阅读说明：本技术 偏氟乙烯共聚物及其制备方法和用途 (Vinylidene fluoride copolymer and preparation method and application thereof ) 是由 陈承镇 杜丽君 李纯婷 蒋文斌 梁聪强 钱勇 朱世炜 于 2020-12-07 设计创作，主要内容包括：公开了偏氟乙烯共聚物及其制备方法和用途。所述偏氟乙烯共聚物包括源自偏氟乙烯的单体单元,和,按共聚物的总重量计,0.1～20.0％至少一种源自氟代丙烯类的单体单元和0.01-5.0％至少一种衣康酸类单体；所述偏氟乙烯共聚物的重均分子量为30-160万克/摩尔。(Vinylidene fluoride copolymers, methods of making them and uses thereof are disclosed. The vinylidene fluoride copolymer comprises monomer units derived from vinylidene fluoride, and 0.1-20.0% of at least one monomer unit derived from fluoropropenes and 0.01-5.0% of at least one itaconic acid monomer by total weight of the copolymer; the weight average molecular weight of the vinylidene fluoride copolymer is 30-160 mug/mol.)

1. A vinylidene fluoride copolymer comprises a monomer unit derived from vinylidene fluoride, and 0.1-20.0% of at least one monomer unit derived from fluoropropenes and 0.01-5.0% of at least one itaconic acid monomer by total weight of the copolymer; the weight average molecular weight of the vinylidene fluoride copolymer is 30-160 mug/mol.

2. Vinylidene fluoride copolymer according to claim 1, wherein the fluoropropene monomer is at least one member selected from the group consisting of 3,3, 3-trifluoropropene, trans-1, 1,1, 3-tetrafluoropropene, 2,3,3, 3-tetrafluoropropene, 1,2,3,3, 3-pentafluoropropene.

3. Vinylidene fluoride copolymer according to claim 1, wherein the fluoropropene monomer is selected from the group consisting of 2,3,3, 3-tetrafluoropropene, trans 1,1,1, 3-tetrafluoropropene and mixtures thereof.

4. Vinylidene fluoride copolymer according to any of claims 1 to 3, wherein the itaconic acid based monomer is selected from at least one of itaconic acid, monomethyl itaconate, dimethyl itaconate, monoethyl itaconate, diethyl itaconate and monopropyl itaconate.

5. Vinylidene fluoride copolymer according to any of claims 1 to 3, wherein the fluorinated propylene-based monomer units represent from 0.5 to 15.0%, preferably from 1.0 to 10.0%, based on the total weight of the copolymer; the itaconic acid monomer unit accounts for 0.05-4.0%, preferably 0.1-3.0%.

6. A process for producing the vinylidene fluoride copolymer according to any one of claims 1 to 5, which comprises:

purging and deoxidizing the closed reaction container filled with the deionized water and the dispersing agent;

adding a mixture containing a vinylidene fluoride monomer and the fluoropropene monomer into the reaction vessel, and adding an itaconic acid monomer;

raising the temperature and the pressure, adding a chain transfer agent and an initiator, and initiating a polymerization reaction;

and adding the rest of vinylidene fluoride monomer, fluoropropene monomer and itaconic acid monomer, maintaining the reaction pressure and supplementing the initiator.

7. The process according to claim 6, wherein the vinylidene fluoride monomer is added to the mixture of vinylidene fluoride and fluoropropene-based monomers in advance in an amount of 40 to 60%, preferably 45 to 55%, based on the total weight of the vinylidene fluoride monomer.

8. The method according to claim 6, wherein the itaconic acid monomer is preliminarily added to the closed reaction vessel in an amount of 5 to 25%, preferably 8 to 18%, based on the total weight of the itaconic acid monomer.

9. The process according to any one of claims 6 to 8, wherein the polymerization temperature is from 45 to 65 ℃, preferably from 50 to 60 ℃; the pressure of the polymerization reaction is 6.0 to 9.0MPa, preferably 6.5 to 8.5 MPa.

