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 that contains a resin having an amide bond. The sodium salt concentration of the electrolyte solution is 2.0 mol / L or higher.

Description

Sodium-ion rechargeable battery

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

[0002] Sodium-ion secondary batteries have long attracted attention because sodium is more abundant and cheaper than lithium. In sodium-ion secondary batteries, manganese is preferably used as the positive electrode active material from a resource perspective.

[0003] Patent documents 1 and 2 specifically disclose a sodium-ion secondary battery. This sodium-ion secondary battery includes a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte. The positive electrode contains manganese(IV) oxide. The separator is formed by laminating a heat-resistant porous layer containing para-oriented aromatic polyamide and a porous polyethylene film. The sodium salt concentration of the non-aqueous electrolyte is 1.0 mol / L.

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

[0005] To improve the energy density of a 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 battery discharge is stopped.

[0006] However, in conventional sodium-ion secondary batteries, when the cutoff voltage is set to a low voltage (for example, 1.5V), the positive electrode active material is trivalent manganese (Mn ３＋ ) is produced. Trivalent manganese (Mn ３＋ The elution of these substances into the non-aqueous electrolyte leads to a decrease in charge-discharge capacity and a deterioration in charge-discharge cycle characteristics.

[0007] Therefore, to prevent manganese from leaching out of the positive electrode active material, it is necessary to set a high cutoff voltage (for example, 2.4V). As a result, it is difficult to increase the sodium utilization rate in the positive electrode active material from the standpoint of charge-discharge cycle characteristics, and the energy density could not be increased.

[0008] One problem to be solved by an embodiment of the present disclosure is to provide a sodium-ion secondary battery having excellent charge-discharge cycle characteristics even when the cut-off voltage is a low voltage (for example, 1.5 V).

[0009] Specific means for achieving the problem are as follows. <1> A sodium-ion secondary battery comprising a positive electrode, a negative electrode, an electrolytic solution containing a non-aqueous solvent and a sodium salt, and a separator containing a resin having an amide bond, wherein the sodium salt concentration of the electrolytic solution is 2.0 mol / L or more. <2> The sodium-ion secondary battery according to <1>, wherein the sodium salt contains sodium bis(fluorosulfonyl)amide. <3> The sodium-ion secondary battery according to <1> or <2>, wherein the separator has a polyolefin microporous membrane and a porous layer containing the resin provided on one or both surfaces of the polyolefin microporous membrane, and the resin is an aromatic polyamide. <4> The sodium-ion secondary battery according to <3>, wherein the aromatic polyamide is a meta-type wholly aromatic polyamide. <5> The sodium-ion secondary battery according to <3> or <4>, wherein the porous layer is provided on both surfaces of the polyolefin microporous membrane. <6> The sodium-ion secondary battery according to any one of <3> to <5>, wherein the porous layer further contains inorganic particles. <7> The sodium-ion secondary battery according to <6>, wherein the inorganic particles contain metal sulfate particles. <8> The sodium-ion secondary battery according to <6> or <7>, wherein the average primary particle diameter of the inorganic particles is 0.3 μm or less. <9> The sodium-ion secondary battery according to <1>, wherein the positive electrode contains a sodium manganese composite oxide as a positive electrode active material.

[0010] According to the present disclosure, there is provided a sodium-ion secondary battery having excellent charge-discharge cycle characteristics even when the cut-off voltage is a low voltage (for example, 1.5 V).

[0011] FIG. 1 is an analysis result of TEM-EDS of a separator, and is an image showing that fibrous meta-type aromatic polyamide is contained in pores of a polyolefin microporous membrane. FIG. 2 is a graph of the charge-discharge curve of Example 1. FIG. 3 is a graph of the charge-discharge curve of Comparative Example 1. FIG. 4 is a photograph of the appearance of a first sample piece (polyolefin microporous membrane of Comparative Example 2) and a second test piece (microporous membrane with a porous layer of Example 1) immediately after a high-salt-concentration electrolyte (concentration of NaFSA: 5 mol / L) was dropped.

