Catalyst for air secondary battery, air electrode and air secondary battery

Abstract

To provide: an air secondary battery catalyst capable of reducing over-voltage in a discharge reaction more than conventional ones to contribute to enhancement of discharge voltage; an air electrode containing the air secondary battery catalyst; and an air secondary battery of the air electrode.SOLUTION: A battery 2 comprises: an electrode group 10 containing an air electrode 16 and a negative electrode 12 overlapped with each other via a separator 14; and a container 4 that houses the electrode group 10 together with an alkaline electrolyte 82. The air electrode 16 contains an air secondary battery catalyst. This air secondary battery catalyst is a pyrochlore type bismuth ruthenium compound oxide, in which a half-value whole width of a diffraction peak with respect to 222 surfaces obtained by a powder X-ray diffraction using a CuKα line as an X-ray is 0.350deg or larger and 0.713deg or less is contained.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a catalyst for an air secondary battery, an air electrode including the catalyst for an air secondary battery, and an air secondary battery including the air electrode. [Background technology]

[0002] In recent years, air batteries, which use atmospheric oxygen as the positive electrode active material, have attracted attention due to their high energy density and ease of miniaturization and lightweight design. Among such air batteries, zinc-air primary batteries have been put to practical use as power sources for hearing aids.

[0003] Additionally, research is being conducted on rechargeable air batteries, which use metals such as Li, Zn, Al, and Mg for the negative electrode. These air secondary batteries are expected to be new secondary batteries with the potential to exceed the energy density of lithium-ion secondary batteries.

[0004] One type of such air secondary battery is known as an air-hydrogen secondary battery, which uses an alkaline aqueous solution (hereinafter also referred to as alkaline electrolyte) as the electrolyte and hydrogen as the negative electrode active material (see, for example, Patent Document 1). Air-hydrogen secondary batteries such as those described in Patent Document 1 use a hydrogen storage alloy as the negative electrode metal. However, the negative electrode active material in air-hydrogen secondary batteries is hydrogen absorbed and released by the hydrogen storage alloy. Therefore, dissolution and precipitation reactions of the hydrogen storage alloy itself do not occur during chemical reactions (hereinafter also referred to as battery reactions) during battery charging and discharging. Therefore, air-hydrogen secondary batteries have the advantage of not being susceptible to problems such as internal short circuits caused by so-called dendrite growth, in which the negative electrode metal precipitates in a dendritic form, or a decrease in battery capacity due to shape change.

[0005] In an air secondary battery that uses an alkaline electrolyte, such as the above-mentioned air hydrogen secondary battery, the following charge and discharge reactions occur at the positive electrode (hereinafter also referred to as the air electrode).

[0006] Charging (oxygen generation reaction): 4OH -→O2+2H2O+4e - (I) Discharge (oxygen reduction reaction): O2 + 2H2O + 4e - →4OH - (II)

[0007] As shown in reaction formula (I), oxygen is generated at the air electrode during charging of an air secondary battery. This oxygen passes through the voids inside the air electrode and is released into the atmosphere from the part of the air electrode that is open to the atmosphere. On the other hand, during discharge, oxygen taken in from the atmosphere is reduced to produce hydroxide ions as shown in reaction formula (II).

[0008] A catalyst that promotes the above-mentioned charge / discharge reactions is used in the air electrode, which is the positive electrode of the above-mentioned air secondary battery. In air secondary batteries, it is desirable to reduce the overvoltage in the charge / discharge reactions of the air electrode in order to improve energy efficiency and increase power output. Therefore, materials that are effective in reducing overvoltage have been investigated as catalyst materials for use in the air electrode. Examples of materials that are effective in reducing overvoltage include noble metals, metal oxides, and metal complexes. Among these materials, pyrochlore-type bismuth ruthenium composite oxides have the dual functions of oxygen reduction and oxygen generation, and are therefore promising as catalysts for air secondary batteries because they can reduce overvoltage during charging and discharging. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Publication No. 2019-179592 Summary of the Invention [Problem to be solved by the invention]

[0010] However, since air secondary batteries are expected to be used in a variety of applications, there is a demand for even higher output. To achieve this, it is particularly necessary to increase the discharge voltage.

[0011] The present invention has been made in light of the above circumstances, and an object of the present invention is to provide a catalyst for air secondary batteries that can contribute to reducing overvoltage in discharge reactions and increasing discharge voltage more than conventional catalysts, an air electrode including this catalyst for air secondary batteries, and an air secondary battery including this air electrode. [Means for solving the problem]

[0012] In order to achieve the above object, the present invention provides a catalyst for an air secondary battery, which contains a pyrochlore-type bismuth-ruthenium composite oxide, in which the full width at half maximum of a diffraction peak corresponding to the 222 plane obtained by powder X-ray diffraction using CuKα rays is 0.350 deg or more and 0.713 deg or less. [Effects of the Invention]

[0013] The catalyst for air secondary batteries according to the present invention is a pyrochlore-type bismuth-ruthenium composite oxide, which contains a bismuth-ruthenium composite oxide having a full width at half maximum of 0.350° or more and 0.713° or less of a diffraction peak corresponding to the 222 plane obtained by powder X-ray diffraction using CuKα radiation as the X-ray. When the full width at half maximum of the diffraction peak corresponding to the 222 plane obtained by powder X-ray diffraction using CuKα radiation is in the range of 0.350° or more and 0.713° or less, catalytic activity is enhanced, and an air secondary battery using this air secondary battery catalyst in the air electrode can reduce overvoltage in the discharge reaction compared to conventional air secondary batteries. Therefore, the present invention can provide an air secondary battery catalyst that can increase discharge voltage compared to conventional air secondary batteries, an air electrode including this air secondary battery catalyst, and an air secondary battery including this air electrode. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a cross-sectional view schematically illustrating an air hydrogen secondary battery according to one embodiment. [Figure 2]1 shows XRD profiles of catalysts for air secondary batteries according to Examples 1 to 7 and Comparative Example 1. [Figure 3] This is an enlarged XRD profile of the area around 2θ=30° in FIG. [Figure 4] 1 is a graph showing the relationship between the full width at half maximum of the 222 plane and the discharge intermediate voltage. DETAILED DESCRIPTION OF THE INVENTION

[0015] An air-hydrogen secondary battery (hereinafter also referred to as battery) 2 including an air electrode catalyst for an air secondary battery according to one embodiment will be described below with reference to the drawings.

