JP5691875B2 - Water-based lithium-air secondary battery - Google Patents

Description

  The present invention relates to an aqueous lithium-air secondary battery.

According to the prior art, in a non-aqueous Li-air secondary battery, the electrolyte is polyacrylonitrile (PAN): ethylene carbonate (EC): propylene carbonate (PC): LiPF ₆ = 12: 40: 40: 8 (mass% ) Based polymer electrolytes are known. Furthermore, a non-aqueous Li-air secondary battery using a hydrophobic room temperature molten salt as an electrolyte is known. Furthermore, a non-aqueous Li-air secondary battery using propylene carbonate (PC) as an electrolyte is also known.

Patent Document 1 discloses a composition of Li _1-x (M, Al, Ga) _x (Ge _1-y Ti _y ) _2−x (PO ₄ ) ₃ adjacent to a metal lithium negative electrode and between electrolytes. Glass ceramic layer (LATP) with NASICON-type crystal structure and water-blocking high lithium ion conductivity, and lithium phosphorous oxynitride to prevent reaction between glass ceramic layer (LATP) and metallic lithium Water-based Li-air that has a lithium protective film made of a reaction-preventing layer such as (LiPON), which separates water and metallic lithium while ensuring the entry and exit of lithium ions, making it possible to use aqueous electrolytes A secondary battery is disclosed.

  Non-Patent Document 1 discloses a technique of using a PEO-based polymer solid electrolyte in order to prevent a reaction between a glass ceramic layer (LATP) and metallic lithium in an aqueous Li-air secondary battery. Non-Patent Document 2 discloses a technique in which a weak acid buffer solution of acetic acid-lithium acetate is used as an electrolyte in order to prevent corrosion of a glass ceramic layer (LATP) in an aqueous Li-air secondary battery. Non-Patent Document 3 discloses a technique of inserting a cation exchange membrane into an aqueous electrolyte solution in order to prevent corrosion of the glass ceramic layer (LATP) in an aqueous Li-air secondary battery. Non-Patent Document 4 describes that in a water-based Li-air secondary battery, in order to prevent LiOH as a discharge product from adhering to the glass ceramic layer (LATP), a cation is formed on the glass ceramic layer (LATP). A technique for arranging an exchange membrane is disclosed.

Special table 2007-513464

Tao Zhang et al., "Li / Polymer Electrolyte / Water Stable Lithium-Conducting Glass Ceramics Composite for Lithium--Air Secondary Batteries with an Aqueous Electrolyte", J. Electrochem. Soc., Vol. 155 (12), pA965-A969 (2008) Tao Zhang et al., "A novel high energy density Rechargeable Lithium / Air Battery", Chem. Commun., Vol. 46, p1661-1663 (2010) What is Flat, "Post Lithium Ion Battery 4 Lithium-Air Fuel Cell with Ion Exchange Membrane", 51st Battery Conference, 3B16 (2010) Philippe Stevens et Al., "Development of an Aqueous, Rechargeable Lithium -Air Battery Operating with Untreated Air", 217th ECS Meeting, B12, ABS # 746 (2010)

Li-air secondary batteries theoretically have a very high energy density. As shown in FIG. 1, a Li-air secondary battery basically includes a negative electrode having Li metal, a positive electrode formed of a gas diffusion electrode for utilizing oxygen in the air, and a positive electrode and a negative electrode. And a cell structure including an electrolytic solution interposed therebetween. Here, the Li-air secondary battery includes a non-aqueous Li-air secondary battery (see FIG. 1) using a non-aqueous solvent electrolyte such as an organic solvent or an ionic liquid as an electrolyte, and HCl, There are two types: an aqueous Li-air secondary battery (see FIG. 2) using an aqueous electrolyte solution of LiCl and LiOH. Here, lithium oxide (Li ₂ O and / or Li ₂ O ₂ or both), which is a discharge product of a non-aqueous Li-air secondary battery, is used as a positive electrode because it is insoluble in the non-aqueous electrolyte. There is a problem that it deposits in the pores of the gas diffusion electrode and seals the pores.

  In contrast, in an aqueous Li-air secondary battery, the discharge product is lithium hydroxide (LiOH), and LiOH dissolves in water, which is a component of the electrolyte. As such, the problem of sealing the pores of the gas diffusion electrode is reduced. Furthermore, by using an aqueous electrolyte, the electrode reaction rate of oxygen at the positive electrode can be made very fast compared to a non-aqueous Li-air secondary battery, so that there is an advantage that the charge / discharge overvoltage can be reduced. . Furthermore, since a large amount of salt can be dissolved in the electrolytic solution, the ionic conductivity of the aqueous electrolytic solution can be increased, and further, there are many advantages such as nonflammability. However, an aqueous Li-air secondary battery has a problem in that the Li metal used for the negative electrode reacts violently with water, so that the aqueous electrolyte cannot be brought into direct contact with the Li metal of the negative electrode. In order to solve this problem, as disclosed in Patent Document 1, a technology for protecting a negative electrode formed of Li metal using a solid electrolyte having water blocking lithium ion conductivity as a reaction preventing layer has been developed. (See FIG. 2). As a characteristic of the solid electrolyte having lithium ion conductivity for protecting the negative electrode including Li metal, it is preferable to satisfy both the water blocking property and the high Li ion conductivity. However, at present, there are very few candidates for solid electrolytes that satisfy such characteristics.

