Air electrode / separator assembly and metal–air secondary battery

Abstract

The purpose of the present invention is to provide an air electrode / separator assembly exhibiting high activity as an electrode while having an extremely thin air electrode layer. This air electrode / separator assembly comprises: a hydroxide ion conduction separator that contains a hydroxide ion conduction solid electrolyte; and an air electrode layer that is provided on one surface side of the hydroxide ion conduction separator, and that contains an air electrode material and has a thickness of 2000 nm or less. The air electrode material is a material having both functions of an air electrode catalyst and an electron conductive material, and includes at least one metal element selected from the group consisting of Ni, Fe, Co, Mn, V, Mo, W, Cr, Pt, and Pd in at least one kind selected from the group consisting of (i) a metal carbide, (ii) a metal in which carbon is solid-dissolved, and (iii) a mixture of amorphous carbon and metal particles.

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] 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.

[0004] 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.

[0005] 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.

[0006] 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.

[0007] It has been proposed that providing an air electrode layer with a thickness of 1000 nm or less on one side of a hydroxide ion-conductive separator such as an LDH separator reduces diffusion resistance and interfacial resistance between solids due to the conduction or diffusion of electrons, gases, and hydroxide ions. Specifically, Patent Document 10 (WO 2023 / 026663) discloses an air electrode / separator assembly comprising a hydroxide ion-conductive separator containing a hydroxide ion-conductive solid electrolyte and an air electrode layer with a thickness of 1000 nm or less provided on one side of the hydroxide ion-conductive separator. The air electrode layer comprises a hydroxide ion-conductive material, an electron-conductive material, and an air electrode catalyst. The hydroxide ion-conductive material may be the same material as the hydroxide ion-conductive solid electrolyte or the air electrode catalyst. Furthermore, the electron-conductive material may be the same material as the air electrode catalyst. This document also discloses that the air electrode / separator assembly further comprises an interfacial layer between the hydroxide ion-conductive separator and the air electrode layer. This interfacial layer is said to include a plurality of plate-like particles made of hydroxide ion conductive solid electrolyte that have grown in a direction away from the surface of the hydroxide ion conductive separator, and an electron conductive material that is provided so as to fill the gaps between the plurality of plate-like particles and / or the irregularities formed by the plurality of plate-like particles.

[0008] WO2013 / 073292WO2016 / 076047WO2016 / 067884WO2019 / 124270WO2015 / 146671 WO2020 / 246177WO2020 / 255856WO2021 / 229916WO2021 / 229917WO2023 / 026663

[0009] In metal-air batteries, the air electrode reaction occurs at a three-phase interface (consisting of a hydroxide ion-conducting phase, an electron-conducting phase, and a gas phase) where hydroxide ions, oxygen, and electrons are present. Therefore, it is desirable to secure as much of this reaction space as possible within the air electrode. In contrast, metal-air batteries using a conventional polymeric porous separator allow the electrolyte to easily penetrate into the air electrode due to the separator's porosity. This allows the electrolyte to handle hydroxide ion conduction within the air electrode, thereby enabling high ionic conductivity. On the other hand, because the electrolyte is highly alkaline, its dissolved oxygen content is low. If the catalyst is covered by the electrolyte, the supply of oxygen to the catalyst is insufficient. As a result, most of the reaction occurs on the catalyst at the interface between the electrolyte and the gas phase, meaning the reaction space is limited to the interface between the electrolyte and the gas phase. Furthermore, because metal-air batteries that use such polymer porous separators are open systems, they have problems such as potassium carbonate being produced in the air electrode by carbon dioxide in the air, blocking the pores, and carbon dioxide permeating the separator gradually increasing the resistance of the electrolyte.

[0010] On the other hand, in metal-air batteries using hydroxide ion-conductive separators such as LDH separators, the denseness of the separator can prevent the electrolyte from penetrating into the air electrode. Therefore, the above-mentioned problems caused by carbon dioxide can be avoided. However, to induce reactions within the air electrode, it is desirable to place a solid hydroxide ion conductor. In this case, while hydroxide ion conduction can expand the reaction field as much as possible, compared to air batteries using electrolyte as a hydroxide ion-conducting medium (air batteries using a polymer porous separator), the resistance of the hydroxide ion conductor itself is higher than that of the electrolyte, and the interfacial resistance between solids is also significant. Therefore, the conduction or diffusion of hydroxide ions may become a bottleneck (rate-limiting step) for the conduction or diffusion of electrons and gases.

[0011] However, as mentioned above, metal-air secondary batteries using hydroxide ion-conductive separators such as LDH separators have the excellent advantage of preventing both short circuits between positive and negative electrodes caused by metal dendrites and carbon dioxide contamination. Furthermore, the denseness of hydroxide ion-conductive separators also has the advantage of suppressing evaporation of water contained in the electrolyte. Therefore, it would be advantageous to reduce problems associated with hydroxide ion conduction or diffusion while utilizing these advantages. In this regard, the air electrode / separator assembly disclosed in Patent Document 10 (WO 2023 / 026663) achieves reduced resistance, etc., due to the conduction or diffusion of electrons, gases, and hydroxide ions by providing an ultrathin air electrode layer with a thickness of 1000 nm or less on one side of the hydroxide ion-conductive separator. However, as the thickness of the air electrode decreases, the catalytic reaction field decreases, which tends to increase resistance. Therefore, the formation of finer reaction fields and high catalyst activation are desired.

[0012] The present inventors have now discovered that by providing an air electrode / separator assembly with a thickness of 2000 nm or less, which contains a predetermined metal element and carbon in a predetermined form, on one side of a hydroxide ion conductive separator, it is possible to provide an air electrode / separator assembly that exhibits high activity as an electrode despite having an extremely thin air electrode layer.

[0013] Therefore, an object of the present invention is to provide an air electrode / separator assembly that has an extremely thin air electrode layer but exhibits high activity as an electrode.

[0014] The present invention provides the following aspects: [Aspect 1] An air electrode / separator assembly comprising: a hydroxide ion-conductive separator containing a hydroxide ion-conductive solid electrolyte; and an air electrode layer having a thickness of 2000 nm or less, provided on one side of the hydroxide ion-conductive separator, the air electrode layer containing an air electrode material, the air electrode layer comprising at least one metal element selected from the group consisting of Ni, Fe, Co, Mn, V, Mo, W, Cr, Pt, and Pd, in the form of at least one selected from the group consisting of (i) metal carbide, (ii) a metal solid-solved with carbon, and (iii) a mixture of amorphous carbon and metal particles, thereby functioning as both an air electrode catalyst and an electron conductor. [Aspect 2] The air electrode / separator assembly according to Aspect 1, further comprising an interfacial layer between the hydroxide ion conductive separator and the air electrode layer, the interfacial layer comprising: a plurality of plate-like particles made of a hydroxide ion conductive solid electrolyte that have grown in a direction away from the surface of the hydroxide ion conductive separator; and an electron conductive material or air electrode material that is provided so as to fill gaps between the plurality of plate-like particles and / or irregularities formed by the plurality of plate-like particles. [Aspect 3] The air electrode / separator assembly according to Aspect 2, wherein the air electrode layer comprises a plurality of air electrode segments that are provided on the interfacial layer with gaps between them and are made of the air electrode material. [Aspect 4] The air electrode / separator assembly according to Aspect 3, wherein the plurality of air electrode segments extend in a direction away from the hydroxide ion conductive separator, and the angle between the extension direction of the plurality of air electrode segments and the normal direction to the hydroxide ion conductive separator is within the range of 0 to 70 degrees. [Aspect 5] The air electrode / separator assembly according to Aspect 3 or Aspect 4, wherein the spacing between adjacent air electrode segments is 0.1 to 100 nm. [Aspect 6] The air electrode / separator assembly according to any one of Aspects 1 to 5, wherein the air electrode segment has a carbon-rich layer on its surface, the carbon-rich layer having a higher carbon atom content than other portions of the air electrode segment.[Aspect 7] The air electrode / separator assembly according to any one of Aspects 1 to 6, wherein the at least one metal element is Ni, or Ni and Fe. [Aspect 8] The air electrode / separator assembly according to any one of Aspects 1 to 7, wherein the at least one metal element is Co. [Aspect 9] The air electrode / separator assembly according to any one of Aspects 1 to 8, wherein the at least one metal element is Mn. [Aspect 10] The air electrode / separator assembly according to any one of Aspects 1 to 9, wherein the at least one metal element is Fe. [Aspect 11] The air electrode / separator assembly according to any one of Aspects 2 to 10, wherein the thickness of the interface layer is 500 nm or less and the thickness of the air electrode layer is 1500 nm or less. [Aspect 12] The air electrode / separator assembly according to any one of Aspects 2 to 11, wherein the hydroxide ion conductive material contained in the interface layer is a layered double hydroxide (LDH) and / or an LDH-like compound. [Aspect 13] The air electrode / separator assembly according to any one of Aspects 1 to 12, wherein the hydroxide ion-conductive separator is a layered double hydroxide (LDH) separator. [Aspect 14] The air electrode / separator assembly according to Aspect 13, wherein the LDH separator is composited with a porous substrate. [Aspect 15] A metal-air secondary battery comprising the air electrode / separator assembly according to any one of Aspects 1 to 14, a metal negative electrode, and an electrolyte, wherein the electrolyte is isolated from the air electrode layer via the hydroxide ion-conductive separator.

