Air electrode / separator junction and metal-air secondary 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

Technical Field

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

Background Art

[0002] A metal-air secondary battery is cited as one of the candidate innovative batteries. Since oxygen, which is the positive electrode active material, is supplied from the air in a metal-air secondary battery, the space in the battery container can be maximally utilized for filling the negative electrode active material, and thus a high energy density can be realized in principle. For example, in a zinc-air secondary battery using zinc as the negative electrode active material, an alkaline aqueous solution such as potassium hydroxide is used as the electrolyte, and a separator (partition wall) is used to prevent a short circuit between the positive and negative electrodes. During discharge, as shown in the following reaction formula, O2 is reduced on the air electrode (positive electrode) side to generate OH - while zinc is oxidized at the negative electrode to generate ZnO. Positive electrode: O2 + 2H2O + 4e - → 4OH - Negative electrode: 2Zn + 4OH - → 2ZnO + 2H2O + 4e -

[0003] Thus, in a metal-air secondary battery, a metal is used for the negative electrode, and oxygen and water in the air are used as active materials for the air electrode (positive electrode). On the air electrode, an oxygen reduction reaction (ORR) that generates hydroxide ions occurs during discharge, and an oxygen evolution reaction (OER) that consumes hydroxide ions to generate oxygen occurs during charging. In order to promote this ORR / OER reaction, a material having hydroxide ion conductivity is used.

[0004] As a kind of hydroxide ion conductive material, layered double hydroxide (LDH) is known. LDH has the general formula [M 2+ 1-x M 3+ x (OH)2][An -x / n ·zH2O] (M 2+ is a divalent metal ion, M 3+Represented as (where A is a trivalent metal ion and An is an anion), LDH has a characteristic layered structure in which negatively charged anions and water molecules are placed between positively charged hydroxide layers. In addition to being applied as an adsorbent and catalytic material, LDH is used as an anion conductive material by substituting or moving the anions between the layers. In particular, LDH is used as a hydroxide ion conductive material when the anions between the layers are hydroxide ions. Recently, an air electrode using LDH as a hydroxide ion conductive material has been proposed.

[0005] Incidentally, in zinc-based secondary batteries such as zinc-air batteries and nickel-zinc batteries, it is known that during charging, metallic zinc deposits in a dendrite-like manner from the negative electrode, penetrates the voids in the separator (such as nonwoven fabric), and reaches the positive electrode, resulting in a short circuit. Such short circuits caused by zinc dendrites shorten the charge-discharge lifespan. Furthermore, in zinc-air batteries, there is also the problem that carbon dioxide in the air passes through the air electrode and dissolves in the electrolyte, depositing alkaline carbonates and degrading battery performance. Similar problems can also occur in lithium-air batteries.

[0006] To address the above problems, batteries equipped with layered double hydroxide (LDH) separators that selectively allow hydroxide ions to permeate while preventing the penetration of zinc dendrites have been proposed. For example, Patent Document 1 (WO2013 / 073292) discloses the provision of an LDH separator 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 due to zinc dendrites and the ingress of carbon dioxide. Furthermore, Patent Document 2 (WO2016 / 076047) discloses a separator structure equipped with an LDH separator fitted or bonded to a resin outer frame, and discloses that the LDH separator has such high density that it is impermeable to gas and / or water. This document also discloses that the LDH separator can be compounded with a porous substrate. Furthermore, Patent Document 3 (WO2016 / 067884) discloses various methods for obtaining a composite material (LDH separator) by forming an LDH density film on the surface of a porous substrate. This method includes the step of uniformly attaching a starting material that can provide a starting point for LDH crystal growth to the porous substrate, and subjecting the porous substrate to hydrothermal treatment in an aqueous raw material solution to form an LDH density film on the surface of the porous substrate. Patent Document 4 (WO2019 / 124270) discloses an LDH separator comprising a porous substrate made of a polymer material and layered double hydroxide (LDH) that seals the pores of the porous substrate, wherein the linear transmittance at a wavelength of 1000 nm is 1% or more.

[0007] Furthermore, in the field of metal-air secondary batteries such as zinc-air secondary batteries, air electrode / separator assemblies in which an air electrode layer is provided on an LDH separator have been proposed. Patent document 5 (WO2015 / 146671) discloses an air electrode / separator assemblies comprising an air electrode layer on an LDH separator containing an air electrode catalyst, an electronically conductive material, and a hydroxide ion conductive material. Patent document 6 (WO2020 / 246177) discloses an air electrode / separator assemblies comprising a hydroxide ion conductive separator, an interface layer covering one side of the separator containing a hydroxide ion conductive material and a conductive material, and an air electrode layer provided on the interface layer and containing an outermost catalyst layer composed of a porous current collector and a layered double hydroxide (LDH) covering its surface. Patent documents 5 and 6 also disclose the use of LDH as the hydroxide ion conductive material.

