JP7249629B2 - Air electrode for lithium-air battery, lithium-air battery, and method for producing air electrode for lithium-air battery - Google Patents

Description

TECHNICAL FIELD The present disclosure relates to air electrodes for lithium-air batteries, lithium-air batteries, and methods of manufacturing air electrodes for lithium-air batteries.

In recent years, in the field of secondary batteries, lithium-air batteries have been studied as batteries having advantages such as high energy density and easy miniaturization and weight reduction. Since lithium-air batteries have significantly higher energy density than lithium-ion batteries, they are expected to be applied to various energy industries, such as electric vehicles that can travel long distances.

A lithium-air battery is a secondary battery that uses oxygen in the air as a positive electrode active material and a material capable of intercalating and deintercalating lithium ions as a negative electrode active material. Reactions at the positive electrode (air electrode) and the negative electrode during discharge of the lithium-air battery are shown below.
Positive electrode: 2Li ⁺⁺ O ₂ + 2e ⁻ → Li ₂ O ₂
Negative electrode: 2Li → 2Li ⁺ + 2e ^-

Also, reactions of the positive electrode (air electrode) and the negative electrode during charging of the lithium-air battery are shown below.
Positive electrode: Li ₂ O ₂ → O ₂ + 2Li ⁺ + 2e ⁻
Negative electrode: 2Li ⁺ + 2e ^- → 2Li

On the other hand, lithium-air batteries have several problems such as electrolyte decomposition and durability due to the generation of superoxide anion radicals (O ₂ ⁻ .) in the positive electrode, and methods to solve these problems are being explored. It is For example, Patent Document 1 discloses a lithium-air battery in which the air electrode of a metal-air battery contains a lithium salt, which is a supporting electrolyte salt. In this method, the electrolyte salt is contained not only in the electrolyte solution but also in the air electrode, thereby increasing the metal ion concentration of the air electrode, suppressing the decomposition of the electrolyte solution by O ₂ ⁻ , and improving the durability of the battery. ing.

Further, lithium peroxide (Li ₂ O ₂ ) deposited on the air electrode by the discharge reaction is decomposed by depriving electrons during charging. At this time, since lithium peroxide is decomposed from the surface of the air electrode, there is a problem that the voltage for decomposing lithium peroxide increases as the distance from the air electrode increases, that is, the overvoltage increases. On the other hand, in Non-Patent Document 1, in a lithium-air battery, an air electrode with a porous three-dimensional network structure is adopted, and LiI, which is a redox mediator (RM), is contained in the electrolyte solution. , the diffusion of each chemical species involved in the charge-discharge reaction from the electrolyte to the air electrode is improved, and LiI efficiently acts to achieve both energy efficiency and capacity characteristics (durability).

WO2012/025975

Kisuk Kang et. al., Superior Rechargeability and Efficiency of Lithium-Oxygen Batteries: Hierarchical Air Electrode Architecture Combined with a Soluble Catalyst, Angew. Chem. Int. Ed., 2014, 53, No.15, 3926-3931.

However, there are still challenges in the energy efficiency and capacity maintenance of lithium-air batteries. For example, in the method described in Patent Document 1, the deterioration of the battery is suppressed by suppressing side reactions such as electrolyte decomposition due to O ₂ ⁻ ·, but the overvoltage in the lithium peroxide decomposition reaction during charging cannot be reduced. energy efficiency remains low. In addition, in the method described in Non-Patent Document 1, the catalytic effect of the oxidation-reduction mediator is not sufficiently efficiently exhibited, and there is room for improvement in energy efficiency and capacity retention.

In view of such circumstances, an object of the present disclosure is to provide a lithium-air battery capable of reducing overvoltage and having improved capacity retention, an air electrode used therein, and a method for manufacturing the same.

Means for solving the above problems include the following aspects.
<1> An air electrode for a lithium-air battery containing a redox mediator.
<2> The air electrode for a lithium-air battery according to <1>, wherein the redox mediator exists in a solid state at -40 to 80°C.
<3> The air electrode for a lithium air battery according to <1> or <2>, wherein the redox mediator contains at least one selected from the group consisting of nitrite, bromide and iodide salts.
<4> The redox mediator is selected from the group consisting of LiNO ₂ , NaNO ₂ , KNO ₂ , RbNO ₂ , CsNO ₂ , LiBr, NaBr, KBr, RbBr, CsBr, LiI, NaI, KI, RbI, and CsI. The air electrode for a lithium-air battery according to <3>, including at least one of
<5> A lithium-air battery comprising the air electrode for a lithium-air battery according to any one of <1> to <4>, a negative electrode, and an electrolyte present between the air electrode and the negative electrode.
<6> The lithium according to <5>, wherein the electrolyte is present in an electrolytic solution, and the content of the oxidation-reduction mediator in the air electrode is an amount corresponding to 10 mmol or more per 1 L of the electrolytic solution. air battery.
<7> The lithium-air battery according to <5> or <6>, wherein the electrolyte is present in an electrolytic solution, and the concentration of the redox mediator in the electrolytic solution is 100 mmol/L or less.
<8> Mixing a redox mediator, a conductive material, a binder, and a solvent to prepare a slurry, and applying the slurry to a current collector to form a catalyst layer. A method for manufacturing an air electrode for a lithium-air battery, comprising:

