JP2019133841A - Water-based lithium-air secondary battery and co2 removal method for water-based lithium-air secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a water-based lithium-air secondary battery capable of suppressing deterioration of battery characteristics due to carbon dioxide in the air, and a CO2 removal method for the water-based lithium-air secondary battery. A water-based lithium-air secondary battery includes a negative electrode containing metallic lithium, an air electrode to which air is supplied, a water-based electrolytic solution, and an electrolytic solution housing space in which the aqueous electrolytic solution is housed. The pH of the aqueous electrolytic solution decreases as the charging progresses and increases as the discharging progresses, and the pH of the aqueous electrolytic solution when fully charged is such that carbon dioxide absorbed in the aqueous electrolytic solution is released from the aqueous electrolytic solution. The first predetermined value is selected from within the easy first range. [Selection diagram] Figure 1

Description

The present invention relates to an aqueous lithium-air secondary battery using lithium as a negative electrode active material and oxygen in the air as a positive electrode active material, and a CO ₂ removal method for the aqueous lithium-air secondary battery.

In recent years, demands for higher capacity of secondary batteries have been increasing, and metal-air secondary batteries that use oxygen (O ₂ ) in the air as a positive electrode active material and have a very high energy density have attracted attention. (For example, see Patent Literature 1 to Patent Literature 5 and Non-Patent Literature 1).

  In a typical metal-air secondary battery, a highly concentrated strong alkaline aqueous solution (for example, NaOH aqueous solution, KOH aqueous solution, or LiOH aqueous solution) is used as an electrolytic solution.

In the metal-air secondary battery, the electrode reactions of the formulas (1) and (2) proceed.

Negative electrode: M Ｍ M ^{n +} + ne ⁻ (1)
(M is a metal (for example, Zn, Fe, Al, Na or Li).)

Air electrode (positive electrode): O ₂ + 2H ₂ O + 4e ^{− ４} 4OH ⁻ (2)

At the time of discharge, at the air electrode, the reaction proceeds to the right in the formula (2), and oxygen (O ₂ ) in the air is reduced to generate hydroxide ions (OH ⁻ ). At the time of charging, at the air electrode, the reaction proceeds to the left in the formula (2), and hydroxide ions (OH ⁻ ) in the electrolytic solution are oxidized to generate oxygen (O ₂ ).

JP 2013-098174 A International Publication No. 2012/046699 International Publication No. 2011/052440 International Publication No. 2009/104570 JP 2012-099266 A

Satoshi Hasegawa et al., “Study on lithium / air secondary batteries Stability of NASICON-type lithium ion conducting glass ceramics with water”, Journal of Power Sources, vol.189 (1), p371-377 (2009)

In the air (atmosphere), for example, weakly acidic carbon dioxide (CO ₂ ) of about 400 ppm is contained. Accordingly, as shown in the following equation (3), carbon dioxide (CO ₂₎ is hydroxide ion alkaline electrolyte reacting with (OH - ^-) and discharge reaction hydroxide ions generated by ^(OH) And carbonate ions (CO ₃ ²⁻ ).

OH ⁻ + 1 / 2CO ₂ → 1 / 2CO ₃ ²⁻ + 1 / 2H ₂ O (3)

Then, the resulting carbonate ion _(CO ^{3 2-)} and cations of the electrolyte solution ^(e.g., Na +, ^{K +} or Li ⁺⁾ and is bound, stable carbonates _(e.g., Na ₂ CO _{3, K 2} CO ₃ or Li ₂ CO ₃ ) is deposited from the electrolyte. The precipitated carbonate is deposited on the surface of the air electrode and inside the pores of the air electrode, and inhibits the battery reaction occurring at the air electrode or precipitates in the electrolyte. As a result, battery characteristics are degraded.

The reaction of the formula (3) is a neutralization reaction between an acid and a base, and the reaction continues as long as the electrolytic solution is alkaline. Therefore, as long as the electrolytic solution is alkaline, the hydroxide ion concentration in the electrolytic solution is reduced by the reaction between the hydroxide ion (OH ⁻ ) of formula (3) and carbon dioxide (CO ₂ ). Keep doing. Thereby, the hydroxide ion conductivity of electrolyte solution will fall. Furthermore, once carbonate ions are generated, they cannot be returned again to carbon dioxide (CO ₂ ) and hydroxide ions (OH ⁻ ) by the electrode reaction, so that the charging efficiency also decreases. As a result, the battery characteristics are degraded.

  As described above, the metal-air secondary battery has a problem that the above-described “deterioration of battery characteristics due to carbon dioxide in the air” occurs.

The present invention has been made to address the above-described problems. That is, one of the objects of the present invention is referred to as an aqueous lithium-air secondary battery (hereinafter referred to as “the present invention aqueous lithium-air secondary battery”) that can suppress deterioration in battery characteristics caused by carbon dioxide in the air. . sometimes that), and aqueous lithium - CO ₂ removal method of an air secondary battery (hereinafter,. which - in some cases be referred to as "the present invention-aqueous lithium CO ₂ removal method of an air secondary battery") provides a There is to do.

In order to solve the above problems,
The aqueous lithium-air secondary battery of the present invention includes a negative electrode containing metallic lithium, an air electrode to which air is supplied, an aqueous electrolyte in which an acid for providing a lithium salt and a proton is dissolved in water, and the aqueous electrolyte. An electrolyte containing space in which the pH of the aqueous electrolyte decreases as charging progresses and increases as discharging progresses, and the pH of the aqueous electrolyte when fully charged is The carbon dioxide contained in the air absorbed in the aqueous electrolyte solution becomes a first predetermined value selected from the first range that is easily released from the aqueous electrolyte solution, and the carbon dioxide released from the aqueous electrolyte solution is: It is discharged outside from the electrolytic solution housing space.

In one aspect of the aqueous lithium-air secondary battery of the present invention,
The pH of the aqueous electrolyte at the time of complete discharge becomes a second predetermined value selected from the second range in which the carbon dioxide is easily absorbed by the aqueous electrolyte.

In one aspect of the aqueous lithium-air secondary battery of the present invention,
Further comprising an exterior body that sheathes a battery constituent member including at least the negative electrode, the air electrode, and the aqueous electrolyte.
The exterior body communicates the electrolytic solution housing space and the outside of the battery, and has a discharge port through which the carbon dioxide released from the electrolytic solution passes and is discharged to the outside of the battery.

In one aspect of the aqueous lithium-air secondary battery of the present invention,
The first range is a range smaller than the value of the first acid dissociation index pKa1 of carbon dioxide.

In one aspect of the aqueous lithium-air secondary battery of the present invention,
The first range is a range of pH 5.587 or less.

In one aspect of the aqueous lithium-air secondary battery of the present invention,
A negative electrode protective layer located between the negative electrode and the aqueous electrolyte solution;
The negative electrode protective layer is LATP,
The lower limit of the first range is pH 2.0.

In one aspect of the aqueous lithium-air secondary battery of the present invention,
The proton ion counter ions of the acid that donates the protons and the lithium ion counter ions of the lithium salt are not decomposed by charging and discharging.

In one aspect of the aqueous lithium-air secondary battery of the present invention,
The proton donating acid is at least one selected from H ₂ SO ₄ , HCl, HNO ₃ and CH ₃ COOH.

In one aspect of the aqueous lithium-air secondary battery of the present invention,
The lithium salt is at least one selected from Li ₂ SO ₄ , LiCl, LiNO ₃ and CH ₃ COOLi.

