Air electrode / separator conjugate and metal–air rechargeable battery

Abstract

Provided is an air electrode / separator conjugate that, when made into a metal–air rechargeable battery, can realize decline in charge / discharge overvoltage. This air electrode / separator conjugate is provided with: a hydroxide-ion conducting separator; a catalyst layer that covers one surface side of the hydroxide-ion conducting separator and that contains an air-electrode catalyst, a hydroxide-ion conducting composite material, a conductive material, and a binder; and a gas-diffusion electrode provided on the side of the catalyst layer opposite from the hydroxide-ion conducting separator. The hydroxide-ion conducting composite material includes hydrophilic fiber, and a plurality of hydroxide-ion conductive particles ranging along and carried mutually on surfaces of the hydrophilic fiber.

Description

Air electrode / separator assembly and metal-air secondary battery

[0001] The present invention relates to an air electrode / separator assembly and a metal-air secondary battery.

[0002] Metal-air secondary batteries are one of the candidates for innovative batteries. In metal-air secondary batteries, oxygen, which is the positive electrode active material, is supplied from the air, so the space inside the battery container can be used to the fullest extent for filling the negative electrode active material, which in principle makes it possible to achieve a high energy density. For example, in zinc-air secondary batteries that use zinc as the negative electrode active material, an alkaline aqueous solution such as potassium hydroxide is used as the electrolyte, and a separator (partition) is used to prevent short circuits between the positive and negative electrodes. During discharge, O is produced on the air electrode (positive electrode) side, as shown in the following reaction formula: ２ is reduced to OH － is produced, while zinc is oxidized at the negative electrode to produce ZnO. ２ +2H ２ O+4e － →4OH － Negative electrode: 2Zn+4OH － → 2ZnO + 2H ２ O+4e －

[0003] Thus, metal-air secondary batteries use a metal for the negative electrode and oxygen and water in the air for the air electrode (positive electrode) as active materials. At the air electrode, an oxygen reduction reaction (ORR) occurs during discharge, producing hydroxide ions, and an oxygen evolution reaction (OER) occurs during charge, consuming hydroxide ions and generating oxygen. Materials with hydroxide ion conductivity are used to promote the ORR / OER reactions.

[0004] Layered double hydroxides (LDHs) are known as a type of hydroxide ion conductive material. LDHs are represented by the general formula [M ２＋ １－ｘ M ３＋ ｘ (OH) ２ ][An －ｘ／ｎ ・zH ２ O] (M ２＋ is a divalent metal ion, M ３＋LDHs are represented by the formula (where A is a trivalent metal ion and An is an anion), and have a characteristic layered structure in which negatively charged anions (negative ions) and water molecules are sandwiched between positively charged hydroxide layers. LDHs are used as adsorbents and catalysts, and are also used as anion-conducting materials by substituting or transferring anions between layers. LDHs are used as hydroxide ion-conducting materials, particularly when the anions between layers are hydroxide ions. In recent years, air electrodes using LDHs as hydroxide ion-conducting materials have been proposed.

[0005] It is known that in zinc secondary batteries, such as zinc-air secondary batteries and nickel-zinc secondary batteries, metallic zinc precipitates in the form of dendrites from the negative electrode during charging, penetrates the pores of separators such as nonwoven fabrics, and reaches the positive electrode, resulting in a short circuit. Such short circuits caused by zinc dendrites shorten the battery's life after repeated charge and discharge. Furthermore, zinc-air secondary batteries also suffer from the problem that carbon dioxide in the air passes through the air electrode and dissolves in the electrolyte, precipitating alkaline carbonates and reducing battery performance. Similar problems can also occur in lithium-air secondary batteries.

[0006] To address the above problems, batteries have been proposed that include a layered double hydroxide (LDH) separator that selectively allows hydroxide ions to pass through while preventing penetration by zinc dendrites. For example, Patent Document 1 (WO 2013 / 073292) discloses that an LDH separator is provided between the air electrode and the negative electrode in a zinc-air secondary battery to prevent both short circuits between the positive and negative electrodes caused by zinc dendrites and the inclusion of carbon dioxide. Furthermore, Patent Document 2 (WO 2016 / 076047) discloses a separator structure that includes an LDH separator fitted or bonded to a resin outer frame, and that the LDH separator has such high density that it is gas-impermeable and / or water-impermeable. This document also discloses that the LDH separator can be composited with a porous substrate. Furthermore, Patent Document 3 (WO 2016 / 067884) discloses various methods for forming a dense LDH membrane on the surface of a porous substrate to obtain a composite material (LDH separator). This method includes a step of uniformly attaching an initiator substance capable of providing a starting point for LDH crystal growth to the porous substrate, and subjecting the porous substrate to hydrothermal treatment in a raw material aqueous solution to form a dense LDH membrane on the surface of the porous substrate. Patent Document 4 (WO 2019 / 124270) discloses an LDH separator that includes a porous substrate made of a polymer material and a layered double hydroxide (LDH) that blocks the pores of the porous substrate, and has an in-line transmittance of 1% or more at a wavelength of 1000 nm.

[0007] In the field of metal-air secondary batteries such as zinc-air secondary batteries, an air electrode / separator assembly has been proposed in which an air electrode layer is provided on an LDH separator. Patent Document 5 (WO 2015 / 146671) discloses an air electrode / separator assembly having an air electrode layer on an LDH separator, the air electrode layer including an air electrode catalyst, an electron conductive material, and a hydroxide ion conductive material. Patent Document 6 (WO 2020 / 246177) also discloses an air electrode / separator assembly including a hydroxide ion conductive separator, an interfacial layer covering one side of the separator and including a hydroxide ion conductive material and an electrically conductive material, and an air electrode layer provided on the interfacial layer and including an outermost catalyst layer composed of a porous current collector and a layered double hydroxide (LDH) covering the surface of the porous current collector. Patent Documents 5 and 6 also disclose the use of LDH as the hydroxide ion conductive material.

