Nano-silicon-loaded porous silicon carbon battery negative electrode material and preparation method thereof

Abstract

The invention belongs to the technical field of battery materials, and discloses a nano-silicon-loaded porous silicon carbon battery negative electrode material and a preparation method thereof. Comprising the steps of raw material mixing reaction, inert gas atmosphere treatment, hydrofluoric acid solution treatment, ethanol solution dispersion and silane coupling, nanometer silicon loading, centrifugal drying, doping agent mixing, inert gas atmosphere high-temperature treatment and optional silicon oxide buffer layer preparation. The preparation process is clear and reasonable in step, various process parameters (such as reaction temperature, time, raw material proportion and the like) are easy to control, the adopted raw materials (coconut shell-based carbon, nano silicon dioxide balls and the like) are wide in source and relatively low in cost, and meanwhile, the whole process is free of operation which is too complicated or difficult to implement, and practical application and operation are facilitated.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of battery materials, in particular to a porous silicon-carbon battery anode material loaded with nano-silicon and a preparation method thereof. BACKGROUND

[0002] Lithium ion batteries have been widely used in portable electronic devices, electric vehicles, large-scale energy storage and other fields due to their high energy density, long cycle life, low self-discharge rate and other advantages.

[0003] However, in actual application, the negative electrode material of lithium ion batteries is mainly graphite, but the theoretical specific capacity of graphite is relatively low (only 372 mAh / g), which is difficult to meet the increasing demand for high energy density batteries. Nano-silicon has a very high theoretical specific capacity (up to 4200 mAh / g), and is a very promising next-generation lithium ion battery anode material; however, nano-silicon will undergo a huge volume change (volume change can reach more than 300%) during charging and discharging. This dramatic volume change will cause the silicon particles to powder and the electrode structure to be damaged, thereby causing the cycle stability of the battery to drop sharply. At the same time, the poor conductivity of nano-silicon will also affect the rate performance of the battery, which is not conducive to actual application and operation. SUMMARY

[0004] One of the purposes of the present application is to provide a porous silicon-carbon battery anode material loaded with nano-silicon and a preparation method thereof.

[0005] To achieve the above purposes, the technical scheme adopted by the present application is as follows: a porous silicon-carbon battery anode material loaded with nano-silicon and a preparation method thereof, comprising the following steps:

[0006] S1, raw material mixing reaction: coconut shell-based carbon, phenolic resin prepolymer, hard template agent, and soft template agent are mixed in proportion and reacted to obtain a porous carbon carrier precursor;

[0007] S2, inert gas atmosphere treatment: the porous carbon carrier precursor is treated in an inert gas atmosphere;

[0008] S3, hydrofluoric acid solution treatment: the substance treated in the inert gas atmosphere is treated with a hydrofluoric acid solution to obtain a porous carbon carrier;

[0009] S4, ethanol solution dispersion and silane coupling: the porous carbon carrier is dispersed in an ethanol solution, a bifunctional silane coupling agent is added, and a reaction is performed;

[0010] S5, nano-silicon loading: hydroxylated nano-silicon is added to the above reaction system, and a reaction is performed;

[0011] S6, centrifugal drying: the system after reaction is subjected to centrifugal and drying operations;

[0012] S7, Doping agent mixing: mixing the dried substance with melamine and ammonium dihydrogen phosphate;

[0013] S8, Inert gas atmosphere high temperature treatment: placing the mixed substance in an inert gas atmosphere for high temperature treatment;

[0014] S9, Optional silicon oxide buffer layer preparation: placing the high temperature treated substance in an oxygen atmosphere for treatment to obtain a porous silicon-carbon battery negative electrode material loaded with nano-silicon and a silicon oxide buffer layer; if this step is not performed, a porous silicon-carbon battery negative electrode material loaded with nano-silicon is directly obtained.

[0015] Preferably, the hard template agent is nano-silicon dioxide spheres and the soft template agent is block copolymer P123.

[0016] Preferably, the bifunctional silane coupling agent is a bifunctional silane containing amino and epoxy groups.

[0017] Preferably, the hydroxylated nano-silicon is obtained by etching treatment of nano-silicon particles with hydrofluoric acid.