INDUSTRIAL APPLICABILITY According to the present disclosure, a lithium-air battery capable of reducing overvoltage and having improved capacity retention, an air electrode used therein, and a method for manufacturing the same are provided.

1 is a schematic perspective view of one embodiment of a lithium-air battery of the present disclosure; FIG. 1 shows potential curves up to 15 cycles when a constant-current discharge-charge cycle test is performed using the lithium-air batteries of Examples 1 to 3 and Comparative Examples 1 and 2. FIG. 1 shows the capacity retention up to a set discharge/charge capacity when a constant-current discharge/charge cycle test is performed using the lithium-air batteries of Examples 1 to 3 and Comparative Examples 1 and 2. FIG. 1 shows potential curves up to 15 cycles when a constant-current discharge-charge cycle test is performed using the lithium-air batteries of Example 4 and Comparative Example 3. FIG. 2 shows capacity retention up to a set discharge-charge capacity when a constant-current discharge-charge cycle test is performed using the lithium-air batteries of Example 4 and Comparative Example 3. FIG.

DETAILED DESCRIPTION OF THE INVENTION Embodiments for carrying out the present invention will be described in detail below. However, the present invention is not limited to the following embodiments. In the following embodiments, the constituent elements (including element steps and the like) are not essential unless otherwise specified. The same applies to numerical values and their ranges, which do not limit the present invention.

In the present disclosure, the numerical range indicated using "-" includes the numerical values before and after "-" as the minimum and maximum values, respectively.
In the numerical ranges described step by step in the present disclosure, the upper limit or lower limit of one numerical range may be replaced with the upper or lower limit of another numerical range described step by step. . Moreover, in the numerical ranges described in the present disclosure, the upper or lower limits of the numerical ranges may be replaced with the values shown in the examples.
In the present disclosure, each component may contain multiple types of applicable substances. When there are multiple types of substances corresponding to each component in the composition, the content rate or content of each component is the total content rate or content of the multiple types of substances present in the composition unless otherwise specified. means quantity.

≪Air electrode for lithium-air battery≫
The air electrode for a lithium-air battery of the present disclosure (hereinafter also simply referred to as air electrode) includes a redox mediator. The air electrode may contain a current collector, and may contain a conductive material, a binder, a catalyst other than a redox mediator, and the like.

The thickness of the air electrode is not particularly limited. Also, the width and length of the air electrode are not particularly limited.

The method of manufacturing the air electrode is not particularly limited. For example, the air electrode is prepared by preparing a slurry containing a redox mediator and other components used as necessary, applying the slurry to a current collector, and drying as necessary to form a catalyst layer. It may be made by In one embodiment, the air electrode is prepared by mixing a redox mediator, a conductive material, a binder, and a solvent to prepare a slurry, and applying the slurry to a current collector to obtain a catalyst. and forming a layer.
Each member or component used in the air electrode will be described below.

<Redox Mediator>
The cathode of the present disclosure includes a redox mediator. The redox mediator may be contained in other parts such as the electrolyte in addition to the air electrode.

In the present disclosure, the redox mediator has an oxidant and a reductant, has an oxidation potential higher than the equilibrium potential (2.96 V) of lithium peroxide, and the oxidant oxidatively decomposes lithium peroxide to Li represents a chemical species that has the ability to generate ⁺ . The redox mediator is first oxidized during charging, and the generated oxidant attacks the surface of lithium peroxide to catalyze the decomposition of lithium peroxide and regenerate itself. As a result, the charging potential during charging is maintained near the potential of the oxidation-reduction mediator, so that the increase in overvoltage is suppressed and the charging reaction tends to proceed stably. Oxidation potentials of various chemical species are described in various documents and can be appropriately selected by those skilled in the art.

