High-strength high-elongation separator and preparation method therefor

Abstract

Disclosed are a high-strength high-elongation separator and a preparation method therefor. The high-strength high-elongation separator comprises ultra-high molecular weight polyolefin and a solubilizer, wherein the solubilizer is one of or a mixture of more of an ethylene-propylene copolymer, an ethylene-butylene copolymer and an ethylene-octene copolymer. The preparation method therefor comprises: mixing ultra-high molecular weight polyolefin, a solubilizer and a pore-forming agent, and stirring same until uniform, so as to obtain a premixed raw material; injecting the premixed raw material into an extruder, heating same to obtain a thermodynamic single-phase melt, and subjecting the thermodynamic single-phase melt to melt casting onto a cooling roller, so as to obtain a crystallized cast sheet; and subjecting the crystallized cast sheet to primary stretching and cooling to obtain a primary stretched film, subjecting the primary stretched film to secondary stretching to obtain a secondary stretched film, performing extraction and washing, and then drying same, so as to obtain a high-strength high-elongation separator. The preparation method utilizes the solubilizing effect of a copolymer and multiple operations of stretching performed in steps, and therefore the separator has good ductility and tensile strength.

Description

High-strength and high-elongation diaphragm and preparation method thereof Technical Field

[0001] The present invention belongs to the technical field of battery separators, and in particular relates to a high-strength and high-elongation separator and a preparation method thereof. Background Art

[0002] Lithium-ion battery separators are crucial safety and functional components of lithium batteries. They have the dual functions of electronic insulation and ion conductivity. The insulation property refers to the separator being sandwiched between the positive and negative electrodes, which can prevent the positive and negative electrodes from short-circuiting and failing. Therefore, the separator needs to have high strength to prevent impurities or burrs on the positive and negative electrodes from puncturing and causing battery short circuits. At the same time, the separator also needs to have a certain degree of ductility. When the electrode material deforms, the separator has ample deformation space to avoid direct rupture of the separator and short circuit of the electrodes. However, the raw material of the separator, the high molecular weight polyolefin, increases its brittleness and deteriorates its ductility due to the molecules being stretched and crystallized in a directional arrangement during the stretching process. That is, the separator has strong rigidity but poor ductility and flexibility, resulting in an increased risk of fracture in the battery cell. In summary, it is difficult to achieve both ductility and strength of the separator. Therefore, how to effectively and simultaneously improve these two properties is a bottleneck problem in the separator manufacturing field. Summary of the Invention

[0003] In view of the shortcomings of the prior art, the object of the present invention is to provide a high-strength and high-elongation diaphragm, which uses a solubilizer to improve the processing fluidity and system compatibility of ultra-high molecular weight polyolefins.

[0004] Another object of the present invention is to provide a method for preparing the high-strength and high-elongation diaphragm.

[0005] The purpose of the present invention is achieved through the following technical solutions.

[0006] A high-strength and high-elongation diaphragm comprises: an ultra-high molecular weight polyolefin and a solubilizer, wherein, by weight, the ratio of the ultra-high molecular weight polyolefin to the solubilizer is (20-40):(2-10), wherein the solubilizer is a mixture of one or more of ethylene-propylene copolymer, ethylene-butene copolymer and ethylene-octene copolymer, and the ultra-high molecular weight polyolefin comprises: ultra-high molecular weight polyethylene and ultra-high molecular weight polypropylene, wherein 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 copolymer is formed by copolymerization of ethylene and propylene.

[0008] Ethylene-butene copolymer is formed by copolymerization of ethylene and butene.

[0009] Ethylene-octene copolymer is formed by copolymerization of 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 index of the ultra-high molecular weight polyethylene at 21.6 kg and 190°C is 0-0.8 g / 10 min; the melt index of the ultra-high molecular weight polypropylene at 21.6 kg and 190°C is 0.1-10 g / 10 min.

