A vinylidene fluoride copolymer and its preparation method and application

Abstract

The present invention relates to a vinylidene fluoride copolymer, which is copolymerized from a polyvinylidene fluoride resin containing carboxyl groups at both ends, trimellitic anhydride, and a diamine monomer containing a phenylene ether group. The present invention also relates to a method for preparing the vinylidene fluoride copolymer and its use as a binder for lithium batteries. The vinylidene fluoride copolymer has strong adhesion and good thermal stability under high voltage conditions, making it suitable for use as a binder for lithium batteries, particularly for positive electrodes in lithium batteries.

Description

Technical Field

[0001] The present invention relates to the technical field of polymers, and in particular to a vinylidene fluoride copolymer and a preparation method and application thereof. Background Art

[0002] Polyvinylidene fluoride (PVDF) is the most widely used binder for cathodes in commercial lithium-ion batteries today. It bonds the active material (lithium iron phosphate, a ternary cathode material), the conductive agent, and the current collector to maintain uniformity and stability during active material slurrying, providing the necessary electron conduction within the electrode, and playing a vital role in maintaining battery capacity, service life, and stability. PVDF exhibits excellent oxidation resistance against oxidizing cathodes. However, PVDF has poor heat resistance and easily loosens in high-voltage batteries and high-temperature conditions, resulting in a weakened bond and a loss of the mechanical strength that binds the active material, carbon black, and current collector components together.

[0003] In order to improve the adhesion of the particles of the positive electrode active material and enhance its thermal stability under high voltage conditions to prevent the failure mode of the positive electrode derived from high cutoff voltage, it is particularly important to develop a new functional PVDF binder. Summary of the Invention

[0004] In order to overcome at least one defect in the prior art, the present invention provides a vinylidene fluoride copolymer, which has strong adhesion and good thermal stability under high voltage conditions and can be used as a binder for lithium batteries.

[0005] The technical solution adopted by the present invention is as follows:

[0006] A vinylidene fluoride copolymer is prepared by copolymerizing a polyvinylidene fluoride resin containing carboxyl groups at both ends, trimellitic anhydride and a diamine monomer containing a phenylene ether group.

[0007] In the design of the vinylidene fluoride copolymer of the present invention, a polyvinylidene fluoride resin containing carboxyl groups at both ends is used so that it can be copolymerized with a diamine monomer. In addition, a diamine monomer containing a phenyl ether group is used, and trimellitic anhydride is added for copolymerization, thereby forming an amide and imide structure, which can effectively improve the thermal stability of the molecule.

[0008] Therefore, the vinylidene fluoride copolymer of the present invention contains more polar groups, can better coat lithium battery electrode materials, has better adhesion under high voltage and high temperature conditions, greatly improves the stability of the electrode material, and improves the mechanical strength of binding the active material, carbon black and current collector components together, especially can improve the adhesion of the particles of the positive electrode active material, while showing excellent antioxidant properties to the oxidizing positive electrode.

[0009] Compared with the prior art that solely uses polyvinylidene fluoride, polyamide, polyimide or polyamide-imide as lithium battery binders, the present invention copolymerizes polyvinylidene fluoride resin, trimellitic anhydride and a diamine monomer containing a phenylene ether group. The obtained vinylidene fluoride copolymer can exert the synergistic effect of the three raw materials. Its thermal stability and positive electrode adhesion strength at high temperature of the battery are better than polyvinylidene fluoride. At the same time, its flexibility and oxidation resistance at high voltage are better than materials such as polyamide and polyimide.

[0010] Performance tests have shown that the vinylidene fluoride copolymer of the present invention generally has better adhesion under high voltage and high temperature conditions, greatly improving the stability of the positive electrode material.

[0011] Specifically, the vinylidene fluoride copolymer has a structure shown in the following formula:

[0012]

[0013] Here, R represents the fluorine-containing unit remaining after the reaction of the polyvinylidene fluoride resin, R1 and R2 represent the phenyl ether-containing units remaining after the reaction of the diamine monomer, and n, m, and p represent the number of repetitions of the corresponding units, with n ranging from 15 to 4000 and the ratio of m to p ranging from 1:1 to 3. If the proportion of m is higher than this, the fluorine content of the overall molecular structure is reduced, resulting in poor oxidation resistance in the positive electrode. If the proportion of m is lower than this, the entire structure has fewer imine bonds, poor thermal stability, and poor adhesion at high temperatures.

