Separator for power storage device and power storage device

Abstract

To provide a separator for a power storage device having a multi-layer structure capable of suppressing clogging in a power storage device with high strength.SOLUTION: In a separator for a power storage device having a microporous layer (X) containing polyolefin (A) as a main component and a microporous layer (Y) containing polyolefin (B) as a main component, the melt flow rate (MFR) of the microporous layer (X) at a load of 2.16 kg and a temperature of 230°C is 0.9 g / 10 min or less, and in the ND-MD cross-sectional observation of the microporous layer (X) and the microporous layer (Y), the average length pore diameter of the pores existing in the microporous layer (X) is 100 nm or more and 400 nm or less, and the average length pore diameter of the pores existing in the microporous layer (Y) is larger than the average length pore diameter of the pores present in the microporous layer (X).SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a separator for an electricity storage device. [Background technology]

[0002] Microporous membranes, particularly polyolefin-based microporous membranes, are used in many technical fields, such as microfiltration membranes, battery separators, capacitor separators, and fuel cell materials, and are particularly used as separators for power storage devices, such as lithium-ion batteries. Lithium-ion batteries are used in a variety of applications, including small electronic devices 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, there has been a demand for lithium ion batteries with high energy capacity, high energy density, and high output characteristics, and this has led to an increasing demand for separators that are thin and have high strength (for example, high puncture strength).

[0004] For example, Patent Document 1 describes a polypropylene resin composition for microporous films, which contains 5 to 30% by weight of a polypropylene resin (X) and 95 to 70% by weight of a polypropylene resin (Y) from the viewpoints of improving the rigidity of the microporous film and reducing the product defect rate at a certain thickness. The polypropylene resin (X) described in Patent Document 1 has a specific melt flow rate (MFR), a specific molecular weight distribution (Mw / Mn) determined by gel permeation chromatography (GPC), and a long chain branching structure. The polypropylene resin (Y) described in Patent Document 1 has a specific MFR and excludes the polypropylene resin (X).

[0005] Patent Document 2 describes a microporous membrane having the following three layers (a) to (c) from the viewpoint of improving the balance of electrochemical stability, high meltdown temperature, high electrolyte affinity, and low water retention of the microporous membrane: (a) 6.0 × 10 5a first layer containing 40.0 to 85.0% by weight of isotactic polypropylene having a weight average molecular weight (Mw) of 10.0 to 15.0% by weight; (b) a second layer containing a polyolefin; and (c) 6.0 × 10 5 A third layer containing 40.0 to 85.0% by weight of isotactic polypropylene having a Mw of at least 100%.

[0006] Patent Document 3 describes a melting point T mA a first microporous film comprising a first resin composition having a melting point T mA Lower melting point T mB and a second microporous film containing a second resin composition having the formula: wherein the extensional viscosity is 18,000 to 40,000 Pa·s and the shear viscosity is 5,000 to 10,000 Pa·s.

[0007] Patent Document 4 discloses a propylene-based resin microporous film made of a propylene-based resin having 25 to 60% by weight of components with a molecular weight of 50,000 or less, 19 to 30% by weight of components with a molecular weight of 700,000 or more, a weight-average molecular weight of 350,000 to 500,000, and a melt tension of 1.1 to 3.2 g.

[0008] Patent Document 5 describes a polypropylene resin composition for forming a microporous film, which contains as an essential component a propylene homopolymer, and which has an intrinsic viscosity [η] of 1 dl / g or more and less than 7 dl / g, a mesopentad fraction in the range of 94.0 to 99.5%, an elution integral up to 100°C during heating of 10% or less, a melting point of 153 to 167°C, and in an elution temperature-elution amount curve, the peak-top temperature of the maximum peak is between 105 and 130°C, and the half-width of the peak is 7.0°C or less. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Publication No. 2018-030992 [Patent Document 2] Special Publication No. 2013-517152 [Patent Document 3] Japanese Patent Application Publication No. 2016-022676 [Patent Document 4] Japanese Patent Application Laid-Open No. 2012-092286 [Patent Document 5] International Publication No. 2010 / 079784 Summary of the Invention [Problem to be solved by the invention]

[0010] It is known that the use of high-molecular-weight polyolefins in microporous membranes can provide high-strength microporous membranes. However, from the viewpoint of the safety of lithium-ion secondary batteries, there is a growing demand for multilayer polyolefin microporous membranes having at least one shutdown layer as separators for lithium-ion secondary batteries.

[0011] However, when a high-molecular-weight polyolefin is used as a constituent material of a microporous membrane, the membrane formability and pore openability during stretching deteriorate, resulting in a small pore size in the resulting microporous membrane. This poses a problem in that, when the microporous membrane is used as a separator in a lithium-ion secondary battery, clogging due to deposits is likely to occur.

[0012] In particular, in the case of a separator containing a multilayer polyolefin microporous membrane, the total thickness of the separator is limited from the viewpoint of battery characteristics, so each layer of the multilayer polyolefin microporous membrane needs to be thin. Under the constraints of the total thickness and the thickness of each layer in the multilayer structure, the pore size of a microporous layer formed using a high-molecular-weight polyolefin becomes even smaller, and clogging by deposits becomes significant.

[0013] For example, in Patent Document 3 cited above, a multilayer separator is produced using a high-molecular-weight polyolefin, and the pore size of the microporous layer formed using the high-molecular-weight polyolefin in this separator is small, and the separator is prone to clogging in a lithium-ion secondary battery.

[0014] Therefore, an object of the present invention is to provide a separator for an electricity storage device having a multilayer structure that has high strength and can suppress clogging within the electricity storage device. [Means for solving the problem]

[0015] As a result of extensive research into solving the above problems, the present inventors have found that using a separator with a multilayer structure containing a polyolefin having a specific melt flow rate (MFR) as a main component and having a microporous layer with a specific average long pore diameter is advantageous in solving the above problems, and have thus completed the present invention. Examples of embodiments of the present invention are listed in the following items [1] to

[10] . [1] A microporous layer (X) mainly composed of polyolefin (A) and a microporous layer (Y) mainly composed of polyolefin (B), the microporous layer (X) has a melt flow rate (MFR) of 0.9 g / 10 min or less under a load of 2.16 kg and at a temperature of 230°C; In ND-MD cross-sectional observation of the microporous layer (X) and the microporous layer (Y) using a scanning electron microscope (SEM), The average long pore diameter of the pores present in the microporous layer (X) is 100 nm or more and 400 nm or less, and the average long pore diameter of the pores present in the microporous layer (Y) is larger than the average long pore diameter of the pores present in the microporous layer (X); Separators for power storage devices. [2] In ND-MD cross-section observation of the microporous layer (X) and the microporous layer (Y) by SEM, the average long pore diameter of pores present in the microporous layer (Y) is 150 nm or more and 2000 nm or less, and is 1.2 times or more and 10 times or less the average long pore diameter of pores present in the microporous layer (X). Item 1. A separator for an electricity storage device according to item 1. [3] The air permeability of the separator for an electricity storage device when converted to a thickness of 16 μm is 250 sec / 100 cm 3 Below is the Item 3. The separator for an electricity storage device according to item 1 or 2. [4] The porosity of the separator for the electricity storage device is 20% or more and 70% or less. Item 3. The separator for an electricity storage device according to any one of items 1 to 3. [5] The main component of the polyolefin (A) is polypropylene. Item 10. The separator for an electricity storage device according to any one of items 1 to 4. [6] The main component of the polyolefin (A) is polypropylene, and The main component of the polyolefin (B) is polyethylene. Item 6. The separator for an electricity storage device according to any one of items 1 to 5. [7] The pentad fraction of the polypropylene is 94.0% or more. Item 7. The separator for an electricity storage device according to item 5 or 6. [8] The polypropylene has an MFR of 0.6 g / 10 min or less under a load of 2.16 kg and a temperature of 230 ° C. 8. The separator for an electricity storage device according to any one of items 5 to 7. [9] The separator for an electricity storage device has a puncture strength of 300 gf or more when converted to a thickness of 16 μm. Item 9. The separator for an electricity storage device according to any one of items 1 to 8.

