High-strength high-ductility separator and method of making same

Abstract

The application discloses a high-strength high-ductility diaphragm and a preparation method thereof. The high-strength high-ductility diaphragm comprises ultrahigh molecular weight polyolefin and a solubilizing agent, wherein the solubilizing agent is a mixture of one or more of ethylene-propylene copolymer, ethylene-butene copolymer and ethylene-octene copolymer. The preparation method comprises the following steps: mixing the ultrahigh molecular weight polyolefin, the solubilizing agent and a pore-forming agent, stirring until uniform to obtain premixed raw materials; injecting the premixed raw materials into an extruder, heating to obtain a thermodynamic single-phase melt, and melt casting the thermodynamic single-phase melt onto a cooling roller to obtain a crystalline casting sheet; once stretching the crystalline casting sheet, cooling to obtain a once-stretched film, twice stretching the once-stretched film to obtain a twice-stretched film, washing, drying to obtain the high-strength high-ductility diaphragm. The preparation method utilizes the solubilizing effect of the copolymer and multiple step-by-step stretching, so that the diaphragm has excellent ductility and tensile strength.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of battery separators, and particularly relates to a high-strength and high-ductility separator and a preparation method thereof. BACKGROUND

[0002] The lithium ion battery separator is a vital safety and functional component of a lithium battery, which has the dual functions of electronic insulation and ionic conductivity. The insulation refers to that the separator is clamped between the positive and negative electrodes to avoid short-circuit failure of the positive and negative electrodes, so the separator needs to have high strength to prevent impurities or positive and negative electrode burrs from being pierced to cause short circuit of the battery. Meanwhile, the separator also needs to have certain ductility, so that when the electrode material deforms, the separator has sufficient deformation space to avoid direct rupture of the separator and short circuit of the electrode. However, the raw material of the separator, high molecular polyolefin, is crystallized due to the straightening and directional arrangement of the molecules in the stretching process, so that the brittleness is enhanced and the ductility is poor. That is, after the separator has strong rigidity strength, the ductility and flexibility are poor, which increases the risk of rupture in the battery cell. In summary, the ductility and strength of the separator are difficult to be considered together, so how to effectively improve the two characteristics simultaneously is a bottleneck problem in the field of separator manufacturing. SUMMARY

[0003] In view of the deficiencies of the prior art, the purpose of the present application is to provide a high-strength and high-ductility separator, which uses a solubilizing agent to improve the processing fluidity and system compatibility of the ultra-high molecular weight polyolefin.

[0004] Another purpose of the present application is to provide a preparation method of the high-strength and high-ductility separator.

[0005] The purpose of the present application is achieved by the following technical solutions.

[0006] A high-strength and high-ductility separator comprises: ultra-high molecular weight polyolefin and solubilizing agent, the ratio of the ultra-high molecular weight polyolefin and the solubilizing agent is (20-40):(2-10) by mass fraction, wherein the solubilizing agent is a mixture of one or more of ethylene-propylene copolymer, ethylene-butene copolymer and ethylene-octene copolymer, the ultra-high molecular weight polyolefin comprises: ultra-high molecular weight polyethylene and ultra-high molecular weight polypropylene, the weight average molecular weight of the ultra-high molecular weight polyethylene is 6-40 million, and the weight average molecular weight of the ultra-high molecular weight polypropylene is 4-26 million.

[0007] The ethylene-propylene copolymer is formed by copolymerization of ethylene and propylene.

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

[0009] The ethylene-octene copolymer is formed by copolymerization of ethylene and octene.

[0010] In the technical scheme, the ratio of the super high molecular weight polyethylene and the super high molecular weight polypropylene is (60-94) : (6-40) by mass fraction.

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

[0012] In the technical scheme, the high-strength high-ductility diaphragm is formed with pores, the pores are formed by filling the pore-forming agent and then washing out the pore-forming agent from the high-strength high-ductility diaphragm, and the ratio of the solubilizing agent and the pore-forming agent is (2-10) : (50-78) by mass fraction.

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

[0014] In the technical scheme, the solubilizing agent has a density of 0.5-1 g / cm 3 , a melting point of 50-90 DEG C, and a crystallization peak temperature of 40-80 DEG C.

