Separator for power storage device and power storage device

Abstract

To provide a separator for a power storage device, the separator being formed as a thin film and being able to have both excellent air permeability and excellent MD tensile strength.SOLUTION: A separator for a power storage device is provided, having separator base material having multilayer structure of: a microporous layer (A) containing polypropylene as a main component; and a microporous layer (B) including polyethylene as a main component and including polypropylene. At least one layer of the microporous layer (A) constitutes an outermost layer on at least one surface of the separator base material. The microporous layer (B) has in the same layer, the polyethylene and the polypropylene included in the microporous layer (B). A melt flow rate (MFR) of the microporous layer (B) measured at a load of 2.16 kg and temperature of 190°C is not less than 0.10 g / 10 min and not more than 1.2 g / 10 min. The polypropylene included in the microporous layer (B) is 4.0% by mass or more and 12% by mass or less based on the total mass of the microporous layer (B).SELECTED DRAWING: None

Description

Technical Field

[0001] The present disclosure relates to a separator for a power storage device, a power storage device, and the like.

Background Art

[0002] Microporous membranes, particularly polyolefin-based microporous membranes, are used in many technical fields such as precision filtration membranes, separators for batteries, separators for capacitors, materials for fuel cells, etc., and are particularly used as separators for power storage devices typified by lithium secondary batteries and lithium-ion secondary batteries. Lithium-ion batteries are applied to various uses, including small electronic device applications such as mobile phones and notebook personal computers, as well as electric vehicles including hybrid vehicles and plug-in hybrid vehicles.

[0003] In recent years, lithium-ion batteries having high energy capacity, high energy density, and high output characteristics have been demanded. Along with this, there is an increasing demand for separators that are thin films and excellent in battery performance, battery reliability, and safety.

[0004] For example, Patent Document 1 describes a separator for a power storage device having a microporous membrane, wherein the microporous membrane has (i) a polymer matrix containing (A) a polypropylene resin and (B) a thermoplastic elastomer, (ii) fibrils extending in the machine direction (MD) of the microporous membrane from the polymer matrix and containing the (A) polypropylene resin, (iii) pores existing between a plurality of the fibrils, and the melt flow rate (MFR) of the microporous membrane is 1.5 g / 10 min or less, a separator for a power storage device, is described.

[0005] Further, Patent Document 2 describes a polyolefin-based microporous membrane having polyethylene and polypropylene, Part or all of the polyethylene is polyethylene having a viscosity average molecular weight (Mv) of 50,000 or more and 300,000 or less, the content of polypropylene is 7 wt% or more and less than 50 wt%, the least-squares approximation linear relationship between the common logarithm value of the molecular weight M(i) determined by GPC / FTIR and the terminal methyl group concentration C(M(i)) is in the molecular weight range of M(i) from 100,000 to 1,000,000, C(M(i)) = A × log(M(i)) + B (A and B are constants) -0.015 ≤ A ≤ 2.000 a microporous polyolefin membrane, is described.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0007] By the way, when producing a separator for a power storage device by laminating a plurality of microporous layers, it has been difficult to thin such a separator. On the other hand, as the separator is thinned, the physical strength of the separator tends to decrease. Therefore, it has also been difficult to ensure excellent MD tensile strength while maintaining excellent air permeability.

[0008] Therefore, an object of the present disclosure is to provide a separator for a power storage device that is a thin film and can achieve both excellent air permeability and excellent MD tensile strength.

Means for Solving the Problems

[0009] Examples of embodiments of the present disclosure are listed in the following items. [1] A separator for a power storage device, comprising a separator substrate having a multilayer structure of a microporous layer (A) mainly composed of polypropylene and a microporous layer (B) mainly composed of polyethylene and containing polypropylene, at least one layer of the microporous layer (A) constitutes the outermost layer on at least one side of the separator substrate, the microporous layer (B) has the polyethylene and the polypropylene contained therein in the same layer, the melt flow rate (MFR) of the microporous layer (B) measured at a load of 2.16 kg and a temperature of 190 °C is 0.10 g / 10 min or more and 1.2 g / 10 min or less, and based on the total mass of the microporous layer (B), the polypropylene contained in the microporous layer (B) is 4.0% by mass or more and 12% by mass or less, A separator for a power storage device. [2] The separator for a power storage device according to item 1, wherein the microporous layer (B) contains 1.0% by mass or more and 6.0% by mass or less of a thermoplastic elastomer. [3] The separator for a power storage device according to item 1 or 2, wherein the ratio (PP / PE) of the melt flow rate (MFR) of polypropylene to polyethylene in the microporous layer (B) is 1.50 or more and 40.0 or less. [4] The separator for a power storage device according to any one of items 1 to 3, wherein the melt flow rate (MFR) of polypropylene in the microporous layer (B) measured at a load of 2.16 kg and a temperature of 230 °C is 0.20 g / 10 min or more and 11.0 g / 10 min or less. [5] The separator for a power storage device according to any one of items 1 to 4, wherein the melt flow rate (MFR) of polyethylene in the microporous layer (B) measured at a load of 2.16 kg and a temperature of 190 °C is 0.15 g / 10 min or more and 0.90 g / 10 min or less. [6] The MFR of the microporous layer (A) when measured at a load of 2.16 kg and a temperature of 230°C is 0.30 g / 10 min or more and 0.90 g / 10 min or more, and the separator for a power storage device according to any one of items 1 to 5. [7] The thickness of the microporous layer (B) is 5.0 μm or less, the thickness of the separator substrate is 15.0 μm or less, and the porosity of the separator substrate is 40% or more and 60% or less, and the separator for a power storage device according to any one of items 1 to 6. [8] A power storage device including a positive electrode, a negative electrode, and the separator for a power storage device according to any one of items 1 to 7 disposed between the positive electrode and the negative electrode. [9] A method for manufacturing the separator for a power storage device according to any one of items 1 to 7, including the following steps (1) to (3): Step (1); the draw ratio (cold draw ratio in a predetermined temperature range) of the resin film in the first drawing step is 10% or more, Step (2); the draw ratio (thermal draw ratio in a predetermined temperature range) of the resin film in the second drawing step is 160% or more, and Step (3); the annealing temperature in the annealing step is 125°C or more, A method for manufacturing a separator for a power storage device.

[10] The method for manufacturing a separator for a power storage device according to item 9, wherein the microporous layer (A) and the microporous layer (B) are coextruded. [Effect of the Invention]

[0010] According to the present disclosure, it is possible to provide a separator for a power storage device that is a thin film and can achieve both excellent air permeability and excellent MD tensile strength. [Embodiments for Carrying Out the Invention]

[0011] In this specification, unless otherwise specified, various measurements are performed based on the methods described in the examples. In this specification, the upper limit or lower limit in the numerical range described step by step may be replaced by the upper limit or lower limit in the corresponding other numerically described step range, and further, it may be replaced by the corresponding value described in the examples. In this specification, the term "step" includes not only cases where it is an independent step but also cases where it cannot be clearly distinguished from other steps as long as the function of the step is achieved.

[0012] 《Separator for Energy Storage Device》 The separator for an energy storage device of the present disclosure includes a separator substrate having a multilayer structure of a microporous layer (A) mainly composed of polypropylene and a microporous layer (B) mainly composed of polyethylene. The separator substrate may further have a coating layer (also referred to as a "surface layer", "coating layer", etc. Hereinafter, simply referred to as a "coating layer") on the microporous layer (A) and / or the microporous layer (B). In the present specification, the "microporous layer" means each microporous layer constituting the substrate of the separator, the "separator substrate" means the substrate of the separator excluding any coating layer, and the "separator" means the entire separator including any coating layer.

[0013] 〈Microporous Layer (A)〉 The separator for a power storage device of the present disclosure has a microporous layer (A). The separator for a power storage device may have only one layer of the microporous layer (A) or two or more layers. At least one layer of the microporous layer (A) constitutes the outermost layer on at least one side of the separator substrate. When the separator for a power storage device has two or more layers of the microporous layer (A), the microporous layer (A) may constitute the outermost layers on both sides of the separator substrate. The microporous layer (A) is mainly composed of polypropylene, whereby good battery performance can be maintained even after storage at a high temperature (for example, 130°C). In the present specification, "mainly composed of" polypropylene means that polypropylene is contained in an amount of 50% by mass or more based on the total mass of the microporous layer (A). The lower limit of the content of polypropylene in the microporous layer (A) is 50% by mass or more, preferably 55% by mass or more, 60% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, or 95% by mass or more, from the viewpoints of the wettability, thinning, and shutdown characteristics of the separator. The upper limit of the content of polypropylene in the microporous layer (A) may be, for example, 60% by mass or less, 70% by mass or less, 80% by mass or less, 90% by mass or less, 95% by mass or less, 98% by mass or less, or 99% by mass or less, and may be 100% by mass.

[0014] <Material of the microporous layer (A)> The microporous layer (A) is mainly composed of polypropylene. Examples of the stereoregularity of polypropylene include atactic, isotactic, or syndiotactic homopolymers. The polypropylene according to the present disclosure is preferably an isotactic or syndiotactic highly crystalline homopolymer.

[0015] The polypropylene of the microporous layer (A) is preferably a homopolymer, and may also be a copolymer obtained by copolymerizing a small amount of comonomer other than propylene, such as an α-olefin comonomer, for example, a block polymer. The amount of the propylene structure contained as a repeating unit in the polypropylene may be, for example, 70 mol% or more, 80 mol% or more, 90 mol% or more, 95 mol% or more, or 99 mol% or more. The amount of the repeating unit derived from the comonomer other than the propylene structure contained in the polypropylene may be, for example, 30 mol% or less, 20 mol% or less, 10 mol% or less, 5 mol% or less, or 1 mol% or less. The polypropylene can be used alone or in combination of two or more kinds.

[0016] From the viewpoint of the strength of the microporous layer, the weight average molecular weight (Mw) of the polypropylene of the microporous layer (A) is preferably 300,000 or more, and from the viewpoint of ensuring good film-forming property and productivity, it is preferably 1,300,000 or less. The Mw of the polypropylene is more preferably 500,000 or more and 1,200,000 or less, still more preferably 650,000 or more and 1,100,000 or less, even more preferably 750,000 or more and 1,000,000 or less, and particularly preferably 800,000 or more and 1,000,000 or less.

[0017] The upper limit of the value (Mw / Mn) obtained by dividing the weight-average molecular weight (Mw) of the polypropylene in the microporous layer (A) by the number-average molecular weight (Mn) is preferably 20 or less, more preferably 18 or less, 16 or less, 14 or less, or 12 or less. By setting Mw / Mn to 20 or less, good film-forming properties and productivity tend to be ensured. Also, Mw / Mn is preferably 3.0 or more, more preferably 3.5 or more, and even more preferably 4.0 or more. As the value of Mw / Mn of the polypropylene increases, the melt tension of the obtained microporous layer also tends to increase, and it is preferable to increase the melt tension of the microporous layer (A) also in enhancing the strength of the microporous layer (A). Therefore, it is preferable that the value of Mw / Mn of the polypropylene is 3.0 or more in order to control the melt tension of the microporous layer (A) at a high level. Note that the weight-average molecular weight, number-average molecular weight, and Mw / Mn of the polyolefin of the present disclosure are molecular weights in terms of polystyrene obtained by GPC (gel permeation chromatography) measurement.