[0008] Furthermore, although they cannot be called LDHs, LDH-like compounds are known as hydroxides and / or oxides with a layered crystal structure similar to LDHs, and they exhibit hydroxide ion conductive properties similar enough to be collectively referred to as hydroxide ion conductive layered compounds together with LDHs. For example, Patent Document 7 (WO 2020 / 255856) discloses a hydroxide ion conductive separator comprising a porous substrate and a layered double hydroxide (LDH)-like compound that plugs the pores of the porous substrate, wherein the LDH-like compound is a hydroxide and / or oxide with a layered crystal structure containing Mg and one or more elements, including at least Ti, selected from the group consisting of Ti, Y, and Al. Patent Document 8 (WO2021 / 229916) discloses an LDH separator using an LDH-like compound containing (i) Ti, Y, and optionally Al and / or Mg, and (ii) an additional element M which is at least one selected from the group consisting of In, Bi, Ca, Sr, and Ba. Furthermore, Patent Document 9 (WO2021 / 229917) discloses an LDH separator using an LDH-like compound and In(OH) ３With regard to an LDH separator containing a mixture of the above, one has been disclosed in which the LDH-like compound is a hydroxide and / or oxide having a layered crystal structure containing Mg, Ti, Y, and optionally Al and / or In. The separators disclosed in Patent Documents 7 to 9 are said to have superior alkali resistance compared to conventional LDH separators and to be able to more effectively suppress short circuits caused by zinc dendrites.

[0009] WO2013 / 073292WO2016 / 076047WO2016 / 067884WO2019 / 124270WO2015 / 146671WO2020 / 246177WO2020 / 255856WO2021 / 229916WO2021 / 229917

[0010] Sun et al., "Single-layer nanosheets with exceptionally high and anisotropic hydroxyl ion conductivity" Sci. Adv. 3, e1602629, 2017

[0011] As mentioned above, metal-air secondary batteries using LDH separators have the excellent advantage of preventing both short circuits between the positive and negative electrodes caused by metal dendrites and the inclusion of carbon dioxide. Furthermore, the denseness of the LDH separator also has the advantage of suppressing evaporation of water contained in the electrolyte. However, since the LDH separator prevents the electrolyte from penetrating into the air electrode, no electrolyte is present in the air electrode layer. Therefore, compared to zinc-air secondary batteries using a general separator (e.g., a porous polymer separator) that allows the electrolyte to penetrate into the air electrode, hydroxide ion conductivity tends to be lower, leading to reduced charge / discharge performance. Therefore, an air electrode / separator assembly that exhibits excellent charge / discharge performance while retaining the advantages of using an LDH separator is desired.

[0012] Therefore, in recent years, the use of LDH as a hydroxide ion conductive material for the air electrode has been proposed (see Patent Documents 5 and 6), but there is still much room for improvement in LDH. LDH generally takes the form of plate-like particles with an arbitrary planar shape. When using LDH plate-like particles as an anion conductive material such as a hydroxide ion conductive material, these plate-like particles are preferably in contact with each other, and particularly preferably continuous in the planar direction by overlapping or contacting each other. In this regard, prior art has reported that the hydroxide ion conductivity in the planar direction was improved by applying a nanoscale exfoliated LDH layer to the surface of a thin film (e.g., Non-Patent Document 1 (Sun et al., "Single-layer nanosheets with exceptionally high and anisotropic hydroxyl ion conductivity," Sci. Adv. 3, e1602629, 2017)). However, when using LDH platelet particles as a hydroxide ion conductive material in the air electrode of a metal-air battery, it is difficult to precisely control the state of the LDH platelet particles by simply mixing them with a raw material powder such as an air electrode catalyst, and it is particularly difficult to arrange the LDH platelet particles continuously in the planar direction. As a result, there has been a problem in that the hydroxide ion conductivity of LDH cannot be fully utilized in the air electrode.

[0013] The present inventors have now discovered that by constructing an air electrode / separator assembly using a hydroxide ion conductive composite material in which a plurality of hydroxide ion conductive particles are supported in a linked relationship on the surface of hydrophilic fibers, a reduction in charge / discharge overvoltage can be achieved when used in a metal-air secondary battery.

[0014] Therefore, an object of the present invention is to provide an air electrode / separator assembly that can reduce charge / discharge overvoltage when used in a metal-air secondary battery.

