JP7486129B2 - Method for producing porous carbon membrane for positive electrode of air battery, and method for producing air battery using the porous carbon membrane obtained by the method as the positive electrode - Google Patents

Description

The present invention relates to a method for producing a porous carbon membrane for use as a positive electrode in an air battery, and a method for producing an air battery using the porous carbon membrane obtained by this method as the positive electrode.

Batteries have been attracting attention as the driving force behind a smart society, and demand for them is increasing rapidly. There are many different types of batteries, but air batteries are attracting a lot of attention because of their small size, light weight, and large capacity.
An air battery is a battery that uses oxygen in the air as the positive electrode active material and a metal as the negative electrode active material, and is also called a metal-air battery and is positioned as a type of fuel cell. One example is a lithium-air battery that uses a metal or compound capable of absorbing and releasing lithium ions as the negative electrode active material. The reaction at each electrode in a lithium-air battery is expressed by the following formula.

In air batteries, the positive electrode active material is oxygen in the air, which can be taken in from outside the battery, so there is no need to place a source of positive electrode active material inside the battery, and the structure is, in principle, suitable for making the battery small and lightweight. Furthermore, being able to take in the positive electrode active material from outside also means that there is no limit to the amount of positive electrode active material that can be used, so the structure is, in principle, suitable for making the battery large capacity.

However, the current potential of air batteries in terms of their small size, light weight, and large capacity has not been fully realized. One of the reasons for this is that the discharge capacity of the positive electrode is not yet high enough.
In an air battery, oxygen in the air is the positive electrode active material, and the reaction at the positive electrode during discharge is that metal ions moving from the negative electrode combine with oxygen and electrons to produce an oxide. Therefore, in order to increase the capacity of an air battery, it is necessary that the metal ions moving from the negative electrode can easily move to every corner of the positive electrode (high ion transport efficiency), that air containing oxygen that reacts with the metal ions can easily penetrate and diffuse into the reaction field in the positive electrode (high oxygen permeability), that there are many reaction fields in the positive electrode where metal ions react with oxygen to produce an oxide, and that the pore volume for storing this oxide is large. In addition, in order to manufacture a small and lightweight air battery at low cost, it is desirable for the positive electrode to be self-supporting as a structure.

In response to these problems, Patent Document 1 proposes a porous carbon for use in a positive electrode, which has pores including mesopores and micropores, and the carbonaceous walls that form the outer periphery of the mesopores have a three-dimensional mesh structure, the micropores are formed in the carbonaceous walls, the mesopores are open pores, the pore portions are continuous and form connected pores, the half-width of the pore size distribution of the pores is 2 nm or less, the half-width of the connected pore size distribution of the connected pores is 50 nm or more, and the pore volume occupied by pores with a pore size of 1 nm or more is 1.0 ml/g or more and 4.0 ml/g or less (Claim 1). Here, it is described that the pore size means a value measured by a nitrogen adsorption method (paragraph [0021]), that a carbon material having many pores in the mesopore region with a pore size of 1 nm or more provides a high capacity as a positive electrode (paragraph [0023]), and that a paint in which a composition containing the porous carbon and a binder that fixes it is dispersed in a solvent can be applied to a positive electrode current collector, or that the composition can be molded by a pressure press to form a positive electrode layer (paragraphs [0033], [0035]).

Patent Document 2 proposes a lithium-air battery in which a first pore volume occupied by pores having a pore diameter of 1 nm or more and 200 nm or less in a positive electrode layer containing a conductive porous body containing carbon is larger than a second pore volume occupied by pores having a pore diameter of more than 200 nm and less than 10,000 nm (claim 1). It also describes that because lithium oxide generated during discharge is generated in the pores of the positive electrode layer, the more first pores with small pore diameters there are, the more small particles are generated, and the smaller the particles are, the more likely the decomposition reaction of lithium oxide that occurs during charging will occur throughout the battery rather than just on the surface, thereby improving the charge-discharge cycle characteristics (paragraph [0035]), and that in consideration of ensuring high capacity and improving charge-discharge cycle characteristics, the preferred range of the first pore volume is 0.1 to 2.4 cm ³ /g (paragraph [0037]). Furthermore, Patent Document 2 exemplifies, as a method for producing the positive electrode layer, a method for applying a coating material, in which a composition containing a conductive porous body and a binder, etc. is dispersed in a solvent, onto a positive electrode current collector by a doctor blade method or the like, or a method for molding the above-mentioned composition by a pressure pressing (paragraph [0042]).

Patent Document 3 proposes a porous carbon structure having specific pore structure and physical properties (claims 1 to 11), a manufacturing method for the porous carbon structure in which the porous carbon structure is carbonized in an oxidizing gas atmosphere having an oxygen concentration in the range of more than 0.03% and less than 5% using porous carbon particles and a polymeric material as raw materials (claims 13 to 15), and a battery including a positive electrode structure having the porous carbon structure (claim 22).

Patent No. 5852548 JP 2018-133168 A International Publication No. 2020/235638

Thus, various attempts have been made to increase the capacity of air batteries by improving the positive electrode, and certain successes have been achieved.
However, in the positive electrode described in Patent Document 1, which uses porous carbon in which the pore volume occupied by pores with a pore diameter of 1 nm or more is 1.0 ml/g or more and 4.0 ml/g or less, a binder is used, and therefore some of the pores in the porous carbon are filled with the binder, and the pore volume of the positive electrode becomes smaller than the pore volume of the porous carbon, so there is a limit to how much capacity can be increased.
Patent Document 2 describes that, in order to improve the charge/discharge cycle characteristics while ensuring a high capacity of the air battery, a conductive porous body containing carbon is used as the positive electrode layer, in which the volume of first pores having a pore size of 1 nm or more and 200 nm or less is larger than the volume of second pores having a pore size of more than 200 nm and 10,000 nm or less. However, the preferred range of the first pore volume is 0.1 to 2.4 ^cm3 /g, and in order to obtain an air battery with a larger discharge capacity, a positive electrode layer having an even larger first pore volume and a method for producing the same are desired.
The present inventors previously proposed to manufacture a porous carbon structure having a large pore volume suitable for the positive electrode of an air battery by performing a carbonization process in an oxidizing gas atmosphere having an oxygen concentration in the range of more than 0.03% and less than 5% (Patent Document 3). However, when oxygen with high oxidizing combustion power is used in the process, there is a risk that the porous film mainly composed of carbon particles will disappear due to oxidizing combustion depending on the process conditions, so it was necessary to manufacture the structure by delicately controlling the oxygen concentration and oxidation temperature within a low range.

The present invention aims to provide a method for producing a porous carbon membrane that does not use oxygen in the carbonization process and that provides high capacity when used in the positive electrode of an air battery, and a method for producing an air battery using the porous carbon membrane in the positive electrode.

In the course of further research into solving the above problems, the inventors discovered that even carbon dioxide, which is generally considered an inert gas, can slowly oxidize and burn a porous membrane made mainly of carbon particles at high temperatures. They then discovered that a porous carbon membrane with a large pore volume can be produced by heat treatment in a carbon dioxide atmosphere, and thus completed the present invention.

