WO2024071438A1

WO2024071438A1 - Electrode, and battery with electrode

Info

Publication number: WO2024071438A1
Application number: PCT/JP2023/036452
Authority: WO
Inventors: 真也福地; 涼一勝見; 慎吾榊原; 桂一渡邉; 祐也薬研地
Original assignee: 高圧ガス工業株式会社; テイカ株式会社
Priority date: 2022-09-30
Filing date: 2023-09-29
Publication date: 2024-04-04

Abstract

Provided is an electrode comprising a current collector and an electrode active material layer disposed on the current collector, wherein the current collector includes carbon nanotubes having regular orientation.

Description

Electrode and battery having the electrode

This disclosure relates to electrodes. This disclosure also relates to batteries using the electrodes.

Batteries such as lithium-ion batteries are widely used as power sources for electronic devices, electric vehicles, etc. As these electronic devices and electric vehicles become smaller and lighter, there is a demand for batteries with higher energy density.

Therefore, attempts have been made to use carbon nanotubes (hereinafter also referred to as "CNTs"), which have low density and excellent conductivity, instead of metals such as aluminum and copper that are commonly used as materials for current collectors in battery electrodes (Patent Documents 1 to 3).

JP 2011-198600 A JP 2021-118135 A International Publication No. 2019/065517

The inventors of this application realized that there were issues with the conventional electrode structure that needed to be overcome, and found it necessary to take measures to address these issues. Specifically, they found the following issues:

For example, in Patent Document 1, a sheet obtained by thinly stretching a product whose main substance is CNT is used as a current collector. Since CNTs have a lower density than metals, electrodes using CNT sheets can improve the weight energy density. However, this CNT sheet is produced by filtering a solution in which a product whose main substance is CNT is dispersed. The CNTs in such a sheet are oriented in random directions, and the arrangement of the CNTs may be non-uniform. An electrode that includes a CNT sheet composed of non-uniformly arranged CNTs as a current collector may not be able to provide the desired electrode characteristics, such as conductivity, when used in a battery or the like.

This disclosure has been made in consideration of these problems. That is, the main objective of this disclosure is to provide an electrode that has a current collector that includes carbon nanotubes (CNTs) and has suitable electrode characteristics.

In order to achieve the above object, in one embodiment of the present disclosure,
A current collector and an electrode active material layer provided on the current collector,
An electrode is provided, in which the current collector includes regularly oriented carbon nanotubes.

An electrode according to one embodiment of the present disclosure has a current collector that includes carbon nanotubes (CNTs) and can provide favorable electrode characteristics.

FIG. 1 is a cross-sectional view illustrating a schematic diagram of an electrode according to an embodiment of the present disclosure. FIG. 2 is a plan view illustrating a schematic diagram of a current collector of an electrode according to an embodiment of the present disclosure. FIG. 3A is an SEM image of a current collector according to one embodiment of the present disclosure. FIG. 3B is an SEM image of a current collector according to one embodiment of the present disclosure. FIG. 4 is a plan view illustrating a schematic diagram of a current collector of an electrode according to an embodiment of the present disclosure. FIG. 5 is a partially cutaway perspective view illustrating a battery according to an embodiment of the present disclosure. FIG. 6 is a schematic diagram of a manufacturing process for a current collector according to one embodiment of the present disclosure. FIG. 7 is a cyclic voltammogram obtained for an electrode of the present disclosure.

Below, an electrode and a battery according to an embodiment of the present disclosure will be described in more detail with reference to the drawings. The various elements in the drawings are shown merely as schematics and examples for the purpose of explaining the present disclosure, and the appearance, dimensional ratios, etc. may differ from the actual ones.

Furthermore, in the following description, terms indicating specific directions or positions are used as necessary. However, the use of these terms is intended to facilitate understanding of the invention with reference to the drawings, and the meanings of these terms do not limit the technical scope of this disclosure. Unless otherwise specified, the same symbols or symbols indicate the same components or parts or the same meanings.

The description of the exemplary embodiments of the present disclosure is intended to be read in conjunction with the accompanying drawings, which are to be considered as part of the entire written description. In the description of the embodiments of the present disclosure disclosed herein, references to directions or orientations are merely for convenience of description and are not intended to limit the scope of the present disclosure. In addition, terms such as "attached," "connected," "coupled," "provided," and "interconnected," and similar terms, unless expressly described otherwise, refer to a relationship in which structures are fixed or attached to each other directly or indirectly by an intervening object, or both are movable or rigidly attached or related to each other, and/or related to each other, unless expressly described otherwise. Furthermore, the features or advantages of the present disclosure are illustrated by reference to preferred embodiments. Such embodiments have been described in sufficient detail to enable one skilled in the art to practice the present disclosure. It is to be understood that other embodiments may be utilized, and that process, electrical, or mechanical changes may be made without departing from the scope of the present disclosure. Thus, the present disclosure is not expressly limited to the preferred embodiments (alone or in combination with other features) which illustrate non-limiting combinations of possible features.

In this specification, "cross-sectional view" refers to the shape of the electrode when cut along a plane approximately parallel to the thickness direction. In addition, "plan view" or "plan view shape" as used in this specification refers to a sketch of the object when viewed from above or below along the thickness direction of the electrode.

In addition, "vertical" and "orthogonal" as used in this specification do not necessarily mean completely "vertical" and include aspects that are slightly deviated from that (for example, within a range of ±10° from completely vertical, such as a range of ±5°).

In addition, "parallel" as used herein does not necessarily mean completely "parallel" and includes slight deviations from that (for example, within a range of ±10° from completely parallel, such as ±5°).

In addition, in this specification, "on" a structure does not only mean that it is in contact with the structure, but also includes cases where it is not in contact with the top surface of the structure. In other words, "on" a structure includes cases where another structure is formed above the structure, and/or cases where another structure is between the structure. Furthermore, "on" does not necessarily mean the upper side in the vertical direction. "On" merely indicates the relative positional relationship between the structures.

Features of the Electrode of the Present Disclosure
FIG. 1 is a cross-sectional view showing a schematic diagram of an electrode according to an embodiment of the present disclosure. The electrode 10 includes a current collector 11 and an electrode active material layer 12 provided on the current collector 11. That is, in the electrode 10 according to an embodiment of the present disclosure, the current collector 11 may support the electrode active material layer 12. More specifically, in the electrode 10, the electrode active material layer 12 may be provided on at least one main surface of the current collector 11. That is, the current collector 11 of the electrode may have the electrode active material layer 12 on either one of its main surfaces, or may have the electrode active material layer 12 on both main surfaces. In this specification, the "main surface" of the current collector means the surface with the largest area in the current collector and the surface opposite to that surface.

The electrode active material layer 12 contains at least an active material involved in the transfer of electrons during the electrode reaction. The current collector 11 contributes to the collection and supply of electrons generated in the electrode active material layer 12 due to the electrode reaction. This disclosure is particularly characterized by such a current collector. The current collector provided in the electrode of this disclosure is described in detail below.