[0016] As shown in FIG. 1, the battery 2 includes a container 4 and an electrode group 10 placed in the container 4 together with an alkaline electrolyte 82 .

[0017] The electrode group 10 is formed by stacking a negative electrode 12 and an air electrode (positive electrode) 16 with a separator 14 interposed therebetween.

[0018] The negative electrode 12 includes a conductive negative electrode substrate having a porous structure and a large number of pores, and a negative electrode mixture supported in the pores and on the surface of the negative electrode substrate. The negative electrode substrate may be, for example, foamed nickel.

[0019] The negative electrode mixture contains a hydrogen storage alloy powder, which is an aggregate of hydrogen storage alloy particles capable of absorbing and releasing hydrogen as a negative electrode active material, a conductive material, and a binder. The conductive material may be graphite powder, carbon black powder, or the like.

[0020] The hydrogen storage alloy constituting the hydrogen storage alloy particles is not particularly limited, but for example, a rare earth-Mg-Ni based hydrogen storage alloy is preferably used. The composition of this rare earth-Mg-Ni based hydrogen storage alloy can be freely selected, but for example, General formula: Ln 1-a Mg a Ni b-c-d Alc M d (III) It is preferable to use one represented by the following formula:

[0021] In general formula (III), Ln represents at least one element selected from the group consisting of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, Y, Zr, and Ti; M represents at least one element selected from the group consisting of V, Nb, Ta, Cr, Mo, Mn, Fe, Co, Ga, Zn, Sn, In, Cu, Si, P, and B; and the subscripts a, b, c, and d represent numbers that satisfy the relationships 0.01≦a≦0.30, 2.8≦b≦3.9, 0.05≦c≦0.30, and 0≦d≦0.50, respectively.

[0022] Here, the hydrogen storage alloy particles can be obtained, for example, as follows. First, metal raw materials are weighed and mixed to obtain a predetermined composition. This mixture is melted in an inert gas atmosphere, for example, in a high-frequency induction melting furnace, and then cooled to form an ingot. The resulting ingot is heated to 900-1200°C in an inert gas atmosphere and homogenized by heat treatment, maintaining the temperature at that temperature for 5-24 hours. The ingot is then crushed and sieved to obtain a hydrogen storage alloy powder, which is an aggregate of hydrogen storage alloy particles of the desired particle size.

[0023] Examples of the binder include sodium polyacrylate, carboxymethyl cellulose, and styrene butadiene rubber.

[0024] Here, the negative electrode 12 can be produced, for example, as follows. First, a negative electrode mixture paste is prepared by kneading a hydrogen storage alloy powder, which is an aggregate of hydrogen storage alloy particles, a conductive material, a binder, and water. The resulting negative electrode mixture paste is filled into a negative electrode substrate, which is then dried. After drying, the negative electrode substrate with the hydrogen storage alloy particles and other components attached is rolled to increase the amount of alloy per unit volume, and then cut to obtain the negative electrode 12. This negative electrode 12 has a plate-like shape overall. The negative electrode mixture layer contained in the negative electrode 12 is formed from hydrogen storage alloy particles, conductive material particles, and other components, and therefore has gaps between the particles, resulting in an overall porous structure.

[0025] Next, the air electrode 16 includes a conductive air electrode substrate having a mesh structure and an air electrode mixture layer (cathode mixture layer) formed of an air electrode mixture (cathode mixture) held on the air electrode substrate. For example, a nickel mesh can be used as the air electrode substrate.

[0026] The air electrode mixture contains a redox catalyst (a catalyst for air secondary batteries), a conductive material, and a binder. Furthermore, it is preferable to add a water repellent agent to the air electrode mixture.

[0027] The redox catalyst used has a dual redox function. Such a dual-function catalyst contributes to reducing the overvoltage of the battery during both the charging and discharging processes. For example, a pyrochlore-type bismuth ruthenium composite oxide is used as such a redox catalyst. This bismuth ruthenium composite oxide has the dual functions of oxygen generation and oxygen reduction.

[0028] Bismuth ruthenium composite oxide has the composition formula Bi 2-x Ru2O 7-z(where 0≦x≦1, and z satisfies the relationship 0≦z≦1.) The bismuth ruthenium composite oxide used in this embodiment has a full width at half maximum of 0.350 deg or more and 0.713 deg or less of a diffraction peak corresponding to the 222 plane obtained by powder X-ray diffraction using CuKα rays as X-rays.

[0029] Generally, catalytic reactions of oxide catalysts occur on solid surfaces, but the surface structure is supported by the bulk crystalline structure, and differences in crystalline structure often have a significant effect on battery performance. However, the optimal conditions for crystallinity and battery performance vary widely depending on the material. For this reason, the relationship between crystallinity and battery performance for all materials has not yet been clarified, and to the knowledge of the present inventors, no research has been conducted on the relationship between the crystallinity and battery performance of bismuth ruthenium composite oxides. Therefore, the present inventors have conducted extensive research into the relationship between the crystal structure and battery performance of bismuth ruthenium composite oxides. As a result, they have found that when the crystallinity of a bismuth ruthenium composite oxide used as an air secondary battery catalyst falls within a certain range, the overvoltage of the discharge reaction of the air secondary battery can be significantly reduced. Here, in this embodiment, crystallinity refers to the proportion of amorphous regions in the crystals of the bismuth ruthenium composite oxide. For example, the higher the crystallinity, the smaller the proportion of amorphous regions, and the lower the crystallinity, the larger the proportion of amorphous regions. As a means of specifically expressing this crystallinity numerically, we decided to use the relationship between the full width at half maximum of the diffraction peak corresponding to the 222 plane obtained by powder X-ray diffraction using CuKα X-rays. In other words, the smaller the full width at half maximum, the higher the crystallinity, and the larger the full width at half maximum, the lower the crystallinity and the more amorphized the material is.

