Lithium ion secondary battery

Abstract

This lithium ion secondary battery comprises a positive electrode, a negative electrode, an electrolytic solution, and a separator. The negative electrode operates by intercalation of lithium ions. The electrolytic solution contains a nonaqueous solvent and a lithium salt, and has a lithium salt concentration of 2.5 mol / L or more. The separator has a polyolefin microporous film, a fibrous meta-type aromatic polyamide included in the pores of the polyolefin microporous film, and a porous layer that is provided to one surface or both surfaces of the polyolefin microporous film and contains a meta-type aromatic polyamide.

Description

Lithium-ion secondary battery

[0001] The present disclosure relates to lithium-ion secondary batteries.

[0002] In order to improve the heat resistance of secondary batteries, separators provided with a porous layer containing a heat-resistant resin such as polyamide, and secondary batteries including such separators have been developed. For example, Patent Document 1 discloses a lithium-ion battery using a porous film containing an aromatic polyamide as a separator. For example, Patent Document 2 discloses a secondary battery including a separator provided with a heat-resistant layer containing a polyamide or the like. For example, Patent Document 3 discloses a separator for an electricity storage device including a heat-resistant porous layer containing a wholly aromatic polyamide or the like.

[0003] High-salt electrolytes with high lithium salt concentrations have been developed for secondary batteries. For example, Patent Document 4 discloses a nonaqueous electrolyte in which the molar ratio of lithium imide salt to solvent is 1:0.8 to 1:2.0. For example, Patent Document 5 discloses an electrolyte containing 3 mol or less of nonaqueous solvent per 1 mol of lithium salt.

[0004] JP 2015-060724 A JP 2013-222582 A WO 2021 / 210318 JP 2019-160723 A WO 2013 / 146714

[0005] High-salinity electrolytes with high lithium salt concentrations generally have high viscosity, and thus, until now, the types of separators that can accommodate high-salinity electrolytes have been limited. Single-layer polyolefin microporous membranes, which are commonly used as battery separators, are difficult to penetrate with high-salinity electrolytes and are therefore unsuitable as separators for lithium-ion secondary batteries containing high-salinity electrolytes. On the other hand, glass fiber nonwoven fabrics have relatively large pore sizes, allowing high-salinity electrolytes to penetrate, making them suitable for use as separators for lithium-ion secondary batteries containing high-salinity electrolytes. However, separators with relatively large pore sizes, such as glass fiber nonwoven fabrics, are relatively thick and therefore unsuitable for increasing the energy density of batteries. Furthermore, separators with relatively large pore sizes have low breakdown voltages, making it relatively difficult to detect insulation defects during the battery manufacturing process.

[0006] In order to commercialize lithium-ion secondary batteries with high-salt electrolytes, separators must have the following characteristics: permeability to the high-salt electrolyte, thinness to enable the battery's energy density, mechanical strength to ensure insulation despite its thinness, ease of handling during battery manufacturing, and high dielectric breakdown voltage.

[0007] The present disclosure has been made under the above circumstances. The present disclosure aims to commercialize a lithium ion secondary battery having a high-salt concentration electrolyte solution with a lithium salt concentration of 2.5 mol / L or more, and an object of the present disclosure is to achieve this.

[0008] Specific means for solving the above problems include the following aspects.

[0009] <1> A lithium ion secondary battery comprising: a positive electrode; a negative electrode that operates by intercalation of lithium ions; an electrolyte solution containing a nonaqueous solvent and a lithium salt, the lithium salt concentration being 2.5 mol / L or more; and a separator, wherein the separator has a polyolefin microporous membrane, a fibrous meta-aromatic polyamide contained in the pores of the polyolefin microporous membrane, and a porous layer containing the meta-aromatic polyamide provided on one or both sides of the polyolefin microporous membrane. <2> The lithium ion secondary battery according to <1>, wherein the lithium salt comprises at least one selected from the group consisting of sulfonamide lithium salts and sulfonimide lithium salts. <3> The lithium ion secondary battery according to <1> or <2>, wherein the separator has the porous layer on both sides of the polyolefin microporous membrane. <4> The lithium ion secondary battery according to any one of <1> to <3>, wherein the porous layer further contains inorganic particles. <5> The lithium ion secondary battery according to <4>, wherein the inorganic particles comprise metal sulfate particles. <6> The lithium ion secondary battery according to <4> or <5>, wherein the inorganic particles include magnesium compound particles. <7> The lithium ion secondary battery according to any one of <4> to <6>, wherein the inorganic particles have an average primary particle size of 0.3 μm or less. <8> The lithium ion secondary battery according to any one of <1> to <7>, wherein the polyolefin microporous membrane is a polyethylene microporous membrane, and when differential scanning calorimetry is performed on the polyethylene microporous membrane as a sample, the melting point in a first heating process is 0.7°C or more higher than the melting point in a second heating process.

[0010] According to the present disclosure, a lithium ion secondary battery including a high salt concentration electrolyte solution with a lithium salt concentration of 2.5 mol / L or more can be put into practical use.

[0011] 1 is an image showing the results of TEM-EDS analysis of a separator, illustrating that fibrous meta-aromatic polyamide is contained in the pores of a polyolefin microporous membrane.

[0012]

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

[0013] In the present disclosure, "A 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, 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.

[0015] In the present disclosure, the term "step" includes not only an independent step but also a step that cannot be clearly distinguished from other steps as long as the purpose of the step is achieved.

[0016] In the present disclosure, when referring to the amount of each component in a composition, if multiple substances corresponding to each component are present in the composition, the total amount of the multiple substances present in the composition is meant unless otherwise specified.

[0017] In the present disclosure, the composition may contain multiple types of particles corresponding to each component. When multiple types of particles corresponding to each component are present in the composition, 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.

[0018] In this disclosure, MD (Machine Direction) refers to the longitudinal direction of a polyolefin microporous membrane and a separator produced in a continuous shape, and TD (Transverse Direction) refers to the plane direction of the polyolefin microporous membrane and the separator perpendicular to the MD. In this disclosure, TD is also referred to as the "width direction."

[0019] In the present disclosure, when the stacking relationship of each layer constituting a separator is expressed as "upper" and "lower," the layer closer to the polyolefin microporous membrane is referred to as "lower," and the layer farther from the polyolefin microporous membrane is referred to as "upper."

[0020] <Lithium-ion secondary battery> The lithium-ion secondary battery of the present disclosure includes a positive electrode, a negative electrode, an electrolyte, and a separator. The negative electrode of the lithium-ion secondary battery of the present disclosure is an electrode that operates by intercalation of lithium ions. The electrolyte of the lithium-ion secondary battery of the present disclosure contains a nonaqueous solvent and a lithium salt, and has a lithium salt concentration of 2.5 mol / L or more. The separator of the lithium-ion secondary battery of the present disclosure is a separator having a polyolefin microporous membrane, a fibrous meta-aromatic polyamide contained in the pores of the polyolefin microporous membrane, and a porous layer containing the meta-aromatic polyamide and provided on one or both sides of the polyolefin microporous membrane. Hereinafter, this separator will also be referred to as a "separator (mA)," and the porous layer containing the meta-aromatic polyamide will also be referred to as a "porous layer (mA)."

[0021] The porous layer (mA) contains a meta-type aromatic polyamide, and the polyolefin microporous membrane contains fibrous meta-type aromatic polyamide in its pores. The meta-type aromatic polyamide contains many polar groups, which is presumed to have a high affinity for high-salt electrolytes. Therefore, high-salt electrolytes easily penetrate the porous layer (mA) and the separator (mA) having fibrous meta-type aromatic polyamide in the pores of the polyolefin microporous membrane. By including the separator (mA), the lithium-ion secondary battery of the present disclosure can employ an electrolyte with a lithium salt concentration of 2.5 mol / L or more.

[0022] The separator (mA) has properties such as permeability to a high-salt concentration electrolyte, thinness that allows the battery to have high energy density, mechanical strength that ensures insulation function even when thin, ease of handling during battery manufacturing, etc. Therefore, the lithium-ion secondary battery of the present disclosure is a secondary battery that can withstand practical use by including the separator (mA).

[0023] The configuration of the lithium ion secondary battery of the present disclosure will be described in detail below.

