Separator for nonaqueous secondary batteries, and nonaqueous secondary battery

Abstract

This separator for nonaqueous secondary batteries includes: a porous base material; a heat-resistant layer that is disposed on one surface or both surfaces of the porous base material and contains inorganic particles and a binder resin; and an adhesive layer that is disposed on one surface or both surfaces of the laminate of the porous base material and the heat-resistant layer and contains a polyvinylidene fluoride-based resin. The mass ratio of the inorganic particles in the heat-resistant layer is 95 mass% or more, the average primary particle diameter of the inorganic particles contained in the heat-resistant layer is 0.01 μm to 0.30 μm, and the adhesive layer has a porous structure in which fibrils containing the polyvinylidene fluoride-based resin are connected in a three-dimensional network form.

Description

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

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

[0002] Patent Document 1 discloses a separator for a non-aqueous secondary battery, the separator comprising: a porous substrate; a heat-resistant porous layer provided on one or both sides of the porous substrate and containing a binder resin and inorganic particles having an average primary particle size of 0.01 μm or more and less than 0.45 μm; and an adhesive layer provided on one or both sides of a laminate of the porous substrate and the heat-resistant porous layer, the adhesive layer having adhesive resin particles adhered to the laminate. Patent Document 2 discloses a separator for a non-aqueous secondary battery, the separator comprising: a porous substrate; and an adhesive layer provided on one or both sides of the porous substrate and containing at least one selected from the group consisting of (i) adhesive resin particles containing a phenyl group-containing acrylic resin and a polyvinylidene fluoride resin, and (ii) a mixture of adhesive resin particles containing a phenyl group-containing acrylic resin and adhesive resin particles containing a polyvinylidene fluoride resin. Patent Document 3 discloses a ceramic-polymer composite-coated lithium ion separator including a polyolefin porous separator, a ceramic coating layer applied to one or both sides of the separator, and a polymer coating layer applied to the ceramic surface or the separator surface.

[0003] International Publication No. 2020 / 246497 Japanese Patent Application Laid-Open No. 2023-040938 Special Publication No. 2019-523518

[0004] An object of the present disclosure is to provide a separator for a non-aqueous secondary battery that has excellent thermal dimensional stability and adhesion to electrodes, and that improves the cycle characteristics and impact resistance of the battery.

[0005] Specific means for solving the above problems include the following aspects. <1> A separator for a non-aqueous secondary battery, comprising: a porous substrate; a heat-resistant layer containing inorganic particles and a binder resin, disposed on one or both sides of the porous substrate; and an adhesive layer containing a polyvinylidene fluoride resin, disposed on one or both sides of a laminate of the porous substrate and the heat-resistant layer, wherein the inorganic particles account for 95 mass% or more of the heat-resistant layer, the inorganic particles contained in the heat-resistant layer have an average primary particle size of 0.01 μm to 0.30 μm, and the adhesive layer has a porous structure in which fibrils containing the polyvinylidene fluoride resin are connected in the form of a three-dimensional network. <2> The separator for a non-aqueous secondary battery according to <1>, wherein the heat-resistant layer is disposed on both sides of the porous substrate. <3> The separator for a non-aqueous secondary battery according to <1> or <2>, wherein the inorganic particles include at least one type selected from the group consisting of metal oxide particles, metal hydroxide particles, and metal sulfate particles. <4> The separator for a nonaqueous secondary battery according to <1> or <2>, wherein the inorganic particles include barium sulfate particles. <5> The separator for a non-aqueous secondary battery according to any one of <1> to <4>, wherein the polyvinylidene fluoride resin comprises the following polyvinylidene fluoride resin A and polyvinylidene fluoride resin B: polyvinylidene fluoride resin A: a polyvinylidene fluoride resin containing vinylidene fluoride and hexafluoropropylene as polymerization components, wherein the proportion of hexafluoropropylene in the total of vinylidene fluoride and hexafluoropropylene is more than 1.5 mol % and not more than 5 mol %; polyvinylidene fluoride resin B: a polyvinylidene fluoride resin containing vinylidene fluoride and hexafluoropropylene as polymerization components, wherein the proportion of hexafluoropropylene in the total of vinylidene fluoride and hexafluoropropylene is more than 5 mol % and not more than 15 mol %. <6> The separator for a nonaqueous secondary battery according to any one of <1> to <5>, wherein the polyvinylidene fluoride resin contains the following polyvinylidene fluoride resin C: polyvinylidene fluoride resin containing vinylidene fluoride, hexafluoropropylene, and a monomer represented by the formula (1) described below as polymerization components.<7> The separator for a non-aqueous secondary battery according to any one of <1> to <6>, wherein the binder resin of the heat-resistant layer comprises at least one selected from the group consisting of a butadiene-based polymer and an acrylic-based resin. <8> The separator for a non-aqueous secondary battery according to any one of <1> to <7>, wherein the thickness of each heat-resistant layer is 0.1 μm to 2 μm. <9> The separator for a non-aqueous secondary battery according to any one of <1> to <8>, wherein the thickness of the porous substrate is 1 μm to 7 μm. <10> The mass per unit area of ​​the adhesive layer on both sides is 0.5 g / m in total. ２ ~3g / m ２ <11> The separator for a nonaqueous secondary battery according to any one of <1> to <10>, wherein a difference between the Gurley value of the laminate and the Gurley value of the separator for a nonaqueous secondary battery is 30 seconds / 100 mL or less. <12> A nonaqueous secondary battery comprising: a positive electrode, a negative electrode, and the separator for a nonaqueous secondary battery according to any one of <1> to <11> disposed between the positive electrode and the negative electrode, wherein an electromotive force is generated by doping and undoping of lithium ions.

[0006] According to the present disclosure, a separator for a non-aqueous secondary battery is provided that has excellent thermal dimensional stability and adhesion to electrodes, and improves the cycle characteristics and impact resistance of the battery.

[0007] Fig. 1 is a schematic cross-sectional view of an example of a separator according to the present disclosure. Fig. 2 is a schematic cross-sectional view of an example of a separator according to the present disclosure. Fig. 3 is a schematic cross-sectional view of an example of a separator according to the present disclosure. Fig. 4 is a schematic cross-sectional view of an example of a separator according to the present disclosure. Fig. 5 is an SEM image of an example embodiment of an adhesive layer provided in a separator according to the present disclosure.

[0008]

[0023] The following describes embodiments of the present disclosure. These descriptions and examples are intended to illustrate the embodiments and are not intended to limit the scope of the embodiments.

[0009] In the present disclosure, a numerical range indicated using "to" indicates a range that includes the numerical values ​​before and after "to" as the minimum and maximum values, respectively. In the numerical ranges described in stages in the present disclosure, the upper or lower limit value described in one numerical range may be replaced with the upper or lower limit value of another numerical range described in stages. Furthermore, in the numerical ranges described in the present disclosure, the upper or lower limit value of that numerical range may be replaced with a value shown in an example.

[0010] In the present disclosure, "A and / or B" is synonymous with "at least one of A and B." In other words, "A and / or B" means that it may be only A, only B, or a combination of A and B.

[0011] In the present disclosure, the term "step" includes not only an independent step but also a step that cannot be clearly distinguished from other steps as long as the purpose of the step is achieved.

[0012] In the present disclosure, when referring to the amount of each component in a composition, if the composition contains multiple substances corresponding to each component, the total amount of the multiple substances present in the composition is referred to unless otherwise specified. In the present disclosure, multiple types of particles corresponding to each component may be included. If the composition contains multiple types of particles corresponding to each component, the particle size of each component refers to the value for a mixture of the multiple types of particles present in the composition, unless otherwise specified.

[0013] In this disclosure, MD (Machine Direction) refers to the longitudinal direction of a separator manufactured in a long shape, and TD (Transverse Direction) refers to the direction perpendicular to the MD in the plane direction of the separator. In this disclosure, TD is also referred to as the "width direction."

[0014] In the present disclosure, when the stacking relationship of each layer constituting a separator is expressed as "upper" and "lower," the layer closer to the porous substrate is referred to as "lower," and the layer farther from the porous substrate is referred to as "upper."

[0015] In the present disclosure, the volume of the porous layer excluding pores is referred to as the "solid content volume."

[0016] In the present disclosure, performing a heat press treatment after impregnating a separator with an electrolyte solution is referred to as "wet heat press," and performing a heat press treatment without impregnating a separator with an electrolyte solution is referred to as "dry heat press."

[0017] In the present disclosure, the term "monomer unit" of a polymer or resin means a structural unit of the polymer or resin, which is formed by polymerization of a monomer. In the present disclosure, the term "(meth)acrylic" means either "acrylic" or "methacrylic".

[0018] <Separator for Non-Aqueous Secondary Battery> A separator for a non-aqueous secondary battery according to the present disclosure (also simply referred to as a "separator" in the present disclosure) includes a porous substrate, a heat-resistant layer containing inorganic particles and a binder resin and disposed on one or both sides of the porous substrate, and an adhesive layer containing a polyvinylidene fluoride resin and disposed on one or both sides of a laminate of the porous substrate and the heat-resistant layer.

[0019] In the separator of the present disclosure, the mass ratio of inorganic particles in the heat-resistant layer is 95 mass% or more, the inorganic particles contained in the heat-resistant layer have an average primary particle size of 0.01 μm to 0.30 μm, and the adhesive layer has a porous structure in which fibrils containing polyvinylidene fluoride resin are connected in the form of a three-dimensional network.

[0020] By virtue of having the above-described configuration, the separator of the present disclosure has excellent thermal dimensional stability and adhesion to electrodes, and also improves the cycle characteristics and impact resistance of the battery.

[0021] The mass proportion of the inorganic particles in the heat-resistant layer is 95% by mass or more, preferably 96% by mass or more, and more preferably 97% by mass or more, from the viewpoint of the thermal dimensional stability of the separator and improving the impact resistance of the battery. The mass proportion of the inorganic particles in the heat-resistant layer is preferably 99% by mass or less, more preferably 98% by mass or less, from the viewpoint of the formability of the heat-resistant layer. In other words, the mass proportion of the binder resin in the heat-resistant layer is preferably 1% by mass or more, more preferably 2% by mass or more, from the viewpoint of the formability of the heat-resistant layer.

[0022] The average primary particle size of the inorganic particles contained in the heat-resistant layer is 0.01 μm or more, preferably 0.05 μm or more, and more preferably 0.08 μm or more, from the viewpoint of ion permeability of the heat-resistant layer. The average primary particle size of the inorganic particles contained in the heat-resistant layer is 0.30 μm or less, preferably 0.20 μm or less, and more preferably 0.15 μm or less, from the viewpoint of thinning the heat-resistant layer and the viewpoint of formability of the heat-resistant layer containing a large amount of inorganic particles.

[0023] The heat-resistant layer contains inorganic particles in an amount of 95% by mass or more for the purpose of improving the thermal dimensional stability of the separator and the impact resistance of the battery. In order to form a heat-resistant layer containing a large amount of inorganic particles and to form it as thin as possible, the inorganic particles have an average primary particle size of 0.30 μm or less.

[0024] The adhesive layer contains a polyvinylidene fluoride resin and has a porous structure in which fibrils containing the polyvinylidene fluoride resin are connected in a three-dimensional network. This structure gives the adhesive layer excellent ion permeability and adhesion to the electrode. This structure also allows the adhesive layer to be made thinner while maintaining mechanical strength, thereby improving the cycle characteristics of the battery.

