JP2021084830A - Carbon sheet - Google Patents

Abstract

To provide a sheet having sufficient strength and conductivity as an electrode and having high gas permeability.SOLUTION: A multilayered carbon sheet comprises (A) carbon nanofibers having an average diameter of less than 50 nm and (B) carbon fibers having an average diameter of 50 nm or more, in which an amount of the carbon nanofibers (A) is 0.1 mass% or more with respect to the carbon sheet. An average aspect ratio of the carbon nanofibers (A) is 50 to 105 and an average aspect ratio of the carbon fibers (B) is 10 to 103.SELECTED DRAWING: Figure 4

Description

The present invention relates to a carbon sheet, an electrode having a carbon sheet, and a metal-air battery having an electrode.

Metal-air batteries, particularly lithium-air batteries, use lithium metal as the negative electrode active material and oxygen as the positive electrode active material. The lithium-air battery has a large electric capacity because oxygen, which is a positive electrode active material, can be taken in from the air and it is not necessary to have the positive electrode active material inside the battery. Lithium-air batteries have the highest theoretical energy density among secondary batteries, and are expected to be put into practical use.

In a lithium-air battery, carbon paper is used to obtain conductivity for the positive electrode, but since the surface area at which the electrode reaction between lithium ions and oxygen takes place is small, the utilization efficiency of the entire electrode for charging and discharging is low. In order to solve this problem, it is common to use a composite positive electrode with high surface area carbon particles (Ketjen black, etc.) placed on the surface of the carbon paper, but again, the carbon paper that hardly contributes to the electrode reaction. Is the majority, so the utilization efficiency of the entire electrode is not sufficient.
Patent Document 1 proposes a porous carbon having pores including mesopores (2 to 50 nm) and micropores (less than 2 nm) having a pore diameter smaller than the mesopores, which is an alternative to Ketjenblack. .. However, since carbon paper is also used in this case, the utilization efficiency of the entire electrode is not sufficient.

As an electrode that does not use carbon paper, the present inventors have found and reported that a large discharge capacity can be obtained by forming a sheet of fibrous carbon having an average diameter of 5 to 30 nm and using it alone as an electrode (). Non-Patent Document 1). However, since the gas permeability is low and the amount of oxygen that can be used by the electrode is not sufficient, there is a limit to the efficiency improvement.

Japanese Unexamined Patent Publication No. 2014-17230 K. Sakai et al., ACS Omega, 2018, 3 (1), 691-697

One object of the present invention is to provide a sheet having sufficient strength and conductivity as an electrode and having high gas permeability.
Another object of the present invention is to provide a metal-air battery having a high discharge capacity.

The present invention is a carbon sheet comprising (A) carbon nanofibers having an average diameter of less than 50 nm and (B) carbon fibers having an average diameter of 50 nm or more.
Provided is a multilayer carbon sheet in which the amount of carbon nanofibers (A) is 0.1% by mass or more with respect to the carbon sheet.
Furthermore, the present invention provides a battery having an electrode having the carbon sheet.

Preferred embodiments of the present invention are as follows.
Aspect 1:
A carbon sheet containing (A) carbon nanofibers having an average diameter of less than 50 nm and (B) carbon fibers having an average diameter of 50 nm or more.
A multilayer carbon sheet in which the amount of carbon nanofibers (A) is 0.1% by mass or more with respect to the carbon sheet.
Aspect 2:
The carbon sheet according to aspect 1, wherein the carbon nanofiber (A) has an average aspect ratio of 50 to 10 ⁵ , and the carbon fiber (B) has an average aspect ratio of 10 to ^3.
Aspect 3:
The carbon sheet according to aspect 1 or 2, wherein the pressure loss due to gas permeation is 700 kPa / ms ^{-1 or less.}
Aspect 4:
The amount of carbon nanofibers (A) is 0.1 to 90% by weight and the amount of carbon fibers (B) is 10 to 99% by weight with respect to the total weight of the carbon nanofibers (A) and the carbon fibers (B). The carbon sheet according to any one of aspects 1 to 3, which is 9% by weight.
Aspect 5:
An electrode having the carbon sheet according to any one of aspects 1 to 4.
Aspect 6:
The electrode according to aspect 5, which is the positive electrode of the battery.
Aspect 7:
A battery comprising the electrodes according to aspect 5 or 6.
Aspect 8:
The battery according to aspect 7, which is a lithium-air battery.
Aspect 9:
The carbon sheet according to any one of aspects 1 to 4, wherein the carbon nanofibers (A) and the carbon fibers (B) are dispersed in a dispersion medium, the dispersion liquid is filtered, and the obtained sheet is dried. Production method.

