JP7591256B2 - Particulate material, its manufacturing method and use - Google Patents

Description

The present invention relates to a particulate material, a method for producing the same, and uses thereof. In particular, the present invention relates to a particulate material containing a metal oxide hydroxide particle mass and a spinel-type metal oxide particle mass in one particle, a method for producing the same, and uses thereof as an air electrode catalyst, etc.

Zinc-air secondary batteries have a theoretical capacity five times that of currently practical lithium-ion batteries, and are attracting attention as the next generation of secondary batteries. There are many challenges to overcome before zinc-air secondary batteries can be put to practical use. In particular, the large overvoltages of the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) at the air electrode significantly reduce efficiency, so there is a demand for the development of a bifunctional air electrode catalyst that exhibits high activity in both reactions. However, to date, no single catalyst material that exhibits high activity in both reactions has been reported. The only known example is a catalyst that combines CoMn ₂ O ₄ and MnOOH and is active in both OER and ORR (Non-Patent Document 1).

Catalysts made of transition metals from the fourth period have been attracting attention from the viewpoint of metal price and production volume, and it has been reported that metal oxide hydroxides show high activity as ORR catalysts (e.g., Patent Document 1 and Non-Patent Document 2). It has been reported that metal oxides with a spinel-type crystal structure show high activity as ORR catalysts (e.g., Non-Patent Document 3).

WO2020/006022

Chem. Commun. ,2018,54,4005 Angew. chem. Int. Ed. 2018, 57, 2672-2676 Adv. Mater. 2014, 26, 2408-2412

However, simply mixing these catalysts as a bifunctional air catalyst has not been able to achieve high activity for both reactions simultaneously, and in order to develop a high-performance air electrode catalyst, it is necessary to synthesize a composite catalyst material in the nano-sized region. However, a method for developing a material in which an oxyhydroxide and a spinel-type oxide, which have different crystal structures, are composited in the nano-sized region has not yet been established.

The object of the present invention is to provide a material in which an oxyhydroxide and a spinel-type oxide, which have different crystal structures, are composited in the nano-sized region, and further to provide an air electrode catalyst, an air electrode for a metal-air secondary battery, and a metal-air secondary battery using this material.

The present invention is as follows.
[1]
A particulate material containing, in each particle, a metal oxide hydroxide particle agglomerate containing Co, Ni and Fe and having a particle size in the range of 1 to 100 nm, and a spinel type metal oxide particle agglomerate containing Co, Ni and Fe and having a particle size in the range of 1 to 100 nm.
[2]
The particulate material according to claim 1, wherein at least a portion of the metal oxide hydroxide particle agglomerates and the spinel-type metal oxide particle agglomerates are present on the surface of the particulate material.
[3]
The particulate material contains 60 to 95 mol % Co atoms, 1 to 25 mol % Ni atoms, and 1 to 20 mol % Fe atoms, and the total content of Ni atoms and Fe atoms is 5 mol % or more, provided that the total of Co atoms, Ni atoms, and Fe atoms is 100 mol %. [4] The material according to [1] or [2]
The particulate material according to [3], wherein the particulate material has a Co atomic content of 70 to 90 mol%, a Ni atomic content of 4 to 25 mol%, and a Fe atomic content of 2 to 14 mol%.
[5]
the metal oxide hydroxide particle agglomerates contain 60 to 95 mol % Co atoms, 1 to 25 mol % Ni atoms, and 1 to 20 mol % Fe atoms, the total content of Ni atoms and Fe atoms being 5 mol % or more;
The particulate material according to any one of [1] to [4], wherein the spinel-type metal oxide particle agglomerates contain 60 to 95 mol % Co atoms, 1 to 25 mol % Ni atoms, and 1 to 20 mol % Fe atoms, with the total content of Ni atoms and Fe atoms being 5 mol % or more, provided that in each case, the total of Co atoms, Ni atoms, and Fe atoms is 100 mol %.
[6]
The particulate material according to any one of [1] to [5], wherein the particle diameter of the metal oxide hydroxide particle agglomerates is in the range of 1 to 20 nm, and the particle diameter of the spinel-type metal oxide particle agglomerates is in the range of 1 to 20 nm.
[7]
The particulate material according to any one of [1] to [6], wherein the particle size of at least a portion of the particulate material is in the range of 0.01 to 50 μm.
[8]
A method for producing a particulate material, comprising calcining a precursor containing a layered double hydroxide containing Co, Ni, and Fe, and a metal hydroxide containing Co, Ni, and Fe, to obtain a particulate material containing, in each particle, a metal oxide hydroxide particle agglomerate containing Co, Ni, and Fe, the particle size of which is in the range of 1 to 100 nm, and a spinel-type metal oxide particle agglomerate containing Co, Ni, and Fe, the particle size of which is in the range of 1 to 100 nm.
[9]
The method according to [8], wherein the precursor is produced by adding an alkali to an aqueous solution containing 70 to 98 mol % of Co ²⁺ , 1 to 20 mol % of Ni ²⁺ , and 1 to 10 mol % of Fe ³⁺ .
[10]
The method according to [8] or [9], wherein the calcination is carried out at a temperature in the range of 110 to 190° C.
[11]
[8] A catalyst for an air electrode, comprising the particulate material according to any one of [1] to [7].
[12]
[1] - [7] Any one of the particulate material of the present invention, the metal-air secondary battery air electrode.
[13]
A metal-air secondary battery comprising an air electrode containing the particulate material according to any one of [1] to [7], a negative electrode containing a negative electrode active material, and an electrolyte interposed between the air electrode and the negative electrode.

According to the present invention, it is possible to provide a material in which an oxyhydroxide and a spinel-type oxide, which have different crystal structures, are composited in the nano-sized region, and this material can be a catalyst with high activity for both OER and ORR. Furthermore, it is possible to provide a catalyst for an air electrode, an air electrode for a metal-air secondary battery, and a metal-air secondary battery using this material.

