JP2019050085A - Lithium ion conductor, method of manufacturing lithium ion conductor, and lithium air battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion conductor capable of improving durability against an alkaline aqueous solution, a method for producing the lithium ion conductor, and a lithium air battery. A lithium ion conductor contains a lithium ion conductive glass ceramics made of LATP, and has an alkali treatment layer containing a crystal phase of LATP and a crystal phase of Li3PO4 on the surface of the lithium ion conductive glass ceramics. .. [Selection diagram] Fig. 1B

Description

  The present invention relates to a lithium ion conductor, a method for producing a lithium ion conductor, and a lithium-air battery.

  In order to increase the energy density of secondary batteries, metal air secondary batteries that use oxygen in the air as a positive electrode active material are attracting attention. The lithium air secondary battery is a type of metal air secondary battery, uses metallic lithium as a negative electrode active material, and has a high energy density.

  2 as a lithium-air secondary battery: a lithium-air secondary battery using a non-aqueous electrolyte solution; and a lithium-air secondary battery using an aqueous electrolyte solution (hereinafter referred to as "aqueous-based lithium-air secondary battery") Types have been proposed. The following patent documents 1 to 3 and non-patent document 1 disclose techniques related to lithium air secondary batteries.

  In the water based lithium air secondary battery, when metal lithium which is the negative electrode active material comes in contact with the water based electrolyte, metal lithium reacts with water, and there is a risk that metal lithium dissolves in the water based electrolyte and generates hydrogen gas. is there. On the other hand, in order to avoid contact between metallic lithium and the aqueous electrolyte, the “negative electrode protective layer” made of a water-impermeable lithium ion conductor is interposed between the metallic lithium and the aqueous electrolyte, It has been done to protect lithium metal.

Patent Document 1 discloses a lithium-air battery (hereinafter referred to as "conventional battery") including a water-impermeable NAICON type high lithium ion conductive glass ceramic called LATP as a "negative electrode protective layer". It is disclosed. LATP is a lithium ion conducting glass-ceramic with relatively high lithium ion conductivity (about 10 ^-4 S / cm). Furthermore, LATP is chemically stable with respect to water, and by adjusting the heat treatment conditions, a glass phase is formed at the grain boundaries, and it is possible to seal the interstices of the grain boundaries. It is one of the few lithium ion conductors that can prevent the infiltration of

Japanese Patent Publication No. 2007-513464 Unexamined-Japanese-Patent No. 2010-192313 JP 2012-033490 A

Ishihara, Tatsuo, ed., "Development of metal and air secondary batteries and the latest technology. Chapter 8 Negative electrode materials of lithium air water system secondary battery", Technology Education Publishing Company (2011)

In the conventional battery, lithium hydroxide generated with the progress of the discharge reaction represented by the formula (1) dissolves in the aqueous electrolytic solution, whereby the strong alkalization of the aqueous electrolytic solution proceeds.
Li + 1/4 O ₂ + 3/2 H ₂ O → LiOH · H ₂ O (1)

  However, LATP used as the “negative electrode protective layer” has a low durability to a strongly alkaline aqueous solution and has a property of corroding when in contact with a strongly alkaline aqueous solution. Therefore, when the strongly alkaline property of the aqueous electrolyte solution of the conventional battery progresses, the LTAP may be deteriorated with time due to the progress of corrosion from the surface in contact with the aqueous electrolyte solution.

  The present invention was made to address the problems described above. That is, one of the objects of the present invention is a lithium ion conductor (hereinafter sometimes referred to as "the present invention lithium ion conductor") capable of improving the durability to an alkaline aqueous solution, and lithium A method of producing an ion conductor (hereinafter sometimes referred to as "the present invention production method"), and a lithium air battery (hereinafter sometimes referred to as "the present invention lithium air battery") are used. It is to provide.

In order to solve the above-mentioned subject,
The lithium ion conductor according to the present invention is Li _{1 + x + y} Al _x (Ti _z Ge _{1 -z} ) _2-x Si _y P _3-y O ₁₂ (wherein 0 ≦ x ≦ 0.8, 0 ≦ y ≦ 0.6) , Lithium ion conductive glass ceramics comprising LATP represented by 0 <z ≦ 1),
The surface of the lithium ion conductive glass ceramic has an alkali-treated layer containing a crystal phase of LATP and a crystal phase of Li ₃ PO ₄ .

In one aspect of the lithium ion conductor of the present invention,
The crystal phase of the Li ₃ PO ₄ is included between the crystal phase of the LATP inside the alkali treatment layer and at least a part of the crystal phase of the LATP exposed to the surface.

In one aspect of the lithium ion conductor of the present invention,
It has a void formed in at least a part of the crystal phase of the LATP inside the alkali treatment layer.

The present invention production _{process, Li 1 + x + y Al} x (Ti z Ge 1-z) 2-x Si y P 3-y O 12 ( where, 0 ≦ x ≦ 0.8,0 ≦ y ≦ 0.6,0 By immersing lithium ion conductive glass-ceramics composed of LATP represented by <z ≦ 1) in an aqueous solution of lithium hydroxide, the crystalline phase of LATP on the surface of the lithium ion conductive glass-ceramics, and , Forming an alkali treatment layer containing a crystal phase of Li ₃ PO ₄ ,
The concentration of the lithium hydroxide in the lithium hydroxide aqueous solution is higher than 0.021 mol / L.

In one aspect of the process of the present invention,
The time for immersing the lithium ion conductive glass ceramic in the aqueous solution is 2 weeks or more.

In one aspect of the process of the present invention,
The lithium hydroxide aqueous solution further contains a phosphate.

In one aspect of the process of the present invention,
The phosphate is Li ₃ PO ₄ .