The CO ₂ removal method of the aqueous lithium-air secondary battery of the present invention includes a negative electrode containing metallic lithium, an air electrode to which air is supplied, an aqueous electrolyte in which an acid for supplying a lithium salt and a proton is dissolved in water, , A CO ₂ removal method for an aqueous lithium-air secondary battery comprising an electrolytic solution housing space in which the aqueous electrolytic solution is accommodated, wherein the pH of the aqueous electrolytic solution that has become larger as discharge progresses, The charge decreases as the charge progresses, and at the time of full charge, the carbon dioxide contained in the air absorbed in the aqueous electrolyte solution becomes a first predetermined value selected from the first range in which the aqueous electrolyte solution is easily released. Then, the carbon dioxide is released from the aqueous electrolyte solution, and the released carbon dioxide is discharged to the outside from the electrolyte solution storage space.

  ADVANTAGE OF THE INVENTION According to this invention, the fall of the battery characteristic resulting from the carbon dioxide in air can be suppressed.

FIG. 1 is a schematic cross-sectional view showing a configuration example of an aqueous lithium-air secondary battery according to an embodiment of the present invention. FIG. 2 is a graph showing the relationship between the pH of the electrolytic solution and the solubility of carbon dioxide (CO ₂ ) in the electrolytic solution. FIG. 3 is a conceptual diagram for explaining the CO ₂ removal characteristics of the electrolytic solution. FIG. 4 is a schematic cross-sectional view of the cell used in the evaluation test 2. FIG. 5 is a graph showing changes in pH and electrode potential due to charging and discharging.

<Technical Background and Summary of the Present Invention>
First, in order to facilitate understanding of the present invention, the technical background of the present invention and the outline of the present invention will be described.

Conventionally, it has been known that a metal-air secondary battery is a battery system having a very high energy density. However, carbon dioxide (CO ₂ ) in the air (atmosphere) has a problem of adversely affecting battery characteristics. It is said that one of the factors that has not led to the practical application of metal-air secondary batteries is that the “problem caused by carbon dioxide in the air (decrease in battery characteristics)” has not been established. Yes.

Here, at the laboratory level, it is relatively easy to eliminate the “problem caused by carbon dioxide in the air” by removing carbon dioxide (CO ₂ ) in the air in advance. For example, carbon dioxide (CO ₂₎ are the weakly acidic pre-dioxide (CO ₂₎ the alkaline CO ₂ removal material (e.g., an alkaline solid and / or alkaline liquid) be a neutralization reaction Can be removed from the air.

Specifically, for example, introducing air into a washing bottle containing an alkaline aqueous solution, introducing air into a filter impregnated with an alkaline aqueous solution, or introducing air into a column charged with an alkaline solid By doing so, carbon dioxide (CO ₂ ) in the air can be removed.

However, since the alkaline CO ₂ removing material is consumed by the neutralization reaction, the amount of CO _{2 that} can be removed is limited. For this reason, at the laboratory level, the CO ₂ removal material is regularly exchanged and used. On the other hand, in a practical metal-air secondary battery, periodic replacement of the CO ₂ removal material forces the battery user to perform complicated replacement work and increase the cost of battery operation. turn into.

Therefore, various techniques have been proposed in order to avoid periodic replacement of the CO ₂ removal material and to solve the “problem caused by carbon dioxide in the air”. For example, Patent Document 1 (Japanese Patent Application Laid-Open No. 2013-098174) proposes a technique for mounting a CO ₂ removal member and its regeneration mechanism on an air battery system. In the technique of Patent Document 1, it is recommended to use a mesoporous material and / or a zeolite adsorbent material in order to facilitate the regeneration of the CO ₂ removing member.

The technique of Patent Document 1 has an advantage that it is not necessary to replace the CO ₂ removal material. However, in the technique of Patent Document 1, by installing the regeneration mechanism for the CO ₂ removal member in the air battery system, the weight and volume of the air battery system increase, and the energy density and output density of the battery decrease. Will cause problems.

Furthermore, the adsorption material of the mesoporous material and the zeolite (CO ₂ removing member), since CO ₂ oxygen addition _{(O 2)} and nitrogen _{(N 2)} is also to some extent adsorbed, reduced adsorption of carbon dioxide (CO ₂₎ Resulting in. As a result, there arises a problem that it becomes necessary to mount more CO ₂ removing members on the air battery system. Furthermore, since the mesoporous material or zeolite adsorption system CO ₂ removal member needs to circulate an inert gas at the time of regeneration, there arises a problem that it is necessary to secure the inert gas.

As another technique, a technique for selectively supplying only oxygen (O ₂ ) in the air into the metal-air secondary battery by using an oxygen selective permeable membrane has been proposed. For example, Patent Document 2 (International Publication No. 2012/046699) proposes a technique of installing and using an oxygen-selective permeable membrane in a flow path for taking in air outside the battery. Patent Document 3 (International Publication 2011/052440) proposes a technique in which an oxygen-selective permeable membrane is used in a battery.

The oxygen selective permeable membrane is a member that separates oxygen (O ₂ ) and carbon dioxide (CO ₂ ) having different molecular sizes by utilizing a “molecular sieve effect”. Patent Document 2 and Patent Document 3 exemplify a silicone resin-based porous membrane as the oxygen selective permeable membrane.

However, the silicone resin-based porous membrane has a problem in that carbon dioxide (CO ₂ ) in the air cannot be completely removed because the separation capability of oxygen (O ₂ ) and carbon dioxide (CO ₂ ) is not so high. End up. As a material for the oxygen selective permeable membrane, a material having a high oxygen selectivity that can be used for a metal-air secondary battery has not been found so far.

Further, as another technique, instead of a highly alkaline aqueous solution having a high concentration such as a NaOH aqueous solution, a KOH aqueous solution, or a LiOH aqueous solution, which is usually used as an electrolyte of a metal-air secondary battery, “having hydroxide ion conductivity” A technique using a “solid electrolyte” has been proposed. With this technique, the reaction between carbon dioxide (CO ₂ ) in the air and hydroxide ions (OH ⁻ ) in the electrolyte can be prevented.

  For example, in the technique of Patent Document 4 (International Publication No. 2009/104570), a “solid electrolyte having hydroxide ion conductivity” is a polymer-based “solid electrolyte having hydroxide ion conductivity”. An anion exchange membrane is used. In the technique of Patent Document 5 (Japanese Patent Application Laid-Open No. 2012-099266), a layered double hydroxide which is an inorganic “solid electrolyte having hydroxide ion conductivity” is used as the “solid electrolyte having hydroxide ion conductivity”. I am using things.

The technique of Patent Document 4 aims to prevent a reaction between carbon dioxide (CO ₂ ) in the air and the electrolytic solution by installing an anion exchange membrane between the air electrode and the electrolytic solution. However, since the anion exchange membrane is weakly alkaline, the anion exchange membrane reacts with carbon dioxide (CO ₂ ), thereby reducing the hydroxide ion conductivity (ion conductivity) of the anion exchange membrane. Will cause problems.

It is said that the layered double hydroxide used in the technique of Patent Document 5 does not decrease the hydroxide ion conductivity (ionic conductivity) even when it comes into contact with carbon dioxide (CO ₂ ). However, since the layered double hydroxide has a much lower hydroxide ion conductivity than the aqueous electrolyte, when it is used as a “solid electrolyte having hydroxide ion conductivity”, the metal-air two The performance of the secondary battery is greatly reduced.

In the techniques of Patent Document 1 to Patent Document 5 described above, the above-described problems are caused by using a CO ₂ removing member, an oxygen-selective permeable membrane, or a “solid electrolyte having hydroxide ion conductivity”.

Therefore, the inventor of the present application diligently studied a solution to the “problem caused by carbon dioxide in the air” without using these members, and found the following. That is, the inventor of the present application reversibly changes the pH of the electrolytic solution with charge and discharge, and returns to a predetermined range in which carbon dioxide (CO ₂ ) is released from the electrolytic solution by charging. The present inventors have found that carbon dioxide (CO ₂ ) absorbed in the electrolytic solution can be released (removed) from the electrolytic solution.

