Separator for power storage device and power storage device

Abstract

To provide a separator for a power storage device, the separator combining high piercing strength, low air permeability, high MD elongation, and low heat shrinkage rate, and being able to be formed thin.SOLUTION: A separator for a power storage device is provided which comprises a separator base material having a microporous layer (A) containing polyolefin as a major component and a thermoplastic elastomer. The microporous layer (A) includes, based on its total mass, the polyolefin of 80.0% by mass or more and 99.5% by mass or less, the thermo-plastic elastomer of 0.5% by mass or more and 20.0% by mass or less, the polypropylene of 80.0% by mass or more and 99.5% by mass or less, and has a final fracture depth of 5.5 mm or more in a piercing strength measurement of the separator for the power storage device.SELECTED DRAWING: Figure 1

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 microfiltration 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 multilayer microporous thin film or membrane capable of improving characteristics including dielectric breakdown and strength. Preferred multilayer microporous membranes include a micro layer and one or more laminated barriers.

[0005] Patent Document 2 describes a separator for a power storage device that can be made thin and has high strength, and discloses a microporous membrane containing polyolefin as a main component, having a melt tension of 30 mN or less when measured at a temperature of 230°C, and a melt flow rate (MFR) of 0.9 g / 10 min or less when measured at a load of 2.16 kg and a temperature of 230°C.

[0006] Patent Document 3 describes a separator for a power storage device that is excellent in product safety and the like, and discloses a microporous membrane containing a polypropylene resin and a thermoplastic elastomer and having specific MFR and morphology.

[0007] Patent Document 4 describes a separator for a lithium-ion lithium battery that has excellent air permeability and shutdown characteristics, and has a mixed resin layer containing a polypropylene-based resin, a polyethylene-based resin, and a thermoplastic resin whose crystal melting peak temperature or glass transition temperature is equal to or lower than the crystal melting peak temperature of the polyethylene-based resin, and a porous film having β activity is disclosed.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0009] In the methods described in Patent Documents 1 to 4, specific film physical properties and resin mixing are used to respectively exhibit thin films, high strength, excellent air permeability, etc. However, for power storage devices, a separator that combines a thin film, high strength, and low air permeability is required, and there is room for further improvement from this perspective.

[0010] Therefore, the problem to be solved by the present invention is to provide a separator for a power storage device that has high puncture strength, low air permeability, high MD elongation, and low thermal shrinkage rate, and can be made into a thin film.

Means for Solving the Problems

[0011] As a result of intensive studies, the inventors of the present invention have found that the above problems can be solved by using a separator substrate having a microporous membrane layer with a specific final fracture depth, which contains a specific amount of polypropylene and a thermoplastic elastomer in the same layer, and have completed the present invention. That is, the present invention is as follows. (1) A separator for a power storage device, comprising a separator substrate having a microporous layer (A) containing a polyolefin as a main component and a thermoplastic elastomer, The microporous layer (A) contains 80.0% by mass or more and 99.5% by mass or less of the polyolefin and 0.5% by mass or more and 20.0% by mass or less of the thermoplastic elastomer based on the total mass of the microporous layer (A), and contains 80.0% by mass or more and 99.5% by mass or less of polypropylene based on the total mass of the polyolefin, A separator for a power storage device, wherein the final fracture depth in the measurement of the puncture strength of the separator for a power storage device is 5.5 mm or more. (2) The separator for a power storage device according to item 1, wherein in the measurement of the puncture strength of the separator for a power storage device, the final fracture point strength is 1.5 times or more the first fracture point strength. (3) The separator for a power storage device according to item 1 or 2, wherein the final fracture depth of the separator for a power storage device is 11 mm or less. (4) The separator for a power storage device according to any one of items 1 to 3, wherein in the measurement of the puncture strength of the separator for a power storage device, the final fracture point strength is 2.5 times or less the first fracture point strength. (5) The separator for a power storage device according to any one of items 1 to 4, wherein the thermoplastic elastomer contains at least one selected from the group consisting of ethylene, propylene, and 1-butene as repeating units. (6) The separator for a power storage device according to any one of Items 1 to 5, wherein the micro-porous layer (A) has a melt flow rate (MFR) of 0.9 g / 10 min or less when measured at a load of 2.16 kg and a temperature of 230°C. (7) The separator for a power storage device according to any one of Items 1 to 6, wherein the content of the thermoplastic elastomer is 3.0 to 10.0% by mass based on the total mass of the micro-porous layer (A). (8) The melt tension Mt of the micro-porous layer (A) at 240°C A is 10 mN or more and 35 mN or less, and the separator for a power storage device according to any one of Items 1 to 7. (9) The separator for a power storage device according to any one of Items 1 to 8, wherein the weight average molecular weight (Mw) of the micro-porous layer (A) is 250,000 or more and 1,500,000 or less. (10) The separator for a power storage device according to Item 9, wherein the molecular weight distribution (Mw / Mn), which is the value obtained by dividing the weight average molecular weight (Mw) of the micro-porous layer (A) by the number average molecular weight (Mn), is 3 or more and 30 or less. (11) The separator for a power storage device according to any one of Items 1 to 10, wherein the micro-porous layer (A) has a melt flow rate (MFR) of 0.3 g / 10 min or more when measured at a load of 2.16 kg and a temperature of 230°C. (12) The separator for a power storage device according to any one of Items 1 to 11, wherein the weight average molecular weight (Mw) of the polypropylene is 300,000 or more and 1,300,000 or less. (13) 13 The separator for a power storage device according to any one of Items 1 to 12, wherein the pentad fraction of the polypropylene measured by C-NMR (nuclear magnetic resonance method) is 94.0% or more. (14) The separator for a power storage device according to any one of Items 1 to 13, wherein the porosity of the separator substrate is 40% or more and 60% or less. (15) The separator for a power storage device according to any one of Items 1 to 14, wherein the thickness of the separator base material is 3 μm or more and 20 μm or less. (16) The separator for a power storage device according to any one of Items 1 to 15, wherein the area-average major pore diameter calculated from the SEM image of the MD-ND cross-section of the microporous layer (A) is 50 nm or more and 500 nm or less. (17) The separator for a power storage device according to any one of Items 1 to 16, wherein the TD thermal shrinkage rate of the separator base material at 105 °C for 1 hour is 5% or less. (18) The separator for a power storage device according to any one of Items 1 to 17, wherein the MD thermal shrinkage rate of the separator base material at 105 °C for 1 hour is 20% or less. (19) The separator for a power storage device according to any one of Items 1 to 18, wherein the MD tensile elongation of the separator base material is 20% or more and 60% or less. (20) The separator for a power storage device according to any one of Items 1 to 19, wherein the MD tensile strength of the separator base material is 2000 to 2500 kgf / cm 2 The separator for a power storage device according to any one of Items 1 to 19. (21) The separator for a power storage device according to any one of Items 1 to 20, wherein the separator base material includes a microporous layer (B) having a melting tension Mt A different from the melting tension Mt B of the microporous layer (A). (22) 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 21 disposed between the positive electrode and the negative electrode, wherein the positive electrode contains lithium iron phosphate as a positive electrode active material.

Advantages of the Invention

[0012] According to the present invention, it is possible to provide a separator for a power storage device that has high puncture strength, low air permeability, high MD elongation, and low thermal shrinkage rate, and can be made into a thin film.

Brief Description of the Drawings

[0013]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

MODE FOR CARRYING OUT THE INVENTION

[0014] In this specification, various measurements are carried out based on the methods described in the examples unless otherwise specified. In this specification, the upper limit value or the lower limit value in a numerical range described step by step may be replaced with the upper limit value or the lower limit value in the corresponding numerical range described in other steps, and further, may be replaced with the corresponding value described in the examples. In this specification, the term "step" includes not only the case of an independent step but also the case where it cannot be clearly distinguished from other steps as long as the function of the step is achieved.

[0015] 《Separator for Energy Storage Device》 The separator for a power storage device of the present disclosure has a separator substrate having a microporous layer (A) containing a polyolefin as a main component and containing a thermoplastic elastomer. The separator substrate may optionally include, separately from the microporous layer (A), a microporous layer (B) having a polyolefin as a main component. Further, the separator substrate may further have a coating layer (also referred to as a "surface layer", a "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 disclosure, 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.

[0016] 〈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. The microporous layer (A) contains a polyolefin as a main component and contains a thermoplastic elastomer, and contains polypropylene in the same layer as the polyolefin. In the present disclosure, the microporous layer (A) contains 80.0% by mass or more and 99.5% by mass or less of a polyolefin and 0.5% by mass or more and 20.0% by mass or less of a thermoplastic elastomer based on the total mass of the microporous layer (A). Further, the microporous layer (A) of the present disclosure contains 80.0% by mass or more and 99.5% by mass or less of polypropylene based on the total mass of the polyolefin.

[0017] As described above, the microporous layer (A) of the present disclosure contains polypropylene and a thermoplastic elastomer as constituent materials. The morphology of the microporous layer (A) of the present disclosure is not limited. For example, when produced by uniaxial stretching in the machine direction (MD), the morphology is schematically as shown in FIG. 1. Between a plurality of polymer matrices (1), a plurality of fibrils (2) extend along the MD of the microporous layer (A). Inside or on the surface of the polymer matrix and / or between the polymer matrices, connecting domains (2) are oriented in parallel to the MD of the microporous layer (A). It is preferable that the portion excluding the polymer matrix (1), fibrils (3), and connecting domains (2) is pores (4). The polymer matrix contains at least polypropylene and has a structure in which lamellar crystals are arranged when produced by uniaxial stretching. The fibrils contain at least polypropylene and are formed by stretching the polymer chains of the polymer matrix during pore formation by stretching when produced by uniaxial stretching. The connecting domain preferably contains a thermoplastic elastomer. The connecting domain may contain polyethylene.

[0018] Although not limited to theory, it is presumed that in the microporous layer (A) of the present disclosure, the formation of connecting domains is important, and by reinforcing the polymer matrix without excessively crushing the pores, it is possible to form a microporous layer (A) having both low air permeability and high puncture strength. As the failure mode of the puncture strength in the microporous layer (A), cracking of the polymer matrix can be mentioned. The presence of connecting domains inside or on the surface of the polymer matrix and / or between the polymer matrices makes it possible to relieve the stress applied to the polymer matrix during fracture, and it is considered that high puncture strength can be exhibited. In the microporous layer (A) of the present disclosure, a small amount of thermoplastic elastomer is added to polypropylene, and if necessary, polyethylene may be added to adjust the domain size.