1 shows XRD patterns of a precursor and a catalyst calcined at 100, 150, and 200° C. in Example 1 (Co73Ni13Fe14 sample). 1 shows (a) ORR and (b) OER activity evaluations of the precursor in Example 1 (Co73Ni13Fe14 sample) and samples fired at various temperatures. 1 shows XRD patterns of a precursor and a catalyst calcined at 100 and 150° C. in Example 2 (Co80Ni6Fe14 sample). 1 shows (a) ORR and (b) OER activity evaluations of the precursor and samples fired at various temperatures in Example 2 (Co80Ni6Fe14 sample). 1 shows XRD patterns of a precursor and a catalyst calcined at 150° C. in Example 3 (Co84Ni12Fe4 sample). 1 shows (a) ORR and (b) OER activity evaluations of the precursor and the sample calcined at 150° C. in Example 3 (Co84Ni12Fe4 sample). 1 shows XRD patterns of a precursor and a catalyst calcined at 100° C. and 150° C. in Reference Example 1 (Co72Ni6Fe22 sample). 1 shows (a) ORR and (b) OER activity evaluations of the precursor in Reference Example 1 (Co72Ni6Fe22 sample) and samples fired at various temperatures. 1 shows XRD patterns of a precursor and a catalyst calcined at 150° C. in Reference Example 2 (Co65Ni20Fe15 sample). 1 shows (a) ORR and (b) OER activity evaluations of the precursor and the sample calcined at 150° C. in Reference Example 2 (Co65Ni20Fe15 sample). 1 shows XRD patterns of a precursor and a catalyst calcined at 100° C. and 150° C. in Reference Example 3 (Co50Ni36Fe14 sample). 1 shows (a) ORR and (b) OER activity evaluations of the precursor and samples fired at various temperatures in Reference Example 3 (Co50Ni36Fe14 sample). TEM images of the Co73Ni13Fe14 sample sintered at 150° C. in Example 1 (a) is a low-magnification image, and (b) is a high-magnification image of the red portion. 4 shows the results of TEM-EDX analysis of the Co73Ni13Fe14 sample sintered at 150° C. in Example 1. In Example 2, a STEM image of a Co80Ni6Fe14 sample sintered at 150° C. is shown: (a) a low-magnification image, and (b) a high-magnification image of the red portion. 1 shows the results of STEM-EDX analysis of the Co80Ni6Fe14 sample sintered at 150° C. in Example 1. 1 shows the results of a cathode durability test when CFCO and a 150° C. sintered Co73Ni13Fe14 sample (Example 1) were used. 1 shows an example of the configuration of a metal-air secondary battery of the present invention.

[Particulate material]
The present invention relates to a particulate material containing, in each particle, a metal oxide hydroxide particle agglomerate containing Co, Ni, and Fe and having a particle size in the range of 1 to 100 nm, and a spinel-type metal oxide particle agglomerate containing Co, Ni, and Fe and having a particle size in the range of 1 to 100 nm.

A metal oxide hydroxide is a compound represented by the composition formula MOOH (M: trivalent metal), and the trivalent metal represented by M can be composed of two or more trivalent metals. For example, when it contains three types of metals, M, M', and M'', it has a composition of M _x M' _y M'' _(1-xy) OOH. In the formula, x, y, and z represent the number of M, M', and M'', and are values of 1 or less. For example, when M, M', and M'' are Co, Ni, and Fe, and the respective numbers are 0.6, 0.3, and 0.1, it is expressed as Co _0.6 Ni _0.3 Fe _0.1 OOH. A metal oxide hydroxide can maintain its structure even if the composition ratio of anions is slightly shifted. For example, a composition ratio such as CoO(OH) _0.99 is also possible. That is, when expressed as MO(OH) _p , it is sufficient that p is less than 1 or more than 1 as long as the crystal structure of the oxide hydroxide is maintained. The metal oxide hydroxide in the present invention is an oxide hydroxide containing Co, Ni and Fe as metals, and has the same crystal structure as the β-CoOOH type crystal structure. Identification of whether or not it is a metal oxide hydroxide can be performed by comparing with the β-CoOOH type crystal in XRD data. When the particulate material of the present invention is used as a catalyst for the air electrode of a metal-air secondary battery, the metal oxide hydroxide mainly exhibits OER activity.

The spinel metal _oxide is represented by the composition formula _AB2O4 (A: divalent metal, B: trivalent _metal ), and two or more metals can coexist in both A and B. Since two or more metals can coexist in A and B, the spinel metal oxide can have a composition such as ( _Ni0.1Co0.9 )( _Fe0.4Co1.6 ) _O4 . However, since the spinel structure can be maintained even if the ratio of elements is slightly different, there are also reports of spinel _oxides with a composition ratio such as _NiCo2O3.99 . The spinel metal oxide _in the present invention is an oxide containing Co, Ni, and Fe as metals, and has a spinel structure _as a crystal structure. Identification of whether or not it is a spinel metal oxide can be performed by comparing with _Co3O4 crystal in XRD data.

The particulate material of the present invention contains, in one particle, a metal oxide hydroxide particle agglomerate containing Co, Ni, and Fe, the particle diameter of which is in the range of 1 to 100 nm, and a spinel-type metal oxide particle agglomerate containing Co, Ni, and Fe, the particle diameter of which is in the range of 1 to 100 nm. The presence of a metal oxide hydroxide particle agglomerate and a spinel-type metal oxide particle agglomerate in one particle can be determined by observing a TEM image. The particulate material of the present invention contains the above-mentioned metal oxide hydroxide particle agglomerate and a spinel-type metal oxide particle agglomerate in one particle, thereby making it possible to obtain a catalyst having high activity in both OER and ORR. The metal oxide hydroxide particle agglomerate has a particle diameter in the range of 1 to 100 nm, preferably in the range of 1 to 50 nm, and more preferably in the range of 1 to 20 nm. The spinel-type metal oxide particle agglomerate has a particle diameter in the range of 1 to 100 nm, preferably in the range of 1 to 50 nm, and more preferably in the range of 1 to 20 nm. When the particulate material of the present invention is used as a catalyst for the air electrode of a metal-air secondary battery, the spinel-type metal oxide primarily exhibits ORR activity.

In the particulate material of the present invention, preferably, at least a portion of the metal oxide hydroxide particle agglomerates and the spinel type metal oxide particle agglomerates are present on the surface of the particulate material. The presence of at least a portion of the metal oxide hydroxide particle agglomerates and the spinel type metal oxide particle agglomerates on the surface of the particulate material can be determined by observing a TEM image. Since it is believed that OER activity occurs mainly in the metal oxide hydroxide particle agglomerates and ORR activity occurs mainly in the spinel type metal oxide particle agglomerates, a catalyst with high activity in both OER and ORR can be obtained by having at least a portion of both of them present on the surface of the particulate material.

The particulate material of the present invention contains Co atoms, Ni atoms and Fe atoms, and the contents of Co atoms, Ni atoms and Fe atoms are, for example, 60 to 95 mol%, 1 to 25 mol%, and 1 to 20 mol%, respectively, and the total content of Ni atoms and Fe atoms is 5 mol% or more. However, the total of Co atoms, Ni atoms and Fe atoms is 100 mol%. By having the particulate material have this content, a catalyst with high activity in both OER and ORR can be obtained. From the viewpoint of a catalyst with high activity in both OER and ORR, the contents of Co atoms, Ni atoms and Fe atoms of the particulate material are preferably 70 to 95 mol%, 4 to 25 mol%, and 2 to 20 mol%, respectively, and more preferably 70 to 85 mol%, 5 to 15 mol%, and 3 to 15 mol%. The total content of Ni atoms and Fe atoms is preferably in the range of 10 to 35 mol%, more preferably 15 to 30 mol%. Metal atoms other than Co, Ni, and Fe may be inevitably contained as impurities in the particulate material of the present invention. Alternatively, metal atoms other than Co, Ni, and Fe, such as atoms of Si, Na, Mg, Ca, Zn, Al, Pb, Cu, Mn, etc., may be added to the particulate material of the present invention within a range that does not impair the effects of the present invention. However, even if the particulate material contains metal atoms other than Co, Ni, and Fe, the content (in mol%) of Co atoms, Ni atoms, and Fe atoms in the particulate material is 100 mol% of the total of Co atoms, Ni atoms, and Fe atoms.

