CN110600658A

CN110600658A - Separator for nonaqueous secondary battery and nonaqueous secondary battery

Info

Publication number: CN110600658A
Application number: CN201910491056.4A
Authority: CN
Inventors: 佐藤恵美; 长尾优
Original assignee: Teijin Ltd
Current assignee: Teijin Ltd
Priority date: 2018-06-13
Filing date: 2019-06-06
Publication date: 2019-12-20
Also published as: KR20190141082A; JP2019216033A

Abstract

The invention provides a separator for a nonaqueous secondary battery and a nonaqueous secondary battery. A separator for a nonaqueous secondary battery, comprising a porous base material and a heat-resistant porous layer provided on one or both surfaces of the porous base material, wherein the heat-resistant porous layer contains a binder resin and first inorganic particles composed of an apatite, and the average primary particle diameter (D50) of the first inorganic particles is 0.01 to 3.0 [ mu ] m.

Description

Separator for nonaqueous secondary battery and nonaqueous secondary battery

Technical Field

The present invention relates to a separator for a nonaqueous secondary battery and a nonaqueous secondary battery.

Background

A nonaqueous secondary battery represented by a lithium ion secondary battery has been widely used as a power source for portable electronic devices such as notebook personal computers, cellular phones, digital cameras, and camcorders. In addition, nonaqueous secondary batteries typified by lithium ion secondary batteries have been studied for use as batteries for power storage and electric vehicles, based on the characteristic of high energy density. With the spread of such nonaqueous secondary batteries, there is an increasing demand for improvement in the safety of the batteries.

In order to ensure the safety of a battery, a separator, which is one of the members constituting a nonaqueous secondary battery, is required to have heat resistance such that film cracking does not easily occur even when the inside of the battery is heated to a high temperature. As a separator having improved heat resistance, a separator including a porous base material and a porous layer containing inorganic particles is known (for example, see patent documents 1 to 7).

Documents of the prior art

Patent document

Patent document 1: japanese patent laid-open publication No. 2011-044419

Patent document 2: japanese patent laid-open publication No. 2015-115321

Patent document 3: international publication No. 2014/148036 pamphlet

Patent document 4: international publication No. 2016/056289 pamphlet

Patent document 5: international publication No. 2016/056288 pamphlet

Patent document 6: international publication No. 2016/031492 pamphlet

Patent document 7: international publication No. 2017/146237 pamphlet

Disclosure of Invention

Problems to be solved by the invention

As an electrolyte for a nonaqueous secondary battery, LiPF is dissolved in a solvent such as a cyclic carbonate, a chain carbonate, or an ether₆Such a nonaqueous electrolyte solution containing a fluorine-containing electrolyte is often used because it is suitable for obtaining a battery with a high voltage and a high capacity, but LiPF is present even in a trace amount of water₆Hydrolysis also occurs to produce Hydrogen Fluoride (HF). In addition, in the conventional separator including the porous layer containing the inorganic filler, it is known that in the battery impregnated with the electrolytic solution, moisture from the filler acts on the electrolyte to generate HF, and the concentration of HF in the nonaqueous secondary battery is further increased. Such a nonaqueous secondary battery having an increased HF concentration causes deterioration, and does not necessarily satisfy battery characteristics such as cycle characteristics, capacitance, and storage stability. In addition, in the case of a conventional separator having a porous layer containing an inorganic filler, the separator is exposed for a long timeAt high temperatures, decomposition of the electrolyte may occur.

In view of the above problems, an object of the present invention is to provide a separator for a nonaqueous secondary battery, which has excellent heat resistance and can reduce the concentration of HF and the amount of gas generated in the battery to constitute a nonaqueous secondary battery having excellent battery characteristics, particularly excellent cycle characteristics.

Means for solving the problems

Specific means for solving the above problems include the following means.

[1] A separator for a nonaqueous secondary battery, comprising a porous base material and a heat-resistant porous layer provided on one or both surfaces of the porous base material, wherein the heat-resistant porous layer contains a binder resin and first inorganic particles composed of an apatite, and the average primary particle diameter (D50) of the first inorganic particles is 0.01 to 3.0 [ mu ] m.

[2] The separator for a nonaqueous secondary battery according to [1], wherein the average primary particle diameter (D50) of the first inorganic particles is 0.01 μm or more and 1.0 μm or less.

[3] The separator for a nonaqueous secondary battery according to the above [1] or [2], wherein the binder resin contains a polyvinylidene fluoride resin.

[4] The separator for a nonaqueous secondary battery according to item [3], wherein the weight average molecular weight of the polyvinylidene fluoride resin is 50 to 300 ten thousand.

[5] The separator for a nonaqueous secondary battery according to the above [1] or [2], wherein the binder resin contains at least 1 selected from the group consisting of wholly aromatic polyamide, polyamideimide, poly-N-vinylacetamide, polyacrylamide, copolyether polyamide, polyimide and polyetherimide.

[6] The separator for a nonaqueous secondary battery according to any one of the above [1] to [5], wherein a volume ratio of the first inorganic particles in the heat-resistant porous layer is 10% by volume to 90% by volume.

[7] The separator for a nonaqueous secondary battery according to any one of the above [1] to [5], wherein the heat-resistant porous layer contains second inorganic particles different from the first inorganic particles in addition to the first inorganic particles.

[8] The separator for a nonaqueous secondary battery according to item [7], wherein the content of the first inorganic particles is 5 to 95 vol% based on the total mass of the first inorganic particles and the second inorganic particles.

[9] The separator for a nonaqueous secondary battery according to the above [7] or [8], wherein the second inorganic particles contain at least 1 kind selected from the group consisting of metal oxides, metal hydroxides, metal nitrides, metal salts, and clays.

[10] The nonaqueous secondary battery separator according to the above [9], wherein the second inorganic particles contain at least 1 selected from the group consisting of alumina, boehmite, zinc oxide, magnesium hydroxide, and barium sulfate.

[11] The separator for a nonaqueous secondary battery according to any one of the above [7] to [10], wherein a total volume ratio of the first inorganic particles and the second inorganic particles in the heat-resistant porous layer is 10% by volume to 90% by volume.

[12] A nonaqueous secondary battery comprising a positive electrode, a negative electrode, and a separator for a nonaqueous secondary battery according to any one of [1] to [11] disposed between the positive electrode and the negative electrode, wherein the nonaqueous secondary battery obtains an electromotive force by doping and dedoping of lithium.

ADVANTAGEOUS EFFECTS OF INVENTION

The present disclosure can provide a separator for a nonaqueous secondary battery which has excellent heat resistance and can reduce the concentration of HF and the amount of gas generated in the battery to form a nonaqueous secondary battery having excellent battery characteristics, particularly excellent cycle characteristics.

Detailed Description

Hereinafter, embodiments of the present disclosure will be described. These descriptions and examples are intended to be illustrative of embodiments and are not intended to limit the scope of embodiments.

In the present disclosure, the numerical range shown by "to" indicates a range in which the numerical values described before and after "to" are included as the minimum value and the maximum value, respectively.

In the present disclosure, the term "step" includes not only an independent step but also a step that is not clearly distinguished from other steps, and is included in the term as long as the desired purpose of the step is achieved.

In the present disclosure, references to the amounts of each ingredient in the composition, and the presence of a plurality of substances belonging to each ingredient in the composition, refer to the total amount of the plurality of substances present in the composition, unless otherwise specified.

In the present disclosure, the "MD direction" refers to the longitudinal direction of the porous base material and the separator manufactured in long strips, and the "TD direction" refers to the direction perpendicular to the "MD direction". In the present disclosure, the "MD direction" is also referred to as the "mechanical direction", and the "TD direction" is also referred to as the "width direction".

In the present disclosure, when the stacking relationship of the layers constituting the separator is represented by "upper" and "lower", a layer closer to the substrate is referred to as "lower", and a layer farther from the substrate is referred to as "upper".

