JP7513411B2 - Air secondary battery - Google Patents

Description

The present invention relates to an air secondary battery.

In recent years, air batteries, which use oxygen in the air as the positive electrode active material, have been attracting attention due to their high energy density and the ease of making them small and lightweight. Among such air batteries, zinc-air primary batteries have been put to practical use as a power source for hearing aids.

Research is also being conducted into rechargeable air batteries that use Li, Zn, Al, Mg, etc. as the negative electrode metal, and these air secondary batteries are expected to become new secondary batteries that may exceed the energy density of lithium-ion secondary batteries.

As one type of such air secondary battery, an air hydrogen secondary battery is known that uses an alkaline aqueous solution (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, but the negative electrode active material in air hydrogen secondary batteries is hydrogen that is absorbed and released by the above-mentioned hydrogen storage alloy, so that the hydrogen storage alloy itself does not undergo dissolution or deposition reactions during chemical reactions (hereinafter also referred to as battery reactions) during charging and discharging in the battery. For this reason, air hydrogen secondary batteries have the advantage that they do not suffer from 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 changes.

JP 2016-152068 A

However, in air batteries that use a high-concentration alkaline aqueous solution as an electrolyte and take in oxygen in the air as an active material, there is a problem that the influence of carbon dioxide (CO ₂ ) in the air is unavoidable. Specifically, air contains about 400 to 500 ppm of CO ₂ , and it is known that when an air secondary battery is operated for a long period of time, the electrolyte deteriorates due to carbon dioxide due to a neutralization reaction with the alkaline electrolyte. For example, when an alkaline electrolyte containing KOH deteriorates due to carbon dioxide, the ionic conductivity decreases and carbonates such as K ₂ CO ₃ precipitate in the pores used for gas diffusion in the air electrode, hindering the diffusibility of oxygen gas in the air electrode, which may increase the overvoltage in the charge and discharge reaction of the air electrode. If the overvoltage in the charge and discharge reaction increases in this way, the discharge voltage will decrease when charging and discharging are repeated. If the discharge voltage decreases, the improvement of energy efficiency and high output will be hindered.

In response to these issues, a method has been proposed in which a carbon dioxide absorbent or an oxygen selective permeable membrane is placed in the air intake of the battery to lower the _CO2 concentration inside the battery in advance.

However, these methods do not provide a complete solution to reduce the _CO2 concentration to zero because the size of the absorbent becomes too large to be practical and because it is difficult to achieve the performance of the separation membrane.

A solid polymer air electrode has also been proposed that uses an alkaline anion exchange membrane as an electrolyte near the air electrode. In this proposal, an alkaline electrolyte is also used, and an anion exchange membrane is present at the interface between the air electrode layer and the alkaline electrolyte. This is expected to prevent cations (e.g., K ⁺ ) in the alkaline aqueous solution from passing through to the air electrode due to electrostatic repulsion with the anion exchange membrane, thereby suppressing the precipitation of carbonates at the air electrode.

However, currently there are no anion exchange membranes that are stable in alkaline aqueous solutions for long periods of time, and they have not yet been put to practical use.

As described above, various attempts have been made to suppress the carbon dioxide degradation of alkaline electrolyte, but satisfactory results have not been obtained. For this reason, in order to further improve the energy efficiency and increase the output of air secondary batteries, it is desirable to develop an air secondary battery that can suppress the increase in overvoltage during the charge and discharge reaction that accompanies the carbon dioxide degradation of alkaline electrolyte.

The present invention was made based on the above circumstances, and its purpose is to provide an air secondary battery that can reduce overvoltage during charge/discharge reactions and suppress the decrease in discharge voltage when repeatedly charged and discharged.

To achieve the above object, the present invention provides an air secondary battery comprising a container, an electrode group disposed in the container, and an alkaline electrolyte injected into the container, the electrode group including an air electrode and a negative electrode stacked with a separator interposed therebetween, the alkaline electrolyte being an alkaline aqueous solution containing at least KOH as a solute, and the container containing at least one of Ca and a Ca compound.

It is preferable that at least one of the Ca and Ca compounds contained in the container is adjacent to the negative electrode.

The negative electrode has a negative electrode mixture, and it is preferable that at least one of the Ca and Ca compounds contained in the container is contained in the negative electrode mixture.

It is preferable that the negative electrode mixture contains a hydrogen storage alloy, and that the Ca contained in the container is Ca dissolved in the hydrogen storage alloy.

It is preferable that the negative electrode has a conductive negative electrode core having a three-dimensional mesh structure, and the negative electrode mixture is filled into the negative electrode core.