The decomposition-promoting reaction of lithium peroxide during charging by a redox mediator will be described by taking LiI as an example of the redox mediator. During charging, the iodide ion is first oxidized to triiodide ion as follows. The oxidant triiodide ion then reacts with lithium peroxide to decompose it and reduce itself back to iodide ion.
3I ⁻ → I ₃ ⁻ + 2e ⁻
I ₃ ⁻ + Li ₂ O ₂ → 3I ⁻ + 2Li ⁺⁺ O ₂

Various researches have so far been conducted on the use of redox mediators dissolved in electrolytes in lithium-air batteries. On the other hand, in the lithium-air battery of the present disclosure, the oxidation-reduction mediator is present in the air electrode, and is abundant in the reaction field where lithium peroxide precipitates. It is believed that the catalysis by the redox mediator is efficiently exhibited. It is presumed that this makes it possible to suppress the increase in overvoltage for a long period of time.

When the redox mediator is dissolved in the electrolyte and used, the redox mediator oxidized at the air electrode during charging diffuses into the negative electrode without decomposing the lithium peroxide, and reacts directly to cause self-discharge. A so-called shuttle effect may occur. On the other hand, when the air electrode contains a redox mediator, the influence of the shuttle effect is suppressed, and it is thought that the capacity retention can be further improved.

The redox mediator is preferably present at the cathode in a solid state at 25°C. In particular, the redox mediator is preferably present at the cathode in a solid state over -40°C to 80°C. In the present disclosure, when "the redox mediator is present in the air electrode in a solid state", at least a portion of the redox mediator may be present in a solid state, and a portion may be in a dissolved state. may For example, when a lithium-air battery is used, 30 mol% or more, preferably 40 mol% or more, more preferably 50 mol% or more, and still more preferably 60 mol% or more of the redox mediator contained in the lithium-air battery is , may be present in the air electrode in a solid state. When the redox mediator is present in the air electrode in a solid state, the effects of reducing overvoltage and improving capacity retention tend to be obtained more favorably. It is believed that when the redox mediator is present in the cathode in a solid state, a higher concentration of the redox mediator can come into contact with the lithium peroxide than when it is present in a dissolved state in the electrolyte. For this reason, it is considered that a particularly good catalytic effect for decomposing lithium peroxide can be obtained. It is also believed that the shuttle effect is better suppressed.

The oxidation potential of the redox mediator need only exceed the equilibrium potential of lithium peroxide. As a result, it can be oxidized prior to lithium peroxide during charging and can function as a redox mediator. From the viewpoint of effectively suppressing an increase in overvoltage, the oxidation potential of the redox mediator is preferably more than 2.96 V, which is the theoretical value of the equilibrium potential of lithium peroxide, and 4.00 V or less. It is more preferably over 96V and 3.80V or less, and more preferably over 2.96V and 3.50V or less.

When the redox mediator is solid, the shape of the redox mediator is not particularly limited, and may be powder (particulate, granular, etc.), gel, sheet, porous, or the like. When the oxidation-reduction mediator is in the form of powder (particulates, etc.), porous, etc., the surface area tends to be increased, and a good oxidation-reduction mediation effect can be obtained. When the redox mediator is in powder form (particulate form), the particle size is not particularly limited.

The redox mediator may be an organic salt or an inorganic salt. Examples of inorganic salts include nitrites, bromide salts, iodide salts and the like. More specifically, LiNO ₂ , NaNO ₂ , KNO ₂ , RbNO ₂ , CsNO ₂ , LiBr, NaBr, KBr, RbBr, CsBr, LiI, NaI, KI, RbI, CsI and the like. Organic salts include tetrathiafulvalene (TTF), ferrocene, 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), tetramethyl-p-phenylenediamine (TMPD), 5 , 10-dimethylphenazine (DMPZ), 1,5-naphthalenediamine (NDA), 4,N,N-trimethylaniline (TMA), or 1-phenylpyrrolidine (PPD), N-methylphenothiazine (MPT), 2, 5-di-t-butyl-1,4-benzquinone (DBBQ) and the like. The redox mediators may be used singly or in combination of two or more.

The redox mediator content in the air electrode is not particularly limited. When an air electrode is produced by forming a catalyst layer on an air electrode current collector using a slurry containing a redox mediator, the solid content of the slurry (i.e., components obtained by removing volatile components from the slurry; catalyst The content of the redox mediator in the layer) is preferably 5% to 80% by mass, more preferably 10% to 70% by mass, from the viewpoint of the balance of properties of the redox mediator and other components. is more preferable, and 15% by mass to 60% by mass is even more preferable.

