Sodium-ion secondary battery

Abstract

A sodium-ion secondary battery according to the present disclosure comprises: a positive electrode; a negative electrode; an electrolyte solution that contains a nonaqueous solvent and a sodium salt; and a separator. The separator has a polyolefin microporous film, a fibrous meta-type wholly aromatic polyamide that is provided in the pores of the polyolefin microporous film, and a porous layer that is provided on one surface or both surfaces of the polyolefin microporous film and contains the meta-type wholly aromatic polyamide.

Description

Sodium-ion rechargeable battery

[0001] This disclosure relates to sodium-ion secondary batteries.

[0002] Sodium-ion rechargeable batteries have long attracted attention because sodium is more abundant and cheaper than lithium.

[0003] Patent documents 1 and 2 specifically disclose sodium-ion secondary batteries. These sodium-ion secondary batteries include a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte. The separator is formed by laminating a heat-resistant porous layer containing para-oriented aromatic polyamide and a porous polyethylene film. The non-aqueous electrolyte includes propylene carbonate (PC) as a non-aqueous solvent.

[0004] Patent Document 1: Patent No. 5158027 Patent Document 2: Patent No. 5493301

[0005] Propylene carbonate offers advantages in terms of safety and low-temperature characteristics. However, propylene carbonate has the drawback of not easily penetrating the separator. In recent years, the application of concentrated electrolytes has also been considered to improve battery characteristics. However, concentrated electrolytes also have high viscosity, making them difficult to penetrate the separator. When the electrolyte does not easily penetrate the separator, it is difficult to obtain excellent battery charge-discharge cycle characteristics.

[0006] One embodiment of this disclosure aims to solve the problem of providing a sodium-ion secondary battery with excellent charge-discharge cycle characteristics.

[0007] The specific means for achieving the objective are as follows: <1> A sodium-ion secondary battery comprising a positive electrode, a negative electrode, an electrolyte containing a non-aqueous solvent and a sodium salt, and a separator, wherein the separator comprises: a polyolefin microporous membrane, fibrous meta-type total aromatic polyamide provided in the pores of the polyolefin microporous membrane, and a porous layer containing the meta-type total aromatic polyamide provided on one or both sides of the polyolefin microporous membrane. <2> The sodium-ion secondary battery according to <1>, wherein the non-aqueous solvent contains propylene carbonate. <3> The sodium-ion secondary battery according to <1> or <2>, wherein the electrolyte has a sodium salt concentration of 2.0 mol / L or more. <4> The sodium-ion secondary battery according to any one of <1> to <3>, wherein the porous layer is provided on both sides of the polyolefin microporous membrane. <5> The sodium-ion secondary battery according to any one of <1> to <4>, wherein the porous layer further contains inorganic particles. <6> The sodium-ion secondary battery according to <5>, wherein the inorganic particles include metal sulfate particles. <7> The sodium-ion secondary battery according to <6>, wherein the average primary particle size of the inorganic particles is 0.3 μm or less. <8> The sodium-ion secondary battery according to any one of <1> to <7>, wherein the positive electrode and the negative electrode are in a combination that allows the cutoff voltage to be set to 2.0 V.

[0008] According to this disclosure, a sodium-ion secondary battery with excellent charge-discharge cycle characteristics is provided.

[0009] Figure 1 shows the TEM-EDS analysis results of the separator, illustrating the presence of fibrous meta-type total aromatic polyamide within the pores of the polyolefin microporous membrane. Figure 2 is a graph of the charge-discharge curve for Example 1. Figure 3 is a graph of the charge-discharge curve for Example 2. Figure 4 is a photograph of the appearance of the first sample piece (polyolefin microporous membrane of Comparative Example 1) and the second test piece (microporous membrane with porous layer of Example 1) immediately after the addition of a high-salt concentration electrolyte (NaFSA concentration: 5 mol / L). Figure 5 shows the appearance of a normal salt concentration electrolyte (NaPF ６Photographs of the appearance of the third sample piece (commercially available polyolefin microporous membrane) and the fourth test piece (microporous membrane with a porous layer of Example 1) immediately after dropping (concentration: 1 mol / L).

[0010] Hereinafter, the content of the present disclosure will be described in detail. The description of the constituent elements described below may be based on representative embodiments of the present disclosure, but the present disclosure is not limited to such embodiments.

[0011] In the present disclosure, the numerical range represented by using "to" means a range including the numerical values described before and after "to" as the lower limit value and the upper limit value. In the present disclosure, in the numerical range described step by step in the "Mode for Carrying Out the Invention", the upper limit value or the lower limit value described in one numerical range may be replaced with the upper limit value or the lower limit value of the numerical range described in other step-by-step descriptions. In the present disclosure, in the numerical range described in the "Mode for Carrying Out the Invention", the upper limit value or the lower limit value of the numerical range may be replaced with the value shown in the "Examples". In the present disclosure, a combination of two or more preferred modes is a more preferred mode. In the drawings, the same or corresponding parts are denoted by the same reference numerals and the description will not be repeated.

[0012] In the present disclosure, MD (Machine Direction) means the longitudinal direction in a polyolefin microporous membrane and a separator manufactured in a long shape. TD (Transverse Direction) means a direction orthogonal to MD in the plane direction of the polyolefin microporous membrane and the separator. In the present disclosure, TD is also referred to as the "width direction".

[0013] (1) Sodium-ion secondary battery The sodium-ion secondary battery of the present disclosure (hereinafter, also simply referred to as "sodium-ion secondary battery") includes a positive electrode, a negative electrode, an electrolytic solution containing a non-aqueous solvent and a sodium salt, and a separator. The separator has a polyolefin microporous membrane, fibrous meta-type wholly aromatic polyamide provided in the pores of the polyolefin microporous membrane, and a porous layer containing meta-type wholly aromatic polyamide provided on one or both sides of the polyolefin microporous membrane.

[0014] "Polyolefin microporous membrane" refers to a microporous membrane mainly composed of polyolefin. "Microporous membrane" refers to a membrane having numerous micropores inside, with these pores interconnected, allowing gas or liquid to pass from one side to the other. "Mainly composed of polyolefin" refers to a microporous membrane in which the polyolefin content relative to the total amount of the polyolefin microporous membrane is 95% by mass or more. "Porous layer" refers to a coating having numerous micropores, allowing gas or liquid to pass from one side to the other. "Meta-type all-aromatic polyamide" refers to an all-aromatic polyamide in which the benzene disubstituted compound constituting the main chain is a meta-disubstituted compound. "All-aromatic polyamide" refers to a polyamide in which the main chain consists only of benzene rings and amide bonds. All-aromatic polyamides may be copolymerized with small amounts of aliphatic monomers.

[0015] The sodium-ion secondary battery of this disclosure has the above configuration and therefore exhibits excellent charge-discharge cycle characteristics. This effect is presumed to be due to, but is not limited to, the following reasons: The porous layer contains meta-type total aromatic polyamide, and the polyolefin microporous membrane contains fibrous meta-type total aromatic polyamide in its pores. Meta-type total aromatic polyamide contains many polar groups and is presumed to exhibit high affinity for the electrolyte. Therefore, the electrolyte easily penetrates the separator having a porous layer and fibrous meta-type total aromatic polyamide in the pores of the polyolefin microporous membrane. For this reason, the sodium-ion secondary battery of this disclosure is presumed to have excellent charge-discharge cycle characteristics. The sodium-ion secondary battery of this disclosure makes it possible to use non-aqueous solvents (e.g., propylene carbonate) that are difficult to penetrate into the polyolefin microporous membrane and are therefore difficult to apply in practice. Furthermore, by providing the separator, the sodium-ion secondary battery of this disclosure can use an electrolyte with a sodium salt concentration of 2.0 mol / L or higher.