In the present disclosure, the expression "(meth) acryl-" means "acryl-" or "methacryl-".

In the present disclosure, the term "monomer unit" of a resin means a structural unit of the resin, and means a structural unit obtained by polymerizing a monomer.

In the present disclosure, the heat-resistant resin refers to a resin having a melting point of 200 ℃ or higher, or a resin having no melting point but having a decomposition temperature of 200 ℃ or higher. That is, the heat-resistant resin in the present disclosure means a resin that does not melt or decompose in a temperature range of less than 200 ℃.

< separator for nonaqueous Secondary Battery >

The separator for a nonaqueous secondary battery (also referred to as a "separator" in the present disclosure) of the present disclosure includes a porous base material and a heat-resistant porous layer provided on one or both surfaces of the porous base material.

In the separator of the present disclosure, the heat-resistant porous layer contains a binder resin and first inorganic particles made of apatite, and the average primary particle diameter (D50) of the first inorganic particles is 0.01 μm or more and 3.0 μm or less.

In the separator of the present disclosure, the apatite includes natural apatite and/or hydroxyapatite. The natural apatite is composed of Ca₅(PO₄)₃Basic calcium phosphate (F, Cl, OH), hydroxyapatite (Hydoroxyapatite) represented by the formula Ca₅(PO₄)₃Basic calcium phosphate represented by (OH) has excellent ion-exchange properties. Specifically, the following properties were exhibited: calcium ion sites are easily exchanged with cations, phosphate and hydroxide ion sites are easily exchanged with anions, and particularly, hydroxide ion sites are easily exchanged with fluoride ions. Since such apatite has excellent HF adsorption ability, HF in the battery can be removed. Therefore, battery characteristics, particularly cycle characteristics, can be improved. Further, apatite is less likely to decompose an electrolytic solution or an electrolyte than magnesium hydroxide and alumina as in the prior art, and therefore, is less likely to generate gas. Therefore, by using the apatite particles as the inorganic filler of the heat-resistant porous layer, a separator in which gas is less likely to be generated and expansion and deformation of the battery are less likely to occur can be obtained.

In the separator of the present disclosure, the average primary particle diameter (D50) of the first inorganic particles made of apatite is 3.0 μm or less from the viewpoint of improving the heat resistance of the heat-resistant porous layer. When the average primary particle diameter of the first inorganic particles is 3.0 μm or less, the heat resistance of the heat-resistant porous layer is improved. The reason for this is considered as follows: since the first inorganic particles have a small particle diameter, the packing density of the particles is increased, and the number of contact points between the first particles and the binder resin is increased, so that the shrinkage of the heat-resistant porous layer when exposed to high temperature can be suppressed. Further, it is considered that since many particles having a small particle diameter are connected to each other, the heat-resistant porous layer is less likely to be broken when exposed to high temperature.

From the above-described viewpoint, the average primary particle diameter of the apatite particles is more preferably 1.0 μm or less, still more preferably 0.6 μm or less, and particularly preferably 0.3 μm or less.

In the separator of the present disclosure, the average primary particle diameter (D50) of the first inorganic particles contained in the heat-resistant porous layer is 0.01 μm or more. When the average primary particle diameter (D50) of the first inorganic particles is 0.01 μm or more, aggregation of the particles is suppressed, and a heat-resistant porous layer having high uniformity can be formed. In addition, from the viewpoint of heat resistance, the smaller the particle size of the first inorganic particles, the better, but on the other hand, the larger the specific surface area of the inorganic particles, the more likely the problem of HF or gas generation in the battery is. However, in the present disclosure, since apatite particles having an average primary particle size (D50) of 0.01 μm or more are used as the first inorganic particles, such a problem is not easily caused, and excellent heat resistance can be achieved at the same time. From such a viewpoint, the average primary particle diameter (D50) of the first inorganic particles is more preferably 0.05 μm or more, and still more preferably 0.10 μm or more.

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

[ porous base Material ]

In the present disclosure, a porous substrate refers to a substrate having pores or voids therein. Examples of such a base material include: a microporous membrane; porous sheets made of fibrous materials such as nonwoven fabrics and paper; a composite porous sheet obtained by laminating 1 or more other porous layers on the microporous membrane or the porous sheet; and so on. In the present disclosure, a microporous membrane is preferable from the viewpoint of making the separator thin and improving the strength. The microporous membrane is a membrane comprising: in the structure in which a large number of fine holes are formed in the inside and the fine holes are connected, a gas or a liquid can pass through from one surface to the other surface.

The material of the porous substrate is preferably a material having electrical insulation properties, and may be any of an organic material and an inorganic material.

In order to impart the shutdown function to the porous base material, the porous base material preferably contains a thermoplastic resin. The shutdown function means the following functions: when the temperature of the battery rises, the constituent material melts to block the pores of the porous base material, thereby blocking the movement of ions and preventing thermal runaway of the battery. As the thermoplastic resin, a thermoplastic resin having a melting point of less than 200 ℃ is preferable. Examples of the thermoplastic resin include polyesters such as polyethylene terephthalate; polyolefins such as polyethylene and polypropylene; and the like, among which polyolefins are preferred.

As the porous substrate, a microporous film containing polyolefin (referred to as "polyolefin microporous film" in the present disclosure) is preferable. The polyolefin microporous membrane is preferably selected from polyolefin microporous membranes having sufficient mechanical properties and ion permeability, for example, which are used in conventional battery separators.

The polyolefin microporous membrane is preferably a microporous membrane containing polyethylene from the viewpoint of exhibiting a shutdown function, and the content of polyethylene is preferably 30% by mass or more with respect to the mass of the entire polyolefin microporous membrane.

The polyolefin microporous membrane may be a microporous membrane containing polypropylene, from the viewpoint of having heat resistance that is less likely to cause membrane rupture when exposed to high temperatures.

The polyolefin microporous membrane may be a polyolefin microporous membrane containing polyethylene and polypropylene, from the viewpoint of having a shutdown function and heat resistance such that the membrane is less likely to break when exposed to high temperatures. Examples of such a polyolefin microporous membrane include a microporous membrane in which polyethylene and polypropylene are mixed in one layer. The microporous membrane preferably contains 95 mass% or more of polyethylene and 5 mass% or less of polypropylene in view of achieving both shutdown function and heat resistance. From the viewpoint of achieving both the shutdown function and the heat resistance, a polyolefin microporous membrane having a laminated structure of 2 or more layers, at least 1 layer containing polyethylene and at least 1 layer containing polypropylene, is also preferable.

The polyolefin contained in the polyolefin microporous membrane is preferably a polyolefin having a weight average molecular weight (Mw) of 10 to 500 ten thousand. When the Mw of the polyolefin is 10 ten thousand or more, sufficient mechanical properties can be imparted to the microporous membrane. On the other hand, when the Mw of the polyolefin is 500 ten thousand or less, not only the shutdown properties of the microporous membrane are good, but also the molding of the microporous membrane is easy.

Examples of the method for producing the polyolefin microporous membrane include the following methods: a method in which a sheet is formed by extruding a molten polyolefin resin from a T-die, and is subjected to crystallization treatment, stretching, and heat treatment to form a microporous film; a method in which a polyolefin resin melted together with a plasticizer such as liquid paraffin is extruded from a T-die, cooled to form a sheet, stretched, extracted with the plasticizer, and heat-treated to form a microporous membrane; and so on.

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

Examples of the composite porous sheet include a sheet obtained by laminating a functional layer on a microporous membrane or a porous sheet made of a fibrous material. Such a composite porous sheet is preferable in that the functional layer can further add a function. Examples of the functional layer include a porous layer made of a heat-resistant resin and an inorganic filler, from the viewpoint of imparting heat resistance. Examples of the heat-resistant resin include 1 or 2 or more heat-resistant resins selected from wholly aromatic polyamides, polyamideimides, polyimides, polyethersulfones, polysulfones, polyetherketones, and polyetherimides. Examples of the inorganic filler include metal oxides such as alumina; metal hydroxides such as magnesium hydroxide; and so on. Examples of the method for forming the composite include a method of coating a functional layer on a microporous membrane or a porous sheet; a method of bonding a microporous film or a porous sheet to a functional layer with an adhesive; and a method of thermocompression bonding the microporous membrane or the porous sheet to the functional layer.

