CN117352953A - Polyolefin film, preparation method thereof, battery separator and battery - Google Patents
Polyolefin film, preparation method thereof, battery separator and battery Download PDFInfo
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- 229920000098 polyolefin Polymers 0.000 title claims abstract description 94
- 238000002360 preparation method Methods 0.000 title claims abstract description 11
- 239000002994 raw material Substances 0.000 claims abstract description 12
- 238000000034 method Methods 0.000 claims description 39
- -1 polyethylene Polymers 0.000 claims description 35
- 230000008569 process Effects 0.000 claims description 28
- YMWUJEATGCHHMB-UHFFFAOYSA-N Dichloromethane Chemical compound ClCCl YMWUJEATGCHHMB-UHFFFAOYSA-N 0.000 claims description 26
- 238000002844 melting Methods 0.000 claims description 23
- 230000008018 melting Effects 0.000 claims description 23
- 239000000463 material Substances 0.000 claims description 21
- 239000004698 Polyethylene Substances 0.000 claims description 19
- 229920000573 polyethylene Polymers 0.000 claims description 19
- 239000003085 diluting agent Substances 0.000 claims description 10
- CJZGTCYPCWQAJB-UHFFFAOYSA-L calcium stearate Chemical compound [Ca+2].CCCCCCCCCCCCCCCCCC([O-])=O.CCCCCCCCCCCCCCCCCC([O-])=O CJZGTCYPCWQAJB-UHFFFAOYSA-L 0.000 claims description 8
- 235000013539 calcium stearate Nutrition 0.000 claims description 8
- 239000008116 calcium stearate Substances 0.000 claims description 8
- 239000003921 oil Substances 0.000 claims description 8
- 230000001360 synchronised effect Effects 0.000 claims description 8
- 230000002457 bidirectional effect Effects 0.000 claims description 7
- 229920001577 copolymer Polymers 0.000 claims description 7
- RTZKZFJDLAIYFH-UHFFFAOYSA-N Diethyl ether Chemical compound CCOCC RTZKZFJDLAIYFH-UHFFFAOYSA-N 0.000 claims description 6
- 239000003963 antioxidant agent Substances 0.000 claims description 6
- 238000000605 extraction Methods 0.000 claims description 6
- 230000003078 antioxidant effect Effects 0.000 claims description 5
- 238000009835 boiling Methods 0.000 claims description 5
- 229920000642 polymer Polymers 0.000 claims description 5
- VYPSYNLAJGMNEJ-UHFFFAOYSA-N Silicium dioxide Chemical compound O=[Si]=O VYPSYNLAJGMNEJ-UHFFFAOYSA-N 0.000 claims description 4
- 239000000919 ceramic Substances 0.000 claims description 4
- 238000001816 cooling Methods 0.000 claims description 4
- 238000009998 heat setting Methods 0.000 claims description 4
- 239000007788 liquid Substances 0.000 claims description 4
- HQKMJHAJHXVSDF-UHFFFAOYSA-L magnesium stearate Chemical compound [Mg+2].CCCCCCCCCCCCCCCCCC([O-])=O.CCCCCCCCCCCCCCCCCC([O-])=O HQKMJHAJHXVSDF-UHFFFAOYSA-L 0.000 claims description 4
- 230000035699 permeability Effects 0.000 claims description 4
- VZGDMQKNWNREIO-UHFFFAOYSA-N tetrachloromethane Chemical compound ClC(Cl)(Cl)Cl VZGDMQKNWNREIO-UHFFFAOYSA-N 0.000 claims description 4
- 239000002033 PVDF binder Substances 0.000 claims description 3
- 239000004743 Polypropylene Substances 0.000 claims description 3
- 239000011248 coating agent Substances 0.000 claims description 3
- 238000000576 coating method Methods 0.000 claims description 3
- 239000002131 composite material Substances 0.000 claims description 3
- 238000001035 drying Methods 0.000 claims description 3
- 229920001155 polypropylene Polymers 0.000 claims description 3
- 229920002981 polyvinylidene fluoride Polymers 0.000 claims description 3
- 239000007787 solid Substances 0.000 claims description 3
- VSAWBBYYMBQKIK-UHFFFAOYSA-N 4-[[3,5-bis[(3,5-ditert-butyl-4-hydroxyphenyl)methyl]-2,4,6-trimethylphenyl]methyl]-2,6-ditert-butylphenol Chemical compound CC1=C(CC=2C=C(C(O)=C(C=2)C(C)(C)C)C(C)(C)C)C(C)=C(CC=2C=C(C(O)=C(C=2)C(C)(C)C)C(C)(C)C)C(C)=C1CC1=CC(C(C)(C)C)=C(O)C(C(C)(C)C)=C1 VSAWBBYYMBQKIK-UHFFFAOYSA-N 0.000 claims description 2
- FKIOYBLZUCCLTL-UHFFFAOYSA-N 4-butyl-2-tert-butyl-5-methylphenol Chemical compound CCCCC1=CC(C(C)(C)C)=C(O)C=C1C FKIOYBLZUCCLTL-UHFFFAOYSA-N 0.000 claims description 2
