Polyolefin microporous membrane

Abstract

To provide a polyolefin microporous film that can balance the battery characteristics of a non-aqueous secondary battery with safety, and a non-aqueous secondary battery separator and a non-aqueous secondary battery each including the same.SOLUTION: A polyolefin microporous film has an average flow diameter of 0.055 μm or less as measured with a Perm Porometer, and also has an air permeability of 60 sec / 100 cm3 or less and a shutdown temperature of 143°C or lower.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This invention relates to a polyolefin microporous membrane. [Background technology]

[0002] Polyolefin microporous membranes exhibit excellent electrical insulation and ion permeability, and are therefore used in battery separators, capacitor separators, fuel cell materials, and microfiltration membranes, with particular use as separators for lithium-ion secondary batteries.

[0003] In recent years, lithium-ion secondary batteries have been used in small electronic devices such as mobile phones and notebook computers, as well as in electric vehicles such as electric cars and small electric motorcycles. Separators for lithium-ion secondary batteries are required to have not only mechanical properties and ion permeability, but also safety in various safety tests. Furthermore, polyolefin microporous membranes are being investigated as separators from the perspective of balancing mechanical properties, heat resistance, compression resistance, dimensional stability, safety, and battery characteristics in environments such as inside batteries equipped with special electrodes, high temperature, high pressure, and large dimensions (Patent Documents 1-2).

[0004] Patent Document 1 describes how increasing the volume proportion of small-diameter pores enhances ion diffusion, enabling a high level of balance between dendrite suppression, high power output, and cycle characteristics. However, prioritizing the balance between intensity and permeability, and improving both, resulted in challenges in suppressing the rise in shutdown temperature.

[0005] Patent Document 2 describes that a pore size distribution obtained by binarizing SEM images shows that a higher proportion of small pores results in superior output characteristics, strength, and shrinkage rate compared to conventional methods. However, Patent Document 2 does not consider the temperature, airflow, and residence time in the biaxial stretching process, nor the strain rate in the heat setting (HS) process. Therefore, it is not possible to suppress the increase in air permeability when reducing pore size, and there is room for improvement from the viewpoint of output. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2021-123614 [Patent Document 2] Japanese Patent Publication No. 2021-105166 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] In recent years, countries have been strengthening their efforts to realize a decarbonized society. Fuel efficiency regulations that reduce greenhouse gas emissions from vehicles while driving are accelerating the spread of hybrid and electric vehicles.

[0008] Lithium-ion batteries used in vehicles outdoors require improved rate characteristics not only at normal temperatures but also in snowy regions, so that they can operate in low-temperature environments. In low-temperature environments, the diffusion rate of lithium ions within the separator decreases, making it impossible to obtain sufficient capacity when the battery is rapidly charged and discharged.

[0009] To improve the ion permeability of a separator, it is possible to reduce air permeability by increasing the pore size of the separator. However, if the pore size becomes too large, the number of pores per unit volume decreases, reducing the pathways through which ions can permeate and degrading the output characteristics. Alternatively, methods to improve the ion permeability of the separator include thinning the film or reducing the amount of resin per unit area. However, this leads to a decrease in the puncture strength of the separator, deterioration of the voltage withstand capability, and an increase in the shutdown temperature, sacrificing battery safety.

[0010] Therefore, conventional non-aqueous secondary batteries that use polyethylene microporous membranes as separators have found it difficult to achieve both battery characteristics, such as output characteristics at room temperature and low-temperature environments, and safety.

[0011] In view of the above circumstances, the present invention aims to provide a polyolefin microporous membrane that can achieve both battery characteristics and safety in a non-aqueous secondary battery, as well as a separator for a non-aqueous secondary battery and a non-aqueous secondary battery containing the same. [Means for solving the problem]

[0012] The inventors have found that the above problems can be solved by specifying the palm-poro average flow diameter, air permeability, and shutdown temperature for polyolefin microporous membranes, and have completed the invention. One aspect of the present invention is illustrated below. [1] The average flow diameter of the palm polo is 0.055 μm or less, and the air permeability is 60 sec / 100 cm. 3 A polyolefin microporous membrane that is as follows, and has a shutdown temperature of 143°C or lower. [2] The polyolefin microporous membrane according to item 1, wherein the viscosity-average molecular weight (Mv) of the polyolefin microporous membrane is 200,000 or more and 500,000 or less. [3] Basis weight 1g / m 2 The withstand voltage converted to this is 0.022kV / (g / m 2 A polyolefin microporous membrane as described in item 1 or 2, which is greater than or equal to the above. [4] A polyolefin microporous membrane as described in any one of items 1 to 3, having a porosity of 40% to 75%. [5] The air permeability after pressing at 30℃ and 2.5MPa for 10 minutes was 20 sec / 100cm 3 Above 100 sec / 100 cm 3 A polyolefin microporous membrane as described in any one of the following items 1 to 4. [6] Air permeability converted to film thickness is 5.0 (sec / 100cm 3 A polyolefin microporous membrane described in any one of items 1 to 5, having a thickness of ) / μm or less. [7] Palm Polo average flow diameter (μm) and air permeability (sec / 100cm) 3 The product of ) is 3.0 (sec / 100cm 3 A polyolefin microporous membrane described in any one of items 1 to 6, having a thickness of ) μm or less. [8] The microporous polyolefin membrane according to any one of items 1 to 7, which has a tortuosity of 1.1 to 1.8. [9] The puncture strength converted to basis weight is 60.0 gf / (g / m 2 ) or more and 110.0 gf / (g / m 2 ) or less, the microporous polyolefin membrane according to any one of items 1 to 8.

[10] The microporous polyolefin membrane according to any one of items 1 to 9, which has a tensile strength of 1000 kgf / cm 2 or more in both MD and TD.

[11] The microporous polyolefin membrane according to any one of items 1 to 10, which has a maximum bubble point pore size of 0.060 µm or less

[12] The microporous polyolefin membrane according to any one of items 1 to 11, wherein a difference between the maximum bubble point pore size and the Palmporos mean flow pore size is 0.007 µm or less.

[13] A separator comprising: the microporous polyolefin membrane according to any one of items 1 to 12, and an inorganic porous layer disposed on at least one surface of the microporous polyolefin membrane.

[14] A separator comprising: the microporous polyolefin membrane according to any one of items 1 to 12, and a thermoplastic resin layer disposed on at least one surface of the microporous polyolefin membrane.

[15] A separator comprising: the microporous polyolefin membrane according to any one of items 1 to 12, and at least one layer selected from the group consisting of a multifunctional layer, an inorganic porous layer, and a thermoplastic resin layer, the at least one layer being disposed on at least one surface of the microporous polyolefin membrane. . Further, as another aspect of the present invention, a method for producing a microporous polyolefin membrane is shown below.

[16] The following steps: (A) a step of extruding a polyolefin composition containing a polyolefin resin and a pore-forming material to form a gel-like sheet; (B) a step of biaxially stretching the gel-like sheet to form a stretched sheet; ​​(C) A step of extracting a pore-forming material from a stretched sheet to form a porous film; and (D) Process of thermally fixing the porous film It includes the following components: (a) In process (A), the proportion of polyethylene with a viscosity-average molecular weight Mv of 500,000 or more and 1,500,000 or less is 10% by mass or more and 45% by mass or less relative to the total polyolefin resin. (i) In process (A), the proportion of polyethylene with a viscosity-average molecular weight Mv of 500,000 or less relative to the total polyolefin resin is 55% by mass or more and 90% by mass or less. (c) In process (B), the stretch ratio of both MD and TD is 5 times or more and 9 times or less. (e) In process (B), the ratio of the stretch coefficient to the heat-fixing coefficient (stretch coefficient / heat-fixing coefficient) is 3.5 or less. (e) In process (D), the relaxation rate is 0.90 or higher, (c) In process (D), the ratio of the strain rate during stretching to the strain rate during relaxation (stretching strain rate % / sec / relaxation strain rate % / sec) is 10 or more. A method for producing a polyolefin microporous membrane, characterized by having at least one of the following. [Effects of the Invention]

[0013] This invention improves the output characteristics of non-aqueous secondary batteries containing polyolefin microporous membranes in room temperature and low-temperature environments by specifying the average palm polo flow diameter, air permeability, and shutdown temperature. Even if an abnormal reaction occurs and the internal battery temperature rises, the separator pores are quickly blocked, stopping the flow of lithium ions between electrodes, thereby improving safety. Furthermore, according to this invention, by specifying the air permeability after pressurization, the high porosity and low air permeability of the polyolefin microporous membrane can be maintained even when electrodes that are prone to expansion and contraction are used in non-aqueous secondary batteries, or when the separator is compressed during modularization of non-aqueous secondary batteries. In addition, by specifying the puncture strength, basis weight equivalent puncture strength, and tensile strength, a membrane with high safety characteristics such as impact resistance can be obtained, and by specifying the average palm polo flow diameter and bubble point, a membrane with excellent voltage resistance characteristics can be obtained. [Modes for carrying out the invention]

[0014] The following describes in detail embodiments for carrying out the present invention (hereinafter abbreviated as "embodiments"). It should be noted that the present invention is not limited to the following embodiments, and can be implemented in various ways within the scope of its gist.

[0015] In this specification, the longitudinal direction (MD) refers to the machine direction of continuous microporous membrane molding, and the width direction (TD) refers to the direction that intersects the MD of the microporous membrane at a 90° angle.

[0016] In this specification, the upper and lower limits of each numerical range can be combined in any way. Furthermore, the presence of a specific component as the main component of a particular component means that the content of that specific component is 50% by mass or more, based on the mass of the component. Unless otherwise specified, the physical properties or numerical values ​​described herein are measured or calculated by the methods described in the examples.

[0017] <Polyolefin microporous membrane> One aspect of the present invention is a polyolefin microporous membrane. The polyolefin microporous membrane contains a polyolefin resin as its main component and can exhibit excellent electrical insulation and ion permeability. Therefore, it can be used, for example, in non-aqueous secondary batteries, specifically as a separator for non-aqueous secondary batteries.

[0018] The polyolefin microporous membrane according to this embodiment has the following characteristics: The average flow diameter of the palm polo is 0.055 μm or less, and the air permeability is 60 sec / 100 cm. 3 The following conditions must be met, and the shutdown temperature must be 143°C or lower.

[0019] The polyolefin microporous membrane according to this embodiment, by specifying the average palm-poro flow diameter, air permeability, and shutdown temperature as described above, can achieve both excellent battery characteristics such as rate characteristics, safety such as safety in heating tests, and superior voltage withstand characteristics in non-aqueous secondary batteries that include the polyolefin microporous membrane as a separator, for example. Furthermore, by improving the porosity and ion permeability of the polyolefin microporous membrane, it is possible to maintain high porosity and low air permeability of the polyolefin microporous membrane even when electrodes that are prone to expansion and contraction are used in non-aqueous secondary batteries, or when the separator is compressed during modularization of non-aqueous secondary batteries.

