JP2016085980A - Polyethylene microporous film, manufacturing method thereof, and lithium ion battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion battery with a microporous film prepared according to the present invention, which has not only an adequate high-speed discharge performance, but also an adequate self discharge performance.SOLUTION: The present invention provides a polyethylene microporous film and its manufacturing method, and a lithium ion battery. The microporous film has front and rear faces; the average pore size of the front face is 100-200 nm, and the average pore size of the rear face is 50-100 nm. The average pore size ratio A:B of the average pore size of the front face and that of the rear face is (1.1-4.0):1. The pore size distribution of each of the front and rear faces is less than 30%. The microporous film is used for a lithium-ion battery. In such a case, the front face is placed closer to a positive electrode of the battery, whereas the rear face is placed closer to the negative electrode of the battery.SELECTED DRAWING: None

Description

  The present invention relates to the technical field of lithium-ion batteries, and more particularly, to a polyethylene microporous membrane, a method for producing the same, and a lithium-ion battery using the polyethylene microporous membrane.

  A polyethylene microporous membrane is a porous membrane having micropores on both sides. When the polyethylene microporous membrane is used as a lithium-ion battery separator, the first requirement for the separator is that it does not degrade the electrochemical properties of the battery. An appropriate pore diameter of sub-micron size of the separator is one of the important factors for meeting various requirements, for example, battery charging and discharging performance, safety performance, and cycle performance.

  When the battery is used for something, it is important to set the pore size to a size of micron or less in order to prevent a short circuit between the anode and the cathode inside the lithium-ion battery. Such properties become increasingly important as battery manufacturers employ thinner separators to increase battery capacity. This pore size directly affects the battery properties. When the separator micropores have a large micropore diameter, the battery exhibits low resistance, good cycle performance and high capacity retention during discharge at high speed. However, if the pore size of the micropores is too large, the active substances react with each other, which reduces the capacity and promotes the battery self-discharge process. Also, the anode and cathode tend to be in direct contact with each other, or tend to be destroyed by the lithium dendrite, which causes a short circuit. When the pore diameter of the separator is extremely small, the battery has high electrical resistance, insufficient cycle performance, and low capacity retention during high-speed discharge. The advantage is that the self-discharge process is slow, the fracture strength is high, a short circuit caused by the breakdown due to lithium dendrites is prevented, and thus the safety performance of the battery is improved. In addition, the uniformity of the micropore pore size distribution of the separator also directly affects the battery properties. If the micropore pore size distribution is non-uniform, large local currents that affect battery properties will occur during operation. Therefore, it is important to strictly control the pore diameter and form pores that are as uniform as possible.

  Considering the separator configuration, the pore size directly affects the air permeability of the separator represented by the Gurley value. For the same separator, its Gurley value can well reflect internal resistance. The larger the pore size of the separator, the higher the porosity and the lower the twist of the void. Therefore, the smaller the Gurley value, the lower the resistance of the separator. In contrast, the smaller the pore size of the separator, the greater the Gurley value and the greater the internal resistance. Separators with good electrical properties generally have a relatively low breathability.

  In addition, the pore size of the separator also directly affects its mechanical properties. In order to meet the requirements of battery assembly and daily charge and discharge cycle processes, the separator must have good mechanical properties. Fracture strength is typically used to evaluate the possibility of a battery short circuit because the rough surface of the electrode is destroyed by the separator during the battery assembly and charge / discharge cycle process. This causes a short circuit of the battery and causes a problem in the safety of the battery. In general, a microporous membrane (separator) having a larger pore diameter has low fracture strength and low safety. On the other hand, a separator having a smaller pore diameter has higher breaking strength and better safety.

  Currently, existing polyolefin microporous membranes have micropores with the same pore diameter on both the front and back surfaces. However, when such a microporous membrane is used in a lithium-ion battery, there are the following problems: When the pore diameter is large, for example, when the average pore diameter exceeds 150 nm, the internal resistance is lowered and the air permeability is reduced. The cycle performance is good, but the fracture strength and safety performance are low, and the self-discharge rate is high; when the pore size is small, for example, when the average pore size is smaller than 70 nm, the fracture strength and safety performance are good, Although the discharge rate is low, the air permeability and cycle performance are insufficient due to the large internal resistance.

  In order to overcome the above disadvantages, one aspect of the present invention provides a novel polyethylene microporous membrane having good breathability and good mechanical properties.

  In another aspect of the present invention, a method for producing a polyethylene microporous membrane is provided.

  In a still further aspect of the present invention, a lithium-ion battery using the polyethylene microporous membrane according to the present invention is provided; the battery using the polyethylene microporous membrane in this case is good not only with good fast discharge performance. Has self-discharge performance.

  A polyethylene micromicroporous membrane having a front and back surface is characterized in that the front micropores have an average pore size different from that of the back micropores; 200 nm, the mean pore diameter of the back micropores is 50-100 nm; the ratio of the mean pore diameter of the front micropores to the mean pore diameter of the back micropores is (1.1-4.0): In addition, the pore size distribution on each of the front surface and the back surface is less than 30%.

