CN114274484A - Polyethylene microporous membrane, preparation method thereof and lithium ion battery - Google Patents

Polyethylene microporous membrane, preparation method thereof and lithium ion battery Download PDF

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Publication number
CN114274484A
CN114274484A CN202111331819.2A CN202111331819A CN114274484A CN 114274484 A CN114274484 A CN 114274484A CN 202111331819 A CN202111331819 A CN 202111331819A CN 114274484 A CN114274484 A CN 114274484A
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stretching
polyethylene
film
membrane
microporous membrane
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宋红芹
王小记
胡玉涛
周世蛟
王月琴
王莉
柳青
李茜茜
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Lucky Film Co Ltd
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Lucky Film Co Ltd
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Abstract

The invention provides a polyethylene microporous membrane, a preparation method thereof and a lithium ion battery, wherein the preparation method of the polyethylene microporous membrane comprises the following steps: (1) carrying out melt extrusion on polyethylene resin and paraffin oil to obtain a melt; (2) casting the melt into slabs; (3) longitudinally drawing and stretching the resin sheet to obtain a longitudinally drawn film; (4) carrying out first transverse stretching on the longitudinally-stretched membrane so as to obtain a stretched oil film; (5) extracting the stretched oil film to remove the paraffin oil in the stretched oil film, and removing an extracting agent to form a microporous film; (6) carrying out secondary transverse stretching on the microporous membrane, wherein the temperature of the secondary transverse stretching is 115-135 ℃, and the magnification of the secondary transverse stretching is 1.1-1.8 times; (7) and (4) performing heat setting on the film subjected to the second transverse stretching. Therefore, the flexibility of the polyethylene microporous membrane has controllability, and the permeability and the membrane resistance of the polyethylene microporous membrane are further improved.

Description

Polyethylene microporous membrane, preparation method thereof and lithium ion battery
Technical Field
The invention relates to the technical field of lithium ion battery diaphragms, in particular to a polyethylene microporous membrane, a preparation method thereof and a lithium ion battery.
Background
The polyethylene microporous membrane is often used as a diaphragm of a lithium ion battery, and has important significance for blocking electrons from passing through, preventing short circuit, ensuring internal ions to permeate through and enabling the battery to operate efficiently, stably and safely. The pore size of the polyethylene microporous film and the tortuosity of the polyethylene microporous film in the thickness direction directly influence the lithium ion passing performance, and further influence the charge and discharge performance, the cycle performance, the safety and the like of the lithium battery. The higher the tortuosity is, the more difficult the lithium ion moves in the micropores is, the higher the resistance is, but the higher the tortuosity is, the advantages of preventing the positive and negative electrodes from contacting, preventing the lithium dendrite from piercing the diaphragm, reducing self-discharge and the like can be realized, and the cycle life of the battery is prolonged. The smaller the tortuosity is, the easier the lithium ion moves in the micropore, and the smaller the resistance is, so that the lithium ion battery is more suitable for the requirement of quick charge and discharge of a high-rate lithium battery. Therefore, the adjustment and control of the tortuosity has important significance for improving the performance of the lithium ion battery.
The wet-process biaxially oriented polyethylene film adopts an extraction process to form pores and prepare a three-dimensional network structure, and compared with the diaphragm prepared by a dry process, the diaphragm prepared by the wet-process biaxially oriented polyethylene film has the characteristics of small pore diameter and high pore tortuosity. How to regulate and control the tortuosity of a wet-process polyethylene diaphragm is a problem which is solved by technical personnel in the field of wet-process diaphragms. In the prior art, the tortuosity of the micropores of the diaphragm is regulated and controlled by adopting different casting sheet temperatures and PE compositions respectively. Although it does produce some effect, the degree of influence is relatively limited.
Therefore, the current method for preparing polyethylene microporous films needs to be further improved.
Disclosure of Invention
The present invention is directed to solving, at least to some extent, one of the technical problems in the related art. Therefore, an object of the present invention is to provide a polyethylene microporous membrane, a preparation method thereof, and a lithium ion battery, wherein the preparation method of the polyethylene microporous membrane can regulate and control the tortuosity of micropores of the polyethylene microporous membrane, and effectively improve the permeability and membrane resistance of the polyethylene microporous membrane.
To this end, in one aspect of the present invention, there is provided a method for preparing a polyethylene microporous membrane, comprising: (1) carrying out melt extrusion on polyethylene resin and paraffin oil so as to obtain a melt; (2) casting the melt into slabs; (3) longitudinally drawing and stretching the thick sheet to obtain a longitudinally drawn film; (4) carrying out first transverse stretching on the longitudinally-stretched membrane so as to obtain a stretched oil film; (5) extracting the stretched oil film to remove the paraffin oil in the stretched oil film, and removing an extracting agent to form a microporous film; (6) performing secondary transverse stretching on the microporous membrane, wherein the temperature of the secondary transverse stretching is 115-135 ℃, and the magnification of the secondary transverse stretching is 1.1-1.8 times; (7) and (4) performing heat setting on the film subjected to the second transverse stretching. Therefore, the flexibility of the prepared polyethylene microporous membrane has controllability, and the permeability and the membrane resistance of the polyethylene microporous membrane are further improved.
In some embodiments of the present invention, in the step (7), the heat-setting temperature is 120 to 135 ℃, preferably 125 to 134 ℃, and most preferably 128 to 133 ℃.
In some embodiments of the present invention, in step (7), the heat-set shrinkage ratio is 8 to 25%, preferably 10 to 23%, and most preferably 15 to 20%.
In some embodiments of the present invention, in step (6), the temperature of the second transverse stretching is 120 to 134 ℃, and most preferably 125 to 133 ℃.
In some embodiments of the present invention, in the step (6), the second transverse stretching ratio is 1.2 to 1.6 times, and most preferably 1.3 to 1.5 times.
