CN113809474B - Polypropylene diaphragm, preparation method thereof and lithium ion battery - Google Patents

Polypropylene diaphragm, preparation method thereof and lithium ion battery Download PDF

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Publication number
CN113809474B
CN113809474B CN202010484271.4A CN202010484271A CN113809474B CN 113809474 B CN113809474 B CN 113809474B CN 202010484271 A CN202010484271 A CN 202010484271A CN 113809474 B CN113809474 B CN 113809474B
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polypropylene
layer
surface layer
polypropylene separator
raw materials
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CN113809474A (en
Inventor
田如辉
秦银银
颜鸿彬
张佳宁
刘孟权
杨雪梅
陈秀峰
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Shenzhen Senior Technology Material Co Ltd
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Shenzhen Senior Technology Material Co Ltd
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B3/00—Layered products comprising a layer with external or internal discontinuities or unevennesses, or a layer of non-planar shape; Layered products comprising a layer having particular features of form
    • B32B3/26—Layered products comprising a layer with external or internal discontinuities or unevennesses, or a layer of non-planar shape; Layered products comprising a layer having particular features of form characterised by a particular shape of the outline of the cross-section of a continuous layer; characterised by a layer with cavities or internal voids ; characterised by an apertured layer
    • B32B3/266—Layered products comprising a layer with external or internal discontinuities or unevennesses, or a layer of non-planar shape; Layered products comprising a layer having particular features of form characterised by a particular shape of the outline of the cross-section of a continuous layer; characterised by a layer with cavities or internal voids ; characterised by an apertured layer characterised by an apertured layer, the apertures going through the whole thickness of the layer, e.g. expanded metal, perforated layer, slit layer regular cells B32B3/12
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B27/00—Layered products comprising a layer of synthetic resin
    • B32B27/06—Layered products comprising a layer of synthetic resin as the main or only constituent of a layer, which is next to another layer of the same or of a different material
    • B32B27/08—Layered products comprising a layer of synthetic resin as the main or only constituent of a layer, which is next to another layer of the same or of a different material of synthetic resin
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B27/00—Layered products comprising a layer of synthetic resin
    • B32B27/32—Layered products comprising a layer of synthetic resin comprising polyolefins
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B33/00—Layered products characterised by particular properties or particular surface features, e.g. particular surface coatings; Layered products designed for particular purposes not covered by another single class
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/052—Li-accumulators
    • H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/058—Construction or manufacture
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B2250/00—Layers arrangement
    • B32B2250/03—3 layers
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B2250/00—Layers arrangement
    • B32B2250/24—All layers being polymeric
    • B32B2250/242—All polymers belonging to those covered by group B32B27/32
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B2307/00—Properties of the layers or laminate
    • B32B2307/20—Properties of the layers or laminate having particular electrical or magnetic properties, e.g. piezoelectric
    • B32B2307/204—Di-electric
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B2307/00—Properties of the layers or laminate
    • B32B2307/50—Properties of the layers or laminate having particular mechanical properties
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B2457/00—Electrical equipment
    • B32B2457/10—Batteries
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10—Energy storage using batteries
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
    • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Manufacturing & Machinery (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Electrochemistry (AREA)
  • General Chemical & Material Sciences (AREA)
  • Materials Engineering (AREA)
  • Cell Separators (AREA)
  • Laminated Bodies (AREA)

Abstract

The invention relates to a polypropylene diaphragm, a preparation method thereof and a lithium ion battery. The polypropylene diaphragm comprises a first surface layer, a middle layer and a second surface layer which are sequentially laminated, wherein the raw materials of the first surface layer and the second surface layer comprise melt flow index of 6.0g/10 min-12.0 g/10min and weight average molecular weight of 2.0x10 5 ～3.5×10 5 The raw materials of the middle layer comprise polypropylene with melt flow index of 0.3g/10 min-1.0 g/10min and weight average molecular weight of 3.5X10 5 ～6.0×10 5 Is a polypropylene of (3). According to the polypropylene diaphragm, through selection of the polypropylene raw materials and design of the film layer, the polypropylene diaphragm has improved mechanical strength, ion migration performance and electrolyte wettability, and the matching degree of the polypropylene diaphragm to the electrical performance is high.

Description

Polypropylene diaphragm, preparation method thereof and lithium ion battery
Technical Field
The invention relates to the field of diaphragms, in particular to a polypropylene diaphragm, a preparation method thereof and a lithium ion battery.
Background
The lithium ion battery is widely applied to industries such as new energy automobiles, energy storage power stations, electric tools, military industry and the like as a new energy source capable of being applied to mass production at present, and the lithium ion battery diaphragm is one of four main materials of the lithium ion battery, and has been very mature after development for decades. However, along with the requirement of increasing the energy density of the lithium ion battery, the thickness of the diaphragm is required to be thinner and thinner from 32 micrometers to 25 micrometers and 20 micrometers, and is 16 micrometers and 12 micrometers which are commonly used at present, and the improvement of the energy density of the battery by thinning the diaphragm is a future trend under the premise of ensuring the safety performance of the battery.
