Separator and method for manufacturing the same

Abstract

One aspect of the present invention provides a separator and a method for producing the same, which comprises a porous film having a hydrophobic region containing a polyolefin and a hydrophilic region containing a hydrophilic polymer dispersed in the hydrophobic region, wherein the content of the hydrophilic region in the porous film is 0.1 to 7.5 wt %.

Description

Technical Field

[0001] The present invention relates to a separator and a method for manufacturing the same, and more particularly, to a separator for a lithium secondary battery having improved impregnation properties with respect to an electrolyte and a method for manufacturing the same.

Background Art

[0002] Lithium secondary batteries are widely used as power sources for various electrical products that require miniaturization and weight reduction, such as smartphones, notebook computers, and tablet PCs. As the application fields expand to include smart grids and medium- to large-sized batteries for electric vehicles, the development of lithium secondary batteries with large capacity, long life, and high stability is required.

[0003] As a means for achieving the above object, a separator having fine pores formed therein to separate the positive electrode and the negative electrode to prevent internal short circuit and to smoothly transfer lithium ions during the charge and discharge process, among which, research and development of a microporous separator using a polyolefin such as polyethylene, which is advantageous for pore formation by thermally induced phase separation, is economical, and easily satisfies the physical properties required for the separator, is active.

[0004] Conventionally, polyolefin-based separators that have been widely used have problems with weak heat resistance and mechanical strength. To complement this, a technique has been proposed to coat the surface of the separator with a heat-resistant layer containing ceramic particles. However, the heat-resistant layer leaves considerable technical problems in relation to air permeability and conductivity (resistance), which are elements that have a very important influence on the performance of the separator. That is, when forming a heat-resistant layer containing ceramic particles on the surface of a porous substrate, the heat resistance of the separator is improved, but the ceramic particles contained in the heat-resistant layer close the pores formed in the porous substrate, reducing the air permeability of the separator. As a result, the ion migration path between the positive electrode and the negative electrode is greatly reduced, and consequently, there is a problem that the charge and discharge performance of the secondary battery is greatly reduced. In addition, when the heat-resistant layer is continuously exposed to the electrolyte inside the battery, the ceramic particles are partially and continuously detached from the porous substrate. In this case, the heat resistance of the separator may also gradually decrease.

[0005] A lithium secondary battery is formed in a structure in which a non-aqueous electrolyte containing a lithium salt is impregnated in an electrode assembly in which a porous separator is interposed between a positive electrode and a negative electrode, on which active materials are respectively coated, on an electrode current collector.

[0006] A lithium secondary battery is manufactured by alternately laminating a positive electrode and a negative electrode, manufacturing an electrode assembly having a structure in which a separator is interposed between the positive electrode and the negative electrode, and then inserting the electrode assembly into a battery case made of a can or a pouch having a certain size and shape, and finally injecting an electrolyte. At this time, the electrolyte enters between the positive electrode, the negative electrode, and the separator by capillary force. However, due to the characteristics of the materials, the positive electrode, the negative electrode, and the separator are hydrophobic, while the electrolyte is hydrophilic. Therefore, in order to improve the impregnation and wetting properties of the electrolyte with respect to the electrodes and the separator, considerable time and strict process conditions are required, and thus there is a limit to achieving and improving the impregnation property of the electrolyte and the productivity of the process in a balanced manner.

[0007] In order to improve the impregnation property of such an electrolytic solution, methods such as injecting the electrolytic solution at a high temperature or injecting the electrolytic solution under a pressurized or depressurized state are used. However, there are problems such as the conventional electrode assembly and the electrolytic solution being deformed by heat and causing internal short circuits. Further, since the above-mentioned process is performed after the electrode assembly is housed in the battery case together with the electrolytic solution, the impregnation property of the electrolytic solution with respect to the electrode assembly may be uneven. In particular, in the case of a jelly roll type electrode assembly, uneven impregnation occurs between the winding center portion and the outer portion, and there is a problem that the life of the battery is shortened.

Summary of the Invention

Problems to be Solved by the Invention

[0008] The present invention is for solving the above-mentioned problems of the prior art, and an object of the present invention is to provide a separator and a method for manufacturing the same that can achieve a balance among mechanical physical properties, appearance physical properties, impregnation property with respect to an electrolytic solution, and productivity of a process.

