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

Abstract

This separator for a non-aqueous secondary battery comprises: a polyolefin microporous film; a heat-resistant porous layer disposed on one or both surfaces of the polyolefin microporous film, the heat-resistant porous layer containing a heat-resistant resin having at least one of an amide bond and an imide bond in each molecule, and inorganic particles; and an adhesive layer disposed on one or both surfaces of a laminate of the polyolefin microporous film and the heat-resistant porous layer, the adhesive layer containing adhesive resin particles. The Gurley value of the polyolefin microporous film is 99 seconds / 100 mL or less, the Gurley value of the separator for a non-aqueous secondary battery is 160 seconds / 100 mL or less, and the difference between the Gurley value of the separator for a non-aqueous secondary battery and the Gurley value of the polyolefin microporous film is 20 seconds / 100 mL to 60 seconds / 100 mL.

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, which includes a porous substrate, a heat-resistant porous layer containing an aromatic resin and inorganic particles, and an adhesive layer in which adhesive resin particles adhere to the heat-resistant porous layer.

[0003] Patent Document 2 discloses a separator for a non-aqueous secondary battery, which includes a porous substrate, a heat-resistant porous layer provided on one or both sides of the porous substrate and containing at least one of a heat-resistant resin having at least one of an amide bond and an imide bond in the molecule and inorganic particles, and an adhesive layer provided on one or both sides of a laminate of the porous substrate and the heat-resistant porous layer, in which adhesive resin particles adhere to the laminate.

[0004] JP 2023-164113 A International Publication No. 2019 / 130994

[0005] A nonaqueous secondary battery separator is known that has a heat-resistant porous layer and an adhesive layer on a polyolefin microporous membrane. In this separator, when the heat-resistant porous layer is formed on the polyolefin microporous membrane, the heat-resistant resin that is the material for the heat-resistant porous layer may penetrate into the pores of the polyolefin microporous membrane and remain there. Because polyolefins are hydrophobic resins, polyolefin microporous membranes are inherently hydrophobic. However, when the heat-resistant resin is a resin having a hydrophilic group (e.g., polyamide, polyimide, polyamideimide), the presence of the heat-resistant resin makes the pores of the polyolefin microporous membrane more susceptible to water. Therefore, during the water washing process after coating and solidifying the material for the heat-resistant porous layer, moisture penetrates into the pores of the polyolefin microporous membrane. Furthermore, when an aqueous dispersion containing adhesive resin particles is used to form the adhesive layer, moisture penetrates into the pores of the heat-resistant porous layer and the polyolefin microporous membrane. This separator requires a large amount of moisture to be removed by drying during the drying process during production, and is also prone to absorbing moisture from the environment even after production.

[0006] If a separator for a non-aqueous secondary battery contains moisture, it can deteriorate the cycle characteristics of the battery and cause internal short circuits. To prevent the separator from bringing moisture into the non-aqueous secondary battery, a step of drying the separator is sometimes included in the battery manufacturing process.

[0007] It is desirable for a separator for a non-aqueous secondary battery to be easy to remove moisture from during the drying process in the separator production and the drying process in the battery production. To facilitate moisture removal by drying, it is conceivable to make the separator's porous structure coarse or thin the separator, but in such cases, concerns arise about the safety of the battery that the separator is supposed to support.

[0008] The present disclosure has been made under the above circumstances, and an object of the present disclosure is to provide a separator for a nonaqueous secondary battery that can easily remove moisture by drying and has excellent short-circuit resistance.

[0009] Specific means for solving the above problems include the following aspects: <1> A separator for a non-aqueous secondary battery comprising: a polyolefin microporous membrane; a heat-resistant porous layer disposed on one or both sides of the polyolefin microporous membrane, the heat-resistant resin having at least one of an amide bond and an imide bond in its molecule and inorganic particles; and an adhesive layer disposed on one or both sides of a laminate of the polyolefin microporous membrane and the heat-resistant porous layer, the adhesive layer containing adhesive resin particles, wherein the polyolefin microporous membrane has a Gurley value of 99 seconds / 100 mL or less, the non-aqueous secondary battery separator has a Gurley value of 160 seconds / 100 mL or less, and a difference between the Gurley value of the non-aqueous secondary battery separator and the Gurley value of the polyolefin microporous membrane is 20 seconds / 100 mL to 60 seconds / 100 mL. <2> The separator for a non-aqueous secondary battery according to <1>, wherein the adhesive resin particles have a surface coverage of 20% to 60% on each side on which the adhesive layer is disposed. <3> The separator for a non-aqueous secondary battery according to <1> or <2>, wherein the separator for a non-aqueous secondary battery has a thickness of 5 μm to 17 μm. <4> The separator for a non-aqueous secondary battery according to any one of <1> to <3>, wherein the polyolefin microporous membrane has a thickness of 4 μm to 9 μm. <5> The separator for a non-aqueous secondary battery according to any one of <1> to <4>, wherein the heat-resistant porous layer is disposed on only one side of the polyolefin microporous membrane, and wherein the heat-resistant porous layer has a thickness of 0.5 μm to 4 μm. <6> The separator for a non-aqueous secondary battery according to any one of <1> to <4>, wherein the heat-resistant porous layers are disposed on both sides of the polyolefin microporous membrane, and the total thickness of the heat-resistant porous layers on both sides is 1 μm to 8 μm. <7> The separator for a non-aqueous secondary battery according to any one of <1> to <6>, wherein the pores of the polyolefin microporous membrane contain a heat-resistant resin having at least one of an amide bond and an imide bond in its molecule. <8> The separator for a non-aqueous secondary battery according to any one of <1> to <7>, wherein the heat-resistant resin includes at least one selected from the group consisting of wholly aromatic polyamides, polyimides, and polyamideimides.<9> The separator for a non-aqueous secondary battery according to any one of <1> to <8>, wherein the inorganic particles comprise at least one selected from the group consisting of metal oxide particles, metal hydroxide particles, and metal sulfate particles. <10> The separator for a non-aqueous secondary battery according to any one of <1> to <9>, wherein the adhesive resin particles comprise at least one selected from the group consisting of acrylic resin particles, polyvinylidene fluoride resin particles, and styrene-butadiene rubber particles. <11> 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 <1> to <10>, disposed between the positive electrode and the negative electrode, wherein an electromotive force is generated by doping and undoping of lithium ions.

[0010] According to the present disclosure, a separator for a nonaqueous secondary battery is provided that can easily remove moisture by drying and has excellent short-circuit resistance.

[0011] 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.

[0012]

[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.

[0013] 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.

[0014] 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.

[0015] 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.

[0016] 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.

[0017] 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."

[0018] 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 polyolefin microporous membrane is referred to as "lower," and the layer farther from the polyolefin microporous membrane is referred to as "upper."

[0019] In the present disclosure, the volume of the porous layer excluding pores is referred to as the "solid content volume."

[0020] In the present disclosure, performing a heat press treatment after the separator is permeated with an electrolyte solution is referred to as "wet heat press," and performing a heat press treatment without permeating the separator with an electrolyte solution is referred to as "dry heat press."

[0021] In the present disclosure, the term "(meth)acrylic" means either "acrylic" or "methacrylic."

[0022] In the present disclosure, the term "monomer unit" of a polymer or resin refers to a structural unit of the polymer or resin, which is formed by polymerization of a monomer.