[0020] A palm-poro average flow diameter of 0.055 μm or less tends to improve the output characteristics of the battery. The palm-poro average flow diameter is measured by the method described in the examples. If the palm-poro average flow diameter is greater than 0.055 μm, the number of pathways for ions to permeate decreases, and the output characteristics deteriorate. On the other hand, if the palm-poro average flow diameter is small, it leads to an increase in air permeability, which reduces the diffusion rate of lithium ions, and good output characteristics cannot be obtained. From this viewpoint, the palm-poro average flow diameter of the polyolefin microporous membrane is preferably 0.020 μm to 0.055 μm, more preferably 0.030 μm to 0.055 μm, even more preferably 0.040 μm to 0.050 μm, and most preferably 0.040 μm to 0.045 μm.

[0021] 60 sec / 100 cm 3 The following air permeability levels tend to improve lithium ion permeability and thus enhance the battery's output characteristics. For example, an air permeability of 60 sec / 100 cm for a polyolefin microporous membrane. 3 If the air permeability exceeds a certain level, the diffusion rate of lithium ions decreases, resulting in poor power characteristics. From this perspective, the air permeability of polyolefin microporous membranes should be 60 sec / 100 cm. 3 The following is preferable: 55 sec / 100 cm 3 The following is more preferable: 50 sec / 100 cm 3 The following is even more preferable: 40 sec / 100 cm 3 The following is most preferable: The air permeability of the polyolefin microporous membrane should be 10 sec / 100 cm, from the viewpoint of ensuring puncture resistance. 3 The above is preferable, 20 sec / 100 cm 3 The above is preferable.

[0022] A shutdown temperature of 143°C or lower means that if some abnormal reaction occurs and the internal temperature of the battery rises, the pores of the separator will close before reaching 143°C, which tends to improve the safety of the battery. On the other hand, from the viewpoint of not degrading battery performance even when exposed to high temperatures close to 100°C, a shutdown temperature of 125°C or higher is preferable for polyolefin microporous membranes. From this viewpoint, the shutdown temperature of polyolefin microporous membranes is preferably 125°C to 143°C, more preferably 125°C to 142°C, even more preferably 125°C to 141°C, even more preferably 125°C to 140°C, particularly preferably 125°C to 139°C, and most preferably 125°C to 138°C.

[0023] From the viewpoint of achieving a certain membrane strength and low air permeability, the porosity of the polyolefin microporous membrane is preferably in the range of 40% to 75%, more preferably 45% to 70%, even more preferably 50% to 65%, and particularly preferably 55% to 62%.

[0024] Air permeability of polyolefin microporous membranes after being pressurized at 30°C and 2.5 MPa for 10 minutes (hereinafter referred to as "air permeability after pressurization") 30℃,2.5MPa (That is) 100 sec / 100 cm 3 The following is preferable because it tends to ensure high ion permeability even under pressure in the thickness direction: 80 sec / 100 cm 3 It is more preferable that the following conditions are met: 60 sec / 100 cm 3 It is even more preferable that the following conditions are met: 40 sec / 100 cm 3 The following is most preferable: Air permeability after pressurization 30℃,2.5MPa From the perspective of ensuring puncture strength, 20 sec / 100 cm 3 The above is preferable.

[0025] From the perspective of improving output characteristics, the air permeability of the polyolefin microporous membrane, converted to film thickness (μm), should be 5.0 (sec / 100cm²). 3 It is preferable that the value is ) / μm or less, and 4.5 (sec / 100cm). 3 It is more preferable that the value is ) / μm or less, and 4.0 (sec / 100cm²). 3 It is even more preferable that the value be less than or equal to ) / μm, and 3.0 (sec / 100cm). 3 It is most preferable that the particle size be ) / μm or less.

[0026] To improve output characteristics, it is important to control both the average flow diameter of the palm polo tube and the air permeability to small values. From this perspective, the average flow diameter of the palm polo tube (μm) and the air permeability (sec / 100cm) are important. 3 The product of ) is 3.0 (sec / 100cm 3 It is preferable that the particle size is ) μm or less, and the time is 2.5 (sec / 100cm). 3 It is more preferable that the particle size be ) μm or less, and the time is 2.0 (sec / 100cm). 3 It is even more preferable that the particle size be 1.5 μm or less, and the ratio is 1.5 (sec / 100cm). 3 It is most preferable that the particle size be ) μm or less.

[0027] To improve the output characteristics, the curvature ratio (τ) calculated by the gas-liquid method described in the examples below is preferably 1.8 or less, more preferably 1.6 or less, even more preferably 1.4 or less, and most preferably 1.3 or less. On the other hand, from the viewpoint of dielectric strength, the curvature ratio of the polyolefin microporous membrane is preferably 1.1 or more, and more preferably 1.2 or more. Particularly preferably, the curvature ratio of the polyolefin microporous membrane is in the range of 1.2 to 1.4.

[0028] When the viscosity-average molecular weight (Mv) of a polyolefin microporous membrane is low, the rate at which the resin occludes the pores at the start of melting is fast, which tends to result in a low shutdown temperature. From this viewpoint, the Mv of the polyolefin microporous membrane is preferably 500,000 or less, more preferably 450,000 or less, and even more preferably 430,000 or less. On the other hand, from the viewpoint of ensuring the strength of the polyolefin microporous membrane, the Mv of the polyolefin microporous membrane is preferably 200,000 or more, more preferably 250,000 or more, even more preferably 300,000 or more, even more preferably 350,000 or more, and even more preferably 380,000 or more.

[0029] From the same viewpoint as above, and from the viewpoint of suppressing short circuits between electrodes or voltage withstand failures caused by the separation of the separator due to foreign matter unintentionally mixed into the inside of the battery, the puncture strength of the polyolefin microporous membrane is preferably 150 gf or more, more preferably 180 gf or more, even more preferably 200 gf or more, particularly preferably 250 gf or more, and most preferably 300 gf or more. The upper limit of the puncture strength is not particularly limited, but can be determined according to the crystallinity of the membrane and the suppressed electrical resistance, and may be, for example, 700 gf or less.

[0030] Puncture strength (gf) is measured in grams (g / m²). 2 The value obtained by dividing by (gf / (g / m)) is the equivalent puncture strength (gf / (g / m) 2The puncture strength equivalent to the base weight was set to 60.0 gf / (g / m²). By increasing the puncture strength equivalent to the base weight, even if the battery is struck, the film will not break and its insulating properties will be maintained, thus ensuring safety by preventing the battery from overheating. From this perspective, the puncture strength equivalent to the base weight was set to 60.0 gf / (g / m²). 2 Preferably, it is 70.0 gf / (g / m³) or higher, and 70.0 gf / (g / m³) 2 It is more preferable that it be 75.0 gf / (g / m³) or higher, and more preferably 75.0 gf / (g / m³). 2 It is even more preferable that it be 80.0 gf / (g / m³) or higher. 2 It is particularly preferable that it be 85.0 gf / (g / m³) or higher, and 85.0 gf / (g / m³) 2 It is most preferable that the pressure be 110.0 gf / (g / m²) or higher. Furthermore, from the viewpoint of reducing the shutdown temperature, the basis weight puncture strength of the polyolefin microporous membrane should be 110.0 gf / (g / m²). 2 Preferably less than 100.0 gf / (g / m³) 2 ) or less is more preferable, and 95.0 gf / (g / m³) 2 ) or less is even more preferable, 90.0 gf / (g / m³). 2 The following are the most preferred.

[0031] The tensile (breaking) strength of microporous membranes is 1000 kgf / cm² for both MD and TD due to their high mechanical strength. 2 Preferably, it should be 1200 kgf / cm² or more. 2 It is more preferable that the load be greater than or equal to 1400 kgf / cm². 2 It is even more preferable that the load be greater than or equal to 1500 kgf / cm². 2 It is even more preferable that the load be greater than or equal to 1600 kgf / cm². 2 It is particularly preferable that the load be 1800 kgf / cm² or higher. 2 It is most preferable if the tensile strength is 1000 kgf / cm². 2 If the pressure is lower than this, the film may not be able to withstand the stress applied during winding of the electrode and the polyolefin microporous film, potentially leading to film rupture. From the perspective of suppressing thermal shrinkage of the polyolefin microporous film, 4000 kgf / cm² is recommended. 2 A value lower than this is preferable.

[0032] From the viewpoint of battery safety and suppression of minor short circuits, the absolute value of the withstand voltage of the polyolefin microporous membrane is preferably 0.5kV or higher, more preferably 0.7kV or higher, even more preferably 0.9kV or higher, even more preferably 1.0kV or higher, particularly preferably 1.1kV or higher, and most preferably 1.2kV or higher.

[0033] From a similar perspective, the dielectric strength per unit basis weight of a polyolefin microporous film is 0.22 kV / (g / m²). 2 Preferably, it is 0.25kV / (g / m 2 ) or more is more preferable, and 0.26kV / (g / m 2 ) More preferably 0.27kV / (g / m 2 ) or more is even more preferable, 0.28kV / (g / m 2 ) More preferably 0.29kV / (g / m 2 ) or more is particularly preferred, and 0.30kV / (g / m 2 The above is the most preferable.

[0034] From the viewpoint of output characteristics, the maximum pore diameter of the bubble point of the polyolefin microporous membrane is preferably 0.060 μm or less, more preferably 0.055 μm or less, and even more preferably 0.050 μm or less. From the viewpoint of suppressing an increase in air permeability, the maximum pore diameter of the bubble point of the polyolefin microporous membrane is preferably 0.020 μm or more, more preferably 0.030 μm or more, and even more preferably 0.040 μm or more.

[0035] From the viewpoint of improving voltage resistance, battery safety, and suppressing minor short circuits, the difference between the maximum bubble point pore diameter and the average palm Polo flow diameter of the polyolefin microporous membrane (maximum bubble point pore diameter - average palm Polo flow diameter) is preferably 0.007 μm or less, more preferably 0.006 μm or less, even more preferably 0.005 μm or less, particularly preferably 0.004 μm or less, and most preferably 0.003 μm or less.

[0036] The palm poro average flow diameter, maximum bubble point pore diameter, air permeability, porosity, shutdown temperature, Mv, dielectric strength, curvature, puncture strength, basis weight equivalent puncture strength, tensile strength, etc., of the polyolefin microporous membrane according to this embodiment can be adjusted to within the numerical range described above by controlling, for example, the molecular weight and blending ratio of polymer raw materials such as polyolefin, the stretching ratio during the biaxial stretching process, the stretching temperature during the biaxial stretching process, the biaxial stretching, and the heat-fixing (HS) ratio during the manufacturing process of the polyolefin microporous membrane.

[0037] Preferred components, or other components, in this embodiment are described below.

