Power storage device separator and power storage device

Abstract

To provide a power storage device separator having a good penetrability, a voltage resistance characteristic, and a tension strength in a thin film.SOLUTION: A power storage device separator includes a multi-layer structure of a fine porous layer (A) in which polypropylene is a main component, and a fine porous layer (B) containing polyethylene as the main component, and containing the polypropylene. At least one of the fine porous layers (A) constructs the outermost layer of at least one surface of a separator base material, and the fine porous layer (B) contains the polyethylene and the polypropylene contained in the fine porous layer (B) at the same layer. An area average long hole diameter calculated by an analysis of a SEM image of a ND-MD cross section in the fine porous layer (B) is 300nm or more and 600nm or less.SELECTED DRAWING: None

Description

Technical Field

[0001] The present disclosure relates to a separator for 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, and accordingly, the demand for separators that are thin films and excellent in battery performance, battery reliability, and safety has been increasing.

[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, and (ii) fibrils extending in the machine direction (MD) of the microporous membrane from the polymer matrix and containing the (A) polypropylene resin, and (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, for example, Patent Document 2 describes (a) polyethylene, and (b)(i) a weight average molecular weight of 6×105 As described above, (ii) polypropylene having a heat of fusion of 90 J / g or more, and containing, based on the total mass of the polypropylene, at least 10% by mass of the polypropylene having a molecular weight of 1.8×10 6 or more, a polyolefin microporous 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 manufacturing a separator for a power storage device by laminating a plurality of microporous layers, it has been difficult to thin such a separator. Here, regarding the manufacturing process of the separator, a coextrusion method capable of forming each microporous layer simultaneously has attracted attention.

[0008] In a separator for a power storage device manufactured using the coextrusion method, when simultaneously forming a heterogeneous polyolefin multilayer separator having a microporous layer mainly composed of polypropylene and a microporous layer mainly composed of polyethylene, there is a possibility that it becomes difficult to control the temperature suitable for each layer, particularly in the microporous layer mainly composed of polyethylene, there is a possibility that large pores are formed. In addition to the formation of large pores in the microporous layer mainly composed of polyethylene, when the separator is thinned, the breakdown voltage characteristics and physical strength may decrease. For this reason, it is often difficult to improve the breakdown voltage and tensile strength while maintaining excellent permeability.

[0009] Accordingly, an object of the present disclosure is to provide a separator for a power storage device that is a thin film and has excellent permeability, excellent withstand voltage characteristics, and excellent tensile strength.

Means for Solving the Problems

[0010] Examples of embodiments of the present disclosure are listed in the following items. [1] A separator for a power storage device 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) contains polyethylene and polypropylene contained therein in the same layer, The area average pore diameter calculated from the analysis of the SEM image of the ND-MD cross section in the microporous layer (B) is 300 nm or more and 600 nm or less, Separator for a power storage device. [2] The separator for a power storage device according to item 1, wherein the length average trunk height calculated from the analysis of the SEM image of the ND-MD cross section in the microporous layer (B) is 700 nm or more and 800 nm or less. [3] The separator for a power storage device according to item 1 or 2, wherein the total thickness of the separator is 4.5 μm or more and 12.0 μm or less, and the thickness of the microporous layer (B) is 1.5 μm or more and 4.0 μm or less. [4] The separator for a power storage device according to any one of items 1 to 3, wherein the porosity of the separator is 40% or more and 60% 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 the microporous layer (A) measured at a load of 2.16 kg and a temperature of 230 ° C is 0.20 g / 10 min or more and 0.90 g / 10 min or less. [6] The melt flow rate (MFR) of the microporous layer (B) when measured at a load of 2.16 kg and a temperature of 190°C is 0.20 g / 10 min or more and 0.60 g / 10 min or less. The separator for a power storage device according to any one of Items 1 to 5. [7] The melt tension Mt at 240°C in the microporous layer (A) A is 10.0 mN or more and 40.0 mN or less. The separator for a power storage device according to any one of Items 1 to 6. [8] The melt tension Mt at 240°C in the microporous layer (B) B is 15.0 mN or more and 30.0 mN or less. The separator for a power storage device according to any one of Items 1 to 7. [9] The heat shrinkage rate in the width direction (TD) of the separator base material at 105°C for 1 hour is -2.0% or more and 3.0% or less. The separator for a power storage device according to any one of Items 1 to 8.

[10] The microporous layer (B) contains 1.0 mass% or more and 6.0 mass% or less of a thermoplastic elastomer. The separator for a power storage device according to any one of Items 1 to 9.

[11] 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 9 disposed between the positive electrode and the negative electrode.

Advantages of the Invention

[0011] According to the present disclosure, it is possible to provide a separator for a power storage device that is a thin film and has excellent permeability, excellent withstand voltage characteristics, and excellent tensile strength.

Embodiments for Carrying Out the Invention

[0012] In this specification, various measurements are performed based on the methods described in the examples unless otherwise specified. In this specification, the upper limit or lower limit in a numerical range described step by step may be replaced by the upper limit or lower limit in another corresponding numerically described step range, and further, may be replaced by the corresponding value described in the examples. In this specification, for a "step", this term 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.

[0013] 《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.

[0014] 〈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 may have 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.

[0015] <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.

[0016] 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 is not limited, but 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 as a mixture of two or more kinds.

[0017] From the viewpoints of the strength of the microporous layer and excellent withstand voltage characteristics, the weight average molecular weight (Mw) of the polypropylene of the microporous layer (A) is preferably 300,000 or more, and from the viewpoints of ensuring good film formability 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.

