JP7703141B2 - Electrodes for power storage devices, secondary batteries - Google Patents

Description

The technology disclosed herein relates to electrodes for electricity storage devices, secondary batteries, methods for manufacturing electrodes for electricity storage devices, and manufacturing apparatuses for electrodes for electricity storage devices.

For example, secondary batteries are known as chargeable and dischargeable electricity storage devices. Typically, in an electrode for an electricity storage device, an active material layer composed of a material involved in the charge and discharge reaction of the electricity storage device is formed on the surface of a current collector. For example, the following Patent Documents 1 and 2 disclose a lithium-ion secondary battery having an active material layer containing lithium (Li) in the positive electrode and an active material layer containing carbon (C) as an active material in the form of graphite, carbon nanowalls, or the like in the negative electrode.

It is generally known that the configuration of the active material layer has a significant effect on the battery performance of an electricity storage device. For example, there is technical knowledge that the battery performance of an electricity storage device is improved by applying a carbon nanostructure such as a carbon nanowall to the active material layer, as in Patent Document 2, rather than applying graphite to the active material layer, as in Patent Document 1.

Patent No. 2668678 JP 2010-9980 A

The conventional structure in which graphite is applied to the active material layer, as in Patent Document 1, may not be able to fully achieve the high battery performance targets required for future electricity storage devices.

On the other hand, when carbon nanowalls are applied to the active material layer as in Patent Document 2, the manufacturing of the carbon nanowalls is time-consuming, and the manufacturing cost of the electricity storage device may increase significantly. In addition, when an electrode in which carbon nanowalls are applied to the active material layer is rolled up for transportation or storage during the manufacturing process of the electricity storage device, the carbon nanowalls on the surface may be destroyed or fall off due to compressive stress or shear stress.

As such, there is still room for improvement in electrodes for electricity storage devices that use carbon as an active material. The purpose of this disclosure is to provide an electrode for electricity storage devices that has a structure different from conventional structures and includes an active material layer that can achieve high battery performance.

Through extensive research into carbon nanostructures that can be used in active material layers, the inventors of the present invention have succeeded in developing an electrode for an electricity storage device that can be easily manufactured using a method different from conventional methods and that can dramatically improve battery performance. The technology disclosed herein can be realized, for example, in the following forms:

One aspect of the present invention is provided as an electrode for an electricity storage device. The electrode for an electricity storage device in this aspect comprises a metal substrate constituting a current collector and having a fine uneven structure formed on its surface, and an active material layer mainly composed of a carbon nanostructure composed of nanographene and amorphous carbon, formed so as to densely cover the surface of the uneven structure, and in which metal atoms involved in the charging and discharging of the electricity storage device are precipitated on the surface when the electricity storage device is charged.

According to this form of the electrode for the electric storage device, the carbon nanostructure composed of nanographene and amorphous carbon is used as the active material, so that the battery performance of the electric storage device can be improved. In addition, since the surface of the active material layer is formed unevenly, the surface area of the active material layer is increased, and the amount of metal precipitated during charging of the electric storage device can be increased, so that the charging capacity of the electric storage device can be increased. In addition, since the uneven structure of the active material layer forms a large number of convex parts on the surface of the active material layer, the precipitation of metal on the surface of the active material layer is promoted during charging of the electric storage device. Therefore, the battery performance of the electric storage device can be further improved. In addition, since the active material layer is formed like a thin film densely covering the surface of the metal substrate, unlike carbon nanowalls that grow to extend from the surface of the metal substrate, it is possible to suppress destruction or falling off during the manufacturing process of the electric storage device.

The present invention can be realized in various forms other than an electrode for an electricity storage device, a secondary battery, a method for manufacturing an electrode for an electricity storage device, and an apparatus for manufacturing an electrode for an electricity storage device. For example, the present invention can be realized in the form of various electricity storage devices other than secondary batteries, active material layers used in the electricity storage devices, methods for manufacturing active material layers, and apparatus for manufacturing active material layers. The present invention can also be realized in the form of an apparatus or system including an electricity storage device.

FIG. 1 is a schematic diagram showing a configuration of a secondary battery. FIG. 2 is a schematic cross-sectional view showing the configuration of a metal substrate and an active material layer. FIG. 2 is a process flow diagram showing a manufacturing process of a secondary battery. 1 is a schematic diagram showing a configuration of a plasma processing apparatus. FIG. 2 is a schematic diagram showing a configuration of an electrode holding unit according to a first configuration example. FIG. 13 is a schematic diagram showing a configuration of an electrode holding unit in a second configuration example. 3A and 3B are schematic diagrams illustrating the mechanism by which an active material layer is formed by plasma treatment. FIG. 4 is an explanatory diagram showing images taken of the process of forming an active material layer according to an embodiment of the present invention. FIG. 4 is an explanatory diagram showing a photographed image of an active material layer according to an embodiment of the present invention. FIG. 4 is an explanatory diagram showing a Raman spectrum of an active material layer according to an embodiment of the present invention. FIG. 11 is an explanatory diagram showing a list of photographed images of active material layers of comparative examples. 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. 13 is an explanatory diagram showing the results of an evaluation test of the battery performance of the third comparative example. FIG. 4 is an explanatory diagram showing the results of evaluation tests of the battery performance of Examples and Comparative Examples.

Below, with reference to the drawings, we will explain embodiments of the electrode for an electricity storage device, the secondary battery, the method for manufacturing an electrode for an electricity storage device, and the manufacturing apparatus for an electrode for an electricity storage device according to the present invention.

1. Embodiments:
1-1. Secondary battery and electrode configuration:
Fig. 1 is a schematic diagram showing the configuration of a secondary battery 10 according to the present embodiment. For convenience, in Fig. 1, a container 11 is shown by a dashed line, and a separator 15 is shown by a dashed double-dashed line.

