JP5755604B2 - Carbon material for air electrode for metal air battery, air electrode for metal air battery and metal air battery - Google Patents

Description

  The present invention relates to a carbon material constituting an air electrode of a metal-air battery, a metal-air battery air electrode, and a metal-air battery.

An air battery using oxygen as a positive electrode active material has advantages such as high energy density, easy size reduction and weight reduction. Therefore, it is attracting attention as a high-capacity battery that exceeds the lithium secondary battery that is currently widely used. As an air battery, metal air batteries, such as a lithium air battery, a magnesium air battery, and a zinc air battery, are known, for example.
The metal-air battery can be charged and discharged by performing an oxygen redox reaction at the air electrode and a metal redox reaction at the negative electrode. For example, in a metal air battery (secondary battery) in which conductive ions are monovalent metal ions, the following charge / discharge reaction is considered to proceed. In the following formula, M represents a metal species.

[During discharge]
Negative electrode: M → M ⁺ + e ⁻
Positive electrode: 2M ⁺ + O ₂ + 2e ⁻ → M ₂ O ₂
[When charging]
Negative electrode: M ⁺ + e ⁻ → M
Positive electrode: M ₂ O ₂ → 2M ⁺ + O ₂ + 2e ⁻

  A metal-air battery generally has a structure comprising an air electrode containing a carbon material and a binder, a negative electrode containing a negative electrode active material (metal, alloy, etc.), and an electrolyte interposed between the air electrode and the negative electrode. Have. The metal-air battery usually further includes an air electrode current collector that collects the air electrode and a negative electrode current collector that collects the negative electrode.

The air electrode is generally formed mainly of a carbon material. Specifically, Patent Document 1 includes a positive electrode mainly composed of a carbonaceous material having a pore volume of 1.0 ml / g or more, and a negative electrode active material that absorbs and releases metal ions. A non-aqueous electrolyte battery comprising a negative electrode and a non-aqueous electrolyte layer sandwiched between the positive electrode and the negative electrode is disclosed.
Patent Document 2 discloses a nonaqueous electrolyte air battery including a positive electrode, a negative electrode, a nonaqueous electrolyte, and a case having an air hole for supplying oxygen to the positive electrode. As the carbonaceous material to be used, ketjen black, acetylene black and the like are used.
Patent Document 3 discloses a structure in which a gas diffusion oxygen electrode mainly composed of carbon is used as a positive electrode, metallic lithium or a lithium-containing material is used as a negative electrode, and a nonaqueous electrolyte is disposed between the positive electrode and the negative electrode. A lithium air battery having In the example of Patent Document 3, ketjen black is used as the carbon constituting the positive electrode.

JP 2002-15737 A JP 2006-286414 A JP 2011-14478 A

  However, as disclosed in Patent Documents 1 to 3, a conventional metal-air battery using a carbon material such as ketjen black has a problem that a sufficient discharge voltage is not obtained.

  The present invention has been accomplished in view of the above circumstances, and an object of the present invention is to provide a carbon material for an air electrode that can improve the discharge voltage of a metal-air battery by reducing the discharge overvoltage. That is.

The carbon material for an air electrode of the present invention constitutes the air electrode of a metal-air battery having an air electrode using oxygen as an active material, a negative electrode, and an electrolyte layer interposed between the air electrode and the negative electrode. Carbon material
In the steam activation treatment, the carbon material as a raw material is obtained by heating at a heating rate of 5 ° C./min, and the oxygen content on the surface is 7 atomic% or more.
According to the carbon material for an air electrode of the present invention, the discharge overvoltage of the metal-air battery can be reduced, and the discharge voltage of the metal-air battery can be improved.