[0018] 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 polypropylene increases, the melt tension of the resulting microporous layer also tends to increase, and it is preferable to increase the melt tension of the microporous layer (A) even in enhancing the strength of the microporous layer (A). Therefore, the value of Mw / Mn of polypropylene being 3.0 or more is preferable for controlling the melt tension of the microporous layer (A) to be high. 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.

[0019] 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.

[0020] 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, but are not limited to, 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).

[0021] 〈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 viewpoints of obtaining a microporous layer (A) with higher strength and excellent withstand voltage characteristics. The lower limit of the MFR (single-layer MFR) of the microporous layer (A) is not limited from the viewpoints of the moldability and thin film forming property of the microporous layer (A), and 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. 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.

[0022] 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 bind crystalline substances to each other increases, so that a microporous layer (A) with high strength tends to be obtained, and a separator substrate with high strength tends to be obtained.

[0023] 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.

[0024] 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 0.90 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 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 polypropylene is not limited, but 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).

[0025] 〈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 polypropylene may be 99.9% or less, 99.8% or less, or 99.5% or less. The pentad fraction of polypropylene is 13 measured by C-NMR (nuclear magnetic resonance method).

[0026] A polypropylene pentad fraction of 94.0% or more indicates high crystallinity of the polypropylene. Since the separator obtained by the stretching and pore-forming method, particularly the dry method, forms pores by stretching the amorphous portion between the crystalline substances, when the polypropylene has high crystallinity, the pore-forming property becomes good and the air permeability can be kept low, enabling high output of the battery.

[0027] 〈Melting tension of the microporous layer (A)〉 The melting tension Mt of the microporous layer (A) at 240 °C A is preferably 10.0 mN or more and 40.0 mN or less. The melting tension Mt A Regarding the lower limit, from the viewpoints of achieving a sufficiently fine pore structure, small pore diameter, improving strength, and exhibiting excellent breakdown voltage characteristics, it is preferably 10.0 mN or more, more preferably 13.0 mN or more, still more preferably 16.0 mN or more, and particularly preferably 20.0 mN or more. The melting tension Mt A Regarding the upper limit, from the viewpoints of good film-forming property and productivity, it is preferably 40.0 mN or less, more preferably 37.0 mN or less, and still more preferably 34.0 mN or less.

[0028] 〈Average area equivalent 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-forming 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. The "major pore diameter" means the pore diameter in the MD. When there are two or more microporous layers (A) and / or 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, most preferably 130 nm or more, from the viewpoint of maintaining excellent air permeability and ensuring good output in the power storage device. 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, most preferably 150 nm or less, from the viewpoint of obtaining a microporous layer (A) with higher strength and excellent withstand voltage characteristics. Note that the "MD-ND cross-section" refers to the cross-section that appears when the microporous layer or the separator substrate is cut by a cut surface formed by the MD direction and the ND direction.

[0029] In a multilayer separator obtained by a dry process 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 by reducing the area-average major pore diameter of the microporous layer (A), the microporous layer (A) and the separator substrate are strengthened, and excellent withstand voltage characteristics are exhibited. The mechanism is presumed to be as follows: By reducing the area-average major pore diameter of the microporous layer (A), the number of pores per unit structure increases. Since each pore is formed by tie molecules that bond crystalline substances together, the number of tie molecules per unit structure increases. Therefore, during the MD tensile test, the tie molecules contribute to improving the stress, and there is a tendency to obtain a microporous layer (A) and a separator substrate with high MD tensile strength. Also, by reducing the area-average major pore diameter of the microporous layer (A), the tortuosity increases, and there is a tendency to obtain a microporous layer (A) having excellent withstand voltage characteristics and a separator substrate.

[0030] The area-average major pore diameter can be measured by image analysis from the obtained image by performing 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 diameter from the cross-sectional SEM image, the number-average pore diameter and the area-average pore diameter can be calculated, but it is preferable to use the area-average pore diameter as the average pore diameter so that the correlation with the physical properties of the separator is better.

[0031] 〈Porosity of the microporous layer (A)〉 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 from the viewpoints of avoiding clogging in the power storage device and obtaining good air permeability of the separator. Also, the porosity of the microporous layer (A) is preferably 70% or less, more preferably 65% or less, and still more preferably 60% or less from the viewpoints of maintaining the strength of the separator and excellent withstand voltage characteristics.

[0032] 〈Compatibility between the MFR of the microporous layer (A) and the porosity of the separator substrate〉 Among the separators for energy storage devices of this 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 a porosity of the separator substrate of 40% or more. Conventionally, in a dry process and a multilayer separator obtained by 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 substrate to 40% or more using the 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 form 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 porosity due to reduced molecular orientation, and the porosity of the microporous layer (B) and the separator substrate tends to decrease.

[0033] Furthermore, 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. It was even more difficult to achieve a high porosity while using a high-molecular-weight polyolefin in such a thin film. In this embodiment, although not limited thereto, by applying precisely controlled annealing and stretching conditions as exemplified in the section of 《Method for Manufacturing Separator for Energy 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.

[0034] 〈Thickness of Microporous Layer (A)〉 From the perspective of increasing the energy density of the energy storage device, etc., 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 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 perspectives of strength and excellent breakdown voltage characteristics. From the perspective of thinning, it is easier to obtain a thinner microporous layer by the coextrusion process than by the lamination process. 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) arranged on both sides of the microporous layer (B), the preferable range is treated as the range obtained by multiplying the above "thickness of the microporous layer (A)" by the total number of layers of the microporous layer (A).