[0019] The lower limit of the content of the polyolefin in the microporous layer (A) is preferably 85.0% by mass or more, more preferably 90.0% by mass or more, still more preferably 92.5% by mass or more, and even more preferably 95.0% by mass or more from the viewpoints of porosity and the like. The upper limit of the content of the polyolefin in the microporous layer (A) is preferably 99.0% by mass or less, more preferably 98.5% by mass or less, still more preferably 98.0% by mass or less, and even more preferably 97.5% by mass or less from the viewpoint of obtaining good battery performance.

[0020] The lower limit of the content of the thermoplastic elastomer in the microporous layer (A) is preferably 1.0% by mass or more, more preferably 2.0% by mass or more, and still more preferably 3.0% by mass or more from the viewpoints of film formability, thinning, low air permeability, and high puncture strength. The upper limit of the content of the thermoplastic elastomer in the microporous layer (A) is preferably 15.0% by mass or less, more preferably 10.0% by mass or less, still more preferably 7.5% by mass or less, and even more preferably 5.0% by mass or less from the viewpoint of maintaining porosity.

[0021] The lower limit of the content of polypropylene in the polyolefin is preferably 85.0% by mass or more, more preferably 90.0% by mass or more, still more preferably 92.5% by mass or more, and even more preferably 95.0% by mass or more from the viewpoint of maintaining good battery performance even after storage at a high temperature (for example, 130°C). The upper limit of the content of polypropylene in the polyolefin is preferably 99.0% by mass or less, more preferably 98.5% by mass or less, still more preferably 98.0% by mass or less, and even more preferably 97.5% by mass or less from the viewpoints of maintaining good puncture strength and air permeability.

[0022] <Material of the microporous layer (A)> The microporous layer (A) is mainly composed of a polyolefin and contains polypropylene in the same layer. By using a polyolefin as the main component, good porosity can be achieved, and good battery performance can be obtained. The microporous layer (A) contains 80.0% by mass or more and 99.5% by mass or less of polypropylene based on the total mass of the polyolefin. Thereby, good battery performance can be maintained even after storage at a high temperature (for example, 130°C). The polypropylene in the microporous layer (A) may be the same material as the polypropylene in the microporous layer (B) described later, or may be polypropylene with a different chemical structure, more specifically, polypropylene with at least one of the monomer composition, stereoregularity, molecular weight, and crystal structure being different. The stereoregularity of the polypropylene is not limited, and examples include atactic, isotactic, or syndiotactic homopolymers. The polypropylene according to the present disclosure is preferably an isotactic or syndiotactic highly crystalline homopolymer.

[0023] The polypropylene in 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 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 propylene structure contained in the polypropylene 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 polypropylene can be used alone or in a mixture of two or more.

[0024] The weight average molecular weight (Mw) of the polypropylene in the microporous layer (A) is preferably 300,000 or more from the viewpoint of the high puncture strength of the microporous layer, etc., and preferably 1,300,000 or less from the viewpoints of ensuring good film-forming properties, productivity, thinning, and low air permeability. 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.

[0025] 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 of the polypropylene to 20 or less, there is a tendency to ensure good film-forming properties, productivity, and thinning. The lower limit of Mw / Mn of the polypropylene is preferably 3 or more, more preferably 4 or more, 4.5 or more, or 5.0 or more. By setting Mw / Mn of the polypropylene to 3 or more, appropriate molecular entanglement is maintained and good film-forming stability is obtained. The weight average molecular weight, number average molecular weight, and Mw / Mn of the polypropylene in the present disclosure are molecular weights in terms of polystyrene obtained by GPC (gel permeation chromatography) measurement.

[0026] 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 / cm3 Hereinafter, 0.93 g / cm 3 or less, or 0.92 g / cm 3 or less may be used. The density of polypropylene is related to the crystallinity of polypropylene. By setting the density of 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.

[0027] The lower limit of the pentad fraction of 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, from the viewpoint of obtaining a microporous layer with low air permeability. The upper limit of the pentad fraction of polypropylene is not limited, but 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).

[0028] When the pentad fraction of polypropylene is 94.0% or more, it indicates that the crystallinity of polypropylene is high. In the stretching pore formation method, particularly in the dry method, the separator obtained has pores formed by stretching the amorphous part between the crystalline substances. Therefore, when the crystallinity of polypropylene is high, the pore formation property is good, and the air permeability can be kept low, enabling high input / output of the battery.

[0029] The melt tension Mt at 240°C of polypropylene in the microporous layer (A) APP (Mt of a single layer APP ) The upper limit is preferably 35 mN or less, more preferably 32 mN or less, still more preferably 30 mN or less, even more preferably 28 mN or less, and particularly preferably 26 mN or less, from the viewpoints of good film-forming property, productivity, thinning, and obtaining a microporous layer (A) with low air permeability. The melt tension Mt of polypropylene in the microporous layer (A) APP (Mt of a single layer APPAs the lower limit value of ), from the viewpoint of obtaining the microporous layer (A) with higher puncture strength, it is preferably 10 mN or more, more preferably 13 mN or more, still more preferably 16 mN or more, even more preferably 18 mN or more, and particularly preferably 19 mN or more.

[0030] The microporous layer (A) mainly consists of polyolefin, but may contain polyolefins other than polypropylene. A polyolefin is a polymer containing a monomer having a carbon-carbon double bond as a repeating unit. Monomers constituting polyolefins other than polypropylene and polyethylene include, but are not limited to, monomers having 4 to 10 carbon atoms with a carbon-carbon double bond, such as 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene. Examples of polyolefins include homopolymers, copolymers, or multi-stage polymerization polymers.

[0031] The microporous layer (A) contains a polyolefin as a main component and a thermoplastic elastomer. Examples of the thermoplastic elastomer include polyolefins, copolymers of 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. Examples of the monomer constituting the polyolefin include, but are not limited to, monomers having 2 to 10 carbon atoms (C2 to C10) with a carbon-carbon double bond, such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene. Examples of the polyolefin include low-crystalline polypropylene having a low stereoregularity region. As the monomers constituting the copolymer of polyolefin, one kind may be used alone or two or more kinds may be used in combination. Examples of the copolymer of polystyrene and polyolefin include styrene-(ethylene-propylene)-styrene copolymer (SEPS), styrene-(ethylene-butene)-styrene copolymer (SEBS), styrene-ethylene-styrene copolymer, styrene-(ethylene-butene)-olefin copolymer (SEBC), olefin-(ethylene-butene)-styrene copolymer (CEBS), etc., and they may be hydrogenated polymers. These copolymers may be random copolymers or block copolymers, and are preferably block copolymers.

[0032] From the viewpoint of obtaining the microporous layer (A) that achieves both high puncture strength and high MD elongation by fulfilling its role, as the thermoplastic elastomer contained in the microporous layer (A) of the present disclosure, a copolymer containing one or more selected from the group consisting of ethylene, propylene, and 1-butene as a repeating unit is preferable.

[0033] Examples of the thermoplastic elastomer include ethylene / α-olefin copolymers, ethylene / styrene copolymers, propylene / α-olefin copolymers, 1-butene / α-olefin copolymers, block copolymers of styrene and butadiene (SBS) and its hydrogenated polymer (SEBS), block copolymers of styrene and isoprene (SIS) and its hydrogenated polymer (SEPS), and the like. Examples of the α-olefin include aliphatic α-olefins such as propylene, 1-butene, 1-hexene, 1-octene, 4-methyl-1-pentene, and the like. High molecular weight polymers obtained by copolymerizing ethylene and α-olefin, high molecular weight polymers obtained by chain transfer during polymerization to copolymerize long chain branches such as linear low density polyethylene or ultra-low density polyethylene, and the like are also included. These thermoplastic elastomers may be used alone or in combination of two or more. Among these, preferred thermoplastic elastomers are those not containing propylene. Thermoplastic elastomers not containing propylene form completely independent domains because of their low interaction with polypropylene, and enter between the lamellar crystals of polypropylene to produce an effect by stress relaxation. More preferred thermoplastic elastomers are polymers having at least one component selected from the group consisting of ethylene / 1-butene copolymers, ethylene / 1-hexene copolymers, and ethylene / 1-octene copolymers. These polymers may be random copolymers or block copolymers, and each component of ethylene / 1-butene copolymers, ethylene / 1-hexene copolymers, and ethylene / 1-octene copolymers has an advantage of high interfacial adhesiveness with polypropylene and being easily finely dispersed. That is, these copolymer components are high molecular weight components that are easily oriented in parallel in the MD direction of the microporous membrane.

[0034] <Melt Flow Rate (MFR) of the Microporous Layer (A)> The melt flow rate (MFR) of the microporous layer (A) of the present disclosure is 1.0 g / 10 min or less. The upper limit of the melt flow rate (MFR) of the microporous layer (A) (single-layer MFR) is preferably 0.9 g / 10 min or less, more preferably 0.8 g / 10 min or less, still more preferably 0.7 g / 10 min or less, and even more preferably 0.6 g / 10 min or less, from the viewpoint of obtaining a microporous layer (A) with higher puncture strength. The lower limit of the MFR of the microporous layer (A) (single-layer MFR) is not limited, but from the viewpoints of obtaining a microporous layer (A) with lower air permeability, film-forming property, and thinner film, it may be, for example, 0.2 g / 10 min or more, 0.25 g / 10 min or more, 0.3 g / 10 min or more, 0.35 g / 10 min or more, 0.4 g / 10 min or more, or 0.45 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. The fact that the MFR of the microporous layer (A) is 1.0 g / 10 min or less means that the molecular weight of the polyolefin contained in the microporous layer (A) is relatively high. When the molecular weight of the polyolefin is high, the number of tie molecules that bond crystalline substances increases, so that a microporous layer (A) with high puncture strength tends to be obtained. Further, when the MFR of the microporous layer (A) is 0.2 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 property, thinner film, and productivity.