[10] The separator for an electricity storage device according to any one of items 1 to 9 is provided. Energy storage device. [Effects of the Invention]

[0016] According to the present invention, it is possible to provide a separator for an electricity storage device that has high strength and can suppress clogging in an electricity storage device such as a lithium ion secondary battery. Note that the above description should not be considered to disclose all embodiments of the present invention and all advantages related to the present invention. Further embodiments and advantages of the present invention will become apparent from the following description. DETAILED DESCRIPTION OF THE INVENTION

[0017] Hereinafter, an embodiment of the present invention (hereinafter referred to as "the present embodiment") will be described in detail for the purpose of illustrating the present invention, but the present invention is not limited to this embodiment. Note that the weight average molecular weight (Mw), number average molecular weight (Mn), and Mw / Mn of the polyolefin of the present embodiment are polystyrene-equivalent molecular weights obtained by GPC (gel permeation chromatography) measurement.

[0018] <Separator for power storage device> <Microporous layer> The separator for an electricity storage device of this embodiment includes a microporous layer mainly composed of a polyolefin. In this embodiment, the microporous layer refers to one microporous layer constituting the separator for an electricity storage device, and may be a multilayer structure formed by stacking two or more layers. In one aspect, the separator for an electricity storage device includes a microporous layer (X) mainly composed of a polyolefin (A) and a microporous layer (Y) mainly composed of a polyolefin (B).

[0019] Even when the separator is composed of multiple microporous layers, each microporous layer can be easily peeled off and collected by applying and pulling an adhesive tape to the edge of the separator, and physical properties such as melt tension, melt flow rate (MFR), molecular weight, long pore diameter, porosity, and thickness, which will be described later, can be measured.

[0020] <Microporous layer (X)> The separator for an electricity storage device of this embodiment has a microporous layer (X). The separator for an electricity storage device may have only one microporous layer (X) or two or more microporous layers. In this embodiment, "containing polyolefin (A) as a main component" means that the microporous layer (X) contains 50 mass% or more of polyolefin (A) based on the total mass of the microporous layer (X). The lower limit of the content of polyolefin (A) in the microporous layer (X) is preferably 55 mass% or more, 60 mass% or more, 70 mass% or more, 80 mass% or more, 90 mass% or more, or 95 mass% or more, from the viewpoints of the wettability of the separator to the electrolyte, thinning of the membrane, shutdown properties, etc. The upper limit of the content of polyolefin (A) in the microporous layer (X) 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.

[0021] <Polyolefin (A)> The polyolefin (A) in this embodiment is a polymer containing a monomer having a carbon-carbon double bond as a repeating unit. Examples of monomers constituting the polyolefin (A) include, but are not limited to, monomers having 1 or more but less than 10 carbon atoms and having a carbon-carbon double bond, such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene. The polyolefin (A) may be, for example, a homopolymer, a copolymer, or a multi-stage polymer, and is preferably a homopolymer.

[0022] Specifically, the polyolefin (A) is preferably polyethylene, polypropylene, a copolymer of ethylene and propylene, or a mixture thereof, from the viewpoint of shutdown characteristics, etc. The polyolefin (A) is more preferably polypropylene.

[0023] The stereoregularity of the polypropylene is not limited, but examples thereof include atactic, isotactic, or syndiotactic polypropylene. The polypropylene according to the present embodiment is preferably a highly crystalline isotactic or syndiotactic homopolymer.

[0024] The polypropylene usable as the polyolefin (A) is preferably a homopolymer, but may also be a copolymer, such as a block polymer, in which a small amount of a comonomer other than propylene, such as an α-olefin comonomer, is copolymerized. The lower limit of the amount of propylene structures contained as repeating units in the polypropylene may be, for example, 70 mol% or more, 80 mol% or more, 90 mol% or more, 95 mol% or more, or 99 mol% or more. The polypropylene may contain repeating units other than the propylene structure. In such cases, the upper limit of the amount of repeating units derived from the comonomer (excluding the propylene structure) 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 may be used alone or in combination of two or more types.

[0025] The lower limit of the weight average molecular weight (Mw) of the polyolefin (A) is preferably 300,000 or more, more preferably 500,000 or more, even more preferably 600,000 or more, still more preferably 700,000 or more, and particularly preferably 800,000 or more, from the viewpoint of the strength of the microporous layer (X). The upper limit of the weight average molecular weight (Mw) of the polyolefin (A) is preferably 1,500,000 or less, more preferably 1,300,000 or less, even more preferably 1,100,000 or less, still more preferably 1,000,000 or less, and particularly preferably 960,000 or less, from the viewpoint of increasing the pore size of the microporous layer (X) and suppressing clogging.

[0026] The upper limit of the value (Mw / Mn) obtained by dividing the weight-average molecular weight (Mw) of the polyolefin (A) by the number-average molecular weight (Mn) is preferably 7 or less, 6.5 or less, 6 or less, 5.5 or less, or 5 or less. The smaller the Mw / Mn value, the less entanglement of molecules there is, and the melt tension of the resulting microporous layer (X) tends to be lower. Therefore, it is preferable that the Mw / Mn value of the polyolefin (A) is 7 or less, because this allows the melt tension of the microporous layer (X) to be controlled low and the microporous layer (X) to be produced more stably as a thinner film. The lower limit of the Mw / Mn of the polyolefin (A) is preferably 1 or more, 1.3 or more, 1.5 or more, 2.0 or more, or 2.5 or more. An Mw / Mn of 1 or more tends to maintain appropriate molecular entanglement and improve stability during film formation.

[0027] When the polyolefin (A) is polypropylene, the lower limit of the weight-average molecular weight (Mw) of the polypropylene is preferably 300,000 or more, more preferably 500,000 or more, even more preferably 600,000 or more, still more preferably 700,000 or more, and particularly preferably 800,000 or more, from the viewpoint of the strength of the microporous layer (X). The upper limit of the Mw of the polypropylene is preferably 1,500,000 or less, more preferably 1,300,000 or less, even more preferably 1,100,000 or less, still more preferably 1,000,000 or less, and particularly preferably 960,000 or less, from the viewpoint of increasing the pore size of the microporous layer (X) and suppressing clogging.

[0028] When the polyolefin (A) is polypropylene, the upper limit of the value (Mw / Mn) obtained by dividing the weight-average molecular weight (Mw) of the polypropylene by the number-average molecular weight (Mn) is preferably 7 or less, 6.5 or less, 6 or less, 5.5 or less, or 5 or less. The smaller the Mw / Mn value of the polypropylene, the lower the melt tension of the resulting microporous layer (X) tends to be. Therefore, a polypropylene Mw / Mn value of 7 or less is preferred for controlling the melt tension of the microporous layer (X) to 30 mN or less. The lower limit of the Mw / Mn value of the polypropylene is preferably 1 or more, 1.3 or more, 1.5 or more, 2.0 or more, or 2.5 or more. When the Mw / Mn of the polypropylene is 1 or more, appropriate molecular entanglement is maintained, and stability during film formation tends to be good.

