Carbon nanowall electrode and method for manufacturing same

Abstract

The present invention addresses the problem of providing a carbon nanowall electrode in which peeling does not readily occur between a substrate and a carbon nanowall, and in which the battery capacity per unit volume (or per unit weight) of a negative electrode material is large even when applied to an all-solid-state battery using a solid electrolyte. The means for solving the problem is that a carbon nanowall (21) is vertically arranged on a substrate (20), and a peak based on carbide exists near the surface of the substrate in narrow scanning of C1s in X-ray photoelectron spectroscopy (XPS) analysis.

Description

Technical Field

[0001] The present invention relates to a carbon nanotube wall electrode having carbon nanotube walls erected on a substrate and a method for manufacturing the same. Further, it relates to an all-solid-state secondary battery using them and a method for manufacturing the same. Background Art

[0002] In recent years, carbon nanotube walls, which are a type of carbon-based nanomaterial, have attracted attention and extensive research has been conducted. Carbon nanotube walls are nanostructures formed by erecting multiple layers of graphene on a substrate and are expected to be applied as electrodes for batteries, capacitors, biosensors, electrochemical sensors, etc. (for example, Non-Patent Documents 1 and 2).

[0003] In addition, Patent Document 1 describes a lithium-ion secondary battery using a carbon nanosheet electrode as a negative electrode. Here, the carbon nanosheet electrode is defined as an electrode in which graphene sheets grow obliquely in various directions from a substrate. This secondary battery uses Li 1- x CoO2 as a positive electrode active material and uses a solution in which LiPF6 is dissolved in a mixed solvent of ethylene carbonate and dimethyl carbonate as an electrolyte, and it has been found that the charge-discharge capacity per unit weight of carbon exceeds the theoretical capacity of graphite and the deterioration due to repeated charge-discharge is small.

[0004] Prior Art Documents

[0005] Patent Documents

[0006] Patent Document 1: WO2014 / 069310

[0007] Non-Patent Documents

[0008] Non-Patent Document 1: M. Hiramatsu, K. Takeda, H. Kondo and M. Hori J. Plasma Fusion Res. Vol. 98, No. 4 (2022) 158 - 164

[0009] Non-Patent Document 2: B. Li, S. Yang et.al. Nano Lett. 2015, 15, 5, 3073 - 3079 Summary of the Invention

[0010] However, the above-mentioned conventional carbon nanotube wall electrodes have the following problems: peeling easily occurs between the copper substrate and the carbon nanotube walls, and the mechanical strength is insufficient. In addition, in the carbon nanosheet electrode described in Patent Document 1, the graphene sheets grow obliquely in various directions from the substrate. Therefore, in the case of a all-solid secondary battery using a solid electrolyte, there is a problem that the solid electrolyte can only contact the outermost graphene sheets. As a result, there are problems that the contact area between the solid electrolyte and the carbon nanosheets becomes small, and the battery capacity per unit volume (or per unit weight) of the negative electrode material becomes small.

[0011] The present invention has been completed in view of the above-mentioned conventional situation, and the problem to be solved is to provide a carbon nanotube wall electrode in which peeling hardly occurs between the substrate and the carbon nanotube walls, and the battery capacity per unit volume (or per unit weight) of the negative electrode material is increased even when applied to an all-solid battery using a solid electrolyte.

[0012] In order to solve the above problems, the present inventors studied carbon nanotube wall electrodes. As a result, it was found that when graphene sheets are formed on a substrate using the microwave plasma CVD method, the graphene sheets grow substantially vertically on the substrate. In addition, it was found that by using a material containing elements such as iron and nickel as the substrate, a carbide layer (i.e., a carbide of a compound of carbon and a positive element; the same shall apply hereinafter) having excellent adhesion to the substrate surface is formed near the substrate surface. Moreover, it was also found that by forming this carbide layer, peeling hardly occurs between the carbon nanotube walls and the substrate, and a carbon nanotube wall electrode having high mechanical strength and excellent durability is obtained. In addition, it was found that if this carbon nanotube wall electrode is applied to an all-solid secondary battery using a solid electrolyte, the capacity per unit volume (or per unit weight) of the negative electrode material can be greatly increased.

[0013] That is, the carbon nanotube wall electrode of the present invention is characterized in that carbon nanotube walls are erected on a substrate, and a peak based on a carbide exists in a narrow scan of C1s in X-ray photoelectron spectroscopy (XPS) analysis near the substrate surface. The center value of the binding energy of the peak based on the carbide varies slightly depending on the type of the carbide, but is a value in the range of 281.0 eV to 284.0 eV.

[0014] In the carbon nanotube wall electrode of the present invention, a peak based on a carbide exists in a narrow scan of C1s in X-ray photoelectron spectroscopy (XPS) analysis near the substrate surface (in other words, a carbide layer exists near the substrate surface). From the viewpoint of improving the adhesion between the substrate and the carbon nanotube walls, the area of the peak based on the carbide in C1s in X-ray photoelectron spectroscopy (XPS) analysis is preferably 2% or more of the total peak area in the C1s spectrum.

