaust air at normal pressure for 30 minutes. Finally, reduce the pressure at 60℃ and -0.04MPa for 15 minutes to obtain composite granules. S6: The composite granules obtained in step S5 are loaded into an integrated sagger hot press mold. After the mold is preheated to 100°C, it is first pressed at 8MPa for 8 minutes, and then completely depressurized for 1 minute. Then the temperature is raised to 125°C and pressed at 15MPa for 12 minutes, and then completely depressurized for 1 minute. Finally, the temperature is raised to 165°C and pressed at 22MPa for 45 minutes. During the 15th minute and 30th minute, the pressure is depressurized for 10 seconds and then immediately restored to 22MPa. After the heat preservation is completed, the temperature is cooled to 80°C and the mold is demolded to obtain the sagger green blank. S7: Place the sagger blank obtained in step S6 on a graphite pad and send it into a carbonization furnace. Carbonize under nitrogen protection with a purity of not less than 99.99% and maintain a nitrogen flow rate of 10 L / min. The heating regime is as follows: from room temperature to 120℃, increase the temperature at 1℃ / min and hold for 2 hours; from 120℃ to 240℃, increase the temperature at 0.5℃ / min and hold for 1 hour; from 240℃ to 420℃, increase the temperature at 0.3℃ / min and hold for 2 hours; from 420℃ to 650℃, increase the temperature at 0.5℃ / min and hold for 1 hour; from 650℃ to 950℃, increase the temperature at 1℃ / min and hold for 2 hours; from 950℃ to 1350℃, increase the temperature at 1℃ / min and hold for 3 hours. Then cool down to below 200℃ and remove from the furnace to obtain a composite graphite sagger for sintering lithium manganese iron phosphate cathode material.

[0023] Example 2: S1: Place 5200g of 300-mesh flake graphite and 3300g of 500-mesh flake graphite in an oven at 115℃ and dry for 2 hours. Remove and seal for later use. Place 900g of phenolic resin in a vacuum drying oven at 48℃ and dry for 2 hours. Place 80g of single-walled carbon nanotubes in a vacuum oven at 75℃ and dry for 1 hour. Sieve 20g of boric acid through a 300-mesh sieve for later use. S2: Dissolve 0.8g of tris(hydroxymethyl)aminomethane in 500g of deionized water, adjust the pH to 8.4 with hydrochloric acid to obtain a buffer solution; add 2000g of anhydrous ethanol, the above 500g buffer solution and 80g of single-walled carbon nanotubes to a dispersion container, disperse under ultrasonic conditions of 28kHz and 600W for 18min, and control the temperature of the solution below 30℃; then add 3.5g of dopamine hydrochloride, stir at 350rpm at 25℃ for 2.5h to form a dopamine site layer on the surface of the single-walled carbon nanotubes; S3: Add 3g of 25wt% ammonia solution to the liquid obtained in step S2, and add 14g of tetraethyl silicate dropwise over 18min. Continue stirring at 25℃ for 1.5h. After the reaction is complete, filter and wash the filter cake twice with 500g of anhydrous ethanol each time. Redisperse the filter cake in 1000g of anhydrous ethanol and 35g of deionized water, add 3g of 3-aminopropyltriethoxysilane, and stir at 25℃ for 20min. Filter again and wash with 700g of anhydrous ethanol. Then, vacuum dry at 45℃ and -0.08MPa for 8h to obtain modified single-walled carbon nanotubes. S4: Add 2000g of anhydrous ethanol to a stirred tank, heat to 48℃, and slowly add 900g of phenolic resin. Keep warm and stir for 45min to obtain phenolic resin ethanol binder. After cooling to 33℃, add 88g of the modified single-walled carbon nanotubes obtained in step S3 and disperse at 2800rpm high shear for 18min. Then add 20g of boric acid and continue stirring for 15min to obtain the modified single-walled carbon nanotube / phenolic resin composite dispersion. S5: Transfer the composite dispersion obtained in step S4 into a kneader. First, add 900g of 300-mesh flake graphite and 600g of 500-mesh flake graphite, and knead at 35℃ for 15 minutes. Then, add 4300g of 300-mesh flake graphite and 2700g of 500-mesh flake graphite in three portions, and continue kneading at 40℃ for 25 minutes. Then, raise the temperature to 50℃ and exhaust air at normal pressure for 25 minutes. Finally, reduce the pressure at 55℃ and -0.035MPa for 10 minutes to obtain composite granules. S6: The composite granules obtained in step S5 are loaded into an integrated sagger hot press mold. After the mold is preheated to 95°C, it is first pressed at 6MPa for 6 minutes, and then completely depressurized for 1 minute. Then the temperature is raised to 120°C and pressed at 12MPa for 10 minutes, and then completely depressurized for 1 minute. Finally, the temperature is raised to 160°C and pressed at 20MPa for 35 minutes. During the 12th minute and 24th minute, the pressure is immediately restored to 20MPa after depressurization for 10 seconds each. After the heat preservation is completed, the temperature is cooled to 80°C and the mold is demolded to obtain the sagger green blank. S7: Place the sagger blank obtained in step S6 on a graphite pad and send it into a carbonization furnace. Carbonize under nitrogen protection with a purity of not less than 99.99% and maintain a nitrogen flow rate of 8 L / min. The heating regime is as follows: from room temperature to 120℃, heat at 1℃ / min and hold for 2 hours; from 120℃ to 240℃, heat at 0.5℃ / min and hold for 1 hour; from 240℃ to 400℃, heat at 0.3℃ / min and hold for 2 hours; from 400℃ to 650℃, heat at 0.5℃ / min and hold for 1 hour; from 650℃ to 930℃, heat at 1℃ / min and hold for 2 hours; from 930℃ to 1300℃, heat at 1℃ / min and hold for 2.5 hours. Then cool down to below 200℃ and remove from the furnace to obtain a composite graphite sagger for sintering lithium manganese iron phosphate cathode material.