Among them, recently, Li and M (M = at least one of Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb) having a NASICON type crystal structure are included as main components. A glass ceramic layer formed of a lithium ion conductive material has excellent characteristics such as high ion conductivity and high barrier property against water. A typical glass ceramic layer includes LATP having a composition of Li _1-x M _x (Ge _1-y Ti _y ) _2-x (PO ₄ ) ₃ . Since the above-described excellent characteristics can be obtained, in the water-based Li-air battery reported so far, the glass ceramic layer is mainly used as a protective part for protecting Li metal of the negative electrode. However, when the glass ceramic layer is brought into direct contact with the Li metal, titanium, which is a constituent component of the glass ceramic layer, may be reduced. Therefore, as shown in FIG. And a reaction prevention layer such as lithium phosphate oxynitride (LiPON, Non-Patent Document 1), polymer solid electrolyte (Non-Patent Document 2), organic electrolyte (Non-Patent Document 3), etc. Is preferred.

By the way, in the water-based Li-air secondary battery, the battery reaction proceeds by the following electrode reaction in the negative electrode and the positive electrode.
^{Anode: Li ⇔ Li + + e -} ( Scheme 1)
Positive electrode: 1/4 O ₂ + 1/2 H ₂ O + Li ⁺ + e ^- ⇔ LiOH (Reaction formula 2)
Overall: Li + 1/4 O ₂ + 1/2 H ₂ O ⇔ LiOH (Scheme 3)
As can be seen from the above reaction formulas 2 and 3, the discharge product of the aqueous Li-air battery is LiOH, which is dissolved and stored in the aqueous electrolyte. At that time, as the discharge reaction progresses, the LiOH concentration of the electrolytic solution increases and gradually shifts to the alkali side. When the discharge reaction of the battery further progresses and exceeds the saturation solubility of LiOH (according to literature, 12.54 g dissolved in 100 mL of water (@ 25 ° C.)), LiOH precipitation occurs, and as a precipitate, it enters the cell. Stored.

On the other hand, the glass-ceramic layer (typical example: LATP) used for the purpose of protecting Li metal, which is the negative electrode, is stable in neutral solutions such as 1M LiNO ₃ and 1M LiCl, but strong acid solution (0.1M HCl) In a strong alkaline solution (for example, 1M LiOH (about 2.4 g of LiOH dissolved in 100 mL of water) is suggested to be unstable (Reference 1: Satoshi Hasegawa et Al., “Study on Lithium / Air secondary batteries stability of NASICON-type lithium ion conducting glass ceramics with water ", Journ of Power Sources, vol. 189 (1), p371-377 (2009)). The stability of the porous glass ceramic layer (LATP) is not always sufficient.

In order to avoid the problem, as shown in Non-Patent Document 2, a technique using an acetic acid electrolytic solution has been proposed. In this case, by adding acetic acid to the electrolytic solution, as a reaction of the whole battery as shown in the reaction formula 3,
Overall: Li + CH ₃ COOH + 1/4 O ₂ ⇔ CH ₃ COOLi + 1/2 H ₂ O (Scheme 4)
The acetic acid used as the electrolyte is
^{_{CH 3 COOH ⇔ CH 3 COO -}} + H + ( Scheme 5)
By acting as a buffer solution by this equilibrium reaction, it is possible to always maintain the pH of the electrolyte in a weak acid environment and prevent corrosion of the glass ceramic layer (LATP). However, in this case, as shown in the reaction formula 4, the battery reaction requires an equimolar amount of acetic acid equivalent to Li that contributes to the battery reaction, and there is a problem that the energy density is lowered accordingly.

As another countermeasure technique, techniques (Non-patent Documents 3 and 4) using a cation exchange membrane as a protective film for a glass ceramic layer (LATP) are known. In this technique, as shown in FIG. 3, a cation exchange membrane is installed in an aqueous electrolyte (Non-patent Document 3) or on the surface of an ion-conductive glass ceramic layer (Non-patent Document 4), and OH ⁻ This is a technique aimed at preventing attacks of ions on the LATP surface (Non-patent Document 3) and preventing deposits of LiOH as discharge products from adhering to the LATP surface (Non-Patent Document 4). However, according to these conventional techniques, there is a limit in suppressing alkali corrosion in the ion conductive glass ceramic layer.

  The present invention has been made in view of the above circumstances, and has water barrier properties and lithium ion conductivity, and Li and M (M = Al, Ga, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Stability and durability of ion-conductive glass ceramic layers by suppressing alkali corrosion in ion-conductive glass ceramic layers based on lithium ion conductive materials containing at least one of Er, Tm, and Yb as main components It is an object of the present invention to provide a water-based lithium-air secondary battery that is advantageous for enhancing the performance.

(1) The present inventor has been diligently developing a water based lithium-air secondary battery based on the above-described problems. And as shown in FIG. 3, it has water barrier property and lithium ion conductivity from the negative electrode side to the positive electrode side between the negative electrode having metallic lithium, the positive electrode supplied with air, and the negative electrode and the positive electrode. And Li and M (M = Al, Ga, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb) as a main component. Regarding the aqueous lithium-air secondary battery including the glass ceramic layer, the part A provided with the first aqueous electrolyte, the cation exchange membrane, and the part B provided with the second aqueous electrolyte, the present applicants Development is in progress. This is because the negatively charged cation exchange membrane suppresses the permeation of OH ^- ions generated by the electrode reaction of the positive electrode from the B part to the A part due to electrostatic repulsion, and the surface of the glass ceramic layer is exposed to an alkaline solution. It is a technology that aims to prevent this from happening. However, as will be described in the detailed experimental results of the present inventor described later, actually, as shown in FIG. 6, the Li ⁺ ion concentration of the aqueous electrolyte in the B part on the positive electrode side separated by the cation exchange membrane. Rises, the protons in the aqueous electrolyte on the glass ceramic layer side separated by the cation exchange membrane undergo ion exchange through the cation exchange membrane, and as a result, the Li ions in the B portion move to the A portion. Then, LiOH is formed by OH ^- ions left in the A part, and the electrolyte solution in the A part becomes strongly alkaline, which may cause alkali corrosion of the glass ceramic layer. The present invention addresses this.