Means for Solving the Problems

[0009] One aspect of the present invention provides a separator made of a porous film having a hydrophobic region containing a polyolefin and a hydrophilic region containing a hydrophilic polymer dispersed in the hydrophobic region, wherein in the porous film, the content of the hydrophilic region is 0.1 to 7.5% by weight.

[0010] In one embodiment, the weight average molecular weight of the polyolefin may be 200,000 to 800,000.

[0011] In one embodiment, the ratio of the content of the hydrophilic region to the weight average molecular weight of the polyolefin may be 0.1×10 -5 ~1.1×10 -5

[0012] In one embodiment, the weight average molecular weight of the polyolefin may be 900,000 to 2,000,000.

[0013] ​ In one embodiment, the ratio of the content of the hydrophilic region to the weight average molecular weight of the polyolefin may be 0.1×10 -5 ~0.75×10 -5 .

[0014] In one embodiment, the polyolefin may include one selected from the group consisting of polyethylene, polypropylene, polybutylene, polymethylpentene, and combinations of two or more thereof.

[0015] In one embodiment, the hydrophilic polymer may be one selected from the group consisting of ethylene vinyl acetate, ethylene vinyl alcohol, polyvinyl alcohol, polyacrylic acid, polyoxyethylene-polyoxypropylene block copolymer, polyethylene glycol, polyethylene oxide, polyvinyl pyrrolidone, polyvinyl acetal, polyvinyl butyral, cellulose derivatives, glycerol, and combinations of two or more thereof.

[0016] In one embodiment, the rate of change in length in the machine direction (MD) from immediately after the addition of the electrolyte droplet dropped on the surface of the separator to the point when 5 minutes have elapsed may be 15 to 50%, and the rate of change in length in the transverse direction (TD) may be 15 to 40%.

[0017] In one embodiment, the number of surface defects present on the surface of the separator, having a different lightness from the periphery and having a size of 2 mm or more, may be 10 pieces / m 2 or less.

[0018] Another aspect of the present invention provides a method for manufacturing the separator, including: (a) a step of charging a composition containing a polyolefin, a hydrophilic polymer, and a pore-forming agent into an extruder to form a base sheet; (b) a step of stretching the base sheet and then extracting the pore-forming agent to manufacture a base film; and (c) a step of thermally fixing the base film.

Advantages of the Invention

[0019] The separator according to one aspect of the present invention is composed of a porous film having a hydrophobic region containing a polyolefin and a hydrophilic region containing a hydrophilic polymer dispersed in the hydrophobic region. In the porous film, by adjusting the content of the hydrophilic region to 0.1 to 7.5% by weight, it is possible to achieve a balance among the mechanical properties, appearance properties, impregnation property with respect to the electrolyte, and productivity of the process of the separator.

[0020] The effects of the present invention are not limited to the above-described effects, and it should be understood that the effects include all effects that can be inferred from the configuration of the invention described in the detailed description or claims of the present invention.

Mode for Carrying Out the Invention

[0021] Hereinafter, the present invention will be described. However, the present invention can be realized in various different forms, and thus is not limited to the embodiments described herein. Throughout the specification, when any part is said to be "connected" to another part, this includes not only the case where it is "directly connected", but also the case where other members are interposed therebetween and it is "indirectly connected". Also, when any part is said to "include" a certain component, this means that, unless otherwise stated to the contrary, it does not exclude other components, and it may further include other components.

[0022] The separator according to one aspect of the present invention is composed of a porous film having a hydrophobic region containing a polyolefin and a hydrophilic region containing a hydrophilic polymer dispersed in the hydrophobic region. In the porous film, the content of the hydrophilic region may be 0.1 to 7.5% by weight.

[0023] In the separator, the hydrophobic region and the hydrophilic region can each form a continuous phase and a discontinuous phase. In the separator, the hydrophilic region is uniformly dispersed in a matrix composed of the hydrophobic region, and can impart substantially uniform hydrophilicity to the entire region in the area and / or thickness direction of the separator, thereby improving the impregnation property of the separator with respect to the electrolytic solution.

