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WO2024077927A1 - 一种聚烯烃多孔膜及其制备方法、电池隔膜、电化学装置 - Google Patents

一种聚烯烃多孔膜及其制备方法、电池隔膜、电化学装置 Download PDF

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Publication number
WO2024077927A1
WO2024077927A1 PCT/CN2023/090556 CN2023090556W WO2024077927A1 WO 2024077927 A1 WO2024077927 A1 WO 2024077927A1 CN 2023090556 W CN2023090556 W CN 2023090556W WO 2024077927 A1 WO2024077927 A1 WO 2024077927A1
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WIPO (PCT)
Prior art keywords
fiber
porous membrane
ratio
polyolefin porous
stretching
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PCT/CN2023/090556
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English (en)
French (fr)
Inventor
薛山
白耀宗
贾国重
刘志刚
郑蕾
董秋春
薛云卿
马源
吴奇阳
孟祥淦
张绪杰
刘杲珺
Original Assignee
中材锂膜(南京)有限公司
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Priority claimed from CN202211244275.0A external-priority patent/CN115312973B/zh
Application filed by 中材锂膜(南京)有限公司 filed Critical 中材锂膜(南京)有限公司
Priority to EP23744028.4A priority Critical patent/EP4376202A1/en
Priority to KR1020237024918A priority patent/KR20240060514A/ko
Publication of WO2024077927A1 publication Critical patent/WO2024077927A1/zh

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
    • H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
    • H01M50/403—Manufacturing processes of separators, membranes or diaphragms
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
    • H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
    • H01M50/409—Separators, membranes or diaphragms characterised by the material
    • H01M50/411—Organic material
    • H01M50/414—Synthetic resins, e.g. thermoplastics or thermosetting resins
    • H01M50/417—Polyolefins

Definitions

  • the present disclosure relates to the technical field of diaphragms for electrochemical devices, and in particular to a polyolefin porous membrane and a preparation method thereof, a battery diaphragm, and an electrochemical device.
  • the main function of the separator is to isolate the positive and negative electrodes to prevent short circuits, while ensuring a certain degree of electrolyte wettability and liquid retention, providing a channel for lithium ion transmission.
  • the performance of the separator directly determines the interface performance and internal resistance of the battery, which in turn affects the battery's charge and discharge performance and cycle performance.
  • the lithium-ion battery separators in the prior art are mainly based on polyolefin porous separators, which are three-dimensional structures containing a large number of tiny through pores inside and supported by polyolefin as a skeleton.
  • the preparation methods are mainly divided into dry method and wet stretching.
  • the dry method is a method of forming micropores by forming a polyolefin film and then stretching the film at low temperature, and the stretching causes microcracks between the thin sheets that are the crystalline part of the polyolefin.
  • the wet method is a method in which a polyolefin-based resin and a diluent are mixed at a high temperature at which the polyolefin-based resin is melted to form a single phase, the polyolefin and the diluent are phase separated during the cooling process, and then the diluent is extracted to form pores therein.
  • the wet method imparts mechanical strength and transparency through a stretching/extraction process after phase separation.
  • the amorphous part in the crystal structure is elongated, and micro-pores are formed between the sheet layers while forming microfibers.
  • the wet method has a thin membrane thickness, uniform pore size, and excellent physical properties.
  • the document with publication number JP2005056851A proposes a microporous membrane of a laminated film.
  • the preparation method does not perform stretching in the TD direction, so it can be said that there is no shrinkage in the TD direction at all; however, only a single axis in the MD direction (mechanical direction, length direction) is used for opening holes, thereby forming a microporous membrane with insufficient strength in the TD direction and extreme anisotropy; therefore, in, for example, battery crush tests, collision tests, etc., it may be easy to crack in one direction.
  • JP2014141644A describes a biaxially oriented porous polypropylene film having a tensile strength ratio between the MD direction and the TD direction of 0.9 or more and less than 1.5.
  • the air barrier is low, and further improvements are needed in terms of elongation at break and toughness.
  • a Chinese patent with publication number CN107250234B discloses a polyolefin microporous membrane with polypropylene as the main component, the ratio of tensile strength in the MD direction to that in the TD direction is greater than 0.4 and less than 2.0, and the ratio of elongation at break in the MD direction to that in the TD direction is greater than 0.6 and less than 1.7.
  • the existing polyolefin porous diaphragms can be maturely applied in lithium-ion batteries, the current technology mainly focuses on the basic physical properties and physicochemical properties of the diaphragm, and pays less attention to the consistency of the microstructure.
  • the object of the present disclosure is to provide a polyolefin porous membrane, which, when used as a battery separator, can provide a battery with excellent battery safety and improve the stability of battery production.
  • the purpose of the present disclosure is also to provide a method for preparing the above-mentioned polyolefin porous membrane.
  • a polyolefin porous membrane with uniform microscopic fiber structure orientation and small difference in macroscopic two-way mechanical properties can be obtained.
  • the polyolefin porous membrane provided by the present disclosure is prepared by wet stretching and has a network fiber structure
  • the polyolefin porous membrane has the following characteristics:
  • R_MD in the MD direction of fibers with a fiber diameter greater than 15 nm satisfies: (R_MD) 2 ⁇ 0.8.
  • R_MD is obtained by the following formula 1:
  • N is the number of fibers on the outermost surface of the observation area, and each fiber is labeled with n; ⁇ n is the angle between fiber n and the MD direction, and ⁇ n is obtained from the left of the MD direction and in the counterclockwise direction.
  • R_TD In a circle with a radius of 1000 nm, the fiber orientation R_TD in the TD direction of fibers with a fiber diameter of more than 15 nm satisfies: (R_TD) 2 ⁇ 0.8.
  • R_TD is obtained by the following formula 2:
  • N is the number of fibers on the outermost surface of the observation area, and each fiber is marked with n; ⁇ n is the angle between fiber n and the TD direction, and ⁇ n starts from the TD direction upward and is obtained in a counterclockwise direction.
  • the product of the length value Ln of fiber n in ⁇ m and cos ⁇ n is Ln cos ⁇ n , and ⁇ n is the diameter value of fiber n in nm.
  • the provided polyolefin porous membrane is prepared by wet stretching and has a network fiber structure
  • the polyolefin porous membrane has the following characteristics:
  • R_MD in the MD direction of fibers with a fiber diameter greater than 15 nm satisfies: (R_MD) 2 ⁇ 0.5.
  • R_MD is obtained by the following formula 1:
  • N is the number of fibers on the outermost surface of the observation area, and each fiber is labeled with n;
  • ⁇ n is the relative position of fiber n to the MD direction.
  • the angle, ⁇ n starts from the left of the MD direction and is obtained in the counterclockwise direction.
  • the product of the length value L n of fiber n in ⁇ m and cos ⁇ n is L n cos ⁇ n , and ⁇ n is the diameter value of fiber n in nm;
  • R_TD In a circle with a radius of 1000 nm, the fiber orientation R_TD in the TD direction of fibers with a fiber diameter of more than 15 nm satisfies: (R_TD) 2 ⁇ 0.5.
  • R_TD is obtained by the following formula 2:
  • N is the number of fibers on the outermost surface of the observation area, and each fiber is marked with n; ⁇ n is the angle between fiber n and the TD direction, and ⁇ n starts from the TD direction upward and is obtained in a counterclockwise direction.
  • the product of the length value Ln of fiber n in ⁇ m and cos ⁇ n is Ln cos ⁇ n , and ⁇ n is the diameter value of fiber n in nm.
  • the ratio of the tensile strength in the MD direction to the tensile strength in the TD direction is the MD/TD tensile strength ratio, and then 0.8 ⁇ MD/TD tensile strength ratio ⁇ 1.2.
  • the ratio of the elongation at break in the MD direction to the elongation at break in the TD direction is the MD/TD elongation at break ratio, and then 0.4 ⁇ MD/TD elongation at break ratio ⁇ 1.2.
  • the polyolefin porous membrane has a thickness of 1 to 30 ⁇ m.
  • the present disclosure also provides a method for preparing the polyolefin porous membrane as described above, comprising the following steps:
  • Step 1 Mixing polyolefin with porogen and additives and extruding the mixture, wherein the mass proportion of polyolefin in the mixture is ⁇ 15%, and the mass proportion of antioxidant in the mixture is 0-0.5%; after high-temperature dispersion mixing and plasticization by a twin-screw extruder, extruding through a die to prepare a cast sheet;
  • Step 2 stretching the cast sheet, during which the temperature of the cast sheet itself does not exceed 140° C., and the temperature difference between different points of the temperature field in the stretching area is less than 0.5° C., and during the stretching process, the ratio of the stretching ratio in the MD direction to the stretching ratio in the TD direction, i.e., the MD/TD stretching ratio ratio, is: 0.9 ⁇ MD/TD stretching ratio ratio ⁇ 1.1, to obtain an oil film;
  • Step three extract the oil film, and then perform heat setting treatment.
  • the original MD and TD stretching ratio relationship needs to be maintained.
  • the ratio of the stretching ratio in the MD direction to the stretching ratio in the TD direction in step two is maintained by expansion stretching treatment.
  • the stretching ratio in the MD direction and the stretching ratio in the TD direction are both greater than 5 times.
  • the viscosity average molecular weight of the polyolefin is between 200,000 and 5,000,000.
  • the porogen is one or more of white oil, paraffin oil, and polyethylene glycol.
  • the additive in step one includes an antioxidant, and the antioxidant is one or more of amines, sulfur-containing compounds, nitrogen-containing compounds, phosphorus-containing compounds, and organic metal salts.
  • the present disclosure also provides a battery separator, comprising the polyolefin porous film as described above.
  • the present disclosure also provides an electrochemical device, comprising the polyolefin porous membrane as described above or the battery separator as described above as a component for separating positive and negative electrodes.
  • the polyolefin porous membrane disclosed in the present invention can achieve consistency in mechanical properties as a separator in a battery, thereby improving the stability of the separator in battery production and the safety of battery application.
  • FIG. 1 is a scanning electron microscope photograph showing the polyolefin porous membrane of Example 1;
  • FIG. 2 is a scanning electron microscope photograph showing the polyolefin porous membrane of Example 2;
  • FIG3 is a scanning electron microscope photograph showing the polyolefin porous membrane of Example 3.
  • FIG4 is a scanning electron microscope photograph showing the polyolefin porous membrane of Example 4.
  • FIG5 is a scanning electron microscope photograph showing the polyolefin porous membrane of Example 5.
  • FIG6 is a scanning electron microscope photograph showing the polyolefin porous membrane of Example 6;
  • FIG. 7 is a scanning electron microscope photograph showing the polyolefin porous membrane of Example 9.
  • the inventors have discovered that the uniformity of the fiber structure in the polyolefin porous membrane is an important guarantee for the performance of the diaphragm.
  • the microscopic fiber structure characteristics determine the macroscopic properties of the polyolefin porous membrane as a battery diaphragm, among which the distribution and orientation uniformity of the fibers determine the uniformity of the mechanical properties of the diaphragm.
  • the polyolefin porous membrane has the following characteristics, it can obtain excellent isotropic uniformity of mechanical properties, thereby having higher safety in battery applications.
  • the polyolefin porous membrane has the following characteristics:
  • R_MD in the MD direction of fibers with a fiber diameter greater than 15 nm satisfies: (R_MD) 2 ⁇ 0.8.
  • R_MD is obtained by the following formula 1:
  • N is the number of fibers on the outermost surface of the observation area, and each fiber is labeled with n; ⁇ n is the angle between fiber n and the MD direction, and ⁇ n is obtained from the left of the MD direction and in the counterclockwise direction.
  • R_TD In a circle with a radius of 1000 nm, the fiber orientation R_TD in the TD direction of fibers with a fiber diameter of more than 15 nm satisfies: (R_TD) 2 ⁇ 0.8.
  • R_TD is obtained by the following formula 2:
  • N is the number of fibers on the outermost surface of the observation area, and each fiber is labeled with n; ⁇ n is the angle between fiber n and the TD direction, and ⁇ n is obtained from the TD direction upward and counterclockwise.