The Ca compound is preferably at least one of Ca salt and Ca oxide.

The air secondary battery according to the present invention comprises a container, an electrode group arranged in the container, and an alkaline electrolyte injected into the container, the electrode group including an air electrode and an anode stacked with a separator interposed therebetween, the alkaline electrolyte being an alkaline aqueous solution containing at least KOH as a solute, and the container containing at least one of Ca and a Ca compound. This embodiment can suppress carbonic acid deterioration of the alkaline electrolyte, and accordingly, can reduce overvoltage in charge and discharge reactions. Thus, the air secondary battery has improved energy efficiency and high output. Therefore, according to the present invention, it is possible to provide an air secondary battery that can reduce overvoltage in charge and discharge reactions and suppress a decrease in discharge voltage when charging and discharging are repeated.

1 is a cross-sectional view illustrating an air hydrogen secondary battery according to an embodiment of the present invention; 1 is a graph showing the relationship between discharge intermediate voltage and number of cycles. 1 is a graph showing the relationship between AC resistance and the number of cycles.

Below, an air hydrogen secondary battery 2 (hereinafter also referred to as battery 2) according to one embodiment will be described with reference to the drawings.

As shown in FIG. 1, the battery 2 includes a container 4, an electrode group 10 disposed in the container 4, and an alkaline electrolyte 82 injected into the container 4.

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 negative electrode core and a negative electrode mixture supported on the negative electrode core.
The negative electrode mixture includes a hydrogen storage alloy powder as a negative electrode active material, which is an aggregate of hydrogen storage alloy particles capable of absorbing and releasing hydrogen, a conductive material, and a binder. The conductive material may be a powder that is an aggregate of particles such as graphite or carbon black.

Here, the hydrogen storage alloy particles can be obtained, for example, as follows.
First, metal raw materials are weighed and mixed to obtain a desired composition, and the 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 is homogenized by heat treatment in which it is held 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.

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 hydrogen storage alloy powder, which is an aggregate of hydrogen storage alloy particles, a conductive material, a binder, and water. The obtained negative electrode mixture paste is filled into a negative electrode core, and then a drying process is performed. After drying, the negative electrode core to which the hydrogen storage alloy particles and the like are attached is rolled to increase the amount of alloy per unit volume, and then cut, thereby obtaining a negative electrode 12 including a negative electrode mixture layer. This negative electrode 12 has a plate shape as a whole. The negative electrode mixture layer included in the negative electrode 12 is formed of hydrogen storage alloy particles, conductive material particles, and the like, and therefore has a porous structure.

Next, the air electrode 16 comprises a conductive air electrode substrate having a porous structure and a large number of pores, and an air electrode mixture layer (positive electrode mixture layer) supported within the pores and on the surface of the air electrode substrate. For example, nickel foam or nickel mesh can be used as the air electrode substrate.

The air electrode mixture contains an air electrode catalyst for an air secondary battery, a conductive material, and a binder.
Bismuth ruthenium composite oxide is used as an air electrode catalyst for air secondary batteries. Bismuth ruthenium composite oxide has the dual functions of oxygen generation and oxygen reduction, and such a catalyst having dual functions contributes to reducing the overvoltage of the battery both during the charging and discharging processes.

In this embodiment, a pyrochlore-type composite oxide represented by the general formula A ₂ B ₂ O _7-z (wherein z satisfies the relationship 0≦z≦1, A represents at least one element selected from Bi, Pb, Tb, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu, Mn, Y, Zn, and Al, and B represents at least one element selected from Ru, Ir, Si, Ge, Ta, Sn, Hf, Zr, Ti, Nb, V, Sb, Rh, Cr, Re, Sc, Co, Cu, In, Ga, Cd, Fe, Ni, W, and Mo) is used as the air electrode catalyst for the air secondary battery.

The pyrochlore-type composite oxide used in this embodiment is preferably a pyrochlore-type bismuth ruthenium composite oxide, which has a crystal structure mainly composed of _{a Bi2Ru2O7 pyrochlore} structure and a crystal structure similar thereto.

Next, a method for producing an air electrode catalyst for an air secondary battery will be specifically described below using a pyrochlore-type bismuth ruthenium composite oxide as an example.

