JP2022181107A - Storage device and separator for storage device - Google Patents

Abstract

Kind Code: A1 An object of the present invention is to provide a separator for an electricity storage device that can control the form of lithium (Li) deposition and suppress Li dendrite formation in the electricity storage device.
A power storage device separator comprising a microporous film, the power storage device separator having a uniform diffusion layer positioned at least on a surface layer, wherein the surface layer has an arithmetic mean roughness ( A power storage device separator having Ra) of 1.20 μm or less is provided.
[Selection figure] None

Description

TECHNICAL FIELD The present invention relates to an electricity storage device separator, an electricity storage device including the same, and the like.

Microporous membranes are widely used for the separation of various substances, or as permselective separation membranes, separators, and the like. Its uses include, for example, microfiltration membranes; separators for batteries such as lithium ion batteries, polymer electrolyte batteries, and fuel cells; separators for capacitors; Base materials for membranes and the like can be mentioned. Among them, polyolefin microporous films are suitably used as separators for power storage devices widely used in mobile phones, smart phones, wearable devices, notebook personal computers (PCs), tablet PCs, digital cameras, and the like.

Conventionally, in a power storage device, an electrolyte is impregnated in a power generation element in which a separator is interposed between a positive electrode plate and a negative electrode plate. In these electric storage devices, under conditions such as charging the device with a relatively large current at a relatively low temperature, lithium (Li) is deposited on the negative electrode plate, and metal Li is formed from the negative electrode plate into dendrites (dendrites). can grow as If such dendrites continue to grow, they break through or penetrate the separator to reach the positive electrode plate, or come close to the positive electrode plate. There is

Moreover, in a battery, when the deposited dendrite (metallic Li) breaks, highly reactive metallic Li may exist in a state where it is not electrically connected to the negative electrode plate. Thus, when a large amount of dendrites or metal Li derived from dendrites is present in the battery, a short circuit may occur in other parts, a short circuit may occur between a plurality of parts, or when heat is generated due to overcharging, reactions in the surroundings may occur. Even the metal Li, which has a high resistance, reacts, which easily leads to problems such as further heat generation, and causes deterioration of the battery capacity.

On the other hand, for example, Patent Document 1 discloses a composite ion-permeable membrane composed of a non-porous ion-permeable membrane formed of a polymer such as aromatic polyamide (aramid) and a porous substrate such as a porous polymer membrane. It is described that when used in an aqueous electrolyte battery, it is excellent in resistance to short circuits caused by dendrites, low resistance, and the like.

In addition, from the viewpoint of battery characteristics such as lowering the resistance of polymer electrolyte batteries and reducing interfacial resistance when electrodes and separators are superimposed, for example, Patent Document 2 discloses an electrolyte solution containing a monomer that can be polymerized by actinic rays. is impregnated into electrodes, separators, etc., and further polymerized and gelled to adjust the surface roughness (Rmax) of electrodes, separators, etc. containing a gel polymer electrolyte to 0.6 μm or less.

WO2016/098660 JP-A-2000-215916

The composite ion permeable membrane described in Patent Document 1 has room for improvement in controlling the form of lithium (Li) deposition and suppressing Li dendrite formation in electrical storage devices. , it may become difficult to withstand cycle deterioration or extend service life. Moreover, the electrode or separator containing the gel polymer electrolyte described in Patent Document 2 does not focus on suppression of Li dendrite formation. Accordingly, an object of the present invention is to provide an electricity storage device separator capable of controlling the form of Li deposition and suppressing Li dendrite formation in an electricity storage device, and an electricity storage device including the same.

The present inventors have elucidated the resistance increase mechanism of an electricity storage device due to lithium (Li) dendrite deposition, and have found that the above problems can be solved by arranging a specific uniform diffusion layer on at least the surface layer of the separator. I discovered it and completed the present invention. Examples of embodiments of the invention are listed below.
<1> A power storage device separator comprising a microporous membrane,
Having a uniform diffusion layer positioned at least on the surface layer of the electricity storage device separator,
An electrical storage device separator, wherein in the surface layer, the uniform diffusion layer has an arithmetic mean roughness (Ra) of 1.20 µm or less.
<2> After impregnating the electricity storage device separator in an electrolytic solution of ethylene carbonate (EC)/diethyl carbonate (DEC) = 1/2 (volume ratio) containing 1 mol/L of LiPF ₆ for 1 day, the uniform 2. The power storage device separator according to item 1, wherein the diffusion layer has a peel strength of 2 N/m or more when press-bonded to lithium (Li) metal at 25° C. and 0.5 MPa.
<3> The electricity storage according to item 1 or 2, wherein the uniform diffusion layer swells twice or more with respect to an electrolytic solution containing 1 mol/L of LiPF ₆ with EC/DEC=1/2 (volume ratio). Device separator.
<4> The power storage device separator according to any one of items 1 to 3, wherein the microporous membrane has an inorganic filler layer on at least one side thereof.
<5> The electricity storage device separator according to any one of items 1 to 4, wherein the uniform diffusion layer is made of an ion conductive material.
<6> The electricity storage device separator 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 electrolytic solution, 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 the 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.

ADVANTAGE OF THE INVENTION According to this invention, the separator for electrical storage devices which can achieve control of a lithium (Li) precipitation form, and suppression of Li dendrite formation, and an electrical storage device containing the same can be provided.

Hereinafter, a mode for carrying out the present invention (hereinafter sometimes abbreviated as "this embodiment") will be described in detail, but the present invention is not limited to this, and is within the scope of the gist. Various modifications are possible.

In this specification, MD is defined as the flow direction of the film during film formation, and TD is defined as the direction intersecting MD at 90 degrees (°) in the plane of the film.

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

When a uniform diffusion layer having an Ra of 1.20 μm or less is positioned at least on the surface layer of the separator according to the present embodiment, it is possible to control the form of lithium (Li) deposition and suppress Li dendrite formation when the separator is used in an electricity storage device. tend to be possible. This tendency is remarkable in an electricity storage device with a charge-discharge mechanism that facilitates deposition of Li, and thus facilitates cycle deterioration or longer life of an electricity storage device including a negative electrode made of Li metal or copper (Cu) foil. .

According to the present invention, the following two patterns were found as the resistance increase mechanism of an electricity storage device caused by Li dendrite precipitation:
(a) Li dendrite layer is thickly deposited on the negative electrode side during charging of the electricity storage device, and compresses 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, when the separator is compressed, the permeability of the separator decreases and the resistance increases.
(a) Li metal has a low potential and a high reducibility, so reductive decomposition of the electrolytic solution easily proceeds on the Li metal surface, and decomposition products are deposited on the negative electrode or the Li dendrite layer. Since Li dendrites precipitate sharply as dendrites, they have a large specific surface area, and the increase in specific surface area due to dendrite formation increases the amount of reductive decomposition products, resulting in an increase in resistance, and the electrolyte solution also dries up. Occur.

In the present embodiment, the smoothness is optimized (Ra ≤ 1.0) from the viewpoint of achieving control of the Li precipitation morphology and suppression of Li dendrite formation based on the above resistance increase mechanisms (a) and (b). 20 μm) A separator having a uniform diffusion layer at least on the surface layer, and an electric storage device including the same. For example, the uniform diffusion layer of the separator according to the present embodiment is arranged so as to contact or face the negative electrode surface (or Li deposition surface) of the electricity storage device, to eliminate the Li ion concentration distribution and control the Li deposition form. (Li dendrite formation can be suppressed).

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

Optionally, the separator according to this embodiment may comprise at least one layer in addition to the substrate such as the microporous membrane and the uniform diffusion layer. In general, at least one layer included in the laminated separator may have, for example, insulating properties, adhesive properties, thermoplasticity, inorganic porosity, etc., and may be formed of a single film or dot-coated. A pattern formed by stripe coating or the like may be used. Therefore, the presence of the uniform diffusion layer on at least the surface layer of the separator according to the present embodiment means not only the layer structure in which the uniform diffusion layer is laminated on one side or both sides of the base material, but also the separator including the base material, or the base material. Structures may also include a uniform diffusion layer partially or entirely exposed from a separator comprising a material and at least one layer.

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

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 morphology and suppressing Li dendrite formation, and reducing the internal resistance of the battery, The thickness ratio of the uniform diffusion layer is preferably substrate: uniform diffusion layer = 1:1 to 10000:1, more preferably 2:1 to 1000:1, and 3:1 to 100:1. It is even more preferable to have In addition, from the viewpoint of suppressing the peeling of the uniform diffusion layer and the substrate and suppressing the increase in internal resistance against the thickness change due to the dissolution and deposition of Li in the Li metal negative electrode, the binding force between the substrate and the uniform diffusion layer is preferably 2 N/m or more and 1000 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.

Components of the separator according to this embodiment will be described below.

<Microporous membrane>
The microporous membrane (hereinafter also referred to as a porous layer) may be one conventionally used as a separator, and preferably has a high resistance to organic solvents and a fine pore size. Microporous membranes include, for example, polyolefins (e.g., polyethylene, polypropylene, polybutene, polyvinyl chloride, etc.), and mixtures thereof or microporous membranes containing resins such as copolymers of a plurality of monomers constituting them as main components. ; Microporous membranes containing resins such as polyethylene terephthalate, polycycloolefin, polyethersulfone, polyamide, polyimide, polyimideamide, polyaramid, polycycloolefin, nylon, polytetrafluoroethylene, etc.; cloth); non-woven fabrics of polyolefin fibers; paper; and aggregates of insulating material particles. Among these, it has excellent chemical and electrical stability, and excellent coatability of the coating liquid when obtaining a polymer layer through the coating process. From the viewpoint of increasing the active material ratio and increasing the capacity per volume, a polyolefin microporous membrane (hereinafter also referred to as a PO microporous membrane) containing a polyolefin resin as a main component is preferred. In the present specification, "containing as a main component" means containing more than 50% by mass of a specific component or member, preferably 75% by mass or more, more preferably 85% by mass or more, and even more preferably contains 90% by weight or more, even more preferably 95% by weight or more, particularly preferably 98% by weight or more, and may be 100% by weight.

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 properties shown below.

(Pore ratio measured by nitrogen gas adsorption test)
The microporous membrane (however, when a microporous laminated membrane is formed by laminating a plurality of porous layers as described above, one porous layer in the laminated membrane) is measured by a nitrogen gas adsorption test. It is preferable that the single-point method total pore volume with a pore diameter of 98 nm or less is 25% or more and 85% or less of the total pore volume.

The nitrogen gas adsorption test can be performed by the method described in Examples below. By defining the adsorption amount at a nitrogen relative pressure (p/p0) of 0.98 as the total pore volume, the one-point method total pore volume (pore diameter of 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 of 98 nm or less) to the total pore volume can be calculated. It can be seen that the higher the ratio, the more the microporous membrane (porous layer) is formed from smaller pores. From the viewpoint that the microporous film (porous layer) becomes more dense and it becomes possible to efficiently delay the growth of lithium dendrites and dendrites derived from foreign substances accompanying the elution of foreign metal substances, the single-point method total pore volume (fine pore diameter of 98 nm or less) is preferably 25% or more, more preferably 30% or more, of the total pore volume. In addition, in the nitrogen gas adsorption test of the microporous membrane (porous layer) by the one-point method, the lower limit of the pore diameter can exceed 0 nm in this technical field.