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

  In a preferred embodiment, the microporous membrane has a thickness of 5-30 μm, a porosity of 35-60%, a breathability of 50-350 s / 100 ml and a breaking strength of 4-10 N / 20 μm.

The process for producing a polyethylene microporous membrane consists of the following steps:
(1) Sheet casting: adding polyethylene resin and a pore-increasing agent to a twin screw extruder, performing melt kneading; extruding from a die head, rapidly cooling, and casting into a thick sheet;
(2) A thick sheet is introduced into the stretcher to perform synchronous or asynchronous biaxial stretching; the temperature between the upper and lower chambers of the stretcher is controlled by controlling the stretching temperature in the upper and lower chambers of the stretcher. Maintaining the difference at 1-10 ° C. and preparing an oil-containing thin film;
(3) extracting the oil-containing thin film with a solvent to remove the pore-forming agent and forming a white microporous film; and (4) subjecting the white microporous film to a heat setting treatment, its front and back surfaces. A step of obtaining a polyethylene microporous membrane having different average pore sizes.

  In a lithium-ion battery including a polyethylene microporous membrane according to the present invention, the front surface of the microporous membrane is close to the anode of the battery, and the back surface of the microporous membrane is close to the cathode of the battery.

  In the lithium-ion battery including the polyethylene microporous membrane, the front surface of the microporous membrane is close to the battery anode, and the back surface of the microporous membrane is close to the battery cathode.

  A lithium-ion battery has a capacity retention rate exceeding 80% when discharged at a 3C rate, and a residual capacity after 30 days of self-discharge exceeds 90%.

The present invention has the following advantages over the prior art.
1. The polyethylene microporous membrane according to the present invention has different average pore sizes on the front and back surfaces, which ensures the good breathability and good mechanical properties exhibited by the microporous membrane.

  2. The method for producing a polyethylene microporous membrane according to the present invention is simple and easy to implement, and by using it, it is possible to obtain a high-quality product by simply changing the control parameters of the process and without requiring additional equipment. Can do.

  3. In a lithium-ion battery comprising a polyethylene microporous membrane according to the present invention, the front surface of the microporous membrane is close to the battery anode, and the back surface of the membrane is close to the battery cathode. Since the hole diameter on the front surface is larger, the internal resistance is low, the cycle performance is good, and as a result, the capacity retention of the battery at high speed discharge is high. On the other hand, since the back hole diameter is smaller, the self-discharge performance of the battery is improved, and the breakdown resistance of the separator is improved, thereby effectively preventing the short circuit due to the destruction of lithium dendrites; Performance is enhanced.

FIG. 1 is a scanning electron micrograph showing the microstructure of the front surface of a polyethylene microporous membrane according to Example 1 of the present invention. FIG. 2 is a scanning electron micrograph showing the microstructure of the back surface of the polyethylene microporous membrane according to Example 1 of the present invention. FIG. 3 is a scanning electron micrograph showing the microstructure of the front surface of the polyethylene microporous membrane according to Example 2 of the present invention. FIG. 4 is a scanning electron micrograph showing the microstructure of the back surface of the polyethylene microporous membrane according to Example 2 of the present invention. FIG. 5 is a graph showing the test results of the average pore diameter of the front surface of the polyethylene microporous membrane according to Example 1 of the present invention. FIG. 6 is a graph showing the test results of the average pore diameter on the back of the polyethylene microporous membrane according to Example 1 of the present invention.

  The present invention will be described in detail below.

1. Polyethylene microporous membrane having an asymmetric structure of the present invention The polyethylene microporous membrane of the present invention has an asymmetric structure. In particular, the front of the membrane has a significant backbone structure on a microscopic scale, the backbone is connected to each other through thin fibers, and there are a number of relatively large micropores interconnected within the fiber, Good air permeability is given to the separator. In contrast, the back of the membrane has a uniform fiber structure on a microscopic scale, and its pore size is relatively small, thereby imparting sufficient mechanical properties to the separator. As a result, such an asymmetric structure of the microporous membrane of the present invention ensures that the separator has both good mechanical properties and good breathability. When the microporous membrane is used in a lithium-ion battery, the front surface of the microporous membrane is close to the battery anode. Since the front hole diameter is larger, the internal resistance of the battery is low and its resistance is small, resulting in good ion permeability and high capacity retention during fast discharge. On the other hand, the back of the membrane with a smaller pore size is close to the battery cathode. Due to the smaller pore size, the internal resistance is larger and the Li-ion transfer rate is slower during the self-discharge process. This leads to the battery having good self-discharge performance and improving the breakdown resistance of the separator, thereby effectively preventing a short circuit due to the destruction of the lithium dendrite and thus the safety performance of the battery. Be enhanced.

