CN114188661A - Low-transmittance film, preparation method thereof and battery - Google Patents
Low-transmittance film, preparation method thereof and battery Download PDFInfo
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- CN114188661A CN114188661A CN202111260414.4A CN202111260414A CN114188661A CN 114188661 A CN114188661 A CN 114188661A CN 202111260414 A CN202111260414 A CN 202111260414A CN 114188661 A CN114188661 A CN 114188661A
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- 238000002834 transmittance Methods 0.000 title claims abstract description 27
- 238000002360 preparation method Methods 0.000 title description 5
- 229920000098 polyolefin Polymers 0.000 claims abstract description 31
- 238000000034 method Methods 0.000 claims abstract description 19
- -1 polyethylene Polymers 0.000 claims abstract description 11
- 239000004743 Polypropylene Substances 0.000 claims abstract description 6
- 229920001155 polypropylene Polymers 0.000 claims abstract description 6
- 239000004698 Polyethylene Substances 0.000 claims abstract description 4
- 229920000573 polyethylene Polymers 0.000 claims abstract description 4
- 239000010410 layer Substances 0.000 claims description 90
- 239000002344 surface layer Substances 0.000 claims description 35
- 239000011148 porous material Substances 0.000 claims description 25
- 238000000137 annealing Methods 0.000 claims description 17
- 238000001125 extrusion Methods 0.000 claims description 14
- 239000000155 melt Substances 0.000 claims description 13
- 239000002994 raw material Substances 0.000 claims description 12
- 238000010622 cold drawing Methods 0.000 claims description 11
- HBBGRARXTFLTSG-UHFFFAOYSA-N Lithium ion Chemical compound [Li+] HBBGRARXTFLTSG-UHFFFAOYSA-N 0.000 claims description 9
- 229910001416 lithium ion Inorganic materials 0.000 claims description 9
- 230000035699 permeability Effects 0.000 claims description 7
- 238000005266 casting Methods 0.000 claims description 6
- 238000010345 tape casting Methods 0.000 claims description 6
- 238000009998 heat setting Methods 0.000 claims description 4
- 238000002844 melting Methods 0.000 claims description 4
- 230000008018 melting Effects 0.000 claims description 4
- VXNZUUAINFGPBY-UHFFFAOYSA-N 1-Butene Chemical compound CCC=C VXNZUUAINFGPBY-UHFFFAOYSA-N 0.000 claims description 3
- FKNQFGJONOIPTF-UHFFFAOYSA-N Sodium cation Chemical compound [Na+] FKNQFGJONOIPTF-UHFFFAOYSA-N 0.000 claims description 2
- 229910001415 sodium ion Inorganic materials 0.000 claims description 2
- 238000012545 processing Methods 0.000 abstract description 5
- 230000006698 induction Effects 0.000 abstract description 4
- 125000000383 tetramethylene group Chemical group [H]C([H])([*:1])C([H])([H])C([H])([H])C([H])([H])[*:2] 0.000 abstract 1
- 239000010408 film Substances 0.000 description 43
- 239000000463 material Substances 0.000 description 12
- 230000008569 process Effects 0.000 description 9
- 239000003792 electrolyte Substances 0.000 description 8
- 238000004804 winding Methods 0.000 description 6
- 238000012360 testing method Methods 0.000 description 5
- 238000010586 diagram Methods 0.000 description 4
- IEJIGPNLZYLLBP-UHFFFAOYSA-N dimethyl carbonate Chemical compound COC(=O)OC IEJIGPNLZYLLBP-UHFFFAOYSA-N 0.000 description 4
- JBTWLSYIZRCDFO-UHFFFAOYSA-N ethyl methyl carbonate Chemical compound CCOC(=O)OC JBTWLSYIZRCDFO-UHFFFAOYSA-N 0.000 description 4
- 238000005259 measurement Methods 0.000 description 4
- WHXSMMKQMYFTQS-UHFFFAOYSA-N Lithium Chemical compound [Li] WHXSMMKQMYFTQS-UHFFFAOYSA-N 0.000 description 2
- 238000013461 design Methods 0.000 description 2
- 238000009826 distribution Methods 0.000 description 2
- 239000007789 gas Substances 0.000 description 2
- 229910052744 lithium Inorganic materials 0.000 description 2
- 239000012528 membrane Substances 0.000 description 2
- 239000000123 paper Substances 0.000 description 2
- 230000000149 penetrating effect Effects 0.000 description 2
- 239000002985 plastic film Substances 0.000 description 2
- 238000001878 scanning electron micrograph Methods 0.000 description 2
- 239000000126 substance Substances 0.000 description 2
