JP7488208B2 - Secondary battery and method for manufacturing electrode for secondary battery - Google Patents

Description

The present invention relates to a secondary battery and a method for manufacturing electrodes for secondary batteries.

A secondary battery is constructed by placing a separator between a positive electrode and a negative electrode, winding or stacking them to form an electrode body, and storing an electrolyte in a battery case. The positive and negative electrodes used in secondary batteries have a current collector foil and an active material layer containing an active material formed on the surface of the current collector foil. Furthermore, a conductive material and a binder may be mixed into the active material layer. The gaps between the materials that make up the active material layer are filled with the electrolyte, allowing smooth ion diffusion at the interface between the active material and the electrolyte.

Lithium ion secondary batteries, which are one type of secondary battery, are used as power sources for driving electric vehicles and hybrid vehicles, which require high energy density and large battery capacity. Patent Document 1 discloses a positive electrode for a lithium secondary battery having a configuration in which a positive electrode active material layer containing a positive electrode active material is held on a positive electrode current collector, and a manufacturing method thereof, in which the positive electrode active material layer comprises carbon nanowalls formed on the positive electrode current collector and a positive electrode active material supported on the carbon nanowalls.

JP 2011-103255 A

In the above-mentioned lithium-ion secondary battery, the electrolyte permeates from the surface and periphery of the active material layer toward the center. It is known that the electrolyte does not easily permeate the center, which is downstream of the permeation path. For example, if a part of the active material layer is depleted of electrolyte, the resistance in that part increases and the battery performance decreases. Therefore, in such a secondary battery, it is preferable to properly secure the permeation path of the electrolyte in the active material layer so that ion diffusion is smooth at the interface between the active material and the electrolyte.

In the technology described in Patent Document 1, the carbon nanowalls formed on the positive electrode current collector maintain an appropriate amount of voids in the positive electrode active material layer, ensuring a permeation path for the electrolyte. However, the technology described in Patent Document 1 does not adequately control the direction of the electrolyte permeation path. As a result, the electrolyte does not permeate uniformly to the center, which can lead to an increase in resistance. As a result, there is a problem in that the battery performance cannot be satisfactorily demonstrated.

The present invention has been made to solve these problems, and aims to provide a secondary battery that improves the permeability of the electrolyte and reduces the internal resistance of the battery, and a method for manufacturing electrodes for the secondary battery.

The secondary battery according to one embodiment has a current collecting foil, thin carbon nanostructures oriented vertically on the surface of the current collecting foil, and an active material held between the carbon nanostructures facing each other on the surface of the current collecting foil, and the carbon nanostructures are arranged in parallel in the first direction, spaced apart from each other, while being oriented in the first direction along the surface of the current collecting foil.

In addition, a method for manufacturing an electrode for a secondary battery according to one embodiment includes the steps of forming a catalyst layer on the surface of a current collector foil, vapor-growth of vertically oriented flaky carbon nanostructures on the surface of the current collector foil starting from the catalyst layer, and forming an active material layer containing at least an active material on the current collector foil, in which the carbon nanostructures are spaced apart from each other and arranged in parallel in the first direction while being oriented in a first direction along the surface of the current collector foil.

The present invention provides a secondary battery that improves the permeability of the electrolyte and reduces the internal resistance of the battery, and a method for manufacturing electrodes for the secondary battery.

1 is a perspective view showing an example of a secondary battery according to a first embodiment; 2 is a perspective view showing a part of an electrode assembly of the secondary battery shown in FIG. 1. 3 is a diagram showing an example in which carbon nanostructures are oriented in a first direction in the secondary battery according to the first embodiment; FIG. 10 is a diagram showing another example in which carbon nanostructures are oriented in a first direction in the secondary battery according to the first embodiment. FIG. 3 is a diagram showing an example in which carbon nanostructures are oriented in a first direction and a second direction in the secondary battery according to the first embodiment; FIG. 1A and 1B are diagrams illustrating the configuration of a carbon nanostructure included in a secondary battery according to a first embodiment. 2 is a flowchart showing an example of a method for manufacturing a secondary battery electrode according to the first embodiment. FIG. 8 is a perspective view showing a catalyst layer forming step in the flowchart shown in FIG. 7 . 8A and 8B are perspective and plan views showing the vapor phase growth process of the flow chart shown in FIG. 7 . 8A and 8B are a perspective view and a plan view showing the active material layer forming step in the flowchart shown in FIG. 7 . 1 is a graph showing the results of comparing the IV resistance of secondary batteries of Example and Comparative Examples 1 and 2. FIG. 4 is a diagram showing the distribution of internal resistance in the secondary battery of Comparative Example 1.

First embodiment
Hereinafter, an embodiment of the present invention will be described with reference to the drawings. However, the present invention is not limited to the following embodiment. In order to clarify the description, the following description and drawings are appropriately simplified. In the drawings, the dimensional relationship of each component is appropriately changed for the convenience of the description, so the dimensional relationship shown in the drawings does not represent the actual dimensional relationship. In the following description, the same or equivalent elements are given the same reference numerals, and duplicated description will be omitted.

