CN116082544A - Polyvinylidene fluoride and preparation method thereof, positive plate and lithium ion battery - Google Patents
Polyvinylidene fluoride and preparation method thereof, positive plate and lithium ion battery Download PDFInfo
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- C—CHEMISTRY; METALLURGY
- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
- C08F114/00—Homopolymers of compounds having one or more unsaturated aliphatic radicals, each having only one carbon-to-carbon double bond, and at least one being terminated by a halogen
- C08F114/18—Monomers containing fluorine
- C08F114/22—Vinylidene fluoride
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- C—CHEMISTRY; METALLURGY
- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
- C08F2/00—Processes of polymerisation
- C08F2/38—Polymerisation using regulators, e.g. chain terminating agents, e.g. telomerisation
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- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
- C08F2/00—Processes of polymerisation
- C08F2/46—Polymerisation initiated by wave energy or particle radiation
- C08F2/48—Polymerisation initiated by wave energy or particle radiation by ultraviolet or visible light
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M10/00—Secondary cells; Manufacture thereof
- H01M10/05—Accumulators with non-aqueous electrolyte
- H01M10/052—Li-accumulators
- H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
- H01M4/621—Binders
- H01M4/622—Binders being polymers
- H01M4/623—Binders being polymers fluorinated polymers
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M2004/026—Electrodes composed of, or comprising, active material characterised by the polarity
- H01M2004/028—Positive electrodes
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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Abstract
The invention relates to the technical field of lithium battery materials, and discloses polyvinylidene fluoride, a preparation method thereof, a positive plate and a lithium ion battery. A process for the preparation of polyvinylidene fluoride comprising: under a light-shielding environment, adding a photoinitiator into a solution dissolved with a RAFT reagent and vinylidene fluoride, and sealing a transparent reaction container; the reaction vessel is irradiated under visible light, so that polymerization reaction occurs in the vessel; after the reaction has ended, the polymerization product is withdrawn from the reaction vessel. Polyvinylidene fluoride is prepared by the preparation method. The positive plate is prepared from the polyvinylidene fluoride. The lithium ion battery comprises the positive plate. The PVDF prepared by the preparation method provided by the invention has the advantages of narrow molecular weight distribution and good consistency of the adhesion force, and the lithium battery prepared by the PVDF has the advantages that the phenomenon of material dropping at certain sites firstly caused by inconsistent adhesion force can not occur under long-cycle conditions.
Description
Technical Field
The invention relates to the technical field of lithium battery materials, in particular to polyvinylidene fluoride, a preparation method thereof, a positive plate and a lithium ion battery.
Background
The traditional PVDF synthesis method is thermal-initiated uncontrollable free radical polymerization, has wider molecular weight distribution, contains a small molecular weight polymer and a high molecular weight polymer, and causes inconsistent adhesion between foil and slurry and uneven stripping force of the pole piece when the material is applied to a lithium battery positive pole piece. In the long-cycle battery, the binding force requirement for the tab binder is further increased. The PVDF with high binding force and high binding force consistency can lead the positive plate of the lithium battery to not be subjected to the phenomenon of material dropping at certain sites firstly due to inconsistent binding force under long circulation, thereby prolonging the circulation period of the battery.
In view of this, the present invention has been made.
Disclosure of Invention
The invention aims to provide polyvinylidene fluoride, a preparation method thereof, a positive plate and a lithium ion battery.
The invention is realized in the following way:
in a first aspect, the present invention provides a method for preparing polyvinylidene fluoride, comprising:
under a light-shielding environment, adding a photoinitiator into a solution dissolved with a RAFT reagent and vinylidene fluoride, and sealing a transparent reaction container;
the reaction vessel is irradiated under visible light, so that polymerization reaction occurs in the vessel;
after the reaction has ended, the polymerization product is withdrawn from the reaction vessel.
In an alternative embodiment, the RAFT agent to VDF to photoinitiator molar ratio is 1:x:0.03 to 0.1, x >0; reaction termination refers to a reaction that is sufficient to no longer occur.
In an alternative embodiment, the RAFT agent to VDF to photoinitiator molar ratio is 1:x:0.03 to 0.1, x >0; the reaction is terminated by opening the reaction vessel to expose the reaction system to air;
preferably, the reaction time has a correlation with the degree of polymerization under the same reaction conditions, and the reaction system is exposed to air at a reaction time node corresponding to the target degree of polymerization to obtain polyvinylidene fluoride of the target degree of polymerization;
preferably, the reaction time corresponding to the target polymerization degree is determined by:
exposing the reaction system to air for terminating the reaction at different time nodes for a plurality of times under the same reaction condition, extracting a product sample to measure the polymerization degree of the product sample, and determining the reaction time required for the reaction to reach the target polymerization degree according to a plurality of test results;
preferably, the polymerization degree is measured by hydrogen nuclear magnetic resonance spectroscopy, and the polymerization degree of the product is calculated using a terminal group method or an internal standard method.