The secondary battery 10 is one type of chargeable and dischargeable electricity storage device, and is a lithium-ion secondary battery in which lithium (Li) 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.

The container 11 has an internal space filled with the electrolyte 12. The container 11 is liquid-tight and made of a material that is not easily reactive with the electrolyte 12. The electrolyte 12 has the property of being able to transmit metal ions involved in charging and discharging between the first electrode 20 and the second electrode 30.

In this embodiment, the electrolyte 12 is a solution in which a lithium salt is dissolved in an organic solvent, and is capable of transmitting lithium ions. For example, lithium hexafluorophosphate (LiPF ₆ ) can be used as the lithium salt of the electrolyte 12. 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 housed and a second electrode chamber 17 in which the second electrode 30 is housed. The separator 15 is made of a thin film member such as a resin film or nonwoven fabric having a porous structure, and has electrical insulation and ion conductivity. The separator 15 electrically insulates the first electrode 20 and the second electrode 30, and is permeable to metal ions transmitted via the electrolyte 12.

The first electrode 20 corresponds to the electrode for the power storage device in this embodiment. In the following description, the first electrode 20 is also simply referred to as "electrode 20." In the secondary battery 10 of this embodiment, the electrode 20 constitutes the negative electrode. The 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) or a Cu alloy. If the metal substrate 21 is made of Cu or a Cu alloy, high battery performance can be achieved in the electricity storage device.

In other embodiments, the metal substrate 21 may be made of a metal other than Cu or a Cu alloy. The metal substrate 21 may be made of, for example, nickel (Ni), a Ni alloy, aluminum (Al), an Al alloy, or the like. 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 metal substrate 21 may have a porous structure.

The active material layer 22 is provided on both the first surface 21a and the second surface 21b of the metal substrate 21. The configuration of the active material layer 22 will be described in detail later. As will be described later, a fine uneven structure is formed on the first surface 21a and the second surface 21b of the metal substrate 21, so for convenience, the first surface 21a and the second surface 21b are illustrated in FIG. 1 by dashed lines.

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 aluminum (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 metal atoms involved in charging and discharging, a conductive assistant, and a binder. The positive electrode active material layer 32 may also contain a thickener.

In this embodiment, the metal atom involved in charging and discharging is a Li atom. As the positive electrode active material, for example, a ternary material such as lithium cobalt oxide (LiCoO ₂ ), lithium manganese oxide (LMO), lithium nickel oxide (NCA), etc. 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 active material layer 22 of the electrode 20 will be described with reference to FIG. 2. FIG. 2 is a schematic cross-sectional view showing the metal substrate 21 and the active material layer 22 at any cut surface along the thickness direction of the metal substrate 21. Although FIG. 2 shows the first surface 21a of the metal substrate 21, the configuration described below is common to both the first surface 21a and the second surface 21b of the metal substrate 21.

A fine uneven structure 23 on the order of microns is formed on the first surface 21a and the second surface 21b of the metal substrate 21. The active material layer 22 is formed as a thin film so as to densely cover the surface of the uneven structure 23. The average thickness of the active material layer 22 may be, for example, 0.1 μm to 15.0 μm.

The active material layer 22 has a surface uneven structure 24 that is finer than the uneven structure 23 of the metal substrate 21. In order to improve the battery performance of the power storage device, it is preferable that the surface of the active material layer 22 has a dense distribution of multiple granular protrusions 24t with a width of 0.1 μm or more and 10.0 μm or less when viewed in the thickness direction of the metal substrate 21. The "width of the protrusion 24t" means the maximum value of the widths measured in all directions perpendicular to the thickness direction of the metal substrate 21 for one protrusion 24t. Note that minute cavities 25 may be formed inside the uneven structure 23 of the metal substrate 21 and the active material layer 22 during the formation process.

The active material layer 22 is mainly composed of a carbon nanostructure made of nanographene and amorphous carbon. The active material layer 22 is configured by depositing nanographene and amorphous carbon particles. The nanographene and amorphous carbon particles include those having a spherical shape and those having a plate-like shape. The protrusions 24t of the active material layer 22 described above include those formed by a single particle and those formed by a random collection of particles.

The active material layer 22 is formed by a plasma treatment, which will be described later, and hydroxyl groups may remain inside the active material layer 22 as impurities introduced by the plasma treatment. The method for forming the active material layer 22 will be described in the description of the method for manufacturing the secondary battery 10, which will be described later.

1-2. Charge/discharge reactions in electricity storage devices:
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)

When the metal involved in charging and discharging is Li, the reaction formula at the negative electrode 20 is expressed by the following formula (2): In the active material layer 22 mainly composed of carbon nanostructures, Li precipitates on the surface of the active material layer 22 of the electrode 20 due to the reaction shown in formula (2) during charging of the secondary battery 10.
Li ⁺ + e ^- ⇔ Li...(2)

In the case of a conventional negative electrode having graphite 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 structure as a comparative example is expressed as the following formula (3).
xLi+ + C ₆ + xe ^- ⇔ Li _x C ₆ ...(3)

As shown in formula (3) of the comparative example above, in the case of a conventional electrode using 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, with the secondary battery 10 having the electrode 20 of this embodiment, as shown in formula (2) above, theoretically, charging is possible as long as Li can be precipitated in the electrode 20. Therefore, with the electrode 20 of this embodiment, a higher charging capacity can be achieved in power storage devices such as the secondary battery 10.

As described above, in the electrode 20 of this embodiment, the active material layer 22 is formed so as to cover the surface of the uneven structure 23 of the metal substrate 21, and has a surface uneven structure 24 that is finer than the uneven structure 23 of the metal substrate 21. This results in a significantly larger surface area of the active material layer 22 on which Li can be deposited. Therefore, the electrode 20 of this embodiment can provide a high charging capacity in the secondary battery 10.