On the other hand, when the single-point method total pore volume (pore diameter of 98 nm or less) of the microporous membrane (porous layer) exceeds 85% of the total pore volume, clogging of the separator causes a point where the resistance increase concentrates, resulting in As a result, it promotes the generation of dendrites. From the above point of view, the total pore volume by one-point method (pore diameter of 98 nm or less) is preferably 85% or less, more preferably 80% or less, of the total pore volume.

From the viewpoint of film-forming properties, conventional microporous membranes containing polyethylene (PE) such as high-density polyethylene (HDPE) as a main component (usually containing 50% by mass or more) generally have a total pore volume of (pore diameter of 98 nm or less) is less than 25%. This is because from the viewpoint of the melt viscosity or the heat setting process, small pores are blocked and the pore size of the membrane as a whole is increased.

The ratio of the single-point total pore volume (pore diameter of 98 nm or less) to the total pore volume can be controlled, for example, by densification of the microporous membrane (porous layer), which will be described in more detail below. In the manufacturing process of a microporous membrane, for example, containing a high molecular weight PE having a molecular weight described later in a content described later; performing a stretching step before the extraction step; Stretching under conditions of temperature 128 ° C. or less; adjusting the draw ratio in each of the vertical and horizontal directions in the stretching process to 5 times or more; Adjusting; 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 combination as appropriate. The use can be controlled to lie within the pore volume percentage ranges described above.

Furthermore, the pore volume ratio can also be controlled by the viscosity-average molecular weight of the PE used. By using high-molecular-weight PE, the pores are made smaller in the stretching process, and the pores are less likely to be clogged in the heat-setting process. It is preferably 500,000 or more, and the content of PE having a Mw of 450,000 or more in the resin composition of the film is preferably 45% by mass or more, and is 50% by mass or more. is more preferred, and this content can be 100% by mass. In order that the pore ratio does not exceed 85%, the Mv of the membrane is preferably 6,000,000 or less, preferably 5,000,000 or less, and 3,000,000 More preferably:

Also, the pore ratio can be controlled within a predetermined range by the molecular structure of the PO used. Although the reason is not clear, the polydispersity of the raw material polymer (weight-average molecular weight Mw/number-average molecular weight Mn and z-average molecular weight Mz/weight-average molecular weight Mw) tends to result in a pore ratio within a predetermined range. It is in.

(Number of holes calculated by gas-liquid method)
The microporous membrane (however, when a microporous laminated membrane is formed by laminating a plurality of porous layers as described above, one porous layer in the laminated membrane) has pores calculated by the gas-liquid method. The number B is preferably 15/μm ² or more and preferably 500/μm ² or less.

The number of pores (B) per cross section calculated by the gas-liquid method has an upper limit of is the diffusion of lithium ions and metal ions accompanying the elution of foreign matter in the microporous film (porous layer), from the viewpoint of being able to efficiently suppress local dendrite generation and suppressing the increase in local resistance increase locations, From the viewpoint of improving cycle characteristics, 15/μm ² or more and 500/μm ² or less, 40/μm ² or more and 480/μm ² or less, 60/μm ² or more and 450/μm ² or less, 80/ It is preferably from ² to 350/μm ² , or ^from 100 to 300/μm ² , and more preferably from 110 to 200/μm ^{2 .} The number of pores per cross section (B) calculated by the gas-liquid method is determined by the plasticizer ratio in all raw materials (that is, resin ratio in all raw materials), heat setting temperature, etc. in the manufacturing process of the PO microporous membrane described later. is adjusted within the above range.

(Porosity)
The microporous membrane (however, when forming a microporous laminated membrane by laminating a plurality of porous layers as described above, one porous layer in the laminated membrane) has a porosity ε of 20% or more. It is preferably in the range of 80% or less. The porosity (ε) may hereinafter be referred to as “porosity (before compression)”.

The porosity (ε) of the microporous membrane (porous layer) reduces the membrane resistance and improves the ion diffusivity, thereby effectively suppressing the generation of dendrites. It is preferably 20% or more, more preferably 40% or more, and even more preferably 45% or more, from the viewpoint of achieving good air permeability and having high ionic conductivity and high output characteristics. , more preferably 47% or more, and particularly preferably 50% or more. In addition, the porosity (ε) is preferably 80% or less, more preferably 70% or less, from the viewpoint of suppressing the growth of dendrites by improving the strength of the film and from the viewpoint of improving the withstand voltage. , is more preferably 65% or less, and particularly preferably 60% or less.

(Average flow diameter measured by half-dry method)
The microporous membrane (however, when forming a microporous laminated membrane by laminating a plurality of porous layers as described above, one porous layer in the laminated membrane) is the average The flow diameter d is preferably 0.010 μm or more and preferably 0.150 μm or less. In this specification, the pore size of the microporous membrane (porous layer) is the average flow rate measured using a perm porometer (Porous Materials, Inc.: CFP-1500AE) in accordance with the half-dry method. means.

By measuring the average flow diameter (d) in the half-dry method, it is possible to know the pore diameter of the portion where the pores are narrowed in the pore structure of the microporous membrane (porous layer). The upper limit of the average flow diameter d measured by the half-dry method is the ability to physically suppress the growth of dendrites by densifying the microporous membrane (porous layer), and the number of pores per cross section (B). From the viewpoint of improving ion diffusibility by increasing the , 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 0.025 μm or more and 0.050 μm or less is more preferable.

As a means for controlling the pore size or porosity (ε) of the microporous membrane (porous layer) within the above numerical range, for example, in the method for producing a PO microporous membrane described later, a high molecular weight Containing PE in a content described later; Performing a stretching step before the extraction step; Adjusting the stretching ratio in each direction in the stretching step to 5 times or more; ; During the heat setting (HS) step, the heat setting temperature is adjusted to 115 ° C. or more and 140 ° C. or less, the HS stretching ratio is adjusted to 1.5 times or more and 2.2 times or less, and the HS relaxation ratio is 0. Adjustment to 0.7 times or more and less than 0.9 times can be used alone or in combination as appropriate.

(Half width of average flow diameter peak measured by half dry method)
The microporous membrane (however, when forming a microporous laminated membrane by laminating a plurality of porous layers as described above, one porous layer in the laminated membrane) is the average It is preferable that the half width of the flow diameter peak is 0.010 μm or less.

It can be said that the narrower the half-value 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 uniformly lithium ions and metal ions accompanying the elution of foreign matter diffuse, and the viewpoint of being able to efficiently suppress local dendrite generation, the viewpoint of being able to physically suppress the growth of dendrites, and the local From the viewpoint of suppressing a significant increase in resistance 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, more preferably 0.002 μm or less. preferable. In addition, the lower limit of the half-value width of the average flow diameter peak measured by the half-dry method can exceed 0 μm in this technical field. The half-value width of the average flow diameter peak can be obtained, for example, by controlling the content of high-molecular-weight PE in the method for producing a microporous membrane, and by adjusting the heat setting temperature to, for example, 140° C. or less during the heat setting (HS) step. For example, it can be adjusted within the above numerical range.

(film thickness)
The thickness of the microporous membrane depends on the capacity of the storage device, the high ion permeability and good rate characteristics, the retardation of dendrite growth, and the volume occupied by the separator when used for high-capacity batteries. is preferably 30 μm or less, 25 μm or less, or 20 μm or less, more preferably 18 μm or less, still more preferably 16 μm or less, and 1 μm or more and 14 μm or less. more preferably 3 μm or more and 13 μm or less, and most preferably 5 μm or more and 12 μm or less. The film thickness of the microporous membrane may be referred to as "average film thickness (before compression)" below.

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

(air permeability)
The air permeability of the microporous membrane is preferably 0 seconds or more and 250 seconds or less, more preferably 0 seconds or more and 250 seconds or less per 100 cm ³ of air, as the upper limit of the electricity storage device from the viewpoint of output characteristics and cycle characteristics, and as the lower limit from the viewpoint of film strength. is 10 seconds or more and 200 seconds or less, more preferably 20 seconds or more and 180 seconds or less, and particularly preferably 25 seconds or more and 170 seconds or less.

(weight conversion conversion piercing strength, piercing strength, basis weight)
The microporous membrane preferably has a per unit weight equivalent puncture strength of 50 gf/(g/m ² ) or more. 50 gf/(g/m ² ) or more in terms of basis weight piercing strength indicates that the film strength per resin basis weight is high and that the film structure is resistant to crushing against compressive stress. Even if the microporous membrane used as the material has a high porosity and a low air permeability, it tends to be difficult to rupture, which tends to improve the safety of the electric storage device. The weight-converted piercing strength is measured by the method described in the Examples, and along the TD of the film, the weight is measured at a total of three points, two points inside 10% of the total width from both ends toward the center and one point in the center. It is obtained by measuring non-converted puncture strength (hereinafter simply referred to as puncture strength) and dividing the average value by the basis weight.

The advantage of controlling the puncture strength is remarkable when using an electrode that easily expands and contracts in the cell of an electricity storage device such as a non-aqueous secondary battery. This is more pronounced when a (Si)-containing negative electrode is used. From this point of view, the microporous membrane has a per unit weight equivalent puncture strength of 50 gf/(g/m ² ) or more and 150 gf/(g/m ² ) or less, more preferably 55 gf/(g/m ² ) or more. It is more preferably 130 gf/(g/m ² ) or less, and particularly preferably 70 gf/(g/m ² ) or more and 120 gf/(g/m ² ) or less.

From the same viewpoint as above, from the viewpoint of achieving a constant film strength and low air permeability, and from the viewpoint of improving short-circuit resistance, the microporous membrane has a basis weight of 1.0 g/m ² or more and 15 g/m ² or less. preferably within the range.

From the same point of view as above, from the point of view of safety when impact is applied to the electricity storage device, and to suppress inter-electrode short circuits or withstand voltage defects caused by separator rupture caused by foreign matter unintentionally entering the electricity storage device. From this point of view, the puncture strength of the microporous membrane is preferably 200 gf or more, more preferably 220 gf or more, still more preferably 250 gf or more, even more preferably 280 gf or more, and 300 gf or more. is particularly preferred. The upper limit of the puncture strength is not particularly limited, but can be determined according to the crystallinity of the film, the heat shrinkability of the film, and the suppressed electrical resistance. Or it may be 680 gf or less.

The pierce strength and per unit area equivalent pierce strength of the microporous membrane are determined, for example, in the manufacturing process of the microporous membrane, by the molecular weight and blending ratio of the polymer raw material such as polyolefin, the stretching ratio during the biaxial stretching process, and the stretching ratio during the biaxial stretching process. By controlling the MD/TD stretching temperature, the heating amount coefficient per unit resin of the resin composition during the biaxial stretching process, the heat setting (HS) ratio, etc., it is adjusted within the numerical range described above. can be done.

(Difference R between maximum and minimum air permeability measured at TD3 points before compression, air permeability distribution before compression)
The difference between the maximum and minimum air permeability ( hereinafter referred to as difference R) is preferably 15 sec/100 cm ³ 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 was measured by the method described in the Examples, and the permeability before the compression test of the microporous membrane was measured. Represents the air distribution. 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, from the viewpoint of air permeability measurement accuracy. The upper limit of the total width W is not particularly limited, and can be determined according to, for example, the film forming device, film forming process, mother roll dimension, slit roll dimension, coating process, etc. For example, 5000 mm or less, or 4000 mm or less. OK.