For the purpose of improving wettability with a coating liquid for forming a heat-resistant porous layer, the surface of the porous substrate may be subjected to various surface treatments within a range that does not impair the properties of the porous substrate. Examples of the surface treatment include corona treatment, plasma treatment, flame treatment, and ultraviolet irradiation treatment.

[ characteristics of porous base Material ]

The thickness of the porous base material is preferably 25 μm or less, more preferably 15 μm or less, from the viewpoint of having good mechanical properties and improving the energy density of the battery, and is preferably 4 μm or more, more preferably 6 μm or more, from the viewpoint of the production yield of the separator and the production yield of the battery.

From the viewpoint of ion permeability or suppression of battery short-circuiting, the porous substrate preferably has a Gurley value (JIS P8117: 2009) of 50 sec/100 mL to 400 sec/100 mL.

The porosity of the porous substrate is preferably 20% to 60% from the viewpoint of obtaining an appropriate membrane resistance and shutdown function. The porosity of the porous substrate was determined by the following calculation method. That is, the constituent materials are a, b, c, …, n, and the masses of the constituent materials are Wa, Wb, Wc, …, Wn (g/cm)²) The true densities of the constituent materials are da, db, dc, …, dn (g/cm)³) When the film thickness is represented by t (cm), the porosity ε (%) can be obtained by the following equation.

ε＝{1-(Wa/da+Wb/db+Wc/dc+…+Wn/dn)/t}×100

The average pore diameter of the porous substrate is preferably 15nm to 100nm from the viewpoint of ion permeability or suppression of battery short-circuiting. The average pore diameter of the porous substrate was measured by using a pore diameter distribution measuring instrument (Perm Porometer) according to astm e 1294-89.

The puncture strength of the porous base material is preferably 200gf (2.0N) or more from the viewpoint of the production yield of the separator and the production yield of the battery. The puncture strength of the porous substrate is: maximum puncture load (gf) measured by performing a puncture test using a Kato Tech KES-G5 hand-held compression tester under the condition that the curvature radius of the needle tip is 0.5mm and the puncture speed is 2 mm/sec.

[ Heat-resistant porous layer ]

In the separator of the present disclosure, the heat-resistant porous layer contains at least a binder resin and first inorganic particles made of apatite. The heat-resistant porous layer is a layer having a large number of fine pores through which a gas or a liquid can pass from one surface to the other surface.

In the separator of the present disclosure, the heat-resistant porous layer may be present on only one side of the porous substrate or may be present on both sides of the porous substrate. When the heat-resistant porous layers are present on both sides of the porous substrate, the separator has more excellent heat resistance, and the safety of the battery can be further improved. In addition, the separator is less likely to curl, and the workability in the production of a battery is excellent. When the heat-resistant porous layer is present on only one surface of the porous substrate, the separator has more excellent ion permeability. Further, the thickness of the entire separator can be suppressed, and a battery with higher energy density can be manufactured.

The type of the binder resin of the heat-resistant porous layer is not particularly limited as long as it is a resin capable of binding inorganic particles. The binder resin of the heat-resistant porous layer is preferably a heat-resistant resin (a resin having a melting point of 200 ℃ or higher, or a resin having no melting point but having a decomposition temperature of 200 ℃ or higher). The binder resin of the heat-resistant porous layer is preferably a resin that is stable to the electrolytic solution and electrochemically stable. The binder resin may be used alone in 1 kind, or may be used in combination in 2 or more kinds.

The binder resin of the heat-resistant porous layer preferably has adhesiveness to the electrode of the battery, and the type of the binder resin may be selected according to the composition of the positive electrode or the negative electrode. When the heat-resistant porous layers are present on both sides of the porous substrate, the binder resin of one heat-resistant porous layer may be the same as or different from the binder resin of the other heat-resistant porous layer.

The binder resin of the heat-resistant porous layer is preferably a polymer containing a functional group or an atomic group having polarity (for example, a hydroxyl group, a carboxyl group, an amino group, an amide group, or a carbonyl group).

Specific examples of the binder resin of the heat-resistant porous layer include polyvinylidene fluoride resin, wholly aromatic polyamide, polyamideimide, polyimide, polyethersulfone, polysulfone, polyetherketone, polyketone, polyetherimide, poly-N-vinylacetamide, polyacrylamide, copolyetherpolyamide, fluorine rubber, acrylic resin, styrene-butadiene copolymer, cellulose, polyvinyl alcohol, and the like.

The binder resin of the heat-resistant porous layer may be a particulate resin, and examples thereof include resin particles such as a polyvinylidene fluoride resin, a fluorine rubber, and a styrene-butadiene copolymer. The binder resin of the heat-resistant porous layer may be a water-soluble resin such as cellulose or polyvinyl alcohol. In the case of using a particulate resin or a water-soluble resin as the binder resin of the heat-resistant porous layer, a coating solution can be prepared by dispersing or dissolving the binder resin in water, and the heat-resistant porous layer can be formed on the porous substrate by a dry coating method using the coating solution.

The binder resin of the heat-resistant porous layer is preferably a wholly aromatic polyamide, polyamideimide, poly-N-vinylacetamide, polyacrylamide, copolyether polyamide, polyimide, or polyetherimide, from the viewpoint of excellent heat resistance. These resins are preferably heat-resistant resins (resins having a melting point of 200 ℃ or higher, or resins having no melting point but a decomposition temperature of 200 ℃ or higher).

Among the heat-resistant resins, wholly aromatic polyamides are preferable from the viewpoint of durability. The wholly aromatic polyamide may be of a meta-type or a para-type. Among the wholly aromatic polyamides, the meta-type wholly aromatic polyamide is preferable from the viewpoint of easy formation of a porous layer and excellent oxidation reduction resistance in the electrode reaction. A small amount of aliphatic monomer may be copolymerized in the wholly aromatic polyamide.

As the wholly aromatic polyamide usable as the binder resin of the heat-resistant porous layer, specifically, polyisophthaloyl metaphenylene diamine, polyparaphenylene terephthalamide or copolyparaphenylene 3, 4' -oxydiphenylene terephthalamide is preferable, and polyisophthaloyl metaphenylene diamine is more preferable.

As the binder resin of the heat-resistant porous layer, a polyvinylidene fluoride-based resin (PVDF-based resin) is preferable from the viewpoint of adhesiveness to the electrode.

From the viewpoint of adhesion to the electrode, PVDF-based resins are suitable as binder resins for the heat-resistant porous layer. By including the PVDF-based resin in the heat-resistant porous layer, the adhesion between the heat-resistant porous layer and the electrode is improved, and as a result, the strength of the battery (cell) is improved.

Examples of the PVDF resin include homopolymers of vinylidene fluoride (i.e., polyvinylidene fluoride); copolymers of vinylidene fluoride with other monomers (polyvinylidene fluoride copolymers); mixtures of polyvinylidene fluoride and polyvinylidene fluoride copolymers. Examples of the monomer copolymerizable with vinylidene fluoride include a carboxyl group-containing monomer such as tetrafluoroethylene, hexafluoropropylene, trifluoroethylene, chlorotrifluoroethylene, trichloroethylene, vinyl fluoride, a perfluoroalkyl vinyl ether, and (meth) acrylic acid, methyl (meth) acrylate, a (meth) acrylic acid ester, vinyl acetate, vinyl chloride, and acrylonitrile. These monomers may be used alone in 1 kind, or in combination of 2 or more kinds.