- 239000010409 thin film Substances 0.000 description 2
- KMTRUDSVKNLOMY-UHFFFAOYSA-N Ethylene carbonate Chemical compound O=C1OCCO1 KMTRUDSVKNLOMY-UHFFFAOYSA-N 0.000 description 1
- 229910013872 LiPF Inorganic materials 0.000 description 1
- 229910001290 LiPF6 Inorganic materials 0.000 description 1
- 101150058243 Lipf gene Proteins 0.000 description 1
- 230000002745 absorbent Effects 0.000 description 1
- 239000002250 absorbent Substances 0.000 description 1
- 230000008878 coupling Effects 0.000 description 1
- 238000010168 coupling process Methods 0.000 description 1
- 238000005859 coupling reaction Methods 0.000 description 1
- 238000002425 crystallisation Methods 0.000 description 1
- 230000008025 crystallization Effects 0.000 description 1
- 238000007599 discharging Methods 0.000 description 1
- 239000004744 fabric Substances 0.000 description 1
- 238000007654 immersion Methods 0.000 description 1
- 239000011261 inert gas Substances 0.000 description 1
- 150000002500 ions Chemical class 0.000 description 1
- 230000001788 irregular Effects 0.000 description 1
- 238000011326 mechanical measurement Methods 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- 239000007773 negative electrode material Substances 0.000 description 1
- 230000003287 optical effect Effects 0.000 description 1
- 239000011087 paperboard Substances 0.000 description 1
- 229920006255 plastic film Polymers 0.000 description 1
- 229920000642 polymer Polymers 0.000 description 1
- 239000002861 polymer material Substances 0.000 description 1
- 239000007774 positive electrode material Substances 0.000 description 1
- 238000003825 pressing Methods 0.000 description 1
- 230000004044 response Effects 0.000 description 1
- 230000035945 sensitivity Effects 0.000 description 1
- 238000000926 separation method Methods 0.000 description 1
- 238000002791 soaking Methods 0.000 description 1
- 239000002904 solvent Substances 0.000 description 1
- 238000006467 substitution reaction Methods 0.000 description 1
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- 230000002087 whitening effect Effects 0.000 description 1
Images
Classifications
-
- 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
-
- 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
- H01M10/054—Accumulators with insertion or intercalation of metals other than lithium, e.g. with magnesium or aluminium
-
- 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
- H01M50/403—Manufacturing processes of separators, membranes or diaphragms
- H01M50/406—Moulding; Embossing; Cutting
-
- 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
- H01M50/449—Separators, membranes or diaphragms characterised by the material having a layered structure
- H01M50/457—Separators, membranes or diaphragms characterised by the material having a layered structure comprising three or more layers
-
- 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
- H01M50/489—Separators, membranes, diaphragms or spacing elements inside the cells, characterised by their physical properties, e.g. swelling degree, hydrophilicity or shut down properties
-
- 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
- H01M50/489—Separators, membranes, diaphragms or spacing elements inside the cells, characterised by their physical properties, e.g. swelling degree, hydrophilicity or shut down properties
- H01M50/491—Porosity
-
- 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
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- Chemical & Material Sciences (AREA)
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- Materials Engineering (AREA)
- Cell Separators (AREA)
Abstract
A low light transmittance film comprises polyolefin, wherein the polyolefin comprises at least one of polyethylene, polypropylene, poly-1-butylene and polypentene. The film effectively reduces the transmittance, and solves the problem of poor battery diaphragm processing procedure induction in the prior art.
Description
Technical Field
The invention relates to the technical field of batteries, in particular to a low-transmittance film, a preparation method thereof and a battery.