<Embodiment>
Hereinafter, a metal-air secondary battery (aqueous lithium-air secondary battery) according to an embodiment of the present invention will be described with reference to the drawings.

(Battery configuration)
FIG. 1 shows a configuration example of a metal-air secondary battery according to an embodiment of the present invention. The metal-air secondary battery according to the embodiment of the present invention uses lithium (Li) as the negative electrode active material, uses oxygen (O ₂ ) in the air as the positive electrode active material, and an aqueous electrolyte (“ It is also referred to as “aqueous electrolyte”.).

  As shown in FIG. 1, the water-based lithium-air secondary battery includes a negative electrode 21, a reaction preventing layer 22, a negative electrode protective layer 23, a water-based electrolyte solution 24, a positive electrode 25, and an exterior in which these are housed. A body 30. Further, the water-based lithium-air secondary battery includes an electrolytic solution housing space 31 that is a space surrounded by the negative electrode protective layer 23, the positive electrode 25, and the exterior body 30, and carbon dioxide that communicates the electrolytic solution housing space 31 with the outside of the battery. And a discharge port 32.

  In the aqueous lithium-air secondary battery, a reaction preventing layer 22, a negative electrode protective layer 23, and an aqueous electrolyte solution 24 are interposed between the negative electrode 21 and the positive electrode 25 (also referred to as “air electrode”). Has a structured. The reaction preventing layer 22, the negative electrode protective layer 23, and the aqueous electrolyte solution 24 are disposed in order of the reaction preventing layer 22, the negative electrode protective layer 23, and the aqueous electrolyte solution 24 from the negative electrode 21 toward the positive electrode 25.

(Negative electrode)
As the negative electrode 21, foil-like metallic lithium can be used.

(Anti-reaction layer)
The reaction preventing layer 22 is provided in order to prevent a reaction between the negative electrode protective layer 23 and the negative electrode 21 (metallic lithium). By providing the reaction preventing layer 22, the negative electrode protective layer 23 comes into contact with metallic lithium, and the constituent component (for example, titanium) of the negative electrode protective layer 23 is reduced, so that the negative electrode protective layer 23 deteriorates. Can be prevented.

As the reaction preventing layer 22, a material having lithium ion conductivity and stable to metallic lithium can be used. The reaction preventing layer 22, for example, a polymer electrolyte, an organic electrolyte (nonaqueous electrolyte solution), or can be used lithium phosphorus oxynitride _{(LiPON (Li 3-x PO} 4-y N y)).

  As the polymer electrolyte, an intrinsic polymer electrolyte (a polymer electrolyte that does not include an electrolytic solution) or a gel electrolyte (a gel-like polymer electrolyte that includes a nonaqueous electrolytic solution) can be used.

As the intrinsic polymer electrolyte, for example, an electrolyte in which a lithium salt is dispersed in a host polymer can be used. As a host polymer, the 1 type (s) or 2 or more types chosen from polyethylene oxide (PEO), polypropylene oxide (PPO), etc. can be used, for example. Examples of the lithium salt include one or more selected from LiPF ₆ , LiClO ₄ , LiBF ₄ , LiTFSI (Li (CF ₃ SO ₂ ) ₂ N), LiBOB (lithium bisoxalate borate), and the like. Can be used.

  As the nonaqueous electrolytic solution, a nonaqueous electrolytic solution in which a lithium salt is dissolved in a nonaqueous solvent can be used. Examples of the non-aqueous solvent include ethylene carbonate, propylene carbonate, tetrahydrofuran, dimethyl sulfoxide, γ-butyrolactone, 1,3-dioxolane, 4-methyl-1,3-dioxolane, 1,2-dimethoxyethane, and 2-methyltetrahydrofuran. , Sulfolane, diethyl carbonate, dimethylformamide, acetonitrile, dimethyl carbonate, or the like can be used. The lithium salt described above can be used as the lithium salt.

  The gel electrolyte has the above-described non-aqueous electrolyte and a host polymer (matrix polymer (polymer compound)) swollen by the non-aqueous electrolyte. As the host polymer, a polymer compound that can swell with a non-aqueous electrolyte can be used. Examples of such a host polymer include one or more selected from polyvinylidene fluoride, a copolymer of vinylidene fluoride and hexafluoropropylene, polyethylene oxide, and the like.

(Negative electrode protective layer)
The negative electrode protective layer 23 can be made of water-impermeable and water-impermeable lithium ion conductive glass ceramics having lithium ion conductivity. Thereby, it can prevent that the metal lithium which is the negative electrode 21 contacts and reacts with the aqueous electrolyte solution 24. FIG. As such a water-impermeable lithium ion conductive glass ceramic, for example, a lithium-substituted NASICON type lithium ion conductive glass ceramic called LATP can be used.

LATP is, for example, a lithium ion conductive glass ceramic including a crystalline phase represented by (Chemical Formula 1) and / or (Chemical Formula 2).
(Chemical formula 1)
Li _{1 + x1} (M, Al, Ga) _x1 (Ge _1-y1 Ti _y1 ) _2-x1 (PO ₄ ) ₃
(In the formula 1, x1 satisfies x1 ≦ 0.8. Y1 satisfies 0 ≦ y1 ≦ 1.0. M is a group consisting of Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb. One or more elements selected from.)
(Chemical formula 2)
Li _{1 + x2 + y2} Q _x2 Ti _2-x2 SiP _3-y2 O ₁₂
(In the chemical formula 2, x satisfies 0 <x2 ≦ 0.4. Y satisfies 0 <y2 ≦ 0.6. Q is Al or Ga.)

  LATP has a relatively high lithium ion conductivity and is chemically stable to water. Furthermore, LATP can prevent gaps in the grain boundary from entering the negative electrode 21 from the outside because the glass phase is formed at the grain boundary by adjusting the heat treatment conditions.

(Aqueous electrolyte)
The aqueous electrolyte 24 is accommodated in the electrolyte accommodating space 31. As the aqueous electrolyte 24, an aqueous electrolyte containing a lithium salt and an acid that donates a proton (H ⁺ ) and water in which these are dissolved is used.

The pH of the aqueous electrolyte solution 24 decreases as charging progresses, and falls within a first range in which carbon dioxide (CO ₂ ) absorbed by the aqueous electrolyte solution 24 is likely to be released. Further, when the charging progresses and becomes fully charged, the pH of the aqueous electrolyte 24 is further reduced to the first predetermined value selected from the first range. Furthermore, the pH of the aqueous electrolyte solution 24 increases as the discharge progresses, and falls within a second range in which carbon dioxide (CO ₂ ) is easily absorbed by the aqueous electrolyte solution 24. Further, when the discharge progresses and complete discharge occurs, the pH of the aqueous electrolyte solution 24 is further increased to a second predetermined value selected from the second range.

Here, the inventor of the present application examined the first range in which carbon dioxide (CO ₂ ) absorbed in the aqueous electrolyte solution 24 is likely to be released (easy to be removed) from the aqueous electrolyte solution 24. Regarding the solubility of the aqueous electrolyte solution 24 in pH and carbon dioxide (CO ₂ ), as described in many books, the acid dissociation constant Ka (acid dissociation index pKa) of carbon dioxide (CO ₂ ) is used. Can be explained.

Carbon dioxide (CO ₂ ), when dissolved in water, produces carbonic acid (H ₂ CO ₃ ) as shown in equation (4).

CO ₂ + H ₂ O⇔H ₂ CO ₃ (4)

However, Formula (4) is greatly biased to the left side, and can be regarded as [CO ₂ ] + [H ₂ CO ₃ ] ≈ [CO ₂ ]. [X] represents the concentration of compound X. For example, [CO ₂ ] represents the CO ₂ concentration, and [H ₂ CO ₃ ] represents the H ₂ CO ₃ concentration.