[0016] The electrode configuration of a sodium-ion secondary battery is not particularly limited and may be monopolar or bipolar. In a monopolar structure, the electrodes may be wound or stacked. In the wound type, a straight-line electrode body is wound, with the negative electrode, separator, and positive electrode stacked in that order. In the stacked type, a single-sheet electrode body is stacked, with the negative electrode, separator, and positive electrode stacked in that order. A sodium-ion secondary battery may contain one cell or multiple cells. When a sodium-ion secondary battery contains multiple cells, the multiple cells may be electrically connected in series or parallel.

[0017] (1.1) Electrolyte A sodium-ion secondary battery is equipped with an electrolyte.

[0018] The electrolyte contains a non-aqueous solvent and a sodium salt. The sodium salt concentration of the electrolyte is not particularly limited. The electrolyte may be an electrolyte with a sodium salt concentration of 2.0 mol / L or higher (hereinafter also referred to as a "high-salt concentration electrolyte") or an electrolyte with a sodium salt concentration of less than 2.0 mol / L (hereinafter also referred to as a "normal-salt concentration electrolyte").

[0019] The sodium salt concentration of the electrolyte is preferably 2.0 mol / L or higher. In this disclosure, the separator has a fibrous meta-type fully aromatic polyamide provided in the pores of the polyolefin microporous membrane, and a porous layer containing the meta-type fully aromatic polyamide provided on one or both sides of the polyolefin microporous membrane. Therefore, since high-salt concentration electrolytes are easily impregnated into the separator, high-salt concentration electrolytes can be easily used. In addition, in order to improve the energy density of the sodium-ion secondary battery, it is preferable to set the cutoff voltage to a low voltage (for example, 1.5V). "Cutoff voltage" refers to the voltage at which the discharge of the battery is terminated. When the positive electrode active material contains manganese and the cutoff voltage is a low voltage (for example, 1.5V), trivalent manganese (Mn ３＋ ) is produced. Typically, trivalent manganese (Mn ３＋ ) is tetravalent manganese (Mn ＋４It is more easily eluted into the electrolyte than ). However, in this disclosure, since a large amount of sodium salt is dissolved in the high-salt concentration electrolyte, trivalent manganese (Mn ３＋ ) does not easily dissolve in high-salt concentration electrolytes. Therefore, degradation is less likely even if the utilization rate of sodium in the positive electrode active material is increased. As a result, it is presumed that the sodium-ion secondary battery of this disclosure has excellent charge-discharge cycle characteristics even when the cutoff voltage is low (e.g., 1.5V). When the electrolyte contains multiple types of sodium salts, the total concentration of the multiple types of sodium salts contained in the electrolyte is 2.0 mol / L or more.

[0020] When the electrolyte is a high-salt concentration electrolyte, the sodium salt concentration of the electrolyte is preferably 2.5 mol / L or more, more preferably 3.0 mol / L or more, even more preferably 4.0 mol / L or more, and particularly preferably 5.0 mol / L or more, from the viewpoint of realizing the characteristics of a high-salt concentration electrolyte. From the viewpoint of improving the charge-discharge cycle characteristics, the sodium salt concentration of the electrolyte is preferably the saturation concentration. The sodium salt concentration of the electrolyte may be 15.0 mol / L or less, 10.0 mol / L or less, 7.0 mol / L or less, or 6.0 mol / L or less. The sodium salt concentration of the electrolyte may be between 2.0 mol / L and 12.0 mol / L.

[0021] When the electrolyte is a normal concentration electrolyte, the sodium salt concentration of the electrolyte is preferably 0.5 mol / L or more, more preferably 1.0 mol / L or more, and may be less than 2.0 mol / L, and may be 1.5 mol / L or less, from the viewpoint of charge-discharge rate characteristics. The sodium salt concentration of the electrolyte may be 0.5 mol / L or more and less than 2.0 mol / L.

[0022] (1.1.1) The non-aqueous solvent electrolyte contains a non-aqueous solvent. The non-aqueous solvent may be a known non-aqueous solvent used in sodium-ion secondary batteries. Examples of non-aqueous solvents include: cyclic carbonates such as ethylene carbonate, propylene carbonate (PC), fluoroethylene carbonate, difluoroethylene carbonate, and vinylene carbonate; linear carbonates such as dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, and their fluorine-substituted derivatives; cyclic esters such as γ-butyrolactone and γ-valerolactone; linear esters such as methyl acetate; ethers such as 1,2-dimethoxyethane (DME), 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. Non-aqueous solvents may be used alone or in combination of two or more. Among them, propylene carbonate is preferable as the non-aqueous solvent because it has a high boiling point, excellent safety, and imparts excellent low-temperature characteristics to sodium-ion secondary batteries.

[0023] (1.1.2) The sodium salt electrolyte contains a sodium salt. The sodium salt may be any known sodium salt used in sodium-ion secondary batteries. Examples of sodium salts include sodium bisfluorosulfonylamide (NaFSA) and sodium hexafluorophosphate (NaPF). ６ ), sodium perchlorate (NaClO ４ ), sodium tetrafluoroborate (NaBF ４Examples include sodium bistrifluoromethanesulfonylimide (NaTFSI) and sodium difluorooxalate borate (NaDFOB). The sodium salt may be used alone or in combination of two or more types. In particular, the sodium salt preferably contains sodium bisfluorosulfonylamide, and more preferably sodium bisfluorosulfonylamide.

[0024] (1.1.3) The additive electrolyte may further contain additives. Examples of additives include vinylene carbonate, propane sultone, tert-butylbenzene, fluoroethylene carbonate, sodium bis(oxalate) borate, succinonitrile, adiponitrile, triisopropoxyboroxine, sulfolane, hydrofluoroether, and vinyl acetate. Additives may be used individually or in combination of two or more.

[0025] (1.2) Separator The separator electrically insulates the positive electrode from the negative electrode and provides a pathway for sodium ions to move between the positive electrode and the negative electrode. The separator is placed between the positive electrode and the negative electrode.

[0026] The separator comprises a polyolefin microporous membrane, fibrous meta-type fully aromatic polyamide provided in the pores of the polyolefin microporous membrane, and a porous layer containing the meta-type fully aromatic polyamide provided on one or both sides of the polyolefin microporous membrane.

[0027] From the viewpoint of mechanical strength, the thickness of the separator 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 is preferably 30 μm or less, more preferably 25 μm or less, and even more preferably 20 μm or less. The thickness of the separator (μm) is the average value obtained by measuring 20 points within a 10 cm square area using a contact-type thickness gauge.

[0028] (1.2.1) Polyolefin microporous membrane The polyolefin microporous membrane may be a known polyolefin microporous membrane used in battery separators. The polyolefin microporous membrane may be a polyethylene microporous membrane from the viewpoint of exhibiting a shutdown function. The polyolefin microporous membrane may be a polypropylene microporous membrane from the viewpoint of having heat resistance that prevents it from easily breaking when exposed to high temperatures.

