Method for preparing high-strength diaphragm by using polyolefin elastomer

Abstract

The invention discloses a method for preparing a high-strength diaphragm by using a polyolefin elastomer, and relates to the technical field of battery diaphragm materials, the method comprises the following steps: (1) mixing polypropylene with the polyolefin elastomer to obtain an outer layer material; (2) mixing polyethylene with a polyolefin elastomer to obtain an inner layer material; (3) respectively melting and plasticizing the outer layer material and the inner layer material, and then carrying out three-layer co-extrusion, traction and cooling to obtain a casting piece; and (4) carrying out annealing treatment, stretching and heat setting on the cast sheet to obtain the diaphragm. By adding the polyolefin elastomer, the transverse tensile strength of the dry-method diaphragm is improved, and the problem that the dry-method diaphragm is easy to tear is effectively solved; and the hole closing temperature of the diaphragm is reduced, rapid temperature rise after the battery is abnormal can be effectively blocked, the diaphragm breaking temperature of the diaphragm can be increased to a certain degree, and the safety of the battery is greatly improved.

Description

Technical Field

[0001] This invention relates to the field of battery separator material technology, and specifically to a method for preparing a high-strength separator using polyolefin elastomers. Background Technology

[0002] The separator is a key component of lithium-ion batteries. Its basic function is to isolate the positive and negative electrodes to prevent short circuits, while allowing lithium ions to pass freely to ensure normal charging and discharging. Currently, commercially available lithium-ion battery separator materials are mainly polyolefins, including polypropylene (PP) films, polyethylene (PE) films, and PP / PE / PP composite films. Among them, PP films have high mechanical strength and good chemical stability, but low porosity, resulting in relatively high internal resistance. PE films have a low melting point and low pore-closing temperature, improving battery safety; and high porosity, resulting in good lithium-ion transport efficiency; however, their mechanical strength is relatively weak. PP / PE / PP composite films combine the advantages of PP and PE films, possessing the advantages of low pore-closing temperature and high melting temperature. At higher temperatures, the separator self-closes its pores without melting, and the outer PP film has antioxidant properties, which can improve the cycle performance and safety performance of the separator.

[0003] There are two main processing methods for separators: dry and wet processes. Each method has its advantages and disadvantages and is suitable for different applications. The wet-process separator production process includes steps such as feeding, casting, stretching, extraction, and winding. It has the advantages of high porosity and uniform pore size, but it requires solvents, resulting in higher costs, complex processes, and low production efficiency. The dry-process separator production process includes steps such as raw material mixing, melt extrusion, stretching, and heat setting. It has the advantages of simple operation, environmental friendliness, lower cost, and suitability for large-scale production. However, dry-process separators have lower transverse tensile strength and lower transverse thermal shrinkage when heated, making them prone to tearing laterally, which can affect battery safety. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a method for preparing a high-strength separator using polyolefin elastomers. By adding an appropriate amount of polyolefin elastomers, the lateral tensile strength of the dry separator is improved, thereby enhancing the safety of the battery.

[0005] The technical problem to be solved by this invention is achieved by the following technical solution:

[0006] This invention provides a method for preparing a high-strength membrane using polyolefin elastomers, comprising the following steps:

[0007] (1) Mix polypropylene with polyolefin elastomer to obtain outer layer material;

[0008] (2) Mix polyethylene with polyolefin elastomer to obtain inner layer material;

[0009] (3) After melting and plasticizing the outer and inner layers respectively, the three layers are co-extruded, drawn, and cooled to obtain a cast sheet;

[0010] (4) Anneal the casting, stretch it, heat set it to obtain a diaphragm.

[0011] Furthermore, the polyolefin elastomer is one or more of ethylene-propylene copolymer (POP) and ethylene-octene copolymer (POE).

[0012] Furthermore, the mass ratio of polypropylene to polyolefin elastomer is (85-95):(5-15). It is necessary to control the amount of polyolefin elastomer added to both polypropylene and polyethylene to reduce costs and improve the processing performance and application effect of the diaphragm.

[0013] Furthermore, the mass ratio of the polyethylene to the polyolefin elastomer is (85-95):(5-15).

[0014] Furthermore, the outer and inner layers also include additives, which are selected from one or more of toughening agents, antioxidants, stabilizers, and slip agents. Commonly used toughening agents, antioxidants, stabilizers, and slip agents in the art can be selected. The addition of additives can improve the processing performance and performance of the three-layer co-extruded diaphragm.

