JP7758545B2 - Catalyst for air electrode, air electrode including this catalyst for air electrode, and air secondary battery including this air electrode - Google Patents

Description

The present invention relates to an air electrode catalyst, an air electrode including this air electrode catalyst, and an air secondary battery including this air electrode.

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 being small and lightweight. Among these air batteries, zinc-air primary batteries have been put to practical use as power sources for hearing aids.

In addition, research is being conducted on rechargeable air batteries that use metals such as Li, Zn, Al, and Mg for the negative electrode, and these air secondary batteries are expected to become new secondary batteries with the potential to exceed the energy density of lithium-ion secondary batteries.

One type of air secondary battery known is the 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.

In air secondary batteries that use alkaline electrolyte, such as the air-hydrogen secondary battery described above, the following charge-discharge reactions occur at the positive electrode (hereinafter also referred to as the air electrode):

Charging (oxygen generation reaction): 4OH ⁻ → O ₂ + 2H ₂ O + 4e ⁻ (I)
Discharge (oxygen reduction reaction): O ₂ + 2H ₂ O + 4e ⁻ → 4OH ⁻ (II)

As shown in reaction formula (I), oxygen and water are generated at the air electrode during charging in an air secondary battery. The generated 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).

A catalyst that promotes the above-mentioned charge and 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 during the charge and discharge reactions of the air electrode in order to improve energy efficiency and increase output. Therefore, materials that are effective in reducing overvoltage are being investigated as catalyst materials for use in the air electrode. Various metal oxides are promising materials for reducing such overvoltage. Among these metal oxides, pyrochlore-type bismuth ruthenium composite oxide has the "dual function" of oxygen reduction and oxygen generation, and is thought to be particularly effective as an air electrode catalyst because it can reduce overvoltage in both charge and discharge reactions.

This bismuth-ruthenium composite oxide is produced, for example, by a coprecipitation method using bismuth nitrate and ruthenium chloride as starting materials to produce a precursor, which is then calcined.

In the above-mentioned manufacturing method of bismuth ruthenium composite oxide, by-products are formed during the process. When an air electrode is fabricated using a catalyst containing these by-products and an air-hydrogen secondary battery equipped with the air electrode undergoes repeated charge-discharge cycles, the bismuth ruthenium oxide itself does not undergo dissolution-precipitation reactions, but the by-products do. Specifically, as the chemical reactions occurring during battery charge and discharge (hereinafter referred to as the battery reaction) occur, the metal components (primarily bismuth) in the by-products undergo repeated dissolution-precipitation reactions, resulting in dendritic growth, or dendritic deposition of the metal components on the electrode plates. This dendritic growth of the metal components extends into the separator and ultimately penetrates the separator. This results in the problem of micro-short circuits. When micro-short circuits occur, not only ionic conductivity via the electrolyte but also electronic conductivity exists between the positive and negative electrodes within the battery. If electronic conductivity exists, the battery is self-discharging. Dendrite growth of the metal components increases with each charge/discharge cycle, so the self-discharge rate also increases as the charge/discharge cycle progresses. As a result, the battery's discharge capacity decreases after a relatively small number of cycles, and the battery's lifespan expires early.

To resolve this problem of shortened cycle life, a manufacturing method such as that disclosed in Patent Document 2 has been proposed. According to Patent Document 2, by-products can be removed by treating bismuth ruthenium composite oxide with acid, thereby suppressing the reduction in cycle life caused by the by-products.

Patent No. 6444205 Japanese Patent Application Laid-Open No. 2019-179592

By adopting the method described in Patent Document 2, by-products are almost entirely removed by acid treatment, and the reduction in cycle life caused by by-products is largely suppressed, thereby extending the life of the air secondary battery. However, there are other factors besides the by-products that impair cycle life characteristics. For example, the formation of a three-phase interface, which serves as the discharge reaction site in the air electrode catalyst, can become difficult as the number of cycles increases, resulting in a decrease in cycle life characteristics. For this reason, it is believed that there is room for further improvement in cycle life characteristics by further investigating and addressing the factors that impair cycle life characteristics.

As air secondary batteries are expected to be used in a variety of applications in the future, further extending their cycle life is an extremely important issue.

The present invention was made based on the above circumstances, and its purpose is to provide an air electrode catalyst that can contribute to improving cycle life characteristics compared to conventional methods, an air electrode containing this air electrode catalyst, and an air secondary battery containing this air electrode.

To achieve the above objective, the present invention provides an air electrode catalyst comprising a bismuth ruthenium composite oxide containing at least Bi, Ru, and Si, in which Si/Bi, representing the atomic ratio of Si to Bi, is 0.007 or less.

