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

Abstract

A separator for a non-aqueous secondary battery according to a first embodiment comprises a porous base material and an adhesive porous layer which is disposed on one surface or both surfaces of the porous base material and which contains a polyvinyl acetal resin, wherein the weight-average molecular weight of the polyvinyl acetal resin contained in the adhesive porous layer is 50,000-200,000.　A separator for a non-aqueous secondary battery according to a second embodiment comprises a porous base material and an adhesive porous layer which is disposed on one surface or both surfaces of the porous base material and which contains a polyvinyl acetal resin and inorganic particles, wherein the volume ratio of the inorganic particles with respect to the polyvinyl acetal resin and the inorganic particles contained in the adhesive porous layer is 5-65 vol%.

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

[0003] Separators containing polyvinylidene fluoride resins are known as battery separators. However, as the production and use of organic fluorine compounds are increasingly restricted, there is an urgent need to develop separators with low or no polyvinylidene fluoride resin content.

[0004] Patent Document 1 discloses a separator protective film resin containing a polyvinyl acetal resin having a content of structural units having an alkylene oxide group of 0.01 mol % to 14 mol %, a hydroxyl group content of 66 mol % to 96 mol %, and a polymerization degree of 250 to 5000, and a separator protective film containing the separator protective film resin.

[0005] Japanese Patent Application Laid-Open No. 2020-87928

[0006] To prevent internal short circuits in a battery, separators are required to have adhesive properties to electrodes. While separators containing a polyvinylidene fluoride resin in the surface layer have excellent adhesive properties to electrodes, separators that have excellent adhesive properties to electrodes even if the content of the polyvinylidene fluoride resin in the surface layer is low or even if the surface layer does not contain a polyvinylidene fluoride resin are desired. Furthermore, to prevent internal short circuits in a battery, separators are required to have thermal dimensional stability so that they do not easily shrink even when the temperature inside the battery becomes high.

[0007] The present disclosure has been made under the above circumstances. An object of the present disclosure, as a first embodiment, is to provide a separator for a nonaqueous secondary battery that has excellent adhesion to electrodes. An object of the present disclosure, as a second embodiment, is to provide a separator for a nonaqueous secondary battery that has adhesion to electrodes and excellent thermal dimensional stability.

[0008] Specific means for solving the above problems include the following aspects: <1> A separator for a non-aqueous secondary battery, comprising: a porous substrate; and an adhesive porous layer containing a polyvinyl acetal resin, disposed on one or both sides of the porous substrate, wherein the polyvinyl acetal resin contained in the adhesive porous layer has a weight average molecular weight of 50,000 to 200,000. <2> A separator for a non-aqueous secondary battery, wherein the mass per unit area of ​​the adhesive porous layer is 0.1 g / m per one side of the separator for a non-aqueous secondary battery. ２ ~2.5g / m ２ <3> The separator for a non-aqueous secondary battery according to <1> or <2>, wherein the separator for a non-aqueous secondary battery has an air permeability of 100 sec / 100 mL to 1000 sec / 100 mL. <4> A separator for a non-aqueous secondary battery, comprising: a porous substrate; and an adhesive porous layer containing a polyvinyl acetal resin and inorganic particles, disposed on one or both surfaces of the porous substrate, wherein the volume ratio of the inorganic particles to the polyvinyl acetal resin and the inorganic particles contained in the adhesive porous layer is 5% by volume to 65% by volume. <5> The separator for a non-aqueous secondary battery according to <4>, wherein the polyvinyl acetal resin contained in the adhesive porous layer has a weight average molecular weight of 50,000 to 200,000. <6> The separator for a nonaqueous secondary battery according to <4> or <5>, wherein the inorganic particles contained in the adhesive porous layer have an average primary particle size of 0.01 μm to 2 μm. <7> The separator for a nonaqueous secondary battery according to any one of <4> to <6>, 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. <8> The separator for a nonaqueous secondary battery according to any one of <4> to <6>, wherein the mass per unit area of ​​the adhesive porous layer is 0.5 g / m per one side of the separator for a nonaqueous secondary battery. ２ ~5g / m２ <9> The separator for a nonaqueous secondary battery according to any one of <4> to <8>, wherein the air permeability of the separator for a nonaqueous secondary battery is 100 sec / 100 mL to 500 sec / 100 mL. <10> The separator for a nonaqueous secondary battery according to any one of <1> to <9>, wherein the polyvinyl acetal resin contained in the adhesive porous layer has a degree of acetalization of 50 mol% to 80 mol%. <11> The separator for a nonaqueous secondary battery according to any one of <1> to <10>, wherein the polyvinyl acetal resin contained in the adhesive porous layer has a hydroxyl group content of 20 mol% to 45 mol%. <12> The separator for a nonaqueous secondary battery according to any one of <1> to <11>, wherein the porous substrate comprises a polyolefin microporous membrane. <13> The separator for a non-aqueous secondary battery according to any one of <1> to <12>, wherein the porous substrate comprises: a polyolefin microporous membrane; and a heat-resistant layer containing at least one of inorganic particles and a heat-resistant resin, disposed on one or both sides of the polyolefin microporous membrane. <14> The separator for a non-aqueous secondary battery according to any one of <1> to <13>, wherein the adhesive porous layer is substantially free of a fluorine-containing resin. <15> 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 <14> disposed between the positive electrode and the negative electrode, wherein electromotive force is generated by doping and dedoping of lithium ions.

[0009] According to a first embodiment of the present disclosure, a separator for a nonaqueous secondary battery having excellent adhesion to electrodes is provided. According to a second embodiment of the present disclosure, a separator for a nonaqueous secondary battery having excellent adhesion to electrodes and excellent thermal dimensional stability is provided.

[0010] 10 is an SEM image of the surface of the separator of Example 3. FIG. 11 is an SEM image of the surface of the separator of Example 13.

[0011]

[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.

[0012] 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.

[0013] 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.

[0014] 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.

[0015] 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.

[0016] 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."

[0017] 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."

[0018] In the present disclosure, the volume of the porous layer excluding pores is referred to as the "solid content volume."

[0019] 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."

[0020] In the present disclosure, the term "(meth)acrylic" means either "acrylic" or "methacrylic."

[0021] In the present disclosure, a separator for a nonaqueous secondary battery is also simply referred to as a "separator." The present disclosure discloses a separator according to a first embodiment (also simply referred to as a "first embodiment") and a separator according to a second embodiment (also simply referred to as a "second embodiment"). In the present disclosure, when describing matters common to the separator according to the first embodiment and the separator according to the second embodiment, both are collectively referred to as the "separator of the present disclosure."

[0022] <Separator for non-aqueous secondary battery> [Separator according to first embodiment] The separator according to the first embodiment comprises a porous substrate and an adhesive porous layer containing a polyvinyl acetal resin disposed on one or both sides of the porous substrate, wherein the weight-average molecular weight of the polyvinyl acetal resin contained in the adhesive porous layer is 50,000 to 200,000. The separator according to the first embodiment has excellent adhesion to electrodes due to the adhesive porous layer having the above configuration.

[0023] If the weight-average molecular weight of the polyvinyl acetal resin contained in the adhesive porous layer is less than 50,000, the adhesive porous layer will have insufficient mechanical strength and will be prone to peeling from the electrode. From the viewpoint of ensuring that the adhesive porous layer adheres to the electrode and is not easily peeled off, the weight-average molecular weight of the polyvinyl acetal resin contained in the adhesive porous layer is 50,000 or more, preferably 70,000 or more, and more preferably 90,000 or more.

[0024] If the weight-average molecular weight of the polyvinyl acetal resin exceeds 200,000, the coating liquid used to form the adhesive porous layer will gel, making it difficult to form the adhesive porous layer. From the viewpoint of keeping the viscosity of the coating liquid relatively low and facilitating the formation of the adhesive porous layer by coating, the weight-average molecular weight of the polyvinyl acetal resin contained in the adhesive porous layer is 200,000 or less, preferably 180,000 or less, and more preferably 150,000 or less. Furthermore, if the weight-average molecular weight of the polyvinyl acetal resin is 200,000 or less, high polymer chain mobility upon heating and ease of swelling in the electrolyte are ensured, making it easy for the adhesive porous layer to adhere to the electrode by both dry heat pressing and wet heat pressing. From this viewpoint as well, the weight-average molecular weight of the polyvinyl acetal resin contained in the adhesive porous layer is 200,000 or less, preferably 180,000 or less, and more preferably 150,000 or less.

[0025] The weight average molecular weight of the polyvinyl acetal resin contained in the adhesive porous layer is a molecular weight in terms of polystyrene measured by gel permeation chromatography (GPC). The polyvinyl acetal resin sampled from the adhesive porous layer or the polyvinyl acetal resin used to form the adhesive porous layer is used as a sample.