According to the present invention, a carbon sheet having sufficient mechanical strength, sufficient conductivity, a large surface area, and high gas permeability can be obtained. The carbon sheet of the present invention has sufficient curvature and sufficient flexibility, and breaks after being stretched. The mechanical strength and conductivity of the carbon sheet of the present invention are sufficient as an electrode. The carbon sheet is a self-supporting film.
The battery using the carbon sheet of the present invention as a positive electrode has a high discharge capacity. A high discharge capacity can be realized without using porous carbon for the positive electrode.

It is a transmission electron micrograph of the carbon sheet obtained in Example 1A, Example 1B and Example 1C. It is a graph which shows the mechanical strength of the carbon sheet obtained in Example 1B, Example 1C and Comparative Example 1. It is a graph which shows the pressure loss of the carbon sheet obtained in Example 1B, Example 1C, Example 2B and Comparative Example 1. It is a graph which shows the discharge capacity of the lithium-air battery which used the carbon sheet obtained in Example 1B, Example 1C and Comparative Example 1 as an air electrode (positive electrode). It is a graph which shows the discharge capacity of the lithium-air battery which used the carbon sheet obtained in Example 2A, Example 2B and Comparative Example 1 as an air electrode (positive electrode).

The carbon sheet of the present invention is a multilayer carbon sheet containing (A) carbon nanofibers having an average diameter of less than 50 nm and (B) carbon fibers having an average diameter of 50 nm or more.

The carbon sheet may consist only of carbon nanofibers (A) and carbon fibers (B), or components other than carbon nanofibers (A) and carbon fibers (B), such as electrode catalysts, binders, conductive aids, etc. May contain the additive of.

The carbon sheet of the present invention contains carbon nanofibers (A) and carbon fibers (B), and forms a multi-layered structure as shown in the TEM image of FIG. The amount of carbon nanofibers (A) is 0.1 to 90% by weight, preferably 1 to 50% by weight, more preferably 5 to 30% by weight, based on the total weight of the carbon nanofibers (A) and the carbon fibers (B). By weight%, the amount of carbon fiber (B) is 10 to 99.9% by weight, preferably 50 to 90% by weight, and more preferably 70 to 95% by weight. The amount of the components other than the carbon nanofibers (A) and the carbon fibers (B) can be contained within a range that does not impair the effects of the present invention, and is 80% by weight or less, for example, 0, based on the weight of the carbon sheet. It may be ~ 70% by weight or 1-50% by weight.

When the carbon nanofibers (A) are used alone, a carbon sheet in which the carbon fiber fibers are uniformly present can be produced, but when the carbon fibers (B) are contained, the carbon nanofibers (A) or the carbon fibers (B) can be produced. ) Is separated. By making the structure of the carbon sheet non-uniform with the carbon fiber (B), the diffusivity inside can be improved, the utilization efficiency (space volume) inside the sheet can be increased, and the discharge capacity of the battery can be improved. .. On the other hand, if the carbon fiber (B) is used alone, it does not become a structure and it tends to be impossible to manufacture an independent sheet. Since the carbon nanofiber (A) is contained, a sheet having excellent mechanical strength can be manufactured.

(A) Carbon nanofibers The average diameter of carbon nanofibers (A) is less than 50 nm. For the carbon nanofiber (A), the upper limit of the average diameter is 45 nm or 40 nm, preferably 35 nm, more preferably 30 nm. For the carbon nanofiber (A), the lower limit of the average diameter may be 0.1 nm, for example 0.5 nm, preferably 1 nm. Examples of average diameters of carbon nanofibers (A) are 0.1-45 nm, 0.2-40 nm, 0.5-35 nm, or 1-30 nm. The carbon nanofiber (A) may be a single fiber fiber or may be formed by aggregating a plurality of fine fiber fibers.
The carbon nanofiber (A) forms an aggregated structure in the carbon sheet of the present invention, and is involved in imparting mechanical strength to the sheet and maintaining or improving conductivity.