  The microporous membrane of the present invention has a larger pore diameter on the front surface, and the average value can be 100 to 200 nm. Such average pore size ensures good air permeability. If this average pore diameter exceeds 200 nm, the mechanical strength of the microporous membrane is greatly reduced, it is easily destroyed by lithium dendrites, causing a short circuit, and causing a safety problem for the battery. . In contrast, the pore size on the back of the microporous membrane is smaller, and its average value can be 50-99 nm. It ensures good mechanical properties and prevents short circuit due to lithium dendrite breakdown, thereby improving the safety performance of the battery. On the other hand, the smaller pore size slows the self-discharge rate and provides good self-discharge performance. When the average pore diameter on the back surface is less than 50 nm, the air permeability of the microporous film is greatly reduced and the internal resistance is increased, but as a result, the cycle performance of the battery is lowered, and the working performance of the battery is remarkably deteriorated. Will be.

In the present invention, the pore size distribution is measured by a mercury penetration method according to the standard of GB / T21650.1-2008 / ISO15901-1: 2006. The maximum pore size and the minimum pore size are obtained from the related figures simulated by data processing. The pore size distribution used herein is a percentage value obtained by dividing the maximum pore size or the difference between the minimum pore size and the average pore size by the average pore size, which is calculated by the following equation: R:
Pore size distribution = (maximum pore size−average pore size) / average pore size × 100%; or pore size distribution = (average pore size−minimum pore size) / average pore size × 100%.

  Each of the front and back surfaces of the microporous membrane of the present invention has 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, it has good current consistency in battery operation and helps maintain good battery properties. If the pore size distribution is greater than 30% or even higher, a very large current flows locally in the battery, resulting in an impact on the safety and cycle performance of the battery.

  In the microporous membrane of the present invention, the ratio of the average pore diameter on the front surface to the average pore diameter on the back surface is (1.1 to 4.0): 1, and this ratio is sufficient in mechanical strength and good for the membrane. Gives both breathability. If the ratio of average pore sizes between the two faces is less than 1.1: 1, the pore size difference between them will not be sufficient to meet the good performance requirements of the present invention. However, if the average pore size ratio between the two surfaces exceeds 4.0: 1, the pore size difference between them is too large, causing non-uniformity and collapse of the heat resistance of the microporous membrane. The objective cannot be achieved.

  The microporous membrane of the present invention has a thickness of 5-30 μm. When the thickness of the film is less than 5 μm, resistance to external stress is poor in battery production, and similarly, the separator can be destroyed by dendritic crystals formed during the battery charging and discharging process. Battery safety cannot be ensured. However, if the thickness of the membrane exceeds 30 μm, the permeability of the membrane decreases, and the thickness and weight of the battery increase.

  The polyethylene microporous membrane of the present invention has a porosity of 35-60%. When the porosity is less than 35%, the permeability of the membrane is insufficient, and the internal resistance is increased. As a result, the properties of the battery are deteriorated. However, if the porosity exceeds 60%, the pores are too large. Therefore, the mechanical strength of the membrane may be reduced, the risk of occurrence of a short circuit may be increased, and the stability and safety performance of the battery cannot be guaranteed.

  The polyethylene microporous membrane of the present invention has an air permeability of 50 to 350 s / 100 ml. When the air permeability exceeds 350 s / 100 ml, the permeability is not good, and the electrical properties of the film are deteriorated. However, if the air permeability is less than 50 s / 100 ml, the membrane pores are too large and a short circuit may occur, so the safety performance of the battery cannot be guaranteed.

  The polyethylene microporous membrane of the present invention has a breaking strength of 4 to 10 N / 20 μm, and has high resistance to the battery against external impact that may occur during battery manufacture and use. Ensure the safety performance of the battery.

  In summary, the polyethylene microporous membrane of the present invention has an asymmetric structure on the front and back, in which case the front pores have a larger average pore diameter of 100-200 nm, while the back pores It has a smaller average pore size of 50-100 nm. The average pore size ratio A: B between the front surface and the back surface is (1.1 to 4.0): 1, and the pore size distribution on each of the front surface and the back surface 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 breaking strength of 4-10 N / 20 μm.

2. Production method of 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 mainly used, but 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 10,000 to 5,000,000, preferably 100,000 to 3,000,000, more preferably 300,000 to 2,000,000.

  The pore-forming agent used in the present invention is not particularly limited, but is mainly a solvent capable of forming pores in the polyolefin resin, for example, liquid paraffin, solid paraffin, dioctyl phthalate, dibutyl phthalate, etc. . The pore-forming agent can be used alone or in combination of two or more of the types. Liquid paraffin and dioctyl phthalate are preferred.

  The extractant used in the present invention is not particularly limited, but is mainly dichloromethane, n-heptane, n-decane or ethanol. Of these, dichloromethane is preferred.

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

  The biaxial stretching process used in the present invention is not particularly limited, and synchronous biaxial stretching or asynchronous biaxial stretching is possible.

  The process parameters applied to the preparation of the polyethylene microporous membrane of the present invention are not particularly limited. Requirements for wet preparation process of polyethylene microporous membranes including melting temperature, extrusion temperature, chill roll temperature, stretching temperature, stretching speed, extraction temperature, extraction speed, heat set temperature, heat set speed, air temperature, air flow and air pressure Conventional parameters that satisfy can be used.