[0014] In some preferred embodiments, the diamine monomer is selected from one or more of 4,4'-diaminodiphenyl ether, 3,3'-diaminodiphenyl ether, 1,3-bis(3-aminophenoxy)benzene, 1,4-bis(4-aminophenoxy)benzene, 4,4'-bis(4-aminophenoxy)biphenyl, and 1,3-bis(4-aminophenoxy)benzene.

[0015] In some preferred embodiments, the polyvinylidene fluoride resin is one or more of a vinylidene fluoride homopolymer, a copolymer of vinylidene fluoride and other fluorine-containing monomers, and the other fluorine-containing monomers are selected from one or more of trifluoroethylene, chlorotrifluoroethylene, tetrafluoroethylene, pentafluoropropylene, hexafluoropropylene, perfluoromethyl vinyl ether or perfluoropropyl vinyl ether.

[0016] In some preferred embodiments, the polyvinylidene fluoride resin is prepared by emulsion polymerization or suspension polymerization.

[0017] The present invention also provides a method for preparing the vinylidene fluoride copolymer, comprising the following steps:

[0018] (1) preparing polyvinylidene fluoride resin by polymerization;

[0019] (2) using the polyvinylidene fluoride resin obtained in step (1) as a raw material to prepare a polyvinylidene fluoride resin containing carboxyl groups at both ends;

[0020] (3) condensing and copolymerizing the polyvinylidene fluoride resin containing carboxyl groups at both ends obtained in step (2), trimellitic anhydride, and a diamine monomer containing a phenylene ether group to obtain a vinylidene fluoride copolymer.

[0021] In some preferred embodiments, the polymerization system of step (1) adopts an initiator and a chain transfer agent, wherein the initiator is an organic peroxide initiator, a persulfate or a persulfate / sodium bisulfite initiation system, and the chain transfer agent is one or more of ethyl acetate, diethyl malonate, diethyl carbonate, dimethyl carbonate, acetone, ethanol, and n-propanol, and the amount thereof is 0.01-1% of the mass of the polymerization monomer.

[0022] In some preferred embodiments, step (2) comprises: dissolving the polyvinylidene fluoride resin obtained in step (1) in an organic solvent, adding an inorganic base and a phase transfer catalyst, and reacting to obtain a polyvinylidene fluoride resin containing carboxyl groups at both ends.

[0023] In some preferred embodiments, the polymerization system of step (3) uses a condensing agent and an organic base, wherein the condensing agent is one or more of triphenylphosphine, triphenylphosphine-polyhalomethane, triphenylphosphine-hexachloroacetone, triphenylphosphine-NBS, 3-acyl-2-thiothiazoline, and tris(2,6-dimethoxyphenyl)bismuth, and the organic base is one or more of pyridine, triethylamine, and triallylamine.

[0024] The present invention also provides the use of the vinylidene fluoride copolymer as a binder for lithium batteries. The vinylidene fluoride copolymer is particularly suitable for use as a binder for positive electrodes of lithium batteries. DETAILED DESCRIPTION

[0025] The vinylidene fluoride copolymer of the present invention is prepared by copolymerizing a polyvinylidene fluoride resin containing carboxyl groups at both ends, trimellitic anhydride and a diamine monomer containing a phenylene ether group.

[0026] Specifically, the vinylidene fluoride copolymer has a structure shown in the following formula:

[0027]

[0028] Among them, R is the unit containing fluorine groups remaining after the reaction of polyvinylidene fluoride resin, R1 and R2 are the units containing phenyl ether groups remaining after the reaction of diamine monomer, n, m and p respectively represent the repetition number (polymerization degree) of the corresponding units, n is 15-4000, and the ratio of m to p is 1:1-3.

[0029] The diamine monomer is selected from one or more of 4,4'-diaminodiphenyl ether, 3,3'-diaminodiphenyl ether, 1,3-bis(3-aminophenoxy)benzene, 1,4-bis(4-aminophenoxy)benzene, 4,4'-bis(4-aminophenoxy)biphenyl, and 1,3-bis(4-aminophenoxy)benzene.