[0030] It was confirmed that if the full width at half maximum is 0.350 deg or more, the effect of reducing the overvoltage in the discharge reaction of the resulting air secondary battery can be obtained, whereas if the full width at half maximum exceeds 0.713 deg, the effect of reducing the overvoltage in the discharge reaction rapidly decreases.

[0031] The pyrochlore-type bismuth ruthenium composite oxide described above can be produced, for example, as follows.

[0032] Bi(NO3)3·5H2O and RuCl3·3H2O are prepared. Then, Bi(NO3)3·5H2O and RuCl3·3H2O are weighed so that the molar ratio of Ru to Bi is 1.00 and greater than or equal to 0.50 but less than 0.80. The weighed Bi(NO3)3·5H2O and RuCl3·3H2O are added to a specified solution and stirred to prepare a mixed aqueous solution of Bi(NO3)3·5H2O and RuCl3·3H2O. The specified solution can be distilled water or a dilute nitric acid solution, and the temperature of the solution should be between 60°C and 90°C. Then, a 1 mol / L to 3 mol / L NaOH solution is added to the mixed aqueous solution to precipitate the precursor (coprecipitation process). After the precursor precipitates, the mixed aqueous solution is stirred. This stirring operation is continued for 12 to 60 hours with oxygen bubbling. During the stirring operation, the pH of the mixed aqueous solution is maintained at 10 to 12, and the temperature is maintained at 60°C or higher and 90°C or lower. After the stirring operation is completed, the mixed aqueous solution is left to stand for 12 to 60 hours. After standing, the resulting precipitate is collected by suction filtration. The collected precipitate is kept at 80°C or higher and 100°C or lower to evaporate some of the water and form a paste. This paste is transferred to an evaporating dish and heated to 100°C or higher and 150°C or lower, and dried by holding at that temperature for 1 hour or higher and 5 hours or lower to obtain a dried paste. The dried paste obtained is placed in a mortar and crushed with a pestle to obtain a precursor powder.

[0033] Next, the precursor powder is heated to a temperature of 400°C or higher and 700°C or lower in an air atmosphere and maintained for 0.5 hours or higher and 4 hours or lower (calcination step). After the heat treatment, the powder is washed with distilled water at a temperature of 60°C or higher and 90°C or lower, and then dried. This drying step is performed by maintaining the washed powder at a temperature of 60°C or higher and 130°C or lower for 1 hour or higher and 12 hours or lower. This produces a pyrochlore-type bismuth ruthenium composite oxide (Bi 2-x Ru2O 7-z ) is obtained.

[0034] Next, it is preferable to immerse the obtained bismuth ruthenium composite oxide in an aqueous nitric acid solution to perform an acid treatment. Specifically, the procedure is as follows.

[0035] First, an aqueous nitric acid solution is prepared. The concentration of the aqueous nitric acid solution is preferably 5 mol / L or less. The amount of the aqueous nitric acid solution is preferably 20 mL per 1 g of bismuth ruthenium composite oxide. The temperature of the aqueous nitric acid solution is preferably set to 20°C or higher and 25°C or lower.

[0036] The bismuth ruthenium composite oxide is then immersed in the prepared nitric acid aqueous solution and stirred for at least one hour and not more than six hours. After stirring for the specified time, the bismuth ruthenium composite oxide is suction filtered from the nitric acid aqueous solution. The filtered bismuth ruthenium composite oxide is then placed in distilled water set to at least 60°C and not more than 80°C and washed.

[0037] The washed bismuth ruthenium composite oxide is dried by being kept at a temperature of 60° C. or higher and 130° C. or lower for 1 hour or longer and 12 hours or shorter.

[0038] In this manner, an acid-treated bismuth ruthenium composite oxide is obtained. By carrying out the acid treatment in this manner, by-products generated in the calcination step of the bismuth ruthenium composite oxide can be removed. The acidic aqueous solution used for the acid treatment is not limited to an aqueous nitric acid solution; in addition to an aqueous nitric acid solution, an aqueous hydrochloric acid solution or an aqueous sulfuric acid solution can also be used. These aqueous hydrochloric acid and aqueous sulfuric acid solutions also have the effect of removing by-products, similar to an aqueous nitric acid solution.

[0039] Here, factors that affect the crystallinity of the bismuth ruthenium composite oxide include the firing conditions in the firing step described above. Specifically, the proportion of amorphous regions can be changed by adjusting the firing temperature and firing time, thereby controlling the crystallinity of the bismuth ruthenium composite oxide.

[0040] The bismuth ruthenium composite oxide obtained as described above is mechanically pulverized as necessary to adjust the particle size to a predetermined value, thereby obtaining a powder of the bismuth ruthenium composite oxide which is an aggregate of particles having the predetermined particle size.

[0041] The pulverization method is not particularly limited, but it is preferable to use a wet bead mill. The procedure for pulverization using a wet bead mill is as follows: first, ion-exchanged water and a dispersant are added to the bismuth ruthenium composite oxide and stirred to prepare a dispersion. Next, this dispersion is pumped into the pulverization chamber of the wet bead mill at a predetermined flow rate. Zirconia beads with a predetermined diameter, for example, 0.1 mm, are placed in the pulverization chamber. The stirring mechanism in the pulverization chamber is driven at a predetermined speed to generate centrifugal force, which energizes the beads, which then act on the bismuth ruthenium composite oxide particles to be pulverized. This pulverization process results in the dispersion being discharged from the pulverization chamber. In the wet bead mill, the dispersion discharged from the pulverization chamber is then pumped back into the pulverization chamber and pulverized again. In this way, the procedure of feeding the dispersion, pulverizing treatment, and discharging is considered as one pass, and by repeating this one pass, the particles of the bismuth ruthenium composite oxide can be pulverized more finely.

