KR20160049492A - Polyethylene Microporous Membrane, Preparation Method Thereof and Lithium-Ion Battery - Google Patents

Abstract

The present invention provides a polyethylene microporous membrane, a preparation method thereof, and a lithium-ion battery. The polyethylene microporous membrane has a front surface and a rear surface, wherein an average porous size on the front surface is from 100 to 200 nm; an average porous size on the rear surface is from 50 to 100 nm; a ratio of the average porous size on the front surface to the average porous size on the rear surface (A:B) is (1.1-4.0):1; and porous size distribution of each of the front and rear surface is 30% or less. The polyethylene microporous membrane is used in a lithium-ion battery, wherein the front surface is close to an anode of the lithium-ion battery, and the rear surface is close to a cathode of the lithium-ion battery. The lithium-ion battery using the polyethylene microporous membrane manufactured according to the present invention can provide excellent self-discharge performance as well as excellent high-efficiency discharge performance.

Description

TECHNICAL FIELD [0001] The present invention relates to a microporous polyethylene membrane, a method of manufacturing the same, and a lithium-ion battery,

TECHNICAL FIELD OF THE INVENTION The present invention relates to the field of lithium ion batteries, particularly to a polyethylene microporous membrane, a method for producing the same, and a lithium ion battery using the polyethylene microporous membrane.

The polyethylene microporous membrane is a porous membrane having a microporous structure on both surfaces. When a polyethylene microporous membrane is used as a separator for a lithium-ion battery, a major requirement for the separator is that it should not degrade the electrochemical properties of the cell. In a separator of less than 1 micron in size, proper pore diameter is one of the important factors to satisfy various requirements such as charging and discharging performance, safety performance, cycle performance and the like for a battery.

For any application of the battery, pore diameters at sub-micron dimensions are important to prevent short circuits between the positive and negative electrodes inside the lithium-ion cell. Such a feature becomes even more important with the selection of a thinner separator to increase the capacity of the battery by the battery manufacturer. The pore size directly affects the properties of the cell. When the micropores of the separator have a large micropore size, the cell has a high capacity retention rate when discharged at low resistance, good cycle performance and high rate. However, if the pore size of the micropores is too large, the active materials will contact and react with one another, which reduces the capacity and accelerates the self-discharge process of the cell. In addition, the positive electrode and the negative electrode tend to be in direct contact with each other or perforated by lithium dendrite, thereby causing a short circuit. If the pore size of the separator is very small, the cell will have high electrical resistance and poor cycle performance, and capacity retention will be low when discharged at high rates. The advantage is that the self-discharging process is slow, the puncture strength is high, the short circuit due to the puncturing by the lithium dendrite can be prevented, and thus the safety performance of the battery is improved. In addition, the uniformity of the pore size distribution of the micropores of the separator also directly affects the properties of the cell. If the pore size distribution of the micropores is not uniform, there will be a large local current that will affect the characteristics of the cell during operation. Therefore, it is important to strictly control the pore size and to form as uniform pores as possible.

Considering the arrangement of the separator, the pore size directly affects the air permeability of the separator, which is indicated by the Gurley value. For the same separator, its Gurley value can well reflect the internal resistance. The larger the pore size of the separator, the greater the porosity and the lower the degree of twisting of the voids, hence the lower the Gurley value and the lower the separator resistance. On the other hand, the smaller the pore size of the separator, the larger the Gurley value and the larger the internal resistance. Separators with good electrical properties generally have a relatively low air permeability.

In addition, the pore size of the separator also directly affects its mechanical properties. In order to meet the requirements of battery assembly and routine charge and discharge cycle processes, the separator should have good mechanical properties. Since the coarse surface of the electrode can be punctured by the separator during the cell assembly, charge and discharge cycle processes, which can lead to short circuits in the cell and cause problems with the safety of the cell, Is used to evaluate the likelihood of the occurrence. In general, a microporous membrane (separator) having a larger pore size has lower piercing strength and lower safety. On the other hand, the separator having a smaller pore size has greater puncture strength and better safety.

At present, conventional polyolefin microporous membranes have micropores of the same pore size on both the front and rear sides. However, when such a microporous membrane is used in a lithium-ion battery, there are the following problems: when the pore size is large, for example, when the average pore size is larger than 150 nm, the internal resistance is low, Performance is good, but the puncture strength and safety performance are low and the self-discharge ratio is larger; When the pore size is small, for example, when the average pore size is smaller than 70 nm, the pore strength and safety performance are excellent and the self-discharge ratio is low, but the air permeability and cycle performance are bad due to the large internal resistance.

The present invention provides a novel polyethylene microporous membrane exhibiting excellent air permeability and excellent mechanical properties, and a method for producing the microporous polyethylene membrane. The present invention provides a method of producing a polyethylene microporous membrane using a polyethylene microporous membrane having excellent self- It is an object of the present invention to provide a lithium-ion battery.

In order to overcome the aforementioned deficiencies, one aspect of the present invention is to provide a novel polyethylene microporous membrane exhibiting excellent air permeability and excellent mechanical properties.

Another aspect of the present invention is to provide a method for producing a polyethylene microporous membrane.

Another aspect of the present invention is to provide a lithium-ion battery using the polyethylene microporous membrane according to the present invention, wherein the battery using the polyethylene microporous membrane has excellent self-discharge performance as well as excellent high rate discharge performance.

Wherein the polyethylene microporous membrane has a front side and a back side and the micropores on the front surface of the microporous membrane have an average pore size different from the micropores on the back surface of the microporous membrane, The average pore size of the micropores on the front surface is 100-200 nm; The average pore size of the micropores on the back surface is 50-100 nm; Wherein the ratio of the average pore size of the micropores on the front surface to the average pore size of the micropores on the back surface is (1.1-4.0): 1; And a pore size distribution on each of the front and back surfaces is less than 30%.