First, Bi(NO ₃ ) _{3.5H 2} O and RuCl _{3.3H 2} O are put into distilled water so that they have the same concentration, 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 60° C. or higher and 90° C. or lower. Then, a 1 mol/L or higher and 3 mol/L or lower NaOH aqueous solution is added to this mixed aqueous solution. The bath temperature is kept at 60° C. or higher and 90° C. or lower, and the mixture is stirred while oxygen bubbling is performed. The solution containing the precipitate generated by this operation is kept at 80° C. or higher and 100° C. or lower to evaporate a part of the water and form a paste. The paste is transferred to an evaporating dish, heated at 100° C. or higher and 150° C. or lower, and dried by keeping it in that state for 10 hours or higher and 20 hours or lower to obtain a dried paste. The dried product is then placed in a mortar and pulverized with a pestle, and then heated to a temperature of 350° C. to 650° C. in an air atmosphere and held for 0.5 hours to 24 hours to obtain a fired product. The fired product is washed with distilled water at a temperature of 60° C. to 90° C. and then dried. This produces a pyrochlore-type bismuth ruthenium composite oxide (Bi ₂ Ru ₂ O ₇ ).

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

First, prepare an aqueous solution of nitric acid. Here, the concentration of the aqueous solution of nitric acid is preferably 5 mol/L or less. The amount of the aqueous solution of nitric acid is preferably 20 mL per 1 g of bismuth ruthenium composite oxide. The temperature of the aqueous solution of nitric acid is preferably set to 20°C or more and 60°C or less.

Then, the bismuth ruthenium composite oxide is immersed in the prepared nitric acid aqueous solution and stirred for 6 hours or less. After a predetermined time has passed, the bismuth ruthenium composite oxide is suction filtered from the nitric acid aqueous solution. The filtered bismuth ruthenium composite oxide is placed in distilled water set at 60°C or higher and 80°C or lower and washed.

The washed bismuth ruthenium composite oxide is then dried by keeping it 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 manner, an acid-treated bismuth ruthenium composite oxide is obtained. By carrying out the acid treatment in this manner, by-products generated during the manufacturing process of the bismuth ruthenium composite oxide can be removed. The acidic aqueous solution used in the acid treatment is not limited to an aqueous nitric acid solution, and 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 the aqueous nitric acid solution.

The bismuth ruthenium composite oxide obtained as described above is mechanically pulverized as necessary to adjust the particle size to a specified size. This results in a powder of bismuth ruthenium composite oxide, which is an aggregate of particles of the specified size.

Next, the conductive material will be described. This conductive material is used to reduce the internal resistance in order to increase the power output of the air secondary battery, and as a carrier for the catalyst described above. For example, nickel powder, which is an aggregate of nickel particles, is preferably used as such a conductive material. For example, the nickel particles described above are preferably particles with an average particle size of 0.1 μm to 10 μm. Here, in the present invention, unless otherwise specified, when referring to the average particle size, it refers to the volume average particle size obtained by measuring the particle size distribution on a volume basis using a laser diffraction/scattering type particle size distribution measuring device for the powder, which is an aggregate of the target particles.

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 nickel powder content is preferably set to 80% by mass or less in relation to the other constituent materials in the air electrode mixture.

The binder binds the constituent materials of the air electrode mixture and also serves to impart appropriate water repellency to the air electrode 16. The binder is not particularly limited, and may be, for example, a fluororesin. A preferred example of the fluororesin is polytetrafluoroethylene (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, the conductive powder, the binder, and the water are kneaded to prepare an air electrode mixture paste.

The obtained air electrode mixture paste is formed into a sheet, for example, by roller pressing, thereby obtaining an air electrode mixture sheet. The air electrode mixture sheet is then press-bonded to a nickel mesh (air electrode substrate). This results in an intermediate air electrode product.

Then, the obtained intermediate product is put into a firing furnace and subjected to firing treatment. This firing treatment is performed in an inert gas atmosphere. For example, nitrogen gas or argon gas is used as the inert gas. The firing treatment conditions are to heat the intermediate product to a temperature of 300°C or more and 400°C or less, and to hold it in this state for 10 minutes or more and 20 minutes or less. After that, the intermediate product is naturally cooled in the firing furnace, and when the temperature of the intermediate product becomes 150°C or less, it is taken out into the air. In this way, an intermediate product that has been subjected to firing treatment is obtained. The intermediate product after this firing treatment is cut into a predetermined shape to obtain an air electrode 16. This air electrode 16 has an air electrode mixture layer formed from an air electrode mixture. Since the air electrode mixture contains particles of bismuth ruthenium composite oxide, the air electrode mixture layer formed from such an air electrode mixture has a porous structure as a whole and has excellent gas diffusion properties.

The air electrode 16 and the negative electrode 12 obtained as described above are stacked via a separator 14, thereby forming an electrode group 10. This separator 14 is arranged to avoid short circuits between the air electrode 16 and the negative electrode 12, and an electrically insulating material is used. 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, etc.

