JP2022181111A - Power storage device and separator for the power storage device - Google Patents

Abstract

To provide a polyolefin microporous film capable of suppressing deterioration due to liquid shortage of a lithium ion battery, a lithium ion battery separator including them, and a lithium ion battery.SOLUTION: The present invention provides a polyolefin microporous film that in a liquid-absorptive test of a propylene carbonate, a liquid height of a longitudinal direction (MD) and a width direction (TD) is 1.0 mm or more and 50 mm or less, a leading extension elastic modulus of the MD and the TD is 3000 kgf/cm2 or large.SELECTED DRAWING: None

Description

TECHNICAL FIELD The present invention relates to a polyolefin microporous membrane, a lithium ion battery separator, and a lithium ion battery.

Polyolefin microporous membranes (hereinafter sometimes abbreviated simply as "PO microporous membranes") are widely used for separating various substances, or as selective permeation 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, PO microporous membranes are suitably used as separators for lithium ion batteries that are widely used in mobile phones, smart phones, wearable devices, notebook personal computers (PCs), tablet PCs, digital cameras, and the like.

Conventionally, in a lithium-ion battery, an electrolyte is impregnated in a power generating element in which a separator is interposed between a positive electrode plate and a negative electrode plate. The separator separates the positive electrode and the negative electrode and can function as a membrane through which electrolyte or ions permeate. It has been proposed to improve battery characteristics such as cycle characteristics, energy density, energy capacity, and charge/discharge characteristics and suppress deterioration of battery characteristics by improving separators containing PO microporous films or negative electrodes (Patent document 1-3).

For example, Patent Document 1 provides a non-aqueous electrolyte secondary battery that has excellent wettability between the separator and the non-aqueous electrolyte, and has excellent battery capacity and charge-discharge characteristics by using a hydrophilic or hydrophilic-treated separator. It is described that Patent Document 2 discusses the white index (whiteness) and diethyl carbonate permeability of a separator containing a PO microporous membrane from the viewpoint of suppressing the deterioration of the battery characteristics of a secondary battery.

For example, in Patent Document 3, the surface of the negative electrode by the negative electrode active layer including a first layer containing Li metal and a second layer consisting of a temporary protective metal that can be alloyed with Li metal or can diffuse into Li metal Treatments have been proposed to facilitate battery fabrication and improve battery cycle life and energy density.

JP-A-6-283153 WO2018/078707 Japanese Patent Publication No. 2003-515893

In a lithium-ion secondary battery, lithium (Li) is deposited on the negative electrode plate under conditions such as charging with a relatively large current at a relatively low temperature, and metal Li grows as dendrites (dendritic crystals) from the negative electrode plate. I have something to do. In addition, the dissolution and deposition reaction of Li ions is repeated in the battery, and each time a new electrode surface is formed, so the reductive decomposition of the electrolytic solution may become intense. As the reductive decomposition of the electrolyte and the formation of dendrites progress during the charging and discharging cycles of the battery, the electrolyte in the battery is consumed and the electrolyte is depleted, which tends to cause not only short circuits or heat generation but also depletion of the electrolyte. be. This tendency is remarkable in a battery system with a charging/discharging mechanism in which Li is likely to deposit, or in a battery system with a negative electrode capable of increasing the capacity.

However, the separators described in Patent Documents 1 and 2 and the negative electrode described in Patent Document 3 do not focus on the liquid drying of the battery due to the formation of Li dendrites, but further improvements in suppressing battery deterioration due to liquid drying. There is room for

Accordingly, an object of the present invention is to provide a polyolefin microporous membrane capable of suppressing deterioration of a lithium ion battery due to dryness of liquid, and a lithium ion battery separator and a lithium ion battery including the same.

The present inventor clarified the mechanism of liquid drying of lithium ion batteries due to the formation of lithium (Li) dendrites, and found that the liquid absorption height and tensile elastic modulus in the liquid absorption test of polyolefin microporous membranes or lithium ion battery separators The inventors have found that the above problems can be solved by specifying the above, and have completed the present invention. Examples of embodiments of the invention are listed below.
<1> Liquid absorption height in the longitudinal direction (MD) and the width direction (TD) in the liquid absorption test of propylene carbonate is 1.0 mm or more and 50 mm or less,
The tensile elastic modulus in MD and TD is 3000 kgf/cm ² or more,
Polyolefin microporous membrane.
<2> The polyolefin microporous membrane according to item 1, having a porous layer having a number of pores calculated by a gas-liquid method of 70/μm ² or more and 500/μm ² or less.
<3> The polyolefin according to item 2, wherein the porous layer has a pore diameter of 98 nm or less and a total pore volume of 20% or more and 85% or less of the total pore volume measured by a nitrogen gas adsorption test. Microporous membrane.
<4> The polyolefin microporous membrane has a compressibility of 35% or less and a porosity after compression of 35% or more in a compression test under conditions of a temperature of 25° C., a pressure of 10 MPa, and a compression time of 3 minutes. The polyolefin microporous membrane according to any one of items 1 to 3.
<5> Items 1 to 4, wherein the main component of the polyolefin microporous membrane is polyethylene, and the crystal length period of the polyethylene measured by a small angle X-ray scattering (SAXS) method is 30.0 nm or more. The polyolefin microporous membrane according to any one of items 1 and 2.
<6> Any of items 1 to 5, wherein the main component of the polyolefin microporous membrane is polyethylene, and the crystallite size of the polyethylene measured by wide-angle X-ray scattering (WAXS) is 35.0 nm or less. 1. The polyolefin microporous membrane according to claim 1.
<7> A polyolefin microporous membrane having a liquid absorption height of 1.0 mm or more and 50 mm or less in MD and TD in a propylene carbonate liquid absorption test and a tensile modulus of elasticity in MD and TD of 3000 kgf/cm ² or more is used as a separator. Lithium-ion battery provided as.
<8> The lithium ion battery according to item 7, wherein an ion conductive layer is present between the separator and the metal negative electrode.
<9> The lithium ion battery according to item 7 or 8, wherein the ion conductive layer is made of an inorganic substance.
<10> 1 to 4 below:
1. Li ion concentration is 1.3 mol / L or more;
2. containing an electrolyte salt having a bisfluorosulfonylimide structure;
3. 4. including carbonates in which hydrogen atoms are replaced with halogen atoms; the electrolyte is an ionic liquid;
10. The lithium ion battery according to any one of items 7 to 9, comprising an electrolyte solution characterized by at least one of
<11> The lithium ion battery according to any one of Items 7 to 10, comprising the polyolefin microporous membrane according to any one of Items 1 to 6 as the separator.

ADVANTAGE OF THE INVENTION According to this invention, the polyolefin microporous membrane which can suppress the deterioration by dryness of a lithium ion battery, and the separator for lithium ion batteries and lithium ion battery containing the same can be provided.

Hereinafter, the mode for carrying out the present invention (hereinafter sometimes abbreviated as "embodiment") will be described in detail, but the present invention is not limited to this, and various deformation is 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. In this specification, the upper and lower limits of each numerical range can be arbitrarily combined. Moreover, containing a specific component as a main component means that the content of the specific component is 50% by mass or more.

<Polyolefin microporous membrane>
In one aspect of the present invention, a polyolefin microporous membrane (PO microporous membrane) is provided. The PO microporous membrane contains polyolefin resin as a main component and can exhibit excellent electrical insulation and ion permeability. It can be used as a separator substrate.

The polyolefin microporous membrane according to the first embodiment of the present invention has the following features:
In a propylene carbonate liquid absorption test, the MD and TD liquid absorption heights are 1.0 mm or more and 50 mm or less; and the MD and TD tensile elastic moduli are 3000 kgf/cm ² or more.

The polyolefin microporous membrane characterized by the above tends to be able to suppress deterioration of the lithium ion battery due to drying up of the electrolyte when used as a separator for a lithium ion battery. This tendency is remarkable in lithium-ion batteries with a charge-discharge mechanism that facilitates deposition of Li or a negative electrode capable of increasing the capacity, and thus the battery characteristics of lithium-ion batteries containing a negative electrode made of Li metal can be easily improved. to

According to the present invention, the following two patterns were found as the mechanisms by which the liquid dry-up of lithium-ion batteries is likely to occur due to the formation of Li dendrites:
(a) Li dendrites deposit irregularities on the surface of the negative electrode, particularly on the surface of the Li metal negative electrode, which increases the specific surface area of the negative electrode and increases the consumption of the electrolytic solution due to reductive decomposition.
(a) Li dendrites formed on the surface of the negative electrode cause compressive deformation of the separator located between the positive and negative electrodes, causing the separator to spit out the electrolyte (that is, the electrolyte impregnated in the separator is spitting out).

In the first embodiment, based on the above mechanisms (a) and (b), from the viewpoint of suppressing the liquid drying of the lithium ion battery, the PO microporous membrane that can be used as a separator has an affinity with the electrolyte. , and to improve compression resistance.

From the viewpoint of improving affinity with the electrolyte, the PO microporous membrane has a liquid absorption height of 1.0 mm or more and 50 mm or less in MD and TD in a liquid absorption test of propylene carbonate. When the PO microporous membrane, whose liquid absorption height is controlled within the numerical range of 1.0 mm or more and 50 mm or less, is included in a lithium ion battery as a separator, the electrolyte decreases due to consumption due to reductive decomposition of the electrolyte and compression deformation. However, since the electrolyte flows smoothly, it is considered that a suitable amount of the electrolyte is retained in the pores of the separator, resulting in excellent cycle characteristics.