[0030] The polyvinylidene fluoride resin may be a vinylidene fluoride homopolymer (formed by homopolymerization of VDF monomer), or a copolymer of vinylidene fluoride and other fluorine-containing monomers, or a mixture thereof with a vinylidene fluoride homopolymer. Preferably, the other fluorine-containing monomers are selected from one or more of trifluoroethylene, chlorotrifluoroethylene, tetrafluoroethylene, pentafluoropropylene, hexafluoropropylene, perfluoromethyl vinyl ether, or perfluoropropyl vinyl ether.

[0031] When the polyvinylidene fluoride resin is a vinylidene fluoride homopolymer, the vinylidene fluoride copolymer has a structure as shown in the following formula:

[0032]

[0033] The polyvinylidene fluoride resin is first prepared by emulsion polymerization or suspension polymerization, and then treated to introduce carboxyl groups at both ends of the molecular chain.

[0034] The preparation method of the vinylidene fluoride copolymer comprises the following steps:

[0035] (1) Preparation of polyvinylidene fluoride resin by polymerization.

[0036] (2) Using the polyvinylidene fluoride resin obtained in step (1) as a raw material, a polyvinylidene fluoride resin containing carboxyl groups at both ends is prepared.

[0037] (3) condensing and copolymerizing the polyvinylidene fluoride resin containing carboxyl groups at both ends obtained in step (2), trimellitic anhydride, and a diamine monomer containing a phenylene ether group to obtain a vinylidene fluoride copolymer.

[0038] Specifically, the step (1) can be carried out by emulsion polymerization or suspension polymerization, preferably emulsion polymerization.

[0039] The initiator used in the polymerization system of step (1) can be an organic peroxide initiator, such as diisopropyl peroxydicarbonate, di-n-propyl peroxydicarbonate, diisobutyryl peroxide, tert-butyl peroxypivalate, tert-amyl peroxypivalate, etc. If an emulsion polymerization system is used, the initiator can also be a persulfate or persulfate / sodium bisulfite redox initiation system, preferably an ammonium persulfate initiation system.

[0040] The polymerization system of step (1) may also use a chain transfer agent to adjust the molecular weight of the polymer. The chain transfer agent is one or more of ethyl acetate, diethyl malonate, diethyl carbonate, dimethyl carbonate, acetone, ethanol, and n-propanol, and its amount is 0.01-1% of the mass of the polymerization monomer.

[0041] Specifically, step (2) comprises: dissolving the polyvinylidene fluoride resin obtained in step (1) in an organic solvent, adding an inorganic base and a phase transfer catalyst, and reacting to obtain a polyvinylidene fluoride resin containing carboxyl groups at both ends. The inorganic base may be sodium hydroxide. The organic solvent may be acetone. The phase transfer catalyst is a quaternary ammonium salt, which may be selected from any one of tetrabutylammonium bromide, tetrabutylammonium chloride, and benzyltributylammonium bromide.

[0042] Specifically, the condensing agent used in the polymerization system of step (3) can be selected from one or more of triphenylphosphine, triphenylphosphine-polyhalomethane, triphenylphosphine-hexachloroacetone, triphenylphosphine-NBS, 3-acyl-2-thiothiazoline, and tris(2,6-dimethoxyphenyl)bismuth. An organic base is also added to the polymerization system of step (3), and the organic base is one or more of pyridine, triethylamine, and triallylamine.

[0043] The step (3) comprises: dissolving the polyvinylidene fluoride resin containing carboxyl groups at both ends obtained in step (2), trimellitic anhydride and a diamine monomer containing a phenylene ether group in an aprotic solvent, heating to 160° C., reacting for 2 hours, adding a condensing agent, an organic base and calcium chloride, continuing the reaction for 3 hours, and then precipitating the obtained polymer solution in methanol to obtain a polyvinylidene fluoride copolymer.

