Separator for nonaqueous secondary battery, and nonaqueous secondary battery

Abstract

Provided is separator for a nonaqueous secondary battery comprising: a polyolefin microporous film; a heat-resistant resin layer that is disposed on one surface or both surfaces of the polyolefin microporous film, and contains more than 10% by mass of a heat-resistant resin; a surface layer (A) that is disposed on one surface of the layered body of the polyolefin microporous film and the heat-resistant resin layer, so as to be in contact with the heat-resistant resin layer, and contains acrylic resin particles; and a surface layer (F) that is disposed on the other surface of the layered body of the polyolefin microporous film and the heat-resistant resin layer, contains a polyvinylidene fluoride resin, and has the three-dimensional network structure of the polyvinylidene fluoride resin.

Description

Separator for non-aqueous secondary batteries and non-aqueous secondary batteries

[0001] This disclosure relates to a separator for non-aqueous secondary batteries and a non-aqueous secondary battery.

[0002] As a measure to suppress internal short circuits in non-aqueous secondary batteries, separators are known that have one or both of the following (1) and (2) provided on a substrate: (1) A heat-resistant layer containing inorganic particles and / or a heat-resistant resin; (2) An adhesive layer with excellent adhesion to electrodes.

[0003] For example, Patent Document 1 discloses a separator for a non-aqueous secondary battery comprising a porous substrate, a heat-resistant layer containing barium sulfate particles and meta-aramid, and an adhesive layer consisting of polyvinylidene fluoride resin particles and acrylic resin particles.

[0004] Japanese Patent Publication No. 2021-192385

[0005] To improve the ability to prevent internal short circuits in batteries, it is desirable to laminate a heat-resistant layer and an adhesive layer on the substrate. However, in this case, the separator tends to become thicker. As a result, the ion mobility of the separator decreases, leading to a problem of reduced battery charge and discharge characteristics.

[0006] This disclosure is made under the circumstances described above. The object of this disclosure is to provide a separator for non-aqueous secondary batteries that, when applied to non-aqueous secondary batteries, exhibits excellent performance in suppressing internal short circuits in the battery while maintaining good battery cycle characteristics.

[0007] Specific means for solving the above problems include the following embodiments: <1> A separator for a non-aqueous secondary battery, comprising: a polyolefin microporous membrane; a heat-resistant resin layer disposed on one or both sides of the polyolefin microporous membrane, containing a heat-resistant resin and comprising more than 10% by mass of the heat-resistant resin; a surface layer (A) containing acrylic resin particles disposed on one side of the laminate of the polyolefin microporous membrane and the heat-resistant resin layer in contact with the heat-resistant resin layer; and a surface layer (F) disposed on the other side of the laminate of the polyolefin microporous membrane and the heat-resistant resin layer, containing a polyvinylidene fluoride resin and having a three-dimensional network structure of the polyvinylidene fluoride resin. <2> The separator for a non-aqueous secondary battery according to claim 1, wherein the heat-resistant resin includes at least one selected from the group consisting of aromatic polyamide, polyimide, and polyamideimide. <3> The separator for a non-aqueous secondary battery according to <1>, wherein the heat-resistant resin layer further contains inorganic particles. <4> The separator for a non-aqueous secondary battery according to <3>, wherein the average primary particle size of the inorganic particles contained in the heat-resistant resin layer is 0.01 μm to 0.3 μm. <5> The separator for a non-aqueous secondary battery according to <3> or <4>, wherein the inorganic particles contain barium sulfate particles. <6> The separator for a non-aqueous secondary battery according to any one of <1> to <5>, wherein the polyvinylidene fluoride resin contains a polyvinylidene fluoride resin having hexafluoropropylene units. <7> The separator for a non-aqueous secondary battery according to any one of <1> to <6>, wherein the surface layer (A) further contains polyvinylidene fluoride resin particles. <8> The separator for a non-aqueous secondary battery according to <7>, wherein the ratio Da / Df of the average primary particle size Da of the acrylic resin particles contained in the surface layer (A) to the average primary particle size Df of the polyvinylidene fluoride resin particles is 1.2 to 2.8. <9> The separator for a non-aqueous secondary battery according to any one of <1> to <8>, further comprising an inorganic particle layer containing inorganic particles, wherein the inorganic particles account for 90% by mass or more, between the polyolefin microporous membrane and the heat-resistant resin layer below the surface layer (A) and / or between the polyolefin microporous membrane and the surface layer (F).<10> The separator for non-aqueous secondary battery according to <9>, wherein the average primary particle diameter of the inorganic particles contained in the inorganic particle layer is 10 nm to 500 nm. <11> The separator for non-aqueous secondary battery according to <9> or <10>, wherein the inorganic particles contain at least one selected from the group consisting of γ-alumina particles, boehmite particles, and barium sulfate particles. <12> A non-aqueous secondary battery comprising a positive electrode, a negative electrode, and the separator for non-aqueous secondary battery according to any one of <1> to <11> disposed between the positive electrode and the negative electrode, and obtaining an electromotive force by doping and de-doping of lithium ions.

[0008] According to the present disclosure, when applied to a non-aqueous secondary battery, there is provided a separator for non-aqueous secondary battery that has excellent performance in suppressing internal short circuit of the battery and good cycle characteristics of the battery.

[0009] It is a schematic cross-sectional view of an example of the separator of the present disclosure. It is a schematic cross-sectional view of another example of the separator of the present disclosure. It is a schematic cross-sectional view of another example of the separator of the present disclosure. It is a schematic cross-sectional view of another example of the separator of the present disclosure. It is an SEM image of the surface of the separator of Example 1. It is an SEM image of the surface of the separator of Example 2. It is a graph showing the internal short circuit resistance of the battery. It is a graph showing the cycle characteristics of the battery.

[0010] Hereinafter, embodiments of the present disclosure will be described. These descriptions and examples are illustrative of the embodiments and do not limit the scope of the embodiments.

[0011] In the present disclosure, a numerical range indicated by using "~" indicates a range including the numerical values described before and after "~" as the minimum value and the maximum value, respectively. In the numerical ranges described stepwise in the present disclosure, the upper limit value or the lower limit value described in one numerical range may be replaced with the upper limit value or the lower limit value of the numerical range described in other stepwise descriptions. Also, in the numerical ranges described in the present disclosure, the upper limit value or the lower limit value of the numerical range may be replaced with the value shown in the examples.

[0012] In the present disclosure, "A and / or B" is synonymous with "at least one of A and B". That is, "A and / or B" means that it may be only A, only B, or a combination of A and B.

[0013] In the present disclosure, the term "step" includes not only an independent step but also the case where it cannot be clearly distinguished from other steps, as long as the purpose of the step is achieved.

[0014] When referring to the amount of each component in a composition in the present disclosure, when there are multiple substances corresponding to each component in the composition, unless otherwise specified, it means the total amount of the multiple substances present in the composition. Each component may include multiple types of particles corresponding thereto. When there are multiple types of particles corresponding to each component in the composition, the particle size of each component means a value for the mixture of the multiple types of particles present in the composition, unless otherwise specified.

[0015] In the present disclosure, MD (Machine Direction) means the longitudinal direction in a separator manufactured in a long shape, and TD (Transverse Direction) means the direction perpendicular to MD in the plane direction of the separator. In the present disclosure, TD is also referred to as the "width direction".

[0016] In the present disclosure, when expressing the lamination relationship of each layer constituting the separator in terms of "upper layer" and "lower layer", the layer closer to the polyolefin microporous membrane is referred to as the "lower layer", and the layer farther from the polyolefin microporous membrane is referred to as the "upper layer".

[0017] In the present disclosure, the volume excluding the pores of the porous layer is referred to as the "solid volume".

[0018] In the present disclosure, performing a heat press treatment after infiltrating an electrolyte into the separator is referred to as "wet heat press", and performing a heat press treatment without infiltrating an electrolyte into the separator is referred to as "dry heat press".

[0019] In this disclosure, "monomer unit" of a polymer or resin means a constituent unit of a polymer or resin, which is formed by the polymerization of monomers. In this disclosure, the notation "(meth)acrylic" means that either "acrylic" or "methacrylic" is acceptable.

[0020] <Separator for Non-Aqueous Secondary Battery> The separator for non-aqueous secondary battery of the present disclosure (also simply referred to as "separator" in this disclosure) comprises a polyolefin microporous membrane, a heat-resistant resin layer containing a heat-resistant resin and comprising more than 10% by mass of the heat-resistant resin disposed on one or both sides of the polyolefin microporous membrane, a surface layer (A) containing acrylic resin particles disposed in contact with the heat-resistant resin layer on one side of the laminate of the polyolefin microporous membrane and the heat-resistant resin layer, and a surface layer (F) containing a polyvinylidene fluoride resin and having a three-dimensional network structure of the polyvinylidene fluoride resin, disposed on the other side of the laminate of the polyolefin microporous membrane and the heat-resistant resin layer.

[0021] In the separator of this disclosure, the laminate of a polyolefin microporous membrane and a heat-resistant resin layer may have the heat-resistant resin layer disposed on only one side of the polyolefin microporous membrane, or it may have the heat-resistant resin layer disposed on both sides of the polyolefin microporous membrane. When the laminate of the polyolefin microporous membrane and the heat-resistant resin layer is a laminate in which the heat-resistant resin layer is disposed on only one side of the polyolefin microporous membrane, a surface layer (A) is disposed in contact with this heat-resistant resin layer.

[0022] The separator of this disclosure has one outermost layer which is a surface layer (A) and the other outermost layer which is a surface layer (F). In a battery equipped with the separator of this disclosure, the surface layer (A) may face the positive electrode and the surface layer (F) may face the negative electrode, or the surface layer (F) may face the positive electrode and the surface layer (A) may face the negative electrode.

[0023] The separator of this disclosure enhances the rigidity of the battery cell by exhibiting adhesion to the electrodes through surface layers (A) and (F), and furthermore, the heat-resistant resin layer provides excellent heat resistance, resulting in superior performance in suppressing internal short circuits in the battery. The separator of this disclosure has excellent ion permeability because surface layer (A) is a layer of resin particles and surface layer (F) is a layer having a three-dimensional network structure, resulting in good battery cycle characteristics. Therefore, when applied to a non-aqueous secondary battery, the separator of this disclosure provides excellent performance in suppressing internal short circuits in the battery while also having good battery cycle characteristics.

[0024] The details of the polyolefin microporous membrane and each layer of the separator of this disclosure are described below.

[0025] [Polyolefin Microporous Membrane] A polyolefin microporous membrane refers to a microporous membrane containing polyolefin. A microporous membrane is a membrane that has a large number of micropores inside, with the micropores interconnected, allowing gas or liquid to pass from one side to the other.

[0026] The separator of this disclosure has a shutdown function by comprising a polyolefin microporous membrane. The shutdown function refers to a function that prevents thermal runaway of the battery by blocking ion movement when the battery temperature rises, as the constituent material of the porous structure melts and the pores of the porous structure become blocked.

[0027] Examples of polyolefin microporous membranes include those used in conventional battery separators, and it is preferable to select one from among these that has sufficient mechanical properties and ion permeability.

