JP7576785B2 - Carbon nanowall electrode and its manufacturing method - Google Patents

Description

Article 30, paragraph 2 of the Patent Act applies. Meeting name: 62nd Battery Symposium, Date: November 30, 2021 (Abstracts will be published on November 29, 2021), Venue: Pacifico Yokohama North, 1-1-1 Minatomirai, Nishi-ku, Yokohama, 220-0012

The present invention relates to a carbon nanowall electrode in which carbon nanowalls are erected on a substrate and a method for manufacturing the same. It also relates to an all-solid-state secondary battery using the same and a method for manufacturing the same.

In recent years, carbon nanowalls, a type of nanomaterial made of carbon, have attracted attention and have been the subject of much research. Carbon nanowalls are nanostructures in which multilayer graphene is erected on a substrate, and are expected to be used as electrodes in batteries, capacitors, biosensors, electrochemical sensors, and the like (e.g., Non-Patent Documents 1 and 2).

Furthermore, Patent Document 1 describes a lithium ion secondary battery with a carbon nanochips electrode as the negative electrode. Here, the carbon nanochips electrode is defined as an electrode in which graphene sheets are grown from a substrate at an inclination in various directions. This secondary battery uses Li1 _{-xCoO2 as} the positive electrode active material and a solution in which _LiPF6 is dissolved in a mixed solvent of ethylene carbonate and dimethyl carbonate as the electrolyte, and it has been found that the charge/discharge capacity per unit weight of carbon exceeds the theoretical capacity of graphite and that there is little deterioration due to repeated charge/discharge.

WO2014/069310

M. Hiramatsu, K. Takeda, H. Kondo and M. Hori J. Plasma Fusion Res. Vol.98, No.4 (2022)158-164 B. Li, S. Yang et. al. Nano Lett. 2015, 15, 5, 3073-3079.

However, the conventional carbon nanowall electrodes described above had problems such as peeling easily occurring between the copper substrate and the carbon nanowalls, and insufficient mechanical strength. In addition, in the carbon nanochips electrode described in Patent Document 1, the graphene sheets grow at an angle in various directions from the substrate, so when used in an all-solid-state secondary battery using a solid electrolyte, the solid electrolyte can only come into contact with the outermost graphene sheet. This reduces the contact area between the solid electrolyte and the carbon nanochips, resulting in a problem of reduced battery capacity per unit volume (or unit weight) of the negative electrode material.

The present invention has been made in consideration of the above-mentioned conventional situation, and aims to provide a carbon nanowall electrode that is less susceptible to peeling between the substrate and the carbon nanowalls, and that, even when applied to an all-solid-state battery using a solid electrolyte, has a large battery capacity per unit volume (or unit weight) of the negative electrode material.

In order to solve the above problems, the present inventors conducted research on carbon nanowall electrodes. As a result, they found that if a graphene sheet is formed on a substrate using a microwave plasma CVD method, the graphene sheet grows almost vertically on the substrate. They also found that by using a material containing elements such as iron and nickel for the substrate, a carbide (i.e., a carbide (hereinafter the same) which is a compound of carbon and an electropositive element) layer with excellent adhesion to the substrate is formed near the substrate surface. They further found that the formation of this carbide layer makes it difficult for the carbon nanowall to peel off from the substrate, resulting in a carbon nanowall electrode with high mechanical strength and excellent durability. They also found that if this carbon nanowall electrode is applied to an all-solid-state secondary battery using a solid electrolyte, the capacity per unit volume (or per unit weight) of the negative electrode material can be extremely increased.

That is, the carbon nanowall electrode of the present invention is characterized in that carbon nanowalls are erected on a substrate, and that a peak based on carbide is present near the substrate surface in a narrow scan of C1s in an X-ray photoelectron spectroscopy (XPS) analysis. The central value of the binding energy of the peak based on carbide varies slightly depending on the type of carbide, but falls within the range of 281.0 eV to 284.0 eV.

In the carbon nanowall electrode of the present invention, a peak based on carbide is present near the substrate surface in a narrow scan of C1s in X-ray photoelectron spectroscopy (XPS) analysis (in other words, a carbide layer is present near the substrate surface). From the viewpoint of improving adhesion between the substrate and the carbon nanowall, it is preferable that the area of the peak based on carbide in C1s in X-ray photoelectron spectroscopy (XPS) analysis is 2% or more of the total peak area in the C1s spectrum.

The substrate material preferably contains at least one of the elements iron, nickel, chromium, cobalt, aluminum, silicon, tungsten, molybdenum, manganese, titanium, and tantalum. These elements react with carbon to easily form carbide. For example, a thin plate or foil made of pure iron, carbon steel, stainless steel, or an iron alloy can be used as the substrate. The substrate may also be a material that doubles as a current collector, or may be a material made of a current collector with a coating layer containing these elements laminated on its surface. Austenitic stainless steel, ferritic stainless steel, and martensitic stainless steel are particularly preferred because they contain iron, chromium, and nickel, which easily form carbide, and are resistant to corrosion.

