A method for preparing porous carbon materials with improved first coulombic efficiency

Abstract

This invention relates to a method for preparing porous carbon materials with improved initial coulombic efficiency, comprising the following steps: uniformly mixing a phenolic resin-based carbon precursor, a phosphorus dispersion solution, and an organic solvent to obtain a suspension of a phenolic resin composite material containing phosphorus phosphorus; mixing the suspension with a fluorocarbon resin solution, spray granulating, and then ball milling and high-temperature carbonizing the resulting particles to obtain secondary carbon particles with a fluorocarbon layer; subsequently performing alkali activation and steam activation to obtain a surface-fluorine-rich three-dimensional phosphorus-anchored porous carbon material. This invention improves the electrochemical performance of a novel silicon-carbon anode by introducing two-dimensional phosphorus phosphorus material into phenolic resin-based porous carbon and achieving three-dimensional anchoring of the phosphorus phosphorus network within the phenolic resin-based porous carbon material through spray granulation.

Description

Technical Field

[0001] This invention belongs to the field of porous carbon material preparation technology, specifically relating to a method for preparing porous carbon materials with improved first coulombic efficiency. Background Technology

[0002] Compared to graphite anodes (specific capacity 372 mAh / g), silicon-based anodes theoretically boast a specific capacity of 4200 mAh / g, and their lithium potential (approximately 0.4 V) is similar to that of graphite anodes (approximately 0.1 V). Therefore, silicon-based anodes are considered the preferred next-generation anode material. However, silicon-based anodes also face several challenges in practical applications, such as severe volume expansion (300%) during lithium-ion insertion / extraction, low electronic and ionic conductivity, and unstable SEI films during electrochemical charge / discharge processes. These issues negatively impact the cycle stability of silicon-based anodes. To address these problems, coating silicon anodes with carbon materials is considered the ultimate solution. Recently, the use of nano-silicon vapor deposition into porous carbon materials to obtain novel silicon-carbon anode materials has attracted significant attention in the industry. However, there is still considerable room for improvement in the regulation of the physicochemical properties of porous carbon (such as specific surface area, pore volume, pore size distribution, and electronic and ionic conductivity). Phenolic resin, as an important raw material for preparing porous carbon frameworks, is widely used in the preparation of porous carbon materials. However, porous carbon anodes prepared using phenolic resin suffer from many problems, such as low electronic conductivity, slow ion diffusion rate, and unstable SEI at the interface. Existing solutions mainly modify the conductivity of phenolic resin through N and P doping. However, this doping modification cannot form a three-dimensional continuous conductive network, has limited global improvement in conductivity, and cannot construct a relatively stable electrode-electrolyte interface SEI film. Existing silicon-carbon anode materials still suffer from low electronic conductivity, poor lithium-ion diffusion capacity, and interface instability. Modification of the phenolic resin-based carbon framework is of great significance for the regulation of the physicochemical properties of novel silicon-carbon anode materials.

[0003] CN117691059A discloses a composite black phosphorus-based phosphorus-carbon anode material and its preparation method. The specific preparation steps are as follows: (1) Preparation of mesoporous carbon material dispersion: The mesoporous carbon material and the solvent are ultrasonically mixed in a certain proportion, and the mixture is stirred for a certain time to finally obtain the mesoporous carbon material dispersion; (2) Preparation of black phosphorus dispersion: The powder obtained by ball milling black phosphorus is dispersed in the solvent while stirring, and finally a uniformly mixed black phosphorus dispersion is obtained; (3) Preparation of composite black phosphorus-based phosphorus-carbon anode material: The above black phosphorus dispersion is slowly added dropwise into the mesoporous carbon material dispersion, followed by ultrasonic vibration and solvent / hydrothermal reaction. The product is collected after centrifugation and washing multiple times and then freeze-dried under vacuum. The freeze-dried final product is placed in an agate mortar for crushing and grinding to obtain the desired composite black phosphorus-based phosphorus-carbon anode material. CN112018363A discloses a composite black phosphorus-based phosphorus-carbon anode material and its preparation method. The carbon material and black phosphorus are covalently bonded to the black phosphorus through high-energy ball milling, forming a black phosphorus-carbon modified composite material. A conductive polymer is continuously or discontinuously coated onto the surface of the black phosphorus-carbon modified composite material through in-situ liquid-phase polymerization, thereby obtaining the desired black phosphorus-based composite anode material. The coating amount of the conductive polymer is 0.01–50 wt% based on the total weight of the black phosphorus-based composite anode material. The conditions for the high-energy ball milling are: a feed mass ratio of black phosphorus to carbon material of 0.05–20:1; a ball-to-material mass ratio of 10–300:1; a ball milling speed of 300–1000 rpm; and a ball milling time of 1–12 h.