The current collector of the present disclosure comprises carbon nanotubes. More specifically, the current collector of the present disclosure comprises carbon nanotubes having regular orientation. FIG. 2 is an enlarged plan view showing a schematic view of a current collector of an electrode according to an embodiment of the present disclosure. As shown in the figure, the carbon nanotubes 11C contained in the current collector 11 may be regularly oriented in the direction X in the current collector 11. Here, the "regular orientation" in this specification means a macroscopic orientation, and not all carbon nanotubes are necessarily oriented in a predetermined direction. In other words, the "regular orientation" in this specification includes a state in which the carbon nanotubes extend toward a predetermined direction while having undulating or meandering portions. Therefore, the long axis direction of the carbon nanotubes 11C contained in the current collector 11 of the present disclosure is not random, but is macroscopically aligned along a predetermined direction. This means that the long axis direction of the carbon nanotubes 11C contained in the current collector 11 is arranged approximately regularly in at least one direction.

With this structure, in the electrode of the present disclosure, the carbon tubes are uniformly arranged in the current collector. This makes it easier to favorably provide the properties of the carbon nanotubes contained in the current collector (excellent thermal conductivity, electrical conductivity, and mechanical strength) compared to when the carbon nanotubes are unevenly arranged in a bent or curved state. As a result, an electrode equipped with a current collector containing such carbon nanotubes can provide favorable electrode properties. In other words, according to the present disclosure, an electrode equipped with a current collector that can more effectively utilize the specific properties of carbon nanotubes can be obtained.

For example, when a sheet of carbon nanotubes is formed by filtering a carbon nanotube dispersion, the carbon nanotubes may be oriented in random directions within the sheet. A sheet formed in this manner may contain bent or curled carbon nanotubes. As a result, there is a risk that the actual length of the carbon nanotubes does not contribute to conductivity, and there is a concern that the electrical resistance of the entire sheet may be high. On the other hand, in the current collector of the present disclosure, the carbon nanotubes have a regular orientation, which makes it possible to uniformly distribute the carbon nanotubes throughout the entire current collector. Therefore, the carbon nanotubes can efficiently contribute to the conductivity of the current collector. As a result, an electrode equipped with such a current collector may be more suitable in terms of conductivity.

In addition, by including oriented carbon nanotubes in the current collector, the current collector can effectively obtain the mechanical strength properties of the carbon nanotubes in the direction of orientation. As a result, an electrode equipped with such a current collector can be more suitable in terms of mechanical strength.

Furthermore, in a current collector containing regularly oriented carbon nanotubes, the carbon nanotubes can be efficiently distributed throughout the current collector. This makes it possible to reduce the amount of carbon nanotubes required to form the current collector. In other words, since the current collector can be formed with a smaller amount of carbon nanotubes, such a structure can also contribute to making the current collector thinner and less dense. Therefore, the present disclosure can provide a smaller and lighter electrode, and can further improve the gravimetric energy density and/or volumetric energy density of a battery equipped with such an electrode.

It is also known that carbon nanotubes have a significant anisotropy in thermal conductivity. Therefore, a current collector containing regularly oriented carbon nanotubes exhibits a higher thermal diffusivity in the orientation direction of the carbon nanotubes. In other words, in the electrode of the present disclosure, the thermal conductivity in the orientation direction of the carbon nanotubes can be further improved. In short, the electrode of the present disclosure can provide anisotropic thermal conductivity in a predetermined direction. Therefore, for example, when the electrode generates heat, the generated heat can be conducted in the orientation direction of the carbon nanotubes and dissipated to the outside of the electrode. In other words, in the electrode of the present disclosure, the regularly oriented carbon nanotubes also contribute to the heat dissipation of the electrode, so that adverse thermal effects on such an electrode itself and on batteries and the like equipped with the electrode can be more effectively suppressed.

2, the carbon nanotubes 11C contained in the current collector 11 may be oriented in the extension direction of the main surface of the current collector 11. In other words, the current collector 11 may contain carbon nanotubes 11C that are regularly oriented approximately parallel to the main surface direction of the current collector 11. This means that the longitudinal axis of the carbon nanotubes 11C is arranged along a direction approximately parallel to the main surface of the current collector 11. In other words, the carbon nanotubes 11C in the current collector 11 may be arranged to extend in a direction approximately perpendicular to the thickness direction of the current collector 11. Such a current collector 11 may have better mechanical properties, electrical properties, and thermal conductivity properties in the direction in which the carbon nanotubes 11C are oriented. For example, the carbon nanotubes 11C oriented along the extension direction of the main surface of the current collector 11 may further improve the mechanical strength of the current collector 11, such as bending strength and tensile strength. In other words, a current collector having such a structure can provide a more suitable electrode with better mechanical strength and handling properties. By having the above-mentioned structure, the electrode of the present disclosure has a current collector containing carbon nanotubes and can have suitable electrode properties.

Furthermore, by including carbon nanotubes oriented in the extension direction of the main surface of the current collector, it becomes possible to more efficiently distribute the carbon nanotubes throughout the entire current collector. Specifically, by having the carbon nanotubes included in the current collector extend along the surface direction of the current collector, it may be possible to make the current collector thinner. Therefore, the present disclosure can provide a more compact electrode, and can further improve the volumetric energy density of a battery equipped with such an electrode.

In one embodiment, the current collector includes a plurality of carbon nanotubes, and the plurality of carbon nanotubes may be oriented in parallel when viewed in the thickness direction of the current collector. In this specification, "parallel" includes a state in which they are completely parallel and a state in which they are approximately parallel. In other words, "parallel" includes a state in which they are macroscopically parallel, but include some details that are not parallel. More specifically, when the current collector is viewed from a direction perpendicular to the longitudinal direction of the carbon nanotubes, the plurality of carbon nanotubes may be arranged to such an extent that the longitudinal directions of the plurality of carbon nanotubes can be visually recognized as being arranged approximately regularly in a predetermined direction. For example, the plurality of carbon nanotubes may be aligned in parallel so as to be regularly oriented in a predetermined direction. In other words, the plurality of carbon nanotubes included in the current collector may be arranged so as to be approximately parallel to each other.

3A and 3B are SEM images of a current collector according to an embodiment of the present disclosure, observed at 1000x and 5000x magnifications from the thickness direction of the current collector. As shown in FIGS. 3A and 3B, the current collector of the present disclosure may include a plurality of carbon nanotubes arranged in parallel so as to be approximately parallel to each other. By orienting the carbon nanotubes in parallel in this manner on the current collector, the number of contact points between the ends of the carbon nanotubes is reduced, thereby reducing the total contact resistance between the carbon nanotubes, and as a result, the electrical resistance value of the entire current collector can be further reduced. Therefore, by providing a current collector containing carbon nanotubes with the above-described structure, an electrode having more suitable electrode characteristics can be provided.

In one embodiment, the carbon nanotubes included in the current collector may be entangled with each other. More specifically, in the carbon nanotubes oriented in parallel, adjacent carbon nanotubes may be entangled with each other. For example, the current collector may include carbon nanotubes that extend while meandering (or undulating) toward the orientation direction X, and adjacent carbon nanotubes may be entangled with each other in the meandering portion. In other words, the carbon nanotubes may have a portion where they are entangled with each other, rather than being aligned in the current collector. The carbon nanotubes arranged in parallel with each other in the current collector can also be considered to be bundled with each other. The entangled or bundled state holds the carbon nanotubes arranged in parallel with each other. In the current collector of the present disclosure, the adjacent carbon nanotubes have a structure in which they are entangled with each other, so that the current collector can more suitably maintain its shape in an oriented state. This means that the entanglement of the parallel carbon nanotubes contributes to improving the mechanical strength in a direction different from the orientation direction X (for example, the direction Y perpendicular to the orientation direction X (see FIG. 2)). Therefore, by providing the above-described structure, an electrode having a current collector containing carbon nanotubes and suitable electrode properties can be provided.