[0023] <Separator for non-aqueous secondary battery> A separator for a non-aqueous secondary battery according to the present disclosure (also simply referred to as "separator" in the present disclosure) includes a polyolefin microporous membrane, a heat-resistant porous layer disposed on one or both sides of the polyolefin microporous membrane and containing a heat-resistant resin having at least one of an amide bond and an imide bond in its molecule and inorganic particles, and an adhesive layer disposed on one or both sides of a laminate of the polyolefin microporous membrane and the heat-resistant porous layer and containing adhesive resin particles.

[0024] In the present disclosure, a heat-resistant resin having at least one of an amide bond and an imide bond in the molecule is referred to as a "heat-resistant resin (AI)."

[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] 1 is a separator in which heat-resistant porous layers 30 are disposed on both sides of a polyolefin microporous membrane 20, and adhesive layers 50 are disposed on both sides of a laminate 40 of the polyolefin microporous membrane 20 and two heat-resistant porous layers 30. One heat-resistant porous layer 30 and the other heat-resistant porous layer 30 may be the same or different in terms of components and / or composition. One adhesive layer 50 and the other adhesive layer 50 may be the same or different in terms of components and / or composition.

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

[0028] 3 is a separator in which a heat-resistant porous layer 30 is disposed on one side of a polyolefin microporous membrane 20, and adhesive layers 50 are disposed on both sides of a laminate 40 of the polyolefin microporous membrane 20 and one heat-resistant porous 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 porous layer 30 is disposed on one side of a polyolefin microporous membrane 20, and an adhesive layer 50 is disposed on one side of a laminate 40 of the polyolefin microporous membrane 20 and one heat-resistant porous layer 30. In separator 10D, adhesive layer 50 is disposed on the surface of heat-resistant porous layer 30.

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

[0031] The heat-resistant porous layer 30 is a layer containing a heat-resistant resin (AI) and inorganic particles, and is disposed on one side of the polyolefin microporous membrane 20. The heat-resistant porous layer 30 may be disposed on only one side of the polyolefin microporous membrane 20, or on both sides of the polyolefin microporous membrane 20. When the heat-resistant porous layer 30 is disposed on both sides of the polyolefin microporous membrane 20, the heat resistance 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 porous layer 30 is disposed on only one side of the polyolefin microporous membrane 20, the ion permeability of the separator is superior. In addition, the overall thickness of the separator can be reduced, allowing the production of a battery with a higher energy density.

[0032] The adhesive layer 50 is a layer disposed on the surface of the polyolefin microporous membrane 20 or the heat-resistant porous layer 30, and exists as the outermost layer of the separator. The adhesive layer 50 may be disposed on only one surface of the laminate 40, or on both surfaces of the laminate 40. When the adhesive layer 50 is disposed on only one surface of the laminate 40, it is preferable that the adhesive layer 50 be disposed on the surface of the heat-resistant porous layer 30. The adhesive layer 50 may be disposed on one or both surfaces 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 surface 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 adhesive layer 50 is a layer containing adhesive resin particles 52. The adhesive layer 50 may be a layer in which the adhesive resin particles 52 adhere to the laminate 40 by their own properties, or may be a layer in which the adhesive resin particles 52 are bound to the laminate 40 by another resin.

[0034] From the viewpoint of excellent adhesion to the electrode, the adhesive layer 50 preferably has a structure in which the adhesive resin particles 52 are arranged adjacent to each other on the surface of the laminate 40. From the viewpoint of permeability of the electrolyte solution into the separator, the adhesive layer 50 preferably has a structure in which the adhesive resin particles 52 are scattered on the surface of the laminate 40. From the viewpoint of increasing the energy density of the battery, the adhesive layer 50 preferably has a structure in which the adhesive resin particles 52 are arranged in a single layer in the thickness direction. The adhesive layer 50 may also have a structure in which multiple layers of the adhesive resin particles 52 are stacked in the thickness direction.

[0035] In a separator adhered to an electrode, the adhesive resin particles 52 contained in the adhesive layer 50 may melt partially or completely when heat is applied to adhere the separator to the electrode, causing adjacent adhesive resin particles 52 to connect to each other, and some or all of the particles may not retain their particle shape.

[0036] In the separator of the present disclosure, the polyolefin microporous membrane has a Gurley value of 99 seconds / 100 mL or less, the separator has a Gurley value of 160 seconds / 100 mL or less, and the difference between the Gurley values ​​of the separator and the polyolefin microporous membrane is 20 seconds / 100 mL to 60 seconds / 100 mL. By virtue of having the above configuration, the separator of the present disclosure is easily capable of removing moisture by drying and has excellent short-circuit resistance.

[0037] If the Gurley value of the polyolefin microporous membrane is 100 seconds / 100 mL or more, moisture may not be sufficiently removed by drying. From the viewpoint of easy moisture removal by drying, the Gurley value of the polyolefin microporous membrane is 99 seconds / 100 mL or less, preferably 95 seconds / 100 mL or less, and more preferably 90 seconds / 100 mL or less. The Gurley value of the polyolefin microporous membrane is preferably in the above range also from the viewpoint of the ion permeability of the separator. From the viewpoint of the short-circuit resistance of the separator, the Gurley value of the polyolefin microporous membrane 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.

[0038] If the Gurley value of the separator exceeds 160 seconds / 100 mL, moisture may not be sufficiently removed by drying. From the viewpoint of facilitating moisture removal by drying, the Gurley value of the separator is 160 seconds / 100 mL or less, preferably 150 seconds / 100 mL or less, and more preferably 140 seconds / 100 mL or less. The Gurley value of the separator is also preferably within the above range from the viewpoint of ion permeability. From the viewpoint of short-circuit resistance, the Gurley value of the separator is preferably 100 seconds / 100 mL or more, more preferably 110 seconds / 100 mL or more, and even more preferably 120 seconds / 100 mL or more.

[0039] It is presumed that the difference between the Gurley value of the separator and that of the polyolefin microporous membrane occurs because the separator of the present disclosure has a heat-resistant porous layer disposed on one or both sides of the polyolefin microporous membrane and the heat-resistant resin (AI), which is the material of the heat-resistant porous layer, penetrates and remains in the pores of the polyolefin microporous membrane. If the difference between the Gurley value of the separator and that of the polyolefin microporous membrane exceeds 60 seconds / 100 mL, the heat-resistant porous layer may be too thick or the amount of heat-resistant resin (AI) present in the pores of the polyolefin microporous membrane may be too large, making it difficult to sufficiently remove moisture by drying. From the viewpoint of easy removal of moisture by drying, the difference between the Gurley value of the separator and that of the polyolefin microporous membrane is 60 seconds / 100 mL or less, preferably 50 seconds / 100 mL or less, and more preferably 40 seconds / 100 mL or less. The difference between the Gurley value of the separator and the Gurley value of the polyolefin microporous membrane is preferably within the above range, also from the viewpoint of the cycle characteristics of the battery. If the difference between the Gurley value of the separator and the Gurley value of the polyolefin microporous membrane is less than 20 seconds / 100 mL, the heat-resistant porous layer may be too thin, causing an internal short circuit in the battery. From the viewpoint of short-circuit resistance, the difference between the Gurley value of the separator and the Gurley value of the polyolefin microporous membrane is 20 seconds / 100 mL or more, preferably 25 seconds / 100 mL or more, and more preferably 30 seconds / 100 mL or more.

