CN107331822B - Polypropylene/polyethylene/polypropylene three-layer co-extrusion lithium ion battery diaphragm and preparation method thereof - Google Patents
Polypropylene/polyethylene/polypropylene three-layer co-extrusion lithium ion battery diaphragm and preparation method thereof Download PDFInfo
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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
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- H—ELECTRICITY
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- H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
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- 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
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Abstract
The invention relates to a polypropylene/polyethylene/polypropylene three-layer co-extrusion lithium battery diaphragm, which is A1/B/A2Three-layer structure, wherein, the surface layer A1、A2All are porous structures formed by polypropylene copolymer; the middle layer B is a porous structure formed by high-density polyethylene; the preparation method comprises the following steps: respectively melting and plasticizing high-density polyethylene and polypropylene copolymer, then synchronously co-extruding from a three-layer composite casting die head with three cavities and drawing to form a film to obtain the product A1/B/A2Co-extrusion casting film with three-layer structure; and then sequentially carrying out annealing treatment, longitudinal cold stretching, hot stretching and heat setting to obtain the polypropylene/polyethylene/polypropylene three-layer co-extrusion lithium ion battery diaphragm. The lithium battery diaphragm provided by the invention has a lower closed pore temperature of the middle layer and a higher melting temperature of the surface layer, so that the melt integrity and the battery safety of the diaphragm under the extremely high temperature condition of the lithium battery are improved; meanwhile, the composite material has a middle layer with a low melt index and a surface layer with a high melt index, and has good middle layer strength and surface layer plasticizing uniformity.
Description
Technical Field
The invention belongs to the technical field of lithium ion battery diaphragms, and particularly relates to a polypropylene (PP)/Polyethylene (PE)/polypropylene (PP) three-layer co-extrusion lithium ion battery diaphragm and a preparation method thereof.
Background
The diaphragm is a key material for determining the interface structure, internal resistance, capacity and cycle performance, particularly the safety performance, in the lithium battery. Under overcharge/overdischarge or other extreme conditions, the internal temperature of the lithium battery may rise very rapidly. When the internal temperature of the battery is close to the melting point of the pore-forming material of the diaphragm, the pore-forming material can be softened and subjected to pore-closing action, so that the ion transmission is blocked to form open circuit, and the effect of safety protection is achieved. However, the temperature of the closed hole of the diaphragm made of the single-layer material is the same as the melting temperature, and the diaphragm is closed and is easy to break due to rapid temperature rise in the process of reaction, so that the anode and the cathode of the battery are in direct contact with each other, and short circuit and explosion are caused.
In order to solve the problem that the closed pore temperature is the same as the melting temperature, researchers adopt heterogeneous materials to compound the diaphragm so as to obtain a higher diaphragm melting point temperature difference. For example, in patent US5952120, Celgard adopts a PE microporous membrane and a PP microporous membrane to perform hot-press compounding to obtain a PE/PP compounded multilayer structure, and the PP membrane does not crack due to a higher melting temperature (about 150 ℃) while ensuring low-temperature closed pores (melting temperature 130-140 ℃) of PE. However, the multilayer film prepared by the ex-situ composite technology of forming the microporous film and then compounding is large in thickness; meanwhile, in order to avoid damaging the pore structure of the diaphragm, the thermal compounding temperature is low, and the adhesion of PE/PP is insufficient, so that the diaphragm can generate the delamination behavior under the working condition of increased temperature. For the non-layered composite technology, Tanghaolin et al (J Power Sources 2013, 241, 203) adopts PVDF-HFP (vinylidene fluoride-hexafluoropropylene copolymer) and ePTFE (expanded polytetrafluoroethylene) porous network to composite, the prepared PVDF-HFP/ePTFE composite membrane has the closed pore temperature of 150 ℃ below zero, and the melting temperature (membrane breaking temperature) reaches 350 ℃ below zero because ePTFE forms a continuous net structure. However, the cost of the composite technology using the porous network as the matrix is too high to meet the requirement of large-scale industrial production.