A well-maintained air permeability distribution that satisfies R≤15 sec/100 cm ³ represents a film structure with small in-plane air permeability variations. , the in-plane electrochemical reaction occurs uniformly, which tends to improve battery characteristics such as rate characteristics and cycle characteristics. This tendency is remarkable when using an electrode that easily expands and contracts in the cell of a non-aqueous secondary battery, and when using a high-capacity electrode used in a vehicle battery or the like, or a silicon (Si)-containing negative electrode , is more pronounced. From such a viewpoint, the difference R is more preferably 0 sec/100 cm ³ or more and 15 sec/100 cm ³ or less, further preferably 0 sec/100 cm ³ or more and 13 sec/100 cm ³ or less, and 0 sec/100 cm ³ or more and 11 sec. /100 cm ³ or less is more preferable, and 0 sec/100 cm ³ or more and 9 sec/100 cm ³ or less is particularly preferable.

The average value of the above three measured values is expressed as air permeability (before compression), from the same viewpoint as above, and from the viewpoint of maintaining high (ion) permeability even in a compressed state and puncture strength from the viewpoint of collateral, it is preferably 0 sec/100 cm ³ or more and 200 sec/100 cm ³ or less, more preferably 30 sec/100 cm ³ or more and 180 sec/100 cm ³ or less, and 40 sec/100 cm ³ or more and 150 sec/100 cm ³ or less. It is even more preferable to have

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

(Membrane properties after compression)
By subjecting the microporous membrane to a compression test under specific conditions, the characteristics of the microporous membrane that can satisfy both the battery characteristics and the safety of the electric storage device are found.
<Compression press test>
Microporous membranes or separators were measured under four different pressure conditions (e.g., 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 four measurements were made. From the results, a relational expression between the porosity and air permeability of the microporous membrane or separator after unloading is obtained.

The air permeability after compression of _{25° C. and 2.5 MPa} ≦180 sec/100 cm ³ is a non-aqueous secondary battery containing a polyolefin microporous membrane as a separator, for example, along with the air permeability distribution or difference R before compression described above. There is a tendency to improve battery characteristics such as rate characteristics and cycle characteristics. This tendency is remarkable for the improvement of the rate characteristics when using electrodes that easily expand and contract within the cell of the non-aqueous secondary battery or when the separator is compressed by pressure from the outside of the cell. This is more pronounced when a high-capacity electrode, a silicon (Si)-containing negative electrode, a Li metal negative electrode, or a Cu foil, which is used in, for example, is used as the negative electrode. From this point of view, the post-compression air permeability of _{25°C and 2.5 MPa} is preferably 200 sec/100 cm ³ or less, more preferably 180 sec/100 cm ³ or less, and still more preferably 170 sec/100 cm ³ or less. It is particularly preferable to be 120 sec/100 cm ³ or less. The lower limit of the air permeability under pressure of _{25° C. and 2.5 MPa} is not particularly limited, but can be determined according to the mechanical strength or puncture strength of the microporous membrane, for example, 0 sec/100 cm ³ or more. , 10 sec/100 cm ³ or more, or 20 sec/100 cm ³ or more.

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

The post-compression air permeability of the microporous membrane of _{25°C and 2.5 MPa} and the post-compression porosity _{of 25°C and 2.5 MPa} are determined, for example, in the production process of the microporous membrane by adjusting the molecular weight and blending ratio of polymeric raw materials such as polyolefin. By controlling the stretching ratio during the biaxial stretching process, the MD/TD stretching temperature during the biaxial stretching process, the heating amount coefficient per unit resin of the resin composition during the biaxial stretching process, the HS ratio, the HS temperature, etc. , can be adjusted within the numerical ranges described above.

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

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

(Tensile breaking strength)
The tensile breaking strength of the microporous membrane is 500 to 3,000 kgf/cm ² in both MD and TD from the viewpoint of ensuring the membrane strength necessary for winding and laminating electrodes and separators in the manufacturing process of electric storage devices. more preferably 700 kgf/cm ² or more and 3,000 kgf/cm ² or less, and even more preferably 1,000 kgf/cm ² or more and 3,000 kgf/cm ² or less.

The closer the MD and TD tensile breaking strength values of the microporous membrane are, the more evenly the membrane breaks during the nail penetration test of the electricity storage device, thereby minimizing the short-circuit area and thus improving the nail penetration test safety. From such a viewpoint, the ratio of the MD tensile strength at break to the TD tensile strength at break (MD/TD tensile strength at break ratio) of the microporous membrane is preferably 0.7 or more and 1.3 or less, more preferably 0.75. It is in the range of 1.25 or less, more preferably in the range of 0.8 or more and 1.2 or less. The MD/TD tensile strength ratio at break of the microporous membrane can be adjusted within the numerical range described above, for example, by controlling the draw ratio, HS ratio, etc. during the biaxial stretching process.

(tensile modulus)
In both MD and TD, the tensile modulus of elasticity of the microporous membrane makes it difficult for the separator to rupture when a nail penetrates the exterior of the electrical storage device and also penetrates and deforms the separator and electrodes during a nail penetration test of the electricity storage device. From the viewpoint of not completing a short circuit and improving safety, it is preferably 1,000 kg/cm ² or more and 10,000 kg/cm ² or less, and 2,000 kg/cm ² or more and 90,000 kg/cm ² or less. is more preferable.

(Thermal shrinkage rate)
Regarding the thermal shrinkage rate of the microporous membrane, the shape stability of the membrane at relatively high temperatures is high, and from the viewpoint of suppressing short circuits in the thermal runaway state of the storage device during a nail penetration test, etc., it is MD at 120 ° C. When measured, it 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. The thermal shrinkage of the microporous membrane is preferably −10% or more and 20% or less, more preferably −5% or more and 18% or less, and 0% or more and 15% when measured in TD at 120° C. % or less, and particularly preferably 0% or more and 10% or less.

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

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

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

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

Among them, polyolefin resins include polyethylene, polypropylene, ethylene-propylene copolymer, ethylene-propylene and other monomers from the viewpoint of reduction or increase suppression of film electrical resistance, compression resistance and structural uniformity of film. and mixtures thereof are preferred.

A polyethylene composition in which polyethylene accounts for 50% by mass or more and 100% by mass or less of the resin component constituting the microporous membrane from the viewpoint of crystallinity, high strength, compression resistance, etc. of the microporous membrane, and from the viewpoint of expressing fuse properties. Microporous membranes formed by matter are preferred. The ratio of polyethylene (PE) in the resin component constituting the microporous membrane is more preferably 60% by mass or more and 100% by mass or less, further preferably 70% by mass or more and 100% by mass or less, and 80% by mass. % or more and 100 mass % or less, and most preferably 90 mass % or more and 100 mass % or less.

In addition, from the viewpoint of high strength, compression resistance, suppression of electrical resistance, etc. when a microporous film is formed as a separator base material, the PE ratio in the polyolefin resin is preferably 30% by mass or more, and 50% by mass or more. It is 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 further preferably 95% by mass or less. preferable. It should be noted that a PE of 50% or more is preferable from the viewpoint of exhibiting fuse behavior with high responsiveness.

The viscosity average molecular weight (Mv) of the polyolefin resin used as the raw material for the microporous membrane is preferably 30,000 or more and 6,000,000 or less, more preferably 80,000 or more and 3,000,000 or less, and even more preferably. is 150,000 or more and 2,000,000 or less. When the Mv is 30,000 or more, the entanglement of the polymers tends to increase the strength, which is preferable. On the other hand, when the Mv is 6,000,000 or less, it is preferable from the viewpoint of improving the moldability in the extrusion and stretching steps.

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

In another aspect, the microporous membrane (porous layer) preferably contains a PE raw material having Mv of 500,000 or more and 6,000,000 or less based on the mass of the microporous membrane, It is more preferably 50% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, and particularly preferably 90% by mass or more. When the microporous layer contains 45% by mass or more of the PE raw material having Mv of 500,000 or more and 6,000,000 or less, the crystals are highly oriented during stretching, and the microporous layer has a small pore size or densification. Tend. From the same point of view, the PO raw material used for the production of the microporous membrane is preferably an ethylene homopolymer, more preferably an ethylene homopolymer having an Mv of 500,000 or more and 6,000,000 or less.

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

In addition, the polyolefin resin is medium density polyethylene (MDPE) having a density of 0.930 g/cm ³ or more and less than 0.942 g/cm ³ from the viewpoint of safety of an electricity storage device having a PO microporous membrane as a separator. For example, PE other than high density polyethylene (HDPE) may be used. Furthermore, from the viewpoint of improving the safety of the electrical storage device even when the PO microporous membrane is a thin film, a medium density polyethylene with a viscosity average molecular weight of less than 1,000,000 and a viscosity average molecular weight of 1,000,000 or more are used. 50% by mass or more, based on the mass of the PO microporous membrane, of at least one selected from ultrahigh molecular weight polyethylene having a density of 2,000,000 or less and a density of 0.930 g/cm ³ or ^more and less than 0.942 g/cm 3 preferably 60% by mass or more, more preferably 70% by mass or more.

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

When PE and PP are used in combination, the polyethylene includes a medium density polyethylene having a viscosity average molecular weight of less than 1 million, and a viscosity average molecular weight of 1 million or more and 2 million or less and a density of 0.930 g/cm ³ or more and 0.942 g/cm ^{3 .} It is preferable to use at least one selected from ultra-high-molecular-weight polyethylene having a molecular weight of less than 100 to balance strength and permeability, and to maintain a proper fuse temperature.

The viscosity-average molecular weight of the polyolefin resin (measured according to the measurement method in Examples described later) is preferably 50,000 or more, more preferably 100,000 or more, more preferably 500,000 or more, and more preferably It is 700,000 or more, more preferably 1,000,000 or more, and the upper limit thereof is preferably 6,000,000 or less, preferably 3,000,000 or less, and preferably 1,900,000 or less. The viscosity-average molecular weight of 50,000 or more is from the standpoint of maintaining a 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 membrane. It is preferable from the viewpoint of enhancement. On the other hand, setting the viscosity average molecular weight to 6,000,000 or less is preferable from the viewpoint of achieving uniform melt-kneading and improving sheet formability, particularly thickness formability.

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 according to the measurement method in Examples described later. Although the reason is not clear, setting the polydispersity of the raw material polymer within the above range tends to make the pore ratio of the PO microporous membrane moderately high and facilitate the formation of a uniform pore structure. From this point of view, 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 (Mz/Mw) of the z-average molecular weight to the weight-average molecular weight 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 Examples described later. Although the reason is not clear, setting the Mz/Mw of the raw material polymer within the above range tends to result in a moderately high pore ratio and a uniform pore structure in the PO microporous membrane. From such a point of view, Mz/Mw of the polyethylene raw material is more preferably 4.0 or more, further preferably 5.0 or more.

The resin composition may optionally contain inorganic particles, phenol-based, phosphorus-based, or sulfur-based antioxidants; metal soaps such as calcium stearate and zinc stearate; ultraviolet absorbers, light stabilizers, and electrifiers. Various known additives such as inhibitors, anti-fogging agents, and color pigments may be mixed. However, by mixing inorganic particles and the like with the resin composition, the resulting film tends to have a smaller pore ratio.