For dry separator, mainly unidirectional stretching product, the mechanical strength is generally weak, and the separator is generally required to be improved in the thinning process. Although the traditional method can improve the mechanical strength of the diaphragm, the ion migration performance and the electrolyte wettability of the diaphragm are reduced, so that the degree of matching of the electrical performance is obviously reduced.
Disclosure of Invention
Based on this, it is necessary to provide a polypropylene separator which can be prepared by a dry process while having improved mechanical strength, ion migration performance and electrolyte wettability, so that the degree of matching to electrical properties is high.
In addition, a preparation method of the polypropylene diaphragm and a lithium ion battery are also provided.
A polypropylene diaphragm comprises a first surface layer, a middle layer and a second surface layer which are sequentially laminated, wherein the raw materials of the first surface layer and the second surface layer comprise melt flow index of 6.0g/10 min-12.0 g/10min and weight average molecular weight of 2.0x10 5 ～3.5×10 5 The raw materials of the intermediate layer comprise polypropylene with melt flow index of 0.3g/10 min-1.0 g/10min and weight average molecular weight of 3.5X10 5 ～6.0×10 5 Is a polypropylene of (3).
In one embodiment, the average pore size of the first surface layer and the second surface layer is 40nm to 80nm.
In one embodiment, the polypropylene separator has a pore tortuosity of 2.0 to 4.5.
In one embodiment, the polypropylene separator comprises 20-70% of the raw materials of the intermediate layer by mass.
In one embodiment, the mass percentage of the raw material of the intermediate layer is 20% -60%.
In one embodiment, the polypropylene separator has a total thickness of 5 μm to 25 μm.
In one embodiment, the polypropylene separator has a porosity of 20% to 70%.
A method for preparing a polypropylene separator, comprising the steps of:
respectively carrying out melt plasticization on raw materials of the first surface layer, the middle layer and the second surface layer, and then sequentially laminating and co-extruding to form a three-layer fluid, wherein the raw materials of the first surface layer and the second surface layer comprise a melt flow index of 6.0g/10 min-12.0 g/10min and a weight average molecular weight of 2.0x10 5 ～3.5×10 5 The raw materials of the intermediate layer comprise polypropylene with melt flow index of 0.3g/10 min-1.0 g/10min and weight average molecular weight of 3.5X10 5 ～6.0×10 5 Polypropylene of (a);
drawing and cooling the three layers of fluid to obtain three layers of nonporous precursor films;
annealing the three-layer nonporous precursor film; a kind of electronic device with high-pressure air-conditioning system
And stretching and pore-forming the three-layer nonporous precursor film after the annealing treatment to obtain the polypropylene diaphragm.
In one embodiment, the traction speed is 60m/min to 150m/min, and the cooling temperature is 40 ℃ to 100 ℃.
In one embodiment, the temperature of the annealing treatment is 120-165 ℃, and the time of the annealing treatment is 0.1-24 h.
In one embodiment, the temperature of the stretching pore-forming is 100-150 ℃, the speed of the stretching pore-forming is 3-15 m/min, and the total multiplying power of the stretching pore-forming is 1.5-3.0 times.
A lithium ion battery comprises an anode, a cathode, electrolyte and a diaphragm, wherein the diaphragm is the polypropylene diaphragm or prepared by the preparation method of the polypropylene diaphragm.
According to the polypropylene diaphragm, through selection of the polypropylene raw materials and design of the diaphragm layer, the first surface layer and the second surface layer of the polypropylene diaphragm have larger pore structures, so that migration capacity of ions in electrolyte can be improved, the pore diameter of the middle layer is smaller, wettability of the electrolyte and the diaphragm can be improved, and meanwhile, mechanical strength is improved. Therefore, the first surface layer, the middle layer and the second surface layer are matched with each other, so that the polypropylene diaphragm has improved mechanical strength, ion migration performance and electrolyte wettability, and the matching degree of the electrical performance is higher.
Drawings
FIG. 1 is a process flow diagram of a method of making a polypropylene separator according to one embodiment;
FIG. 2 is a cross-sectional SEM image of a polypropylene separator membrane prepared according to example 1.
Detailed Description
In order that the invention may be readily understood, a more complete description of the invention will be rendered by reference to specific embodiments that are now described. Preferred embodiments of the invention are given in the detailed description. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete.
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.
The polypropylene separator according to one embodiment comprises a first surface layer, an intermediate layer and a second surface layer which are laminated in this order. The polypropylene diaphragm is a three-layer diaphragm with a symmetrical structure.