[0025] The layer structure of the separator of the present disclosure will be described with reference to the drawings. FIGS. 1 to 5 are each a schematic cross-sectional view of an embodiment of the separator of the present disclosure. FIGS. 1 to 5 are schematic cross-sectional views primarily for explaining the stacking order of layers, and the structure of each layer is omitted or simplified. In FIGS. 1 to 5, layers having similar functions will be assigned the same reference numerals.

[0026] The separator 10A shown in Fig. 1 is a separator in which heat-resistant layers 30 are disposed on both sides of a porous substrate 20, and adhesive layers 50 are disposed on both sides of a laminate 40 of the porous substrate 20 and two heat-resistant layers 30. One heat-resistant layer 30 and the other heat-resistant layer 30 may be the same or different in components and / or composition. One adhesive layer 50 and the other adhesive layer 50 may be the same or different in components and / or composition.

[0027] 2 is a separator in which heat-resistant layers 30 are disposed on both sides of a porous substrate 20, and an adhesive layer 50 is disposed on one side of a laminate 40 of the porous substrate 20 and two heat-resistant layers 30. One heat-resistant layer 30 and the other heat-resistant layer 30 may be the same or different in components and / or composition.

[0028] 3 is a separator in which a heat-resistant layer 30 is disposed on one side of a porous substrate 20, and adhesive layers 50 are disposed on both sides of a laminate 40 of the porous substrate 20 and one heat-resistant layer 30. One adhesive layer 50 and the other adhesive layer 50 may be the same or different in components and / or composition.

[0029] 4 is a separator in which a heat-resistant layer 30 is disposed on one surface of a porous substrate 20, and an adhesive layer 50 is disposed on one surface of a laminate 40 of the porous substrate 20 and one heat-resistant layer 30. In the separator 10D, the adhesive layer 50 is disposed on the surface of the heat-resistant layer 30.

[0030] 5 is a separator in which a heat-resistant layer 30 is disposed on one surface of a porous substrate 20, and an adhesive layer 50 is disposed on one surface of a laminate 40 of the porous substrate 20 and one heat-resistant layer 30. In the separator 10E, the adhesive layer 50 is disposed on the surface of the porous substrate 20.

[0031] The heat-resistant layer 30 is a layer containing inorganic particles and a binder resin, and is disposed on the surface of the porous substrate 20. The heat-resistant layer 30 may be present on only one side of the porous substrate 20, or on both sides of the porous substrate 20. When the heat-resistant layer 30 is present on both sides of the porous substrate 20, the thermal dimensional stability of the separator is superior, and the safety of the battery can be further improved. In addition, the separator is less likely to curl, and handling during battery production is excellent. When the heat-resistant layer 30 is present on only one side of the porous substrate 20, the ion permeability of the separator is superior. In addition, the overall thickness of the separator can be reduced, and a battery with a higher energy density can be produced.

[0032] The adhesive layer 50 is a layer containing a polyvinylidene fluoride resin and is disposed on the surface of the porous substrate 20 or the heat-resistant layer 30, and exists as the outermost layer of the separator. The adhesive layer 50 may be disposed on only one side of the laminate 40, or on both sides of the laminate 40. When the adhesive layer 50 is disposed on only one side of the laminate 40, it is preferable that the adhesive layer 50 be disposed on the surface of the heat-resistant layer 30. The adhesive layer 50 may be disposed on one or both sides of the laminate 40 depending on the composition or surface properties of the positive or negative electrode of the battery. When the adhesive layer 50 is disposed on only one side of the laminate 40, the overall thickness of the separator can be reduced, and a battery with a higher energy density can be manufactured.

[0033] The porous substrate, heat-resistant layer, and adhesive layer of the separator of the present disclosure will be described in detail below.

[0034] [Porous substrate] In the present disclosure, the term "porous substrate" refers to a substrate having pores or gaps therein. Examples of such substrates include microporous membranes; porous sheets made of fibrous materials, such as nonwoven fabrics and paper; and composite porous sheets obtained by laminating one or more other porous layers on these microporous membranes or porous sheets. In the present disclosure, microporous membranes are preferred from the viewpoint of thinning and strength of the separator. The term "microporous membrane" refers to a membrane having a large number of micropores therein, a structure in which the micropores are connected, and which allows gas or liquid to pass from one surface to the other.

[0035] The material of the porous substrate is preferably an electrically insulating material.

[0036] The porous substrate preferably contains a thermoplastic resin to impart a shutdown function to the porous substrate. The shutdown function refers to a function in which, when the battery temperature rises, the constituent materials dissolve and block the pores of the porous substrate, thereby blocking the movement of ions and preventing thermal runaway of the battery. The thermoplastic resin preferably has a melting point of less than 200°C. Examples of the thermoplastic resin include polyesters such as polyethylene terephthalate; polyolefins such as polyethylene and polypropylene; and polyolefins are particularly preferred.

[0037] The porous substrate is preferably a microporous membrane containing polyolefin (referred to as a "polyolefin microporous membrane" in the present disclosure). Examples of polyolefin microporous membranes include those used in conventional battery separators, and it is preferable to select one having sufficient mechanical properties and ion permeability from these.

[0038] From the viewpoint of exhibiting a shutdown function, the polyolefin microporous membrane is preferably a microporous membrane containing polyethylene, and the polyethylene content is preferably 95% by mass or more based on the total mass of the polyolefin microporous membrane.

[0039] The polyolefin microporous film is preferably a microporous film containing polypropylene, from the viewpoint of heat resistance that prevents the film from easily breaking when exposed to high temperatures.

[0040] From the viewpoint of providing a shutdown function and heat resistance that does not easily rupture when exposed to high temperatures, the polyolefin microporous membrane is preferably a polyolefin microporous membrane containing polyethylene and polypropylene. An example of a polyolefin microporous membrane containing polyethylene and polypropylene is a microporous membrane in which polyethylene and polypropylene are mixed in one layer. From the viewpoint of achieving both the shutdown function and heat resistance, the microporous membrane preferably contains 95% by mass or more of polyethylene and 5% by mass or less of polypropylene. Also, from the viewpoint of achieving both the shutdown function and heat resistance, a polyolefin microporous membrane having a laminate structure of two or more layers, at least one layer containing polyethylene and at least one layer containing polypropylene, is preferred.

[0041] The polyolefin contained in the polyolefin microporous membrane preferably has a weight-average molecular weight (Mw) of 100,000 to 5,000,000. When the Mw of the polyolefin is 100,000 or more, the microporous membrane can be imparted with sufficient mechanical properties. On the other hand, when the Mw of the polyolefin is 5,000,000 or less, the microporous membrane has good shutdown properties and is easy to mold.

[0042] Examples of methods for producing a polyolefin microporous membrane include a method in which a molten polyolefin resin is extruded through a T-die to form a sheet, which is crystallized, stretched, and then heat-treated to form a microporous membrane; and a method in which a molten polyolefin resin together with a plasticizer such as liquid paraffin is extruded through a T-die, cooled to form a sheet, stretched, the plasticizer is extracted, and then heat-treated to form a microporous membrane.

[0043] Examples of porous sheets made of fibrous materials include porous sheets such as nonwoven fabrics and papers made of fibrous materials such as polyesters such as polyethylene terephthalate; polyolefins such as polyethylene and polypropylene; heat-resistant resins such as wholly aromatic polyamide, polyamideimide, polyimide, polyethersulfone, polysulfone, polyetherketone, and polyetherimide; and cellulose.

[0044] In the present disclosure, a heat-resistant resin refers to a resin having a melting point of 200° C. or higher, or a resin having no melting point and a decomposition temperature of 200° C. or higher. In other words, a heat-resistant resin in the present disclosure refers to a resin that does not melt or decompose in a temperature range below 200° C.

[0045] Examples of composite porous sheets include sheets in which a functional layer is laminated on a porous sheet made of a microporous membrane or a fibrous material. Such composite porous sheets are preferred because the functional layer allows for additional functionality to be added. For example, from the viewpoint of imparting heat resistance, the functional layer may be a porous layer made of a heat-resistant resin. Examples of heat-resistant resins include one or more heat-resistant resins selected from wholly aromatic polyamides, polyamideimides, polyimides, polyethersulfones, polysulfones, polyetherketones, and polyetherimides. Examples of composite methods include coating a functional layer on a microporous membrane or porous sheet, bonding a microporous membrane or porous sheet and a functional layer with an adhesive, and thermocompression bonding a microporous membrane or porous sheet and a functional layer.

[0046] The surface of the porous substrate may be subjected to various surface treatments to improve wettability with the coating liquid for forming the heat-resistant layer or adhesive layer, as long as the properties of the porous substrate are not impaired. Examples of surface treatments include corona treatment, plasma treatment, flame treatment, and ultraviolet irradiation treatment.

[0047] -Characteristics of porous substrate- From the viewpoint of mechanical strength, the thickness of the porous substrate is preferably 1 μm or more, more preferably 3 μm or more, and even more preferably 5 μm or more. From the viewpoint of increasing the energy density of the battery, the thickness of the porous substrate is preferably 7 μm or less, and more preferably 6 μm or less.

[0048] The thickness of the porous substrate is determined by measuring 20 points within a 10 cm square area with a contact type thickness meter and averaging the measurements.

[0049] The Gurley value of the porous substrate is preferably 30 seconds / 100 mL or more, more preferably 50 seconds / 100 mL or more, and even more preferably 70 seconds / 100 mL or more, from the viewpoint of suppressing short-circuiting of the battery. The Gurley value of the porous substrate 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, from the viewpoint of ion permeability and suppressing clogging of the porous structure at the boundary between the porous substrate and the heat-resistant layer or adhesive layer when exposed to high temperatures. The Gurley value of the porous substrate is measured using a Gurley densometer according to JIS P8117:2009.

[0050] From the viewpoint of ion permeability, the porosity of the porous substrate is preferably 30% to 60%. The porosity ε (%) of the porous substrate is calculated by the following formula: ε = {1 - Ws / (ds·t)} × 100, where Ws is the basis weight (g / m ２ ), ds is the true density of the porous substrate (g / cm ３ ), t is the thickness (μm) of the porous substrate. Basis weight is the mass per unit area.

[0051] [Heat-Resistant Layer] The heat-resistant layer contains at least inorganic particles and a binder resin.

[0052] - 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.

[0053] Examples of metal oxides constituting the metal oxide particles include silica, alumina, boehmite (alumina monohydrate), titania, zirconia, magnesium oxide, barium oxide, etc., with alumina being preferred. Examples of metal hydroxides constituting the metal hydroxide particles include magnesium hydroxide, aluminum hydroxide, calcium hydroxide, chromium hydroxide, zirconium hydroxide, cerium hydroxide, nickel hydroxide, boron hydroxide, etc., with magnesium hydroxide being preferred. Examples of metal sulfates constituting the metal sulfate particles include barium sulfate, calcium sulfate, etc., with barium sulfate being preferred. Examples of metal carbonates constituting the metal carbonate particles include calcium carbonate, magnesium carbonate, barium carbonate, etc. Examples of metal nitrides constituting the metal nitride particles include boron nitride, aluminum nitride, etc. Examples of clay mineral particles include calcium silicate, talc, etc.

[0054] The inorganic particles may be surface-modified with a silane coupling agent or the like.