10. Use of the vinylidene fluoride copolymer of any one of claims 1 to 5 as a binder for lithium ion batteries.

Technical Field

The present invention relates to a vinylidene fluoride copolymer having excellent adhesive properties. The invention also relates to a method for producing said vinylidene fluoride copolymers and to the use thereof as binders for lithium battery electrodes.

Background

In recent years, the rapid development of electric automobiles, unmanned aerial vehicles, communication products, energy storage systems and the like puts forward higher requirements on lithium batteries, particularly power lithium batteries, such as energy density, power density, cycle life, use safety and the like. The positive electrode material, the negative electrode material, the diaphragm and the electrolyte are the core of the lithium battery, and the adhesive which is one of the key auxiliary materials is a main carrier for connecting the electrode active material, the conductive agent and the electrode current collector, so that all components in the electrode have integrity, a stable electrode structure is formed, the volume deformation of the electrode of the lithium ion battery in the charging and discharging process is favorably relieved, the separation of the active material is prevented, the mechanical integrity of the electrode is kept, and the electrochemical performance of the battery is improved. In addition, the high-viscosity adhesive can reduce the use amount, and is beneficial to preparing a battery with higher capacity.

Polyvinylidene fluoride (PVDF) has good electrochemical stability and is a common binder for lithium ion batteries. For example, chinese patent CN1489231A discloses a battery with modified lithium ion polymer, which uses a binder comprising a binder mainly comprising 0.5-95 wt% polyvinylidene fluoride, 1-90 wt% modified polyacrylate, and 0.5-85 wt% modified polyethylene or polydiene. The binder is said to absorb a certain electrolyte to form a gel, and has good lithium ion conductivity, low moisture sensitivity, good ductility, and excellent high and low temperature characteristics (Tg of-40, thermal cracking temperature of 300), while the binder has good adhesion to copper foil/aluminum foil, and does not cause a phenomenon that active materials of positive and negative electrodes fall off from a current collector due to the electrolyte, and the binder has excellent flexibility for the positive and negative electrode sheets during the manufacturing process.

Although the existing PVDF homopolymer adhesive has a plurality of advantages, the crystallinity is higher, and under the common use temperature of the battery, the circulation of electrolyte has larger resistance, and the charge and discharge load is increased; further, the high crystallinity of PVDF homopolymer causes a large difference between the shrinkage rate and the shrinkage rate of the current collector, and the active material-containing coating film is detached from the current collector by internal stress of the electrode with time transition, thereby deteriorating load characteristics.

In order to solve the problems caused by the PVDF homopolymer, a technical scheme for modifying the PVDF homopolymer by using a comonomer is provided. For example:

EP1311566B1 discloses a process for obtaining ultra-high molecular weight polymers by copolymerizing vinylidene fluoride, chlorotrifluoroethylene and hexafluoropropylene using a suspension polymerization process. The total content of chlorotrifluoroethylene and hexafluoropropylene monomers in the polymer is up to 18% by mole. Although such polymers have low melting points and crystallinities, there is still room for improvement in adhesion.

CN103270058A discloses a binder for manufacturing battery electrodes or separators, which uses acrylic acid as a modified monomer to be copolymerized with vinylidene fluoride by an emulsion polymerization method, and the polymerization medium contains a fluorinated surfactant. The content of modifying monomers in the resulting copolymer was less than 0.06% by mole. Although the adhesive has improved thermal stability, there is still room for improvement in adhesion.

CN110183562B discloses a vinylidene fluoride polymer which can be used as a lithium battery binder, and the preparation process comprises three stages: firstly, homopolymerizing vinylidene fluoride monomer to obtain ultrahigh molecular weight homopolymer, secondly, copolymerizing vinylidene fluoride and a second monomer to obtain high molecular weight copolymer, and finally, mixing the vinylidene fluoride homopolymer and the copolymer. Although the vinylidene fluoride mixture resin prepared by the method has better solution viscosity, adhesive property and flexibility, the production process is complicated, the product cost is high, the difficulty of mass production is high, and in addition, the problem caused by the vinylidene fluoride homopolymer is still difficult to completely overcome due to the existence of the high molecular weight vinylidene fluoride homopolymer in the mixture.