[0015] 5A is a schematic cross-sectional view conceptually illustrating an air electrode / separator assembly according to one embodiment of the present invention, and an enlarged view thereof. FIG. 5B is a schematic cross-sectional view conceptually illustrating a hydroxide ion conductive separator used in the present invention. FIG. 5C is a schematic cross-sectional view illustrating an example of an air electrode segment having a carbon-rich layer on its surface. FIG. 5D is a diagram illustrating the positional relationship between a target, an LDH separator, and a substrate holder in a film formation apparatus for oblique film formation. FIG. 5E is a conceptual diagram illustrating an example of a He permeability measurement system. FIG. 5F is a schematic cross-sectional view of a sample holder and its peripheral configuration used in the measurement system shown in FIG. 5A. FIG. 5C is a charge / discharge curve measured for the zinc-air battery prepared in Example 1. FIG. 5D is a TEM image of the air electrode layer prepared in Example 1. FIG. 5E is a BF-STEM image of the air electrode layer prepared in Example 1. FIG. 5F is a BF-STEM image of the air electrode layer prepared in Example 1. FIG. 5E is an EDX elemental mapping image of the air electrode layer prepared in Example 1.

[0016] Air Electrode / Separator Assembly Figure 1 shows one embodiment of an air electrode / separator assembly according to the present invention. The air electrode / separator assembly 10 shown in Figure 1 comprises a hydroxide ion-conductive separator 12 and an air electrode layer 14 provided on one side of the hydroxide ion-conductive separator 12. Optionally, an interface layer 13 may be interposed between the hydroxide ion-conductive separator 12 and the air electrode layer 14. The hydroxide ion-conductive separator 12 contains a hydroxide ion-conductive solid electrolyte. The air electrode layer 14 is a layer containing a air electrode material and having a thickness of 2000 nm or less. The air electrode material is a material that functions as both an air electrode catalyst and an electron conductor, and contains at least one metal element selected from the group consisting of Ni, Fe, Co, Mn, V, Mo, W, Cr, Pt, and Pd, in the form of at least one selected from the group consisting of (i) metal carbide, (ii) a metal solid-solved with carbon, and (iii) a mixture of amorphous carbon and metal particles. By providing an air electrode layer 14 having a thickness of 2000 nm or less and containing a predetermined metal element and carbon in a predetermined form on one side of the hydroxide ion-conductive separator 12, it is possible to provide an air electrode / separator assembly 10 that exhibits high activity as an electrode despite having an extremely thin air electrode layer 14.

[0017] That is, as mentioned above, in metal-air batteries using a hydroxide ion-conductive separator such as an LDH separator, the denseness of the separator prevents the electrolyte from penetrating into the air electrode. Therefore, the above-mentioned problems caused by carbon dioxide can be avoided. However, to induce reactions within the air electrode, it is desirable to provide a solid-phase hydroxide ion conductor. While hydroxide ion conduction can expand the reaction field as much as possible, compared to air batteries using an electrolyte as a hydroxide ion-conducting medium (air batteries employing a polymer porous separator), the resistance of the hydroxide ion conductor itself is higher than that of the electrolyte, and the interfacial resistance between the solids is also significant. This may result in the conduction or diffusion of hydroxide ions becoming a bottleneck (rate-limiting step) relative to the conduction or diffusion of electrons and gases. The air electrode / separator assembly 10 advantageously solves this problem. This is because the air electrode layer 14 can be made extremely thin (2000 nm or less), allowing the air electrode reaction to be completed within the microspace within the air electrode layer 14. That is, the travel distances of electrons, gases, and hydroxide ions within such a microspace (particularly the diffusion distances of gases and hydroxide ions, which are prone to diffusion resistance) can be shortened, resulting in reduced diffusion resistance and interfacial resistance between solids. However, as mentioned above, a thinner air electrode reduces the catalytic reaction field, which in turn tends to increase resistance. In this regard, the air electrode / separator assembly 10 of the present invention employs an air electrode layer 14 containing a predetermined metal element and carbon in a predetermined form, allowing it to exhibit high electrode activity despite having an extremely thin air electrode layer 14. This allows batteries incorporating the air electrode / separator assembly 10 to achieve reduced battery resistance. Furthermore, because the air electrode layer 14 is extremely thin, there is no waste of material constituting the air electrode layer 14, and the air electrode layer 14 can be formed using a very small amount of material, allowing even expensive catalysts to be used effectively.

[0018] Furthermore, since the air electrode / separator assembly 10 can be made very thin, it can be made flexible. In this case, because the air electrode / separator assembly 10 can bend even when pressurized, it can be housed in a battery container and pressurized together with other battery elements (such as a negative electrode) in a direction that brings the battery elements into close contact with each other. This type of pressurization is particularly advantageous when multiple air electrode / separator assemblies 10 are alternately assembled into a battery container along with multiple metal negative electrodes to form a stacked battery. It is also advantageous when multiple stacked batteries are housed in a single module container to form a battery module. For example, pressurizing a zinc-air secondary battery can minimize (and preferably eliminate) the gap between the negative electrode and the hydroxide ion-conducting separator 12 that allows zinc dendrite growth, thereby more effectively preventing zinc dendrite extension. For example, the thickness of the air electrode / separator assembly 10 is preferably 7 to 102 μm, more preferably 7 to 82 μm, and even more preferably 7 to 62 μm.

[0019] The hydroxide ion-conductive separator 12 is defined as a separator containing a hydroxide ion-conductive solid electrolyte that selectively transmits hydroxide ions solely by utilizing the hydroxide ion conductivity of the hydroxide ion-conductive solid electrolyte. Therefore, the hydroxide ion-conductive separator is gas-impermeable and / or water-impermeable, particularly gas-impermeable. That is, the hydroxide ion-conductive material is dense enough to exhibit gas-impermeability and / or water-impermeability, and constitutes all or part of the hydroxide ion-conductive dense separator. The hydroxide ion-conductive dense separator may be composited with a porous substrate.

[0020] Preferably, the hydroxide ion-conductive solid electrolyte is a layered double hydroxide (LDH) and / or an LDH-like compound (hereinafter collectively referred to as hydroxide ion-conductive layered compounds), thereby forming the hydroxide ion-conductive separator 12 as an LDH separator. That is, an LDH separator is defined as a separator containing LDH and / or an LDH-like compound (hereinafter collectively referred to as hydroxide ion-conductive layered compounds) that selectively transmits hydroxide ions solely by utilizing the hydroxide ion conductivity of the hydroxide ion-conductive layered compounds. In this specification, "LDH-like compounds" refer to hydroxides and / or oxides with a layered crystalline structure that have hydroxide ion conductivity and are equivalent to LDH, even if they may not be called LDHs. However, in a broad sense, "LDH" can be interpreted to encompass not only LDHs but also LDH-like compounds. Such LDH separators can be known, as disclosed in Patent Documents 1 to 10, and are preferably LDH separators composited with a porous substrate. A particularly preferred LDH separator, the hydroxide ion-conductive separator 12, includes a porous substrate 12a made of a polymer material and a hydroxide ion-conductive layered compound 12b that plugs the pores P of the porous substrate, as conceptually shown in FIG. 2 . This type of LDH separator will be described later. The inclusion of a porous substrate made of a polymer material allows the separator 12 to bend and resist cracking even under pressure. This allows the separator 12 to be housed in a battery container and pressurized together with other battery elements (e.g., negative electrodes) in a direction that brings the battery elements into close contact with each other. This type of pressurization is particularly advantageous when multiple air electrode / separator assemblies 10 are alternately assembled with multiple metal negative electrodes into a battery container to form a stacked battery. Similarly, it is also advantageous when multiple stacked batteries are housed in a single module container to form a battery module. For example, pressurizing a zinc-air secondary battery can minimize (preferably eliminate) the gap that allows zinc dendrite growth between the negative electrode and the LDH separator, thereby more effectively preventing zinc dendrite extension. For example, known LDH separators such as those disclosed in Patent Documents 1 to 10 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.

[0021] The air electrode layer 14 is a layer containing a cathode material and is provided on one side of the hydroxide ion-conductive separator (or on the surface of the interfacial layer 13, if present). The air electrode material contains at least one metal element selected from the group consisting of Ni, Fe, Co, Mn, V, Mo, W, Cr, Pt, and Pd, in the form of at least one selected from the group consisting of (i) metal carbide, (ii) a metal solid-solved with carbon, and (iii) a mixture of amorphous carbon and metal particles. This air electrode material functions as both an air electrode catalyst and an electron conductor. At least one form of the air electrode material selected from the group consisting of (i) to (iii) above can be deposited as the air electrode layer 14 by vapor deposition, such as sputtering, using a target and / or atmosphere containing these components, as described below. In the air electrode layer formed by this method, the carbon and the predetermined metal element can be said to exist in at least one of the forms (i) to (iii) above, but should not be limited to any one form and may be any combination of (i) to (iii) above. The at least one metal element in the air electrode layer 14 is preferably Ni, or Ni and Fe, but also preferably Co, Mn, or Fe.