Carbon average aspect ratio of nanofibers (A) (= fiber average length / fiber average diameter) is 50 to 10 ^5. Further, the average aspect ratio of the carbon nanofiber (A) is preferably ^{10 2} to 10 ^4. The carbon nanofiber (A) having a high average aspect ratio is likely to aggregate even when mixed with the carbon fiber (B), and the effect of the present invention is likely to be exhibited in the produced carbon sheet.

FIG. 1 is a cross-sectional view of the carbon sheet of the present invention, and in the carbon nanofiber (A), an agglomerated region is observed in the TEM image. It is considered that this agglomerated portion binds the carbon fibers (B) and enhances the mechanical strength of the carbon sheet.
The average diameter and average length of the fibers are arithmetic mean values for 200 fibers measured using a transmission electron microscope (or optical microscope).

The carbon nanofiber (A) may be a carbon nanofiber (that is, a raw material carbon nanofiber) obtained by a known method for producing the carbon nanofiber, but the raw material carbon nanofiber is irradiated with microwaves. The carbon nanofibers attached to (that is, the irradiated carbon nanofibers) are preferable.

As a method for producing raw carbon nanofibers, a method for producing carbon nanofibers by carbonizing organic fibers and a method for growing carbon nanofibers by contacting a carbon compound with metal fine particles as a catalyst (gas phase method). Can be mentioned. The vapor phase method, particularly the liquid pulse injection method (LPI method), is a method suitable for industrial production of carbon nanofibers.

The raw material carbon nanofibers may be irradiated with microwaves. The microwave irradiation conditions are not particularly limited, and are appropriately set according to the output method of the microwave irradiation device, the presence or absence of microwave irradiation unevenness, the amount of carbon nanofibers, the microwave absorption rate of carbon nanofibers, and the like. Can be done. The output of the microwave irradiation device is not particularly limited, but is preferably large, for example, 100 W or more, and preferably 500 W or more. For example, the output of the microwave irradiation device may be about 100 to 1500 W. The irradiation time of the microwave is also not particularly limited, but may be, for example, 0.1 to 100 minutes, particularly 1 to 10 minutes. The frequency of the microwave may be 300 MHz to 300 GHz (wavelength 1 m to 1 mm), preferably 1 GHz to 100 GHz, for example 2.45 GHz. Microwave irradiation may be performed continuously or intermittently, but when the average output is the same, continuous irradiation is preferable.

By irradiating with microwaves, the impurity (particularly ash content) content of the carbon nanofibers becomes small, the oxidation resistance of the carbon nanofibers becomes high, and the hydrophilicity of the carbon nanofibers becomes high. The higher the hydrophilicity, the better the carbon nanofibers are dispersed in water, and the higher the degree of dispersion of the carbon nanofibers in the obtained carbon sheet. Furthermore, the degree of graphitization of carbon nanofibers is increased by irradiation with microwaves.

The carbon nanofiber (A) can be produced by the above-mentioned production method, but can also be obtained from, for example, single-walled carbon nanotubes (Meijo Nanocarbon Co., Ltd., etc.).

(B) Carbon fiber The average diameter of the carbon fiber (B) is 50 nm or more. The carbon fiber (B) may be classified as carbon nanofiber (average diameter less than 1000 nm) or carbon microfiber (average diameter 1000 nm or more) depending on the average diameter. For carbon fiber (B), the lower limit of the average diameter may be 60 nm or 70 nm. The upper limit of the average diameter may be, for example, 100 μm, preferably 1 μm, more preferably 500 nm, and even more preferably 200 nm. Examples of the average diameter of the carbon fibers (B) are 50 nm to 100 μm, 55 nm to 1 μm, or 60 nm to 200 μm. The carbon fiber (B) may be a single fiber fiber or may be formed by aggregating a plurality of fine fiber fibers.

By mixing the carbon fiber (B), the utilization efficiency of the manufactured sheet as an electrode can be improved, and a high discharge capacity can be expected.
The average aspect ratio of the carbon fiber (B) (= average fiber length / average fiber diameter) is 10 to ³ , but the average aspect ratio of the carbon fiber (B) is preferably 10 to 100. If the average aspect ratio is high, it tends to aggregate itself, which may impair gas permeability.
FIG. 1 is a cross-sectional view of the carbon sheet of the present invention, and it is observed that the carbon fibers (B) are uniformly distributed in the TEM image without the fibers being entangled. It is considered that this region improves the gas permeability of the carbon sheet and enhances the utilization efficiency as an electrode.