In a preferred embodiment, the average pore size of the micropores on the surface is 120-180 nm; The average pore size of the micropores on the back surface is 60-80 nm; The ratio of the average pore size of the micropores on the front surface to the average pore size of the micropores on the back surface is (1.5-3.0): 1; And a pore size distribution on each of the front and back surfaces is less than 20%.

In a preferred embodiment, the microporous membrane has a thickness of 5-30 μm, a porosity of 35-60%, an air permeability of 50-350 s / 100 ml and a puncture strength of 4-10 N / strength.

A method for producing a polyethylene microporous membrane includes:

(1) A polyethylene resin and a pore-forming agent are added to a twin screw extruder to perform melt kneading, extrusion through a die head, and rapid cooling to produce a thick sheet by casting Sheet casting;

(2) introducing the thick sheet into a stretcher to effect synchronous or asynchronous biaxial stretching, and controlling the stretching temperature of the upper and lower chambers of the stretcher to form an oil- Maintaining a temperature difference between the upper and lower chambers of the stretcher at 1-10 占 폚 to produce the thin film containing the thin film;

(3) an extraction step of extracting the oil-containing thin film with a solvent to remove the pore-forming agent to form a white microporous membrane; And

(4) introducing the white microporous membrane into a heat setting treatment to obtain a polyethylene microporous membrane having different average pore sizes on the front and back surfaces of the polyethylene microporous membrane.

The lithium-ion battery includes the polyethylene microporous membrane according to the present invention, wherein the front surface of the microporous membrane is close to the anode of the battery, and the rear surface of the microporous membrane is close to the cathode of the battery.

The lithium-ion battery includes the polyethylene microporous membrane, wherein the front surface of the microporous membrane is close to the anode of the battery, and the rear surface of the microporous membrane is close to the cathode of the battery.

Lithium-ion batteries have a capacity retention rate greater than 80% when discharged at a 3C rate and greater than 90% after 30 days of self-discharge.

The present invention has the following advantages over the prior art:

1. The polyethylene microporous membrane according to the present invention has different mean pore sizes on its front and back sides, which ensures that the microporous membrane exhibits good air permeability and good mechanical properties.

2. The process for producing the polyolefin microporous membrane according to the invention is simple and easy to carry out, since the qualified product can be obtained by only changing the control parameters of the process without additional equipment.

3. In a lithium-ion battery including a polyolefin microporous membrane according to the present invention, the front surface of the microporous membrane is close to the anode of the battery, and the rear surface of the membrane is close to the cathode of the battery. Since the pore size is larger on the front surface, the internal resistance is low and the cycle performance is excellent. As a result, the capacity retention ratio of the battery in the high rate discharge is high. On the other hand, since the pore size on the rear surface is smaller, the self-discharging performance of the battery is excellent and the puncture resistance of the separator is improved, whereby the short circuit due to the puncturing of the lithium dendrite is efficiently prevented and thus the safety performance is improved.

1 is a photograph of a scanning electron microscope showing the microstructure of the entire surface of the polyethylene microporous membrane according to Example 1 of the present invention;
2 is a photograph of a scanning electron microscope showing the microstructure of the rear surface of the polyethylene microporous membrane according to Example 1 of the present invention;
3 is a photograph of a scanning electron microscope showing the microstructure of the entire surface of the polyethylene microporous membrane according to Example 2 of the present invention;
4 is a photograph of a scanning electron microscope showing the microstructure of the rear surface of the polyethylene microporous membrane according to Example 2 of the present invention;
5 is a graph showing a test result of the average pore size of the front surface of the polyethylene microporous membrane according to Example 1 of the present invention; And
6 is a graph showing the test results of the average pore size of the rear surface of the polyethylene microporous membrane according to Example 1 of the present invention

Hereinafter, the present invention will be described in detail.

1. Polyethylene microporous membrane having asymmetric structure of the present invention

The polyethylene microporous membrane of the present invention has an asymmetric structure. In particular, the front side of the membrane has important backbone structures on a microscopic scale, where the backbone is interconnected through finer fibers and has a number of relatively large There are micropores. Alternatively, the backside of the membrane has a uniform fiber structure at a fine scale, where the pore size is relatively small, thereby producing a separator with sufficient mechanical properties. As a result, such an asymmetric structure of the microporous membrane of the present invention ensures that the separator possesses both good mechanical properties and good air permeability. When the above-mentioned microporous membrane is used in a lithium-ion battery, the front surface of the microporous membrane is close to the anode of the battery. Because of the larger pore size on the front side, the internal resistance of the cell is low and the resistivity is small, which leads to high ion retention and high capacity retention in high rate discharges. On the other hand, the back surface having a smaller pore size of the membrane is close to the cathode of the electrode. Because of the smaller pore size, the internal resistance is larger and the Li-ion migration rate during the self-discharge process is slower. This makes the battery excellent in self-discharging performance, which improves the puncture resistance of the separator, whereby the short circuit due to the puncturing of the lithium dendrite is effectively prevented and thus the safety performance of the battery is enhanced.

The microporous membrane of the present invention has a larger pore size on the front side, which can be on average 100-200 nm. Such an average pore size ensures excellent air permeability. If the average pore size is larger than 200 nm, the mechanical strength of the microporous membrane is greatly reduced, and it is easily punctured by the lithium dendrite to cause a short circuit and cause a safety problem in the battery. On the contrary, the pore size on the back surface of the microporous membrane is smaller, which can be on average 50-99 nm. It ensures excellent mechanical properties and prevents short circuits due to perforation of the lithium dendrite, thereby improving the safety performance of the battery. On the other hand, the above-mentioned smaller pore size also slows the self-discharge rate and leads to excellent self-discharge performance. If the average pore size on the back surface is smaller than 50 nm, the air permeability of the microporous membrane will be greatly reduced and the internal resistance will be increased, which results in a deterioration of the cycle performance of the battery, .