[0022] In addition to the above-described cathode material, the cathode layer 14 may further include a hydroxide ion conductive material, an electron conductive material, and / or an cathode catalyst as additional components. The hydroxide ion conductive material may be the same material as the hydroxide ion conductive solid electrolyte or the cathode catalyst, and the electron conductive material may be the same material as the cathode catalyst. Examples of additional components that can be used include the hydroxide ion conductive material, electron conductive material, and / or cathode catalyst disclosed in Patent Document 10.

[0023] The thickness of the air electrode layer 14 is 2000 nm or less, preferably 1500 nm or less, more preferably 1000 nm or less, still more preferably 5 to 1000 nm, particularly preferably 5 to 800 nm, and most preferably 20 to 800 nm.

[0024] The air electrode / separator assembly 10 preferably further includes an interface layer 13 between the hydroxide ion conductive separator 12 and the air electrode layer 14. The interface layer 13 includes a plurality of plate-like particles 12p composed of a hydroxide ion conductive solid electrolyte that have grown in a direction away from the surface of the hydroxide ion conductive separator 12 (perpendicular or oblique to the surface), and an electron conductive material 16 or an air electrode material that is provided so as to fill gaps between the plurality of plate-like particles 12p and / or irregularities formed by the plurality of plate-like particles 12p. By filling the gaps and irregularities caused by the plate-like particles 12p that have grown in a direction away from the hydroxide ion conductive separator 12 with the electron conductive material 16 or air electrode material, the interface layer 13 can be used to provide electron conduction in the in-plane direction of the hydroxide ion conductive separator 12 and hydroxide ion conduction in a direction perpendicular to the main surface of the hydroxide ion conductive separator 12 (thickness direction of the hydroxide ion conductive separator 12 and the air electrode layer 14). In particular, the plate-like particles 12p of hydroxide ion-conductive solid electrolytes such as LDH and / or LDH-like compounds have the property of conducting hydroxide ions in the plate surface direction (the (003) surface direction in the case of LDH), and it is therefore believed that arranging the plate-like particles 12p in a direction away from the surface of the LDH separator 12 reduces the interfacial resistance between the air cathode layer 14 and the LDH separator 12. In particular, when observing the surface microstructure of an LDH separator 12 produced according to a known method, the LDH plate-like particles 12p typically grow in a direction away from the surface of the LDH separator 12, as shown in Figure 1 . In the present invention, the presence of plate-like particles 12p (hydroxide ion-conductive material) and electron-conductive material 16 or air cathode material in such a state between the LDH separator 12 and the air cathode layer 14 significantly reduces the interfacial resistance.

[0025] The thickness of the interface layer 13 is preferably 500 nm or less, more preferably 150 nm or less, more preferably 5 to 150 nm, and even more preferably 5 to 130 nm.

[0026] The air electrode layer 14 preferably includes a plurality of air electrode segments 14s made of an air electrode material and spaced apart from one another on the interface layer 13. The shape of the air electrode segments 14s is not particularly limited, but is typically columnar. This allows air to be efficiently introduced into the air electrode layer 14, thereby increasing the area of ​​the reaction field (a three-phase interface consisting of a hydroxide ion-conducting phase, an electron-conducting phase, and a gas phase). In this regard, in the air electrode / separator assembly 10, the air electrode layer 14 is extremely thin. This is thought to result in hydroxide ion conduction due to the plate-like particles 12p (hydroxide ion-conducting solid electrolyte) on the surface of the hydroxide ion-conducting separator 12, and the interaction between the metal source uniformly exposed on the surface of the air electrode layer 14 (particularly the air electrode segments 14s) and the hydroxide generated when the metal source comes into contact with humidified air, thereby providing a hydroxide ion-conducting phase and forming a favorable reaction field (the above-mentioned three-phase interface). The air electrode segments 14s extend in a direction away from the hydroxide ion conductive separator 12, and the angle formed between the extension direction of the air electrode segments 14s and the normal direction to the hydroxide ion conductive separator 12 is preferably within a range of 0 to 70 degrees, more preferably 10 to 65 degrees, and even more preferably 30 to 65 degrees. The spacing between adjacent air electrode segments 14s is preferably 0.1 to 100 nm, more preferably 2 to 50 nm, and even more preferably 5 to 40 nm.

[0027] As shown in FIG. 3 , the air electrode segment 14s preferably has a carbon-rich layer 14c on its surface. The carbon-rich layer 14c is defined as a layer with a higher carbon content than the rest of the air electrode segment 14s (i.e., a carbon-rich surface layer). The presence of the carbon-rich layer 14c allows the electrode to exhibit higher activity. While the reason for this is not entirely clear, it is presumed that the carbon-rich layer 14c protects the reactive active sites, preventing excessive exposure of the reactive active sites, thereby suppressing undesirable side reactions at those sites and preferentially inducing the desired catalytic reaction. The carbon-rich layer 14c can be observed by obtaining cross-sectional TEM images using an electron microscope, as exemplified in the examples described below. Furthermore, the fact that the carbon content in the carbon-rich layer 14c is higher than the rest of the air electrode segment 14s can be confirmed by obtaining EDX elemental mapping of various elements for a cross section of the air electrode segment 14s, as exemplified in the examples described below. That is, by acquiring EDX elemental mapping images of various metal elements (e.g., Ni), carbon (C), oxygen (O), and other elements contained in the air electrode segment 14s for the same cross-sectional field of view including the surface of the air electrode segment 14s and comparing them, the relative abundance ratio of carbon atoms can be evaluated. For example, as shown in the rightmost part of Figure 9 (described later), when an EDX elemental mapping image of C is superimposed on an EDX elemental mapping image of a metal element such as Ni, if C is distributed beyond the region occupied by the metal element such as Ni, the region where C is distributed can be determined to be a layer with a higher abundance ratio of carbon atoms than the rest of the air electrode segment 14s (i.e., the carbon-rich layer 14c). Therefore, the above determination can be made without quantifying the abundance ratio of carbon atoms. The carbon-rich layer 14c may contain the aforementioned metal elements (i.e., at least one selected from the group consisting of Ni, Fe, Co, Mn, V, Mo, W, Cr, Pt, and Pd), but preferably contains Ni, Co, Mn, Fe, or a combination thereof.The carbon-rich layer 14 c may contain oxygen (O), but is preferably not oxygen-rich. The carbon-rich layer 14 c preferably covers the entire surface of the cathode segment 14 s, but may cover only a portion of the surface of the cathode segment 14 s.

[0028] The metal constituting the air electrode layer 14 may be oxidized on the surface of the air electrode segment 14s (which may be the carbon-rich layer 14c). Furthermore, the metal and / or carbon constituting the air electrode layer 14 may be hydroxylated (i.e., may have hydroxyl groups attached) on the surface of the air electrode segment 14s (which may be the carbon-rich layer 14c).

[0029] Manufacturing Method The air electrode / separator assembly 10 can be preferably manufactured by performing vapor deposition, in which components of the air electrode layer 14 are deposited on a hydroxide ion conductive separator 12, such as an LDH separator. Such vapor deposition is preferably performed by: (a) using a metal target composed of at least one metal selected from the group consisting of Ni, Fe, Co, Mn, V, Mo, W, Cr, Pt, and Pd in ​​combination with a carbon target; (b) using a composite target composed of at least one metal selected from the group consisting of Ni, Fe, Co, Mn, V, Mo, W, Cr, Pt, and Pd and carbon; or (c) using a metal target composed of at least one metal selected from the group consisting of Ni, Fe, Co, Mn, V, Mo, W, Cr, Pt, and Pd in ​​an atmosphere containing gaseous carbon-containing molecules.

[0030] Preferred examples of vapor deposition methods include sputtering and laser ablation, and particularly preferred are sputtering, such as bipolar sputtering and magnetron sputtering. According to the vapor deposition method described above, the gaps between the plurality of plate-like particles 12 p and / or the irregularities formed by the plurality of plate-like particles 12 p on the surface of the hydroxide ion conductive separator 12 are filled with the electron conductive material 16 or the air electrode material to form the interface layer 13, and then the air electrode material is deposited on the interface layer 13 to form a plurality of spaced-apart air electrode segments 14 s, thereby forming the air electrode layer 14.

[0031] Any technique may be used to form a film that leaves a gap between the air electrode segments 14s, but as shown in Figure 4, it is preferable to perform film formation in a vapor phase film formation device such as a sputtering device, with the hydroxide ion conductive separator 12 positioned at an angle to the direction D in which film formation particles fly from the target 20. By forming the film in this manner with the hydroxide ion conductive separator 12 positioned at an angle, it is easier to form a gap (compared to film formation in a vertical position).