[0015] The present invention provides the following aspects. [Aspect 1] A cathode / separator assembly comprising: a hydroxide ion-conductive separator; a catalyst layer covering one side of the hydroxide ion-conductive separator, the catalyst layer including a cathode catalyst, a hydroxide ion-conductive composite material, a conductive material, and a binder; and a gas diffusion electrode provided on the catalyst layer opposite the hydroxide ion-conductive separator, wherein the hydroxide ion-conductive composite material comprises hydrophilic fibers and a plurality of hydroxide ion-conductive particles supported in a connected relationship on the surface of the hydrophilic fibers. [Aspect 2] The cathode / separator assembly according to Aspect 1, wherein the hydrophilic fibers are at least one selected from the group consisting of cellulose nanofibers, chitin nanofibers, and chitosan nanofibers. [Aspect 3] The cathode / separator assembly according to Aspect 1 or 2, wherein the hydrophilic fibers are cellulose nanofibers. [Aspect 4] The cathode / separator assembly according to any one of Aspects 1 to 3, wherein the hydrophilic fibers have a length of 0.1 to 100 μm. [Aspect 5] The air electrode / separator assembly according to any one of Aspects 1 to 4, wherein the hydrophilic fibers have a length of 0.2 to 50 μm. [Aspect 6] The air electrode / separator assembly according to any one of Aspects 1 to 5, wherein the hydrophilic fibers have a length of 5 to 50 μm. [Aspect 7] The air electrode / separator assembly according to any one of Aspects 1 to 6, wherein the hydroxide ion conductive particles are composed of layered double hydroxides (LDHs). [Aspect 8] The air electrode / separator assembly according to any one of Aspects 1 to 7, wherein the layered double hydroxides (LDHs) contain, as constituent elements, at least two elements selected from the group consisting of Ni, Fe, Mg, Al, and Ti. [Aspect 9] The air electrode / separator assembly according to Aspect 8, wherein the at least two elements include Mg and Al. [Aspect 10] The air electrode / separator assembly according to any one of Aspects 1 to 9, wherein the content of the hydroxide ion-conducting composite material in the catalyst layer is 10 to 40 volume % relative to the total amount of the air electrode catalyst, the hydroxide ion-conducting composite material, the electrically conductive material, and the binder in terms of solid matter.[Aspect 11] The air electrode / separator assembly according to any one of Aspects 1 to 10, wherein the content of the hydroxide ion-conducting composite material in the catalyst layer is 10 to 30 volume % relative to the total amount of the air electrode catalyst, the hydroxide ion-conducting composite material, the electrically conductive material, and the binder, calculated on a solid basis. [Aspect 12] The air electrode / separator assembly according to any one of Aspects 1 to 11, wherein the hydroxide ion-conducting separator is a layered double hydroxide (LDH) separator. [Aspect 13] The air electrode / separator assembly according to Aspect 12, wherein the LDH separator is composited with a porous substrate. [Aspect 14] A metal-air secondary battery comprising the air electrode / separator assembly according to any one of Aspects 1 to 13, a metal negative electrode, and an electrolyte, wherein the electrolyte is separated from the air electrode layer via the hydroxide ion-conducting separator.

[0016] 1 is a schematic cross-sectional view conceptually illustrating an air electrode / separator assembly according to one embodiment of the present invention.

[0034] FIG. 1 is a diagram conceptually illustrating the microstructure of the catalyst layer in an air electrode / separator assembly according to one embodiment of the present invention. In this figure, other components such as the gas diffusion electrode and current collector are omitted in order to focus on the microstructure of the catalyst layer. For the same purpose, the LDH separator is also depicted with a portion removed near the catalyst layer.

[0035] FIG. 1 is an XRD profile obtained for the Mg-Al-LDH prepared in Example 1.

[0036] FIG. 2 is an SEM image (magnification: 25,000x) of the Mg-Al-LDH-supported cellulose nanofiber (CNF) prepared in Example 1.

[0037] FIG. 3 is an SEM image (magnification: 50,000x) of the Mg-Al-LDH-supported CNF prepared in Example 1.

[0038] FIG. 4 is an SEM image showing measurement points in EDX quantitative analysis of the Mg-Al-LDH-supported CNF prepared in Example 1.

[0039] FIG. 5 is an EDX profile obtained for the Mg-Al-LDH prepared in Example 1. The results of charge / discharge evaluation of the zinc-air secondary batteries prepared in Examples 1 to 3 are shown below.

[0017] One embodiment of a cathode / separator assembly is shown in Figure 1. The cathode / separator assembly 10 shown in Figure 1 comprises a hydroxide ion-conductive separator 12 and an cathode layer 13. The cathode layer 13 comprises a catalyst layer 14, a gas diffusion electrode 16, and, optionally, an cathode current collector 18. As conceptually shown in Figure 2, the catalyst layer 14 comprises a cathode catalyst 20, a hydroxide ion-conductive composite material 22, a conductive material 24, and a binder 26, and covers one side of the hydroxide ion-conductive separator 12. The hydroxide ion-conductive composite material 22 comprises hydrophilic fibers 28 and a plurality of hydroxide ion-conductive particles 30 supported in a connected relationship on the surfaces of the hydrophilic fibers 28. In this way, by constructing the air electrode / separator assembly 10 using the hydroxide ion conductive composite material 22 in which a plurality of hydroxide ion conductive particles 30 are supported in a connected relationship on the surface of hydrophilic fibers 28, it is possible to achieve a reduction in charge / discharge overvoltage when used as a metal-air secondary battery.

