JP7675352B2 - ELECTRODE FOR ELECTRIC STORAGE DEVICE, SECONDARY BATTERY, AND METHOD FOR MANUFACTURING ELECTRODE FOR ELECTRIC STORAGE DEVICE - Google Patents

Description

This application relates to an electrode for an electricity storage device, a secondary battery, and a method for manufacturing an electrode for an electricity storage device.

Examples of chargeable and dischargeable electricity storage devices include secondary batteries and electric double layer capacitors. In electrodes for these electricity storage devices, carbon may be used as the active material placed on the surface of the current collector. For example, Patent Documents 1 and 2 below disclose a technique in which the active material layer of the negative electrode of a lithium ion secondary battery is made of graphite or carbon nanowalls. When carbon nanowalls are used in the active material layer, it is expected that the battery performance of the electricity storage device, such as the charging capacity and charging speed, will be improved compared to when graphite is used.

Patent No. 2668678 JP 2010-9980 A

However, the conventional active material layers described above have still not been able to fully achieve the high battery performance required for electricity storage devices. The present application aims to provide an electrode for an electricity storage device having a new active material layer that can improve the performance of the electricity storage device.

Through extensive research into electrodes for electricity storage devices that use carbon as an active material, the inventors of the present invention have succeeded in developing an active material layer that has a different structure from conventional graphite and carbon nanowalls and can dramatically improve the performance of electricity storage devices. The present invention can be realized, for example, in the following forms:

A first aspect of the present invention is provided as an electrode for an electricity storage device. The electrode for an electricity storage device of this aspect includes a metal substrate constituting a current collector, and an active material layer formed on a surface of the metal substrate and containing carbon as an active material, the active material layer being composed of a carbon nanostructure having a base portion formed of stacked graphene extending in an elongated manner in a thickness direction of the metal substrate, and a plurality of extension portions composed of the carbon and extending from the base portion.

In this form of electrode for an electricity storage device, the active material layer is composed of a carbon nanostructure with a novel structure. With this active material layer, when the electricity storage device is charged, a dense, flat, and well-formed metal layer is formed on the surface of the active material layer, on which metal ions involved in the charging reaction are precipitated. This makes it possible to improve the battery performance of the electricity storage device.

The present invention can be realized in various forms other than an electrode for an electricity storage device. For example, it can be realized in the form of an electricity storage device such as a secondary battery equipped with an electrode for an electricity storage device, a manufacturing method for an electrode for an electricity storage device, a manufacturing method for an electricity storage device, a manufacturing method for an active material layer, a manufacturing apparatus suitable for carrying out these manufacturing methods, a control program for controlling the manufacturing apparatus, a recording medium on which the control program is recorded, etc.

FIG. 1 is a schematic diagram showing the configuration of a secondary battery including an electrode for an electricity storage device. FIG. 2 is a schematic cross-sectional view showing a cross-sectional structure of an electrode. FIG. 1 is a schematic diagram showing the configuration of a carbon nanostructure. FIG. 2 is a schematic diagram showing the deposition of metal atoms in an active material layer during charging. FIG. 2 is a process flow diagram showing a manufacturing process of a secondary battery. FIG. 2 is a schematic diagram showing a configuration of a first processing apparatus. FIG. 4 is a schematic diagram showing a configuration of a second processing apparatus. Schematic diagram showing a process of generating an extension portion of a carbon nanostructure. FIG. 4 is an explanatory diagram showing a photographed image in an embodiment. 1A to 1C are explanatory diagrams showing, by photographic images, the growth process of a carbon nanostructure according to an embodiment of the present invention. FIG. 11 is an explanatory diagram showing photographed images of an example and a comparative example. FIG. 4 is an explanatory diagram showing the results of an evaluation test of the battery performance of the examples. FIG. 4 is an explanatory diagram showing the results of an evaluation test of the battery performance of the first comparative example. FIG. 11 is an explanatory diagram showing the results of an evaluation test of the battery performance of the second comparative example. FIG. 4 is an explanatory diagram showing the results of evaluation tests of the battery performance of Examples and Comparative Examples. FIG. 4 is an explanatory diagram showing photographed images of precipitated layers of metal atoms in an example and a comparative example. FIG. 4 is an explanatory diagram showing photographed images of active material layers of an example and a comparative example on whose surfaces water droplets have been dropped. FIG. 11 is an explanatory diagram showing a photographed image of an active material layer of a comparative example produced by a production method different from that of the example.

Hereinafter, an embodiment of an electrode for an electricity storage device according to the present invention will be described with reference to the drawings. In this specification, the term "electricity storage device" refers to a chargeable and dischargeable electricity storage body, such as a secondary battery or an electric double layer capacitor, and an apparatus including the electricity storage body.

1. Embodiments:
1-1. Structure of secondary battery:
Fig. 1 is a schematic diagram showing the configuration of a secondary battery 10 according to the present embodiment. The secondary battery 10 according to the present embodiment is a lithium ion secondary battery in which lithium ions are involved in charging and discharging. The secondary battery 10 includes a container 11, an electrolyte 12, a separator 15, a first electrode 20, and a second electrode 30. In Fig. 1, for convenience, the container 11 is shown by a dashed line, and the separator 15 is shown by a broken line.

The container 11 has an internal space filled with the electrolyte solution 12. The container 11 is liquid-tightly formed from a material that is difficult to react with the electrolyte solution 12. The electrolyte solution 12 has a property of being capable of transmitting metal ions involved in charging and discharging between the first electrode 20 and the second electrode 30. In this embodiment, the electrolyte solution 12 is formed from a solution in which a lithium salt is dissolved in an organic solvent, and is capable of transmitting lithium ions between the first electrode 20 and the second electrode 30. For example, lithium hexafluorophosphate (LiPF ₆ ) can be used as the lithium salt of the electrolyte solution 12. In addition, for example, dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), etc. can be used as the organic solvent.

The separator 15 divides the internal space of the container 11 into a first electrode chamber 16 in which the first electrode 20 is accommodated, and a second electrode chamber 17 in which the second electrode 30 is accommodated. The separator 15 has electrical insulation and ion conductivity, electrically insulates the first electrode 20 from the second electrode 30, and allows the lithium ions transmitted through the electrolyte 12 to pass through. The separator 15 is made of, for example, a resin film or nonwoven fabric having a porous structure.