[0035] 〈Additives in the microporous layer (A)〉 The microporous layer (A) mainly composed of polypropylene may further contain additives such as elastomers, crystal nucleating agents, antioxidants, and fillers 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).

[0036] 〈Microporous layer (B)〉 The separator for a power storage device of the present disclosure has a microporous layer (B). The separator for a power storage device may have only one layer or two or more layers of the microporous layer (B). The microporous 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 microporous layer (B). The lower limit of the content of polyethylene in the microporous 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 film, etc. The upper limit of the content of polyethylene in the microporous layer (B) is not limited, and 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, or may be 100% by mass.

[0037] 〈Material of the microporous layer (B)〉 The microporous layer (B) is mainly composed of polyethylene. The polyethylene of the microporous layer (B) is preferably a homopolymer, and may be a copolymer obtained by copolymerizing a small amount of comonomer other than ethylene, for example, an α-olefin comonomer, for example, a block polymer. The amount of the ethylene structure contained as a repeating unit in the polyethylene is not limited, and 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 the polyethylene is not limited, and 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 polyethylene can be used alone or in combination of two or more.

[0038] The density of the polyethylene of 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 3It may be as described above. 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.

[0039] 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 highly oriented crystallization, and is preferably 700,000 or less from the viewpoint of improving the thin film formability 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.

[0040] 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. 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 polyethylene increases, the melt tension of the resulting 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 the polyethylene is 3.0 or more in order to highly control the melt tension of the microporous layer (B). 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.

[0041] In addition to polyethylene, the microporous layer (B) may contain other resins. Examples of other resins include polyolefins other than polyethylene (also referred to as "other polyolefins"). A polyolefin is a polymer containing a monomer having a carbon-carbon double bond as a repeating unit. Examples of monomers constituting polyolefins other than polyethylene include, but are not limited to, 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.

[0042] 〈MFR of Microporous Layer (B)〉 The upper limit of the MFR (single-layer MFR) of the microporous layer (B) is preferably 0.60 g / 10 min or less, more preferably 0.50 g / 10 min or less, and still more preferably 0.40 g / 10 min or less, from the viewpoints of obtaining a microporous layer (B) with higher strength and excellent withstand voltage characteristics. The lower limit of the MFR (single-layer MFR) of the microporous layer (B) is preferably 0.20 g / 10 min or more, more preferably 0.25 g / 10 min or more, and still more preferably 0.30 g / 10 min or more, from the viewpoints of good pore opening property and clogging suppression. 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.

[0043] 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 0.80 g / 10 min or less, and particularly preferably 0.50 g / 10 min or less, from the viewpoints of obtaining a microporous layer (B) with higher strength and excellent withstand voltage characteristics. 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 suppression. 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.

[0044] <Melting Tension of the Microporous Layer (B)> The melting tension Mt at 240°C of the microporous layer (B) B is preferably 15.0 mN or more and 30.0 mN or less. The melting tension Mt B The lower limit of is preferably 15.0 mN or more, more preferably 16.0 mN or more, still more preferably 17.0 mN or more, and particularly preferably 18.0 mN or more, from the viewpoints of achieving a sufficiently fine pore structure and small pore diameter, improving strength, and exhibiting excellent withstand voltage characteristics. The melting tension Mt A The upper limit of is preferably 30.0 mN or less, more preferably 27.0 mN or less, and still more preferably 24.0 mN or less, from the viewpoints of good film-forming property and productivity.

[0045] 〈Average pore length diameter of the microporous layer (B)〉 The average pore length diameter of the microporous layer (B) in the MD-ND cross-section is preferably 300 nm or more and 600 nm or less, more preferably 350 nm or more and 550 nm or less, and still more preferably 400 nm or more and 500 nm or less. When the average pore length diameter of the microporous layer (B) is within this range, excellent permeability can be maintained while exhibiting withstand voltage characteristics and high MD tensile strength.

[0046] 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 by reducing the average pore length diameter of the microporous layer (B), the microporous layer (B) and the separator substrate are strengthened, and excellent withstand voltage characteristics are exhibited. The mechanism is presumed to be as follows: By reducing the average pore length diameter of the microporous layer (B), the number of pores per unit structure increases. Since each pore is formed by tie molecules that bond crystalline substances together, the number of tie molecules per unit structure increases. Therefore, during the MD tensile test, the tie molecules contribute to improving the stress, and there is a tendency to obtain a microporous layer (B) with high MD tensile strength and a separator substrate. Also, by reducing the average pore length diameter of the microporous layer (B), the tortuosity increases, and there is a tendency to obtain a microporous layer (B) with excellent withstand voltage characteristics and a separator substrate.

[0047] 〈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 viewpoints of maintaining the strength of the separator and excellent withstand voltage characteristics. 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.

[0048] 〈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. The lower limit 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 viewpoints of strength and excellent withstand voltage characteristics. From the viewpoint of thinning, a coextrusion process is more likely to obtain a thinner microporous layer than a lamination process. By extruding the microporous layer (B) into a thin film by the coextrusion method, it can be efficiently quenched to achieve high orientation and easily reduce the pore diameter. Therefore, it is possible to exhibit excellent withstand voltage characteristics and high MD tensile strength. Note that the "thickness of the microporous layer (B)" mentioned 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).