(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:
a mixing step (a) of mixing a resin composition containing a polyolefin resin, a pore-forming material, and optionally various additives;
an extrusion step (b) of melt-kneading and extruding the mixture obtained in step (a);
A sheet forming step (c) for forming the extrudate obtained in the step (b) into a sheet;
A primary stretching step (d) of stretching the sheet-shaped molding 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 primary stretched membrane obtained in step (d);
and a heat setting step (f) of heat setting (HS) the extracted membrane obtained in step (e) at a predetermined temperature.

When used as a separator for lithium-ion secondary batteries and other power storage devices, the above-mentioned PO microporous film production method can improve the ability to suppress lithium dendrites, the ability to suppress chemical micro-shorts caused by metallic foreign matter, high output characteristics, and cycle characteristics. It is possible to provide a PO microporous membrane compatible with Among them, the method of stretching in the MD and TD in the primary stretching step (d), followed by the extraction step (e) and then heat-setting in the TD in the heat-setting step (f) is a method in which the pore ratio of the resulting PO microporous membrane is It tends to be easy to adjust the hole diameter and the like within the numerical range described above. The method for producing the PO microporous membrane of the present embodiment is not limited to the above-described production 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 optionally various additives. In addition, in the mixing step (a), other components may be mixed with the resin composition, if necessary.

The pore-forming material can be any, for example a plasticizer, as long as it is distinguished from the material of the PO resin and the inorganic particles. Examples of plasticizers include nonvolatile solvents capable of forming a uniform solution above the melting point of the PO resin, such as liquid paraffin (LP), hydrocarbons such as paraffin wax; esters such as dioctyl phthalate and dibutyl phthalate; Higher alcohols such as alcohol and stearyl alcohol may be used.

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, still more preferably 73% by mass to 85% by mass, particularly preferably 75% by mass. % 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 membrane increases, the ion diffusibility increases, the generation of dendrites can be efficiently suppressed, and the cycle characteristics are improved. Since the melt viscosity of the resin composition is lowered and melt fracture is suppressed, there is a tendency that the film formability during extrusion is improved. On the other hand, by adjusting the content of the plasticizer to 90% by mass or less, it is possible to suppress the stretching of the original fabric during the film-forming process.

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

The method of mixing in step (a) is not particularly limited, but includes, for example, a method of premixing part or all of the raw materials using a Henschel mixer, ribbon blender, tumbler blender, or the like, if necessary. Among them, a method of mixing using a Henschel mixer is preferable.

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

The method of melt-kneading in step (b) is not particularly limited, but for example, all raw materials including the mixture mixed in step (a) are screw extruders such as single-screw extruders and twin-screw extruders; kneaders; A method of melt-kneading with a mixer or the like can be mentioned. Among them, it is preferable to perform melt-kneading using a twin-screw extruder using a screw. Further, when performing melt-kneading, it is preferable to add the plasticizer in two or more batches. It is preferable to adjust the amount to 80% by weight or less from the viewpoint of suppressing aggregation of the contained components and uniformly dispersing them. This is preferable from the viewpoint of suppressing heat generation by shutting down the resulting microporous membrane 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 from the viewpoint of uniformly kneading the resin composition. The lower limit of the temperature of the melt-kneading section is the melting point of the polyolefin or higher from the viewpoint of uniformly dissolving the polyolefin resin into the pore-forming material such as the plasticizer.

At the time of kneading, although not particularly limited, it is possible to mix an antioxidant in a predetermined concentration with PO as a raw material, replace the surroundings of the mixture with a nitrogen atmosphere, and perform melt kneading while maintaining the nitrogen atmosphere. preferable. The temperature during melt-kneading is preferably 160°C or higher, more preferably 180°C or higher, and preferably lower than 300°C.

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

[Sheet forming step (c)]
The sheet forming step (c) is a step of forming the extrudate obtained in the extruding step (b) into a sheet. The sheet-like molding obtained in the sheet forming step (c) may be a single layer or a laminate. The method of sheet molding is not particularly limited, but an example thereof includes a method of solidifying an extrudate by compression cooling.

The compression cooling method is not particularly limited, but includes, for example, a method in which the extrudate is brought into direct contact with a cooling medium such as cold air and cooling water; be done. Among these, the method of bringing the extrudate into contact with a metal roll cooled with a refrigerant, a press, or the like is preferable in terms of easy film thickness control.

After the melt-kneading step (b), the set temperature in the step of forming the melt into a sheet is preferably set higher than the set temperature of the extruder. The upper limit of the set temperature for sheet molding is preferably 300° C. or lower, more preferably 260° C. or lower, from the viewpoint of thermal deterioration of the polyolefin resin. For example, when producing a sheet-shaped molding continuously from an extruder, after the melt-kneading step, the step of molding into a sheet shape, that is, the path from the extruder outlet to the T die and the set temperature of the T die are extruded. When the temperature is set higher than the set temperature of the process, the resin composition and the pore-forming material do not separate, and the molten material can be formed into a sheet, which is preferable. From the viewpoint of the film thickness of the PO microporous membrane to be obtained, it is preferable to control the cast clearance and the like.

[Primary stretching step (d)]
The primary stretching step (d) is a step of stretching the sheet-shaped molding obtained in the sheet forming step (c) at least once in at least one axial direction. This stretching step (the stretching step performed before the next extraction step (e)) is called "primary stretching", and the film obtained by the primary stretching is called "primary stretching film". In the primary stretching, the sheet-like molding can be stretched in at least one direction, and may be stretched in both MD and TD directions, or may be stretched in either MD or TD.

The stretching method of the primary stretching is not particularly limited, but for example, uniaxial stretching with a roll stretching machine; TD uniaxial stretching with a tenter; sequential biaxial stretching with a combination of a roll stretching machine and a tenter, or a plurality of tenters; simultaneous biaxial tenter Or simultaneous biaxial stretching by inflation molding, etc. are mentioned. Among them, simultaneous biaxial stretching is preferable from the viewpoint of physical property stability of the resulting PO microporous membrane.

The MD and/or TD draw ratio in the primary drawing is preferably 5 times or more, more preferably 6 times or more. When the MD and/or TD draw ratio in the primary drawing is 5 times or more, the resulting PO microporous membrane tends to form dense fibrils to have small pore diameters and improve strength. In addition, the MD and/or TD draw ratio of the primary drawing is preferably 9 times or less, more preferably 8 times or less or 7 times or less. When the MD and/or TD draw ratio in the primary drawing is 9 times or less, breakage during drawing tends to be suppressed. When performing biaxial stretching, sequential stretching or simultaneous biaxial stretching may be used. The stretching ratio in each axial direction is preferably 5 times or more and 9 times or less, and 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 raw material resin composition and concentration contained in the PO resin composition. The MD and/or TD stretching temperature is preferably 110° C. or higher, more preferably 115° C. or higher, from the viewpoint of reducing the pore size and suppressing breakage. The MD and/or TD stretching temperature is preferably 128° C. or lower, more preferably 126° C. or lower, and 124° C. or lower from the viewpoint of increasing the film strength or reducing the pore size. is more preferable, and 122° C. or less is particularly preferable.

[Extraction step (e)]
The extraction step (e) is a step of extracting the pore-forming material from the primary stretched membrane obtained in the primary stretching step (d) to obtain an extracted membrane. As a method for removing the pore-forming material, for example, a method of immersing the primary stretched membrane in an extraction solvent to extract the pore-forming material, and thoroughly drying it, and the like can be mentioned. The method of extracting the pore-forming material may be batch or continuous. Moreover, the residual amount of the pore-forming material, particularly the plasticizer, in the porous membrane is preferably less than 1% by mass.

The extraction solvent used for extracting the pore-forming material is a solvent that is a poor solvent for the polyolefin resin, a good solvent for the pore-forming material or the plasticizer, and has a boiling point lower than the melting point of the polyolefin resin. is preferred. Examples of such extraction solvents include, but are not limited to, hydrocarbons such as n-hexane and cyclohexane; halogenated hydrocarbons such as methylene chloride and 1,1,1-trichloroethane; 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 by an operation such as distillation and reused.

[Heat setting step (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 method of heat treatment at this time is not particularly limited, but includes a heat setting method in which stretching and relaxation operations are performed using a tenter or a roll stretching machine.

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 MD and TD directions, or in either MD or TD. You can just go

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

The stretching temperature in this stretching operation is preferably 115° C. or higher, more preferably 120° C. or higher, from the viewpoint of suppressing breakage during stretching. The stretching temperature in the heat setting step (f) is preferably 140° C. or lower, more preferably 138° C. or lower, and 134° C. or lower from the viewpoint of increasing the porosity and porosity. is more preferably 131° C. or lower, more preferably 128° C. or lower, or more preferably 125° C. or lower. Further, when the stretching temperature is within the above numerical range, the pore size of the resulting 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 may be performed in MD or TD. You can go to only one of The relaxation ratio in the heat setting step (f) is preferably 0.95 times or less, more preferably 0.90 times or less, and still more preferably 0.85 times or less. When the relaxation ratio in step (f) is 0.95 times or less, thermal shrinkage of the film tends to be suppressed. Moreover, the relaxation ratio is preferably 0.5 or more, more preferably 0.7 or more, from the viewpoint of increasing the relaxation temperature or increasing the porosity and porosity. Here, the "relaxation factor" is a value obtained by dividing the dimension of the membrane after the relaxation operation by the dimension of the membrane before the relaxation operation. is the value multiplied by the relaxation factor of
Relaxation magnification = (membrane dimension after relaxation operation (m))/(membrane dimension before relaxation operation (m))

The relaxation temperature in this relaxation operation is preferably 115° C. or higher, more preferably 120° C. or higher, from the viewpoint of suppression of breakage. In addition, from the viewpoint of increasing the number of porosity and increasing the porosity, the temperature is preferably 140° C. or less, more preferably 138° C. or less, more preferably 134° C. or less, and more preferably 131° C. or less. It is preferably 128° C. or lower, or more preferably 125° C. or lower. Further, when the relaxation temperature is within the above numerical range, the pore size of the resulting PO microporous membrane tends to be small and easily controlled to be uniform.

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

[Other processes]
The method for producing the PO microporous membrane can include steps other than the above steps (a) to (f). Other steps are not particularly limited, but for example, in addition to the heat setting step, as a step for obtaining a laminated PO microporous membrane, a plurality of monolayer PO microporous membranes are laminated. A lamination process can be mentioned. In addition, from the viewpoint of forming a uniform diffusion layer, which will be described later, the method for producing a PO microporous membrane includes electron beam irradiation, plasma irradiation, corona discharge treatment, surface active agent coating, and surface active agent coating. A surface treatment step of applying a surface treatment such as chemical modification; and a step of providing an adhesive layer containing a thermoplastic resin on the surface of the PO microporous membrane. Furthermore, the material for the inorganic porous layer or the organic (adhesion) layer may be coated on one or both sides of the PO microporous membrane to obtain a PO microporous membrane with at least one layer.

[Formation of inorganic coating layer]
From the viewpoint of safety, dimensional stability, heat resistance, etc., an inorganic coating layer can be provided on the surface of the PO microporous membrane. The inorganic coating layer is a layer containing an inorganic component such as inorganic particles, and may optionally contain a binder resin that binds the inorganic particles together, a dispersant that disperses the inorganic particles in a solvent, and the like.