[0024] Example 3: S1: Place 4700g of 300-mesh flake graphite and 3800g of 500-mesh flake graphite in an oven at 125℃ and dry for 2 hours. Remove and seal for later use. Place 1100g of phenolic resin in a vacuum drying oven at 52℃ and dry for 2 hours. Place 120g of single-walled carbon nanotubes in a vacuum oven at 85℃ and dry for 1 hour. Sieve 40g of boric acid through a 300-mesh sieve for later use. S2: Dissolve 1.2g of tris(hydroxymethyl)aminomethane in 700g of deionized water, adjust the pH to 8.6 with hydrochloric acid to obtain a buffer solution; add 2800g of anhydrous ethanol, the above 700g buffer solution and 120g of single-walled carbon nanotubes to a dispersion container, disperse under ultrasonic conditions of 28kHz and 600W for 22min, and control the temperature of the solution below 30℃; then add 6.5g of dopamine hydrochloride, stir at 450rpm for 3.5h at 25℃ to form a dopamine site layer on the surface of the single-walled carbon nanotubes; S3: Add 5g of 25wt% ammonia solution to the liquid obtained in step S2, and add 24g of tetraethyl silicate dropwise over 22min. Continue stirring at 25℃ for 2.5h. After the reaction is complete, filter and wash the filter cake twice with 700g of anhydrous ethanol each time. Redisperse the filter cake in 1400g of anhydrous ethanol and 65g of deionized water, add 7g of 3-aminopropyltriethoxysilane, and stir at 25℃ for 25min. Filter again and wash with 900g of anhydrous ethanol. Then, vacuum dry at 48℃ and -0.08MPa for 8h to obtain modified single-walled carbon nanotubes. S4: Add 2400g of anhydrous ethanol to a stirred tank, heat to 52℃, and slowly add 1100g of phenolic resin. Keep warm and stir for 55min to obtain phenolic resin ethanol binder. After cooling to 36℃, add 132g of the modified single-walled carbon nanotubes obtained in step S3 and disperse at 3200rpm for 22min under high shear. Then add 40g of boric acid and continue stirring for 25min to obtain the modified single-walled carbon nanotube / phenolic resin composite dispersion. S5: Transfer the composite dispersion obtained in step S4 into a kneader. First, add 1400g of 300-mesh flake graphite and 1000g of 500-mesh flake graphite, and knead at 45℃ for 25 minutes. Then, add 3300g of 300-mesh flake graphite and 2800g of 500-mesh flake graphite in three portions, and continue kneading at 50℃ for 35 minutes. Then, raise the temperature to 60℃ and exhaust air at normal pressure for 35 minutes. Finally, reduce the pressure at 65℃ and -0.045MPa for 20 minutes to obtain composite granules. S6: The composite granules obtained in step S5 are loaded into an integrated sagger hot press mold. The mold is preheated to 105°C and then pressed at 10MPa for 10 minutes, and then completely depressurized for 1 minute. Then the temperature is raised to 130°C and pressed at 18MPa for 15 minutes, and then completely depressurized for 1 minute. Finally, the temperature is raised to 170°C and pressed at 24MPa for 50 minutes. The pressure is depressurized for 10 seconds at the 18th minute and the 36th minute and then immediately restored to 24MPa. After the heat preservation is completed, the temperature is cooled to 80°C and the mold is demolded to obtain the sagger green blank. S7: The green sagger obtained in step S6 is placed on a graphite pad and fed into a carbonization furnace. Carbonization is carried out under nitrogen protection with a purity of not less than 99.99%, and the nitrogen flow rate is maintained at 12L / min. The heating regime is as follows: from room temperature to 120℃, the temperature is increased at 1℃ / min and held for 2 hours; from 120℃ to 240℃, the temperature is increased at 0.5℃ / min and held for 1 hour; from 240℃ to 430℃, the temperature is increased at 0.3℃ / min and held for 2 hours; from 430℃ to 650℃, the temperature is increased at 0.5℃ / min and held for 1 hour; from 650℃ to 960℃, the temperature is increased at 1℃ / min and held for 2 hours; from 960℃ to 1400℃, the temperature is increased at 1℃ / min and held for 3.5 hours. Then the temperature is lowered to below 200℃ and the sagger is removed from the furnace to obtain a composite graphite sagger for sintering lithium manganese iron phosphate cathode material.