[0026] The separator according to the first embodiment has an adhesive porous layer on one or both sides of a porous substrate, the adhesive porous layer containing a polyvinyl acetal resin, and the weight-average molecular weight of the entire polyvinyl acetal resin being 50,000 to 200,000. Examples of the embodiment include the following embodiments (1) to (3).

[0027] Form (1): A separator having adhesive porous layers on both sides of a porous substrate as the outermost layers of the separator, the adhesive porous layers containing a polyvinyl acetal resin and having a weight average molecular weight of the entire polyvinyl acetal resin of 50,000 to 200,000. In the separator, the adhesive porous layer on one side and the adhesive porous layer on the other side may be the same or different in components and / or composition.

[0028] Form (2): A separator having an adhesive porous layer containing a polyvinyl acetal resin as the outermost layer of the separator on one side of a porous substrate, the adhesive porous layer containing the polyvinyl acetal resin as a whole having a weight average molecular weight of 50,000 to 200,000, and another layer on the other side of the porous substrate.

[0029] Form (3): A separator having an adhesive porous layer, which contains a polyvinyl acetal resin and has a weight-average molecular weight of the entire polyvinyl acetal resin of 50,000 to 200,000, as the outermost layer of the separator on one side of the porous substrate, and no layer on the other side of the porous substrate (i.e., the surface of the porous substrate is exposed).

[0030] [Separator according to a second embodiment] A separator according to a second embodiment includes a porous substrate and an adhesive porous layer containing a polyvinyl acetal resin and inorganic particles, the adhesive porous layer containing the polyvinyl acetal resin and inorganic particles having a volume ratio of 5 to 65 volume % relative to the volume of the polyvinyl acetal resin and inorganic particles contained in the adhesive porous layer. In other words, the separator according to the second embodiment has a volume ratio of polyvinyl acetal resin to inorganic particles of polyvinyl acetal resin:inorganic particles=95:5 to 35:65.

[0031] The separator according to the second embodiment has excellent adhesiveness to the electrode and thermal dimensional stability, i.e., resistance to thermal shrinkage even at high temperatures, due to the adhesive porous layer having the above-described configuration. If the volume ratio of inorganic particles to the polyvinyl acetal resin and inorganic particles contained in the adhesive porous layer exceeds 65 volume %, the adhesive porous layer is less likely to adhere to the electrode. To ensure adhesion of the adhesive porous layer to the electrode, the volume ratio of inorganic particles to the polyvinyl acetal resin and inorganic particles is 65 volume % or less, preferably 60 volume % or less, and more preferably 55 volume % or less. If the volume ratio of inorganic particles to the polyvinyl acetal resin and inorganic particles contained in the adhesive porous layer is less than 5 volume %, the thermal dimensional stability of the separator is insufficient. From the viewpoint of the thermal dimensional stability of the separator, the volume ratio of inorganic particles to the polyvinyl acetal resin and inorganic particles is 5 volume % or more, preferably 10 volume % or more, and more preferably 15 volume % or more.

[0032] The separator according to the second embodiment has an adhesive porous layer on one or both surfaces of a porous substrate, the adhesive porous layer containing polyvinyl acetal resin and inorganic particles, and the volume ratio of the inorganic particles to the polyvinyl acetal resin and inorganic particles is 5% by volume to 65% by volume. Examples of the embodiment include the following embodiments (11) to (13).

[0033] Form (11): A separator having adhesive porous layers on both sides of a porous substrate as outermost layers of the separator, the adhesive porous layers containing polyvinyl acetal resin and inorganic particles, with the volume ratio of the inorganic particles to the polyvinyl acetal resin and inorganic particles being 5% to 65% by volume. In the separator, the adhesive porous layer on one side and the adhesive porous layer on the other side may be the same or different in components and / or composition.

[0034] Form (12): A separator having an adhesive porous layer on one surface of a porous substrate as the outermost layer of the separator, the adhesive porous layer containing polyvinyl acetal resin and inorganic particles, in which the volume ratio of the inorganic particles to the polyvinyl acetal resin and inorganic particles is 5% by volume to 65% by volume, and another layer on the other surface of the porous substrate.

[0035] Form (13): A separator having an adhesive porous layer on one side of a porous substrate as the outermost layer of the separator, the adhesive porous layer containing polyvinyl acetal resin and inorganic particles, in which the volume ratio of the inorganic particles to the polyvinyl acetal resin and inorganic particles is 5% by volume to 65% by volume, and no layer on the other side of the porous substrate (i.e., the surface of the porous substrate is exposed).

[0036] The porous substrate and adhesive porous layer of the separator of the present disclosure will be described in detail below.

[0037] [Porous substrate] In the present disclosure, the porous substrate refers to a substrate having pores or voids therein. Examples of such substrates include a microporous membrane; a porous sheet made of a fibrous material, such as a nonwoven fabric or paper; and a composite porous sheet obtained by laminating one or more other porous layers on the microporous membrane or porous sheet.

[0038] The material of the porous substrate is preferably an electrically insulating material.

[0039] From the viewpoint of thinning and strength of the separator, the porous substrate is preferably a microporous membrane. A microporous membrane refers to a membrane having a large number of micropores therein, with the micropores interconnected, allowing gas or liquid to pass through from one surface to the other.

[0040] From the viewpoint of thermal dimensional stability, the porous substrate is preferably a composite porous substrate in which one or more porous heat-resistant layers are laminated on a microporous membrane. The porous heat-resistant layer is preferably a heat-resistant layer containing at least one of inorganic particles and a heat-resistant resin. In the present disclosure, the 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, the heat-resistant resin in the present disclosure refers to a resin that does not melt or decompose in a temperature range below 200°C.

[0041] 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.

[0042] As the porous substrate, from the viewpoint of imparting a shutdown function to the porous substrate, a porous substrate containing a microporous membrane containing polyolefin (referred to as "polyolefin microporous membrane" in the present disclosure) is preferred. Porous substrates containing a polyolefin microporous membrane include porous substrates consisting only of a polyolefin microporous membrane (i.e., a polyolefin microporous membrane) and composite porous substrates in which a porous heat-resistant layer is disposed on one or both sides of a polyolefin microporous membrane. Here, the porous heat-resistant layer is preferably a heat-resistant layer containing at least one of inorganic particles and a heat-resistant resin.

[0043] Examples of the polyolefin microporous membrane include polyolefin microporous membranes that are used in conventional battery separators, and it is preferable to select one from these that has sufficient mechanical properties and ion permeability.

[0044] 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.

[0045] 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.

[0046] 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. Examples of the polyolefin microporous membrane containing polyethylene and polypropylene include 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.

[0047] 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.

[0048] 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.

[0049] Examples of porous sheets made of fibrous materials include porous sheets such as nonwoven fabrics and paper. Examples of fibrous materials include polyesters such as polyethylene terephthalate, polyolefins such as polyethylene and polypropylene, heat-resistant resins such as wholly aromatic polyamide, polyamideimide, polyimide, polyethersulfone, polysulfone, polyetherketone, and polyetherimide, and cellulose.

[0050] An example of a composite porous sheet is a sheet in which a functional layer is laminated on a microporous membrane or a porous sheet made of a fibrous material. Such a composite porous sheet is preferable from the viewpoint that the functional layer can add further functions. For example, an example of the functional layer is a porous heat-resistant layer from the viewpoint of imparting heat resistance to the composite porous sheet. Methods for combining a microporous membrane or a porous sheet with a functional layer include a method of coating the functional layer on the surface of the microporous membrane or porous sheet, a method of bonding the microporous membrane or porous sheet and the functional layer with an adhesive, and a method of thermocompression bonding the microporous membrane or porous sheet and the functional layer.

[0051] An example of an embodiment of a composite porous sheet is a composite porous substrate comprising a polyolefin microporous membrane and a heat-resistant layer containing at least one of inorganic particles and a heat-resistant resin, disposed on one or both sides of the polyolefin microporous membrane. The heat-resistant layer is a porous layer. Examples of inorganic particles include metal oxide particles (silica, alumina, boehmite, titania, zirconia, magnesium oxide, barium oxide, etc.), metal hydroxide particles (magnesium hydroxide, aluminum hydroxide, calcium hydroxide, chromium hydroxide, zirconium hydroxide, cerium hydroxide, nickel hydroxide, boron hydroxide, etc.), metal sulfate particles (barium sulfate, calcium sulfate, etc.), metal carbonate particles (calcium carbonate, magnesium carbonate, barium carbonate, etc.), metal nitride particles (boron nitride, aluminum nitride, etc.), and clay mineral particles (calcium silicate, talc, etc.). The inorganic particles may be surface-modified with a silane coupling agent or the like. Examples of heat-resistant resins include wholly aromatic polyamide, polyamideimide, polyimide, polyethersulfone, polysulfone, polyetherketone, and polyetherimide.