[0018] Preferably, the following steps are included:

[0019] SR1, raw material mixing reaction: mixing coconut shell-based carbon, phenolic resin prepolymer, hard template agent, and soft template agent in proportion, stirring and reacting at 80-100°C for 4-6h to obtain a porous carbon carrier precursor;

[0020] SR2, inert gas atmosphere treatment: raising the temperature of the porous carbon carrier precursor to 600°C at a temperature raising rate of 5°C / min in an inert gas atmosphere, maintaining the temperature for 2h, and then raising the temperature to 800-1000°C at a temperature raising rate of 3°C / min, maintaining the temperature for 3-5h;

[0021] SR3, hydrofluoric acid solution treatment: treating the substance treated in the inert gas atmosphere with a 15-20% mass fraction hydrofluoric acid solution at 50-60°C for 3-4h to obtain a porous carbon carrier;

[0022] SR4, ethanol solution dispersion and silane coupling: adding the porous carbon carrier to a mixed solution of ethanol and deionized water in a volume ratio of 3:1-5:1, adding 8-15% of the mass of the porous carbon carrier of bifunctional silane coupling agent, and stirring and reacting at 60-80°C for 3-4h;

[0023] SR5, nano-silicon loading: adding 20-40% of the mass of the porous carbon carrier of hydroxylated nano-silicon to the above reaction system, and stirring and reacting at 80-100°C for 5-6h;

[0024] SR6, centrifugal drying: centrifugal and drying operation are performed on the system after reaction;

[0025] SR7, dopant mixing: the dried material is mixed with melamine and ammonium dihydrogen phosphate at a mass ratio of 10:3:1;

[0026] SR8, inert gas atmosphere high temperature treatment: the mixed material is heated to 600℃ at a heating rate of 4℃ / min, and then a mixed gas of hydrogen and inert gas with a hydrogen volume ratio of 5-10% is introduced, and the temperature is kept for 0.5-1h, and then the temperature is continuously increased to 850-950℃ at a heating rate of 4℃ / min, and the temperature is kept for 2-3h;

[0027] SR9, optional silicon oxide buffer layer preparation: the material obtained by high temperature treatment is placed in an oxygen atmosphere and kept at 200-300℃ for 1-2h to obtain a porous silicon-carbon battery negative electrode material loaded with nano-silicon containing a silicon oxide buffer layer; if this step is not performed, a porous silicon-carbon battery negative electrode material loaded with nano-silicon is directly obtained.

[0028] Preferably, the preparation process of the phenolic resin prepolymer is as follows: phenol and formaldehyde are mixed at a molar ratio of 1:1.2-1:1.5, a sodium hydroxide solution with a mass fraction of 20-30% is added as a catalyst, the amount of catalyst is 5-8% of the mass of phenol, and the reaction is carried out at 70-80℃ for 1.5-2h.

[0029] Preferably, the bifunctional silane coupling agent is a mixture of γ-(2,3-epoxypropoxy) propyl trimethoxysilane and 3-aminopropyl triethoxysilane, and the molar ratio of the two is 1:1-1:2.

[0030] Preferably, the preparation process of the hydroxylated nano-silicon is as follows: nano-silicon particles are added to a hydrofluoric acid solution with a mass fraction of 5-10%, ultrasonic treatment is carried out at room temperature for 1-2h, centrifugal and washing are carried out until neutral, and drying is carried out.

[0031] Compared with the prior art, the present application has the following advantages:

[0032] (1) The porous carbon carrier constructed by the hard template agent and the soft template agent has abundant multi-level pores. These pores provide sufficient space for the uniform dispersion and large loading of nano-silicon, which can improve the utilization rate of silicon. On the other hand, the pores can serve as channels for the rapid transmission of lithium ions during battery charging and discharging, reducing the ion transmission resistance and improving the rate performance of the battery. In addition, the porous structure can effectively alleviate the volume expansion of nano-silicon during charging and discharging, avoiding the destruction of the structure due to excessive volume change.

[0033] (2) The nano-silicon and the porous carbon carrier are tightly connected together by the way of chemical bonding through the bifunctional silane coupling agent, so that the interface bonding force between the two is greatly enhanced. The strong bonding force can prevent the nano-silicon from falling off from the carbon carrier during the circulation process, ensure the integrity of the material structure, and further maintain the stability of the battery performance.