[0004] In the aforementioned mixing method, black phosphorus material is filled into the mesopores of carbon material. The black phosphorus material occupies a certain amount of void space, reducing the space available for CVD vapor deposition of silicon and thus decreasing the amount of silicon deposited, leading to a decrease in the capacity of the anode material. Furthermore, when added to the mesopores as black phosphorus, the black phosphorus in each void is in a discrete state, unable to form a continuous network, resulting in the added black phosphorus not fully exerting its chemical properties. Summary of the Invention

[0005] Compared with existing technologies, this technical solution mainly solves the following problems: 1) how to improve the electronic and ionic conductivity of phenolic resin-based porous carbon substrates; 2) how to anchor phosphorene in phenolic resin-based porous carbon substrates to promote the improvement of conductivity after silicon deposition; 3) how to construct a fluorine-rich interface SEI layer through fluorocarbon resin to stabilize the interface stability between silicon-carbon anode and electrolyte.

[0006] To achieve the above objectives, this technical solution mainly adopts the following technical means:

[0007] A method for preparing porous carbon materials with improved initial coulombic efficiency includes the following steps:

[0008] (S1) Phenolic resin is pre-carbonized at high temperature to obtain phenolic resin-based carbon precursor.

[0009] (S2 is prepared by adding anionic surfactant to deoxygenated water to form a solution, adding black phosphorus crystals, using a probe ultrasonic instrument to peel off the black phosphorus crystals, peeling the phosphorene from the black phosphorus crystals, separating the unpeeled black phosphorus crystals at the bottom by low-speed centrifugation, and obtaining a phosphorene dispersion solution by liquid phase high-speed centrifugation.)

[0010] (S3) Phenolic resin-based carbon precursor, phosphorene dispersion solution and organic solvent are mixed evenly to obtain a suspension of phenolic resin composite material containing phosphorus phosphorene.

[0011] (S4) The suspension and fluorocarbon resin solution are mixed, the mixture is spray granulated, and the resulting particles are ball-milled and carbonized at high temperature to obtain secondary carbon particles containing fluorocarbon layer.

[0012] (S5) The secondary carbon particles containing the fluorocarbon layer are activated for the first time in the presence of an alkaline activator to obtain the first activated carbon material;

[0013] (S6) The first activated carbon material is activated a second time in the presence of water vapor as the activating agent to obtain the second activated carbon material; the second activated carbon material is subjected to acid washing, washing and drying to obtain porous carbon material.

[0014] Furthermore, in step (S1), the molecular weight of the phenolic resin is 5000-20000, and the high-temperature pre-carbonization is performed at 500-800℃ for 2-3 hours.

[0015] Further, in step (S2), the anionic surfactant is selected from at least one of sodium dodecyl sulfate and sodium dodecylbenzene sulfonate; the content of the anionic surfactant is 2-5 wt%; and the mass ratio of black phosphorus crystals to anionic surfactant solution is 10-18:100.

[0016] Furthermore, in step (S2), the operating parameters of the probe-type ultrasonic instrument are 20-50 kHz, the ultrasonic time is 1-2 h, the low-speed centrifugation is 500-5000 rpm for 5-10 min, and the high-speed centrifugation is 10000-30000 rpm for 10-30 min; the solid content of the obtained phosphorene dispersion solution is 5-10 wt%.

[0017] Further, in step (S3), the mass ratio of the phenolic resin-based carbon precursor, the phosphorus dispersion solution, and the organic solvent is 100:20-30:150-200.

[0018] Further, in step (S3), the organic solvent is selected from at least one of ethanol, NMP, DMF, and DMSO.

