Wet-process polypropylene membrane, its preparation method, and secondary batteries made from it.

Abstract

This invention provides a wet-process polypropylene membrane, its preparation method, and a secondary battery made from it, belonging to the field of battery separator technology. The separator has a thickness of 3.5-30 μm, a tensile strength ≥150 MPa, a puncture strength ≥450 gf, and a rupture temperature ≥170℃. The preparation method involves: mixing and melting 20-60 parts by weight of polypropylene main material, 1-5 parts by weight of polymer copolymer elastomer, 30-80 parts by weight of solvent, 0.1-5 parts by weight of nucleating agent, and / or 0.1-1 parts by weight of antioxidant, followed by twin-screw extrusion and thermal phase separation to obtain a cast sheet; then, the cast sheet is stretched, extracted, and post-treated, or directly extracted and post-treated to obtain the final product, which can be applied to the preparation of secondary batteries. This invention effectively improves the puncture strength and rupture temperature of the separator, while also increasing the air permeability and ionic conductivity of the separator.

Description

Technical Field

[0001] This invention relates to the field of secondary battery separators, and more particularly to a wet-process polypropylene membrane;

[0002] The present invention also provides a method for preparing the wet-process polypropylene film;

[0003] The present invention also provides a secondary battery made from the aforementioned wet-process polypropylene film. Background Technology

[0004] In recent years, lithium-ion batteries have been widely used in mobile electronic devices, electric vehicles, and energy storage due to their advantages such as high energy density, long cycle life, low self-discharge rate, no memory effect, and environmental friendliness. Simultaneously, the rapid development of lithium-ion batteries has also driven the development of other rechargeable batteries. The separator is a crucial component of lithium-ion batteries; its function is to isolate electrons and provide a transport channel for lithium ions. Therefore, the performance of the separator directly affects the development of the battery industry.

[0005] Currently, lithium-ion battery separators are mainly made of PE (polyethylene) and PP (polypropylene). PP membranes have a higher melting point, which significantly improves battery safety. There are two existing methods for manufacturing lithium battery separators: dry and wet processes. Dry-process separators are thicker and have poorer pore size uniformity, while wet-process separators are thinner, have more uniform pore size, and higher strength. PE membranes are primarily wet-processed, which is not conducive to improving battery safety; PP membranes are primarily dry-processed, which is not conducive to improving battery energy density.

[0006] Existing wet-process technology uses polyethylene as raw material. The melting point of polyethylene (130℃~140℃) results in lower heat resistance than PP separators, reducing battery safety. Dry-process PP membranes, also using PP as raw material, offer advantages such as high temperature resistance, but suffer from poor pore size uniformity and a thicker membrane. This makes stable, high-speed, high-volume production impossible, and results in high energy consumption during production, which runs counter to the development of the new energy industry.

[0007] In view of the above problems, there is an urgent need to develop a high-temperature resistant wet-process PP membrane and its application in secondary batteries. Chinese patent CN114512767B discloses a lithium-ion battery separator with high-temperature resistance, the first to use a wet process to prepare a PP membrane. This separator possesses characteristics such as high temperature resistance, bidirectional high strength, uniform pore size, and high specific resistivity. Furthermore, thanks to the high-temperature resistance, high porosity, and easily adjustable pore size of this invention, batteries manufactured using this separator exhibit higher safety and better electrochemical performance. However, the puncture strength and rupture temperature of the separator prepared by this method still need improvement.

[0008] In view of the above, this application is hereby submitted. Summary of the Invention

[0009] The purpose of this invention is to provide a wet-process polypropylene film, its preparation method, and a secondary battery made therefrom, in order to solve the above-mentioned problems.

[0010] To achieve the above objectives, the present invention adopts the following technical solution:

[0011] A wet-process polypropylene membrane, wherein the membrane thickness is 3.5-30μm, tensile strength is ≥150MPa, puncture strength is ≥450gf, and membrane rupture temperature is ≥170℃.

[0012] The present invention also provides a method for preparing the wet-process polypropylene film: by weight, 20-60 parts of polypropylene main material, 1-5 parts of polymer copolymer elastomer, 30-80 parts of solvent, 0.1-5 parts of nucleating agent and / or 0.1-1 parts of antioxidant are mixed, melt-plasticized, extruded by twin screw extrusion, and thermally separated to obtain a cast sheet; then the cast sheet is stretched, extracted and post-treated, or directly extracted and post-treated to obtain the final product.