In order to solve the above problems, the present invention employs the following means.
[1] A method for producing a porous carbon membrane for an air battery positive electrode, comprising the steps of:
Preparing a mixture slurry containing porous carbon particles and a polymeric binder material;
Molding the mixture slurry;
immersing the sample obtained by the molding in a solvent in which the binder polymer material has a low solubility;
drying the sample obtained by immersion;
carbonizing the dried sample in an inert gas atmosphere; and
and heat-treating the sample obtained by the carbonization in a carbon dioxide atmosphere.
[2] The method for producing a porous carbon membrane according to [1] above, further comprising infusibly treating the sample obtained by drying in air prior to the carbonization.
[3] The method for producing a porous carbon membrane according to any one of [1] and [2] above, wherein the heat treatment in the carbon dioxide atmosphere is carried out until a pore volume occupied by pores having a diameter of 1 nm or more and 1000 nm or less in the porous carbon membrane becomes 1.02 times or more of the porous carbon particles used as a raw material.
[4] The method for producing a porous carbon membrane according to any one of [1] to [3] above, wherein the heat treatment in the carbon dioxide atmosphere is carried out at a temperature of 500° C. or higher and 1000° C. or lower.
[5] The method for producing a porous carbon membrane according to any one of [1] to [4], wherein the heat treatment in a carbon dioxide atmosphere is carried out in an atmosphere having a carbon dioxide concentration of 50% or more.
[6] The method for producing a porous carbon membrane according to any one of [1] to [5] above, wherein the porous carbon membrane satisfies all of the following conditions (a) to (d):
(a) the pore volume occupied by pores having a diameter of 1 nm or more and 1000 nm or less is 3.5 cm ³ /g or more and 7.0 cm ³ /g or less; (b) the pore volume occupied by pores having a diameter of 1 nm or more and 200 nm or less is 2.5 cm ³ /g or more and 7.0 cm ³ /g or less; (c) the pore volume occupied by pores having a diameter of 200 nm or more and 10,000 nm or less is 2.2 cm ³ /g or more and 7.0 cm ³ /g or less; and (d) the specific surface area occupied by pores having a diameter of 1 nm or more and 1000 nm or less is 900 m ² /g or more and 2000 m ² /g or less. [7] The method for producing a porous carbon membrane according to [6] above, wherein the porous carbon membrane further satisfies the following condition (e):
(e) an apparent density of 0.10 g/cm ³ or more and 0.20 g/cm ³ or less. [8] The method for producing a porous carbon membrane according to the above [6] or [7], wherein the porous carbon membrane further satisfies the following condition (f):
(f) a porosity of 90% or more and 99% or less. [9] A method for producing an air battery, comprising the steps of:
Producing a porous carbon membrane by the method for producing a porous carbon membrane according to any one of claims 1 to 8;
forming a positive electrode from the porous carbon membrane and preparing a negative electrode and a separator;
stacking the positive electrode and the negative electrode with a separator interposed therebetween; and filling the separator with an electrolyte.

According to the present invention, a porous carbon membrane for an air battery positive electrode having a large pore volume can be obtained, resulting in high oxygen permeability and high ion transport efficiency, and also providing many reaction sites and reactant storage spaces, making it possible to provide a small, lightweight air battery suitable for large capacity.

FIG. 1 is a flow chart showing a method for producing a porous carbon membrane according to one aspect of the present invention. A schematic cross-sectional view showing the structure of a coin cell, which is an example of an air battery equipped with a porous carbon membrane. FIG. 1 is a schematic cross-sectional view showing the structure of a stacked air battery, which is another example of an air battery equipped with a porous carbon membrane.

The configuration and effects of the present invention will be explained below with reference to the drawings, together with technical concepts. However, the mechanism of action includes assumptions, and the correctness of such assumptions does not limit the present invention. Furthermore, among the components in the following embodiments, those components that are not described in the claims that show the highest concept will be described as optional components. Furthermore, descriptions of numerical ranges (two numerical values connected with "~") are meant to include the numerical values described as the lower and upper limits.

[Method of manufacturing a porous carbon membrane for an air battery positive electrode]
One aspect of the present invention is a method for producing a porous carbon membrane for an air battery positive electrode. As shown in FIG. 1, an embodiment according to this aspect (hereinafter, sometimes referred to as "the present embodiment") includes preparing a mixture slurry containing porous carbon particles and a polymeric material for a binder (S1), molding the mixture slurry (S2), immersing the molded body in a solvent having a low solubility for the polymeric material for a binder (S3), drying the immersed molded body (S4), carbonizing the dried molded body in an inert gas atmosphere (S6), and heat-treating the carbonized molded body in a carbon dioxide atmosphere (S7). Note that, in this embodiment, the sample obtained in S4 may further be infusible in air (S5) before S6.
Each step will be described in detail below.

(S1: Preparation of mixture slurry)
The composite slurry contains porous carbon particles and a polymeric binder material.
As the porous carbon particles, carbon black such as Ketjen Black (registered trademark) and other carbon particles formed by a template method can be used.
As the polymeric material for the binder, polymeric materials such as polyacrylonitrile (PAN), polyvinylidene fluoride, polysulfone, and solvent-soluble polyimide can be used.
As a solvent for dispersing the porous carbon particles and the binder polymer, for example, dimethylsulfoxide (DMSO), N-methylpyrrolidone (NMP), dimethylformamide (DMF), dimethylacetamide (DMA), etc. can be used.
The mixture slurry may contain carbon fibers and/or carbon nanotubes. When the mixture slurry contains carbon fibers and/or carbon nanotubes, the strength of the resulting porous carbon membrane can be increased.
The carbon fiber may have a diameter of 0.1 μm to 20 μm and a length of 1 mm to 20 mm, and the carbon nanotube may have a diameter of 0.5 nm to 20 nm and a length of 1 μm to 20 μm.
The contents of the components mixed with the solvent when preparing the mixture slurry can be, for example, 50% by mass to 85% by mass of porous carbon particles, 5% by mass to 49% by mass of polymeric material for binder, 1% by mass to 15% by mass of carbon fibers, and 0% by mass to 10% by mass of carbon nanotubes. These are uniformly dispersed in the solvent to prepare the mixture slurry.

(S2: Molding)
The method for molding the mixture slurry is not limited as long as it can obtain a molded product of a predetermined size, and examples of the method include a wet film-forming method using a doctor blade, etc. In addition, a roll coater method, a die coater method, a spin coat method, a spray coating method, etc. may also be used.
The shape of the molded body after molding may be any shape depending on the purpose, but in order to reduce the size of the air battery, it is preferable to form the molded body in a sheet shape with a uniform thickness.

(S3: Solvent immersion)
The solvent immersion is performed by a non-solvent induced phase separation method in which the sample molded in step S2 is immersed in a solvent in which the binder polymer material has low solubility. The non-solvent induced phase separation method is a method in which a polymer solution is immersed in a poor solvent for the polymer, thereby causing the polymer to undergo phase separation and precipitation. By the non-solvent induced phase separation method, the binder polymer material undergoes phase separation and precipitation in a state in which the porous carbon particles are bonded to each other, and a porous carbon membrane having a skeleton of the porous carbon particles and the optional components of carbon fibers and/or carbon nanotubes can be produced. At this time, most of the solvent of the mixture slurry is compatible with the poor solvent.
Examples of the solvent in which the binder polymer material has low solubility include water, alcohols such as ethyl alcohol, methyl alcohol, isopropyl alcohol, and butyl alcohol, and mixtures of these.

(S4: Drying)
The drying step is performed to volatilize and remove the solvent used in the preparation of the mixture slurry and the poor solvent used in step S3. Examples of the drying method include a method of placing the mixture in a dry air environment, a reduced pressure drying method, a vacuum drying method, etc. In this drying step, the mixture may be heated to a temperature exceeding the boiling point of the solvent in order to increase the drying speed.

(S5: Infusibility)
The infusible treatment is performed for the purpose of preventing the binder polymer material from melting and separating in the carbonization treatment in the next step S6, damaging the bond between the carbon particles, and causing the shape of the molded body to collapse. The infusible treatment may not be performed depending on the type of binder polymer material used. The binder polymer material that requires the infusible treatment is a polymer that melts and liquefies when heated as is, and in order to prevent the melting and liquefying, it is necessary to promote intermolecular crosslinking with oxygen and change the structure to one that does not melt and liquefy when heated. Examples of binder polymer materials that require the infusible treatment include polyvinyl chloride (PVC) and polyvinyl acetate. On the other hand, the binder polymer material that does not require the infusible treatment is a material that, even without oxygen being introduced, undergoes intermolecular crosslinking due to its own functional groups when heated, softening, but does not become liquid, and carbonization proceeds. Examples of binder polymeric materials that do not require infusible treatment include phenol formaldehyde resin (PF), polyvinylidene chloride (PVDC), polyphenylene (PP), and cellulose. Among the binder polymeric materials, polyacrylonitrile (PAN) is a polymer that softens but does not become liquid when heated, and carbonization proceeds. When carbon fiber is industrially produced from PAN, infusible treatment is performed at 200 to 400°C in air to promote crosslinking reaction, and carbonization is performed after changing the PAN into fibers that do not melt even at high temperatures. In the examples described below, PAN is used because of the ease of preparing a mixture slurry and the ease of non-solvent-induced phase separation, and infusible treatment is performed following the industrial production method of carbon fiber.