On the surface of the active material layer 22, numerous protrusions 24t are formed that constitute the surface uneven structure 24. According to nucleation theory, the protrusions 24t on the surface of the active material layer 22 serve as starting points for Li precipitation, which promotes Li precipitation on the surface of the active material layer 22. Therefore, the electrode 20 of this embodiment can improve the battery performance of the secondary battery 10, such as the charge capacity and specific capacity.

Furthermore, if the protrusions 24t are uniformly distributed on the surface of the active material layer 22, Li can be uniformly precipitated on the surface of the active material layer 22. This makes it possible to prevent dendrites from being generated in the electrode 20 due to repeated charging and discharging of the secondary battery 10, and to prevent damage and deterioration of the electricity storage device caused by dendrites.

1-3. Manufacturing method of secondary battery:
3 is a process flow diagram showing the manufacturing process of the secondary battery 10. Processes P1 to P5 are manufacturing processes for the electrode 20. In this embodiment, a plurality of electrodes 20 are manufactured continuously.

In process P1, a base material BM of the metal substrate 21 is prepared. In this embodiment, the base material BM has a belt-like configuration in which multiple pieces are connected together so that multiple electrodes 20 can be continuously manufactured. The fine uneven surface structure 23 described above has not yet been formed on the base material BM, and the surface of the base material BM is configured to be flat.

In steps P2 to P4, an active material layer 22 is formed on the surface of the substrate BM by plasma treatment, which will be described below. In this plasma treatment, a fine uneven structure 23 is formed on the surface of the substrate BM, and the active material layer 22 is formed so as to cover the surface of the uneven structure 23.

In this embodiment, the plasma treatment is performed by the plasma treatment device 50 of the first configuration example or the plasma treatment device 50a of the second configuration example. The plasma treatment devices 50 and 50a are each an example of a device that forms an active material layer 22 on the base material BM of the metal substrate 21, and correspond to an electrode 20 manufacturing device. Below, the plasma treatment device 50 of the first configuration example will be described, followed by the plasma treatment device 50a of the second configuration example, and then the contents of steps P2 to P4 for the plasma treatment will be described.

Figure 4 is a schematic diagram showing the configuration of a plasma processing device 50 of a first configuration example. The plasma processing device 50 performs plasma processing to form an active material layer 22 on a strip-shaped substrate BM transported in the longitudinal direction. The plasma processing device 50 includes a reaction chamber 51, a liquid supply section 52, a liquid drainage section 53, a substrate transport section 55, and a plasma generation section 60.

The reaction chamber 51 is configured to be liquid-tight so that the solution LQ can be stored therein. The reaction chamber 51 is also configured to be airtight so that plasma can be generated in the region above the stored solution LQ.

The solution LQ contains an alcohol. The alcohol concentration of the solution LQ may be, for example, about 50 to 100 mass %. The C atoms contained in the solution LQ constitute the active material of the active material layer 22. The alcohol contained in the solution LQ may be, for example, a primary alcohol. In this embodiment, the alcohol contained in the solution LQ is ethanol (C ₂ H ₆ O).

The alcohol of the solution LQ is not limited to ethanol. In other embodiments, the alcohol of the solution LQ may be, for example, butanol (C ₄ H ₁₀ O) or hexanol (C ₆ H ₁₄ O). The solution LQ may also be a mixture of multiple types of alcohol. The solution LQ may be, for example, a mixture of ethanol and methanol (CH ₃ OH). Alternatively, the solution LQ may be a mixture of a hydrocarbon such as hexane (C ₆ H ₁₄ ) or benzene (C ₆ H ₆ ) with an alcohol such as ethanol.

A liquid supply unit 52 and a liquid drainage unit 53 are connected to the bottom of the reaction chamber 51. The liquid supply unit 52 supplies solution LQ to the reaction chamber 51, and the liquid drainage unit 53 drains the solution LQ to the outside of the reaction chamber 51. During plasma processing, the amount of solution LQ stored in the reaction chamber 51 and the concentration of the solution LQ are controlled to predetermined target values by controlling the liquid supply unit 52 and the liquid drainage unit 53.

The substrate conveying section 55 conveys the substrate BM of the metal substrate 21. As described above, in this embodiment, the substrate BM of the metal substrate 21 is strip-shaped, and the substrate conveying section 55 conveys the substrate BM in its longitudinal direction. An arrow indicating the conveying direction MD of the substrate BM is shown in FIG. 4.

The substrate transport unit 55 includes a substrate supply unit 56, a plurality of transport rollers 57, and a recovery unit 58. The substrate supply unit 56 stores a roll BMr in which substrate BM is wound in a roll shape in the longitudinal direction, and unwinds a strip of substrate BM from the roll BMr and sends it into the reaction chamber 51.

The multiple transport rollers 57 are installed in the reaction chamber 51 and form a transport path for the substrate BM. The substrate BM sent out from the substrate supply unit 56 is introduced into the area above the solution LQ in the reaction chamber 51. The multiple transport rollers 57 are installed to transport the substrate BM into the solution LQ, immerse it in the solution LQ, and then transport it again to the area above the solution LQ in the reaction chamber 51. The transport rollers 57 in the solution LQ are also installed below the plasma electrode 65 so that the substrate BM is positioned along the liquid surface of the solution LQ.

As will be described in detail later, in the plasma processing apparatus 50, while the substrate is immersed in the solution LQ, the upper surface is irradiated with plasma generated by the plasma electrode 65 to form an active material layer 22. After the active material layer 22 is formed, the substrate BM is sent out of the reaction chamber 51 by the transport rollers 57 and collected by the collection section 58. The collection section 58 is configured to wind up the substrate BM on which the active material layer 22 has been formed into a roll and collect it.