[0042] In a wet bead mill, the degree of pulverization of the bismuth ruthenium composite oxide particles can be controlled by the drive speed of the stirring mechanism and the number of times the above-mentioned single pass is repeated. Here, when strong pulverization is performed, which results in a high degree of pulverization, the bismuth ruthenium composite oxide is subjected to a load, and the amorphous region increases. In other words, the proportion of the amorphous region can be changed by adjusting the drive speed of the stirring mechanism and the number of passes, so the crystallinity of the bismuth ruthenium composite oxide can also be controlled by the pulverization conditions.

[0043] Next, the conductive material will be described. The conductive material is used to reduce the internal resistance in order to increase the output of the air secondary battery, and as a support for supporting the above-mentioned oxidation-reduction catalyst.

[0044] Examples of such conductive materials (catalyst-supporting conductive materials) include graphite and nickel. It is particularly preferable to use nickel powder made of nickel particles. The average particle size of the nickel particles is not particularly limited, but it is preferable to set the size so as to impart the desired conductivity to the air electrode.

[0045] The nickel powder is preferably contained in the air electrode mixture in an amount of 60% by mass or more. The upper limit of the content of this nickel powder is preferably 80% by mass or less in relation to the other constituent materials in the air electrode mixture. Carbonyl nickel powder is preferably used as the nickel powder. More preferably, filamentous nickel powder is used.

[0046] The binder functions to bind the constituent materials of the air electrode mixture, and it is preferable to use a commonly used binder as the binder.

[0047] The water repellent imparts appropriate water repellency to the air electrode 16. The water repellent is not particularly limited, but examples of the water repellent include fluororesins such as FEP (perfluoroethylene propene copolymer), PTFE (polytetrafluoroethylene), PFA (perfluoroalkoxyalkane polymer), ETFE (ethylene-tetrafluoroethylene copolymer), PVDF (polyvinylidene fluoride), and PVF (polyvinyl fluoride). PTFE is a preferred fluororesin. PTFE has the property of becoming fibrous under shear stress and also functions to bind the air electrode composite, so it can also serve as a binder.

[0048] The air electrode 16 can be manufactured, for example, as follows. First, a catalyst powder which is an aggregate of bismuth ruthenium composite oxide particles, a conductive powder which is an aggregate of Ni particles as a conductive material, a water repellent, and water are prepared.The catalyst powder, conductive powder, water repellent, and water are then kneaded together to prepare an air electrode mixture paste.

[0049] The obtained air electrode mixture paste is formed into a sheet, for example, by roller pressing, and dried at room temperature of about 25°C. This produces an air electrode mixture sheet. The air electrode mixture sheet is then press-bonded to a nickel mesh (air electrode substrate), thereby producing an intermediate air electrode product.

[0050] The resulting intermediate product is then placed in a heat treatment furnace and subjected to heat treatment (firing). This heat treatment is carried out in an inert gas atmosphere. Examples of the inert gas include nitrogen gas and argon gas. The heat treatment is carried out by heating the intermediate product to a temperature of 200°C or higher and 400°C or lower and maintaining this temperature for 10 minutes or longer and 40 minutes or shorter. The intermediate product is then naturally cooled in the heat treatment furnace and removed into the atmosphere when its temperature reaches 150°C or lower. This results in a heat-treated intermediate product. The heat-treated intermediate product is then cut into a predetermined shape to obtain an air electrode 16. The air electrode 16 includes an air electrode mixture layer formed from an air electrode mixture. The air electrode mixture layer formed from such an air electrode mixture has a porous structure including numerous pores overall and has excellent gas diffusion properties.

[0051] The air electrode 16 and negative electrode 12 obtained as described above are stacked with a separator 14 interposed therebetween, thereby forming an electrode group 10. This separator 14 is disposed to prevent short circuits between the air electrode 16 and the negative electrode 12, and is made of an electrically insulating material. Materials that can be used for this separator 14 include, for example, a polyamide fiber nonwoven fabric to which hydrophilic functional groups have been added, or a polyolefin fiber nonwoven fabric such as polyethylene or polypropylene to which hydrophilic functional groups have been added.

[0052] The formed electrode group 10 is placed in a container 4. There are no particular limitations on the container 4 as long as it can accommodate the electrode group 10 and the alkaline electrolyte, and for example, an acrylic box-shaped container 4 is used. For example, as shown in FIG. 1 , the container 4 includes a container body 6 and a lid 8.

[0053] The container body 6 is box-shaped and has a bottom wall 18 and side walls 20 extending upward from the peripheral edge of the bottom wall 18. The portion surrounded by the upper edge 21 of the side wall 20 is open. In other words, an opening 22 is provided on the opposite side of the bottom wall 18. Furthermore, the side wall 20 has through holes at predetermined positions on the right wall 20R and the left wall 20L, respectively, which serve as lead wire outlets 24, 26, which will be described later.

[0054] Furthermore, an electrolyte solution storage unit 80 is attached to the container body 6. This electrolyte solution storage unit 80 is a container that stores an alkaline electrolyte solution 82, and is attached, for example, via a connecting unit 84 that communicates with a through-hole 19 provided in the bottom wall 18. The connecting unit 84 is a flow path for the alkaline electrolyte solution 82 that communicates between the interior of the container 4 and the electrolyte solution storage unit 80. Since the interior of the container 4 and the electrolyte solution storage unit 80 are thus connected, the alkaline electrolyte solution 82 can move between the interior of the container 4 and the electrolyte solution storage unit 80.

[0055] The lid 8 has the same shape as the container body 6 in a plan view, and is placed on top of the container body 6 to close the opening 22. The gap between the lid 8 and the upper edge 21 of the side wall 20 is liquid-tightly sealed.

[0056] An air vent 30 is provided in the inner surface 28 of the lid 8, which faces the inside of the container body 6. The air vent 30 is open at the portion facing the inside of the container body 6, and has a serpentine shape overall. Furthermore, an inlet air hole 32 and an outlet air hole 34 are provided at predetermined positions in the lid 8, penetrating the lid 8 in the thickness direction. The inlet air hole 32 communicates with one end of the air vent 30, and the outlet air hole 34 communicates with the other end of the air vent 30. In other words, the air vent 30 is open to the atmosphere via the inlet air hole 32 and the outlet air hole 34. It is preferable to attach a pressure pump (not shown) to the inlet air hole 32. By driving this pressure pump, air can be sent from the inlet air hole 32 to the air vent 30.