[0024] As used herein, the term "matrix" means a component that forms a continuous phase in a separator containing two or more components. That is, in the separator, the hydrophobic region containing the polyolefin can exist as a continuous phase, and the hydrophilic region containing the hydrophilic polymer can be dispersed and exist as a discontinuous phase therein.

[0025] In the porous film, the content of the hydrophilic region may be 0.1 to 7.5% by weight, preferably 1 to 6% by weight, and more preferably 3 to 6% by weight. If the content of the hydrophilic region is less than 0.1% by weight, the required level of electrolytic solution impregnation property cannot be achieved, and if it is greater than 7.5% by weight, although the electrolytic solution impregnation property can be further improved, the mechanical properties and heat resistance of the separator that can be achieved through the polyolefin may decrease. Further, if the content of the hydrophilic region is greater than 7.5% by weight, the dispersibility of the hydrophilic polymer decreases, the separator has a different lightness from the periphery on the surface, the number of surface defects having a size of 2 mm or more increases, and the appearance quality may deteriorate. There may be a sudden change in resistance at a site and / or region where the hydrophilic polymer aggregates arbitrarily on the surface and / or inside of the separator, which may adversely affect the electrochemical characteristics of the battery.

[0026] The polyolefin may include one selected from the group consisting of polyethylene, polypropylene, polybutylene, polymethylpentene, and combinations of two or more thereof, preferably may include at least one of polyethylene and polypropylene, and more preferably may include polyethylene, but is not limited thereto.

[0027] Conventionally, a separator made only of polyolefin is essentially hydrophobic, but by melting and kneading a certain amount of hydrophilic polymer with the polyolefin during the production of the separator, a predetermined hydrophilicity can be imparted to the separator. In this case, the molecular weight of the polyolefin and the content of the hydrophilic polymer in the separator are derived and combined with variables to optimize the electrolyte impregnation property at a required level, whereby the productivity of the process for melting and kneading the hydrophilic polymer with the polyolefin, the hydrophilicity of the separator, and the resulting electrolyte impregnation property can be realized in a balanced manner.

[0028] First, when the polyolefin is high-density polyethylene (HDPE), the weight-average molecular weight of the polyethylene may be 200,000 to 800,000, preferably 250,000 to 600,000, and more preferably 300,000 to 500,000. The ratio of the content of the hydrophilic region to the weight-average molecular weight of the high-density polyethylene is 0.1×10 -5 ~1.1×10 -5 , preferably 0.2×10 -5 ~1×10 -5 , more preferably 0.5×10 -5 ~1×10 -5 and may be.

[0029] If the ratio of the content of the hydrophilic region to the weight-average molecular weight of the high-density polyethylene is less than 0.1×10 -5 , the required level of electrolyte impregnation property cannot be realized, and if it is 1.1×10 -5If it is larger, not only does the electrolyte impregnation property decrease, but also the mechanical properties and heat resistance of the separator that can be realized through the high-density polyethylene may decrease. Further, the ratio of the content of the hydrophilic region to the weight-average molecular weight of the high-density polyethylene is 1.1×10 -5 If it is larger, the dispersibility of the hydrophilic polymer decreases, the separator has a different brightness from the periphery on the surface, the number of surface defects having a size of 2 mm or more increases, and the appearance quality may deteriorate. At a site and / or region where the hydrophilic polymer is arbitrarily aggregated on the surface and / or inside of the separator, the resistance changes abruptly, which may adversely affect the electrochemical characteristics of the battery.

[0030] In the case where the polyolefin is ultra-high molecular weight polyethylene (UHMWPE), the weight-average molecular weight of the polyethylene may be 900,000 to 2,000,000, preferably 1,000,000 to 1,500,000, and the ratio of the content of the hydrophilic region to the weight-average molecular weight of the ultra-high molecular weight polyethylene is 0.1×10 -5 ~0.75×10 -5 , preferably 0.15×10 -5 ~0.6×10 -5 , more preferably 0.3×10 -5 ~0.6×10 -5 and may be.