The air electrode catalyst according to the present invention is an air electrode catalyst made of a bismuth ruthenium composite oxide containing at least Bi, Ru, and Si, in which the Si/Bi ratio, representing the atomic ratio of Si to Bi, is 0.007 or less. When Si/Bi is 0.007 or less, a stable three-phase interface can be formed in the air electrode catalyst. Therefore, an air secondary battery using an air electrode containing the air electrode catalyst according to the present invention can discharge stably even with an increased number of cycles, improving cycle life characteristics. Therefore, the present invention can provide an air electrode catalyst that can contribute to improved cycle life characteristics compared to conventional methods, an air electrode containing this air electrode catalyst, and an air secondary battery containing this air electrode.

1 is a cross-sectional view schematically illustrating an air hydrogen secondary battery according to one embodiment. 1 is an X-ray diffraction (XRD) profile of the air electrode catalyst of Example 1.

An air-hydrogen secondary battery (hereinafter also referred to as a 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.

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

The electrode group 10 is formed by stacking a negative electrode 12 and an air electrode (positive electrode) 16 with a separator 14 interposed between them.

The negative electrode 12 includes a conductive negative electrode substrate with a porous structure and numerous pores, and a negative electrode mixture supported within the pores and on the surface of the negative electrode substrate. For example, foamed nickel can be used as the negative electrode substrate.

The negative electrode mixture contains hydrogen storage alloy powder, which serves as the negative electrode active material and is an aggregate of hydrogen storage alloy particles capable of absorbing and releasing hydrogen, a conductive material, and a binder. Examples of conductive materials that can be used here include graphite powder, which is an aggregate of graphite particles, and carbon black powder, which is an aggregate of carbon black particles.

The hydrogen storage alloy constituting the hydrogen storage alloy particles is not particularly limited, but it is preferable to use, for example, a rare earth-Mg-Ni based hydrogen storage alloy. 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 Al _c M _d ...(III)
It is preferable to use one represented by the following formula:

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 satisfying the relationships 0.01≦a≦0.30, 2.8≦b≦3.9, 0.05≦c≦0.30, and 0≦d≦0.50, respectively.

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 to 1200°C in an inert gas atmosphere and homogenized by heat treatment, holding at that temperature for 5 to 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.

Examples of binders that can be used include sodium polyacrylate, carboxymethyl cellulose, and styrene-butadiene rubber.

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 and 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 as a whole. The negative electrode mixture layer included 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, forming an overall porous structure.

Next, the air electrode 16 comprises a conductive air electrode substrate with a mesh structure and an air electrode mixture layer (cathode mixture layer) formed from an air electrode mixture (cathode mixture) supported on the air electrode substrate. For example, nickel mesh can be used as the air electrode substrate.

The air electrode mixture contains an oxidation-reduction catalyst (air electrode catalyst), a conductive material, and a binder.
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.

The bismuth ruthenium composite oxide in this embodiment is an air electrode catalyst made of a bismuth ruthenium composite oxide containing at least Bi, Ru, and Si, in which the Si/Bi atomic ratio of Si to Bi is 0.007 or less. Preferably, the Si/Bi ratio is 0.002 or more.

The inventors of the present application conducted extensive research into the factors that hinder the improvement of the cycle life characteristics of air secondary batteries, and discovered that a high amount of Si contained in the air electrode catalyst deteriorates the cycle life characteristics, and that the cycle life characteristics can be improved by minimizing the amount of Si. More details are as follows.

Si is contained as an impurity in the bismuth ruthenium composite oxide and exists mainly in the form of Si or SiO ₂ (silica). When such a bismuth ruthenium composite oxide containing excess Si or SiO ₂ comes into contact with an alkaline electrolyte (e.g., a KOH aqueous solution), the Si in the Si or SiO ₂ forms a silanol bond (Si—OH), which further neutralizes hydrogen ions, forming Si—O—K, and producing alkaline silica gel. This alkaline silica gel has the property of absorbing moisture and expands while absorbing surrounding moisture. The alkaline silica gel formed on the bismuth ruthenium composite oxide mainly absorbs and stores water generated during the charging reaction of the air secondary battery, so the more cycles are performed, the more moisture can be stored. In this way, as the amount of moisture-rich portions of the bismuth ruthenium composite oxide increases, the formation of a three-phase interface becomes more difficult, and the area of the three-phase interface decreases. When the three-phase interface, which serves as a discharge reaction field, becomes fewer, discharge becomes more difficult, causing the cycle life to end prematurely. In other words, the inventors have found that the amount of Si affects the cycle life characteristics, and that the cycle life characteristics of air secondary batteries can be improved by regulating the amount of Si.