[0008] Furthermore, LDH-like compounds are known as hydroxides and / or oxides with a layered crystalline structure that are similar to LDH, although they cannot be called LDH themselves. These compounds exhibit hydroxide ion conductivity characteristics so similar to LDH that they can be collectively referred to as hydroxide ion-conducting layered compounds. For example, Patent Document 7 (WO2020 / 255856) discloses a hydroxide ion-conducting separator comprising a porous substrate and a layered double hydroxide (LDH)-like compound that seals the pores of the porous substrate, wherein the LDH-like compound is a hydroxide and / or oxide with a layered crystalline structure containing Mg and at least one element, including at least Ti, selected from the group consisting of Ti, Y, and Al. Patent Document 8 (WO2021 / 229916) also discloses an LDH separator using an LDH-like compound comprising (i) Ti, Y, and optionally Al and / or Mg, and (ii) at least one additive element M selected from the group consisting of In, Bi, Ca, Sr, and Ba. Furthermore, Patent Document 9 (WO2021 / 229917) discloses an LDH separator containing a mixture of an LDH-like compound and In(OH)3, wherein the LDH-like compound is a layered crystalline hydroxide and / or oxide containing Mg, Ti, Y, and optionally Al and / or In. According to the separators disclosed in Patent Documents 7 to 9, compared to conventional LDH separators, they have superior alkali resistance and can more effectively suppress short circuits caused by zinc dendrites. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] WO2013 / 073292 [Patent Document 2] WO2016 / 076047 [Patent Document 3] WO2016 / 067884 [Patent Document 4] WO2019 / 124270 [Patent Document 5] WO2015 / 146671 [Patent Document 6] WO2020 / 246177 [Patent Document 7] WO2020 / 255856 [Patent Document 8] WO2021 / 229916 [Patent Document 9] WO2021 / 229917 [Non-Patent Document]

[0010] [Non-Patent Document 1] Sun et al., "Single-layer nanosheets with exceptionally high and anisotropic hydroxyl ion conductivity" Sci. Adv. 3, e1602629, 2017 [Summary of the Invention]

[0011] As described above, the metal-air secondary battery using the LDH separator has excellent advantages of being able to prevent both short-circuiting between the positive and negative electrodes due to metal dendrites and the mixing of carbon dioxide. In addition, due to the denseness of the LDH separator, there is also an advantage that evaporation of moisture contained in the electrolyte can be suppressed. However, since the LDH separator blocks the penetration of the electrolyte into the air electrode, there will be no electrolyte in the air electrode layer. Therefore, compared with a zinc-air secondary battery using a general separator (for example, a porous polymer separator) that allows the penetration of the electrolyte into the air electrode, the hydroxide ion conductivity tends to be low, leading to a decrease in charge-discharge performance. Therefore, an air electrode / separator conjugate that exhibits excellent charge-discharge performance while having the advantages of using the LDH separator is desired. [[ID=--]]

[0012] [[ID=--]] Therefore, in recent years, the use of LDH as a hydroxide ion conductive material for an air electrode has been proposed (see Patent Documents 5 and 6), but there is still much room for improvement in LDH. LDH is generally in the form of platelet particles having an arbitrary planar shape. When using LDH platelet particles as an anion conductive material such as a hydroxide ion conductive material, it is desirable that these platelet particles are in contact with each other, and particularly desirable that they are continuous in the planar direction in a form that overlaps or contacts each other. In this regard, in the prior art, there has been a report of improving the hydroxide ion conductivity in the planar direction by applying a material obtained by exfoliating the layers of LDH on a nanoscale to the surface of a thin film (for example, 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 existence of the LDH platelet particles simply by mixing them with raw material powders such as an air electrode catalyst, and particularly difficult to continuously arrange them so that the LDH platelet particles are connected in the planar direction. For this reason, there has been a problem that the hydroxide ion conductivity of LDH cannot be fully utilized in the air electrode.

[0013] The present inventors have now found that, when a metal-air secondary battery is formed 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 on the surface of a hydrophilic fiber so as to be connected to each other, a decrease in charge-discharge overvoltage can be achieved.