[0012] 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.

[0013] In the present disclosure, a numerical range represented by using "~" means a range including the numerical values described before and after "~" as the lower limit value and the upper limit value. In the present disclosure, in the numerical ranges described step by step in "Modes 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 ranges described in "Modes 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 "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 thereof will not be repeated.

[0014] 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".

[0015] (1) Sodium-ion secondary battery The sodium-ion secondary battery of this disclosure (hereinafter also simply referred to as "sodium-ion secondary battery") comprises a positive electrode, a negative electrode, an electrolyte containing a non-aqueous solvent and a sodium salt, and a separator containing a resin having an amide bond. The sodium salt concentration of the electrolyte is 2.0 mol / L or higher.

[0016] The sodium-ion secondary battery of this disclosure has the above configuration and therefore exhibits excellent charge-discharge cycle characteristics even when the cutoff voltage is low (e.g., 1.5V). This effect is presumed to be due to, but is not limited to, the following reasons. In this disclosure, the sodium salt concentration of the electrolyte is 2.0 mol / L or higher. A sodium salt concentration of 2.0 mol / L or higher indicates that it is a high-salt concentration electrolyte. Hereinafter, an electrolyte with a sodium salt concentration of 2.0 mol / L or higher will also be referred to as a "high-salt concentration electrolyte". In this disclosure, the separator contains a resin having an amide bond. Therefore, the high-salt concentration electrolyte is easily impregnated into the separator, and the high-salt concentration electrolyte can be easily used. For example, if the positive electrode active material contains manganese and the cutoff voltage is low (e.g., 1.5V), trivalent manganese (Mn ３＋ ) is produced. Typically, trivalent manganese (Mn ３＋ ) is tetravalent manganese (Mn ＋４ It 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 electrolytes. Therefore, even if the utilization rate of sodium in the positive electrode active material is increased, degradation is less likely. As a result, it is presumed that the sodium-ion secondary battery of this disclosure will have excellent charge-discharge cycle characteristics even when the cutoff voltage is low (e.g., 1.5V).

[0017] 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.

[0018] (1.1) Electrolyte A sodium-ion secondary battery is equipped with an electrolyte.

[0019] The electrolyte contains a non-aqueous solvent and a sodium salt. The sodium salt concentration of the electrolyte is 2.0 mol / L or higher. If 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 higher.

[0020] From the viewpoint of realizing the characteristics of 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 improving the charge-discharge cycle characteristics, the sodium salt concentration of the electrolyte is preferably at 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.

[0021] The sodium salt concentration of the electrolyte may be between 2.0 mol / L and 12.0 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 a sodium-ion secondary battery. As the non-aqueous solvent, cyclic carbonates such as ethylene carbonate, propylene carbonate (PC), fluoroethylene carbonate, difluoroethylene carbonate, and vinylene carbonate; chain carbonates such as dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, and fluorine-substituted products thereof; cyclic esters such as γ-butyrolactone and γ-valerolactone; chain 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; fluorinated alkanes; dimethyl sulfoxide; and sulfolane can be mentioned. The non-aqueous solvent may be used alone or in combination of two or more kinds.

[0023] (1.1.2) The sodium salt electrolyte contains a sodium salt. The sodium salt may be a known sodium salt used in a sodium-ion secondary battery. As the sodium salt, sodium bis(fluorosulfonyl)amide (NaFSA), sodium hexafluorophosphate (NaPF ６ ), sodium perchlorate (NaClO ４ ), sodium tetrafluoroborate (NaBF ４ ), sodium bis(trifluoromethanesulfonyl)imide (NaTFSI), and sodium difluorooxalate borate (NaDFOB) can be mentioned. The sodium salt may be used alone or in combination of two or more kinds. Among them, the sodium salt preferably contains sodium bis(fluorosulfonyl)amide, and more preferably is sodium bis(fluorosulfonyl)amide.