[0024] [Positive Electrode] The positive electrode includes, for example, a current collector and a positive electrode active material layer disposed on one or both surfaces of the current collector.

[0025] The positive electrode current collector is preferably a metal foil, such as aluminum foil, titanium foil, or stainless steel foil. The thickness of the positive electrode current collector is preferably 5 μm to 20 μm.

[0026] The positive electrode active material layer preferably contains a positive electrode active material and a resin. The positive electrode active material layer may further contain a conductive additive.

[0027] The positive electrode active material is preferably a lithium-containing active material that electrochemically dopes and dedopes lithium. Examples of the lithium-containing active material include lithium-containing transition metal oxides and metal phosphates. Examples of the lithium-containing transition metal oxides and metal phosphates include LiCoO ２ , LiCoPO ４ , LiCo １／２ Ni １／２ O ２ , LiNiO ２ , Li ０．９６ NiO ２ , LiNiPO ４ , LiNi １／２ Mn １／２ O ２ , LiNi ０．５ Mn １．５ O ４ , LiCo １／３ Ni １／３ Mn １／３ O ２ , LiCo ０．２ Ni ０．４ Mn ０．４ O ２ , LiMn ２ O ４ , Li ２ MnO ３ , LiMnPO ４ , LiFeO ２ , LiFePO ４ , LiAl １／４ Ni ３／４ O ２ , Li ４ Ti５ O １２ , Li ８／７ Ti ２／７ V ４／７ O ２ , Li １．１ Nb ０．１ Mn ０．８ O ２ , Li ２ MoO ４ These may be used alone or in combination.

[0028] Examples of resins include polyvinylidene fluoride resins, alginates, etc. These may be used alone or in combination.

[0029] Examples of the conductive additive include carbon materials such as acetylene black, ketjen black, and carbon fiber. These may be used alone or in combination.

[0030] [Negative Electrode] The negative electrode is an anode that operates by intercalation of lithium ions and includes, for example, a current collector and an anode active material layer disposed on one or both sides of the current collector.

[0031] The negative electrode current collector is preferably a metal foil, such as copper foil, nickel foil, or stainless steel foil. The positive electrode current collector preferably has a thickness of 5 μm to 20 μm.

[0032] The negative electrode active material layer preferably contains a negative electrode active material and a resin, and may further contain a conductive additive.

[0033] The negative electrode active material is a material that intercalates lithium ions, and specific examples thereof include carbon materials such as graphite, hard carbon, and soft carbon; alloys of lithium with silicon, silicon compounds, tin, aluminum, and the like; Wood's alloy; etc. These may be used alone or in combination.

[0034] Examples of resins include polyvinylidene fluoride resins, styrene-butadiene copolymers, carboxymethyl cellulose, etc. These may be used alone or in combination.

[0035] Examples of the conductive additive include carbon materials such as acetylene black, ketjen black, and carbon fiber. These may be used alone or in combination.

[0036] [Electrolyte Solution] The electrolyte solution contains a non-aqueous solvent and a lithium salt, and has a lithium salt concentration of 2.5 mol / L or more. When the electrolyte solution contains multiple types of lithium salts, the total concentration of the multiple types of lithium salts contained in the electrolyte solution is 2.5 mol / L or more.

[0037] It has been reported that high-salt electrolytes with high lithium salt concentrations have a unique solution structure (Y. Yamada, et al., J. Am. Chem. Soc., 136, 5039-5046, 2014). This solution structure is characterized by the fact that all solvent molecules are lithium ions (Li ＋ ) and there are no uncoordinated solvent molecules, and still Li ＋ Since the stable solvation state of tetracoordination has not been achieved, the counter anion of the lithium salt is Li ＋ The high salt concentration electrolyte having the above solution structure exhibits properties such as high electrochemical stability, reduced volatility and flammability, an expanded potential window, and suppression of elution of transition metals in the positive electrode active material.

[0038] From the viewpoint of realizing the properties of a high-salt-concentration electrolyte solution, the lithium salt concentration of the electrolyte solution is 2.5 mol / L or more, preferably 3.0 mol / L or more, more preferably 4.0 mol / L or more, and even more preferably 5.0 mol / L or more. From the viewpoint of suppressing the viscosity of the electrolyte solution, the lithium salt concentration of the electrolyte solution is preferably 12.0 mol / L or less, more preferably 10.0 mol / L or less, even more preferably 7.0 mol / L or less, and even more preferably 6.0 mol / L or less.

[0039] From the viewpoint of achieving both the properties of a high-salt-concentration electrolyte and suppressing viscosity, the lithium salt concentration of the electrolyte is preferably 2.5 mol / L to 12.0 mol / L, more preferably 3.0 mol / L to 10.0 mol / L, even more preferably 4.0 mol / L to 7.0 mol / L, and still more preferably 5.0 mol / L to 6.0 mol / L.

[0040] Examples of the nonaqueous solvent include any known nonaqueous solvent used in lithium ion secondary batteries. Specific examples 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; cyclic esters such as γ-butyrolactone and γ-valerolactone; chain esters such as methyl acetate; ethers such as 1,2-dimethoxyethane, ethyl methyl ether, dipropyl ether, and tetrahydrofuran; nitriles such as acetonitrile and methoxypropionitrile; amines such as triethylamine; alcohols such as methanol; ketones such as acetone; fluorine-containing alkanes; dimethyl sulfoxide; sulfolane; and the like. These may be used alone or in combination.

[0041] As the nonaqueous solvent, a solvent having a relatively low viscosity, such as a chain carbonate, for example, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, or a fluorine-substituted derivative thereof, is preferred, and dimethyl carbonate is more preferred. An electrolyte solution using a solvent having a relatively low viscosity has a relatively low viscosity even when it contains a high concentration of lithium salt, and has high permeability into the separator.

[0042] The lithium salt may be any known lithium salt used in lithium ion secondary batteries. Specifically, Li(FSO ２ ) ２ N (also known as "LiFSA" or "LiFSI"), Li(CF ３ SO ２ ) ２ N (also known as "LiTFSA" or "LiTFSI"), Li(C ２ F ５ SO ２ ) ２ N (also known as "LiBETA" or "LiBETI"), Li(CF ３ SO ２ ) (C ２ F ５ SO ２ )N, Li(CF ３ SO ２ ) (C３ F ７ SO ２ )N, Li(CF ３ SO ２ ) (C ４ F ９ SO ２ Lithium sulfonamide salts and lithium sulfonimide salts such as Li(CF) ３ SO ２ ) ３ Lithium salts of sulfonmethides such as C; LiCF ３ SO ３ , LiC ４ F ９ SO ３ Lithium sulfonate salts such as LiPF ６ , LiBF ４ , LiClO ４ These may be used alone or in combination.

[0043] As the lithium salt, at least one selected from the group consisting of sulfonamide lithium salts and sulfonimide lithium salts is preferred from the viewpoint of electrochemical stability and thereby excellent cycle characteristics of the secondary battery. Specifically, Li(FSO ２ ) ２ N (also known as "LiFSA" or "LiFSI"), Li(CF ３ SO ２ ) ２ N (also known as "LiTFSA" or "LiTFSI"), Li(C ２ F ５ SO ２ ) ２ N (also known as "LiBETA" or "LiBETI"), Li(CF ３ SO ２ ) (C ２ F ５ SO ２ )N, Li(CF ３ SO ２ ) (C ３ F ７ SO ２ )N, Li(CF ３ SO ２ ) (C ４ F ９ SO ２ )N and the like are preferred.

[0044] The electrolyte preferably contains a non-aqueous solvent of dimethyl carbonate, at least one lithium salt selected from the group consisting of lithium sulfonamide salts and lithium sulfonimide salts, and has a lithium salt concentration of 2.5 mol / L to 7.0 mol / L, more preferably 3.0 mol / L to 6.5 mol / L, and even more preferably 4.0 mol / L to 6.0 mol / L.

[0045] The electrolyte may contain additives such as vinylene carbonate, propane sultone, tert-butylbenzene, fluoroethylene carbonate, lithium bis(oxalate)borate, succinonitrile, adiponitrile, triisopropoxyboroxine, sulfolane, hydrofluoroether, and vinyl acetate. These additives may be used alone or in combination.