[0018] That is, as mentioned above, LDH is generally in the form of plate-like particles having an arbitrary planar shape. When LDH plate-like particles are used as an anion-conducting material such as a hydroxide ion-conducting material, it is desirable that these plate-like particles be in contact with each other, and in particular, that they be continuous in the planar direction by overlapping or contacting each other. However, when LDH plate-like particles are used as a hydroxide ion-conducting material in the air electrode of a metal-air battery, it is difficult to precisely control the state of existence of the LDH plate-like particles by simply mixing them with a raw material powder such as an air electrode catalyst, and it is particularly difficult to arrange the LDH plate-like particles so that they are continuous in the planar direction. For this reason, there was a problem in that the hydroxide ion conductivity of LDH could not be fully utilized in the air electrode. In this regard, in the present invention, hydroxide ion conductive particles 30 (e.g., LDH platelet particles) are synthesized and supported on the surfaces of hydrophilic fibers 28 so that they are connected to one another (e.g., so that the planes of the LDH platelet particles are parallel to one another), thereby enabling the hydroxide ion conductive particles 30 (e.g., LDH platelet particles) to be formed into a continuum (i.e., an aggregate of particles continuously connected to one another in the planar direction). This makes it possible to utilize hydroxide ion conductive particles such as LDH particles as a continuum, which was difficult to achieve with conventional techniques, and achieves excellent anion conductivity (especially hydroxide ion conductivity). Therefore, by using a hydroxide ion conductive composite material 22 with this configuration as a hydroxide ion conductive material for the air electrode of a metal-air battery, the hydroxide ion conductivity of the air electrode layer 13 (especially the catalyst layer 14) can be improved. Furthermore, when an air electrode / separator assembly 10 including such an air electrode layer 13 (particularly the catalyst layer 14) is constructed and used as a metal-air secondary battery, the reaction rate of the charge / discharge reaction increases due to the improved hydroxide ion conductivity, and as a result, it is thought that a decrease in charge / discharge overvoltage is achieved.

[0019] The hydroxide ion-conductive separator 12 is not particularly limited as long as it is a separator capable of separating the air cathode layer 13 and the anode layer in a manner that allows hydroxide ions to be conductively separated in a zinc-air secondary battery. Typically, however, it is a separator that includes a hydroxide ion-conductive solid electrolyte and selectively passes hydroxide ions solely by utilizing hydroxide ion conductivity. A preferred hydroxide ion-conductive solid electrolyte is a layered double hydroxide (LDH) and / or an LDH-like compound. Therefore, the hydroxide ion-conductive separator 12 is preferably an LDH separator. In this specification, an "LDH separator" is defined as a separator that includes an LDH and / or an LDH-like compound and selectively passes hydroxide ions solely by utilizing the hydroxide ion conductivity of the LDH and / or LDH-like compound. In this specification, an "LDH-like compound" is a hydroxide and / or oxide with a layered crystal structure that has hydroxide ion conductivity, even if it may not be called an LDH, and can be considered an equivalent of an LDH. However, in a broad sense, "LDH" can be interpreted as encompassing not only LDH but also LDH-like compounds. The LDH separator is preferably composited with a porous substrate. Therefore, the LDH separator preferably further comprises a porous substrate, and is composited with the porous substrate in a form in which the pores of the porous substrate are filled with LDH and / or LDH-like compounds. That is, in a preferred LDH separator, the pores of the porous substrate are filled with LDH and / or LDH-like compounds so as to exhibit hydroxide ion conductivity and gas impermeability (and thus function as an LDH separator exhibiting hydroxide ion conductivity). The porous substrate is preferably made of a polymer material, and it is particularly preferred that the LDH and / or LDH-like compounds are incorporated throughout the entire thickness of the porous substrate made of a polymer material. For example, known LDH separators such as those disclosed in Patent Documents 1 to 9 can be used. The thickness of the LDH separator is preferably 5 to 100 μm, more preferably 5 to 80 μm, even more preferably 5 to 60 μm, and particularly preferably 5 to 40 μm.

[0020] The catalyst layer 14 includes a cathode catalyst 20 (eg, a charge catalyst and a discharge catalyst), a hydroxide ion conducting composite material 22 , an electrically conductive material 24 , and a binder 26 .

[0021] The air electrode catalyst 20 contained in the catalyst layer 14 has a spherical, plate-like, or fibrous form and is dispersed throughout the catalyst layer 14. Separate catalysts may be used for charging and discharging, or a single catalyst may be used for both charging and discharging. The catalyst 20 may also serve as the conductive material 24 or the hydroxide ion conductive composite material 22. The catalyst 20 is not particularly limited as long as it has catalytic activity for each reaction. For discharging, a carbon-based catalyst, an oxide catalyst, or a metal catalyst is desirable, while for charging, a hydroxide catalyst, an oxide catalyst, or a carbon-based catalyst is desirable. The catalyst 20 is preferably in the form of fine particles to increase the reaction field. Specifically, the particle size of the catalyst 20 is preferably 5 μm or less, more preferably 0.5 nm to 3 μm, and even more preferably 1 nm to 3 μm.

[0022] The hydroxide ion conductive composite material 22 contained in the catalyst layer 14 is a composite material that forms hydroxide ion conductive paths throughout the catalyst layer 14. The hydroxide ion conductive composite material 22 includes hydrophilic fibers 28 and a plurality of hydroxide ion conductive particles 30 that are supported on the surfaces of the hydrophilic fibers 28 and are connected to one another. The "a plurality of hydroxide ion conductive particles that are supported on the surfaces of the hydrophilic fibers 28 and are connected to one another" can be specified as hydroxide ion conductive particles 30 that are supported on the surfaces of the hydrophilic fibers 28 and are in contact with adjacent hydroxide ion conductive particles 30 at at least one location. The hydrophilic fibers 28 are not particularly limited as long as hydroxyl groups (OH groups) are coordinated or can be coordinated to the surfaces of the hydrophilic fibers 28. Hydroxide ion conductive particles 30 (e.g., LDH platelet-shaped particles) can be synthesized and supported on the surfaces of the hydrophilic fibers 28 by a coprecipitation method or the like so that the hydroxide ion conductive particles 30 are connected to one another (e.g., so that the faces of the LDH platelet-shaped particles are parallel to one another). That is, by using the hydrophilic fibers 28, it is possible to form the hydroxide ion conductive particles 30 (e.g., LDH platelet particles) into a continuum (i.e., an aggregate of particles continuously linked together in the planar direction). This is presumably because the hydroxyl groups (OH groups) coordinated on the surfaces of the hydrophilic fibers 28 are converted into a form (O-groups) in which hydrogen ions have been extracted by the action of a strong base such as NaOH in a coprecipitation method or the like, and these groups are electrostatically attracted to metal ions in the raw aqueous solution containing the constituent elements of LDH, resulting in the deposition of LDH platelet particles on the surfaces.