[0029] The "shutdown function" refers to a feature that prevents thermal runaway of the battery by preventing sodium ion movement when the battery temperature rises, as the constituent materials melt and block the pores in the polyolefin microporous membrane.

[0030] The surface of the polyolefin microporous film may be surface-treated to improve wettability with the coating liquid for forming the porous layer. Examples of surface treatments include corona treatment, plasma treatment, flame treatment, and ultraviolet irradiation treatment.

[0031] The thickness of the polyolefin microporous membrane is not particularly limited. From the viewpoint of the manufacturing yield of the separator and the manufacturing yield of the battery, 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 of the polyolefin microporous membrane (μm) is the average value obtained by measuring 20 points within a 10 cm square area using a contact-type thickness gauge.

[0032] The porosity of the polyolefin microporous membrane can be the same as that of a typical porosity used in battery separators. From the viewpoint of obtaining appropriate membrane resistance and shutdown function, the porosity of the polyolefin microporous membrane is preferably 20% to 60%, and more preferably 30% to 50%. The method for measuring the porosity ε (%) of the polyolefin microporous membrane is the same as that described in the examples.

[0033] The Gahl value (JIS P8117:2009) of the polyolefin microporous membrane is preferably 20 sec / 100 cc or higher, more preferably 30 sec / 100 cc or higher, and even more preferably 50 sec / 100 mL or higher, from the viewpoint of suppressing battery short circuits. The Gahl value (JIS P8117:2009) of the polyolefin microporous membrane is preferably 200 sec / 100 cc or lower, more preferably 180 sec / 100 cc or lower, and even more preferably 165 sec / 100 cc or lower, from the viewpoint of ion permeability. The method for measuring the Gahl value of the polyolefin microporous membrane is the same as the method described in the examples.

[0034] It is preferable that part or all of the walls of the pores in the polyolefin microporous membrane are coated with meta-type fully aromatic polyamide. This allows high-salt concentration electrolytes to easily penetrate the polyolefin microporous membrane.

[0035] Methods for producing polyolefin microporous membranes include a first production method and a second production method. The first production method involves extruding molten polyolefin from a T-die to form a sheet, crystallizing it, stretching it, and then heat-treating it to form a microporous membrane. The second production method involves extruding molten polyolefin together with a plasticizer such as liquid paraffin from a T-die, cooling it to form a sheet, stretching it, extracting the plasticizer, and then heat-treating it to form a microporous membrane.

[0036] (1.2.2) Porous layer The porous layer facilitates the impregnation of the polyolefin microporous membrane with a high salt concentration electrolyte.

[0037] It is preferable that the porous layer be provided on both sides of the polyolefin microporous membrane. The polyolefin microporous membrane alone tends to repel high-salt concentration electrolytes, and high-salt concentration electrolytes do not easily penetrate the polyolefin microporous membrane alone. By providing the porous layer on both sides of the polyolefin microporous membrane, the high-salt concentration electrolyte becomes easier to penetrate the polyolefin microporous membrane alone than when the porous layer is provided on only one side. In addition, excellent electrolyte retention at the positive electrode interface and negative electrode interface makes it easier to obtain good cycle characteristics.

[0038] From the viewpoint of handling ease during battery manufacturing, the thickness of the porous layer is preferably 0.1 μm or more, more preferably 0.5 μm or more, and even more preferably 1.0 μm or more. From the viewpoint of increasing ion permeability and the energy density of the battery, the thickness of the porous layer is preferably 10.0 μm or less, more preferably 8.0 μm or less, and even more preferably 6.0 μm or less.

[0039] When porous layers are provided on both sides of a polyolefin microporous membrane, the total thickness of the porous layers may be 1.0 μm or more, 2.0 μm or more, 3.0 μm or more, 20.0 μm or less, 16.0 μm or less, or 12.0 μm or less. The thickness of the porous layers (total thickness of 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 (μm).

[0040] When porous layers are provided on both sides of a polyolefin microporous membrane, it is preferable that the difference (μm) between the thickness of one porous layer and the thickness of the other porous layer be as small as possible.

[0041] The porosity of the porous layer only needs to be sufficient for it to function as a battery separator.

[0042] (1.2.2.1) The meta-type all-aromatic polyamide porous layer contains a meta-type all-aromatic polyamide. The porous layer may consist of a meta-type all-aromatic polyamide. Meta-type all-aromatic polyamide has excellent moldability. For example, the porous layer can be obtained by applying a coating solution onto a polyolefin microporous membrane, solidifying the coating layer by immersion in a solidification solution, removing it from the solidification solution, washing it with water, and drying it, as described later. In the case of para-type all-aromatic polyamide, the intermolecular forces are strong, making it difficult to produce a coating solution containing para-type all-aromatic polyamide unless an ionic substance (e.g., calcium chloride) is added. On the other hand, because meta-type all-aromatic polyamide has a weak molecular weight, it is easy to produce a coating solution containing meta-type all-aromatic polyamide with low viscosity without adding an ionic substance. A low-viscosity coating solution easily penetrates the pores of a polyolefin microporous membrane. Therefore, the moldability of a porous layer using meta-type all-aromatic polyamide is superior to that of a porous layer using para-type all-aromatic polyamide. Furthermore, since the presence of ionic substances in the porous layer raises concerns about their impact on battery performance, meta-type total aromatic polyamides are preferred from this viewpoint as well.

[0043] Examples of meta-type all-aromatic polyamides include polymetaphenylene isophthalamide. Examples of para-type all-aromatic polyamides include coplyparaphenylene 3,4'-oxydiphenylene terephthalamide and polyparaphenylene terephthalamide. All-aromatic polyamides may be commercially available. Examples of commercially available products include Conex (registered trademark; meta-type), Technora (registered trademark; para-type), and Twaron (registered trademark; para-type), all manufactured by Teijin Limited.

[0044] The content of meta-type total aromatic polyamide in the porous layer is preferably 85% to 100% by mass, more preferably 90% to 100% by mass, even more preferably 95% to 100% by mass, and particularly preferably 100% by mass, relative to the total amount of resin in the porous layer.

[0045] When porous layers are provided on both sides of a polyolefin microporous membrane, at least one of the types and content of meta-type total aromatic polyamides contained in one porous layer and at least one of the types and content of meta-type total aromatic polyamides contained in the other porous layer may be the same or different.

[0046] (1.2.2.2) Other porous resin layers may contain other resins different from meta-type total aromatic polyamides. Examples of other resins include para-type total aromatic polyamides, polyamide-imides, poly-N-vinylacetamide, polyacrylamide, copolymer polyether polyamides, polyimides, polyetherimides, polyvinylidene fluoride resins, acrylic resins, fluororubbers, styrene-butadiene copolymers, homopolymers or copolymers of vinyl nitrile compounds (e.g., acrylonitrile and methacrylonitrile), carboxymethylcellulose, hydroxyalkylcellulose, polyvinyl alcohol, polyvinyl butyral, polyvinylpyrrolidone, polyethers (e.g., polyethylene oxide and polypropylene oxide), polysulfones, polyketones, polyetherketones, and polyethersulfones. Other resins may be used individually or in combination of two or more types.

[0047] The content of other resins in the porous layer is preferably 0% to 15% by mass, more preferably 0% to 10% by mass, even more preferably 0% to 5% by mass, and particularly preferably 0% by mass, relative to the total amount of resin in the porous layer. Ideally, the resin component of the porous layer should consist solely of meta-type total aromatic polyamide.