  • the length value Ln of fiber n in ⁇ m is related to cos ⁇ n
  • the product is L n cos ⁇ n , where ⁇ n is the diameter of fiber n in nm.
  • a polyolefin porous membrane which is prepared by wet stretching and has a network fiber structure
  • the polyolefin porous membrane has the following characteristics:
  • R_MD in the MD direction of fibers with a fiber diameter greater than 15 nm satisfies: (R_MD) 2 ⁇ 0.5.
  • R_MD is obtained by the following formula 1:
  • N is the number of fibers on the outermost surface of the observation area, and each fiber is labeled with n; ⁇ n is the angle between fiber n and the MD direction, and ⁇ n is obtained from the left of the MD direction and in the counterclockwise direction.
  • R_TD In a circle with a radius of 1000 nm, the fiber orientation R_TD in the TD direction of fibers with a fiber diameter of more than 15 nm satisfies: (R_TD) 2 ⁇ 0.5.
  • R_TD is obtained by the following formula 2:
  • N is the number of fibers on the outermost surface of the observation area, and each fiber is marked with n; ⁇ n is the angle between fiber n and the TD direction, and ⁇ n starts from the TD direction upward and is obtained in a counterclockwise direction.
  • the product of the length value Ln of fiber n in ⁇ m and cos ⁇ n is Ln cos ⁇ n , and ⁇ n is the diameter value of fiber n in nm.
  • Polyolefin resins used as raw materials include, for example, polyethylene, polypropylene, etc.
  • the polyethylene is not particularly limited, and various polyethylenes can be used, for example, ultra-high molecular weight polyethylene (UHMwPE), high-density polyethylene (HDPE), medium-density polyethylene, branched low-density polyethylene, linear low-density polyethylene, etc.
  • UHMwPE ultra-high molecular weight polyethylene
  • HDPE high-density polyethylene
  • medium-density polyethylene medium-density polyethylene
  • branched low-density polyethylene branched low-density polyethylene
  • linear low-density polyethylene linear low-density polyethylene
  • polyethylene may be a homopolymer of ethylene or a copolymer of ethylene and other ⁇ -olefins.
  • ⁇ -olefins include propylene, 1-butene, 1-hexene, 1-pentene, 4-methyl-1-pentene, octene, vinyl acetate, methyl methacrylate, styrene, and the like.
  • the polyolefin porous membrane refers to a microporous membrane having a polyolefin resin as a main component, for example, a microporous membrane in which the polyolefin resin accounts for 90% by mass or more of the total amount of the microporous membrane.
  • the physical properties of the polyolefin porous membrane of this embodiment are described below.
  • the ratio of the tensile strength in the MD direction and the TD direction of the polyolefin porous membrane disclosed herein i.e., the MD/TD tensile strength ratio
  • the MD/TD tensile strength ratio can be selected to be greater than 0.8 and less than 1.2.
  • the membrane can be subjected to impact in all directions more evenly, so that the membrane can be used as a separator in a battery with improved impact resistance, more stable, and can suppress membrane breakage and short circuit.
  • the MD/TD tensile strength ratio is preferably close to 1.
  • the strength of the polyolefin porous membrane does not have direction dependence. Therefore, when stress is applied to the membrane, it has excellent toughness and will not crack in a specific direction. Even if foreign matter penetrates the microporous membrane, the expansion of the through-holes can be avoided. Otherwise, when the tensile strength in the MD direction is too large, sometimes When the tensile strength in the TD direction is too high, tearing in the TD direction or the bonding of the electrode tabs may be detached, which may easily cause a short circuit.
  • the ratio of the elongation at break in the MD direction and the TD direction of the polyolefin porous membrane disclosed herein i.e., the MD/TD elongation at break ratio
  • the MD/TD elongation at break ratio can be selected to be greater than 0.8 and less than 1.2.
  • the membrane can be subjected to impact in all directions more evenly, so that the membrane can be used as a separator in a battery with improved impact resistance, more stable, and can suppress membrane breakage and short circuit.
  • the MD/TD elongation at break ratio is optionally close to 1.
  • the strength of the polyolefin microporous membrane has no direction dependence. Therefore, when stress is applied to the membrane, it has excellent toughness and will not crack in a specific direction. Even if foreign matter penetrates the microporous membrane, the expansion of the through holes can be avoided.
  • the thickness of the polyolefin porous membrane there is no particular limitation on the thickness of the polyolefin porous membrane. From the perspective of membrane strength, the thickness of the polyolefin porous membrane disclosed herein is 2 ⁇ m or more, and preferably 5 ⁇ m or more. In addition, from the perspective of permeability, the thickness of the microporous membrane is 50 ⁇ m or less, and preferably 30 ⁇ m or less.
  • the tensile strength of the polyolefin porous membrane disclosed herein is 150 MPa or more in both MD and TD directions, and optionally 200 MPa or more. If the tensile strength is weak, the winding property of the battery will be deteriorated or a short circuit may easily occur due to a battery collision test from the outside, foreign matter in the battery, etc.
  • the upper limit of the tensile strength in the MD direction and the TD direction there is no particular limit on the upper limit of the tensile strength in the MD direction and the TD direction. From actual production experience, the upper limit of the tensile strength in the MD direction and the TD direction may be 700 MPa or less, 600 MPa or less, or 550 MPa or less.
  • the elongation at break of the polyolefin porous membrane in the MD direction and the TD direction of the present invention can be 90% or more, respectively.
  • the elongation at break is within the above range, when the battery is impacted, its flexibility is used to suppress the rupture of the diaphragm and the occurrence of short circuit.
  • the upper limit of the elongation at break in the MD direction and the TD direction for example, it is 400% or less, optionally 300% or less, and optionally 200% or less.
  • the elongation at break is within the above range, when the electrode is wound, the diaphragm will not stretch and deform, and the winding property is good.
  • the MD and TD elongations at break are 10 to 200%, 10 to 150%, or 10 to 120%, respectively.
  • the microporous membrane having MD and TD elongations at break within the above ranges not only has good battery winding properties, but is also less likely to deform in battery collision tests.
  • the polyolefin porous film as a separator is also required to have excellent heat shrinkage characteristics at high temperatures, and, for example, to show excellent results in high-temperature storage tests, high-temperature cycle tests, baking tests, etc. when manufactured into a battery.
  • the heat shrinkage rate in the MD direction is less than 5% and the heat shrinkage rate in the TD direction is less than 6.5%.
  • the MD heat shrinkage rate is 4% or less and the TD heat shrinkage rate is 4% or less.
  • the MD heat shrinkage rate is 3% or less and the TD heat shrinkage rate is 2.5% or less.
  • the MD heat shrinkage rate is 1% or less and the TD heat shrinkage rate is 1% or less.
  • the polyolefin porous membrane disclosed in the present invention is prepared by wet stretching, and its preparation method includes: (1) using one or more polyolefin resin materials as the main raw materials, mixing the above raw materials with a porogen and an antioxidant, and extruding the mixture through a die to prepare a cast sheet; (2) biaxially stretching the cast sheet to obtain an oil film; (3) extracting the oil film and then performing a heat setting treatment.
  • the control of the temperature and stretching ratio ratio of the cast sheet during the stretching process is one of the key factors for obtaining a polyolefin porous membrane with the above-mentioned characteristics; the temperature field and the stretching field need to be controlled during the biaxial stretching process, the temperature of the cast sheet itself shall not exceed 140°C and the temperature difference between different points in the temperature field in the stretching area is less than 0.5°C, and it is necessary that the ratio of the stretching ratio in the MD direction to the stretching ratio in the TD direction is greater than 0.9 and less than 1.1.
  • the ratio of the stretching ratio in the MD direction to the stretching ratio in the TD direction is maintained to be the ratio of the stretching ratio in the biaxial stretching in step (2), that is, greater than 0.9 and less than 1.1, which is the second key factor.
  • the stretching ratio in the MD direction and the stretching ratio in the TD direction are actually the stretching ratio in the MD direction and the actual stretching ratio in the TD direction of the separator.
  • the stretching ratio in the MD direction and the stretching ratio in the TD direction can be directly obtained from the actual settings of the equipment, and can also be obtained in the following way: the stretching ratio in the synchronous MD direction is equal to the ratio of the speed of the oil film in the MD direction obtained after synchronous stretching to the running speed of the sheet before stretching; the stretching ratio in the synchronous TD direction is equal to the ratio of the width of the oil film in the TD direction obtained after synchronous stretching to the width of the TD stretched area of the sheet before stretching. Note that the width length of the clamping part of the clamp should be deducted.
  • the stretching ratio in the asynchronous MD direction can be directly obtained from the actual setting of the equipment, or it can be obtained by the ratio of the sheet rate before TD stretching after MD stretching to the rate before stretching; the stretching ratio in the asynchronous TD direction is the ratio of the actual width of the oil film after TD stretching is completed to the actual width of the gel sheet before MD stretching.
  • polyolefin porous membrane As long as a polyolefin porous membrane with the above-mentioned properties can be obtained, there are no special limitations on the types and ratios of polyolefin, porogen and antioxidant, the mixing and extrusion methods, the type of extractant and whether to use it, the process of moderate heat setting, etc.
  • Polyolefin resins used as raw materials include, for example, polyethylene and polypropylene.
  • Polyethylene is not particularly limited, and various polyethylenes can be used, for example, ultra-high molecular weight polyethylene (UHMWPE), high density polyethylene (HDPE), medium density polyethylene, branched low density polyethylene, linear low density polyethylene, etc.
  • UHMWPE ultra-high molecular weight polyethylene
  • HDPE high density polyethylene
  • polyethylene can be a homopolymer of ethylene or a copolymer of ethylene and other ⁇ -olefins.
  • ⁇ -olefins include propylene, 1-butene, 1-hexene, 1-pentene, 4-methyl-1-pentene, octene, vinyl acetate, methyl methacrylate, styrene, etc.
  • the viscosity average molecular weight of the polyolefin resin is 200,000 to 5,000,000 from the viewpoint of easy processing of the polymer melt and preventing film breakage. When the viscosity average molecular weight of the polyolefin resin is within the above range, film forming properties become good.
  • the film-forming material may contain polymers of other resin components other than polyolefin materials as required, for example, high temperature resistant resins such as polypropylene, polyimide, polyamide, fluororesin, etc.
  • high temperature resistant resins such as polypropylene, polyimide, polyamide, fluororesin, etc.
  • the mass proportion of polyolefin in the mixed material is ⁇ 15%, and the mass proportion of high temperature resistant resin in the mixed material is ⁇ 10%.
  • the porogen can be: a liquid hydrocarbon mixture such as white oil, an organic acid ester such as dioctyl phthalate (DOP), an inorganic acid ester such as trioctyl phosphate (TOP), an organic substance that can form a uniform solution with the polyolefin resin, or a combination of the above porogens. Mixtures, etc.
  • DOP dioctyl phthalate
  • TOP trioctyl phosphate
  • the antioxidant may be a single main antioxidant or a combination of multiple antioxidants. Adding antioxidants can slow down the aging and degradation of polymers and porogens during shearing, and effectively improve the color, smoothness and strength of the membrane surface.
  • the stretching temperature can be optionally within the range of above the softening point (Tcd) of the fully molten mixture of the polyolefin resin and the pore-forming agent and below the melting point of the polyolefin resin.
  • Tcd softening point
  • the melting point of the polyolefin resin refers to the melting point of the polyolefin resin in the gel-like sheet.
  • the stretching temperature when the stretching temperature is above the softening point (Tcd), the polyolefin resin in the gel-like sheet can be fully softened, and the stretching tension can be reduced. Therefore, the film-forming property becomes good, the film breakage during stretching can be suppressed, and it can be stretched at a high ratio.
  • the stretching temperature can be set, for example, above 50°C and below 140°C, and optionally above 60°C and below 130°C.
  • the stretching temperature refers to the temperature of the gel sheet.
  • a polyolefin porous membrane with uniform microscopic fiber structure orientation and small difference in macroscopic two-dimensional mechanical properties can be obtained.
  • the stability of the diaphragm in battery production and the safety of battery application can be improved.