The contents of Co atoms, Ni atoms, and Fe atoms in the metal oxide hydroxide particle agglomerates and spinel-type metal oxide particle agglomerates are approximately the same as the contents of Co atoms, Ni atoms, and Fe atoms (expressed in mol%) in the particulate material. The contents of Co atoms, Ni atoms, and Fe atoms in the metal oxide hydroxide particle agglomerates are, for example, 60 to 95 mol%, 1 to 25 mol%, and 1 to 20 mol%, respectively, and the total content of Ni atoms and Fe atoms is 5 mol% or more. The contents of Co atoms, Ni atoms, and Fe atoms in the spinel-type metal oxide particle agglomerates are, for example, in the ranges of 60 to 95 mol%, 1 to 25 mol%, and 1 to 20 mol%, respectively, and the total content of Ni atoms and Fe atoms is suitably 5 mol% or more. Either or both of the metal oxide hydroxide particle mass and the spinel-type metal oxide particle mass may contain metal atoms other than Co, Ni, and Fe, such as atoms of Si, Na, Mg, Ca, Zn, Al, Pb, Cu, Mn, etc., as unavoidable impurities or as additives within a range that does not impair the effects of the present invention. In this case, as in the case of the particulate material, the content (expressed in mol%) of Co atoms, Ni atoms, and Fe atoms is 100 mol% in total of Co atoms, Ni atoms, and Fe atoms.

At least a portion of the particulate material of the present invention may have a particle size in the range of, for example, 0.01 to 50 μm, preferably in the range of 0.01 to 20 μm, and more preferably in the range of 0.01 to 10 μm. However, the particle size of the particulate material can be appropriately adjusted by crushing or classification.

TEM and STEM images of an example of the particulate material of the present invention are shown in Figures 13-1 and 14-1. The TEM image shown in Figure 13-1 is a TEM image of a Co73Ni13Fe14 sample prepared in Example 1 and calcined at 150°C. The STEM image shown in Figure 14-1 is a STEM image of a Co80Ni6Fe14 sample prepared in Example 2 and calcined at 150°C. As can be seen from these images, the particulate material of the present invention contains spinel-type metal oxide particle clusters and oxyhydroxide particle clusters in one particle. TEM-EDX analysis and STEM-EDX analysis of these particles showed that the spinel-type metal oxide contains Co, Ni, and Fe as metals, and the metal oxyhydroxide also contains Co, Ni, and Fe as metals, as shown in Figures 13-2 and 14-2.

Furthermore, the TEM and STEM images shown in Figures 13-1 and 14-1 show that spinel-type oxide particle agglomerates and oxyhydroxide particle agglomerates are present on the surface of the particulate material in at least some regions.

[Method of producing particulate material]
The present invention includes a method for producing a particulate material, which comprises calcining a precursor containing a layered double hydroxide containing Co, Ni, and Fe and a metal hydroxide containing Co, Ni, and Fe to obtain a particulate material containing a metal oxide hydroxide particle mass containing Co, Ni, and Fe and a spinel-type metal oxide particle mass containing Co, Ni, and Fe in each particle. The particulate material of the present invention can be produced by this method.

The precursors used in the manufacturing method of the present invention include a layered double hydroxide containing Co, Ni, and Fe, and a metal hydroxide containing Co, Ni, and Fe.

Metal hydroxides are represented by the formula M(OH) ₂ (M: divalent metal), where M can be composed of multiple divalent metals. Metal hydroxides containing Co, Ni, and Fe contain Co, Ni, and Fe as metals, and therefore have a composition of _{CoxNiyFe (1-xy)} (OH) ₂ , where x, _y , and z vary depending on the ratio of Co, Ni, and Fe in the raw materials used to prepare the precursor. The structure can be maintained even if the composition ratio of the anions is slightly shifted. Metal hydroxides can be specified by a hexagonal cadmium hydroxide type structure (cadmium iodide structure) or a brucite type crystal structure.

Layered double hydroxides are non-stoichiometric compounds with the formula [M _1-x M' _x (OH) ₂ ][A _x/ n.yH ₂ O]. M is a monovalent or divalent metal, and M' is a trivalent or tetravalent metal. Two or more metals can coexist as M and M'. Layered double hydroxides containing Co, Ni, and Fe are expressed as [Co _0.6 Ni _0.2 Co _0.1 Fe _0.1 (OH) ₂ ][(CO ₃ ) _{0.2.0.5H 2} O] when x=0.2. The combination of valences of M and M' includes M-M'=1-3, 2-3, and 2-4, but in most cases M-M'=2-3. A is a monovalent or divalent anion. Generally, x=0.2-0.33, and n is the valence of the anion. The sheet has a layered structure in which anion A and water molecules are inserted between the layers of the sheet having a composition of M _1-x M' _x (OH) ₂ .

The precursor can be produced, for example, by adding an alkali to an aqueous solution containing 70 to 98 mol% Co ²⁺ , 1 to 20 mol% Ni ^{2+ ,} and 1 to 10 mol% Fe ³⁺ . By adding an alkali to an aqueous solution containing metal ions of this composition, a precursor containing both a layered double hydroxide containing Co, Ni, and Fe and a metal hydroxide containing Co, Ni, and Fe is precipitated. The raw materials of Co, Ni, and Fe can be salts of these metals, such as nitrates, acetates, citrates, etc. The alkali is not particularly limited, but can be, for example, an alkali metal hydroxide, ammonia water, etc., and potassium hydroxide can be used as the alkali metal hydroxide. The precipitate is separated into solid and liquid by a conventional method, and the obtained solid can be washed and dried appropriately. It is preferable to wash with water and alcohol (e.g., ethanol) to remove the raw materials and alkali, and then dry.

The precursor contains both the layered double hydroxide and the metal hydroxide, and the layered double hydroxide becomes a spinel-type metal oxide and the metal hydroxide becomes a metal oxyhydroxide by firing, which will be described later. As described above, a precursor containing both the layered double ^hydroxide and the metal hydroxide in appropriate amounts is obtained by adding an alkali to an aqueous solution containing 70 to 98 mol% Co 2+, 1 to 20 mol% Ni ²⁺ , and 1 to 10 mol% Fe ³⁺ . The composition ratio of Co ²⁺ , Ni ^2+, and Fe ³⁺ in the aqueous solution can be appropriately adjusted so that both the layered double hydroxide and the metal hydroxide are contained in appropriate amounts. The precursor produced by this method has, for example, the contents of Co atoms, Ni atoms, and Fe atoms in the ranges of 60 to 95 mol%, 1 to 25 mol%, and 1 to 20 mol%, respectively. The contents (expressed in mol %) of Co atoms, Ni atoms and Fe atoms in the precursor are such that the total of Co atoms, Ni atoms and Fe atoms is 100 mol %.

