Method for preparing sodium-ion battery negative electrode material

Abstract

The application discloses a preparation method of a sodium ion battery negative electrode material, and specifically implements the following steps: step 1, pre-carbonization of raw materials under a protective atmosphere to obtain reaction product I; step 2, crushing of the reaction product I to obtain reaction product II; step 3, activation and pore forming of the reaction product II to obtain reaction product III; step 4, water washing, acid washing and impurity removal of the reaction product III, ash reduction and drying to obtain reaction product IV; step 5, mixing of the reaction product IV and metal ions to obtain reaction product V; step 6, high-temperature calcination of the reaction product V under a protective atmosphere to obtain reaction product VI; and step 7, crushing of the reaction product VI to obtain the final product VII. The application solves the problems of low energy density and low coulomb efficiency of the sodium ion battery prepared from the sodium ion battery negative electrode material in the prior art, and improves the full-electricity-cycle stability and rate performance of the material.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of sodium ion battery electrode material preparation methods, and relates to a preparation method of a sodium ion battery negative material. BACKGROUND

[0002] Energy storage is the key and core of supporting new power systems dominated by new energy. Lithium ion resources are limited and unevenly distributed, while sodium ion resources are abundant and low in cost. Lithium ion batteries and sodium ion batteries have similar working principles and energy storage mechanisms. Sodium ion batteries have great application scenarios in the field of energy storage. Developing sodium ion battery negative materials with high energy density, high rate performance, high initial efficiency and excellent cycle performance is the key. Therefore, it is crucial to reduce the ash content of the material, improve the internal sodium storage active sites of the material, control the defects, and reduce the specific surface area of the material.

[0003] Carbon-based materials are considered the most promising negative materials for sodium ion batteries due to their natural abundance, stable electrochemical performance, excellent electrical conductivity, and low sodium storage potential. Although soft carbon has certain sodium storage capacity, it has low sodium storage capacity and high charging potential. Hard carbon has more disordered and chaotic structures (such as defects and vacancies) and larger carbon layer spacing than soft carbon, and contains disordered micropores, which have more sodium storage sites. The sodium ion battery negative materials prepared in the current reports have low reversible specific capacity, low initial coulombic efficiency, and poor cycle and rate performance when used in sodium ion batteries. SUMMARY

[0004] The purpose of the present application is to provide a preparation method of a sodium ion battery negative material, which solves the problems of low energy density and low coulombic efficiency of sodium ion batteries prepared by the sodium ion battery negative material in the prior art.

[0005] The technical solution adopted by the present application is a preparation method of a sodium ion battery negative material, which is implemented according to the following steps:

[0006] Step 1: Pre-carbonization of raw materials under a protective atmosphere to obtain reaction product I;

[0007] Step 2: Crushing the reaction product I to obtain reaction product II;

[0008] Step 3: Activating and pore-forming the reaction product II to obtain reaction product III;

[0009] Step 4: Washing, acid washing, and drying the reaction product III to reduce the ash content and obtain reaction product IV;

[0010] Step 5: Mixing the reaction product IV with metal ions to obtain reaction product V;

[0011] Step 6, the reaction product V is calcined at high temperature under a protective atmosphere to obtain a reaction product VI;

[0012] Step 7, the reaction product VI is crushed and sieved to obtain a final product VII.

[0013] The application is also characterized in that,

[0014] The raw material in step 1 is one of corn cob, coconut shell, sand willow, fruit wood, lump coal or raw coal.

[0015] The pre-carbonization is specifically as follows: under a protective atmosphere, the temperature is increased from room temperature to 300-600 DEG C at a rate of 1-5 DEG C / min, and the temperature is kept for 1-4 h.

[0016] The crushing in step 2 is specifically as follows: the reaction product I is crushed by an air flow crusher to a particle size D50 of 5-7 um to obtain a reaction product II.

[0017] The activation and pore forming in step 3 are specifically as follows: KOH or K2CO3 or a mixture of KOH and K2CO3 is used as an activating agent, the activating agent is 10-40% of the mass of the reaction product II, a VC mixer is used for solid mixing, the mixing time is 20-40 min, the activating agent is mixed with the reaction product II, then the temperature is increased from room temperature to 700-900 DEG C under a protective atmosphere at a rate of 1-5 DEG C / min, the temperature is kept for 1-3 h, and a reaction product III is obtained.