[0015] The preparation method of the high-strength high-ductility diaphragm includes the following steps:

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

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

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

[0019] In the technical scheme, the nucleating agent and the solubilizing agent have a ratio of (0.5-1) : (2-10) by mass fraction.

[0020] In the technical scheme, the nucleating agent is a mixture of heptanedioic acid and calcium stearate, and the ratio of heptanedioic acid and calcium stearate is (0.8-3.8) : (1.2-4.2) by mass fraction.

[0021] In the technical scheme, the antioxidant and the solubilizing agent have a ratio of (0.05-0.1) : (2-10) by mass fraction.

[0022] In the technical scheme, the antioxidant is tetra[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionic acid]pentaerythritol ester (antioxidant 1010).

[0023] In step 2, the pre-mixed raw materials are injected into an extruder and heated to melt to obtain a thermodynamic single-phase melt, and the thermodynamic single-phase melt is melt-cast onto a cooling roller to obtain a crystalline cast sheet.

[0024] In step 2, the ratio of the linear speed of the cooling roller 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.

[0025] In step 2, the temperature of the heating 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 rotation speed of the co-rotating twin-screw extruder is 100-240 rpm, and the screw length-diameter ratio is 56-68.

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

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

[0030] In step 3, the crystalline cast sheet is subjected to primary stretching, cooled to obtain a primary stretched film, and the primary stretched film is subjected to secondary stretching to obtain a secondary stretched film, washed, dried to obtain a high-strength high-elongation separator.

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

[0032] In the technical scheme, the stretching ratio of the longitudinal stretching and the transverse stretching in the primary stretching is each 2-5 times.

[0033] In the technical scheme, the primary stretching is preceded by preheating, the temperature of the preheating is 140-150°C, and the preheating time is 180-300 s.

[0034] In step 3, the secondary stretching comprises: synchronous transverse and longitudinal stretching or stepwise transverse and longitudinal stretching, wherein the synchronous transverse and longitudinal stretching comprises: simultaneously stretching in the transverse and longitudinal directions at 90-110 DEG C at a stretching speed of 10-150 % / s, the stepwise transverse and longitudinal stretching comprises: longitudinally stretching at 90-110 DEG C at a stretching speed of 10-150 % / s, heat setting for 60-120 s at 90-110 DEG C, and then transversely stretching at 90-110 DEG C at a stretching speed of 10-150 % / s, and in the secondary stretching, the transverse stretching is performed before or after the longitudinal stretching, and the heat setting is performed between the longitudinal stretching and the transverse stretching.

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

[0036] In the above technical solution, the stretching ratio of the longitudinal stretching and the transverse stretching in the secondary stretching is 2-5 times.

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

[0038] In step 3, the extraction and washing is performed by extracting in an extracting agent at 25-55 DEG C for 60-240 s, the extracting agent is dichloromethane, and the purity of the dichloromethane is >99 wt%.

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

[0040] The preparation method of the present application utilizes the solubilization effect of the copolymer to make the ultra-high molecular weight polyethylene and the ultra-high molecular weight polypropylene melt into a uniform single phase at high temperature, so that the ultra-high molecular weight polyethylene and the ultra-high molecular weight polypropylene are compatible, and higher strength is provided, and meanwhile, the differences in the crystalline structures and densities of different polyolefins are utilized to form crystalline regions with different textures, the ultra-high molecular weight polypropylene forms folded chain crystal structure (the ultra-high molecular weight polypropylene has lower crystallinity, better flexibility, and better ductility, while the ultra-high molecular weight polyethylene forms straight chain crystal structure, which is more rigid and has poorer ductility than the ultra-high molecular weight polypropylene), so that the diaphragm has higher ductility; in the stretching process, the first stretching makes the polyolefin molecules regularly arranged and locally crystallized (crystallinity 50-70 %), so as to provide ultra-high strength, and the second stretching makes the amorphous regions crystallize to form semi-crystalline regions, and the ductility of these semi-crystalline regions is better than that of the crystalline regions, so as to provide higher ductility. The tensile strength of the high-strength and high-ductility diaphragm is increased by about 89 % compared with that of the conventional lithium ion battery diaphragm, and the ductility is increased by about 76 %. DETAILED DESCRIPTION

[0041] The technical solution of the present application will be further described below in combination with specific embodiments.