Therefore, regarding the solubility of carbon dioxide (CO ₂ ), it can be said that the equilibrium between CO ₂ , HCO ₃ ⁻ and CO ₃ ²⁻ (see the formulas (5) and (6)) may be considered.

CO ₂ + H ₂ O⇔H ⁺ + HCO ^3- (5)

HCO ³ −⇔ H ⁺ + CO ₃ ²⁻ (6)

The acid dissociation constant (first acid dissociation constant Ka1 (also simply referred to as “Ka1”) and the second acid dissociation constant Ka2 (simply “Ka2”) of the carbon dioxide (CO ₂ ) in the formulas (5) and (6) )) Is defined by the equations (7) and (8), and the respective values are known as follows.
Ka1 = ([H ⁺ ] [HCO ₃ ⁻ ]) / [CO ₂ ] = 10 ^−6.34 (7)
The first acid dissociation index pKa1 (also simply referred to as “pKa1”) of carbon dioxide (CO ₂ ) is pKa1 = −logKa1 = 6.34.
The expression (7) ′ is derived from the expression (7).
log ([CO ₂ ] / [HCO ₃ ⁻ ]) = pKa 1 -pH (7) ′

Ka2 = [H ⁺ ] [CO ₃ ²⁻ ] / [HCO ₃ ⁻ ] = ^10−10.25 (8)
The second acid dissociation index pKa2 (also simply referred to as “pKa2”) of carbon dioxide (CO ₂ ) is pKa2 = −logKa2 = 10.25.
Equation (8) is derived from Equation (8) ′.
log ([CO ₃ ²⁻ ] / [HCO ₃ ⁻ ]) = pKa ² −pH (8) ′

If the total concentration of CO ₂ , HCO ^3- and CO ₃ ^2- does not change and the total concentration is 1,
[CO ₂ ] + [HCO ₃ ⁻ ] + [CO ₃ ²⁻ ] = 1 (9)
It is said.

Solve the simultaneous equations (7), (8) and (9),
F = 1 + Ka1 / [H ⁺ ] + Ka1 * Ka2 / [H ⁺ ] ² (10)
Then,
[CO ₂ ] = 1 / F (11)
[HCO ₃ ] = Ka1 / ([H ⁺ ] * F) (12)
[CO ₃ ²⁻ ] = (K 1 * Ka 2) / ([H ⁺ ] ² * F) (13)
Can be written as:

When the concentrations of CO ₂ , HCO ₃ ⁻ and CO ₃ ²⁻ are plotted against pH from the equations (11), (12) and (13), the concentration profile shown in FIG. 2 is obtained.

In FIG. 2, the intersection P1 between the curve a1 indicating “CO ₂ ] (CO ₂ abundance ratio) and the curve a2 indicating [HCO ₃ ⁻ ] (HCO ₃ ⁻ abundance ratio) is the CO ₂ concentration and the HCO ₃ ⁻ concentration. Are equal to each other, and in the formula (7), the concentration is [HCO ₃ ⁻ ] / [CO ₂ ] = 1, and the pH at that time is equal to the first acid dissociation index pKa1, and pH = 6. 34 (25 ° C.).

Similarly, the pH of the intersection P2 between the curve a2 indicating [HCO ₃ ⁻ ] (HCO ₃ ⁻ abundance ratio) and the curve a3 indicating [CO ₃ ²⁻ ] (CO ₃ ^{2 −} abundance ratio) is the second acid dissociation. This corresponds to the value of the index pKa2 (= 10.25 (25 ° C.)).

When the pH of the aqueous electrolyte solution 24 is made acidic (lower) than the value of the first acid dissociation index pKa1, CO ₂ absorbed (dissolved) in more aqueous electrolyte solution 24 is converted into the aqueous electrolyte solution 24. Can be released (removed).

For example, as can be seen from the equation (7 ′), when the pH of the aqueous electrolyte solution 24 is set to pH 5.34, which is smaller by 1 than the value of the first acid dissociation index pKa1, it remains in the aqueous electrolyte solution 24 [HCO ₃ ⁻ ] And [CO ₂ ] released from the aqueous electrolyte solution 24 (released outside the battery) is [CO ₂ ] / [HCO ₃ ⁻ ] = 10. When the pH of the aqueous electrolyte 24 is set to pH 4.34, which is 2 lower than the value of the first acid dissociation index pKa1, [HCO ₃ ⁻ ] remaining in the aqueous electrolyte 24 and the aqueous electrolyte 24 are released (battery The ratio to [CO ₂ ] released to the outside is [CO ₂ ] / [HCO ₃ ⁻ ] = 100.

From the above, it can be seen that the first range is preferably a range smaller than the value of the first acid dissociation index pKa1 of carbon dioxide (CO ₂ ) (first range <pKa1).

  For example, when the first range is an aqueous electrolyte solution having an ordinary temperature (25 ° C.) and a predetermined lithium salt concentration (for example, 0.05 mol / L to 2.0 mol / less), the pH is preferably 5.587 or less. The pH is more preferably 5.5 or less, and further preferably pH 4.0 or less.

  Since the value of the first acid dissociation index pKa1 changes as the temperature of the aqueous electrolyte solution 24, the lithium salt concentration, and the like change, the first range changes accordingly. Therefore, the first range changes when used at a temperature other than room temperature.

  The lower limit of the first range is preferably pH 2.0. LATP constituting the anode protective layer 23 tends to be corroded in an acidic solution having a pH of less than 2.0 (for example, Non-Patent Document 1: Satoshi Hasegawa et al., “Study on lithium / air secondary batteries Stability of NASICON -type lithium ion conducting glass ceramics with water ”, Journal of Power Sources, vol.189 (1), p371-377 (2009)). Therefore, considering the effect of inhibiting LATP corrosion, the lower limit of the first range is preferably pH 2.0.

That is, in consideration of both the CO ₂ removal effect and the LATP corrosion inhibiting effect, the first range is preferably pH 2.0 or more and pH 5.587 or less, and pH 2.0 or more and pH 5.5 or less. Is more preferable, and it is still more preferable that it is pH2.0 or more and pH4.0 or less.

In the embodiment of the present invention, the pH of the aqueous electrolyte solution 24 changes as follows along with charging and discharging, and carbon dioxide (CO ₂ ) absorbed by the aqueous electrolyte solution 24 can be discharged outside the battery.

That is, as indicated by the line b1 in FIG. 3, the pH of the aqueous electrolyte solution 24 becomes the first predetermined value (y1) selected from the first range when not discharged. Thereafter, when the discharge is started, the non-discharged aqueous electrolyte 24 after the preparation generates hydroxide ions (OH ⁻ ) ((2) in the right direction of the formula (O ₂ + 2H ₂ O + 4e ⁻ ⇔ 4OH ⁻ ). The reaction proceeds.) The generated hydroxide ions (OH ⁻ ) neutralize the “acid that provides protons” in the aqueous electrolyte solution 24, and the aqueous electrolyte solution 24 gradually changes to alkalinity.

Even after the aqueous electrolyte solution 24 is changed to alkaline, if the discharge reaction is further continued, the pH of the aqueous electrolyte solution 24 increases as the discharge proceeds. When the pH of the aqueous electrolyte 24 falls within a second range in which carbon dioxide (CO ₂ ) is easily absorbed by the aqueous electrolyte 24 (for example, a range larger than pKa2), carbon dioxide in the air is added to the aqueous electrolyte 24. (CO ₂ ) is absorbed and the concentration of carbonate ions (CO ₃ ²⁻ ) increases. The produced carbonate ion (CO ₃ ²⁻ ) reacts with LiOH to precipitate Li ₂ CO ₃ having low solubility in water. Further, when the discharge proceeds and complete discharge is reached, the pH of the aqueous electrolyte solution 24 is further increased to a second predetermined value (y2) selected from the second range.