[0046] [Separator (mA)] The separator (mA) comprises a polyolefin microporous membrane, a fibrous meta-aromatic polyamide contained in the pores of the polyolefin microporous membrane, and a porous layer (mA) provided on one or both sides of the polyolefin microporous membrane. The porous layer (mA) is a porous layer containing a meta-aromatic polyamide. The porous layer (mA) is preferably the outermost layer of the separator on one or both sides of the polyolefin microporous membrane.

[0047] The separator (mA) may be embodied in the following forms (a) to (c).

[0048] Form (a): A separator having porous layers (mA) on both sides of a polyolefin microporous membrane, wherein the porous layer (mA) on one side of the separator and the porous layer (mA) on the other side of the separator may be the same or different in components and / or composition.

[0049] Form (b): A separator having a porous layer (mA) on one side of a polyolefin microporous membrane and another porous layer (i.e., a porous layer containing no meta-aromatic polyamide) on the other side of the polyolefin microporous membrane. The other porous layer may be, for example, an adhesive layer intended to bond an electrode and the separator (mA).

[0050] Form (c): A separator having a porous layer (mA) on one side of a polyolefin microporous membrane and no layer on the other side of the polyolefin microporous membrane (i.e., the surface of the polyolefin microporous membrane is exposed).

[0051] The separator (mA) is preferably in the form (a) from the viewpoint of better permeability to a high-salt concentration electrolyte.

[0052] The separator (mA) is preferably of form (c) from the viewpoint of reducing the overall thickness of the separator and obtaining a secondary battery with a higher energy density.

[0053] The polyolefin microporous film, the fibrous meta-aromatic polyamide contained in the pores of the polyolefin microporous film, and the porous layer (mA) contained in the separator (mA) will be described in detail below.

[0054] -Polyolefin microporous membrane- In this disclosure, a polyolefin microporous membrane refers to a microporous membrane containing a polyolefin. In this disclosure, a microporous membrane refers to a membrane having numerous internal micropores, with the micropores interconnected, allowing gas or liquid to pass from one surface to the other.

[0055] The polyolefin microporous membrane may be any known polyolefin microporous membrane used for battery separators. The polyolefin microporous membrane preferably contains polyethylene to exhibit a shutdown function. The polyolefin microporous membrane preferably contains polypropylene to provide heat resistance that prevents the membrane from rupturing easily when exposed to high temperatures.

[0056] The polyolefin microporous membrane preferably contains polyethylene and polypropylene from the viewpoint of providing a shutdown function and heat resistance that prevents the membrane from easily rupturing when exposed to high temperatures. An example of a polyolefin microporous membrane containing polyethylene and polypropylene is a microporous membrane in which polyethylene and polypropylene are mixed in one layer. From the viewpoint of achieving both the shutdown function and heat resistance, this microporous membrane preferably contains a mixture of 95% by mass or more of polyethylene and 5% by mass or less of polypropylene. Furthermore, 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 also preferred.

[0057] An example of an embodiment of the polyolefin microporous membrane is a polyethylene microporous membrane whose main component is polyethylene. In the polyethylene microporous membrane, the mass of polyethylene preferably accounts for 95 mass% or more of the total mass.

[0058] The polyolefin contained in the polyolefin microporous membrane is preferably a polyolefin having a weight average molecular weight (Mw) of 100,000 to 5,000,000. When the Mw of the polyolefin is 100,000 or more, the microporous membrane can be imparted with sufficient mechanical properties. When the Mw of the polyolefin is 5,000,000 or less, the shutdown properties of the microporous membrane are good and the microporous membrane is easy to form. The Mw of the polyolefin is the molecular weight in terms of polystyrene measured by gel permeation chromatography (GPC). The measurement is performed using polyolefin extracted from the microporous membrane or polyolefin used to form the microporous membrane as a sample.

[0059] Examples of methods for producing a polyolefin microporous membrane include a method in which molten polyolefin is extruded through a T-die to form a sheet, which is crystallized, stretched, and then heat-treated to form a microporous membrane; and a method in which molten polyolefin together with a plasticizer such as liquid paraffin is extruded through a T-die, cooled to form a sheet, stretched, the plasticizer is extracted, and then heat-treated to form a microporous membrane.

[0060] The surface of the polyolefin microporous membrane may be subjected to various surface treatments, such as corona treatment, plasma treatment, flame treatment, and ultraviolet irradiation treatment, to improve wettability with a coating liquid for forming the porous layer (mA), provided that the properties of the polyolefin microporous membrane are not impaired.

[0061] -Properties of polyolefin microporous membrane- From the viewpoints of separator production yield and battery production yield, the thickness of the polyolefin microporous membrane is preferably 3 μm or more, more preferably 5 μm or more, and even more preferably 6 μm or more. From the viewpoint of increasing the energy density of the battery, the thickness of the polyolefin microporous membrane is preferably 25 μm or less, more preferably 20 μm or less, and even more preferably 15 μm or less. The thickness (μm) of the polyolefin microporous membrane is measured at 20 points within a 10 cm square using a contact thickness meter, and the average value is calculated.

[0062] The Gurley value (JIS P8117:2009) of the polyolefin microporous membrane is preferably 20 seconds / 100 mL or more, more preferably 30 seconds / 100 mL or more, and even more preferably 50 seconds / 100 mL or more, from the viewpoint of suppressing short circuits in the battery. The Gurley value (JIS P8117:2009) of the polyolefin microporous membrane is preferably 200 seconds / 100 mL or less, more preferably 180 seconds / 100 mL or less, and even more preferably 165 seconds / 100 mL or less, from the viewpoint of ion permeability. The Gurley value of the polyolefin microporous membrane is measured using a Gurley densometer according to JIS P8117:2009.

[0063] The porosity of the polyolefin microporous membrane is preferably 20% to 60%, more preferably 30% to 50%, from the viewpoint of obtaining appropriate membrane resistance and shutdown function. The porosity ε (%) of the polyolefin microporous membrane is calculated by the following formula: ε = {1 - Ws / (ds t)} × 100, where Ws is the basis weight (g / m ) of the polyolefin microporous membrane. ２ ), ds is the true density of the polyolefin microporous membrane (g / cm ３ ), t is the thickness (μm) of the polyolefin microporous membrane. Basis weight is the mass per unit area.

[0064] The average pore size of the polyolefin microporous membrane is preferably 15 to 100 nm from the viewpoint of achieving both ion permeability and suppression of short circuits in the battery. The average pore size of the polyolefin microporous membrane is measured using a perm porometer (CFP-1500-A, PMI) in accordance with ASTM E1294-89.

[0065] A preferred embodiment of the polyolefin microporous membrane is one in which the walls of the pores of the polyolefin microporous membrane are partially or entirely coated with a meta-aromatic polyamide, which allows high-salt electrolyte to easily penetrate the membrane.

[0066] A preferred embodiment of the polyethylene microporous membrane is one in which, when differential scanning calorimetry is performed on a polyethylene microporous membrane sample, the melting point in the first heating step is at least 0.7°C higher than the melting point in the second heating step. Although the detailed mechanism is unknown, when a polyethylene microporous membrane has the above thermal properties, during the formation of the porous layer (mA), the meta-aromatic polyamide tends to penetrate into the pores of the microporous membrane and adhere to the wall surfaces of the pores, or fibrous meta-aromatic polyamide tends to be formed in the pores. The melting point of the polyethylene microporous membrane in the first heating step is preferably at least 0.7°C and no more than 12°C higher, more preferably at least 1.5°C and no more than 10°C higher, and even more preferably at least 1.8°C and no more than 5°C higher than the melting point in the second heating step.