[0038] (Components) Examples of polyolefin microporous membranes include: porous membranes containing polyolefin resin; porous membranes containing polyolefin resin in addition to resins such as polyethylene terephthalate, polycycloolefin, polyethersulfone, polyamide, polyimide, polyimidamide, polyaramid, polycycloolefin, nylon, and polytetrafluoroethylene; woven fabrics made from polyolefin fibers; and nonwoven fabrics made from polyolefin fibers. Among these, from the viewpoint of reducing membrane resistance, improving membrane compression resistance, and ensuring structural uniformity, microporous membranes containing polyolefin resin (hereinafter referred to as polyolefin resin porous membranes) are preferred, and microporous membranes containing polyethylene as the main component are more preferred.

[0039] A porous polyolefin resin membrane will be described. From the viewpoint of improving the shutdown performance when forming a polyolefin microporous membrane for non-aqueous secondary batteries, it is preferable that the porous membrane is formed from a polyolefin resin composition in which polyolefin resin accounts for 50% to 100% by mass of the resin components constituting the porous membrane. More preferably, the proportion of polyolefin resin in the polyolefin resin composition is 60% to 100% by mass, even more preferably 70% to 100% by mass, and most preferably 95% to 100% by mass.

[0040] The polyolefin resin contained in the polyolefin resin composition is not particularly limited, and examples include homopolymers, copolymers, or multi-stage polymers obtained using ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene as monomers. Furthermore, these polyolefin resins may be used individually or as a mixture of two or more.

[0041] In particular, from the viewpoint of reducing film resistance, improving film compression resistance, and ensuring structural uniformity, polyethylene, polypropylene, ethylene-propylene copolymers, ethylene-propylene-polymers of other monomers, and mixtures thereof are preferred as polyolefin resins.

[0042] Specific examples of polyethylene include low-density polyethylene, linear low-density polyethylene, medium-density polyethylene, high-density polyethylene, and ultra-high molecular weight polyethylene. Specific examples of polypropylene include isotactic polypropylene, syndiotactic polypropylene, and atactic polypropylene. Specific examples of copolymers include ethylene-propylene random copolymer and ethylene-propylene rubber.

[0043] From the viewpoint of crystallinity, high strength, compressibility, and shutdown characteristics when forming a polyolefin microporous membrane for non-aqueous secondary batteries, it is preferable that the porous membrane is formed from a polyethylene composition in which polyethylene accounts for 50% to 100% by mass of the resin components constituting the microporous membrane. More preferably, the proportion of polyethylene in the resin components constituting the porous membrane is 60% to 100% by mass, even more preferably 70% to 100% by mass, and most preferably 90% to 100% by mass.

[0044] From the viewpoint of high strength, compressibility, and low resistance when forming a polyolefin microporous membrane as a separator for non-aqueous secondary batteries, the proportion of polyethylene in the polyolefin resin is preferably 30% by mass or more, more preferably 50% by mass or more, even more preferably 70% by mass or more, particularly preferably 80% by mass or more, and most preferably 90% by mass or more.

[0045] The polyolefin resin composition may contain any additives. Examples of additives include polymers other than polyolefin resins; inorganic fillers; antioxidants such as phenolic, phosphorus-based, and sulfur-based agents; metal soaps such as calcium stearate and zinc stearate; ultraviolet absorbers; light stabilizers; antistatic agents; antifogging agents; and coloring pigments. From the viewpoint of improving shutdown performance, the total amount of these additives added is preferably 20% by mass or less, more preferably 10% by mass or less, and even more preferably 5% by mass or less, relative to 100% by mass of the polyolefin resin.

[0046] When the microporous membrane is a polyolefin resin porous membrane, the viscosity-average molecular weight (Mv) of the polyolefin resin used as a raw material is preferably 30,000 to 6,000,000, more preferably 80,000 to 3,000,000, and even more preferably 150,000 to 1,500,000. A viscosity-average molecular weight of 30,000 or more is preferable because it tends to result in high strength due to the entanglement of polymers. On the other hand, a viscosity-average molecular weight of 6,000,000 or less is preferable from the viewpoint of improving moldability in the extrusion and stretching processes.

[0047] The type, molecular weight, and composition of the polyolefin resin constituting the polyolefin resin porous membrane can be adjusted as described above, for example, by controlling the type, molecular weight, and blending ratio of polymer raw materials such as polyolefins in the manufacturing process of the polyolefin microporous membrane. Furthermore, multilayer polyolefin resin microporous membranes having a structure in which two or more layers of the same or different types of polyolefin resin microporous membranes are laminated can also be adjusted as described above.

[0048] (Details of microporous membranes) Polyolefin microporous membranes have a porous structure in which a large number of very small pores come together to form dense interconnected pores. As a result, they exhibit excellent ion permeability and high strength when containing electrolytes.

[0049] The thickness of the polyolefin microporous film is preferably 2 μm to 15 μm, more preferably 3 μm to 13 μm, even more preferably 4 μm to 12 μm, and most preferably 5 μm to 12 μm, from the viewpoint of high ion permeability and good rate characteristics, and from the viewpoint of reducing the volume occupied by the separator and contributing to an improvement in battery capacity when used for high-capacity batteries. The average thickness of the polyolefin microporous film can be adjusted within the above numerical range by controlling the distance between the cast rolls, the cast clearance, the stretching ratio during the biaxial stretching process, the HS ratio, the HS temperature, etc.

[0050] The closer the tensile breaking strength values ​​of the longitudinal direction (MD) and TD of a polyolefin microporous membrane, the more uniformly the membrane will break during nail penetration tests of non-aqueous secondary batteries, thereby minimizing the short-circuit area and improving impact safety. From this viewpoint, the ratio of the tensile breaking strength in the longitudinal direction (MD) to the tensile breaking strength in the TD direction (MD / TD tensile breaking strength ratio) of a polyolefin microporous membrane is preferably in the range of 0.80 to 1.20, more preferably in the range of 0.85 to 1.15, even more preferably in the range of 0.90 to 1.10, and most preferably in the range of 0.95 to 1.05. The MD / TD tensile breaking strength ratio of a polyolefin microporous membrane can be adjusted to the numerical range described above by controlling, for example, the stretching ratio and HS ratio during the biaxial stretching process.

[0051] The basis weight of the polyolefin microporous membrane is 0.1 g / m², from the viewpoint of puncture strength and dielectric strength. 2 ~10g / m 2 It is preferable that it be within the range.

[0052] (Multilayer porous membrane) In this embodiment, a multilayer porous membrane is also provided, having the polyolefin microporous membrane described above and at least one layer disposed on at least one side thereof. Depending on the properties of the at least one layer, the multilayer porous membrane can impart one or more functions to the polyolefin microporous membrane and can also be used as a separator for non-aqueous secondary batteries.

[0053] Specifically, a multilayer porous film can have any of the following layer configurations: Layer configuration 1: comprising a polyolefin microporous film and an inorganic porous layer disposed on at least one side of the polyolefin microporous film; Layer configuration 2: comprising a polyolefin microporous film and a thermoplastic resin layer disposed on at least one side of the polyolefin microporous film; and Layer configuration 3: comprises a polyolefin microporous membrane and at least one layer selected from the group consisting of a multifunctional layer, an inorganic porous layer, and a thermoplastic resin layer, which is disposed on at least one side of the polyolefin microporous membrane.

[0054] (Inorganic porous layer) The inorganic porous layer contains inorganic particles and a binder polymer. Because the multilayer porous film containing the inorganic porous layer has a porous structure, it maintains ion permeability while exhibiting excellent thermal shrinkage suppression even in thin films.

[0055] While the inorganic particles are not particularly limited, those with high heat resistance and electrical insulation properties, and that are electrochemically stable within the operating range of non-aqueous secondary batteries, are preferred.

[0056] Examples of inorganic particle materials include oxide ceramics such as alumina, silica, titania, zirconia, magnesia, ceria, yttria, zinc oxide, and iron oxide; nitride ceramics such as silicon nitride, titanium nitride, and boron nitride; ceramics such as silicon carbide, calcium carbonate, magnesium sulfate, aluminum sulfate, barium sulfate, aluminum hydroxide, aluminum hydroxide oxide or boehmite, potassium titanate, talc, kaolinite, decite, nacrite, halloysite, pyrophyllite, montmorillonite, sericite, mica, amethyst, bentonite, asbestos, zeolite, calcium silicate, magnesium silicate, diatomaceous earth, and silica sand; and glass fibers. Among these, at least one selected from the group consisting of alumina, boehmite, and barium sulfate is preferred from the viewpoint of stability in non-aqueous secondary batteries. Furthermore, as boehmite, synthetic boehmite that can reduce ionic impurities that adversely affect the properties of electrochemical elements is preferred. Inorganic particles may be used individually or in combination.

[0057] Examples of inorganic particle shapes include plate-like, flaky, polyhedral, needle-like, columnar, granular, spherical, spindle-shaped, and block-like shapes, and multiple types of inorganic particles having the above shapes may be used in combination. Among these, block-like shapes are preferred from the viewpoint of balancing permeability and heat resistance.

[0058] The aspect ratio of the inorganic particles is preferably 1.0 to 3.0, and more preferably 1.1 to 2.5. An aspect ratio of 3.0 or less is preferable from the viewpoint of suppressing the amount of moisture adsorption of the multilayer porous film and suppressing capacity degradation when repeated cycles are performed, and from the viewpoint of suppressing deformation of the PO microporous film at temperatures exceeding its melting point.

[0059] The proportion of inorganic particles in the inorganic porous layer is preferably 90% by mass or more and 99% by mass or less, more preferably 91% by mass or more and 98% by mass or less, and even more preferably 92% by mass or more and 98% by mass or less. A proportion of inorganic particles of 90% by mass or more is preferable from the viewpoint of ion permeability and from the viewpoint of suppressing deformation at temperatures exceeding the melting point of the polyolefin microporous film. Furthermore, a proportion of 99% by mass or less is preferable from the viewpoint of maintaining the binding force between inorganic particles or the interfacial binding force between inorganic particles and the polyolefin microporous film.

[0060] Binder polymers are materials that bind multiple inorganic particles together in an inorganic porous layer, or that bind an inorganic porous layer to a polyolefin microporous membrane. When a multilayer porous membrane is used as a separator, it is preferable to use a binder polymer that is insoluble in the electrolyte of a non-aqueous secondary battery and is electrochemically stable within the operating range of a non-aqueous secondary battery.

[0061] Specific examples of binder polymers include the following 1) to 7). 1) Polyolefins: for example, polyethylene, polypropylene, ethylene propylene rubber, and modified versions thereof; 2) Conjugated diene polymers: for example, styrene-butadiene copolymers and their hydrides, acrylonitrile-butadiene copolymers and their hydrides, acrylonitrile-butadiene-styrene copolymers and their hydrides; 3) Acrylic polymers: for example, methacrylic acid ester-acrylic acid ester copolymers, styrene-acrylic acid ester copolymers, acrylonitrile-acrylic acid ester copolymers; 4) Polyvinyl alcohol-based resins: for example, polyvinyl alcohol, polyvinyl acetate; 5) Fluorine-containing resins: for example, polyvinylidene fluoride, polytetrafluoroethylene, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer, ethylene-tetrafluoroethylene copolymer; 6) Cellulose derivatives: for example, ethylcellulose, methylcellulose, hydroxyethylcellulose, carboxymethylcellulose; 7) Resins with a melting point and / or glass transition temperature of 180°C or higher, or polymers that do not have a melting point but have a decomposition temperature of 200°C or higher: for example, polyphenylene ether, polysulfone, polyethersulfone, polyphenylene sulfide, polyetherimide, polyamideimide, polyamide, polyester.