[0019] Further, in step (S4), the fluorocarbon resin is selected from at least one of polytetrafluoroethylene (PTFE), polychlorotrifluoroethylene (PCTFE), polyvinylidene fluoride (PVDF), ethylene-tetrafluoroethylene copolymer (ETFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), and polyvinyl fluoride (PVF); the solvent of the fluorocarbon resin solution is N-methylpyrrolidone (NMP), and the concentration of the fluorocarbon resin solution is 10-30 wt%; the mass ratio of the suspension to the fluorocarbon resin solution is 100:2.5-5.

[0020] Further, in step (S4), the process parameters for spray granulation are: pressure controlled at 0.08-0.1 MPa, feed rate controlled at 100-300 mL / min, and spray granulation process conditions controlled so that the D50 of the obtained secondary carbon spheres is 5-10 μm; high-temperature curing is performed at 700-1000℃ for 2-5 hours under a carbonization inert atmosphere; preferably, the inert atmosphere is at least one of nitrogen, argon, and helium.

[0021] Further, in step (S5), the alkaline activator is potassium hydroxide and / or sodium hydroxide, and the amount of alkaline activator is 100-300% of the mass of the secondary carbon particles in the fluorocarbon layer; the first activation is performed at 700-800℃ for 2-4 hours.

[0022] Further, in step (S6), the ratio of steam flow rate to the mass of the first activated carbon material is 0.01-0.1 L / min / g, preferably 0.03-0.045 L / min / g.

[0023] Furthermore, in steps (S5) and (S6), the first activation and the second activation are carried out under a protective atmosphere, which is at least one of nitrogen, argon, and helium.

[0024] Further, in step (S6), the acid washing is done with a 1-10% HCl solution. The purpose of acid washing is to remove the KOH components involved in alkali activation and some ash. The washing is done with water until the filtrate is neutral (pH = 7 ± 0.5). The drying is done by centrifugal drying, oven drying, vacuum drying, etc.

[0025] The present invention achieves the following technical effects:

[0026] I. This invention introduces two-dimensional phosphorene materials into phenolic resin-based porous carbon and achieves three-dimensional anchoring of the phosphorene network in the phenolic resin-based porous carbon material through spray granulation, thereby improving the capacity and first coulombic efficiency of silicon-carbon anodes.

[0027] Second, this invention achieves uniform coating of fluorocarbon resin on the surface of porous carbon materials, improving product performance. Furthermore, by controlling the molecular weight and type of the phenolic resin, as well as its mass ratio with black phosphorus material, precise control can be achieved over parameters such as pore size, pore volume, and specific surface area of ​​the carbon material, meeting diverse application requirements.

[0028] III. The porous carbon products prepared by the method of the present invention have excellent performance, with high specific surface area, good electrical conductivity and chemical stability, and are suitable for applications such as batteries, supercapacitors, and catalyst supports. Attached Figure Description

[0029] Figure 1 This is a SEM image of the fluorine-rich three-dimensional phosphorus-anchored porous carbon material prepared in Example 1.

[0030] Figure 2 This is a comparison of XRD patterns of fluorine-rich three-dimensional phosphorus-bonded porous carbon before and after activation and pore formation in Example 1.

[0031] Figure 3 This is the elemental mapping diagram of porous carbon anchored to fluorine-rich three-dimensional phosphorus olefin in Example 1.

[0032] Figure 4 This is a high-resolution XPS image of F element in porous carbon with fluorine-rich three-dimensional phosphorus-bonded structure, as shown in Example 1.

[0033] Figure 5 This is a high-resolution XPS image of the fluorine-rich three-dimensional phosphorus-bonded porous carbon P element from Example 1.

[0034] Figure 6 This is the adsorption curve of carbon and nitrogen gas in the porous structure anchored with fluorine-rich three-dimensional phosphorus ene, as shown in Example 1.

[0035] Figure 7 This is a pore size distribution diagram of fluorine-rich three-dimensional phosphorus riveting porous carbon in Example 1.

[0036] Figure 8 This is the powder resistivity curve of fluorine-rich three-dimensional phosphorus riveted porous carbon pores in Example 1 at 10MPa-100MPa.

[0037] Figure 9 The resistivity curves of porous carbon pores in Comparative Example 1 are shown in the range of 10 MPa to 100 MPa. Detailed Implementation

[0038] Phenolic resin is prepared by mixing formaldehyde and phenol in a molar ratio of 1.3:1.

[0039] Example 1

[0040] (S1) Select a phenolic resin with a molecular weight of 5000 and perform high-temperature pre-carbonization at a temperature of 500°C for 2 hours to obtain a phenolic resin-based carbon precursor for later use.