The multilayer co-extrusion is a production mode for obtaining the ultrathin film with high efficiency and low cost, and because the layers are co-extruded under the condition of higher than the melting temperature, the interlayer combination is very tight, and the layers are not easy to delaminate under the condition of high-temperature use. ZL201410427455.1 discloses a preparation technology of a PP/EVA/PVDC/EVA/PE five-layer co-extrusion film, the multilayer film is prepared by adopting a multilayer die for extruding different materials, and the extrusion layer is provided with a plurality of flow channels connected with an extruder. ZL201410497379.1 discloses a method for preparing a biaxially oriented polypropylene antifogging film by five-layer co-extrusion, wherein each layer containing different additives is collected from each layer extruder to a die head through different temperatures of 180-250 ℃ and is jointly sent out to obtain a five-layer composite casting film, and the total thickness of the five-layer composite casting film is 17-49 mu m. For lithium battery separators with extremely high requirements on pore structure indexes, multilayer coextrusion presents certain technical complexity due to the differences of melt flowability, crystallization and crystal transformation into pore properties of each component at similar temperatures.
ZL201510269976.3 adopts two kinds of polypropylene with more than 98 percent of isotacticity and different melt indexes to melt and co-extrude, and the diaphragm with a composite structure and good mechanical property is obtained. ZL201410247897.8 discloses a polypropylene microporous membrane with a three-layer composite structure and a preparation method thereof, wherein a surface layer and a middle layer respectively adopt polypropylene copolymer and polypropylene homopolymer; because the melting points and the flowability of the two polypropylenes are relatively close, the coextrusion rheological property is relatively easy to solve; the pore-forming consistency is solved by adding beta crystal form nucleating agent and biaxial stretching. ZL201410292515 discloses a preparation method of a polypropylene/polyethylene/polypropylene three-layer co-extrusion composite membrane, and in the preparation process, polypropylene and polyethylene with low melt index are adopted to enhance the rheological property of the membrane. ZL201510366657.4, 201510366133.5 used a method of adding a metal or semiconductor compound to the surface layer resin to increase the pore-forming properties of the surface layer. However, in the above patent, the addition of the heterogeneous material in the surface layer or the intermediate layer affects the stability of the separator during use.
Disclosure of Invention
The invention aims to solve the technical problem of providing a polypropylene/polyethylene/polypropylene three-layer co-extrusion lithium ion battery diaphragm and a preparation method thereof aiming at the defects in the prior art, which have the advantages of synchronous casting and synchronous stretching pore-forming, and can obtain a uniform composite high-porosity diaphragm structure without depending on an additive.
The technical scheme adopted by the invention for solving the problems is as follows:
a polypropylene/polyethylene/polypropylene three-layer co-extrusion lithium ion battery diaphragm is A1/B/A2Three-layer structure, wherein, the surface layer A1、A2All are porous structures formed by polypropylene copolymer (PP); the middle layer B is a porous structure formed by high-density Polyethylene (PE).
According to the scheme, A is1/B/A2The total thickness of the three-layer structure is 9-50 μm.
According to the scheme, the thickness of the middle layer B is the surface layer A1Or surface layer A21-4 times of thickness, surface layer A1、A2Equal or unequal.
According to the scheme, the molecular weight of the co-polypropylene is 20-40 ten thousand, the content of a Polyethylene (PE) block is 0.1-2%, and the melt index at 230 ℃ is 2-8.
According to the scheme, the molecular weight of the high-density polyethylene is 40-60 ten thousand, and the melt index at 190 ℃ is 0.1-1.