[0028] From the viewpoint of exhibiting a shutdown function, a polyethylene-containing microporous membrane (hereinafter referred to as "polyethylene microporous membrane") is preferred among the polyolefin microporous membranes. The polyethylene content is preferably 95% by mass or more relative to the mass of the polyethylene microporous membrane.

[0029] From the viewpoint of possessing heat resistance that prevents the film from easily breaking when exposed to high temperatures, polyolefin microporous membranes containing polypropylene are preferred.

[0030] From the viewpoint of possessing both a shutdown function and heat resistance that prevents easy rupture when exposed to high temperatures, polyolefin microporous membranes containing polyethylene and polypropylene are preferred. Examples of polyolefin microporous membranes containing polyethylene and polypropylene include microporous membranes in which polyethylene and polypropylene are mixed in a single layer. In such microporous membranes, from the viewpoint of achieving both a shutdown function and heat resistance, it is preferable to include 95% by mass or more of polyethylene and 5% by mass or less of polypropylene. From the viewpoint of achieving both a shutdown function and heat resistance, polyolefin microporous membranes having a laminated structure of two or more layers, in which at least one layer contains polyethylene and at least one layer contains polypropylene, are also preferred.

[0031] The polyolefin used in the polyolefin microporous membrane is preferably one with a weight-average molecular weight (Mw) of 100,000 to 5,000,000. A polyolefin with an Mw of 100,000 or more provides sufficient mechanical properties to the microporous membrane. A polyolefin with an Mw of 5,000,000 or less exhibits good shutdown characteristics and facilitates the molding of the microporous membrane.

[0032] Methods for producing polyolefin microporous membranes include: a method in which molten polyolefin resin is extruded from a T-die to form a sheet, which is then crystallized, stretched, and subsequently heat-treated to form a microporous membrane; and a method in which molten polyolefin resin together with a plasticizer such as liquid paraffin is extruded from a T-die, which is then cooled to form a sheet, stretched, the plasticizer is extracted, and then heat-treated to form a microporous membrane.

[0033] - Characteristics of Polyolefin Microporous Films - From the viewpoint of mechanical strength, the thickness of the polyolefin microporous film 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 polyolefin microporous film is preferably 10 μm or less, more preferably 9 μm or less, and even more preferably 8 μm or less. The thickness of the polyolefin microporous film is determined by measuring 20 points within a 10 cm square area using a contact-type length measuring instrument and averaging the results.

[0034] The porosity of the polyolefin microporous membrane is preferably 30% to 60% from the viewpoint of excellent electrolyte permeability and ion permeability. The porosity ε (%) of the polyolefin microporous membrane is calculated by the following formula: ε = {1 - Ws / (ds・t)} × 100 where Ws is the mass per unit area (i.e., basis weight) (g / m²) of the polyolefin microporous membrane. ２ ), ds is the true density (g / cm³) of the polyolefin microporous membrane. ３ ), t is the thickness of the polyolefin microporous membrane (μm).

[0035] From the viewpoint of suppressing internal short circuits in the battery, the air permeability of the polyolefin microporous membrane is preferably 80 seconds / 100 mL or more, more preferably 90 seconds / 100 mL or more, and even more preferably 100 seconds / 100 mL or more. From the viewpoint of excellent electrolyte permeability and ion permeability, the air permeability of the polyolefin microporous membrane is preferably 200 seconds / 100 mL or less, more preferably 180 seconds / 100 mL or less, and even more preferably 160 seconds / 100 mL or less. The air permeability of the polyolefin microporous membrane is the value measured using a Gurley densometer in accordance with JIS P8117:2009 "Paper and cardboard - Test methods for air permeability and air permeability resistance (intermediate region) - Gurley method".

[0036] [Surface layer (A)] The surface layer (A) contains at least acrylic resin particles. The surface layer (A) may contain other components besides acrylic resin particles. The surface layer (A) may contain resin particles other than acrylic resin particles. Examples of resin particles other than acrylic resin particles include polyvinylidene fluoride resin particles.

[0037] In the separator of this disclosure, the surface layer (A) is a layer in contact with the heat-resistant resin layer. Preferably, the resin particles contained in the surface layer (A) are attached to the heat-resistant resin layer.

[0038] The surface layer (A) has a porous structure. Examples of the porous structure of the surface layer (A) include the following forms.

[0039] - A layered structure in which acrylic resin particles are arranged adjacent to each other in two or three dimensions, with voids between the resin particles. - A layered structure in which acrylic resin particles and polyvinylidene fluoride resin particles are arranged adjacent to each other in two or three dimensions, with voids between the resin particles. - A layered structure in which acrylic resin particles are scattered on a heat-resistant resin layer. - A layered structure in which acrylic resin particles and polyvinylidene fluoride resin particles are scattered on a heat-resistant resin layer.

[0040] In a separator that is bonded to an electrode, some or all of the resin particles contained in the surface layer (A) may melt due to the heat applied to bond the separator to the electrode, causing adjacent resin particles to connect and resulting in some or all of them not maintaining their particle shape.

[0041] - Acrylic Resin Particles - Examples of acrylic resins constituting acrylic resin particles include homopolymers or copolymers of acrylic monomers; and copolymers of acrylic monomers and styrene monomers. Copolymers include alternating copolymers, random copolymers, block copolymers, and graft copolymers. Acrylic resins may be used individually or mixed together in groups of two or more.

[0042] Examples of acrylic monomers for acrylic resins include (meth)acrylic acid, methyl (meth)acrylate, ethyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate, n-hexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate, stearyl (meth)acrylate, cyclohexyl (meth)acrylate, dicyclopentanyl (meth)acrylate, isobornyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate. Alkyl (meth)acrylate esters are preferred as the acrylic monomers. The alkyl group in the ester portion of the alkyl (meth)acrylate ester is preferably a C1-C10 alkyl group, and more preferably a C1-C8 alkyl group. The acrylic monomers may be used individually or in combination of two or more.

[0043] Examples of styrene monomers for acrylic resins include styrene, α-methylstyrene; alkyl-substituted styrenes such as 2-methylstyrene, 3-methylstyrene, 4-methylstyrene, 2-ethylstyrene, 3-ethylstyrene, and 4-ethylstyrene; halogen-substituted styrenes such as 2-chlorostyrene, 3-chlorostyrene, and 4-chlorostyrene; and fluorine-substituted styrenes such as 4-fluorostyrene and 2,5-difluorostyrene. Styrene and α-methylstyrene are preferred as styrene monomers, with styrene being more preferred. Styrene monomers may be used individually or in combination of two or more.

[0044] From the viewpoint of ion permeability of the surface layer (A), the average primary particle size of the acrylic resin particles is preferably 50 nm or more, more preferably 100 nm or more, and even more preferably 200 nm or more. From the viewpoint of thinning the surface layer (A) and suppressing delamination between the surface layer (A) and the heat-resistant resin layer, and maintaining adhesion of the separator to the electrode, the average primary particle size of the acrylic resin particles is preferably 1000 nm or less, more preferably 800 nm or less, and even more preferably 500 nm or less.

[0045] The average primary particle size of acrylic resin particles is determined by measuring the major axis of 100 randomly selected acrylic resin particles during scanning electron microscopy (SEM) observation and averaging the major axes of these 100 particles. The sample used for SEM observation is either acrylic resin particles that form the surface layer (A), or acrylic resin particles extracted from the surface layer (A).

[0046] -Polyvinylidene fluoride resin particles- The surface layer (A) preferably contains polyvinylidene fluoride resin particles, from the viewpoint of having better adhesion to the heat-resistant resin layer and to the electrodes.

[0047] Examples of polyvinylidene fluoride resins that constitute polyvinylidene fluoride resin particles include: a homopolymer of vinylidene fluoride (i.e., polyvinylidene fluoride); a copolymer of vinylidene fluoride and a halogen-containing monomer; a copolymer of vinylidene fluoride and other monomers other than halogen-containing monomers; a copolymer of vinylidene fluoride, a halogen-containing monomer, and other monomers other than halogen-containing monomers; and mixtures thereof. Examples of halogen-containing monomers include hexafluoropropylene, tetrafluoroethylene, trifluoroethylene, chlorotrifluoroethylene, vinyl fluoride, and trichloroethylene. Copolymers include alternating copolymers, random copolymers, block copolymers, and graft copolymers. Polyvinylidene fluoride resins may be used individually or as a mixture of two or more types.

[0048] From the viewpoint of ion permeability of the surface layer (A), the average primary particle size of the polyvinylidene fluoride resin particles is preferably 50 nm or more, more preferably 100 nm or more, and even more preferably 200 nm or more. When the surface layer (A) contains polyvinylidene fluoride resin particles, from the viewpoint of making the surface layer (A) thinner, the average primary particle size of the polyvinylidene fluoride resin particles is preferably 350 nm or less, more preferably 300 nm or less, and even more preferably 250 nm or less.

[0049] The average primary particle size of polyvinylidene fluoride resin particles is determined by measuring the major axis of 100 randomly selected polyvinylidene fluoride resin particles observed using a scanning electron microscope (SEM) and averaging the major axes of these 100 particles. The sample used for SEM observation is either polyvinylidene fluoride resin particles that form the surface layer (A), or polyvinylidene fluoride resin particles extracted from the surface layer (A).

[0050] The ratio Da / Df of the average primary particle size Da of the acrylic resin particles contained in the surface layer (A) to the average primary particle size Df of the polyvinylidene fluoride resin particles is preferably 1.2 to 2.8. When the ratio Da / Df is 1.5 or higher, the resin particles are more easily filled on the surface of the heat-resistant resin layer, and the adhesion of the surface layer (A) to the heat-resistant resin layer and the adhesion of the surface layer (A) to the electrode are better. When the ratio Da / Df is 2.5 or lower, voids are more easily formed between the resin particles, and the ion permeability of the surface layer (A) is better. From the above viewpoint, the ratio Da / Df is more preferably 1.5 to 2.5, and even more preferably 1.8 to 2.2.

[0051] The surface layer (A) preferably contains acrylic resin particles and polyvinylidene fluoride resin particles. The proportion of acrylic resin particles to the total amount of acrylic resin particles and polyvinylidene fluoride resin particles contained in the surface layer (A) is preferably 15% to 50% by mass, more preferably 20% to 45% by mass, and even more preferably 25% to 40% by mass.

[0052] The ratio of the total amount of acrylic resin particles and polyvinylidene fluoride resin particles to the total amount of surface layer (A) is preferably 90% to 100% by mass, more preferably 95% to 100% by mass, and even more preferably 99% to 100% by mass.

[0053] - Characteristics of Surface Layer (A) - The mass per unit area (i.e., basis weight) of Surface Layer (A) is 0.2 g / m² from the viewpoint of adhesion to the electrode. ２ Preferably, it is 0.3 g / m ２ The above is more preferable, 0.4 g / m ２ The above is even more preferable. The mass per unit area (i.e., basis weight) of the surface layer (A) is 1.0 g / m² from the viewpoint of the energy density of the battery. ２ Preferably, it is 0.8 g / m ２ The following is more preferable: 0.6 g / m ２ The following is even more preferable.