[0012] In the above technical solution, multiple pores are formed in the high-strength and high-elongation diaphragm, and the multiple pores are first filled with a pore-forming agent and then the pore-forming agent is extracted and washed out of the high-strength and high-elongation diaphragm. In terms of mass, the ratio of the solubilizer and the pore-forming agent is (2 to 10): (50 to 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 50-90℃, and the crystallization peak temperature is 40-80℃.

[0015] The method for preparing the high-strength and high-elongation diaphragm comprises the following steps:

[0016] Step 1, mixing an ultra-high molecular weight polyolefin, a solubilizer, and a pore-forming agent, and stirring until uniform to obtain a premixed raw material;

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

[0018] In step 1, the premixed raw materials further include: an antioxidant and a nucleating agent.

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

[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 is (0.8-3.8): (1.2-4.2) by mass.

[0021] The above technical solution further comprises: an antioxidant, wherein 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: injecting the premixed raw material into an extruder, heating it until it melts to obtain a thermodynamic single-phase melt, and melt-casting the thermodynamic single-phase melt onto a cooling roll to obtain a crystalline cast sheet;

[0024] In step 2, the ratio of the linear velocity of the cooling roller 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 220-260°C.

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

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

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

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

[0030] Step 3: stretch the crystallized cast sheet once, cool it to obtain a primary stretched film, stretch the primary stretched film twice to obtain a secondary stretched film, wash it, and dry it to obtain a high-strength and high-elongation diaphragm.

[0031] In step 3, the primary stretching includes: longitudinal stretching at a stretching speed of 10 to 150% / s at 140 to 150° C., heat setting at 140 to 150° C. for 60 to 120 seconds, and transverse stretching at a stretching speed of 10 to 150% / s at 140 to 150° C., wherein the 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 of the longitudinal stretching and the transverse stretching in the one stretching are 2 to 5 times respectively.

[0033] In the above technical solution, preheating is performed before the primary 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 step-by-step stretching in the transverse and longitudinal directions, wherein the synchronous stretching in the transverse and longitudinal directions includes: simultaneous stretching in the transverse and longitudinal directions at a stretching speed of 10 to 150% / s at 90 to 110°C, and the step-by-step stretching in the transverse and longitudinal directions includes: longitudinal stretching at a stretching speed of 10 to 150% / s at 90 to 110°C, heat setting at 90 to 110°C for 60 to 120s, and transverse stretching at a stretching speed of 10 to 150% / s at 90 to 110°C. In the secondary stretching, transverse stretching is performed before or after longitudinal stretching, and heat setting is performed between longitudinal stretching and transverse stretching.

[0035] In the above technical solution, preheating is performed before the secondary stretching, the preheating temperature is 90 to 110° C., and the preheating time is 180 to 300 seconds.

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

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

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

[0039] Compared with the prior art, the present invention has the following beneficial effects:

[0040] The preparation method of the present invention utilizes the solubilizing effect of the copolymer in raw materials, allowing ultra-high molecular weight polyethylene (UHMWPE) and ultra-high molecular weight polypropylene (UHMWPP) to melt into a homogeneous single phase at high temperature, making the UHMWPE and UHMWPP compatible and providing higher strength. Furthermore, the differences in the crystal structure and density of different polyolefins are utilized to form crystalline regions with different textures. The UHMWPP forms a folded chain crystal structure (UHMWPP has lower crystallinity, better flexibility, and better ductility, while the UHMWPE forms a straight chain crystal structure, which is more rigid but less ductile than UHMWPP). This can give the separator higher ductility. During the stretching process, the primary stretching process first causes the polyolefin molecules to be regularly arranged and locally uniformly crystallize (crystallinity of 50% to 70%), providing ultra-high strength. The secondary stretching process causes the amorphous regions to produce some crystals, forming semi-crystalline regions. These semi-crystalline regions have better ductility than crystalline regions, providing higher ductility. The high-strength and high-ductility separator has a tensile strength approximately 89% higher than conventional lithium-ion battery separators, and an ductility increase of approximately 76%. DETAILED DESCRIPTION

[0041] The technical solution of the present invention is further described below with reference to specific embodiments.