The carbon fiber (B) can be produced, for example, by the same method as the carbon nanofiber (A) when the diameter is small (for example, less than 1 μm). Generally, the carbon fiber (B) is not irradiated with microwaves, but the carbon fiber (B) may be irradiated with microwaves.
The carbon fiber (B) can be produced by a usual method for producing carbon fiber when the diameter is large (for example, 1 μm or more).
The carbon fiber (B) can be produced by the above method, and for example, Dialead and K223HM (manufactured by Mitsubishi Chemical Corporation) can be obtained as products.

In the present invention, the carbon nanofibers (A) and the carbon fibers (B) are used to produce a sheet by freely selecting the average diameter thereof within the range described for the purpose of producing the carbon nanofibers (A). The difference in average diameter between the carbon fiber (B) and the carbon fiber (B) is preferably 10 to 100,000 nm, for example, 20 to 10000 nm or 50 to 1000 nm.
As a preferable combination of the carbon nanofibers (A) and the carbon fibers (B), the carbon nanofibers (A) have an average diameter of 1 to 40 nm and an average aspect ratio of 100 to 10 ⁵ , and the average of the carbon fibers (B). The diameter is 60 to 100 nm and the average aspect ratio is 10 to ³ , more preferably, the average diameter of the carbon nanofiber (A) is 2 to 30 nm and the average aspect ratio is 1000 to 10 ⁴ , and the average diameter of the carbon fiber (B). The average aspect ratio is 100 to 90 nm and the average aspect ratio is 100 to ³ .

The carbon nanofibers (A) and carbon fibers (B) may be processed fibers. The processing may vary, for example, doping carbon nanofibers with dissimilar elements, placing metal or metal oxides on carbon nanofibers, such that the carbon nanofibers have pores (eg, porous). To do. Examples of substances (to be doped or loaded) are iron, cobalt, nickel, gold, platinum, palladium, ruthenium, manganese, boron, nitrogen, phosphorus, sulfur, titanium, vanadium, oxides thereof and the like.

The carbon sheet can be produced as follows. The carbon nanofibers (A) and the carbon fibers (B) are dispersed in a dispersion medium. An example of a dispersion medium is an organic solvent (eg, toluene, ethylene glycol). Fiber dispersion is improved by using ultrasonic waves. The fiber dispersion is filtered, especially suction filtered. The resulting sheet is dried to remove the dispersion medium. The sheet is a self-supporting paper that retains the state of the sheet even when it is gripped and lifted with tweezers. It is not necessary to use a binder in the production of the sheet. In the absence of binder, the fibers in the sheet are fully entangled with each other to form the sheet.

The resistance of the carbon sheet due to gas permeation may be 700 Pacm ² / (cm ³ h -1 cm) or less, for example, ^{1 to 600 Pacm 2} / (cm ³ h ^-1 cm). When the resistance due to gas permeation is low, that is, the gas permeation is high, the amount of oxygen provided to the electrode becomes sufficient, and the reaction efficiency at the electrode tends to be high.
The electrical conductivity σ (S / cm) of the carbon sheet may be 0.1 S / cm or more, for example, 1 to 300 S / cm. Higher conductivity is preferable, but in these ranges, the conductivity is sufficient for the reaction at the electrode.
The mechanical strength of the carbon sheet is preferably such that the breaking stress is 0.001 MPa or more. Sufficient mechanical strength to stand on its own as a carbon sheet can be obtained.
The thickness of the carbon sheet may be 0.001 to 5 mm, for example 0.01 to 1 mm.

<Use of carbon sheet>
The carbon sheet of the present invention can be used as an electrode material, a plastic reinforced material, and a gas storage material. The carbon sheet is preferably an electrode material. Since the carbon sheet has a space between the carbon fibers, it is possible to form an electrode for supplying air (oxygen), that is, an air electrode. The carbon sheet can be used as an electrode for various devices, but the device is preferably a battery. Examples of batteries are primary or secondary batteries. The battery is preferably a metal-air battery or a fuel cell. Lithium-air batteries are particularly preferred.