[0031] If the ratio of the content of the hydrophilic region to the weight-average molecular weight of the ultra-high molecular weight polyethylene is less than 0.1×10 -5 , the required level of electrolyte impregnation property cannot be realized. If it is larger than 0.75×10 -5 , not only does the electrolyte impregnation property decrease, but also the mechanical properties and heat resistance of the separator that can be realized through the ultra-high molecular weight polyethylene may decrease. Further, the ratio of the content of the hydrophilic region to the weight-average molecular weight of the ultra-high molecular weight polyethylene is 0.75×10 -5If it is larger, the dispersibility of the hydrophilic polymer decreases, the separator surface has a different brightness from the periphery, the number of surface defects having a size of 2 mm or more increases, and the appearance quality may deteriorate. At the site and / or region where the hydrophilic polymer aggregates arbitrarily on the surface and / or inside of the separator, the resistance changes suddenly, which may adversely affect the electrochemical characteristics of the battery.

[0032] The hydrophilic polymer may be one selected from the group consisting of ethylene vinyl acetate, ethylene vinyl alcohol, polyvinyl alcohol, polyacrylic acid, polyoxyethylene-polyoxypropylene block copolymer, polyethylene glycol, polyethylene oxide, polyvinyl pyrrolidone, polyvinyl acetal, polyvinyl butyral, cellulose derivatives, glycerol, and combinations of two or more thereof. Preferably, it may be ethylene vinyl acetate. More preferably, it may be ethylene vinyl acetate having a vinyl acetate content of 15 to 30% by weight, but is not limited thereto. If the vinyl acetate content in ethylene vinyl acetate is less than 15% by weight, the mechanical properties and hydrophilicity of the separator may decrease. If it is more than 30% by weight, the processability and the dispersibility of the hydrophilic polymer may decrease. In addition to the above-mentioned ones, in the hydrophilic polymer, various types of polymers having hydrophilic functional groups such as amine groups, amide groups, hydroxyl groups, and carboxylic acid groups in the main chain and / or side chain can also be applied as the hydrophilic polymer.

[0033] The length change rate in the longitudinal direction (MD) from immediately after the addition of the electrolyte droplet dropped on the surface of the separator to the point when 5 minutes have elapsed may be 15 to 50%, preferably 20 to 45%. The length change rate in the transverse direction (TD) may be 15 to 40%, preferably 20 to 30%.

[0034] When the maximum lengths in the longitudinal direction (MD) and the transverse direction (TD) of the electrolyte droplet immediately after being dropped onto the surface of the separator are MD1 and TD1, and the maximum lengths in the longitudinal direction (MD) and the transverse direction (TD) at the time when 5 minutes have elapsed therefrom are MD2 and TD2, the length change rate can be calculated by the following formulas 1 and 2. <Formula 1> Length change rate in the longitudinal direction (MD) (%) = (MD2 - MD1) / (MD1) × 100 <Formula 2> Length change rate in the transverse direction (TD) (%) = (TD2 - TD1) / (TD1) × 100

[0035] The electrolyte may be one selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and combinations of two or more thereof. Preferably, it may be a mixture of ethylene carbonate (EC), diethyl carbonate (DEC), and dimethyl carbonate (DMC) mixed at a predetermined ratio, but is not limited thereto.

[0036] When the electrolyte is, for example, a mixed electrolyte of ethylene carbonate (EC), diethyl carbonate (DEC), and dimethyl carbonate (DMC) (EC:DEC:DMC = 2:2:1 (v / v)) in which the concentration of LiPF6 is 1.5 M and the content of vinylene carbonate (VC) is 1.5% by weight, if the length change rates in the longitudinal direction (MD) and the transverse direction (TD) are outside the above range, it can be evaluated that the required level of electrolyte impregnation property cannot be achieved.

[0037] The polyolefin is high-density polyethylene (HDPE), and the ratio of the content of the hydrophilic region to the weight-average molecular weight of the high-density polyethylene is 0.1×10 -5 ~1.1×10 -5When it is, the ratio of the rate of change in the length in the machine direction (MD) to the rate of change in the length in the transverse direction (TD) of the electrolyte droplet may be 0.9 to 1.06, preferably 1 to 1.04.

[0038] Further, when the polyolefin is ultra-high molecular weight polyethylene (UHMWPE) and the ratio of the content of the hydrophilic region to the weight average molecular weight of the ultra-high molecular weight polyethylene is 0.1×10 -5 ~0.75×10 -5 When it is, the ratio of the rate of change in the length in the machine direction (MD) to the rate of change in the length in the transverse direction (TD) of the electrolyte droplet may be 0.9 to 1.8, preferably 1 to 1.7.