[0029] The lower limit of the density of the polyolefin (A) is preferably 0.85 g / cm 3 More than 0.88g / cm 3 More than 0.89g / cm 3 or more, or 0.90 g / cm 3 The upper limit of the density of the polyolefin (A) is preferably 1.1 g / cm 3 Below 1.0g / cm 3 Below, 0.98g / cm 3 Below, 0.97g / cm 3 Below, 0.96g / cm 3 Below, 0.95g / cm 3 Below, 0.94g / cm 3 Below, 0.93g / cm 3 or less than 0.92 g / cm 3 The density of the polyolefin (A) is related to the crystallinity of the polyolefin (A), and when the density of the polyolefin (A) is 0.85 g / cm or less, 3 If the above conditions are met, the productivity of the microporous layer (X) is improved, and this is particularly advantageous in a method for making a resin raw sheet porous by a dry method (hereinafter referred to as a dry method).

[0030] <Melt tension of the microporous layer (X)> The upper limit of the melt tension of the microporous layer (X) (melt tension of a single layer) measured at 230°C is preferably 30 mN or less, more preferably 25 mN or less, from the viewpoint of formability of the microporous layer (X). The lower limit of the melt tension of the microporous layer (X) (melt tension of a single layer) is preferably 10 mN or more, more preferably 15 mN or more, and even more preferably 20 mN or more, from the viewpoint of strength of the microporous layer (X) or microporous membrane.

[0031] When the polyolefin (A) is polypropylene, the upper limit of the melt tension of the polypropylene measured at 230°C is preferably 30 mN or less, more preferably 25 mN or less, from the viewpoint of the formability of the microporous layer (X). The lower limit of the melt tension of the polypropylene is preferably 10 mN or more, more preferably 15 mN or more, and even more preferably 20 mN or more, from the viewpoint of the strength of the microporous layer (X).

[0032] <Melt flow rate (MFR) of microporous layer (X)> In order to obtain a microporous layer (X) having higher strength, the upper limit of the melt flow rate (MFR) of the microporous layer (X) of the present embodiment (i.e., the MFR of a single layer) is, in one aspect, 0.9 g / 10 min or less, and may be, for example, 0.85 g / 10 min or less, 0.7 g / 10 min or less, 0.65 g / 10 min or less, or 0.5 g / 10 min or less. The lower limit of the MFR of the microporous layer (X) (MFR of a single layer) is not limited from the viewpoint of formability of the microporous layer (X), and may be, for example, 0.15 g / 10 min or more, 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, 0.38 g / 10 min or more, 0.4 g / 10 min or more, 0.45 g / 10 min or more, 0.5 g / 10 min or more, 0.55 g / 10 min or more, 0.6 g / 10 min or more, 0.65 g / 10 min or more, or 0.7 g / 10 min or more. The MFR in this embodiment is measured under conditions of a load of 2.16 kg and a temperature of 230°C.

[0033] A microporous layer (X) with an MFR of 0.9 g / 10 min or less means that the molecular weight of the polyolefin contained in the microporous layer (X) is high. A high molecular weight polyolefin tends to result in a high strength microporous layer (X) because there are more tie molecules bonding crystalline substances together. A microporous layer (X) with an MFR of 0.2 g / 10 min or more tends to prevent the melt tension of the microporous layer (X) from becoming too low, resulting in a high strength, thin microporous layer (X) or microporous membrane.

[0034] When the polyolefin (A) is polypropylene, the upper limit of the MFR of the polypropylene measured under a load of 2.16 kg and at a temperature of 230°C is preferably 0.9 g / 10 min or less, 0.85 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, 0.55 g / 10 min or less, or 0.5 g / 10 min or less, from the viewpoint of obtaining a microporous layer (X) with even higher strength. From the viewpoint of the formability of the microporous layer (X), the lower limit of the MFR of the polypropylene is preferably 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, 0.5 g / 10 min or more, 0.55 g / 10 min or more, 0.6 g / 10 min or more, 0.65 g / 10 min or more, or 0.7 g / 10 min or more.

[0035] <Pentad fraction> When the polyolefin (A) is polypropylene, in this embodiment, 13 From the viewpoint of obtaining a microporous layer (X) with low air permeability, the lower limit of the pentad fraction of the polypropylene measured by C-NMR (nuclear magnetic resonance) is preferably 94.0% or more, 95.0% or more, 96.0% or more, 96.5% or more, 97.0% or more, 97.5% or more, 98.0% or more, 98.5% or more, or 99.0% or more. The upper limit of the pentad fraction of the polypropylene is not limited, but may be, for example, 99.9% or less, 99.8% or less, or 99.5% or less.

[0036] When the pentad fraction of the polypropylene is 94.0% or higher, the crystallinity of the polypropylene is high. Separators obtained by the stretching method, particularly the dry method, have pores formed by stretching the amorphous portions between multiple crystalline portions. Therefore, high crystallinity of the polypropylene results in good pore opening in separators having a microporous layer (X). This allows for a larger average pore diameter of the pores present in the microporous layer (X) in ND-MD cross-sectional observation by scanning electron microscope (SEM), thereby reducing clogging in separators having the microporous layer (X). Furthermore, increasing the average pore diameter of the pores present in the microporous layer (X) in ND-MD cross-sectional observation by SEM can also reduce air permeability, enabling higher power output for energy storage devices.

[0037] <Average pore diameter of the microporous layer (X)> In the electricity storage device separator of this embodiment, when ND-MD cross section is observed by SEM, the average long pore diameter of the pores present in the microporous layer (X) is 100 nm or more and 400 nm or less. Here, the normal direction (ND) refers to the thickness direction of the microporous layer, and the machine direction (MD) refers to the deposition direction of the microporous layer. The MD of a separator having a microporous layer is the longitudinal direction if it is a roll. The present inventors have found that the average long pore diameter of pores present in this ND-MD cross section affects the prevention of clogging in electricity storage devices such as lithium ion secondary batteries and the adjustment of the separator's air permeability.

[0038] When used in the power storage device separator of this embodiment, the average pore diameter of the pores present in the microporous layer (X) in ND-MD cross-section observation by SEM is 100 nm or more from the viewpoint of suppressing clogging in the lithium ion secondary battery, and 400 nm or less from the viewpoint of suppressing short-circuiting in the lithium ion secondary battery. The lower limit of the average pore diameter of the pores present in the microporous layer (X) in ND-MD cross-section observation by SEM is preferably 130 nm or more, more preferably 140 nm or more. The upper limit of the average pore diameter of the pores present in the microporous layer (X) in ND-MD cross-section observation by SEM is preferably 350 nm or less, more preferably 300 nm or less.

[0039] The average pore diameter is the area average of the major axes calculated based on the area of ​​each pore present in the microporous layer when the ND-MD cross section of the microporous layer is observed by SEM. The average pore diameter can be measured by performing cross-sectional SEM observation of a sample prepared by cutting out the MD-ND cross section of the microporous layer from the separator, and image analysis of the obtained cross-sectional SEM image in a 4 μm × 4 μm range. Detailed conditions are shown in the Examples.

[0040] <Achieving both MFR and average pore diameter in the microporous layer (X)> In the separator for an electricity storage device of this embodiment, the microporous layer (X) has an MFR of 0.9 g / 10 min or less, and the average pore diameter of pores present in the ND-MD cross section when observed by cross-sectional SEM is 100 nm or more and 400 nm or less. Conventionally, in separators having a multilayer structure, particularly multilayer separators obtained by a dry process, it has been very difficult to reduce the MFR to 0.9 g / 10 min or less and increase the average pore diameter to 100 nm or more because the dry process does not contain a plasticizer. When the MFR is as low as 0.9 g / 10 min or less, i.e., when the molecular weight of the polyolefin is high, pores are very difficult to open during stretching, and even when pores are opened, the average pore diameter tends to be small. Furthermore, even in conventional techniques in which the air permeability of the separator is adjusted to about 200 sec, a small average pore diameter of the separator tends to make it difficult to suppress clogging in the electricity storage device. Furthermore, in a multilayer structure, the thickness of each layer needs to be further reduced. For example, in the case of a separator with a three-layer structure of 18 μm, if the thickness ratio of each layer is 1:1:1, the thickness of each layer needs to be as thin as 6 μm. When each layer is formed thin like this and perforated by stretching, the perforation property of the layer with a low MFR becomes very poor, and only a very small average long pore diameter can be obtained.