Wherein the melt flow index of the polypropylene raw material of the first surface layer and the second surface layer is 6.0g/10 min-12.0 g/10min, and the weight average molecular weight is 2.0x10 5 ～3.5×10 5 . In one embodiment, the polypropylene material of the first skin layer and the second skin layer has a melt flow index of 6g/10min, 7g/10min, 8g/10min, 9g/10min, 10g/10min, 11g/10min or 12g/10min. The polypropylene raw material of the first surface layer and the second surface layer has a weight average molecular weight of 2.0X10 5 、2.2×10 5 、2.5×10 5 、2.8×10 5 、3.0×10 5 、3.2×10 5 Or 3.5X10 5 。
The melt flow index of the polypropylene raw material of the middle layer is 0.3g/10 min-1.0 g/10min, and the weight average molecular weight is 3.5X10 5 ～6.0×10 5 . In one embodiment, the polypropylene of the middle layer has a melt flow index of 0.3g/10min, 0.4g/10min, 0.5g/10min, 0.6g/10min, 0.7g/10min, 0.8g/10min, 0.9g/10min, or 1.0g/10min. The polypropylene of the middle layer had a weight average molecular weight of 3.5X10 5 、3.8×10 5 、4.0×10 5 、4.2×10 5 、4.5×10 5 、5.0×10 5 、5.2×10 5 、5.5×10 5 、5.8×10 5 Or 6.0X10 5 。
In the embodiment, the melt flow index of the polypropylene raw material used for the first surface layer and the second surface layer is higher, so that the processing difficulty is reduced, and the production efficiency is improved. And the raw materials of the first surface layer, the middle layer and the second surface layer are selected, so that the defect of poor mechanical strength caused by an outer layer macroporous structure is avoided, the polypropylene diaphragm has improved mechanical strength, ion migration performance and electrolyte wettability, and the matching degree of the diaphragm to the electrical performance is improved.
Preferably, the first skin layer and the second skin layer have the same composition. The polypropylene of the first surface layer and the polypropylene of the second surface layer have the same composition, so that the performance is similar, and the obtained membrane layer has similar structure under the same process condition, so that the polypropylene membrane does not need to be distinguished between an inner layer and an outer layer when in use, is more convenient to use, and can avoid the safety risk caused by the internal and external layer misplacement.
The average pore diameters of the first surface layer and the second surface layer are the same and are larger than the average pore diameter of the middle layer. Specifically, the average pore diameter of the first surface layer and the second surface layer ranges from 40nm to 80nm. The average pore diameter of the intermediate layer is 20 nm-35 nm. The average pore diameters of the first surface layer and the second surface layer are in the above range, and have a large pore structure, and can improve the ion mobility and electrolyte wettability. And the pore diameter of the middle layer is smaller, so that the mechanical strength of the separator can be improved, and the self-discharge property of the battery can be improved.
Specifically, the polypropylene separator has a pore curvature of 2.0 to 4.5. Preferably, the polypropylene separator has a pore curvature of 2.5 to 4.0. The inventors have found through a large number of experiments that too large a pore curvature can reduce the migration ability of ions, while too small a pore curvature can affect the wettability of the electrolyte, especially the liquid retention. The curvature of the holes can enable the polypropylene diaphragm to have good electrolyte wettability and ion migration capability.
Specifically, the polypropylene raw material of the middle layer is 20-70% by mass of the polypropylene raw material of the polypropylene diaphragm. Further, the mass percentage of the polypropylene raw material of the middle layer is 20% -60%. Further, the mass percentage of the polypropylene raw material of the middle layer is 20-50%. In one embodiment, the intermediate layer comprises 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, or 70% by mass of the raw material. By controlling the ratio of the polypropylene raw material of the middle layer, the mechanical strength and the surface resistance of the polypropylene diaphragm can reach better performance.
Further, the mass percentages of the raw materials of the first surface layer and the second surface layer are the same.
In the present embodiment, the number of layers of the polypropylene separator is not limited to three, but may be six, nine, or the like. When the polypropylene diaphragm is six layers, the first surface layer, the middle layer and the second surface layer which are sequentially laminated are taken as a film layer unit, and the two film layer units are laminated, so that the six-layer polypropylene diaphragm is obtained. When the polypropylene diaphragm is nine layers, the first surface layer, the middle layer and the second surface layer which are sequentially laminated are taken as a film layer unit, and the three film layer units are laminated, so that the nine-layer polypropylene diaphragm is obtained.
Specifically, the total thickness of the polypropylene separator is 5-25 μm. In one embodiment, the polypropylene separator has a total thickness of 5 μm, 8 μm, 10 μm, 12 μm, 15 μm, 16 μm, 20 μm, or 25 μm.
Experiments prove that the polypropylene diaphragm has the following properties: the Grignard value is 30sec/100 mL-800 sec/100mL, the porosity is 20% -70%, and the longitudinal tensile strength is more than 1700kgf/cm 2 The thermal shrinkage at 105 ℃ is less than or equal to 3 percent, and the puncture strength per unit thickness is more than or equal to 20 g/mu m. Further, the porosity of the polypropylene separator is 30% -60%.
An improved low electrical impedance microporous battery separator membrane is disclosed in one conventional technique, comprising: extruding polypropylene having a melt flow index of less than 1.0g/10 minutes to form a single layer nonporous precursor film, stretching the nonporous polypropylene precursor film longitudinally to form a semi-porous intermediate film having a puncture strength of >350gf and a TD elongation of >600%, stretching the semi-porous intermediate film transversely, stretching using a stretch ratio of 15% to 400%, preferably using a stretch ratio of 25% to 100%, to form a microporous membrane. When polypropylene with a melt flow index smaller than 1.0g/10min is adopted in the method, the production efficiency of the product is obviously affected to a certain extent through a process method of stretching step by step for multiple times.