The inventors of the present application believe that restricting the amount of Si would improve the cycle life characteristics of air secondary batteries, and have conducted more specific research into the relationship between the cycle life and Si in air secondary batteries. As a result, they have discovered that there is a relationship between the amount of Si contained in bismuth ruthenium composite oxide and cycle life, and that a significant improvement in cycle life characteristics can be achieved when Si/Bi, which represents the atomic ratio of Si to Bi in bismuth ruthenium composite oxide, is 0.007 or less. While minimizing the amount of Si is thought to be effective in improving cycle life characteristics, during the course of this research, they have confirmed that an improvement in cycle life characteristics can be achieved at least up to a Si/Bi ratio of 0.002.

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

Bi( _NO3 ) _{3.5H2O and RuCl3.3H2O} are prepared. Then, Bi( _NO3 ) _3.5H2O and _RuCl3.3H2O are weighed out to predetermined amounts, respectively. Here, _it is preferable _that Bi( _NO3 ) _{3.5H2O and RuCl3.3H2O} are weighed out so that the _atomic ratio of _Ru to _Bi is 1.00 to _1.00 or less. Note that in the _{bismuth ruthenium} composite oxide finally obtained after the acid treatment described below, it is preferable that the atomic ratio of Bi to Ru is 0.70 to _0.78 .

Next, the measured Bi(NO ₃ ) ₃ ·5H ₂ O and RuCl ₃ ·3H ₂ O are added to distilled water and stirred to prepare a mixed aqueous solution of Bi(NO ₃ ) ₃ ·5H ₂ O and RuCl ₃ ·3H ₂ O. The temperature of the distilled water is set to 60°C or higher and 90°C or lower. Then, this mixed aqueous solution and an NaOH aqueous solution of 1 mol/L or higher and 3 mol/L or lower are simultaneously added dropwise to a glass reaction vessel. The mixed aqueous solution and the NaOH aqueous solution are added dropwise until the reaction vessel overflows. At this time, a precursor is formed in the reaction vessel. The mixed aqueous solution and the NaOH aqueous solution stored in the reaction vessel are then transferred to a glass stirring vessel and stirred.

This stirring operation is carried out for 12 to 48 hours with oxygen bubbling. During the stirring operation, the pH of the mixed aqueous solution is maintained at 11, 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 allowed to stand for 12 to 48 hours. After standing, the resulting precipitate is recovered by suction filtration. The recovered 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, heated to 100°C or higher and 150°C or lower, and held at that temperature for 1 hour to 5 hours to dry, thereby obtaining a dried paste. The dried paste obtained is placed in a mortar and crushed with a pestle to obtain a powder. This powder is then fired by heating to 400°C or higher and 700°C or lower in an air atmosphere and held for 0.5 hours to 4 hours. The powder after the calcination treatment is washed with distilled water at a temperature of 60°C or higher and 90°C or lower, and then dried. This results in a pyrochlore-type bismuth ruthenium composite oxide. This bismuth ruthenium composite oxide is expressed as Bi2 _{- xRu2O7 -z} (where x satisfies the relationships 0≦x≦1 and z satisfies the relationships 0≦z≦1).

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

First, prepare an aqueous nitric acid solution. The concentration of the aqueous nitric acid solution is preferably 5 mol/L or less, and more preferably 2 mol/L or less. The amount of aqueous nitric acid solution prepared 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.

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

The washed bismuth ruthenium composite oxide is then dried by being held in an environment of 100°C or higher and 130°C or lower for 1 hour or longer and 4 hours or shorter.

In this way, an acid-treated bismuth ruthenium composite oxide is obtained. By performing the acid treatment in this manner, by-products generated during the production process 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 sulfuric acid solutions also have the effect of removing by-products, just like an aqueous nitric acid solution.

In this way, bismuth ruthenium composite oxide powder is obtained, which is an aggregate of bismuth ruthenium composite oxide particles from which by-products have been removed.

The resulting bismuth ruthenium composite oxide contains Si on its surface or in the bulk. The Si in the bulk is thought to be in a state where it has substituted for some of the Bi.

There are various possible routes by which Si may become mixed into bismuth ruthenium composite oxide, including dissolution from the glass reaction vessel, contamination during the drying process, dissolution from the glass funnel used for filtration, and dissolution from the crucible used during firing. Because these routes may be combined, it is difficult to pinpoint the exact route. Of the routes mentioned above, dissolution from the glass reaction vessel is considered to be the most common route by which Si may become mixed in. When glass comes into contact with a strongly alkaline aqueous solution such as an aqueous sodium hydroxide solution, the Si and silica contained in the glass components are eluted. This eluted Si and the Si contained in the eluted silica become incorporated into the bismuth ruthenium composite oxide. For this reason, controlling the elution of Si from the glass reaction vessel described above is considered to be the most effective way to minimize the amount of Si in the bismuth ruthenium composite oxide.