Accordingly, there is still a need to provide a vinylidene fluoride polymer which is useful as a binder for lithium batteries, which is required to have high viscosity, low crystallinity, and simple manufacturing process in order to reduce the cost of the binder.

Disclosure of Invention

An object of the present invention is to provide a vinylidene fluoride polymer which is useful as a binder for lithium batteries, has high viscosity, low crystallinity, and is simple in manufacturing process, thereby reducing the cost of the binder.

Another object of the present invention is to provide a method for preparing the vinylidene fluoride polymer which can be used as a binder for lithium batteries, which has the advantage of simple process compared with the prior art, thereby reducing the cost of the binder.

Accordingly, one aspect of the present invention relates to a vinylidene fluoride copolymer comprising monomer units derived from vinylidene fluoride, and, based on the total weight of the copolymer, from 0.1 to 20.0% of at least one monomer unit derived from fluoropropenes and from 0.01 to 5.0% of at least one itaconic monomer; the weight average molecular weight of the vinylidene fluoride copolymer is 30-160 mug/mol.

Another aspect of the present invention relates to a method for producing the vinylidene fluoride copolymer, which comprises:

purging and deoxidizing the closed reaction container filled with the deionized water and the dispersing agent;

adding a mixture containing a vinylidene fluoride monomer and the fluoropropene monomer into the reaction vessel, and adding an itaconic acid monomer;

raising the temperature and the pressure, adding a chain transfer agent and an initiator, and initiating a polymerization reaction;

and adding the rest of vinylidene fluoride monomer, fluoropropene monomer and itaconic acid monomer, maintaining the reaction pressure and supplementing the initiator.

The invention also relates to the use of said vinylidene fluoride copolymers as binders for lithium ion batteries.

Detailed Description

The vinylidene fluoride copolymers of the present invention comprise predominantly monomer units derived from vinylidene fluoride.

The copolymers also comprise from 0.1 to 20.0%, preferably from 0.5 to 18%, more preferably from 1 to 16%, preferably from 1.5 to 14%, preferably from 2 to 12%, by weight of the total vinylidene fluoride copolymer, of at least one monomer unit derived from a fluoropropene.

In one embodiment of the present invention, the fluoropropene monomer is selected from 3,3, 3-trifluoropropene (chemical formula CF)₃-CH＝CH₂) Trans-1, 1,1, 3-tetrafluoropropene (chemical structural formula is CF)₃-CH ═ CHF), 2,3,3, 3-tetrafluoropropene, 1,2,3,3, 3-pentafluoropropene (chemical structural formula CF)₃-CF ═ CHF). Preferably, the fluoropropylene monomer is selected from 2,3,3, 3-tetrafluoropropene, trans 1,1,1, 3-tetrafluoropropene or a mixture thereof in any proportion.

The copolymer also comprises from 0.01 to 5.0%, preferably from 0.08 to 4.5%, more preferably from 0.12 to 4.0%, preferably from 0.18 to 3.5%, preferably from 0.25 to 3.0%, by weight of the total vinylidene fluoride copolymer, of at least one itaconic acid monomer.

In an embodiment of the present invention, the itaconic acid-type monomer is at least one selected from itaconic acid, monomethyl itaconate, dimethyl itaconate, monoethyl itaconate, diethyl itaconate and monopropyl itaconate. Preferably, the itaconic acid monomer is selected from itaconic acid, monomethyl itaconate or mixtures thereof in any ratio.

The weight average molecular weight of the vinylidene fluoride copolymer of the present invention is 30 to 160 kg/mol, preferably 35 to 158 kg/mol, more preferably 40 to 155 kg/mol, preferably 45 to 152 kg/mol, and preferably 50 to 150 kg/mol.

In one embodiment of the present invention, the melting point of the vinylidene fluoride copolymer is 140-.

In one embodiment of the invention, the temperature at which the vinylidene fluoride copolymer loses 1% weight by mass is higher than 350 ℃, preferably higher than 370 ℃, more preferably higher than 390 ℃, preferably higher than 400 ℃.