That is, the water based lithium-air secondary battery according to aspect 1 of the present invention has a negative electrode having metallic lithium, a positive electrode to which air is supplied, water blocking property and lithium ion conductivity, and Li and M (M = A glass ceramic layer formed of a lithium ion conductive material containing at least one of Al, Ga, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb as a main component; A part provided with an aqueous electrolyte (hereinafter also referred to as a first electrolyte), a B part provided with a cation exchange membrane and a second aqueous electrolyte (hereinafter also referred to as a first electrolyte) in this order. The first aqueous electrolyte solution of Part A contains a neutral lithium salt and is set to a pH of 12 or less and a pH of 4 or more, and the second aqueous electrolyte solution of Part B is alkaline, and Lithium ion concentration in part A of the first aqueous electrolyte ≧ lithium ion concentration in part B of the second aqueous electrolyte And characterized in that.

  According to the water-based Li-air secondary battery according to aspect 1 of the present invention, a glass-ceramic layer having a water barrier property and high lithium ion conductivity is used as a protective layer for protecting the Li metal negative electrode. According to this aspect, it is possible to obtain an advantage that LiOH as a discharge product can be dissolved, precipitated and stored in an aqueous electrolyte. However, since the glass ceramic layer may cause alkali corrosion when it comes into contact with a strong alkaline solution, there is a situation that the concentration of LiOH in the electrolytic solution cannot be so high.

Therefore, according to the aspect 1 of the present invention, the aqueous electrolyte portion interposed between the negative electrode and the positive electrode is separated into the A portion and the B portion by the cation exchange membrane. This suppresses permeation of OH ⁻ ions generated in the B part by the discharge reaction of the positive electrode to the A part. For this reason, it is suppressed that the 1st electrolyte solution of A part becomes a strong alkali. Therefore, it is possible to suppress alkali corrosion on the surface of the glass ceramic layer, which is not necessarily sufficiently stable against alkali. That is, the negatively charged cation exchange membrane suppresses permeation of OH ⁻ ions generated by the electrode reaction of the positive electrode from the B part to the A part due to electrostatic repulsion, and the LiOH in the first aqueous electrolyte solution of the A part. Therefore, the surface of the glass ceramic layer can be prevented from being exposed to the alkaline solution. The surface stability and durability of the glass ceramic layer are improved.

Furthermore, according to this aspect, the A part and the B part are separated by installing a cation exchange membrane in the aqueous electrolyte. In this case, Li ⁺ ions generated in the A part on the negative electrode side in the discharge reaction of the battery can pass through the cation exchange membrane and move to the B part. However, OH ⁻ ions in the second electrolyte solution of the B part generated by the discharge reaction of the positive electrode are restricted from moving from the B part to the A part due to electrostatic repulsion of the cation exchange membrane, and the second B part of the B part. The LiOH concentration in the electrolyte gradually increases. Thus, when the LiOH concentration in the second electrolyte solution of part B gradually increases and the alkali becomes stronger, the Li ⁺ ions in the second electrolyte solution of part B and the H ⁺ ions in the first aqueous electrolyte solution of part A become positive. It is conceivable that the ions are exchanged through the ion exchange membrane and moved to the opposite tanks by an equal amount (see FIG. 6). For this reason, the neutralization reaction of the alkali by H ^<+> ion which becomes an acidic factor arises in the B part side. In contrast, on the first electrolyte side of part A, the Li ⁺ ion concentration increases, and the OH ^- ions remaining in part A increase the LiOH concentration. As a result, the electrolyte in part A is strongly alkaline ( It is considered that the stability of the glass ceramic layer is lowered by changing to a pH value of 13 or more. Therefore, according to this aspect, in order to avoid this phenomenon, the Li electrolyte in the neutral region is preliminarily dissolved in the first electrolyte solution of the A part, and the Li ⁺ ion concentration of the first electrolyte solution of the A part ≧ The Li ⁺ ion concentration of the second electrolytic solution in part B is set , and the pH value of the first electrolytic solution in part A is in the vicinity of the neutral region in the range of 4 or more and 12 or less. The pH value of the first electrolyte solution of part A can be 4 or more, 5 or more, 6 or more. Further, the pH value can be 11 or less, 10 or less, or 9 or less.

Since there is a Li ⁺ ion concentration of the second electrolyte Li ⁺ ion concentration ≧ B portion of the first electrolytic solution of Part A, from the second aqueous electrolytic solution of the Part B of the first aqueous electrolytic solution A portion Li ⁺ movement is suppressed. For this reason, it is suppressed that the LiOH density | concentration in the 1st electrolyte solution of A part becomes high, and also that the 1st electrolyte solution of A part becomes a strong alkali. Therefore, the stability and durability of the glass ceramic layer is ensured.
The second aqueous electrolyte in part B is alkaline. If the Li ⁺ ion concentration a relationship of A portion> B part, by the concentration gradient of Li ⁺ ions from Li ⁺ ion concentration is high A part, Li ⁺ ions move to the Li ⁺ ion concentration is low B side There is a fear. However, if the second electrolyte solution of part B is kept alkaline, the H ⁺ concentration of the second electrolyte solution of part B decreases, and as a result, Li ⁺ ions in part A and H ⁺ ions in part B Ion exchange is limited. In this case, it is considered that the movement of Li ⁺ ions from the A part to the B part having a high Li ⁺ ion concentration is limited.