[0052] When the composite porous substrate contains inorganic particles in the heat-resistant layer, the heat-resistant layer preferably also contains a binder resin that binds the inorganic particles. The binder resin may be a heat-resistant resin or a non-heat-resistant resin. Examples of non-heat-resistant resins include butadiene-based polymers (e.g., butadiene homopolymers, styrene-butadiene copolymers), and acrylic resins (e.g., homopolymers or copolymers of acrylic monomers, copolymers of acrylic monomers and styrene monomers).

[0053] A method for disposing a heat-resistant layer containing at least one of inorganic particles and a heat-resistant resin on one or both sides of a polyolefin microporous membrane includes coating one or both sides of the polyolefin microporous membrane with a coating liquid containing at least one of inorganic particles and a heat-resistant resin.

[0054] In this disclosure, the term "porous substrate" includes "composite porous substrate."

[0055] The surface of the porous substrate may be subjected to various surface treatments to improve wettability with the coating liquid for forming the adhesive porous layer, as long as the properties of the porous substrate are not impaired. Examples of surface treatments include corona treatment, plasma treatment, flame treatment, and ultraviolet irradiation treatment.

[0056] -Characteristics of porous substrate- From the viewpoint of mechanical strength, the thickness of the porous substrate is preferably 1 μm or more, more preferably 3 μm or more, and even more preferably 5 μm or more. From the viewpoint of increasing the energy density of the battery, the thickness of the porous substrate is preferably 10 μm or less, more preferably 9 μm or less, and even more preferably 8 μm or less. The thickness of the porous substrate is measured at 20 points within a 10 cm square using a contact thickness meter and the average is determined.

[0057] The air permeability of the porous substrate is preferably 50 seconds / 100 mL or more, more preferably 70 seconds / 100 mL or more, and even more preferably 90 seconds / 100 mL or more from the viewpoint of suppressing internal short circuits in the battery. The air permeability of the porous substrate is preferably 220 seconds / 100 mL or less, more preferably 200 seconds / 100 mL or less, and even more preferably 180 seconds / 100 mL or less from the viewpoint of excellent electrolyte permeability and ion permeability. The air permeability of the porous substrate is measured using a digital Oken air permeability tester in accordance with JIS P8117:2009.

[0058] When the porous substrate is a porous substrate consisting solely of a polyolefin microporous membrane (i.e., a polyolefin microporous membrane), the air permeability is preferably 50 seconds / 100 mL to 180 seconds / 100 mL, more preferably 70 seconds / 100 mL to 160 seconds / 100 mL, and even more preferably 90 seconds / 100 mL to 140 seconds / 100 mL. When the porous substrate is a composite porous substrate in which a porous heat-resistant layer is disposed on one or both sides of a polyolefin microporous membrane, the air permeability is preferably 90 seconds / 100 mL to 220 seconds / 100 mL, more preferably 100 seconds / 100 mL to 210 seconds / 100 mL, and even more preferably 110 seconds / 100 mL to 200 seconds / 100 mL.

[0059] The porosity of the porous substrate is preferably 30% to 60% from the viewpoint of excellent electrolyte permeability and ion permeability. The porosity ε (%) of the porous substrate is calculated by the following formula: ε = {1 - Ws / (ds·t)} × 100, 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.

[0060] [Adhesive Porous Layer] The adhesive porous layer is a layer disposed on the surface of the porous substrate and is the outermost layer of the separator. The adhesive porous layer has a large number of gaps or micropores, allowing gas or liquid to pass through from one surface to the other.

[0061] In the first embodiment, the adhesive porous layer preferably has a mesh-like structure. The mesh-like structure of the adhesive porous layer means a structure in which the resin is continuously connected in a network form and has a large number of pores. The mesh-like structure of the adhesive porous layer may be a planar mesh-like structure in the surface direction of the separator, or a three-dimensional mesh-like structure in the surface direction and thickness direction of the separator. The three-dimensional mesh-like structure of the adhesive porous layer may be flattened by heat pressing to bond the separator to the electrode, and part or all of the separator in the state bonded to the electrode may have a planar mesh-like structure.

[0062] Examples of the network structure of the adhesive porous layer include a porous structure in which fibrils containing polyvinyl acetal resin are connected in a two-dimensional or three-dimensional network, and a network-like microporous structure containing polyvinyl acetal resin.

[0063] The network structure of the adhesive porous layer can be confirmed by observing the surface of the separator with a scanning electron microscope (SEM).

[0064] In the second embodiment, the adhesive porous layer contains a polyvinyl acetal resin and inorganic particles. Examples of the adhesive porous layer include a structure in which inorganic particles are bound or trapped in a porous structure in which fibrils containing a polyvinyl acetal resin 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 polyvinyl acetal resin, and a layer structure in which a large number of inorganic particles are connected to each other by the polyvinyl acetal resin to form voids between the inorganic particles.

[0065] -Polyvinyl acetal resin- A polyvinyl acetal resin is a resin obtained by reacting polyvinyl alcohol with an aldehyde in the presence of an acid catalyst to acetalize some or most of the hydroxyl groups of the polyvinyl alcohol. Thus, a polyvinyl acetal resin has a structural unit represented by the following formula (1) and a structural unit represented by the following formula (2). In the present disclosure, the structural unit represented by the following formula (1) is also referred to as a "structural unit having an acetal group," and the structural unit represented by the following formula (2) is also referred to as a "structural unit having a hydroxyl group."

[0066]

[0067] In formula (1), R is an arbitrary group derived from an aldehyde. The structural unit represented by formula (1) is a vinyl alcohol unit (—CH ２ The structural unit represented by formula (2) is formed by acetalization of two vinyl alcohol units (-CHOH-) of polyvinyl alcohol. ２ The structural unit represented by formula (1) is a unit that has not been acetalized among the vinyl alcohol units (-CHOH-). Since the structural unit represented by formula (1) is formed by acetalizing two vinyl alcohol units, one structural unit represented by formula (1) is counted as two structural units in calculating the proportions of various structural units (i.e., the degree of acetalization, the amount of hydroxyl groups, the amount of acetyl groups, and the amount of other structural units; unit: mol %).

[0068] The polyvinyl acetal resin is preferably a resin obtained by reacting polyvinyl alcohol with at least one selected from the group consisting of formaldehyde and alkylaldehyde in the presence of an acid catalyst to acetalize a portion or most of the hydroxyl groups of the polyvinyl alcohol. Therefore, R in formula (1) is preferably a hydrogen atom or an alkyl group. Here, the alkylaldehyde is preferably an alkylaldehyde having an alkyl group containing 1 to 4 carbon atoms, more preferably an alkylaldehyde having a linear alkyl group containing 1 to 4 carbon atoms. Therefore, when R in formula (1) is an alkyl group, it is preferably an alkyl group containing 1 to 4 carbon atoms, more preferably a linear alkyl group containing 1 to 4 carbon atoms. Examples of alkylaldehydes having an alkyl group containing 1 to 4 carbon atoms include acetaldehyde, propionaldehyde, n-butyraldehyde, isobutyraldehyde, n-valeraldehyde, isovaleraldehyde, and mixtures thereof. When acetaldehyde is used, R in formula (1) is a methyl group. When propionaldehyde is used, R in formula (1) is an ethyl group. When n-butylaldehyde is used, R in formula (1) is an n-propyl group. When isobutyraldehyde is used, R in formula (1) is an isopropyl group. When n-valeraldehyde is used, R in formula (1) is an n-butyl group. When isovaleraldehyde is used, R in formula (1) is an isobutyl group.

[0069] The polyvinyl acetal resin preferably has an acetalization degree of 50 mol% to 80 mol%, more preferably 55 mol% to 78 mol%, and even more preferably 60 mol% to 75 mol%. When the polyvinyl acetal resin has an acetalization degree of 50 mol% or more, the coating liquid used to form the adhesive porous layer is easily prepared. From this viewpoint, the acetalization degree of the polyvinyl acetal resin is more preferably 55 mol% or more, and even more preferably 60 mol% or more. When the acetalization degree of the polyvinyl acetal resin is 80 mol% or less, the polyvinyl acetal resin is less likely to dissolve into the electrolyte solution. From this viewpoint, the acetalization degree of the polyvinyl acetal resin is more preferably 78 mol% or less, and even more preferably 75 mol% or less. The acetalization degree of the polyvinyl acetal resin is the proportion (mol%) of structural units having an acetal group to all structural units of the polyvinyl acetal resin. The degree of acetalization of the polyvinyl acetal resin can be measured by NMR (Nuclear Magnetic Resonance).

