WO2025178012A1 - Separator for nonaqueous secondary battery, and nonaqueous secondary battery - Google Patents

Abstract

This separator for a nonaqueous secondary battery is provided with: a porous base material; and an adhesive porous layer that is disposed on one surface or both surfaces of the porous base material and contains an acrylic resin (1). The acrylic resin (1) contains, as polymerization components, a (meth)acrylic acid, a (meth)acrylamide-based monomer, and at least one of methyl (meth)acrylate and butyl (meth)acrylate. The total proportion of the (meth)acrylic acid and the (meth)acrylamide-based monomer in all polymerization components is 20 mol% or more, and the molar ratio of the (meth)acrylic acid to the (meth)acrylamide-based monomer is 40:60 to 60:40.

Description

Separator for non-aqueous secondary battery and non-aqueous secondary battery

This disclosure relates to a separator for a non-aqueous secondary battery and a non-aqueous secondary battery.

Organofluorine compounds have useful properties such as heat resistance, chemical resistance, and surface activity, and have been used in a wide range of manufacturing and industrial applications. In recent years, reports have been released about the ecotoxicity and human toxicity of organofluorine compounds, leading to stricter restrictions on the production and use of these compounds worldwide.

Separators containing polyvinylidene fluoride resin are known as battery separators. As restrictions on the production and use of organic fluorine compounds are coming into effect, there is an urgent need to develop separators that contain little or no polyvinylidene fluoride resin.

Patent Document 1 discloses a binder composition for lithium ion secondary batteries containing: (A) component: a particulate copolymer containing, as monomer units, a monomer containing at least one aldehyde group or ketone group, a (meth)acrylic acid ester monomer, a carboxylic acid group-containing monomer, and a crosslinkable monomer; (B) component: a hydrazine-based crosslinking agent; and/or (C) component: a semicarbazide compound derived from hydrazine and at least one isocyanate compound selected from the group consisting of aliphatic isocyanates and alicyclic isocyanates.

Patent Document 2 discloses an aqueous resin composition for use in a heat-resistant layer binder for a lithium-ion secondary battery separator, which contains a radical polymer (A) whose essential raw materials are an acrylic monomer (a1) having an alkyl group with 4 to 18 carbon atoms, at least one monomer (a2) selected from diacetone(meth)acrylamide and N-methylol(meth)acrylamide, an unsaturated monomer (a3) having a carboxyl group, and acrylonitrile (a4), and an aqueous medium (B).

Patent Document 3 discloses a composition for a non-aqueous secondary battery functional layer, which contains non-conductive particles, a water-soluble polymer containing (meth)acrylamide monomer units in a proportion of 40 mass% or more and having a weight-average molecular weight of less than 3.0 × 10 ⁵ , and a particulate polymer.

Japanese Patent Application Laid-Open No. 2021-157895 International Publication No. 2021/251092 International Publication No. 2017/026095

In order to prevent internal short circuits in batteries, separators are required to have adhesive properties to electrodes. Separators that contain polyvinylidene fluoride resin in their surface layers have excellent adhesive properties to electrodes, but there is a demand for separators that have adhesive properties to electrodes even if the surface layer contains a low amount of polyvinylidene fluoride resin or does not contain any polyvinylidene fluoride resin at all.

In the battery manufacturing process, in order to increase productivity and improve battery performance, the separator is impregnated with electrolyte and then hot-pressed together with the electrodes (this process is called "wet heat pressing"). During this process, if the resin on the separator surface swells excessively due to the electrolyte, the separator will not adhere well to the electrodes, or even if it does adhere to the electrodes, it will easily peel off.

It is against this background that the present disclosure has been made.
An object of the present disclosure is to provide a separator for a non-aqueous secondary battery that is bonded to an electrode by wet heat pressing.

Specific means for solving the above problems include the following aspects.
<1>
A porous substrate;
An adhesive porous layer containing an acrylic resin (1) arranged on one or both sides of the porous substrate,
The acrylic resin (1) contains, as polymerization components, (meth)acrylic acid, a (meth)acrylamide-based monomer, and at least one of methyl (meth)acrylate and butyl (meth)acrylate, wherein the total proportion of the (meth)acrylic acid and the (meth)acrylamide-based monomer in all polymerization components is 20 mol % or more, and the molar ratio of the (meth)acrylic acid to the (meth)acrylamide-based monomer is 40:60 to 60:40.
Separator for non-aqueous secondary batteries.
<2>
The separator for a non-aqueous secondary battery according to <1>, wherein the acrylic resin (1) contains methyl (meth)acrylate and butyl (meth)acrylate in a total proportion of 10 mol % to 80 mol % of all polymer components.
<3>
The separator for a non-aqueous secondary battery according to <1> or <2>, wherein the adhesive porous layer further contains inorganic particles.
＜4＞
<1><2> The separator for a non-aqueous secondary battery according to any one of <1> to <2>, wherein the separator for a non-aqueous secondary battery has an air permeability of 100 seconds/100 mL to 1000 seconds/100 mL.
<5>
<4> The separator for a non-aqueous secondary battery according to any one of <1> to <4>, wherein the porous substrate includes a polyolefin microporous membrane.
<6>
The porous substrate is
a polyolefin microporous membrane;
a heat-resistant layer containing at least one of inorganic particles and a heat-resistant resin, disposed on one or both sides of the polyolefin microporous membrane;
<4> The separator for a non-aqueous secondary battery according to any one of <1> to <4>.
＜７＞
<6> The separator for a non-aqueous secondary battery according to any one of <1> to <6>, wherein the adhesive porous layer does not substantially contain a fluorine-containing resin.
＜8＞
A nonaqueous secondary battery separator according to any one of <1> to <7>, which is disposed between the positive electrode and the negative electrode;
Electromotive force is generated by doping and dedoping of lithium ions.
Non-aqueous secondary battery.

This disclosure provides a separator for a non-aqueous secondary battery that is bonded to an electrode by wet heat pressing.

1 is an SEM image of the surface of the separator of Example 1. 1 is an SEM image of the surface of the separator of Example 3.

The following describes embodiments of the present disclosure. These descriptions and examples are intended to illustrate the embodiments and do not limit the scope of the embodiments.

In the present disclosure, a numerical range indicated using "to" indicates a range that includes the numerical values before and after "to" as the minimum and maximum values, respectively.
In the numerical ranges described in stages in this disclosure, the upper or lower limit value described in one numerical range may be replaced with the upper or lower limit value of another numerical range described in stages. Furthermore, in the numerical ranges described in this disclosure, the upper or lower limit value of that numerical range may be replaced with a value shown in the examples.

In this disclosure, "A and/or B" is synonymous with "at least one of A and B." In other words, "A and/or B" means that it may be A alone, B alone, or a combination of A and B.

In this disclosure, the term "process" includes not only independent processes, but also processes that cannot be clearly distinguished from other processes, as long as the purpose of the process is achieved.

In the present disclosure, when referring to the amount of each component in a composition, if multiple substances corresponding to each component are present in the composition, the total amount of the multiple substances present in the composition is meant unless otherwise specified.
In the present disclosure, the composition may contain multiple types of particles corresponding to each component. When multiple types of particles corresponding to each component are present in the composition, the particle size of each component refers to the value for a mixture of the multiple types of particles present in the composition, unless otherwise specified.

In this disclosure, MD (Machine Direction) refers to the longitudinal direction of a separator manufactured in a long shape, and TD (Transverse Direction) refers to the direction perpendicular to the MD in the plane direction of the separator. In this disclosure, TD is also referred to as the "width direction."

In this disclosure, when the stacking relationship of the layers constituting the separator is expressed as "top" and "bottom," the layer closer to the porous substrate is referred to as "bottom," and the layer farther from the porous substrate is referred to as "top."

In this disclosure, the volume of the porous layer excluding the pores is referred to as the "solids volume."

In this disclosure, performing heat pressing after permeating the separator with electrolyte is referred to as "wet heat pressing," and performing heat pressing without permeating the separator with electrolyte is referred to as "dry heat pressing."

In this disclosure, the term "(meth)acrylic" means either "acrylic" or "methacrylic."

In this disclosure, a "monomer unit" of a polymer or resin means a structural unit of the polymer or resin formed by polymerization of a monomer.

<Non-aqueous secondary battery separator>
The separator for a nonaqueous secondary battery according to the present disclosure (also simply referred to as a "separator" in the present disclosure) comprises a porous substrate and an adhesive porous layer containing an acrylic resin (1) disposed on one or both sides of the porous substrate.