The preparation method of the polypropylene/polyethylene/polypropylene three-layer co-extrusion lithium ion battery diaphragm mainly comprises the following steps:
(1) a melting step: respectively adding high-density polyethylene and polypropylene copolymer into two extruders for melting and plasticizing;
(2) casting: synchronously co-extruding the melt-plasticized high-density polyethylene and the co-polypropylene obtained in the step (1) from a three-layer composite casting die head of a three-layer co-extrusion casting machine, and drawing the co-extruded high-density polyethylene and the co-polypropylene into a film under a high-power drawing ratio to obtain the polyethylene/polypropylene composite material A1/B/A2Co-extrusion casting film with three-layer structure;
(3) and (3) annealing: subjecting the product of step (2) to reaction with A1/B/A2Annealing the co-extrusion casting film with the three-layer structure, and naturally cooling to room temperature;
(4) a stretching step: sequentially carrying out longitudinal cold stretching and hot stretching on the cast film subjected to annealing treatment obtained in the step (3) to form a porous membrane structure;
(5) heat setting: and (4) cooling the porous membrane obtained in the step (4) to room temperature after heat setting to obtain the polypropylene/polyethylene/polypropylene three-layer co-extrusion lithium ion battery diaphragm.
According to the scheme, in the step (1), the temperature of a conveying section of the high-density polyethylene extruder is 50-75 ℃, the temperature of a compression section is 140-195 ℃, the temperature of a melting section is 170-195 ℃, the temperature of a metering section is 170-220 ℃, and the rotating speed of a screw is 50-300 rpm; the temperature of the conveying section of the copolymerization polypropylene extruder is 50-75 ℃, the temperature of the compression section is 140-225 ℃, the temperature of the melting section is 170-225 ℃, the temperature of the metering section is 170-215 ℃ and the rotating speed of a screw is 50-300 rpm.
According to the scheme, in the step (2), the die head temperature is 190-215 ℃, the opening of the die orifice is 2.0-3.0 mm, the casting roll speed is 25-80 m/min, and the cooling roll temperature is 35-100 ℃.
According to the scheme, in the step (3), the annealing temperature is 110-130 ℃, and the annealing time is 8-16 h.
According to the scheme, in the step (4), the cold stretching temperature is 30-50 ℃, and the stretching ratio is 1.2-1.5; the hot stretching temperature is 120 to 150 ℃, and the stretching ratio is 1.5 to 3.0.
According to the scheme, in the step (5), the heat setting temperature is 125-140 ℃ and the time is 10 s-5 min.
Preferably, the invention provides a more detailed preparation method of a polypropylene/polyethylene/polypropylene three-layer co-extrusion lithium ion battery separator, which comprises the following steps:
(1) a melting step: respectively adding high-density polyethylene and polypropylene copolymer into two extruders for melting and plasticizing, wherein the temperature of a conveying section of the high-density polyethylene extruder is 50-75 ℃, the temperature of a compression section is 140-195 ℃, the temperature of a melting section is 170-195 ℃, the temperature of a metering section is 170-220 ℃, and the rotating speed of a screw is 50-300 rpm; the temperature of a conveying section of the copolymerization polypropylene extruder is 50-75 ℃, the temperature of a compression section is 140-225 ℃, the temperature of a melting section is 170-225 ℃, the temperature of a metering section is 170-215 ℃, and the rotating speed of a screw is 50-300 rpm;
(2) casting: synchronously coextruding the melted and plasticized high-density polyethylene and the copolymerized polypropylene obtained in the step (1) from a three-layer composite casting die head with three cavities, and drawing the mixture into a film under a high-power drawing ratio to obtain the product A1/B/A2The co-extrusion casting film with the three-layer structure is characterized in that the temperature of a die head is 190-215 ℃, the opening of a die opening is 2.0-3.0 mm, the casting roll speed is 25-80 m/min, and the temperature of a cooling roll is 35-100 ℃;
(3) and (3) annealing: subjecting the product of step (2) to reaction with A1/B/A2Annealing the co-extrusion casting film with the three-layer structure at the annealing temperature of 110-130 ℃ for 8-16 h, and naturally cooling to room temperature;
(4) a stretching step: sequentially carrying out longitudinal cold stretching and hot stretching on the cast film subjected to annealing treatment obtained in the step (3), wherein the cold stretching temperature is 30-50 ℃, and the stretching ratio is 1.2-1.5; the hot-drawing temperature is 120-150 ℃, the drawing ratio is 1.5-3.0, and a porous structure is formed;
(5) heat setting: and (3) carrying out heat setting on the stretched porous membrane at 125-140 ℃ for 10 s-5 min, and then cooling to room temperature to obtain the polypropylene/polyethylene/polypropylene three-layer co-extrusion lithium ion battery diaphragm.