[0015] As the material of the substrate, it is preferable to contain at least one element selected from iron, nickel, chromium, cobalt, aluminum, silicon, tungsten, molybdenum, manganese, titanium, and tantalum. This is because these elements easily form carbides when reacting with carbon. For example, as the substrate, thin plates, foils, etc. made of pure iron, carbon steel, stainless steel, and ferroalloys can be used. Additionally, the substrate can be a material that also serves as a current collector, or a material formed by laminating a film layer containing these elements on the surface of a material composed of a current collector. Austenitic stainless steel, ferritic stainless steel, and martensitic stainless steel contain iron, chromium, and nickel that easily form carbides and are not easily corroded, so they are particularly preferred.

[0016] In addition, the average distance between adjacent carbon nanotube walls is preferably 1.2 μm or more. This is because when using the carbon nanotube wall electrode in an all-solid-state lithium-ion battery, if the average distance between the carbon nanotube walls is less than 1.2 μm, it is difficult for the solid electrolyte to fill the gaps between the carbon nanotube walls, and the contact area between carbon and lithium decreases, resulting in a smaller charge capacity.

[0017] The carbon nanotube wall electrode of the present invention can be manufactured as follows.

[0018] That is, the manufacturing method of the carbon nanotube wall electrode of the present invention is characterized by including a CVD process, in which carbon nanotube walls are formed on a substrate by performing microwave plasma CVD while supplying a mixed gas containing at least hydrogen and hydrocarbons; the temperature of the stage on which the substrate is placed is 400 - 600 °C. In addition to hydrogen and hydrocarbons, the mixed gas can also contain rare gases such as helium and argon.

[0019] According to this manufacturing method of the carbon nanotube wall electrode, a carbon layer containing carbides can be formed on the substrate. The carbon layer containing carbides formed in this way is thin and has good adhesion, so it is possible to prevent the carbon layer from peeling or bending due to strain caused by the difference in the thermal expansion coefficients of the substrate and the carbon layer. It should be noted that if the temperature of the stage on which the substrate is placed is less than 400 °C, the surface side of the substrate is heated more strongly than the back side due to the radiant heat from the plasma, so it becomes higher than the temperature of the stage on which the substrate is placed. Therefore, a temperature difference is generated in the thickness direction of the substrate, resulting in problems such as substrate bending or warping.

[0020] The all-solid-state secondary battery of the present invention is characterized by using the carbon nanotube wall electrode of the present invention as the negative electrode for the secondary battery. Since the all-solid-state secondary battery of the present invention uses a carbon nanotube wall electrode with graphene sheets erected on the substrate, the solid electrolyte contacts the surface of the graphene sheets of each carbon nanotube wall, thereby increasing the contact area. In addition, during the charging process, not only is lithium occluded between the layers of the carbon nanotube walls by intercalation, but several layers of lithium are also deposited on the surface of the carbon nanotube walls. Therefore, the battery capacity per unit volume or per unit weight of the negative electrode material is significantly increased.

[0021] The manufacturing method of the all-solid secondary battery of the present invention is characterized by having a process of forming a solid electrolyte layer on the carbon nanotube wall electrode of the present invention by physical vapor deposition and a process of forming a positive electrode material layer on the above solid electrolyte by physical vapor deposition.

[0022] According to the manufacturing method of the all-solid secondary battery of the present invention, since each manufacturing process is carried out in a vacuum device, there is no risk of mixing moisture and impurities in the atmosphere, and manufacturing can be carried out by automation.

[0023] The inventors of the present invention confirmed that the use of the carbon nanotube wall electrode of the present invention to fabricate an all-solid lithium-ion battery exhibits excellent battery characteristics. Compared with the case of using an electrolyte solution, this all-solid lithium-ion battery has the advantages of being able to increase the operating temperature and having no risk of liquid leakage, thus being safe.

[0024] As the solid electrolyte, LiPON etc. can be used. In addition, as the positive electrode active material, positive electrode active materials such as LiMnO that are commonly used in lithium-ion batteries using electrolyte solutions can be used. Description of the Drawings

[0025] Figure 1 It is a schematic diagram showing the process of carbon nanotube wall growth on a substrate.

[0026] Figure 2 It is a schematic cross-sectional view of the microwave plasma CVD apparatus used in the examples.

[0027] Figure 3 It is a scanning electron microscope photograph of the surface and cross-section of the carbon nanotube wall electrode of Example 1.

[0028] Figure 4 It is a transmission electron microscope photograph of the cross-section of the carbon nanotubes in the carbon nanotube wall electrode of Example 1.

[0029] Figure 5 It is the X-ray photoelectron spectroscopy (XPS) measurement result of the carbon nanotube wall electrode and graphite of Example 1.

[0030] Figure 6 It is the X-ray photoelectron spectroscopy (XPS) measurement results before and after argon etching (after 5 minutes and 10 minutes) of the carbon nanotube wall electrode of Example 4.

[0031] Figure 7 It is a schematic cross-sectional view of the carbon nanotube wall electrode.

[0032] Figure 8 It is the Raman spectrum of the carbon nanotube wall electrode of Example 1.