[0044] Example 1

[0045] The polyvinylidene fluoride copolymer of the present invention is prepared by the following steps:

[0046] (1) Add 11.5 kg of deionized water to a 20 L vertical polymerization reactor. Close the reactor, evacuate the air, and replace the air with nitrogen several times until the oxygen content in the reactor is less than 10 ppm. Heat the reactor to 85 ° C, start stirring the reactor at a speed of 400 r / min. Inject a certain amount of VDF monomer until the reactor pressure reaches 4.1 MPa. Add 0.64 g of ammonium persulfate and 3.48 g of diethyl malonate to the reactor via an auxiliary pump at a rate of 150 g / hour. At the same time, the polymerization reaction begins, and the reaction pressure is maintained constant by continuously adding VDF monomer. After 1 hour of reaction, add 0.28 g of ammonium persulfate, and after 2 hours of reaction, add 0.28 g of ammonium persulfate and 0.93 g of diethyl malonate. When the amount of VDF monomer reaction reaches 3 kg, stop the reaction. The reaction time is 4.7 hours. The reactor was depressurized, the emulsion was collected, demulsified, and washed repeatedly until the conductivity of the filtrate dropped below 0.1. Finally, the product was dried in an oven at 95°C for 24 hours to obtain PVDF resin powder.

[0047] (2) PVDF resin powder was dissolved in acetone, and NaOH and a phase transfer catalyst were added. After the reaction for 10 h, the powder was dried to obtain PVDF resin powder with carboxyl groups at both ends of the molecular chain. The molecular weight Mw was determined to be 100,000 g / mol by GPC.

[0048] (3) 0.1 mol of PVDF resin powder with carboxyl groups at both ends of the molecular chain, 0.1 mol of trimellitic anhydride and 0.2 mol of 4,4'-diaminodiphenyl ether were dissolved in an aprotic solvent, heated to 160°C, and reacted for 2 hours. Then, 0.05 mol of triphenylphosphine, 0.05 mol of pyridine and 0.1 mol of calcium chloride were added and the reaction was continued for 3 hours. The resulting polymer solution was precipitated in methanol to obtain a polyvinylidene fluoride copolymer, and its molecular weight Mw was determined to be 421000 g / mol by GPC.

[0049] Example 2

[0050] The polyvinylidene fluoride copolymer of the present invention is prepared by the following steps:

[0051] (1) Add 11.5 kg of deionized water to a 20 L vertical polymerization reactor. Close the reactor, evacuate the reactor, and replace it with nitrogen several times until the oxygen content in the reactor is less than 10 ppm. Heat the reactor to 85 ° C, start the reactor stirring, and rotate at 400 r / min. Inject a certain amount of VDF monomer until the reactor pressure reaches 4.1 MPa. Add 6.4 g of ammonium persulfate and 34.8 g of diethyl malonate to the reactor through an auxiliary pump at a rate of 150 g / hour. At the same time, the polymerization reaction begins, and the reaction pressure is kept constant by continuously adding VDF monomer. After 1 hour of reaction, add 2.8 g of ammonium persulfate, and after 2 hours of reaction, add 2.8 g of ammonium persulfate and 9.3 g of diethyl malonate. When the amount of VDF monomer reaction reaches 3 kg, stop the reaction. The reaction time is 4.7 hours. Depressurize the reactor, collect the emulsion, break the emulsion, and wash repeatedly until the conductivity of the filtrate drops below 0.1. Finally, the product was placed in an oven at 95°C and dried for 24 h to obtain PVDF resin powder.

[0052] (2) PVDF resin powder was dissolved in acetone, and NaOH and a phase transfer catalyst were added. After the reaction for 10 h, the powder was dried to obtain PVDF resin powder with carboxyl groups at both ends of the molecular chain. The molecular weight Mw was determined to be 10,000 g / mol by GPC.

[0053] (3) 0.1 mol of PVDF resin powder with carboxyl groups at both ends of the molecular chain, 0.1 mol of trimellitic anhydride and 0.2 mol of 4,4'-diaminodiphenyl ether were dissolved in an aprotic solvent, heated to 160°C, and reacted for 2 hours. Then, 0.05 mol of triphenylphosphine, 0.05 mol of pyridine and 0.1 mol of calcium chloride were added and the reaction was continued for 3 hours. The resulting polymer solution was precipitated in methanol to obtain a polyvinylidene fluoride copolymer, and its molecular weight Mw was determined to be 324000 g / mol by GPC.