[0042] The related instruments and equipment used in the embodiment of the present application are as follows:

[0043] A co-rotating twin-screw extruder, model MT-52, screw diameter 52 mm, length-diameter ratio 68, purchased from Jiangsu Meizhi Long Machinery Co., Ltd.;

[0044] A melt line and a T-shaped die, T-shaped die width 250 mm, purchased from Zhejiang Jingcheng Die & Mold Machinery Co., Ltd.;

[0045] A cooling roller, purchased from Anhui Dongsheng Machinery Co., Ltd.;

[0046] A static biaxial stretching machine, model JTL-10, purchased from Weihai Haichao Machinery Co., Ltd.;

[0047] An oven, model 101-4B, Shanghai Shandao Instrument Manufacturing Co., Ltd.

[0048] The related raw materials used in the embodiment of the present application are as follows:

[0049] Ultra-high molecular weight polyethylene and ultra-high molecular weight polypropylene, purchased from Daehan Yuhwa (Shanghai) Co., Ltd.; the weight average molecular weight of the ultra-high molecular weight polyethylene is 1.5 million, and the melt index is 0 g / 10 min (under the condition of 21.6 kg and 190℃); the weight average molecular weight of the ultra-high molecular weight polypropylene is 1.3 million, and the melt index is 0.20 g / 10 min (under the condition of 21.6 kg and 190℃).

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

[0051] Methylene chloride, purchased from Shandong Luhua Chemical Co., Ltd.;

[0052] Ethylene-propylene copolymer, ethylene-butene copolymer and ethylene-octene copolymer, with the Engage 8411, Engage 8230 and Engage 8150 as the grades respectively, purchased from Jiangsu Feigu Plastic Co., Ltd.; the density of the ethylene-propylene copolymer is 0.868 g / cm 3 , the melting point is 55.0℃, and the crystallization peak temperature is 42.0℃; the density of the ethylene-butene copolymer is 0.842 g / cm 3 , the melting point is 57.2℃, and the crystallization peak temperature is 45.1℃; the density of the ethylene-octene copolymer is 0.852 g / cm 3 , the melting point is 60.2℃, and the crystallization peak temperature is 50.4℃;

[0053] DOTP, dioctyl terephthalate, purchased from Rui Feng Nano Co., Ltd.

[0054] The tensile strength and elongation test methods refer to the national standard "Polyolefin Separator for Lithium Ion Battery" (GB / T 36363-2018).

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

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

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

[0058] Example 1

[0059] A high-strength and high-elongation separator, comprising: an ultrahigh molecular weight polyolefin and a solubilizing agent, a plurality of pores are formed in the high-strength and high-elongation separator, the plurality of pores are formed by filling a pore former and then extracting the pore former from the high-strength and high-elongation separator.

[0060] The preparation method of the high-strength and high-elongation separator, comprising the following steps:

[0061] Step 1, mixing ultrahigh molecular weight polyolefin, solubilizing agent and pore former in a raw material premixing tank, stirring at 85°C and 50rpm for 30min to uniform, obtaining premixing raw material, the ratio of ultrahigh molecular weight polyolefin, solubilizing agent and pore former is 30:5:65 by mass fraction, wherein the solubilizing agent is ethylene-butene copolymer, the ultrahigh molecular weight polyolefin is a mixture of ultrahigh molecular weight polyethylene and ultrahigh molecular weight polypropylene, the ratio of ultrahigh molecular weight polyethylene and ultrahigh molecular weight polypropylene is 70:30 by mass fraction, and the pore former is white oil;

[0062] Step 2, injecting the premixing raw material into a co-rotating twin screw extruder, heating to melt at 170rpm and 240°C in the co-rotating twin screw extruder, obtaining a thermodynamic single-phase melt, extruding the 245°C thermodynamic single-phase melt from a T-shaped die through a melt line, melt casting onto a cooling roller at 80°C (the ratio of the cooling roller linear speed to the melt casting sheet flow speed is 1, the unit of linear speed is m / min, and the unit of flow speed is m / min), cooling to room temperature 20-25°C, obtaining a crystalline casting sheet with a thickness of 1.4mm;