  • the gel was extruded using a T-die, the extruded melt temperature was controlled below 220°C, and then rapidly cooled on a thermostatic metal pair of rollers with a surface temperature of 15°C and rolled into a sheet with a thickness of 1.5 mm.
  • the sheet i.e. the cast sheet
  • Rapid stretching is performed at 120°C at a speed of 40m/min, wherein both the MD direction and the TD direction are stretched at a ratio of 6.5.
  • the uniformity of the temperature field needs to be controlled during stretching, and the temperature difference between different points in the temperature field in the stretching area is less than 0.3°C, and an oil film is obtained after stretching.
  • the oil film is pulled into dichloromethane for extraction to extract the paraffin oil in the oil film. Then, the dichloromethane remaining in the diaphragm is taken out by constant temperature hot air at 40°C. Due to the extraction and drying process, the TD direction shrinks a little under the action of MD traction, and the shrinkage is 15%. Therefore, after heat setting and expansion at 131°C, the ratio of the stretching ratio in the original MD direction to the stretching ratio in the TD direction is achieved, and finally a polyolefin porous membrane with uniform fiber structure orientation is obtained.
  • the gel was extruded using a T-die, the extruded melt temperature was controlled below 220°C, and then rapidly cooled on a thermostatic metal pair of rollers with a surface temperature of 15°C and rolled into a sheet with a thickness of 1.5 mm.
  • the sheet i.e. the cast sheet
  • Rapid stretching is performed at 117°C at a speed of 40 m/min, with a stretching ratio of 7.5 in the MD direction and 7.2 in the TD direction.
  • the uniformity of the temperature field needs to be controlled during stretching, and the temperature difference between different points in the temperature field in the stretching area is less than 0.3°C.
  • an oil film is obtained. The oil film is pulled into the extraction, and the paraffin oil in the oil film is extracted.
  • ultra-high molecular weight polyethylene powder GUR2122, Ticona Celanese
  • 5 parts by weight of high-density polyethylene powder GUR4116, Ticona Celanese
  • 80 parts by weight of paraffin oil 60#, Zhejiang Zhengxin
  • add 0.3 parts by weight of antioxidant Irg1010 (Ciba Specialty Chemicals BASF)
  • 0.1 parts by weight of antioxidant P168 (Ciba Specialty Chemicals Co., Ltd.)
  • the ultra-high molecular weight polyethylene material after mixing the auxiliary agent is put into a twin-screw extruder together with the paraffin oil for mixing, and finally a mixture melt gel is made.
  • the gel was extruded using a T-die, the extruded melt temperature was controlled below 220°C, and then rapidly cooled on a thermostatic metal pair of rollers with a surface temperature of 15°C and rolled into a sheet with a thickness of 1.5 mm.
  • the sheet i.e. the cast sheet
  • Rapid stretching is performed at 122°C at a speed of 40m/min, with a stretching ratio of 7.2 in the MD direction and 7.1 in the TD direction.
  • the uniformity of the temperature field needs to be controlled during stretching, and the temperature difference between different points in the temperature field in the stretching area is less than 0.3°C.
  • An oil film is obtained after stretching.
  • the oil film is pulled into the extraction to extract the paraffin oil in the oil film.
  • the dichloromethane remaining in the diaphragm is taken out by constant temperature hot air at 40°C. Due to the extraction and drying process, a small amount of shrinkage occurs in the TD direction under the action of MD traction, and the shrinkage is 20%. Therefore, after heat setting and expansion at 131°C, the ratio of the stretching ratio in the original MD direction to the stretching ratio in the TD direction is achieved, and finally a polyolefin porous membrane with uniform fiber structure orientation is obtained.
  • ultra-high molecular weight polyethylene powder GUR2122, Ticona Celanese
  • 10 parts by weight of high-density polyethylene powder VH200U, KPIC
  • 5 parts by weight of polyethylene powder VH035, KPIC
  • paraffin oil 60#, Zhejiang Zhengxin
  • Add 0.3 parts by weight of antioxidant Irg1010 (Ciba Specialty Chemicals BASF)
  • 0.1 parts by weight of antioxidant P168 (Ciba Specialty Chemicals Co., Ltd.)
  • the gel was extruded using a T-die, the extruded melt temperature was controlled below 220°C, and then rapidly cooled on a thermostatic metal pair of rollers with a surface temperature of 15°C and rolled into a sheet with a thickness of 1.5 mm.
  • the sheet i.e. the cast sheet
  • Rapid stretching is performed at 117°C at a speed of 40m/min, with a stretching ratio of 6.8 in the MD direction and 6.8 in the TD direction.
  • the uniformity of the temperature field needs to be controlled during stretching, and the temperature difference between different points in the temperature field in the stretching area is less than 0.3°C.
  • An oil film is obtained after stretching.
  • the oil film is pulled into the extraction to extract the paraffin oil in the oil film.
  • the dichloromethane remaining in the diaphragm is taken out by constant temperature hot air at 40°C.
  • a small amount of shrinkage occurs in the TD direction under the action of traction in the MD direction, and the shrinkage is 21%. Therefore, after heat setting and expansion at 131°C, the ratio of the stretching ratio in the original MD direction to the stretching ratio in the TD direction is achieved, and finally a polyolefin porous membrane with uniform fiber structure orientation is obtained.
  • ultra-high molecular weight polyethylene powder GUR2122, Ticona Celanese
  • 10 parts by weight of high-density polyethylene powder VH200U, KPIC
  • 5 parts by weight of polyethylene powder VH035, KPIC
  • paraffin oil 60#, Zhejiang Zhengxin
  • Add 0.3 parts by weight of antioxidant Irg1010 (Ciba Specialty Chemicals BASF)
  • 0.1 parts by weight of antioxidant P168 (Ciba Specialty Chemicals Co., Ltd.)
  • the gel was extruded using a T-die, the extruded melt temperature was controlled below 220°C, and then rapidly cooled on a thermostatic metal pair of rollers with a surface temperature of 15°C and rolled into a sheet with a thickness of 1.5 mm.
  • the sheet i.e. the cast sheet
  • Rapid stretching is performed at 116°C at a speed of 40m/min, with a stretching ratio of 8 in the MD direction and 8 in the TD direction.
  • the uniformity of the temperature field needs to be controlled during stretching, and the temperature difference between different points in the temperature field in the stretching area is less than 0.3°C.
  • An oil film is obtained after stretching.
  • the oil film is pulled into the extraction to extract the paraffin oil in the oil film.
  • the dichloromethane remaining in the diaphragm is taken out by constant temperature hot air at 40°C.
  • a small amount of shrinkage occurs in the TD direction under the action of traction in the MD direction, and the shrinkage is 21%. Therefore, after heat setting and expansion at 131°C, the ratio of the stretching ratio in the original MD direction to the stretching ratio in the TD direction is achieved, and finally a polyolefin porous membrane with uniform fiber structure orientation is obtained.
  • ultra-high molecular weight polyethylene powder GUR2122, Ticona Celanese
  • 10 parts by weight of high-density polyethylene powder VH200U, KPIC
  • 5 parts by weight of polyethylene powder VH035, KPIC
  • paraffin oil 60#, Zhejiang Zhengxin
  • Add 0.3 parts by weight of antioxidant Irg1010 (Ciba Specialty Chemicals BASF)
  • 0.1 parts by weight of antioxidant P168 (Ciba Specialty Chemicals Co., Ltd.)
  • the gel was extruded using a T-die, the extruded melt temperature was controlled below 220°C, and then rapidly cooled on a thermostatic metal pair of rollers with a surface temperature of 15°C and rolled into a sheet with a thickness of 1.5 mm.
  • the sheet i.e. the cast sheet
  • Rapid stretching is performed at 115°C at a speed of 40m/min, with a stretching ratio of 10 in the MD direction and 10 in the TD direction.
  • the uniformity of the temperature field must be controlled during stretching, and the temperature difference between different points in the temperature field in the stretching area is less than 0.3°C.
  • An oil film is obtained after stretching.
  • the oil film is pulled into the extraction to extract the paraffin oil in the oil film.
  • the residual dichloromethane in the diaphragm is taken out by constant temperature hot air at 40°C. Due to the effect of traction of the TD direction in the MD direction during the extraction and drying process, A small amount of shrinkage occurred, which was 21%. Therefore, after heat setting and expansion at 131°C, the ratio of the stretching ratio in the original MD direction to the stretching ratio in the TD direction was achieved, and finally a polyolefin porous membrane with uniform fiber structure orientation was obtained.
  • ultra-high molecular weight polyethylene powder GUR2122, Ticona Celanese
  • 10 parts by weight of high-density polyethylene powder VH200U, KPIC
  • 5 parts by weight of polyethylene powder VH035, KPIC
  • paraffin oil 60#, Zhejiang Zhengxin
  • Add 0.3 parts by weight of antioxidant Irg1010 (Ciba Specialty Chemicals BASF)
  • 0.1 parts by weight of antioxidant P168 (Ciba Specialty Chemicals Co., Ltd.)
  • the gel was extruded using a T-die, the extruded melt temperature was controlled below 220°C, and then rapidly cooled on a thermostatic metal pair of rollers with a surface temperature of 15°C and rolled into a sheet with a thickness of 1.5 mm.
  • the sheet i.e. the cast sheet
  • Rapid stretching is performed at 115°C at a speed of 40m/min, with a stretching ratio of 10 in the MD direction and 11 in the TD direction.
  • the uniformity of the temperature field needs to be controlled during stretching, and the temperature difference between different points in the temperature field in the stretching area is less than 0.3°C.
  • An oil film is obtained after stretching.
  • the oil film is pulled into the extraction to extract the paraffin oil in the oil film.
  • the dichloromethane remaining in the diaphragm is taken out by constant temperature hot air at 40°C.
  • a small amount of shrinkage occurs in the TD direction under the action of traction in the MD direction, and the shrinkage is 21%. Therefore, after heat setting and expansion at 131°C, the ratio of the stretching ratio in the original MD direction to the stretching ratio in the TD direction is achieved, and finally a polyolefin porous membrane with uniform fiber structure orientation is obtained.
  • ultra-high molecular weight polyethylene powder GUR2122, Ticona Celanese
  • 10 parts by weight of high-density polyethylene powder VH200U, KPIC
  • 5 parts by weight of polyethylene powder VH035, KPIC
  • paraffin oil 60#, Zhejiang Zhengxin
  • Add 0.3 parts by weight of antioxidant Irg1010 (Ciba Specialty Chemicals BASF)
  • 0.1 parts by weight of antioxidant P168 (Ciba Specialty Chemicals Co., Ltd.)
  • the gel was extruded using a T-die, the extruded melt temperature was controlled below 220°C, and then rapidly cooled on a thermostatic metal pair of rollers with a surface temperature of 15°C and rolled into a sheet with a thickness of 1.5 mm.
  • the sheet i.e. the cast sheet
  • Rapid stretching is performed at 115°C at a speed of 40m/min, with a stretching ratio of 10 in the MD direction and 9.1 in the TD direction.
  • the uniformity of the temperature field needs to be controlled during stretching, and the temperature difference between different points in the temperature field in the stretching area is less than 0.3°C.
  • An oil film is obtained after stretching.
  • the oil film is pulled into the extraction to extract the paraffin oil in the oil film.
  • the dichloromethane remaining in the diaphragm is taken out by constant temperature hot air at 40°C.
  • a small amount of shrinkage occurs in the TD direction under the action of traction in the MD direction, and the shrinkage is 21%. Therefore, after heat setting and expansion at 131°C, the ratio of the stretching ratio in the original MD direction to the stretching ratio in the TD direction is achieved, and finally a polyolefin porous membrane with uniform fiber structure orientation is obtained.
  • Ultra-high molecular weight polyethylene powder VH200U, LPIC
  • 5 parts by weight of high-density polyethylene powder VH035, KPIC
  • paraffin oil 60#, Zhejiang Zhengxin
  • Add 0.3 parts by weight of antioxidant Irg1010 (Ciba Specialty Chemicals BASF)
  • 0.1 parts by weight of antioxidant P168 (Ciba Specialty Chemicals Co., Ltd.)
  • the gel was extruded using a T-die, the extruded melt temperature was controlled below 220°C, and then rapidly cooled on a thermostatic metal pair of rollers with a surface temperature of 15°C and rolled into a sheet with a thickness of 1.5 mm.