[0032] The carbon-rich layer 14c may be formed by any method, but is preferably formed by a vapor deposition method such as the sputtering method described above. For example, in the case of vapor deposition by (a) or (c) above, it is preferable to use a smaller metal target and / or a larger carbon target to increase the carbon content in the vapor deposition atmosphere. In addition, in the case of vapor deposition by (b) above, it is preferable to increase the carbon content in the vapor deposition atmosphere by increasing the carbon content in the metal-carbon composite target. In this way, by increasing the carbon content in the vapor deposition atmosphere, the carbon-rich layer 14c can be formed on the surface of the cathode segment 14s.

[0033] Metal-Air Secondary Battery As mentioned 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 comprises the air electrode / separator assembly 10, a metal negative electrode, and an electrolyte, with the electrolyte being separated from the air electrode 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.

[0034] An LDH separator 12 according to a preferred embodiment of the present invention will be described below. As described above, the LDH separator 12 of this embodiment includes a porous substrate 12a and a hydroxide ion-conducting layered compound 12b, which is an LDH and / or an LDH-like compound, as conceptually shown in Figure 2. While Figure 2 depicts the regions of the hydroxide ion-conducting layered compound 12b as being disconnected between the upper and lower surfaces of the LDH separator 12, this is because the cross section is depicted two-dimensionally. Taking depth into consideration, the regions of the hydroxide ion-conducting layered compound 12b are connected three-dimensionally between the upper and lower surfaces of the LDH separator 12, thereby ensuring the hydroxide ion conductivity of the LDH separator 12. The porous substrate 12a is made of a polymer material, and the pores of the porous substrate 12a are blocked by the hydroxide ion-conducting layered compound 12b. However, the pores of the porous substrate 12a do not need to be completely blocked, and there may be a small number of residual pores P. By blocking the pores of the polymeric porous substrate 12a with the hydroxide ion-conductive layered compound 12b in this manner to achieve high densification, it is possible to provide an LDH separator 12 that can more effectively suppress short circuits caused by zinc dendrites.

[0035] Furthermore, the LDH separator 12 of this embodiment not only has the desired ion conductivity required of a separator due to the hydroxide ion conductivity of the hydroxide ion-conducting layered compound 12b, but also has excellent flexibility and strength. This is due to the flexibility and strength of the polymeric porous substrate 12a itself contained in the LDH separator 12. That is, the LDH separator 12 is densified in such a manner that the pores of the polymeric porous substrate 12a are sufficiently blocked with the hydroxide ion-conducting layered compound 12b, so that the polymeric porous substrate 12a and the hydroxide ion-conducting layered compound 12b are intimately integrated as a highly composite material. Therefore, it can be said that the rigidity and brittleness caused by the hydroxide ion-conducting layered compound 12b, which is a ceramic material, are offset or reduced by the flexibility and strength of the polymeric porous substrate 12a.

[0036] The LDH separator 12 of this embodiment desirably has very few residual pores P (pores not blocked by the hydroxide ion-conducting layered compound 12b). Due to the residual pores P, the LDH separator 12 has an average porosity of, for example, 0.03% or more and less than 1.0%, preferably 0.05% or more and 0.95% or less, more preferably 0.05% or more and 0.9% or less, even more preferably 0.05 to 0.8%, and most preferably 0.05 to 0.5%. With an average porosity within the above range, the pores of the porous substrate 12a are sufficiently blocked by the hydroxide ion-conducting layered compound 12b, resulting in an extremely high degree of denseness, which can more effectively suppress short circuits caused by zinc dendrites. Furthermore, significantly high ionic conductivity can be achieved, allowing the LDH separator 12 to fully function as a dense hydroxide ion-conducting separator. The average porosity can be measured by a) polishing the cross section of the LDH separator using a cross-section polisher (CP), b) obtaining two cross-sectional images of the functional layer at a magnification of 50,000 times using an FE-SEM (field emission scanning electron microscope), and c) calculating the porosity of each of the two fields using image inspection software (e.g., HDevelop, manufactured by MVTec Software) based on the image data of the obtained cross-sectional images, and then calculating the average value of the obtained porosities.

[0037] The LDH separator 12 is a separator containing a hydroxide ion-conducting layered compound 12b. When incorporated into a zinc secondary battery, the LDH separator 12 separates the positive and negative electrode plates in a manner that allows hydroxide ion conductivity. That is, the LDH separator 12 functions as a hydroxide ion-conducting dense separator. Therefore, the LDH separator 12 is gas-impermeable and / or water-impermeable. Therefore, the LDH separator 12 is preferably densified to the extent that it is gas-impermeable and / or water-impermeable. As used herein, "gas-impermeable" means that, as described in Patent Documents 2 and 3, even when helium gas is brought into contact with one side of a test object in water at a differential pressure of 0.5 atm, no bubbles are generated from the other side due to helium gas. As used herein, "water-impermeable" means that water in contact with one side of a test object does not permeate to the other side, as described in Patent Documents 2 and 3. That is, the LDH separator 12 being gas-impermeable and / or water-impermeable means that the LDH separator 12 is dense enough to prevent gas or water from passing through, and is not a porous film or other porous material that is water- or gas-permeable. This allows the LDH separator 12 to selectively pass only hydroxide ions due to its hydroxide ion conductivity, allowing it to function as a battery separator. This configuration is extremely effective in physically preventing zinc dendrites generated during charging from penetrating the separator, thereby preventing short circuits between the positive and negative electrodes. Because the LDH separator 12 has hydroxide ion conductivity, it allows the efficient movement of hydroxide ions required between the positive and negative electrode plates, thereby realizing charge / discharge reactions in the positive and negative electrode plates.

[0038] The LDH separator 12 preferably has a He permeability per unit area of ​​3.0 cm / min·atm or less, more preferably 2.0 cm / min·atm or less, and even more preferably 1.0 cm / min·atm or less. A separator with a He permeability of 3.0 cm / min·atm or less can extremely effectively suppress Zn permeation (typically, permeation of zinc ions or zincate ions) in an electrolyte. Thus, the separator of this embodiment is thought in principle to be able to effectively suppress the growth of zinc dendrites when used in a zinc secondary battery by significantly suppressing Zn permeation. The He permeability is measured through a process of supplying He gas to one side of the separator to allow He gas to permeate the separator, and a process of calculating the He permeability to evaluate the denseness of the hydroxide ion conductive dense separator. The He permeability is calculated by the formula F / (P×S), where F is the amount of He gas permeated per unit time, P is the differential pressure applied to the separator when He gas permeates, and S is the membrane area through which He gas permeates. By evaluating gas permeability using He gas in this way, it is possible to evaluate the presence or absence of denseness at an extremely high level, and as a result, it is possible to effectively evaluate a high level of denseness, such as preventing the permeation of substances other than hydroxide ions (especially Zn, which causes zinc dendrite growth) as much as possible (permeating only trace amounts). This is because He gas has the smallest structural unit among the wide variety of atoms or molecules that can constitute gas, and is also extremely low in reactivity. In other words, He does not form molecules, but constitutes He gas as a single He atom. In this regard, hydrogen gas is H ２ Because it is composed of molecules, a single He atom is smaller as a gas constituent unit. ２ He gas is flammable and therefore dangerous. By using the He gas permeability index defined by the above formula, it is possible to easily and objectively evaluate the density of a separator, regardless of the size of the sample or the measurement conditions. In this way, it is possible to easily, safely, and effectively evaluate whether a separator has a sufficiently high density suitable for use in a zinc secondary battery.

[0039] Measurement of He permeability can be preferably carried out according to the following procedure. First, a He permeability measurement system 310 shown in Figures 5A and 5B is constructed. The He permeability measurement system 310 is configured so that He gas from a gas cylinder filled with He gas is supplied to a sample holder 316 via a pressure gauge 312 and a flow meter 314 (digital flow meter), and the He gas permeates from one side to the other side of an LDH separator 318 held by the sample holder 316 and is then discharged.

[0040] The sample holder 316 has a structure including a gas supply port 316a, a sealed space 316b, and a gas exhaust port 316c, and is assembled as follows. First, adhesive 322 is applied along the outer periphery of the LDH separator 318, and the LDH separator 318 is attached to a jig 324 (made of ABS resin) with a central opening. Butyl rubber packings are placed at the upper and lower ends of the jig 324 as sealing members 326a, 326b. Furthermore, the sealing members 326a, 326b are sandwiched from the outside by support members 328a, 328b (made of PTFE) with flanged openings. Thus, the LDH separator 318, jig 324, sealing member 326a, and support member 328a define a sealed space 316b. The support members 328a and 328b are tightly fastened together with screw fastening means 330 so that He gas does not leak from any part other than the gas exhaust port 316c. A gas supply pipe 334 is connected via a joint 332 to the gas supply port 316a of the sample holder 316 thus assembled.

