High strength, high elongation separator and its manufacturing method

Abstract

The present invention discloses a high-strength, high-elongation separator and a method for producing the same. [Solution] The high-strength, high-elongation separator comprises an ultra-high molecular weight polyolefin and a solubilizing agent, the solubilizing agent being a mixture of one or more of an ethylene-propylene copolymer, an ethylene-butene copolymer, and an ethylene-octene copolymer. The manufacturing method includes the steps of: mixing the ultra-high molecular weight polyolefin, the solubilizing agent, and the pore-forming agent and stirring until homogeneous to obtain a premixed raw material; injecting the premixed raw material into an extruder and heating it to obtain a thermodynamic single-phase melt; melt-casting the thermodynamic single-phase melt onto a cooling roll to obtain a crystallized cast sheet; primarily stretching the crystallized cast sheet and cooling it to obtain a primarily stretched film; secondarily stretching the primarily stretched film to obtain a secondarily stretched film; extracting, washing, and drying the resulting high-strength, high-elongation separator. The manufacturing method of the present invention utilizes the compatibilizing effect of the copolymer and multiple stepwise stretching processes to provide the separator with excellent ductility and tensile strength.

Description

[Technical Field]

[0001] The present invention relates to the technical field of battery separators, and more particularly to a high-strength, high-elongation separator and a method for producing the same. [Background technology]

[0002] Lithium-ion battery separators are crucial safety and functional components for lithium batteries, possessing the dual functions of electronic insulation and ionic conductivity. Insulation refers to the separator's ability to prevent short-circuiting between the positive and negative electrodes. Therefore, separators must have relatively high strength to prevent impurities or burrs from penetrating the separator and causing short-circuiting. At the same time, separators must also have a certain degree of ductility, allowing sufficient deformation space in the separator when the electrode material deforms to prevent direct separator rupture or electrode short-circuiting. However, the polymer polyolefins used to make separators become brittle but less ductile as their molecules are stretched, oriented, and crystallized during stretching. This means that separators with high rigidity and strength have lower ductility and flexibility, increasing the risk of fracture within the battery cell. For these reasons, it is difficult to achieve both ductility and strength in a separator, and how to effectively and simultaneously improve these two properties is a bottleneck in the field of separator manufacturing. Summary of the Invention [Problem to be solved by the invention]

[0003] In view of the drawbacks of the prior art, an object of the present invention is to provide a high-strength, high-elongation separator in which the processing flowability and system compatibility of ultra-high molecular weight polyolefins are improved by using a solubilizing agent.

[0004] Another object of the present invention is to provide a method for producing the high strength and high elongation separator. [Means for solving the problem]

[0005] The object of the present invention is achieved by the following technical solutions:

[0006] The high-strength, high-elongation separator comprises an ultra-high molecular weight polyolefin and a solubilizing agent, wherein the ratio of the ultra-high molecular weight polyolefin to the solubilizing agent is, in parts by mass, (20-40):(2-10), the solubilizing agent is a mixture of one or more of an ethylene-propylene copolymer, an ethylene-butene copolymer, and an ethylene-octene copolymer, the ultra-high molecular weight polyolefin comprises an ultra-high molecular weight polyethylene and an ultra-high molecular weight polypropylene, the weight-average molecular weight of the ultra-high molecular weight polyethylene is 600,000 to 4,000,000, and the weight-average molecular weight of the ultra-high molecular weight polypropylene is 400,000 to 2,600,000.

[0007] Ethylene-propylene copolymers are formed by copolymerizing ethylene and propylene.

[0008] Ethylene-butene copolymers are formed by copolymerizing ethylene and butene.

[0009] Ethylene-octene copolymer is formed by copolymerizing ethylene and octene.

[0010] In the above technical solution, the ratio of the ultra-high molecular weight polyethylene to the ultra-high molecular weight polypropylene is (60-94):(6-40) in parts by mass.