The formed electrode group 10 is placed in a container 4 together with an alkaline electrolyte. The container 4 is not particularly limited 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.

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 walls 20 is open. In other words, an opening 22 is provided on the opposite side of the bottom wall 18. In addition, the side walls 20 have through holes at predetermined positions on the right side wall 20R and the left side wall 20L, and these through holes become the lead wire withdrawal openings 24, 26 described below.

Furthermore, an electrolyte storage unit 80 is attached to the container body 6. This electrolyte storage unit 80 is a container that contains an alkaline electrolyte 82, and is attached, for example, via a connection unit 84 that communicates with a through hole 19 provided in the bottom wall 18. The connection unit 84 is a flow path for the alkaline electrolyte 82 that communicates between the inside of the container 4 and the electrolyte storage unit 80. In this way, since the inside of the container 4 and the electrolyte storage unit 80 are connected, the alkaline electrolyte 82 can move between the inside of the container 4 and the electrolyte storage unit 80.

The lid 8 has the same shape as the container body 6 when viewed from above, and is placed on 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-tightly sealed.

The lid 8 has an air passage 30 on the inner surface 28 facing the inside of the container body 6. The air passage 30 is open at the portion facing the inside of the container body 6, and has a serpentine shape as a whole. Furthermore, an inlet air hole 32 and an outlet air hole 34 are provided at a predetermined position of the lid 8, which penetrate in the thickness direction. The inlet air hole 32 communicates with one end of the air passage 30, and the outlet air hole 34 communicates with the other end of the air passage 30. In other words, the air passage 30 is open to the atmosphere through 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 passage 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 inside the container 4. For example, a foamed nickel sheet is used as the adjustment member 36.

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.

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 a water-repellent effect 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 path 30, the inlet ventilation hole 32, and the outlet ventilation hole 34 of the lid 8.

The container body 6 housing the electrode group 10, the adjustment member 36, and the water-repellent and breathable member 40 as described above is covered with the lid 8. Then, as shown diagrammatically in FIG. 1, the 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. After that, a predetermined amount of alkaline electrolyte 82 is injected from the electrolyte storage portion 80, and the container 4 is filled with the alkaline electrolyte 82. In this manner, the battery 2 is formed.

In the battery 2, the ventilation passage 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 through the water-repellent ventilation member 40, the ventilation passage 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 a negative electrode lead 56 is electrically connected to the negative electrode 12. These air electrode lead 54 and negative electrode lead 56 are illustrated diagrammatically in FIG. 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 a negative electrode terminal 60 is provided at the tip of the negative electrode lead 56. Therefore, in the battery 2, the air electrode terminal 58 and negative electrode terminal 60 are used to input and output current during charging and discharging.

Here, the alkaline electrolyte 82 is an alkaline aqueous solution containing at least KOH as a solute.

An alkaline aqueous solution containing KOH reacts with carbon dioxide (CO ₂ ) in the air, consuming KOH and generating K ₂ CO ₃ as shown in formula (I) described below. This not only reduces the ionic conductivity, but also causes K ₂ CO ₃ to be generated in excess of its solubility and precipitate in the pores for gas diffusion in the air electrode, potentially impeding the diffusibility of oxygen gas.

In this manner, in order to suppress the deterioration of the alkaline electrolyte due to reaction with _CO2 , in this embodiment, a substance that suppresses the deterioration of the alkaline electrolyte 82 described above (hereinafter, also referred to as a deterioration suppressing substance) is disposed within the container 4 of the battery 2.

As the deterioration inhibitor, at least one of Ca and a Ca compound is used. As the Ca compound, at least one of Ca salt and Ca oxide is preferably used. More preferably, Ca(OH) ₂ is used.

Here, from Table 1, which summarizes the solubilities of potassium salts and calcium salts in water at ₂₅ °C, it can be seen that the solubility of KOH and _K2CO3 produced by the reaction of KOH with carbon dioxide is relatively high. On the other hand, the solubility of Ca(OH) ₂ and _CaCO3 produced by the reaction of Ca(OH) ₂ with carbon dioxide is low. It can be said that Ca(OH) ₂ itself is a salt that is poorly soluble in alkaline aqueous solutions. This is because, compared to alkali metals such as K ⁺ , alkaline earth metal ions such as ^{Ca2+ are} less likely to ionize due to the covalent bond with OH-, and in a strong alkaline aqueous solution such as an alkaline electrolyte, the equilibrium is biased toward the non-ionization side.