[0055] The inorganic particles may be used alone or in combination of two or more kinds.

[0056] From the viewpoint of stability in the electrolyte and electrochemical stability, the inorganic particles are preferably at least one type selected from the group consisting of metal oxide particles, metal hydroxide particles, and metal sulfate particles.

[0057] As the inorganic particles, metal sulfate particles are preferred, and barium sulfate particles are more preferred, from the viewpoint that they are less likely to decompose the electrolytic solution or electrolyte and therefore are less likely to cause gas generation inside the battery.

[0058] The particle shape of the inorganic particles is not limited, and may be any of spherical, elliptical, plate-like, needle-like, and amorphous. From the viewpoint of suppressing short circuits in the battery, the inorganic particles contained in the heat-resistant layer are preferably plate-like particles or non-aggregated primary particles.

[0059] The average primary particle size of the inorganic particles contained in the heat-resistant layer is 0.01 μm to 0.30 μm, preferably 0.05 μm to 0.20 μm, and more preferably 0.08 μm to 0.15 μm.

[0060] The average primary particle size of the inorganic particles is determined by measuring the long diameters of 100 randomly selected inorganic particles in observation using a scanning electron microscope (SEM) and averaging the long diameters of the 100 particles. The sample used for SEM observation is inorganic particles that are the material forming the heat-resistant layer, or inorganic particles removed from the heat-resistant layer. There are no limitations on the method for removing the inorganic particles from the heat-resistant layer. Examples of such methods include immersing the heat-resistant layer peeled off from the separator in an organic solvent that dissolves the binder resin to remove the inorganic particles; or heating the heat-resistant layer peeled off from the separator to about 800°C to remove the binder resin and remove the inorganic particles.

[0061] Density of inorganic particles (g / cm ３ ) is preferably 1.5 to 11.0, more preferably 2.0 to 5.5, and even more preferably 3.5 to 5.0, from the viewpoint of making the inorganic particle content of the heat-resistant layer 95% by mass or more and from the viewpoint of the stability over time and coatability of the coating solution for forming the heat-resistant layer.

[0062] The mass proportion of the inorganic particles in the heat-resistant layer is 95 mass % or more, preferably 96 mass % to 99 mass %, and more preferably 97 mass % to 98 mass %.

[0063] When the inorganic particles are barium sulfate particles, the mass proportion of the barium sulfate particles in the heat-resistant layer is preferably 95 mass % or more, more preferably 96 mass % to 99 mass %, and even more preferably 97 mass % to 98 mass %.

[0064] When heat-resistant layers are present on both sides of the porous substrate, the type and / or content of inorganic particles contained in one heat-resistant layer may be the same as or different from the type and / or content of inorganic particles contained in the other heat-resistant layer.

[0065] -Organic Particles- The heat-resistant layer may contain organic particles. Examples of organic particles include particles made of crosslinked polymers such as crosslinked poly(meth)acrylic acid, crosslinked poly(meth)acrylic acid ester, crosslinked polysilicone, crosslinked polystyrene, crosslinked polydivinylbenzene, styrene-divinylbenzene copolymer crosslinked product, melamine resin, phenol resin, and benzoguanamine-formaldehyde condensate; and particles made of heat-resistant polymers such as polysulfone, polyacrylonitrile, aramid, and polyacetal. The resin constituting the organic particles may be a mixture, modified product, derivative, copolymer (random copolymer, alternating copolymer, block copolymer, graft copolymer), or crosslinked product of the above-mentioned exemplified materials.

[0066] The organic particles may be used alone or in combination of two or more kinds.

[0067] - Binder Resin - The binder resin contained in the heat-resistant layer has the function of binding the inorganic particles contained in the heat-resistant layer together, as well as the function of adhering the heat-resistant layer to the porous substrate, the function of adhering the heat-resistant layer to the electrode, the function of improving the heat resistance of the heat-resistant layer, etc. The binder resin contained in the heat-resistant layer may have a particle shape in the heat-resistant layer, or may not have a specific shape, and may have any form as long as it can bind the inorganic particles together.

[0068] The binder resin contained in the heat-resistant layer preferably contains at least one selected from the group consisting of butadiene-based polymers and acrylic-based resins, from the viewpoint of binding a large amount of inorganic particles.

[0069] Examples of butadiene-based polymers include homopolymers of butadiene and copolymers of butadiene and styrene-based monomers (i.e., styrene-butadiene copolymers). These resins may be used alone or in combination of two or more.

[0070] Examples of styrene-based monomers constituting the styrene-butadiene copolymer include styrene and 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. As the styrene-based monomer, styrene and α-methylstyrene are preferred, and styrene is more preferred. One type of styrene-based monomer may be used alone, or two or more types may be used in combination.

[0071] The styrene-butadiene copolymer may be copolymerized with a small amount of an acrylic monomer. The acrylic monomer is preferably a (meth)acrylic acid alkyl ester. The alkyl group at the ester moiety of the (meth)acrylic acid alkyl ester is preferably an alkyl group having 1 to 10 carbon atoms, and more preferably an alkyl group having 1 to 8 carbon atoms. Specific examples of the acrylic monomer include methyl (meth)acrylate, ethyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate. One type of acrylic monomer may be used alone, or two or more types may be used in combination.

[0072] Examples of the acrylic resin include a homopolymer or copolymer of an acrylic monomer, and a copolymer of an acrylic monomer and a styrene monomer. These resins may be used alone or in combination of two or more.

[0073] 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. Preferred acrylic monomers are alkyl (meth)acrylates. The alkyl group at the ester moiety of the alkyl (meth)acrylate is preferably an alkyl group having 1 to 10 carbon atoms, and more preferably an alkyl group having 1 to 8 carbon atoms. One type of acrylic monomer may be used alone, or two or more types may be used in combination.

[0074] Examples of styrene-based monomers for acrylic resins include styrene and 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. As the styrene-based monomer, styrene and α-methylstyrene are preferred, and styrene is more preferred. One type of styrene-based monomer may be used alone, or two or more types may be used in combination.

[0075] The heat-resistant layer may contain a resin other than the butadiene-based polymer and the acrylic resin. Examples of other resins include wholly aromatic polyamide (also known as aramid), polyamideimide, poly-N-vinylacetamide, polyacrylamide, copolymerized polyetherpolyamide, polyimide and polyetherimide, fluorine-based resin, homopolymer or copolymer of vinyl nitrile compounds (such as acrylonitrile and methacrylonitrile), carboxymethyl cellulose, hydroxyalkyl cellulose, polyvinyl alcohol, polyvinyl butyral, polyvinylpyrrolidone, polyether (such as polyethylene oxide and polypropylene oxide), polysulfone, polyketone, polyether ketone, polyether sulfone, and mixtures thereof. These resins may be used alone or in combination of two or more.

[0076] The mass proportion of other resins in the total resin of the heat-resistant layer is preferably 10 mass% or less, more preferably 5 mass% or less, even more preferably 1 mass% or less, and particularly preferably substantially none. The mass proportion of the butadiene-based polymer and acrylic resin in the total resin of the heat-resistant layer is preferably 90 mass% or more, more preferably 95 mass% or more, even more preferably 99 mass% or more, and particularly preferably 100 mass%.

[0077] The mass proportion of the binder resin in the heat-resistant layer is preferably 1 mass % or more, more preferably 2 mass % or more, from the viewpoint of formability of the heat-resistant layer, and is preferably 5 mass % or less, more preferably 4 mass % or less, and more preferably 3 mass % or less, from the viewpoint of increasing the content of inorganic particles in the heat-resistant layer.

[0078] When heat-resistant layers are present on both sides of the porous substrate, the type and / or content of the binder resin contained in one heat-resistant layer may be the same as or different from the type and / or content of the binder resin contained in the other heat-resistant layer.

[0079] -Other Components- The heat-resistant layer may contain additives such as a dispersant such as a surfactant, a wetting agent, an antifoaming agent, and a pH adjuster. The dispersant is added to the coating liquid for forming the heat-resistant layer for the purpose of improving dispersibility, coatability, or storage stability. The wetting agent, antifoaming agent, and pH adjuster are added to the coating liquid for forming the heat-resistant layer for the purpose of, for example, improving compatibility with the porous substrate, suppressing air entrapment in the coating liquid, or adjusting the pH.

[0080] -Characteristics of Heat-Resistant Layer- From the viewpoint of heat resistance of the battery, the thickness of each heat-resistant layer is preferably 0.1 μm or more, more preferably 0.5 μm or more, and even more preferably 0.8 μm or more. From the viewpoint of ion permeability and energy density of the battery, the thickness of each heat-resistant layer is preferably 2 μm or less, more preferably 1.5 μm or less, and even more preferably 1.2 μm or less.

[0081] When the heat-resistant layer is present on both sides of the porous substrate, the thickness of the heat-resistant layer is preferably 0.2 μm or more, more preferably 1.0 μm or more, and even more preferably 1.5 μm or more, and is preferably 4 μm or less, more preferably 3 μm or less, and even more preferably 2.5 μm or less, in total on both sides.

[0082] The thickness of the heat-resistant layer is the thickness of the flat membrane after removing the adhesive layer from the separator minus the thickness of the porous substrate. The thickness of the flat membrane after peeling the adhesive layer from the separator is measured at 20 points within a 10 cm square using a contact thickness meter and the average is calculated.

[0083] The porosity of the heat-resistant layer is preferably 20% to 70% from the viewpoint of ion permeability. The porosity ε (%) of the heat-resistant layer is calculated by the following formula.

[0084] Here, for the constituent material 1, constituent material 2, constituent material 3, ..., constituent material n of the heat-resistant layer, 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 heat-resistant layer is t (cm).

[0085] [Adhesive Layer] The adhesive layer is a layer disposed on the surface of the heat-resistant layer or the porous substrate, and exists as the outermost layer of the separator. The adhesive layer has numerous gaps or micropores, allowing gas or liquid to pass from one surface to the other.

[0086] -Polyvinylidene fluoride resin- The adhesive layer contains a polyvinylidene fluoride resin. Examples of polyvinylidene fluoride resins include homopolymers of vinylidene fluoride (i.e., polyvinylidene fluoride); copolymers of vinylidene fluoride and halogen-containing monomers such as hexafluoropropylene, tetrafluoroethylene, trifluoroethylene, chlorotrifluoroethylene, vinyl fluoride, and trichloroethylene; copolymers of vinylidene fluoride and monomers other than halogen-containing monomers; copolymers of vinylidene fluoride, halogen-containing monomers, and monomers other than halogen-containing monomers; and mixtures thereof. The polyvinylidene fluoride resins may be used alone or in combination of two or more.

[0087] As the polyvinylidene fluoride resin, from the viewpoint of adhesion to electrodes, a copolymer of vinylidene fluoride (VDF) and hexafluoropropylene (HFP) (VDF-HFP copolymer) is preferred. In the present disclosure, the VDF-HFP copolymer includes both a copolymer obtained by polymerizing only VDF and HFP, and a copolymer obtained by polymerizing VDF, HFP, and other monomers. By increasing or decreasing the HFP content of the VDF-HFP copolymer, the crystallinity, heat resistance, resistance to solubility in the electrolyte, and other properties of the copolymer can be controlled within appropriate ranges.