[0018] The protective atmosphere in step 1, step 3 and step 6 is any one of nitrogen, helium or argon.

[0019] The water washing in step 4 is specifically as follows: the water washing is performed until the pH is 7.

[0020] The acid washing is specifically as follows: hydrochloric acid or dilute sulfuric acid or hydrofluoric acid is used for acid washing, the concentration is 1 mol / L, the stirring time is 6-12 h, the product is washed and filtered after stirring until the pH is 7.

[0021] The drying is specifically as follows: the product after acid washing is placed in an oven, the oven temperature is 80-100 DEG C, and a reaction product IV is obtained.

[0022] The metal ion doping in step 5 uses a metal chloride salt, the metal chloride salt is one of MnCl2, FeCl3, NiCl2 and CoCl2, and the process is specifically as follows: deionized water, the metal chloride salt and the reaction product IV are fully stirred to obtain a mixed solution, the concentration of the metal chloride salt is 0.02-0.04 mol / L, the mixed solution is transferred to a stainless steel autoclave, and a hydrothermal reaction is performed, the hydrothermal reaction conditions are as follows: 120-180 DEG C for 8-12 h, the product of the hydrothermal reaction is collected by centrifugation and dried at 80 DEG C, and a reaction product V is obtained.

[0023] The high-temperature calcination in step 6 is specifically as follows: the reaction product V is raised from room temperature to 1000-1200 DEG C under a protective atmosphere at a temperature raising rate of 3 DEG C / min-6 DEG C / min, and then kept for 1-4 hours, and then raised to 1300 DEG C-1600 DEG C at a temperature raising rate of 1 DEG C / min-3 DEG C / min, and then kept for 1-4 hours, to obtain a reaction product VI.

[0024] The pulverization in step 7 is specifically as follows: the reaction product VI is pulverized by a mechanical pulverizer or an air flow pulverizer, and the particle size D50 is controlled to be 7-9 um.

[0025] The beneficial effects of the present application are as follows:

[0026] 1. The sodium ion battery negative electrode material prepared by the preparation method has a reversible specific capacity of 343.6 mAh / g, a first charge-discharge efficiency of 92.1%, a high energy density, and stable cycle performance, and the prepared sodium ion soft package battery has a capacity retention rate of 93.4% after 669 cycles at a 0.5C / 1C rate and a capacity retention rate of 83.7% after 884 cycles at a 1C / 1C rate.

[0027] 2. The low-temperature pre-carbonization in the present application can make the volatile components in the precursor release in the form of small molecules, increase the disorder of the structure, increase the internal disorder degree of the material by activation and pore making, increase the proportion of internal micropores of the material, increase the internal sodium storage active sites and Na + ion diffusion channels of the material, and improve the diffusion capacity of the material in the pore channel, which is beneficial to improve the rate performance; at the same time, K + plays a role in expanding the layer spacing of the material, and then improves the sodium storage of the material layer. By removing impurities in the material through acid washing and water washing, the carbon content is increased, the ash content is reduced, the fixed carbon content is increased, and the cycle stability of the material is improved; metal ion doping, through the coordination between metal ions and oxygen-containing functional group defects, catalyzing the graphitization of the material surface, providing an open channel for Na + , and improving the first coulombic efficiency. At the same time, graphitization is beneficial to increase the conductivity of the material, reduce the specific surface area, improve the first efficiency of the material, reduce the loss of the first irreversible capacity, and improve the rate performance of the material.

[0028] 3. The high-temperature pyrolysis in the present application changes the layer spacing of the graphite-like structure and the surface structure of the material. As the temperature increases, the relative carbon content of the material increases and tends to be stable, the open pores gradually close, the micropore sodium storage increases, and the platform capacity increases. Metal ion doping and graphitization regulate defects and reduce the specific surface area of the material. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 is a flow chart of the preparation method of the sodium ion battery negative electrode material of the present application.