[0034] (3) The nitrogen and phosphorus heteroatoms introduced by melamine and ammonium dihydrogen phosphate have a significant improvement effect on the conductivity of the carbon material. The existence of the heteroatoms adjusts the electron cloud distribution of the carbon, reduces the resistance of the electron transmission, improves the charge and discharge efficiency of the battery, and the heteroatoms can also interact with lithium ions and the like, affect the storage and release of ions, and further optimize the electrochemical performance of the battery.

[0035] (4) The silicon oxide buffer layer provides more effective buffering for the volume change of the nano-silicon. When the silicon expands in volume during charging, the silicon oxide layer can accommodate the volume change of the silicon through its own elastic deformation and the like, avoiding the direct breakage of the silicon particles due to excessive stress; when discharging, the silicon contracts in volume, and the silicon oxide layer can maintain the stability of the structure to prevent the electrode from powdering, thereby significantly improving the cycle stability of the battery negative electrode material and prolonging the service life of the battery.

[0036] (5) The coconut shell-based carbon is used as the raw material, the coconut shell is widely available and low in price, and belongs to renewable resources, which meets the concept of green environmental protection. Each step of the entire preparation process, such as mixing reaction, high-temperature treatment, etching, and loading, has good operability, and the process parameters are relatively easy to control, facilitating large-scale production. BRIEF DESCRIPTION OF DRAWINGS

[0037] Figure 1 It is the overall flowchart of the present application. DETAILED DESCRIPTION

[0038] In the following, the present application will be further described in conjunction with specific embodiments, and it should be noted that the embodiments described below or the technical features between the embodiments can be combined in any manner to form new embodiments without conflict.

[0039] In the description of the present application, it should be noted that for orientation words, such as terms "center", "transverse", "longitudinal", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", etc. indicate the orientation and positional relationship based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present application and simplifying the description, and cannot be understood as indicating or implying that the devices or elements referred to must have a specific orientation, be constructed and operated in a specific orientation, and cannot be understood as limiting the specific protection scope of the present application.

[0040] It should be noted that the terms "first", "second", etc. in the specification and claims of the present application are used to distinguish similar objects, and do not necessarily describe a specific order or sequence.

[0041] One preferred embodiment of the present application is shown in Figure 1 A porous silicon-carbon battery anode material loaded with nanosilicon and a preparation method thereof, comprising the following steps:

[0042] S1, raw material mixing reaction: coconut shell-based carbon, phenolic resin prepolymer, hard template agent, and soft template agent are mixed in proportion and reacted to obtain a porous carbon carrier precursor;

[0043] S2, inert gas atmosphere treatment: the porous carbon carrier precursor is treated under an inert gas atmosphere;

[0044] S3, hydrofluoric acid solution treatment: the substance treated under the inert gas atmosphere is treated with a hydrofluoric acid solution to obtain a porous carbon carrier;

[0045] S4, ethanol solution dispersion and silane coupling: the porous carbon carrier is dispersed in an ethanol solution, a bifunctional silane coupling agent is added, and a reaction is performed;

[0046] S5, nanosilicon loading: hydroxylated nanosilicon is added to the above reaction system and a reaction is performed;

[0047] S6, centrifugal drying: the system after the reaction is subjected to centrifugal and drying operations;

[0048] S7, dopant mixing: the dried substance is mixed with melamine and ammonium dihydrogen phosphate;

[0049] S8, inert gas atmosphere high-temperature treatment: the mixed substance is subjected to high-temperature treatment under an inert gas atmosphere;

[0050] S9, optional silicon oxide buffer layer preparation: the substance obtained by high-temperature treatment is treated under an oxygen atmosphere to obtain a porous silicon-carbon battery anode material loaded with nanosilicon containing a silicon oxide buffer layer; if this step is not performed, a porous silicon-carbon battery anode material loaded with nanosilicon is directly obtained.

[0051] The hard template agent is nanosilica spheres, and the soft template agent is a block copolymer P123.

[0052] The bifunctional silane coupling agent is a bifunctional silane containing amino and epoxy groups.

[0053] The hydroxylated nanosilicon is obtained by treating nanosilicon particles with hydrofluoric acid etching.