[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 contains a resin having amide bonds. Examples of resins having amide bonds include aromatic polyamides (e.g., fully aromatic polyamides and semi-aromatic polyamides, etc.) and aliphatic polyamides (e.g., nylon 6, nylon 6,6 and nylon 6,10, etc.). Examples of semi-aromatic polyamides include polymetaxylylene adipamide (MXD6), polyhexamethylene terephthalamide (6T), and copolymer polyamides containing these units.

[0027] "Total aromatic polyamide" refers to a polyamide whose main chain consists only of benzene rings and amide bonds. Total aromatic polyamide may also have small amounts of aliphatic monomers copolymerized into it.

[0028] Examples of fully aromatic polyamides include meta-type fully aromatic polyamides, para-type fully aromatic polyamides, and polybenzamides. Examples of meta-type fully aromatic polyamides include polymetaphenylene isophthalamide. Examples of para-type fully aromatic polyamides include coplyparaphenylene-3,4'-oxydiphenylene terephthalamide and polyparaphenylene terephthalamide. Fully 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.

[0029] 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.

[0030] The separator configuration is not particularly limited as long as it includes a resin having amide bonds. The separator configuration may be, for example, any of the following first to third configurations. The first configuration separator comprises a polyolefin microporous membrane and a porous layer containing a resin having amide bonds. The porous layer is provided on one or both sides of the polyolefin microporous membrane. The second configuration separator comprises a nonwoven fabric and a porous layer containing a resin having amide bonds. The porous layer may be provided on one or both sides of the nonwoven fabric, or the nonwoven fabric may be embedded inside the porous layer to form a composite. The third configuration separator consists only of a single layer membrane containing a resin having amide bonds. From the viewpoint of improving charge-discharge cycle characteristics, the first configuration is preferred.

[0031] "Polyolefin microporous membrane" refers to a microporous membrane whose main component is polyolefin. "Microporous membrane" refers to a membrane that has a large number of micropores inside, in which these pores are 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 that has a large number of micropores, allowing gas or liquid to pass from one side to the other.

[0032] The first configuration of the above will be explained below.

[0033] The separator comprises a polyolefin microporous membrane and a porous layer containing a resin having amide bonds. The porous layer is provided on one or both sides of the polyolefin microporous membrane.

[0034] (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.

[0035] The "shutdown function" refers to a function 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] It is preferable that part or all of the walls of the pores in the polyolefin microporous membrane are coated with meta-aromatic polyamide. This allows high-salt concentration electrolytes to easily penetrate the polyolefin microporous membrane.

[0041] 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.

[0042] (1.2.2) Porous layer The porous layer facilitates the impregnation of the polyolefin microporous membrane with a high salt concentration electrolyte.

[0043] 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 penetrates the polyolefin microporous membrane alone more easily than when the porous layer is provided on only one side. As a result, the capacity (mAh / g) of the sodium-ion secondary battery is improved. From this viewpoint, it is more preferable that the porous layer be provided on the entire surface of both sides of the polyolefin microporous membrane.

[0044] 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.

[0045] 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).

[0046] 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.

[0047] The porosity of the porous layer only needs to be sufficient for it to function as a battery separator.

[0048] (1.2.2.1) The porous resin layer having amide bonds contains a resin having amide bonds. The porous layer may consist of a resin having amide bonds. Examples of resins having amide bonds are the same as those exemplified above as resins having amide bonds. In particular, from the viewpoint of facilitating the impregnation of high-salt concentration electrolytes by the polyolefin microporous membrane, the resin having amide bonds is preferably an aromatic polyamide.