[0063] Step 3: The crystallized cast sheet was cut into 100 mm x 100 mm square samples for primary stretching: The square sample was placed on a static biaxial stretching machine, the clamps were flattened to hold the square sample, and then the sample was preheated in a heating oven at 145 °C for 240 s. The sample was first stretched longitudinally at 145 °C at a stretching speed of 80 % / s to 5 times (stretching ratio of 5 times), heat set at 145 °C for 90 s, and then stretched transversely at 145 °C at a stretching speed of 80 % / s to 5 times (stretching ratio of 5 times). The sample was cooled to room temperature to obtain a primary stretched film. The primary stretched film was cut into 100 mm x 100 mm square samples for secondary stretching (transverse-longitudinal step stretching): The square sample was placed on a static biaxial stretching machine, the clamps were flattened to hold the square sample, and then the sample was preheated in a heating oven at 100 °C for 240 s. The sample was first stretched longitudinally at 100 °C at a stretching speed of 80 % / s to 5 times (stretching ratio of 5 times), heat set at 100 °C for 90 s, and then stretched transversely at 100 °C at a stretching speed of 80 % / s to 5 times (stretching ratio of 5 times). The sample was cooled to room temperature to obtain a secondary stretched film. The secondary stretched film was flattened and held by clamps, and then extracted in dichloromethane (purity > 99 wt.%) at 25 °C for 180 s. The sample was dried in an oven at 40 °C for 5 min to obtain a high-strength high-ductility separator.

[0064] Example 2

[0065] A method for preparing a high-strength high-ductility separator was substantially the same as that of Example 1, except that the stretching ratio of the longitudinal stretching and the transverse stretching in the secondary stretching was 3 times in this example. That is, the secondary stretching in this example was as follows: The square sample was placed on a static biaxial stretching machine, the clamps were flattened to hold the square sample, and then the sample was preheated in a heating oven at 100 °C for 240 s. The sample was first stretched longitudinally at 100 °C at a stretching speed of 80 % / s to 3 times, heat set at 100 °C for 90 s, and then stretched transversely at 100 °C at a stretching speed of 80 % / s to 3 times. The sample was cooled to room temperature to obtain a secondary stretched film.

[0066] Example 3

[0067] A method for preparing a high-strength high-ductility separator was substantially the same as that of Example 1, except that the secondary stretching in this example was transverse-longitudinal simultaneous stretching. That is, the secondary stretching in this example was as follows: The square sample was placed on a static biaxial stretching machine, the clamps were flattened to hold the square sample, and then the sample was preheated in a heating oven at 100 °C for 240 s. The sample was simultaneously stretched transversely and longitudinally at 100 °C at a stretching speed of 80 % / s to 5 times (stretching ratio of 5 times). The sample was cooled to room temperature to obtain a secondary stretched film.

[0068] Example 4

[0069] A method for preparing a high-strength high-ductility separator, which is substantially the same as that of Example 3, with the only difference being that the stretching ratio of the longitudinal stretching and the transverse stretching in the secondary stretching is 3 times each.

[0070] Comparative Example 1

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

[0072] Step 1, add ultra-high molecular weight polyethylene and pore-forming agent into the raw material premixing tank, stir at 90℃ and 50rpm for 30min to uniform, obtain the premixing raw material, wherein the ratio of ultra-high molecular weight polyethylene and pore-forming agent is 30:65 by mass fraction, and the pore-forming agent is white oil;

[0073] Step 2, inject the premixing raw material into the co-rotating twin-screw extruder, heat to melt at 170rpm and 240℃ in the co-rotating twin-screw extruder, obtain the thermodynamic single-phase melt, extrude the 245℃ thermodynamic single-phase melt from the T-shaped die through the melt line, flow cast onto the cooling roller at 80℃ (the ratio of the linear speed of the cooling roller to the flow speed of the melt cast sheet is 1, the unit of linear speed is m / min, and the unit of flow speed is m / min), cool to room temperature 20-25℃, obtain the crystalline cast sheet with a thickness of 1.3mm;

