Lithium secondary battery, separator for lithium secondary battery, and method for suppressing porosity of negative electrode

Abstract

Provided are a lithium secondary battery, a separator for a lithium secondary battery, and a method for suppressing the porosity of a negative electrode, the lithium secondary battery comprising: a positive electrode; a negative electrode operated by dissolution and precipitation of metallic lithium; an electrolyte containing a non-aqueous solvent and a lithium salt and having a lithium salt concentration of at least 2.0 mol / L; and a separator having a polyolefin microporous membrane and a porous layer provided on one surface or both surfaces of the polyolefin microporous membrane, wherein the porous layer contains a wholly aromatic polyamide, and the film thickness of the porous layer is 5-20 μm for each surface of the separator.

Description

Lithium secondary battery, separator for lithium secondary battery, and method for preventing negative electrode from becoming porous

[0001] The present disclosure relates to a lithium secondary battery, a separator for a lithium secondary battery, and a method for suppressing porosity in a negative electrode.

[0002] The use of high-salinity electrolytes with high lithium salt concentrations has been investigated in lithium secondary batteries. High-salinity electrolytes generally have high viscosity, and the types of separators that can accommodate these electrolytes have traditionally been limited. Single-layer polyolefin microporous membranes, commonly used as battery separators, are impervious to high-salinity electrolytes and are therefore unsuitable for use in 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 in secondary batteries containing high-salinity electrolytes. However, separators with relatively large pore sizes, such as glass fiber nonwoven fabrics, are prone to the formation and growth of lithium dendrites on the electrode. When glass fiber nonwoven fabrics are used in lithium secondary batteries, the formation and growth of lithium dendrites on the negative electrode is difficult to suppress, resulting in a significant decrease in cycle performance and an increased risk of short-circuiting.

[0003] It is known that the use of an aramid-coated polyolefin microporous membrane as a separator for a lithium secondary battery using a high-salt electrolyte improves the wettability of the separator and suppresses the generation of dendrites (Non-Patent Document 1).

[0004] Non-patent document 1: Energy Advances, (UK), 2023, 2, 503-507

[0005] The present inventors have found for the first time that in lithium secondary batteries in which performance such as cycle characteristics has deteriorated due to the generation of lithium dendrites or the like, the negative electrode may become porous.

[0006] An object of the present disclosure is to provide a lithium secondary battery that effectively prevents the negative electrode from becoming porous and has excellent cycle characteristics, a separator for the lithium secondary battery, and a method for preventing the negative electrode from becoming porous.

[0007] Specific means for solving the problems include the following aspects.

[0008] <1> A lithium secondary battery comprising a positive electrode, a negative electrode that operates by dissolution and precipitation of metallic lithium, an electrolyte solution containing a non-aqueous solvent and a lithium salt and having a lithium salt concentration of 2.0 mol / L or more, a polyolefin microporous membrane, and a separator having a porous layer provided on one or both sides of the polyolefin microporous membrane, wherein the porous layer contains a wholly aromatic polyamide and the thickness of the porous layer is 5 μm to 20 μm per side of the separator. <2> The lithium secondary battery according to <1>, wherein the porous layer further contains inorganic particles. <3> The lithium secondary battery according to <2>, wherein the porous layer contains inorganic particles in a proportion of 40% by volume to 80% by volume, based on the volume of the entire solid content constituting the porous layer. <4> The lithium secondary battery according to <2> or <3>, wherein the inorganic particles are barium sulfate. <5> The lithium secondary battery according to any one of <1> to <4>, wherein the porosity of the porous layer is 60% or more and less than 85%. <6> The lithium secondary battery according to any one of <1> to <5>, wherein the pores of the polyolefin microporous membrane contain a fibrous wholly aromatic polyamide. <7> The lithium secondary battery according to any one of <1> to <6>, wherein the lithium salt concentration of the electrolyte solution is 2.5 mol / L or more. <8> The lithium secondary battery according to any one of <1> to <7>, wherein the lithium salt contains at least one selected from the group consisting of sulfonamide lithium salts and sulfonimide lithium salts. <9> The lithium secondary battery according to any one of <1> to <8>, wherein the non-aqueous solvent is an ether-based solvent. <10> A separator for a lithium secondary battery, the separator comprising: a polyolefin microporous membrane; and a porous layer provided on one or both sides of the polyolefin microporous membrane and containing a wholly aromatic polyamide, wherein the porous layer has a thickness of 5 μm to 20 μm per side of the separator; the lithium secondary battery comprising: a positive electrode; a negative electrode that operates by dissolution and precipitation of metallic lithium; and an electrolyte containing a non-aqueous solvent and a lithium salt, the lithium salt concentration being 2.0 mol / L or more.<11> A method for suppressing the formation of porosity in a negative electrode in a lithium secondary battery including a positive electrode, a negative electrode that operates by dissolution and precipitation of metallic lithium, an electrolyte, and a separator, the method comprising: disposing, between the positive electrode and the negative electrode, a polyolefin microporous membrane; and a separator having a porous layer that is provided on one or both sides of the polyolefin microporous membrane and contains a wholly aromatic polyamide, wherein the porous layer has a thickness of 5 μm to 20 μm per side of the separator.

[0009] According to the present disclosure, there are provided a lithium secondary battery that effectively prevents the negative electrode from becoming porous and has excellent cycle characteristics, a separator for the lithium secondary battery, and a method for preventing the negative electrode from becoming porous.