[0011] In the above technical solution, the melt flow index of the ultra-high molecular weight polyethylene is 0-0.8 g / 10 min under the condition of 21.6 kg and 190°C, and the melt flow index of the ultra-high molecular weight polypropylene is 0.1-10 g / 10 min under the condition of 21.6 kg and 190°C.

[0012] In the above technical solution, a porous structure is formed in the high-strength, high-elongation separator, and the porous structure is formed by first filling the separator with a pore-forming agent, and then extracting and washing the pore-forming agent from the separator, and the ratio of the solubilizing agent to the pore-forming agent, in parts by mass, is (2-10):(50-78).

[0013] In the above technical solution, the pore-forming agent is white oil and / or dioctyl terephthalate (DOTP).

[0014] In the above technical solution, the density of the solubilizer is 0.5 to 1 g / cm 3 The melting point is 50 to 90°C, and the crystallization peak temperature is 40 to 80°C.

[0015] The method for producing the high strength and high elongation separator includes the following steps.

[0016] Step 1: Mix and stir the ultra-high molecular weight polyolefin, solubilizer, and pore-forming agent until homogeneous to obtain a pre-mixed raw material.

[0017] In step 1, the stirring temperature is 60 to 110° C., the stirring rotation speed is 40 to 60 rpm, and the stirring time is 20 to 40 minutes.

[0018] In step 1, the premixed raw material further includes an antioxidant and a nucleating agent.

[0019] The above technical solution further comprises a nucleating agent, and the ratio of the nucleating agent to the solubilizing agent in parts by mass is (0.5-1):(2-10).

[0020] In the above technical solution, the nucleating agent is a mixture of pimelic acid and calcium stearate, and the ratio of pimelic acid to calcium stearate in parts by mass is (0.8~3.8):(1.2~4.2).

[0021] The above technical solution further comprises an antioxidant, and the ratio of the antioxidant to the solubilizer is (0.05-0.1):(2-10) in parts by mass.

[0022] In the above technical solution, the antioxidant is pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] (antioxidant 1010).

[0023] Step 2: The premixed raw materials are injected into an extruder and heated until melted to obtain a thermodynamic single-phase melt, and the thermodynamic single-phase melt is melt-cast onto a chill roll to obtain a crystallized cast sheet.

[0024] In step 2, the ratio of the linear velocity of the cooling roll to the flow velocity of the melt-cast sheet is greater than 0.8, the unit of the linear velocity is m / min, and the unit of the flow velocity is m / min.

[0025] In step 2, the heating temperature is 220 to 260°C.

[0026] In step 2, the temperature of the cooling roll is 70 to 90°C.

[0027] In step 2, the extruder is a co-rotating twin-screw extruder, and the screw rotation speed of the co-rotating twin-screw extruder is 100-240 rpm and the screw length ratio is 56-68.

[0028] In step 2, the temperature of the thermodynamic single-phase melt during melt casting is 225 to 265°C.

[0029] In step 2, the thickness of the crystallized cast sheet is 0.8 to 2.0 mm.

[0030] Step 3: The crystallized cast sheet is first stretched and cooled to obtain a first stretched film, and the first stretched film is second stretched to obtain a second stretched film, which is extracted, washed, and dried to obtain a high-strength, high-elongation separator.

[0031] In step 3, the primary stretching includes longitudinal stretching at 140 to 150°C and a stretching speed of 10 to 150% / s, heat setting at 140 to 150°C for 60 to 120 seconds, and transverse stretching at 140 to 150°C and a stretching speed of 10 to 150% / s, and in the primary stretching, transverse stretching is performed before or after the longitudinal stretching, and heat setting is performed between the longitudinal stretching and the transverse stretching.

[0032] In the above technical solution, the stretching ratios in the longitudinal direction and transverse direction in the first stretching are each 2 to 5 times.

[0033] In the above technical solution, preheating is carried out before the first stretching, the preheating temperature is 140-150°C, and the preheating time is 180-300 seconds.