[0041] (S2) Add sodium dodecyl sulfate (SDS) to deoxygenated water to prepare a 2.5 wt% surfactant solution. Add black phosphorus crystals. The mass ratio of black phosphorus crystals to surfactant solution is 15:100. Use a probe-type ultrasonic instrument to peel off the black phosphorus crystals (ultrasonic frequency 20 kHz, ultrasonic time 1-2 h). First, centrifuge at 3000 rpm for 5 min to separate the black phosphorus crystals that have not been peeled off at the bottom. Then, centrifuge at 10000 rpm for 20 min to obtain a phosphorus dispersion solution.

[0042] (S3) Phenolic resin-based carbon precursor, phosphorene dispersion solution and ethanol are mixed evenly at a mass ratio of 100:20:150 to obtain a suspension of phenolic resin composite material containing phosphorus phosphorene.

[0043] (S4) Prepare a 10wt% polyvinylidene fluoride (PVDF) / NMP solution system. Mix the suspension obtained in step (S3) and the PVDF / NMP solution system at a mass ratio of 100:5. Spray granulate the mixture (0.08MPa, feed rate 100mL / min), and then ball mill it to make the D50 of the secondary carbon particles 5-10μm. Then, perform high-temperature carbonization in a tube furnace (argon protection) at a carbonization temperature of 800℃ for 2 hours to obtain secondary carbon particles with a uniform fluorinated carbon layer.

[0044] (S5) The above-mentioned secondary carbon spheres and potassium hydroxide are mixed evenly in a mass ratio of 1:1 and activated at high temperature (nitrogen protection) for 2 hours to obtain the first activated carbon material.

[0045] (S6) The carbon material was first activated and placed in a tube furnace (argon protection). Secondary activation and pore-forming were performed using steam at a flow rate of 0.045 L / min / g, an activation temperature of 900℃, and a time of 3 hours. Finally, after acidification, washing, centrifugal drying, and other treatments, a fluorine-rich three-dimensional phosphorus-bonded porous carbon material was obtained.

[0046] Figure 1 The image shows the SEM image of the fluorine-rich three-dimensional phosphorus-anchored porous carbon material prepared in Example 1. It can be seen that the material exhibits spherical and near-spherical shapes. The phenolic resin carbon precursor, black phosphorus, and fluorocarbon resin form relatively uniform spherical shapes, which then undergo condensation reaction to form spherical carbon during the high-temperature carbonization process.

[0047] Figure 2The image shows a comparison of XRD patterns of fluorine-rich three-dimensional phosphorus-bonded porous carbon before and after activation and pore formation in Example 1. It can be seen that the reactants before activation are mainly carbides. Due to the small amount of P and F elements added, they are not obvious in the XRD. With the high-temperature activation reaction, the C peak in the XRD gradually becomes obvious. This is because during the high-temperature activation process, the resin material that was not completely carbonized was completely carbonized and presented a stable carbon structure.

[0048] Figure 3 This is the elemental mapping diagram of the fluorine-rich three-dimensional phosphorus-bonded porous carbon in Example 1, which shows that P and N elements are doped in the carbon material and that the P and N elements are evenly distributed.

[0049] Figure 4 This is a high-resolution XPS image of F element in porous carbon with fluorine-rich three-dimensional phosphorus-bonded structure, as shown in Example 1.

[0050] Figure 5 This is a high-resolution XPS image of the fluorine-rich three-dimensional phosphorus-bonded porous carbon P element from Example 1. Figure 4 , Figure 5 This method can be used to detect the F and P content in porous carbon materials. As shown in Tables 2 and 3, the phosphorus content in the carbon material exhibits a linear trend with increasing black phosphorus mass ratio. When the raw material mass ratio is below 10%, the conversion rate is approximately 75%. When it is above 10%, the conversion rate of black phosphorus to P in the carbon material is less than 50%. This is because with excessive addition of black phosphorus, P becomes supersaturated in the carbon material, and some black phosphorus does not participate in the construction of the three-dimensional network structure of phosphorene. The addition of F shows a stable linear trend, and the addition of F helps to form a stable SEI film. Analysis of specific surface area, pore volume, and pore size shows that the specific surface area of ​​the porous carbon decreases with increasing P content. This is because black phosphorus has a very stable structure; the high-temperature activation process does not etch the phosphorene structure, but only activates and etches the internal resin-based carbon material. Phosphene acts as a stable phenolic resin carbon linking matrix in the porous carbon structure. Simultaneously, the increased phosphorus content leads to more phosphorene acting as interlocking elements between carbon materials, increasing the distance between carbon atoms and significantly improving pore volume. The addition of element F will not have a significant impact on pore volume or pore size.

