JP7570280B2 - Electricity storage device and separator for electricity storage device - Google Patents

Description

The present invention relates to a separator for an electricity storage device and an electricity storage device including the same.

Microporous membranes are widely used for separating various substances, or as selective permeation separation membranes, separators, etc. Applications include, for example, microfiltration membranes; separators for batteries such as lithium ion batteries, polymer electrolyte batteries, and fuel cells; separators for capacitors; and base materials for functional membranes in which functional materials are filled into the pores to create new functions. In particular, polyolefin microporous membranes are preferably used as separators for electricity storage devices, which are widely used in mobile phones, smartphones, wearable devices, notebook personal computers (PCs), tablet PCs, digital cameras, etc.

Conventionally, in electricity storage devices, a power generating element having a separator between a positive plate and a negative plate is impregnated with an electrolyte. In these electricity storage devices, under conditions such as charging the device at a relatively low temperature with a relatively large current, lithium (Li) precipitates on the negative plate, and metallic Li may grow from this negative plate as dendrites (branched crystals). If such dendrites continue to grow, they may break through or penetrate the separator and reach the positive plate, or may approach the positive plate, causing the dendrites themselves to become a pathway, resulting in a short circuit.

In addition, when precipitated dendrites (metallic Li) break in a battery, highly reactive metallic Li may be present without electrical continuity with the negative plate. In this way, if a battery contains a large amount of dendrites or metallic Li resulting from them, it can easily lead to problems such as short-circuiting other parts, short-circuiting multiple parts together, or in the event of heat generation due to overcharging, the surrounding highly reactive metallic Li may react, causing further heat generation, and it can also cause deterioration of the battery capacity.

In response to this, for example, Patent Document 1 describes that when a composite ion-permeable membrane made of a non-porous ion-permeable membrane formed of a polymer such as aromatic polyamide (aramid) and a porous substrate such as a polymer porous membrane is used in a non-aqueous electrolyte battery, it has excellent resistance to short circuits caused by dendrites, low resistance, etc.

In addition, from the viewpoint of battery characteristics such as lowering the resistance of the polymer electrolyte battery and reducing the interfacial resistance when the electrodes and separators are stacked, for example, Patent Document 2 describes a method in which an electrolyte solution containing a monomer that can be polymerized by actinic rays is impregnated into electrodes, separators, etc., and then polymerized to form a gel, thereby adjusting the surface roughness (Rmax) of the electrodes, separators, etc. that contain the gelled polymer electrolyte to 0.6 μm or less.

International Publication No. 2016/098660 JP 2000-215916 A

The composite ion-permeable membrane described in Patent Document 1 has room for improvement in controlling the lithium (Li) deposition morphology and suppressing Li dendrite formation in an electricity storage device, and in particular in a battery system with a charge/discharge mechanism that is prone to Li deposition, it may be difficult to achieve cycle deterioration resistance or long life. In addition, the electrode or separator containing the gel polymer electrolyte described in Patent Document 2 does not focus on suppressing Li dendrite formation. Therefore, the present invention aims to provide a separator for an electricity storage device that can achieve control of Li deposition morphology and suppression of Li dendrite formation in an electricity storage device, and an electricity storage device including the same.

The present inventors have elucidated the mechanism of the increase in resistance of an electricity storage device caused by lithium (Li) dendrite precipitation, and have found that the above-mentioned problem can be solved by disposing a specific uniform diffusion layer at least on the surface layer of a separator, thereby completing the present invention. Examples of embodiments of the present invention are listed below.
<1> A separator for an electricity storage device comprising a microporous membrane,
The separator for the electric storage device has a uniform diffusion layer located at least on a surface layer thereof,
The separator for an electricity storage device, wherein the uniform diffusion layer in the surface layer has an arithmetic mean roughness (Ra) of 1.20 μm or less.
<2> The separator for an electricity storage device according to item 1, wherein the separator for an electricity storage device is immersed in an electrolyte solution of ethylene carbonate (EC)/diethyl carbonate (DEC)=1/2 (volume ratio) containing 1 _mol /L LiPF6 for one day, and then the uniform diffusion layer is pressure-bonded to lithium (Li) metal at 25° C. and 0.5 MPa, and the peel strength is 2 N/m or more.
<3> The separator for an electric storage device according to item 1 or 2, wherein the degree of swelling of the uniform diffusion layer in an electrolyte containing 1 mol/L _LiPF6 and having an EC/DEC ratio of 1/2 (volume ratio) is two or more times.
<4> The separator for an electricity storage device according to any one of items 1 to 3, further comprising an inorganic filler layer on at least one side of the microporous membrane.
<5> The separator for an electricity storage device according to any one of items 1 to 4, wherein the uniform diffusion layer is made of an ion-conductive material.
<6> The separator for an electricity storage device according to any one of items 1 to 5, wherein the uniform diffusion layer is non-porous.
<7> An electricity storage device comprising a positive electrode, a negative electrode, an electrolyte, and the separator for an electricity storage device according to any one of items 1 to 6.
<8> The electricity storage device according to item 7, wherein a peel strength between the negative electrode and the electricity storage device separator is 2 N/m or more.
<9> The electricity storage device according to item 7 or 8, wherein the negative electrode is Li metal or copper (Cu) foil.

The present invention provides a separator for an electricity storage device that can control the morphology of lithium (Li) precipitation and suppress Li dendrite formation, as well as an electricity storage device that includes the separator.

The following provides a detailed explanation of the embodiment of the present invention (hereinafter, sometimes abbreviated as "present embodiment"); however, the present invention is not limited to this embodiment, and various modifications are possible without departing from the gist of the present invention.

In this specification, the flow direction of the membrane during membrane production is defined as MD, and the direction that intersects with MD at 90 degrees (°) within the membrane plane is defined as TD.

<Separator for power storage device>
The separator for an electricity storage device according to this embodiment (hereinafter referred to as "separator") includes a microporous membrane and has a uniform diffusion layer located at least on the surface layer of the separator, and the arithmetic mean roughness (Ra) of the uniform diffusion layer on the surface layer is 1.20 μm or less.

When a uniform diffusion layer with an Ra of 1.20 μm or less is located at least on the surface layer of the separator according to this embodiment, when the separator is used in an electricity storage device, it tends to be possible to control the lithium (Li) deposition form and suppress the formation of Li dendrites. This tendency is remarkable in electricity storage devices with a charge/discharge mechanism that is prone to Li deposition, and it also makes it easier to improve the cycle deterioration resistance or extend the life of electricity storage devices that include a negative electrode made of Li metal or copper (Cu) foil.

According to the present invention, the following two patterns have been found as mechanisms of the increase in resistance of an electricity storage device caused by Li dendrite precipitation:
(A) When the power storage device is charged, a thick Li dendrite layer is deposited on the negative electrode side, compressing the separator between the positive and negative electrodes. Since the Li dendrite layer has a high porosity as a layer and a large deposition thickness, the separator is compressed, decreasing its permeability and increasing its resistance.
(a) Because Li metal has a low potential and is highly reductive, the reductive decomposition of the electrolyte tends to proceed on the Li metal surface, and the decomposition products accumulate on the negative electrode or Li dendrite layer. Li dendrites precipitate sharply as dendritic crystals, so they have a large specific surface area. The increase in specific surface area due to dendrite formation increases the amount of reductive decomposition products, which results in an increase in resistance and also causes the electrolyte to dry up.

In this embodiment, from the viewpoint of achieving control of Li deposition morphology and suppression of Li dendrite formation based on the above resistance increase mechanisms (a) and (b), a separator having a uniform diffusion layer with optimized smoothness (Ra≦1.20 μm) at least on the surface, and an electric storage device including the separator are provided. For example, the uniform diffusion layer of the separator according to this embodiment can be arranged to abut or face the negative electrode surface (or Li deposition surface) of the electric storage device to eliminate the Li ion concentration distribution and control the Li deposition morphology (suppress Li dendrite formation).

From the viewpoint of controlling the Li deposition form and suppressing Li dendrite formation, the separator according to this embodiment is impregnated with an electrolyte solution of ethylene carbonate (EC)/diethyl carbonate (DEC)=1/2 (volume ratio) containing 1 _mol /L LiPF 6 for one day, and then the uniform diffusion layer of the separator is pressed against Li metal at 25° C. and 0.5 MPa, and the peel strength is preferably 2 N/m or more, more preferably 3 N/m to 60 N/m or less, and even more preferably 10 N/m to 40 N/m or less. The peel strength when the uniform diffusion layer of the separator is pressed against Li metal at 25° C. and 0.5 MPa can be adjusted within the above numerical range, for example, by forming the uniform diffusion layer using a resin that has high affinity with Li metal and is moderately soft.

If desired, the separator according to this embodiment may include at least one layer in addition to the substrate such as a microporous membrane and the uniform diffusion layer. In general, the at least one layer included in the laminated separator may have, for example, insulating properties, adhesive properties, thermoplastic properties, inorganic porosity, etc., and may be formed of a single film or a pattern formed by dot coating, stripe coating, etc. Therefore, the position of the uniform diffusion layer on at least the surface layer of the separator according to this embodiment may include not only a layer structure in which the uniform diffusion layer is laminated on one or both sides of the substrate, but also a structure in which the uniform diffusion layer is partially or entirely exposed from a separator including a substrate, or a separator including a substrate and at least one layer.

From the viewpoint of arranging the uniform diffusion layer of the separator so as to abut or face the negative electrode surface (or Li deposition surface) of the electricity storage device, a preferred layer configuration is one in which the uniform diffusion layer is located on at least the surface of the separator, and at least one side of the microporous membrane has an inorganic filler layer.

In relation to the layer structure of the separator, from the viewpoint of eliminating the Li ion concentration gradient on the negative electrode surface, controlling the Li deposition form and suppressing Li dendrite formation, and simultaneously reducing the internal resistance of the battery, the thickness ratio of the substrate to the uniform diffusion layer is preferably substrate:uniform diffusion layer = 1:1 to 10,000:1, more preferably 2:1 to 1,000:1, and even more preferably 3:1 to 100:1. In addition, from the viewpoint of suppressing peeling between the uniform diffusion layer and the substrate and suppressing an increase in internal resistance due to thickness changes accompanying dissolution and deposition of Li in the Li metal negative electrode, the adhesive strength between the substrate and the uniform diffusion layer is preferably 2 N/m or more and 1,000 N/m or less, more preferably 10 N/m or more and 600 N/m or less, and even more preferably 40 N/m or more and 400 N/m or less.

The components of the separator according to this embodiment are described below.

<Microporous membrane>
The microporous film (hereinafter also referred to as porous layer) may be one that has been used as a conventional separator, and is preferably one that has high resistance to organic solvents and has a fine pore size. Examples of the microporous film include a microporous film that mainly contains a resin such as polyolefin (e.g., polyethylene, polypropylene, polybutene, polyvinyl chloride, etc.), a mixture thereof, or a copolymer of a plurality of monomers that constitutes them; a microporous film that mainly contains a resin such as polyethylene terephthalate, polycycloolefin, polyethersulfone, polyamide, polyimide, polyimideamide, polyaramid, polycycloolefin, nylon, polytetrafluoroethylene, etc.; a woven fabric (woven cloth) of polyolefin fiber; a nonwoven fabric of polyolefin fiber; paper; and an aggregate of insulating material particles. Among these, from the viewpoints of chemical electrical stability, excellent applicability of the coating liquid when a polymer layer is obtained through a coating process, and making the separator thinner to increase the active material ratio in an electricity storage device such as a battery and increasing the capacity per volume, a polyolefin microporous film (hereinafter also referred to as a PO microporous film) containing a polyolefin resin as a main component is preferred. Note that, in this specification, "containing as a main component" means containing a specific component or member in more than 50 mass%, preferably 75 mass% or more, more preferably 85 mass% or more, even more preferably 90 mass% or more, still more preferably 95 mass% or more, particularly preferably 98 mass% or more, and may be 100 mass%.

If desired, an inorganic coating layer or an organic (adhesive) layer can be formed on the surface of the PO microporous membrane. Also, one PO microporous membrane can be laminated with another porous layer.

The microporous membrane according to this embodiment can have the following characteristics:

(Porosity measured by nitrogen gas adsorption test)
The microporous membrane (however, when a microporous laminate membrane is formed by laminating a plurality of porous layers as described above, one of the porous layers in the laminate membrane) preferably has a single-point total pore volume having a pore diameter of 98 nm or less, measured by a nitrogen gas adsorption test, of 25% to 85% of the total pore volume.

The nitrogen gas adsorption test can be performed by the method described in the Examples below. The total pore volume is defined as the amount of adsorption at a nitrogen relative pressure (p/p0) of 0.98, and the single-point total pore volume (pore diameter 98 nm or less) is derived. By comparing this value with the total pore volume of the microporous membrane (porous layer), the ratio of the pore volume (pore diameter 98 nm or less) to the total pore volume can be calculated. The higher this ratio, the smaller the pores the microporous membrane (porous layer) is formed from. From the viewpoint of making the microporous membrane (porous layer) more dense and efficiently delaying the growth of lithium dendrites and foreign matter-derived dendrites associated with the elution of metal foreign matter, the single-point total pore volume (pore diameter 98 nm or less) is preferably 25% or more of the total pore volume, and more preferably 30% or more. In addition, in the nitrogen gas adsorption test of the microporous membrane (porous layer) by the single-point method, the lower limit of the pore diameter can exceed 0 nm in this technical field.

On the other hand, if the single-point total pore volume (pore diameter 98 nm or less) of the microporous membrane (porous layer) exceeds 85% of the total pore volume, the separator will become clogged, causing points of concentrated resistance increase, which will ultimately promote the generation of dendrites. From the above perspective, the single-point total pore volume (pore diameter 98 nm or less) is preferably 85% or less of the total pore volume, and more preferably 80% or less.

From the viewpoint of film formability, conventional microporous membranes that contain polyethylene (PE), for example, high density polyethylene (HDPE), as a main component (usually at 50% by mass or more) generally have a single-point total pore volume (pore diameter of 98 nm or less) of less than 25%. This is because, from the viewpoint of melt viscosity or heat setting process, small pores are blocked, resulting in a large pore size in the membrane as a whole.

The ratio of the one-point total pore volume (pore diameter 98 nm or less) to the total pore volume can be controlled, for example, by densifying the microporous membrane (porous layer). More specifically, in the manufacturing process of the microporous membrane described below, for example, by including a high molecular weight PE having a molecular weight described below in a content described below; performing a stretching process before the extraction process; stretching under conditions of a stretching areal ratio of 50 times or more and/or a stretching temperature of 128°C or less; adjusting the longitudinal and lateral stretching ratios in the stretching process to 5 times or more; adjusting the heat setting temperature during the heat setting (HS) process to 115°C or more and 140°C or less; adjusting the HS stretching ratio to 1.5 times or more and 2.2 times or less; adjusting the HS relaxation ratio to 0.7 times or more and less than 0.9 times, etc., alone or in appropriate combination, it can be controlled to be within the range of the pore volume ratio described above.