[0013] Furthermore, the polypropylene is a single component with a melt index of less than 20 g / 10 min, or it is a blend of multiple polypropylenes with different melt indices.

[0014] Preferably, the polypropylene is a single component with a melt index of less than 2 g / 10 min, or it is compounded with polypropylene with a melt index of less than 0.5 g / 10 min.

[0015] Optionally, the solvent includes one or more of alkanes, esters, ethers, and aromatic compounds, or a mixture thereof.

[0016] Preferably, the solvent includes one or more of the following: liquid paraffin, solid paraffin, paraffin oil, natural vegetable oil, dimethyl phthalate, diethyl phthalate, dibutyl phthalate, dioctyl phthalate, dioctyl sebacate, methyl salicylate, diphenyl ether, and diphenylmethane.

[0017] More preferably, the solvent includes white oil or dioctyl sebate. White oil is a type of paraffin oil.

[0018] Optionally, the polymer copolymer elastomer includes polyethylene octene copolymer elastomer and / or polyethylene propylene copolymer elastomer;

[0019] Preferably, the polymer copolymer elastomer is added by premixing it with the solvent beforehand or by adding it directly.

[0020] Furthermore, when the addition method is to add the polymer copolymer elastomer and solvent after premixing them, it also includes heating the mixture of the polymer copolymer elastomer and solvent to ≥150°C during or after addition, stirring it evenly, and then cooling it to ≤105°C.

[0021] Optionally, the nucleating agent includes one or more of adipic acid, calcium stearate, aluminum stearate, sorbitol benzylidene derivative, sodium benzoate, and bis(p-tert-butylbenzoic acid)carboxylated aluminum.

[0022] Optionally, the antioxidant adjuvant includes one of antioxidant 1076, antioxidant 1010, and antioxidant 168.

[0023] Preferably, after obtaining the casting, the casting is cooled and shaped, then heat-treated at 150-170°C for 10-20 seconds, cooled again, and then stretched.

[0024] Furthermore, the pre-extrusion feeding method adopts synchronous feeding and / or asynchronous feeding.

[0025] Preferably, during the twin-screw extrusion process, the screw temperature is 140℃-240℃, the melt pipe temperature is 190℃-230℃, and the die temperature is 180℃-220℃.

[0026] Preferably, the thickness of the extruded melt is 0.7 mm to 5 mm.

[0027] Preferably, during the preparation of the casting, the twin screws rotate at a speed of 60-100 rpm.

[0028] Preferably, the extrusion process further includes a cooling process, wherein the cooling method is: cooling of the quench roll at 10℃-80℃, cooling of the quench roll + cooling of the bottom roll at 10℃-80℃, cooling of the quench roll + water cooling at 5℃-80℃, water cooling + cooling of the bottom roll at 5℃-80℃, cooling of the quench roll + oil cooling at 5℃-80℃, and / or oil cooling + cooling of the bottom roll at 5℃-80℃.

[0029] Preferably, during the stretching process of the cast sheet, the air temperature is maintained at 130℃-165℃ and the film surface temperature is maintained at 124℃-140℃. The stretching is performed at a speed of 3m / min-40m / min, with longitudinal stretching of 1-30 times, transverse stretching of 1-30 times, and / or bidirectional synchronous stretching of (1-30)*(1-30) times. "Stretching 1 time" indicates that no stretching is performed in that direction. The stretching process can achieve the stretching of crystals within the cast sheet through different combinations of longitudinal stretching, transverse stretching, simultaneous longitudinal and transverse stretching, and multiple longitudinal and transverse stretching operations, along with different air temperatures, air speeds, and stretching speeds. This allows the lamellar crystals to unfold, dislocate, and slip, ultimately achieving a fibrous spatial network structure, thereby obtaining oil film precursors for high-temperature resistant polyolefin porous membranes with different porosities, pore sizes, and strengths.

[0030] Preferably, the stretched edge material is not removed prematurely during extraction. During extraction drying, the stretched edge material is not removed prematurely. The greater tension of the thicker edge material suppresses lateral shrinkage of the film during extraction, as extraction shrinkage leads to increased lateral variation and poor uniformity of the film, severely affecting the stability of the polypropylene film.

[0031] Preferably, the post-processing includes biaxial stretching of the dry film and shrinkage heat setting.