  • the sheet i.e., the cast sheet
  • the sheet is pulled and stretched in steps.
  • the cast sheet is stretched along the MD direction, wherein the stretching ratio in the MD direction is 9 times; then, at 119°C, the TD direction is rapidly stretched at a speed of 60m/min, and the stretching ratio in the TD direction is 8.3 times (the ratio of its outlet width to the width of the original casting sheet is 8.2 times).
  • the uniformity of the temperature field needs to be controlled during stretching, and the temperature difference between different points of the temperature field in the stretching area is less than 0.3°C.
  • an oil film is obtained. The oil film is pulled into the extraction to extract the paraffin oil in the oil film.
  • the dichloromethane remaining in the diaphragm is taken out by constant temperature hot air at 40°C. Due to the extraction and drying process, the TD direction shrinks a little under the action of traction in the MD direction, and the shrinkage is 22%. Therefore, after heat setting and expansion at 131°C, the ratio of the stretching ratio in the original MD direction to the stretching ratio in the TD direction is achieved, and finally a polyolefin porous membrane with uniform fiber structure orientation is obtained.
  • ultra-high molecular weight polyethylene powder GUR4116, Ticona Celanese
  • paraffin oil 60#, Zhejiang Zhengxin
  • the ultra-high molecular weight polyethylene material after mixing the auxiliary agent is put into a twin-screw extruder together with the paraffin oil for mixing, and finally a mixture melt gel is made.
  • the gel was extruded using a T-die, the extruded melt temperature was controlled below 220°C, and then rapidly cooled on a thermostatic metal pair of rollers with a surface temperature of 15°C and rolled into a sheet with a thickness of 1.5 mm.
  • the sheet i.e. the cast sheet
  • Rapid stretching is performed at 117°C at a speed of 40m/min, with a stretching ratio of 7.9 in the MD direction and 6.5 in the TD direction.
  • the uniformity of the temperature field needs to be controlled during stretching, and the temperature difference between different points in the temperature field in the stretching area is less than 0.3°C.
  • An oil film is obtained after stretching.
  • the oil film is pulled into the extraction to extract the paraffin oil in the oil film.
  • the dichloromethane remaining in the diaphragm is taken out by constant temperature hot air at 40°C. Due to the extraction and drying process, a small amount of shrinkage occurs in the TD direction under the action of MD traction, and the shrinkage is 18%. Therefore, after heat setting and expansion at 131°C, the ratio of the stretching ratio in the original MD direction to the stretching ratio in the TD direction is achieved, and finally a polyolefin porous membrane is obtained.
  • ultra-high molecular weight polyethylene powder GUR2122, Ticona Celanese
  • 10 parts by weight of high-density polyethylene powder VH200U, KPIC
  • 5 parts by weight of polyethylene powder VH035, KPIC
  • 80 parts by weight of paraffin oil 60#, Zhejiang Zhengxin
  • 0.3 parts by weight of antioxidant Irg1010 (Ciba Specialty Chemicals BASF)
  • 0.1 parts by weight of antioxidant P168 (Ciba Specialty Chemicals Co., Ltd.)
  • the ultra-high molecular weight polyethylene material after mixing the additives is put into a twin-screw extruder together with paraffin oil for mixing, and finally a mixture melt gel is prepared.
  • the gel was extruded using a T-die, the extruded melt temperature was controlled below 220°C, and then rapidly cooled on a thermostatic metal pair of rollers with a surface temperature of 15°C and rolled into a sheet with a thickness of 1.5 mm.
  • the sheet i.e. the cast sheet
  • Rapid stretching is performed at 116°C at a speed of 40m/min, with a stretching ratio of 7.1 in the MD direction and 5.4 in the TD direction.
  • the uniformity of the temperature field needs to be controlled during stretching, and the temperature difference between different points in the temperature field in the stretching area is less than 0.3°C.
  • An oil film is obtained after stretching.
  • the oil film is pulled into the extraction to extract the paraffin oil in the oil film.
  • the dichloromethane remaining in the diaphragm is taken out by constant temperature hot air at 40°C.
  • a small amount of shrinkage occurs in the TD direction under the action of traction in the MD direction, and the shrinkage is 21%. Therefore, after heat setting and expansion at 131°C, the ratio of the stretching ratio in the original MD direction to the stretching ratio in the TD direction is achieved, and finally a polyolefin porous membrane is obtained.
  • the thickness of the polyolefin porous membrane obtained in the example was measured using a micrometer.
  • a Nova Nano SEM 450 field emission scanning electron microscope produced by FEI Company of the United States was used to obtain a 20,000-fold SEM image of the polyolefin porous membrane, and the observation was carried out using a circle with a radius of 1000 nm as the observation area.
  • the orientation result R_MD of the fibers in the MD direction and the orientation result R_TD of the fibers in the TD direction of the polyolefin porous membrane are obtained.
  • the fibers taken are the fibers on the outermost surface of the polyolefin porous membrane, N is the number of fibers on the outermost surface of the observation area, and each fiber is identified by n (n is a natural number).