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

[0015] According to the present invention, the following aspects are provided. [Aspect 1] A hydroxide ion conductive separator, and A catalyst layer comprising an air electrode catalyst, a hydroxide ion conductive composite material, a conductive material, and a binder covers one side of the hydroxide ion conductive separator. A gas diffusion electrode is provided on the side of the catalyst layer opposite to the hydroxide ion conductive separator, The hydroxide ion conductive composite material is provided with, Hydrophilic fibers, A plurality of hydroxide ion conductive particles are supported on the surface of the hydrophilic fiber in a connected manner, An air electrode / separator assembly, including an air electrode / separator joint. [Aspect 2] The air electrode / separator assembly according to embodiment 1, wherein the hydrophilic fiber is at least one selected from the group consisting of cellulose nanofibers, chitin nanofibers, and chitosan nanofibers. [Aspect 3] The air electrode / separator joint according to embodiment 1 or 2, wherein the hydrophilic fiber is cellulose nanofiber. [Aspect 4] The air electrode / separator joint according to any one of embodiments 1 to 3, wherein the hydrophilic fiber has a length of 0.1 to 100 μm. [Aspect 5] The air electrode / separator joint according to any one of embodiments 1 to 4, wherein the hydrophilic fiber has a length of 0.2 to 50 μm. [Aspect 6] The air electrode / separator joint according to any one of embodiments 1 to 5, wherein the hydrophilic fiber has a length of 5 to 50 μm. [Aspect 7] The air electrode / separator assembly according to any one of embodiments 1 to 6, wherein the hydroxide ion conductive particles are composed of layered double hydroxides (LDH). [Aspect 8] The layered double hydroxide (LDH) comprises at least two elements selected from the group consisting of Ni, Fe, Mg, Al, and Ti as constituent elements, 7 The air electrode / separator assembly described above. [Aspect 9] The air electrode / separator assembly according to embodiment 8, wherein the aforementioned at least two elements include Mg and Al. [Aspect 10] The air electrode / separator assembly according to any one of embodiments 1 to 9, wherein in the catalyst layer, the content of the hydroxide ion conductive composite material relative to the total amount of the air electrode catalyst, the hydroxide ion conductive composite material, the conductive material, and the binder in terms of solid matter is 10 to 40 volume percent. [Aspect 11] The air electrode / separator assembly according to any one of embodiments 1 to 10, wherein the content of the hydroxide ion conductive composite material in the catalyst layer is 10 to 30 volume percent relative to the total amount of the air electrode catalyst, the hydroxide ion conductive composite material, the conductive material, and the binder in terms of solid matter. [Aspect 12] The air electrode / separator assembly according to any one of embodiments 1 to 11, wherein the hydroxide ion conductive separator is a layered double hydroxide (LDH) separator. [Aspect 13] The air electrode / separator joint according to embodiment 12, wherein the LDH separator is compounded with a porous substrate. [Aspect 14] A metal-air secondary battery comprising an air electrode / separator junction according to any one of embodiments 1 to 13, a metal negative electrode, and an electrolyte, wherein the electrolyte is isolated from the air electrode layer via the hydroxide ion conductive separator. [Brief explanation of the drawing]

[0016] [Figure 1] This is a schematic cross-sectional view conceptually illustrating an air electrode / separator joint according to one aspect of the present invention. [Figure 2] This figure conceptually illustrates the microstructure of the catalyst layer in an air electrode / separator assembly according to one aspect of the present invention. To focus on the microstructure of the catalyst layer, other components such as gas diffusion electrodes and current collectors are omitted. For a similar purpose, the LDH separator is also depicted partially excluded near the catalyst layer. [Figure 3]This is the XRD profile obtained for the Mg-Al-LDH prepared in Example 1. [Figure 4] This is an SEM image (magnification: 25,000x) of the Mg-Al-LDH-supported cellulose nanofiber (CNF) prepared in Example 1. [Figure 5] This is an SEM image (magnification: 50,000x) of the Mg-Al-LDH-supported CNF prepared in Example 1. [Figure 6] This is an SEM image showing the measurement points in the EDX quantitative analysis of Mg-Al-LDH-supported CNF prepared in Example 1. [Figure 7] This is the EDX profile obtained for the Mg-Al-LDH prepared in Example 1. [Figure 8] The charge-discharge evaluation results for the zinc-air secondary batteries fabricated in Examples 1-3 are shown. [Modes for carrying out the invention]

[0017] Air electrode / separator assembly Figure 1 shows one embodiment of an air electrode / separator assembly. The air electrode / separator assembly 10 shown in Figure 1 comprises a hydroxide ion conductive separator 12 and an air electrode layer 13, the air electrode layer 13 comprising a catalyst layer 14, a gas diffusion electrode 16, and optionally an air electrode current collector 18. The catalyst layer 14, as conceptually shown in Figure 2, includes an air electrode 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 includes hydrophilic fibers 28 and a plurality of hydroxide ion conductive particles 30 supported on the surface of the hydrophilic fibers 28 in a connected manner. Thus, by constructing the air electrode / separator assembly 10 using a hydroxide ion conductive composite material 22 in which multiple hydroxide ion conductive particles 30 are supported on the surface of a hydrophilic fiber 28 in a linked manner, it is possible to reduce the charge and discharge overvoltage when used as a metal-air secondary battery.