In addition, a three-layer coextrusion dry PP-PE-PP diaphragm, specifically an A-B-C asymmetric structure, is disclosed in the technology, the raw material of the A film layer is melt index of 2.0-6.0 g/10min, and the weight average molecular weight is 2×10 5 ～3.5×10 5 The raw material of the film layer B is melt index of 0.2-1.0 g/10min, and weight average molecular weight is 1X 10 5 ～2.5×10 5 The raw material of the high-density polyethylene of the film layer C is melt index of 0.3-1.0 g/10min, and the weight average molecular weight is 4 multiplied by 10 5 ～6×10 5 When the product is used, the A film layer is used as an inner layer, the C film layer is used as an outer layer and faces the negative electrode, and the A film layer is used as an inner layer, so that the aperture is large and the ion migration is realizedThe capability is strong, the aperture of the C film layer used as an outer layer is small, the contact angle is small, and the wettability to electrolyte is good. Although the asymmetric structure improves the mechanical property defect of the macropores to a certain extent, when the product is used, the product is required to be distinguished between the inner layer polypropylene and the outer layer polypropylene, and the misplacement easily causes safety accidents. And the melt flow index of the raw materials adopted by the diaphragm is still low, so that the processing difficulty is increased.
Therefore, the conventional polypropylene separator cannot balance the mechanical strength, ion migration performance and electrolyte wettability of the separator with a thinner thickness, so that the application of the separator is limited.
In addition, compared with the polypropylene/polyethylene/polypropylene composite membrane, the polypropylene composite membrane of the embodiment has no composite polyethylene layer, because the polypropylene and the polyethylene have different properties, the compatibility in composite molding is poor, synchronous casting and synchronous stretching pore forming cannot be considered, and therefore, the technical defects that uniform composite and high-pore membrane is obtained by adding other additives are overcome, the technical difficulty is high, and the processing cost is high. And under the condition of the same melt flow index, the pore diameter of the polyethylene membrane is larger than that of the polypropylene membrane, and the self-discharge phenomenon of the battery is easy to cause. Therefore, in the embodiment, polypropylene with different melt flow indexes and molecular weights is used as a raw material, the problem of poor compatibility during product compounding is solved, the obtained polypropylene diaphragm has better compatibility, the process difficulty is low, and the aperture of the middle layer is smaller.
The polypropylene separator according to the present embodiment has at least the following advantages:
(1) The polypropylene diaphragm is formed into the dry multilayer co-extrusion polypropylene diaphragm with improved mechanical strength, ion migration performance and electrolyte wettability by selecting polypropylene materials and adopting the structural design of the polypropylene diaphragm and matching polypropylene raw materials with different melt indexes.
(2) The first surface layer and the second surface layer of the polypropylene diaphragm are macroporous structures with symmetrical structures, and the polypropylene diaphragm does not need to be distinguished between an inner layer and an outer layer when in use, so that the polypropylene diaphragm is simple and convenient, and the safety risk of a product is avoided.
(3) The polypropylene diaphragm uses the polypropylene raw materials with different melt indexes in a matching way, so that the raw material consumption of the low melt flow index is less, the use of the polypropylene material with high melt flow index further reduces the processing difficulty of the product, and the hole structure of the composite product is ensured when the polypropylene diaphragm is used in a matching way, so that the product can meet the performance requirement.
Referring to fig. 1, a method for preparing a polypropylene separator according to an embodiment includes the following steps:
step S110: and respectively carrying out melting plasticization on the raw materials of the first surface layer, the middle layer and the second surface layer, and then sequentially laminating and coextruding to form the three-layer fluid.
Wherein the three-layer fluid has a structure of a first surface layer, a middle layer and a second surface layer.
Specifically, the raw materials of the first surface layer and the second surface layer comprise materials with a melt flow index of 6.0g/10 min-12.0 g/10min and a weight average molecular weight of 2.0X10 5 ～3.5×10 5 The raw materials of the middle layer comprise polypropylene with melt flow index of 0.3g/10 min-1.0 g/10min and weight average molecular weight of 3.5X10 5 ～6.0×10 5 Is a polypropylene of (3).
In step S110, the raw materials of the surface layer and the intermediate layer are melt-plasticized in different extruders. The raw materials on the surface layer are uniformly distributed into two different flow channels through the sub-flow channels. The first skin-middle layer-second skin structure may be formed by a dispenser that merges with the middle layer stock material within the die cavity. Or two layers of fluids separated from the surface layer before extrusion of the die head through a cavity-dividing die head are combined with the raw material fluid of the middle layer to form a first surface layer-middle layer-second surface layer structure.
Specifically, the processing temperature of the raw materials of the first surface layer and the second surface layer is 170 ℃ to 250 ℃. The processing temperature of the raw materials of the middle layer is 170-250 ℃.
Step S120: the three layers of fluid are drawn and cooled to produce three layers of nonporous precursor film.
Wherein the traction speed is 60-150 m/min, and the cooling temperature is 40-100 ℃. Specifically, the three-layer fluid is drawn and cooled by a chill roll.
Specifically, in step S120, the three-layer fluid may be drawn and cooled by an extrusion blow molding method or a casting method. The extrusion blow molding and casting methods may be methods commonly used in the art and are not described in detail herein.
Step S130: the three-layer nonporous precursor film is annealed.
Wherein the temperature of the annealing treatment is 120-165 ℃, and the time of the annealing treatment is 0.1-24 h.