  • the fiber length taken refers to the same thickness segment with the same extension direction on the same fiber as one length. When the thickness on the same fiber changes, it is not counted as one length and needs to be compared in time.
  • the same thickness segment refers to the increase in the width of the thickest part of the segment compared to the width of the thinnest part is within 5%.
  • the diameter of the fiber refers to the average value of the width of the widest part and the width of the thinnest part on the same thickness segment.
  • the angle between the fiber and the MD direction or the TD direction is the angle between the extension direction of the same thickness segment and the MD direction or the TD direction.
  • the test was carried out according to the requirements of GB 1040.3-2006.
  • the specimen was prepared by cutting method and the specimen type was Type 2.
  • the specimen was in the shape of a strip with a length of 200 mm and a width of 25 mm.
  • the distance between the fixtures was (100 ⁇ 5) mm and the test speed was (250 ⁇ 10) mm/min.
  • the test was carried out according to the requirements of GB 1040.3-2006.
  • the specimen was prepared by cutting method and the specimen type was Type 2.
  • the specimen was in the shape of a strip with a length of 200 mm and a width of 25 mm.
  • the distance between the fixtures was (100 ⁇ 5) mm and the test speed was (250 ⁇ 10) mm/min.
  • the test was conducted according to the requirements of GB/T12027-2004.
  • a sample of 100 mm ⁇ 100 mm was taken from the polyolefin microporous separator and placed in a blast oven at 105°C for 1 hour.
  • the heat shrinkage rate can be calculated by the following formula.
  • MD heat shrinkage (%)
  • TD heat shrinkage rate (%)
  • Table 1-2 the performance test results of the above embodiments and comparative examples are as follows:
  • Table 2 Relevant performance parameter indicators of the polyolefin porous membrane of Comparative Examples 1-2
  • Table 1 and Table 2 respectively show the relevant performance indicators of the polyolefin porous membranes of Examples 1-9 and Comparative Examples 1-2. It can be seen that when the fiber orientation result R_MD in the MD direction of the polyolefin porous membrane satisfies (R_MD) 2 ⁇ 0.8 and the fiber orientation result R_TD in the TD direction satisfies (R_TD) 2 ⁇ 0.8, the separator can obtain excellent isotropic uniformity of mechanical properties, and thus have higher safety in battery applications.

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Abstract

本公开涉及电化学装置用隔膜技术领域，公开了一种聚烯烃多孔膜及其制备方法、电池隔膜、电化学装置。本公开的聚烯烃多孔膜，具有网络纤维结构，对其2万倍SEM图像进行观察，其具有如下特性：(1)以1000nm为半径的圆圈内，纤维直径＞15nm的纤维在MD方向上的纤维取向结果R_MD满足：(R_MD)2＜0.8；(2)以1000nm为半径的圆圈内，纤维直径＞15nm的纤维在TD方向上的纤维取向结果R_TD满足：(R_TD)2＜0.8；因此，本公开的聚烯烃多孔膜，可以实现其作为隔膜在电池中的力学性能一致性，进而提高隔膜在电池生产的稳定性和电池应用的安全性。

Description

一种聚烯烃多孔膜及其制备方法、电池隔膜、电化学装置
相关申请的交叉引用
本公开要求于2022年10月28日提交中国专利局作为受理局的申请号为PCTCN2022128427、名称为“一种聚烯烃多孔膜及其制备方法、电池隔膜、电化学装置”的PCT专利申请的优先权，其全部内容通过引用结合在本公开中。
技术领域
本公开涉及电化学装置用隔膜技术领域，特别涉及一种聚烯烃多孔膜及其制备方法、电池隔膜、电化学装置。
背景技术
隔膜作为锂离子电池中的关键主材，主要作用是隔绝正、负极，防止短路，同时保证具有一定的电解质浸润性和保液性，为锂离子传输提供通道。隔膜各项性能直接决定电池的界面性能及内阻，进而影响到电池充放电性能及循环性能。
现有技术的锂离子电池隔膜主要以聚烯烃多孔隔膜为主，聚烯烃多孔隔膜是一种内部含有大量微小贯通的细孔，以聚烯烃为骨架支撑大量细孔的三维立体结构，其制备方法主要分为干法、湿法拉伸。其中，干法是通过形成聚烯烃膜、然后在低温下拉伸该膜而形成微孔的方法，所述拉伸导致作为聚烯烃的结晶部分的薄片之间的微裂纹。湿法是将聚烯烃基树脂和稀释剂在聚烯烃基树脂熔融形成单相的高温下进行混炼、聚烯烃和稀释剂在冷却过程中进行相分离、然后稀释剂被提取以在其中形成孔隙的方法。
湿法在相分离处理后通过拉伸/提取工艺赋予机械强度和透明性，通过对已成型为片状的聚乙烯坯料片进行拉伸，调节速度、倍率、温度等拉伸条件，将晶体结构中的非晶质部分拉长，在形成微纤维的同时在片状层之间形成微细孔。与干法相比，湿法的膜厚度薄，孔径均匀，物理性能优异。
公开号为JP2005056851A的文献提出一种层压膜的微多孔膜，为降低隔膜TD方向(与机械垂直的方向，宽度方向)的热收缩率，其制备方法不进行在TD方向的拉伸，因此可以说完全没有在TD方向的收缩；但是，仅利用在MD方向(机械方向，长度方向)的单轴进行开孔，从而形成TD方向强度不充分的具有极端各向异性的微多孔膜；因此，在例如电池压坏试验、碰撞试验等中，可能容易在一个方向开裂等。
公开号为JP2014141644A的文献记载了一种MD方向与TD方向的拉伸强度之比为0.9以上小于1.5的双向取向多孔性聚丙烯膜。但是，由于该聚丙烯膜利用干式法制备，因此，气阻度低，需要对断裂伸长率、韧性进行进一步改良。
公开号为CN107250234B的中国专利公开了一种聚烯烃微多孔膜，以聚丙烯为主成分，MD方向与TD方向的拉伸强度之比为0.4以上2.0以下，MD方向与TD方向的断裂伸长率之比为0.6以上1.7以下，通过添加对聚烯烃树脂的结晶化进行促进或抑制的添加剂，成核剂以及结晶化阻滞剂，以使聚烯烃多孔膜的细孔结构变得均匀且微细，耐冲击性得以进一步提高。
因此，提升隔膜的性能对提升电池的安全性能至关重要。虽然现有聚烯烃多孔隔膜在锂离子电池中能够成熟应用，但目前技术主要关注隔膜的基本物性以及理化性能，对于微观结构的一致性关注较少。为了提高锂离子电池湿法隔膜的性能，需要从微观结构方面进行优化，以提升其性能，优化其力学性能的各向均一性。
发明内容
本公开的目的在于提供一种聚烯烃多孔膜，在作为电池隔膜使用的情况下，可以提供具有优异电池安全性的电池，提高电池生产的稳定性。
本公开的目的还在于提供一种上述聚烯烃多孔膜的制备方法，通过控制并减小隔膜拉伸过程中取向的差异，并控制成型温度，可以获得微观纤维结构取向均匀，进而宏观两向力学性能差异小的聚烯烃多孔膜。
本公开提供的聚烯烃多孔膜，通过湿法拉伸制得，具有网络纤维结构；
对其2万倍SEM图像进行观察，所述聚烯烃多孔膜具有如下特性：
(1)以1000nm为半径的圆圈内，纤维直径＞15nm的纤维在MD方向上的纤维取向R_MD满足：(R_MD)2＜0.8，R_MD由以下式1得出：
其中，N为观察区域内最外侧表面上纤维的数量，各纤维以n标识；θn为纤维n与MD方向的夹角，θn由MD方向向左为起始，逆时针方向得到，以μm为单位的纤维n的长度数值Ln与cosθn的乘积为Lncosθn，φn为以nm为单位的纤维n的直径数值；
(2)以1000nm为半径的圆圈内，纤维直径＞15nm的纤维在TD方向上的纤维取向R_TD满足：(R_TD)2＜0.8，R_TD由以下式2得出：
其中，N为观察区域内最外侧表面上纤维的数量，各纤维以n标识；βn为纤维n与TD方向的夹角，βn由TD方向向上为起始，逆时针方向得到，以μm为单位的纤维n的长度数值Ln与cosβn的乘积为Lncosβn，φn为以nm为单位的纤维n的直径数值。
可选地，提供的聚烯烃多孔膜，通过湿法拉伸制得，具有网络纤维结构；
对其2万倍SEM图像进行观察，所述聚烯烃多孔膜具有如下特性：
(1)以1000nm为半径的圆圈内，纤维直径＞15nm的纤维在MD方向上的纤维取向R_MD满足：(R_MD)2＜0.5，R_MD由以下式1得出：
其中，N为观察区域内最外侧表面上纤维的数量，各纤维以n标识；θn为纤维n与MD方向的 夹角，θn由MD方向向左为起始，逆时针方向得到，以μm为单位的纤维n的长度数值Ln与cosθn的乘积为Lncosθn，φn为以nm为单位的纤维n的直径数值；
(2)以1000nm为半径的圆圈内，纤维直径＞15nm的纤维在TD方向上的纤维取向R_TD满足：(R_TD)2＜0.5，R_TD由以下式2得出：
其中，N为观察区域内最外侧表面上纤维的数量，各纤维以n标识；βn为纤维n与TD方向的夹角，βn由TD方向向上为起始，逆时针方向得到，以μm为单位的纤维n的长度数值Ln与cosβn的乘积为Lncosβn，φn为以nm为单位的纤维n的直径数值。
所述的聚烯烃微多孔膜，还具有下述的特性；
可选的，MD方向上的拉伸强度与TD方向上的拉伸强度之比为MD/TD拉伸强度比，则0.8≤MD/TD拉伸强度比≤1.2。
可选的，MD方向上的断裂伸长率与TD方向上的断裂伸长率之比为MD/TD断裂伸长率比，则0.4≤MD/TD断裂伸长率比≤1.2。
可选的，所述聚烯烃多孔膜的膜厚为1～30μm。
本公开还提供一种如上所述聚烯烃多孔膜的制备方法，包括如下步骤：
步骤一、将聚烯烃与致孔剂、添加剂混练挤出，其中聚烯烃在混炼物中的质量占比≥15％，抗氧化剂在混炼物中的质量占比为0～0.5％；将经过双螺杆高温分散混合塑化后，经口模挤出制备流延片；