Furthermore, the pore volume ratio can also be controlled by the viscosity average molecular weight of the PE used. From the viewpoint that the use of high molecular weight PE makes the pores smaller in diameter during the stretching process and makes the pores less likely to be blocked during the heat setting process, the viscosity average molecular weight (Mv) of the PO microporous membrane is preferably 450,000 or more, more preferably 500,000 or more, and the content of PE with Mw of 450,000 or more in the resin composition of the membrane is preferably 45% by mass or more, more preferably 50% by mass or more, and this content can be 100% by mass. In order to prevent the pore ratio from exceeding 85%, the Mv of the membrane is preferably 6,000,000 or less, preferably 5,000,000 or less, and more preferably 3,000,000 or less.

The pore ratio can also be controlled within a specified range by the molecular structure of the PO used. Although the reason is unclear, the pore ratio tends to fall within a specified range as the polydispersity (weight average molecular weight Mw/number average molecular weight Mn and z average molecular weight Mz/weight average molecular weight Mw) of the raw polymer increases.

(Number of holes calculated by gas-liquid method)
The microporous membrane (however, in the case where a microporous laminate membrane is formed by laminating a plurality of porous layers as described above, one of the porous layers in the laminate membrane) preferably has a pore number B calculated by the gas-liquid method of 15 pores/ ^μm2 or more and 500 pores/μm2 or ^less .

The upper limit of the number of pores (B) per cross section calculated by the gas-liquid method is preferably 15 pores/μm2 or more and 500 pores/μm2 or less, 40 pores/μm2 or more and 480 pores/μm2 or less, 60 pores/μm2 or more and 450 pores/μm2 or less, 80 pores/μm2 or more and 350 pores/μm2 or less, or 100 pores/ ^μm2 or more and 300 pores/μm2 ^{or less} , and more preferably 110 pores/μm2 or more and 200 pores/μm2 or ^less , from the viewpoint of being able to physically suppress the growth of dendrites due to the high strength ^of the microporous membrane (porous layer), and from the viewpoint of being able to efficiently suppress local dendrite generation by diffusing lithium ions ^and metal ions accompanying the elution of ^foreign matter in the microporous membrane ( ^{porous layer)} and suppressing ^an increase in locations with ^local increased resistance ^and improving ^cycle characteristics. The number of pores per cross section (B) calculated by the gas-liquid method is adjusted to be within the above range by adjusting the plasticizer ratio in all raw materials (i.e., the resin ratio in all raw materials), the heat setting temperature, etc. in the production process of the PO microporous membrane described below.

(Porosity)
The microporous film (however, when a microporous laminate film is formed by laminating a plurality of porous layers as described above, one of the porous layers in the laminate film) preferably has a porosity ε in the range of 20% to 80%. Hereinafter, the porosity (ε) may be referred to as "porosity (before compression)".

The porosity (ε) of the microporous membrane (porous layer) is preferably 20% or more, more preferably 40% or more, even more preferably 45% or more, even more preferably 47% or more, and particularly preferably 50% or more, from the viewpoint of being able to efficiently suppress the generation of dendrites by reducing the membrane resistance and improving ion diffusivity, from the viewpoint of achieving a certain membrane strength and low air permeability, and from the viewpoint of having high ion conductivity and high output characteristics. Furthermore, from the viewpoint of being able to suppress the growth of dendrites by improving the membrane strength, and from the viewpoint of improving the withstand voltage, the porosity (ε) is preferably 80% or less, more preferably 70% or less, even more preferably 65% or less, and particularly preferably 60% or less.

(Mean flow diameter measured by half-dry method)
The microporous membrane (when a microporous laminate membrane is formed by laminating a plurality of porous layers as described above, one of the porous layers in the laminate membrane) preferably has a mean flow diameter d measured by the half-dry method of 0.010 μm or more and 0.150 μm or less. In this specification, the pore diameter of the microporous membrane (porous layer) means the mean flow diameter measured by the half-dry method using a perm porometer (Porous Materials, Inc.: CFP-1500AE).

By measuring the average flow diameter (d) by the half-dry method, the pore diameter of the narrowed portion of the pore structure of the microporous membrane (porous layer) can be known. The average flow diameter d measured by the half-dry method is preferably 0.010 μm or more and 0.150 μm or less, 0.015 μm or more and 0.100 μm or less, 0.020 μm or more and 0.080 μm or less, 0.020 μm or more and 0.055 μm or less, and more preferably 0.025 μm or more and 0.050 μm or less, from the viewpoint of physically suppressing the growth of dendrites by densifying the microporous membrane (porous layer) and from the viewpoint of increasing the number of pores (B) per cross section and improving ion diffusivity, and from the viewpoint of preventing the occurrence of a concentrated portion of increased resistance due to clogging, which results in the promotion of dendrite generation, and from the viewpoint of improving cycle characteristics.

Methods for controlling the pore size or porosity (ε) of the microporous membrane (porous layer) within the above numerical ranges can be used alone or in appropriate combination, for example, in the manufacturing method of the PO microporous membrane described below, by including a high molecular weight PE having a molecular weight described below in an amount described below; performing a stretching process before the extraction process; adjusting the longitudinal and lateral stretching ratios in the stretching process to 5 times or more, and performing stretching at a temperature of 128°C or less; adjusting the heat setting temperature in the heat setting (HS) process to 115°C or more and 140°C or less, adjusting the HS stretching ratio to 1.5 times or more and 2.2 times or less, and adjusting the HS relaxation ratio to 0.7 times or more and less than 0.9 times.

(Half-width of mean flow diameter peak measured by half-dry method)
The microporous membrane (however, when a microporous laminate membrane is formed by laminating a plurality of porous layers as described above, one of the porous layers in the laminate membrane) preferably has a half-value width of the mean flow diameter peak of 0.010 μm or less as measured by the half-dry method.

The narrower the half-width of the average flow diameter peak in the half-dry method, the more uniform the pore structure of the microporous membrane (porous layer). The more uniform the pore structure, the more uniform the lithium ions and metal ions associated with the elution of foreign matter are diffused, and from the viewpoint of efficiently suppressing local dendrite generation, physically suppressing dendrite growth, and suppressing local resistance increase and improving cycle characteristics, the half-width of the average flow diameter peak measured by the half-dry method is preferably 0.005 μm or less, and more preferably 0.002 μm or less. Note that the lower limit of the half-width of the average flow diameter peak measured by the half-dry method can exceed 0 μm in this technical field. The half-width of the average flow diameter peak can be adjusted to within the above numerical range, for example, by controlling the content ratio of high molecular weight PE in the manufacturing method of the microporous membrane, or by adjusting the heat setting temperature during the heat setting (HS) process to, for example, 140° C. or less.

(Film Thickness)
From the viewpoints of the capacity of the power storage device, high ion permeability and good rate characteristics, retarding the growth of dendrites, and reducing the volume occupied by the separator to contribute to improving the battery capacity when used for a high-capacity battery, the thickness of the microporous membrane is preferably 30 μm or less, 25 μm or less, or 20 μm or less, more preferably 18 μm or less, even more preferably 16 μm or less, even more preferably 1 μm or more and 14 μm or less, particularly preferably 3 μm or more and 13 μm or less, and most preferably 5 μm or more and 12 μm or less. The thickness of the microporous membrane is hereinafter sometimes referred to as the "average thickness (before compression)".

The average thickness of the microporous membrane (before compression) can be adjusted within the above numerical range by controlling the distance between the cast rolls, the cast clearance, the stretch ratio during the biaxial stretching process, the HS ratio, the HS temperature, etc. in the manufacturing process of the microporous membrane.

(Air permeability)
The upper limit of the air permeability of the microporous membrane is from the viewpoint of output characteristics and cycle characteristics of the electricity storage device, and ^the lower limit is from the viewpoint of membrane strength, and is preferably 0 seconds or more and 250 seconds or less, more preferably 10 seconds or more and 200 seconds or less, even more preferably 20 seconds or more and 180 seconds or less, and particularly preferably 25 seconds or more and 170 seconds or less, per 100 cm3 of air.

(Puncture strength converted to basis weight, Puncture strength, basis weight)
The microporous membrane preferably has a puncture strength calculated based on basis weight of 50 gf/(g/m ² ) or more. A puncture strength calculated based on basis weight of 50 gf/(g/m ² ) or more indicates a membrane structure with high membrane strength per resin basis weight and difficult to be crushed by compressive stress, and tends to improve the safety of the electric storage device, for example, during a nail penetration test or pressure test, even if the microporous membrane used as a separator substrate has high porosity and low air permeability, it is difficult to break, and the puncture strength calculated based on basis weight is measured by the method described in the Examples, and is obtained by measuring the puncture strength (hereinafter simply referred to as puncture strength) not converted to basis weight at a total of three points, two points 10% inside the full width from both ends toward the center and one point in the center along the TD of the membrane, and dividing the average value by basis weight.

The advantage of controlling the pin puncture strength is remarkable when an electrode that easily expands and contracts in a cell of an electricity storage device such as a nonaqueous secondary battery is used, and is even more remarkable when a high-capacity electrode or a silicon (Si)-containing negative electrode used in an in-vehicle battery is used. From this viewpoint, the pin puncture strength converted into basis weight of the microporous membrane is more preferably 50 gf/(g/m ² ) to 150 gf/(g/m ² ), even more preferably 55 gf/(g/m ² ) to 130 gf/(g/m ² ), and particularly preferably 70 gf/(g/m ² ) to 120 gf/(g/m ² ).

From the same viewpoint as above, from the viewpoint of achieving a certain membrane strength and low air permeability, and from the viewpoint of improving short circuit resistance, the basis weight of the microporous membrane is preferably in the range of 1.0 g/m2 or ^more and 15 g/m2 or ^less .

From the same viewpoint as above, from the viewpoint of safety when an impact is applied to the electric storage device, and from the viewpoint of suppressing short circuit between electrodes or voltage resistance failure caused by the separator being ruptured by a foreign object unintentionally mixed inside the electric storage device, the puncture strength of the microporous membrane is preferably 200 gf or more, more preferably 220 gf or more, even more preferably 250 gf or more, even more preferably 280 gf or more, and particularly preferably 300 gf or more. The upper limit of the puncture strength is not particularly limited, but can be determined according to the crystallinity of the membrane, the heat shrinkability of the membrane, and the suppressed electrical resistance, and may be, for example, 900 gf or less, 850 gf or less, 700 gf or less, or 680 gf or less.

The puncture strength and basis weight equivalent puncture strength of the microporous membrane can be adjusted within the above-described numerical ranges by, for example, controlling the molecular weight and blending ratio of the polymer raw material such as polyolefin, the stretching ratio during the biaxial stretching process, the MD/TD stretching temperature during the biaxial stretching process, the heat amount coefficient per unit resin of the resin composition during the biaxial stretching process, the heat setting (HS) ratio, etc., during the manufacturing process of the microporous membrane.

(The difference R between the maximum and minimum air permeability values measured at three TD points before compression, air permeability distribution before compression)
The microporous membrane preferably has a difference between the maximum and minimum air permeability values (hereinafter referred to as difference R) measured at three points along the TD: two points 10% inward of the overall width from both ends toward the center and one point at the center, of 15 sec/100 cm3 or ^less .

The difference R between the maximum and minimum air permeability values measured at three points along the TD of the microporous membrane as described above is measured by the method described in the Examples and represents the air permeability distribution of the microporous membrane before the compression test. From the viewpoint of the accuracy of the air permeability measurement, the total width W of the microporous membrane is preferably 50 mm or more, more preferably 100 mm or more, and even more preferably 300 mm or more. The upper limit of the total width W is not particularly limited and can be determined depending on, for example, the film-forming device, the film-forming process, the mother roll dimensions, the slit roll dimensions, the coating process, etc., and may be, for example, 5000 mm or less, or 4000 mm or less.

A well-maintained air permeability distribution satisfying R≦15 sec/100 ^cm3 represents a membrane structure with small in-plane variations in air permeability, and for example, in an electricity storage device including a microporous membrane as a separator substrate, the in-plane electrochemical reaction occurs uniformly, tending to improve battery characteristics such as rate characteristics and cycle characteristics. This tendency is prominent when an electrode that is prone to expansion and contraction in a cell of a nonaqueous secondary battery is used, and is even more prominent when a high-capacity electrode used in an on-board battery or the like, or a silicon (Si)-containing negative electrode is used. From such a viewpoint, the difference R is more preferably 0 sec/100 cm3 or ^more and 15 sec/100 ^cm3 or less, even more preferably 0 sec/100 cm3 or ^more and 13 sec/100 cm3 or ^less , still more preferably 0 sec/100 cm3 or ^more and 11 sec/100 ^cm3 or less, and particularly preferably 0 sec/100 ^cm3 or more and 9 sec/100 ^cm3 or less.

The average of the measured values at the above three points is expressed as the air permeability (before compression), and from the same viewpoint as above, as well as from the viewpoint of maintaining high (ion) permeability even in a compressed state, and from the viewpoint of ensuring puncture strength, it is preferably 0 sec/100 cm3 ^or more and 200 sec/100 cm3 ^or less, more preferably 30 sec/100 cm3 or ^more and 180 sec/100 cm3 or ^less , and even more preferably 40 sec/100 ^cm3 or more and 150 sec/100 ^cm3 or less.

The difference R and air permeability (before compression) of the microporous membrane can be adjusted within the numerical ranges described above, for example, by controlling the molecular weight and blending ratio of the polymer raw material such as polyolefin, the stretching ratio during the biaxial stretching process, the MD/TD stretching temperature during the biaxial stretching process, the heat coefficient per unit resin of the resin composition during the biaxial stretching process, the HS magnification, etc., during the manufacturing process of the microporous membrane.

(Membrane properties after compression)
By subjecting the microporous membrane to a compression test under specific conditions, the properties of the microporous membrane that can achieve both the battery properties and safety of the electricity storage device can be discovered.
<Compression press test>
The microporous membrane or separator is measured under four different pressure conditions (for example, pressures of 2.5 MPa, 5 MPa, 7.5 MPa, and 10 MPa at 25°C) by the method described later in the Examples, and a relational equation between the porosity and air permeability of the microporous membrane or separator after unloading is obtained from the four measurement results.