Background
The lithium ion battery is mainly formed by combining four basic parts, namely a positive electrode material, a negative electrode material, electrolyte, a separation film and a battery container, wherein a diaphragm of the lithium battery is one of key inner layer components of the lithium battery, the performance of the diaphragm determines the interface structure, the internal resistance and the like of the battery, the characteristics of the battery, such as capacity, cycle performance, safety performance and the like, are directly influenced, the diaphragm with excellent performance has an important function of improving the comprehensive performance of the battery, and the most important function of the lithium ion battery is to separate the positive electrode and the negative electrode of the battery, prevent the short circuit caused by the contact of the two electrodes, and have the function of enabling electrolyte ions to pass through.
The aperture of the diaphragm is a key index in the performance of the diaphragm, if the aperture is large, the charging and discharging efficiency is high, but the self-discharge is large, so that the battery capacity is influenced; if the aperture is small, the safety is high, but the transmittance is high, and the process sensitivity is poor. In view of the fact that the refractive index of the polymer body is maximum, light is totally transmitted without reflection, if the transmittance of the diaphragm is high, the film is transparent or close to transparent, the edge of the diaphragm cannot be detected by a deviation rectifier (the deviation rectifier is an instrument for correcting the winding uniformity of the diaphragm), so that deviation rectification failure is caused, and the winding of the diaphragm is irregular in the process of winding the battery. The polymer materials have stress whitening phenomenon, and the holes on the film influence the refractive index, thereby causing the difference of light transmittance. At present, the aperture of a pp (polypropylene) diaphragm is generally distributed in the range of 20-60 nm, wherein when the aperture is 20-30 nm, the transmittance of the diaphragm is high, the process is difficult to induce, and meanwhile, the aperture is small, the lithium ion passing efficiency is influenced, and quick charge and discharge cannot be realized; when the aperture of the diaphragm is 50-60 nm, the self-discharge of the battery is large, and the short-circuit rate is increased.
Disclosure of Invention
According to a first aspect, in an embodiment, there is provided a film comprising a polyolefin comprising at least one of polyethylene, polypropylene, poly-1-butene, polypentene. The film has lower transmittance, and solves the problem of poor battery diaphragm process induction in the prior art.
According to a second aspect, in an embodiment, there is provided a method for preparing the thin film of the first aspect, comprising:
an extrusion step, which comprises respectively melting and plasticizing two polyolefin raw materials, and then carrying out tape casting extrusion to obtain a three-layer co-extrusion tape casting film;
annealing, namely annealing the three-layer co-extruded casting film;
and stretching and forming holes, namely performing cold drawing, hot drawing and hot setting on the annealed film to obtain the film.
According to a third aspect, in an embodiment, there is provided a battery comprising the film of the first aspect.
According to the low-transmittance film, the preparation method thereof and the battery, the transmittance of the film is effectively reduced and the battery performance is improved by adjusting the hole structure of the film.
Drawings
Fig. 1 is an SEM image of the separator of example 2.
Fig. 2 is a light-transmitting diagram of the diaphragm of example 2.
FIG. 3 is a light-transmitting diagram of the diaphragm of example 4.
Detailed Description
The present invention will be described in further detail with reference to the following detailed description and accompanying drawings. Wherein like elements in different embodiments are numbered with like associated elements. In the following description, numerous details are set forth in order to provide a better understanding of the present application. However, those skilled in the art will readily recognize that some of the features may be omitted or replaced with other elements, materials, methods in different instances. In some instances, certain operations related to the present application have not been shown or described in detail in order to avoid obscuring the core of the present application from excessive description, and it is not necessary for those skilled in the art to describe these operations in detail, so that they may be fully understood from the description in the specification and the general knowledge in the art.
Furthermore, the features, operations, or characteristics described in the specification may be combined in any suitable manner to form various embodiments. Also, the various steps or actions in the method descriptions may be transposed or transposed in order, as will be apparent to one of ordinary skill in the art. Thus, the various sequences in the specification and drawings are for the purpose of describing certain embodiments only and are not intended to imply a required sequence unless otherwise indicated where such sequence must be followed.
The numbering of the components as such, e.g., "first", "second", etc., is used herein only to distinguish the objects as described, and does not have any sequential or technical meaning. The terms "connected" and "coupled" when used herein, unless otherwise indicated, include both direct and indirect connections (couplings).