Furthermore, the carbon nanotubes included in the current collector of the present disclosure may be arranged in series along the orientation direction X. More specifically, in one embodiment of the present disclosure, the carbon nanotubes may be arranged in series along the main surface of the current collector. This means that the carbon nanotubes are arranged so as to be continuous in the long axis direction. In other words, the current collector may include a plurality of carbon nanotubes that are connected in a substantially straight line along the long axis direction of the carbon nanotubes. The carbon nanotubes arranged in series may be connected by intertwining the ends of the carbon nanotubes with each other. When more importance is placed on improving the mechanical strength and electrical properties of the current collector, the carbon nanotubes arranged in series may be connected to each other by van der Waals forces. In this way, by including carbon nanotubes oriented in series in the current collector, the number of junctions between the carbon nanotubes in the current collector can be reduced. With such a structure, the increase in the electrical resistance value of the entire current collector caused by the junctions can be suppressed, and an electrode including a current collector with better electrical properties can be provided.

Furthermore, according to the present disclosure, such entanglement (or bundling) or bonding by van der Waals forces makes it possible to maintain the shape of the current collector without using a binder to bond the carbon nanotubes together. In other words, it is possible to form a current collector that is substantially free of binder, which further reduces the weight of the current collector and can result in an electrode with a high weight energy density. Furthermore, since it is possible to prevent an increase in electrical resistance due to the inclusion of a binder, an electrode with excellent conductivity can be provided.

Therefore, the current collector of the present disclosure may be a carbon nanotube sheet made of carbon nanotubes. This means that the current collector is a sheet made only of carbon nanotubes, substantially free of fixing materials such as dispersants or resins. On the other hand, the current collector may contain components that are inevitably contained during the manufacture of the current collector, the electrode and the battery that include the current collector. For example, the current collector may contain other components to the extent that the effect of the present invention is not lost. Examples of other components include colorants such as organic dyes and organic pigments, organic fillers such as organic particles and organic fibers, anti-aging agents/antioxidants, ultraviolet absorbers, light stabilizers, etc.

As shown in FIG. 2, in one embodiment of the present disclosure, the current collector 11 is long. For example, the carbon nanotubes 11C contained in such a long current collector 11 may be oriented along at least one of the longitudinal and lateral directions of the main surface of the current collector 11. That is, the carbon nanotubes 11C in the current collector 11 may be oriented approximately parallel to the long side and/or short side of the current collector 11. In the current collector 11, multiple carbon nanotubes 11C adjacent to each other in the diameter direction of the carbon nanotube 11C (i.e., direction Y perpendicular to the orientation direction X) may be entangled with each other. On the other hand, two or more carbon nanotubes 11C adjacent to each other in the longitudinal direction of the carbon nanotube 11C (i.e., orientation direction X) may be bonded by strong van der Waals forces. As a result, the carbon nanotubes 11C arranged approximately parallel to the long and/or short sides of the current collector 11 are more preferably entangled and/or bonded to the adjacent carbon nanotubes 11C, and the falling off of the carbon nanotubes 11C from the current collector 11 can be more preferably prevented. Furthermore, since the carbon nanotubes adjacent to each other in the orientation direction X are firmly bonded by the van der Waals force acting between the carbon nanotubes, they can have a higher mechanical strength (e.g., tensile strength) than the entanglement of the carbon nanotubes 11C adjacent to each other in the direction Y perpendicular to the orientation direction X. Therefore, when the mechanical strength of the current collector 11 is more important, it is more preferable that the current collector 11 includes at least the carbon nanotubes 11C oriented along the longitudinal direction of the current collector 11.

At least one of the thermal conductivity and electrical resistance of the current collector 11 provided in the electrode of the present disclosure may be anisotropic. More specifically, since the current collector 11 contains carbon nanotubes 11C having regular orientation, the current collector 11 may have thermal conductivity and/or electrical properties oriented macroscopically along the orientation direction of the carbon nanotubes 11C contained in the current collector 11. In other words, the electrical properties and thermal conduction properties of the current collector 11 may be particularly prominent in the direction in which the carbon nanotubes 11C are oriented. For example, the thermal conductivity in the orientation direction X of the carbon nanotubes 11C contained in the current collector 11 is relatively higher than the thermal conductivity in the direction Y (see FIG. 2) perpendicular to the orientation direction X of the carbon nanotubes 11C. Since the current collector 11 has orientation in thermal conductivity, it is possible to more efficiently dissipate heat generated in the electrode to the electrode and outside the battery.

Similarly, the electrical resistance value in the orientation direction X of the carbon nanotubes 11C contained in the current collector 11 may be relatively smaller than the electrical resistance value in the direction Y perpendicular to the orientation direction X of the carbon nanotubes 11C. In short, the current collector 11 of the present disclosure has a smaller electrical resistivity in the orientation direction X. This allows a more energy-efficient battery to be obtained by connecting a connection member (not shown) such as a lead or tab that can be attached to the current collector 11 to extract current from the electrode along the orientation direction X. More specifically, by attaching a connection member to the end of the current collector in the orientation direction X, the electrical resistance between the connection member and the current collector 11 is reduced, and a more suitable electrode can be provided.

The thermal conductivity in the orientation direction of the carbon nanotubes contained in the current collector may be, for example, 10 W/mK or more, 20 W/mK or more, 30 W/mK or more, 40 W/mK or more, or 50 W/mK or more, and is preferably greater than 50 W/mK. The thermal conductivity in the orientation direction of the carbon nanotubes contained in the current collector may be 1000 W/mK or less, 800 W/mK or less, 700 W/mK or less, 600 W/mK or less, or 500 W/mK or less.

The thermal conductivity in a direction perpendicular to the orientation direction of the carbon nanotubes contained in the current collector may be, for example, 0.5 W/mK or more, 1 W/mK or more, 2 W/mK or more, 3 W/mK or more, or 5 W/mK or more. The thermal conductivity in a direction perpendicular to the orientation direction of the carbon nanotubes contained in the current collector may be 50 W/mK or less, 40 W/mK or less, or 30 W/mK or less.

The thermal conductivity in the orientation direction of the carbon nanotubes contained in the current collector may be 1.2 times or more, 1.5 times or more, 2.0 times or more, 2.5 times or more, 3.0 times or more, 3.5 times or more, or 4.0 times or more, and is preferably 2.0 times or more, as compared to the thermal conductivity in the direction perpendicular to the orientation direction. The thermal conductivity in the orientation direction of the carbon nanotubes in the current collector may be, for example, 50 times or less, 25 times or less, or 10 times or less, as compared to the thermal conductivity in the direction perpendicular to the orientation direction of the carbon nanotubes in the current collector.

The electrical resistance value in the orientation direction of the carbon nanotubes contained in the current collector may be, for example, 0.5 Ω or more, 1 Ω or more, 2 Ω or more, 5 Ω or more, or 10 Ω or more. The electrical resistance value in the orientation direction of the carbon nanotubes contained in the current collector may be, for example, 50 Ω or less, 40 Ω or less, 30 Ω or less, or 20 Ω or less, and is preferably 30 Ω or less.