2.2. Method for Producing Polyethylene Microporous Membrane The polyethylene microporous membrane is prepared by a wet process which consists essentially of the following four steps: sheet casting, stretching, extraction and heat setting. Here, the stretching process is the most important process for the orientation of polyethylene molecular chains, and is the central process for producing microporous membranes. In this step, a thick sheet is stretched by a force applied from the outside in a highly elastic state having a melting point or lower and a glass transition temperature or higher, and is directed in the direction of the external force. Thereby, the application performance of the product is changed and improved. The stretched oil-containing thin film molecular chains are oriented in both the longitudinal and transverse directions, and the pore-forming agent is evenly distributed within the oriented molecular chains to form a special water-oil mixed structure. .

  The main factors affecting the stretching process are the stretching ratio, stretching speed and stretching temperature. In the case of the same apparatus, under conditions where the draw ratio and draw speed are constant during the drawing process, a higher drawing temperature will form larger pores on the oil-containing thin film and a larger pore size after extraction. As a result, the air permeability of the membrane is improved. On the other hand, when the stretching temperature is relatively low, the pores formed on the oil film become smaller and the pore diameter after extraction becomes smaller, and as a result, the mechanical properties of the film become better. Inside the stretcher, the chamber is effectively divided into two parts, an upper chamber and a lower chamber, by an oil-containing film as a dividing line. If the two parts have the same temperature, a microporous membrane with a uniform microstructure is formed with the same pore formation conditions. When the temperatures of the two portions are controlled to be different, that is, when the temperatures are different between the two portions, microporous membranes having different microstructures are formed with different pore formation conditions.

  In the present invention, the stretching temperature of the upper chamber can be 117 to 127 ° C, preferably 120 to 125 ° C, and the stretching temperature of the lower chamber can be 110 to 120 ° C, preferably 115 to 119 ° C. By controlling the temperature difference between the upper chamber and the lower chamber to be 1 to 10 ° C., microporous membranes having different microstructures on the two surfaces are obtained. When the temperature difference is less than 1 ° C., the microstructures of the two surfaces of the microporous membrane are not so different from each other because the pore formation conditions are similar, and the object of the present invention cannot be achieved. On the other hand, when the temperature difference is larger than 10 ° C., the microstructure on the two surfaces is significantly different in stretching ratio and stretching speed from each other as in the case where the pore formation conditions are greatly different, and the heat resistance of the microporous membrane Unevenness and degeneration can be caused and the object of the present invention cannot be achieved. The temperature difference in the present invention for obtaining a polyethylene microporous membrane exhibiting excellent overall performance is preferably 2-8 ° C, most preferably 3-5 ° C.

  The draw ratio in the present invention is 10 to 50, preferably 20 to 40.

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

3. Method for Manufacturing a Lithium-Ion Battery The polyethylene microporous membrane prepared as described above is cut according to battery size requirements and then assembled in the same manner as lithium lithium ferrous phosphate / graphite batteries are manufactured; The front of the microporous membrane is brought closer to the battery anode and the back is brought closer to the battery cathode.

  When the microporous membrane is used in a lithium-ion battery, the front surface of the microporous membrane is brought close to the battery anode. Since the front hole diameter is larger, the internal resistance of the battery is lower and the resistance is lower; as a result, the ion permeability is better, the battery cycle performance is better, and the capacity retention rate at high speed discharge Becomes higher. On the other hand, the back of the membrane is close to the cathode of the battery. Since the pore size is smaller, the internal resistance is greater and the Li-ion transfer rate is slower during the self-discharge process. This gives the battery good self-discharge performance and improves the separator's fracture resistance, thereby preventing short circuits due to lithium dendrite destruction.

  The lithium-ion battery using the microporous membrane prepared according to the present invention has not only good discharge performance but also good self-discharge performance.

Hereinafter, the present invention will be described in more detail with reference to examples.
<Example 1>

  (1) Extrusion of cast sheet: Polyethylene resin and liquid paraffin were poured into a twin screw extruder and melt-kneaded in a twin screw extruder to obtain a uniform melt. Subsequently, the melt was continuously extruded from the die head, rapidly cooled, and formed by a cooling roll. In the obtained thick sheet, the weight average molecular weight of the polyethylene resin is 500,000, the viscosity grade of liquid paraffin is 40, the mass ratio of polyethylene resin and liquid paraffin is 1: 3, and the melting temperature is 200 ° C. The temperature of the cooling roll was 30 ° C.

  (2) Biaxial stretching: A thick sheet was stretched in a preheated stretcher to obtain an oil-containing thin film; here, the stretching temperature of the upper chamber was 120 ° C., and the stretching temperature of the lower chamber was 115 ° C. The temperature difference between the upper and lower chambers was 5 ° C.