[0033] Figure 9It is a scanning electron microscope photograph of the surface of the carbon nanotube wall electrode of Example 5-4, Example 5-5, and Example 5-6.

[0034] Figure 10 It is a scanning electron microscope photograph of the surface of the carbon nanotube wall electrode of Example 6-1, Example 6-2, and Example 6-3.

[0035] Figure 11 It is a scanning electron microscope photograph of the surface of the carbon nanotube wall electrode of Comparative Example 2.

[0036] Figure 12 It is a graph of the weight change of the substrate before and after plasma irradiation.

[0037] Figure 13 It is a graph of the scanning electron microscope photograph and Raman spectrum of the surface of the carbon nanotube wall electrode fabricated in Examples 7-1 to 7-4.

[0038] Figure 14 It is a graph showing the XPS measurement results of the carbon nanotube wall electrode fabricated in Example 7-1.

[0039] Figure 15 It is a schematic cross-sectional view of a lithium-ion all-solid-state secondary battery.

[0040] Figure 16 It is a graph showing the charge-discharge characteristics of a lithium-ion all-solid-state secondary battery.

[0041] Figure 17 It is a scanning electron microscope photograph of the cross-section of a lithium-ion all-solid-state secondary battery. Detailed implementation mode

[0042] In the carbon nanotube wall electrode of the present invention, carbon nanotube walls are erected on a substrate. The carbon nanotube wall is a highly perfect crystal composed of nanosized graphite microcrystals, and is a plate-shaped nanostructure formed by overlapping several to about 100 graphene sheets.

[0043] As the material of the substrate, a material containing at least one element of iron, nickel, chromium, cobalt, aluminum, silicon, tungsten, molybdenum, manganese, titanium, and tantalum can be used. In addition, it can also be a substrate obtained by forming a film composed of at least one element of iron, nickel, chromium, cobalt, aluminum, silicon, tungsten, molybdenum, manganese, titanium, and tantalum on the surface of a metal substrate such as copper, copper alloy, nickel, nickel alloy, iron, iron alloy, stainless steel, molybdenum, tungsten, and tantalum, which have been used as current collectors in the past, by surface treatment methods such as plating and evaporation plating.

[0044] As the material of the substrate, austenitic stainless steel, martensitic stainless steel containing iron, nickel and chromium, and ferritic stainless steel containing iron and chromium are particularly preferred because of their excellent corrosion resistance. For example, austenitic (SUS304, SUS304-L, SUS302, SUS301, SUS310S, SUS321, SUS316, SUS316-L, etc.), ferritic (SUS430, SUS434, etc.), martensitic (SUS410S, SUS420J2, etc.), precipitation hardening type (SUS631, ASL-350, etc.) stainless steel of any steel type specified in JIS G4305:2005 "Cold-rolled stainless steel sheets and strips" can be used. In addition, it can be a semiconductor substrate such as Si, SiC, AlGaAs, or AlGaN.

[0045] The thickness of the substrate is not particularly limited, but from the viewpoint of reducing the weight of the electrode, it is preferably about 1 mm or less, more preferably a foil material of about 5 μm to 100 μm which has been used as a current collector in the past, and further preferably a foil material of 5 μm to 20 μm in practical use.

[0046] In addition, it is required that there is a carbide-based peak in the narrow scan of C1s in X-ray photoelectron spectroscopy (XPS) analysis near the substrate surface in the carbon nanotube wall electrode of the present invention. Here, the vicinity of the substrate surface refers to the range within 100 nm from the substrate surface. When growing carbon nanotube walls on the substrate using the microwave plasma CVD method, carbides are generated near the substrate surface, which plays a role in improving the adhesion between the substrate and the carbon nanotube walls. From the viewpoint of improving the adhesion, the area of the carbide-based peak 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.

[0047] The average distance between adjacent carbon nanotube walls is preferably 1.2 μm or more. This is because when the carbon nanotube wall electrode is used in an all-solid-state lithium-ion battery, if the average distance between the carbon nanotube walls is less than 1.2 μm, it is difficult for the solid electrolyte to fill the narrow gap between the carbon nanotube walls, and the contact area between carbon and lithium decreases, resulting in a reduction in the charging capacity.

[0048] <Manufacturing method of carbon nanotube wall electrode>

[0049] Next, the manufacturing method of the carbon nanotube wall electrode of the present invention will be described.

[0050] First, prepare a substrate containing at least one element of iron, nickel, chromium, cobalt, aluminum, silicon, tungsten, molybdenum, manganese, titanium, and tantalum, cut it into the required size, and clean the surface. As the cleaning method, cleaning using a surfactant, cleaning using an organic solvent, electrolytic cleaning in an alkaline solution, plasma treatment, etc. can be used.

[0051] Next, place the cleaned substrate on the specimen stage of a microwave plasma CVD apparatus and perform the microwave plasma CVD method. Here, the microwave plasma CVD method refers to a method of chemically vapor growing by plasmaizing a source gas using microwave power in order to activate a chemical reaction. The excitation method for generating the microwave plasma is not particularly limited, and examples thereof include a microwave surface wave plasma CVD method, an ECR plasma CVD method in which microwaves and an ECR magnetic field are applied, and the like.