[0023] Preferred examples of hydrophilic fibers 28 include cellulose nanofibers, chitin nanofibers, chitosan nanofibers (CNF), and combinations thereof, with cellulose nanofibers (CNF) being more preferred. The length of hydrophilic fibers 28 is not particularly limited, but is preferably 0.1 to 100 μm, more preferably 0.1 to 50 μm, even more preferably 0.2 to 50 μm, particularly preferably 5 to 50 μm, and most preferably 10 to 50 μm. That is, depending on the size of the raw material available, hydrophilic fibers 28 can be used with fiber lengths ranging from short (e.g., several hundred nanometers) to long (e.g., several tens of μm), with longer fiber lengths being preferred. This is because longer fiber lengths increase the hydroxide ion conduction distance, which is believed to ensure that the hydroxide ion conduction paths necessary for the charge / discharge reaction in catalyst layer 14 are not interrupted and hydroxide ions are sufficiently distributed throughout catalyst layer 14.

[0024] The hydroxide ion conductive particles 30 are not particularly limited as long as they have hydroxide ion conductivity. However, as described above, they are preferably composed of layered double hydroxides (LDHs). In this case, the hydroxide ion conductive particles 30 may be LDH plate-like particles. The LDH constituting the hydroxide ion conductive particles 30 preferably contains at least two elements selected from the group consisting of Ni, Fe, Mg, Al, and Ti as constituent elements, and more preferably, these at least two elements include Mg and Al. By containing at least the two elements Mg and Al, better anion conductivity (e.g., hydroxide ion conductivity) can be achieved. In this case, the Al / Mg atomic ratio of the Mg-Al-LDH determined by energy dispersive X-ray spectroscopy (EDX) is preferably 0.30 to 0.55, more preferably 0.40 to 0.55. When Mg-Al-LDH having an atomic ratio within this range is used as a hydroxide ion conductive material for the air electrode of a metal-air battery, the hydroxide ion conductivity can be particularly effectively improved, and as a result, the reaction rate of the charge / discharge reaction can be further increased, and thereby the charge / discharge overvoltage can be further reduced. In the LDH, the anion between the layers is preferably a hydroxide ion. That is, the LDH has a structure in which the anion between the layers is a hydroxide ion. ２＋１－ｘ M ３＋ ｘ (OH) ２ ][An －ｘ／ｎ ・zH ２ O] (M ２＋ is Mg ２＋ Including M ３＋ is Al ３＋ An －ｘ／ｎ OH － Preferably, the LDH is represented by the general formula: (x≦x≦0.4, z is any real number greater than 0). LDH can be synthesized by coprecipitation. For example, a raw material aqueous solution containing the constituent elements of LDH may be dropped into an aqueous solution containing carbonate ions and a fibrous material such as cellulose nanofibers (CNF) at a pH of 9.5 to 12, followed by hydrothermal treatment. For example, an aqueous NaOH solution may be used to adjust the pH. The crystal size, crystallinity, and / or orientation of the resulting reaction product can be controlled by subjecting it to aging treatment such as stirring, heating, or pressurization, as necessary.

[0025] The content of the hydroxide ion conductive composite material 22 contained in the catalyst layer 14 is preferably an amount that allows the formation of an ion conduction path within the catalyst layer 14. Specifically, in the catalyst layer 14, the content of the hydroxide ion conductive composite material 22 relative to the total amount (defined as 100% by volume) of the cathode catalyst 20, the hydroxide ion conductive composite material 22, the conductive material 24, and the binder 26 in terms of solid matter is preferably 10 to 40% by volume, and more preferably 10 to 30% by volume.

[0026] The conductive material 24 contained in the catalyst layer 14 is preferably at least one selected from the group consisting of conductive ceramics and carbon-based materials. A preferred example of the conductive ceramic is LaNiO. ３ , LaSr ３ Fe ３ O １０ Examples of carbon-based materials include carbon black, graphite, carbon nanotubes, graphene, reduced graphene oxide, ketjen black, and any combination thereof.

[0027] A known binder resin can be used as the binder 26 contained in the catalyst layer 14. Examples of organic polymers include butyral resins, vinyl alcohol resins, celluloses, vinyl acetal resins, polytetrafluoroethylene, and polyvinylidene fluoride, with butyral resins, polytetrafluoroethylene, and polyvinylidene fluoride being preferred. As conceptually shown in FIG. 2 , the binder 26 is preferably present so as to bind the air electrode catalyst 20, hydroxide ion conductive composite material 22, and conductive material 24 together and to adequately expose these components so that they can come into contact with air.

[0028] The catalyst layer 14 can be manufactured by preparing a paste containing the air electrode catalyst 20, the hydroxide ion conductive composite material 22, the conductive material 24, and the binder 26, and applying the paste to the surface of the hydroxide ion conductive separator 12. The paste can be prepared by adding an organic polymer (binder resin) and an organic solvent to a mixture of the air electrode catalyst 20, the hydroxide ion conductive composite material 22, and the conductive material 24, as appropriate, and mixing the mixture using a known kneading machine such as a three-roll mill or a jet mill. Preferred examples of the organic solvent include alcohols such as butyl carbitol and terpineol, and acetate ester solvents such as butyl acetate. The paste can be applied to the hydroxide ion conductive separator 12 by printing. While various known printing methods can be used for this printing, screen printing is preferred.