[0018] The density of the polypropylene in the microporous layer (A) is preferably 0.85 g / cm 3 or more, for example 0.88 g / cm 3 or more, 0.89 g / cm 3 or more, or 0.90 g / cm 3 or more. The density of the polypropylene is preferably 1.1 g / cm 3 or less, for example 1.0 g / cm 3 or less, 0.98 g / cm 3 or less, 0.97 g / cm 3 or less, 0.96 g / cm 3 or less, 0.95 g / cm 3 or less, 0.94 g / cm 3 or less, 0.93 g / cm 3 or less, or 0.92 g / cm 3 or less. The density of the polyolefin is related to the crystallinity of the polypropylene, and by setting the density of the polypropylene to 0.85 g / cm 3 or more, the productivity of the microporous layer is improved, which is particularly advantageous in the dry process.

[0019] As long as the microporous layer (A) is mainly composed of polypropylene, it may contain other resins. Examples of other resins include polyolefins other than polypropylene (also referred to as "other polyolefins"), and copolymers of polystyrene and polyolefins. A polyolefin is a polymer containing a monomer having a carbon-carbon double bond as a repeating unit. Monomers constituting polyolefins other than polypropylene include monomers having 2 or 4 to 10 carbon atoms with a carbon-carbon double bond, such as ethylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene. Polyolefins are, for example, homopolymers, copolymers, or multi-stage polymerization polymers, and as an example, it is also possible to contain polyethylene. Preferred examples of copolymers of polystyrene and polyolefins include styrene-(ethylene-propylene)-styrene copolymer (SEPS), styrene-(ethylene-butene)-styrene copolymer, and styrene-ethylene-styrene copolymer. Particularly preferred is styrene-(ethylene-propylene)-styrene copolymer (SEPS).

[0020] 〈MFR of Microporous Layer (A)〉 The upper limit of the MFR (single-layer MFR) of the microporous layer (A) is 0.90 g / 10 min or less, for example, 0.60 g / 10 min or less, 0.55 g / 10 min or less, or 0.50 g / 10 min or less, from the viewpoint of obtaining a microporous layer (A) with higher strength. The lower limit of the MFR (single-layer MFR) of the microporous layer (A) is, for example, 0.20 g / 10 min or more, 0.25 g / 10 min or more, 0.30 g / 10 min or more, 0.35 g / 10 min or more, or 0.50 g / 10 min or more, from the viewpoints of the moldability and thin-film forming property of the microporous layer (A). The MFR of the microporous layer (A) is measured under the conditions of a load of 2.16 kg and a temperature of 230°C.

[0021] The fact that the MFR of the microporous layer (A) is 0.90 g / 10 min or less means that the molecular weight of the polyolefin contained in the microporous layer (A) is relatively high. When the molecular weight of the polyolefin is high, the number of tie molecules that bond crystalline substances increases, so that a microporous layer (A) with high strength tends to be obtained, and a separator substrate with high strength also tends to be obtained.

[0022] When the MFR of the microporous layer (A) is 0.20 g / 10 min or more, the melt tension of the microporous layer (A) does not become too high, and it becomes possible to ensure good film-forming properties and productivity.

[0023] From the viewpoint of obtaining a microporous layer (A) with high strength and high melt tension, the MFR of the polypropylene in the microporous layer (A) is preferably 0.20 g / 10 min or more and 0.9 g / 10 min or less when measured under the conditions of a load of 2.16 kg and a temperature of 230 °C. The upper limit value of the MFR of the polypropylene may be, for example, 0.90 g / 10 min or less, 0.60 g / 10 min or less, 0.55 g / 10 min or less, or 0.50 g / 10 min or less from the viewpoint of obtaining a microporous layer with higher strength. The lower limit value of the MFR of the polypropylene may be, for example, 0.20 g / 10 min or more, 0.25 g / 10 min or more, 0.30 g / 10 min or more, 0.35 g / 10 min or more, or 0.50 g / 10 min or more from the viewpoints of the moldability and thin film-forming properties of the microporous layer (A).

[0024] 〈Pentad fraction of the microporous layer (A)〉 From the viewpoint of obtaining a microporous layer with low air permeability, the lower limit value of the pentad fraction of the polypropylene in the microporous layer (A) is preferably 94.0% or more, for example, 95.0% or more, 96.0% or more, 96.5% or more, 97.0% or more, 97.5% or more, 98.0% or more, 98.5% or more, or 99.0% or more. The upper limit value of the pentad fraction of the polypropylene may be 99.9% or less, 99.8% or less, or 99.5% or less. The pentad fraction of the polypropylene is 13 measured by C-NMR (nuclear magnetic resonance method).

[0025] When the pentad fraction of polypropylene is 94.0% or more, it indicates that the crystallinity of polypropylene is high. In the stretched pore-forming method, particularly the dry method, the separator obtained has pores formed by stretching the amorphous part between the crystalline parts. Therefore, when the crystallinity of polypropylene is high, the pore-forming property is good, and the air permeability can be kept low, enabling the high output of the battery.

[0026] 〈Melting tension of the microporous layer (A)〉 The melting tension Mt of the microporous layer (A) at 240 °C A is preferably 10 mN or more and 40 mN or less. The melting tension Mt A As the lower limit of the melting tension Mt, from the viewpoint of achieving a sufficiently fine pore structure and small pore diameter and expressing the strength improvement effect, it is preferably 10 mN or more, more preferably 13 mN or more, still more preferably 16 mN or more, particularly preferably 20 mN or more, and most preferably 23 mN or more. The melting tension Mt A As the upper limit of the melting tension Mt, from the viewpoints of good film-forming property and productivity, it is preferably 40 mN or less, more preferably 37 mN or less, still more preferably 34 mN or less, and most preferably 32 mN or less.

[0027] 〈Area-average long pore diameter of the microporous layer (A)〉 The area-average major pore diameter (hereinafter, also simply referred to as "area-average major pore diameter") in the MD-ND cross-section of the microporous layer (A) is preferably 50 nm or more and 400 nm or less. In the present specification, "ND" indicates the thickness direction of the microporous layer, and "MD" indicates the film formation direction of the microporous layer. For example, in the case of a separator having a microporous layer, the MD is the longitudinal direction if it is a roll. "Major pore diameter" means the pore diameter in the MD. Further, when there are two or more layers of the microporous layer (A) and / or the microporous layer (B), the area-average major pore diameters of the microporous layer (A) and the microporous layer (B) are compared based on the average area-average major pore diameter values of each microporous layer. The lower limit of the area-average pore diameter of the microporous layer (A) is preferably 50 nm or more, more preferably 80 nm or more, still more preferably 100 nm or more, particularly preferably 120 nm or more, and most preferably 130 nm or more, from the viewpoint of maintaining excellent air permeability and ensuring good output in the power storage device. Further, the upper limit of the area-average pore diameter of the microporous layer (A) is preferably 400 nm or less, more preferably 250 nm or less, still more preferably 200 nm or less, particularly preferably 170 nm or less, and most preferably 150 nm or less, from the viewpoint of obtaining a microporous layer (A) with higher strength.

[0028] In a multilayer separator obtained by a dry method and a coextrusion process, which has a multilayer structure of a microporous layer (A) mainly composed of polypropylene and a microporous layer (B) mainly composed of polyethylene, it has been clarified that the area-average major pore diameter of the microporous layer (A) becomes smaller, thereby increasing the strength of the microporous layer (A) and the separator substrate. The mechanism is presumed to be as follows: When the area-average major pore diameter of the microporous layer (A) becomes smaller, the number of pores per unit area increases. Since each pore is formed by tie molecules that bond crystals to each other, the number of tie molecules per unit area increases. Therefore, when the area-average major pore diameter of the microporous layer (A) becomes smaller, there is a tendency to obtain a microporous layer (A) and a separator substrate with high strength. Further, when the area-average major pore diameter becomes smaller, the stress applied to the ends of each pore during the piercing test decreases, so there is a tendency to obtain a microporous layer (A) and a separator substrate with high strength.

[0029] The area-average pore size can be measured by image analysis from the obtained image through cross-sectional SEM observation of the MD-ND cross-section of the separator. The detailed conditions are shown in the examples. When measuring the average pore size from the cross-sectional SEM image, the number-average pore size and the area-average pore size can be calculated, but it is preferable to use the area-average pore size as the average pore size so that a better correlation with the physical properties of the separator can be obtained.

[0030] <Porosity of the microporous layer (A)> From the viewpoint of avoiding clogging in the power storage device and obtaining good air permeability of the separator, the porosity of the microporous layer (A) is preferably 30% or more, more preferably 40% or more, still more preferably 45% or more, and particularly preferably 50% or more. Also, from the viewpoint of maintaining the strength of the separator, the porosity of the microporous layer (A) is preferably 70% or less, more preferably 65% or less, and still more preferably 60% or less.

[0031] <Compatibility between the MFR of the microporous layer (A) and the porosity of the separator base material> Among the separators for power storage devices of the present embodiment, an exemplary preferred embodiment is that the microporous layer (A) has an MFR of 0.90 g / 10 min or less, a film thickness of 15 μm or less, and the porosity of the separator base material is 40% or more. Conventionally, in a multilayer separator obtained by a dry method and a coextrusion process having a multilayer structure of a microporous layer mainly composed of polypropylene and a microporous layer mainly composed of polyethylene, it was very difficult to adjust the porosity of the separator base material to 40% or more using a microporous layer (A) with an MFR of 0.90 g / 10 min or less. When the MFR of the microporous layer (A) mainly composed of polypropylene is as low as 0.90 g / 10 min or less, the melt viscosity of polypropylene is significantly higher than that of polyethylene. Therefore, when the resin composition is extruded from the extruder in a film shape during extrusion film formation, the stress concentrates on the high-viscosity polypropylene resin and is not sufficiently transmitted to the low-viscosity polyethylene resin. The microporous layer (B) mainly composed of polyethylene with insufficient stress transmission has significantly reduced pore-forming properties due to reduced molecular orientation, and the porosity of the microporous layer (B) and the separator base material tend to decrease.

[0032] Furthermore, for the separator with a multilayer structure, it is necessary to make the film thickness of each microporous layer particularly thin. For example, for a separator with a three-layer structure having a thickness of 13 μm, if the thickness ratio of each microporous layer is 1:1:1, the thickness of each microporous layer needs to be about 4.3 μm. It has been even more difficult to achieve a high porosity while using a high molecular weight polyolefin in such a thin film. In the present embodiment, by applying precisely controlled annealing and stretching conditions as exemplified in the section of "Method for Manufacturing a Separator for a Power Storage Device" described later, even when the MFR of the microporous layer (A) mainly composed of polypropylene is as low as 0.90 g / 10 min or less, the porosity of the separator substrate can be controlled within the above range.