(2) According to the water based lithium-air secondary battery according to aspect 2 of the present invention, in the above aspect, the base material of the lithium ion conductive material (lithium ion conductive layer) is Li _1-x M _x (Ge _1- having _{y Ti y) 2-x (} PO 4) 3 (x = 0~0.8, y = 0~1.0) composition formula. Such a glass ceramic layer is also called LATP. M = At least one of Al, Ga, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb. Li _1-x M _x (Ge _1-y Ti _y ) _2-x (PO ₄ ) ₃ x is preferably in the range of 0 to 0.8, particularly preferably in the range of 0.1 to 0.7. y is preferably in the range of 0 to 1.0, particularly preferably in the range of 0.1 to 0.9. Li _1.35 Ti _1.75 Al _0.25 P _0.9 Si _0.1 O ₁₂ is exemplified. The lithium ion conductive layer is preferably formed of a base material (LATP) having a NASICON type crystal structure and having a water blocking property and high lithium ion conductivity.

In this glass ceramic layer, the glass phase fills the grain boundaries of the ceramic particles. Such an ion conductive glass ceramic layer is an oxide inorganic solid electrolyte. The ion conductivity in the thickness direction is 1 × 10 ⁻⁵ S / cm (25 ° C.) or more, 1 × 10 ⁻⁴ S / cm (25 ° C.) or more, and 1 × 10 ⁻³ S / cm (25 ° C.) or more. it can. Examples of the pH of the neutral range lithium salt include 4 to 10. The thickness of the ion conductive glass ceramic layer is preferably 10 to 1,000 μm.

(3) According to the water based lithium-air secondary battery according to aspect 3 of the present invention, the lithium salt is at least one of LiCl, LiBr, LiI, and LiNO _{3 in} the above aspect. To do. It is advantageous to make the pH value of the first aqueous electrolyte solution of Part A near the neutral range within the range of 4-12.

(4) The aqueous lithium-air secondary battery according to aspect 4 of the present invention is characterized in that in the above aspect, a reaction preventing layer is provided between the negative electrode and the glass ceramic layer. When the glass ceramic layer described above is brought into direct contact with the negative electrode including Li metal, lithium has a strong reducing power and may affect the glass ceramic layer. Specifically, titanium which is a constituent component of the glass ceramic layer may be reduced. Therefore, a reaction preventing layer is interposed between the negative electrode including Li metal and the glass ceramic layer. The reaction preventing layer has lithium ion conductivity and is preferably stable to metallic lithium. The reaction preventing layer of lithium phosphate oxynitride (LiPON, the composition _{formula: Li 3-x PO 4-} y N y), a polymer solid electrolyte, organic electrolyte, and the like. Examples of the polymer solid electrolyte include polyethylene oxide containing bis (trifluoromethanesulfonyl) imide lithium salt. Examples of the solvent for the organic electrolyte include ethylene carbonate and propylene carbonate.

  According to the water-based Li-air secondary battery of the present invention, the negative electrode is protected because the glass-ceramic layer having high lithium ion conductivity that is water-blocking is used as the protective part of the Li metal negative electrode.

However, since the glass ceramic layer described above may cause alkaline corrosion, there is a situation that the concentration of LiOH in the electrolytic solution cannot be made very high. In order to solve this problem, the aqueous electrolytic solution interposed between the positive electrode and the negative electrode is separated into a first aqueous electrolytic solution of part A and a second aqueous electrolytic solution of part B by a cation exchange membrane, so that the positive electrode discharge OH to produce the second aqueous electrolytic solution of the part B by a reaction ^- that ions are transmitted to the first aqueous electrolytic solution a portion was suppressed by cation exchange membranes, the concentration of the LiOH in the first aqueous electrolytic solution of the part a Can be suppressed, and the surface of the glass ceramic layer, which is not necessarily sufficiently stable against strong alkali, can be protected.

It is sectional drawing which concerns on a prior art and shows the concept of a non-aqueous lithium-air battery typically. It is sectional drawing which shows the concept of a water-system lithium-air battery typically. It is sectional drawing which shows typically the concept of the water-system lithium-air battery which concerns on Embodiment 1 of this invention. It is sectional drawing which shows typically the test apparatus which evaluates the alkali inhibitory effect of a cation exchange membrane. It is a graph which shows the test result about the alkali inhibitory effect of a cation exchange membrane. It is sectional drawing which shows typically the problem in an aqueous lithium-air battery. It is a figure which shows typically the state with little hydrogen ion concentration of B tank, since B tank in a water-system lithium-air battery is a strong alkali. It is a graph which shows the test result about the alkali inhibitory effect of a cation exchange membrane and LiCl addition. It is sectional drawing which shows typically the concept which concerns on another form of the water-system lithium-air battery which concerns on this invention. It is a figure which concerns on this invention and a comparative example, and shows the electrolyte solution immersion test of LATP. (A) is based on the data disclosed in Reference Document 1, and is a graph showing the results of X-ray diffraction (XRD) measurement of LATP powder immersed in an electrolytic solution, and (B) is an electrolysis in a test example. It is a graph which shows the X-ray-diffraction (XRD) measurement result of the LATP powder immersed in the liquid. (A) is a graph which shows the X-ray-diffraction (XRD) measurement result of the LATP powder before implementation of the test example immersed in electrolyte solution, (B) is the LATP powder after implementation of the test example immersed in electrolyte solution. It is a graph which shows a XRD measurement result.