The infusibilization treatment is preferably performed in an atmosphere under air circulation or air flow from the viewpoint of cost, but it is also possible to use an inert gas such as nitrogen or argon mixed with oxygen. In the case of air flow, the flow rate is preferably 0.1 mL/min or more, more preferably 1 mL/min or more, from the viewpoint of sufficiently progressing the infusibilization. Moreover, in terms of reducing the production cost, it is preferably ¹⁰⁰ m3/min or less, more preferably ⁵⁰ m3/min or less, and even more preferably ¹⁰ m3/min or less.

The temperature at which the infusible treatment is carried out is preferably 250°C or higher, more preferably 260°C or higher, and even more preferably 270°C or higher, in order to sufficiently advance the infusible treatment. When the infusible treatment temperature is equal to or higher than the lower limit, the binder polymer does not melt and separate from the carbon particles in the next carbonization step due to sufficient infusibility, and the strength of the porous carbon membrane can be maintained. In addition, the infusible treatment temperature is preferably 350°C or lower, more preferably 340°C or lower, and even more preferably 330°C or lower. When the infusible treatment temperature is equal to or lower than the upper limit, a moderate crosslinking reaction can be caused, and the binder polymer is lost by oxidative combustion through an oxidation reaction with oxygen present at 21% in the air, which goes beyond the crosslinking reaction, and the porous membrane is less likely to be damaged.
The holding time at a predetermined treatment temperature for infusibility is preferably 1 min or more, more preferably 10 min or more, and even more preferably 20 min or more, from the viewpoint of sufficiently progressing the infusibility, and the upper limit of the holding time is preferably 100 hr or less, more preferably 10 hr or less, and even more preferably 5 hr or less, from the viewpoint of reducing the production cost.
The upper limit of the heating rate in the infusible treatment is preferably 100° C./min or less, more preferably 50° C./min or less, and even more preferably 30° C./min or less, from the viewpoint of sufficiently infusible the polymer mixed as a binder. The lower limit of the heating rate is not particularly limited, but is preferably 0.01° C./min or more, from the viewpoint of shortening the heating time and completing the infusible treatment in a short time.

Examples of equipment used for the infusibility treatment include ovens and infrared furnaces.

(S6: Carbonization)
The purpose of the carbonization process is to convert the binder polymer material into carbon, which improves electrical conductivity and increases the binding strength with porous carbon particles, carbon fibers, and carbon nanotubes, thereby increasing the strength of the porous carbon film.
The carbonization treatment can be carried out using, for example, an oven furnace, a tubular furnace, a box furnace, an infrared furnace, a bake furnace, etc. The lower limit of the temperature of the carbonization treatment is preferably 600° C. or higher, more preferably 700° C. or higher, and even more preferably 800° C. or higher, from the viewpoint of sufficiently carbonizing the binder polymer material. On the other hand, from the viewpoint of reducing the processing cost by using a general-purpose processing device, the temperature of the carbonization treatment is preferably 3000° C. or lower, more preferably 2000° C. or lower, and even more preferably 1500° C. or lower.
The upper limit of the heating rate in the carbonization treatment is preferably 100° C./min or less, more preferably 50° C./min or less, and even more preferably 30° C./min or less in order to sufficiently carbonize the porous carbon membrane and ensure electronic conductivity. There is no particular lower limit for the heating rate, but in order to shorten the carbonization time and reduce the production cost, it is preferably 0.01° C./min or more.
The carbonization treatment atmosphere is preferably an inert atmosphere such as argon (Ar) gas or nitrogen (N ₂ ) gas. The inert gas may be sealed in the furnace, or may be flowed during the treatment. When the inert gas is flowed, the flow rate is not particularly limited, but from the viewpoint of suppressing the influence of the mixing of outside air due to the structure of the furnace, it is preferably 0.1 mL/min or more, more preferably 1 mL/min or more, and even more preferably 10 mL/min or more. On the other hand, from the viewpoint of reducing the treatment cost, the flow rate of the inert gas is preferably 100 m ³ /min or less, more preferably 50 m ³ /min or less, and even more preferably 10 m ³ /min or less.

The above steps produce a porous carbon membrane that is self-supporting and has practical mechanical strength. However, in this porous carbon membrane, the surfaces of the porous carbon particles are covered with a carbonized product of the binder polymer material. Therefore, the porous carbon membrane produced has a reduced pore volume and specific pore surface area compared to the porous carbon particles used as raw material, and the inherent properties of the porous carbon particles used as raw material cannot be fully utilized. However, since the porous carbon particles themselves are powder, they cannot stand on their own and cannot be used as an electrode as is, so it is essential to mold them using a binder polymer material.

In a porous carbon film in which the surfaces of porous carbon particles are coated with a carbonized product of a polymeric binder material, the carbonized product present at the junctions between the porous carbon particles is necessary as a binder to maintain the structure of the porous carbon film, but most of the rest exists in a state of being absorbed and coated on the surface of the porous carbon particles and does not contribute to maintaining the structure of the porous carbon film. Therefore, in this embodiment, carbon dioxide treatment, which will be described later, is carried out as a means of reducing the amount of carbonized product that does not contribute to maintaining the structure of the porous carbon film.

(S7: Heat treatment in a carbon dioxide atmosphere (carbon dioxide treatment))
The carbon dioxide treatment is an essential step for achieving the effects of the present invention.
The carbon dioxide treatment can be carried out using, for example, an oven furnace, a tubular furnace, a box furnace, an infrared furnace, a bake furnace, etc. The temperature of the carbon dioxide treatment is preferably 500°C or more and 1000°C or less. When the temperature of the carbon dioxide treatment is 500°C or more, the pore volume of the produced porous carbon membrane can be increased. The lower limit of the temperature of the carbon dioxide treatment is preferably 600°C or more, more preferably 700°C or more, and particularly preferably 800°C or more. In addition, when the treatment temperature is 1000°C or less, the carbonized material present at the joint between the porous carbon particles and the weight loss due to the oxidative combustion of the porous carbon particles can be effectively suppressed. The upper limit of the treatment temperature is preferably 980°C or less, more preferably 950°C or less, and particularly preferably 900°C or less.

The retention time at the carbon dioxide treatment temperature is preferably 1 minute or more, more preferably 10 minutes or more, even more preferably 20 minutes or more, and particularly preferably 30 minutes or more. By having a retention time at or above this lower limit, the oxidative combustion of the carbonized material by carbon dioxide proceeds, and the pore volume of the resulting porous carbon membrane can be increased. In addition, the retention time is preferably 24 hours or less, more preferably 10 hours or less, even more preferably 5 hours or less, and particularly preferably 2 hours or less. By having a retention time not exceeding this upper limit, the production time can be shortened and weight loss due to oxidative combustion of the carbonized material present at the junctions between the porous carbon particles can be effectively suppressed.

From the viewpoint of production cost, carbon dioxide is preferably used without dilution, but may be used after dilution with an inert gas such as argon (Ar) gas or nitrogen (N ₂ ) gas. When used after dilution, the carbon dioxide concentration is preferably 50% or more, more preferably 80% or more, and even more preferably 90% or more. By increasing the carbon dioxide concentration, the oxidative combustion of the carbonized material is facilitated, and the pore volume of the obtained porous carbon membrane can be increased. The carbon dioxide treatment may be performed in a state where carbon dioxide is sealed in the furnace, or may be performed while the carbon dioxide gas is flowing. When the carbon dioxide gas is flowed, the flow rate is not particularly limited, but is preferably 0.1 mL/min or more, more preferably 1 mL/min or more, and even more preferably 10 mL/min or more. By setting the flow rate of the carbon dioxide gas to this lower limit or more, the oxidative combustion of the carbonized material can be smoothly promoted. In addition, the upper limit of the flow rate is not particularly limited, but from the viewpoint of reducing the amount of carbon dioxide that does not contribute to the oxidative combustion of the carbonized material and thereby suppressing production costs, it is preferably 100 m ³ /min or less, more preferably 50 m ³ /min or less, and even more preferably 10 m ³ /min or less.