The liquid absorption test of propylene carbonate can be performed by the method described in Examples. From the viewpoint of further improving the affinity between the PO microporous membrane and the electrolytic solution, the liquid absorption height is preferably in the range of 1.0 mm or more and 50 mm or less for each of MD and TD, and is preferably 2.0 mm. It is more preferably in the range of 45 mm or less, more preferably 3.0 mm or more and 40 mm or less, and particularly preferably 4.0 mm or more and 30 mm or less.

A lithium-ion battery may change its volume, and the electrolyte held in the pores of the separator flows. When the contact angle to propylene carbonate is within the above numerical range, the electrolytic solution flows smoothly in response to the volume change as described above, so that a suitable amount of electrolytic solution is retained in the pores of the separator. Therefore, it is considered that the cycle characteristics are excellent.

The liquid absorption height in the propylene carbonate liquid absorption test of the PO microporous membrane depends on, for example, the content of inorganic fillers or inorganic particles in the PO microporous membrane, and the combination of polyolefin (PO) and polar groups in the production process of the PO microporous membrane. It can be adjusted within the above numerical range by mixing a polymer having the above, surface treatment of the PO microporous membrane, or the like.

The PO microporous membrane has a tensile elastic modulus of 3000 kgf/cm ² or more in MD and TD from the viewpoint of improving compression resistance. When the PO microporous membrane whose MD and TD tensile elastic moduli are controlled within the above numerical ranges is placed between the positive and negative electrodes of a lithium ion battery as a separator, even if Li dendrites are deposited on the electrode surface, it is difficult to collapse. , it is difficult to be compressed and deformed, and it is possible to suppress the discharge of the electrolytic solution.

The tensile modulus of the PO microporous membrane can be measured by the method described in Examples. From the viewpoint of further improving the compression resistance of the PO microporous membrane, the MD and TD tensile elastic moduli of the PO microporous membrane should be in the range of 3,000 kgf/cm ² or more and 15,000 kgf/cm ² or less. is preferably in the range of 3,200 kgf/cm ² or more and 12,000 kgf/cm ² or less, and more preferably in the range of 3,500 kgf/cm ² or more and 10,000 kgf/cm ² or less. , 3,800 kgf/cm ² or more and 9,000 kgf/cm ² or less.

The tensile elastic modulus of the PO microporous membrane is determined by, for example, the selection of the raw material composition, the ratio of the plasticizer, the biaxial stretching temperature, the biaxial stretching ratio, the heat setting temperature, the stretching ratio during heat setting, and the relaxation rate during heat setting. The control can be adjusted within the above numerical range.

From the viewpoint of further improving compression resistance, the PO microporous membrane preferably has a compressibility of 35% or less in a compression test under conditions of a temperature of 25° C., a pressure of 10 MPa, and a compression time of 3 minutes. , more preferably in the range of 3% or more and 34% or less, more preferably in the range of 5% or more and 33% or less, even more preferably in the range of 7% or more and 32% or less, It is particularly preferably within the range of 10% or more and 31% or less, and most preferably within the range of 11% or more and 30% or less. From the same point of view, the PO microporous membrane preferably has a post-compression porosity of 35% or more, preferably 36% or more, in a compression test under conditions of a temperature of 25°C, a pressure of 10 MPa, and a compression time of 3 minutes. is more preferably 37% or more, even more preferably 38% or more, even more preferably 39% or more, and particularly preferably 40% or more.

If desired, an ion-conducting layer, an inorganic coating layer, or an organic (adhesive) layer may be formed on the surface of the PO microporous membrane.

If desired, multiple PO microporous membranes can be laminated, and one PO microporous membrane can be laminated with another porous membrane.

Preferred properties and constituent elements of the PO microporous membrane are described below.

(Pore ratio measured by nitrogen gas adsorption test)
The PO microporous membrane preferably has a one-point method total pore volume with a pore diameter of 98 nm or less measured by a nitrogen gas adsorption test of 20% or more and 85% or less of the total pore volume. In addition, when the PO microporous membrane has a porous layer, the total pore volume of the single-point method of 98 nm or less in pore diameter measured by a nitrogen gas adsorption test of the porous layer is 20% or more and 85% of the total pore volume. It is also preferable that:

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, 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 smaller the pores of the microporous membrane. The single-point method total pore volume (pore diameter 98 nm or less) from the viewpoint that the microporous membrane becomes more dense and the growth of lithium dendrites and dendrites derived from foreign substances accompanying the elution of metallic foreign substances can be efficiently delayed. is preferably 20% or more, more preferably 25% or more, and even more preferably 30% or more of the total pore volume. In addition, in the nitrogen gas adsorption test of the microporous membrane or 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 or porous layer exceeds 85% of the total pore volume, clogging of the separator causes a point where the resistance increase concentrates, resulting in It leads to promotion of dendrite generation and deterioration of the capacity maintenance cycle. 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.

The ratio of the single-point method 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. In the production process of, 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 areal ratio 50 times or more and / or stretching temperature 128 ° C. or less Stretching under the conditions of; adjusting the draw ratio in each of the vertical and horizontal directions in the stretching step to 5 times or more; By 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 0.95 times, etc., alone or in combination as appropriate. , can be controlled to lie within the range of pore volume percentages 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 30% by mass or more, and is 40% by mass or more. is more preferable, and 50% by mass or more is even more preferable. This content can be 100% by weight. 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 number of pores B of the PO microporous membrane calculated by the gas-liquid method is preferably 70/μm ² or more and preferably 500/μm ² or less. Further, when the PO microporous membrane has a porous layer, the number of pores of the porous layer calculated by the gas-liquid method is preferably 70/μm ² or more and 500/μm ² or less.

The number of pores per cross section (B) calculated by the gas-liquid method effectively suppresses local dendrite generation by diffusing lithium ions and metal ions associated with elution of foreign matter in the microporous membrane or porous layer. 70/μm ² or more is preferable, 80/μm ² or more is more preferable, and 90/μm It is more preferably ² or more, even more preferably 100/μm ² or more, and particularly preferably 110/μm ² or more. In addition, the number of pores (B) is preferably 500/μm ² or less, and 350/μm ² or less, from the viewpoint that the growth of dendrites can be suppressed by the high strength of the microporous membrane or porous layer. is more preferable. 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 PO microporous membrane preferably has a porosity ε of 42% or more. The porosity (ε) may hereinafter be referred to as “porosity (before compression)”.

The porosity (ε) of the PO microporous membrane reduces the membrane resistance and improves the ion diffusivity, thereby effectively suppressing the generation of dendrites, achieving a certain level of membrane strength and low air permeability. and from the viewpoint of having high ionic conductivity and high output characteristics, it is preferably 42% or more, more preferably 44% or more, even more preferably 47% or more, and 50% or more. is particularly preferred. 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 PO microporous membrane preferably has an average flow diameter d of 0.020 μm or more and preferably 0.070 μm or less as measured by the half-dry method. As used herein, the pore diameter of the PO microporous membrane means the average flow diameter measured using a perm porometer (Porous Materials, Inc.: CFP-1500AE) according to the half-dry method.

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. The average flow diameter d measured by the half-dry method is from the viewpoint that the densification of the microporous membrane can physically suppress the growth of dendrites, and the number of pores per cross section (B) increases to improve ion diffusivity. From the viewpoint of The following are particularly preferred. In addition, from the viewpoint of preventing the formation of dendrites from being promoted as a result of the occurrence of places where resistance increases due to clogging, and from the viewpoint of improving cycle characteristics, the average flow diameter is preferably 0.020 μm or more. It is more preferably 0.025 μm or more.

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

(film thickness)
The film thickness of the PO microporous membrane is determined from the viewpoint of lithium ion battery capacity, from the viewpoint of high ion permeability and good rate characteristics, from the viewpoint of retarding the growth of dendrites, and from the viewpoint of the separator when used for high capacity batteries. From the viewpoint of reducing the occupied volume and contributing to the improvement of the battery capacity, it is preferably 20 μm or less, more preferably 18 μm or less, still more preferably 16 μm or less, and even more preferably 1 μm to 14 μm, Particularly preferred is between 3 μm and 13 μm, most preferred between 5 μm and 12 μm. The film thickness of the PO microporous membrane may be referred to as "average film thickness (before compression)" below.

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

(air permeability)
The air permeability of the PO microporous membrane is preferably 200 seconds or less, more preferably 190 seconds or less per 100 cm ³ of air, from the viewpoint of the output characteristics and cycle characteristics of the lithium ion battery. From the viewpoint of film strength, the lower limit of the air permeability is preferably 20 seconds or more per 100 cm ³ of air.

(weight conversion conversion piercing strength, piercing strength, basis weight)
It is preferable that the PO microporous membrane 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 PO microporous membrane used as the material has a high porosity and a low air permeability, it is difficult to rupture, which tends to improve the safety of the lithium ion battery. 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 a lithium ion battery. more pronounced when used. From this point of view, the PO microporous membrane has a per unit area equivalent puncture strength of preferably 40 gf/(g/m ² ) to 150 gf/(g/m ² ), more preferably 50 gf/(g/m ² ) to More preferably 140 gf/(g/m ² ), even more preferably 55 gf/(g/m ² ) to 130 gf/(g/m ² ), even more preferably 70 gf/(g/m ² ) to 120 gf /(g/m ² ) is particularly preferred.