[0062] From the viewpoint of safety during short circuits, 3) acrylic polymers, 5) fluororesins, and 7) polyamides as polymers are preferred. As for polyamides, from the viewpoint of durability, fully aromatic polyamides, especially polymetaphenylene isophthalamide, are preferred.

[0063] From the viewpoint of compatibility between the binder polymer and the electrode, 2) conjugated diene polymers are preferred, and from the viewpoint of voltage resistance, 3) acrylic polymers and 5) fluororesins are preferred.

[0064] The above 2) Conjugated diene polymers are polymers that contain conjugated diene compounds as monomer units.

[0065] Examples of the above-mentioned conjugated diene compounds include 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, 2-chlor-1,3-butadiene, substituted linear-conjugated pentadienes, substituted and side-conjugated hexadienes, etc. These may be used individually or in combination of two or more. Among these, 1,3-butadiene is particularly preferred.

[0066] The above 3) Acrylic polymer is a polymer that contains a (meth)acrylic compound as a monomer unit. The above (meth)acrylic compound refers to at least one selected from the group consisting of (meth)acrylic acid and (meth)acrylic acid esters.

[0067] Examples of (meth)acrylic acid used in the acrylic polymers described in 3) above include acrylic acid and methacrylic acid.

[0068] Examples of (meth)acrylic acid esters used in the acrylic polymers described in 3) above include alkyl (meth)acrylic acid esters, such as methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate, butyl methacrylate, 2-ethylhexyl acrylate, and 2-ethylhexyl methacrylate; and epoxy group-containing (meth)acrylic acid esters, such as glycidyl acrylate and glycidyl methacrylate. These may be used individually or in combination of two or more. Among these, 2-ethylhexyl acrylate (EHA) and butyl acrylate (BA) are particularly preferred.

[0069] From the viewpoint of safety for non-aqueous secondary batteries, acrylic polymers are preferably polymers that contain EHA or BA as the main constituent units. The main constituent units refer to monomers that account for 40 mol% or more of the total raw materials for forming the polymer and the corresponding polymer portion.

[0070] The above 2) conjugated diene polymers and 3) acrylic polymers may also be obtained by copolymerizing them with other monomers that can copolymerize with them. Examples of other copolymerizable monomers that can be used include unsaturated alkyl carboxylates, aromatic vinyl monomers, vinyl cyanide monomers, unsaturated monomers containing hydroxyalkyl groups, unsaturated carboxylic acid amide monomers, crotonic acid, maleic acid, maleic anhydride, fumaric acid, itaconic acid, etc. These may be used individually or in combination of two or more. Among the above, unsaturated alkyl carboxylate monomers are particularly preferred. Examples of unsaturated alkyl carboxylate monomers include dimethyl fumarate, diethyl fumarate, dimethyl maleate, diethyl maleate, dimethyl itaconate, monomethyl fumarate, monoethyl fumarate, etc. These may be used individually or in combination of two or more.

[0071] Furthermore, the conjugated diene polymer described in 2) above may be obtained by copolymerizing the above (meth)acrylic compound as another monomer.

[0072] The binder polymer is preferably in the form of latex, and more preferably an acrylic polymer latex, from the viewpoint of having strong binding force between multiple inorganic particles even at high temperatures exceeding room temperature, and suppressing thermal shrinkage.

[0073] A dispersant such as a surfactant may be added to the coating liquid for forming the inorganic porous layer to stabilize dispersion or improve coating properties. The dispersant is adsorbed onto the surface of inorganic particles in the slurry and stabilizes the inorganic particles by electrostatic repulsion, etc. Examples include polycarboxylates, sulfonates, and polyoxyethers. The amount of dispersant added is preferably 0.2 parts by weight or more and 5.0 parts by weight or less on a solid content basis, and more preferably 0.3 parts by weight or more and 1.0 part by weight or less.

[0074] The total thickness of the inorganic porous layer is preferably 0.1 μm to 10 μm, more preferably 0.2 μm to 7 μm, and even more preferably 0.3 μm to 4 μm. The total thickness of the inorganic porous layer refers to the thickness of the inorganic porous layer when it is formed on one side of the polyolefin microporous film, and the sum of the thicknesses of both inorganic porous layers when it is formed on both sides of the PO microporous film. A total thickness of 0.1 μm or more is preferable from the viewpoint of suppressing deformation at temperatures exceeding the melting point of the polyolefin microporous film, and a total thickness of 10 μm or less is preferable from the viewpoint of improving battery capacity.

[0075] (thermoplastic resin layer) The thermoplastic resin layer is a layer mainly composed of thermoplastic resin, and may contain other components as desired. From the viewpoint of high adhesion, it is preferable that the thermoplastic resin layer and the polyolefin microporous film are in direct contact.

[0076] From the viewpoint of adhesion to the electrode, the proportion of thermoplastic resin in the thermoplastic resin layer is preferably more than 3% by mass, more preferably 10% by mass or more, even more preferably 20% by mass or more, 40% by mass or more, 60% by mass or more, or 80% by mass or more, and particularly preferably 90% by mass or more.

[0077] Examples of thermoplastic resins include binder polymers contained in the inorganic porous layer mentioned above. Among these, from the viewpoint of adhesion and safety during nail puncture tests or short circuits of non-aqueous secondary batteries, 2) conjugated diene polymers, 3) acrylic polymers, 5) fluororesins, and 7) polyamides as polymers are preferred.

[0078] The area ratio of the thermoplastic resin layer to the total surface area of ​​the polyolefin microporous membrane is preferably 100% or less, 95% or less, 80% or less, 75% or less, or 70% or less, and more preferably 5% or more, 10% or more, or 15% or more. Setting this area ratio to 100% or less is preferable from the viewpoint of suppressing blockage of the pores of the polyolefin microporous membrane by the thermoplastic resin and further improving the permeability of the separator. Setting this area ratio to 5% or more is preferable from the viewpoint of further improving adhesion to the electrode.

[0079] When a thermoplastic resin layer is placed on a portion of the surface of a polyolefin microporous film or an inorganic porous layer, possible arrangement patterns for the thermoplastic resin layer include, for example, dots, diagonal lines, stripes, grids, stripes, tortoiseshell patterns, random patterns, and combinations thereof.

[0080] The thickness of the thermoplastic resin layer is preferably 0.1 μm or more per side of the polyolefin microporous film, more preferably 0.2 μm or more, even more preferably 0.3 μm or more, preferably 10 μm or less, more preferably 7 μm or less, and even more preferably 4 μm or less. A thickness of 0.1 μm or more for the thermoplastic resin layer is preferable from the viewpoint of uniformly developing adhesion between the electrode and the multilayer porous film, and as a result, battery characteristics can be improved. A thickness of 10 μm or less for the thermoplastic resin layer is preferable from the viewpoint of suppressing a decrease in ion permeability.

[0081] (Multifunctional layer) The multifunctional layer is a layer that imparts numerous functions to the polyolefin microporous film or separator, and can, for example, have the functions of both the inorganic porous layer and the thermoplastic resin layer described above. More specifically, the multifunctional layer comprises the binder polymer or thermoplastic resin and inorganic particles as described above, and may optionally contain additional components such as dispersants. The thickness of the multifunctional layer is not limited but can be determined according to the functions to be imparted to the polyolefin microporous film and the coating conditions.

[0082] <Method for manufacturing polyolefin microporous membranes> The method for producing a polyolefin microporous membrane according to this embodiment is not particularly limited, but one example includes the following steps: (A) A step of extruding a polyolefin composition containing a polyolefin resin and a pore-forming agent to form a gel-like sheet; (B) A step of forming a stretched sheet by biaxially stretching a gel-like sheet; (C) A step of extracting a pore-forming material from a stretched sheet to form a porous film; and (D) A process of heat-fixing a porous film.

[0083] Furthermore, in another aspect of the present invention, a method for producing a polyolefin microporous membrane comprising the above steps (A) to (D) is provided, comprising the following configuration: (a) In process (A), the proportion of polyethylene with a viscosity-average molecular weight Mv of 500,000 or more and 1,500,000 or less is 10% by mass or more and 45% by mass or less relative to the total polyolefin resin. (i) In process (A), the proportion of polyethylene with a viscosity-average molecular weight Mv of 500,000 or less relative to the total polyolefin resin is 55% by mass or more and 90% by mass or less. (c) In process (B), the stretch ratio of both MD and TD is 5 times or more and 9 times or less. (e) In process (B), the ratio of the stretch coefficient to the heat-fixing coefficient (stretch coefficient / heat-fixing coefficient) is 3.5 or less. (e) In process (D), the relaxation rate is 0.90 or higher, (c) In process (D), the ratio of the strain rate during stretching to the strain rate during relaxation (stretching strain rate % / sec / relaxation strain rate % / sec) is 10 or more. A method for producing a polyolefin microporous membrane is provided, characterized by having at least one of the following.

[0084] In the above-described method for producing a polyolefin microporous membrane, it is preferable to have all of the above configurations (a) to (f), and it is more preferable to have any or both of the following configurations in addition to configurations (a) to (f): (k) In process (D), the stretch ratio is 1.7 times or more; (k) In process (D), the relaxation temperature is 115°C or higher and 130°C or lower.

[0085] The manufacturing process and preferred embodiments of polyolefin microporous membranes are described below.

[0086] [Extrusion Process (A)] In step (A), the polyolefin composition is extruded to form a gel-like sheet. The polyolefin composition may contain a polyolefin resin, a pore-forming agent, etc. It is preferable that the resin contained in the polyolefin composition consists only of polyolefin, without containing non-resin components such as fine particles or high heat-resistant resins with significantly different melting points, from the viewpoint of uniformizing the tensile stress and improving the air permeability and air permeability distribution of the resulting film. The gel-like sheet can be obtained by melt-kneading the polyolefin resin and the pore-forming agent and forming it into a sheet.

[0087] First, the polyolefin resin and the pore-forming agent are melt-mixed. One method of melt-mixing is to put the polyolefin resin and, if necessary, other additives into a resin mixing device such as an extruder, kneader, laboplast mill, mixing roll, or Banbury mixer, and mix the resin components while introducing the pore-forming agent in a desired ratio.

[0088] The polyolefin resin contained in the polyolefin composition can be determined according to the predetermined resin raw material of the resulting polyolefin microporous membrane. Specifically, the polyolefin resin used in the extrusion process (A) may be one of the polyolefin resins described above as a component of the polyolefin microporous membrane.