[0054] Example 3

[0055] The polyvinylidene fluoride copolymer of the present invention is prepared by the following steps:

[0056] (1) Add 11.5 kg of deionized water to a 20 L vertical polymerization reactor. Close the reactor, evacuate the reactor, and replace it with nitrogen several times until the oxygen content in the reactor is less than 10 ppm. Heat the reactor to 85 ° C, start the reactor stirring, and rotate at 400 r / min. Inject a certain amount of VDF monomer until the reactor pressure reaches 4.1 MPa. Add 6.4 g of ammonium persulfate and 34.8 g of diethyl malonate to the reactor through an auxiliary pump at a rate of 150 g / hour. At the same time, the polymerization reaction begins, and the reaction pressure is kept constant by continuously adding VDF monomer. After 1 hour of reaction, add 2.8 g of ammonium persulfate, and after 2 hours of reaction, add 2.8 g of ammonium persulfate and 9.3 g of diethyl malonate. When the amount of VDF monomer reaction reaches 3 kg, stop the reaction. The reaction time is 4.7 hours. Depressurize the reactor, collect the emulsion, break the emulsion, and wash repeatedly until the conductivity of the filtrate drops below 0.1. Finally, the product was placed in an oven at 95°C and dried for 24 h to obtain PVDF resin powder.

[0057] (2) PVDF resin powder was dissolved in acetone, and NaOH and a phase transfer catalyst were added. After the reaction for 10 h, the powder was dried to obtain PVDF resin powder with carboxyl groups at both ends of the molecular chain. The molecular weight Mw was determined to be 10,000 g / mol by GPC.

[0058] (3) 0.125 mol of PVDF resin powder with carboxyl groups at both ends of the molecular chain, 0.075 mol of trimellitic anhydride and 0.2 mol of 4,4'-diaminodiphenyl ether were dissolved in an aprotic solvent, heated to 160°C, and reacted for 2 hours. Then, 0.05 mol of triphenylphosphine, 0.05 mol of pyridine and 0.1 mol of calcium chloride were added and the reaction was continued for 3 hours. The resulting polymer solution was precipitated in methanol to obtain a polyvinylidene fluoride copolymer, and its molecular weight Mw was determined to be 321000 g / mol by GPC.

[0059] Example 4

[0060] The polyvinylidene fluoride copolymer of the present invention is prepared by the following steps:

[0061] (1) Add 11.5 kg of deionized water to a 20 L vertical polymerization reactor. Close the reactor, evacuate the reactor, and replace it with nitrogen several times until the oxygen content in the reactor is less than 10 ppm. Heat the reactor to 85 ° C, start the reactor stirring, and rotate at 400 r / min. Inject a certain amount of VDF monomer until the reactor pressure reaches 4.1 MPa. Add 6.4 g of ammonium persulfate and 34.8 g of diethyl malonate to the reactor through an auxiliary pump at a rate of 150 g / hour. At the same time, the polymerization reaction begins, and the reaction pressure is kept constant by continuously adding VDF monomer. After 1 hour of reaction, add 2.8 g of ammonium persulfate, and after 2 hours of reaction, add 2.8 g of ammonium persulfate and 9.3 g of diethyl malonate. When the amount of VDF monomer reaction reaches 3 kg, stop the reaction. The reaction time is 4.7 hours. Depressurize the reactor, collect the emulsion, break the emulsion, and wash repeatedly until the conductivity of the filtrate drops below 0.1. Finally, the product was placed in an oven at 95°C and dried for 24 h to obtain PVDF resin powder.

[0062] (2) PVDF resin powder was dissolved in acetone, and NaOH and a phase transfer catalyst were added. After the reaction for 10 h, the powder was dried to obtain PVDF resin powder with carboxyl groups at both ends of the molecular chain. The molecular weight Mw was determined to be 10,000 g / mol by GPC.

[0063] (3) 0.15 mol of PVDF resin powder with carboxyl groups at both ends of the molecular chain, 0.05 mol of trimellitic anhydride and 0.2 mol of 4,4'-diaminodiphenyl ether were dissolved in an aprotic solvent, heated to 160°C, and reacted for 2 hours. Then, 0.05 mol of triphenylphosphine, 0.05 mol of pyridine and 0.1 mol of calcium chloride were added and the reaction was continued for 3 hours. The resulting polymer solution was precipitated in methanol to obtain a polyvinylidene fluoride copolymer, and its molecular weight Mw was determined to be 303000 g / mol by GPC.