[0049] 〈Additives in 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 content is 4.0% by mass or more, "twisted lamellae" that are likely to break the crystals of polyethylene are formed, resulting in an increase in the amount of cracks during cold stretching and a tendency to have a smaller pore diameter. The pore diameter of the microporous layer (B) can be controlled by the addition amount of the polyethylene added. Specifically, as the addition amount of the polyethylene added increases, the proportion of "twisted lamellae", that is, the amount of cracks during cold stretching increases, and it becomes easier to have a smaller pore diameter. As a result of forming a smaller pore diameter, in addition to improving the withstand voltage characteristics of the microporous layer (B), the amount of tie molecules connecting between the lamellae increases, and the MD tensile strength tends to improve. Also, due to the domain of polypropylene extending in the MD direction in the polyethylene, it contributes to stress improvement, and there is a tendency to obtain a microporous layer (B) and a separator substrate with high MD tensile strength. When the polypropylene content is 12.0% by mass or less, since the amount of the component to be melted decreases during the solidification of polyethylene during film formation, the crystal orientation of polyethylene progresses, and it is easy to obtain a lower air permeability.

[0050] 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.

[0051] The polypropylene of 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, for example, a block polymer. The amount of the propylene structure contained as a repeating unit in the polypropylene is not limited, but 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 is not limited, but 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.

[0052] From the viewpoints of the strength of the microporous layer and excellent withstand voltage characteristics, the weight average molecular weight (Mw) of the polypropylene of the microporous layer (B) is preferably 300,000 or more, and from the viewpoints of ensuring good film-forming properties and productivity, it is preferably 1,300,000 or less. The Mw of the polypropylene is more preferably 400,000 or more and 1,200,000 or less, still more preferably 500,000 or more and 1,100,000 or less, even more preferably 600,000 or more and 1,000,000 or less, and particularly preferably 650,000 or more and 950,000 or less.

[0053] 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 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 or more. As the value of Mw / Mn of polypropylene increases, the melt tension of the resulting microporous layer also tends to increase, and it is preferable to increase the melt tension of the microporous layer (B) even in enhancing the strength of the microporous layer (B). Therefore, it is preferable that the value of Mw / Mn of polypropylene is 3 or more in order to highly control the melt tension of the microporous layer (B). 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.

[0054] 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 / 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.

[0055] The MFR of the polypropylene in the microporous layer (B) is preferably 0.20 g / 10 min or more and 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 is not limited, but 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).

[0056] 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 is not limited, but 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).

[0057] In addition to polyethylene and polypropylene, the microporous layer (B) may further contain a thermoplastic elastomer for the purpose of compatibilization to improve dispersibility, if necessary. The amount of the thermoplastic elastomer may be, for example, 1.0% by mass or more, 6.0% by mass or less, or 2.0% by mass or more, 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 formation, and low air permeability is easily obtained. When the thermoplastic elastomer is 6.0% by mass or less, the proportion of highly crystalline polyethylene and polypropylene increases, so lamellar pore formation is promoted and low air permeability is easily obtained.

[0058] 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 preferable. 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 preferable, 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 preferable. Here, since the ethylene-butene structure is structurally similar to polypropylene, it shows a high affinity for polypropylene. The thermoplastic elastomer can be used alone or in combination of two or more.

[0059] Here, the microporous layer (B) has polyethylene and polypropylene contained therein in the same layer. "Having in the same layer" means that 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 layers of the microporous layer (B) has both polyethylene and polypropylene, which means. Therefore, for example, in the case of a separator substrate having a three-layer structure composed of an A / B / A layer, the middle 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 substrate of the separator for the power storage device (also simply referred to as "separator substrate" in the present specification) has at least one layer each of the microporous layer (A) and the microporous layer (B). The separator substrate 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 the microporous layer (A) / the microporous layer (B), a three-layer structure of the microporous layer (A) / the microporous layer (B) / the microporous layer (A), etc. can be mentioned. Further, the separator substrate may have a layer other than the microporous layer (A) and the microporous layer (B). For example, examples of the layer other than the microporous layer (A) and the microporous layer (B) include a microporous layer mainly composed of a polyolefin other than (A)(B), a layer containing an inorganic substance, and a layer containing a heat-resistant resin. For example, the separator substrate may have a multilayer structure of four or more layers such as the microporous layer (A) / the microporous layer (B) / the microporous layer (C) / the microporous layer (A). From the viewpoints of ease of manufacture, curl suppression of the separator, etc., 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 0.5 to 1.0 from the viewpoints of thinning the separator substrate 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 substrate of the separator for the power storage device, it is preferable that the microporous layer (A) constitutes the outermost layer on at least one side of the separator substrate, and more preferably constitutes the outermost layers on both sides of the separator substrate. When the microporous layer (A) constitutes the outermost layer, the strength tends to be easily improved.

[0063] 〈Area average major axis to minor axis ratio (B layer / A layer) of the separator substrate〉 The area average major axis to minor axis ratio (B layer / A layer) in the MD-ND cross-section of the separator for the power storage device is preferably 2.0 or more and 4.0 or less, more preferably 2.2 or more and 3.8 or less, still more preferably 2.4 or more and 3.6 or less, and even more preferably 2.6 or more and 3.8 or less. When the area average major axis to minor axis ratio (B layer / A layer) is within this range, excellent permeability can be maintained while exhibiting withstand voltage characteristics and high MD tensile strength.

[0064] 〈Trunk height of the separator substrate〉 The trunk 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. When the trunk height increases, the tortuosity of the pores decreases, and when the trunk 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 trunk height is preferably 890 nm or less, more preferably 880 nm or less, still more preferably 860 nm or less, 840 nm or less from the viewpoints of high strength and withstand voltage characteristics. The lower limit of the trunk height is preferably 660 nm or more, more preferably 670 nm or more, still more preferably 680 nm or more, 690 nm or more, 700 nm or more from the viewpoint of permeability.