[0029] The gas diffusion electrode 16 preferably comprises a microporous layer (MPL) and a gas diffusion substrate, and is formed on one side of the catalyst layer 14 so that the microporous layer (MPL) is in contact with the catalyst layer 14. The gas diffusion substrate is not particularly limited as long as it is a porous material that has electronic conductivity and can diffuse oxygen throughout the electrode, but carbon paper or a porous metal is preferred. The thickness of the gas diffusion substrate is preferably 0.4 μm or less, more preferably 0.1 to 0.3 μm, from the viewpoint of ensuring gas diffusibility while reducing energy density. Furthermore, the porosity of the gas diffusion substrate is preferably 70% or more, more preferably 70 to 90%, and particularly preferably 75 to 85%, from the viewpoint of gas permeability. With this porosity, excellent gas diffusivity can be ensured and a wide reaction region can be secured. Furthermore, the large amount of pore space reduces clogging with generated water. Porosity can be measured by mercury intrusion porosimetry. The microporous layer is not particularly limited as long as it has electronic conductivity and water repellency to the extent that water produced in the air electrode reaction does not penetrate into the gas diffusion substrate, but it preferably contains a carbon material and polytetrafluoroethylene (PTFE).

[0030] The cathode current collector 18 can be made of a typical conductive porous material, preferably metal. Preferred examples of metals constituting the cathode current collector 18 include stainless steel, titanium, nickel, brass, and copper. When made of metal, the shape of the cathode current collector 18 is not particularly limited as long as it ensures conductivity and breathability. Preferred examples include porous metal, metal mesh, and textured metal plate. Examples of porous metal include metal products with open pores, such as metal foam and sintered porous metal. Examples of metal mesh include metal mesh laminates and laminated metal mesh. Corrugated porous metal plates, such as punched metal, may also be used as textured metal plates.

[0031] As described above, the air electrode / separator assembly 10 is preferably used in a metal-air secondary battery. That is, according to a preferred embodiment of the present invention, a metal-air secondary battery is provided, which includes the air electrode / separator assembly 10, a metal negative electrode, and an electrolyte, with the electrolyte being separated from the catalyst layer 14 by a hydroxide ion-conductive separator 12. A zinc-air secondary battery using a zinc electrode as the metal negative electrode is particularly preferred. Alternatively, a lithium-air secondary battery using a lithium electrode as the metal negative electrode may be used.

[0032] The present invention will be further illustrated by the following examples.

[0033] Example 1 (1) Preparation of hydroxide ion conductive composite material Mg-Al-LDH-supported cellulose nanofibers (CNF) (fiber length: 2-50 μm) were prepared as hydroxide ion conductive composite materials by the following procedure. Here, "fiber length: 2-50 μm" means that various fiber lengths are distributed in the range of 2 μm to 50 μm, including significant amounts (non-negligible amounts) of fiber lengths of 5-50 μm and 10-50 μm.

[0034] (1a) Preparation of raw material aqueous solution Magnesium nitrate (Mg(NO ３ ) ２ ・6H ２ O, manufactured by Kanto Chemical Co., Ltd.) 0.025 mol, aluminum nitrate (Al(NO ３ ) ３ ・6H ２ O, manufactured by Kanto Chemical Co., Ltd.) 0.0125 mol, and cellulose nanofiber (CNF) ((C ６ H １０ O ５ 0.01 g of cellulose nitrile (IMA-1002, manufactured by Sugino Machine Co., Ltd., fiber length: 2 to 50 μm) was weighed and placed in a beaker, and ion-exchanged water was added to make up a total volume of 200 ml. The resulting solution was stirred for 30 minutes to prepare a raw material aqueous solution.

[0035] (1b) Preparation of basic aqueous solution Sodium carbonate (Na ２ CO ３0.05 mol of ammonium hydroxide (manufactured by Kanto Chemical Co., Inc.) and 0.15 mol of sodium hydroxide (NaOH, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were dissolved in the solution, and the mixture was stirred for 10 minutes to prepare a basic aqueous solution.

[0036] (1c) Coprecipitation Method While stirring at 400 rpm, 200 mL of the raw material aqueous solution was added dropwise to 100 mL of ion-exchanged water at a constant rate using a diaphragm-type metering pump. At this time, a pH meter was placed in the reaction solution, and a basic aqueous solution was added dropwise as needed to adjust the pH to 9.5 to 11. After all of the raw material aqueous solution had been added dropwise, the reaction solution was stirred at 60°C for 5 hours. Thereafter, the reaction solution was placed in an autoclave and subjected to hydrothermal treatment at 100°C for 12 hours to synthesize LDH on the surface of the CNF. After that, purification was performed by centrifugation to obtain Mg-Al-LDH-supported CNF.

[0037] (1d) Identification of LDH by XRD The crystalline phase of Mg—Al-LDH supported on CNF was measured using an X-ray diffractometer (Rigaku Corporation, RINT TTR III) under the measurement conditions of voltage: 50 kV, current value: 300 mA, and measurement range: 10 to 70°, to obtain the XRD profile shown in FIG. 3. The obtained XRD profile was identified using the diffraction peak of LDH (hydrotalcite-type compound) listed in JCPDS card No. 35-0964. The Mg—Al-LDH supported on CNF was identified as LDH (hydrotalcite-type compound).