[0035] The MFR of the polypropylene in the microporous layer (A) is preferably 0.2 to 0.9 g / 10 min when measured under the conditions of a load of 2.16 kg and a temperature of 230°C from the viewpoints of obtaining a high puncture strength, a low air permeability, and the microporous layer (A) of the thin film. The upper limit value of the MFR of the polypropylene may be, for example, 0.8 g / 10 min or less, 0.7 g / 10 min or less, 0.65 g / 10 min or less, 0.6 g / 10 min or less, or 0.55 g / 10 min or less from the viewpoint of obtaining a microporous layer (A) with a higher puncture strength. The lower limit value of the MFR of the polypropylene is not limited, but may be, for example, 0.2 g / 10 min or more, 0.25 g / 10 min or more, 0.3 g / 10 min or more, 0.35 g / 10 min or more, 0.4 g / 10 min or more, or 0.45 g / 10 min or more from the viewpoints of obtaining a microporous layer (A) with a lower air permeability, film-forming property, and thinning of the thin film.

[0036] The MFR of the thermoplastic elastomer in the microporous layer (A) is preferably 0.1 to 100.0 g / 10 min when measured under the conditions of a load of 2.16 kg and a temperature of 230°C from the viewpoints of obtaining a high puncture strength, a low air permeability, the thin film, and good film-forming stability of the microporous layer (A). The upper limit value of the MFR of the thermoplastic elastomer may be, for example, 80.0 g / 10 min or less, 60.0 g / 10 min or less, 40.0 g / 10 min or less, 30.0 g / 10 min or less, 20.0 g / 10 min or less, 15.0 g / 10 min or less, 10.0 g / 10 min or less, 8.0 g / 10 min or less, 6.0 g / 10 min or less, or 5.0 g / 10 min or less from the viewpoint of being uniformly kneaded with the polyolefin and obtaining a microporous layer (A) with a high puncture strength and good film-forming stability. The lower limit value of the MFR of the thermoplastic elastomer is not limited, but may be, for example, 0.5 g / 10 min or more, 1.0 g / 10 min or more, 1.5 g / 10 min or more, 2.0 g / 10 min or more, 2.5 g / 10 min or more, or 3.0 g / 10 min or more from the viewpoints of obtaining a microporous layer (A) with a lower air permeability, film-forming property, and thinning of the thin film.

[0037] 〈Mw, Mw / Mn of the microporous layer (A)〉 The weight average molecular weight (Mw) of the microporous layer (A) is preferably 250,000 or more from the viewpoint of obtaining a microporous layer (A) with higher puncture strength, and preferably 1,500,000 or less from the viewpoints of ensuring good film-forming properties, productivity, thinning, and low air permeability. More preferably, the Mw of the microporous layer (A) is 400,000 or more and 1,300,000 or less, still more preferably 500,000 or more and 1,200,000 or less, even more preferably 600,000 or more and 1,100,000 or less, and particularly preferably 700,000 or more and 1,000,000 or less.

[0038] The upper limit value of the value (Mw / Mn) obtained by dividing the weight average molecular weight (Mw) of the microporous layer (A) by the number average molecular weight (Mn) is preferably 30 or less, more preferably 25 or less, 20 or less, 18 or less, 16 or less, 14 or less, 12 or less, or 10 or less. By setting Mw / Mn of the microporous layer (A) to 30 or less, there is a tendency that good film-forming properties, productivity, and thinning can be ensured. Further, the lower limit value of Mw / Mn of the microporous layer (A) is preferably 3 or more, more preferably 4 or more, 4.5 or more, 5.0 or more. By setting Mw / Mn of the microporous layer (A) to 3 or more, appropriate molecular entanglement is maintained and good film-forming stability is obtained. The weight average molecular weight, number average molecular weight, and Mw / Mn of the microporous layer (A) of the present disclosure are molecular weights in terms of polystyrene obtained by GPC (gel permeation chromatography) measurement. Further, since the microporous layer (A) of the present disclosure contains polyolefin containing polypropylene and polyethylene as a main component and contains a thermoplastic elastomer in addition, the Mw and Mw / Mn of the above-described microporous layer (A) are values reflecting the influence of the combination of these constituent materials.

[0039] 〈Melting Tension of Microporous Layer (A)〉 The melting tension Mt of the microporous layer (A) at 240°C A (Mt of a single layer AAs the upper limit value of , from the viewpoints of obtaining a microporous layer (A) with good film-forming properties, productivity, thin film formation, and low air permeability, it is preferably 35 mN or less, more preferably 32 mN or less, still more preferably 30 mN or less, even more preferably 28 mN or less, and particularly preferably 26 mN or less. The melt tension Mt of the microporous layer (A) A (The Mt of a single layer A ) As the lower limit value, from the viewpoint of obtaining a microporous layer (A) with higher puncture strength, it is preferably 10 mN or more, more preferably 13 mN or more, still more preferably 16 mN or more, even more preferably 18 mN or more, and particularly preferably 19 mN or more.

[0040] 〈Average pore length diameter of the microporous layer (A)〉 The average pore length diameter in the MD-ND cross-section of the microporous layer (A) (hereinafter, also simply referred to as "average pore length diameter") is preferably 50 nm or more and 500 nm or less. In the present disclosure, "ND" indicates the thickness direction of the microporous layer, and "MD" indicates the film-forming direction of the microporous layer. For example, the MD of the separator having the microporous layer is the longitudinal direction if it is a roll. "Pore length diameter" means the pore diameter in the MD. Further, when there are two or more layers of the microporous layer (A) and / or the microporous layer (B), the average pore length diameters of the microporous layer (A) and the microporous layer (B) are compared based on the average pore length diameter values of each layer. The lower limit of the average pore length 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, even more preferably 120 nm or more, and particularly preferably 130 nm or more, from the viewpoints of ensuring high input / output in the power storage device and obtaining a microporous layer (A) with low air permeability. Also, the upper limit of the average pore length diameter of the microporous layer (A) is preferably 500 nm or less, more preferably 400 nm or less, still more preferably 350 nm or less, even more preferably 300 nm or less, particularly preferably 250 nm or less, and most preferably 210 nm or less, from the viewpoint of obtaining a microporous layer (A) with high puncture strength.

[0041] The area-average pore size 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 size from the cross-sectional SEM image, the number-average pore size and the area-average pore size can be calculated. However, in the present disclosure, the area-average pore size is used as the average pore size so that a better correlation with the physical properties of the separator can be obtained.

[0042] <Porosity of the microporous layer (A)> The porosity of the microporous layer (A) is preferably 30% or more from the viewpoint of avoiding clogging in the power storage device and obtaining a microporous layer (A) with low air permeability, and preferably 60% or less from the viewpoint of obtaining a microporous layer (A) with high puncture strength. The porosity of the microporous layer (A) is more preferably 30% or more and 55% or less, still more preferably 35% or more and 50% or less, and particularly preferably 40% or more and 45% or less.

[0043] <Thickness of the microporous layer (A)> When the base material of the separator for a power storage device is composed of a single-layer structure having only one layer of the microporous layer (A), the upper limit value of the thickness of the microporous layer (A) is preferably 20 μm or less, for example, 18 μm or less, 16 μm or less, 14 μm or less, 13 μm or less, 12 μm or less, 11 μm or less, or 10 μm or less, from the viewpoints of increasing the energy density of the power storage device and the low air permeability of the microporous layer (A). The lower limit value of the thickness of the microporous layer (A) in the case of a single-layer structure is preferably 3 μm or more, for example, 4 μm or more, 5 μm or more, 6 μm or more, 7 μm or more, or 8 μm or more, from the viewpoint of obtaining a microporous layer (A) with high puncture strength.

[0044] When the base material of the separator for the energy storage device has a multilayer structure including one or more microporous layers (A), the upper limit value of the thickness of the microporous layer (A) is preferably 10 μm or less, for example, 8 μm or less, 7 μm or less, 6 μm or less, 5 μm or less, 4.5 μm or less, or 4 μm or less from the viewpoints of increasing the energy density of the energy storage device and reducing the air permeability of the separator base material. The lower limit value of the thickness of the microporous layer (A) when having a multilayer structure is preferably 1 μm or more, for example, 2 μm or more, 3 μm or more, or 3.5 μm or more from the viewpoint of obtaining a separator base material with high puncture strength.

[0045] 〈Additives in the microporous layer (A)〉 The microporous layer (A) mainly composed of polyolefin may further contain additives such as elastomers, crystal nucleating agents, antioxidants, and fillers, in addition to polypropylene, polyethylene, and thermoplastic elastomers, if necessary. The amount of the additive is not particularly limited, but may be, for example, 0.01% by mass or more, 0.1% by mass or more, or 1% by mass or more, and 20% by mass or less, 10% by mass or less, or 7% by mass or less based on the total mass of the microporous layer (A).

[0046] 〈Microporous layer (B)〉 The separator for the energy storage device of the present disclosure may have a microporous layer (B) if desired. The separator for the energy storage device may have only one layer of the microporous layer (B) or may have two or more layers. The microporous layer (B) is also mainly composed of polyolefin, more preferably mainly composed of polypropylene and / or polyethylene. Thereby, the porosity is good and good battery performance can be obtained. The microporous layer (B) is more preferably 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 disclosure, that the microporous layer (B) has polypropylene as the "main component" means that polypropylene 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 polypropylene 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 polypropylene in the microporous layer (B) is not limited, but 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.

[0047] 〈Material of the microporous layer (B)〉 The polypropylene of the microporous layer (B) may be the same material as the polypropylene of the above-mentioned microporous layer (A), or may be a polypropylene having a chemically different structure, more specifically, a polypropylene having at least one of the monomer composition, stereoregularity, molecular weight, and crystal structure different. The stereoregularity of the polypropylene of the microporous layer (B) is not limited, and examples thereof include atactic, isotactic, or syndiotactic homopolymers. The polypropylene according to the present disclosure is preferably an isotactic or syndiotactic highly crystalline homopolymer.

[0048] 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.

[0049] From the viewpoint of the strength of the microporous layer, the weight average molecular weight (Mw) of the polypropylene of the microporous layer (B) is preferably 250,000 or more, and from the viewpoint of increasing the pore diameter of the microporous layer and exhibiting good air permeability, it is preferably 1,000,000 or less. The Mw of the polypropylene is more preferably 400,000 or more and 950,000 or less, still more preferably 550,000 or more and 900,000 or less, even more preferably 600,000 or more and 900,000 or less, and particularly preferably 700,000 or more and 900,000 or less.