[0034] In step 3, the secondary stretching includes synchronous stretching in the transverse and longitudinal directions, or stepwise stretching in the transverse and longitudinal directions, the synchronous stretching in the transverse and longitudinal directions includes simultaneous stretching in the transverse and longitudinal directions at a stretching rate of 10 to 150% / s at 90 to 110°C, and the stepwise stretching in the transverse and longitudinal directions includes longitudinal stretching at a stretching rate of 10 to 150% / s at 90 to 110°C, heat setting at 90 to 110°C for 60 to 120 seconds, and transverse stretching at a stretching rate of 10 to 150% / s at 90 to 110°C, and in the secondary stretching, transverse stretching is performed before or after longitudinal stretching, and heat setting is performed between the longitudinal stretching and the transverse stretching.

[0035] In the above technical solution, preheating is carried out before the second stretching, the preheating temperature is 90-110°C, and the preheating time is 180-300 seconds.

[0036] In the above technical solution, the stretching ratios of the longitudinal stretching and the transverse stretching in the second stretching are each 2 to 5 times.

[0037] In step 3, the drying temperature is 40 to 60° C., and the drying time is 4 to 6 minutes.

[0038] In step 3, the extraction and washing is performed in an extractant at 25 to 55°C for 60 to 240 seconds, and the extractant is dichloromethane, and the purity of the dichloromethane is >99 wt%.

[0039] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0040] The manufacturing method of the present invention utilizes the compatibilizing effect of copolymers in the raw materials. Ultra-high molecular weight polyethylene and ultra-high molecular weight polypropylene are melted at high temperatures to form a homogeneous single phase, improving the compounding and compatibility of the two, providing higher strength. At the same time, the differences in the crystalline structure and density of the different polyolefins are utilized to form crystalline regions with different properties. The ultra-high molecular weight polypropylene forms a folded-chain crystalline structure (ultra-high molecular weight polypropylene has a lower crystallinity and is flexible and ductile, while the ultra-high molecular weight polyethylene forms an extended-chain crystalline structure, which is stiffer and slightly less ductile than ultra-high molecular weight polypropylene), resulting in higher ductility of the separator. During the stretching process, the primary stretching first aligns the polyolefin molecules, resulting in localized, uniform crystallization (crystallinity 50%-70%) and imparting ultra-high strength. The secondary stretching induces a degree of crystallization in the amorphous regions, forming semi-crystalline regions. These semi-crystalline regions have superior ductility to the crystalline regions, providing higher ductility. The high-strength, high-elongation separator has an approximately 89% improvement in tensile strength and an approximately 76% improvement in elongation compared to conventional lithium-ion battery separators. DETAILED DESCRIPTION OF THE INVENTION

[0041] The technical solutions of the present invention will be further described below in conjunction with specific examples.

[0042] The relevant instruments and devices used in specific embodiments of the present invention are as follows:

[0043] Co-rotating twin screw extruder: MT-52 type, with a screw diameter of 52 mm and a length ratio of 68, purchased from Jiangsu Meizhilong Machinery Co., Ltd.

[0044] Melting pipeline and T-die: The width of the T-die is 250 mm and it was purchased from Zhejiang Jingcheng Mould Machinery Co., Ltd.

[0045] Cooling roll: purchased from Anhui Dongsheng Machinery Co., Ltd.

[0046] Static biaxial stretching machine: JTL-10 type, purchased from Weihai Haichao Machinery Co., Ltd.

[0047] Oven: Model 101-4B, manufactured by Shanghai Shangdao Instrument Manufacturing Co., Ltd.

[0048] The relevant ingredients used in specific embodiments of the present invention are as follows:

[0049] Ultra-high molecular weight polyethylene, ultra-high molecular weight polypropylene: manufactured by Daihan Petrochemical (Shanghai) Co., Ltd. The weight-average molecular weight of the ultra-high molecular weight polyethylene was 1.5 million, and the melt flow index was 0 g / 10 min (21.6 kg, 190°C). The weight-average molecular weight of the ultra-high molecular weight polypropylene was 1.3 million, and the melt flow index was 0.20 g / 10 min (21.6 kg, 190°C).