[0054] The mass per unit area (i.e., basis weight) of the surface layer (A) is calculated by subtracting the mass of the flat film from which the surface layer (A) has been removed from the mass of a separator cut to 20 cm x 20 cm, and then dividing the difference in mass by the area.

[0055] [Surface layer (F)] The surface layer (F) contains at least a polyvinylidene fluoride resin. The surface layer (F) may contain other components besides the polyvinylidene fluoride resin. The surface layer (F) may contain inorganic particles.

[0056] The surface layer (F) is a porous layer having a three-dimensional network structure of polyvinylidene fluoride resin. Examples of the morphology of the surface layer (F) are shown below.

[0057] - A structure in which fibrils containing polyvinylidene fluoride resin are linked together in a three-dimensional network. - A structure in which fibrils containing polyvinylidene fluoride resin are linked together in a three-dimensional network, and inorganic particles are bound to or trapped in the fibrils.

[0058] -Polyvinylidene fluoride resins- Examples of polyvinylidene fluoride resins include: homopolymers of vinylidene fluoride (i.e., polyvinylidene fluoride); copolymers of vinylidene fluoride and halogen-containing monomers; 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. Examples of halogen-containing monomers include hexafluoropropylene, tetrafluoroethylene, trifluoroethylene, chlorotrifluoroethylene, vinyl fluoride, and trichloroethylene. Copolymers include alternating copolymers, random copolymers, block copolymers, and graft copolymers.

[0059] Polyvinylidene fluoride resins may be used individually or in mixtures of two or more types.

[0060] The total weight-average molecular weight (Mw) of the polyvinylidene fluoride resin contained in the surface layer (F) is preferably 600,000 or more, more preferably 700,000 or more, and even more preferably 800,000 or more, from the viewpoint of preventing blockage of the pores in the surface layer (F) when heat is applied to the separator during battery manufacturing. The total Mw of the polyvinylidene fluoride resin contained in the surface layer (F) is preferably 2,000,000 or less, more preferably 1,500,000 or less, and even more preferably 1,200,000 or less, from the viewpoint of allowing the polyvinylidene fluoride resin to soften appropriately when heat is applied to the separator during battery manufacturing, and ensuring good adhesion between the surface layer (F) and the electrodes.

[0061] The total Mw of the polyvinylidene fluoride resin contained in the surface layer (F) is the molecular weight in polystyrene equivalent, measured by gel permeation chromatography (GPC). The polyvinylidene fluoride resin extracted from the surface layer (F) or the polyvinylidene fluoride resin used to form the surface layer (F) is used as the sample.

[0062] As a polyvinylidene fluoride-based resin, a copolymer of vinylidene fluoride (VDF) and hexafluoropropylene (HFP) (VDF-HFP copolymer) is preferred from the viewpoint of adhesion to electrodes. In this disclosure, the VDF-HFP copolymer includes both copolymers obtained by polymerizing only VDF and HFP, and copolymers obtained by polymerizing VDF, HFP, and other monomers. By increasing or decreasing the HFP content, the crystallinity, heat resistance, and degree of swelling in the electrolyte of the VDF-HFP copolymer can be controlled to an appropriate range.

[0063] The polyvinylidene fluoride resin preferably contains the following polyvinylidene fluoride resin (1) and polyvinylidene fluoride resin (2).

[0064] - Polyvinylidene fluoride resin (1): A polyvinylidene fluoride resin containing VDF and HFP as polymerization components, in which the proportion of HFP in the total of VDF and HFP is greater than 1.5 mol% and 5 mol% or less. - Polyvinylidene fluoride resin (2): A polyvinylidene fluoride resin containing VDF and HFP as polymerization components, in which the proportion of HFP in the total of VDF and HFP is greater than 5 mol% and 15 mol% or less.

[0065] Polyvinylidene fluoride resins (1) and (2) may each contain monomers other than VDF and HFP as polymerization components. Examples of other monomers include halogen-containing monomers such as tetrafluoroethylene, trifluoroethylene, chlorotrifluoroethylene, vinyl fluoride, and trichloroethylene.

[0066] The polyvinylidene fluoride resins (1) and (2) each preferably have a total of 80 mol% or more of VDF and HFP in relation to the total of halogen-containing monomers, more preferably 90 mol% or more, and even more preferably 100 mol%.

[0067] The polyvinylidene fluoride resin (1) facilitates the development of a fine porous structure in the surface layer (F), and also suppresses excessive swelling of the polyvinylidene fluoride resin in relation to the electrolyte, thus preventing pore blockage. The polyvinylidene fluoride resin (2) prevents the porosity and average pore diameter of the surface layer (F) from becoming too large, and ensures high mobility of the polymer chains when heated and easy swelling in relation to the electrolyte, resulting in good adhesion between the surface layer (F) and the electrode.

[0068] The polyvinylidene fluoride resins (1) and (2) each preferably have a weight-average molecular weight (Mw) of 600,000 to 2,000,000. When the Mw of the polyvinylidene fluoride resins (1) and (2) is 600,000 or more, the surface layer (F) can be made to have mechanical properties that can withstand bonding treatment with the electrode, resulting in superior adhesion to the electrode. From this viewpoint, the Mw of the polyvinylidene fluoride resins (1) and (2) is more preferably 700,000 or more, and even more preferably 800,000 or more. When the Mw of the polyvinylidene fluoride resins (1) and (2) each is 2,000,000 or less, the resin softens easily by hot pressing, making it easier for the surface layer (F) to adhere to the electrode. Furthermore, the viscosity of the coating liquid used for coating and molding the surface layer (F) does not become too high, resulting in good moldability and crystal formation, and good porosity of the surface layer (F). From this perspective, the Mw of polyvinylidene fluoride resins (1) and (2) is more preferably 1.5 million or less, and even more preferably 1.2 million or less.

[0069] The weighted average of the Mw of polyvinylidene fluoride resin (1) and polyvinylidene fluoride resin (2) contained in the surface layer (F) (the average weighted by the content ratio (mass basis) of the Mw of both resins) is preferably 600,000 to 2,000,000 from the viewpoint of the ion permeability of the surface layer (F) and adhesion to the electrode. The lower limit of the weighted average is more preferably 700,000 or more, and even more preferably 800,000 or more, and the upper limit of the weighted average is more preferably 1,500,000 or less, and even more preferably 1,200,000 or less.

[0070] Preferably, the proportion of polyvinylidene fluoride resin (1) in the total amount of polyvinylidene fluoride resin (1) and polyvinylidene fluoride resin (2) contained in the surface layer (F) is 15% to 85% by mass (the proportion of polyvinylidene fluoride resin (2) is 15% to 85% by mass). When polyvinylidene fluoride resin (1) accounts for 15% by mass or more of the total amount of polyvinylidene fluoride resin (1) and polyvinylidene fluoride resin (2), a fine porous structure is easily developed in the surface layer (F), and excessive swelling of the polyvinylidene fluoride resin in relation to the electrolyte is suppressed, making pore blockage less likely. On the other hand, if the polyvinylidene fluoride resin (2) is 15% by mass or more of the total amount of polyvinylidene fluoride resin (1) and polyvinylidene fluoride resin (2), then high mobility of the polymer chains when heated and ease of swelling in the electrolyte are ensured. From the above viewpoint, the proportion of polyvinylidene fluoride resin (1) to the total amount of polyvinylidene fluoride resin (1) and polyvinylidene fluoride resin (2) is preferably 15% by mass to 85% by mass, preferably 30% by mass to 85% by mass, and more preferably 45% by mass to 85% by mass.

[0071] In a form in which the surface layer (F) contains polyvinylidene fluoride resins (1) and (2), the total proportion of polyvinylidene fluoride resins (1) and (2) to the total resin contained in the surface layer (F) is preferably 85% to 100% by mass, more preferably 90% to 100% by mass, and even more preferably 95% to 100% by mass.

[0072] -Other Resins- The surface layer (F) may contain resins other than polyvinylidene fluoride resins. Examples of other resins include acrylic resins, butadiene-acrylonitrile resins, fluororubber, homopolymers or copolymers of vinyl nitrile compounds (acrylonitrile, methacrylonitrile, etc.), carboxymethylcellulose, hydroxyalkylcellulose, polyvinyl alcohol, polyvinyl butyral, polyvinylpyrrolidone, and polyethers (polyethylene oxide, polypropylene oxide, etc.). These resins may be used individually or in mixtures of two or more.

[0073] The mass ratio of other resins in the total resin of the surface layer (F) is preferably 10% by mass or less, more preferably 5% by mass or less, still more preferably 1% by mass or less, and particularly preferably substantially not contained. The mass ratio of the polyvinylidene fluoride-based resin in the total resin of the surface layer (F) is preferably 90% by mass or more, more preferably 95% by mass or more, still more preferably 99% by mass or more, and particularly preferably 100% by mass.

[0074] The surface layer (F) may contain inorganic particles. In the form where the surface layer (F) does not contain inorganic particles, the mass ratio of the polyvinylidene fluoride-based resin in the surface layer (F) is preferably 85% by mass to 100% by mass, more preferably 90% by mass to 100% by mass, and still more preferably 95% by mass to 100% by mass.

[0075] - Other components - The surface layer (F) 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 to the coating liquid for forming the surface layer (F) for the purpose of improving dispersibility, coating property, or storage stability. The wetting agent, antifoaming agent, and pH adjuster are added to the coating liquid for the purpose of improving the compatibility with the polyolefin microporous membrane or other layers, suppressing air entrainment into the coating liquid, or adjusting the pH.

[0076] - Characteristics of the surface layer (F) - From the viewpoint of adhesion to the electrode, the thickness of the surface layer (F) is preferably 0.5 μm or more, more preferably 0.8 μm or more, and still more preferably 1 μm or more. From the viewpoint of the energy density of the battery, the thickness of the surface layer (F) is preferably 3 μm or less, more preferably 2.5 μm or less, and still more preferably 2 μm or less.

[0077] The thickness of the surface layer (F) is a value obtained by subtracting the thickness of the flat film from which the surface layer (F) has been removed from the thickness of the separator. The thicknesses of the separator and the flat film are determined by measuring 20 points within a 10 cm square using a contact type length measuring machine and averaging them.

[0078] The mass per unit area (i.e., basis weight) of the surface layer (F) is 0.4 g / m ２Preferably, it is 0.5 g / m ２ The above is more preferable, 0.6 g / m ２ The above is even more preferable. The mass per unit area (i.e., basis weight) of the surface layer (F) is 1.8 g / m² from the viewpoint of the energy density of the battery. ２ Preferably, it is 1.5 g / m ２ The following is more preferable: 1.2 g / m ２ The following is even more preferable.

[0079] The mass per unit area (i.e., basis weight) of the surface layer (F) is calculated by subtracting the mass of the flat film with the surface layer (F) removed from the mass of a separator cut to 20 cm x 20 cm, and then dividing the difference in mass by the area.

[0080] [Heat-resistant resin layer] The separator of this disclosure comprises a heat-resistant resin layer on one or both sides of a polyolefin microporous membrane.