[0050] The upper limit of the value (Mw / Mn) obtained by dividing the weight average molecular weight (Mw) of the polypropylene in the microporous layer (B) by the number average molecular weight (Mn) is preferably 7 or less, more preferably 6.5 or less, 6 or less, 5.5 or less, or 5 or less. The smaller the value of Mw / Mn of the polypropylene, the smaller the melt tension of the resulting microporous layer tends to be. Therefore, it is preferable that the value of Mw / Mn of the polypropylene is 7 or less in order to control the melt tension of the microporous layer (B) to be small. Further, Mw / Mn may preferably be 1.1 or more, for example 1.3 or more, 1.5 or more, 2.0 or more, or 2.5 or more. When Mw / Mn is 1.1 or more, appropriate molecular entanglement may be maintained and the stability during film formation may be improved. The weight average molecular weight, number average molecular weight, and Mw / Mn of the polyolefin of the present disclosure are molecular weights in terms of polystyrene obtained by GPC (gel permeation chromatography) measurement.

[0051] The density of the polypropylene in the microporous layer (B) is preferably 0.85 g / cm 3 or more, for example 0.88 g / cm 3 or more, 0.89 g / cm 3 or more, or 0.90 g / cm 3 or more. The density of the polypropylene is preferably 1.1 g / cm 3 or less, for example 1.0 g / cm 3 or less, 0.98 g / cm 3 or less, 0.97 g / 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.

[0052] In addition to polypropylene, the microporous layer (B) may contain other resins. Examples of the other resins include polyolefins other than polypropylene (also referred to as "other polyolefins"). A polyolefin is a polymer containing a monomer having a carbon-carbon double bond as a repeating unit. The monomers constituting the polyolefin 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.

[0053] In addition to polypropylene, the microporous layer (B) may contain a thermoplastic elastomer. The thermoplastic elastomer is not particularly limited, and examples thereof include polypropylene, polyolefins other than polypropylene (also referred to as "other polyolefins"), and copolymers of polystyrene and polyolefins. Examples of polypropylene include low-crystalline polypropylene having a low stereoregularity region. A polyolefin is a polymer containing a monomer having a carbon-carbon double bond as a repeating unit. The monomers constituting polyolefins other than polypropylene are not limited, and examples thereof include monomers having 2 or 4 to 10 carbon atoms and having a carbon-carbon double bond, such as ethylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene. The polyolefin is, for example, a homopolymer, a copolymer, or a multi-stage polymerization polymer, and as an example, it is also possible to contain polyethylene. Examples of copolymers of polystyrene and polyolefins include styrene-(ethylene-propylene)-styrene copolymer (SEPS), styrene-(ethylene-butene)-styrene copolymer (SEBS), hydrogenated-styrene-(ethylene-butene)-styrene copolymer (hydrogenated SEBS), styrene-ethylene-styrene copolymer, olefin crystal-(ethylene-butene)-olefin crystal copolymer (CEBC), hydrogenated-olefin crystal-(ethylene-butene)-olefin crystal copolymer (hydrogenated CEBC), styrene-(ethylene-butene)-olefin crystal copolymer (SEBC), hydrogenated-styrene-(ethylene-butene)-olefin crystal copolymer (hydrogenated SEBC), and the like. Particularly preferred are hydrogenated-styrene-(ethylene-butene)-styrene copolymer (hydrogenated SEBS), hydrogenated-styrene-(ethylene-butene)-olefin crystal copolymer (hydrogenated SEBC), and styrene-(ethylene-propylene)-styrene copolymer (SEPS).

[0054] As the thermoplastic elastomer contained in the microporous layer (B), an elastomer incompatible with polypropylene is more preferable from the viewpoints of pore-opening property and increasing the pore diameter. The elastomer incompatible with polypropylene is not particularly limited, but preferably includes a copolymer of polyethylene and another polyolefin, and a copolymer of polystyrene and a polyolefin. Examples of the copolymer of polystyrene and a polyolefin preferably include styrene-(ethylene-propylene)-styrene copolymer (SEPS), styrene-(ethylene-butene)-styrene copolymer (SEBS), hydrogenated-styrene-(ethylene-butene)-styrene copolymer (hydrogenated SEBS), styrene-ethylene-styrene copolymer, olefin crystal-(ethylene-butene)-olefin crystal copolymer (CEBC), hydrogenated-olefin crystal-(ethylene-butene)-olefin crystal copolymer (hydrogenated CEBC), styrene-(ethylene-butene)-olefin crystal copolymer (SEBC), hydrogenated-styrene-(ethylene-butene)-olefin crystal copolymer (hydrogenated SEBC), and the like. Particularly preferably, they are hydrogenated-styrene-(ethylene-butene)-styrene copolymer (hydrogenated SEBS), hydrogenated-styrene-(ethylene-butene)-olefin crystal copolymer (hydrogenated SEBC), and styrene-(ethylene-propylene)-styrene copolymer (SEPS).

[0055] 〈Melt Flow Rate (MFR) of Microporous Layer (B)〉 The upper limit of the melt flow rate (MFR) (single-layer MFR) of the microporous layer (B) is preferably 8.0 g / 10 min or less, for example, 6.0 g / 10 min or less, 4.0 g / 10 min or less, 3.0 g / 10 min or less, 2.0 g / 10 min or less, or 1.1 g / 10 min or less, from the viewpoint of obtaining a microporous layer (B) with higher strength. The lower limit of the MFR (single-layer MFR) of the microporous layer (B) is not limited from the viewpoint of exhibiting good air permeability, but may be, for example, 0.3 g / 10 min or more, 0.35 g / 10 min or more, 0.4 g / 10 min or more, 0.45 g / 10 min or more, or 0.5 g / 10 min or more. The MFR of the microporous layer (B) is measured under the conditions of a load of 2.16 kg and a temperature of 230°C. The fact that the MFR of the microporous layer (B) is 8.0 g / 10 min or less means that the molecular weight of the polyolefin contained in the microporous layer (B) is relatively high. Since the increase in the molecular weight of the polyolefin results in an increase in the number of tie molecules that bond crystalline substances together, a microporous layer (B) with higher strength tends to be obtained. When the MFR of the microporous layer (B) is 0.3 g / 10 min or more, the melt tension of the microporous layer (B) does not become too high, and a separator that exhibits good air permeability is more easily obtained.

[0056] The MFR of the polypropylene in the microporous layer (B) is preferably 8.0 g / 10 min or less, for example, 6.0 g / 10 min or less, 4.0 g / 10 min or less, 3.0 g / 10 min or less, 2.0 g / 10 min or less, or 1.1 g / 10 min or less, from the viewpoint of obtaining a microporous layer (B) with higher strength. The lower limit of the MFR (single-layer MFR) of the microporous layer (B) is not limited from the viewpoint of exhibiting good air permeability, but may be, for example, 0.3 g / 10 min or more, 0.35 g / 10 min or more, 0.4 g / 10 min or more, 0.45 g / 10 min or more, or 0.5 g / 10 min or more.

[0057] The MFR of the microporous layer (B) is preferably higher than the MFR of the microporous layer (A). By making the MFR of the microporous layer (B) higher than the MFR of the microporous layer (A), the pore diameter of the microporous layer (B) of the obtained separator can be controlled to be larger than the pore diameter of the microporous layer (A).

[0058] 〈Pentad fraction of the microporous layer (B)〉 The lower limit of the pentad fraction of 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 of the pentad fraction of polypropylene is not limited, but may be 99.9% or less, 99.8% or less, or 99.5% or less. The pentad fraction of polypropylene is 13 measured by 13C-NMR (nuclear magnetic resonance method).

[0059] That the pentad fraction of polypropylene is 94.0% or more indicates high crystallinity of polypropylene. For separators obtained by the stretching and pore-forming method, especially the dry method, pores are formed by stretching the amorphous part between the crystalline parts. Therefore, when the crystallinity of polypropylene is high, the pore-forming property is good, the air permeability can be kept low, and the high output of the battery can be achieved.

[0060] The melt tension Mt of polypropylene in the microporous layer (B) at 240 °C BPP is preferably 4 mN or more and 30 mN or less. The lower limit of the melt tension Mt BPP is preferably 4 mN or more, more preferably 7 mN or more, still more preferably 10 mN or more, particularly preferably 13 mN or more, and most preferably 16 mN or more, from the viewpoints of good film-forming property and productivity of the separator substrate having the microporous layer (A) and the microporous layer (B). The upper limit of the melt tension Mt BPP is preferably 30 mN or less, more preferably 28 mN or less, still more preferably 26 mN or less, and most preferably 24 mN or less, from the viewpoint of achieving a large pore diameter and exhibiting good air permeability. The melt tensions Mt APP and Mt BPP of polypropylene in the microporous layers (A) and (B) at 240 °C may be different from each other from the viewpoint of the area-average long pore diameter of the microporous membrane.

[0061] <Melting Tension of the Microporous Layer (B)> The melting tension Mt of the microporous layer (B) at 240 °C B is preferably 4 mN or more and 30 mN or less. The melting tension Mt B As the lower limit of, from the viewpoints of good film formability and productivity of the separator substrate having the microporous layer (A) and the microporous layer (B), it is preferably 4 mN or more, more preferably 7 mN or more, still more preferably 10 mN or more, particularly preferably 13 mN or more, and most preferably 16 mN or more. The melting tension Mt B As the upper limit of, from the viewpoint of achieving a large pore diameter and exhibiting good air permeability, it is preferably 30 mN or less, more preferably 28 mN or less, still more preferably 26 mN or less, and most preferably 24 mN or less. The melting tension Mt of the microporous layers (A) and (B) at 240 °C A and Mt B may be different from each other from the viewpoint of the area-average major pore diameter of the microporous membrane.

[0062] <Area-Average Major Pore Diameter of the Microporous Layer (B)> The area-average major pore diameter in the MD-ND cross-section of the microporous layer (B) (hereinafter, also simply referred to as "area-average major pore diameter") is preferably larger than the area-average major pore diameter of the microporous layer (A). For details regarding the relationship with the area-average major pore diameter of the microporous layer (A), refer to the column of <Area-Average Major Pore Diameter of the Microporous Layer (A)>.

[0063] The area-average major pore diameter in the MD-ND cross-section of the microporous layer (B) is preferably 100 nm or more and 600 nm or less, more preferably 120 nm or more and 500 nm or less, still more preferably 140 nm or more and 400 nm or less, and even more preferably 160 nm or more and 350 nm or less. When the area-average major pore diameter of the microporous layer (B) is within this range, good puncture strength and air permeability can be obtained.

[0064] <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 70% or less from the viewpoint of maintaining the strength of the separator. The porosity of the microporous layer (B) is more preferably 25% or more and 65% or less, still more preferably 30% or more and 60% or less, and particularly preferably 35% or more and 60% or less.