In one embodiment of the present invention, the heat of crystallization of the vinylidene fluoride copolymer is from 20 to 45J/g, preferably from 22 to 43J/g, more preferably from 25 to 40J/g, preferably from 28 to 38J/g, and preferably from 30 to 35J/g.

In one embodiment of the present invention, the vinylidene fluoride copolymer has a viscosity of greater than 6000cp, preferably greater than 6500cp, more preferably greater than 7000cp, preferably greater than 7500cp, and preferably greater than 8000cp in an 8.8% solution of N-methylpyrrolidone.

The invention also relates to a method for producing the vinylidene fluoride copolymer. The method comprises the following steps:

a) purging and deoxidizing the closed reaction container filled with deionized water and a dispersant

In one embodiment of the present invention, deionized water is used to have a resistivity of 10 M.OMEGA.cm and water is used in an amount of 150 to 350% by mass based on the mass of the vinylidene fluoride monomer.

In one embodiment of the present invention, the dispersant is used in an amount such that the mass ratio of the dispersant to the deionized water is greater than 0.015%, preferably greater than 0.020%, and more preferably greater than 0.025%.

The dispersant suitable for use in the process of the present invention is not particularly limited and may be any conventional dispersant known in the art. In one embodiment of the present invention, the dispersant is selected from one or more of methyl cellulose, carboxymethyl cellulose, ethyl cellulose, hydroxyethyl cellulose, hydroxypropyl methyl cellulose, cyanoethyl cellulose, polyvinyl alcohol and polyethylene oxide.

Purging to remove oxygen is a routine operation of polymerization reactions. In one embodiment of the invention, a nitrogen purge is used to remove oxygen from the reaction vessel.

In one embodiment of the invention, the reaction vessel is provided with a stirring device. The method of the present invention further comprises the step of turning on the stirring device after purging to remove oxygen.

In one example of the invention, the method steps include sequentially adding deionized water and a dispersant to a closed reaction kettle, purging nitrogen to remove oxygen, and starting stirring.

In one embodiment of the present invention, the stirring rate is 300 to 600rpm, preferably 350 to 550rpm, more preferably 400 to 500rpm, and is maintained until the reaction is stopped.

b) Adding a mixture containing vinylidene fluoride monomer and the fluoropropene monomer into the reaction vessel, and adding itaconic acid monomer

In one embodiment of the present invention, the content of the fluoropropene monomer is 0.1 to 15.0%, preferably 0.5 to 12.5%, more preferably 1.0 to 10.0% by weight of the mixture of the vinylidene fluoride and the fluoropropene monomer.

In one embodiment of the present invention, the fluoropropene monomer is selected from 3,3, 3-trifluoropropene (chemical formula CF)₃-CH＝CH₂) Trans-1, 1,1, 3-tetrafluoropropene (chemical structural formula is CF)₃-CH ═ CHF), 2,3,3, 3-tetrafluoropropene, 1,2,3,3, 3-pentafluoropropene (chemical structural formula CF)₃-CF ═ CHF). Preferably, the fluoropropylene monomer is selected from 2,3,3, 3-tetrafluoropropene, trans 1,1,1, 3-tetrafluoropropene or a mixture thereof in any proportion.

In one embodiment of the present invention, the vinylidene fluoride monomer is added in step b) in an amount of 40 to 60%, preferably 45 to 55%, based on the total weight of the vinylidene fluoride monomer.

In an embodiment of the present invention, the itaconic acid-type monomer is at least one selected from itaconic acid, monomethyl itaconate, dimethyl itaconate, monoethyl itaconate, diethyl itaconate and monopropyl itaconate. Preferably, the itaconic acid monomer is selected from itaconic acid, monomethyl itaconate or mixtures thereof in any ratio.