[0081] In the separator of this disclosure, the heat-resistant resin layer is a porous layer in which the mass ratio of the heat-resistant resin to the total mass of the layer is greater than 10% by mass. In this disclosure, a porous layer is a layer having a large number of micropores inside, with a structure in which the micropores are connected, and through which gas or liquid can pass from one surface to the other.

[0082] In this disclosure, a heat-resistant resin refers to a resin with a melting point of 200°C or higher, or a resin that does not have a melting point but has a thermal decomposition temperature of 200°C or higher. In other words, a heat-resistant resin in this disclosure is a resin that does not melt or decompose in the temperature range below 200°C.

[0083] In the separator of this disclosure, the surface layer (A) is a layer in contact with the heat-resistant resin layer. Examples of embodiments relating to the heat-resistant resin layer of the separator of this disclosure include the following embodiments (1) and (2).

[0084] Embodiment (1): A separator having a heat-resistant resin layer between a polyolefin microporous membrane and a surface layer (A), and having a heat-resistant resin layer between a polyolefin microporous membrane and a surface layer (F). In this separator, one heat-resistant resin layer and the other heat-resistant resin layer may be the same or different in terms of components and / or composition.

[0085] Form (2): A separator having a heat-resistant resin layer between the polyolefin microporous membrane and the surface layer (A), and not having a heat-resistant resin layer between the polyolefin microporous membrane and the surface layer (F).

[0086] The heat-resistant resin layer contains at least a heat-resistant resin and may contain other components.

[0087] The mass ratio of heat-resistant resin to the heat-resistant resin layer is more than 10% by mass, preferably 15% by mass or more, and more preferably 20% by mass or more, from the viewpoint of improving the heat resistance of the separator and suppressing internal short circuits in the battery. The mass ratio of heat-resistant resin to the heat-resistant resin layer may be 100% by mass.

[0088] The heat-resistant resin layer may contain inorganic particles from the viewpoint of increasing the rigidity of the separator or from the viewpoint of easily making the layer porous. In a form in which the heat-resistant resin layer contains inorganic particles, the mass ratio of inorganic particles to the heat-resistant resin layer is preferably 50% by mass or more and less than 90% by mass, more preferably 60% by mass to 85% by mass, and even more preferably 70% by mass to 85% by mass.

[0089] - Heat-resistant resins - Examples of heat-resistant resins include aromatic polyamides, polyimides, polyamide-imides, polyethersulfones, polysulfones, polyetherketones, and polyetherimides. Heat-resistant resins may be used individually or in mixtures of two or more types.

[0090] As for the heat-resistant resin, at least one selected from the group consisting of aromatic polyamides, polyimides, and polyamide-imides is preferred from the viewpoint of durability and heat resistance.

[0091] As a heat-resistant resin, all-aromatic polyamides (also known as aramids) are more preferred from the viewpoint of durability and heat resistance. All-aromatic polyamides may be meta-type or para-type. Among all-aromatic polyamides, meta-type all-aromatic polyamides are preferred from the viewpoint of easily forming porous layers and having excellent oxidation-reduction resistance in electrode reactions. Specifically, polymetaphenylene isophthalamide or polyparaphenylene terephthalamide are preferred as all-aromatic polyamides, and polymetaphenylene isophthalamide is more preferred.

[0092] In a form in which the heat-resistant resin layer contains a total aromatic polyamide, the mass percentage of the total aromatic polyamide in the total resin of the heat-resistant resin layer is preferably 85% to 100% by mass, more preferably 90% to 100% by mass, and even more preferably 95% to 100% by mass.

[0093] - Inorganic Particles - The heat-resistant resin layer may contain inorganic particles. From the viewpoint of increasing the rigidity of the separator, it is preferable that the heat-resistant resin layer contains inorganic particles.

[0094] Examples of inorganic particles include metal oxide particles, metal hydroxide particles, metal sulfate particles, metal carbonate particles, metal nitride particles, and clay mineral particles.

[0095] Examples of metal oxides constituting 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 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 metal sulfate particles include barium sulfate and calcium sulfate, with barium sulfate being preferred. Examples of metal carbonates constituting metal carbonate particles include calcium carbonate, magnesium carbonate, and barium carbonate. Examples of metal nitrides constituting metal nitride particles include boron nitride and aluminum nitride. Examples of clay mineral particles include calcium silicate and talc.

[0096] The inorganic particles may be inorganic particles whose surface has been modified with a silane coupling agent or the like.

[0097] Inorganic particles may be used individually or in combination of two or more types.

[0098] As inorganic particles, at least one selected from the group consisting of metal oxide particles, metal hydroxide particles, and metal sulfate particles is preferred from the viewpoint of stability with respect to the electrolyte and electrochemical stability. Among these, at least one selected from the group consisting of alumina particles (aluminum oxide particles), magnesium hydroxide particles, and barium sulfate particles is more preferred.

[0099] From the viewpoint of being able to obtain relatively small particle sizes inexpensively as inorganic particles, and from the viewpoint of not generating gas inside the battery, metal sulfate particles are preferred, and barium sulfate particles are more preferred.

[0100] There are no limitations on the particle shape of the inorganic particles; they may be spherical, elliptical, plate-shaped, needle-shaped, or irregularly shaped. From the viewpoint of suppressing internal short circuits in the battery, the inorganic particles contained in the inorganic particle layer are preferably plate-shaped particles or non-aggregated primary particles.

[0101] The average primary particle size of the inorganic particles contained in the heat-resistant resin layer is preferably 0.3 μm or less, more preferably 0.1 μm or less, and even more preferably 0.08 μm or less, from the viewpoint of suppressing the peeling of the surface layer (A) from the heat-resistant resin layer. The average primary particle size of the inorganic particles contained in the heat-resistant resin layer is preferably 0.01 μm or more, more preferably 0.02 μm or more, and even more preferably 0.03 μm or more, from the viewpoint of forming a good porous structure without aggregation.

[0102] The average primary particle size of inorganic particles is determined by measuring the major axis of 100 randomly selected inorganic particles observed using a scanning electron microscope (SEM) and averaging the major axes of these 100 particles. The sample used for SEM observation is inorganic particles that form the heat-resistant resin layer, or inorganic particles extracted from the heat-resistant resin layer. There are no restrictions on the method for extracting inorganic particles from the heat-resistant resin layer. For example, this method involves immersing the heat-resistant resin layer peeled from the separator in an organic solvent that dissolves the heat-resistant resin, thereby dissolving the resin and extracting the inorganic particles; or heating the heat-resistant resin layer peeled from the separator to approximately 800°C to remove the heat-resistant resin and extract the inorganic particles; and so on.

[0103] -Other Components- The heat-resistant resin layer may contain additives such as dispersants (including surfactants), wetting agents, defoamers, and pH adjusters. Dispersants are added to the coating solution for forming the heat-resistant resin layer to improve dispersibility, coating properties, or storage stability. Wetting agents, defoamers, and pH adjusters are added to the coating solution for forming the heat-resistant resin layer to improve compatibility with the polyolefin microporous membrane or other layers, to suppress air entrapment in the coating solution, or to adjust the pH.

[0104] - Characteristics of the heat-resistant resin layer - In a configuration where the heat-resistant resin layer is present on only one side of the polyolefin microporous membrane, the thickness of the heat-resistant resin layer is preferably 0.5 μm to 4 μm, more preferably 1 μm to 3.5 μm, and even more preferably 1.5 μm to 3 μm, from the viewpoint of balancing the heat resistance and energy density of the battery.

[0105] In a configuration where the heat-resistant resin layer is present on both sides of the polyolefin microporous film, the thickness of the heat-resistant resin layer is preferably 1 μm to 5 μm in total on both sides, more preferably 1.5 μm to 4 μm, and even more preferably 2 μm to 3 μm, from the viewpoint of balancing the heat resistance and energy density of the battery.

[0106] The thickness of the heat-resistant resin layer is obtained by subtracting the thickness of the flat film after removing the heat-resistant resin layer from the thickness of the flat film obtained by removing the upper layer from the separator to expose the heat-resistant resin layer. The thickness of the flat film is determined by measuring 20 points within a 10 cm square area using a contact-type length measuring instrument and averaging the results.

[0107] In a configuration where the heat-resistant resin layer is present on only one side of the polyolefin microporous membrane, the mass per unit area (i.e., basis weight) of the heat-resistant resin layer is 1.5 g / m², from the viewpoint of balancing the heat resistance and energy density of the battery. ２ ~3.8 g / m ２ Preferably, it is 1.8 g / m ２ ~3.5 g / m ２ More preferably, 2.0 g / m ２ ~3.2 g / m ２ That is even more preferable.

[0108] In a configuration where the heat-resistant resin layer is present on both sides of the polyolefin microporous film, the mass per unit area (i.e., basis weight) of the heat-resistant resin layer should be 1.5 g / m² in total for both sides, from the viewpoint of balancing the heat resistance and energy density of the battery. ２ ~3.8 g / m ２ Preferably, it is 1.8 g / m ２ ~3.5 g / m ２ More preferably, 2.0 g / m ２ ~3.2 g / m ２ That is even more preferable.

[0109] The mass per unit area (i.e., basis weight) of the heat-resistant resin layer is calculated by subtracting the mass of the flat film obtained by removing the heat-resistant resin layer from the mass of the flat film obtained by removing the heat-resistant resin layer from a separator cut to 20 cm x 20 cm, and then dividing the difference in mass by the area.

[0110] [Inorganic Particle Layer] The separator of this disclosure may include an inorganic particle layer containing inorganic particles between the polyolefin microporous membrane and the heat-resistant resin layer beneath the surface layer (A), and / or between the polyolefin microporous membrane and the surface layer (F).

[0111] In the separator of this disclosure, the inorganic particle layer is a porous layer in which the mass ratio of inorganic particles to the total mass of the layer is 90% by mass or more. In this disclosure, a porous layer is a layer having a large number of micropores inside, with a structure in which the micropores are connected, and through which a gas or liquid can pass from one surface to the other.

[0112] Examples of embodiments of a separator comprising an inorganic particle layer include the following embodiments (11) to (13).

[0113] Embodiment (11): A separator having an inorganic particle layer between a polyolefin microporous membrane and a heat-resistant resin layer beneath a surface layer (A), and having an inorganic particle layer between the polyolefin microporous membrane and a surface layer (F). In this separator, one inorganic particle layer and the other inorganic particle layer may be the same or different in terms of components and / or composition.

[0114] Morphology (12): A separator having an inorganic particle layer between the polyolefin microporous membrane and the heat-resistant resin layer beneath the surface layer (A), and not having an inorganic particle layer between the polyolefin microporous membrane and the surface layer (F).

[0115] Morphology (13): A separator having an inorganic particle layer between the polyolefin microporous membrane and the surface layer (F), and not having an inorganic particle layer between the polyolefin microporous membrane and the heat-resistant resin layer beneath the surface layer (A).

[0116] The inorganic particle layer contains at least inorganic particles and optionally a binder resin.