The plasma generating unit 60 is provided at the top of the reaction chamber 51 and generates plasma within the reaction chamber 51. The plasma generating unit 60 includes a gas supply unit 61, an exhaust unit 62, a plasma electrode 65, an electrode holder 66, and a plasma power supply unit 67.

The gas supply unit 61 is connected to the top of the reaction chamber 51 and supplies a reactive gas for generating plasma in the region above the solution LQ in the reaction chamber 51. In this embodiment, the gas supply unit 61 supplies argon (Ar) as the reactive gas. Note that the reactive gas is not limited to argon. For example, hydrogen may be used as the reactive gas.

The gas supply unit 61 supplies the reaction gas at a flow rate of, for example, about 1 to 10 slm. The unit "slm" indicates the flow rate per minute at 1 atmosphere (atm) and 0°C.

The exhaust unit 62 is connected to the top of the reaction chamber 51, and exhausts gas from the region above the solution LQ within the reaction chamber 51. During plasma processing, the exhaust unit 62 controls the air pressure within the reaction chamber 51 to a predetermined target value. The target value of the air pressure within the reaction chamber 51 may be, for example, approximately 0.5×10 ⁵ to 1.5×10 ⁵ Pa.

The plasma electrode 65 is installed in the region above the solution LQ in the reaction chamber 51. The plasma electrode 65 is composed of a carbon substrate such as a graphite sintered body. The plasma electrode 65 has a cylindrical shape, and is installed so that its tip faces the surface of the substrate BM immersed in the solution LQ above the liquid surface of the solution LQ.

The distance Da between the tip of the plasma electrode 65 and the liquid surface of the solution LQ may be, for example, about 1 to 10 mm. The distance Db between the tip of the plasma electrode 65 and the surface of the substrate BM may be, for example, about 5 to 20 mm. The plasma electrode 65 is held by an electrode holder 66. The electrode holder 66 will be described later.

The substrate BM, which is immersed in the solution LQ and being transported, is grounded via the transport roller 57. The plasma electrode 65 is connected to a plasma power supply unit 67. The plasma power supply unit 67 applies a high-frequency voltage to generate plasma between the plasma electrode 65 and the substrate BM. The high-frequency voltage may be, for example, 5 to 15 kV, and the frequency may be 50 to 80 Hz. The frequency to generate plasma is not limited to the above-mentioned frequencies, and may be, for example, a frequency within a range of several Hz to several hundred Hz. It may also be a frequency in GHz units, which corresponds to microwaves.

The electrode holder 66 in the first configuration example will be described with reference to Figure 5A. Figure 5A shows a schematic diagram of the electrode holder 66 as viewed from above and the substrate BM being transported below it in the transport direction MD. In Figure 5A, the position of the tip of the plasma electrode 65 is shown by a dashed line, and its direction of movement is shown by a dashed arrow.

In the plasma processing apparatus 50 of the first configuration example, the electrode holding unit 66 is configured as a moving mechanism that can move one plasma electrode 65 along the surface of the base material BM in the width direction of the base material BM. This makes it possible to move the plasma irradiation area on the base material BM in the width direction of the base material BM, making it possible to form the active material layer 22 over the entire width direction of the base material BM.

In plasma processing by the plasma processing device 50 of the first configuration example, the process of forming an active material layer 22 across the width of the substrate BM by scanning the plasma electrode 65 and the process of transporting the substrate BM in the transport direction MD by the substrate transport unit 55 are repeated alternately. This allows the active material layer 22 to be efficiently formed over the entire surface of the substrate BM.

The configuration of the plasma processing device 50a of the second configuration example will be described with reference to FIG. 5B. The configuration of the plasma processing device 50a of the second configuration example is almost the same as the configuration of the plasma processing device 50 of the first configuration example, except that it is equipped with multiple plasma electrodes 65 and the configuration of the electrode holding portion 66a is different. Like FIG. 5A, FIG. 5B shows a schematic diagram of the electrode holding portion 66a of the second configuration example when viewed from above and below, and the substrate BM being transported below it in the transport direction MD.

The electrode holding portion 66a holds multiple plasma electrodes 65 arranged in the width direction of the substrate BM. The plasma electrodes 65 are arranged at a predetermined interval so that the active material layer 22 can be formed without gaps during plasma processing. The plasma electrodes 65 are arranged at intervals of, for example, 1 to 50 mm. In this embodiment, the plasma electrodes 65 are arranged at equal intervals.

In other embodiments, the multiple plasma electrodes 65 do not have to be arranged in a row. The multiple plasma electrodes 65 may be arranged, for example, in a staggered pattern. Furthermore, the multiple plasma electrodes 65 do not have to be arranged parallel to the width direction of the substrate BM, and may be arranged, for example, in a direction that diagonally intersects the transport direction MD. The multiple plasma electrodes 65 do not have to be arranged at equal intervals.

In the plasma processing apparatus 50a of the second configuration example, during plasma processing, each of the multiple plasma electrodes 65 can simultaneously generate plasma between the substrate BM. Therefore, the active material layer 22 can be formed at once across the arrangement direction of the plasma electrodes 65. This makes it possible to shorten the time required to form the active material layer 22.

In plasma processing by the plasma processing device 50a of the second configuration example, the active material layer 22 is formed by the multiple plasma electrodes 65 in the arrangement direction of the plasma electrodes 65, while the substrate BM is transported in the transport direction MD by the substrate transport unit 55. This allows the active material layer 22 to be efficiently formed over the entire surface of the substrate BM.