[0033] 〈Thickness of Microporous Layer (A)〉 From the viewpoint of increasing the energy density of the power storage device and the like, the thickness of the microporous layer (A) is preferably 6.0 μm or less, for example, 5.5 μm or less, 5.0 μm or less, 4.5 μm or less, 4.0 μm or less, 3.5 μm or less, or 3.0 μm or less. The lower limit value of the thickness of the microporous layer (A) is preferably 1.0 μm or more, for example, 2.0 μm or more, 2.5 μm or more, or 3.0 μm or more from the viewpoint of strength and the like. Note that the "thickness of the microporous layer (A)" mentioned here is the thickness of each microporous layer (A). Therefore, regarding the total thickness when laminating different microporous layers (A) and the total thickness of each microporous layer (A) disposed on both sides of the microporous layer (B), the preferable range is treated as the range obtained by multiplying the total number of layers of the microporous layer (A) by the "thickness of the microporous layer (A)".

[0034] 〈Additives in Microporous Layer (A)〉 The microporous layer (A) mainly composed of polypropylene may further contain additives such as an elastomer, a crystal nucleating agent, an antioxidant, and a filler as needed in addition to polypropylene. The amount of the additive may be, for example, 0.01% by mass or more, 0.1% by mass or more, or 1.0% by mass or more, and 20% by mass or less, 10% by mass or less, or 7.0% by mass or less based on the total mass of the microporous layer (A).

[0035] <Micro-porous layer (B)> The separator for a power storage device of the present disclosure has a micro-porous layer (B). The separator for a power storage device may have only one layer of the micro-porous layer (B) or may have two or more layers. The micro-porous layer (B) is mainly composed of polyethylene, whereby the shutdown performance can be exhibited at a low temperature during thermal runaway of the battery. In the present specification, "mainly composed of" polyethylene means that polyethylene is contained in an amount of 50% by mass or more based on the total mass of the micro-porous layer (B). The lower limit of the content of polyethylene in the micro-porous layer (B) is preferably 55% by mass or more, 60% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, or 95% by mass or more from the viewpoints of wettability of the separator, thinning of the thin film, etc. The upper limit of the content of polyethylene in the micro-porous layer (B) may be, for example, 60% by mass or less, 70% by mass or less, 80% by mass or less, 90% by mass or less, 95% by mass or less, 98% by mass or less, or 99% by mass or less, and may be less than 100% by mass.

[0036] <Material of the micro-porous layer (B)> The micro-porous layer (B) is mainly composed of polyethylene. The polyethylene of the micro-porous layer (B) is preferably a homopolymer, and may be a copolymer obtained by copolymerizing a small amount of comonomer other than ethylene, such as an α-olefin comonomer, such as a block polymer. The amount of the ethylene structure contained as a repeating unit in polyethylene may be, for example, 70 mol% or more, 80 mol% or more, 90 mol% or more, 95 mol% or more, or 99 mol% or more. The amount of the repeating unit derived from the comonomer other than the ethylene structure contained in polyethylene may be, for example, 30 mol% or less, 20 mol% or less, 10 mol% or less, 5 mol% or less, or 1 mol% or less. Polyethylene can be used alone or in combination of two or more.

[0037] The density of the polyethylene of the micro-porous layer (B) is preferably 0.85 g / cm 3 or more, for example, 0.88 g / cm 3 or more, 0.89 g / cm 3or more, or 0.90 g / cm 3 or more. The density of polyethylene is preferably 1.1 g / cm 3 or less, for example 1.0 g / cm 3 or less, 0.98 g / cm 3 or less, 0.97 g / cm 3 or less, 0.96 g / cm 3 or less, 0.95 g / cm 3 or less, 0.94 g / cm 3 or less, 0.93 g / cm 3 or less, or 0.92 g / cm 3 or less. The density of polyethylene is related to the crystallinity of polyethylene. By setting the density of polyethylene to 0.85 g / cm 3 or more, the productivity of the microporous layer is improved, which is particularly advantageous in the dry process.

[0038] The weight average molecular weight (Mw) of the polyethylene in the microporous layer (B) is preferably 50,000 or more from the viewpoints of promoting pore formation, reducing air permeability, and increasing strength due to the high orientation of crystals, and preferably 700,000 or less from the viewpoint of improving the film-forming property of the thin film due to improved fluidity. The Mw of polyethylene is more preferably 100,000 or more and 600,000 or less, still more preferably 200,000 or more and 500,000 or less, and particularly preferably 300,000 or more and 400,000 or less.

[0039] The upper limit of the value (Mw / Mn) obtained by dividing the weight average molecular weight (Mw) of the polyethylene in the microporous layer (B) by the number average molecular weight (Mn) is preferably 20 or less, more preferably 18 or less, 16 or less, 14 or less, or 12 or less. By setting Mw / Mn to 20 or less, there is a tendency to ensure good film-forming properties and productivity. Further, Mw / Mn is preferably 3.0 or more, more preferably 3.5 or more, still more preferably 4.0 or more. As the value of Mw / Mn of polypropylene increases, the melt tension of the obtained microporous layer also tends to increase, and it is preferable to increase the melt tension of the microporous layer (B) also in enhancing the strength of the microporous layer (B). Therefore, it is preferable that the value of Mw / Mn of polypropylene is 3.0 or more in order to control the melt tension of the microporous layer (B) at a high level. The weight average molecular weight, number average molecular weight, and Mw / Mn of the polyolefin of the present disclosure are molecular weights in terms of polystyrene obtained by GPC (gel permeation chromatography) measurement.

[0040] In addition to polyethylene, the microporous layer (B) may contain other resins. Examples of other resins include polyolefins other than polyethylene (other polyolefins). A polyolefin is a polymer containing a monomer having a carbon-carbon double bond as a repeating unit. Examples of the monomer constituting the polyolefin other than polyethylene include monomers having 3 to 10 carbon atoms having a carbon-carbon double bond, such as propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene. In particular, the microporous layer (B) of the present disclosure contains polypropylene in addition to polyethylene. The microporous layer (B) of the present disclosure may contain other resins in addition to polyethylene and polypropylene.

[0041] <MFR of the microporous layer (B)> The upper limit of the MFR (MFR of a single layer) of the microporous layer (B) is preferably 2.0 g / 10 min or less, more preferably 1.2 g / 10 min or less or 1.0 g / 10 min or less, still more preferably 0.80 g / 10 min or less, and particularly preferably 0.50 g / 10 min or less, from the viewpoint of obtaining a microporous layer (B) with higher strength. The lower limit of the MFR (MFR of a single layer) of the microporous layer (B) is preferably 0.10 g / 10 min or more, more preferably 0.15 g / 10 min or more, still more preferably 0.18 g / 10 min or more, and particularly preferably 0.20 g / 10 min or more, from the viewpoints of good pore-opening property and clogging prevention. The MFR of the microporous layer (B) is measured under the conditions of a load of 2.16 kg and a temperature of 190°C.

[0042] The upper limit of the MFR of the polyethylene of the microporous layer (B) is preferably 2.0 g / 10 min or less, more preferably 1.0 g / 10 min or less, still more preferably 0.90 g / 10 min or less or 0.80 g / 10 min or less, and particularly preferably 0.50 g / 10 min or less, from the viewpoint of obtaining a microporous layer (B) with higher strength. The lower limit of the MFR of the polyethylene of the microporous layer (B) is preferably 0.10 g / 10 min or more, more preferably 0.15 g / 10 min or more, still more preferably 0.18 g / 10 min or more, and particularly preferably 0.20 g / 10 min or more, from the viewpoints of good pore-opening property and clogging prevention. The MFR of the polyethylene of the microporous layer (B) is measured under the conditions of a load of 2.16 kg and a temperature of 190°C.

[0043] 〈Average major pore diameter of the microporous layer (B)〉 The average major pore diameter in the MD-ND cross-section of the microporous layer (B) is preferably 300 nm or more and 600 nm or less, more preferably 350 nm or more and 550 nm or less, still more preferably 400 nm or more and 500 nm or less. When the average major pore diameter of the microporous layer (B) is within this range, it is possible to achieve both air permeability and high strength while maintaining a high porosity.

[0044] 〈Porosity of the microporous layer (B)〉 The porosity of the microporous layer (B) is preferably 20% or more from the viewpoint of avoiding clogging in the power storage device and obtaining good air permeability of the separator, and preferably 90% or less from the viewpoint of maintaining the strength of the separator. The porosity of the microporous layer (B) is more preferably 25% or more and 85% or less, still more preferably 25% or more and 80% or less, and particularly preferably 30% or more and 80% or less. In the present embodiment, by applying precisely controlled annealing and stretching conditions as exemplified in the section of 《Method for manufacturing a separator for a power storage device》 described later, even when the MFR of the microporous layer (A) mainly composed of polypropylene is as low as 0.90 g / 10 min or less, the porosity of the microporous layer (B) can be controlled within the above range.

[0045] 〈Thickness of the microporous layer (B)〉 The thickness of the microporous layer (B) of the present disclosure is preferably 10.0 μm or less, for example, 8.0 μm or less, 6.0 μm or less, 5.0 μm or less, 4.5 μm or less, 4.0 μm or less, 3.0 μm or less from the viewpoint of increasing the energy density of the power storage device and the like. The lower limit value of the thickness of the microporous layer (B) is preferably 1.0 μm or more, for example, 1.5 μm or more, 2.0 μm or more, or 2.5 μm or more from the viewpoint of strength and the like. Note that the "thickness of the microporous layer (B)" referred to here is the thickness of each microporous layer (B). Therefore, when there are a plurality of different microporous layers (B), the preferable range for the total thickness is treated as the range obtained by multiplying the above "thickness of the microporous layer (B)" by the total number of layers of the microporous layer (B).

[0046] 〈Additive of the microporous layer (B)〉 The microporous layer (B) mainly composed of polyethylene contains 4.0% by mass or more and 12.0% by mass or less of polypropylene, preferably 5.0% by mass or more and 11.0% by mass or less, more preferably 6.0% by mass or more and 10.0% by mass or less. When the polypropylene in the microporous layer (B) is 4.0% by mass or more, "twist" that easily breaks the crystals of PE is formed, resulting in a large amount of cracks during cold stretching and an easy formation of small pore diameters. As a result of forming small pore diameters, the amount of tie molecules connecting between lamellae increases, and the MD tensile strength tends to improve. When the polypropylene in the microporous layer (B) is 12.0% by mass or less, when the polyethylene solidifies during film formation, the amount of melted components decreases, so the crystal orientation of polyethylene progresses and low air permeability is easily obtained.

[0047] The polypropylene in the microporous layer (B) is mainly composed of polypropylene. Examples of the stereoregularity of polypropylene include atactic, isotactic, or syndiotactic homopolymers, etc. The polypropylene according to the present disclosure is preferably an isotactic or syndiotactic highly crystalline homopolymer.