[0065] The stem height can be calculated by observing the cross-section of the separator substrate in the MD-ND plane using SEM, removing fibrils by image analysis, and measuring the length in the ND direction of the remaining lamellae by image analysis. The detailed conditions are shown in the examples. When measuring the stem height from the cross-sectional SEM image, the number-average stem height and the length-average stem height can be calculated. In this specification, the length-average stem height is used as the stem height to better correlate with the physical properties of the separator.

[0066] 〈Thickness of the separator substrate〉 From the perspective of increasing the energy density of the energy storage device, the upper limit of the thickness of the separator substrate is 15.0 μm or less, 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 perspective of strength and the like, the lower limit of the thickness of the separator substrate is preferably 5.0 μm or more, for example, it may be 6.0 μm or more, 7.0 μm or more, 8.0 μm or more, or 9.0 μm or more. From the perspective of manufacturing cost and thinning, the coextrusion process can obtain a separator substrate with a thinner film at a lower cost than the lamination process.

[0067] 〈Air permeability of the separator substrate〉 When the thickness of the separator substrate is converted to 12 μm, the upper limit of the air permeability of the separator substrate 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 3 or less, 150 sec / 100 cm 3 or less, or 140 sec / 100 cm 3 or less. The lower limit 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, the air permeability of the separator base material (air permeability in terms of 12-μm film thickness) when the thickness of the separator base material is converted to 12 μm is calculated using the following formula: (Air permeability in terms of 12-μm film thickness [sec / 100 cm 3 ) = {(Air permeability of the separator base material [sec / 100 cm 3 ) / (Thickness of the separator base material [μm])} × (12 [μm]) is used for the calculation.

[0068] 〈Porosity of the separator base material〉 The lower limit of the porosity of the separator base material 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. Also, the upper limit of the porosity of the separator base material is preferably 70% or less, more preferably 65% or less, still more preferably 60% or less, and particularly preferably 55% or less, from the viewpoints of the strength of the separator and the withstand voltage characteristics.

[0069] 〈Puncture strength of the separator base material〉 The lower limit of the puncture strength of the separator base material is preferably 145 gf or more (about 1.42 N or more), more preferably 150 gf or more, still more preferably 160 gf or more, and particularly preferably 170 gf or more, when the thickness of the separator base material is converted to 12 μm, from the viewpoint of suppressing short circuits during physical damage of the battery. The upper limit of the puncture strength of the separator base material is not limited, but when the thickness of the separator base material is converted to 12 μm, it may be preferably 500 gf or less, for example, 480 gf or less, or 450 gf or less. In addition, the puncture strength of the separator base material (puncture strength in terms of 12-μm film thickness) when the thickness of the separator base material is converted to 12 μm is calculated using the following formula: (Puncture strength in terms of 12-μm film thickness [gf]) = {(Puncture strength of the separator base material [gf]) / (Thickness of the separator base material [μm])} × (12 [μm]) is used for the calculation.

[0070] 〈Thermal shrinkage rate of the separator base material〉 The separator base material preferably has a heat shrinkage rate in the transverse direction (TD) of -2.0% or more and 3.0% or less after heat treatment at 105°C for 1 hour. That is, it means that the separator base material has very small heat shrinkage in the TD direction even at high temperatures. By having the heat shrinkage rate of 3.0% or less, short circuits at high temperatures can be effectively suppressed. The reason for the heat shrinkage rate being -2.0% or more is that when measuring the heat shrinkage rate, the base material may expand in the TD direction, and the heat shrinkage rate may become smaller than 0% and be a negative value. The heat shrinkage rate may be 0% or more, or may be greater than 0%. As a method for manufacturing a separator base material having a heat shrinkage rate of -2.0% or more and 3.0% or less, for example, a method for manufacturing a separator by uniaxial stretching in the machine direction (MD) can be mentioned, and preferably a method of manufacturing by the dry method of uniaxial stretching can be mentioned. In the method for manufacturing a separator by biaxial stretching in the MD and TD directions, typified by a wet separator, generally, the heat shrinkage in the TD direction becomes very large, whereas in a dry uniaxial stretched separator, it is easy to obtain a separator base material having a heat shrinkage rate of -2.0% or more and 3.0% or less.

[0071] 〈Tensile Strength of Separator Base Material〉 From the viewpoints of operability during battery winding and high puncture strength, the tensile strength of the separator base material 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, most preferably 1900 kgf / cm 2 or more. The upper limit value of the tensile strength of the separator base material in the MD is not limited, but is preferably 4000 kgf / cm 2 or less, for example 3800 kgf / cm 2 or less, 3500 kgf / cm 2 or less, 3200 kgf / cm 2 or less, or 3000 kgf / cm 2 or less.

[0072] 〈Dielectric Withstanding Voltage of Separator Base Material〉 From the perspective of suppressing short circuits in energy storage devices, the withstand voltage of the separator substrate 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, most preferably 1.05 kV or more when the thickness of the separator substrate is converted to 12 μm.

[0073] 《Method for Manufacturing Separator for Energy Storage Device》 The method for manufacturing a separator for an energy 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 perforating and making porous 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.

[0074] Examples of the dry method include a method of melt-kneading and extruding a polyolefin resin composition and then peeling the polyolefin crystal interface by heat treatment and stretching, and a method of melt-kneading a polyolefin resin composition and an inorganic filler and molding them into a film shape and then peeling the interface between the polyolefin and the inorganic filler by stretching.