Returning to FIG. 3, the process of forming an active material layer 22 on the substrate BM by plasma treatment in the plasma treatment device 50, 50a will be described. In process P2, the substrate BM is transported by the substrate transport unit 55, and the substrate BM is immersed in the solution LQ in the reaction chamber 51.

In process P3, the plasma generating unit 60 of the plasma processing device 50, 50a generates plasma in the reaction chamber 51. First, the gas supply unit 61 supplies a reactive gas into the reaction chamber 51. Next, when the concentration of the reactive gas in the reaction chamber 51 reaches a predetermined concentration and pressure by controlling the gas supply unit 61 and the exhaust unit 62, the plasma power supply unit 67 applies a high-frequency voltage between the plasma electrode 65 and the substrate BM to generate a discharge from the plasma electrode 65. This generates plasma from the tip of the plasma electrode 65 toward the surface of the substrate BM in the solution LQ, and as shown by the dashed line in FIG. 4, the plasma is irradiated onto the surface of the substrate BM in the solution LQ. In process P4, the active material layer 22 is formed on the surface of the substrate BM by the irradiation of the plasma.

With reference to Figure 6, the mechanism by which the active material layer 22 is formed by irradiating the substrate BM with plasma will be described. Figures 6(a) to (c) show schematic diagrams of the solution LQ after plasma treatment has begun and the change over time in the appearance of the surface of the substrate BM in the solution LQ. Note that the mechanism described below is based on the considerations derived by the inventor of the present invention from the examples described below.

When the plasma irradiation of the substrate BM in the solution LQ starts, as shown in FIG. 6(a), the alcohol and hydrocarbon in the solution LQ are decomposed, and hydrogen atoms H and radicals RD are generated in the solution LQ. Then, as shown in FIG. 6(b), the surface of the substrate BM is eroded by the hydrogen atoms H and radicals RD, and a fine uneven structure 23 is formed. While the plasma is being irradiated, the erosion of the surface of the substrate BM by the hydrogen atoms H and radicals RD continues, and the concaves and conves of the uneven structure 23 become larger, as shown in FIG. 6(b) and FIG. 6(c). During this time, hydrogen atoms H may enter the interior of the metal substrate 21, forming minute cavities 25 inside the uneven structure 23.

During the formation of the uneven structure 23, as shown in FIG. 6(b), carbon atoms generated in the solution LQ by decomposition of the alcohol due to plasma irradiation precipitate as nanographene and amorphous carbon particles CP, and begin to adhere to the surface of the uneven structure 23 of the substrate BM. Then, as shown in FIGS. 6(b) and 6(c), in parallel with the growth of the uneven structure 23 on the surface of the substrate BM, nanographene and amorphous carbon particles CP are deposited to cover the surface of the uneven structure 23, and the active material layer 22 is formed. During this growth process, minute cavities 25 may also be formed inside the active material layer 22.

The degree of unevenness of the active material layer 22 and the amount of carbon nanostructures deposited can be adjusted by the plasma irradiation time. The plasma irradiation time for forming the active material layer 22 should be at least 1 second. The plasma irradiation time may be, for example, about 1 to 60 minutes.

The active material layer 22 can be formed more efficiently and in a shorter time in the area directly below the plasma electrode 65 of the substrate BM. In the plasma processing device 50, 50a, the area in which the active material layer 22 is formed is expanded by alternately repeating the movement of the plasma irradiation area on the surface of the substrate BM and the irradiation of plasma for a predetermined period of time.

Refer to FIG. 4. In the plasma processing device 50, 50a, the first plasma treatment is performed on the substrate BM to form an active material layer 22 on one side of the substrate BM. To form the active material layer 22 on both sides of the substrate BM, the substrate BM with the active material layer 22 formed on only one side recovered in the recovery section 58 is set in the substrate supply section 56 as a roll body BMr, and a second plasma treatment is performed.

When the substrate BM is subjected to the second plasma treatment, it is transported so that the side opposite to that of the first transport faces upward in the solution LQ. Therefore, during the first plasma treatment, in the recovery section 58, the substrate BM is preferably wound up so that the front and back are reversed from when it was in the roll body BMr stored in the substrate supply section 56.

See FIG. 3. In process P5, a plurality of first electrodes 20 are individually cut out from a strip-shaped substrate BM having active material layers 22 formed on both sides. Through the above processes P1 to P5, the first electrodes 20 are completed.

In process P6, the second electrode 30, which will become the positive electrode, is manufactured. In the following process P7, 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.

1-4. Summary:
As described above, the electrode 20 of the present embodiment includes an active material layer 22 having a novel structure, which is mainly composed of a carbon nanostructure constituted by nanographene and amorphous carbon and is formed so as to cover the surface of the uneven structure 23 of the metal substrate 21. With this electrode 20, the charge/discharge performance of the secondary battery 10 can be significantly improved, as will be shown in the examples described later.

In addition, according to the manufacturing method of the electrode 20 of this embodiment and the plasma processing apparatus 50, 50a which is the manufacturing apparatus of the electrode 20, the active material layer 22 can be easily formed by plasma processing together with the uneven structure 23 of the metal substrate 21. Therefore, the electrode 20 can be manufactured efficiently. According to the plasma processing apparatus 50 of the first configuration example, the active material layer 22 can be formed on the entire substrate BM for forming the multiple electrodes 20 by transporting the strip-shaped substrate BM and moving the plasma electrode 65 by the electrode holding unit 66. According to the plasma processing apparatus 50a of the second configuration example, the active material layer 22 can be formed on the entire substrate BM for forming the multiple electrodes 20 by irradiating plasma with each of the multiple plasma electrodes 65 and transporting the strip-shaped substrate BM. Therefore, by using the plasma processing apparatus 50, 50a, it is possible to continuously manufacture multiple electrodes 20, and the electrodes 20 can be manufactured even more efficiently.