[0070] The polyvinyl acetal resin preferably has a hydroxyl group content of 20 mol% to 45 mol%, more preferably 22 mol% to 42 mol%, and even more preferably 25 mol% to 40 mol%. When the hydroxyl group content of the polyvinyl acetal resin is 20 mol% or more, the polyvinyl acetal resin is less likely to dissolve in the electrolyte solution. From this viewpoint, the hydroxyl group content of the polyvinyl acetal resin is more preferably 22 mol% or more, and even more preferably 25 mol% or more. When the hydroxyl group content of the polyvinyl acetal resin is 45 mol% or less, the coating liquid used to form the adhesive porous layer is easily prepared. From this viewpoint, the hydroxyl group content of the polyvinyl acetal resin is more preferably 42 mol% or less, and even more preferably 40 mol% or less. The hydroxyl group content of the polyvinyl acetal resin is the proportion (mol%) of structural units having hydroxyl groups to all structural units of the polyvinyl acetal resin. The hydroxyl group content of the polyvinyl acetal resin can be measured by NMR.

[0071] The sum of the acetalization degree and the hydroxyl group content in the polyvinyl acetal resin is preferably 75 mol% or more, more preferably 85 mol% or more, and even more preferably 95 mol% or more, and is, for example, 100 mol% or less, or 99 mol% or less.

[0072] The polyvinyl acetal resin may have a structural unit represented by the following formula (3): Since polyvinyl alcohol, which is a raw material for the polyvinyl acetal resin, may have a small amount of acetyl groups, the polyvinyl acetal resin may have a structural unit represented by the following formula (3): In the present disclosure, the structural unit represented by the following formula (3) is also referred to as a "structural unit having an acetyl group."

[0073]

[0074] The amount of acetyl groups in the polyvinyl acetal resin is, for example, 0 mol % to 10 mol %, 0 mol % to 5 mol %, or 0 mol % to 1 mol %. The amount of acetyl groups in the polyvinyl acetal resin is the proportion (mol %) of structural units having acetyl groups to all structural units of the polyvinyl acetal resin. The amount of acetyl groups in the polyvinyl acetal resin can be measured by NMR.

[0075] The polyvinyl acetal resin may contain structural units other than structural units having an acetal group, structural units having a hydroxyl group, and structural units having an acetyl group. Examples of such structural units include structural units having an alkylene oxide group. Examples of the alkylene oxide group of the structural unit having an alkylene oxide group include an ethylene oxide group, a propylene oxide group, a butylene oxide group, a pentylene oxide group, and a hexylene oxide group. The proportion of structural units having alkylene oxide groups in all structural units of the polyvinyl acetal resin is, for example, 0 mol % to 15 mol %. The proportion of structural units having alkylene oxide groups in all structural units of the polyvinyl acetal resin can be measured by NMR.

[0076] The polyvinyl acetal resin may have a structural unit having an ionic functional group. Examples of the ionic functional group of the structural unit include a carboxy group, a sulfonic acid group, a phosphoric acid group, an amino group, and salts thereof (e.g., sodium salts and potassium salts).

[0077] The adhesive porous layer may contain one type of polyvinyl acetal resin or two or more types of polyvinyl acetal resins.

[0078] As the polyvinyl acetal resin, polyvinyl butyral resin is preferred from the viewpoint of adhesion of the adhesive porous layer to the electrode. Polyvinyl butyral resin is a resin obtained by reacting polyvinyl alcohol with butyralaldehyde in the presence of an acid catalyst to butyralize a portion or most of the hydroxyl groups of the polyvinyl alcohol. Here, the butyralaldehyde is at least one of n-butyralaldehyde and isobutyralaldehyde, and n-butyralaldehyde is preferred. The polyvinyl butyral resin preferably has a butyralization degree of 50 mol% to 80 mol%, more preferably 55 mol% to 78 mol%, and even more preferably 60 mol% to 75 mol%. The polyvinyl butyral resin preferably has a hydroxyl group content of 20 mol% to 45 mol%, more preferably 22 mol% to 42 mol%, and even more preferably 25 mol% to 40 mol%. The sum of the butyralization degree and the hydroxyl group amount in the polyvinyl butyral resin is preferably 75 mol% or more, more preferably 85 mol% or more, and even more preferably 95 mol% or more. The sum of the butyralization degree and the hydroxyl group amount in the polyvinyl butyral resin is, for example, 100 mol% or less, or 99 mol% or less. The polyvinyl butyral resin may have a structural unit having an acetyl group. The acetyl group amount in the polyvinyl butyral resin is, for example, 0 mol% to 10 mol%, 0 mol% to 5 mol%, or 0 mol% to 1 mol%.

[0079] The weight-average molecular weight (Mw) of the entire polyvinyl acetal resin contained in the adhesive porous layer is 50,000 to 200,000, preferably 70,000 to 180,000, and more preferably 90,000 to 150,000. When the Mw of the polyvinyl acetal resin is 50,000 or more, the adhesive porous layer has excellent mechanical strength and is less likely to peel off from the electrode. From this perspective, the Mw of the polyvinyl acetal resin is 50,000 or more, preferably 70,000 or more, and more preferably 90,000 or more. When the Mw of the polyvinyl acetal resin is 200,000 or less, high polymer chain mobility upon heating and ease of swelling in the electrolyte are ensured, making the adhesive porous layer easily adhered to the electrode by both dry heat pressing and wet heat pressing. Furthermore, when the Mw of the polyvinyl acetal resin is 200,000 or less, the viscosity of the coating liquid used to form the adhesive porous layer is relatively low, resulting in excellent coating properties of the coating liquid. Furthermore, when the Mw of the polyvinyl acetal resin is 200,000 or less, a porous structure is easily formed in the adhesive porous layer, and the adhesive porous layer has excellent electrolyte permeability and ion permeability. From these viewpoints, the Mw of the polyvinyl acetal resin is 200,000 or less, preferably 180,000 or less, and more preferably 150,000 or less.

[0080] When the adhesive porous layer contains a polyvinyl butyral resin, the weight average molecular weight (Mw) of the entire polyvinyl butyral resin contained in the adhesive porous layer is preferably 50,000 to 200,000, more preferably 70,000 to 180,000, and even more preferably 90,000 to 150,000.

[0081] In the first embodiment, the mass proportion of the polyvinyl acetal resin in the adhesive porous layer is preferably 90 mass% or more, more preferably 95 mass% or more, and even more preferably 99 mass% or more, from the viewpoint of the adhesion of the layer to the electrode.

[0082] In the first embodiment, when the adhesive porous layer contains a polyvinyl butyral resin, the mass proportion of the polyvinyl butyral resin in the adhesive porous layer is preferably 90 mass% or more, more preferably 95 mass% or more, and even more preferably 99 mass% or more, from the viewpoint of the adhesion of the layer to the electrode.

[0083] In the second embodiment, the volume ratio of the polyvinyl acetal resin to the solid content volume of the adhesive porous layer is preferably 35% by volume to 95% by volume, more preferably 40% by volume to 90% by volume, and even more preferably 45% by volume to 85% by volume, from the viewpoint of achieving a good balance between the adhesiveness of the separator to the electrodes and the thermal dimensional stability of the separator.

[0084] In the second embodiment, when the adhesive porous layer contains a polyvinyl butyral resin, the volume ratio of the polyvinyl butyral resin to the solid volume of the adhesive porous layer is preferably 35% by volume to 95% by volume, more preferably 40% by volume to 90% by volume, and even more preferably 45% by volume to 85% by volume, from the viewpoint of achieving a good balance between the adhesiveness of the separator to the electrodes and the thermal dimensional stability of the separator.

[0085] When adhesive porous layers are present on both sides of the porous substrate, the type and / or content of the polyvinyl acetal resin contained in one adhesive porous layer may be the same as or different from the type and / or content of the polyvinyl acetal resin contained in the other adhesive porous layer.

[0086] -Other Resins- The adhesive porous layer may contain other resins in addition to polyvinyl acetal resin. Examples of other resins include acrylic resins, butadiene-acrylonitrile resins, homopolymers or copolymers of vinyl nitrile compounds (such as acrylonitrile and methacrylonitrile), carboxymethyl cellulose, hydroxyalkyl cellulose, polyvinyl alcohol, polyvinylpyrrolidone, 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.

[0087] The mass proportion of other resins in the total resin of the adhesive porous layer is preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 1% by mass or less. The mass proportion of polyvinyl acetal resin in the total resin of the adhesive porous layer is preferably 90% by mass or more, more preferably 95% by mass or more, even more preferably 99% by mass or more, and particularly preferably 100% by mass. When the adhesive porous layer contains a polyvinyl butyral resin, the mass proportion of polyvinyl butyral resin in the total resin of the adhesive porous layer is preferably 90% by mass or more, more preferably 95% by mass or more, even more preferably 99% by mass or more, and particularly preferably 100% by mass.

[0088] Preferably, the adhesive porous layer does not substantially contain fluorine-containing resin.Examples of fluorine-containing resins include polyvinylidene fluoride resins and fluorine-containing rubber.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 other monomers other than halogen-containing monomers; copolymers of vinylidene fluoride, halogen-containing monomers, and other monomers other than halogen-containing monomers; and mixtures thereof.