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

From the same viewpoint as above, from the viewpoint of safety when impact is applied to the lithium-ion battery, and from the separator membrane breakage due to foreign substances that are unintentionally mixed into the lithium-ion battery From the viewpoint of suppression, the puncture strength of the PO microporous membrane is preferably 200 gf or more, more preferably 220 gf or more, still more preferably 250 gf or more, and even more preferably 280 gf or more. , 300 gf or more. The upper limit of the puncture strength is not particularly limited, but can be determined according to the crystallinity of the film, the heat shrinkability of the film, and the suppressed electrical resistance. Or it may be 680 gf or less.

The puncture strength and per unit area equivalent puncture strength of the PO microporous membrane are determined, for example, in the production process of the PO microporous membrane, depending on the molecular weight and compounding ratio of the polymer raw material such as polyolefin, the draw ratio during the biaxial stretching process, and the biaxial stretching process. It can be adjusted within the numerical range described above by controlling the MD/TD stretching temperature at the time, the heat setting (HS) ratio, and the like.

(Component)
PO microporous membranes include, for example, porous membranes containing polyolefin resin; Porous membranes also containing resins such as polyimideamide, polyaramid, nylon, and polytetrafluoroethylene; woven fabrics of polyolefin fibers; and non-woven fabrics of polyolefin fibers. Among these, a microporous membrane containing a polyolefin resin is preferable, and a microporous membrane containing polyethylene as a main component is more preferable, from the viewpoints of reduction or increase suppression of the electrical resistance of the membrane, compression resistance of the membrane, and structural uniformity. .

(Ingredients)
The PO microporous membrane is formed from a resin composition containing a polyolefin resin. If desired, the resin composition may further contain inorganic fillers or inorganic particles, resins other than polyolefins, and the like.

The PO 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 lithium ion battery. is preferably 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.

In addition, the polyolefin resin contained in the PO microporous membrane preferably has a melting point of 120° C. or higher and 150° C. or lower, more preferably 125° C. or higher and 140° C. or lower, from the viewpoint of making the membrane rigid and improving compression resistance. and/or the DSC 1st peak temperature is preferably within the range of 136°C to 144°C.

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.

From the viewpoint of the crystallinity, high strength, compression resistance, etc. of the PO microporous membrane, and from the viewpoint of expressing fuse properties, polyethylene occupies 50% by mass or more and 100% by mass or less of the resin component constituting the microporous membrane. A microporous membrane formed by the composition is 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.

WAXS measurements of polyolefin microporous membranes are detailed in the Examples, and the crystallite sizes obtained by WAXS measurements indicate the structural uniformity and compression resistance of the membranes and the reaction uniformity in non-aqueous secondary batteries. It is thought to be related to the structure of polyethylene which improves. When the main component of the PO microporous membrane is polyethylene (PE), the polyethylene contained as the main component in the PO microporous membrane has a crystal MDND plane (110) crystallite size of 35.0 nm or less. preferable. The MDND plane (110) crystallite size of polyethylene can be measured by X-ray diffraction (XRD) or wide angle X-ray scattering (WAXS) methods, as detailed in the Examples.

When the MDND plane (110) crystallite size of polyethylene is 35.0 nm or less, the polyolefin microporous membrane containing the polyethylene tends to be rigid, and the compression resistance of the membrane tends to be improved. Even if the volume of the electrode changes during use of the secondary battery, both high output and high cycle characteristics can be achieved. From this point of view, the MDND plane (110) crystallite size of polyethylene, which is the main component of the film, is more preferably 33.0 nm or less, and still more preferably 10.0 nm to 32.0 nm, or 15.0 nm. ~31.0 nm, or 15.0 nm to 30.0 nm, or 15.0 nm to 25.0 nm. In particular, when the MDND plane (110) crystallite size of polyethylene is 25.0 nm or less, the rigidity of the film is enhanced and the compression resistance is improved.

The MDND plane (110) crystallite size of polyethylene, which is the main component of the polyolefin microporous membrane, is determined by, for example, the molecular weight of the polyolefin raw material, the molecular weight of the polyethylene raw material, and the draw ratio during the biaxial stretching process in the manufacturing process of the polyolefin microporous membrane. , by controlling the draw ratio and temperature during the biaxial stretching process, the draw ratio and temperature during the biaxial stretching process, etc., so that the values can be adjusted within the above-described numerical ranges.

While not wishing to be bound by theory, it is believed that the crystal length period of the polyolefin microporous membrane correlates with the post-compression porosity of the membrane. From the viewpoint described above, the crystal long period of the polyolefin microporous membrane is 30.0 nm or more, or 30.0 nm to 60.0 nm, or 31.0 nm to 55.0 nm, or 32.0 nm to 50.0 nm, or 33.0 nm to 50.0 nm, or 35.0 nm to 50.0 nm, or 37.0 nm to 50.0 nm.

The crystal long period of the polyolefin microporous membrane can be measured, for example, by a small angle X-ray scattering (SAXS) method. By controlling the draw ratio during the stretching process, the draw ratio and temperature during the biaxial stretching process, the stretch ratio and temperature during the biaxial stretching process, etc., it can be adjusted within the numerical range described above. .

In addition, from the viewpoint of high strength, compression resistance, suppression of electrical resistance, etc. when a PO microporous membrane 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. 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 95% by mass or less. More preferred. 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 PO 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 further It is preferably 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 PO microporous membrane contains polyethylene (PE), the Mv of at least one type of PE is preferably 600,000 or more, preferably 700,000 or more, from the viewpoint of film orientation and rigidity. More preferably, the Mv upper limit of PE may be, for example, 2,000,000 or less. From the standpoint of reduced fluidity when the film 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 PO microporous film should be 30% by mass or more. is preferably 50% by mass or more, more preferably 60% by mass or more, particularly preferably 70% by mass or more, and may be 100% by mass.

In another aspect, the PO microporous membrane (PO microporous layer) contains 45% by mass or more of a PE raw material having Mv of 500,000 or more and 6,000,000 or less based on the mass of the PO microporous membrane. 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 PO microporous layer contains 45% by mass or more of the PE raw material having an Mv of 500,000 or more and 6,000,000 or less, the crystals are highly oriented during stretching, and the PO microporous layer has a small pore size or a dense structure. tend to become From the same point of view, the PO raw material used for producing the PO microporous layer or PO microporous membrane is preferably an ethylene homopolymer, and an ethylene homopolymer having a Mv of 500,000 or more and 6,000,000 or less. is more preferable.

The content of the polyolefin resin contained in the PO microporous membrane (PO microporous layer) is 50% by mass or more, preferably 60% by mass or more, based on the mass of the PO microporous membrane (PO microporous layer). is preferably 70% by mass or more, preferably 80% by mass or more, and may be 90% by mass or more or 100% by mass 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 a lithium ion battery 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 lithium ion batteries 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 50% by mass or more of at least one selected from ultrahigh molecular weight polyethylenes having a density of 0.930 g/cm ³ or more and 0.942 g/cm ³ or more and a density of 0.930 g/cm 3 or more and less than 0.942 g/cm 3 , based on the mass of the PO microporous membrane It is preferably contained, more preferably 60% by mass or more, even 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.

In the above resin composition, if necessary, inorganic fillers or inorganic particles; resins other than polyolefin having a polar group; antioxidants such as phenol-based, phosphorus-based, or sulfur-based; Metal soaps; Various known additives such as ultraviolet absorbers, light stabilizers, antistatic agents, antifogging agents, and coloring pigments may be mixed.

The inorganic filler for the PO microporous membrane is not particularly limited, and examples thereof include oxide-based ceramics such as alumina, silica (silicon oxide), titania, zirconia, magnesia, ceria, yttria, zinc oxide, and iron oxide; silicon nitride; , titanium nitride, boron nitride and other nitride ceramics; silicon carbide, calcium carbonate, aluminum sulfate, barium sulfate, aluminum hydroxide, potassium titanate, talc, kaolin clay, kaolinite, halloysite, pyrophyllite, montmorillonite, celery ceramics such as site, mica, amesite, bentonite, asbestos, zeolite, calcium silicate, magnesium silicate, diatomaceous earth, silica sand; and glass fiber. These are used individually by 1 type or in combination of 2 or more types. Among these, at least one selected from the group consisting of silica, alumina, and barium sulfate is preferable as the inorganic particles from the viewpoint of electrochemical stability.

When the PO microporous membrane contains an inorganic filler, it is preferably 1% by mass or more and 50% by mass or less, more preferably 3% by mass or more and 30% by mass or less, further preferably 5% by mass or more and 20% by mass or less, and 10% by mass. More than 18% by mass or less is particularly preferable. When the inorganic filler is contained in an amount of 1% by mass or more, the affinity with a highly polar electrolytic solution is increased, and the liquid absorbency of propylene carbonate can be improved. When the inorganic filler is 50% by mass or less, it is possible to prevent excessive enlargement of the pore size of the PO microporous membrane and suppress overcharge during charging.