The PVDF resin contained in the heat-resistant porous layer is more preferably a copolymer (VDF-HFP copolymer) containing a vinylidene fluoride monomer unit (VDF unit) and a hexafluoropropylene monomer unit (HFP unit) from the viewpoint of adhesiveness to an electrode. When the VDF-HFP copolymer is used as the binder resin of the heat-resistant porous layer, the crystallinity and heat resistance of the binder resin can be easily controlled to an appropriate range, and the flow of the heat-resistant porous layer can be suppressed during heat pressure treatment for bonding the separator to the electrode.

The VDF-HFP copolymer contained in the heat-resistant porous layer may be a copolymer composed of only VDF units and HFP units, or may be a copolymer further containing monomer units other than VDF units and HFP units. Examples of the other monomer include tetrafluoroethylene, trifluoroethylene, chlorotrifluoroethylene, trichloroethylene, and vinyl fluoride.

From the viewpoint of controlling the crystallinity and heat resistance of the VDF-HFP copolymer to be within appropriate ranges, the content of VDF units in the VDF-HFP copolymer is preferably 91 mol% or more.

The content of HFP units in the VDF-HFP copolymer is preferably 0.2 mol% or more, more preferably 0.5 mol% or more, from the viewpoint of appropriate swelling when impregnated with an electrolytic solution, and is preferably 7 mol% or less, more preferably 6 mol% or less, from the viewpoint of being not easily soluble in an electrolytic solution.

The PVDF resin contained in the heat-resistant porous layer preferably has a weight average molecular weight (Mw) of 50 to 300 ten thousand. When the Mw of the PVDF resin is 50 ten thousand or more, the heat-resistant porous layer having mechanical properties that can withstand heat and pressure treatment when bonding the separator and the electrode is easily obtained, and the adhesiveness between the electrode and the heat-resistant porous layer is improved. From this viewpoint, the Mw of the PVDF resin is more preferably 55 ten thousand or more, and still more preferably 100 ten thousand or more. On the other hand, if the Mw of the PVDF resin is 300 ten thousand or less, the viscosity of the heat-resistant porous layer during molding becomes too high, moldability and crystal formation become good, and the heat-resistant porous layer is likely to be porous. From this viewpoint, the Mw of the PVDF resin is more preferably 250 ten thousand or less, and still more preferably 230 ten thousand or less.

The acid value of the PVDF resin contained in the heat-resistant porous layer is preferably 2mgKOH/g to 30 mgKOH/g. The acid value of the PVDF resin can be controlled by introducing a carboxyl group into the PVDF resin, for example. The amount of the carboxyl group introduced into the PVDF resin can be controlled by adjusting the polymerization ratio of a monomer having a carboxyl group (for example, (meth) acrylic acid, (meth) acrylic ester, maleic acid, maleic anhydride, maleic acid ester, and fluorine substitution products thereof) used as the polymerization component of the PVDF resin.

In the separator of the present disclosure, the heat-resistant porous layer may contain a resin other than the binder resin. The other resin may be used for the purpose of improving the adhesion of the heat-resistant porous layer to the electrode, adjusting the ion permeability or the membrane resistance of the heat-resistant porous layer, or the like. Examples of the other resin include homopolymers and copolymers of vinyl nitrile compounds (such as acrylonitrile and methacrylonitrile), carboxymethyl cellulose, hydroxyalkyl cellulose, polyvinyl butyral, polyvinyl pyrrolidone, and polyethers (such as polyethylene oxide and polypropylene oxide).

In the separator of the present disclosure, the total content of the other resins than the binder resin contained in the heat-resistant porous layer is preferably 5% by mass or less, more preferably 3% by mass or less, and still more preferably 1% by mass or less, relative to the total amount of the resins contained in the heat-resistant porous layer.

The separator of the present disclosure contains first inorganic particles formed of apatite in the heat-resistant porous layer. The average primary particle diameter of the first inorganic particles contained in the heat-resistant porous layer is 0.01 to 3.0 [ mu ] m. The lower limit is more preferably 0.05 μm or more, and still more preferably 0.10 μm or more, and the upper limit is more preferably 2 μm or less, and still more preferably 1 μm or less.

The particle shape of the first inorganic particles is not limited, and may be any of spherical, elliptical, plate-like, needle-like, and amorphous. From the viewpoint of increasing the packing density of the particles and improving the heat resistance, the first inorganic particles are preferably unagglomerated primary particles.

From the viewpoint of heat resistance, the volume ratio of the first inorganic particles in the heat-resistant porous layer is preferably 10 vol% or more, more preferably 30 vol% or more, and still more preferably 50 vol% or more. The volume ratio of the first inorganic particles in the heat-resistant porous layer is preferably 90 vol% or less, more preferably 85 vol% or less, and still more preferably 80 vol% or less, from the viewpoint that the heat-resistant porous layer is not easily peeled from the porous substrate.

In the separator of the present disclosure, the heat-resistant porous layer may contain second inorganic particles different from the first inorganic particles in addition to the first inorganic particles.

In general, inorganic particles have been conventionally included in nonaqueous secondary battery separators for the purpose of improving heat resistance and strength of batteries, but particularly inorganic particles such as metal hydroxides and metal oxides act on electrolytes to impair liquid stability, and may cause decomposition of the electrolytes and gases to be generated. However, in the separator of the present disclosure, since the heat-resistant porous layer contains apatite particles having a specific particle diameter as the first inorganic particles, the generation of gas due to the decomposition of the electrolyte solution and the electrolyte induced by the second inorganic particles such as a metal hydroxide and a metal oxide can be suppressed, and the expansion (deformation of the battery) due to the generated gas can be suppressed.

The second inorganic particles are added to the heat-resistant porous layer in order to impart a desired function, and preferably include at least 1 selected from the group consisting of metal oxides, metal hydroxides, metal nitrides, metal salts, and clays, for example. Examples of the metal oxide include silicon dioxide, aluminum oxide, titanium dioxide, zirconium dioxide, and magnesium oxide. Examples of the metal hydroxide include aluminum hydroxide, magnesium hydroxide, calcium hydroxide, chromium hydroxide, zirconium hydroxide, cerium hydroxide, nickel hydroxide, and boron hydroxide. Examples of the metal nitride include boron nitride and silicon nitride. Examples of the metal salt include carbonates such as calcium carbonate and magnesium carbonate; sulfates such as calcium sulfate and barium sulfate. Examples of the clay include clay minerals such as calcium silicate and talc. The second inorganic particles are preferably particles of a metal hydroxide, a metal oxide, or a metal salt, from the viewpoint of stability to the electrolytic solution and electrochemical stability. In particular, from the viewpoint of suitably obtaining the effect of preventing HF gas generation by the first inorganic particles, the second inorganic particles preferably contain at least 1 selected from the group consisting of alumina, boehmite, zinc oxide, magnesium hydroxide, and barium sulfate. The other inorganic particles may be inorganic particles surface-modified with a silane coupling agent or the like.

The particle shape of the second inorganic particles is not limited, and may be any of spherical, elliptical, plate-like, needle-like, and amorphous. The second inorganic particles are preferably unagglomerated primary particles. The second inorganic particles may be used alone in 1 kind, or in combination with 2 or more kinds.

The average primary particle diameter (D50) of the second inorganic particles is preferably 0.01 to 5 μm. The lower limit is more preferably 0.1 μm or more, and the upper limit is more preferably 1 μm or less.

In the heat-resistant porous layer, the content of the first inorganic particles is preferably 5 to 95 vol% based on the total mass of the first inorganic particles and the second inorganic particles. When the content of the first inorganic particles is 5% by volume or more, the effect of suppressing gas generation accompanying decomposition of the electrolyte solution and the electrolyte induced by the heat-resistant filler can be remarkably obtained. From such a viewpoint, the content of the first inorganic particles is more preferably 20 vol% or more, and still more preferably 30 vol% or more. On the other hand, when the content of the first inorganic particles is 90% by volume or less, it is preferable from the viewpoint of imparting a shape effect by the second inorganic particles. From such a viewpoint, the content of the first inorganic particles is more preferably 90% by volume or less, and still more preferably 85% by volume or less.