<Air electrode current collector>
The air electrode may include an air electrode current collector. The material of the air electrode current collector is not particularly limited as long as it has conductivity, and examples thereof include metals such as nickel and aluminum, alloys such as stainless steel, and carbon materials. The air electrode current collector may be used alone or in combination of two or more.

The shape of the air electrode current collector is not particularly limited, and may be sheet-like, plate-like, mesh-like, fiber-like, or the like. The air electrode current collector may or may not have a porous structure, and preferably has a porous structure from the viewpoint of increasing reaction fields and improving charge/discharge efficiency.

<Conductive material>
The air electrode may contain a conductive material in addition to the air electrode current collector. The conductive material may function as a conductive aid or may function as a carrier for the catalyst. Examples of conductive materials include carbon black such as ketjen black and acetylene black, natural graphite, graphite, carbon nanotube (CNT), graphene, and the like. The conductive material may be used singly or in combination of two or more. The shape of the conductive material may be, for example, particulate, flat, fibrous, or the like. The conductive material may or may not have a porous structure, and preferably has a porous structure from the viewpoint of increasing reaction fields and improving charge/discharge efficiency.

The content of the conductive material in the catalyst layer in the air layer is not particularly limited. When an air electrode is produced by forming a catalyst layer on an air electrode current collector using a slurry containing a redox mediator and a conductive material, the amount of the conductive material with respect to the solid content of the slurry (catalyst layer) The content of is preferably 20% by mass to 80% by mass, more preferably 30% by mass to 70% by mass, from the viewpoint of the balance of properties due to various components contained in the slurry, and 40% by mass to It is more preferably 60% by mass.

<Catalysts other than redox mediators>
The cathode may contain a catalyst other than the redox mediator. Catalysts other than redox mediators are not particularly limited as long as they promote charge-discharge reactions. For example, a catalyst that lowers the activation energy of lithium peroxide decomposition during charge, and a catalyst that lowers the activation energy of lithium peroxide formation during discharge.
Specific examples of catalysts include noble metals such as platinum and gold; metals such as manganese, cobalt, nickel, iron, ruthenium, and iridium, and metal oxides or metal complexes thereof; and the materials described above as conductive materials. . These catalysts may be used alone or in combination of two or more.
The shape of the catalyst other than the redox mediator is not particularly limited, and may be nanoparticles, nanowires, nanoflakes, nanorods, nanosheets, porous bodies, or the like.

When the air electrode contains a catalyst other than the redox mediator, the content of the catalyst other than the redox mediator is not particularly limited. When an air electrode is produced by forming a catalyst layer on an air electrode current collector using a slurry containing a redox mediator and other catalysts, the solid content (catalyst layer) of the slurry is subjected to redox The content of the catalyst other than the mediator may be in the range of 5% to 60% by mass, or 10% to 50% by mass, from the viewpoint of suitably exhibiting the effect of promoting the discharge and charge reaction by the catalyst. may be in the range of

<Binder>
The air electrode may further contain a binder. When the air electrode contains a binder, the redox mediator and other components that are used as necessary are well fixed to the air electrode, which tends to further improve the discharge and charge efficiency. The type of binder is not particularly limited, and fluorine-based resins such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and fluororubber; thermoplastic resins such as polypropylene, polyethylene, and polyacrylonitrile; styrene-butadiene rubber ( rubber such as SBR), polyimide resin, and the like. One type of binder may be used alone, or two or more types may be used in combination.

When the air electrode contains a binder, the content of the binder in the air electrode is not particularly limited. When an air electrode is produced by forming a catalyst layer on an air electrode current collector using a slurry containing a redox mediator and a binder, the binder for the solid content (catalyst layer) of the slurry The content is preferably 1% by mass to 20% by mass, preferably 2% by mass to 15% by mass, from the viewpoint of the balance of the properties and dispersibility of various components contained in the slurry, and 3% by mass. % to 10 mass %.

<Solvent>
When an air electrode is produced by preparing a slurry containing a redox mediator and applying it to a current collector, the slurry may contain a solvent. The solvent is not particularly limited as long as it can disperse or dissolve each component contained in the slurry, and may be an aqueous solvent or an organic solvent. Examples of solvents include N-methylpyrrolidone, dimethylacetamide, ethanol, propanol, isopropanol, 1-methoxy-2-propanol, acetone, methylethylketone, methylisobutylketone, dimethylformamide, cyclohexanone, dioxolane, and water. A solvent may be used individually by 1 type, or may use 2 or more types together.

The content of the solvent in the slurry is not particularly limited, and is preferably 60% by mass to 95% by mass relative to the mass of the slurry from the viewpoint of viscosity, uniformity of application, and the like.