[0050] White oil: 50#, purchased from Zhejiang Zhengxin Petroleum Technology Co., Ltd.

[0051] Dichloromethane: purchased from Shandong Luhua Chemical Co., Ltd.

[0052] Ethylene-propylene copolymer, ethylene-butene copolymer, and ethylene-octene copolymer: the grades were Engage 8411, Engage 8230, and Engage 8150, respectively, purchased from Jiangsu Feiyong Plastics Co., Ltd. The density of the ethylene-propylene copolymer was 0.868 g / cm 3 The melting point is 55.0°C, the crystallization peak temperature is 42.0°C, and the density of the ethylene-butene copolymer is 0.842 g / cm 3 The melting point is 57.2°C, the crystallization peak temperature is 45.1°C, and the density of the ethylene-octene copolymer is 0.852 g / cm 3 The melting point is 60.2°C and the crystallization peak temperature is 50.4°C.

[0053] DOTP: dioctyl terephthalate, purchased from Yufeng Nano Co., Ltd.

[0054] The test methods for tensile strength and elongation comply with the Chinese national standard "Polyolefin separators for lithium-ion batteries" (GB / T36363-2018).

[0055] % / s: Percentage of stretching relative to the original dimension per second, stretching rate = (dimension after stretching - dimension before stretching) / (dimension before stretching x stretching time).

[0056] Single micron needle puncture strength = needle puncture strength / separator thickness.

[0057] Test temperature for ionic conductivity: room temperature.

[0058] Example 1 The high-strength, high-elongation separator contains an ultra-high molecular weight polyolefin and a solubilizing agent, and a porous structure is formed within the high-strength, high-elongation separator. The porous structure is formed by first filling the separator with a pore-forming agent, and then extracting and washing the pore-forming agent from the high-strength, high-elongation separator.

[0059] The method for producing the high strength and high elongation separator includes the following steps.

[0060] Step 1: Ultra-high molecular weight polyolefin, solubilizer, and pore-forming agent were mixed in a raw material premixing tank and stirred at 85°C and 50 rpm for 30 minutes until homogeneous, to obtain a premixed raw material. The ratio of ultra-high molecular weight polyolefin, solubilizer, and pore-forming agent, by parts by weight, was 30:5:65, where the solubilizer was ethylene-butene copolymer, the ultra-high molecular weight polyolefin was a mixture of ultra-high molecular weight polyethylene and ultra-high molecular weight polypropylene, the ratio of ultra-high molecular weight polyethylene and ultra-high molecular weight polypropylene, by parts by weight, was 70:30, and the pore-forming agent was white oil.

[0061] Step 2: The premixed raw materials were injected into a co-rotating twin-screw extruder and heated to 240°C at 170 rpm until melted, yielding a thermodynamic single-phase melt. The 245°C thermodynamic single-phase melt was extruded through a T-die through a melt pipeline and melt-cast onto an 80°C cooling roll (the ratio of the cooling roll linear velocity to the melt-cast sheet flow rate was 1, the linear velocity was in m / min, and the flow rate was in m / min). The melt was then cooled to room temperature (20-25°C) to obtain a crystallized cast sheet with a thickness of 1.4 mm.