Acrylic resin (1) is an acrylic resin containing (meth)acrylic acid, a (meth)acrylamide monomer, and at least one of methyl (meth)acrylate and butyl (meth)acrylate as polymerization components, in which the total proportion of (meth)acrylic acid and (meth)acrylamide monomers in all polymerization components is 20 mol% or more, and the molar ratio of (meth)acrylic acid to (meth)acrylamide monomer is 40:60 to 60:40.

The separator of the present disclosure adheres to the electrode by wet heat pressing because the adhesive porous layer contains acrylic resin (1). The mechanism is presumed to be as follows:

Wet heat pressing is a process in which the separator is permeated with an electrolyte solution and then heat-pressed. During this process, if the resin on the separator surface is excessively swollen by the electrolyte solution, the separator will not adhere well to the electrode, or even if it does adhere to the electrode, the separator will easily peel off.
It is believed that when the electrolyte solution penetrates into the acrylic resin (1), hydrogen bonds are formed between the carboxyl groups of the (meth)acrylic acid units and the amide bonds of the (meth)acrylamide monomer units. The formation of these hydrogen bonds between the molecules of the acrylic resin (1) prevents the acrylic resin (1) from excessively swelling with the electrolyte solution. As a result, it is presumed that the adhesive porous layer containing the acrylic resin (1) adheres to the electrode by wet heat pressing and is not easily peeled off from the electrode.

The following structural formula shows an example of the hydrogen bond formed between the carboxy group of a (meth)acrylic acid unit and the amide bond of a (meth)acrylamide-based monomer unit. This example shows the hydrogen bond formed between the carboxy group of a methacrylic acid unit and the amide bond of an N-methylmethacrylamide unit.

In the acrylic resin (1), from the viewpoint of ensuring the number of hydrogen bonds formed between the carboxy groups of the (meth)acrylic acid units and the amide bonds of the (meth)acrylamide-based monomer units, the total proportion of (meth)acrylic acid and (meth)acrylamide-based monomers in all polymerization components is 20 mol % or more, preferably 25 mol % or more, more preferably 30 mol % or more, and even more preferably 35 mol % or more.
The total proportion of (meth)acrylic acid and (meth)acrylamide monomers in all polymerization components of the acrylic resin (1) may be, for example, 90 mol% or less, 70 mol% or less, 50 mol% or less, 45 mol% or less, or 40 mol% or less.

In order to efficiently form hydrogen bonds between the carboxy groups of the (meth)acrylic acid units and the amide bonds of the (meth)acrylamide-based monomer units, the acrylic resin (1) has a molar ratio of (meth)acrylic acid to (meth)acrylamide-based monomer, which are polymerization components, of 40:60 to 60:40, preferably 45:55 to 60:40, and more preferably 50:50 to 60:40.

The acrylic resin (1) contains at least one of methyl (meth)acrylate and butyl (meth)acrylate as a polymerization component from the viewpoints of solubility in an organic solvent constituting a coating liquid for forming an adhesive porous layer, control of the glass transition temperature, etc. From the above viewpoints, the acrylic resin (1) preferably contains both methyl (meth)acrylate and butyl (meth)acrylate as polymerization components.
From the above viewpoints, the total proportion of methyl (meth)acrylate and butyl (meth)acrylate in all polymerization components of the acrylic resin (1) is preferably 10 mol % to 80 mol %, more preferably 30 mol % to 70 mol %, and even more preferably 50 mol % to 65 mol %.

Analysis of the structural units of acrylic resin (1) is possible using nuclear magnetic resonance (NMR). The acrylic resin (1) extracted from the adhesive porous layer or the acrylic resin (1) used to form the adhesive porous layer is used as a sample for analysis.

The separator of the present disclosure has an adhesive porous layer containing acrylic resin (1) on one or both sides of a porous substrate. Examples of embodiments of the separator of the present disclosure include the following forms (1) to (3).

Configuration (1): A separator having adhesive porous layers containing acrylic resin (1) on both sides of a porous substrate as the outermost layers of the separator. The adhesive porous layer on one side of the separator and the adhesive porous layer on the other side may be the same or different in terms of components and/or composition.

Configuration (2): A separator having an adhesive porous layer containing acrylic resin (1) as the outermost layer of the separator on one side of the porous substrate, and another layer on the other side of the porous substrate.

Configuration (3): A separator having an adhesive porous layer containing acrylic resin (1) on one side of a porous substrate as the outermost layer of the separator, and no layer on the other side of the porous substrate (i.e., the surface of the porous substrate is exposed).

The porous substrate and adhesive porous layer of the separator of the present disclosure are described in detail below.

[Porous base material]
In the present disclosure, the term "porous substrate" refers to a substrate having internal pores or voids. Examples of such substrates include microporous membranes, porous sheets made of fibrous materials such as nonwoven fabrics and paper, and composite porous sheets obtained by laminating one or more other porous layers on these microporous membranes or porous sheets.

It is preferable that the material of the porous substrate be an electrically insulating material.

From the perspective of thinning the separator and increasing its strength, a microporous membrane is preferred as the porous substrate. A microporous membrane is a membrane that has numerous micropores inside, with interconnected micropores that allow gas or liquid to pass from one side to the other.

From the viewpoint of thermal dimensional stability, the porous substrate is preferably a composite porous substrate in which one or more porous heat-resistant layers are laminated on a microporous membrane. The porous heat-resistant layer is preferably a heat-resistant layer containing at least one of inorganic particles and a heat-resistant resin.
In the present disclosure, a heat-resistant resin refers to a resin having a melting point of 200° C. or higher, or a resin having no melting point and a decomposition temperature of 200° C. or higher. In other words, a heat-resistant resin in the present disclosure refers to a resin that does not melt or decompose in a temperature range below 200° C.

The porous substrate preferably contains a thermoplastic resin to impart a shutdown function to the porous substrate. The shutdown function refers to the function in which, when the battery temperature rises, the constituent materials dissolve and block the pores of the porous substrate, thereby blocking the movement of ions and preventing thermal runaway of the battery. Thermoplastic resins with a melting point of less than 200°C are preferred. Examples of thermoplastic resins include polyesters such as polyethylene terephthalate; and polyolefins such as polyethylene and polypropylene; with polyolefins being particularly preferred.

As the porous substrate, a porous substrate containing a microporous membrane containing polyolefin (referred to as a "polyolefin microporous membrane" in the present disclosure) is preferred from the viewpoint of imparting a shutdown function to the porous substrate.
Examples of porous substrates containing a polyolefin microporous membrane include a porous substrate consisting of only a polyolefin microporous membrane (i.e., a polyolefin microporous membrane) and a composite porous substrate in which a porous heat-resistant layer is disposed on one or both sides of a polyolefin microporous membrane. Here, the porous heat-resistant layer is preferably a heat-resistant layer containing at least one of inorganic particles and a heat-resistant resin.

Examples of polyolefin microporous membranes include polyolefin microporous membranes used in conventional battery separators, and it is preferable to select one from these that has sufficient mechanical properties and ion permeability.

In order to exhibit a shutdown function, the polyolefin microporous membrane is preferably a microporous membrane containing polyethylene, and the polyethylene content is preferably 95% by mass or more based on the total mass of the polyolefin microporous membrane.

A polyolefin microporous membrane containing polypropylene is preferred from the viewpoint of heat resistance, which means that the membrane does not easily rupture when exposed to high temperatures.

From the viewpoint of providing a shutdown function and heat resistance that prevents the film from easily rupturing when exposed to high temperatures, the polyolefin microporous film is preferably a polyolefin microporous film containing polyethylene and polypropylene. Examples of polyolefin microporous films containing polyethylene and polypropylene include microporous films in which polyethylene and polypropylene are mixed in one layer. From the viewpoint of achieving both shutdown function and heat resistance, the microporous film preferably contains 95% by mass or more polyethylene and 5% by mass or less polypropylene. Also from the viewpoint of achieving both shutdown function and heat resistance, a polyolefin microporous film having a laminate structure of two or more layers, at least one layer containing polyethylene and at least one layer containing polypropylene, is preferred.

The polyolefin contained in the polyolefin microporous membrane preferably has a weight-average molecular weight (Mw) of 100,000 to 5,000,000. Polyolefins with an Mw of 100,000 or more can impart sufficient mechanical properties to the microporous membrane. On the other hand, polyolefins with an Mw of 5,000,000 or less provide good shutdown properties to the microporous membrane, and the microporous membrane is easy to mold.