≪Lithium air battery≫
The lithium-air battery of the present disclosure comprises the air electrode of the present disclosure described above, a negative electrode, and an electrolyte present between the air electrode and the negative electrode. A lithium-air battery may further have other members such as a separator.

A specific example of a lithium-air battery in one embodiment of the present disclosure will be described with reference to FIG. Note that the lithium-air battery of the present disclosure is not limited to the configuration shown in FIG. In addition, the sizes of the members in FIG. 1 are conceptual, and the relative relationship between the sizes of the members is not limited to this.
FIG. 1 depicts one embodiment of a lithium-air battery during charging. In FIG. 1 , a lithium-air battery 100 has an air electrode 2 and a negative electrode 4 , and an electrolytic solution 6 containing an electrolyte is stored between the air electrode 2 and the negative electrode 4 . A separator 8 is provided between the air electrode 2 and the negative electrode 4 to provide insulation between the two electrodes. The positive electrode electrolyte present on the air electrode side of the separator 8 and the negative electrode electrolyte present on the negative electrode side of the separator 8 may be the same electrolyte or different electrolytes. During charging, the power source 10 electrically connects the air electrode 2 and the negative electrode 4 . A load (not shown) is connected instead of the power supply 10 during discharging.

In the lithium-air battery of the present disclosure, the redox mediator may be present in the air electrode or dissolved in the electrolyte. When the redox mediator is dissolved in the electrolyte, the concentration of the redox mediator in the electrolyte may be 100 mmol/L or less, or 50 mmol/L, from the viewpoint of suitably suppressing the shuttle effect. L or less, or 30 mmol/L or less. The redox mediator may have desirable properties such as dendrite generation suppression and decomposition of lithium oxide that may accumulate on the negative electrode. From the viewpoint of expecting such properties, the concentration of the redox mediator in the electrolytic solution may be, for example, 1 mmol/L or more. Moles or concentrations of redox mediators in this disclosure refer to moles or molarities as salts when the redox mediator can be present as an inorganic or organic salt.

The total content of redox mediators in the lithium-air battery (ie, the total amount of redox mediators present in the solid state and dissolved in the electrolyte) is not particularly limited. From the viewpoint of satisfactorily exhibiting the effect of the redox mediator, the content of the redox mediator in the lithium-air battery is preferably an amount corresponding to 10 mmol or more, preferably 50 mmol or more, per 1 L of the electrolytic solution. A corresponding amount is more preferable, and an amount corresponding to 100 mmol or more is even more preferable. The upper limit of the content of the redox mediator is not particularly limited, and from the viewpoint of securing an appropriate reaction field, the content of the redox mediator may be an amount equivalent to 2000 mmol or less per 1 L of the electrolytic solution. good.

In the lithium-air battery, when the electrolyte is present in the electrolytic solution, the solubility of the redox mediator in the electrolytic solution is not particularly limited. There may be. In the lithium-air battery of the present disclosure, since the oxidation-reduction mediator is present in the air electrode, even if the solubility in the electrolyte is relatively low, it is thought that the catalytic action is efficiently exhibited.

Each member or component of the lithium-air battery will be described below.

<Air electrode>
The details of the air electrode used in the lithium-air battery of the present disclosure are as described above.

<Negative Electrode>
A lithium-air battery includes a negative electrode. The negative electrode includes a negative electrode active material capable of intercalating and deintercalating lithium ions. The negative electrode may be formed by forming a layer containing a negative electrode active material on a negative electrode current collector.

Examples of negative electrode active materials include metallic lithium, lithium alloys, metallic oxides, metallic sulfides, metallic nitrides, and carbon materials. Lithium alloys include alloys of aluminum, tin, silicon and the like with lithium. Metal oxides include oxides of tin, silicon, and the like. Metal sulfides include sulfides of tin and titanium. Metal nitrides include nitrides of lithium manganese, lithium cobalt, and the like. Carbon materials include graphite, coke, spherical carbon, and the like. A negative electrode active material may be used individually by 1 type, or may use 2 or more types together. From the viewpoint of battery capacity, it is preferable to use metallic lithium as the negative electrode active material.

The material of the negative electrode current collector is not particularly limited as long as it has conductivity, and examples thereof include metals such as nickel and copper, alloys such as stainless steel, and carbon materials. The shape of the negative electrode current collector is not particularly limited, and may be sheet-like, plate-like, mesh-like, fiber-like, or the like. The negative electrode current collector may or may not have a porous structure.