[0062] Step 3: The crystallized cast sheet was cut into a 100mm x 100mm square specimen and subjected to a primary stretching, i.e., the square specimen was placed on a static biaxial stretching machine and clamped flat with clips. Then, the square specimen was placed in a heating oven and preheated at 145°C for 240 seconds. The specimen was first stretched 5 times in the machine direction at a stretching speed of 80% / s at 145°C (stretching ratio: 5 times), heat-set at 145°C for 90 seconds, and then stretched 5 times in the transverse direction at a stretching speed of 80% / s at 145°C (stretching ratio: 5 times), and cooled to room temperature to obtain a primary stretched film. The first-stretched film was recut into 100mm x 100mm square specimens and subjected to secondary stretching (stepwise stretching in the transverse and longitudinal directions). The square specimens were placed on a static biaxial stretching machine, clamped flat with clips, and then placed in a heated oven at 100°C for 240 seconds. The specimens were then stretched to 5x in the longitudinal direction at a stretching rate of 80% / s at 100°C (stretch ratio of 5x). The film was then heat-set at 100°C for 90 seconds, and then stretched to 5x in the transverse direction at a stretching rate of 80% / s at 100°C (stretch ratio of 5x). The film was then cooled to room temperature to obtain a second-stretched film. The second-stretched film was then clamped flat in an extraction jig and extracted by immersion in dichloromethane (purity >99wt%) at 25°C for 180 seconds. The film was then dried in an oven at 40°C for 5 minutes to obtain a high-strength, high-elongation separator.

[0063] Example 2 This is a manufacturing method of a high-strength, high-elongation separator, and is basically the same as Example 1, with the only difference being that the stretching ratios in the longitudinal and transverse directions in the secondary stretching in this example were each 3x. That is, in the secondary stretching in this example, the square test specimen was placed on a static biaxial stretching machine, flatly clamped with clips, and then placed in a heating oven and preheated at 100°C for 240 seconds. The square test specimen was first stretched to 3x in the longitudinal direction at a stretching rate of 80% / s at 100°C, heat-set at 100°C for 90 seconds, and then stretched to 3x in the transverse direction at a stretching rate of 80% / s at 100°C. The specimen was then cooled to room temperature to obtain a secondary-stretched film.

[0064] Example 3 This is a manufacturing method of a high-strength, high-elongation separator, and is basically the same as in Example 1, with the only difference being that the secondary stretching in this example was synchronous stretching in the transverse and longitudinal directions. That is, in the secondary stretching in this example, the square test piece was placed on a static biaxial stretching machine, and the square test piece was clamped flat with clips. Then, the square test piece was placed in a heating oven and preheated at 100°C for 240 seconds, and simultaneously stretched to 5 times in the transverse and longitudinal directions at a stretching rate of 80% / s at 100°C (stretching ratio: 5 times), and then cooled to room temperature to obtain a secondarily stretched film.

[0065] Example 4 This is a manufacturing method of a high-strength, high-elongation separator, and is basically the same as in Example 3, except that the stretching ratios in the longitudinal and transverse directions in the second stretching in this example were each 3 times.

[0066] Comparative Example 1 A method for manufacturing a separator for a lithium ion battery, comprising the following steps:

[0067] Step 1: The ultra-high molecular weight polyethylene and pore-forming agent were added to a raw material pre-mixing tank and stirred at 90°C and 50 rpm for 30 minutes until homogeneous, to obtain a pre-mixed raw material. Here, the ratio of ultra-high molecular weight polyethylene to pore-forming agent was 30:65, and the pore-forming agent was white oil.

[0068] Step 2: The premixed raw materials were injected into a co-rotating twin-screw extruder and heated to 240°C at 170 rpm until melted, yielding a thermodynamic single-phase melt. The 245°C thermodynamic single-phase melt was extruded through a T-die through a melt pipeline and melt-cast onto an 80°C cooling roll (the ratio of the cooling roll linear velocity to the melt-cast sheet flow rate was 1, the linear velocity unit was m / min, and the flow rate unit was m / min). The melt was then cooled to room temperature (20-25°C) to obtain a crystallized cast sheet with a thickness of 1.3 mm.