步骤二、将流延片进行拉伸，拉伸过程中流延片材自身的温度不超过140℃，且拉伸区域温度场不同点之间的温差小于0.5℃，且拉伸过程中MD方向上的拉伸倍率与TD方向上的拉伸倍率之比即MD/TD拉伸倍率比为：0.9≤MD/TD拉伸倍率比≤1.1，制得油膜；
步骤三、将油膜萃取，然后进行热定型处理，热定型处理过程中，需维持原有MD与TD的拉伸倍率关系，所述热定型处理过程中通过扩幅拉伸处理来维持步骤二中MD方向上的拉伸倍率与TD方向上的拉伸倍率之比。
可选的，所述步骤二中，MD方向上的拉伸倍率和TD方向上的拉伸倍率均为5倍以上。
可选的，所述聚烯烃的粘均分子量介于20万至500万之间。
可选的，所述致孔剂为白油、石蜡油、聚乙二醇中的一种或几种。
可选的，所述步骤一中添加剂包含抗氧化剂，所述抗氧化剂为胺类、含硫化合物、含氮化合物、含磷化合物、有机金属盐中的一种或几种。
本公开还提供一种电池隔膜，包含如上所述的聚烯烃多孔膜。
本公开还提供一种电化学装置，包含如上所述的聚烯烃多孔膜或如上所述的电池隔膜作为将正负两极分开的元件。
本公开具有以下有益效果：
本公开的聚烯烃多孔膜，可以实现其作为隔膜在电池中的力学性能一致性，进而提高隔膜在电池生产的稳定性和电池应用的安全性。
附图说明
为了更清楚地说明本公开实施例的技术方案，下面将对实施例中所需要使用的附图作简单地介绍，应当理解，以下附图仅示出了本公开的某些实施例，因此不应被看作是对范围的限定，对于本领域普通技术人员来讲，在不付出创造性劳动的前提下，还可以根据这些附图获得其他相关的附图。
图1为示出实施例1的聚烯烃多孔膜的扫描电镜照片；
图2为示出实施例2的聚烯烃多孔膜的扫描电镜照片；
图3为示出实施例3的聚烯烃多孔膜的扫描电镜照片；
图4为示出实施例4的聚烯烃多孔膜的扫描电镜照片；
图5为示出实施例5的聚烯烃多孔膜的扫描电镜照片；
图6为示出实施例6的聚烯烃多孔膜的扫描电镜照片；
图7为示出实施例9的聚烯烃多孔膜的扫描电镜照片。
具体实施方式
为使本公开实施例的目的、技术方案和优点更加清楚，下面将对本公开实施例中的技术方案进行清楚、完整地描述。实施例中未注明条件者，按照常规条件或制备商建议的条件进行。所用试剂或仪器未注明生产厂商者，均为可以通过市售购买获得的常规产品。
本发明人发现，聚烯烃多孔膜中纤维结构的均一性是隔膜性能的重要保证，微观纤维结构特性决定着聚烯烃多孔膜作为电池隔膜的各项宏观性能，其中纤维的分布及取向均匀程度决定了隔膜的力学性能缩均匀性，当聚烯烃多孔膜具有以下特性时，能够获取优异的力学性能的各向均一性，进而在电池应用中具有更高安全性。
对其2万倍SEM图像进行观察，所述聚烯烃多孔膜具有以下特性：
(1)以1000nm为半径的圆圈内，纤维直径＞15nm的纤维在MD方向上的纤维取向R_MD满足：(R_MD)2＜0.8，R_MD由以下式1得出：
其中，N为观察区域内最外侧表面上纤维的数量，各纤维以n标识；θn为纤维n与MD方向的夹角，θn由MD方向向左为起始，逆时针方向得到，以μm为单位的纤维n的长度数值Ln与cosθn的乘积为Lncosθn，φn为以nm为单位的纤维n的直径数值；
(2)以1000nm为半径的圆圈内，纤维直径＞15nm的纤维在TD方向上的纤维取向R_TD满足：(R_TD)2＜0.8，R_TD由以下式2得出：
其中，N为观察区域内最外侧表面上纤维的数量，各纤维以n标识；βn为纤维n与TD方向的夹角，βn由TD方向向上为起始，逆时针方向得到，以μm为单位的纤维n的长度数值Ln与cosβn 的乘积为Lncosβn，φn为以nm为单位的纤维n的直径数值。
在其中一个方案中，提供的聚烯烃多孔膜，通过湿法拉伸制得，具有网络纤维结构；
对其2万倍SEM图像进行观察，所述聚烯烃多孔膜具有如下特性：
(1)以1000nm为半径的圆圈内，纤维直径＞15nm的纤维在MD方向上的纤维取向R_MD满足：(R_MD)2＜0.5，R_MD由以下式1得出：
其中，N为观察区域内最外侧表面上纤维的数量，各纤维以n标识；θn为纤维n与MD方向的夹角，θn由MD方向向左为起始，逆时针方向得到，以μm为单位的纤维n的长度数值Ln与cosθn的乘积为Lncosθn，φn为以nm为单位的纤维n的直径数值；
(2)以1000nm为半径的圆圈内，纤维直径＞15nm的纤维在TD方向上的纤维取向R_TD满足：(R_TD)2＜0.5，R_TD由以下式2得出：
其中，N为观察区域内最外侧表面上纤维的数量，各纤维以n标识；βn为纤维n与TD方向的夹角，βn由TD方向向上为起始，逆时针方向得到，以μm为单位的纤维n的长度数值Ln与cosβn的乘积为Lncosβn，φn为以nm为单位的纤维n的直径数值。
[聚烯烃树脂]
用作原料的聚烯烃树脂，例如，可以使用聚乙烯、聚丙烯等。作为聚乙烯，没有特别限定，可以使用各种聚乙烯，例如，可使用：超高分子量聚乙烯(UHMwPE)、高密度聚乙烯(HDPE)、中密度聚乙烯、支链状低密度聚乙烯、直链状低密度聚乙烯等。
需要说明的是，聚乙烯可以是乙烯的均聚物，也可以是乙烯与其它α-烯烃的共聚物。作为α-烯烃，可举出：丙烯、1-丁烯、1-己烯、1-戊烯、4-甲基-1-戊烯、辛烯、乙酸乙烯酯、甲基丙烯酸甲酯、苯乙烯等。
在本说明书中，所述聚烯烃多孔膜是指以聚烯烃树脂作为主要成分的微多孔膜，例如，聚烯烃树脂占微多孔膜总量为90质量％以上的微多孔膜。以下，对本实施方式的聚烯烃多孔膜的物性进行说明。
[MD/TD拉伸强度比]
本公开聚烯烃多孔膜的MD方向及TD方向的拉伸强度之比即MD/TD拉伸强度比可选地为0.8以上且1.2以下。当拉伸强度之比为上述范围时，能够针对全方向的冲击，更均匀地受力，因此，作为隔膜应用于电池中的耐冲击性提高，更稳定，可抑制破膜及短路。
出于在电池应用中更安全、更稳定的观点考虑，可选地，MD/TD拉伸强度比接近1，此时，聚烯烃多孔膜的强度不存在方向依赖性，因此，在对膜施加应力时，韧性优异，不会在特定方向上开裂，即使异物贯穿微多孔膜，也能避免贯通孔的扩大。否则，当MD方向的拉伸强度过大时，有时 会发生MD方向的撕裂。当TD方向的拉伸强度过大时，有时会发生TD方向的撕裂或电极接片粘接部分的结合脱离而变得容易发生短路。
[MD/TD断裂伸长率比]
本公开聚烯烃多孔膜的MD方向及TD方向的断裂伸长率之比即MD/TD断裂伸长率比可选地为0.8以上且1.2以下。当断裂伸长率之比为上述范围时，能够针对全方向的冲击，更均匀地受力，因此，作为隔膜应用于电池中的耐冲击性提高，更稳定，可抑制破膜及短路。
出于在电池应用中更安全、更稳定的观点考虑，可选地，MD/TD断裂伸长率比接近1，此时，聚烯烃微多孔膜的强度不存在方向依赖性，因此，在对膜施加应力时，韧性优异，不会在特定方向上开裂，即使异物贯穿微多孔膜，也能避免贯通孔的扩大。
[膜厚]
对聚烯烃多孔膜的膜厚的没有特别限定。从膜强度的观点出发，本公开的聚烯烃多孔膜的厚度为2μm以上、可选地为5μm以上。另外，从透过性的观点出发，微多孔膜的厚度为50μm以下、可选地为30μm以下。
[拉伸强度]
本公开聚烯烃多孔膜拉伸强度在MD、TD两方向为150MPa以上、可选地为200MPa以上。拉伸强度弱的话，电池卷绕性将变差或由于从外部进行的电池碰撞试验、电池内的异物等而容易产生短路。另外，对MD方向及TD方向的拉伸伸强度的上限没有特别限定，从实际生产经验来看，MD方向及TD方向的拉伸强度的上限可选地为700MPa以下、可选地为600MPa以下、可选地为550MPa以下。
[断裂伸长率]
本公开聚烯烃多孔膜的MD方向及TD方向的断裂伸长率分别可选地为90％以上。当断裂伸长率为上述范围时，在电池内受到冲击时，利用其柔软性来抑制隔膜的破膜及短路的发生。另外，对MD方向及TD方向的断裂伸长率的上限没有特别限定，例如为400％以下，可选地为300％以下，可选地为200％以下。当断裂伸长率为上述范围时，在电极卷绕时，隔膜不会伸长而发生变形，卷绕性良好。
MD以及TD断裂伸长率可选地分别为10～200％、可选地为10～150％、可选地为10～120％。MD以及TD断裂伸长率在上述范围内的微多孔膜不仅电池卷绕性良好、而且在电池碰撞试验等中不易引起变形。
[热收缩率]
聚烯烃多孔膜作为隔膜还需要在高温下的热收缩特性也优异，如在做成电池的状态下在高温储藏试验、高温循环试验、烘烤试验等中显示出优异的结果等。
从充放电过程中隔离膜尺寸稳定性出发，MD方向上的热收缩率小于5％且TD方向上的热收缩率小于6.5％，可选地，MD热收缩率为4％以下且TD热收缩率为4％以下，可选地，MD热收缩率为3％以下且TD热收缩率为2.5％以下，可选地，MD热收缩率为1％以下且TD热收缩率为1％以下。当热收缩率大于上述可选的范围的话，微多孔膜在电池干燥工序、电池高温循环试验、电池高温保存试验等中容易在宽度方向收缩，故不优选。
[聚烯烃多孔膜的制备方法]
本公开聚烯烃多孔膜通过湿法拉伸制得，其制备方法包括：(1)使用一种或一种以上的聚烯烃树脂材料作为主要原料，将上述原料与致孔剂、抗氧化剂混练，将混炼物经口模挤出制备成流延片；(2)将流延片进行双向拉伸，制得油膜；(3)将油膜萃取，然后进行热定型处理。
其中，流延片在拉伸过程中的温度和拉伸倍率比的控制是获得具有上述特性聚烯烃多孔膜的关键因素之一；双向拉伸过程中需控制温度场及拉伸场，流延片材自身的温度不得超过140℃且拉伸区域温度场不同点之间的温差小于0.5℃，MD方向上的拉伸倍率与TD方向上的拉伸倍率之比为0.9以上且1.1以下是必要的，当实际拉伸倍率之比为上述范围时，所获得的聚烯烃多孔膜的拉伸强度或断裂伸长率，MD方向及TD方向的平衡变得良好，可以进一步提高膜强度，提高耐冲击性。
其次，在热定型处理过程中，需维持原有MD与TD方向的拉伸倍率关系，热定型处理过程中维持MD方向上的拉伸倍率与TD方向上的拉伸倍率之比为步骤(2)中双向拉伸的拉伸倍率之比即0.9以上且1.1以下则是关键因素之二。
其中，MD方向上的拉伸倍率、TD方向上的拉伸倍率是实际上隔膜MD方向上的拉伸倍率和TD方向上的实际拉伸倍率。
对同步拉伸而言，MD方向上的拉伸倍率和TD方向上的拉伸倍率可由设备实际设置直接得到，也可以由以下方式得到：同步MD方向上的拉伸倍率等于同步拉伸后制得油膜在MD方向上的速率与进入拉伸前片材的运行速率的比值；同步TD方向上的拉伸倍率等于同步拉伸后制得的油膜在TD方向上的幅宽与进入拉伸前片材的TD被拉伸区的幅宽之比，注意要扣除夹子夹持部分的幅宽长度。
对异步拉伸而言，异步MD方向上的拉伸倍率可由设备实际设置直接得到，也可以由MD方向拉伸后，进入TD拉伸之前的片材速率与拉伸前的速率比值得到；异步TD方向上的拉伸倍率为，油膜在TD拉伸完毕后的实际幅宽与MD拉伸前凝胶片材的实际幅宽之比。
对于上述聚烯烃多孔膜的制备方法，只要可获得具有上述特性的聚烯烃多孔膜，则聚烯烃、致孔剂和抗氧化剂的种类及配比，混炼、挤出的方式方法，萃取剂的种类及使用与否，缓和热定形的工艺等就没有特别限定。
用作原料的聚烯烃树脂，例如，可以使用聚乙烯、聚丙烯等。作为聚乙烯，没有特别限定，可以使用各种聚乙烯，例如，可使用：超高分子量聚乙烯(UHMWPE)、高密度聚乙烯(HDPE)、中密度聚乙烯、支链状低密度聚乙烯、直链状低密度聚乙烯等。需要说明的是，聚乙烯可以是乙烯的均聚物，也可以是乙烯与其它α-烯烃的共聚物。作为α-烯烃，可举出：丙烯、1-丁烯、1-己烯、1-戊烯、4-甲基-1-戊烯、辛烯、乙酸乙烯酯、甲基丙烯酸甲酯、苯乙烯等。
从聚合物熔体易加工且不破膜的角度出发，聚烯烃树脂的粘均分子量为20万以上500万以下；当聚烯烃树脂的粘均分子量为上述范围时，制膜性变得良好。
另外，成膜材料可以根据需要包含聚烯烃材料以外的其它树脂成分的聚合物，例如，可以使用耐高温树脂等，例如：聚丙烯、聚酰亚胺、聚酰胺、氟树脂等。其中，聚烯烃在混炼物中的质量占比≥15％，耐高温树脂在混炼物中的质量占比＜10％。
其中，所述致孔剂可以为：如白油的液态烃混合物，如邻苯二甲酸二辛酯(DOP)的有机酸酯，如磷酸三辛酯(TOP)的无机酸酯，能与聚烯烃树脂形成均一溶液的有机物，或上述多种致孔剂的 混合物等。
其中，所述抗氧化剂既可只使用一种主抗氧化剂，也可将多种抗氧化剂复配使用。添加抗氧化剂可以减缓聚合物和致孔剂在剪切加工过程中发生的老化降解，以及有效改善膜面的色泽、平整和强度。
拉伸温度可选地为聚烯烃树脂与制孔剂充分熔融混合物的软化点(Tcd)以上、聚烯烃树脂的熔点以下的范围内。需要说明的是，在此，聚烯烃树脂的熔点是指凝胶状片材中的聚烯烃树脂的熔点。当拉伸温度为聚烯烃树脂的熔点以下时，抑制凝胶状片材中的聚烯烃树脂的熔融，通过延伸，可以使分子链有效地取向。另外，当拉伸温度为软化点(Tcd)以上时，可以使凝胶状片材中的聚烯烃树脂充分软化，降低延伸张力，因此，制膜性变得良好，可抑制延伸时的破膜，能够以高倍率延伸。拉伸温度可以设为例如50℃以上，140℃以下，可选地为60℃以上，130℃以下。在此，拉伸温度是指凝胶片材的温度。