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

[0043] Co-rotating twin-screw extruder: MT-52 model, screw diameter 52 mm, aspect ratio 68, purchased from Jiangsu Meizhilong Machinery Co., Ltd.

[0044] Melt line and T-die: The T-die width is 250 mm and was purchased from Zhejiang Jingcheng Mould Machinery Co., Ltd.

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

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

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

[0048] The relevant raw materials used in the specific implementation of the present invention are as follows:

[0049] Ultra-high molecular weight polyethylene, ultra-high molecular weight polypropylene: Korea Petrochemical (Shanghai) Co., Ltd.; the weight-average molecular weight of ultra-high molecular weight polyethylene is 1.5 million, and the melt index is 0 g / 10 min (at 21.6 kg and 190°C); the weight-average molecular weight of ultra-high molecular weight polypropylene is 1.3 million, and the melt index is 0.20 g / 10 min (at 21.6 kg and 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 brands are Engage8411, Engage8230 and Engage8150 respectively, purchased from Jiangsu Feiou Plastic Co., Ltd. The density of ethylene-propylene copolymer is 0.868 g / cm 3 , melting point is 55.0℃, crystallization peak temperature is 42.0℃; density of ethylene-butene copolymer is 0.842g / cm 3 , melting point is 57.2℃, crystallization peak temperature is 45.1℃; density of ethylene-octene copolymer is 0.852g / cm 3 , melting point is 60.2℃, crystallization peak temperature is 50.4℃;

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

[0054] The tensile strength and elongation test methods are based on the national standard "Polyolefin Separators for Lithium-ion Batteries" (GB / T36363-2018).

[0055] % / s: The percentage of stretching relative to the original size per second. Stretching speed = (size after stretching - size before stretching) / (size before stretching * stretching time).

[0056] Single micron puncture strength = puncture strength / diaphragm thickness.

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

[0058] Example 1

[0059] A high-strength and high-elongation diaphragm comprises ultra-high molecular weight polyolefin and a solubilizer. The high-strength and high-elongation diaphragm is formed with multiple pores, which are first filled with a pore-forming agent and then the pore-forming agent is extracted and washed out of the high-strength and high-elongation diaphragm.

[0060] The method for preparing the high-strength and high-elongation diaphragm comprises the following steps:

[0061] Step 1, mixing an ultra-high molecular weight polyolefin, a solubilizer and a pore-forming agent in a raw material premixing tank, stirring at 85° C. and a speed of 50 rpm for 30 minutes until uniform, to obtain a premixed raw material, wherein the ratio of the ultra-high molecular weight polyolefin, the solubilizer and the pore-forming agent is 30:5:65 by weight, wherein the solubilizer is ethylene-butene copolymer, the ultra-high molecular weight polyolefin is 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 is 70:30 by weight, and the pore-forming agent is white oil;

[0062] Step 2: injecting the premixed raw material into a co-rotating twin-screw extruder, heating the mixture at 240° C. at a speed of 170 rpm in the co-rotating twin-screw extruder until it melts to obtain a thermodynamically single-phase melt, extruding the 245° C. thermodynamically single-phase melt through a melt line from a T-die, and melt-casting the melt onto a cooling roll at 80° C. (the ratio of the linear velocity of the cooling roll to the flow velocity of the melt-cast sheet is 1, the unit of the linear velocity is m / min, and the unit of the flow velocity is m / min), and cooling the mixture to room temperature of 20 to 25° C. to obtain a crystalline cast sheet with a thickness of 1.4 mm;