[0030] Figure 2 is the XRD graph of the negative electrode material prepared by the preparation method of the sodium ion battery negative electrode material of the application embodiment 1;

[0031] Figure 3 is the Raman graph of the negative electrode material prepared by the preparation method of the sodium ion battery negative electrode material of the application embodiment 1;

[0032] Figure 4 is the first circle charge-discharge curve of the negative electrode material prepared by the preparation method of the sodium ion battery negative electrode material of the application embodiment 1;

[0033] Figure 5 is the performance curve of the negative electrode material prepared by the preparation method of the sodium ion battery negative electrode material of the application embodiment 1;

[0034] Figure 6a is the full electric cycle performance curve of the negative electrode material prepared by the preparation method of the sodium ion battery negative electrode material of the application embodiment 1 under 0.5C / 1C rate cycle 669 times;

[0035] Figure 6b is the full electric cycle performance curve of the negative electrode material prepared by the preparation method of the sodium ion battery negative electrode material of the application embodiment 1 under 1C / 1C rate cycle 884 times. DETAILED DESCRIPTION

[0036] The application will be described in detail below in combination with the drawings and specific embodiments.

[0037] The preparation method of the sodium ion battery negative electrode material of the application, the flow is as shown in Figure 1 The specific implementation is as follows:

[0038] Step 1, the raw material is pre-carbonized under a protective atmosphere to obtain a reaction product I; wherein one of the corn cob, coconut shell, sand willow, fruit wood, lump coal or raw coal, the protective atmosphere is any one of nitrogen, helium or argon, and the pre-carbonization is specifically as follows: under the protective atmosphere, the temperature is raised from room temperature to 300-600 DEG C at a rate of 1-5 DEG C / min, and the holding time is 1-4 h;

[0039] Step 2, the reaction product I is crushed by an air flow crusher to a particle size D50 of 5-7 um to obtain a reaction product II;

[0040] Step 3, using KOH or K2CO3 or a mixture of KOH and K2CO3 as an activator, the activator is 10%~40% of the mass of the reaction product II, the activator is mixed with the reaction product II, solid mixing is carried out by using a VC mixer, the mixing time is 20~40min, then the mixture is heated from room temperature to 700~900℃ under a protective atmosphere, the heating rate is 1~5℃ / min, the holding time is 1~3h, reaction product III is obtained, the protective atmosphere is any one of nitrogen, helium or argon;

[0041] Step 4, reaction product III is washed with water until PH=7, then it is pickled with hydrochloric acid or dilute sulfuric acid or hydrofluoric acid, the concentration is 1mol / L, stirring for 6~12h, after stirring, water washing and filtration are carried out until PH=7, after impurity removal and ash reduction by water washing and pickling, reaction product IV is obtained in an oven, the oven temperature is 80~100℃;

[0042] Step 5, deionized water, metal chloride salt and reaction product IV are stirred to obtain a mixed solution, the concentration of the metal chloride salt is 0.02~0.04mol / L, the mixed solution is transferred to a stainless steel autoclave, and a hydrothermal reaction is carried out, the hydrothermal reaction conditions are: 120℃~180℃ for 8~12h, the hydrothermal reaction product is collected by centrifugation and dried at 80℃, reaction product V is obtained, wherein the metal chloride salt is one of MnCl2, FeCl3, NiCl2 and CoCl2;

[0043] Step 6, reaction product V is heated from room temperature to 1000~1200℃ under a protective atmosphere, the heating rate is 3℃ / min~6℃ / min, the holding time is 1h~4h, then it is heated to 1300℃~1600℃ at a heating rate of 1℃ / min~3℃ / min, the holding time is 1h~4h, reaction product VI is obtained, wherein the protective atmosphere is any one of nitrogen, helium or argon;

[0044] Step 7, reaction product VI is crushed by a mechanical crusher or an air flow crusher, the particle size D50 is controlled at 7~9um, and a sodium ion battery negative electrode material is obtained.