Step S140: and stretching and pore-forming the annealed three-layer nonporous precursor film to obtain the polypropylene diaphragm.
Specifically, in step S140, the temperature of the stretching and pore-forming is 100 ℃ to 150 ℃. The speed of stretching and pore-forming is 3 m/min-15 m/min. The total multiplying power of stretching pore-forming is 1.5 times to 3.0 times.
The preparation method of the polypropylene diaphragm is characterized in that the polypropylene diaphragm is obtained through extrusion, cooling forming, annealing and stretching pore-forming treatment. The polypropylene separator has improved separator mechanical strength, ion migration performance and electrolyte wettability.
Specifically, the average pore size of the first surface layer and the second surface layer is the same and larger than the average pore size of the intermediate layer. Specifically, the average pore diameter of the first surface layer and the second surface layer ranges from 40nm to 80nm. The average pore diameter of the intermediate layer is 20 nm-35 nm. The average pore diameters of the first surface layer and the second surface layer are in the above range, and have a large pore structure, and the migration ability of ions in the electrolyte can be improved. The aperture of the middle layer is smaller, so that the wettability of the electrolyte and the diaphragm can be improved, and the mechanical strength of the diaphragm can be improved.
Specifically, the polypropylene separator has a pore curvature of 2.0 to 4.5. Preferably, the polypropylene separator has a pore curvature of 2.5 to 4.0. The inventors have found through a large number of experiments that too large a pore curvature reduces the mobility of ions, while too small a pore curvature affects the wettability of the electrolyte. The curvature of the holes can enable the polypropylene diaphragm to have good electrolyte wettability and ion migration capability.
Specifically, the polypropylene raw material of the middle layer is 20-70% by mass of the polypropylene raw material of the polypropylene diaphragm. Further, the mass percentage of the polypropylene raw material of the middle layer is 20% -60%. Further, the mass percentage of the polypropylene raw material of the middle layer is 20-50%. In one embodiment, the intermediate layer comprises 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, or 70% by mass of the raw material. By controlling the ratio of the polypropylene raw material of the middle layer, the mechanical strength and the surface resistance of the polypropylene diaphragm can reach better performance.
Further, the mass percentages of the raw materials of the first surface layer and the second surface layer are the same.
In the present embodiment, the number of layers of the polypropylene separator is not limited to three, but may be six, nine, or the like. When the polypropylene diaphragm is six layers, the first surface layer, the middle layer and the second surface layer which are sequentially laminated are taken as a film layer unit, and the two film layer units are laminated, so that the six-layer polypropylene diaphragm is obtained. When the polypropylene diaphragm is nine layers, the first surface layer, the middle layer and the second surface layer which are sequentially laminated are taken as a film layer unit, and the three film layer units are sequentially laminated, so that the nine-layer polypropylene diaphragm is obtained. The step of laminating the film units may be a step commonly used in the art.
Specifically, the total thickness of the polypropylene separator is 5-25 μm. In one embodiment, the polypropylene separator has a total thickness of 5 μm, 8 μm, 10 μm, 12 μm, 15 μm, 16 μm, 20 μm, or 25 μm.
Experiments prove that the polypropylene diaphragm has the following properties: the Grignard value is 30sec/100 mL-800 sec/100mL, the porosity is 20% -70%, and the longitudinal tensile strength is more than 1700kgf/cm 2 The thermal shrinkage at 105 ℃ is less than or equal to 3 percent, and the puncture strength per unit thickness is more than or equal to 20 g/mu m. Further, the porosity of the polypropylene separator is 30% -60%.
The polypropylene microporous membrane prepared by the method has the structure of the first surface layer, the middle layer and the second surface layer, so that the surface layer structure with large surface pore structure and the middle layer structure for improving the mechanical strength are prepared while the production efficiency is realized, the mechanical strength of the membrane is improved, meanwhile, the ion migration capacity in electrolyte is improved through the surface macropores, and the wettability of the electrolyte and the membrane is improved through the middle micropore structure, so that the polypropylene membrane has the improved mechanical strength, ion migration performance and electrolyte wetting performance.
The following is a detailed description of embodiments. The following examples are merely illustrative of the present invention and should not be construed as limiting the invention.
Example 1
The polypropylene separator of this example is prepared from the following raw materials: the first surface layer and the second surface layer have a melt flow index of 8.0g/10min and a weight average molecular weight of 3.0X10 5 The raw material of the middle layer is melt flow index 0.5g/10min and weight average molecular weight 5.0X10 5 Is a polypropylene of (3). The mass percentage of the raw materials of the middle layer is 50%, and the mass percentage of the raw materials of the first surface layer and the second surface layer is 25% and 25% respectively.
The preparation process of the polypropylene separator of the embodiment is specifically as follows:
(1) Extrusion: and respectively melting and plasticizing the raw materials of the first surface layer and the middle layer in different extruders, and converging two fluids in the die head through a three-cavity structure of the split runner and the die head to form a circulation structure of the first surface layer, the middle layer and the second surface layer. Wherein the extruder processing temperature of the raw material of the first skin layer is 200 ℃ and the extruder processing temperature of the raw material of the intermediate layer is 210 ℃.