[0088] The polyvinylidene fluoride resin preferably includes polyvinylidene fluoride resin A and polyvinylidene fluoride resin B. In the present disclosure, polyvinylidene fluoride resin A refers to 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 more than 1.5 mol% and not more than 5 mol%. In the present disclosure, polyvinylidene fluoride resin B refers to 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 more than 5 mol% and not more than 15 mol%.

[0089] The polyvinylidene fluoride resin A may contain a monomer other than VDF and HFP as a polymerization component. Examples of the other monomer include halogen-containing monomers such as tetrafluoroethylene, trifluoroethylene, chlorotrifluoroethylene, vinyl fluoride, and trichloroethylene, and a monomer represented by the formula (1) described below. When the polyvinylidene fluoride resin A contains a monomer represented by the formula (1) as a polymerization component, the polyvinylidene fluoride resin A is also the polyvinylidene fluoride resin C.

[0090] VDF and HFP are preferably the main monomers of the halogen-containing monomers constituting the polyvinylidene fluoride resin A. The total of VDF and HFP constituting the polyvinylidene fluoride resin A is preferably 80 mol % or more, more preferably 90 mol % or more, and even more preferably 100 mol %, based on the total amount of the halogen-containing monomers.

[0091] The proportion of HFP in the polyvinylidene fluoride resin A relative to the total of VDF and HFP is preferably more than 1.5 mol % and not more than 5 mol %, more preferably more than 1.5 mol % and not more than 4 mol %. When the proportion of HFP in polyvinylidene fluoride resin A is within this range, a fine porous structure is likely to develop in the adhesive layer, and excessive swelling of the VDF-HFP copolymer in the electrolyte is suppressed, making pore blockage less likely to occur, resulting in excellent battery cycle characteristics.

[0092] The polyvinylidene fluoride resin B may contain a monomer other than VDF and HFP as a polymerization component. Examples of the other monomer include halogen-containing monomers such as tetrafluoroethylene, trifluoroethylene, chlorotrifluoroethylene, vinyl fluoride, and trichloroethylene, and a monomer represented by the formula (1) described below. When the polyvinylidene fluoride resin B contains a monomer represented by the formula (1) as a polymerization component, the polyvinylidene fluoride resin B is also the polyvinylidene fluoride resin C.

[0093] VDF and HFP are preferably the main monomers of the halogen-containing monomers constituting the polyvinylidene fluoride resin B. The total of VDF and HFP constituting the polyvinylidene fluoride resin A is preferably 80 mol % or more, more preferably 90 mol % or more, and even more preferably 100 mol %, based on the total amount of the halogen-containing monomers.

[0094] The proportion of HFP in the polyvinylidene fluoride resin B relative to the total of VDF and HFP is more than 5 mol% and not more than 15 mol%, and preferably more than 5 mol% and not more than 10 mol%. When the proportion of HFP in polyvinylidene fluoride resin B is within this range, the porosity and average pore size of the adhesive layer do not become too large, and high mobility of the polymer chains when heated and ease of swelling in the electrolyte solution are ensured, resulting in excellent adhesion between the adhesive layer and the electrode in both dry heat pressing and wet heat pressing.

[0095] The polyvinylidene fluoride resins A and B each preferably have a weight-average molecular weight (Mw) of 400,000 to 2,000,000. When the Mw of each of the polyvinylidene fluoride resins A and B is 400,000 or more, the adhesive layer can ensure mechanical properties that enable it to withstand adhesion treatment with the electrode, resulting in better adhesion to the electrode. From this perspective, the Mw of each of the polyvinylidene fluoride resins A and B is more preferably 500,000 or more, even more preferably 600,000 or more, and even more preferably 700,000 or more. When the Mw of each of the polyvinylidene fluoride resins A and B is 2,000,000 or less, the resins are easily softened by heat pressing, making it easy for the adhesive layer to adhere to the electrode. In addition, the viscosity of the coating liquid for coating and molding the adhesive layer does not become too high, resulting in good moldability and crystal formation, and good porosity of the adhesive layer. From this viewpoint, the Mw of each of the polyvinylidene fluoride resins A and B is more preferably 1.5 million or less, and even more preferably 1.2 million or less.

[0096] Both polyvinylidene fluoride resin A and polyvinylidene fluoride resin B preferably have an Mw of 600,000 or more. When the Mw of both resins is 600,000 or more, the adhesive layer has excellent thermal dimensional stability, and as a result, deformation during storage of the battery at high temperatures is suppressed. From this viewpoint, the Mw of both resins is more preferably 700,000 or more, and even more preferably 800,000 or more.

[0097] From the viewpoint of excellent adhesion to the electrode, the weighted average of the Mw of the polyvinylidene fluoride resin A contained in the adhesive layer and the Mw of the polyvinylidene fluoride resin B contained in the adhesive layer (the average of the Mw of both resins weighted by the content ratio (mass basis)) is preferably 600,000 or more and 2,000,000 or less. The lower limit of the weighted average is more preferably 700,000 or more, even more preferably 800,000 or more, and even more preferably 900,000 or more, and the upper limit of the weighted average is more preferably 1,500,000 or less, even more preferably 1,200,000 or less, and even more preferably 1,100,000 or less.

[0098] The content ratio of polyvinylidene fluoride resin A to polyvinylidene fluoride resin B in the adhesive layer (polyvinylidene fluoride resin A:polyvinylidene fluoride resin B) is preferably 15:85 to 85:15 by mass. That is, the proportion of polyvinylidene fluoride resin A in the total amount of polyvinylidene fluoride resin A and polyvinylidene fluoride resin B is preferably 15% by mass to 85% by mass (the proportion of polyvinylidene fluoride resin B is 15% by mass to 85% by mass). When polyvinylidene fluoride resin A is 15% by mass or more of the total amount of polyvinylidene fluoride resin A and polyvinylidene fluoride resin B, a fine porous structure is likely to develop in the adhesive layer, and excessive swelling of the VDF-HFP copolymer in response to the electrolyte can be suppressed, making pore blockage less likely to occur. On the other hand, when the polyvinylidene fluoride resin B is 15% by mass or more of the total amount of polyvinylidene fluoride resin A and polyvinylidene fluoride resin B, high polymer chain mobility when heated and ease of swelling in the electrolyte are ensured. Therefore, when the content ratio of polyvinylidene fluoride resin A to polyvinylidene fluoride resin B is in the range of 15:85 to 85:15, the behavior of the VDF-HFP copolymer as a polymer and the surface morphology of the adhesive layer are well balanced, the adhesion between the adhesive layer and the electrode is excellent in both dry heat pressing and wet heat pressing, and the battery cycle characteristics and dimensional stability are excellent. From the above viewpoints, the proportion of polyvinylidene fluoride resin A is preferably 15% by mass or more, more preferably 20% by mass or more, even more preferably 25% by mass or more, even more preferably 30% by mass or more, and even more preferably 40% by mass or more. From the above viewpoints, the proportion of polyvinylidene fluoride resin B is preferably 15% by mass or more, more preferably 20% by mass or more, even more preferably 25% by mass or more, even more preferably 30% by mass or more, and still more preferably 40% by mass or more. The content ratio of polyvinylidene fluoride resin A to polyvinylidene fluoride resin B in the adhesive layer (polyvinylidene fluoride resin A:polyvinylidene fluoride resin B) is, on a mass basis, more preferably 20:80 to 80:20, even more preferably 25:75 to 75:25, even more preferably 30:70 to 70:30, and still more preferably 40:60 to 60:40.

[0099] When the adhesive layer contains polyvinylidene fluoride resins A and B, the total proportion of the polyvinylidene fluoride resins A and B in the entire polyvinylidene fluoride resins contained in the adhesive layer is preferably 20% by mass to 80% by mass, more preferably 30% by mass to 70% by mass, and even more preferably 40% by mass to 60% by mass.

[0100] From the viewpoint of achieving superior adhesion to electrodes by wet heat pressing, the polyvinylidene fluoride resin preferably contains polyvinylidene fluoride resin C. In the present disclosure, polyvinylidene fluoride resin C refers to a polyvinylidene fluoride resin containing vinylidene fluoride, hexafluoropropylene, and a monomer represented by the following formula (1) as polymerization components. Polyvinylidene fluoride resin C may be polyvinylidene fluoride resin A or polyvinylidene fluoride resin B, or it may not be polyvinylidene fluoride resin A or polyvinylidene fluoride resin B.

[0101]

[0102] In formula (1), R １ , R ２ and R ３ each independently represents a hydrogen atom, a halogen atom, an alkyl group having 1 to 5 carbon atoms, a carboxy group, or a derivative of a carboxy group; X represents a single bond, an alkylene group having 1 to 5 carbon atoms, or an alkylene group having 1 to 5 carbon atoms and having a substituent; Y represents a hydrogen atom, an alkyl group having 1 to 5 carbon atoms, an alkyl group having 1 to 5 carbon atoms substituted with at least one hydroxy group, an alkyl group having 1 to 5 carbon atoms substituted with at least one carboxy group, or -R-O-C(=O)-(CH ２ ) ｎ It represents —C(═O)—OH (R represents an alkylene group having 1 to 5 carbon atoms, and n represents an integer of 0 or more).

[0103] In formula (1), R １ , R ２ and R ３ The halogen atom represented by may be any of a fluorine atom, a chlorine atom, a bromine atom and an iodine atom, and is preferably a fluorine atom.

[0104] In formula (1), R１ , R ２ and R ３ Examples of the alkyl group having 1 to 5 carbon atoms represented by R include linear alkyl groups such as methyl, ethyl, n-propyl, n-butyl, and n-pentyl groups; and branched alkyl groups such as isopropyl, isobutyl, sec-butyl, tert-butyl, isopentyl, neopentyl, and tert-pentyl groups. １ , R ２ and R ３ As the alkyl group having 1 to 5 carbon atoms, an alkyl group having 1 to 4 carbon atoms is preferred, and an alkyl group having 1 to 3 carbon atoms is more preferred.

[0105] In formula (1), R １ , R ２ and R ３ Examples of the derivative of the carboxy group represented by the formula include -C(=O)-OR ４ (R ４ represents an alkyl group. ４ Examples of R include linear alkyl groups such as methyl, ethyl, n-propyl, n-butyl, and n-pentyl groups; and branched alkyl groups such as isopropyl, isobutyl, sec-butyl, tert-butyl, isopentyl, neopentyl, and tert-pentyl groups. ４ As the alkyl group, an alkyl group having 1 to 5 carbon atoms is preferable, an alkyl group having 1 to 4 carbon atoms is more preferable, and an alkyl group having 1 to 3 carbon atoms is even more preferable.

[0106] In formula (1), examples of the alkylene group having 1 to 5 carbon atoms represented by X include linear alkylene groups such as methylene, ethylene, n-propylene, n-butylene, and n-pentylene; and branched alkylene groups such as isopropylene, isobutylene, sec-butylene, tert-butylene, isopentylene, neopentylene, and tert-pentylene. The alkylene group having 1 to 5 carbon atoms represented by X is preferably an alkylene group having 1 to 4 carbon atoms, and more preferably an alkylene group having 1 to 3 carbon atoms.