The lithium-air battery of the present invention
A negative electrode containing metallic lithium,
An air electrode supplied with air;
An electrolyte comprising a water-based electrolyte solution;
An anode protective layer interposed between the anode and the electrolyte;
The negative electrode protective layer is formed of Li _{1 + x + y} Al _x (Ti _z Ge ₁ -z) _2-x Si _y P _3-y O ₁₂ (wherein 0 ≦ x ≦ 0.8, 0 ≦ y ≦ 0.6, 0 Containing lithium ion conductive glass ceramics composed of LATP represented by <z ≦ 1),
The surface of the lithium ion conductive glass ceramic facing the electrolyte is made of a lithium ion conductor having an alkali treatment layer containing a crystal phase of LATP and a crystal phase of Li ₃ PO ₄ .

  According to the present invention, the durability to an alkaline aqueous solution can be improved.

FIG. 1A is a schematic cross-sectional view showing a configuration example of lithium ion conductive glass ceramics. FIG. 1B is a schematic cross-sectional view showing a configuration example of the lithium ion conductor according to the first embodiment of the present invention. FIG. 2A is a schematic view for explaining the formation mechanism of the alkali treatment layer. FIG. 2B is a schematic view for explaining the formation mechanism of the alkali treatment layer. FIG. 2C is a schematic view for explaining the formation mechanism of the alkali treatment layer. FIG. 3A is a schematic view for explaining a method of manufacturing a lithium ion conductor according to the first embodiment of the present invention. FIG. 3B is a schematic view for explaining the method of manufacturing a lithium ion conductor according to the first embodiment of the present invention. FIG. 3C is a schematic view for explaining the method for producing a lithium ion conductor according to the first embodiment of the present invention. FIG. 3D is a schematic view for explaining the method for producing a lithium ion conductor according to the first embodiment of the present invention. FIG. 4 is a schematic cross-sectional view showing a configuration example of a lithium-air battery according to a second embodiment of the present invention. FIG. 5 is a SEM photograph of lithium ion conductors of Examples 1-1 to 1-4 and Comparative Example 1-1. FIG. 6 is a graph plotting corrosion depth against immersion time. FIG. 7 is an X-ray diffraction profile of lithium ion conductors of Examples 1-1 to 1-4 and Comparative Example 1-1. FIG. 8 is a schematic diagram for explaining impedance measurement. FIG. 9 is a Cole-Cole plot obtained by measuring the impedance of the lithium ion conductors of Examples 1-1 to 1-4 and Comparative Example 1-1. FIG. 10 is a SEM photograph of lithium ion conductors of Example 2-1 to Example 2-2 and Comparative Example 2-1. FIG. 11 is a graph in which the corrosion depth is plotted against the LiOH concentration of the LiOH aqueous solution. FIG. 12 is a SEM photograph of the lithium ion conductor of Example 3-1 to Example 3-2. FIG. 13 is a Cole-Cole plot obtained by measuring the impedance of the lithium ion conductors of Examples 3-1 to 3-4 and Comparative Example 1-1. FIG. 14A is a photograph of a lithium ion conductor of Example 4-1. FIG. 14B is a photograph of a lithium ion conductor of Example 4-2. FIG. 14C is a photograph of a lithium ion conductor of Comparative Example 4-1.

<Technical background and outline of the present invention>
First, in order to facilitate the understanding of the present invention, the technical background of the present invention and the outline of the present invention will be described. The LATP described in Patent Document 1 (Japanese Patent Application Publication No. 2007-513464) mentioned above has a relatively high lithium ion conductivity, is chemically stable to water, and further adjusts the heat treatment conditions to A glass phase is formed at the grain boundaries, and it is possible to seal the interstices of the grain boundaries. Therefore, in a lithium-air battery, LATP is used as a "negative electrode protective layer" to be interposed between metallic lithium and an aqueous electrolyte.

  However, in a lithium air battery using LATP as a "negative electrode protective layer", the following problems may occur.

It has been found that LATP is corroded in a strongly acidic or strongly alkaline environment. In a lithium-air secondary battery, strongly alkaline-ization of the aqueous electrolyte is achieved by lithium hydroxide generated along with the discharge reaction (reaction in the direction of the right arrow) of the charge-discharge reaction of the lithium-air battery represented by the following formula (2) Progress.
Li + 1/4 O ₂ +3/2 H ₂ O ⇔ LiOH · H ₂ O (2)
(Theoretical capacity: 1119 Ah / kg)

  Generally, since the stable region of LATP is about pH 2 to pH 10, there is a possibility that the problem of deterioration of LATP in contact with the aqueous electrolyte as “negative electrode protective layer” may occur with the progress of strong alkalization of the aqueous electrolyte. is there.

  On the other hand, Patent Document 2 (Japanese Unexamined Patent Application Publication No. 2010-192313) discloses using a pH buffer solution in which acetic acid and lithium acetate are dissolved as an aqueous electrolyte solution of an aqueous lithium-air battery (hereinafter referred to as For convenience, it may be called "the first prior art".

This aqueous lithium-air battery operates in the charge / discharge reaction of the following formula (3).
Li + 1/4 O ₂ + CH ₃ COOH⇔1⁄2 H ₂ O + CH ₃ COOLi (3)
(Theoretical capacity: 357 Ah / kg)

  In this aqueous lithium-air battery, the aqueous electrolytic solution can be maintained at a weakly acidic state of about pH 2 to pH 7 by the pH buffering action, so that the strong alkalization of the aqueous electrolytic solution can be suppressed.

  However, in this water based lithium-air battery, there is a problem of a decrease in theoretical capacity due to a decrease in active oxygen due to the liquid property (oxidation property) of the water-based electrolyte and an increase in weight due to acetic acid.

  In addition, Non-Patent Document 1 (Tsumi Ishihara, "Development of metal-air secondary batteries and latest technology Chapter 8 Lithium-Aqueous Water Secondary Battery Anode Material", Technology Education and Publishing Company (2011)) A water-based lithium-air battery is disclosed in which an aqueous solution is made to coexist with lithium chloride, which is more easily ionized and dissolved than lithium hydroxide, to suppress the ionization of lithium hydroxide and to prevent the corrosion of LATP. Furthermore, Patent Document 3 (Japanese Patent Application Laid-Open No. 2012-033490) describes lithium hydroxide and lithium halide (lithium halide: lithium fluoride (LiF), lithium chloride (LiCl), lithium bromide (LiBr), lithium iodide) A water-based lithium-air battery is disclosed that prevents the corrosion of LATP by using a water-based electrolyte in which (LiI) is dissolved. The techniques disclosed in Non-Patent Document 1 and Patent Document 3 may be collectively referred to as “second prior art” for convenience.