[0038] (1e) SEM observation and EDX quantitative analysis When the Mg—Al-LDH-supported CNF was observed with an SEM at 25,000x and 50,000x magnification, it was observed that a large number of LDH plate-like particles were supported on the surface of the CNF with their faces parallel to each other, as shown in Figures 4 and 5, respectively. In addition, quantitative analysis was performed on the measurement points shown in Figure 6 (positions where the LDH plate-like particles were present) using SEM-EDX (apparatus name: JSM-7900F, manufactured by JEOL Ltd.) under the following conditions: acceleration voltage: 10.0 kV, irradiation current: 7.47500 nA, PHA mode: T3, elapsed time: 108.24 sec, effective time: 100.00 sec, dead time: 7%, coefficient rate: 16622 cps, energy range: 0 to 20 keV, and the results shown in Figure 7 and Table 1 below were obtained. From these results, it was confirmed that the LDH supported on CNF was an LDH containing Mg and Al as constituent elements in an atomic ratio of Al / Mg=0.51 (ie, Mg-Al-LDH).

[0039]

[0040] (1f) Confirmation of Contact of Hydroxide Ion Conductive Particles A cross-sectional sample of the catalyst layer was prepared using a cross-section polisher (device name: IB-19520CCP, manufactured by JEOL Ltd.) and observed with an SEM at 25,000x and 50,000x magnifications. It was confirmed that the hydroxide ion conductive particles (Mg-Al-LDH particles) in the catalyst layer were in contact with adjacent hydroxide ion conductive particles at one or more locations and were present in a connected state.

[0041] (2) Preparation of air electrode / separator assembly 14 parts by weight (40 parts by volume) of carbon powder (Tokai Carbon Co., Ltd., Toka Black #3855), 8 parts by weight (14 parts by volume) of LDH powder (Ni—Fe-LDH powder prepared by coprecipitation method), 12 parts by weight (26 parts by volume) of the hydroxide ion conductive composite material (Mg—Al-LDH-supported CNF) prepared in (1) above, and 13 parts by weight (12 parts by volume) of platinum-supported carbon (Toyo Corporation, EC-20-PTC) were mixed with 2 parts by weight (8 parts by volume) of 25 wt% butyral resin (a viscous material obtained by dissolving BL-s manufactured by Sekisui Chemical Co., Ltd. in butyl carbitol) and 50 parts by weight (271 parts by volume) of butyl carbitol, and kneaded with a three-roll and rotation / revolution mixer (Thinky Corporation, ARE-310) to form a paste. The proportion of Mg-Al-LDH-supported CNF in the total solid content of the obtained paste (components excluding butyl carbitol) was 26% by volume. This paste was applied by screen printing to the surface of an LDH separator (a polyethylene microporous membrane in which Ni-Al-Ti-LDH was deposited by hydrothermal synthesis within the pores and on the surface and then roll-pressed; thickness: 20 μm) to form a catalyst layer. Before the prepared paste dried, a gas diffusion electrode (SIGRACET28BC) was placed on the catalyst layer, and a weight was placed on top of it and dried in air at 80°C for 12 hours to form an air electrode. In this way, an air electrode / separator assembly was obtained.

[0042] (3) Preparation of Zinc Oxide Negative Electrode 100 parts by weight of ZnO powder (manufactured by Seido Chemical Industry Co., Ltd., JIS Class 1 grade, average particle size D50: 0.2 μm) was added to 5 parts by weight of metallic Zn powder (manufactured by Mitsui Mining & Smelting Co., Ltd., doped with Bi and In, Bi: 1000 ppm by weight, In: 1000 ppm by weight, average particle size D50: 100 μm), and 1.26 parts by weight of a polytetrafluoroethylene (PTFE) aqueous dispersion (manufactured by Daikin Industries, Ltd., solids content 60%) was added, and the mixture was kneaded with propylene glycol. The resulting kneaded mixture was rolled using a roll press to obtain a 0.4 mm negative electrode active material sheet. The negative electrode active material sheet was then pressure-bonded to a tin-plated copper expand metal and dried in a vacuum dryer at 80°C for 14 hours. After drying, the portion of the negative electrode sheet coated with the active material was cut into a 2 cm square, and copper foil was welded to the current collector portion to obtain a zinc oxide negative electrode.

[0043] (4) Assembly and Evaluation of Evaluation Cell A zinc oxide negative electrode was laminated on the LDH separator side of the air electrode / separator assembly. The resulting laminate was clamped between a holding jig with a sealing member tightly attached to the outer periphery of the LDH separator, and firmly fixed with screws. This holding jig had an oxygen inlet on the air electrode side and a liquid inlet through which the electrolyte could be introduced on the zinc oxide negative electrode side. A 5.4 M KOH aqueous solution saturated with zinc oxide was added to the negative electrode side of the resulting assembly to form an evaluation cell.

[0044] The charge / discharge characteristics of the evaluation cell were measured using an electrochemical measurement device (HZ-Pro S12, manufactured by Hokuto Denko Corporation) under the following conditions: Air electrode gas: saturated water vapor (25°C) oxygen (flow rate 200 cc / min) Charge / discharge current density: 2 mA / cm ２ Measurement was performed using the following cycles: charge / discharge time: 60 minutes charge / 60 minutes discharge; number of cycles: 4. The results are shown in FIG.