[0025] Example 4: S1: Place 4900g of 300-mesh flake graphite and 3600g of 500-mesh flake graphite in an oven at 118℃ and dry for 2 hours. Remove and seal for later use. Place 950g of phenolic resin in a vacuum drying oven at 49℃ and dry for 2 hours. Place 90g of single-walled carbon nanotubes in a vacuum oven at 78℃ and dry for 1 hour. Sieve 25g of boric acid through a 300-mesh sieve for later use. S2: Dissolve 0.9g of tris(hydroxymethyl)aminomethane in 550g of deionized water, adjust the pH to 8.5 with hydrochloric acid to obtain a buffer solution; add 2200g of anhydrous ethanol, the above 550g buffer solution and 90g of single-walled carbon nanotubes to a dispersion container, disperse under ultrasonic conditions of 28kHz and 600W for 19min, and control the temperature of the solution below 30℃; then add 4.5g of dopamine hydrochloride, stir at 380rpm for 3h at 25℃ to form a dopamine site layer on the surface of the single-walled carbon nanotubes; S3: Add 4g of 25wt% ammonia solution to the liquid obtained in step S2, and add 18g of tetraethyl silicate dropwise over 20min. Continue stirring at 25℃ for 2h. After the reaction is complete, filter and wash the filter cake twice with 550g of anhydrous ethanol each time. Redisperse the filter cake in 1100g of anhydrous ethanol and 45g of deionized water, add 4g of 3-aminopropyltriethoxysilane, and stir at 25℃ for 20min. Filter again and wash with 750g of anhydrous ethanol. Then, vacuum dry at 46℃ and -0.08MPa for 8h to obtain modified single-walled carbon nanotubes. S4: Add 2100g of anhydrous ethanol to a stirred tank, heat to 49℃, and slowly add 950g of phenolic resin. Keep warm and stir for 50min to obtain phenolic resin ethanol binder. After cooling to 34℃, add 99g of the modified single-walled carbon nanotubes obtained in step S3 and disperse at 2900rpm for 20min with high shear. Then add 25g of boric acid and continue stirring for 18min to obtain the modified single-walled carbon nanotube / phenolic resin composite dispersion. S5: Transfer the composite dispersion obtained in step S4 into a kneader. First, add 1200g of 300-mesh flake graphite and 700g of 500-mesh flake graphite, and knead at 38℃ for 18 minutes. Then, add 3700g of 300-mesh flake graphite and 2900g of 500-mesh flake graphite in three portions, and continue kneading at 43℃ for 28 minutes. Then, raise the temperature to 53℃ and exhaust air at normal pressure for 30 minutes. Finally, reduce the pressure at 58℃ and -0.04MPa for 12 minutes to obtain composite granules. S6: The composite granules obtained in step S5 are loaded into an integrated sagger hot press mold. The mold is preheated to 98°C and then pressed at 7MPa for 7 minutes, followed by complete depressurization for 1 minute. Then the temperature is raised to 123°C and pressed at 14MPa for 11 minutes, followed by complete depressurization for 1 minute. Finally, the temperature is raised to 163°C and pressed at 21MPa for 40 minutes. The pressure is depressurized for 10 seconds at the 14th minute and the 28th minute and then immediately restored to 21MPa. After the heat preservation is completed, the material is cooled to 80°C and demolded to obtain the sagger green blank. S7: Place the sagger blank obtained in step S6 on a graphite pad and send it into a carbonization furnace. Carbonize under nitrogen protection with a purity of not less than 99.99% and maintain a nitrogen flow rate of 9 L / min. The heating regime is as follows: from room temperature to 120℃, heat at 1℃ / min and hold for 2 hours; from 120℃ to 240℃, heat at 0.5℃ / min and hold for 1 hour; from 240℃ to 410℃, heat at 0.3℃ / min and hold for 2 hours; from 410℃ to 650℃, heat at 0.5℃ / min and hold for 1 hour; from 650℃ to 940℃, heat at 1℃ / min and hold for 2 hours; from 940℃ to 1330℃, heat at 1℃ / min and hold for 3 hours. Then cool down to below 200℃ and remove from the furnace to obtain a composite graphite sagger for sintering lithium manganese iron phosphate cathode material.