[0049] Aromatic polyamides are more preferably meta-type fully aromatic polyamides. This results in superior moldability. For example, a porous layer is 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, and then washing and drying it, as described later. Para-type fully aromatic polyamides have strong intermolecular forces, making it difficult to produce a coating solution containing them unless ionic substances (e.g., calcium chloride) are added. On the other hand, meta-type fully aromatic polyamides have a weak molecular weight, making it easier to produce a coating solution containing them with low viscosity without adding ionic substances. Low viscosity coating solutions easily penetrate the pores of polyolefin microporous membranes. Therefore, the moldability of a porous layer using meta-type fully aromatic polyamides is superior to that of a porous layer using para-type fully aromatic polyamides. Furthermore, since residual ionic substances in the porous layer may affect battery characteristics, meta-type fully aromatic polyamides are preferred from this viewpoint as well.

[0050] The content of the resin having amide bonds 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.

[0051] When porous layers are provided on both sides of a polyolefin microporous membrane, at least one of the types and amounts of resin having amide bonds contained in one porous layer and at least one of the types and amounts of resin having amide bonds contained in the other porous layer may be the same or different.

[0052] (1.2.2.2) Other porous resin layers may contain resins other than those having amide bonds. Examples of other resins include 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.

[0053] 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.

[0054] (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.

[0055] 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.

[0056] 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.

[0057] 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.

[0058] (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.

[0059] 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".

[0060] (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.

[0061] (1.2.3) The fibrous meta-aromatic polyamide separator preferably contains fibrous meta-aromatic polyamide in the pores of the polyolefin microporous membrane. Since the meta-aromatic polyamide is in the form of fine fibers, the meta-aromatic polyamide 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.

[0062] Meta-aromatic polyamides exhibit high affinity for high-salt concentration electrolytes. High-salt concentration electrolytes easily penetrate polyolefin microporous membranes containing fibrous meta-aromatic polyamides in their pores. From the viewpoint of the penetration of high-salt concentration electrolytes into polyolefin microporous membranes, it is preferable that fibrous meta-aromatic polyamides are contained in the pores at least in the region close to the surface of the polyolefin microporous membrane, and it is more preferable that fibrous meta-aromatic polyamides are contained in the pores of the entire polyolefin microporous membrane.

[0063] The fibrous meta-aromatic polyamide may form a three-dimensional network structure within the polyolefin microporous membrane. This allows high-salt concentration electrolytes to penetrate more easily through the separator than if the fibrous meta-aromatic polyamide did not form a three-dimensional network structure.

[0064] When the separator contains fibrous meta-aromatic polyamide, from the viewpoint of separator manufacturing, it is preferable that the resin having amide bonds contained in the porous layer is a meta-aromatic polyamide.

[0065] The presence of fibrous meta-aromatic polyamides within the pores of the polyolefin microporous membrane in the separator can be confirmed by elemental imaging combining TEM (transmission electron microscopy) with EDS (energy dispersive X-ray spectroscopy). The meta-aromatic polyamides are ruthenium tetroxide (RuO ４ By utilizing its readily stainable properties, meta-type aromatic polyamides are detected by Ru imaging. RuO2 is used for TEM observation. ４ The method for preparing stained samples is as follows:

[0066] (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.

[0067] 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-aromatic polyamides. It can be seen that the fibrous meta-aromatic polyamides constitute a three-dimensional network structure inside the polyolefin microporous membrane.

[0068] (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.

[0069] The following describes a wet coating method for a separator having a polyolefin microporous membrane, a porous layer containing meta-aromatic polyamide, and fibrous meta-aromatic polyamide.

[0070] 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.

[0071] The coating solution for forming the porous layer is prepared by dissolving meta-aromatic polyamide in a solvent. Other components besides meta-aromatic polyamide may be dissolved or dispersed in the coating solution as needed.

[0072] The solvent used in preparing the coating solution includes a solvent that dissolves meta-type aromatic polyamides (hereinafter also referred to as a "good solvent"). Examples of good solvents include polar amide solvents (for example, N-methylpyrrolidone, dimethylacetamide, and dimethylformamide).

[0073] 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.

[0074] 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.

[0075] 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.

[0076] 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.

[0077] 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.

[0078] The fibrous meta-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-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 be in a dry state 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.