By producing the porous carbon film by the above method, the amount of carbonized binder polymer material that covers the surface of the porous carbon particles and does not contribute to maintaining the structure of the porous carbon film can be reduced, and the pore volume and pore specific surface area that the porous carbon particles originally had can be revealed. Furthermore, by oxidizing a part of the porous carbon particles themselves, the pore volume and pore specific surface area are increased compared to those of the original porous carbon particles. In addition, by burning off a part of the carbonized binder polymer material and the porous carbon particles, the gaps between the particles also increase accordingly. Moreover, the obtained porous carbon film has sufficient and practical mechanical strength to stand on its own. This indicates that the carbonized binder polymer material that exists between the porous carbon particles after the carbonization process is not burned off and contributes to the binding of the porous carbon particles. This can be considered to be because carbon dioxide is unlikely to penetrate between the porous carbon particles that are densely bound together via the carbonized material. On the other hand, it is thought that carbon dioxide reaches the carbonized polymer material present on the surface of the porous carbon particles after the carbonization process through the mesopores and macropores in the porous carbon film, which does not contribute to bonding between the porous carbon particles, causing weight loss due to oxidative combustion.

In addition, by carrying out the oxidation treatment with carbon dioxide, rather than oxygen or air, the oxidation treatment proceeds mildly. Since oxygen gas has a high oxidation combustion power, if the oxygen concentration is not lowered to reduce the oxidation power, the carbon porous film being treated will disappear due to oxidation combustion. Therefore, it is necessary to delicately control the oxygen concentration within the range of more than 0.03% and less than 5%. It is also sensitive to the oxidation treatment temperature, and if the temperature is higher than the set temperature, the oxidation power becomes strong and the porous film disappears due to oxidation combustion, while if the temperature is lower than the set temperature, the oxidation power weakens and the porous carbon film may not have the required pores. In contrast, in the treatment with carbon dioxide gas of the present invention, oxidation proceeds mildly, so delicate control of the gas concentration is not necessary, and it is possible to manufacture a porous carbon film with a preferable pore structure even with treatment with 100% carbon dioxide. In addition, since delicate control of the oxidation treatment temperature is not required, the manufacturing equipment can be simplified. From this point of view, it can be said that oxidation treatment with carbon dioxide can significantly reduce costs in the manufacture of porous carbon films compared to treatment with oxygen.

[Physical properties of porous carbon membrane]
In the porous carbon membrane obtained by this embodiment, the pore volume occupied by the pores having a diameter of 1 nm to 1000 nm is preferably larger than the pore volume occupied by the pores having a diameter of 1 nm to 1000 nm in the raw material porous carbon particles. At this time, the increase factor, which is the ratio of the pore volume occupied by the pores having a diameter of 1 nm to 1000 nm in the porous carbon membrane to the pore volume occupied by the pores having a diameter of 1 nm to 1000 nm in the porous carbon particles, is preferably 1.02 times or more, more preferably 1.04 times or more, and even more preferably 1.05 times or more. By the increase factor being equal to or more than this lower limit, a high capacity can be obtained when the porous carbon membrane is used as a positive electrode of an air battery. In addition, the increase factor is preferably 10.0 times or less, more preferably 8.0 times or less, and even more preferably 5.0 times or less in order to maintain the strength of the porous carbon membrane and maintain its self-supporting property.

Moreover, it is preferable that the porous carbon membrane obtained by this embodiment has a large number of pores and satisfies all of the following conditions (a) to (d).
(a) the pore volume occupied by pores having a diameter of 1 nm or more and 1000 nm or less is 3.5 cm ³ /g or more and 7.0 cm ³ /g or less; (b) the pore volume occupied by pores having a diameter of 1 nm or more and 200 nm or less is 2.5 cm ³ /g or more and 7.0 cm ³ /g or less; (c) the pore volume occupied by pores having a diameter of 200 nm or more and 10,000 nm or less is 2.2 cm ³ /g or more and 7.0 cm ³ /g or less; (d) the specific surface area occupied by pores having a diameter of 1 nm or more and 1000 nm or less is 900 m ² /g or more and 2000 m ² /g or less. Each of the conditions (a) to (d) is described in detail below.

Condition (a) indicates that the pore volume in the pore diameter range of 1 nm to 1000 nm is considerably large. This pore volume is obtained by using the Barrett-Joyner-Hallenda (BJH) method from an adsorption isotherm obtained by nitrogen adsorption measurement.
When the porous carbon membrane satisfying the condition (a) is used as the positive electrode of an air battery, it is possible to store more metal oxides (lithium peroxide in a lithium-air battery) generated by discharge, and thus a battery having high discharge capacity characteristics can be provided. In addition, the total amount of air passing through the pores in this pore size range increases in proportion to the pore volume, which facilitates the permeation and diffusion of air within the porous carbon membrane. Therefore, air introduced from outside the battery into the positive electrode can reach every corner of the carbon particles forming the carbon skeleton at high speed. Furthermore, since the pores in this pore size range serve as a migration path for metal ions (Li ions in a lithium-air battery), the large volume allows the metal ions to move smoothly, and combined with the high permeation and diffusion of air and oxygen, an air battery having excellent high-speed discharge characteristics, i.e., high-load characteristics, can be provided.
Therefore, from the viewpoint of obtaining an air battery having a larger discharge capacity and excellent high-load characteristics, the pore volume is preferably ^3.5 cm3/g or more, more preferably ^3.6 cm3/g or more, and even more preferably ^3.7 cm3/g or more. Also, from the viewpoint of maintaining the strength of the porous carbon membrane, the pore volume is preferably ^7.0 cm3/g or less, and more preferably ^6.0 cm3/g or less.

Condition (b) indicates that the pore volume occupied by pores having a diameter in the range of 1 nm to 200 nm is large, and the number of smaller pores is large. This pore volume is also obtained by using the BJH (Barrett-Joyner-Hallenda) method from the adsorption isotherm obtained by nitrogen adsorption measurement.
When a porous carbon membrane satisfying the condition (b) is used for the positive electrode of an air battery, it becomes possible to store a large amount of oxide generated by the reaction of metal ions, oxygen, and electrons during discharge, and it is possible to provide a battery exhibiting a high discharge capacity.
Therefore, in order to obtain a large discharge capacity, the pore volume in the above pore diameter range is preferably 2.5 cm ³ /g or more, and more preferably 2.6 cm ³ /g or more.
In order to ensure a volume of pores with a diameter exceeding 200 nm, which have higher oxygen permeability and diffusivity, and to have excellent discharge characteristics under high loads, i.e., a large discharge capacity at high speed, the pore volume in the above pore diameter range is preferably 7.0 cm ³ /g or less, more preferably 6.0 cm ³ /g or less, and even more preferably 5.0 cm ³ /g or less.

Condition (c) indicates that the pore volume occupied by pores having a diameter of 200 nm or more and 10,000 nm or less is sufficiently large. The pore volume in this pore size range is obtained using a value measured by mercury intrusion porosimetry.
The pores with diameters in this range mainly function to allow oxygen from outside the battery to penetrate into the porous carbon membrane, which is the positive electrode. Therefore, if a porous carbon membrane with a sufficiently large pore volume having a diameter in this range is used as the positive electrode of an air battery, the oxygen required for the positive electrode reaction during discharge can penetrate at high speed, resulting in a battery with a large discharge capacity at high current density, i.e., excellent high-load characteristics. During charging, the metal oxide transfers electrons to the positive electrode to become metal ions and oxygen, and if the pore volume with a diameter in this range is sufficiently large, the generated oxygen can easily escape from the porous carbon membrane, enabling high-speed charging.
Therefore, in order to realize high speed intrusion and release of oxygen into and from the positive electrode, the lower limit of the pore volume is preferably 2.2 cm ³ /g or more, more preferably 2.3 cm ³ /g or more, and even more preferably 2.4 cm ³ /g or more.
Furthermore, since the strength of the porous carbon membrane can be maintained by not having the pore volume be too large, the upper limit of the pore volume is preferably 7.0 cm ³ /g or less, more preferably 6.0 cm ³ /g or less, and even more preferably 5.0 cm ³ /g or less.