In one embodiment of the present invention, the amount of the itaconic acid monomer pre-charged into the closed reaction vessel is 5 to 25%, preferably 8 to 18%, based on the total weight of the itaconic acid monomer.

c) Raising the temperature and the pressure, adding a chain transfer agent and an initiator to initiate polymerization reaction

The temperature and pressure used for the polymerization reaction are not particularly limited and may be conventional temperatures and pressures known in the art. In one embodiment of the invention, the temperature is from 45 to 65 ℃, preferably from 50 to 60 ℃; the pressure is 6.0-9.0MPa, preferably 6.5-8.5 MPa.

The chain transfer agent suitable for use in the process of the present invention is not particularly limited and may be a conventional chain transfer agent known in the art. In one embodiment of the present invention, the chain transfer agent is selected from one or more of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, ethylene carbonate, diethyl malonate, ethyl acetate, methyl propionate, ethyl propionate, diethyl succinate, acetone, diethyl ether, methyl tert-butyl ether, and isopropanol.

The amount of the chain transfer agent is not particularly limited and may be a conventional amount known in the art. In one embodiment of the present invention, the amount of the chain transfer agent is 0.01 to 2.0%, preferably 0.03 to 1.8%, and more preferably 0.05 to 1.6% by weight of the vinylidene fluoride monomer.

The initiator suitable for use in the process of the present invention is not particularly limited and may be a conventional initiator known in the art. In one embodiment of the invention, the initiator is selected from the group consisting of organic peroxy-type initiators and azo-type initiators.

Non-limiting examples of the organic peroxy initiator include diisopropyl peroxydicarbonate, dibutyl peroxydicarbonate, diethylhexyl peroxydicarbonate, dicyclohexyl peroxydicarbonate, di-t-butylcyclohexyl peroxydicarbonate, dicetyl peroxydicarbonate, t-amyl peroxypivalate, t-butyl peroxypivalate, bis (2-methylbenzoyl) peroxide, dioctanoyl peroxide, dilauroyl peroxide, dibenzoyl peroxide, t-butyl peroxyisopropylcarbonate, t-butyl peroxyacetate, t-butyl peroxybenzoate, dicumyl peroxide, t-butyl cumyl peroxide, and di-t-butyl peroxide, or a mixture of two or more thereof. Non-limiting examples of the organic azo initiator include, for example, one or more of azobisisobutyronitrile and azobisisoheptonitrile.

The amount of the initiator is not particularly limited and may be an initiating effective amount. In one embodiment of the present invention, the amount of the initiator is 0.02 to 1.6%, preferably 0.03 to 1.5%, and more preferably 0.04 to 1.4% by weight of the vinylidene fluoride monomer.

In one embodiment of the present invention, the method step comprises raising the temperature of the reaction vessel to 45-65 ℃ within 180-240 minutes, adding a chain transfer agent and an initiator, and initiating the polymerization reaction.

d) And adding the rest of vinylidene fluoride monomer, fluoropropene monomer and itaconic acid monomer, maintaining the reaction pressure and supplementing the initiator.

In the polymerization reaction process, the vinylidene fluoride monomer, the fluoropropene monomer and the itaconic acid monomer are supplemented, the pressure of a reaction container is 6.0-9.0MPa, and the free radical polymerization reaction is maintained by supplementing the initiator.

In one embodiment of the present invention, the vinylidene fluoride monomer, the fluoropropene-based monomer and the itaconic acid-based monomer are added when the pressure in the reaction vessel is less than 6.0 MPa.

In one embodiment of the present invention, the amount of the supplementary initiator is 10 to 80% by mass of the amount of the initiator added for the first time. In one embodiment of the invention, the amount of initiator is the same for each additional initiator.

In one embodiment of the present invention, the polymerization reaction is stopped after 5 to 10 hours.

Examples

The present invention will be described in more detail with reference to the following specific examples, but the scope of the present invention is not limited to these specific examples.

The test methods and test conditions in the following examples and comparative examples were as follows:

1. weight average molecular weight

Weight average molecular weight (M) of the Polymer_w) The polymer was completely dissolved in HPLC grade Dimethylformamide (DMF) using a solution of 0.1 mol% lithium bromide in dimethylformamide as eluent as measured by Gel Permeation Chromatography (GPC) under the following specific test conditions: the flow rate was 1mL/min, the column temperature was 50 ℃ and the sample concentration was 2 mg/mL.