The first electrode 20 corresponds to the electrode for the power storage device in this embodiment. In the secondary battery 10 of this embodiment, the first electrode 20 constitutes the negative electrode. The first electrode 20 includes a metal substrate 21 and an active material layer 22.

The metal substrate 21 constitutes a current collector. In this embodiment, the metal substrate 21 is made of a metal foil of copper (Cu). The metal substrate 21 may be made of a metal other than Cu. In other embodiments, the metal substrate 21 may be made of, for example, any one of a Cu alloy, aluminum (Al), and an Al alloy. The metal substrate 21 does not have to be made of a metal foil, and may be made of, for example, a thin metal plate. The metal substrate 21 does not have to be made of a flat plate, and may be bent into various shapes, such as a cylindrical shape or a corrugated shape.

The active material layer 22 is provided on both the first surface 21a and the second surface 21b of the metal substrate 21. The active material layer 22 contains carbon (C) as an active material. Details of the configuration of the metal substrate 21 and the active material layer 22 of the first electrode 20, as well as the manufacturing method, will be described later. In the following description, the first electrode 20 will also be simply referred to as "electrode 20."

The second electrode 30 constitutes the positive electrode of the secondary battery 10. The second electrode 30 has a positive electrode collector 31 and a positive electrode active material layer 32. The positive electrode collector 31 is made of a metal foil such as Al or titanium (Ti). The positive electrode collector 31 may be made of other metals or may have a form other than a metal foil. The positive electrode collector 31 does not have to be made in a flat shape, and may be bent into various shapes such as a cylindrical shape or a wavy shape.

The positive electrode active material layer 32 is formed on each of the first surface 31a and the second surface 31b of the positive electrode current collector 31. The positive electrode active material layer 32 contains a positive electrode active material containing Li atoms, a conductive assistant, and a binder. The positive electrode active material layer 32 may contain a thickener. As the positive electrode active material, for example, a ternary substance can be used, and lithium cobalt oxide (LiCoO ₂ ), lithium manganate (LMO), or lithium nickel oxide (NCA) can be used. As the conductive assistant, for example, acetylene black or carbon black can be used. As the binder, for example, polyvinylidene fluoride (PVDF) or styrene butadiene rubber (SBR) can be used. As the thickener, for example, carboxymethyl cellulose (CMC) can be used.

The configuration of the electrode 20 of this embodiment will be described with reference to Figures 2 and 3 in order.

Figure 2 is a schematic cross-sectional view illustrating the cross-sectional structure of the electrode 20 at any cut surface along the thickness direction. In Figure 2, the configuration of the electrode 20 on the first surface 21a side of the metal substrate 21 is illustrated. The configuration of the electrode 20 on the second surface 21b side of the metal substrate 21 is the same as the configuration on the first surface 21a side shown in Figure 2.

As described above, an active material layer 22 is provided on the surface of the metal substrate 21. The active material layer 22 has an amorphous carbon layer 23 that covers the surface of the metal substrate 21, and a carbon nanostructure 25 formed on the amorphous carbon layer 23.

The amorphous carbon layer 23 is a thin film layer made of carbon, and is the starting point for the growth of the carbon nanostructures 25. The amorphous carbon layer 23 is well formed, thereby improving the formation state of the carbon nanostructures 25. The thickness of the amorphous carbon layer 23 is preferably, for example, 10 nm or more and 300 nm or less. The thickness of the amorphous carbon layer 23 is more preferably 10 nm or more and 100 nm or less, and even more preferably 12 nm or more and 30 nm or less.

The carbon nanostructures 25 are tiny structures made entirely of carbon, and are formed so as to be distributed over the entire surface of the amorphous carbon layer 23. The carbon nanostructures 25 have a base 26 that extends in an elongated manner in the thickness direction of the metal substrate 21, and a number of extensions 27 that extend from the base 26. Many of the extensions 27 are formed so as to branch out in a branch-like manner at the upper end side of the base 26. The carbon nanostructures 25 are graphite-like substances, and therefore have a higher electrical conductivity than carbon materials such as activated carbon.

Figure 3 is a schematic diagram showing the configuration of a single carbon nanostructure 25. For convenience, the graphene GS constituting the base 26 is shown in FIG. 3 as a roughly rectangular sheet. Also, for convenience, multiple extensions 27 are shown in FIG. 3 with dashed lines.

The base 26 has a multi-layer structure in which multiple graphene GS are stacked in the thickness direction. Graphene GS, also known as a graphene sheet, is a sheet-like material with a thickness of one carbon atom, composed of a six-membered carbon ring structure, that is, a hexagonal lattice structure with carbon atoms as vertices.

The number of layers of graphene GS in the base 26 varies, and the base 26 is formed in a needle, plate, or pleat shape. The base 26 can also be interpreted as a structure similar to a carbon nanowall or the similar carbon nanoflake or carbon nanoflower. The base 26 is arranged in a random mesh pattern on the surface of the metal substrate 21, as will be shown in the photographed image later.

The base 26 does not have to be entirely made of single crystals of carbon six-membered rings. The graphene GS constituting the base 26 may not have a complete graphene structure, but may be a thin film mainly composed of carbon with a six-membered ring structure. The graphene GS may have a mosaic structure mainly composed of carbon with a six-membered ring structure. The mosaic structure means a structure in which multiple regions composed of carbon six-membered ring structures are discretely arranged.

The height Ha of the base 26 corresponds to the length from the lower end of the base 26 on the metal substrate 21 side to the apex, that is, the height from the surface of the amorphous carbon layer 23 to the upper end of the graphene GS. The average height Ha of the base 26 is preferably 0.1 μm or more, and more preferably 0.5 μm or more. The average height Ha of the base 26 is even more preferably 0.8 μm or more. The average height Ha of the base 26 may be 50.0 μm or less. The average height Ha of the base 26 is preferably 25.0 μm or less, and more preferably 10.0 μm or less. The average height Ha of the base 26 is even more preferably 5.0 μm or less. The average height Ha of the base 26 is even more preferably 2.5 μm or less, and even more preferably 2.0 μm or less.

The average thickness W of the base 26 is preferably, for example, 0.5 nm or more and 100.0 nm or less. The average thickness W is more preferably 1.0 nm or more and 50.0 nm or less, and even more preferably 1.5 nm or more and 30.0 nm or less.

Each extension 27 is made of carbon and is formed in a needle, column, plate, or pleat shape. Many of the extensions 27 are randomly distributed on the surface of the upper end of the base 26. By forming each extension 27 on the base 26, the gaps between the carbon nanostructures 25 near the surface of the active material layer 22 are reduced.