[0048] The polypropylene in the microporous layer (B) is preferably a homopolymer, and may also be a copolymer obtained by copolymerizing a small amount of comonomer other than propylene, such as an α-olefin comonomer, such as a block polymer. The amount of the propylene structure contained as a repeating unit in the polypropylene may be, for example, 70 mol% or more, 80 mol% or more, 90 mol% or more, 95 mol% or more, or 99 mol% or more. The amount of the repeating unit derived from the comonomer other than the propylene structure contained in the polypropylene may be, for example, 30 mol% or less, 20 mol% or less, 10 mol% or less, 5 mol% or less, or 1 mol% or less. The polypropylene can be used alone or in combination of two or more.

[0049] The weight average molecular weight (Mw) of the polypropylene in the microporous layer (B) is preferably 300,000 or more from the viewpoint of the strength of the microporous layer, etc., and preferably 1,300,000 or less from the viewpoint of ensuring good film-forming properties and productivity. More preferably, the Mw of the polypropylene is 500,000 or more and 1,200,000 or less, still more preferably 650,000 or more and 1,100,000 or less, even more preferably 750,000 or more and 1,000,000 or less, and particularly preferably 800,000 or more and 1,000,000 or less.

[0050] The upper limit of the value (Mw / Mn) obtained by dividing the weight average molecular weight (Mw) of the polypropylene in the microporous layer (B) by the number average molecular weight (Mn) is preferably 20 or less, more preferably 18 or less, 16 or less, 14 or less, or 12 or less. By setting Mw / Mn to 20 or less, there is a tendency to be able to ensure good film-forming properties and productivity. Also, Mw / Mn is preferably 3 or more, more preferably 3.5 or more, and still more preferably 4.0 or more. The larger the value of Mw / Mn of the polypropylene, the greater the melt tension of the obtained microporous layer tends to be, and it is preferable to increase the melt tension of the microporous layer (B) also in enhancing the strength of the microporous layer (B). Therefore, it is preferable that the value of Mw / Mn of the polypropylene is 3.0 or more in order to highly control the melt tension of the microporous layer (B). The weight average molecular weight, number average molecular weight, and Mw / Mn of the polyolefin of the present disclosure are molecular weights in terms of polystyrene obtained by GPC (gel permeation chromatography) measurement.

[0051] The density of the polypropylene in the microporous layer (B) is preferably 0.85 g / cm 3 or more, for example 0.88 g / cm 3 or more, 0.89 g / cm 3 or more, or 0.90 g / cm 3 or more. The density of the polypropylene is preferably 1.1 g / cm 3 or less, for example 1.0 g / cm 3 or less, 0.98 g / cm 3 or less, 0.97 g / cm3 The following is 0.96 g / cm 3 The following is 0.95 g / cm 3 The following is 0.94 g / cm 3 The following is 0.93 g / cm 3 The following, or 0.92 g / cm 3 The following may be sufficient. The density of the polyolefin is related to the crystallinity of the polypropylene. By setting the density of the polypropylene to 0.85 g / cm 3 or higher, the productivity of the microporous layer is improved, which is particularly advantageous in the dry process.

[0052] The MFR of the polypropylene in the microporous layer (B) is preferably 0.20 g / 10 min or more, 11.0 g / 10 min or less, or 0.90 g / 10 min or less when measured under the conditions of a load of 2.16 kg and a temperature of 230 °C from the viewpoint of obtaining a microporous layer (B) having high strength and high melt tension. The upper limit value of the MFR of the polypropylene may be, for example, 0.90 g / 10 min or less, 0.60 g / 10 min or less, 0.55 g / 10 min or less, or 0.50 g / 10 min or less from the viewpoint of obtaining a microporous layer with higher strength. The lower limit value of the MFR of the polypropylene may be, for example, 0.20 g / 10 min or more, 0.25 g / 10 min or more, 0.30 g / 10 min or more, 0.35 g / 10 min or more, or 0.50 g / 10 min or more from the viewpoints of the moldability and thin film formability of the microporous layer (B).

[0053] The lower limit value of the pentad fraction of the polypropylene in the microporous layer (B) is preferably 94.0% or more, for example, 95.0% or more, 96.0% or more, 96.5% or more, 97.0% or more, 97.5% or more, 98.0% or more, 98.5% or more, or 99.0% or more from the viewpoint of obtaining a microporous layer with low air permeability. The upper limit value of the pentad fraction of the polypropylene may be 99.9% or less, 99.8% or less, or 99.5% or less. The pentad fraction of the polypropylene is 13 Measured by C-NMR (nuclear magnetic resonance method).

[0054] The melt tension Mt at 240 °C of the microporous layer (B) Bis preferably 10 mN or more and 40 mN or less. The melt tension Mt B As the lower limit of, from the viewpoint of achieving a sufficiently fine pore structure and a small pore diameter and exhibiting the strength improving effect, it is preferably 10 mN or more, more preferably 13 mN or more, still more preferably 16 mN or more, particularly preferably 20 mN or more, and most preferably 23 mN or more. The melt tension Mt B As the upper limit of, from the viewpoint of good film formability and productivity, it is preferably 40 mN or less, more preferably 37 mN or less, still more preferably 34 mN or less, and most preferably 32 mN or less.

[0055] The ratio (PP / PE) of the MFR of polypropylene and polyethylene in the microporous layer (B) is, for example, 0.50 to 40.0, 0.80 to 20.0, 1.20 to 10.0, or 1.50 to 5.00.

[0056] In addition to polyethylene and polypropylene, the microporous layer (B) may further contain a thermoplastic elastomer for the purpose of compatibilization to improve dispersibility as needed. The amount of the thermoplastic elastomer may be, for example, 1.0% by mass or more and 6.0% by mass or less, or 2.0% by mass or more and 4.0% by mass or less, 2.5% by mass or less, or 3.5% by mass or less based on the total mass of the microporous layer (B). When the thermoplastic elastomer is 1.0% by mass or more, polypropylene is uniformly dispersed in polyethylene, so it is difficult to form a large polypropylene domain that inhibits pore opening, and it is easy to obtain a low air permeability. When the thermoplastic elastomer is 6.0% by mass or less, the ratio of highly crystalline polyethylene and polypropylene increases, so lamellar pore opening is promoted and a low air permeability is easily obtained.

[0057] As the thermoplastic elastomer contained in the microporous layer (B) of the present disclosure, as described above, from the viewpoint of achieving both high MD tensile strength and low air permeability by improving the dispersibility of polyethylene and polypropylene or by playing a role of connecting polypropylene and polyethylene to strengthen the connection of domains, a copolymer containing at least one selected from the group consisting of ethylene, propylene, and 1-butene as a repeating unit and being compatible with polypropylene and / or polyethylene is preferred. Among them, ethylene / propylene (C2C3) copolymer, ethylene / 1-hexene (C2C6) copolymer, ethylene / 1-octene (C2C8) copolymer, olefin-(ethylene-butene)-olefin copolymer (CEBC), and olefin-(ethylene-butene)-styrene copolymer (CEBS) are more preferred, and ethylene / propylene (C2C3) copolymer, olefin-(ethylene-butene)-olefin copolymer (CEBC) with an ethylene end, and olefin-(ethylene-butene)-styrene copolymer (CEBS) with an ethylene end are even more preferred. Here, since the ethylene-butene structure is structurally similar to propylene, it shows a high affinity for polypropylene. The thermoplastic elastomer can be used alone or in combination of two or more.

[0058] The MFR of the thermoplastic elastomer in the microporous layer (B) is preferably 0.1 to 100.0 g / 10 min when measured under the conditions of a load of 2.16 kg and a temperature of 230 °C from the viewpoints of obtaining high puncture strength, low air permeability, a thin film, and good film-forming stability. The upper limit value of the MFR of the thermoplastic elastomer may be, for example, 80.0 g / 10 min or less, 60.0 g / 10 min or less, 40.0 g / 10 min or less, 30.0 g / 10 min or less, 20.0 g / 10 min or less, 15.0 g / 10 min or less from the viewpoints of being uniformly kneaded with the polyolefin and obtaining MD tensile strength and good film-forming stability. The lower limit value of the MFR of the thermoplastic elastomer may be, for example, 0.5 g / 10 min or more, 1.0 g / 10 min or more, 1.5 g / 10 min or more, 2.0 g / 10 min or more, 2.5 g / 10 min or more, or 3.0 g / 10 min or more from the viewpoints of obtaining a microporous layer (B) with lower air permeability, film-forming property, and thin film formation.

[0059] Here, the microporous layer (B) has polyethylene and polypropylene contained therein in the same layer. "Having in the same layer" means when the microporous layer (B) is a single layer, the single-layer microporous layer (B) has both polyethylene and polypropylene, and when the microporous layer (B) is a plurality of layers, at least one of the plurality of microporous layers (B) has both polyethylene and polypropylene. means. Therefore, for example, in the case of a separator substrate having a three-layer structure composed of an A / B / A layer, the intermediate layer B has both polyethylene and polypropylene.

[0060] 〈Relationship between the microporous layer (A) and the microporous layer (B)〉 〈Layer structure of the separator substrate〉 The base material of the separator for the power storage device (also simply referred to as "separator base material" in this specification) has at least one layer each of the microporous layer (A) and the microporous layer (B). The separator base material may have a multilayer structure of three or more layers having at least two layers of at least one of the microporous layer (A) and / or the microporous layer (B). For example, a two-layer structure of microporous layer (A) / microporous layer (B), a three-layer structure of microporous layer (A) / microporous layer (B) / microporous layer (A), etc. can be mentioned. Further, the separator base material may have a layer other than the microporous layer (A) and the microporous layer (B). For example, as the layer other than the microporous layer (A) and the microporous layer (B), for example, a microporous layer mainly composed of a polyolefin other than (A)(B), a layer containing an inorganic substance, a layer containing a heat-resistant resin, etc. can be mentioned. For example, the separator base material may have a multilayer structure of four or more layers such as microporous layer (A) / microporous layer (B) / microporous layer (C) / microporous layer (A). From the viewpoints of ease of manufacture and suppression of curling of the separator, a symmetric laminated structure is preferable.

[0061] The ratio of the thickness of the microporous layer (B) to the thickness of the microporous layer (A) is preferably in the range of 0.2 to 1.5, 0.35 to 1.25, and 0.5 to 1.0 from the viewpoints of thinning the separator base material as the total thickness and achieving both air permeability and puncture strength. When the microporous layer (A) or the microporous layer (B) is multilayered, this ratio shall be calculated by converting it to the thickness per single layer.

[0062] As the layer structure of the base material of the separator for the power storage device, at least one layer of the microporous layer (A) constitutes the outermost layer on at least one side of the separator base material. Here, it is preferable that the microporous layer (A) constitutes the outermost layer on both sides of the separator base material. When the microporous layer (A) constitutes the outermost layer, the strength tends to be easily improved.

[0063] 〈Thickness of the separator base material〉 The upper limit value of the thickness of the separator base material is preferably 15.0 μm or less, more preferably 14.0 μm or less, for example, it may be 13.0 μm or less, 12.0 μm or less, 11.0 μm or less, 10.0 μm or less, or 9.5 μm or less, from the viewpoint of increasing the energy density of the power storage device. The lower limit value of the thickness of the separator base material is preferably 4.5 μm or more, for example, it may be 5 μm or more, 6 μm or more, 7 μm or more, or 8 μm or more, from the viewpoint of strength.