[0065] 〈Thickness of the microporous layer (B)〉 From the viewpoint of increasing the energy density of the power storage device, etc., the thickness of the microporous layer (B) is preferably 10 μm or less, for example, 8 μm or less, 7 μm or less, 6 μm or less, 5 μm or less, 4.5 μm or less, or 4 μm or less. The lower limit of the thickness of the microporous layer (B) is preferably 1 μm or more, for example, 2 μm or more, 3 μm or more, or 3.5 μm or more from the viewpoint of strength, etc.

[0066] 〈Additives in the microporous layer (B)〉 In addition to polyolefin, the microporous layer (B) may further contain additives such as elastomers, crystal nucleating agents, antioxidants, and fillers as needed. The amount of the additive is not particularly limited, but based on the total mass of the microporous layer (B), for example, it may be 0.01% by mass or more, 0.1% by mass or more, or 1% by mass or more, and 10% by mass or less, 7% by mass or less, or 5% by mass or less.

[0067] Although not wishing to be bound by theory, in order to further improve the safety of the separator and the power storage device, the pore size ratio (a) / (b) between the pore size (a) in the direction perpendicular to the major axis diameter and the pore size (b) is preferably 1.5 or more and 30 or less, more preferably 3 or more and 20 or less. Furthermore, it is preferable that the major axis diameters of the microporous membrane are aligned in one direction, and it is more preferable that the direction is the MD direction. By achieving such pore sizes, it is possible to ensure good air permeability resistance, exhibit high puncture strength, and reduce unsafe modes such as dendrite generation in the power storage device.

[0068] [MD / TD strength ratio] The tensile strength of the separator can be measured as detailed in the examples. The MD / TD tensile strength ratio of the separator is preferably from 16 to 30, more preferably from 18 to 28, and even more preferably from 20 to 26. When the MD / TD tensile strength ratio is 16 or more, the thermal shrinkage rate of TD can be reduced, so that the resistance to short circuits due to the thermal shrinkage of TD during battery winding is increased. On the other hand, when the MD / TD tensile strength ratio is 30 or less, cracks are less likely to occur in MD, and it is easy to obtain high puncture strength, and it is less likely that the separator will tear longitudinally (MD) during handling in the battery manufacturing process.

[0069] 〈Relationship between the microporous layer (A) and the microporous layer (B)〉 The melt tension Mt at 240 °C of the microporous layer (A) A and the melt tension Mt at 240 °C of the microporous layer (B) B The ratio Mt A / Mt B is preferably from 1.05 or more to 4.0 or less. Here, Mt A / Mt B By setting it to 1.05 or more, the pore diameter of the microporous layer (A) of the obtained separator can be sufficiently reduced, and the pore diameter of the microporous layer (B) can be sufficiently increased, and good withstand voltage and air permeability can be achieved at the same time. Mt A / Mt B By setting it to 4.0 or less, it becomes possible to obtain a separator having good pore formation and air permeability. Incidentally, Mt A / Mt B is more preferably from 1.1 or more to 3.5 or less, still more preferably from 1.15 or more to 3.3 or less, even more preferably from 1.2 or more to 3.0 or less, and particularly preferably from 1.25 or more to 2.5 or less.

[0070] The ratio of the MFR (MFR B ) of the microporous layer (B) to the MFR (MFR A ) of the microporous layer (A), MFR B / MFR A is preferably from 1.02 or more to 10.0 or less. MFR B / MFR ABy setting it to 1.02 or more, the pore diameter of the microporous layer (A) of the obtained separator can be made sufficiently small, and the pore diameter of the microporous layer (B) can be sufficiently controlled to be large, enabling both good withstand voltage and air permeability. MFR B / MFR A By setting it to 10.0 or less, it becomes possible to obtain a separator having stable film-forming properties, productivity, and good pore-forming properties and air permeability. Note that MFR B / MFR A is more preferably 1.05 or more and 6.0 or less, still more preferably 1.1 or more and 5.0 or less, even more preferably 1.1 or more and 4.0 or less, and particularly preferably 1.1 or more and 3.0 or less.

[0071] The weight average molecular weight Mw of the polypropylene in the microporous layer (A) A and the weight average molecular weight Mw of the polypropylene in the microporous layer (B) B The ratio Mw A / Mw B is preferably 1.02 or more and 2.0 or less. Mw A / Mw B By setting Mw / Mw to 1.02 or more, the melt tension ratio Mt A / Mt B of the microporous layer (A) and the microporous layer (B) can be highly controlled, and as a result, it becomes possible to favorably exhibit the dendrite suppression effect in the power storage device. Mt A / Mt B By setting it to 2.0 or less, it becomes possible to obtain a separator having stable film-forming properties, productivity, and good pore-forming properties and air permeability. Note that Mw A / Mw B is preferably 1.02 or more and 1.8 or less, more preferably 1.03 or more and 1.6 or less, and most preferably 1.05 or more and 1.4 or less.

[0072] 〈Layer structure of separator substrate〉 The base material of the separator for the energy storage device (also simply referred to as "separator base material" in the present disclosure) has at least one microporous layer (A), or has at least one layer each of the microporous layer (A) and the microporous layer (B). The separator base material may have a multilayer structure of three or more layers having at least two layers of at least one of the microporous layer (A) and / or the microporous layer (B). For example, a two-layer structure of the microporous layer (A) / microporous layer (B), a three-layer structure of the microporous layer (A) / microporous layer (B) / microporous layer (A), etc. can be mentioned. Further, the separator base material may have a layer other than the microporous layer (A) and the microporous layer (B). For example, as the layer other than the microporous layer (A) and the microporous layer (B), for example, a microporous layer mainly composed of a polyolefin other than (A)(B), a layer containing an inorganic substance, a layer containing a heat-resistant resin, etc. can be mentioned. For example, the separator base material may have a multilayer structure of four or more layers such as the microporous layer (A) / microporous layer (B) / microporous layer (C) / microporous layer (A). From the viewpoints of ease of manufacture, curl suppression of the separator, etc., a symmetric laminated structure is preferable.

[0073] 〈Height of the core of the separator base material〉 The height of the core in the MD-ND cross-section of the separator base material is 500 nm or more and 1000 nm or less. The core height is correlated with the tortuosity of the pores and is a numerical value indicating a pore structure different from the pore diameter. When the core height increases, the tortuosity of the pores decreases, and when the core height is small, the tortuosity of the pores increases, and good withstand voltage can be obtained. The upper limit of the core height is preferably 950 nm or less, more preferably 920 nm or less, still more preferably 900 nm or less, particularly preferably 850 nm or less, and most preferably 820 nm or less from the viewpoints of withstand voltage and high puncture strength. The lower limit of the core height is preferably 520 nm or more, more preferably 540 nm or more, still more preferably 560 nm or more, particularly preferably 580 nm or more, and most preferably 600 nm or more from the viewpoint of obtaining good air permeability.

[0074] The stem height can be calculated by observing the cross-section of the separator base material in the MD-ND plane using SEM, removing fibrils through image analysis, and measuring the length of the remaining lamella in the ND direction by image analysis. The detailed conditions are shown in the examples. When measuring the 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.

[0075] 〈Thickness of the separator base material〉 From the perspective of increasing the energy density and power input / output of the energy storage device, the upper limit value of the thickness of the separator base material is preferably 20 μm or less, for example, it may be 18 μm or less, 16 μm or less, 14 μm or less, 13 μm or less, 12 μm or less, 11 μm or less, or 10 μm or less. From the perspective of obtaining a microporous layer (A) with high puncture strength, the lower limit value of the thickness of the separator base material is preferably 3 μm or more, for example, it may be 4 μm or more, 5 μm or more, 6 μm or more, 7 μm or more, or 8 μm or more.

[0076] 〈Air permeability (air resistance) of the separator base material〉 From the perspective of ensuring good output in the energy storage device, the upper limit value of the air permeability of the separator base material is preferably 400 s / 100 cm 3 Hereinafter, more preferably 350 s / 100 cm 3 Hereinafter, even more preferably 300 s / 100 cm 3 Hereinafter, even more preferably 250 s / 100 cm 3 Hereinafter, particularly preferably 220 s / 100 cm 3 Hereinafter, most preferably 200 s / 100 cm 3 Hereinafter. The lower limit value of the air permeability of the separator base material is not limited, for example, 10 s / 100 cm 3 or more, 20 s / 100 cm 3 or more, or 30 s / 100 cm 3 or more.

[0077] 〈Porosity of the separator base material〉 The porosity of the separator substrate is preferably 30% or more from the viewpoint of avoiding clogging in the power storage device and obtaining good air permeability of the separator, and preferably 60% or less from the viewpoint of maintaining the puncture strength of the separator. The porosity of the separator substrate is more preferably 35% or more and 57% or less, still more preferably 40% or more and 57% or less, and particularly preferably 47% or more and 53% or less.

[0078] 〈Puncture Strength and Puncture Depth of Separator Substrate〉 The lower limit value of the puncture strength of the separator substrate is preferably 150 gf or more (about 1.47 N or more), more preferably 180 gf or more, 200 gf or more, 220 gf or more, or 240 gf or more when the thickness of the separator substrate is converted to 10 μm, and particularly preferably 260 gf or more, 270 gf or more, or 280 gf or more. The upper limit value of the puncture strength of the separator substrate is not limited, but may be preferably 500 gf or less, for example, 450 gf or less, or 400 gf or less when the thickness of the separator substrate is converted to 10 μm.

[0079] In the present disclosure, the puncture depth means the moving distance (depth) from when a needle of a specific size is punctured in the thickness direction of the separator from the outer surface of the separator with the periphery of the separator fixed until a hole is formed after the needle contacts the separator.

[0080] In the puncture test of the separator for the power storage device of the present disclosure, when the final fracture depth is 5.5 mm or more, the flexibility of the pore structure of the separator is increased, stress concentration is less likely to occur, and high puncture strength is easily obtained due to the improved toughness. From such a tendency, the final fracture depth is preferably in the numerical range of 5.5 mm to 12.4 mm, and more preferably within the numerical range of 5.5 mm or more and 11 mm or less from the viewpoint of puncture strength.

[0081] Although not wishing to be bound by theory, when the final breaking depth is 13 mm or more, polypropylene often does not orient sufficiently, fibrils cannot extend uniformly, it is difficult to achieve a high magnification, it is difficult to form stretched chains, resulting in a decrease in rigidity, the pore structure easily deforms under slight stress, and there is a tendency not to obtain high puncture strength.