[0074] Step 3, cut the crystalline cast sheet into 100mm×100mm square sample, place the square sample on the static two-way stretching machine, flatten and clamp the square sample with the clamp, then enter the heating oven at 145℃ for preheating for 240s, simultaneously stretch in the transverse and longitudinal directions at 145℃ with a stretching speed of 80% / s, each stretching to 10 times (the stretching ratio of the transverse and longitudinal stretching is 10 times each), cool to room temperature, obtain the stretched separator, flatten and clamp the stretched separator with the extraction tooling, place it in dichloromethane (the concentration of dichloromethane is >99wt%) at 25℃ for extraction for 180s, dry in the oven at 40℃ for 5min, obtain the lithium ion battery separator.

[0075] Comparative Example 2

[0076] A method for preparing a lithium ion battery separator, which is substantially the same as that of Comparative Example 1, with the only difference being that the "simultaneously stretch in the transverse and longitudinal directions at 145℃ with a stretching speed of 80% / s, each stretching to 10 times (the stretching ratio of the transverse and longitudinal stretching is 10 times each)" in Comparative Example 1 is replaced by "simultaneously stretch in the transverse and longitudinal directions at 145℃ with a stretching speed of 80% / s, each stretching to 8 times".

[0077] Comparative Example 3

[0078] A method for preparing a lithium ion battery separator is substantially the same as that of Example 1, the only difference being that in the present comparative example, the ratio of the super high molecular weight polyolefin, the solubilizing agent and the pore-forming agent is 30:5:65 by mass fraction, the super high molecular weight polyolefin is a mixture of super high molecular weight polyethylene and super high molecular weight polypropylene, the ratio of the super high molecular weight polyethylene and the super high molecular weight polypropylene is 28.5:1.5 by mass fraction, and the thickness of the crystalline casting sheet is 1.6 mm.

[0079] Comparative Example 4

[0080] A method for preparing a lithium ion battery separator is substantially the same as that of Example 1, the only difference being that in the present comparative example, the ratio of the super high molecular weight polyolefin, the solubilizing agent and the pore-forming agent is 30:5:65 by mass fraction, the super high molecular weight polyolefin is a mixture of super high molecular weight polyethylene and super high molecular weight polypropylene, the ratio of the super high molecular weight polyethylene and the super high molecular weight polypropylene is 16.5:13.5 by mass fraction.

[0081] In the present comparative example, melt fracture occurred during melt casting, and a crystalline casting sheet could not be formed.

[0082] Comparative Example 5

[0083] A method for preparing a lithium ion battery separator is substantially the same as that of Example 1, the only difference being that in the present comparative example, the solubilizing agent is not included, i.e., the super high molecular weight polyolefin and the pore-forming agent are mixed in the raw material premixing tank, the ratio of the super high molecular weight polyolefin and the pore-forming agent is 30:70 by mass fraction, the super high molecular weight polyolefin is a mixture of super high molecular weight polyethylene and super high molecular weight polypropylene, the ratio of the super high molecular weight polyethylene and the super high molecular weight polypropylene is 70:30 by mass fraction, and the pore-forming agent is white oil.

[0084] In the present comparative example, melt fracture occurred during melt casting, and a crystalline casting sheet could not be formed.

[0085] Comparative Example 6

[0086] A preparation method of a lithium ion battery separator is basically the same as that of Example 1, the only difference is that in the present comparative example, the super high molecular weight polyethylene, the super high molecular weight polypropylene, the antioxidant, the solubilizing agent, the pore forming agent and the nucleating agent are mixed, and the mass ratio of the super high molecular weight polyethylene, the super high molecular weight polypropylene, the antioxidant, the solubilizing agent, the pore forming agent and the nucleating agent is 21:9:0.05:5:65:0.5, wherein the nucleating agent is a mixture of pimelic acid and calcium stearate, and the mass ratio of pimelic acid to calcium stearate is 1.2:3.8, the antioxidant is tetra[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionic acid]pentaerythritol ester (antioxidant 1010), and the pore forming agent and the solubilizing agent are the same as those in Example 1, and the thickness of the crystalline casting sheet is 1.4 mm.