[0057] An adjustment member 36 is placed on the bottom wall 18 of the container body 6 as needed. The adjustment member 36 is used to align the electrode group 10 in the height direction inside the container 4. As the adjustment member 36, for example, a foamed nickel sheet is used.

[0058] The electrode group 10 is disposed on the adjustment member 36. At this time, the negative electrode 12 of the electrode group 10 is disposed so as to be in contact with the adjustment member 36.

[0059] On the other hand, a water-repellent ventilation member 40 is disposed on the air electrode 16 side of the electrode group 10 so as to be in contact with the air electrode 16. This water-repellent ventilation member 40 is a combination of a PTFE porous membrane 42 and a nonwoven diffusion paper 44. The water-repellent ventilation member 40 exhibits a water-repellent effect due to the PTFE and also allows gas to pass through. The water-repellent ventilation member 40 is interposed between the lid 8 and the air electrode 16 and is in close contact with both the lid 8 and the air electrode 16. This water-repellent ventilation member 40 is large enough to cover the entire ventilation channel 30, inlet vent 32, and outlet vent 34 of the lid 8.

[0060] The container body 6, which houses the electrode group 10, the adjusting member 36, and the water-repellent and breathable member 40 as described above, is covered with the lid 8. Then, as schematically depicted in FIG. 1 , peripheral edge portions 46, 48 of the container 4 (container body 6 and lid 8) are sandwiched from above and below by connectors 50, 52. Thereafter, a predetermined amount of alkaline electrolyte 82 is poured from the electrolyte reservoir 80, and the container 4 is filled with the alkaline electrolyte 82. In this manner, the battery 2 is formed.

[0061] As the alkaline electrolyte 82, a general alkaline electrolyte used in alkaline secondary batteries is suitably used, and specifically, an aqueous solution containing at least one of NaOH, KOH, and LiOH as a solute is used.

[0062] In the battery 2, the ventilation channel 30 of the lid 8 faces the water-repellent ventilation member 40. The water-repellent ventilation member 40 allows gas to pass through but blocks moisture, so the air electrode 16 is open to the atmosphere via the water-repellent ventilation member 40, the ventilation channel 30, the inlet ventilation hole 32, and the outlet ventilation hole 34. In other words, the air electrode 16 comes into contact with the atmosphere through the water-repellent ventilation member 40.

[0063] In addition, in this battery 2, an air electrode lead (positive electrode lead) 54 is electrically connected to the air electrode (positive electrode) 16, and an anode lead 56 is electrically connected to the anode 12. Although these air electrode lead 54 and negative electrode lead 56 are depicted schematically in FIG. 1 , they are drawn out of the container 4 from the outlets 24, 26 while maintaining airtightness and liquid tightness. An air electrode terminal (positive electrode terminal) 58 is provided at the tip of the air electrode lead 54, and an anode terminal 60 is provided at the tip of the negative electrode lead 56. Therefore, in the battery 2, current is input and output during charging and discharging using these air electrode terminal 58 and negative electrode terminal 60.

[0064] [Example] 1. Battery manufacturing Example 1 (1) Synthesis of catalyst for air secondary battery 1) Co-precipitation process Bi(NO3)3·5H2O and RuCl3·3H2O were prepared. Bi(NO3)3·5H2O and RuCl3·3H2O were weighed to a molar ratio of 1.00 Ru to 0.75 Bi. The weighed Bi(NO3)3·5H2O and RuCl3·3H2O were added to a dilute nitric acid solution at 75°C and stirred to prepare a mixed solution of Bi(NO3)3·5H2O and RuCl3·3H2O. A 2 mol / L NaOH solution was then gradually added to the resulting mixed solution to precipitate the precursor. After the precursor precipitated, the mixed solution was stirred. This stirring operation was continued for 48 hours while bubbling oxygen. During this stirring operation, the pH of the mixed solution was maintained at 10.7 and the temperature was maintained at 75°C. After stirring, the mixed solution was left to stand for 48 hours. After leaving it to stand, the resulting precipitate was collected by filtration. The collected precipitate was kept at 85°C to evaporate some of the water and form a paste. The obtained paste was transferred to an evaporating dish, heated to 120°C, and kept at that state for 3 hours for drying treatment, thereby obtaining a dried precursor. The obtained dried precursor was placed in a mortar and crushed with a pestle to obtain a powder.

[0065] 2) Firing process The obtained precursor powder was subjected to a calcination treatment in which it was heated to a calcination temperature of 500°C in an air atmosphere and maintained for 3 hours. After the calcination treatment, the precursor powder was washed with distilled water at 70°C, filtered by suction, and then dried at 120°C for 3 hours. This yielded a pyrochlore-type bismuth ruthenium composite oxide (catalyst for air secondary batteries).

[0066] 3) Acid treatment process A nitric acid solution was prepared at a ratio of 20 mL per 1 g of bismuth ruthenium composite oxide powder. Then, this nitric acid solution and the bismuth ruthenium composite oxide powder were placed in a stirring tank, and the nitric acid solution was stirred for 1 hour while maintaining the temperature at 25°C, for acid treatment. Here, the concentration of the nitric acid solution was 2 mol / L.

[0067] After stirring was completed, the bismuth ruthenium composite oxide powder was removed from the nitric acid aqueous solution by suction filtration. The removed bismuth ruthenium composite oxide powder was washed with 1 liter of distilled water heated to 75°C. After washing, the bismuth ruthenium composite oxide powder was dried by being kept in an atmosphere of 120°C for 3 hours.

