Lithium battery electrolyte and preparation method thereof

Abstract

The invention relates to the field of lithium batteries, in particular to a lithium battery electrolyte and a preparation method thereof. The invention provides a lithium battery electrolyte. The lithium battery electrolyte is composed of the following components in percentage by mass: 65-75% of polycarbonate; 1.9 to 2.1% of ethylene sulfate; 15-20% of a lithium salt; 1-2% of polyethylene glycol; and 0.5 to 1.5 percent of 2, 2-diethoxy-1-(trimethylsilyl)-1-aza-2-silacyclopentane and 0.5 to 1.5 percent of 2, 2-diethoxy-1-(trimethylsilyl)-1-aza-2-silacyclopentane. The invention also provides a preparation method of the lithium battery electrolyte. The preparation method comprises the following steps: S1, preparing lithium salt; S2, preparing the lithium battery electrolyte. In the preparation process, proper mixing reaction temperature and pressure are controlled, so that the raw materials can be fully dissolved and mixed; the organic solvent needs to be dried in advance, so that the influence of moisture on the performance of the electrolyte is avoided.

Description

Technical Field

[0001] The present invention relates to the field of lithium batteries, and in particular to a lithium battery electrolyte and a preparation method thereof. Background Art

[0002] In the context of the rapid development of industries such as electric vehicles, electric light vehicles, power tools, consumer electronics and new energy storage, the comprehensive advantages of lithium batteries are consistent with the increasing demand for large capacity, high power, service life and environmental protection in downstream fields, and there is a broad market application prospect. The advantages of lithium batteries mainly include high energy density, long cycle life, low self-discharge rate, etc. These advantages make lithium batteries an indispensable power source for modern electronic devices. With the rapid development of electric vehicles and energy storage systems, the demand for lithium batteries will continue to grow. As an important component of lithium batteries, electrolyte additives will play an important role in improving battery performance, reliability and safety.

[0003] Lithium battery electrolyte is the carrier of ion transmission in the battery and is the guarantee for lithium-ion batteries to obtain advantages such as high voltage and high specific energy. The electrolyte is generally made of high-purity organic solvents, electrolyte lithium salts, necessary additives and other raw materials, prepared under certain conditions and in a certain proportion.

[0004] With the advancement of science and technology, the electrolyte industry is also constantly innovating and upgrading, and has made breakthroughs in the technology of high-temperature and high-pressure electrolytes. However, there are still some problems. The electrolyte is highly corrosive, flammable, and prone to bulging and rupture. Therefore, it is necessary to invent an electrolyte product with better performance, lower cost, and greater environmental protection. Summary of the invention

[0005] In view of the above-mentioned existing technical problems, the present invention provides a lithium battery electrolyte and a preparation method thereof. The lithium battery electrolyte prepared by this method has better performance, lower cost and is more environmentally friendly.

[0006] The present invention discloses a lithium battery electrolyte, which is composed of the following components in mass ratio:

[0007] Polycarbonate 65-75%; vinyl sulfate 1.9-2.1%; lithium salt 15-20%; polyethylene glycol 1-2%; 2,2-diethoxy-1-(trimethylsilyl)-1-aza-2-silacyclopentane 0.5-1.5%.

[0008] The present invention also discloses a method for preparing a lithium battery electrolyte, comprising the following steps:

[0009] S1 Preparation of lithium salt: Add lithium tetrafluoroborate to a high-pressure reactor, then add acetonitrile and mix well, then add oxalic acid while stirring, after the addition is complete, seal the reactor and evacuate, set the reaction temperature and reaction pressure to react, after the reaction is complete, take out the reaction solution, separate, purify and dry it to obtain lithium salt;

[0010] Preparation of S2 lithium battery electrolyte: add the lithium salt prepared in S1 to the reactor, add dried polycarbonate and vinyl sulfate, and then heat and stir. After the lithium salt is completely dissolved, add polyethylene glycol and 2,2-diethoxy-1-(trimethylsilyl)-1-aza-2-silacyclopentane in sequence for mixed reaction to obtain a reaction mixture. After the reaction mixture is cooled, it is filtered and purified to obtain a lithium battery electrolyte.

[0011] Preferably, in the S1 lithium salt preparation step, the molar ratio of lithium tetrafluoroborate to oxalic acid is 1:(0.8-1.3).