[0051] Figure 6 This is the adsorption curve of carbon and nitrogen gas in the porous structure anchored with fluorine-rich three-dimensional phosphorus ene, as shown in Example 1.

[0052] Figure 7 This is a pore size distribution diagram of fluorine-rich three-dimensional phosphorus riveting porous carbon in Example 1.

[0053] Figure 8 This is the powder resistivity curve of fluorine-rich three-dimensional phosphorus riveted porous carbon pores in Example 1 at 10MPa-100MPa.

[0054] Figure 9 The resistivity curves of porous carbon pores in Comparative Example 1 are shown in the range of 10 MPa to 100 MPa.

[0055] Examples 2-9

[0056] Examples 1-10 are the same as Example 1, except that the mass ratio of black phosphorus crystals to surfactant solution in step (S2) is different (Examples 2-4), and the concentration of fluorocarbon resin solution in step (S4) is different (Examples 5-10); Examples 11-13 are the same as Example 10, except that Example 11 only undergoes KOH activation, and Example 12 only undergoes steam activation; Example 13 has a primary activation of steam activation and a secondary activation of KOH activation; The preparation process parameters of the examples and comparative examples are listed in Table 1 below;

[0057] Table 1 Process Parameters

[0058]

[0059] The specific surface area, pore size distribution, pore volume, phosphorus and fluorine content, and resistivity of the porous carbon prepared in the above examples and comparative examples were tested. The results are shown in Table 2.

[0060] Table 2. Parameter Testing of Porous Carbon

[0061]

[0062] Application examples

[0063] The porous carbon from the above examples and comparative examples was used to prepare silicon-carbon anode materials, and batteries were assembled to test the first charge-discharge specific capacity and first efficiency. Specifically, 1 kg of porous carbon was first placed in a rotary kiln, and Ar was introduced as a protective gas. The rotary kiln speed was 60 r / min, and the temperature was raised to 500°C and kept stable. Then, silane gas was introduced at a flow rate of 3 L / min for 4.5 h. The silane decomposed into Si and deposited into the pores of the porous carbon to form silicon-carbon material. The introduction of silane gas was then stopped. Then, the temperature was raised to 900°C, and acetylene gas was introduced at a flow rate of 3 L / min for 2 h. The acetylene decomposed into carbon and coated the deposited silicon-carbon surface to form the active material of the lithium battery anode material. The obtained silicon-carbon active material was used as the battery anode for electrochemical testing. Specifically, the active material, polyacrylic acid (PAA), and superP were mixed in deionized water at a mass ratio of 8:1:1, and after thorough grinding, the mixture was placed in a homogenizer to prepare a slurry. The slurry was then uniformly coated onto copper foil to a thickness of 200 μm and dried in a vacuum drying oven. Subsequently, 12 mm electrodes were formed using a punching machine and placed in a vacuum glove box. A 16 mm lithium sheet was selected as the symmetrical electrode, and Celgard 2600 was used, cut to 18 mm diameter as the separator. A CR2032 coin cell was prepared using a Krohde 1M LiPF6 electrolyte (solvent with a volume ratio of EC:EDC:EMC of 1:1:1 and 5% FEC added). Electrochemical testing was performed using a Blue Electric system. Specifically, the battery was allowed to stand for 1 hour, then discharged at a 0.1C rate until the voltage dropped to 0.005 V, allowed to stand for another hour, then discharged at a 0.02C rate until the voltage dropped to 0.005 V, allowed to stand for another hour, then discharged at a 0.01C rate until the voltage dropped to 0.005 V, and finally charged at a 0.1C rate until the voltage reached 1.5 V. The results are shown in Table 3.