Hereinafter, Embodiment 1 of the present invention will be described. FIG. 3 schematically shows a cross-sectional view of a lithium-air secondary battery. As shown in FIG. 3, a negative electrode 1 made of metallic lithium, a positive electrode 2 to which air is supplied, and lithium having water blocking properties from the negative electrode side to the positive electrode side between the negative electrode 1 and the positive electrode 2. A tank 51 (A part) provided with the ion conductive glass ceramic layer 3, the first aqueous electrolyte 42 (hereinafter also referred to as the first electrolyte), the cation exchange membrane 102, and the second aqueous electrolyte 42 ( Hereinafter, it has B tank 52 (B part) provided with the 2nd electrolyte solution 42) in this order. The negative electrode 1 is preferably coated with the above-described reaction preventing layer. The reaction preventing layer of lithium phosphate oxynitride (LiPON, the composition _{formula: Li 3-x PO 4-} y N y), a polymer solid electrolyte, organic electrolyte, and the like.

The composition formula of the base material of the lithium ion conductive glass ceramic layer 3 is Li _1-x (M, Al, Ga) _x (Ge _1-y Ti _y ) _2-x (PO ₄ ) ₃ , and PO ₄ including. x is preferably 0 to 0.8, particularly preferably 0 to 0.5. y is preferably 0 to 1.0, particularly preferably 0.1 to 0.9. Examples include at least one of Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb other than M = Al and Ga. Here include _{Li 1-x M x (Ge} 1-y Ti y) 2-x (PO 4) 3. x is preferably in the range of 0 to 0.8, particularly preferably in the range of 0.1 to 0.7. y is preferably in the range of 0 to 1.0, particularly preferably in the range of 0.1 to 0.9.

The first electrolyte solution 41 of part A contains a lithium salt in a neutral region and is set to a pH of 12 or less and a pH of 4 or more , the second electrolyte solution 42 of the B tank 52 is alkaline, and A The lithium ion concentration of the first electrolytic solution 41 in the tank 51 ≧ the lithium ion concentration of the second electrolytic solution 42 in the B tank 52 .

A glass ceramic layer 3 (LATP) having high lithium ion conductivity that is water-blocking is used as a protective part of the Li metal negative electrode 1. In this case, there is an advantage that LiOH as a discharge product can be dissolved, precipitated and stored in the aqueous electrolyte. However, the glass ceramic layer 3 (LATP) may cause alkaline corrosion when exposed to a strong alkaline solution, and therefore there is a situation that the concentration of LiOH in the electrolytic solution cannot be made very high. Therefore, according to this embodiment, as shown in FIG. 3, the aqueous electrolyte portion interposed between the negative electrode 1 and the positive electrode 2 is separated into the A tank 51 and the B tank 52 by the cation exchange membrane 102. ing. This suppresses permeation of OH ⁻ ions generated in the B tank 52 by the positive electrode discharge reaction to the A tank 51 . For this reason, it is suppressed that the aqueous 1st electrolyte solution 41 of A tank 51 becomes a strong alkali, and can protect the surface of the glass-ceramics layer (LATP) 3 whose stability is not enough with respect to an alkali. That is, the negatively charged cation exchange membrane 102 suppresses permeation of OH ⁻ ions generated by the electrode reaction of the positive electrode from the B tank 52 to the A tank 51 due to electrostatic repulsion, and the first electrolytic solution in the A tank 51 The concentration of LiOH in 41 can be suppressed, so that the surface of the glass ceramic layer (LATP) 3 is not exposed to the alkaline solution.

According to the present embodiment, as shown in FIG. 3, the first electrolytic solution 41 in the A tank 51 and the second electrolytic solution 42 in the B tank 52 are separated by the cation exchange membrane 102. In this case, Li ⁺ ions generated in the negative electrode side A tank 51 in the discharge reaction can pass through the cation exchange membrane 102 and move to the B tank 52. However, OH ⁻ ions in the second electrolytic solution 42 of the B tank 52 generated by the positive electrode discharge reaction are restricted from moving from the B tank 52 to the A tank 51 due to electrostatic repulsion of the cation exchange membrane 102, As a result, the LiOH concentration in the second electrolytic solution 42 in the B tank 52 gradually increases. When the LiOH concentration in the second electrolytic solution 42 in the B tank 52 increases, Li ⁺ ions in the second electrolytic solution 42 in the B tank 52 and H ⁺ ions in the first electrolytic solution 41 in the A tank 51 become cation exchange membranes 102. It is conceivable that the ions are exchanged via the same amount and moved in equal amounts to the opposite tanks. For this reason, an alkali neutralization reaction by H ⁺ ions occurs on the B tank 52 side. On the other hand, on the A tank 51 side, the Li ⁺ ion concentration increases, and the LiOH concentration increases due to the OH ⁻ ions remaining in the A tank 51. As a result, the electrolyte in the A tank 51 changes to strong alkalinity. It is considered that the stability of the glass ceramic layer 3 is lowered. In order to avoid this phenomenon, according to the present embodiment, the first electrolyte solution 41 in the A tank 51 is preliminarily dissolved with a Li salt in the neutral region, and the pH of the first electrolyte solution 41 in the A tank 51 is reduced. The value is in the vicinity of the neutral range within the range of 4-12. Furthermore, there is a Li ⁺ ion concentration of the second electrolyte 42 of Li ⁺ ion concentration ≧ B tank 52 of the first electrolytic solution 41 in tank A 51. Thereby, the movement of Li + from the second electrolytic solution 42 in the B tank 52 to the first electrolytic solution 41 in the A tank 51 is suppressed. For this reason, it is suppressed that the LiOH density | concentration in the 1st electrolyte solution 41 of A tank 51 becomes high, and it is suppressed that the 1st electrolyte solution 41 of A tank 51 becomes a strong alkali by extension. As a result, the stability of the glass ceramic layer (LATP) 3 that is not sufficiently stable against strong alkali is improved.
The second electrolytic solution 42 in the B tank 52 is made alkaline. If the Li ⁺ ion concentration a relationship of A tank 51> B tank 52, the concentration gradient of Li ⁺ ions from Li ⁺ ion concentration higher A tank 51, the Li ⁺ ion concentration is low B tank 52 side Li ⁺ There is a possibility that ions may move. However, if the second electrolytic solution 42 in the B tank 52 is kept alkaline, the H ⁺ concentration of the second electrolytic solution 42 in the B tank 52 is lowered, and consequently the Li ⁺ ions in the A tank 51 and the B tank. Ion exchange with 52 H ⁺ ions is limited. In this case, it is considered that the movement of Li ⁺ ions from the A tank 51 having a high Li ⁺ ion concentration to the B tank 52 is limited.