[0052] As the supply gas in the microwave plasma CVD, a gas in which a hydrocarbon gas (such as methane, ethane, acetylene, etc.) as a carbon source for the carbon nanowalls and hydrogen are mixed in an inert gas such as argon can be used. It is inferred that the supply gas flowing into the chamber of the plasma CVD apparatus becomes argon ions and carbon positive ions, and these ions further react to become argon gas, hydrogen, and carbon nanowalls. Figure 1 This shows the growth process of the carbon nanowalls on the substrate. First, initially (during about 30 seconds to 2 minutes), after forming several carbon layers 101 that grow laterally on the surface of the substrate 100 (refer to Figure 1 a), a part of its surface becomes nuclei and carbon layers 102 grow in the vertical direction, thereby forming carbon nanowalls (refer to Figure 1 b). Here, it doesn't matter whether the carbide exists in an island-like separation in the carbon layer 101 or exists as a carbide layer between the carbon layer 101 and the substrate 100. In any case, the formation of the carbide is a factor for improving the adhesion between the substrate 100 and the carbon layers 101 and 102.

[0053] <Full solid-state lithium ion battery>

[0054] Using the carbon nanowall electrode of the present invention, making it contact with a solid electrolyte through which lithium ions can move, and further laminating a positive electrode material and a current collector that have been used in a lithium ion battery in the past, a full solid-state lithium ion battery can be constructed in this way. As the solid electrolyte through which lithium ions can move, a lithium-based oxide (such as lithium phosphonitride (LiPON), Li3PO4, LiBO3, etc.) can be used. In addition, as the positive electrode active material, a lithium-based oxide (such as lithium manganate (LiMnO), lithium cobaltate (LiCoO2), lithium nickelate (LiNiO2), etc.) can be used. These solid electrolytes and positive electrode active materials are formed in a vacuum apparatus using physical vapor deposition methods such as sputtering and evaporation. When laminating the solid electrolyte on the carbon nanowall electrode by sputtering, a lithium-based oxide or the like can be used as the target electrode. The power per unit 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 preferably about 0.2 to 5 W / cm 2 or so.

[0055] The all-solid-state lithium-ion battery using the carbon nanotube wall electrode of the present invention is stable with almost no change in the structure of the negative electrode surface even after repeated charge and discharge.

[0056] In addition, during the charging process of this battery, not only does lithium intercalate and be occluded between the layers of the carbon nanotube walls, but several layers of lithium also precipitate on the surface of the carbon nanotube walls. Therefore, not only the lithium occluded between the layers but also the lithium precipitated on the surface of the carbon nanotube walls contributes to the charge and discharge capacity, far exceeding the charge and discharge capacity theoretically calculated based on the amount of lithium occluded between the layers. Therefore, significant downsizing and weight reduction of the battery can be achieved.

[0057] In addition, similar to conventional lithium-ion secondary batteries, high-potential driving around 3.2 V to 4.2 V can be performed.

[0058] Furthermore, since a solid electrolyte is used, there is no worry about liquid leakage, and the operating temperature can also be increased.

[0059] Examples

[0060] <Fabrication of Carbon Nanotube Wall Electrode>

[0061] (Example 1)

[0062] A stainless steel foil with a thickness of 40 μm made of SUS304 was prepared and cut into 50 mm × 50 mm as a substrate. SUS304 is a type of austenitic stainless steel. This substrate was placed on Figure 2 the pedestal 6 in the reaction vessel 1 of the microwave surface wave plasma CVD apparatus 200 shown. After performing microwave plasma CVD under the following conditions, nitrogen was introduced into the reaction vessel 1 and the pressure was returned to atmospheric pressure, and then the substrate was taken out and used as the negative electrode for a lithium-ion secondary battery.

[0063] Flow gas: A mixed gas of CH4:H2:Ar = 5:3:3

[0064] Temperature of pedestal 6: 500 °C, Process pressure: 10 Pa,

[0065] Plasma irradiation time: 3 minutes, 8 minutes, and 10 minutes, Applied microwave power: 1 kW,

[0066] Applied microwave frequency: 2.45 GHz,

[0067] Distance from plasma excitation plate to substrate: 55 mm

[0068] (Structure of Plasma CVD Apparatus 200)

[0069] The plasma CVD apparatus 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 plurality of minute recesses 30 are formed on the CVD reaction vessel 1 side of the plasma excitation plate 3. By the electric field concentrating on the recesses 30, the recesses 30 become the generation starting points of the plasma, and it is easy to generate the plasma with low power. A slot antenna 4 is provided on the lower side of the waveguide 2. Microwaves of 2.45 GHz are 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.

[0070] Inside the CVD reaction vessel 1, a susceptor 6 for placing a substrate 5 and a heating device 7 for heating the susceptor 6 are provided. The supply amounts of the mixed gases are adjusted by mass flow controllers (not shown) for respective gas components, and are introduced into the CVD reaction vessel 1 from an inlet 11 provided on the upper side surface of the CVD reaction vessel 1. An outlet 12 for discharging gases is provided at the lower part of the side surface of the CVD reaction vessel 1. In addition, the inside of the CVD reaction vessel 1 can be depressurized to about 10 -3 torr by a vacuum pump (not shown).