Compared with the prior art, the invention has the beneficial effects that:
firstly, the lithium battery diaphragm prepared by the invention is of a three-layer composite structure with a PP surface layer and a PE middle layer, and has lower closed pore temperature of the middle layer and higher melting temperature of the surface layer, so that the integrity and the battery safety of the lithium battery diaphragm under the extremely high temperature condition are improved; meanwhile, the middle layer with low melt index and the surface layer with high melt index have good strength and surface plasticizing uniformity.
Secondly, aiming at the technical defect that the existing three-layer co-extrusion heterogeneous composite lithium battery diaphragm cannot realize synchronous casting and synchronous stretching pore-forming simultaneously so as to obtain a uniform composite and high-pore diaphragm structure by adding other additives, the invention provides a polypropylene/polyethylene/polypropylene three-layer co-extrusion lithium ion battery diaphragm and a preparation method thereof.
Detailed Description
In order to better understand the present invention, the following examples are further provided to illustrate the present invention, but the present invention is not limited to the following examples.
Example 1
A polypropylene/polyethylene/polypropylene three-layer co-extrusion lithium ion battery diaphragm is prepared by the following specific steps:
(1) a melting step: respectively adding high-density polyethylene with the molecular weight of 40-50 ten thousand and the melt index of 0.3 at 190 ℃ and copolymerized polypropylene with the molecular weight of 20-30 ten thousand, the PE block content of 0.5 percent and the melt index of 4 at 230 ℃ into two extruders for melting and plasticizing; wherein the temperature of the conveying section of the high-density polyethylene extruder is 50-65 ℃, the temperature of the compression section is 140-180 ℃, the temperature of the melting section is 170-200 ℃, the temperature of the metering section is 170-190 ℃, and the rotating speed of a screw is 200 rpm; the temperature of the conveying section of the copolymerization polypropylene extruder is 50-60 ℃, the temperature of the compression section is 140-175 ℃, the temperature of the melting section is 170-210 ℃, the temperature of the metering section is 170-195 ℃ and the rotating speed of a screw is 250 rpm;
(2) casting: synchronously coextruding the melted and plasticized high-density polyethylene and the copolymerized polypropylene obtained in the step (1) from a three-layer composite casting die head with three cavities, and drawing the mixture to form a film under a high draw ratio to obtain the product A1/B/A2The three-layer co-extrusion casting film has the structure, wherein the die head temperature is 200 ℃, the die opening is 2.5mm, the casting roll speed is 70m/min, and the cooling roll temperature is 40 ℃;
(3) and (3) annealing: subjecting the product of step (2) to reaction with A1/B/A2Annealing the three-layer co-extrusion casting film with the structure at 125 ℃ for 16h, and naturally cooling to room temperature;
(4) a stretching step: sequentially carrying out longitudinal cold stretching and hot stretching on the annealed casting film obtained in the step (3), wherein the cold stretching temperature is 40 ℃, and the stretching speed ratio is 1.4; the hot-drawing temperature is 140 ℃, and the drawing speed ratio is 2.0, so that a porous membrane structure is formed;
(5) heat setting: and (4) carrying out heat setting on the porous membrane obtained in the step (4) at 130 ℃ for 10 s-5 min, and then cooling to room temperature to obtain the polypropylene/polyethylene/polypropylene three-layer co-extrusion lithium ion battery diaphragm.
The total thickness of the polypropylene/polyethylene/polypropylene three-layer co-extrusion lithium ion battery separator prepared in the embodiment is 20 micrometers, the thickness of the middle layer B is 10 micrometers, and the surface layer A is1And A2The thickness was 5 μm.