From the viewpoint of obtaining a particulate material containing metal oxide hydroxide particle agglomerates and spinel-type metal oxide particle agglomerates in one particle, the calcination of the precursor is suitably carried out at a temperature in the range of, for example, 110 to 190°C, although this depends on the ratio of Co, Ni, and Fe in the precursor. The calcination of the precursor is preferably carried out in the range of 120 to 180°C, more preferably in the range of 130 to 170°C. The calcination time of the precursor is not particularly limited as long as the particulate material obtained by calcination contains metal oxide hydroxide particle agglomerates and spinel-type metal oxide particle agglomerates, and although it varies depending on the temperature, it can be, for example, 1 hour to 24 hours. The calcination atmosphere may be an oxygen-containing atmosphere, and from the viewpoint of ease of operation, it is suitable to carry out the calcination in the air. The particulate material obtained by calcination can be used as a catalyst as it is, or after appropriate crushing and classification.

<Electrode catalyst>
The present invention includes a cathode catalyst comprising the particulate material of the present invention.

The surface area of the air electrode catalyst and water electrolysis anode catalyst containing the particulate material of the present invention can be, for example, in the range of 1 to 100 m ² /g, and is preferably in the range of 10 to 100 m ² /g, although it is not intended to be limited to this range.

The particulate material of the present invention is extremely useful as an air electrode, and is extremely promising for hydrogen production by photochemical water splitting, and as an air electrode for metal-air secondary batteries, which are expected to be the next generation of high-capacity secondary batteries.

The particulate material of the present invention also has excellent catalytic activity for the ORR reaction. The ORR reaction is represented by the following reaction formula:
_O2 + 4H ⁺ + ^4e- → _2H2O (neutral to acidic)
O ₂ +2H ₂ O+4e ^- →4OH ^- (basic)
The particulate material of the present invention has excellent both OER activity and ORR activity and is extremely useful as an air electrode in a secondary battery.

<Air electrode>
The air electrode functions as a positive electrode. The air electrode usually has a porous structure and contains an oxygen reaction catalyst as well as a conductive material. The air electrode may also contain a binder, etc., as necessary. The air electrode may contain an OER catalyst or an ORR catalyst, or both, other than the particulate material of the present invention. The air electrode in a secondary battery is required to have OER catalytic activity as a function during charging and ORR catalytic activity as a function during discharging. The catalyst of the present invention is a catalyst for both OER and ORR, and therefore can be used alone as a catalyst for the air electrode. The chemical formulas of the air electrode during charging and discharging are shown below.

The content of the catalyst of the present invention in the air electrode is not particularly limited, but from the viewpoint of improving the oxygen reaction performance and oxygen reduction performance of the air electrode, it is preferably, for example, 1 to 90 mass%, particularly preferably 10 to 80 mass%, and more preferably 30 to 60 mass%.

There are no particular limitations on the OER catalyst other than the particulate material of the present invention. The OER catalyst can be, for example, a Brownmillerite-type transition metal oxide described in WO2015/115592. However, it is not intended to be limited thereto.

Examples of ORR catalysts other than the particulate material of the present invention are not particularly limited, but include, for example, Pt or Pt-based materials (e.g., PtCo, PtCoCr, Pt- _W2C , Pt-RuOx, etc.), Pd-based materials (e.g., PdTi, PdCr, PdCo, PdCoAu, etc.), metal oxides (e.g., ZrO2 _{-x , TiOx} , _TaNxOy , IrMOx _, etc.), complex systems (Co-porphyrin complexes), and others ( _PtMoRuSeOx , RuSe, etc.). Further examples include LaNiO ₃ (Nat. Chem. 3, 546 (2011)) reported by Suntivich et al. as being highly active, and CoO/N-doped CNT (Nat. Commun. 4, 1805 (2013)) reported by Li et al. However, it is not intended to be limited to these. In addition, a combination of multiple catalysts can be used in consideration of the performance and properties of each catalyst. Furthermore, the above catalyst can be used in combination with a co-catalyst (e.g., TiO _x , RuO ₂ , SnO ₂ , etc.). The content when an additional ORR catalyst is used in combination can be appropriately determined in consideration of the type and catalytic activity of the ORR catalyst, and can be, for example, 1 to 90 mass %. However, it is not intended to be limited to this numerical range.

The conductive material is not particularly limited, and may be any material that can be generally used as a conductive assistant, but a suitable example is conductive carbon. Specific examples include mesoporous carbon, graphite, acetylene black, carbon nanotubes, and carbon fibers. As the conductive material, a conductive assistant for oxygen generating electrodes containing pCNF (loop edge-pCNF (lpe-pCNF)) having a platelet structure of a carbon hexagonal mesh surface described in WO2019/039538, in which at least a part of the edge of the carbon hexagonal mesh surface exposed on the side surface of the fiber has a loop structure between the edge of the adjacent carbon hexagonal mesh surface, can also be used. Since it provides many reaction fields at the air electrode, conductive carbon with a large specific surface area is preferred. Specifically, conductive carbon with a specific surface area of 1 to 3000 m ² /g, particularly 500 to 1500 m ² /g is preferred. The catalyst of the air electrode may be supported on a conductive material.

The amount of conductive material contained in the air electrode is not particularly limited, but from the viewpoint of increasing the discharge capacity, it is preferably, for example, 10 to 99 mass%, more preferably 20 to 80 mass%, and even more preferably 20 to 60 mass%.

By incorporating a binder into the air electrode, the catalyst and conductive material can be immobilized, improving the cycle characteristics of the battery. There are no particular limitations on the binder, and examples include polyvinylidene fluoride (PVDF) and its copolymers, polytetrafluoroethylene (PTFE) and its copolymers, and styrene butadiene rubber (SBR). The content of the binder in the air electrode is not particularly limited, but from the viewpoint of the binding strength between the carbon (conductive material) and the catalyst, it is preferably, for example, 1 to 40 mass%, particularly preferably 5 to 35 mass%, and more preferably 10 to 35 mass%.

The air electrode can be formed, for example, by dispersing the air electrode constituent materials in a suitable solvent to prepare a slurry, which is then applied to a substrate and dried. The solvent is not particularly limited, and examples include acetone, N,N-dimethylformamide, and N-methyl-2-pyrrolidone (NMP). The air electrode constituent materials and the solvent are usually mixed for 3 hours or more, and preferably 4 hours. There are no particular limitations on the mixing method, and a general method can be used.

The substrate to which the slurry is applied is not particularly limited, and examples include a glass plate, a Teflon (registered trademark) plate, etc. These substrates are peeled off from the resulting air electrode after the slurry is dried. Alternatively, the current collector of the air electrode or the solid electrolyte layer can be treated as the substrate. In this case, the substrate is not peeled off and is used as it is as a component of the metal-air secondary battery.

The method of applying the slurry and the method of drying are not particularly limited, and any common method can be used. For example, application methods such as spraying, doctor blade, and gravure printing, and drying methods such as heat drying and reduced pressure drying can be used.