[0064] 〈Dry height of the separator base material〉 The dry height in the MD-ND cross-section of the separator for the power storage device is correlated with the tortuosity of the pores and is a numerical value indicating a pore structure different from the pore diameter. As the dry height increases, the tortuosity of the pores decreases, and as the dry height decreases, the tortuosity of the pores increases, and high strength and good withstand voltage characteristics can be exhibited. Therefore, the upper limit of the dry height is preferably 890 nm or less, more preferably 880 nm or less, 860 nm or less, 840 nm or less, from the viewpoints of high strength and withstand voltage characteristics. The lower limit of the dry height is preferably 700 nm or more, more preferably 720 nm or more, 740 nm or more, 760 nm or more, from the viewpoint of permeability.

[0065] The dry height can be calculated by observing the cross-section of the MD-ND cross-section of the separator base material by SEM, removing the fibrils by image analysis, and measuring the length of the remaining lamella in the ND direction by image analysis. The detailed conditions are shown in the examples. When measuring the dry height from the cross-sectional SEM image, the number average dry height and the length average dry height can be calculated. In this specification, the length average dry height is used as the dry height so that the correlation with the physical properties of the separator can be better obtained.

[0066] 〈Air permeability of the separator base material〉 The upper limit value of the air permeability of the separator base material is preferably 250 sec / 100 cm 3 or less, more preferably 200 sec / 100 cm 3 or less, for example, 180 sec / 100 cm 3 or less, 160 sec / 100 cm 3The following, 150 sec / 100 cm 3 The following, or 140 sec / 100 cm 3 It may be the following. The lower limit value of the air permeability of the separator substrate is, for example, 10 sec / 100 cm 3 or more, 20 sec / 100 cm 3 or more, or 30 sec / 100 cm 3 or more. In addition, when the thickness of the separator substrate is converted to 12 μm, the air permeability of the separator substrate (air permeability converted to 12 μm film thickness) is represented by the following formula: (Air permeability converted to 12 μm film thickness [sec / 100 cm 3 ) = (Air permeability of the separator substrate [sec / 100 cm 3 ) / (Thickness of the separator substrate [μm]) × (12 [μm]) is calculated using.

[0067] 〈Porosity of the separator substrate〉 The lower limit value of the porosity of the separator substrate is 40% or more, preferably 42% or more, more preferably 44% or more, and particularly preferably 45% or more, from the viewpoints of avoiding clogging in the power storage device and obtaining good air permeability of the separator. Further, the upper limit value of the porosity of the separator substrate is preferably 70% or less, more preferably 65% or less, still more preferably 60% or less, and particularly preferably 55% or less, from the viewpoint of maintaining the strength of the separator.

[0068] 〈Puncture strength of the separator substrate〉 The lower limit value of the puncture strength of the separator substrate is preferably 210 gf or more (about 1.42 N or more), more preferably 150 gf or more, still more preferably 230 gf or more, and particularly preferably 250 gf or more, when the thickness of the separator substrate is converted to 12 μm, from the viewpoint of suppressing short - circuit during physical damage of the battery. The upper limit value of the puncture strength of the separator substrate is preferably 500 gf or less, for example, 480 gf or less, or 450 gf or less, when the thickness of the separator substrate is converted to 12 μm. In addition, when the thickness of the separator substrate is converted to 12 μm, the puncture strength of the separator substrate (puncture strength converted to 12 μm film thickness) is represented by the following formula: (12μm film thickness conversion puncture strength [gf]) = (Puncture strength of separator substrate [gf]) / (Thickness of separator substrate [μm]) × (12 [μm]) It is calculated using.

[0069] 〈Thermal shrinkage rate of separator substrate〉 The separator substrate preferably has a thermal shrinkage rate in the width direction (TD) of -1.0% or more and 3.0% or less after heat treatment at 150°C for 1 hour. That is, it means that the separator substrate has very little thermal shrinkage in the TD even at high temperatures. By having the thermal shrinkage rate of 3.0% or less, short circuits at high temperatures can be effectively suppressed. The reason for the thermal shrinkage rate of -1.0% or more is that during the measurement of the thermal shrinkage rate, the substrate expands in the TD and the thermal shrinkage rate may become smaller than 0% and a negative value. The thermal shrinkage rate may be 0% or more, or greater than 0%. Examples of the method for manufacturing a separator substrate having a thermal shrinkage rate of -1.0% or more and 3.0% or less include, for example, a method for manufacturing a separator by uniaxial stretching in the MD direction, preferably a method for manufacturing by the dry method of uniaxial stretching. In the method for manufacturing a separator by biaxial stretching in the MD and TD directions represented by a wet separator, generally, the thermal shrinkage in the TD becomes very large, whereas in a dry separator by uniaxial stretching, it is easy to obtain a separator substrate having a thermal shrinkage rate of -1.0% or more and 3.0% or less.

[0070] 〈Tensile strength of separator substrate〉 From the viewpoints of the operability during battery winding and high puncture strength, the tensile strength of the separator substrate in the MD is preferably 1500 kgf / cm 2 or more (about 14.7 kN / cm 2 or more), more preferably 1600 kgf / cm 2 or more, still more preferably 1700 kgf / cm 2 or more, particularly preferably 1800 kgf / cm 2 or more, and most preferably 1900 kgf / cm 2 or more. The upper limit value of the tensile strength of the separator substrate in the MD is preferably 4000 kgf / cm 2 or less, for example 3800 kgf / cm 23500 kgf / cm or less 2 3200 kgf / cm or less 2 or 3000 kgf / cm or less 2 may be used.

[0071] <Dielectric strength of separator substrate> From the viewpoint of suppressing short circuits in the power storage device, the dielectric strength of the separator substrate, when the thickness of the separator substrate is converted to 12 μm, is preferably 0.80 kV or more, more preferably 0.90 kV or more, still more preferably 0.95 kV or more, particularly preferably 1.00 kV or more, and most preferably 1.05 kV or more.

[0072] <Method for manufacturing a separator for a power storage device> The method for manufacturing a separator for a power storage device includes a melt extrusion step of melt extruding a resin composition mainly composed of polypropylene or polyethylene (hereinafter also referred to as "polyolefin resin composition") to obtain a resin film (precursor film), and a pore formation step of forming pores by perforating the obtained precursor film. The method for manufacturing the microporous layer is roughly classified into a dry method that does not use a solvent in the pore formation step and a wet method that uses a solvent.

[0073] Examples of the dry method include a method of peeling the polyolefin crystal interface by heat treatment and stretching after melt kneading and extruding the polyolefin resin composition, and a method of peeling the interface between the polyolefin and the inorganic filler by stretching after melt kneading the polyolefin resin composition and the inorganic filler and forming it into a film.

[0074] Examples of the wet method include a method of melt kneading the polyolefin resin composition and the pore former, forming it into a film, stretching it as necessary, and then extracting the pore former, and a method of dissolving the polyolefin resin composition and immersing it in a poor solvent for polyolefin to coagulate the polyolefin and simultaneously removing the solvent.

[0075] For the melt-kneading of the polyolefin resin composition, a single-screw extruder and a twin-screw extruder can be used. In addition to these, for example, a kneader, a lab plastomill, a kneading roll, a Banbury mixer, etc. can also be used.

[0076] The polyolefin resin composition may optionally contain a resin other than polypropylene, a resin other than polyethylene, an additive, etc., depending on the manufacturing method of the microporous layer or the physical properties of the target microporous layer. Examples of the additive include a pore-forming material, a fluorine-based flow modifier, a wax, a crystal nucleating agent, an antioxidant, metal soaps such as aliphatic carboxylic acid metal salts, an ultraviolet absorber, a light stabilizer, an antistatic agent, an antifogging agent, and a coloring pigment. Examples of the pore-forming material include a plasticizer, an inorganic filler, or a combination thereof.

[0077] Examples of the plasticizer 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.

[0078] Examples of the inorganic filler include 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, amesite, bentonite, asbestos, zeolite, calcium silicate, magnesium silicate, diatomaceous earth, and silica sand; and glass fiber.

[0079] As a method for manufacturing the separator substrate, a dry lamellar crystal opening process that peels the polyolefin crystal interface by heat treatment and stretching is preferable. Here, as a method for manufacturing a separator substrate having a microporous layer (A) and a microporous layer (B), the following methods (i) and (ii) are known: (i) A method for manufacturing a separator substrate by coextrusion film formation, comprising coextruding and forming a microporous layer (A) and a microporous layer (B) into a film, and subjecting the film to annealing, cold stretching, hot stretching, and heat relaxation processes; and (ii) A method for manufacturing a separator substrate by lamination, comprising extruding and forming the microporous layer (A) and the microporous layer (B) separately into films, laminating and bonding the films, and then subjecting the laminated film to annealing, cold stretching, hot stretching, and heat relaxation processes.

[0080] Among the above coextrusion process (i) and lamination process (ii), from the viewpoints of thin film formability and manufacturing cost, etc., the coextrusion process (i) is used. The reasons for not using the lamination process (ii) are described below: For a separator with a multilayer structure, it is necessary to make the film thickness of each microporous layer particularly thin. For example, for a three-layer separator with a thickness of 13 μm, if the thickness ratio of each microporous layer is 1:1:1, the thickness of each microporous layer needs to be about 4.3 μm. Also, for example, for a three-layer separator with a thickness of 9 μm, if the thickness ratio of each microporous layer is 1:1:1, the thickness of each microporous layer needs to be 3 μm. Also, for example, for a three-layer separator with a thickness of 9.5 μm, if the thickness ratio of each microporous layer is 1:0.5:1, the thickness of the thinnest microporous layer needs to be 1.9 μm. In the lamination process (ii), it is necessary to extrude and form each microporous layer separately into a film. However, it is difficult to stably produce a resin film with a thickness of, for example, 4.3 μm, or 3 μm, or 1.9 μm. In particular, when the MFR of the microporous layer (A) mainly composed of polypropylene is 0.90 g / 10 min or less, that is, when a high molecular weight polyolefin is used, it is extremely difficult to stably produce a resin film with a thickness of, for example, 4.3 μm, or 3 μm, or 1.9 μm. In addition, the lamination process has a process of extruding and forming each microporous layer separately into a film and a process of laminating and bonding them. Therefore, compared with the coextrusion process, the number of processes is larger and the manufacturing cost increases.

[0081] In the co-extrusion process (i), as the extrusion film-forming conditions for the microporous layers (A) and (B), it is preferable to discharge the resin at as low a temperature as possible and effectively quench it by blowing low-temperature air. After film formation, it is preferable to quench with air. The temperature of the blown air is preferably 20°C or lower, more preferably 15°C or lower. By blowing such cold air controlled to such a low temperature, the resin after film formation is uniformly oriented in the MD direction.