In the following, examples of the device having an electrode include a metal-air battery, particularly a lithium-air battery, but the device is not limited to the metal-air battery and may be various devices.

Generally, a metal-air battery, particularly a lithium-air battery, is composed of a positive electrode, a negative electrode, an electrolyte arranged between the positive electrode and the negative electrode, and a separator. The oxygen permeable membrane may be laminated on the positive electrode, but in the present invention, the oxygen permeable membrane can be omitted. This is because the carbon sheet functions as an oxygen permeable membrane.

The positive electrode is composed of a carbon sheet. The positive electrode may have a positive electrode current collector in addition to the carbon sheet. The positive electrode may further have a gas diffusion layer. Examples of the material of the positive electrode current collector include metal materials such as aluminum, copper, nickel, titanium, stainless steel, and nickel-plated steel. The positive electrode current collector may be a mesh, but is preferably a sheet. For example, a positive electrode can be manufactured by crimping a carbon sheet to a positive electrode current collector.

The negative electrode is not particularly limited as long as it is used for a metal-air battery. The negative electrode is generally composed of a negative electrode active material and a negative electrode current collector holding the negative electrode active material. The negative electrode active material is not particularly limited as long as it has a metal such as lithium, silicon, or manganese, and examples thereof include simple metals, alloys, metal oxides, and metal nitrides. The material of the negative electrode current collector is not particularly limited as long as it has conductivity, and examples thereof include copper, stainless steel, nickel, and carbon. Examples of the shape of the negative electrode current collector include a foil shape, a plate shape, a mesh (grid) shape, and the like.

The electrolyte is not particularly limited as long as it is used for a metal-air battery. The electrolyte layer may be in the form of either a solution-based electrolyte represented by an organic solvent system containing a metal (lithium) salt or a solid-based electrolyte represented by a polymer solid electrolyte containing a metal (lithium) salt.

The separator is installed between the positive electrode and the negative electrode. The separator is not particularly limited as long as it has a function of electrically separating the positive electrode and the negative electrode. Examples of separators are porous membranes and non-woven fabrics made of resins such as polyethylene and polypropylene, and insulating materials such as non-woven fabrics made of glass fibers.

Hereinafter, the present disclosure will be described in detail with reference to examples, but the present disclosure is not limited to these examples.
In the following, parts or% or ratios represent parts by weight or% by weight or ratios by weight unless otherwise specified.
The average diameter and average aspect ratio of the fibers are arithmetic mean values of numerical values measured using a transmission electron microscope (or optical microscope) for 200 fibers.

In the production example, carbon nanofibers were produced by the liquid pulse injection method (LPI method).
Production Example 1: Production of CNF20 An initial solution was prepared which was a mixture of benzene, ferrocene, thiophene and methanol (weight ratio 14: 1.5: 0.3: 84.2). Using a vertical ceramic reactor equipped with an inlet and an electric furnace ^{, hydrogen gas was circulated at a flow rate of 1000 cm 3} / min and heated until the temperature at the center of the reactor reached 1473 K. At this temperature, 100 μL of the initial solution is injected into the reactor from the injection port using a syringe, and injection is repeated 100 times at 60-second intervals to obtain carbon nanofibers (CNF20) with an average diameter of 20 nm (average aspect ratio 10 ³ ). Was generated.

Production Example 2: Production of CNF80 Same as Production Example 1 except that a mixture of benzene, ferrocene, and thiophene (weight ratio 94: 5: 1) is used as the initial solution and the flow rate of hydrogen gas is 400 cm ^{3 / min.} The procedure was repeated to produce carbon nanofibers (CNF80) (average aspect ratio 100) having an average diameter of 80 nm.