[0048] (1.2.2.3) The inorganic particle porous layer preferably contains inorganic particles from the viewpoint of heat resistance and porousness of the layer. Examples of inorganic particles include metal sulfate particles (e.g., barium sulfate particles, strontium sulfate particles, and calcium sulfate particles), metal hydroxide particles (e.g., magnesium hydroxide, aluminum hydroxide, and calcium hydroxide), metal oxide particles (e.g., barium titanate particles, magnesium oxide particles, and alumina particles), metal carbonate particles (e.g., calcium carbonate particles, and magnesium carbonate particles), metal nitride particles (e.g., magnesium nitride, aluminum nitride, and calcium nitride), metal fluoride particles (e.g., magnesium fluoride particles, and calcium fluoride particles), and clay mineral particles (e.g., calcium silicate particles, calcium phosphate particles, and apatite particles). One type of inorganic particle may be used alone, or two or more types may be used in combination. In particular, inorganic particles are less likely to decompose the electrolyte or electrolyte solution, and therefore less likely to cause gas generation inside the battery. From this viewpoint, it is preferable to include metal sulfate particles, and more preferably to include barium sulfate particles.

[0049] From the viewpoint of suppressing gas generation inside the battery, the amount of inorganic particles in the porous layer 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.

[0050] The average primary particle size of the inorganic particles is preferably 0.01 μm to 0.3 μm or less, from the viewpoint of making the layer porous and forming a dense porous layer with high uniformity. The average primary particle size of the inorganic particles is determined by measuring the major axis of 100 randomly selected inorganic particles observed with a scanning electron microscope (SEM) and averaging the major axes of the 100 particles.

[0051] The mass percentage of inorganic particles in the porous layer is preferably 60% to 95% by mass, more preferably 70% to 90% by mass, and even more preferably 75% to 85% by mass.

[0052] (1.2.2.4) The organic filler porous layer may further contain organic fillers. Examples of organic fillers include particles made of crosslinked polymers and particles made of heat-resistant polymers. The organic fillers may be used individually or in combination of two or more types.

[0053] Examples of crosslinked polymers include crosslinked poly(meth)acrylic acid, crosslinked poly(meth)acrylic acid esters, crosslinked polysilicone, crosslinked polystyrene, crosslinked polydivinylbenzene, styrene-divinylbenzene copolymer crosslinked products, polyimides, melamine resins, phenolic resins, and benzoguanamine-formaldehyde condensates. Examples of heat-resistant polymers include polysulfone, polyacrylonitrile, aramid, polyacetal, and thermoplastic polyimides. "(meth)acrylic" refers to at least one of "acrylic" and "methacrylic".

[0054] (1.2.2.5) The additive porous layer may further contain additives. Examples of additives include dispersants (e.g., surfactants), wetting agents, defoaming agents, and pH adjusters. Additives may be used individually or in combination of two or more types.

[0055] (1.2.3) The fibrous meta-type all-aromatic polyamide separator preferably contains fibrous meta-type all-aromatic polyamide in the pores of the polyolefin microporous membrane. Since the meta-type all-aromatic polyamide is in the form of fine fibers, it does not block the pores of the polyolefin microporous membrane. Therefore, gas or liquid can pass from one side of the polyolefin microporous membrane to the other.

[0056] Meta-type wholly aromatic polyamide shows a high affinity with high-salt-concentration electrolytes. High-salt-concentration electrolytes can easily penetrate into a polyolefin microporous membrane containing fibrous meta-type wholly aromatic polyamide in its pores. From the perspective of the permeability of high-salt-concentration electrolytes to the polyolefin microporous membrane, it is preferable that fibrous meta-type wholly aromatic polyamide is contained in the pores of at least the region close to the surface of the polyolefin microporous membrane, and it is more preferable that fibrous meta-type wholly aromatic polyamide is contained in all the pores of the polyolefin microporous membrane.

[0057] The fibrous meta-type wholly aromatic polyamide may form a three-dimensional network structure inside the polyolefin microporous membrane. Thereby, high-salt-concentration electrolytes can penetrate more easily through the separator than when the fibrous meta-type wholly aromatic polyamide does not form a three-dimensional network structure.

[0058] When the separator contains fibrous meta-type wholly aromatic polyamide, from the perspective of the manufacture of the separator, the meta-type wholly aromatic polyamide contained in the porous layer is preferably meta-type wholly aromatic polyamide.

[0059] The fact that fibrous meta-type wholly aromatic polyamide is contained in the pores of the polyolefin microporous membrane in the separator can be confirmed by elemental imaging combining TEM (transmission electron microscope) with EDS (Energy Dispersive X-ray Spectroscopy). Utilizing the fact that meta-type wholly aromatic polyamide is easily stainable with ruthenium tetroxide (RuO ４ ), meta-type wholly aromatic polyamide is detected by Ru imaging. The method for preparing the RuO ４ -stained sample for TEM observation is as follows.

[0060] (i) Cut out the separator into a triangle (base about 0.5 mm × height about 1 mm, MD as the base). (ii) Attach the triangular sample to a slide glass and leave it standing in a sealed container with a capacity of about 50 ml. 0.5 mass% RuO ４(iii) Add approximately 0.5 ml of aqueous solution to a sealed container and perform vapor staining at room temperature for 15 minutes. This staining condition does not stain polyolefins. (iii) Impregnate the stained sample with embedding resin. A stepwise substitution treatment is performed to impregnate the embedded resin into the porous interior of the sample. Specifically, the sample is placed in n-butyl glycidyl ether, n-butyl glycidyl ether:embedding resin = 1:1, and n-butyl glycidyl ether:embedding resin = 1:3 sequentially for 30 minutes each, and then placed in the embedding resin overnight. (iv) Place the sample on an embedding plate, inject the embedding resin and allow it to harden completely. (v) Using an ultramicrotome, prepare ultrathin sections of approximately 80 nm at room temperature. Cut the sample so that the MD of the separator is visible in the cross-section. Attach a carbon support film to the ultrathin section and place it on a Cu grid.

[0061] Figure 1 shows an example of TEM-EDS analysis results. This example is an analysis of a separator equipped with a polyethylene microporous membrane as the polyolefin microporous membrane. The three images in Figure 1 are: the left is a TEM image, the center is a STEM-HAADF image of a portion of the left image, and the right is a Ru imaging image of the same field of view as the center image. In the TEM image, the cells are the pores of the polyolefin microporous membrane, and the partitions between cells are the pore walls of the polyolefin microporous membrane. From the TEM and STEM-HAADF images, it can be seen that there are fine fibers inside the pores. From the STEM-HAADF and Ru imaging images, it can be seen that the fine fibers inside the pores are meta-type total aromatic polyamides. It can be seen that the fibrous meta-type total aromatic polyamides constitute a three-dimensional network structure inside the polyolefin microporous membrane.

[0062] (1.2.3.1) Meta-type total aromatic polyamides In fibrous meta-type total aromatic polyamides, the meta-type total aromatic polyamides are the same as those exemplified as meta-type total aromatic polyamide amides contained in the porous layer. The meta-type total aromatic polyamides in fibrous meta-type total aromatic polyamides may be the same as or different from the meta-type total aromatic polyamide amides contained in the porous layer.