[0107] In formula (1), examples of the substituent in the alkylene group having 1 to 5 carbon atoms represented by X include a halogen atom, which may be any of a fluorine atom, a chlorine atom, a bromine atom, and an iodine atom. Examples of the alkylene group having 1 to 5 carbon atoms substituted by X include linear alkylene groups such as a methylene group, an ethylene group, an n-propylene group, an n-butylene group, and an n-pentylene group; and branched alkylene groups such as an isopropylene group, an isobutylene group, a sec-butylene group, a tert-butylene group, an isopentylene group, a neopentylene group, and a tert-pentylene group. The alkylene group having 1 to 5 carbon atoms substituted by X is preferably an alkylene group having 1 to 4 carbon atoms, and more preferably an alkylene group having 1 to 3 carbon atoms.

[0108] In formula (1), examples of the alkyl group having 1 to 5 carbon atoms represented by Y include linear alkyl groups such as methyl, ethyl, n-propyl, n-butyl, and n-pentyl groups, and branched alkyl groups such as isopropyl, isobutyl, sec-butyl, tert-butyl, isopentyl, neopentyl, and tert-pentyl groups. As the alkyl group having 1 to 5 carbon atoms represented by Y, an alkyl group having 1 to 4 carbon atoms is preferred, and an alkyl group having 1 to 3 carbon atoms is more preferred.

[0109] In formula (1), in the alkyl group having 1 to 5 carbon atoms substituted with at least one hydroxy group represented by Y, examples of the substituted alkyl group having 1 to 5 carbon atoms include linear alkyl groups such as methyl, ethyl, n-propyl, n-butyl, and n-pentyl; and branched alkyl groups such as isopropyl, isobutyl, sec-butyl, tert-butyl, isopentyl, neopentyl, and tert-pentyl. The substituted alkyl group having 1 to 5 carbon atoms represented by Y is preferably an alkyl group having 1 to 4 carbon atoms, more preferably an alkyl group having 1 to 3 carbon atoms. The number of hydroxy groups substituted is preferably one or two, and more preferably one.

[0110] In formula (1), examples of the alkyl group having 1 to 5 carbon atoms substituted with at least one hydroxy group represented by Y include a 2-hydroxyethyl group, a 2-hydroxypropyl group, and a 4-hydroxybutyl group.

[0111] In formula (1), in the alkyl group having 1 to 5 carbon atoms substituted with at least one carboxy group represented by Y, examples of the substituted alkyl group having 1 to 5 carbon atoms include linear alkyl groups such as methyl, ethyl, n-propyl, n-butyl, and n-pentyl; and branched alkyl groups such as isopropyl, isobutyl, sec-butyl, tert-butyl, isopentyl, neopentyl, and tert-pentyl. The substituted alkyl group having 1 to 5 carbon atoms represented by Y is preferably an alkyl group having 1 to 4 carbon atoms, more preferably an alkyl group having 1 to 3 carbon atoms. The number of substituted carboxy groups is preferably one or two, and more preferably one.

[0112] In formula (1), examples of the alkyl group having 1 to 5 carbon atoms substituted with at least one carboxy group represented by Y include a 2-carboxyethyl group, a 2-carboxypropyl group, and a 4-carboxybutyl group.

[0113] In formula (1), Y represents —R—O—C(═O)—(CH ２ ) ｎ In —C(═O)—OH, R represents an alkylene group having 1 to 5 carbon atoms, and n represents an integer of 0 or more. Examples of R include linear alkylene groups such as methylene, ethylene, n-propylene, n-butylene, and n-pentylene; and branched alkylene groups such as isopropylene, isobutylene, sec-butylene, tert-butylene, isopentylene, neopentylene, and tert-pentylene. R is preferably an alkylene group having 1 to 4 carbon atoms, and more preferably an alkylene group having 1 to 3 carbon atoms. n is preferably an integer of 0 to 5, more preferably an integer of 1 to 4, and even more preferably 2 or 3. Specific examples of such groups include, for example, —(CH ２ ) ２ -OC(=O)-(CH ２ )２ -C(=O)-OH is an example.

[0114] Examples of the monomer represented by formula (1) include R １ , R ２ and R ３ are each independently a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, X is a single bond, and Y is an alkyl group having 1 to 4 carbon atoms or an alkyl group having 1 to 3 carbon atoms substituted with at least one hydroxy group.

[0115] Examples of the monomer represented by formula (1) include acrylic monomers, unsaturated dibasic acids, and monoesters of unsaturated dibasic acids.

[0116] Examples of acrylic monomers include (meth)acrylic acid, methyl (meth)acrylate, ethyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, tert-butyl (meth)acrylate, pentyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 2-carboxyethyl (meth)acrylate, 2-carboxypropyl (meth)acrylate, 4-carboxybutyl (meth)acrylate, butenoic acid, pentenoic acid, hexenoic acid, and (meth)acryloyloxyethyl succinate. The term "(meth)acrylic" refers to either "acrylic" or "methacrylic".

[0117] Examples of unsaturated dibasic acids include unsaturated dicarboxylic acids, more specifically, maleic acid, maleic anhydride, citraconic acid, itaconic acid, and the like.

[0118] Examples of monoesters of unsaturated dibasic acids include maleic acid monomethyl ester, maleic acid monoethyl ester, citraconic acid monomethyl ester, citraconic acid monoethyl ester, itaconic acid monomethyl ester, and itaconic acid monoethyl ester. Of these, maleic acid monomethyl ester and citraconic acid monomethyl ester are preferred.

[0119] In the polyvinylidene fluoride resin C, the proportion of the monomer represented by formula (1) in all polymerization components is preferably 0.1 mol % or more, more preferably 0.2 mol % or more, and even more preferably 0.5 mol % or more, from the viewpoint of adhesiveness to an electrode. In the polyvinylidene fluoride resin C, the proportion of the monomer represented by formula (1) in all polymerization components is preferably 5.0 mol % or less, more preferably 4.0 mol % or less, and even more preferably 3.0 mol % or less, from the viewpoint of low influence on the active material contained in the electrode.

[0120] The polyvinylidene fluoride resin C can be adjusted within appropriate ranges in terms of its crystallinity, adhesion to electrodes, and resistance to dissolution in an electrolyte solution by increasing or decreasing the proportion of hexafluoropropylene (HFP) in all polymerization components. In the polyvinylidene fluoride resin C, the proportion of HFP in all polymerization components is preferably 0.5 mol % to 5.0 mol %, more preferably 0.8 mol % to 4.0 mol %, and even more preferably 1.0 mol % to 3.0 mol %.

[0121] The polyvinylidene fluoride resin C may contain, as a polymerization component, a monomer other than vinylidene fluoride (VDF), hexafluoropropylene (HFP), and the monomer represented by formula (1). Examples of the other monomer include halogen-containing monomers such as tetrafluoroethylene, trifluoroethylene, chlorotrifluoroethylene, vinyl fluoride, and trichloroethylene.

[0122] The polyvinylidene fluoride resin C is preferably a terpolymer composed of VDF, HFP, and a monomer represented by formula (1). As the terpolymer, a VDF-HFP-acrylic acid terpolymer is preferred.

[0123] The weight average molecular weight (Mw) of the polyvinylidene fluoride resin C is preferably 300,000 to 3,000,000, more preferably 500,000 to 2,500,000, even more preferably 650,000 to 2,300,000, and still more preferably 850,000 to 2,000,000.

[0124] When the adhesive layer contains polyvinylidene fluoride resin C, the proportion of polyvinylidene fluoride resin C in the entire polyvinylidene fluoride resin contained in the adhesive layer is preferably 20% by mass to 80% by mass, more preferably 30% by mass to 70% by mass, and even more preferably 40% by mass to 60% by mass, from the viewpoint of keeping the acid value of the entire polyvinylidene fluoride resin in an appropriate range.

[0125] The weight average molecular weight (Mw) of the entire polyvinylidene fluoride resin contained in the adhesive layer is preferably 300,000 or more, more preferably 500,000 or more, even more preferably 650,000 or more, and even more preferably 850,000 or more, from the viewpoint of preventing blockage of pores in the adhesive layer when heat is applied to the adhesive layer during battery production. The Mw of the entire polyvinylidene fluoride resin contained in the adhesive layer is preferably 3,000,000 or less, more preferably 2,500,000 or less, even more preferably 2,300,000 or less, and even more preferably 2,000,000 or less, from the viewpoint of appropriately softening the polyvinylidene fluoride resin when heat is applied to the adhesive layer during battery production, and achieving good adhesion between the adhesive layer and the electrode.

[0126] The Mw of the entire polyvinylidene fluoride resin contained in the adhesive layer is a molecular weight in terms of polystyrene measured by gel permeation chromatography (GPC). The polyvinylidene fluoride resin extracted from the adhesive layer or the polyvinylidene fluoride resin used to form the adhesive layer is used as a sample.

[0127] The content of the polyvinylidene fluoride resin in the adhesive layer is preferably 85% by mass to 100% by mass, more preferably 90% by mass to 100% by mass, and even more preferably 95% by mass to 100% by mass, based on the total mass of the adhesive layer.

[0128] -Other Resins- The adhesive layer may contain a resin other than polyvinylidene fluoride resin. Examples of other resins include acrylic resins, butadiene-acrylonitrile resins, fluorine-containing rubbers, homopolymers or copolymers of vinyl nitrile compounds (such as acrylonitrile and methacrylonitrile), carboxymethyl cellulose, hydroxyalkyl cellulose, polyvinyl alcohol, polyvinyl butyral, polyvinyl pyrrolidone, polyethers (such as polyethylene oxide and polypropylene oxide), and mixtures of two or more of these. These resins may be used alone or in combination of two or more.

[0129] The mass proportion of other resins in the total resin of the adhesive layer is preferably 10 mass% or less, more preferably 5 mass% or less, even more preferably 1 mass% or less, and particularly preferably substantially none. The mass proportion of polyvinylidene fluoride resin in the total resin of the adhesive layer is preferably 90 mass% or more, more preferably 95 mass% or more, even more preferably 99 mass% or more, and particularly preferably 100 mass%.

[0130] -Other Components- The adhesive layer may contain additives such as a dispersant such as a surfactant, a wetting agent, an antifoaming agent, and a pH adjuster. The dispersant is added, for example, to the resin particle dispersion for forming the adhesive layer in order to improve dispersibility, coatability, or storage stability. The wetting agent, antifoaming agent, and pH adjuster are added, for example, to the resin particle dispersion for forming the adhesive layer in order to improve compatibility with the heat-resistant layer or the porous substrate, to suppress air entrapment in the resin particle dispersion, or to adjust the pH.

[0131] -Characteristics of the adhesive layer- The adhesive layer has a porous structure in which fibrils containing polyvinylidene fluoride resin are connected in a three-dimensional network pattern. This porous structure can be confirmed using a scanning electron microscope (SEM).

[0132] The mass per unit area of ​​the adhesive layer (total of both sides) is 0.5 g / m from the viewpoint of adhesion to the electrode. ２ More than 0.8 g / m ２ More preferably, 1 g / m or more ２The mass per unit area of ​​the adhesive layer (total of both sides) is more preferably 3 g / m from the viewpoint that the adhesive layer is thin and the battery has excellent cycle characteristics. ２ Preferably, 2.5 g / m or less ２ More preferably, 2 g / m or less ２ The following is even more preferred:

[0133] The mass per unit area of ​​the adhesive layer is determined by cutting the separator into a 20 cm x 20 cm piece, measuring the mass of the layer portion corresponding to the adhesive layer, and dividing the mass by the area to determine the total for both sides.

[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 12 μm or less, and even more preferably 10 μm or less.