[0067] Differential scanning calorimetry (DSC) of a polyethylene microporous membrane is performed as follows. 5.0 mg±0.3 mg of the polyethylene microporous membrane is placed in an aluminum sample pan and set in a measuring device. Thermal analysis of the sample is performed in a nitrogen atmosphere by sequentially performing the following three steps: Step 1: Heat from 30°C to 200°C at a rate of 5°C / min. Step 2: Cool from 200°C to 30°C at a rate of 5°C / min. Step 3: Heat from 30°C to 200°C at a rate of 5°C / min. Based on the results of the DSC thermal analysis, a DSC curve is drawn, with the horizontal axis representing temperature (°C) and the vertical axis representing heat flow (W / g). The melting point in the first heating process is the temperature of the endothermic peak observed in the DSC curve in Step 1. If the DSC curve in Step 1 has multiple endothermic peaks, the temperature of the lowest endothermic peak is taken as the melting point. The melting point in the second temperature-raising process is the temperature of the endothermic peak observed in the DSC curve in Step 3. If there are multiple endothermic peaks in the DSC curve in Step 3, the temperature of the endothermic peak at the lowest temperature is taken as the melting point.

[0068] Examples of methods for producing a polyethylene microporous membrane having the above thermal properties include a method in which molten polyethylene is extruded through a T-die to form a sheet, which is crystallized, stretched, and then heat-treated to form a microporous membrane, in which the stretching step is sequential biaxial stretching in which longitudinal stretching and transverse stretching are carried out separately. The polyethylene microporous membrane having the above thermal properties is easy to produce by sequential biaxial stretching, but can also be produced by simultaneous biaxial stretching.

[0069] - Fibrous meta-aromatic polyamide - In the separator (mA), fibrous meta-aromatic polyamide is contained in the pores of the polyolefin microporous membrane. Because the meta-aromatic polyamide is in a fine fibrous form, it does not clog the pores of the polyolefin microporous membrane, allowing gas or liquid to pass from one side of the polyolefin microporous membrane to the other side.

[0070] The meta-aromatic polyamide refers to a polyamide in which the benzene disubstitution constituting the main chain is a meta-disubstitution. However, a small amount of a para-disubstitution may be copolymerized as the benzene disubstitution in the meta-aromatic polyamide. The details and preferred form of the meta-aromatic polyamide constituting the fibrous meta-aromatic polyamide are the same as those of the meta-aromatic polyamide contained in the porous layer (mA) (described later).

[0071] Because meta-aromatic polyamides have a high affinity for high-salt electrolytes, high-salt electrolytes easily penetrate into polyolefin microporous membranes containing fibrous meta-aromatic polyamides in their pores. From the viewpoint of the permeability of high-salt electrolytes into the polyolefin microporous membrane, it is preferable that the fibrous meta-aromatic polyamide be contained in at least the pores in the region close to the surface of the polyolefin microporous membrane, and it is more preferable that the fibrous meta-aromatic polyamide be contained in the entire pores of the polyolefin microporous membrane.

[0072] The presence of fibrous meta-aromatic polyamide in the pores of the polyolefin microporous membrane in the separator (mA) can be confirmed by elemental imaging using a combination of TEM (transmission electron microscope) and EDS (energy dispersive X-ray spectroscopy). ４ ) is easily stained by Ru, and meta-type aromatic polyamides are detected by Ru imaging. ４ The method for preparing the stained sample is as follows.

[0073] (1) Cut the separator into a triangle (base approximately 0.5 mm x height approximately 1 mm, MD is the base). (2) Affix the triangular sample to a slide glass and place it in a sealed container with a capacity of approximately 50 ml. ４Approximately 0.5 ml of the aqueous solution was introduced into a sealed container and subjected to steam staining at room temperature for 15 minutes. These staining conditions do not stain polyolefins. (3) The stained sample was impregnated with embedding resin. A stepwise substitution process was performed to ensure that the embedding resin penetrated the porous interior of the sample. Specifically, the sample was placed in n-butyl glycidyl ether, n-butyl glycidyl ether:embedding resin = 1:1, and n-butyl glycidyl ether:embedding resin = 1:3, successively for 30 minutes each, and then left in the embedding resin overnight. (4) The sample was placed on an embedding plate, and the embedding resin was injected and allowed to fully harden. (5) Ultrathin sections approximately 80 nm thick were prepared using an ultramicrotome at room temperature. The sample was cut so that the separator's MD was visible on the cross section. A carbon support film was attached to the ultrathin section and mounted on a Cu grid.

[0074] FIG. 1 shows an example of the results of TEM-EDS analysis. This example shows the results of analysis of a separator (mA) comprising a polyethylene microporous membrane as the polyolefin microporous membrane. The three images shown in FIG. 1 are a TEM image on the left, a STEM-HAADF image of a portion of the left in the center, and a Ru imaging image of the same field of view as the center in the right. In the TEM image, the cells are pores in the polyolefin microporous membrane, and the partitions between the cells are pore walls of the polyolefin microporous membrane. The TEM image and STEM-HAADF image show that there are fine fibers inside the pores. The STEM-HAADF image and Ru imaging image show that the fine fibers inside the pores are meta-type aromatic polyamide.

[0075] -Porous Layer (mA)- In the present disclosure, a porous layer is a layer having a large number of micropores therein, the micropores being interconnected, and allowing gas or liquid to pass through from one surface to the other.

[0076] The porous layer (mA) contains a meta-type aromatic polyamide. The meta-type aromatic polyamide refers to a polyamide in which the benzene disubstitution constituting the main chain is a meta-type aromatic polyamide. However, the meta-type aromatic polyamide may contain a small amount of para-type disubstitution as the benzene disubstitution copolymerized therein.

[0077] Meta-aromatic polyamides are polymers with higher flexibility than para-aromatic polyamides, and therefore tend to penetrate into the pores of the polyolefin microporous membrane during the formation of the porous layer (mA) and adhere to the wall surfaces of the pores or form a fibrous structure. When meta-aromatic polyamides are adhered to the wall surfaces of the pores of the polyolefin microporous membrane or when fibrous meta-aromatic polyamides are contained in the pores, a high-salt electrolyte solution can easily penetrate the polyolefin microporous membrane.

[0078] The meta-type aromatic polyamide may be a polyamide copolymerized with an aliphatic monomer, a polyamide not copolymerized with an aliphatic monomer, or a mixture of both.

[0079] From the viewpoint of the heat resistance of the porous layer (mA), the meta-type aromatic polyamide is preferably a wholly aromatic polyamide, and polymetaphenylene isophthalamide is particularly preferred. A wholly aromatic polyamide refers to a polyamide whose main chain is composed only of benzene rings and amide bonds. However, a small amount of an aliphatic monomer may be copolymerized in the wholly aromatic polyamide. Wholly aromatic polyamides are also called aramids.

[0080] The content of the meta-type aromatic polyamide contained in the porous layer (mA) is preferably 85% by mass to 100% by mass, more preferably 90% by mass to 100% by mass, even more preferably 95% by mass to 100% by mass, and particularly preferably 100% by mass, based on the total amount of resin contained in the porous layer (mA).

[0081] When the porous layers (mA) are present on both sides of the polyolefin microporous membrane, the type and / or content of the meta-aromatic polyamide contained in one porous layer (mA) may be the same as or different from the type and / or content of the meta-aromatic polyamide contained in the other porous layer (mA).

[0082] The porous layer (mA) may contain a resin other than meta-type aromatic polyamide. Examples of the other resin include para-type aromatic polyamide, polyamideimide, poly-N-vinylacetamide, polyacrylamide, copolymer polyether polyamide, polyimide, polyetherimide, polyvinylidene fluoride resin, acrylic resin, fluorine-containing rubber, styrene-butadiene copolymer, homopolymer or copolymer of vinyl nitrile compound (such as acrylonitrile or methacrylonitrile), carboxymethyl cellulose, hydroxyalkyl cellulose, polyvinyl alcohol, polyvinyl butyral, polyvinyl pyrrolidone, polyether (such as polyethylene oxide or polypropylene oxide), polysulfone, polyketone, polyether ketone, polyether sulfone, and mixtures thereof.

[0083] The content of the other resin contained in the porous layer (mA) is preferably 0% by mass to 15% by mass, more preferably 0% by mass to 10% by mass, even more preferably 0% by mass to 5% by mass, and particularly preferably 0% by mass, based on the total amount of resin contained in the porous layer (mA). Ideally, the porous layer (mA) does not contain any other resins other than the meta-type aromatic polyamide.

[0084] The porous layer (mA) preferably contains inorganic particles from the viewpoint of heat resistance and porosity of the layer.