[0089] The PC (polypolymer) content of the resin composition is preferably 20% to 40% by mass, more preferably 22% to 37% by mass, and even more preferably 24% to 33% by mass, from the viewpoint of uniformly dispersing high molecular weight resins to uniformly apply tensile stress and improve the ion permeability and air permeability distribution of the resulting film. Here, PC refers to the "proportion of polymer components in the extruded components."

[0090] When the polyolefin resin porous membrane contains polyethylene, the Mv of at least one type of polyethylene is preferably 500,000 or more, more preferably 600,000 or more, even more preferably 700,000 or more, and most preferably 800,000 or more, from the viewpoint of membrane orientation and rigidity. Furthermore, from the viewpoint of keeping the average flow diameter and air permeability of the palm polo within the above range and reducing the shutdown temperature, the Mv of the polyethylene is preferably 1,500,000 or less, more preferably 1,200,000 or less, even more preferably 1,000,000 or less, and particularly preferably 900,000 or less. From the viewpoint of short-circuit resistance during impact testing, the proportion of polyethylene with an Mv of 500,000 or more in the polyolefin resin constituting the polyolefin resin porous membrane is preferably 10% by mass or more, more preferably 20% by mass or more, and most preferably 25% by mass or more, relative to the total polyolefin resin. On the other hand, from the standpoint of ensuring battery safety by promptly shutting down the battery, the proportion of polyethylene with an Mv of 500,000 or more in the polyolefin resin constituting the porous polyolefin resin membrane is preferably 45% by mass or less, more preferably 40% by mass or less, and most preferably 35% by mass or less, relative to the total polyolefin resin.

[0091] When the polyolefin resin porous membrane contains polyethylene, the Mv of at least one type of polyethylene is preferably less than 500,000, more preferably 450,000 or less, even more preferably 400,000 or less, even more preferably 350,000 or less, and most preferably 300,000 or less, from the viewpoint of good shutdown characteristics. The lower limit of the polyethylene's Mv may be, for example, 50,000 or more. From the viewpoint of ensuring battery safety by prompt shutdown, the proportion of polyethylene with an Mv of less than 500,000 in the polyolefin resin constituting the polyolefin resin porous membrane is preferably 55% by mass or more, more preferably 60% by mass or more, and most preferably 65% ​​by mass or more, relative to the total polyolefin resin. On the other hand, from the viewpoint of short-circuit resistance during impact tests, the proportion of polyethylene with an Mv of less than 500,000 in the polyolefin resin constituting the porous polyolefin resin film is preferably 90% by mass or less, more preferably 80% by mass or less, and most preferably 75% by mass or less, relative to the total polyolefin resin.

[0092] Furthermore, a mixture of polyethylene and polypropylene may be used as the polyolefin resin to improve the heat resistance of the microporous membrane and improve moldability. From the viewpoint of not worsening the shutdown temperature, the proportion of polypropylene to the total polyolefin resin in the polyolefin resin composition is preferably 0% by mass or more and 10% by mass or less, more preferably 0% by mass or more and 7% by mass or less, and even more preferably 0% by mass or more and 5% by mass or less.

[0093] Pore-forming materials include plasticizers, inorganic materials, or combinations thereof.

[0094] While the plasticizer is not particularly limited, it is preferable to use a non-volatile solvent that can form a homogeneous solution above the melting point of the polyolefin. Specific examples of non-volatile solvents include hydrocarbons such as liquid paraffin and paraffin wax; esters such as dioctyl phthalate and dibutyl phthalate; and higher alcohols such as oleyl alcohol and stearyl alcohol. These plasticizers may be recovered and reused after extraction by operations such as distillation.

[0095] Among plasticizers, liquid paraffin is preferred because, when the polyolefin resin is polyethylene or polypropylene, it has high compatibility with these materials, and when the molten mixture is stretched, interfacial delamination between the resin and the plasticizer is less likely to occur, making it easier to perform uniform stretching.

[0096] The inorganic material is not particularly limited and includes, for example, oxide-based ceramics such as alumina, silica (silicon oxide), titania, zirconia, magnesia, ceria, yttria, zinc oxide, and iron oxide; nitride-based ceramics such as silicon nitride, titanium nitride, and boron nitride; ceramics such as silicon carbide, calcium carbonate, aluminum sulfate, aluminum hydroxide, potassium titanate, talc, kaolin clay, kaolinite, halloysite, pyrophyllite, montmorillonite, sericite, mica, amethyst, bentonite, asbestos, zeolite, calcium silicate, magnesium silicate, diatomaceous earth, and silica sand; and glass fibers. These can be used individually or in combination of two or more. Among these, silica is particularly preferred due to its ease of extraction.

[0097] From the viewpoint of obtaining good segregation properties, the ratio of the polyolefin resin composition to the inorganic material is preferably such that the inorganic material is 3% by mass or more, more preferably 10% by mass or more, relative to the total mass of the two materials. From the viewpoint of ensuring high strength, it is preferably 60% by mass or less, and more preferably 50% by mass or less.

[0098] Next, the molten mixture is formed into a sheet to obtain a gel-like sheet. When molten mixing is performed using an extruder, the ratio of the extrusion speed of the polyolefin composition (i.e., the discharge rate Q of the extruder: kg / hour) to the screw rotation speed N (rpm) of the extruder (Q / N, unit: kg / (h·rpm)) is preferably 0.1 or more and 7.0 or less, more preferably 0.5 or more and 6.0 or less, and even more preferably 1.0 or more and 5.0 or less. When molten mixing is performed under Q / N conditions of 0.1 or more and less than 7.0, the liquid paraffin that has phase-separated from the resin disperses more easily, resulting in a denser pore structure and a tendency to increase strength.

[0099] One method for manufacturing a sheet-like molded product is to extrude a molten mixture into a sheet shape through a T-die or the like, and then cool and solidify it by contacting it with a heat conductor to a temperature sufficiently lower than the crystallization temperature of the resin component. Examples of heat conductors used for cooling and solidification include metals, water, air, and plasticizers. Among these, it is preferable to use metal rolls because they have high heat conduction efficiency. Furthermore, when contacting the extruded gel-like sheet with metal rolls, sandwiching it between the rolls is even more preferable because it further increases the efficiency of heat conduction, the sheet becomes oriented, increasing film strength and improving the surface smoothness of the sheet.

[0100] From the viewpoint of controlling the cast clearance when extruding the molten mixture from the T-die into a sheet, and adjusting the average film thickness (before compression) of the resulting microporous film to within the numerical range described above, the distance between rolls is preferably 200 μm to 3,000 μm, and more preferably 500 μm to 2,500 μm. Furthermore, from the viewpoint of obtaining a thin film and achieving the stretching ratio necessary to improve compressibility by enhancing surface orientation and crystallinity, the cast thickness is preferably 500 μm to 2,200 μm, and more preferably 700 μm to 2,000 μm.

[0101] Furthermore, the extruded sheet-like molded body or gel-like sheet may be rolled. Rolling can be carried out, for example, by a method using rolls. Rolling can increase the orientation, particularly of the surface layer. The rolling surface ratio is preferably greater than 1 and less than or equal to 3, and more preferably greater than 1 and less than or equal to 2. When the rolling surface ratio exceeds 1, the surface orientation increases, and the final film strength tends to increase. When the rolling surface ratio is 3 or less, the difference in orientation between the surface layer and the central interior is small, and a uniform porous structure can be formed in the thickness direction of the film.

[0102] [Biaxial stretching process (B)] In step (B), the gel-like sheet obtained in step (A) is stretched. Step (B) is performed before step (C), in which the pore-forming material is extracted from the sheet. In step (B), the stretching of the gel-like sheet is performed at least once each in the MD and TD directions (i.e., by biaxial stretching) from the viewpoint of controlling the strength of the polyolefin microporous membrane, and after stretching, heat setting is performed to relieve the stress caused during stretching.

[0103] Examples of stretching methods include simultaneous biaxial stretching, sequential biaxial stretching, multi-stage stretching, and multiple-pass stretching. Among these, simultaneous biaxial stretching is preferred from the viewpoint that the trunk structure tends to become isotropic in plane, and stress is isotropically distributed during collision tests, resulting in good collision safety. Simultaneous biaxial stretching refers to a stretching method in which MD stretching and TD stretching are performed simultaneously, and the stretching ratios in each direction may be different. Sequential biaxial stretching refers to a stretching method in which MD and TD stretching are performed independently, and when stretching is performed in MD or TD, the other direction is in an unconstrained state or fixed to a fixed length.

[0104] In the MD stretching of step (B), the MD stretching ratio is preferably 5 times or more, and more preferably 6 times or more, from the viewpoint of increasing the strength of the film by stretching orientation, improving compressibility by increasing crystallinity, and balancing pore size and air permeability of the microporous film. On the other hand, from the viewpoint of suppressing the increase in the shutdown temperature due to the rise in the crystal melting point caused by increasing the stretching ratio, the MD stretching ratio is preferably 9 times or less, more preferably 8 times or less, and most preferably 7 times or less. Furthermore, from the viewpoint of uniformly applying stress even to high molecular weight resins and improving the permeability and air permeability distribution of the resulting film, the MD stretching temperature is preferably 115°C to 140°C, and more preferably 120°C to 130°C.

[0105] In the TD stretching of step (B), the TD stretching ratio is preferably 5 times or more, and more preferably 6 times or more, from the viewpoint of increasing the strength of the film by stretching orientation, improving compressibility by increasing crystallinity, and lowering the pore size or palm polo average flow diameter of the film. On the other hand, from the viewpoint of suppressing the increase in the melting point of the crystal and the shutdown temperature caused by increasing the stretching ratio, the TD stretching ratio is preferably 9 times or less, more preferably 8 times or less, and most preferably 7 times or less. Furthermore, from the viewpoint of uniformly applying stress even to high molecular weight resins and improving the permeability and air permeability distribution of the resulting film, the TD stretching temperature is preferably 115°C to 140°C, and more preferably 120°C to 130°C.

[0106] In step (B), while it is desirable not to be bound by theory, as it is desirable to suppress the shrinkage of the resulting microporous membrane to enable a uniform pore size distribution and to adjust the pore size, palm polo average flow diameter, air permeability, etc., to within the range described above, it is preferable to make the ratio of the stretching coefficient to the heat fixing coefficient (stretching coefficient / heat fixing coefficient) 3.5 or less, and more preferably 3.0 or less. The stretching coefficient is a value obtained by multiplying the stretching temperature by the stretching wind speed and the residence time of the membrane in the stretching process, and is preferably in the range of 20000℃·m to 50000℃·m, and more preferably in the range of 30000℃·m to 50000℃·m. The heat fixing coefficient is a value obtained by multiplying the heat fixing temperature by the heat fixing wind speed and the residence time of the membrane in the heat fixing process, and is preferably in the range of 5000℃·m to 14000℃·m, and more preferably in the range of 6000℃·m to 14000℃·m.