Example 2
A polypropylene/polyethylene/polypropylene three-layer co-extrusion lithium ion battery diaphragm is prepared by the following specific steps:
(1) a melting step: respectively adding high-density polyethylene with the molecular weight of 40-50 ten thousand and the melt index of 0.1 at 190 ℃ and copolymerized polypropylene with the molecular weight of 20-30 ten thousand, the PE block content of 0.1% and the melt index of 2 at 230 ℃ into two extruders for melting and plasticizing; wherein the temperature of the conveying section of the high-density polyethylene extruder is 60-75 ℃, the temperature of the compression section is 140-195 ℃, the temperature of the melting section is 175-195 ℃, the temperature of the metering section is 175-215 ℃ and the rotating speed of a screw is 50 rpm; the temperature of a conveying section of the copolymerization polypropylene extruder is 60-75 ℃, the temperature of a compression section is 140-195 ℃, the temperature of a melting section is 170-225 ℃, the temperature of a metering section is 170-215 ℃, and the rotating speed of a screw is 50 rpm;
(2) casting: synchronously coextruding the melted and plasticized high-density polypropylene and the copolymerized polypropylene obtained in the step (1) from a three-layer composite casting die head with three cavities, and drawing the mixture to form a film under high multiplying power to obtain the polypropylene material A1/B/A2The three-layer co-extrusion casting film has the structure that the die head temperature is 190 ℃, the die opening is 2.0mm, the casting roll speed is 25m/min, and the cooling roll temperature is 35 ℃;
(3) and (3) annealing: subjecting the product of step (2) to reaction with A1/B/A2Annealing the three-layer co-extrusion casting film with the structure at the annealing temperature of 120 ℃ for 18h, and then naturally cooling to room temperature;
(4) a stretching step: sequentially carrying out longitudinal cold stretching and hot stretching on the annealed casting film obtained in the step (3), wherein the cold stretching temperature is 30 ℃, and the stretching speed ratio is 1.2; the hot-drawing temperature is 120 ℃, and the drawing speed ratio is 1.5, so that a porous membrane structure is formed;
(5) heat setting: and (4) carrying out heat setting on the porous membrane obtained in the step (4) at 125 ℃ for 10 s-5 min, and then cooling to room temperature to obtain the polypropylene/polyethylene/polypropylene three-layer co-extrusion lithium ion battery diaphragm.
The total thickness of the polypropylene/polyethylene/polypropylene three-layer co-extrusion lithium ion battery separator prepared in the embodiment is 50 micrometers, the thickness of the middle layer B is 30 micrometers, and the surface layer A is1And A2The thickness was 10 μm.
Example 3
A polypropylene/polyethylene/polypropylene three-layer co-extrusion lithium ion battery diaphragm is prepared by the following specific steps:
(1) a melting step: respectively adding 50-60 ten thousand high-density polyethylene with the molecular weight of 1 and the melt index of 190 ℃ of 30-40 ten thousand, 2 percent of PE block content and 8 percent of polypropylene copolymer with the melt index of 230 ℃ into two extruders for melting and plasticizing; wherein the temperature of the conveying section of the high-density polyethylene extruder is 60-70 ℃, the temperature of the compression section is 160-190 ℃, the temperature of the melting section is 170-210 ℃, the temperature of the metering section is 170-195 ℃ and the rotating speed of a screw is 300 rpm; the temperature of a conveying section of the copolymerization polypropylene extruder is 60-70 ℃, the temperature of a compression section is 160-220 ℃, the temperature of a melting section is 190-215 ℃, the temperature of a metering section is 190-205 ℃, and the rotating speed of a screw is 300 rpm;
(2) casting: synchronously coextruding the melted and plasticized high-density polyethylene and the copolymerized polypropylene obtained in the step (1) from a three-layer composite casting die head with three cavities, and drawing the mixture to form a film under a high draw ratio to obtain the product A1/B/A2The three-layer co-extrusion casting film has the structure that the die head temperature is 215 ℃, the die opening is 3.0mm, the casting roll speed is 80m/min, and the cooling roll temperature is 80 ℃;
(3) and (3) annealing: subjecting the product of step (2) to reaction with A1/B/A2Annealing the three-layer co-extrusion casting film with the structure at the annealing temperature of 130 ℃ for 20 hours, and then naturally cooling to room temperature;
(4) a stretching step: sequentially carrying out longitudinal cold stretching and hot stretching on the annealed casting film obtained in the step (3), wherein the cold stretching temperature is 50 ℃, and the stretching speed ratio is 1.5; the hot-drawing temperature is 150 ℃, and the drawing speed ratio is 3.0, so that a porous structure is formed;
(5) heat setting: and (4) carrying out heat setting on the porous membrane obtained in the step (4) at 140 ℃ for 10 s-5 min, and then cooling to room temperature to obtain the polypropylene/polyethylene/polypropylene three-layer co-extrusion lithium ion battery diaphragm.