Methods for producing microporous polyolefin membranes include: extruding molten polyolefin resin through a T-die to form a sheet, which is then crystallized, stretched, and then heat-treated to form a microporous membrane; and extruding molten polyolefin resin together with a plasticizer such as liquid paraffin through a T-die, cooling it to form a sheet, stretching it, extracting the plasticizer, and heat-treating it to form a microporous membrane.

Examples of porous sheets made of fibrous materials include porous sheets such as nonwoven fabric and paper. Examples of fibrous materials include polyesters such as polyethylene terephthalate; polyolefins such as polyethylene and polypropylene; heat-resistant resins such as wholly aromatic polyamide, polyamideimide, polyimide, polyethersulfone, polysulfone, polyetherketone, and polyetherimide; cellulose; and others.

An example of a composite porous sheet is a sheet in which a functional layer is laminated onto a microporous membrane or a porous sheet made of a fibrous material. Such composite porous sheets are preferable because the functional layer allows for the addition of additional functionality. An example of a functional layer is a porous heat-resistant layer, which provides heat resistance to the composite porous sheet. Methods for combining a microporous membrane or porous sheet with a functional layer include coating the functional layer on the surface of the microporous membrane or porous sheet, bonding the microporous membrane or porous sheet and functional layer with an adhesive, and thermocompression bonding the microporous membrane or porous sheet and functional layer.

An example of an embodiment of the composite porous sheet is a composite porous substrate comprising a polyolefin microporous membrane and a heat-resistant layer containing at least one of inorganic particles and a heat-resistant resin, the heat-resistant layer being disposed on one or both sides of the polyolefin microporous membrane. The heat-resistant layer is a porous layer.
Examples of inorganic particles include metal oxide particles (silica, alumina, boehmite, titania, zirconia, magnesium oxide, barium oxide, etc.), metal hydroxide particles (magnesium hydroxide, aluminum hydroxide, calcium hydroxide, chromium hydroxide, zirconium hydroxide, cerium hydroxide, nickel hydroxide, boron hydroxide, etc.), metal sulfate particles (barium sulfate, calcium sulfate, etc.), metal carbonate particles (calcium carbonate, magnesium carbonate, barium carbonate, etc.), metal nitride particles (boron nitride, aluminum nitride, etc.), and clay mineral particles (calcium silicate, talc, etc.). The inorganic particles may be surface-modified with a silane coupling agent or the like.
Examples of heat-resistant resins include wholly aromatic polyamide, polyamideimide, polyimide, polyethersulfone, polysulfone, polyetherketone, and polyetherimide.

When the composite porous substrate described above contains inorganic particles in the heat-resistant layer, the heat-resistant layer preferably also contains a binder resin that binds the inorganic particles. The binder resin may be a heat-resistant resin or a non-heat-resistant resin. Examples of non-heat-resistant resins include butadiene-based polymers (e.g., butadiene homopolymers, styrene-butadiene copolymers) and acrylic resins (e.g., homopolymers or copolymers of acrylic monomers, copolymers of acrylic monomers and styrene monomers).

One method for disposing a heat-resistant layer containing at least one of inorganic particles and a heat-resistant resin on one or both sides of a polyolefin microporous membrane is to coat one or both sides of the polyolefin microporous membrane with a coating liquid containing at least one of inorganic particles and a heat-resistant resin.

In this disclosure, the term "porous substrate" includes "composite porous substrate."

The surface of the porous substrate may be subjected to various surface treatments to improve wettability with the coating liquid used to form the adhesive porous layer, as long as the properties of the porous substrate are not impaired. Examples of surface treatments include corona treatment, plasma treatment, flame treatment, and ultraviolet irradiation treatment.

- Characteristics of porous substrate -
From the viewpoint of mechanical strength, the thickness of the porous substrate is preferably 1 μm or more, more preferably 3 μm or more, and even more preferably 5 μm or more.
From the viewpoint of increasing the energy density of the battery, the thickness of the porous substrate is preferably 10 μm or less, more preferably 9 μm or less, and even more preferably 8 μm or less.
The thickness of the porous substrate is determined by measuring 20 points within a 10 cm square area with a contact type thickness meter and averaging the measurements.

From the viewpoint of suppressing internal short circuits in the battery, the air permeability of the porous substrate is preferably 50 seconds/100 mL or more, more preferably 70 seconds/100 mL or more, and even more preferably 90 seconds/100 mL or more.
The air permeability of the porous substrate is preferably 220 seconds/100 mL or less, more preferably 200 seconds/100 mL or less, and even more preferably 180 seconds/100 mL or less, from the viewpoint of excellent electrolyte permeability and ion permeability.
The air permeability of the porous substrate is measured using a digital Oken air permeability tester in accordance with JIS P8117:2009.

When the porous substrate is a porous substrate consisting solely of a polyolefin microporous membrane (i.e., a polyolefin microporous membrane), the air permeability is preferably 50 sec/100 mL to 180 sec/100 mL, more preferably 70 sec/100 mL to 160 sec/100 mL or less, and even more preferably 90 sec/100 mL to 140 sec/100 mL.
When the porous substrate is a composite porous substrate in which a porous heat-resistant layer is disposed on one or both sides of a polyolefin microporous membrane, the air permeability is preferably 90 sec/100 mL to 220 sec/100 mL, more preferably 100 sec/100 mL to 210 sec/100 mL or less, and even more preferably 110 sec/100 mL to 200 sec/100 mL.

The porosity of the porous substrate is preferably 30% to 60% from the viewpoint of excellent electrolyte permeability and ion permeability.
The porosity ε (%) of the porous substrate is calculated by the following formula.
ε={1-Ws/(ds・t)}×100
Here, Ws is the basis weight (g/m ² ) of the porous substrate, ds is the true density (g/cm ³ ) of the porous substrate, and t is the thickness (μm) of the porous substrate. Basis weight is the mass per unit area.

[Adhesive porous layer]
The adhesive porous layer is a layer disposed on the surface of the porous substrate and is the outermost layer of the separator. The adhesive porous layer has numerous gaps or micropores, allowing gas or liquid to pass through from one surface to the other.

In the separator of the present disclosure, the adhesive porous layer preferably has a network structure containing the acrylic resin (1). The network structure of the adhesive porous layer means a structure in which the resin is continuously connected in a network shape and has a large number of pores. The network structure of the adhesive porous layer may be a planar network structure in the surface direction of the separator, or a three-dimensional network structure in the surface direction and thickness direction of the separator.
The three-dimensional mesh structure of the adhesive porous layer may be flattened by the heat press used to bond the separator to the electrode, and part or all of the separator may have a planar mesh structure when bonded to the electrode.

The mesh structure of the adhesive porous layer can be confirmed by observing the separator surface with a scanning electron microscope (SEM).

Examples of the mesh-like structure of the adhesive porous layer include a porous structure in which fibrils containing acrylic resin (1) are connected in a two-dimensional or three-dimensional mesh-like structure; a mesh-like microporous structure containing acrylic resin (1); etc.

When the adhesive porous layer contains inorganic particles, examples of the adhesive porous layer's form include a structure in which inorganic particles are bound or trapped in a porous structure in which fibrils containing acrylic resin (1) are connected in a two-dimensional or three-dimensional network; a structure in which inorganic particles are bound or trapped in a network-like microporous structure containing acrylic resin (1); and a layered structure in which acrylic resin (1) connects a large number of inorganic particles together, forming voids between the inorganic particles.

- Acrylic resin (1) -
The acrylic resin (1) contains at least (meth)acrylic acid, a (meth)acrylamide monomer, and at least one of methyl (meth)acrylate and butyl (meth)acrylate as polymerization components. The acrylic resin (1) may contain other monomers as polymerization components.

The (meth)acrylamide-based monomer is preferably a (meth)acrylamide-based monomer represented by the following formula (1):

In formula (1), ^R1 is a hydrogen atom or a methyl group, and ^R2 and ^R3 are each independently a hydrogen atom or an alkyl group having 1 to 4 carbon atoms. The alkyl group may be linear, branched, or cyclic.
^R2 and ^R3 are each independently preferably a hydrogen atom or an alkyl group having 1 to 3 carbon atoms, more preferably a hydrogen atom, a methyl group or an ethyl group, and even more preferably a hydrogen atom or a methyl group.

The (meth)acrylamide monomer is preferably at least one selected from the group consisting of acrylamide, methacrylamide, N-methylacrylamide, N-methylmethacrylamide, N,N-dimethylacrylamide, and N,N-dimethylmethacrylamide.