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 claims description 2
- 239000005662 Paraffin oil Substances 0.000 claims description 2
- 239000004696 Poly ether ether ketone Substances 0.000 claims description 2
- 239000004952 Polyamide Substances 0.000 claims description 2
- 239000004695 Polyether sulfone Substances 0.000 claims description 2
- 239000004642 Polyimide Substances 0.000 claims description 2
- 150000001336 alkenes Chemical class 0.000 claims description 2
- PNEYBMLMFCGWSK-UHFFFAOYSA-N aluminium oxide Inorganic materials [O-2].[O-2].[O-2].[Al+3].[Al+3] PNEYBMLMFCGWSK-UHFFFAOYSA-N 0.000 claims description 2
- CVJYOKLQNGVTIS-UHFFFAOYSA-K aluminum;lithium;titanium(4+);phosphate Chemical compound [Li+].[Al+3].[Ti+4].[O-]P([O-])([O-])=O CVJYOKLQNGVTIS-UHFFFAOYSA-K 0.000 claims description 2
- 229910001593 boehmite Inorganic materials 0.000 claims description 2
- 229910052799 carbon Inorganic materials 0.000 claims description 2
- 238000006243 chemical reaction Methods 0.000 claims description 2
- 238000007334 copolymerization reaction Methods 0.000 claims description 2
- FAHBNUUHRFUEAI-UHFFFAOYSA-M hydroxidooxidoaluminium Chemical compound O[Al]=O FAHBNUUHRFUEAI-UHFFFAOYSA-M 0.000 claims description 2
- 229910000664 lithium aluminum titanium phosphates (LATP) Inorganic materials 0.000 claims description 2
- 235000019359 magnesium stearate Nutrition 0.000 claims description 2
- JRZJOMJEPLMPRA-UHFFFAOYSA-N olefin Natural products CCCCCCCC=C JRZJOMJEPLMPRA-UHFFFAOYSA-N 0.000 claims description 2
- 229920003229 poly(methyl methacrylate) Polymers 0.000 claims description 2
- 229920002492 poly(sulfone) Polymers 0.000 claims description 2
- 229920002647 polyamide Polymers 0.000 claims description 2
- 229920001083 polybutene Polymers 0.000 claims description 2
- 229920006393 polyether sulfone Polymers 0.000 claims description 2
- 229920002530 polyetherether ketone Polymers 0.000 claims description 2
- 229920001721 polyimide Polymers 0.000 claims description 2
- 239000004926 polymethyl methacrylate Substances 0.000 claims description 2
- 239000004800 polyvinyl chloride Substances 0.000 claims description 2
- 229920000915 polyvinyl chloride Polymers 0.000 claims description 2
- 239000000377 silicon dioxide Substances 0.000 claims description 2
- RYYKJJJTJZKILX-UHFFFAOYSA-M sodium octadecanoate Chemical compound [Na+].CCCCCCCCCCCCCCCCCC([O-])=O RYYKJJJTJZKILX-UHFFFAOYSA-M 0.000 claims description 2
- XOOUIPVCVHRTMJ-UHFFFAOYSA-L zinc stearate Chemical compound [Zn+2].CCCCCCCCCCCCCCCCCC([O-])=O.CCCCCCCCCCCCCCCCCC([O-])=O XOOUIPVCVHRTMJ-UHFFFAOYSA-L 0.000 claims description 2
- XBDQKXXYIPTUBI-UHFFFAOYSA-N dimethylselenoniopropionate Natural products CCC(O)=O XBDQKXXYIPTUBI-UHFFFAOYSA-N 0.000 claims 2
- WPMYUUITDBHVQZ-UHFFFAOYSA-N 3-(3,5-ditert-butyl-4-hydroxyphenyl)propanoic acid Chemical compound CC(C)(C)C1=CC(CCC(O)=O)=CC(C(C)(C)C)=C1O WPMYUUITDBHVQZ-UHFFFAOYSA-N 0.000 claims 1
- 229920006231 aramid fiber Polymers 0.000 claims 1
- 238000006116 polymerization reaction Methods 0.000 claims 1
- 239000000843 powder Substances 0.000 claims 1
- 235000019260 propionic acid Nutrition 0.000 claims 1
- IUVKMZGDUIUOCP-BTNSXGMBSA-N quinbolone Chemical compound O([C@H]1CC[C@H]2[C@H]3[C@@H]([C@]4(C=CC(=O)C=C4CC3)C)CC[C@@]21C)C1=CCCC1 IUVKMZGDUIUOCP-BTNSXGMBSA-N 0.000 claims 1
- 235000012239 silicon dioxide Nutrition 0.000 claims 1
- HBBGRARXTFLTSG-UHFFFAOYSA-N Lithium ion Chemical compound [Li+] HBBGRARXTFLTSG-UHFFFAOYSA-N 0.000 abstract description 7
- 229910001416 lithium ion Inorganic materials 0.000 abstract description 7
- 230000009467 reduction Effects 0.000 abstract description 2
- 230000007547 defect Effects 0.000 abstract 1
- 238000004519 manufacturing process Methods 0.000 description 7
- 239000000758 substrate Substances 0.000 description 7
- 230000000052 comparative effect Effects 0.000 description 6
- 238000010438 heat treatment Methods 0.000 description 6
- 239000012528 membrane Substances 0.000 description 6