The electrical resistance value in a direction perpendicular to the orientation direction of the carbon nanotubes contained in the current collector may be, for example, 70 Ω or more, 80 Ω or more, 100 Ω or more, or 200 Ω or more. The electrical resistance value in a direction perpendicular to the orientation direction of the carbon nanotubes contained in the current collector may be, for example, 500 Ω or less, 400 Ω or less, or 300 Ω or less.

The electrical resistance value in the orientation direction of the carbon nanotubes contained in the current collector may be, for example, 0.9 times or less, 0.8 times or less, 0.7 times or less, or 0.6 times or less, and is preferably 0.6 times or less, of the electrical resistance value in the direction perpendicular to the orientation direction. The electrical resistance value in the orientation direction of the carbon nanotubes contained in the current collector may be, for example, 0.01 times or more, 0.05 times or more, 0.1 times or more, 0.2 times or more, or 0.5 times or more, of the electrical resistance value in the direction perpendicular to the orientation direction.

The average diameter of the carbon nanotubes contained in the current collector may be 0.5 nm or more, 3 nm or more, 5 nm or more, 10 nm or more, 30 nm or more, 50 nm or more, or 100 nm or more, preferably 5 nm or more, and more preferably 10 nm or more. The diameter of the carbon nanotubes may be 500 nm or less, 300 nm or less, 100 nm or less, 80 nm or less, 60 nm or less, 40 nm or less, or 20 nm or less, for example, 100 nm or less. In this disclosure, "average diameter" refers to the average value of the diameters of five carbon nanotubes, and "diameter" refers to the width perpendicular to the length direction of the carbon nanotubes when the carbon nanotubes are observed in a SEM image.

The carbon nanotubes contained in the current collector may be single-walled or multi-walled, but are more preferably multi-walled. The number of layers of the carbon nanotubes may be 1 or more, 2 or more, 3 or more, 5 or more, 7 or more, 10 or more, or 20 or more, and is preferably 2 or more. The number of layers of the carbon nanotubes may be 55 or less, 45 or less, 35 or less, 25 or less, 15 or less, or 5 or less. Multi-walled carbon nanotubes having the number of layers in the above ranges allow multiple carbon nanotubes to be more suitably bundled together, resulting in a current collector with superior mechanical strength.

The average length of the carbon nanotubes contained in the current collector may be 0.05 mm or more, 0.1 mm or more, 0.2 mm or more, 0.3 mm or more, 0.5 mm or more, 1.0 mm or more, 1.5 mm or more, preferably 0.1 mm or more, and more preferably 0.3 mm or more. The average length of the carbon nanotubes may be 20 mm or less, 15 mm or less, 10 mm or less, 5.0 mm or less, 2.5 mm or less, 2.0 mm or less, 1.5 mm or less, for example, 5.0 mm or less.

The "average length" in the present invention is the average value of the lengths of multiple (e.g., five) carbon nanotubes measured using a microscope. For example, the "average length" can be the average value of the lengths of multiple (e.g., five) carbon nanotubes grown on a substrate by CVD, measured using a microscope. The length of the carbon nanotubes grown on the substrate and the length of one carbon nanotube included in the current collector are assumed to be the same or approximately the same. If the average length of the carbon nanotubes is within the above range, other performances such as mechanical properties, chemical or physical durability, and ease of handling can also be excellent. For example, if the average length of the carbon nanotubes is within the above range, the contact points between the carbon nanotubes can be reduced, thereby reducing electrical resistance. Furthermore, if the average length of the carbon nanotubes is within the above range, the current collector containing the carbon nanotubes can have excellent thermal conductivity. Furthermore, the greater the average length of the carbon nanotubes, the easier it is for multiple carbon nanotubes to be bundled and more suitably entangled with each other, thereby improving the mechanical strength of the current collector.

The average aspect ratio of the carbon nanotubes contained in the current collector of the present disclosure, i.e., the ratio of the average length to the average diameter of a single carbon nanotube oriented in the current collector of the present disclosure, may be 500 or more, 1000 or more, 1500 or more, 2000 or more, 2500 or more, or 3000 or more, for example, 2000 or more. The average aspect ratio of the carbon nanotubes may be 100,000 or less, 90,000 or less, 80,000 or less, 70,000 or less, 65,000 or less, 60,000 or less, or 55,000 or less, for example, 60,000 or less. When the average aspect ratio of the carbon nanotubes is within the above-mentioned range, multiple carbon nanotubes can be easily bundled and more suitably entangled with each other, and therefore the mechanical strength of the current collector can be improved.

The G/D ratio of the carbon nanotubes may be 1.0 or more, 1.5 or more, 2.0 or more, or 2.5 or more, and is preferably 2.0 or more. The G/D ratio of the carbon nanotubes may be 10.0 or less, 8.0 or less, 6.0 or less, or 4.0 or less. The G/D ratio is an index of the crystallinity of the carbon nanotubes determined by Raman spectroscopy.

The carbon nanotubes constituting the current collector in this disclosure may be synthesized by the CVD method. The CVD method is a type of deposit formation method that uses a chemical reaction in the deposit formation process. CVD methods include thermal CVD, in which the temperature is raised and the raw material is decomposed by heat, photo-CVD, in which light is irradiated to promote a chemical reaction, and plasma CVD, in which a gas is excited into a plasma state, but typically, thermal CVD is preferred in the carbon nanotube synthesis process in this disclosure. Examples of thermal CVD methods include the DIPS method, the CoMoCAT method, the HiPCO method, the super-growth CVD method, the solid-phase catalyst method, and the gas-phase catalyst method.

For example, as shown in FIG. 6, the carbon nanotubes contained in the current collector of the present disclosure may be synthesized on a substrate by a CVD method. The ends of the carbon nanotubes grown from the substrate after synthesis are pinched and pulled out, and the sheet-like carbon nanotubes obtained by winding them up using a winding roll may be used as the current collector. Such a carbon nanotube sheet (hereinafter also referred to as a "CNT sheet") is formed by the dry spinning phenomenon of carbon nanotubes. More specifically, when the carbon nanotubes are pinched and pulled out from the ends of an aggregate of carbon nanotubes (so-called CNT forest) vertically aligned on a substrate by a CVD method, the carbon nanotubes spontaneously bond to each other through van der Waals forces, resulting in a CNT web in which the carbon nanotubes are connected. The obtained CNT web can also be interpreted as an aggregate in which the carbon nanotubes form a two-dimensional network.

The CNT sheet may be formed by stacking CNT webs. A manufacturing process for a CNT sheet utilizing the dry spinning phenomenon is known, for example, Y. Inoue, K. Kakihata, Y. Hirono, T. Horie, A. Ishida & H. Mimura: Appl. Phys. Lett., 92, 21 (2008), 213113.; Y. Inoue, Y. Suzuki, Y. Minami, J. Muramatsu, Y. Shimamura, K. Suzuki, A. Ghemes, M. Okada, S. Sakakibara, H. Mimura & K. Naito: Carbon, 49, 7 (2011), 2437-2443. and the like can be referred to. In this way, in the CNT web formed by the dry spinning phenomenon, it is not necessary to twist the carbon nanotubes or to add a separate binder to the carbon nanotubes. Therefore, by using a CNT sheet made of the above-mentioned CNT web as a current collector, an electrode with excellent electrical conductivity that is substantially free of additives that may cause a decrease in electrical resistance can be provided.