In another aspect of the present invention, a polyethylene microporous membrane is provided, which is prepared by the aforementioned method. Therefore, the method has all the characteristics and advantages of the polyethylene microporous membrane prepared by the method, and the detailed description is omitted. Overall, there is at least the advantage of a controllable degree of tortuosity.
In some embodiments of the invention, the polyethylene microporous membrane has a thickness of 5 to 25 μm. Thereby, the permeability and the service life of the polyethylene microporous membrane are improved.
In some embodiments of the invention, the micropores of the polyethylene microporous membrane have an average pore size of 30 to 80 nm. This improves the permeability of the polyethylene microporous membrane.
In some embodiments of the invention, the micropores of the polyethylene microporous membrane have a tortuosity of 1.5 to 3.5. Therefore, the requirements of batteries with different characteristics are met.
In some embodiments of the invention, the polyethylene microporous membrane has an air permeability of 80 to 500s/100 mL. Therefore, the requirements of batteries with different characteristics are met.
In some embodiments of the invention, the polyethylene microporous membrane has a porosity of 30% to 48%. Therefore, the requirements of batteries with different characteristics are met.
In another aspect of the present invention, a lithium ion battery is provided, wherein the foregoing polyethylene microporous membrane is used as a separator. Therefore, the lithium ion battery has all the characteristics and advantages of the polyethylene microporous membrane, and the description is omitted. Generally speaking, the method has the advantage that the flexibility of the polyethylene microporous membrane is adjustable, so that the cycle characteristic, the self-discharge and the rate characteristic of the lithium ion battery are improved, and the requirements of batteries with different characteristics are met.
Drawings
The above and/or additional aspects and advantages of the present invention will become apparent and readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which:
fig. 1 shows a schematic flow diagram of a method for preparing a polyethylene microporous membrane according to an embodiment of the present invention.
Detailed Description
The following describes embodiments of the present invention in detail. The following examples are illustrative only and are not to be construed as limiting the invention. The examples, where specific techniques or conditions are not indicated, are to be construed according to the techniques or conditions described in the literature in the art or according to the product specifications. The reagents or instruments used are not indicated by the manufacturer, and are all conventional products commercially available.
In one aspect of the present invention, a method for preparing a polyethylene microporous membrane is provided, comprising: the method comprises the steps of performing melt extrusion on polyethylene resin and paraffin oil to obtain a melt, casting the melt into a thick sheet, performing longitudinal drawing and stretching on the thick sheet to obtain a longitudinal drawing membrane, performing first transverse drawing on the longitudinal drawing membrane to obtain a drawing oil film, extracting the drawing oil film to remove the paraffin oil in the drawing oil film, removing an extracting agent to form a microporous membrane, performing second transverse drawing on the microporous membrane at the drawing temperature of 115-135 ℃, wherein the drawing multiplying factor is 1.1-1.8 times, and finally performing heat setting on the microporous membrane subjected to the second transverse drawing. Therefore, the flexibility of the prepared polyethylene microporous membrane has controllability, so that the permeability and the membrane resistance of the polyethylene microporous membrane are further improved.
For convenience of understanding, the following is a brief description of the principle by which the method can achieve the above-described advantageous effects:
as described above, the pore size and the tortuosity of the polyethylene microporous membrane have an important influence on the lithium ion passage performance, and the tortuosity is a characteristic of the degree to which a single micropore is bent back in the thickness direction. The larger the tortuosity is, the larger the bending degree of the micropores along the thickness direction of the polyethylene microporous membrane is, the more difficult the lithium ions move in the micropores, and the larger the membrane resistance is; the smaller the tortuosity is, the smaller the degree of bending and folding back of the micropores along the thickness direction of the polyethylene microporous membrane is, the easier the lithium ions move in the micropores, and the smaller the corresponding membrane resistance is, so that the regulation and control of the tortuosity of the polyethylene microporous membrane has significance for improving the performance of the lithium ion battery. The inventor of the present invention found through research that, in the wet-process separator production process, after the casting, the longitudinal stretching and the first transverse stretching, liquid drops of paraffin oil are distributed among molecular chains of the polyethylene resin, and thus they have an important influence on the formation of the original microporous structure. However, since the extractant displaces the paraffin oil in the subsequent extraction stage and then is replaced by air in the drying stage, a white microporous membrane with a microporous structure is formed. The micropores formed primarily in this process are affected by volatilization and drying of the extractant, and therefore, the micropores are small in size and poor in air permeability, and are difficult to play a role of the separator in the battery. Therefore, the microporous structure needs to be adjusted in the subsequent second transverse stretching and heat setting stage, so that the size and the tortuosity of the micropores meet the design requirements, and the requirements of lithium ion batteries with different functions are met. The invention controls the second transverse stretching and heat setting process to make the micropore generate different structural changes in the membrane surface direction and the thickness direction, and change the tortuosity of the micropore, thereby achieving the purpose of controlling the tortuosity of the micropore. Specifically, the structure of micropores in the film surface direction can be obviously improved by regulating and controlling the stretching temperature and stretching ratio of the second transverse stretching, so that the smaller micropores formed preliminarily after extraction are obviously enlarged; by regulating and controlling the retraction ratio and the heat setting temperature in the heat setting process, the micropores shrink and collapse in different degrees in the film surface direction and the film thickness direction, and further the tortuosity of the polyethylene microporous film is changed, so that the polyethylene microporous film with different tortuosity is obtained, and the design requirements are met.
The individual steps of the method are described in detail below according to an embodiment of the invention. Referring to fig. 1, the method may include:
s100: the polyethylene resin and the paraffin oil are melted and extruded
In this step, a polyethylene resin and paraffin oil are melt-extruded to obtain a melt.