In addition, it is preferable that the average distance between adjacent carbon nanowalls is 1.2 μm or more. When using a carbon nanowall electrode in an all-solid-state lithium-ion battery, if the average distance between the carbon nanowalls is less than 1.2 μm, it becomes difficult for the solid electrolyte to fill the gaps between the carbon nanowalls, and the contact area between the carbon and lithium is reduced, resulting in a smaller charging capacity.

The carbon nanowall electrode of the present invention can be produced as follows.
That is, the method for producing a carbon nanowall electrode of the present invention is characterized in that it includes a CVD step of forming carbon nanowalls on a substrate by performing a microwave plasma CVD method while supplying a mixed gas containing at least hydrogen and a hydrocarbon, and the temperature of a stage on which the substrate is placed is set to 400 to 600° C. The mixed gas may contain a rare gas such as helium gas or argon gas in addition to hydrogen and a hydrocarbon.

According to this method for manufacturing a carbon nanowall electrode, a carbon layer containing carbide can be formed on a substrate. The carbide-containing carbon layer thus formed is thin and has good adhesion, so that it is possible to prevent the carbon layer from peeling off or bending due to distortion caused by the difference in thermal expansion coefficient between the substrate and the carbon layer. If the temperature of the table on which the substrate is placed is set to less than 400°C, the front side of the substrate is heated relatively more strongly than the back side by the radiant heat from the plasma, and the temperature becomes higher than that of the table on which the substrate is placed. This causes a temperature difference in the thickness direction of the substrate, which causes the substrate to bend or warp.

The all-solid-state secondary battery of the present invention is characterized in that the carbon nanowall electrode of the present invention is used as the negative electrode for the secondary battery. The all-solid-state secondary battery of the present invention uses a carbon nanowall electrode in which graphene sheets are erected on a substrate, so that the solid electrolyte comes into contact with the graphene sheet surface of each carbon nanowall, thereby increasing the contact area. In addition, not only is lithium absorbed between the layers of the carbon nanowall by intercalation during the charging process, but lithium is also precipitated in several layers on the carbon nanowall surface. As a result, the battery capacity per unit volume or unit weight of the negative electrode material becomes significantly large.

The method for producing an all-solid-state secondary battery of the present invention is characterized by comprising the steps of forming a solid electrolyte layer on the carbon nanowall electrode of the present invention by a physical vapor deposition method, and forming a positive electrode material layer on the solid electrolyte by a physical vapor deposition method.
According to the method for producing an all-solid-state secondary battery of the present invention, each production step is performed in a vacuum device, so there is no risk of contamination with moisture or impurities from the atmosphere, and production can be automated.

The inventors have fabricated an all-solid-state lithium ion battery using the carbon nanowall electrode of the present invention and confirmed that it exhibits excellent battery characteristics. This all-solid-state lithium ion battery has the advantages of being able to operate at a higher temperature and being safer without the risk of electrolyte leakage, compared to batteries using a liquid electrolyte.
The solid electrolyte may be LiPON, etc. The positive electrode active material may be LiMnO, etc., which is a positive electrode active material that is commonly used in lithium ion batteries that use an electrolyte.

FIG. 2 is a schematic diagram showing the process of carbon nanowall growth on a substrate. FIG. 2 is a schematic cross-sectional view of a microwave plasma CVD apparatus used in the examples. 2A and 2B are scanning electron microscope photographs of the surface and cross section of the carbon nanowall electrode of Example 1. 2 is a transmission electron microscope photograph of a cross section of a carbon nanowall in the carbon nanowall electrode of Example 1. 1 shows the results of X-ray photoelectron spectroscopy (XPS) measurement of the carbon nanowall electrode and graphite of Example 1. 1 shows the results of measuring X-ray photoelectron spectroscopy (XPS) of the carbon nanowall electrode of Example 4 before and after argon etching (5 minutes and 10 minutes after etching). FIG. 2 is a schematic cross-sectional view of a carbon nanowall electrode. 1 shows a Raman spectrum of the carbon nanowall electrode of Example 1. 11 shows scanning electron microscope photographs of the carbon nanowall electrode surfaces of Examples 5-4, 5-5, and 5-6. 1 shows scanning electron microscope photographs of the surfaces of carbon nanowall electrodes in Examples 6-1, 6-2, and 6-3. 1 is a scanning electron microscope photograph of the surface of the carbon nanowall electrode of Comparative Example 2. 1 is a graph showing the change in weight of a substrate before and after plasma exposure. 1 shows scanning electron microscope photographs and Raman spectrum graphs of the surfaces of the carbon nanowall electrodes produced in Examples 7-1 to 7-4. 1 is a graph showing the results of XPS measurement of the carbon nanowall electrode prepared in Example 7-1. FIG. 1 is a schematic cross-sectional view of a lithium-ion all-solid-state secondary battery. 1 is a graph showing charge/discharge characteristics of a lithium-ion all-solid-state secondary battery. 1 is a scanning electron microscope photograph of a cross section of a lithium-ion all-solid-state secondary battery.