Materials for the inorganic particles contained in the inorganic coating layer include, for example, oxide-based ceramics such as alumina, silica, titania, zirconia, magnesia, ceria, yttria, zinc oxide, and iron oxide; silicon nitride, titanium nitride, and Nitride ceramics such as boron nitride; silicon carbide, calcium carbonate, magnesium sulfate, aluminum sulfate, barium sulfate, aluminum hydroxide, aluminum hydroxide oxide (AlOOH, alumina monohydrate, or boehmite), potassium titanate, talc , kaolinite, dakite, nacrite, halloysite, pyrophyllite, montmorillonite, sericite, mica, amesite, bentonite, asbestos, zeolite, calcium silicate, magnesium silicate, diatomaceous earth, and silica sand; Glass fiber etc. are mentioned. The inorganic particles may be used singly or in combination.

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

The dispersant is adsorbed on the surfaces of the inorganic particles in the slurry and stabilizes the inorganic particles by electrostatic repulsion. Examples thereof include polycarboxylates, sulfonates, polyoxyethers, and surfactants. .

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

[Formation of adhesive layer]
An adhesive layer containing a thermoplastic resin is provided on the surface of the PO microporous membrane to prevent deformation or swelling due to gas generation in laminated batteries, which are increasingly being used in automotive batteries in order to increase energy density. be able to.

The thermoplastic resin contained in the adhesive layer is not particularly limited, and examples include polyolefins such as polyethylene or polypropylene; fluorine-containing resins such as polyvinylidene fluoride and polytetrafluoroethylene; vinylidene fluoride-hexafluoropropylene copolymer; Fluorine-containing rubbers such as vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene fluoride-hexafluoropropylenetetrafluoroethylene copolymer, ethylene-tetrafluoroethylene copolymer; styrene-butadiene copolymer and its hydride, Acrylonitrile-butadiene copolymer and its hydride, acrylonitrile-butadiene-styrene copolymer and its hydride, (meth)acrylic acid ester copolymer, styrene-acrylic acid ester copolymer, acrylonitrile-acrylic acid ester copolymer Rubbers such as union, ethylene propylene rubber, polyvinyl alcohol, polyvinyl acetate; cellulose derivatives such as ethyl cellulose, methyl cellulose, hydroxyethyl cellulose, carboxymethyl cellulose; Examples thereof include resins such as polyamide and polyester.

Furthermore, after the heat setting step (f), the lamination step or the surface treatment step, the master roll wound with the PO microporous membrane is subjected to aging treatment under a predetermined temperature condition, and then the master roll is rewound. You can also perform operations. This tends to make it easier to obtain a PO microporous membrane having higher thermal stability than the PO microporous membrane before rewinding. When the master roll is aged and rewound, 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 still more preferably 60° C. or higher. be. From the viewpoint of maintaining the permeability of the PO microporous membrane, the temperature for aging the master roll is preferably 120° C. or less. The time required for the aging treatment is not particularly limited, but is preferably 24 hours or more because the above effects are likely to be exhibited.

<Uniform diffusion layer>
A uniform diffusion layer is a layer in which uniform diffusion of lithium (Li) ions is possible when an electrolytic solution is included. The uniform diffusion layer according to the present embodiment is located at least on the surface layer of the separator and has an Ra of 1.20 μm or less, thereby eliminating the Li ion concentration distribution in the electricity storage device including the separator, controlling the Li deposition form and Li Allows suppression of dendritization.

The arithmetic average roughness (Ra) of the uniform diffusion layer is 0.001 μm from the viewpoint of uniform diffusion of Li ions and the optimization of smoothness for suppressing local Li ion concentration concentration on the negative electrode surface. It is preferably at least 1.200 μm, more preferably at least 0.010 μm and at most 1.000 μm, even more preferably at least 0.015 μm and at most 0.950 μm. From the same point of view, the ten-point average 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. It is more preferably 015 μm or more and 2.000 μm or less. From the same viewpoint, Rms (average length of elements) 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. More preferably, it is 3.0 μm or more and 70.0 μm or less.

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

In addition, from the viewpoint of improving the cycle characteristics of the electricity storage device by reducing the interfacial resistance between the negative electrode containing Li metal or copper (Cu) foil and the separator, EC / DEC = 1/2 containing 1 mol / L LiPF ₆ (volume ratio) after impregnating the separator according to the present embodiment for one day, the peel strength when the uniform diffusion layer of the separator is pressed against the Li metal at 25 ° C. and 0.5 MPa is 2N. /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 moderately soft resin that has a high affinity with Li metal. A material having polarity is preferable as a material having a high affinity with Li metal. Examples of polar materials include, but are not limited to, olefin copolymers such as ethylene/vinyl acetate copolymers and ethylene/acrylic acid/maleic anhydride copolymers; Polyvinyl acetates; Polyvinyl alcohols; Polyvinyl acetals; Polyvinyl butyrals; Polyfluoropolybenzoxazole; Acrylic resins; Acrylonitonol copolymers such as polymers; styrene copolymers such as sterene-methacrylic acid copolymers and styrene-acrylonitrile copolymers; ionic polymers such as sodium polystyrene sulfonate and sodium polyacrylate; acetal resins; Polyamides such as 66; gelatin; gum arabic; polycarbonates, polyester carbonates; cellulose resins; melamine resins; polyurethanes; diaryl phthalate resins; polyphenylene oxides; polyphenylene sulfide polysulfones; ether sulfones; polyether ketones; polyether imides and the like. From the viewpoint of affinity with the electrolytic solution, fluorine-containing resins, polycarbonates, polyketones, polyethers, and polyacrylonitriles are preferred. Moderately soft resins may include, for example, resins having a lower Tg than aramid resins, which are considered in the art to have a glass transition temperature (Tg) greater than about 300°C. 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. Moreover, Tg may be a value in a state where the uniform diffusion layer material is plasticized including an electrolytic solution (EC/DEC=1/2 (volume ratio), etc.). Tg is determined from a DSC curve obtained by differential scanning calorimetry (DSC). Specifically, the temperature at the intersection of a straight line obtained by extending the base line on the low temperature side of the DSC curve to the high temperature side and the tangent line at the inflection point of the stepwise change portion of the glass transition can be used as the glass transition temperature. can. "Glass transition" refers to a change in the amount of heat generated on the endothermic side due to a change in the state of the polymer, which is the test piece, in DSC. Such a heat quantity change is observed as a shape of step-like change in the DSC curve. A “step-like change” refers to a portion of a DSC curve from a baseline on the low temperature side to a transition to a new baseline on the high temperature side. A combination of a stepped change and a peak is also included in the stepped change. Furthermore, the “point of inflection” indicates the point at which the gradient of the DSC curve of the stepwise change portion is maximized. In addition, in the stepwise change portion, when the upper side is the heat generation side, it can be expressed as a point at which the upwardly convex curve changes to the downwardly convex curve. "Peak" refers to the portion of the DSC curve from when the curve departs from the baseline on the low temperature side to when it returns to the same baseline again. "Baseline" refers to the DSC curve in the temperature range where no transition or reaction occurs in the specimen.

The degree of swelling of the uniform diffusion layer in an electrolytic solution containing 1 mol/L of LiPF ₆ with EC/DEC=1/2 (volume ratio) is determined from the viewpoint of the strength of the uniform diffusion layer as the upper limit, and the amount of Li ions as the lower limit. From the viewpoint of uniform diffusion, it 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. This degree of swelling can be adjusted within the above numerical range, for example, by controlling the solubility parameter (SP value) of the raw material or contained components of the uniform diffusion layer.

The uniform diffusion layer has the following (1) to (3) from the viewpoint of optimizing the smoothness, and from the viewpoint of controlling the Li deposition morphology and suppressing Li dendrite formation in the electric storage device:
(1) 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 in observation of the surface of the uniform diffusion layer when the electrolyte solution is included;
(2) When the electrolyte solution is included, the total area (S _total ) of the region where Li ions cannot diffuse is the upper limit when observing the surface in the range of 3.0 μm × 4.0 μm at any point of the uniform diffusion layer. and ( ³ ) when the electrolyte solution is included, the area ratio of the region where Li ion diffusion is possible to the region where Li ion diffusion is not possible in the surface observation of the uniform diffusion layer is Area: possible area = 60:40 to 0:100;
It is preferable to satisfy one or more of the above.

Regarding (1) to (3) above, the region where Li ion diffusion is impossible 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. It can be regarded as satisfying (1). Moreover, the case where the uniform diffusion layer is a filler layer may also be considered in the same manner as above. As a method of distinguishing the regions where Li ion diffusion is possible and the regions where Li ion diffusion is not possible in surface observation, for example, by appropriately combining EDX, NMR, XRD, XPS, XRF, etc., ion-conducting materials or non-ion-conducting materials can be identified. , mapping, etc., to clarify the distribution of these materials on the film surface. Note that the void portion is a region in which Li ions can diffuse because it is possible to diffuse Li ions by being filled with the electrolytic solution.

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, and 20 nm or less, Most preferably, it does not contain non-diffusing Li-ion regions.

Regarding (2) above, the total area S _total is preferably 7.2 μm ² or less, 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. is more preferable, and it is most preferable not to include a region where Li ions cannot diffuse.

Regarding (3) above, it is preferable that the area ratio of the region where Li ion diffusion is possible to the region where Li ion diffusion is impossible is impossible region:possible region = 60:40 to 0:100. :70 to 0:100, 20:80 to 0:100, 10:90 to 0:100, and most preferably does not contain a region where Li ions cannot diffuse.

The thinner the uniform diffusion layer, the lower the electrical resistance, while the thicker the uniform diffusion layer, the easier it is for Li ions to uniformly diffuse on the negative electrode surface of the electricity storage device. From such a viewpoint, the thickness of the uniform diffusion layer is preferably in the range of more than 0 μm and 10 μm or less, more preferably 0.5 to 7.0 μm, and 1.0 to 6.0 μm. is more preferable, and 2.0 to 5.0 μm is even more preferable. When the uniform diffusion layer is formed as a pattern, or when the thickness of the uniform diffusion layer is not uniform, the thickness of the uniform diffusion layer is determined so as to maximize along the thickness direction of the microporous membrane. can do.

The constituent material of the uniform diffusion layer preferably contains at least one ion-conducting material, and may contain both ion-conducting and non-ion-conducting materials. More preferably, the uniform diffusion layer is made of an ion-conductive material, more preferably made of only an ion-conductive material. It is particularly preferred that the uniform diffusion layer is a non-porous layer made of only an ion-conductive material.

The ion conductive material may be any material that exhibits an ion conductivity of 10 ⁻⁷ Scm ⁻¹ or more at 25° C. The ion conductivity is 10 ⁻⁶ Scm ⁻¹ or more, 10 ⁻⁴ Scm ⁻¹ or more, Alternatively, it is preferably 10 ⁻³ Scm ⁻¹ or more. The ionic conductivity may be the value of the material alone or the value of the material swollen in an electrolytic solution or the like, but from the viewpoint of resistance, the value of the material alone is more preferable. The ionic conductivity can be easily determined by a person skilled in the art from literature or the like if the material is known. If it is difficult to judge, it is also possible to measure by the method described in Examples. Examples of materials that satisfy the ionic conductivity include organic materials and inorganic materials, such as solid electrolytes and gel electrolytes. The gel electrolyte may be a swollen electrolytic solution, or may exhibit conductivity by containing a Li salt. Examples include materials having polar functional groups such as polyethylene oxide, polyvinylidene fluoride, polyacrylonitrile, polymethacrylic acid, polycarbonate, and polyacetal, and copolymers of a plurality of monomers constituting these may be used. Preferably, the ion-conducting material comprises polyethylene oxide, polyacrylonitrile, polymethacrylic acid, more preferably polyethylene oxide, polyacrylonitrile. Solid electrolytes include, for example, 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 _{2O12 , Li3BO3 - Li2SO4 , Li2.9PO3.3N0.46 , Li1.07Al0.69Ti1.46 ( PO4 ) 3 , Li9.54Si 1.74P1.44S11.7Cl0.3 , Li10GeP2S12 , Li3.25Ge0.25P0.75S4 , Li6PS5Cl , Li2SB2 _ _ _ _ _ _ S3} -LI, _Li2SP2S5 - _LiBH4 , _{Li2S - SiS2 - Li3PO4 , Li2S} - _{SiS2 - Li4SiO4 , Li2SP2S5 ,} Materials such as Li ₇ P ₃ S ₁₁ , Li _3.25 P _0.95 S ₄ , and Li ₃ N can be mentioned, and the composition ratio is not limited to the above.