According to a first aspect, in an embodiment, there is provided a film comprising a polyolefin, said polyolefin comprising at least one of polyethylene, polypropylene, poly-1-butene, polypentene. The film effectively reduces the transmittance, and solves the problem of poor battery diaphragm processing procedure induction in the prior art.
In one embodiment, the film has a light transmittance of 10% or less, including but not limited to 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, and the like.
In one embodiment, the transmittance of the film is 4 to 10%, preferably 5 to 10%.
In one embodiment, the film is a three-layer structure comprising two skin layers including a first skin layer and a second skin layer, and an intermediate layer disposed between the two skin layers, the intermediate layer and the two skin layers independently comprising the polyolefin.
In one embodiment, the melt index of the polyolefin contained in the middle layer < the melt index of the polyolefin contained in any of the skin layers.
In one embodiment, the polyolefin with different melt indexes is used as the material of the surface layer and the middle layer, the cross-sectional aperture of the prepared film is of a small-size structure, the transmittance is effectively reduced, and the prepared diaphragm has the characteristic of low transmittance, so that the diaphragm has more excellent processing capability in the winding of the battery.
In one embodiment, the film is used as a battery diaphragm, the transmittance of the diaphragm is low, so that the edge of the diaphragm can be smoothly detected by the deviation rectifier, the deviation rectification failure is effectively avoided, and the winding of the diaphragm is more orderly in the winding of the battery.
In one embodiment, the melt index of the polyolefin contained in the intermediate layer is 0.3 to 2.0g/10min, preferably 0.3 to 1.5g/10 min. The melt index of the polyolefin contained in the intermediate layer includes, but is not limited to, 0.3g/10min, 0.4g/10min, 0.5g/10min, 0.6g/10min, 0.7g/10min, 0.8g/10min, 0.9g/10min, 1.0g/10min, 1.1g/10min, 1.2g/10min, 1.3g/10min, 1.4g/10min, 1.5g/10min, 1.6g/10min, 1.7g/10min, 1.8g/10min, 1.9g/10min, 2.0g/10min, and the like.
In one embodiment, the first and second skin layers independently have a melt index of 3.0 to 6.0g/10min, preferably 3.0 to 5.0g/10 min. The melt index of the polyolefin contained in the first and second skin layers includes, but is not limited to, 3.0g/10min, 3.1g/10min, 3.2g/10min, 3.3g/10min, 3.4g/10min, 3.5g/10min, 3.6g/10min, 3.7g/10min, 3.8g/10min, 3.9g/10min, 4.0g/10min, 4.1g/10min, 4.2g/10min, 4.3g/10min, 4.4g/10min, 4.5g/10min, 4.6g/10min, 4.7g/10min, 4.8g/10min, 4.9g/10min, 5.0g/10min, 5.1g/10min, 5.2g/10min, 5.3g/10min, 5.4g/10min, 5.5.9 g/10min, 5.5.5 g/10min, 5.0g/10min, 5.1g/10min, 5.2g/10min, 5.9g/10min, 6.0g/10min, and so on.
In one embodiment, the invention uses polyolefins with different melt indexes as surface layer and middle layer materials, and combines high-temperature stretching and large cold-drawing speed ratio to prepare the film with low transmittance, thereby effectively improving the processing capacity.
In one embodiment, the mass of the raw material of the middle layer accounts for 20-50% of the total mass of the raw materials of the middle layer and the two surface layers, including but not limited to 20%, 25%, 30%, 35%, 40%, 45%, 50%, and the like.
In one embodiment, the average pore size of the first surface layer and the average pore size of the second surface layer are smaller than the average pore size of the intermediate layer.
In one embodiment, the average pore diameter of the first surface layer and the second surface layer is independently 10 to 30nm, preferably 15 to 17 nm.
In one embodiment, the average pore diameter of the intermediate layer is 30 to 40nm, preferably 35 to 38 nm.
In one embodiment, the average pore size of the first surface layer is the same as that of the second surface layer.
In one embodiment, the mass ratio of the raw materials of the first surface layer and the second surface layer is 1: 1.
In one embodiment, the polyolefins contained in the first and second skin layers have the same melt index.
In one embodiment, the first skin layer and the second skin layer comprise the same polyolefin.