In the present invention, the pore size distribution is measured using the mercury intrusion method according to the standard GB / T21650.1-2008 / ISO15901-1: 2006. The maximum pore size and the minimum pore size can be obtained through a related plot simulated by data processing. The pore size distribution used herein is the percentage value obtained by dividing the difference between the maximum pore size or the minimum pore size and the average pore size by the average pore size, which is calculated by the following equation:

Pore size distribution = (maximum pore size - average pore size) / average pore size x 100%, or

Pore size distribution = (average pore size - minimum pore size) / average pore size x 100%.

The front and back sides of the microporous membrane of the present invention each have a pore size distribution of less than 30%, preferably less than 20%, and most preferably less than 10%. Since the microporous membrane has a uniform pore size distribution, the current consistency in the operation of the battery is excellent, which helps to maintain good battery characteristics. When the pore size distribution is greater than or greater than 30%, there will be a very high local current in the cell and thus will affect the safety performance and cycle performance of the cell.

The microporous membrane of the present invention has a ratio of mean pore size on the front side to (1.1-4.0): 1 versus average pore size on the back side, which imparts both sufficient mechanical strength and good air permeability to the membrane. If the ratio of the average pore size between the two faces is less than 1.1: 1, the pore size difference between them is not sufficient to satisfy the requirements for the excellent performance of the present invention. However, if the ratio of the average pore size between the two faces is more than 4.0: 1, the pore size difference between them is too large to cause imbalance and drop in the thermal resistance of the microporous membrane and the object of the present invention may not be achieved have.

The polyethylene microporous membrane of the present invention has a thickness of 5-30 탆. If the film thickness is less than 5 占 퐉, the safety of the battery is not guaranteed due to the poor resistance to external stress in the production of the battery as well as the potential puncturing of the separator by the dentritic crystal formed during the charging and discharging process of the battery . However, if the membrane thickness exceeds 30 占 퐉, the permeability of the membrane will decrease, and the thickness and weight of the cell will increase.

The polyethylene microporous membrane of the present invention has a porosity of 35-60%. If the porosity is less than 35%, the porosity of the membrane will be bad, the internal resistance will increase, and the cell characteristics will be degraded. However, if the porosity exceeds 60%, the pores are too large. Therefore, the mechanical strength of the membrane will decrease, the risk of short-circuiting may increase, and the safety and safety performance of the battery can not be guaranteed.

The polyethylene microporous membrane of the present invention has an air permeability of 50-350 s / 100 ml. If the air permeability exceeds 350 s / 100 ml, the permeability is not excellent and the electrical characteristics of the membrane will be lowered. However, if the air permeability is less than 50 s / 100 ml, the safety performance of the battery can not be guaranteed because a short circuit may occur due to too large pores of the membrane.

The polyethylene microporous membrane of the present invention has a puncture strength of 4-10 N / 20 μm to ensure that the battery has high resistance to external impacts that may occur during the manufacture and use of the battery, .

Briefly, the polyethylene microporous membrane according to the present invention has an asymmetric structure on the front and back surfaces, wherein: the pores on the front surface are larger with an average pore size of 100-200 nm, while the pores on the back surface are 50 It is smaller with an average pore size of -100 nm. The average pore size ratio A: B between (1.1-4.0): 1 between the front and back surfaces, and the pore size distribution on each of the front and back surfaces is less than 30%. The microporous membrane has a thickness of 5-30 μm, a porosity of 35-60%, an air permeability of 50-350 s / 100 ml, and a piercing strength of 4-10 N / 20 μm.

2. Method for producing polyethylene microporous membrane

2.1 Description of raw materials and process parameters

The polyethylene resin used in the present invention is not particularly limited. A single resin is typically used and two or more resins may be used in combination.

The weight average molecular weight of the polyethylene resin used in the present invention is not particularly limited, but is generally from 10,000 to 5,000,000, preferably from 100,000 to 3,000,000, more preferably from 300,000 to 2,000,000.

The pore-forming agent used in the present invention is not particularly limited, but typically includes liquid paraffin, solid paraffin, dioctyl phthalate, dibutyl phthalate and the like which can form pores on the polyolefin resin. Solvent. The pore-forming agents may be used alone or in combination of two or more types thereof. Liquid paraffin and dioctyl phthalate may be preferred.

The extraction solvent used in the present invention is not particularly limited, but is typically dichloromethane, n-heptane, n-decane or ethanol. Of these, dichloromethane may be preferred.

Additives such as antioxidants, crosslinking agents, nucleating agents, heat stabilizers, lubricants and the like are not particularly limited in the present invention, and conventionally used additives may be used.

The biaxial stretching process used in the present invention is not particularly limited, but may be synchronous biaxial stretching or asynchronous biaxial stretching.

The process parameters applied to the production of the polyethylene microporous membrane of the present invention are not particularly limited. The extrusion temperature, the cooling roll temperature, the stretching temperature, the stretching speed, the extraction temperature, the extraction speed, the heat setting temperature, the temperature and the like, which satisfy the requirements of the wet process of the polyethylene microporous membrane, Conventional parameters such as heat setting speed, air temperature, air flow and air pressure can be used.

2.2 Method for producing polyethylene microporous membrane

The polyethylene microporous membrane is produced by a wet process, which involves substantially the following four steps: a sheet casting step, a stretching step, an extraction step and a heat setting step. Here, the stretching step is the most important step for the orientation of the polyethylene molecular chain and is a key step in the production of the microporous membrane. At this stage, the thick sheet is stretched to a high elastic state below the melting point and above the glass transition temperature by an external force applied thereon, since it is oriented according to the direction of the external force. Thereby, the application performance of the product is changed and improved. The molecular chains of the stretched oil-containing thin film are oriented in both the longitudinal and transverse directions, and the pore-former is uniformly distributed among the oriented molecular chains to form a specific water-oil mixed structure.