[0018] In other words, as mentioned above, LDH generally exists in the form of plate-like particles having any planar shape. When LDH plate-like particles are used as anion-conducting materials such as hydroxide ion-conducting materials, it is desirable that these plate-like particles are in contact with each other, and in particular, it is desirable that they are continuous in the planar direction in a manner that overlaps or contacts each other. However, when using LDH plate-like particles as a hydroxide ion-conducting material in the air electrode of a metal-air battery, it is difficult to precisely control the state of the LDH plate-like particles by simply mixing them with raw material powders such as air electrode catalysts, and it has been particularly difficult to arrange the LDH plate-like particles continuously in the planar direction. For this reason, there has been a problem in that the hydroxide ion conductivity of LDH cannot be fully utilized in the air electrode. In this regard, the present invention makes it possible to form a continuum of hydroxide ion conductive particles 30 (e.g., LDH plate-shaped particles) on the surface of a hydrophilic fiber 28 by synthesizing and supporting the hydroxide ion conductive particles 30 (e.g., LDH plate-shaped particles) so that they are interconnected (for example, so that the surfaces of the LDH plate-shaped particles are parallel to each other), thereby enabling the hydroxide ion conductive particles 30 (e.g., LDH plate-shaped particles) to be formed into a continuum (i.e., an aggregate of particles that are continuously connected to each other in the plane direction). This makes it possible to utilize hydroxide ion conductive particles such as LDH particles as a continuum, which was difficult in the conventional art, and enables the use of excellent anions. transmission This allows for the realization of properties (particularly hydroxide ion conductivity). Therefore, by using such a hydroxide ion conductive composite material 22 as the hydroxide ion conductive material for the air electrode of a metal-air battery, the hydroxide ion conductivity of the air electrode layer 13 (particularly the catalyst layer 14) can be improved. Furthermore, by constructing an air electrode / separator assembly 10 equipped with such an air electrode layer 13 (particularly the catalyst layer 14), it is believed that when used as a metal-air secondary battery, the reaction rate of the charge-discharge reaction will increase due to the improved hydroxide ion conductivity, resulting in a reduction in charge-discharge overvoltage.

[0019] The hydroxide ion conductive separator 12 is not particularly limited as long as it is a separator capable of separating the air electrode layer 13 and the negative electrode layer in a zinc-air secondary battery in a manner that allows hydroxide ions to conduct. Typically, however, it is a separator that contains a hydroxide ion conductive solid electrolyte and selectively allows hydroxide ions to pass through by exclusively utilizing its hydroxide ion conductivity. Preferred hydroxide ion conductive solid electrolytes are layered double hydroxides (LDH) and / or LDH-like compounds. Therefore, it is preferable that the hydroxide ion conductive separator 12 is an LDH separator. In this specification, "LDH separator" is defined as a separator containing LDH and / or an LDH-like compound that selectively allows hydroxide ions to pass through by exclusively utilizing the hydroxide ion conductivity of LDH and / or an LDH-like compound. In this specification, "LDH-like compound" is a layered crystalline hydroxide and / or oxide that has hydroxide ion conductivity but may not be called LDH, and can be considered an equivalent of LDH. However, in a broader definition, "LDH" can also be interpreted to include not only LDH but also LDH-like compounds. The LDH separator is preferably compounded with a porous substrate. Therefore, the LDH separator is preferably compounded with the porous substrate in a form in which LDH and / or an LDH-like compound fills the pores of the porous substrate. That is, in a preferred LDH separator, the LDH and / or LDH-like compound fills the pores of the porous substrate so as to exhibit hydroxide ion conductivity and gas impermeability (and thus function as a hydroxide ion conductive LDH separator). The porous substrate is preferably made of a polymer material, and it is particularly preferable that the LDH and / or LDH-like compound is incorporated throughout the entire thickness of the polymer material porous substrate. 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 an air electrode catalyst 20 (e.g., a charging catalyst and a discharging catalyst), a hydroxide ion conductive composite material 22, a conductive material 24, and a binder 26.

