A polyvinylidene fluoride material, its preparation method and uses

Abstract

This invention provides a method for preparing polyvinylidene fluoride (PVDF) material, comprising adding deionized water, a chain transfer agent, a surfactant, and an initiator to a reaction vessel, stirring until homogeneous, evacuating and deoxygenating, heating to 65-110°C, then adding PVDF until the pressure inside the reaction vessel reaches 1.5-3.5 MPa to initiate the polymerization reaction, then continuing to add tetrafluoroethylene until the pressure reaches 4.8-5.5 MPa and heating to 125-142°C, until the reaction ends and a polymer emulsion is obtained; the obtained polymer emulsion is then coagulated, washed, dried, and granulated to obtain the final product. The PVDF material obtained by this method exhibits excellent bonding properties for battery applications and can ensure the electrical performance of lithium batteries, showing promising application prospects.

Description

Technical Field

[0001] This invention relates to the field of organic material synthesis, and more specifically, to a polyvinylidene fluoride material, its preparation method, and its uses. Background Technology

[0002] Polyvinylidene fluoride (PVDF) is a highly non-reactive thermoplastic fluoropolymer, soluble in strong polar solvents such as dimethylacetamide, and can be synthesized through the polymerization of 1,1-difluoroethylene. PVDF possesses excellent properties such as anti-aging, chemical resistance, weather resistance, and UV radiation resistance, and is widely used in coatings, lithium batteries, filter membranes, films, and wires and cables. Coatings, filter membranes, and lithium batteries are the three most important products, accounting for over 50% of global PVDF production. In recent years, the rapid development of the new energy vehicle industry has spurred the simultaneous growth of the lithium battery industry. PVDF is a popular material for binders, dispersants, and electrolytes in lithium batteries. Therefore, major manufacturers are accelerating the deployment of new production capacity and optimizing product performance, primarily focusing on improving production process yield, controlling molecular weight distribution, and enhancing stability.

[0003] Patent CN106632770B discloses a method for preparing polyvinylidene fluoride (PVDF). The method involves adding PVDF, deionized water, an initial chain transfer agent, and a dispersant to a reaction vessel, evacuating and deoxygenating the vessel, heating to 20-150°C, and adding PVDF monomer to bring the pressure inside the vessel to 1.0-6.0 MPa. An initial initiator is then added, and the polymerization reaction begins. Further chain transfer agents and initiators are then added. This patent is the first to use two chain transfer agents in the polymerization process, resulting in a product with uniform molecular weight and narrow distribution, and high production efficiency. However, it does not address improvements to the bonding properties when used as a binder. The bonding properties of PVDF are also a crucial factor affecting battery performance. If the bonding performance is unsuitable, over time, the electrode binder layer may partially or completely peel off from the current collector due to internal stress in the electrode, leading to poor load characteristics and capacity degradation.

[0004] In addition, polyvinylidene fluoride is an important binder in the lithium battery industry. However, within the temperature range commonly used in lithium batteries, it may still produce a large amount of crystals, which can hinder the flow of electrolyte molecules, increase charge and discharge charge, and lead to poor performance of lithium batteries.

[0005] Therefore, how to produce polyvinylidene fluoride materials suitable for use in lithium batteries is an important research topic with very promising prospects. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides a method for preparing polyvinylidene fluoride (PVDF) material. The PVDF material obtained by this method exhibits excellent bonding properties for lithium batteries and ensures the electrical performance of lithium batteries.

[0007] This invention provides a method for preparing polyvinylidene fluoride (PVDF) material, comprising:

[0008] Deionized water, chain transfer agent, surfactant and initiator are added to the reaction vessel and stirred evenly. After vacuuming and deoxygenation, the temperature is raised to 65-110℃. Then, vinylidene fluoride is added until the pressure in the reaction vessel is 1.5-3.5 MPa to start the polymerization reaction. Tetrafluoroethylene is then added until the pressure is 4.8-5.5 MPa and the temperature is raised to 125-142℃. The reaction ends and a polymer emulsion is obtained.

[0009] The obtained polymer emulsion is then coagulated, washed, dried, and granulated to obtain the final product.