The main factors affecting the stretching process include the stretching ratio, the stretching speed, and the stretching temperature. For the same equipment, under conditions where the stretching ratio and the stretching speed are constant during the stretching process, a higher stretching temperature leads to larger pores formed on the oil-containing thin film, resulting in a larger pore size after extraction, Will be excellent. On the other hand, when the stretching temperature is relatively low, the pores formed on the oil film are smaller and the pore size is smaller after extraction, so that the mechanical properties of the membrane will be excellent. Inside the stretcher, the chamber is actually divided into two parts, namely an upper chamber and a lower chamber, by the oil-containing thin film as a boundary line. When the two portions have the same temperature, the microporous membrane having a uniform microstructure will be formed because the pore forming conditions are the same. If the temperature of the two parts is controlled differently, that is, if there is a temperature difference between the two parts, the microporous membrane with different microstructure will be formed because the pore forming conditions are different.

In the present invention, the stretching temperature of the upper chamber may be 117-127 占 폚, preferably 120-125 占 폚, and the stretching temperature of the lower chamber may be 110-120 占 폚, preferably 115-119 占 폚. The temperature difference between the upper chamber and the lower chamber can be controlled at 1-10 占 폚 to obtain a microporous membrane having different microstructures on two surfaces. When the temperature difference is less than 1 占 폚, the microstructure on both sides of the microporous membrane is less different from each other because the pore forming conditions are similar, and the object of the present invention can not be achieved. On the other hand, when the temperature difference is higher than 10 DEG C, the microstructure on the two surfaces is significantly different from each other at the same stretch ratio and stretching speed because the pore-forming conditions are very different and causes the imbalance and drop in the thermal resistance of the microporous membrane And the object of the present invention can not also be achieved. In order to obtain a polyethylene microporous membrane exhibiting excellent overall performance, the temperature difference in the present invention is preferably 2-8 占 폚, most preferably 3-5 占 폚.

In the present invention, the stretching ratio may be 10-50, preferably 20-40.

The polyethylene microporous membrane obtained by the method of the present invention has an asymmetric structure on the front and back surfaces, wherein: the pores on the front surface are larger with an average pore size of 100-200 nm, Is smaller with an average pore size of 50-100 nm. The average pore size ratio A: B between (1.1-4.0): 1 between the front and back surfaces, and the pore size distribution on each of the front and back surfaces is less than 30%. The microporous membrane has a thickness of 5-30 μm, a porosity of 35-60%, an air permeability of 50-350 s / 100 ml, and a piercing strength of 4-10 N / 20 μm.

3. Manufacturing Method of Lithium-ion Battery

As prepared above, the polyethylene microporous membrane is cut according to the requirements of the cell size and then assembled to produce a lithium ferrous phosphate / graphite battery, wherein the front surface of the microporous membrane comprises Close to the anode of the battery, and close to the cathode of the battery.

When the above-described microporous membrane is used for a lithium-ion battery, the front surface of the microporous membrane is close to the anode of the battery. Because of the larger pore size on the surface, the internal resistance of the cell is lower and the resistivity is lower, leading to superior ion permeability, superior battery cycle performance and high capacity retention at high rate discharges. On the other hand, the rear surface of the membrane is close to the cathode of the battery. Because of the smaller pore size, the internal resistance is larger and the Li-ion migration rate during the self-discharge process is slower. This makes the battery have excellent self-discharge performance and improves the puncture resistance of the separator, thereby preventing short-circuit by lithium dendrites.

The lithium ion battery using the microporous membrane manufactured according to the present invention has excellent self discharge performance as well as excellent high rate discharge performance.

Example

Hereinafter, the present invention will be described in further detail with reference to examples.

Example 1

(1) Casting sheet extrusion step: Polyethylene resin and liquid paraffin were injected into a twin screw extruder and were melted and kneaded with a homogeneous melt in the biaxial extruder. The melt was then extruded continuously through a die head and rapidly cooled to be molded by a chill roll. Wherein the weight average molecular weight of the polyethylene resin was 500,000, the viscosity grade of the liquid paraffin was 40, the mass ratio of polyethylene resin to liquid paraffin was 1: 3, the melting temperature was 200 ° C, Was 30 < 0 > C.

(2) biaxial stretching step: the thick sheet was stretched in a stretcher after pre-heating to obtain an oil-containing thin film; Where the stretching temperature of the upper chamber was 120 ° C, the stretching temperature of the lower chamber was 115 ° C, and the temperature difference between the upper and lower chambers was 5 ° C.

(3) Extraction step: Liquid paraffin was extracted with dichloromethane from the oil-containing thin film to form a white thin film having a microporous structure.

(4) Heat setting step: The white microporous membrane was heat-set at 125 占 폚 for 2 minutes to obtain a polyethylene microporous membrane having an asymmetric structure.

Polyethylene microporous membranes having an asymmetric structure were prepared according to the above method and their physical and chemical properties were tested. The test results are shown in Table 1.

(5) As prepared above, the polyethylene microporous membrane was cut according to cell size requirements and then assembled, wherein the front surface of the microporous membrane was close to the anode of the cell, and the rear surface was attached to the cathode of the cell It was close. The electrical characteristics of the cell were tested and the test results are shown in Table 1.

Example 2

(1) Casting sheet extrusion step: Polyethylene resin and liquid paraffin were injected into a twin-screw extruder and melt-kneaded with a homogeneous melt in the twin-screw extruder. The melt was then continuously extruded through a die head and rapidly cooled to be molded by a chill roll. A thick sheet was obtained in which the weight average molecular weight of the polyethylene resin was 1,000,000, the viscosity grade of the liquid paraffin was 50, the mass ratio of the polyethylene resin to liquid paraffin was 1: 4, the melting temperature was 200 ° C, Was 30 < 0 > C.