  For example, the aqueous lithium-air battery described in Patent Document 3 adds a high concentration of lithium chloride at a concentration of 8 mol / L or more to a 5 mol / L aqueous solution of lithium hydroxide to control pH 10 or less, which is a stable region of LATP. The corrosion of LATP is suppressed by using the aqueous electrolyte solution of

However, cathode blockage caused by solid lithium hydroxide formation due to ionization suppression of LiOH and oxygen supply failure due to the cathode blockage, oxygen generation reaction inhibition by high concentration lithium chloride (chlorination reaction (Cl ⁻ → 1 / 2Cl ₂ + e ^-) progression), as well as a high concentration LiCl content, capacity deterioration of by the entire mass of the aqueous electrolytic solution is increased occurs.

  As described above, in the lithium-air battery using LATP as the “negative electrode protective layer”, when trying to suppress the corrosion of LATP using the first prior art, the oxygen activity is reduced and the theoretical capacity is significantly increased. Problems will occur. If it is intended to suppress the corrosion of LATP by using the second prior art, problems such as cathode clogging and oxygen supply failure, oxygen generation reaction inhibition, and capacity reduction occur.

  Then, when the present inventor examined the technique which suppresses corrosion of LATP without giving rise to these problems, the durability with respect to the alkaline aqueous solution of LATP can be improved by performing the alkali treatment which immerses LATP in LiOH aqueous solution. Found out.

  Hereinafter, each embodiment of the present invention will be described.

First Embodiment
A lithium ion conductor according to a first embodiment of the present invention will be described with reference to the drawings.

(Composition of lithium ion conductor)
The lithium ion conductor according to the first embodiment of the present invention is obtained by subjecting the thin plate-shaped lithium ion conductive glass ceramic 11 shown in FIG. 1A to an alkaline treatment to immerse it in an immersion liquid (alkaline treatment aqueous solution) Can. As shown in FIG. 1B, this lithium ion conductor has an alkali treatment layer 12 on at least a part of the surface of the lithium ion conductive glass ceramic 11. In the present example, the alkali treatment layer 12 is provided on the entire surface of the lithium ion conductive glass ceramic 11.

(Lithium ion conductive glass ceramics)
As lithium ion conductive glass ceramics 11, for example, a material which is water impermeable and has lithium ion conductivity can be used.

As the lithium ion conductive glass ceramics _{11, Li 1 + x + y} Al x (Ti z Ge 1-z) 2-x Si y P 3-y O 12 ( where, 0 ≦ x ≦ 0.8,0 ≦ y ≦ 0 And lithium ion conductive glass ceramics (referred to as “LATP”) of the lithium substitution type NASICON type represented by the following formula (6, 0 <z ≦ 1) can be used.

(Alkali treatment layer)
The alkali treatment layer 12 is a layer formed by altering the surface of the lithium ion conductive glass ceramic 11 before the alkali treatment by the alkali treatment. The alkali treatment layer 12 contains at least a crystal phase of LATP and a crystal phase of Li ₃ PO ₄ . In addition, the alkali treatment layer 12 may contain the glass phase of LATP and the other compound phase formed by the alkali treatment.

The solubility of Li ₃ PO ₄ per 100 g of water is 0.027 wt%, and Li ₃ PO ₄ tends to have low solubility in an aqueous alkali solution. The alkali-treated layer 12 includes a crystal phase of Li ₃ PO ₄ having a low solubility in an alkaline aqueous solution as compared to the crystal phase of LATP, and therefore, the durability to an alkaline aqueous solution is higher than LATP.

  As shown in FIG. 2A, the surface of the lithium ion conductive glass ceramic 11 before alkali treatment has a crystal phase 11a of LATP and a glass phase 11b of LATP. The glass phase 11b of LATP is more easily corroded by the alkali treatment than the crystal phase 11a of LATP. Therefore, on the surface of the lithium ion conductive glass ceramic 11 before the alkali treatment, a difference occurs in the degree of corrosion between the portion of the crystal phase of LATP and the portion of the glass phase of LATP.

When alkali treatment is performed, as shown in FIG. 2B, corrosion (decomposition) proceeds from the portion of the glass phase 11b of the LATP between the crystal phase 11a of the surface LATP and the glass phase 11b of the LATP is corroded as shown in FIG. 2B A crystal phase of Li ₃ PO ₄ is formed on the crystal phase 11 a of LATP exposed from the part and the surface. Furthermore, as time passes, as shown in FIG. 2C, the corrosion of the glass phase 11b of LATP further progresses (grain boundary) between the crystal phases 11a of LATP, and at least a part of the crystal phase of LATP inside The crystal phase 12a of Li ₃ PO ₄ is formed in The alkali-treated layer 12 has a rougher shape than the surface of the lithium ion conductive glass ceramic 11 before alteration by alkali treatment, because the corrosion selectively proceeds from the portion of the glass phase 11b of the surface LATP. . Furthermore, the alkali treatment layer 12 may have a void (not shown) formed by corrosion of LATP in at least a part of the crystal phase 11a of LATP inside.

Rough shape of the surface of the alkali-treated layer 12, and / or, by an air gap, it is possible to improve the contact area between the crystal phase 11a and the electrolyte solution LATP, lithium ion due to the crystalline phase 12a of _Li 3 PO ₄ is formed A significant rise in the resistance of the conductor can be suppressed. That is, while the crystal phase 12a of Li ₃ PO ₄ has characteristics (resistance lower in lithium ion conductivity) higher in resistance than the crystal phase 11a of LATP, the crystal phase 12a of Li ₃ PO ₄ is A significant increase in resistance of the lithium ion conductor due to formation can be suppressed.