[0010] In one embodiment, the initiator is a composition of tert-amyl hydrogen peroxide, tert-dodecyl mercaptan and potassium persulfate in a mass ratio of (3-7):(2-5):(0.01-0.6).

[0011] As one implementation method, the amount of initiator added is 0.08 to 1.8 parts by weight, based on 100 parts by weight of deionized water.

[0012] In one embodiment, the chain transfer agent is a combination of mercaptoethanol and tert-butanol in a mass ratio of (1-2):(3-7).

[0013] In one embodiment, the amount of chain transfer agent added is 0.2 to 4.0 parts by weight, based on 100 parts by weight of deionized water.

[0014] In one embodiment, the surfactant is a composition of perfluoroethylene propylene micro powder and perfluorooctanoic acid or ammonium perfluorooctanoate; preferably, the surfactant is a composition of perfluoroethylene propylene micro powder and ammonium perfluorooctanoate in a mass ratio of (4-7):1.

[0015] In one embodiment, the amount of surfactant added is 0.2 to 1.5 parts per 100 parts by weight of deionized water.

[0016] In one implementation method, a vacuum is drawn and oxygen is removed, the temperature is raised to 98-110°C, and then vinylidene fluoride is added until the pressure inside the reaction vessel is 2.1-3.3 MPa to start the polymerization reaction. Then, tetrafluoroethylene is added until the pressure is 5.1-5.5 MPa and the temperature is raised to 130-140°C.

[0017] The present invention also provides vinylidene fluoride materials prepared by any of the above methods.

[0018] The present invention further provides the use of the above-mentioned polyvinylidene fluoride material, using the polyvinylidene fluoride material as a binder for lithium batteries.

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

[0020] 1. The PVDF material prepared by the method of the present invention has suitable adhesion and is particularly suitable for use in lithium batteries. The electrode adhesive layer formed thereon is not easily peeled off from the current collector, either partially or completely.

[0021] 2. When the PVDF material prepared by the method of this invention is used in lithium batteries, it can ensure that the load characteristics and capacity are maintained well, and the battery performance is more reliable.

[0022] 3. The raw materials used in the preparation method of the present invention are simpler than those disclosed in other existing technologies, which can reduce costs and make it more industrially applicable. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention are described clearly and completely below. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the described embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0024] Unless otherwise specified, the materials or equipment used in the following cases are all general materials obtained from the market through commercial procurement, and different manufacturers or batches do not affect the realization of the excellent effects of the present invention.

[0025] In the case study, the specific ratios of the initiator, chain transfer agent, and surfactant were all mass ratios.

[0026] Example 1

[0027] Add 100 parts of deionized water, 0.2 parts of chain transfer agent (mercaptoethanol: tert-butanol = 1:7), 0.2 parts of surfactant (poly(perfluoroethylene propylene) micro powder: ammonium perfluorooctanoate = 4:1), and 0.08 parts of initiator (tert-amyl hydrogen peroxide: tert-dodecyl mercaptan: potassium persulfate = 3:2:0.6) to the reaction vessel. After stirring evenly, evacuate and deoxygenate until the oxygen content drops below 30 ppm. Raise the temperature to 65°C, then add vinylidene fluoride until the pressure in the reaction vessel reaches 3.5 MPa to start the polymerization reaction. Continue to add tetrafluoroethylene until the pressure reaches 4.8 MPa and raise the temperature to 142°C. The reaction ends, and a polymer emulsion is obtained. The obtained polymer emulsion is then coagulated, washed, dried, and granulated to obtain the final product.

[0028] Example 2

[0029] Add 100 parts of deionized water, 0.4 parts of chain transfer agent (mercaptoethanol: tert-butanol = 2:3), 0.4 parts of surfactant (poly(perfluoroethylene propylene) micro powder: ammonium perfluorooctanoate = 5:1), and 0.1 parts of initiator (tert-amyl hydrogen peroxide: tert-dodecyl mercaptan: potassium persulfate = 7:5:0.01) to the reaction vessel. After stirring evenly, evacuate and deoxygenate until the oxygen content drops below 30 ppm. Raise the temperature to 80°C, then add vinylidene fluoride until the pressure in the reaction vessel reaches 1.5 MPa to start the polymerization reaction. Continue to add tetrafluoroethylene until the pressure reaches 5.0 MPa and raise the temperature to 142°C. The reaction ends, and a polymer emulsion is obtained. The obtained polymer emulsion is then coagulated, washed, dried, and granulated to obtain the final product.