[0010] FIG. 1 shows the results of TEM-EDS analysis of the separator, and is an image showing the presence of fibrous wholly aromatic polyamide in the pores of the polyolefin microporous membrane. FIG. 2 shows charge / discharge curves for two-electrode cells of Example 1 and Comparative Example 1. FIG. 3 shows X-ray CT images of the cross section of the negative electrode in cycle tests at 0, 50, and 100 cycles in Example 1. FIG. 4 shows X-ray CT images of the cross section of the negative electrode in cycle tests at 0, 50, and 100 cycles in Comparative Example 1. FIG. 5 is a graph showing the thickness of the negative electrode in cycle tests at 100 cycles in Example 1, Comparative Example 1, and Reference Example 1. FIG. 6 is a graph showing the cycle characteristics of two-electrode cells of Example 2, Example 3, Comparative Example 2, Comparative Example 3, Comparative Example 4, and Reference Example 1. FIG. 7 shows X-ray CT images of the cross section of the negative electrode in the cycle test of 50 cycles in each of Reference Example 2, Example 2, Example 3, Comparative Example 2, and Comparative Example 4.

[0011]

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

[0012] In the present disclosure, a numerical range indicated using "to" indicates a range that includes the numerical values ​​before and after "to" as the minimum and maximum values, respectively. In the numerical ranges described in stages in the present disclosure, the upper or lower limit value described in one numerical range may be replaced with the upper or lower limit value of another numerical range described in stages. Furthermore, in the numerical ranges described in the present disclosure, the upper or lower limit value of that numerical range may be replaced with a value shown in an example.

[0013] In the present disclosure, 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.

[0014] In the present disclosure, when an embodiment is described with reference to drawings, the configuration of the embodiment is not limited to the configuration shown in the drawings.

[0015] 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.

[0016] 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.

[0017] <Lithium Secondary Battery> The lithium secondary battery of the present disclosure includes a positive electrode, a negative electrode, an electrolyte, and a separator. The negative electrode included in the lithium secondary battery of the present disclosure is an electrode that operates by dissolution and precipitation of metallic lithium. The electrolyte included in the lithium secondary battery of the present disclosure contains a non-aqueous solvent and a lithium salt, and has a lithium salt concentration of 2.0 mol / L or more. The separator included in the lithium secondary battery of the present disclosure includes a polyolefin microporous membrane and a porous layer provided on one or both sides of the polyolefin microporous membrane, the porous layer containing a wholly aromatic polyamide, and having a thickness of 5 μm to 20 μm per side of the separator. Hereinafter, this separator will also be referred to as "separator (A)," and the porous layer containing the wholly aromatic polyamide will also be referred to as "porous layer (A)."

[0018] The porous layer (A) contains a wholly aromatic polyamide. The wholly aromatic polyamide contains many polar groups, which is presumed to have a high affinity for high-salt electrolytes. Therefore, the separator (A) having the porous layer (A) allows the high-salt electrolyte to penetrate. The lithium secondary battery of the present disclosure, which includes the separator (A), can employ a high-salt electrolyte having a lithium salt concentration of 2.0 mol / L or more.

[0019] The polyolefin microporous membrane and porous layer (A) of the separator (A) are membranes and layers with smaller pore sizes and more uniform pore sizes than glass fiber nonwoven fabric. In other words, the separator (A) has a denser porous structure than glass fiber nonwoven fabric. The thickness of the porous layer (A) is 5 μm to 20 μm per side of the separator.

[0020] The present inventors have found that repeated dissolution and precipitation of lithium during charge and discharge cycles can, for example, cause dendrites to form, making the negative electrode porous, and thereby deteriorating cycle performance. However, no prior art has focused on the porosity of the negative electrode, or the relationship between the porosity of the negative electrode and cycle performance.

[0021] The inventors speculate that repeated dissolution and precipitation of lithium during charge and discharge cycles leads to localized lithium precipitation, resulting in, for example, the formation of dendrites and the resulting porosity of the negative electrode. The inventors speculate that, in a specific lithium secondary battery, adjusting the film thickness of the porous layer (A) may suppress localized lithium precipitation and result in planar lithium precipitation, although the mechanism is unclear. The inventors have found that this effectively suppresses the formation of dendrites and effectively prevents the negative electrode from becoming porous, resulting in excellent cycle characteristics of the lithium secondary battery. The lithium secondary battery of the present disclosure, which includes the separator (A), effectively prevents the negative electrode from becoming porous and exhibits excellent cycle characteristics.

[0022] The configuration of the lithium secondary battery of the present disclosure will be described in detail below.

[0023] [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.

[0024] 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.

[0025] 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.

[0026] 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 Mn１／２ 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 Ti ５ O １２ , Li ８／７ Ti ２／７ V ４／７ O ２ , Li １．１４ Ti ０．２９ Mn ０．５７ 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.

[0027] Examples of resins include polyvinylidene fluoride resins, alginates, etc. These may be used alone or in combination.

[0028] 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.

[0029] [Negative Electrode] The negative electrode is an anode that operates by dissolution and precipitation of metallic lithium. The negative electrode is preferably in either form (1) or form (2) below.

[0030] Form (1): A negative electrode having a metallic lithium layer. Form (2): A negative electrode having a current collector on the surface of which metallic lithium is deposited.

[0031] - Form (1) - The negative electrode of form (1) includes, for example, a current collector and a metallic lithium layer disposed on one or both sides of the current collector.

[0032] The current collector in embodiment (1) is preferably a metal foil. Examples of metal foil include copper foil, silver foil, stainless steel foil, and palladium foil. The current collector in embodiment (1) is preferably a copper foil. The thickness of the current collector in embodiment (1) is preferably 3 μm to 20 μm.

[0033] The metallic lithium layer in form (1) is a layer of simple lithium. The thickness of the metallic lithium layer is preferably 0.1 μm to 100 μm. Commercially available metallic lithium foil can be used as the metallic lithium layer. The metallic lithium layer may be formed on the current collector by vapor deposition.

[0034] -Configuration (2)- The negative electrode of Configuration (2) does not require a negative electrode active material layer to be provided on the current collector in advance. In a lithium secondary battery employing Configuration (2), lithium ions dedoped from the lithium-containing active material of the positive electrode are deposited as metallic lithium on the negative electrode current collector during charging.