[0045] Example 1

[0046] The raw material in this embodiment is fruitwood. A certain amount of fruitwood is laid flat in a crucible and placed in a box furnace. Under a nitrogen atmosphere, the temperature is raised from room temperature to 450°C at a rate of 3°C / min, and the temperature is maintained for 2 hours to obtain reaction product I. After cooling to room temperature, the fruitwood precursor is removed and subjected to airflow crushing. The median particle size of the crushed particles is about 6 um. The crushed product is mixed with KOH solid, with the mass of KOH solid accounting for 10% of the mass of the crushed product. A VC mixer is used, and the mixing time is 30 min. After mixing, the reaction product is placed in a box furnace for activation. The activation conditions are as follows: under a nitrogen atmosphere, the temperature is raised from room temperature to 900°C at a rate of 2°C / min, and the temperature is maintained for 1 hour to obtain activated product III. The activated product is placed in water and washed until the pH is neutral (pH = 7). The washed product is placed in 1 mol / L hydrochloric acid and stirred for 12 hours, then washed and filtered until the pH of the filtrate is 7. The filtered product is dried in an oven at a temperature of 80°C to obtain reaction product IV. Reaction product IV, deionized water, and metal chloride salt MnCl2 are thoroughly stirred, with the concentration of the metal salt being 0.04 mol / L. The above mixture is transferred to a stainless steel autoclave for hydrothermal reaction at 180°C for 8 hours. The reaction product is collected by centrifugation and dried at 80°C to obtain reaction product V. The reaction product V is calcined at high temperature under a nitrogen atmosphere, with the temperature being raised from room temperature to 1000°C at a rate of 3°C / min, and the temperature being maintained for 2 hours. The temperature is then raised to 1400°C at a rate of 2°C / min, and the temperature is maintained for 2 hours to obtain reaction product VI. The reaction product VI is mechanically crushed, and the particle size D50 after crushing is about 8 um.

[0047] Example 2

[0048] The preparation method is the same as that of Example 1, except that the raw material is coconut shell.

[0049] Example 3

[0050] The preparation method is the same as that of Example 1, except that the activator is K2CO3, the mass ratio of the activator is 40%, the mixing time is 20 min, the activation temperature is 800°C, the temperature is raised at a rate of 1°C / min, and the temperature is maintained for 2 hours. The metal salt for metal doping is NiCl2, the concentration is unchanged, the reaction is carried out at 180°C for 12 hours, the high-temperature carbonization is carried out by raising the temperature from room temperature to 1200°C at a rate of 6°C / min, maintaining the temperature for 1 hour, then raising the temperature to 1600°C at a rate of 2°C / min, and maintaining the temperature for 1 hour.

[0051] Example 4

[0052] The preparation method is the same as that of Example 1, except that the activating agent is a mixture of K2CO3 and KOH, the ratio is 1:1, the total mass of the activating agent accounts for 20%, the mixing time is 40 min. The activation temperature is 700℃, the heating rate is 3℃ / min, and the holding time is 3h. The metal salt is MnCl2 when the metal is mixed, the concentration is 0.02mol / L, the reaction temperature is 120℃, the reaction time is 12h, the high-temperature carbonization is from room temperature to 1100℃, the heating rate is 5℃ / min, and the holding time is 4h. Then the temperature is increased to 1300℃, the heating rate is 1℃ / min, and the holding time is 4h.

[0053] Example 5

[0054] The preparation method is the same as that of Example 1, except that the concentration of the metal salt is 0.03mol / L when the metal is mixed.

[0055] Example 6

[0056] The preparation method is the same as that of Example 1, except that 1mol / L hydrofluoric acid is used for acid washing, and the stirring time is 6h.

[0057] Comparative Example 1

[0058] Compared with Example 1, the step of mixing the reaction product IV with metal ions is omitted, and the other steps remain unchanged.

[0059] Comparative Example 2

[0060] Compared with Example 1, the alkali activation step is omitted, and the reaction product II is directly acid washed and dried. The stirring time and concentration of the acid washing are the same as those of Example 1, and the other steps remain unchanged.

[0061] The samples obtained in Examples 1-6 and Comparative Examples are assembled into button cells, and the assembly and test method is as follows: the negative electrode material, conductive agent and binder are mixed in a solvent according to a mass percentage of 8:1:1, the solid content of the slurry is controlled to be 42.3%, and the slurry is coated on a copper foil current collector, dried, and cut to obtain a negative electrode sheet. Then, a 2025 button cell is assembled with metal sodium as the counter electrode. The button cell is tested at room temperature using a LAND battery test system of Wuhan Jinuo Electronics Co., Ltd., and the test conditions are as follows: the first charge and discharge I=0.1C, the cycle I=0.1C, and the voltage range 0.005-2.0V vs Na / Na + , and the test results are shown in Table 1. The sample obtained in Example 1 is assembled into a soft package battery, and the test method is as follows: the positive electrode material of Xiangying is selected, a soft package battery is assembled, and the 1.5-3.95V voltage, 0.5C / 1C, 1C / 1C rate performance is tested after liquid injection, formation, aging and capacity distribution, and the results are shown in Figure 5 .