As the non-Li ion conductive material, materials other than the Li ion conductive material can be used, and specific examples include polyolefins such as polyethylene and polypropylene, and organic and inorganic materials such as polystyrene and polyvinyl chloride. mentioned.

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

(Method for Forming Uniform Diffusion Layer)
The method of forming the uniform diffusion layer is particularly limited as long as the uniform diffusion layer is positioned at least on the surface layer of the separator and the arithmetic mean roughness (Ra) of the uniform diffusion layer is adjusted within the above numerical range. However, for example, partial molding or processing of a separator base material such as a microporous film or laminated film, or coating of a paint containing a constituent material of a uniform diffusion layer on the separator base material may be used.

When forming a uniform diffusion layer by partially forming a separator base material such as a microporous film or laminated film, the forming process should be performed so that the thickness of the stem on the surface layer of the base material satisfies the condition (1) above. preferably.

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

In order to improve the smoothness of the uniform diffusion layer, the solvent suppresses excessive penetration of the paint into the microporous membrane, suppresses paint repelling on the surface of the microporous membrane, and promotes leveling of the paint during coating. It is preferable to select a solvent that can Solvents can be selected in terms of viscosity and surface tension based on the compatibility of the paint and the microporous membrane to which it is applied. Solvents that satisfy the above aspects are not particularly limited, but examples include N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, dimethylsulfoxide, water, ethanol, methanol, n-propanol, acetone, and toluene. , xylene, n-hexane, cyclohexane, chloroform, dichloromethane, ethyl acetate, methyl ethyl ketone, and the like.

Examples of coating methods include 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 them, methods other than dipping are preferred. From the viewpoint 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 according to, for example, the boiling point of the solvent of the paint. From the viewpoint of smoothness, it is preferably lower than the boiling point of the solvent, more preferably lower than 15°C from the boiling point, and particularly preferably lower than 50°C.

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

<Power storage device>
An electricity storage device including a positive electrode, a negative electrode, an electrolytic solution, and the separator described above is also an aspect of the present invention. Examples of electricity storage devices include non-aqueous electrolyte batteries, non-aqueous electrolyte batteries, non-aqueous lithium ion secondary batteries, non-aqueous gel secondary batteries, non-aqueous solid secondary batteries, lithium ion capacitors, electric double layer capacitors, and the like. mentioned.

The electricity storage device may include a separator, a positive electrode plate, a negative electrode plate, and a non-aqueous electrolyte (including 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 the like and a negative electrode plate capable of absorbing and releasing lithium ions and the like are wound so as to face each other with a separator interposed therebetween. Alternatively, it is laminated, holds a non-aqueous electrolyte, and is housed in a device container or a device outer package.

The positive plate will be described below. As the positive electrode active material, for example, lithium composite metal oxides such as lithium nickelate, lithium manganate or lithium cobaltate, lithium composite metal phosphates such as lithium iron phosphate, and the like can be used. The positive electrode active material is kneaded with a conductive agent and a binder, applied as a positive electrode paste to a positive electrode current collector such as an aluminum foil, dried, rolled to a predetermined thickness, and 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, such as 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, such as polyvinylidene fluoride, modified acrylic rubber, or polytetrafluoroethylene, can be used.

The negative plate will be described below. As the negative electrode active material, lithium (Li) metal, copper (Cu) foil, or a material capable of intercalating lithium can be used. Furthermore, for example, at least one material selected from the group consisting of graphite, silicide, and titanium alloy materials can be used for the negative electrode. In addition, as the negative electrode active material of the non-aqueous electrolyte secondary battery, for example, metals, metal fibers, carbon materials, oxides, nitrides, silicon (Si) compounds, tin (Sn) compounds, various alloy materials, etc. are used. be able to.

Examples of carbon materials that can be used for the negative electrode include various natural graphites, cokes, graphitizable carbons, non-graphitizable carbons, carbon fibers, spherical carbons, various artificial graphites, and amorphous carbons.

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

Among the materials for the negative electrode described above, it is preferable that the negative electrode is composed of Li metal or copper (Cu) foil, as the effect of the uniform diffusion layer of the separator according to the present embodiment is manifested remarkably. In addition, the uniform diffusion layer of the separator is arranged so as to contact or face the negative electrode surface (or Li deposition surface) of the electricity storage device to eliminate the Li ion concentration distribution and control the Li deposition form (Li dendrite formation suppression) is also preferred.

The non-aqueous electrolyte will be described below. A non-aqueous electrolyte generally contains a non-aqueous solvent and a metal salt such as a lithium salt, a sodium salt, or a calcium salt dissolved therein. As the non-aqueous solvent, a cyclic carbonate, a chain carbonate, a cyclic carboxylate, or the like is used, and those having improved oxidation resistance by partially fluorinating them may be used. Lithium salts include, 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 , lower aliphatic carbon Lithium oxide, LiCl, LiBr, LiI, borates, imide salts and the like can be mentioned. Alternatively, an ionic liquid may be used, or a high-concentration Li salt non-aqueous electrolyte solution in which 1 mol/L or more of a lithium salt is dissolved in a non-aqueous solvent may be used.

Unless otherwise specified, the various parameters described above are measured according to the measurement methods in Examples described later.

Next, the present embodiment will be described in more detail with reference to examples and comparative examples, but the present embodiment is not limited to the following examples as long as the gist thereof is not exceeded. Physical properties in the examples were measured by the following methods. Unless otherwise specified, the measurement was performed at room temperature of 23° C. and humidity of 40%.

(1) Single-point method total pore volume and pore ratio (1a) Single-point method total pore volume (also known as single-point method volume adsorption amount) (pore diameter 98 nm) (cm ³ /g)
A nitrogen gas adsorption test was performed by a constant volume method based on JIS Z8831-2:2010 Powder (Solid) Pore Distribution and Pore Characteristics "Method for Measuring Mesopores and Macropores by Gas Adsorption, Part 2". Specifically, the gas adsorption amount in terms of temperature and pressure (STP) in the standard state where the nitrogen relative pressure (p/p0) is 0.98 is defined as the volume equivalent of the liquid volume of the adsorbate, A one-point method volume 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 cylindrical pores and rp as the pore radius, which corresponds to a pore diameter of 98.8 nm. Here, rk is the radius of curvature of the adsorbate condensed in the pores and is expressed by the following equation.
rk = -0.953/ln (p/p0).
Further, t is the average thickness of the nitrogen multilayer adsorption film at the relative pressure obtained from the reference adsorption isotherm determined experimentally or theoretically, and is expressed by the following equation.
t=0.354*[-5/ln(p/p0)]^1/3

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

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

(3) Film thickness (μm)
The thickness of the PO microporous membrane, uniform diffusion layer, inorganic filler layer, or separator was measured at room temperature of 23±2° C. using a micro thickness 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 film thickness (μm) of microporous membrane (before compression)]
After sampling the microporous membrane to 10 cm × 10 cm, multiple microporous membranes are stacked so that the thickness is 15 μm or more, 9 points are measured, the average is taken, and the average value is divided by the number of stacked layers. was taken as the thickness of one sheet.

[Thickness (μm) of multilayer porous membrane and coating layer (before compression)]
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 a multilayer porous film, it is also possible to measure the thickness of each layer using a cross-sectional SEM image.

(4) Porosity (%)
A sample of 10 cm×10 cm square was cut out, its volume (cm ³ ) and mass (g) were determined, and from these values and true density (g/cm ³ ), calculation was performed using the following formula. In a separator with a laminated structure formed by coating a microporous film, etc., the porosity of the separator refers to the average porosity of the entire separator, and the porosity of the microporous film is calculated only from the microporous film and formed by coating, etc. The porosity of the laminated portion 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 x 100
As the density of the mixed composition, a value calculated from the density of each material used and the mixing ratio was used.

(5) Air permeability (sec/100 cm ³ ) Air permeability was measured in an atmosphere of 23°C temperature and 40% humidity using Oken type air permeability measuring machine "EGO2" manufactured by Asahi Seiko Co., Ltd.
The measured value of the air permeability is measured at a total of 3 points, 2 points inside 10% of the total width from both ends toward the center along the width direction (TD) of the membrane, and 1 point at the center, It is the value which calculated those average values.

[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 are laminated in this order, and the resulting laminate is left to stand. Then, a compression test was performed by applying pressure to the cushioning material surface on one side of the laminate. Here, the microporous membrane used was 5×5 cm square, and the average film thickness (average of 9 points), basis weight, and air permeability were measured before use in the press test. Also, the porosity before the compression press test was calculated from the basis weight and the average film thickness.
The compression test was performed using a press at pressures of 2.5 MPa, 5 MPa, 7.5 MPa, and 10 MPa under conditions of a temperature of 25° C. and a compression time of 3 minutes. In addition, one hour after the unloading, the microporous membrane was removed from the laminate, and the average film thickness after compression (average of 9 points) and the air permeability after compression were measured. Also, the porosity after compression was calculated from the basis weight and the average film thickness after compression.

(6) basis weight (g/m ² ), puncture strength (gf) and basis weight equivalent puncture strength (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×1 m, the weight was measured with an electronic balance (AUW120D) manufactured by Shimadzu Corporation. In addition, when the sample could not be sampled to 1 m×1 m, it was cut out to an appropriate area, weighed, and then converted to 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 in a sample holder having an opening with a diameter of 11.3 mm. Next, the central portion of the fixed microporous membrane was subjected to a puncture test at a needle tip radius of curvature of 0.5 mm and a puncture speed of 2 mm/sec in an atmosphere of room temperature of 23°C and humidity of 40%. Puncture strength (gf) was measured as a load. The measured value of the puncture test is a value obtained by measuring a total of 3 points along the TD of the membrane, 2 points inside 10% of the total width from both ends toward the center and 1 point in the center, and calculating their average value. is.
The weight-converted piercing strength is obtained by the following formula.
Puncture strength converted to basis weight [gf/(g/m ² )] = Puncture strength [gf]/weight basis [g/m ² ]
Here, regarding the pin puncture strength and per unit area equivalent pin puncture strength of a multi-layer porous membrane in which at least one or more layers are provided on a polyolefin microporous membrane substrate, from the viewpoint of evaluating the strength of the resin and the strength per unit area, the polyolefin microporous membrane The properties were evaluated based on the puncture strength of the membrane substrate and the per unit area-converted puncture strength.