The carbon nanowall electrode of the present invention has carbon nanowalls standing on a substrate. Carbon nanowalls are relatively perfect crystals composed of nano-sized graphite crystallites, and are plate-like nanostructures in which several to about 100 graphene sheets are stacked.

The substrate may be made of a material containing at least one of the elements iron, nickel, chromium, cobalt, aluminum, silicon, tungsten, molybdenum, manganese, titanium, and tantalum. Alternatively, the substrate may be made of a metal substrate such as copper, copper alloy, nickel, nickel alloy, iron, iron alloy, stainless steel, molybdenum, tungsten, or tantalum that has been conventionally used as a current collector, and a coating of at least one of the elements iron, nickel, chromium, cobalt, aluminum, silicon, tungsten, molybdenum, manganese, titanium, or tantalum formed on the surface of the substrate by a surface treatment method such as plating or vapor deposition.

As substrate materials, austenitic stainless steel, martensitic stainless steel containing iron, nickel and chromium, and ferritic stainless steel containing iron and chromium are particularly preferred because they have excellent corrosion resistance. For example, any type of stainless steel can be used, such as austenitic stainless steel (SUS304, SUS304-L, SUS302, SUS301, SUS310S, SUS321, SUS316, SUS316-L, etc.), ferritic stainless steel (SUS430, SUS434, etc.), martensitic stainless steel (SUS410S, SUS420J2, etc.), and precipitation hardened stainless steel (SUS631, ASL-350, etc.) stainless steel, as specified in JIS G4305:2005 "Cold-rolled stainless steel sheets and strips." Semiconductor substrates such as Si, SiC, AlGaAs, and AlGaN may also be used.

There are no particular limitations on the thickness of the substrate, but it is preferable that it be less than about 1 mm in order to reduce the weight of the electrode, and a foil material of about 5 μm to 100 μm that is conventionally used as a current collector is more preferable, and from a practical standpoint, a foil material of 5 μm to 20 μm is even more preferable.

In addition, in the carbon nanowall electrode of the present invention, a peak based on carbide must be present in the vicinity of the substrate surface in a narrow scan of C1s in an X-ray photoelectron spectroscopy (XPS) analysis. Here, the vicinity of the substrate surface refers to a range within 100 nm from the substrate surface. When carbon nanowalls are grown on a substrate using a microwave plasma CVD method, carbide is generated in the vicinity of the substrate surface, which plays a role in increasing the adhesion between the substrate and the carbon nanowall. From the viewpoint of increasing the adhesion, the area of the peak based on carbide is preferably 2% or more of the total peak area in the C1s spectrum, more preferably 3% or more, and most preferably 5% or more.

It is preferable that the average distance between adjacent carbon nanowalls is 1.2 μm or more. When using a carbon nanowall electrode in an all-solid-state lithium-ion battery, if the average distance between the carbon nanowalls is less than 1.2 μm, it becomes difficult for the solid electrolyte to fill the narrow gaps between the carbon nanowalls, and the contact area between the carbon and lithium is reduced, resulting in a decrease in charging capacity.

<Method of Manufacturing Carbon Nanowall Electrode>
Next, a method for producing the carbon nanowall electrode of the present invention will be described.
First, a substrate containing at least one element selected from the group consisting of iron, nickel, chromium, cobalt, aluminum, silicon, tungsten, molybdenum, manganese, titanium, and tantalum is prepared, cut to a required size, and the surface is cleaned. The cleaning method may be a method using a surfactant, an organic solvent, electrolytic cleaning in an alkaline solution, or plasma treatment.
Next, the cleaned substrate is placed on the sample stage of the microwave plasma CVD apparatus, and the microwave plasma CVD method is performed. Here, the microwave plasma CVD method refers to a method in which a raw material gas is turned into plasma by microwave power to activate a chemical reaction, and chemical vapor deposition is performed. There is no particular limitation on the excitation method for generating microwave plasma, but examples include microwave surface wave plasma CVD method and ECR plasma CVD method in which microwaves and an ECR magnetic field are applied.