The height Hb of the carbon nanostructure 25 corresponds to the height from the lower end of the base 26 on the metal substrate 21 side to the upper end of the extension 27. The height Hb of the carbon nanostructure 25 is preferably 0.5 μm or more, and more preferably 0.8 μm or more. Furthermore, the height Hb of the carbon nanostructure 25 is even more preferably 1.0 μm or more, and even more preferably 1.2 μm or more.

However, if one attempts to form a taller carbon nanostructure 25, the time required to form the carbon nanostructure 25 increases accordingly. Therefore, from the viewpoint of increasing the productivity of the electrode 20, it is preferable that the average height Hb of the carbon nanostructures 25 is 10.0 μm or less. It is more preferable that the average height Hb of the carbon nanostructures 25 is 8.0 μm or less, and even more preferable that the average height Hb of the carbon nanostructures 25 is 5.0 μm or less. It is even more preferable that the average height Hb of the carbon nanostructures 25 is 3.0 μm or less.

Here, the greater the density of the carbon nanostructures 25 in the active material layer 22, the higher the battery performance of the secondary battery 10 can be. In this specification, the density of the carbon nanostructures 25 in the active material layer 22 is a value determined as follows. Each carbon nanostructure 25 is divided into a plurality of unitary units for each predetermined volume. The number of unitary units present per unit area in an image taken directly facing the surface of the active material layer 22 is measured as the density.

In the active material layer 22, the density of the carbon nanostructures 25 is preferably 50 or more per 1 μm ² , more preferably 60 or more per 1 μm ² , and further preferably 70 or more per 1 μm 2 .

However, if an attempt is made to excessively increase the density of the carbon nanostructures 25 in the active material layer 22, it will take time and effort to form the carbon nanostructures 25, which may reduce the productivity of the electrode 20. The density of the carbon nanostructures 25 in the active material layer 22 is preferably 200 or less ^per 1 μm2, and more preferably 100 or less.

The density of the carbon nanostructures 25 in the active material layer 22 increases as the average spacing between adjacent carbon nanostructures 25 in the active material layer 22 decreases, and decreases as the average spacing increases. The average spacing here is calculated as the average value of the measured lengths of the line segments partitioned by adjacent carbon nanostructures 25 on each line, by drawing straight lines in a certain direction at a number of predetermined locations on a captured image in which the carbon nanostructures 25 are projected in the height direction.

The average spacing of the carbon nanostructures 25 in the active material layer 22 is preferably 40 nm or less, and more preferably 30 nm or less. It is even more preferable that the average spacing of the carbon nanostructures 25 is 20 nm or less. However, it is preferable that the average spacing is 10 nm or more.

1-2. Battery reaction in secondary batteries:
The chemical reactions occurring during charging and discharging in the secondary battery 10 can be expressed, for example, by the following reaction formula: When the positive electrode material is _LiCoO2 , the reaction formula at the positive electrode, the second electrode 30, is expressed by the following formula (1): x represents the proportion of reacting atoms and is a real number greater than 0 and less than 1.
Li _1−x CoO ₂ + xLi ⁺ + xe− ⇔ LiCoO ₂ …(1)

In contrast, the reaction formula at the negative electrode 20 is expressed by the following formula (2): As shown in formula (2), lithium is deposited on the surface layer of the electrode 20 when the secondary battery 10 is charged.
Li ⁺ + e ^- ⇔ Li...(2)

Here, in the case of a conventional negative electrode configuration in which graphite is disposed as an active material on the surface of a flat metal substrate, in principle, one lithium ion is absorbed for every six carbon atoms during charging, forming lithium carbide (LiC ₆ ). Therefore, the reaction formula for the conventional configuration as a comparative example is expressed as the following formula (3).
Reaction formula in conventional configuration: xLi ⁺⁺ _C6 +xe ^- _⇔LixC6 ...( ₃ )

As shown in formula (3) above, in the case of a conventional electrode that uses graphite as the negative electrode active material, the number of lithium ions that can be absorbed by charging is determined by the number of carbon atoms contained in the active material layer.

In contrast, in the case of the electrode 20 used in the secondary battery 10 of this embodiment, as shown in the above formula (2), theoretically, charging is possible as long as lithium can be precipitated without being limited by the number of carbon atoms in the active material layer 22. In other words, according to the electrode 20 of this embodiment, two or more lithium ions per carbon atom can be involved in the charge/discharge reaction in one charge or discharge. Therefore, the secondary battery 10 of this embodiment has an increased charge capacity by having the electrode 20 as the negative electrode.

Furthermore, in the secondary battery 10 of this embodiment, the carbon nanostructures 25 of the active material layer 22 of the electrode 20 have the extensions 27, which improves the battery performance compared to a case in which the carbon nanostructures 25 do not have the extensions 27. The inventor of the present invention speculates that the reason for this is as follows.

Figure 4(a) is a schematic diagram showing how metal atoms 28 are precipitated in the active material layer 22 of the electrode 20 upon charging in the secondary battery 10 of this embodiment. As described above, the metal atoms 28 are lithium atoms.

Figure 4(b) is a schematic diagram showing how metal atoms 28 are precipitated in the active material layer 22a of the electrode 20a upon charging in the secondary battery 10a of the comparative example. The secondary battery 10a of the comparative example has a configuration substantially the same as that of the secondary battery 10 of the present embodiment, except that the active material layer 22a of the electrode 20a does not have an extension 27 and is composed of a carbon nanostructure 25a composed only of a portion corresponding to the base 26.

According to nucleation theory, in the active material layer 22 of the secondary battery 10 of this embodiment, the multiple extensions 27 of the carbon nanostructure 25 can function as protrusions that serve as starting points for promoting the precipitation of metal atoms 28. Therefore, with the active material layer 22 of this embodiment, which has many tiny protrusions, the metal atoms 28 are efficiently precipitated during charging.

In contrast, as shown in FIG. 4(b), the carbon nanostructure 25a constituting the active material layer 22a of the secondary battery 10a of the comparative example has many flat surfaces, and the efficiency of the deposition of metal atoms 28 is lower than that of the carbon nanostructure 25 of the present embodiment. Therefore, according to the secondary battery 10 of the present embodiment, it is possible to deposit metal atoms 28 more efficiently and in a shorter time during charging than the secondary battery 10a of the comparative example.