[0075] Examples of the wet method include a method of melt-kneading a polyolefin resin composition and a pore-forming material, molding them into a film shape, stretching them as necessary, and then extracting the pore-forming material, and a method of dissolving a polyolefin resin composition and immersing it in a poor solvent for polyolefin to solidify the polyolefin and simultaneously remove the solvent.

[0076] For the melt-kneading of the polyolefin resin composition, a single-screw extruder and a twin-screw extruder can be used, and in addition to these, for example, a kneader, a labo plastomill, a kneading roll, and a Banbury mixer can also be used.

[0077] The polyolefin resin composition may optionally contain resins other than polypropylene, resins other than polyethylene, additives, etc., depending on the method for manufacturing the microporous layer or the physical properties of the target microporous layer. Examples of the additives include pore formers, fluorine-based flow modifiers, waxes, crystal nucleating agents, antioxidants, metal soaps such as aliphatic carboxylic acid metal salts, ultraviolet absorbers, light stabilizers, antistatic agents, antifogging agents, and coloring pigments. Examples of the pore formers include plasticizers, inorganic fillers, or combinations thereof.

[0078] Examples of the plasticizers 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.

[0079] Examples of the inorganic fillers 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 fibers.

[0080] As a method for manufacturing the separator substrate, a dry lamellar crystal opening process for peeling the polyolefin crystal interface by heat treatment and stretching is preferred. Here, as methods 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, in which the microporous layer (A) and the microporous layer (B) are coextruded into a film and subjected to an annealing, cold stretching, hot stretching, and heat relaxation process; and (ii) A method for manufacturing a separator substrate by lamination, which comprises separately extruding and forming the microporous layer (A) and the microporous layer (B), laminating them together, and then subjecting them to annealing, cold stretching, hot stretching, and heat relaxation processes.

[0081] Among the above co-extrusion process (i) and lamination process (ii), from the viewpoints of thin film formability and manufacturing cost, etc., the co-extrusion 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. However, for example, it is difficult to stably produce a resin film with a thickness of 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 4.3 μm, or 3 μm, or 1.9 μm. Also, since the lamination process has a process of separately extruding and forming each microporous layer and a process of laminating and bonding them together, compared with the co-extrusion process, the number of processes is larger and the manufacturing cost increases.

[0082] 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 cold air controlled to such a low temperature, the resin after film formation is uniformly oriented in the MD direction.

[0083] Regarding the method of 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 locations were observed in 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.

[0084] The method for manufacturing a separator substrate 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) tend to grow and the porosity tends to improve. By applying annealing at a specific temperature for a predetermined time, it is possible to obtain a good area-average pore diameter for both the microporous layers (A) and (B). The reason is considered to be that the crystals grow without the crystal structure being disrupted, and high porosity is obtained.

[0085] 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 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 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.

[0086] The lower limit of the annealing time is preferably 10 minutes 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.

[0087] The annealing process may be performed while running the resin film obtained by coextrusion film formation, or may be performed in a state where the resin film is wound into a roll.

[0088] The method for manufacturing the separator base material 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 thermal shrinkage of TD, etc., uniaxial stretching is preferable. From the viewpoint of improving the strength etc. of the obtained separator base material, biaxial stretching is preferable. Examples of the biaxial stretching include methods such as simultaneous biaxial stretching, sequential biaxial stretching, multi-stage stretching, and multiple stretching.

[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 the first stretching process.

[0090] In the first stretching process, by separating the lamellae formed 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 process, uniaxial stretching in the MD direction is performed.

[0091] In the first stretching process, the lower limit of the temperature of the resin film is preferably -20°C or higher, more preferably 0°C or higher. In the first stretching process, the upper limit of the temperature of the resin film is preferably 110°C or lower, more preferably 80°C or lower. 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 the "cold draw ratio") is preferably 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, the formation of micropores hardly progresses, and it is difficult to exhibit excellent permeability. This is a phenomenon peculiar to a dry process and a multilayer separator obtained by a coextrusion process having 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 the pore diameter is easily reduced. Therefore, excellent permeability, withstand voltage characteristics, and high MD tensile strength can be exhibited. 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 represented by the following formula: Cold draw ratio (%) = { (length of resin film after cold drawing / length of resin film before cold drawing) - 1} × 100 as represented by.

[0093] 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 easily formed uniformly 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.

[0094] The stretching method of the resin film in the above-mentioned first stretching step is not particularly limited as long as the resin film can be uniaxially stretched. For example, there is a method of stretching the resin film at a predetermined temperature using a uniaxial stretching device.

[0095] Next, preferably, the atmosphere temperature inside the apparatus is higher than the atmosphere temperature during the uniaxial stretching in the first stretching step and lower than the melting point of the resin film (the resin film with the lowest melting point in the case of a multilayer structure) 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), and a second stretching step of stretching is performed on the resin film after uniaxial stretching in the first stretching step (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. Thus, by performing a stretching treatment on the resin film at an atmosphere temperature higher than the atmosphere 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 above 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 below 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.

[0096] 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 process 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 obtained 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 close, and the air permeability of the obtained 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

[0097] In the second stretching process, from the viewpoint of the MD tensile strength, the stretching temperature of the resin film is preferably 135°C or lower, or 130°C or lower. From the viewpoint of uniformly expanding the micropores formed in the first stretching process, the stretching temperature is preferably 110°C or higher, or 115°C or higher.