(2) biaxial stretching step: the thick sheet was stretched in a stretcher after pre-heating to obtain an oil-containing thin film; Where the stretching temperature of the upper chamber was 125 占 폚, the stretching temperature of the lower chamber was 115 占 폚, and the temperature difference between the upper and lower chambers was 10 占 폚.

(3) Extraction step: Liquid paraffin was extracted with dichloromethane from the oil-containing thin film to form a white thin film having a microporous structure.

(4) Heat setting step: White microporous membrane was heat-set at 128 占 폚 for 2 minutes to obtain a polyethylene microporous membrane having an asymmetric structure.

Polyethylene microporous membranes having an asymmetric structure were prepared according to the above method and their physical and chemical properties were tested. The test results are shown in Table 1.

(5) As prepared above, the polyethylene microporous membrane was cut according to cell size requirements and then assembled, wherein the front surface of the microporous membrane was close to the anode of the cell, and the rear surface was attached to the cathode of the cell It was close. The electrical characteristics of the cell were tested and the test results are shown in Table 1.

Example 3

(1) Casting sheet extrusion step: Polyethylene resin and liquid paraffin were injected into a twin-screw extruder and melt-kneaded with a homogeneous melt in the twin-screw extruder. The melt was then continuously extruded through a die head and rapidly cooled to be molded by a chill roll. Wherein the weight average molecular weight of the polyethylene resin was 500,000, the viscosity grade of the liquid paraffin was 40, the mass ratio of the polyethylene resin to the liquid paraffin was 1: 3, the melting temperature was 210 ° C, Was 30 < 0 > C.

(2) biaxial stretching step: the thick sheet was stretched in a stretcher after pre-heating to obtain an oil-containing thin film; Where the stretching temperature of the upper chamber was 120 占 폚, the stretching temperature of the lower chamber was 119 占 폚, and the temperature difference between the upper and lower chambers was 1 占 폚.

(3) Extraction step: Liquid paraffin was extracted with dichloromethane from the oil-containing thin film to form a white thin film having a microporous structure.

(4) Heat setting step: The white microporous membrane was heat-set at 125 占 폚 for 2 minutes to obtain a polyethylene microporous membrane having an asymmetric structure.

Polyethylene microporous membranes having an asymmetric structure were prepared according to the above method and their physical and chemical properties were tested. The test results are shown in Table 1.

(5) As prepared above, the polyethylene microporous membrane was cut according to cell size requirements and then assembled, wherein the front surface of the microporous membrane was close to the anode of the cell, and the rear surface was attached to the cathode of the cell It was close. The electrical characteristics of the cell were tested and the test results are shown in Table 1.

Example 4

(1) Casting sheet extrusion step: Polyethylene resin and liquid paraffin were injected into a twin-screw extruder and melt-kneaded with a homogeneous melt in the twin-screw extruder. The melt was then continuously extruded through a die head and rapidly cooled to be molded by a chill roll. Wherein the weight average molecular weight of the polyethylene resin was 1,000,000, the viscosity grade of the liquid paraffin was 50, the mass ratio of polyethylene resin to liquid paraffin was 1: 3, the melting temperature was 200 ° C, Was 30 < 0 > C.

(2) biaxial stretching step: the thick sheet was stretched in a stretcher after pre-heating to obtain an oil-containing thin film; Where the stretching temperature of the upper chamber was 122 占 폚, the stretching temperature of the lower chamber was 119 占 폚, and the temperature difference between the upper and lower chambers was 3 占 폚.

(3) Extraction step: Liquid paraffin was extracted with dichloromethane from the oil-containing thin film to form a white thin film having a microporous structure.

(4) Heat setting step: White microporous membrane was heat-set at 128 占 폚 for 2 minutes to obtain a polyethylene microporous membrane having an asymmetric structure.

Polyethylene microporous membranes having an asymmetric structure were prepared according to the above method and their physical and chemical properties were tested. The test results are shown in Table 1.

(5) As prepared above, the polyethylene microporous membrane was cut according to cell size requirements and then assembled, wherein the front surface of the microporous membrane was close to the anode of the cell, and the rear surface was attached to the cathode of the cell It was close. The electrical characteristics of the cell were tested and the test results are shown in Table 1.

Example 5

(1) Casting sheet extrusion step: Polyethylene resin and liquid paraffin were injected into a twin-screw extruder and melt-kneaded with a homogeneous melt in the twin-screw extruder. The melt was then continuously extruded through a die head and rapidly cooled to be molded by a chill roll. Wherein the weight average molecular weight of the polyethylene resin (1) was 2,000,000, the weight average molecular weight of the polyethylene resin (2) was 500,000 and the mass ratio of the polyethylene resin (1) to the polyethylene resin (2) was 7 : 3; The liquid paraffin had a viscosity grade of 70, the mass ratio of the mixture of polyethylene resin to liquid paraffin was 1: 4, the melting temperature was 220 ° C, and the temperature of the cooling roll was 30 ° C.

(2) biaxial stretching step: the thick sheet was stretched in a stretcher after pre-heating to obtain an oil-containing thin film; Where the stretching temperature of the upper chamber was 123 占 폚, the stretching temperature of the lower chamber was 115 占 폚, and the temperature difference between the upper and lower chambers was 8 占 폚.

(3) Extraction step: Liquid paraffin was extracted with dichloromethane from the oil-containing thin film to form a white thin film having a microporous structure.

(4) Heat setting step: White microporous membrane was heat-set at 130 占 폚 for 2 minutes to obtain a polyethylene microporous membrane having an asymmetric structure.