In addition, in the active material layer 22 of the secondary battery 10 of this embodiment, the presence of multiple extensions 27 reduces the gaps between each carbon nanostructure 25. Therefore, when the secondary battery 10 is charged, the formation of a layer of metal atoms 28 having a denser and smoother surface is promoted, the thickness of the layer of metal atoms 28 increases, and the amount of precipitation of metal atoms 28 can be increased.

In contrast, in the active material layer 22a of the secondary battery 10a of the comparative example, the gaps between the carbon nanostructures 25a are larger because the active material layer 22a does not have the extensions 27. Therefore, the metal atoms 28 do not precipitate sufficiently between the carbon nanostructures 25a, and there is a possibility that voids VS will occur inside the layer of the metal atoms 28.

In this way, the secondary battery 10 of this embodiment can increase the amount of metal atoms 28 precipitated at the electrode 20 compared to the secondary battery 10a of the comparative example when charging is performed under the same charging conditions. Therefore, the secondary battery 10 of this embodiment can increase the charging capacity compared to the secondary battery 10a of the comparative example.

Furthermore, according to the secondary battery 10 of this embodiment, as described above, the precipitation of the metal atoms 28 at the electrode 20 is promoted, making it easier to form a uniform layer of the metal atoms 28, and enabling stable and smooth charge and discharge reactions. In addition, according to the secondary battery 10 of this embodiment, the layer of the metal atoms 28 is formed more evenly, which prevents the amount of precipitation of the metal atoms 28 from becoming locally large, thereby suppressing the occurrence of dendrites.

1-3. Manufacturing method of secondary battery:
5 is a process flow diagram showing the manufacturing process of the secondary battery 10. Processes P1 and P2 are manufacturing processes of the electrode 20. In processes P1 and P2, the carbon nanostructures 25 constituting the active material layer 22 of the electrode 20 are formed by two types of CVD (chemical vapor deposition) methods. The two types of CVD methods in this embodiment are a radical-injection plasma-enhanced (RI-PE) CVD method and a capacitively coupled plasma (CPP) CVD method.

Step P1 corresponds to a step of growing the base 26 of the carbon nanostructure 25 on the surface of the metal substrate 21 using a RI-PECVD apparatus.

Figure 6 is a schematic diagram showing the configuration of the first processing apparatus 40, which is the RI-PECVD apparatus used in process P1. The first processing apparatus 40 includes, at its upper stage, a plasma generation chamber 41, a waveguide 42, a quartz window 43, a slot antenna 44, a microwave generator 45, and a radical source supply unit 46.

The plasma generation chamber 41 is a space where surface wave plasma (SWP) is generated by microwaves. The waveguide 42 is a space for transmitting microwaves to the plasma generation chamber 41. The waveguide 42 is partitioned by a quartz window 43 at the upper stage of the plasma generation chamber 41. Slot antennas 44 are arranged on the quartz window 43.

The microwave generating unit 45 introduces microwaves MW into the waveguide 42. The microwave generating unit 45 can generate microwaves with a frequency of 2.00 to 3.00 GHz, for example, with a power of 300 to 500 W. The microwaves MW introduced into the waveguide 42 are introduced into the plasma generation chamber 41 by the slot antenna 44.

When microwaves MW are introduced into the plasma generation chamber 41, high density plasma (HDP) is generated at the quartz window 43. The high density plasma HDP diffuses into the plasma generation chamber 41 and becomes surface wave plasma SWP.

The radical source supply unit 46 supplies a gas that serves as a radical source to the plasma generation chamber 41. For example, hydrogen can be used as the radical source. Note that the gas that serves as the radical source is not limited to hydrogen, and may be, for example, oxygen, nitrogen, or other gases. Ions of the radical source gas are mixed into the surface wave plasma SWP of the plasma generation chamber 41.

The first processing device 40 further includes a reaction chamber 50, a partition wall 51, a raw material gas supply unit 54, a power supply unit 55, a support base 56, a heater 57, and an exhaust unit 58 below the plasma generation chamber 41.

The reaction chamber 50 is partitioned below the plasma generation chamber 41 by a metal partition wall 51. The partition wall 51 has a number of through holes 52 for allowing the surface wave plasma SWP generated in the plasma generation chamber 41 to flow into the reaction chamber 50.

As described above, in the plasma generation chamber 41, ions of the radical source are mixed into the surface wave plasma SWP. Many of the ions of the radical source collide with the partition wall 51 provided between the reaction chamber 50 and the plasma generation chamber 41, becoming radicals by being neutralized, and are introduced into the reaction chamber 50 together with the surface wave plasma SWP.

A gas flow passage 53 for supplying raw material gas to the reaction chamber 50 is provided inside the partition wall 51. The raw material gas supply unit 54 is connected to the gas flow passage 53 and supplies the raw material gas into the reaction chamber 50 through the gas flow passage 53.

The raw material gas includes a carbon-based gas containing the carbon that constitutes the carbon nanostructure 25, and a reaction contributing gas that contributes to the growth of the carbon nanostructure 25. For convenience, detailed illustration is omitted in FIG. 6, but the raw material gas supply unit 54 has a configuration similar to the raw material gas supply unit 65 of the second processing device 60 shown in FIG. 7, and the flow rates of the carbon-based gas and the reaction contributing gas can be adjusted.

As the carbon-based gas, for example, methane ( _CH4 ) or _{hexafluoroethane} ( _C2F6 ) can be used. As the reaction contributing gas, for example, hydrogen ( _H2 ) or argon (Ar) can be used. The reaction contributing gas promotes the plasmatization of the carbon-based gas by the Penning effect in the plasma processing. In addition, the reaction contributing gas is ionized in the plasma processing and collides with the metal substrate 21 to be processed, thereby forming nuclei that become the growth starting points of the carbon nanostructures 25.

The power supply unit 55 is connected to the partition 51 and applies a high-frequency voltage for plasma processing to the partition 51. The power supply unit 55 is capable of applying a power of 300 to 400 W and a high-frequency voltage with a frequency of 80 to 120 MHz. The partition 51 functions as an electrode for plasma processing.

The support base 56 is provided below the partition wall 51 and has a mounting surface 56s facing the partition wall 51. The metal substrate 21 is placed on the mounting surface 56s. The support base 56 is earthed and functions as an electrode for plasma processing. The heater 57 is provided inside the support base 56 and heats the metal substrate 21 placed on the mounting surface 56s of the support base 56 to a predetermined temperature.