[0068] In this manner, an acid-treated bismuth ruthenium composite oxide powder, i.e., an air electrode catalyst powder for an air secondary battery, was obtained. By subjecting the obtained air secondary battery catalyst to the acid treatment as described above, by-products generated in the process of producing the bismuth ruthenium composite oxide were removed. 4) Analysis The resulting bismuth ruthenium composite oxide powder was analyzed by powder X-ray diffraction (XRD). A parallel-beam X-ray diffractometer was used for this X-ray diffraction (XRD) analysis. The analytical conditions were a CuKα X-ray source, a tube voltage of 40 kV, a tube current of 15 mA, a scan speed of 1 degree / min, and a step width of 0.01 degree. Figure 2 shows the resulting XRD profile, and Figure 3 shows an enlarged view of the XRD profile near 2θ = 30° in Figure 2. The peak positions of Bi2Ru2O7 are also indicated in Figures 2 and 3. The XRD profile shows diffraction peaks at positions corresponding to those of Bi2Ru2O7, confirming that the main phase of the calcined powder is a crystalline structure composed primarily of pyrochlore-type Bi2Ru2O7 crystals and similar crystal structures.

[0069] Furthermore, the full width at half maximum of the diffraction peak of the strongest line corresponding to the 222 plane located near 2θ = 30° was determined from the obtained XRD profile. As a result, the full width at half maximum of the diffraction peak corresponding to the 222 plane was 0.676 deg.

[0070] (2) Manufacturing of the air electrode A nickel powder was prepared, which is an aggregate of nickel particles. Carbonyl nickel particles were used as the nickel particles. The carbonyl nickel particles had a volume average particle size (MV) of 3 μm and an apparent density of 0.50 to 0.65 g / mL, as measured using a laser diffraction / scattering particle size distribution analyzer.

[0071] Furthermore, polytetrafluoroethylene (PTFE) dispersion (31-JR, manufactured by Mitsui-Chemours Fluoroproducts) and ion-exchanged water were prepared.

[0072] Nickel powder (carbonyl nickel powder), polytetrafluoroethylene (PTFE) dispersion, and ion-exchanged water were added to and mixed with the bismuth ruthenium composite oxide powder (air secondary battery catalyst) obtained as described above. At this time, 20 parts by mass of the bismuth ruthenium composite oxide powder, 70 parts by mass of the carbonyl nickel powder, 10 parts by mass of the PTFE dispersion, and 10 parts by mass of the ion-exchanged water were uniformly mixed to produce an air electrode mixture paste.

[0073] The obtained air electrode mixture paste was formed into a sheet using a roller press, and this sheet-shaped air electrode mixture paste was dried at room temperature of 25°C and then press-bonded to a nickel mesh with a mesh count of 60, a wire diameter of 0.08 mm, and an opening ratio of 60%, thereby obtaining an intermediate air electrode product.

[0074] Next, the obtained intermediate product was heat-treated (calcined). The heat-treatment (calcination) conditions were that the intermediate product was heated to a heat treatment temperature of 340°C in a nitrogen gas atmosphere and held at this temperature for 13 minutes. The heat-treated intermediate product was cut into a length of 40 mm and a width of 40 mm, thereby obtaining an air electrode 16. The thickness of this air electrode 16 was 0.24 mm. In the obtained air electrode 16, the amount of bismuth ruthenium composite oxide powder (catalyst for air secondary batteries) was 0.28 g.

[0075] (3) Manufacturing of the negative electrode The metal materials Nd, Mg, Ni, and Al were mixed to a predetermined molar ratio, then placed in a high-frequency induction melting furnace and melted under an argon gas atmosphere. The resulting molten metal was poured into a mold and cooled to room temperature of 25°C to produce an ingot.

[0076] The ingot was then heat-treated at 1000°C in an argon gas atmosphere for 10 hours, and then cooled to room temperature of 25°C. After cooling, the ingot was mechanically pulverized in an argon gas atmosphere to obtain a rare earth-Mg-Ni-based hydrogen storage alloy powder. The volume average particle size (MV) of the obtained rare earth-Mg-Ni-based hydrogen storage alloy powder was measured using a laser diffraction / scattering particle size distribution analyzer. The volume average particle size (MV) was found to be 60 μm.

[0077] The composition of this hydrogen storage alloy powder was analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES), and the composition was found to be Nd 0.89 Mg 0.11 Ni 3.33 Al 0.17 It was.

[0078] To 100 parts by mass of the obtained hydrogen storage alloy powder, 0.2 parts by mass of sodium polyacrylate powder, 0.04 parts by mass of carboxymethyl cellulose powder, 3.0 parts by mass of a styrene butadiene rubber dispersion, 0.5 parts by mass of carbon black powder, and 22.4 parts by mass of water were added and kneaded in an environment of 25°C to prepare a negative electrode mixture paste.

[0079] This negative electrode mixture paste is applied to a substrate with an areal density (weight per unit area) of approximately 300 g / m 2 The mixture was then filled into a foamed nickel sheet approximately 0.6 mm thick. The negative electrode mixture paste was then dried to obtain a foamed nickel sheet filled with the negative electrode mixture. The obtained sheet was rolled to increase the amount of alloy per unit volume, and then cut into a length of 40 mm and a width of 40 mm. In this way, a negative electrode 12 was obtained. The thickness of the negative electrode 12 was 0.77 mm. The design capacity of the negative electrode was 2500 mAh.

[0080] (4) Manufacturing of air hydrogen secondary batteries The obtained air electrode 16 and negative electrode 12 were stacked with a separator 14 sandwiched between them to produce an electrode group 10. The separator 14 used in producing this electrode group 10 was formed from a nonwoven fabric made of polypropylene fibers having sulfonic groups, and had a thickness of 0.1 mm (basis weight 53 g / m 2 ) was.

[0081] Next, a container body 6 was prepared, and the above-described electrode group 10 was housed in this container body 6. At this time, a foamed nickel sheet serving as an adjusting member 36 was placed on the bottom wall 18 of the container body 6, and the electrode group 10 was placed on this adjusting member 36. Here, the foamed nickel sheet serving as the adjusting member 36 was 1 mm thick and had a square shape measuring 40 mm long and 40 mm wide.