[0021] The catalyst 20 for the air electrode contained in the catalyst layer 14 has a spherical, plate-like, or fibrous form and is dispersed in the catalyst layer 14. Separate catalysts may be used for charging and discharging, or a single catalyst may be responsible for both charging and discharging reactions. The catalyst 20 may also be combined with 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, but carbon-based catalysts, oxide catalysts, or metal catalysts are preferable for discharging, while hydroxide catalysts, oxide catalysts, or carbon-based catalysts are preferable for charging. The catalyst 20 is preferably in the form of fine particles in order 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 conduction 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 supported on the surface of the hydrophilic fibers 28 in a connected manner. These "a plurality of hydroxide ion conductive particles supported in a connected manner" can be specified as hydroxide ion conductive particles 30 supported on the surface of the hydrophilic fibers 28 that are in contact with adjacent hydroxide ion conductive particles 30 at least one location. The hydrophilic fibers 28 are not particularly limited as long as hydroxyl groups (OH groups) are coordinated to or can coordinate to their surface. On the surface of such hydrophilic fibers 28, hydroxide ion conductive particles 30 (e.g., LDH plate-like particles) can be synthesized and supported on the surface of the hydrophilic fibers 28 in a connected manner (e.g., so that the surfaces of the LDH plate-like particles are parallel to each other) by coprecipitation or the like. In other words, by using hydrophilic fibers 28, it becomes possible to form hydroxide ion conductive particles 30 (e.g., LDH plate-like particles) into a continuum (i.e., an aggregate of particles continuously connected to each other in the planar direction). This is presumed to be because the hydroxyl groups (OH groups) coordinated to the surface of the hydrophilic fibers 28 are changed to a form in which hydrogen ions are abstracted (O- groups) by the action of a strong base such as NaOH in a coprecipitation method, and these groups electrostatically attract metal ions in the raw material aqueous solution containing the constituent elements of LDH, causing LDH plate-like particles to precipitate on the surface.

[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 the 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 nm) to long (e.g., several tens of μm), but longer fiber lengths are preferred. This is because longer fiber lengths result in longer hydroxide ion conduction distances, which allows hydroxide ions to spread sufficiently throughout the catalyst layer 14 without interruption of the hydroxide ion conduction path necessary for the charge-discharge reaction in the catalyst layer 14.

[0024] The hydroxide ion conductive particles 30 are not particularly limited as long as they are particles that conduct hydroxide ions, but as described above, they are preferably composed of layered double hydroxides (LDH). In this case, the hydroxide ion conductive particles 30 may be LDH plate-shaped 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 it is more preferable that these at least two elements include Mg and Al. By including at least two elements, Mg and Al, better anions are formed. transmission Properties (e.g., hydroxide ion conductivity) can be achieved. In this case, the Al / Mg atomic ratio determined by energy-dispersive X-ray spectroscopy (EDX) of Mg-Al-LDH is preferably 0.30 to 0.55, and more preferably 0.40 to 0.55. When Mg-Al-LDH with 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, resulting in a further increase in the reaction rate of the charge-discharge reaction and a further decrease in the charge-discharge overpotential. In LDH, it is preferable that the interlayer anions are hydroxide ions. That is, LDH is [M2+ 1-x M 3+ x (OH)2][An -x / n ·zH2O](M 2+ is Mg 2+ Includes M 3+ is Al 3+ Includes, An -x / n OH - It is preferable that the LDH is expressed by the general formula (where 0.2 ≤ x ≤ 0.4, and z is any real number greater than 0). LDH can be synthesized by coprecipitation. For example, an aqueous solution of raw materials containing the constituent elements of LDH can be dropped into an aqueous solution containing carbonate ions and fibrous material such as cellulose nanofiber (CNF) under conditions of pH 9.5 to 12, and hydrothermal treatment can be performed. For pH adjustment, for example, an aqueous NaOH solution can be used. The resulting reaction product can be subjected to maturation treatment such as stirring, heating, and pressurization as needed to control the crystal size, crystallinity, and / or orientation.

[0025] The amount of hydroxide ion conductive composite material 22 contained in the catalyst layer 14 is preferably such that ion conduction paths can be formed within the catalyst layer 14. Specifically, in the catalyst layer 14, the content of hydroxide ion conductive composite material 22 relative to the total amount of solid matter equivalent of the air electrode catalyst 20, hydroxide ion conductive composite material 22, conductive material 24, and binder 26 (assuming this is 100 volume%) is preferably 10 to 40 volume%, and more preferably 10 to 30 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. Preferred examples of conductive ceramics include LaNiO3 and LaSr3Fe3O 10 Examples include carbon black, graphite, carbon nanotubes, graphene, reduced graphene oxide, Ketjenblack, and any combination thereof.

[0027] As the binder 26 contained in the catalyst layer 14, known binder resins can be used. Examples of organic polymers include butyral resins, vinyl alcohol resins, celluloses, vinyl acetal resins, polytetrafluoroethylene, polyvinylidene fluoride, etc., with butyral resins, polytetrafluoroethylene, and polyvinylidene fluoride being preferred. The binder 26 preferably exists such that it binds the air electrode catalyst 20, hydroxide ion conductive composite material 22, and conductive material 24 together, and allows these components to be adequately exposed to contact with air, as conceptually shown in Figure 2.