[0015] In one specific embodiment, the toughening agent accounts for 5-10% of the mass of the outer and inner layers, and the toughening agent is zinc dipalmitate.

[0016] In one specific implementation, the stabilizer accounts for 0.1% to 1% of the mass of the outer and inner layers, and the stabilizer is propionyl ferrocene.

[0017] Furthermore, the melt index of the polypropylene at a temperature of 230°C and a load of 2.16 kg is less than 1 g / 10 min.

[0018] Furthermore, the melt index of the polyethylene at a temperature of 230°C and a load of 2.16 kg is less than 1 g / 10 min.

[0019] Furthermore, the melt index of the polyolefin elastomer is less than 1 g / 10 min under the conditions of temperature 230°C and load 2.16 kg.

[0020] On the one hand, this invention selects low melt index polypropylene and polyethylene as raw materials. Low melt index separators can quickly close pores when the battery overheats, blocking ion conduction and effectively preventing short circuits. Furthermore, they exhibit good dimensional stability at high temperatures and maintain stable performance over a wide temperature range, further enhancing battery safety. On the other hand, this invention improves the transverse tensile strength of the separator by adding an appropriate amount of polyolefin elastomer to the polypropylene and polyethylene, effectively solving the problem of easy tearing in dry-process separators.

[0021] Furthermore, the traction speed is 50–100 m / min.

[0022] Furthermore, the annealing temperature is 110–140°C, and the annealing time is 8–24 hours. Annealing can eliminate stress within the film, enhance its stability, and improve its mechanical properties.

[0023] Furthermore, the stretching includes cold stretching and hot stretching. The cold stretching temperature is 30–60°C, and the stretching ratio is 1.2–1.5. The hot stretching temperature is 120–150°C, and the stretching ratio is 1.5–3. Stretching peels away the crystal interface, forming a porous structure.

[0024] Furthermore, the heat setting temperature is 120–150°C, and the heat setting time is 1–5 minutes. Heat setting can improve the dimensional stability of the film, eliminate internal stress, improve surface smoothness, enhance heat resistance, and improve mechanical properties.

[0025] The beneficial effects of this invention are: by adding polyolefin elastomer, this invention improves the transverse tensile strength of dry-process separators, effectively solving the problem of easy tearing of dry-process separators; and reduces the pore-closing temperature of the separator, effectively preventing the rapid temperature rise after the battery malfunctions, and also increasing the membrane rupture temperature to a certain extent, greatly improving battery safety. Detailed Implementation

[0026] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below with reference to specific embodiments.

[0027] The following descriptions of the raw materials used in the examples and comparative examples are as follows:

[0028] PP: CNOOC Shell 2420D, with a melt index of 0.25 g / 10 min at a temperature of 230℃ and a load of 2.16 kg.

[0029] PE: Yanshan Petrochemical 2F0.4A-1, with a melt index of 0.4 g / 10 min at a temperature of 230℃ and a load of 2.16 kg.

[0030] POP: Dow Affinity POP EG 8150G(2), with a melt index of 0.5 g / 10 min at a temperature of 230 °C and a load of 2.16 kg.

[0031] POE: SABIC FORTIFYPOE C0560, with a melt index of 0.5 g / 10 min at a temperature of 230 °C and a load of 2.16 kg.

[0032] Example 1

[0033] (1) Mix PP and POP at a mass ratio of 85:15 evenly to obtain the outer layer material.

[0034] (2) Mix PE and POP at a mass ratio of 85:15 evenly to obtain the inner layer material.

[0035] (3) After melting and plasticizing the outer layer material and the inner layer material respectively, three-layer co-extrusion is carried out. The melting and plasticizing temperature of the outer layer material is 140-200℃ and the screw speed is 150rpm; the melting and plasticizing temperature of the inner layer material is 150-210℃ and the screw speed is 150rpm; the die temperature of the three-layer co-extrusion is 190-210℃, and it is pulled at a speed of 100m / min and cooled on a 30℃ casting roller to obtain a casting sheet.

[0036] (4) The casting was annealed at 120°C for 12 hours, cold stretched at 40°C and stretching ratio of 1.2, hot stretched at 150°C and stretching ratio of 2.5, and heat-set at 120°C for 4 minutes to obtain the diaphragm.

[0037] Example 2

[0038] (1) Mix PP and POE at a mass ratio of 90:10 evenly to obtain the outer layer material.

[0039] (2) Mix PE and POE at a mass ratio of 90:10 evenly to obtain the inner layer material.