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

  (4) Heat setting: The white microporous membrane was subjected to a heat setting treatment at 125 ° C. for 2 minutes to obtain a polyethylene microporous membrane having an asymmetric structure.

  A polyethylene microporous membrane with an asymmetric structure was prepared according to the method described above and its physicochemical properties were tested. The test results are shown in Table 1.

(5) The polyethylene microporous membrane prepared as above was cut according to battery size requirements and then assembled; with the front of the microporous membrane approaching the battery anode and the back being the battery cathode Approached. The electrical properties of the battery were tested and the test results are shown in Table 1.
<Example 2>

  (1) Extrusion of cast sheet: Polyethylene resin and liquid paraffin were poured into a twin screw extruder and melt-kneaded in a twin screw extruder to obtain a uniform melt. Subsequently, the melt was continuously extruded from the die head, rapidly cooled, and formed by a cooling roll. In the resulting thick sheet, the weight average molecular weight of the polyethylene resin is 1,000,000, the viscosity grade of liquid paraffin is 50, the mass ratio of polyethylene resin to liquid paraffin is 1: 4, and the melting temperature is It was 200 degreeC and the temperature of the cooling roll was 30 degreeC.

  (2) Biaxial stretching: A thick sheet was stretched in a preheated stretcher to obtain an oil-containing thin film; here, the stretching temperature of the upper chamber was 125 ° C and the stretching temperature of the lower chamber was 115 ° C. The temperature difference between the upper and lower chambers was 10 ° C.

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

  (4) Heat setting: The white microporous membrane was subjected to a heat setting treatment at 128 ° C. for 2 minutes to obtain a polyethylene microporous membrane having an asymmetric structure.

  A polyethylene microporous membrane with an asymmetric structure was prepared according to the method described above and its physicochemical properties were tested. The test results are shown in Table 1.

(5) The polyethylene microporous membrane prepared as above was cut according to battery size requirements and then assembled; with the front of the microporous membrane approaching the battery anode and the back being the battery cathode Approached. The electrical properties of the battery were tested and the test results are shown in Table 1.
<Example 3>

  (1) Extrusion of cast sheet: Polyethylene resin and liquid paraffin were poured into a twin screw extruder and melt-kneaded in a twin screw extruder to obtain a uniform melt. Subsequently, the melt was continuously extruded from the die head, rapidly cooled, and formed by a cooling roll. In the obtained thick sheet, the weight average molecular weight of the polyethylene resin is 500,000, the viscosity grade of liquid paraffin is 40, the mass ratio of polyethylene resin and liquid paraffin is 1: 3, and the melting temperature is 210 ° C. The temperature of the cooling roll was 30 ° C.

  (2) Biaxial stretching: A thick sheet was stretched in a preheated stretcher to obtain an oil-containing thin film; here, the stretching temperature of the upper chamber was 120 ° C., and the stretching temperature of the lower chamber was 119 ° C. The temperature difference between the upper and lower chambers was 1 ° C.

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

  (4) Heat setting: The white microporous membrane was subjected to a heat setting treatment at 125 ° C. for 2 minutes to obtain a polyethylene microporous membrane having an asymmetric structure.

  A polyethylene microporous membrane with an asymmetric structure was prepared according to the method described above and its physicochemical properties were tested. The test results are shown in Table 1.

(5) The polyethylene microporous membrane prepared as above was cut according to battery size requirements and then assembled; with the front of the microporous membrane approaching the battery anode and the back being the battery cathode Approached. The electrical properties of the battery were tested and the test results are shown in Table 1.
<Example 4>

(1) Extrusion of cast sheet: Polyethylene resin and liquid paraffin were poured into a twin screw extruder and melt-kneaded in a twin screw extruder to obtain a uniform melt. Subsequently, the melt was continuously extruded from the die head, rapidly cooled, and formed by a cooling roll. In the obtained thick sheet, the weight average molecular weight of the polyethylene resin is 1,000,000, the viscosity grade of liquid paraffin is 50, the mass ratio of polyethylene resin and liquid paraffin is 1: 3, and the melting temperature is It was 200 degreeC and the temperature of the cooling roll was 30 degreeC.

  (2) Biaxial stretching: A thick sheet was stretched in a preheated stretcher to obtain an oil-containing thin film; here, the stretching temperature of the upper chamber was 122 ° C., and the stretching temperature of the lower chamber was 119 ° C. The temperature difference between the upper and lower chambers was 3 ° C.

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

  (4) Heat setting: The white microporous membrane was subjected to a heat setting treatment at 128 ° C. for 2 minutes to obtain a polyethylene microporous membrane having an asymmetric structure.

  A polyethylene microporous membrane with an asymmetric structure was prepared according to the method described above and its physicochemical properties were tested. The test results are shown in Table 1.