Here, for example, in the manufacturing process of a conventional secondary battery having an active material layer of carbon nanowalls, when the electrode on which the active material layer is formed is rolled up for transportation, storage, etc., some of the carbon nanowalls may be damaged or fall off. In contrast, according to the electrode 20 of this embodiment, the active material layer 22 has a thin-film configuration, so that even if the electrode is rolled up and transported or stored in the manufacturing process of the secondary battery 10, deterioration or fall-off of the active material layer 22 is suppressed. Therefore, according to the electrode 20 of this embodiment, it is easy to handle, which makes it easy to mass-produce the electrode 20.

2. Working Example:
2-1. Plasma treatment:
An example of the process for forming the active material layer 22 by the plasma treatment described in the above embodiment will be described with reference to Fig. 7. Fig. 7 shows images taken for each plasma irradiation time when the plasma treatment described in the above embodiment is performed under the treatment conditions shown in Table 1 below. Each image in Fig. 7 is an image of the surface of the base material BM of the metal substrate 21 taken from the side. The plasma irradiation time is displayed in the upper left corner of each image.

When the plasma irradiation started, the surface of the substrate BM was flat. 0.5 minutes after the plasma irradiation started, unevenness began to form on the surface of the substrate BM. This is presumably due to erosion by hydrogen atoms and radicals in the solution LQ. As the plasma irradiation time continued, the unevenness on the surface of the substrate BM gradually increased. When the plasma irradiation time had reached 10 minutes, several tiny cavities were observed in the uneven structure. During this experiment, it was confirmed that the concentrations of C atoms and O atoms increased near the surface of the substrate BM when the plasma irradiation time was between 1 and 10 minutes. This indicates that the active material layer 22 was formed while the uneven structure 23 was formed on the surface of the substrate BM.

2-2. Examples of active material layers:
8 shows an image of the active material layer 22 of the electrode 20 of Example E1 taken in the thickness direction of the metal substrate 21. The active material layer 22 of Example E1 was formed by plasma treatment under the conditions shown in Table 1 above. The plasma irradiation time was 30 minutes. As shown in this image, in the active material layer 22 of Example E1, a plurality of granular protrusions 24t having a width of 0.1 μm or more and 10.0 μm or less were densely distributed when viewed in the thickness direction of the metal substrate 21.

9 is a graph showing the Raman spectrum of the active material layer 22 of Example E1. In this graph, peaks of the D band (1340 cm ⁻¹ ) and the G band (1580 cm ⁻¹ ) were observed. This result indicates that the active material layer 22 is formed mainly from a carbon nanostructure composed of nanographene and amorphous carbon.

2-3. Comparative Example of Active Material Layer:
FIG. 10 shows a list of photographed images of the electrodes of the comparative examples C1, C2, and C3.

10 (a) and (b) show images of the surface of the electrode of Comparative Example C1. Fig. 10 (a) is an image taken by a scanning electron microscope facing the surface of the electrode of Comparative Example C1. Fig. 10 (b) is an image taken at a higher magnification than the image of Fig. 10 (a).

The electrode of Comparative Example C1 did not have an active material layer, and was composed only of a metal substrate constituting a current collector. The metal substrate of Comparative Example C1 was composed of the same copper foil as the base material BM of the metal substrate 21 of Example E1 before the active material layer 22 was formed. Therefore, the surface of Comparative Example C1 was composed of a flat surface as shown in the images of Figures 10 (a) and (b).

Figure 10 (c) shows an image of the electrode of Comparative Example C2. Figure 10 (c) is an image taken by a scanning electron microscope facing directly on the surface of the active material layer of Comparative Example C2. The electrode of Comparative Example C2 is configured by providing a graphite active material layer on the surface of a metal substrate having a flat surface similar to that of Comparative Example C1. Figure 10 (c) shows the granular graphite that constitutes the active material layer.

10 (d) and (e) show images of the electrode of Comparative Example C3. Fig. 10 (d) is an image of the surface of the electrode of Comparative Example C1 taken with a scanning electron microscope from a direction perpendicular to the thickness direction of the electrode. Fig. 10 (e) is an image of the surface of the electrode of Comparative Example C1 taken with a scanning electron microscope directly facing the electrode.

The electrode of Comparative Example C3 was configured by providing carbon nanowalls as an active material layer on the surface of a metal substrate having a flat surface similar to that of Comparative Example C1. The pleated white image in the image of Fig. 10 (d) and the mesh-like white streaky image in the image of Fig. 10 (e) are carbon nanowalls. The carbon nanowalls were formed by the CVD method in a random mesh pattern over the entire surface of the metal substrate with a nearly uniform height of about 1.0 μm.

2-4. Battery performance of Examples and Comparative Examples:
11 to 15, the results of evaluation tests of the battery performance of secondary batteries using the electrodes of Example E1 and Comparative Examples C1, C2, and C3 as negative electrodes will be described.

Figure 11 shows a graph obtained for a secondary battery using the electrode of Example E1. Figures 12, 13, and 14 show graphs obtained for secondary batteries using the electrodes of Comparative Examples C1, C2, and C3, respectively. In Figures 11 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 the secondary batteries using Example E1 and Comparative Examples C2 and C3 in a bar graph. 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, C2, and C3 were lithium ion secondary batteries and were fabricated with the configurations shown in Table 2 below.