Next, we will explain the conductive material. The conductive material is used to reduce internal resistance in order to increase the output of the air secondary battery, and as a support for the redox catalyst mentioned above.

As such a conductive material, it is preferable to use, for example, nickel powder made of nickel particles. The average particle size of the nickel particles is not particularly limited, but it is preferable to set it to a size that can impart the desired conductivity to the air electrode.

The above-mentioned nickel powder is preferably contained in an amount of 60% by mass or more in the air electrode mixture. The upper limit of the nickel powder content is preferably set to 80% by mass or less, taking into account the relationship with other constituent materials in the air electrode mixture.

The binder binds the constituent materials of the air electrode mixture and also provides the air electrode 16 with appropriate water repellency. The binder is not particularly limited, and examples include fluororesin. A preferred example of a fluororesin is polytetrafluoroethylene (hereinafter also referred to as PTFE).

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 binder, and water are prepared. Then, the catalyst powder, conductive powder, binder, and water are kneaded together to prepare an air electrode mixture paste.

The resulting air electrode mixture paste is formed into a sheet, for example by roller pressing, to obtain an air electrode mixture sheet. The air electrode mixture sheet is then press-bonded to a nickel mesh (air electrode substrate). This produces an intermediate air electrode product.

The resulting intermediate product is then placed in a firing furnace and fired. This firing is carried out in an inert gas atmosphere, such as nitrogen or argon gas. The firing conditions include heating to a temperature of 200°C to 400°C and maintaining this temperature for 10 to 40 minutes. The intermediate product is then allowed to cool naturally in the firing furnace and removed into the atmosphere when its temperature drops to 150°C or below. This results in a fired intermediate product. The fired intermediate product is then cut into a predetermined shape to obtain the air electrode 16. This air electrode 16 includes an air electrode mixture layer formed from an air electrode mixture. Because the air electrode mixture contains particles of bismuth ruthenium composite oxide, the air electrode mixture layer formed from this air electrode mixture has a porous structure with numerous pores overall, providing excellent gas diffusion properties.

The air electrode 16 and negative electrode 12 obtained as described above are stacked with a separator 14 interposed between them, thereby forming an electrode group 10. This separator 14 is arranged to prevent short circuits between the air electrode 16 and 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.

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

The container body 6 is box-shaped and has a bottom wall 18 and side walls 20 that extend upward from the periphery 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 side wall 20R and the left side wall 20L, which serve as lead wire outlets 24, 26, which will be described later.

Furthermore, an electrolyte storage unit 80 is attached to the container body 6. This electrolyte storage unit 80 is a container that stores alkaline electrolyte 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 82 that connects the interior of the container 4 with the electrolyte storage unit 80. In this way, because the interior of the container 4 and the electrolyte storage unit 80 are connected, the alkaline electrolyte 82 can move between the interior of the container 4 and the electrolyte storage unit 80.

The lid 8 has the same shape as the container body 6 in a plan view, and is placed over the 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-tight.

The lid 8 has an air vent 30 on its inner surface 28 facing the inside of the container body 6. The air vent 30 is open at the portion facing the inside of the container body 6, forming a single serpentine shape overall. Furthermore, an inlet air vent 32 and an outlet air vent 34 are provided at predetermined positions on the lid 8, penetrating the thickness direction. The inlet air vent 32 is connected to one end of the air vent 30, and the outlet air vent 34 is connected to the other end of the air vent 30. In other words, the air vent 30 is open to the atmosphere via the inlet air vent 32 and the outlet air vent 34. It is preferable to attach a pressure pump (not shown) to the inlet air vent 32. Driving this pressure pump allows air to be sent from the inlet air vent 32 to the air vent 30.

If necessary, an adjustment member 36 is placed on the bottom wall 18 of the container body 6. The adjustment member 36 is used to align the electrode group 10 in the height direction within the container 4. For example, a foamed nickel sheet is used as the adjustment member 36.

The electrode group 10 is placed on top of the adjustment member 36. At this time, the negative electrode 12 of the electrode group 10 is placed so that it contacts the adjustment member 36.

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 contact 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 water-repellent properties due to the PTFE and 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.