The carbon material for an air electrode of the present invention is preferably heated to 800 ° C. in the steam activation treatment.
When the carbon material for an air electrode of the present invention reaches 800 ° C. in the steam activation treatment, it is preferable to supply 10 ml / min of water vapor to 50 g of the carbon material as the raw material.
The carbon material for an air electrode of the present invention preferably supplies the water vapor for 15 minutes or more in the water vapor activation treatment.
Carbon material for the air electrode of the present invention is preferably a pore volume is 1.5 ml / g or more.
In the present invention, the oxygen content is preferably 20 atomic% or less.

An air electrode for a metal-air battery of the present invention is an air electrode constituting a metal-air battery having an air electrode using oxygen as an active material, a negative electrode, and an electrolyte layer interposed between the air electrode and the negative electrode. However, the carbon material for an air electrode according to the present invention is included.
The metal-air battery of the present invention is a metal-air battery having an air electrode using oxygen as an active material, a negative electrode, and an electrolyte layer interposed between the air electrode and the negative electrode. The electrode includes the carbon material for an air electrode of the present invention.
The metal-air battery of the present invention is a lithium-air battery, and the negative electrode of the lithium-air battery preferably includes lithium metal, an alloy material containing a lithium element, or a lithium compound as a negative electrode active material.

  According to the present invention, it is possible to improve the discharge voltage characteristics of a metal-air battery.

It is a cross-sectional schematic diagram which shows one example of a metal air battery. It is a figure which shows the manufacturing process flow of the air electrode in an Example and a comparative example. It is a graph which shows the relationship between the surface oxygen content of the carbon material of an Example and a comparative example, and discharge voltage.

[Carbon material for air electrode]
The carbon material for an air electrode of the present invention constitutes the air electrode of a metal-air battery having an air electrode using oxygen as an active material, a negative electrode, and an electrolyte layer interposed between the air electrode and the negative electrode. Carbon material
The surface oxygen content is 7 atomic% or more.

The carbon material of the present invention constitutes an air electrode of a metal-air battery. Hereinafter, a metal air battery including an air electrode containing the carbon material of the present invention will be described with reference to FIG. 1 by taking a lithium air battery as an example.
In the metal-air battery 9 shown in FIG. 1, an air electrode (positive electrode) 1 using oxygen as an active material, a negative electrode 2 made of metal (for example, Li metal), and metal ions (for example, between the air electrode 1 and the negative electrode 2) The electrolyte layer 3 responsible for the conduction of (Li ions) is accommodated in a battery case composed of an air electrode can 5 and a negative electrode can 6. The air electrode can 5 and the negative electrode can 6 are fixed by a gasket 7, and the inside of the battery case is sealed.

  The air electrode 1 is an oxygen redox reaction field, and is supplied with air (oxygen) taken from an air hole 8 provided in the air electrode can 5. The air electrode 1 contains the carbon material of the present invention and a binder (for example, polytetrafluoroethylene). The air electrode 1 is provided with an air electrode current collector 4 for collecting the air electrode 1. The air electrode current collector 4 is made of a conductive material (for example, a metal mesh) having a porous structure, and air (oxygen) taken in from the air holes 8 passes through the air current collector 4 to form an air electrode. 1 can be supplied.

  The negative electrode 2 is made of a metal (for example, Li metal) that is an electrode active material. That is, the negative electrode 2 can occlude / release metal ions (Li ions) which are conductive ion species.

The electrolyte layer 3 is composed of a supporting electrolyte salt (for example, a Li salt such as LiN (SO ₂ CF ₃ ) ₂ ) and a nonaqueous solvent (for example, N, N-diethyl-N-methyl-N- (2-methoxyethyl) ammonium). An electrolyte solution dissolved in bis (trifluoromethanesulfonyl) amide). A separator having an insulating property and a porous structure is arranged between the air electrode 1 and the negative electrode 2 (not shown), and the electrolyte is impregnated in the porous portion of the separator.