[0026] Example 5: S1: Place 5100g of 300-mesh flake graphite and 3400g of 500-mesh flake graphite in an oven at 122℃ and dry for 2 hours. Remove and seal for later use. Place 1050g of phenolic resin in a vacuum drying oven at 51℃ and dry for 2 hours. Place 110g of single-walled carbon nanotubes in a vacuum oven at 82℃ and dry for 1 hour. Sieve 35g of boric acid through a 300-mesh sieve for later use. S2: Dissolve 1.1g of tris(hydroxymethyl)aminomethane in 650g of deionized water, adjust the pH to 8.5 with hydrochloric acid to obtain a buffer solution; add 2600g of anhydrous ethanol, the above 650g buffer solution and 110g of single-walled carbon nanotubes to a dispersion container, disperse under ultrasonic conditions of 28kHz and 600W for 21min, and control the temperature of the solution below 30℃; then add 5.5g of dopamine hydrochloride, stir at 420rpm for 3h at 25℃ to form a dopamine site layer on the surface of the single-walled carbon nanotubes; S3: Add 4.5g of 25wt% ammonia solution to the liquid obtained in step S2, and add 22g of tetraethyl orthosilicate dropwise over 20 min. Continue stirring at 25℃ for 2 h. After the reaction is complete, filter the solution and wash the filter cake twice with 650g of anhydrous ethanol each time. Redisperse the filter cake in 1300g of anhydrous ethanol and 55g of deionized water, add 6g of 3-aminopropyltriethoxysilane, and stir at 25℃ for 22 min. Filter again and wash with 850g of anhydrous ethanol. Then, vacuum dry at 47℃ and -0.08MPa for 8 h to obtain modified single-walled carbon nanotubes. S4: Add 2300g of anhydrous ethanol to a stirred tank, heat to 51℃, and slowly add 1050g of phenolic resin. Keep warm and stir for 52min to obtain phenolic resin ethanol binder. After cooling to 35℃, add 121g of the modified single-walled carbon nanotubes obtained in step S3 and disperse at 3100rpm high shear for 22min. Then add 35g of boric acid and continue stirring for 22min to obtain modified single-walled carbon nanotube / phenolic resin composite dispersion. S5: Transfer the composite dispersion obtained in step S4 into a kneader. First, add 1000g of 300-mesh flake graphite and 800g of 500-mesh flake graphite, and knead at 42℃ for 22 minutes. Then, add 4100g of 300-mesh flake graphite and 2600g of 500-mesh flake graphite in three portions, and continue kneading at 47℃ for 32 minutes. Then, raise the temperature to 57℃ and exhaust air at normal pressure for 32 minutes. Finally, reduce the pressure at 62℃ and -0.042MPa for 18 minutes to obtain composite granules. S6: The composite granules obtained in step S5 are loaded into an integrated sagger hot press mold. The mold is preheated to 102°C and then pressed at 9MPa for 9 minutes, followed by complete depressurization for 1 minute. Then the temperature is raised to 128°C and pressed at 16MPa for 13 minutes, followed by complete depressurization for 1 minute. Finally, the temperature is raised to 168°C and pressed at 23MPa for 48 minutes. The pressure is depressurized for 10 seconds at the 16th minute and 32nd minute and then immediately restored to 23MPa. After the heat preservation is completed, the material is cooled to 80°C and demolded to obtain the sagger green blank. S7: The green sagger obtained in step S6 is placed on a graphite pad and fed into a carbonization furnace. Carbonization is carried out under nitrogen protection with a purity of not less than 99.99%, and the nitrogen flow rate is maintained at 11 L / min. The heating regime is as follows: from room temperature to 120℃, the temperature is increased at 1℃ / min and held for 2 hours; from 120℃ to 240℃, the temperature is increased at 0.5℃ / min and held for 1 hour; from 240℃ to 420℃, the temperature is increased at 0.3℃ / min and held for 2 hours; from 420℃ to 650℃, the temperature is increased at 0.5℃ / min and held for 1 hour; from 650℃ to 950℃, the temperature is increased at 1℃ / min and held for 2 hours; from 950℃ to 1380℃, the temperature is increased at 1℃ / min and held for 3 hours. Then the temperature is lowered to below 200℃ and the sagger is removed from the furnace to obtain a composite graphite sagger for sintering lithium manganese iron phosphate cathode material.