The thickness of the negative electrode is not particularly limited, and is preferably 10 μm or more, more preferably 50 μm or more, and even more preferably 100 μm or more from the viewpoint of battery capacity. From the viewpoint of miniaturization, it is preferably 500 μm or less, more preferably 300 μm or less, and even more preferably 200 μm or less. The width and length of the negative electrode are not particularly limited.

The method for producing the negative electrode is not particularly limited. For example, as the negative electrode, a sheet-like, plate-like negative electrode active material may be used as it is. In addition, for the negative electrode, the negative electrode active material and optionally used conductive materials, binders, and other components are mixed to prepare a negative electrode mixture slurry, the slurry is applied to a current collector, and dried. , and optionally compacting. The details of the conductive substance and binder that may be contained in the negative electrode can be applied to the details of the conductive substance and binder that may be contained in the air electrode. A solvent for dispersing or dissolving each component may be used to prepare the slurry. Specific examples of the solvent include those described above as the solvent used for preparing the positive electrode slurry. Also, the negative electrode may be produced by press-bonding a sheet-like, plate-like negative electrode active material to a negative electrode current collector.

<Electrolyte>
A lithium-air battery of the present disclosure includes an electrolyte. The electrolyte may exist in the form of an electrolytic solution in which an electrolyte salt is dissolved in a non-aqueous solvent, or may exist in the form of a solid electrolyte. An electrolytic solution and a solid electrolyte may be used in combination.

Any electrolyte can be used as long as it can conduct lithium ions between the air electrode and the negative electrode, and a lithium salt can be used. The lithium salt may be an organic lithium salt or an inorganic lithium salt. Examples of inorganic lithium salts include LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiClO ₄ , LiNO ₃ and the like. Examples of organic lithium salts include lithium bistrifluoromethanesulfonylimide (LiTFSI), lithium bisfluoromethanesulfonylimide (LiFSI), lithium trifluoromethanesulfonate (LiOTf), and the like. One type of electrolyte may be used alone, or two or more types may be used in combination.

The type of solvent for the electrolyte is not particularly limited as long as it can dissolve the electrolyte.
Non-aqueous solvents include ethers such as diethylene glycol dimethyl ether, diethylene glycol diethyl ether, triethylene glycol dimethyl ether, triethylene glycol diethyl ether, tetraethylene glycol dimethyl ether, and tetraethylene glycol diethyl ether; ethylene carbonate, propylene carbonate, dimethyl carbonate, ethyl methyl Carbonates such as carbonate and diethyl carbonate; esters such as methyl acetate, ethyl acetate, methyl propionate and ethyl propionate; ionic liquids, dimethoxyethane, dimethylsulfoxide, ethylmethylsulfone, sulfolane and the like. Among them, the group consisting of ethers such as diethylene glycol dimethyl ether, diethylene glycol diethyl ether, triethylene glycol dimethyl ether, triethylene glycol diethyl ether, tetraethylene glycol dimethyl ether, and tetraethylene glycol diethyl ether from the viewpoint of decomposition stability during discharge and charge. At least one more selected is preferable. Solvents for the electrolyte may be used singly or in combination of two or more.

Solid electrolytes include Li ₇ La ₃ Zr ₂ O ₁₂ (LLZ), LISICON (Lithium Super Ionic Conductor), and the like.

When the electrolyte is present in the electrolytic solution, the concentration of the electrolyte in the electrolytic solution is not particularly limited, and from the viewpoint of the balance of the electric conductivity, viscosity, etc. of the electrolytic solution, it is 0.05 mol / L to 2.00 mol / L. , more preferably 0.10 mol/L to 1.50 mol/L, even more preferably 0.15 mol/L to 1.00 mol/L.

<Separator>
A lithium-air battery may include a separator. The separator is not particularly limited as long as it has lithium ion permeability and can maintain insulation between the air electrode and the negative electrode. Examples of materials for the separator include polyolefin polymers such as polyethylene and polypropylene, cellulose polymers, fluorine polymers, glass, and paper. The shape of the separator may be a porous membrane, a non-woven fabric, or the like.

The thickness of the separator is not particularly limited, and may be, for example, 10 μm to 1000 μm.

[Applications of lithium-air batteries]
Applications of the lithium-air battery of the present disclosure are not particularly limited, and include driving power sources for electric vehicles, stationary storage batteries for next-generation power grids (smart grids), power leveling in power generation facilities using natural energies such as solar and wind power. Alternatively, it can be applied to the energy industry in general, such as storage batteries and mobile large-capacity batteries.