[0062] Table 1: Electrochemical performance test results of negative electrode materials

[0063]

[0064] The negative electrode material obtained by the application has lower ash content, higher reversible specific capacity and first efficiency, smaller specific surface area, and higher capacity retention rate and stable cycle performance after 100 cycles, due to the addition of the acid pickling, activation pore forming, and metal ion blending technologies.

[0065] As can be seen from the test results of Examples 1-2, the carbon content, internal structure of the material, and the large pores on the surface of the coconut shell are different due to the different biomass raw materials. Under the same experimental conditions, the performance of the negative electrode material prepared from fruit wood is slightly better than that of the negative electrode material prepared from coconut shell.

[0066] As can be seen from the test results of Examples 1-3-4, the activation effect of the activator K2CO3 is not as good as that of KOH, and the use of K2CO3 requires an increase in the amount of activator. The better the activation effect of the alkali, the lower the ash content of the material after acid pickling, and the more complete the removal of SiO2 impurities. At the same time, the interlayer spacing of the material is larger. The activation temperature also affects the activation effect of the material. If the activation temperature is too high or the holding time is too long, the proportion of micropores in the material may change, thereby affecting the activation sites in the material. When metal ions are blended, the type of metal ions affects the defect regulation and specific surface area of the material. During the high-temperature stage, as the temperature increases, the interlayer spacing of the material gradually decreases, the ash content of the material decreases, the specific surface area decreases, and the first efficiency increases.

[0067] As can be seen from the test results of Example 1-5, when metal ions are blended, the ion concentration affects the catalytic graphitization of the material, thereby affecting the electrochemical performance of the material.

[0068] As can be seen from the test results of Example 1-6, different acids have different acid pickling effects, and the ash content of the material is different. The ash content mainly affects the cycle stability of the material. The acid pickling effect of HF is not as good as that of HCl. HF is a weak acid that mainly removes SiO2 in the material, and HCl is a strong acid that mainly removes impurity metal ions in the material. However, after alkali activation and acid pickling and water washing, SiO2 can also be removed, the ash content of the prepared material is lower, and the cycle performance is better.

[0069] As compared with Example 1, the metal ion blending step is omitted in Comparative Example 1, and the specific surface area of the material is increased. After alkali activation and pore forming, the proportion of micropores in the material increases, and the specific surface area increases. Although high-temperature calcination can shrink the pores and reduce the specific surface area, there are still many defects, and the electrolyte has many side reactions at the interface. The obtained material has a lower first coulomb efficiency and lower cycle stability. This shows that metal ions can catalyze graphitization, regulate material defects, and increase the first coulomb efficiency of the material.

[0070] Compared with Example 1, the reversible specific capacity of the material is reduced by omitting the alkali activation step. The activation pore-forming can increase the active sodium storage sites and sodium ion diffusion channels inside the material, and increase the interlayer spacing, thereby increasing the interlayer sodium storage. At the same time, the ash content of the material can be reduced, and the cycle stability of the material can be increased.

[0071] Figure 2 is the XRD pattern of the negative electrode material obtained in Example 1 of the present application. As can be seen in the figure, two weak broad diffraction peaks appear at 24° and 43°, which correspond to the (002) crystal plane and the (100) crystal plane of the amorphous carbon material respectively, showing the typical characteristics of amorphous carbon material. The interlayer spacing D002 is calculated by the Scherrer formula to be 0.385 nm

[0072] Figure 3 is the Raman pattern of the negative electrode material obtained in Example 1 of the present application. The peak area ratio ID / IG of D peak and G peak is 1.42, which reflects the disorder of the hard carbon;

[0073] Figure 4 is the first cycle charge-discharge curve of the negative electrode material obtained in Example 1 of the present application, wherein the discharge specific capacity is 373.0 mAh g -1 , the first reversible specific capacity is 342.6 mAh g -1 , and the first efficiency is 92.1%;

[0074] Figure 5 is the cycle performance curve of the negative electrode material obtained in Example 1 of the present application. After 100 cycles, the capacity retention rate is 98.9%.