[0040] The Gurley values ​​of the polyolefin microporous membrane and separator are values ​​measured using a Gurley densometer in accordance with JIS P8117:2009 "Paper and paperboard -- Test method for air permeability and air resistance (intermediate range) -- Gurley method."

[0041] The Gurley value of the polyolefin microporous membrane can be controlled by adjusting the stretch ratio during membrane formation. The Gurley value of the separator can be controlled by the Gurley value of the polyolefin microporous membrane and the thickness and porosity of the heat-resistant porous layer. The porosity of the heat-resistant porous layer can be controlled by the primary particle size and content of the inorganic particles contained in the heat-resistant porous layer, the viscosity of the coating liquid for forming the heat-resistant porous layer, the resin concentration, the solidification conditions, etc.

[0042] The polyolefin microporous film, heat-resistant porous layer, and adhesive layer contained in the separator of the present disclosure will be described in detail below.

[0043] [Polyolefin microporous membrane] In the present disclosure, the term "polyolefin microporous membrane" refers to a microporous membrane containing a polyolefin, which has a structure in which numerous micropores are connected inside and allows gas or liquid to pass from one side to the other side.

[0044] The polyolefin microporous membrane may be a microporous membrane made of only polyolefin, or a microporous membrane made of polyolefin and a material other than polyolefin (e.g., a resin such as an acrylic resin, a styrene resin, or a butadiene rubber). The polyolefin microporous membrane preferably contains polyolefin at 95% by mass or more, more preferably 99% by mass or more, of the total mass.

[0045] Examples of the polyolefin microporous membrane include polyolefin microporous membranes that are conventionally used in battery separators, and it is preferable to select one from these that has sufficient mechanical properties and ion permeability.

[0046] From the viewpoint of exhibiting a shutdown function (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 polyolefin microporous membrane is preferably a microporous membrane containing polyethylene, and the polyethylene preferably accounts for 95 mass% or more of the total mass of the polyolefin microporous membrane. In the present disclosure, a microporous membrane in which polyethylene is the resin that accounts for the largest mass proportion of all the resins that constitute the microporous membrane is referred to as a polyethylene microporous membrane.

[0047] The polyolefin microporous film is preferably a microporous film containing polypropylene, from the viewpoint of heat resistance that prevents film rupture when exposed to high temperatures.

[0048] The polyolefin microporous membrane is preferably a polyolefin microporous membrane containing polyethylene and polypropylene, from the viewpoint of providing a shutdown function and heat resistance such that it does not easily rupture when exposed to high temperatures. 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. Another example of a polyolefin microporous membrane containing polyethylene and polypropylene is 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.

[0049] 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. When the Mw of the polyolefin is 5,000,000 or less, the microporous membrane has good shutdown properties and is easy to mold.

[0050] The weight-average molecular weight of the polyolefin constituting the polyolefin microporous membrane is determined by dissolving the polyolefin microporous membrane in o-dichlorobenzene under heating and measuring the molecular weight by gel permeation chromatography (GPC). Polystyrene or the like is used for molecular weight calibration.

[0051] Examples of methods for producing a polyolefin microporous membrane include a method in which molten polyolefin 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 molten polyolefin 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.

[0052] The surface of the polyolefin microporous membrane may be subjected to various surface treatments, such as corona treatment, plasma treatment, flame treatment, and ultraviolet irradiation treatment, to improve wettability with a coating liquid for forming a heat-resistant porous layer or an adhesive layer, provided that the properties of the polyolefin microporous membrane are not impaired.

[0053] -Properties of polyolefin microporous membrane- From the viewpoint of mechanical strength, the thickness of the polyolefin microporous membrane is preferably 3 μm or more, more preferably 4 μm or more, and even more preferably 5 μm or more. From the viewpoint of increasing the energy density of the battery, the thickness of the polyolefin microporous membrane is preferably 10 μm or less, more preferably 9 μm or less, and even more preferably 8 μm or less. The thickness of the polyolefin microporous membrane is determined by measuring 20 points within a 10 cm square using a contact-type measuring device and averaging the measurements.

[0054] From the viewpoint of ion permeability, the porosity of the polyolefin microporous membrane is preferably 30% to 60%. The porosity ε (%) of the polyolefin microporous membrane is calculated by the following formula: ε = {1 - W / (d t)} × 100, where W is the mass per unit area of ​​the polyolefin microporous membrane (g / m ２ ), and d is the true density (g / cm) of the polyolefin microporous membrane. ３ ) and t is the thickness (μm) of the polyolefin microporous membrane.

[0055] An example of the form of the polyolefin microporous membrane is a form having a heat-resistant resin (AI) in the pores. The heat-resistant resin (AI) is a heat-resistant resin (AI) that has entered the pores of the polyolefin microporous membrane when the heat-resistant porous layer is formed. The details and preferred form of the heat-resistant resin (AI) present in the pores of the polyolefin microporous membrane are the same as those of the heat-resistant resin (AI) contained in the heat-resistant porous layer (described below).

[0056] Examples of the form of the polyolefin microporous membrane having the heat-resistant resin (AI) in the pores include a polyolefin microporous membrane having the heat-resistant resin (AI) in the pores in the region close to the surface of the microporous membrane; and a polyolefin microporous membrane having the heat-resistant resin (AI) in the pores throughout the microporous membrane. Examples of the form of the heat-resistant resin (AI) in the pores include a form in which the heat-resistant resin (AI) covers part or all of the wall surface of the pores; a form in which fibrous heat-resistant resin (AI) is contained in the pores; and combinations thereof. If the heat-resistant resin (AI) in the pores has these forms, it is preferred from the viewpoint that the electrolyte solution can easily penetrate into the polyolefin microporous membrane.

[0057] The presence of the heat-resistant resin (AI) in the pores of the polyolefin microporous membrane can be confirmed by elemental imaging using a transmission electron microscope in combination with energy dispersive X-ray spectroscopy. ４ ) is easily stained by Ru, and heat-resistant resin (AI) is detected by Ru imaging.

[0058] In this disclosure, the Gurley value of a polyolefin microporous membrane is a value measured without a heat-resistant resin (AI) in the pores, i.e., using only the polyolefin microporous membrane. In this disclosure, the porosity of a polyolefin microporous membrane is a value calculated without a heat-resistant resin (AI) in the pores, i.e., using only the polyolefin microporous membrane.

[0059] [Heat-resistant porous layer] The porous layer means a layer having a large number of fine pores through which gas or liquid can pass from one surface to the other. The heat-resistant porous layer is a porous layer that is endowed with heat resistance by containing a heat-resistant material (e.g., a heat-resistant resin and / or inorganic particles).

[0060] 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.

[0061] In the present disclosure, the heat-resistant porous layer is a porous layer containing a heat-resistant resin (AI) and inorganic particles. The heat-resistant porous layer may be a porous layer made only of the heat-resistant resin (AI) and inorganic particles, or a porous layer made of the heat-resistant resin (AI), inorganic particles, and other materials.

[0062] Examples of the form of the heat-resistant porous layer include a structure in which inorganic particles are bound or trapped in a porous structure in which fibrils containing a heat-resistant resin (AI) are connected in a two-dimensional or three-dimensional network; a structure in which inorganic particles are bound or trapped in a network-like microporous structure containing a heat-resistant resin (AI); and a layered structure in which a large number of inorganic particles are connected to each other by a heat-resistant resin (AI), forming voids between the inorganic particles.