Thereafter, when charging is started, hydroxide ions (OH ⁻ ) are consumed (the reaction in the left direction of the formula (2) proceeds), so that the pH of the aqueous electrolyte solution 24 decreases as the charging progresses. , Within the first range. Further, when the charging progresses and the fully charged state is reached, the pH of the aqueous electrolyte 24 further decreases and returns to the first predetermined value (y1) selected from the first range. Further, when discharging and charging are performed similarly thereafter, the pH of the aqueous electrolyte solution 24 is similarly changed.

When the pH of the aqueous electrolyte solution 24 falls within the first range, carbon dioxide (CO ₂ ) absorbed by the aqueous electrolyte solution 24 is released from the aqueous electrolyte solution 24. Even when Li ₂ CO ₃ is deposited, the deposited Li ₂ CO ₃ is decomposed and released (removed) from the aqueous electrolyte 24 as carbon dioxide (CO ₂ ). The released carbon dioxide (CO ₂ ) is discharged outside the battery through the carbon dioxide outlet 32. Thus, in an embodiment of the present invention, the carbon dioxide absorbed by the aqueous electrolyte 24 (CO ₂₎ it can be discharged to the outside of the battery.

(Lithium salt)
The lithium salt is used to increase the concentration of lithium in the aqueous electrolyte solution 24. The lithium salt is preferably a lithium salt in which counter ions (anions) of lithium ions are not decomposed by charging and discharging.

(Acids that donate protons)
As an acid for donating protons, it is preferable that protons (H ⁺ ) can be supplied into the aqueous electrolyte solution 24 and the proton counter ions (anions) are not decomposed by charging and discharging.

  By using such a lithium salt and an acid that donates a proton, even when the pH rises to a value greater than the first range due to discharge, the pH of the aqueous electrolyte solution 24 is reduced when the battery is fully charged again by charging. It is possible to return to a first predetermined value selected from within one range.

For example, a lithium salt such as lithium citrate and an acid such as citric acid are formed by LiOH (lithium hydroxide) generated by an electrode reaction during discharge because anions are oxidized and decomposed by charge after discharge. Is less likely to be oxidized by charging (the reaction to the left in equation (2) is less likely to proceed). Thereby, even when charged after discharging, the hydroxide ions (OH ⁻ ) in the aqueous electrolyte solution 24 tend not to decrease. As a result, the charge after discharge tends to make it difficult for the pH of the aqueous electrolyte 24 to return to the first predetermined value selected from the first range during full charge.

As the lithium salt, for example, Li ₂ SO ₄ (lithium sulfate), LiCl (lithium chloride), LiNO ₃ (lithium nitrate) or CH ₃ COOLi (lithium acetate) can be used. The lithium salt is not limited to these lithium salts, and various lithium salts can be used. Furthermore, lithium salt may be used by 1 type and may be used in mixture of 2 or more types.

As an acid for donating protons, for example, H ₂ SO ₄ (sulfuric acid), HCl (hydrochloric acid) or HNO ₃ (nitric acid) can be used as a strong acid. As the weak acid, for example, CH ₃ COOH (acetic acid) can be used. The acid for donating protons is not limited to these acids, and various acids can be used. Furthermore, the acid which provides a proton may be used by 1 type, and 2 or more types may be mixed and used for it.

(Method for preparing electrolyte)
The aqueous electrolyte solution 24 described above can be prepared as follows. That is, first, in order to ensure lithium ion conductivity of the aqueous electrolyte solution 24, a lithium salt is added to water. The concentration of the lithium salt is preferably 0.05 mol / L or more and 2.0 mol / L or less, for example, from the viewpoint of ensuring lithium ion conductivity of the aqueous electrolyte solution 24. Next, the pH of the aqueous electrolyte solution 24 is adjusted to a first predetermined value selected from the first range by further adding an acid for donating protons to the water to which the lithium salt has been added. Thereby, the aqueous electrolyte solution 24 can be obtained.

(Reason why electrolyte can be used suitably for lithium-air secondary battery)
Since the above-mentioned aqueous electrolyte solution 24 is used for an aqueous lithium-air secondary battery, it can be preferably used, and cannot be suitably used for other metal-air secondary batteries. The reason is described below.

For example, among fuel cells that are a type of air cell, an alkaline fuel cell (AFC) uses high-concentration KOH or NaOH aqueous solution as an electrolyte, so that the atmosphere is similar to that of a metal-air secondary cell. It absorbs carbon dioxide (CO ₂ ) in it and is greatly affected by it. On the other hand, since a solid acid fuel cell (PEFC) uses an acidic electrolyte such as Nafion (registered trademark) as an electrolyte, carbon dioxide (CO ₂ ) is not absorbed, and the atmosphere It is considered that the influence of carbon dioxide (CO ₂ ) inside can be eliminated.

However, metals such as zinc, which are most studied as negative electrodes for metal-air secondary batteries, are dissolved in a strongly acidic electrolyte. Even when a weakly acidic electrolyte solution that does not dissolve the negative electrode metal is used, the positive electrode (air electrode) generates hydroxide ions (OH ⁻ ) during discharge according to the formula (2). The electrolytic solution shifts to alkaline and eventually absorbs carbon dioxide (CO ₂ ).

Furthermore, the ionic species responsible for charge transfer in the electrolyte of a normal metal-air secondary battery is hydroxide ion (OH ⁻ ), and when an acidic electrolyte is used instead of a strong alkaline electrolyte, This causes a decrease in the hydroxide ion conductivity of the liquid. As a result, the problem that battery performance falls will arise.

  On the other hand, in the case of using an aqueous lithium-air secondary battery instead of a strong alkaline electrolyte, an electrolyte that becomes acidic when fully charged (aqueous electrolyte 24) is used. Less susceptible to decline in rate.

  That is, the difference between the water-based lithium-air secondary battery and other metal-air secondary batteries is that the metal hydroxide (M (OH) n), which is the discharge product of the other metal-air secondary batteries, is on the negative electrode. In contrast, in the case of an aqueous lithium-air secondary battery, lithium hydroxide (LiOH), which is a discharge product, is generated on the positive electrode (air electrode).

This means that in the aqueous electrolyte of the aqueous lithium-air secondary battery, lithium ions (Li ⁺ ) generated on the negative electrode side have moved to the positive electrode (air electrode) side due to the discharge reaction. ing.

Therefore, in other metal-air secondary batteries, the battery performance is greatly deteriorated due to a decrease in the hydroxide ion (OH ⁻ ) concentration of the electrolytic solution (in other words, a decrease in pH), whereas an aqueous lithium-air secondary battery. In secondary batteries, if lithium ions (Li ⁺ ) (preferably high-concentration lithium ions (Li ⁺ )) are present in the aqueous electrolyte, changes in the pH of the aqueous electrolyte will affect changes in ionic conductivity. Hard to give. For this reason, in an aqueous lithium-air secondary battery, even when an acidic aqueous electrolyte is used during full charge, the battery performance is unlikely to deteriorate. Therefore, in the aqueous lithium-air secondary battery, an electrolytic solution (aqueous electrolytic solution 24) that becomes acidic when fully charged can be preferably used.

In the case of an aqueous lithium-air secondary battery, even if an acidic aqueous electrolyte is used at full charge, the positive electrode (air The aqueous electrolyte solution is neutralized by the hydroxide ions (OH ⁻ ) generated in the electrode), and the aqueous electrolyte solution is eventually changed to alkaline. Since the aqueous electrolyte changed to alkaline absorbs carbon dioxide (CO ₂ ) in the atmosphere, there arises a problem of generating poorly soluble Li ₂ CO ₃ .