[0089] The adhesive porous layer being substantially free of a fluorine-containing resin means that the mass proportion of the fluorine-containing resin in the adhesive porous layer is 1 mass% or less. The mass proportion of the fluorine-containing resin in the adhesive porous layer is preferably as small as possible, and is preferably 0.5 mass% or less, more preferably 0.1 mass% or less, and particularly preferably 0 mass%. In other words, it is particularly preferable that the adhesive porous layer is free of a fluorine-containing resin.

[0090] Particles In the first embodiment, the adhesive porous layer may further contain particles. Examples of the particles include organic particles and / or inorganic particles. When the adhesive porous layer contains particles in the first embodiment, the volume ratio of the particles to the polyvinyl acetal resin and particles contained in the adhesive porous layer is preferably 65% ​​by volume or less, more preferably 60% by volume or less, even more preferably 55% by volume or less, and even more preferably 50% by volume or less, from the viewpoint of the adhesion of the adhesive porous layer to the electrode. In the first embodiment, the volume ratio of the particles to the polyvinyl acetal resin and particles contained in the adhesive porous layer is, for example, 0% by volume to 65% by volume, 0% by volume to 60% by volume, 0% by volume to 55% by volume, or 0% by volume to 50% by volume.

[0091] An example of the adhesive porous layer in the first embodiment is a layer that does not substantially contain particles. The adhesive porous layer being substantially free of particles means that the volume ratio of particles to the polyvinyl acetal resin and particles contained in the adhesive porous layer is less than 5 volume %. In this embodiment, the volume ratio of particles to the polyvinyl acetal resin and particles contained in the adhesive porous layer is 0 volume % or more and less than 5 volume %.

[0092] -Organic Particles- The adhesive 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 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.

[0093] The organic particles may be used alone or in combination of two or more kinds.

[0094] In the first embodiment, when the adhesive porous layer contains organic particles, the volume ratio of the organic particles to the polyvinyl acetal resin and organic particles contained in the adhesive porous layer is preferably 65% ​​by volume or less, more preferably 60% by volume or less, even more preferably 55% by volume or less, and even more preferably 50% by volume or less, from the viewpoint of the adhesiveness of the adhesive porous layer to the electrode. The volume ratio of the organic particles to the polyvinyl acetal resin and organic particles contained in the adhesive porous layer is, for example, 0% by volume to 65% by volume, 0% by volume to 60% by volume, 0% by volume to 55% by volume, or 0% by volume to 50% by volume.

[0095] An example of the adhesive porous layer in the first embodiment is a layer that does not substantially contain organic particles. The adhesive porous layer being substantially free of organic particles means that the volume ratio of organic particles to the polyvinyl acetal resin and organic particles contained in the adhesive porous layer is less than 5 volume %. In this embodiment, the volume ratio of organic particles to the polyvinyl acetal resin and organic particles contained in the adhesive porous layer is 0 volume % or more and less than 5 volume %.

[0096] - Inorganic Particles - The adhesive porous layer in the first embodiment may contain inorganic particles. The adhesive porous layer in the second embodiment contains inorganic particles.

[0097] Examples of inorganic particles include metal oxide particles, metal hydroxide particles, metal sulfate particles, metal carbonate particles, metal nitride particles, and clay mineral particles.

[0098] Examples of metal oxides constituting the metal oxide particles include silica (silicon dioxide), alumina (aluminum oxide), boehmite (alumina monohydrate), titania (titanium oxide), zirconia (zirconium oxide), magnesium oxide, 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.

[0099] The inorganic particles may be surface-modified with a silane coupling agent or the like.

[0100] The inorganic particles may be used alone or in combination of two or more kinds.

[0101] 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.

[0102] 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.

[0103] 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 circuits in the battery, the inorganic particles contained in the adhesive porous layer are preferably plate-like particles or non-aggregated primary particles.

[0104] The average primary particle size of the inorganic particles contained in the adhesive porous layer is preferably 0.01 μm to 2 μm, more preferably 0.05 μm to 1 μm, and even more preferably 0.1 μm to 0.5 μm. When the average primary particle size of the inorganic particles is 0.01 μm or more, a porous structure is easily formed in the adhesive porous layer, and the adhesive porous layer has excellent electrolyte permeability and ion permeability. From this perspective, the average primary particle size of the inorganic particles is more preferably 0.05 μm or more, and even more preferably 0.1 μm or more. When the average primary particle size of the inorganic particles is 2 μm or less, the adhesive porous layer easily adheres to the electrode and is less likely to peel off from the electrode. From this perspective, the average primary particle size of the inorganic particles is more preferably 1 μm or less, and even more preferably 0.5 μm or less.

[0105] The average primary particle size of inorganic particles is determined by measuring the long diameters of 100 randomly selected inorganic particles in scanning electron microscope (SEM) observation 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 adhesive porous layer, or inorganic particles removed from the adhesive porous layer. There are no limitations on the method for removing inorganic particles from the adhesive porous layer. Examples of such methods include immersing the adhesive porous layer peeled off from the separator in an organic solvent that dissolves the binder resin to remove the inorganic particles; or heating the adhesive porous layer peeled off from the separator to about 800°C to remove the binder resin and remove the inorganic particles.

[0106] In the first embodiment, when the adhesive porous layer contains inorganic particles, the volume ratio of the inorganic particles to the polyvinyl acetal resin and inorganic particles contained in the adhesive porous layer is preferably 65% ​​by volume or less, more preferably 60% by volume or less, even more preferably 55% by volume or less, and even more preferably 50% by volume or less, from the viewpoint of the adhesiveness of the adhesive porous layer to the electrode. The volume ratio of the inorganic particles to the polyvinyl acetal resin and inorganic particles contained in the adhesive porous layer is, for example, 0 to 65% by volume, 0 to 60% by volume, 0 to 55% by volume, or 0 to 50% by volume.

[0107] An example of the adhesive porous layer in the first embodiment is a layer that does not substantially contain inorganic particles. The adhesive porous layer being substantially free of inorganic particles means that the volume ratio of inorganic particles to the polyvinyl acetal resin and inorganic particles contained in the adhesive porous layer is less than 5 volume %. In this embodiment, the volume ratio of inorganic particles to the polyvinyl acetal resin and inorganic particles contained in the adhesive porous layer is 0 volume % or more and less than 5 volume %.

[0108] In the second embodiment, the volume ratio of the inorganic particles to the solid content volume of the adhesive porous layer is preferably 5 vol% to 65 vol%, more preferably 10 vol% to 60 vol%, and even more preferably 15 vol% to 55 vol%, from the viewpoint of achieving a good balance between the adhesiveness of the separator to the electrode and the thermal dimensional stability of the separator.

[0109] When adhesive porous layers are present on both sides of the porous substrate, the type and / or content of inorganic particles contained in one adhesive porous layer may be the same as or different from the type and / or content of inorganic particles contained in the other adhesive porous layer.

[0110] -Other Components- The adhesive porous layer may contain additives such as a dispersant such as a surfactant, a wetting agent, an antifoaming agent, and a pH adjuster. Dispersants are added, for example, to a coating liquid for forming the adhesive porous layer for the purpose of improving dispersibility, coatability, or storage stability. Wetting agents, antifoaming agents, and pH adjusters are added, for example, to a coating liquid for forming the adhesive porous layer for the purpose of improving compatibility with the porous substrate, preventing air entrapment in the coating liquid, or adjusting the pH.

[0111] -Characteristics of adhesive porous layer- In the first embodiment, the mass per unit area of ​​the adhesive porous layer is 0.1 g / m per one side of the separator. ２ ~2.5g / m ２ It is preferable that the density is 0.2 g / m ２ ~2.0 g / m ２ More preferably, 0.3 g / m ２ ~1.5g / m ２It is more preferable that the mass per unit area of ​​the adhesive porous layer is 0.1 g / m per one side of the separator. ２ From this viewpoint, the mass per unit area of ​​the adhesive porous layer is 0.2 g / m per one side of the separator. ２ More preferably, it is 0.3 g / m or more. ２ It is more preferable that the mass per unit area of ​​the adhesive porous layer is 2.5 g / m or more per one side of the separator. ２ From this viewpoint, the mass per unit area of ​​the adhesive porous layer is 2.0 g / m per one side of the separator. ２ More preferably, it is 1.5 g / m or less. ２ It is more preferable that:

[0112] In the first embodiment, when the adhesive porous layer is on both sides of the separator, the mass per unit area of ​​the adhesive porous layer is 0.2 g / m in total on both sides. ２ ~5g / m ２ is preferred, and 0.4 g / m ２ ~4g / m ２ More preferably, 0.6 g / m ２ ~3g / m ２ is more preferred.