[0063] Step 3, the crystallized cast sheet is cut into 100mm × 100mm square specimens for primary stretching: the square specimen is placed on a static biaxial stretching machine, the clamp is flattened and clamped to hold the square specimen, and then placed in a heating oven and preheated at 145°C for 240s, first longitudinally stretched to 5 times at a stretching speed of 80% / s at 145°C (stretching ratio is 5 times), heat-set at 145°C for 90s, and then transversely stretched to 5 times at a stretching speed of 80% / s at 145°C (stretching ratio is 5 times), cooled to room temperature, and a primary stretched film is obtained; the primary stretched film is cut into 100mm × 100mm square specimens for secondary stretching (stretching in steps of transverse and longitudinal directions) Stretching): The square sample is placed on a static biaxial stretching machine, and after the clips are flattened and clamped, the square sample is placed in a heating oven and preheated at 100°C for 240s. It is first longitudinally stretched to 5 times at a stretching speed of 80% / s at 100°C (stretching ratio is 5 times), heat-set at 100°C for 90s, and then transversely stretched to 5 times at a stretching speed of 80% / s at 100°C (stretching ratio is 5 times), cooled to room temperature, and a secondary stretched film is obtained. The secondary stretched film is flattened and clamped with an extraction tool, placed in dichloromethane (purity>99wt%) at 25°C for extraction for 180s, and dried in an oven at 40°C for 5min to obtain a high-strength and high-elongation diaphragm.

[0064] Example 2

[0065] A method for preparing a high-strength and high-elongation diaphragm is basically the same as that in Example 1, with the only difference being that the stretching ratios of the longitudinal stretching and the transverse stretching in the secondary stretching of this embodiment are each 3 times, that is, the secondary stretching of this embodiment is as follows: the square sample is placed on a static biaxial stretching machine, the clamps are flattened and clamped to clamp the square sample, and then placed in a heating oven and preheated at 100°C for 240s, first longitudinally stretched to 3 times at a stretching speed of 80% / s at 100°C, heat-set at 100°C for 90s, and then transversely stretched to 3 times at a stretching speed of 80% / s at 100°C, cooled to room temperature, to obtain a secondary stretched film.

[0066] Example 3

[0067] A method for preparing a high-strength and high-elongation diaphragm is basically the same as that in Example 1, with the only difference being that the secondary stretching in this embodiment is synchronous stretching in the transverse and longitudinal directions, that is, the secondary stretching in this embodiment is as follows: the square sample is placed on a static biaxial stretching machine, the clamps are flattened and clamped to clamp the square sample, and then placed in a heating oven and preheated at 100°C for 240s, and stretched simultaneously in the transverse and longitudinal directions at 100°C at a stretching speed of 80% / s to 5 times (stretching ratio is 5 times), and then cooled to room temperature to obtain a secondary stretched film.

[0068] Example 4

[0069] A method for preparing a high-strength and high-elongation diaphragm is basically the same as that in Example 3, with the only difference being that the stretching ratios of the longitudinal stretching and the transverse stretching in the secondary stretching of this embodiment are each 3 times.

[0070] Comparative Example 1

[0071] A method for preparing a lithium-ion battery separator comprises the following steps:

[0072] Step 1: adding ultra-high molecular weight polyethylene and a pore-forming agent into a raw material premixing tank, stirring at 90° C. and 50 rpm for 30 minutes until uniform, to obtain a premixed raw material, wherein the ratio of ultra-high molecular weight polyethylene to the pore-forming agent is 30:65 by mass, and the pore-forming agent is white oil;

[0073] Step 2: injecting the premixed raw material into a co-rotating twin-screw extruder, heating the mixture at 240° C. at a speed of 170 rpm in the co-rotating twin-screw extruder until it melts to obtain a thermodynamically single-phase melt, extruding the 245° C. thermodynamically single-phase melt through a melt line from a T-die, and melt-casting the melt onto a cooling roll at 80° C. (the ratio of the linear velocity of the cooling roll to the flow velocity of the melt-cast sheet is 1, the unit of the linear velocity is m / min, and the unit of the flow velocity is m / min), and cooling the mixture to room temperature of 20 to 25° C. to obtain a crystalline cast sheet with a thickness of 1.3 mm;