[0012] Preferably, in the S1 lithium salt preparation step, the mass ratio of lithium tetrafluoroborate to acetonitrile is 1:(15-20).

[0013] Preferably, in the step of preparing the S1 lithium salt, the reaction temperature is 120-150°C.

[0014] Preferably, in the S1 lithium salt preparation step, the reaction pressure is 2-3 MPa.

[0015] Preferably, in the S1 lithium salt preparation step, the drying is vacuum drying with a vacuum degree of 0.5 to 1 MPa.

[0016] Preferably, in the step of preparing the S2 lithium battery electrolyte, the temperature of heating and stirring is 50-80° C.; the rate of heating and stirring is 100-500 rpm.

[0017] Preferably, in the step of preparing the S2 lithium battery electrolyte, the temperature of the mixing reaction is 40-60°C.

[0018] Preferably, in the step of preparing the S2 lithium battery electrolyte, the reaction mixture is cooled to 20-25° C.; the filtration is performed by vacuum filtration, and the mesh size of the filter used is 200-300 meshes.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] The invention provides a lithium battery electrolyte. The organic solvent used in the raw materials is a mixed solvent of polycarbonate and vinyl sulfate, which can be used in combination with electrode materials, thereby improving the reaction activity and structural stability of the electrode and effectively reducing the problem of battery bulging. 2,2-diethoxy-1-(trimethylsilyl)-1-aza-2-silacyclopentane is added to the raw materials to reduce the viscosity of the electrolyte, improve the fluidity, and improve the wettability of the electrode, thereby improving the charge and discharge performance of the battery. LiDFBOP is selected as the lithium salt, which can be reduced preferentially to the electrolyte solvent and the lithium salt to form a thin and uniform solid electrolyte interface (SEI) film with higher mechanical strength. The SEI film has a lower interface impedance, is conducive to the transmission of lithium ions, thereby improving the cycle stability and low-temperature performance of the battery.

[0021] The present invention also provides a method for preparing a lithium battery electrolyte. In the S2 lithium battery electrolyte preparation step, the appropriate mixing reaction temperature and pressure are controlled so that the raw materials can be fully dissolved and mixed; the organic solvent needs to be dried in advance to avoid the influence of moisture on the performance of the electrolyte. DETAILED DESCRIPTION

[0022] The following examples are provided for a better understanding of the present invention, but are not intended to limit the best mode of implementation, nor to limit the content and protection scope of the present invention. Any product identical or similar to the present invention obtained by anyone under the inspiration of the present invention or by combining the features of the present invention with other prior arts shall fall within the protection scope of the present invention.

[0023] If no specific experimental steps or conditions are specified in the examples, the conventional experimental steps or conditions described in the literature in the field can be used. If no manufacturer is specified for the reagents or instruments used, they are all conventional reagent products that can be obtained commercially.

[0024] Embodiment 1: A lithium battery electrolyte, comprising the following components in mass ratio:

[0025] Polycarbonate 65%; vinyl sulfate 1.9%; lithium salt 15%; polyethylene glycol 1%; 2,2-diethoxy-1-(trimethylsilyl)-1-aza-2-silacyclopentane 0.5%.

[0026] A method for preparing a lithium battery electrolyte comprises the following steps:

[0027] Preparation of S1 lithium salt: Add lithium tetrafluoroborate to a high-pressure reactor, and then add acetonitrile and mix evenly, wherein the mass ratio of lithium tetrafluoroborate to acetonitrile is 1:15; then add oxalic acid while stirring, and the molar ratio of lithium tetrafluoroborate to oxalic acid is 1:0.8. After the addition is completed, seal the reactor and evacuate it, set the reaction temperature to 120°C, and the reaction pressure to 2MPa for reaction. After the reaction is completed, take out the reaction liquid for separation and filtration, and collect the filtrate. The collected filtrate is first concentrated under reduced pressure, and then vacuum dried to obtain lithium salt, wherein the vacuum degree of vacuum drying is 0.5MPa, and the drying temperature is 50-55°C.