[0082] Regarding the method for distinguishing co-extrusion and lamination from the product, For a product made of lamination, when observing any 40 points by cross-sectional SEM at a magnification of 5000 times, two or more peeling portions (portions with the same contrast as pores) with a length of 5000 nm or more in the MD direction were observed. On the other hand, no peeling points were observed for the product made of co-extrusion. For a product made of lamination, an adhesive tape (Nichiban Co., Ltd. Cellotape (registered trademark) No. 405) was attached to the front and back surfaces of the product, and in this state, it was left in an environment of 23°C and a relative humidity of 50% for 20 minutes. Then, the adhesive tape was peeled off at a peeling angle of 180°. This operation was repeated 40 times, and the A layer and the B layer were peeled off 10 times or more. On the other hand, when the same operation was performed on a product made of co-extrusion, the A layer and the B layer were peeled off 1 time or less.

[0083] The method for manufacturing the separator base material by the above co-extrusion process (i) may include an annealing step after extrusion film formation. By performing the annealing step, the crystal structures of the microporous layers (A) and (B) grow, and the porosity tends to be improved. By applying annealing at a specific temperature for a predetermined time, it is possible to obtain a good area-average pore size and pore length for both the microporous layers (A) and (B). The reason is considered to be that the crystals grow without the crystal structure being disturbed, and high porosity can be obtained. Since polyethylene with a thickness of 4 μm or less per layer is extruded as a thin film by co-extrusion, it is efficiently quenched and highly oriented. The microporous layer (B) mainly composed of polyethylene forms small pores and the MD strength increases.

[0084] The lower limit of the annealing temperature is preferably 125 °C or higher, more preferably 128 °C or higher, and particularly preferably 130 °C or higher. When the MFR of the microporous layer (A) mainly composed of polypropylene is as low as 0.90 g / 10 min or less, the molecular orientation of the microporous layer (B) mainly composed of polyethylene decreases, resulting in a significant decrease in the porosity, and the porosity of the microporous layer (B) and the separator substrate tends to decrease. This is a phenomenon peculiar to the dry method and the multilayer separator obtained by the co-extrusion process, which has a multilayer structure of a microporous layer mainly composed of polypropylene and a microporous layer mainly composed of polyethylene. When the annealing temperature is within this range, the porosity can be improved by the growth of the crystal structure, and the porosity can be increased. The upper limit of the annealing temperature is preferably 150 °C or lower, more preferably 140 °C or lower, and particularly preferably 135 °C or lower. When the annealing temperature is within this range, the disorder of the crystal structure caused by the melting of polyethylene is suppressed, and the porosity is improved.

[0085] The lower limit of the annealing time is preferably 1 minute or longer, more preferably 15 minutes or longer, 20 minutes or longer, 25 minutes or longer, or 30 minutes or longer, and particularly preferably 60 minutes or longer. When the annealing time is within this range, the porosity can be improved by the growth of the crystal structure, and the porosity can be increased. The upper limit of the annealing time is preferably 600 minutes or shorter, more preferably 300 minutes or shorter. When the annealing time is within this range, the porosity is improved while maintaining a high production rate of the separator.

[0086] The annealing process may be performed while running the resin film obtained by co-extrusion film formation, or may be performed in a state where the resin film is wound into a roll.

[0087] The manufacturing method of the separator substrate may include a stretching process after the annealing process. As the stretching treatment, either uniaxial stretching or biaxial stretching can be used. From the viewpoints of manufacturing cost when using the dry method, reduction of TD heat shrinkage, etc., uniaxial stretching is preferable. From the viewpoint of improving the strength, etc. of the obtained separator substrate, biaxial stretching is preferable. Examples of biaxial stretching include methods such as simultaneous biaxial stretching, sequential biaxial stretching, multi-stage stretching, and multiple stretching.

[0088] Here, the manufacturing method of the separator for the power storage device includes the following steps (1) to (3): Step (1); the stretching ratio (cold stretching ratio) of the resin film in the first stretching step is 10% or more, Step (2); the stretching ratio (hot stretching ratio) of the resin film in the second stretching step is 160% or more, and Step (3); the annealing temperature in the annealing step is 125°C or more. It is preferably included.

[0089] The stretching process preferably includes a first stretching process (hereinafter, also referred to as "cold stretching") and a second stretching process (hereinafter, also referred to as "hot stretching") following this first stretching process.

[0090] In the first stretching step, by separating the lamellae generated in the resin film from each other, fine cracks are generated in the amorphous portion between the lamellae, and a large number of micropores are formed starting from these cracks. In the first stretching step, uniaxial stretching in the MD direction is performed.

[0091] In the first stretching step, the lower limit of the temperature of the resin film is preferably -20°C or more, more preferably 0°C or more. In the first stretching step, the upper limit of the temperature of the resin film is preferably 110°C or less, more preferably 80°C or less. When the temperature is above the above lower limit, breakage of the resin film during stretching is suppressed, and when the temperature is below the above upper limit, cracks are favorably generated in the amorphous portion between the lamellae, and neck-in of the film is suppressed.

[0092] The lower limit of the draw ratio of the resin film in the first stretching process (hereinafter also referred to as "cold draw ratio") is preferably 10% or more, or 20% or more, more preferably 25% or more, and still more preferably 30% or more. When the MFR of the microporous layer (A) mainly composed of polypropylene is as low as 0.90 g / 10 min or less, the molecular orientation of the microporous layer (B) mainly composed of polyethylene tends to be low. Therefore, in the amorphous part between the lamellae, it is difficult for the formation of micropores to proceed, and it is difficult to exhibit excellent permeability. This is a phenomenon peculiar to the dry method and the multilayer separator obtained by the coextrusion process, which has a multilayer structure of a microporous layer mainly composed of polypropylene and a microporous layer mainly composed of polyethylene. When the cold draw ratio is equal to or higher than the above lower limit, more pore initiation points are generated in the amorphous part between the lamellae, and it becomes easier to reduce the pore diameter. Therefore, excellent permeability, withstand voltage characteristics, and high MD tensile strength can be exhibited.

[0093] Also, the upper limit of the cold draw ratio is preferably 60% or less, more preferably 50% or less, and still more preferably 40% or less. When the cold draw ratio is equal to or lower than the above upper limit, micropores are not excessively formed and the pore diameter does not become too small, so the air permeability can be lowered. In the present disclosure, the cold draw ratio is defined by the following formula: Cold draw ratio (%) = { (length of the resin film after cold drawing / length of the resin film before cold drawing) - 1} × 100 as represented by.

[0094] The stretching speed of the resin film in the first stretching process is preferably 10% / min or more, more preferably 50% / min or more, and preferably 1000% / min or less, more preferably 600% / min or less. When the stretching speed is equal to or higher than the above lower limit, micropores are likely to be uniformly formed in the amorphous part between the lamellae, and when it is equal to or lower than the above upper limit, breakage of the resin film can be suppressed. In the present disclosure, the stretching speed of the resin film refers to the rate of change in the dimension in the stretching direction of the resin film per unit time.

[0095] As for the method of stretching the resin film in the above-described first stretching step, if the resin film can be uniaxially stretched, for example, a method of stretching the resin film at a predetermined temperature using a uniaxial stretching device can be mentioned.

[0096] Next, preferably, the resin film after uniaxial stretching in the first stretching step is stretched in a second stretching step such that the ambient temperature inside the apparatus is higher than the ambient temperature during uniaxial stretching in the first stretching step and lower than the melting point of the resin film (in the case of a multilayer structure, the resin film with the lowest melting point) by 1°C or more and 60°C or less (in the first aspect), or 1°C or more and less than 40°C (in the second aspect) (that is, a first stretching step as cold stretching and a second stretching step as hot stretching) are performed. Also in the second stretching step, the resin film is preferably uniaxially stretched only in the machine direction. In this way, by performing a stretching treatment on the resin film at an ambient temperature higher than the ambient temperature inside the apparatus in the first stretching step, a large number of micropores formed in the resin film in the first stretching step can be grown. When the temperature is equal to or higher than the above lower limit, the micropores formed in the resin film in the first stretching step are likely to grow, and the air permeability of the separator for the power storage device obtained can be lowered. When the temperature is equal to or lower than the above upper limit, the micropores formed in the resin film in the first stretching step are difficult to close, and the air permeability of the separator for the power storage device obtained can be lowered.

[0097] The lower limit of the draw ratio of the resin film in the second stretching process (hereinafter also referred to as "thermal draw ratio") is preferably 160% or more, more preferably 170% or more, and still more preferably 180% or more. When the MFR of the microporous layer (A) mainly composed of polypropylene is as low as 0.90 g / 10 min or less, the molecular orientation of the microporous layer (B) mainly composed of polyethylene decreases, resulting in a significant decrease in the porosity, and the porosity and air permeability of the microporous layer (B) and the separator substrate tend to decrease. This is a phenomenon peculiar to the dry method and the multilayer separator obtained by the coextrusion process, which has a multilayer structure of a microporous layer mainly composed of polypropylene and a microporous layer mainly composed of polyethylene. When the thermal draw ratio is equal to or higher than the above lower limit, the micropores formed in the resin film during cold stretching are likely to grow, and the air permeability of the resulting separator for a power storage device can be lowered. Also, the upper limit of the draw ratio of the resin film in the second stretching process is preferably 300% or less, more preferably 260% or less, and still more preferably 220% or less. When the thermal draw ratio is equal to or lower than the above upper limit, the micropores formed in the resin film during cold stretching are difficult to block, and the air permeability of the resulting separator for a power storage device can be lowered. In the present disclosure, the thermal draw ratio is represented by the following formula: Thermal draw ratio (%) = {((length of the resin film after thermal stretching) / (length of the resin film before thermal stretching)) - 1} × (cold draw ratio + 100) as represented by

[0098] In the second stretching process, from the viewpoint of uniformly expanding the micropores formed in the resin film in the first stretching process, the stretching speed of the resin film is preferably 60% / min or less, or 30% / min or less. From the viewpoint of process efficiency, the stretching speed may be, for example, 2% / min or more, or 3% / min or more.

[0099] As a method for stretching the resin film in the second stretching process, as long as the resin film can be uniaxially stretched, for example, a method of uniaxially stretching at a predetermined temperature using a uniaxial stretching device can be mentioned.

[0100] After the stretching process (preferably after uniaxially stretching in the second stretching process), a heat relaxation process for relaxing the residual stress by heating may be applied to the resin film. Residual stress may be generated in the resin film by the stretching in the second stretching process. The heat relaxation process is performed to relax the residual stress and suppress the heat shrinkage of the resulting resin microporous layer due to heating other than the heat relaxation process, thereby improving the safety of the separator for the power storage device obtained. The heat relaxation process can be performed using a tenter or a roll stretcher.

[0101] As described above, in order to improve the dimensional stability of the resin microporous layer during heating, it is necessary to relax the residual stress of the resin film. For this purpose, the ambient temperature in the apparatus in the heat relaxation process is preferably 40 °C or more lower than the melting point of the resin film (in the case of a multilayer structure, the resin film with the lowest melting point), or 20 °C or more lower. Also, from the viewpoint of suppressing the clogging of the micropores formed in the stretching process, the above temperature is preferably 1 °C or more, or 4 °C or more, lower than the melting point of the resin film (in the case of a multilayer structure, the resin film with the lowest melting point).