[0064] Example 5

[0065] The polyvinylidene fluoride copolymer of the present invention is prepared by the following steps:

[0066] (1) Add 11.5 kg of deionized water to a 20 L vertical polymerization reactor. Close the reactor, evacuate, and replace with nitrogen several times until the oxygen content in the reactor is less than 10 ppm. Heat the reactor to 85 ° C, start the reactor stirring, and rotate at 400 r / min. Inject a certain amount of VDF monomer and hexafluoropropylene monomer until the reactor pressure reaches 4.1 MPa. Add 6.4 g of ammonium persulfate and 34.8 g of diethyl malonate to the reactor through an auxiliary pump at a rate of 150 g / hour. At the same time, the polymerization reaction begins, and the reaction pressure is maintained constant by continuously adding VDF monomer. After 1 hour of reaction, 2.8 g of ammonium persulfate is added. After 2 hours of reaction, 2.8 g of ammonium persulfate and 9.3 g of diethyl malonate are added. When the amount of VDF monomer reaction reaches 3 kg, the reaction is stopped. The reaction time is 4.7 hours. The reactor was depressurized, the emulsion was collected, demulsified, and repeatedly washed until the conductivity of the filtrate dropped below 0.1. Finally, the product was dried in an oven at 95°C for 24 hours to obtain PVDF copolymer resin powder.

[0067] (2) PVDF copolymer resin powder was dissolved in acetone, and NaOH and a phase transfer catalyst were added. After the reaction for 10 h, the powder was dried to obtain PVDF copolymer resin powder with carboxyl groups at both ends of the molecular chain. The molecular weight Mw was determined to be 10,000 g / mol by GPC.

[0068] (3) 0.15 mol of PVDF copolymer resin powder with carboxyl groups at both ends of the molecular chain, 0.05 mol of trimellitic anhydride and 0.2 mol of 4,4'-diaminodiphenyl ether were dissolved in an aprotic solvent, heated to 160°C, and reacted for 2 hours. Then, 0.05 mol of triphenylphosphine, 0.05 mol of pyridine and 0.1 mol of calcium chloride were added and the reaction was continued for 3 hours. The resulting polymer solution was precipitated in methanol to obtain a polyvinylidene fluoride copolymer, and its molecular weight Mw was determined to be 305000 g / mol by GPC.

[0069] Comparative Example 1

[0070] Different from the above examples, this comparative example only prepares polyvinylidene fluoride resin according to the following steps:

[0071] In a 20L vertical polymerization reactor, add 11.5kg of deionized water. Close the reactor, evacuate, and replace the atmosphere with nitrogen several times until the oxygen content in the reactor is less than 10ppm. Heat the reactor to 85°C and start stirring at 400 rpm. Pump a predetermined amount of VDF monomer until the reactor pressure reaches 4.1 MPa. Add 0.24g of ammonium persulfate and 3.48g of diethyl malonate to the reactor via an auxiliary pump at a rate of 150g / hour. Simultaneously, the polymerization reaction begins, and the reaction pressure is maintained constant by continuously adding VDF monomer. After 1 hour of reaction, add 0.18g of ammonium persulfate, and after 2 hours of reaction, add another 0.18g of ammonium persulfate and 0.93g of diethyl malonate. When the amount of VDF monomer reacted reaches 3kg, stop the reaction. The reaction time is 4.7 hours. Depressurize the reactor, collect the emulsion, break the emulsion, and repeatedly wash it until the filtrate conductivity drops below 0.1. Finally, the product was placed in an oven at 95° C. and dried for 24 h to obtain polyvinylidene fluoride copolymer resin powder. GPC analysis showed that the molecular weight Mw was 312,000 g / mol.