2. Melting point and enthalpy of crystallization

Melting point and enthalpy of crystallization were determined by Differential Scanning Calorimetry (DSC) according to ASTM D3418, with a temperature program of: heating from 40 ℃ to 200 ℃ at a heating rate of 10 ℃/min, preserving heat at 200 ℃ for 10 minutes, cooling to 40 ℃ at a cooling rate of 20 ℃/min, preserving heat at 40 ℃ for 10 minutes, and heating from 40 ℃ to 200 ℃ at a heating rate of 10 ℃/min. Record 3 DSC spectra of the scan.

3. Thermogravimetric analysis (TGA)

Thermogravimetric analysis (TGA) was performed according to the ISO11358 standard. The relative weight of the polymer at different temperatures was recorded under nitrogen and in dynamic mode. The temperature required to obtain a weight loss of 1 wt.% of the polymer is higher, the better the thermal stability of the polymer.

4. Rotational viscosity

Rotational viscosity was measured using a digital viscometer, equipped with a # 3 spindle. 1g of the polymer was dissolved in 10mL of N-methylpyrrolidone and the test temperature was 50 ℃.

5. Peel strength

The peel strength test requires that the polymer and electrode active material be formed into an electrode, as measured according to standard GB/T2790-1995.

Example 1

6000 g of deionized water and 3 g of hydroxypropyl methyl cellulose are added into a 10L vertical high-pressure reaction kettle, and nitrogen is purged to remove oxygen. The stirring was started at 500 rpm/min. 1683 g of vinylidene fluoride was added to the closed reaction vessel from a VDF storage tank by a membrane pump, and 117 g of 2,3,3, 3-tetrafluoropropene was added to the closed reaction vessel from a 2,3,3, 3-tetrafluoropropene storage tank by a membrane pump. 1.3 g of monomethyl itaconate (medium: 6.0 wt% aqueous monomethyl itaconate solution, the same applies below) was added by a metering pump, and the temperature in the reaction vessel was raised to 53 ℃ and the pressure was 6.5 MPa. The reaction was started by adding 3 g of ethyl acetate and 1.3 g of tert-amyl peroxypivalate by means of a metering pump. When the pressure in the reaction kettle is lower than 6.0MPa, 125 g of vinylidene fluoride and 8.7 g of 2,3,3, 3-tetrafluoropropene are added by a membrane pump, 2.7 g of monomethyl itaconate and 1.5 g of tert-amyl peroxypivalate are added by a metering pump. The feeding mode at this stage was repeated, and the reaction was completed after 8.6 hours. The polymer slurry is degassed in a degassing tank to recover unreacted monomers. Washing the polymer slurry with deionized water, and spray drying to obtain the final product. The test results are shown in Table 1.

Example 2

6000 g of deionized water and 4.5 g of hydroxypropyl methyl cellulose are added into a 10L vertical high-pressure reaction kettle, and nitrogen is purged to remove oxygen. The stirring was started at 500 rpm/min. 1655 g of vinylidene fluoride was added to the closed reaction vessel from a VDF storage tank by means of a membrane pump, and 55 g of 1,1,1, 3-tetrafluoropropene was added to the closed reaction vessel from a 1,1,1, 3-tetrafluoropropene storage tank by means of a membrane pump. 1.2 g of monomethyl itaconate was added by means of a metering pump, and the temperature in the reaction vessel was raised to 53 ℃ and the pressure reached 6.2 MPa. The reaction was started by adding 2.5 g of diethyl malonate and 1.3 g of tert-amyl peroxypivalate by a metering pump. When the pressure in the reaction kettle is lower than 6.0MPa, 135 g of vinylidene fluoride and 4.5 g of 1,1,1, 3-tetrafluoropropene are added by a membrane pump, 2.8 g of monomethyl itaconate and 1.5 g of tert-amyl peroxypivalate are added by a metering pump. The feeding mode at this stage was repeated, and the reaction was completed after 8.9 hours. The polymer slurry is degassed in a degassing tank to recover unreacted monomers. Washing the polymer slurry with deionized water, and spray drying to obtain the final product. The test results are shown in Table 1.