[0030] Example 3

[0031] Add 100 parts of deionized water, 0.8 parts of chain transfer agent (mercaptoethanol: tert-butanol = 1:4), 0.6 parts of surfactant (poly(perfluoroethylene propylene) micro powder: ammonium perfluorooctanoate = 5:1), and 0.3 parts of initiator (tert-amyl hydrogen peroxide: tert-dodecyl mercaptan: potassium persulfate = 3:3:0.3) to the reaction vessel. After stirring evenly, evacuate and deoxygenate until the oxygen content drops below 30 ppm. Raise the temperature to 75°C, then add vinylidene fluoride until the pressure in the reaction vessel reaches 2.0 MPa to start the polymerization reaction. Continue to add tetrafluoroethylene until the pressure reaches 5.4 MPa and raise the temperature to 128°C. The reaction ends, and a polymer emulsion is obtained. The obtained polymer emulsion is then coagulated, washed, dried, and granulated to obtain the final product.

[0032] Example 4

[0033] Add 100 parts of deionized water, 1.5 parts of chain transfer agent (mercaptoethanol: tert-butanol = 2:5), 0.8 parts of surfactant (poly(perfluoroethylene propylene) micro powder: ammonium perfluorooctanoate = 7:1), and 0.7 parts of initiator (tert-amyl hydrogen peroxide: tert-dodecyl mercaptan: potassium persulfate = 4:5:0.5) to the reaction vessel. After stirring evenly, evacuate and deoxygenate until the oxygen content drops below 30 ppm. Raise the temperature to 105°C, then add vinylidene fluoride until the pressure in the reaction vessel reaches 3.5 MPa to start the polymerization reaction. Continue to add tetrafluoroethylene until the pressure reaches 5.2 MPa and raise the temperature to 135°C. The reaction ends, and a polymer emulsion is obtained. The obtained polymer emulsion is then coagulated, washed, dried, and granulated to obtain the final product.

[0034] Example 5

[0035] Add 100 parts of deionized water, 2.0 parts of chain transfer agent (mercaptoethanol: tert-butanol = 1:6), 1.0 part of surfactant (poly(perfluoroethylene propylene) micro powder: ammonium perfluorooctanoate = 7:1), and 1.1 parts of initiator (tert-amyl hydrogen peroxide: tert-dodecyl mercaptan: potassium persulfate = 7:4:0.2) to the reaction vessel. After stirring evenly, evacuate and deoxygenate until the oxygen content drops below 30 ppm. Raise the temperature to 98°C, then add vinylidene fluoride until the pressure inside the reaction vessel reaches 3.3 MPa to start the polymerization reaction. Continue to add tetrafluoroethylene until the pressure reaches 5.3 MPa and raise the temperature to 130°C. The reaction ends, and a polymer emulsion is obtained. The obtained polymer emulsion is then coagulated, washed, dried, and granulated to obtain the final product.

[0036] Example 6

[0037] Add 100 parts of deionized water, 2.8 parts of chain transfer agent (mercaptoethanol: tert-butanol = 1:3), 1.2 parts of surfactant (poly(perfluoroethylene propylene) micro powder: ammonium perfluorooctanoate = 5:1), and 1.3 parts of initiator (tert-amyl hydrogen peroxide: tert-dodecyl mercaptan: potassium persulfate = 6:4:0.1) to the reaction vessel. After stirring evenly, evacuate and deoxygenate until the oxygen content drops below 30 ppm. Raise the temperature to 106°C, then add vinylidene fluoride until the pressure inside the reaction vessel reaches 2.8 MPa to start the polymerization reaction. Continue to add tetrafluoroethylene until the pressure reaches 5.4 MPa and raise the temperature to 139°C. The reaction ends, and a polymer emulsion is obtained. The obtained polymer emulsion is then coagulated, washed, dried, and granulated to obtain the final product.