[0039] When the ratio of the rate of change in the length in the machine direction (MD) to the rate of change in the length in the transverse direction (TD) of the electrolyte droplet is out of the above range, the impregnation of the electrolyte into the separator may be uneven depending on the direction, and the electrochemical characteristics of the battery may deteriorate.

[0040] The number of surface defects (white dots and / or black dots) present on the surface of the separator, having a different brightness from the periphery and having a size of 2 mm or more, is 10 pieces / m 2 or less, preferably 8 pieces / m 2 or less, more preferably 6 pieces / m 2 or less. If the number of the surface defects is more than 10 pieces / m 2 or more, the appearance quality may deteriorate, and the resistance may change abruptly at the site and / or region where the polyolefin and / or the hydrophilic polymer are arbitrarily aggregated on the surface of the separator, which may adversely affect the electrochemical characteristics of the battery.

[0041] The separator can be manufactured by a method including: (a) a step of charging a composition containing a polyolefin, a hydrophilic polymer, and a pore former into an extruder to form a base sheet; (b) a step of stretching the base sheet and then extracting the pore former to produce a base film; and (c) a step of thermally fixing the base film.

[0042] In the step (a), a composition containing 20 to 40% by weight of the polyolefin, 0.1 to 5% by weight of the hydrophilic polymer, and the balance of the pore-forming agent can be fed into an extruder to form a base sheet. The types, physical properties, functions and effects of the polyolefin and the hydrophilic polymer are as described above. The amounts of the polyolefin and the hydrophilic polymer added can be adjusted so that the content of the hydrophilic region containing the hydrophilic polymer in the separator is 0.1 to 7.5% by weight.

[0043] The pore-forming agent may be selected from the group consisting of paraffin oil, paraffin wax, mineral oil, solid paraffin, soybean oil, rapeseed oil, palm oil, coconut oil, di-2-ethylhexyl phthalate, dibutyl phthalate, diisononyl phthalate, diisodecyl phthalate, bis(2-propylheptyl) phthalate, naphthenic oil, and combinations of two or more thereof, preferably paraffin oil, and more preferably paraffin oil having a kinematic viscosity of 50 to 100 cSt at 40°C, but is not limited thereto.

[0044] In the step (b), after stretching the base sheet, the pore-forming agent can be extracted to produce a base film. The stretching can be carried out by a known method such as uniaxial stretching or biaxial stretching (sequential or simultaneous biaxial stretching). In the case of sequential biaxial stretching, the stretching ratio may be 4 to 20 times in the transverse direction (MD) and the longitudinal direction (TD), respectively, and the resulting area ratio may be 16 to 400 times.

[0045] In the step (c), the base film can be heat-fixed. Heat-fixing is to fix the film, apply heat, and forcibly hold the film that attempts to shrink to remove residual stress. A high heat-fixing temperature is advantageous for reducing the shrinkage rate. However, if the temperature is excessively high, the film may partially melt, the formed pores may be closed, and the permeability may decrease. The heat-fixing temperature is preferably selected in the range where 10 to 30% by weight of the crystalline portion of the base film melts. If the heat-fixing temperature is selected within the above range, it is possible to prevent the problem that the rearrangement of polyolefin molecules in the base film is insufficient and there is no effect of removing the residual stress of the film, and the problem that the pores are closed due to partial melting and the permeability decreases. For example, the heat-fixing temperature may be 120 to 150 °C, preferably 130 to 145 °C, and the heat-fixing time may be 5 seconds to 10 minutes, preferably 10 seconds to 1 minute. Hereinafter, embodiments of the present invention will be described in detail.

[0046] Example 1-1 31.5 parts by weight of polyethylene (PE1, V600, HTC) with a weight average molecular weight (Mw) of 600,000, 0.5 parts by weight of ethylene vinyl acetate (EVA, HTC) with a vinyl acetate content of 28% by weight, and 68 parts by weight of paraffin oil with a kinematic viscosity of 70 cSt at 40 °C were mixed and charged into a twin-screw extruder (inner diameter 58 mm, L / D = 56). After discharging from the twin-screw extruder through a T-die with a width of 300 mm under the conditions of a screw rotation speed of 40 rpm and 200 °C, it was passed through a casting roll at a temperature of 40 °C to produce a base sheet with a thickness of 800 μm.