[0063] -Heat-resistant resin (AI)- The heat-resistant resin (AI) may be a homopolymer or a copolymer. Examples of the heat-resistant resin (AI) include polyamide, polyimide, polyamideimide, polyacrylamide, polyacrylimide, polyetherpolyamide, polyetherpolyimide, and poly-N-vinylacetamide. These resins may be used alone or in combination of two or more.

[0064] As the heat-resistant resin (AI), polyamide, polyimide, and polyamideimide are preferred from the viewpoints of heat resistance, stability against the electrolyte, and electrochemical stability. As the polyamide, fully aromatic polyamide is preferred from the viewpoint of durability. Fully aromatic polyamide means a polyamide whose main chain is composed only of benzene rings and amide bonds. However, a small amount of aliphatic monomer may be copolymerized in the fully aromatic polyamide. Fully aromatic polyamide is also called aramid.

[0065] The wholly aromatic polyamide may be meta-type or para-type. Among wholly aromatic polyamides, meta-type wholly aromatic polyamides are preferred from the viewpoints of ease of forming a porous layer and excellent oxidation-reduction resistance in electrode reactions. Specific wholly aromatic polyamides are preferably polymetaphenylene isophthalamide or polyparaphenylene terephthalamide, and more preferably polymetaphenylene isophthalamide.

[0066] Meta-type wholly aromatic polyamides are polymers with higher flexibility than para-type wholly aromatic polyamides, and therefore, when a heat-resistant porous layer is formed, they penetrate into the pores of the polyolefin microporous membrane and tend to adhere to the pore wall surfaces or form a fibrous structure. When meta-type aromatic polyamides are adhered to the pore wall surfaces of the polyolefin microporous membrane or when fibrous meta-type aromatic polyamides are contained in the pores, the electrolyte solution tends to penetrate the polyolefin microporous membrane.

[0067] The weight average molecular weight of the heat-resistant resin (AI) contained in the heat-resistant porous layer is 1×10 ３ ~1 x 10 ７ Preferably, it is 5×10 ３ ~5 x 10 ６ is more preferable, and 1×10 ４ ~1 x 10 ６ is more preferred.

[0068] The weight average molecular weight of the heat-resistant resin (AI) is a molecular weight measured by GPC in terms of polystyrene. The measurement is performed using the heat-resistant resin (AI) extracted from the heat-resistant porous layer or the heat-resistant resin (AI) used to form the heat-resistant porous layer as a sample.

[0069] The mass proportion of the heat-resistant resin (AI) in the total resin of the heat-resistant porous layer is preferably 90 mass% or more, more preferably 95 mass% or more, even more preferably 99 mass% or more, and particularly preferably 100 mass%.

[0070] The mass proportion of the heat-resistant resin (AI) in the heat-resistant porous layer is preferably 5% by mass to 50% by mass, more preferably 10% by mass to 40% by mass, and even more preferably 15% by mass to 30% by mass, from the viewpoint of forming a good porous structure and suppressing interlayer delamination with the polyolefin microporous membrane.

[0071] When heat-resistant porous layers are present on both sides of the polyolefin microporous membrane, the type and / or content of the heat-resistant resin (AI) contained in one heat-resistant porous layer may be the same as or different from the type and / or content of the heat-resistant resin (AI) contained in the other heat-resistant porous layer.

[0072] -Other Resins- The heat-resistant porous layer may contain other resins in addition to the heat-resistant resin (AI). Examples of other resins include acrylic resins, polyvinylidene fluoride resins, styrene-butadiene copolymers, butadiene-acrylonitrile resins, cellulose, polyvinylpyrrolidone, polyether, polysulfone, polyethersulfone, polyketone, and polyetherketone. These resins may be used alone or in combination of two or more.

[0073] The mass proportion of other resins in the total resin of the heat-resistant porous layer is preferably 10 mass % or less, more preferably 5 mass % or less, and even more preferably 1 mass % or less.

[0074] - 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.

[0075] Examples of metal oxides constituting metal oxide particles include silica (silicon dioxide), alumina (aluminum oxide), boehmite (alumina monohydrate), titania (titanium oxide), zirconia (zirconium oxide), magnesium oxide, and barium oxide, with alumina being preferred. Examples of metal hydroxides constituting metal hydroxide particles include magnesium hydroxide, aluminum hydroxide, calcium hydroxide, chromium hydroxide, zirconium hydroxide, cerium hydroxide, nickel hydroxide, and boron hydroxide, with magnesium hydroxide being preferred. Examples of metal sulfates constituting metal sulfate particles include barium sulfate and calcium sulfate, with barium sulfate being preferred. Examples of metal carbonates constituting metal carbonate particles include calcium carbonate, magnesium carbonate, and barium carbonate. Examples of metal nitrides constituting metal nitride particles include boron nitride and aluminum nitride. Examples of clay mineral particles include calcium silicate and talc.

[0076] The inorganic particles may be surface-modified with a silane coupling agent or the like.

[0077] The inorganic particles may be used alone or in combination of two or more kinds.

[0078] From the viewpoints of stability in the electrolyte and electrochemical stability, the inorganic particles are preferably at least one selected from the group consisting of metal oxide particles, metal hydroxide particles, and metal sulfate particles, and more preferably at least one selected from the group consisting of alumina particles (aluminum oxide particles), magnesium hydroxide particles, and barium sulfate particles.

[0079] 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 internal short-circuiting in the battery, the inorganic particles contained in the heat-resistant porous layer are preferably plate-like particles or non-aggregated primary particles.

[0080] The average primary particle size of the inorganic particles contained in the heat-resistant porous layer is preferably 0.01 μm to 2 μm, more preferably 0.02 μm to 1 μm, and even more preferably 0.03 μm to 0.5 μm, from the viewpoint of forming a good porous structure and suppressing interlayer delamination with the polyolefin microporous membrane.

[0081] 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 for forming the heat-resistant porous layer, or inorganic particles removed from the heat-resistant porous layer. There are no limitations on the method for removing the inorganic particles from the heat-resistant porous layer. Examples of such methods include immersing the heat-resistant porous layer peeled off from the separator in an organic solvent that dissolves the heat-resistant resin to remove the inorganic particles; or heating the heat-resistant porous layer peeled off from the separator to about 800°C to remove the heat-resistant resin and remove the inorganic particles.

[0082] The mass proportion of the inorganic particles in the heat-resistant porous layer is preferably 50% by mass to 95% by mass, more preferably 60% by mass to 90% by mass, and even more preferably 70% by mass to 85% by mass, from the viewpoint of forming a good porous structure and suppressing interlayer delamination with the polyolefin microporous membrane.

[0083] When heat-resistant porous layers are present on both sides of the polyolefin microporous membrane, the type and / or content of inorganic particles contained in one heat-resistant porous layer may be the same as or different from the type and / or content of inorganic particles contained in the other heat-resistant porous layer.

[0084] -Organic Particles- The heat-resistant porous 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 products, 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.

[0085] The organic particles may be used alone or in combination of two or more kinds.

[0086] -Other Components- The heat-resistant porous layer may contain additives such as a dispersant such as a surfactant, a wetting agent, an antifoaming agent, and a pH adjuster. Dispersants are added to the coating liquid for forming the heat-resistant porous layer, for example, to improve dispersibility, coatability, or storage stability. Wetting agents, antifoaming agents, and pH adjusters are added to the coating liquid for forming the heat-resistant porous layer, for example, to improve compatibility with the polyolefin microporous membrane, to suppress air entrapment in the coating liquid, or to adjust the pH.