[0087] Comparative Example 7

[0088] A preparation method of a lithium ion battery separator is basically the same as that of Example 1, the only difference is that in the present comparative example, the super high molecular weight polyethylene, the super high molecular weight polypropylene, the antioxidant, the solubilizing agent, the pore forming agent and the nucleating agent are mixed, and the mass ratio of the super high molecular weight polyethylene, the super high molecular weight polypropylene, the antioxidant, the solubilizing agent, the pore forming agent and the nucleating agent is 21:9:0.05:5:65:0.5, wherein the nucleating agent is a mixture of pimelic acid and calcium stearate, and the mass ratio of pimelic acid to calcium stearate is 1.2:3.8, the antioxidant is tetra[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionic acid]pentaerythritol ester (antioxidant 1010), and the pore forming agent and the solubilizing agent are the same as those in Example 1, and the thickness of the crystalline casting sheet is 1.4 mm.

[0089] Comparative Example 8

[0090] A preparation method of a lithium ion battery separator is basically the same as that of Example 1, the only difference is that in the present comparative example, the super high molecular weight polyethylene, the super high molecular weight polypropylene, the antioxidant, the solubilizing agent, the pore forming agent and the nucleating agent are mixed, and the mass ratio of the super high molecular weight polyethylene, the super high molecular weight polypropylene, the antioxidant, the solubilizing agent, the pore forming agent and the nucleating agent is 21:9:0.05:5:65:0.5, wherein the nucleating agent is a mixture of pimelic acid and calcium stearate, and the mass ratio of pimelic acid to calcium stearate is 1.2:3.8, the antioxidant is tetra[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionic acid]pentaerythritol ester (antioxidant 1010), and the pore forming agent and the solubilizing agent are the same as those in Example 1, and the thickness of the crystalline casting sheet is 1.4 mm.

[0091] Comparative Example 9

[0092] A preparation method of a lithium ion battery separator is basically the same as that of Example 1, the only difference is that in the present comparative example, the super high molecular weight polyethylene, the super high molecular weight polypropylene, the antioxidant, the solubilizing agent, the pore forming agent and the nucleating agent are mixed, and the mass ratio of the super high molecular weight polyethylene, the super high molecular weight polypropylene, the antioxidant, the solubilizing agent, the pore forming agent and the nucleating agent is 21:9:0.05:5:65:0.5, wherein the nucleating agent is a mixture of pimelic acid and calcium stearate, and the mass ratio of pimelic acid to calcium stearate is 1.2:3.8, the antioxidant is tetra[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionic acid]pentaerythritol ester (antioxidant 1010), and the pore forming agent and the solubilizing agent are the same as those in Example 1, and the thickness of the crystalline casting sheet is 1.4 mm.

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

[0094] Table 1

[0095]

[0096]

[0097] From Example 1 and Example 2, it can be seen that as the draw ratio of the secondary stretching increases, the tensile strength and the single micron needle puncture strength gradually increase, but the ductility decreases. The main reason is that as the draw ratio of the secondary stretching increases, the oriented crystallization in the unshaped area gradually increases, which increases the rigid strength of the separator and deteriorates the toughness and ductility. From Comparative Example 1 and Comparative Example 2, it can be seen that a single polyolefin after single stretching has low tensile strength and ductility due to the single crystalline morphology. From Comparative Example 3 to Comparative Example 5, it can be seen that when the content of polypropylene in the ultra-high molecular weight polyolefin is ≤5% (Comparative Example 3), the tensile strength, needle puncture strength and ductility of the lithium ion battery separator do not significantly increase due to the small amount of addition. When the content of polypropylene in the ultra-high molecular weight polyolefin is ≥45% (Comparative Example 4) and no solubilizing agent is added (Comparative Example 5), the two phases cannot be fused due to the difference in the compatibility coefficient of polyethylene, polypropylene and white oil, which results in the failure to form a uniform melt and the failure to form a cast piece. From Comparative Example 6 to Comparative Example 7, it can be seen that adding a nucleating agent and an antioxidant or performing high-ratio stretching on the cast film has no significant effect on the tensile strength, needle puncture strength, ion conductivity and ductility of the separator. From Comparative Example 8 and Comparative Example 9, it can be seen that the factors affecting the high strength and high ductility of the separator are multiple stretching and the interaction of the polyolefin raw materials. When only single stretching is performed, the tensile strength and ductility of the separator are both poor. When only ultra-high molecular weight polyethylene is used but multiple stretching is performed, the tensile strength slightly increases, but the ductility significantly decreases. This is because multiple stretching increases the crystallinity of the separator and increases the rigidity, but the flexible crystalline region formed by the ultra-high molecular weight polypropylene component deteriorates the ductility. Therefore, only the technical solution of the present application can achieve significant performance consideration and improvement.