Condition (d) indicates that the specific surface area occupied by pores with a diameter of 1 nm or more and 1000 nm or less is large. The positive electrode reaction during discharge of the air battery is a reaction in which metal ions that have migrated from the negative electrode and oxygen that has migrated and diffused from the outside air side of the positive electrode receive electrons from the inner surface of the pores inside the positive electrode to generate metal oxide. Therefore, by having a large specific surface area, there are more places where this reaction occurs when used in the positive electrode of the air battery, and discharge is performed more smoothly, resulting in a battery with excellent high-load characteristics.
The lower limit of the specific surface area is preferably 900 m ² /g or more, more preferably 1000 m ² /g or more, from the viewpoint of ensuring a reaction field, and the upper limit of the specific surface area is preferably 2000 m ² /g or more, more preferably 1600 m ² /g or less, from the viewpoint of maintaining the strength of the porous carbon membrane.

The porous carbon membrane obtained by this embodiment preferably satisfies the following condition (e) in addition to the above conditions (a) to (d).
(e) Apparent density is 0.10 g/cm ³ or more and 0.20 cm ³ /g or less If the apparent density of the porous carbon membrane is in this range, the porous carbon membrane has sufficient pores necessary for oxygen to permeate and diffuse, and has sufficient strength. Therefore, it can be made into a self-supporting porous carbon membrane, and can be used as the positive electrode of an air battery without using a metal mesh or the like. This enables the battery to be made lighter, smaller, more productive, and less expensive. The apparent density can be calculated by dividing the mass of the porous carbon membrane by its volume.
When the apparent density is not too small, the strength of the porous carbon membrane is increased, and the porous carbon membrane can be used as a positive electrode without being attached to a current collector such as a metal mesh. From this point of view, the lower limit of the apparent density is preferably 0.10 g/cm ³ or more, more preferably 0.11 g/cm ³ or more, and even more preferably 0.12 g/cm ³ or more. This makes it possible to obtain a porous carbon membrane having pores and high strength.
In addition, when the apparent density is small, the amount of pores present in the porous carbon membrane is large, and an air battery having a large capacity and excellent load characteristics is obtained. From this point of view, the upper limit of the apparent density is preferably 0.20 g/ ^cm3 or less, more preferably 0.19 g/ ^cm3 or less, and further preferably 0.18 g/ ^cm3 or less.

The porous carbon membrane obtained by this embodiment preferably satisfies the following condition (f) in addition to the above conditions (a) to (d) or the above conditions (a) to (e).
(f) Porosity is 90% or more and 99% or less The porosity of the porous carbon membrane can be calculated from the apparent density and the true density by (1-apparent density of the porous carbon membrane/true density of the material constituting the porous carbon membrane). The porosity being equal to or greater than the lower limit indicates that the membrane has a sufficiently large porosity. When such a porous carbon membrane is used in the positive electrode of an air battery, a large amount of metal oxide generated by discharge can be stored, and air and oxygen can smoothly invade the positive electrode from the outside of the battery with little resistance, so that a battery having a high discharge capacity and capable of high-speed discharge can be provided.
Furthermore, by keeping the porosity at or below the above upper limit, the rigidity and strength of the porous carbon membrane are ensured, and the membrane can stand on its own.The upper limit of the porosity is preferably 98% or less.

[Method of manufacturing air battery]
Another aspect of the present invention is a method for producing an air battery, comprising the steps of producing a porous carbon membrane by the above-mentioned method, forming a positive electrode from the porous carbon membrane, preparing a negative electrode and a separator, stacking the porous carbon membrane and the negative electrode with the separator interposed therebetween, and filling the separator with an electrolyte. Hereinafter, this aspect will be described with reference to the drawings, taking as examples a method for producing a coin cell and a stacked battery.

(Coin Cell)
FIG. 2 is a schematic diagram showing an example of a coin cell.
The coin cell 600 is composed of a laminated structure in which a negative electrode structure 610 and a positive electrode structure 620 are laminated with a separator 660 interposed therebetween. This laminated structure is restrained by a coin cell type restraining device 630. An insulating O-ring (not shown) is disposed between the coin cell type restraining device 630 and the metal mesh 680, ensuring insulation between the restraining device 630 and the positive electrode structure 620.

The negative electrode structure 610 is composed of a current collector 635, a metal layer 640 arranged on the current collector 635, and columnar spacers 650 arranged on both ends of the current collector 635. A space 670 is provided between the metal layer 640 and the separator 660, and is filled with an electrolyte.
The metal layer 640 preferably contains an alkali metal and/or an alkaline earth metal, and is particularly preferably a layer made of lithium metal.

The positive electrode structure 620 includes a porous carbon membrane 690 in mechanical and electrical contact with a metal-containing mesh (metal mesh) 680, which is a current collector. In this case, the metal mesh 680 serves as a positive electrode substrate and also functions as a flow path through which air or oxygen passes.
A separator 660 is disposed between the negative electrode structure 610 and the positive electrode structure 620 .

An example of a method for manufacturing the coin cell 600 will be described below.
First, a negative electrode structure 610 is prepared. A disk-shaped metal layer 640 made of lithium or the like is laminated on a disk-shaped current collector 635, the metal layer 640 being concentric with the current collector 635 and having a smaller diameter than the current collector 635, and a columnar spacer 650 is pressed onto the current collector 635, thereby obtaining a negative electrode structure 610.

The spacer 650 is an insulator. The material may be a metal oxide, a metal nitride, a metal oxynitride, or the like. For example, Al ₂ O ₃ , Ta ₂ O ₅ , TiO ₂ , ZnO, ZrO ₂ , SiO ₂ , B ₂ O ₃ , P ₂ O ₅ , GeO ₂ , Li ₂ O, Na ₂ O, K ₂ O, MgO, CaO, SrO, BaO, Si ₃ N ₄ , AlN, and AlO _x N _1-x (0<x<1), or the like. Among them, Al ₂ O ₃ and SiO ₂ are preferable because they are easily available and have excellent processability.

The spacer 650 may be made of a resin. Examples of the resin include polyolefin resin, polyester resin, polyimide resin, and polyether ether ketone (PEEK) resin. Examples of the polyolefin resin include polyethylene and polypropylene. Examples of the polyester resin include polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyethylene naphthalate (PEN), and polytributylene terephthalate (PTT). These resins are preferred because they are easily available and have excellent processability.

The separator 660 is then pressed onto the spacer 650 .
A space 670 is provided between the metal layer 640 , the spacer 650 and the separator 660 .
The separator 660 is a porous insulator that allows alkali metal ions and/or alkaline earth metal ions to pass through it. The separator 660 is any inorganic material (including metallic materials) or organic material that is non-reactive with the metal layer 640 and the electrolyte.
The material of the separator 660 may be a resin such as polyethylene, polypropylene, or polyolefin, glass, etc. The separator 660 may be a woven fabric or a nonwoven fabric.

Then, the separator 660 is filled with electrolyte. At this time, the space 670 is also filled with electrolyte.

The electrolyte may be any aqueous or non-aqueous electrolyte containing an alkali metal salt and/or an alkaline earth metal salt. When the aqueous electrolyte contains a lithium salt, the lithium salt may be, for example, LiOH, LiCl, LiNO ₃ , or Li ₂ SO ₄ , and the solvent may be water or a water-soluble solvent.

When the non-aqueous electrolyte (nonaqueous electrolyte) contains a lithium salt, examples of the lithium salt that can _be used include _LiPF6 , _LiBF4 , _LiSbF6 , _LiSiF6 , _LiAsF6 , LiN( _SO2C2F5 ) ₂ , Li( _FSO2 ) _{2N , LiCF3SO3 ( LiTfO )} , Li( _{CF3SO2 ) 2N (} LiTFSI), _LiC4F9SO3 , _LiClO4 , _LiAlO2 , _LiAlCl4 , and _LiB ( _C2O4 ) ₂ .