The total proportion of (meth)acrylic acid and (meth)acrylamide monomers in all polymerization components of the acrylic resin (1) is 20 mol% or more, preferably 20 mol% to 90 mol%, more preferably 20 mol% to 70 mol%, even more preferably 20 mol% to 50 mol%, still more preferably 25 mol% to 45 mol%, still more preferably 30 mol% to 40 mol%, and particularly preferably 35 mol% to 40 mol%.
The proportion of (meth)acrylic acid in all polymerization components of the acrylic resin (1) is preferably 10 mol% to 50 mol%, more preferably 15 mol% to 40 mol%, still more preferably 20 mol% to 30 mol%, and particularly preferably 20 mol% to 25 mol%.
The proportion of the (meth)acrylamide monomer in all polymerization components of the acrylic resin (1) is preferably 10 mol% to 40 mol%, more preferably 10 mol% to 30 mol%, still more preferably 10 mol% to 25 mol%, and particularly preferably 15 mol% to 20 mol%.

The molar ratio of (meth)acrylic acid to (meth)acrylamide monomer, which are the polymerization components of acrylic resin (1), is 40:60 to 60:40, preferably 45:55 to 60:40, and more preferably 50:50 to 60:40.

The total proportion of methyl (meth)acrylate and butyl (meth)acrylate in all polymerization components of the acrylic resin (1) is preferably 10 mol% to 80 mol%, more preferably 30 mol% to 70 mol%, and even more preferably 50 mol% to 65 mol%.
The proportion of methyl (meth)acrylate in all polymer components of the acrylic resin (1) is preferably 10 mol % to 65 mol %, more preferably 25 mol % to 60 mol %, and even more preferably 40 mol % to 55 mol %.
The proportion of butyl (meth)acrylate in all polymer components of the acrylic resin (1) is preferably 5 mol % to 25 mol %, more preferably 10 mol % to 20 mol %, and even more preferably 10 mol % to 15 mol %.

The butyl (meth)acrylate is at least one selected from the group consisting of n-butyl acrylate, n-butyl methacrylate, sec-butyl acrylate, sec-butyl methacrylate, isobutyl acrylate, isobutyl methacrylate, t-butyl acrylate, and t-butyl methacrylate, and is preferably at least one of n-butyl acrylate and n-butyl methacrylate.

Examples of such monomers include lower alkyl (meth)acrylate esters (having an alkyl group containing 8 or fewer carbon atoms) other than methyl (meth)acrylate and butyl (meth)acrylate. Examples of such lower alkyl (meth)acrylate esters include ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, n-pentyl (meth)acrylate, isopentyl (meth)acrylate, neopentyl (meth)acrylate, n-hexyl (meth)acrylate, isohexyl (meth)acrylate, n-heptyl (meth)acrylate, isoheptyl (meth)acrylate, n-octyl (meth)acrylate, isooctyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate.

Examples of such monomers include styrene-based monomers. Examples of styrene-based monomers include styrene, α-methylstyrene, and alkyl-substituted styrenes such as 2-methylstyrene, 3-methylstyrene, 4-methylstyrene, 2-ethylstyrene, 3-ethylstyrene, and 4-ethylstyrene. Preferred styrene-based monomers are styrene and α-methylstyrene, with styrene being more preferred.

Examples of such monomers include vinyl nitrile compounds. Examples of vinyl nitrile compounds include acrylonitrile and methacrylonitrile.

The total proportion of (meth)acrylic acid, (meth)acrylamide monomers, methyl (meth)acrylate, and butyl (meth)acrylate in all polymer components of the acrylic resin (1) is preferably 30 mol% to 100 mol%, more preferably 60 mol% to 100 mol%, and even more preferably 90 mol% to 100 mol%.

An example of an embodiment of the acrylic resin (1) is an acrylic resin (1) containing, as polymerization components, (meth)acrylic acid, a (meth)acrylamide monomer represented by formula (1), methyl (meth)acrylate, and butyl (meth)acrylate. The total proportion of the above monomers in all polymerization components of the acrylic resin (1) is preferably 90 mol % to 100 mol %.
The proportion of each monomer in the total polymerization components of the acrylic resin (1) is preferably 20 mol % to 25 mol % for (meth)acrylic acid, preferably 15 mol % to 20 mol % for the (meth)acrylamide monomer represented by formula (1), preferably 40 mol % to 55 mol % for methyl (meth)acrylate, and preferably 10 mol % to 15 mol % for butyl (meth)acrylate.

An example of an embodiment of the acrylic resin (1) is an acrylic resin (1) containing, as polymerization components, (meth)acrylic acid, at least one of N-methyl(meth)acrylamide and N,N-dimethyl(meth)acrylamide, methyl (meth)acrylate, and n-butyl (meth)acrylate. The total proportion of the above monomers in all polymerization components of the acrylic resin (1) is preferably 90 mol % to 100 mol %.
The proportion of each monomer in the total polymerization components of the acrylic resin (1) is preferably 20 mol % to 25 mol % for (meth)acrylic acid, preferably 15 mol % to 20 mol % for the total of N-methyl(meth)acrylamide and N,N-dimethyl(meth)acrylamide, preferably 40 mol % to 55 mol % for methyl (meth)acrylate, and preferably 10 mol % to 15 mol % for n-butyl (meth)acrylate.

The weight average molecular weight of the acrylic resin (1) is preferably 50,000 to 500,000, and more preferably 80,000 to 400,000.
The weight average molecular weight of the acrylic resin (1) is a molecular weight in terms of polystyrene measured by gel permeation chromatography (GPC). The measurement is performed using the acrylic resin (1) extracted from the adhesive porous layer or the acrylic resin (1) used to form the adhesive porous layer as a sample.

-Other resins-
The adhesive porous layer may contain a resin other than the acrylic resin (1). Examples of the other resin include acrylic resins other than the acrylic resin (1), butadiene-acrylonitrile resins, homopolymers or copolymers of vinyl nitrile compounds (such as acrylonitrile and methacrylonitrile), carboxymethyl cellulose, hydroxyalkyl cellulose, polyvinyl alcohol, polyvinyl butyral, polyvinyl pyrrolidone, polyethers (such as polyethylene oxide and polypropylene oxide), and mixtures of two or more of these. These resins may be used alone or in combination.

The mass proportion of the other resins in the total resin of the adhesive porous layer is preferably 10 mass % or less, more preferably 5 mass % or less, and even more preferably 1 mass % or less.
The mass proportion of the acrylic resin (1) in the total resin of the adhesive porous layer is preferably 90 mass % or more, more preferably 95 mass % or more, even more preferably 99 mass % or more, and particularly preferably 100 mass %.

It is preferable that the adhesive porous layer contains substantially no fluorine-containing resin. Examples of fluorine-containing resins include polyvinylidene fluoride resins and fluorine-containing rubbers. Examples of polyvinylidene fluoride resins include homopolymers of vinylidene fluoride (i.e., polyvinylidene fluoride); copolymers of vinylidene fluoride and halogen-containing monomers such as hexafluoropropylene, tetrafluoroethylene, trifluoroethylene, chlorotrifluoroethylene, vinyl fluoride, and trichloroethylene; copolymers of vinylidene fluoride and monomers other than halogen-containing monomers; copolymers of vinylidene fluoride, halogen-containing monomers, and monomers other than halogen-containing monomers; and mixtures thereof.

The adhesive porous layer being substantially free of fluorine-containing resin means that the mass ratio of the fluorine-containing resin in the adhesive porous layer is 1 mass % or less.
The mass proportion of the fluorine-containing resin in the adhesive porous layer is preferably as small as possible, and is preferably 0.5 mass% or less, more preferably 0.1 mass% or less, and particularly preferably 0 mass%. In other words, it is particularly preferable that the adhesive porous layer does not contain a fluorine-containing resin.

-particle-
The adhesive porous layer may further contain particles, which may include inorganic particles and/or organic particles.

One example of an embodiment of the adhesive porous layer is one that is substantially free of particles. An adhesive porous layer that is substantially free of particles means that the volume ratio of particles to the acrylic resin (1) and particles contained in the adhesive porous layer is less than 5 volume %. In this embodiment, the volume ratio of particles to the acrylic resin (1) and particles contained in the adhesive porous layer is 0 volume % or more and less than 5 volume %.