- 230000000704 physical effect Effects 0.000 description 6
- 230000000630 rising effect Effects 0.000 description 6
- 230000014759 maintenance of location Effects 0.000 description 5
- 238000001125 extrusion Methods 0.000 description 4
- 239000012982 microporous membrane Substances 0.000 description 4
- 230000001276 controlling effect Effects 0.000 description 3
- 238000002425 crystallisation Methods 0.000 description 3
- 230000008025 crystallization Effects 0.000 description 3
- 239000000155 melt Substances 0.000 description 3
- 238000001878 scanning electron micrograph Methods 0.000 description 3
- 230000009286 beneficial effect Effects 0.000 description 2
- 238000005266 casting Methods 0.000 description 2
- 230000004927 fusion Effects 0.000 description 2
- GELKBWJHTRAYNV-UHFFFAOYSA-K lithium iron phosphate Chemical compound [Li+].[Fe+2].[O-]P([O-])([O-])=O GELKBWJHTRAYNV-UHFFFAOYSA-K 0.000 description 2
- 238000012986 modification Methods 0.000 description 2
- 230000004048 modification Effects 0.000 description 2
- 239000011885 synergistic combination Substances 0.000 description 2
- 229910010707 LiFePO 4 Inorganic materials 0.000 description 1
- WHXSMMKQMYFTQS-UHFFFAOYSA-N Lithium Chemical compound [Li] WHXSMMKQMYFTQS-UHFFFAOYSA-N 0.000 description 1
- XBDQKXXYIPTUBI-UHFFFAOYSA-M Propionate Chemical compound CCC([O-])=O XBDQKXXYIPTUBI-UHFFFAOYSA-M 0.000 description 1
- 239000007983 Tris buffer Substances 0.000 description 1
- 239000011149 active material Substances 0.000 description 1
- 239000000654 additive Substances 0.000 description 1
- 230000000996 additive effect Effects 0.000 description 1
- 239000004760 aramid Substances 0.000 description 1
- 229920003235 aromatic polyamide Polymers 0.000 description 1
- 239000006229 carbon black Substances 0.000 description 1
- 230000002079 cooperative effect Effects 0.000 description 1
- 239000013078 crystal Substances 0.000 description 1
- 238000011161 development Methods 0.000 description 1
- 238000009826 distribution Methods 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 239000003792 electrolyte Substances 0.000 description 1
- 238000004146 energy storage Methods 0.000 description 1
- 238000005516 engineering process Methods 0.000 description 1
- 239000000835 fiber Substances 0.000 description 1
- 238000005242 forging Methods 0.000 description 1
- 229910052744 lithium Inorganic materials 0.000 description 1
- 230000003446 memory effect Effects 0.000 description 1
- 238000009828 non-uniform distribution Methods 0.000 description 1
- NFHFRUOZVGFOOS-UHFFFAOYSA-N palladium;triphenylphosphane Chemical compound [Pd].C1=CC=CC=C1P(C=1C=CC=CC=1)C1=CC=CC=C1.C1=CC=CC=C1P(C=1C=CC=CC=1)C1=CC=CC=C1.C1=CC=CC=C1P(C=1C=CC=CC=1)C1=CC=CC=C1.C1=CC=CC=C1P(C=1C=CC=CC=1)C1=CC=CC=C1 NFHFRUOZVGFOOS-UHFFFAOYSA-N 0.000 description 1
- WXZMFSXDPGVJKK-UHFFFAOYSA-N pentaerythritol Chemical compound OCC(CO)(CO)CO WXZMFSXDPGVJKK-UHFFFAOYSA-N 0.000 description 1
- 239000011148 porous material Substances 0.000 description 1
- 238000012545 processing Methods 0.000 description 1
- 238000012827 research and development Methods 0.000 description 1
- 238000012216 screening Methods 0.000 description 1
- 239000002904 solvent Substances 0.000 description 1
- 230000002195 synergetic effect Effects 0.000 description 1
- 238000012360 testing method Methods 0.000 description 1
Abstract
The invention provides a polyolefin film and a preparation method thereof, a battery diaphragm and a battery, and relates to the field of lithium ion batteries. According to the invention, the polyolefin with high crystallinity is selected as a raw material, the stretching multiplying power is controlled, particularly the first stretching is controlled to be low-multiplying power stretching, and then the subsequent high-multiplying power stretching is carried out twice, so that the ultrathin high-strength polyolefin film can be obtained. The invention overcomes the defects of film fracture and strength reduction caused by the stretching of the traditional polyolefin film preparation method.