In one embodiment, the film has a thickness of 10 to 18 μm, a porosity of 38 to 45%, a gas permeability of 100 to 350s/100mL, and a Machine Direction (MD) tensile strength of 1800 to 2500kgf/cm2The transverse (TD direction) tensile strength is 120 to 160kgf/cm2The puncture strength is 220-500 g, the longitudinal heat shrinkage rate is less than or equal to 5%, and the transverse heat shrinkage rate is less than or equal to 1%.
In one embodiment, the ratio of the thickness of the middle layer to the thickness of any surface layer is 1 (1-3).
In one embodiment, the first skin layer and the second skin layer have the same thickness.
According to a second aspect, in an embodiment, there is provided a method for preparing the thin film of the first aspect, comprising:
an extrusion step, which comprises respectively melting and plasticizing two polyolefin raw materials, and then carrying out tape casting extrusion to obtain a three-layer co-extrusion tape casting film;
annealing, namely annealing the three-layer co-extruded casting film;
and stretching and forming holes, namely performing cold drawing, hot drawing and hot setting on the annealed film to obtain the film.
In one embodiment, the annealing temperature in the annealing step is 100 ℃ to 145 ℃.
In one embodiment, the annealing time in the annealing step is 0.1-24 h, including but not limited to 0.1h, 0.2h, 0.3h, 0.4h, 0.5h, 0.6h, 0.7h, 0.8h, 0.9h, 1h, 5h, 10h, 15h, 20h, 21h, 22h, 23h, 24h, and the like.
In one embodiment, the cold drawing temperature is normal temperature, and the cold drawing speed ratio is 0.8-1.5.
In one embodiment, the hot-drawing temperature is 100 ℃ to 130 ℃, and the hot-drawing speed ratio is 1.8 to 2.48.
In one embodiment, the heat setting temperature is 110-130 ℃ and the heat setting speed ratio is 0.3-0.9.
According to a third aspect, in an embodiment, there is provided a battery comprising the film of the first aspect.
In one embodiment, the film can be used as a battery diaphragm for isolating the positive electrode and the negative electrode of the battery and preventing the positive electrode and the negative electrode from being short-circuited.
In one embodiment, the battery includes, but is not limited to, a lithium ion battery, a sodium ion battery.
In one embodiment, an ABA three-layer structure separator (a is a surface layer, and B is a middle layer) is provided, and based on the existing process, polyolefins with different melt indexes are used as materials of the middle layer and the surface layer of the three-layer co-extrusion lithium ion battery separator, so that the pore structure design of the three-layer film with small pore diameter is realized, and the battery performance and the processing requirements are considered. As a lithium ion battery diaphragm material, the lithium ion battery diaphragm material has a microporous structure, the size of the pore diameter and the uniformity of the distribution have direct influence on the performance of the lithium ion battery, the pore diameter is large, the charge and discharge efficiency is high, but the self-discharge is large, and the battery capacity is influenced; small aperture, high safety, high transmittance, and poor process response. The aperture of the middle layer of the ABA three-layer structure is large, and the aperture of the surface layer is small. The mass ratio of the raw materials of the middle layer and the surface layer can be adjusted.
In one embodiment, the invention designs a three-layer structure with small surface pore size and large middle layer pore size by three-layer extrusion, thereby effectively solving the problems of poor performance and poor process induction of the existing film.
Examples 1 to 5
The polyolefins used to make the skin layers (i.e., layer a) and the intermediate layers (i.e., layer B) are all polypropylene.
The preparation method of the diaphragm comprises the following steps:
extruding: taking the material A for preparing the layer A and the material B for preparing the layer B, respectively adding the materials A and B into two different extruders according to different mass ratios for melting and plasticizing, pressing the materials into a uniform melt through a die, and extruding the melt through a casting die head to obtain a three-layer co-extrusion casting film, wherein the three-layer structure comprises two surface layers and a middle layer, and the middle layer is positioned between the two surface layers.
Annealing: and annealing the three-layer co-extrusion casting film at the annealing temperature of 100-145 ℃ for 0.1-24 h to further perfect the crystallization.