The total thickness of the polypropylene/polyethylene/polypropylene three-layer co-extrusion lithium ion battery diaphragm prepared in the embodiment is 9 microns, the thickness of the middle layer B is 3 microns, and the surface layer A is1And A2The thickness was 3 μm.
Example 4
A polypropylene/polyethylene/polypropylene three-layer co-extrusion lithium ion battery diaphragm is prepared by the following specific steps:
(1) a melting step: respectively adding high-density polyethylene with the molecular weight of 50-60 ten thousand and the melt index of 0.7 at 190 ℃ and copolymerized polypropylene with the molecular weight of 30-40 ten thousand, the PE block content of 1.5 percent and the melt index of 5.8 at 230 ℃ into two extruders for melting and plasticizing; wherein the temperature of the conveying section of the high-density polyethylene extruder is 55-65 ℃, the temperature of the compression section is 160-185 ℃, the temperature of the melting section is 170-210 ℃, the temperature of the metering section is 170-200 ℃, and the rotating speed of a screw is 100 rpm; the temperature of a conveying section of the copolymerization polypropylene extruder is 55-70 ℃, the temperature of a compression section is 150-210 ℃, the temperature of a melting section is 170-230 ℃, the temperature of a metering section is 170-210 ℃, and the rotating speed of a screw is 100 rpm;
(2) casting: synchronously coextruding the melted and plasticized high-density polyethylene and the copolymerized polypropylene obtained in the step (1) from a three-layer composite casting die head with three cavities, and drawing the mixture to form a film under a high draw ratio to obtain the product A1/B/A2The three-layer co-extrusion casting film has the structure that the die head temperature is 200 ℃, the die opening is 2.4mm, the casting roll speed is 50m/min, and the cooling roll temperature is 70 ℃;
(3) and (3) annealing: subjecting the product of step (2) to reaction with A1/B/A2Annealing the three-layer co-extrusion casting film with the structure at 125 ℃ for 10 s-5 min, and then cooling to room temperature;
(4) a stretching step: sequentially carrying out longitudinal cold stretching and hot stretching on the annealed casting film obtained in the step (3), wherein the cold stretching temperature is 45 ℃ and the stretching speed ratio is 1.3; forming a porous membrane structure at a hot-drawing temperature of 120-150 ℃ and a tensile speed ratio of 2.5;
(5) heat setting: and (4) carrying out heat setting on the porous membrane obtained in the step (4) at 135 ℃ for 10 s-5 min, and then cooling to room temperature to obtain the polypropylene/polyethylene/polypropylene three-layer co-extrusion lithium ion battery diaphragm.
The total thickness of the polypropylene/polyethylene/polypropylene three-layer co-extrusion lithium ion battery separator prepared in the embodiment is 30 micrometers, the thickness of the middle layer B is 20 micrometers, and the surface layer A is1And A2The thickness was 5 μm.
The polypropylene/polyethylene/polypropylene three-layer co-extruded membrane prepared in examples 1 to 4 was tested under the same conditions using a new three-si universal tester, a Gurley permeameter, and a conta PoreMaster-60 mercury porosimeter, and the data results are shown in table 1.