The exhaust unit 58 is connected to the reaction chamber 50. The exhaust unit 58 is equipped with a vacuum pump, which controls the pressure inside the reaction chamber 50 during plasma processing to a predetermined pressure. In addition to the components described above, the first processing device 40 may also be equipped with a measuring unit equivalent to the vacuum gauge 81 and thermocouple 82 of the second processing device 60 shown in FIG. 7.

In step P1, first, the metal substrate 21 is placed in the reaction chamber 50 of the first processing device 40. The metal substrate 21 is placed on the mounting surface 56s of the support base 56. The heater 57 heats the metal substrate 21 to a temperature of, for example, about 600 to 800°C.

In step P1, the microwave generator 45 generates microwaves with a frequency of 2.00 to 3.00 GHz, for example, at a power of 300 to 500 W. The microwaves are introduced into the plasma generation chamber 41 through the waveguide 42 and the slot antenna 44. A radical source gas is also supplied to the plasma generation chamber 41 from the radical source supply unit 46. As a result, a surface wave plasma SWP containing radicals is generated in the plasma generation chamber 41 as described above, and is introduced into the reaction chamber 50 through the through hole 52 in the partition wall 51.

Furthermore, in process P1, the raw material gas supply unit 54 supplies raw material gas to the reaction chamber 50. The raw material gas supply unit 54 supplies, as the raw material gas, for example, a carbon-based gas at a flow rate of 80 to 120 sccm, and also supplies a reaction contributing gas at a flow rate of 40 to 60 sccm. The pressure inside the reaction chamber 50 is controlled by the exhaust unit 58 to, for example, about 0.5 to 1.5 Pa.

While the raw material gas is being supplied to the reaction chamber 50, the power supply unit 55 applies a high-frequency voltage to the partition wall 51. The power supply unit 55 applies a high-frequency voltage with a frequency of 80 to 120 MHz, for example, at a power of 300 to 500 W. The plasma processing in the first processing device 40 continues for, for example, about 5 to 15 minutes.

In the plasma treatment, a capacitively coupled plasma (CCP) is generated between the partition wall 51 and the metal substrate 21 on the support base 56. When the capacitively coupled plasma (CCP) is generated, an amorphous carbon layer 23 is first formed to cover the entire surface of the metal substrate 21. Then, bases 26 of carbon nanostructures 25 are generated to be distributed over the entire surface of the metal substrate 21. Each base 26 grows to extend upward from the amorphous carbon layer 23.

Returning to FIG. 5, process P2 corresponds to a process of forming multiple extensions 27 on the base 26 generated in process P1 using a CCP-CVD device.

Figure 7 is a schematic diagram showing the configuration of the second processing apparatus 60, which is a CCP-CVD apparatus used in process P2. The second processing apparatus 60 comprises a reaction chamber 61, an exhaust section 62, a source gas supply section 65, a support base 70, an upper electrode 75, and a power supply section 76. The second processing apparatus 60 further comprises a vacuum gauge 81 and a thermocouple 82 as measuring sections.

The reaction chamber 61 has an airtight structure capable of maintaining a vacuum. The reaction chamber 61 is connected to an exhaust unit 62 and a raw material gas supply unit 65. The exhaust unit 62 is equipped with a vacuum pump, and controls the reaction chamber 61 to a predetermined pressure (degree of vacuum) during plasma processing. The degree of vacuum in the reaction chamber 61 is measured by a vacuum gauge 81.

The raw gas supply unit 65 supplies the raw gas to the reaction chamber 61. The raw gas supply unit 65 includes a storage unit 66, a flow control unit 67, and an introduction pipe 68. The storage unit 66 stores multiple types of gases that make up the raw gas, sorted by type. The raw gas used in the second processing device 60 is the same as that described for the second processing device 60.

In the storage section 66, the multiple types of gas are stored individually in tanks (not shown). The flow rate control section 67 includes a gas delivery device 67a and a flow rate adjustment valve 67b. The gas delivery device 67a and the flow rate adjustment valve 67b are provided in the pipes for the multiple types of gas connected to the storage section 66. The gas delivery device 67a is composed of, for example, an ejector or a pump, and delivers the gas from the storage section 66 at a predetermined pressure. The flow rate of the gas delivered by the gas delivery device 67a is controlled by the flow rate adjustment valve 67b. The various gases whose flow rates are adjusted by the flow rate adjustment valve 67b join together in the introduction pipe 68. The introduction pipe 68 delivers the raw material gas, in which the multiple types of gas are mixed in a predetermined ratio, into the reaction chamber 61.

A support base 70 is provided in the reaction chamber 61. The support base 70 includes a susceptor 71, a quartz cover 72, and a heater 73. The susceptor 71 constitutes a mounting surface for the metal substrate 21. The metal substrate 21 is placed on the susceptor 71. The susceptor 71 is earthed. The susceptor 71 may be floating.

The susceptor 71 is placed on the quartz cover 72, and the heater 73 is provided below the quartz cover 72. The quartz cover 72 protects the heater 73. The quartz cover 72 also supports the susceptor 71 and prevents the heater 73 from directly contacting the susceptor 71.

During plasma processing, the metal substrate 21 is heated by radiant heat from the heater 73 via the susceptor 71. The thermocouple 82 measures the heating temperature of the heater 73. During plasma processing, the heating of the metal substrate 21 by the heater 73 is controlled based on the temperature measured by the thermocouple 82.

An upper electrode 75 is installed at the top of the reaction chamber 61. The upper electrode 75 is installed on the susceptor 71 so as to face the metal substrate 21 to be processed. The upper electrode 75 is preferably installed so as to be approximately parallel to the metal substrate 21. The distance between the upper electrode 75 and the susceptor 71 is, for example, approximately 1 to 5 cm. The upper electrode 75 is connected to a power supply unit 76 installed outside the reaction chamber 61.

The power supply unit 76 is a high-frequency power supply device. The power supply unit 76 can apply a high-frequency voltage with a frequency of 12 to 15 MHz, for example, with a power of 2000 to 3000 W, to the upper electrode 75. When the raw material gas is being supplied to the reaction chamber 61 by the storage unit 66, the power supply unit 76 applies a high-frequency voltage to the upper electrode 75, generating a high-density capacitively coupled plasma in the reaction chamber 61.

In step P2, first, the metal substrate 21 on which the base 26 of the carbon nanostructure 25 was generated in step P1 is placed on the support base 70 in the reaction chamber 61. The heater 73 heats the metal substrate 21 to a temperature of, for example, 600 to 800°C.