[0082] Next, a water-repellent ventilation member 40 was disposed on the electrode group 10 (on the air electrode 16). Here, the water-repellent ventilation member 40 was formed by combining a PTFE porous membrane 42 having a length of 45 mm, a width of 45 mm, and a thickness of 0.1 mm with a nonwoven fabric diffusion paper 44 having a length of 40 mm, a width of 40 mm, and a thickness of 0.2 mm.

[0083] Next, the lid 8 was placed on the container body 6 to close the opening 22. At this time, the water-repellent ventilation member 40 was brought into close contact with the inner surface 28 of the lid 8 so that the entire area including the ventilation channel 30, the inlet ventilation hole 32, and the outlet ventilation hole 34 was covered with the water-repellent ventilation member 40. Here, the ventilation channel 30 has a single serpentine shape as a whole. The cross section of the ventilation channel 30 is rectangular, with a vertical dimension of 1 mm and a horizontal dimension of 1 mm. The ventilation channel 30 is open on the water-repellent ventilation member 40 side.

[0084] The container 4 is formed by combining the container body 6 and the lid 8, and its peripheral edges 46, 48 are sandwiched from above and below by connectors 50, 52. A resin packing (not shown) is provided at the contact point between the container body 6 and the lid 8 to prevent leakage of the alkaline electrolyte.

[0085] Next, a 5 mol / L KOH aqueous solution was poured as alkaline electrolyte 82 into electrolyte reservoir 80. The amount of the poured KOH aqueous solution was 50 mL. In this manner, a battery 2 as shown in FIG. 1 was produced.

[0086] An air electrode lead 54 is electrically connected to the air electrode 16, and an anode lead 56 is electrically connected to the anode 12, and these air electrode lead 54 and anode lead 56 appropriately extend from the lead wire outlets 24, 26 to the outside of the container 4 while maintaining the airtightness and liquid-tightness of the container 4. An air electrode terminal 58 is attached to the tip of the air electrode lead 54, and an anode terminal 60 is attached to the tip of the anode lead 56.

[0087] Example 2 An air-hydrogen secondary battery was produced in the same manner as in Example 1, except that the bismuth ruthenium composite oxide powder obtained after the acid treatment step was subjected to a pulverization treatment using a wet bead mill.

[0088] Here, the milling process will be described. First, ion-exchanged water was prepared in an amount such that the solid content ratio per weight of the bismuth ruthenium composite oxide powder that had undergone the acid treatment process was 10 wt %, and a dispersant (SN Dispersant 5468, San Nopco) was prepared in an amount such that the solid content ratio per weight of the bismuth ruthenium composite oxide powder that had also undergone the acid treatment process was 1 wt %. The ion-exchanged water and dispersant prepared as described above were then added to a predetermined amount of bismuth ruthenium composite oxide powder, and the mixture was mixed to produce a catalyst dispersion. The resulting catalyst dispersion was then pumped at a predetermined flow rate into the milling chamber of a wet bead mill (Ashizawa Finetech, Labostar Mini, DMS65) through the milling chamber inlet. Zirconia beads with a diameter of 0.1 mm had been placed in the milling chamber beforehand. The stirring mechanism in the milling chamber was then driven at a peripheral speed of 8 m / s, thereby subjecting the bismuth ruthenium composite oxide particles to a first-stage milling process. Thereafter, the catalyst dispersion discharged from the outlet of the pulverization chamber was again introduced into the pulverization chamber through the inlet of the pulverization chamber, and a second stage of pulverization treatment was carried out. This procedure of introducing the catalyst dispersion, pulverization treatment, and discharging was counted as one pass, and this one pass was repeated a total of 20 times (20 passes). The catalyst dispersion was then dried by being held in an atmosphere of 70°C for 12 hours, to obtain a powdered bismuth ruthenium composite oxide.

[0089] Here, the full width at half maximum of the diffraction peak corresponding to the 222 plane in the bismuth ruthenium composite oxide (catalyst for air secondary batteries) of Example 2 was 0.648 deg.

[0090] Example 3 An air-hydrogen secondary battery was produced in the same manner as in Example 2, except that the cycle of adding the catalyst dispersion, crushing treatment, and discharging was repeated five times (five passes).

[0091] Here, the full width at half maximum of the diffraction peak corresponding to the 222 plane in the bismuth ruthenium composite oxide (catalyst for air secondary batteries) of Example 3 was 0.635 deg. Example 4 An air-hydrogen secondary battery was produced in the same manner as in Example 1, except that the precursor powder was fired at 460°C in the firing step.

[0092] Here, the full width at half maximum of the diffraction peak corresponding to the 222 plane in the bismuth ruthenium composite oxide (catalyst for air secondary batteries) of Example 4 was 0.687 deg. Example 5 An air-hydrogen secondary battery was produced in the same manner as in Example 1, except that the precursor powder was fired at 540°C in the firing step.

[0093] Here, the full width at half maximum of the diffraction peak corresponding to the 222 plane in the bismuth ruthenium composite oxide (catalyst for air secondary batteries) of Example 5 was 0.553 deg. Example 6 An air-hydrogen secondary battery was produced in the same manner as in Example 1, except that the precursor powder was fired at 600°C in the firing step.

[0094] Here, the full width at half maximum of the diffraction peak corresponding to the 222 plane in the bismuth ruthenium composite oxide (catalyst for air secondary batteries) of Example 6 was 0.350 deg. Example 7 An air-hydrogen secondary battery was produced in the same manner as in Example 2, except that a high-load type wet bead mill (Ashizawa Finetech, Labostar Mini, LMZ015) was used, the peripheral speed of the stirring mechanism was set to 12 m / s, and the number of times the catalyst dispersion was charged, crushed, and discharged was repeated was set to 10 times (10 passes).