-Inorganic particles-
The adhesive porous layer may contain inorganic particles. From the viewpoint of thermal dimensional stability, i.e., resistance to thermal shrinkage even at high temperatures, the separator preferably includes an adhesive porous layer containing inorganic particles.

Examples of inorganic particles include metal oxide particles, metal hydroxide particles, metal sulfate particles, metal carbonate particles, metal nitride particles, and clay mineral particles.

Examples of metal oxides constituting the metal oxide particles include silica (silicon dioxide), alumina (aluminum oxide), boehmite (alumina monohydrate), titania (titanium oxide), zirconia (zirconium oxide), magnesium oxide, and barium oxide, with alumina being preferred.
Examples of metal hydroxides constituting the metal hydroxide particles include magnesium hydroxide, aluminum hydroxide, calcium hydroxide, chromium hydroxide, zirconium hydroxide, cerium hydroxide, nickel hydroxide, and boron hydroxide, with magnesium hydroxide being preferred.
Examples of metal sulfates constituting the metal sulfate particles include barium sulfate and calcium sulfate, with barium sulfate being preferred.
Examples of metal carbonates constituting the metal carbonate particles include calcium carbonate, magnesium carbonate, and barium carbonate.
Examples of metal nitrides that constitute the metal nitride particles include boron nitride and aluminum nitride.
Examples of clay mineral particles include calcium silicate and talc.

The inorganic particles may be surface-modified with a silane coupling agent or the like.

One type of inorganic particle may be used alone, or two or more types may be used in combination.

From the standpoint of stability in the electrolyte and electrochemical stability, at least one type of inorganic particle selected from the group consisting of metal oxide particles, metal hydroxide particles, and metal sulfate particles is preferred. Among these, at least one type selected from the group consisting of alumina particles (aluminum oxide particles), magnesium hydroxide particles, and barium sulfate particles is more preferred.

As inorganic particles, metal sulfate particles are preferred, and barium sulfate particles are more preferred, from the viewpoint that they are less likely to decompose the electrolytic solution or electrolyte and therefore less likely to cause gas generation inside the battery.

There are no limitations on the particle shape of the inorganic particles, and they may be spherical, elliptical, plate-like, needle-like, or irregular. From the perspective of suppressing internal short circuits in the battery, it is preferable that the inorganic particles contained in the adhesive porous layer be plate-like particles or non-agglomerated primary particles.

The average primary particle size of the inorganic particles contained in the adhesive porous layer is preferably 0.01 μm to 2 μm, more preferably 0.05 μm to 1 μm, and even more preferably 0.1 μm to 0.5 μm.
When the average primary particle size of the inorganic particles is 0.01 μm or more, a porous structure is easily formed in the adhesive porous layer, and the adhesive porous layer has excellent electrolyte permeability and ion permeability. From this viewpoint, the average primary particle size of the inorganic particles is more preferably 0.05 μm or more, and even more preferably 0.1 μm or more.
When the average primary particle size of the inorganic particles is 2 μm or less, the adhesive porous layer is easily adhered to the electrode and is not easily peeled off from the electrode. From this viewpoint, the average primary particle size of the inorganic particles is more preferably 1 μm or less, and even more preferably 0.5 μm or less.

The average primary particle size of inorganic particles is determined by measuring the long diameter of 100 randomly selected inorganic particles during observation with a scanning electron microscope (SEM) and averaging the long diameters of the 100 particles. The sample used for SEM observation is inorganic particles that are the material forming the adhesive porous layer, or inorganic particles extracted from the adhesive porous layer. There are no restrictions on the method for extracting inorganic particles from the adhesive porous layer. Examples of such methods include immersing the adhesive porous layer peeled off from the separator in an organic solvent that dissolves the binder resin, thereby dissolving the binder resin with the organic solvent and extracting the inorganic particles; or heating the adhesive porous layer peeled off from the separator to approximately 800°C to eliminate the binder resin and extract the inorganic particles.

When the adhesive porous layer contains inorganic particles, the volume ratio of the inorganic particles to the acrylic resin (1) and inorganic particles contained in the adhesive porous layer is preferably 5% to 50% by volume, more preferably 10% to 40% by volume, and even more preferably 15% to 30% by volume, from the viewpoint of achieving a good balance between the adhesiveness of the separator to the electrode and the thermal dimensional stability of the separator.

When adhesive porous layers are present on both sides of the porous substrate, the type and/or content of inorganic particles contained in one adhesive porous layer may be the same as or different from the type and/or content of inorganic particles contained in the other adhesive porous layer.

One example of an embodiment of the adhesive porous layer is one that contains substantially no inorganic particles. An adhesive porous layer that contains substantially no inorganic particles means that the volume ratio of inorganic particles to the acrylic resin (1) and inorganic particles contained in the adhesive porous layer is less than 5 volume %. In this embodiment, the volume ratio of inorganic particles to the acrylic resin (1) and inorganic particles contained in the adhesive porous layer is 0 volume % or more and less than 5 volume %.

-Organic particles-
The adhesive porous layer may contain organic particles, such as particles made of crosslinked polymers such as crosslinked poly(meth)acrylic acid, crosslinked poly(meth)acrylic acid ester, crosslinked polysilicone, crosslinked polystyrene, crosslinked polydivinylbenzene, crosslinked styrene-divinylbenzene copolymer, melamine resin, phenol resin, and benzoguanamine-formaldehyde condensate; and particles made of heat-resistant polymers such as polysulfone, polyacrylonitrile, aramid, and polyacetal.
The resin constituting the organic particles may be a mixture, modified product, derivative, copolymer (random copolymer, alternating copolymer, block copolymer, graft copolymer) or crosslinked product of the above-exemplified materials.

One type of organic particle may be used alone, or two or more types may be used in combination.

One example of an embodiment of the adhesive porous layer is one that contains substantially no organic particles. An adhesive porous layer that contains substantially no organic particles means that the volume ratio of organic particles to the acrylic resin (1) and organic particles contained in the adhesive porous layer is less than 5 volume %. In this embodiment, the volume ratio of organic particles to the acrylic resin (1) and organic particles contained in the adhesive porous layer is 0 volume % or more and less than 5 volume %.

-Other ingredients-
The adhesive porous layer may contain additives such as a dispersant such as a surfactant, a wetting agent, an antifoaming agent, and a pH adjuster. The dispersant is added, for example, to the coating liquid for forming the adhesive porous layer for the purpose of improving dispersibility, coatability, or storage stability. The wetting agent, antifoaming agent, and pH adjuster are added, for example, to the coating liquid for forming the adhesive porous layer for the purpose of improving compatibility with the porous substrate, preventing air entrapment in the coating liquid, or adjusting the pH.

- Characteristics of the adhesive porous layer -
When the adhesive porous layer does not substantially contain inorganic particles, the mass per unit area of the adhesive porous layer is preferably 0.3 g/m 2 to 2 g/m 2 per side of the separator, more preferably 0.4 g/m ² to 1.5 g/m ² , and even more preferably 0.5 g/m ² to 1 g/m ² , from the viewpoint of achieving a good balance between adhesion to the electrode, permeability ^of the electrolyte solution, and ion ^permeability .

When adhesive porous layers that do not substantially contain inorganic particles are present on both sides of the separator, the mass per unit area of the adhesive porous layers, in total for both sides, is preferably 0.6 g/m ² to 4 g/m ² , more preferably 0.8 g/m ² to 3 g/m ² , and even more preferably 1 g/m ² to 2 g/m ² .

When the adhesive porous layer contains inorganic particles, the mass per unit area of the adhesive porous layer is preferably 0.5 g/m 2 to 3 g/m ² per side of the separator, more preferably 0.8 g/m ² to 2.5 g/m 2, and even more preferably 1 g/m ² to 2 g/m ² , from the viewpoint of achieving a good balance between adhesion to the electrode, ^{permeability of} the electrolyte solution, and ion ^permeability .

When adhesive porous layers containing inorganic particles are present on both sides of the separator, the mass per unit area of the adhesive porous layers, in total for both sides, is preferably 1 g/m ² to 6 g/m ² , more preferably 1.5 g/m ² to 5 g/m ² , and even more preferably 2 g/m ² to 4 g/m ² .

The mass per unit area of the adhesive porous layer is calculated by cutting a 20cm x 20cm piece of separator, peeling off the adhesive porous layer, measuring the mass, and dividing the mass by the area.