(Test Example 1)
In order to confirm the effect of preventing permeation of OH ^- ions in the cation exchange membrane 102, the test was performed using the test apparatus shown in FIG. As shown in FIG. 4, the test apparatus sandwiches a cation exchange membrane 102 in a U-shaped glass container 100, and water is placed in one side container 100 </ b> A (hereinafter abbreviated as A ′ tank) of the U-shaped glass container 100. A pH meter 106 for pH measurement is inserted, a cation exchange membrane 102 (material: fluorine-based cation exchange membrane or hydrocarbon-based cation exchange membrane) is sandwiched, and a U-shaped glass container 100 on the opposite side to the A ′ tank. An aqueous LiOH solution was placed in one side container 100B (abbreviated as B 'tank). The test which monitors the change of pH value of the water in the one side container 100A which is A 'tank was done.

The one side container 100A (A ′ tank) and the one side container 100B (B ′ tank) in this test simulate the A tank 51 and the B tank 52 in FIG. The effect of the cation exchange membrane 102 can be confirmed by measuring the pH value of the water in the one-side container 100A (A ′ tank). That is, it is possible to confirm the effect of suppressing the transmission of OH ⁻ ions generated in the one-side container 100A (A ′ tank) through the cation exchange membrane 102 into the one-side container 100B (B ′ tank). In this test, whether or not the pH value of the first electrolyte solution 41 in the A ′ tank (corresponding to the A tank 51) rises above the pH value at which alkali corrosion on the surface of the glass ceramic layer (LATP) becomes a problem is evaluated. To do.

  In this test, as the cation exchange membrane 102, a fluorine-based cation exchange membrane (DuPont: Nafion 112 membrane, thickness 50 μm) and a hydrocarbon cation exchange membrane (Astom: Neosepta CMB membrane, thickness 210 μm) And tested. In the test, the cation exchange membrane 102 was sandwiched between the one-side container 100A (A ′ tank) and the one-side container 100B (B ′ tank). One side container 100A (A ′ tank) was charged with water, and one side container 100B (B ′ tank) was charged with a saturated (5.23 mol / L @ 25 ° C.) LiOH aqueous solution. The test results are shown in FIG. FIG. 5 shows the time change of the pH value in the A 'tank. In FIG. 5, the horizontal axis indicates time, and the vertical axis indicates pH value. A characteristic line W10 (▽ mark) in FIG. 5 shows a case where a fluorine-based cation exchange membrane is used as the cation exchange membrane 102 and water (without LiCl) is used. A characteristic line W20 in FIG. 5 shows a case where a hydrocarbon cation exchange membrane is used as the cation exchange membrane 102 and water (without LiCl) is used. As can be understood from the characteristic lines W10 and W20 in FIG. 5, as the cation exchange membrane 102, a fluorine-based cation exchange membrane (characteristic line W10 in FIG. 5) and a hydrocarbon-based cation exchange membrane (characteristic line in FIG. 5). In any of the cases of using W20), the pH value of the one-side container 100A (A 'tank) rises quickly regardless of the rising speed, and the pH value at which alkali corrosion occurs in the glass ceramic layer (LATP) 12 It turned out that it reached above. Therefore, it is not sufficient to simply divide the A tank 51 and the B tank 52 by the cation exchange membrane 102.

Thus, even if the cation exchange membrane 102 is simply used, it is not possible to prevent the pH value of the A ′ tank (corresponding to the A tank 51 in FIG. 3) from increasing. As a reason for this, the mechanism shown in FIG. 6 can be considered. That is, by installing the cation exchange membrane 102 in the electrolytic solution, Li + ions generated on the negative electrode side in the discharge reaction can be transferred from the A tank 51 to the B tank 52 through the cation exchange membrane 102. . However, the OH ⁻ ions in the B tank 52 generated by the discharge reaction of the positive electrode are restricted from passing through the cation exchange membrane 102 and moving from the B tank 52 to the A tank 51, and the LiOH concentration in the B tank 52 is reduced. To increase. However, when the LiOH concentration in the B tank 52 is increased, the Li ⁺ ions in the B tank 52 and the H ⁺ ions in the A tank 51 are ion-exchanged through the cation exchange membrane 102 and are respectively transferred to the opposite tanks. As a result, the neutralization reaction of alkali by H ⁺ ions occurs on the B tank 52 side. On the other hand, since the Li ⁺ ions move from the B tank 52 to the A tank 51 on the A tank 51 side, the Li ⁺ ion concentration in the A tank 51 increases, and the OH ⁻ ions and Li ⁺ remaining in the A tank 51 increase. It is considered that the ions react with each other, and as a result, LiOH is generated in the A tank 51, and the electrolytic solution in the A tank 51 changes to strong alkalinity.