  In the present invention, the metal-air battery has an air electrode using oxygen as an active material, a negative electrode, and an electrolyte layer interposed between the air electrode and the negative electrode. And is not particularly limited as long as it can be charged and discharged by performing a metal redox reaction at the negative electrode. For example, lithium-air battery, sodium-air battery, potassium-air battery, magnesium-air battery, calcium Examples include an air battery, an aluminum air battery, a zinc air battery, and an iron air battery.

  Conventionally, powdery carbon materials such as ketjen black and acetylene black have been used as carbon materials forming the air electrode. Although surface structures such as specific surface area and pore volume of powdered carbon materials suitable as air electrode forming materials have been studied, improvement of air electrode characteristics by surface modification of carbon materials has been studied in the past. It wasn't.

The present inventors performed a surface modification treatment on a powdery carbon material conventionally used as a carbon material constituting the air electrode, and examined the relationship between the oxygen content on the surface and the air electrode characteristics. However, it has been found that the discharge overvoltage can be reduced by using a carbon material having an oxygen content of 7 atomic% or more.
The reason why the discharge overvoltage of the metal-air battery can be reduced by forming the air electrode using a carbon material having an oxygen content of 7 atomic% or more on the surface is presumed as follows.
That is, it is considered that the content of oxygen atoms on the surface of the carbon material is derived from the properties (functional groups) on the surface of the carbon material. As the oxygen atom content on the surface of the carbon material increases, the surface properties of the carbon material change, and it is presumed that the discharge overvoltage decreases due to improvement in wettability with the electrolytic solution and increase in active sites.

  The oxygen content (atomic%) on the surface of the air electrode carbon material of the present invention (hereinafter sometimes simply referred to as “carbon material”) is CHNOS commonly used (carbon, hydrogen, nitrogen, oxygen, sulfur). It can be measured by elemental analysis.

  The oxygen content (hereinafter sometimes referred to as surface oxygen content) on the surface of the carbon material of the present invention is not particularly limited as long as it is 7 atomic% or more, but is preferably 20 atomic% or less.

A carbon material having a surface oxygen content of 7 atomic% or more can be produced by, for example, a steam activation method. Specific production methods by the steam activation method include the following methods. For example, there is a method in which a carbon material put into a rotary heat treatment is heated to 800 ° C. in an argon atmosphere, and when reaching 800 ° C., water vapor is supplied. The oxygen content on the surface can be controlled by adjusting the supply rate and supply time of the water vapor. The temperature increase rate at the time of a heating is not specifically limited, For example, it can be set to 5 degrees C / min. Moreover, supply of water vapor | steam can be performed at 10 ml / min with respect to 50 g of carbon materials, for example. Examples of the carbon material to be subjected to the steam activation treatment include those conventionally used as conductive carbon materials constituting the air electrode, for example, carbon blacks such as ketjen black and acetylene black, and carbon materials such as carbon nanotubes. .
The method for controlling the surface oxygen content of the carbon material is not limited to the above method, and for example, a method of subjecting the carbon material as a raw material to acid treatment can also be mentioned.
The carbon material whose oxygen content is controlled is preferably subjected to pulverization. This is because agglomeration of the carbon material is suppressed and a carbon material having a small particle size distribution can be obtained. A general method can be adopted as the pulverization treatment.

The carbon material preferably has a pore volume of 1.5 ml / g or more. The pore volume of the carbon material can be measured by, for example, a known N ₂ gas adsorption method such as the BJH method.

Although the shape of the carbon material of the present invention is not particularly limited, it is preferably in the form of particles since it can be highly dispersed in the slurry when it is slurried in the air electrode manufacturing process.
The size of the particulate carbon material is not particularly limited, but for example, the average primary particle diameter is preferably 100 nm or less. The average primary particle diameter of the particulate carbon material can be measured by an observation image of a transmission electron microscope. Specifically, the diameter of the primary particles of the observed image can be measured, and the calculated average value can be used as the average primary particle diameter. The magnification of the transmission electron microscope is desirably, for example, 400,000 times or more.