[0082] The puncture test for measuring the puncture depth and puncture strength will be described in the examples.

[0083] When the final breaking point strength is 1.5 times or more the first breaking point strength in puncture strength measurement, destruction is difficult to progress due to stress relaxation by the thermoplastic polymer, elongation of the polypropylene component is enabled, and as a result, a highly oriented structure of lamellar crystals in the high strain region is achieved, so there is a tendency to have high puncture strength.

[0084] When the final breaking point strength is 2.5 times or less the first breaking point strength of the puncture strength, extreme high orientation of lamellar crystals is suppressed, and since the elastomer easily forms a structure connecting between lamellae, destruction is less likely to occur with respect to strain, and it is likely to have a high MD elongation. Also, when the final breaking point strength is 2.5 times or less the first breaking point strength, the stress applied to polypropylene during stretching decreases, so the heat shrinkage rate also tends to decrease.

[0085] 〈Thermal shrinkage rate of separator substrate〉 The separator substrate preferably has a heat shrinkage rate in the transverse direction (TD) after heat treatment at 105°C for 1 hour of 5% or less, more preferably -1.0% or more and 3.0% or less. That is, a separator substrate with a TD heat shrinkage rate of 5% or less at 105°C means that the heat shrinkage in the TD is very small even at high temperatures. By having the heat shrinkage rate be 5% or less or 3.0% or less, short circuits at high temperatures can be effectively suppressed. The reason for the heat shrinkage rate being -1.0% or more is that during the measurement of the heat shrinkage rate, the substrate may expand in the TD and the heat shrinkage rate may become less than 0% and 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 substrate having a heat shrinkage rate of 5% or less or -1.0% 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 a 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 becomes very large, whereas in a dry separator by uniaxial stretching, it is easy to obtain a separator substrate having a heat shrinkage rate of 5% or less or -1.0% or more and 3.0% or less.

[0086] The heat shrinkage rate of the separator substrate in the machine direction (MD) after heat treatment at 105°C for 1 hour is preferably 20% or less, more preferably 15% or less, still more preferably 10% or less, even more preferably 8% or less, particularly preferably 6% or less, and most preferably 5.5% or less from the viewpoints of the productivity of the power storage device and suppression of short circuits at high temperatures. The lower limit value of the heat shrinkage rate is not limited, but is preferably 0.1% or more, for example, 0.3% or more, or 0.5% or more. In the separator substrate of the present disclosure, as described above, when manufactured by uniaxial stretching in the MD and having a connected domain parallel to the MD in the microporous layer (A), it is considered that the stress applied to the polymer matrix can be relaxed even during heat shrinkage in the MD, and a low MD heat shrinkage rate can be achieved.

[0087] The tensile strength of the separator substrate in the MD is preferably 1500 kgf / cm from the viewpoints of the operability during battery winding and high puncture strength. 2Above (about 14.7 kN / cm 2 above), more preferably 1600 kgf / cm 2 above, even more preferably 1700 kgf / cm 2 above, still more preferably 1800 kgf / cm 2 above, particularly preferably 1900 kgf / cm 2 above, most preferably 2000 kgf / cm 2 above. The upper limit of the tensile strength of the MD of the separator substrate 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, 3000 kgf / cm 2 or less, 2800 kgf / cm 2 or less, or 2500 kgf / cm 2 or less may be sufficient.

[0088] The tensile elongation of the MD of the separator substrate is preferably 20% or more, more preferably 24% or more, even more preferably 26.5% or more, still more preferably 28% or more, particularly preferably 30% or more from the viewpoints of the productivity of the power storage device and high puncture strength. The upper limit of the tensile elongation of the MD of the separator substrate is preferably 60% or less, more preferably 55% or less, even more preferably 50% or less, still more preferably 45% or less, particularly preferably 40% or less from the viewpoint of workability. In the separator substrate of the present disclosure, as described above, when it is manufactured by uniaxial stretching in the MD and there are connection domains parallel to the MD in the microporous layer (A), it is considered that the stress applied to the polymer matrix can be relaxed even during tensile of the MD, and a high MD tensile elongation can be realized.

[0089] 《Method for Manufacturing Separator for Power Storage Device》 The manufacturing method of the separator for the energy storage device includes a melt extrusion step of melt extruding a resin composition mainly composed of polypropylene (hereinafter, also referred to as "polypropylene-based resin composition") to obtain a resin sheet (precursor sheet), and a pore formation step of forming pores in the obtained precursor sheet to make it porous. The manufacturing method of 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.

[0090] Examples of the dry method include a method of peeling the polypropylene crystal interface by heat treatment and stretching after melt kneading and extruding the polypropylene-based resin composition, and a method of peeling the interface between polypropylene and an inorganic filler by stretching after melt kneading the polypropylene-based resin composition and an inorganic filler and forming it into a film.

[0091] Examples of the wet method include a method of melt kneading the polypropylene-based resin composition and a pore-forming material, forming it into a film, stretching it if necessary, and then extracting the pore-forming material, and a method of dissolving the polypropylene-based resin composition and immersing it in a poor solvent for polypropylene to solidify the polypropylene and remove the solvent at the same time.

[0092] For the melt kneading of the polypropylene-based resin composition, a single-screw extruder and a twin-screw extruder can be used. In addition to these, for example, a kneader, a lab plastomill, a kneading roll, a Banbury mixer, etc. can also be used.

[0093] The polypropylene-based resin composition may optionally contain resins other than polypropylene, additives, etc. according to the manufacturing method of the microporous layer or according to the physical properties of the target microporous layer. Examples of the additives include pore-forming materials, fluorine-based flow improvers, 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-forming material include plasticizers, inorganic fillers, or combinations thereof.

[0094] Examples of the plasticizer include hydrocarbons such as liquid paraffin and paraffin wax; esters such as dioctyl phthalate and dibutyl phthalate; and higher alcohols such as oleyl alcohol and stearyl alcohol.

[0095] Examples of the inorganic filler include oxide-based ceramics such as alumina, silica (silicon oxide), titania, zirconia, magnesia, ceria, yttria, zinc oxide, and iron oxide; nitride-based ceramics such as silicon nitride, titanium nitride, and boron nitride; ceramics such as silicon carbide, calcium carbonate, aluminum sulfate, aluminum hydroxide, potassium titanate, talc, kaolin clay, kaolinite, halloysite, pyrophyllite, montmorillonite, sericite, mica, amesite, bentonite, asbestos, zeolite, calcium silicate, magnesium silicate, diatomaceous earth, and silica sand; and glass fiber.

[0096] As the method for manufacturing the separator base material, a dry lamellar crystal pore-forming process for peeling the polypropylene crystal interface by heat treatment and stretching is preferable. Here, as the method for manufacturing the separator base material having the microporous layer (A) and the microporous layer (B), it is preferable to use at least one of the following methods (i) and (ii): (i) A method for manufacturing a separator base material by coextrusion film formation, in which the microporous layer (A) and the microporous layer (B) are coextruded into a film and subjected to annealing, cold stretching, hot stretching, and heat relaxation processes; and (ii) A method for manufacturing a separator base material by lamination, in which the microporous layer (A) and the microporous layer (B) are separately extruded into films, laminated and bonded, and then subjected to annealing, cold stretching, hot stretching, and heat relaxation processes.

[0097] Among the above co-extrusion process (i) and lamination process (ii), from the perspective of manufacturing costs and the like, the co-extrusion process (i) is preferred. 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 preferably quenched by air, and the temperature of the blown air is preferably 20°C or lower, more preferably 15°C or lower. By blowing such cold air controlled at a low temperature, the resin after film formation is uniformly oriented in the MD direction.

[0098] In both the above co-extrusion process (i) and lamination process (ii), the method for manufacturing the separator substrate 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 is improved. By applying annealing at a specific temperature for a predetermined time, it is possible to obtain a good area-average pore size, high porosity, low air permeability, and high puncture strength 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. In the annealing step, the annealing treatment is preferably performed in a temperature range of 115°C or higher and 160°C or lower, more preferably 135°C or higher and 160°C or lower, and preferably for 20 minutes or longer, more preferably for 60 minutes or longer. This makes the crystals of the main component, polypropylene, highly oriented, and high porosity can be obtained during the stretching in the subsequent process, realizing a separator with low air permeability and high puncture strength due to a high elastic modulus, which is preferable from the perspective of achieving high input / output and high energy density of the power storage device.

[0099] The manufacturing method of the separator substrate may include a stretching process after the annealing process. As the stretching treatment, either uniaxial stretching or biaxial stretching can be used. Although not limited, from the viewpoints of manufacturing cost when using the dry method, reduction of TD thermal shrinkage, etc., uniaxial stretching is preferred. The elongation rate of MD in cold stretching ((dimension after stretching - dimension before stretching) / dimension before stretching × 100 (%)) is preferably in the range of 5% or more and 50% or less, more preferably 20% or more and 45% or less, still more preferably 25% or more and 45% or less. By increasing the amount of crack generation during cold stretching, it is easier to obtain a small pore diameter during hot stretching, and an appropriate and good stalk height can be obtained without being excessive. Furthermore, due to the formation of a smaller pore diameter, the domains (thermoplastic elastomers incompatible with polypropylene (PP)) are connected, and even at the time of fracture, the stress relaxation of the connected domains makes it difficult for the fracture to progress, enabling the elongation of polypropylene, which is the main component. As a result, a high-lamellar crystal highly oriented structure in the high-strain region is achieved, and a separator with high puncture strength can be obtained. The temperature of cold stretching is preferably 10°C or more and 50°C or less, more preferably 20°C or more and 30°C or less, and may be carried out at room temperature (23 ± 2°C) from the viewpoint of manufacturing cost. From the viewpoints of improving both the puncture strength and low air permeability of the obtained separator substrate, manufacturing cost, reduction of TD thermal shrinkage, etc., uniaxial stretching is preferred.