According to some embodiments of the present invention, the content of the polyethylene resin in the melt is 20 wt% to 35 wt%, specifically, 22 wt%, 24 wt%, 26 wt%, 28 wt%, 30 wt%, 32 wt%, 34 wt%, etc., based on the total mass of the polyethylene resin and the paraffin oil, and thus, the content of the polyethylene resin in the above range can ensure good melt plasticization of the resin, form a sufficient number of primary micropores with the paraffin oil, and the melt can maintain sufficient strength to form a stable slab for subsequent drawing. The preparation method of the melt is not particularly limited, and those skilled in the art can freely select the melt according to actual needs, for example, the polyethylene resin and paraffin oil can be fed into a melting device commonly used in the art, such as a twin-screw extruder, to be melted and extruded to obtain the melt.
S200: casting the melt into slabs
In the step, the blended melt can be conveyed to an extrusion die head and other common equipment in the field through a metering pump for extrusion, and after a cast film is extruded, the blended melt is cooled and rolled through a casting sheet roller and other common rolling equipment in the field to be cast into a thick sheet, wherein the thickness of the thick sheet is 500-2500 microns. When the thickness of the thick sheet is within the range, on one hand, the melt can be fully cooled, and on the other hand, certain thickness is more beneficial to subsequent high-magnification stretching.
S300: subjecting the thick slices to vertical drawing
In this step, the thick sheet is subjected to longitudinal drawing by a longitudinal drawing machine to obtain a longitudinally drawn film. The longitudinal drawing machine consists of a preheating section, a stretching section and a shaping section. The stretching section consists of 3-5 stretching rollers, the roller speed of the next stretching roller is greater than that of the previous stretching roller, and the longitudinal stretching ratio is the ratio of the roller speed of the last stretching roller to that of the first stretching roller. After entering a longitudinal drawing machine, the thick sheet is drawn into a longitudinal drawing film under certain drawing temperature and drawing multiplying power, and molecular chains of the polyethylene resin are strongly oriented in the longitudinal direction, so that the microporous film has certain strength in the longitudinal direction. According to some embodiments of the present invention, the drawing temperature of the longitudinal drawing is 90 to 110 ℃, and when the longitudinal drawing temperature is in the above range, the polyethylene resin can be sufficiently softened, and drawing can be more easily performed in this state to obtain a certain degree of longitudinal orientation and crystallization. The stretching ratio is 5-15 times, if the stretching ratio is in the range, the micropores can be fully oriented in the longitudinal direction, on one hand, the microporous membrane can obtain enough high longitudinal stretching strength to meet the requirement of the product on the strength, and on the other hand, the size of the micropores in the longitudinal direction is lengthened.
S400: subjecting the longitudinally-stretched film to first transverse stretching
In this step, the longitudinally-stretched film is subjected to a first transverse stretching by a transverse stretching box to obtain a stretched oil film, and the paraffin oil droplets are distributed among molecular chains of the polyethylene resin. The transverse drawing box also comprises a preheating section, a stretching section and a shaping section, wherein the longitudinal drawing film is clamped into the transverse drawing box by a chain clamp at the inlet of the transverse drawing box, the width of the last stretching track is larger than that of the previous stretching track, and the transverse drawing multiplying power is the ratio of the width of the last stretching track to that of the first stretching track. And longitudinally drawing the membrane to finish the first transverse drawing under a certain drawing ratio and drawing temperature.
According to some embodiments of the present invention, the stretching ratio and the stretching temperature in the first transverse stretching are not particularly limited, and can be selected by those skilled in the art according to actual needs, and according to some embodiments of the present invention, the stretching temperature in the first transverse stretching can be 100 ℃ to 130 ℃, and when the temperature in the first transverse stretching is in the above range, the film sheet can make the polyethylene molecular chains more easily stretched and oriented in the transverse direction in a relatively soft state, so that a sufficiently high transverse stretching strength can be obtained. The stretching ratio can be 4-12 times, when the stretching ratio is in the range, the polyethylene molecular chains obtain enough stretching and orientation in the transverse direction, on one hand, the microporous film obtains enough transverse direction stretching strength to meet the requirement of the product on transverse direction strength, on the other hand, the size of the microporous film in the transverse direction is enlarged, and a foundation is laid for obtaining good air permeability subsequently.
S500: extracting the stretched oil film
In the step, the stretched oil film is extracted through an extraction tank, paraffin oil in the stretched oil film is removed, and then the extraction agent is dried and removed, according to some embodiments of the invention, the type of the extraction agent is not particularly limited as long as the paraffin oil in the oil-containing base film can be effectively removed, and for example, the extraction agent may include but is not limited to at least one of dichloromethane, acetone and ethanol, at this time, air replaces the paraffin oil to occupy the positions of micropores, so as to form a white microporous film, the average pore diameter of the micropores in the microporous film is 20-35 nm, and the tortuosity of the micropores is 1.2-1.8.
S600: second transverse stretching
In this step, the white microporous film is subjected to a second transverse stretching at a stretching temperature and a stretching ratio to expand the microporous structure in the film surface direction, wherein the stretching temperature of the second transverse stretching is 115 to 135 ℃, specifically, 118 ℃, 120 ℃, 122 ℃, 124 ℃, 126 ℃, 128 ℃, 130 ℃, 132 ℃, 134 ℃ and the like, preferably, the temperature of the second transverse stretching is 120 ℃ to 134 ℃, and more preferably, the temperature of the second transverse stretching is 125 ℃ to 133 ℃. The stretching temperature of the second transverse stretching is limited in the range, so that on one hand, the polyethylene resin is convenient to stretch and orient at a higher temperature, and micropores are convenient to expand in the film surface direction; the inventor finds that if the stretching temperature of the second transverse stretching is too high, the molecular chains of the polyethylene are over-oriented in the transverse direction, and simultaneously, the molecular chains are longitudinally de-oriented, so that the longitudinal orientation is weakened, and the microporous membrane is easy to tear; if the stretching temperature of the second transverse stretching is too low, the difficulty of stretching the polyethylene molecular chains is increased, the stretching and the orientation in the transverse direction are not facilitated, and the micropores are smaller. The stretch ratio is 1.1 to 1.8 times, specifically, the stretch ratio may be 1.2 times, 1.3 times, 1.4 times, 1.5 times, 1.6 times, 1.7 times, or the like, preferably, the stretch ratio in the second transverse stretch is 1.2 to 1.6 times, more preferably, the stretch ratio is 1.3 to 1.5 times. By limiting the stretch ratio of the second transverse stretching to the above range, the polyethylene resin can be sufficiently oriented, and the micropores can be enlarged in the film surface direction, thereby obtaining excellent air permeability. The inventor finds that if the magnification of the second transverse stretching is too large, the microporous membrane is easy to tear due to the fact that the micropores are stretched in the membrane surface direction too much, molecular chains are subjected to longitudinal disorientation, and the longitudinal tensile strength is reduced; if the magnification of the second transverse stretching is too small, the micropores stretch insufficiently in the film surface direction, and are smaller, so that the air permeability of the product is poor.