(5) The polyethylene microporous membrane prepared as above was cut according to battery size requirements and then assembled; with the front of the microporous membrane approaching the battery anode and the back being the battery cathode Approached. The electrical properties of the battery were tested and the test results are shown in Table 1.
<Example 5>

  (1) Extrusion of cast sheet: Polyethylene resin and liquid paraffin were poured into a twin screw extruder and melt-kneaded in a twin screw extruder to obtain a uniform melt. Subsequently, the melt was continuously extruded from the die head, rapidly cooled, and formed by a cooling roll. In the obtained thick sheet, the weight average molecular weight of polyethylene resin 1 was 2,000,000, and the weight average molecular weight of polyethylene resin 2 was 500,000; the mass ratio of polyethylene resin 1 and polyethylene resin 2 was 7 The viscosity grade of liquid paraffin was 70; the mass ratio of the polyethylene resin mixture and liquid paraffin was 1: 4; the melting temperature was 220 ° C., and the temperature of the chill roll was 30 ° C. there were.

  (2) Biaxial stretching: A thick sheet was stretched in a preheated stretcher to obtain an oil-containing thin film; here, the stretching temperature of the upper chamber was 123 ° C., and the stretching temperature of the lower chamber was 115 ° C. The temperature difference between the upper and lower chambers was 8 ° C.

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

  (4) Heat setting: The white microporous membrane was subjected to a heat setting treatment at 130 ° C. for 2 minutes to obtain a polyethylene microporous membrane having an asymmetric structure.

  A polyethylene microporous membrane with an asymmetric structure was prepared according to the method described above and its physicochemical properties were tested. The test results are shown in Table 1.

(5) The polyethylene microporous membrane prepared as above was cut according to battery size requirements and then assembled; with the front of the microporous membrane approaching the battery anode and the back being the battery cathode Approached. The electrical properties of the battery were tested and the test results are shown in Table 1.
<Comparative Example 1>

(1) Extrusion of cast sheet: Polyethylene resin and liquid paraffin were poured into a twin screw extruder and melt-kneaded in a twin screw extruder to obtain a uniform melt. Subsequently, the melt was continuously extruded from the die head, rapidly cooled, and formed by a cooling roll. In the obtained thick sheet, the weight average molecular weight of the polyethylene resin is 500,000, the viscosity grade of liquid paraffin is 40, the mass ratio of polyethylene resin and liquid paraffin is 1: 3, and the melting temperature is 200 ° C. The temperature of the cooling roll was 30 ° C.

  (2) Biaxial stretching: A thick sheet was stretched in a preheated stretcher to obtain an oil-containing thin film; here, the stretching temperature of the upper chamber was 120 ° C., and the stretching temperature of the lower chamber was 120 ° C. The temperature difference between the upper and lower chambers was 0 ° C.

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

  (4) Heat setting: The white microporous membrane was subjected to a heat setting treatment at 125 ° C. for 2 minutes to obtain a polyethylene microporous membrane having an asymmetric structure.

  A polyethylene microporous membrane with an asymmetric structure was prepared according to the method described above and its physicochemical properties were tested. The test results are shown in Table 2.

(5) The polyethylene microporous membrane prepared as above was cut according to battery size requirements and then assembled; with the front of the microporous membrane approaching the battery anode and the back being the battery cathode Approached. The electrical properties of the battery were tested and the test results are shown in Table 2.
<Comparative Example 2>

  (1) Extrusion of cast sheet: Polyethylene resin and liquid paraffin were poured into a twin screw extruder and melt-kneaded in a twin screw extruder to obtain a uniform melt. Subsequently, the melt was continuously extruded from the die head, rapidly cooled, and formed by a cooling roll. In the resulting thick sheet, the weight average molecular weight of the polyethylene resin is 1,000,000, the viscosity grade of liquid paraffin is 50, the mass ratio of polyethylene resin to liquid paraffin is 1: 4, and the melting temperature is It was 210 degreeC and the temperature of the cooling roll was 30 degreeC.

  (2) Biaxial stretching: A thick sheet was stretched in a preheated stretcher to obtain an oil-containing thin film; here, the stretching temperature of the upper chamber was 125 ° C., and the stretching temperature of the lower chamber was 125 ° C. The temperature difference between the upper and lower chambers was 0 ° C.

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

  (4) Heat setting: The white microporous membrane was subjected to a heat setting treatment at 128 ° C. for 2 minutes to obtain a polyethylene microporous membrane having an asymmetric structure.

  A polyethylene microporous membrane with an asymmetric structure was prepared according to the method described above and its physicochemical properties were tested. The test results are shown in Table 2.

(5) The polyethylene microporous membrane prepared as above was cut according to battery size requirements and then assembled; with the front of the microporous membrane approaching the battery anode and the back being the battery cathode Approached. The electrical properties of the battery were tested and the test results are shown in Table 2.
<Comparative Example 3>

  (1) Extrusion of cast sheet: Polyethylene resin and liquid paraffin were poured into a twin screw extruder and melt-kneaded in a twin screw extruder to obtain a uniform melt. Subsequently, the melt was continuously extruded from the die head, rapidly cooled, and formed by a cooling roll. In the resulting thick sheet, the weight average molecular weight of the polyethylene resin is 500,000, the viscosity grade of liquid paraffin is 40, the mass ratio of polyethylene resin and liquid paraffin is 1: 2, and the melting temperature is 200 ° C. The temperature of the cooling roll was 30 ° C.