The supply gas in microwave plasma CVD can be a mixture of an inert gas such as argon, a hydrocarbon gas (methane, ethane, acetylene, etc.) that serves as a carbon source for carbon nanowalls, and hydrogen. It is estimated that the supply gas that flows into the chamber of the plasma CVD device becomes argon ions and carbonium ions, and these ions further react to turn into argon gas, hydrogen, and carbon nanowalls. Figure 1 shows the growth process of carbon nanowalls on a substrate. First (approximately 30 seconds to 2 minutes), several carbon layers 101 are formed by growing laterally on the surface of the substrate 100 (see Figure 1a), and then a carbon layer 102 grows vertically from a part of the surface as a nucleus, forming a carbon nanowall (see Figure 1b). Here, it does not matter whether the carbide exists in the carbon layer 101 as islands or as a carbide layer between the carbon layer 101 and the substrate 100. In any case, the formation of carbide is a factor in improving the adhesion between the substrate 100 and the carbon layers 101 and 102 .

<All-solid-state lithium-ion battery>
A solid-state lithium ion battery can be constructed by contacting the carbon nanowall electrode of the present invention with a solid electrolyte capable of transferring lithium ions, and further laminating a cathode material and a current collector that have been conventionally used in lithium ion batteries. As the solid electrolyte capable of transferring lithium ions, lithium oxides (e.g., lithium phosphate oxynitride (LiPON), Li ₃ PO ₄ , LiBO ₃ , etc.) can be used. As the cathode active material, lithium oxides (e.g., lithium manganate (LiMnO), lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), etc.) can be used. These solid electrolytes and cathode active materials are formed in a vacuum device using a physical vapor deposition method such as a sputtering method or a vapor deposition method. When a solid electrolyte is laminated on a carbon nanowall electrode by a sputtering method, a lithium oxide or the like can be used as a target electrode. The output per target area in the sputtering method can be appropriately selected in consideration of the adhesion between the carbon nanowall and the solid electrolyte, but it is usually preferable to set it to about 0.2 to 5 W/cm ² .

In an all-solid-state lithium ion battery using the carbon nanowall electrode of the present invention, the structure of the negative electrode surface remains stable and hardly changes even after repeated charge and discharge.
In addition, in the charging process, not only is lithium absorbed between the carbon nanowalls by intercalation, but lithium is also precipitated over several layers on the surface of the carbon nanowalls. Therefore, not only the lithium absorbed between the layers but also the lithium precipitated on the surface of the carbon nanowalls contributes to the charge and discharge capacity, far exceeding the charge and discharge capacity theoretically calculated from the amount of lithium absorbed between the layers. This allows the battery to be significantly more compact and lightweight.
Moreover, like conventional lithium ion secondary batteries, it is possible to operate at a high potential in the range of about 3.2 V to 4.2 V.
Furthermore, since a solid electrolyte is used, there is no need to worry about liquid leakage, and the operating temperature can be made higher.

<Fabrication of carbon nanowall electrodes>
Example 1
A 40 μm thick stainless steel foil made of SUS304 was prepared and cut to 50 mm x 50 mm to serve as a substrate. SUS304 is a type of austenitic stainless steel. This substrate was placed on the susceptor 6 in the reaction vessel 1 of the microwave surface wave plasma CVD device 200 shown in Figure 2, and microwave plasma CVD was performed under the following conditions. After that, nitrogen was introduced into the reaction vessel 1 to return it to atmospheric pressure, and the substrate was removed, and this was used as a negative electrode for a lithium ion secondary battery.
Flow gas: _CH4 : _H2 :Ar=5:3:3 mixed gas. Temperature of susceptor 6: 500°C, process pressure: 10 Pa.
Plasma irradiation time: 3 minutes, 8 minutes and 10 minutes, applied microwave power: 1 kW,
Applied microwave frequency: 2.45 GHz,
Distance from plasma excitation plate to substrate: 55 mm

(Structure of Plasma CVD Apparatus 200)
The plasma CVD device 200 is provided with a reaction vessel 1 and a waveguide 2 disposed above the CVD reaction vessel 1. A plasma excitation plate 3 made of quartz is provided between the CVD reaction vessel 1 and the waveguide 2, and a number of minute recesses 30 are formed on the plasma excitation plate 3 on the CVD reaction vessel 1 side. By concentrating an electric field in the recesses 30, the recesses 30 become the plasma generation starting points, making it easy to generate plasma at low power. A slot antenna 4 is provided below the waveguide 2. A microwave of 2.45 GHz is supplied to the waveguide 2 from a microwave generator (not shown), and electromagnetic waves are supplied to the inside of the CVD reaction vessel 1 and the plasma excitation plate 3 via the slot antenna 4.
A susceptor 6 for placing a substrate 5 thereon and a heating device 7 for heating the susceptor 6 are provided inside the CVD reaction vessel 1. The supply amounts of the mixed gas are adjusted by mass flow controllers (not shown) for each gas component, and the gases are introduced into the CVD reaction vessel 1 through an inlet 11 provided on the upper side of the CVD reaction vessel 1. An outlet 12 for discharging the gas is provided at the lower part of the side of the CVD reaction vessel 1. The inside of the CVD reaction vessel 1 can be depressurized to about ^10-3 torr by a vacuum pump (not shown).