The thickness of the air electrode is not particularly limited and may be set appropriately depending on the application of the metal-air secondary battery, but is usually preferably 5 to 100 μm, 10 to 80 μm, and particularly preferably 20 to 70 μm.

An air electrode current collector that collects current from the air electrode is usually connected to the air electrode. The material and shape of the air electrode current collector are not particularly limited. Examples of materials for the air electrode current collector include stainless steel, aluminum, iron, nickel, titanium, and carbon. The shape of the air electrode current collector can be foil, plate, mesh (grid), fiber, etc., and among these, a porous shape such as a mesh is preferable. This is because a porous current collector has excellent oxygen supply efficiency to the air electrode.

<Metal-air secondary battery>
The metal-air secondary battery of the present invention has an air electrode containing a catalyst including the particulate material of the present invention, a negative electrode containing a negative electrode active material, and an electrolyte interposed between the air electrode and the negative electrode. The air electrode of the metal-air secondary battery of the present invention contains a catalyst including the particulate material of the present invention, and this catalyst exhibits excellent OER catalyst and ORR catalyst properties. Therefore, by using an air electrode using this catalyst, the overvoltage required for charging can be reduced, the electromotive force of the battery during discharge can be improved, and high efficiency can be achieved, so that the metal-air secondary battery of the present invention has excellent charge and discharge voltages.

The air electrode can also contain a catalyst having OER catalytic activity or a catalyst having ORR catalytic activity, as described above.

An example of the configuration of the metal-air secondary battery of the present invention will be described below. The metal-air secondary battery of the present invention is not limited to the following configuration. FIG. 16 is a cross-sectional view showing an example of the metal-air secondary battery of the present invention. The metal-air secondary battery 1 is composed of an air electrode 2, which is a positive electrode using oxygen as an active material, a negative electrode 3 containing a negative electrode active material, an electrolyte 4 that is responsible for ion conduction between the air electrode 2 and the negative electrode 3, an air electrode current collector 5 that collects current from the air electrode 2, and a negative electrode current collector 6 that collects current from the negative electrode 3, and these are housed in a battery case (not shown). The air electrode 2 is electrically connected to the air electrode current collector 5 that collects current from the air electrode 2, and the air electrode current collector 5 has a porous structure that can supply oxygen to the air electrode 2. The negative electrode 3 is electrically connected to the negative electrode current collector 6 that collects current from the negative electrode 3, and one of the ends of the air electrode current collector 5 and the negative electrode current collector 6 protrudes from the battery case. They function as a positive electrode terminal (not shown) and a negative electrode terminal (not shown), respectively.

(Negative electrode)
The negative electrode contains a negative electrode active material. The negative electrode active material may be a general air battery negative electrode active material, and is not particularly limited. The negative electrode active material is usually capable of absorbing and releasing metal ions. Specific examples of the negative electrode active material include metals such as Li, Na, K, Mg, Ca, Zn, Al, and Fe, alloys, oxides, and nitrides of these metals, and carbon materials.

Among them, zinc-air secondary batteries are expected to be the next generation secondary batteries because of their superior safety. From the viewpoint of high voltage and high output, lithium-air secondary batteries and magnesium-air secondary batteries are promising.
An example of a zinc-air secondary battery will be explained below. The reaction formula is as follows:

In the zinc-air secondary battery of the present invention, a material capable of absorbing and releasing zinc ions is used as the negative electrode. In addition to metallic zinc, a zinc alloy can also be used as such a negative electrode. Examples of zinc alloys include zinc alloys containing one or more elements selected from aluminum, indium, magnesium, tin, titanium, and copper.

Examples of negative electrode active materials for lithium-air secondary batteries include metallic lithium; lithium alloys such as lithium aluminum alloys, lithium tin alloys, lithium lead alloys, and lithium silicon alloys; metal oxides such as tin oxide, silicon oxide, lithium titanium oxide, niobium oxide, and tungsten oxide; metal sulfides such as tin sulfide and titanium sulfide; metal nitrides such as lithium cobalt nitride, lithium iron nitride, and lithium manganese nitride; and carbon materials such as graphite, with metallic lithium being preferred.

Furthermore, a material capable of absorbing and releasing magnesium ions is used as the negative electrode active material for magnesium-air secondary batteries. In addition to metallic magnesium, magnesium alloys such as magnesium aluminum, magnesium silicon, and magnesium gallium can also be used for such negative electrodes.

When a foil- or plate-shaped metal, alloy, or the like is used as the negative electrode active material, the foil- or plate-shaped negative electrode active material can be used as the negative electrode itself.

The negative electrode needs to contain at least the negative electrode active material, but may contain a binder to fix the negative electrode active material if necessary. The type and amount of binder used are the same as those for the air electrode described above, so a description of them will be omitted here.

A negative electrode current collector that collects current from the negative electrode is usually connected to the negative electrode. The material and shape of the negative electrode current collector are not particularly limited. Examples of materials for the negative electrode current collector include stainless steel, copper, nickel, etc. Also, examples of shapes of the negative electrode current collector include foil, plate, mesh (grid), etc.

(Electrolytes)
The electrolyte is disposed between the air electrode and the negative electrode. Metal ion conduction between the negative electrode and the air electrode occurs via the electrolyte. The form of the electrolyte is not particularly limited, and examples thereof include a liquid electrolyte, a gel electrolyte, and a solid electrolyte.

For example, when the negative electrode is zinc or an alloy thereof, the electrolyte may be an alkaline aqueous solution such as an aqueous potassium hydroxide solution or an aqueous sodium hydroxide solution containing zinc oxide, an aqueous solution containing zinc chloride or zinc perchlorate, or a non-aqueous solvent containing zinc perchlorate or a non-aqueous solvent containing zinc bis(trifluoromethylsulfonyl)imide. For example, when the negative electrode is magnesium or an alloy thereof, a non-aqueous solvent containing magnesium perchlorate or magnesium bis(trifluoromethylsulfonyl)imide may be used. Here, examples of non-aqueous solvents include organic solvents used in conventional secondary batteries and capacitors, such as ethylene carbonate (EC), propylene carbonate (PC), γ-butyrolactone (γ-BL), diethyl carbonate (DEC), and dimethyl carbonate (DMC). These may be used alone or in combination. Alternatively, an ionic liquid such as N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide (am) can be used.

In the secondary battery of the present invention, the electrolyte preferably contains a dendrite formation inhibitor. The dendrite formation inhibitor is believed to suppress the generation of dendrites by adsorbing to the negative electrode surface during charging, reducing the energy difference between crystal planes, and preventing preferred orientation. There are no particular limitations on the dendrite formation inhibitor, but it can be, for example, at least one selected from the group consisting of polyalkyleneimines, polyallylamines, and asymmetric dialkylsulfones (see, for example, JP 2009-93983 A). The amount of the dendrite formation inhibitor used is also not particularly limited, but it may be used in an amount that saturates the electrolyte at room temperature and normal pressure, or it may be used as a solvent.