[0027] Comparative Example 1: The difference from Example 1 is that in step S4, 110g of the modified single-walled carbon nanotubes obtained in step S3 are not added, but 110g of 500-mesh flake graphite is added to make up the solid feed amount. Steps S2 and S3 are not performed, and the remaining conditions are the same as in Example 1. The sagger described in Comparative Example 1 is obtained by preparing according to the method of Example 1.

[0028] Comparative Example 2: The difference from Example 1 is that 5g of dopamine hydrochloride is not added in step S2, while the other conditions are the same as in Example 1. That is, after ultrasonic dispersion, the liquid obtained in step S2 is directly used for subsequent processing in step S3. The crucible described in Comparative Example 2 is obtained by preparing the solution according to the method of Example 1.

[0029] Comparative Example 3: The difference from Example 1 is that 20g of tetraethyl silicate is not added in step S3, while the other conditions are the same as in Example 1. That is, after adding 4g of 25wt% ammonia water, tetraethyl silicate is not added dropwise, and then the process is carried out according to the steps of filtration, washing, redispersing, adding 3-aminopropyltriethoxysilane, filtration again, and drying as in Example 1. The crucible described in Comparative Example 3 was obtained by preparing the crucible according to the method of Example 1.

[0030] Comparative Example 4: The difference from Example 1 is that in step S3, 5g of 3-aminopropyltriethoxysilane is not added after washing and redispersing, but is added directly to the liquid obtained in step S2 after adding 4g of 25wt% ammonia water and before adding 20g of tetraethyl silicate. The other conditions are the same as in Example 1. The crucible described in Comparative Example 4 was obtained by preparing the crucible according to the method of Example 1.

[0031] Comparative Example 5: The difference from Example 1 is that the 30g boric acid in step S4 is not added after the modified single-walled carbon nanotubes are added to the phenolic resin ethanol binder, but is added simultaneously to the dispersion container at the beginning of step S2 along with 2400g anhydrous ethanol, 600g buffer solution, and 100g single-walled carbon nanotubes. The remaining conditions are the same as in Example 1. The crucible described in Comparative Example 5 was obtained by preparing the crucible according to the method of Example 1.

[0032] Comparative Example 6: The difference from Example 1 is that in step S5, 1100g of 300-mesh flake graphite, 800g of 500-mesh flake graphite, 3900g of 300-mesh flake graphite, and 2700g of 500-mesh flake graphite are all added to the kneader at once, instead of the two-stage feeding method of first forming a small amount of high-viscosity capturing phase and then adding a large amount in batches. All other conditions are the same as in Example 1. The sagger described in Comparative Example 6 was obtained by preparing the sagger according to the method of Example 1.

[0033] Sample preparation process before performance testing: Samples used for performance testing were prepared according to Examples 1-5 and Comparative Examples 1-6. For each formulation, three integral saggers were prepared using the same batch of raw materials, the same kneader, the same hot-pressing mold, and the same carbonization furnace. The integral saggers had uniform dimensions of 320mm × 220mm × 95mm, a wall thickness of 22mm, and a bottom thickness of 25mm. Simultaneously, six accompanying sample bricks and three solid plates were prepared using the same batch of composite granules, the same hot-pressing regime, and the same carbonization regime as the integral saggers. The accompanying sample bricks had uniform dimensions of 230mm × 114mm × 65mm. The solid plates were dry-cut with diamond tools into 150mm × 25mm × 25mm flexural strength test specimens, 50mm × 50mm × 50mm compressive strength test specimens, and 150mm × 75mm × 25mm thermal conductivity test specimens. All samples were dried at 110℃ for 2 hours, then placed in an environment of 23±2℃ and 50±10% relative humidity for 24 hours before testing. For each test item, at least 5 parallel samples should be taken, and the arithmetic mean should be taken after removing the highest and lowest values.