EXAMPLES Next, the present invention will be specifically described with reference to Examples, but the present invention is not limited to these Examples.

<Example 1>
[Production of air electrode]
LiI (redox mediator), Ketjenblack (KB: conductive material), and polyvinylidene fluoride (PVDF: binder) were dissolved in N-methylpyrrolidone (NMP: solvent) at a mass ratio of 50:50. :6 to prepare a catalyst layer slurry for an air electrode. The prepared slurry was applied to the surface of carbon paper (air electrode current collector) and dried to obtain an air electrode with a catalyst layer formed thereon.

[Fabrication of lithium-air battery]
An electrolytic solution (concentration: 0.2 mol/L) was prepared by dissolving LiTFSI (electrolyte salt) in diethylene glycol dimethyl ether (solvent) in an argon glove box (dew point <−90° C.). Using a metal lithium foil (thickness: 500 μm) as the negative electrode, the prepared electrolyte, a separator (Celgard 2400 (thickness: 25 μm)), the electrolyte, and the air electrode obtained above are sandwiched in this order to produce a lithium-air battery. bottom.

<Example 2>
An air electrode and a lithium-air battery were produced in the same manner as in Example 1, except that LiBr (redox mediator) was used instead of LiI.

<Example 3>
An air electrode and a lithium-air battery were produced in the same manner as in Example 1, except that LiI was changed to NaNO ₂ (redox mediator).

<Comparative Example 1>
An air electrode and a lithium-air battery were produced in the same manner as in Example 1, except that LiI was not used.

<Comparative Example 2>
An air electrode and a lithium-air battery were produced in the same manner as in Example 1, except that LiI was changed to LiCl. LiCl is a lithium salt that does not function as a redox mediator.

<Comparative Example 3>
A lithium-air battery was fabricated in the same manner as in Comparative Example 1, except that LiBr (oxidation-reduction mediator) was further added to the electrolytic solution so as to have a concentration of 50 mmol/L.

<Example 4>
LiBr in the same amount as LiBr contained in the electrolytic solution in Comparative Example 3 was contained in the air electrode in a solid state instead of being contained in the electrolytic solution, thereby producing a lithium-air battery.

Tables 1 and 2 show outlines of the configurations of the lithium-air batteries of Examples 1 to 4 and Comparative Examples 1 to 3. "-" in the table indicates that it does not correspond to each item.

[Evaluation of lithium-air battery]
Using the lithium-air batteries obtained in Examples and Comparative Examples, in a pure oxygen (99.9% by volume) atmosphere at 30° C., an applied current of 200 mA/g-KB, a discharge charge capacity of 500 mAh/g-KB, A constant-current discharge-charge cycle test was conducted under the conditions of a cut-off potential of 2.0V to 4.5V. The evaluation results are shown in FIGS. 2 to 5. FIG.

FIG. 2 shows potential curves up to 15 cycles when a constant current discharge/charge cycle test was performed using the lithium-air batteries of Examples 1 to 3 and Comparative Examples 1 and 2. FIG.
The overvoltage during discharge and during charge is a potential difference from the theoretical potential of the following reaction (ie, 2.96 V (V vs. Li/Li ⁺ )), and the smaller the better.
During discharge: 2Li ⁺⁺ O ₂ + 2e ⁻ → Li ₂ O ₂
During charging: Li ₂ O ₂ → 2Li ⁺⁺ O ₂ + 2e ⁻
As shown in FIG. 2, in the lithium-air batteries of Examples 1 to 3, in which the redox mediator was contained in the air electrode, the overvoltage was reduced compared to the lithium-air batteries of Comparative Examples 1 and 2. .

FIG. 3 shows capacity retention up to a set discharge/charge capacity when a constant current discharge/charge cycle test was performed using the lithium-air batteries of Examples 1 to 3 and Comparative Examples 1 and 2. FIG. The higher the number of cycles in which the capacity up to 500 mAh/g-KB is maintained, the better.
As shown in FIG. 3, in the lithium-air batteries of Examples 1-3, compared with the lithium-air batteries of Comparative Examples 1 and 2, the capacity retention is improved.

FIG. 4 shows a constant-current discharge-charge cycle test using the lithium-air batteries of Example 4, in which the redox mediator was contained in the air electrode, and Comparative Example 3, in which the electrolyte solution contained the same amount of the redox mediator. shows the potential curve up to 15 cycles when performing As shown in FIG. 4 , the overvoltage is reduced in Example 4 compared to Comparative Example 3. Further, in Comparative Example 3, a shuttle effect was observed in which the redox mediator contained in the electrolytic solution diffused to the negative electrode to cause self-discharge, but in Example 4, the shuttle effect was suppressed.