[0035] The current collector in embodiment (2) is preferably a metal foil. Examples of metal foil include copper foil, silver foil, stainless steel foil, and palladium foil. The current collector in embodiment (2) is preferably a copper foil. The thickness of the current collector in embodiment (2) is preferably 3 μm to 20 μm.

[0036] The negative electrode of form (2) is thinner than the negative electrode of form (1), which is advantageous from the viewpoint of increasing the energy density of the battery.

[0037] [Electrolyte Solution] The electrolyte solution contains a non-aqueous solvent and a lithium salt, and has a lithium salt concentration of 2.0 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 2.0 mol / L or more.

[0038] 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 coordinate to lithium ions (Li+), leaving no uncoordinated solvent molecules. However, the stable solvation state of Li+, tetracoordination, is not yet achieved, resulting in the lithium salt counteranion coordinating with Li+. High-salt electrolytes with the above solution structure exhibit properties such as high electrochemical stability, reduced volatility and flammability, a wide potential window, and suppression of transition metal leaching in the positive electrode active material.

[0039] The lithium salt concentration of the electrolyte solution is 2.0 mol / L or more, preferably 2.5 mol / L or more, and more preferably 3.0 mol / L or more, from the viewpoint of realizing the properties of a high-salt-concentration electrolyte solution. The lithium salt concentration of the electrolyte solution is preferably 10.0 mol / L or less, more preferably 7.0 mol / L or less, and even more preferably 6.0 mol / L or less, from the viewpoint of suppressing the viscosity of the electrolyte solution.

[0040] The lithium salt concentration of the electrolyte is preferably 2.0 mol / L to 7.0 mol / L, more preferably 2.5 mol / L to 6.0 mol / L, from the viewpoint of achieving both the properties of a high-salt-concentration electrolyte and suppressing viscosity.

[0041] Examples of the nonaqueous solvent include any known nonaqueous solvent used in lithium 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; and sulfolane. These may be used alone or in combination.

[0042] The non-aqueous solvent is preferably an ether-based solvent. Examples of the ether-based solvent include any of the known non-aqueous solvents used in lithium secondary batteries. Specific examples include ethers such as 1,2-dimethoxyethane, 1,2-diethoxyethane, diglyme, triglyme, tetraglyme, ethyl methyl ether, dipropyl ether, and tetrahydrofuran. Electrolytes using ether-based solvents tend to have a wide potential window on the reduction side, and are therefore preferred when used with metallic lithium negative electrodes.

[0043] As the nonaqueous solvent, a chain carbonate such as dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, or a fluorine-substituted derivative thereof, which is a solvent having a relatively low viscosity, is also 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.

[0044] The lithium salt may be any known lithium salt used in lithium secondary batteries. Specifically, Li(FSO ２ ) ２ N (also known as "LiFSA" or "LiFSI"), Li(CF ３ SO２ ) ２ N (also known as "LiTFSA" or "LiTFSI"), Li(C ２ F ５ SO ２ ) 2N (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 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.

[0045] 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 that the secondary battery has excellent cycle characteristics due to the fact that it contains a bulky anion, is easily dissociated, and is electrochemically stable. 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(CF３ 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.

[0046] The electrolyte preferably contains dimethyl carbonate as a non-aqueous solvent, 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 2.0 mol / L to 7.0 mol / L, more preferably 2.5 mol / L to 6.0 mol / L.

[0047] 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.

[0048] [Separator (A)] The separator (A) has a polyolefin microporous membrane and a porous layer (A) provided on one or both sides of the polyolefin microporous membrane. The porous layer (A) is a porous layer containing a wholly aromatic polyamide. The thickness of the porous layer is 5 μm to 20 μm per side of the separator. The porous layer (A) is preferably the outermost layer of the separator on one or both sides of the polyolefin microporous membrane.

[0049] The separator (A) may be embodied in the following forms (a) to (c).

[0050] Form (a): A separator having porous layers (A) on both sides of a polyolefin microporous membrane, wherein the porous layer (A) on one side of the separator and the porous layer (A) on the other side of the separator may be the same or different in components and / or composition.

[0051] Form (b): A separator having a porous layer (A) on one side of a polyolefin microporous membrane and another porous layer (i.e., a porous layer containing no wholly aromatic polyamide, a porous layer having a thickness of less than 5 μm or more than 20 μm) on the other side of the polyolefin microporous membrane. An example of the another porous layer is an adhesive layer intended to bond an electrode and separator (A).

[0052] Form (c): A separator having a porous layer (A) 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).

[0053] The separator (A) is preferably in the form (a) from the viewpoint of better permeability to a high-salt concentration electrolyte.

[0054] The separator (A) is preferably in the form (c) from the viewpoint of reducing the thickness of the entire separator and obtaining a secondary battery with a higher energy density.

[0055] Hereinafter, the polyolefin microporous film of the separator (A) and the porous layer (A) containing a wholly aromatic polyamide provided on one or both sides of the polyolefin microporous film will be described in detail.

[0056] -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.

[0057] 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.

[0058] 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.

[0059] 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.

[0060] 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 provided 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 a microporous membrane or polyolefin used to form the microporous membrane as a sample.

[0061] 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.

[0062] The surface of the polyolefin microporous membrane may be subjected to various surface treatments without impairing the properties of the polyolefin microporous membrane in order to improve wettability with the coating liquid for forming the porous layer (A). Examples of surface treatments include corona treatment, plasma treatment, flame treatment, and ultraviolet irradiation treatment.

[0063] -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.

[0064] 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.

[0065] 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.

[0066] 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.

[0067] A preferred embodiment of the polyolefin microporous membrane is one in which the wall surfaces of the pores of the polyolefin microporous membrane are partially or entirely coated with a wholly aromatic polyamide, which allows high-salt electrolyte to easily penetrate the membrane.