ここで、表１中の単位ｗは飽和溶液１００ｇ中に含まれる無水物の質量[ｇ]（質量％）を表し、表１中の単位ｓは飽和溶液１ｄｍ^３中に含まれる無水物の質量[ｇ]を表す。

Here, the unit w in Table 1 represents the mass [g] (mass%) of the anhydride contained in 100 g of saturated solution, and the unit s in Table 1 represents the mass [g] of the anhydride contained in ^{1 dm3} of saturated solution.

In this embodiment, the difference in solubility described above is utilized to suppress the decrease in ionic conductivity of the alkaline electrolyte and the precipitation of carbonates in the air electrode, thereby suppressing the associated decrease in discharge voltage.

Specifically, when Ca(OH) ₂ is added to the negative electrode and a high-concentration KOH aqueous solution is used as the alkaline electrolyte, Ca(OH) ₂ is hardly dissolved in the alkaline electrolyte. Furthermore, when part of the KOH aqueous solution is carbonated by CO ₂ in the air (reaction of formula (I)), KOH is generated according to the following reaction formula (II), and the alkaline electrolyte is regenerated. This suppresses the deterioration of the alkaline electrolyte. Here, Ca(OH) ₂ acts as a buffer for OH ^- , so to speak.
2KOH+ _CO2 (gas)→ _K2CO3 + _{H2O ...} (I)
_K2CO3 +Ca(OH) ₂ (solid)→2KOH _{+ CaCO3} (solid)...(II)

The location where the deterioration inhibitor is present is not particularly limited as long as it is basically inside the container 4. However, since carbonate precipitation occurs at the location where Ca(OH) ₂ is added, it is preferable to avoid the inside of the air electrode. In this way, if the inside of the air electrode is avoided as the location where Ca(OH) ₂ is added, the pores in the air electrode are not blocked.

A preferred embodiment of the deterioration inhibitor is adjacent to the negative electrode. In other words, it is preferable to dispose the deterioration inhibitor on the surface of the negative electrode. Another preferred embodiment is to mix the deterioration inhibitor into the negative electrode mixture. These embodiments are preferred because ion species in the electrolyte are transported between the air electrode and the negative electrode in accordance with charging and discharging, and therefore CO ₃ ^2- can be efficiently removed by adding the deterioration inhibitor to the negative electrode.

A more preferred embodiment is one in which Ca is dissolved in the hydrogen storage alloy. In order to process a large amount of CO ₃ ^2-, it is necessary to add a large amount of Ca, and the embodiment in which Ca is dissolved in the hydrogen storage alloy has the advantage of increasing the battery capacity, compared to adding Ca(OH) ₂ or the like, which does not contribute to the battery reaction, to the negative electrode. Furthermore, when Ca (zero valence) in the hydrogen storage alloy reacts with water, the following reaction occurs.
Ca(0 valence)+ _2H2O → _H2 (gas)+Ca(OH) ₂ ...(III)

In the sum of the above reaction formulas (I) and (II), there is concern that the increase in water will cause a decrease in ionic conductivity, but by adding reaction formula (III), the decrease in ionic conductivity can also be suppressed.

By the way, hydrogen storage alloys with Ca dissolved in solid solution have the advantage of improving the hydrogen storage capacity due to the presence of Ca, and therefore have the advantage of being able to increase the capacity of the battery. However, hydrogen storage alloys with Ca dissolved in solid solution also have the disadvantage of being easily pulverized by the hydrogen absorption and release reaction. In detail, there is a risk that the capacity of the negative electrode will decrease due to the loss of electrical conductivity caused by the pulverization of the hydrogen storage alloy. For this reason, particularly when using a negative electrode mixture containing a hydrogen storage alloy with Ca dissolved in solid solution, it is preferable to use a conductive negative electrode core having a three-dimensional mesh structure and a large number of voids, specifically, foamed nickel. In other words, it is preferable to fill the foamed nickel with a negative electrode mixture containing a hydrogen storage alloy with Ca dissolved in solid solution to improve current collection. This allows the hydrogen storage alloy to maintain high electrical conductivity even if it is pulverized.

Here, the hydrogen storage alloy constituting the hydrogen storage alloy particles in which Ca is dissolved is not particularly limited, but for example, a rare earth-Mg-Ca-Ni based hydrogen storage alloy is preferably used. The composition of this rare earth-Mg-Ca-Ni based hydrogen storage alloy can be freely selected, but for example,
General formula: Ln _1-x-y Mg _x Ca _y Ni _a-b-c Al _b M _c ... (IV)
It is preferable to use one represented by the following formula:

In the general formula (IV), 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 x, y, a, b, and c represent numbers that satisfy the relationships 0.01≦x≦0.30, 0.01≦y≦0.30, 2.8≦a≦3.9, 0≦b≦0.30, and 0≦c≦0.50, respectively.