[Separator characteristics]
From the viewpoint of mechanical strength, the thickness of the separator is preferably 5 μm or more, more preferably 7 μm or more, and even more preferably 9 μm or more.
From the viewpoint of the energy density of the battery, the thickness of the separator is preferably 15 μm or less, more preferably 12 μm or less, and even more preferably 10 μm or less.
The thickness of the separator is determined by measuring 20 points within a 10 cm square area with a contact type thickness meter and averaging the measurements.

From the viewpoint of suppressing internal short circuits in the battery, the air permeability of the separator is preferably 100 seconds/100 mL or more, more preferably 110 seconds/100 mL or more, and even more preferably 120 seconds/100 mL or more.
From the viewpoint of ion permeability, the air permeability of the separator is preferably 1000 seconds/100 mL or less, more preferably 700 seconds/100 mL or less, and even more preferably 500 seconds/100 mL or less.
The air permeability of the separator is measured using a digital Oken air permeability tester in accordance with JIS P8117:2009.

The porosity of the separator is preferably 30% to 60% from the viewpoint of ion permeability.
The porosity ε (%) of the separator is calculated by the following formula.

Here, for the separator's constituent material 1, constituent material 2, constituent material 3, ..., constituent material n, the mass per unit area of each constituent material is _W1 , W2 _{, W3} , ..., _Wn (g/ ^cm2 ), the true density of each constituent material is _d1 , _d2 , _d3 , ..., _dn (g/ ^cm3 ), and the thickness of the separator is t (cm).

[Separator manufacturing method]
The separator of the present disclosure can be manufactured, for example, by forming an adhesive porous layer on a porous substrate by a wet coating method or a dry coating method. In this disclosure, a wet coating method is a method in which a coating layer is solidified in a coagulation liquid, and a dry coating method is a method in which a coating layer is solidified by drying. An embodiment of the wet coating method is described below. In the following description, the "adhesive porous layer" will be simply referred to as the "porous layer."

Wet coating methods include, for example, a step of applying a coating liquid to one or both sides of a porous substrate to form a coating layer; a step of immersing the porous substrate with the coating layer in a coagulating liquid to solidify the coating layer and form a porous layer; and a step of removing the laminate consisting of the porous substrate and porous layer from the coagulating liquid, washing with water, and drying.

The coating liquid for forming the porous layer is prepared by dissolving the acrylic resin (1) in a solvent. If necessary, other components (e.g., inorganic particles) besides the acrylic resin (1) may be dissolved or dispersed in the coating liquid.

The solvent used to prepare the coating liquid includes a solvent that dissolves the acrylic resin (1) (hereinafter also referred to as a "good solvent"). Examples of good solvents include polar amide solvents such as N-methylpyrrolidone, dimethylacetamide, and dimethylformamide.

The solvent used to prepare the coating liquid may contain a phase separation agent that induces phase separation, in order to form a porous layer with a good porous structure. Therefore, the solvent used to prepare the coating liquid may be a mixed solvent of a good solvent and a phase separation agent. It is preferable to mix the phase separation agent with the good solvent in an amount that ensures a viscosity appropriate for coating. Examples of phase separation agents include water, butanediol, and ethylene glycol.

If the solvent used to prepare the coating liquid is a mixed solvent of a good solvent and a phase separation agent, from the perspective of forming a good porous structure, a mixed solvent containing 60% by mass or more of the good solvent and 5% to 40% by mass of the phase separation agent is preferred.

The resin concentration of the coating liquid is preferably 1% to 20% by mass in order to form a good porous structure. If the porous layer contains inorganic particles, the inorganic particle concentration of the coating liquid is preferably 0.5% to 50% by mass in order to form a good porous structure.

The coating liquid may contain dispersants such as surfactants, wetting agents, antifoaming agents, pH adjusters, etc. These additives may remain in the porous layer as long as they are electrochemically stable within the range of use of the non-aqueous secondary battery and do not inhibit reactions within the battery.

Means for applying the coating liquid to the porous substrate include a Mayer bar, die coater, reverse roll coater, roll coater, gravure coater, etc. When forming a porous layer on both sides of the porous substrate, it is preferable from the standpoint of productivity to apply the coating liquid to both sides of the porous substrate simultaneously.

The coating layer is solidified by immersing the porous substrate with the coating layer formed in a coagulation liquid, which induces phase separation in the coating layer while solidifying the resin. This results in a laminate consisting of the porous substrate and porous layer.

The coagulation liquid generally contains the good solvent and phase separation agent used in preparing the coating liquid, as well as water. From a production standpoint, it is preferable for the mixing ratio of the good solvent and phase separation agent to match the mixing ratio of the mixed solvent used in preparing the coating liquid. From the perspectives of forming a porous structure and productivity, it is preferable for the water content in the coagulation liquid to be 40% to 90% by mass. The temperature of the coagulation liquid is, for example, 20°C to 50°C.

After the coating layer is solidified in the coagulating liquid, the laminate is lifted out of the coagulating liquid and washed with water. The coagulating liquid is removed from the laminate by washing with water. The water is then removed from the laminate by drying. Washing with water is performed, for example, by transporting the laminate in a water bath. Drying is performed, for example, by transporting the laminate in a high-temperature environment, by blowing air on the laminate, or by bringing the laminate into contact with a heat roll. The drying temperature is preferably 40°C to 80°C.

The separator of the present disclosure can also be manufactured using a dry coating method. The dry coating method involves applying a coating liquid to a porous substrate, drying the coating layer, and volatilizing and removing the solvent, thereby forming a porous layer on the porous substrate.

The separator of the present disclosure can also be manufactured by creating a porous layer as an independent sheet, overlaying this porous layer on a porous substrate, and combining them using thermocompression bonding or an adhesive. Examples of methods for creating a porous layer as an independent sheet include forming the porous layer on a release sheet using the wet coating method or dry coating method described above.

<Nonaqueous secondary battery>
The nonaqueous secondary battery of the present disclosure is a nonaqueous secondary battery that generates electromotive force by doping and dedoping of lithium ions, and includes a positive electrode, a negative electrode, and the separator of the present disclosure. "Doping" refers to the phenomenon of lithium ions entering the active material of the electrode.

The nonaqueous secondary battery disclosed herein has a structure in which, for example, a battery element in which a negative electrode and a positive electrode face each other with a separator interposed between them is sealed together with an electrolyte in an exterior packaging material. The nonaqueous secondary battery disclosed herein is suitable for nonaqueous electrolyte secondary batteries, particularly lithium-ion secondary batteries.

By incorporating the separator of the present disclosure, the nonaqueous secondary battery of the present disclosure is less likely to separate from the electrodes. Therefore, the nonaqueous secondary battery of the present disclosure is less likely to develop internal short circuits.

Below, examples of the positive electrode, negative electrode, electrolyte, and exterior material included in the nonaqueous secondary battery of the present disclosure are described.

An example of a positive electrode is a structure in which an active material layer containing a positive electrode active material and a binder resin is disposed on a current collector. The active material layer may further contain a conductive additive. Examples of positive electrode active materials include lithium-containing transition metal oxides, such as LiCoO ₂ , LiNiO ₂ , LiMn _1/2 Ni _1/2 O ₂ , LiCo _1/3 Mn _1/3 Ni _1/3 O ₂ , LiMn ₂ O ₄ , LiFePO ₄ , LiCo _1/2 Ni _1/2 O ₂ , and LiAl _1/4 Ni _3/4 O ₂ . Examples of binder resins include polyvinylidene fluoride resins and styrene-butadiene copolymers. Examples of the conductive additive include carbon materials such as acetylene black, ketjen black, graphite powder, etc. Examples of the current collector include aluminum foil, titanium foil, stainless steel foil, etc., each having a thickness of 5 μm to 20 μm.

An example of an embodiment of the negative electrode is a structure in which an active material layer containing a negative electrode active material and a binder resin is disposed on a current collector. The active material layer may further contain a conductive additive. Examples of negative electrode active materials include materials capable of electrochemically absorbing lithium ions, such as carbon materials; alloys of lithium with silicon, tin, aluminum, etc.; and Wood's alloy. Examples of binder resins include polyvinylidene fluoride resins and styrene-butadiene copolymers. Examples of conductive additives include carbon materials such as acetylene black, ketjen black, graphite powder, and ultrafine carbon fibers. Examples of the current collector include copper foil, nickel foil, stainless steel foil, etc., with a thickness of 5 μm to 20 μm. Alternatively, metallic lithium foil may be used as the negative electrode.