[0085] Examples of inorganic particles include metal sulfate particles, metal hydroxide particles, metal oxide particles, metal carbonate particles, metal nitride particles, metal fluoride particles, clay mineral particles, etc. One type of inorganic particle may be used alone, or two or more types may be used in combination.

[0086] Examples of metal sulfates that constitute the metal sulfate particles include barium sulfate, strontium sulfate, calcium sulfate, calcium sulfate dihydrate, alum, and jarosite.

[0087] Examples of metal hydroxides that constitute the metal hydroxide particles include magnesium hydroxide, aluminum hydroxide, calcium hydroxide, chromium hydroxide, zirconium hydroxide, cerium hydroxide, and nickel hydroxide.

[0088] As the metal oxide constituting the metal oxide particles, barium titanate (BaTiO ３ ), magnesium oxide, alumina (Al ２ O ３ ), boehmite (alumina monohydrate), titania (TiO ２ ), silica (SiO ２ ), zirconia (ZrO ２ ), zinc oxide, etc.

[0089] Examples of metal carbonates constituting the metal carbonate particles include calcium carbonate and magnesium carbonate.

[0090] Examples of metal nitrides that constitute the metal nitride particles include magnesium nitride, aluminum nitride, calcium nitride, and titanium nitride.

[0091] Examples of metal fluorides constituting the metal fluoride particles include magnesium fluoride and calcium fluoride.

[0092] Examples of clay minerals that make up the clay mineral particles include calcium silicate, calcium phosphate, apatite, and talc.

[0093] The inorganic particles may be surface-modified with a silane coupling agent or the like.

[0094] 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.

[0095] When metal sulfate particles are used as the inorganic particles, the amount of the metal sulfate particles relative to the total amount of inorganic particles contained in the porous layer (mA) is preferably 80% by mass or more, more preferably 85% by mass or more, even more preferably 90% by mass or more, even more preferably 95% by mass or more, and most preferably 100% by mass, from the viewpoint of suppressing gas generation inside the battery.

[0096] When barium sulfate particles are used as the inorganic particles, the amount of barium sulfate particles relative to the total amount of inorganic particles contained in the porous layer (mA) is preferably 80% by mass or more, more preferably 85% by mass or more, even more preferably 90% by mass or more, even more preferably 95% by mass or more, and most preferably 100% by mass, from the viewpoint of suppressing gas generation inside the battery.

[0097] As the inorganic particles, from the viewpoint of high electrochemical stability, magnesium compound particles such as magnesium oxide particles, magnesium hydroxide particles, magnesium carbonate particles, magnesium nitride particles, and magnesium fluoride particles are preferred, and at least one type selected from the group consisting of magnesium oxide particles and magnesium hydroxide particles is more preferred.

[0098] When magnesium compound particles are used as the inorganic particles, the amount of magnesium compound particles relative to the total amount of inorganic particles contained in the porous layer (mA) is preferably 80% by mass or more, more preferably 85% by mass or more, even more preferably 90% by mass or more, even more preferably 95% by mass or more, and most preferably 100% by mass, from the viewpoint of high electrochemical stability.

[0099] When the porous layers (mA) are present on both sides of the polyolefin microporous membrane, the type and / or content of inorganic particles contained in one porous layer (mA) may be the same as or different from the type and / or content of inorganic particles contained in the other porous layer (mA).

[0100] The particle shape of the inorganic particles is not limited, and may be any of spherical, plate-like, cubic, needle-like, and irregular shapes. From the viewpoint of suppressing short circuits in the battery and forming a highly uniform and dense porous layer, the inorganic particles are preferably spherical, plate-like, or cubic particles, and are non-aggregated primary particles.

[0101] The average primary particle size of the inorganic particles contained in the porous layer (mA) is preferably 0.3 μm or less, more preferably 0.01 μm or more and 0.2 μm or less, and even more preferably 0.03 μm or more and 0.15 μm or less, from the viewpoint of making the layer porous and forming a highly uniform and dense porous layer.

[0102] When metal sulfate particles are used as the inorganic particles, the average primary particle size of the metal sulfate particles contained in the porous layer (mA) is preferably 0.3 μm or less, more preferably 0.01 μm or more and 0.2 μm or less, and even more preferably 0.03 μm or more and 0.15 μm or less, from the viewpoint of making the layer porous and forming a highly uniform and dense porous layer.

[0103] When barium sulfate particles are used as the inorganic particles, the average primary particle size of the barium sulfate particles contained in the porous layer (mA) is preferably 0.3 μm or less, more preferably 0.01 μm or more and 0.2 μm or less, and even more preferably 0.03 μm or more and 0.15 μm or less, from the viewpoint of making the layer porous and forming a highly uniform and dense porous layer.

[0104] When magnesium compound particles are used as the inorganic particles, the average primary particle size of the magnesium compound particles contained in the porous layer (mA) is preferably 0.3 μm or less, more preferably 0.01 μm or more and 0.2 μm or less, and even more preferably 0.03 μm or more and 0.15 μm or less, from the viewpoint of making the layer porous and forming a highly uniform and dense porous layer.

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

[0106] When the porous layers (mA) are present on both sides of the polyolefin microporous membrane, the average primary particle size of the inorganic particles contained in one porous layer (mA) may be the same as or different from the average primary particle size of the inorganic particles contained in the other porous layer (mA).

[0107] The volume ratio of the inorganic particles to the solid volume of the porous layer (mA) is preferably 10% by volume or more and 90% by volume or less, more preferably 20% by volume or more and 80% by volume or less, and even more preferably 30% by volume or more and 70% by volume or less. In the present disclosure, the solid volume of the porous layer means the volume excluding the pores of the porous layer.

[0108] When metal sulfate particles are used as the inorganic particles, the volume ratio of the metal sulfate particles to the solid volume of the porous layer (mA) is preferably 10 vol% or more and 90 vol% or less, more preferably 20 vol% or more and 80 vol% or less, and even more preferably 30 vol% or more and 70 vol% or less.

[0109] When barium sulfate particles are used as the inorganic particles, the volume ratio of the barium sulfate particles to the solid volume of the porous layer (mA) is preferably 10 vol% or more and 90 vol% or less, more preferably 20 vol% or more and 80 vol% or less, and even more preferably 30 vol% or more and 70 vol% or less.

[0110] When magnesium compound particles are used as the inorganic particles, the volume ratio of the magnesium compound particles to the solid volume of the porous layer (mA) is preferably 10 vol% or more and 90 vol% or less, more preferably 20 vol% or more and 80 vol% or less, and even more preferably 30 vol% or more and 70 vol% or less.

[0111] The volume ratio V (vol %) of the inorganic particles to the solid volume of the porous layer is calculated by the following formula: V = {(Xa / Da) / (Xa / Da + Xb / Db + Xc / Dc + ... + Xn / Dn)} x 100, where a represents the inorganic particles and b, c, ..., n represent the other constituent materials of the porous layer, the masses of the constituent materials contained in a predetermined area of ​​the porous layer are Xa, Xb, Xc, ..., Xn (g), and the true densities of the constituent materials are Da, Db, Dc, ..., Dn (g / cm ３ ) Xa and the like substituted in the above formula are the mass (g) of the constituent material used to form a porous layer of a predetermined area, or the mass (g) of the constituent material removed from a porous layer of a predetermined area. Da and the like substituted in the above formula are the true density (g / cm) of the constituent material used to form the porous layer. ３ ), or the true density (g / cm 3 ) of the constituent material extracted from the porous layer３ )

[0112] When the porous layers (mA) are present on both sides of the polyolefin microporous membrane, the volume ratio of the inorganic particles to the solid content volume of one porous layer (mA) may be the same as or different from the volume ratio of the inorganic particles to the solid content volume of the other porous layer (mA).

[0113] The mass ratio of inorganic particles in the porous layer (mA) is preferably 60% by mass or more and 95% by mass or less, more preferably 70% by mass or more and 90% by mass or less, and even more preferably 75% by mass or more and 85% by mass or less. When metal sulfate particles are used as the inorganic particles, the mass ratio of metal sulfate particles in the porous layer (mA) is preferably 60% by mass or more and 95% by mass or less, more preferably 70% by mass or more and 90% by mass or less, and even more preferably 75% by mass or more and 85% by mass or less. When barium sulfate particles are used as the inorganic particles, the mass ratio of barium sulfate particles in the porous layer (mA) is preferably 60% by mass or more and 95% by mass or less, more preferably 70% by mass or more and 90% by mass or less, and even more preferably 75% by mass or more and 85% by mass or less. When magnesium compound particles are used as the inorganic particles, the mass proportion of the magnesium compound particles in the porous layer (mA) is preferably 60 mass% or more and 95 mass% or less, more preferably 70 mass% or more and 90 mass% or less, and even more preferably 75 mass% or more and 85 mass% or less.