The post-compression air permeability of _{2.5 MPa at 25° C.} ≦180 sec/100 cm ³ , together with the pre-compression air permeability distribution or difference R described above, tends to improve battery characteristics such as rate characteristics and cycle characteristics for non-aqueous secondary batteries containing a polyolefin microporous membrane as a separator, etc. This tendency is remarkable in improving rate characteristics when an electrode that is prone to expand and contract within a cell of a non-aqueous secondary battery is used or when the separator is compressed by pressure from outside the cell, and is more remarkable when a high-capacity electrode, silicon (Si)-containing negative electrode, Li metal negative electrode, or Cu foil used in an on-board battery, etc. is used as the negative electrode. From this viewpoint, the post-compression air permeability at _{25°C, 2.5 MPa} is preferably 200 sec/100 ^cm3 or less, more preferably 180 sec/100 ^cm3 or less, even more preferably 170 sec/100 cm3 or ^less , and particularly preferably 120 sec/100 cm3 or ^less . The lower limit of the pressurized air permeability at _{25°C, 2.5 MPa} is not particularly limited, but can be determined according to the mechanical strength or puncture strength of the microporous membrane, and may be, for example, 0 sec/100 ^cm3 or more, 10 sec/100 cm3 ^or more, or 20 sec/100 ^cm3 or more.

From the same viewpoint as above, and from the viewpoint of achieving high porosity in a state in which the electrodes are expanded and contracted or the separator is compressed in the cell, and thereby improving the cycle characteristics of the electricity storage device, the porosity of the microporous membrane after compression at 25° C. and 2.5 MPa (hereinafter referred to as post-compression porosity _{25° C., 2.5 MPa} ) is preferably 40% or more, more preferably 40% or more and 80% or less, even more preferably 45% or more and 70% or less, still more preferably 47% or more and 70% or less, and particularly preferably 50% or more and 65% or less.

The compressed air permeability of the microporous membrane _{, 2.5 MPa at 25°C,} and the compressed porosity of the microporous membrane _{, 2.5 MPa at 25°C} , can be adjusted within the above-described numerical ranges by controlling, for example, the molecular weight and blending ratio of the polymer raw material such as polyolefin, the stretching ratio in the biaxial stretching process, the MD/TD stretching temperature in the biaxial stretching process, the heating coefficient per unit resin of the resin composition in the biaxial stretching process, the HS magnification, the HS temperature, etc. in the production process of the microporous membrane.

(Melt Flow Index)
The melt flow index (MI) of the microporous membrane is preferably 1.0 or less, more preferably 0.001 to 1.0, even more preferably 0.005 to 0.8, and particularly preferably 0.01 to 0.4, from the viewpoint of reducing the fluidity of the membrane when melted and suppressing inter-electrode short circuit due to the flow of the separator in a heat generation state during a nail penetration test. The MI of the microporous membrane can be adjusted to within the above numerical range, for example, by controlling the molecular weight and blending ratio of the polymer raw material such as polyolefin.

(Molecular weight measured by GPC)
Regarding the molecular weight of the microporous membrane measured by GPC, from the viewpoint of the decrease in flowability of the membrane when melted and the short circuit resistance during nail penetration test, the polyethylene component having a weight average molecular weight (Mw) of 1,000,000 or more is preferably 7% or more of the total eluted components, more preferably 9% or more, even more preferably 12% or more, and particularly preferably 15% or more. Also, from the viewpoint of suppressing excessive stress when the membrane shrinks in a high temperature environment, the polyethylene component having a Mw of 1,000,000 or more is preferably 57% or less of the total eluted components, more preferably 42% or less, even more preferably 33% or less, and particularly preferably 27% or less. The molecular weight of the PO microporous membrane can be adjusted within the above numerical range, for example, by controlling the type, molecular weight and compounding ratio of the polyolefin raw material.

(Tensile Breaking Strength)
The tensile breaking strength of the microporous membrane in both MD and TD is preferably 500 to 3,000 kgf/ ^cm2 , more preferably 700 kgf/cm2 ^{or more} and 3,000 kgf/cm2 or ^less , and even more preferably 1,000 kgf/cm2 or more and 3,000 kgf/ ^cm2 or ^less , from the viewpoint of ensuring the membrane strength necessary for winding and laminating electrodes and separators in the production process of an electricity storage device.

The closer the values of the MD and TD tensile breaking strengths of the microporous membrane are, the more evenly the membrane breaks during a nail penetration test of an electricity storage device, minimizing the short circuit area and thus improving the safety of the nail penetration test. From this perspective, the ratio of the MD tensile breaking strength to the TD tensile breaking strength of the microporous membrane (MD/TD tensile breaking strength ratio) is preferably 0.7 to 1.3, more preferably 0.75 to 1.25, and even more preferably 0.8 to 1.2. The MD/TD tensile breaking strength ratio of the microporous membrane can be adjusted to within the numerical range described above, for example, by controlling the stretching ratio and HS ratio during the biaxial stretching process.

(Tensile Modulus)
The tensile modulus of the microporous membrane in both MD and TD is preferably 1,000 kg/cm2 or more and 10,000 kg/cm2 or less, and more preferably 2,000 kg/cm2 or ^more and 90,000 kg/cm2 or less, from the viewpoint of making the separator less likely to rupture and not completing a short circuit when a nail penetrates the device exterior and also penetrates and deforms the separator and electrodes during a ^nail penetration test ^of an electricity storage device ^, thereby improving safety.

(Thermal shrinkage rate)
From the viewpoints of high shape stability of the membrane at relatively high temperatures and suppressing a short circuit in a thermal runaway state of an electricity storage device during a nail penetration test, etc., the heat shrinkage rate of the microporous membrane is preferably -10% or more and 20% or less, more preferably -5% or more and 15% or less, and even more preferably 0% or more and 10% or less, when measured in MD at 120°C. The heat shrinkage rate of the microporous membrane is preferably -10% or more and 20% or less, more preferably -5% or more and 18% or less, even more preferably 0% or more and 15% or less, and especially preferably 0% or more and 10% or less, when measured in TD at 120°C.

(Shutdown temperature)
The shutdown (Fuse) temperature of the microporous membrane is preferably 130°C or more and 150°C or less, more preferably 135°C or more and 149°C or less. A shutdown temperature of 150°C or less means that when some abnormal reaction occurs and the temperature inside the battery rises, the pores of the separator substrate are blocked before reaching 150°C. Therefore, the lower the shutdown temperature, the more quickly the flow of lithium ions between the electrodes stops at a low temperature, improving safety. On the other hand, from the viewpoint of not deteriorating the battery performance even when exposed to a high temperature exceeding 100°C, the shutdown temperature is preferably 130°C or more.

(Ingredients)
The microporous film (however, when a plurality of porous layers are laminated as described above to form a microporous laminate film, one of the porous layers in the laminate film) is preferably formed from a resin composition containing a polyolefin resin. If desired, the resin composition may further contain inorganic particles, resins other than polyolefin, etc., but the inclusion of inorganic particles tends to reduce the pore ratio.

From the viewpoint of improving the shutdown performance of the power storage device, the microporous membrane is preferably a porous membrane formed from a polyolefin resin composition in which polyolefin resin accounts for 50% by mass or more and 100% by mass or less of the resin components constituting the porous membrane. The proportion of polyolefin resin in the polyolefin resin composition is more preferably 60% by mass or more and 100% by mass or less, even more preferably 70% by mass or more and 100% by mass or less, and most preferably 95% by mass or more and 100% by mass or less.

The polyolefin resin used is not particularly limited, and examples thereof include polymers (e.g., homopolymers, copolymers, multi-stage polymers, etc.) obtained by polymerizing monomers such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene. These polymers can be used alone or in combination of two or more types.

Among them, from the viewpoints of reducing or suppressing an increase in the electrical resistance of the membrane, and of the compression resistance and structural uniformity of the membrane, the polyolefin resins preferably used are polyethylene, polypropylene, ethylene-propylene copolymers, copolymers of ethylene-propylene and other monomers, and mixtures of these.

From the viewpoints of the crystallinity, high strength, compression resistance, etc. of the microporous membrane, and from the viewpoint of expressing fuse properties, a microporous membrane formed from a polyethylene composition in which polyethylene accounts for 50% by mass or more and 100% by mass or less of the resin components constituting the microporous membrane is preferred. The proportion of polyethylene (PE) in the resin components constituting the microporous membrane is more preferably 60% by mass or more and 100% by mass or less, more preferably 70% by mass or more and 100% by mass or less, even more preferably 80% by mass or more and 100% by mass or less, and most preferably 90% by mass or more and 100% by mass or less.

In addition, from the viewpoint of high strength, compression resistance, suppression of electrical resistance, etc., when a microporous membrane is formed as a separator substrate, the PE ratio in the polyolefin resin is preferably 30% by mass or more, more preferably 50% by mass or more, even more preferably 70% by mass or more, particularly preferably 80% by mass or more, and preferably 100% by mass or less, more preferably 95% by mass or less, and even more preferably 95% by mass or less. In addition, if the PE ratio is 50% or more, it is also preferable from the viewpoint of exhibiting high responsiveness in fuse behavior.

The viscosity average molecular weight (Mv) of the polyolefin resin used as the raw material for the microporous membrane is preferably 30,000 to 6,000,000, more preferably 80,000 to 3,000,000, and even more preferably 150,000 to 2,000,000. When Mv is 30,000 or more, it is preferred because the polymers tend to be entangled to provide high strength. On the other hand, when Mv is 6,000,000 or less, it is preferred from the viewpoint of improving moldability in the extrusion and stretching processes.

In one embodiment, when the microporous membrane contains polyethylene (PE), the Mv of at least one type of PE is preferably 600,000 or more, more preferably 700,000 or more, from the viewpoint of membrane orientation and rigidity, and the upper limit of the Mv of the PE may be, for example, 2,000,000 or less. From the viewpoint of the decrease in fluidity of the membrane when melted and the short circuit resistance in a nail penetration test, the proportion of PE with an Mv of 600,000 or more in the polyolefin resin constituting the microporous membrane is preferably 30% by mass or more, more preferably 50% by mass or more, even more preferably 60% by mass or more, particularly preferably 70% by mass or more, and may be 100% by mass.

In another embodiment, the microporous membrane (porous layer) preferably contains 45% by mass or more, more preferably 50% by mass or more, even more preferably 70% by mass or more, even more preferably 80% by mass or more, and particularly preferably 90% by mass or more of PE raw material having an Mv of 500,000 to 6,000,000 based on the mass of the microporous membrane. If the microporous layer contains 45% by mass or more of PE raw material having an Mv of 500,000 to 6,000,000, the crystals tend to be highly oriented during stretching, and the microporous layer tends to have a small pore size or become dense. From the same viewpoint, the PO raw material used in the production of the microporous membrane is preferably an ethylene homopolymer, and more preferably an ethylene homopolymer having an Mv of 500,000 to 6,000,000.

Examples of polyolefin resins include low density polyethylene (density 0.910 g/cm ³ or more and less than 0.930 g/cm ³ ), linear low density polyethylene (density 0.910 g/cm ³ or more and less than 0.940 g/cm ³ ), medium density polyethylene (density 0.930 g/cm ³ or more and less than 0.942 g/cm ³ ), high density polyethylene (density 0.942 g/cm ³ or more), ultra-high molecular weight polyethylene (density 0.910 g/cm ³ or more and less than 0.970 g/cm ³ ), isotactic polypropylene, atactic polypropylene, polybutene, ethylene propylene rubber, etc. These can be used alone or in combination of two or more. Among them, it is preferable to use either polyethylene alone, polypropylene alone, or a mixture of polyethylene and polypropylene from the viewpoint of obtaining a uniform film.

From the viewpoint of the safety of an electricity storage device having a PO microporous membrane as a separator, the polyolefin resin is preferably a medium density polyethylene (MDPE) having a density of 0.930 g/cm ³ or more and less than 0.942 g/cm ³ , and may be, for example, PE other than high density polyethylene (HDPE). From the viewpoint of improving the safety of an electricity storage device even when the PO microporous membrane is a thin film, the polyolefin resin preferably contains at least one selected from medium density polyethylene having a viscosity average molecular weight of less than 1,000,000 and ultra-high molecular weight polyethylene having a viscosity average molecular weight of 1,000,000 or more and 2,000,000 or less and a density of 0.930 g/cm ³ or more and less than 0.942 g/cm ³ in an amount of 50 mass% or more, more preferably 60 mass% or more, and even more preferably 70 mass% or more based on the mass of the PO microporous membrane.

The polyolefin resin preferably contains polypropylene (PP) from the viewpoint of ensuring safety in the high temperature range (160°C or higher) where it is difficult to ensure safety with PE. As the polypropylene, a propylene homopolymer is preferable from the viewpoint of heat resistance. From the viewpoint of further improving heat resistance, it is more preferable that the polyolefin resin contains polyethylene and polypropylene as main components. Therefore, from the viewpoint of film formability and film rupture resistance in the stretching process, the proportion of PP raw material in the PO raw material is preferably more than 0 mass% and 10 mass% or less.

When PE and PP are used in combination, it is preferable to use, as the polyethylene, at least one selected from medium-density polyethylene having a viscosity-average molecular weight of less than 1 million, and ultra-high molecular weight polyethylene having a viscosity-average molecular weight of 1 million or more and 2 million or less and a density of 0.930 g/ ^cm3 or more and less than 0.942 g/ ^cm3 , from the viewpoints of balancing strength and permeability and maintaining an appropriate fuse temperature.

The viscosity average molecular weight of the polyolefin resin (measured in accordance with the measurement method in the examples described later) is preferably 50,000 or more, more preferably 100,000 or more, more preferably 500,000 or more, more preferably 700,000 or more, more preferably 1,000,000 or more, and the upper limit is preferably 6,000,000 or less, preferably 3,000,000 or less, preferably 1,900,000 or less. A viscosity average molecular weight of 50,000 or more is preferable from the viewpoint of maintaining high melt tension during melt molding to ensure good moldability, or from the viewpoint of imparting sufficient entanglement to the resin to increase the strength of the microporous film. On the other hand, a viscosity average molecular weight of 6,000,000 or less is preferable from the viewpoint of realizing uniform melt kneading and improving the moldability of the sheet, especially the thickness moldability.

The polydispersity (Mw/Mn) of the polyethylene raw material is preferably 4.0 or more and 10.0 or less. The polydispersity (Mw/Mn) is measured in accordance with the measurement method in the examples described later. Although the reason is unclear, setting the polydispersity of the raw polymer within the above range tends to make the PO microporous membrane have a moderately high pore ratio and a uniform pore structure. From this perspective, the polydispersity of the polyethylene raw material is more preferably 6.0 or more, and even more preferably 7.0 or more.

Furthermore, the ratio of z-average molecular weight to weight-average molecular weight (Mz/Mw) of the polyethylene raw material is preferably 2.0 or more and 7.0 or less. Mz/Mw is measured according to the measurement method in the examples described later. Although the reason is unclear, setting the Mz/Mw of the raw material polymer within the above range tends to make the PO microporous membrane have a moderately high pore ratio and a uniform pore structure. From this perspective, the Mz/Mw of the polyethylene raw material is more preferably 4.0 or more, and even more preferably 5.0 or more.