Description
Technical Field
The invention relates to the field of lithium ion batteries, in particular to an ultrathin high-strength polyolefin film, a preparation method thereof, a battery diaphragm and a battery.
Background
The lithium ion battery has the advantages of high working voltage, high energy density, long cycle life, small self-discharge rate, low pollution, no memory effect and the like, and is widely applied to the fields of new energy automobiles, new energy storage, consumer electronics and the like. Further improving the energy density, the cycle life and the safety of the lithium ion battery are key research and development directions in the field of new energy. The separator is one of important components of the lithium ion battery, plays a role of separating positive and negative electrode conducting electrolyte, and is used as an inactive material in the battery, so that the thinner the separator is, the better the energy density of the battery is necessarily required, however, the strength reduction caused by the thinning of the separator can further increase the potential safety hazard in the use process of the battery, and therefore, the development of an ultrathin high-strength polyolefin separator product is needed.
Currently, the conventional wet process diaphragm manufacturing process flow mainly comprises: high temperature melting, die extrusion, casting sheet, MD stretching, 1TD stretching, extraction, 2TD stretching, heat setting, or high temperature melting, die extrusion, casting sheet, synchronous biaxial stretching, extraction, 2TD stretching and heat setting, the thickness and strength of the conventional diaphragm product are limited due to insufficient substrate stretching under the process, and the process is not suitable for manufacturing ultrathin high-strength diaphragm products.
Chinese patent No. 109517210A discloses an ultrathin high-strength polyolefin microporous membrane and a preparation method thereof, compared with the conventional wet membrane process, the process has the advantages that a stretching step is added after the steps of extraction and drying, the thickness of 5-100 um is obtained, the puncture strength is more than 300gf,a polyolefin microporous membrane having a tensile strength of 170MPa or more; chinese patent No. CN 112592500B discloses a polyolefin microporous membrane and its production system, battery separator, electrochemical device, which has increased extractant temperature compared with conventional wet separator process to obtain polyolefin microporous membrane with thickness of 2-30 um and puncture strength of 1000-2000 gf; chinese patent No. CN 112290164B discloses a high performance additive suitable for producing ultra-thin lithium battery separator, and preparation and application thereof, by adding low surface energy polymer, the surface energy of the melt is reduced, lubricity in the extrusion process of the melt is improved, plasticization and rheological properties of the melt are enhanced, and a polyethylene separator with a thickness of 3um is prepared; the Chinese patent No. 114204213A discloses an ultrathin polyolefin membrane and application, wherein the ultrathin polyolefin membrane with the thickness less than or equal to 7um is obtained by screening polyethylene materials with specific molecular weight and melting characteristics and controlling extrusion quantity, stretching temperature and stretching multiplying power in the processing process; the Chinese patent No. 114914631A discloses an ultra-high strength diaphragm and a preparation method thereof, wherein the stretching multiplying power is increased by adding a section of second longitudinal stretching (second transverse stretching or synchronous bidirectional stretching) before extraction, thus obtaining the diaphragm with the thickness of 4-8um and the transverse stretching strength of 4000kgf/cm 2 Tensile strength in machine direction > 4000kgf/cm 2 A separator with a needling strength of not less than 100 gf/um; chinese patent No. 115020909A discloses a diaphragm for lithium ion battery and its preparation method, the diaphragm is stretched in two directions four times to obtain a diaphragm with a thickness of 3-8 um and a tensile strength of > 5000kgf/cm 2 A lithium ion battery separator with needling strength of more than 120 gf/um; chinese patent No. CN 110815763B discloses an apparatus and a method for preparing a high-strength high-modulus polyolefin film, and a high-strength high-modulus polyolefin film, wherein 10000 times of region stretching is achieved by arranging a cooling system and a multi-stage stretching machine, so as to obtain a polyolefin film with a thickness of 2-100 um, a tensile strength of 300-2000 MPa, and a puncture strength of 50-300 gf/um.
At present, the disclosed ultra-thin high-strength polyolefin membrane manufacturing technology basically adopts a multi-step collaborative high-rate stretching strategy, which is easy to cause frequent membrane breakage caused by uneven stress of a polyolefin substrate crystallization area and an amorphous area in the high-rate stretching process, so that the production efficiency is reduced, and the membrane thinning often causes lower puncture and tensile strength. Therefore, there is a need to develop a method for preparing an ultra-thin high strength polyolefin separator with high efficiency.
Disclosure of Invention
The invention aims to prepare an ultrathin high-strength polyolefin film by controlling the temperature-rising crystallinity and the stretching multiplying power; and the battery diaphragm is manufactured by using the film, and the battery diaphragm is used for a primary battery or a secondary battery, so that the performance of the battery can be improved.
In order to achieve the above purpose, the present invention adopts the following technical scheme:
the invention provides a polyolefin film, which has a thickness of 1-5 um, a puncture strength of 300-1000 gf, a MD tensile strength of more than or equal to 300MPa, and a TD tensile strength of more than or equal to 300MPa.