[0028] The catalyst layer 14 can be manufactured by preparing a paste containing an air electrode catalyst 20, a hydroxide ion conductive composite material 22, a conductive material 24, and a binder 26, and applying it 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 using a known kneader such as a three-roll mill or a jet mill. Preferred examples of organic solvents include alcohols such as butyl carbitol and terpineol, and acetic acid ester solvents such as butyl acetate. The paste can be applied to the hydroxide ion conductive separator 12 by printing. This printing can be carried out by various known printing methods, but screen printing is preferred.

[0029] The gas diffusion electrode 16 comprises a microporous layer (MPL) and a gas diffusion substrate, and is preferably formed on one side of the catalyst layer 14 such 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 body is desirable. 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 diffusion while lowering the 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 the above porosity, excellent gas diffusion can be ensured and a wide reaction region can be secured. Also, because there is a large amount of pore space, clogging by generated water is less likely to occur. The porosity can be measured by the mercury intrusion method. The microporous layer is not particularly limited as long as it has electronic conductivity and water repellency sufficient to prevent water generated by the air electrode reaction from penetrating the gas diffusion substrate, but it is preferable that it contains a carbon material and polytetrafluoroethylene (PTFE).

[0030] The air electrode current collector 18 can be made of a porous material with general conductivity, preferably a metal. Preferred examples of metals that make up the air electrode current collector 18 include stainless steel, titanium, nickel, brass, and copper. When the air electrode current collector 18 is made of metal, the form is not particularly limited as long as conductivity and breathability can be ensured, but preferred examples include porous metal, metal mesh, and metal plates with an uneven shape. Examples of porous metals include metal products with open pores such as foamed metal and sintered porous metal. Examples of metal meshes include laminated metal mesh or metal mesh in a laminated form. As a metal plate with an uneven shape, a porous metal plate such as perforated metal may be used that has been processed into a corrugated shape.

[0031] As described above, the air electrode / separator junction 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, comprising an air electrode / separator junction 10, a metal anode, and an electrolyte, wherein the electrolyte is isolated from the catalyst layer 14 via a hydroxide ion conductive separator 12. A zinc-air secondary battery using a zinc electrode as the metal anode is particularly preferred. Alternatively, a lithium-air secondary battery using a lithium electrode as the metal anode may also be used. [Examples]

[0032] The present invention will be further described in detail by the following examples.

[0033] Example 1 (1) Fabrication of hydroxide ion conductive composite materials As a hydroxide ion conductive composite material, Mg-Al-LDH-supported cellulose nanofibers (CNF) (fiber length: 2-50 μm) were fabricated using the following procedure. Here, "fiber length: 2-50 μm" means that it includes various fiber lengths distributed in the range from 2 μm to 50 μm, with significant amounts (non-negligible amounts) of fiber lengths of 5-50 μm and 10-50 μm included within this range.

[0034] (1a) Preparation of aqueous solution of raw materials The raw materials are 0.025 mol of magnesium nitrate (Mg(NO3)2·6H2O, manufactured by Kanto Chemical Co., Ltd.), 0.0125 mol of aluminum nitrate (Al(NO3)3·6H2O, manufactured by Kanto Chemical Co., Ltd.), and cellulose nanofiber (CNF)((C6H 10 0.01 g of O5)n (manufactured by Sugino Machine Industry Co., Ltd., IMA-1002, fiber length: 2-50 μm) was weighed and placed in a beaker, and deionized water was added to make a total volume of 200 ml. The resulting solution was stirred for 30 minutes to prepare the raw material aqueous solution.

[0035] (1b) Preparation of basic aqueous solution A basic aqueous solution was prepared by dissolving 0.05 mol of sodium carbonate (Na2CO3, manufactured by Kanto Chemical Co., Ltd.) and 0.15 mol of sodium hydroxide (NaOH, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) in 100 mL of deionized water and stirring for 10 minutes.

[0036] (1c) Co-precipitation method While stirring 200 mL of the raw material aqueous solution at 400 rpm, centre The reaction solution was added dropwise to 100 mL of deionized water at a constant rate using a ram-type metering pump. At this time, a pH meter was placed in the reaction solution, and basic aqueous solutions were added dropwise as needed to maintain a pH of 9.5 to 11. After all of the starting material aqueous solutions had been added dropwise, the reaction solution was stirred at 60°C for 5 hours. Subsequently, the reaction solution was placed in an autoclave container and subjected to hydrothermal treatment at 100°C for 12 hours to synthesize LDH on the surface of the CNF. After that, the solution was purified by centrifugation to obtain Mg-Al-LDH-supported CNF.