[0038] Example 7

[0039] Add 100 parts of deionized water, 3.6 parts of chain transfer agent (mercaptoethanol: tert-butanol = 1:5), 1.4 parts of surfactant (poly(perfluoroethylene propylene) micro powder: ammonium perfluorooctanoate = 6:1), and 1.5 parts of initiator (tert-amyl hydrogen peroxide: tert-dodecyl mercaptan: potassium persulfate = 5:5:0.5) to the reaction vessel. After stirring evenly, evacuate and deoxygenate until the oxygen content drops below 30 ppm. Raise the temperature to 110°C, then add vinylidene fluoride until the pressure in the reaction vessel reaches 3.3 MPa to start the polymerization reaction. Continue to add tetrafluoroethylene until the pressure reaches 5.1 MPa and raise the temperature to 133°C. The reaction ends, and a polymer emulsion is obtained. The obtained polymer emulsion is then coagulated, washed, dried, and granulated to obtain the final product.

[0040] Example 8

[0041] Add 100 parts of deionized water, 4.0 parts of chain transfer agent (mercaptoethanol: tert-butanol = 1:3), 1.5 parts of surfactant (poly(perfluoroethylene propylene) micro powder: ammonium perfluorooctanoate = 4:1), and 1.8 parts of initiator (tert-amyl hydrogen peroxide: tert-dodecyl mercaptan: potassium persulfate = 4:4:0.4) to the reaction vessel. After stirring evenly, evacuate and deoxygenate until the oxygen content drops below 30 ppm. Raise the temperature to 98°C, then add vinylidene fluoride until the pressure inside the reaction vessel reaches 2.9 MPa to start the polymerization reaction. Continue to add tetrafluoroethylene until the pressure reaches 5.1 MPa and raise the temperature to 135°C. The reaction ends, and a polymer emulsion is obtained. The obtained polymer emulsion is then coagulated, washed, dried, and granulated to obtain the final product.

[0042] Comparative Example 1

[0043] The difference from Example 1 is that a vacuum was drawn and oxygen was removed until the oxygen content was reduced to below 30 ppm, the temperature was raised to 65°C, and then vinylidene fluoride was added until the pressure inside the reaction vessel reached 3.5 MPa to start the polymerization reaction. Vinylidene fluoride was then added again until the pressure reached 4.8 MPa and the temperature was raised to 142°C. Everything else remained the same.

[0044] Comparative Example 2

[0045] The difference from Example 1 is that, in this case, a vacuum was drawn and oxygen was removed until the oxygen content was reduced to below 30 ppm, the temperature was directly raised to 142°C, and then vinylidene fluoride was added until the pressure inside the reaction vessel reached 3.5 MPa to begin the polymerization reaction. Vinylidene fluoride was then added further until the pressure reached 4.8 MPa. Everything else remained the same.

[0046] Experimental Example 1 Adhesion Test

[0047] The PVDF materials obtained in Examples 1-8, the PVDF materials obtained in Comparative Examples 1-2, and the commercially available PVDF product JH-D2500 homopolymer resin were dissolved in DMAc solvent to prepare solutions with a mass fraction of 8 wt%. The solutions were then coated onto a clean copper plate using a coater and placed at 60°C for 24 hours. After film formation, the films were adhered to the surface of the positive electrode with transparent tape and cut into 200*40mm strips for 180° peel strength testing.

[0048] Table 1

[0049]

[0050] Experiment 2: Lithium-ion Battery Performance Testing

[0051] (1) Preparation of lithium-ion batteries

[0052] Step 1: Preparation of the positive electrode sheet for lithium-ion batteries: The positive electrode active material LiCoO2, PVDF binder, and conductive carbon black are mixed in an N-methylpyrrolidone solvent at a mass ratio of 95:3:2 and stirred until homogeneous to obtain a positive electrode slurry. The obtained positive electrode slurry is coated onto a 0.2 mm thick positive electrode current collector, dried, and cold-pressed to obtain a compaction density of 1.6 g / cm³. 3 The electrode sheet is then cut and the tabs are welded to obtain the positive electrode sheet;

[0053] Step 2: Preparation of lithium-ion battery negative electrode sheet

[0054] Carbon anode material, PVDF binder and conductive agent are mixed in N-methylpyrrolidone solvent at a mass ratio of 95:3:2. After uniform mixing, anode slurry is obtained. The anode slurry is then coated onto copper foil of anode current collector. After drying, anode film is formed. After cold pressing, slitting and welding of tabs, anode sheet is obtained.