S700: heat setting
In the step, the membrane after the second transverse stretching is subjected to high-temperature heat setting at a certain temperature and retraction ratio, so that the micropores generate structural changes to a certain degree in the membrane surface and the membrane thickness direction, the tortuosity of the micropores is changed, and the final polyethylene microporous membrane is obtained after cooling.
According to some embodiments of the present invention, the heat-setting temperature during the heat-setting process is 120 ℃ to 135 ℃, for example, 122 ℃, 124 ℃, 126 ℃, 128 ℃, 130 ℃, 132 ℃, etc., preferably, the heat-setting temperature is 125 ℃ to 134 ℃, and more preferably, the heat-setting temperature is 128 ℃ to 133 ℃. When the heat-setting temperature is limited to the above range, on the one hand, the micropores can be appropriately shrunk in the film surface direction, collapsed to some extent in the thickness direction, and the tortuosity can be increased to obtain desired air permeability. On the other hand, the internal stress generated in the previous stretching orientation process can be fully eliminated, and the transverse and longitudinal heat shrinkage rate can be reduced. The inventor finds that if the temperature is too high in the heat setting process, micropores excessively shrink in the transverse direction, the pore diameter is too small, and the micropores excessively collapse in the thickness direction, so that the tortuosity is too high, and the air permeability of a product is reduced; if the temperature is too low among the heat setting process, the micropore aperture is big, and the tortuosity is low, though air permeability is good, but be unfavorable for reducing the internal stress, the horizontal and vertical thermal shrinkage rate of product can be on the high side, and puncture strength can obviously descend simultaneously, brings the potential safety hazard for the battery.
According to some embodiments of the present invention, the shrinkage ratio is 8-25%, specifically, 10%, 12%, 14%, 16%, 18%, 20%, 22%, 24%, etc., preferably, the shrinkage ratio of the heat setting is 10-23%, and more preferably, the shrinkage ratio of the heat setting is 15-20%. By limiting the shrinkage ratio in the heat setting process to the above range, the micropores can be appropriately shrunk in the film surface direction to obtain a desired pore diameter. On the other hand, the internal stress generated in the previous stretching orientation process can be fully eliminated, and the transverse and longitudinal heat shrinkage rates are reduced. The inventor finds that if the retraction ratio is less than 8%, the aperture of the micropores is too large, so that the puncture strength is reduced, meanwhile, the transverse internal stress is not sufficiently eliminated, the transverse heat shrinkage rate of the product is higher, and potential safety hazards exist in the use process of the battery; if the retraction ratio is more than 25%, the aperture of the micropores is too small, the air permeability of the product is greatly reduced, and the requirement of the battery on the air permeability of the diaphragm cannot be met.
According to the preparation method of the polyethylene microporous membrane, the structure and the tortuosity of the polyethylene microporous membrane are effectively regulated and controlled by regulating and controlling the secondary transverse stretching process and the heat setting process, so that the permeability and the membrane resistance of the membrane can be effectively improved, and the requirements of batteries with different characteristics are met.
According to the embodiment of the present invention, the thickness of the polyethylene microporous membrane is 5 to 25 μm, specifically, 8 μm, 10 μm, 12 μm, 15 μm, 18 μm, 21 μm, 23 μm, etc., and the inventors found that if the thickness of the polyethylene microporous membrane is less than 5 μm, the microporous membrane is easily broken, and safety is poor; if the thickness of the polyethylene microporous film is more than 25 μm, the distance of lithium ions passing through the microporous film becomes large, and the internal resistance of the microporous film increases.
According to the embodiment of the invention, the average pore diameter of the polyethylene microporous membrane is 30-80nm, specifically, 40nm, 50nm, 60nm, 70nm and the like, if the average pore diameter of the polyethylene microporous membrane is less than 30nm, the internal resistance of the microporous membrane is increased, and the permeability is reduced; if the average pore diameter of the polyethylene microporous membrane is larger than 80nm, the contact between a positive electrode and a negative electrode is easily caused, so that micro short circuit is caused, and the self-discharge phenomenon of the battery is increased.
According to an embodiment of the present invention, the tortuosity of the polyethylene microporous membrane is 1.5 to 3.5, and specifically, may be 2.0, 2.5, 3.0, or the like. According to some embodiments of the present invention, if the tortuosity of the polyethylene microporous membrane is less than 1.5, the microporous membrane approaches the straight through hole, and the positive electrode and the negative electrode are easy to contact, causing a micro short circuit; if the tortuosity of the polyethylene microporous membrane is more than 3.5, the internal resistance of the microporous membrane is increased, the penetration capacity of lithium ions is reduced, and the permeability of the microporous membrane is reduced.