Stretching to form a hole: carrying out three-step treatment on the annealed sample, wherein the three-step treatment comprises cold drawing, hot drawing and hot setting, the cold drawing is carried out at normal temperature (23 +/-2 ℃), the cold drawing speed ratio is 0.8-1.5, the hot drawing temperature is 100-130 ℃, the hot drawing speed ratio is 1.8-2.48, the hot setting temperature is 110-130 ℃, the hot setting speed ratio is 0.3-0.9, a three-layer co-extruded film with better pore forming is obtained, the aperture is 18 nm-30 nm, the thickness is 10 mu m-18 mu m, the porosity is 38-45%, the ventilation value is 100-350 s/100mL, the tensile strength in the MD direction is 1800-2500 kgf/cm2The tensile strength in the TD direction is 120 to 160kgf/cm2The puncture strength is 220-500 g, the longitudinal heat shrinkage rate is less than or equal to 5%, and the transverse heat shrinkage rate is less than or equal to 1%.
In examples 1 to 5, the melt index of the polyolefin material of the intermediate layer was 1.5g/10min, and the thickness of the intermediate layer of the obtained film was 4 μm. The melt index of the polyolefin material of the surface layer was 3.8g/10min, and the thickness of the surface layer of the obtained film was 5 μm.
The process parameters for each example are shown in Table 2.
The specific process conditions of each example are as follows:
TABLE 1
TABLE 2
In table 2, "pore size of layer a" means the average pore size of one of the skin layers. The average pore size of both surface layers is the same.
"A: the "B mass ratio" means the ratio of the total mass of the raw materials of the two surface layers to the mass of the raw material of the intermediate layer.
The thickness testing method is carried out according to GB/T6672-2001 plastic film and sheet thickness measuring mechanical measurement method, a Mark thickness gauge with a flat head contact head is adopted for measurement, the gauge is calibrated and cleared before measurement, the contact surface is kept clean, one point is taken along the TD direction of the film every 5cm for measurement, and the average value of 5 points is measured to be the thickness of the film.
Air permeability value (i.e. air permeability) test: according to GB/T458 + 2008 paper and paperboard air permeability determination, 5 samples are taken and tested by an air permeability tester, and the average value of the measurement is taken as the air permeability value of the sample to be tested.
The porosity was tested as follows: completely soaking the membrane in electrolyte for 1h, taking out, sucking the electrolyte on the surface of the diaphragm by using absorbent paper, weighing, wherein the calculation formula of the porosity p is as follows: p ═ p (ρ)c-ρe)/ρc*100％。ρcDensity of the separator after immersion in electrolyte, peThe density of the separator when not impregnated with the electrolyte.
The electrolyte consists of LiPF6EC (ethylene carbonate), EMC (methyl ethyl carbonate) and DMC (dimethyl carbonate), wherein the EC, EMC and DMC are solvents. LiPF in electrolyte6Is 1mol/L, EC: EMC: DMC 1: 1: 1.
the pore size was measured as follows: the capillary flow analyzer is used for testing, and is carried out by a bubble point method, namely, inert gas is adopted to break a wetted diaphragm, the pressure value of gas outflow is measured, and the aperture parameter is obtained through calculation according to GB/T14041.1-2007 hydraulic filter element structure integrity verification and initial bubble point determination and GB/T24219-2009 woven filter cloth bubble point aperture determination.
The transmittance was measured as follows: the sample is placed in a test groove, a purple light source, a red light source and a visible light source are adopted to irradiate the transparent substance to be tested, the sensor respectively detects the incident light intensity of the three light sources and the light intensity after penetrating through the transparent substance to be tested, and the ratio of the penetrating light intensity to the incident light intensity is the transmittance and is expressed by percentage. The instrument has three displays, and during testing, the instrument can simultaneously display the values of three light sources, and in examples 1-5, the values of the visible light sources are taken.
As can be seen from Table 2, the diaphragm has high porosity, and the prepared film with low transmittance has good performances, and the transmittance is preferably 5-10%.
The three-layer co-extrusion diaphragm can flexibly adjust the micropore structure, and different pore distributions have different optical properties and battery properties. The surface layer (layer A) is small holes, the safety of the battery is high, the transmittance of the film is extremely high, the film surface is transparent, the middle layer (layer B) is large holes, the multiplying power performance of the battery is good, the film surface is white, and the picture contrast is obvious.