[0074] Step 3, cut the crystallized cast into 100mm × 100mm square specimens, place the square specimens on a static biaxial stretching machine, flatten and clamp the square specimens with a clamp, and then put them into a 145°C heating oven for preheating for 240s. At 145°C, stretch them simultaneously in the transverse and longitudinal directions at a stretching speed of 80% / s, and stretch them to 10 times each (the stretching ratios of the transverse and longitudinal stretching are 10 times each), cool them to room temperature, and obtain a stretched diaphragm. The stretched diaphragm is flattened and clamped with an extraction tool, placed in 25°C dichloromethane (the concentration of dichloromethane is >99wt%) and extracted for 180s. Dry them in an oven at 40°C for 5 minutes to obtain a lithium-ion battery separator.

[0075] Comparative Example 2

[0076] A method for preparing a lithium-ion battery separator is basically the same as that of Comparative Example 1, with the only difference being that: the "stretching simultaneously in the transverse and longitudinal directions at a stretching rate of 80% / s at 145°C, each stretched to 10 times (the stretching ratios of the transverse and longitudinal stretching are each 10 times)" in Comparative Example 1 is replaced by "stretching simultaneously in the transverse and longitudinal directions at a stretching rate of 80% / s at 145°C, each stretched to 8 times".

[0077] Comparative Example 3

[0078] A method for preparing a lithium-ion battery separator is basically the same as that in Example 1, except that: in this comparative example, the ratio of ultra-high molecular weight polyolefin, solubilizer and pore-forming agent is 30:5:65, calculated by mass; the ultra-high molecular weight polyolefin is 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 is 28.5:1.5, calculated by mass; and the thickness of the crystallized sheet is 1.6 mm.

[0079] Comparative Example 4

[0080] A method for preparing a lithium-ion battery separator is basically the same as that in 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 is 30:5:65, calculated by mass; the ultra-high molecular weight polyolefin is 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 is 16.5:13.5, calculated by mass.

[0081] In this comparative example, melt fracture occurred during the melt casting process and no crystalline casting sheet could be formed.

[0082] Comparative Example 5

[0083] A method for preparing a lithium-ion battery separator is basically the same as that in Example 1, except that: this comparative example does not contain a solubilizer, that is, an ultra-high molecular weight polyolefin and a pore-forming agent are mixed in a raw material premixing tank, and the ratio of the ultra-high molecular weight polyolefin to the pore-forming agent is 30:70 by mass. The ultra-high molecular weight polyolefin is a mixture of ultra-high molecular weight polyethylene and ultra-high molecular weight polypropylene, and the ratio of the ultra-high molecular weight polyethylene to the ultra-high molecular weight polypropylene is 70:30 by mass. The pore-forming agent is white oil.

[0084] In this comparative example, melt fracture occurred during the melt casting process and no crystalline casting sheet could be formed.

[0085] Comparative Example 6

[0086] A method for preparing a lithium ion battery separator is basically the same as that in Example 1, except that: in this comparative example, ultra-high molecular weight polyethylene, ultra-high molecular weight polypropylene, an antioxidant, a solubilizer, a pore former and a nucleating agent are mixed, and the ratio of ultra-high molecular weight polyethylene, ultra-high molecular weight polypropylene, antioxidant, solubilizer, pore former and nucleating agent is 21:9:0.05:5:65:0.5 by mass, wherein the nucleating agent is a mixture of pimelic acid and calcium stearate, and the ratio of pimelic acid to calcium stearate is 1.2:3.8 by mass, and the antioxidant is pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate (antioxidant 1010) (the pore former and the solubilizer are the same as in Example 1), and the thickness of the crystallized sheet is 1.4 mm.