[0079] 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-aromatic polyamide contained within the pores of the polyolefin microporous membrane, and a porous layer.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] (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.

[0084] 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.

[0085] 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.

[0086] 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.

[0087] 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 Ni０．３１７ 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 oxides (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 V ２ (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 the viewpoint of the effects and resources of this disclosure, it is more preferable that the sodium-containing transition metal compound includes a sodium-manganese composite oxide.

[0088] 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.

[0089] 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.

[0090] (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.

[0091] The negative electrode may be one that operates by sodium ion intercalation, alloying reaction, or sodium dissolution reaction.

[0092] (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.

[0093] 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.

[0094] 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.

[0095] 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.

[0096] 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.

[0097] (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.

[0098] 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).

[0099] (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.

[0100] (1.5) 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.).

[0101] (1.6) Preferred Embodiments The sodium-ion secondary battery of the present disclosure is preferably satisfied with a first condition. The "first condition" indicates that the sodium salt contains sodium bis(fluorosulfonyl)amide. This sodium salt is readily soluble in non-aqueous solvents and is therefore suitable for increasing the concentration of the sodium salt. When the sodium-ion secondary battery of the present disclosure satisfies the first condition, the charge-discharge cycle characteristics are more easily improved than when the first condition is not satisfied.

[0102] The sodium-ion secondary battery of this disclosure preferably satisfies a second condition. The "second condition" indicates that the separator comprises a polyolefin microporous membrane and a porous layer containing the resin provided on one or both sides of the polyolefin microporous membrane, and the resin is an aromatic polyamide. Since the separator of this configuration readily impregnates an electrolyte in which sodium salt is dissolved at a high concentration, the charge-discharge cycle characteristics of the sodium-ion secondary battery of this disclosure are improved when the second condition is satisfied compared to when the second condition is not satisfied.

[0103] The sodium-ion secondary battery of this disclosure preferably satisfies the second and third conditions. The "third condition" indicates that the aromatic polyamide is a meta-type total aromatic polyamide. The sodium-ion secondary battery of this disclosure that satisfies the second and third conditions has better moldability than the case where the third condition is not met.

[0104] The sodium-ion secondary battery of this disclosure preferably satisfies the second and fourth conditions. The fourth condition indicates that the porous layer is provided on both sides of the polyolefin microporous membrane. By satisfying the second and fourth conditions, the sodium-ion secondary battery of this disclosure can retain the electrolyte well at the positive and negative electrode interfaces, thereby improving the charge-discharge cycle characteristics compared to when the fourth condition is not met.

[0105] The sodium-ion secondary battery of this disclosure preferably satisfies the second, fourth, and fifth conditions. The fifth condition indicates that the porous layer further contains inorganic particles. By satisfying the second, fourth, and fifth conditions, the sodium-ion secondary battery of this disclosure exhibits superior heat resistance and porousness of the porous layer compared to when the fifth condition is not satisfied.

[0106] The sodium-ion secondary battery of this disclosure preferably satisfies the second, fourth, fifth, and sixth conditions. The sixth condition indicates that the inorganic particles include metal sulfate particles. By satisfying the second, fourth, fifth, and sixth conditions, the sodium-ion secondary battery of this disclosure is less likely to generate gas inside the battery than if the sixth condition is not satisfied.

[0107] The sodium-ion secondary battery of this disclosure preferably satisfies the second, fourth, fifth, sixth, and seventh conditions. The seventh 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 second, fourth, fifth, sixth, and seventh conditions, it exhibits superior porousness of the porous layer compared to when the seventh condition is not met, and the porous layer becomes a dense film with high uniformity.

[0108] The sodium-ion secondary battery of this disclosure preferably satisfies the eighth condition. The eighth condition indicates that the positive electrode contains a sodium-manganese composite oxide as the positive electrode active material. When the sodium-ion secondary battery of this disclosure satisfies the eighth condition, it is easier to obtain an improvement in charge-discharge cycle characteristics compared to when the eighth condition is not met.