The resin composition may be mixed with various known additives, such as inorganic particles, phenol-based, phosphorus-based, or sulfur-based antioxidants, metal soaps such as calcium stearate and zinc stearate, ultraviolet absorbers, light stabilizers, antistatic agents, antifogging agents, and coloring pigments, as necessary. However, mixing inorganic particles or the like with the resin composition tends to reduce the pore ratio of the resulting film.

(Method for producing microporous membrane)
The method for producing the microporous membrane is not particularly limited. For example, in the case of a method for producing a PO microporous membrane using a polyolefin (PO) resin, the following method is used:
A mixing step (a) of mixing a resin composition containing a polyolefin resin, a pore-forming material, and various additives as desired;
an extrusion step (b) of melt-kneading and extruding the mixture obtained in the step (a);
a sheet forming step (c) of forming the extrudate obtained in step (b) into a sheet;
A primary stretching step (d) of stretching the sheet-like molded product obtained in the step (c) at least once in at least one axial direction;
an extraction step (e) of extracting the pore-forming material from the first stretched membrane obtained in step (d);
and a heat fixing step (f) of heat fixing (HS) the extracted film obtained in the step (e) at a predetermined temperature.

The above-mentioned method for producing a microporous PO membrane can provide a microporous PO membrane that, when used as a separator for a lithium-ion secondary battery or other power storage device, can highly simultaneously suppress lithium dendrites and chemical micro-short circuits caused by metal foreign matter, as well as high output characteristics and cycle characteristics. In particular, the method of stretching in MD and TD in the primary stretching step (d), and then heat-setting in TD in the heat-setting step (f) after passing through the extraction step (e) tends to make it easier to adjust the pore ratio and pore size of the resulting microporous PO membrane to within the numerical ranges described above. The method for producing a microporous PO membrane of this embodiment is not limited to the above-mentioned method, and various modifications are possible without departing from the gist of the method.

[Mixing step (a)]
The mixing step (a) is a step of mixing a resin composition containing a polyolefin resin, a pore-forming material, and various additives as desired. In the mixing step (a), other components may be mixed with the resin composition as necessary.

The pore-forming material may be any material, for example a plasticizer, as long as it is distinguished from the PO resin and inorganic particle materials. The plasticizer may be a non-volatile solvent capable of forming a homogeneous solution at or above the melting point of the PO resin, such as hydrocarbons such as liquid paraffin (LP) and paraffin wax; esters such as dioctyl phthalate and dibutyl phthalate; and higher alcohols such as oleyl alcohol and stearyl alcohol.

The content of the plasticizer in the resin composition is preferably 60% by mass to 90% by mass, more preferably 71% by mass to 85% by mass, even more preferably 73% by mass to 85% by mass, and particularly preferably 75% by mass to 85% by mass. By adjusting the content of the plasticizer to 60% by mass or more, the number of pores in the film increases, ion diffusivity increases, and the generation of dendrites can be efficiently suppressed. In addition, the melt viscosity of the resin composition decreases, and melt fracture is suppressed, which tends to improve the film formability during extrusion. On the other hand, by adjusting the content of the plasticizer to 90% by mass or less, the elongation of the original roll during the film formation process can be suppressed.

In step (a), the resin composition containing PO may contain any additive. The additives are not particularly limited, but may include, for example, polymers other than polyolefin resins; antioxidants such as phenolic compounds, phosphorus compounds, and sulfur compounds; metal soaps such as calcium stearate and zinc stearate; ultraviolet absorbers; light stabilizers; antistatic agents; antifogging agents; coloring pigments, and the like. The total amount of these additives added is preferably more than 0 parts by mass and not more than 20 parts by mass, more preferably not more than 10 parts by mass, and even more preferably not more than 5 parts by mass, relative to 100 parts by mass of the polyolefin resin.

The method of mixing in step (a) is not particularly limited, but examples include a method in which some or all of the raw materials are premixed as necessary using a Henschel mixer, ribbon blender, tumbler blender, etc. Among these, a method of mixing using a Henschel mixer is preferred.

[Extrusion step (b)]
The extrusion step (b) is a step of melt-kneading and extruding the resin composition obtained in the step (a). In the extrusion step (b), components other than the polyolefin resin may be mixed with the resin composition as necessary.

The method of melt kneading in step (b) is not particularly limited, but examples include a method in which all raw materials including the mixture mixed in step (a) are melt kneaded using a screw extruder such as a single screw extruder or a twin screw extruder; a kneader; a mixer, etc. Among them, it is preferable to perform melt kneading using a screw of a twin screw extruder. In addition, when performing melt kneading, it is preferable to add the plasticizer in two or more separate steps, and further, when adding the additive in multiple separate steps, it is preferable to adjust the amount added in the first step to 80% by weight or less of the total amount added, from the viewpoint of suppressing aggregation of the contained components and dispersing them uniformly. As a result, the obtained microporous film is preferable from the viewpoint of suppressing heat generation by shutting down over a large area and improving the safety of the cell as a separator.

When a pore-forming material is used in step (b), the temperature of the melt-kneading section is preferably less than 200°C in order to uniformly knead the resin composition. The lower limit of the temperature of the melt-kneading section is equal to or higher than the melting point of the polyolefin in order to uniformly dissolve the polyolefin resin in the pore-forming material such as a plasticizer.

During kneading, although there are no particular limitations, it is preferable to mix the antioxidant with the raw material PO at a predetermined concentration, replace the atmosphere around the mixture with a nitrogen atmosphere, and perform melt kneading while maintaining the nitrogen atmosphere. The temperature during melt kneading is preferably 160°C or higher, more preferably 180°C or higher, and preferably less than 300°C.

In step (b), the kneaded product obtained through the above kneading is extruded by an extruder such as a T-die or an annular die. At this time, it may be a single-layer extrusion or a co-extrusion. The conditions for extrusion are not particularly limited, and for example, a known method can be used. In addition, it is preferable to control the (die) lip clearance, etc., from the viewpoint of the film thickness of the obtained PO microporous film.

[Sheet molding step (c)]
The sheet forming step (c) is a step of forming the extrudate obtained in the extrusion step (b) into a sheet. The sheet-shaped molded product obtained by the sheet forming step (c) may be a single layer or a laminate. The method of forming the sheet is not particularly limited, but for example, a method of solidifying the extrudate by compression and cooling can be mentioned.

The compression cooling method is not particularly limited, but examples include a method in which the extrudate is directly contacted with a cooling medium such as cold air or cooling water; a method in which the extrudate is contacted with a metal roll, a press, or the like cooled with a refrigerant, and the like. Among these, the method in which the extrudate is contacted with a metal roll, a press, or the like cooled with a refrigerant is preferred because it is easy to control the film thickness.

After the melt-kneading step (b), the set temperature in the step of forming the molten material into a sheet is preferably set to a temperature higher than the set temperature of the extruder. The upper limit of the set temperature for sheet forming is preferably 300°C or lower, more preferably 260°C or lower, from the viewpoint of thermal degradation of the polyolefin resin. For example, when continuously producing a sheet-shaped molded product from an extruder, if the set temperature of the step of forming the sheet after the melt-kneading step, i.e., the set temperature of the path from the extruder outlet to the T-die and the T-die, is set to a temperature higher than the set temperature of the extrusion step, it is preferable because the molten material can be formed into a sheet without separation of the resin composition and the pore-forming material. In addition, it is preferable to control the cast clearance, etc., from the viewpoint of the film thickness of the resulting PO microporous film.

[Primary stretching step (d)]
The primary stretching step (d) is a step of stretching the sheet-like molded product obtained in the sheet molding step (c) at least once in at least one uniaxial direction. The stretching step performed before the primary stretching is called "primary stretching", and the membrane obtained by the primary stretching is called "primary stretched membrane". In the primary stretching, the sheet-like molded product is stretched in at least one direction. The cutting may be performed in both the MD and TD directions, or in only one of the MD and TD directions.

The stretching method for the primary stretching is not particularly limited, but examples include uniaxial stretching using a roll stretching machine; TD uniaxial stretching using a tenter; sequential biaxial stretching using a roll stretching machine and a tenter, or a combination of multiple tenters; simultaneous biaxial stretching using a simultaneous biaxial tenter or inflation molding. Among these, simultaneous biaxial stretching is preferred from the viewpoint of the stability of the physical properties of the resulting PO microporous membrane.

The stretching ratio in the MD and/or TD of the first stretching is preferably 5 times or more, more preferably 6 times or more. When the stretching ratio in the MD and/or TD of the first stretching is 5 times or more, the resulting PO microporous membrane tends to form dense fibrils to have small pores and to have improved strength. In addition, the stretching ratio in the MD and/or TD of the first stretching is preferably 9 times or less, more preferably 8 times or less or 7 times or less. When the stretching ratio in the MD and/or TD of the first stretching is 9 times or less, breakage during stretching tends to be suppressed. When performing biaxial stretching, either sequential stretching or simultaneous biaxial stretching may be used, but the stretching ratio in each axial direction is preferably 5 times or more and 9 times or less, more preferably 6 times or more and 8 times or less, or 6 times or more and 7 times or less.

The primary stretching temperature can be selected with reference to the composition and concentration of the raw resins contained in the PO resin composition. The stretching temperature in the MD and/or TD is preferably 110°C or higher from the viewpoint of reducing the pore size and suppressing breakage, and more preferably 115°C or higher. In addition, the stretching temperature in the MD and/or TD is preferably 128°C or lower from the viewpoint of increasing the film strength or reducing the pore size, more preferably 126°C or lower, even more preferably 124°C or lower, and particularly preferably 122°C or lower.

[Extraction step (e)]
The extraction step (e) is a step of extracting the pore-forming material from the first stretched film obtained in the first stretching step (d) to obtain an extracted film. For example, the method of removing the pore-forming material includes a method of immersing the first stretched film in an extraction solvent to extract the pore-forming material and drying it thoroughly. The method of extracting the pore-forming material may be either a batch method or a continuous method. In addition, the amount of the pore-forming material, particularly the plasticizer, remaining in the porous film is preferably less than 1% by mass.

The extraction solvent used to extract the pore-forming material is preferably a poor solvent for the polyolefin resin, a good solvent for the pore-forming material or plasticizer, and has a boiling point lower than the melting point of the polyolefin resin. Such extraction solvents are not particularly limited, but examples include hydrocarbons such as n-hexane and cyclohexane; halogenated hydrocarbons such as methylene chloride and 1,1,1-trichloroethane; non-chlorine halogenated solvents such as hydrofluoroethers and hydrofluorocarbons; alcohols such as ethanol and isopropanol; ethers such as diethyl ether and tetrahydrofuran; and ketones such as acetone and methyl ethyl ketone. These extraction solvents may be recovered and reused by operations such as distillation.

[Heat setting process (f)]
The heat setting step (f) is a step of heat setting the extracted film obtained in the extraction step (e) at a predetermined temperature. The heat treatment method at this time is not particularly limited, but may be a tenter or roll stretching method. A heat fixing method in which a stretching and relaxation operation is performed using a machine is exemplified.

The stretching operation in the heat setting step (f) is an operation of stretching the PO microporous membrane in at least one of the MD and TD directions, and may be performed in both the MD and TD directions, or in only one of the MD or TD directions.

The stretching ratios in the MD and TD in the heat setting step (f) are preferably 1.5 to 2.2 times, and more preferably 1.7 to 2.1 times. The stretching ratios in the MD and TD in step (f) are preferably 1.5 times or more from the viewpoint of developing the strength of the film, and are preferably 2.2 times or less from the viewpoint of preventing breakage.

The stretching temperature in this stretching operation is preferably 115°C or higher from the viewpoint of suppressing breakage during stretching, and more preferably 120°C or higher. In addition, the stretching temperature in the heat setting step (f) is preferably 140°C or lower from the viewpoint of increasing the number of holes and the porosity, and more preferably 138°C or lower, more preferably 134°C or lower, more preferably 131°C or lower, more preferably 128°C or lower, or more preferably 125°C or lower. In addition, by having the stretching temperature within the above numerical range, the pore size of the obtained PO microporous membrane tends to be easily controlled.

The relaxation operation in the heat setting step (f) is an operation of shrinking the PO microporous membrane in at least one of MD and TD, and may be performed in both MD and TD, or only in MD or TD. The relaxation ratio in the heat setting step (f) is preferably 0.95 times or less, more preferably 0.90 times or less, and even more preferably 0.85 times or less. When the relaxation ratio in step (f) is 0.95 times or less, the thermal shrinkage of the membrane tends to be suppressed. In addition, from the viewpoint of increasing the relaxation temperature or increasing the number of holes and the porosity, the relaxation ratio is preferably 0.5 times or more, more preferably 0.7 times or more. Here, the "relaxation ratio" refers to the value obtained by dividing the dimensions of the membrane after the relaxation operation by the dimensions of the membrane before the relaxation operation, and when both MD and TD are relaxed, it refers to the value obtained by multiplying the relaxation ratio in MD and the relaxation ratio in TD.
Relaxation ratio=(membrane dimension after relaxation (m))/(membrane dimension before relaxation (m))

The relaxation temperature in this relaxation operation is preferably 115°C or higher from the viewpoint of preventing breakage, and more preferably 120°C or higher. Also, from the viewpoint of increasing the number of holes and the porosity, it is preferably 140°C or lower, and more preferably 138°C or lower, more preferably 134°C or lower, more preferably 131°C or lower, more preferably 128°C or lower, or more preferably 125°C or lower. Also, by having the relaxation temperature within the above numerical range, the pore size of the obtained PO microporous membrane tends to be easily controlled to be small and uniform.

The order of the above steps (a) to (f) can be changed as desired as long as the effect of the present invention is not impaired. After the above steps (a) to (f), the total stretch ratio of the PO microporous membrane is preferably in the range of 50 to 100 times, more preferably in the range of 70 to 100 times. Here, the "total stretch ratio" refers to the value obtained by multiplying the MD and/or TD stretch ratio in the primary stretching step (d) by the stretch ratio and/or relaxation ratio in the heat setting step.

[Other steps]
The method for producing a PO microporous membrane may include other steps other than the above steps (a) to (f). The other steps are not particularly limited, but may include, for example, a lamination step of laminating a plurality of monolayer PO microporous membranes as a step for obtaining a laminated PO microporous membrane in addition to the above heat fixing step. The method for producing a PO microporous membrane may also include, from the viewpoint of forming a uniform diffusion layer described later, a surface treatment step of subjecting the surface of the PO microporous membrane to surface treatment such as electron beam irradiation, plasma irradiation, corona discharge treatment, application of a surfactant, chemical modification, etc.; a step of providing an adhesive layer containing a thermoplastic resin on the surface of the PO microporous membrane, etc. Furthermore, a material for an inorganic porous layer or an organic (adhesive) layer may be applied to one or both sides of the PO microporous membrane to obtain a PO microporous membrane having at least one layer.