Further, the polyolefin film has a porosity of 25% to 45% and a permeability of 20 to 80s/100cc.
Further, the primary temperature rise crystallinity of the polyolefin film measured by a differential scanning calorimeter is more than or equal to 80%, and the secondary temperature rise crystallinity is more than or equal to 60%.
The invention also provides a preparation method of the polyolefin film, which comprises the following steps:
(1) Adding polyolefin, antioxidant and diluent into a screw extruder, melting into uniform liquid at the temperature of T1, extruding through a die head, and cooling at the temperature of T2 to form a cast sheet;
(2) Carrying out primary stretching on the cast sheet at the temperature of T3, and controlling the multiplying power of the primary stretching to be 1-2.5 times to obtain a primary stretched film; the first stretching refers to sequentially performing first longitudinal stretching (1 MD) and first transverse stretching (1 TD) or performing first bidirectional synchronous stretching (1 SBS);
(3) Carrying out secondary stretching on the primary stretched film at the temperature of T4 to obtain a secondary stretched film; the second stretching means sequentially performing a second longitudinal stretching (2 MD) and a second transverse stretching (2 TD), or performing a second bidirectional synchronous stretching (2 SBS);
(4) The secondarily stretched film enters an extraction tank with the temperature of T5 to completely elute the diluent, then enters a drying oven with the temperature of T6 to be dried, and then is subjected to tertiary stretching at the temperature of T7 to obtain a tertiary stretched film; the third stretching means that the third longitudinal stretching (3 MD) and the third transverse stretching (3 TD) are sequentially carried out, or the third bidirectional synchronous stretching (3 SBS) is carried out;
(5) And (3) carrying out heat setting on the three-time stretched film at the temperature of T8 to obtain the polyolefin film.
Further, the polyolefin includes, but is not limited to, one or more of polyethylene, polypropylene, polybutene, polypentene, ethylene-propylene copolymer, ethylene-butene copolymer, ethylene-pentene copolymer, etc. synthesized from olefin raw materials having 2 to 10 carbon numbers in the molecule of ethylene, propylene, butene, etc. by a homo-or copolymerization method, and the total solid content is 10 to 60% of the total mass of the polyolefin and the diluent.
Still further, the polyolefin is preferably one or more of polyethylene, polypropylene, and ethylene-propylene copolymer, and the total solid content is 20% to 40% of the total mass of the polyolefin and the diluent.
Further, the polyolefin has a viscosity average molecular weight of 350 to 2000kDa.
Further, the primary temperature rise crystallinity of the polyolefin measured by a differential scanning calorimeter is more than or equal to 65%, and the secondary temperature rise crystallinity is more than or equal to 45%.
Further, the antioxidants include, but are not limited to, one or more of calcium stearate, magnesium stearate, zinc stearate, sodium stearate, 1, 3-tris (2-methyl-4-hydroxy-5-t-butylphenyl) butane, 1,3, 5-trimethyl-2, 4, 6-tris (3, 5-di-t-butyl-4-hydroxybenzyl) benzene, pentaerythritol tetrakis [ beta- (3, 5-di-t-butyl-4-hydroxyphenyl) propionate ], n-stearyl beta- (3, 5-di-t-butyl-4-hydroxyphenyl) propionate.
Further, the addition amount of the antioxidant is 0.01-1.0% of the mass of the polyolefin material.
Further, the diluent includes, but is not limited to, one or more of white oil, paraffin oil, alkane compounds or ester compounds, and can be fully melt-mixed with polyolefin into uniform liquid at the temperature of the melting point of the polyolefin material of +50-100 ℃ (namely 50-100 ℃ higher than the melting point of the polyolefin material) without chemical reaction.
Further, the extractant includes, but is not limited to, methylene chloride, carbon tetrachloride, diethyl ether, and the like, solvents that are fully miscible with the diluent and are volatile.
Further, the T1 temperature is the melting point of the polyolefin material and is between +50 and 100 ℃.
Further, the T2 temperature is 5-30 ℃.
Further, the T3 temperature is the melting point of the polyolefin material of-10 to 40 ℃ (namely, 10 to 40 ℃ lower than the melting point of the polyolefin material).
Further, the temperature T4 is the melting point of the polyolefin material and is between-10 and 40 ℃.
Further, the second stretching magnification is 7-15 times.
Further, the temperature T5 is the boiling point of the extractant and is between-10 and 30 ℃.
Further, the temperature T6 is the boiling point of the extractant and is between 0 and 20 ℃.
Further, the T7 temperature is the melting point of the polyolefin material and is between-10 and 40 ℃.
Further, the third stretching magnification is 3-7 times.
Further, the temperature T8 is the melting point of the polyolefin material and is between 40 ℃ below zero and 60 ℃.
The invention also provides a battery separator, which comprises the polyolefin film.
The battery separator is a ceramic composite polyolefin separator or a polymer composite polyolefin separator coated on one or both sides of the polyolefin film.