[0041] Next, He gas was supplied to the He permeability measurement system 310 via a gas supply pipe 334 and allowed to permeate through an LDH separator 318 held in a sample holder 316. At this time, the gas supply pressure and flow rate were monitored by a pressure gauge 312 and a flow meter 314. After permeating the He gas for 1 to 30 minutes, the He permeability was calculated. The He permeability was calculated based on the amount of He gas permeated per unit time F (cm ３ / min), the differential pressure P (atm) applied to the LDH separator during He gas permeation, and the membrane area S (cm ２ ) was used to calculate the He gas permeation amount F (cm ３ / min) was read directly from the flow meter 314. The differential pressure P was measured using a gauge pressure read from the pressure gauge 312. The He gas was supplied so that the differential pressure P was in the range of 0.05 to 0.90 atm.

[0042] In the LDH separator 12, the pores of the porous substrate 12a are filled with hydroxide ion-conducting layered compounds 12b, which are LDHs and / or LDH-like compounds. As is generally known, LDHs are composed of multiple hydroxide base layers and intermediate layers interposed between these multiple hydroxide base layers. The hydroxide base layers are mainly composed of metal elements (typically metal ions) and OH groups. The intermediate layers of the LDHs are composed of anions and H ２ The anion is a monovalent or higher anion, preferably a monovalent or divalent ion. Preferably, the anion in LDH is OH. － and / or CO ３ ２－ Furthermore, LDH has excellent ionic conductivity due to its inherent properties.

[0043] Generally, LDH is ２＋ １－ｘ M ３＋ ｘ (OH) ２ A ｎ－ ｘ／ｎ ・mH ２ O (in the formula, M ２＋ is a divalent cation, and M ３＋ is a trivalent cation, and A ｎ－ is an n-valent anion, n is an integer of 1 or more, x is 0.1 to 0.4, and m is 0 or more. ２＋ can be any divalent cation, but preferred examples include Mg ２＋ , Ca ２＋ and Zn ２＋ More preferably, Mg ２＋ It is. ３＋ can be any trivalent cation, but preferred examples include Al ３＋ or Cr ３＋ More preferably, Al ３＋ It is. Aｎ－ can be any anion, but preferred examples include OH － and CO ３ ２－ Therefore, in the above basic composition formula, M ２＋ is Mg ２＋ Including M ３＋ Al ３＋ Including A ｎ－ OH － and / or CO ３ ２－ n is an integer of 1 or more, preferably 1 or 2. x is 0.1 to 0.4, preferably 0.2 to 0.35. m is an arbitrary number representing the number of moles of water, and is a real number of 0 or more, typically greater than 0 or 1 or more. However, the above basic composition formula is merely a formula of a "basic composition" typically exemplified for LDH, and the constituent ions can be replaced as appropriate. For example, in the above basic composition formula, M ３＋ may be partially or entirely replaced with a cation having a valence of 4 or more. In this case, the anion A ｎ－ The coefficient x / n may be changed as appropriate.

[0044] For example, the hydroxide base layer of the LDH may contain Ni, Al, Ti and OH groups. The intermediate layer may contain anions and H as described above. ２ O. The alternately laminated structure of the hydroxide base layer and intermediate layer itself is basically the same as the alternately laminated structure of commonly known LDHs, but the LDH of this embodiment can exhibit excellent alkaline resistance by configuring the hydroxide base layer of the LDH with predetermined elements or ions including Ni, Al, Ti, and OH groups. The reason for this is not necessarily clear, but it is thought that in the LDH of this embodiment, Al, which was previously thought to be easily eluted in alkaline solutions, becomes less eluted in alkaline solutions due to some kind of interaction with Ni and Ti. Nevertheless, the LDH of this embodiment can also exhibit high ionic conductivity suitable for use as a separator for alkaline secondary batteries. Ni in the LDH can take the form of nickel ions. Nickel ions in the LDH are typically Ni ２＋ It is thought that Ni３＋ The valence of Al in LDH is not particularly limited, as it may be other valences such as Al. The Al in LDH may take the form of an aluminum ion. The aluminum ion in LDH is typically Al ３＋ However, other valences are possible, so there is no particular limitation. Ti in LDH can take the form of a titanium ion. Titanium ions in LDH are typically Ti ４＋ However, Ti ３＋ The hydroxide basic layer may have other valences such as Ni, Al, Ti, and OH groups, and is not particularly limited. The hydroxide basic layer may contain other elements or ions as long as it contains Ni, Al, Ti, and OH groups. However, it is preferable that the hydroxide basic layer contains Ni, Al, Ti, and OH groups as its main components. That is, it is preferable that the hydroxide basic layer is mainly composed of Ni, Al, Ti, and OH groups. Therefore, the hydroxide basic layer is typically composed of Ni, Al, Ti, OH groups, and, in some cases, inevitable impurities. The inevitable impurities are any elements that may be inevitably mixed in during the manufacturing process, and may be mixed into the LDH from, for example, the raw materials or substrate. As mentioned above, the valences of Ni, Al, and Ti are not necessarily certain, so it is impractical or impossible to strictly specify the LDH by a general formula. Even if the hydroxide basic layer is mainly composed of Ni, Al, Ti, and OH groups, ２＋ , Al ３＋ , Ti ４＋ and OH groups, the corresponding LDH has the general formula: Ni ２＋ １－ｘ－ｙ Al ３＋ ｘ Ti ４＋ ｙ (OH) ２ A ｎ－ （ｘ＋２ｙ）／ｎ ・mH ２ O (in the formula, A ｎ－ is an n-valent anion, n is an integer of 1 or more, preferably 1 or 2, 0<x<1, preferably 0.01≦x≦0.5, 0<y<1, preferably 0.01≦y≦0.5, 0<x+y<1, m is 0 or more, typically a real number greater than 0 or 1 or more. However, the above general formula should be understood as merely a "basic composition", and Ni ２＋ , Al ３＋ , Ti４＋ It should be understood that these elements can be replaced with other elements or ions (including elements or ions of the same element with different valencies and elements or ions that may be unavoidably mixed in during the manufacturing process) to the extent that the basic properties of LDH are not impaired.

[0045] The LDH-like compound may not be called LDH, but is a hydroxide and / or oxide with a layered crystal structure similar to LDH. Preferred LDH-like compounds will be described later. By using an LDH-like compound, which is a hydroxide and / or oxide with a layered crystal structure having a predetermined composition described later, as a hydroxide ion conductive material instead of conventional LDH, it is possible to provide a hydroxide ion conductive separator that has excellent alkali resistance and can more effectively suppress short circuits caused by zinc dendrites.

[0046] As described above, the LDH separator 12 comprises a hydroxide ion-conducting layered compound 12b and a porous substrate 12a (typically consisting of the porous substrate 12a and the hydroxide ion-conducting layered compound 12b), with the hydroxide ion-conducting layered compound filling the pores of the porous substrate so that the LDH separator 12 is hydroxide ion-conductive and gas-impermeable (and therefore functions as an LDH separator exhibiting hydroxide ion conductivity). It is particularly preferred that the hydroxide ion-conducting layered compound 12b be incorporated throughout the entire thickness of the polymeric porous substrate 12a.

[0047] The porous substrate 12a is made of a polymer material. The polymer porous substrate 12a has the following advantages: 1) flexibility (hence, it is less likely to crack even when thinned); 2) ease of achieving high porosity; 3) ease of achieving high conductivity (because the thickness can be reduced while increasing porosity); and 4) ease of manufacturing and handling. Furthermore, by utilizing the advantage of flexibility (1), 5) an LDH separator including a porous substrate made of a polymer material can be easily folded or sealed. Preferred examples of polymer materials include polystyrene, polyethersulfone, polypropylene, epoxy resin, polyphenylene sulfide, fluororesin (e.g., tetrafluororesin: PTFE), cellulose, nylon, polyethylene, and any combination thereof. More preferred examples of thermoplastic resins suitable for hot pressing include polystyrene, polyethersulfone, polypropylene, epoxy resin, polyphenylene sulfide, fluororesin (e.g., tetrafluororesin: PTFE), nylon, polyethylene, and any combination thereof. All of the above-mentioned preferred materials are alkali-resistant to the battery electrolyte. Particularly preferred polymeric materials are polyolefins such as polypropylene and polyethylene, with polypropylene or polyethylene being the most preferred, due to their excellent hot water resistance, acid resistance, and alkali resistance, and low cost. When the porous substrate is made of a polymeric material, it is particularly preferred that the hydroxide ion-conducting layered compound is incorporated throughout the entire thickness of the porous substrate (e.g., most or almost all of the pores inside the porous substrate are filled with the hydroxide ion-conducting layered compound). Commercially available microporous polymeric membranes can be preferably used as such polymeric porous substrates.