[0032] More preferably, the post-processing includes sequentially performing biaxial stretching of the dry film in both longitudinal and transverse directions, biaxial shrinkage treatment in both longitudinal and transverse directions, and heat setting. By stretching and shrinking the extracted dry film, secondary crystallization of microfibrillated polypropylene, refinement of microfibers, and elimination of internal stress can be achieved, thereby realizing the purpose of pore expansion and heat setting, improving membrane permeability and thermal stability. The biaxial shrinkage treatment in both longitudinal and transverse directions and heat setting, based on the traditional transverse uniaxial shrinkage treatment method, allows the membrane material to be shrunken in the longitudinal direction simultaneously, which can simultaneously improve biaxial thermal shrinkage stability. The specific methods are as follows: After extraction, the longitudinal and transverse stretching of the dry film needs to maintain an air temperature of 130℃-175℃ and a film surface temperature of 125℃-150℃. The film is stretched longitudinally (1-30) times, transversely (1-30) times, and / or bidirectionally simultaneously (1-30)*(1-30) times at a stretching speed of 3m / min-40m / min. "Stretching 1 time" means that no stretching is performed in this direction. The stretching process can achieve the required stretching multiple through different combinations of longitudinal stretching, transverse stretching, longitudinal and transversely simultaneously stretching, and multiple longitudinal and transverse stretching superimposed methods. The longitudinal and transverse bidirectional retraction needs to maintain an air temperature of 130℃-175℃ and a film surface temperature of 125℃-150℃. The film is retracted longitudinally 1-3 times, transversely 1-3 times, and / or bidirectionally simultaneously (1-3)*(1-3) times at a retraction speed of 3m / min-20m / min. "Retraction 1 time" means that no retraction is performed in this direction.

[0033] The present invention also provides a secondary battery, which is prepared using the aforementioned wet-process polypropylene membrane as a separator.

[0034] Optionally, the secondary battery includes at least a lithium-ion secondary battery, a lithium metal secondary battery, a sodium-ion secondary battery, a potassium-ion secondary battery, a zinc-ion secondary battery, a calcium-ion secondary battery, a lithium-sulfur secondary battery, or a sodium-sulfur secondary battery.

[0035] In the secondary battery, there are no particular restrictions on the types and contents of current collectors, conductive agents, binders, positive electrode active materials, etc. contained in the positive electrode, and they can be selected according to requirements.

[0036] The positive electrode active material may include at least one of the following materials: positive electrode active materials for lithium-ion secondary batteries, including lithium phosphates, lithium transition metal oxides and their respective modified compounds; positive electrode active materials for sodium-ion secondary batteries, including Prussian series (blue / white), polyanionic series, transition metal oxide series, organic series and their respective modified compounds. This application is not limited to these materials. Among them, lithium phosphates may include, but are not limited to, at least one of lithium iron phosphate (such as LiFePO4 (also abbreviated as LFP)), lithium iron phosphate and carbon composites, lithium manganese phosphate (such as LiMnPO4), lithium manganese phosphate and carbon composites, lithium iron manganese phosphate, and lithium iron manganese phosphate and carbon composites. Lithium transition metal oxides may include, but are not limited to, lithium cobalt oxides (such as LiCoO2), lithium nickel oxides (such as LiNiO2), lithium manganese oxides (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxides, lithium manganese cobalt oxides, lithium nickel manganese oxides, and lithium nickel cobalt manganese oxides (such as LiNi). 1 / 3 Co 1 / 3Mn 1 / 3 O2 (also known as NCM333), LiNi 0.5 Co 0.2 Mn 0.3 O2 (also known as NCM523), LiNi 0.5 Co 0.25 Mn 0.25 O2 (also known as NCM211), LiNi 0.6 Co 0.2 Mn 0.2 O2 (also known as NCM622), LiNi 0.8 Co 0.1 Mn 0.1 O2 (also known as NCM811), lithium nickel cobalt aluminum oxide (such as LiNi) 0.85 Co 0.15 A1 0.05At least one of O2 and its modified compounds. The Prussian series (blue / white) (AxM[M'(CN)6]·z H2O (A = alkali metal ions such as Na, K; M / M' = transition metal ions such as Fe, Co, Mn, Ni)) may include, but is not limited to, Na2Mn[Fe(CN)6xH2O], Na2Fe[Fe(CN)6], Na2Mn[Fe(CN)6], Na... 0.66 Ti[Fe(CN)6] 0.92 Na 0.84 Ni[Fe(CN)6] 0.71 Polyanionic series may include, but are not limited to, phosphates (such as Na3V2(PO4)), fluorophosphates (such as NaV2(PO4)2F3), pyrophosphates (such as Na2Fe(P2O7)), sulfates (such as Na2Fe2(SO4)3), etc., and transition metal oxide series (NaxMO2 (M=Co, Mn, Fe, Ni, etc.)) may include, but are not limited to, sodium copper iron manganate (Na 0.9 [Cu 0.22 Fe 0.3 Mn 0.45 O2), sodium nickel iron manganate (NaNi) 1 / 3 Fe 1 / 3 Mn 1 / 3 O2), sodium manganate (Na) 0.44 MnO2, Na 0.7 MnO2, etc.