According to some embodiments of the present invention, the polyethylene microporous membrane has an air permeability of 80 to 500s/100mL, and specifically, may be 100s/100mL, 150s/100mL, 200s/100mL, 250s/100mL, 300s/100mL, 350s/100mL, 400s/100mL, 450s/100mL, or the like. If the air permeability of the polyethylene microporous membrane is less than 80s/100mL, the time required for air to pass through the microporous membrane is longer, and the internal resistance of the microporous membrane is higher; if the air permeability of the polyethylene microporous membrane is more than 500s/100mL, the contact between a positive electrode and a negative electrode is easily caused, so that micro short circuit is caused, and the self-discharge phenomenon of the battery is increased.
According to some embodiments of the present invention, the porosity of the polyethylene microporous membrane is 30% to 48%, and specifically, may be 32%, 34%, 36%, 38%, 40%, 42%, 44%, 46%, and the like. If the porosity of the polyethylene microporous membrane is less than 30%, the time required for air to pass through the microporous membrane is longer, and the internal resistance of the microporous membrane is higher; if the porosity of the polyethylene microporous membrane is more than 48%, the contact between a positive electrode and a negative electrode is easily caused, so that micro short circuit is caused, and the self-discharge phenomenon of the battery is increased.
In another aspect of the present invention, a polyethylene microporous membrane is provided, which is prepared by the aforementioned method. Therefore, the method has all the characteristics and advantages of the polyethylene microporous membrane prepared by the method, and the detailed description is omitted. Overall, there is at least the advantage of a controllable degree of tortuosity.
In another aspect of the present invention, a lithium ion battery is provided, wherein the foregoing polyethylene microporous membrane is used as a separator. Therefore, the lithium ion battery has all the characteristics and advantages of the polyethylene microporous membrane, and the description is omitted. Generally speaking, the method has the advantage that the flexibility of the polyethylene microporous membrane is adjustable, so that the cycle characteristic, the self-discharge and the rate characteristic of the lithium ion battery are improved, and the requirements of batteries with different characteristics are met.
Performance testing
Thickness: measuring by using an automatic thickness gauge, randomly selecting 30 parts for measurement, and averaging;
average pore diameter: testing by adopting a bubble point PB filter membrane aperture analyzer, and processing an analysis result according to the analyzer to obtain an average aperture;
air permeability: testing by using an automatic air permeability instrument, randomly selecting 15 parts for measurement, and averaging;
porosity: weighing method, and calculating according to formula to obtain: p ═ 1-m/(s ═ d-0)]100%, wherein: p is the membrane porosity, expressed in%; m is the diaphragm mass in g; s is the area of the diaphragm in cm2(ii) a d is the thickness of the diaphragm in cm; rho0Is the density of the raw material and has the unit of g/cm3
Tortuosity: according to an empirical formula, the following calculation results:
Figure BDA0003349158920000071
in the formula: t is air permeability with the unit of s/100 mL; l is the thickness of the diaphragm, and the unit is mum; d is the micropore diameter, unit: nm; ε is the porosity; τ is the pore tortuosity.
Example 1
(1) Melt extrusion: and (2) performing melt extrusion on the polyethylene resin and paraffin oil through a double-screw extruder to obtain a melt, wherein the mass fraction of the polyethylene resin is 25% and the mass fraction of the paraffin oil is 75% based on the total mass of the polyethylene resin and the paraffin oil.
(2) Cooling the cast sheet: cooling the melt through a cooling roller, and casting the melt into a thick sheet, wherein the thickness of the thick sheet is 1000 mu m;
(3) longitudinal stretching: longitudinally drawing and stretching the thick sheet by a longitudinal drawing machine at the stretching temperature of 92 ℃ and the stretching ratio of 8 times to obtain a longitudinally drawn film;
(4) first transverse stretching: carrying out first transverse stretching on the longitudinally-stretched film through a transverse stretching box, wherein the stretching temperature is 115 ℃, and the stretching magnification is 8.5, so as to obtain a stretched oil film;
(5) extracting the stretched oil film by an extraction tank, wherein an extracting agent is dichloromethane, removing paraffin oil in the stretched oil film, and drying and removing the extracting agent to form a white microporous film, wherein the aperture of micropores on the white microporous film is 35nm, and the tortuosity is 1.6;
(6) performing secondary transverse stretching on the white microporous membrane, wherein the stretching temperature is 130 ℃, and the stretching magnification is 1.5 times;
(7) and (3) carrying out high-temperature heat setting on the microporous membrane subjected to the second transverse stretching, wherein the heat setting temperature is 132 ℃, and the retraction ratio is 22%.
Example 2
(1) Melt extrusion: and (2) performing melt extrusion on the polyethylene resin and paraffin oil through a double-screw extruder to obtain a melt, wherein the mass fraction of the polyethylene resin is 28% and the mass fraction of the paraffin oil is 72% based on the total mass of the polyethylene resin and the paraffin oil.
(2) Cooling the cast sheet: cooling the melt through a cooling roller, and casting the melt into a thick sheet with the thickness of 2000 mu m;
(3) longitudinal stretching: longitudinally drawing and stretching the thick sheet by a longitudinal drawing machine, wherein the drawing temperature is 102 ℃; the stretching multiplying power is 6.5 times, and a longitudinally-stretched membrane is obtained;
(4) first transverse stretching: carrying out primary transverse stretching on the longitudinally-stretched membrane through a transverse stretching box, wherein the stretching temperature is 116 ℃; the stretching multiplying power is 7, and a stretching oil film is obtained;
(5) extracting the stretched oil film by an extraction tank, wherein an extracting agent is dichloromethane, removing paraffin oil in the stretched oil film, and drying and removing the extracting agent to form a white microporous film, wherein the aperture of micropores on the white microporous film is 30nm, and the tortuosity is 1.3;
(6) performing secondary transverse stretching on the white microporous membrane, wherein the stretching temperature is 132 ℃, and the stretching magnification is 1.7 times;
(7) and (3) performing high-temperature heat setting on the microporous membrane subjected to the second transverse stretching, wherein the heat setting temperature is 128 ℃, and the retraction ratio is 20%.