In step P2, the raw material gas supply unit 65 supplies raw material gas to the reaction chamber 50. The raw material gas supply unit 65 supplies, as the raw material gas, for example, a carbon-based gas at a flow rate of 80 to 120 sccm, and also supplies a reaction contributing gas at a flow rate of 40 to 60 sccm. The exhaust unit 62 controls the pressure inside the reaction chamber 50 to, for example, 5 to 15 Pa.

While the raw material gas is being supplied to the reaction chamber 61, the power supply unit 76 applies a high-frequency voltage to the upper electrode 75. The power supply unit 76 applies a high-frequency voltage with a frequency of 12 to 15 MHz, for example, at a power of 2000 to 3000 W. The power supply unit 76 gradually increases the power to the target power over a period of several tens of seconds to several minutes.

The plasma treatment in the second processing device 60 continues for, for example, about 5 to 15 minutes. By the plasma treatment in the second processing device 60, multiple extensions 27 are formed on the surface of each base 26 formed on the metal substrate 21 in process P1.

Figure 8 is a schematic diagram showing the process of forming multiple extensions 27 by plasma treatment in process P2. In Figure 8, the state of the metal substrate 21 is shown diagrammatically at elapsed times t1, t2, t3, and t4 during the plasma treatment in process P2.

At time t1, when the plasma treatment of process P2 is started, as described above, a base 26 extending from the amorphous carbon layer 23 is formed on the metal substrate 21. At time t2, intermediate carbon nanostructures CM made of carbon are formed on the surface of the base 26. Most of the intermediate carbon nanostructures CM are formed on the upper end side of the base 26. Between times t3 and t4, protrusions PM gradually extend from the intermediate carbon nanostructures CM to form extensions 27. In this way, the carbon nanostructures 25 of this embodiment are formed on the metal substrate 21.

Returning to FIG. 5, in process P3, the second electrode 30, which will become the positive electrode, is manufactured. In process P4, as shown in FIG. 1, the first electrode 20 and the second electrode 30 are assembled into a container 11 filled with an electrolyte 12. Through these processes, the secondary battery 10 is completed.

The above process makes it possible to form a carbon nanostructure 25 having a plurality of extensions 27 formed on a base 26, and to obtain a secondary battery 10 having the carbon nanostructure 25 as the active material layer 22 of an electrode 20. As described above, if the carbon nanostructure 25 is used as the active material layer 22 of a secondary battery 10, the charging capacity and charging speed can be increased, and the battery performance can be improved.

Figure 9 is an explanatory diagram showing photographed images of the embodiment E1 of the active material layer 22 described above. Figures 9(a1) and (b1) show photographed images showing the base 26 of the carbon nanostructure 25 formed by process P1. Figures 9(a2) and (b2) show photographed images showing the carbon nanostructure 25 with the extension 27 formed by process P2. The photographed images of Figures 9(a1) and (a2) were each taken by a scanning electron microscope in the thickness direction of the metal substrate 21, and the photographed images of Figures 9(a1) and (a2) were each taken by a scanning electron microscope in a direction perpendicular to the thickness direction of the metal substrate 21.

In Example E1, copper foil was used as the metal substrate 21. Example E1 was produced by carrying out the plasma treatments of steps P1 and P2 under the conditions shown in Table 1 below. In step P2, the power of the power supply unit 76 was gradually increased from 0 W to 2000 W over a period of about 1 minute.

As shown in the photographed image of FIG. 9 (b1), the average height Ha of the base 26 formed in step P1 was approximately 1.0 μm. Also, as shown in the photographed image of FIG. 9 (b2), the average height Hb of the carbon nanostructure 25 in which the extension 27 was formed in step P2 was approximately 1.3 μm. In this way, the height of the carbon nanostructure 25 was approximately 30% higher than the base 26 due to the formation of the extension 27.

The average spacing between the bases 26 formed in step P1 was about 73 nm, and the density of the bases 26 was about 17 per μm ^2. Due to the formation of the extensions 27 in step P2, the average spacing between the carbon nanostructures 25 became about 20 nm, and the density of the carbon nanostructures 25 became about 74 per μm ^2. In this way, the average spacing between the carbon nanostructures 25 was reduced by about 30% compared to the average spacing between the bases 26, and the density of the carbon nanostructures 25 was increased by about 50% compared to the density of the bases 26.

In Example E1, an amorphous carbon layer 23 was formed under the carbon nanostructure 25. The thickness of the amorphous carbon layer 23 was generally in the range of 10 nm to 300 nm.

Figure 10 is a set of photographed images showing the growth process of the extension portion 27 in process P2. The photographed images in Figure 10 were taken of multiple samples S1 to S4 in which the processing time of process P2 was changed. The upper photographed image was taken with a scanning electron microscope in the thickness direction of the metal substrate 21, and the lower photographed image was taken with a scanning electron microscope in a direction perpendicular to the thickness direction of the metal substrate 21. The processing conditions for processes P1 and P2 were the same as those of Example E1 above.

Sample S1 corresponds to the case where only the base 26 is formed before the execution of process P2. Sample S2 shows the state one minute after the start of the execution of process P2. In the image of sample S2, it was confirmed that the intermediate carbon nanostructure CM described in Figure 8 has begun to form on the surface of the base 26.

Sample S3 shows the state 5 minutes after the start of execution of process P2. In the image of sample S3, it was confirmed that the protrusion PM described in FIG. 8 had begun to form from the intermediate carbon nanostructure CM. Sample S4 shows the state 10 minutes after the start of execution of process P2. In the image of sample S4, it was confirmed that the protrusion PM had grown to become the extension 27.

Referring to Figures 11 to 15, the evaluation results of the battery performance of secondary batteries using the electrodes of Example E1 and Comparative Examples C1 and C2 are described.

Figure 11 is an explanatory diagram showing photographed images of the active material layers of Example E1 and Comparative Examples C1 and C2. The photographed images in Figure 11 were taken by a scanning electron microscope facing directly at the surface of the active material layer.

Example E1 is the same as that described in FIG. 9. In Comparative Examples C1 and C2, the same copper foil as in Example E1 was used as the metal substrate. Comparative Example C1 was produced under the same conditions as Example E1, except that step P2 for forming extension portion 27 was not performed. The active material layer in Comparative Example C1 was composed of carbon nanowalls. In Comparative Example C2, an active material layer of uniform thickness was provided using graphite.