Furthermore, in the current collector according to one embodiment of the present disclosure, the carbon nanotubes may be regularly oriented in multiple directions. In other words, the current collector includes regularly oriented carbon nanotubes, and the orientation direction of the carbon nanotubes may be multiple directions. FIG. 4 is a schematic plan view of a current collector according to one embodiment of the present disclosure. As shown in the figure, carbon nanotubes 11C included in the current collector 11 may cross each other when viewed from the thickness direction of the current collector 11. By including carbon nanotubes 11C oriented in multiple directions in the current collector 11, the mechanical strength of the current collector is made more isotropic, and the durability of the electrode as a whole may be improved.

In the current collector, the crossing angle formed by the carbon nanotubes having different orientation directions is not particularly limited, but may be, for example, 30° to 150°, 40° to 140°, or 45° to 135°. If greater importance is placed on the mechanical strength of the current collector, it is more preferable that the crossing angle be 90°.

For example, the current collector may include carbon nanotubes oriented in a first direction and carbon nanotubes oriented in a second direction, and the first direction and the second direction may cross each other. That is, the carbon nanotubes may be oriented in a matrix in the current collector. In other words, the current collector may include a plurality of carbon nanotubes arranged in a matrix. For example, the carbon nanotubes may be arranged in a lattice when viewed from the thickness direction of the current collector. This structure includes a plurality of groups of carbon nanotubes oriented in a specific direction, and can also be interpreted as the carbon nanotubes being arranged in a matrix. This structure provides higher mechanical and electrical properties in the orientation direction of the carbon nanotubes, and can provide an electrode including a current collector with improved overall mechanical strength.

The current collector having carbon nanotubes oriented in a matrix may be formed, for example, by stacking CNT sheets oriented in a predetermined direction. Specifically, it may be formed by stacking sheets containing carbon nanotubes oriented in a predetermined direction with different orientation directions. In other words, the current collector is composed of multiple sheets, and the multiple sheets may be stacked so that the orientation directions of the sheets cross each other. For example, the current collector may be formed by alternately stacking a first sheet containing carbon nanotubes oriented in a first direction and a second sheet containing carbon nanotubes oriented in a second direction. The sheet stacking configuration is not limited to this, and the sheet stacking configuration, such as the number of stacked sheets and the arrangement direction of the carbon nanotubes, may be appropriately selected depending on the required characteristics of the electrode.

The thickness of the current collector may be 1 to 200 μm, 1 to 180 μm, or 1 to 150 μm, for example, 1 to 130 μm. When emphasis is placed on increasing the weight energy density of the battery by reducing the weight, it is preferable that the thickness of the current collector is thinner. Specifically, the thickness of the current collector is preferably 1 to 150 μm, more preferably 1 to 100 μm, and even more preferably 1 to 80 μm. By having the current collector have a thickness within the above range, an electrode with excellent weight energy density can be provided.

<Batteries>
The present disclosure also relates to a battery comprising a positive electrode, a negative electrode, and an electrolyte, characterized in that at least one of the positive electrode and the negative electrode contains the above-mentioned electrode. In this specification, the term "battery" includes not only a primary battery that can only be discharged, but also a secondary battery that can be repeatedly charged and discharged. The term "secondary battery" is not limited to the name, and may also include, for example, a power storage device.

First, the basic configuration of a secondary battery according to one embodiment of the present disclosure will be described below. The configuration of the battery described here is merely an example for understanding the invention, and does not limit the invention. For example, as shown in FIG. 5, the battery may have a structure in which a wound electrode body formed by winding a sheet-shaped positive electrode and a sheet-shaped negative electrode with a separator interposed therebetween is housed in an outer case, or a laminated electrode body formed by repeatedly stacking positive electrodes and negative electrodes in this order with a separator interposed therebetween is housed in an outer case. In addition, the battery according to this embodiment may be a cylindrical battery or a square battery with a cylindrical or rectangular parallelepiped outer case, or may be in various forms such as a small coin battery or a button battery.

<Basic battery structure>
5 is a schematic diagram of a secondary battery according to an embodiment of the present disclosure. As shown in the figure, the battery 100 may include a separator 130, a positive electrode 110 and a negative electrode 120 arranged to face each other with the separator 130 interposed therebetween, and a non-aqueous electrolyte (not shown). The electrode of the present disclosure may be used for either or both of the positive electrode 110 and the negative electrode 120 of the battery 100. In such applications, the electrode of the present disclosure containing carbon nanotubes in the current collector may be used as the positive electrode and/or the negative electrode to improve the performance of the battery (e.g., lower resistance, higher energy density, and higher durability).

The positive electrode 110 may be an electrode in which a positive electrode active material layer containing at least a positive electrode active material is applied onto a current collector. For example, positive electrode active materials used in batteries include lithium cobalt oxide ( _LiCoO2 ), lithium nickel oxide ( _LiNiO2 ), lithium manganese oxide ( _LiMnO2 ), lithium manganese spinel ( _LiMn2O4 ), and composite metal oxides represented by the general formula: LiNi _x Co _y Mn _z M _a _O2 (x+y+z+a=1, ₀ ≦x<1, 0≦y<1, 0≦z<1, 0≦a<1, M is one or more elements selected from Al, Mg, Nb, Ti, Cu, Zn, and Cr), lithium vanadium compound ( _LiV2O5 ), olivine-type _LiMPO4 (wherein M is one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, and Zr, or VO), and spinel-type lithium titanate ( _Li4O5 ) _. The material may be a composite metal oxide such as _Ti5O12 ), _{LiNixCoyAlzO2} (0.9<x+y+z< _1.1 ) _, _{polyacetylene} , polyaniline, _polypyrrole , polythiophene, and/or polyacene.

In addition, as the negative electrode 120, an electrode in which an active material layer containing at least a negative electrode active material is applied onto a current collector can be used. The negative electrode active material used in the battery is not particularly limited as long as it is a material that contributes to the absorption and release of ions (e.g., lithium ions). For example, the negative electrode active material can be an alkali or alkaline earth metal such as metallic lithium, a carbon material such as graphite (natural graphite, artificial graphite) capable of absorbing and releasing ions, carbon nanotubes, non-graphitizable carbon, easily graphitizable carbon, and low-temperature fired carbon, a metal that can combine with a metal such as lithium, such as aluminum, silicon, or tin, an amorphous compound mainly composed of an oxide such as SiO _x (0<x<2) or tin dioxide, and/or particles containing lithium titanate (Li ₄ Ti ₅ O ₁₂ ), etc., can be mentioned.

The electrode active material layer may contain other substances if necessary. For example, it may contain a conductive additive, a binder, etc.

The conductive additive is an additive that is blended to improve the conductivity of the active material layer. Examples of conductive additives include, but are not limited to, carbon powders such as acetylene black, carbon black, ketjen black, and graphite, various carbon fibers such as vapor grown carbon fiber (VGCF; registered trademark), and/or expanded graphite.

Binders include, but are not limited to, fluororesins such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), polyvinyl fluoride (PVF), and/or acrylonitrile butadiene rubber (NBR).

The compounding ratio of the components contained in the electrode active material layer is not particularly limited. The compounding ratio may be adjusted by appropriately referring to known knowledge about batteries. There is also no particular limit to the thickness of the electrode active material layer, and known knowledge about batteries may be appropriately referred to. As an example, the thickness of the electrode active material layer may be, for example, about 10 to about 100 μm. If the active material layer is about 10 μm or more, the battery capacity can be sufficiently secured. On the other hand, if the electrode active material layer is about 100 μm or less, the occurrence of the problem of increased internal resistance due to the difficulty of lithium ions diffusing deep into the electrode (collector side) can be suppressed. When placing more importance on the miniaturization and weight reduction of the battery, it is more preferable that the thickness of the electrode active material layer is about 20 to about 50 μm.