Comparative example 1

6000 g of deionized water and 3.3 g of hydroxypropyl methyl cellulose are added into a 10L vertical high-pressure reaction kettle, and nitrogen is purged to remove oxygen. The stirring was started at 500 rpm/min. 1800 g of vinylidene fluoride was added to the closed reaction vessel from a VDF storage tank using a diaphragm pump. 1.3 g of monomethyl itaconate was added by means of a metering pump, and the temperature in the reaction vessel was raised to 53 ℃ and the pressure reached 6.5 MPa. The reaction was started by adding 3 g of ethyl acetate and 1.3 g of tert-amyl peroxypivalate by means of a metering pump. When the pressure in the reaction kettle is lower than 6.0MPa, 133 g of vinylidene fluoride is added by a membrane pump, 2.7 g of monomethyl itaconate is added by a metering pump, and 1.5 g of tert-amyl peroxypivalate is added. The feeding mode at this stage was repeated, and the reaction was completed after 8.5 hours. The polymer slurry is degassed in a degassing tank to recover unreacted monomers. Washing the polymer slurry with deionized water, and spray drying to obtain the final product. The test results are shown in Table 1.

Comparative example 2

6000 g of deionized water and 3.6 g of hydroxypropyl methyl cellulose are added into a 10L vertical high-pressure reaction kettle, and nitrogen is purged to remove oxygen. The stirring was started at 500 rpm/min. 2000 g of vinylidene fluoride was added to the reactor from a VDF storage tank using a membrane pump. The temperature in the reaction vessel was raised to 52 ℃ and the pressure reached 6.8 MPa. The reaction was started by adding 2.8 g of ethyl acetate and 2 g of tert-amyl peroxypivalate by means of a metering pump. When the pressure in the reaction kettle is lower than 6.0MPa, 200 g of vinylidene fluoride is added by a membrane pump. The feeding mode at this stage is repeated, and the reaction is finished after 6 hours. The polymer slurry is degassed in a degassing tank to recover unreacted monomers. Washing the polymer slurry with deionized water, and spray drying to obtain the final product. The test results are shown in Table 1.

Comparative example 3

6000 g of deionized water and 3 g of hydroxypropyl methyl cellulose are added into a 10L vertical high-pressure reaction kettle, and nitrogen is purged to remove oxygen. The stirring was started at 500 rpm/min. 1683 g of vinylidene fluoride was added to the closed reaction vessel from a VDF storage tank by a membrane pump, and 117 g of 2,3,3, 3-tetrafluoropropene was added to the closed reaction vessel from a 2,3,3, 3-tetrafluoropropene storage tank by a membrane pump. 1.0 g of methyl acrylate was added by means of a metering pump, and the temperature in the reaction vessel was raised to 53 ℃ and the pressure was 6.5 MPa. The reaction was started by adding 3.6 g of ethyl acetate and 1.4 g of tert-amyl peroxypivalate by means of a metering pump. When the pressure in the reaction kettle is lower than 6.0MPa, 125 g of vinylidene fluoride and 8.7 g of 2,3,3, 3-tetrafluoropropene are added by a membrane pump, 2.0 g of methyl acrylate and 1.7 g of tert-amyl peroxypivalate are added by a metering pump. The feeding mode at this stage was repeated, and the reaction was completed after 8.3 hours. The polymer slurry is degassed in a degassing tank to recover unreacted monomers. Washing the polymer slurry with deionized water, and spray drying to obtain the final product. The test results are shown in Table 1.

TABLE 1 Performance testing of vinylidene fluoride polymers

The results in table 1 show that the vinylidene fluoride copolymer provided by the invention has the characteristics of low crystallinity, good flexibility, excellent chemical stability and the like, the adhesive property is greatly improved, and the acting force between the active substance and the metal pole piece is effectively improved.