[0037] Preferably, the above-mentioned positive electrode active material of the battery can be selected from at least one of lithium iron phosphate, lithium cobalt oxide, lithium nickel cobalt manganese oxide, sodium copper iron manganese oxide, sodium nickel iron manganese oxide, and sodium vanadium phosphate.

[0038] Positive electrode binders typically include fluorinated polyolefin binders. These fluorinated polyolefin binders may include, but are not limited to, polyvinylidene fluoride (PVDF), vinylidene fluoride copolymers, polyacrylic acid, or modified derivatives thereof.

[0039] The conductive agent for the positive electrode can be any conductive agent suitable for secondary batteries in this field. For example, the conductive agent can be one or more of the following, including but not limited to acetylene black, conductive carbon black, Ketjen black, conductive graphite, graphene, carbon nanotubes, and carbon fiber (VGCF).

[0040] In the secondary battery, there are no particular restrictions on the types and contents of current collectors, conductive agents, binders, and negative electrode active materials contained in the negative electrode, and they can be selected according to requirements.

[0041] There are no specific limitations on the types of current collectors, conductive agents, binders, and negative electrode active materials mentioned above. For example, negative electrode active materials may include natural graphite, artificial graphite, modified graphite, mesophase carbon microspheres, elemental silicon, silicon oxides, silicon-carbon composites, lithium metal sheets, lithium titanate, hard carbon, soft carbon, alloy materials, and organic materials, etc., and may be any one of them or a mixture of two or more in any proportion.

[0042] Preferably, the above-mentioned battery negative electrode active material can be selected from at least one of natural graphite, artificial graphite, hard carbon, soft carbon, and silicon-carbon composite materials.

[0043] In the aforementioned secondary battery, the electrolyte can be any electrolyte suitable for secondary batteries in the art. Taking lithium-ion and sodium-ion secondary batteries as examples, the electrolyte includes an electrolyte salt, a solvent, and additives. The electrolyte salt is not particularly limited and may include, but is not limited to, one or more of lithium hexafluorophosphate, lithium perchlorate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate; or, but is not limited to, one or more of sodium hexafluorophosphate, sodium perchlorate, sodium tetrafluoroborate, sodium hexafluoroarsenate, sodium bis(fluorosulfonyl)imide, sodium bis(trifluoromethanesulfonyl)imide, sodium trifluoromethanesulfonate, sodium difluorophosphate, sodium difluorooxalate borate, sodium dioxalate borate, sodium difluorodioxalate phosphate, and sodium tetrafluorooxalate phosphate. The solvent is not particularly limited, and may be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, 1,2-dimethoxypropane, dimethoxymethane, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, tetrahydrofuran, and 2-methyltetrahydrofuran. The additives are not particularly limited and may include, but are not limited to, one or more of ethylene carbonate, fluoroethylene carbonate, propane sulfonate lactone, ethylene sulfate, lithium difluorooxalate borate, lithium difluorophosphate, and lithium bis(trifluoromethanesulfonyl)imide.

[0044] Preferably, the electrolyte salt used in the above-mentioned battery electrolyte is selected from at least one of lithium hexafluorophosphate, lithium perchlorate, sodium hexafluorophosphate, and sodium perchlorate. Preferably, the solvent used in the above-mentioned battery electrolyte is at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, 1,3-dioxolane, ethylene glycol dimethyl ether, and diethylene glycol dimethyl ether. Preferably, the additive used in the above-mentioned battery electrolyte is at least one of vinylene carbonate, fluoroethylene carbonate, and propane sulfonate lactone.