Non-aqueous solvents used in non-aqueous electrolytes include glymes (monoglyme, diglyme, triglyme, tetraglyme), methyl butyl ether, diethyl ether, ethyl butyl ether, dibutyl ether, polyethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, cyclohexanone, dioxane, dimethoxyethane, 2-methyltetrahydrofuran, 2,2-dimethyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, tetrahydrofuran, methyl acetate, ethyl acetate, n-propyl acetate, dimethyl acetate, methyl propionate, ethyl propionate, methyl formate, ethyl formate, dimethyl carbonate, diethyl carbonate, ethane, etc. These include ethyl methyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, ethylene carbonate, propylene carbonate, butylene carbonate, polyethylene carbonate, gamma-butyrolactone, decanolide, valerolactone, mevalonolactone, caprolactone, acetonitrile, benzonitrile, nitromethane, nitrobenzene, triethylamine, triphenylamine, tetraethylene glycol diamine, dimethylformamide, diethylformamide, N-methylpyrrolidone, dimethylsulfone, tetramethylene sulfone, triethylphosphine oxide, 1,3-dioxolane, and sulfolane.

Thereafter, a positive electrode structure 620 is prepared in which a metal mesh 680 is disposed on a porous carbon membrane 690 .
As the metal mesh 680, for example, a mesh having at least one metal selected from the group consisting of copper (Cu), tungsten (W), aluminum (Al), nickel (Ni), titanium (Ti), gold (Au), silver (Ag), platinum (Pt), and palladium (Pd) can be used. That is, examples include a mesh made of a simple metal selected from this group, an alloy containing a metal selected from this group, and a compound of a metal selected from this group and carbon (C) or nitrogen (N). In the case of an alloy, it may also contain iron (Fe) or chromium (Cr). The mesh may have a thickness of 0.2 mm and a mesh size of 1 mm, for example.

Thereafter, the positive electrode structure 620 is attached to the negative electrode structure 610 filled with the electrolyte via a separator 660, and the assembly is restrained by a restraining tool 630 to obtain a coin cell 600. Here, the mounting is preferably performed in dry air, for example, in dry air with a dew point temperature of −50° C. or lower.
Through the above steps, the coin cell 600 is manufactured.
Although the coin cell 600 has a porous carbon membrane 690 and a metal mesh 680 as the positive electrode structure 620, the air battery of the present invention is not limited to the above and may have only the porous carbon membrane 690 as the positive electrode structure 620.

The manufactured coin cell 600 has a positive electrode structure 620 using a porous carbon membrane 690 that has excellent air or oxygen permeability, can take in a large amount of oxygen, has high ion transport efficiency, and has a large reaction field, making it an excellent air battery that is small, lightweight, and has a large capacity.

(Stacked air battery)
FIG. 3 is a schematic diagram showing an example of a stacked-type air battery.
The stacked air battery 500 has a stacked structure in which a positive electrode structure 510 and a negative electrode structure 100 are stacked via a separator 540. The number of stacked pairs may be one or more pairs, with one positive electrode structure 510 and one negative electrode structure 100 being regarded as one pair, and there is no particular upper limit to the number of pairs.

The negative electrode structure 100 is composed of a pair of negative electrode active material layers (metal layers) and a negative electrode current collector 520 sandwiched between them.

On the other hand, the positive electrode structure 510 is configured such that a pair of laminates consisting of a porous carbon membrane 550 and a gas diffusion layer 560 sandwich the positive electrode current collector 525. The gas diffusion layer 560 and the porous carbon membrane 550 are arranged in this order from the positive electrode current collector 525 side.

The gas diffusion layer 560 of the positive electrode structure 510 allows air, oxygen, and other gases to pass between the outside of the battery and the porous carbon membrane 550. The gas diffusion layer also acts as a path for electrons to move between the porous carbon membrane 550 and the positive electrode current collector 525. Since the gas diffusion layer acts as a path for the gases, it is necessary for it to have air-permeable communicating holes and to have electronic conductivity. For example, Toray's carbon paper TGP-H and Kureha's KRECA (registered trademark) E704 can be used as the gas diffusion layer.

The positive electrode current collector 525 functions as an air or oxygen flow path in addition to being electrically connected to the outside, so that the air battery 500 has a simpler structure and can achieve a larger capacity.

For the negative electrode current collector 520 and the positive electrode current collector 525, for example, metals such as copper (Cu), tungsten (W), aluminum (Al), nickel (Ni), titanium (Ti), gold (Au), silver (Ag), platinum (Pt), and palladium (Pd), as well as alloys and compounds of these (e.g., compounds with carbon and/or nitrogen) can be used. In the case of alloys, iron (Fe) and chromium (Cr) can also be included.

Next, a method for manufacturing the stacked air battery 500 will be described.
The negative electrode structure 100 is composed of a pair of negative electrode metals and a negative electrode current collector 520 sandwiched between them, and the negative electrode current collector 520 is surrounded by a separator 540 so as to extend to the outside, and the space within the separator is filled with an electrolyte.
The positive electrode structure 510 is composed of a pair of laminates each made of a porous carbon membrane 550 and a gas diffusion layer 560, and a positive electrode current collector 525 sandwiched therebetween. The gas diffusion layer 560 and the porous carbon membrane 550 are arranged in this order from the positive electrode current collector 525 side. The direction in which the positive electrode current collector 525 and the negative electrode current collector 520 are drawn out is not particularly limited.
The negative electrode structure 100 and the positive electrode structure 510 are laminated with a separator 540 interposed therebetween to produce a stacked type air battery 500. This stacked type air battery may be housed in a storage container (not shown).

In this specification, the method for manufacturing an air battery has been described in detail with reference to Figures 2 and 3, but the positive electrode using the porous carbon membrane obtained by this embodiment can be applied to any other method for manufacturing an air battery.

The present embodiment will be described in detail below with reference to examples, but the present embodiment is not limited to these.

The physical properties of the porous carbon particles used in the following Examples and Comparative Examples, and the properties of the resulting porous carbon membranes were measured by the following methods.
(1) Pore volume occupied by pores having a diameter of 1 nm or more and 1000 nm or less: This was determined by the BJH (Barrett-Joyner-Hallenda) method from an adsorption isotherm obtained by a nitrogen adsorption method using a 3Flex (manufactured by Micromeritics Instrument Corp.).
(2) Pore volume occupied by pores having a diameter of 1 nm or more and 200 nm or less: The pore volume was determined by the BJH method from an adsorption isotherm obtained by a nitrogen adsorption method using a 3Flex (manufactured by Micromeritics Instrument Corp.).
(3) Specific surface area occupied by pores having a diameter of 1 nm or more and 1000 nm or less: This was determined by the BJH method from an adsorption isotherm obtained by a nitrogen adsorption method using a 3Flex (manufactured by Micromeritics Instrument Corp.).
(4) Specific surface area occupied by pores having a diameter of 1 nm or more and 200 nm or less: This was determined by the BJH method from an adsorption isotherm obtained by a nitrogen adsorption method using a 3Flex (manufactured by Micromeritics Instrument Corp.).
(5) Pore volume occupied by pores with diameters of 200 nm to 10,000 nm The pore volume in the pore diameter range of 10 nm to 200,000 nm (0.01 μm to 200 μm) was measured by mercury intrusion using an AutoPoreIV (manufactured by Micromeritics Instrument Corp.), and the pore volume in the pore diameter range of 200 nm to 10,000 nm was calculated from the measurement results.
(6) BET specific surface area: The BET specific surface area was determined from an adsorption isotherm obtained by a nitrogen adsorption method using a 3Flex (Micromeritics Instrument Corp.) according to the BET (Brunauer-Emmett-Teller) method.
(7) Apparent Density of Porous Carbon Membrane The apparent density was calculated by dividing the mass of the porous carbon membrane by its volume.
(8) Porosity (%)
This was calculated using the following formula:
(1-apparent density of porous carbon membrane/true density of material constituting porous carbon membrane) x 100
(9) Carbonization yield (%)
This was calculated using the following formula:
Mass of porous carbon membrane after carbonization (g)/mass of porous membrane after curing (g)×100
(10) Carbon dioxide treatment yield (%)
This was calculated using the following formula:
Mass of porous carbon membrane after carbon dioxide treatment (g)/mass of porous carbon membrane after carbonization (g)×100

In the following Examples and Comparative Examples, the battery characteristics of the air batteries using the porous carbon membranes as the positive electrodes were evaluated by measuring the discharge capacity of coin cells prepared by the following method under the following conditions.
A positive electrode cut into a size of φ16 mm from a porous carbon membrane, metallic lithium (φ16 mm, thickness 0.2 mm) as a negative electrode, and a glass fiber paper separator (Whatman (registered trademark), GF/A) soaked in 100 μL of a 1 M tetraethylene glycol dimethyl ether solution of LiTFS (lithium trifluoromethanesulfonate) as an electrolyte were mounted in a coin cell case (CR2032 type) in a dry room (in dry air) with a dew point temperature of −50° C. or lower to produce a coin cell 600 as shown in FIG. 2.
This coin cell 600 was discharged in a pure oxygen atmosphere at a current density of 0.4 mA/ ^cm2 , and the point at which the voltage dropped to 2.3 V was set as the discharge end point. The discharge capacity thus obtained was divided by the mass of the porous carbon membrane used as the positive electrode to calculate the discharge capacity per mass of the positive electrode (hereinafter referred to as the "discharge capacity of the porous carbon membrane").