  (2) Biaxial stretching: A thick sheet was stretched in a preheated stretcher to obtain an oil-containing thin film; here, the stretching temperature of the upper chamber was 115 ° C., and the stretching temperature of the lower chamber was 115 ° C. The temperature difference between the upper and lower chambers was 0 ° C.

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

  (4) Heat setting: The white microporous membrane was subjected to a heat setting treatment at 125 ° C. for 2 minutes to obtain a polyethylene microporous membrane having an asymmetric structure.

  A polyethylene microporous membrane with an asymmetric structure was prepared according to the method described above and its physicochemical properties were tested. The test results are shown in Table 2.

(5) The polyethylene microporous membrane prepared as above was cut according to battery size requirements and then assembled; with the front of the microporous membrane approaching the battery anode and the back being the battery cathode Approached. The electrical properties of the battery were tested and the test results are shown in Table 2.
<Comparative example 4>

  (1) Extrusion of cast sheet: Polyethylene resin and liquid paraffin were poured into a twin screw extruder and melt-kneaded in a twin screw extruder to obtain a uniform melt. Subsequently, the melt was continuously extruded from the die head, rapidly cooled, and formed by a cooling roll. In the obtained thick sheet, the weight average molecular weight of the polyethylene resin is 1,000,000, the viscosity grade of liquid paraffin is 50, the mass ratio of polyethylene resin and liquid paraffin is 1: 3, and the melting temperature is It was 210 degreeC and the temperature of the cooling roll was 30 degreeC.

  (2) Biaxial stretching: A thick sheet was stretched in a preheated stretcher to obtain an oil-containing thin film; here, the stretching temperature of the upper chamber was 122 ° C., and the stretching temperature of the lower chamber was 122 ° C. The temperature difference between the upper and lower chambers was 0 ° C.

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

  (4) Heat setting: The white microporous membrane was subjected to a heat setting treatment at 128 ° C. for 2 minutes to obtain a polyethylene microporous membrane having an asymmetric structure.

  A polyethylene microporous membrane with an asymmetric structure was prepared according to the method described above and its physicochemical properties were tested. The test results are shown in Table 2.

(5) The polyethylene microporous membrane prepared as above was cut according to battery size requirements and then assembled; with the front of the microporous membrane approaching the battery anode and the back being the battery cathode Approached. The electrical properties of the battery were tested and the test results are shown in Table 2.
<Comparative Example 5>

(1) Extrusion of cast sheet: Polyethylene resin and liquid paraffin were poured into a twin screw extruder and melt-kneaded in a twin screw extruder to obtain a uniform melt. Subsequently, the melt was continuously extruded from the die head, rapidly cooled, and formed by a cooling roll. In the obtained thick sheet, the weight average molecular weight of polyethylene resin 1 was 2,000,000, and the weight average molecular weight of polyethylene resin 2 was 500,000; the mass ratio of polyethylene resin 1 and polyethylene resin 2 was 7 The viscosity grade of liquid paraffin was 70; the mass ratio of the polyethylene resin mixture and liquid paraffin was 1: 4; the melting temperature was 220 ° C., and the temperature of the chill roll was 30 ° C. there were.

  (2) Biaxial stretching: A thick sheet was stretched in a preheated stretcher to obtain an oil-containing thin film; here, the stretching temperature of the upper chamber was 123 ° C., and the stretching temperature of the lower chamber was 123 ° C. The temperature difference between the upper and lower chambers was 0 ° C.

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

  (4) Heat setting: The white micro-microporous membrane was subjected to a heat-setting treatment at 130 ° C. for 2 minutes to obtain a polyethylene microporous membrane having an asymmetric structure.

  A polyethylene microporous membrane with an asymmetric structure was prepared according to the method described above and its physicochemical properties were tested. The test results are shown in Table 2.

  (5) The polyethylene microporous membrane prepared as above was cut according to battery size requirements and then assembled; with the front of the microporous membrane approaching the battery anode and the back being the battery cathode Approached. The electrical properties of the battery were tested and the test results are shown in Table 2.

  While preferred embodiments of the invention have been disclosed for purposes of illustration, various modifications, additions and changes may be made by those skilled in the art without departing from the scope and spirit disclosed in the appended claims. It should be understood.

The method for measuring the properties of the separator in the present invention is as follows:
1. Average pore diameter: GB / T21650.1-2008 / ISO15901-1: Measured by mercury penetration method according to 2006 standard. Mercury was injected into the test tube under vacuum conditions and analyzed at a high pressure station with a maximum pressure of 227.5 MPa. During the mercury intrusion process, mercury was injected into the pores of the specimen while increasing the pressure. The resulting electrical signal was input to a computer via a data processing sensor, thereby simulating the associated graphics and calculating the pore size data. The average pore size was obtained by averaging data processing.