[0063] (1.2.4) Method for manufacturing a separator A separator can be manufactured, for example, by forming a porous layer on a polyolefin microporous membrane using a wet coating method or a dry coating method. In this disclosure, "wet coating method" refers to a method of solidifying the coating layer in a solidifying liquid. "Dry coating method" refers to a method of solidifying the coating layer by drying.

[0064] The following describes a wet coating method for a separator having a polyolefin microporous membrane, a porous layer containing meta-type fully aromatic polyamide, and fibrous meta-type fully aromatic polyamide.

[0065] In the wet coating method, a coating solution for forming a porous layer is applied to a polyolefin microporous membrane, the coating layer is solidified by immersion in a solidifying solution, and then the membrane is removed from the solidifying solution and washed with water and dried.

[0066] The coating solution for forming the porous layer is prepared by dissolving meta-type total aromatic polyamide in a solvent. If necessary, other components besides meta-type total aromatic polyamide may be dissolved or dispersed in the coating solution.

[0067] The solvent used in preparing the coating solution includes a solvent that dissolves meta-type total aromatic polyamides (hereinafter also referred to as a "good solvent"). Examples of good solvents include polar amide solvents (e.g., N-methylpyrrolidone, dimethylacetamide, and dimethylformamide).

[0068] The solvent used in preparing the coating solution may contain a phase-separating agent to induce phase separation, from the viewpoint of forming a porous layer with a good porous structure. It is preferable to mix the phase-separating agent with a good solvent in an amount that ensures a viscosity suitable for coating. Examples of phase-separating agents include water, methanol, ethanol, propyl alcohol, butyl alcohol, butanediol, ethylene glycol, propylene glycol, and tripropylene glycol.

[0069] When the solvent used to prepare the coating solution is a mixed solvent of a good solvent and a phase separating agent, a mixed solvent containing 60% by mass or more of the good solvent and 5% to 40% by mass of the phase separating agent is preferred from the viewpoint of forming a good porous structure.

[0070] The resin concentration of the coating solution is preferably 1% to 20% by mass, from the viewpoint of forming a good porous structure. The inorganic particle concentration of the coating solution is preferably 0.5% to 50% by mass, from the viewpoint of forming a good porous structure.

[0071] The coating solution may contain dispersants such as surfactants, wetting agents, defoaming agents, and pH adjusters. These additives may remain in the porous layer as long as they are electrochemically stable within the operating range of the secondary battery and do not inhibit the reactions inside the battery.

[0072] Methods for applying coating liquid to polyolefin microporous films include Meyer bars, die coaters, reverse roll coaters, roll coaters, and gravure coaters. When forming porous layers on both sides of a polyolefin microporous film, it is preferable from a productivity viewpoint to apply the coating liquid to both sides of the polyolefin microporous film simultaneously.

[0073] The fibrous meta-type fully aromatic polyamide contained within the pores of the polyolefin microporous membrane is formed when the coating solution penetrates into the pores of the polyolefin microporous membrane and the meta-type fully aromatic polyamide solidifies within the pores. To allow the coating solution to penetrate into the pores of the polyolefin microporous membrane, it is preferable that the polyolefin microporous membrane is dry at the time of coating. Conventionally, a step of applying or impregnating the polyolefin microporous membrane with the solvent of the coating solution before coating may be performed for purposes such as improving the wettability of the coating solution, but in the manufacture of separators, it is preferable not to perform this step.

[0074] The coating layer is solidified by immersing the polyolefin microporous membrane on which the coating layer is formed in a solidification solution, thereby inducing phase separation in the coating layer and solidifying the resin. This yields a laminate consisting of a polyolefin microporous membrane, fibrous meta-type fully aromatic polyamide contained within the pores of the polyolefin microporous membrane, and a porous layer.

[0075] The solidification solution generally contains the good solvent and phase separating agent used in the preparation of the coating solution, along with water. From the viewpoint of productivity, it is preferable that the mixing ratio of the good solvent and the phase separating agent match the mixing ratio of the mixed solvent used in the preparation of the coating solution. From the viewpoint of forming a porous structure and productivity, it is preferable that the water content in the solidification solution be 40% to 90% by mass. The temperature of the solidification solution is, for example, 20°C to 50°C.

[0076] After the coating layer is solidified in the solidification solution, the laminate is removed from the solidification solution and washed with water. The solidification solution is removed from the laminate by washing with water. Further water is removed from the laminate by drying. Washing is performed, for example, by transporting the laminate in a water bath. Drying is performed, for example, by transporting the laminate in a high-temperature environment, blowing air on the laminate, or bringing the laminate into contact with a heat roll. The drying temperature is preferably 40°C to 80°C.

[0077] The separator can also be manufactured by a dry coating method. The dry coating method involves applying a coating solution to a polyolefin microporous membrane, drying the coating layer, and removing the solvent by evaporation to form a porous layer on the polyolefin microporous membrane.

[0078] (1.3) A positive-electrode sodium-ion secondary battery is provided with a positive electrode. The positive electrode may be a known positive electrode used in sodium-ion secondary batteries.

[0079] The positive electrode has a positive electrode mixture layer and may further have a positive electrode current collector. The positive electrode mixture layer may be arranged on one or both sides of the positive electrode current collector.

[0080] A metal foil is preferred as the positive electrode current collector. Examples of metal foils include aluminum foil, titanium foil, and stainless steel foil. The thickness of the positive electrode current collector is, for example, 5 μm to 20 μm.

[0081] The positive electrode mixture layer contains a positive electrode active material. The positive electrode active material releases or absorbs sodium ions. A sodium-containing transition metal compound is preferred as the positive electrode active material. The sodium-containing transition metal compound may, for example, contain at least one element from among Mn, Ni, and Co, along with Na and O as constituent elements. The structure of the sodium-containing transition metal compound is not particularly limited and may be a P2 type structure, an O2 type structure, or an O3 type structure, etc.

[0082] Examples of sodium-containing transition metal compounds include sodium manganese complex oxides (e.g., P'2-Na). ２／３ MnO ２ ), sodium nickel manganese magnesium titanium composite oxide (e.g., O3 / P2-Na ０．８３３ Ni ０．３１７ Mn ０．４６７ Mg ０．１ Ti ０．１１７ O ２ O3-Na ０．９５ Ni ０．３１６７ Mn ０．３１６７ Mg ０．１５８３ Ti ０．２０８３ O ２ , and P2-Na ２／３ Ni １／３ Mn １／２ Ti １／６ O ２ (e.g., sodium iron manganese complex oxide (e.g., P2-Na) ２／３ (Fe １／２ Mn １／２ ) O ２ (e.g., sodium manganese nickel composite oxide (e.g., P3-Na) ０．６７ Mn ０．６７ Ni ０．３３ O ２ P2-Na ０．６７ Mn ０．６７ Ni ０．３３ O ２ (e.g., Na) ３ V ２ (PO ４ ) ３ (etc.), and sodium vanadium phosphate fluorine compounds (e.g., NaVPO) ４ F and Na ３ V２ (PO ４ ) ２ F ３ Examples include the following. The sodium-containing transition metal compound may be used alone or in combination of two or more types. In particular, from a resource viewpoint, it is more preferable that the sodium-containing transition metal compound contains a sodium-manganese composite oxide.