[0047] The base sheet was longitudinally (MD) stretched 8 times with a roll stretching machine at 110 °C and transversely (TD) stretched 9 times with a tenter stretching machine at 125 °C to produce a film. The film was impregnated in a dichloromethane leaching tank at 25 °C, the paraffin oil was extracted and removed for 1 minute, and then dried at 50 °C for 5 minutes. The film was heat-fixed in a state where it was relaxed 25% transversely (TD) at 140 °C to produce a porous separator.

[0048] Example 1-2 A porous separator was produced in the same manner as in Example 1-1, except that the amounts of PE1 and EVA charged were changed to 31 parts by weight and 1 part by weight, respectively.

[0049] Example 1-3 A porous separator was produced in the same manner as in Example 1-1, except that the amounts of PE1 and EVA charged were changed to 30.4 parts by weight and 1.6 parts by weight, respectively.

[0050] Example 1-4 A porous separator was produced in the same manner as in Example 1-1, except that the amounts of PE1 and EVA charged were changed to 30 parts by weight and 2 parts by weight, respectively.

[0051] Comparative Example 1-1 A porous separator was produced in the same manner as in Example 1-1, except that the amount of PE1 charged was changed to 32 parts by weight and no EVA was charged.

[0052] Comparative Example 1-2 A porous separator was produced in the same manner as in Example 1-1, except that the amounts of PE1 and EVA charged were changed to 29.8 parts by weight and 2.2 parts by weight, respectively. The weight-average molecular weight of the polyethylene constituting the separators according to the above Examples and Comparative Examples and the content of ethylene vinyl acetate in the separators (the content of EVA in PE1 and EVA) are shown in Table 1 below.

[0053]

Table 1

[0054] Experimental Example 1 The electrolyte impregnation properties of the separators produced in the above Examples and Comparative Examples were measured by the following method. As the electrolyte, a mixed electrolyte of ethylene carbonate (EC), diethyl carbonate (DEC), and dimethyl carbonate (DMC) (EC:DEC:DMC = 2:2:1 (v / v)) with a LiPF6 concentration of 1.5 M and a vinylene carbonate (VC) content of 1.5 wt% was used. Immediately after dropping 2 μl of the mixed electrolyte onto the surface of the separator, the sizes of the liquid droplets (MD1, TD1) and the sizes of the liquid droplets (MD2, TD2) diffused after 5 minutes were measured along the vertical direction (MD) and the horizontal direction (TD). Also, the number (ea / m 2 ) of non-uniform fine points (surface defects) with a size of 2 mm or more and a significant difference in brightness from the surrounding area on the separator surface was visually measured, and the results are shown in Table 2 below.

[0055]

Table 2

[0056] Example 2-1 27.5 parts by weight of polyethylene (PE2, VH100U, KPIC Co., Ltd.) with a weight average molecular weight (Mw) of 1,000,000, 0.5 parts by weight of ethylene vinyl acetate (EVA, HTC Co., Ltd.) with a vinyl acetate content of 28 wt%, and 72 parts by weight of paraffin oil with a kinematic viscosity of 70 cSt at 40°C were mixed and fed into a twin-screw extruder (inner diameter 58 mm, L / D = 56). After discharging from the twin-screw extruder through a T-die with a width of 300 mm under the conditions of a screw rotation speed of 40 rpm and a temperature of 200°C, it was passed through a casting roll at a temperature of 40°C to produce a base sheet with a thickness of 800 μm.

[0057] The base sheet was stretched 8 times in the machine direction (MD) using a roll stretching machine at 110 °C and 9 times in the transverse direction (TD) using a tenter stretching machine at 125 °C to produce a film. The film was impregnated in a dichloromethane leaching bath at 25 °C to extract and remove paraffin oil for 1 minute, and then dried at 50 °C for 5 minutes. The film was heat-fixed in a state where it was relaxed by 15% in the transverse direction (TD) at 135 °C to produce a porous separator.