[0054] comprising the following steps:

[0055] SR1, raw material mixing reaction: mixing coconut shell-based carbon, phenolic resin prepolymer, hard template agent, and soft template agent in proportion, stirring and reacting at 80-100℃ for 4-6h to obtain a porous carbon carrier precursor;

[0056] SR2, inert gas atmosphere treatment: raising the temperature of the porous carbon carrier precursor to 600℃ at a temperature raising rate of 5℃ / min under an inert gas atmosphere, maintaining the temperature for 2h, and then raising the temperature to 800-1000℃ at a temperature raising rate of 3℃ / min, maintaining the temperature for 3-5h;

[0057] SR3, hydrofluoric acid solution treatment: treating the substance after the inert gas atmosphere treatment with a hydrofluoric acid solution with a mass fraction of 15-20% at 50-60℃ for 3-4h to obtain a porous carbon carrier;

[0058] SR4, ethanol solution dispersion and silane coupling: adding the porous carbon carrier to a mixed solution of ethanol and deionized water with a volume ratio of 3:1-5:1, adding a bifunctional silane coupling agent in an amount of 8-15% of the mass of the porous carbon carrier, and stirring and reacting at 60-80℃ for 3-4h;

[0059] SR5, nano-silicon loading: adding hydroxylated nano-silicon in an amount of 20-40% of the mass of the porous carbon carrier to the above reaction system, and stirring and reacting at 80-100℃ for 5-6h;

[0060] SR6, centrifugal drying: performing centrifugal and drying operations on the reaction system;

[0061] SR7, dopant mixing: mixing the dried substance with melamine and ammonium dihydrogen phosphate in a mass ratio of 10:3:1;

[0062] SR8, inert gas atmosphere high-temperature treatment: raising the temperature of the mixed substance to 600℃ at a temperature raising rate of 4℃ / min under an inert gas atmosphere, introducing a mixed gas of hydrogen and inert gas with a hydrogen volume ratio of 5-10% at this temperature, maintaining the temperature for 0.5-1h, and then continuously raising the temperature to 850-950℃ at a temperature raising rate of 4℃ / min, maintaining the temperature for 2-3h;

[0063] SR9, optional silicon oxide buffer layer preparation: placing the substance after the high-temperature treatment in an oxygen atmosphere, maintaining the temperature at 200-300℃ for 1-2h to obtain a porous silicon-carbon battery negative electrode material loaded with nano-silicon and containing a silicon oxide buffer layer; if this step is not performed, a porous silicon-carbon battery negative electrode material loaded with nano-silicon is directly obtained.

[0064] The preparation process of the phenolic resin prepolymer is as follows: phenol and formaldehyde are mixed in a molar ratio of 1:1.2-1:1.5, a sodium hydroxide solution with a mass fraction of 20-30% is added as a catalyst, the catalyst is used in an amount of 5-8% of the mass of the phenol, and the mixture is reacted at 70-80°C for 1.5-2h.

[0065] The bifunctional silane coupling agent is a mixture of γ-(2,3-epoxypropoxy) propyl trimethoxysilane and 3-aminopropyl triethoxysilane, and the molar ratio of the two is 1:1-1:2.

[0066] The preparation process of the hydroxylated nanosilicon is as follows: nanosilicon particles are added to a hydrofluoric acid solution with a mass fraction of 5-10%, ultrasonic treatment is performed at room temperature for 1-2h, centrifugation and washing are performed until neutralization, and drying is performed.

[0067] Working principle:

[0068] In use, the synergistic effect of the carbon source and the template agent. The coconut shell-based carbon itself has a good carbon basis, and the phenolic resin prepolymer serves as another carbon source and can be further carbonized to form a carbon structure in subsequent high-temperature treatment. The hard template agent nanosilica spheres and the soft template agent block copolymer P123 cooperate with each other, the hard template agent provides rigid pore support, and the soft template agent assists in building a hierarchical pore structure by virtue of its self-assembly characteristics. After mixing and reacting these raw materials in a certain proportion, a porous carbon carrier precursor is obtained;

[0069] Subsequently, the precursor is subjected to high-temperature treatment under an inert gas atmosphere, which promotes the carbonization reaction of the carbon source such as the phenolic resin prepolymer to form a relatively stable carbon skeleton, and the structure of the template agent also lays the foundation for the formation of subsequent pores. Then, the precursor is treated with a hydrofluoric acid solution, the hydrofluoric acid reacts with the hard template agent nanosilica spheres to etch and remove them, thereby leaving a large number of uniformly distributed pores in the carbon carrier, and a porous carbon carrier is obtained. These pores not only provide space for the loading of nanosilicon, but also provide channels for ion transport during battery charging and discharging.