[0028] Preparation of S2 lithium battery electrolyte: Add the lithium salt prepared in S1 to the reactor, add dried polycarbonate and vinyl sulfate, and then heat and stir. The heating and stirring temperature is 50°C and the stirring rate is 100rpm. After the lithium salt is completely dissolved, polyethylene glycol and 2,2-diethoxy-1-(trimethylsilyl)-1-aza-2-silacyclopentane are added in sequence for mixed reaction. The reaction temperature of the mixed reaction is 40°C to obtain a reaction mixture. The reaction mixture is cooled to 20°C and filtered for purification. The filter mesh used for filtration is 200 mesh. After filtering 2 to 3 times, the combined filtrate is collected. The collected filtrate is the lithium battery electrolyte.

[0029] Embodiment 2: A lithium battery electrolyte, comprising the following components in mass ratio:

[0030] Polycarbonate 67%; vinyl sulfate 1.95%; lithium salt 16%; polyethylene glycol 1.2%; 2,2-diethoxy-1-(trimethylsilyl)-1-aza-2-silacyclopentane 0.7%.

[0031] A method for preparing a lithium battery electrolyte comprises the following steps:

[0032] Preparation of S1 lithium salt: Add lithium tetrafluoroborate to a high-pressure reactor, and then add acetonitrile and mix evenly, wherein the mass ratio of lithium tetrafluoroborate to acetonitrile is 1:16; then add oxalic acid while stirring, and the molar ratio of lithium tetrafluoroborate to oxalic acid is 1:0.9. After the addition is completed, seal the reactor and evacuate it, set the reaction temperature to 125°C, and the reaction pressure to 2.2MPa for reaction. After the reaction is completed, take out the reaction liquid for separation and filtration, and collect the filtrate. The collected filtrate is first concentrated under reduced pressure, and then vacuum dried to obtain lithium salt, wherein the vacuum degree of vacuum drying is 0.6MPa, and the drying temperature is 50-55°C.

[0033] Preparation of S2 lithium battery electrolyte: add the lithium salt prepared in S1 to the reactor, add dried polycarbonate and vinyl sulfate, and then heat and stir. The heating and stirring temperature is 55°C and the stirring rate is 200rpm. After the lithium salt is completely dissolved, polyethylene glycol and 2,2-diethoxy-1-(trimethylsilyl)-1-aza-2-silacyclopentane are added in sequence for mixed reaction. The reaction temperature of the mixed reaction is 45°C to obtain a reaction mixture. The reaction mixture is cooled to 21°C and filtered for purification. The filter mesh used for filtration is 220 mesh. After filtering 2 to 3 times, the combined filtrate is collected. The collected filtrate is the lithium battery electrolyte.

[0034] Embodiment 3: A lithium battery electrolyte, comprising the following components in mass ratio:

[0035] Polycarbonate 69%; vinyl sulfate 2%; lithium salt 17%; polyethylene glycol 1.4%; 2,2-diethoxy-1-(trimethylsilyl)-1-aza-2-silacyclopentane 0.9%.

[0036] A method for preparing a lithium battery electrolyte comprises the following steps:

[0037] Preparation of S1 lithium salt: Add lithium tetrafluoroborate to a high-pressure reactor, and then add acetonitrile and mix evenly, wherein the mass ratio of lithium tetrafluoroborate to acetonitrile is 1:17; then add oxalic acid while stirring, and the molar ratio of lithium tetrafluoroborate to oxalic acid is 1:1. After the addition is completed, the reactor is sealed and evacuated, and the reaction temperature is set to 130°C and the reaction pressure is set to 2.4MPa for reaction. After the reaction is completed, the reaction liquid is taken out for separation and filtration, and the filtrate is collected. The collected filtrate is first concentrated under reduced pressure and then vacuum dried to obtain lithium salt, wherein the vacuum degree of vacuum drying is 0.7MPa and the drying temperature is 50-55°C.

[0038] Preparation of S2 lithium battery electrolyte: Add the lithium salt prepared in S1 to the reactor, add dried polycarbonate and vinyl sulfate, and then heat and stir. The heating and stirring temperature is 60°C and the stirring rate is 300rpm. After the lithium salt is completely dissolved, polyethylene glycol and 2,2-diethoxy-1-(trimethylsilyl)-1-aza-2-silacyclopentane are added in sequence for mixed reaction. The reaction temperature of the mixed reaction is 50°C to obtain a reaction mixture. The reaction mixture is cooled to 22°C and filtered for purification. The mesh size of the filter used for filtration is 240 mesh. After filtering 2 to 3 times, the combined filtrate is collected. The collected filtrate is the lithium battery electrolyte.