From the viewpoint of heat resistance, the total volume ratio of the first inorganic particles and the second inorganic particles in the heat-resistant porous layer is preferably 10 vol% or more, more preferably 20 vol% or more, and still more preferably 30 vol% or more. The total volume ratio of the first inorganic particles and the second inorganic particles is preferably 90 vol% or less, more preferably 85 vol% or less, and even more preferably 80 vol% or less, from the viewpoint that the heat-resistant porous layer is not easily peeled from the porous substrate.

In the separator of the present disclosure, the heat-resistant porous layer may further contain an organic filler. Examples of the organic filler include particles formed of crosslinked polymers such as crosslinked poly (meth) acrylic acid, crosslinked poly (meth) acrylate, crosslinked polysiloxane, crosslinked polystyrene, crosslinked polydivinylbenzene, a crosslinked product of a styrene-divinylbenzene copolymer, a melamine resin, a phenol resin, and a benzoguanamine-formaldehyde condensate; particles made of a heat-resistant polymer such as polysulfone, polyacrylonitrile, aramid, or polyacetal; and so on. These organic fillers may be used alone in 1 kind, or may be used in combination in 2 or more kinds.

In the separator of the present disclosure, the heat-resistant porous layer may contain additives such as a dispersant such as a surfactant, a wetting agent, an antifoaming agent, and a pH adjuster. The dispersant may be added to the coating liquid for forming the heat-resistant porous layer for the purpose of improving dispersibility, coatability, or storage stability. The wetting agent, the defoaming agent, and the pH adjuster may be added to the coating liquid for forming the heat-resistant porous layer for the purpose of, for example, improving affinity with the porous base material, suppressing entrainment of air bubbles into the coating liquid, or adjusting pH.

[ Properties of Heat-resistant porous layer ]

The thickness of the heat-resistant porous layer is preferably 0.5 μm or more on one side, more preferably 1 μm or more on one side, from the viewpoint of heat resistance and handling properties of the separator, and is preferably 5 μm or less on one side, more preferably 4 μm or less on one side, from the viewpoint of handling properties of the separator and energy density of the battery. The thickness of the heat-resistant porous layer is preferably 1 μm or more, more preferably 2 μm or more, preferably 10 μm or less, and more preferably 8 μm or less, in terms of the total of both surfaces, regardless of whether the heat-resistant porous layer is present only on one surface of the porous substrate or on both surfaces.

The mass of the heat-resistant porous layer per unit area is preferably 1.0g/m in terms of the sum of both surfaces from the viewpoint of heat resistance and handling properties of the separator²Above, more preferably 2.0g/m²From the viewpoint of handling of the separator and energy density of the battery, the total of both surfaces is preferably 10.0g/m²Hereinafter, more preferably 8.0g/m²The following.

When the heat-resistant porous layer is present on both sides of the porous substrate, the difference in mass between the one side and the other side of the heat-resistant porous layer is preferably 20 mass% or less with respect to the total of both sides from the viewpoint of suppressing curling of the separator.

The porosity of the heat-resistant porous layer is preferably 30% or more from the viewpoint of ion permeability of the separator, and is preferably 80% or less from the viewpoint of thermal dimensional stability of the separator. The method of solving the porosity of the heat-resistant porous layer is the same as the method of solving the porosity of the porous substrate.

The average pore diameter of the heat-resistant porous layer is preferably 10nm to 200 nm. When the average pore diameter is 10nm or more, pores are less likely to be blocked even if the resin contained in the heat-resistant porous layer swells when the heat-resistant porous layer is impregnated with the electrolyte solution. When the average pore diameter is 200nm or less, the uniformity of ion movement is high, and the cycle characteristics and load characteristics of the battery are excellent.

The average pore diameter (nm) of the heat-resistant porous layer was calculated by the following formula assuming that all pores were cylindrical.

d＝4V/S

Wherein d represents the average pore diameter (diameter) of the heat-resistant porous layer, and V represents 1m per unit²The pore volume of the heat-resistant porous layer, S represents 1m per unit²The pore surface area of the heat-resistant porous layer.

Every 1m²The pore volume V of the heat-resistant porous layer was calculated from the porosity of the heat-resistant porous layer.

Every 1m²The pore surface area S of the heat-resistant porous layer was determined by the following method.

First, the BET formula was applied by the nitrogen adsorption method to calculate the specific surface area (m) of the porous substrate from the nitrogen adsorption amount²Specific surface area of the separator (m)²In terms of/g). Their specific surface area (m)²Multiplied by the respective weights per unit area (g/m)²) Calculating each 1m²Pore surface area of (a). Then, from every 1m²Pore surface area of the separator minus 1m per²The pore surface area of the porous substrate was calculated for each 1m²The heat-resistant porous layer has a pore surface area S.

The peel strength between the porous base material and the heat-resistant porous layer is preferably 0.1N/10mm or more, more preferably 0.2N/10mm or more, and even more preferably 0.3N/10mm or more, from the viewpoint of the adhesion strength of the separator to the electrode. From the above-mentioned viewpoint, the higher the peel strength between the porous substrate and the heat-resistant porous layer is, the better, but the peel strength is usually 2N/10mm or less. In the case where the separator of the present disclosure has the heat-resistant porous layers on both surfaces of the porous substrate, the peel strength between the porous substrate and the heat-resistant porous layer is preferably in the above range on both surfaces of the porous substrate.

[ characteristics of separator ]

The thickness of the separator of the present disclosure is preferably 6 μm or more, more preferably 7 μm or more from the viewpoint of mechanical strength of the separator, and is preferably 25 μm or less, more preferably 20 μm or less from the viewpoint of energy density of the battery.

The puncture strength of the separator of the present disclosure is preferably 160gf (1.6N) to 1000gf (9.8N), and more preferably 200gf (2.0N) to 600gf (5.9N), from the viewpoint of mechanical strength of the separator and short circuit resistance of the battery. The puncture strength of the separator is measured by the same method as the puncture strength of the porous substrate.

The porosity of the separator of the present disclosure is preferably 30% to 60% from the viewpoint of the operability, ion permeability, or mechanical strength of the separator.

The Gurley value (JIS P8117: 2009) of the separator of the present disclosure is preferably 50 seconds/100 mL to 800 seconds/100 mL, and more preferably 100 seconds/100 mL to 450 seconds/100 mL, from the viewpoint of balance between mechanical strength and ion permeability.

In the separator of the present disclosure, from the viewpoint of ion permeability, a value obtained by subtracting the Gurley value of the porous substrate from the Gurley value of the separator is preferably 300 seconds/100 mL or less, more preferably 150 seconds/100 mL or less, and still more preferably 100 seconds/100 mL or less. The lower limit of the value obtained by subtracting the Gurley value of the porous substrate from the Gurley value of the separator is not particularly limited, and is usually 10 seconds/100 mL or more in the separator of the present disclosure.

The membrane resistance of the separator of the present disclosure is preferably 1 Ω · cm from the viewpoint of the load characteristics of the battery²～10Ω·cm². The membrane resistance of the separator is a resistance value in a state where the separator is impregnated with an electrolyte, and 1mol/L LiBF is used₄1, 2-propylene carbonate: ethylene carbonate (1: 1 by mass) was used as an electrolyte, and the value was measured by the AC method at 20 ℃.

The tortuosity (tortuosity) of the separator of the present disclosure is preferably 1.2 to 2.8 from the viewpoint of ion permeability.

The amount of water contained in the separator of the present disclosure (on a mass basis) is preferably 1000ppm or less. The smaller the moisture content of the separator, the more the reaction between the electrolyte and water can be suppressed and the more the gas generation in the battery can be suppressed in the case of constituting the battery, and the cycle characteristics of the battery can be improved. From this viewpoint, the amount of water contained in the separator is more preferably 800ppm or less, and still more preferably 500ppm or less.