[0113] In the second embodiment, the mass per unit area of ​​the adhesive porous layer is 0.5 g / m per one side of the separator. ２ ~5g / m ２ It is preferable that the density is 0.6 g / m ２ ~3g / m ２ More preferably, 0.7 g / m ２ ~2g / m ２ It is more preferable that the mass per unit area of ​​the adhesive porous layer is 0.5 g / m per one side of the separator. ２ From this viewpoint, the mass per unit area of ​​the adhesive porous layer is 0.6 g / m per one side of the separator. ２ More preferably, it is 0.7 g / m or more. ２It is more preferable that the mass per unit area of ​​the adhesive porous layer is 5 g / m or more per one side of the separator. ２ From this viewpoint, the mass per unit area of ​​the adhesive porous layer is 3 g / m or less per one side of the separator, and the permeability of the electrolyte solution and the ion permeability are excellent. ２ More preferably, it is 2 g / m or less. ２ It is more preferable that:

[0114] In the second embodiment, when the adhesive porous layer is present on both sides of the separator, the mass per unit area of ​​the adhesive porous layer is 1.0 g / m2 in total. ２ ~10g / m ２ is preferred, and 1.2 g / m ２ ~6g / m ２ More preferably, 1.4 g / m ２ ~4g / m ２ is more preferred.

[0115] The mass per unit area of ​​the adhesive porous layer is determined by cutting the separator into a 20 cm x 20 cm piece, peeling off the adhesive porous layer, measuring the mass, and dividing the mass by the area.

[0116] [Separator Properties] From the viewpoint of mechanical strength, the thickness of the separator is preferably 5 μm or more, more preferably 7 μm or more, and even more preferably 9 μm or more. From the viewpoint of the energy density of the battery, the thickness of the separator is preferably 15 μm or less, more preferably 12 μm or less, and even more preferably 10 μm or less. The thickness of the separator is determined by measuring 20 points within a 10 cm square using a contact thickness meter and averaging the measurements.

[0117] The separator according to the first embodiment has an air permeability of preferably 100 sec / 100 mL or more, more preferably 120 sec / 100 mL or more, and even more preferably 150 sec / 100 mL or more from the viewpoint of suppressing an internal short circuit in the battery. The separator according to the first embodiment has an air permeability of preferably 1000 sec / 100 mL or less, more preferably 700 sec / 100 mL or less, and even more preferably 600 sec / 100 mL or less from the viewpoint of ion permeability.

[0118] The separator according to the second embodiment has an air permeability of preferably 100 sec / 100 mL or more, more preferably 110 sec / 100 mL or more, and even more preferably 120 sec / 100 mL or more from the viewpoint of suppressing an internal short circuit in the battery. The separator according to the second embodiment has an air permeability of preferably 500 sec / 100 mL or less, more preferably 400 sec / 100 mL or less, and even more preferably 300 sec / 100 mL or less from the viewpoint of ion permeability.

[0119] The air permeability of the separator is measured using a digital Oken air permeability tester in accordance with JIS P8117:2009.

[0120] From the viewpoint of ion permeability, the porosity of the separator is preferably 30% to 60%. The porosity ε (%) of the separator is calculated by the following formula:

[0121]

[0122] Here, for the separator's constituent material 1, constituent material 2, constituent material 3, ..., constituent material n, the mass per unit area of ​​each constituent material is W １ , W ２、 W ３ , ..., W ｎ (g / cm ２ ) and the true density of each constituent material is d １ , d ２ , d ３ , ..., d ｎ (g / cm ３ ) and the thickness of the separator is t (cm).

[0123] [Method for Manufacturing Separator] The separator of the present disclosure can be manufactured, for example, by forming an adhesive porous layer on a porous substrate by a wet coating method or a dry coating method. In the present disclosure, the wet coating method is a method in which a coating layer is solidified in a coagulation liquid, and the dry coating method is a method in which a coating layer is solidified by drying. An embodiment of the wet coating method will be described below. In the following description, the "adhesive porous layer" will be simply referred to as the "porous layer."

[0124] The wet coating method includes, for example, a step of applying a coating liquid to one or both sides of a porous substrate to form a coating layer, a step of immersing the porous substrate having the coating layer in a coagulation liquid to solidify the coating layer and form a porous layer, and a step of lifting the laminate consisting of the porous substrate and the porous layer out of the coagulation liquid, washing with water, and drying.

[0125] The coating liquid for forming the porous layer according to the first embodiment is prepared by dissolving a polyvinyl acetal resin in a solvent. If necessary, other components besides the polyvinyl acetal resin may be dissolved or dispersed in the coating liquid.

[0126] The coating liquid for forming the porous layer according to the second embodiment is prepared by dissolving and dispersing a polyvinyl acetal resin and inorganic particles in a solvent. If necessary, other components besides the polyvinyl acetal resin and inorganic particles may be dissolved or dispersed in the coating liquid.

[0127] The solvent used in preparing the coating solution includes a solvent that dissolves the polyvinyl acetal resin (hereinafter also referred to as a "good solvent"). Examples of the good solvent include polar amide solvents such as N-methylpyrrolidone, dimethylacetamide, and dimethylformamide.

[0128] The solvent used to prepare the coating solution may contain a phase separation agent that induces phase separation in order to form a porous layer with a good porous structure. Therefore, the solvent used to prepare the coating solution may be 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, butanediol, and ethylene glycol.

[0129] When the solvent used to prepare the coating liquid is a mixed solvent of a good solvent and a phase separation agent, from the viewpoint of forming a good porous structure, a mixed solvent containing 60% by mass or more of the good solvent and 5% by mass to 40% by mass of the phase separation agent is preferred.

[0130] The resin concentration of the coating liquid is preferably 1% by mass to 20% by mass from the viewpoint of forming a good porous structure. When the coating liquid contains inorganic particles, the inorganic particle concentration of the coating liquid is preferably 0.5% by mass to 50% by mass from the viewpoint of forming a good porous structure.

[0131] The coating liquid may contain a dispersant such as a surfactant, a wetting agent, an antifoaming agent, a pH adjuster, etc. These additives may remain in the porous layer as long as they are electrochemically stable within the range of use of the nonaqueous secondary battery and do not inhibit the reaction within the battery.

[0132] Examples of means for applying the coating liquid to the porous substrate include a Mayer bar, a die coater, a reverse roll coater, a roll coater, a gravure coater, etc. When forming a porous layer on both sides of the porous substrate, it is preferable from the viewpoint of productivity to apply the coating liquid to both sides of the porous substrate simultaneously.

[0133] The coating layer is solidified by immersing the porous substrate with the coating layer formed thereon in a coagulation liquid to induce phase separation in the coating layer while solidifying the resin, thereby obtaining a laminate consisting of the porous substrate and the porous layer.

[0134] The coagulation liquid generally contains the good solvent and phase separation agent used in preparing the coating liquid, as well as water. It is preferable from a production standpoint 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.

[0135] After the coating layer is solidified in the coagulating liquid, the laminate is lifted out of the coagulating liquid and washed with water. The coagulating liquid is removed from the laminate by washing with water. The water is then removed from the laminate by drying. 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.

[0136] The separator of the present disclosure can also be manufactured by a dry coating method, which is a method of applying a coating liquid to a porous substrate and drying the coating layer to volatilize and remove the solvent, thereby forming a porous layer on the porous substrate.

[0137] The separator of the present disclosure can also be produced by a method in which the porous layer is produced as an independent sheet, and this porous layer is then superimposed on a porous substrate and combined with thermocompression bonding or an adhesive. Examples of a method for producing the porous layer as an independent sheet include a method in which the porous layer is formed on a release sheet by applying the above-mentioned wet coating method or dry coating method.

[0138] <Non-aqueous secondary battery> The present disclosure discloses a non-aqueous secondary battery according to a first embodiment and a non-aqueous secondary battery according to a second embodiment. In the present disclosure, when describing matters common to the non-aqueous secondary battery according to the first embodiment and the non-aqueous secondary battery according to the second embodiment, both are collectively referred to as the "non-aqueous secondary battery of the present disclosure."

[0139] The nonaqueous secondary battery of the present disclosure is a nonaqueous secondary battery that generates electromotive force by doping and dedoping of lithium ions, and includes a positive electrode, a negative electrode, and the separator of the present disclosure. "Doping" refers to the phenomenon of lithium ions entering the active material of the electrode.

[0140] 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.

[0141] The nonaqueous secondary battery according to the first embodiment includes the separator according to the first embodiment. By including the separator according to the first embodiment, the nonaqueous secondary battery according to the first embodiment is less likely to have the electrodes and the separator peel off. Therefore, the nonaqueous secondary battery according to the first embodiment is less likely to have an internal short circuit.