[0083] The positive electrode mixture layer may further contain a positive electrode binder. Examples of positive electrode binders include polyvinylidene fluoride (PVDF) and fluoride-based binders (e.g., polyvinylidene fluoride-polyhexafluoropropylene copolymer (PVDF-HFP) and polytetrafluoroethylene (PTFE), etc.). The positive electrode binder may be used alone or in combination of two or more types.

[0084] The positive electrode mixture layer may further contain a positive electrode conductive additive. Examples of positive electrode conductive additives include carbon blacks (e.g., acetylene black and Ketjenblack), carbon fibers, metal fibers, metal powders, and organic conductive materials (e.g., polyphenylene derivatives). The positive electrode conductive additive may be used alone or in combination of two or more types.

[0085] (1.4) The negative electrode sodium-ion secondary battery is equipped with a negative electrode. The negative electrode may be a known negative electrode used in sodium-ion secondary batteries.

[0086] The negative electrode may be one that operates by sodium ion intercalation, alloying reaction, or sodium dissolution reaction.

[0087] (1.4.1) An intercalation of sodium ions is used to operate the negative electrode, for example, by intercalation of sodium ions, which comprises a negative electrode current collector and a negative electrode mixture layer disposed on one or both sides of the negative electrode current collector.

[0088] A metal foil is preferred as the negative electrode current collector. Examples of metal foils include copper foil, aluminum foil, nickel foil, and stainless steel foil. The thickness of the negative electrode current collector is, for example, 5 μm to 20 μm.

[0089] The negative electrode mixture layer contains a negative electrode active material that performs an intercalation reaction of sodium ions. Examples of such negative electrode active materials include hard carbon (i.e., carbon that is not easily graphitized), soft carbon (i.e., carbon that is easily graphitized), and graphite. The negative electrode active material may be used alone or in combination of two or more types.

[0090] The negative electrode mixture layer may further contain a negative electrode binder. Examples of negative electrode binders include those similar to those exemplified as positive electrode binders, as well as rubber-based binders (e.g., carboxymethylcellulose and styrene-butadiene rubber). The negative electrode binder may be the same as or different from the positive electrode binder.

[0091] The negative electrode mixture layer may further contain a negative electrode conductive additive. Examples of negative electrode conductive additives are the same as those exemplified for positive electrode conductive additives. The negative electrode conductive additive may be the same as or different from the positive electrode conductive additive.

[0092] (1.4.2) Alloying reaction The negative electrode that operates by an alloying reaction comprises, for example, a negative electrode current collector and a negative electrode mixture layer disposed on one or both sides of the negative electrode current collector. The negative electrode that operates by an alloying reaction may be the same as the negative electrode that operates by sodium ion intercalation, except that the negative electrode active material contained in the negative electrode mixture layer is different.

[0093] The negative electrode mixture layer contains a negative electrode active material that undergoes an alloying reaction with sodium. Examples of such negative electrode active materials include Si, Ge, Sn, Pb, P, Sb, and Bi. The negative electrode active material may be used alone or in combination of two or more types. The negative electrode active material may be mixed with inert elements (for example, Co, Ni, Zn, Mo, Cu, Ti, Te, and F).

[0094] (1.4.3) Sodium Dissolution and Extraction Reaction The negative electrode, which operates by the sodium dissolution and extraction reaction, includes a negative electrode current collector and does not have a negative electrode mixture layer. The negative electrode current collector includes a main surface on which sodium metal is deposited during charging. Specifically, sodium ions contained in the electrolyte receive electrons on the negative electrode current collector during charging, causing sodium metal to be deposited. The deposited sodium metal dissolves into the electrolyte as sodium ions during discharge. Examples of negative electrode current collectors include copper foil, aluminum foil, nickel foil, stainless steel foil, and sodium foil.

[0095] (1.5) Preferred combination of positive and negative electrodes It is preferable that the positive electrode and the negative electrode are in a combination that allows the cutoff voltage to be set to 2.0V. A "combination that allows the cutoff voltage to be set to 2.0V" refers to a combination in which, when the cutoff voltage is set to 2.0V, the positive electrode active material contained in the positive electrode is less likely to dissolve into the electrolyte (i.e., a combination in which the capacity (mAh / g) in the charge-discharge curve of the positive electrode active material is less likely to decrease). By having a combination of positive and negative electrodes that allows the cutoff voltage to be set to 2.0V, the generation of trivalent manganese is suppressed, the dissolution of manganese into the electrolyte is suppressed, and good cycle characteristics can be obtained.

[0096] Combinations that allow the cutoff voltage to be set to 2.0V include, for example, a case where the negative electrode is metallic sodium or hard carbon, the positive electrode is a sodium-manganese composite oxide, and the current capacity ratio of the negative electrode to the positive electrode is 1 to 2. The cutoff voltage of a sodium-ion secondary battery when the positive and negative electrodes are in a combination that allows the cutoff voltage to be set to 2.0V may be 2.0V or greater than 2.0V, and is appropriately selected depending on the combination of the positive and negative electrodes.

[0097] (1.6) Enclosure A sodium-ion secondary battery usually has an enclosure. The enclosure houses the positive electrode, negative electrode, electrolyte, and separator. Examples of enclosures include laminate film (e.g., aluminum sheet, etc.) and battery cans (e.g., cylindrical, prismatic, and coin-shaped, etc.).

[0098] (1.7) Preferred Embodiments The sodium-ion secondary battery of the present disclosure is preferably satisfied with a first condition. The "first condition" indicates that the non-aqueous solvent contains propylene carbonate. When the sodium-ion secondary battery of the present disclosure satisfies the first condition, better cycle characteristics can be obtained than when the first condition is not satisfied.

[0099] The sodium-ion secondary battery of this disclosure preferably satisfies a second condition. The "second condition" indicates that the electrolyte has a sodium salt concentration of 2.0 mol / L or higher. By satisfying the second condition, the sodium-ion secondary battery of this disclosure can obtain better cycle characteristics than when the second condition is not met, as it suppresses the elution of manganese into the electrolyte.

[0100] The sodium-ion secondary battery of this disclosure preferably satisfies a third condition. The "third condition" indicates that the porous layer is provided on both sides of the polyolefin microporous membrane. By satisfying the third condition, the sodium-ion secondary battery of this disclosure can retain the electrolyte well at the positive electrode and negative electrode interface, thereby improving the charge-discharge cycle characteristics compared to when the third condition is not met.

[0101] The sodium-ion secondary battery of this disclosure preferably satisfies the fourth condition. The "fourth condition" indicates that the porous layer further contains inorganic particles. When the sodium-ion secondary battery of this disclosure satisfies the fourth condition, the heat resistance and porousness of the porous layer are superior to when the fourth condition is not satisfied.

[0102] The sodium-ion secondary battery of this disclosure preferably satisfies the fourth and fifth conditions. The fifth condition indicates that the inorganic particles include metal sulfate particles. When the sodium-ion secondary battery of this disclosure satisfies the fourth and fifth conditions, it is less likely to generate gas inside the battery than when the fifth condition is not met.

[0103] The sodium-ion secondary battery of this disclosure preferably satisfies the fourth and sixth conditions. The sixth condition indicates that the average primary particle size of the inorganic particles is 0.3 μm or less. When the sodium-ion secondary battery of this disclosure satisfies the fourth and sixth conditions, it exhibits superior porosity of the porous layer compared to when the sixth condition is not met, and the porous layer becomes a dense film with high uniformity.