Figures 12 to 14 show graphs obtained by charging and discharging secondary batteries using the electrodes of Example E1 and Comparative Examples C1 and C2 as the negative electrodes. In Figures 12 to 14, the relationship between the voltage and charge capacity of the secondary battery is shown by a solid line graph during charging and by a dashed line graph during discharging. Figure 15 shows the charge capacity and specific capacity of secondary batteries using Example E1 and Comparative Examples C1 and C2, respectively, as bar graphs. In this evaluation test, the charge current and discharge current were both set to 0.5 mA.

The secondary batteries used in Example E1 and Comparative Examples C1 and C2 were lithium ion secondary batteries and were fabricated with the configurations shown in Table 2 below.

The secondary battery using Example E1 had a charge capacity of 16.8 [mAh] and a specific capacity of 12.7 [mAh/ ^cm2 ]. In contrast, the secondary battery using Comparative Example C1 had a charge capacity of 12.6 [mAh] and a specific capacity of 9.4 [mAh/ ^cm2 ]. The secondary battery using Comparative Example C2 had a charge capacity of 4.0 [mAh] and a specific capacity of 2.0 [mAh/ ^cm2 ].

A secondary battery using Comparative Example C1, in which the active material layer was made of carbon nanowalls, had a charge capacity and specific capacity that were more than three times higher than Comparative Example C2, in which the active material layer was made of graphite. Example E1, in which the active material layer was made of carbon nanostructures 25 with extensions 27 formed on the base 26, had charge capacity and specific capacity that were 30% or more higher than Comparative Example C1. This shows that the use of carbon nanostructures 25 with extensions 27, as described in the above embodiment, as the active material layer dramatically improves the battery performance of secondary batteries.

Figure 16 shows images of the secondary batteries using the electrodes of Example E1 and Comparative Example C1 after charging was completed, taken by a scanning electron microscope in a direction perpendicular to the thickness direction of the metal substrate. In both Example E1 and Comparative Example C1, a layer of Li precipitated in the active material layer due to the charging reaction was confirmed. However, as can be seen from these images, the Li layer of Example E1 was denser than the Li layer of Comparative Example C1. In addition, the thickness Ta of the Li layer of Example E1 was about 50 μm, while the thickness Tb of the Li layer of Comparative Example C1 was about 32 μm, so that the thickness of the Li layer of Example E1 was significantly larger than that of Comparative Example C1. This also shows that the charging capacity of the secondary battery using the electrode of Example E1 is dramatically improved compared to the secondary battery using the electrode of Comparative Example C1.

As described above, the electrode for an electricity storage device according to the present invention can significantly improve the battery performance of the electricity storage device.

The results of evaluation of the surfaces of the active material layers of Example E1 and Comparative Examples C3 and C4 using the sessile drop method will be described with reference to Figure 17. Comparative Example C3 corresponds to an active material layer of carbon nanowalls formed by the CCP-CVD method, and Comparative Example C4 corresponds to an active material layer of carbon nanowalls formed by the RI-PECVD method. The CVD method conditions for Comparative Examples C3 and C4 are approximately the same as those shown in Table 1.

Figure 17 (a) is an explanatory diagram for explaining a method for measuring a contact angle using the sessile drop method. In the sessile drop method, a water droplet LQ is dropped onto the surface SP to be evaluated, and the contact angle θ is determined as the angle on the water droplet LQ side between the tangent L to the surface of the water droplet LQ, which passes through the intersection of the surface of the water droplet LQ and the surface SP to be evaluated. The larger the contact angle θ, the higher the water repellency of the surface SP to be evaluated.

Figures 17(b), (c), and (d) are images taken of the state when a water droplet LQ was dropped onto the surface of the active material layer for Example E1, Comparative Example C3, and Comparative Example C4, respectively. The contact angle θ in Example E1 was approximately 150°, the contact angle θ in Comparative Example C3 was approximately 138°, and the contact angle θ in Comparative Example C4 was approximately 134°. Thus, the contact angle θ in Example E1 was the largest, indicating super-water repellency. Carbon nanostructures 25 that exhibit such super-water repellency can be applied to coating technologies, such as oil separation and corrosion prevention technologies.

Figure 18 is a photograph showing Comparative Examples C5 to C9 of active material layers produced by a manufacturing method different from that of the Examples. The carbon nanostructures of the active material layers of Comparative Examples C5 to C9 were formed by performing plasma treatment once or twice by RI-PECVD or CCP-CVD on the surface of copper foil as a metal substrate. The treatment conditions for the CVD method are the same as those shown in Table 1.

The carbon nanostructures of Comparative Example C5 were formed by a single plasma treatment in a CCP-CVD apparatus. In Comparative Example C5, carbon nanowalls were formed that were smaller in thickness, height, and length, and had a finer, more random structure.

The carbon nanostructure of Comparative Example C6 was formed by performing plasma treatment twice using a CCP-CVD apparatus. In Comparative Example C6, the second plasma treatment increased the amount of carbon nanowalls obtained in the first treatment, but did not produce a structure equivalent to the extension portion 27 described in the above embodiment.

The carbon nanostructure of Comparative Example C7 was formed by a single plasma treatment in a RI-PECVD apparatus. The carbon nanostructure of Comparative Example C7 had a configuration corresponding to the base 26 described in the above embodiment.

The carbon nanostructure of Comparative Example C8 was formed by performing plasma treatment twice using an RI-PECVD apparatus. In Comparative Example C8, the height of the carbon nanowalls formed in the first plasma treatment increased after the second plasma treatment, but a structure equivalent to the extension portion 27 described in the above embodiment was not obtained.

The carbon nanostructure of Comparative Example C9 was formed by performing a first plasma treatment using a CCP-CVD apparatus followed by a second plasma treatment using a RI-PECVD apparatus. In Comparative Example C9, as in Comparative Example C8, the height of the carbon nanowalls increased due to the second plasma treatment, but a structure equivalent to the extension portion 27 described in the above embodiment was not obtained.

The results of Comparative Examples C5 to C9 show that in order to obtain the carbon nanostructure 25 described in the above embodiment, it is preferable to perform a first plasma treatment using an RI-PECVD apparatus and then a second plasma treatment using a CPC-CVD apparatus.

As described above, the manufacturing method according to the present invention makes it possible to obtain an electrode for an electricity storage device that can significantly improve the battery performance of the electricity storage device.