[0114] When the porous layers (mA) are present on both sides of the polyolefin microporous membrane, the mass ratio of the inorganic particles in one porous layer (mA) may be the same as or different from the mass ratio of the inorganic particles in the other porous layer (mA).

[0115] The porous layer (mA) may contain an organic filler. Examples of the organic filler include particles made of crosslinked polymers such as crosslinked poly(meth)acrylic acid, crosslinked poly(meth)acrylic acid ester, crosslinked polysilicone, crosslinked polystyrene, crosslinked polydivinylbenzene, styrene-divinylbenzene copolymer crosslinked products, polyimide, melamine resin, phenol resin, and benzoguanamine-formaldehyde condensate; and particles made of heat-resistant polymers such as polysulfone, polyacrylonitrile, aramid, polyacetal, and thermoplastic polyimide. The term "(meth)acrylic" means either "acrylic" or "methacrylic."

[0116] The resin constituting the organic filler 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. One type of organic filler may be used alone, or two or more types may be used in combination.

[0117] The porous layer (mA) may contain additives such as a dispersant such as a surfactant, a wetting agent, an antifoaming agent, and a pH adjuster. The dispersant is added to the coating liquid for forming the porous layer (mA) for the purpose of improving dispersibility, coatability, or storage stability. The wetting agent, antifoaming agent, and pH adjuster are added to the coating liquid for forming the porous layer (mA), for example, to improve compatibility with the polyolefin microporous membrane, to suppress air entrapment in the coating liquid, or to adjust the pH.

[0118] -Characteristics of Porous Layer (mA)- From the viewpoint of ease of handling during battery production, the thickness of the porous layer (mA) is preferably 0.1 μm or more on one side, more preferably 0.5 μm or more on one side, and even more preferably 1.0 μm or more on one side. From the viewpoint of increasing ion permeability and the energy density of the battery, the thickness of the porous layer (mA) is preferably 10.0 μm or less on one side, more preferably 8.0 μm or less on one side, and even more preferably 6.0 μm or less on one side.

[0119] When the porous layer (mA) is present on both sides of the polyolefin microporous membrane, the total thickness of the porous layer (mA) on both sides is preferably 1.0 μm or more, more preferably 2.0 μm or more, and even more preferably 3.0 μm or more, and is preferably 20.0 μm or less, more preferably 16.0 μm or less, and even more preferably 12.0 μm or less. The thickness of the porous layer (mA) (total thickness on both sides of the polyolefin microporous membrane, μm) is the value obtained by subtracting the thickness of the polyolefin microporous membrane (μm) from the thickness of the separator (mA) (μm).

[0120] When the porous layer (mA) is present on both sides of the polyolefin microporous membrane, the smaller the difference (μm) between the thickness of one porous layer (mA) and the thickness of the other porous layer (mA) is, the better, and it is preferably 20% or less of the total thickness (μm) of both sides.

[0121] The mass per unit area of ​​the porous layer (mA) is 1.0 g / m in total for both sides, whether the porous layer (mA) is on one side or both sides of the polyolefin microporous membrane, from the viewpoint of handling during battery production. ２ More than 2.0 g / m is preferable. ２ More preferably, 3.0 g / m or more ２ The mass per unit area of ​​the porous layer (mA) is more preferably 30.0 g / m in total for both sides, from the viewpoints of ion permeability and battery energy density, whether the porous layer (mA) is on one side or both sides of the polyolefin microporous membrane. ２ Preferably, 20.0 g / m or less ２ More preferably, 10.0 g / m or less ２ The following is even more preferred:

[0122] When the porous layer (mA) is present on both sides of the polyolefin microporous membrane, the difference (g / m) between the mass per unit area of ​​one porous layer (mA) and the mass per unit area of ​​the other porous layer (mA) ２ ) is preferably as small as possible from the viewpoint of suppressing curling of the separator or improving the cycle characteristics of the battery, and the total amount (g / m ２ ) is preferably 20% or less.

[0123] From the viewpoint of ion permeability, the porosity of the porous layer (mA) is preferably 30% or more, more preferably 35% or more, and even more preferably 40% or more. From the viewpoint of the mechanical strength of the porous layer (mA), the porosity of the porous layer (Am) is preferably 80% or less, more preferably 70% or less, and even more preferably 60% or less. The porosity ε (%) of the porous layer is calculated by the following formula:

[0124] Here, for constituent material 1, constituent material 2, constituent material 3, ..., constituent material n of the porous layer, the mass per unit area of ​​each constituent material is W １ , W ２、 W ３ , ..., W ｎ (g / cm ２ ) and the true density of each constituent material is d １ , d ２ , d ３ , ..., d ｎ (g / cm ３ ) and the thickness of the porous layer is t (cm).

[0125] [Characteristics of Separator (mA)] From the viewpoint of mechanical strength, the thickness of the separator (mA) is preferably 5 μm or more, more preferably 10 μm or more, and even more preferably 12 μm or more. From the viewpoint of increasing the energy density of the battery, the thickness of the separator (mA) is preferably 30 μm or less, more preferably 25 μm or less, and even more preferably 20 μm or less. The thickness (μm) of the separator (mA) is the average value measured at 20 points within a 10 cm square using a contact thickness meter.

[0126] The Gurley value (JIS P8117:2009) of the separator (mA) is preferably 40 seconds / 100 mL or more, more preferably 50 seconds / 100 mL or more, and even more preferably 60 seconds / 100 mL or more, from the viewpoint of suppressing short circuits in the battery. The Gurley value (JIS P8117:2009) of the separator (mA) is preferably 230 seconds / 100 mL or less, more preferably 220 seconds / 100 mL or less, and even more preferably 215 seconds / 100 mL or less, from the viewpoint of ion permeability. The Gurley value of the separator is measured using a Gurley densometer in accordance with JIS P8117:2009.

[0127] [Method for producing separator (mA)] The separator (mA) can be produced, for example, by forming a porous layer (mA) on a polyolefin microporous membrane 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 example embodiment of the wet coating method will be described below.

[0128] The wet coating method is a method in which a coating liquid for forming a porous layer is applied onto a polyolefin microporous membrane, the membrane is immersed in a coagulating liquid to solidify the coating layer, and the membrane is then pulled out of the coagulating liquid, washed with water, and dried.

[0129] The coating liquid for forming the porous layer (mA) is prepared by dissolving the meta-type aromatic polyamide in a solvent. If necessary, other components besides the meta-type aromatic polyamide may be dissolved or dispersed in the coating liquid.

[0130] The solvent used in preparing the coating solution includes a solvent that dissolves meta-type aromatic polyamide (hereinafter also referred to as a "good solvent"), such as polar amide solvents such as N-methylpyrrolidone, dimethylacetamide, and dimethylformamide.

[0131] 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, methanol, ethanol, propyl alcohol, butyl alcohol, butanediol, ethylene glycol, propylene glycol, and tripropylene glycol.

[0132] 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, the mixed solvent preferably contains 60% by mass or more of the good solvent and 5% by mass to 40% by mass of the phase separation agent.

[0133] The resin concentration in the coating liquid is preferably 1% by mass to 20% by mass from the viewpoint of forming a good porous structure, and the inorganic particle concentration in the coating liquid is preferably 0.5% by mass to 50% by mass from the viewpoint of forming a good porous structure.

[0134] 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 secondary battery and do not inhibit the reaction within the battery.

[0135] Examples of means for applying the coating liquid to the polyolefin microporous membrane include a Mayer bar, a die coater, a reverse roll coater, a roll coater, a gravure coater, etc. When porous layers are formed on both sides of the polyolefin microporous membrane, it is preferable from the viewpoint of productivity to apply the coating liquid to both sides of the polyolefin microporous membrane simultaneously.