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

[0038] (1e) SEM observation and EDX quantitative analysis When Mg-Al-LDH-supported CNF was observed by SEM at 25,000x and 50,000x magnification, as shown in Figures 4 and 5, 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, using SEM-EDX (instrument name: JSM-7900F, manufactured by JEOL Ltd.) at the measurement point shown in Figure 6 (the location where the LDH plate-like particles are present), the following settings were used: acceleration voltage: 10.0kV, irradiation current: 7.47500nA, PHA mode: T3, elapsed time: 108.24sec, effective time: 100.00sec, dead time: 7%, Counting Quantitative analysis was performed under the conditions of a rate of 16622 cps and an energy range of 0 to 20 keV, yielding the results shown in Figure 7 and Table 1 below. These results confirmed that the LDH supported on the CNF is LDH containing Mg and Al as constituent elements with an atomic ratio of Al / Mg = 0.51 (i.e., Mg-Al-LDH).

[0039] [Table 1]

[0040] (1f) Confirmation of the contact between hydroxide ion conductive particles Cross-sectional samples of the catalyst layer were prepared using a cross-section polisher (device name: IB-19520CCP, manufactured by JEOL Ltd.). Observation with a SEM at 25,000x and 50,000x magnification revealed that 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 points, and were found to exist in a connected state.

[0041] (2) Fabrication of the air electrode / separator assembly 14 parts by weight (40 volumes) of carbon powder (Tokai Carbon Co., Ltd., Toka Black #3855), 8 parts by weight (14 volumes) of LDH powder (Ni-Fe-LDH powder produced by coprecipitation), 12 parts by weight (26 volumes) of hydroxide ion conductive composite material (Mg-Al-LDH supported CNF) prepared in (1) above, and 13 parts by weight (12 volumes) of platinum-supported carbon (Toyo Technica Co., Ltd., EC-20-PTC) were mixed with 2 parts by weight (8 volumes) of 25% butyral resin (a viscous substance obtained by dissolving BL-s, manufactured by Sekisui Chemical Co., Ltd., in butyl carbitol) and 50 parts by weight (271 volumes) of butyl carbitol, and kneaded with a three-roll and rotation / revolution mixer (Sinky Co., Ltd., ARE-310) to form a paste. The proportion of Mg-Al-LDH-supported CNF in the total solid content (excluding butyl carbitol) of the obtained paste was 26% by volume. This paste was screen-printed onto the surface of an LDH separator (a polyethylene microporous membrane with Ni-Al-Ti-LDH deposited in the pores and on the surface by hydrothermal synthesis and 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, a weight was placed on top, and it was dried in air at 80°C for 12 hours to form an air electrode. Thus, an air electrode / separator assembly was obtained.

[0042] (3) Preparation of zinc oxide anode 100 parts by weight of ZnO powder (manufactured by Seido Chemical Industry Co., Ltd., JIS standard Grade 1, average particle size D50: 0.2 μm) was mixed with 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). Furthermore, 1.26 parts by weight (based on solid content) of polytetrafluoroethylene (PTFE) dispersion aqueous solution (manufactured by Daikin Industries, Ltd., 60% solid content) was added and kneaded together with propylene glycol. The resulting 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 pressed onto tin-plated copper expanded metal and dried in a vacuum dryer at 80°C for 14 hours. After drying, the negative electrode sheet was cut so that the area coated with the active material was 2 cm square, and copper foil was welded to the current collector portion to obtain a zinc oxide negative electrode.

[0043] (4) Assembling and evaluating evaluation cells A zinc oxide negative electrode was laminated on the LDH separator side of the air electrode / separator assembly. The resulting laminate was clamped in a clamping jig with a sealing member tightly fitted around the outer circumference of the LDH separator, and then firmly fixed with screws. This clamping jig has an oxygen inlet on the air electrode side and an electrolyte injection port 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 create an evaluation cell.

[0044] Using an electrochemical measuring device (Hokuto Denko Co., Ltd., HZ-Pro S12), the charge-discharge characteristics of the evaluation cell were measured under the following conditions: • Air electrode gas: Water vapor saturated (25°C) oxygen (flow rate 200cc / min) ·Charge / discharge current density: 2mA / cm 2 ·Charge / discharge time: 60 minutes charge / 60 minutes discharge • Number of cycles: 4 cycles The measurements were taken using [specific method / tool]. The results are shown in Figure 8.