As shown in Fig. 11 and Fig. 15, the secondary battery using Example E1 had a charge capacity of 14.2 [mAh] and a specific capacity of 10.8 [mAh/cm ² ]. In contrast, as shown in Fig. 12, the secondary battery using Comparative Example C1, which has an electrode made of only a metal substrate having a flat surface as the negative electrode, was unable to charge and discharge. Also, as shown in Fig. 13 and Fig. 15, the secondary battery using Comparative Example C2 having a graphite active material layer had a charge capacity of 4.0 [mAh] and a specific capacity of 2.0 [mAh/cm ² ]. As shown in Fig. 14 and Fig. 15, the secondary battery using Comparative Example C3 having a carbon nanowall active material layer had a charge capacity of 12.6 [mAh] and a specific capacity of 9.4 [mAh/cm ² ].

Thus, the secondary battery 10 using the electrode 20 of Example E1 exhibited significantly improved battery performance compared to not only the secondary battery using the electrode of Comparative Example C1, which does not have an active material layer, but also the secondary batteries using Comparative Examples C2 and C3, which have carbon active material layers.

As described above, the electrode for an electricity storage device according to the present invention can significantly improve the battery performance of electricity storage devices, including secondary batteries. Furthermore, the manufacturing method and manufacturing apparatus for an electrode for an electricity storage device according to the present invention can easily manufacture electrodes for electricity storage devices.

3. Other embodiments:
The present invention is not limited to the configurations of the above-mentioned embodiment and examples, and can be realized, for example, in the following forms. Any configurations described below as other embodiments are positioned as one example form for implementing the present invention, similar to the above-mentioned embodiment and the configurations and examples described as other embodiments within the above-mentioned embodiment.

3-1. 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.

3-2. Other embodiment 2:
In the electrode 20, the active material layer 22 may be formed on at least one of the first surface 21a and the second surface 21b of the metal substrate 21, and may be formed only on the first surface 21a, for example. The electrode 20 may be applied to an electricity storage device other than a secondary battery. For example, the electrode 20 may be applied to a capacitor.

3-3. Other embodiment 3:
The plasma processing apparatus 50, 50a may not have a configuration for continuously manufacturing a plurality of electrodes 20, and may be configured to manufacture the electrodes 20 one by one. In this case, the plasma processing apparatus 50, 50a may not have a configuration for transporting the strip-shaped substrate BM by the transport roller 57. For example, the plasma processing apparatus 50, 50a may be configured to support the substrate BM of one metal substrate 21 by a support portion installed in the solution LQ. Even with such a configuration, it is possible to manufacture the electrode 20 of the secondary battery 10 having a small size.

3-4. Other embodiment 4:
The configuration of the plasma processing apparatus that performs the plasma processing to form the active material layer 22 is not limited to the first configuration example or the second configuration example described in the above embodiment. The plasma processing apparatus may have, for example, a configuration in which a plurality of electrode holding units 66a of the second configuration example are arranged in the transport direction MD of the substrate BM. In addition, the plasma processing apparatus may be provided with a moving mechanism that holds a group of a plurality of plasma electrodes 65 movably in a direction intersecting the transport direction MD of the substrate BM.

3-5. Other embodiment 5:
The plasma treatment described in the above embodiment may be applied to a method for manufacturing an electric storage device other than a secondary battery. In addition, the plasma treatment may be applied to a method for manufacturing various electrodes, not limited to an electric storage device.

4. 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 comprises a metal substrate constituting a current collector and having a fine uneven structure formed on its surface, and an active material layer mainly composed of a carbon nanostructure composed of nanographene and amorphous carbon, formed so as to densely cover the surface of the uneven structure, and in which metal atoms involved in the charging and discharging of the electricity storage device are precipitated on the surface when the electricity storage device is charged.
According to the first embodiment of the electrode for the electric storage device, the carbon nanostructure composed of nanographene and amorphous carbon is used as the active material, so that the battery performance of the electric storage device can be improved. In addition, since the surface of the active material layer is formed unevenly, the surface area of the active material layer is increased, and the amount of metal precipitated during charging of the electric storage device can be increased, so that the charging capacity of the electric storage device can be increased. Furthermore, since the uneven structure of the active material layer forms a large number of convex portions on the surface of the active material layer, the precipitation of metal on the surface of the active material layer is promoted during charging of the electric storage device. Therefore, the battery performance of the electric storage device can be further improved. In addition, since the active material layer is formed like a thin film densely covering the surface of the metal substrate, unlike carbon nanowalls that grow to extend from the surface of the metal substrate, it is possible to suppress the active material layer from being destroyed or falling off during the manufacturing process of the electric storage device.

[Second embodiment] In the electrode for an electricity storage device of the first embodiment, a surface of the active material layer may have a plurality of protrusions, each having a width of 0.1 μm or more and 10.0 μm or less, densely distributed thereon when viewed in the thickness direction of the metal substrate.
According to the second embodiment of the electrode for an electricity storage device, the surface area of the active material layer can be further increased and more protrusions can be formed on the surface of the active material layer, thereby further improving the battery performance of the electricity storage device.

[Third Aspect] In the electricity storage device electrode of the third aspect, the metal substrate may be made of copper or a copper alloy.
The electricity storage device electrode of the third aspect can achieve higher battery performance in an electricity storage device.

[Fourth embodiment] The fourth embodiment is provided as a secondary battery. The secondary battery of the fourth embodiment may include a container filled with an electrolytic solution, a separator having electrical insulation and ion conductivity and dividing an internal space of the container into a first electrode chamber and a second electrode chamber, a first electrode contained in the first electrode chamber and configured by the electricity storage device electrode according to any one of the above embodiments, and a second electrode contained in the second electrode chamber and containing metal atoms that are ionized and move to the first electrode during charging.
According to the secondary battery of the fourth aspect, the battery performance can be improved by the electrode for an electricity storage device of the above aspect.