Polyethylene microporous membranes having an asymmetric structure were prepared according to the above method and their physical and chemical properties were tested. The test results are shown in Table 1.

(5) As prepared above, the polyethylene microporous membrane was cut according to cell size requirements and then assembled, wherein the front surface of the microporous membrane was close to the anode of the cell, and the rear surface was attached to the cathode of the cell It was close. The electrical characteristics of the cell were tested and the test results are shown in Table 1.

Comparative Example 1

(1) Casting sheet extrusion step: Polyethylene resin and liquid paraffin were injected into a twin-screw extruder and melt-kneaded with a homogeneous melt in the twin-screw extruder. The melt was then continuously extruded through a die head and rapidly cooled to be molded by a chill roll. Wherein the weight average molecular weight of the polyethylene resin was 500,000, the viscosity grade of the liquid paraffin was 40, the mass ratio of polyethylene resin to liquid paraffin was 1: 3, the melting temperature was 200 ° C, Was 30 < 0 > C.

(2) biaxial stretching step: the thick sheet was stretched in a stretcher after pre-heating to obtain an oil-containing thin film; Where the stretching temperature of the upper chamber was 120 占 폚, the stretching temperature of the lower chamber was 120 占 폚, and the temperature difference between the upper and lower chambers was 0 占 폚.

(3) Extraction step: Liquid paraffin was extracted with dichloromethane from the oil-containing thin film to form a white thin film having a microporous structure.

(4) Heat setting step: The white microporous membrane was heat-set at 125 占 폚 for 2 minutes to obtain a polyethylene microporous membrane having an asymmetric structure.

Polyethylene microporous membranes having an asymmetric structure were prepared according to the above method and their physical and chemical properties were tested. The test results are shown in Table 2.

(5) As prepared above, the polyethylene microporous membrane was cut according to cell size requirements and then assembled, wherein the front surface of the microporous membrane was close to the anode of the cell, and the rear surface was attached to the cathode of the cell It was close. The electrical characteristics of the battery were tested, and the test results are shown in Table 2.

Comparative Example 2

(1) Casting sheet extrusion step: Polyethylene resin and liquid paraffin were injected into a twin-screw extruder and melt-kneaded with a homogeneous melt in the twin-screw extruder. The melt was then continuously extruded through a die head and rapidly cooled to be molded by a chill roll. A thick sheet was obtained in which the weight average molecular weight of the polyethylene resin was 1,000,000, the viscosity grade of the liquid paraffin was 50, the mass ratio of the polyethylene resin to liquid paraffin was 1: 4, the melting temperature was 210 ° C, Was 30 < 0 > C.

(2) biaxial stretching step: the thick sheet was stretched in a stretcher after pre-heating to obtain an oil-containing thin film; Where the stretching temperature of the upper chamber was 125 占 폚, the stretching temperature of the lower chamber was 125 占 폚, and the temperature difference between the upper and lower chambers was 0 占 폚.

(3) Extraction step: Liquid paraffin was extracted with dichloromethane from the oil-containing thin film to form a white thin film having a microporous structure.

(4) Heat setting step: White microporous membrane was heat-set at 128 占 폚 for 2 minutes to obtain a polyethylene microporous membrane having an asymmetric structure.

Polyethylene microporous membranes having an asymmetric structure were prepared according to the above method and their physical and chemical properties were tested. The test results are shown in Table 2.

(5) As prepared above, the polyethylene microporous membrane was cut according to cell size requirements and then assembled, wherein the front surface of the microporous membrane was close to the anode of the cell, and the rear surface was attached to the cathode of the cell It was close. The electrical characteristics of the battery were tested, and the test results are shown in Table 2.

Comparative Example 3

(1) Casting sheet extrusion step: Polyethylene resin and liquid paraffin were injected into a twin-screw extruder and melt-kneaded with a homogeneous melt in the twin-screw extruder. The melt was then continuously extruded through a die head and rapidly cooled to be molded by a chill roll. A thick sheet was obtained in which the weight average molecular weight of the polyethylene resin was 500,000, the viscosity grade of the liquid paraffin was 40, the mass ratio of the polyethylene resin to liquid paraffin was 1: 2, the melting temperature was 200 ° C, Was 30 < 0 > C.

(2) biaxial stretching step: the thick sheet was stretched in a stretcher after pre-heating to obtain an oil-containing thin film; Where the stretching temperature of the upper chamber was 115 ° C, the stretching temperature of the lower chamber was 115 ° C, and the temperature difference between the upper and lower chambers was 0 ° C.

(3) Extraction step: Liquid paraffin was extracted with dichloromethane from the oil-containing thin film to form a white thin film having a microporous structure.

(4) Heat setting step: The white microporous membrane was heat-set at 125 占 폚 for 2 minutes to obtain a polyethylene microporous membrane having an asymmetric structure.

Polyethylene microporous membranes having an asymmetric structure were prepared according to the above method and their physical and chemical properties were tested. The test results are shown in Table 2.

(5) As prepared above, the polyethylene microporous membrane was cut according to cell size requirements and then assembled, wherein the front surface of the microporous membrane was close to the anode of the cell, and the rear surface was attached to the cathode of the cell It was close. The electrical characteristics of the battery were tested, and the test results are shown in Table 2.

Comparative Example 4

(1) Casting sheet extrusion step: Polyethylene resin and liquid paraffin were injected into a twin-screw extruder and melt-kneaded with a homogeneous melt in the twin-screw extruder. The melt was then continuously extruded through a die head and rapidly cooled to be molded by a chill roll. A thick sheet was obtained in which the weight average molecular weight of the polyethylene resin was 1,000,000, the viscosity grade of the liquid paraffin was 50, the mass ratio of the polyethylene resin to liquid paraffin was 1: 3, the melting temperature was 210 ° C, Was 30 < 0 > C.