[0041] In this embodiment, by using a polyolefin (A) having specific physical properties and controlling the film formation and stretching conditions more precisely than in conventional methods, it is possible to obtain a separator with large pores in which the average long pore diameter of the pores present in the microporous layer (X) is 100 nm or more when observed in ND-MD cross section by SEM, even if the MFR of the microporous layer (X) is as low as 0.9 g / 10 min or less.

[0042] <Porosity of the microporous layer (X)> The lower limit of the porosity of the microporous layer (X) according to this embodiment is preferably 20% or more, more preferably 25% or more, even more preferably 30% or more, and particularly preferably 35% or more, from the viewpoint of suppressing clogging in the electricity storage device and controlling the air permeability of the separator. The upper limit of the porosity of the microporous layer (X) is preferably 70% or less, more preferably 65% ​​or less, even more preferably 60% or less, and particularly preferably 55% or less, from the viewpoint of maintaining the strength of the separator. The porosity is measured by the method described in the Examples.

[0043] <Thickness of the microporous layer (X)> The upper limit of the thickness of the microporous layer (X) according to this embodiment is preferably 10 μm or less, 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 viewpoint of increasing the energy density of the electricity storage device, etc. The lower limit of the thickness of the microporous layer (X) is preferably 1 μm or more, 2 μm or more, 3 μm or more, 3.5 μm or more, 4 μm or more, or 4.5 μm or more, from the viewpoint of strength, etc.

[0044] <Additives> In this embodiment, the microporous layer (X) containing polyolefin (A) as a main component may further contain, in addition to polyolefin (A), additives such as elastomers, fluorine-containing flow modifiers, waxes, crystal nucleating agents, antioxidants, metal soaps such as metal salts of aliphatic carboxylic acids, ultraviolet absorbers, light stabilizers, antistatic agents, antifogging agents, color pigments, and fillers, as needed.

[0045] <Microporous layer (Y)> The separator for an electricity storage device of this embodiment has a microporous layer (Y) containing polyolefin (B) as a main component. The separator for an electricity storage device of this embodiment may have only one microporous layer (Y) or two or more microporous layers. In this embodiment, containing polyolefin (B) as a main component means containing 50 mass% or more of polyolefin (B) based on the total mass of the microporous layer (Y). The lower limit of the content of polyolefin (B) in the microporous layer (Y) is preferably 55 mass% or more, 60 mass% or more, 70 mass% or more, 80 mass% or more, 90 mass% or more, or 95 mass% or more, from the viewpoints of the wettability of the separator to the electrolyte, thinning of the membrane, shutdown properties, etc. The upper limit of the content of polyolefin (B) in the microporous layer (Y) 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.

[0046] <Polyolefin (B)> The polyolefin (B) in this embodiment is a polymer containing a monomer having a carbon-carbon double bond as a repeating unit. Monomers constituting the polyolefin (B) are not limited, but include monomers having 1 to 10 carbon atoms and a carbon-carbon double bond, such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene. The polyolefin (B) may be, for example, a homopolymer, a copolymer, or a multi-stage polymerization polymer, and is preferably a homopolymer.

[0047] Specifically, the polyolefin (B) is preferably polyethylene, polypropylene, a copolymer of ethylene and propylene, or a mixture thereof, from the viewpoint of shutdown characteristics, etc. Polyolefin (B) is more preferably polyethylene.

[0048] When polyethylene is used as the polyolefin (B), the lower limit of the MFR of the polyethylene is preferably 0.1 or more, more preferably 0.15 or more, even more preferably 0.18 or more, and particularly preferably 0.2 or more, from the viewpoint of good pore opening property and suppression of clogging. The upper limit of the MFR of the polyethylene is preferably 2.0 or less, more preferably 1.0 or less, even more preferably 0.8 or less, and particularly preferably 0.5 or less, from the viewpoint of separator strength.

[0049] <Average pore diameter of the microporous layer (Y)> In this embodiment, in the case of ND-MD cross-section observation by SEM, the lower limit of the average pore diameter of the pores present in the microporous layer (Y) is preferably 150 nm or more, more preferably 200 nm or more, from the viewpoints of preventing clogging and achieving good air permeability of the separator. In the ND-MD cross-section observation by SEM, the upper limit of the average pore diameter of the pores present in the microporous layer (Y) is preferably 2000 nm or less, more preferably 1000 nm or less, from the viewpoint of preventing short circuits in the electricity storage device.

[0050] <Ratio of average pore diameter of microporous layer (X) to that of microporous layer (Y)> In this embodiment, when ND-MD cross sections of the microporous layer (X) and the microporous layer (Y) are observed by SEM, the average pore diameter of the pores present in the microporous layer (Y) is preferably 1.2 to 10 times the average pore diameter of the pores present in the microporous layer (X). This means that the average pore diameter of the pores present in the ND-MD cross section of the microporous layer (Y) is larger than the average pore diameter of the pores present in the ND-MD cross section of the microporous layer (X) by a certain range. When observed by SEM, the ND-MD cross section is observed. By providing a separator with a microporous layer (Y) having an average pore diameter that is 1.2 times or more larger than the average pore diameter of the pores present in the microporous layer (X), clogging can be more effectively suppressed. From the viewpoint of suppressing contact short circuits in the power storage device, when observed by SEM, the average pore diameter of the pores present in the microporous layer (Y) is preferably 10 times or less the average pore diameter of the pores present in the microporous layer (X). In ND-MD cross section observation by SEM, the ratio of the average long pore diameter of the pores present in the microporous layer (Y) to the average long pore diameter of the pores present in the microporous layer (X) is more preferably 1.4 to 8 times.

[0051] <Thickness of the microporous layer (Y)> The upper limit of the thickness of the microporous layer (Y) according to this embodiment is preferably 10 μm or less, 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 viewpoint of increasing the energy density of the electricity storage device, etc. The lower limit of the thickness of the microporous layer (Y) is preferably 1 μm or more, 2 μm or more, 3 μm or more, 3.5 μm or more, 4 μm or more, or 4.5 μm or more, from the viewpoint of strength, etc.

[0052] <Multilayer structure> The separator for an electricity storage device in this embodiment has at least a microporous layer (X) and a microporous layer (Y). Here, the separator in this embodiment has at least one microporous layer (X) and one microporous layer (Y). The separator in this embodiment may have one microporous layer (X) and one microporous layer (Y). Furthermore, a separator having two or more microporous layers (X) and / or two or more microporous layers (Y) may have a three or more layer structure. For example, a two-layer structure of microporous layer (X) / microporous layer (Y), a three-layer structure of microporous layer (X) / microporous layer (Y) / microporous layer (X), a three-layer structure of microporous layer (Y) / microporous layer (X) / microporous layer (Y), and the like are preferred. The electricity storage device separator may also have a layer other than the microporous layer (X) and the microporous layer (Y). Examples of the layer other than the microporous layer (X) and the microporous layer (Y) include a layer containing an inorganic substance or a layer containing a heat-resistant resin.

[0053] <Microporous layer (X) and microporous layer (Y)> In the power storage device separator according to this embodiment, when the ND-MD cross section is observed by SEM, the layer with the smallest average pore diameter is the microporous layer (X), and the layer with the largeest average pore diameter is the microporous layer (Y). By making the average pore diameter of the pores present in the ND-MD cross section of the microporous layer (Y) larger than the average pore diameter of the pores present in the ND-MD cross section of the microporous layer (X), clogging can be more effectively suppressed. Furthermore, when there are two or more microporous layers (X) or microporous layers (Y), the average pore diameter of the pores present in the ND-MD cross section of the microporous layer (X or Y) converted to the arithmetic mean value of the layer thicknesses of the two or more layers may be used. This allows the average pore diameters of the pores present in the microporous layer (X) and the microporous layer (Y) to be appropriately compared in ND-MD cross section observation by SEM, even when there are two or more microporous layers (X or Y).