[0100] In order to suppress the thermal shrinkage of the separator substrate, a heat treatment step may be performed after the stretching step or after the pore formation step for the purpose of heat fixation. The heat treatment step may include a hot stretching operation performed at a predetermined temperature and a predetermined stretching ratio for the purpose of adjusting physical properties, and / or a heat relaxation operation performed at a predetermined temperature and a predetermined relaxation ratio for the purpose of reducing the shrinkage stress applied during film formation and stretching. The heat relaxation operation may be performed after the hot stretching operation. In hot stretching and heat relaxation, taking the dimension of MD before stretching as 100%, it is preferably stretched to 140% or more and 280% or less, more preferably 160% or more and 260% or less, and still more preferably 180% or more and 240% or less. Thereby, stretched-out chains are formed and turned into strong fibrils, and a separator with high puncture strength can be obtained. Also, in the above heat treatment step, it tends to be possible to obtain a good stem height by performing a stretching operation at a specific stretching ratio or more. The reason is considered to be that below a specific stretching ratio, pore formation is prioritized, and after the pore formation process, a structural change accompanied by a change in the stem height occurs. In heat relaxation after hot stretching, it is preferably relaxed by 10% or more and 50% or less in MD, more preferably 20% or more and 45% or less. These heat treatment steps can be performed using a tenter or a roll stretcher. The temperature of the heat treatment step is preferably 120°C or more and 160°C or less, and more preferably 130°C or more and 155°C or less. By performing the above heat treatment at a temperature equal to or higher than the melting point of the thermoplastic elastomer, the stress applied to the polypropylene backbone, which is the main component during hot stretching, is relaxed, and fibrils are uniformly extended in MD, thereby obtaining a separator with high puncture strength, improved rigidity and toughness.

[0101] The obtained separator substrate can be used as a separator for a power storage device as it is. 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.

[0102] 《Power Storage Device》 The power storage device of the present disclosure includes a separator for the 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 the power storage device of the present disclosure is disposed between the positive electrode and the negative electrode.

[0103] Examples of the power storage device include, but are not limited to, 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 a lithium-ion secondary battery.

[0104] For example, the power storage device can be manufactured by superposing a positive electrode and a negative electrode via the separator described above, winding them as 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.

[0105] More preferably, the present power storage device is a lithium-ion secondary battery, and here, a preferred embodiment of the lithium-ion secondary battery will be described.

[0106] 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 0.5 Spinel-type lithium nickel manganese oxide represented by O4, lithium nickel oxide represented by LiNiO2, lithium-containing composite metal oxide represented by LiMO2 (M represents two or more elements selected from the group consisting of Ni, Mn, Co, Al, and Mg), and lithium iron phosphate compound represented by LiFePO4 can be mentioned. Among these, from the viewpoints of high safety and long-term stability, more preferably, lithium cobalt oxide represented by LiCoO2, lithium nickel oxide represented by LiNiO2, lithium-containing composite metal oxide represented by LiMO2 (M represents two or more elements selected from the group consisting of Ni, Mn, Co, Al, and Mg), and lithium iron phosphate compound represented by LiFePO4 can be mentioned, and particularly preferably, lithium iron phosphate compound represented by LiFePO4.

[0107] The negative electrode is not particularly limited as long as it functions as the negative electrode of a lithium-ion secondary battery, and known ones may be used. The negative electrode preferably contains one or more materials selected from the group consisting of a material capable of occluding and releasing lithium ions as a negative electrode active material and metallic lithium. That is, 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 represented by hard carbon, soft carbon, artificial graphite, natural graphite, graphite, pyrolytic carbon, coke, vitreous carbon, fired bodies of organic polymer compounds, mesocarbon microbeads, carbon fibers, activated carbon, graphite, carbon colloids, and carbon black.

Example

[0108] Hereinafter, the measurement method and evaluation method adopted in this example will be described. In this example, since the "separator base material" corresponds to the "separator", in the following description, the "separator base material" may be read as the "separator".

[0109] 《Measurement and Evaluation Method》 [Measurement of Melt Flow Rate (MFR)] The melt flow rate (MFR) of the microporous layer (A) and the microporous layer (B) was measured in accordance with JIS K 7210 under the conditions of a temperature of 230°C and a load of 2.16 kg (the unit is g / 10 min). The MFR of polypropylene was measured in accordance with JIS K 7210 under the conditions of a temperature of 230°C and a load of 2.16 kg. The melt flow rate (MFR) of polyethylene was measured in accordance with JIS K 7210 under the conditions of a temperature of 190°C and a load of 2.16 kg.

[0110] [Measurement of Mw and Mn by GPC (Gel Permeation Chromatography)] Using Agilent PL-GPC220, standard polystyrene was measured under the following conditions to create a calibration curve. The chromatograph of the sample polymer was also measured under the same conditions, and based on the calibration curve, the weight average molecular weight (Mw), number average molecular weight (Mn), and MWD (Mw / Mn) obtained by dividing the weight average molecular weight (Mw) of the polymer by the number average molecular weight (Mn) were calculated under the following conditions. Column: TSKgel GMHHR-H(20) HT(7.8 mm I.D.×30 cm) 2 columns Mobile phase: 1,2,4-trichlorobenzene Detector: RI Column temperature: 160°C Sample concentration: 1 mg / ml Calibration curve: Polystyrene

[0111] [Measurement of Melt Tension] Using a Capillograph manufactured by Toyo Seiki Seisaku-sho, the melt tension (mN) of the microporous layer (A) and the microporous layer (B) was measured under the following conditions. · Capillary: diameter 1.0 mm, length 20 mm · Cylinder extrusion speed: 2 mm / min · Withdrawal speed: 60 m / min · Temperature: 240 °C

[0112] [Measurement of pentad fraction] The pentad fraction of polypropylene was assigned based on the description in the Polymer Analysis Handbook (edited by the Japanese Society for Analytical Chemistry). 13 It was calculated by the peak height method from the C-NMR spectrum. 13 The C-NMR spectrum was measured using a JEOL-ECZ500. Polypropylene pellets were dissolved in o-dichlorobenzene-d, and the measurement was carried out under the conditions of a measurement temperature of 145 °C and an integration number of 25,000 times.

[0113] [Measurement of DSC] Using a DSC-60 manufactured by Shimadzu Corporation, the DSC measurement of the microporous layer (A) was carried out under the following conditions. · Sample amount: approximately 5 mg · Cell used: aluminum crimp cell (diameter 5.8 mm) · Atmosphere used: nitrogen (flow rate 50 mL / min) · Temperature program: First step: Heat from room temperature to 230 °C at 10 °C / min and hold for 5 min. Second step: Cool to 20 °C at 10 °C / min and hold for 5 min. Third step: Heat to 200 °C at 10 °C / min and hold for 5 min. From the DSC curve (with the vertical axis being the heat flow and the horizontal axis being the temperature) during the heating process of the above third step, using the analysis software TA-60 attached to the apparatus, the peak temperatures of the endothermic peaks A and B were read, and the area S of the endothermic peak A A and the area S of the endothermic peak B B were calculated, and S B / S A was determined.

[0114] [Measurement of Thickness (μm)] Using a Mitutoyo Digimatic Indicator IDC112, the thickness (μm) of the separator substrate was measured at room temperature of 23 ± 2°C. The thickness of each microporous layer was calculated from the image data obtained by cross-sectional SEM acquired by the evaluation method of the area-average pore size described below.

[0115] [Measurement of Porosity (%)] A sample with dimensions of 10 cm × 10 cm square was cut from the separator or the microporous layer, and its volume (cm 3 ) and mass (g) were determined. From these and the density (g / cm 3 ), the porosity was calculated using the following formula. Porosity (%) = (Volume - Mass / Density) / Volume × 100

[0116] [Air Permeability (seconds / 100 cm 3 )] Using a Gurley-type air permeability meter compliant with JIS P-8117, the air permeability (seconds / 100 cm 3 ) of the separator substrate was measured. By dividing the air permeability (seconds / 100 cm 3 ) by the separator substrate thickness (μm) and multiplying by 10 μm, the air permeability (seconds / 100 cm 3 ) when the separator substrate thickness was converted to 10 μm was determined.

[0117] [MD Thermal Shrinkage Rate (%), TD Thermal Shrinkage Rate (%)] Samples obtained by cutting out the separator substrate into squares of 50 mm each in MD / TD were placed on copy paper and put into a hot air dryer (manufactured by Yamato Scientific Co., Ltd., DF1032), and heat treatment was performed at 105°C for 1 hour under normal pressure in the air. The samples were taken out from the hot air dryer, allowed to cool at 25°C for 10 minutes, and then the dimensional shrinkage rate was determined. Thermal Shrinkage Rate (%) = (Dimension before heating (mm) - Dimension after heating (mm)) / (Dimension before heating (mm)) × 100

[0118] [Puncture Strength and Puncture Depth] With reference to FIGS. 3 to 5, the measurement of the puncture strength and puncture depth of the present disclosure will be described below. As illustrated in FIG. 1, a needle (8) with a hemispherical tip having a radius of 0.5 mm was prepared, a separator (7) was sandwiched between two plates (9, 9) having an opening with a diameter (dia.) of 11 mm, and the needle (8), separator (7), and plates (9, 9) were set. Using "MX2-50N" manufactured by IMADA Co., Ltd., a puncture test was conducted under the conditions of a needle tip curvature radius of 0.5 mm, an opening diameter of 11 mm of the separator holding plate, and a puncture speed of 25 mm / min. The needle (8) was brought into contact with the separator (7), and as illustrated in FIG. 5, the maximum puncture load (i.e., puncture strength (gf)) was measured. By dividing the puncture strength (gf) by the separator base material thickness (μm) and multiplying by 10 μm, the puncture strength (gf) when the separator base material thickness was converted to 10 μm was obtained. The load when the separator (7) first split longitudinally in the MD direction, on one or both sides of the needle (8) simultaneously, and the load suddenly dropped, as shown in FIG. 4, was defined as the first breaking point strength. In FIG. 3(c), the depth from when the needle (8) touched the separator (7) to the first breaking point was defined as the first breaking depth (D). Continuing the puncture, as illustrated in FIG. 4(c), the load when the separator (7) finally completely broke in the TD direction was measured as the final breaking point strength. As illustrated in FIGS. 4 and 5, the displacement (mm) of the needle from when it touched the separator (7) to reaching the final break was measured as the final breaking depth.

[0119] [MD Tensile Strength, MD Tensile Elongation] The tensile strength of the separator was measured using a tensile testing machine (TG-1kN type manufactured by Minebea Co., Ltd.) with a sample length before the test set to 35 mm and the sample being 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 (breaking) if it broke before yielding, by the cross-sectional area of the test piece was defined as the tensile strength (kgf / cm 2 ). Also, the elongation (rate of elongation from before the test) when the sample broke was defined as the tensile elongation (%). The tensile strength and tensile elongation in the MD direction of the separator were measured.