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

[0099] As a method for stretching a resin film in the second stretching step, it is not particularly limited 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] A heat relaxation step of relaxing residual stress by heating may be applied to the resin film after the stretching step (preferably after uniaxially stretching in the second stretching step). Residual stress may be generated in the resin film by the stretching in the second stretching step. The heat relaxation step is performed to relax the residual stress and suppress the heat shrinkage of the resulting resin microporous layer by heating other than the heat relaxation step, thereby improving the safety of the separator for the power storage device obtained. The heat relaxation step can be performed using a tenter or a roll stretching machine.

[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 temperature of the atmosphere inside the device in the heat relaxation step 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 step, 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 (hereinafter also referred to as "heat relaxation ratio") of the resin film in the heat relaxation step is preferably 25% or more, more preferably 30% or more. When the heat relaxation ratio is equal to or higher than 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 a power storage device such as a lithium-ion secondary battery at high temperature is good. Also, the upper limit of the heat relaxation ratio is preferably 80% or less, more preferably 60% or less, and even more preferably 50% or less. When the heat relaxation ratio is equal to or lower than the above upper limit, sagging is less likely to occur 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 resin film after heat relaxation process / Length of resin film before heat relaxation process) - 1} × (Cold stretching ratio + Heat stretching ratio + 100) It is represented by

[0103] In the heat stretching and heat relaxation processes, from the viewpoint of sufficiently relaxing the stress and reducing the heat relaxation ratio, it is preferable to adjust the stretching speed and the conveying 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 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 problem that "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 the air permeability and the 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, but are not limited to, 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 preferable, 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 is not particularly limited as long as it functions as the positive electrode of a lithium-ion secondary battery, and known ones 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 a 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 (M represents two or more elements selected from the group consisting of Ni, Mn, Co, AI, 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 (M represents two or more elements selected from the group consisting of Ni, Mn, Co, AI, 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 is not particularly limited as long as it functions as the negative electrode of a lithium-ion secondary battery, and it may be a known one. The negative electrode preferably contains one or more materials selected from the group consisting of materials capable of occluding and releasing lithium ions as a negative electrode active material and metallic lithium. That is, the negative electrode preferably 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 an alloy with lithium, and lithium-containing compounds. Such materials include, in addition to metallic lithium, for example, carbon materials typified 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 Method and Evaluation Method》 Hereinafter, the measurement method and evaluation method 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". [Melt Flow Rate (MFR; g / 10min)] In accordance with JIS K 7210, the MFR of the microporous layer (A) was measured under the conditions of a temperature of 230°C and a load of 2.16 kg. In accordance with JIS K 7210, the MFR of the microporous layer (B) was measured under the conditions of a temperature of 190°C and a load of 2.16 kg.

[0114] [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

[0115] [Melt tension (mN)] Using a Capirograph manufactured by Toyo Seiki Seisakusho, the melt tension of the separator substrate or the microporous layer was measured under the following conditions. (Measurement conditions) Capillary: Diameter 1.0 mm, length 20 mm Cylinder extrusion speed: 2 mm / min Withdrawal speed: 60 m / min Temperature: 240°C

[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 later.

[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 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 )] Using a Gurley-type air permeability meter compliant with JIS P-8117, the air permeability of the separator substrate was measured. 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 (%)] Samples were obtained by cutting out the separator substrate into a square shape with MD and TD each being 50 mm. The obtained samples were 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 105°C in the air for 1 hour. After the heat treatment, the samples were taken out from the hot air dryer and allowed to cool at 25°C for 10 minutes. 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 of TD.

[0120] [Puncture strength (gf)] A needle with a hemispherical tip having a radius of 0.5 mm was prepared. Also, two plates having an opening with a diameter (dia.) of 11 mm were prepared, a separator substrate was sandwiched between these plates, and the needle, the separator substrate, and the plates were set on "MX2-50N" manufactured by IMADA Co., Ltd. Using "MX2-50N" manufactured by IMADA Co., Ltd., a piercing test was conducted under the conditions of a curvature radius of 0.5 mm at the needle tip, an opening diameter of 11 mm of the plate holding the separator substrate, and a piercing speed of 25 mm / min. The needle was brought into contact with the separator substrate, and the maximum piercing load (i.e., piercing strength (gf)) was measured. The obtained piercing strength (gf) was divided by the separator thickness (μm) and then multiplied by 12 μm to obtain the piercing strength when the thickness of the separator substrate was converted to 12 μm.

[0121] [Average length pore diameter (nm)] The average length 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 an SEM sample stage for cross-sectional observation with a conductive adhesive (carbon-based) and dried. Then, 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.

[0122] Next, using a scanning electron microscope (S-4800 manufactured by Hitachi High-Technologies Corporation), for each cross-section of each microporous layer (microporous layer (A) and microporous layer (B)) in the microscopy sample, eight arbitrary points were observed 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.

[0123] The obtained observation images were trimmed in the environment of the programming language Python using functions of OpenCV, which is a library for image analysis, 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.

[0124] After that, binarization was performed using the Otsu method, thereby separating the resin part and the hole part, and calculating the average major axis length of the hole part. At this time, among the holes existing across the imaging range and outside the imaging range, the hole area included in the imaging range was 0.001 nm 2 The following holes were excluded from the measurement targets. Then, the average diameter was calculated by area averaging from the area of each hole. 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).

[0125] [Average trunk height (nm)] The trunk height was measured by image analysis in cross-sectional SEM observation. Similar to when calculating the area-averaged major axis pore diameter, a cross-sectional sample was prepared and an inspection sample was prepared. Then, for the cross-section of the inspection sample, three arbitrary points were observed under the conditions of an acceleration voltage of 1 kV, a detection signal LA10, an operating distance of 5 mm, and a magnification of 5000 times. Thereby, observation images were obtained for the microporous layer (A) and the microporous layer (B), respectively.