As the separator 130, a separator similar to that used in the past can be used. For example, a porous sheet (porous film) made of a polyolefin resin such as polyethylene (PE) and/or polypropylene (PP) can be used. Alternatively, a nonwoven fabric made of fibers such as cellulose, glass, polyethylene terephthalate (PET), polyamide, polyimide, polyamideimide, polyacrylonitrile, and/or wholly aromatic polyester can be used.

In the battery, the tabs (positive electrode tab 113 and negative electrode tab 123) electrically connected to the current collector are electrically connected to the positive electrode terminal 150 or the negative electrode terminal (or the exterior material 140) for the purpose of extracting current outside the battery. The current collector is electrically connected to the positive electrode terminal 150 or the negative electrode terminal (or the exterior material 140) via the positive electrode tab 113 and the negative electrode tab 123, respectively. The material constituting the tab is not particularly limited, and a known highly conductive material conventionally used as a tab for a secondary battery can be used. As the material constituting the tab, for example, a metal material such as aluminum, copper, titanium, nickel, stainless steel (SUS), and/or an alloy thereof is preferable. From the viewpoints of light weight, corrosion resistance, and high conductivity, aluminum or copper is more preferable, and aluminum is particularly preferable. The same material may be used for the positive electrode tab and the negative electrode tab, or different materials may be used.

<Characteristics of the battery disclosed herein>
In the battery having the above-mentioned structure, the electrode of the present disclosure, which has low resistance and excellent mechanical properties, can be used as the positive electrode 110 and/or the negative electrode 120. The carbon nanotubes constituting the current collector of the electrode of the present disclosure are much lighter than conventionally used metals such as aluminum and copper, so that it is possible to further improve the weight energy density of the battery. Furthermore, in the current collector provided in the electrode of the present disclosure, a plurality of carbon nanotubes oriented in a predetermined direction are suitably held in the orientation direction and in a direction perpendicular to the orientation direction by van der Waals forces and by entanglement of the carbon nanotubes. Therefore, even if a volume change occurs in the battery due to charging and discharging, the carbon nanotubes constituting the current collector are prevented from falling off, and a battery having better cycle life characteristics can be provided.

Furthermore, in the electrode of the present disclosure, since the carbon nanotubes have an orientation, multiple carbon nanotubes are more uniformly arranged throughout the current collector, and each carbon nanotube can contribute more efficiently to conduction. Furthermore, by uniformly arranging the carbon nanotubes, it is possible to form a current collector with fewer carbon nanotubes, and voids can be formed between each carbon nanotube. Such voids can provide space into which the electrode active material layer provided on the current collector can be filled. In other words, a battery equipped with the electrode of the present disclosure can increase the amount of filling of the electrode active material layer, which can be more advantageous in increasing the battery's output and capacity.

As mentioned above, when the battery is used, a current is taken out of the battery from the electrode terminal electrically connected to the current collector. The electrode of the present disclosure has a lower electrical resistance in the orientation direction of the carbon nanotubes contained in the current collector. Therefore, the carbon nanotubes contained in the current collector of the present disclosure may be oriented toward the connection point with the electrode terminal. Specifically, the current collector may contain carbon nanotubes that are oriented in a generally regular pattern toward a tab attached to the current collector to electrically connect the current collector and the electrode terminal. In other words, in the current collector, the carbon nanotubes may extend toward the location where the tab is installed. This structure further reduces the electrical resistance in the connection direction between the current collector and the tab, making it possible to improve the performance of the battery (e.g., reduce resistance).

In conventional batteries that use electrodes with current collectors such as aluminum foil or copper foil, when a high voltage (e.g., 4.0 V or higher) is applied, the electrolyte and small amounts of water present decompose to generate hydrocarbon gases and hydrofluoric acid, which can corrode the current collector. In the present disclosure, carbon nanotubes are used as the current collector, so the current collector is not corroded by these gases, making it more advantageous in terms of safety and corrosion resistance than previous batteries that used aluminum foil, copper foil, or the like as a current collector.

Furthermore, by using a carbon nanotube sheet as the current collector, the carbon nanotubes can serve as an anchor for holding the electrode active material, which expands and contracts when the battery is used, on the current collector. Generally, a coating agent containing carbon nanotubes is used on the current collector as an anchor for the electrode active material in an electrode (e.g., a silicon negative electrode). On the other hand, in the electrode disclosed herein, the current collector itself is carbon nanotubes, so a coating agent is not required and there is an effect of suppressing peeling between the current collector and the electrode active material.

The present disclosure will be explained in more detail below with examples and comparative examples, but the present disclosure is not limited to these examples.

(Example of carbon nanotube synthesis)
Using a thermal CVD apparatus including a reaction chamber containing a substrate, a gas inlet, and a gas outlet, carbon nanotubes were grown on the substrate for 10 to 30 minutes depending on the desired length (see FIG. 6). As a result, multi-walled carbon nanotubes were obtained that were grown almost perpendicular to the substrate surface.

After synthesis, the ends of the carbon nanotubes grown from the substrate were pinched and pulled out, and then wound up using a winding roll to stack approximately 10 layers of CNT webs, resulting in a CNT sheet composed of carbon nanotubes oriented in one direction. The thickness of the resulting CNT sheet was approximately 5.0 μm. The carbon nanotubes contained in the CNT sheet had a length of approximately 0.7 mm, a diameter of approximately 50 nm, and a G/D ratio of approximately 2.8. The length and diameter of the carbon nanotubes were determined from SEM photographs. The G/D ratio was determined by Raman analysis.

The electrical resistance of the carbon nanotubes was measured using Loresta (Nitto Seiko Analytech Co., Ltd.) and found to be 13 Ω in the orientation direction and 100 Ω in the direction perpendicular to the orientation direction. Similarly, the thermal conductivity was measured using the laser flash method (Bethel Co., Ltd.) and found to be 134 W/mK in the orientation direction and 39 W/mK in the direction perpendicular to the orientation direction. Thus, the CNT sheet exhibited particularly excellent electrode properties in the orientation direction of the carbon nanotubes. The following evaluation tests were carried out using such a CNT sheet as a current collector.