[0098] The above has exemplarily described the present application. It should be noted that any simple modification, change or equivalent replacement without creative labor by those skilled in the art without departing from the core of the present application falls within the protection scope of the present application.

Claims

1. A method for producing a high-strength high-ductility separator, characterized by, The method comprises the following steps: Step 1, mixing the ultra-high molecular weight polyolefin, the solubilizing agent and the pore-forming agent to obtain a premix, wherein the ultra-high molecular weight polyolefin comprises: ultra-high molecular weight polyethylene and ultra-high molecular weight polypropylene, the weight average molecular weight of the ultra-high molecular weight polyethylene is 6-40 million, the weight average molecular weight of the ultra-high molecular weight polypropylene is 4-26 million, the solubilizing agent is a mixture of one or more of ethylene-propylene copolymer, ethylene-butene copolymer and ethylene-octene copolymer, and the pore-forming agent is white oil and / or dioctyl terephthalate, wherein the ratio of the ultra-high molecular weight polyolefin and the solubilizing agent is (20-40):(2-10) by mass fraction, the ratio of the solubilizing agent and the pore-forming agent is (2-10):(50-78) by mass fraction, and the ratio of the ultra-high molecular weight polyethylene and the ultra-high molecular weight polypropylene is (60-94):(6-40) by mass fraction; Step 2, injecting the premix into an extruder and heating to melt to obtain a thermodynamic single-phase melt, and melt casting the thermodynamic single-phase melt onto a cooling roller to obtain a crystallized cast sheet; Step 3, stretching the crystallized cast sheet once, cooling to obtain a first-stretching film, stretching the first-stretching film twice to obtain a second-stretching film, washing, drying to obtain a high-strength high-elongation separator; The first stretching comprises: longitudinal stretching at 140-150℃, heat setting for 60-120 s at 140-150℃ and transverse stretching at 140-150℃, wherein the transverse stretching is performed before or after the longitudinal stretching, and the heat setting is performed between the longitudinal stretching and the transverse stretching; The second stretching comprises: synchronous transverse and longitudinal stretching or stepwise transverse and longitudinal stretching, wherein the synchronous transverse and longitudinal stretching comprises: simultaneous transverse and longitudinal stretching at 90-110℃, and the stepwise transverse and longitudinal stretching comprises: longitudinal stretching at 90-110℃, heat setting for 60-120 s at 90-110℃ and transverse stretching at 90-110℃, wherein the transverse stretching is performed before or after the longitudinal stretching, and the heat setting is performed between the longitudinal stretching and the transverse stretching; The stretching ratio of the longitudinal stretching and the transverse stretching in the first stretching is each 2-5 times, and the stretching ratio of the longitudinal stretching and the transverse stretching in the second stretching is each 2-5 times.

2. The method of claim 1, wherein the high-strength high-ductility separator is prepared by the steps of: The solubilizing agent has a density of 0.5-1 g / cm 3 , a melting point of 50-90°C, and a crystallization peak temperature of 40-80°C.

3. The method of claim 2, wherein the high-strength high-ductility separator is prepared by the steps of: In step 2, the ratio of the linear speed of the cooling roller to the flow speed of the melt casting sheet is greater than 0.8, the unit of the linear speed is m / min, and the unit of the flow speed is m / min.

4. The method of claim 3, wherein the high-strength high-ductility separator is prepared by the steps of: In step 2, the temperature of the thermodynamic single-phase melt during melt casting is 225-265℃.

5. The high-strength high-elongation separator prepared by the method according to any one of claims 1-4, wherein a plurality of pores are formed in the high-strength high-elongation separator.

Images