[0102] The lower limit of the heat shrinkage rate of the resin film in the heat relaxation process (hereinafter, also referred to as "heat relaxation ratio") is preferably 25% or more, more preferably 30% or more. When the heat relaxation ratio is at least the above lower limit, the residual stress of the resin film is sufficiently relaxed, the dimensional stability of the resulting resin microporous layer during heating is good, and the safety of power storage devices such as lithium-ion secondary batteries at high temperatures is good. Also, the upper limit of the heat relaxation ratio is preferably 80% or less, more preferably 60% or less, still more preferably 50% or less. When the heat relaxation ratio is at most the above upper limit, sagging hardly occurs in the resin film, and poor winding of the roll or deterioration of uniformity is suppressed. In the present disclosure, the heat relaxation ratio is represented by the following formula: Heat relaxation ratio (%) = { (length of the resin film after the heat relaxation process / length of the resin film before the heat relaxation process) - 1} × (cold stretching ratio + hot stretching ratio + 100) is represented by.

[0103] In the heat stretching and heat relaxation processes, from the viewpoint of sufficiently relaxing stress and reducing the heat relaxation ratio, it is preferable to adjust the stretching speed and the conveyance speed so that they do not become excessively large.

[0104] Further, after the heat relaxation process, it is preferable to apply heat relaxation again at the same temperature as or higher than the temperature of the heat relaxation process and at a temperature 20°C higher than or equal to the temperature of the heat relaxation process. By applying this process, it becomes possible to obtain a separator for a power storage device having better heat shrinkage physical properties.

[0105] By adopting the manufacturing conditions of the present invention described above as described above, when producing a multilayer separator including a PE layer by a coextrusion method, the problems caused by using a high MwPP resin, specifically, "the difference in melt viscosity between the PP layer and the PE layer becomes large, resulting in poor film formation of the PE layer and a significant decrease in porosity" can be solved, and it is possible to provide a separator that is a thin film and satisfies both air permeability and puncture strength at the same time.

[0106] The obtained separator substrate can be used as it is as a separator for a power storage device. Optionally, a further layer such as a coating layer may be provided on one or both sides of the separator substrate, and surface treatment such as corona treatment may be performed as necessary.

[0107] 《Power Storage Device》 The power storage device of the present disclosure includes the separator for a power storage device of the present disclosure. The power storage device of the present disclosure has a positive electrode and a negative electrode, and it is preferable that the separator for a power storage device of the present disclosure is disposed between the positive electrode and the negative electrode.

[0108] Examples of the power storage device include, for example, lithium secondary batteries (including all-solid-state lithium batteries, lithium-sulfur batteries, and lithium-air batteries), lithium-ion secondary batteries, sodium secondary batteries, sodium-ion secondary batteries, magnesium secondary batteries, magnesium-ion secondary batteries, calcium secondary batteries, calcium-ion secondary batteries, aluminum secondary batteries, aluminum-ion secondary batteries, nickel-metal hydride batteries, nickel-cadmium batteries, electric double-layer capacitors, lithium-ion capacitors, redox flow batteries, and zinc-air batteries. Among these, from the viewpoints of high energy density, low cost, and durability, lithium secondary batteries, lithium-ion secondary batteries, or lithium-ion capacitors are preferred, and more preferably lithium-ion secondary batteries.

[0109] For example, the power storage device can be manufactured by superimposing a positive electrode and a negative electrode via the separator described above, winding them if necessary to form a laminated electrode body or a wound electrode body, then loading this into an exterior body, connecting the positive and negative electrodes to the positive and negative electrode terminals of the exterior body via a lead body or the like, and further injecting a non-aqueous electrolyte solution containing a non-aqueous solvent such as a chain or cyclic carbonate and an electrolyte such as a lithium salt into the exterior body and then sealing the exterior body.

[0110] This power storage device is more preferably a lithium-ion secondary battery, and here, preferred embodiments of the lithium-ion secondary battery will be described.

[0111] The positive electrode may be any material that functions as the positive electrode of a lithium-ion secondary battery, and known materials can be used. The positive electrode preferably contains one or more selected from the group consisting of materials capable of occluding and releasing lithium ions as the positive electrode active material. As the positive electrode, from the viewpoints of battery capacity and safety, preferably, lithium cobalt oxide represented by LiCoO2, spinel-type lithium manganese oxide represented by Li2Mn2O4, Li2Mn 1.5 Ni 0.5Spinel-type lithium nickel manganese oxides represented by O4, lithium nickel oxides represented by LiNiO2, lithium-containing composite metal oxides represented by LiMO2 (where M represents two or more elements selected from the group consisting of Ni, Mn, Co, Al, and Mg), and lithium iron phosphate compounds represented by LiFePO4 can be mentioned. Among these, from the viewpoints of high safety and long-term stability, more preferably, lithium cobalt oxides represented by LiCoO2, lithium nickel oxides represented by LiNiO2, lithium-containing composite metal oxides represented by LiMO2 (where M represents two or more elements selected from the group consisting of Ni, Mn, Co, Al, and Mg), and lithium iron phosphate compounds represented by LiFePO4 can be mentioned, and particularly preferably, lithium iron phosphate compounds represented by LiFePO4.

[0112] The negative electrode may be any material that acts as the negative electrode of a lithium-ion secondary battery and may be a known one. It is preferable that the negative electrode contains one or more materials selected from the group consisting of a material capable of occluding and releasing lithium ions as a negative electrode active material and metallic lithium. That is, it is preferable that the negative electrode contains, as a negative electrode active material, one or more materials selected from the group consisting of metallic lithium, carbon materials, materials containing elements capable of forming alloys with lithium, and lithium-containing compounds. Such materials include, in addition to metallic lithium, for example, carbon materials represented by hard carbon, soft carbon, artificial graphite, natural graphite, graphite, pyrolytic carbon, coke, vitreous carbon, fired bodies of organic polymer compounds, mesocarbon microbeads, carbon fibers, activated carbon, carbon colloids, and carbon black.

Examples

[0113] 《Measurement Methods and Evaluation Methods》 Hereinafter, the measurement methods and evaluation methods adopted in this example will be described. In this example, since the "separator substrate" corresponds to the "separator", in the following description, the "separator substrate" may be read as the "separator".

[0114] [Melt Flow Rate (MFR; g / 10 min)] In accordance with JIS K 7210, and under the conditions of a temperature of 230°C and a load of 2.16 kg, the MFR of the microporous layer (A) was measured. In accordance with JIS K 7210, and under the conditions of a temperature of 190°C and a load of 2.16 kg, the MFR of the microporous layer (B) was measured. In accordance with JIS K 7210, and under the conditions of a temperature of 230°C and a load of 2.16 kg, the MFR of polypropylene was measured. In accordance with JIS K 7210, and under the conditions of a temperature of 190°C and a load of 2.16 kg, the MFR of polyethylene was measured.

[0115] [Weight-average molecular weight (Mw) and number-average molecular weight (Mn) by gel permeation chromatography (GPC)] Using Agilent PL-GPC220, standard polystyrene was measured under the following conditions, and a calibration curve was created. For each polymer as a sample, GPC was also used under the same conditions to create a calibration curve. Subsequently, based on the above calibration curve, for each polymer as a sample, each value in terms of polystyrene {weight-average molecular weight (Mw) and number-average molecular weight (Mn)} was calculated under the following conditions. Furthermore, the molecular weight distribution (MWD; Mw / Mn) was calculated by dividing the weight-average molecular weight (Mw) by the number-average molecular weight (Mn). (Measurement conditions) Column: Two TSKgel GMHHR-H(20) HT (7.8 mm I.D. × 30 cm) Mobile phase: 1,2,4-Trichlorobenzene Detector: RI Column temperature: 160°C Sample concentration: 1 mg / ml Calibration curve: Polystyrene

[0116] [Thickness (μm)] Using a Mitutoyo Digimatic Indicator IDC112, the thickness of the separator substrate was measured at room temperature of 23 ± 2°C. Also, the thickness of each microporous layer was calculated from the image data obtained by cross-sectional SEM using the evaluation method of the area-average pore diameter described below.

[0117] [Porosity (%)] Samples having a size of 10 cm × 10 cm square were cut out from the separator substrate and the microporous layer, respectively. Then, their volume (cm 3 ) and mass (g) were determined, and using these and the density (g / cm 3 ), the following formula: Porosity (%) = [{Volume - (Mass / Density)} / Volume] × 100 was used to calculate the porosity.

[0118] [Air permeability (sec / 100 cm 3 )] The air permeability of the separator substrate was measured using a Gurley-type air permeability meter conforming to JIS P-8117. Also, the measured air permeability was divided by the thickness (μm) of the separator substrate, and then multiplied by 12 μm to obtain the air permeability when the thickness of the separator substrate was converted to 12 μm.

[0119] [TD thermal shrinkage rate (%)] A sample was obtained by cutting out the separator substrate into a square shape with MD and TD each being 50 mm. The obtained sample was placed on copy paper and put into a hot air dryer (manufactured by Yamato Scientific Co., Ltd., DF1032). Then, heat treatment was performed at normal pressure and 150 °C in the air for 1 hour. After the heat treatment, the sample was taken out from the hot air dryer and allowed to cool at 25 °C for 10 minutes, and then the following formula: Thermal shrinkage rate (%) = [{Dimension before heating (mm) - Dimension after heating (mm)} / (Dimension before heating (mm)] × 100 was used to determine the dimensional shrinkage rate.

[0120] [Area-average pore diameter (nm)] The area-average pore diameter was measured by image analysis in cross-sectional SEM observation. As a pretreatment, the separator substrate was ruthenium-stained, and then a cross-sectional sample was prepared by freezing and cutting. The cross-section was treated as the "MD-ND plane". The above cross-sectional sample was fixed to a SEM sample stage for cross-section observation with a conductive adhesive (carbon-based) and dried. Thereafter, as a conductive treatment, osmium coating was performed using an osmium coater (HPC-30W, manufactured by Vacuum Devices Co., Ltd.) under the conditions of an applied voltage adjustment knob setting of 4.5 and a discharge time of 0.5 seconds, thereby preparing a microscopy sample.

[0121] Next, using a scanning electron microscope (S-4800 manufactured by Hitachi High-Technologies), for the cross-sections of each microporous layer (microporous layer (A) and microporous layer (B)) in the microscopy sample, eight arbitrary points were observed for each of the microporous layer (A) and the microporous layer (B) under the conditions of an acceleration voltage of 1 kV, a detection signal LA10, an operating distance of 5 mm, and a magnification of 30,000 times.

[0122] The obtained observation images were trimmed using functions of OpenCV, an image analysis library, in an environment of the programming language Python so that only the cross-section of one microporous layer was included in the observation region, and the surface, the outside, and other microporous layers were excluded from the observation region.