[0045] Example 2 Cellulose nanofiber ((C6H10 Except for using AFo-1002 (fiber length: 0.2~3μm) manufactured by Sugino Machine Industry Co., Ltd. as O5)n) instead of IMA-1002, Mg-Al-LDH-supported CNF (fiber length: 0.2~3μm) was prepared and evaluated in the same manner as in Example 1. As a result, XRD identified the Mg-Al-LDH supported on the CNF as LDH (hydrotalcite compounds). SEM observation revealed that numerous LDH plate-like particles were supported on the surface of the CNF with their faces parallel to each other. Furthermore, SEM-EDX confirmed that the LDH supported on the CNF was LDH (i.e., Mg-Al-LDH) containing Mg and Al as constituent elements in an atomic ratio of Al / Mg = 0.51. Furthermore, when the contact between hydroxide ion conductive particles was examined in the same manner as in Example 1, it was confirmed that in the catalyst layer prepared in this example, hydroxide ion conductive particles (Mg-Al-LDH particles) were in contact with adjacent hydroxide ion conductive particles at one or more points within the catalyst layer, and existed in a connected state.

[0046] Next, using Mg-Al-LDH-supported CNF, air electrode / separator assemblies and evaluation cells were fabricated in the same manner as in Example 1, and evaluated. The charge-discharge evaluation results are shown in Figure 8. From Figure 8, it was found that the evaluation cells (zinc-air secondary batteries) fabricated in Examples 1 and 2, which have a configuration using Mg-Al-LDH-supported CNF as a hydroxide ion conductive composite material in the catalyst layer of the air electrode, suppressed the increase in charge-discharge overvoltage compared to Example 3, which uses Mg-Al-LDH (without CNF), as described later. In particular, since the charge-discharge overvoltage was smaller in Example 1 (CNF fiber length: 2-50 μm) than in Example 2 (CNF fiber length: 0.2-3 μm), it was found that a longer CNF fiber length better exhibits the effect of the hydroxide ion conductive composite material, further suppressing the increase in charge-discharge overvoltage.

[0047] Example 3 (comparison) Except for preparing Mg-Al-LDH powder (not supported on CNF) without adding cellulose nanofibers and using it in place of Mg-Al-LDH-supported CNF, the hydroxide ion conductive material, air electrode / separator assembly, zinc oxide negative electrode, and evaluation cell were prepared and evaluated in the same manner as in Example 1. When the contact of 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) did not necessarily come into contact with other hydroxide ion conductive particles, but existed in a dispersed state. Furthermore, the charge-discharge evaluation results are shown in Figure 8. From Figure 8, it was found that 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 with their surfaces parallel to each other, resulting in a large increase in charge-discharge overpotential during the charge-discharge evaluation.

Claims

1. A layered double hydroxide (LDH) separator, A catalyst layer covering one side of the LDH separator, comprising an air electrode catalyst, a hydroxide ion conductive composite material, a conductive material, and a binder, A gas diffusion electrode is provided on the side of the catalyst layer opposite to the LDH separator, The hydroxide ion conductive composite material is provided with, Hydrophilic fibers having hydroxyl groups on the fiber surface, Hydroxide ion conductive particles composed of multiple layered double hydroxides (LDHs) are supported on the surface of the hydrophilic fiber in a connected manner, An air electrode / separator assembly, including the air electrode.

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 fiber is cellulose nanofiber.

4. The air electrode / separator joint according to any one of claims 1 to 3, wherein the hydrophilic fiber has a length of 0.1 to 100 μm.

5. The air electrode / separator joint according to any one of claims 1 to 3, wherein the hydrophilic fiber has a length of 0.2 to 50 μm.

6. The air electrode / separator joint according to any one of claims 1 to 3, wherein the hydrophilic fiber has a length of 5 to 50 μm.

7. The air electrode / separator assembly according to any one of claims 1 to 3, wherein the layered double hydroxide (LDH) constituting the hydroxide ion conductive particles contains at least two elements selected from the group consisting of Ni, Fe, Mg, Al, and Ti as constituent elements.

8. The air electrode / separator assembly according to claim 7, wherein the at least two elements include Mg and Al.

9. The air electrode / separator assembly according to any one of claims 1 to 3, wherein in the catalyst layer, the content of the hydroxide ion conductive composite material relative to the total amount of the air electrode catalyst, the hydroxide ion conductive composite material, the conductive material, and the binder in terms of solid matter is 10 to 40 volume percent.

10. The air electrode / separator assembly according to any one of claims 1 to 3, wherein in the catalyst layer, the content of the hydroxide ion conductive composite material relative to the total amount of the air electrode catalyst, the hydroxide ion conductive composite material, the conductive material, and the binder in terms of solid matter is 10 to 30 volume percent.

11. The air electrode / separator joint according to any one of claims 1 to 3, wherein the LDH separator is compounded with a porous substrate.

12. A metal-air secondary battery comprising an air electrode / separator junction 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 LDH separator.

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