[0136] The fibrous meta-aromatic polyamide contained in the pores of the polyolefin microporous membrane is formed by the penetration of a coating liquid into the pores of the polyolefin microporous membrane and the solidification of the meta-aromatic polyamide in the pores. In order to allow the coating liquid to penetrate into the pores of the polyolefin microporous membrane, it is preferable that the polyolefin microporous membrane is in a dry state when the coating liquid is applied. Conventionally, for the purpose of improving the wettability of the coating liquid, a step of applying or impregnating the polyolefin microporous membrane with a solvent for the coating liquid before coating has sometimes been carried out, but in the production of the separator (mA), it is preferable not to carry out this step.

[0137] The coating layer is solidified by immersing the polyolefin microporous membrane with the coating layer formed thereon in a coagulation liquid to solidify the resin while inducing phase separation in the coating layer, thereby obtaining a laminate comprising the polyolefin microporous membrane, the fibrous meta-aromatic polyamide contained in the pores of the polyolefin microporous membrane, and the porous layer.

[0138] 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.

[0139] 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.

[0140] The separator (mA) can also be produced by a dry coating method, in which a coating liquid is applied to a polyolefin microporous membrane and the coating layer is dried to volatilize and remove the solvent, thereby forming a porous layer on the polyolefin microporous membrane.

[0141] [Shape and Manufacturing Method of Lithium Ion Secondary Battery] The shape of the lithium ion secondary battery may be any of a square, cylindrical, coin, pouch, etc.

[0142] Examples of packaging materials for lithium ion secondary batteries include metal cans and packs made of aluminum laminated film.

[0143] A lithium ion secondary battery is manufactured, for example, through the steps of: producing a laminate in which a separator is disposed between a positive electrode and a negative electrode; housing the laminate and an electrolytic solution in an exterior packaging material and allowing the electrolytic solution to permeate the laminate; and creating a vacuum inside the exterior packaging material and sealing the exterior packaging material.

[0144] 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.

[0145] An example of an embodiment of the lithium ion secondary battery includes a laminated body in which a positive electrode, a separator, and a negative electrode are wound together, and an electrolyte solution, in a pack made of an aluminum laminate film.

[0146] The lithium ion 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 lithium ion secondary battery of the present disclosure should not be construed as being limited by the specific examples shown below.

[0147] <Measurement Methods and Evaluation Methods> The measurement methods and evaluation methods used in the examples and comparative examples are as follows.

[0148] [Thickness of Polyolefin Microporous Membrane and Separator] The thickness (μm) of the polyolefin microporous membrane and separator was determined by measuring 20 points within a 10 cm square using a contact thickness meter (Mitutoyo Corporation, LITEMATIC VL-50S) and averaging the measurements. A spherical probe with a sphere radius of 10 mm (Mitutoyo Corporation) was used as the measurement terminal, and was adjusted so that a load of 0.19 N was applied during measurement.

[0149] [Thickness of Porous Layer] The thickness of the porous layer (total of both sides, μm) was determined by subtracting the thickness (μm) of the polyolefin microporous film from the thickness (μm) of the separator.

[0150] [Porosity of Polyolefin Microporous Membrane] The porosity ε (%) of a polyolefin microporous membrane was determined by the following formula: ε = {1 - Ws / (ds·t)} × 100, where Ws is the basis weight (g / m) of the polyolefin microporous membrane. ２ ), ds is the true density of the polyolefin microporous membrane (g / cm ３), and t is the thickness (μm) of the polyolefin microporous membrane.

[0151] [Gurley Values ​​of Polyolefin Microporous Membrane and Separator] The Gurley values ​​(seconds / 100 mL) of the polyolefin microporous membrane and separator were measured using a Gurley densometer (Toyo Seiki Co., Ltd., G-B2C) in accordance with JIS P8117:2009.

[0152] [Breakdown voltage of polyolefin microporous membrane and separator] The breakdown voltage of the polyolefin microporous membrane and separator was measured in accordance with JIS C 2110-1:2016.

[0153] [Melting point of polyethylene microporous membrane] DSC on the polyethylene microporous membrane was performed as follows. 5.0 mg±0.3 mg of polyethylene microporous membrane was placed in an aluminum sample pan and set in the measuring device. Thermal analysis of the sample was performed in a nitrogen atmosphere by successively performing the following three steps. Step 1: Heat from 30°C to 200°C at a rate of 5°C / min. Step 2: Cool from 200°C to 30°C at a rate of 5°C / min. Step 3: Heat from 30°C to 200°C at a rate of 5°C / min. Based on the results of the DSC thermal analysis, a DSC curve was drawn, with the horizontal axis representing temperature (°C) and the vertical axis representing heat flow (W / g). The temperature of the endothermic peak observed in the DSC curve for Step 1 was taken as the melting point in the first heating process. When there were multiple endothermic peaks in the DSC curve for Step 1, the temperature of the lowest endothermic peak was taken as the melting point. The temperature of the endothermic peak observed in the DSC curve of Step 3 was taken as the melting point in the second heating process. When there were multiple endothermic peaks in the DSC curve of Step 3, the temperature of the lowest endothermic peak was taken as the melting point.

[0154] [Average Primary Particle Size of Inorganic Particles] The average primary particle size of the inorganic particles was determined by SEM observation of the inorganic particles used to form the porous layer as a sample, measuring the major axes of 100 randomly selected inorganic particles, and averaging the major axes of the 100 particles.

[0155] [TEM-EDS Analysis of Separator] TEM-EDS analysis of the separator was carried out as follows to observe the inside of the pores of the polyolefin microporous membrane. ４Preparation of stained sample: RuO for TEM observation was prepared according to the following steps (1) to (5). ４ A dyed sample was prepared. (1) The separator was cut into a triangle (base approximately 0.5 mm x height approximately 1 mm, MD is the base). (2) The triangular sample was attached to a slide glass and placed in a sealed container with a capacity of approximately 50 ml. ４ Approximately 0.5 ml of the aqueous solution was introduced into a sealed container and subjected to steam staining at room temperature for 15 minutes. These staining conditions do not stain polyolefins. (3) The stained sample was impregnated with embedding resin. A stepwise substitution process was performed to ensure that the embedding resin penetrated the porous interior of the sample. Specifically, the sample was placed in n-butyl glycidyl ether, n-butyl glycidyl ether:embedding resin = 1:1, and n-butyl glycidyl ether:embedding resin = 1:3, successively for 30 minutes each, and then left in the embedding resin overnight. (4) The sample was placed on an embedding plate, and the embedding resin was injected and allowed to fully harden. (5) Ultrathin sections approximately 80 nm thick were prepared using an ultramicrotome at room temperature. The sample was cut so that the separator's MD was visible on the cross section. A carbon support film was attached to the ultrathin section and mounted on a Cu grid. TEM observation was performed using a transmission electron microscope (JEM-2100F, JEOL Ltd.) at an accelerating voltage of 120 kV. Elemental analysis was performed using the EDS detector attached to the JEM-2100F, and elemental imaging was performed using the STEM function.

[0156] <Production of Lithium-Ion Secondary Battery> [Example 1] - Preparation of Positive Electrode - 94 parts by mass of lithium cobalt oxide powder, 3 parts by mass of acetylene black, 3 parts by mass of polyvinylidene fluoride, and an appropriate amount of N-methyl-2-pyrrolidone were mixed by stirring in a twin-arm mixer to prepare a positive electrode slurry. The positive electrode slurry was applied to one 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. The weight of the positive electrode active material layer was 12 mg / cm. ２ It was.

[0157] - Preparation of Negative Electrode - 96.2 parts by mass of artificial graphite, 2.8 parts by mass of styrene butadiene rubber, 1.0 part by mass of carboxymethyl cellulose, and an appropriate amount of water were mixed and stirred in a twin-arm mixer to prepare a negative electrode slurry. 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. The weight of the negative electrode active material layer was 6.5 mg / cm. ２ It was.

[0158] -Preparation of electrolyte- LiFSI (i.e., Li(FSO ２ ) ２ N) was dissolved to prepare an electrolyte solution with a LiFSI concentration of 5.6 mol / L.