(Formation of Carbon Nanowalls by CVD)
Next, a method for forming carbon nanowalls on the substrate 5 will be described.
The substrate 5 was ultrasonically cleaned in methanol, and then ultrasonically cleaned in acetone, and then dried and placed on the susceptor 6. After the pressure inside the CVD reaction vessel 1 was reduced to a predetermined pressure by the vacuum pump, the heating device 7 was turned on, and the temperature of the susceptor 6 was controlled to a predetermined temperature. In this state, the gas flow rates of methane, hydrogen, and argon were supplied to the inside of the CVD reaction vessel 1 while being controlled by the mass flow controller, and the inside of the CVD reaction vessel 1 was controlled to a predetermined pressure. Next, the microwave generator was driven to supply microwaves of 2.45 GHz to the waveguide 2, and plasma of the mixed gas was generated inside the CVD reaction vessel 1. As for the plasma, microwave surface wave plasma was generated above the substrate 5. After a predetermined time, the microwave generator was stopped to drive, and the heating device 7 was turned off, and after a certain time, the inside of the CVD reaction vessel 1 was returned to atmospheric pressure, and the substrate 5 on whose surface the carbon nanowalls were formed was taken out, and this was used as the carbon nanowall electrode of Example 1.

Example 2
In Example 2, the same SUS304 substrate as in Example 1 was used, and the conditions for microwave plasma CVD were as follows: The other steps were the same as in Example 1, and therefore the description will be omitted.
Flow gas: Acetylene: Hydrogen: Argon = 3:20:20 mixed gas,
Temperature of susceptor 6: 500° C., process pressure: 7 Pa,
Plasma exposure time: 5 minutes
Applied microwave power: 1.5 kW, applied microwave frequency: 2.45 GHz,
・Distance from plasma excitation plate to substrate: 50 mm

Example 3
In Example 3, an austenitic stainless steel (SUS316) foil having a thickness of 40 μm was used as the substrate. The other steps were the same as those in Example 2, and therefore the description thereof will be omitted.

Example 4
A ferritic stainless steel (SUS430) foil having a thickness of 50 μm was used as the substrate in Example 4. The other steps were the same as those in Example 2, and therefore the description thereof will be omitted.

(Comparative Example 1)
A copper foil having a thickness of 40 μm was used as the substrate in Comparative Example 1. The other steps were the same as those in Example 1, and therefore the description thereof will be omitted.

-evaluation-
The carbon nanowall electrodes of Examples 1 to 4 and Comparative Example 1 prepared as described above were subjected to the following analyses.

Observation by Scanning Electron Microscope Scanning electron microscope photographs of the surface and cross section of the carbon nanowall electrode of Example 1 (plasma exposure time: 8 minutes) are shown in Figure 3. From these, it can be seen that flat carbon crystals, which are characteristic of carbon nanowalls, grow in a direction approximately perpendicular to the substrate surface. Similar carbon nanowalls were observed in Examples 2, 3, and 4, and no peeling of the carbon nanowalls occurred.
On the other hand, in Comparative Example 1, in which copper foil was used as the substrate, carbon nanowalls were formed on the copper foil, but they were observed to peel off from the substrate during the subsequent substrate processing stage, indicating that they would be difficult to use as a carbon nanowall electrode.

Observation by Transmission Electron Microscope For the carbon nanowall electrode of Example 1, the carbon nanowalls were peeled off from the substrate 1, solidified with a hardener, and then thinned to enable cross-section observation using a microtome or the like to prepare a sample, and a transmission electron microscope photograph was taken (see FIG. 4). As a result, it was found that the thickness of the carbon nanowall was about 4.5 nm, and that about 10 or so graphene sheets were overlapped. Similar carbon nanowalls were also observed in Examples 2, 3, and 4.