[0072] Comparative Example 2

[0073] Different from the above-mentioned embodiment, this comparative example only uses a polyvinylidene fluoride resin containing carboxyl groups at both ends and a diamine monomer containing a phenylene ether group to copolymerize a polyvinylidene fluoride copolymer different from the present invention, and is carried out according to the following steps:

[0074] In a 20L vertical polymerization reactor, add 11.5kg of deionized water. Close the reactor, evacuate, and replace the atmosphere with nitrogen several times until the oxygen content in the reactor is less than 10ppm. Heat the reactor to 85°C and start stirring at 400 rpm. Pump a predetermined amount of VDF monomer until the reactor pressure reaches 4.1 MPa. Add 6.4g of ammonium persulfate and 34.8g of diethyl malonate to the reactor via an auxiliary pump at a rate of 150g / hour. Simultaneously, the polymerization reaction begins, and the reaction pressure is maintained constant by continuously adding VDF monomer. After 1 hour of reaction, add 2.8g of ammonium persulfate, and after 2 hours, add another 2.8g of ammonium persulfate and 9.3g of diethyl malonate. When the amount of VDF monomer reacted reaches 3kg, terminate the reaction. The reaction time is 4.7 hours. Depressurize the reactor, collect the emulsion, break the emulsion, and repeatedly wash it until the filtrate conductivity drops below 0.1. Finally, the product was placed in an oven at 95°C and dried for 24 h to obtain PVDF resin powder.

[0075] PVDF resin powder was dissolved in acetone, and NaOH and a phase transfer catalyst were added. After the reaction for 10 hours, the powder was dried to obtain PVDF resin powder with carboxyl groups at both ends of the molecular chain. The molecular weight Mw was determined to be 10,000 g / mol by GPC.

[0076] 0.2 mol of PVDF resin powder with carboxyl groups at both ends of the molecular chain and 0.2 mol of 4,4'-diaminodiphenyl ether were dissolved in an aprotic solvent, heated to 160°C, and reacted for 2 hours. Then, 0.05 mol of triphenylphosphine, 0.05 mol of pyridine and 0.1 mol of calcium chloride were added, and the reaction was continued for 3 hours. The resulting polymer solution was precipitated in methanol to obtain a polyvinylidene fluoride copolymer, whose molecular weight Mw was 301200 g / mol as determined by GPC.

[0077] Comparative Example 3

[0078] Different from the above examples, this comparative example uses only trimellitic anhydride and a diamine monomer containing a phenylene ether group to synthesize a polyamide-imide resin in the following steps:

[0079] 0.2 mol of trimellitic anhydride and 0.2 mol of 4,4'-diaminodiphenyl ether were dissolved in an aprotic solvent, heated to 160°C, and reacted for 2 hours. Then, 0.05 mol of triphenylphosphine, 0.05 mol of pyridine, and 0.1 mol of calcium chloride were added, and the reaction was continued for 3 hours. The resulting polymer solution was precipitated in methanol to obtain a polyamide-imide resin, whose molecular weight Mw was 55,000 g / mol as determined by GPC.

[0080] Performance Testing

[0081] The materials prepared in Examples 1-5 and Comparative Examples 1-3 were used as binders to prepare positive electrodes of lithium batteries, and then performance tests were performed.

[0082] The electrode preparation process is as follows: 2g of the material to be tested is dissolved in 100g of NMP (N-methylpyrrolidone) solution and stirred to dissolve thoroughly. Then, 2.8g of conductive agent carbon black and 56.2g of lithium-rich manganese-based positive electrode material Li are added under stirring. 1.13 Mn 0.463 Ni 0.203 Co 0.203 O2(0.3Li2MnO3·0.7LiMn 1 / 3 Ni 1 / 3 Co 1 / 3 O2) and then ultrasonically stirred for 2 hours to produce a uniform slurry. The resulting slurry was coated onto a 12μm thick electrode aluminum foil using a coater. The slurry-coated electrode foil was then dried in a vacuum oven at 100°C for 12 hours to obtain a positive electrode.

[0083] The positive electrodes prepared using the materials prepared in Examples 1-5 and Comparative Examples 1-3 were subjected to peel strength tests and high temperature resistance tests, respectively. The peel strength tests were carried out in accordance with the test method in the national standard GB / T2790-1995 "Adhesive 180° Peel Strength Test Method - Flexible Material to Rigid Material".