[0034] Performance testing: Flexural strength at room temperature: According to GB / T 3001-2017, five 150mm×25mm×25mm specimens were taken for each group, with a uniform support span of 125mm. The three-point bending method was used for loading, and the displacement rate of the crossbeam was controlled at 0.5mm / min until the specimen broke. The breaking load was recorded and the flexural strength at room temperature was calculated according to the standard.

[0035] Room temperature compressive strength: According to GB / T 5072-2023, five 50mm×50mm×50mm specimens were taken for each group. Before loading, the upper and lower pressure surfaces were lightly ground to ensure flatness. Uniaxial compression was performed using a universal testing machine, and the loading rate was controlled at 0.5MPa / s until the specimen was damaged. The maximum load was recorded and the room temperature compressive strength was calculated according to the standard.

[0036] Thermal conductivity: According to GB / T 5990-2021, each group of 150mm×75mm×25mm thermal conductivity samples were taken. The thermal conductivity was tested at 25℃ and 800℃ using the hot wire method. After the samples reached the target temperature, they were kept at the temperature for 30 minutes before the test was started. The average value was taken after 3 consecutive measurements.

[0037] Thermal shock resistance: According to GB / T 30873-2014, five 230mm×114mm×65mm accompanying sample bricks were taken for each group. The air rapid cooling method was used for testing: the sample was placed in a furnace at 950℃ and kept at that temperature for 30 minutes, then quickly removed and placed on a refractory pad at room temperature and cooled in air for 20 minutes, which was recorded as one thermal shock cycle. After 20 consecutive thermal shock cycles, the surface cracks of the sample were inspected and the longest crack length was recorded. Then, a 150mm×25mm×25mm strip sample was cut from the middle of the sample, and the residual room temperature flexural strength was determined according to GB / T3001-2017.

[0038] The permanent linear change under heating was carried out in accordance with GB / T 5988-2022. Five accompanying sample bricks were taken for each group. A reference length of 100 mm was marked in the middle of the sample. The temperature was raised to 1350℃ under nitrogen protection and held for 3 hours. After naturally cooling to room temperature, the reference length was measured again. The rate of change of permanent linear change under heating was calculated according to the standard.

[0039] Cyclic sintering test of lithium manganese iron phosphate: The integral saggers obtained in Examples 1 to 5 and Comparative Examples 1 to 6 were subjected to working condition simulation. The lithium manganese iron phosphate precursor mixture used was self-made material from the same batch, passed through a 100-mesh sieve, and the moisture content was controlled below 0.2%. Each integral sagger was loaded with 3 kg of precursor mixture, and the thickness of the material was controlled to be 35 mm. Cyclic sintering was carried out under the condition of nitrogen flow rate of 15 L / min: room temperature was raised to 250℃ and held for 1 h, 250℃ was raised to 450℃ and held for 2 h, 450℃ was raised to 780℃ and held for 8 h, and then cooled to below 200℃ before being removed from the furnace. This was recorded as one cycle. Each group was continuously subjected to 30 cycles. The maximum deformation at the bottom of the sagger and the maximum width of the edge defect were recorded. After 30 cycles, the residual room temperature flexural strength was determined according to GB / T 3001-2017.

[0040] Table 1 Performance Test Results

[0041] Data Analysis: As can be seen from the data in Table 1, the composite graphite sagger for sintering lithium manganese iron phosphate cathode material prepared by the present invention exhibits good synergy in indicators such as room temperature flexural strength, room temperature compressive strength, thermal conductivity, residual flexural strength after thermal shock, and residual strength after loading cycle. At the same time, the permanent linear change rate of heating, the amount of bottom deformation, and the maximum edge defect width remain at a low level, indicating that the sagger does not rely solely on high graphite content to obtain performance, but forms a stable structure that takes into account heat transfer, load bearing, and crack resistance. The possible reason is that after the single-walled carbon nanotubes are site-modified with dopamine, they further form discrete silica anchors. After being locally end-capped with 3-aminopropyltriethoxysilane in a low-water-content ethanol system, they are more likely to form continuous bridging with the pyrolysis carbon in phenolic resin and the micro-gap of flake graphite. At the same time, the addition of boric acid after the modified single-walled carbon nanotubes enter the phenolic resin phase is beneficial to the ordering of the pyrolysis carbon in the bridging region, thereby reducing the interfacial thermal resistance and inhibiting crack propagation. Ultimately, the overall sagger has high thermal conductivity, structural stability and service reliability under continuous sintering conditions.