通过控制并减小隔膜拉伸过程中取向的差异，并控制成型温度，可以获得微观纤维结构取向均匀，进而宏观两向力学性能差异小的聚烯烃多孔膜，在用作电池用隔膜的情况下，可以提高隔膜在电池生产的稳定性和电池应用的安全性。
实施例
以下，通过实施例详细地说明本公开。需要说明的是，本公开并不受这些实例的限定。
实施例1
采用聚乙烯粉末(GUR4116，Ticona塞拉尼斯)25重量份，聚丙烯(T30S，镇海炼化)5重量份，石蜡油(60#，浙江正信)70重量份，相对于该聚烯烃组合物与石蜡油的合计100重量份，加入抗氧化剂(Irg1010(Ciba Specialty Chemicals巴斯夫))0.3重量份、抗氧化剂(P168(Ciba Specialty Chemicals株式会社制))0.1重量份，将它们按顺序进行混合。将混合助剂之后的超高分子量聚乙烯物料与石蜡油一起投入双螺杆挤出机进行混练，最终制成混合物熔体凝胶。
使用T型模头，将该凝胶挤出，挤出熔体温度控制在220℃以下，然后在表面温度为15℃的恒温金属对辊上快速冷却，并轧制成片材，片材厚度为1.5mm。
将片材即流延片牵引并进行拉伸。在120℃，以40m/min的速度进行快速拉伸，其中MD方向、TD方向均进行6.5倍率的拉伸。拉伸时需控制温度场的均匀性，拉伸区域温度场不同点之间的温差小于0.3℃，拉伸后制得油膜。将油膜牵引入二氯甲烷中进行萃取，将油膜中的石蜡油萃取出。然后经过40℃的恒温热风将隔膜中残留的二氯甲烷带出，由于萃取及干燥过程中，TD方向在MD牵引的作用发生了少量收缩，其收缩为15％，因此，再经过131℃的热定型和扩幅，达到原始MD方向上的拉伸倍率和TD方向上的拉伸倍率之比，最终得到纤维结构取向均匀的聚烯烃多孔膜。
实施例2
采用超高分子量聚乙烯粉末(GUR4012，Ticona塞拉尼斯)23重量份，石蜡油(60#，浙江正信)77重量份，相对于该高分子量聚乙烯与石蜡油的合计100重量份，加入抗氧化剂(Irg1010(Ciba Specialty Chemicals巴斯夫))0.3重量份、抗氧化剂(P168(Ciba Specialty Chemicals株式会社制)) 0.1重量份，将它们按顺序进行混合。将混合助剂之后的超高分子量聚乙烯物料与石蜡油一起投入双螺杆挤出机进行混练，最终制成混合物熔体凝胶。
使用T型模头，将该凝胶挤出，挤出熔体温度控制在220℃以下，然后在表面温度为15℃的恒温金属对辊上快速冷却，并轧制成片材，片材厚度为1.5mm。
将片材即流延片牵引并进行拉伸。在117℃，以40m/min的速度进行快速拉伸，其中MD方向7.5倍率、TD方向进行7.2倍率的拉伸。拉伸时需控制温度场的均匀性，拉伸区域温度场不同点之间的温差小于0.3℃，拉伸后制得油膜。将油膜牵引入萃取，将油膜中的石蜡油萃取出。
然后经过40℃的恒温热风将隔膜中残留的二氯甲烷带出，由于萃取及干燥过程中，TD方向在MD牵引的作用发生了少量收缩，其收缩为18％，因此，再经过131℃的热定型和扩幅，达到原始MD方向上的拉伸倍率和TD方向上的拉伸倍率之比，最终得到纤维结构取向均匀的聚烯烃多孔膜。
实施例3
采用超高分子量聚乙烯粉末(GUR2122，Ticona塞拉尼斯)15重量份，高密度聚乙烯粉末(GUR4116，Ticona塞拉尼斯)5重量份，石蜡油(60#，浙江正信)80重量份，相对于该高分子量聚乙烯与石蜡油的合计100重量份，加入抗氧化剂(Irg1010(Ciba Specialty Chemicals巴斯夫))0.3重量份、抗氧化剂(P168(Ciba Specialty Chemicals株式会社制))0.1重量份，将它们按顺序进行混合。将混合助剂之后的超高分子量聚乙烯物料与石蜡油一起投入双螺杆挤出机进行混练，最终制成混合物熔体凝胶。
使用T型模头，将该凝胶挤出，挤出熔体温度控制在220℃以下，然后在表面温度为15℃的恒温金属对辊上快速冷却，并轧制成片材，片材厚度为1.5mm。
将片材即流延片牵引并进行拉伸。在122℃，以40m/min的速度进行快速拉伸，其中MD方向7.2倍率、TD方向均7.1倍率的拉伸。拉伸时需控制温度场的均匀性，拉伸区域温度场不同点之间的温差小于0.3℃，拉伸后制得油膜。将油膜牵引入萃取，将油膜中的石蜡油萃取出。然后经过40℃的恒温热风将隔膜中残留的二氯甲烷带出，由于萃取及干燥过程中，TD方向在MD牵引的作用发生了少量收缩，其收缩为20％，因此，再经过131℃的热定型和扩幅，达到原始MD方向上的拉伸倍率和TD方向上的拉伸倍率之比，最终得到纤维结构取向均匀的聚烯烃多孔膜。
实施例4
采用超高分子量聚乙烯粉末(GUR2122，Ticona塞拉尼斯)5重量份，高密度聚乙烯粉末(VH200U，KPIC)10重量份，聚乙烯粉末(VH035，KPIC)5重量份，石蜡油(60#，浙江正信)80重量份，相对于该高分子量聚乙烯与石蜡油的合计100重量份，加入抗氧化剂(Irg1010(Ciba Specialty Chemicals巴斯夫))0.3重量份、抗氧化剂(P168(Ciba Specialty Chemicals株式会社制))0.1重量份，将它们按顺序进行混合。将混合助剂之后的超高分子量聚乙烯物料与石蜡油一起投入双螺杆挤出机进行混练，最终制成混合物熔体凝胶。
使用T型模头，将该凝胶挤出，挤出熔体温度控制在220℃以下，然后在表面温度为15℃的恒温金属对辊上快速冷却，并轧制成片材，片材厚度为1.5mm。
将片材即流延片牵引并进行拉伸。在117℃，以40m/min的速度进行快速拉伸，其中MD方向6.8倍率、TD方向6.8倍率的拉伸。拉伸时需控制温度场的均匀性，拉伸区域温度场不同点之间的温差小于0.3℃，拉伸后制得油膜。将油膜牵引入萃取，将油膜中的石蜡油萃取出。然后经过40℃的恒温热风将隔膜中残留的二氯甲烷带出，由于萃取及干燥过程中，TD方向在MD方向牵引的作用下发生了少量收缩，其收缩为21％，因此，再经过131℃的热定型和扩幅，达到原始MD方向上的拉伸倍率和TD方向上的拉伸倍率之比，最终得到纤维结构取向均匀的聚烯烃多孔膜。
实施例5
采用超高分子量聚乙烯粉末(GUR2122，Ticona塞拉尼斯)5重量份，高密度聚乙烯粉末(VH200U，KPIC)10重量份，聚乙烯粉末(VH035，KPIC)5重量份，石蜡油(60#，浙江正信)80重量份，相对于该高分子量聚乙烯与石蜡油的合计100重量份，加入抗氧化剂(Irg1010(Ciba Specialty Chemicals巴斯夫))0.3重量份、抗氧化剂(P168(Ciba Specialty Chemicals株式会社制))0.1重量份，将它们按顺序进行混合。将混合助剂之后的超高分子量聚乙烯物料与石蜡油一起投入双螺杆挤出机进行混练，最终制成混合物熔体凝胶。
使用T型模头，将该凝胶挤出，挤出熔体温度控制在220℃以下，然后在表面温度为15℃的恒温金属对辊上快速冷却，并轧制成片材，片材厚度为1.5mm。
将片材即流延片牵引并进行拉伸。在116℃，以40m/min的速度进行快速拉伸，其中MD方向8倍率、TD方向8倍率的拉伸。拉伸时需控制温度场的均匀性，拉伸区域温度场不同点之间的温差小于0.3℃，拉伸后制得油膜。将油膜牵引入萃取，将油膜中的石蜡油萃取出。然后经过40℃的恒温热风将隔膜中残留的二氯甲烷带出，由于萃取及干燥过程中，TD方向在MD方向牵引的作用下发生了少量收缩，其收缩为21％，因此，再经过131℃的热定型和扩幅，达到原始MD方向上的拉伸倍率和TD方向上的拉伸倍率之比，最终得到纤维结构取向均匀的聚烯烃多孔膜。
实施例6
采用超高分子量聚乙烯粉末(GUR2122，Ticona塞拉尼斯)5重量份，高密度聚乙烯粉末(VH200U，KPIC)10重量份，聚乙烯粉末(VH035，KPIC)5重量份，石蜡油(60#，浙江正信)80重量份，相对于该高分子量聚乙烯与石蜡油的合计100重量份，加入抗氧化剂(Irg1010(Ciba Specialty Chemicals巴斯夫))0.3重量份、抗氧化剂(P168(Ciba Specialty Chemicals株式会社制))0.1重量份，将它们按顺序进行混合。将混合助剂之后的超高分子量聚乙烯物料与石蜡油一起投入双螺杆挤出机进行混练，最终制成混合物熔体凝胶。
使用T型模头，将该凝胶挤出，挤出熔体温度控制在220℃以下，然后在表面温度为15℃的恒温金属对辊上快速冷却，并轧制成片材，片材厚度为1.5mm。
将片材即流延片牵引并进行拉伸。在115℃，以40m/min的速度进行快速拉伸，其中MD方向10倍率、TD方向10倍率的拉伸。拉伸时需控制温度场的均匀性，拉伸区域温度场不同点之间的温差小于0.3℃，拉伸后制得油膜。将油膜牵引入萃取，将油膜中的石蜡油萃取出。然后经过40℃的恒温热风将隔膜中残留的二氯甲烷带出，由于萃取及干燥过程中，TD方向在MD方向牵引的作用 下发生了少量收缩，其收缩为21％，因此，再经过131℃的热定型和扩幅，达到原始MD方向上的拉伸倍率和TD方向上的拉伸倍率之比，最终得到纤维结构取向均匀的聚烯烃多孔膜。
实施例7
采用超高分子量聚乙烯粉末(GUR2122，Ticona塞拉尼斯)5重量份，高密度聚乙烯粉末(VH200U，KPIC)10重量份，聚乙烯粉末(VH035，KPIC)5重量份，石蜡油(60#，浙江正信)80重量份，相对于该高分子量聚乙烯与石蜡油的合计100重量份，加入抗氧化剂(Irg1010(Ciba Specialty Chemicals巴斯夫))0.3重量份、抗氧化剂(P168(Ciba Specialty Chemicals株式会社制))0.1重量份，将它们按顺序进行混合。将混合助剂之后的超高分子量聚乙烯物料与石蜡油一起投入双螺杆挤出机进行混练，最终制成混合物熔体凝胶。
使用T型模头，将该凝胶挤出，挤出熔体温度控制在220℃以下，然后在表面温度为15℃的恒温金属对辊上快速冷却，并轧制成片材，片材厚度为1.5mm。
将片材即流延片牵引并进行拉伸。在115℃，以40m/min的速度进行快速拉伸，其中MD方向10倍率、TD方向11倍率的拉伸。拉伸时需控制温度场的均匀性，拉伸区域温度场不同点之间的温差小于0.3℃，拉伸后制得油膜。将油膜牵引入萃取，将油膜中的石蜡油萃取出。然后经过40℃的恒温热风将隔膜中残留的二氯甲烷带出，由于萃取及干燥过程中，TD方向在MD方向牵引的作用下发生了少量收缩，其收缩为21％，因此，再经过131℃的热定型和扩幅，达到原始MD方向上的拉伸倍率和TD方向上的拉伸倍率之比，最终得到纤维结构取向均匀的聚烯烃多孔膜。
实施例8
采用超高分子量聚乙烯粉末(GUR2122，Ticona塞拉尼斯)5重量份，高密度聚乙烯粉末(VH200U，KPIC)10重量份，聚乙烯粉末(VH035，KPIC)5重量份，石蜡油(60#，浙江正信)80重量份，相对于该高分子量聚乙烯与石蜡油的合计100重量份，加入抗氧化剂(Irg1010(Ciba Specialty Chemicals巴斯夫))0.3重量份、抗氧化剂(P168(Ciba Specialty Chemicals株式会社制))0.1重量份，将它们按顺序进行混合。将混合助剂之后的超高分子量聚乙烯物料与石蜡油一起投入双螺杆挤出机进行混练，最终制成混合物熔体凝胶。
使用T型模头，将该凝胶挤出，挤出熔体温度控制在220℃以下，然后在表面温度为15℃的恒温金属对辊上快速冷却，并轧制成片材，片材厚度为1.5mm。
将片材即流延片牵引并进行拉伸。在115℃，以40m/min的速度进行快速拉伸，其中MD方向10倍率、TD方向9.1倍率的拉伸。拉伸时需控制温度场的均匀性，拉伸区域温度场不同点之间的温差小于0.3℃，拉伸后制得油膜。将油膜牵引入萃取，将油膜中的石蜡油萃取出。然后经过40℃的恒温热风将隔膜中残留的二氯甲烷带出，由于萃取及干燥过程中，TD方向在MD方向牵引的作用下发生了少量收缩，其收缩为21％，因此，再经过131℃的热定型和扩幅，达到原始MD方向上的拉伸倍率和TD方向上的拉伸倍率之比，最终得到纤维结构取向均匀的聚烯烃多孔膜。
实施例9
采用超高分子量聚乙烯粉末(VH200U，LPIC)15重量份，高密度聚乙烯粉末(VH035，KPIC)5重量份，石蜡油(60#，浙江正信)80重量份，相对于该高分子量聚乙烯与石蜡油的合计100重量份，加入抗氧化剂(Irg1010(Ciba Specialty Chemicals巴斯夫))0.3重量份、抗氧化剂(P168(Ciba Specialty Chemicals株式会社制))0.1重量份，将它们按顺序进行混合。将混合助剂之后的超高分子量聚乙烯物料与石蜡油一起投入双螺杆挤出机进行混练，最终制成混合物熔体凝胶。
使用T型模头，将该凝胶挤出，挤出熔体温度控制在220℃以下，然后在表面温度为15℃的恒温金属对辊上快速冷却，并轧制成片材，片材厚度为1.5mm。