The resin having a polar group other than polyolefin is not particularly limited, but olefin copolymers such as ethylene/vinyl acetate copolymer and ethylene/acrylic acid/maleic anhydride copolymer; Polyesters such as phthalates; Polyvinyl acetates; Polyvinyl alcohols; Polyvinyl acetals; Polyvinyl butyrals; Acrylonitonol copolymers such as sterene copolymers; 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 nylon 66; gelatin; gum arabic; polycarbonates, polyester carbonates; cellulose resins; Alkyd resins; Melamine resins; Polyurethanes; Diaryl phthalate resins; Polyphenylene oxides; Polyphenylene sulfide polysulfones; amides; polyethersulfones; polyetherketones; polyetherimides and the like. From the viewpoint of affinity with the electrolytic solution, fluorine-containing resins, polycarbonates, polyketones, polyethers, and polyacrylonitriles are preferred.

When the PO microporous membrane contains a polymer having a polar group, it is preferably 1% by mass or more and 50% by mass or less, more preferably 3% by mass or more and 35% by mass or less, even more preferably 5% by mass or more and 30% by mass or less. More than mass % and below 23 mass % are especially preferable. By containing 1% by mass or more of a polymer having a polar group, the affinity with a highly polar electrolytic solution is increased, and the liquid absorbency of propylene carbonate can be improved. When the polymer having a polar group is 50% by mass or less, it is possible to impart appropriate strength to the PO microporous membrane.

(Method for producing PO microporous membrane)
The method for producing the PO microporous membrane is not particularly limited, but for example:
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 a lithium ion battery, the PO microporous film production method described above makes it possible to achieve a high degree of compatibility between the ability to suppress lithium dendrites and the ability to suppress chemical micro-shorts due to metallic foreign matter, as well as high output characteristics and cycle characteristics. A PO microporous membrane can be provided. 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. 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, resins or polymers other than polyolefin resins; inorganic fillers or inorganic particles; antioxidants such as phenolic compounds, phosphorus compounds and sulfur compounds; calcium stearate and zinc stearate. UV absorbers; light stabilizers; antistatic agents; antifogging agents;

From the viewpoint of adjusting the MD and TD liquid absorption heights in the propylene carbonate liquid absorption test of the resulting PO microporous membrane within the above numerical ranges, inorganic fillers such as silica and barium sulfate, and/or polyketones , maleic anhydride-modified polyolefin, polycarbonate, polyethylene oxide and the like are preferably mixed with the resin composition.

The total amount of these additives added is preferably more than 0 parts by mass and 50 parts by mass or less, more preferably 40 parts by mass or less, and even more preferably 35 parts by mass or less, relative 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.

Further, in the method for producing the PO microporous membrane, from the viewpoint of controlling the liquid absorption height in the propylene carbonate liquid absorption test described above, the surface of the PO microporous membrane is irradiated with an electron beam or plasma. , F ₂ O ₂ gas treatment, corona discharge treatment, UV treatment, UV ozone treatment, sulfurous acid gas treatment, oxidation treatment with potassium dichromate solution or potassium permanganate solution, etching treatment with sodium treatment solution, surface activation A surface treatment step of applying an agent, surface treatment such as chemical modification, and the like may also be included. Among them, F ₂ O ₂ gas treatment or plasma irradiation is preferable from the viewpoint of excellent treatment up to the inside of the PO microporous membrane in the thickness direction. 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; Resins having a melting point and/or a glass transition temperature of 180° C. or higher, such as polyamides and polyesters, may be mentioned.

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.

<Separator for Lithium Ion Battery>
The lithium ion battery separator may be a single layer separator consisting of the PO microporous membrane described above, or a laminated separator comprising a substrate such as a PO microporous membrane and at least one layer disposed on the substrate. OK. 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.

The separator is provided on at least a part of at least one side of the PO microporous membrane from the viewpoint of improving the Li ion diffusibility in the lithium ion battery, flattening the Li deposition on the electrode surface, and suppressing the battery from drying up. It preferably has an ion-conducting layer disposed thereon. The ion-conducting layer may be organic or inorganic, but more preferably inorganic. The configuration and formation method of the ion-conducting layer made of inorganic material may be the same as those of the inorganic coating layer described above.

In addition to the inorganic coating layers described above, materials for the ion-conducting layer made of inorganic substances include lithium silicate, lithium borate, lithium aluminate, lithium phosphate, lithium nitride, phosphorus-lithium oxynitride, and siliculfide. selected from the group consisting of lithium, lithium germano sulfide, lithium lanthanum oxide, lithium tantalum oxide, lithium niobium oxide, lithium titanium oxide, lithium fluoride, lithium borosulfide, lithium aluminosulfide, lithium phosphorous sulfide, graphene oxide and combinations thereof; and glass.

Examples of ion-conducting layers made of organic substances are not particularly limited, but polyesters such as polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate; polyvinyl acetates; polyvinyl alcohols; polyvinyl acetals; polyvinyl butyrals; benzoxazole; acrylic resins; methacrylic resins such as polymethyl methacrylate; polyacrylonitriles; acrylonitrile copolymers such as acrylonitrile-butadiene-sterene copolymer; styrene copolymers such as sodium polystyrene sulfonate, ionic polymers such as sodium polyacrylate; acetal resins; polyamides such as nylon 66; gelatin; gum arabic; polycarbonates, polyester carbonates; polyketones; polyethers; phenolic resins; urea resins; epoxy resins; unsaturated polyester resins; alkyd resins; sulfones; polyphenylsulfones; polyimides; bismaleimide triazine resins; fluorine-containing resins; polyimideamides; From the viewpoint of affinity with the electrolytic solution, fluorine-containing resins, polycarbonates, polyketones, polyethers, and polyacrylonitriles are preferred.

In addition, from the viewpoint of flattening Li deposition on the electrode surface, the ion conductive layer of the separator is arranged so as to contact or face the negative electrode surface (or Li deposition surface) of the lithium ion battery, and the Li ion concentration distribution It is also preferable to eliminate and control the Li precipitation form (suppress Li dendrite formation).

<Lithium-ion battery>
In another aspect of the invention, a lithium ion battery is provided. Lithium ion batteries are generally composed of a positive electrode, a negative electrode, a liquid containing an electrolyte, a gel or a solid, a separator, and an outer casing. Examples of lithium ion batteries include non-aqueous electrolyte lithium ion secondary batteries, non-aqueous gel secondary batteries, non-aqueous solid secondary batteries, lithium ion capacitors, electric double layer capacitors, and the like.

A lithium ion battery may comprise a separator, a positive plate, a negative 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 lithium ion battery according to the second embodiment of the present invention has a liquid absorption height of 1.0 mm or more and 50 mm or less in MD and TD in a propylene carbonate liquid absorption test, and a tensile modulus of elasticity in MD and TD of 3000 kgf. /cm ² or more is provided as a separator. Moreover, in the lithium ion battery according to the second embodiment, the ion conductive layer is preferably present between the separator and the metal negative electrode. A lithium-ion battery characterized by the above can have a high capacity by including a metal negative electrode, and can suppress deterioration due to drying up of an electrolytic solution.

In the second embodiment, the affinity of the PO microporous membrane that can be used as a separator with the electrolyte is based on the above-described (a) and (b) as the mechanism by which the liquid drying of the lithium ion battery is likely to occur. In addition, the ion-conducting layer present between the separator and the metal negative electrode increases the Li ion diffusibility and planarizes the Li deposition.

The affinity of the PO microporous membrane that can be used as a separator with the electrolyte is specified by the liquid absorption height of MD and TD being 1.0 mm or more and 50 mm or less in a propylene carbonate liquid absorption test, Compression resistance is specified by having a tensile modulus of elasticity in MD and TD of 3000 kgf/cm ² or more. The separator according to the second embodiment may be a single-layer separator made of the PO microporous membrane according to the first embodiment, or a base material such as a PO microporous membrane, the inorganic coating layer described above, It may be a multi-layer separator comprising at least one layer such as an adhesive layer, an ion-conducting layer, or the like.

The ion-conducting layer present between the separator and the metal negative electrode is preferably made of an inorganic material from the viewpoint of further increasing Li ion diffusibility and planarizing Li deposition. The configuration and formation method of the ion-conducting layer according to the second embodiment may be the same as the ion-conducting layer that can be formed on the PO microporous membrane according to the first embodiment, as described above.

The positive electrode will be explained 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 metal negative electrode will be explained below. As the negative electrode active material, lithium (Li) metal, copper (Cu) foil, or a metal material capable of intercalating lithium can be used, and from the viewpoint of compatibility with the separator, Li metal is preferable. Further, in the second embodiment, the separator is provided with an ion-conducting layer, and the metal negative electrode may also be provided with an ion-conducting layer.

It is preferable to provide an ion-conducting layer on the surface of the negative electrode facing the separator. The ion-conducting layer may be organic or inorganic, but more preferably inorganic. The configuration of the ion-conducting layer includes, for example, the same ion-conducting layer as the ion-conducting layer on the separator described above, but alumina, titanium oxide, lithium phosphate, lithium nitride, lithium silicate, and the like are particularly preferable.