[0070] After that, the silicon loading stage is entered, and the bifunctional silane coupling agent (such as a mixture of γ-(2,3-epoxypropoxy) propyl trimethoxysilane and 3-aminopropyl triethoxysilane) plays a key role. One end of the coupling agent can be combined with the active sites on the surface of the porous carbon carrier, and the other end can react with the hydroxyl groups of the hydroxylated nanosilicon (which is obtained by treating nanosilicon particles with a hydrofluoric acid etching solution, and hydroxyl groups are generated on the surface). Through this chemical bonding action, the hydroxylated nanosilicon is firmly loaded into the pores and surface of the porous carbon carrier, forming a preliminary form of silicon-carbon composite structure.

[0071] Melamine and ammonium dihydrogen phosphate are added for doping, and in the subsequent high-temperature treatment in an inert gas atmosphere, melamine will decompose to produce nitrogen elements, and ammonium dihydrogen phosphate will introduce phosphorus elements, which are doped into the carbon structure, can adjust the electronic structure of the carbon material, enhance its conductivity, and also can optimize the surface chemical environment of the material, which is beneficial to the adsorption and desorption of ions in the battery charging and discharging process. Moreover, hydrogen-containing mixed gas is introduced at a specific stage of high-temperature treatment, and hydrogen can reduce the material, further optimize the microstructure and performance of the material;

[0072] Finally, the optional silicon oxide buffer layer preparation step is to place the high-temperature treated material in an oxygen atmosphere, so that part of the nanosilicon is oxidized to form a silicon oxide layer. Since silicon changes greatly in volume during charging and discharging, and silicon oxide has good structural stability and certain flexibility, it can play a buffering role when silicon changes in volume, inhibit the pulverization and rupture of the electrode, and thus improve the cycle stability of the battery anode material; if this step is not performed, the obtained porous silicon-carbon battery anode material loaded with nanosilicon can also rely on the pore structure of the porous carbon carrier to alleviate the volume expansion problem of silicon;

[0073] The above describes the basic principles, main features and advantages of the present application. Those skilled in the art should understand that the present application is not limited to the above-mentioned embodiments, and the above-mentioned embodiments and descriptions in the specification are only the principles of the present application. Without departing from the spirit and scope of the present application, various changes and improvements can be made to the present application, and these changes and improvements all fall within the scope of the claimed present application. The scope of protection claimed by the present application is defined by the appended claims and their equivalents.

Claims

1. A porous silicon-carbon battery anode material loaded with nanosilicon, characterized in that, The method comprises the following steps: S1, raw material mixing reaction: mixing coconut shell-based carbon, phenolic resin prepolymer, hard template agent, and soft template agent according to a proportion, and reacting to obtain a porous carbon carrier precursor; S2, inert gas atmosphere treatment: treating the porous carbon carrier precursor under an inert gas atmosphere; S3, hydrofluoric acid solution treatment: treating the substance treated under the inert gas atmosphere with a hydrofluoric acid solution to obtain a porous carbon carrier; S4, ethanol solution dispersion and silane coupling: dispersing the porous carbon carrier in an ethanol solution, adding a bifunctional silane coupling agent, and reacting; S5, nano-silicon loading: adding hydroxylated nano-silicon to the above reaction system and reacting; S6, centrifugal drying: performing centrifugal and drying operations on the reaction system; S7, dopant mixing: mixing the dried substance with melamine and ammonium dihydrogen phosphate; S8, inert gas atmosphere high-temperature treatment: placing the mixed substance under an inert gas atmosphere for high-temperature treatment; S9, optional silicon oxide buffer layer preparation: placing the substance obtained by high-temperature treatment under an oxygen atmosphere for treatment to obtain a porous silicon-carbon battery negative electrode material loaded with nano-silicon and containing a silicon oxide buffer layer; if this step is not performed, a porous silicon-carbon battery negative electrode material loaded with nano-silicon is directly obtained.