FIG. 1 is an SEM image of the membrane of example 2 showing a three-layer structure with "small-size" pore structure, i.e., the surface layer with smaller pores and the middle layer with larger pores.
Fig. 2 is a light-transmitting diagram of the diaphragm of example 2.
FIG. 3 is a light-transmitting diagram of the diaphragm of example 4.
The present invention has been described in terms of specific examples, which are provided to aid understanding of the invention and are not intended to be limiting. For a person skilled in the art to which the invention pertains, several simple deductions, modifications or substitutions may be made according to the idea of the invention.
Claims (10)
1. A film comprising a polyolefin, wherein the polyolefin comprises at least one of polyethylene, polypropylene, poly-1-butene, and polypentene.
2. The film of claim 1, wherein the film has a light transmittance of 10% or less.
3. The film of claim 2, wherein the film has a transmittance of 4 to 10%, preferably 5 to 10%.
4. The film of claim 1, wherein the film is a three-layer structure comprising two skin layers including a first skin layer, a second skin layer, and an intermediate layer disposed between the two skin layers, the intermediate layer, the two skin layers independently comprising the polyolefin;
the melt index of the polyolefin contained in the middle layer is less than the melt index of the polyolefin contained in any of the surface layers;
the melt index of the polyolefin contained in the middle layer is 0.3-2.0 g/10min, preferably 0.3-1.5 g/10 min;
the melt index of the polyolefin contained in the first surface layer and the second surface layer is independently 3.0-6.0 g/10min, preferably 3.0-5.0 g/10 min.
5. The film of claim 4, wherein the first skin layer and the second skin layer each have an average pore size smaller than the average pore size of the intermediate layer;
and/or the average pore diameter of the first surface layer and the second surface layer is independently 10-30 nm, preferably 15-17 nm;
and/or the aperture of the middle layer is 30-40 nm, preferably 35-38 nm;
and/or the average pore diameter of the first surface layer and the second surface layer is the same;
and/or the mass ratio of the raw materials of the first surface layer and the second surface layer is 1: 1;
and/or the polyolefins contained in the first skin layer and the second skin layer have the same melt index;
and/or the first skin layer and the second skin layer contain the same polyolefin;
and/or the mass of the raw material of the middle layer accounts for 20-50% of the total mass of the raw materials of the middle layer and the two surface layers;
and/or the ratio of the thickness of the intermediate layer to the thickness of any surface layer is 1: (1: 3);
and/or the first skin layer and the second skin layer have the same thickness.
6. The film of claim 1, wherein the film has a thickness of 10 to 18 μm, a porosity of 38 to 45%, a gas permeability of 100 to 350s/100mL, and a machine direction tensile strength of 1800 to 2500kgf/cm2The transverse tensile strength is 120 to 160kgf/cm2The puncture strength is 220-500 g, the longitudinal heat shrinkage rate is less than or equal to 5%, and the transverse heat shrinkage rate is less than or equal to 1%.
7. The method for preparing a film according to any one of claims 1 to 6, comprising:
an extrusion step, which comprises respectively melting and plasticizing two polyolefin raw materials, and then carrying out tape casting extrusion to obtain a three-layer co-extrusion tape casting film;
annealing, namely annealing the three-layer co-extruded casting film;
and stretching and forming holes, namely performing cold drawing, hot drawing and hot setting on the annealed film to obtain the film.
8. The method according to claim 7, wherein in the annealing step, the annealing temperature is 100 ℃ to 145 ℃;
and/or in the annealing step, the annealing time is 0.1-24 h;
and/or the cold drawing temperature is normal temperature, and the cold drawing speed ratio is 0.8-1.5;
and/or the hot-drawing temperature is 100-130 ℃, and the hot-drawing speed ratio is 1.8-2.48;
and/or the heat setting temperature is 110-130 ℃, and the heat setting speed ratio is 0.3-0.9.
9. A battery comprising the film according to any one of claims 1 to 6.
10. The battery of claim 9, wherein the film is used to separate the positive and negative electrodes of the battery;
and/or the battery comprises a lithium ion battery and a sodium ion battery.
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