[0068] 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 the polyethylene microporous membrane has the above thermal properties, during the formation of the porous layer (A), the wholly aromatic polyamide tends to penetrate into the pores of the microporous membrane and adhere to the wall surfaces of the pores, or fibrous wholly 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.

[0069] 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.

[0070] 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.

[0071] - Fibrous wholly aromatic polyamide - In separator (A), the pores of the polyolefin microporous membrane preferably contain a fibrous wholly aromatic polyamide. Because the wholly 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.

[0072] The details and preferred forms of the wholly aromatic polyamide constituting the fibrous wholly aromatic polyamide are the same as those of the wholly aromatic polyamide contained in the porous layer (A). The wholly aromatic polyamide will be described later.

[0073] Because wholly aromatic polyamides exhibit high affinity for high-salt electrolytes, high-salt electrolytes easily permeate into polyolefin microporous membranes containing fibrous wholly 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 wholly aromatic polyamide be contained in the pores at least in the region close to the surface of the polyolefin microporous membrane, and it is more preferable that the fibrous wholly aromatic polyamide be contained in the entire pores of the polyolefin microporous membrane.

[0074] The presence of fibrous wholly aromatic polyamide in the pores of the polyolefin microporous membrane in separator (A) 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 the wholly aromatic polyamide is detected by Ru imaging. ４ The method for preparing the stained sample is as follows.

[0075] (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.

[0076] FIG. 1 shows an example of the results of TEM-EDS analysis. This example shows the results of analysis of a separator (A) 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 partition walls 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 wholly aromatic polyamide.

[0077] -Porous Layer (A)- In the present disclosure, a porous layer refers to a layer having a large number of micropores therein, with the micropores being interconnected, and allowing gas or liquid to pass through from one surface to the other.

[0078] The porous layer (A) contains a wholly aromatic polyamide. 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. The wholly aromatic polyamide may be a polyamide copolymerized with an aliphatic monomer, a polyamide not copolymerized with an aliphatic monomer, or a mixture of both. Aromatic polyamides are also called aramids.

[0079] Specifically, the wholly aromatic polyamide is preferably polymetaphenylene isophthalamide or polyparaphenylene terephthalamide, and more preferably polymetaphenylene isophthalamide. The weight average molecular weight (Mw) of the wholly aromatic polyamide contained in the porous layer is 1×10 ３ ~1 x 10 ７ is preferred, and 5 × 10 ３ ~5 x 10 ６ is more preferable, and 1×10 ４ ~1 x 10 ６ The Mw of the wholly aromatic polyamide is the molecular weight in terms of polystyrene measured by gel permeation chromatography (GPC).

[0080] The wholly aromatic polyamide may be meta-type or para-type. Among wholly aromatic polyamides, meta-type wholly aromatic polyamides are preferred from the viewpoints of ease of forming a porous layer and excellent oxidation-reduction resistance in electrode reactions.

[0081] The meta-type wholly aromatic polyamide refers to a wholly aromatic polyamide in which the benzene disubstitution constituting the main chain is a meta-disubstitution, although a small amount of a para-disubstitution may be copolymerized as the benzene disubstitution in the meta-type wholly aromatic polyamide.

[0082] Meta-type wholly aromatic polyamides are polymers with higher flexibility than para-type wholly aromatic polyamides, and therefore tend to penetrate into the pores of the polyolefin microporous membrane and adhere to the wall surfaces of the pores or form a fibrous structure during the formation of the porous layer (A). When meta-type wholly aromatic polyamides are adhered to the wall surfaces of the pores of the polyolefin microporous membrane or when fibrous meta-type wholly aromatic polyamides are contained in the pores, a high-salt concentration electrolyte solution easily penetrates the polyolefin microporous membrane.

[0083] The meta-type wholly aromatic polyamide may be a polyamide copolymerized with an aliphatic monomer, a polyamide not copolymerized with an aliphatic monomer, or a mixture of both.

[0084] The content of the wholly aromatic polyamide contained in the porous layer (A) 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 (A).

[0085] When the porous layers (A) are present on both sides of the polyolefin microporous membrane, the type and / or content of the wholly aromatic polyamide contained in one porous layer (A) may be the same as or different from the type and / or content of the wholly aromatic polyamide contained in the other porous layer (A).

[0086] The porous layer (A) may contain a resin other than the wholly aromatic polyamide. Examples of the other resin include aromatic polyamides other than wholly aromatic polyamides, polyamideimides, poly-N-vinylacetamides, polyacrylamides, copolymerized polyetherpolyamides, polyimides, polyetherimides, polyvinylidene fluoride resins, acrylic resins, fluorine-containing rubbers, styrene-butadiene copolymers, homopolymers or copolymers of vinyl nitrile compounds (such as acrylonitrile and methacrylonitrile), carboxymethyl cellulose, hydroxyalkyl cellulose, polyvinyl alcohol, polyvinyl butyral, polyvinylpyrrolidone, polyethers (such as polyethylene oxide and polypropylene oxide), polysulfones, polyketones, polyether ketones, polyether sulfones, and mixtures thereof.

[0087] The content of the other resin contained in the porous layer (A) is preferably 0% by mass to 15% by mass, more preferably 0% by mass to 10% by mass, still 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 (A). Ideally, the porous layer (A) does not contain any other resins than the meta-type aromatic polyamide.

[0088] From the viewpoint of heat resistance and porosity of the layer, it is preferable that the porous layer (A) further contains inorganic particles.

[0089] 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.

[0090] Examples of metal sulfates that constitute the metal sulfate particles include barium sulfate, strontium sulfate, calcium sulfate, calcium sulfate dihydrate, alum, and jarosite.

[0091] 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.

[0092] 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.

[0093] Examples of metal carbonates constituting the metal carbonate particles include calcium carbonate and magnesium carbonate.