[0045] The secondary battery includes, but is not limited to, any one of cylindrical, prismatic, and pouch cells. The bare cell forming method can be any one of laminated or pouch cells.

[0046] Compared with existing technologies, this invention, by combining the advantages of the good heat resistance of dry-process polypropylene separators and the high strength and uniform microstructure of wet-process biaxially oriented polyethylene separators, innovatively achieves the production of wet-process biaxially oriented polypropylene separators with high ratios. This achieves a balance between heat resistance, mechanical strength, and controllable microstructure uniformity, resulting in a novel lithium-ion battery separator with characteristics such as high temperature resistance, high biaxial strength, uniform pore size, and high resistivity. Therefore, it improves both the safety performance of the separator and the cycle performance of the battery. Furthermore, by incorporating the polymer copolymer elastomer and premixing it with a solvent and heating it, this invention effectively improves the puncture strength and rupture temperature of the separator; and by using a casting heat treatment method, it enhances the gas permeability and ionic conductivity of the separator. Attached Figure Description

[0047] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0048] Figure 1 This is a scanning electron microscope image of the wet-process polypropylene membrane of Example 2 of the present invention. Detailed Implementation

[0049] The embodiments of the present invention are described in detail below. All substances and raw materials in the embodiments are commercially available products. Unless otherwise specified, all ratios are by mass or arbitrary proportions.

[0050] As used in this article:

[0051] "Prepared from" is synonymous with "comprising". The terms "comprising", "including", "having", "containing", or any other variations thereof as used herein are intended to cover non-exclusive inclusion. For example, a composition, step, method, article, or apparatus that includes the listed elements is not necessarily limited to those elements, but may include other elements not expressly listed or elements inherent to such composition, step, method, article, or apparatus.

[0052] The conjunction "composed of..." excludes any unspecified elements, steps, or components. If used in a claim, this phrase makes the claim closed, excluding materials other than those described, except for associated conventional impurities. When the phrase "composed of..." appears in a clause of the body of a claim rather than immediately following it, it limits only the elements described in that clause; other elements are not excluded from the claim as a whole.

[0053] When a quantity, concentration, or other value or parameter is expressed as a range, a preferred range, or a range defined by a series of upper and lower preferred values, this should be understood as specifically disclosing all ranges formed by any pair of any upper or preferred value with any lower or preferred value, regardless of whether the range is disclosed individually. For example, when the range “1–5” is disclosed, the described range should be interpreted as including ranges “1–4”, “1–3”, “1–2”, “1–2 and 4–5”, “1–3 and 5”, etc. When numerical ranges are described herein, unless otherwise stated, the range is intended to include its endpoints and all integers and fractions within that range.

[0054] In these embodiments, unless otherwise specified, the portions and percentages are all by weight.

[0055] "And / or" is used to indicate that one or both of the described situations may occur, for example, A and / or B includes (A and B) and (A or B).

[0056] Example 1

[0057] One part by weight of polyethylene octene copolymer elastomer was added to a white oil kettle containing 65 parts by weight of 100# white oil. The mixture was stirred and heated to 150°C. After thorough stirring, the temperature was lowered to 100°C. 35 parts by weight of polypropylene with a melt index of 0.5 g / 10 min was added to the white oil kettle, along with one part by weight of 3988 nucleating agent (a sorbitol benzyl derivative) and 0.2 parts by weight of 1076 antioxidant. After high-speed stirring for 20 min, the mixture was melt-plasticized using a twin-screw extruder at 190°C. The melt was extruded from the die to form a sheet. The gel-like sheet was immediately passed through a pre-set gap between the shaping roller and the guide roller of a casting machine (the surface temperature of the shaping roller and the guide roller was set to 15°C). A cooling tank (cooling medium: water) was added to force-cool the reverse side of the melt at 20°C, forming a 1.5 mm thick cast sheet. The cast sheet was stretched 6 times along the machine direction (MD) at 130°C using a longitudinal stretching machine, and then stretched 6 times along the width direction (TD) at 140°C using a transverse stretching machine. The resulting oil film was then trimmed and slit into three 800mm wide sections. The stretched oil film was then passed through an extraction tank containing dichloromethane to extract the white oil, and the film was dried. The resulting dried microporous membrane was then fed into a transverse stretching machine and stretched 1.2 times along the TD direction at 135°C, and then retracted to 1.1 times along the TD direction, while simultaneously heat-setting at 135°C. Finally, it was wound up using a take-up roller to obtain a wet-process polypropylene film with a thickness of 14.1μm.