Further, the ceramic includes, but is not limited to, one or more of alumina, boehmite, lithium aluminum titanium phosphate, and silica.
Further, the polymer includes, but is not limited to, one or more of polyvinylidene fluoride, polymethyl methacrylate, aramid, polyimide, polyamide, polyoxazole, polysulfone, polyethersulfone, polyetheretherketone, and polyvinyl chloride.
Further, the coating thickness is 0.2-1.5 um.
The invention also provides a battery, which comprises the battery separator as an element for separating the positive electrode from the negative electrode, and is a primary battery or a secondary battery.
Compared with the prior art, the invention has the beneficial effects that:
(1) The invention has the innovation that the polyolefin with high crystallinity is selected as the basic raw material, specifically, the polyolefin with the primary temperature rising crystallinity of more than or equal to 65 percent and the secondary temperature rising crystallinity of more than or equal to 45 percent is selected, compared with the polyolefin with low crystallinity, the invention can improve the crystallinity and orientation of the polyolefin substrate molecular chain segments of each process section, increase the stress consistency of a crystal area and an amorphous area of the polyolefin substrate in the stretching process, further improve the stretchability of the polyolefin substrate, reduce the film breaking risk of the stretching step and improve the production efficiency.
(2) The invention is also innovative in that a low-rate stretching strategy, specifically 1-2.5-rate stretching is adopted in the first stretching process, compared with high-rate stretching (more than 3 times), the crystallinity of the polyolefin base material can be more effectively improved, the orientation of polyolefin molecular chain forging is improved, the guarantee is provided for the second-step and third-step high-rate stretching, the film breaking risk in the second-step and third-step high-rate stretching process is reduced, and the production efficiency is improved.
(3) The invention is also innovative in that a three-step cooperative combination stretching strategy of high-crystallinity polyolefin raw materials and low multiplying power/high multiplying power is adopted, the high-crystallinity polyolefin raw materials are selected, the first low multiplying power stretching is adopted, an environment beneficial to improving crystallinity and orientation is maintained on the basis of the original high crystallinity, the crystallinity and orientation are further improved through the cooperative effect, the polyolefin film with the high crystallinity, the primary temperature rise crystallinity of which is more than or equal to 80 percent and the secondary temperature rise crystallinity of which is more than or equal to 60 percent is obtained, the thickness of the polyolefin film is controlled to be 1-5 mu m, the puncture strength of the polyolefin film is tested to be 300-1000 gf, the MD direction tensile strength of the polyolefin film is more than or equal to 300MPa, and the TD direction tensile strength of the polyolefin film is more than or equal to 300MPa, and the polyolefin film has the characteristics of ultra-thin, high puncture and high tensile strength.
(4) The prepared polyolefin film is made into a battery diaphragm and applied to primary or secondary batteries, so that the capacity of the battery active material can be effectively increased, the internal resistance of the battery can be reduced, the energy density and the cycle performance of the secondary battery can be improved, and the safety risk of the battery caused by mechanical failure can be effectively reduced.
Drawings
Fig. 1 is a surface scanning electron micrograph of example 1.
Fig. 2 is a surface scanning electron micrograph of comparative example 1.
Detailed Description
In order to make the technical features and advantages or technical effects of the technical scheme of the invention more obvious and understandable, the following detailed description is given with reference to the accompanying drawings.
Example 1
The physical properties of the film were as shown in Table 1, and the properties of the film were as shown in Table 3, which were obtained by adding polyethylene, calcium stearate and white oil to a screw extruder and using methylene chloride as an extractant under the process parameters shown in Table 2.
Example 2
The physical properties of the film were as shown in Table 1, and the properties of the film were as shown in Table 3, which were obtained by adding polyethylene, calcium stearate and white oil to a screw extruder and using methylene chloride as an extractant under the process parameters shown in Table 2.
Example 3
The physical properties of the film were as shown in Table 1, and the properties of the film were as shown in Table 3, which were obtained by adding polyethylene, calcium stearate and white oil to a screw extruder and using methylene chloride as an extractant under the process parameters shown in Table 2.
Example 4 as table 1 physical properties, polyethylene, calcium stearate, white oil were added to a screw extruder and methylene chloride was used as an extractant to obtain ultra-thin high strength polyolefin films with properties as shown in table 3 under the process parameters of table 2.
Example 5
According to the physical parameters shown in Table 1, polyethylene, calcium stearate and white oil were added into a screw extruder, and methylene dichloride was used as an extractant, and film samples were obtained under the process parameters shown in Table 2, and the properties thereof are shown in Table 3.
Comparative example 1
The physical properties of the film were as shown in Table 1, and the properties of the film were as shown in Table 3, which were obtained by adding polyethylene, calcium stearate and white oil to a screw extruder and using methylene chloride as an extractant under the process parameters shown in Table 2.
In the above embodiment, the polyethylene has a melting point of 137℃and the methylene chloride has a boiling point of 40 ℃.