[0040] (3) After melting and plasticizing the outer layer material and the inner layer material respectively, three-layer co-extrusion is carried out. The melting and plasticizing temperature of the outer layer material is 140-200℃ and the screw speed is 50rpm; the melting and plasticizing temperature of the inner layer material is 150-210℃ and the screw speed is 50rpm; the die temperature of the three-layer co-extrusion is 190-210℃, and it is pulled at a speed of 50m / min and cooled on a 50℃ casting roller to obtain a casting sheet.

[0041] (4) The casting was annealed at 130°C for 8 hours, cold stretched at 30°C and stretching ratio of 1.2, hot stretched at 130°C and stretching ratio of 2, and heat-set at 130°C for 2 minutes to obtain the diaphragm.

[0042] Example 3

[0043] (1) Mix PP and POP at a mass ratio of 95:5 evenly to obtain the outer layer material.

[0044] (2) Mix PE and POP at a mass ratio of 95:5 evenly to obtain the inner layer material.

[0045] (3) After melting and plasticizing the outer layer material and the inner layer material respectively, three-layer co-extrusion is carried out. The melting and plasticizing temperature of the outer layer material is 140-200℃ and the screw speed is 100rpm; the melting and plasticizing temperature of the inner layer material is 150-210℃ and the screw speed is 100rpm; the die temperature of the three-layer co-extrusion is 190-210℃, and it is pulled at a speed of 50m / min and cooled on a 40℃ casting roller to obtain a casting sheet.

[0046] (4) The casting was annealed at 110°C for 24 hours, cold-stretched at 50°C and stretching ratio of 1.3, hot-stretched at 120°C and stretching ratio of 3, and heat-set at 120°C for 5 minutes to obtain the diaphragm.

[0047] Example 4

[0048] (1) Mix PP and POE at a mass ratio of 88:12 evenly to obtain the outer layer material.

[0049] (2) Mix PE and POE at a mass ratio of 88:12 evenly to obtain the inner layer material.

[0050] (3) After the outer layer material and the inner layer material are melted and plasticized, they are co-extruded in three layers. The melting and plasticizing temperature of the outer layer material is 140-200℃ and the screw speed is 100rpm; the melting and plasticizing temperature of the inner layer material is 150-210℃ and the screw speed is 100rpm; the die temperature of the three-layer co-extrusion is 190-210℃, and it is pulled at a speed of 80m / min and cooled on a 50℃ casting roller to obtain a cast sheet.

[0051] (4) The casting was annealed at 120°C for 10 hours, cold-stretched at 60°C and stretching ratio of 1.5, hot-stretched at 140°C and stretching ratio of 2, and heat-set at 150°C for 1 minute to obtain the diaphragm.

[0052] Example 5

[0053] (1) Mix PP and POP at a mass ratio of 90:10 evenly to obtain the outer layer material.

[0054] (2) Mix PE and POP at a mass ratio of 95:5 evenly to obtain the inner layer material.

[0055] (3) After melting and plasticizing the outer layer material and the inner layer material respectively, three-layer co-extrusion is carried out. The melting and plasticizing temperature of the outer layer material is 140-200℃ and the screw speed is 80rpm; the melting and plasticizing temperature of the inner layer material is 150-210℃ and the screw speed is 80rpm; the die temperature of the three-layer co-extrusion is 190-210℃, and it is pulled at a speed of 50m / min and cooled on a 35℃ casting roller to obtain a casting sheet.

[0056] (4) The casting was annealed at 130°C for 8 hours, cold stretched at 55°C and a stretching ratio of 1.4, hot stretched at 130°C and a stretching ratio of 2.5, and heat-set at 140°C for 3 minutes to obtain the diaphragm.

[0057] Example 6

[0058] The method is the same as in Example 1, except that in Example 6, 5% zinc dipalmitate was added as a toughening agent to both the outer and inner layers.

[0059] (1) PP, POP and zinc dipalmitate are mixed evenly to obtain the outer layer material. The mass ratio of PP to POP is 85:15, and the mass content of zinc dipalmitate in the outer layer material is 5%.

[0060] (2) Mix PE, POP and zinc dipalmitate evenly to obtain the inner layer material. The mass ratio of PE to POP is 85:15, and the mass content of zinc dipalmitate in the inner layer material is 5%.

[0061] (3) After melting and plasticizing the outer layer material and the inner layer material respectively, three-layer co-extrusion is carried out. The melting and plasticizing temperature of the outer layer material is 140-200℃ and the screw speed is 150rpm; the melting and plasticizing temperature of the inner layer material is 150-210℃ and the screw speed is 150rpm; the die temperature of the three-layer co-extrusion is 190-210℃, and it is pulled at a speed of 100m / min and cooled on a 30℃ casting roller to obtain a casting sheet.