<Electrochemical evaluation>
(Example of electrode preparation)
The electrodes of Examples 1 to 11 and Comparative Examples 1 to 8 used in the evaluation of the electrodes of the present disclosure will be described below.
[Examples 1 and 2]
An electrode mixture slurry was prepared using N-methyl _-2 -pyrrolidone as a dispersion medium _so as to contain 90 wt% of spinel-type lithium titanate _Li4Ti5O12 (manufactured by Teika Co., Ltd.) as an active material, 5 wt% of acetylene black (Denka Black Powder, Denki Kagaku Kogyo Co., Ltd.) as a conductive assistant, and 5 wt% of polyvinylidene fluoride (PVDF, KF Polymer, Kureha Co., Ltd.) as a binder. The electrode mixture slurry was applied and dried on a CNT sheet (Koatsu Gas Kogyo Co., Ltd., thickness approximately 5.0 μm) used as a current collector, and then roll pressed to prepare an electrode.
[Example 3]
An electrode was produced in the same manner as in Example 1, except that a CNT sheet (manufactured by Koatsu Gas Kogyo Co., Ltd., thickness approximately 10.0 μm) was used as the current collector.
[Example 4]
An electrode was produced in the same manner as in Example 1, except that a CNT sheet (manufactured by Koatsu Gas Kogyo Co., Ltd., thickness: approximately 20.0 μm) was used as the current collector.
[Example 5]
An electrode was produced in the same manner as in Example 1, except that a CNT sheet (manufactured by Koatsu Gas Kogyo Co., Ltd., thickness approximately 40.0 μm) was used as the current collector.
[Example 6]
An electrode was prepared in the same manner as in Example 1, except that a Ni-based positive electrode material LiNi _0.5 Co _0.2 Mn _0.3 O ₂ (manufactured by Hohsen Co., Ltd.) was used as the active material.
[Example 7]
An electrode was prepared in the same manner as in Example 1, except that lithium cobalt oxide LiCoO ₂ (CELLSEED, manufactured by Nippon Chemical Industry Co., Ltd.) was used as the active material.
[Example 8]
An electrode was prepared in the same manner as in Example 1, except that lithium iron phosphate LiFePO ₄ (manufactured by Teika Corporation) was used as the active material.
[Example 9]
An electrode was prepared in the same manner as in Example 1, except that lithium nickel oxide LiNiO ₂ (manufactured by Teika Corporation) was used as the active material.
[Example 10]
An electrode was prepared in the same manner as in Example 1, except that natural graphite (CGB-20, manufactured by Nippon Graphite Industries Co., Ltd.) was used as the active material.
[Example 11]
An electrode was prepared in the same manner as in Example 1, except that silicon (Silgrain-Si, manufactured by Elkem Japan Co., Ltd.) was used as the active material.

[Comparative Example 1]
An electrode was prepared in the same manner as in Example 1, except that an etched aluminum foil (manufactured by Nippon Chemi-Con Corporation, JCC-20CB) was used as the current collector.
[Comparative Example 2]
An electrode was prepared in the same manner as in Example 1, except that copper foil (manufactured by Fukuda Metal Foil & Powder Co., Ltd.) was used as the current collector.
[Comparative Example 3]
An electrode was prepared in the same manner as in Example 6, except that an etched aluminum foil (manufactured by Nippon Chemi-Con Corporation, JCC-20CB) was used as the current collector.
[Comparative Example 4]
An electrode was prepared in the same manner as in Example 7, except that an etched aluminum foil (manufactured by Nippon Chemi-Con Corporation, JCC-20CB) was used as the current collector.
[Comparative Example 5]
An electrode was prepared in the same manner as in Example 8, except that an etched aluminum foil (manufactured by Nippon Chemi-Con Corporation, JCC-20CB) was used as the current collector.
[Comparative Example 6]
An electrode was prepared in the same manner as in Example 9, except that an etched aluminum foil (manufactured by Nippon Chemi-Con Corporation, JCC-20CB) was used as the current collector.
[Comparative Example 7]
An electrode was prepared in the same manner as in Example 10, except that copper foil (manufactured by Fukuda Metal Foil & Powder Co., Ltd.) was used as the current collector.
[Comparative Example 8]
An electrode was prepared in the same manner as in Example 11, except that copper foil (manufactured by Fukuda Metal Foil & Powder Co., Ltd.) was used as the current collector.

(Preparation of coin-type batteries for electrochemical evaluation)
The electrode prepared as described above was punched out into a coin shape with a diameter of 1.5 cm to prepare an electrode for evaluation (positive electrode). Metallic lithium (Honjo Metal Co., Ltd.) was used as the counter electrode, and a polyethylene separator was sandwiched between the electrode for evaluation and the counter electrode to prepare a battery. An organic electrolyte solution in which 1M LiPF ₆ was dissolved in a mixed solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed at a volume ratio of EC:DEC = 1:1 was injected, and then the battery container was sealed to prepare a coin-type battery for electrochemical evaluation.

(Charge/discharge test)
Using the coin-type battery prepared as described above, a three-cycle charge/discharge test was performed at a charge/discharge rate of 1C (current density 0.53mA/ ^cm2 ). In the test, the voltage range was changed depending on the type of electrode active material used. The voltage ranges applied to each Example and Comparative Example are shown in Table 1. The discharge capacity obtained in the third cycle was taken as the capacity (mAh) at 1C. The measurement was performed using a charge/discharge tester (HJ1001SD8, manufactured by Hokuto Denko Corporation). The capacity per weight was calculated using the following calculation formula (I).
Capacity at 1C (mAh) ÷ Weight of positive electrode (g) = Capacity per weight (mAh/g) (I)

(Rate Evaluation)
Using the coin-type battery prepared as described above, charge/discharge tests were performed at charge/discharge rates of 10C, 50C, and 100C in the voltage ranges shown in Table 1, and the obtained discharge capacities were taken as the capacities (mAh) at each C rate, and the capacity retention rates (%) at each C rate were calculated using the following calculation formula (II). The measurements were performed using a charge/discharge tester (HJ1001SD8, manufactured by Hokuto Denko Corporation).
Capacity (mAh) at each C rate ÷ Capacity (mAh) at 1C × 100 = Capacity maintenance rate (%) at each C rate (II)

Table 1 below shows the electrodes for each example and comparative example, as well as the voltage ranges applied in the evaluation tests.

The results of various evaluation tests obtained for the electrodes of each example and comparative example are shown in Table 2 below.

According to the above results, in the batteries of Examples 1 to 11 in which a CNT sheet was used as the positive electrode current collector, the capacity at 1C and the capacity retention rate at each C rate were approximately the same as those of the batteries of Comparative Examples 1 to 8 in which the current collector was made of a conventional material. Therefore, it was found that the electrode of the present disclosure, which uses a CNT sheet as a current collector, exhibits electrode characteristics that are comparable to those of electrodes that use conventional materials.

In addition, the weight of the current collector in Examples 1 to 11 is much lighter than that in Comparative Examples 1 to 8. Therefore, the capacity per weight of the battery equipped with the electrode of the present disclosure is about 1.9 to about 12.3 times that of Comparative Examples 1 to 8, and the results showed a higher value than that of the electrodes of the comparative examples regardless of the type of electrode active material. For example, in an electrode using lithium titanate as the electrode active material, the electrode of Example 4 had a capacity at 1C and a capacity retention rate at each C rate that was comparable to that of Comparative Examples 1 and 2 using a current collector of the same thickness (20 μm) of the conventional material. On the other hand, the weight of the current collector in Example 4 was about 3% to about 9% of the conventional material, and the capacity per weight in Example 4 was improved by about 2.2 to about 5.0 times compared to the conventional material. In other words, by providing a current collector composed of carbon nanotubes, an electrode with excellent weight energy density is provided. Therefore, it was found that the electrode of the present disclosure equipped with a current collector containing carbon nanotubes can provide a predetermined electrode characteristic more favorably.

In addition, although the electrodes of Examples 1 to 3 and 6 to 11 had a smaller thickness than the etched aluminum foil and copper foil used as the comparative examples, the capacity at 1C and the capacity retention rate at each C rate showed results that were comparable to those of the comparative examples using the same electrode active material. Therefore, by using a CNT sheet as the current collector, it is possible to reduce the thickness of the current collector, and the present disclosure may also contribute to the miniaturization of electrodes and batteries equipped with such electrodes.