As described above, the carbon material of the present invention can be used as a material constituting the air electrode of a metal-air battery. Hereinafter, taking the lithium air battery as an example, the metal-air battery air electrode of the present invention including the carbon material and the metal-air battery of the present invention including the air electrode including the carbon material will be described.
[Air electrode for metal-air battery]
An air electrode for a metal-air battery of the present invention is an air electrode constituting a metal-air battery having an air electrode using oxygen as an active material, a negative electrode, and an electrolyte layer interposed between the air electrode and the negative electrode. And the carbon material for air electrodes of the said invention is included, It is characterized by the above-mentioned.
The air electrode contains at least the carbon material of the present invention, and may further contain other components such as a binder as necessary. The air electrode can be provided with an air electrode current collector that normally collects the air electrode.
The carbon material of the present invention is the same as that described in the above [Carbon material for air electrode], and the description thereof is omitted here.

  The content of the carbon material of the present invention in the air electrode is not particularly limited. For example, when the mass of the entire air electrode is 100 wt%, it is preferably 10 to 99 wt%, and particularly 50 to 95 wt%. Preferably there is. When the content ratio of the carbon material is less than 10 wt%, the reaction field at the air electrode is reduced, which may cause a reduction in battery capacity.

  The air electrode preferably contains a binder that immobilizes the carbon material. Examples of the binder include rubber resins such as polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), and styrene-butadiene rubber (SBR rubber). The content ratio of the binder in the air electrode is not particularly limited, but when the mass of the entire air electrode is 100 wt%, it is preferably less than 40 wt%, and preferably 1 to 20 wt%. More preferred.

The air electrode may further contain a conductive material other than the carbon material, a catalyst, or the like as necessary. Examples of the conductive material include carbon blacks such as ketjen black and acetylene black, carbonaceous materials such as carbon nanotubes, and conductive materials other than the above carbon materials, such as conductive polymers such as polythiazyl and polyacetylene. Examples of the catalyst include a platinum group such as nickel, palladium and platinum; a perovskite oxide containing a transition metal such as cobalt, manganese or iron; an inorganic compound containing a noble metal oxide such as ruthenium, iridium or palladium; Examples thereof include metal coordination organic compounds having a skeleton or a phthalocyanine skeleton; inorganic ceramics such as manganese dioxide (MnO ₂ ) and cerium oxide (CeO ₂ ); and composite materials obtained by mixing these materials.

  The thickness of the air electrode varies depending on the use of the metal-air battery, but is preferably 2 μm to 500 μm, particularly preferably 5 μm to 300 μm.

The air electrode current collector collects the air electrode current. The material for the air electrode current collector is not particularly limited as long as it has conductivity, and examples thereof include stainless steel, nickel, aluminum, iron, titanium, and carbon. Examples of the shape of the air electrode current collector include a foil shape, a plate shape, and a fiber shape, and a porous shape such as a nonwoven fabric and a mesh (grid). Among these, a porous current collector is preferable from the viewpoint of high oxygen supply performance and excellent current collection efficiency.
When a porous current collector is used, the air electrode and the current collector may be laminated as shown in FIG. 1, or the current collector may be disposed inside the air electrode. Moreover, the battery case mentioned later may have the function of the air electrode current collector.
The thickness of the air electrode current collector is, for example, preferably in the range of 10 μm to 1000 μm, and more preferably in the range of 20 μm to 400 μm.

  The method for producing the air electrode is not particularly limited. For example, the air electrode paste mixed with at least the carbon material of the present invention and, if necessary, the binder is applied to the surface of the air electrode current collector and dried. Thus, an air electrode can be formed on the air electrode current collector. Alternatively, the air electrode paste can also be formed on the air electrode current collector by rolling the air electrode paste on the air electrode current collector and drying it. In addition, the air electrode obtained by applying or rolling the air electrode paste and drying it is overlapped with the air electrode current collector, and appropriately subjected to pressurization, heating, etc. It is also possible to laminate the electric body.