In the above embodiments, the polyolefin materials and films were tested for crystallinity using a Differential Scanning Calorimeter (DSC). And (3) under the condition of 25 ℃, heating the sample to above 40 ℃ of the melting point of the material at the first time at 10 ℃/min to obtain the first heating crystallization degree, cooling to below 25 ℃ at the second time at 10 ℃/min, and heating to above 40 ℃ of the melting point of the material at the second time at the temperature of 10 ℃/min to obtain the second heating crystallization degree. Specifically, the primary temperature rise crystallinity of the polyolefin is obtained by dividing the measured enthalpy of fusion of the polyolefin during the primary temperature rise by the standard enthalpy of fusion of the polyolefin. Wherein the polyethylene has a standard melting enthalpy of 293J/g.
In the above embodiments, polyolefin film thickness, air permeability, porosity, puncture strength, tensile strength were tested according to GB/T36363-2018.
In the above embodiment, the specific discharge capacity and the capacity retention rate after 100 cycles of 1C at room temperature were tested by assembling the film sample into LFP// separator// Li battery and performing a charge-discharge test between 2.5 and 4.0V at 0.1C at room temperature. Lithium iron phosphate (LFP) positive electrode was composed of 80wt% LiFePO 4 10wt% PVDF and 10wt% carbon black.
Physical properties and proportions of raw materials in Table 1
Table 2 process parameters of the embodiment
Table 3 performance of the embodiment
As shown in FIG. 1, the ultrathin high-strength film (example 1) prepared by the invention adopts a three-step synergetic combination stretching strategy of high crystallinity polyolefin raw material and low multiplying power/high multiplying power, and the surface fiber diameter distribution is uniform and the pores are rich, which indicates that the polyethylene molecular chain is uniformly stressed in the stretching process, and the substrate is fully stretched in all directions. FIG. 2 is a surface SEM image of a polyethylene film (comparative example 1) prepared from a low crystallinity polyolefin feedstock and a two-step high magnification stretching strategy, with less surface porosity and non-uniform distribution, indicating that the polyolefin substrate is not sufficiently stretched under the strategy.
As shown in table 3, in example 1, polyethylene with a primary heating crystallinity of 68.7% and a secondary heating crystallinity of 57.1% is used as a raw material, and a film sample with a thickness of 5 micrometers, a puncture strength of 1000gf, an md tensile strength of 410MPa and a td tensile strength of 435MPa is obtained through a process of three-step small multiplying power and large multiplying power synergistic combination stretching of 1.5×1.5+10×10+5×5, and is applied to a secondary battery using LFP as an anode, wherein the specific discharge capacity is 152.1mAh/g, and the capacity retention rate after 100 cycles is 83.6%; in comparative example 1, polyethylene with a primary temperature rising crystallinity of 45.3% and a secondary temperature rising crystallinity of 27.1% is used as a raw material, and a film sample with a thickness of 5 micrometers, a puncture strength of 435gf, an MD tensile strength of 198MPa and a TD tensile strength of 215MPa is obtained through a 10 x 10+5 x 5 two-step high-magnification stretching process, and is applied to a secondary battery taking LFP as a positive electrode, wherein the specific discharge capacity is 151.2mAh/g, and the capacity retention rate after 100 circles is 80.1%. As can be seen from the above data of example 1 and comparative example 1, the use of the high crystallinity feedstock in combination with the three-step co-stretching process significantly improved the strength and battery performance of the polyethylene film.
As shown in table 3, in example 5, polyethylene with a primary temperature rising crystallinity of 65.2% and a secondary temperature rising crystallinity of 45.6% was used as a raw material, and a film sample with a thickness of 1 μm, a puncture strength of 310gf, a md tensile strength of 311mpa, and a td tensile strength of 315MPa was obtained by a process of combining three steps of small magnification and large magnification of 2×2+15×15+3×3, and was applied to a secondary battery using LFP as a positive electrode, the specific discharge capacity was 153.6mAh/g, the capacity retention after 100 cycles was 84.7%, and the capacity was increased by 2.4mAh/g and the circulating capacity retention was increased by 4.6% as compared with comparative example 1.
According to the embodiment of the invention, the polyolefin film with ultra-thin high strength characteristic is obtained by selecting the high crystallinity raw material and combining the synergistic combination stretching process, and the battery performance can be obviously improved when the polyolefin film is applied to the secondary battery.
Although the present invention has been described with reference to the above embodiments, it should be understood that the invention is not limited thereto, and that modifications and equivalents may be made thereto by those skilled in the art, which modifications and equivalents are intended to be included within the scope of the present invention as defined by the appended claims.
Claims (10)
1. A polyolefin film is characterized in that the thickness is 1-5 um, the puncture strength is 300-1000 gf, the MD tensile strength is more than or equal to 300MPa, the TD tensile strength is more than or equal to 300MPa, the porosity is 25-45%, and the air permeability is 20-80 s/100cc; the primary temperature rise crystallinity measured by a differential scanning calorimeter is more than or equal to 80 percent, and the secondary temperature rise crystallinity is more than or equal to 60 percent.