Any known method can be used as a method for providing an ion-conducting layer on the metal negative electrode. Examples include physical vapor deposition, chemical vapor deposition, extrusion, and electroplating. Examples of suitable physical or chemical vapor deposition methods include, but are not limited to, thermal evaporation (e.g., resistive heating, inductive heating, radiation heating, electron beam heating, etc.), sputtering (e.g., diode, DC magnetron, RF, RF magnetron, pulsed, dual magnetron, AC, MF, and reactive sputtering, etc.), plasma chemical vapor deposition, laser chemical vapor deposition, ion plating, cathodic arc, jet deposition, and laser ablation. Alternatively, a method of treating a metal negative electrode with a gas or liquid to form an ion-conducting layer through a chemical reaction, or a method of forming an ion-conducting layer through a reaction during battery charge/discharge may be used.

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.

In the second embodiment, the following 1 to 4:
1. Li ion concentration is 1.3 mol / L or more;
2. containing an electrolyte salt having a bisfluorosulfonylimide structure;
3. 4. including carbonates in which hydrogen atoms are replaced with halogen atoms; the electrolyte is an ionic liquid;
Preferred electrolytes are characterized by at least one of:

The Li ion concentration in the non-aqueous electrolyte is preferably 1.3 mol/L or more, more preferably 1.8 or more and 5.0 mol/L or less, and 2.8 or more and 3.5 mol/L or less. is more preferable. When the Li ion concentration is 1.3 mol/L or more, the amount of solvated solvent molecules increases, suppressing the generation of reduction products with high resistance, keeping the metal deposition surface flat, and reducing the specific surface area. to suppress the consumption of the electrolyte. When the Li ion concentration is 5.0 mol/L or less, sufficient ionic conductivity can be ensured.

Examples of electrolyte salts used in the non-aqueous electrolyte include LiCl, LiBr, LiI, _LiClO4 , _LiBF4 , _LiB10Cl10 , _LiPF6 , _{LiCF3SO3 , LiCF3CO2} , _{LiAsF6 , LiSbF6 ,} LiAlCl ₄ , _{CH3SO3Li , CF3SO3Li} , LiSCN, LiC( _{CF3SO2 ) 3} , ( _{CF3SO2 ) 2NLi} , ( _{FSO2 ) 2NLi} , lithium chloroborane, lower aliphatic carboxylic acid Lithium, tetraphenyllithium borate, lithium imide, lithium bistrifluoromethanesulfonylimide represented by (CF ₃ SO ₂ ) ₂ NLi (LiTFSI, lithium bis(trifluoromethane sulfonyl) imide) represented by (FSO ₂ ) ₂ NLi and lithium bis(fluorosulfonyl)imide (LiFSI). Among them, those having a bisfluorosulfonylimide structure such as lithium bistrifluoromethanesulfonylimide with high chemical stability are preferable from the viewpoint that it is difficult to generate voids in which an electrolytic solution that inhibits ion permeation is not present due to gas generation.

Examples of non-aqueous electrolyte solvents include carbonate solvents (e.g., propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, butylene carbonate, vinylene carbonate, ethyl methyl carbonate, methyl propyl carbonate, butyl methyl carbonate, ethyl propyl carbonate, butyl ethyl carbonate, , dipropyl carbonate, etc.), halogenated carbonate solvents (e.g., fluoroethylene carbonate, difluoroethylene carbonate, etc.) in which some or more of the hydrogen atoms of the carbonate solvent are substituted with halogen atoms, lactone solvents (e.g., γ-butyrolactone, etc.) , ester solvents (e.g., methyl acetate, ethyl acetate, etc.), linear ether solvents (e.g., 1,2-dimethoxyethane, dimethyl ether, diethyl ether, etc.), cyclic ether solvents (e.g., tetrahydrofuran, 2-methyltetrahydrofuran, dioxolane, 4-methyldioxolane, etc.), ketone solvents (e.g., cyclopentanone, etc.), sulfolane solvents (e.g., sulfolane, 3-methylsulfolane, 2,4-dimethylsulfolane, etc.), sulfoxide solvents (e.g., dimethylsulfoxide, etc.), nitriles Solvents (e.g., acetonitrile, propionitrile, benzonitrile, etc.), amide solvents (e.g., N,N-dimethylformamide, N,N-dimethylacetamide, etc.), urethane solvents (e.g., 3-methyl-1,3-oxazolidine -2-one, etc.), and aprotic solvents such as polyoxyalkylene glycol solvents (eg, diethylene glycol, etc.). Among them, reduction products with low resistance are likely to be generated, the metal deposition surface is kept flat by suppressing non-uniformity of ion permeation, and the consumption of the electrolyte is suppressed by reducing the specific surface area. The non-aqueous electrolyte solvent preferably contains halogenated carbonate solvents, and more preferably contains fluoroethylene carbonate or difluoroethylene carbonate.

When the nonaqueous electrolyte solvent contains a halogenated carbonate solvent, the content of the halogenated carbonate solvent is preferably 5% by volume or more and 50% by volume or less with respect to the total volume of the nonaqueous electrolyte solvent.

An ionic liquid can also be used as the non-aqueous electrolyte. Ionic liquids have high ionic conductivity, and are preferable from the viewpoint of suppressing consumption of the electrolyte by increasing the diffusivity of ions, thereby keeping the metal deposition surface flat by suppressing non-uniformity, and reducing the specific surface area. . Ionic liquids are composed of cations and anions.

Ionic liquids are classified into imidazolium-based, ammonium-based, pyrrolidinium-based, piperidinium-based, pyridinium-based, morpholinium-based, phosphonium-based, sulfonium-based, and the like, depending on the cation species. Examples of cations constituting imidazolium-based ionic liquids include alkylimidazolium cations such as 1-butyl-3-methylimidazolium (BMI).

Examples of cations constituting ammonium-based ionic liquids include alkylammonium cations such as N,N,N-trimethyl-N-propylammonium, in addition to tetraamylammonium and the like. Examples of cations constituting pyrrolidinium-based ionic liquids include alkylpyrrolidinium cations such as N-methyl-N-propylpyrrolidinium (Py13) and 1-butyl-1-methylpyrrolidinium. Cations constituting piperidinium-based ionic liquids include, for example, alkylpiperidinium cations such as N-methyl-N-propylpiperidinium (PP13) and 1-butyl-1-methylpiperidinium. Cations constituting pyridinium-based ionic liquids include, for example, alkylpyridinium cations such as 1-butylpyridinium and 1-butyl-4-methylpyridinium. Examples of cations constituting morpholinium-based ionic liquids include alkylmorpholinium such as 4-ethyl-4-methylmorpholinium. Examples of cations constituting phosphonium-based ionic liquids include alkylphosphonium cations such as tetrabutylphosphonium and tributylmethylphosphonium. Examples of cations constituting sulfonium-based ionic liquids include alkylsulfonium cations such as trimethylsulfonium and tributylsulfonium. Anions paired with these cations include, for example, bis(trifluoromethanesulfonyl)imide (TFSI), bis(fluorosulfonyl)imide, tetrafluoroborate (BF ₄ ), hexafluorophosphate (PF ₆ ), bis(penta fluoroethanesulfonyl)imide (BETI), trifluoromethanesulfonate (triflate), acetate, dimethylphosphate, dicyanamide, trifluoro(trifluoromethyl)borate, and the like. These ionic liquids may be used singly or in combination.

The electrolyte salt may be included in the non-aqueous solvent or ionic liquid. As the electrolyte salt, one that can be uniformly dispersed in the solvent can be used. A cation consisting of lithium and the anion described above can be used as a lithium salt. sulfonyl)imide (LiBETI), lithium tetrafluoroborate (LiBF4), lithium hexafluorophosphate (LiPF6), lithium triflate, and the like. These electrolyte salts may be used singly or in combination.

Ether-based solvents constitute solvated electrolyte salts and solvated ionic liquids. As an ether solvent, a known glyme (R—O(CH ₂ CH ₂ O) _n —R′ {wherein R and R′ are saturated hydrocarbons and n is an integer is a general term for symmetrical glycol diethers represented by ) can be used. From the viewpoint of ion conductivity, tetraglyme (tetraethylene dimethyl glycol, G4), triglyme (triethylene glycol dimethyl ether, G3), pentaglyme (pentaethylene glycol dimethyl ether, G5), hexaglyme (hexaethylene glycol dimethyl ether, G6). It can be preferably used. As an ether solvent, crown ethers (generic term for macrocyclic ethers represented by (--CH ₂ --CH ₂ --O) _n {wherein n is an integer}) can be used. Specifically, 12-crown-4, 15-crown-5, 18-crown-6, dibenzo-18-crown-6 and the like can be preferably used, but are not limited thereto. These ether solvents may be used singly or in combination. Tetraglyme and triglyme are preferably used because they can form a complex structure with the solvated electrolyte salt.

Lithium salts such as LiFSI, LiTFSI, and LiBETI can be used as solvated electrolyte salts, but are not limited to these. As non-aqueous solvents, mixtures of ether solvents and solvated electrolyte salts may be used singly or in combination.

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 was measured at a 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 porous membrane (before compression) were measured as follows.

[Average film thickness (μm) of microporous membrane (before compression)]
The thickness was measured at an ambient temperature of 23±2° C. using a micro thickness gauge manufactured by Toyo Seiki (type KBM, terminal diameter Φ5 mm). In addition, when measuring the thickness, after sampling the microporous membrane to 10 cm × 10 cm, a plurality of microporous membranes are stacked so that the thickness is 15 μm or more, 9 points are measured, and the average is taken. The thickness of one sheet is obtained by dividing the value by the number of stacked sheets.