The solvent for the air electrode paste is not particularly limited as long as it has volatility, and can be appropriately selected. Specific examples include ethanol, acetone, N, N-dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP) and the like.
The method for applying the air electrode paste is not particularly limited, and general methods such as a doctor blade and a spray method can be used.

[Metal-air battery]
The metal-air battery of the present invention is a metal-air battery having an air electrode using oxygen as an active material, a negative electrode, and an electrolyte layer interposed between the air electrode and the negative electrode, wherein the air electrode The carbon material for an air electrode according to the present invention is included.
The carbon material of the present invention and the air electrode including the carbon material are the same as those described in the above [Carbon material for air electrode] and [Air electrode for metal-air battery], and thus the description thereof is omitted here. To do.
(Negative electrode)
The negative electrode is a layer capable of releasing / capturing metal ions (for example, Li ions), and usually contains a negative electrode active material capable of releasing / capturing metal ions (for example, Li ions). The negative electrode may include a negative electrode current collector that collects current from the negative electrode, if necessary.
Examples of the negative electrode active material include metals such as Li, Na, K, Mg, Ca, Al, Zn, and Fe, or compounds such as oxides, sulfides, and nitrides of these metals, or carbon materials. .

Specifically, as a negative electrode active material of a lithium air battery, for example, a known negative electrode active material such as lithium metal, an alloy material containing a lithium element, a lithium compound (for example, oxide, sulfide, nitride, etc.) Can be used.
Examples of the alloy containing lithium element include a lithium aluminum alloy, a lithium tin alloy, a lithium lead alloy, and a lithium silicon alloy. Examples of the lithium compound include oxides such as lithium titanium oxide, nitrides such as lithium cobalt nitride, lithium iron nitride, and lithium manganese nitride.

  The negative electrode may contain only the negative electrode active material, or may contain at least one of a conductive material and a binder in addition to the negative electrode active material. For example, when the negative electrode active material has a foil shape, a negative electrode containing only the negative electrode active material can be obtained. On the other hand, when the negative electrode active material is in a powder form, a negative electrode containing a negative electrode active material and a binder can be obtained. Note that the conductive material and the binder are the same as those described in the above-mentioned section “Air electrode for metal-air battery”, and thus the description thereof is omitted here.

  The material for the negative electrode current collector is not particularly limited as long as it has conductivity, and examples thereof include copper, stainless steel, nickel, carbon, etc. Among them, stainless steel and nickel are preferable. Examples of the shape of the negative electrode current collector include a foil shape, a plate shape, and a mesh (grid) shape. Moreover, the battery case mentioned later may have the function of the negative electrode collector.

  The manufacturing method of a negative electrode is not specifically limited. For example, a method may be mentioned in which a foil-like negative electrode active material and a negative electrode current collector are superposed and pressurized as necessary. Another method includes preparing a negative electrode material mixture containing a negative electrode active material and a binder, applying or rolling the mixture onto a negative electrode current collector, and drying the mixture.

(Electrolyte layer)
The electrolyte layer is held between the air electrode and the negative electrode, and has a function of exchanging metal ions between the air electrode and the negative electrode. The electrolyte layer is not particularly limited as long as it can conduct metal ions (typically, metal ions derived from the negative electrode active material), such as an electrolytic solution, a solid electrolyte, and a gel electrolyte. You may combine electrolyte solution, gel electrolyte, and solid electrolyte.
The electrolytic solution is obtained by dissolving an electrolyte salt in a solvent, and either a non-aqueous electrolytic solution obtained by dissolving an electrolyte salt in a non-aqueous solvent or an aqueous electrolytic solution obtained by dissolving an electrolyte salt in an aqueous solvent may be used. Since the side reaction can be suppressed, the solvent of the electrolytic solution preferably has high oxygen radical resistance.
Hereinafter, the electrolyte layer will be described taking an electrolyte having lithium ion conductivity as an example.