(Method of manufacturing lithium ion conductor)
The manufacturing method of the lithium ion conductor which concerns on 1st Embodiment of this invention is demonstrated. This lithium ion conductor can be produced by immersing the thin plate-like lithium ion conductive glass ceramic 11 in an aqueous alkali treatment solution.

  Specifically, as shown in FIG. 3A, after immersing the thin plate lithium ion conductive glass ceramic 11 in the alkali treatment aqueous solution 16 contained in the container 15, it is shown in FIG. 3B. As described above, the lid 17 is put on the opening of the container 15 to seal the container 15. Next, as shown in FIG. 3C, the sealed container 15 is placed in the thermostatic bath 18 for a predetermined time. Thereby, as shown in FIG. 3D, the alkali treatment layer 12 is formed on the surface of the lithium ion conductive glass ceramic 11. By the above, the lithium ion conductor according to the first embodiment of the present invention can be manufactured.

As the alkali treatment aqueous solution 16, a LiOH aqueous solution can be used. The LiOH concentration of the LiOH aqueous solution is preferably higher than 0.021 mol / L. The third step acid dissociation equilibrium reaction of phosphoric acid in an aqueous solution is represented by equation (4), and in this case, the acid dissociation constant is pKa3 = 12.32. Based on this, the LiOH concentration of the aqueous solution required to increase the amount of PO ₄ ^3- required to form Li ₃ PO ₄ needs to be higher than 0.021 mol / L corresponding to pH 12.32. It is because
HPO ₄ ^2- ⇔ H ⁺ + PO ₄ ^3- (4)

  The upper limit of the concentration of the LiOH aqueous solution is preferably a concentration corresponding to a saturated solution of the LiOH aqueous solution (for example, 5.23 mol / L at 25 ° C.).

The immersion time is preferably 1 week or more, more preferably 2 weeks or more, from the viewpoint of being able to form an alkali-treated layer 12 having more excellent alkali resistance by generating more Li ₃ PO _4. Is more preferred.

The LiOH aqueous solution preferably further contains a phosphate in addition to LiOH. As a result, the elution of PO ₄ ^3- from LATP into the LiOH aqueous solution can be suppressed, so that Li ₃ PO ₄ can be efficiently formed on the surface of LATP. The phosphates, for _{example, Li 3 PO 4, Na 3} PO 4, and _can be used alone or in combination of two or more selected from K 3 PO ₄ and the like.

(effect)
The lithium ion conductor according to the first embodiment is formed by the surface of the lithium ion conductive glass ceramic 11 by the highly alkali-resistant alkali-treated layer 12 including the crystal phase of LATP and the crystal phase of Li ₃ PO ₄ It is possible to suppress the progress of alkaline corrosion when in contact with an alkaline aqueous solution. As a result, the durability to an aqueous alkaline solution can be improved.

Second Embodiment
Next, a lithium-air battery according to a second embodiment of the present invention will be described with reference to the drawings. In the present example, the lithium air battery is a chargeable and dischargeable lithium air secondary battery. The lithium air battery may be a lithium air primary battery. The lithium-air battery according to the second embodiment includes the lithium ion conductor according to the above-described first embodiment as a negative electrode protective layer.

(Battery configuration)
FIG. 4 shows a configuration example of a lithium-air battery according to a second embodiment. As shown in FIG. 4, in the lithium-air battery, the reaction prevention layer 22, the negative electrode protective layer 23, and the aqueous electrolyte are disposed between the negative electrode 21 and the positive electrode 25 (also referred to as “air electrode”). Layer 24 is interposed. The reaction prevention layer 22, the negative electrode protection layer 23, and the aqueous electrolyte layer 24 are disposed in the order of the reaction prevention layer 22, the negative electrode protection layer 23, the aqueous electrolyte layer 24, and the positive electrode 25 from the negative electrode 21 toward the positive electrode 25. There is.

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

(Reaction prevention layer)
The reaction prevention layer 22 is provided to prevent a reaction between the negative electrode protective layer 23 and the negative electrode 21 (metallic lithium). By providing the reaction prevention layer 22, the negative electrode protective layer 23 is in contact with metal lithium, and titanium of the constituent component of the negative electrode protective layer 23 is reduced to prevent deterioration of the negative electrode protective layer 23. be able to.

For the reaction prevention layer 22, a material having lithium ion conductivity and stable to metallic lithium can be used. As the reaction prevention layer 22, for example, a polymer electrolyte, an organic electrolytic solution (non-aqueous electrolytic solution), or lithium phosphoric oxynitride (LiPON (Li _3-x PO _4-y N _y) ) can be used.

  As the polymer electrolyte, an intrinsic polymer electrolyte (polymer electrolyte not containing an electrolytic solution) or a gel electrolyte (gel-like polymer electrolyte containing an 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, 1 type (s) or 2 or more types selected from polyethylene oxide (PEO), a polypropylene oxide (PPO), etc. can be used, for example. As a lithium salt, 1 type or 2 types or more selected from LiPF ₆ , LiClO ₄ , LiBF ₄ , LiTFSI (Li (CF ₃ SO ₂ ) ₂ N), LiBOB (lithium bis oxalate borate), etc., for example Can be used.

  As the non-aqueous electrolyte, a non-aqueous electrolyte in which a lithium salt is dissolved in a non-aqueous solvent can be used. As the non-aqueous solvent, for example, ethylene carbonate, propylene carbonate, tetrahydrofuran, dimethyl sulfoxide, γ-butyrolactone, 1,3-dioxolane, 4-methyl-1,3-dioxolane, 1,2-dimethoxyethane, 2-methyltetrahydrofuran One or more selected from sulfolane, diethyl carbonate, dimethylformamide, acetonitrile, dimethyl carbonate and the like can be used. The lithium salt described above can be used as the lithium salt.

  The gel electrolyte comprises the non-aqueous electrolyte described above and a host polymer (matrix polymer (polymer compound)) swollen by the non-aqueous electrolyte. As the host polymer, a polymer compound which can be swelled by a non-aqueous electrolyte can be used. As such a host polymer, 1 type (s) or 2 or more types selected from polyvinylidene fluoride, the copolymer of vinylidene fluoride and hexafluoropropylene, polyethylene oxide etc. can be mentioned, for example.