(2) biaxial stretching step: the thick sheet was stretched in a stretcher after pre-heating to obtain an oil-containing thin film; Where the stretching temperature of the upper chamber was 122 占 폚, the stretching temperature of the lower chamber was 122 占 폚, and the temperature difference between the upper and lower chambers was 0 占 폚.

(3) Extraction step: Liquid paraffin was extracted with dichloromethane from the oil-containing thin film to form a white thin film having a microporous structure.

(4) Heat setting step: White microporous membrane was heat-set at 128 占 폚 for 2 minutes to obtain a polyethylene microporous membrane having an asymmetric structure.

Polyethylene microporous membranes having an asymmetric structure were prepared according to the above method and their physical and chemical properties were tested. The test results are shown in Table 2.

(5) As prepared above, the polyethylene microporous membrane was cut according to cell size requirements and then assembled, wherein the front surface of the microporous membrane was close to the anode of the cell, and the rear surface was attached to the cathode of the cell It was close. The electrical characteristics of the battery were tested, and the test results are shown in Table 2.

Comparative Example 5

(1) Casting sheet extrusion step: Polyethylene resin and liquid paraffin were injected into a twin-screw extruder and melt-kneaded with a homogeneous melt in the twin-screw extruder. The melt was then continuously extruded through a die head and rapidly cooled to be molded by a chill roll. Wherein the weight average molecular weight of the polyethylene resin (1) was 2,000,000, the weight average molecular weight of the polyethylene resin (2) was 500,000 and the mass ratio of the polyethylene resin (1) to the polyethylene resin (2) was 7 : 3; The liquid paraffin had a viscosity grade of 70, the mass ratio of the mixture of polyethylene resin to liquid paraffin was 1: 4, the melting temperature was 220 ° C, and the temperature of the cooling roll was 30 ° C.

(2) biaxial stretching step: the thick sheet was stretched in a stretcher after pre-heating to obtain an oil-containing thin film; Where the stretching temperature of the upper chamber was 123 ° C, the stretching temperature of the lower chamber was 123 ° C, and the temperature difference between the upper and lower chambers was 0 ° C.

(3) Extraction step: Liquid paraffin was extracted with dichloromethane from the oil-containing thin film to form a white thin film having a microporous structure.

(4) Heat setting step: White microporous membrane was heat-set at 130 占 폚 for 2 minutes to obtain a polyethylene microporous membrane having an asymmetric structure.

Polyethylene microporous membranes having an asymmetric structure were prepared according to the above method and their physical and chemical properties were tested. The test results are shown in Table 2.

(5) As prepared above, the polyethylene microporous membrane was cut according to cell size requirements and then assembled, wherein the front surface of the microporous membrane was close to the anode of the cell, and the rear surface was attached to the cathode of the cell It was close. The electrical characteristics of the battery were tested, and the test results are shown in Table 2.

Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those of ordinary skill in the art will recognize that various changes, additions and substitutions can be made without departing from the scope and spirit of what is disclosed by the appended claims I have to understand.

In the present invention, the method of measuring the characteristics of the separator is as follows:

1. Average pore size: Measured using the mercury indentation method according to standard GB / T21650.1-2008 / ISO15901-1: 2006. The mercury was injected into the sample tube under vacuum conditions and analyzed at a high pressure station at a maximum pressure of 227.5 MPa. During the mercury intrusion process, mercury was pressurized into the pores of the sample with increasing pressure. The generated electrical signal was input to the computer via a sensor for data processing, whereby the relevant plot was simulated and the data of the pore size were calculated. The average pore size was then obtained by averaging the data processing.

2. Pore size distribution: Measured by using the mercury indentation method according to standard GB / T21650.1-2008 / ISO15901-1: 2006. The maximum pore size and the minimum pore size can be obtained through a related plot simulated by data processing. The pore size distribution used herein is the percentage value obtained by dividing the difference between the maximum pore size or the minimum pore size and the average pore size by the average pore size, which is calculated by the following equation:

Pore size distribution = (maximum pore size - average pore size) / average pore size x 100%, or

Pore size distribution = (average pore size - minimum pore size) / average pore size x 100%.

3. Thickness: Thicknesses of 10 or more positions obtained in the respective MD and TD directions were individually measured by using a 1/1000 mm desk type thickness gauge and their average values were calculated to represent the thickness of the film.

4. Porosity: A square sample of 100 mm x 100 mm was cut from the microporous membrane and its actual mass W ₁ was measured. Then, the mass W ₀ of the sample having the porosity of 0% was calculated on the basis of the density and the thickness of the resin composition, and as a result, the porosity was determined by the following expression: porosity = (W ₀ - W ₁ ) / W ₀ x 100.

5. Air permeability: The Gurley air permeability gauge was used to measure air permeability at 25 ° C in an air atmosphere according to standard JIS P8117.

6. Perforation Strength: A needle tip having a diameter of 1.0 mm and a curvature radius of 0.5 mm was fitted on a universal tensile machine and when the needle tip penetrated it at a speed of 120 mm / min , And the puncture strength of the separator was measured at 23 ° C.

7. Microstructure: Scanning electron microscopy was used to observe the film and take microstructural photographs.

8. Internal resistance: The BT-2000 electrochemical tester was used to test the electrical properties.

9. High-rate discharging performance: Under standard ambient conditions, the battery is charged to 4.2 V at a constant current of 1.0 C, according to the method described in "General specification for the lithium-ion battery" And then discharged to 3.0V at a current of 3.0C.

10. Self-Discharge Performance: Under standard atmospheric conditions, the battery is charged to a constant voltage and constant voltage of 4.2 V at a current of 1.0 C according to the method described in the "General specification for the lithium-ion battery" , After which the remaining capacity was tested 30 days after static placement.