The non-aqueous electrolyte contains a lithium salt and a non-aqueous solvent.
Examples of the lithium salt include inorganic lithium salts such as LiPF ₆ , LiBF ₄ , LiClO _4, and LiAsF ₆ ; LiCF ₃ SO ₃ , LiN (SO ₂ CF ₃ ) ₂ [abbreviation Li-TFSA], LiN (SO ₂ C ₂ F ₅ ) Organic lithium salts such as ₂ and LiC (SO ₂ CF ₃ ) ₃ can be mentioned.
Examples of the non-aqueous solvent include ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethyl carbonate, butylene carbonate, γ-butyrolactone, sulfolane, Examples thereof include acetonitrile, 1,2-dimethoxymethane, 1,3-dimethoxypropane, diethyl ether, tetrahydrofuran, 2-methyltetrahydrofuran, and mixtures thereof.
Moreover, an ionic liquid can also be used as a non-aqueous solvent. Examples of the ionic liquid include N, N, N-trimethyl-N-propylammonium bis (trifluoromethanesulfonyl) amide [abbreviation: TMPA-TFSA], N-methyl-N-propylpiperidinium bis (trifluoromethanesulfonyl). ) Amide [abbreviation: PP13-TFSA], N-methyl-N-propylpyrrolidinium bis (trifluoromethanesulfonyl) amide [abbreviation: P13-TFSA], N-methyl-N-butylpyrrolidinium bis (trifluoromethanesulfonyl) ) Aliphatic quaternary ammonium such as amide [abbreviation: P14-TFSA], N, N-diethyl-N-methyl-N- (2-methoxyethyl) ammonium bis (trifluoromethanesulfonyl) amide [abbreviation: DEME-TFSA] Salt, 1-ethyl-3-methyl Dazo potassium fluorohydrocarbon oxygenate (trifluoromethanesulfonyl) amide _{[abbreviation: emim (HF) 2,3 F-} TFSA] , and the like.
The concentration of the lithium salt in the non-aqueous electrolyte solution can be set in the range of 0.5 mol / L to 3 mol / L, for example.

  A non-aqueous gel electrolyte can be obtained by adding a polymer to the non-aqueous electrolyte and gelling. Examples of the polymer used for gelation of the non-aqueous electrolyte include polyethylene oxide (PEO), polyacrylonitrile (PAN), and polymethyl methacrylate (PMMA).

Examples of the aqueous electrolyte include an aqueous electrolytic solution in which lithium salt is contained in water. Examples of the lithium salt include LiOH, LiCl, LiNO ₃ , and CH ₃ CO ₂ Li.

  As the solid electrolyte, for example, a Li—La—Ti—O based solid electrolyte can be used.

(Separator)
In order to ensure the insulation between the air electrode and the negative electrode, a separator made of an insulating porous material can be disposed between the air electrode and the negative electrode. Typically, by impregnating a separator made of an insulating porous body with an electrolyte, insulation between the air electrode and the negative electrode and metal ion conductivity can be ensured.
Moreover, when taking the structure which laminates | stacks the layered structure arrange | positioned in order of an air electrode-electrolyte layer-negative electrode repeatedly, it is between the air electrode and negative electrode which belong to a different laminated body from a safety viewpoint. It is preferable to have a separator.
Examples of the separator include porous films such as polyethylene and polypropylene; and nonwoven fabrics such as a resin nonwoven fabric and a glass fiber nonwoven fabric.