(4) Porosity of whole membrane (%)
A sample of 10 cm×10 cm square was cut from the microporous membrane, its volume (cm ³ ) and mass (g) were obtained, and the porosity was calculated from these and density (g/cm ³ ) using the following formula.
Porosity (%) = (volume - mass / density) / volume x 100

(5) Air permeability (sec/100 cm ³ )
Air permeation resistance according to JIS P-8117 was defined as air permeability.
The air permeability of the microporous membrane is measured in accordance with JIS P-8117 using a Gurley air permeability meter G-B2 (trademark) manufactured by Toyo Seiki Co., Ltd. at a temperature of 23 ° C. and a humidity of 40%. The air permeability of the multi-layer porous membrane or the air permeability resistance of the PO microporous membrane was measured in an atmosphere of 100.degree.

However, apart from (4) and (5) above, measurement of porosity (%) (before compression), air permeability (before compression) (sec/100 cm ³ ), and compression press test are as follows. measured to

[(Before compression) porosity (%)]
A sample of 3 cm x 3 cm square, 1 cm x 1 cm square, 5 cm x 5 cm square, or 10 cm x 10 cm square is cut from the polyolefin microporous membrane, and the volume (cm ³ ) and mass (g) thereof are calculated from the measurement results of the film thickness. Based on these values and the density (g/cm ³ ), calculation was performed using the following formula.
Porosity (%) = (volume - mass / density of mixed composition) / volume x 100
As the density of the mixed composition, a value calculated from the density and mixing ratio of each of the polyolefin resin used and the other components was used.
Also, the porosity (before compression) of the multilayer porous membrane is determined by the following formula.
Porosity of multilayer porous membrane = (porosity of polyolefin resin microporous membrane as substrate) x (average thickness of polyolefin resin microporous membrane as substrate) ÷ (thickness of entire multilayer porous membrane) + (coating Porosity of layer) x (thickness of coating layer) ÷ (thickness of entire multilayer porous membrane)
Here, the porosity of the multilayer porous membrane was calculated assuming that the porosity of the coating layer was 50%. When the porosity of the coating layer is not 50%, the porosity of the coating layer can be calculated in the same manner as the porosity of the polyolefin microporous membrane, if necessary. Specifically, in the coating film, the thickness of the coating layer is measured by direct observation with SEM or from the change in film thickness before and after coating, and the volume of the coating layer sample of a specific area is obtained. The porosity of the coating layer is calculated using the mass-average density of the coating components calculated from the material ratios of the constituent components of the coating layer.

[(Before compression) air permeability (sec/100 cm ³ )]
The air permeability was measured with an 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, the PET film, and the cushioning material are laminated in this order, and the resulting laminate is allowed to stand, 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 a temperature of 25° C. and a compression time of 3 minutes at a pressure of 10 MPa. 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 formula.
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 (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

(9) 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 equation.
[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) 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.

(11) Crystal structure analysis Regarding the crystal long period in the polyolefin microporous film, small-angle X-ray scattering measurement of transmission method was performed using NANOPIX manufactured by Rigaku. A sample was irradiated with CuKα rays, and scattering was detected by a semiconductor detector HyPix-6000. The measurement was performed under the conditions of a sample-detector distance of 1312 mm and an output of 40 kV and 30 mA. A point-focus optical system was employed, and the slit diameters were 1st slit: φ=0.55 mm, 2nd slit: open, guard slit: φ=0.35 mm. The sample was set so that the sample surface was perpendicular to the X-ray incident direction.

In addition, regarding the polyethylene MDND plane (110) crystallite size in the polyolefin microporous film, wide-angle X-ray scattering measurement of transmission method was performed using NANOPIX manufactured by Rigaku. The sample was irradiated with CuKα rays and scattering was detected by an imaging plate. The measurement was performed under the conditions of a sample-detector distance of 110 mm and an output of 40 kV and 30 mA. A point focus was used for the optical system, and the slit diameter was 1st slit: φ=1.2 mm and the guard slit: φ=0.35 mm. The sample was set so that the sample surface and the X-ray incident direction formed an angle of 11.0°. At this time, the direction of X-ray incidence and the MD of the sample were set to be perpendicular to each other.

(Crystal long period [nm])
A SAXS profile I(q) was obtained by ring averaging for the X-ray scattering pattern obtained from the HyPix-6000. A linear baseline was drawn in the range of 0.1 nm ⁻¹ < q < 0.6 nm ⁻¹ in the Linear-Linear plot of the obtained one-dimensional profile I(q), and fitting was performed using a Gaussian function. The crystal long period was calculated from Equation 4 with the peak position _qm derived from the crystal long period being the position of maximum intensity.

d = 2π/q _m Equation 4

{In the formula, d (nm): crystal long period
q _m (mm ⁻¹ ): lamella-derived peak position in the SAXS profile}

(crystallite size)
The range of 2θ = 10.0° to 2θ = 30.0° in the obtained XRD profile is separated into three peaks: an orthorhombic (110) plane diffraction peak, an orthorhombic (200) plane diffraction peak, and an amorphous peak. Then, the crystallite size was calculated from the full width at half maximum of the (110) plane diffraction peak according to Scherrer's formula (Formula 1). The (110) plane diffraction peak and the (200) plane diffraction peak were approximated by a Voigt function, and the amorphous peak was approximated by a Gaussian function. The peak position of the amorphous peak was fixed at 2θ=19.6° and the full width at half maximum was fixed at 6.3°, and the peak position and full width at half maximum of the crystalline peak were not fixed, and the peak separation was performed. From the full width at half maximum of the (110) plane diffraction peak calculated by peak separation, the crystallite size was calculated by Scherrer's formula (Formula 1).

D(110)=Kλ/(β cos θ) Equation 1

{In the formula, D (110): crystallite size (nm)
K: 0.9 (constant)
λ: X-ray wavelength (nm)
β: (β ₁ ² - β ₂ ² ) ^0.5
β ₁ : full width at half maximum (rad) of peak (hkl) calculated as a result of peak separation
β ₂ : Full width at half maximum (rad) of divergence of incident beam
θ: Bragg angle}

(12) Liquid Absorbency Test of Propylene Carbonate An MD sample (MD50 mm×TD10 mm) and a TD sample (MD10 mm×TD50 mm) were cut out from the microporous membrane. Next, a horizontal bar of a predetermined height was placed on a glass water tank containing propylene carbonate, and the end of the strip-shaped sample was pinned to the horizontal bar so that the long side was parallel to the vertical direction. Then, the horizontal bar was lowered to immerse the end of the sample piece in propylene carbonate by 10 mm, and the maximum height (mm) from the liquid surface at the position where the propylene carbonate raised the microporous membrane for 60 minutes after this immersion. was measured, and the measured value was taken as the liquid absorption height of propylene carbonate in MD and TD. The measurement was carried out in an environment of 25° C., and the average value of 3 measurements each in MD and TD was adopted.

(13) Battery test

<Cycle test>
Each battery assembled in Examples and Comparative Examples described below was charged at 25° C. with a current value of 3 mA (about 0.5 C) to a battery voltage of 4.3 V, and the current was kept at 4.3 V. A cycle of charging the battery for the first time after fabrication was performed for a total of about 3 hours by reducing the value from 3 mA, and then discharging to a battery voltage of 3.0 V at a current value of 3 mA was repeated. Then, the cycle characteristics were evaluated according to the following criteria, using the number of cycles at which the capacity retention rate fell below 80% for the first time with respect to the initial capacity (capacity in the first cycle).
Evaluation criteria for cycle characteristics A: 300 cycles or more B: 200 cycles or more and less than 300 cycles C: 100 cycles or more and less than 200 cycles D: 50 cycles or more and less than 100 cycles E: 1 cycle or more and less than 50 cycles

<Initial overcharge test>
A constant current (CC)-constant voltage (CV) charge of 4.3 V at a set current value of 0.1 C in a cell assembled by the method described in the example separately from the cell for the cycle test Convergence of the Cut Off condition A current value of 0.03 mA) was performed to measure the normal charge amount (i).
A new cell was prepared separately from the cell for which the normal charge amount (i) was measured, and a CC-CV charge of 4.3 V with a set current value of 20 mA / cm (Cut Off condition: 25 mAh or a convergence current value of 0.03 mA). was performed to measure the overloaded rechargeable battery (ii).
The value of (ii)-(i) was evaluated according to the following criteria as an overcharge value due to dendrite short-circuiting.
[Evaluation Rank]
A: Less than 0.05 mAh B: 0.05 mAh or more and less than 0.1 C: 0.1 mAh or more and less than 0.5 mAh D: 0.5 mAh or more and less than 1.0 mAh E: 1.0 mAh or more and 20.0 mAh or less

a. Preparation of Polyolefin Microporous Film Polyolefin microporous films a1 to a12 were prepared under the following conditions. Table 1 shows the properties of the polyolefin microporous membrane.