[0045] Example 2 Cellulose nanofiber ((C ６ H １０ O ５)n), instead of IMA-1002, AFo-1002 (fiber length: 0.2-3 μm) manufactured by Sugino Machine Co., Ltd. was used, and Mg—Al-LDH-supported CNF (fiber length: 0.2-3 μm) was produced and evaluated in the same manner as in Example 1. As a result, by XRD, the Mg—Al-LDH supported on CNF was identified as LDH (hydrotalcite-type compound). Furthermore, by SEM observation, it was observed that numerous LDH plate-like particles were supported on the surface of the CNF with their faces parallel to each other. Furthermore, by SEM-EDX, it was confirmed that the LDH supported on CNF was LDH containing Mg and Al as constituent elements in an atomic ratio of Al / Mg = 0.51 (i.e., Mg—Al-LDH). Furthermore, when the state of contact between hydroxide ion conductive particles was confirmed in the same manner as in Example 1, it was confirmed that in the catalyst layer produced in this example, hydroxide ion conductive particles (Mg-Al-LDH particles) were in contact with adjacent hydroxide ion conductive particles at one or more locations in the catalyst layer and existed in a connected state.

[0046] Subsequently, air electrode / separator assemblies and evaluation cells were fabricated using the Mg—Al-LDH-supported CNF in the same manner as in Example 1, and were evaluated. The results of the charge / discharge evaluation were as shown in FIG. 8 . From FIG. 8 , it was found that the evaluation cells (zinc-air secondary batteries) fabricated in Examples 1 and 2, which had a configuration in which Mg—Al-LDH-supported CNF was used as a hydroxide ion conductive composite material in the catalytic layer of the air electrode, exhibited a suppressed increase in charge / discharge overvoltage compared to Example 3, which used Mg—Al-LDH (without CNF), as described below. In particular, Example 1 (CNF fiber length: 2 to 50 μm) exhibited a lower charge / discharge overvoltage than Example 2 (CNF fiber length: 0.2 to 3 μm). This indicates that longer CNF fiber lengths better exhibit the effect of the hydroxide ion conductive composite material and further suppress the increase in charge / discharge overvoltage.

[0047] Example 3 (Comparative) A hydroxide ion conductive material, an air electrode / separator assembly, a zinc oxide anode, and an evaluation cell were prepared and evaluated in the same manner as in Example 1, except that Mg—Al-LDH powder (not supported on CNF) was prepared without adding cellulose nanofibers and used instead of the Mg—Al-LDH-supported CNF. The contact state of the hydroxide ion conductive particles was confirmed in the same manner as in Example 1. It was confirmed that in the catalyst layer prepared in this comparative example, the hydroxide ion conductive particles (Mg—Al-LDH particles) were not necessarily in contact with other hydroxide ion conductive particles but were present in a dispersed state. Furthermore, the charge / discharge evaluation results were as shown in FIG. 8. As shown in FIG. 8, in the evaluation cell prepared in this example, the hydroxide ion conductive material (Mg—Al-LDH) was not supported on CNF, and the LDH plate-like particles were not continuous so that their surfaces were parallel to each other, resulting in a large increase in charge / discharge overvoltage when the charge / discharge evaluation was performed.

Claims

1. a hydroxide ion conducting separator; a catalyst layer covering one side of the hydroxide ion conductive separator, the catalyst layer including an air electrode catalyst, a hydroxide ion conductive composite material, a conductive material, and a binder; a gas diffusion electrode provided on the catalyst layer opposite the hydroxide ion conductive separator; and the hydroxide ion conducting composite material comprises: A hydrophilic fiber; a plurality of hydroxide ion conductive particles supported in a connected state on the surface of the hydrophilic fiber; A cathode / separator assembly comprising:

2. 2. The air electrode / separator assembly according to claim 1, wherein the hydrophilic fiber is at least one selected from the group consisting of cellulose nanofibers, chitin nanofibers, and chitosan nanofibers.

3. The air electrode / separator assembly according to claim 1 , wherein the hydrophilic fibers are cellulose nanofibers.

4. 4. The air electrode / separator assembly according to claim 1, wherein the hydrophilic fibers have a length of 0.1 to 100 μm.

5. 4. The air electrode / separator assembly according to claim 1, wherein the hydrophilic fibers have a length of 0.2 to 50 μm.

6. 4. The air electrode / separator assembly according to claim 1, wherein the hydrophilic fibers have a length of 5 to 50 μm.

7. 4. The air electrode / separator assembly according to claim 1, wherein the hydroxide ion conductive particles are composed of layered double hydroxides (LDHs).

8. 8. The air electrode / separator assembly according to claim 7, wherein the layered double hydroxide (LDH) contains, as constituent elements, at least two elements selected from the group consisting of Ni, Fe, Mg, Al, and Ti.

9. 9. The cathode / separator assembly of claim 8, wherein the at least two elements include Mg and Al.

10. 4. The air electrode / separator assembly according to claim 1, wherein the content of the hydroxide ion-conducting composite material in the catalyst layer is 10 to 40% by volume relative to the total amount of the air electrode catalyst, the hydroxide ion-conducting composite material, the electrically conductive material, and the binder in terms of solid matter.

11. 4. The air electrode / separator assembly according to claim 1, wherein the content of the hydroxide ion-conducting composite material in the catalyst layer is 10 to 30% by volume relative to the total amount of the air electrode catalyst, the hydroxide ion-conducting composite material, the electrically conductive material, and the binder in terms of solid matter.

12. 4. The air electrode / separator assembly according to claim 1, wherein the hydroxide ion-conducting separator is a layered double hydroxide (LDH) separator.

13. The air electrode / separator assembly according to claim 12 , wherein the LDH separator is composited with a porous substrate.

14. A metal-air secondary battery comprising the air electrode / separator assembly according to any one of claims 1 to 3, a metal negative electrode, and an electrolyte, wherein the electrolyte is isolated from the air electrode layer via the hydroxide ion-conductive separator.