The electrolyte solution is preferably a solution in which a lithium salt is dissolved in a non-aqueous solvent. Examples of lithium salts include LiPF ₆ , LiBF ₄ , and LiClO ₄ . Examples of non-aqueous solvents include cyclic carbonates such as ethylene carbonate, propylene carbonate, fluoroethylene carbonate, difluoroethylene carbonate, and vinylene carbonate; chain carbonates such as dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, and fluorine-substituted derivatives thereof; and cyclic esters such as γ-butyrolactone and γ-valerolactone. These may be used alone or in combination.
The electrolyte solution is preferably a solution in which a cyclic carbonate and a chain carbonate are mixed in a mass ratio (cyclic carbonate:chain carbonate) of 20:80 to 40:60, and a lithium salt is dissolved in the range of 0.5 mol/L to 1.5 mol/L.

Examples of exterior materials include aluminum laminate film packs and metal cans. Battery shapes include rectangular, cylindrical, and coin shapes, and the separators disclosed herein are suitable for all of these shapes.

The nonaqueous secondary battery of the present disclosure is preferably produced by first producing a laminate in which the separator of the present disclosure is disposed between a positive electrode and a negative electrode, and then using this laminate by either production method (1) or production method (2) described below.

Manufacturing method (1): The laminate is dry heat pressed to temporarily bond the electrodes and separator, then placed in an exterior packaging (e.g., an aluminum laminate film pack; the same applies below) and electrolyte is injected into it. The laminate is then wet heat pressed onto the exterior packaging to bond the electrodes and separator and seal the exterior packaging.

Manufacturing method (2): The laminate is placed in an exterior packaging and electrolyte is injected into it. The laminate is then wet heat pressed onto the exterior packaging to bond the electrodes and separator and seal the exterior packaging.

In manufacturing method (1) or manufacturing method (2), the pressing temperature of the wet heat press is preferably 50°C to 90°C, more preferably 60°C to 80°C. The pressing pressure of the wet heat press is preferably 0.1 MPa to 2 MPa, more preferably 0.5 MPa to 1.5 MPa. The pressing time of the wet heat press is preferably adjusted according to the pressing temperature and pressing pressure, for example, within the range of 1 minute to 12 hours.

In manufacturing method (1), the pressing temperature of the dry heat press is preferably 60°C to 90°C, more preferably 70°C to 85°C. The pressing pressure of the dry heat press is preferably 0.5 MPa to 5 MPa, more preferably 0.5 MPa to 3 MPa. The pressing time of the dry heat press is preferably adjusted according to the pressing temperature and pressing pressure, for example, within the range of 0.5 minutes to 1 hour.

When manufacturing a laminate in which a separator is placed between a positive electrode and a negative electrode, the method of placing the separator between the positive electrode and the negative electrode may be a method in which at least one layer of positive electrode, separator, and negative electrode are stacked in that order (the so-called stack method), or a method in which a positive electrode, separator, negative electrode, and separator are stacked in that order and wound up lengthwise.

The separator and nonaqueous secondary battery of the present disclosure will be described in more detail below with reference to the following examples. The materials, amounts used, ratios, processing procedures, etc. shown in the following examples can be modified as appropriate without departing from the spirit of the present disclosure. Therefore, the scope of the separator and nonaqueous secondary battery of the present disclosure should not be interpreted as being limited by the specific examples shown below.

In the following description, synthesis, processing, manufacturing, etc. were carried out at room temperature (25°C ± 3°C) unless otherwise noted.

<Measurement and evaluation methods>
The measurement and evaluation methods used in the examples and comparative examples are as follows.

[Thickness of porous substrate and separator]
The thicknesses of the porous substrate and the separator were measured at 20 points within a 10 cm square using a contact thickness meter (Mitutoyo Corporation, LITEMATIC VL-50S) and averaged. A spherical probe with a sphere radius of 10 mm (Mitutoyo Corporation) was used as the measurement terminal, and a load of 0.19 N was applied during the measurement.

[Air permeability of porous substrate and separator]
The air permeability (seconds/100 mL) of the porous substrate and the separator was measured according to JIS P8117:2009 using a digital Oken air permeability tester (Asahi Seiko Co., Ltd., model EG01).

[Analysis of acrylic resin (1)]
The acrylic resin (1) used to form the adhesive porous layer was used as a sample and the constituent units of the resin were analyzed by NMR.
The weight average molecular weight of the acrylic resin (1) used for forming the adhesive porous layer was measured by GPC using polystyrene as a standard sample.

[Solubility of resin in dimethylacetamide]
The resin used to form the adhesive porous layer was used as a sample. The resin was added to dimethylacetamide at a concentration of 20% by mass and stirred at room temperature for 5 hours. After stirring, the test solution was visually observed, and the solubility of the resin was classified as follows.
G: The resin was completely dissolved.
NG: Resin remained undissolved.

[Electrolyte resistance of resin]
The resin used to form the adhesive porous layer was used as a sample. The resin and dimethylacetamide were stirred and mixed to prepare a resin solution with a concentration of 20% by mass. The resin solution was applied to a glass plate and dried to prepare a resin film with a thickness of 200 μm. The resin film was immersed in an electrolyte solution (1 mol/L LiPF ₆ -ethylene carbonate:ethyl methyl carbonate [mass ratio 3:7]) at room temperature for 72 hours. The resin film after immersion was visually observed, and the shape was classified as follows:
G: The resin film maintained its shape.
NG: The resin film was deformed.

[Average primary particle size of inorganic particles]
The inorganic particles used to form the adhesive porous layer were observed under a scanning electron microscope (SEM) to determine the average primary particle size. The major axes of 100 randomly selected inorganic particles on the SEM image were measured, and the average of the major axes of the 100 particles was taken as the average primary particle size (μm).

[Mass per unit area of adhesive porous layer]
The separator was cut into a size of 20 cm x 20 cm, the adhesive porous layer was peeled off, the mass was measured, and the mass was divided by the area to determine the mass per unit area (g/m ² ) of both sides combined.

[Wet adhesion to electrodes]
A positive electrode slurry was prepared by mixing 89.5 parts by mass of lithium cobalt oxide powder as a positive electrode active material, 4.5 parts by mass of acetylene black as a conductive additive, 6 parts by mass of polyvinylidene fluoride as a binder resin, and an appropriate amount of N-methyl-2-pyrrolidone using a twin-arm mixer. The positive electrode slurry was applied to one side of a 20 μm-thick aluminum foil, dried, and pressed to obtain a positive electrode having a positive electrode active material layer on one side.
Hereinafter, in the description of this test, "electrode" means "positive electrode."

The electrode was cut into a rectangle measuring 15 mm wide x 70 mm long. The separator was cut into a rectangle measuring 18 mm TD x 74 mm MD. Release paper measuring 15 mm wide x 70 mm long was prepared. The separator was layered on the active material layer of the electrode, and then the release paper was layered on top of the separator to create a laminate.

The laminate was inserted into an aluminum laminate film pack, and an electrolyte solution (1 mol/L LiPF ₆ -ethylene carbonate:ethyl methyl carbonate [mass ratio 3:7]) was poured into the laminate, allowing the electrolyte solution to soak into the laminate. Next, the pack and the laminate were heat-pressed in the stacking direction using a heat press machine (wet heat press) to bond the electrodes and separators. The heat press conditions were a temperature of 85°C, a pressure of 1 MPa, and a time of 5 minutes. After heat pressing, the laminate was removed from the pack, and the release paper was peeled off to obtain a wet adhesive test piece.

The uncoated side of the test specimen's electrode was fixed to a metal plate with double-sided tape, and the metal plate was fixed to the lower chuck of a Tensilon (A&D Corporation, STB-1225S). The metal plate was fixed to the Tensilon so that the longitudinal direction of the test specimen (i.e., the separator's MD) was aligned with the direction of gravity. The separator was peeled away from the electrode by approximately 2 cm from the bottom edge, and this edge was fixed to the upper chuck, and a 180° peel test was performed. The tensile speed for the 180° peel test was 20 mm/min, and loads (N) were collected at 0.4 mm intervals from 10 mm to 40 mm after the start of measurement, and the average was calculated. The loads for 10 test specimens were then averaged to determine the adhesive strength (N/15 mm) between the electrode and separator.