(Battery case)
A metal-air battery usually has a battery case that houses an air electrode, a negative electrode, an electrolyte layer, and the like. Specific examples of the shape of the battery case include a coin type, a flat plate type, a cylindrical type, and a laminate type. As long as oxygen can be supplied to the air electrode, the battery case may be an open type having holes (air holes) that can take in oxygen from the outside, or may be a sealed type.
The open battery case has a structure in which at least the air electrode can sufficiently contact the oxygen-containing gas. Further, an oxygen permeable film or a water repellent film may be provided in the air hole. On the other hand, the sealed battery case is preferably provided with an oxygen-containing gas introduction pipe and an exhaust pipe. Examples of the oxygen-containing gas supplied to the air electrode include pure oxygen and air, and pure oxygen is preferred because it is preferable to have a high oxygen concentration. The air is preferably dry air with low humidity.

[Example 1]
(Production of carbon material for air electrode)
Ketjen black (ECP600JD) was subjected to a steam activation treatment as follows to control the oxygen content on the surface. That is, first, the ketjen black put into the rotary heat treatment was heated to 800 ° C. at a heating rate of 5 ° C./min in an argon atmosphere. When the temperature reached 800 ° C., water vapor was supplied to 50 g of the ketjen black at 10 ml / min for 15 minutes.

<Measurement of surface oxygen content>
The oxygen content on the surface of the carbon material obtained by the steam activation treatment was measured by CHNOS elemental analysis. The results are shown in Table 1.
<Measurement of pore volume>
The pore volume of the carbon material obtained by the steam activation treatment was evaluated by an N ₂ gas adsorption method (BJH method) using a pore distribution measuring device (Belsorb-max, manufactured by Nippon Bell).
The results are shown in Table 1.

(Production of air electrode)
Using the obtained carbon material, an air electrode was produced according to the flow shown in FIG. That is, first, the carbon material was mixed with polytetrafluoroethylene (PTFE, binder) in ethanol (solvent) to prepare an air electrode slurry. Carbon material: PTFE = 90 wt%: 10 wt% in the air electrode slurry.
The air electrode slurry was applied on a substrate and then rolled with a roll press. Next, the rolled body was cut and dried. The obtained air electrode was peeled from the substrate. A mesh made of SUS304 was attached to the air electrode as a current collector.

(Evaluation of air electrode)
Using the air electrode produced above, a lithium air battery test cell was produced as follows.
First, metallic lithium (made by Honjo Metal, thickness 250 μm, φ20 mm) was prepared as a negative electrode. A SUS304 plate was attached to the negative electrode as a current collector.
Moreover, the separator made from porous polyolefin was prepared as a separator.
On the other hand, Li-TFSA (manufactured by Kishida Chemical Co., Ltd.) and DEME-TFSA were mixed so that the LiTFSA concentration was 0.35 mol / kg, and stirred overnight in an argon atmosphere to prepare an electrolytic solution.
Next, an air electrode with a current collector, a separator, and a negative electrode with a current collector were accommodated in a test cell (manufactured by Nippon Tom Cell Co., Ltd.), and a lithium air battery test cell having the structure shown in FIG. 1 was produced. . The separator and the air electrode were impregnated with an electrolytic solution.

  The obtained lithium-air battery test cell was accommodated in a glass desiccator (500 mL) with a gas replacement cock. The glass desiccator has a structure in which oxygen can be introduced and oxygen can be supplied to the air electrode through the air hole of the test cell.

A discharge test was conducted under the following conditions to evaluate discharge overvoltage characteristics. The results are shown in FIG.
<Discharge test>
・ Charge / discharge test equipment: Multi-channel potentiostat / galvanostat (manufactured by Bio-Logic)
・ Discharge current density: 0.1 mA / cm ²
・ Atmosphere temperature (temperature in glass desiccator): 60 ° C. (3 hours in a thermostatic bath before starting the test)
-In-cell pressure: 1 atmosphere of O _2- Reading point of discharge overvoltage: discharge voltage value at 200 mAh / g

[Example 2]
(Production of carbon material for air electrode)
Except for changing the steam supply time to 30 minutes, a steam activation treatment was performed in the same manner as in Example 1 to prepare a carbon material.
The oxygen content and pore volume on the surface of the obtained carbon material were determined in the same manner as in Example 1. The results are shown in Table 1.