[0042] As can be seen from the data in Example 1 and Comparative Example 1 in Table 1, when the solid feed amount is supplemented with flake graphite without the addition of modified single-walled carbon nanotubes, the room temperature strength, thermal conductivity, residual flexural strength after thermal shock, and edge integrity after cycling are all significantly worse. The main reason is that although the surface contact between flake graphite can provide a certain thermal conductivity channel, it is difficult to form a stable bridge across the micro-gap between particles, making it more prone to interlayer slip and crack penetration under stress.

[0043] As can be seen from the data in Table 1 for Example 1 and Comparative Example 2, simply omitting the dopamine hydrochloride site formation step resulted in a decrease in the material's room-temperature mechanical properties, residual strength after thermal shock, and cyclic service stability. This may be because the single-walled carbon nanotubes without dopamine site layer pretreatment lack sufficient surface active sites, making it difficult to effectively construct the subsequent discrete silica anchors and 3-aminopropyltriethoxysilane at suitable locations. This leads to decreased dispersion uniformity, poor interfacial wetting, and affects both the number and stability of bridging structures.

[0044] As can be seen from the data in Table 1 for Example 1 and Comparative Examples 3 and 4, when tetraethyl silicate is not added, or when 3-aminopropyltriethoxysilane is added before the addition of tetraethyl silicate, the thermal conductivity, flexural strength, and thermal shock resistance of the material are all adversely affected, with the latter showing a more significant decrease. This is because, in the former case, there are no discrete silica anchor points, resulting in insufficient anti-slip ability of the bridging nodes; in the latter case, the premature participation of 3-aminopropyltriethoxysilane in the system reaction easily causes the effective end cap to deviate from the bridging node and increases the interfacial thermal resistance.

[0045] As can be seen from the data in Table 1 for Example 1 and Comparative Example 5, simply changing the timing of boric acid addition resulted in significant differences in the high-temperature thermal conductivity, residual flexural strength after thermal shock, and dimensional retention after cycling. The main reason is that if boric acid enters the front-end ethanol-buffered solution too early, it is difficult to effectively serve the subsequent structural regulation of the phenolic resin-pyrolyzed carbon, and it may interfere with the surface treatment process of single-walled carbon nanotubes, leading to insufficient formation of ordered carbon structures near the bridging region. In contrast, adding boric acid after the modified single-walled carbon nanotubes have entered the phenolic resin phase is more conducive to forming a stable and continuous thermal conductivity and load-bearing path around the bridging nodes.

[0046] As can be seen from the data in Table 1 for Example 1 and Comparative Example 6, after keeping the basic formulation unchanged and only eliminating the two-stage feeding, the room temperature strength, thermal conductivity, residual flexural strength after thermal shock, and bottom deformation and edge defects after charging cycles all deteriorated significantly. This is because when all the flake graphite is added at once, the modified single-walled carbon nanotube / phenolic resin bridge formed at the front end is difficult to be fully incorporated into the micro-interstices of the flake graphite during the high-viscosity capture stage. This leads to local enrichment or detachment from the effective bonding area, resulting in similar macroscopic graphite content but insufficient microscopic interparticle bridging.

[0047] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention is limited to these examples; within the framework of the invention, the technical features of the above embodiments or different embodiments can also be combined, and there are many other variations of the different aspects of the invention as described above, which are not provided in detail for the sake of brevity.