2. Other embodiments:
The present invention is not limited to the configurations of the above-mentioned embodiments and examples, and can be realized, for example, in the following forms. In the following, any configuration described as another embodiment is positioned as an example form for implementing the present invention, similar to the above-mentioned embodiments and examples.

2-1. Other embodiment 1:
The secondary battery using the electrode 20 of the above embodiment may be configured to involve metal ions other than lithium ions in charging and discharging. For example, the secondary battery using the electrode 20 of the above embodiment may be configured to involve sodium (Na) ions, potassium (K) ions, magnesium (Mg) ions, etc. in charging and discharging.

2-2. Other embodiment 2:
The electrode 20 of the above embodiment may be used in an electricity storage device other than a secondary battery. For example, the electrode 20 of the above embodiment may be used in an electric double layer capacitor. In this case, the electrode 20 may be used as both the positive electrode and the negative electrode of the electric double layer capacitor.

3. Example of form:
The present invention can be realized in the following forms.

[First embodiment] The first embodiment is provided as an electrode for an electricity storage device. The electrode for an electricity storage device of the first embodiment includes a metal substrate constituting a current collector, and an active material layer formed on the surface of the metal substrate and containing carbon as an active material, the active material layer being composed of a carbon nanostructure having a base portion in which graphene extending in an elongated manner in the thickness direction of the metal substrate is laminated, and a plurality of extension portions composed of the carbon and extending from the base portion.
According to the first embodiment of the electrode for an electricity storage device, the active material layer is composed of a carbon nanostructure having a novel structure. This active material layer can increase the amount of metal ions involved in the charging reaction that precipitate as metal when the electricity storage device is charged. Thus, the charging capacity of the electricity storage device can be increased.

[Second Mode] In the electricity storage device electrode of the first mode, the carbon nanostructure may have a height of 0.5 μm or more.
According to the electricity storage device electrode of the second embodiment, the battery performance of the electricity storage device to which it is applied can be further improved.

[Third aspect] In the power storage device electrode of the first or second aspect, the average spacing between the carbon nanostructures in the active material layer may be 40 nm or less.
According to the third embodiment of the electrode for an electricity storage device, the density of the carbon nanostructures in the active material layer is increased, and therefore the battery performance of the electricity storage device to which it is applied can be further improved.

[Fourth aspect] The fourth aspect is provided as a secondary battery. The secondary battery of the fourth aspect includes a first electrode constituted by the electrode for an electric storage device according to any one of the first, second and third aspects, and a second electrode containing metal atoms that are ionized and move to the first electrode.
According to the secondary battery of the fourth aspect, high battery performance can be obtained by applying an active material layer constituted by a carbon nanostructure having a novel structure.

[Fifth aspect] The fifth aspect is provided as a method for producing an electrode for an electrical device, comprising the steps of growing, on a surface of a metal substrate, a base portion in which graphene is laminated and which extends in an elongated manner in a thickness direction of the metal substrate, by using a radical injection plasma CVD apparatus, and forming, by using a capacitively coupled plasma CVD apparatus, a plurality of extension portions which are made of the carbon and extend from a surface layer of the base portion of the metal substrate.
According to the fifth embodiment of the manufacturing method, it is possible to obtain an electrode having an active material layer composed of a carbon nanostructure having a novel structure. The carbon nanostructure has an extension portion, which can dramatically improve the battery performance of the power storage device.

REFERENCE SIGNS LIST 10... secondary battery, 10a... comparative secondary battery, 11... container, 12... electrolyte, 15... separator, 16... first electrode chamber, 17... second electrode chamber, 20... first electrode (electrode for electricity storage device), 20a... comparative electrode, 21... metal substrate, 21a... first surface, 21b... second surface, 22... active material layer, 22a... comparative active material layer, 23... amorphous carbon layer, 25... carbon Nanostructure, 25a... carbon nanostructure of comparative example, 26... base, 27... extension, 28... metal atoms, 30... second electrode, 31... positive electrode current collector, 31a... first surface, 31b... second surface, 32... positive electrode active material layer, 40... first processing device, 41... plasma generation chamber, 42... waveguide, 43... quartz window, 44... slot antenna, 45... micro generation section, 46... radical source supply section, 5 0... reaction chamber, 51... partition wall, 52... through hole, 53... gas flow path, 54... raw material gas supply unit, 55... power supply unit, 56... support base, 56s... placement surface, 57... heater, 58... exhaust unit, 60... second processing device, 61... reaction chamber, 62... exhaust unit, 65... raw material gas supply unit, 66... storage unit, 67... flow rate control unit, 67a... gas delivery device, 67b... flow rate adjustment valve, 68... introduction pipe, 70 ...Support base, 71...susceptor, 72...quartz cover, 73...heater, 75...upper electrode, 76...power supply unit, 81...vacuum gauge, 82...thermocouple, CCP...capacitively coupled plasma, CM...intermediate carbon nanostructure, GS...graphene, HDP...high density plasma, LQ...water droplet, MW...microwave, PM...projection, SP...evaluation target surface, SWP...surface wave plasma, VS...gap

Claims (5)

An electrode for an electricity storage device,
A metal substrate constituting a current collector;
an active material layer formed on a surface of the metal substrate and containing carbon as an active material;
Equipped with
The active material layer is an electrode for an electricity storage device, the active material layer being composed of a carbon nanostructure having a base formed of laminated graphene extending in an elongated manner in the thickness direction of the metal substrate, and a plurality of extension portions composed of the carbon and extending from the base. The electrode for an electricity storage device according to claim 1 ,
The height of the carbon nanostructure is 0.5 μm or more. The electrode for an electricity storage device according to claim 1 ,
An electrode for an electricity storage device, wherein the average spacing between the carbon nanostructures in the active material layer is 40 nm or less. A secondary battery,
A first electrode constituted by the electrode for an electricity storage device according to any one of claims 1 to 3;
a second electrode containing metal atoms that ionize and migrate to the first electrode;
A secondary battery comprising: A method for producing an electrode for an electricity storage device, comprising the steps of:
A step of growing a base portion on a surface of a metal substrate, the base portion being composed of graphene that is elongated in a thickness direction of the metal substrate, by using a radical injection type plasma CVD apparatus;
forming a plurality of extensions made of carbon and extending from a surface layer of the base of the metal substrate by a capacitively coupled plasma CVD apparatus;
A manufacturing method comprising:

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