On the other hand, in the embodiment of the present invention, as described above, by returning the aqueous electrolyte solution 24 to acidity (acidity at which the pH falls within the first range) again at the time of charging (at the time of full charge), Li ₂ CO ₃ can be easily decomposed into carbon dioxide (CO ₂ ). Thereby, in the embodiment of the present invention, carbon dioxide (CO ₂ ) absorbed in the aqueous electrolyte solution 24 is released (removed) from the aqueous electrolyte solution 24, and the released carbon dioxide (CO ₂ ) is discharged into carbon dioxide. It can be discharged to the outside through the outlet 32.

(Positive electrode)
The positive electrode 25 includes a catalyst layer 25a and a gas diffusion layer 25b. The positive electrode 25 is supplied with oxygen (air) as an active material.

The catalyst layer 25a includes a catalyst-supporting carbon particle (platinum-supporting carbon) as a discharge catalyst, an iridium oxide (IrO ₂ ) particle (powder) as a charging catalyst, and a binder (fluorine-based resin (for example, polytetrafluoroethylene ( PTFE)).

  The gas diffusion layer 25b is a conductive porous layer (a porous layer mainly composed of a conductive material) that supplies a reaction gas (air) to the catalyst layer 25a and collects charges generated in the catalyst layer 25a. . One main surface outside the gas diffusion layer 25b is in contact with air, and a catalyst layer 25a is formed on the other main surface inside the gas diffusion layer 25b. As the gas diffusion layer 25b, for example, carbon paper subjected to water repellent treatment can be used.

(Battery operation)
In the water-based lithium-air secondary battery, the following reaction occurs at the negative electrode 21 and the positive electrode 25 with charging and discharging.
(During discharge)
Negative electrode reaction: Li → Li ⁺ + e ⁻
Positive electrode reaction: Li ⁺ + 1 / 4O ₂ + 3 / 2H ₂ O → LiOH · H ₂ O
(When charging)
Negative electrode reaction: Li ⁺ + e ⁻ → Li
Positive electrode reaction: LiOH.H ₂ O → Li ⁺ + 1 / 4O ₂ + 3 / 2H ₂ O

That is, at the time of discharging, in the water-based lithium-air secondary battery, at the negative electrode 21, the metal lithium (Li) moves as lithium ions (Li ⁺ ) through the reaction preventing layer 22 and the negative electrode protective layer 23 to the water-based electrolyte 24. To do. At this time, the electrons are supplied to an external circuit (not shown) connected to the negative electrode 21. At the positive electrode 25, electrons are supplied from an external circuit connected to the positive electrode 25, and oxygen (O ₂ ) and water (H ₂ O) in the air react with each other in the catalyst layer 25 a to react with hydroxide ions (OH ⁻ (I.e., a rightward reaction of formula (2) (O ₂ + 2H ₂ O + 4e ⁻ ⇔ 4OH ⁻ ) occurs). In the aqueous electrolyte solution 24, lithium ions (Li ⁺ ) and hydroxide ions (OH ⁻ ) react to produce water-soluble LiOH.

On the other hand, at the time of charging, in the water-based lithium-air secondary battery, electrons are supplied to the negative electrode 21 from an external circuit, and lithium ions (Li ⁺ ) present in the water-based electrolyte 24 pass through the negative electrode protective layer 23 and the reaction preventing layer 22. It moves to the surface of the negative electrode 21, and lithium ions (Li ⁺ ) receive electrons on the surface of the negative electrode 21 to generate metallic lithium (Li). In the catalyst layer 25a of the positive electrode 25, hydroxide ions (OH ⁻ ) react to generate oxygen (O ₂ ) and generate electrons (that is, a leftward reaction of the formula (2) occurs). Electrons are supplied to an external circuit.

(effect)
In the aqueous lithium-air secondary battery according to the embodiment of the present invention, carbon dioxide (CO ₂ ) absorbed by the aqueous electrolyte 24 can be released (removed) from the aqueous electrolyte 24 and discharged to the outside of the battery. Furthermore, in the aqueous lithium-air secondary battery according to the embodiment of the present invention, the ion species responsible for charge transfer in the aqueous electrolyte solution 24 is lithium ions (Li ⁺ ), so carbon dioxide (CO ₂ ) is aqueous. By being absorbed in the electrolytic solution 24, it is difficult to be affected by a decrease in the ionic conductivity of the aqueous electrolytic solution 24 resulting from a decrease in hydroxide ions (OH ⁻ ). As a result, it is possible to suppress a decrease in battery characteristics caused by carbon dioxide in the air.

  EXAMPLES Hereinafter, although an Example demonstrates this invention concretely, this invention is not limited only to these Examples.

In order to evaluate the water based lithium-air secondary battery according to the embodiment of the present invention, the following evaluation test 1 and evaluation test 2 were performed.
(Evaluation Test 1) Carbon dioxide (CO ₂ ) removability evaluation due to pH difference of electrolyte (Li ₂ CO ₃ solubility evaluation)
(Evaluation Test 2) Measurement of pH change of electrolytic solution accompanying discharge and charge

(Evaluation Test 1)
As described above, the value of the first acid dissociation index pKa1 of carbon dioxide (CO ₂ ) is reported to be 6.34. This is a value in pure water at room temperature (25 ° C.). However, as the aqueous electrolyte solution 24 of the aqueous lithium-air secondary battery, an aqueous solution in which a lithium salt is dissolved in water is used in order to impart lithium ion conductivity.

It is expected that the value of the first acid dissociation index pKa1 of carbon dioxide (CO ₂ ) in the aqueous solution in which the lithium salt is dissolved in water is deviated from 6.34. Therefore, it is conceivable that the pH range (first range) of the electrolytic solution that can remove carbon dioxide (CO ₂ ) from the electrolytic solution is changed. Therefore, as an electrolytic solution, lithium sulfate as a lithium salt (Li 2 _{SO 4)} is with an aqueous solution dissolved in water, for the removal of carbon dioxide (CO ₂₎ of the electrolyte solution was evaluated as follows .

Procedure 1-1: In order to ensure the lithium ion conductivity of the electrolyte, lithium sulfate monohydrate (manufactured by Wako Pure Chemical Industries, Ltd., special grade) is dissolved in ion-exchanged water, and 0.5 mol / L A Li ₂ SO ₄ aqueous solution having a concentration of ₅ was prepared.

Procedure 1-2: 5 mL of the 0.5 mol / L Li ₂ SO ₄ aqueous solution prepared in Procedure 1-1 was placed in a glass sample bottle. Next, in order to simulate the state of the electrolytic solution that has absorbed carbon dioxide (CO ₂ ) in the atmosphere, lithium carbonate Li ₂ CO ₃ (Wako Pure Chemical Industries, Ltd.) was added to the Li ₂ SO ₄ aqueous solution in the glass sample bottle. A small amount (about 70 mg) of microspatel was added. At this time, Li ₂ CO ₃ was not dissolved but precipitated in the Li ₂ SO ₄ aqueous solution.

Procedure 1-3: By adding dropwise 0.5 mol / L H ₂ SO ₄ (manufactured by Wako Pure Chemical Industries, Ltd., analytical reagent grade) in a Li ₂ SO ₄ aqueous solution to which a small amount of Li ₂ CO ₃ has been added. A plurality of electrolyte samples (Samples 1 to 13) were prepared by changing the pH in the Li ₂ SO ₄ aqueous solution.

Procedure 1-4: Immediately after adding 0.5 mol / L of H ₂ SO ₄ , the solution was stirred, and the dissolution state of Li ₂ CO ₃ when left at room temperature (25 ° C.) for 1 hour was visually confirmed. The pH of the solution at that time was measured with a pH meter (manufactured by HORIBA, F-52).

(Evaluation results)
The evaluation results are shown in Table 1.