[0094] Examples of metal nitrides that constitute the metal nitride particles include magnesium nitride, aluminum nitride, calcium nitride, and titanium nitride.

[0095] Examples of metal fluorides constituting the metal fluoride particles include magnesium fluoride and calcium fluoride.

[0096] Examples of clay minerals that make up the clay mineral particles include calcium silicate, calcium phosphate, apatite, and talc.

[0097] The inorganic particles may be surface-modified with a silane coupling agent or the like.

[0098] 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.

[0099] 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 (A) is preferably 80 mass% or more, more preferably 85 mass% or more, even more preferably 90 mass% or more, even more preferably 95 mass% or more, and most preferably 100 mass% from the viewpoint of suppressing gas generation inside the battery.

[0100] 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 (A) 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.

[0101] 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.

[0102] 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 (A) 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.

[0103] When the porous layers (A) are present on both sides of the polyolefin microporous membrane, the type and / or content of inorganic particles contained in one porous layer (A) may be the same as or different from the type and / or content of inorganic particles contained in the other porous layer (A).

[0104] 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.

[0105] The average primary particle size of the inorganic particles contained in the porous layer (A) 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.

[0106] 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 (A) 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.

[0107] 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 (A) 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.

[0108] 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 (A) 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.

[0109] 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.

[0110] When the porous layers (A) are present on both sides of the polyolefin microporous membrane, the average primary particle size of the inorganic particles contained in one porous layer (A) may be the same as or different from the average primary particle size of the inorganic particles contained in the other porous layer (A).

[0111] The porous layer (A) preferably contains inorganic particles in a proportion of 40% by volume or more and 80% by volume or less, based on the total volume of the solid content constituting the porous layer (A). More preferably, it is 30% by volume or more and 70% by volume or less. In the present disclosure, the solid content volume of the porous layer means the volume excluding the pores of the porous layer.

[0112] 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 (A) is preferably 40% by volume or more and 80% by volume or less, and more preferably 30% by volume or more and 70% by volume or less.

[0113] When barium sulfate particles are used as the inorganic particles, the volume ratio of the barium sulfate particles to the solid content volume of the porous layer (A) is preferably 40% by volume or more and 80% by volume or less, more preferably 30% by volume or more and 70% by volume or less.

[0114] When magnesium compound particles are used as the inorganic particles, the volume ratio of the magnesium compound particles to the solid content volume of the porous layer (A) is preferably 40% by volume or more and 80% by volume or less, more preferably 30% by volume or more and 70% by volume or less.

[0115] 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 layer ３ )

[0116] When the porous layers (A) 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 (A) 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 (A).

[0117] The mass ratio of inorganic particles in the porous layer (A) 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 (A) 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 (A) 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 (A) 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.

[0118] When the porous layers (A) are present on both sides of the polyolefin microporous membrane, the mass ratio of the inorganic particles in one porous layer (A) may be the same as or different from the mass ratio of the inorganic particles in the other porous layer (A).

[0119] The porous layer (A) 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."

[0120] 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.

[0121] The porous layer (A) 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 (A) 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 (A), for example, to improve compatibility with the polyolefin microporous membrane, to suppress air entrapment in the coating liquid, or to adjust the pH.

[0122] Characteristics of Porous Layer (A) The thickness of the porous layer (A) is preferably 5.0 μm or more on one side from the viewpoints of effectively suppressing the negative electrode from becoming porous and of handling ease during battery production. The thickness of the porous layer (A) is preferably 20.0 μm or less on one side, more preferably 10.0 μm or less on one side, and even more preferably 8.0 μm or less on one side, from the viewpoints of effectively suppressing the negative electrode from becoming porous and of increasing ion permeability and the energy density of the battery.

[0123] When the porous layer (A) is present on both sides of the polyolefin microporous membrane, the total thickness of the porous layer (A) on both sides is preferably 10.0 μm or more, and 40.0 μm or less, more preferably 20.0 μm or less, and even more preferably 16.0 μm or less. The thickness of the porous layer (A) (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 (A) (μm).

[0124] When porous layers (A) are present on both sides of the polyolefin microporous membrane, the smaller the difference (μm) between the thickness of one porous layer (A) and the thickness of the other porous layer (A) is, the better, and it is preferably 20% or less of the total thickness (μm) of both sides.

[0125] The porosity of the porous layer (A) is preferably 60% or more from the viewpoints of suppressing the formation of pores in metallic lithium and ion permeability. The porosity of the porous layer (Am) is preferably 85% or less from the viewpoint of the mechanical strength of the porous layer (A). The porosity ε (%) of the porous layer is calculated by the following formula:

[0126]

[0127] 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 W1, W2, W3, ..., Wn (g / cm ２ ), and the true densities of the constituent materials are d1, d2, d3, ..., dn (g / cm ３ ) and the thickness of the porous layer is t (cm).

[0128] [Characteristics of Separator (A)] From the viewpoint of mechanical strength, the thickness of the separator (A) is preferably 8 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 (A) is preferably 45 μm or less, more preferably 30 μm or less, and even more preferably 25 μm or less. The thickness (μm) of the separator (A) is the average value measured at 20 points within a 10 cm square using a contact thickness meter.

[0129] The Gurley value (JIS P8117:2009) of the separator (A) 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 (A) 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.

[0130] As a preferred example of the separator (A), the following separator (A) can be mentioned. That is, the separator (A) for a lithium secondary battery comprises a polyolefin microporous membrane and a porous layer (A) containing a wholly aromatic polyamide provided on one or both sides of the polyolefin microporous membrane, the porous layer (A) having a thickness of 5 μm to 20 μm per side of the separator (A). The lithium secondary battery comprises a positive electrode, a negative electrode that operates by dissolution and precipitation of metallic lithium, and an electrolyte containing a nonaqueous solvent and a lithium salt, the lithium salt concentration of which is 2.0 mol / L or more. A lithium secondary battery using such a separator (A) is preferred because it suppresses the formation of pores in the negative electrode and has excellent cycle characteristics.