[Formation of inorganic coating layer]
An inorganic coating layer can be provided on the surface of the PO microporous membrane from the viewpoints of safety, dimensional stability, heat resistance, etc. The inorganic coating layer is a layer containing inorganic components such as inorganic particles, and may optionally contain a binder resin that bonds the inorganic particles together, a dispersant that disperses the inorganic particles in a solvent, etc.

Examples of materials for the inorganic particles contained in the inorganic coating layer include oxide ceramics such as alumina, silica, titania, zirconia, magnesia, ceria, yttria, zinc oxide, and iron oxide; nitride ceramics such as silicon nitride, titanium nitride, and boron nitride; ceramics such as silicon carbide, calcium carbonate, magnesium sulfate, aluminum sulfate, barium sulfate, aluminum hydroxide, aluminum oxide hydroxide (AlOOH, alumina monohydrate, or boehmite), potassium titanate, talc, kaolinite, dickite, nacrite, halloysite, pyrophyllite, montmorillonite, sericite, mica, amesite, bentonite, asbestos, zeolite, calcium silicate, magnesium silicate, diatomaceous earth, and silica sand; and glass fibers. The inorganic particles may be used alone or in combination.

Examples of binder resins include conjugated diene polymers, acrylic polymers, polyvinyl alcohol resins, and fluorine-containing resins. The binder resin may be in the form of a latex and may contain water or a water-based solvent.

Dispersants are substances that adsorb to the surfaces of inorganic particles in the slurry and stabilize the inorganic particles through electrostatic repulsion, etc. Examples of dispersants include polycarboxylates, sulfonates, polyoxyethers, and surfactants.

The inorganic coating layer can be formed, for example, by applying a slurry of the components described above to the surface of the PO microporous membrane and drying it.

[Formation of adhesive layer]
In order to prevent deformation or swelling due to gas generation in laminated batteries, which have recently been increasingly adopted in vehicle-mounted batteries to increase energy density, an adhesive layer containing a thermoplastic resin can be provided on the surface of the PO microporous membrane.

The thermoplastic resin contained in the adhesive layer is not particularly limited, and examples thereof include polyolefins such as polyethylene or polypropylene; fluorine-containing resins such as polyvinylidene fluoride and polytetrafluoroethylene; fluorine-containing rubbers such as vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene fluoride-hexafluoropropylene tetrafluoroethylene copolymer, and ethylene-tetrafluoroethylene copolymer; rubbers such as styrene-butadiene copolymer and its hydrogenated product, acrylonitrile-butadiene copolymer and its hydrogenated product, acrylonitrile-butadiene-styrene copolymer and its hydrogenated product, (meth)acrylic acid ester copolymer, styrene-acrylic acid ester copolymer, acrylonitrile-acrylic acid ester copolymer, ethylene propylene rubber, polyvinyl alcohol, and polyvinyl acetate; cellulose derivatives such as ethyl cellulose, methyl cellulose, hydroxyethyl cellulose, and carboxymethyl cellulose; resins such as polyphenylene ether, polysulfone, polyether sulfone, polyphenylene sulfide, polyetherimide, polyamideimide, polyamide, and polyester.

Furthermore, after the heat setting step (f), lamination step or surface treatment step, the master roll on which the PO microporous membrane is wound can be aged under a predetermined temperature condition, and then the master roll can be rewound. This tends to make it easier to obtain a PO microporous membrane with higher thermal stability than the PO microporous membrane before rewinding. When aging and rewinding the master roll, the temperature at which the master roll is aged is not particularly limited, but is preferably 35°C or higher, more preferably 45°C or higher, and even more preferably 60°C or higher. In addition, from the viewpoint of maintaining the permeability of the PO microporous membrane, the temperature at which the master roll is aged is preferably 120°C or lower. The time required for the aging treatment is not particularly limited, but is preferably 24 hours or more, since the above-mentioned effect is easily manifested.

<Uniform diffusion layer>
The uniform diffusion layer is a layer in which lithium (Li) ions can be uniformly diffused when an electrolyte solution is included. The uniform diffusion layer according to the present embodiment is located at least on the surface of the separator and has an Ra of 1.20 μm or less, thereby eliminating the Li ion concentration distribution in the power storage device including the separator, and enabling control of the Li precipitation morphology and suppression of Li dendrite formation.

From the viewpoint of uniform diffusion of Li ions and optimization of smoothness to suppress local Li ion concentration concentration on the negative electrode surface, the arithmetic mean roughness (Ra) of the uniform diffusion layer is preferably 0.001 μm or more and 1.200 μm or less, more preferably 0.010 μm or more and 1.000 μm or less, and even more preferably 0.015 μm or more and 0.950 μm or less. From the same viewpoint, the ten-point mean roughness (Rz) of the uniform diffusion layer is preferably 0.001 μm or more and 4.000 μm or less, more preferably 0.010 μm or more and 3.000 μm or less, and even more preferably 0.015 μm or more and 2.000 μm or less. From the same viewpoint, the Rms (average element length) of the uniform diffusion layer is preferably 1.0 μm or more and 100.0 μm or less, more preferably 2.0 μm or more and 80.0 μm or less, and even more preferably 3.0 μm or more and 70.0 μm or less.

The Ra, Rz or Rms of the uniform diffusion layer can be adjusted to within the above numerical ranges, for example, by controlling the coating conditions in the process of forming the uniform diffusion layer, such as the combination of the uniform diffusion layer material and the solvent used, the coating method, the drying process, etc.

In addition, from the viewpoint of reducing the interface resistance between the negative electrode containing Li metal or copper (Cu) foil and the separator to improve the cycle characteristics of the power storage device, the separator according to this embodiment is impregnated with an electrolyte containing 1 mol/L LiPF ₆ and having an EC/DEC=1/2 (volume ratio) for one day, and then the uniform diffusion layer of the separator is pressed against Li metal at 25 ° C. and 0.5 MPa. The peel strength is preferably 2 N/m or more and 200 N/m or less, more preferably 3 N/m or more and 60 N/m or less, and even more preferably 10 N/m or more and 40 N/m or less. The peel strength can be adjusted within the above numerical range by forming a uniform diffusion layer using a resin that has a high affinity with Li metal and is moderately soft. As a material with high affinity with Li metal, a material having polarity is preferable. Examples of polar materials include, but are not limited to, olefin copolymers such as ethylene-vinyl acetate copolymer and ethylene-acrylic acid-maleic anhydride copolymer; polyesters such as polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate; polyvinyl acetates; polyvinyl alcohols; polyvinyl acetals; polyvinyl butyrals; fluorinated polybenzoxazole; acrylic resins; methacrylic resins such as polymethyl methacrylate; polyacrylonitriles; acrylonitrile-butadiene-styrene copolymer and other acrylonitrile copolymers; styrene copolymers such as styrene-methacrylic acid copolymer and styrene-acrylonitrile copolymer; and polystyrene sulfone. Examples of the polymerizable polymer include ionic polymers such as sodium polyacrylate and sodium polyacrylate, acetal resins, polyamides such as nylon 66, gelatin, gum arabic, polycarbonates, polyester carbonates, cellulose resins, polyketones, polyethers, phenolic resins, urea resins, epoxy resins, unsaturated polyester resins, alkyd resins, melamine resins, polyurethanes, diaryl phthalate resins, polyphenylene oxides, polyphenylene sulfide polysulfones, polyphenylsulfones, polyimides, bismaleimide triazine resins, fluorine-containing resins, polyimide amides, polyether sulfones, polyether ketones, polyether imides, etc. From the viewpoint of affinity with the electrolytic solution, fluorine-containing resins, polycarbonates, polyketones, polyethers, and polyacrylonitriles are preferred. As a moderately soft resin, for example, a resin having a lower Tg than aramid resin, which is considered to have a glass transition temperature (Tg) of more than about 300°C in the present technical field, may be used. The range of Tg is preferably less than 100°C, more preferably 25 to 100°C, and even more preferably less than 25°C, from the viewpoint of forming a good bond with a negative electrode containing Li metal or copper (Cu) foil. In addition, Tg may be a value in a state in which a uniform diffusion layer material containing an electrolyte (EC/DEC=1/2 (volume ratio) or the like) is plasticized. Tg is determined from a DSC curve obtained by differential scanning calorimetry (DSC). Specifically, the temperature at the intersection of a straight line extending the low-temperature side baseline to the high-temperature side in the DSC curve and a tangent at the inflection point of the stepwise change part of the glass transition can be adopted as the glass transition temperature. In addition, "glass transition" refers to a heat quantity change occurring on the endothermic side due to a change in the state of a polymer, which is a test piece, in DSC. Such a heat quantity change is observed as a stepwise change shape in the DSC curve. The term "step change" refers to the portion of the DSC curve where the curve leaves the previous low-temperature baseline and transitions to a new high-temperature baseline. The step change also includes a combination of a step change and a peak. Furthermore, the term "inflection point" refers to the point where the gradient of the DSC curve in the step change portion is maximum. It can also be expressed as the point where the upward convex curve changes to a downward convex curve, with the upper side being the heat generation side in the step change portion. The term "peak" refers to the portion of the DSC curve where the curve leaves the low-temperature baseline and transitions back to the same baseline. The term "baseline" refers to the DSC curve in the temperature range where no transition or reaction occurs in the test specimen.

The swelling degree of the uniform diffusion layer with respect to an electrolyte containing 1 mol/L _LiPF6 and having an EC/DEC ratio of 1/2 (volume ratio) is preferably 2 times or more, more preferably 3 times or more and 10 times or less, and even more preferably 3.5 times or more and 8 times or less, from the viewpoint of the strength of the uniform diffusion layer as an upper limit and the uniform diffusion of Li ions as a lower limit. This swelling degree can be adjusted within the above numerical range by, for example, controlling the solubility parameters (SP values) of the raw materials or components of the uniform diffusion layer.

From the viewpoint of optimizing smoothness, controlling the Li deposition morphology in the electricity storage device, and suppressing Li dendrite formation, the uniform diffusion layer is formed by satisfying the following (1) to (3):
(1) When an electrolyte solution is contained, in a surface observation of the uniform diffusion layer, the maximum diameter of an inscribed circle that can be drawn in a region in which Li ions cannot diffuse is 100 nm or less;
(2) when the uniform diffusion layer contains an electrolyte, the total area (S _total ) of regions where Li ion diffusion is impossible is up to 7.2 ^μm2 in a surface observation of a range of 3.0 μm × 4.0 μm at any point of the uniform diffusion layer; and (3) when the uniform diffusion layer contains an electrolyte, the area ratio of regions where Li ion diffusion is possible to regions where Li ion diffusion is impossible is 60:40 to 0:100.
It is preferable that one or more of the above conditions are satisfied.

In the above (1) to (3), the region where Li ions cannot diffuse refers to, for example, a region containing a non-ion conductive material. Specifically, the polyolefin (PO) portion of the PO microporous membrane corresponds to a region where Li ions cannot diffuse, but when the thickness of the PO trunk in the surface layer of the PO microporous membrane is 100 nm or less, it can be considered as satisfying the above (1). The same can be said when the uniform diffusion layer is a filler layer. In surface observation, the region where Li ions can diffuse and the region where Li ions cannot diffuse can be distinguished by, for example, appropriately combining EDX, NMR, XRD, XPS, XRF, etc. to identify ion conductive materials or non-ion conductive materials, and clarifying the distribution of these materials on the membrane surface by mapping, etc. Note that the voids are regions where Li ions can diffuse because they are filled with an electrolyte.

Regarding (1) above, the maximum diameter of the inscribed circle that can be drawn in the region where Li ion diffusion is impossible is preferably 100 nm or less, more preferably 80 nm or less, 60 nm or less, 40 nm or less, or 20 nm or less, and most preferably does not include any region where Li ion diffusion is impossible.

Regarding (2) above, the total area S _total is preferably 7.2 μm ² or less, more preferably 6.0 μm ² or less, 4.8 μm ² or less, 3.6 μm ² or less, or 2.4 μm ² or less, and most preferably does not include any region into which Li ions cannot diffuse.

Regarding (3) above, it is preferable that the area ratio of the area in which Li ions can diffuse to the area in which Li ions cannot diffuse is 60:40 to 0:100, more preferably 30:70 to 0:100, 20:80 to 0:100, or 10:90 to 0:100, and most preferably no area in which Li ions cannot diffuse.

The thinner the uniform diffusion layer, the lower the electrical resistance, while a thicker uniform diffusion layer facilitates uniform diffusion of Li ions on the negative electrode surface of the power storage device. From this perspective, the thickness of the uniform diffusion layer is preferably in the range of more than 0 μm and not more than 10 μm, more preferably 0.5 to 7.0 μm, even more preferably 1.0 to 6.0 μm, and even more preferably 2.0 to 5.0 μm. Note that when the uniform diffusion layer is formed as a pattern or when the thickness of the uniform diffusion layer is not constant, the thickness of the uniform diffusion layer can be determined so as to be maximum along the thickness direction of the microporous membrane.

The constituent material of the uniform diffusion layer preferably contains at least one ion-conductive material, and may contain both ion-conductive and non-ion-conductive materials, but is preferably made of an ion-conductive material, more preferably made substantially of an ion-conductive material, and even more preferably made only of an ion-conductive material, and it is particularly preferable that the uniform diffusion layer is a non-porous layer made only of an ion-conductive material.