[0071] (Forming carbon nanotube walls by CVD)

[0072] Next, a method for forming carbon nanotube walls on the substrate 5 will be described.

[0073] The substrate 5 is ultrasonically cleaned in methanol, further ultrasonically cleaned in acetone, and then dried, and is set on the susceptor 6. After the inside of the CVD reaction vessel 1 is depressurized to a prescribed pressure by a vacuum pump, the heating device 7 is turned ON, and the temperature of the susceptor 6 is controlled to a prescribed temperature. In this state, methane, hydrogen, and argon are supplied to the inside of the CVD reaction vessel 1 while controlling the gas flow rates of respective gases by mass flow controllers, and the inside of the CVD reaction vessel 1 is controlled to a prescribed pressure. Next, the microwave generator is driven to supply microwaves of 2.45 GHz to the waveguide 2, and a plasma of the mixed gases is generated inside the CVD reaction vessel 1. Microwave surface wave plasma is generated above the substrate 5. After a prescribed time, the driving of the microwave generator is stopped, and after the heating device 7 is turned OFF, after a certain time, the inside of the CVD reaction vessel 1 is restored to atmospheric pressure, and the substrate 5 having carbon nanotube walls formed on the surface is taken out and used as the carbon nanotube wall electrode of Example 1.

[0074] (Example 2)

[0075] In Example 2, the same SUS304 substrate as in Example 1 was used, and the conditions in microwave plasma CVD were as follows. Other processes were the same as in Example 1, and their descriptions were omitted.

[0076] · Flow gas: A mixed gas of acetylene: hydrogen: argon = 3:20:20,

[0077] · Temperature of the susceptor 6: 500 °C, process pressure 7 Pa,

[0078] · Plasma irradiation time: 5 minutes,

[0079] · Applied microwave power 1.5 kW, applied microwave frequency 2.45 GHz,

[0080] · Distance from the plasma excitation plate to the substrate 50 mm

[0081] (Example 3)

[0082] In Example 3, an austenitic stainless steel (SUS316) foil with a thickness of 40 μm was used as the substrate. Other processes were the same as in Example 2, and their descriptions were omitted.

[0083] (Example 4)

[0084] In Example 4, a ferritic stainless steel (SUS430) foil with a thickness of 50 μm was used as the substrate. Other processes were the same as in Example 2, and their descriptions were omitted.

[0085] (Comparative Example 1)

[0086] In Comparative Example 1, a copper foil with a thickness of 40 μm was used as the substrate. Other processes were the same as in Example 1, and their descriptions were omitted.

[0087] - Evaluation -

[0088] The carbon nanotube wall electrodes of Examples 1 to 4 and Comparative Example 1 prepared as described above were analyzed as follows.

[0089] · Observation using a scanning electron microscope

[0090] For the carbon nanotube wall electrode of Example 1 (plasma irradiation time: 8 minutes), scanning electron microscope photos of the surface and cross-section are shown in Figure 3 . From this, it can be seen that the flat carbon crystals characteristic of carbon nanotube walls grow in a direction substantially perpendicular to the substrate surface. The same carbon nanotube walls were also observed for Examples 2, 3, and 4, and no peeling of the carbon nanotube walls occurred.

[0091] On the other hand, in Comparative Example 1 with a copper foil as the substrate, although carbon nanotubes were formed on the copper foil, peeling from the substrate was observed at the stage of subsequent substrate processing, indicating that it is difficult to utilize it as a carbon nanotube electrode.

[0092] · Observation using a transmission electron microscope

[0093] For the carbon nanotube electrode of Example 1, the carbon nanotubes were peeled off from the substrate 1, fixed with a curing agent, and then further thinned by a microtome or the like in a manner that enables cross-sectional observation to prepare a specimen, and a transmission electron microscope photograph was taken (see Figure 4 ). As a result, it was found that the thickness of the carbon nanotubes was approximately 4.5 nm, and about 10 or more graphene sheets were stacked. In addition, the same carbon nanotubes were also observed for Example 2, Example 3, and Example 4.

[0094] · Measurement based on X-ray photoelectron spectroscopy (XPS)

[0095] Two carbon nanotube electrodes were fabricated under the same conditions as in Example 1 except that the plasma irradiation time was 3 minutes or 10 minutes, and X-ray photoelectron spectroscopy (XPS) measurement was performed. In addition, for comparison, highly crystalline graphite was also measured.

[0096] The measurement results are shown in Figure 5 . When the plasma irradiation time was 3 minutes, a broad peak was observed around 283 - 284 eV. It was found that this broad peak was not detected in highly crystalline graphite, and from the value of the chemical shift, it was a carbide-based peak in C1s. In addition, waveform separation was performed, and the area of the broad peak was determined around 283 - 284 eV. As a result, it was 2.5% of the total peak area in the C1s spectrum. In contrast, when the plasma irradiation time was 10 minutes, no carbide-based peak around 283 - 284 eV was observed. From the above results, it can be seen that carbide exists within a range of 100 nm or less from the substrate surface in the carbon nanotubes.