[0058] Example 2-2 A porous separator was produced in the same manner as in Example 2-1, except that the input amounts of PE2 and EVA were changed to 27 parts by weight and 1 part by weight, respectively.

[0059] Example 2-3 A porous separator was produced in the same manner as in Example 2-1, except that the input amounts of PE2 and EVA were changed to 26.4 parts by weight and 1.6 parts by weight, respectively.

[0060] Example 2-4 A porous separator was produced in the same manner as in Example 2-1, except that the input amounts of PE2 and EVA were changed to 26 parts by weight and 2 parts by weight, respectively.

[0061] Comparative Example 2-1 A porous separator was produced in the same manner as in Example 2-1, except that the input amount of PE2 was changed to 28 parts by weight and no EVA was added.

[0062] Comparative Example 2-2 A porous separator was produced in the same manner as in Example 2-1, except that the input amounts of PE2 and EVA were changed to 25.8 parts by weight and 2.2 parts by weight, respectively. The weight average molecular weight of the polyethylene constituting the separators according to the above Examples and Comparative Examples and the content of ethylene vinyl acetate in the separators (the content of EVA in PE2 and EVA) are shown in Table 3 below.

[0063]

Table 3

[0064] Experimental Example 2 The electrolyte impregnation property and the number of surface defects of the separators produced in the above Examples and Comparative Examples were measured in the same manner as in Experimental Example 1, and the results are shown in Table 4 below.

[0065]

Table 4

[0066] The foregoing description of the present invention is for illustrative purposes, and those having ordinary knowledge in the technical field to which the present invention pertains can understand that it can be easily modified into other specific forms without changing the technical idea and essential features of the present invention. Therefore, it should be understood that the embodiments described above are illustrative in all respects and not restrictive. For example, each component described as a single type can also be implemented dispersedly, and similarly, the components described as dispersed can also be implemented in a combined form.

[0067] The scope of the present invention is indicated by the claims described below, and it should be understood that all changes or modified forms derived from the meaning and scope of the claims and their equivalent concepts are included in the scope of the present invention.

Claims

1. A separator comprising a porous film having a hydrophobic region containing a polyolefin and a hydrophilic region containing a hydrophilic polymer dispersed in the hydrophobic region, wherein in the porous film, the content of the hydrophilic region is 0.1 to 7.5% by weight, and the length change rate in the longitudinal direction (MD) from immediately after the addition of an electrolytic solution droplet dropped on the surface of the separator to the time when 5 minutes have elapsed is 15 to 50%, and the length change rate in the transverse direction (TD) is 15 to 40%.

2. The separator according to claim 1, wherein the weight average molecular weight of the polyolefin is 200,000 to 800,000.

3. The ratio of the content of the hydrophilic region to the weight average molecular weight of the polyolefin is 0.1 × 10 －５ ~1.1 x 10 －５ The separator according to claim 2 ,

4. The separator according to claim 1, wherein the weight average molecular weight of the polyolefin is 900,000 to 2,000,000.

5. The ratio of the content of the hydrophilic region to the weight average molecular weight of the polyolefin is 0.1×10 －５ to 0.75×10 －５ The separator according to claim 4, which is such.

6. The separator according to claim 1, wherein the polyolefin contains one selected from the group consisting of polyethylene, polypropylene, polybutylene, polymethylpentene, and combinations of two or more thereof.

7. The separator according to claim 1, wherein the hydrophilic polymer is one selected from the group consisting of ethylene vinyl acetate, ethylene vinyl alcohol, polyvinyl alcohol, polyacrylic acid, polyoxyethylene-polyoxypropylene block copolymer, polyethylene glycol, polyethylene oxide, polyvinyl pyrrolidone, polyvinyl acetal, polyvinyl butyral, cellulose derivatives, glycerol, and combinations of two or more thereof.

8. The number of surface defects that exist on the surface of the separator, have a lightness different from the periphery, and have a size of 2 mm or more is 10 pieces / m ２ The separator according to claim 1, which is as follows.

9. A method for manufacturing the separator according to any one of claims 1 to 8, comprising: (a) feeding a composition containing a polyolefin, a hydrophilic polymer, and a pore-forming agent into an extruder to form a base sheet; (b) stretching the base sheet and then extracting the pore-forming agent to produce a base film; and (c) heat-fixing the base film.

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