(Production and evaluation of air electrode)
Using the obtained carbon material, an air electrode was produced in the same manner as in Example 1, and the air electrode was evaluated. The results are shown in FIG.

[Comparative Example 1]
(Production of carbon material for air electrode)
The ketjen black used in Example 1 was directly prepared as a carbon material without performing the steam activation treatment.
The oxygen content and pore volume on the surface of the carbon material were determined in the same manner as in Example 1. The results are shown in Table 1.

(Production and evaluation of air electrode)
Using the obtained carbon material, an air electrode was produced in the same manner as in Example 1, and the air electrode was evaluated. The results are shown in FIG.

As shown in Table 1 and FIG. 3, the air electrode of Example 1 and Example 2 containing a carbon material having a surface oxygen content of 7 atomic% or more is Comparative Example 1 having a surface oxygen content of less than 7 atomic%. The discharge voltage at 200 mAh / g was higher than that.
Also, when the surface oxygen content is in the range up to about 7 atomic%, the discharge voltage tends to increase when the surface oxygen content is increased. When the surface oxygen content is in the range of 7 atomic% or more, the discharge voltage is reduced. There was a tendency for the increase to decrease and to saturate. That is, when the surface oxygen content of the carbon material is in the range of up to about 7 atomic%, there is a change in which the discharge overvoltage decreases when the surface oxygen content is increased, and in the range of 7 atomic% or more, the reduction amount of the discharge overvoltage. The tendency to decrease and saturate was confirmed.
From the above results, the discharge overvoltage can be reduced by using a carbon material having a surface oxygen content of 7 atomic% or more as a material constituting the air electrode of the metal-air battery. It can be seen that the discharge voltage characteristics can be improved.

DESCRIPTION OF SYMBOLS 1 ... Air electrode 2 ... Negative electrode 3 ... Electrolyte layer 4 ... Air electrode current collector 5 ... Air electrode can 6 ... Negative electrode can 7 ... Gasket 8 ... Air hole 9 ... Air metal battery

Claims (9)

A carbon material constituting the air electrode of a metal-air battery comprising an air electrode having oxygen as an active material, a negative electrode, and an electrolyte layer interposed between the air electrode and the negative electrode,
A carbon material for an air electrode obtained by heating a carbon material as a raw material at a heating rate of 5 ° C./min in a steam activation treatment, and having a surface oxygen content of 7 atomic% or more . The carbon material for an air electrode according to claim 1, wherein the carbon material is heated to 800 ° C. in the steam activation treatment. The carbon material for an air electrode according to claim 2, wherein when the temperature reaches 800 ° C in the steam activation treatment, 10 ml / min of steam is supplied to 50 g of the carbon material as the raw material. The carbon material for an air electrode according to claim 3, wherein the water vapor is supplied for 15 minutes or more in the water vapor activation treatment. The carbon material for an air electrode according to any one of claims 1 to 4 , wherein the pore volume is 1.5 ml / g or more. The carbon material for an air electrode according to any one of claims 1 to 5 , wherein the oxygen content is 20 atomic% or less. An air electrode constituting a metal-air battery having an air electrode using oxygen as an active material, a negative electrode, and an electrolyte layer interposed between the air electrode and the negative electrode,
An air electrode for a metal-air battery, comprising the carbon material for an air electrode according to any one of claims 1 to 6 . A metal-air battery comprising an air electrode having oxygen as an active material, a negative electrode, and an electrolyte layer interposed between the air electrode and the negative electrode,
A metal-air battery, wherein the air electrode includes the carbon material for an air electrode according to any one of claims 1 to 7 . The metal air battery is a lithium air battery,
The metal-air battery according to claim 8, wherein the negative electrode of the lithium-air battery includes, as a negative electrode active material, lithium metal, an alloy material containing a lithium element, or a lithium compound.

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