[0097] For the specimens of Example 2 - 4, argon ion etching was performed in the XPS apparatus to etch away the carbon nanotubes on the spot and XPS measurement was carried out. The measurement results of Example 4 (plasma irradiation time was 5 minutes) are shown in Figure 6 . In the specimen before etching, the peaks were the same as those of graphite, so it was known that it was composed of carbon nanotubes. When argon sputter etching was performed on the surface of this specimen, it was found that as the etching time passed, the peak based on the carbon nanotubes in the C1s spectrum at 284.4 eV became smaller, and a peak based on carbide appeared at 283.2 eV. The same results were also obtained for Example 2 and Example 3. From this, it can also be seen that carbide exists near the very surface of the substrate.

[0098] On the other hand, in Comparative Example 1 with a copper foil as the substrate, no carbide-based peak was observed in the C1s spectrum at all.

[0099] · Measurement based on energy dispersive X-ray analysis (EDX)

[0100] Energy dispersive X-ray analysis (EDX) was performed on Examples 1 to 4. As a result, in addition to carbon, Fe and Cr were also detected.

[0101] From the above results and the XPS measurement results, it can be seen that in Examples 1 to 4, in the carbon nanotube walls near the substrate surface, Fe and Cr combine with carbon to form carbides.

[0102] From the results of the measurement based on X-ray photoelectron spectroscopy (XPS) and the measurement based on energy dispersive X-ray analysis (EDX), as Figure 7 shown, a carbide-containing carbon layer 22 was formed near the interface between the stainless steel foil 20 as the substrate and the carbon nanotube walls 21. The good adhesion between the stainless steel foil 20 and the carbon nanotube walls 21 is due to the formation of this carbide-containing carbon layer 22.

[0103] · Measurement of Raman spectrum

[0104] For the sample of Example 1, the Raman spectrum was measured using a micro laser Raman spectrometer (InVia Raman microscope manufactured by Renishaw). The laser was an argon laser with a wavelength of 532 nm. Figure 8 The measurement results are shown. Peaks represented by D, G, and 2D were observed near 1350 cm -1 , 1580 cm -1 and 2682 cm -1 respectively. Here, G is a peak from the graphite structure, and a small peak G′ is observed at the shoulder of this peak. This G′ peak indicates that the formed carbon nanotube walls extend in the vertical direction with respect to the substrate.

[0105] <Influence of gas composition in microwave plasma CVD>

[0106] In order to investigate the influence of the gas composition in microwave plasma CVD, the following experiment was conducted.

[0107] (Examples 5-1 to 5-6)

[0108] In Examples 5-1 to 5-6, the same SUS304 substrate as in Example 1 was used, and the conditions in microwave plasma CVD were changed in various ways to investigate the influence.

[0109] Flow gas conditions: shown in Table 1.

[0110] [Table 1]

[0111] <![CDATA[CH4]]> <![CDATA[CH4 / H2]]> Example 5-1 Flow rate constant* 0.5 Example 5-2 ″ 1.0 Example 5-3 ″ 1.6 Example 5-4 Flow rate variable** 1.7 Example 5-5 ″ 2.6 Example 5-6 ″ 3.3

[0112] *: Keep the flow rate of CH4 constant, change the flow rate of H2, and adjust the value of CH4 / H2.

[0113] **: Keep the flow rate of H2 constant, change the flow rate of CH4, and adjust the value of CH4 / H2.

[0114] The following conditions are the same as those in Examples 5-1 to 5-6.

[0115] Temperature of the base 6: 500 °C, process pressure: 10 Pa,

[0116] Plasma irradiation time: 10 minutes

[0117] Applied microwave power: 1 kW, applied microwave frequency: 2.45 GHz,

[0118] Distance from the plasma excitation plate to the substrate: 55 mm

[0119] - Results -

[0120] (Observation using a scanning electron microscope)

[0121] Figure 9 Scanning electron microscope photographs showing the surfaces of the carbon nanotube walls fabricated in Examples 5-4, 5-5, and 5-6 are presented. Based on these photographs, the average distance between adjacent carbon nanotube walls was calculated. The results were 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 can be seen that by controlling the value of CH4 / H2 in the supply gas for plasma CVD, the distance between adjacent carbon nanotube walls can be controlled.

[0122] (Examples 6-1 to 6-3 and Comparative Examples 2 and 3)

[0123] In Examples 6-1 to 6-3 and Comparative Examples 2 and 3, SUS304 was used as the substrate, and the temperature of the base 6 was set to various temperatures (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). The plasma irradiation time in microwave plasma CVD was 10 minutes. The rest was the same as in Example 1, and the description thereof is omitted.

[0124] <Results>

[0125] Scanning electron microscope photographs of the surfaces of the carbon nanotube wall electrodes fabricated in Examples 6-1, 6-2, and 6-3, and Comparative Example 2 are shown in Figure 10 and Figure 11。In Comparative Example 2 where the temperature of the susceptor 6 was 200 °C, a layer of carbon nanotube walls as shown in Figure 11 was formed, but there was a problem that the substrate warped due to strain, and the layer of carbon nanotube walls was also easily peeled off. In addition, in Comparative Example 3 where the temperature of the susceptor 6 was 700 °C, there was also a problem that the substrate warped due to strain.