[0054] <Separator thickness> The upper limit of the thickness of the separator for an electricity storage device according to this embodiment is preferably 25 μm or less, 22 μm or less, 20 μm or less, 18 μm or less, 17 μm or less, 16.5 μm or less, 16 μm or less, 15.5 μm or less, 15 μm or less, 14.5 μm or less, 14 μm or less, or 12 μm or less, from the viewpoint of increasing the energy density of the electricity storage device, etc. The lower limit of the thickness of the separator for an electricity storage device according to this embodiment is preferably 6 μm or more, 7 μm or more, 8 μm or more, 9 μm or more, 10 μm or more, or 11 μm or more, from the viewpoint of strength, etc.

[0055] <Air permeability (air resistance)> The upper limit of the air permeability of the separator for an electricity storage device according to this embodiment is preferably 250 seconds / 100 cm when converted into an air permeability of the separator for an electricity storage device having a thickness of 16 μm. 3 Below, 240 seconds / 100cm 3 Below, 230 seconds / 100cm 3 Below, 200 seconds / 100cm 3 or less, or 180 seconds / 100cm 3 The lower limit of the air permeability of the separator for an electricity storage device having a multilayer structure is not limited, but when the thickness of the separator for an electricity storage device is converted to 16 μm, it is, for example, 50 seconds / 100 cm 3 Over 60 seconds / 100cm 3 or more, or 70 seconds / 100cm 3 That's all.

[0056] <Piercing strength> The lower limit of the puncture strength of the electricity storage device separator according to this embodiment, when converted to a thickness of 16 μm, is preferably 230 gf or more, 240 gf or more, 250 gf or more, 260 gf or more, 280 gf or more, 300 gf or more, or 320 gf or more. The upper limit of the puncture strength of the electricity storage device separator, when converted to a thickness of 16 μm, is preferably 550 gf or less, 500 gf or less, or 480 gf or less.

[0057] <<Method for manufacturing separator for electricity storage device>> The method for producing a microporous layer containing a polyolefin as a main component generally includes a melt-extrusion step in which a resin composition containing the above-described polyolefin (hereinafter referred to as a polyolefin resin composition) is melt-extruded to obtain a resin film, and a hole-forming step in which the obtained resin film is perforated to make it porous, and may further include an annealing step, a stretching step, a heat-relaxing step, etc., after the hole-forming step, as needed. The method for producing a microporous layer is broadly divided into a dry method in which no solvent is used in the hole-forming step, and a wet method in which a solvent is used. Examples of the melt-extrusion method include a T-die method and an inflation method.

[0058] Dry methods include a method in which a polyolefin resin composition is melt-kneaded and extruded, and then the polyolefin crystal interface is peeled off by heat treatment and stretching, and a method in which a polyolefin resin composition and an inorganic filler are melt-kneaded and formed into a film, and then the interface between the polyolefin and the inorganic filler is peeled off by stretching.

[0059] Examples of the wet method include a method in which a polyolefin resin composition and a pore-forming material are melt-kneaded to form a film, which is stretched as necessary, and then the pore-forming material is extracted; and a method in which a polyolefin resin composition is dissolved, and then the film is immersed in a poor solvent for the polyolefin to solidify the polyolefin and simultaneously remove the solvent.

[0060] A single-screw extruder or a twin-screw extruder can be used for melt-kneading the polyolefin resin composition, and other devices such as a kneader, a Laboplastomill, a kneading roll, and a Banbury mixer can also be used.

[0061] The polyolefin resin composition may optionally contain resins other than polyolefins, additives, etc., depending on the method for producing the microporous layer or the desired physical properties of the microporous layer. Examples of additives include pore-forming materials, fluorine-based flow modifiers, elastomers, waxes, crystal nucleating materials, antioxidants, metal soaps such as metal salts of aliphatic carboxylic acids, ultraviolet absorbers, light stabilizers, antistatic agents, antifogging agents, and coloring pigments. Examples of pore-forming materials include plasticizers, inorganic fillers, and combinations thereof.

[0062] 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.

[0063] Examples of inorganic fillers include oxide ceramics such as alumina, silica (silicon oxide), titania, zirconia, magnesia, ceria, yttria, zinc oxide, and iron oxide; nitride ceramics such as silicon nitride, titanium nitride, and boron nitride; ceramics such as silicon carbide, calcium carbonate, 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.

[0064] The method for producing the separator for an electricity storage device according to this embodiment is preferably a dry lamellar crystal opening process in which the polyolefin crystal interface is peeled off by heat treatment and stretching. Here, the method for producing a separator having a microporous layer (X) and a microporous layer (Y) is preferably at least one of the following methods (A) and (B): (A) A method for producing a separator by co-extrusion, which comprises co-extrusion-forming a microporous layer (X) and a microporous layer (Y), followed by annealing, cold stretching, hot stretching, and heat relaxation; and (a) A method for producing a separator by lamination, in which the microporous layer (X) and the microporous layer (Y) are extruded separately, laminated together, and then subjected to annealing, cold stretching, hot stretching, and heat relaxation steps.

[0065] From the viewpoint of imparting a sufficient average pore size and pore size ratio between the microporous layer (X) and the microporous layer (Y) and suppressing clogging in the electricity storage device, the lamination process (a) is more preferable. Without wishing to be bound by theory, it is believed that the lamination process allows the microporous layer (X) and the microporous layer (Y) to be formed separately, which enables more strict temperature control and the imparting of orientation during film formation, and as a result, tends to achieve a good average pore size and pore size ratio and suppress clogging.

[0066] In the lamination process (a), the extrusion film formation conditions for the microporous layer (X) are preferably such that the resin is discharged as slowly as possible and effectively quenched by blowing low-temperature air. While a high film formation rate is usually preferred, it is believed that by deliberately forming the film at a low film formation rate and quenching it by blowing air at 15°C or below, the microporous layer (X) can achieve both a good average pore diameter and high strength. To achieve a good average pore diameter, the upper limit of the discharge rate is preferably 20 kg / h or less, more preferably 15 kg / h or less, even more preferably 10 kg / h or less, and particularly preferably 8 kg / h or less per meter of die width. From the viewpoint of stability during film formation, the lower limit of the discharge rate is preferably 4 kg / h or more. After film formation, it is preferable to quench the film with air, and the upper limit of the temperature of the blown air is preferably 20°C or less, more preferably 15°C or less. By blowing cold air controlled at such a low temperature, the resin after film formation is rapidly cooled and uniformly oriented in the MD, which is thought to result in good pore opening in the microporous layer (X) and an average pore diameter within the range described above, even when a resin with a relatively high molecular weight, such as an MFR of 0.9 g / 10 min or less at a load of 2.16 kg and a temperature of 230°C, is used.

[0067] In the method for producing the separator for an electricity storage device of this embodiment, a lamination step may be performed after extrusion film formation. Any lamination step can be performed as long as it can bond the microporous layer (X) and the microporous layer (Y), and it is preferable to perform at least one thermal lamination step.

[0068] In the method for producing a separator for an electricity storage device according to this embodiment, an annealing step may be performed after extrusion film formation or after the lamination step. The annealing step tends to grow the crystal structure of the microporous layer (X) and the microporous layer (Y), improving pore openness. In particular, in this embodiment, annealing at a specific temperature for a long period of time allows crystals to grow without disturbing the crystal structure, resulting in high pore openness, which tends to enable good average pore diameters to be obtained in both the microporous layer (X) and the microporous layer (Y). In the annealing step, from the viewpoint of obtaining a good average pore diameter and suppressing clogging in the electricity storage device, the annealing is preferably performed at a temperature ranging from 115°C to 130°C, preferably for 30 minutes or more, more preferably 60 minutes or more.