[0087] -Characteristics of heat-resistant porous layer- When the heat-resistant porous layer is present on only one side of the polyolefin microporous membrane, the thickness of the heat-resistant porous layer is preferably 4 μm or less, more preferably 3.5 μm or less, and even more preferably 3 μm or less, from the viewpoint of easy moisture removal by drying. When the heat-resistant porous layer is present on only one side of the polyolefin microporous membrane, the thickness of the heat-resistant porous layer is preferably 0.5 μm or more, more preferably 1 μm or more, and even more preferably 1.5 μm or more, from the viewpoint of short-circuit resistance of the separator.

[0088] When the heat-resistant porous layer is provided on both sides of the polyolefin microporous membrane, the thickness of the heat-resistant porous layer on both sides in total is preferably 8 μm or less, more preferably 7 μm or less, and even more preferably 6 μm or less, from the viewpoint of easy moisture removal by drying. When the heat-resistant porous layer is provided on both sides of the polyolefin microporous membrane, the thickness of the heat-resistant porous layer on both sides in total is preferably 1 μm or more, more preferably 2 μm or more, and even more preferably 3 μm or more, from the viewpoint of short-circuit resistance of the separator.

[0089] The thickness of the heat-resistant porous layer is the value obtained by subtracting the thickness of the polyolefin microporous membrane from the thickness of the flat membrane obtained by removing the adhesive layer from the separator. The thickness of the flat membrane obtained by removing the adhesive layer from the separator is measured at 20 points within a 10 cm square using a contact-type measuring device and the average of the measurements is obtained.

[0090] When the heat-resistant porous layer is present on only one side of the polyolefin microporous membrane, the mass per unit area of ​​the heat-resistant porous layer (i.e., basis weight) is 1 g / m ２ ~4.5g / m ２ It is preferable that the density is 1.5 g / m ２ ~4g / m ２ More preferably, 2 g / m ２ ~3.5g / m ２ is more preferred.

[0091] When the heat-resistant porous layer is provided on both sides of the polyolefin microporous film, the mass per unit area (i.e., basis weight) of the heat-resistant porous layer is 2 g / m in total on both sides. ２ ~9g / m ２ It is preferable that the density is 3 g / m ２ ~8g / m ２ More preferably, 4 g / m ２ ~7g / m ２ is more preferred.

[0092] The mass per unit area of ​​the heat-resistant porous layer (i.e., basis weight) is the value obtained by subtracting the basis weight of the polyolefin microporous membrane from the basis weight of the flat membrane obtained by removing the adhesive layer from the separator. The basis weight of the flat membrane obtained by removing the adhesive layer from the separator is determined by cutting the separator into a 20 cm x 20 cm piece, removing the adhesive layer, measuring the mass, and dividing the mass by the area.

[0093] The porosity of the heat-resistant porous layer is preferably 20% to 70% from the viewpoint of ion permeability. The porosity ε (%) of the heat-resistant porous layer is calculated by the following formula.

[0094]

[0095] Here, for constituent material 1, constituent material 2, constituent material 3, ..., constituent material n of the heat-resistant porous 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 porous layer is t (cm).

[0096] [Adhesive Layer] The adhesive layer is a layer disposed on the surface of the heat-resistant porous layer or the polyolefin microporous membrane, 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.

[0097] The adhesive layer may be a layer made of adhesive resin particles alone, or may be a layer made of adhesive resin particles and materials other than adhesive resin particles, such as non-particulate resins, dispersants, wetting agents, antifoaming agents, and pH adjusters.

[0098] From the viewpoint of adhesion to the electrode, the mass proportion of the adhesive resin particles in the adhesive layer is preferably 90 mass% or more, more preferably 95 mass% or more, and even more preferably 99 mass% or more. The mass proportion of the adhesive resin particles in the adhesive layer may be 100 mass%.

[0099] -Adhesive Resin Particles- Examples of adhesive resin particles include acrylic resin particles, polyvinylidene fluoride resin particles, styrene butadiene rubber particles, fluorine-containing rubber particles, butadiene-acrylonitrile resin particles, and cellulose particles. Among these, at least one selected from the group consisting of acrylic resin particles, polyvinylidene fluoride resin particles, and styrene butadiene rubber particles is preferred from the viewpoint of excellent adhesion to electrodes.

[0100] Examples of the acrylic resin constituting the acrylic resin particles 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.

[0101] 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 (meth)acrylic acid alkyl esters. 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, 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.

[0102] 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.

[0103] The polyvinylidene fluoride resin constituting the polyvinylidene fluoride resin particles can be exemplified by a homopolymer of vinylidene fluoride (i.e., polyvinylidene fluoride); a copolymer of vinylidene fluoride and other monomers (polyvinylidene fluoride copolymer); or a mixture of polyvinylidene fluoride and polyvinylidene fluoride copolymer. Examples of monomers copolymerizable with vinylidene fluoride include tetrafluoroethylene, hexafluoropropylene, trifluoroethylene, chlorotrifluoroethylene, trichloroethylene, vinyl fluoride, trifluoroperfluoropropyl ether, ethylene, (meth)acrylic acid, methyl (meth)acrylate, (meth)acrylic acid ester, vinyl acetate, vinyl chloride, acrylonitrile, etc. These monomers can be used alone or in combination of two or more.

[0104] The polyvinylidene fluoride copolymer is preferably a copolymer containing 50 mol% or more of vinylidene fluoride units, from the viewpoint of imparting mechanical strength to the adhesive resin particles that can withstand pressure and heat during battery production. The polyvinylidene fluoride copolymer is preferably a copolymer of vinylidene fluoride and hexafluoropropylene. The copolymer preferably contains 0.1 mol% to 10 mol% (preferably 0.5 mol% to 5 mol%) of hexafluoropropylene units.

[0105] The styrene-butadiene rubber constituting the styrene-butadiene rubber particles may be an emulsion polymer or a solution polymer, or may be a random copolymer or a block copolymer. The styrene content of the styrene-butadiene rubber is preferably 20% by mass to 50% by mass.

[0106] The average primary particle size of the adhesive resin particles is preferably 0.05 μm or more, more preferably 0.1 μm or more, and even more preferably 0.2 μm or more, from the viewpoint of thinning the adhesive layer, suppressing interlayer delamination between the adhesive layer and the heat-resistant porous layer or the polyolefin microporous membrane, and maintaining adhesion of the separator to the electrode, and is preferably 1 μm or less, more preferably 0.8 μm or less, and even more preferably 0.6 μm or less.

[0107] The average primary particle size of the adhesive resin particles is determined by measuring the major axes of 100 adhesive resin particles randomly selected in observation with a scanning electron microscope (SEM) and averaging the major axes of the 100 particles. The sample used for SEM observation is adhesive resin particles that are a material for forming the adhesive layer, or adhesive resin particles removed from the adhesive layer.

[0108] The coverage of the separator surface by the adhesive resin particles is preferably 20% or more, more preferably 25% or more, and even more preferably 30% or more, on each side on which the adhesive layer is disposed, from the viewpoints of excellent adhesion to the electrode and excellent cycle characteristics of the battery. The coverage of the separator surface by the adhesive resin particles is preferably 60% or less, more preferably 55% or less, and even more preferably 50% or less, on each side on which the adhesive layer is disposed, from the viewpoints of easy removal of moisture from the separator by drying and excellent cycle characteristics of the battery.