In an alternative embodiment, the total solids content of the reaction system is from 10 to 50%.
In an alternative embodiment, the method further comprises removing oxygen from the solution in the reaction vessel prior to sealing the transparent reaction vessel;
preferably, the removal method is a bubbling method or a freeze-degassing method by passing inert gas.
In an alternative embodiment, the RAFT agent is selected from at least one of bis/trithioester derivatives;
preferably, the bis/trithioester derivative is 2-mercapto-S-thiobenzoylacetic acid, tetramethylthiuram disulfide, S-dibenzyl trithiocarbonate or 2- (dodecylthio thiocarbonylthio) -2-methylpropanoic acid;
preferably, the photoinitiator is selected from at least one of photoinitiators TPO, TPO-L, BAPO;
preferably, the solvent used in the reaction system is at least one selected from DMC, DMF, DMAc and NMP.
In an alternative embodiment, the means for withdrawing the polymerization product from the reaction vessel comprises:
distilling the mixture in the reaction vessel to remove the solvent after the reaction is finished, or removing the solvent in vacuum to obtain a crude product;
dissolving the crude product in tetrahydrofuran to obtain PVDF solution;
dropping PVDF solution into frozen ethane, and precipitating PVDF to obtain a solid-liquid mixture;
filtering and drying the solid-liquid mixture to obtain a product;
preferably, the temperature of the frozen ethane is 0-4 ℃;
preferably, the temperature of the tetrahydrofuran is 35-45 ℃.
In a second aspect, the present invention provides a polyvinylidene fluoride produced by the method of any one of the preceding embodiments.
In a third aspect, the present invention provides a positive electrode sheet, the preparation raw material of which comprises the polyvinylidene fluoride material according to the foregoing embodiment.
In a fourth aspect, the present invention provides a lithium ion battery comprising the positive electrode sheet of the foregoing embodiment.
The invention has the following beneficial effects:
according to the preparation method of polyvinylidene fluoride, the PVDF prepared by the preparation method is narrow in molecular weight distribution, good in consistency of bonding force, and the lithium battery prepared by the PVDF is free from the phenomenon that the positive plate is firstly dropped in certain sites due to inconsistent bonding force under long circulation. In addition, the polymerization mode of the preparation method provided by the application is visible light room temperature active polymerization, the polymerization rate is high, the initiation mode is visible light initiation, the light source is a light source harmful to the body such as visible light and non-ultraviolet light, the initiation temperature is room temperature, and no additional heat source is required to be acquired.
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present invention more clear, the technical solutions of the embodiments of the present invention will be clearly and completely described below. The specific conditions are not noted in the examples and are carried out according to conventional conditions or conditions recommended by the manufacturer. The reagents or apparatus used were conventional products commercially available without the manufacturer's attention.
The polyvinylidene fluoride, the preparation method thereof, the positive plate and the lithium ion battery provided by the embodiment of the application are specifically described below.
The preparation method of polyvinylidene fluoride provided by the embodiment of the application comprises the following steps:
under a light-shielding environment, adding a photoinitiator into a solution dissolved with a RAFT reagent and vinylidene fluoride, and sealing a transparent reaction container;
the reaction vessel is irradiated under visible light, so that polymerization reaction occurs in the vessel;
after the reaction has ended, the polymerization product is withdrawn from the reaction vessel.
According to the preparation method of polyvinylidene fluoride, the PVDF prepared by the preparation method is narrow in molecular weight distribution, good in consistency of PVDF adhesive force, and the lithium battery prepared by the PVDF adhesive force can not be subjected to material dropping phenomenon at certain sites due to inconsistent adhesive force under long circulation. In addition, the polymerization mode of the preparation method provided by the application is visible light room temperature active polymerization, the polymerization rate is high, the initiation mode is visible light initiation, the light source is a light source harmful to the body such as visible light and non-ultraviolet light, the initiation temperature is room temperature, and no additional heat source is required to be acquired.
Specifically, the preparation method of polyvinylidene fluoride provided by the embodiment of the application comprises the following preparation steps:
s1, preparing materials
VDF (vinylidene fluoride), solvent, transparent reaction vessel, photoinitiator, RAFT agent, and visible light source (mercury lamp) were prepared.