[0069] In the method for producing a separator for a power storage device according to this embodiment, a stretching step may be performed after the annealing step, during the hole-forming step, or before or after the hole-forming step. Either uniaxial stretching or biaxial stretching can be used as the stretching treatment. While not limited thereto, uniaxial stretching is preferred from the viewpoint of production costs when using a dry method. Biaxial stretching is preferred from the viewpoint of improving the strength of the resulting separator. Examples of biaxial stretching include simultaneous biaxial stretching, sequential biaxial stretching, multistage stretching, and multiple stretching. Simultaneous biaxial stretching is preferred from the viewpoint of improving puncture strength, stretching uniformity, and shutdown properties. Furthermore, sequential biaxial stretching is preferred from the viewpoint of ease of control of planar orientation. When a sheet-shaped product is stretched biaxially at a high magnification, the molecules are oriented in the planar direction, which tends to result in a separator that is less susceptible to tearing and has high puncture strength.

[0070] To suppress shrinkage of the separator, a heat treatment step may be performed for the purpose of heat setting after the stretching step or the hole-forming step. The heat treatment step may include a stretching operation performed in a predetermined temperature atmosphere at a predetermined stretch ratio for the purpose of adjusting physical properties, and / or a relaxation operation performed in a predetermined temperature atmosphere at a predetermined relaxation ratio for the purpose of reducing stretching stress. The relaxation operation may be performed after the stretching operation. These heat treatment steps may be performed using a tenter or roll stretching machine.

[0071] <Energy storage device> The electricity storage device of this embodiment includes the electricity storage device separator of this embodiment. The electricity storage device of this embodiment has a positive electrode and a negative electrode. The electricity storage device separator is preferably laminated between the positive electrode and the negative electrode, positioned outside the positive electrode or negative electrode within the battery exterior, or envelops the electrode. If desired, lead bodies may be connected to the positive electrode and the negative electrode, respectively, so that the positive electrode and the negative electrode can be connected to external equipment, etc.

[0072] Examples of the power storage device include, but are not limited to, lithium secondary 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, lithium-sulfur batteries, lithium-air batteries, and zinc-air batteries. Among these, from the viewpoint of compatibility with the separator according to this embodiment, lithium secondary batteries, lithium ion secondary batteries, nickel-metal hydride batteries, and lithium ion capacitors are preferred, and lithium ion secondary batteries are more preferred.

[0073] The electricity storage device can be produced, for example, by the following method: The positive and negative electrodes are stacked with the separator described above interposed therebetween and wound as necessary to form a stacked electrode body or a wound electrode body. The stacked or wound electrode body is then loaded into an exterior housing. At this time, lead bodies are connected to the positive and negative electrodes within the exterior housing, and the ends of the lead bodies can be arranged so that they extend outside the exterior housing. When there are multiple positive and negative electrodes, tabs of the same electrode may be joined by welding or other means to form a single lead body that extends outside the exterior housing. The tabs of the same electrode may be formed from exposed portions of the current collectors, or may be formed by welding metal pieces to exposed portions of the current collectors. The positive and negative electrodes are connected to the positive and negative electrode terminals of the exterior housing via lead bodies or other means. At this time, portions of the lead body and portions of the exterior housing may be joined by heat fusion or other means. A nonaqueous electrolyte solution containing a nonaqueous solvent such as a chain and / or cyclic carbonate and an electrolyte such as a lithium salt is then injected into the exterior housing. The exterior housing is then sealed to produce an electricity storage device. [Example]

[0074] EXAMPLES The present invention will be specifically explained below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0075] <<Measurement and Evaluation Methods>> [Melt flow rate (MFR) measurement] The melt flow rate (MFR) of the microporous layer (X) was measured (unit: g / 10 min) at a temperature of 230°C and a load of 2.16 kg in accordance with JIS K 7210. The MFR of polypropylene was measured at a temperature of 230°C and a load of 2.16 kg in accordance with JIS K 7210. The melt flow rate (MFR) of polyethylene was measured at a temperature of 190°C and a load of 2.16 kg in accordance with JIS K 7210.

[0076] [GPC (gel permeation chromatography) measurement] A calibration curve was created by measuring standard polystyrene under the following conditions using an Agilent PL-GPC220 from Agilent Technologies, Inc. Chromatography was also performed on the sample polymer under the same conditions, and based on the calibration curve, the weight average molecular weight (Mw), number average molecular weight (Mn), and the value obtained by dividing the weight average molecular weight (Mw) by the number average molecular weight (Mn) of the polymer in terms of polystyrene (Mw / Mn) were calculated under the following conditions. Column: TSKgel GMHHR-H(20) HT (7.8mm I.D. x 30 cm) x 2 Mobile phase: 1,2,4-trichlorobenzene Detector: RI Column temperature: 160℃ Sample concentration: 1mg / ml Calibration curve: Polystyrene

[0077] [Melt tension measurement] The melt tension (mN) of the microporous layer was measured under the following conditions using a Capillograph manufactured by Toyo Seiki Seisaku-sho, Ltd. Capillary: diameter 1.0 mm, length 20 mm Cylinder extrusion speed: 2mm / min Take-up speed: 60m / min ·Temperature: 230℃

[0078] [Measurement of pentad fraction] The pentad fraction of polypropylene was assigned based on the description in the Polymer Analysis Handbook (edited by the Japan Society for Analytical Chemistry). 13 Calculations were made from the C-NMR spectrum using the peak height method. 13 The C-NMR spectrum was measured using a JEOL-ECZ500 manufactured by JEOL Ltd., by dissolving polypropylene pellets in o-dichlorobenzene-d at a measurement temperature of 145°C and accumulating 25,000 times.

[0079] [Thickness (μm) measurement] The thickness (μm) of the separator was measured at room temperature of 23±2°C using a Digimatic Indicator IDC112 manufactured by Mitutoyo Corporation.

[0080] [Porosity (%) measurement] A sample measuring 10 cm x 10 cm was cut from the separator and its volume (cm 3 ) and mass (g), and then calculate the density (g / cm 3 ) and the porosity was calculated.

[0081] [Air permeability (sec / 100cm) 3 )] The separator's air permeability resistance (sec / 100cm) was measured using a Gurley air permeability meter conforming to JIS P-8117. 3 ) was measured, divided by the thickness in μm, and then multiplied by 16 to calculate the air permeability converted into a 16 μm thickness.

[0082] [Evaluation of puncture strength] A needle with a hemispherical tip and a radius of 0.5 mm was prepared, and a separator was sandwiched between two plates with openings of 11 mm diameter (dia.), and the needle, separator, and plates were set in place. A puncture test was performed using an Imada MX2-50N instrument under the following conditions: a needle tip curvature radius of 0.5 mm, an opening diameter of 11 mm in the separator holding plate, and a puncture speed of 25 mm / min. The needle and separator were brought into contact, and the maximum puncture load (i.e., puncture strength (gf)) was measured. The puncture strength obtained was divided by the thickness and then multiplied by 16 to calculate the puncture strength equivalent to a thickness of 16 μm.