[0126] The obtained observation image was trimmed using a function of OpenCV, which is an image analysis library, in the environment of the programming language Python so that only the cross-section of one microporous layer is included in the observation region, and the surface, the outside, and other microporous layers were removed from the observation region.

[0127] After that, binarization was performed using the Otsu method, thereby separating the resin part (trunk part + fibril part) and the hole part. Then, "blurring processing" was repeatedly performed only in the ND direction (thickness direction), thereby removing the fibril part (fibril removal processing). At this time, the above fibril removal processing was performed by repeating 100 times using a Gaussian filter in a rod-shaped processing range of 3 pixels in the ND direction and 1 pixel in the MD direction.

[0128] After blurring processing, opening processing and closing processing were sequentially performed in an elliptical processing range with a major axis of 7 pixels in the ND direction and a minor axis of 3 pixels in the MD direction to remove noise, thereby obtaining a fibril-removed image.

[0129] Based on the above fibril-removed image, the trunk height was calculated. That is, first, a fibril-removed 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 repeated so as to include all of the MD direction, and the lengths of the resin parts in the ND direction over the entire range of the fibril-removed image 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] It is calculated by. By using a weighted average using n lengths L, a numerical value highly correlated with the breakdown voltage is likely to be obtained.

[0130] [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 further an electrode pressure piece was placed thereon. 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.

[0131] [MD tensile strength (kgf / cm 2 )] The tensile strength of the separator substrate was measured using a tensile testing machine (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 the strength (tensile load value) at the time of cutting (fracture) if it was cut before yielding, 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.

[0132] 《Example 1》 [Production of microporous layer] As the resin of the microporous layer (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-layer three-layer co-extrusion inflation die at a discharge rate of 6 kg / h using a gear pump.

[0133] Also, as the resin of the microporous layer (B), 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, 1.3% by mass of ethylene-propylene block copolymer (the "compatibilizer" in the table) (MFR (230 °C) = 9.5 g / 10 min, density = 0.91 g / cm 3 )} were dry-blended and mixed to obtain a resin material. The obtained resin material was melted with 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.

[0134] The obtained resin material was melted with 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.

[0135] As the temperature of the inflation die, 230 °C was used. After discharging the melted resin from the inflation die, the resin (the discharged resin) was wound around a roll while being cooled by blown air. As a result, a precursor film having a three-layer structure composed of an A / B / A layer, which has a microporous layer (B) as an intermediate layer and a microporous layer (A) as an 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.

[0136] 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 the stretched film. Then, hot stretching was performed to a hot-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, which has a microporous layer (B) as an intermediate layer and a microporous layer (A) as an outermost layer, was obtained. The structure and physical properties of the obtained separator substrate are shown in the following table.

[0137] 《Examples 2 to 8, Comparative Examples 1 to 4》 As shown in the following table, except for changing the raw materials and composition, etc., a microporous layer and a separator substrate were obtained according to the same method as in Example 1, and they were evaluated.

[0138] 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)

[0139] 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 intermediate 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 form after the decimal point.

[0140] In addition, 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 above compatibilizer together with polyethylene and polypropylene.

[0141] [Table 1]

[0142] In the examples, the composition of each layer is strictly controlled. From the above table, it can be seen that by such examples, it is possible to control the area-average pore size of the microporous layer (B) to be small and to control the stem height of the separator substrate to be small. Also, by adopting such a separator configuration, it can be realized that it is a thin film and has excellent permeability, excellent withstand voltage characteristics, and excellent tensile strength.

Industrial Applicability

[0143] 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 or the like.

Claims

1. A separator for a power storage device 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) contains polyethylene and polypropylene contained therein in the same layer, the area-average major pore diameter calculated from the SEM image analysis of the ND-MD cross-section in the microporous layer (B) is 300 nm or more and 600 nm or less, A separator for a power storage device.

2. The separator for a power storage device according to claim 1, wherein the length-average trunk height calculated from the SEM image analysis of the ND-MD cross-section in the microporous layer (B) is 700 nm or more and 800 nm or less.

3. The separator for a power storage device according to claim 1 or 2, wherein the total thickness of the separator is 4.5 μm or more and 12.0 μm or less, and the thickness of the microporous layer (B) is 1.5 μm or more and 4.0 μm or less.

4. The separator for a power storage device according to claim 1 or 2, wherein the porosity of the separator is 40% or more and 60% or less.

5. The separator for a power storage device according to claim 1 or 2, wherein the melt flow rate (MFR) of the microporous layer (A) measured at a load of 2.16 kg and a temperature of 230°C is 0.20 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 melt flow rate (MFR) of the microporous layer (B) measured at a load of 2.16 kg and a temperature of 190°C is 0.20 g / 10 min or more and 0.60 g / 10 min or less.

7. The melt tension Mt at 240°C in the microporous layer (A) Ａ is 10.0 mN or more and 40.0 mN or less, and the separator for a power storage device according to claim 1 or 2.

8. The melt tension Mt at 240°C in the microporous layer (B) Ｂ is 15.0 mN or more and 30.0 mN or less, and the separator for a power storage device according to claim 1 or 2.

9. The separator for a power storage device according to claim 1 or 2, wherein the heat shrinkage rate in the width direction (TD) at 105°C for 1 hour in the separator substrate is -2.0% or more and 3.0% or less.

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

11. 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.

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