[0109] The coverage of the separator surface by the adhesive resin particles is the area ratio of the separator surface covered by the adhesive resin particles when viewed from above. The coverage is determined by imaging the separator surface from a direction perpendicular to the surface using a scanning electron microscope (SEM), randomly selecting 10 square regions, determining the coverage of each region, and then calculating the average of the coverages for the 10 regions.

[0110] The coverage of the separator surface with the adhesive resin particles can be controlled by the resin particle concentration and coating amount of the adhesive resin particle dispersion liquid for forming the adhesive layer.

[0111] When adhesive layers are present on both sides of the separator, the type and / or coverage of adhesive resin particles contained in one adhesive layer may be the same as or different from the type and / or coverage of adhesive resin particles contained in the other adhesive layer.

[0112] Other Components The adhesive layer may contain a binder resin that binds the adhesive resin particles to the laminate. The binder resin is preferably the same type of resin as that contained in the heat-resistant porous layer.

[0113] 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 porous layer or the polyolefin microporous membrane, to suppress air entrapment in the resin particle dispersion, or to adjust the pH.

[0114] [Separator Characteristics] From the viewpoint of short-circuit resistance, 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 ease of moisture removal by drying and the viewpoint of the energy density of the battery, the thickness of the separator is preferably 17 μm or less, more preferably 15 μm or less, and even more preferably 13 μm or less. The thickness of the separator is determined by measuring 20 points within a 10 cm square using a contact-type measuring device and averaging the measurements.

[0115] The mass per unit area of ​​the separator (i.e., basis weight) is set to 6 g / m from the viewpoint of facilitating the removal of moisture from the separator by drying and from the viewpoint of the short-circuit resistance of the separator. ２ ~13g / m ２ It is preferable that the density is 7 g / m ２ ~12g / m ２ More preferably, 8 g / m ２~11g / m ２ The mass per unit area of ​​the separator (i.e., basis weight) is determined by cutting the separator into a 20 cm x 20 cm piece, measuring the mass, and dividing the mass by the area.

[0116] [Separator Manufacturing Method] The separator of the present disclosure can be manufactured by, for example, the following manufacturing method (A) or manufacturing method (B). In manufacturing methods (A) and (B), a heat-resistant porous layer is formed on a polyolefin microporous membrane by a wet coating method, and an adhesive layer is formed on a laminate by a dry coating method. In the present disclosure, the wet coating method is a method in which a coating layer is solidified in a coagulation liquid. In the present disclosure, the dry coating method is a method in which a coating layer is solidified by drying.

[0117] Production method (A) (continuous production method): A heat-resistant porous layer is formed by wet coating on a polyolefin microporous membrane unwound from a roll to obtain a laminate of the polyolefin microporous membrane and the heat-resistant porous layer, and then an adhesive layer is formed on the laminate by dry coating to obtain a separator, and the completed separator is taken up on another roll.

[0118] Production method (B) (discontinuous production method): A heat-resistant porous layer is formed on a polyolefin microporous membrane unwound from a roll by a wet coating method to obtain a laminate of the polyolefin microporous membrane and the heat-resistant porous 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 dry coating method to obtain a separator, and the completed separator is then wound up on another roll.

[0119] The steps included in the production method (A) are described in detail below. The production method (A) includes the following steps (1) to (7).

[0120] -Step (1): Preparation of Coating Liquid for Forming Heat-Resistant Porous Layer- The coating liquid for forming the heat-resistant porous layer is prepared by dissolving and dispersing the heat-resistant resin (AI) and inorganic particles in a solvent. If necessary, other components besides the heat-resistant resin (AI) and inorganic particles may be dissolved or dispersed in the coating liquid.

[0121] The solvent used in preparing the coating liquid includes a solvent that dissolves the heat-resistant resin (AI) (hereinafter also referred to as a "good solvent"). Examples of the good solvent include polar amide solvents. Examples of the polar amide solvent include dimethylacetamide, dimethylformamide, and N-methylpyrrolidone.

[0122] From the viewpoint of forming 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 phase separation agents include water, methanol, ethanol, propyl alcohol, butyl alcohol, butanediol, ethylene glycol, propylene glycol, and tripropylene glycol.

[0123] From the viewpoint of forming a good porous structure, the solvent used to prepare the coating liquid is preferably a mixed solvent of a good solvent and a phase separation agent, containing 60% by mass or more of the good solvent and 5% by mass to 40% by mass of the phase separation agent.

[0124] 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, and the inorganic particle concentration in the coating liquid is preferably 0.5% by mass to 50% by mass from the viewpoint of forming a good porous structure.

[0125] -Step (2): Preparation of adhesive resin particle dispersion- The adhesive layer-forming coating liquid is preferably an adhesive resin particle dispersion. The adhesive resin particle dispersion is prepared by dispersing adhesive resin particles in an appropriate dispersion medium (e.g., water). A surfactant may be added to the adhesive resin particle dispersion to enhance the dispersibility of the adhesive resin particles in the dispersion medium. The adhesive resin particle dispersion may be a commercially available product or a diluted solution of a commercially available product.

[0126] From the viewpoint of coating suitability, the concentration of the adhesive resin particles in the adhesive resin particle dispersion is preferably 1% by mass to 60% by mass.

[0127] -Step (3): Coating of heat-resistant porous layer-forming coating liquid- The heat-resistant porous layer-forming coating liquid is coated on at least one surface of the polyolefin microporous membrane to form a coating layer on the polyolefin microporous membrane. Examples of coating methods for the coating liquid include knife coating, Mayer bar coating, die coating, reverse roll coating, roll coating, and gravure coating. When forming heat-resistant porous layers on both surfaces of the polyolefin microporous membrane, it is preferable from the viewpoint of productivity to coat the polyolefin microporous membrane with the coating liquid simultaneously on both surfaces.

[0128] -Step (4): Solidification of coating layer- The polyolefin microporous membrane with the coating layer that will become the heat-resistant porous layer formed thereon is immersed in a coagulation liquid to solidify the heat-resistant resin (AI) while inducing phase separation in the coating layer, thereby obtaining a laminate consisting of the polyolefin microporous membrane and the heat-resistant porous layer.

[0129] 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.

[0130] - Step (5): Washing and drying of coating layer - The laminate is pulled out of the coagulating solution and washed with water. By washing with water, the coagulating solution is removed from the laminate. Furthermore, by drying, water is removed from the laminate. The washing with water is carried out, for example, by transporting the laminate in a water bath. The drying is carried out, for example, by transporting the laminate in a high-temperature environment, by blowing air on the laminate, or by bringing the laminate into contact with a heat roll. The drying temperature is preferably 40°C to 80°C.

[0131] -Step (6): Coating of adhesive resin particle dispersion- The adhesive resin particle dispersion is coated on at least one surface of the laminate. Examples of coating methods for the adhesive resin particle dispersion 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 adhesive resin particle dispersion on both surfaces of the laminate simultaneously.

[0132] Step (7): Drying of adhesive resin particle dispersion The adhesive resin particle dispersion on the laminate is dried to adhere the adhesive resin particles to the surface of the laminate. Drying is performed, for example, by transporting the laminate in a high-temperature environment or by blowing air onto the laminate. The drying temperature is preferably 40°C to 100°C.