The ion conductive material may be a material that exhibits an ion conductivity of 10 ⁻⁷ Scm ⁻¹ or more at 25° C., and the ion conductivity is preferably 10 ⁻⁶ Scm ⁻¹ or more, 10 ⁻⁴ Scm ⁻¹ or more, or 10 ⁻³ Scm ⁻¹ or more. The ion conductivity may be the value of the material alone, or the value of the material in a state in which the material is swollen in an electrolyte solution or the like, but from the viewpoint of resistance, the value of the material alone is more preferable. If the material is known, the ion conductivity can be easily determined by a person skilled in the art from literature or the like. If it is difficult to determine, it can also be measured by the method described in the examples. Examples of materials that satisfy the ion conductivity include organic materials and inorganic materials, such as solid electrolytes and gel electrolytes. The gel electrolyte may be one that is swollen with an electrolyte solution, or one that exhibits conductivity by containing Li salt. Examples of the material include materials having polar functional groups such as polyethylene oxide, polyvinylidene fluoride, polyacrylonitrile, polymethacrylic acid, polycarbonate, and polyacetal, and may be copolymers of a plurality of monomers constituting these. The ion conductive material preferably includes polyethylene oxide, polyacrylonitrile, polymethacrylic acid, and further polyethylene oxide and polyacrylonitrile. Examples of the solid electrolyte include oxides, sulfides, and nitrides. Specifically, Li _1.3 Al _0.3 Ti _1.7 (PO ₄ ) ₃ , Li _0.34 La _0.51 TiO _2.94 , Li ₄ SiO ₄ -Li ₂ BO ₃ , Li ₇ La ₃ Zr ₂ O ₁₂ , Li ₃ BO ₃ -Li ₂ SO ₄ , Li _2.9 PO _3.3 N _0.46 , Li _1.07 Al _0.69 Ti _1.46 (PO ₄ ) ₃ , Li _9.54 Si _1.74 P _1.44 S _11.7 Cl _0.3 , Li ₁₀ GeP ₂ S ₁₂ , Li _{3.2 5} Ge Examples _of materials that _can be _{used include Li2S} - _0.25P0.75S4 , _Li6PS5Cl , _Li2S - _B2S3 - _LI , _Li2S -P2S5- _{LiBH4 , Li2S - SiS2} - _Li3PO4 , _Li2S - _{SiS2 -Li4SiO4 , Li2S - P2S5 , Li7P3S11} , _{Li3.25P0.95S4 ,} and _Li3N , and the composition ratio _is not limited to _the above.

As the non-Li ion conductive material, it is possible to use materials other than the Li ion conductive materials described above, and specific examples include organic materials such as polyolefins such as polyethylene and polypropylene, polystyrene, and polyvinyl chloride, and inorganic materials.

The uniform diffusion layer may be in the form of particles, a membrane, a film, or a composite thereof, and is preferably in the form of a membrane or a film. From the viewpoint of uniform diffusion of Li ions, the uniform diffusion layer is preferably non-porous, and more preferably non-porous in the form of a membrane.

(Method of forming a uniform diffusion layer)
The method for forming the uniform diffusion layer is not particularly limited as long as the uniform diffusion layer is located at least on the surface layer of the separator and the arithmetic mean roughness (Ra) of the uniform diffusion layer is adjusted to be within the above-mentioned numerical range. For example, the method may include partial molding or processing of a separator substrate such as a microporous membrane or a laminated membrane, or coating the separator substrate with a paint containing a constituent material of the uniform diffusion layer.

When forming a uniform diffusion layer by partial molding of a separator substrate such as a microporous membrane or laminated membrane, it is preferable to perform the molding process so that the thickness of the stem on the substrate surface satisfies the above condition (1).

When applying paint containing the constituent materials of the uniform diffusion layer to the separator substrate, it is preferable to optimize the paint solvent, application method, drying process, etc., from the viewpoint of the surface smoothness of the uniform diffusion layer.

In order to improve the smoothness of the uniform diffusion layer, it is preferable to select a solvent that can suppress excessive penetration of the paint into the microporous membrane, suppress paint repellency on the microporous membrane surface, and promote leveling of the paint during coating. The solvent can be selected from the viewpoints of viscosity and surface tension based on the affinity between the paint and the microporous membrane to be coated. There are no particular limitations on the solvent as long as it satisfies the above-mentioned requirements, but examples of the solvent include N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, dimethylsulfoxide, water, ethanol, methanol, n-propanol, acetone, toluene, xylene, n-hexane, cyclohexane, chloroform, dichloromethane, ethyl acetate, methyl ethyl ketone, etc.

Coating methods include, for example, bar coating, gravure direct printing, gravure reverse printing, dipping, die coating, knife coating, air doctor coating, blade coating, squeeze coating, cast coating, screen printing, and spray coating. Among these, methods other than dipping are preferred. From the viewpoints of thin coating and smoothness, bar coating, gravure reverse printing, die coating, and knife coating are more preferred, and gravure reverse printing is particularly preferred.

In the drying process, the drying temperature can be determined depending on, for example, the boiling point of the paint solvent. From the viewpoint of smoothness, it is preferable that the temperature is lower than the boiling point of the solvent, and it is even more preferable to dry at a temperature that is 15°C lower than the boiling point, and it is particularly preferable that the temperature is lower than 50°C.

<Separator manufacturing method>
The separator according to the present embodiment can be produced, for example, by combining the above-described method for producing a microporous membrane and the method for forming a uniform diffusion layer. The resulting separator can be incorporated into an electricity storage device to suppress the generation of Li dendrites and improve cycle characteristics.

<Electricity storage device>
An embodiment of the present invention is an electric storage device comprising a positive electrode, a negative electrode, an electrolytic solution, and the separator described above. Examples of the electric storage device include a non-aqueous electrolyte battery, a non-aqueous electrolyte battery, a non-aqueous lithium ion secondary battery, a non-aqueous gel secondary battery, a non-aqueous solid secondary battery, a lithium ion capacitor, and an electric double layer capacitor.

The power storage device may include a separator, a positive electrode plate, a negative electrode plate, and a non-aqueous electrolyte (containing a non-aqueous solvent and a metal salt dissolved therein). Specifically, for example, a positive electrode plate containing a transition metal oxide capable of absorbing and releasing lithium ions and a negative electrode plate capable of absorbing and releasing lithium ions are wound or stacked so as to face each other with a separator interposed therebetween, retaining the non-aqueous electrolyte, and housed in a device container or device exterior body.

The positive electrode plate will be described below. As the positive electrode active material, for example, lithium composite metal oxides such as lithium nickel oxide, lithium manganate, or lithium cobalt oxide, or lithium composite metal phosphates such as lithium iron phosphate can be used. The positive electrode active material is mixed with a conductive agent and a binder, and applied as a positive electrode paste to a positive electrode collector such as aluminum foil, dried, rolled to a predetermined thickness, and then cut to a predetermined size to form a positive electrode plate. Here, as the conductive agent, a metal powder that is stable under the positive electrode potential, for example, carbon black such as acetylene black, or a graphite material can be used. As the binder, a material that is stable under the positive electrode potential, for example, polyvinylidene fluoride, modified acrylic rubber, or polytetrafluoroethylene can be used.

The negative electrode plate will be described below. The negative electrode active material may be lithium (Li) metal, copper (Cu) foil, or a material capable of absorbing lithium. In addition, at least one material selected from the group consisting of graphite, silicide, and titanium alloy materials may be used for the negative electrode. In addition, the negative electrode active material for the non-aqueous electrolyte secondary battery may be, for example, metal, metal fiber, carbon material, oxide, nitride, silicon (Si) compound, tin (Sn) compound, or various alloy materials.

Carbon materials that can be used for the negative electrode include, for example, various types of natural graphite, coke, graphitizable carbon, non-graphitizable carbon, carbon fiber, spherical carbon, various types of artificial graphite, and amorphous carbon.

As the negative electrode active material, one of the above materials may be used alone, or two or more may be used in combination. The negative electrode active material is mixed with a binder, and applied as a negative electrode paste to a negative electrode current collector such as copper foil, which is then dried and rolled to a specified thickness, and then cut to a specified size to form a negative electrode plate. Here, as the binder, a material that is stable under the negative electrode potential, such as PVDF or a styrene-butadiene rubber copolymer, can be used.

Among the negative electrode materials described above, the effect of the uniform diffusion layer of the separator according to this embodiment is most pronounced, and it is therefore preferable that the negative electrode is made of Li metal or copper (Cu) foil. It is also preferable to arrange the uniform diffusion layer of the separator so that it abuts or faces the negative electrode surface (or Li deposition surface) of the power storage device, thereby eliminating the Li ion concentration distribution and controlling the Li deposition form (suppressing Li dendrite formation).

The nonaqueous electrolyte will be described below. The nonaqueous electrolyte generally includes a nonaqueous solvent and a metal salt such as a lithium salt, a sodium salt, or a calcium salt dissolved therein. As the nonaqueous solvent, a cyclic carbonate ester, a chain carbonate ester, a cyclic carboxylate ester, or the like is used, and a part of these may be fluorinated to improve oxidation resistance. As the lithium salt, for example, LiPF ₆ , LiClO ₄ , LiBF ₄ , LiAlCl ₄ , LiSbF ₆ , LiSCN, LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , Li(CF ₃ SO ₂ ) ₂ , LiAsF ₆ , a lower aliphatic lithium carboxylate, LiCl, LiBr, LiI, borates, imide salts, or the like may be used. In addition, an ionic liquid may be used, or a high-concentration Li salt nonaqueous electrolyte in which a lithium salt is dissolved in a nonaqueous solvent at 1 mol/L or more may be used.

Unless otherwise specified, the measurement methods for the various parameters mentioned above are based on the measurement methods in the examples described below.

Next, the present embodiment will be described in more detail with examples and comparative examples, but the present embodiment is not limited to the following examples as long as it does not deviate from the gist of the embodiment. The physical properties in the examples were measured by the following methods. Unless otherwise specified, measurements were taken in an environment at room temperature of 23°C and humidity of 40%.

(1) Single-point total pore volume and pore ratio (1a) Single-point total pore volume (also known as single-point volumetric adsorption amount) (pore diameter 98 nm) (cm ³ /g)
Nitrogen gas adsorption test was performed by constant volume method based on JIS Z8831-2:2010 Powder (solid) pore distribution and pore characteristics "Part 2: Method for measuring mesopores and macropores by gas adsorption". Specifically, the gas adsorption amount converted to standard state temperature and pressure (STP) with nitrogen relative pressure (p/p0) of 0.98 was defined as the volumetric equivalent of the liquid volume of the adsorbate, and the one-point method volumetric adsorption amount (pore diameter 98 nm) was derived. At this time, the relative pressure (p/p0) of 0.98 is expressed as rp = rk + t, assuming a cylindrical pore and the pore radius rp, which corresponds to a pore diameter of 98.8 nm. Here, rk is the radius of curvature of the adsorbate condensed in the pore, and is expressed by the following formula.
rk=-0.953/ln(p/p0).
Furthermore, t is the average thickness of the multilayer adsorption film of nitrogen at a relative pressure obtained from an experimentally or theoretically determined reference adsorption isotherm, and is expressed by the following formula:
t=0.354*[-5/ln(p/p0)]^1/3

(1b) Pore ratio (pore diameter 98 nm or less) (%)
The porosity consisting of pores with a pore diameter of 98 nm or less was calculated using the above one-point total pore volume (pore diameter of 98 nm or less) A and the resin raw material density using the following formula.
Porosity P (%) consisting of pores with a pore diameter of 98 nm or less = A/(A+1/density of resin raw material) x 100
The pore ratio was calculated by calculating the ratio of the above porosity to the porosity of the entire membrane, which will be described later.
Pore ratio (pore diameter 98 nm or less) (%) = (P / porosity of entire membrane) x 100

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

(3) Film thickness (μm)
The thickness of the PO microporous membrane, the uniform diffusion layer, the inorganic filler layer, or the separator was measured at room temperature of 23±2° C. using a microthickness gauge, KBM (trademark), manufactured by Toyo Seiki Co., Ltd. However, the average thickness (μm) of the microporous membrane (before compression) and the thickness (μm) of the multilayer porous membrane and coating layer (before compression) were measured as follows.

[Average thickness of microporous membrane (before compression) (μm)]
The microporous membrane was sampled in an area of 10 cm x 10 cm, and multiple sheets of the microporous membrane were stacked to have a thickness of 15 µm or more. Measurements were taken at nine points and an average was calculated. The average was divided by the number of sheets stacked to obtain the thickness of one sheet.

[Thickness of multilayer porous membrane and coating layer (before compression) (μm)]
The thickness of the coating layer was calculated from the average thickness of the microporous membrane (before compression) and the thickness of each of the multilayer porous membranes (before compression). From the viewpoint of detection from the multilayer porous membrane, it is also possible to measure the thickness of each layer using a cross-sectional SEM image.

(4) Porosity (%)
A sample was cut into a 10 cm x 10 cm square, and its volume ( ^cm3 ) and mass (g) were determined, and the volume (cm3) and mass (g) were used to calculate the true density (g/ ^cm3 ) using the following formula: In a separator having a laminated structure formed on a microporous membrane by coating or the like, the porosity of the separator refers to the average porosity of the entire separator, the porosity of the microporous membrane was calculated from only the microporous membrane, and the porosity of the laminated portion formed by coating or the like was calculated using the difference in volume and mass between the separator and the microporous membrane, and the true density of the material constituting the laminated portion.
Porosity (%)=(volume−(mass/true density of mixed composition))/volume×100
The density of the mixed composition was calculated from the density of each of the materials used and the mixing ratio.

(5) Air permeability (sec/100 cm ³ ) The air permeability was measured using an Oken air permeability measuring instrument "EGO2" manufactured by Asahi Seiko Co., Ltd. under an atmosphere having a temperature of 23° C. and a humidity of 40%.
The air permeability was measured at three points along the width direction (TD) of the membrane: two points 10% inward of the total width from both ends toward the center, and one point in the center, and the average of these values was calculated.

[Compression press test]
A rubber cushioning material with a thickness of 0.8 mm, a PET film with a thickness of 0.1 mm, two microporous membranes or separators, the PET film, and the cushioning material were laminated in this order, and the resulting laminate was left stationary, and a compression test was performed by applying pressure to one side of the cushioning material surface of the laminate. Here, the microporous membrane used was 5 x 5 cm square, and the average thickness (average of 9 points), basis weight, and air permeability were measured before using it in the press test. In addition, the porosity before the compression press test was calculated from the basis weight and average thickness.
The compression test was performed using a press at a temperature of 25° C. for a compression time of 3 minutes at pressures of 2.5 MPa, 5 MPa, 7.5 MPa, and 10 MPa. One hour after unloading, the microporous membrane was removed from the laminate, and the average membrane thickness after compression (average of 9 points) and the air permeability after compression were measured. The porosity after compression was calculated from the basis weight and the average membrane thickness after compression.

(6) Basis weight (g/m ² ), puncture strength (gf) and puncture strength converted to basis weight (gf/(g/m ² ))
The basis weight is the weight (g) of the polyolefin microporous membrane per unit area (1 m ² ). After sampling to 1 m x 1 m, the weight was measured using an electronic balance (AUW120D) manufactured by Shimadzu Corporation. When it was not possible to sample to 1 m x 1 m, a sample was cut to an appropriate area, the weight was measured, and then converted to the weight (g) per unit area (1 m ² ).