[Example 1]
A porous carbon membrane for an air battery positive electrode was prepared by the following method, and evaluated by the above-mentioned method.
(Combination slurry preparation process)
65 parts by mass of Ketjen Black EC600JD as porous carbon particles, 12 parts by mass of carbon fiber, 23 parts by mass of polyacrylonitrile (PAN) as a polymeric material for binder, and N-methylpyrrolidone as a solvent for uniformly dispersing them were added and mixed in a Thinky Corporation ARE310 rotary kneader to prepare a mixture slurry. The carbon fiber used had an average fiber diameter of 6 μm and an average length of 3 mm. The physical properties of Ketjen Black EC600JD are shown in Table 1 below.

(Molding process)
The mixture slurry was formed into a sheet having a thickness of 500 μm by a wet film forming method using a doctor blade.

(Solvent immersion process)
The molded sheet was placed in a tray, 220 g of methanol was added thereto, and the sheet was allowed to stand. After 2 hours, the methanol in the tray was discharged, and 220 g of fresh methanol was added. After the sheet was allowed to stand for 17 hours, the methanol in the tray was discharged.
In this example, the above-mentioned molded sheet in which the porous carbon particles and carbon fibers were dispersed in the N-methylpyrrolidone solution of PAN was immersed in methanol, which is a non-solvent (poor solvent), whereby the PAN was phase-separated and precipitated in a state in which the PAN was bound to the carbon particles and carbon fibers, and a porous film having the carbon particles as a skeleton was produced, and most of the N-methylpyrrolidone was compatible with methanol.

(Drying process)
The porous membrane was taken out of the tray and dried at 50° C. for 2 hours and then at 80° C. for 10 hours to remove the volatile solvent contained in the porous membrane.

(Infusible process)
The dried porous membrane was subjected to an infusible heat treatment at 320° C. for 3 hours in an air circulating atmosphere, and the PAN in the dried porous membrane was converted into an infusible resin.

(Carbonization process)
The infusible porous membrane having a length of 90 mm and a width of 80 mm obtained by the infusible treatment was heated to 1,050° C. at a heating rate of 10° C./min using a box furnace manufactured by Denken Hydental Co., Ltd. while flowing nitrogen gas at 600 mL/min, and then held at 1,050° C. for 3 hours and then allowed to cool to room temperature, thereby carbonizing the infusible PAN and obtaining a porous carbon membrane composed entirely of carbon.

(Heat treatment in carbon dioxide (carbon dioxide treatment) step)
The carbonized porous carbon membrane was heated to 850°C at a heating rate of 10°C/min using a box furnace manufactured by Denken Hydental Co., Ltd. while flowing carbon dioxide gas at 200 mL/min, and then held at 850°C for 1 hour and then allowed to cool to room temperature to obtain a carbon dioxide-treated porous carbon membrane.

(Physical properties of porous carbon membrane)
The porous carbon membrane obtained by carbon dioxide treatment had a pore volume of 4.3 ^cm3 /g for pores with diameters of 1 nm to 1000 nm, a pore volume of 2.8 ^cm3 /g for pores with diameters of 1 nm to 200 nm, a pore volume of 2.6 ^cm3 /g for pores with diameters of 200 nm to 10,000 nm, and a specific surface area of 1 nm to ¹⁰⁰⁰ nm.

(Discharge capacity of air battery using porous carbon membrane as positive electrode)
Next, a coin cell using this porous carbon membrane as a positive electrode was produced by the above-mentioned method, and the discharge capacity of the porous carbon membrane was calculated. As a result, the discharge capacity showed a large value of 3554 mAh/g.

[Comparative Example 1]
The porous carbon membrane was prepared in the same manner as in Example 1, except that the carbonization step was performed without the carbon dioxide treatment.
The resulting porous carbon membrane had a pore volume of 3.1 ^cm3 /g for pores with diameters of 1 nm to 200 nm, a pore volume of 2.0 ^cm3 /g for pores with diameters of 200 nm to 10,000 nm, and a specific surface area of ^{655 m2} /g for pores with diameters of 1 nm to 1000 nm. The discharge capacity of this porous carbon membrane was 2488 mAh/g, which was smaller than the 3544 mAh/g of the carbon dioxide-treated porous carbon membrane positive electrode of Example 1.

[Example 2]
A porous carbon membrane for an air battery positive electrode was produced and evaluated in the same manner as in Example 1, except that 68 parts by mass of Ketjen Black EC600JD was used as the porous carbon particles, 9 parts by mass of carbon fiber, 23 parts by mass of polyacrylonitrile (PAN) was used as the polymer material for binder, and the sheet thickness was 300 μm.
The resulting porous carbon membrane had a pore volume of 3.8 ^cm3 /g for pores with diameters of 1 nm to 1000 nm, a pore volume of 2.7 ^cm3 /g for pores with diameters of 1 nm to 200 nm, a pore volume of 2.4 ^cm3 /g for pores with diameters of 200 nm to 10,000 nm, and a specific surface area of 1008 ^m2 /g for pores with diameters of 1 nm to 1000 nm. The discharge capacity of this porous carbon membrane was 3508 mAh/g, which was a large discharge capacity similar to that of Example 1.

[Comparative Example 2]
The porous carbon membrane was prepared in the same manner as in Example 2, except that the carbonization step was not performed and carbon dioxide treatment was not performed. The obtained porous carbon membrane had a pore volume of 3.4 cm ³ /g for pores with a diameter of 1 nm to 1000 nm, a pore volume of 2.1 cm ³ /g for pores with a diameter of 1 nm to 200 nm, a pore volume of 1.5 cm ³ /g for pores with a diameter of 200 nm to 10,000 nm, and a specific surface area of 771 m ² /g for pores with a diameter of 1 nm to 1000 nm. The discharge capacity of this porous carbon membrane was 2563 mAh/g, which was smaller than the 3508 mAh/g of the carbon dioxide-treated porous carbon membrane of Example 2.

[Example 3]
A porous carbon membrane for an air battery positive electrode was produced and evaluated in the same manner as in Example 2, except that a mixture slurry was produced using 75 parts by mass of Black Pearl 2000 as the porous carbon particles, 5 parts by mass of carbon fiber, 5 parts by mass of carbon nanotubes, 15 parts by mass of polyacrylonitrile (PAN) as a polymeric binder material, and N-methylpyrrolidone as a solvent for uniformly dispersing them.
The physical properties of the Black Pearl 2000 used are shown in Table 1. The carbon fibers used had an average fiber diameter of 6 μm and an average length of 3 mm, and the carbon nanotubes used had an average diameter of 1.6 nm and a length of about 5 μm.
The resulting porous carbon membrane had a pore volume of 3.9 ^cm3 /g for pores with diameters of 1 nm to 1000 nm, a pore volume of 3.3 ^cm3 /g for pores with diameters of 1 nm to 200 nm, a pore volume of 2.5 ^cm3 /g for pores with diameters of 200 nm to 10,000 nm, and a specific surface area of 1323 ^m2 /g for pores with diameters of 1 nm to 1000 nm. The discharge capacity of this porous carbon membrane was 2518 mAh/g.

[Comparative Example 3]
The porous carbon membrane was prepared in the same manner as in Example 3, except that the carbonization step was performed without carrying out the carbon dioxide treatment.
The resulting porous carbon membrane had a pore volume of 2.6 ^cm3 /g for pores with diameters of 1 nm to 1000 nm, a pore volume of 2.3 ^cm3 /g for pores with diameters of 1 nm to 200 nm, a pore volume of 1.9 ^cm3 /g for pores with diameters of 200 nm to 10,000 nm, and a specific surface area of 835 m2/g for pores with diameters of 1 nm to ¹⁰⁰⁰ nm. The discharge capacity of this porous carbon membrane was 1463 mAh/g, which was smaller than the 2518 mAh/g of the carbon dioxide-treated porous carbon membrane positive electrode of Example 3.