Measurement by X-ray photoelectron spectroscopy (XPS) Two types of carbon nanowall electrodes were prepared under the same conditions as in Example 1, except that the plasma exposure time was set to 3 or 10 minutes, and X-ray photoelectron spectroscopy (XPS) measurements were performed. For comparison, highly crystallized graphite was also measured.
The measurement results are shown in Figure 5. When the plasma exposure time was 3 minutes, a broad peak was observed around 283 to 284 eV. This broad peak was not detected in highly crystallized graphite, and from the chemical shift value, it was found to be a peak due to carbide in C1s. In addition, when the area of the broad peak around 283 to 284 eV was calculated by waveform separation, it was 2.5% of the total peak area in the C1s spectrum. In contrast, when the plasma exposure time was 10 minutes, no peak due to carbide was observed around 283 to 284 eV. From the above results, it was found that carbide exists within a range of 100 nm from the substrate surface in the carbon nanowalls.
For the samples of Examples 2 to 4, the carbon nanowalls were etched away in situ by argon ion etching in the XPS apparatus, and XPS measurements were performed. The measurement results for Example 4 (plasma exposure time 5 minutes) are shown in FIG. 6. The sample before etching had a peak similar to that of graphite, and it was found to be made of carbon nanowalls. When the surface of this sample was argon sputter etched, it was found that the peak due to carbon nanowalls in the C1s spectrum, which appeared at 284.4 eV, became smaller with the passage of etching time, and a peak due to carbide appeared at 283.2 eV. The same results were obtained for Examples 2 and 3. This also showed that carbide was present near the very surface of the substrate.
On the other hand, in Comparative Example 1 in which a copper foil was used as the substrate, no peaks due to carbide were observed in the C1s spectrum.

Measurement by Energy Dispersive X-ray Analysis (EDX) As a result of carrying out energy dispersive X-ray analysis (EDX) on Examples 1 to 4, Fe and Cr were detected in addition to carbon.
From the above results and the XPS measurement results, it was found that in Examples 1 to 4, Fe and Cr were bonded to carbon in the carbon nanowalls near the substrate surface to form carbides.

The results of measurements using X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray analysis (EDX) showed that a carbide-containing carbon layer 22 was formed near the interface between the stainless steel foil 20 substrate and the carbon nanowalls 21, as shown in Figure 7. The good adhesion between the stainless steel foil 20 and the carbon nanowalls 21 is due to the formation of this carbide-containing carbon layer 22.

Measurement of Raman spectrum The Raman spectrum of the sample of Example 1 was measured using a microscopic laser Raman spectrometer (Renishaw InVia Raman microscope). The laser light was an argon laser with a wavelength of 532 nm. The measurement results are shown in FIG. 8. Peaks indicated by D, G, and 2D were observed near 1350 cm ^-1 , 1580 cm ^-1 , and 2682 cm ^-1 , respectively. Here, G is a peak derived from the graphite structure, and a small peak, G' peak, was observed on the shoulder of this peak. This G' peak indicates that the formed carbon nanowalls extend in a direction perpendicular to the substrate.

<Effect of Gas Composition in Microwave Plasma CVD>
In order to investigate the effect of gas composition in microwave plasma CVD, the following experiment was carried out.
(Examples 5-1 to 5-6)
In Examples 5-1 to 5-6, the same SUS304 substrate as in Example 1 was used, and the conditions in the microwave plasma CVD were variously changed to examine the effects thereof.
Flow gas conditions: as shown in Table 1.

The following conditions were the same for Examples 5-1 to 5-6.
Temperature of susceptor 6: 500° C., process pressure: 10 Pa,
Plasma exposure time: 10 minutes
Applied microwave power: 1 kW, applied microwave frequency: 2.45 GHz,
Distance from plasma excitation plate to substrate: 55 mm

-Results-
(Observation by scanning electron microscope)
9 shows scanning electron microscope photographs of the surfaces of the carbon nanowalls produced in Examples 5-4, 5-5, and 5-6. From these photographs, the average distance between adjacent carbon nanowalls was calculated to be 1.2 μm in Example 5-4, 1.5 μm in Example 5-5, and 1.9 μm in Example 5-6. From these results, it was found that the distance between adjacent carbon nanowalls can be controlled by controlling the _CH4 / _H2 ratio in the plasma CVD supply gas.

(Examples 6-1 to 6-3 and Comparative Examples 2 and 3)
In Examples 6-1 to 6-3 and Comparative Examples 2 and 3, SUS304 was used as the substrate, the temperature of the susceptor 6 was varied (400° C. in Example 6-1, 500° C. in Example 6-2, 600° C. in Example 6-3, 200° C. in Comparative Example 2, and 700° C. in Comparative Example 3), and the plasma irradiation time in microwave plasma CVD was 10 minutes. The rest is the same as in Example 1, and the description will be omitted.

<Results>
Scanning electron microscope photographs of the surfaces of the carbon nanowall electrodes prepared in Examples 6-1, 6-2, and 6-3, and Comparative Example 2 are shown in Figures 10 and 11. In Comparative Example 2, in which the temperature of the susceptor 6 was set to 200°C, a layer of carbon nanowalls was formed as shown in Figure 11, but the substrate was warped due to distortion, and the carbon nanowall layer was also easily peeled off. In Comparative Example 3, in which the temperature of the susceptor 6 was set to 700°C, the substrate was warped due to distortion, and this was also a problem.
On the other hand, when the temperature of the susceptor 6 was in the range of 400 to 600° C., a layer of carbon nanowalls that was not easily peeled off was formed, and the substrate was not warped due to distortion.
FIG. 12 shows a graph of the weight change of the substrate before and after plasma exposure.