[0126] On the other hand, when fabricated in the temperature range of 400 to 600 °C for the susceptor 6, a layer of carbon nanotube walls that was not easily peeled off was formed, and the substrate did not warp due to strain.

[0127] In addition, the graphs of the weight change of the substrate before and after plasma irradiation are shown in Figure 12 。

[0128] (Examples 7-1 to 7-4)

[0129] In Examples 7-1 to 7-4, an aluminum plate with a thickness of 100 μm was used as the substrate, and the temperature of the stage on which the substrate was placed and the plasma irradiation time in microwave plasma CVD were set to the conditions shown in Table 2. The rest was the same as in Example 2, and the description thereof was omitted.

[0130] [Table 2]

[0131] Temperature of pedestal 6 Plasma irradiation time Applied microwave power Example 7-1 500℃ 5 min. 1.5 kW Example 7-2 ″ 20 min. ″ Example 7-3 400℃ 20 min. ″ Example 7-4 ″ 20 min. 1.0 kW

[0132] <Results>

[0133] The scanning electron microscope photographs of the surfaces of the carbon nanotube wall electrodes fabricated in Examples 7-1 to 7-4 are shown in Figure 13 。From the Figure 13 scanning electron microscope photographs, it can be seen that even when an aluminum plate was used as the substrate, carbon nanotube walls were formed. The carbon nanotube wall electrodes fabricated in Examples 7-1 to 7-4 did not deform due to strain, and the layer of carbon nanotube walls did not peel off.

[0134] In addition, the XPS measurement results of the surface of the carbon nanotube wall electrode fabricated in Example 7-1 are shown in Figure 14 。In the XPS before argon sputter etching, the peaks were the same as those of graphite, indicating that it was composed of carbon nanotube walls. On the other hand, in the case of argon sputter etching, it was found that as the etching time passed, the peak based on the carbon nanotube walls in the C1s spectrum at 284.4 eV became smaller, and a peak based on carbide appeared at 282 eV. From this, it can be seen that carbide exists near the outermost surface of the substrate.

[0135] <Fabrication of all-solid-state lithium-ion secondary battery>

[0136] Using the negative electrode for a lithium-ion secondary battery of Example 1 fabricated as described above, an all-solid-state lithium-ion secondary battery was fabricated through the following steps.

[0137] In the chamber of a magnetron sputtering apparatus, the negative electrode for a lithium-ion secondary battery of Example 1 was placed on a specimen stage with the carbon nanotube wall on the upper side, and a solid electrolyte layer was laminated by sputtering. A lithium phosphate sintered body was used as the sputtering target, and argon (Ar) and nitrogen (N2) were used as the sputtering gases. The gas introduction amounts were 30 sccm and 30 sccm respectively, and the power per unit target area was 2 W / cm 2 , and the sputtering time was 20 hours. Thus, a lithium nitride phosphate-based ceramic (hereinafter referred to as LiPON layer), which is a solid electrolyte through which lithium ions can move, was laminated on the carbon nanotube wall that is the negative electrode for the lithium-ion secondary battery of Example 1.

[0138] Next, a positive electrode was laminated on the LiPON layer by sputtering. A lithium manganate (LiMnO) sintered body was used as the sputtering target, Ar was used as the sputtering gas, the gas introduction amount was 60 sccm, and the power per unit target area was 2 W / cm 2 , and the sputtering time was 6 hours.

[0139] Thus, after further laminating LiMnO, which is a positive electrode active material, on the LiPON layer, gold evaporation was performed on the LiMnO in an evaporation apparatus to obtain an all-solid-state lithium-ion secondary battery. The structure of this battery is shown in Figure 15 . That is, in this battery, carbon nanotube walls 21 are vertically provided on a substrate 20 made of stainless steel SUS304 with a carbide-containing carbon layer 22 interposed therebetween, and a LiPON layer 23, a LiMnO layer 24, and a gold evaporation layer 25 are further laminated in sequence. The substrate 20 is a current collector on the negative electrode side, the carbon nanotube walls 21 are negative electrode active materials, the LiPON layer 23 is a solid electrolyte through which lithium ions can move, and the gold evaporation layer 25 is a current collector on the positive electrode side.

[0140] The LiPON layer 23 as the solid electrolyte can be formed by separating the walls of the carbon nanotube walls to a certain extent so as to be in contact with the wall surfaces of the carbon nanotube walls. Finally, a gold electrode was formed on the surface of the positive electrode to complete the all-solid-state battery structure.

[0141] -Evaluation of battery characteristics-

[0142] (Charge and discharge characteristics)

[0143] The charge and discharge characteristics of the lithium-ion all-solid-state secondary battery fabricated as described above were measured.

[0144] ·Measurement conditions

[0145] At a temperature of 25°C, charge and discharge were performed at a constant current of 5 μA. The voltage range was 0.5 V to 4.2 V, and charge and discharge were repeated 15 times.