[0133] The manufacturing method (B) can be carried out by carrying out steps (1) to (5), then temporarily winding the laminate onto a roll, and then unwinding the laminate from the roll to carry out steps (6) and (7).

[0134] The nonaqueous secondary battery of 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 of 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 the electrode.

[0135] 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.

[0136] In the nonaqueous secondary battery of the present disclosure, the separator of the present disclosure has adhesive properties to the electrode, so that the electrode and the separator are less likely to peel off, and internal short circuits are less likely to occur.In the nonaqueous secondary battery of the present disclosure, the separator of the present disclosure is easily dried to remove moisture, so that internal short circuits are less likely to occur and the battery has excellent cycle characteristics.

[0137] 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.

[0138] 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.

[0139] 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 negative electrode active materials include materials capable of electrochemically absorbing lithium ions, such as carbon materials; alloys of lithium with silicon, tin, aluminum, etc.; and Wood's alloy. Examples of binder resins include polyvinylidene fluoride resins and styrene-butadiene copolymers. Examples of conductive additives include carbon materials such as acetylene black, ketjen black, graphite powder, and ultrafine carbon fibers. Examples of current collectors 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.

[0140] 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.

[0141] Examples of the exterior packaging include aluminum laminate film packs, metal cans, etc. Battery shapes include prismatic, cylindrical, coin-shaped, etc., and the separator of the present disclosure is suitable for any of these shapes.

[0142] 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, any of the following production methods (1) to (3).

[0143] 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.

[0144] Manufacturing method (2): The laminate is placed in an exterior packaging material, and an electrolyte solution is poured into the exterior 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.

[0145] Manufacturing method (3): The laminate is dry-heat pressed to bond the electrodes and the separator, and then the laminate is housed in an exterior packaging material, into which an electrolyte solution is injected, and the exterior packaging material is then sealed.

[0146] In Production Methods (1) to (3), the pressing temperatures of the wet heat press and dry heat press are each preferably 50°C to 100°C, more preferably 60°C to 90°C. The pressing pressures of the wet heat press and dry heat press are each preferably 0.1 MPa to 2 MPa, more preferably 0.5 MPa to 1.5 MPa. The pressing time is preferably adjusted depending on the pressing temperature and pressing pressure, for example, within a range of 5 seconds to 20 hours.

[0147] 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.

[0148] 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.

[0149] In the following description, synthesis, processing, manufacturing, testing, etc. were carried out at room temperature (25° C.±3° C.) unless otherwise specified.

[0150] <Measurement Methods and Evaluation Methods> The measurement methods and evaluation methods used in the examples and comparative examples are as follows.

[0151] [Thickness of polyethylene microporous membrane and separator] The thicknesses (μm) of the polyethylene microporous membrane and separator were measured at 20 points within a 10 cm square using a contact-type measuring instrument LITEMATIC VL-50S (Mitutoyo Corporation) and the average values ​​were determined. A spherical measuring probe with a sphere radius of 10 mm (Mitutoyo Corporation) was used, and the measurement was adjusted so that a load of 0.19 N was applied during measurement.

[0152] [Thickness of Heat-Resistant Porous Layer] The thickness (μm) of the polyethylene microporous membrane was calculated by subtracting the thickness (μm) of the flat membrane obtained by removing the adhesive layer from the separator. The thickness of the flat membrane was measured at 20 points within a 10 cm square using a contact-type measuring instrument LITEMATIC VL-50S (Mitutoyo Corporation) and the average was calculated. A spherical probe with a sphere radius of 10 mm (Mitutoyo Corporation) was used, and the measurement was adjusted so that a load of 0.19 N was applied during measurement.

[0153] [Basis Weight of Polyethylene Microporous Membrane and Separator] Each of the polyethylene microporous membrane and the separator was cut into a 20 cm x 20 cm piece, and the mass was measured. The mass was divided by the area to determine the basis weight (g / m ２ ) was sought.

[0154] [Basis Weight of Heat-Resistant Porous Layer] The basis weight (g / m) of the flat membrane after removing the adhesive layer from the separator ２ ) to calculate the basis weight (g / m) of the polyethylene microporous membrane. ２The basis weight of the flat membrane was determined by cutting the separator into a size of 20 cm x 20 cm, removing the adhesive layer, measuring the mass, and dividing the mass by the area.

[0155] [Gurley values ​​of polyethylene microporous membrane and separator] The Gurley values ​​(seconds / 100 mL) of the polyethylene microporous membrane and separator were measured using a Gurley densometer G-B2C (Toyo Seiki Seisaku-Sho, Ltd.) in accordance with JIS P8117: 2009. The Gurley value difference was determined by subtracting the Gurley value of the polyethylene microporous membrane from the Gurley value of the separator.

[0156] [Average primary particle size of inorganic particles] The inorganic particles used to form the heat-resistant porous layer were used as samples and observed with a SEM to determine the average primary particle size. 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).

[0157] [Average Primary Particle Size of Adhesive Resin Particles] A dried adhesive resin particle dispersion liquid used for forming an adhesive layer was used as a sample and observed with an SEM to determine the average primary particle size. The major axes of 100 adhesive resin particles randomly selected in the SEM observation were measured, and the average value of the major axes of the 100 particles was taken as the average primary particle size (μm).

[0158] [Coverage of Separator Surface by Adhesive Resin Particles] The coverage of the separator surface by the adhesive resin particles was determined by taking an image of the separator surface from the direction perpendicular to the surface using an SEM, randomly selecting 10 square regions, determining the coverage of each region, and then calculating the average value of the 10 regions.

[0159] [Separator Drying Suitability] The separator was immersed in a water bath while being transported, and after being removed from the water bath, it was passed through nip rolls to squeeze out the water. Subsequently, while the separator was being transported, air at a temperature of 70°C was blown onto one side, and the other side was brought into contact with a heat roll with a surface temperature of 70°C for 30 seconds. While being transported, one side of 1000 m of the separator (the side not in contact with the heat roll) was visually observed to check for the presence or absence of drying irregularities, such as scattered circular or oval spots. After transporting 1000 m of the separator, the heat roll surface was visually observed to check for the presence or absence of deposits (film-like substances peeled off from the separator and attached thereto) with a major axis length of 1 cm or more. The presence or absence of drying irregularities and deposits was classified as follows:

[0160] A: There is no drying unevenness on the separator surface and no deposits on the heat roll surface. B: There is drying unevenness on the separator surface and no deposits on the heat roll surface. C: There is drying unevenness on the separator surface and deposits on the heat roll surface.

[0161] [Battery Short-Circuit Resistance] One hundred nonaqueous secondary batteries, as described below, were prepared. The batteries were charged at a constant current and constant voltage of 1 C / 4.2 V in a 25°C environment. The charged batteries were placed on a horizontal table with the positive electrode side facing up and secured with adhesive tape. A cylindrical terminal with a diameter of 10 mm was placed on the center of the battery, and a load of 5 kgf was applied. A battery was judged to have a short circuit when its voltage dropped to 3.5 V or less. The number of short-circuited batteries out of 100 was classified as follows:

[0162] A: 0 pieces B: 1 to 5 pieces C: 6 to 9 pieces D: 10 or more pieces

[0163] [Battery Cycle Characteristics] One hundred nonaqueous secondary batteries, as described below, were prepared. The batteries were subjected to 100 charge / discharge cycles at a temperature of 25°C. Charging was performed at a constant current and constant voltage of 1C / 4.2V, and discharging was performed at a constant current with a cutoff of 1C / 2.75V. The discharge capacity at the 100th cycle was divided by the initial discharge capacity to calculate the capacity retention rate (%). The average value for the 100 batteries was then calculated, and the average capacity retention rate was classified as follows:

[0164] A: 90% or more B: 80% or more but less than 90% C: 70% or more but less than 80% D: Less than 70%

[0165] <Manufacture of Separator and Battery> [Example 1] - Preparation of Separator - Dimethylacetamide (DMAc) and tripropylene glycol (TPG) were mixed in a mass ratio of 95:5 to prepare a mixed solvent. Meta-aramid (polymetaphenylene isophthalamide) and alumina particles (average primary particle size 0.04 μm) were added to the mixed solvent and stirred to prepare coating liquid (1). Coating liquid (1) had a meta-aramid concentration of 5 mass% and a mass ratio of meta-aramid to alumina particles of 20:80.