[0131] [Method for producing separator (A)] The separator (A) can be produced, for example, by forming a porous layer (A) 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 embodiment of the wet coating method will be described below.

[0132] 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.

[0133] The coating liquid for forming the porous layer (A) is prepared by dissolving a wholly aromatic polyamide in a solvent. If necessary, other components besides the wholly aromatic polyamide may be dissolved or dispersed in the coating liquid.

[0134] The solvent used in preparing the coating liquid includes a solvent that dissolves the wholly aromatic polyamide (hereinafter also referred to as a "good solvent"), such as polar amide solvents such as N-methylpyrrolidone, dimethylacetamide, and dimethylformamide.

[0135] 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.

[0136] 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.

[0137] 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.

[0138] 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.

[0139] 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.

[0140] The fibrous wholly aromatic polyamide contained in the pores of the polyolefin microporous membrane is formed by penetration of a coating liquid into the pores of the polyolefin microporous membrane and solidification of a 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 separator (A), this step is preferably not carried out.

[0141] 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 wholly aromatic polyamide contained in the pores of the polyolefin microporous membrane, and the porous layer.

[0142] 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.

[0143] 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.

[0144] The separator (A) 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.

[0145] [Shape and Manufacturing Method of Lithium Secondary Battery] The shape of the lithium secondary battery may be any of a square type, a cylindrical type, a coin type, a pouch type, and the like.

[0146] Examples of the exterior packaging of lithium secondary batteries include metal cans and aluminum laminate film packs.

[0147] A lithium 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.

[0148] 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.

[0149] An example of an embodiment of a lithium secondary battery includes a laminate in which a positive electrode, a separator, and a negative electrode are wound together, and an electrolyte solution, in a cylindrical metal can.

[0150] [Method for Suppressing Porosity in Negative Electrode] A method for suppressing porosity in a negative electrode includes a lithium secondary battery including a positive electrode, a negative electrode that operates by dissolution and precipitation of metallic lithium, an electrolyte, and a separator, the method comprising the steps of: disposing, between the positive electrode and the negative electrode, a polyolefin microporous membrane; and a separator having a porous layer that contains a wholly aromatic polyamide and is provided on one or both sides of the polyolefin microporous membrane, wherein the porous layer has a thickness of 5 μm to 20 μm per side of the separator.

[0151] In the method for suppressing the porosity of the negative electrode, the positive electrode, negative electrode, electrolyte, separator, various manufacturing methods, etc. are the same as those described above, and therefore will not be described here. According to the above method, the porosity of the negative electrode can be effectively suppressed in a lithium secondary battery. By effectively suppressing the porosity of the negative electrode, the cycle characteristics of the lithium secondary battery can be improved.

[0152] The electrolyte is preferably a high-salt electrolyte. Specifically, the lithium salt concentration in the electrolyte is preferably 2.0 mol / L or more, more preferably 2.5 mol / L or more. By using a high-salt electrolyte, it is possible to more effectively suppress the formation of porosity in the negative electrode and improve the cycle characteristics of the lithium secondary battery.

[0153] The lithium 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 secondary battery of the present disclosure should not be construed as being limited by the specific examples shown below.

[0154] <Measurement Methods and Evaluation Methods> The measurement methods and evaluation methods used in the examples and comparative examples are as follows.

[0155] [Thickness of Polyolefin Microporous Membrane and Separator] The thicknesses (μm) of the polyolefin microporous membrane and separator were measured at 20 points within a 10 cm square using a contact thickness meter (Mitutoyo Corporation, LITEATIC VL-50S) and averaged. 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.

[0156] [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.

[0157] [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.

[0158] [Mass Proportion of Inorganic Particles] The mass ratio was calculated from the mass of the wholly aromatic polyamide and the mass of the inorganic particles used in the coating liquid for forming the porous layer.

[0159] [Volume Proportion of Inorganic Particles] The volume proportion V (vol %) of inorganic particles in the solid volume of the porous layer was calculated by the following formula: V = {(Xa / Da) / (Xa / Da + Xb / Db + Xc / Dc + ... + Xn / Dn)} x 100, where, among the constituent materials of the porous layer, the inorganic particles are a, the other constituent materials are b, c, ..., n, 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 ) where Xa and the like substituted in the above formula are the mass (g) of the constituent material used to form the 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 )

[0160] [Porosity of Porous Layer] The porosity ε (%) of the porous layer was calculated by the following formula: ε={1-(Wa / da+Wb / db+Wc / dc+...+Wn / dn) / t}×100, where the constituent materials of the porous layer are a, b, c,..., n, and the masses per unit area of ​​each constituent material are Wa, Wb, Wc,..., Wn (g / cm ２ ), and the true densities of the constituent materials are da, db, dc, ..., dn (g / cm ３ ) and the thickness of the porous layer is t (cm).

[0161] [Cycle Characteristics] A cycle evaluation test was carried out on the manufactured two-electrode lithium secondary battery under the following conditions: Test temperature: room temperature Charge / discharge current density: 0.6 mA / cm ２ Discharge: 6mA / cm ２ ・Voltage range: 2.0V to 4.2V

[0162] In the cycle evaluation test, batteries with a capacity retention of 80% or more after 250 cycles were rated as "A (best)", batteries with a capacity retention of 60% or more were rated as "B (good)", batteries with a capacity retention of 40% or more were rated as "C (fair)", and batteries with a capacity retention of less than 40% were rated as "D (bad)".

[0163] [X-ray CT Analysis] X-ray CT analysis was performed using an Xradia 520 Versa (manufactured by ZEISS). The power supply voltage and power were 140 kV and 10 W, respectively. For each battery prepared in the following Examples and Comparative Examples, after a predetermined number of charge / discharge tests, the battery was attached to a sample holder, rotated 360° with an exposure time of 10 seconds, and 4501 scans (sinograms) were taken. The pixel resolution was 3.385 μm.