[0117] From the viewpoint of improving the impact resistance of the battery, the mass ratio of inorganic particles in the inorganic particle layer is 90% by mass or more, preferably 92% by mass or more, more preferably 95% by mass or more, and even more preferably 98% by mass or more. The mass ratio of inorganic particles in the inorganic particle layer may be 100% by mass.

[0118] The inorganic particle layer may contain a binder resin to form the layer and maintain its structure. The mass percentage of inorganic particles in the inorganic particle layer is, for example, 10% by mass or more, 8% by mass or more, or 5% by mass or more.

[0119] - 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.

[0120] Examples of metal oxides constituting 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 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 metal sulfate particles include barium sulfate and calcium sulfate, with barium sulfate being preferred. Examples of metal carbonates constituting metal carbonate particles include calcium carbonate, magnesium carbonate, and barium carbonate. Examples of metal nitrides constituting metal nitride particles include boron nitride and aluminum nitride. Examples of clay mineral particles include calcium silicate and talc.

[0121] The inorganic particles may be inorganic particles whose surface has been modified with a silane coupling agent or the like.

[0122] Inorganic particles may be used individually or in combination of two or more types.

[0123] As inorganic particles, at least one selected from the group consisting of metal oxide particles, metal hydroxide particles, and metal sulfate particles is preferred from the viewpoint of stability with respect to the electrolyte and electrochemical stability. Among these, at least one selected from the group consisting of alumina particles (aluminum oxide particles), magnesium hydroxide particles, and barium sulfate particles is more preferred.

[0124] From the viewpoint of being able to obtain relatively small particle sizes inexpensively as inorganic particles, at least one selected from the group consisting of γ-alumina particles, boehmite particles, and barium sulfate particles is more preferable.

[0125] There are no limitations on the particle shape of the inorganic particles; they may be spherical, elliptical, plate-shaped, needle-shaped, or irregularly shaped. From the viewpoint of suppressing internal short circuits in the battery, the inorganic particles contained in the inorganic particle layer are preferably plate-shaped particles or non-aggregated primary particles.

[0126] The average primary particle size of the inorganic particles contained in the inorganic particle layer is preferably 10 nm to 500 nm, more preferably 10 nm to 300 nm, even more preferably 20 nm to 200 nm, and still more preferably 30 nm to 100 nm, from the viewpoint of forming a good porous structure without aggregation and suppressing delamination between the inorganic particles and the polyolefin microporous membrane or other layers.

[0127] The average primary particle size of inorganic particles is determined by measuring the major axis of 100 randomly selected inorganic particles observed using a scanning electron microscope (SEM) and averaging the major axes of these 100 particles. The sample used for SEM observation is inorganic particles that form the inorganic particle layer, or inorganic particles extracted from the inorganic particle layer. There are no restrictions on the method for extracting inorganic particles from the inorganic particle layer. For example, this method involves immersing the inorganic particle layer peeled from the separator in an organic solvent that dissolves the binder resin, thereby dissolving the binder resin and extracting the inorganic particles; or heating the inorganic particle layer peeled from the separator to about 800°C to remove the binder resin and extract the inorganic particles; and so on.

[0128] -Binding Resin- The binding resin has the function of binding inorganic particles contained in the inorganic particle layer together, as well as adhering the inorganic particle layer to a polyolefin microporous membrane or other layers. The binding resin contained in the inorganic particle layer may have a particle shape, or it may not have a specific shape, and any form is acceptable as long as it can bind the inorganic particles together.

[0129] The binder resin may be a heat-resistant resin or a non-heat-resistant resin. Examples of heat-resistant resins include aromatic polyamides, polyimides, polyamide-imides, polyethersulfones, polysulfones, polyetherketones, and polyetherimides. Examples of non-heat-resistant resins include butadiene polymers (e.g., butadiene homopolymers, styrene-butadiene copolymers) and acrylic resins (e.g., acrylic monomer homopolymers or copolymers, copolymers of acrylic monomers and styrene monomers). These resins may be used individually or in mixtures of two or more.

[0130] -Other Components- The inorganic particle layer may contain additives such as dispersants (including surfactants), wetting agents, defoamers, and pH adjusters. Dispersants are added to the coating solution for forming the inorganic particle layer to improve dispersibility, coating properties, or storage stability. Wetting agents, defoamers, and pH adjusters are added to the coating solution for forming the inorganic particle layer to improve compatibility with the polyolefin microporous membrane or other layers, to suppress air entrapment in the coating solution, or to adjust the pH.

[0131] - Characteristics of the inorganic particle layer - In a configuration in which the inorganic particle layer is present on only one side of the polyolefin microporous membrane, the thickness of the inorganic particle layer is preferably 0.5 μm to 3 μm, more preferably 0.8 μm to 2.5 μm, and even more preferably 1 μm to 2 μm, from the viewpoint of the battery's impact resistance and the permeability of the electrolyte.

[0132] In a configuration where the inorganic particle layer is present on both sides of the polyolefin microporous membrane, the total thickness of the inorganic particle layer on both sides is preferably 0.5 μm to 5 μm, more preferably 0.8 μm to 4 μm, and even more preferably 1 μm to 3 μm, from the viewpoint of the battery's impact resistance and the permeability of the electrolyte.

[0133] The thickness of the inorganic particle layer is obtained by subtracting the thickness of the flat film after removing the inorganic particle layer from the thickness of the flat film obtained by removing the upper layer from the separator to expose the inorganic particle layer. The thickness of the flat film is determined by measuring 20 points within a 10 cm square area using a contact-type length measuring instrument and averaging the results.

[0134] [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 14 μm or less, and even more preferably 13 μm or less.

[0135] The thickness of the separator is determined by measuring 20 points within a 10 cm square area using a contact-type length measuring device and averaging the results.

[0136] From the viewpoint of ion permeability, the porosity of the separator is preferably 30% to 60%. The porosity ε (%) of the separator is calculated using the following formula.

[0137]

[0138] 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 W. １ , W ２、 W ３ ..., W ｎ (g / cm ２ ) and the true density of each constituent material is d １ d ２ d ３ , ..., d ｎ (g / cm ３ ) and the thickness of the separator is t (cm).

[0139] From the viewpoint of suppressing internal short circuits in the battery, the air permeability of the separator is preferably 80 seconds / 100 mL or more, more preferably 90 seconds / 100 mL or more, and even more preferably 100 seconds / 100 mL or more. From the viewpoint of ion permeability, the air permeability of the separator is preferably 200 seconds / 100 mL or less, more preferably 180 seconds / 100 mL or less, and even more preferably 160 seconds / 100 mL or less.

[0140] The air permeability of the separator was measured using a Gurley densometer in accordance with JIS P8117:2009 "Paper and cardboard - Test methods for air permeability and air resistance (intermediate region) - Gurley method".

[0141] The thickness and mass per unit area (i.e., basis weight) of the heat-resistant resin layer placed on the separator are preferably set using the thermal shrinkage rate of the separator as an indicator. In a configuration in which an inorganic particle layer is placed on the separator, the thickness and mass per unit area (i.e., basis weight) of the inorganic particle layer are preferably set using the thermal shrinkage rate of the separator as an indicator. From the viewpoint of suppressing internal short circuits in the battery, the thermal shrinkage rate when the separator is heat-treated at a temperature of 150°C for 60 minutes is preferably 10% or less for both MD and TD, and more preferably 5% or less.

[0142] The thermal shrinkage rate of the separator is measured by the following method. Cut the separator into a rectangle with dimensions TD 60 mm x MD 180 mm to make a test specimen. Mark the specimen at points 20 mm and 170 mm from one end on the line that bisects TD (referred to as points A and B, respectively). Furthermore, mark the specimen at points 10 mm and 50 mm from one end on the line that bisects MD (referred to as points C and D, respectively). Attach a clip to the specimen (the clip should be placed between the end closest to point A and point A), and suspend it in an oven at 150°C for 60 minutes under no tension to perform heat treatment. Measure the lengths between A and B and between C and D before and after heat treatment, and calculate the thermal shrinkage rate using the following formula.

[0143] Thermal shrinkage rate of MD (%) = {(Length of AB before heat treatment - Length of AB after heat treatment) ÷ Length of AB before heat treatment} × 100 Thermal shrinkage rate of TD (%) = {(Length of CD before heat treatment - Length of CD after heat treatment) ÷ Length of CD before heat treatment} × 100

[0144] [Laminated Structure of Separators] The laminated structure of the separators of this disclosure will be described with reference to the drawings. Figures 1 to 4 are schematic cross-sectional views of embodiments of the separators of this disclosure. Figures 1 to 4 are schematic cross-sectional views mainly for explaining the layering order, and the structure of each layer is abstracted or simplified. In Figures 1 to 4, layers having similar functions are denoted by the same reference numerals and described accordingly.

[0145] Figures 1 and 2 are schematic cross-sectional views of a separator without an inorganic particle layer. Figures 3 and 4 are schematic cross-sectional views of a separator with an inorganic particle layer.

[0146] The separator shown in Figure 1 is a separator in which a surface layer (F) 80, a polyolefin microporous film 20, a heat-resistant resin layer 60, and a surface layer (A) 70 are laminated in this order.

[0147] The separator shown in Figure 2 is a separator in which a surface layer (F) 80, a heat-resistant resin layer 60, a polyolefin microporous film 20, another heat-resistant resin layer 60, and a surface layer (A) 70 are laminated in this order. In the separator shown in Figure 2, one heat-resistant resin layer 60 and the other heat-resistant resin layer 60 may be the same or different in terms of components and / or composition.

[0148] The separator shown in Figure 3 is a separator in which a surface layer (F) 80, an inorganic particle layer 30, a polyolefin microporous film 20, a heat-resistant resin layer 60, and a surface layer (A) 70 are laminated in this order.

[0149] The separator shown in Figure 4 is a separator in which a surface layer (F) 80, an inorganic particle layer 30, a polyolefin microporous film 20, another inorganic particle layer 30, a heat-resistant resin layer 60, and a surface layer (A) 70 are laminated in this order. In the separator shown in Figure 4, one inorganic particle layer 30 and the other inorganic particle layer 30 may be the same or different in terms of components and / or composition.

[0150] As shown in Figures 1 to 4, the surface layer (A) 70 is a layer in contact with the heat-resistant resin layer 60.

[0151] The lamination configuration of the separator of this disclosure is not limited to the configurations shown in Figures 1 to 4. Other configurations of the separator of this disclosure include an example in which an inorganic particle layer and a heat-resistant resin layer are arranged on both sides of a polyolefin microporous membrane; and an example in which an inorganic particle layer is arranged on one side of a polyolefin microporous membrane and heat-resistant resin layers are arranged on both sides of the polyolefin microporous membrane.

[0152] [Method for Manufacturing Separators] The separators of this disclosure can be manufactured by forming each layer on a polyolefin microporous membrane using a wet coating method or a dry coating method. In this disclosure, a wet coating method is a method of solidifying the coating layer in a solidifying liquid, and a dry coating method is a method of solidifying the coating layer by drying it.

[0153] Surface layer (A) can be formed by a dry coating method. Surface layer (F) can be formed by a wet coating method. The heat-resistant resin layer can be formed by either a wet or dry coating method, but it is preferable to form it by a wet coating method. The inorganic particle layer can be formed by either a wet or dry coating method.