[a1]
A polyolefin microporous membrane was produced by the following procedure. The composition of the resin raw material was 93 parts by mass of high-density polyethylene with a melting point of 135° C. and a viscosity average molecular weight of 900,000, and 7 parts by mass of isotactic polypropylene with a melting point of 161° C. and a viscosity average molecular weight of 400,000.
An appropriate amount of pentaerythrityl-tetrakis-[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate] as an antioxidant was added to the resin composition and mixed with a Henschel mixer. The resulting mixture was fed by a feeder to the twin co-rotating screw extruder feed throat. In addition, liquid paraffin is divided into two times so that the liquid paraffin (LP) amount ratio in the total mixture (100 parts by mass) to be melted and extruded is 77.0 parts by mass. Side to the twin screw extruder cylinder. Added in the feed. The set temperatures were 160° C. for the kneading section and 200° C. for the T-die. Subsequently, the melt-kneaded product was extruded into a sheet form from a T-die and cooled with a cooling roll whose surface temperature was controlled to 90° C. to obtain a sheet-like molded product.

The obtained sheet-like molding was introduced into a simultaneous biaxial stretching machine to obtain a primary stretched film (primary stretching step). The stretching conditions were set to an MD stretching ratio of 7 times, a TD stretching ratio of 6.4 times, and a biaxial stretching temperature of 121°C in MD and TD. Next, the obtained primarily stretched membrane was introduced into a methylene chloride tank and fully immersed to extract and remove liquid paraffin, which is a plasticizer, and then methylene chloride was removed by drying to obtain an extracted membrane.

Subsequently, the extracted membrane was led to a TD uniaxial tenter for heat setting. As a heat setting step, after stretching under the conditions of a TD stretching temperature of 125 ° C. and a TD stretching ratio of 1.92 times, a relaxation operation is performed under the conditions of a relaxation temperature of 125 ° C. and a relaxation ratio of 0.78 times, and the polyolefin is finely divided. A porous membrane a1 was obtained.

[a2]
After producing a polyolefin microporous membrane in the same manner as in a1, the polyolefin microporous membrane is treated with a mixed gas in which the partial pressure ratio of the concentrations of F ₂ , O ₂ and N ₂ gases is adjusted to obtain a polyolefin microporous membrane a2. got The mixed gas of O _{2 and} N ₂ mixed with F 2 is mixed in a ratio of 2:8, similar to the composition of air, and the partial pressure of F ₂ and mixed gas of O ₂ and N ₂ is 1 :9 and mixed in the buffer tank to produce a mixed process gas. The prepared polyolefin microporous membrane was placed in a reaction chamber, and the mixed process gas was added. A gas valve between the buffer tank and the evacuated reaction chamber adjusted the concentration of the mixed process gas to a pressure condition of 30 KPa, and the reaction time was adjusted to 20 minutes.

[a3]
The composition of the resin raw material was 70 parts by mass of high-density polyethylene with a melting point of 135 ° C. and a viscosity average molecular weight of 900,000, a melting point of 220 ° C., a temperature of 240 ° C. measured according to ASTM D1238, and a melt index of 3.0 g / 10 min at a load of 2.16 kg. 20 parts by mass of a certain polyketone, 10 parts by mass of maleic anhydride-modified polyethylene having a melting point of 131° C. and a viscosity average molecular weight of 70,000, and the amount ratio of liquid paraffin (LP) in the total mixture (100 parts by mass) to be extruded is 65.5 parts. A polyolefin microporous membrane a3 was produced in the same manner as for a1, except that the content was 0 parts by mass.

[a4]
The composition of the resin raw material is 70 parts by mass of high density polyethylene with a melting point of 135 ° C. and a viscosity average molecular weight of 900,000, and 30 parts by mass of high density polyethylene with a melting point of 135 ° C. and a viscosity average molecular weight of 250,000, and the entire mixture (100 parts by mass) to be extruded. Liquid paraffin (LP) accounts for 75.0 parts by mass, the biaxial stretching temperature is 126 ° C., and as a heat setting step, the stretching operation is performed under the conditions of a TD stretching ratio of 1.90 times, followed by relaxation. A polyolefin microporous membrane a4 was produced in the same manner as a1, except that the relaxation operation was performed under the conditions of a temperature of 136° C. and a relaxation ratio of 0.84.

[a5]
40 parts by mass of high density polyethylene with a melting point of 135°C and a viscosity average molecular weight of 700,000; 40 parts by mass of high density polyethylene with a melting point of 135°C and a viscosity average molecular weight of 250,000; 5 parts by weight of isotactic polypropylene, 15 parts by weight of silica "DM10C" (trademark, manufactured by Tokuyama, subjected to hydrophobic treatment with dimethyldichlorosilane) having an average primary particle size of 15 nm, and extruded total mixture (100 parts by weight) The amount ratio of liquid paraffin (LP) is 70.0 parts by mass, the biaxial stretching temperature is 127 ° C., and the heat setting process is performed at a stretching temperature of 129 ° C. and a TD stretching ratio of 1.90 times. After the operation, a polyolefin microporous membrane a5 was produced in the same manner as a1, except that the relaxation operation was performed under the conditions of a relaxation temperature of 136° C. and a relaxation ratio of 0.79.

[a6]
The composition of the resin raw material is 30 parts by mass of high density polyethylene with a melting point of 135 ° C. and a viscosity average molecular weight of 900,000, and 70 parts by mass of high density polyethylene with a melting point of 135 ° C. and a viscosity average molecular weight of 250,000, and the entire mixture (100 parts by mass) to be extruded. Liquid paraffin (LP) accounts for 71.0 parts by mass, the biaxial stretching temperature is 128 ° C., and the heat setting step is performed at a stretching temperature of 128 ° C. and a TD stretching ratio of 1.70 times. After the operation, a microporous polyolefin membrane a6 was produced in the same manner as a1, except that the relaxation operation was performed under the conditions of a relaxation temperature of 128° C. and a relaxation ratio of 0.94.

[a7]
After producing a polyolefin microporous membrane in the same manner as for a4, the produced polyolefin microporous membrane was plasma-treated to obtain a polyolefin microporous membrane a7. The plasma treatment utilizes dielectric barrier discharge type electrodes under atmospheric pressure, injecting 300 ml/min of nitrogen as carrier gas and 1 ml/min of oxygen as reaction gas at a power of 3.6 kW and a voltage of was set to 12 kV and the plasma was discharged, and both sides of the polyolefin microporous membrane were brought into contact with the plasma simultaneously for 3 seconds. The distance between the electrode and the microporous membrane during the plasma discharge was fixed at 3 mm.

[a8]
47.5 parts by mass of high-density polyethylene with a melting point of 135°C and a viscosity average molecular weight of 700,000; 47.5 parts by mass of high-density polyethylene with a melting point of 135°C and a viscosity average molecular weight of 250,000; 5 parts by mass of isotactic polypropylene with a molecular weight of 400,000, the amount ratio of liquid paraffin (LP) in the total mixture (100 parts by mass) to be extruded is 68.0 parts by mass, the biaxial stretching temperature is 120 ° C., and heat As the fixing step, after the stretching operation under the conditions of the stretching temperature of 130 ° C. and the TD stretching ratio of 1.75 times, the relaxation operation was performed under the conditions of the relaxation temperature of 131 ° C. and the relaxation ratio of 0.86 times. A polyolefin microporous membrane a8 was obtained by treating a polyolefin microporous membrane prepared in the same manner as in a2 with a mixed gas of F ₂ , O ₂ and N ₂ in the same manner as in a2.

[a9]
The composition of the resin raw material is as follows: 70 parts by mass of high density polyethylene with a melting point of 135°C and a viscosity average molecular weight of 700,000; 23 parts by mass of high density polyethylene with a melting point of 135°C and a viscosity average molecular weight of 250,000; 7 parts by mass of isotactic polypropylene with a molecular weight distribution of 6.0, the amount ratio of liquid paraffin (LP) in the total mixture (100 parts by mass) to be extruded is 71.0 parts by mass, and the biaxial stretching temperature is 125 ° C. , As the heat setting step, the stretching operation was performed under the conditions of a stretching temperature of 130 ° C. and a TD stretching ratio of 1.90 times, followed by a relaxation operation under the conditions of a relaxation temperature of 128 ° C. and a relaxation ratio of 0.83 times. prepared a polyolefin microporous membrane a9 in the same manner as a1.

[a10]
A microporous polyolefin membrane a10 was obtained in the same manner as a8, except that the treatment was not performed with a mixed gas of _F2 , _O2 and _N2 .

[a11]
Polypropylene (PP) resin with a melt index of 0.25 g/10 min at a temperature of 190° C. and a load of 2.16 kg measured according to ASTM D1238 was melted in a 2.5 inch extruder and fed to an annular die using a gear pump. . The temperature of the die was set at 260° C. and the molten polymer was cooled by blown air before being wound on a roll. Similarly, a high-density polyethylene (PE) resin with a melt index of 0.38 g/10 min at a temperature of 190° C. and a load of 2.16 kg is melted in a 2.5-inch extruder and fed to an annular die using a gear pump. did. The temperature of the die was set at 230° C. and the molten polymer was cooled by blown air before being wound on a roll. The polypropylene and polyethylene precursors (original films) wound into rolls each have a thickness of 5 μm, and then the PP and PE precursors are bound to form PP/PE/PP, and PP/ A raw film having a three-layer structure of PE/PP was obtained. This raw film was annealed at 125° C. for 20 minutes. The annealed film was then cold stretched to 15% at room temperature, then hot stretched to 1.5 times MD at 115°C, and after relaxation to 0.69 times MD at 125°C, similar to a2. A polyolefin microporous membrane a11 was obtained by treating with F ₂ , O ₂ and N ₂ gases by the method of .