2. The porous silicon-carbon battery anode material loaded with nanosilicon according to claim 1, and a preparation method thereof, characterized in that: The hard template agent is nano-silicon dioxide balls, and the soft template agent is a block copolymer P123.

3. The porous silicon-carbon battery anode material loaded with nanosilicon according to claim 1, and a preparation method thereof, characterized in that: The bifunctional silane coupling agent is a bifunctional silane containing amino and epoxy groups.

4. The porous silicon-carbon battery anode material loaded with nanosilicon according to claim 1, and a preparation method thereof, characterized in that: The hydroxylated nano-silicon is obtained by etching treatment of nano-silicon particles with hydrofluoric acid.

5. A method for preparing a porous silicon-carbon battery anode material loaded with nanosilicon, characterized in that: The method comprises the following steps: SR1, raw material mixing reaction: mixing coconut shell-based carbon, phenolic resin prepolymer, hard template agent, and soft template agent according to a proportion, and stirring and reacting at 80-100°C for 4-6h to obtain a porous carbon carrier precursor; SR2, inert gas atmosphere treatment: raising the temperature of the porous carbon carrier precursor under an inert gas atmosphere to 600°C at a temperature raising rate of 5°C / min, maintaining the temperature for 2h, and then raising the temperature to 800-1000°C at a temperature raising rate of 3°C / min and maintaining the temperature for 3-5h; SR3, hydrofluoric acid solution treatment: treating the substance treated under the inert gas atmosphere with a hydrofluoric acid solution with a mass fraction of 15-20% at 50-60°C for 3-4h to obtain a porous carbon carrier; SR4, ethanol solution dispersion and silane coupling: adding the porous carbon carrier to a mixed solution of ethanol and deionized water with a volume ratio of 3:1-5:1, adding a bifunctional silane coupling agent in an amount of 8-15% of the mass of the porous carbon carrier, and stirring and reacting at 60-80°C for 3-4h; SR5, nano-silicon loading: adding hydroxylated nano-silicon in an amount of 20-40% of the mass of the porous carbon carrier to the above reaction system and stirring and reacting at 80-100°C for 5-6h; SR6, centrifugal drying: performing centrifugal and drying operations on the reaction system; SR7, dopant mixing: mixing the dried substance with melamine and ammonium dihydrogen phosphate according to a mass ratio of 10:3:1; SR8, inert gas atmosphere high temperature treatment: the mixed substance is heated to 600℃ at a rate of 4℃ / min, and then a mixed gas of hydrogen and inert gas with a hydrogen volume ratio of 5-10% is introduced, and the temperature is kept for 0.5-1h, then the temperature is continuously increased to 850-950℃ at a rate of 4℃ / min, and the temperature is kept for 2-3h; SR9, optional silicon oxide buffer layer preparation: the high temperature treated substance is placed in an oxygen atmosphere and kept at 200-300℃ for 1-2h to obtain a porous silicon-carbon battery negative electrode material loaded with nano-silicon containing a silicon oxide buffer layer; if this step is not performed, a porous silicon-carbon battery negative electrode material loaded with nano-silicon is directly obtained.

6. A porous silicon-carbon battery anode material loaded with nanosilicon according to claim 5, characterized in that: The preparation process of the phenolic resin prepolymer is as follows: phenol and formaldehyde are mixed at a molar ratio of 1:1.2-1:1.5, a sodium hydroxide solution with a mass fraction of 20-30% is added as a catalyst, the catalyst is used in an amount of 5-8% of the mass of the phenol, and the reaction is carried out at 70-80℃ for 1.5-2h.

7. The porous silicon-carbon battery anode material loaded with nanosilicon according to claim 5, characterized in that: The bifunctional silane coupling agent is a mixture of γ-(2,3-epoxypropoxy) propyl trimethoxysilane and 3-aminopropyl triethoxysilane, and the molar ratio of the two is 1:1-1:

2.

8. The porous silicon-carbon battery anode material loaded with nanosilicon according to claim 5, characterized in that: The preparation process of the hydroxylated nano-silicon is as follows: nano-silicon particles are added to a hydrofluoric acid solution with a mass fraction of 5-10%, ultrasonic treatment is carried out at room temperature for 1-2h, centrifugation and washing are carried out until neutral, and drying is carried out.

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