[0107] [Extraction process (C)] In step (C), the pore-forming material is removed from the sheet-like molded body to obtain a porous membrane. Methods for removing the pore-forming material include, for example, immersing the sheet-like molded body in an extraction solvent to extract the pore-forming material and then thoroughly drying it. The extraction method for the pore-forming material may be either batch or continuous. To suppress shrinkage of the porous membrane, it is preferable to restrain the edges of the sheet-like molded body during the immersion and drying process. Furthermore, it is preferable that the amount of pore-forming material remaining in the porous membrane is less than 1% by mass of the total mass of the porous membrane.

[0108] When extracting the pore-forming material, it is preferable to use an extraction solvent that is a poor solvent for the polyolefin resin, a good solvent for the pore-forming material, and has a boiling point lower than the melting point of the polyolefin resin. Examples of such extraction solvents include hydrocarbons such as n-hexane and cyclohexane; halogenated hydrocarbons such as methylene chloride and 1,1,1-trichloroethane; non-chlorinated halogenated solvents such as hydrofluoroethers and hydrofluorocarbons; alcohols such as ethanol and isopropanol; ethers such as diethyl ether and tetrahydrofuran; and ketones such as acetone and methyl ethyl ketone. These extraction solvents may be recovered and reused by operations such as distillation. Furthermore, when using an inorganic material as the pore-forming material, aqueous solutions of sodium hydroxide, potassium hydroxide, etc., can be used as the extraction solvent.

[0109] [Heat setting process (D)] In the heat-setting process (D), the microporous film is heat-treated for heat-setting (HS) after the plasticizer extraction in process (C) in order to suppress shrinkage of the polyolefin microporous film. Examples of heat treatments for the porous film include stretching operations performed in a predetermined temperature atmosphere and at a predetermined stretching ratio for the purpose of adjusting physical properties, and / or relaxation operations performed in a predetermined temperature atmosphere and at a predetermined relaxation rate for the purpose of reducing stretching stress. These heat treatments can be carried out using a tenter or roll stretcher. It is preferable to perform the heat-setting, including stretching and relaxation operations after plasticizer extraction, in step TD.

[0110] The stretching ratio in step (D) is preferably 1.7 times or more, more preferably 1.8 times or more, and most preferably 2.0 times or more, from the viewpoint of obtaining a microporous film with even higher strength and high porosity or low air permeability. In this specification, the stretching ratio refers to the value obtained by dividing the maximum dimension (mm) during stretching in the heat setting step by the width dimension (mm) of the film at the inlet of the stretcher.

[0111] The heat-fixing temperature in step (D) is preferably 115°C to 130°C, more preferably 118°C to 127°C, and even more preferably 120°C to 125°C, from the viewpoint of adjusting the porosity, average palm polo flow diameter, and air permeability of the resulting microporous membrane to within the numerical range described above, as well as from the viewpoint of achieving both pore size and air permeability of the microporous membrane.

[0112] In step (D), the TD stretching operation is carried out such that the TD stretching temperature is preferably 110°C to 150°C, more preferably 115°C to 140°C, and even more preferably 120°C to 135°C, from the viewpoint of obtaining a microporous film with even higher strength and higher porosity or lower air permeability.

[0113] The relaxation operation in step (D) is the reduction operation of the film to the MD and / or TD after the stretching operation. The relaxation operation may be performed in both the longitudinal and width directions, or in only one of the longitudinal and width directions.

[0114] From the viewpoint of improving the balance between the average flow diameter of the palm polo and the air permeability, and from the viewpoint of adjusting the curvature ratio and air permeability to within the numerical range described above, the relaxation ratio is preferably 0.90 or higher, more preferably 0.92 or higher, and even more preferably 0.95 or higher. The upper limit of the relaxation ratio may be 1.00. In this specification, the relaxation ratio refers to the value obtained by dividing the width dimension (mm) at the stretching outlet of the heat-setting process by the maximum dimension (mm) during stretching in the heat-setting process.

[0115] The relaxation temperature in step (D) is preferably 115°C to 130°C, more preferably 118°C to 127°C, and even more preferably 120°C to 125°C, from the viewpoint of improving the balance between the average palm polo flow diameter and air permeability of the resulting microporous membrane, and from the viewpoint of reducing TD thermal shrinkage.

[0116] If the ratio of the strain rate during stretching to the strain rate during relaxation in process (D) (stretching strain rate % / sec / relaxation strain rate % / sec) is 10 or greater, it is possible to achieve both a relatively small pore size and low air permeability, although we do not wish to be constrained by theory. Therefore, from the viewpoint of improving output characteristics, it is preferable that the ratio of the strain rate during stretching to the strain rate during relaxation (stretching strain rate % / sec / relaxation strain rate % / sec) is 10 or greater, more preferably 12 or greater, and most preferably 15 or greater. Note that strain rate refers to the rate of change per unit time of an object before and after being subjected to a specific process.

[0117] <Method for manufacturing multilayer porous membranes> The method for manufacturing the multilayer porous film according to this embodiment is not particularly limited, but as an example, it may include the step of placing at least one layer selected from the group consisting of a multifunctional layer, an inorganic porous layer, and a thermoplastic resin layer on at least one side of the polyolefin microporous film manufactured above. The method of placing the multifunctional layer, inorganic porous layer, or thermoplastic resin layer is not particularly limited, and for example, a coating liquid containing the components of any of these layers can be applied to one or both sides of the polyolefin microporous film, or onto a layer formed on the polyolefin microporous film. The thickness of the coating layer is preferably 0.1 to 10 μm, more preferably 0.2 to 7 μm, and even more preferably 0.3 to 4 μm. The number of coating layers is preferably 0 to 5, and more preferably 0 to 3. By appropriately controlling the thickness of the coating layer, the battery capacity can be increased. Inorganic coating has the effect of suppressing shrinkage of the substrate and improving the safety of the battery, while organic coating has the effect of improving adhesion to electrodes and improving processability. By mixing inorganic and organic polymer components, it is possible to achieve a good balance of the characteristics of both.

[0118] Regarding the coating method, there are no particular limitations as long as it can achieve the desired coating pattern, coating film thickness, and coating area. Examples include gravure coater method, small-diameter gravure coater method, reverse roll coater method, transfer roll coater method, kiss coater method, dip coater method, knife coater method, air doctor coater method, blade coater method, rod coater method, squeeze coater method, cast coater method, die coater method, screen printing method, spray coating method, inkjet coating method, etc.

[0119] Water or a mixed solvent of water and a water-soluble organic medium is preferred as the medium for the coating solution. The water-soluble organic medium is not particularly limited, but examples include ethanol and methanol.

[0120] It is preferable to surface-treat the polyolefin microporous film prior to coating, as this facilitates the application of the coating solution and improves the adhesion between the polyolefin microporous film and the coating layer. Examples of surface treatment methods include corona discharge treatment, plasma treatment, mechanical roughening, solvent treatment, acid treatment, and ultraviolet oxidation.

[0121] After coating, the solvent may be removed from the coated film by drying at a temperature below the melting point of the polyolefin microporous film, vacuum drying, or solvent extraction.

[0122] Alternatively, a polyolefin microporous membrane and at least one layer selected from the group consisting of a multifunctional layer, an inorganic porous layer, and a thermoplastic resin layer may be manufactured separately and then integrated by bonding, lamination, adhesion, fusion, or the like.

[0123] <Separators for non-aqueous secondary batteries, and non-aqueous secondary batteries> The polyolefin microporous membrane and multilayer porous membrane according to this embodiment can be used, for example, in non-aqueous secondary batteries, specifically as separators for non-aqueous secondary batteries. Examples of non-aqueous secondary batteries include lithium-ion secondary batteries. By incorporating the polyolefin microporous membrane according to this embodiment into a lithium-ion secondary battery, it not only suppresses thermal runaway of the lithium-ion secondary battery, but also enables both high power output characteristics and high cycle characteristics, as well as safety, even when equipped with easily shrinkable electrodes, high-capacity electrodes, or Si-containing negative electrodes. [Examples]

[0124] Next, this embodiment will be described in more detail with reference to examples and comparative examples, but this embodiment is not limited to the following examples unless it exceeds the gist of the embodiment. The physical properties in the examples were measured by the following methods. Unless otherwise specified, each measurement was performed in an environment of room temperature 23℃±2℃ and humidity 40%±5%.

[0125] [Viscosity average molecular weight] Based on ASTM-D4020, the intrinsic viscosity [η] (dl / g) at 135°C in decalin solvent was determined. For polyolefin microporous membranes and polyethylene, the calculations were performed using the following formula. [η] = 6.77 × 10 -4 Mv 0.67 For polypropylene, Mv was calculated using the following formula. [η] = 1.10 × 10 -4 Mv 0.80

[0126] [Weight-average molecular weight and number-average molecular weight] Using a Waters ALC / GPC 150C™ (trademark), standard polystyrene was measured under the following conditions to create a calibration curve. Similarly, chromatograms were measured for each of the polymers listed below under the same conditions, and the weight-average molecular weight of each polymer was calculated based on the calibration curve using the method described below. Columns: Tosoh GMH6-HT (trademark) x 2 + GMH6-HTL (trademark) x 2 Mobile phase: o-dichlorobenzene Detector: Differential refractometer Flow rate: 1.0ml / min Column temperature: 140℃ Sample concentration: 0.1 wt% (Weight-average molecular weight and number-average molecular weight of polyethylene and polypropylene) By multiplying each molecular weight component in the obtained calibration curve by 0.43 (Q factor of polyethylene / Q factor of polystyrene = 17.7 / 41.3) or 0.64 (Q factor of polypropylene / Q factor of polystyrene = 26.4 / 41.3), molecular weight distribution curves in polyethylene equivalent or polypropylene equivalent were obtained, and the weight-average molecular weight and number-average molecular weight were calculated. (Weight-average molecular weight and number-average molecular weight of resin composition or resin microporous membrane) The Q-factor value of the polyolefin with the largest mass fraction was used, and the weight-average molecular weight and number-average molecular weight were calculated in the same manner as for polyethylene.

[0127] [Density (g / cm 3 )] The density of the sample was measured using the density gradient tube method (23°C) in accordance with JIS K7112:1999.

[0128] [Basis weight (g / m²) 2 )] Basis is measured per unit area (1 m²). 2 This is the weight (g) of the polyolefin microporous membrane per unit area (1m x 1m). After sampling a 1m x 1m area, the weight was measured using a Shimadzu Corporation electronic balance (AUW120D). If sampling a 1m x 1m area is not possible, the weight was measured by cutting out an appropriate area and then measured per unit area (1m). 2 The weight was converted to grams per unit.