[Comparative Example 4]
The same procedure as in Example 1 was carried out except that the temperature for the carbon dioxide treatment was 1050° C. However, since the porous carbon membrane was burned away by the carbon dioxide treatment, the subsequent evaluations were not carried out.

[Comparative Example 5]
The same procedure as in Example 1 was carried out except that the treatment was carried out at 850° C. using air instead of carbon dioxide. However, since the porous carbon membrane was burned away, the subsequent evaluations were not carried out.

The physical properties of the carbon particles, Ketjen Black EC600JD and Black Pearl 2000, used in the above examples and comparative examples are shown in Table 1, and the manufacturing conditions of the porous carbon membrane are shown in Table 2. The physical properties of the obtained porous carbon membrane and the discharge capacity of the battery are shown in Table 3.

The above examples and comparative examples reveal the following.
Regarding the porous carbon membrane obtained by treating the porous carbon membrane after the carbonization treatment in a carbon dioxide atmosphere, the pore volume occupied by pores with a diameter of 1 nm or more and 1000 nm or less in the porous carbon membrane of Example 1 was 4.3 cm ³ /g, and the pore volume occupied by pores with a diameter of 1 nm or more and 200 nm or less was 2.8 cm ³ /g, while the pore volumes of the porous carbon membrane of Example 2 were 3.8 cm ³ /g and 2.7 cm ³ /g, respectively, which were larger than the pore volumes of the carbon particles Ketjen black used as the raw material shown in Table 1, which were 3.5 cm ³ /g and 2.6 cm ³ /g.
In addition, the porous carbon membrane of Example 3 also had a pore volume of 3.9 cm ³ /g for pores with diameters of 1 nm or more and 1000 nm or less, and a pore volume of 3.3 cm ³ /g for pores with diameters of 1 nm or more and 200 nm or less, which were larger than the pore volumes of the raw material black pearls, 3.2 cm ³ /g and 2.8 cm ³ /g, respectively.
In contrast, the porous carbon membranes according to Comparative Examples 1 to 3, which were produced in the same manner as in Examples 1 to 3, except that they were not subjected to carbon dioxide treatment, had a pore volume occupied by pores having a diameter of 1 nm or more and 1,000 nm or less and a pore volume occupied by pores having a diameter of 1 nm or more and 200 nm or less that were lower than the pore volumes of the raw materials Ketjen Black and Black Pearl, respectively.

This is presumably because in Examples 1 to 3, in which carbon dioxide treatment was performed, most of the PAN and PAN carbonized material, which were the polymeric binder material covering the porous carbon particles, were burned off, exposing the surface of the porous carbon particles, and part of the surface of the porous carbon particles was also burned off, increasing the pore volume, which is believed to have contributed to the increase in battery capacity when the positive electrode structure of the air battery was formed. In contrast, in Comparative Examples 1 to 3, in which carbon dioxide treatment was not performed, the PAN-derived carbonized material used as the polymeric binder material remained covering the porous carbon particles, and therefore the increase in battery capacity was not achieved.

In addition, in Examples 1 to 3, the pore volume of pores with diameters of 200 nm or more and 10,000 nm or less was larger than that of Comparative Examples 1 to 3. As a result, in Examples 1 to 3, oxygen present outside the film penetrates the porous carbon film at high speed during discharge, and oxygen escapes at high speed during charge, so that it is expected to provide a battery with high discharge capacity and excellent high load characteristics.

Furthermore, in Examples 1 to 3, the specific surface area of the pores with diameters of 1 nm to 1000 nm was also larger than those in Comparative Examples 1 to 3. This means that the reaction field in which the metal ions that migrate from the negative electrode during discharge and the oxygen that enters from the outside react by receiving electrons from the inner surface of the pores is large, and the reaction proceeds smoothly, so it is expected that a battery with high discharge capacity and excellent high-load characteristics can be provided.

In addition, it was confirmed that the porous carbon membranes in Examples 1 to 3 had large pores with apparent densities of 0.16 to 0.19 g/ ^cm and porosities of 90.6 to 91.8%, while still maintaining their self-supporting and strength. This is believed to be because the PAN in the portion bonding the carbon particles together was not burned away because carbon dioxide was less likely to penetrate it, and instead remained carbonized, thereby maintaining its binding properties.
In contrast, in Comparative Example 4, the temperature of the carbon dioxide treatment was too high, and in Comparative Example 5, the oxidizing power of the air treatment was too strong, so the porous carbon membrane was burned away in both cases.

According to the present invention, when compared with air batteries having the same structure and size, it is possible to achieve a high discharge capacity that was previously unattainable. Therefore, the present invention is useful in that it can provide air batteries with a high capacity that was previously unattainable, and in that it can make batteries with the same discharge capacity smaller.

600 coin cell 610 negative electrode structure 620 positive electrode structure 630 coin cell type restraining device 635 current collector 640 metal layer 650 spacer 660 separator 670 space (space for filling electrolyte)
680 Metal mesh 690 Porous carbon membrane 100 Negative electrode structure 500 Stacked air battery 510 Positive electrode structure 520 Negative electrode current collector 525 Positive electrode current collector 540 Separator 550 Porous carbon membrane 560 Gas diffusion layer

Claims (7)

A method for producing a porous carbon membrane for an air battery positive electrode, comprising the steps of:
Preparing a mixture slurry containing porous carbon particles and a polymeric binder material;
Molding the mixture slurry;
immersing the sample obtained by the molding in a solvent in which the binder polymer material has a low solubility;
drying the sample obtained by immersion;
carbonizing the dried sample in an inert gas atmosphere; and
The method for producing a porous carbon membrane for an air battery positive electrode includes heat treating the sample obtained by the carbonization in a carbon dioxide atmosphere having a carbon dioxide concentration of 50% or more at a temperature of 500° C. or more and 1000° C. or less . The method for producing the porous carbon membrane according to claim 1, further comprising infusibly in air the sample obtained by drying before the carbonization. The method for producing a porous carbon membrane according to claim 1 or 2, wherein the heat treatment in the carbon dioxide atmosphere is carried out until the pore volume occupied by the pores having a diameter of 1 nm or more and 1000 nm or less in the porous carbon membrane is 1.02 times or more the pore volume occupied by the pores having a diameter of 1 nm or more and 1000 nm or less in the porous carbon particles used as a raw material. The method for producing a porous carbon membrane according to any one of claims 1 to 3 , wherein the porous carbon membrane satisfies all of the following conditions (a) to (d):
(a) the pore volume occupied by pores having a diameter of 1 nm or more and 1000 nm or less is 3.5 cm ³ /g or more and 7.0 cm ³ /g or less; (b) the pore volume occupied by pores having a diameter of 1 nm or more and 200 nm or less is 2.5 cm ³ /g or more and 7.0 cm ³ /g or less; (c) the pore volume occupied by pores having a diameter of 200 nm or more and 10,000 nm or less is 2.2 cm ³ /g or more and 7.0 cm ³ /g or less; (d) the specific surface area occupied by pores having a diameter of 1 nm or more and 1000 nm or less is 900 m ² /g or more and 2000 m ² /g or less. The method for producing a porous carbon membrane according to claim 4 , wherein the porous carbon membrane further satisfies the following condition (e):
(e) Apparent density is 0.10 g/ ^cm3 or more and 0.20 g/ ^cm3 or less. The method for producing a porous carbon membrane according to claim 4 or 5 , wherein the porous carbon membrane further satisfies the following condition (f):
(f) Porosity is 90% or more and 99% or less. A method for producing an air battery, comprising the steps of:
Producing a porous carbon membrane by the method for producing a porous carbon membrane according to any one of claims 1 to 6 ;
forming a positive electrode from the porous carbon membrane and preparing a negative electrode and a separator;
stacking the positive electrode and the negative electrode with a separator interposed therebetween; and filling the separator with an electrolyte.