In the case of a lithium-air secondary battery, the liquid electrolyte having lithium ion conductivity is usually a non-aqueous electrolyte solution containing a lithium salt and a non-aqueous solvent. Examples of the lithium salt include inorganic lithium salts such as _{LiPF6 , LiBF4} , _LiClO4 , and _LiAsF6 , and organic lithium salts such as _LiCF3SO3 , LiN( _CF3SO2 ) ₂ , _{LiN ( C2F5SO2 ) 2} , and LiC _{( CF3SO2} ) ₃ .

Examples of the non-aqueous solvent include ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), butylene carbonate, γ-butyrolactone, sulfolane, acetonitrile, 1,2-dimethoxymethane, 1,3-dimethoxypropane, diethyl ether, tetrahydrofuran, 2-methyltetrahydrofuran, and mixtures thereof. Ionic liquids can also be used as the non-aqueous solvent.

The concentration of the lithium salt in the non-aqueous electrolyte is not particularly limited, but is preferably within the range of 0.1 mol/L to 3 mol/L, and is preferably 1 mol/L. In the present invention, a low-volatility liquid such as an ionic liquid may be used as the non-aqueous electrolyte.

A gel electrolyte having lithium ion conductivity can be obtained, for example, by adding a polymer to the non-aqueous electrolyte solution and gelling it. Specifically, gelling can be achieved by adding a polymer such as polyethylene oxide (PEO), polyvinylidene fluoride (PVDF, such as Kynar, a product of Arkema), polyacrylonitrile (PAN), or polymethyl methacrylate (PMMA) to the non-aqueous electrolyte solution.

The solid electrolyte having lithium ion conductivity is not particularly limited, and may be any general solid electrolyte that can be used in lithium metal air secondary batteries, such as oxide solid electrolytes such as _{Li1.5Al0.5Ge1.5} ( _PO4 ) ₃ , and sulfide solid electrolytes such as _Li2S - _P2S5 compound, _Li2S - _{SiS2 compound} , _{and Li2S - GeS2} compound.

The thickness of the electrolyte varies greatly depending on the battery configuration, but it is preferably within the range of 10 μm to 5000 μm, for example.

(Additional components)
In the metal-air secondary battery of the present invention, a separator is preferably disposed between the air electrode and the negative electrode in order to ensure electrical insulation between these electrodes. The separator is not particularly limited as long as it has a structure that can ensure electrical insulation between the air electrode and the negative electrode and allows an electrolyte to be interposed between the air electrode and the negative electrode.

Examples of separators include porous membranes such as polyethylene, polypropylene, cellulose, polyvinylidene fluoride, and glass ceramics; and nonwoven fabrics such as resin nonwoven fabrics and glass fiber nonwoven fabrics. Among these, separators made of glass ceramics are preferred.

As a battery case for housing the metal-air secondary battery, a battery case for a general metal-air secondary battery can be used. The shape of the battery case is not particularly limited as long as it can hold the air electrode, negative electrode, and electrolyte described above, but specific examples include a coin type, a flat type, a cylindrical type, and a laminate type.

The metal-air secondary battery of the present invention is capable of discharging when oxygen, which is an active material, is supplied to the air electrode. Examples of oxygen supply sources include air and oxygen gas, and oxygen gas is preferred. There are no particular limitations on the pressure of the air or oxygen gas supplied, and it may be set appropriately.

The present invention will be described in more detail below with reference to examples. However, the examples are merely illustrative of the present invention, and the present invention is not intended to be limited to the examples.

(1) Synthesis of particulate material A precursor was synthesized by adding 15 ml of 1 M potassium hydroxide aqueous solution to 150 ml of an aqueous solution (total metal concentration 0.1 M) in which nitrates of iron group metals (Fe, Co, Ni) were dissolved in the molar ratios shown in Table 1. The precursor was calcined in an _O2 atmosphere at 100, 150, and 200 °C for 12 hours to synthesize a catalyst.

(2) Electrochemical Measurement The obtained powder sample was dispersed in ethanol with acetylene black (AB) and Nafion in a weight ratio of sample:AB:Nafion = 5:1:1, and applied to a glassy carbon (GC) electrode in an amount of oxide catalyst of 10 mg cm ^-2 . Electrochemical measurements were performed in a 4M KOH aqueous solution using a VersaStat4 (manufactured by AMETEC) and a rotating ring-disk electrode device (manufactured by Pine Research) with a Hg/HgO electrode as the reference electrode and a Pt wire as the counter electrode.

Carbon black powder (TB300) manufactured by Tokai Carbon and PTFE dispersion liquid were mixed in a ratio of carbon:PTFE = 70:30, and rolled into a sheet of about 0.3 mm thickness using a roll press. The obtained sheet was cut into a circle of φ12 mm and heat-treated at 335 ° C under a nitrogen atmosphere to prepare a gas diffusion layer. The catalyst sample was dispersed in a solvent of ultrapure water:isopropanol = 1: 1 with a carbon conductive assistant (TB3800 or carbon nanochip) and Nafion so that the weight ratio of sample:carbon:Nafion = 3:1:1, and the catalyst was applied to the gas diffusion layer so that the amount of oxide catalyst was 10 mg cm ^-2 as a catalyst layer. The gas diffusion layer to which the catalyst layer was applied was hot pressed together with a nickel mesh at 130 ° C and 0.7 t to prepare an air electrode. The air electrode was placed in a PFA cell, and the air electrode was evaluated at 40 ° C using a 4 M KOH aqueous solution as an electrolyte.

(3) Elemental ratios in raw materials and products The feed ratios during synthesis of the synthesized catalyst samples and the obtained samples (calcined at 150°C) were subjected to elemental analysis using SEM-EDX (Sigma-500, Carl Zeiss), and the contents of each metal atom are shown in Table 1. In all samples, the content ratios of Fe and Ni were greater than the feed ratios. Hereinafter, the names of the samples (calcined products) will be expressed as CoxNiyFez, using the numerical values (x, y, z) of the ratios of each element.

Example 1
The XRD patterns of the precursor and the catalyst calcined at 100°C, 150°C or 200°C in the Co73Ni13Fe14 sample are shown in FIG. 1. The precursor showed peaks derived from LDH (layered double hydroxide) and hydroxide, and was found to be a composite of particles having these crystal structures. The catalyst calcined from the precursor at 100°C showed the peaks of hydroxide disappeared and the peaks of LDH and oxyhydroxide. The catalyst calcined at 150°C showed the peaks of LDH disappeared and the peaks of oxyhydroxide and spinel oxide. The catalyst calcined at 200°C showed the peaks of oxyhydroxide disappeared and only the peaks of spinel oxide.