[0039] Embodiment 4: A lithium battery electrolyte, comprising the following components in mass ratio:

[0040] Polycarbonate 71%; vinyl sulfate 2.05%; lithium salt 18%; polyethylene glycol 1.6%; 2,2-diethoxy-1-(trimethylsilyl)-1-aza-2-silacyclopentane 1.1%.

[0041] A method for preparing a lithium battery electrolyte comprises the following steps:

[0042] Preparation of S1 lithium salt: lithium tetrafluoroborate is added to a high-pressure reactor, and then acetonitrile is added and mixed evenly, wherein the mass ratio of lithium tetrafluoroborate to acetonitrile is 1:18; then oxalic acid is added while stirring, and the molar ratio of lithium tetrafluoroborate to oxalic acid is 1:1.1. After the addition is completed, the reactor is sealed and evacuated, and the reaction temperature is set to 135°C and the reaction pressure is set to 2.6MPa for reaction. After the reaction is completed, the reaction liquid is taken out for separation and filtration, and the filtrate is collected. The collected filtrate is first concentrated under reduced pressure and then vacuum dried to obtain lithium salt, wherein the vacuum degree of vacuum drying is 0.8MPa and the drying temperature is 50-55°C.

[0043] Preparation of S2 lithium battery electrolyte: add the lithium salt prepared in S1 to the reactor, add dried polycarbonate and vinyl sulfate, and then heat and stir. The heating and stirring temperature is 65°C and the stirring rate is 400rpm. After the lithium salt is completely dissolved, polyethylene glycol and 2,2-diethoxy-1-(trimethylsilyl)-1-aza-2-silacyclopentane are added in sequence for mixed reaction. The reaction temperature of the mixed reaction is 55°C to obtain a reaction mixture. The reaction mixture is cooled to 23°C and filtered for purification. The mesh size of the filter used for filtration is 260 mesh. After filtering 2 to 3 times, the combined filtrate is collected. The collected filtrate is the lithium battery electrolyte.

[0044] Embodiment 5: A lithium battery electrolyte, comprising the following components in mass ratio:

[0045] Polycarbonate 73%; vinyl sulfate 2.1%; lithium salt 19%; polyethylene glycol 1.8%; 2,2-diethoxy-1-(trimethylsilyl)-1-aza-2-silacyclopentane 1.3%.

[0046] A method for preparing a lithium battery electrolyte comprises the following steps:

[0047] Preparation of S1 lithium salt: add lithium tetrafluoroborate to a high-pressure reactor, and then add acetonitrile and mix evenly, wherein the mass ratio of lithium tetrafluoroborate to acetonitrile is 1:19; then add oxalic acid while stirring, and the molar ratio of lithium tetrafluoroborate to oxalic acid is 1:1.2. After the addition is completed, seal the reactor and evacuate it, set the reaction temperature to 140°C, and the reaction pressure to 2.8MPa for reaction. After the reaction is completed, take out the reaction liquid for separation and filtration, and collect the filtrate. The collected filtrate is first concentrated under reduced pressure, and then vacuum dried to obtain lithium salt, wherein the vacuum degree of vacuum drying is 0.9MPa, and the drying temperature is 50-55°C.

[0048] Preparation of S2 lithium battery electrolyte: Add the lithium salt prepared in S1 to the reactor, add dried polycarbonate and vinyl sulfate, and then heat and stir. The heating and stirring temperature is 70°C and the stirring rate is 500rpm. After the lithium salt is completely dissolved, polyethylene glycol and 2,2-diethoxy-1-(trimethylsilyl)-1-aza-2-silacyclopentane are added in sequence for mixed reaction. The reaction temperature of the mixed reaction is 55°C to obtain a reaction mixture. The reaction mixture is cooled to 24°C and filtered for purification. The filter mesh used for filtration is 280 mesh. After filtering 2 to 3 times, the combined filtrate is collected. The collected filtrate is the lithium battery electrolyte.

[0049] Embodiment 6: A lithium battery electrolyte, comprising the following components in mass ratio:

[0050] Polycarbonate 75%; vinyl sulfate 2.1%; lithium salt 20%; polyethylene glycol 2%; 2,2-diethoxy-1-(trimethylsilyl)-1-aza-2-silacyclopentane 1.5%.