[0135] The thickness of the separator is determined by measuring 20 points within a 10 cm square area with a contact type thickness meter and averaging the measurements.

[0136] The Gurley value of the separator is preferably 60 seconds / 100 mL or more, more preferably 70 seconds / 100 mL or more, and even more preferably 80 seconds / 100 mL or more from the viewpoint of suppressing short circuits in the battery. From the viewpoint of ion permeability, the Gurley value of the separator is preferably 260 seconds / 100 mL or less, more preferably 250 seconds / 100 mL or less, and even more preferably 240 seconds / 100 mL or less.

[0137] The difference between the Gurley value of the laminate (a laminate consisting of a porous substrate and a heat-resistant layer disposed on one or both sides of the porous substrate) and the Gurley value of the separator is preferably 30 seconds / 100 mL or less, more preferably 25 seconds / 100 mL or less, and even more preferably 20 seconds / 100 mL or less.

[0138] The Gurley value of the separator is measured using a Gurley densometer in accordance with JIS P8117:2009. The Gurley value of the laminate is the Gurley value of a flat membrane obtained by removing the adhesive layer from the separator. The Gurley value of the flat membrane is measured using a Gurley densometer in accordance with JIS P8117:2009.

[0139] The porosity of the separator is preferably 30% to 60% from the viewpoint of ion permeability. The porosity of the laminate is preferably 30% to 60% from the viewpoint of ion permeability. The porosity ε (%) of the separator (or laminate) is calculated by the following formula:

[0140] Here, for constituent material 1, constituent material 2, constituent material 3, ..., constituent material n of the separator (or laminate), 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 (or laminate) is t (cm).

[0141] [Method for manufacturing separator] The separator of the present disclosure is manufactured, for example, by the following manufacturing method A or manufacturing method B. In manufacturing methods A and B, a heat-resistant layer is formed on a porous substrate by a dry coating method, and an adhesive layer is formed on a laminate by a wet coating method. In the present disclosure, the dry coating method is a method in which a coating layer is dried to solidify the coating layer. In the present disclosure, the wet coating method is a method in which a coating layer is solidified in a coagulation liquid.

[0142] Manufacturing method A (discontinuous manufacturing method): A heat-resistant layer is formed on a porous substrate unwound from a roll by a dry coating method to obtain a laminate of the porous substrate and the heat-resistant layer, and the laminate is then temporarily wound up on another roll. Next, an adhesive layer is formed on the laminate unwound from the roll by a wet coating method to obtain a separator, and the completed separator is then wound up on another roll.

[0143] Manufacturing method B (continuous manufacturing method): A heat-resistant layer is formed by a dry coating method on a porous substrate unwound from a roll to obtain a laminate of the porous substrate and the heat-resistant layer, and then an adhesive layer is formed by a wet coating method on the laminate to obtain a separator, and the completed separator is wound up on another roll.

[0144] The following describes in detail the steps included in Production Method B. Production Method B includes the following steps (1) to (7), which are carried out in sequence.

[0145] Step (1): Preparation of Coating Liquid for Forming Heat-Resistant Layer The coating liquid for forming the heat-resistant layer is prepared by dispersing the binder resin and inorganic particles of the heat-resistant layer in a dispersion medium. If necessary, other components besides the binder resin and inorganic particles may be dissolved or dispersed in the coating liquid.

[0146] The dispersion medium used in preparing the coating solution is preferably water. The solid content of the coating solution may be set within a range that provides an appropriate viscosity from the viewpoint of coatability, and is preferably approximately 10% by mass to 60% by mass.

[0147] Step (2): Preparation of Coating Liquid for Forming Adhesive Layer The coating liquid for forming the adhesive layer is prepared by dissolving a polyvinylidene fluoride resin in a solvent. If necessary, other components besides the polyvinylidene fluoride resin may be dissolved or dispersed in the coating liquid.

[0148] The solvent used in preparing the coating solution includes a solvent that dissolves polyvinylidene fluoride resin (hereinafter also referred to as a "good solvent"). Examples of the good solvent include polar amide solvents such as N-methylpyrrolidone, dimethylacetamide, dimethylformamide, and dimethylformamide.

[0149] From the viewpoint of forming an adhesive layer having a good porous structure, the solvent used in preparing the coating liquid preferably contains a phase separation agent that induces phase separation. Therefore, the solvent used in preparing the coating liquid is preferably a mixed solvent of a good solvent and a phase separation agent. The phase separation agent is preferably mixed with the good solvent in an amount that ensures a viscosity appropriate for coating. Examples of the phase separation agent include water, methanol, ethanol, propyl alcohol, butyl alcohol, butanediol, ethylene glycol, propylene glycol, and tripropylene glycol.

[0150] As the solvent used for preparing the coating liquid, from the viewpoint of forming a good porous structure, a mixed solvent of a good solvent and a phase separation agent is preferred, which mixed solvent contains 60% by mass or more of the good solvent and 5% by mass to 40% by mass of the phase separation agent.

[0151] The resin concentration in the coating liquid is preferably 1% by mass to 20% by mass from the viewpoint of forming a good porous structure.

[0152] Step (3): Coating of the coating liquid for forming a heat-resistant layer The coating liquid for forming a heat-resistant layer is coated on at least one side of the porous substrate, forming a coating layer on the porous substrate. Methods for applying the coating liquid to the porous substrate include knife coating, Mayer bar coating, die coating, reverse roll coating, roll coating, gravure coating, screen printing, inkjet printing, and spraying. When forming a heat-resistant layer on both sides of the porous substrate, it is preferable from the viewpoint of productivity to simultaneously apply the coating liquid to both sides of the porous substrate.

[0153] Step (4): Solidification of the Coating Layer The porous substrate on which the coating layer that will become the heat-resistant layer has been formed is heated, and the dispersion medium (e.g., water) of the coating liquid is dried to form the heat-resistant layer. Drying is performed, for example, by transporting the porous substrate on which the coating layer has been formed into a high-temperature environment or by blowing air onto the porous substrate on which the coating layer has been formed. The drying temperature is preferably 40°C to 100°C. This results in a laminate consisting of the porous substrate and the heat-resistant layer.

[0154] Step (5): Coating of adhesive layer-forming coating liquid The adhesive layer-forming coating liquid is coated on at least one surface of the laminate. Methods for coating the coating liquid include knife coating, gravure coating, Mayer bar coating, die coating, reverse roll coating, roll coating, screen printing, inkjet printing, and spraying. When adhesive layers are formed on both surfaces of the laminate, it is preferable from the viewpoint of productivity to coat the coating liquid on both surfaces of the laminate simultaneously.

[0155] Step (6): Solidification of the Coating Layer The laminate with the coating layer formed thereon is immersed in a coagulation liquid to induce phase separation in the coating layer while solidifying the polyvinylidene fluoride resin into fibrils, thereby forming an adhesive layer containing the polyvinylidene fluoride resin, thereby obtaining a separator having a porous substrate, a heat-resistant layer, and an adhesive layer.

[0156] The coagulation liquid generally contains the good solvent and phase separation agent used in preparing the coating liquid, as well as water. From a productivity perspective, it is preferable that the mixing ratio of the good solvent and the phase separation agent be the same as the mixing ratio of the mixed solvent used in preparing the coating liquid. From the viewpoints of forming a porous structure and productivity, the water content in the coagulation liquid is preferably 40% by mass to 90% by mass. The temperature of the coagulation liquid is, for example, 20°C to 50°C.

[0157] Step (7): Washing and drying of the coating layer The separator is lifted out of the coagulating liquid and washed with water. By washing with water, the coagulating liquid is removed from the separator. Furthermore, by drying, water is removed from the separator. Washing with water is performed, for example, by transporting the separator in a water washing bath. Drying is performed, for example, by transporting the separator in a high-temperature environment, by blowing air on the separator, or by bringing the separator into contact with a heat roll. The drying temperature is preferably 40°C to 80°C.

[0158] Manufacturing method A can be carried out by carrying out steps (1) to (4), then temporarily winding the laminate onto a roll, and then unwinding the laminate from the roll and carrying out steps (5) to (7).

[0159] The nonaqueous secondary battery according to the present disclosure is a nonaqueous secondary battery that generates electromotive force by doping and dedoping lithium ions, and includes a positive electrode, a negative electrode, and the separator according to the present disclosure. "Doping" refers to occlusion, support, adsorption, or insertion, and refers to the phenomenon in which lithium ions enter the active material of an electrode such as a positive electrode.

[0160] The nonaqueous secondary battery of the present disclosure has a structure in which, for example, a battery element in which a negative electrode and a positive electrode face each other with a separator interposed therebetween is enclosed in an exterior material together with an electrolyte solution. The nonaqueous secondary battery of the present disclosure is suitable for nonaqueous electrolyte secondary batteries, particularly lithium ion secondary batteries.

[0161] The nonaqueous secondary battery of the present disclosure has excellent impact resistance and cycle characteristics due to the inclusion of the separator of the present disclosure.

[0162] Hereinafter, examples of the positive electrode, negative electrode, electrolyte, and exterior material included in the nonaqueous secondary battery of the present disclosure will be described.

[0163] An example of the positive electrode is a structure in which an active material layer containing a positive electrode active material and a binder resin is disposed on a current collector. The active material layer may further contain a conductive additive. Examples of the positive electrode active material include lithium-containing transition metal oxides, specifically 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 of binder resins include polyvinylidene fluoride resins and styrene-butadiene copolymers. Examples of conductive additives include carbon materials such as acetylene black, ketjen black, and graphite powder. Examples of current collectors include aluminum foil, titanium foil, and stainless steel foil, each having a thickness of 5 μm to 20 μm.

[0164] An example of an embodiment of the negative electrode includes a structure in which an active material layer containing a negative electrode active material and a binder resin is disposed on a current collector. The active material layer may further include a conductive additive. Examples of the negative electrode active material include materials capable of electrochemically absorbing lithium ions, such as carbon materials; alloys of lithium with silicon, tin, aluminum, etc.; and Wood's alloy. Examples of the binder resin include polyvinylidene fluoride resins and styrene-butadiene copolymers. Examples of the conductive additive include carbon materials such as acetylene black, ketjen black, graphite powder, and ultrafine carbon fibers. Examples of the current collector include copper foil, nickel foil, stainless steel foil, and the like, each having a thickness of 5 μm to 20 μm. Alternatively, a metallic lithium foil may be used as the negative electrode instead of the above-described negative electrode.

[0165] The electrolyte is preferably a solution in which a lithium salt is dissolved in a non-aqueous solvent. Examples of the lithium salt include LiPF 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; chain carbonates such as dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, and fluorine-substituted derivatives thereof; and cyclic esters such as γ-butyrolactone and γ-valerolactone; which may be used alone or in combination. A suitable electrolyte solution is a solution in which a cyclic carbonate and a chain carbonate are mixed in a mass ratio (cyclic carbonate:chain carbonate) of 20:80 to 40:60, and a lithium salt is dissolved in the range of 0.5 mol / L to 1.5 mol / L.

[0166] Examples of the exterior packaging include aluminum laminate film packs, metal cans, etc. The shape of the battery may be rectangular, cylindrical, coin-shaped, etc., and the separator of the present disclosure is suitable for any shape.

[0167] The nonaqueous secondary battery of the present disclosure can be produced by producing a laminate in which the separator of the present disclosure is disposed between a positive electrode and a negative electrode, and then using this laminate by, for example, Production Method 1 or Production Method 2 described below.