[0164] Example 1 Production of Separator (A) Polymetaphenylene isophthalamide was dissolved in dimethylacetamide (DAc) to a resin concentration of 4.0% by mass, and barium sulfate particles (average primary particle size 0.05 μm) were further stirred and mixed to obtain a coating solution (1). An appropriate amount of the coating solution (1) was placed on a Mayer bar, and the coating solution (1) was applied to both sides of a polyethylene microporous membrane (thickness 9 μm, porosity 53%). The coating was applied so that the coating amount was equal on both sides of the polyethylene microporous membrane. The membrane was immersed in a coagulation liquid (DAc:water = 50:50 [mass ratio], liquid temperature 40°C) to solidify the coating layer, followed by washing in a water washing tank at a water temperature of 40°C and drying. In this way, a separator was obtained in which porous layers of equal thickness were formed on both sides of the polyethylene microporous membrane. In this separator, the thickness (average thickness) of the porous layer on both sides was 13.3 μm in total, and the volume ratio of barium sulfate particles to the solid volume of the porous layer was 64 volume %. Hereinafter, this separator will be referred to as "separator (A1)". Separator (A1) is shown in Table 1.

[0165] <Manufacturing of lithium secondary batteries>

[0166] A two-electrode cell was produced using the separator (A1). The two-electrode cell had the following configuration: Single-layer laminate cell Positive electrode: NMC811 (30 mg / cm) was deposited on a current collector made of aluminum foil. ２ ) as a composite layer containing acetylene black as a conductive additive and polyvinylidene fluoride as a binder. Negative electrode: A metal lithium foil (thickness 50 μm) is laminated on a current collector made of copper foil. Electrolyte: 4 mol / L LiFSI in DME (1,2-dimethoxyethane), 10 μL / cm ２

[0167] The following treatments were all carried out in an argon gas atmosphere. The positive electrode was cut into a rectangle measuring 40 mm in length and 30 mm in width. The negative electrode was cut into a rectangle measuring 42 mm in length and 32 mm in width. The separator was cut into a rectangle measuring 46 mm in length and 36 mm in width, placed in an aluminum laminate pack, and a predetermined amount of electrolyte was poured into it and sealed.

[0168] The assembled two-electrode cell was subjected to a cycle evaluation test. The evaluation results are shown in Table 1.

[0169] [Comparative Example 1] <Production of Separator (B)> A separator was produced in the same manner as in Example 1, except that the film thickness (average thickness) of the porous layer was changed to 1.3 μm in total on both sides. Hereinafter, this separator will be referred to as "separator (B1)". The separator (B1) is shown in Table 1.

[0170] <Manufacturing of lithium secondary batteries>

[0171] A two-electrode cell was produced using the separator (B1) in the same manner as in Example 1. A cycle evaluation test was carried out using the produced two-electrode cell. The evaluation results are shown in Table 1.

[0172] 2 shows the charge / discharge curves of the two-electrode cells of Example 1 and Comparative Example 1. As can be seen from FIG. 2, the two-electrode cell of Example 1 has better cycle characteristics than the two-electrode cell of Comparative Example 1.

[0173] For the separator (A1) of Example 1 and the separator (B1) of Comparative Example 1, X-ray CT analysis was performed on the cross sections of the negative electrodes when the cycle evaluation test was performed at 0 cycle (the separator before the cycle evaluation test), 50 cycles, and 100 cycles. The results of the X-ray CT analysis of the separator (A1) are shown in FIG. 3. The results of the X-ray CT analysis of the separator (B1) are shown in FIG. 4. Furthermore, as a result of the X-ray CT analysis, the thickness of the negative electrode when the cycle evaluation test was performed at 100 cycles was calculated from the image obtained by X-ray CT analysis, and was 98 μm in Example 1 and 160 μm in Comparative Example 1. The thicknesses of these negative electrodes are shown in FIG. 5.

[0174] [Reference Example 1] A separator (C) was prepared by stacking three polyethylene microporous membranes (thickness 7 μm, porosity 53%). The thickness of the separator (C) was 22 μm. A two-electrode cell was assembled using the separator (C) in the same manner as in Example 1, and a cycle evaluation test was performed. The charge / discharge curve of the two-electrode cell of Reference Example 1 is shown in FIG. 6 together with the charge / discharge curves of the two-electrode cells of Example 1 and Comparative Example 1. As in Example 1 and Comparative Example 1, the thickness of the negative electrode when a cycle evaluation test was performed for 100 cycles was calculated from an image obtained by X-ray CT analysis and is shown in FIG. 5.

[0175] [Example 2] A separator (A2) was produced in the same manner as in Example 1, except that the film thickness (average thickness) of the porous layer was 23.2 µm in total on both sides. The separator (A2) is shown in Table 1. A two-electrode cell was assembled using the separator (A2) in the same manner as in Example 1, and a cycle evaluation test was performed. The charge / discharge curve in the cycle evaluation test is shown in Figure 6. The evaluation results of the cycle evaluation test are also shown in Table 1. In addition, X-ray CT analysis was performed on the cross section of the negative electrode when the cycle evaluation test was performed for 50 cycles in the same manner as in Example 1. The results of the X-ray CT analysis of the separator (A2) are shown in Figure 7.