[0166] A resin particle dispersion (1) was prepared by dispersing acrylic resin particles (glass transition temperature: 55° C.) in water. The resin particle dispersion (1) had a resin particle concentration of 7% by mass and an average primary particle size of 0.4 μm.

[0167] A polyethylene microporous membrane (thickness 5.9 μm, Gurley coefficient 90 sec / 100 mL) was passed through a pair of Mayer bars carrying an appropriate amount of coating liquid (1), and equal amounts of coating liquid (1) were applied to both sides of the polyethylene microporous membrane. The membrane was then immersed in a coagulation liquid (DMAc:TPG:water = 38:2:60 [mass ratio], liquid temperature 25°C) to solidify the coating layer. The membrane was then washed in a water washing tank at a water temperature of 25°C and dried to obtain a laminate comprising heat-resistant porous layers on both sides of the polyethylene microporous membrane. The laminate was then passed through a pair of bar coaters carrying an appropriate amount of resin particle dispersion liquid (1), and equal amounts of resin particle dispersion liquid (1) were applied to both sides of the laminate, followed by drying. Thus, a separator comprising a heat-resistant porous layer and an adhesive layer on both sides of the polyethylene microporous membrane was obtained. The coverage of the separator surface with acrylic resin particles was 40% on both sides.

[0168] - 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 and stirred in a twin-arm mixer to prepare a positive electrode slurry. The positive electrode slurry was applied to both sides of a 20 μm thick aluminum foil, dried, and pressed to obtain a positive electrode having positive electrode active material layers on both sides.

[0169] - Preparation of negative electrode - 300 parts by mass of artificial graphite as a negative electrode active material, 7.5 parts by mass of an aqueous dispersion containing 40% by mass of a modified styrene-butadiene copolymer as a binder resin, 3 parts by mass of carboxymethyl cellulose as a thickener, and an appropriate amount of water were mixed by stirring using a twin-arm mixer to prepare a negative electrode slurry. The negative electrode slurry was applied to both sides of a 10 μm thick copper foil, dried, and pressed to obtain a negative electrode having a negative electrode active material layer on both sides.

[0170] - Preparation of Battery - The positive electrode and the negative electrode were each cut into a rectangle of 30 mm x 50 mm, and a lead tab was welded to each. The separator was cut into a rectangle of TD 35 mm x MD 55 mm. These were stacked so that the positive electrode and the negative electrode alternated and the separator was sandwiched between the positive electrode and the negative electrode, to prepare a stack consisting of three positive electrodes, three negative electrodes, and five separators. The stack was inserted into an aluminum laminate film pack, 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 separator. The heat press conditions were a press temperature of 85°C, a press pressure of 1 MPa, and a press time of 5 minutes. A nonaqueous secondary battery was thus obtained. This nonaqueous secondary battery was subjected to an evaluation test for short-circuit resistance and an evaluation test for cycle characteristics.

[0171] [Examples 2 to 9, Comparative Examples 1 to 3] Separators were produced in the same manner as in Example 1, except that at least one of the polyethylene microporous membrane, the heat-resistant porous layer, and the adhesive layer was changed as shown in Table 1. The coverage of the separator surface with the acrylic resin particles was controlled by the coating amount of the resin particle dispersion (1). Non-aqueous secondary batteries were produced using each separator.

[0172] The basis weight of the heat-resistant porous layer shown in Table 1 is the total basis weight of both sides of the separator. When heat-resistant porous layers are present on both sides of the polyethylene microporous membrane, the basis weight of the heat-resistant porous layer per side is half the basis weight of the heat-resistant porous layer shown in Table 1.

[0173]

[0174] 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.

[0175] The disclosure of Japanese Application No. 2024-045644, filed on March 21, 2024, is incorporated herein by reference in its entirety.

[0176] 10A, 10B, 10C, 10D, 10E Separator 20 Polyolefin microporous film 30 Heat-resistant porous layer 40 Laminate 50 Adhesive layer 52 Adhesive resin particles

Claims

1. A separator for a non-aqueous secondary battery comprising: a polyolefin microporous membrane; a heat-resistant porous layer disposed on one or both sides of the polyolefin microporous membrane, the heat-resistant resin having at least one of an amide bond and an imide bond in its molecule and inorganic particles; and an adhesive layer disposed on one or both sides of a laminate of the polyolefin microporous membrane and the heat-resistant porous layer, the adhesive layer containing adhesive resin particles, wherein the polyolefin microporous membrane has a Gurley value of 99 seconds / 100 mL or less, the non-aqueous secondary battery separator has a Gurley value of 160 seconds / 100 mL or less, and the difference between the Gurley value of the non-aqueous secondary battery separator and the Gurley value of the polyolefin microporous membrane is 20 seconds / 100 mL to 60 seconds / 100 mL.

2. The non-aqueous secondary battery separator according to claim 1, wherein the coverage of the adhesive resin particles on the surface of the non-aqueous secondary battery separator is 20% to 60% on each side on which the adhesive layer is disposed.

3. The non-aqueous secondary battery separator according to claim 1, wherein the thickness of the non-aqueous secondary battery separator is 5 μm to 17 μm.

4. The separator for a non-aqueous secondary battery according to claim 1, wherein the thickness of the polyolefin microporous film is 4 μm to 9 μm.

5. The separator for a non-aqueous secondary battery according to claim 1, wherein the heat-resistant porous layer is disposed on only one side of the polyolefin microporous membrane, and the heat-resistant porous layer has a thickness of 0.5 μm to 4 μm.

6. The separator for a non-aqueous secondary battery according to claim 1, wherein the heat-resistant porous layer is disposed on both sides of the polyolefin microporous membrane, and the total thickness of the heat-resistant porous layer on both sides is 1 μm to 8 μm.

7. The separator for a non-aqueous secondary battery according to claim 1, wherein the pores of the polyolefin microporous film contain a heat-resistant resin having at least one of an amide bond and an imide bond in the molecule.

8. The separator for a non-aqueous secondary battery according to claim 1, wherein the heat-resistant resin comprises at least one selected from the group consisting of wholly aromatic polyamides, polyimides, and polyamideimides.

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

10. The separator for a non-aqueous secondary battery according to claim 1, wherein the adhesive resin particles include at least one selected from the group consisting of acrylic resin particles, polyvinylidene fluoride resin particles, and styrene-butadiene rubber particles.

11. 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 10, disposed between the positive electrode and the negative electrode, wherein an electromotive force is generated by doping and dedoping of lithium ions.

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