[0104] The sodium-ion secondary battery of this disclosure preferably satisfies the seventh condition. The seventh condition indicates that the positive electrode and the negative electrode are in a combination that allows the cutoff voltage to be set to 2.0V. By satisfying the seventh condition, the sodium-ion secondary battery of this disclosure can obtain better cycle characteristics than when the seventh condition is not satisfied.

[0105] The present disclosure will be described in detail below with reference to examples, but the present disclosure is not limited in any way by these examples, and other embodiments may also fall within the scope of the present disclosure as long as they are consistent with the spirit of the present disclosure.

[0106] [1] Measurement Method The measurement methods applied to the examples and comparative examples are as follows.

[0107] [1.1] Thickness of Polyolefin Microporous Film and Separator The thickness (μm) of the polyolefin microporous film and separator was determined by measuring 20 points within a 10 cm square area using a contact-type thickness gauge (Mitutoyo Corporation, LITEMATIC VL-50S) and averaging the results. A spherical measuring probe with a radius of 10 mm (Mitutoyo Corporation) was used as the measuring terminal, and the device was adjusted so that a load of 0.19 N was applied during measurement.

[0108] [1.2] Average primary particle size of inorganic particles The average primary particle size of inorganic particles was determined by performing SEM observation on inorganic particles used to form the porous layer, measuring the major axis of 100 randomly selected inorganic particles, and averaging the major axes of the 100 particles.

[0109] [1.3] TEM-EDS analysis of the separator TEM-EDS analysis of the separator was performed as described below, and the pores of the polyolefin microporous membrane were observed.

[0110] [1.3.1] RuO ４Preparation of stained samples Follow the procedures (i) to (v) below to prepare RuO for TEM observation. ４ Stained samples were prepared. (i) Cut the separator into a triangle (base approximately 0.5 mm x height approximately 1 mm, MD is the base). (ii) Attach the triangular sample to a glass slide and leave it undisturbed in a sealed container with a capacity of approximately 50 ml. 0.5 mass% RuO ４ (iii) Add approximately 0.5 ml of aqueous solution to a sealed container and perform vapor staining at room temperature for 15 minutes. This staining condition does not stain polyolefins. (iii) Impregnate the stained sample with embedding resin. A stepwise substitution treatment is performed to impregnate the embedded resin into the porous interior of the sample. Specifically, the sample is placed in n-butyl glycidyl ether, n-butyl glycidyl ether:embedding resin = 1:1, and n-butyl glycidyl ether:embedding resin = 1:3 sequentially for 30 minutes each, and then placed in the embedding resin overnight. (iv) Place the sample on an embedding plate, inject the embedding resin and allow it to harden completely. (v) Using an ultramicrotome, prepare ultrathin sections of approximately 80 nm at room temperature. Cut the sample so that the MD of the separator is visible in the cross-section. Attach a carbon support film to the ultrathin section and place it on a Cu grid.

[0111] [1.3.2] TEM-EDS TEM observation was performed using a transmission electron microscope (JEM-2100F, JEOL Ltd.) at an acceleration 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.

[0112] [1.3.3] Porosity of Polyolefin Microporous Membranes The porosity ε (%) of polyolefin microporous membranes was calculated using the following formula: ε = {1 - Ws / (ds・t)} × 100 Ws is the basis weight (g / m) of the polyolefin microporous membrane. ２ ), ds is the true density (g / cm³) of the polyolefin microporous membrane. ３ ), t is the thickness of the polyolefin microporous membrane (μm).

[0113] [1.3.4] Gahl values ​​of polyolefin microporous membranes and separators The Gahl values ​​(seconds / 100 mL) of polyolefin microporous membranes and separators were measured in accordance with JIS P8117:2009 using a Gahl type densometer (Toyo Seiki Co., Ltd., G-B2C).

[0114] [2] Examples and Comparative Examples [2.1] Example 1 A sodium-ion secondary battery was prepared as follows.

[0115] [2.1.1] A separator meta-type total aromatic polyamide ("Conex®," manufactured by Teijin Limited) was dissolved in N,N-dimethylacetamide (DMAc) to obtain a solution. The concentration of the meta-type total aromatic polyamide relative to the total volume of the solution was 4% by mass. Barium sulfate particles (average primary particle size: 0.1 μm) were prepared. The barium sulfate particles were dispersed in the solution to prepare a slurry for separator coating. The mass ratio of solids between the meta-type total aromatic polyamide and the barium sulfate particles (meta-type total aromatic polyamide: barium sulfate particles) was 15:85.

[0116] A polyethylene microporous membrane with a thickness of 7 μm was prepared as the polyolefin microporous membrane. The thickness of this polyethylene microporous membrane was 7 μm, the porosity was 37%, and the Gaule value was 127 sec / 100 cc. The prepared slurry was coated onto both sides of the polyethylene microporous membrane to obtain a coated film. The amount of coating on both sides of the polyethylene microporous membrane was the same.

[0117] A coagulation solution was obtained by mixing DMAc and water. The mass ratio of DMAc to water in the coagulation solution (DMAc:water) was 50:50. By immersing the coated film in the coagulation solution at 25°C, porous layers were formed on both sides of the polyethylene microporous membrane. The porous layers consisted of meta-type fully aromatic polyamide and barium sulfate particles. This resulted in a microporous membrane (separator) with a porous layer and a film thickness of 11 μm. The microporous membrane with a porous layer comprises a polyethylene microporous membrane and porous layers provided on both sides of the polyethylene microporous membrane.

[0118] Furthermore, the central part of the polyethylene microporous membrane of the separator is RuO ４TEM observation after staining confirmed the presence of fibrous meta-type total aromatic polyamides within the polyethylene microporous membrane. Similar to Figure 1, the fibrous meta-type total aromatic polyamides formed a three-dimensional network structure within the polyethylene microporous membrane, consisting of numerous interconnected fibers.

[0119] [2.1.2] Sodium manganate (P'2-Na) as positive electrode active material ２／３ MnO ２ A positive electrode was prepared using PVdF, acetylene black (AB) as a conductive additive, polyvinylidene fluoride as a binder, and aluminum foil as a current collector. Specifically, P'2-Na in an N-methylpyrrolidone solution in which PVdF was dissolved. ２／３ MnO ２ A slurry for cathode coating was prepared by dispersing AB. P'2-Na ２／３ MnO ２ The mass ratio of the solid content of AB and PVdF (P'2-Na ２／３ MnO ２ The ratio of AB to PVdF was 90:10:10. The prepared slurry was applied to aluminum foil and then dried to obtain a positive electrode. The positive electrode consisted of aluminum foil (current collector) and a positive electrode mixture layer. The positive electrode mixture layer was P'2-Na ２／３ MnO ２ It contained (positive electrode active material), PVdF (binder), and AB (conductive additive).

[0120] [2.1.3] A sodium foil was prepared as the negative electrode.

[0121] [2.1.4] Sodium bis(fluorosulfonyl)amide (NaFSA) as the sodium salt of the first electrolyte and dimethoxyethane (DME) as a non-aqueous solvent were prepared. The first electrolyte was obtained by dissolving NaFSA in DME. The concentration of NaFSA in the first electrolyte was 5 mol / L.

[0122] [2.1.5] A sodium-ion secondary battery was obtained by stacking the positive electrode, separator, and negative electrode in this order. The separator was placed between the positive electrode mixture layer and the sodium foil (negative electrode). The stack was inserted into the outer casing and the first electrolyte was impregnated into the stack to obtain a sodium-ion secondary battery.