[0048] The LDH separator of this embodiment can be manufactured by (i) preparing a hydroxide ion-conducting layered compound-containing composite material using a polymeric porous substrate according to a known method (see, for example, Patent Documents 1 to 3), and then (ii) pressing the hydroxide ion-conducting layered compound-containing composite material. The pressing method may be, for example, roll pressing, uniaxial pressing, cold isostatic pressing (CIP), or the like, but is not particularly limited. Roll pressing is preferred. This pressing is preferably performed while heating, as this softens the polymeric porous substrate, thereby allowing the pores of the porous substrate to be sufficiently blocked with the hydroxide ion-conducting layered compound. For example, in the case of polypropylene or polyethylene, heating to a temperature of 60 to 200°C is preferred as the temperature at which the substrate is sufficiently softened. Pressing, such as by roll pressing, within this temperature range significantly reduces the average porosity of the LDH separator due to residual pores. As a result, the LDH separator can be extremely densified, thereby more effectively suppressing short circuits caused by zinc dendrites. When performing roll pressing, the shape of the residual pores can be controlled by appropriately adjusting the roll gap and roll temperature, thereby making it possible to obtain an LDH separator with the desired density or average porosity.

[0049] The method for producing the hydroxide ion conductive layered compound-containing composite material (i.e., crude LDH separator) before pressing is not particularly limited, and it can be produced by appropriately modifying the conditions of the known methods for producing LDH-containing functional layers and composite materials (i.e., LDH separators) (see, for example, Patent Documents 1 to 3). For example, (1) a porous substrate is prepared, (2) a titanium oxide sol or a mixed sol of alumina and titania is applied to the porous substrate and heat-treated to form a titanium oxide layer or an alumina-titania layer, and (3) nickel ions (Ni ２＋(4) immersing the porous substrate in a raw material aqueous solution containing ammonium hydroxide and urea; and (5) hydrothermally treating the porous substrate in the raw material aqueous solution to form a hydroxide ion-conducting layered compound-containing functional layer on and / or within the porous substrate. This allows the production of a hydroxide ion-conducting layered compound-containing functional layer and composite material (i.e., LDH separator). In particular, forming a titanium oxide layer or an alumina-titania layer on the porous substrate in step (2) not only provides the raw material for the hydroxide ion-conducting layered compound, but also serves as a starting point for the crystal growth of the hydroxide ion-conducting layered compound, resulting in the formation of a highly densified hydroxide ion-conducting layered compound-containing functional layer in the porous substrate. Furthermore, the presence of urea in step (3) generates ammonia in the solution via hydrolysis of the urea, thereby raising the pH value. This causes coexisting metal ions to form hydroxides, thereby producing a hydroxide ion-conducting layered compound. Furthermore, the hydrolysis is accompanied by the generation of carbon dioxide, allowing the production of a hydroxide ion-conducting layered compound whose anions are carbonate ions.

[0050] In particular, when preparing a composite material (i.e., an LDH separator) in which the porous substrate is made of a polymer material and the functional layer is incorporated throughout the entire thickness of the porous substrate, it is preferable to apply the mixed sol of alumina and titania to the substrate in (2) above using a method that allows the mixed sol to penetrate the entire or most of the substrate. This allows the hydroxide ion-conducting layered compound to ultimately fill most or almost all of the pores inside the porous substrate. Preferred application methods include dip coating and filter coating, with dip coating being particularly preferred. The amount of mixed sol attached can be adjusted by adjusting the number of applications of dip coating or the like. The substrate to which the mixed sol has been applied by dip coating or the like may be dried and then subjected to the above steps (3) and (4).

[0051] LDH-Like Compound According to a preferred embodiment of the present invention, the LDH separator may contain an LDH-like compound. The definition of the LDH-like compound is as described above. Preferred LDH-like compounds are: (a) hydroxides and / or oxides having 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; or (b) hydroxides and / or oxides having a layered crystal structure containing (i) Ti, Y, and optionally Al and / or Mg, and (ii) at least one additional element M selected from the group consisting of In, Bi, Ca, Sr, and Ba; or (c) hydroxides and / or oxides having a layered crystal structure containing Mg, Ti, Y, and optionally Al and / or In, wherein the LDH-like compound in (c) is In(OH). ３ It exists in the form of a mixture with

[0052] According to a preferred aspect (a) of the present invention, the LDH-like compound may be a hydroxide and / or oxide having 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. Therefore, a typical LDH-like compound is a composite hydroxide and / or composite oxide of Mg, Ti, optionally Y, and optionally Al. While the above elements may be replaced with other elements or ions to the extent that the basic properties of the LDH-like compound are not impaired, it is preferable that the LDH-like compound does not contain Ni. For example, the LDH-like compound may further contain Zn and / or K. This can further improve the ionic conductivity of the LDH separator.

[0053] LDH-like compounds can be identified by X-ray diffraction. Specifically, when X-ray diffraction is performed on the surface of an LDH separator, peaks attributable to LDH-like compounds are typically detected in the range of 5°≦2θ≦10°, more typically in the range of 7°≦2θ≦10°. As described above, LDHs are formed by stacking hydroxide base layers, with exchangeable anions and H as intermediate layers. ２O is present. In this regard, when LDH is measured by X-ray diffraction, a peak due to the crystalline structure of LDH (i.e., the (003) peak of LDH) is inherently detected at 2θ = 11 to 12°. In contrast, when an LDH-like compound is measured by X-ray diffraction, a peak is typically detected in the above-mentioned range, which is shifted to a lower angle than the peak position of LDH. Furthermore, the interlayer distance of the layered crystalline structure can be determined by the Bragg equation using 2θ corresponding to the peak due to the LDH-like compound in X-ray diffraction. The interlayer distance of the layered crystalline structure constituting the LDH-like compound determined in this manner is typically 0.883 to 1.8 nm, more typically 0.883 to 1.3 nm.

[0054] In the LDH separator according to the above aspect (a), the atomic ratio of Mg / (Mg + Ti + Y + Al) in the LDH-like compound, as determined by energy dispersive X-ray analysis (EDS), is preferably 0.03 to 0.25, more preferably 0.05 to 0.2. The atomic ratio of Ti / (Mg + Ti + Y + Al) in the LDH-like compound is preferably 0.40 to 0.97, more preferably 0.47 to 0.94. The atomic ratio of Y / (Mg + Ti + Y + Al) in the LDH-like compound is preferably 0 to 0.45, more preferably 0 to 0.37. The atomic ratio of Al / (Mg + Ti + Y + Al) in the LDH-like compound is preferably 0 to 0.05, more preferably 0 to 0.03. Within the above range, the alkali resistance is more excellent, and the effect of suppressing short circuits caused by zinc dendrites (i.e., dendrite resistance) can be more effectively realized. ２＋ １－ｘ M ３＋ ｘ (OH) ２ A ｎ－ ｘ／ｎ ・mH ２ O (in the formula, M ２＋ is a divalent cation, M ３＋ is a trivalent cation, and A ｎ－is an n-valent anion, n is an integer of 1 or more, x is 0.1 to 0.4, and m is 0 or more). In contrast, the atomic ratio in the LDH-like compound generally deviates from the general formula of LDH. Therefore, it can be said that the LDH-like compound in this embodiment generally has a composition ratio (atomic ratio) different from that of conventional LDH. Note that EDS analysis is preferably performed using an EDS analyzer (e.g., X-act, manufactured by Oxford Instruments) by 1) capturing an image at an accelerating voltage of 20 kV and a magnification of 5,000x, 2) performing three-point analysis in point analysis mode with an interval of about 5 μm, 3) repeating the above 1) and 2) once more, and 4) calculating the average value of a total of six points.

[0055] According to another preferred aspect (b) of the present invention, the LDH-like compound may be a hydroxide and / or oxide having a layered crystal structure containing (i) Ti, Y, and optionally Al and / or Mg, and (ii) an additional element M. Thus, a typical LDH-like compound is a composite hydroxide and / or composite oxide of Ti, Y, the additional element M, optionally Al, and optionally Mg. The additional element M is In, Bi, Ca, Sr, Ba, or a combination thereof. While the above elements may be replaced with other elements or ions to the extent that the basic properties of the LDH-like compound are not impaired, it is preferred that the LDH-like compound does not contain Ni.

[0056] In the LDH separator according to the above aspect (b), the atomic ratio of Ti / (Mg+Al+Ti+Y+M) in the LDH-like compound, as determined by energy dispersive X-ray analysis (EDS), is preferably 0.50 to 0.85, more preferably 0.56 to 0.81. The atomic ratio of Y / (Mg+Al+Ti+Y+M) in the LDH-like compound is preferably 0.03 to 0.20, more preferably 0.07 to 0.15. The atomic ratio of M / (Mg+Al+Ti+Y+M) in the LDH-like compound is preferably 0.03 to 0.35, more preferably 0.03 to 0.32. The atomic ratio of Mg / (Mg+Al+Ti+Y+M) in the LDH-like compound is preferably 0 to 0.10, more preferably 0 to 0.02. The atomic ratio of Al / (Mg+Al+Ti+Y+M) in the LDH-like compound is preferably 0 to 0.05, more preferably 0 to 0.04. Within the above range, the alkali resistance is more excellent, and the effect of suppressing short circuits caused by zinc dendrites (i.e., dendrite resistance) can be more effectively realized. Incidentally, LDHs conventionally known for LDH separators have the general formula: M ２＋ １－ｘ M ３＋ ｘ (OH) ２ A ｎ－ ｘ／ｎ ・mH ２ O (in the formula, M ２＋ is a divalent cation, M ３＋ is a trivalent cation, and A ｎ－is an n-valent anion, n is an integer of 1 or more, x is 0.1 to 0.4, and m is 0 or more). In contrast, the atomic ratio in the LDH-like compound generally deviates from the general formula of LDH. Therefore, it can be said that the LDH-like compound in this embodiment generally has a composition ratio (atomic ratio) different from that of conventional LDH. Note that EDS analysis is preferably performed using an EDS analyzer (e.g., X-act, manufactured by Oxford Instruments) by 1) capturing an image at an accelerating voltage of 20 kV and a magnification of 5,000x, 2) performing three-point analysis in point analysis mode with an interval of about 5 μm, 3) repeating the above 1) and 2) once more, and 4) calculating the average value of a total of six points.