(7) Number of pores of porous membrane or porous layer determined by gas-liquid method (number/μm ² )
It is known that the fluid inside the capillary follows the Knudsen flow when the mean free path of the fluid is larger than the pore diameter of the capillary, and follows the Poiseuille flow when it is smaller. Therefore, it is assumed that the air flow in the air permeability measurement of the porous membrane or layer follows the Knudsen flow, and the water flow in the water permeability measurement of the porous membrane or porous layer follows the Poiseuille flow.
In this case, the average pore diameter d (μm) and tortuosity τ _a (dimensionless) of the porous membrane are 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)), air molecular velocity ν (m/sec), water viscosity η (Pa·sec), standard pressure P _s (=101325 Pa), porosity ε ( %) and the film 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 obtained from the air permeability (sec) using the following equation.
R _gas = 0.0001/(air permeability x (6.424 x 10 ^-4 ) x (0.01276 x 101325))
Also, R _liq was obtained from the water permeability (cm ³ /(cm ² ·sec · Pa)) using the following equation.
R _liq = Permeability/100
The water permeability was determined as follows. A porous membrane or porous layer pre-soaked in ethanol is set in a stainless steel permeable cell with a diameter of 41 mm, and after washing the ethanol of the membrane or layer with water, water permeates at a differential pressure of about 50000 Pa. Then, the water permeation rate per unit time/unit pressure/unit area was calculated from the water permeation rate (cm ³ ) after 120 seconds, and this was taken as the water permeability.
Also, ν is the gas constant R (=8.314), the 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: asked.
ν=((8R×T)/(π×M)) ^1/2
Furthermore, the number of holes B (holes/μm ² ) was obtained from the following formula.
B=4×(ε/100)/(π×d ² ×τ _a )

(8) Pore 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) according to the half-dry method. Perfluoropolyester manufactured by the same company (trade name “Galwick”, surface tension 15.6 dyn/cm) was used as the immersion liquid. For the drying curve and the wetting curve, the applied pressure and the amount of air permeation are measured, and from the pressure PHD (Pa) at which 1/2 of the obtained drying curve and the wetting curve intersect, the average pore diameter dHD is obtained by the following equation. (μm) was obtained and taken as the average flow diameter.
dHD=2860×γ/PHD

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

(9) Molecular weight and viscosity average molecular weight (9a) Measurement of polydispersity (Mw/Mn) and (Mz/Mw) of polyethylene by GPC-light scattering Auction chromatography), under the following conditions, 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 ) was measured. Specifically, Agilent's PL-GPC200 with built-in differential refractometer (RI) and light scattering detector (PD2040) was used. As a column, two pieces of Agilent PLgel MIXED-A (13 μm, 7.5 mm ID×30 cm) were connected and used. At a column temperature of 160° C., 1,2,4-trichlorobenzene (0.05 wt % of 4,4′-Thiobis (containing 6-tert-butyl-3-methylphenol) was used as the eluent at a flow rate of 1.0 ml. /min and an injection volume of 500 μL to obtain an RI chromatogram and a light scattering chromatogram at scattering angles of 15° and 90°.From the obtained chromatogram, using Cirrus software, the number average molecular weight ( Mn), weight average molecular weight (Mw), and z-average molecular weight (Mz) were obtained, and the Mz and Mw values were used to obtain the molecular weight distribution (Mz/Mw), and the Mw and Mn values were used to obtain the molecular weight distribution. (Mw/Mn), where 0.053 ml/g was used as the refractive index increment value of polyethylene.

(9b) viscosity average molecular weight (Mv)
Based on ASTM-D4020, the intrinsic viscosity [η] (dl/g) at 135°C in decalin solvent was determined. For the PO microporous membrane and polyethylene raw material, Mv was calculated by the following formula.
[η]=6.77×10 ⁻⁴ Mv ^0.67
For 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 (Plastics--Melt Mass Flow Rate (MFR) and Melt Volume Flow Rate (MVR) of Thermoplastics). A load of 21.6 kgf was applied to the membrane at 190 ° C., and the amount of resin (g) that flowed out in 10 minutes from an orifice with a diameter of 2 mm and a length of 10 mm was measured, and the value rounded to the first decimal place was taken as MI. .

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

(12) The difference between the maximum and minimum air permeability at 3 points in the TD direction (2 points inside 10% of the total width from both ends toward the center and 1 point in the center) Asahi Seiko Co., Ltd. Using an Oken type air permeability measuring machine (measurement part diameter 30mmφ), when the left width of the membrane is 0% and the right end is 100%, one center point at the 50% position and the center 10% from the left end (10% position) and 10% center side from the right end (90% position), the air permeability was measured at a total of three points, and the difference R between the largest value and the smallest value 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 portion having a diameter of 13 mmφ.

(13) Thermal shrinkage rate (%) at 120°C for 1 hour
As a sample, the porous membrane was cut to a length (mm) 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 before heating, and placed in an oven at 120°C for 1 hour. left undisturbed. At this time, the sample was sandwiched between 10 sheets of paper so that warm air would not hit the sample directly. After the sample was taken out of the oven and cooled, the length was measured to determine the length (mm) after heating, and the thermal shrinkage rate was calculated by the following formula. Measurements were made in MD and TD, and the larger value was taken as the thermal shrinkage rate.
Thermal 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, using a tensile tester manufactured by Shimadzu Corporation, Autograph AG-A type (trademark), MD and TD samples (shape ; width 10 mm x length 100 mm). The distance between the chucks of the tensile tester is 50 mm, and cellophane (registered trademark) tape (manufactured by Nitto Denko Packaging Systems Co., Ltd., product name: N.29) is attached to one side of both ends (25 mm each) of the sample. board. Furthermore, in order to prevent the sample from slipping during the test, a fluororubber with a thickness of 1 mm was attached to the inner side of the chuck of the tensile tester.
The measurement was performed under 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 obtained by dividing the strength at break of the polyolefin microporous membrane by the cross-sectional area of the sample before the test.
The tensile strength at break was obtained for each of MD and TD, and the ratio of MD tensile strength at break to TD tensile strength at break (MD/TD tensile strength at break ratio) was also calculated.

(15) MD and TD tensile modulus (MPa)
For MD and TD measurements, MD samples (MD120 mm x TD10 mm) and TD samples (MD10 mm x TD120 mm) were cut. In accordance with JIS K7127 at an ambient temperature of 23 ± 2 ° C and a humidity of 40 ± 2%, using a tensile tester manufactured by Shimadzu Corporation, Autograph AG-A type (trademark), MD and TD tension of the sample Elastic modulus was measured. The sample was set so that the chuck-to-chuck distance was 50 mm, and the sample was stretched at a tensile speed of 200 mm/min until the chuck-to-chuck distance reached 60 mm, that is, the strain reached 20.0%. Tensile modulus (MPa) was determined from the slope of the resulting stress-strain curve from 1.0% to 4.0% strain.

(16) Arithmetic average roughness Ra, ten-point average roughness Rz, average length RSm
(Compliant with JIS B0601-1994)
A battery separator is cut out with a width of 10 mm and a length of 50 mm. Double-sided tape (double-sided adhesive tape No. 501F manufactured by Nitto Denko Co., Ltd., 5 mm width × 20 m ). At this time, due to the height of the double-faced tape, the central portion of the separator is fixed in a floating state without being in direct contact with the glass plate.
For the sample in this state, the surface roughness of the uniform diffusion layer was measured using a laser microscope (manufactured by Keyence Corporation, VK-8500). At this time, the roughness was measured in a range of 110 μm×150 μm, and the measurement was performed five times at different locations. Arithmetic average roughness Ra, ten-point average roughness Rz, and average length RSm were used.

(17) Surface observation of uniform diffusion layer Using a scanning electron microscope SEM (model S-4800, manufactured by HITACHI), the surface of the uniform diffusion layer was photographed at a size of 3.0 µm × 4.0 µm at any 10 locations. was read into the image analysis software "ImageJ", and the following three items were confirmed.
(I) The maximum diameter of the inscribed circle that can be drawn in the region where Li ion diffusion is impossible when the electrolyte solution is included (II) The total area of the region where Li ion diffusion is impossible when the electrolyte solution is included (Stotal )
(III) Area ratio of the area where Li ion diffusion is possible to the area where Li ion diffusion is impossible when the electrolyte solution is included

(18) Measurement of adhesive strength with Li (18a) Adhesive strength with Li (before electrolyte injection)
The separator is cut into a rectangular shape with a width of 20 mm and a length of 70 mm, and is overlapped with a Li foil (manufactured by Honjo Metal Co., Ltd.) cut into a size of 15 mm x 60 mm to form a laminate, and this laminate is pressed under the following conditions. did.
Temperature: 25°C
Press pressure: 0.5 MPa
Pressing time: 5 seconds The peel strength between the separator and the Li foil after pressing was tested using force gauges ZP5N and MX2-500N (product name) manufactured by Imada Co., Ltd. at a peel speed of 50 mm/min and a 90° peel test. and the peel strength was measured. 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 adhesive strength with Li (before injection of the electrolytic solution).

(18b) Adhesive strength with Li (after electrolyte injection)
Cut the separator into a rectangular shape with a width of 20 mm × length of 70 mm, and overlap it with a Li foil cut into a size of 15 mm × 60 mm to form a laminate of the separator and the electrode. (EC/DEC=1/2 mixture containing 1 mol/L of LiPF ₆ ) was added in an amount of 0.4 ml. After sealing this, it was allowed 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 tested using force gauges ZP5N and MX2-500N (product name) manufactured by Imada Co., Ltd., and a 90° peel test was performed at a peel speed of 50 mm/min. , the peel strength was measured. 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 adhesive strength with Li (after electrolyte injection).

(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 to completely remove the solvent to obtain a dry uniform diffusion layer material. About 0.5 g of the dried product was weighed and used as the weight before immersion (WA). This dried product was placed in a 50 mL vial with 10 g of an electrolytic 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 after the weight was measured, it was taken as the weight after immersion (W _B ).
The electrolyte swelling degree of the uniform diffusion layer was calculated from the following formula.
Degree of swelling (%) = W _B /W _A
In the above formula, if the material of the uniform diffusion layer neither swells nor dissolves in the electrolytic solution, the degree of swelling is 100%.

(20) Measurement of binding force between microporous membrane and uniform diffusion layer A separator is pasted on a slide glass of width 76 mm x length 126 mm with double-sided tape (Nicetack NWBB-15) so that the uniform diffusion layer faces upward. rice field.
A mending tape (Scotch MP-12) was attached to the uniform diffusion layer side of the test piece. A slide glass was fixed in a flat state, one end of the mending tape was folded back, and a 90° peel test was performed by pulling the tape horizontally against the slide glass at a rate of 100 mm/min to measure the peel strength. At this time, the average value of the peel strength in the 40 mm long peel test conducted under the above conditions was adopted as the binding force between the microporous membrane and the uniform diffusion layer.