(2) Forming a nonporous precursor film: the three-layer fluid of the first skin layer-middle layer-second skin layer flowing out of the die head was drawn and cooled by a cooling roll having a temperature of 80 c and a drawing speed of 100m/min, to obtain a three-layer nonporous precursor film.
(3) Annealing: and (3) annealing the three-layer nonporous precursor film, wherein the annealing temperature is 135 ℃ and the time is 12 hours.
(4) Stretching and pore-forming: and directly carrying out high-temperature stretching pore-forming on the annealed three-layer nonporous precursor film to obtain the polypropylene diaphragm. Wherein the high-temperature stretching temperature is 140 ℃, the stretching speed is 6m/min, and the total stretching multiplying power is 2.2 times.
Example 2
The polypropylene separator of this example is prepared from the following raw materials: the raw materials of the first surface layer and the second surface layer have melt flow index of 6.0g/10min and weight average molecular weight of 3.3X10 5 The raw material of the middle layer is melt flow index 0.8g/10min and weight average molecular weight 4.5X10 5 Is a polypropylene of (3). The mass percentage of the raw materials of the middle layer is 50%, and the mass percentage of the raw materials of the first surface layer and the second surface layer is 25% and 25% respectively.
The preparation process of the polypropylene separator of this embodiment is the same as that of the polypropylene separator of embodiment 1, and will not be described here again.
Example 3
The polypropylene separator of this example is prepared from the following raw materials: the first surface layer and the second surface layer have a melt flow index of 12.0g/10min and a weight average molecular weight of 2.0X10 5 Is a polypropylene of (3). The intermediate layer has a melt flow index of 0.3g/10min and a weight average molecular weight of 5.5X10 5 Is a polypropylene of (3). The mass percentage of the raw materials of the middle layer is 50%, and the mass percentage of the raw materials of the first surface layer and the second surface layer is 25% and 25% respectively.
The preparation process of the polypropylene separator of this embodiment is the same as that of the polypropylene separator of embodiment 1, and will not be described here again.
Example 4
The polypropylene separator of this example is prepared from the following raw materials: the raw materials of the first surface layer and the second surface layer have melt flow index of 6.0g/10min and weight average molecular weight of 3.3X10 5 Is a polypropylene of (3). The intermediate layer has a melt flow index of 0.8g/10min and a weight average molecular weight of 4.5X10 5 Is a polypropylene of (3). The mass percentage of the raw materials of the middle layer is 20%, and the mass percentage of the raw materials of the first surface layer and the second surface layer is 40% and 40% respectively.
The preparation process of the polypropylene separator of this embodiment is the same as that of the polypropylene separator of embodiment 1, and will not be described here again.
Example 5
The polypropylene separator of this example is prepared from the following raw materials: the raw materials of the first surface layer and the second surface layer are melt flow indexes6.0g/10min, weight average molecular weight 3.3X10 5 Is a polypropylene of (3). The intermediate layer has a melt flow index of 0.8g/10min and a weight average molecular weight of 4.5X10 5 Is a polypropylene of (3). The mass percentage of the raw materials of the middle layer is 35%, and the mass percentages of the raw materials of the first surface layer and the second surface layer are 32.5% and 32.5%, respectively.
The preparation process of the polypropylene separator of this embodiment is the same as that of the polypropylene separator of embodiment 1, and will not be described here again.
Comparative example 1
The polypropylene separator of comparative example 1 is specifically prepared from the following raw materials: the raw materials of the first surface layer, the second surface layer and the middle layer all adopt melt flow index of 2.0g/10min and weight average molecular weight of 3.5X10 5 Is a polypropylene of (3).
The preparation process of the polypropylene separator of comparative example 1 is the same as that of the polypropylene separator of example 1, and will not be described again.
Comparative example 2
The polypropylene separator of comparative example 2 is specifically prepared from the following raw materials: the first surface layer and the second surface layer are prepared from materials with melt flow index of 4.0g/10min and weight average molecular weight of 3.0X10 5 Is a polypropylene of (3). The intermediate layer is prepared from melt flow index of 0.5g/10min and weight average molecular weight of 5.0X10 5 Is a polypropylene of (3). The mass percentage of the raw materials of the middle layer is 50%, and the mass percentage of the raw materials of the first surface layer and the second surface layer is 25% and 25% respectively.
The preparation process of the polypropylene separator of comparative example 2 is the same as that of the polypropylene separator of example 1, and will not be described again.
Comparative example 3
The polypropylene separator of comparative example 3 is specifically prepared from the following raw materials: the first surface layer and the second surface layer are prepared from materials with melt flow index of 14.0g/10min and weight average molecular weight of 3.0X10 5 Is a polypropylene of (3). The intermediate layer is prepared from melt flow index of 0.5g/10min and weight average molecular weight of 5.0X10 5 Is a polypropylene of (3). The mass percentage of the raw materials of the middle layer is 50%, and the mass percentage of the raw materials of the first surface layer and the second surface layer is 25% and 25% respectively.
The preparation process of the polypropylene separator of comparative example 3 is the same as that of the polypropylene separator of example 1, and will not be described again.