[0142] The nonaqueous secondary battery according to the second embodiment includes the separator according to the second embodiment. In the nonaqueous secondary battery according to the second embodiment, the separator according to the second embodiment has adhesive properties to the electrodes, so the electrodes and the separator are less likely to peel off, and internal short circuits are less likely to occur. In the nonaqueous secondary battery according to the second embodiment, the separator according to the second embodiment has excellent thermal dimensional stability, so internal short circuits are less likely to occur.

[0143] 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.

[0144] 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.

[0145] 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.

[0146] 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.

[0147] 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.

[0148] 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).

[0149] 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.

[0150] 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.

[0151] 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.

[0152] In the production method (1) or (2), the pressing temperature of the wet heat press is preferably 50°C to 90°C, more preferably 60°C to 80°C. The pressing pressure of the wet heat press is preferably 0.1 MPa to 2 MPa, more preferably 0.5 MPa to 1.5 MPa. The pressing time of the wet heat press is preferably adjusted according to the pressing temperature and pressing pressure, for example, within a range of 1 minute to 12 hours.

[0153] In the production method (1) or (3), the pressing temperature of the dry heat press is preferably 60°C to 90°C, more preferably 70°C to 85°C. The pressing pressure of the dry heat press is preferably 0.5 MPa to 5 MPa, more preferably 0.5 MPa to 3 MPa. The pressing time of the dry heat press is preferably adjusted according to the pressing temperature and pressing pressure, for example, within a range of 0.5 minutes to 1 hour.

[0154] 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.

[0155] 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.

[0156] In the following description, syntheses, treatments, manufacturing, etc. were carried out at room temperature (25° C.±3° C.) unless otherwise specified.

[0157] <Measurement Methods and Evaluation Methods> The measurement methods and evaluation methods used in the examples and comparative examples are as follows.

[0158] [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 a load of 0.19 N was applied during the measurement.

[0159] [Air Permeability of Porous Substrate and Separator] The air permeability (seconds / 100 mL) of the porous substrate and the separator was measured according to JIS P8117:2009 using a digital Oken air permeability tester (Asahi Seiko Co., Ltd., model EG01).

[0160] [Average Primary Particle Size of Inorganic Particles] The inorganic particles used to form the adhesive porous layer were observed under a scanning electron microscope (SEM) to determine the average primary particle size. The major axes of 100 randomly selected inorganic particles on the SEM image were measured, and the average of the major axes of the 100 particles was taken as the average primary particle size (μm).

[0161] [Weight per unit area of ​​adhesive porous layer] The separator was cut into a size of 20 cm x 20 cm, the adhesive porous layer was peeled off, and the weight was measured. The weight was divided by the area to obtain the weight per unit area (g / m) of both sides in total. ２ ) was sought.

[0162] [Adhesion to Electrode] 89.5 parts by mass of lithium cobalt oxide powder as a positive electrode active material, 4.5 parts by mass of acetylene black as a conductive additive, 6 parts by mass of polyvinylidene fluoride as a binder resin, and an appropriate amount of N-methyl-2-pyrrolidone were mixed and stirred in a twin-arm mixer to prepare a positive electrode slurry. The positive electrode slurry was applied to one side of a 20 μm thick aluminum foil, dried, and pressed to obtain a positive electrode having a positive electrode active material layer on one side.

[0163] A negative electrode slurry was prepared by mixing 300 parts by mass of artificial graphite as a negative electrode active material, 7.5 parts by mass of an aqueous dispersion containing 40% by mass of a modified styrene-butadiene copolymer as a binder resin, 3 parts by mass of carboxymethyl cellulose as a thickener, and an appropriate amount of water using a twin-arm mixer. The 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 on one side.

[0164] The electrodes (positive and negative electrodes) were cut into rectangles measuring 15 mm wide x 70 mm long. The separator was cut into a rectangle measuring 18 mm in diameter x 74 mm in length. Release paper measuring 15 mm wide x 70 mm long was prepared. The separator was placed on the active material layer of the electrode (positive or negative electrode), and then release paper was placed on top of the separator to produce a laminate.

[0165] The laminate was inserted into an aluminum laminate film pack, 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 soaked into the laminate. Next, the pack and the laminate were heat-pressed in the stacking direction using a heat press machine (wet heat press) to bond the electrode (positive electrode or negative electrode) and the separator. The heat press conditions were a temperature of 85°C, a pressure of 1 MPa, and a time of 5 minutes. After heat pressing, the laminate was removed from the pack, and the release paper was peeled off to obtain a wet adhesive test piece.

[0166] The laminate was inserted into an aluminum laminate film pack and heat-pressed (dry heat press) using a heat press machine in the stacking direction of the laminate together with the pack to bond the electrode (positive electrode or negative electrode) to the separator. The heat press conditions were a temperature of 85°C, a pressure of 1 MPa, and a time of 0.5 minutes. After heat pressing, the laminate was removed from the pack, and the release paper was peeled off to obtain a dry adhesive test piece.

[0167] The uncoated surface of the electrode of the test specimen was fixed to a metal plate with double-sided tape, and the metal plate was fixed to the lower chuck of a Tensilon (A&D Co., Ltd., STB-1225S). The metal plate was fixed to the Tensilon so that the longitudinal direction of the test specimen (i.e., the MD of the separator) was the direction of gravity. The separator was peeled from the electrode by approximately 2 cm from the lower end, and this end was fixed to the upper chuck, and a 180° peel test was performed. The tensile speed for the 180° peel test was 20 mm / min, and loads (N) were collected from 10 mm to 40 mm after the start of the measurement at 0.4 mm intervals, and the average was calculated. The loads for 10 test specimens were then averaged to determine the adhesive strength (N / 15 mm) between the electrode and the separator.

[0168] [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 test specimen in half in TD (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 test specimen in half in MD (referred to as points C and D, respectively). A clip was attached to the test specimen (the clip was attached between the end closest to point A and point A), and the specimen was hung in an oven at 105°C and subjected to heat treatment for 60 minutes under no tension. The lengths between A and B and between CD were measured before and after heat treatment, and the heat shrinkage was calculated using the following formula. The heat shrinkage of the three test specimens was then averaged.

[0169] MD heat shrinkage rate (%) = {(length AB before heat treatment - length AB after heat treatment) ÷ length AB before heat treatment} × 100

[0170] TD heat shrinkage rate (%) = {(CD length before heat treatment - CD length after heat treatment) ÷ CD length before heat treatment} × 100

[0171] <Preparation of Separator and Battery> In the following examples and comparative examples, the weight average molecular weight, degree of acetalization (i.e., degree of butyralization), and amount of hydroxyl groups of the polyvinyl butyral resin (PVB) used to form the adhesive porous layer are all values ​​from the manufacturer's catalog.

[0172] [First Embodiment] [Example 1] - Preparation of Separator - PVB (weight average molecular weight 56,000) was dissolved in dimethylacetamide (DMAc) to prepare a coating solution (1) with a PVB concentration of 6.0 mass %. An appropriate amount of coating solution (1) was placed on a Mayer bar, and the coating solution (1) was applied to both sides of a polyethylene microporous membrane (thickness 6 μm, air permeability 100 sec / 100 mL). The coating was performed so that the coating amount was equal on both sides of the polyethylene microporous membrane. The polyethylene microporous membrane with the coating layer formed thereon was immersed in a coagulation liquid (DMAc:water=50:50 [mass ratio], liquid temperature 40°C) to solidify the coating layer. The polyethylene microporous membrane was then washed in a water washing tank at a water temperature of 40°C and dried. In this way, a separator was obtained in which adhesive porous layers were formed on both sides of the polyethylene microporous membrane.

[0173] - 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.

[0174] - 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.

[0175] - 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 to allow the electrolyte to penetrate 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 85°C, a press pressure of 1 MPa, and a press time of 5 minutes. A non-aqueous secondary battery was thus obtained.

[0176] [Examples 2 to 6] Separators were produced in the same manner as in Example 1, except that at least one of the weight average molecular weight of the PVB and the coating amount of the adhesive porous layer was changed as shown in Table 1. Non-aqueous secondary batteries were produced using each separator.

[0177] Example 7 A separator was prepared in the same manner as in Example 1, except that the polyethylene microporous membrane was replaced with a composite porous substrate (thickness: 8 μm, air permeability: 140 sec / 100 mL) having porous heat-resistant layers on both sides of the polyethylene microporous membrane, and the weight-average molecular weight of the PVB was changed to 92,000. A nonaqueous secondary battery was prepared using this separator. The composite porous substrate had a polyethylene microporous membrane with a thickness of 6 μm and an air permeability of 100 sec / 100 mL, and a heat-resistant layer containing γ-alumina (average primary particle size: 0.01 μm) and styrene-butadiene rubber, with a γ-alumina content of 97% by mass and a thickness of 1 μm per side.