Example 3
(1) Melt extrusion: and (2) performing melt extrusion on the polyethylene resin and paraffin oil through a double-screw extruder to obtain a melt, wherein the mass fraction of the polyethylene resin is 27% and the mass fraction of the paraffin oil is 73% based on the total mass of the polyethylene resin and the paraffin oil.
(2) Cooling the cast sheet: cooling the melt through a cooling roller, and casting into a thick sheet with the thickness of 1500 mu m;
(3) longitudinal stretching: longitudinally drawing and stretching the thick sheet by a longitudinal drawing machine at the stretching temperature of 100 ℃ and the stretching ratio of 7 times to obtain a longitudinally drawn film;
(4) first transverse stretching: carrying out first transverse stretching on the longitudinally-stretched film through a transverse stretching box at the stretching temperature of 122 ℃ and the stretching magnification of 7 to obtain a stretched oil film;
(5) extracting the stretched oil film by an extraction tank, wherein an extracting agent is dichloromethane, removing paraffin oil in the stretched oil film, and drying and removing the extracting agent to form a white microporous film, wherein the aperture of micropores on the white microporous film is 32nm, and the tortuosity is 1.4;
(6) performing secondary transverse stretching on the white microporous membrane, wherein the stretching temperature is 133 ℃, and the stretching magnification is 1.8 times;
(7) and (3) performing high-temperature heat setting on the microporous membrane subjected to the second transverse stretching, wherein the heat setting temperature is 133 ℃, and the retraction ratio is 25%.
Example 4
(1) Melt extrusion: and (2) performing melt extrusion on the polyethylene resin and paraffin oil through a double-screw extruder to obtain a melt, wherein the mass fraction of the polyethylene resin is 25% and the mass fraction of the paraffin oil is 75% based on the total mass of the polyethylene resin and the paraffin oil.
(2) Cooling the cast sheet: cooling the melt through a cooling roller, and casting the melt into a thick sheet, wherein the thickness of the thick sheet is 1300 mu m;
(3) longitudinal stretching: longitudinally drawing and stretching the thick sheet by a longitudinal drawing machine at the stretching temperature of 96 ℃ and the stretching magnification of 6.6 times to obtain a longitudinally drawn membrane;
(4) first transverse stretching: carrying out first transverse stretching on the longitudinally-stretched membrane through a transverse stretching box at the stretching temperature of 120 ℃ and the stretching magnification of 6.2 to obtain a stretched oil film;
(5) extracting the stretched oil film by an extraction tank, wherein an extracting agent is dichloromethane, removing paraffin oil in the stretched oil film, and drying and removing the extracting agent to form a white microporous film, wherein the aperture of micropores on the white microporous film is 28nm, and the tortuosity is 1.3;
(6) carrying out secondary transverse stretching on the white microporous membrane, wherein the stretching temperature is 125 ℃, and the stretching magnification is 1.5 times;
(7) and (3) carrying out high-temperature heat setting on the microporous membrane subjected to the second transverse stretching, wherein the heat setting temperature is 130 ℃, and the retraction ratio is 20%.
Example 5
(1) Melt extrusion: and (2) performing melt extrusion on the polyethylene resin and paraffin oil through a double-screw extruder to obtain a melt, wherein the mass fraction of the polyethylene resin is 24% and the mass fraction of the paraffin oil is 76% based on the total mass of the polyethylene resin and the paraffin oil.
(2) Cooling the cast sheet: cooling the melt through a cooling roller, and casting the melt into a thick sheet with the thickness of 800 mu m;
(3) longitudinal stretching: longitudinally drawing and stretching the thick sheet by a longitudinal drawing machine at the stretching temperature of 90 ℃ and the stretching ratio of 9 times to obtain a longitudinally drawn film;
(4) first transverse stretching: carrying out first transverse stretching on the longitudinally-stretched film through a transverse stretching box at the stretching temperature of 108 ℃ and the stretching magnification of 9 to obtain a stretched oil film;
(5) extracting the stretched oil film by an extraction tank, wherein an extracting agent is dichloromethane, removing paraffin oil in the stretched oil film, and drying and removing the extracting agent to form a white microporous film, wherein the aperture of micropores on the white microporous film is 32nm, and the tortuosity is 1.5;
(6) performing secondary transverse stretching on the white microporous membrane, wherein the stretching temperature is 120 ℃, and the stretching magnification is 1.4 times;
(7) and (3) performing high-temperature heat setting on the microporous membrane subjected to the second transverse stretching, wherein the heat setting temperature is 135 ℃, and the retraction ratio is 25%.
Example 6
(1) Melt extrusion: and (2) performing melt extrusion on the polyethylene resin and paraffin oil through a double-screw extruder to obtain a melt, wherein the mass fraction of the polyethylene resin is 28% and the mass fraction of the paraffin oil is 72% based on the total mass of the polyethylene resin and the paraffin oil.
(2) Cooling the cast sheet: cooling the melt through a cooling roller, and casting into a thick sheet with the thickness of 1100 mu m;
(3) longitudinal stretching: longitudinally drawing and stretching the thick sheet by a longitudinal drawing machine at the stretching temperature of 95 ℃ and the stretching ratio of 7.8 times to obtain a longitudinally drawn film;
(4) first transverse stretching: carrying out first transverse stretching on the longitudinally-stretched membrane through a transverse stretching box at the stretching temperature of 114 ℃ and the stretching magnification of 7.5 to obtain a stretched oil film;
(5) extracting the stretched oil film by an extraction tank, wherein an extracting agent is dichloromethane, removing paraffin oil in the stretched oil film, and drying and removing the extracting agent to form a white microporous film, wherein the aperture of micropores on the white microporous film is 24nm, and the tortuosity is 1.2;
(6) carrying out secondary transverse stretching on the white microporous membrane, wherein the stretching temperature is 125 ℃, and the stretching magnification is 1.3 times;
(7) and (3) carrying out high-temperature heat setting on the microporous membrane subjected to the second transverse stretching, wherein the heat setting temperature is 120 ℃, and the retraction ratio is 16%.