The solvent may be at least one of DMC (dimethyl carbonate), DMF, DMAc and NMP. The photoinitiator may be at least one of a photoinitiator TPO (2, 4, 6-trimethylbenzoyl diphenylphosphine oxide), TPO-L (ethyl 2,4, 6-trimethylbenzoyl phenylphosphonate) and BAPO (bis (2, 4, 6-trimethylbenzoyl) diphenylphosphine oxide).
The RAFT agent is selected from at least one of bis/trithioester derivatives; the bis/trithio ester derivative is 2-mercapto-S-thiobenzoylacetic acid, tetramethylthiuram disulfide, S-dibenzyl trithiocarbonate or 2- (dodecyl thio thiocarbonylthio) -2-methylpropanoic acid.
S2, reaction
Adding a solvent and a RAFT reagent into a transparent reaction container, filling VDF gas into the transparent container, and stirring the magnetic particles to fully dissolve the VDF gas; adding a photoinitiator in a light-shielding environment, and removing oxygen in the reaction bottle by a bubbling method or a freezing-degassing method by introducing inert gas (such as nitrogen or argon) into the mixed solution; finally, the reaction bottle is exposed to the irradiation of a visible light source mercury lamp, and then the polymerization reaction can be initiated.
The temperature has a certain influence on the reaction rate, and the reaction can be accelerated by increasing the reaction temperature or can be slowed down by decreasing the reaction temperature.
In addition, the intensity of visible light and the amount of photoinitiator also have a certain effect on the reaction rate, the higher the intensity of visible light, the faster the reaction rate, and conversely, the slower the reaction rate; the higher the concentration of photoinitiator in the range of RAFT agent to 1:0.03-0.1, the faster the reaction, and vice versa. Thus, the visible light intensity and the concentration of the photoinitiator added can also be adjusted if the reaction speed is to be controlled.
Preferably, the total solids content of the reaction system is 10 to 50% (e.g., 10%, 20%, 30%, 40% or 50) in order that the reaction may proceed smoothly and normally.
S3, termination reaction
The RAFT reagent is designed, the molar ratio of VDF to photoinitiator is 1:x:0.03-0.1, x is >0. Under the reaction conditions designed in the present application, if VDF is added to the reaction system, the polymerization reaction proceeds. The amount to be added can be arbitrarily selected as required.
There are two ways to terminate the reaction:
first kind: the reaction is sufficient until no more polymerization takes place.
Second kind: the reaction vessel was opened and the reaction was terminated by exposing the reaction system to air.
Preferably, the reaction time has a correlation with the degree of polymerization under the same reaction conditions, and the reaction system is exposed to air at a reaction time node corresponding to the target degree of polymerization to obtain polyvinylidene fluoride of the target degree of polymerization;
preferably, the reaction time corresponding to the target polymerization degree is determined by:
exposing the reaction system to air for terminating the reaction at different time nodes for a plurality of times under the same reaction condition, extracting a product sample to measure the polymerization degree of the product sample, and determining the reaction time required for the reaction to reach the target polymerization degree according to a plurality of test results;
further, the manner of determining the reaction time required for the reaction to the target polymerization degree from the plurality of test results may be, for example: and drawing a standard curve according to a plurality of test results, and obtaining the reaction time corresponding to the target polymerization degree according to the standard curve.
Preferably, the polymerization degree is measured by hydrogen nuclear magnetic resonance spectroscopy, and the polymerization degree of the product is calculated using a terminal group method or an internal standard method.
S4, purifying
Distilling the mixture in the reaction vessel to remove the solvent after the reaction is finished, or removing the solvent in vacuum to obtain a crude product;
dissolving the crude product in tetrahydrofuran to obtain PVDF solution;
dropping PVDF solution into frozen ethane, and precipitating PVDF to obtain a solid-liquid mixture;
and filtering and drying the solid-liquid mixture to obtain the product.
Preferably, to ensure adequate precipitation of PVDF, the temperature of the frozen ethane is between 0 and 4deg.C (e.g., 0deg.C, 2deg.C, 3deg.C, or 4deg.C);
preferably, to ensure a dissolution rate of the crude product, the temperature of tetrahydrofuran is 35 to 45 ℃ (e.g., 35 ℃, 40 ℃ or 45 ℃).
The polyvinylidene fluoride provided by the embodiment of the invention is prepared by adopting the preparation method provided by the embodiment of the invention. The polyvinylidene fluoride has the characteristic of narrow molecular weight distribution.
The preparation raw materials of the positive plate provided by the embodiment of the invention comprise the polyvinylidene fluoride material provided by the embodiment of the application. The positive plate can not be subjected to the phenomenon of material dropping at certain positions firstly because of inconsistent PVDF adhesive force.