[0123] Thereafter, binarization processing was performed using the Otsu method, thereby separating the resin part and the pore part, and calculating the average major axis diameter of the pore part. At this time, among the pores existing across the shooting range and outside the shooting range, pores with a pore area included in the shooting range of 0.001 nm 2 The following pores were excluded from the measurement targets. Then, the average diameter was calculated by area averaging from the area of each pore. Here, the area-averaged major axis pore diameter in the microporous layer (A) was calculated by the above operation using the observation image obtained for the microporous layer (A), and the area-averaged major axis pore diameter in the microporous layer (B) was calculated by the above operation using the observation image obtained for the microporous layer (B).

[0124] [Average trunk height (nm)] The trunk height was measured by image analysis in cross-sectional SEM observation. Similar to when calculating the area-average major pore diameter, a cross-sectional sample was prepared and a microscopy sample was prepared. Then, for the cross-section of the microscopy sample, three arbitrary points were observed under the conditions of an acceleration voltage of 1 kV, a detection signal of LA10, an operating distance of 5 mm, and a magnification of 5000 times. As a result, an observation image was obtained.

[0125] The obtained observation image was trimmed in the environment of the programming language Python using functions of OpenCV, which is an image analysis library, so that only the cross-section of one microporous layer was included in the observation region, and the surface, the outside, and other microporous layers were removed from the observation region.

[0126] After that, binarization was performed using the Otsu method, and thereby, the resin part (trunk part + fibril part) and the pore part were separated. Then, the "blurring process" was repeatedly performed only in the ND direction (thickness direction), and thereby, the fibril part was removed (fibril removal process). At this time, the above fibril removal process was performed by repeating 100 times using a Gaussian filter within a bar-shaped processing range of 3 pixels in the ND direction and 1 pixel in the MD direction.

[0127] After the blurring process, opening and closing processes were sequentially performed within an elliptical processing range of a major diameter of 7 pixels in the ND direction and a minor diameter of 3 pixels in the MD direction to remove noise, and thereby, a fibril removal image was obtained.

[0128] Based on the above fibril removal image, the trunk height was calculated. That is, first, the fibril removal image was cut out by 1 pixel in the MD direction, and the lengths of the resin parts in the ND direction were all detected. Further, the above operation was repeatedly performed so as to include all of the MD direction, and for the fibril removal image, the lengths of the resin parts in the ND direction over the entire range were detected. For the obtained numerical values of the lengths of the resin parts in the ND direction, a weighted average with the length of the resin part in the ND direction as the weight was calculated, and the obtained value was treated as the average trunk height. When the length of the resin part in the ND direction is treated as L, the average trunk height H is given by the following formula:

Equation

[0129] [Breakdown voltage (kV)] A separator substrate sandwiched between aluminum foils was set on the sample stage of a breakdown voltage tester (grade) manufactured by Kikusui Electronics Industry Co., Ltd., and an electrode pressure piece was further placed on it. The voltage was increased at a starting voltage of 0 kV and a voltage increase rate of 0.025 kV / second, and the voltage value when a current of 0.2 mA flowed between the electrodes was treated as the breakdown voltage value. Also, the obtained breakdown voltage (kV) was divided by the thickness (μm) of the separator substrate and then multiplied by 12 μm to obtain the breakdown voltage (kV / 12 μm) when the thickness of the separator substrate was converted to 12 μm.

[0130] [MD tensile strength (kgf / cm 2 )] The tensile strength of the separator substrate was measured using a tensile tester (TG-1kN type manufactured by Minebea Co., Ltd.). The length of the sample (separator substrate) before the test was adjusted to 35 mm, and the sample was pulled at a speed of 100 mm / min. The value obtained by dividing the strength (tensile load value) when the sample yielded or, if it was cut (broken) before yielding, the strength (tensile load value) at the time of cutting by the cross-sectional area of the test piece was treated as the tensile strength (kgf / cm2). Here, the tensile strength was measured in the MD direction of the separator substrate.

[0131] 《Example 1》 [Production of microporous layer] As the resin of the microporous layer (A), a polypropylene resin {MFR (230 °C) = 0.90 g / 10 min, density = 0.91 g / cm 3} was prepared. 100% by mass of such resin was melted with a 2.5-inch extruder and supplied to both outer layers of a two-kind three-layer coextrusion inflation die at a discharge rate of 6 kg / h using a gear pump.

[0132] Also, as the resin of the microporous layer (B), a polyethylene resin {MFR (190 °C) = 0.32 g / 10 min, density = 0.96 g / cm 3}, and polypropylene resin {MFR (230 °C) = 0.90 g / 10 min, density = 0.91 g / cm 3} were prepared. 94.7% by mass of polyethylene resin, 4.0% by mass of polypropylene resin, ethylene-propylene block copolymer (the "compatibilizer" in the table) {MFR (230 °C) = 9.5 g / 10 min, density = 0.91 g / cm 3} 1.3% by mass, were melted in a 2.5-inch extruder and supplied to the inner layer of the above two-layer three-layer co-extrusion inflation die at a discharge rate of 3 kg / h using a gear pump.

[0133] As the temperature of the inflation die, 240 °C was used. After discharging the melted resin from the inflation die, the resin (discharged resin) was cooled by blowing air and wound up on a roll. As a result, a precursor film having a three-layer structure composed of an A / B / A layer, with a microporous layer (B) as the intermediate layer and a microporous layer (A) as the outermost layer, was obtained. The thickness of the precursor film was about 14 μm. Here, the distance between the lips (lip clearance) of the inflation die was set to 1.8 mm. Also, the melted resin was discharged at a total discharge rate of 9 kg / h.

[0134] The obtained precursor film was put into a dryer and annealed at 130 °C for 180 minutes. The annealed precursor film was cold-stretched at room temperature (for example, 25 °C) to a cold-stretching ratio of 30%, and the stretched film was put into an oven at 120 °C without shrinking. Then, heat stretching was performed to a heat-stretching ratio of 180%, and thereafter, heat relaxation was carried out to a heat relaxation ratio of 40% in an oven at 130 °C. As a result, a separator substrate having a three-layer structure composed of an A / B / A layer, with a microporous layer (B) as the intermediate layer and a microporous layer (A) as the outermost layer, was obtained. The structure and physical properties of the obtained separator substrate are shown in the following table.

[0135] <<Examples 2 to 10, Comparative Examples 1 to 5>> As shown in the following table, except for changing the raw materials, discharge amount, thickness of the precursor film, temperature in the annealing process, cold drawing ratio in the stretching process, and heat drawing ratio, etc., a microporous layer and a separator substrate were obtained in the same manner as in Example 1 and evaluated.

[0136] The abbreviations in the following table are as follows. MFR: Melt Flow Rate PP: Polypropylene PE: Polyethylene Mw: Weight-average molecular weight Mn: Number-average molecular weight MwD: Molecular weight distribution (Mw / Mn) C2C3 elastomer: A thermoplastic elastomer that is an ethylene / propylene copolymer CEBS: Olefin-(ethylene-butene)-styrene copolymer

[0137] In the following table, for the microporous layer (A), the "total thickness value" means the total thickness of each outermost microporous layer (A) arranged on both sides of the middle microporous layer (B). Note that the numerical values shown in the following table as the "total thickness value" may not simply match the value obtained by multiplying the "thickness of the microporous layer" by the "number of layers of the microporous layer" due to the relationship of the notation format after the decimal point.

[0138] Also, the polyethylene resin employed as the resin of the microporous layer (B) contains polypropylene in addition to polyethylene as the main component. The mass percentages of polyethylene and polypropylene in the microporous layer (B) are as shown in the following table. Note that the microporous layer (B) also contains the compatibilizer described in the following table together with polyethylene and polypropylene.

[0139]

Table 1

[0140]

Table 2

[0141] In the examples, the composition of each layer is strictly controlled, and the conditions of the annealing treatment and the stretching conditions are strictly controlled. From the above table, it can be seen that by such examples, even when the MFR of the microporous layer (A) mainly composed of polypropylene is low, it is possible to control the porosity of the separator substrate, and by adopting such a separator configuration, it is found that a thin film can be obtained, and excellent air permeability and excellent MD tensile strength can be achieved simultaneously.

Industrial Applicability

[0142] The separator for a power storage device of the present disclosure can be suitably used as a separator for a power storage device, such as a lithium ion secondary battery.

Claims

1. A separator for a power storage device, comprising a separator substrate having a multilayer structure of a microporous layer (A) mainly composed of polypropylene and a microporous layer (B) mainly composed of polyethylene and containing polypropylene, at least one layer of the microporous layer (A) constitutes the outermost layer on at least one side of the separator substrate, the microporous layer (B) has the polyethylene and the polypropylene contained therein in the same layer, the melt flow rate (MFR) of the microporous layer (B) measured at a load of 2.16 kg and a temperature of 190°C is 0.10 g / 10 min or more and 1.2 g / 10 min or less, and based on the total mass of the microporous layer (B), the polypropylene contained in the microporous layer (B) is 4.0% by mass or more and 12% by mass or less, a separator for a power storage device.

2. The separator for a power storage device according to claim 1, wherein the microporous layer (B) contains 1.0% by mass or more and 6.0% by mass or less of a thermoplastic elastomer.

3. The separator for a power storage device according to claim 1 or 2, wherein the ratio (PP / PE) of the melt flow rate (MFR) of polypropylene to polyethylene in the microporous layer (B) is 1.50 or more and 40.0 or less.

4. The separator for a power storage device according to claim 1 or 2, wherein the melt flow rate (MFR) of the polypropylene in the microporous layer (B) measured at a load of 2.16 kg and a temperature of 230°C is 0.20 g / 10 min or more and 11.0 g / 10 min or less.

5. The separator for a power storage device according to claim 1 or 2, wherein the melt flow rate (MFR) of the polyethylene in the microporous layer (B) measured at a load of 2.16 kg and a temperature of 190°C is 0.15 g / 10 min or more and 0.90 g / 10 min or less.

6. The separator for a power storage device according to claim 1 or 2, wherein the MFR of the microporous layer (A) measured at a load of 2.16 kg and a temperature of 230°C is 0.30 g / 10 min or more and 0.90 g / 10 min or more.

7. the thickness of the microporous layer (B) is 5.0 μm or less, the thickness of the separator substrate is 15.0 μm or less, and the porosity of the separator substrate is 40% or more and 60% or less, The separator for a power storage device according to claim 1 or 2. A power storage device comprising a positive electrode, a negative electrode, and the separator for a power storage device according to claim 1 or 2 disposed between the positive electrode and the negative electrode.

9. A method for manufacturing the separator for a power storage device according to claim 1 or 2, comprising the following steps (1) to (3): Steps (1) to (3) below: Step (1); The draw ratio (cold draw ratio in a predetermined temperature range) of the resin film in the first drawing step is 10% or more. Step (2); The draw ratio (hot draw ratio in a predetermined temperature range) of the resin film in the second drawing step is 160% or more, and Step (3); The annealing temperature in the annealing step is 125°C or more. A method for manufacturing a separator for a power storage device, including these steps.

10. The method for manufacturing the separator for a power storage device according to claim 9, wherein the microporous layer (A) and the microporous layer (B) are co-extruded.

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