[0058] The thickness, tensile strength, puncture strength, porosity, air permeability, and heat shrinkage rate of the prepared wet-process polypropylene film were measured. The results of the measured properties of the wet-process polypropylene film are shown in Table 1.

[0059] Meanwhile, using this wet-process polypropylene membrane, corresponding secondary batteries can be produced, including but not limited to lithium-ion secondary batteries, lithium metal secondary batteries, sodium-ion secondary batteries, potassium-ion secondary batteries, zinc-ion secondary batteries, calcium-ion secondary batteries, lithium-sulfur secondary batteries, or sodium-sulfur secondary batteries.

[0060] Example 2

[0061] Three parts by weight of polyethylene octene copolymer elastomer were added to a white oil kettle containing 63 parts by weight of 100# white oil. The mixture was stirred and heated to 150°C. After stirring until homogeneous, the temperature was lowered to 100°C. 37 parts by weight of polypropylene (8:2 ratio) with melt index of 0.5 g / 10 min and 2 g / 10 min were added to the 100# white oil containing polyethylene octene copolymer elastomer via a powder scale and a plunger pump, respectively, and fed into a twin-screw extruder. 1.1 parts by weight of 3988 nucleating agent and 0.35 parts by weight of 1076 antioxidant were added, and the mixture was melt-plasticized at 185°C. The melt is extruded from the die to form a sheet. The gel-like sheet is immediately passed through a pre-set gap between the shaping roller and the guide roller of the casting machine (the surface temperature of the shaping roller and the guide roller is set to 10℃). A cooling tank (cooling medium is water) is added to force-cool the reverse side of the melt at 15℃, forming a 0.8mm thick casting sheet. This casting sheet is then treated in a 167℃ hot oil bath for 13 seconds, and then cooled again in a 15℃ cooling water bath. It is then stretched 9 times along the mechanical direction (MD) at 130℃ using a longitudinal stretching machine, and then stretched 7 times along the width direction (TD) at 140℃ using a transverse stretching machine. The stretched oil film is then trimmed and slit into three 800mm wide sections. The stretched oil film is then passed through an extraction tank containing dichloromethane to extract the white oil from the film, and the film is dried. The dried microporous membrane was fed into a transverse stretching machine and stretched 1.2 times along the TD direction at 135°C, then shrank back to 1.1 times along the TD direction while being heat-set at 135°C. Next, it was wound up using a take-up roller to obtain a wet-process polypropylene film with a thickness of 9.0 μm.

[0062] The thickness, tensile strength, puncture strength, porosity, air permeability, and heat shrinkage rate of the prepared wet-process polypropylene membrane were measured. The results of the measured properties of the obtained wet-process polypropylene membrane are shown in Table 1. Microscopic photographs of the obtained wet-process polypropylene membrane are shown below. Figure 1 As shown.

[0063] Meanwhile, using this wet-process polypropylene membrane, corresponding secondary batteries can be produced, including but not limited to lithium-ion secondary batteries, lithium metal secondary batteries, sodium-ion secondary batteries, potassium-ion secondary batteries, zinc-ion secondary batteries, calcium-ion secondary batteries, lithium-sulfur secondary batteries, or sodium-sulfur secondary batteries.

[0064] Example 3

[0065] The difference from Example 1 is that the polyethylene octene copolymer elastomer was added directly without premixing with white oil or undergoing a heating treatment.

[0066] The thickness, tensile strength, puncture strength, porosity, air permeability, and heat shrinkage rate of the prepared wet-process polypropylene film were measured. The results of the measured properties of the wet-process polypropylene film are shown in Table 1.

[0067] Meanwhile, using this wet-process polypropylene membrane, corresponding secondary batteries can be produced, including but not limited to lithium-ion secondary batteries, lithium metal secondary batteries, sodium-ion secondary batteries, potassium-ion secondary batteries, zinc-ion secondary batteries, calcium-ion secondary batteries, lithium-sulfur secondary batteries, or sodium-sulfur secondary batteries.