(Anode protective layer)
The negative electrode protective layer 23 is made of the lithium ion conductor according to the first embodiment described above. The negative electrode protective layer 23 (lithium ion conductor) has a lithium ion conductive glass ceramic 11 and an alkali treatment layer 12 formed on the surface of the lithium ion conductive glass ceramic 11. The alkali treatment layer 12 is formed on the surface of the lithium ion conductive glass ceramic 11 facing at least the aqueous electrolyte layer 24. The alkali treatment layer 12 may be formed on the entire surface of the lithium ion conductive glass ceramic 11 as in the first embodiment.

(Water-based electrolyte layer)
The aqueous electrolyte layer 24 is composed of a separator (not shown) and an aqueous electrolytic solution (aqueous electrolytic solution) impregnated in the separator. A hydrophilic porous membrane can be used as a separator. As such a porous membrane, a polyolefin film (for example, polyethylene or polypropylene film) subjected to a hydrophilization treatment, or a non-woven fabric can be used. An aqueous alkaline solution (for example, an aqueous solution of lithium hydroxide (LiOH)) can be used as the aqueous electrolytic solution.

(Positive electrode)
The positive electrode 25 is composed of a catalyst layer 31 and a gas diffusion layer 32. The positive electrode 25 is supplied with oxygen (air) which is an active material.

The catalyst layer 31 includes catalyst-supporting carbon particles (platinum-supporting carbon) as a discharge catalyst, iridium oxide (IrO ₂ ) particles (powder) as a charge catalyst, and a binder (a fluorine-based resin (for example, polytetrafluoroethylene (eg, polytetrafluoroethylene PTFE)) and.

  The gas diffusion layer 32 supplies a reaction gas (air (oxygen)) to the catalyst layer 31 and collects a charge generated in the catalyst layer 31. The conductive porous layer (a porous layer mainly composed of a conductive material) ). One main surface on the outer side of the gas diffusion layer 32 is in contact with air, and the catalyst layer 31 is formed on the other main surface on the inner side of the gas diffusion layer 32. As the gas diffusion layer 32, for example, carbon paper subjected to water repellent treatment can be used.

(Battery operation)
In the lithium air battery (secondary battery), the following reaction occurs at the negative electrode 21 and the positive electrode 25 along 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 discharge, in the lithium-air battery, metal lithium is transferred as lithium ions at the negative electrode 21 through the reaction prevention layer 22 and the negative electrode protective layer 23 to the aqueous electrolyte layer 24 (electrolyte solution). At this time, the electrons are supplied to an external circuit (not shown) connected to the negative electrode 21. Electrons are supplied from the external circuit connected to the positive electrode 25 at the positive electrode 25, and oxygen in the air reacts with water at the catalyst layer 31 to generate hydroxide ions. In the aqueous electrolyte layer 24 (electrolytic solution), lithium ions and hydroxide ions react with each other to produce water-soluble lithium hydroxide (LiOH).

  On the other hand, at the time of charging, in the lithium-air battery, electrons are supplied from the external circuit to the negative electrode 21, and lithium ions present in the aqueous electrolyte layer 24 (electrolyte solution) pass through the negative electrode protective layer 23 and the reaction prevention layer 22. It moves to the surface, and lithium ions receive electrons at the surface of the negative electrode 21 to generate metallic lithium. In the catalyst layer 31 of the positive electrode 25, the hydroxide ions react to generate oxygen and generate electrons, and the generated electrons are supplied to the external circuit.

(effect)
In the lithium-air battery according to the second embodiment, the negative electrode protective layer 23 is the lithium ion conductor according to the first embodiment, and the alkali treatment layer 12 on the surface of the lithium ion conductor faces and contacts the water-based electrolyte layer 24 Is configured as. As a result, the durability of the negative electrode protective layer 23 to the alkaline aqueous solution can be improved, and the negative electrode protective layer 23 can be made strongly alkaline even when the aqueous electrolyte solution contained in the aqueous electrolyte layer 24 becomes strongly alkaline due to the discharge reaction of the lithium air battery. Can control the progress of corrosion. As a result, it is possible to suppress the deterioration of the battery performance due to the progress of the corrosion of the negative electrode protective layer 23.

  EXAMPLES Hereinafter, the present invention will be specifically described by way of examples, but the present invention is not limited to only these examples.

Embodiments of the present invention will be described in the following order.
(1) Evaluation of the influence of alkali treatment time (2) Evaluation of the influence of LiOH concentration of LiOH aqueous solution which is the immersion liquid (3) evaluation of the influence of Li ₃ PO ₄ addition to LiOH aqueous solution which is the immersion liquid (4) evaluation of alkali resistance

(1) Evaluation of influence of alkali treatment time <Example 1-1 to Example 1-4>
The lithium ion conductors of Examples 1-1 to 1-4 different in alkali treatment time as follows were produced.

Example 1-1
First, the thickness of 150μm, 1 inch × 1 inch size of LATP thin (Ohara Inc., product name: LICGC ^TM AG-01) was prepared. Next, this LATP thin plate was immersed in a saturated LiOH aqueous solution for one week in a constant temperature bath at a temperature of 60.degree. Thus, an alkali-treated LATP thin plate (lithium ion conductor) of Example 1-1 was produced.

Example 1-2
A lithium ion conductor was produced in the same manner as Example 1-1 except that the immersion time of the LATP thin plate in the saturated LiOH aqueous solution was changed to 2 weeks.

Example 1-3
A lithium ion conductor was produced in the same manner as Example 1-1 except that the immersion time of the LATP thin plate in the saturated LiOH aqueous solution was changed to 4 weeks.

Example 1-4
A lithium ion conductor was produced in the same manner as Example 1-1 except that the immersion time of the LATP thin plate in the saturated LiOH aqueous solution was changed to 10 weeks.

Comparative Example 1-1
The LATP thin plate not immersed in the LiOH aqueous solution was used as a lithium ion conductor of Comparative Example 1-1.