Here, the portion in the battery 2 where the deterioration suppressing substance is disposed is not limited to any one of the above-mentioned aspects, and it is also possible to adopt an aspect in which the deterioration suppressing substance is disposed in a plurality of portions. For example, it is possible to adopt an aspect in which a hydrogen storage alloy containing Ca as a solid solution is included in the negative electrode mixture and Ca(OH) ₂ is added to the negative electrode mixture, an aspect in which a hydrogen storage alloy containing Ca as a solid solution is included in the negative electrode mixture and Ca(OH) ₂ is adjacent to the negative electrode, etc.

[Example]
1. Battery Production (Example 1)

(1) Production of hydrogen storage alloy powder After preparing a mixed metal material by mixing the metal materials of La, Ca, Mg, and Ni in a predetermined molar ratio, the mixed metal material 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 1025°C for 10 hours in an argon gas atmosphere to homogenize the alloy phase, and then mechanically pulverized in an argon gas atmosphere to obtain rare earth-Mg-Ca-Ni hydrogen storage alloy powder. The volume average particle size (MV) of the obtained hydrogen storage alloy powder was measured using a laser diffraction/scattering particle size distribution measuring device. The volume average particle size (MV) was 60 μm.

The composition of this hydrogen storage alloy powder was analyzed by inductively coupled plasma spectrometry (ICP) and found to be La _0.50 Ca _0.25 Mg _0.25 Ni _3.50 .

(2) Production of Negative Electrode 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, 1.0 part by mass of a styrene butadiene rubber dispersion, 0.3 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.

This negative electrode mixture paste was filled into a foamed nickel sheet with an areal density (weight per unit area) of 300 g/m ² and a thickness of about 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 volume, and then cut into a length of 40 mm and a width of 40 mm to obtain the negative electrode 12. The thickness of the negative electrode 12 was 0.80 mm, the weight of the negative electrode 12 was 7.72 g, the weight of the hydrogen storage alloy was 7.17 g, and the weight ratio of Ca in the negative electrode was 3.2 wt%.

(3) Synthesis of air electrode catalyst A predetermined amount of Bi(NO ₃ ) _{3.5H 2} O and RuCl _{3.3H 2} O were prepared, and these Bi(NO ₃ ) _{3.5H 2} O and RuCl _{3.3H 2} O were put into distilled water at 75 ° C. so that they had the same concentration, and stirred to prepare a mixed aqueous solution of Bi(NO ₃ ) _{3.5H 2} O and RuCl _{3.3H 2} O. Then, 2 mol/L of NaOH aqueous solution was added to this mixed aqueous solution. The bath temperature at this time was set to 75 ° C., and stirring was performed while performing oxygen bubbling. The solution containing the precipitate generated by this operation was kept at 85 ° C. to evaporate part of the water and form a paste. This paste was transferred to an evaporating dish, heated to 120 ° C., and kept in that state for 12 hours to dry, and a dried paste (precursor) was obtained.

The resulting dried product was placed in a mortar and crushed with a pestle, then heated to 600°C in an air atmosphere and held for 1 hour to obtain a fired product. The fired product was washed with distilled water at 70°C, filtered under suction, and dried at 120°C. This resulted in a fired product.

When the secondary electron image of this fired powder was observed using a scanning electron microscope, the particle size of the fired powder was found to be 0.1 μm or less.

1 g of the fired powder was placed in a stirrer tank together with 20 mL of aqueous nitric acid solution, and stirred for 1 hour while maintaining the temperature of the aqueous nitric acid solution at room temperature of 25°C. Here, the concentration of the aqueous nitric acid solution was 2 mol/L.

After the stirring was completed, the fired powder was removed from the nitric acid aqueous solution by suction filtration. The removed fired powder was washed with 1 liter of distilled water heated to 70°C. After washing, the fired powder was heated to 120°C and dried.

This resulted in the production of an air electrode catalyst (pyrochlore-type composite oxide catalyst) for air-hydrogen secondary batteries.

(4) Manufacturing of the air electrode Nickel powder, which is an aggregate of nickel particles, was prepared. Here, nickel powder made of filamentary nickel particles was used as the nickel powder. The nickel particles had an average particle size of 2 to 3 μm.