Comparative example 4
The polypropylene separator of comparative example 4 is specifically prepared from the following raw materials: the first surface layer and the second surface layer are prepared from materials with melt flow index of 8.0g/10min and weight average molecular weight of 3.0X10 5 Is a polypropylene of (3). The intermediate layer is prepared from melt flow index 1.5g/10min and weight average molecular weight 5.0X10 5 Is a polypropylene of (3). The mass percentage of the raw materials of the middle layer is 50%, and the mass percentage of the raw materials of the first surface layer and the second surface layer is 25% and 25% respectively.
The preparation process of the polypropylene separator of comparative example 4 is the same as that of the polypropylene separator of example 1, and will not be described again.
The properties of the respective layer raw materials of the polypropylene separators of the above examples and comparative examples are specifically shown in table 1 below:
table 1 properties of the respective layer raw materials of examples and comparative examples
The following are the test parts:
1. the cross section of the polypropylene separator prepared in example 5 was tested by using an electron scanning microscope (SEM), and a cross-sectional SEM image as shown in fig. 2 was obtained.
2. The polypropylene separators prepared in examples 1 to 5 and comparative examples 1 to 4 were subjected to the following performance tests:
(1) Thickness: reference is made to the specification of GB/T6672-2001, in which the resolution of the thickness gauge should be no more than 0.1 μm, and no less than 3 points are measured equidistantly in the width direction, and the average is taken.
(2) Porosity: the length, width and thickness of the diaphragm were measured as specified in GB/T6673-2001 and GB/T6672-2001, the mass of the sample was weighed with an analytical balance having a resolution of 0.0001g, and the porosity was calculated according to the following formula
P＝(1-m/(L×b×d×ρ))×100％
Wherein: p is the porosity of the diaphragm in units of; m is the mass of the diaphragm, and the unit is g; l is the length of the diaphragm in cm; b is the width of the diaphragm in cm; d is the thickness of the diaphragm, with the unit being um; ρ is the density of the raw material in g/cm 2 。
(3) Tensile strength: the test was carried out in accordance with GB/T1040.3-2006, using a type 2 sample having a width of (15.+ -. 0.1) mm, an initial distance between the clamps of (100.+ -. 5) mm and a test speed of (250.+ -. 10) mm/min.
(4) Puncture strength: with reference to the specification of GB/T6672-2001, the resolution of the load cell is 0.1N, the diameter of the puncture needle is 1.0mm, the inner diameter of the sample fixing clamp is 10mm, the diaphragm is flattened and clamped in the clamp, puncture is performed at a speed of (100+/-10) mm/min, at least 3 points are measured, and the average value of the values is taken.
(5) Average pore diameter: average pore size data were obtained using PMI instrument measurements, pore size being expressed in nm.
(6) Liquid absorption and preservation: a diaphragm sample with the size of 150mm multiplied by 150mm is cut, marked and weighed m 1 The sample was then developed (to fully absorb the electrolyte) and immersed in the electrolyte for 1h. Then taking out the sample, wiping the electrolyte on the surface of the diaphragm with dust-free cloth until the granular electrolyte is not seen by the naked eye, and weighing the weight m of the wiped sample 2 The liquid absorption was calculated as follows: liquid absorption= (m 2 -m 1 )/m 1 ×100％；
Spreading the weighed diaphragm, standing for 1h, and weighing the sample weight m 3 The liquid retention rate was calculated according to the following formula: retention = (m) 3 -m 1 )/m 1 ×100％。
(7) Ion conductivity: the sample membrane is punched into a sample of 50mm multiplied by 50mm by adopting a sample preparation mould, 4 samples are taken and placed in electrolyte, sealing and soaking are carried out for 1 hour, then 1-4 layers of membranes are sequentially placed, the resistance value is tested, the membrane resistance is taken as a vertical coordinate as a horizontal coordinate, the curve is taken as a vertical coordinate, the slope of the curve is obtained to be the ion conductivity, and the migration capacity of lithium ions in the membrane can be represented.
(8) Hole curvature: the pore curvature can be determined by the following formula: τ= { (rm×ε)/(ρ×t) } 1/2 Wherein τ is tortuosity and Rm is film resistance (Ω·cm) 2 ) Epsilon is the porosity (%), ρ is the resistivity (Ω·cm) of the electrolyte, and t is the film thickness (μm). In this context, 1M LiBF at 20℃was used 4 The film resistance was measured using 1, 2-propylene carbonate/ethylene carbonate (mass ratio of 1/1) as an electrolyte, and ρ in this case was 2.663 ×10 -2 Ω·cm。
(9) Surface resistance: and (3) measuring the resistance value of a layer of diaphragm by the ion conductivity test method (7), and obtaining by the following formula: rm=R/S, where Rm is the film resistance (Ω·cm 2 ) R is the resistance value (omega) of the diaphragm, S is the area (m) of the diaphragm to be tested 2 ) The method comprises the steps of carrying out a first treatment on the surface of the The surface resistance is the surface resistance value of the composite diaphragm surface layer.
The test results of the polypropylene separators prepared in the above examples 1 to 5 and comparative examples 1 to 4 are shown in the following table 2.