(21) Battery test a. Preparation of positive electrode 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 each of flake graphite and acetylene black as a conductive material, and a binder A slurry is prepared by dispersing 3.2% by mass of polyvinylidene fluoride (PVDF) in N-methylpyrrolidone (NMP). This slurry is 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. At this time, the active material coating amount of the positive electrode is 125 g/m ² and the bulk density of the active material is 3.00 g/cm ³ . A circle having an area of 2.00 cm ² was punched out of the prepared positive electrode.

b. Preparation of Negative Electrode Lithium metal was prepared as the negative electrode and punched into a circle having an area of 2.05 cm ² .

c. Non-aqueous Electrolyte Solution Prepared by dissolving LiPF ₆ 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, the separator, and the positive electrode are stacked in this order from the bottom along the vertical direction so that the active material surfaces of the positive electrode and the negative electrode face each other, and placed in a stainless steel container with a lid. The container and the lid are insulated, and the container is in contact with the copper foil of the negative electrode, and the lid is in contact with the aluminum foil of the positive electrode. The prepared non-aqueous electrolytic solution was poured into this container, sealed, and left to stand for one day.
The simple battery assembled as described above was charged to a battery voltage of 4.3 V at a current value of 0.3 mA/cm ² in an atmosphere of 25 ° C., and further the current value was changed to 0.3 mA/cm2 while maintaining 4.3 V. The battery was conditioned by first charging the battery after fabrication for a total of about 12 hours by starting the squeeze from cm ² , and then discharging to a battery voltage of 3.0 V at a current value of 0.3 mA/cm ² .
Next, in an atmosphere of 25° C., the battery was charged to a battery voltage of 4.3 V at a current value of 3.0 mA/cm ² , and further the current value was reduced from 3 mA/cm ² while maintaining 4.3 V. A cycle of charging for 3 hours and discharging to a battery voltage of 3.0 V at a current value of 3.0 mA/cm ² was repeated.
In this cycle, the ratio of the discharge capacity at each cycle number to the discharge capacity at the first cycle was obtained as the capacity retention rate (%), and the number of cycles where the capacity retention rate fell below 80% was evaluated.

(22) Measurement of ionic conductivity A cell was produced in the same manner as in item (21) above, except that the positive electrode and negative electrode were changed to stainless steel plates, and a nonporous 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 the conditions of a voltage amplitude of 10 mV and a frequency of 10 Hz to 5,000 kHz in an atmosphere of 25 ° C., and the intersection of the real axes of the Cole-Cole plot, or the real axis extrapolated from the Cole-Cole plot. The film resistance (Ωcm) was obtained from the intersection point with the following calculation formula. At this time, when the Cole-Cole plot was nearly semicircular, the intersection point was obtained by extrapolation of circular arcs, and when the Cole-Cole plot was nearly linear, the intersection point was obtained by straight line extrapolation. Then, the reciprocal of the membrane resistance was calculated as the ionic conductivity.
・Membrane resistance (Ωcm): resistance value (Ω) obtained from Cole-Cole plot × membrane 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 uniform diffusion layer material was placed in 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 uniform diffusion layer material does not thermally decompose to obtain a dry body. . 10 mg of the dried product was filled in an aluminum container for measurement, and a DSC curve under a nitrogen atmosphere and a DSC curve were obtained with a DSC measurement device (manufactured by Shimadzu Corporation, model name "DSC6220"). The measurement conditions were as follows.
1st stage temperature decreasing program: Temperature decreased from 25°C at a rate of 15°C per minute. Hold for 5 minutes after reaching -50°C.
2nd step temperature rising program: Temperature rising from -50°C to 400°C at a rate of 15°C/min. Acquire DSC and DDSC data during this second stage temperature rise.
The intersection of the baseline (a straight line obtained by extending the baseline in the obtained DSC curve to the high temperature side) and the tangent line at the inflection point (the point where the upwardly convex curve changes to the downwardly convex curve) is the glass transition temperature ( Tg).

[Example 1]
<Preparation of polyolefin porous substrate>
45 parts by mass of homopolymer high-density polyethylene with Mv of 700,000, 45 parts by mass of homopolymer high-density polyethylene with Mv of 300,000, and 5 parts by mass of homopolymer polypropylene with 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 as an antioxidant was added to 99 parts by mass of the resulting polyolefin mixture, and the mixture was again blended in a tumbler. A mixture was obtained by dry blending using

The resulting mixture was fed through a feeder to a twin-screw extruder under a nitrogen atmosphere.

Liquid paraffin (kinematic viscosity at 37.78° C. of 7.59×10 ⁻⁵ m ² /s) was also injected into the extruder cylinder by means of a plunger pump.

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

Then, they are melt-kneaded while being heated to 230°C in a twin-screw extruder, and the resulting melt-kneaded product is extruded through a T-die onto a cooling roll whose surface temperature is controlled to 80°C, and the extrudate is obtained. A sheet-like molded product was obtained by contacting with a cooling roll, molding (casting), and solidifying by cooling.

This sheet was stretched by a simultaneous biaxial stretching machine at a temperature of 112° C. at a magnification of 7×6.4. Thereafter, the stretched product was immersed in methylene chloride to remove the liquid paraffin by extraction, dried, and further stretched twice in the horizontal direction at a temperature of 130° C. using a tenter stretching machine.

After that, the stretched sheet was relaxed about 10% in the width direction and heat-treated to obtain a polyolefin porous substrate A1. Table 1 shows the physical properties of the obtained substrate.

<Preparation of Material for Uniform Diffusion Layer>
(Production of catalyst)
10 g of tributyltin chloride and 35 g of tributyl phosphate are 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 distill off the distillate, A solid condensate was obtained as a residue. In the following, this organotin-phosphate condensate 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-liter glass four-necked flask was purged with nitrogen, and 0.3 g of the above organic tin-phosphate ester condensate was added as a catalyst, and the water content was adjusted to 10 ppm or less.

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

The polyether multi-element copolymer thus obtained had a glass transition temperature of -70°C and a weight average molecular weight of 1.0 x 10 6 as measured by gel permeation chromatography. No heat of fusion was present. Further, the composition of this polyether multi-element copolymer in terms of monomers according to the proton NMR spectrum was said glycidyl ether compound (16):ethylene oxide:allyl glycidyl ether=20:80:2 mol %.

2.0 g of the polyether multi-element copolymer, 0.4 g of the cross-linking agent diethylene glycol dimethacrylate (Blenmer PDE-100 manufactured by NOF Co., Ltd.) and 0.02 g of the polymerization initiator benzoyl peroxide were dissolved in 67.0 g of water. Then, a coating liquid was prepared.

<Supporting Uniform Diffusion Layer on Substrate Surface Layer>
After subjecting base material A1 to 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 at 85° C. for 2 hours in an inert gas atmosphere. , to obtain a separator having a uniform diffusion layer on the surface layer by cross-linking the polyether multi-element copolymer. Each evaluation result is shown in Table 2. The uniform diffusion layer was arranged so as to be in contact with the negative electrode.

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

[Example 3]
4,4′-diaminodiphenyl ether (manufactured by Tokyo Chemical Industry Co., Ltd.) was dissolved as a diamine in dehydrated N-methyl-2-pyrrolidone (NMP, manufactured by Mitsubishi Chemical Corp.) under a stream of nitrogen and cooled to 30° C. or less. To this, 2-chloroterephthaloyl chloride (manufactured by Nippon Light Metal Co., Ltd.) corresponding to 99 mol% of the total amount of diamine was added over 30 minutes while the inside of the system was maintained at 30°C or lower under a nitrogen stream. After the total amount was added, the aromatic polyamide (A) was polymerized by stirring for about 2 hours. The resulting polymerization solution was neutralized with 97 mol% lithium carbonate (manufactured by Honjo Chemical Co., Ltd.) and 6 mol% diethanolamine (manufactured by Tokyo Kasei Co., Ltd.) with respect to the total amount of acid chloride to obtain aromatic polyamide (A). A solution was obtained.

The obtained aromatic polyamide solution was coated on a stainless steel (SUS316) belt as a support in the form of a film using a bar coater. After drying until the film was self-supporting, the film was peeled from the support. Then, the solution was introduced into a water bath at 60° C. to extract the solvent, neutralized salts, and the like. The stretching from peeling to after water bathing was 1.1 times in the longitudinal direction (MD) of the film, and was not gripped in the width direction (TD). Subsequently, the resulting hydrous film was subjected to heat treatment for 2 minutes in a tenter chamber at a temperature of 280° C. while being stretched 1.15 times in the TD at a fixed length to form a uniform diffusion layer with a thickness of 4 μm. Obtained.

The resulting uniform diffusion layer was placed on the release surface of the stainless steel support and the substrate A1 to obtain a separator having a uniform diffusion layer. At this time, the Ra of the uniform diffusion layer was the surface opposite to the peeled surface from the stainless steel support, and this surface was arranged so as to be in contact with the negative electrode. Each evaluation result is shown in Table 2.

[Example 4]
Plate-like boehmite (average particle size 1.0 μm) 96.0 parts by mass, acrylic polymer latex (solid concentration 40%, average particle size 145 nm, Tg = -10°C) 4.0 parts by mass, and polycarboxylic acid A coating liquid was prepared by uniformly dispersing 1.0 part by mass of an aqueous ammonium solution (SN Dispersant 5468, manufactured by San Nopco) in 100 parts by mass of water, and the coating liquid was coated on the substrate A1 to a thickness of 4 μm. Next, a uniform diffusion layer was formed in the same manner as in Example 2 on the surface opposite to the boehmite-coated surface. Each evaluation result is shown in Table 2. The uniform diffusion layer was arranged so as to be in contact with the negative electrode.

[Comparative Example 1]
Table 2 shows each evaluation result of the base material A1 as a separator having no uniform diffusion layer.

[Comparative Example 2]
A separator having a uniform diffusion layer on the surface layer was prepared in the same manner as in Example 1, except that 21.6 g of toluene was used in place of water to prepare the paint, and the base material A1 was not subjected to corona treatment. got Each evaluation result is shown in Table 2.

[Comparative Example 3]
A separator having a uniform diffusion layer on the surface layer was obtained in the same manner as in Example 4 except that the drying conditions after application of the aromatic polyamide solution in Example 3 were changed only to 120°C. Each evaluation result is shown in Table 2.

Claims (9)

A power storage device separator comprising a microporous membrane,
Having a uniform diffusion layer positioned at least on the surface layer of the electricity storage device separator,
An electrical storage device separator, wherein in the surface layer, the uniform diffusion layer has an arithmetic mean roughness (Ra) of 1.20 µm or less. After impregnating the electricity storage device separator in an electrolytic solution of ethylene carbonate (EC)/diethyl carbonate (DEC)=1/2 (volume ratio) containing 1 mol/L of LiPF ₆ for one day, the uniform diffusion layer was formed. 2. The electricity storage device separator according to claim 1, wherein the separator has a peel strength of 2 N/m or more when pressure-bonded to lithium (Li) metal at 25[deg.] C. and 0.5 MPa. 3. The power storage device according to claim 1, wherein the uniform diffusion layer swells twice or more in an electrolytic solution containing 1 mol/L of LiPF ₆ with EC/DEC=1/2 (volume ratio). separator. The electricity storage device separator according to any one of claims 1 to 3, which has an inorganic filler layer on at least one side of said microporous membrane. The electricity storage device separator according to any one of claims 1 to 4, wherein said uniform diffusion layer is made of an ion conductive material. The electricity storage device separator according to any one of claims 1 to 5, wherein said uniform diffusion layer is non-porous. An electricity storage device comprising a positive electrode, a negative electrode, an electrolytic solution, and the separator for an electricity storage device according to any one of claims 1 to 6. The electricity storage device according to claim 7, wherein a peel strength between said negative electrode and said electricity storage device separator is 2 N/m or more. The electricity storage device according to claim 7 or 8, wherein the negative electrode is Li metal or copper (Cu) foil.