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

Nickel powder, polytetrafluoroethylene (PTFE) dispersion, and ion-exchanged water were added to the air electrode catalyst powder obtained as described above and mixed. At this time, 20 parts by mass of the air electrode catalyst powder, 70 parts by mass of the nickel powder, 10 parts by mass of the PTFE dispersion, and 30 parts by mass of the ion-exchanged water were mixed uniformly to produce an air electrode mixture paste.

The obtained air electrode mixture paste was formed into a sheet and dried in an atmosphere at 25°C. The sheet-shaped air electrode mixture paste was then pressed onto a nickel mesh with a mesh number of 60, a wire diameter of 0.08 mm, and an opening ratio of 60%.

The paste of the air electrode mixture pressed onto the nickel mesh was heated to 340°C under a nitrogen gas atmosphere and held at this temperature for 13 minutes for firing. The fired sheet of the air electrode mixture was cut into a sheet 40 mm long and 40 mm wide, thereby obtaining an air electrode 23. The thickness of this air electrode 23 was 0.23 mm. The amount of the air electrode catalyst (pyrochlore-type composite oxide catalyst) in the obtained air electrode 23 was 0.24 g.

(5) 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 the production of this electrode group 10 was formed of a nonwoven fabric made of polypropylene fibers having sulfone groups, and had a thickness of 0.1 mm (basis weight 53 g/ ^m2 ).

Next, the container body 6 was prepared, and the above-mentioned electrode group 10 was housed in this container body 6. At this time, a sheet of foamed nickel was placed as an adjustment member 36 on the bottom wall 18 of the container body 6, 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 of 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 is formed by combining a PTFE porous membrane 42 that is 45 mm long, 45 mm wide, and 0.1 mm thick, and a nonwoven fabric diffusion paper 44 that is 40 mm long, 40 mm wide, and 0.2 mm thick.

Then, 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 area including the ventilation passage 30, the inlet ventilation hole 32, and the outlet ventilation hole 34 on the inner surface 28 of the lid 8 so that the area was entirely covered with the water-repellent ventilation member 40. Here, the ventilation passage 30 has a single serpentine shape as a whole. The cross section of the ventilation passage 30 is rectangular, with the vertical dimension of the rectangle being 1 mm and the horizontal dimension being 1 mm. The ventilation passage 30 is open on the water-repellent ventilation member 40 side.

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

Next, a 5 mol/L KOH aqueous solution was injected as the alkaline electrolyte 82 into the electrolyte storage unit 80. The amount of the injected KOH aqueous 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. The air electrode lead 54 and the 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.

(6) Measurement of negative electrode capacity The manufactured air hydrogen secondary battery 2 was subjected to aging treatment in an atmosphere of 60°C for 12 hours, and then cooled to 25°C. Thereafter, the air hydrogen secondary battery 2 was charged at 200mA for 10 hours, and discharged at 400mA until 0.4V was performed for 5 cycles to activate the negative electrode. Thereafter, the air hydrogen secondary battery 2 was repeatedly discharged at 400mA until 0.4V, with the charge time at 200mA being extended by 1 hour each time, to examine the maximum capacity of the negative electrode. As a result, the capacity became constant when the charge was 14 hours or more, and the capacity at that time was 2652mAh.

(Comparative Example 1)
An air hydrogen secondary battery was manufactured in the _{same manner as in Example 1, except that a hydrogen storage alloy in which Ca was not dissolved and having a composition represented by (Nd0.99Zr0.01)0.89Mg0.11Ni3.23Al0.17 was used as the} hydrogen storage alloy. Here, the thickness of the manufactured negative electrode was 0.79 _mm , the weight of the negative electrode was 8.42 g, and the weight of the hydrogen storage alloy was 7.85 g. The negative electrode capacity obtained in the same manner as in Example 1 was 2576 mAh.

(Comparative Example 2)
An air hydrogen secondary battery was manufactured in the _same manner as in _{Example 1} , except that a hydrogen storage alloy in which Ca was not dissolved and having a composition represented by ( _La0.20Sm0.80 ) _{0.98Mg0.02Ni3.19Al0.25Cr0.01} was used as the hydrogen storage alloy. Here, the thickness of the manufactured negative electrode was _{0.78 mm} , the weight of the negative electrode was 8.51 g, and the weight of the hydrogen storage alloy was 7.94 g. The negative electrode capacity obtained in the same manner as in Example 1 was 2502 mAh.