[0084] The test results are shown in the table below:

[0085]

[0086] As can be seen from the above table, compared with the polyvinylidene fluoride resin prepared in Comparative Example 1 and the polyvinylidene fluoride copolymer prepared in Comparative Example 2, the polyvinylidene fluoride copolymers prepared in Examples 1-5 of the present invention have stronger adhesion at high temperatures, as well as higher melting points and thermal decomposition temperatures, indicating that they have better high temperature resistance; at the same time, compared with the polyamide-imide resin prepared in Comparative Example 3, the polyvinylidene fluoride copolymers prepared in Examples 1-5 have higher molecular weights and stronger adhesion.

[0087] The above-described embodiments merely illustrate several implementations of the present invention. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the spirit of the present invention, and all such variations and improvements fall within the scope of protection of the present invention.

Claims

1. A vinylidene fluoride copolymer, characterized in that It is copolymerized by polyvinylidene fluoride resin with carboxyl groups at both ends, trimellitic anhydride and diamine monomer containing phenyl ether groups; The vinylidene fluoride copolymer has a structure shown in the following formula: Among them, R is the unit containing fluorine groups remaining after the reaction of polyvinylidene fluoride resin, R1 and R2 are the units containing phenyl ether groups remaining after the reaction of diamine monomer, n, m and p respectively represent the repetition number of the corresponding units, n is 15-4000, and the ratio of m to p is 1:1-3.

2. The vinylidene fluoride copolymer according to claim 1, characterized in that The diamine monomer is selected from one or more of 4,4'-diaminodiphenyl ether, 3,3'-diaminodiphenyl ether, 1,3-bis(3-aminophenoxy)benzene, 1,4-bis(4-aminophenoxy)benzene, 4,4'-bis(4-aminophenoxy)biphenyl, and 1,3-bis(4-aminophenoxy)benzene.

3. The vinylidene fluoride copolymer according to claim 1, characterized in that The polyvinylidene fluoride resin is one or more of a vinylidene fluoride homopolymer and a copolymer formed by vinylidene fluoride and other fluorine-containing monomers, and the other fluorine-containing monomers are selected from one or more of trifluoroethylene, chlorotrifluoroethylene, tetrafluoroethylene, pentafluoropropylene, hexafluoropropylene, perfluoromethyl vinyl ether or perfluoropropyl vinyl ether.

4. The vinylidene fluoride copolymer according to claim 1, characterized in that The polyvinylidene fluoride resin is prepared by emulsion polymerization or suspension polymerization.

5. The method for preparing the vinylidene fluoride copolymer according to any one of claims 1 to 4, comprising the steps of: (1) preparing polyvinylidene fluoride resin by polymerization; (2) using the polyvinylidene fluoride resin obtained in step (1) as a raw material to prepare a polyvinylidene fluoride resin containing carboxyl groups at both ends; (3) condensing and copolymerizing the polyvinylidene fluoride resin containing carboxyl groups at both ends obtained in step (2), trimellitic anhydride, and a diamine monomer containing a phenylene ether group to obtain a vinylidene fluoride copolymer.

6. The method according to claim 5, characterized in that The polymerization system of step (1) adopts an initiator and a chain transfer agent, wherein the initiator is an organic peroxide initiator, a persulfate or a persulfate / sodium bisulfite initiation system, and the chain transfer agent is one or more of ethyl acetate, diethyl malonate, diethyl carbonate and dimethyl carbonate, and the amount thereof is 0.01-1% of the mass of the polymerization monomer.

7. The preparation method according to claim 5, characterized in that Step (2) comprises: dissolving the polyvinylidene fluoride resin obtained in step (1) in an organic solvent, adding an inorganic base and a phase transfer catalyst, and reacting to obtain a polyvinylidene fluoride resin containing carboxyl groups at both ends.

8. The preparation method according to claim 5, characterized in that The polymerization system of step (3) adopts a condensing agent and an organic base, wherein the condensing agent is one or more of triphenylphosphine, triphenylphosphine-polyhalomethane, triphenylphosphine-hexachloroacetone, triphenylphosphine-NBS, 3-acyl-2-thiothiazoline, and tris(2,6-dimethoxyphenyl)bismuth, and the organic base is one or more of pyridine, triethylamine, and triallylamine.

9. Use of the vinylidene fluoride copolymer according to any one of claims 1 to 4 as a binder for lithium batteries.

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