[0064] Table 3 Electrochemical Performance Tests

[0065]

Claims

1. A method for preparing porous carbon materials with improved initial coulombic efficiency, characterized in that, Includes the following steps: (S1) Phenolic resin is pre-carbonized at high temperature to obtain phenolic resin-based carbon precursor; the high temperature pre-carbonization is 500-800℃ for 2-3 hours. (S2) Add anionic surfactant to deoxygenated water to prepare a solution, add black phosphorus crystals, use a probe ultrasonic instrument to peel off the black phosphorus crystals, peel the phosphorene from the black phosphorus crystals, separate the unpeeled black phosphorus crystals at the bottom by low-speed centrifugation, and obtain the phosphorene dispersion solution by liquid phase high-speed centrifugation. (S3) Phenolic resin-based carbon precursor, phosphorus dispersion solution and organic solvent are mixed evenly to obtain a suspension of phosphorus-containing phenolic resin composite material. (S4) The suspension and fluorocarbon resin solution are mixed, the mixture is spray granulated, and the resulting particles are ball-milled and carbonized at high temperature to obtain secondary carbon particles containing fluorocarbon layer; the high temperature carbonization is to treat at 700-1000℃ for 2-5 hours under carbonization inert atmosphere. (S5) The secondary carbon particles containing the fluorocarbon layer are activated for the first time in the presence of an alkaline activator to obtain the first activated carbon material; (S6) The first activated carbon material is activated a second time in the presence of water vapor as the activating agent to obtain the second activated carbon material; The second activated carbon material is subjected to acid washing, washing, and drying to obtain porous carbon material.

2. The preparation method according to claim 1, characterized in that, In step (S1), the molecular weight of the phenolic resin is 5000-20000.

3. The preparation method according to claim 1, characterized in that, In step (S2), the anionic surfactant is selected from at least one of sodium dodecyl sulfate and sodium dodecylbenzene sulfonate; the content of the anionic surfactant is 2-5 wt%; and the mass ratio of black phosphorus crystals to anionic surfactant solution is 10-18:

100.

4. The preparation method according to claim 1, characterized in that, In step (S2), the operating parameters of the probe-type ultrasonic instrument are 20-50kHz, the ultrasonic time is 1-2h, the low-speed centrifugation is 500-5000rpm for 5-10min, and the high-speed centrifugation is 10000-30000rpm for 10-30min; the solid content of the resulting phosphorene dispersion solution is 5-10wt%.

5. The preparation method according to claim 1, characterized in that, In step (S3), the mass ratio of phenolic resin-based carbon precursor, phosphorus dispersion solution and organic solvent is 100:20-30:150-200.

6. The preparation method according to claim 1, characterized in that, In step (S4), the fluorocarbon resin is selected from at least one of polytetrafluoroethylene (PTFE), polychlorotrifluoroethylene (PCTFE), polyvinylidene fluoride (PVDF), ethylene-tetrafluoroethylene copolymer (ETFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), and polyvinyl fluoride (PVF); the solvent of the fluorocarbon resin solution is N-methylpyrrolidone (NMP), and the concentration of the fluorocarbon resin solution is 10-30 wt%; the mass ratio of the suspension to the fluorocarbon resin solution is 100:2.5-5.

7. The preparation method according to claim 1, characterized in that, In step (S4), the process parameters for spray granulation are: pressure controlled at 0.08-0.1 MPa, feed rate controlled at 100-300 mL / min, and spray granulation process conditions controlled so that the D50 of the obtained secondary carbon sphere particles is 5-10 μm.

8. The preparation method according to claim 1, characterized in that, In step (S4), the inert atmosphere is at least one of nitrogen, argon, and helium.

9. The preparation method according to claim 1, characterized in that, In step (S5), the alkaline activator is potassium hydroxide and / or sodium hydroxide, and the amount of alkaline activator is 100-300% of the mass of the secondary carbon particles in the fluorocarbon layer; the first activation is performed at 700-800℃ for 2-4 hours.

10. The preparation method according to claim 1, characterized in that, In step (S6), the ratio of steam flow rate to the mass of the first activated carbon material is 0.01-0.1 L / min / g.

11. The preparation method according to claim 1, characterized in that, In step (S6), the ratio of steam flow rate to the mass of the first activated carbon material is 0.03-0.045 L / min / g.

12. The preparation method according to claim 1, characterized in that, In step (S6), the acid washing is done with a 1-10% HCl solution; the washing is done with water until the filtrate is neutral; and the drying is done by centrifugal drying, oven drying, or vacuum drying.

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