[0120] [Area-Average Long Hole Diameter (nm)] The area-average major pore diameter was measured by image analysis of cross-sectional SEM observations. As a pretreatment, the separator was ruthenium-stained, and a cross-sectional sample was prepared by cryo-fracture. The cross-section was 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), dried, and then 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 to obtain a sample for microscopy. Next, using a scanning electron microscope (S-4800 manufactured by Hitachi High-Technologies), eight arbitrary points on the cross-section of each microporous layer of the microporous membrane 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.

[0121] The observed image was trimmed in the environment of the programming language Python using functions of OpenCV, an image analysis library, so that only the cross-section of any one microporous layer was included. After removing the surface, the outside, and other microporous layers, binarization was performed using the Otsu method to separate the resin part and the pore part, and the average major pore diameter of the pore part was calculated. At this time, pore areas existing across the imaging range and outside the imaging range with an area of 0.001 nm 2 The following pores were excluded from the measurement targets. The average diameter was calculated by area averaging from the area of each pore.

[0122] [Trunk height (nm)] The trunk height was measured by image analysis of cross-sectional SEM observations. Similar to the calculation of the area-average major pore diameter, after preparing the cross-sectional sample and the sample for microscopy, three arbitrary points on the cross-section of the microporous membrane 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 5,000 times.

[0123] The observation image was binarized using the Otsu method with functions of OpenCV, an image analysis library, in the environment of the programming language Python to separate the resin part and the hole part. Further, blurring processing was repeatedly performed only on ND to remove the fibril part. The above fibril removal process was performed by repeating 100 times using a Gaussian filter with a bar-shaped processing range of 3 pixels for ND and 1 pixel for MD. After the blurring process, opening and closing processes were sequentially performed in an elliptical processing range with a major axis of 7 pixels for ND and a minor axis of 3 pixels for MD to remove noise, and a fibril removal image was obtained.

[0124] The trunk height was calculated from the above fibril removal image. The fibril removal image was cut out by 1 pixel of MD, the lengths of the resin parts of ND were all detected, and this was repeated so as to include all of MD, and the lengths of the resin parts of ND in the entire range of the fibril removal image were detected. Figure 2 is a schematic diagram showing a part of the fibril removal image cut out. The bright part in the figure is the resin part (5), the dark part is the hole part (6), and an example of the part to be cut out and detected by 1 pixel of MD is the part indicated by the double arrow. Regarding the numerical values of the lengths of the resin parts of ND obtained, a weighted average with the lengths of the resin parts of ND as weights was calculated, and the obtained value was taken as the trunk height. If the length of the resin part of ND is L, the trunk height H is given by the following mathematical formula:

Equation

[0125] 《Example 1》 [Preparation of Polypropylene Resin Composition] Pellets of the high molecular weight polypropylene resin (PP1, MFR = 0.51) and ethylene / 1-butene copolymer (C2C4, MFR = 6.7) shown in Table 1 were dry blended at a mass ratio of PP1:C2C4 = 88.0:12.0 (mass %), and then melt kneaded using TEM26SS (manufactured by Toshiba Machine Co., L / D = 48.5). After melt kneading, strands were drawn from the die (3 holes) and cooled in a water-cooled bath, and then cut using a pelletizer to obtain polypropylene resin composition pellets.

[0126] [Preparation of microporous layer] As the resin of the microporous layer (A), the high molecular weight polypropylene resin (PP1, MFR = 0.51) shown in Table 1 and the above polypropylene resin composition pellets were dry blended at a mass ratio of PP1: polypropylene resin composition pellets = 73.0:27.0 (mass %), then melted using a 2.5-inch extruder and supplied to a single-layer inflation die using a gear pump. The temperature of the inflation die was set at 240 °C. After the molten polymer was discharged from the inflation die, the discharged resin was cooled by blowing air and wound around a roll to obtain a precursor sheet having a single-layer structure composed of the microporous layer (A) with a thickness of about 12 μm. Here, the distance between the lips (lip clearance) of the inflation die was set at 1.8 mm, and discharge was carried out under a discharge rate condition of 9 kg / h.

[0127] Next, the obtained precursor sheet was put into a dryer and annealed at 150 °C for 180 minutes. Then, the annealed precursor sheet was cold drawn by 30% in the MD at room temperature, and the drawn film was put into an oven at 135 °C without shrinkage, and hot drawn to 200% in the MD with the dimension before drawing as 100%, and then heat relaxed by 44% in the MD to obtain a separator substrate having a single-layer structure composed of the microporous layer (A). The evaluation results of the structure and physical properties of the obtained separator substrate are shown in Table 1.

[0128] The lower limit thickness of the precursor sheet was determined by thinning the precursor sheet with a thickness of about 12 μm by gradually increasing the take-up speed while keeping the discharge amount constant after manufacturing the precursor sheet, and measuring the film thickness immediately before film breakage. Further, the obtained precursor sheet with the lower limit thickness was stretched and perforated under the above annealing and stretching conditions (annealed at 150 °C for 180 minutes, cold-rolled at room temperature by 30%, hot-rolled at 135 °C up to 200%, and relaxed by 44%) to obtain a separator substrate having a single-layer structure, and the lower limit thickness of the separator was determined by measuring the film thickness. The obtained results are shown in Table 1.

[0129] 《Examples 2 to 12, Comparative Example 2》 As shown in Table 1, the raw materials were changed, the mass ratio of the polypropylene resin and the polypropylene resin composition pellets was adjusted as shown in the composition of Table 1, and a separator substrate having a single-layer structure was obtained in the same manner as in Example 1 except that the extrusion temperature at which the die and the discharge were stable was finely adjusted. The evaluation results of the obtained separator substrate are shown in Table 1.

[0130] 《Comparative Example 1》 A polypropylene resin composition was not prepared, and 100% by mass of the high molecular weight polypropylene resin shown in Table 1 was melted as the resin of the microporous layer (A) using a 2.5-inch extruder, and a separator substrate having a single-layer structure was obtained in the same manner as in Example 1 except that the extrusion temperature at which the die and the discharge were stable was finely adjusted. The evaluation results of the obtained separator substrate are shown in Table 1.

[0131]

Table 1-1

[0132]

Table 1-2

Industrial Applicability

[0133] 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.

Explanation of Symbols

[0134] 1 Polymer matrix 2 Domain that becomes the light-colored part after dyeing 3 Fiber 4 Hole 5 Resin part (light-colored part) 6 Hole part (dark-colored part) 7 Separator 8 Needle 9 Plate D First fracture depth dia. Diameter

Claims

1. A separator for a power storage device, comprising a separator substrate having a microporous layer (A) containing a polyolefin as a main component and containing a thermoplastic elastomer, wherein the microporous layer (A) contains 80.0% by mass or more and 99.5% by mass or less of the polyolefin and 0.5% by mass or more and 20.0% by mass or less of the thermoplastic elastomer based on the total mass of the microporous layer (A), and contains 80.0% by mass or more and 99.5% by mass or less of polypropylene based on the total mass of the polyolefin, wherein the separator for a power storage device has a final fracture depth in the measurement of the puncture strength of 5.5 mm or more.

2. The separator for a power storage device according to claim 1, wherein in the measurement of the puncture strength of the separator for a power storage device, the final fracture point strength is 1.5 times or more the first fracture point strength.

3. The separator for a power storage device according to claim 1 or 2, wherein the final fracture depth of the separator for a power storage device is 11 mm or less.

4. The separator for a power storage device according to claim 1 or 2, wherein in the measurement of the puncture strength of the separator for a power storage device, the final fracture point strength is 2.5 times or less the first fracture point strength.

5. The separator for a power storage device according to claim 1 or 2, wherein the thermoplastic elastomer contains at least one selected from the group consisting of ethylene, propylene, and 1-butene as repeating units.

6. The separator for a power storage device according to claim 1 or 2, wherein the melt flow rate (MFR) measured at a load of 2.16 kg and a temperature of 230°C of the microporous layer (A) is 0.9 g / 10 min or less.

7. The separator for a power storage device according to claim 1 or 2, wherein the content of the thermoplastic elastomer is 3.0 to 10.0% by mass based on the total mass of the microporous layer (A).

8. The melt tension Mt at 240°C of the microporous layer (A) Ａ is 10 mN or more and 35 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 weight average molecular weight (Mw) of the microporous layer (A) is 250,000 or more and 1,500,000 or less.

10. The separator for a power storage device according to claim 9, wherein the molecular weight distribution (Mw / Mn), which is a value obtained by dividing the weight average molecular weight (Mw) of the microporous layer (A) by the number average molecular weight (Mn), is 3 or more and 30 or less.

11. The separator for a power storage device according to claim 1 or 2, wherein the microporous layer (A) has a melt flow rate (MFR) of 0.3 g / 10 min or more when measured at a load of 2.16 kg and a temperature of 230°C.

12. The separator for a power storage device according to claim 1 or 2, wherein the weight average molecular weight (Mw) of the polypropylene is 300,000 or more and 1,300,000 or less.

13. １３ The separator for a power storage device according to claim 1 or 2, wherein the pentad fraction of the polypropylene measured by C-NMR (nuclear magnetic resonance method) is 94.0% or more.

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

15. The separator for a power storage device according to claim 1 or 2, wherein the thickness of the separator base material is 3 μm or more and 20 μm or less.

16. The separator for a power storage device according to claim 1 or 2, wherein the area average pore size calculated from the SEM image of the MD-ND cross section of the microporous layer (A) is 50 nm or more and 500 nm or less.

17. The separator for a power storage device according to claim 1 or 2, wherein the TD thermal shrinkage rate of the separator base material at 105°C for 1 hour is 5% or less.

18. The separator for a power storage device according to claim 1 or 2, wherein the MD thermal shrinkage rate of the separator base material at 105°C for 1 hour is 20% or less.

19. The separator for a power storage device according to claim 1 or 2, wherein the MD tensile elongation of the separator base material is 20% or more and 60% or less.

20. The MD tensile strength of the separator base material is 2000 to 2500 kgf / cm ２ The separator for a power storage device according to claim 1 or 2.

21. The separator base material has a melt tension Mt at 240°C of the microporous layer (A) Ａ and a different melt tension Mt Ｂ The separator for a power storage device according to claim 1 or 2, comprising a microporous layer (B).

22. 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, wherein the positive electrode contains lithium iron phosphate as a positive electrode active material.

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