[0129] [Average film thickness of microporous membrane (μm)] The thickness was measured using a micro-thickness gauge (Type KBN, terminal diameter Φ5mm) manufactured by Toyo Seiki at an ambient temperature of 23±2℃. To measure the thickness, a 10cm × 10cm sample of the microporous film was taken, and multiple films were stacked to a thickness of 15μm or more. Measurements were taken at nine points, the average was calculated, and the average value was divided by the number of stacked films to determine the thickness of a single film.

[0130] [Porosity (%)] A 5cm x 5cm or 10cm x 10cm sample is cut from the polyolefin microporous film, and its volume (cm³) is determined from the measurement results of the film thickness. 3 Calculate the mass (g) and density (g / cm³), and then compare them with the density (g / cm³). 3 The calculation was performed using the following formula. Porosity (%) = (Volume - Mass / Density of mixed composition) / Volume × 100 The density of the mixed composition was calculated using the density of the polyolefin resin and other components used, along with their respective mixing ratios.

[0131] [Air permeability (sec / 100cm) 3 )] The air permeability was measured using the "EGO2" air permeability measuring instrument of Asahi Seiko Co., Ltd. The air permeability measurement is obtained by taking measurements at three points along the width direction (TD) of the membrane: two points 10% inward from both ends towards the center, and one point in the center. The average of these measurements is then calculated.

[0132] [Compression press test] A 0.8 mm thick rubber cushioning material, a 0.1 mm thick PET film, two microporous membranes, the aforementioned PET film, and the aforementioned cushioning material were laminated in that order. The resulting laminate was left to stand, and a compression test was performed by applying pressure to one side of the cushioning material surface of the laminate. The compression test was performed using a press machine at a temperature of 30°C and a compression time of 10 minutes at a pressure of 2.5 MPa. After unloading, the microporous membranes were removed from the laminate one hour later, and the air permeability after compression was measured.

[0133] [Average flow diameter of palm polo (μm)] Following the half-dry method, the average flow diameter (μm) of the palm porometer (CFP-1500AE, Porous Materials, Inc.) was measured. Perfluoropolyester (product name "Galwick", surface tension 15.6 dyn / cm) manufactured by the same company was used as the immersion solution. The applied pressure and air permeability were measured for both the dry and wet curves. From the pressure PHD (Pa) at which the half-moon curve of the obtained dry curve intersected the wet curve, the average pore diameter dHD (μm) was calculated using the following formula and defined as the average flow diameter (μm) of the palm porometer. dHD = 2860 × γ / PHD

[0134] <Maximum pore size of bubble point (μm)> The maximum pore size (μm) was measured using a palm porometer (Porous Materials, Inc.: CFP-1500AE) in accordance with the bubble point method. Perfluoropolyester (product name "Galwick", surface tension 15.6 dyn / cm) manufactured by the same company was used as the immersion solution. For the wet curve, the applied pressure and air permeability were measured in pressurized mode, and the maximum pore size dBP (μm) was calculated from the pressure PBP (Pa) at which the first bubble appeared in the obtained wet curve using the following formula, and this was defined as the maximum pore size at the bubble point. dBP = 2860 × γ / PBP

[0135] [Curvature of porous membranes determined by the gas-liquid method] It is known that the fluid inside a capillary follows a Knudsen flow when its mean free path is greater than the capillary's pore diameter, and a Poiseuille flow when it is smaller. Therefore, we assume that the airflow in the permeability measurement of a porous membrane follows a Knudsen flow, and the water flow in the permeability measurement of a porous membrane follows a Poiseuille flow. In this case, the mean pore diameter d (μm) and curvature τ of the porous membrane are... a (Dimensionless) is the air permeation rate constant R gas (m 3 / (m 2 ·sec·Pa)) Water permeability constant R liq (m 3 / (m 2·s·Pa), molecular velocity of air ν (m / s), viscosity of water η (Pa·s), standard pressure P s (=101325 Pa), porosity ε (%), and film thickness L (μm) using the following formula. d=2ν×(R liq / R gas )×(16η / 3P s )×10 6 τ a =(d×(ε / 100)×ν / (3L×P s ×R gas )) 1 / 2 Here, R gas is obtained from air permeability (sec) using the following formula. R gas =0.0001 / (air permeability×(6.424×10 -4 )×(0.01276×101325)) Also, R liq is obtained from water permeability (cm 3 / (cm 2 ·s·Pa) using the following formula. R liq = water permeability / 100 The water permeability is obtained as follows. A porous membrane, which has been immersed in ethanol in advance, is set in a liquid permeation cell made of stainless steel with a diameter of 41 mm. After washing ethanol off the membrane with water, water is allowed to permeate under a differential pressure of about 50000 Pa, and the water permeation amount per unit time, unit pressure and unit area is calculated from the water permeation amount (cm 3 ) after 120 seconds have elapsed, and this value is taken as the water permeability. Also, ν is obtained from the gas constant R (=8.314), absolute temperature T (K), pi π, and average molecular weight of air M (=2.896×10 -2 kg / mol) using the following formula. ν=((8R×T) / (π×M)) 1 / 2

[0136] [Shutdown Temperature] Two 10 μm thick nickel foils (A and B) were prepared. One nickel foil A was masked with Teflon® tape, leaving a rectangular section of 15 mm vertically and 10 mm horizontally. The separator of the sample to be measured was placed on the other nickel foil B, and both ends of the separator were secured with Teflon® tape. Nickel foil B was immersed in a 1 mol / L lithium borofluoride solution (solvent: propylene carbonate / ethylene carbonate / γ-butyllactone = mixed solvent in a volume ratio of 1 / 1 / 2) to impregnate the separator with the electrolyte. After that, nickel foils (A and B) were bonded together and both sides were held in place with clips using two glass plates. The nickel foil electrode thus prepared was placed in a 25°C oven and heated to 200°C at a rate of 2°C / min. The impedance change during this process was measured using an electrical resistance measuring device "AG-4311" (manufactured by Ando Electric Co., Ltd.) under conditions of 1V and 1kHz. In this measurement, the temperature at which the impedance value reached 1000Ω was defined as the shutdown temperature (°C).

[0137] [Withstand Voltage Measurement] A section of the polyolefin microporous membrane was cut to a size of MD10cm × TD10cm at the center of its width, sandwiched between 5mm diameter aluminum plates, and measured using a Kikusui Electronics TOS9201 voltage withstand tester. The measurement conditions were as follows: a DC voltage was applied starting from 0V, increasing at a boost rate of 100V / sec, and the voltage value when a current of 0.2mA flowed was taken as the voltage withstand measurement of the microporous membrane. A total of 25 points were measured at 15mm intervals (5 MD points × 5 TD points), and the average value was taken as the voltage withstand measurement. Furthermore, the dielectric strength per unit basis was calculated as follows: Dielectric strength per unit film thickness [V / (g / m 2 ) = Withstand voltage [V] / Weight [g / m 2 ]

[0138] [Puncture strength and puncture strength equivalent to basis weight] A microporous membrane was fixed using a Kato Tech KES-G5 (trademark) handy compression tester with a sample holder having an opening diameter of 11.3 mm. Next, a puncture test was performed on the central part of the fixed microporous membrane with a needle tip radius of curvature of 0.5 mm and a puncture speed of 2 mm / sec under room temperature of 23°C and 40% humidity. The puncture strength (gf) was measured as the maximum puncture load. The measured values ​​for the puncture test were taken at three points along the TD of the membrane: two points 10% inward from both ends toward the center, and one point in the center. The average value of these measurements was then calculated. The puncture strength converted to base weight is calculated using the following formula. Penetration strength converted to basis weight [gf / (g / m 2 )]=Piercing strength [gf] / Weight [g / m 2 ] Here, regarding the puncture strength and basis weight equivalent puncture strength of a multilayer porous film having at least one layer on a polyolefin microporous film substrate, the properties were evaluated using the puncture strength and basis weight equivalent puncture strength of the polyolefin microporous film substrate, from the viewpoint of evaluating the strength of the resin and the strength per basis weight.

[0139] [Tensile breaking strength (MPa) and MD / TD tensile breaking strength ratio] In accordance with JIS K7127, measurements were taken on MD and TD samples (shape: 10 mm wide x 100 mm long) using a Shimadzu Corporation Autograph AG-A tensile testing machine. The chucks of the tensile testing machine were set to 50 mm apart, and cellophane tape (registered trademark) (manufactured by Nitto Denko Packaging Systems Co., Ltd., product name: N.29) was attached to one side of both ends (25 mm each) of the sample. Furthermore, to prevent the sample from slipping during testing, a 1 mm thick fluororubber was attached to the inside of the chuck of the tensile testing machine. The measurements were performed under the following conditions: temperature 23±2℃, chuck pressure 0.40MPa, and tensile speed 100mm / min. The tensile breaking strength (MPa) was determined by dividing the strength of the polyolefin microporous membrane at the time of rupture by the cross-sectional area of ​​the sample before testing. The tensile breaking strength was determined for both MD and TD, and the ratio of the MD tensile breaking strength to the TD tensile breaking strength (MD / TD tensile breaking strength ratio) was also calculated.

[0140] [Battery Testing] a. Fabrication of the positive electrode A slurry was prepared by dispersing lithium nickel cobalt manganese oxide (NCM111) as the positive electrode active material and carbon black as the conductive material in polyvinylidene fluoride (PVDF) and N-methylpyrrolidone (NMP) as binders. This slurry was applied to a 15 μm thick aluminum foil, which would serve as the positive electrode current collector, using a die coater. After drying at 130°C for 3 minutes, it was compression molded using a roll press. The resulting molded body was slit to a width of 57.0 mm to obtain the positive electrode.

[0141] b. Fabrication of the negative electrode A slurry was prepared by dispersing artificial graphite as the negative electrode active material and a binder consisting of ammonium salt of carboxymethylcellulose and styrene-butadiene copolymer latex in purified water. This slurry was applied to copper foil, which would serve as the negative electrode current collector, using a die coater, dried at 120°C for 3 minutes, and then compressed and molded using a roll press. The resulting molded body was slit to a width of 58.5 mm to obtain the negative electrode.

[0142] c. Preparation of non-aqueous electrolyte A non-aqueous electrolyte was prepared by dissolving LiPF6 as a solute to a concentration of 1 mol / L in a mixed solvent of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate in a volume ratio of 1:1:2.

[0143] d. Battery assembly After winding the positive electrode, the porous film obtained in the example or comparative example, and the negative electrode, a wound electrode body was fabricated by a conventional method and pressed with a press machine so that it would fit into the outer casing. The number of windings was adjusted according to the thickness of the polyolefin microporous film and the degree of springback. The outermost end of the obtained wound electrode body was fixed by applying insulating tape. The negative electrode lead was welded to the battery casing and the positive electrode lead was welded to the safety valve, and the wound electrode body was inserted into the battery casing. Then, 5 g of non-aqueous electrolyte was injected into the battery casing, and the lid was crimped to the battery casing via a gasket to obtain a rectangular secondary battery with a width of 42.0 mm, a height of 63.0 mm, and a thickness of 10.5 mm. This rectangular secondary battery was charged in a 25°C atmosphere with a current value of 0.2C (current of 0.2 times the hourly rate (1C) of the rated electrical capacity) until the battery voltage reached 4.0V, and after reaching 4.0V, the current value was reduced to maintain the voltage, and the charging was carried out for a total of 3 hours. Next, the battery was discharged to a voltage of 3.0V at a current of 0.2C.