The lithium ion battery provided by the embodiment of the invention comprises the positive plate provided by the embodiment of the application. The lithium ion battery has good performance.
The features and capabilities of the present invention are described in further detail below in connection with the examples.
Example 1
VDF, solvent (DMC), transparent reaction vessel, photo initiator TPO, RAFT reagent (2-mercapto-S-thiobenzoylacetic acid) and visible light source (mercury lamp) were prepared.
The RAFT reagent and VDF are designed to have a molar ratio of 1:5000:0.03.
Adding a solvent and a RAFT reagent into a transparent reaction container, filling VDF gas, and stirring the mixture with a magnet to fully dissolve the mixture; adding a photoinitiator in a light-shielding environment, and removing oxygen in the reaction bottle by introducing nitrogen into the mixed solution by a bubbling method; finally, the reaction bottle is exposed to the irradiation of a visible light source mercury lamp, and then the polymerization reaction can be initiated.
After 12h of reaction, the closed vessel was opened and the mixture was exposed to air, thereby stopping the reaction. .
Distilling the mixture in the reaction vessel to remove the solvent after the reaction is finished, or removing the solvent in vacuum to obtain a crude product;
dissolving the crude product in tetrahydrofuran at 40 ℃ to obtain PVDF solution;
dropping PVDF solution into frozen ethane at 4 ℃ to precipitate PVDF to obtain a solid-liquid mixture;
and filtering and drying the solid-liquid mixture to obtain the PVDF product.
Example 2
VDF, solvent (DMF), transparent reaction vessel, photoinitiator TPO-L, RAFT reagent (S, S-dibenzyl trithiocarbonate) and visible light source (mercury lamp) were prepared.
The RAFT reagent to VDF/photoinitiator molar ratio was designed to be 1:7000:0.1.
Adding a solvent and a RAFT reagent into a transparent reaction container, filling VDF gas, and stirring the mixture with a magnet to fully dissolve the mixture; adding a photoinitiator in a light-shielding environment, and removing oxygen in the reaction bottle by a freezing and degassing method into the mixed solution; finally, the reaction bottle is exposed to the irradiation of a visible light source mercury lamp, and then the polymerization reaction can be initiated.
After 12h of reaction, the closed vessel was opened and the mixture was exposed to air, thereby stopping the reaction. .
Distilling the mixture in the reaction vessel to remove the solvent after the reaction is finished, or removing the solvent in vacuum to obtain a crude product;
dissolving the crude product in tetrahydrofuran at 40 ℃ to obtain PVDF solution;
dropping PVDF solution into frozen ethane at 0 ℃ to precipitate PVDF to obtain a solid-liquid mixture;
and filtering and drying the solid-liquid mixture to obtain the PVDF product.
Example 3
VDF, solvent (NMP), transparent reaction vessel, photoinitiator BAPO, RAFT agent (tetramethylthiuram disulfide) and visible light source (mercury lamp) were prepared.
The RAFT reagent to VDF/photoinitiator molar ratio was designed to be 1:15000:0.06.
Adding a solvent and a RAFT reagent into a transparent reaction container, filling VDF gas, and stirring the mixture with a magnet to fully dissolve the mixture; adding a photoinitiator in a light-shielding environment, and removing oxygen in the reaction bottle by introducing inert gas into the mixed solution by a bubbling method; finally, the reaction bottle is exposed to the irradiation of a visible light source mercury lamp, and then the polymerization reaction can be initiated.
After 24 hours of reaction, the closed vessel was opened and the mixture was exposed to air, thereby stopping the reaction. .
Distilling the mixture in the reaction vessel to remove the solvent after the reaction is finished, or removing the solvent in vacuum to obtain a crude product;
dissolving the crude product in tetrahydrofuran at 40 ℃ to obtain PVDF solution;
dropping PVDF solution into frozen ethane at 0 ℃ to precipitate PVDF to obtain a solid-liquid mixture;
and filtering and drying the solid-liquid mixture to obtain the PVDF product.
Comparative example
An existing thermally initiated polymerization process is provided. The method comprises the following steps:
the PVDF emulsion polymerization process is as follows: vacuumizing the autoclave, and charging nitrogen and discharging oxygen for a plurality of times. Adding a certain amount of deionized water, a certain amount of initiator and auxiliary agent, and pressing a small amount of VDF monomer. Heating to the reaction temperature, keeping the pressure in the kettle along with the reaction, and continuously adding VDF monomer until the pressure of the monomer tank is almost unchanged, and ending the reaction. And demulsifying, washing and drying the polymer to obtain the PVDF product.