[0154] Examples of wet coating and dry coating methods are described below. In the following description, layers other than the polyolefin microporous membrane of the separator are collectively referred to as "porous layers."

[0155] -Wet Coating Method- An example of an embodiment of the wet coating method includes the steps of: applying a coating liquid to one or both sides of a substrate to form a coating layer; immersing the substrate having the coating layer in a solidifying solution to solidify the coating layer and form a porous layer; and removing the laminate consisting of the substrate and the porous layer from the solidifying solution and washing and drying it.

[0156] The coating solution is prepared by dissolving or dispersing the porous layer material in a solvent.

[0157] The solvent used in preparing the coating solution includes a solvent that dissolves the resin contained in the porous layer (hereinafter also referred to as the "good solvent"). A good solvent common to polyvinylidene fluoride resins and polyamides is a polar amide solvent. Examples of polar amide solvents include dimethylacetamide, dimethylformamide, and N-methylpyrrolidone.

[0158] The solvent used in preparing the coating solution preferably contains a phase-separating agent that induces phase separation, from the viewpoint of forming a good porous structure in the porous layer. Therefore, the solvent used in preparing the coating solution is preferably a mixed solvent of a good solvent and a phase-separating agent. The phase-separating agent is preferably mixed with the good solvent in an amount that ensures a viscosity suitable for coating. Examples of phase-separating agents common to polyvinylidene fluoride resins and polyamides include water, methanol, ethanol, propyl alcohol, butyl alcohol, butanediol, ethylene glycol, propylene glycol, and tripropylene glycol.

[0159] The solvent used in preparing the coating solution is preferably a mixed solvent of a good solvent and a phase separating agent, from the viewpoint of forming a good porous structure in the porous layer, and is preferably a mixed solvent containing 60% by mass or more of the good solvent and 5% to 40% by mass of the phase separating agent.

[0160] The resin concentration of the coating solution is preferably 1% to 20% by mass, from the viewpoint of forming a good porous structure in the porous layer. If the coating solution contains inorganic particles, the inorganic particle concentration of the coating solution is preferably 0.5% to 50% by mass, from the viewpoint of forming a good porous structure in the porous layer.

[0161] The coating solution may contain dispersants such as surfactants, wetting agents, defoamers, pH adjusters, etc. These additives may remain in the porous layer as long as they are electrochemically stable within the operating range of non-aqueous secondary batteries and do not inhibit reactions within the battery.

[0162] Methods for applying the coating liquid to the substrate include Meyer bar, die coater, reverse roll coater, roll coater, and gravure coater. When forming a porous layer on both sides of the substrate, it is preferable from a productivity standpoint to apply the coating liquid to both sides of the substrate simultaneously.

[0163] The coating layer is solidified by immersing the substrate on which the coating layer is formed in a solidifying solution, thereby inducing phase separation in the coating layer and solidifying the resin. This results in a laminate consisting of the substrate and the porous layer.

[0164] The solidification solution generally contains the good solvent and phase separating agent used in the preparation of the coating solution, along with water. From a production standpoint, it is preferable that the mixing ratio of the good solvent and the phase separating agent match the mixing ratio of the mixed solvent used in the preparation of the coating solution. From the viewpoint of forming a porous structure and productivity, the water content in the solidification solution is preferably 40% to 90% by mass. The temperature of the solidification solution is, for example, 20°C to 50°C.

[0165] After the coating layer is solidified in the solidification solution, the laminate is removed from the solidification solution and washed with water. Washing removes the solidification solution from the laminate. Further drying removes water from the laminate. Washing 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, blowing air on the laminate, or bringing the laminate into contact with a heat roll. In one embodiment, the drying temperature is set to a temperature that does not melt the resin (for example, a temperature of (glass transition temperature of the resin) or less).

[0166] - Dry Coating Method - An example of a dry coating method includes the steps of: applying a coating liquid to one or both sides of a substrate to form a coating layer; and drying and solidifying the coating layer on the substrate to form a porous layer.

[0167] The coating solution is prepared by dissolving or dispersing the porous layer material in a solvent.

[0168] Water is an example of a solvent used in the preparation of coating solutions. Water is preferred because it is easily removed by drying and has no toxicity to humans or the environment.

[0169] The resin concentration of the coating solution is preferably 1% to 20% by mass, from the viewpoint of coating properties and productivity of the porous layer. If the coating solution contains inorganic particles, the inorganic particle concentration of the coating solution is preferably 0.5% to 50% by mass, from the viewpoint of coating properties and productivity of the porous layer.

[0170] The coating solution may contain dispersants such as surfactants, wetting agents, defoamers, pH adjusters, etc. These additives may remain in the resin particle layer as long as they are electrochemically stable within the operating range of non-aqueous secondary batteries and do not inhibit reactions within the battery.

[0171] Methods for applying the coating liquid to the substrate include Meyer bar, die coater, reverse roll coater, roll coater, and gravure coater. When forming a porous layer on both sides of the substrate, it is preferable from a productivity standpoint to apply the coating liquid to both sides of the substrate simultaneously.

[0172] The coating layer solidifies by drying (i.e., removing the solvent). Drying is performed, for example, by transporting the substrate with the coating layer in a high-temperature environment, by blowing air onto the substrate with the coating layer, or by bringing the substrate with the coating layer into contact with a heat roll. In one embodiment, the drying temperature is set to a temperature that does not melt the resin (for example, a temperature of (glass transition temperature of the resin + 20°C) or lower).

[0173] The separator can also be manufactured by preparing the porous layer as an independent sheet and then combining it with a substrate by heat compression or adhesive. One method for preparing the porous layer as an independent sheet is to apply the wet coating method or dry coating method described above to form the porous layer on a release sheet.

[0174] The method for manufacturing the separator may be a discontinuous method or a continuous method.

[0175] • Discontinuous manufacturing method: A lower layer is formed on a substrate unwound from a roll to obtain a laminate of the substrate and the lower layer, and then the laminate is wound onto another roll. Next, an upper layer is formed on the laminate unwound from the roll to obtain a separator, and the resulting separator is wound onto another roll.

[0176] • Continuous manufacturing method: A lower layer is formed on the substrate unwound from a roll to obtain a laminate of the substrate and the lower layer, then an upper layer is formed on the laminate to obtain a separator, and the resulting separator is wound onto another roll.

[0177] <Non-aqueous secondary battery> The non-aqueous secondary battery of this disclosure is a non-aqueous secondary battery that obtains electromotive force by doping and dedoping of lithium ions, and comprises a positive electrode, a negative electrode, and a separator of this disclosure. Doping means absorption, support, adsorption, or insertion, and refers to the phenomenon in which lithium ions enter the active material of the electrode.

[0178] The non-aqueous secondary battery of this disclosure has a structure in which, for example, a battery element in which a negative electrode and a positive electrode face each other via a separator is sealed together with an electrolyte in an outer casing. The non-aqueous secondary battery of this disclosure is suitable for non-aqueous electrolyte secondary batteries, particularly lithium-ion secondary batteries.

[0179] The non-aqueous secondary battery of this disclosure, by comprising the separator of this disclosure, is less prone to internal short circuits and has good cycle characteristics.

[0180] The following describes examples of the forms of the positive electrode, negative electrode, electrolyte, and outer casing material of the non-aqueous secondary battery of this disclosure.

[0181] An example of a positive electrode embodiment is a configuration in which an active material layer containing a positive electrode active material and a binder resin is arranged 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. Examples of lithium-containing transition metal oxides include 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 LiAl １／４ Ni ３／４ O ２ Examples include polyvinylidene fluoride resins and styrene-butadiene copolymers as binder resins. Examples of conductive additives include acetylene black, Ketjen black, graphite powder, and carbon materials such as ultrafine carbon fibers. Examples of current collectors include aluminum foil, titanium foil, and stainless steel foil with a thickness of 5 μm to 20 μm.

[0182] An example of a negative electrode embodiment is a configuration in which an active material layer containing a negative electrode active material and a binder resin is arranged on a current collector. The active material layer may further contain a conductive additive. Examples of negative electrode active materials include materials that can electrochemically absorb lithium ions. Examples of such materials include carbon materials; alloys of silicon, tin, aluminum, etc. with lithium; Wood's alloys; etc. 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 current collectors include copper foil, nickel foil, and stainless steel foil with a thickness of 5 μm to 20 μm. Instead of the above negative electrode, metallic lithium foil may be used as the negative electrode.

[0183] The electrolyte is preferably a solution of lithium salt dissolved in a non-aqueous solvent. For example, LiPF4 is used as the lithium salt. ６ LiBF ４ LiClO ４ Examples of non-aqueous solvents include cyclic carbonates such as ethylene carbonate, propylene carbonate, fluoroethylene carbonate, difluoroethylene carbonate, and vinylene carbonate; linear carbonates such as dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, and their fluorine-substituted derivatives; and cyclic esters such as γ-butyrolactone and γ-valerolactone. Non-aqueous solvents may be used individually or in mixtures of two or more.

[0184] As the electrolyte, a solution is preferred in which cyclic carbonate and linear carbonate are mixed in a mass ratio (cyclic carbonate:linear carbonate) of 20:80 to 40:60, and a lithium salt is dissolved in it at a concentration of 0.5 mol / L to 1.5 mol / L.

[0185] Examples of exterior materials include aluminum laminate film packs and metal cans. Examples of battery shapes include rectangular, cylindrical, and coin-shaped batteries. The separator of this disclosure is suitable for any of the exterior materials and shapes.

[0186] The non-aqueous secondary battery of this disclosure can be manufactured, for example, by manufacturing a laminate in which the separator of this disclosure is placed between the positive electrode and the negative electrode, and then subjecting this laminate to a wet heat press. Dry heat pressing may be performed in the process of manufacturing the laminate for the purpose of integrating the laminate. Integrating the laminate by dry heat pressing is preferable from the viewpoint of the manufacturing yield and transportability of the separator.

[0187] The non-aqueous secondary battery of this disclosure can be manufactured by first creating a laminate in which the separator of this disclosure is placed between the positive electrode and the negative electrode, and then using this laminate by, for example, the following manufacturing method (1) or manufacturing method (2).

[0188] Manufacturing method (1): After temporarily bonding the electrodes and separators by dry heat pressing the laminate, it is placed in an outer casing and the electrolyte is injected. Next, the laminate is wet heat pressed from above the outer casing to bond the electrodes and separators and seal the outer casing.

[0189] Manufacturing method (2): The laminate is placed in an outer material and the electrolyte is injected. Next, the laminate is wet-heat-pressed from above the outer material to bond the electrodes to the separator and seal the outer material.

[0190] In manufacturing methods (1) and (2), the press temperature, press pressure, and press time of the wet heat press should be set according to the type of exterior material, the size and application of the battery, etc. Examples of wet heat press embodiments include a press temperature of 50°C to 95°C, a press pressure of 0.1 MPa to 2 MPa, and a press time of 1 minute to 20 hours.