(Examples 7-1 to 7-4)
In Examples 7-1 to 7-4, an aluminum plate having a thickness of 100 μm was used as the substrate, and the temperature of the table on which the substrate was placed and the plasma exposure time in microwave plasma CVD were set as conditions shown in Table 2. The rest of the conditions were the same as in Example 2, and therefore a description thereof will be omitted.

<Results>
Scanning electron microscope photographs of the surfaces of the carbon nanowall electrodes prepared in Examples 7-1 to 7-4 are shown in Figure 13. The scanning electron microscope photograph in Figure 13 shows that carbon nanowalls are formed even when an aluminum plate is used as the substrate. The carbon nanowall electrodes prepared in Examples 7-1 to 7-4 were not deformed by strain, and the carbon nanowall layer was not peeled off.
14 shows the results of XPS measurement of the surface of the carbon nanowall electrode prepared in Example 7-1. In the XPS before argon sputter etching, the peak was similar to that of graphite, and it was found that the electrode was made of carbon nanowalls. On the other hand, when argon sputter etching was performed, it was found that the peak due to carbon nanowalls in the C1s spectrum, which appeared at 284.4 eV, became smaller with the passage of etching time, and a peak due to carbide appeared at 282 eV. This shows that carbide is present near the very surface of the substrate.

<Preparation of Lithium-Ion All-Solid-State Secondary Battery>
Using the negative electrode for a lithium ion secondary battery of Example 1 prepared as described above, an all-solid-state lithium ion secondary battery was prepared by the following steps.
The negative electrode for lithium ion secondary battery of Example 1 was placed on a sample stage in a chamber of a magnetron sputtering device with the carbon nanowalls facing up, and a solid electrolyte was laminated by sputtering. A lithium phosphate sintered body was used as the sputtering target, argon (Ar) and nitrogen ( _N2 ) were used as the sputtering gas, the gas introduction rates were 30sccm and 30sccm, respectively, the output per target area was 2W/ ^cm2 , and the sputtering time was 20 hours. In this way, a lithium phosphate nitride ceramic (hereinafter referred to as LiPON layer), which is a solid electrolyte capable of lithium ion migration, was laminated on the carbon nanowalls, which are the negative electrode for lithium ion secondary battery of Example 1.

Next, a positive electrode was laminated on the LiPON layer by sputtering. A sintered lithium manganese oxide (LiMnO) was used as the sputtering target, Ar was used as the sputtering gas, the gas introduction rate was 60 sccm, the output per target area ^was 2 W/cm2, and the sputtering time was 6 hours.
In this way, LiMnO, a positive electrode active material, was further laminated on the LiPON layer, and then gold was evaporated on the LiMnO in an evaporation device to obtain an all-solid-state lithium-ion secondary battery. The structure of this battery is shown in Fig. 15. That is, in this battery, carbon nanowalls 21 are erected on a substrate 20 made of stainless steel SUS304 with a carbide-containing carbon layer 22 sandwiched therebetween, and a LiPON layer 23, a LiMnO layer 24, and a gold evaporated layer 25 are further laminated in this order. The substrate 20 is a negative electrode side current collector, the carbon nanowalls 21 are a negative electrode active material, the LiPON layer 23 is a solid electrolyte in which lithium ions can move, and the gold evaporated film 25 is a positive electrode side current collector.
The LiPON layer 23, which is a solid electrolyte, is formed by leaving a certain amount of space between the walls of the carbon nanowalls so that it can come into contact with the walls of the carbon nanowalls. Finally, a gold electrode is formed on the surface of the positive electrode to complete the all-solid-state battery structure.

- Evaluation of battery characteristics -
(Charge/discharge characteristics)
The charge/discharge characteristics of the lithium ion all-solid-state secondary battery prepared as described above were measured.
Measurement conditions: Charge and discharge were performed at a constant current of 5 μA at a temperature of 25° C. The voltage range was from 0.5 V to 4.2 V, and charge and discharge were repeated 15 times.
Measurement results The measurement results are shown in FIG. 16. The charge/discharge characteristics were relatively stable against repetition, and the discharge capacity was 14.4 μAh/cm ^2. Since the mass of the carbon nanowalls was 13 μg/cm ² , the discharge capacity per unit weight of the negative electrode material based on the discharge capacity was calculated to be 1100 mAh/g. This discharge capacity per unit weight is a value that greatly exceeds the theoretical discharge capacity of the carbon raw material, 372 mAh/g, calculated from the amount of lithium that can be intercalated between the layers of the graphene sheet (one lithium atom per six carbon atoms). The reason for this is that not only the charge/discharge reaction involving graphene, but also the lithium precipitated on the graphene surface is thought to be involved in the charge/discharge reaction.