[0168] Manufacturing method 1: The laminate is dry-heat pressed to temporarily bond the electrodes and separator, and then housed in an exterior packaging (e.g., an aluminum laminate film pack; the same applies hereinafter), and an electrolyte solution is poured into it. Next, the laminate is wet-heat pressed from above the exterior packaging to bond the electrodes and separator and seal the exterior packaging.

[0169] Manufacturing method 2: The laminate is placed in an exterior packaging material, and an electrolyte solution is poured into the packaging material. The laminate is then wet heat pressed onto the exterior packaging material to bond the electrodes and separator together and seal the exterior packaging material.

[0170] In Production Method 1 and Production Method 2, the pressing temperature of the wet heat press is preferably 50°C to 95°C, more preferably 60°C to 90°C. The pressing pressure of the wet heat press is preferably 0.1 MPa to 2 MPa, more preferably 0.5 MPa to 1.5 MPa. The pressing time of the wet heat press is preferably adjusted according to the pressing temperature and pressing pressure, for example, within the range of 1 minute to 20 hours.

[0171] When manufacturing a laminate in which a separator is disposed between a positive electrode and a negative electrode, the method of disposing the separator between the positive electrode and the negative electrode may be a method of stacking at least one layer of a positive electrode, a separator, and a negative electrode in this order (so-called stack method), or a method of stacking a positive electrode, a separator, a negative electrode, and a separator in this order and winding them in the length direction.

[0172] The separator and nonaqueous secondary battery of the present disclosure will be described in more detail below with reference to examples. The materials, amounts used, ratios, processing procedures, etc. shown in the following examples can be changed as appropriate without departing from the spirit of the present disclosure. Therefore, the scope of the separator and nonaqueous secondary battery of the present disclosure should not be construed as being limited by the specific examples shown below.

[0173] In the following description, syntheses, treatments, manufacturing, etc. were carried out at room temperature (25° C.±3° C.) unless otherwise specified.

[0174] <Measurement Methods and Evaluation Methods> The measurement methods and evaluation methods used in the examples and comparative examples are as follows.

[0175] [Thickness of porous substrate and separator] The thickness of the porous substrate and separator was measured at 20 points within a 10 cm square using a contact thickness meter (Mitutoyo Corporation, LITEMATIC VL-50S) and the average was calculated. A spherical probe with a sphere radius of 10 mm (Mitutoyo Corporation) was used as the measurement terminal, and the measurement was adjusted so that a load of 0.19 N was applied during measurement.

[0176] [Thickness of Heat-Resistant Layer] The thickness of the flat membrane was determined by subtracting the thickness of the porous substrate from the thickness of the flat membrane obtained by removing the adhesive layer from the separator. The thickness of the flat membrane was determined by measuring 20 points within a 10 cm square using a contact thickness meter (Mitutoyo Corporation, LITEMATIC VL-50S) and averaging the measurements. A spherical probe with a sphere radius of 10 mm (Mitutoyo Corporation) was used as the measurement terminal, and the measurement was adjusted so that a load of 0.19 N was applied during measurement.

[0177] [Mass per unit area of ​​adhesive layer] The separator was cut into a piece of 20 cm x 20 cm, the mass of the layer portion corresponding to the adhesive layer was measured, and the mass was divided by the area to obtain the total basis weight (g / m) of both sides. ２ ) was sought.

[0178] [Gurley Values ​​of Porous Substrate, Laminate, and Separator] The Gurley values ​​(seconds / 100 mL) of the porous substrate, laminate, and separator were measured using a Gurley densometer (Toyo Seiki Co., Ltd., G-B2C) in accordance with JIS P8117: 2009. The Gurley value of the laminate was subtracted from the Gurley value of the separator to determine the difference in Gurley values.

[0179] [Average Primary Particle Size of Inorganic Particles] The inorganic particles used to form the heat-resistant layer were used as samples and observed with a scanning electron microscope (SEM) to determine the average primary particle size. Specifically, the major axes of 100 randomly selected inorganic particles were measured during the SEM observation, and the average value of the major axes of the 100 particles was taken as the average primary particle size (μm).

[0180] [Heat Shrinkage] The separator was cut into a rectangle measuring 60 mm in TD x 180 mm in MD to prepare a test specimen. Marks were made on the test specimen at 20 mm and 170 mm from one end on the line dividing the TD in half (referred to as points A and B, respectively). Furthermore, marks were made on the test specimen at 10 mm and 50 mm from one end on the line dividing the MD in half (referred to as points C and D, respectively). A clip was attached to the test specimen (the clip was attached between the end closest to point A and point A), and the specimen was hung in an oven at 150°C and subjected to a heat treatment for 30 minutes under no tension. The lengths between A and B and between CD were measured before and after the heat treatment, and the heat shrinkage was calculated using the following formula. The heat shrinkage of the three test specimens was then averaged.

[0181] MD heat shrinkage rate (%) = {(length AB before heat treatment - length AB after heat treatment) ÷ length AB before heat treatment} × 100

[0182] TD heat shrinkage rate (%) = {(CD length before heat treatment - CD length after heat treatment) ÷ CD length before heat treatment} × 100

[0183] [Adhesion to electrode (1)] 300 g of artificial graphite as a negative electrode active material, 7.5 g of an aqueous dispersion containing 40% by mass of a modified styrene-butadiene copolymer as a binder, 3 g of carboxymethyl cellulose as a thickener, and an appropriate amount of water were mixed by stirring using a twin-arm mixer to prepare a negative electrode slurry. This negative electrode slurry was applied to one side of a 10 μm thick copper foil, dried, and pressed to obtain a negative electrode having a negative electrode active material layer.

[0184] The negative electrode obtained above was cut into a rectangle measuring 15 mm wide x 70 mm long, and the separator was cut into a rectangle measuring 18 mm TD x 74 mm MD. Release paper measuring 15 mm wide x 70 mm long was prepared. A laminate consisting of the negative electrode, separator, and release paper stacked in this order was inserted into a pack made of aluminum laminate film, and the pack was heat-pressed in the stacking direction of the laminate using a heat press machine, thereby bonding the negative electrode and separator. The heat press conditions were a temperature of 90 ° C, a pressure of 9 MPa, and a time of 10 seconds. The laminate was then removed from the pack, and the release paper was peeled off to form a test piece.

[0185] The uncoated surface of the negative electrode of the test piece 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 Co., Ltd., STB-1225S). At this time, the metal plate was fixed to the Tensilon so that the length direction of the test piece (i.e., the MD of the separator) was the direction of gravity. The separator was peeled from the negative electrode about 2 cm from the lower end, and the end was fixed to the upper chuck, and a 180° peel test was performed. The tensile speed of the 180° peel test was 20 mm / min, and the load (N) from 10 mm to 40 mm after the start of the measurement was collected at 0.4 mm intervals, and the average was calculated. Furthermore, the load of 10 test pieces was averaged to determine the adhesive strength (N / 15 mm) between the electrode and the separator.

[0186] [Adhesion to Electrode (2)] A test secondary battery described below was prepared. A compression bending test (three-point bending measurement) was performed on the battery. The measurement was performed using a Tensilon (A&D Co., Ltd., STB-1225S) equipped with a compression bending test jig. The distance between the support stands was 4 cm, and the battery was placed on the support stand so that the short side of the battery was parallel to the longitudinal direction of the indenter and the compression position during measurement was at the center of the longitudinal direction of the electrode in the battery. The measurement was started by setting the displacement when the indenter was lowered until a load of 0.1 N was applied as 0. The compression speed during measurement was 2 mm / min, and the measurement was performed up to a displacement of 2 mm. The elastic modulus gradient (unit: N / mm) in the load-displacement curve obtained from this result was taken as the adhesive strength.

[0187] [Battery Cycle Characteristics (Capacity Retention Rate)] Ten test secondary batteries, as described below, were prepared. Under a temperature of 20°C, the batteries were subjected to three cycles of constant-current, constant-voltage charging at 0.1 C, 4.2 V, and 15 hours, followed by constant-current discharging at 0.1 C and a cutoff voltage of 2.5 V. The discharge capacity after three cycles was recorded as the cell capacity. Next, under a temperature of 20°C, the batteries were subjected to 300 cycles of constant-current charging at 3 C and a cutoff voltage of 4.2 V, followed by constant-current discharging at 3 C and a cutoff voltage of 2.5 V. The charge capacity at the 300th cycle was divided by the cell capacity to calculate the average for the ten batteries, and the resulting value (%) was recorded as the capacity retention rate.

[0188] [Battery Impact Resistance (Compression Breakdown Strength)] Ten test secondary batteries, as described below, were prepared. The test secondary batteries were subjected to constant current and constant voltage charging in an environment at 20°C. Specifically, the test secondary batteries were charged to 4.2 V at 0.1 C and maintained at 4.2 V for 3 hours. The test secondary batteries were then placed on a horizontal table with the positive electrode facing up and secured with adhesive tape. A spherical terminal with a diameter of 5 mm was placed at the center of the test secondary battery and gradually lowered at a speed of 100 mm / min to apply a load to the test secondary battery. The load value (unit: N) was determined at the moment when the voltage of the test secondary battery dropped to 3.5 V, and the average of the 10 batteries was calculated.

[0189] <Preparation of Separator and Battery> [Example 1] -Preparation of Separator- Styrene butadiene rubber (SBR) and barium sulfate particles (average primary particle size 0.10 μm) were added to water and stirred to obtain coating solution 1. Coating solution 1 had an SBR concentration of 3.5 mass % and a barium sulfate particle:SBR ratio of 97:3 (mass ratio).

[0190] A binary copolymer (weight average molecular weight 1,130,000) with a polymerization ratio (molar ratio) of VDF:HFP = 97.6:2.4 was prepared as polyvinylidene fluoride resin A. A binary copolymer (weight average molecular weight 860,000) with a polymerization ratio (molar ratio) of VDF:HFP = 94.3:5.7 was prepared as polyvinylidene fluoride resin B. Polyvinylidene fluoride resin A and polyvinylidene fluoride resin B were mixed in a mass ratio A:B = 70:30.

[0191] A mixed solvent of dimethylacetamide (DMAc) and tripropylene glycol (TPG) (DMAc:TPG=70:30 [mass ratio]) was prepared, and a polyvinylidene fluoride resin (the mixture of polyvinylidene fluoride resin A and polyvinylidene fluoride resin B) was dissolved in the mixed solvent to a concentration of 5.0 mass % to obtain a coating solution (2).

[0192] An appropriate amount of coating liquid (1) was placed on a pair of Mayer bars, and a polyethylene microporous membrane (thickness 5.6 μm, Gurley value 120 seconds / 100 mL) was passed between the Mayer bars to coat both sides with equal amounts of coating liquid (1) and dry. Next, an appropriate amount of coating liquid (2) was placed on a pair of Mayer bars, and a polyethylene microporous membrane having a coating layer of coating liquid (1) was passed between the Mayer bars to coat both sides with equal amounts of coating liquid (2). The membrane was immersed in a coagulation liquid (DMAc:TPG:water=30:8:62 [mass ratio], liquid temperature 40°C) to solidify the coating layer, washed in a water washing tank at a water temperature of 40°C, and dried. In this way, a separator having a heat-resistant layer and an adhesive layer formed on both sides of the polyethylene microporous membrane was obtained.