The area shrinkage of the separator of the present disclosure when heat-treated at 135 ℃ for 0.5 hour is preferably 35% or less, more preferably 30% or less, and still more preferably 20% or less.

The separator of the present disclosure preferably has a shrinkage ratio in the MD direction when heat-treated at 135 ℃ for 0.5 hours of 25% or less, more preferably 20% or less, still more preferably 15% or less, and still more preferably 10% or less.

The separator of the present disclosure preferably has a shrinkage ratio in the TD direction when heat-treated at 135 ℃ for 0.5 hours of 25% or less, more preferably 20% or less, further preferably 15% or less, and further preferably 10% or less.

The shrinkage rate when the separator of the present disclosure is subjected to heat treatment can be controlled by, for example, the content of the inorganic particles in the heat-resistant porous layer, the thickness of the heat-resistant porous layer, the porosity of the heat-resistant porous layer, and the like.

The separator of the present disclosure may further include a porous substrate and a layer other than the heat-resistant porous layer. As the other layer, an adhesive layer provided as an outermost layer mainly for adhesion to an electrode can be given.

[ method for producing separator ]

The separator of the present disclosure can be produced by forming a heat-resistant porous layer on a porous substrate by, for example, a wet coating method or a dry coating method. In the present disclosure, the wet coating method refers to a method of curing a coating layer in a solidification liquid, and the dry coating method refers to a method of drying and curing a coating layer. Hereinafter, an embodiment of the wet coating method will be described.

The wet coating method is the following method: the coating liquid containing a binder resin and inorganic particles is applied to a porous substrate, the porous substrate is immersed in a coagulating liquid to cure the coating layer, and the coating layer is taken out from the coagulating liquid, washed with water, and dried.

The coating liquid for forming the heat-resistant porous layer is prepared by dissolving or dispersing a binder resin and inorganic particles in a solvent. If necessary, other components than the binder resin and the inorganic particles are dissolved or dispersed in the coating liquid.

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

The solvent used for preparing the coating liquid preferably contains a phase separation agent that induces phase separation from the viewpoint of forming a porous layer having a good porous structure. Therefore, the solvent used for preparing the coating liquid is preferably a mixed solvent of a good solvent and a phase-separating agent. The phase separation agent is preferably mixed with the good solvent in an amount that can ensure a viscosity suitable for coating. Examples of the phase separating agent include water, methanol, ethanol, propanol, butanol, butanediol, ethylene glycol, propylene glycol, and tripropylene glycol.

The solvent used for preparing the coating liquid is preferably a mixed solvent of a good solvent and a phase-separating agent, which contains 60 mass% or more of the good solvent and 40 mass% or less of the phase-separating agent, from the viewpoint of forming a good porous structure.

The resin concentration of the coating liquid is preferably 1 to 20% by mass from the viewpoint of forming a good porous structure. From the viewpoint of forming a good porous structure, the inorganic particle concentration of the coating liquid is preferably 2% by mass to 50% by mass.

Examples of means for applying the coating liquid to the porous substrate include a meyer bar, a die coater, a reverse roll coater, a roll coater, and a gravure coater. When the heat-resistant porous layer is formed on both sides of the porous base material, the coating liquid is preferably applied to the porous base material on both sides simultaneously from the viewpoint of productivity.

Curing of the coating layer may be carried out by: the porous substrate having the coating layer formed thereon is immersed in a coagulating liquid, and the binder resin is cured while phase separation is induced in the coating layer. In this way, a laminate composed of the porous substrate and the heat-resistant porous layer was obtained.

The coagulation liquid generally contains a good solvent and a phase-separating agent used for preparation of the coating liquid, and water. In production, the mixing ratio of the good solvent to the phase-separating agent is preferably the same as the mixing ratio of the mixed solvent used in the preparation of the coating liquid. From the viewpoint of formation of a porous structure and productivity, the content of water in the coagulation liquid is preferably 40% by mass to 90% by mass. The temperature of the solidification solution is, for example, 20 ℃ to 50 ℃.

After the coating layer is cured in the solidification liquid, the laminate is taken out of the solidification liquid and washed with water. The solidified liquid was removed from the laminate by washing with water. Further, water was removed from the laminate by drying. The water washing is performed by, for example, transporting the stacked body in a water bath. The drying is performed, for example, by conveying the laminate in a high-temperature environment, blowing air to the laminate, and bringing the laminate into contact with a heating roller. The drying temperature is preferably 40 ℃ to 80 ℃.

The separator of the present disclosure may also be manufactured using a dry coating method. The dry coating method is the following method: the coating liquid is applied to the porous substrate, the coating layer is dried, and the solvent is evaporated and removed, thereby forming a heat-resistant porous layer on the porous substrate. However, the dry coating method is more likely to make the dried coating layer denser than the wet coating method, and therefore, the wet coating method is preferable in terms of obtaining a good porous structure.

The separator of the present disclosure may also be manufactured using the following method: the heat-resistant porous layer is formed as an independent sheet, and the heat-resistant porous layer and the porous base material are stacked and then combined by thermocompression bonding or an adhesive. Examples of a method for producing the heat-resistant porous layer as an independent sheet include the following methods: the heat-resistant porous layer is formed on the release sheet by applying the wet coating method or the dry coating method described above.

< nonaqueous Secondary Battery

The nonaqueous secondary battery of the present disclosure is a nonaqueous secondary battery that obtains electromotive force by doping and dedoping lithium, and includes a positive electrode, a negative electrode, and the separator for a nonaqueous secondary battery of the present disclosure. Doping refers to absorption, carrying, adsorption, or intercalation, and refers to a phenomenon in which lithium ions enter an active material of an electrode such as a positive electrode.

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 sealed in an outer package together with an electrolyte solution. The nonaqueous secondary battery of the present disclosure is suitable for a nonaqueous electrolyte secondary battery, particularly a lithium ion secondary battery.

The nonaqueous secondary battery of the present disclosure has excellent safety because the separator of the present disclosure not only suppresses gas generation inside the battery, but also has excellent heat resistance. In addition, since the separator of the present disclosure reduces the HF concentration inside the battery, the battery characteristics such as cycle characteristics are also excellent.

Hereinafter, examples of the positive electrode, the negative electrode, the electrolyte solution, and the outer material of the nonaqueous secondary battery according to the present disclosure will be described.

An example of the embodiment of the positive electrode is a structure in which an active material layer containing a positive electrode active material and a binder resin is molded on a current collector. The active material layer may further include a conductive aid. Examples of the positive electrode active material include lithium-containing transition metal oxidesSpecific examples of the compound include LiCoO₂、LiNiO₂、LiMn_1/2Ni_1/2O₂、LiCo_1/3Mn_1/ ₃Ni_1/3O₂、LiMn₂O₄、LiFePO₄、LiCo_1/2Ni_1/2O₂、LiAl_1/4Ni_3/4O₂And the like. Examples of the binder resin include a polyvinylidene fluoride resin and a styrene-butadiene copolymer. Examples of the conductive aid include carbon materials such as acetylene black, ketjen black, and graphite powder. Examples of the current collector include an aluminum foil, a titanium foil, and a stainless steel foil having a thickness of 5 to 20 μm.

In the nonaqueous secondary battery of the present disclosure, since the heat-resistant porous layer is also excellent in oxidation resistance, LiMn that can be operated at a high voltage of 4.2V or more can be easily applied by disposing the heat-resistant porous layer in contact with the positive electrode of the nonaqueous secondary battery_1/2Ni_1/2O₂、LiCo_1/3Mn_1/3Ni_1/3O₂And the like as the positive electrode active material.