Table 1 shows the amount of 0.5 mol / L H ₂ SO ₄ added (mL) dropwise into a 0.5 mol / L Li ₂ SO ₄ aqueous solution to which a small amount of Li ₂ CO ₃ was added, and stirring and leaving for 1 hour. The dissolution status of Li ₂ CO ₃ after the preparation was summarized.

The evaluation in Table 1 was determined based on the following criteria.
(Time to complete dissolution)
The time until complete dissolution was determined based on the following criteria.
Long: 55 minutes <T minutes ≦ 60 minutes (= 1 hour)
Normal: 1 min ≦ T min ≦ 55 min Instantaneous: 0 min <T min <1 min Note that the time until complete dissolution is T min.

(Comprehensive evaluation)
Comprehensive evaluation was determined based on the following criteria.
☆: Instantly and completely melts.
(Double-circle): It melts normally and completely.
◯: Long and completely soluble.
X: It does not melt completely or does not melt at all.
In addition, *, (double-circle), and (circle) are a pass, and x is a disqualification. The disappearance (in other words, CO ₂ removability) of Li ₂ CO ₃ is excellent in the order of ☆, ◎, ○.

As can be seen from Table 1, when a 0.5 mol / L Li ₂ SO ₄ electrolyte was used at room temperature (25 ° C.), it was absorbed from the atmosphere if the acidity of the electrolyte was set to pH 5.587 or lower. It has been found that the influence of carbon dioxide (CO ₂ ) can be completely eliminated.

(Evaluation Test 2) Measurement of pH Change of Electrolytic Solution Accompanying Discharge and Charging Operation In the aqueous lithium-air secondary battery according to the embodiment of the present invention, the pH of the electrolytic solution is in the first range (for example, weak acidity) when fully charged. ) To remove (release) carbon dioxide (CO ₂ ) absorbed in the electrolytic solution from the electrolytic solution. Therefore, the battery using an undischarged weakly acidic electrolyte was discharged and charged, and it was confirmed whether or not it was possible to return to the initial pH of the electrolyte (when not discharged).

Procedure 2-1: Preparation of Electrolytic Solution Lithium sulfate monohydrate Li ₂ SO ₄ .H ₂ O (manufactured by Wako Pure Chemical Industries, Ltd., special grade) is dissolved in ion-exchanged water, and 0.5 mol / L 250 mL of a Li ₂ SO ₄ aqueous solution having a concentration was prepared. Further, 2.5 mL of 0.5 mol / L H ₂ SO ₄ (manufactured by Wako Pure Chemical Industries, Ltd., analytical reagent grade) is added therein, and an acidic electrolytic solution (evaluation electrolytic solution) of pH 2.6 is added. ) Was prepared. When a small amount (about 70 mg) of Li ₂ CO ₃ (manufactured by Wako Pure Chemical Industries, Ltd., special grade) was added to the electrolytic solution for evaluation, it was confirmed that it was dissolved and disappeared.

Procedure 2-2: Air electrode (preparation of positive electrode)
The catalyst layer of the air electrode is formed by integrating a discharge catalyst layer that reduces oxygen (O ₂ ) in the air and a charge catalyst layer that decomposes LiOH as a discharge product and generates oxygen (O ₂ ). .

  0.5 g of platinum-supported carbon powder (manufactured by Tanaka Kikinzoku Kogyo Co., Ltd., TECEA50E) as a discharge catalyst for forming a discharge catalyst layer, and 20 g of anion exchange ionomer (manufactured by Tokuyama Co., Ltd., AS-4) as a binder resin. And 0.5 g of a tetrafluoroethylene (PTFE) dispersion (Daikin Kogyo Co., Ltd., D-210C) as a water repellent material, and a vapor grown carbon fiber (Showa Denko Co., Ltd.) (VGCF) 0.5 g was mixed and dispersed to prepare a discharge catalyst paste.

  Next, the prepared discharge catalyst paste is coated on the surface of carbon paper (TGP-120, manufactured by Toray Industries, Inc.) and fired at 350 ° C., so that carbon paper (TGP-120, manufactured by Toray Industries, Inc.) is used. ) Was formed on one side of the discharge catalyst layer.

Next, to form a charge catalyst layer, 0.25 g of IrO ₂ powder (manufactured by Tanaka Kikinzoku Kogyo Co., Ltd.) as the charge catalyst and 10 g of anion exchange ionomer (manufactured by Tokuyama Co., Ltd., AS-4) as the binder resin Then, 0.5 g of vapor grown carbon fiber (VGCF, manufactured by Showa Denko KK) was mixed and dispersed as a conductive auxiliary agent to prepare a charging catalyst paste. Thereafter, the prepared charge catalyst paste was applied to the back surface of the previously prepared discharge catalyst layer, and dried at room temperature, thereby preparing a catalyst layer in which the discharge catalyst layer and the charge catalyst layer were laminated and integrated.

  A carbon paper (TGP-120, manufactured by Toray Industries, Inc.) subjected to water repellency treatment with PTFE dispersion as a gas diffusion layer was joined by hot pressing to produce an air electrode.

Procedure 2-3: Configuration and Evaluation Conditions of Evaluation Cell In order to simulate the situation of the aqueous electrolyte solution 24 in the aqueous lithium-air secondary battery, the evaluation cell 101 made of acrylic whose schematic configuration is shown in FIG. Evaluation was performed at room temperature (25 ° C.). The material of the member constituting the evaluation cell 101 in contact with the electrolytic solution is only a resin member of acrylic, PTFE, a metal member made of titanium and a noble metal, and a carbon member, and a member that does not cause corrosion during this evaluation was selected. .

The evaluation cell 101 is separated into an electrolytic solution tank 103A and an electrolytic solution tank 103B through the LATP window 102, and the electrolytic solution tank 103A and the electrolytic solution tank 103B are moved except for lithium ions (Li ⁺ ). In the electrolyte bath 103A, it is possible to cause the same electrode reaction as that of the aqueous electrolyte solution 24 in the aqueous lithium-air secondary battery.

In the electrolytic solution tank 103A, 46 g of the acidic evaluation electrolytic solution 104A described in "Procedure 2-1: Preparation of electrolytic solution" was placed. In the electrolytic solution tank B, 46 g of a 0.5 mol / L Li ₂ SO ₄ aqueous solution was put as the counter electrode electrolytic solution 104B. As the working electrode 105, the air electrode described in Procedure 2-2: Preparation procedure of air electrode (positive electrode) was used, as the counter electrode 106, a platinum mesh was used, and as the reference electrode 107, an Hg / HgO electrode was used.

The working electrode 105 (air electrode), the counter electrode 106, and the reference electrode 107 were connected to a potentio galvanostat 110 (manufactured by Solartron Analytical, SI1287). At a flow rate of 300 ccm, air 111 is passed through a washing bottle 113 containing 1 mol / L of lithium hydroxide 112 as a CO ₂ removal agent to remove carbon dioxide (CO ₂ ) present in the air in advance. During the evaluation, the gas diffusion layer 105b of the electrode 105 was continuously supplied and supplied.

  The pH change of the evaluation electrolytic solution 104A due to the discharging / charging operation was monitored by the pH electrode 120 inserted into the evaluation electrolytic solution 104A. At this time, the stirrer rod 121 was placed so that the difference between the pH of the electrolytic electrolyte 104A for evaluation in the vicinity of the working electrode 105 and the pH of the bulk was small, and stirring was continued during the evaluation.

As an evaluation procedure, in order to simulate the discharge operation, the working electrode 105 is dispersed negatively, and a constant current of −8 mA (−2 mA / cm ² when converted to the area of the catalyst layer 105a) is allowed to flow for 170 minutes, (2) The reaction proceeded to the right of the equation. At that time, hydroxide ions (OH ⁻ ) are generated by the reaction, and the pH of the evaluation electrolytic solution 104A is increased. Thereafter, in order to similarly simulate the charging operation, the working electrode 105 is positively inverted and polarized, and a constant current of +8 mA (+2 mA / cm ² in terms of the area of the catalyst layer 105a) is allowed to flow for 170 minutes, (2) The reaction was allowed to proceed to the left of the equation. At that time, it was considered that hydroxide ions (OH ⁻ ) generated by simulating the discharge reaction were consumed and the pH of the evaluation electrolytic solution 104A returned to the initial pH.