The lid 8 is placed on the container body 6, which houses the electrode group 10, adjustment member 36, and water-repellent and breathable member 40 as described above. Then, as shown schematically in Figure 1, the peripheral edges 46, 48 of the container 4 (container body 6 and lid 8) are sandwiched from above and below by connectors 50, 52. A predetermined amount of alkaline electrolyte 82 is then injected from the electrolyte reservoir 80, filling the container 4 with alkaline electrolyte 82. In this manner, the battery 2 is formed.

The alkaline electrolyte 82 is preferably a typical alkaline electrolyte used in alkaline secondary batteries, specifically an aqueous solution containing at least one of NaOH, KOH, and LiOH as a solute.

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.

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. These air electrode lead 54 and anode lead 56 are depicted schematically in Figure 1, but 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 anode lead 56. Therefore, in the battery 2, current is input and output during charging and discharging using these air electrode terminal 58 and anode terminal 60.

[Example]
1. Battery Production (Example 1)
(1) Synthesis of cathode catalyst 1) Coprecipitation process Bi( _NO3 ) _3.5H2O and _RuCl3.3H2O were prepared. Bi( _NO3 ) _{3.5H2O and RuCl3.3H2O were} weighed so that the atomic ratio of Ru was 1.00 and Bi was 0.75. The Bi/Ru ratio at this time was the Bi/ _Ru value at the time of preparation, which was 0.75. _The weighed Bi( _NO3 ) _{3.5H2O and RuCl3.3H2O} were combined and poured into distilled _water at 70°C and stirred to prepare a mixed aqueous solution of Bi _{(NO3)3.5H2O and RuCl3.3H2O . Furthermore} , a 2 mol/L _NaOH aqueous solution was prepared. The mixed aqueous solution and the NaOH aqueous solution were then simultaneously dropped into a glass reaction vessel. The reaction vessel includes a cylindrical body with an open top and a closed bottom, and a discharge pipe connected to the interior of the body at a sidewall near the top of the body and used to discharge the solution accumulated in the body. The discharge pipe extends diagonally downward from the sidewall. Therefore, when a certain amount of the solution accumulates in the body and the liquid level reaches the discharge pipe, the solution overflows from the discharge pipe and is discharged to the outside. The opening of the discharge pipe on the body side (hereinafter referred to as the body-side opening) is positioned at a predetermined height so that the solution overflows when the volume of the injected solution reaches 100 mL. In other words, the reaction vessel can store solution up to a volume of 100 mL. Therefore, the volume of the reaction vessel is 100 mL.

In Example 1, the dripping time was adjusted so that the mixed aqueous solution and NaOH aqueous solution would overflow after an average residence time of 50 minutes in the reaction vessel. Here, the average residence time is calculated as follows: average residence time = reaction vessel volume [mL] / dripping rate [mL/min], and dripping rate = drip amount [mL] / dripping time [min]. In other words, when dripping a fixed amount of solution into a reaction vessel of a certain volume, the dripping time of the solution was adjusted so that the liquid level of the solution dripped at a specified drip amount would reach the opening on the main body side of the outlet tube and overflow. Specifically, the average residence time is 50 minutes when dripping into a 100 mL reaction vessel at a drip rate that will cause overflow after 50 minutes.

As described above, the precursor was precipitated by reacting the mixed aqueous solution with the NaOH aqueous solution. The overflowed mixed aqueous solution containing the precursor was then placed in a stirring vessel and stirred. This stirring was carried out for 24 hours while oxygen bubbling was performed. During this stirring, the pH of the mixed aqueous solution was maintained at 11, and the temperature was maintained at 70°C. After stirring, the mixed aqueous solution was allowed to stand for 24 hours. After standing, the resulting precipitate was recovered by suction filtration. The recovered precipitate was kept at 85°C to evaporate some of the water, forming a paste. The resulting paste was transferred to an evaporating dish, heated to 120°C, and dried by holding it there for 3 hours to obtain a dried precursor.

2) Calcination Step The dried precursor was placed in a mortar and crushed with a pestle to form a powder. The resulting precursor powder was subjected to a calcination treatment in which it was heated to 500°C in an air atmosphere and maintained for 3 hours. After the calcination treatment, the precursor was washed with distilled water at 70°C, filtered by suction, and then maintained at 120°C for 3 hours to be dried. This yielded a bismuth ruthenium composite oxide (air electrode catalyst).

3) Acid Treatment Step The bismuth ruthenium composite oxide was placed in a stirring tank with a stirrer together with an aqueous nitric acid solution, and the nitric acid solution was stirred for 1 hour while maintaining the temperature at 25°C, thereby carrying out an acid treatment. The amount of the aqueous nitric acid solution was 20 mL per 1 g of the bismuth ruthenium composite oxide powder. The concentration of the aqueous nitric acid solution was 2 mol/L.