[0057] According to yet another preferred embodiment (c) of the present invention, the LDH-like compound is a hydroxide and / or oxide of a layered crystal structure containing Mg, Ti, Y, and optionally Al and / or In, and the LDH-like compound is In(OH) ３ The LDH-like compound of this embodiment is a hydroxide and / or oxide having a layered crystal structure containing Mg, Ti, Y, and optionally Al and / or In. Therefore, a typical LDH-like compound is a composite hydroxide and / or composite oxide of Mg, Ti, Y, optionally Al, and optionally In. Note that the In that can be contained in the LDH-like compound is not only that which is intentionally added to the LDH-like compound, but also In(OH) ３ The above elements may be replaced with other elements or ions to the extent that the basic properties of the LDH-like compound are not impaired, but it is preferable that the LDH-like compound does not contain Ni. Incidentally, LDHs conventionally known for LDH separators are represented by the general formula: M ２＋ １－ｘ M ３＋ ｘ (OH) ２ A ｎ－ ｘ／ｎ ・mH ２ O (in the formula, M ２＋ is a divalent cation, M ３＋ is a trivalent cation, and A ｎ－is an n-valent anion, n is an integer of 1 or more, x is 0.1 to 0.4, and m is 0 or more. In contrast, the atomic ratios in LDH-like compounds generally deviate from the above general formula of LDH. Therefore, it can be said that the LDH-like compounds in this embodiment generally have composition ratios (atomic ratios) different from those of conventional LDHs.

[0058] The mixture according to the above embodiment (c) contains not only LDH-like compounds but also In(OH) ３ (typically LDH-like compounds and In(OH) ３ It is composed of In(OH) ３ The inclusion of In(OH) in the mixture can effectively improve the alkali resistance and dendrite resistance of the LDH separator. ３ The content of In(OH) is preferably an amount that can improve the alkali resistance and dendrite resistance of the LDH separator without substantially impairing the hydroxide ion conductivity, and is not particularly limited. ３ may have a cubic crystal structure, and In(OH) ３ The crystal may be surrounded by an LDH-like compound. ３ can be identified by X-ray diffraction.

[0059] The present invention will be explained in more detail by the following examples, but the present invention is not limited to the following examples.

[0060] Example 1 (1) Preparation of LDH separator An LDH separator (thickness: 20 μm) was prepared by hydrothermally precipitating an Mg-Al-Ti-LDH compound in the pores and on the surface of a polyethylene microporous membrane and then roll-pressing the membrane. This LDH separator was produced by the methods described in Patent Documents 1 to 4 and 7 to 9, and the surface of the LDH separator contained a plurality of plate-like particles composed of the Mg-Al-Ti-LDH compound that had grown in a direction away from the surface of the LDH separator.

[0061] (2) Formation of Interface Layer and Air Electrode Layer The interface layer and air electrode layer were formed by sputtering using the following procedure. First, an LDH separator was placed on a substrate holder in a magnetron sputtering device (ULVAC, Inc., VTR-151M / SRF), and a nickel plate was placed on a target holder arranged parallel to the substrate holder. In this state, argon (Ar) gas was introduced into the chamber of the sputtering device as the sputtering gas, and sputtering was performed for 10 minutes. Ni (electron conductive material) was deposited in the gaps and irregularities between the multiple plate-like particles on the surface of the LDH separator, forming a 30 nm thick interface layer. Next, the target was changed to a graphite plate and a nickel plate (i.e., the graphite plate and the nickel plate were repositioned in the target holder), and the LDH separator with the formed interface layer was placed at an angle to the substrate holder. Argon (Ar) gas was introduced as the sputtering gas, and sputtering was performed for 2 hours to form a 60 nm thick air cathode layer composed of Ni and C. In this way, an air electrode / separator assembly was obtained. In the obtained air cathode layer, Ni was considered to be present in at least one form selected from the group consisting of (i) metal carbide, (ii) metal with carbon as a solid solution, and (iii) a mixture of amorphous carbon and metal particles.

[0062] (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 in solids content equivalent, followed by kneading 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 at 80°C for 14 hours in a vacuum dryer. Cu foil was welded to the current collector portion of the dried negative electrode sheet to obtain a zinc oxide negative electrode.

[0063] (4) Assembly and Evaluation of Evaluation Cell of Separator / Air Electrode Assembly The air electrode / separator assembly prepared in (1) above, a platinum wire (Pt counter electrode, manufactured by BAS Inc.), and an Hg / HgO reference electrode (RE-61AP, manufactured by BAS Inc.) were immersed in a 0.1 mol / L KOH aqueous solution. After bubbling oxygen into the electrolyte for 30 minutes, the discharge and charge characteristics were evaluated under the following conditions. (Discharge Characteristics) Measurement method: Linear sweep voltammetry (LSV) Voltage range: 1.2 to 0 V vs. RHE Sweep rate: 10 mV / sec (Charge Characteristics) Measurement method: Linear sweep voltammetry (LSV) Voltage range: 1.2 to 1.8 V vs. RHE Sweep rate: 10 mV / sec

[0064] The results obtained are shown in Table 1, with a discharge characteristic of 0.01 mA / cm ２ The potential (V vs. RHE) and charging characteristics at 10 mA / cm ２ The graph shows the potential (V vs. RHE) at the time of charging and discharging. The electrode activity of the air electrode was determined based on the oxidation-reduction potential of water (1.23 V vs. RHE), and the smaller the overpotential, the higher the activity was determined to be. In other words, in the case of charging, the lower the potential at which current flows, the higher the charge activity. Conversely, in the case of discharging, the higher the potential at which current flows, the higher the discharge activity. Table 1 shows that the air electrode / separator assembly produced in this example had higher electrode activity in both discharge and charge characteristics than the air electrode / separator assembly (Example 7) in which only carbon was formed as a film.

[0065] (5) Assembly and Evaluation of Zinc-Air Battery Evaluation Cell A zinc oxide negative electrode impregnated with a 5.4 mol / L KOH aqueous solution 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 clamped to the outer periphery of the LDH separator, and tightly fixed with screws to form an evaluation cell. This holding jig had an oxygen inlet on the air electrode side, allowing oxygen to be supplied to the air electrode. The charge / discharge characteristics of the evaluation cell were evaluated under the following conditions using an electrochemical measurement device (VMP-3, manufactured by BioLogic). Air electrode gas: water vapor saturated oxygen (25°C) Charge / discharge current density: 0.01 mA / cm ２ ・Charge / discharge time: 5 minutes charge / 5 minutes discharge

[0066] The resulting charge-discharge curves are shown in Figure 6. From Figure 6, it was found that the evaluation cell prepared in this example could be charged and discharged at a low overvoltage.

[0067] (6) Microstructure Observation of the Air Electrode 1 An AFM (atomic force microscope) cantilever was placed on a substrate holder in a magnetron sputtering apparatus (ULVAC, Inc., VTR-151M / SRF) so that the Si probe of the cantilever was oblique to the substrate holder, while a graphite plate and a nickel plate were placed on the target holder. In this state, argon (Ar) gas was introduced into the chamber of the sputtering apparatus as the sputtering gas, and sputtering was performed for 2 hours to form an air electrode layer. The cross section of the air electrode layer thus formed was observed with an electron microscope to obtain a cross-sectional TEM image. As a result, a microstructure with gaps, as shown in FIG. 7, was observed, specifically a microstructure consisting of multiple air electrode segments spaced 0.1 to 100 nm apart from each other. It was also confirmed that the multiple air electrode segments extended in a direction away from the Si probe, and that the angle between the extension direction of the multiple air electrode segments and the normal direction to the Si probe surface was within the range of 0 to 70 degrees. From these results, it was inferred that the air electrode layer or air electrode segment formed on the LDH separator also had a similar structure.