[0075] Figure 6a and Figure 6b is the cycle performance curve of the soft pack battery of the negative electrode material obtained in Example 1 of the present application, Figure 6a can be seen in which the capacity retention rate is 93.4% after 669 cycles at 0.5C / 1C rate, Figure 6b can be seen in which the capacity retention rate is 83.7% after 884 cycles at 1C / 1C rate.

Claims

1. A method for preparing a sodium-ion battery negative electrode material, characterized in that, The specific steps are as follows: Step 1: Pre-carbonize the raw materials under a protective atmosphere to obtain reaction product I. The raw materials are one of corn cobs, coconut shells, sand willows, fruit wood, lump coal, or raw coal. Step 2: Crush reaction product I to obtain reaction product II; Step 3: Activate reaction product II to form pores, thereby obtaining reaction product III. The activation and pore-forming process uses KOH or a mixture of KOH and K2CO3 as the activating agent, and the activating agent accounts for 10% to 40% of the mass of reaction product II. Step 4: Wash reaction product III with water and acid to remove impurities, reduce ash content, and dry to obtain reaction product IV; Step 5: Mix reaction product IV with metal ions to obtain reaction product V. The metal ions used for mixing are metal chlorides, specifically one of MnCl2, FeCl3, NiCl2, and CoCl2. The mixture is prepared by thoroughly stirring deionized water, metal chloride, and reaction product IV to obtain a mixed solution. The concentration of the metal chloride is 0.02~0.04 mol / L. The mixed solution is then transferred to a stainless steel autoclave for hydrothermal reaction. Step 6: Calcining reaction product V at high temperature under a protective atmosphere to obtain reaction product VI; Step 7: Crush reaction product VI and sieve it to obtain final product VII.

2. The method for preparing the sodium-ion battery negative electrode material according to claim 1, characterized in that, The pre-carbonization process specifically involves heating from room temperature to 300℃~600℃ under a protective atmosphere at a heating rate of 1℃ / min~5℃ / min and holding for 1h~4h.

3. The method for preparing the sodium-ion battery negative electrode material according to any one of claims 1-2, characterized in that, The pulverization in step 2 specifically involves pulverizing reaction product I using an air jet mill to a particle size D50 of 5-7 μm, thereby obtaining reaction product II.

4. The method for preparing the sodium-ion battery negative electrode material according to any one of claims 1-2, characterized in that, The activation and pore-forming process in step 3 specifically involves mixing the activator with reaction product II using a VC mixer for solid mixing for 20-40 minutes. Then, the mixture is heated from room temperature to 700-900°C under a protective atmosphere at a heating rate of 1-5°C / min for 1-3 hours to obtain reaction product III.

5. The method for preparing the sodium-ion battery negative electrode material according to claim 4, characterized in that, The protective atmosphere in steps 1, 3, and 6 is any one of nitrogen, helium, or argon.

6. The method for preparing the sodium-ion battery negative electrode material according to any one of claims 1-2, characterized in that, The water washing in step 4 specifically involves washing with water until the pH value is 7. The pickling process is as follows: use hydrochloric acid, dilute sulfuric acid, or hydrofluoric acid to pickle at a concentration of 1 mol / L, stir for 6-12 hours, and then wash with water and filter until pH=7. The drying process specifically involves placing the acid-washed product in an oven at a temperature of 80-100°C to obtain reaction product IV.

7. The method for preparing the sodium-ion battery negative electrode material according to any one of claims 1-2, characterized in that, In step 5, the hydrothermal reaction conditions are: 120℃~180℃ for 8~12h. The hydrothermal reaction product is collected by centrifugation and dried at 80℃ to obtain reaction product V.

8. The method for preparing the sodium-ion battery negative electrode material according to any one of claims 1-2, characterized in that, The high-temperature calcination in step 6 specifically involves: raising the reaction product V from room temperature to 1000~1200℃ under a protective atmosphere at a heating rate of 3℃ / min~6℃ / min, holding it at that temperature for 1h~4h, then raising the temperature to 1300℃~1600℃ at a heating rate of 1℃ / min~3℃ / min, and holding it at that temperature for 1h~4h to obtain the reaction product VI.

9. The method for preparing the sodium-ion battery negative electrode material according to any one of claims 1-2, characterized in that, The pulverization in step 7 specifically involves pulverizing the reaction product VI using a mechanical pulverizer or an air jet mill, with the particle size D50 controlled at 7~9 μm.

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