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 distilled water heated to 70°C. After washing, the bismuth ruthenium composite oxide powder was dried by holding it in an atmosphere at 120°C for 3 hours.

In this way, acid-treated bismuth ruthenium composite oxide powder, i.e., air electrode catalyst powder, was obtained.

(2) Fabrication of the Air Electrode Ni powder was prepared, which was an aggregate of Ni particles. The Ni particles were filament-shaped and had an average particle size of 10 to 20 μm.

In addition, polytetrafluoroethylene (PTFE) dispersion and ion-exchanged water were prepared.

The bismuth ruthenium composite oxide (air electrode catalyst) powder obtained as described above was mixed with nickel powder, polytetrafluoroethylene (PTFE) dispersion, and ion-exchanged water. 20 parts by weight of the bismuth ruthenium composite oxide powder, 70 parts by weight of the nickel powder, 10 parts by weight of the PTFE dispersion, and 10 parts by weight of the ion-exchanged water were uniformly mixed to produce a paste of the air electrode mixture.

The resulting air electrode mixture paste was formed into a sheet and dried at room temperature of 25°C to obtain an air electrode mixture sheet. The resulting air electrode mixture sheet was then press-bonded to a nickel mesh with a mesh count of 60, a wire diameter of 0.08 mm, and an opening rate of 60%. This resulted in an intermediate air electrode product.

Next, the intermediate air electrode product was subjected to a calcination process in a calcination furnace. The calcination process was performed by heating the intermediate air electrode product to a calcination temperature of 340°C in a nitrogen gas atmosphere and holding it at this temperature for 13 minutes. The calcined 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 0.23 mm. The amount of bismuth ruthenium composite oxide powder (air electrode catalyst) in the resulting air electrode 16 was 0.23 g.

(3) Production of Negative Electrode After mixing the metal materials Nd, Mg, Ni, and Al to a predetermined molar ratio, the mixture was charged into a high-frequency induction melting furnace and melted in 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.

The ingot was then heat-treated at 1000°C in an argon gas atmosphere for 10 hours, after which it was cooled to room temperature of 25°C. After cooling, the ingot was mechanically pulverized in an argon gas atmosphere to obtain 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.

The composition of this hydrogen storage alloy powder was analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES) and found to be Nd _0.89 Mg _0.11 Ni _3.33 Al _0.17 .

The electrochemical alloy capacity of the obtained hydrogen storage alloy was then measured. Specifically, a measurement sample was prepared by setting aside a portion of the hydrogen storage alloy powder obtained as described above, and nickel powder. 0.25 g of the hydrogen storage alloy powder used as the measurement sample was mixed with 0.75 g of nickel powder to produce a mixed powder, which was then molded to produce a circular pellet electrode with a diameter of 10 mm.

Next, 100 mL of an 8 mol/L KOH aqueous solution was poured into a cylindrical resin container, and a pellet electrode and a mercury oxide reference electrode were placed in the center of the container within the KOH aqueous solution. Furthermore, a nickel hydroxide counter electrode with a capacity sufficiently large relative to the negative electrode (pellet electrode) was placed along the edge of the container. In this way, a battery with a negative electrode capacity limitation was formed. A charge-discharge test was conducted on this battery, in which the battery was charged at 0.5 It for 200 minutes and discharged at 0.5 It until the negative electrode potential reached -0.3 V relative to the mercury oxide reference electrode, and the electrochemical alloy capacity was determined. Note that in the charge-discharge operation using the pellet electrode described above, the negative electrode capacity was calculated as 1 It, assuming an alloy capacity of 300 mAh/g.

To 100 parts by weight of the obtained hydrogen storage alloy powder, 0.2 parts by weight of sodium polyacrylate powder, 0.04 parts by weight of carboxymethyl cellulose powder, 1.0 part by weight of styrene butadiene rubber dispersion, 0.3 parts by weight of carbon black powder, and 22.4 parts by weight of water were added and kneaded in an environment of 25°C to prepare a negative electrode mixture paste.

This negative electrode mixture paste was filled into a foamed nickel sheet with an areal density (weight per unit area) of approximately 300 g/m ² and a thickness of approximately 1.7 mm. 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 0.75 mm. The negative electrode capacity calculated from the above electrochemical alloy capacity was 2500 mAh.

Next, the obtained negative electrode 12 was subjected to activation treatment. The procedure of this activation treatment is as follows.
First, a typical sintered nickel hydroxide positive electrode was prepared. The positive electrode capacity of this nickel hydroxide positive electrode was sufficiently greater than the negative electrode capacity of the negative electrode 12. This nickel hydroxide positive electrode and the resulting negative electrode 12 were then stacked together with a separator made of polyethylene nonwoven fabric interposed therebetween to form an electrode group for activation treatment. This electrode group for activation treatment was then placed in an acrylic resin container together with a predetermined amount of alkaline electrolyte. This resulted in the formation of a single-electrode cell for a nickel-metal hydride secondary battery with a negative electrode capacity restriction.