[0083] [Evaluation of average long hole diameter] The average pore diameter was measured by image analysis of cross-sectional SEM observations. As a pretreatment, the separator was stained with ruthenium, then impregnated with epoxy resin and cured at 60°C for at least 12 hours. The ruthenium-stained separator was then embedded in the epoxy resin. After embedding in the epoxy resin, the cross section was roughly machined with a razor blade and then milled using an ion milling machine (E3500 Plus, Hitachi High-Tech Corporation) to prepare a cross-sectional specimen. The cross section was in the ND-MD plane. The cross-sectional specimen was fixed to the SEM specimen stage for cross-sectional observation with a conductive adhesive (carbon-based), dried, and then subjected to electrical conductivity treatment using an osmium coater (HPC-30W, Vacuum Device Co., Ltd.). Osmium coating was then performed with the applied voltage adjustment knob set to 4.5 and a discharge time of 0.5 seconds. The specimen was then used for microscopic examination. Next, using a scanning electron microscope (S-4800 manufactured by Hitachi High-Technologies Corporation), cross-sections of the microporous layer were cut out and a sample was prepared. Three arbitrary points within a 4 μm x 4 μm area of ​​the sample were subjected to cross-sectional SEM observation at an accelerating voltage of 1 kV, a working distance of 5 mm, and a magnification of 30,000 times.

[0084] The obtained cross-sectional SEM observation image was binarized using the image processing software ImageJ and the Otsu method to separate the resin part from the pore part, and the average long diameter of the pore part was calculated. At this time, micropores existing across the photographed range and outside the photographed range, and pores with an area of ​​0.001 μm were not included. 2 The following pores were excluded from the measurement: The average elongated pore diameter was calculated by the area-weighted average of the pores.

[0085] The longest pore diameter of each pore in the 4 μm × 4 μm area of ​​the obtained cross-sectional SEM image was measured, and the area-weighted average value of these was calculated to obtain the average long pore diameter.

[0086] [Battery performance evaluation and clogging observation] An electrolyte solution for evaluating battery performance and observing clogging was prepared by mixing ethylene carbonate and ethyl methyl carbonate in a volume ratio of 1:2 and adding 1 mol / L of LiPF6 as a lithium salt.

[0087] The positive electrode active material was lithium nickel manganese cobalt composite oxide (LiNi 0.5 Co 0.2 Mn 0.3 O2), carbon black powder (manufactured by Timcal Corporation, product name: SuperP Li) as a conductive additive, and PVDF as a binder were mixed in a mass ratio of composite oxide:conductive additive:binder = 100:3.5:3. The resulting mixture was dispersed in N-methylpyrrolidone solvent to form a dispersion. The dispersion was applied to both sides of a 15 μm-thick aluminum foil serving as a positive electrode current collector, and after the solvent was dried and removed, the foil was pressed with a roll press to produce a double-sided coated positive electrode.

[0088] A graphite powder (Hitachi Chemical Co., Ltd., product name: MAG) with a particle diameter of 22 μm (D50) was used as the negative electrode active material. A binder (Zeon Corporation, product name: BM400B) and carboxymethyl cellulose (Daicel Corporation, product name: #2200) were mixed in a mass ratio of graphite powder:binder:thickener = 100:1.5:1.1. The mixture was dispersed in water to prepare an aqueous dispersion. The aqueous dispersion was applied to one side of a 10 μm-thick copper foil serving as a negative electrode current collector to prepare a single-sided coated negative electrode. Separately from the single-sided coated negative electrode, the aqueous dispersion was applied to both sides of a 10 μm-thick copper foil serving as a negative electrode current collector to prepare a double-sided coated negative electrode. The solvent (water) was dried and removed from the single-sided coated negative electrode and the double-sided coated negative electrode. The coated copper foil was then pressed using a roll press to prepare a single-sided coated negative electrode and a double-sided coated negative electrode, respectively.

[0089] The resulting positive and negative electrodes were stacked in the following order: single-sided coated negative electrode / double-sided coated positive electrode / double-sided coated negative electrode / double-sided coated positive electrode / single-sided coated negative electrode, with the separator prepared below sandwiched between the opposing surfaces of the active materials. The resulting stack was then inserted into a bag (battery exterior) made of a laminate film in which both sides of aluminum foil (40 μm thick) were coated with a resin layer. Terminals of the inserted electrodes protruded from the battery exterior. 0.8 mL of the electrolyte prepared as described above was then poured into the bag, and the bag was vacuum-sealed to produce a sheet-shaped lithium-ion secondary battery.

[0090] The resulting sheet-shaped lithium-ion secondary battery was placed in a thermostatic chamber (Futaba Scientific Co., Ltd., product name: PLM-73S) set at 25°C, connected to a charge / discharge device (Asuka Electronics Co., Ltd., product name: ACD-01), and left to stand for 16 hours. The battery was then charged at a constant current of 0.05 C, and after the voltage reached 4.35 V, it was charged at a constant voltage of 4.35 V for 2 hours, and then discharged at a constant current of 0.2 C to 3.0 V. This charge / discharge cycle was repeated three times to perform initial charging and discharging of the battery. 1 C refers to the current value when the battery's full capacity is discharged in 1 hour.

[0091] After the initial charge and discharge, the battery was placed in a thermostatic chamber set at 50°C and charged at a constant current of 1 C. After the voltage reached 4.35 V, the battery was charged at a constant voltage of 4.35 V for 1 hour, and then discharged at a constant current of 1 C to 3.0 V. This charge and discharge cycle was repeated 100 times. The battery cycle test involved repeating the above charge and discharge cycle 100 times.

[0092] The cycle capacity retention rate was determined as a percentage by dividing the discharge capacity (mAh) at the 100th cycle by the discharge capacity (mAh) at the 1st cycle. After 100 cycles, the sheet-shaped lithium-ion secondary battery was disassembled under an argon atmosphere, and the separator was removed. The first immersion cleaning was performed by immersing the separator in an ethyl methyl carbonate-containing bath for 30 seconds and then removing it. A total of three immersion cleanings were performed. The ethyl methyl carbonate in the bath was replaced during the second and third immersion cleanings. The negative electrode side surface of the separator was then observed in a 1 mm square area using an optical microscope at 100-1000x magnification to check for clogging. Table 1 shows the presence or absence of clogging.

[0093] Example 1 [Preparation of microporous layer] As polyolefin (A), a high molecular weight polypropylene resin (PP, MFR (230°C) = 0.51 g / 10 min, density = 0.91 g / cm 3The polymer (Mw / Mn = 5.2, pentad fraction = 99.3%) was melted in a 2.5-inch extruder and fed to a T-die using a gear pump. The T-die temperature was set to 230°C. The molten polymer was extruded from the T-die. The extruded resin was then rapidly cooled to 10°C using a chiller by blowing in 800 L / min of air while being wound around a roll at a speed of 25 m / min to obtain a precursor (X') (precursor of the microporous layer (X)) with a thickness of approximately 6 μm. The T-die TD lip width was set to 500 mm, the distance between the T-die lips (lip clearance) was set to 2.4 mm, and the extrusion rate was 3.8 kg / h. Similarly, as polyolefin (B), polyethylene resin (PE, MFR (190°C) = 0.38 g / 10 min, density = 0.96 g / cm 3 ) was melted in a 2.5-inch extruder and fed to a T-die using a gear pump. The temperature of the T-die was set to 210°C. The molten polymer was extruded from the T-die. The extruded resin was then rapidly cooled by blowing in 800 L / min of air cooled to 10°C using a chiller, while being wound around a roll at a roll speed of 25 m / min to obtain a precursor (Y') (precursor of the microporous layer (Y)) with a thickness of approximately 6 μm. Here, the TD lip width of the T-die was set to 500 mm, the distance between the lips of the T-die (lip clearance) was set to 2.4 mm, and extrusion was performed at a rate of 3.8 kg / h.