[0069] Step 3: The crystallized cast sheet was cut into 100mm x 100mm square specimens, placed on a static biaxial stretching machine, and clamped flat with clips. Then, the specimens were placed in a 145°C oven for 240 seconds. They were then simultaneously stretched at 145°C at a stretching rate of 80% / s to 10x in both the transverse and longitudinal directions (stretching ratios of 10x in both the transverse and longitudinal directions), cooled to room temperature, and then extracted. The stretched separators were then clamped flat in an extraction tool and extracted by immersion in dichloromethane (dichloromethane concentration >99wt%) at 25°C for 180 seconds. They were then dried in an oven at 40°C for 5 minutes to obtain lithium-ion battery separators.

[0070] Comparative Example 2 This is a manufacturing method of a lithium-ion battery separator, and is basically the same as Comparative Example 1, except that the "simultaneous stretching in the transverse and longitudinal directions up to 10 times at a stretching rate of 80% / s at 145°C (stretching ratios in the transverse and longitudinal directions are each 10 times)" in Comparative Example 1 is replaced with "simultaneous stretching in the transverse and longitudinal directions up to 8 times at a stretching rate of 80% / s at 145°C."

[0071] Comparative Example 3 This is a manufacturing method of a lithium-ion battery separator, and is basically the same as Example 1, with the only difference being that in this comparative example, the ratio of ultra-high molecular weight polyolefin, solubilizer, and pore-forming agent, by parts by mass, was 30:5:65, the ultra-high molecular weight polyolefin was a mixture of ultra-high molecular weight polyethylene and ultra-high molecular weight polypropylene, the ratio of ultra-high molecular weight polyethylene to ultra-high molecular weight polypropylene, by parts by mass, was 28.5:1.5, and the thickness of the crystallized cast sheet was 1.6 mm.

[0072] Comparative Example 4 This is a method for manufacturing a lithium-ion battery separator, and is basically the same as Example 1, with the only difference being that in this comparative example, the ratio of ultra-high molecular weight polyolefin, solubilizer, and pore-forming agent, by parts by mass, was 30:5:65, the ultra-high molecular weight polyolefin was a mixture of ultra-high molecular weight polyethylene and ultra-high molecular weight polypropylene, and the ratio of ultra-high molecular weight polyethylene to ultra-high molecular weight polypropylene, by parts by mass, was 16.5:13.5.

[0073] In this comparative example, melt fracture occurred during the melt casting process, and a crystallized cast sheet could not be formed.

[0074] Comparative Example 5 This is a manufacturing method for a lithium-ion battery separator, and is basically the same as Example 1, with the only difference being that in this comparative example, no solubilizer was used; that is, ultra-high molecular weight polyolefin and pore-forming agent were mixed in a raw material pre-mixing tank, and the ratio of ultra-high molecular weight polyolefin to pore-forming agent was 30:70, by parts by mass. The ultra-high molecular weight polyolefin was a mixture of ultra-high molecular weight polyethylene and ultra-high molecular weight polypropylene, and the ratio of ultra-high molecular weight polyethylene to ultra-high molecular weight polypropylene was 70:30, by parts by mass. The pore-forming agent was white oil.

[0075] In this comparative example, melt fracture occurred during the melt casting process, and a crystallized cast sheet could not be formed.

[0076] Comparative Example 6 This comparative example is a method for producing a lithium-ion battery separator, and is basically the same as Example 1, except that in this comparative example, ultra-high molecular weight polyethylene, ultra-high molecular weight polypropylene, antioxidant, solubilizer, pore-forming agent, and nucleating agent were mixed in a ratio of 21:9:0.05:5:65:0.5 by weight, where the nucleating agent was a mixture of pimelic acid and calcium stearate in a ratio of 1.2:3.8 by weight, and the antioxidant was pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] (antioxidant 1010) (the pore-forming agent and solubilizing agent were the same as in Example 1), and the crystallized cast sheet had a thickness of 1.4 mm.

[0077] Comparative Example 7 This is a manufacturing method of a lithium-ion battery separator, which is basically the same as Example 1, except that the ratio of the linear velocity of the cooling roll to the flow velocity of the melt-cast sheet was set to 30, and the sheet was stretched by casting, and then cooled to room temperature to form a crystallized cast sheet with a thickness of 1.3 mm.