  "Confirmation of the corrosion state of the surface and the cross section by SEM observation" was performed as follows about the produced lithium ion conductor of Example 1-1 to Example 1-4, and Comparative Example 1-1.

(Confirmation of corrosion state by SEM observation of surface and cross section)
Using a field emission electron probe microanalyzer (FE-EPMA, manufactured by Nippon Denshi Co., Ltd., product name: JXA-8530F), morphological observation (SEM observation) of the surface and the cross section was performed at an acceleration voltage of 7 kV. In addition, cross-sectional observation observed the cross section after plane ion milling.

  The photograph (secondary electron image) of the surface of the lithium ion conductor of Example 1-1 to Example 1-4, and Comparative Example 1-1 is shown in FIG. FIG. 6 shows a graph in which the corrosion depth (the thickness of the alkali-treated layer) is plotted against the immersion time. In FIG. 6, the thick line along the ordinate indicates the distribution of the measured value of the corrosion depth in the observation area, and the point on the thick line indicates the most frequent corrosion depth (mode). .

  Table 1 shows the evaluation results of the corrosion state. In addition, corrosion state evaluation in Table 1 is performed based on the following judgment criteria.

(Surface corrosion evaluation)
"Available": Corrosion (deterioration) was observed on the surface in SEM observation.
"None": No corrosion (deterioration) was observed on the surface in SEM observation.
(Evaluation of corrosion depth)
"Shallow": The corrosion depth mode was less than 600 nm.
"Deep": The mode of corrosion depth was 600 nm or more and less than 1200 nm.
"Very deep": The mode of corrosion depth was 1200 nm or more.
In Table 1, in Examples 1-1 to 1-4 and Comparative Example 1-1, no example or comparative example corresponding to “very deep” was found.

  As shown in FIG. 5, on the surface of the lithium ion conductor (alkali-treated LATP) of Examples 1-1 to 1-4, several hundreds of crystalline LATP (crystal phase of LATP) are considered. A large number of square-shaped lumps of about nm could be confirmed. That is, it could be confirmed that the surface of the lithium ion conductor of Example 1-1 to Example 1-4 was corroded by the alkali treatment.

  Furthermore, as shown in FIG. 6, the corrosion depth of the lithium ion conductor of Example 1-2 to Example 1-4 in which the immersion time is 2 weeks or more remains at about 900 nm to 1200 nm, The rate of increase of corrosion depth over time was almost constant. That is, it was found that the progress of LATP corrosion almost stopped when the immersion time was 2 weeks or more.

(Thin film X-ray diffraction analysis)
Furthermore, thin film X-ray diffraction analysis is performed on the lithium ion conductors of Examples 1-1 to 1-4 and Comparative Example 1-1 prepared to confirm the components generated by the corrosion of LATP. The

  Thin film X-ray diffraction analysis was performed as follows. Using a thin film XRD apparatus (Rigaku Co., Ltd., product name: RINT2500HLB), by performing thin film X-ray diffraction analysis at a tube voltage of 50 kV and a tube current of 250 mA, the surface layer portion of the lithium ion conductor (alkali treated layer The composition analysis of. An X-ray diffraction profile is shown in FIG.

As shown in FIG. 7, crystalline peaks derived from Li ₃ PO ₄ could be confirmed in the X-ray diffraction profiles for Example 1-1 to Example 1-4. That is, it was found that the component produced by the LATP being corroded by immersion in a saturated LiOH aqueous solution contains the crystal phase of Li ₃ PO ₄ . Furthermore, as shown in the integrated intensity ratio of Table B1 in FIG. 7, the proportion of generated Li ₃ PO ₄ (ie, the amount of Li ₃ PO ₄ as the immersion time for immersing LATP in a saturated LiOH aqueous solution becomes longer ) Was found to increase.

(Evaluation of resistance)
Since the lithium ion conductivity of Li ₃ PO ₄ itself produced by the alkali treatment is very low compared to LATP, there is a concern that the resistance of the lithium ion conductor will increase. Therefore, the impedance of Example 1-1 to Example 1-4 and Comparative Example 1-1 was measured. Thereby, the impedance change (The impedance change from the impedance of Comparative Example 1-1 before alkali treatment) of the lithium ion conductor of Example 1-1 to Example 1-4 was evaluated.

(Impedance measurement)
The impedance measurement was performed as follows. As shown in FIG. 8, the manufactured sample 101 (lithium ion conductor) was attached to the central part of a glass U-shaped tube 100. A 1 mol / L LiOH aqueous solution was placed in each of the one (tank A) and the other (tank B) of the glass U-shaped tube 100 partitioned into two by the sample 101.

  A platinum black electrode was set as the electrode 102A in the LiOH aqueous solution put in the A layer, and a platinum black electrode was set as the electrode 102B in the LiOH aqueous solution put in the B tank. The potentio / galvanostat (manufactured by BioLogic, product name: VMP3) was connected to the electrode 102A and the electrode 102B, and AC impedance measurement between two electrodes was performed.

  FIG. 9 shows a Call-Cole plot for Examples 1-1 to 1-4 and Comparative Example 1-1 obtained by impedance measurement. Table 1 shows the total resistance value (the total resistance value of the lithium ion conductor and the LiOH aqueous solution) determined based on the Cole-Cole plot, and the evaluation based on the total resistance value.

In addition, evaluation of resistance in Table 1 was performed based on the following judgment criteria by using the total resistance value of Comparative Example 1-1 as a reference value.
"Decrease (◎)": The total resistance value is smaller than the reference value.
"No significant increase (o)": The total resistance value has not increased significantly compared to the reference value (specifically, the difference between the total resistance value and the reference value is less than 20 Ωcm ² ).

  As shown in FIG. 9 and Table 1, no significant increase in resistance was observed regardless of the alkali treatment time (duration of immersion time). It is considered that the increase in resistance can be suppressed by the residual crystalline LATP that contributes to lithium ion conductivity even after the LATP corrosion. It is considered that the increase in the resistance can be suppressed by the increase in the contact area between the electrolytic solution and the crystalline LATP due to the surface becoming rough after corrosion of LATP and the formation of voids in the corrosion portion.