Furthermore, when lithium nickel oxide is used as the electrode active material, in the electrode of Comparative Example 6 in which the current collector is etched aluminum foil, corrosion occurred in the current collector, and the capacity at 1C and the capacity retention rate at each C rate were low. On the other hand, in the electrode of Example 9 in which the CNT sheet of the present disclosure was used, no corrosion of the current collector was observed, and a high capacity was obtained. Therefore, it was found that the electrode of the present disclosure can provide a more suitable battery that can suppress corrosion of the current collector during battery use.

<Evaluation of oxidation-reduction resistance>
(Cyclic Voltammetry (CV) Test)
A coin-type battery was fabricated using the CNT sheet of the present disclosure as the working electrode and metallic lithium (Honjo Metals) as the counter electrode. An organic electrolyte solution in which 1M _LiBF4 was dissolved in a propylene carbonate (PC) solvent was used as the electrolyte. Cyclic voltammetry tests were performed on the fabricated battery under the following conditions. The voltammogram obtained by the test is shown in FIG. 7.
Upper limit voltage: 4.3 V (vs Li ⁺ /Li)
Lower limit voltage: 0.3 V (vs Li ⁺ /Li)
Sweep speed: 2.0 mV/s
Temperature: 25°C

In the cyclic voltammetry test of the CNT sheet, the peak top observed near 0.5 V is derived from the organic electrolyte. In addition, since no peak top is observed in the cyclic voltammetry test of the CNT sheet of the present disclosure, it is recognized that the CNT sheet of the present disclosure has oxidation resistance and reduction resistance in the voltage range of 0.3 to 4.3 V (vs Li ⁺ /Li). Therefore, it was found that the electrode of the present disclosure having a current collector containing carbon nanotubes can be suitably used as an electrode having oxidation and reduction resistance.

The above describes embodiments of the present disclosure, but these are merely typical examples. Those skilled in the art will easily understand that the present disclosure is not limited to these, and that various embodiments are possible without departing from the spirit of the present disclosure.

Note that the embodiment of the present disclosure as described above includes the following preferred aspects.
First aspect:
A current collector and an electrode active material layer provided on the current collector,
An electrode, wherein the current collector comprises regularly oriented carbon nanotubes.
Second aspect:
In the first aspect, the carbon nanotubes are oriented along an extending direction of the main surface of the current collector.
Third aspect:
In the first or second aspect, the aspect ratio of the carbon nanotubes is 2,000 to 60,000.
Fourth aspect:
In any one of the first to third aspects, the electrode comprises a plurality of the carbon nanotubes, the plurality of carbon nanotubes being oriented in parallel when viewed from the thickness direction of the current collector.
Fifth aspect:
In the fourth aspect, in the electrode, adjacent carbon nanotubes in the parallel oriented carbon nanotubes are entangled with each other.
Sixth aspect:
In any one of the first to fifth aspects, the electrode comprises a plurality of the carbon nanotubes, the plurality of carbon nanotubes being aligned in series along a main surface of the current collector.
Seventh aspect:
In any one of the first to sixth aspects, the electrode has an elongated current collector, and the carbon nanotubes are oriented along at least one of the longitudinal and lateral directions of the current collector.
Eighth aspect:
In any one of the first to seventh aspects, the electrode wherein at least one of the thermal conductivity and the electrical resistance value of the current collector is anisotropic.
Ninth aspect:
In the eighth aspect, in the electrode, the thermal conductivity of the current collector in the alignment direction of the carbon nanotubes is relatively higher than the thermal conductivity in a direction perpendicular to the alignment direction.
Tenth aspect:
In the eighth or ninth aspect, an electric resistance value of the current collector in an alignment direction of the carbon nanotubes is relatively smaller than an electric resistance value in a direction perpendicular to the alignment direction.
Eleventh aspect:
The electrode according to any one of the first to tenth aspects, wherein a plurality of the carbon nanotubes are present, and the plurality of carbon nanotubes intersect with each other when viewed from the thickness direction of the current collector.
Twelfth aspect:
The electrode according to any one of the first to eleventh aspects, wherein the carbon nanotubes are oriented in a matrix form when viewed in a thickness direction of the current collector.
Thirteenth aspect:
In any one of the first to twelfth aspects, the electrode, wherein the current collector is constituted only by the carbon nanotubes.
Fourteenth aspect:
In any one of the first to thirteenth aspects, the carbon nanotubes have a length of 0.3 mm or more.
Fifteenth aspect:
A battery comprising the electrode according to any one of the first to fourteenth aspects.
Sixteenth aspect:
In the fifteenth aspect, the present invention further includes an electrode terminal electrically connected to the current collector,
The carbon nanotubes are oriented toward a connection point between the electrode terminal and the current collector.

The electrodes disclosed herein can be used in a variety of fields where battery use or power storage is anticipated.

REFERENCE SIGNS LIST 10 Electrode 11 Current collector 12 Electrode active material layer 100 Battery 110 Positive electrode 113 Positive electrode tab 120 Negative electrode 123 Negative electrode tab 130 Separator 140 Exterior material 150 Positive electrode terminal

Claims

A current collector and an electrode active material layer provided on the current collector,
An electrode, wherein the current collector comprises regularly oriented carbon nanotubes.
The electrode according to claim 1, wherein the carbon nanotubes are oriented along the extension direction of the main surface of the current collector.
The electrode according to claim 1, wherein the aspect ratio of the carbon nanotubes is 2,000 to 60,000.
The electrode according to claim 1, wherein the carbon nanotubes are present in a plurality of layers, and the plurality of carbon nanotubes are oriented in parallel when viewed in the thickness direction of the current collector.
The electrode according to claim 4, wherein adjacent carbon nanotubes in the plurality of carbon nanotubes aligned in parallel are entangled with each other.
The electrode according to claim 1, wherein a plurality of the carbon nanotubes are present and the plurality of the carbon nanotubes are aligned in series along the main surface of the current collector.
The electrode of claim 1, wherein the current collector is elongated and the carbon nanotubes are oriented along at least one of the longitudinal and lateral directions of the current collector.
The electrode according to claim 1, wherein at least one of the thermal conductivity and electrical resistance of the current collector is anisotropic.
The electrode of claim 8, wherein the thermal conductivity of the current collector in the orientation direction of the carbon nanotubes is relatively greater than the thermal conductivity in a direction perpendicular to the orientation direction.
The electrode according to claim 8, wherein the electrical resistance of the current collector in the orientation direction of the carbon nanotubes is relatively smaller than the electrical resistance in a direction perpendicular to the orientation direction.
The electrode according to claim 1, wherein a plurality of the carbon nanotubes are present, and the plurality of the carbon nanotubes intersect with each other when viewed in the thickness direction of the current collector.
The electrode according to claim 1, wherein the carbon nanotubes are oriented in a matrix when viewed in the thickness direction of the current collector.
The electrode according to claim 1, wherein the current collector is composed only of the carbon nanotubes.
The electrode according to claim 1, wherein the carbon nanotubes have a length of 0.3 mm or more.
A battery comprising the electrode according to claim 1.
further comprising an electrode terminal electrically connected to the current collector;
The battery according to claim 15 , wherein the carbon nanotubes are oriented toward a connection point between the electrode terminal and the current collector.