TABLE 1
| Example 1 | Example 2 | Example 3 | Example 4 | |
| Porosity% | 42 | 39 | 45 | 43 |
| Air permeability s/100mL | 346 | 593 | 325 | 457 |
| Puncture strength, gf | 678 | 743 | 585 | 646 |
| Longitudinal tensile strength, MPa | 148 | 165 | 136 | 143 |
| Shrinkage (90 ℃,2h) | 3 | 2.5 | 2.2 | 2.9 |
| Obturator temperature, C | 133 | 136 | 129 | 131 |
| Melting temperature of | 153 | 156 | 149 | 151 |
As shown in Table 1, the polypropylene/polyethylene/polypropylene three-layer co-extrusion diaphragm prepared by the invention has high porosity, good air permeability, proper puncture strength, longitudinal tensile strength and shrinkage rate, lower closing temperature of the middle layer and higher melting temperature of the surface layer, and improves the melt integrity and battery safety of the diaphragm under the extremely high temperature condition of a lithium battery.
The above description is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, many modifications and changes can be made without departing from the inventive concept of the present invention, and these modifications and changes are within the protection scope of the present invention.
Claims (1)
1. A polypropylene/polyethylene/polypropylene three-layer co-extrusion lithium ion battery diaphragm is characterized in that the diaphragm is of an A1/B/A2 three-layer structure, wherein the surface layers A1 and A2 are both porous structures formed by polypropylene copolymer; the middle layer B is a porous structure formed by high-density polyethylene; the total thickness of the A1/B/A2 three-layer structure is 9 mu m, and the thicknesses of the middle layer B and the surface layers A1 and A2 are equal;
the preparation method of the polypropylene/polyethylene/polypropylene three-layer co-extrusion lithium ion battery diaphragm is characterized by comprising the following specific steps:
(1) a melting step: respectively adding high-density polyethylene with the molecular weight of 50-60 ten thousand and the melt index of 1 at 190 ℃ and copolymerized polypropylene with the molecular weight of 30-40 ten thousand, the PE block content of 2 percent and the melt index of 8 at 230 ℃ into two extruders for melting and plasticizing; wherein the temperature of the conveying section of the high-density polyethylene extruder is 60-70 ℃, the temperature of the compression section is 160-190 ℃, the temperature of the melting section is 170-210 ℃, the temperature of the metering section is 170-195 ℃ and the rotating speed of a screw is 300 rpm; the temperature of a conveying section of the copolymerization polypropylene extruder is 60-70 ℃, the temperature of a compression section is 160-220 ℃, the temperature of a melting section is 190-215 ℃, the temperature of a metering section is 190-205 ℃, and the rotating speed of a screw is 300 rpm;
(2) casting: synchronously co-extruding the melted and plasticized high-density polyethylene and the copolymerized polypropylene obtained in the step (1) from a three-layer composite casting die head with three cavities, and drawing the mixture to form a film under a high draft ratio to obtain a three-layer co-extrusion casting film with an A1/B/A2 structure, wherein the die head temperature is 215 ℃, the die opening is 3.0mm, the casting roller speed is 80m/min, and the cooling roller temperature is 80 ℃;
(3) and (3) annealing: annealing the three-layer co-extrusion casting film with the A1/B/A2 structure obtained in the step (2), wherein the annealing temperature is 130 ℃, the annealing time is 20 hours, and then naturally cooling to the room temperature;
(4) a stretching step: sequentially carrying out longitudinal cold stretching and hot stretching on the annealed casting film obtained in the step (3), wherein the cold stretching temperature is 50 ℃, and the stretching speed ratio is 1.5; the hot-drawing temperature is 150 ℃, and the drawing speed ratio is 3.0, so that a porous structure is formed;
(5) heat setting: and (4) carrying out heat setting on the porous membrane obtained in the step (4) at 140 ℃ for 10 s-5 min, and then cooling to room temperature to obtain the polypropylene/polyethylene/polypropylene three-layer co-extrusion lithium ion battery diaphragm.
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Denomination of invention: Polypropylene / polyethylene / polypropylene three-layer coextrusion lithium ion battery diaphragm and its preparation method Effective date of registration: 20220622 Granted publication date: 20201020 Pledgee: Bank of China Limited Wuhan Jianghan sub branch Pledgor: HUIQIANG (WUHAN) NEW ENERGY MATERIAL TECHNOLOGY CO.,LTD. Registration number: Y2022420000176 |
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