[Heat shrinkage rate]
The separator was cut into a rectangle measuring 60 mm in TD x 180 mm in MD to prepare a test specimen. Marks were made on the test specimen at 20 mm and 170 mm from one end on the line dividing the test specimen in half in TD (referred to as points A and B, respectively). Marks were also made on the test specimen at 10 mm and 50 mm from one end on the line dividing the test specimen in half in MD (referred to as points C and D, respectively). A clip was attached to the test specimen (the clip was attached between the end closest to point A and point A), and the specimen was hung in an oven at 105°C and subjected to a heat treatment for 60 minutes under no tension. The lengths between A and B and between CD were measured before and after the heat treatment, and the heat shrinkage was calculated using the following formula. The heat shrinkage of the three test specimens was then averaged.

MD heat shrinkage (%) = {(length A and B before heat treatment - length A and B after heat treatment) ÷ length A and B before heat treatment} x 100

TD heat shrinkage (%) = {(CD length before heat treatment - CD length after heat treatment) ÷ CD length before heat treatment} x 100

<Preparation of Separator and Battery>
[Example 1]
-Separator production-
The acrylic resin (1) was dissolved in dimethylacetamide (DMAc) to prepare a coating solution (1) having an acrylic resin (1) concentration of 8.0 mass %. The monomer composition of the acrylic resin (1) used in Example 1 is as shown in Table 1.

An appropriate amount of coating liquid (1) was placed on a Mayer bar, and coating liquid (1) was applied to both sides of a polyethylene microporous membrane (thickness 6 μm, air permeability 100 sec/100 mL). The coating was applied so that the amount of coating on both sides of the polyethylene microporous membrane was equal. The polyethylene microporous membrane with the coating layer formed was immersed in a coagulation liquid (DMAc:water = 50:50 [mass ratio], liquid temperature 40°C) to solidify the coating layer. It was then washed in a water washing tank with water at a temperature of 40°C and dried. In this way, a separator was obtained in which adhesive porous layers were formed on both sides of the polyethylene microporous membrane.

- Preparation of the positive electrode -
A positive electrode slurry was prepared by mixing 89.5 parts by mass of lithium cobalt oxide powder as a positive electrode active material, 4.5 parts by mass of acetylene black as a conductive additive, 6 parts by mass of polyvinylidene fluoride as a binder resin, and an appropriate amount of N-methyl-2-pyrrolidone using a twin-arm mixer. The positive electrode slurry was applied to both sides of a 20 μm-thick aluminum foil, dried, and pressed to obtain a positive electrode having positive electrode active material layers on both sides.

- Preparation of negative electrode -
A negative electrode slurry was prepared by mixing 300 parts by mass of artificial graphite as a negative electrode active material, 7.5 parts by mass of an aqueous dispersion containing 40% by mass of a modified styrene-butadiene copolymer as a binder resin, 3 parts by mass of carboxymethyl cellulose as a thickener, and an appropriate amount of water with a twin-arm mixer. The negative electrode slurry was applied to both sides of a 10 μm thick copper foil, dried, and pressed to obtain a negative electrode having a negative electrode active material layer on both sides.

- Battery construction -
The positive and negative electrodes were each cut into a 30 mm x 50 mm rectangle, and a lead tab was welded to each. The separator was cut into a 35 mm x 55 mm rectangle. These were stacked so that the positive and negative electrodes alternated and a separator was sandwiched between the positive and negative electrodes, producing a laminate consisting of three positive electrodes, three negative electrodes, and five separators. The laminate was inserted into an aluminum laminate film pack, and an electrolyte solution (1 mol/L LiPF ₆ -ethylene carbonate:ethyl methyl carbonate [mass ratio 3:7]) was injected into the pack, allowing the electrolyte solution to penetrate the laminate. Next, the pack and the laminate were heat-pressed in the stacking direction using a heat press (wet heat press) to bond the electrodes and separators. The heat press conditions were a press temperature of 85°C, a press pressure of 1 MPa, and a press time of 5 minutes. In this way, a nonaqueous secondary battery was obtained.

[Example 2]
The acrylic resin (1) was dissolved in dimethylacetamide (DMAc), and barium sulfate particles were further stirred and dispersed to prepare a coating solution (2). The coating solution (2) had an acrylic resin (1) concentration of 8.0 mass%, and the acrylic resin (1):barium sulfate particles ratio was 80:20 [volume ratio]. The acrylic resin (1) used in Example 2 was the same as the acrylic resin (1) used in Example 1.

A separator was prepared in the same manner as in Example 1, except that coating liquid (1) was replaced with coating liquid (2). A nonaqueous secondary battery was prepared using this separator.

[Example 3]
A separator was prepared in the same manner as in Example 1, except that the acrylic resin (1) was changed to an acrylic resin (1) having the monomer composition shown in Table 1. A nonaqueous secondary battery was prepared using this separator.

[Example 4]
A separator was prepared in the same manner as in Example 2, except that the acrylic resin (1) was changed to an acrylic resin (1) having the monomer composition shown in Table 1. A nonaqueous secondary battery was prepared using this separator. The acrylic resin (1) used in Example 4 was the same as the acrylic resin (1) used in Example 3.

[Comparative Example 1]
A separator was produced in the same manner as in Example 2, except that the acrylic resin (1) was changed to a polyvinylidene fluoride resin (a binary copolymer of vinylidene fluoride and hexafluoropropylene, weight-average molecular weight 1,400,000, and hexafluoropropylene 1.5 mol%).

[Comparative Example 2]
A separator was produced in the same manner as in Example 1, except that the acrylic resin (1) was changed to polymethyl methacrylate resin.

In each of the examples and comparative examples, the adhesive porous layer was coated so that the separator had a thickness of 9 μm.
The mass of the adhesive porous layer shown in Table 1 is the total mass of both sides of the separator. The mass of the adhesive porous layer per separator side in each example and comparative example is half the mass of the adhesive porous layer shown in Table 1.

The abbreviations in Table 1 have the following meanings.
Mw: Weight average molecular weight; DMAc: Dimethylacetamide; VDF-HFP: Dipolymer of vinylidene fluoride and hexafluoropropylene; PMMA: Polymethyl methacrylate resin; MAA: Methacrylic acid; NMMAm: N-methylmethacrylamide; DMMAm: N,N-dimethylmethacrylamide; MMA: Methyl methacrylate; BA: n-butyl acrylate

The surfaces of the separators of Examples 1 and 3 were observed with a scanning electron microscope (SEM).
FIG. 1 shows an SEM image (magnification: 20,000) of the surface of the separator of Example 1.
FIG. 2 shows an SEM image (magnification: 20,000) of the surface of the separator of Example 3.

All publications, patent applications, and technical standards mentioned in this specification are incorporated by reference herein to the same extent as if each individual publication, patent application, and technical standard was specifically and individually indicated to be incorporated by reference.

The disclosure of Japanese Application No. 2024-023072, filed February 19, 2024, is incorporated herein by reference in its entirety.

Claims (8)

A porous substrate;
An adhesive porous layer containing an acrylic resin (1) arranged on one or both sides of the porous substrate,
The acrylic resin (1) contains, as polymerization components, (meth)acrylic acid, a (meth)acrylamide-based monomer, and at least one of methyl (meth)acrylate and butyl (meth)acrylate, wherein the total proportion of the (meth)acrylic acid and the (meth)acrylamide-based monomer in all polymerization components is 20 mol % or more, and the molar ratio of the (meth)acrylic acid to the (meth)acrylamide-based monomer is 40:60 to 60:40.
Separator for non-aqueous secondary batteries. The separator for a non-aqueous secondary battery according to claim 1, wherein the acrylic resin (1) contains methyl (meth)acrylate and butyl (meth)acrylate in a total proportion of 10 mol% to 80 mol% of all polymer components. The separator for a non-aqueous secondary battery according to claim 1, wherein the adhesive porous layer further contains inorganic particles. The non-aqueous secondary battery separator according to claim 1, wherein the air permeability of the non-aqueous secondary battery separator is 100 sec/100 mL to 1000 sec/100 mL. The separator for a non-aqueous secondary battery according to claim 1, wherein the porous substrate comprises a polyolefin microporous membrane. The porous substrate is
a polyolefin microporous membrane;
a heat-resistant layer containing at least one of inorganic particles and a heat-resistant resin, disposed on one or both sides of the polyolefin microporous membrane;
The separator for a non-aqueous secondary battery according to claim 1 . The separator for a non-aqueous secondary battery according to claim 1, wherein the adhesive porous layer is substantially free of fluorine-containing resin. A non-aqueous secondary battery separator according to any one of claims 1 to 7, wherein the separator is disposed between the positive electrode and the negative electrode;
Electromotive force is generated by doping and dedoping of lithium ions.
Non-aqueous secondary battery.