Using a handy compression tester KES-G5 (trademark) manufactured by Kato Tech, the microporous membrane was fixed with a sample holder having an opening diameter of 11.3 mm. Next, a puncture test was performed on the center of the fixed microporous membrane at a needle tip curvature radius of 0.5 mm, a puncture speed of 2 mm/sec, and an atmosphere of room temperature of 23°C and humidity of 40%, to measure the puncture strength (gf) as the maximum puncture load. The measured value of the puncture test was measured along the TD of the membrane at three points, two points 10% inside the full width from both ends toward the center and one point in the center, and the average value was calculated.
The puncture strength converted into basis weight is calculated using the following formula.
Piercing strength converted to fabric weight [gf/(g/m ² )] = Piercing strength [gf]/ fabric weight [g/m ² ]
Here, with regard to the pin puncture strength and basis weight equivalent pin puncture strength of a multilayer porous membrane having at least one layer provided on a polyolefin microporous membrane base, from the viewpoint of evaluating the strength of the resin and the strength per basis weight, the properties were evaluated based on the pin puncture strength and basis weight equivalent pin puncture strength of the polyolefin microporous membrane base.

(7) Number of pores in the porous membrane or porous layer determined by the gas-liquid method (pores/μm ² )
It is known that the fluid inside a capillary follows Knudsen flow when the mean free path of the fluid is larger than the pore diameter of the capillary, and follows Poiseuille flow when it is smaller. Therefore, it is assumed that the air flow in the air permeability measurement of a porous membrane or porous layer follows Knudsen flow, and the water flow in the water permeability measurement of a porous membrane or porous layer follows Poiseuille flow.
In this case, the average pore size d (μm) and tortuosity τ _a (dimensionless) of the porous membrane were calculated from the air permeation rate constant R _gas (m ³ /(m ² sec Pa)), the water permeation rate constant R _liq (m ³ /(m ² sec Pa)), the air molecular velocity ν (m/sec), the water viscosity η (Pa sec), the standard pressure P _s (=101,325 Pa), the porosity ε (%), and the membrane thickness L (μm) using the following formula:
d=2ν×(R _liq /R _gas )×(16η/3Ps)×10 ⁶
τ _a = (d×(ε/100)×ν/(3L×P _s ×R _gas )) ^1/2
Here, R _gas was calculated from the air permeability (sec) using the following formula.
R _gas =0.0001/(Air permeability x (6.424 x ^10-4 ) x (0.01276 x 101325))
Furthermore, R _liq was calculated from the water permeability (cm ³ /(cm ² ·sec·Pa)) using the following formula.
R _liq = water permeability/100
The water permeability was determined as follows: A porous membrane or porous layer, which had been soaked in ethanol beforehand, was placed in a stainless steel liquid permeation cell with a diameter of 41 mm, and the ethanol in the membrane or layer was washed with water. After that, water was passed through the membrane or layer at a pressure difference of about 50,000 Pa, and the water permeability per unit time, unit pressure, and unit area was calculated from the water permeation amount ( ^cm3 ) after 120 seconds had elapsed, and this was taken as the water permeability.
Moreover, ν was calculated from the gas constant R (=8.314), absolute temperature T (K), the circular constant π, and the average molecular weight of air M (=2.896×10 ⁻² kg/mol) using the following formula.
ν=((8R×T)/(π×M)) ^1/2
Furthermore, the number of holes B (holes/μm ² ) was calculated from the following formula.
B=4×(ε/100)/(π× ^d2 × _τa )

(8) Hole diameter and half width (8a) Average flow diameter (μm) by half dry method
The average pore size (μm) was measured using a perm porometer (Porous Materials, Inc.: CFP-1500AE) in accordance with the half-dry method. Perfluoropolyester (trade name "Galwick", surface tension 15.6 dyn/cm) manufactured by the same company was used as the immersion liquid. The applied pressure and air permeability were measured for the dry curve and the wet curve, and the average pore size dHD (μm) was calculated from the pressure PHD (Pa) at which the curve of 1/2 of the obtained dry curve and the wet curve intersected, using the following formula, and was taken as the average flow diameter.
dHD=2860×γ/PHD

(8b) Half-width (μm) of pore size peak by half dry method
The difference between the pore size values corresponding to half the value of the pore size distribution peak top showing the mean flow diameter was calculated and taken as the half-value width.

(9) Molecular weight and viscosity average molecular weight (9a) Measurement of polyethylene polydispersity (Mw/Mn) and (Mz/Mw) by GPC-light scattering Using GPC (gel permeation chromatography) connected to a differential refractometer and a light scattering detector, the number average molecular weight (Mn), weight average molecular weight (Mw), z-average molecular weight (Mz), molecular weight distribution (Mw/Mn), and (Mz/Mw) of each resin were measured under the following conditions. Specifically, a PL-GPC200 manufactured by Agilent, equipped with a differential refractometer (RI) and a light scattering detector (PD2040), was used. Two Agilent PLgel MIXED-A (13 μm, 7.5 mm I.D.×30 cm) were connected together and used as columns. Measurements were performed under conditions of a column temperature of 160° C., 1,2,4-trichlorobenzene (containing 0.05 wt % 4,4'-thiobis (6-tert-butyl-3-methylphenol) as an eluent, a flow rate of 1.0 ml/min, and an injection amount of 500 μL, to obtain an RI chromatogram and light scattering chromatograms at scattering angles of 15° and 90°. From the obtained chromatograms, the number average molecular weight (Mn), weight average molecular weight (Mw), and z-average molecular weight (Mz) were obtained using Cirrus software. The molecular weight distribution (Mz/Mw) was obtained using the values of Mz and Mw, and the molecular weight distribution (Mw/Mn) was obtained using the values of Mw and Mn. The refractive index increment value of polyethylene used was 0.053 ml/g.

(9b) Viscosity average molecular weight (Mv)
The intrinsic viscosity [η] (dl/g) in decalin solvent at 135° C. was determined according to ASTM-D4020. For the microporous PO membrane and the raw polyethylene material, Mv was calculated according to the following formula.
[η]=6.77× ^10-4 Mv ^0.67
For the polypropylene raw material, Mv was calculated by the following formula:
[η]=1.10×10 ⁻⁴ Mv ^0.80

(10) Melt flow index (MI)
The melt flow index (MI) of the microporous membrane was measured according to JIS K7210:1999 (Melt mass flow rate (MFR) and melt volume flow rate (MVR) of plastics-thermoplastics). A load of 21.6 kgf was applied to the membrane at 190°C, and the amount of resin (g) flowing out from an orifice with a diameter of 2 mm and a length of 10 mm in 10 minutes was measured, and the value rounded off to one decimal place was taken as the MI.

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

(12) Difference between maximum and minimum air permeability at three points in the TD direction (two points 10% inward of the overall width from both ends toward the center and one central point, for a total of three points) Using an Oken-type air permeability meter (diameter of the measuring part 30 mmφ) manufactured by Asahi Seiko Co., Ltd., the air permeability was measured at a total of three points, namely, the central point, which is 50% of the width of the membrane when the left width is 0% and the right end is 100%, the 10% central point from the left end (10% position), and the 10% central point from the right end (90% position), when the left width is 0% and the right end is 100%, and the difference R between the maximum and minimum values among the three points was obtained.
Depending on the width of the sample to be measured, specifically when the sample width is 150 mm or less, the air permeability distribution in the width direction is similarly measured using a nozzle with a measuring part having a diameter of 13 mmφ.

(13) Heat shrinkage rate (%) at 120°C for 1 hour
As a sample, the porous membrane was cut into a length (mm) before heating of 100 mm in MD and 100 mm in TD, 50 mm in MD and 50 mm in TD, or 30 mm in MD and 30 mm in TD, and left to stand in an oven at 120 ° C for 1 hour. At this time, the sample was sandwiched between 10 sheets of paper so that the hot air would not directly hit the sample. After removing the sample from the oven and cooling, the length was measured and used as the length (mm) after heating, and the heat shrinkage was calculated using the following formula. Measurements were performed in MD and TD, respectively, and the larger value was used as the heat shrinkage.
Heat shrinkage rate (%) = {(length before heating - length after heating) / length before heating} x 100

(14) Tensile Breaking Strength (MPa) and MD/TD Tensile Breaking Strength Ratio According to JIS K7127, MD and TD samples (shape: width 10 mm x length 100 mm) were measured using a tensile tester, Autograph AG-A type (trademark), manufactured by Shimadzu Corporation. The chuck distance of the tensile tester was set to 50 mm, and samples were used in which Cellophane (registered trademark) tape (manufactured by Nitto Denko Packaging Systems Corporation, product name: N.29) was attached to one side of both ends (each 25 mm). Furthermore, in order to prevent the sample from slipping during the test, a fluororubber having a thickness of 1 mm was attached to the inside of the chuck of the tensile tester.
The measurement was carried out under the conditions of a temperature of 23±2° C., a chuck pressure of 0.40 MPa, and a tensile speed of 100 mm/min.
The tensile strength at break (MPa) was determined by dividing the strength of the polyolefin microporous membrane at break by the cross-sectional area of the sample before the test.
The tensile breaking strength was determined for each of the MD and TD, and the ratio of the MD tensile breaking strength to the TD tensile breaking strength (MD/TD tensile breaking strength ratio) was also calculated.

(15) MD and TD tensile modulus (MPa)
For the measurement of MD and TD, MD samples (MD 120 mm x TD 10 mm) and TD samples (MD 10 mm x TD 120 mm) were cut out. The tensile modulus of the samples in MD and TD was measured using a Shimadzu tensile tester, Autograph AG-A (trademark), in accordance with JIS K7127 under conditions of an atmospheric temperature of 23±2°C and humidity of 40±2%. The sample was set so that the chuck distance was 50 mm, and the sample was stretched at a tensile speed of 200 mm/min until the chuck distance reached 60 mm, i.e., the strain reached 20.0%. The tensile modulus (MPa) was determined from the slope of the resulting stress-strain curve from 1.0% to 4.0% strain.

(16) Arithmetic mean roughness Ra, ten-point mean roughness Rz, average length RSm
(Compliant with JIS B0601-1994)
A battery separator is cut into a size of 10 mm wide x 50 mm long. The cut separator is attached to a double-sided tape (Nitto Denko double-sided adhesive tape No. 501F, 5 mm wide x 20 m) that is attached parallel to a glass plate (Matsunami Glass Industry Co., Ltd., microslide glass S1225, 76 mm x 26 mm) at a distance of 15 mm or more. At this time, due to the height of the double-sided tape, the center of the separator is fixed in a floating state without being directly attached to the glass plate.
The surface roughness of the uniform diffusion layer of the sample in this state was measured using a laser microscope (Keyence Corporation, VK-8500). The roughness was measured over an area of 110 μm × 150 μm, and the measurement was performed five times at different locations. The average values of the arithmetic mean roughness Ra, ten-point mean roughness Rz, and mean length RSm were determined as the arithmetic mean roughness Ra, ten-point mean roughness Rz, and mean length RSm of the uniform diffusion layer.

(17) Surface Observation of Uniform Diffusion Layer Using a scanning electron microscope (SEM, model S-4800, manufactured by Hitachi, Ltd.), images of 3.0 μm × 4.0 μm were taken at 10 randomly selected points on the surface of the uniform diffusion layer. The images were then loaded into the image analysis software “Image J” to confirm the following three items.
(I) The maximum diameter of the inscribed circle that can be drawn in the region where Li ions cannot diffuse when the electrolyte is contained. (II) The total area (Stotal) of the region where Li ions cannot diffuse when the electrolyte is contained.
(III) Area ratio of the region in which Li ions can diffuse to the region in which Li ions cannot diffuse when an electrolyte is contained

(18) Measurement of Adhesion Strength with Li (18a) Adhesion Strength with Li (Before Injecting Electrolyte)
The separator was cut into a rectangular shape of 20 mm width x 70 mm length, and layered with Li foil (manufactured by Honjo Metals Co., Ltd.) cut to a size of 15 mm x 60 mm to form a laminate, and this laminate was pressed under the following conditions.
Temperature: 25°C
Press pressure: 0.5 MPa
Press time: 5 seconds The peel strength between the separator and the Li foil after pressing was measured by a 90° peel test at a peel speed of 50 mm/min using force gauges ZP5N and MX2-500N (product name) manufactured by Imada Co., Ltd. At this time, the average value of the peel strength in the peel test for a length of 40 mm conducted under the above conditions was adopted as the adhesive strength with Li (before injection of the electrolyte solution).

(18b) Adhesion strength with Li (after electrolyte injection)
The separator was cut into a rectangular shape of 20 mm wide x 70 mm long, and was layered with Li foil cut to a size of 15 mm x 60 mm to form a separator-electrode laminate, and this laminate was inserted into an aluminum laminate film, and 0.4 ml of electrolyte (a mixture of EC/DEC = 1/2 containing 1 mol/L _LiPF6 ) was added. After sealing, this was left to stand for 12 hours and pressed under the following conditions.
Temperature: 25°C
Press pressure: 0.5 MPa
Pressing time: 1 minute The peel strength between the separator and the electrode after pressing was measured by a 90° peel test at a peeling speed of 50 mm/min using force gauges ZP5N and MX2-500N (product name) manufactured by Imada Co., Ltd. At this time, the average value of the peel strength in the peel test for a length of 40 mm conducted under the above conditions was adopted as the adhesive strength with Li (after injection of the electrolyte).

(19) Measurement of swelling degree The material used for the uniform diffusion layer was vacuum dried at a temperature below the melting point for 12 hours, and the solvent was completely removed to obtain a dried material of the uniform diffusion layer material. Approximately 0.5 g of the obtained dried material was weighed and used as the pre-immersion mass (WA). This dried material was placed in a 50 mL vial together with 10 g of an electrolyte solution of ethylene carbonate (EC):diethyl carbonate (DEC) = 1:2 (volume ratio) containing 1 mol/L LiPF ₆ at 25 ° C., and immersed for 24 hours. After that, the sample was taken out, wiped off with towel paper, and immediately thereafter the mass was measured and used as the post-immersion mass ( _WB ).
The degree of swelling of the uniform diffusion layer with the electrolyte was calculated from the following formula.
Swelling degree (%) = W _B / W _A
In the above formula, when the material of the uniform diffusion layer does not swell or dissolve in the electrolyte, the degree of swelling is 100%.

(20) Measurement of adhesion strength between microporous membrane and uniform diffusion layer A separator was attached to a slide glass of 76 mm width × 126 mm length with double-sided tape (Nistack NWBB-15) so that the uniform diffusion layer side faced upward.
A mending tape (Scotch MP-12) was applied to the surface of the uniform diffusion layer side of the test piece. The slide glass was fixed in a flat state, and one end of the mending tape was folded back. A 90° peel test was performed in the horizontal direction against the slide glass at a pulling speed of 100 mm/min to measure the peel strength. At this time, the average value of the peel strength in the peel test for a length of 40 mm performed under the above conditions was adopted as the binding strength between the microporous membrane and the uniform diffusion layer.