Therefore, in order to avoid the phenomenon that the first electrolytic solution 41 in the A tank 51 changes to strong alkalinity, the present inventor previously dissolved a neutral Li salt in the A tank 51, The pH value of the electrolytic solution 41 was set in the range of 4-12. As described above, the Li ⁺ ion concentration in the first electrolytic solution 41 of the A tank 51 was set higher than the Li ⁺ ion concentration in the second electrolytic solution 42 of the B tank 52 in advance. Thereby, the movement of Li ⁺ ions from the B tank 52 to the A tank 51 can be prevented.

Incidentally, as described above, the Li ⁺ ion concentration by keeping the relation of A tank 51> B tank 52, the concentration gradient of Li ⁺ ions from Li ⁺ ion concentration higher A tank 51, Li ⁺ ion concentration Therefore, there is a possibility that Li + ions may move to the B tank 52 side where the temperature is low. However, if the second electrolytic solution 42 in the B tank 52 is kept alkaline, the H ⁺ concentration of the second electrolytic solution 42 in the B tank 52 is lowered, and consequently the Li ⁺ ions in the A tank 51 and the B tank. Ion exchange with 52 H ⁺ ions is limited. In this case, it is considered that the movement of Li ⁺ ions from the A tank 51 having a high Li ⁺ ion concentration to the B tank 52 is limited. Therefore, it is preferable to make the electrolytic solution in the B tank 52 alkaline. The pH value of the second electrolytic solution 42 in the B tank 52 is preferably in the range of 8 to 15 and in the range of 12 to 15.

  In order to confirm this effect, in the A ′ tank of the U-shaped glass container 100 shown in FIG. 4, instead of the above water, a very high concentration 12.8 M LiCl solution equal to or higher than the saturated LiOH concentration was used for the first electrolysis. The solution was previously placed as a liquid 41, and a saturated LiOH solution was placed as a second electrolytic solution 42 in a B ′ tank. And the change of pH value of A 'tank was measured. The measurement results are shown in FIG. In FIG. 8, the horizontal axis indicates time, and the vertical axis indicates pH value. The characteristic line W21 in FIG. 8 uses a hydrocarbon cation exchange membrane (Neosepta CMB membrane) as the cation exchange membrane 102, and an electrolytic solution in which 12.8M (12 mol / L) LiCl is dissolved is used as the first electrolytic solution 41. The case where it is used is shown. A characteristic line W11 shows a case where a fluorine-based cation exchange membrane (Nafion 112 membrane) is used as the cation exchange membrane 102, 1M (1 mol / L) LiCl is dissolved, and the electrolytic solution is used as the first electrolytic solution 41. A characteristic line W12 shows a case where a fluorine-based cation exchange membrane (Nafion 112 membrane) is used as the cation exchange membrane 102, 12.8M (12 mol / L) LiCl is dissolved, and the electrolytic solution is used as the first electrolytic solution 41. As shown by characteristic lines W10 and W20 in FIG. 8, when the A ′ tank is water, the first electrolyte solution 41 in the A ′ portion has a pH value of 12 or more and becomes a strong alkali.

  However, when the first electrolytic solution 41 in the A ′ portion is a 12.8 M (12.8 mol / L) neutral LiCl solution (characteristic lines W21 and W12), the first electrolytic solution 41 in the A ′ portion is It was found that the steady-state pH value could be maintained at a neutral pH of about 8.5, and the first electrolyte solution 41 did not become strongly alkaline even after a lapse of time. This indicates that it is advantageous for stabilizing the glass ceramic layer 3. Further, in order to see the effect of the concentration of LiCl in the neutral range to be placed in the A ′ tank, when comparing the characteristic line W11 and the characteristic line W12, the above-described high concentration LiCl solution of 12.8M (12.8mol / L) is obtained. In the case where it was added, it was possible to confirm an excellent effect of suppressing the change to strong alkali in the A ′ tank.

Based on the above results, the cell configuration shown in FIG. 3 can solve the problem of alkali corrosion in the glass ceramic layer (LATP) 3 covering the negative electrode. Therefore, the Li ⁺ ion concentration of the first electrolytic solution 41 in the A tank 51 must always be higher than the Li ⁺ ion concentration of the second electrolytic solution 42 in the B tank 52. As the discharge reaction proceeds, the LiOH concentration in the second electrolyte solution 42 in the B tank 52 gradually increases, but the Li ⁺ ion concentration in the B tank 52 does not exceed the saturated solution concentration of LiOH due to precipitation. It is preferable to previously dissolve a neutral lithium salt in the first electrolytic solution 41 of the A tank 51 so that the Li ⁺ ion concentration is equal to or higher than the LiOH concentration.