FIG. 5 shows capacity retention up to a set discharge/charge capacity when the lithium-air batteries of Example 4 and Comparative Example 3 were subjected to a constant current discharge/charge cycle test. As shown in FIG. 5, in the lithium-air battery of Example 4, compared with the lithium-air battery of Comparative Example 3, the capacity retention is improved.

As described above, in the lithium-air batteries of Examples, the overvoltage is reduced in long-term charge-discharge cycles, and excellent capacity retention is exhibited. Although the mechanism of this action is not necessarily clear, it is presumed as follows.

In the method of dissolving the redox mediator in the electrolyte, the amount of the redox mediator supplied to the air electrode depends on the adsorption equilibrium between the redox mediator and the air electrode and the diffusion rate in the electrolyte. On the other hand, when the redox mediator is included in the air electrode in advance, it is presumed that a large amount of the redox mediator is present in the reaction field, and its effect is more efficiently exhibited. be done.

In addition, when the lithium peroxide deposited during discharge is decomposed by charging, the redox mediator is released not only from the electrolyte side (i.e., the outer surface of lithium peroxide) but also from the air electrode side (i.e., lithium peroxide). It is suggested that it also functions from the inside) and that there is also an effect of accelerated decomposition from both.

In addition, by including the oxidation-reduction mediator in the air electrode, the oxidation-reduction mediator covers the catalyst layer and the current collector of the air electrode, and the superoxide anion radicals (O ₂ ⁻ .) generated during charging and discharging process the catalyst layer. It is also expected that the current collector can be prevented from being attacked and corroded.

Furthermore, it is expected that the deposition of lithium peroxide on the surface of the air electrode is made uniform, and that clogging due to non-uniform deposition is alleviated.

Furthermore, when the redox mediator is included in the cathode, it is possible to include a larger amount of the redox mediator regardless of the solubility of the redox mediator in the electrolyte. For example, the content of the redox mediator in Example 1 is about 190 mmol/L in terms of the volume of the electrolytic solution. Therefore, it is considered possible to obtain a highly sustained catalytic effect.

As described above, in the lithium-air batteries of the examples, the oxidation-reduction mediator functions locally and effectively in the vicinity of the air electrode, so that the overvoltage can be effectively reduced and excellent capacity retention can be exhibited. Conceivable.

2 positive electrode (air electrode)
4 Negative Electrode 6 Electrolyte 8 Separator 10 Power Supply 100 Lithium Air Battery

Claims (8)

at _least one redox mediator selected from the group consisting of LiNO2, NaNO2, KNO2, RbNO2, CsNO2, LiBr, NaBr, KBr, RbBr, CsBr, LiI , NaI _, KI _, RbI _, and _CsI ; wherein said at least one redox mediator is present in the lithium air battery cathode in a solid state at -40 to 80°C. Lithium air according to claim 1, comprising a catalyst layer containing the at least one redox mediator, wherein the content of the at least one redox mediator with respect to the catalyst layer is 5 wt% to 80 wt%. Air electrode for batteries. 3. The lithium-air battery of claim 1 or 2, wherein the selected at least one redox mediator comprises at least one selected from the group consisting of _LiNO2 , _NaNO2 , LiBr, NaBr, LiI, NaI. air electrode. an air electrode for a lithium-air battery according to any one of claims 1 to 3 ;
a negative electrode;
an electrolyte present between the air electrode and the negative electrode;
Lithium air battery with. the electrolyte is present in an electrolytic solution,
5. The lithium air battery according to claim 4 , wherein the content of said oxidation-reduction mediator in said air electrode is an amount corresponding to 10 mmol or more per 1 L of electrolytic solution. the electrolyte is present in an electrolytic solution,
6. The lithium-air battery according to claim 4 , wherein the concentration of said redox mediator in said electrolyte is 100 mmol/L or less. at least one redox mediator selected from the group consisting of LiNO2, NaNO2, KNO2, RbNO2, CsNO2, LiBr, NaBr, KBr, RbBr, CsBr, LiI , NaI _, KI _, RbI _, and _{CsI ;} , mixing a conductive material, a binder, and a solvent to prepare a slurry;
applying the slurry to a current collector to form a catalyst layer;
wherein said at least one redox mediator becomes present in said catalyst layer in a solid state at -40 to 80°C. 8. The method for producing an air electrode for a lithium air battery according to claim 7, wherein the content of said at least one redox mediator with respect to the solid content of said slurry is 5% by mass to 80% by mass.

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