(21) Battery Test a. Preparation of Positive Electrode A slurry was prepared by dispersing 92.2% by mass of lithium nickel cobalt manganese composite oxide (LiNi ₅ Co ₂ Mn ₃ O ₂ ) as a positive electrode active material, 2.3% by mass of flake graphite and acetylene black as conductive materials, and 3.2% by mass of polyvinylidene fluoride (PVDF) as a binder in N-methylpyrrolidone (NMP). This slurry was applied to one side of a 20 μm thick aluminum foil serving as a positive electrode current collector with a die coater, dried at 130 ° C for 3 minutes, and then compression molded with a roll press machine. At this time, the amount of active material applied to the positive electrode was 125 g / m ² , and the bulk density of the active material was 3.00 g / cm ^3. The prepared positive electrode was punched into a circle with an area of 2.00 cm ² .

b. Preparation of negative electrode Lithium metal was prepared as a negative electrode and punched out into a circle having an area of 2.05 ^cm2 .

c. Non-aqueous electrolyte: A non-aqueous electrolyte was prepared by dissolving _LiPF6 as a solute in a mixed solvent of ethylene carbonate:diethyl carbonate=1:2 (volume ratio) to a concentration of 1.0 ml/L.

d. Battery assembly and evaluation The negative electrode, separator, and positive electrode were stacked vertically from the bottom so that the active material surfaces of the positive and negative electrodes faced each other, and then placed in a stainless steel metal container with a lid. The container and the lid were insulated, and the container was in contact with the copper foil of the negative electrode, and the lid was in contact with the aluminum foil of the positive electrode. The prepared nonaqueous electrolyte was poured into this container, which was then sealed and left to stand for one day.
The simple battery assembled as described above was charged in an atmosphere of 25° C. at a current value of 0.3 mA/ ^cm2 up to a battery voltage of 4.3 V, and then the current value was tapered from 0.3 mA/ ^cm2 while maintaining 4.3 V for a total of about 12 hours, for the first charge after the battery was fabricated. The battery was then discharged at a current value of 0.3 mA/ ^cm2 down to a battery voltage of 3.0 V to condition the battery.
Next, the battery was charged at a current value of 3.0 mA/ ^cm2 in an atmosphere of 25°C up to a battery voltage of 4.3 V, and then the current value was tapered from 3 mA/ ^cm2 to maintain 4.3 V. In this manner, charging was continued for a total of about 3 hours, and then discharged at a current value of 3.0 mA/ ^{cm2 down} to a battery voltage of 3.0 V. This cycle was repeated.
The ratio of the discharge capacity at each cycle to the discharge capacity at the first cycle was calculated as the capacity retention rate (%), and the number of cycles at which the capacity retention rate fell below 80% was evaluated.

(22) Measurement of ionic conductivity A cell was prepared in the same manner as in item (21) above, except that the positive and negative electrodes were replaced with stainless steel plates, and a non-perforated film or plate made of a uniform diffusion layer material was used instead of the separator. The AC impedance of the prepared cell was measured under conditions of a 25°C atmosphere, a voltage amplitude of 10 mV, and a frequency of 10 Hz to 5,000 kHz, and the membrane resistance (Ωcm) was calculated from the intersection of the real axis of the Cole-Cole plot, or the intersection of the real axis extrapolated from the Cole-Cole plot, using the following formula. In this case, when the Cole-Cole plot was close to a semicircle, the intersection was calculated from the extrapolation of the arc, and when the Cole-Cole plot was close to a straight line, the intersection was calculated from the extrapolation of the straight line. The reciprocal of the membrane resistance was then calculated as the ionic conductivity.
Film resistance (Ωcm): resistance value (Ω) calculated from Cole-Cole plot × film area (cm ² )/thickness (cm)
Ionic conductivity (Scm ⁻¹ ): 1/membrane resistance (Ωcm)

(23) Measurement of glass transition temperature Tg An appropriate amount of the homogeneous diffusion layer material was placed on an aluminum dish, and dried for 30 minutes at a temperature higher than the boiling point of the solvent used and at a temperature at which the homogeneous diffusion layer material does not thermally decompose, to obtain a dried product. 10 mg of the dried product was filled into an aluminum container for measurement, and a DSC curve under a nitrogen atmosphere and a DSC curve were obtained using a DSC measurement device (manufactured by Shimadzu Corporation, model name "DSC6220"). The measurement conditions were as follows.
First stage temperature drop program: Drop temperature from 25°C at a rate of 15°C per minute. After reaching -50°C, maintain for 5 minutes.
Second-stage heating program: Heat from -50°C to 400°C at a rate of 15°C per minute. DSC and DDSC data were obtained during this second-stage heating.
The intersection point between the baseline (a straight line extending the baseline of the obtained DSC curve toward the higher temperature side) and the tangent at the inflection point (the point where the upward convex curve changes to a downward convex curve) was determined as the glass transition temperature (Tg).

[Example 1]
<Preparation of polyolefin porous substrate>
45 parts by mass of homopolymer high-density polyethylene having an Mv of 700,000, 45 parts by mass of homopolymer high-density polyethylene having an Mv of 300,000, and 5 parts by mass of homopolymer polypropylene having an Mv of 400,000 were dry blended using a tumbler blender.

1 part by mass of tetrakis-[methylene-(3',5'-di-t-butyl-4'-hydroxyphenyl)propionate]methane was added as an antioxidant to 99 parts by mass of the resulting polyolefin mixture, and the mixture was dry-blended again using a tumbler blender to obtain a mixture.

The resulting mixture was fed into a twin-screw extruder using a feeder under a nitrogen atmosphere.

Furthermore, liquid paraffin (kinematic viscosity at 37.78° C.: 7.59×10 ⁻⁵ m ² /s) was injected into the extruder cylinder by a plunger pump.

The operating conditions of the feeder and pump were adjusted so that the proportion of liquid paraffin in the total mixture being extruded was 65 parts by mass and the polymer concentration was 35 parts by mass.

Next, they were melt-kneaded in a twin-screw extruder while being heated to 230°C, and the resulting molten mixture was extruded through a T-die onto a cooling roll whose surface temperature was controlled at 80°C. The extrudate was brought into contact with the cooling roll and cast, and then cooled and solidified to obtain a sheet-like molded product.

This sheet was stretched to a ratio of 7x6.4x at 112°C using a simultaneous biaxial stretching machine. The stretched sheet was then immersed in methylene chloride to extract and remove the liquid paraffin, then dried, and further stretched 2x in the transverse direction at 130°C using a tenter stretching machine.

The stretched sheet was then relaxed by about 10% in the width direction and heat-treated to obtain polyolefin porous substrate A1. The physical properties of the obtained substrate are shown in Table 1.

<Preparation of material for uniform diffusion layer>
(Catalyst Production)
10 g of tributyltin chloride and 35 g of tributyl phosphate were placed in a three-necked flask equipped with a stirrer, a thermometer, and a distillation apparatus, and heated at 250° C. for 20 minutes while stirring under a nitrogen stream to remove the distillate and obtain a solid condensation product as a residue. In the following, this organotin-phosphate ester condensation product was used as a catalyst.

で表されるグリシジルエーテル化合物３００ｇとアリルグリシジルエーテル２７ｇと溶媒ｎ－ヘキサン２０００ｇを仕込み、これにエチレンオキシド３２０ｇを上記グリシジルエーテル化合物の重合率をガスクロマトグラフィーで追跡しながら、逐次添加した。重合反応はメタノールで停止した。重合反応終了後、生成したポリマーをデカンテーションにて取り出した後、常圧下、４０℃で２４時間、更に、減圧下、４５℃で１０時間乾燥して、反応性モノマー成分としてアリルグリシジルエーテル成分を有するポリマー５９５ｇを得た。 (Production of polyether multicomponent copolymer)
The inside of a 3 L four-necked glass flask was replaced with nitrogen, and 0.3 g of the organotin-phosphate ester condensation product as a catalyst and a solution of the following formula (1) adjusted to a moisture content of 10 ppm or less were added thereto:

300 g of a glycidyl ether compound represented by the formula (I), 27 g of allyl glycidyl ether, and 2000 g of n-hexane solvent were charged, and 320 g of ethylene oxide was successively added thereto while monitoring the polymerization rate of the glycidyl ether compound by gas chromatography. The polymerization reaction was stopped with methanol. After the polymerization reaction was completed, the produced polymer was taken out by decantation, and then dried at normal pressure at 40° C. for 24 hours and further at reduced pressure at 45° C. for 10 hours to obtain 595 g of a polymer having an allyl glycidyl ether component as a reactive monomer component.

The polyether multicomponent copolymer thus obtained had a glass transition temperature of -70°C and a weight average molecular weight of 1.0 x 106 as measured by gel permeation chromatography. There was no heat of fusion. The monomer-equivalent composition of this polyether multicomponent copolymer as measured by proton NMR spectrum was the above glycidyl ether compound (16): ethylene oxide: allyl glycidyl ether = 20: 80: 2 mol%.

2.0 g of the above polyether multicomponent copolymer, 0.4 g of the crosslinking aid diethylene glycol dimethacrylate (Blenmer PDE-100, manufactured by Nippon Oil & Fats Co., Ltd.), and 0.02 g of the polymerization initiator benzoyl peroxide were dissolved in 67.0 g of water to prepare a coating liquid.

<Supporting the uniform diffusion layer on the substrate surface>
After the substrate A1 was subjected to a corona treatment under the conditions of an output of 100 W and a speed of 2.0 m/min, the above coating liquid was applied using a bar coater and heated in an inert gas atmosphere at 85° C. for 2 hours to crosslink the polyether multicomponent copolymer, thereby obtaining a separator having a uniform diffusion layer on the surface layer. The evaluation results are shown in Table 2. The uniform diffusion layer was disposed so as to be in contact with the negative electrode.

[Example 2]
A separator having a uniform diffusion layer on the surface was obtained in the same manner as in Example 1, except that the coating was performed using a reverse gravure machine instead of a bar coater, and the heating was performed at 50° C. for 10 minutes and then at 85° C. for 2 hours. The evaluation results are shown in Table 2. The uniform diffusion layer was disposed so as to be in contact with the negative electrode.

[Example 3]
In dehydrated N-methyl-2-pyrrolidone (NMP, manufactured by Mitsubishi Chemical Corporation), 4,4'-diaminodiphenyl ether (manufactured by Tokyo Chemical Industry Co., Ltd.) was dissolved as a diamine under a nitrogen stream, and cooled to 30°C or less. 2-chloroterephthaloyl chloride (manufactured by Nippon Light Metal Co., Ltd.) equivalent to 99 mol% based on the total amount of diamine was added thereto over 30 minutes while maintaining the system at 30°C or less under a nitrogen stream, and after the entire amount was added, the mixture was stirred for about 2 hours to polymerize aromatic polyamide (A). The obtained polymerization solution was neutralized with 97 mol% lithium carbonate (manufactured by Honjo Chemical Co., Ltd.) and 6 mol% diethanolamine (manufactured by Tokyo Chemical Industry Co., Ltd.) based on the total amount of acid chloride, to obtain a solution of aromatic polyamide (A).

The obtained aromatic polyamide solution was applied in the form of a film on a stainless steel (SUS316) belt support using a bar coater, and dried at 80°C for 10 minutes, 100°C for 10 minutes, and finally at 120°C until the film had self-supporting properties, after which the film was peeled off from the support. The film was then introduced into a water bath at 60°C to extract the solvent and neutralization salts. Note that the film was stretched 1.1 times in the machine direction (MD) and ungripped in the transverse direction (TD) from the time of peeling until after the water bath. The obtained water-containing film was then heat-treated for 2 minutes in a tenter chamber at 280°C while being stretched 1.15 times in the TD at a constant length, to obtain a uniform diffusion layer with a thickness of 4 μm.

The obtained uniform diffusion layer was placed on the peeled surface of the stainless steel support and on the substrate A1 to obtain a separator with a uniform diffusion layer. The Ra of the uniform diffusion layer in this case was the surface opposite to the peeled surface of the stainless steel support, and this surface was placed in contact with the negative electrode. The evaluation results are shown in Table 2.

[Example 4]
96.0 parts by mass of plate-shaped boehmite (average particle size 1.0 μm), 4.0 parts by mass of acrylic polymer latex (solid content concentration 40%, average particle size 145 nm, Tg = -10 ° C.), and 1.0 part by mass of an aqueous solution of ammonium polycarboxylate (SN Dispersant 5468 manufactured by San Nopco) were uniformly dispersed in 100 parts by mass of water to prepare a coating solution, which was then applied to a thickness of 4 μm on the substrate A1. Next, a uniform diffusion layer was formed on the surface opposite to the surface coated with boehmite in the same manner as in Example 2. The evaluation results are shown in Table 2. The uniform diffusion layer was disposed so as to be in contact with the negative electrode.

[Comparative Example 1]
Table 2 shows the evaluation results of the substrate A1, which is a separator having no uniform diffusion layer.

[Comparative Example 2]
A separator having a uniform diffusion layer on the surface was obtained in the same manner as in Example 1, except that the paint was prepared by using 21.6 g of toluene instead of water and the substrate A1 was not subjected to corona treatment. The evaluation results are shown in Table 2.

[Comparative Example 3]
A separator having a uniform diffusion layer on the surface was obtained in the same manner as in Example 4, except that the drying condition after coating of the aromatic polyamide solution in Example 3 was changed to 120° C. The evaluation results are shown in Table 2.

Claims (6)

A separator for an electricity storage device comprising a microporous membrane,
The separator for the electric storage device has a uniform diffusion layer located at least on a surface layer thereof,
the uniform diffusion layer is a non-porous layer made of an ion-conductive material containing polyethylene oxide or a copolymer of ethylene oxide and a monomer;
The arithmetic mean roughness (Ra) of the uniform diffusion layer in the surface layer is 1.20 μm or less,
The separator for an electric storage device is impregnated with an electrolyte solution of ethylene carbonate (EC)/diethyl carbonate (DEC)=1/2 (volume ratio) containing 1 mol/L LiPF6 _for one day, and then the uniform diffusion layer is pressure-bonded to lithium (Li) metal at 25° C. and 0.5 MPa, and the peel strength of the separator for an electric storage device is 2 N/m or more . 2. The separator for an electric storage device according to claim 1 , wherein the degree of swelling of the uniform diffusion layer in an electrolyte containing 1 mol/L _LiPF6 and having an EC/DEC ratio of 1/2 (volume ratio) is two or more times. The separator for an electricity storage device according to claim 1 or 2 , which has an inorganic filler layer on at least one side of the microporous membrane. An electricity storage device comprising a positive electrode, a negative electrode, an electrolyte, and the separator for an electricity storage device according to any one of claims 1 to 3 . The electricity storage device according to claim 4 , wherein a peel strength between the negative electrode and the electricity storage device separator is 2 N/m or more. The electricity storage device according to claim 4 or 5 , wherein the negative electrode is Li metal or copper (Cu) foil.