2. Evaluation of air-hydrogen secondary batteries (1) Battery characteristic evaluation The air-hydrogen secondary batteries of Example 1, Comparative Example 1, and Comparative Example 2 were subjected to an aging treatment in which they were held in an atmosphere of 60° C. for 12 hours. After this aging treatment, each battery was cooled to 25° C. Then, the air-hydrogen secondary batteries of Example 1, Comparative Example 1, and Comparative Example 2 were charged at 0.1 It for 10 hours via the air electrode terminal 58 and the negative electrode terminal 60, and discharged at 0.2 It until the battery voltage reached 0.4 V, which constituted one cycle, and such charging and discharging was repeated 40 cycles. 2 Ah, which corresponds to 75% of the negative electrode capacity, was defined as 1 It.

When carrying out the above-mentioned charge/discharge cycle, air was continuously supplied to the ventilation path 30 at a rate of 33 mL/min, by introducing air from the inlet ventilation hole 32 and discharging air from the outlet ventilation hole 34, regardless of whether the battery was being charged or discharged. Here, the air supplied to the ventilation path 30 was air ( _CO2 concentration of about 100 ppm) that had been bubbled through 100 mL of a 5 mol/L KOH aqueous solution. In other words, air with a _CO2 concentration reduced to about 100 ppm was continuously supplied.

During the above-mentioned charge/discharge operation, the discharge capacity and battery voltage were measured for each cycle. The battery voltage when the discharge capacity value was half of the total discharge capacity was calculated as the discharge intermediate voltage. The relationship between the number of cycles and the discharge intermediate voltage was then calculated. The results are shown in Figure 2.

Also, the alternating current resistance (AC resistance) at 1 kHz was measured for the air hydrogen secondary batteries of Example 1, Comparative Example 1, and Comparative Example 2. The relationship between the AC resistance and the number of cycles was then determined. The results are shown in Figure 3.

(2) Observations The change in discharge intermediate voltage over the charge/discharge cycle can be seen from Fig. 2. It can be seen from Fig. 2 that Example 1, which contains a degradation inhibitor, has a higher discharge intermediate voltage than Comparative Examples 1 and 2, which do not contain a degradation inhibitor. It can also be seen that the discharge intermediate voltage of Example 1 is almost constant. From this, it can be said that the increase in overvoltage during discharge is suppressed in the battery of Example 1 compared to the batteries of Comparative Examples 1 and 2.

Also, from Figure 3, the AC resistance at 1 kHz at 0 cycles can be compared with the AC resistance at 1 kHz at 40 cycles. From Figure 3, it can be seen that Example 1 has the smallest increase in AC resistance. AC resistance at high frequencies reflects electrolyte resistance and electrical resistance that are not dependent on the exchange of electrons. For this reason, the battery of Example 1 shows less degradation of the alkaline electrolyte compared to the batteries of Comparative Examples 1 and 2, as the resistance does not increase significantly even with an increase in the number of charge/discharge cycles.

From the above, it is believed that by including Ca in the battery as a degradation inhibitor, the increase in electrical conductivity associated with carbonic acid degradation of the alkaline electrolyte can be suppressed, and the precipitation of carbonates in the pores of the air electrode and the blockage of those pores can be suppressed, thereby suppressing the increase in the discharge overvoltage of the battery.

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 (6)

A container;
An electrode group disposed in the container;
and an alkaline electrolyte injected into the container;
The electrode group includes an air electrode and a negative electrode stacked with a separator interposed therebetween,
the negative electrode has a negative electrode mixture containing a hydrogen storage alloy,
The alkaline electrolyte is an alkaline aqueous solution containing at least KOH as a solute,
The container contains at least one of Ca and a Ca compound. The air secondary battery according to claim 1, wherein at least one of the Ca and Ca compounds contained in the container is adjacent to the negative electrode. 3 . The air secondary battery according to claim 1 , wherein at least one of the Ca and the Ca compound contained in the container is contained in the negative electrode mixture. 4. The air secondary battery according to claim 3, wherein the Ca contained in the container is Ca dissolved in the hydrogen storage alloy. The hydrogen storage alloy is
General formula: Ln _1-x-y Mg _x Ca _y Ni _a-b-c A _b M _c 5. The air secondary battery according to claim 4, having a composition represented by the formula: (wherein in the general formula, 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 x, y, a, b, and c represent numbers satisfying the relationships: 0.01≦x≦0.30, 0.01≦y≦0.30, 2.8≦a≦3.9, 0≦b≦0.30, and 0≦c≦0.50, respectively). The negative electrode has a negative electrode core body having a three-dimensional mesh structure and is conductive,
The air secondary battery according to any one of claims 1 to 5, wherein the negative electrode mixture is filled in the negative electrode core.

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