将片材即流延片牵引并分步骤进行拉伸。在90℃条件下，将流延片材沿着MD方向进行拉伸，其中MD方向拉伸倍率为9倍；然后在119℃，以60m/min的速度进行TD方向的快速拉伸，TD方向拉伸倍率为8.3倍率(其出口幅宽与原始铸片幅宽之比为8.2倍)。拉伸时需控制温度场的均匀性，拉伸区域温度场不同点之间的温差小于0.3℃。完成拉伸后制得油膜。将油膜牵引入萃取，将油膜中的石蜡油萃取出。然后经过40℃的恒温热风将隔膜中残留的二氯甲烷带出，由于萃取及干燥过程中，TD方向在MD方向牵引的作用下发生了少量收缩，其收缩为22％，因此，再经过131℃的热定型和扩幅，达到原始MD方向上的拉伸倍率和TD方向上的拉伸倍率之比，最终得到纤维结构取向均匀的聚烯烃多孔膜。
对比例1
采用超高分子量聚乙烯粉末(GUR4116，Ticona塞拉尼斯)25重量份，石蜡油(60#，浙江正信)77重量份，相对于该高分子量聚乙烯与石蜡油的合计100重量份，加入抗氧化剂(Irg1010(Ciba Specialty Chemicals巴斯夫))0.3重量份、抗氧化剂(P168(Ciba Specialty Chemicals株式会社制))0.1重量份，将它们按顺序进行混合。将混合助剂之后的超高分子量聚乙烯物料与石蜡油一起投入双螺杆挤出机进行混练，最终制成混合物熔体凝胶。
使用T型模头，将该凝胶挤出，挤出熔体温度控制在220℃以下，然后在表面温度为15℃的恒温金属对辊上快速冷却，并轧制成片材，片材厚度为1.5mm。
将片材即流延片牵引并进行拉伸。在117℃，以40m/min的速度进行快速拉伸，其中MD方向7.9倍率、TD方向进行6.5倍率的拉伸。拉伸时需控制温度场的均匀性，拉伸区域温度场不同点之间的温差小于0.3℃，拉伸后制得油膜。将油膜牵引入萃取，将油膜中的石蜡油萃取出。然后经过40℃的恒温热风将隔膜中残留的二氯甲烷带出，由于萃取及干燥过程中，TD方向在MD牵引的作用发生了少量收缩，其收缩为18％，因此，再经过131℃的热定型和扩幅，达到原始MD方向上的拉伸倍率和TD方向上的拉伸倍率之比，最终得到聚烯烃多孔膜。
对比例2
采用超高分子量聚乙烯粉末(GUR2122，Ticona塞拉尼斯)5重量份，高密度聚乙烯粉末(VH200U，KPIC)10重量份，聚乙烯粉末(VH035，KPIC)5重量份，石蜡油(60#，浙江正信)80重量份，相对于该高分子量聚乙烯与石蜡油的合计100重量份，加入抗氧化剂(Irg1010(Ciba Specialty Chemicals巴斯夫))0.3重量份、抗氧化剂(P168(Ciba Specialty Chemicals株式会社制))0.1重 量份，将它们按顺序进行混合。将混合助剂之后的超高分子量聚乙烯物料与石蜡油一起投入双螺杆挤出机进行混练，最终制成混合物熔体凝胶。
使用T型模头，将该凝胶挤出，挤出熔体温度控制在220℃以下，然后在表面温度为15℃的恒温金属对辊上快速冷却，并轧制成片材，片材厚度为1.5mm。
将片材即流延片牵引并进行拉伸。在116℃，以40m/min的速度进行快速拉伸，其中MD方向7.1倍率、TD方向5.4倍率的拉伸。拉伸时需控制温度场的均匀性，拉伸区域温度场不同点之间的温差小于0.3℃，拉伸后制得油膜。将油膜牵引入萃取，将油膜中的石蜡油萃取出。然后经过40℃的恒温热风将隔膜中残留的二氯甲烷带出，由于萃取及干燥过程中，TD方向在MD方向牵引的作用下发生了少量收缩，其收缩为21％，因此，再经过131℃的热定型和扩幅，达到原始MD方向上的拉伸倍率和TD方向上的拉伸倍率之比，最终得到聚烯烃多孔膜。
<评价方法>
(1)膜厚测试
采用万分尺测试实施例所获得的聚烯烃多孔膜的厚度。
(2)纤维取向度测试
采用美国FEI公司Nova Nano SEM 450型场发射扫描电子显微镜，获得聚烯烃多孔膜的2万倍SEM图像，以1000nm为半径的圆圈作为观察区域，进行观察。
按照上述式1和式2，获取聚烯烃多孔膜在MD方向上纤维的取向结果R_MD以及在TD方向上纤维的取向结果R_TD。
需要注意的是，在计算R_MD以及R_TD时，所取的纤维是聚烯烃多孔膜最外侧表面的纤维，N为观察区域内最外侧表面上纤维的数量，各纤维以n(n为自然数)标识，所取的纤维长度是指同一纤维上具有同一延伸方向的相同粗细段为一个长度，当同一纤维上粗细发生变化，则不算一个长度，需及时较差，所述的相同粗细段是指该段内最粗部分宽度相比于最细部分宽度的增幅在5％以内，纤维的直径是指相同粗细段上最宽部分宽度与最细部分宽度的平均值，纤维与MD方向或TD方向的角度为该相同粗细段的延伸方向与MD方向或TD方向的角度。
(3)拉伸强度测试
按GB 1040.3-2006的要求进行测试，采用切割法制备试样，试样的类型为2型试样。试样采用长200mm、宽25mm的长条形，夹具间距离为(100±5)mm，试验速度为(250±10)mm/min。
(4)断裂伸长率测试
按GB 1040.3-2006的要求进行测试，采用切割法制备试样，试样的类型为2型试样。试样采用长200mm、宽25mm的长条形，夹具间距离为(100±5)mm，试验速度为(250±10)mm/min。
(5)热收缩率测试
按GB/T12027-2004的要求进行测试，从聚烯烃微多孔隔离膜上取下100mm×100mm的样品，放入鼓风烘箱中105℃静止1小时，可由下式求得热收缩率。
MD热收缩率(％)＝|(加热前MD长度-加热后MD长度)÷加热前MD长度|×100
TD热收缩率(％)＝|(加热前TD长度-加热后TD长度)÷加热前TD长度|×100
参见表1-2，以上实施例及对比例性能测试结果如下：
表1：实施例1-9的聚烯烃多孔膜的相关性能参数指标
表2：对比例1-2的聚烯烃多孔膜的相关性能参数指标

表1和表2分别为实施例1-9、对比例1-2的聚烯烃多孔膜的相关性能指标，由此可见，当聚烯烃多孔膜中MD方向上纤维的取向结果R_MD满足(R_MD)2＜0.8，TD方向上纤维的取向结果R_TD满足(R_TD)2＜0.8，便能够使隔膜获取优异的力学性能的各向均一性，进而可在电池应用中具有更高安全性。
以上所述仅为本公开的可选的实施例而已，并不用于限制本公开，对于本领域的技术人员来说，本公开可以有各种更改和变化。凡在本公开的精神和原则之内，所作的任何修改、等同替换、改进等，均应包含在本公开的保护范围之内。

Claims (13)

  1. 一种聚烯烃多孔膜，其特征在于，具有网络纤维结构；
    对其2万倍SEM图像进行观察，所述聚烯烃多孔膜具有如下特性：
    (1)以1000nm为半径的圆圈内，纤维直径＞15nm的纤维在MD方向上的纤维取向R_MD满足：(R_MD)2＜0.8，R_MD由以下式1得出：
    其中，θn为纤维与MD方向的夹角，θn由MD方向向左为起始，逆时针方向得到，以μm为单位的纤维长度数值Ln与cosθn的乘积为Lncosθn，为以nm为单位的纤维直径数值；
    (2)以1000nm为半径的圆圈内，纤维直径＞15nm的纤维在TD方向上的纤维取向R_TD满足：(R_TD)2＜0.8，R_TD由以下式2得出：
    其中，βn为纤维与TD方向的夹角，βn由TD方向向上为起始，逆时针方向得到，以μm为单位的纤维长度数值Ln与cosβn的乘积为Lncosβn，为以nm为单位的纤维直径数值。
  2. 根据权利要求1所述一种聚烯烃多孔膜，其特征在于，具有网络纤维结构；
    对其2万倍SEM图像进行观察，所述聚烯烃多孔膜具有如下特性：
    (1)以1000nm为半径的圆圈内，纤维直径＞15nm的纤维在MD方向上的纤维取向R_MD满足：(R_MD)2＜0.5，R_MD由以下式1得出：
    其中，θn为纤维与MD方向的夹角，θn由MD方向向左为起始，逆时针方向得到，以μm为单位的纤维长度数值Ln与cosθn的乘积为Lncosθn，为以nm为单位的纤维直径数值；
    (2)以1000nm为半径的圆圈内，纤维直径＞15nm的纤维在TD方向上的纤维取向R_TD满足：(R_TD)2＜0.5，R_TD由以下式2得出：
    其中，βn为纤维与TD方向的夹角，βn由TD方向向上为起始，逆时针方向得到，以μm为单位的纤维长度数值Ln与cosβn的乘积为Lncosβn，为以nm为单位的纤维直径数值。
  3. 根据权利要求1或2中任一项所述的聚烯烃多孔膜，其特征在于，MD方向上的拉伸强度与TD方向上的拉伸强度之比即MD/TD拉伸强度比为：
    0.8≤MD/TD拉伸强度比≤1.2。
  4. 根据权利要求1或2中任一项所述的聚烯烃多孔膜，其特征在于，MD方向上的断裂伸长率与TD方向上的断裂伸长率之比即MD/TD断裂伸长率比为：
    0.75≤MD/TD断裂伸长率比≤1.34。
  5. 根据权利要求1或2中任一项所述的聚烯烃多孔膜，其特征在于，所述聚烯烃多孔膜的膜厚为1～30μm。
  6. 一种聚烯烃多孔膜的制备方法，其特征在于，所述聚烯烃多孔膜具有网络纤维结构；对其2万倍SEM图像进行观察，所述聚烯烃多孔膜具有如下特性：
    (1)以1000nm为半径的圆圈内，纤维直径＞15nm的纤维在MD方向上的纤维取向R_MD满足：(R_MD)2＜0.8，R_MD由以下式1得出：
    其中，θn为纤维与MD方向的夹角，θn由MD方向向左为起始，逆时针方向得到，以μm为单位的纤维长度数值Ln与cosθn的乘积为Lncosθn，为以nm为单位的纤维直径数值；
    (2)以1000nm为半径的圆圈内，纤维直径＞15nm的纤维在TD方向上的纤维取向R_TD满足：(R_TD)2＜0.8，R_TD由以下式2得出：
    其中，βn为纤维与TD方向的夹角，βn由TD方向向上为起始，逆时针方向得到，以μm为单位的纤维长度数值Ln与cosβn的乘积为Lncosβn，为以nm为单位的纤维直径数值；
    所述制备方法包括如下步骤：
    步骤一、将聚烯烃、致孔剂与添加剂混练挤出，制备流延片；
    步骤二、将流延片进行拉伸，拉伸过程中流延片材自身的温度不超过140℃，且拉伸区域温度场不同点之间的温差小于0.5℃，且MD方向上的拉伸倍率与TD方向上的拉伸倍率之比即MD/TD拉伸倍率比为：0.9≤MD/TD拉伸倍率比≤1.1，制得油膜；
    步骤三、将油膜萃取，然后进行热定型处理，热定型处理过程中通过扩幅拉伸处理维持步骤二中MD方向上的拉伸倍率与TD方向上的拉伸倍率之比。
  7. 根据权利要求6所述一种聚烯烃多孔膜的制备方法，一种聚烯烃多孔膜的制备方法，其特征在于，所述聚烯烃多孔膜具有网络纤维结构；对其2万倍SEM图像进行观察，所述聚烯烃多孔膜具有如下特性：
    (1)以1000nm为半径的圆圈内，纤维直径＞15nm的纤维在MD方向上的纤维取向R_MD满足：(R_MD)2＜0.5，R_MD由以下式1得出：
    其中，θn为纤维与MD方向的夹角，θn由MD方向向左为起始，逆时针方向得到，以μm为单位的纤维长度数值Ln与cosθn的乘积为Lncosθn，为以nm为单位的纤维直径数值；
    (2)以1000nm为半径的圆圈内，纤维直径＞15nm的纤维在TD方向上的纤维取向R_TD满足：(R_TD)2＜0.5，R_TD由以下式2得出：
    其中，βn为纤维与TD方向的夹角，βn由TD方向向上为起始，逆时针方向得到，以μm为单位的纤维长度数值Ln与cosβn的乘积为Lncosβn，为以nm为单位的纤维直径数值；
    所述制备方法包括如下步骤：
    步骤一、将聚烯烃、致孔剂与添加剂混练挤出，制备流延片；
    步骤二、将流延片进行拉伸，拉伸过程中流延片材自身的温度不超过140℃，且拉伸区域温度场不同点之间的温差小于0.5℃，且MD方向上的拉伸倍率与TD方向上的拉伸倍率之比即MD/TD拉伸倍率比为：0.9≤MD/TD拉伸倍率比≤1.1，制得油膜；
    步骤三、将油膜萃取，然后进行热定型处理，热定型处理过程中通过扩幅拉伸处理维持步骤二中MD方向上的拉伸倍率与TD方向上的拉伸倍率之比。
  8. 根据权利要求6或7中任一项所述的聚烯烃多孔膜的制备方法，其特征在于，所述步骤二中，MD方向上的拉伸倍率和TD方向上的拉伸倍率均为5倍以上。
  9. 根据权利要求6或7中任一项所述的聚烯烃多孔膜的制备方法，其特征在于，所述步骤一中，所述聚烯烃在混炼物中的质量占比≥15％，所述聚烯烃的粘均分子量介于20万～500万之间。
  10. 根据权利要求6或7中任一项所述的聚烯烃多孔膜的制备方法，其特征在于，所述添加剂包含抗氧化剂，所述抗氧化剂在混炼物中质量占比为0～0.5％，所述步骤一中抗氧化剂为胺类、含硫化合物、含氮化合物、含磷化合物、有机金属盐中的一种或几种。
  11. 根据权利要求6或7中任一项所述的聚烯烃多孔膜的制备方法，其特征在于，所述致孔剂为白油、石蜡油、聚乙二醇中的一种或几种。
  12. 一种电池隔膜，其特征在于，包含权利要求1～5中任一项所述的聚烯烃多孔膜。
  13. 一种电化学装置，其特征在于，包含权利要求1～5中任一项所述的聚烯烃多孔膜或权利要求12所述的电池隔膜作为将正负两极分开的元件。
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