[0055] Step 3: Preparation of electrolyte for lithium-ion batteries

[0056] Ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate were mixed evenly in a mass ratio of 2:1:7, and 16 wt% lithium hexafluorophosphate was added as a solute to prepare an electrolyte.

[0057] Step 4: The diaphragm is made of polyethylene porous membrane with a thickness of 16μm;

[0058] Step 5: Assembly of lithium batteries. The obtained positive electrode, negative electrode and separator are wound into a cell in sequence. The top and sides of the cell are sealed with aluminum mold film, leaving the electrolyte injection port. The lithium-ion battery is then produced through formation, capacity and other processes.

[0059] The PVDF adhesives mentioned above are the PVDF materials obtained in Examples 1-8, the PVDF materials obtained in Comparative Examples 1-2, and the commercially available PVDF product JH-D2500 homopolymer resin, which are used to manufacture electrode sheets, assemble batteries, and test their electrical performance.

[0060] The method for detecting the internal resistance of batteries is as follows: The internal resistance of each battery is tested by AC voltage drop internal resistance measurement method. That is, a small current of 50mA with a frequency of 1kHz is applied to the lithium battery, and its voltage is sampled. After a series of processes such as rectification and filtering, the internal resistance of the lithium battery is calculated by the operational amplifier circuit.

[0061] The method for testing battery cycle life is as follows: the test conditions are 1C charge and discharge, the capacity of the battery after 300 cycles is tested using an electrochemical workstation, and the capacity retention rate is calculated.

[0062] The specific performance results obtained from the tests are shown in Table 2.

[0063] Table 2

[0064] PVDF material used Battery internal resistance mΩ Capacity retention rate % Example 1 43 80 Example 2 41 83 Example 3 39 81 Example 4 40 82 Example 5 35 86 Example 6 37 85 Example 7 34 89 Example 8 36 87 Comparative Example 1 47 62 Comparative Example 2 45 69 JH-D2500 38 32

[0065] While the embodiments disclosed in this invention are as described above, the content is merely for the purpose of facilitating understanding of the invention and is not intended to limit the invention. Any person skilled in the art to which this invention pertains may make any modifications and changes to the form and details of the implementation without departing from the spirit and scope disclosed herein; however, the scope of patent protection of this invention shall still be determined by the scope defined in the appended claims.

Claims

1. A method for preparing polyvinylidene fluoride material, comprising: Deionized water, chain transfer agent, surfactant and initiator are added to the reaction vessel and stirred evenly. After vacuuming and deoxygenation, the temperature is raised to 98~110℃. Then, vinylidene fluoride is added until the pressure in the reaction vessel is 2.1~3.3MPa to start the polymerization reaction. Tetrafluoroethylene is then added until the pressure is 5.1~5.5MPa and the temperature is raised to 130~140℃. The reaction ends and a polymer emulsion is obtained. The obtained polymer emulsion is then coagulated, washed, dried, and granulated to obtain the final product. The initiator is a composition of tert-amyl hydrogen peroxide, tert-dodecyl mercaptan, and potassium persulfate in a mass ratio of (3~7): (2~5): (0.01~0.6). The chain transfer agent is a combination of mercaptoethanol and tert-butanol in a mass ratio of (1~2):(3~7); The surfactant is a composition of poly(perfluoroethylene propylene) micro powder and ammonium perfluorooctanoate in a mass ratio of (4~7):1; Based on 100 parts by weight of deionized water, the amount of chain transfer agent added is 0.2 to 4.0 parts by weight, the amount of surfactant added is 0.2 to 1.5 parts by weight, and the amount of initiator added is 0.08 to 1.8 parts by weight.

2. The polyvinylidene fluoride material obtained by the preparation method described in claim 1.

3. The use of the polyvinylidene fluoride material as described in claim 2, characterized in that, The polyvinylidene fluoride material is used as a binder for lithium batteries.

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