[0051] A method for preparing a lithium battery electrolyte comprises the following steps:

[0052] Preparation of S1 lithium salt: Add lithium tetrafluoroborate to a high-pressure reactor, and then add acetonitrile and mix evenly, wherein the mass ratio of lithium tetrafluoroborate to acetonitrile is 1:20; then add oxalic acid while stirring, and the molar ratio of lithium tetrafluoroborate to oxalic acid is 1:1.3. After the addition is completed, seal the reactor and evacuate it, set the reaction temperature to 150°C, and the reaction pressure to 3MPa for reaction. After the reaction is completed, take out the reaction liquid for separation and filtration, and collect the filtrate. The collected filtrate is first concentrated under reduced pressure, and then vacuum dried to obtain lithium salt, wherein the vacuum degree of vacuum drying is 1MPa, and the drying temperature is 50-55°C.

[0053] Preparation of S2 lithium battery electrolyte: add the lithium salt prepared in S1 to the reactor, add dried polycarbonate and vinyl sulfate, and then heat and stir. The heating and stirring temperature is 80°C and the stirring rate is 500rpm. After the lithium salt is completely dissolved, polyethylene glycol and 2,2-diethoxy-1-(trimethylsilyl)-1-aza-2-silacyclopentane are added in sequence for mixed reaction. The reaction temperature of the mixed reaction is 60°C to obtain a reaction mixture. The reaction mixture is cooled to 25°C and filtered for purification. The filter mesh used for filtration is 300 mesh. After filtering 2 to 3 times, the combined filtrate is collected. The collected filtrate is the lithium battery electrolyte.

[0054] Comparative Example 1: A lithium battery electrolyte, comprising the following components in mass ratio:

[0055] Polycarbonate 71%; lithium salt 18%; polyethylene glycol 1.6%; 2,2-diethoxy-1-(trimethylsilyl)-1-aza-2-silacyclopentane 1.1%.

[0056] The preparation method and condition parameters of the lithium battery electrolyte are the same as those in Example 4.

[0057] Comparative Example 2: A lithium battery electrolyte, comprising the following components in mass ratio:

[0058] Polycarbonate 71%; Vinyl sulfate 2.05%; Lithium salt 18%; Polyethylene glycol 1.6%;

[0059] The preparation method and condition parameters of the lithium battery electrolyte are the same as those in Example 4.

[0060] The properties of the lithium battery electrolytes prepared in Examples 1 to 6 and Comparative Examples 1 to 2 were tested, and the test results are shown in the following table:

[0061] Examples and Comparative Examples Moisture content (%) <![CDATA[H + Concentration (mol / L)]]> <![CDATA[Density (g / cm 3 )]]> Conductivity (S / cm) Example 1 1.52 0.16 1.25 <![CDATA[10 -4 ]]> Example 2 1.45 0.15 1.20 <![CDATA[10 -3 ]]> Example 3 1.32 0.13 1.16 <![CDATA[10 -2 ]]> Example 4 1.23 0.11 1.12 <![CDATA[10 -2 ]]> Example 5 1.24 0.12 1.11 <![CDATA[10 -2 ]]> Example 6 1.22 0.11 1.13 <![CDATA[10 -2 ]]> Comparative Example 1 1.26 0.13 1.22 <![CDATA[10 -5 ]]> Comparative Example 2 1.30 0.15 1.32 <![CDATA[10 -4 ]]>

[0062] From the test data in the above table, it can be seen that the performance of the electrolytes prepared in Examples 1 to 6 is different due to the different raw material ratios and different conditions and parameters in the preparation process. Among them, the electrolyte prepared in Example 4 has a low moisture content, a moderate H+ concentration and density, and a high conductivity, which indicates that this electrolyte has the best performance. The electrolyte prepared in Comparative Example 2 has a high density and poor conductivity. This is because the raw materials in Comparative Example 2 lack 2,2-diethoxy-1-(trimethylsilyl)-1-aza-2-silacyclopentane.

[0063] Adding this compound to the raw materials can reduce the viscosity of the electrolyte, increase fluidity, and improve the wettability of the electrode, thereby improving the charge and discharge performance of the battery.