[0176] [Example 3] A separator (A3) was produced in the same manner as in Example 1, except that the film thickness (average thickness) of the porous layer was 36.3 μm in total on both sides. The separator (A3) is shown in Table 1. A two-electrode cell was assembled using the separator (A3) in the same manner as in Example 1, and a cycle evaluation test was performed. The charge / discharge curve in the cycle evaluation test is shown in FIG. 6. The evaluation results of the cycle evaluation test are also shown in Table 1. X-ray CT analysis was performed on the cross section of the negative electrode when the cycle evaluation test was performed for 50 cycles in the same manner as in Example 1. The results of the X-ray CT analysis of the separator (A3) are shown in FIG. 7. In FIG. 7, the separator (D) is Reference Example 2, and is a polyethylene microporous membrane (thickness 9 μm, porosity 53%).

[0177] Comparative Example 2 A separator (B2) was produced in the same manner as in Example 1, except that the film thickness (average thickness) of the porous layer was 3.1 μm in total on both sides. The separator (B2) is shown in Table 1. A two-electrode cell was assembled using the separator (B2) in the same manner as in Example 1, and a cycle evaluation test was performed. The charge / discharge curve in the cycle evaluation test is shown in FIG. 6. The evaluation results of the cycle evaluation test are also shown in Table 1. X-ray CT analysis was also performed on the cross section of the negative electrode when the cycle evaluation test was performed for 50 cycles in the same manner as in Example 1. The results of the X-ray CT analysis of the separator (B2) are shown in FIG. 7.

[0178] [Comparative Example 3] A separator (B3) was produced in the same manner as in Example 1, except that the thickness (average thickness) of the porous layer was 9.0 μm in total on both sides. The separator (B3) is shown in Table 1. A two-electrode cell was assembled using the separator (B3) in the same manner as in Example 1, and a cycle evaluation test was performed. The charge / discharge curve in the cycle evaluation test is shown in FIG. 6. The evaluation results of the cycle evaluation test are also shown in Table 1.

[0179] Comparative Example 4 A separator (B4) was produced in the same manner as in Example 1, except that the film thickness (average thickness) of the porous layer on both sides was 44.7 μm in total. The separator (B4) is shown in Table 1. A two-electrode cell was assembled using the separator (B4) in the same manner as in Example 1, and a cycle evaluation test was performed. The charge / discharge curve in the cycle evaluation test is shown in FIG. 6. The evaluation results of the cycle evaluation test are also shown in Table 1. X-ray CT analysis was also performed on the cross section of the negative electrode when the cycle evaluation test was performed for 50 cycles in the same manner as in Example 1. The results of the X-ray CT analysis of the separator (B4) are shown in FIG. 7.

[0180]

[0181] The results shown in Table 1 and FIG. 2 reveal that Example 1, which used separator (A1), had improved cycle characteristics compared to the comparative example using separator (B1). The results shown in FIGS. 3 and 4 reveal that Example 1, which used separator (A1), suppressed the formation of porosity in the negative electrode compared to Comparative Example 1 using separator (B1). The results shown in FIG. 5 reveal that Example 1, which used separator (A1), suppressed the formation of thicker negative electrode films compared to Comparative Example 1 using separator (B1). The results shown in FIG. 6 reveal that Example 2, which used separator (A2), and Example 3, which used separator (A3), had improved cycle characteristics compared to Comparative Example 2, which used separator (B2), Comparative Example 2, which used separator (B3), and Comparative Example 4, which used separator (B4). The results shown in FIG. 7 reveal that Example 2, which used separator (A2), and Example 3, which used separator (A3), suppressed the formation of porosity in the negative electrode.

[0182] The disclosure of Japanese Patent Application No. 2024-073231, filed on April 26, 2024, is incorporated herein by reference in its entirety. All documents, patent applications, and technical standards mentioned herein are incorporated herein by reference to the same extent as if each individual document, patent application, and technical standard was specifically and individually indicated to be incorporated by reference.

Claims

1. A lithium secondary battery comprising: a positive electrode; a negative electrode that operates by dissolution and precipitation of metallic lithium; an electrolyte solution containing a non-aqueous solvent and a lithium salt, the lithium salt concentration being 2.0 mol / L or more; a polyolefin microporous membrane; and a separator having a porous layer provided on one or both sides of the polyolefin microporous membrane, wherein the porous layer contains a wholly aromatic polyamide, and the thickness of the porous layer is 5 μm to 20 μm per side of the separator.

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

3. The lithium secondary battery according to claim 2, wherein the porous layer contains the inorganic particles in a proportion of 40% by volume or more and 80% by volume or less, based on the volume of the entire solid content constituting the porous layer.

4. The lithium secondary battery according to claim 2, wherein the inorganic particles are barium sulfate.

5. The lithium secondary battery according to claim 1, wherein the porosity of the porous layer is 60% or more and less than 85%.

6. The lithium secondary battery according to claim 1, wherein the pores of the polyolefin microporous film contain the fibrous wholly aromatic polyamide.

7. The lithium secondary battery according to claim 1, wherein the lithium salt concentration of the electrolyte is 2.5 mol / L or more.

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

9. The lithium secondary battery according to claim 1, wherein the non-aqueous solvent is an ether-based solvent.

10. A separator for a lithium secondary battery comprising: a polyolefin microporous membrane; and a porous layer containing a wholly aromatic polyamide and provided on one or both sides of the polyolefin microporous membrane, wherein the porous layer has a thickness of 5 μm to 20 μm per side of the separator; and the lithium secondary battery comprises a positive electrode, a negative electrode that operates by dissolution and precipitation of metallic lithium, and an electrolyte containing a non-aqueous solvent and a lithium salt, the lithium salt concentration being 2.0 mol / L or more.

11. A method for suppressing porosity in a negative electrode of a lithium secondary battery comprising a positive electrode, a negative electrode that operates by dissolution and precipitation of metallic lithium, an electrolyte, and a separator, the method comprising the steps of: disposing, between the positive electrode and the negative electrode, a polyolefin microporous membrane; and the separator having a porous layer containing a wholly aromatic polyamide and provided on one or both sides of the polyolefin microporous membrane, wherein the porous layer has a thickness of 5 μm to 20 μm per side of the separator.

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