2. Pore size distribution: Measured by mercury penetration method according to GB / T21650.1-2008 / ISO 15901-1: 2006 standard. The maximum pore size and the minimum pore size are obtained from the related figures simulated by data processing. The pore size distribution used herein is a percentage value obtained by dividing the maximum pore size or the difference between the minimum pore size and the average pore size by the average pore size, which is calculated by the following equation: R:
Pore size distribution = (maximum pore size−average pore size) / average pore size × 100%; or pore size distribution = (average pore size−minimum pore size) / average pore size × 100%.

  3. Thickness: 10 or more thicknesses employed in each of the MD direction and the TD direction were measured using a 1/1000 mm desktop gap gauge, and the average value was calculated to display the film thickness.

4). Porosity: A square sample of 100 mm × 100 mm was cut out from the microporous membrane, and its substantial amount W ₁ was measured. Subsequently, the mass W ₀ of the standard with a porosity of 0% was calculated based on the density and thickness of the resin composition, and the porosity was determined by the following formula:
Porosity = (W ₀ −W ₁ ) / W ₀ × 100

  5. Breathability: Breathability was measured at 25 ° C. at atmospheric pressure using a Gurley breathability meter in accordance with JIS standard P8117.

  6). Breaking strength: A needle tip having a diameter of 1.0 mm and a radius of curvature of 0.5 mm was attached to a universal tension machine, and the needle tip was passed through a separator at a speed of 120 mm / min, and the breaking strength at that time was measured at 23 ° C.

  7). Microstructure: Observe the film using a scanning electron microscope and take a picture of the microstructure.

  8). Internal resistance: Electrical properties are tested using a BT-2000 electrochemical tester.

  9. Fast discharge performance: According to the method described in “Common Test for Lithium-Ion Battery Testing”, the battery is charged to 4.2 V with a constant current of 1.0 C and then 3.0 C under standard ambient conditions. The battery was discharged to 3.0 V with a current of.

  10. Self-discharge performance: Under standard ambient conditions, the battery is charged at a constant current and a constant voltage to a current of 1.0 C to 4.2 V, and after 30 days of standing, “for testing lithium-ion batteries Residual capacity was tested according to the method described in “Common Specifications”.

  11. Cycle performance: According to the method described in “Common Test for Lithium-Ion Battery Tests”, the battery is charged at a constant current and a constant voltage to 4.2 V at a current of 1.0 C under standard external conditions. The battery was discharged to 3.0 V with a current of 1.0 C. The above process was cycled 300 times to test the residual capacity.

Abbreviations:
MD: machine direction; vertical direction TD: horizontal direction

Claims (6)

  A polyethylene microporous membrane having a front surface and a back surface, wherein the micropores on the front surface of the microporous membrane have an average pore diameter different from the micropores on the back surface, in which case the average of the micropores on the front surface The pore diameter is 100 to 200 nm, the average pore diameter of the back micropores is 50 to 100 nm, and the ratio of the average pore diameter of the front micropores to the average pore diameter of the back micropores is (1.1 to 4). 0.0): 1, a polyethylene microporous membrane characterized in that the pore size distribution on each of the front and back surfaces is less than 30%.   The average pore diameter of the front micropores is 120 to 180 nm, the average pore diameter of the back micropores is 60 to 80 nm, and the ratio between the average pore diameter of the front micropores and the average pore diameter of the back micropores The polyethylene microporous membrane according to claim 1, wherein (1.5 to 3.0): 1 and the pore size distribution on each of the front surface and the back surface is less than 20%.   2. The microporous membrane having a thickness of 5 to 30 [mu] m, a porosity of 35 to 60%, a gas permeability of 50 to 350 s / 100 ml, and a breaking strength of 4 to 10 N / 20 [mu] m. Polyethylene microporous membrane. The method for producing a polyethylene microporous membrane according to any one of claims 1 to 3, comprising the following steps:
(1) Sheet casting: adding polyethylene resin and a pore-increasing agent to a twin screw extruder, performing melt kneading; extruding from a die head, rapidly cooling, and casting into a thick sheet;
(2) A thick sheet is introduced into the stretcher to perform synchronous or asynchronous biaxial stretching; the temperature between the upper and lower chambers of the stretcher is controlled by controlling the stretching temperature in the upper and lower chambers of the stretcher. Maintaining the difference at 1-10 ° C. and preparing an oil-containing thin film;
(3) extracting the oil-containing thin film with a solvent to remove the pore-forming agent and forming a white microporous film; and (4) subjecting the white microporous film to a heat setting treatment, its front and back surfaces. A step of obtaining a polyethylene microporous membrane having different average pore sizes.   The polyethylene microporous membrane according to any one of claims 1 to 3, comprising the front surface of the microporous membrane approaching the battery anode and the back surface of the microporous membrane approaching the battery cathode. A lithium-ion battery.   6. The lithium-ion battery according to claim 5, wherein the capacity retention rate when discharged at a 3C rate exceeds 80% and the residual capacity after 30 days of self-discharge exceeds 90%.