[0094] The resulting precursor (X') and precursor (Y') were then thermocompressed at 120°C at 4 m / min using a thermocompression laminator to form a precursor (X') / precursor (Y') / precursor (X') structure, resulting in a three-layer precursor (Z). The resulting precursor (Z) was then placed in a dryer and annealed at 120°C for 1 hour. The annealed precursor (Z) was then cold-stretched by 8% at room temperature, placed in a 125°C oven without shrinking the stretched film, and hot-stretched to 140%, followed by a 15% relaxation, resulting in a separator with a three-layer structure consisting of microporous layer (X) / microporous layer (Y) / microporous layer (X). The structure, physical properties, and battery performance evaluation results of the resulting separator are shown in Table 1.

[0095] Example 2, Example 3, Example 6, Comparative Example 1, Comparative Example 3 A microporous layer was obtained in the same manner as in Example 1 except that the raw materials were changed as shown in Table 1, and the microporous layer and the separator obtained using the same were evaluated.

[0096] Example 4 The raw materials were changed as shown in Table 1, and in the film formation of precursor (X'), the temperature of the blown air was changed to 25°C and the amount of blown air was changed to 600 L / min, and film formation was performed to obtain precursor (X'). The other conditions were the same as those described in Example 1.

[0097] Example 5 The raw materials were changed as shown in Table 1, and in the film formation of precursor (X'), the temperature of the blown air was changed to 0°C and the amount of blown air was changed to 1000 L / min, and film formation was performed to obtain precursor (X'). In addition, in the annealing step of Example 5, annealing treatment was performed at 125°C for 3 hours. The other conditions were the same as those described in Example 1.

[0098] Comparative Example 4 As polyolefin (A), a high molecular weight polypropylene resin (PP, MFR (230°C) = 0.25 g / 10 min, density = 0.91 g / cm 3 The polymer (Mw / Mn = 5.0, pentad fraction = 97.5%) was melted in a 2.5-inch extruder and fed to a T-die using a gear pump. The T-die temperature was set to 230°C. The molten polymer was extruded from the T-die. The extruded resin was then rapidly cooled to 10°C using a chiller by blowing in 800 L / min of air while being wound around a roll at a speed of 25 m / min to obtain a precursor (X') (precursor of the microporous layer (X)) with a thickness of approximately 6 μm. The T-die TD lip width was set to 500 mm, the distance between the T-die lips (lip clearance) was set to 2.4 mm, and the extrusion rate was 3.8 kg / h. Similarly, as the polyolefin resin (B), a high molecular weight polypropylene resin (PP, MFR (230°C) = 0.25 g / 10 min, density = 0.91 g / cm3 The polymer (Mw / Mn = 5.0, pentad fraction = 97.5%) was melted in a 2.5-inch extruder and fed to a T-die using a gear pump. The T-die temperature was set to 230°C. The molten polymer was extruded from the T-die. The extruded resin was then rapidly cooled to 10°C using a chiller by blowing in 800 L / min of air while being wound around a roll at a speed of 25 m / min to obtain a precursor (Y') (precursor of the microporous layer (Y)) with a thickness of approximately 6 μm. The lip width of the T-die TD was set to 500 mm, the distance between the lips of the T-die (lip clearance) was set to 2.4 mm, and the extrusion rate was 3.8 kg / h.

[0099] The resulting precursor (X') and precursor (Y') were then thermocompressed at 135°C and 2 m / min using a thermocompression laminator to form a precursor (X') / precursor (Y') / precursor (X') structure, resulting in a three-layer precursor (Z). The resulting precursor (Z) was then placed in a dryer and annealed at 120°C for 1 hour. The annealed precursor (Z) was then cold-stretched by 8% at room temperature, placed in a 125°C oven without shrinking the stretched film, and hot-stretched to 140%, followed by a 15% relaxation, resulting in a separator with a three-layer structure consisting of microporous layer (X) / microporous layer (Y) / microporous layer (X). The structure, physical properties, and battery performance evaluation results of the resulting separator are shown in Table 1.

[0100] Comparative Example 2 The raw materials were changed as shown in Table 1, and film formation was performed by a coextrusion process rather than a lamination process. Specifically, using a 2.5-inch extruder, the polypropylene resin shown in Table 1 was melted at 220°C, and the polyethylene resin shown in Table 1 was melted at 200°C. The molten polypropylene resin and molten polyethylene resin were then fed into a coextrusion T-die (220°C) at a discharge rate ratio of 1:1:1 to form a three-layer structure of polypropylene resin / polyethylene resin / polypropylene resin. The molten polymer was then extruded from the T-die at 11.4 kg / h. The extruded resin was then rapidly cooled to 10°C by blowing in 800 L / min of air using a chiller, while being wound around a roll at a roll speed of 25 m / min to obtain a three-layer precursor (Z) (precursor of the microporous membrane (Z)) with a thickness of approximately 16 μm. The resulting precursor (Z) was then placed in a dryer and annealed at 120°C for 1 hour. The annealed precursor (Z) was then cold-stretched by 8% at room temperature, and the stretched film was placed in a 125°C oven without shrinkage and hot-stretched to 140%, followed by relaxation by 15% to obtain a separator having a three-layer structure consisting of microporous layer (X) / microporous layer (Y) / microporous layer (X). The structure, physical properties, and battery performance evaluation results of the obtained separator are shown in Table 1.

[0101] Table 1 shows that by using a polyolefin resin with an MFR of 0.9 g / 10 min or less at a load of 2.16 kg and a temperature of 230°C for the microporous layer (X) and strictly controlling the film formation conditions, it is possible to produce a separator that has a sufficiently large average pore diameter, sufficiently low air permeability, and high pin puncture strength. It also shows that using a separator with a sufficient average pore diameter and a sufficient ratio of the average pore diameter between the microporous layer (X) and the microporous layer (Y) suppresses separator clogging in a lithium ion secondary battery and improves cycle performance.

[0102] [Table 1] [Industrial Applicability]

[0103] The separator for an electricity storage device of the present embodiment has high strength and low air permeability, and can suppress clogging within the electricity storage device, and can be suitably used as a separator for an electricity storage device, such as a lithium ion secondary battery.

Claims

1. The microporous layer (X) mainly contains polyolefin (A) and the microporous layer (Y) mainly contains polyolefin (B), the microporous layer (X) has a melt flow rate (MFR) of 0.9 g / 10 min or less under a load of 2.16 kg and at a temperature of 230°C; In ND-MD cross-section observation of the microporous layer (X) and the microporous layer (Y) using a scanning electron microscope (SEM), The average long pore diameter of the pores present in the microporous layer (X) is 140 nm or more and 400 nm or less, and the average long pore diameter of the pores present in the microporous layer (Y) is larger than the average long pore diameter of the pores present in the microporous layer (X); and The main component of the polyolefin (A) is polypropylene, The main component of the polyolefin (B) is polyethylene, and The pentad fraction of the polypropylene is 98.5% or more. Separators for power storage devices.

2. In ND-MD cross-section observation of the microporous layer (X) and the microporous layer (Y) by SEM, the average long pore diameter of pores present in the microporous layer (Y) is 150 nm or more and 2000 nm or less and is 1.2 times or more and 10 times or less the average long pore diameter of pores present in the microporous layer (X). The separator for an electricity storage device according to claim 1 .

3. The air permeability when the thickness of the separator for an electricity storage device is converted to 16 μm is 250 sec / 100 cm ３ Below is the The separator for an electricity storage device according to claim 1 or 2.

4. The porosity of the separator for the electricity storage device is 20% or more and 70% or less. The separator for an electricity storage device according to any one of claims 1 to 3.

5. The polypropylene has an MFR of 0.6 g / 10 min or less under a load of 2.16 kg and at a temperature of 230°C. The separator for an electricity storage device according to any one of claims 1 to 4.

6. The separator for an electricity storage device has a puncture strength of 300 gf or more when converted to a thickness of 16 μm. The separator for an electricity storage device according to any one of claims 1 to 5.

7. The separator for an electricity storage device according to any one of claims 1 to 6 is provided. Energy storage device.

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