[0144] [Rate characteristic test at room temperature (25℃)] The prismatic secondary batteries assembled in the same manner as described in d. above and selected for evaluation were charged with a constant current of 1C under a 30°C, 3MPa pressurized environment until they reached 4.0V, and then charged at a constant voltage of 4.0V for a total of 3 hours. After charging, the 1C discharge capacity up to the discharge termination voltage of 3V and the 20C discharge capacity of the batteries were measured under a constant temperature environment of 25°C, and the ratio of 20C capacity to 1C capacity was defined as the output characteristic value. The rate characteristics were evaluated according to the following criteria. A: Output characteristic value is 50% or higher. B: Output characteristic value is 45% or more but less than 50%. C: Output characteristic value is between 40% and 45%. D: Output characteristic value is 35% or more but less than 40%. E: Output characteristic value is less than 35%.

[0145] [Rate characteristics at low temperatures (-20℃)] The prismatic secondary batteries assembled in the same manner as described in d. above and selected for evaluation were charged with a constant current of 1C under a 30°C, 3MPa pressurized environment until they reached 4.0V, and then charged at a constant voltage of 4.0V for a total of 3 hours. After charging, the 1C discharge capacity and 20C discharge capacity were measured up to the discharge termination voltage of 3V in a constant temperature environment of -20°C, and the ratio of 20C capacity to 1C capacity was defined as the output characteristic value. The rate characteristics were evaluated according to the following criteria. A: Output characteristic value is 45% or higher. B: Output characteristic value is 40% or more but less than 45%. C: Output characteristic value is between 35% and 40%. D: Output characteristic value is 30% or more but less than 35%. E: Output characteristic value is less than 30%.

[0146] [Heating test] Using batteries assembled and selected for evaluation in the same manner as described later in the crash test, prismatic rechargeable batteries were charged at a constant current of 1C in a 25°C environment until they reached 4.0V, and then charged at a constant voltage of 4.0V for a total of 3 hours. After charging, the batteries were heated from room temperature to a predetermined temperature at a rate of 5°C / min, left at the predetermined temperature for 60 minutes, and the ignition status was checked. Three batteries were prepared, and the results were evaluated according to the following criteria. A: None of the batteries ignited at 136℃. B: None of the batteries caught fire at 134℃. None of the batteries caught fire at C:132℃. D: None of the batteries caught fire at 130℃. E: At 130°C, at least one battery ignited.

[0147] [Crash test] In the impact test, a round rod (φ=15.8mm) is placed on top of a rectangular battery sample positioned on a test stand, with the sample and the round rod roughly perpendicular to each other. An 18.2kg weight is then dropped onto the top surface of the round rod from a height of 61cm to observe the effect of the impact on the sample. Under a 25°C environment, a prismatic secondary battery obtained using the same method as in item d. above was charged with a constant current of 1C until it reached 4.0V, and then charged at a constant voltage of 4.0V for a total of 3 hours. Next, under a 25°C environment, the secondary battery was placed horizontally on a flat surface, and a stainless steel rod with a diameter of 15.8 mm was placed across the center of the secondary battery. The rod was positioned so that its long axis was parallel to the longitudinal direction (MD) of the separator. An 18.2 kg weight was dropped from a height of 61 cm so that the impact was applied perpendicular to the vertical axis of the secondary battery from the rod placed in the center of the secondary battery. The surface temperature of the secondary battery was measured 3 seconds and 3 minutes after the impact. The test was performed on 5 cells at a time and evaluated according to the following criteria. For this evaluation item, A, B, and C were used as the passing criteria. Note that the surface temperature of a secondary battery is the temperature measured using a thermocouple (K-type seal type) at a position 1 cm from the bottom of the battery's casing. A: In all cells, the surface temperature is 30°C or lower. B: In all cells, the surface temperature is 50°C or lower. C: Surface temperature of all cells is 70°C or lower. D: Surface temperature is 100°C or lower in all cells. E: One or more cells exceed a surface temperature of 100°C or ignite.

[0148] [Slight Short Circuit Test] Using batteries assembled and selected for evaluation in the same manner as in the crash test, the battery voltage was adjusted to 4.0V by a method of constant current constant voltage (CCCV) charging for 3 hours under conditions of a terminal battery voltage of 4.0V in a 25°C environment, followed by continuing 4.0V constant voltage charging for 2 hours. Subsequently, the batteries were left to stand for 1 hour in a constant temperature bath set at 25°C under a pressure of 10kPa, and those whose voltage dropped to 3.7V or below were considered to have a minor short circuit. Ten batteries were prepared, and the results of the 4.0V minor short circuit test were evaluated according to the following criteria based on the number of batteries that exhibited a minor short circuit. A: The number of batteries with a slight short circuit is 0. B: Number of batteries with a slight short circuit: 1 C: The number of batteries that are slightly short-circuited is 2 to 4. D: The number of batteries that experienced a slight short circuit was 5 to 7. E: The number of batteries that are slightly short-circuited is 8 or more.

[0149] [Example 1] <Manufacturing of polyolefin microporous membranes> A polyolefin microporous membrane was fabricated using the following procedure. (A) As shown in Table 1, 25 parts by mass of polyethylene with an Mv of 900,000 and 75 parts by mass of polyethylene with an Mv of 300,000 were dry-blended, and 0.3 parts by mass of tetrakis-(methylene-3-(3',5'-di-t-butyl-4'-hydroxyphenyl)propionate)methane was added as an antioxidant to obtain a raw material composition. Next, the obtained composition was fed into a twin-screw extruder via a feeder. Next, liquid paraffin (kinematic viscosity of 75.90 cSt at 37.78°C) was injected into the extruder via a side feed, with a total of 100 parts by mass of resin raw material + liquid paraffin, so that the "percentage of polymer components in the extruded components (PC)" shown in Table 1 was 31%. After kneading at a kneading temperature of 200°C, the material was extruded from a T-die installed at the tip of the extruder. (B) After extrusion, the sheet was formed by cooling and solidifying it on a cast roll. This sheet was stretched to MD7 times × TD6 times using a simultaneous biaxial stretcher, and the ratio of stretch coefficient / thermal fixation coefficient was set to 3.0. (C) After stretching, the liquid paraffin was extracted and removed by immersion in methylene chloride. (D) The sheet was then dried and stretched 1.8 times in the width direction (TD) using a tenter stretcher. Subsequently, the stretched sheet was subjected to a heat treatment at 124°C to relax in the width direction (TD) to a relaxation rate of 0.96, thereby obtaining a polyolefin microporous film. The ratio of the strain rate during stretching to the strain rate during relaxation (stretching strain rate % / sec / relaxation strain rate % / sec) was set to 15. (E) The obtained polyolefin microporous membrane was evaluated according to the above method, and a battery equipped with the polyolefin microporous membrane was also evaluated.

[0150] [Examples 2-22 and Comparative Examples 1-12] Polyolefin microporous films and their coated films were obtained and evaluated using the same method as in Example 1, except that the resin raw materials, manufacturing conditions, and coating conditions shown in Tables 1 and 2 were used. The evaluation results are shown in Tables 3 and 4. In Examples 20-22, the obtained polyolefin microporous film was further coated with a first layer and / or a second layer, respectively, to achieve the coating thickness shown in Table 1.

[0151] In Examples 20-22, boehmite was used as the inorganic filler and acrylic latex and sodium carboxymethylcellulose as the binder for the inorganic coating; acrylic latex was used for the organic coating; and alumina was used as the inorganic filler and PVdF as the binder for the organic-inorganic mixed coating. For coating the substrate, the coating solution was applied to the substrate surface after corona discharge treatment using a gravure coater, and then the coating solution was dried to obtain the separator of Example 20-22 having a coating layer. The evaluation results of Example 20-22 are shown in Table 3.

[0152] [Table 1-1]

[0153] [Table 1-2]

[0154] [Table 2-1]

[0155] [Table 2-2]

[0156] [Table 3-1]

[0157] Table 3-2

[0158] Table 4-1

[0159] Table 4-2

Claims

1. A biaxially oriented polyolefin microporous membrane, wherein, with respect to the total polyolefin resin, the proportion of polyethylene having a viscosity-average molecular weight Mv of 500,000 or more and 1,500,000 or less is 10% by mass or more and 45% by mass or less, the proportion of polyethylene having a viscosity-average molecular weight Mv of 500,000 or less is 55% by mass or more and 90% by mass or less, the proportion of polypropylene is 0% by mass or more and 10% by mass or less, the average flow diameter of palm polo is 0.020 μm or more and 0.055 μm or less, and the air permeability is 10 sec / 100 cm ３ Over 60sec / 100cm ３ The following conditions must be met, and the shutdown temperature must be between 125°C and 143°C, and the air permeability converted to film thickness must be 5.0 (sec / 100cm²). ３ A biaxially oriented polyolefin microporous membrane having a diameter of 0.007 μm or less, and a difference of 0.007 μm or less between the maximum pore diameter at the bubble point and the average flow diameter at the palm poro.

2. The polyolefin microporous membrane according to claim 1, wherein the viscosity-average molecular weight (Mv) of the polyolefin microporous membrane is 200,000 or more and 500,000 or less.

3. Weight: 1 g / m ２ The converted withstand voltage is 0.22 kV / (g / m ２ The polyolefin microporous membrane according to claim 1 or 2, wherein the above is true.

4. A polyolefin microporous membrane according to claim 1 or 2, wherein the porosity is 40% or more and 75% or less.

5. The air permeability after pressing at 30°C and 2.5 MPa for 10 minutes is 20 sec / 100 cm. ３ Above 100 sec / 100 cm ３ The polyolefin microporous membrane according to claim 1 or 2, which is as follows:

6. The product of the mean pore flow diameter (μm) and the air permeability (sec / 100 cm ３ ) is 3.0 (sec / 100 cm ３ ) μm or less, the microporous polyolefin membrane according to claim 1 or 2.

7. A polyolefin microporous membrane according to claim 1 or 2, wherein the curvature ratio is 1.1 to 1.

8.

8. The puncture strength, converted to basis weight, is 60.0 gf / (g / m). ２ ) Above 110.0 gf / (g / m ２ The polyolefin microporous membrane according to claim 1 or 2, wherein the following conditions apply.

9. Both MD and TD have a tensile strength of 1000 kgf / cm². ２ The polyolefin microporous membrane according to claim 1 or 2.

10. A polyolefin microporous membrane according to claim 1 or 2, wherein the maximum pore diameter of the bubble point is 0.060 μm or less.

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