[0068] Example 4

[0069] The difference from Example 2 is that, after being formed into a casting, the casting was not treated in a 167°C hot oil bath and cooled again in a 15°C cooling water bath.

[0070] The thickness, tensile strength, puncture strength, porosity, air permeability, and heat shrinkage rate of the prepared wet-process polypropylene film were measured. The results of the measured properties of the wet-process polypropylene film are shown in Table 1.

[0071] Meanwhile, using this wet-process polypropylene membrane, corresponding secondary batteries can be produced, including but not limited to lithium-ion secondary batteries, lithium metal secondary batteries, sodium-ion secondary batteries, potassium-ion secondary batteries, zinc-ion secondary batteries, calcium-ion secondary batteries, lithium-sulfur secondary batteries, or sodium-sulfur secondary batteries.

[0072] Example 5

[0073] The difference from Example 1 is that the polyethylene octene copolymer elastomer is replaced with a polyethylene propylene copolymer elastomer.

[0074] The thickness, tensile strength, puncture strength, porosity, air permeability, and heat shrinkage rate of the prepared wet-process polypropylene film were measured. The results of the measured properties of the wet-process polypropylene film are shown in Table 1.

[0075] Meanwhile, using this wet-process polypropylene membrane, corresponding secondary batteries can be produced, including but not limited to lithium-ion secondary batteries, lithium metal secondary batteries, sodium-ion secondary batteries, potassium-ion secondary batteries, zinc-ion secondary batteries, calcium-ion secondary batteries, lithium-sulfur secondary batteries, or sodium-sulfur secondary batteries.

[0076] Comparative Example 1

[0077] Compared to Example 1, no polyethylene octene copolymer elastomer was added to the solvent white oil.

[0078] The thickness, tensile strength, puncture strength, porosity, air permeability, and heat shrinkage rate of the prepared wet-process polypropylene film were measured. The results of the measured properties of the wet-process polypropylene film are shown in Table 1.

[0079] Comparative Example 2

[0080] Compared to Example 2, no polyethylene octene copolymer elastomer was added to the solvent white oil. Furthermore, after molding into a cast sheet, the cast sheet was not subjected to a 167°C hot oil bath treatment or a second cooling in a 15°C cooling water bath.

[0081] The thickness, tensile strength, puncture strength, porosity, air permeability, and heat shrinkage rate of the prepared wet-process polypropylene film were measured. The results of the measured properties of the wet-process polypropylene film are shown in Table 1.

[0082] Table 1 Comparison of test performance of Examples 1-5 and Comparative Examples 1-2

[0083]

[0084]

[0085] As can be seen from the data comparison in Table 1, the wet-process polypropylene membrane of the present invention can effectively improve the mechanical properties of puncture and tensile strength by adding thermoplastic elastomer, and can reduce the air permeability value of the membrane and improve the air permeability of the membrane by using heat treatment process, and both can be used simultaneously.

[0086] Test Example 1

[0087] The wet-process polypropylene membranes obtained in Examples 1-5 and Comparative Examples 1-2 were applied to the following secondary battery preparations.

[0088] Preparation of the positive electrode: The positive electrode active material LiNi... 0.5 Co 0.2 Mn 0.3 O2, conductive agent Super P, and binder polyvinylidene fluoride (PVDF) are dissolved in N-methylpyrrolidone (NMP) to prepare the positive electrode slurry. The solid content of the positive electrode slurry is 63 wt%, and the solid component is LiNi. 0.5 Co 0.2 Mn 0.3 The mass ratio of O2, Super P, and PVDF is 92:5:3. The positive electrode slurry is coated onto the current collector aluminum foil, dried at 120°C, rolled, cut into sheets, and dried under vacuum at 120°C for 12 hours to produce the positive electrode sheet.

[0089] Preparation of the negative electrode sheet: Graphite (negative electrode active material), Super P (conductive agent), styrene-butadiene rubber (SBR) (binder), and carboxymethyl cellulose (CMC) (thickener) are added to deionized water and mixed evenly to form a negative electrode slurry. The negative electrode slurry is coated onto a copper foil current collector and dried at 85°C. After rolling and cutting, it is dried under vacuum at 80°C for 12 hours to form the negative electrode sheet.