11. Cycle performance: Under standard atmospheric conditions, the cell was charged to 4.2 V with a constant current and a constant voltage at a current of 1.0 C according to the method described in " General specification for the test of the lithium-ion battery & To 3.0V. This step was cycled 300 times and the remaining capacity was tested.

Abbreviation

MD: machine direction, longitudinal (longitudinal)

TD: transverse direction

Characteristic test result of the embodiment characteristic Example 1 Example 2 Example 3 Example 4 Example 5 Of polyethylene resin
Weight average molecular weight 500,000 1,000,000 500,000 1,000,000 2,000,000 + 500,000 Resin versus pore-forming agent
Mass ratio 1: 3 1: 4 1: 3 1: 3 1: 4 The upper chamber
Stretching temperature (캜) 120 125 120 122 123 The lower chamber
Stretching temperature (캜) 115 115 119 119 115 Between the upper and lower chambers
Temperature difference (℃) 5 10 One 3 8 Heat-setting temperature (℃) 125 128 125 128 130 In MD × TD
Stretching ratio 5 × 5 5 × 5 5 × 5 5 × 5 5 × 5 Microporous
Average thickness (占 퐉) 30 16 12 5 20 On the front
Average pore size (nm) 100 200 110 150 180 On the rear
Average pore size (nm) 50 50 100 90 60 Between two sides
Pore size ratio 2.0: 1 4.0: 1 1.1: 1 1.67: 1 3.0: 1 Porosity (%) 50 45 35 60 40 Air permeability (s / 100ml) 350 180 90 50 240 Perforation strength (N / 20 탆) 10 7 5.5 4 8 Internal resistance (Ω) 41 31 28 25 37 At discharge at 3C ratio
Capacity retention rate (%) 80 91 85 93 88 After 30 days self-discharge
Remaining capacity (%) 96 93 90 95 92 Cycle performance (%) 82 85 84 87 85

Characteristic test result of comparative example characteristic Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example
4 Comparative Example
5 Of polyethylene resin
Weight average molecular weight 500,000 1,000,000 500,000 1,000,000 2,000,000
+ 500,000 Resin versus pore-forming agent
Mass ratio 1: 3 1: 4 1: 2 1: 3 1: 4 The upper chamber
Stretching temperature (캜) 120 125 115 122 123 The lower chamber
Stretching temperature (캜) 120 125 115 122 123 The upper and lower chambers
(° C) 0 0 0 0 0 Heat-setting temperature (℃) 125 128 125 128 130 In MD × TD
Stretching ratio 5 × 5 5 × 5 5 × 5 5 × 5 5 × 5 Microporous
Average thickness (占 퐉) 30 16 12 5 20 On the front
Average pore size (nm) 100 200 50 150 180 On the rear
Average pore size (nm) 100 200 50 150 180 Between two sides
Pore size ratio 1.0: 1 1.0: 1 1.0: 1 1.0: 1 1.0: 1 Porosity (%) 52 70 35 60 65 Air permeability (s / 100ml) 340 150 280 35 210 Perforation strength (N / 20 탆) 8 5 7 3 6 Internal resistance (Ω) 47 20 56 18 30 At discharge at 3C ratio
Capacity retention rate (%) 79 87 75 90 82 After 30 days self-discharge
Remaining capacity (%) 91 70 95 81 75 Cycle performance (%) 80 87 69 82 85

Claims (6)

Wherein the microporous membrane has a front surface and a rear surface, the micropores on the front surface of the microporous membrane have an average pore size different from the micropores on the rear surface of the microporous membrane, The average pore size of the pores is 100-200 nm; The average pore size of the micropores on the back surface is 50-100 nm; Wherein the ratio of the average pore size of the micropores on the front surface to the average pore size of the micropores on the back surface is (1.1-4.0): 1; And a pore size distribution on each of the front and back sides of less than 30%. The method according to claim 1,
The average pore size of the micropores on the front surface is 120-180 nm; The average pore size of the micropores on the back surface is 60-80 nm; The ratio of the average pore size of the micropores on the front surface to the average pore size of the micropores on the back surface is (1.5-3.0): 1; And a pore size distribution on each of the front and back sides of the microporous membrane is less than 20%. The method according to claim 1,
The microporous membrane has a thickness of 5-30 μm, a porosity of 35-60%, an air permeability of 50-350 s / 100 ml and a puncture strength of 4-10 N / 20 μm It is characterized by a polyethylene microporous membrane. A process for producing a polyethylene microporous membrane according to any one of claims 1 to 3,
(1) A polyethylene resin and a pore-forming agent are added to a twin screw extruder to perform melt kneading, extrusion through a die head, and rapid cooling to produce a thick sheet by casting Sheet casting;
(2) introducing the thick sheet into a stretcher to effect synchronous or asynchronous biaxial stretching, and controlling the stretching temperature of the upper and lower chambers of the stretcher to form an oil- Maintaining a temperature difference between the upper and lower chambers of the stretcher at 1-10 占 폚 to produce a thin film containing the thin film;
(3) an extraction step of extracting the oil-containing thin film with a solvent to remove the pore-forming agent to form a white microporous membrane; And
(4) a heat setting treatment step of introducing the white microporous membrane into a heat setting treatment to obtain a polyethylene microporous membrane having different average pore sizes on the front and back surfaces of the polyethylene microporous membrane By weight based on the total weight of the polyolefin microporous membrane. A lithium-ion battery, wherein the lithium-ion battery includes the polyethylene microporous membrane according to any one of claims 1 to 3, wherein the front surface of the microporous membrane is close to the anode of the battery, Lt; RTI ID = 0.0 > Li-ion < / RTI > The method of claim 5,
Wherein the capacity retention rate is greater than 80% when discharged at a 3C rate and the remaining capacity is greater than 90% after 30 days of self-discharge.

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