[0109] 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.

[0110] [1] Measurement Method The measurement methods applied to the examples and comparative examples are as follows.

[0111] [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.

[0112] [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.

[0113] [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.

[0114] [1.3.1] RuO ４ Preparation 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.

[0115] [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.

[0116] [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).

[0117] [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).

[0118] [2] Examples and Comparative Examples [2.1] Example 1 A sodium-ion secondary battery was prepared as follows.

[0119] [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.

[0120] 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 slurry prepared above 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.

[0121] 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.

[0122] Furthermore, the central part of the polyethylene microporous membrane of the separator is RuO ４ TEM observation after staining confirmed the presence of fibrous meta-type total aromatic polyamide within the polyethylene microporous membrane. Similar to Figure 1, the fibrous meta-type aromatic polyamide was found to constitute a three-dimensional network structure within the polyethylene microporous membrane, with numerous interconnected fibers.

[0123] [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 slurry prepared above was applied to aluminum foil and then dried to obtain the 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).

[0124] [2.1.3] A sodium foil was prepared as the negative electrode.

[0125] [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.

[0126] [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.

[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 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.

[0128] 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 even when the cutoff voltage is 1.5V".

[0129] [2.2] Comparative Example 1 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. A charge-discharge cycle characteristic test was performed in the same manner as in Example 1. The results are shown in Figure 3.

[0130] 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.

[0131] As shown in Figure 3, in Comparative Example 1, the capacity (mAh / g) decreased as the number of cycles increased. In Comparative Example 1, the capacity (mAh / g) after 100 cycles was less than 40% of the capacity (mAh / g) after 1 cycle. As a result, it was found that the sodium-ion secondary battery of Comparative Example 1 is not a "sodium-ion secondary battery that has excellent charge-discharge cycle characteristics even when the cutoff voltage is low (e.g., 1.5V)".

[0132] [2.3] 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 used as a separator was changed to a polyolefin microporous membrane. In Comparative Example 2, 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 2 was not a "sodium-ion secondary battery with excellent charge-discharge cycle characteristics even when the cutoff voltage is low (for example, 1.5V)".

[0133] [2.4] Comparative Example 3 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 changed to a glass filter. A charge-discharge cycle characteristic test was performed in the same manner as in Example 1. In Comparative Example 3, 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 3 is not a "sodium-ion secondary battery that has excellent charge-discharge cycle characteristics even when the cutoff voltage is low (for example, 1.5V)".

[0134] [3] Reference Example A polyolefin microporous membrane (separator) similar to the one used in Comparative Example 2 was prepared, and a circular piece was cut from the polyolefin microporous membrane to obtain a white first sample piece.

[0135] 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.

[0136] 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.

[0137] A high-salt concentration electrolyte was dropped onto the first and second sample pieces. Figure 4 shows the 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.

[0138] The disclosure of Japanese Patent Application No. 2024-164406, 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 containing a resin having an amide bond, wherein the sodium salt concentration of the electrolyte is 2.0 mol / L or higher.

2. The sodium-ion secondary battery according to claim 1, wherein the sodium salt comprises sodium bis(fluorosulfonyl)amide.

3. The sodium-ion secondary battery according to claim 1, wherein the separator comprises a polyolefin microporous membrane and a porous layer containing the resin provided on one or both sides of the polyolefin microporous membrane, and the resin is an aromatic polyamide.

4. The sodium-ion secondary battery according to claim 3, wherein the aromatic polyamide is a meta-type total aromatic polyamide.

5. The sodium-ion secondary battery according to claim 3, wherein the porous layer is provided on both sides of the polyolefin microporous membrane.

6. The sodium-ion secondary battery according to claim 3, wherein the porous layer further contains inorganic particles.

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

8. 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.

9. The sodium-ion secondary battery according to claim 1, wherein the positive electrode contains a sodium-manganese composite oxide as the positive electrode active material.

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