The results of ORR and OER activity evaluation of the precursor and samples calcined at each temperature in the Co73Ni13Fe14 sample are shown in FIG. 2. The precursor showed the same ORR activity as CFCO and higher OER activity than CFCO. CFCO is _Ca2FeCoO5 prepared in Example 1 of WO2015/115592. The 100°C calcined catalyst also showed activity similar to that of the precursor. The LDH, hydroxide, and _oxyhydroxide , which are the crystal structures of the precursor and the 100°C sample, are known as materials that show low ORR activity but high OER activity, and it is considered that this sample also showed low ORR activity and high OER activity. The 150°C calcined sample showed improved ORR activity than the 100°C calcined sample and showed higher ORR activity than CFCO. The OER activity was also lower than that of the 100°C calcined sample, and showed the same OER activity as CFCO. The 200°C sample had improved ORR activity and significantly decreased OER activity compared to the 150°C calcined sample. The spinel-type oxides found in the samples calcined at 150°C or higher are generally known as materials with high ORR activity, but their OER activity is lower than that of LDH, hydroxide, and oxyhydroxide. In this sample, the LDH and oxyhydroxide found in the 100°C calcined sample were partially converted to spinel-type oxides at 150°C, which is thought to have improved the ORR activity and decreased the OER activity. At 200°C, the entire sample was converted to a spinel structure, which is thought to have improved the ORR activity but significantly decreased the OER activity. From the above, it is thought that the optimal temperature for a bifunctional catalyst is, for example, about 125-175°C.

Example 2
The XRD patterns of the precursor and the catalyst calcined at 100°C or 150°C for the Co80Ni6Fe14 sample are shown in Figure 3. The Co80Ni6Fe14 sample showed similar changes in crystal structure to the Co73Ni13Fe14 sample, i.e., the precursor showed LDH and hydroxide peaks, the catalyst calcined at 100°C showed LDH and oxyhydroxide peaks with the disappearance of the hydroxide peak, and the catalyst calcined at 150°C showed oxyhydroxide and spinel oxide peaks with the disappearance of the LDH peak.

Figure 4 shows the results of ORR and OER activity evaluation of the precursor and samples fired at each temperature for the Co80Ni6Fe14 sample. The Co80Ni6Fe14 sample showed a similar trend to the Co73Ni13Fe14 sample. The precursor and the sample fired at 100°C showed ORR and OER activity at the same level as CFCO. The 150°C sample showed improved ORR activity and a decrease in OER activity.

Example 3
The XRD patterns of the precursor and 150°C calcined catalyst for the Co84Ni12Fe4 sample are shown in Figure 5. The Co84Ni12Fe4 sample showed similar crystal structure changes as the Co73Ni13Fe14 sample, i.e., the precursor showed LDH and hydroxide peaks, and the catalyst calcined at 150°C showed oxyhydroxide and spinel oxide peaks.

Figure 6 shows the results of ORR and OER activity evaluation of the precursor and samples fired at each temperature for the Co84Ni12Fe4 sample. The precursor sample showed ORR activity similar to that of CFCO, but lower activity in OER than CFCO. The 150°C sample showed improved ORR activity and almost the same activity in OER.

Reference Example 1
The XRD patterns of the precursor and the catalyst calcined at 100°C or 150°C for the Co72Ni6Fe22 sample are shown in Figure 7. The precursor showed peaks of LDH and hydroxide. In the sample calcined at 100°C, the hydroxide peak disappeared, and peaks due to oxyhydroxide and spinel oxide were shown. In the sample calcined at 150°C, the LDH and oxyhydroxide peaks disappeared, and only the peak of spinel oxide was shown.

Figure 8 shows the results of ORR and OER activity evaluation of the precursor and samples fired at each temperature for the Co72Ni6Fe22 sample. The precursor and the sample fired at 100°C showed low ORR activity similar to that of CFCO, and higher OER activity than CFCO. The 150°C sample showed a slight improvement in ORR activity, but showed lower ORR activity than the 150°C fired Co73Ni13Fe14 sample and the 150°C fired Co80Ni6Fe14 sample. The OER activity of the 150°C sample was lower than that of the precursor and the 100°C fired sample, and showed the same level of OER activity as CFCO.

The 150°C calcined Co72Ni6Fe22 sample exhibits only a spinel structure, and so is expected to exhibit high ORR activity, but in reality it exhibited low ORR activity. In addition, when comparing the ORR activity of the 150°C calcined catalysts of the above four compositions, the samples with a higher Fe content exhibited lower ORR activity. From this, it is believed that the higher the Fe ratio, the lower the ORR activity of the spinel-type oxide. On the other hand, it was found that the higher the Fe ratio, the higher the OER activity.

Reference Example 2
The XRD patterns of the precursor and the catalyst calcined at 150° C. in the Co65Ni20Fe15 sample are shown in Figure 9. In the Co65Ni20Fe15 sample, the precursor showed a peak only for LDH, and the catalyst calcined at 150° C. showed a peak only for spinel-type oxide.

Figure 10 shows the results of ORR and OER activity evaluation of the precursor and samples fired at each temperature for the Co65Ni20Fe15 sample. The precursor sample showed ORR and OER activity comparable to that of CFCO. The 150°C sample showed a slight improvement in ORR activity, but showed lower ORR activity than the 150°C fired Co73Ni13Fe14 sample, the 150°C fired Co80Ni6Fe14 sample, and the 150°C fired Co84Ni12Fe4 sample. In OER, it showed lower activity than the precursor.

Reference Example 3
The XRD patterns of the precursor and the catalyst calcined at 100° C. or 150° C. in the Co50Ni36Fe14 sample are shown in Figure 11. Both the precursor and the calcined sample of the Co50Ni36Fe14 sample showed only the LDH peak.

Figure 12 shows the results of ORR and OER activity evaluation of the precursor and samples calcined at each temperature for the Co50Ni36Fe14 sample. For the Co65Ni5Fe10 sample, both the precursor and the calcined sample showed almost the same activity, with low ORR activity similar to that of CFCO and higher OER activity than CFCO. LDH is a material that shows high activity in OER, and since the crystal structure was the same even after calcination, it is believed to have shown almost the same activity.

[TEM image]
FIG. 13-1 shows a TEM image of the Co73Ni13Fe14 sample sintered at 150°C in Example 1. The TEM was a JEM-2000FX (JEOL Ltd.). It was found that the sample was in a state of secondary particles with a particle size of several hundred nm, which were formed by agglomeration of primary particles with a particle size of about 10 nm. When the crystal structure was identified from the lattice spacing of the primary particles (metal oxide hydroxide particle agglomerates and spinel-type metal oxide particle agglomerates) observed with a high magnification TEM, it was found that the particulate material, which is a secondary particle, is in a state in which spinel-type metal oxide particle agglomerates, which are primary particles of spinel oxide, and metal oxide hydroxide particle agglomerates, which are primary particles of oxide hydroxide, are highly dispersed and composited. The average particle size of the spinel-type oxide particle agglomerates was 5.1 nm, and the average particle size of the oxide hydroxide particle agglomerates was 4.9 nm. In addition, TEM-EDX analysis revealed that the metal contents in the spinel oxide were 60-85 mol% Co atoms, 7-20 mol% Ni atoms, and 3-18 mol% Fe atoms, while the metal contents in the oxyhydroxide were 64-85 mol% Co atoms, 8-24 mol% Ni atoms, and 5-20 mol% Fe atoms, indicating that all nanoparticles contained all three metal elements with similar ratios. The results of the TEM-EDX analysis are shown in Figure 13-2.