[0087] Comparative Example 7

[0088] A method for preparing a lithium-ion battery separator is basically the same as that in Example 1, with the only difference being that the ratio of the cooling roller linear velocity to the melt cast sheet flow rate is set to 30, stretching is performed by casting, and cooling is performed to room temperature to form a crystalline cast sheet with a thickness of 1.3 mm.

[0089] Comparative Example 8

[0090] A method for preparing a lithium-ion battery separator is basically the same as that in Example 1, with the only difference being step 3: Step 3 of this comparative example: cut the crystallized cast sheet into a 100 mm × 100 mm square sample and place it on a static biaxial stretching machine. After the clips are flattened and clamped to clamp the square sample, enter the heating oven and preheat at 145°C for 240s, first longitudinally stretch it to 10 times at a stretching speed of 80% / s at 145°C (stretching ratio of 10 times), heat-set at 145°C for 90s, and then transversely stretch it to 10 times at a stretching speed of 80% / s at 145°C (stretching ratio of 10 times), and cool to room temperature to obtain a lithium-ion battery separator, that is, this comparative example only performs a single 10-fold stretching.

[0091] Comparative Example 9

[0092] A method for preparing a lithium-ion battery separator is basically the same as that in Example 1, with the only difference being that the ultra-high molecular weight polyolefin in this comparative example is ultra-high molecular weight polyethylene, that is, ultra-high molecular weight polypropylene is not added in this comparative example.

[0093] Table 1 shows the performance parameters of the high-strength and high-elongation diaphragms prepared in Examples 1 to 4 and the lithium-ion battery diaphragms prepared in Comparative Examples 1 to 9 (the diaphragm thickness in Table 1 is the thickness of the high-strength and high-elongation diaphragm / lithium-ion battery diaphragm).

[0094] Table 1

[0095] It can be seen from Examples 1 and 2 that as the stretching ratio of the secondary stretching increases, the tensile strength and single-micron needle punch strength gradually increase, but the elongation decreases. The main reason is that as the stretching ratio of the secondary stretching increases, the directional crystallization of the amorphous zone gradually increases, which increases the rigidity strength of the diaphragm, while the toughness and ductility deteriorate. It can be seen from Comparative Examples 1 and 2 that a single polyolefin has a low tensile strength and elongation due to its single crystal morphology after a single stretch; it can be seen from Comparative Examples 3 to 5 that when the content of polypropylene in the ultra-high molecular weight polyolefin is ≤5% (Comparative Example 3), the addition amount is too small, and the tensile strength, needle puncture strength and elongation of the lithium ion battery separator are not significantly improved; when the content of polypropylene in the ultra-high molecular weight polyolefin is ≥45% (Comparative Example 4) and no solubilizer is added (Comparative Example 5), due to the difference in the compatibility coefficients of polyethylene, polypropylene and white oil, the two phases cannot be fused and a uniform melt cannot be formed, and the melt fractures and cannot form a casting sheet; it can be seen from Comparative Examples 6 to 7 that the addition of nucleating agents and antioxidants or convection High-ratio stretching has no significant effect on the tensile strength, needle puncture strength, ionic conductivity and elongation of the diaphragm. It can be seen from Comparative Examples 8 and 9 that the factors that make the diaphragm have high strength and high ductility at the same time are the interaction between multiple stretching and polyolefin raw materials. When only a single stretching is performed, the tensile strength and ductility of the diaphragm are very poor. When only ultra-high molecular weight polyethylene is used but multiple stretching is performed, the tensile strength is slightly improved, but the ductility is significantly reduced. This is because multiple stretching increases the crystallinity of the diaphragm and enhances the rigidity, but there is no ultra-high molecular weight polypropylene component to form a flexible crystalline area, resulting in poor ductility. Therefore, only the technical solution of the embodiment of the present invention can achieve significant characteristics and improvements.