[a12]
The composition of the resin raw material is 70 parts by mass of ultra-high molecular weight polyethylene with a melting point of 134 ° C. and a viscosity average molecular weight of 4.5 million, 30 parts by mass of polyethylene wax with a melting point of 113 ° C. and a viscosity average molecular weight of 1000, and the proportion of the total volume is 36% by volume. Calcium carbonate having an average pore size of 0.1 μm and a BET specific surface area of 11.6 m ² /g was added so as to obtain the desired amount. An appropriate amount of pentaerythrityl-tetrakis-[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate] as an antioxidant was added to the resin composition and mixed with a Henschel mixer. The resulting mixture was supplied to the feed port of a twin co-rotating screw type extruder using a feeder and extruded into a sheet from a T-die set at a temperature of 200°C. A rolled sheet was prepared by rolling this sheet using a pair of rolls at a surface temperature of 150°C. Subsequently, the rolled sheet was immersed in hydrochloric acid (4 mol / L) containing 0.5% by weight of a nonionic surfactant to remove calcium carbonate, then stretched 7.0 times in the MD, and further The magnification was fixed and heat setting was performed at 123° C. to obtain a polyolefin microporous membrane a12.

b. Production of Positive Electrode A positive electrode b1 was produced under the following conditions.
91.2 parts by mass of lithium-nickel-manganese-cobalt composite oxide (Li[Ni _1/3 Mn _1/3 Co _1/3 ]O ₂ ) as a positive electrode active material, and flaky graphite and acetylene black as conductive materials. 2.3 parts by mass and 4.2 parts by mass of polyvinylidene fluoride (PVdF) as a resin binder were prepared, and these were dispersed in N-methylpyrrolidone (NMP) to prepare a slurry. This slurry was applied to one side of a 20 μm-thick aluminum foil serving as a positive electrode current collector using a die coater so that the positive electrode active material coating amount was 120 g/m ² . After drying at 130° C. for 3 minutes, it was compression molded using a roll press so that the bulk density of the positive electrode active material was 2.90 g/cm ³ to obtain a positive electrode. This was punched into a circle with an area of 2.00 cm ² .

c. Production of Negative Electrode Negative electrodes c1 and c2 were produced under the following conditions.

[c1]
A circular lithium metal (Honjo Metal) with a thickness of 0.5 mm and an area of 2.00 cm ² is placed in a SUS glove box under a nitrogen atmosphere, and high-purity nitrogen gas (99.99%) is constantly supplied, By leaving the metallic lithium at a temperature of 60° C. for 2 hours, a metallic lithium negative electrode c1 having lithium nitride formed on the surface was produced.

[c2]
A circular lithium metal (Honjo Metals Co., Ltd.) having a thickness of 0.5 mm and an area of 2.00 cm ² was used as the negative electrode c2.

d. Preparation of Nonaqueous Electrolyte Electrolyte solutions d1 to d8 were prepared under the following conditions.

[d1]
An electrolytic solution d1 was prepared by dissolving LiPF ₆ as a solute in a mixed solvent of ethylene carbonate:ethyl methyl carbonate=1:2 (volume ratio) so as to have a concentration of 2.0 mol/L.

[d2]
An electrolytic solution d2 was prepared by dissolving LiPF ₆ as a solute in a mixed solvent of ethylene carbonate:ethyl methyl carbonate=1:2 (volume ratio) so as to have a concentration of 1.3 mol/L.

[d3]
An electrolytic solution d3 was prepared by dissolving LiPF ₆ as a solute in a mixed solvent of fluoroethylene carbonate:ethyl methyl carbonate=1:2 (volume ratio) so as to have a concentration of 1.0 mol/L.

[d4]
An electrolytic solution d4 was prepared by dissolving LiTFSI as a solute in a mixed solvent of ethylene carbonate:ethyl methyl carbonate=1:2 (volume ratio) so as to have a concentration of 1.0 mol/L.

[d5]
An electrolytic solution d5 was prepared by dissolving LiPF ₆ as a solute in a mixed solvent of ethylene carbonate:ethyl methyl carbonate=1:2 (volume ratio) so as to have a concentration of 3.0 mol/L.

[d6]
An electrolytic solution was prepared by dissolving LiPF ₆ in ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF ₄ ) to a concentration of 1.0 mol/L to obtain electrolytic solution d6.

[d7]
Tetraglyme (G4) and LiTFSI were put into a beaker so that the molar ratio was 1:1, and mixed until a homogeneous solvent was obtained to prepare an electrolytic solution d7.

[d8]
An electrolytic solution d8 was prepared by dissolving LiPF ₆ as a solute in a mixed solvent of ethylene carbonate:ethyl methyl carbonate=1:2 (volume ratio) so as to have a concentration of 1.0 mol/L.

e. Battery Assembly <Examples 1 to 11 and Comparative Examples 1 to 4>
Batteries of Examples 1 to 11 and Comparative Examples 1 to 4 were produced under the following conditions by changing the combination of the polyolefin microporous film, the positive electrode, the negative electrode, and the electrolytic solution prepared in the steps a to b as shown in Table 2. .
In an argon box, the negative electrode, the separator, and the positive electrode were stacked in this order so that the active material of the positive electrode and the negative electrode faced each other. A cell is obtained by housing this laminate in a lidded stainless metal container in which the container body and the lid are insulated so that the lithium foil of the negative electrode and the aluminum foil of the positive electrode are in contact with the container body and the lid, respectively. rice field. The cell was dried at 25° C. for 10 hours under reduced pressure. Thereafter, a non-aqueous electrolyte was injected into this container in an argon box and the container was sealed to obtain a battery. Table 2 shows the combinations of the polyolefin microporous membranes, positive electrodes, negative electrodes, and electrolytes of Examples 1 to 11, Examples 12 and Comparative Examples 1 to 4. When the negative electrode c1 was used, the battery was assembled with the lithium nitride formation surface facing the polyolefin microporous membrane side.

<Example 12>
Polyvinylidene fluoride resin particles (melting point: 140° C., particle size: 0.2 μm) were dissolved in acetone to a concentration of 8.0 wt % to prepare a coating liquid. The prepared coating liquid was applied to one side of a polyolefin microporous membrane prepared in the same manner as in a1 above with a bar coater and dried at 40° C. for 1 hour to obtain a separator coated with PVdF on one side. The coating weight of the PVdF layer was 3.2 g/m ² . Batteries were assembled in the same manner as in Examples 1 to 11 and Comparative Examples 1 to 4 using the obtained PVdF-coated separator, positive electrode b1, negative electrode c2, and electrolytic solution d2. The PVdF-coated surface of the separator was arranged so as to face the negative electrode.

Table 2 shows the configurations and battery evaluation results of Examples 1 to 12 and Comparative Examples 1 to 4.

Claims (11)

In the liquid absorption test of propylene carbonate, the liquid absorption height in the longitudinal direction (MD) and the width direction (TD) is 1.0 mm or more and 50 mm or less,
The tensile elastic modulus in MD and TD is 3000 kgf/cm ² or more,
Polyolefin microporous membrane. 2. The polyolefin microporous membrane according to claim 1, having a porous layer having a number of pores calculated by a gas-liquid method of 70/μm ² or more and 500/μm ² or less. The polyolefin microporous according to claim 2, wherein the porous layer has a pore diameter of 98 nm or less and a total pore volume of 20% or more and 85% or less of the total pore volume measured by a nitrogen gas adsorption test. film. The polyolefin microporous membrane has a compression rate of 35% or less and a porosity after compression of 35% or more in a compression test under conditions of a temperature of 25° C., a pressure of 10 MPa, and a compression time of 3 minutes. The polyolefin microporous membrane according to any one of items 1 to 3. Any one of claims 1 to 4, wherein the main component of the polyolefin microporous membrane is polyethylene, and the crystal length period of the polyethylene measured by a small angle X-ray scattering (SAXS) method is 30.0 nm or more. 2. The polyolefin microporous membrane according to item 1. Any one of claims 1 to 5, wherein the main component of the polyolefin microporous membrane is polyethylene, and the crystallite size of the polyethylene measured by a wide-angle X-ray scattering (WAXS) method is 35.0 nm or less. 2. The polyolefin microporous membrane according to item 1. A polyolefin microporous membrane having a liquid absorption height of 1.0 mm or more and 50 mm or less in MD and TD in a propylene carbonate liquid absorption test and a tensile modulus of elasticity in MD and TD of 3000 kgf/cm ² or more is provided as a separator. Lithium-ion battery. 8. The lithium ion battery of claim 7, wherein an ion-conducting layer is present between the separator and the metal negative electrode. 9. The lithium ion battery according to claim 7, wherein said ion conductive layer is made of inorganic material. 1 to 4 below:
1. Li ion concentration is 1.3 mol / L or more;
2. containing an electrolyte salt having a bisfluorosulfonylimide structure;
3. 4. including carbonates in which hydrogen atoms are replaced with halogen atoms; the electrolyte is an ionic liquid;
Lithium ion battery according to any one of claims 7 to 9, comprising an electrolyte characterized by at least one of The lithium ion battery according to any one of claims 7 to 10, comprising the polyolefin microporous membrane according to any one of claims 1 to 6 as the separator.