This single-electrode cell was left to stand for 5 hours in an environment at 25°C, then charged at 0.5 It for 2.8 hours, and then discharged at 0.5 It until the battery voltage reached 0.70 V. This charge-discharge cycle was repeated five times to activate the negative electrode 12.

Then, the negative electrode 12 was charged at 0.5 It for 2.8 hours, and then removed from the single-electrode cell. In this way, an activated and charged negative electrode 12 was obtained.

(4) Production of Air-Hydrogen Secondary Battery 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 0.2 mm (basis weight 100 g/m ² ).

Next, the container body 6 was prepared, and the electrode group 10 described above was placed inside this container body 6. At this time, a foamed nickel sheet was placed on the bottom wall 18 of the container body 6 as the adjustment member 36, and the electrode group 10 was placed on this adjustment member 36. Here, the foamed nickel sheet was 1 mm thick and had a square shape measuring 40 mm in length and 40 mm in width.

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

The lid 8 was then placed over the container body 6 to close the opening 22. At this time, the water-repellent ventilation member 40 was brought into tight contact with the inner surface 28 of the lid 8 so that the entire area including the ventilation channel 30, inlet ventilation hole 32, and outlet ventilation hole 34 was covered with the water-repellent ventilation member 40. Here, the ventilation channel 30 has a single serpentine shape overall. 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.

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

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

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. These air electrode lead 54 and anode lead 56 extend appropriately 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.

Example 2
Except for adjusting the average residence time in the coprecipitation step to 30 minutes, an air-hydrogen secondary battery was produced in the same manner as in Example 1. The amount of bismuth ruthenium composite oxide powder (air electrode catalyst) in Example 2 was 0.24 g.

Example 3
An air-hydrogen secondary battery was produced in the same manner as in Example ₁ , except that in the coprecipitation step, Bi( _NO3 ) _3.5H2O and _RuCl3.3H2O were weighed so that the atomic ratio of Ru to _Bi was 1.00. Here, the Bi/Ru value at the time of preparation in Example 3 was 1.00. The amount of bismuth ruthenium composite oxide powder (air electrode catalyst) in Example 3 was 0.24 g.

(Comparative Example 1)
Except for adjusting the average residence time in the coprecipitation step to 2 minutes, an air-hydrogen secondary battery was produced in the same manner as in Example 1. The amount of bismuth ruthenium composite oxide powder (air electrode catalyst) in Comparative Example 1 was 0.25 g.

(Comparative Example 2)
An air-hydrogen secondary battery was manufactured in the same manner as in Example 1, except that in the coprecipitation step, the average residence time was adjusted to 5 minutes and the production amount was tripled. Here, "triple scale" means that the dropwise addition time was tripled and the quantity per lot was tripled. The amount of bismuth ruthenium composite oxide powder (air electrode catalyst) in Comparative Example 1 was 0.24 g.

(Comparative Example 3)
An air-hydrogen secondary battery was produced in the same manner as in Example 1, except that in the coprecipitation step, the reaction vessel was changed to one with a volume of 200 mL and the average residence time was adjusted to 5 minutes. The amount of bismuth ruthenium composite oxide powder (air electrode catalyst) in Comparative Example 1 was 0.25 g.

2. Analysis of Air Electrode Catalysts (1) Analysis of Composition Ratios Quantitative analysis of Bi, Ru, and Si was performed on analytical samples of the air electrode catalyst powders obtained in Examples 1 to 3 and Comparative Examples 1 to 3 by X-ray fluorescence analysis using the fundamental parameter (FP) method. An energy-dispersive X-ray fluorescence analyzer was used for the X-ray fluorescence analysis. The analytical conditions were a CuKα X-ray source, a tube voltage of 50 kV, and a tube current of 60 mA for Bi and Ru, and a tube voltage of 30 kV and a tube current of 100 mA for Si. Specifically, the analytical samples were irradiated with X-rays, and the intensity of the characteristic X-rays (fluorescent X-rays) generated by the irradiation was measured. The content of each element was determined using the FP method. Based on the obtained Bi, Ru, and Si contents, Si/Bi, which represents the atomic ratio of Si to Bi, and Bi/Ru, which represents the atomic ratio of Bi to Ru, were calculated. The results are shown in Table 1.