Example 7
(1) Melt extrusion: and (2) performing melt extrusion on the polyethylene resin and paraffin oil through a double-screw extruder to obtain a melt, wherein the mass fraction of the polyethylene resin is 27% and the mass fraction of the paraffin oil is 73% based on the total mass of the polyethylene resin and the paraffin oil.
(2) Cooling the cast sheet: cooling the melt through a cooling roller, and casting the melt into a thick sheet, wherein the thickness of the thick sheet is 650 mu m;
(3) longitudinal stretching: longitudinally drawing and stretching the thick sheet by a longitudinal drawing machine at the stretching temperature of 90 ℃ and the stretching ratio of 7 times to obtain a longitudinally drawn film;
(4) first transverse stretching: carrying out first transverse stretching on the longitudinally-stretched film through a transverse stretching box at the stretching temperature of 107 ℃ and the stretching magnification of 7 to obtain a stretched oil film;
(5) extracting the stretched oil film by an extraction tank, wherein an extracting agent is dichloromethane, removing paraffin oil in the stretched oil film, and drying and removing the extracting agent to form a white microporous film, wherein the aperture of micropores on the white microporous film is 22nm, and the tortuosity is 1.2;
(6) transversely stretching the white microporous membrane for the second time at the stretching temperature of 115 ℃ at the stretching magnification of 1.1 times;
(7) and (3) performing high-temperature heat setting on the microporous membrane subjected to the second transverse stretching, wherein the heat setting temperature is 128 ℃, and the retraction ratio is 8%.
Example 8
(1) Melt extrusion: and (2) performing melt extrusion on the polyethylene resin and paraffin oil through a double-screw extruder to obtain a melt, wherein the mass fraction of the polyethylene resin is 27% and the mass fraction of the paraffin oil is 73% based on the total mass of the polyethylene resin and the paraffin oil.
(2) Cooling the cast sheet: cooling the melt through a cooling roller, and casting the melt into a thick sheet with the thickness of 2500 mu m;
(3) longitudinal stretching: longitudinally drawing and stretching the thick sheet by a longitudinal drawing machine at the drawing temperature of 1058 ℃ and the drawing magnification of 7 times to obtain a longitudinally drawn film;
(4) first transverse stretching: carrying out first transverse stretching on the longitudinally-stretched film through a transverse stretching box at the stretching temperature of 123 ℃ and the stretching magnification of 7 to obtain a stretched oil film;
(5) extracting the stretched oil film by an extraction tank, wherein an extracting agent is dichloromethane, removing paraffin oil in the stretched oil film, and drying and removing the extracting agent to form a white microporous film, wherein the aperture of micropores on the white microporous film is 33nm, and the tortuosity is 1.6;
(6) transversely stretching the white microporous membrane for the second time, wherein the stretching temperature is 130 ℃, and the stretching magnification is 1.6 times;
(7) and (3) performing high-temperature heat setting on the microporous membrane subjected to the second transverse stretching, wherein the heat setting temperature is 135 ℃, and the retraction ratio is 16%.
Example 9
(1) Melt extrusion: and (2) performing melt extrusion on the polyethylene resin and paraffin oil through a double-screw extruder to obtain a melt, wherein the mass fraction of the polyethylene resin is 30% and the mass fraction of the paraffin oil is 70% based on the total mass of the polyethylene resin and the paraffin oil.
(2) Cooling the cast sheet: cooling the melt through a cooling roller, and casting into a thick sheet, wherein the thickness of the thick sheet is 2300 mu m;
(3) longitudinal stretching: longitudinally drawing and stretching the thick sheet by a longitudinal drawing machine at the stretching temperature of 108 ℃ and the stretching ratio of 7.8 times to obtain a longitudinally drawn film;
(4) first transverse stretching: carrying out first transverse stretching on the longitudinally-stretched membrane through a transverse stretching box, wherein the stretching temperature is 125 ℃, and the stretching magnification is 8, so as to obtain a stretched oil film;
(5) extracting the stretched oil film by an extraction tank, wherein an extracting agent is dichloromethane, removing paraffin oil in the stretched oil film, and drying and removing the extracting agent to form a white microporous film, wherein the aperture of micropores on the white microporous film is 35nm, and the tortuosity is 1.3;
(6) performing secondary transverse stretching on the white microporous membrane, wherein the stretching temperature is 132 ℃, and the stretching magnification is 1.8 times;
(7) and (3) performing high-temperature heat setting on the microporous membrane subjected to the second transverse stretching, wherein the heat setting temperature is 125 ℃, and the retraction ratio is 10%.
Comparative example 1
The difference from example 3 is that: (6) the white microporous film was subjected to a second transverse stretching at a stretching magnification of 2.0 times, and the rest was the same as in example 3.
Comparative example 2
The difference from example 3 is that: (6) the white microporous film was subjected to a second transverse stretching at a stretching temperature of 110 ℃ and otherwise the same as in example 3.
Comparative example 3
The difference from example 3 is that: (7) the microporous film after the second transverse stretching was subjected to high-temperature heat setting at 138 ℃, and the rest was the same as in example 3.
Comparative example 4
The difference from example 3 is that (7) the microporous film after the second transverse stretching was subjected to high-temperature heat setting, and the retraction ratio was 28%, which was otherwise the same as in example 3.
The process conditions of the above examples 1 to 9 and comparative examples 1 to 4 are shown in table 1, and the polyethylene microporous membranes prepared in the respective examples and comparative examples were respectively tested for thickness, average pore diameter, air permeability and porosity, and the resulting tortuosity was calculated, and the test and calculation results are shown in table 2.