2. A process for the preparation of a polyolefin film comprising the steps of:
(1) Adding polyolefin, antioxidant and diluent into a screw extruder, melting into uniform liquid at the temperature of T1, extruding through a die head, and cooling at the temperature of T2 to form a cast sheet;
(2) Carrying out primary stretching on the cast sheet at the temperature of T3, and controlling the multiplying power of the primary stretching to be 1-2.5 times to obtain a primary stretched film; the first stretching refers to sequentially performing first longitudinal stretching and first transverse stretching or performing first bidirectional synchronous stretching;
(3) Carrying out secondary stretching on the primary stretched film at the temperature of T4 to obtain a secondary stretched film; the second stretching refers to sequentially performing the second longitudinal stretching and the second transverse stretching or performing the second bidirectional synchronous stretching;
(4) The secondarily stretched film enters an extraction tank with the temperature of T5 to completely elute the diluent, then enters a drying oven with the temperature of T6 to be dried, and then is subjected to tertiary stretching at the temperature of T7 to obtain a tertiary stretched film; the third stretching means that the third longitudinal stretching and the third transverse stretching are sequentially carried out, or the third bidirectional synchronous stretching is carried out;
(5) And (3) carrying out heat setting on the three-time stretched film at the temperature of T8 to obtain the polyolefin film.
3. The method according to claim 2, wherein the polyolefin is one or more of polyethylene, polypropylene, polybutene, polypentene, ethylene-propylene copolymer, ethylene-butene copolymer, ethylene-pentene copolymer synthesized from olefin raw material with 2-10 carbon number by homo-polymerization or copolymerization method, and the total solid content is 20-40% of the total mass of polyolefin and diluent; the viscosity average molecular weight of the polyolefin is 350-2000 kDa.
4. The method of claim 2, wherein the polyolefin has a primary temperature-rising crystallinity of greater than or equal to 65% and a secondary temperature-rising crystallinity of greater than or equal to 45% as measured by differential scanning calorimeter.
5. The method of claim 2, wherein,
the antioxidant is one or more of calcium stearate, magnesium stearate, zinc stearate, sodium stearate, 1, 3-tri (2-methyl-4-hydroxy-5-tertiary butyl phenyl) butane, 1,3, 5-trimethyl-2, 4, 6-tri (3, 5-di-tertiary butyl-4-hydroxybenzyl) benzene, tetra [ beta- (3, 5-di-tertiary butyl-4-hydroxyphenyl) propionic acid ] pentaerythritol ester and beta- (3, 5-di-tertiary butyl-4-hydroxyphenyl) propionic acid n-stearyl alcohol ester; the addition amount of the antioxidant is 0.01-1.0% of the mass of the polyolefin;
the diluent is one or more of white oil, paraffin oil, alkane compounds and ester compounds, and can be fully mixed with polyolefin to form uniform liquid at the temperature of +50-100 ℃ of the melting point of the polyolefin material without chemical reaction;
the extractant is one of dichloromethane, carbon tetrachloride and diethyl ether.
6. The method of claim 2, wherein,
the temperature T1 is the melting point of the polyolefin material and is +50-100 ℃;
the temperature of T2 is 5-30 ℃;
the temperature T3 is the melting point of the polyolefin material and is between 10 ℃ below zero and 40 ℃;
the temperature T4 is the melting point of the polyolefin material and is between 10 ℃ below zero and 40 ℃;
the temperature T5 is the boiling point of the extractant and is between 10 ℃ below zero and 30 ℃;
the temperature T6 is the boiling point of the extractant and is between +0 and 20 ℃;
t7 is the melting point of the polyolefin material at-10-40 ℃;
the temperature T8 is the melting point of the polyolefin material of-40 to 60 ℃.
7. The method of claim 2, wherein,
the second stretching multiplying power is 7-15 times;
the third stretching multiplying power is 3-7 times.
8. The method of claim 2, wherein the polyolefin film prepared has a thickness of 1 to 5um, a puncture strength of 300 to 1000gf, a tensile strength in md of 300MPa or more, a tensile strength in td of 300MPa or more, a porosity of 25 to 45%, and a permeability of 20 to 80s/100cc; the primary temperature rise crystallinity measured by a differential scanning calorimeter is more than or equal to 80 percent, and the secondary temperature rise crystallinity is more than or equal to 60 percent.
9. A battery separator characterized by being a composite polyolefin separator produced by coating a ceramic or polymer powder on one or both sides of the polyolefin film produced by the method of any one of claims 2 to 7;
the ceramic is one or more of alumina, boehmite, lithium aluminum titanium phosphate and silicon dioxide;
the polymer is one or more of polyvinylidene fluoride, polymethyl methacrylate, aramid fiber, polyimide, polyamide, polyoxazole, polysulfone, polyether sulfone, polyether ether ketone and polyvinyl chloride;
the coating thickness is 0.2-1.5 um.
10. A battery comprising the battery separator according to claim 9 as a separator for separating positive and negative electrodes, the battery being a primary battery or a secondary battery.
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