The analyzed Bi/Ru value is smaller than the Bi/Ru value at the time of preparation. This is because the Bi/Ru value at the time of preparation is roughly maintained up until the co-precipitation and calcination processes, but the subsequent acid treatment process and the washing performed during the acid treatment process remove Bi and Bi compounds that are not incorporated into the bismuth ruthenium composite oxide crystals, so the Bi/Ru value in the final air electrode catalyst (bismuth ruthenium composite oxide) is smaller than the Bi/Ru value at the time of preparation.

(2) Measurement of Lattice Constants The analytical samples of the air electrode catalyst powders obtained in Examples 1 to 3 and Comparative Examples 1 to 3 were analyzed by X-ray diffraction (XRD). A parallel-beam X-ray diffractometer was used for the XRD analysis. The analytical conditions were a CuKα X-ray source, a tube voltage of 15 kV, a tube current of 15 mA, a scan speed of 1 degree/min, and a step width of _0.01 degrees. From the diffraction chart patterns of the obtained XRD profiles, the air electrode catalysts were confirmed to be _{Bi2Ru2O7 ,} which has a pyrochlore-type crystal structure and a crystal structure similar thereto. As an example, the XRD profile of Example 1 is shown in Figure 2. Next, the lattice constants were determined from the peaks of the (111), (222), (400), (331), (440), (622), and (444) planes in the obtained diffraction chart patterns. The results are shown in Table 1.

3. Evaluation of Battery Characteristics The air-hydrogen secondary batteries of Examples 1 to 3 and Comparative Examples 1 to 3 were repeatedly charged and discharged in an atmosphere of 25° C. via the air electrode terminal 58 and the negative electrode terminal 60 at 0.1 It for 10 hours, and then discharged at 0.2 It until the battery voltage reached 0.4 V, forming one cycle. In the charge and discharge operations of the air-hydrogen secondary batteries described above, the rated capacity was 2.0 Ah, which corresponds to 80% of the negative electrode capacity.

During the above-mentioned charge/discharge operations, a 10-minute rest period was provided between charge and discharge, and between discharge and charge.

The discharge capacity was determined for each of the above charge/discharge cycles. The number of cycles was then counted, and the number of cycles at which the percentage of discharge capacity/rated capacity fell below 70% was recorded. This number of cycles is shown in Table 1. The higher the number of cycles, the longer the cycle life.

During the above-described charge/discharge operation, 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. The air supplied to the ventilation path 30 was air bubbling through a KOH aqueous solution ( _CO2 concentration: approximately 100 ppm).

4. Discussion Table 1 shows that the number of cycles for the batteries of Examples 1 to 3 was 49 to 58, while the number of cycles for the batteries of Comparative Examples 1 to 3 was 13 to 18. This indicates that the cycle life of the batteries of Examples 1 to 3 was significantly improved compared to the batteries of Comparative Examples 1 to 3. The batteries of Examples 1 to 3 had Si/Bi values of 0.002 to 0.007, while the batteries of Comparative Examples 1 to 3 had Si/Bi values of 0.011 to 0.014. In other words, the amount of Si relative to Bi in the batteries of Examples 1 to 3 was kept lower than that in the batteries of Comparative Examples 1 to 3. This suggests that setting the amount of Si relative to Bi contained in the air electrode catalyst to 0.007 or less can contribute to improving the cycle life characteristics of air secondary batteries.

Example 3 has a higher Bi content than Example 1. It is believed that within the bulk of bismuth ruthenium composite oxide, Si substitutes for Bi sites in the crystal, so if there is a lot of Bi, it is thought that Si is less likely to penetrate into the bulk. Therefore, since the Bi content in Example 3 is higher than in Example 1, it is thought that the amount of Si incorporated into the bulk is correspondingly less, resulting in a Si/Bi value of 0.002. As a result, the number of cycles in Example 3 was 58, and it is believed that Example 3 exhibits particularly excellent cycle life characteristics.

2. Battery (air hydrogen secondary battery)
4 Container 6 Container body 8 Lid 10 Electrode group 12 Negative electrode 14 Separator 16 Air electrode (positive electrode)
30 Ventilation path 40 Water-repellent ventilation member

Claims (5)

An air electrode catalyst made of a bismuth ruthenium composite oxide containing at least Bi, Ru, and Si, in which the Si/Bi atomic ratio of Si to Bi is 0.007 or less. The air electrode catalyst according to claim 1, wherein the Si/Bi ratio is 0.002 or greater. an air electrode substrate;
an air electrode mixture held by the air electrode substrate,
The air electrode mixture comprises the air electrode catalyst according to claim 1 or 2. 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 claim 3 . The air secondary battery described in claim 4, wherein the negative electrode contains a hydrogen storage alloy.