[0191] When manufacturing a laminate with a separator placed between the positive electrode and the negative electrode, the method of placing the separator between the positive electrode and the negative electrode may be a method in which the positive electrode, separator, and negative electrode are stacked in that order in at least one layer each (the so-called stack method), or a method in which the positive electrode, separator, negative electrode, and separator are stacked in that order and wound in the length direction.

[0192] The separator and non-aqueous secondary battery of this disclosure will be described in more detail below with reference to examples. The materials, amounts used, proportions, processing procedures, etc., shown in the following examples can be modified as appropriate without departing from the spirit of this disclosure. Therefore, the scope of the separator and non-aqueous secondary battery of this disclosure should not be interpreted as being limited by the specific examples shown below.

[0193] In the following descriptions, synthesis, processing, and manufacturing were carried out at room temperature (25°C ± 3°C) unless otherwise specified.

[0194] <Measurement Methods and Evaluation Methods> The measurement and evaluation methods applied to the examples and comparative examples are as follows.

[0195] [Average Primary Particle Size of Inorganic Particles] The average primary particle size of inorganic particles was determined by performing SEM observation on inorganic particles used to form the porous layer, measuring the major axis of 100 randomly selected inorganic particles, and averaging the major axes of these 100 particles.

[0196] [Thickness of Polyolefin Microporous Film, Substrate, and Separator] The thickness (μm) of the polyolefin microporous film, substrate, and separator was determined by measuring 20 points within a 10 cm square area using a contact-type length measuring instrument (Mitutoyo Corporation, LITEMATIC VL-50S) and averaging the results. A spherical measuring probe with a radius of 10 mm (Mitutoyo Corporation) was used, and a load of 0.19 N was applied during measurement.

[0197] [Porosity of Polyolefin Microporous Membrane] The porosity ε (%) of the polyolefin microporous membrane was calculated using the following formula: ε = {1 - Ws / (ds・t)} × 100 where Ws is the basis weight (g / m) of the polyolefin microporous membrane. ２ ), ds is the true density (g / cm³) of the polyolefin microporous membrane. ３ ), t is the thickness of the polyolefin microporous membrane (μm).

[0198] [Air permeability of polyolefin microporous membranes and separators] The air permeability (seconds / 100 mL) of polyolefin microporous membranes and separators was measured in accordance with JIS P8117:2009 using a Gaale densometer (Toyo Seiki Co., Ltd., G-B2C).

[0199] [Porosity of the separator] The porosity ε (%) of the separator was calculated using the following formula.

[0200]

[0201] 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 W. １ , W ２、 W ３ ..., W ｎ (g / cm ２ ) and the true density of each constituent material is d １ d ２ d ３ , ..., d ｎ (g / cm ３ ) and the thickness of the separator is t (cm).

[0202] [Heat Shrinkage Rate of Separator] A rectangle of separator with dimensions TD 60 mm x MD 180 mm was cut out to form a test specimen. Marks were made on the specimen at points 20 mm and 170 mm from one end, along the line that bisects TD (referred to as points A and B, respectively). Furthermore, marks were made at points 10 mm and 50 mm from one end, along the line that bisects MD (referred to as points C and D, respectively). A clip was attached to the specimen (the clip was attached between the end closest to point A and point A), and the specimen was suspended in an oven at 150°C and heat-treated for 60 minutes under no tension. The lengths between A and B and between C and D were measured before and after the heat treatment, and the heat shrinkage rate was calculated using the following formula. The heat shrinkage rates of the three specimens were then averaged.

[0203] Thermal shrinkage rate of MD (%) = {(Length of AB before heat treatment - Length of AB after heat treatment) ÷ Length of AB before heat treatment} × 100 Thermal shrinkage rate of TD (%) = {(Length of CD before heat treatment - Length of CD after heat treatment) ÷ Length of CD before heat treatment} × 100

[0204] [Dry Adhesion to Electrodes] A slurry for the positive electrode was prepared by stirring and mixing 94 parts by mass of lithium cobalt oxide powder, which is the positive electrode active material, 3 parts by mass of acetylene black, which is a conductive additive, 3 parts by mass of polyvinylidene fluoride, which is a binder resin, and an appropriate amount of N-methyl-2-pyrrolidone in a double-arm mixer. The slurry for the positive electrode was applied to one side of a 20 μm thick aluminum foil, dried, and then pressed to obtain a positive electrode having a positive electrode active material layer on one side.

[0205] A slurry for the negative electrode was prepared by mixing 96.2 parts by mass of artificial graphite, which is the negative electrode active material, 7 parts by mass of a water-soluble dispersion containing 40% by mass of a modified styrene-butadiene copolymer, which is the binder resin, 1 part by mass of carboxymethylcellulose, which is the thickener, and an appropriate amount of water using a double-arm mixer. The slurry for the negative electrode was applied to one side of a 10 μm thick copper foil, dried, and then pressed to obtain a negative electrode having a negative electrode active material layer on one side.

[0206] The electrodes (positive and negative electrodes) were cut into rectangles measuring 15 mm wide x 70 mm long. The separator was cut into a rectangle measuring TD 18 mm x MD 74 mm. A release paper measuring 15 mm wide x 70 mm long was prepared. A separator was placed on top of the active material layer of the electrode (positive or negative electrode), and then the release paper was placed on top of the separator to create a laminate. For the positive electrode, the separator was placed so that the surface layer (A) was in contact with the positive electrode, and for the negative electrode, the separator was placed so that the surface layer (F) was in contact with the negative electrode. The laminate was placed in an aluminum laminate film pack, and the pack and the laminate were heat-pressed in the direction of the laminate using a heat press (dry heat press) to bond the electrodes (positive or negative electrode) and the separator. The heat pressing conditions were a temperature of 85°C, a pressure of 2 MPa, and a time of 5 minutes. After heat pressing, the laminate was removed from the pack, the release paper was peeled off, and a dry adhesive test specimen was obtained.

[0207] The uncoated surface of the electrode of the test specimen 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). At this time, the metal plate was fixed to the Tensilon so that the length direction of the test specimen (i.e., the MD of the separator) was in the direction of gravity. The separator was peeled off the electrode by about 2 cm from the lower end, and that end was fixed to the upper chuck, and a 180° peel test was performed. The tensile speed of the 180° peel test was set to 20 mm / min, and the load (N) from 10 mm to 40 mm after the start of measurement was taken at 0.4 mm intervals, and the average was calculated. Furthermore, the load of 10 test specimens was averaged to obtain the adhesive strength (N / 15 mm) between the electrode and the separator.

[0208] [Internal Short-Circuit Resistance of the Battery] A test secondary battery, as described below, was prepared. At room temperature, the battery was charged with a constant current and constant voltage of 0.2C and 4.2V. Next, the battery was placed in an oven at a temperature of 160°C, a pressure of 170kPa was applied, and the battery voltage was measured for up to 150 minutes.

[0209] [Battery Cycle Characteristics] A test secondary battery, as described below, was prepared. At room temperature, the battery was subjected to 300 cycles of constant current charging at 3C and a 4.2V cutoff, and constant current discharge at 3C and a 2.5V cutoff.

[0210] <Manufacturing of Separators and Batteries> [Example 1] - Manufacturing of Separators - Meta-aramid was dissolved in dimethylacetamide (DMAc), and barium sulfate particles (average primary particle size 0.05 μm) were dispersed to obtain coating solution (1). Coating solution (1) had a meta-aramid concentration of 4.5% by mass, and the mass ratio of meta-aramid to barium sulfate particles was meta-aramid:barium sulfate particles = 20:80.

[0211] Two types of PVDF-based resins were prepared as follows: • PVDF-based resin (1): VDF-HFP binary copolymer, VDF:HFP = 97.6:2.4 (molar ratio), weight-average molecular weight 1.13 million • PVDF-based resin (2): VDF-HFP binary copolymer, VDF:HFP = 94.3:5.7 (molar ratio), weight-average molecular weight 860,000

[0212] PVDF resin (1) and PVDF resin (2) were dissolved in DMAc to obtain coating solution (2). In coating solution (2), the mass ratio of PVDF resin (1) to PVDF resin (2) was (1):(2) = 70:30, and the PVDF resin concentration was 5% by mass.

[0213] A resin particle dispersion (1) was prepared in which acrylic resin particles and polyvinylidene fluoride resin particles were dispersed in water. In the resin particle dispersion (1), the mass ratio of acrylic resin particles to polyvinylidene fluoride resin particles was 30:70, and the resin particle concentration was 10% by mass. The acrylic resin particles were copolymer particles of acrylic monomer and styrene monomer, with a polymerization ratio (mass ratio) of acrylic monomer:styrene monomer = 38:62, a glass transition temperature of 52°C, and an average primary particle size of 500 nm. The polyvinylidene fluoride resin particles had a melting point of 140°C and an average primary particle size of 250 nm.

[0214] A polyethylene microporous membrane (6 μm thick, 41% porosity, 100 seconds / 100 mL air permeability) was coated with coating liquid (1) on one side and with coating liquid (2) on the other side using a gravure coater. The coated polyethylene microporous membrane was then immersed in a solidification solution (DMAc:water = 50:50 [mass ratio], liquid temperature 40°C) to solidify the coating layer. Next, it was washed in a water washing tank at 40°C and dried. Then, resin particle dispersion (1) was applied onto the porous layer formed with coating liquid (1) and dried to solidify the coating layer. Thus, the separator of Example 1 was obtained.

[0215] Figure 5 shows an SEM image (magnification 10,000) of the separator surface of Example 1. The left side is the surface of surface layer (A), and the right side is the surface of surface layer (F). Surface layer (F) had a three-dimensional network structure of polyvinylidene fluoride resin.

[0216] - Manufacturing of the positive electrode - 94 parts by mass of lithium cobalt oxide powder, which is the positive electrode active material, 3 parts by mass of acetylene black, which is a conductive additive, 3 parts by mass of polyvinylidene fluoride, which is a binder resin, and an appropriate amount of N-methyl-2-pyrrolidone were stirred and mixed in a double-arm mixer to prepare a slurry for the positive electrode. The slurry for the positive electrode was applied to both sides or one side of a 20 μm thick aluminum foil, dried, and then pressed to obtain a positive electrode having a positive electrode active material layer on both sides or one side.

[0217] - Manufacturing of the negative electrode - 96.2 parts by mass of artificial graphite, which is the negative electrode active material, 7 parts by mass of a water-soluble dispersion containing 40% by mass of a modified styrene-butadiene copolymer, which is the binder resin, 1 part by mass of carboxymethylcellulose, which is the thickener, and an appropriate amount of water were mixed by stirring in a double-arm mixer to prepare a negative electrode slurry. The negative electrode slurry was applied to both sides or one side of a 10 μm thick copper foil, dried, and then pressed to obtain a negative electrode having a negative electrode active material layer on both sides or one side.