[0095] Here, the full width at half maximum of the diffraction peak corresponding to the 222 plane in the bismuth ruthenium composite oxide (catalyst for air secondary batteries) of Example 7 was 0.713 deg. (Comparative Example 1) An air-hydrogen secondary battery was produced in the same manner as in Example 2, except that a high-load type wet bead mill (Ashizawa Finetech, Labostar Mini, LMZ015) was used, the peripheral speed of the stirring mechanism was set to 12 m / s, and the number of times the catalyst dispersion was charged, crushed, and discharged was repeated was set to 60 times (60 passes).

[0096] Here, the full width at half maximum of the diffraction peak corresponding to the 222 plane in the bismuth ruthenium composite oxide (catalyst for air secondary batteries) of Comparative Example 1 was 0.784 deg.

[0097] 2. Evaluation of air-hydrogen secondary batteries (1) Evaluation of battery characteristics For the air-hydrogen secondary batteries of Examples 1 to 7 and Comparative Example 1, the batteries having the above configuration were aged at 60°C for 12 hours, then cooled to room temperature. 2000 mAh, equivalent to 80% of the negative electrode capacity, was defined as 1 It, and charging was performed for 0.1 It x 10 hours, followed by repeated discharging at 0.2 It (discharge cut-off voltage EV = 0.4 V). The discharge capacity at the first cycle was determined. Furthermore, the battery voltage at the first cycle when the battery capacity reached half of the first-cycle discharge capacity was measured as the discharge intermediate voltage. The results of the discharge capacity and discharge intermediate voltage at the first cycle are summarized in Table 1. The full width at half maximum (FWHM) obtained by XRD is also shown in Table 1. Furthermore, the relationship between the discharge intermediate voltage and the FWHM is summarized in FIG. 4.

[0098] During the above-described charging and discharging operations, air was continuously supplied to the ventilation path 30 at a rate of 33 mL / min by introducing air through the inlet vent 32 and discharging air through the outlet vent 34, regardless of whether the battery was being charged or discharged.

[0099] [Table 1]

[0100] (2) Consideration 2, which shows the XRD profiles of the air secondary battery catalysts according to Examples 1 to 7 and Comparative Example 1, and FIG. 3, which shows enlarged XRD profiles near the strongest lines of the air secondary battery catalysts according to Examples 1 to 7 and Comparative Example 1, show that diffraction peaks are present at positions corresponding to the diffraction peaks of Bi2Ru2O7, indicating that the catalysts of Examples 1 to 7 and Comparative Example 1 all have a crystal structure primarily composed of pyrochlore-type crystals of Bi2Ru2O7 and crystal structures similar thereto. Furthermore, as shown in FIGS. 2 and 3, the peak intensities and full widths at half maximum of the bismuth ruthenium composite oxides of Examples 1 to 7 and Comparative Example 1, which were produced under different conditions, each change sharply, indicating that the crystallinity varies greatly depending on the production conditions.

[0101] Table 1, which shows the results of the FWHM and battery characteristics, shows that the batteries of Examples 1 to 7 were able to discharge stably, but the battery of Comparative Example 1 had a large discharge overvoltage and barely discharged before reaching the discharge cutoff voltage. Furthermore, Figure 4, which summarizes the correlation between the FWHM and the discharge midpoint voltage, shows a volcanic-like sequence between the FWHM and the discharge midpoint voltage, with the discharge midpoint voltage gradually increasing from 0.350 to 0.676° but rapidly dropping once it exceeded 0.676°. Furthermore, at 0.784°, where amorphization was most advanced, the overvoltage was high and almost no discharge was possible. From the above, it is clear that the FWHM of the 222 plane of the bismuth ruthenium composite oxide catalyst has a significant impact on battery characteristics, and it was found that the discharge overvoltage can be reduced by appropriately amorphizing the crystal structure.

[0102] The oxygen reduction reaction at the air electrode is thought to occur via two-electron and four-electron reaction pathways. In the case of a two-electron reaction, oxygen adsorbs onto the active site of the catalyst, receives an electron without dissociating, and reacts with water to form hydrogen peroxide ions and hydroxide ions. On the other hand, in the case of a four-electron reaction, only hydroxide ions are produced, and the bonds between oxygen atoms must be broken during the reaction process. By making the crystal structure of the bismuth ruthenium composite oxide catalyst moderately amorphous, lattice defects such as disruption of the regularity of the atomic arrangement on the surface of the active site are created, and it is thought that this change in electronic state promotes the breaking of oxygen molecules and the decomposition of hydrogen peroxide ions, the reaction intermediate. [Explanation of symbols]

[0103] 2. Battery (air hydrogen secondary battery) 4 containers 6 Container body 8 Lid 10 electrode groups 12 Negative electrode 14 Separator 16 Air electrode (positive electrode) 30 Ventilation channel 40 Water-repellent and breathable material

Claims

1. A catalyst for an air secondary battery, comprising a pyrochlore-type bismuth ruthenium composite oxide having a composition formula of Bi2-xRu2O7-z (where 0≦x≦1, z satisfies the relationship 0≦z≦1), wherein the full width at half maximum of the diffraction peak corresponding to the 222 plane obtained by powder X-ray diffraction using CuKα rays is 0.350 deg or more and 0.713 deg or less.

2. a cathode substrate; an air electrode mixture held on the air electrode substrate, An air electrode, wherein the air electrode mixture comprises the catalyst for an air secondary cell according to claim 1 and a catalyst-supporting conductive material that supports the catalyst for an air secondary cell.

3. 3. The air electrode according to claim 2, wherein the catalyst-supporting conductive material is nickel.

4. The air electrode according to claim 2 or 3, wherein the air electrode mixture further contains a water repellent agent.

5. The air electrode according to claim 4 , wherein the water repellent agent is a fluororesin.

6. 6. The air electrode according to claim 5, wherein the fluororesin is polytetrafluoroethylene.

7. A container and an electrode group disposed in the container; an alkaline electrolyte injected into the container; the electrode group includes an air electrode and a negative electrode stacked together with a separator interposed therebetween, An air secondary battery, wherein the air electrode is the air electrode according to any one of claims 2 to 6.

8. The air secondary battery according to claim 7 , wherein the negative electrode contains a hydrogen storage alloy.

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