[0062] (4) The casting was annealed at 120°C for 12 hours, cold stretched at 40°C and stretching ratio of 1.2, hot stretched at 150°C and stretching ratio of 2.5, and heat-set at 120°C for 4 minutes to obtain the diaphragm.

[0063] Example 7

[0064] The method is the same as in Example 1, except that in Example 7, 10% zinc dipalmitate was added as a toughening agent to both the outer and inner layers.

[0065] (1) PP, POP and zinc dipalmitate are mixed evenly to obtain the outer layer material. The mass ratio of PP to POP is 85:15, and the mass content of zinc dipalmitate in the outer layer material is 10%.

[0066] (2) Mix PE, POP and zinc dipalmitate evenly to obtain the inner layer material. The mass ratio of PE to POP is 85:15, and the mass content of zinc dipalmitate in the inner layer material is 10%.

[0067] (3) After melting and plasticizing the outer layer material and the inner layer material respectively, three-layer co-extrusion is carried out. The melting and plasticizing temperature of the outer layer material is 140-200℃ and the screw speed is 150rpm; the melting and plasticizing temperature of the inner layer material is 150-210℃ and the screw speed is 150rpm; the die temperature of the three-layer co-extrusion is 190-210℃, and it is pulled at a speed of 100m / min and cooled on a 30℃ casting roller to obtain a casting sheet.

[0068] (4) The casting was annealed at 120°C for 12 hours, cold stretched at 40°C and stretching ratio of 1.2, hot stretched at 150°C and stretching ratio of 2.5, and heat-set at 120°C for 4 minutes to obtain the diaphragm.

[0069] Example 8

[0070] The method of Example 1 is followed, except that in Example 8, 0.5% propionyl ferrocene was added as a stabilizer to both the outer and inner layers.

[0071] (1) PP, POP and propionyl ferrocene are mixed evenly to obtain the outer layer material. The mass ratio of PP to POP is 85:15, and the mass content of propionyl ferrocene in the outer layer material is 0.5%.

[0072] (2) PE, POP and propionyl ferrocene are mixed evenly to obtain the inner layer material. The mass ratio of PE to POP is 85:15, and the mass content of propionyl ferrocene in the inner layer material is 0.5%.

[0073] (3) After melting and plasticizing the outer layer material and the inner layer material respectively, three-layer co-extrusion is carried out. The melting and plasticizing temperature of the outer layer material is 140-200℃ and the screw speed is 150rpm; the melting and plasticizing temperature of the inner layer material is 150-210℃ and the screw speed is 150rpm; the die temperature of the three-layer co-extrusion is 190-210℃, and it is pulled at a speed of 100m / min and cooled on a 30℃ casting roller to obtain a casting sheet.

[0074] (4) The casting was annealed at 120°C for 12 hours, cold stretched at 40°C and stretching ratio of 1.2, hot stretched at 150°C and stretching ratio of 2.5, and heat-set at 120°C for 4 minutes to obtain the diaphragm.

[0075] Example 9

[0076] The method of Example 1 is followed, except that in Example 8, 0.8% propionyl ferrocene was added as a stabilizer to both the outer and inner layers.

[0077] (1) PP, POP and propionyl ferrocene are mixed evenly to obtain the outer layer material. The mass ratio of PP to POP is 85:15, and the mass content of propionyl ferrocene in the outer layer material is 0.8%.

[0078] (2) PE, POP and propionyl ferrocene are mixed evenly to obtain the inner layer material. The mass ratio of PE to POP is 85:15, and the mass content of propionyl ferrocene in the inner layer material is 0.8%.

[0079] (3) After melting and plasticizing the outer layer material and the inner layer material respectively, three-layer co-extrusion is carried out. The melting and plasticizing temperature of the outer layer material is 140-200℃ and the screw speed is 150rpm; the melting and plasticizing temperature of the inner layer material is 150-210℃ and the screw speed is 150rpm; the die temperature of the three-layer co-extrusion is 190-210℃, and it is pulled at a speed of 100m / min and cooled on a 30℃ casting roller to obtain a casting sheet.

[0080] (4) The casting was annealed at 120°C for 12 hours, cold stretched at 40°C and stretching ratio of 1.2, hot stretched at 150°C and stretching ratio of 2.5, and heat-set at 120°C for 4 minutes to obtain the diaphragm.