[0159] - Preparation of Separator - Polymetaphenylene isophthalamide was dissolved in dimethylacetamide (DMAc) to a resin concentration of 4.5% by mass, and then barium sulfate particles (average primary particle size 0.1 μm) were stirred and mixed to obtain coating liquid (1). The composition of coating liquid (1) was polymetaphenylene isophthalamide:barium sulfate particles = 20:80 (mass ratio). 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 9 μm, porosity 41%, Gurley value 139 seconds / 100 mL, breakdown voltage 0.7 kV). The coating was performed so that the coating amount on the front and back of the polyethylene microporous membrane was equal. The coated film was immersed in a coagulation liquid (DMAc:water = 50:50 [mass ratio], liquid temperature 40°C) to solidify the coating layer, then washed in a water washing tank with water at 40°C, and dried. In this way, a separator was obtained in which porous layers were formed on both sides of the polyethylene microporous membrane. The weight of the porous layers on both sides was 6.3 g / m in total. ２ The thickness of the separator was 14 μm (therefore, the total thickness of the porous layer on both sides was 5 μm), and the Gurley value of the separator was 194 seconds / 100 mL.

[0160] The polyethylene microporous membrane used in Example 1 had a melting point of 137.7°C in the first heating process and a melting point of 135.7°C in the second heating process, as observed in the DSC curve. The melting point in the first heating process was 2.0°C higher than the melting point in the second heating process.

[0161] From the separator of Example 1, RuO ４ A stained sample was prepared and subjected to TEM-EDS. The TEM-EDS image confirmed that fibrous meta-aromatic polyamide was present in the pores of the polyethylene microporous membrane.

[0162] The separator of Example 1 had a breakdown voltage of 2.0 kV, which was an increase of 1.3 kV from the breakdown voltage of the polyethylene microporous membrane, 0.7 kV. When the breakdown voltage of the separator of Example 1, 2.0 kV, was compared with the breakdown voltage of the separator of Comparative Example 1 described below, 1.5 kV, the separator of Example 1 had a breakdown voltage higher by 0.5 kV, even though both separators had the same thickness.

[0163] - Fabrication of Battery - The positive electrode and negative electrode were each cut into a 30 mm x 50 mm rectangle, and a lead tab was welded to each. The separator was cut into a 34 mm x 54 mm rectangle. The positive electrode, separator, and negative electrode were stacked in this order to fabricate a laminate. The laminate was inserted into a pack made of aluminum laminate film, and an electrolyte was injected into the pack. A vacuum was created inside the pack using a vacuum sealer. The pack was then heat-pressed in the stacking direction of the laminate using a heat press machine, and the pack was sealed.

[0164] - Battery charge / discharge test - One charge / discharge cycle was performed on the battery. Charging was performed at a constant current / constant voltage of 0.1 C / 4.2 V for 15 hours, and discharging was performed at a constant current / constant voltage of 0.1 C / 2.5 V cutoff. A 10-minute interval was allowed between charges. The battery of Example 1 was able to obtain a discharge capacity of 4.5 mAh as designed. When the battery was charged at a constant current / constant voltage of 0.1 C / 4.2 V for 15 hours and then discharged at a constant current with a cutoff of 1 C / 2.5 V, the capacity retention rate based on a 0.1 C discharge current was 87%.

[0165] [Example 2] A separator was prepared in the same manner as in Example 1, except that the polyethylene microporous membrane used to prepare the separator was changed, and a battery was fabricated. The polyethylene microporous membrane used to prepare the separator in Example 2 had a thickness of 9 μm, a porosity of 36%, and a Gurley value of 161 seconds / 100 mL. The polyethylene microporous membrane used to prepare the separator in Example 2 had a melting point of 136.4°C in the first heating process and a melting point of 135.6°C in the second heating process, as observed in the DSC curve. The difference between the two melting points was 0.8°C.

[0166] The separator of Example 2 has a porous layer weight of 6.3 g / m on both sides in total. ２ The thickness of the separator was 14 μm (thus, the total thickness of the porous layer on both sides was 5 μm), and the Gurley value of the separator was 211 seconds / 100 mL.

[0167] From the separator of Example 2, RuO ４ A stained sample was prepared and subjected to TEM-EDS. The TEM-EDS image confirmed that fibrous meta-aromatic polyamide was present in the pores of the polyethylene microporous membrane.

[0168] The battery of Example 2 was subjected to a charge-discharge test under the same conditions as in Example 1. The battery of Example 2 was able to obtain a discharge capacity of 4.5 mAh as designed. The capacity retention rate based on a discharge current of 0.1 C was 34%.

[0169] The capacity retention rate in Example 1 was 87%, and the capacity retention rate in Example 2 was 34%. These two examples differed in the polyethylene microporous membrane used to produce the separator. Although the detailed mechanism is unknown, it is thought that during the formation of the porous layer, the degree to which the meta-aromatic polyamide penetrated into the pores of the microporous membrane differed depending on the thermal properties of the polyethylene microporous membrane (the degree of difference between the melting point in the first heating process and the melting point in the second heating process), resulting in a difference in the ease with which the high-salt concentration electrolyte solution penetrated the separator.

[0170] Comparative Example 1 A battery was fabricated in the same manner as in Example 1, except that in the preparation of a separator, the polyethylene microporous membrane was impregnated with dimethylacetamide before being coated with Coating Liquid (1), and then both surfaces of the polyethylene microporous membrane were coated with Coating Liquid (1) to prepare a separator.

[0171] The separator of Comparative Example 1 has a porous layer weight of 6.3 g / m on both sides in total. ２ The thickness of the separator was 14 μm (therefore, the total thickness of the porous layer on both sides was 5 μm), and the Gurley value of the separator was 185 seconds / 100 mL.

[0172] RuO from the separator of Comparative Example 1 ４ A stained sample was prepared and subjected to TEM-EDS. No meta-aromatic polyamide was confirmed in the pores of the polyethylene microporous membrane.

[0173] The dielectric breakdown voltage of the separator of Comparative Example 1 was 1.5 kV, and the increase from the dielectric breakdown voltage of the polyethylene microporous film, 0.7 kV, was 0.8 kV.

[0174] The battery of Comparative Example 1 was subjected to a charge-discharge test under the same conditions as in Example 1. The battery of Comparative Example 1 failed to achieve the designed discharge capacity. It was presumed that because the separator of the battery of Comparative Example 1 did not contain meta-type aromatic polyamide in the pores of the polyethylene microporous membrane, the high-salt concentration electrolyte did not sufficiently penetrate the polyethylene microporous membrane, and a practical battery could not be fabricated.

[0175] 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.

[0176] The disclosure of Japanese Application No. 2023-185069, filed on October 27, 2023, is incorporated herein by reference in its entirety.

Claims

1. A lithium ion secondary battery comprising: a positive electrode; a negative electrode that operates by intercalation of lithium ions; an electrolyte solution containing a non-aqueous solvent and a lithium salt, the lithium salt concentration being 2.5 mol / L or more; and a separator, wherein the separator has a polyolefin microporous membrane, a fibrous meta-type aromatic polyamide contained in the pores of the polyolefin microporous membrane, and a porous layer containing a meta-type aromatic polyamide and provided on one or both sides of the polyolefin microporous membrane.

2. The lithium ion secondary battery according to claim 1, wherein the lithium salt includes at least one selected from the group consisting of a sulfonamide lithium salt and a sulfonimide lithium salt.

3. The lithium ion secondary battery according to claim 1, wherein the separator has the porous layer on both sides of the polyolefin microporous film.

4. The lithium ion secondary battery according to claim 1, wherein the porous layer further contains inorganic particles.

5. The lithium ion secondary battery of claim 4, wherein the inorganic particles comprise metal sulfate particles.

6. The lithium ion secondary battery according to claim 4, wherein the inorganic particles include magnesium compound particles.

7. The lithium ion secondary battery according to any one of claims 4 to 6, wherein the inorganic particles have an average primary particle size of 0.3 μm or less.

8. The lithium ion secondary battery according to claim 1, wherein the polyolefin microporous membrane is a polyethylene microporous membrane, and when differential scanning calorimetry is performed using the polyethylene microporous membrane as a sample, the melting point in a first heating process is 0.7°C or more higher than the melting point in a second heating process.

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