[STEM image]
FIG. 14-1 shows an STEM image of the Co80Ni6Fe14 sample sintered at 150° C. in Example 2. The STEM was a JEM-ARM200F (JEOL Ltd.). As in Example 1, the sample was found to be a particle mass having a particle size of several hundred nm, which was formed by agglomeration of primary particles having a particle size of about 10 nm. When the crystal structure was identified from the lattice spacing of the primary particles by high-magnification STEM observation, it was found that the particle mass was in a state where primary particles of spinel oxide and oxyhydroxide were highly dispersed and composited. It was found that the average particle size of the spinel oxide particle mass was 6.3 nm, and the average particle size of the oxyhydroxide particle mass was 5.9 nm. In addition, the STEM-EDX analysis showed that the metal contents in the spinel oxide were Co: 71-87 mol%, Ni: 4-13 mol%, and Fe: 6-20 mol%, and the metal contents in the oxyhydroxide were Co: 69-90 mol%, Ni: 5-16 mol%, and Fe: 6-18 mol%, indicating that all nanoparticles contained all three metal elements with the same ratio distribution. The results of the STEM-EDX analysis are shown in Figure 14-2.

[Relationship between composition ratio and ORR, OER activity]
When the composition ratios of the samples synthesized in Examples 1 to 3 and Reference Examples 1 to 3 and the tendency of the ORR activity and the OER activity were examined, it was found that the higher the content ratio of Ni atoms and Fe atoms (in mol%), the higher the OER activity. On the other hand, in the ORR activity, when the crystal structure of a spinel-type oxide is present, the higher the Ni atom content ratio and the lower the Fe content ratio, the higher the ORR activity. When the correlation between the composition ratio and the crystal structure of the synthesized sample was examined, it was found that when the Ni atom content is more than 25 mol%, no hydroxide is generated in the precursor and only LDH is generated, and after calcination, no oxyhydroxide is generated and only spinel-type oxide is generated or the crystal structure of LDH remains unchanged. From the above results, it was found that a bifunctional catalyst in which oxyhydroxide and spinel-type oxide coexist can be obtained by calcination treatment in the range of Co atom content 60 to 95, Ni atom content 1 to 25 mol%, and Fe content 1 to 20 mol%. In particular, it was found that the Co73Ni13Fe14 sample calcined at 150°C exhibited the highest activity as a bifunctional catalyst among the samples obtained in the examples.

[Air electrode durability test results]
The Co73Ni13Fe14 sample (Example 1) calcined at 150°C was applied to a gas diffusion electrode and subjected to a cycle durability test as an air electrode. Figure 15 shows the electrode potential over the number of cycles when OER and ORR were performed for 1 hour each per cycle under a constant current condition of 20 mA cm ^-2 . During the test, the OER activity was almost constant and stable, and the ORR activity gradually improved with the increase in the number of cycles. The overall overvoltage decreased with the increase in the number of cycles. It was revealed that the Co73Ni13Fe14 sample calcined at 150°C showed good bifunctional performance and showed high durability of 450 cycles (1.5 months) as an air electrode catalyst.

The present invention is useful in the field of secondary batteries, particularly metal-air secondary batteries, which are expected to be the next generation of high-capacity secondary batteries.

1 Metal-air secondary battery 2 Air electrode 3 Negative electrode 4 Electrolyte 4
5 Air electrode current collector 6 Negative electrode current collector 6

Claims (11)

The metal oxide hydroxide particle agglomerate containing Co, Ni and Fe and having a particle size in the range of 1 to 100 nm, and a spinel-type metal oxide particle agglomerate containing Co, Ni and Fe and having a particle size in the range of 1 to 100 nm are contained in one particle,
A particulate material containing 70 to 95 mol % Co atoms, 1 to 25 mol % Ni atoms, and 1 to 20 mol % Fe atoms, with the total content of Ni atoms and Fe atoms being 5 mol % or more, with the proviso that the total of Co atoms, Ni atoms, and Fe atoms is 100 mol %, and peaks of the metal oxide hydroxide and the spinel metal oxide are detected by XRD measurement . The particulate material according to claim 1, wherein at least a portion of the metal oxide hydroxide particle agglomerates and the spinel-type metal oxide particle agglomerates are present on the surface of the particulate material. 2. The particulate material according to claim 1 , wherein the particulate material has a Co atomic content of 70-90 mol%, a Ni atomic content of 4-25 mol% and a Fe atomic content of 2-14 mol%. the metal oxide hydroxide particle agglomerates contain 70 to 95 mol % of Co atoms, 1 to 25 mol % of Ni atoms, and 1 to 20 mol % of Fe atoms, the total content of Ni atoms and Fe atoms being 5 mol % or more;
The particulate material according to any one of claims 1 to 3, wherein the spinel-type metal oxide particle agglomerates contain 70 to 95 mol % Co atoms, 1 to 25 mol % Ni atoms, and 1 to 20 mol % Fe atoms, with the total content of Ni atoms and Fe atoms being 5 mol % or more, provided that in each case , the total of Co atoms, Ni atoms, and Fe atoms is 100 mol %. 5. The particulate material according to claim 1, wherein the particle size of the metal oxide hydroxide particle agglomerates is in the range of 1 to 20 nm, and the particle size of the spinel type metal oxide particle agglomerates is in the range of 1 to 20 nm. A particulate material according to any one of claims 1 to 5 , wherein at least a portion of the particulate material has a particle size in the range 0.01 to 50 µm. a precursor containing a layered double hydroxide containing Co, Ni and Fe and a metal hydroxide containing Co, Ni and Fe is fired at a temperature in the range of 110 to 190°C to produce a metal oxide hydroxide particle agglomerate containing Co, Ni and Fe having a particle size in the range of 1 to 100 nm and a spinel type metal oxide particle agglomerate containing Co, Ni and Fe having a particle size in the range of 1 to 100 nm,
A method for producing a particulate material comprising obtaining a particulate material containing 70 to 95 mol % Co atoms, 1 to 25 mol % Ni atoms, and 1 to 20 mol % Fe atoms, with the total content of Ni atoms and Fe atoms being 5 mol % or more, with the proviso that the total of Co atoms, Ni atoms, and Fe atoms is 100 mol %, and in which peaks of the metal oxide hydroxide and the spinel metal oxide are detected by XRD measurement . 8. The method according to claim 7 , wherein the precursor is produced by adding an alkali to an aqueous solution containing 70 to 98 mol % of Co ²⁺ , 1 to 20 mol % of Ni ²⁺ , and 1 to 10 mol % of Fe ³⁺ . A catalyst for an air electrode comprising the particulate material according to any one of claims 1 to 6 . A metal-air secondary battery air electrode comprising the particulate material according to any one of claims 1 to 6 . A metal-air secondary battery comprising an air electrode containing the particulate material according to any one of claims 1 to 6 , a negative electrode containing a negative electrode active material, and an electrolyte interposed between the air electrode and the negative electrode.