A small amount of Li ₂ CO ₃ was added to the electrolytic solution 104A for evaluation after the charging operation (discharge operation → charging operation) was repeated once after the discharging operation, and it was confirmed whether it could be dissolved and disappeared.

(Evaluation results)
FIG. 5 shows changes in pH (see line c1 and line c2) and electrode potential (see line d1 and line d2) of the electrolytic electrolyte 104A for evaluation by discharging and charging operations. As a result, as shown by the line c1, during the discharge operation, the charge amount exceeded 40000 mA · sec, and when the pH of the electrolyte solution 104A for evaluation exceeded 4, the pH rose significantly and shifted to the alkali side. I found out that When the discharge operation is further continued, the pH of the evaluation electrolytic solution 104A exceeds 11, and the evaluation electrolytic solution 104A absorbs carbon dioxide (CO ₂ ) in the atmosphere and easily generates insoluble Li ₂ CO _3. It turned out that it will change to electrolyte solution.

  Next, when the current value is reversed and the charging operation is performed, as shown by a line c2, the pH of the evaluation electrolytic solution 104A rapidly decreases near the pH of the evaluation electrolytic solution 104A lower than 10. It was confirmed that when the charge operation was performed with the same amount of charge as that during the discharge operation, the pH returned to the acidic pH of the initial evaluation electrolyte solution 104A (when not discharged).

When a charging operation is performed after the discharging operation (discharging operation → charging operation), and a small amount of Li ₂ CO ₃ is added to the electrolytic electrolyte 104A for evaluation that has been returned to the acidic pH of the electrolytic electrolyte for initial evaluation, it is easy Li ₂ CO ₃ could be dissolved and disappeared.

From these results, as the aqueous electrolyte 24 of the aqueous lithium-air secondary battery when not discharged (fully charged), the weakly acidic aqueous electrolyte (pH when not discharged (fully charged) is within the first range. It was found that the following can be achieved by using an aqueous electrolyte solution having a first predetermined value selected from: That is, even when the aqueous electrolyte becomes alkaline due to the discharge reaction, and the aqueous electrolyte absorbs carbon dioxide (CO ₂ ) in the atmosphere and generates insoluble Li ₂ CO ₃ , the aqueous electrolyte is weakened by the charging operation. It was found that it can be returned to acidity again. As a result, it was found that carbon dioxide (CO ₂ ) absorbed in the aqueous electrolyte solution was released again from the aqueous electrolyte solution, and the released carbon dioxide (CO ₂ ) could be discharged outside the battery.

<Modification>
Although the embodiments and examples of the present invention have been specifically described above, the present invention is not limited to the above-described embodiments and examples, and various modifications based on the technical idea of the present invention are possible. It is.

  For example, the configurations, methods, steps, shapes, materials, numerical values, and the like given in the above-described embodiments and examples are merely examples, and different configurations, methods, steps, shapes, materials, numerical values, etc. are necessary as necessary. May be used.

  The configurations, methods, processes, shapes, materials, numerical values, and the like of the above-described embodiments and examples can be combined with each other without departing from the gist of the present invention.

  For example, the position where the carbon dioxide discharge port 32 is provided is not limited to the position of the above-described embodiment. For example, as long as the electrolytic solution housing space 31 and the outside of the battery are communicated, they can be provided at various positions.

  The electrolytic solution storage space 31 may be configured to store the separator and the aqueous electrolytic solution 24 impregnated in the separator. In this case, a hydrophilic porous membrane can be used as the separator. As such a porous film, a polyolefin film (for example, polyethylene or polypropylene film) subjected to a hydrophilic treatment or a nonwoven fabric can be used.

  DESCRIPTION OF SYMBOLS 21 ... Negative electrode, 22 ... Reaction prevention layer, 23 ... Negative electrode protective layer, 24 ... Aqueous electrolyte, 25 ... Positive electrode, 25a ... Catalyst layer, 25b ... Gas diffusion layer, 31 ... Electrolyte accommodation space, 32 ... Carbon dioxide discharge port

Claims (10)

A negative electrode containing metallic lithium;
An air electrode to which air is supplied;
An aqueous electrolyte in which a lithium salt and an acid that donates a proton are dissolved in water;
An electrolyte storage space in which the aqueous electrolyte solution is stored;
With
The pH of the aqueous electrolyte decreases as charging progresses and increases as discharging progresses,
The pH of the aqueous electrolyte at full charge is a first predetermined value selected from the first range in which carbon dioxide contained in the air absorbed in the aqueous electrolyte is likely to be released from the aqueous electrolyte, The carbon dioxide released from the aqueous electrolyte is discharged from the electrolyte storage space to the outside.
Water-based lithium-air secondary battery. The water based lithium-air secondary battery according to claim 2,
The pH of the aqueous electrolyte at the time of complete discharge is a second predetermined value selected from the second range in which the carbon dioxide is easily absorbed by the aqueous electrolyte.
Water-based lithium-air secondary battery. The water based lithium-air secondary battery according to claim 1 or 2,
Further comprising an exterior body that sheathes a battery constituent member including at least the negative electrode, the air electrode, and the aqueous electrolyte.
The exterior body communicates the electrolytic solution housing space and the outside of the battery, and has a discharge port through which the carbon dioxide released from the electrolytic solution passes and is discharged to the outside of the battery.
Water-based lithium-air secondary battery. The water based lithium-air secondary battery according to any one of claims 1 to 3,
The first range is a range smaller than the value of the first acid dissociation index pKa1 of carbon dioxide.
Water-based lithium-air secondary battery. The water based lithium-air secondary battery according to claim 4,
The first range is a range of pH 5.587 or less.
Water-based lithium-air secondary battery. The water based lithium-air secondary battery according to claim 4 or 5,
A negative electrode protective layer located between the negative electrode and the aqueous electrolyte solution;
The negative electrode protective layer is LATP,
The lower limit of the first range is pH 2.0.
Water-based lithium-air secondary battery. The water based lithium-air secondary battery according to any one of claims 1 to 6,
The counter ion of the proton of the acid that donates the proton and the counter ion of the lithium ion of the lithium salt are not decomposed by charging and discharging.
Water-based lithium-air secondary battery. The water based lithium-air secondary battery according to claim 7,
The proton donating acid is at least one selected from H ₂ SO ₄ , HCl, HNO ₃ and CH ₃ COOH.
Water-based lithium-air secondary battery. The water based lithium-air secondary battery according to claim 7 or 8,
The lithium salt is at least one selected from Li ₂ SO ₄ , LiCl, LiNO ₃ and CH ₃ COOLi.
Water-based lithium-air secondary battery. A negative electrode containing metallic lithium, an air electrode to which air is supplied, an aqueous electrolyte solution in which an acid for supplying a lithium salt and a proton is dissolved in water, and an electrolytic solution storage space in which the aqueous electrolyte solution is stored aqueous lithium with - a CO ₂ removal method of an air secondary battery,
The pH of the aqueous electrolyte that has become larger as the discharge progresses becomes smaller as charging progresses, and carbon dioxide contained in the air absorbed in the aqueous electrolyte is released from the aqueous electrolyte when fully charged. The carbon dioxide is released from the aqueous electrolyte solution so as to be a first predetermined value selected from the first range that is likely to be obtained,
Exhausting the released carbon dioxide to the outside from the electrolytic solution housing space,
A method for removing CO _{2 from an} aqueous lithium-air secondary battery.

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