[0146] ·Measurement results

[0147] The measurement results are shown in Figure 16 . The charge-discharge characteristics are relatively stable for repeated cycles, and the discharge capacity is 14.4 μAh / cm 2 . The mass of the carbon nanotube walls is 13 μg / cm 2 . Therefore, the discharge capacity per unit weight of the negative electrode material based on the discharge capacity is calculated to be 1100 mAh / g. This discharge capacity per unit weight is significantly higher than the theoretical discharge capacity of 372 mAh / g of the carbon raw material calculated based on the amount of lithium that can be intercalated between the layers of graphene sheets (one lithium atom per six carbon atoms). For this reason, it is considered that not only the charge-discharge reaction involving only graphene but also the lithium deposited on the surface of graphene participates in the charge-discharge reaction.

[0148] (Observation of the battery cross-section using a scanning electron microscope)

[0149] For the lithium-ion all-solid-state secondary battery fabricated as described above, the cross-section exposed by etching using the ion milling method was observed with a scanning electron microscope. The results are shown in Figure 17 . It can be seen that carbon nanotube walls, LiPON, LiMnO, and gold are laminated on the stainless steel serving as the substrate, respectively. In addition, no peeling was observed between the stainless steel and the carbon nanotube walls. It can be seen that the adhesion is also good during the process of the all-solid-state battery and the charge-discharge operation.

[0150] In addition, the carbon nanotube walls are preferably erected substantially perpendicular to the substrate, but Figure 17 as can be seen, since the film formation of the solid electrolyte can also be carried out by detouring from an inclined direction, it can also be inclined at about 45 degrees with respect to the substrate.

[0151] The present invention is not limited by any of the descriptions of the above-described embodiments of the invention. Various modification methods within the scope of the claims and easily conceivable by those skilled in the art are also included in the present invention.

[0152] Industrial applicability

[0153] The carbon nanotube wall electrode of the present invention can be a negative electrode active material composed only of carbon nanotube walls as a carbon raw material without using any conventional binders or conductive aids. Therefore, the manufacturing process is simple, the battery capacity can be increased compared with the conventional graphite negative electrode, and thus a compact and high-capacity battery can be fabricated. In addition, in the case of an electrode used for a capacitor, the surface area of the electrode also becomes larger, so a high-capacity capacitor is obtained.

[0154] Symbol description

[0155] 100… Substrate, 101, 102… Carbon layer, 200… Plasma CVD apparatus, 1… CVD reaction vessel, 2… Waveguide, 3… Plasma excitation plate, 30… Recess, 4… Slot antenna, 5… Substrate, 6… Base (stage for placing the substrate), 7… Heating device, 11… Inlet, 12… Outlet, 20… Stainless steel foil, 21… Carbon nanotube wall, 22… Carbide-containing carbon layer, 23… LiPON layer, 24… Lithium manganate layer, 25… Gold evaporation coating layer.

Claims

1. A carbon nanotube wall electrode, in which carbon nanotube walls are erected on a substrate, and near the surface of the substrate, there is a carbide-based peak in the narrow scan of C1s in X-ray photoelectron spectroscopy (XPS) analysis.

2. The carbon nanotube wall electrode according to claim 1, wherein The substrate contains at least one element of iron, nickel, chromium, cobalt, aluminum, silicon, tungsten, molybdenum, manganese, titanium, and tantalum.

3. The carbon nanotube wall electrode according to claim 1, wherein, The substrate is austenitic stainless steel or ferritic stainless steel.

4. The carbon nanotube wall electrode according to claim 1, wherein The carbide exists in the carbon nanotube walls near the surface of the substrate.

5. The carbon nanotube wall electrode according to claim 1, wherein, The area of the carbide-based peak is 2% or more of the total peak area in the C1s spectrum.

6. The carbon nanotube wall electrode according to claim 1, wherein, The average distance between adjacent carbon nanotube walls is 1.2 μm or more.

7. A method for manufacturing a carbon nanotube wall electrode, which is a method for manufacturing the carbon nanotube wall electrode according to any one of claims 1 to 6, and includes: A CVD process, in which carbon nanotube walls are formed on a substrate by performing microwave plasma CVD while supplying a mixed gas containing at least hydrogen and hydrocarbons; wherein the temperature of the stage on which the substrate is placed is 400 to 600 °C.

8. An all-solid-state secondary battery, which uses the carbon nanotube wall electrode according to any one of claims 1 to 6 as a negative electrode for a secondary battery.

9. A method for manufacturing an all-solid-state secondary battery, having the following processes: A process of forming a solid electrolyte layer on the carbon nanotube wall electrode according to any one of claims 1 to 6 by physical vapor deposition, and A process of forming a positive electrode material layer on the solid electrolyte by physical vapor deposition.

10. A method for manufacturing an all-solid-state secondary battery, having the following processes: A process of forming a carbon nanotube wall electrode by the manufacturing method according to claim 7, A process of forming a solid electrolyte layer on the carbon nanotube wall electrode by physical vapor deposition, and A process of forming a positive electrode material layer on the solid electrolyte by physical vapor deposition.

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