TABLE 1 second transverse drawing and Heat-setting Process conditions
Transverse drawing temperature DEG C Transverse drawing magnification Heat setting temperature C Percent retraction
Example 1 130 1.5 132 22
Example 2 132 1.7 128 20
Example 3 133 1.8 133 25
Example 4 125 1.5 130 20
Example 5 120 1.4 135 25
Example 6 125 1.3 120 16
Example 7 115 1.1 128 8
Example 8 130 1.6 135 16
Example 9 132 1.8 125 10
Comparative example 1 133 2.0 133 25
Comparative example 2 110 1.8 133 25
Comparative example 3 133 1.8 138 25
Comparative example 4 133 1.8 133 28
TABLE 2 test results
Figure BDA0003349158920000131
As can be seen from the data in table 2, in examples 1 to 9, the size of the micropores is first enlarged in the film surface direction by adjusting the second transverse stretching process and the heat setting process and using the corresponding stretching temperature and stretching magnification, and then the size of the micropores in the film surface direction is reduced and the collapse width in the thickness direction is controlled by using the corresponding heat setting temperature and retraction ratio, so that the pore diameter and the tortuosity of the micropores are controlled. Meanwhile, the internal stress of the polyethylene film is eliminated, the expected air permeability is obtained, and the heat resistance of the product is also controlled, so that the foundation is laid for the subsequent successful application of the product in the battery. In contrast, in comparative example 1, the microporous membrane broke during the second transverse drawing due to the excessively large draw ratio; in comparative example 2, the micropores were not easily expanded due to too low stretching temperature, and the microporous membrane had poor air permeability; in the comparative example 3, due to the overhigh heat setting temperature, the obvious closed pore phenomenon appears, the air permeability is more than 800s/100mL, and the microporous membrane cannot be used; in comparative example 4, the pore diameter of the micropores was small and the gas permeability of the microporous film was too poor due to an excessively large heat-set shrinkage ratio. Thus, comparative examples 1 to 4 do not achieve the technical effects of the present invention.
When the microporous polyethylene film is applied to lithium ion batteries, polyethylene microporous films with different tortuosity can be adopted according to the requirements of battery characteristics, so that the cycle life and the rate capability of the batteries can be effectively improved, the self-discharge can be reduced, and the requirements of the batteries with different characteristics can be met.
Specific applications are shown in table 3 below:
TABLE 3 field of application of polyethylene microporous films of different tortuosity
Average pore diameter nm Tortuosity of Field of application of products Require that
40~60 2.0~3.0 Mobile phone, electric automobile and the like General of
40~60 2.5~3.0 Submarine, aviation, etc High safetyProperty of (2)
60~80 1.5~2.5 Large power equipment and the like Fast charge and discharge
30~50 3.0~3.5 Space station storage battery Ultra-long cycle life
In the description herein, references to the description of the term "one embodiment," "some embodiments," "an example," "a specific example," or "some examples," etc., mean that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the invention. In this specification, the schematic representations of the terms used above are not necessarily intended to refer to the same embodiment or example. Furthermore, the particular features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples. Furthermore, various embodiments or examples and features of different embodiments or examples described in this specification can be combined and combined by one skilled in the art without contradiction.
Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention, and that variations, modifications, substitutions and alterations can be made to the above embodiments by those of ordinary skill in the art within the scope of the present invention.

Claims (10)

1. A method of making a polyethylene microporous membrane, comprising:
(1) carrying out melt extrusion on polyethylene resin and paraffin oil so as to obtain a melt;
(2) casting the melt into slabs;
(3) longitudinally drawing and stretching the thick sheet to obtain a longitudinally drawn film;
(4) carrying out first transverse stretching on the longitudinally-stretched membrane so as to obtain a stretched oil film;
(5) extracting the stretched oil film to remove the paraffin oil in the stretched oil film, and removing an extracting agent to form a microporous film;
(6) performing secondary transverse stretching on the microporous membrane, wherein the temperature of the secondary transverse stretching is 115-135 ℃, and the magnification of the secondary transverse stretching is 1.1-1.8 times;
(7) and (4) performing heat setting on the film subjected to the second transverse stretching.
2. The process according to claim 1, wherein in step (7), the heat-setting temperature is 120 to 135 ℃, preferably 125 to 134 ℃, and most preferably 128 to 133 ℃.
3. The method according to claim 1, wherein in step (7), the heat-set shrinkage ratio is 8 to 25%, preferably 10 to 23%, and most preferably 15 to 20%.
4. A process according to any one of claims 1 to 3, characterized in that in step (6) the temperature of the second transverse stretching is 120 to 134 ℃, most preferably 125 to 133 ℃.
5. A process according to any one of claims 1 to 3, characterized in that in step (6), the magnification of the second transverse stretching is 1.2 to 1.6 times, most preferably 1.3 to 1.5 times.
6. A microporous polyethylene membrane produced by the method of any one of claims 1 to 5.
7. The microporous polyethylene membrane of claim 6, wherein the microporous polyethylene membrane has a thickness of 5 to 25 μm.
8. The microporous polyethylene membrane of claim 6, wherein the micropores in the microporous polyethylene membrane have an average pore size of 30 to 80 nm.
9. The microporous polyethylene membrane of any of claims 6-8, wherein the micropores in the microporous polyethylene membrane have a tortuosity of 1.5 to 3.5;
optionally, the polyethylene microporous membrane has an air permeability of 80-500s/100 mL;
optionally, the polyethylene microporous membrane has a porosity of 30% to 48%.
10. A lithium ion battery is characterized in that the polyethylene microporous membrane prepared by the method of any one of claims 1 to 5 or the polyethylene microporous membrane of any one of claims 6 to 9 is used as a diaphragm.
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