[0178] Comparative Example 1 A separator was produced in the same manner as in Example 1, except that the PVB was changed to a polyvinylidene fluoride resin (a binary copolymer of vinylidene fluoride and hexafluoropropylene, weight average molecular weight 1,400,000, and hexafluoropropylene 1.5 mol%).

[0179] Comparative Example 2 A separator was produced in the same manner as in Example 1, except that the weight average molecular weight of PVB was changed to 40,000.

[0180] Comparative Example 3 An attempt was made to produce a separator in the same manner as in Example 1, except that the weight-average molecular weight of PVB was changed to 230,000. However, the coating liquid for forming the adhesive porous layer gelled and could not be applied, so a separator could not be produced.

[0181] Table 1 shows the configurations of the porous substrate and the adhesive porous layer, and Table 2 shows the physical properties and evaluation results of the separator. The mass of the adhesive porous layer shown in Table 1 is the total mass of both sides of the separator. The mass of the adhesive porous layer per separator side in each example and comparative example was half the mass of the adhesive porous layer shown in Table 1.

[0182] The abbreviations in Table 1 have the following meanings: Mw: weight average molecular weight PVB: polyvinyl butyral VDF-HFP: copolymer of vinylidene fluoride and hexafluoropropylene

[0183]

[0184]

[0185] The surface of the separator of Example 3 was observed with a scanning electron microscope (SEM). Figure 1 shows SEM images of the surface of the separator of Example 3 (left: magnification 10,000, right: magnification 20,000).

[0186] [Second Embodiment] [Example 11] - Preparation of Separator - PVB (weight average molecular weight 130,000) was dissolved in dimethylacetamide (DMAc), and barium sulfate particles were further dispersed by stirring to prepare coating liquid (1). Coating liquid (1) had a PVB concentration of 6.0 mass%, with a PVB:barium sulfate particle volume ratio of 80.4:19.6. An appropriate amount of coating liquid (1) was placed on a Mayer bar, and coating liquid (1) was applied to both sides of a polyethylene microporous membrane (thickness 6 μm, air permeability 100 sec / 100 mL). The coating was performed so that the coating amount was equal on both sides of the polyethylene microporous membrane. The polyethylene microporous membrane with the coating layer formed thereon was immersed in a coagulation liquid (DMAc:water = 50:50 [mass ratio], liquid temperature 40°C) to solidify the coating layer. The polyethylene microporous membrane was then washed in a water washing tank at a water temperature of 40°C and dried. In this way, a separator having adhesive porous layers formed on both sides of a polyethylene microporous membrane was obtained. Using this separator, a nonaqueous secondary battery was fabricated in the same manner as in Example 1.

[0187] [Examples 12 to 19] Separators were produced in the same manner as in Example 11, except that at least one of the weight average molecular weight of PVB, the type of inorganic particles, the average primary particle size and volume ratio, and the coating amount of the adhesive porous layer was changed as shown in Table 3. Non-aqueous secondary batteries were produced using each separator.

[0188] Example 20 A separator was prepared in the same manner as in Example 11, except that the polyethylene microporous membrane was replaced with a composite porous substrate (thickness 8 μm, air permeability 140 sec / 100 mL) having porous heat-resistant layers on both sides of the polyethylene microporous membrane, and the volume ratio of the inorganic particles was changed as shown in Table 3. A nonaqueous secondary battery was prepared using this separator. The composite porous substrate had a polyethylene microporous membrane with a thickness of 6 μm and an air permeability of 100 sec / 100 mL, and a heat-resistant layer containing γ-alumina (average primary particle size 0.01 μm) and styrene-butadiene rubber, with a γ-alumina content of 97% by mass and a thickness of 1 μm per side.

[0189] Comparative Example 11 A separator was produced in the same manner as in Example 11, except that the PVB was changed to a polyvinylidene fluoride resin (a binary copolymer of vinylidene fluoride and hexafluoropropylene, weight-average molecular weight 1,400,000, hexafluoropropylene 1.5 mol%), and the volume ratio of the inorganic particles was changed as shown in Table 3.

[0190] Comparative Example 12 A separator was produced in the same manner as in Example 11, except that no inorganic particles were used to form the adhesive porous layer, and only PVB was used.

[0191] Comparative Example 13 A separator was produced in the same manner as in Example 11, except that the volume ratio of the inorganic particles was changed as shown in Table 3.

[0192] In each of Examples 11 to 19 and Comparative Examples 11 to 13, the adhesive porous layer was coated so that the separator had a thickness of 9 μm. In Example 20, the adhesive porous layer was coated so that the separator had a thickness of 11 μm.

[0193] The configurations of the porous substrate and adhesive porous layer are shown in Table 3, and the physical properties and evaluation results of the separator are shown in Table 4. The mass of the adhesive porous layer shown in Table 3 is the total mass of both sides of the separator. The mass of the adhesive porous layer per separator side in each example and comparative example was half the mass of the adhesive porous layer shown in Table 3.

[0194] The abbreviations in Table 3 have the following meanings: Mw: weight average molecular weight PVB: polyvinyl butyral VDF-HFP: copolymer of vinylidene fluoride and hexafluoropropylene

[0195]

[0196]

[0197] The surface of the separator of Example 13 was observed with a scanning electron microscope (SEM). Figure 2 shows SEM images of the surface of the separator of Example 13 (left: magnification 10,000, right: magnification 20,000).

[0198] 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.

[0199] The disclosure of Japanese Application No. 2024-007563, filed on January 22, 2024, is incorporated herein by reference in its entirety. The disclosure of Japanese Application No. 2024-007564, filed on January 22, 2024, is incorporated herein by reference in its entirety.

Claims

1. A separator for a non-aqueous secondary battery, comprising a porous substrate and an adhesive porous layer containing a polyvinyl acetal resin disposed on one or both sides of the porous substrate, wherein the weight average molecular weight of the polyvinyl acetal resin contained in the adhesive porous layer is from 50,000 to 200,000.

2. The mass per unit area of the adhesive porous layer is 0.1 g / m per side of the separator for non-aqueous secondary batteries ２ to 2.5 g / m ２ The separator for non-aqueous secondary batteries according to claim 1.

3. The separator for a non-aqueous secondary battery according to claim 1, wherein the air permeability of the separator for a non-aqueous secondary battery is from 100 seconds / 100 mL to 1000 seconds / 100 mL.

4. A separator for a non-aqueous secondary battery, comprising a porous substrate and an adhesive porous layer containing a polyvinyl acetal resin and inorganic particles disposed on one or both sides of the porous substrate, wherein the volume ratio of the inorganic particles in the polyvinyl acetal resin and the inorganic particles contained in the adhesive porous layer is from 5% by volume to 65% by volume.

5. The separator for a non-aqueous secondary battery according to claim 4, wherein the weight average molecular weight of the polyvinyl acetal resin contained in the adhesive porous layer is from 50,000 to 200,000.

6. The separator for a non-aqueous secondary battery according to claim 4, wherein the average primary particle size of the inorganic particles contained in the adhesive porous layer is from 0.01 μm to 2 μm.

7. The separator for a non-aqueous secondary battery according to claim 4, wherein the inorganic particles contain at least one selected from the group consisting of metal oxide particles, metal hydroxide particles, and metal sulfate particles.

8. The mass per unit area of the adhesive porous layer is 0.5 g / m per side of the separator for non-aqueous secondary batteries ２ to 5 g / m ２ The separator for non-aqueous secondary batteries according to claim 4, wherein the separator is as described above.

9. The separator for a non-aqueous secondary battery according to claim 4, wherein the air permeability of the separator for a non-aqueous secondary battery is from 100 seconds / 100 mL to 500 seconds / 100 mL.

10. The separator for a non-aqueous secondary battery according to claim 1 or claim 4, wherein the degree of acetalization of the polyvinyl acetal resin contained in the adhesive porous layer is from 50 mol% to 80 mol%.

11. The separator for a non-aqueous secondary battery according to claim 1 or claim 4, wherein the amount of hydroxyl groups of the polyvinyl acetal resin contained in the adhesive porous layer is from 20 mol% to 45 mol%.

12. The separator for a non-aqueous secondary battery according to claim 1 or claim 4, wherein the porous substrate includes a polyolefin microporous membrane.

13. The separator for a non-aqueous secondary battery according to claim 1 or claim 4, wherein the porous substrate includes a polyolefin microporous membrane and a heat-resistant layer containing at least one of inorganic particles and a heat-resistant resin disposed on one or both sides of the polyolefin microporous membrane.

14. The separator for a non-aqueous secondary battery according to claim 1 or claim 4, wherein the adhesive porous layer substantially does not contain a fluorine-containing resin.

15. A non-aqueous secondary battery comprising a positive electrode, a negative electrode, and the separator for a non-aqueous secondary battery according to claim 1 or claim 4 disposed between the positive electrode and the negative electrode, and obtaining an electromotive force by doping and dedoping of lithium ions.

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