Experimental example
The PVDF prepared in examples 1 to 4 and comparative examples were tested for molecular weight distribution.
The testing method comprises the following steps: the molecular weight and molecular weight distribution of the polymers were tested using DMF-GPC. The test results are recorded in table 1.
TABLE 1 statistics of the molecular weight distribution of PVDF obtained in comparative examples
* Comparative example data picking
PVDF Design&Processing Guide
As can be seen from the above table, examples 1-2 of the present application produced a narrower molecular weight distribution of PVDF at a higher molecular weight than the comparative example.
In conclusion, the PVDF prepared by the preparation method of polyvinylidene fluoride provided by the application has the advantages of narrow molecular weight distribution and good consistency of the bonding force, and the positive plate of the lithium battery prepared by the PVDF does not have the phenomenon of material dropping at certain sites due to inconsistent bonding force under long circulation. In addition, the polymerization mode of the preparation method provided by the application is visible light room temperature active polymerization, the polymerization rate is high, the initiation mode is visible light initiation, the light source is a light source harmful to the body such as visible light and non-ultraviolet light, the initiation temperature is room temperature, and no additional heat source is required to be acquired.
The above is only a preferred embodiment of the present invention, and is not intended to limit the present invention, but various modifications and variations can be made to the present invention by those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (10)
1. A method for preparing polyvinylidene fluoride, comprising the steps of:
under a light-shielding environment, adding a photoinitiator into a solution dissolved with a RAFT reagent and vinylidene fluoride, and sealing a transparent reaction container;
the reaction vessel is irradiated under visible light, so that polymerization reaction occurs in the vessel;
after the reaction has ended, the polymerization product is withdrawn from the reaction vessel.
2. The preparation method according to claim 1, wherein the molar ratio of RAFT agent to VDF to photoinitiator is 1:x:0.03-0.1, x >0; reaction termination refers to a reaction that is sufficient to no longer occur.
3. The preparation method according to claim 1, wherein the molar ratio of RAFT agent to VDF to photoinitiator is 1:x:0.03-0.1, x >0; the reaction is terminated by opening the reaction vessel to expose the reaction system to air;
preferably, the reaction time has a correlation with the degree of polymerization under the same reaction conditions, and the reaction system is exposed to air at a reaction time node corresponding to the target degree of polymerization to obtain polyvinylidene fluoride of the target degree of polymerization;
preferably, the method for determining the reaction time corresponding to the target polymerization degree is as follows:
exposing the reaction system to air for terminating the reaction at different time nodes for a plurality of times under the same reaction condition, extracting a product sample to measure the polymerization degree of the product sample, and determining the reaction time required for the reaction to reach the target polymerization degree according to a plurality of test results;
preferably, the polymerization degree is measured by hydrogen nuclear magnetic resonance spectroscopy, and the polymerization degree of the product is calculated using a terminal group method or an internal standard method.
4. The process according to claim 1, wherein the total solids content of the reaction system is from 10 to 50%.
5. The method of claim 1, further comprising removing oxygen from the solution in the reaction vessel prior to sealing the transparent reaction vessel;
preferably, the removal method is a bubbling method or a freeze-degassing method by passing inert gas.
6. The method of claim 1, wherein the RAFT agent is selected from at least one of bis/trithioester derivatives;
preferably, the bis/trithioester derivative is 2-mercapto-S-thiobenzoylacetic acid, tetramethylthiuram disulfide, S-dibenzyl trithiocarbonate or 2- (dodecylthio thiocarbonylthio) -2-methylpropanoic acid;
preferably, the photoinitiator is selected from at least one of photoinitiators TPO, TPO-L, BAPO;
preferably, the solvent used in the reaction system is at least one selected from DMC, DMF, DMAc and NMP.
7. The process of claim 1, wherein the means for withdrawing the polymerization product from the reaction vessel comprises:
distilling the mixture in the reaction container to remove the solvent after the reaction is finished, or removing the solvent in vacuum to obtain a crude product;
dissolving the crude product in tetrahydrofuran to obtain PVDF solution;
dropping the PVDF solution into frozen ethane, and precipitating PVDF to obtain a solid-liquid mixture;
filtering and drying the solid-liquid mixture to obtain a product;
preferably, the temperature of the frozen ethane is 0-4 ℃;
preferably, the temperature of the tetrahydrofuran is 35-45 ℃.
8. Polyvinylidene fluoride produced by the process according to any one of claims 1 to 7.
9. A positive electrode sheet, characterized in that the raw material for its production comprises the polyvinylidene fluoride according to claim 8.
10. A lithium ion battery comprising the positive electrode sheet according to claim 9.
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