[0068] (7) Microstructural Observation of the Air Electrode 2 A notched mesh for FIB (focused ion beam) was placed on a substrate holder in a Magnelon sputtering apparatus (ULVAC, Inc., VTR-151M / SRF) so that the mesh was oblique to the substrate holder, while a graphite plate and a nickel plate were placed on the target holder. In this state, argon (Ar) gas was introduced into the chamber of the sputtering apparatus as the sputtering gas, and sputtering was performed for 2 hours to form an air electrode layer. The cross section of the air electrode layer thus formed was observed using an electron microscope (JEOL Ltd., JEM-ARM300F2) at an acceleration voltage of 200 kV, and a cross-sectional TEM image was obtained. As a result, the BF-STEM images shown in FIGS. 8A and 8B and the EDX elemental mapping image shown in FIG. 9 confirmed that a carbon-rich layer 14c (i.e., a carbon-rich surface layer) was formed on the surface of the air electrode segment 14s.

[0069] Example 2 An air electrode / separator assembly was fabricated and evaluated in the same manner as in Example 1, except that a nickel-iron alloy plate (78 Permalloy (78NiFe)) was used instead of a nickel plate as the target, and an interface layer containing Ni and Fe (electron-conducting materials) was formed between the plate-like particles, etc., and an air electrode layer composed of Ni, Fe, and C was formed. In the resulting air electrode layer, Ni and Fe were considered to be present in at least one form selected from the group consisting of (i) metal carbide, (ii) a metal solid solution containing carbon, and (iii) a mixture of amorphous carbon and metal particles. The results are shown in Table 1. The air electrode / separator assembly fabricated in this example was found to have higher electrode activity in both discharge and charge characteristics than an air electrode / separator assembly (Example 7) in which only carbon was formed as the air electrode layer. Furthermore, observation of the air electrode microstructure revealed a microstructure similar to that of Example 1.

[0070] Example 3: An air electrode / separator assembly was fabricated and evaluated in the same manner as in Example 1, except that a nickel-iron alloy plate (45 permalloy (45NiFe)) was used instead of a nickel plate as the target, and an interface layer containing Ni and Fe (electron-conducting materials) was formed between the plate-like particles, etc., and an air electrode layer composed of Ni, Fe, and C was formed. In the resulting air electrode layer, Ni and Fe were considered to be present in at least one form selected from the group consisting of (i) metal carbide, (ii) a metal containing carbon as a solid solution, and (iii) a mixture of amorphous carbon and metal particles. The results are shown in Table 1. The air electrode / separator assembly fabricated in this example was found to have higher electrode activity in both discharge and charge characteristics than an air electrode / separator assembly (Example 7) in which only carbon was formed as the air electrode layer. Furthermore, observation of the air electrode microstructure revealed a microstructure similar to that of Example 1.

[0071] Example 4: An air electrode / separator assembly was fabricated and evaluated in the same manner as in Example 1, except that an iron plate was used as the target instead of a nickel plate, and an interface layer containing Fe (electron conductive material) and an air electrode layer composed of Fe and C were formed between the plate-like particles. In the resulting air electrode layer, Fe was believed to be present in at least one form selected from the group consisting of (i) metal carbide, (ii) a metal solid solution containing carbon, and (iii) a mixture of amorphous carbon and metal particles. The results are shown in Table 1. The air electrode / separator assembly fabricated in this example was found to have higher electrode activity in both discharge and charge characteristics than an air electrode / separator assembly (Example 7) in which only carbon was deposited as the air electrode layer. Furthermore, observation of the air electrode microstructure revealed a microstructure similar to that of Example 1.

[0072] Example 5: An air electrode / separator assembly was fabricated and evaluated in the same manner as in Example 1, except that a manganese plate was used instead of a nickel plate as the target, and an interface layer containing Mn (an electron conductive material) was formed between the plate-like particles, and an air electrode layer composed of Mn and C was formed. In the resulting air electrode layer, Mn was believed to be present in at least one form selected from the group consisting of (i) metal carbide, (ii) a metal solid-dissolved with carbon, and (iii) a mixture of amorphous carbon and metal particles. The results are shown in Table 1. The air electrode / separator assembly fabricated in this example was found to have higher electrode activity in both discharge and charge characteristics than an air electrode / separator assembly (Example 7) in which only carbon was formed as the air electrode layer. Furthermore, observation of the air electrode microstructure revealed a microstructure similar to that of Example 1.

[0073] Example 6: An air electrode / separator assembly was fabricated and evaluated in the same manner as in Example 1, except that a cobalt plate was used instead of a nickel plate as the target, and an interface layer containing Co (an electron conductive material) and an air electrode layer composed of Co and C were formed between the plate-like particles. In the resulting air electrode layer, Co was believed to be present in at least one form selected from the group consisting of (i) metal carbide, (ii) a metal solid solution containing carbon, and (iii) a mixture of amorphous carbon and metal particles. The results are shown in Table 1. The air electrode / separator assembly fabricated in this example was found to have higher electrode activity in both discharge and charge characteristics than an air electrode / separator assembly (Example 7) in which only carbon was deposited as the air electrode layer. Furthermore, observation of the air electrode microstructure revealed a microstructure similar to that of Example 1.

[0074] Example 7 (Comparative) An air electrode / separator assembly was fabricated and evaluated in the same manner as in Example 1, except that an air electrode layer consisting only of C was formed using only a graphite plate instead of a nickel plate as the target. An air electrode / separator assembly was fabricated and evaluated in the same manner as in Example 1. The results are shown in Table 1, and the air electrode / separator assembly fabricated in this example had lower activity in both discharge and charge characteristics than the air electrode / separator assemblies fabricated in Examples 1 to 6.

[0075]

[0076] REFERENCE SIGNS LIST 10 Air electrode / separator assembly 12 Hydroxide ion conductive separator 12a Porous substrate 12b Hydroxide ion conductive layered compound 12p Plate-like particle 13 Interface layer 14 Air electrode layer 14s Air electrode segment 14c Carbon-rich layer 16 Electron conductive material 20 Target 22 Substrate holder

Claims

1. An air electrode / separator assembly comprising: a hydroxide ion-conductive separator containing a hydroxide ion-conductive solid electrolyte; and an air electrode layer having a thickness of 2000 nm or less, provided on one side of the hydroxide ion-conductive separator, the air electrode layer comprising an air electrode material, the air electrode layer having a thickness of 2000 nm or less, wherein the air electrode material comprises at least one metal element selected from the group consisting of Ni, Fe, Co, Mn, V, Mo, W, Cr, Pt, and Pd, in the form of at least one selected from the group consisting of (i) metal carbide, (ii) a metal solid-solved with carbon, and (iii) a mixture of amorphous carbon and metal particles, and thus functions as both an air electrode catalyst and an electron conductor.

2. The air electrode / separator assembly according to claim 1, further comprising an interfacial layer between the hydroxide ion conductive separator and the air electrode layer, the interfacial layer comprising: a plurality of plate-like particles made of a hydroxide ion conductive solid electrolyte grown in a direction away from the surface of the hydroxide ion conductive separator; and an electron conductive material or air electrode material provided so as to fill gaps between the plurality of plate-like particles and / or irregularities formed by the plurality of plate-like particles.

3. The cathode / separator assembly of claim 2, wherein said cathode layer comprises a plurality of cathode segments formed of said cathode material and spaced apart from one another on said interface layer.

4. The air electrode / separator assembly according to claim 3, wherein the plurality of air electrode segments extend in a direction away from the hydroxide ion conductive separator, and the angle formed between the extension direction of the plurality of air electrode segments and the normal direction of the hydroxide ion conductive separator is within the range of 0 to 70 degrees.

5. The air electrode / separator assembly according to claim 3 or 4, wherein the distance between adjacent air electrode segments is 0.1 to 100 nm.

6. The air electrode / separator assembly according to any one of claims 1 to 4, wherein the air electrode segment has a carbon-rich layer on its surface, the carbon-rich layer being a layer having a higher carbon atom content than other portions of the air electrode segment.

7. The air electrode / separator assembly according to any one of claims 1 to 4, wherein the at least one metal element is Ni, or Ni and Fe.

8. The air electrode / separator assembly according to any one of claims 1 to 4, wherein the at least one metal element is Co.

9. The air electrode / separator assembly according to any one of claims 1 to 4, wherein the at least one metal element is Mn.

10. The air electrode / separator assembly according to any one of claims 1 to 4, wherein the at least one metal element is Fe.

11. The air electrode / separator assembly according to any one of claims 2 to 4, wherein the thickness of the interface layer is 500 nm or less, and the thickness of the air electrode layer is 1500 nm or less.

12. The air electrode / separator assembly according to any one of claims 2 to 4, wherein the hydroxide ion conductive material contained in the interfacial layer is a layered double hydroxide (LDH) and / or an LDH-like compound.

13. The air electrode / separator assembly according to any one of claims 1 to 4, wherein the hydroxide ion-conducting separator is a layered double hydroxide (LDH) separator.

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

15. A metal-air secondary battery comprising the air electrode / separator assembly according to any one of claims 1 to 4, a metal negative electrode, and an electrolyte, the electrolyte being isolated from the air electrode layer via the hydroxide ion-conducting separator.

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