[0078] Comparative Example 8 This comparative example was basically the same as Example 1, except for step 3, in which the crystallized cast sheet was cut into a 100 mm × 100 mm square specimen, placed on a static biaxial stretching machine, and clamped flat with clips. The specimen was then placed in a heating oven and preheated at 145°C for 240 seconds. The specimen was then stretched to 10 times its original length at a stretching rate of 80% / s at 145°C (stretch ratio: 10 times), heat-set at 145°C for 90 seconds, and then stretched to 10 times its original length at a stretching rate of 80% / s at 145°C (stretch ratio: 10 times). The specimen was then cooled to room temperature to obtain a lithium-ion battery separator. In other words, in this comparative example, 10-fold stretching was performed only once.

[0079] Comparative Example 9 This is a method for producing a lithium-ion battery separator, which is basically the same as Example 1, except that the ultra-high molecular weight polyolefin in this comparative example is ultra-high molecular weight polyethylene, i.e., no ultra-high molecular weight polypropylene was added in this comparative example.

[0080] Table 1 shows the performance parameters of the high-strength, high-elongation separators produced in Examples 1 to 4 and the lithium-ion battery separators produced in Comparative Examples 1 to 9 (the separator thickness in Table 1 is the thickness of the high-strength, high-elongation separator / lithium-ion battery separator). [Table 1]

[0081] As can be seen from Examples 1 and 2, as the stretch ratio in the secondary stretching increases, the tensile strength and single-micron needle puncture strength gradually increase, but the elongation decreases. This is mainly because, as the stretch ratio in the secondary stretching increases, the oriented crystallization of the amorphous regions gradually increases, increasing the rigidity strength of the separator while decreasing its toughness and ductility. As can be seen from Comparative Examples 1 and 2, when a single type of polyolefin is stretched once, the crystalline form is uniform, resulting in low tensile strength and elongation. As can be seen from Comparative Examples 3 to 5, when the polypropylene content in the ultra-high molecular weight polyolefin is ≦5% (Comparative Example 3), the amount added is too small, and no significant improvement is observed in the tensile strength, needle puncture strength, and elongation of the lithium-ion battery separator. When the polypropylene content in the ultra-high molecular weight polyolefin was ≥ 45% (Comparative Example 4) and no solubilizer was added (Comparative Example 5), the difference in the compatibility coefficients of polyethylene, polypropylene, and white oil prevented the two phases from fusing, preventing the formation of a homogeneous melt. This resulted in melt fracture, making it impossible to form a cast sheet. As can be seen from Comparative Examples 6 and 7, the addition of a nucleating agent and antioxidant, or high-magnification stretching during casting, did not significantly affect the separator's tensile strength, pinpoint strength, ionic conductivity, or elongation. As can be seen from Comparative Examples 8 and 9, the factors that influence the separator's high strength and high ductility are the interaction between multiple stretching cycles and the polyolefin raw materials. When stretching was performed only once, both the tensile strength and ductility of the separator were significantly poor. When only ultra-high molecular weight polyethylene was used and stretching was performed multiple times, the tensile strength improved slightly, but the ductility significantly decreased. This is because, although multiple stretching increases the crystallinity of the separator and strengthens its rigidity, the flexible crystalline regions due to the ultra-high molecular weight polypropylene component are not formed, resulting in poor ductility. Therefore, only the technical solution of the embodiment of the present invention can achieve both significant property improvement and compatibility.

[0082] The present invention has been described above by way of example, and it should be understood that, as long as it does not deviate from the essence of the present invention, any simple variations, modifications, or other equivalent replacements that can be made by those skilled in the art without expending any creative effort, are all included in the protection scope of the present invention.