[0090] Preparation of the secondary battery: The wet-process PP film of this application is used as the separator. The above-mentioned positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes, and wound to obtain a bare cell. The bare cell is then placed in the battery outer packaging. The electrolyte is a 1 mol / L LiPF6 solution dissolved in a mixed solution of ethylene carbonate, dimethyl carbonate, and methyl ethyl carbonate (volume ratio 1:1:1). The electrolyte is injected into the dry cell, sealed, and the electrode sheets are completely wetted. Formation and capacity testing are then performed. The performance of the resulting battery is shown in Table 2.

[0091] Table 2 Comparison of the test performance of secondary batteries prepared by wet-process polypropylene membranes obtained in Examples 1-5 and Comparative Examples 1-2

[0092]

[0093] As shown in Table 2, the secondary battery using the high-temperature resistant wet-process PP membrane of this application as the separator exhibits better cycle performance and superior safety performance compared to the batteries in the comparative examples. In particular, the battery performance of Example 5 is better than that of Examples 1-4, which is due to the better mechanical strength and air permeability of its separator.

[0094] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0095] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A process for the production of a wet polypropylene film, characterized in that, The polypropylene main material 20-60 parts, the polymer copolymer elastomer 1-5 parts, the solvent 30-80 parts, the nucleating aid 0.1-5 parts and the antioxidant 0.1-1 part are mixed and melt plasticized, and the cast sheet is obtained by double screw extrusion and thermal phase separation; Then the cast sheet is stretched, extracted and post-processed to obtain the wet polypropylene film. The polymer copolymer elastomer includes polyethylene octene copolymer elastomer and / or polyethylene propylene copolymer elastomer. The polymer copolymer elastomer is added in advance after being premixed with the solvent, and the mixture of the polymer copolymer elastomer and the solvent is heated to ≥ 150℃ after being added, stirred uniformly, and then cooled to ≤ 105℃, and then mixed with other components; The film thickness is 3.5-30 µm, the tensile strength is ≥ 150 MPa, the puncture strength is ≥ 450 gf, and the film breaking temperature is ≥ 170℃. The solvent is white oil. The stretching process of the cast sheet is performed at a wind temperature of 130°C to 165°C and a film surface temperature of 124°C to 140°C, at a stretching speed of 3 m / min to 40 m / min, with a longitudinal stretching of 1 to 30 times, a transverse stretching of 1 to 30 times, and / or a simultaneous biaxial stretching of 1 to 30 times (1-30) times. The post-processing includes longitudinal and transverse double stretching of dry film, longitudinal and transverse double shrinkage treatment and heat setting. The stretching of the extracted dry film in both longitudinal and transverse directions requires air temperature of 130-175°C and film surface temperature of 125-150°C, and the film is stretched longitudinally by (1-30) times, transversely by (1-30) times and / or synchronously in both directions by (1-30) times at a stretching speed of 3-40 m / min (1-30) times. The longitudinal and transverse two-way retraction requires air temperature of 130°C-175°C and film surface temperature of 125°C-150°C, with retraction speed of 3 m / min-20 m / min, longitudinal retraction of 1-3 times, transverse retraction of 1-3 times and / or two-way synchronous retraction (1-3) times.

2. The production method according to claim 1, characterized by, The polypropylene is a single composition with a melt index of ≤ 20 g / 10 min, or a plurality of polypropylenes with different melt indexes.

3. The preparation method according to claim 2, characterized in that, The polypropylene is a single composition with a melt index of ≤ 2 g / 10 min, or a plurality of polypropylenes with different melt indexes.

4. The method of claim 1, wherein, The nucleating aid includes a mixture of one or more of adipic acid, calcium stearate, aluminum stearate, sodium benzoate and bis(p-tert-butylbenzoic acid) aluminum carboxylate; and the antioxidant includes one of antioxidant 1076, antioxidant 1010 and antioxidant 168.

5. The preparation method according to claim 1, characterized in that, After the cast sheet is obtained, the cast sheet is cooled and shaped, then heat treated at 150-170℃ for 10-20s, and then stretched again after being cooled.

6. A secondary battery characterized by comprising: The wet polypropylene film prepared by the preparation method of any one of claims 1-5 is used as a separator.

7. The secondary battery according to claim 6, characterized by The secondary battery includes at least lithium ion secondary battery, lithium metal secondary battery, sodium ion secondary battery, potassium ion secondary battery, zinc ion secondary battery, calcium ion secondary battery, lithium-sulfur secondary battery or sodium-sulfur secondary battery.

Images