[0064] The lithium battery electrolytes prepared in Examples 1 to 6 and Comparative Examples 1 to 2 were injected into the outer packaging, and after vacuum packaging, standing and other processes, lithium batteries were prepared. The prepared lithium batteries were subjected to performance tests, and the test results are shown in the following table:

[0065]

[0066] It can be seen from the test results in the above table that compared with Examples 1 to 6, the lithium batteries made from the electrolytes of Comparative Examples 1 and 2 have obvious changes in internal resistance, reduced discharge capacity, and reduced median voltage after high-temperature storage. This is because the raw materials of Comparative Example 2 lack 2,2-diethoxy-1-(trimethylsilyl)-1-aza-2-silacyclopentane; and when the mixed solvent of polycarbonate and vinyl sulfate can be used with the electrode material, the reaction activity and structural stability of the electrode can be improved, and the problem of battery bulging can be effectively reduced; adding 2,2-diethoxy-1-(trimethylsilyl)-1-aza-2-silacyclopentane to the raw materials can reduce the viscosity of the electrolyte, improve the fluidity, and improve the wettability of the electrode, thereby improving the charge and discharge performance of the battery.

[0067] Obviously, the above embodiments are merely examples for the purpose of clear explanation, and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived from these are still within the scope of protection of the invention.

Claims

1. A lithium battery electrolyte, characterized in that: It is composed of the following ingredients in mass ratio: Polycarbonate 65-75%; vinyl sulfate 1.9-2.1%; lithium salt 15-20%; polyethylene glycol 1-2%; 2,2-diethoxy-1-(trimethylsilyl)-1-aza-2-silacyclopentane 0.5-1.5%.

2. A method for preparing a lithium battery electrolyte, characterized in that: The following steps are involved: S1 Preparation of lithium salt: Add lithium tetrafluoroborate to a high-pressure reactor, then add acetonitrile and mix well, then add oxalic acid while stirring, after the addition is complete, seal the reactor and evacuate, set the reaction temperature and reaction pressure to react, after the reaction is complete, take out the reaction solution, separate, purify and dry it to obtain lithium salt; Preparation of S2 lithium battery electrolyte: add the lithium salt prepared in S1 to the reactor, add dried polycarbonate and vinyl sulfate, and then heat and stir. After the lithium salt is completely dissolved, add polyethylene glycol and 2,2-diethoxy-1-(trimethylsilyl)-1-aza-2-silacyclopentane in sequence for mixed reaction to obtain a reaction mixture. After the reaction mixture is cooled, it is filtered and purified to obtain a lithium battery electrolyte.

3. The method for preparing a lithium battery electrolyte according to claim 2, characterized in that: In the S1 lithium salt preparation step, the molar ratio of lithium tetrafluoroborate to oxalic acid is 1:(0.8-1.3).

4. The method for preparing a lithium battery electrolyte according to claim 2, characterized in that: In the S1 lithium salt preparation step, the mass ratio of lithium tetrafluoroborate to acetonitrile is 1:(15-20).

5. The method for preparing a lithium battery electrolyte according to claim 2, characterized in that: In the S1 lithium salt preparation step, the reaction temperature is 120-150°C.

6. The method for preparing a lithium battery electrolyte according to claim 2, characterized in that: In the S1 lithium salt preparation step, the reaction pressure is 2 to 3 MPa.

7. The method for preparing a lithium battery electrolyte according to claim 2, characterized in that: In the S1 lithium salt preparation step, the drying is vacuum drying with a vacuum degree of 0.5 to 1 MPa.

8. The method for preparing a lithium battery electrolyte according to claim 2, characterized in that: In the S2 lithium battery electrolyte preparation step, the heating and stirring temperature is 50 to 80° C.; the heating and stirring rate is 100 to 500 rpm.

9. The method for preparing a lithium battery electrolyte according to claim 2, characterized in that: In the S2 lithium battery electrolyte preparation step, the temperature of the mixing reaction is 40 to 60°C.

10. The method for preparing a lithium battery electrolyte according to claim 2, characterized in that: In the step of preparing the S2 lithium battery electrolyte, the reaction mixture is cooled to 20-25° C.; the filtration is performed by vacuum filtration, and the mesh size of the filter used is 200-300 meshes.

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