(2) Evaluation of influence of LiOH concentration of LiOH aqueous solution which is immersion liquid <Example 2-1 to Example 2-2, and Comparative Example 2-1>
Next, lithium ion conductors of Examples 2-1 to 2-2 and Comparative Example 2-1, which differ only in the LiOH concentration of the LiOH aqueous solution as the immersion liquid, were produced as follows.

Example 2-1
A lithium ion conductor of Example 2-1 was produced in the same manner as in Example 1-3.

Example 2-2
A lithium ion conductor was obtained in the same manner as in Example 2-1 except that a 1 mol / L LiOH aqueous solution was used instead of the saturated LiOH aqueous solution.

Comparative Example 2-1
A lithium ion conductor was obtained in the same manner as in Example 2-1 except that a 0.01 mol / L LiOH aqueous solution was used instead of the saturated LiOH aqueous solution.

(Confirmation of corrosion state by SEM observation of surface and cross section)
“Confirmation of the corrosion state of the surface and the cross section by SEM observation” of the prepared lithium ion conductors of Example 2-1 to Example 2-2 and Comparative Example 2-1 in the same manner as Example 1-1. went.

  FIG. 10 shows photographs (secondary electron images) of the surfaces and cross sections of the lithium ion conductors of Example 2-1 to Example 2-2 and Comparative Example 2-1. FIG. 11 shows a graph in which the mode value of the corrosion depth (the thickness of the alkali-treated layer) is plotted against the concentration of LiOH. Table 2 shows the evaluation results of the corrosion state.

  As shown in FIG. 10, the surface of the lithium ion conductor of Example 2-1 and Example 2-2 obtained by immersing in an aqueous solution of 1 mol / L or more of LiOH seems to be crystalline LATP. A large number of square-shaped lumps having a size of several hundred nm were identified. On the other hand, on the surface of the lithium ion conductor of Comparative Example 2-1 obtained by immersing in a dilute 0.01 mol / L aqueous solution of LiOH, a square-shaped aggregate which is considered to be crystalline LATP should be confirmed. I could not Furthermore, on the surface of the lithium ion conductor of Comparative Example 2-1, a large number of pores (voids) of about several nm were confirmed.

As shown in FIG. 11, in the lithium ion conductor of Comparative Example 2-1 in which the LiOH concentration in the LiOH aqueous solution is 0.01 mol / L, the concentration of the aqueous solution is too low, so the reaction of dissolving LATP in the aqueous solution proceeds As it became easy to do, while the corrosion depth became very deep, the further increase of the amount of void | holes (void amount) was confirmed. As described above, it is considered that the LiOH concentration of the LiOH aqueous solution for producing low solubility Li ₃ PO ₄ needs to be higher than 0.021 mol / L corresponding to pH 12.32. Thus, this result is consistent with the discussion.

From the above evaluation results and consideration, it was found that in order to form an alkali-treated layer containing Li ₃ PO ₄ , it is necessary to treat with a LiOH aqueous solution having a concentration higher than 0.021 mol / L.

(3) Evaluation of the influence of addition of Li ₃ PO _{4 on the} aqueous solution of LiOH which is the immersion liquid <Examples 3-1 to 3-2>
Next, the lithium ion conductors of Example 3-1 and Example 3-2 which differ only in the presence or absence of addition of Li ₃ PO _{4 with} respect to the LiOH aqueous solution which is an immersion liquid were produced as follows.

Example 3-1
A lithium ion conductor of Example 3-1 was produced in the same manner as in Example 1-3.

Example 3-2
A lithium ion conductor was prepared in the same manner as in Example 3-1 except that a saturated LiOH aqueous solution was added in which the Li ₃ PO ₄ was added so that the Li ₃ PO ₄ was 5% by mass instead of the saturated LiOH aqueous solution. Made.

(Confirmation of corrosion state by SEM observation of surface and cross section)
About the produced lithium ion conductor of Example 3-1 and Example 3-2, it carried out similarly to Example 1-1, "confirmed the corrosion state of the surface and cross section by SEM observation" was performed. The photograph (secondary electron image) of the surface and cross section of the lithium ion conductor of Example 3-1 and Example 3-2 is shown in FIG. The evaluation results of the corrosion state are shown in Table 3.

(Impedance measurement)
Furthermore, "impedance measurement" was performed about the lithium ion conductor of Example 3-1 and Example 3-2 which were produced similarly to Example 1-1. FIG. 13 shows a Call-Cole plot of Example 3-1 to Example 3-2 obtained by impedance measurement. In addition, the call-call plot about Comparative Example 1-1 is also shown in FIG. 13 for comparison. Table 3 shows the total resistance value (the total resistance value of the lithium ion conductor and the LiOH aqueous solution) determined based on the Cole-Cole plot, and the evaluation based on the total resistance value.

As shown in FIG. 12, the surface condition of the alkali-treated LATP of Example 3-2 was a fine granular product of 100 nm or less and a several hundred nm granular product that the product was considered to be aggregated. And were confirmed. The product is believed to _Li 3 PO _4, by adding in advance to an aqueous LiOH solution is dip the _Li 3 PO _4, it is possible to prevent the _PO ^{4 3-} elution from LATP in aqueous LiOH, LATP surface It is considered that Li ₃ PO ₄ could be formed efficiently.

Furthermore, the corrosion depth of the lithium ion conductor of Example 3-2 is as shallow as about 300 nm, and it is compared with the lithium ion conductor of Example 3-1 using an immersion liquid to which Li ₃ PO ₄ is not added. The alkali-treated layer could be formed with a thickness of about 1/3.

Furthermore, as shown in Table 3, the total resistance value of the impedance for the lithium ion conductor of Example 3-2 is lower than the total resistance value of the impedance for the lithium ion conductor of Example 3-1. It was found that the addition of Li ₃ PO ₄ to the immersion liquid LiOH aqueous solution is a more preferable method due to the formation of a thin alkali-resistant coating (alkali-treated layer) on the LATP surface.