[0193] When the surface of the separator was observed with an SEM, it was found that the adhesive layer had a porous structure in which fibrils were connected in a three-dimensional network. Figure 6 shows a scanning electron microscope (SEM) image of the adhesive layer of the separator of Example 1.

[0194] - Preparation of Positive Electrode - 89.5 parts by mass of lithium cobalt oxide powder as the positive electrode active material, 4.5 parts by mass of acetylene black as the conductive additive, 6 parts by mass of polyvinylidene fluoride as the binder resin, and an appropriate amount of N-methyl-2-pyrrolidone were mixed by stirring in a twin-arm mixer to prepare a positive electrode slurry. The positive electrode slurry was applied to one or both sides of a 20 μm thick aluminum foil, dried, and pressed to obtain a positive electrode having a positive electrode active material layer on one or both sides.

[0195] - Preparation of Negative Electrode - 300 parts by mass of artificial graphite as the negative electrode active material, 7.5 parts by mass of an aqueous dispersion containing 40% by mass of a modified styrene-butadiene copolymer as the binder resin, 3 parts by mass of carboxymethyl cellulose as a thickener, and an appropriate amount of water were mixed by stirring using a twin-arm mixer to prepare a negative electrode slurry. The negative electrode slurry was applied to one or both sides of a 10 μm thick copper foil, dried, and pressed to obtain a negative electrode having a negative electrode active material layer on one or both sides.

[0196] - Preparation of a battery for evaluating adhesion to electrodes - The double-sided positive electrode and double-sided negative electrode were each cut into a rectangle measuring 30 mm x 70 mm. The separator was cut into a rectangle measuring TD 35 mm x MD 75 mm. These were stacked so that the positive and negative electrodes alternated and the separator was sandwiched between the positive and negative electrodes, producing a laminate consisting of three positive electrodes, three negative electrodes, and five separators. The laminate was inserted into a pack made of aluminum laminate film, and an electrolyte (1 mol / L LiPF ６ -ethylene carbonate:ethyl methyl carbonate [mass ratio 3:7]) was poured into the laminate, and the electrolyte solution was allowed to soak into the laminate. Next, the pack and the laminate were heat-pressed in the stacking direction using a heat press (wet heat press) to bond the electrodes and separators. The heat press conditions were a press temperature of 85°C, a press pressure of 1 MPa, and a press time of 2 minutes. The test secondary battery thus obtained was subjected to evaluation of adhesion to the electrodes (2).

[0197] - Preparation of a battery for evaluating cycle characteristics and impact resistance - A single-sided positive electrode and a single-sided negative electrode were each cut into a rectangle measuring 14 mm x 20 mm. A separator was cut into a rectangle measuring 20 mm in diameter x 26 mm in length. 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 electrode and the negative electrode to prepare a laminate consisting of one positive electrode, one negative electrode, and one separator. The laminate was inserted into a pack made of aluminum laminate film, and an electrolyte (1 mol / L LiPF ６ A mixture of ethylene carbonate and ethyl methyl carbonate (mass ratio 3:7) was poured into the laminate, allowing the electrolyte to soak into the laminate. The laminate was then heat-pressed (wet heat press) along the stacking direction of the pack using a heat press machine to bond the electrodes and separators. The heat press conditions were a press temperature of 80°C, a press pressure of 1 MPa, and a press time of 2 minutes. The test secondary batteries thus obtained were subjected to evaluation of the battery's cycle characteristics and impact resistance.

[0198] [Example 2] A separator was produced in the same manner as in Example 1, except that barium sulfate particles (average primary particle size 0.10 μm) were replaced with γ-alumina particles (average primary particle size 0.01 μm). A test secondary battery was produced using this separator.

[0199] [Example 3] A separator was produced in the same manner as in Example 1, except that the barium sulfate particles (average primary particle size 0.10 μm) were replaced with boehmite particles (average primary particle size 0.10 μm). A test secondary battery was produced using this separator.

[0200] [Example 4] A separator was fabricated in the same manner as in Example 1, except that the barium sulfate particles (average primary particle size 0.10 μm) were replaced with boehmite particles (average primary particle size 0.10 μm), and the heat-resistant layer was coated on one side of a polyethylene microporous membrane. A test secondary battery was fabricated using this separator.

[0201] [Example 5] A separator was fabricated in the same manner as in Example 1, except that the barium sulfate particles (average primary particle size 0.10 μm) were replaced with boehmite particles (average primary particle size 0.10 μm) and the coating amount of the adhesive layer was changed. A test secondary battery was fabricated using this separator.

[0202] [Example 6] A separator was fabricated in the same manner as in Example 1, except that the barium sulfate particles (average primary particle size 0.10 μm) were replaced with boehmite particles (average primary particle size 0.10 μm) and the coating amount of the adhesive layer was changed. A test secondary battery was fabricated using this separator.

[0203] Example 7 A separator was fabricated in the same manner as in Example 1, except that SBR was replaced with poly(n-butyl acrylate) and barium sulfate particles (average primary particle size 0.10 μm) were replaced with boehmite particles (average primary particle size 0.10 μm). A test secondary battery was fabricated using this separator.

[0204] Example 8 A separator was produced in the same manner as in Example 1, except that the average primary particle size of the barium sulfate particles was changed from 0.10 μm to 0.30 μm. A test secondary battery was produced using this separator.

[0205] [Example 9] A separator was produced in the same manner as in Example 1, except that the composition of the coating solution (1) was changed to barium sulfate particles:SBR = 95:5 [mass ratio]. A test secondary battery was produced using this separator.

[0206] Comparative Example 1 A separator was prepared in the same manner as in Example 1, except that the composition of the coating solution (1) was changed to barium sulfate particles:SBR = 80:20 [mass ratio]. A test secondary battery was prepared using this separator.

[0207] Comparative Example 2 A separator was fabricated in the same manner as in Example 1, except that the barium sulfate particles (average primary particle size 0.10 μm) were replaced with boehmite particles (average primary particle size 2.3 μm), the heat-resistant layer was coated on one side of a polyethylene microporous membrane, and the thickness of the heat-resistant layer was changed. A test secondary battery was fabricated using this separator.

[0208] Comparative Example 3 A separator was prepared in the same manner as in Example 1, except that the coating solution (2) was changed to a PVDF particle dispersion (particle volume average particle size: 0.2 μm, dispersion medium: water, solid content concentration: 7 mass%), and the PVDF particle dispersion was dry-coated on the heat-resistant layer. A test secondary battery was prepared using this separator.

[0209] When the surface of the separator of Comparative Example 3 was observed with an SEM, it was found that the adhesive layer had a structure in which resin particles were aligned adjacently in the surface direction.

[0210] The separator of Comparative Example 3 was not evaluated for adhesiveness to the electrode (1) because it was not adhered to the test negative electrode when hot-pressed (dry-heat-pressed) in the absence of an electrolyte solution.

[0211] The abbreviations in Table 1 have the following meanings: PE: polyethylene SBR: styrene butadiene rubber PVDF: polyvinylidene fluoride

[0212]

[0213] All publications, patent applications, and technical standards mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication, patent application, or technical standard was specifically and individually indicated to be incorporated by reference.

[0214] The disclosure of Japanese Application No. 2023-188885, filed on November 2, 2023, is incorporated herein by reference in its entirety.

[0215] 10A, 10B, 10C, 10D, 10E Separator 20 Porous substrate 30 Heat-resistant layer 40 Laminate 50 Adhesive layer

Claims

1. A separator for a non-aqueous secondary battery comprising: a porous substrate; a heat-resistant layer containing inorganic particles and a binder resin, the heat-resistant layer being disposed on one or both sides of the porous substrate; and an adhesive layer containing a polyvinylidene fluoride resin, the heat-resistant layer being disposed on one or both sides of a laminate of the porous substrate and the heat-resistant layer, wherein the inorganic particles account for 95 mass% or more of the heat-resistant layer, the inorganic particles contained in the heat-resistant layer have an average primary particle size of 0.01 μm to 0.30 μm, and the adhesive layer has a porous structure in which fibrils containing the polyvinylidene fluoride resin are connected in a three-dimensional network.

2. The separator for a non-aqueous secondary battery according to claim 1, wherein the heat-resistant layers are disposed on both sides of the porous substrate.

3. The separator for a non-aqueous secondary battery according to claim 1, wherein the inorganic particles include at least one type selected from the group consisting of metal oxide particles, metal hydroxide particles and metal sulfate particles.

4. The separator for a non-aqueous secondary battery according to claim 1, wherein the inorganic particles include barium sulfate particles.

5. The separator for a non-aqueous secondary battery according to claim 1, wherein the polyvinylidene fluoride resin comprises the following polyvinylidene fluoride resin A and polyvinylidene fluoride resin B: Polyvinylidene fluoride resin A: A polyvinylidene fluoride resin containing vinylidene fluoride and hexafluoropropylene as polymerization components, with the proportion of hexafluoropropylene in the total of vinylidene fluoride and hexafluoropropylene being more than 1.5 mol% and not more than 5 mol%, Polyvinylidene fluoride resin B: A polyvinylidene fluoride resin containing vinylidene fluoride and hexafluoropropylene as polymerization components, with the proportion of hexafluoropropylene in the total of vinylidene fluoride and hexafluoropropylene being more than 5 mol% and not more than 15 mol%.

6. The separator for a non-aqueous secondary battery according to claim 1, wherein the polyvinylidene fluoride resin comprises the following polyvinylidene fluoride resin C: polyvinylidene fluoride resin containing vinylidene fluoride, hexafluoropropylene, and a monomer represented by the following formula (1) as polymerization components. In formula (1), R １ , R ２ and R ３ each independently represents a hydrogen atom, a halogen atom, an alkyl group having 1 to 5 carbon atoms, a carboxy group, or a derivative of a carboxy group; X represents a single bond, an alkylene group having 1 to 5 carbon atoms, or an alkylene group having 1 to 5 carbon atoms and having a substituent; Y represents a hydrogen atom, an alkyl group having 1 to 5 carbon atoms, an alkyl group having 1 to 5 carbon atoms substituted with at least one hydroxy group, an alkyl group having 1 to 5 carbon atoms substituted with at least one carboxy group, or -R-O-C(=O)-(CH ２ ) ｎ It represents —C(═O)—OH (R represents an alkylene group having 1 to 5 carbon atoms, and n represents an integer of 0 or more).

7. The separator for a non-aqueous secondary battery according to claim 1, wherein the binder resin of the heat-resistant layer contains at least one resin selected from the group consisting of butadiene-based polymers and acrylic-based resins.

8. The separator for a non-aqueous secondary battery according to claim 1, wherein the heat-resistant layer has a thickness of 0.1 μm to 2 μm per layer.

9. The separator for a non-aqueous secondary battery according to claim 1, wherein the porous substrate has a thickness of 1 μm to 7 μm.

10. The mass per unit area of ​​the adhesive layer is 0.5 g / m2 on both sides in total. ２ ~3g / m ２ The separator for a non-aqueous secondary battery according to claim 1 , 11. The separator for a nonaqueous secondary battery according to claim 1, wherein the difference between the Gurley value of the laminate and the Gurley value of the separator for a nonaqueous secondary battery is 30 seconds / 100 mL or less.

12. A non-aqueous secondary battery comprising a positive electrode, a negative electrode, and the 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, and which generates an electromotive force by doping and dedoping lithium ions.

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