Claims

1. A high-strength, high-elongation separator comprising an ultra-high molecular weight polyolefin and a solubilizing agent, wherein the ratio of the ultra-high molecular weight polyolefin to the solubilizing agent is, in parts by mass, (20 to 40):(2 to 10), wherein the solubilizing agent is a mixture of one or more of an ethylene-propylene copolymer, an ethylene-butene copolymer, and an ethylene-octene copolymer, the ultra-high molecular weight polyolefin comprises an ultra-high molecular weight polyethylene and an ultra-high molecular weight polypropylene, the ultra-high molecular weight polyethylene having a weight-average molecular weight of 600,000 to 4,000,000, and the ultra-high molecular weight polypropylene having a weight-average molecular weight of 400,000 to 2,600,000.

2. 2. The high-strength, high-elongation separator according to claim 1, wherein a porous structure is formed within the high-strength, high-elongation separator, the porous structure being formed by first filling the separator with a pore-forming agent, and then extracting and washing the pore-forming agent from the high-strength, high-elongation separator, and the ratio of the solubilizing agent to the pore-forming agent is (2-10):(50-78) parts by mass.

3. The density of the solubilizer is 0.5 to 1 g / cm ３ 3. The high-strength, high-elongation separator according to claim 1, wherein the melting point is 50 to 90°C and the crystallization peak temperature is 55 to 80°C.

4. 3. The high-strength, high-elongation separator according to claim 1, wherein the ratio of the ultra-high molecular weight polyethylene to the ultra-high molecular weight polypropylene is (60 to 94):(6 to 40), in parts by mass.

5. 3. The high strength and high elongation separator according to claim 2, wherein the pore-forming agent is white oil and / or dioctyl terephthalate.

6. A method for producing a high-strength, high-elongation separator, comprising: Step 1: Mixing and stirring an ultra-high molecular weight polyolefin, a solubilizing agent, and a pore-forming agent until uniform, to obtain a pre-mixed raw material; Step 2: injecting the premixed material into an extruder and heating it until melted to obtain a thermodynamic single-phase melt, and melt-casting the thermodynamic single-phase melt onto a chill roll to obtain a crystallized cast sheet; Step 3: A method for producing a high-strength, high-elongation separator, comprising: first stretching the crystallized cast sheet, cooling the crystallized cast sheet to obtain a first-stretched film; secondly stretching the first-stretched film to obtain a second-stretched film; extracting, washing, and drying the second-stretched film to obtain a high-strength, high-elongation separator.

7. 7. The method according to claim 6, wherein in step 3, the primary stretching comprises longitudinal stretching at 140 to 150°C, heat setting at 140 to 150°C for 60 to 120 seconds, and transverse stretching at 140 to 150°C, wherein in the primary stretching, transverse stretching is performed before or after the longitudinal stretching, and heat setting is performed between the longitudinal stretching and the transverse stretching.

8. The method according to claim 6 or 7, wherein the secondary stretching comprises synchronous stretching in the transverse and longitudinal directions or stepwise stretching in the transverse and longitudinal directions, wherein the synchronous stretching in the transverse and longitudinal directions comprises simultaneous stretching in the transverse and longitudinal directions at 90 to 110°C, and the stepwise stretching in the transverse and longitudinal directions comprises longitudinal stretching at 90 to 110°C, heat setting at 90 to 110°C for 60 to 120 seconds, and transverse stretching at 90 to 110°C, and wherein in the secondary stretching, transverse stretching is performed before or after longitudinal stretching, and heat setting is performed between the longitudinal stretching and the transverse stretching.

9. The method according to claim 7, wherein the stretching ratios in the longitudinal direction and transverse direction in the first stretching are each 2 to 5 times, and the stretching ratios in the longitudinal direction and transverse direction in the second stretching are each 2 to 5 times.

10. 8. The method according to claim 7, wherein in step 2, the ratio of the linear velocity of the chill roll to the flow velocity of the melt-cast sheet is 0.8-1.5, the unit of the linear velocity is m / min, and the unit of the flow velocity is m / min; and in step 2, the temperature of the thermodynamic single-phase melt during melt-casting is 225-265°C.

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