[Fifth embodiment] The fifth embodiment is provided as a method for producing an electrode for an electric storage device. The fifth embodiment includes the steps of: immersing a base material of a metal substrate constituting a current collector in an alcohol solution; applying a high-frequency voltage between a plasma electrode arranged on the solution and the base material in the solution while supplying a reactive gas for generating plasma onto the solution, thereby generating plasma; and forming an active material layer, which is mainly composed of a carbon nanostructure composed of nanographene and amorphous carbon and in which metal atoms involved in the charging and discharging of the electric storage device precipitate on the surface during charging of the electric storage device, so as to densely cover the surface of the uneven structure while forming a fine uneven structure on the surface of the base material by the plasma.
According to the fifth embodiment, the active material layer of carbon nanostructures can be formed while forming a concave-convex structure on the surface of the metal substrate by plasma treatment, and thus an electrode for an electricity storage device that can improve the battery performance of the electricity storage device can be easily and efficiently produced.

[Sixth aspect] The manufacturing method of the sixth aspect may further include a step of moving the plasma electrode along the surface of the substrate in the solution to move the plasma irradiation area on the surface of the substrate.
According to the manufacturing method of the sixth embodiment, the plasma irradiated area where the active material layer is formed on the substrate surface can be moved, so that an electrode having a large area can be manufactured more efficiently.

[Seventh embodiment] The seventh embodiment is provided as an apparatus for manufacturing an electrode for an electric storage device. The manufacturing apparatus of the seventh embodiment includes a reaction chamber in which an alcohol solution in which a base material of a metal substrate constituting a current collector is immersed is stored, a plasma electrode arranged in an area above the solution in the reaction chamber so as to face the surface of the base material immersed in the solution, a gas supply unit that supplies a reactive gas for generating plasma to an area above the solution in the reaction chamber, and a plasma power supply unit that applies a high-frequency voltage for generating the plasma between the plasma electrode and the base material, and has a configuration in which a fine uneven structure is formed on the surface of the base material by the plasma generated between the plasma electrode and the base material, and an active material layer is formed so as to cover the surface of the uneven structure, the active material layer being mainly composed of a carbon nanostructure composed of nanographene and amorphous carbon, and in which metal atoms involved in the charging and discharging of the electric storage device are precipitated on the surface during charging of the electric storage device.
According to the seventh embodiment of the manufacturing apparatus, it is possible to form an active material layer of carbon nanostructures while forming a concave-convex structure on the surface of a metal substrate by plasma treatment, and thus it is possible to easily manufacture an electrode for an electricity storage device that can improve the battery performance of the electricity storage device.

[Eighth aspect] The manufacturing apparatus of the seventh aspect may further include a moving mechanism for moving the plasma electrode along the surface of the substrate in the solution.
According to the eighth embodiment of the manufacturing apparatus, the plasma electrode is moved by the moving mechanism, so that the plasma irradiation area where the active material layer is formed on the surface of the substrate can be moved. Therefore, an electrode with a large area can be manufactured more efficiently.

[9th Form] The manufacturing apparatus of the 7th or 8th form may have a plurality of the plasma electrodes arranged along a surface of the substrate, and may be configured to be capable of simultaneously generating plasma between each of the plurality of plasma electrodes and the substrate.
According to the manufacturing apparatus of the ninth embodiment, a wide area on the surface of the base material can be irradiated with plasma at once by using a plurality of plasma electrodes, and therefore an electrode having a large area can be manufactured more efficiently.

10... secondary battery, 11... container, 12... electrolyte, 15... separator, 16... first electrode chamber, 17... second electrode chamber, 20... first electrode, 21... metal substrate, 21a... first surface, 21b... second surface, 22... active material layer, 23... uneven structure, 24... surface uneven structure, 24t... convex portion, 25... cavity, 30... second electrode, 31... positive electrode current collector, 31a... first surface, 31b... second surface, 32... positive electrode active material layer, 50, 50a... plasma plasmon processing device, 51... reaction chamber, 52... liquid supply section, 53... liquid drainage section, 55... substrate transport section, 56... substrate supply section, 57... transport roller, 58... recovery section, 60... plasma generation section, 61... gas supply section, 62... exhaust section, 65... plasma electrode, 66, 66a... electrode holding section, 67... plasma power supply section, BM... substrate, BMr... roll body, CP... particle, H... hydrogen atom, LQ... solution, MD... transport direction, RD... radical

Claims (4)

An electrode for an electricity storage device,
A metal substrate constituting a current collector and having a fine uneven structure formed on its surface;
an active material layer comprising a carbon nanostructure composed of nanographene and amorphous carbon as a main component, the active material layer being formed so as to densely cover the uneven surface of the metal substrate , the active material layer having a thin film configuration in which particles of the nanographene and particles of the amorphous carbon are deposited, the active material layer having a fine uneven structure on its surface , and metal atoms involved in charging and discharging of the electricity storage device being precipitated on its surface when the electricity storage device is charged;
An electrode for an electricity storage device comprising: The electrode for an electricity storage device according to claim 1 ,
The electrode for an electricity storage device, wherein a plurality of protrusions each having a width of 0.1 μm or more and 10.0 μm or less are densely distributed on a surface of the active material layer when viewed in a thickness direction of the metal substrate. The electrode for an electricity storage device according to claim 1 ,
The electrode for an electricity storage device, wherein the metal substrate is made of copper or a copper alloy. A secondary battery,
a container filled with an electrolyte;
a separator having electrical insulation and ion conductivity, which divides an internal space of the container into a first electrode chamber and a second electrode chamber;
a first electrode that is accommodated in the first electrode chamber and is configured by the electricity storage device electrode according to any one of claims 1 to 3;
A second electrode that is accommodated in the second electrode chamber and includes metal atoms that are ionized and move to the first electrode upon charging;
A secondary battery comprising:

Images