[0096] The above is an exemplary description of the present invention. It should be noted that, without departing from the core of the present invention, any simple deformation, modification or other equivalent replacement that can be made by other skilled in the art without expending creative labor falls within the scope of protection of the present invention.

Claims

1. A high-strength and high-elongation-rate diaphragm, characterized in that Comprising: Ultra-high molecular weight polyolefin and a solubilizer. By mass parts, the ratio of the ultra-high molecular weight polyolefin to the solubilizer is (20 - 40):(2 - 10). Among them, the solubilizer is one or a mixture of ethylene-propylene copolymer, ethylene-butene copolymer, and ethylene-octene copolymer. The ultra-high molecular weight polyolefin includes ultra-high molecular weight polyethylene and ultra-high molecular weight polypropylene. The weight-average molecular weight of the ultra-high molecular weight polyethylene is 600,000 - 4,000,000, and the weight-average molecular weight of the ultra-high molecular weight polypropylene is 400,000 - 2,600,000.

2. The high-strength and high-elongation-rate separator according to claim 1, wherein Pores are formed in the high-strength and high-elongation-rate separator. The pores are formed by first filling with a pore-forming agent and then extracting and washing the pore-forming agent out of the high-strength and high-elongation-rate separator. By mass parts, the ratio of the solubilizer to the pore-forming agent is (2 - 10):(50 - 78).

3. The high-strength and high-elongation-rate separator according to claim 1 or 2, wherein 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.

4. The high-strength and high-elongation-rate diaphragm according to claim 1 or 2, characterized in that, By mass parts, the ratio of the ultra-high molecular weight polyethylene to the ultra-high molecular weight polypropylene is (60 - 94):(6 - 40).

5. The high-strength and high-elongation-rate separator according to claim 2, characterized in that The pore-forming agent is white oil and / or dioctyl terephthalate.

6. A method for preparing a high-strength and high-elongation-rate diaphragm, characterized in that, Including the following steps: Step 1: Mix the ultra-high molecular weight polyolefin, solubilizer, and pore-forming agent, and stir until uniform to obtain a premixed raw material. Step 2: Inject the premixed raw material into an extruder, heat it to melting to obtain a thermodynamically single-phase melt, and perform melt casting of the thermodynamically single-phase melt onto a cooling roll to obtain a crystalline cast sheet. Step 3: Perform a first stretching on the crystalline cast sheet, cool it to obtain a first-stretched film, perform a second stretching on the first-stretched film to obtain a second-stretched film, extract and wash, and dry to obtain a high-strength and high-elongation-rate separator.

7. The preparation method according to claim 6, characterized in that, In Step 3, the first stretching includes: longitudinally stretching at 140 - 150°C, heat-setting at 140 - 150°C for 60 - 120 s, and transversely stretching at 140 - 150°C. In the first stretching, transverse stretching is performed before or after longitudinal stretching, and heat-setting is performed between longitudinal stretching and transverse stretching.

8. The preparation method according to claim 6 or 7, characterized in that, The second stretching includes: simultaneous transverse and longitudinal stretching or step-by-step transverse and longitudinal stretching. Among them, the simultaneous transverse and longitudinal stretching includes: simultaneously stretching transversely and longitudinally at 90 - 110°C. The step-by-step transverse and longitudinal stretching includes: longitudinally stretching at 90 - 110°C, heat-setting at 90 - 110°C for 60 - 120 s, and transversely stretching at 90 - 110°C. In the second stretching, transverse stretching is performed before or after longitudinal stretching, and heat-setting is performed between longitudinal stretching and transverse stretching.

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

10. The preparation method according to claim 7, wherein In Step 2, the ratio of the linear speed of the cooling roll to the flow rate of the melt-cast sheet is greater than 0.

8. The unit of the linear speed is m / min, and the unit of the flow rate is m / min; in Step 2, the temperature of the thermodynamically single-phase melt during melt casting is 225 - 265°C.

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