An example of the embodiment of the negative electrode is a structure in which an active material layer containing a negative electrode active material and a binder resin is molded on a current collector. The active material layer may further include a conductive aid. Examples of the negative electrode active material include materials capable of electrochemically occluding lithium, and specific examples thereof include carbon materials; alloys of silicon, tin, aluminum, etc. with lithium; a wood alloy; and so on. Examples of the binder resin include a polyvinylidene fluoride resin and a styrene-butadiene copolymer. Examples of the conductive aid include carbon materials such as acetylene black, ketjen black, and graphite powder. Examples of the current collector include copper foil, nickel foil, and stainless steel foil having a thickness of 5 to 20 μm. In addition, a metal lithium foil may be used as the negative electrode instead of the negative electrode described above.

The electrolyte is a solution obtained by dissolving a lithium salt in a nonaqueous solvent. Examples of the lithium salt include LiPF₆、LiBF₄、LiClO₄And the like. As the nonaqueous solvent, there may be mentionedExamples thereof include cyclic carbonates such as ethylene carbonate, 1, 2-propylene carbonate, fluoroethylene carbonate, difluoroethylene carbonate and vinylene carbonate; chain carbonates such as dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, and fluorine substitutes thereof; cyclic esters such as γ -butyrolactone and γ -valerolactone; and the like, and they may be used alone or in combination. The electrolyte solution is preferably prepared by mixing a cyclic carbonate and a chain carbonate in a ratio of 20: 80-40: 60 (cyclic carbonate: chain carbonate) and a lithium salt dissolved therein in a range of 0.5 to 1.5 mol/L.

Examples of the outer packaging material include a metal can and an aluminum laminated film package. The shape of the battery is square, cylindrical, button-shaped, etc., and the separator of the present disclosure is suitable for any shape.

The nonaqueous secondary battery of the present disclosure may be manufactured by: the separator of the present disclosure is disposed between a positive electrode and a negative electrode, wound in a longitudinal direction to produce a wound body, and then the wound body is put into an outer packaging material, further impregnated with an electrolyte solution, and sealed. The same applies to the case where an element manufactured by stacking at least 1 layer each of the positive electrode, the separator, and the negative electrode in this order (so-called stacking method) is used instead of the roll.

Examples

The separator and the nonaqueous secondary battery according to the present disclosure will be described in more detail below with reference to examples. The materials, the amounts used, the ratios, the processing steps, and the like shown in the following examples may be appropriately changed without departing from the gist of the present disclosure. Therefore, the scope of the separator and the nonaqueous secondary battery of the present disclosure should not be construed as being limited to the specific examples shown below.

< measuring method, evaluation method >

The measurement methods and evaluation methods applied in examples and comparative examples are as follows.

[ average Primary particle diameter of inorganic particles ]

In the observation by a Scanning Electron Microscope (SEM), the major diameters of 100 arbitrarily selected particles were measured, and the average value thereof was calculated as the average primary particle diameter (μm) of the inorganic particles.

[ weight average molecular weight of resin ]

The weight average molecular weight (Mw) of the resin was measured as a molecular weight in terms of polystyrene using a gel permeation chromatography analyzer (GPC-900, Japan Spectroscopy Co., Ltd.), 2 TSKgel SUPER AWM-H, Tosoh Co., Ltd., a column, N-dimethylformamide as a solvent, under conditions of a temperature of 40 ℃ and a flow rate of 10 ml/min.

[ film thickness ]

The film thicknesses of the porous substrate and the separator were measured by a contact type thickness meter (LITEMATIC manufactured by Mitutoyo Corporation). A cylindrical measuring terminal having a diameter of 5mm was used, and the measurement was performed while applying a load of 7g, and 20 arbitrary points within 10cm × 10cm were measured, and the average value thereof was calculated.

The thickness of the heat-resistant porous layer is determined by subtracting the thickness of the porous base material from the thickness of the separator.

[ porosity ]

The porosity of the porous substrate and the separator was determined by the following calculation method.

The constituent materials are a, b, c, …, n, the mass of each constituent material is Wa, Wb, Wc, …, Wn (g/cm)²) The true densities of the constituent materials are da, db, dc, …, dn (g/cm)³) When the film thickness is represented by t (cm), the porosity ε (%) is obtained by the following equation.

ε＝{1-(Wa/da+Wb/db+Wc/dc+…+Wn/dn)/t}×100

[ Gurley value ]

Gurley values of the porous substrate and the separator are determined in accordance with JIS P8117: 2009. measured by a Gurley air permeability measuring apparatus (G-B2C, Toyo Seiki Seisaku-Sho Ltd.).

[ area shrinkage ratio based on Heat treatment ]

The separator was cut out to have a width of 180mm in the MD direction and 60mm in the TD direction to prepare a test piece. On the test piece, marks (referred to as a point a and a point B, respectively) were marked on a line bisecting the TD direction at positions 20mm and 170mm from one end. Further, marks (referred to as a point C and a point D, respectively) are marked on a line bisecting the MD direction at positions 10mm and 50mm from one end. The sheet was held by a jig (the portion where the jig was fixed was between the end nearest to the point A and the point A), suspended in a temperature-controlled oven, subjected to heat treatment at 120 ℃ for 0.5 hour under a tensionless condition, and then the shrinkage was measured. Then, the test piece heat-treated at 120 ℃ was again suspended in a temperature-controlled oven and heat-treated at 135 ℃ for 0.5 hour. In the measurement of the shrinkage, the lengths between AB and CD were measured before and after each heat treatment, and the area shrinkage was calculated by the following formula, and the average value of the area shrinkage of 10 test pieces was obtained.

Area shrinkage (%) {1- (length of AB after heat treatment ÷ length of AB before heat treatment) × (length of CD after heat treatment ÷ length of CD before heat treatment) } × 100

[ Spot heating ]

The separator was cut out to 50mm in the MD direction and 50mm in the TD direction to prepare a test piece. The test piece was placed on a horizontal table, an iron having a tip diameter of 2mm was heated, and the tip of the iron was brought into point contact with the surface of the separator for 60 seconds in a state where the tip temperature had been brought to 260 ℃. The area (mm) of the hole produced in the diaphragm by point contact was measured²) Further, the average value of the hole areas of the 10 test pieces was determined. The higher the heat resistance of the separator, the smaller the area of the pores formed in the separator.

[ high temperature storage test (gas generation amount) ]

The separator was cut into a predetermined size, placed in an aluminum laminate film package, an electrolyte was injected into the package to impregnate the separator with the electrolyte, and the package was sealed to obtain a test cell. LiPF is used as an electrolyte at a concentration of 1mol/L₆-ethylene carbonate: 0.1 wt% of ion-exchanged water was added to ethyl methyl carbonate (mass ratio: 3: 7) to obtain an electrolyte solution. The test unit cells were left to stand at 85 ℃ for 7 days, and the volume of the test unit cells before and after the heat treatment was measured. The volume V1 of the test cell before the heat treatment was subtracted from the volume V2 of the test cell after the heat treatment to determine the gas generation amount V (V2 to V1, unit: mL). Divide V by the area of the diaphragm, calculate every 1m²Gas of diaphragmVolume production.

[ HF concentration ]

The electrolyte recovered from the unit cell after the high-temperature storage test was centrifuged (3000rpm, 10 minutes), and the supernatant was recovered, diluted with ultrapure water, and stirred. The liquid was treated with a solid phase extraction column, and the fluorine ion (F-) was quantitatively analyzed by Ion Chromatography (IC).

[ circulation characteristics ]

94g of lithium cobaltate powder as a positive electrode active material, 3g of acetylene black as a conductive aid, and 3g of polyvinylidene fluoride as a binder were dissolved in N-methylpyrrolidone so that the concentration of polyvinylidene fluoride became 5 mass%, and the resulting solution was stirred with a double arm mixer to prepare a slurry for a positive electrode. This slurry for a positive electrode was applied to one surface of an aluminum foil having a thickness of 20 μm, dried, and then pressurized to obtain a positive electrode having a positive electrode active material layer.