Example 1A: Production of 20% mixture of carbon sheet CNF80 CNF20 and CNF80 (weight ratio of CNF20 to CNF80 80:20) (total 10 mg) were placed in a 100 mL Erlenmeyer flask, 20 mL of ethylene glycol was added thereto, and a stirrer (stir bar). The mixture was stirred with C8 × 30; AS ONE) for 2 to 3 hours. The stirrer was rotated at about 1000 rpm and applied to the wall surface of a 100 mL Erlenmeyer flask to apply shear stress and disperse fibrous carbon.
The stirred dispersion is suction-filtered using a membrane filter (PTFE, pore diameter 0.1 μm, diameter 25 mm, H010A025A; ADVANTEC), and the dispersion in the filter holder is completely eliminated from the triangular flask. Was replenished as appropriate so that it would not be interrupted.
The sample was molded into a sheet in the filter holder, and after confirming that no filtrate remained, ethylene glycol used as the dispersion solvent was thoroughly washed by flowing 20 mL of ethanol.
After that, take out the molded sample while it is placed on the membrane filter, fix the 3 or 4 sides of the membrane filter to the Teflon (registered trademark) sheet with cellophane tape so as not to touch the sample, and naturally prevent shrinkage or deformation. After drying, a CNF sheet (thickness: 200 μm) having a surface density of about 6 mg / cm ^{2 was obtained.}

Example 1B: Production of 50% mixture of carbon sheet CNF80 Carbon sheet CNF80 50% by repeating the same procedure as in Example 1A except that the amount of CNF80 is 50% by weight based on the total amount of CNF20 and CNF80. A mixture was produced.

Example 1C: Production of 80% mixture of carbon sheet CNF80 Carbon sheet CNF80 80% by repeating the same procedure as in Example 1A except that the amount of CNF80 is 80% by weight based on the total amount of CNF20 and CNF80. A mixture was produced.

Example 2A: Production of 20% mixture of carbon sheet CF10 Example 1A except that carbon fiber (Mitsubishi Chemical Dialead, K223HM, average aspect ratio 10) (CF10) having an average diameter of 10 microns is used instead of CNF80. The same procedure as in the above was repeated to produce a carbon sheet CF10 20% mixture.

Example 2B: Production of carbon sheet CF10 50% mixture A carbon sheet CF10 50% mixture was produced by repeating the same procedure as in Example 1B except that CF10 was used instead of CNF80.

Example 2C: Production of carbon sheet CF10 80% mixture A carbon sheet CF10 80% mixture was produced by repeating the same procedure as in Example 1C except that CF10 was used instead of CNF80.

Comparative Example 1
The carbon sheet CNF20 (CNF20 only) was produced by repeating the same procedure as in Example 1A except that the CNF80 was omitted, that is, only the CNF20 was used.

Test Example 1: Observation with a transmission electron microscope Carbon obtained in Example 1A (carbon sheet CNF80 20% mixture), Example 1B (carbon sheet CNF80 50% mixture) and Example 1C (carbon sheet CNF80 80% mixture) The sheet was observed with a transmission electron microscope (JSM-6500F; JEOL Ltd.).
The result (micrograph) is shown in FIG. In FIG. 1, carbon nanofibers (CNFs) forming aggregates are laminated to form a layered sheet structure, and CNF 20 is present in the sheet in an aggregated state (dotted circled portion). Shown.

Test Example 2: Measurement of Mechanical Strength With respect to the carbon sheets obtained in Example 1B (carbon sheet CNF80 50% mixed) and Example 1C (carbon sheet CNF80 80% mixed), the mechanical strength was measured as follows. did.
A sheet electrode having a diameter of 35 mm was prepared, and three test pieces having a diameter of 30 mm × 5 mm were cut out from the sheet electrode. The test piece was placed on a universal material testing machine (AG-100kNXplus; Shimadzu Corporation) with a margin of 10 mm above and below the test piece, and tensile strength was measured until the sample broke, and displacement and stress were measured.
The result (graph of mechanical strength) is shown in FIG. FIG. 2 shows that the carbon sheet CNF80 50% mixture and the carbon sheet CNF80 80% mixture have breaking stresses of 0.95 MPa and 0.46 MPa, respectively, and have sufficient mechanical strength. The carbon sheet of the present invention is self-supporting and has no problem in handling.

Test Example 3:
Measurement of pressure loss (measurement of gas permeability)
Obtained in Example 1B (carbon sheet CNF80 50% mixed), Example 1C (carbon sheet CNF80 80% mixed), Example 2B: carbon sheet CF10 50% mixed and Example 3B (carbon sheet CNF30 50% mixed). Regarding the carbon sheet, the pressure loss was determined by fixing the carbon sheet to a cell having an inner diameter of 11 mm and allowing air to permeate at an air ^{flow rate of 100 to 400 cm 3 / min.}
The result (graph of pressure loss) is shown in FIG. ^{FIG. 3 shows the resistance values of 91 Pacm 2} / (cm ³ h ^-1 cm) and 51 Pacm ² / (cm ³ h ^-1 cm) for gas permeation for the carbon sheet CNF80 50% mixture and the carbon sheet CNF80 80% mixture, respectively. The carbon sheet of the present invention has sufficiently high gas permeability.