(Scanning electron microscope observation of battery cross section)
The cross section of the lithium ion all-solid-state secondary battery prepared as described above, which was etched by ion milling, was observed with a scanning electron microscope. The results are shown in FIG. 17. It was found that the carbon nanowalls, LiPON, LiMnO, and gold were laminated on the stainless steel substrate. In addition, no peeling was observed between the stainless steel and the carbon nanowalls. It was found that the adhesion was good during the process of the all-solid-state battery and the charge and discharge operation.
Furthermore, it is desirable to set the carbon nanowalls almost vertically relative to the substrate, but since it is also possible for the solid electrolyte film to be formed from an oblique direction, it is also possible to set the carbon nanowalls at an angle of about 45 degrees relative to the substrate, as shown in FIG. 17 .

This invention is not limited in any way to the above description of the embodiment of the invention. Various modifications that do not deviate from the scope of the claims and that can be easily conceived by a person skilled in the art are also included in this invention.

The carbon nanowall electrode of the present invention can be made into a negative electrode active material consisting of only carbon nanowalls, which are carbon materials, without using any conventional binders or conductive additives. This simplifies the manufacturing process, and allows for a higher battery capacity than conventional graphite negative electrodes, making it possible to produce a small, high-capacity battery. Furthermore, when used as a capacitor electrode, the electrode has a large surface area, resulting in a capacitor with a large capacity.

100...substrate, 101, 102...carbon layer, 200...plasma CVD device, 1...CVD reaction vessel, 2...waveguide, 3...plasma excitation plate, 30...recess, 4...slot antenna, 5...substrate, 6...susceptor (table for placing substrate), 7...heating device, 11...inlet, 12...outlet, 20...stainless steel foil, 21...carbon nanowall, 22...carbide-containing carbon layer, 23...LiPON layer, 24...lithium manganate layer, 25...gold vapor deposition layer

Claims (9)

A carbon nanowall electrode in which carbon nanowalls are erected on a substrate, and a peak due to carbide is present near the substrate surface in a narrow scan of C1s in an X-ray photoelectron spectroscopy (XPS) analysis, and the substrate is any one of austenitic stainless steel, ferritic stainless steel, and aluminum . A carbon nanowall electrode in which carbon nanowalls are erected on a substrate, a peak due to carbide is present in a narrow scan of C1s in an X-ray photoelectron spectroscopy (XPS) analysis near the substrate surface , and the average distance between adjacent carbon nanowalls is 1.2 μm or more . 3. The carbon nanowall electrode of claim 2 , wherein the substrate comprises at least one element selected from the group consisting of iron, nickel, chromium, cobalt, aluminum, silicon, tungsten, molybdenum, manganese, titanium, and tantalum. 3. The carbon nanowall electrode according to claim 1, wherein the carbide is present in the carbon nanowalls in the vicinity of the substrate surface. 3. The carbon nanowall electrode of claim 1, wherein the area of the peak due to the carbide is 2% or more of the total peak area in the C1s spectrum. A method for producing a carbon nanowall electrode, comprising: a substrate having carbon nanowalls standing on the substrate; and a peak due to carbide present in a narrow scan of C1s in an X-ray photoelectron spectroscopy (XPS) analysis in the vicinity of the substrate surface, the method comprising the steps of :
The method includes a CVD process of forming carbon nanowalls on a substrate by performing a microwave plasma CVD process while supplying a mixed gas containing at least hydrogen and a hydrocarbon,
The temperature of the table on which the substrate is placed is set to 400 to 600° C. An all-solid-state secondary battery that uses a carbon nanowall electrode as the negative electrode for the secondary battery, in which carbon nanowalls are erected on a substrate and a carbide-based peak is present in a narrow scan of C1s in X-ray photoelectron spectroscopy (XPS) analysis near the substrate surface. forming a solid electrolyte layer on a carbon nanowall electrode by physical vapor deposition, the carbon nanowall electrode having a substrate on which a carbon nanowall is erected and a carbide-based peak is present in a narrow scan of C1s in an X-ray photoelectron spectroscopy (XPS) analysis in the vicinity of the substrate surface;
forming a positive electrode material layer on the solid electrolyte by physical vapor deposition;
A method for producing an all-solid-state secondary battery comprising the steps of: forming a carbon nanowall electrode by the method of claim 6 ;
forming a solid electrolyte layer on the carbon nanowall electrode by physical vapor deposition;
forming a positive electrode material layer on the solid electrolyte by physical vapor deposition;
A method for producing an all-solid-state secondary battery comprising the steps of:

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