[0081] Comparative Example 1

[0082] The method of Example 1 was followed, except that no polyolefin elastomer was added in Comparative Example 1.

[0083] (1) After PP and PE are melted and plasticized separately, they are co-extruded in three layers. The melting and plasticizing temperature of PP is 140-200℃ and the screw speed is 150rpm; the melting and plasticizing temperature of PE is 150-210℃ and the screw speed is 150rpm; the die temperature of the three-layer co-extrusion is 190-210℃, and it is pulled at a speed of 100m / min and cooled on a casting roller at 30℃ to obtain a cast sheet.

[0084] (2) The casting was annealed at 120°C for 12 hours, cold stretched at 40°C and stretching ratio of 1.2, hot stretched at 150°C and stretching ratio of 2.5, and heat-set at 120°C for 4 minutes to obtain the diaphragm.

[0085] The membranes (with a thickness of 5 μm) prepared in Examples 1 to 9 and Comparative Example 1 were subjected to performance tests, and the test results are shown in Table 1.

[0086] The transverse tensile strength and longitudinal heat shrinkage rate of the diaphragm were tested according to standard GB / T 36363-2018 (105℃, 1h).

[0087] Table 1 Performance test results of the diaphragm

[0088] Tensile strength (MPa) Heat shrinkage rate (%) Example 1 14.7 1.68 Example 2 12.8 1.72 Example 3 13.2 1.70 Example 4 14.3 1.75 Example 5 13.9 1.79 Example 6 16.4 1.61 Example 7 17.0 1.59 Example 8 14.2 1.04 Example 9 13.8 0.98 Comparative Example 1 10.5 1.93

[0089] As can be seen from Table 1, the present invention can substantially improve the transverse tensile strength of the diaphragm by adding polyolefin elastomer and improve the thermal stability of the diaphragm to a certain extent; and can further improve the transverse tensile strength of the diaphragm by adding zinc dipalmitate as a toughening agent, and can further improve the thermal stability of the diaphragm by adding propionyl ferrocene as a stabilizer.

[0090] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a high-strength membrane using polyolefin elastomers, characterized in that, Includes the following steps: (1) Mix polypropylene with polyolefin elastomer to obtain outer layer material; (2) Mix polyethylene with polyolefin elastomer to obtain inner layer material; (3) After melting and plasticizing the outer and inner layers respectively, the three layers are co-extruded, drawn, and cooled to obtain a cast sheet; (4) Anneal the casting, stretch it, heat set it to obtain a diaphragm.

2. The method for preparing a high-strength membrane using polyolefin elastomer according to claim 1, characterized in that: The polyolefin elastomer is one or more of ethylene-propylene copolymer and ethylene-octene copolymer.

3. The method for preparing a high-strength membrane using polyolefin elastomer according to claim 1, characterized in that: The mass ratio of the polypropylene to the polyolefin elastomer is (85-95):(5-15); Preferably, the mass ratio of polyethylene to polyolefin elastomer is (85-95):(5-15).

4. The method for preparing a high-strength membrane using polyolefin elastomer according to claim 1, characterized in that: The outer and inner layers also include additives, which are selected from one or more of toughening agents, antioxidants, stabilizers, and slip agents.

5. The method for preparing a high-strength membrane using polyolefin elastomer according to claim 1, characterized in that: The polypropylene has a melt flow index of less than 1 g / 10 min at a temperature of 230°C and a load of 2.16 kg. Preferably, the polyethylene has a melt index of less than 1 g / 10 min under the conditions of 230°C and 2.16 kg load.

6. The method for preparing a high-strength membrane using polyolefin elastomer according to claim 1, characterized in that: The polyolefin elastomer has a melt index of less than 1 g / 10 min at a temperature of 230°C and a load of 2.16 kg.

7. The method for preparing a high-strength membrane using polyolefin elastomer according to claim 1, characterized in that: The traction speed is 50–100 m / min.

8. The method for preparing a high-strength membrane using polyolefin elastomer according to claim 1, characterized in that: The annealing temperature is 110–140°C, and the annealing time is 8–24 hours.

9. The method for preparing a high-strength membrane using polyolefin elastomer according to claim 1, characterized in that: The stretching includes cold stretching and hot stretching. The cold stretching temperature is 30-60℃ and the stretching ratio is 1.2-1.

5. The hot stretching temperature is 120-150℃ and the stretching ratio is 1.5-3.

10. The method for preparing a high-strength membrane using polyolefin elastomer according to claim 1, characterized in that: The heat setting temperature is 120–150°C, and the heat setting time is 1–5 min.