Claims

1. A method for preparing a composite graphite sagger for sintering lithium manganese iron phosphate cathode material, characterized in that, Includes the following steps: (1) Disperse single-walled carbon nanotubes in anhydrous ethanol and buffer solution, and add dopamine hydrochloride to form a dopamine site layer on the surface of the single-walled carbon nanotubes; (2) Add ammonia to the liquid obtained in step (1) and add tetraethyl silicate dropwise to react. After separating the solid obtained, redisperse it in anhydrous ethanol and water, and add 3-aminopropyltriethoxysilane to react to obtain modified single-walled carbon nanotubes. (3) Dissolve phenolic resin in anhydrous ethanol, add the modified single-walled carbon nanotubes obtained in step (2) for dispersion, and then add boric acid to obtain a composite dispersion. (4) The composite dispersion obtained in step (3) is kneaded with flake graphite. First, some flake graphite is added and kneaded, then the remaining flake graphite is added and kneaded again to obtain composite granules. (5) The composite granules obtained in step (4) are hot-pressed to form a sagger green blank; (6) The green sagger obtained in step (5) is carbonized under nitrogen protection to obtain a composite graphite sagger for sintering lithium manganese iron phosphate cathode material.

2. The method for preparing the composite graphite sagger for sintering lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The single-walled carbon nanotubes have a diameter of 1-2 nm and a length greater than 98 μm.

3. The method for preparing the composite graphite sagger for sintering lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The phenolic resin is a linear phenolic resin system with a softening point of 110°C, free phenol content not exceeding 3.5%, and volatile matter content not exceeding 2%.

4. The method for preparing the composite graphite sagger for sintering lithium manganese iron phosphate cathode material according to claim 1, characterized in that, In step (1), the buffer solution is obtained by dissolving tris(hydroxymethyl)aminomethane in deionized water and adjusting the pH to 8.4-8.6 with hydrochloric acid.

5. The method for preparing the composite graphite sagger for sintering lithium manganese iron phosphate cathode material according to claim 1, characterized in that, In steps (1) and (2), the mass ratio of the single-walled carbon nanotubes, dopamine hydrochloride, tetraethyl silicate and 3-aminopropyltriethoxysilane is 80-120:3.5-6.5:14-24:3-7.

6. The method for preparing the composite graphite sagger for sintering lithium manganese iron phosphate cathode material according to claim 1, characterized in that, In steps (3) and (4), the mass ratio of the phenolic resin, modified single-walled carbon nanotubes, boric acid and flake graphite is 900-1100:88-132:20-40:8000-9000.

7. The method for preparing the composite graphite sagger for sintering lithium manganese iron phosphate cathode material according to claim 1, characterized in that, In step (4), the flake graphite includes flake graphite with a particle size of 300 mesh and flake graphite with a particle size of 500 mesh, and the mass ratio of flake graphite with a particle size of 300 mesh to flake graphite with a particle size of 500 mesh is 4700-5200:3300-3800.

8. The method for preparing the composite graphite sagger for sintering lithium manganese iron phosphate cathode material according to claim 1, characterized in that, In step (4), the first batch of flake graphite includes 300-mesh flake graphite, which accounts for 15wt%-30wt% of the total 300-mesh flake graphite; and 500-mesh flake graphite, which accounts for 15wt%-30wt% of the total 500-mesh flake graphite.

9. The method for preparing the composite graphite sagger for sintering lithium manganese iron phosphate cathode material according to claim 1, characterized in that, In step (5), the hot pressing process is as follows: press at 6-10 MPa for 6-10 min, then completely depressurize for 1 min; then heat up to 120-130℃ and press at 12-18 MPa for 10-15 min, then completely depressurize for 1 min; finally heat up to 160-170℃ and press at 20-24 MPa for 35-50 min, wherein in the 160-170℃ stage, after depressurizing for 10 s in the 12-18 min and 24-36 min stages, immediately restore to 20-24 MPa, and after heat preservation, cool to 80℃ for demolding.

10. The method for preparing the composite graphite sagger for sintering lithium manganese iron phosphate cathode material according to claim 1, characterized in that, In step (6), the nitrogen flow rate for carbonization is 8-12 L / min, and the heating regime is as follows: from room temperature to 120℃, the temperature is increased at 1℃ / min and held for 2 hours; from 120℃ to 240℃, the temperature is increased at 0.5℃ / min and held for 1 hour; from 240℃ to (400-430)℃, the temperature is increased at 0.3℃ / min and held for 2 hours. The temperature is increased from 400-430℃ to 650℃ at a rate of 0.5℃ / min and held for 1 hour; then increased from 650℃ to 930-960℃ at a rate of 1℃ / min and held for 2 hours; then increased from 930-960℃ to 1300-1400℃ at a rate of 1℃ / min and held for 2.5-3.5 hours, followed by cooling to below 200℃ before being removed from the furnace.

Images