Table 1 shows the saturation concentration of alkali salts. As shown in Table 1, the saturation concentration of saturated LiOH is 5.23 mol / L (25 ° C.). As a typical example of a neutral Li salt that can be dissolved at a concentration higher than that, a lithium salt containing at least one of LiCl, LiBr, LiI, LiNO _{3 and the} like can be given as shown in FIG. As described above, the tank A 51 needs to have a sufficiently high Li ⁺ ion concentration, but does not necessarily have a large thickness. Actually, as in many embodiments shown in FIG. 9, it is preferable to impregnate and use a porous body 45 such as a non-woven fabric with the first electrolytic solution 41 formed of the neutral Li salt aqueous solution. . The B tank 52 is preferably made alkaline (pH value: 12 to 15) in order to prevent the movement of Li + ions from the A tank 51. In this case, it is preferable to add LiOH to the B tank 52 in advance.

(Test Example 2)
In order to accelerate the corrosion of the glass ceramic layer (LATP) 3 in an alkaline solution, a commercially available glass ceramic plate (LATP plate, made of OHARA glass, thickness: 150 μm) is crushed with an agate mortar, and a powdery LATP sample is prepared. An evaluation test was performed. In this test, in order to simulate the cell structure of the present invention, as shown in FIG. 10, an opening window 301 is opened in the resin container 300, and a cation exchange membrane 102 (Nafion 112 film) is pasted on the opening window 301. And fixed with a sealing material. A 10M (10 mol / L) LiCl aqueous solution was placed in a container 300 provided with the cation exchange membrane 102, and the LATP powder 250 was immersed therein. Thus, the container 300 in which the LATP powder 250 was immersed was immersed in a 4M (4 mol / L) LiOH aqueous solution in the container 400. In this case, the container 300 was immersed in the container 400 so that the cation exchange membrane 102 was completely in contact with the LiOH aqueous solution in the container 400. As a comparative example, the LATP powder 250 was directly immersed in a 4M (4 mol / L) LiOH aqueous solution as shown in the right diagram of FIG. The LATP powder 250 was allowed to stand at room temperature for 40 days in the container prepared by the above procedure, and then filtered out from the solution. After sufficient water washing, the sample was dried in air to obtain an evaluation sample.

In order to confirm the corrosion state of the LATP powder 250, measurement by X-ray diffraction (XRD) was performed. According to Reference Document 1, as in this test, when LATP powder is immersed and held in strong alkali, as shown in FIG. 11 (A), it is newly around 2θ = 22-24 °. A diffraction peak appears. It has been reported that the peak is attributed to lithium phosphate (Li ₃ PO ₄ ), which is a decomposition product of LATP powder. Also in the experiment of the present inventors, the LATP powder 250 of the comparative example immersed and held in a 4M LiOH aqueous solution contains Li ₃ PO ₄ which is a decomposition product of the LATP powder as shown in FIG. 11 (B). A new assigned diffraction peak was observed around 2θ = 22-24 °.

In contrast, as shown in FIG. 12A, it was found that the diffraction peak corresponding to Li ₃ PO ₄ was not observed for the LATP powder 250 in the initial state before immersion. Similarly, it was found that no diffraction peak corresponding to Li ₃ PO ₄ which is a decomposition product of LATP powder was observed for LATP powder 250 after immersion, as shown in FIG. 12 (B). From this, in the present invention, unlike the comparative example, the decomposition of the glass ceramic layer (LATP) in strong alkali was not observed, and the effect of the present invention was confirmed.

(Other)
The lithium ion concentration of the first aqueous electrolyte solution 41 in the A tank 51 is preferably higher than the lithium ion concentration of the second aqueous electrolyte solution 42 in the B tank 52, but may be equivalent. The first electrolyte solution 41 of part A contains a neutral range lithium salt and is set to a pH of 12 or less and a pH of 4 or more. The pH value of the first electrolyte solution 41 in part A may be 11 or less, 10 or less, pH 4 or more, or pH 5 or more. The present invention is not limited to the embodiments described above and shown in the drawings, and can be implemented with appropriate modifications within the scope not departing from the gist.

  1 denotes a negative electrode, 2 denotes a positive electrode, 102 denotes a cation exchange membrane, 41 denotes a first electrolytic solution, 42 denotes a second electrolytic solution, 51 denotes an A tank (A part), and 52 denotes a B tank (B part).

Claims (4)

A negative electrode having metallic lithium;
A positive electrode supplied with air;
Li and M (M = Al, Ga, Nd, Sm, Eu, Gd, Tb and water blocking property and lithium ion conductivity from the negative electrode side to the positive electrode side between the negative electrode and the positive electrode , Dy, Ho, Er, Tm, Yb) as a main component, a glass ceramic layer formed of a lithium ion conductive material, a part A including a first aqueous electrolyte, and a cation exchange membrane And B part provided with the second aqueous electrolyte solution in this order,
The first aqueous electrolyte of the part A contains a neutral lithium salt and is set to a pH of 12 or less and a pH of 4 or more, and the second aqueous electrolyte of the B part is alkaline, and An aqueous lithium-air secondary battery , wherein the lithium ion concentration of the first aqueous electrolyte solution in the part A ≧ the lithium ion concentration of the second aqueous electrolyte solution in the part B. 2. The base material of the lithium ion conductive material according to claim 1, wherein Li _1−x M _x (Ge _1−y Ti _y ) _2−x (PO ₄ ) ₃ (x = 0 to 0.8, y = A water-based lithium-air secondary battery having a composition formula of 0 to 1.0 and at least one of M = Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb. _3. The water based lithium-air secondary battery according to claim 1, wherein the neutral range lithium salt is at least one of LiCl, LiBr, LiI, and LiNO ₃ .   4. The water based lithium-air secondary battery according to claim 1, wherein a reaction preventing layer is provided between the negative electrode and the glass ceramic layer.