Table 2 comparison of the properties of polypropylene separators of examples and comparative examples
As can be seen from the above Table 2, the MD tensile strength of the polypropylene separators prepared in examples 1 to 5 was more than 1700Kgf/cm 2 Even up to 2100Kgf/cm 2 Has higher mechanical strength. And the ionic conductivity is higher, and the ionic migration capability is better. The pore diameter is proper, the liquid absorption rate is high, and the electrolyte wettability is good. From this, it can be seen that the polypropylene separator prepared in the examples has both improved mechanical strength and ion migrationThe performance and electrolyte wettability have higher matching degree to the electrical performance.
Comparative example 1 used polypropylene with a melt index of 2.0, and the resulting composite separator had a much smaller pore diameter and a large pore tortuosity than the composite separator of example 1 using a different melt index, resulting in poor liquid absorption and retention, low ionic conductivity, and large sheet resistance.
The comparative example 2 uses polypropylene with a melt index of 4.0 as the surface layer, and the obtained composite membrane has a smaller pore diameter than that of example 1, poor liquid absorption and retention properties, and large pore curvature, so that the ionic conductivity is low and the surface resistance is large.
In comparative examples 3 and 4, polypropylene with high melt index was used in the surface layer and the middle layer, respectively, to make the pore structure too large, and although the liquid absorption and retention properties were good, the mechanical strength of the composite separator was significantly reduced.
The technical features of the above-described embodiments may be arbitrarily combined, and all possible combinations of the technical features in the above-described embodiments are not described for brevity of description, however, as long as there is no contradiction between the combinations of the technical features, they should be considered as the scope of the description.
The above examples illustrate only a few embodiments of the invention, which are described in detail and are not to be construed as limiting the scope of the invention. It should be noted that it will be apparent to those skilled in the art that several variations and modifications can be made without departing from the spirit of the invention, which are all within the scope of the invention. Accordingly, the scope of protection of the present invention is to be determined by the appended claims.

Claims (12)

1. A polypropylene separator is characterized by comprising a first surface layer, a middle layer and a second surface layer which are sequentially laminated, wherein the raw materials of the first surface layer and the second surface layer comprise a melt flow index of 6.0g/10 min-12.0 g/10min and a weight average molecular weight of 2.0x10 5 ～3.5×10 5 The raw materials of the intermediate layer comprise polypropylene with melt flow index of 0.3g/10 min-1.0 g/10min and weight average molecular weight of 3.5X10 5 ～6.0×10 5 Polypropylene of (2)The average pore diameter of the first surface layer and the second surface layer is 40 nm-80 nm, and the average pore diameter of the intermediate layer is 20 nm-35 nm.
2. The polypropylene separator according to claim 1, wherein the first skin layer and the second skin layer are identical in composition.
3. The polypropylene separator according to claim 1 or 2, wherein the polypropylene separator has a pore tortuosity of 2.0 to 4.5.
4. The polypropylene separator according to claim 1, wherein the mass percentage of the raw material of the intermediate layer is 20% to 70% of the raw material of the polypropylene separator.
5. The polypropylene separator according to claim 4, wherein the mass percentage of the raw material of the intermediate layer is 20% -60%.
6. The polypropylene separator according to claim 1, wherein the total thickness of the polypropylene separator is 5 μm to 25 μm.
7. The polypropylene separator according to claim 1, wherein the polypropylene separator has a porosity of 20% to 70%.
8. The preparation method of the polypropylene diaphragm is characterized by comprising the following steps:
respectively carrying out melt plasticization on raw materials of the first surface layer, the middle layer and the second surface layer, and then sequentially laminating and co-extruding to form a three-layer fluid, wherein the raw materials of the first surface layer and the second surface layer comprise a melt flow index of 6.0g/10 min-12.0 g/10min and a weight average molecular weight of 2.0x10 5 ～3.5×10 5 The raw materials of the intermediate layer comprise polypropylene with melt flow index of 0.3g/10 min-1.0 g/10min and weight average molecular weight of 3.5X10 5 ～6.0×10 5 Polypropylene of (a);
drawing and cooling the three layers of fluid to obtain three layers of nonporous precursor films;
annealing the three-layer nonporous precursor film; a kind of electronic device with high-pressure air-conditioning system
Stretching and pore-forming the three-layer nonporous precursor film after the annealing treatment to obtain a polypropylene diaphragm;
the average pore diameter of the first surface layer and the second surface layer is 40 nm-80 nm, and the average pore diameter of the intermediate layer is 20 nm-35 nm.
9. The method for producing a polypropylene separator according to claim 8, wherein the drawing speed is 60m/min to 150m/min, and the cooling temperature is 40 ℃ to 100 ℃.
10. The method for producing a polypropylene separator according to claim 8, wherein the annealing treatment is performed at a temperature of 120 to 165 ℃ for a time of 0.1 to 24 hours.
11. The method for preparing a polypropylene separator according to claim 8, wherein the temperature of the stretching and pore-forming is 100 ℃ to 150 ℃, the speed of the stretching and pore-forming is 3m/min to 15m/min, and the total multiplying power of the stretching and pore-forming is 1.5 times to 3.0 times.
12. A lithium ion battery, comprising a positive electrode, a negative electrode, an electrolyte and a separator, wherein the separator is the polypropylene separator according to any one of claims 1 to 7 or the polypropylene separator prepared by the method for preparing the polypropylene separator according to any one of claims 8 to 11.
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