[0123] A charge-discharge cycle characteristic test was conducted using a sodium-ion secondary battery with a charge-discharge current of 40 mA / g (positive electrode active material) and a charge-discharge voltage range of 1.5 V to 4.0 V. In the charge-discharge cycle characteristic test, the sodium-ion secondary battery was repeatedly charged and then discharged (hereinafter simply referred to as "cycle"). The results are shown in Figure 2.

[0124] As shown in Figure 2, in Example 1, the capacity (mAh / g) after 100 cycles was 90% or more of the capacity (mAh / g) after 1 cycles. As a result, it was found that the sodium-ion secondary battery of Example 1 is a "sodium-ion secondary battery with excellent charge-discharge cycle characteristics."

[0125] [2.2] Example 2 A sodium-ion secondary battery was obtained in the same manner as in Example 1, except that the first electrolyte was changed to the second electrolyte described below.

[0126] The second electrolyte is sodium hexafluorophosphate (NaPF). ６ The electrolyte is obtained by dissolving ) in propylene carbonate (PC). NaPF in the second electrolyte ６ The concentration was 1 mol / L.

[0127] A charge-discharge cycle characteristic test was conducted using a sodium-ion secondary battery with a charge-discharge current of 40 mA / g (positive electrode active material) and a charge-discharge voltage range of 2.4 V to 4.0 V. The results are shown in Figure 3.

[0128] As shown in Figure 3, in Example 2, the capacity (mAh / g) after 30 cycles was 90% or more of the capacity (mAh / g) after 1 cycle. As a result, it was found that the sodium-ion secondary battery of Example 2 is a "sodium-ion secondary battery with excellent charge-discharge cycle characteristics."

[0129] [2.3] Comparative Example 1 A sodium-ion secondary battery was obtained in the same manner as in Example 1, except that the porous membrane with a porous layer used as a separator was changed to a polyolefin microporous membrane. In Comparative Example 1, the electrolyte did not permeate the polyolefin microporous membrane (separator), so charging and discharging was not possible. As a result, it was found that the sodium-ion secondary battery of Comparative Example 1 was not a "sodium-ion secondary battery with excellent charge-discharge cycle characteristics".

[0130] [2.4] Comparative Example 2 A sodium-ion secondary battery was obtained in the same manner as in Example 1, except that the porous membrane with a porous layer, which serves as the separator, was replaced with a glass filter. A charge-discharge cycle characteristic test was performed in the same manner as in Example 1. In Comparative Example 2, a short circuit occurred during charge-discharge, making it impossible to perform sufficient measurements. As a result, it was found that the sodium-ion secondary battery of Comparative Example 2 was not a "sodium-ion secondary battery with excellent charge-discharge cycle characteristics".

[0131] [3] Reference Example 1 A polyolefin microporous membrane (separator) similar to the one used in Comparative Example 1 was prepared, and a circular piece was cut from a portion of the polyolefin microporous membrane to obtain a white first sample piece.

[0132] A microporous membrane with a porous layer (separator) similar to the one used in Example 1 was prepared, and a circular section of the microporous membrane with a porous layer was cut out to obtain a second white sample piece.

[0133] An electrolyte solution similar to the first electrolyte solution (NaFSA concentration: 5 mol / L) used in Example 1 (hereinafter also referred to as the "high-salt concentration electrolyte solution") was prepared.

[0134] A high-salt concentration electrolyte was dropped onto the first and second sample pieces. Figure 4 shows top views of the first and second sample pieces immediately after the drop of the high-salt concentration electrolyte. In Figure 4, the left (Polyolefin) is the first test piece, and the right (Aramid coated polyolefin) is the second test piece. As shown in Figure 4, in the first sample piece, the entire surface in contact with the water droplet of the high-salt concentration electrolyte remained white. Therefore, it was found that the high-salt concentration electrolyte did not penetrate into the interior of the first test piece. As shown in Figure 4, in the second sample piece, most of the surface in contact with the water droplet of the high-salt concentration electrolyte turned black. Therefore, it was found that the high-salt concentration electrolyte penetrated into the interior of the second test piece. As a result of these findings, it was found that when a high-salt concentration electrolyte is used as the electrolyte, a polyolefin microporous membrane alone cannot be used as the separator for the sodium-ion secondary battery of this disclosure.

[0135] [4] Reference Example 2 A polyolefin microporous membrane (manufactured by Cellgard LLC) was prepared. A circular section of the polyolefin microporous membrane was cut out to obtain a white third sample piece.

[0136] A microporous membrane with a porous layer (separator) similar to the one used in Example 1 was prepared, and a circular section of the microporous membrane with a porous layer was cut out to obtain a fourth white sample piece.

[0137] The second electrolyte (NaPF) used in Example 2 ６ An electrolyte solution similar to the one used (at a concentration of 1 mol / L) (hereinafter also referred to as "normal concentration electrolyte solution") was prepared.

[0138] 10 μL of normal concentration electrolyte was dropped onto the third and fourth sample pieces. Figure 5 shows the top views of the third and fourth sample pieces immediately after dropping the normal concentration electrolyte. In Figure 5, the left is the third test piece and the right is the fourth test piece. As shown in Figure 5, droplets of the normal concentration electrolyte were present on the third sample piece. Therefore, it was found that the normal concentration electrolyte had not permeated into the interior of the third test piece. As shown in Figure 5, no droplets of the normal concentration electrolyte were present on the fourth sample piece. Therefore, it was found that the normal concentration electrolyte had permeated into the interior of the fourth test piece. These results show that when using a normal salt concentration electrolyte, a polyolefin microporous membrane alone cannot be used as the separator for the sodium-ion secondary battery of this disclosure.

[0139] The disclosure of Japanese Patent Application No. 2024-164407, filed on September 20, 2024, is incorporated herein by reference in its entirety. All documents, patent applications, and technical standards described herein are incorporated herein by reference to the same extent as if each individual document, patent application, and technical standard were specifically and individually noted to be incorporated by reference.

Claims

1. A sodium-ion secondary battery comprising a positive electrode, a negative electrode, an electrolyte containing a non-aqueous solvent and a sodium salt, and a separator, wherein the separator comprises: a polyolefin microporous membrane; fibrous meta-type total aromatic polyamide provided in the pores of the polyolefin microporous membrane; and a porous layer containing meta-type total aromatic polyamide provided on one or both sides of the polyolefin microporous membrane.

2. The sodium-ion secondary battery according to claim 1, wherein the non-aqueous solvent comprises propylene carbonate.

3. The sodium ion secondary battery according to claim 1, wherein the electrolyte has a sodium salt concentration of 2.0 mol / L or more.

4. The sodium-ion secondary battery according to claim 1, wherein the porous layer is provided on both sides of the polyolefin microporous membrane.

5. The sodium-ion secondary battery according to claim 1, wherein the porous layer further contains inorganic particles.

6. The sodium-ion secondary battery according to claim 5, wherein the inorganic particles include metal sulfate particles.

7. The sodium-ion secondary battery according to claim 6, wherein the average primary particle size of the inorganic particles is 0.3 μm or less.

8. The sodium-ion secondary battery according to claim 1, wherein the positive electrode and the negative electrode are in a combination that allows the cutoff voltage to be set to 2.0V.

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