300g of artificial graphite as a negative electrode active material, 7.5g of a water-soluble dispersion containing a modified styrene-butadiene copolymer as a binder in an amount of 40 mass%, 3g of carboxymethyl cellulose as a thickener, and an appropriate amount of water were stirred and mixed by a double arm mixer to prepare a slurry for a negative electrode. The slurry for a negative electrode was applied to a copper foil having a thickness of 10 μm as a negative electrode current collector, dried, and then pressurized to obtain a negative electrode having a negative electrode active material layer.

The positive electrode and the negative electrode (length 70mm, width 30mm) were stacked on each other with the separator obtained in the following examples and comparative examples interposed therebetween, and the battery element was obtained by welding a tab. The battery element was housed in an aluminum laminated film package, impregnated with an electrolyte, and then subjected to hot pressing under a pressure of 1MPa, a temperature of 90 ℃, and a time of 2 minutes, and the exterior was sealed to obtain a secondary battery for test (thickness of 1.1 mm). Here, 1mol/L LiPF was used as the electrolyte₆-ethylene carbonate: and methyl ethyl carbonate (mass ratio of 3: 7).

Using the prepared secondary battery for testing, 500 cycles of charge and discharge cycles were repeated under the conditions of 4.2V constant current at 1C, 2 hours of constant voltage charge, and 3V off constant current discharge at 1C in an environment of 45 ℃, and the rate of discharge capacity obtained after 500 cycles (%), (discharge capacity after 500 cycles/discharge capacity at first cycle × 100) was determined as a percentage using the discharge capacity obtained in the first cycle as a reference, and this value was used as an index for evaluating cycle characteristics.

< production of diaphragm >

[ example 1]

Polyvinylidene fluoride resin (VDF-HFP copolymer, VDF: HFP (molar ratio) 97.6: 2.4, weight average molecular weight 113 ten thousand) was dissolved in a mixed solvent of dimethylacetamide (DMAc) and tripropylene glycol (TPG) (DMAc: TPG 90: 10[ mass ratio ]) so that the resin concentration became 4 mass%, and natural apatite particles (average primary particle diameter 2.7 μm) were further stirred and mixed to obtain a coating liquid (P).

An appropriate amount of the coating liquid (P) was placed on a pair of mayer rods, and a polyethylene microporous membrane (thickness 9 μm, porosity 36%, Gurley 168 sec/100 mL) was passed between the mayer rods, and an equal amount of the coating liquid (P) was applied to both surfaces. The coating layer was solidified by immersing the coating layer in a solidifying solution (DMAc: TPG: water 30: 8: 62[ mass ratio ], liquid temperature 40 ℃), followed by washing in a water washing bath at 40 ℃ and drying. In this way, a separator in which heat-resistant porous layers were formed on both sides of the polyethylene microporous membrane was obtained.

[ example 2]

A separator was produced in the same manner as in example 1, except that the natural apatite particles were changed to other natural apatite particles (average primary particle size was 1.0 μm).

[ example 3]

A separator was produced in the same manner as in example 1, except that the natural apatite particles were changed to hydroxyapatite particles (average primary particle size of 0.3 μm).

[ example 4]

A separator was produced in the same manner as in example 1, except that the natural apatite particles were changed to hydroxyapatite particles (average primary particle size of 0.05 μm).

[ example 5]

A separator was produced in the same manner as in example 1, except that the natural apatite particles were changed to hydroxyapatite particles (average primary particle size of 0.01 μm).

[ example 6]

The meta-type wholly aromatic polyamide was dissolved in a mixed solvent of dimethylacetamide (DMAc) and tripropylene glycol (TPG) (DMAc: TPG: 80: 20[ mass ratio ]) so that the resin concentration became 4 mass%, and hydroxyapatite particles (average primary particle diameter: 0.3 μm) were further stirred and mixed to obtain coating liquid (a).

An appropriate amount of coating liquid (a) was placed on a meyer rod, and the coating liquid (a) was applied to one side of a polyethylene microporous membrane (thickness: 9 μm, porosity: 36%, Gurley: 168 sec/100 mL). The coating layer was solidified by immersing the coating layer in a solidifying solution (DMAc: TPG: water 30: 8: 62[ mass ratio ], liquid temperature 40 ℃), followed by washing in a water washing bath at 40 ℃ and drying. In this way, a separator in which a heat-resistant porous layer was formed on one surface of a polyethylene microporous membrane was obtained.

[ examples 7 to 8]

A separator was produced in the same manner as in example 3, except that the volume ratio of the hydroxyapatite particles was changed as described in table 1.

Comparative example 1

A separator was produced in the same manner as in example 1, except that the natural apatite particles were changed to other hydroxyapatite particles (average primary particle size was 9.2 μm).

Comparative example 2

A separator was produced in the same manner as in example 1, except that the natural apatite particles were changed to other hydroxyapatite particles (average primary particle size was less than 0.01 μm).

Comparative example 3

A separator was produced in the same manner as in example 1, except that the natural apatite particles were changed to magnesium hydroxide (average primary particle diameter: 0.9 μm).

[ examples 9 to 12]

A separator was produced in the same manner as in example 1, except that the natural apatite particles were changed to a mixture of hydroxyapatite particles (average primary particle size of 0.3 μm) and magnesium hydroxide (average primary particle size of 0.5 μm), and the mixing ratio of the hydroxyapatite particles and the magnesium hydroxide was adjusted as described in table 1.

The compositions, physical properties and evaluation results of the separators of examples 1 to 12 and comparative examples 1 to 3 are shown in table 1.

[ Table 1]

Claims

1. A separator for a nonaqueous secondary battery, comprising:

a porous substrate; and

a heat-resistant porous layer provided on one or both surfaces of the porous substrate, the heat-resistant porous layer containing a binder resin and first inorganic particles made of apatite,

the first inorganic particles have an average primary particle diameter (D50) of 0.01 to 3.0 [ mu ] m.

2. The separator for a nonaqueous secondary battery according to claim 1, wherein the average primary particle diameter (D50) of the first inorganic particles is 0.01 μm or more and 1.0 μm or less.

3. The separator for a nonaqueous secondary battery according to claim 1 or 2, wherein the binder resin contains a polyvinylidene fluoride-based resin.

4. The nonaqueous secondary battery separator according to claim 3, wherein the weight average molecular weight of the polyvinylidene fluoride resin is 50 to 300 ten thousand.

5. The separator for a nonaqueous secondary battery according to claim 1 or 2, wherein the binder resin contains at least 1 selected from the group consisting of wholly aromatic polyamide, polyamideimide, poly-N-vinylacetamide, polyacrylamide, copolyether polyamide, polyimide, and polyetherimide.

6. The nonaqueous secondary battery separator according to claim 1 or 2, wherein a volume ratio of the first inorganic particles in the heat-resistant porous layer is 10 to 90 vol%.

7. The nonaqueous secondary battery separator according to claim 1 or 2, wherein the heat-resistant porous layer contains second inorganic particles different from the first inorganic particles in addition to the first inorganic particles.

8. The nonaqueous secondary battery separator according to claim 7, wherein the content of the first inorganic particles is 5 to 95 vol% based on the total mass of the first inorganic particles and the second inorganic particles.

9. The nonaqueous secondary battery separator according to claim 7, wherein the second inorganic particles contain at least 1 selected from the group consisting of metal oxides, metal hydroxides, metal nitrides, metal salts, and clays.

10. The nonaqueous secondary battery separator according to claim 9, wherein the second inorganic particles contain at least 1 selected from the group consisting of aluminum oxide, boehmite, zinc oxide, magnesium hydroxide, and barium sulfate.

11. The nonaqueous secondary battery separator according to claim 7, wherein the total volume ratio of the first inorganic particles and the second inorganic particles in the heat-resistant porous layer is 10 to 90 vol%.

12. A nonaqueous secondary battery comprising a positive electrode, a negative electrode, and the separator for a nonaqueous secondary battery according to any one of claims 1 to 11 disposed between the positive electrode and the negative electrode, wherein the nonaqueous secondary battery obtains an electromotive force by doping/dedoping lithium.