[0218] - Manufacturing of a secondary battery for internal short-circuit resistance evaluation - A single-sided positive electrode and a single-sided negative electrode were cut into rectangles of 14 mm x 20 mm. A separator was cut into a rectangle of TD 20 mm x MD 26 mm. These were stacked so that the positive electrode active material layer and the negative electrode active material layer faced each other, and the separator was sandwiched between the positive and negative electrodes, to create a laminate consisting of one positive electrode, one negative electrode, and one separator. The separator was positioned so that its surface layer (A) was in contact with the negative electrode and its surface layer (F) was in contact with the positive electrode. The laminate was inserted into an aluminum laminate film pack, and an electrolyte (1 mol / L LiPF) was placed inside the pack. ６ Ethylene carbonate:ethyl methyl carbonate (mass ratio 3:7) was injected to impregnate the laminate with the electrolyte. Next, the entire pack was heat-pressed in the direction of the laminate using a hot press (wet heat press) to bond the electrodes to the separator. The heat-pressing conditions were a press temperature of 85°C, a press pressure of 1 MPa, and a press time of 5 minutes. The resulting test secondary batteries were used to evaluate the battery's internal short-circuit resistance.

[0219] - Manufacturing of a secondary battery for cycle characteristic evaluation - A single-sided positive electrode was cut into a rectangle of 30 mm x 50 mm. A single-sided negative electrode was cut into a rectangle of 31 mm x 51 mm. A separator was cut into a rectangle of TD 32 mm x MD 52 mm. These were stacked so that the positive electrode active material layer and the negative electrode active material layer faced each other and the separator was sandwiched between the positive and negative electrodes to create a laminate consisting of one positive electrode, one negative electrode, and one separator. The separator was positioned so that its surface layer (A) was in contact with the negative electrode and its surface layer (F) was in contact with the positive electrode. The laminate was inserted into an aluminum laminate film pack, and an electrolyte (1 mol / L LiPF) was placed inside the pack. ６ Ethylene carbonate:ethyl methyl carbonate (mass ratio 3:7) was injected to impregnate the laminate with the electrolyte. Next, the entire pack was heat-pressed in the direction of the laminate using a hot press (wet heat press) to bond the electrodes to the separator. The heat-pressing conditions were a press temperature of 85°C, a press pressure of 1 MPa, and a press time of 5 minutes. The resulting test secondary battery was used to evaluate the battery's cycle characteristics.

[0220] [Example 2] - Separator Production - A substrate (6 μm thick, porosity 38%, air permeability 105 seconds / 100 mL) was prepared, having an inorganic particle layer (1 μm thick, boehmite particles:styrene-butadiene copolymer = 99:1 [mass ratio]) containing boehmite particles (average primary particle size 100 nm) and styrene-butadiene copolymer on one side of a polyethylene microporous membrane (5 μm thick). Coating liquid (1) was applied to the polyethylene microporous membrane of the substrate, and coating liquid (2) was applied to the inorganic particle layer using a gravure coater. The coated substrate was immersed in a solidification solution (DMAc:water = 50:50, liquid temperature 40°C) to solidify the coating layer. Next, it was washed in a water washing tank at a water temperature of 40°C and dried. Next, resin particle dispersion (1) was applied to the porous layer formed with coating liquid (1), dried, and the coating layer was solidified. Thus, the separator of Example 2 was obtained.

[0221] Figure 6 shows an SEM image (magnification 10,000) of the separator surface of Example 2. The left side is the surface of surface layer (A), and the right side is the surface of surface layer (F). Surface layer (F) had a three-dimensional network structure of polyvinylidene fluoride resin.

[0222] -Battery Manufacturing- Using the separator described above, a test secondary battery was manufactured in the same manner as in Example 1.

[0223] [Comparative Example 1] - Separator Production - Meta-aramid was dissolved in a mixed solvent of dimethylacetamide (DMAc) and tripropylene glycol (TPG) (DMAc:TPG = 80:20 [mass ratio]), and magnesium hydroxide particles (average primary particle size 0.8 μm) were further dispersed to obtain coating solution (3). Coating solution (3) had a meta-aramid concentration of 5% by mass, and the mass ratio of meta-aramid to magnesium hydroxide particles was meta-aramid:magnesium hydroxide particles = 20:80.

[0224] A resin particle dispersion (2) was prepared in which acrylic resin particles and polyvinylidene fluoride resin particles were dispersed in water. In the resin particle dispersion (2), the mass ratio of acrylic resin particles to polyvinylidene fluoride resin particles was 30:70, and the resin particle concentration was 10% by mass. The acrylic resin particles were acrylic resin particles with a glass transition temperature of 59°C and an average primary particle size of 500 nm. The polyvinylidene fluoride resin particles had a melting point of 140°C and an average primary particle size of 250 nm.

[0225] Equal amounts of coating solution (3) were applied to both sides of a polyethylene microporous membrane (thickness 8 μm, porosity 38%, air permeability 163 seconds / 100 mL) using a gravure coater. The coated polyethylene microporous membrane was then immersed in a solidification solution (DMAc:TPG:water = 39:4:57 [mass ratio], liquid temperature 40°C) to solidify the coating layer. Next, it was washed in a water washing tank at a water temperature of 40°C and dried. Then, resin particle dispersion (2) was applied onto the porous layer formed by coating solution (3) and dried to solidify the coating layer. Thus, the separator of Comparative Example 1 was obtained.

[0226] -Battery Manufacturing- Using the separator described above, a test secondary battery was manufactured in the same manner as in Example 1.

[0227] [Comparative Example 2] - Separator Production - PVDF resin (1) and PVDF resin (2) were dissolved in a mixed solvent of DMAc and TPG (DMAc:TPG = 80:20 [mass ratio]), and magnesium hydroxide particles (average primary particle size 0.8 μm) were further dispersed to obtain coating solution (4). In coating solution (4), the mass ratio of PVDF resin (1) to PVDF resin (2) was (1):(2) = 70:30, the PVDF resin concentration was 4% by mass, and the mass ratio of PVDF resin to magnesium hydroxide particles was PVDF resin:magnesium hydroxide particles = 40:60.

[0228] Equal amounts of coating solution (4) were applied to both sides of a polyethylene microporous membrane (thickness 8 μm, porosity 38%, air permeability 163 seconds / 100 mL) using a gravure coater. The coated polyethylene microporous membrane was then immersed in a solidification solution (DMAc:TPG:water = 34:9:57 [mass ratio], liquid temperature 40°C) to solidify the coating layer. Next, it was washed in a water washing tank at a water temperature of 40°C and dried. Thus, the separator of Comparative Example 2 was obtained.

[0229] -Battery Manufacturing- Using the separator described above, a test secondary battery was manufactured in the same manner as in Example 1. However, the wet heat press conditions for manufacturing the battery were changed to a press temperature of 80°C, a press pressure of 1 MPa, and a press time of 2 minutes.

[0230] Table 1 shows the composition of the polyolefin microporous membrane, inorganic particle layer, and heat-resistant resin layer; Table 2 shows the composition of the surface layer; and Table 3 shows the physical properties and evaluation results of the separator. The abbreviations in Tables 1 and 2 have the following meanings: PE: polyethylene; AC: acrylic resin; PVDF: polyvinylidene fluoride resin; VDF-HFP: binary copolymer of vinylidene fluoride and hexafluoropropylene.

[0231]

[0232]

[0233]

[0234] [Internal Short-Circuit Resistance of Electrodes] Figure 7 shows a graph relating to the internal short-circuit resistance of the battery. In Figure 7, (1) is the voltage-time curve of Example 1, (2) is the voltage-time curve of Example 2, (3) is the voltage-time curve of Comparative Example 1, and (4) is the voltage-time curve of Comparative Example 2. As can be seen from Figure 7, Examples 1 and 2 have excellent performance in suppressing internal short circuits in the battery.

[0235] [Battery Cycle Characteristics] Figure 8 shows a graph relating to the battery's cycle characteristics. In Figure 8, (1) is the cycle life curve for Example 1, (2) is the cycle life curve for Example 2, (3) is the cycle life curve for Comparative Example 1, and (4) is the cycle life curve for Comparative Example 2. As can be seen from Figure 8, Examples 1 and 2 have good cycle characteristics.

[0236] All documents, patent applications, and technical standards described herein are incorporated by reference to the same extent as if each individual document, patent application, and technical standard were specifically and individually noted to be incorporated by reference.

[0237] The disclosure of Japanese application number 2024-231305, filed on 26 December 2024, is incorporated herein by reference in its entirety.

[0238] 20 Polyolefin microporous membrane 30 Inorganic particle layer 60 Heat-resistant resin layer 70 Surface layer (A) 80 Surface layer (F)

Claims

1. A separator for a non-aqueous secondary battery, comprising: a polyolefin microporous membrane; a heat-resistant resin layer disposed on one or both sides of the polyolefin microporous membrane, containing a heat-resistant resin and comprising more than 10% by mass of the heat-resistant resin; a surface layer (A) containing acrylic resin particles, disposed in contact with the heat-resistant resin layer on one side of the laminate of the polyolefin microporous membrane and the heat-resistant resin layer; and a surface layer (F) disposed on the other side of the laminate of the polyolefin microporous membrane and the heat-resistant resin layer, containing a polyvinylidene fluoride resin and having a three-dimensional network structure of the polyvinylidene fluoride resin.

2. The separator for a non-aqueous secondary battery according to claim 1, wherein the heat-resistant resin comprises at least one selected from the group consisting of aromatic polyamide, polyimide, and polyamideimide.

3. The separator for a non-aqueous secondary battery according to claim 1, wherein the heat-resistant resin layer further contains inorganic particles.

4. The separator for a non-aqueous secondary battery according to claim 3, wherein the average primary particle size of the inorganic particles contained in the heat-resistant resin layer is 0.01 μm to 0.3 μm.

5. The separator for a non-aqueous secondary battery according to claim 3, wherein the inorganic particles include barium sulfate particles.

6. The separator for a non-aqueous secondary battery according to claim 1, wherein the polyvinylidene fluoride resin comprises a polyvinylidene fluoride resin having hexafluoropropylene units.

7. The separator for a non-aqueous secondary battery according to claim 1, wherein the surface layer (A) further contains polyvinylidene fluoride resin particles.

8. The separator for a non-aqueous secondary battery according to claim 7, wherein the ratio Da / Df of the average primary particle size Da of the acrylic resin particles contained in the surface layer (A) to the average primary particle size Df of the polyvinylidene fluoride resin particles is 1.2 to 2.

8.

9. The separator for a non-aqueous secondary battery according to claim 1, further comprising an inorganic particle layer containing inorganic particles, wherein the inorganic particles constitute 90% by mass or more, between the polyolefin microporous membrane and the heat-resistant resin layer below the surface layer (A) and / or between the polyolefin microporous membrane and the surface layer (F).

10. The separator for a non-aqueous secondary battery according to claim 9, wherein the average primary particle size of the inorganic particles contained in the inorganic particle layer is 10 nm to 500 nm.

11. The separator for a non-aqueous secondary battery according to claim 9, wherein the inorganic particles include at least one selected from the group consisting of γ-alumina particles, boehmite particles, and barium sulfate particles.

12. A non-aqueous secondary battery comprising a positive electrode, a negative electrode, and a separator for a non-aqueous secondary battery according to any one of claims 1 to 11 disposed between the positive electrode and the negative electrode, wherein electromotive force is obtained by doping and dedoping lithium ions.

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