(4) Alkali Tolerance Evaluation Next, lithium ion conductors of Example 4-1 to Example 4-3 and Comparative Example 4-1 were produced as follows.

Example 4-1
The lithium ion conductor of Example 4-1 was produced in the same manner as in Example 1-3.

Example 4-2
A lithium ion conductor of Example 4-2 was produced in the same manner as in Example 3-2.

Comparative Example 4-1
A lithium ion conductor was produced in the same manner as in Example 4-1 except that the saturated LiOH aqueous solution was used instead of the saturated LiOH aqueous solution and LiCl was added to 10 mol / L.

Comparative Example 4-2
The LATP thin plate not immersed in the LiOH aqueous solution was used as a lithium ion conductor of Comparative Example 4-2.

(Alkali resistance evaluation)
After immersion in a 1 mol / L aqueous solution of NaOH for 24 hours at room temperature to confirm the alkali resistance of the alkali-treated layer, the surface shape change of the lithium ion conductor (LATP) was observed using an optical microscope.

  The observation results of the lithium ion conductors of Example 4-1, Example 4-2, and Comparative Example 4-1 are shown in FIGS. 14A to 14C, and the evaluation results are shown in Table 4.

As shown in FIG. 14C, the untreated LATP not subjected to the alkali treatment (Comparative Example 4-2) was corroded to such an extent that the plate shape could not be maintained and became powder. In addition, although not shown in the photograph, in Comparative Example 4-1, LATP was similarly pulverized. On the other hand, the lithium ion conductor of Example 4-2 using the lithium ion conductor of Example 4-1 using a saturated LiOH aqueous solution as the immersion liquid and the LiOH aqueous solution containing 5% by mass Li ₃ PO ₄ , Could hold the plate shape. Incidentally, as shown in FIG. 14A, the lithium ion conductor of Example 4-1 is shown in FIG. 14B while a crack can be confirmed in the places shown by arrows a1 to a3 on the surface. Thus, the lithium ion conductor of Example 4-2 did not show any cracks on its surface, and it could be confirmed that a dense alkali resistant film (alkali treated layer) could be formed.

<Modification>
As mentioned above, although each embodiment and each example of the present invention were explained concretely, the present invention is not limited to each above-mentioned embodiment and each example, and various kinds based on the technical idea of the present invention Variations of are possible.

  For example, the configurations, methods, processes, shapes, materials, numerical values, and the like described in the above-described embodiments and examples are merely examples, and different configurations, methods, processes, shapes, materials, and the like may be used as necessary. You may use a numerical value etc.

  In addition, the configurations, methods, processes, shapes, materials, numerical values, and the like of the above-described embodiments and examples can be combined with one another without departing from the spirit of the present invention.

DESCRIPTION OF SYMBOLS 11 lithium ion conductive glass ceramics, 12 alkali treatment layer 21 negative electrode 22 reaction prevention layer 23 negative electrode protective layer 24 water-based electrolyte layer 25 positive electrode 31 catalyst layer 32 gas diffusion layer

Claims (8)

Li _{1 + x + y} Al _x (Ti _z Ge _1-z ) _2-x Si _y P _3-y O ₁₂ (wherein 0 ≦ x ≦ 0.8, 0 ≦ y ≦ 0.6, 0 <z ≦ 1) Containing lithium ion conductive glass-ceramics consisting of LATP,
It has an alkali treatment layer containing a crystal phase of LATP and a crystal phase of Li ₃ PO _{4 on} the surface of the lithium ion conductive glass ceramic.
Lithium ion conductor. In the lithium ion conductor according to claim 1,
The crystal phase of the Li ₃ PO ₄ is contained between at least a part of the crystal phase of the LATP inside the alkali treatment layer and on the crystal phase of the LATP exposed to the surface.
Lithium ion conductor. In the lithium ion conductor according to claim 1 or 2,
It has a void formed at least in part between crystal phases of the LATP inside the alkali treatment layer,
Lithium ion conductor. Li _{1 + x + y} Al _x (Ti _z Ge _1-z ) _2-x Si _y P _3-y O ₁₂ (wherein 0 ≦ x ≦ 0.8, 0 ≦ y ≦ 0.6, 0 <z ≦ 1) By immersing lithium ion conductive glass-ceramics composed of LATP represented by L.) in an aqueous solution of lithium hydroxide, the crystal phase of LATP and Li ₃ PO _{4 on} the surface of the lithium ion conductive glass-ceramics. Forming an alkali treatment layer containing a crystal phase,
The manufacturing method of a lithium ion conductor whose density | concentration of the said lithium hydroxide of the said lithium hydroxide aqueous solution is higher than 0.021 mol / L. In the method for producing a lithium ion conductor according to claim 4,
The method for producing a lithium ion conductor, wherein the time for immersing the lithium ion conductive glass ceramic in the aqueous solution is 2 weeks or more. In the method for producing a lithium ion conductor according to claim 4 or 5,
The method for producing a lithium ion conductor, wherein the lithium hydroxide aqueous solution further comprises a phosphate. In the method for producing a lithium ion conductor according to claim 6,
The method for producing a lithium ion conductor, wherein the phosphate is Li ₃ PO ₄ . A negative electrode containing metallic lithium,
An air electrode supplied with air;
An electrolyte comprising a water-based electrolyte solution;
An anode protective layer interposed between the anode and the electrolyte;
The negative electrode protective layer is
Li _{1 + x + y} Al _x (Ti _z Ge _1-z ) _2-x Si _y P _3-y O ₁₂ (wherein 0 ≦ x ≦ 0.8, 0 ≦ y ≦ 0.6, 0 <z ≦ 1) Containing lithium ion conductive glass-ceramics consisting of LATP,
A lithium ion conductor comprising an alkali-treated layer containing a crystal phase of LATP and a crystal phase of Li ₃ PO _{4 on} the surface of the lithium ion conductive glass ceramic facing the electrolyte.
Lithium air battery.