Test Example 4: Measurement of conductivity The carbon sheets obtained in Example 1A (carbon sheet CNF80 20% mixed), Example 1B (carbon sheet CNF80 50% mixed) and Example 1C (carbon sheet CNF80 80% mixed) were used. The conductivity was measured using.
Both ends of the carbon sheet cut into a rectangular (10 mm × 20 mm) shape were fixed to the current collector with a conductive paste (Dotite D-500, manufactured by Fujikura Kasei Co., Ltd.). Both ends of the carbon sheet were connected to potentiostats (HA-151, manufactured by Hokuto Denko Co., Ltd.), different voltages were applied between both ends of the carbon sheet, and the resulting current was measured to obtain an IV plot. The conductivity of the carbon sheet was calculated from the gradient of the IV plot. The results are shown in Table 1. In Table 1, each is in the range of 30 to 300 S / cm, indicating that the carbon sheet of the present invention has sufficient conductivity as an electrode.

Test Example 4: Measurement of discharge capacity A carbon sheet was punched into a circle with a diameter of 16 mm using a punching tool. 2032 type coin batteries with a small hole in the cathode cap that permits O ₂ diffusion was assembled filled glove box Ar. Lithium metal, fiberglass filter (GF / A, Whatman), and CF ₃ SO ₃ Li TEGDME solution (TEGDME / CF ₃ SO ₃ Li molar ratio = 4: 1) were applied to the counter electrode (negative electrode), separator, and separator, respectively. Used as an electrolyte. In an airtight box where oxygen is ^{supplied at a flow rate of 10 cm 3} / min, the battery is held in an oxygen atmosphere for 10 hours and then at a current density of 50 mA / g (0.3 mA / cm ² ) with respect to Li / Li ^+. It was discharged to 2.0V.
The results are shown in FIGS. 4-5. FIGS. 4 to 5 show the present invention because the CNF80 80% mixed sheet and the CF10 50% mixed sheet have discharge capacities of about 2.2 times and about 1.4 times that of the CNF20 sheet of Comparative Example 1, respectively. It is shown that the lithium-air battery using the carbon sheet of No. 1 as an air electrode (positive electrode) has a sufficient discharge capacity.

The carbon sheet of the present invention can be used as an electrode material, a plastic reinforced material, and a gas storage material. The carbon sheet is preferably an electrode material, particularly an air electrode material.
Carbon sheets can be used as electrodes for various devices, including batteries, but examples of batteries are primary or secondary batteries. The battery is preferably a metal-air battery such as a lithium-air battery or a fuel cell.

Claims (9)

A carbon sheet containing (A) carbon nanofibers having an average diameter of less than 50 nm and (B) carbon fibers having an average diameter of 50 nm or more.
A multilayer carbon sheet in which the amount of carbon nanofibers (A) is 0.1% by mass or more with respect to the carbon sheet. The carbon sheet according to claim 1, wherein the carbon nanofiber (A) has an average aspect ratio of 50 to 10 ⁵ , and the carbon fiber (B) has an average aspect ratio of 10 to ^3. The carbon sheet according to claim 1 or 2, wherein the pressure loss due to gas permeation is 700 kPa / ms ^{-1 or less.} The amount of carbon nanofibers (A) is 0.1 to 90% by weight and the amount of carbon fibers (B) is 10 to 99% by weight with respect to the total weight of the carbon nanofibers (A) and the carbon fibers (B). The carbon sheet according to any one of claims 1 to 3, which is 9% by weight. An electrode having the carbon sheet according to any one of claims 1 to 4. The electrode according to claim 5, which is the positive electrode of the battery. A battery comprising the electrode according to claim 5 or 6. The battery according to claim 7, which is a lithium-air battery. The carbon sheet according to any one of claims 1 to 4, wherein the carbon nanofibers (A) and the carbon fibers (B) are dispersed in a dispersion medium, the dispersion liquid is filtered, and the obtained sheet is dried. Manufacturing method.

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