In the secondary battery 1 according to this embodiment, one of the features is the negative electrode plate 20 that constitutes the electrode body 15, so the negative electrode plate 20 will be described in particular detail. However, the present invention is also applicable to the structure of the positive electrode plate that constitutes the electrode body 15.

In this embodiment, the secondary battery 1 will be described as a lithium ion secondary battery. First, an overview of the secondary battery 1 according to this embodiment will be described with reference to FIG. 1. FIG. 1 is a perspective view showing an example of a secondary battery according to the first embodiment. FIG. 1 shows the external appearance of the secondary battery 1.

As shown in FIG. 1, the secondary battery 1 mainly has a battery case 10 consisting of a case body 11 and a lid 12, a negative terminal 13, and a positive terminal 14. A battery pack can be formed by combining multiple secondary batteries 1, and the secondary battery 1 can be one cell of the battery pack.

The case body 11 has a flat rectangular parallelepiped shape with an open top. The lid 12 has a rectangular plate shape and seals the opening of the case body 11. The lid 12 is provided with a liquid injection port and the like that is used when injecting an electrolyte into a battery case formed inside the battery case 10. The battery case 10 is made of, for example, aluminum or an alloy thereof. The electrolyte and the electrode body 15 are stored inside the battery case. The secondary battery 1 according to this embodiment is a lithium ion secondary battery that uses a nonaqueous electrolyte solution containing a supporting salt in an organic solvent as the electrolyte solution.

Examples of the organic solvent contained in the electrolyte include ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), _ethyl methyl carbonate (EMC), etc. Examples of the supporting salt include lithium salts such as _{LiPF6 , LiBF4} , LiN( _{SO2CF3 ) 2} , LiN _{( SO2C2F5 ) 2} , _LiCF3SO3 , _LiC4F9SO3 , LiC( _{SO2CF3 ) 3} , _{and LiClO4} .

The negative terminal 13 and the positive terminal 14 penetrate the lid 12 and are electrically connected to the electrode body 15 housed in the battery case. The negative terminal 13 and the positive terminal 14 are electrodes for inputting and outputting current to and from the electrode body 15. In addition, the negative terminal 13 and the positive terminal 14 each have a pole portion that protrudes outward from the battery case 10.

The electrode body 15 has a positive electrode plate, a negative electrode plate 20, and a separator. The electrode body 15 is a wound electrode body in which strip-shaped positive and negative electrode plates 20 are stacked with strip-shaped separators between them, wound in the length direction, and then compressed into a flat shape.

The positive electrode plate and the negative electrode plate 20 each have a current collector foil and an active material layer containing an active material. Each active material layer constituting the positive electrode plate and the negative electrode plate 20 is formed on at least one surface of each current collector foil along the length of the current collector foil. On one edge in the width direction of each current collector foil, no active material layer is formed, and an exposed portion is provided in which the current collector foil is exposed along the length of the current collector foil. In the electrode body 15, the exposed portion of the positive electrode plate is arranged on one side of the axial direction of the winding axis of the electrode body 15, and the exposed portion 23 of the negative electrode plate 20 is arranged on the other side. It is preferable that the axial length of the active material layer 22 in the negative electrode plate 20 is greater than the axial length of the active material layer in the positive electrode plate.

The positive electrode plate is electrically connected to the positive electrode terminal 14 via a positive electrode collector plate attached to the exposed portion of the positive electrode plate. The negative electrode plate 20 is electrically connected to the negative electrode terminal 13 via a negative electrode collector plate attached to the exposed portion 23 of the negative electrode plate 20.

The separator is a porous insulating sheet that allows lithium ions to pass through. The separator is made of an insulating resin material such as polyethylene (PE) or polypropylene (PP). The separator is arranged so as to insulate the active material layer of the positive electrode plate from the active material layer 22 of the negative electrode plate 20. It is preferable that the axial length of the separator is greater than the axial length of the active material layer 22 of the negative electrode plate 20.

The active material layer of the positive electrode plate, the active material layer 22 of the negative electrode plate 20, and the separator are impregnated with an electrolyte in the battery container. For example, in these active material layers, the electrolyte permeates the minute gaps between the materials that make up the active material, and lithium ions, which are charge carriers, are transferred between the active material and the electrolyte. The electricity stored in the positive electrode plate and the negative electrode plate 20 is collected by the positive electrode current collector plate and the negative electrode current collector plate, respectively, and output from the positive electrode terminal 14 and the negative electrode terminal 13.

The current collector foil constituting the positive electrode plate is a metal foil made of a long strip-shaped conductive material. For example, aluminum or an alloy thereof can be used for the current collector foil of the positive electrode plate. The active material layer constituting the positive electrode plate has at least an active material. The active material layer may contain a conductive material and a binder, etc.

The positive electrode active material is a material capable of absorbing and releasing lithium ions, and examples of the material that can be used include lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), and lithium manganese oxide (LiMn ₂ O ₄ ).

As the conductive material, for example, carbon black such as acetylene black, activated carbon, graphite, and other carbon materials can be used. As the binder, for example, polyvinylidene fluoride (PVdF), polyethylene oxide (PEO), styrene butadiene rubber (SBR), butyl rubber (BR), etc. can be used.

When forming such a positive electrode plate, the active material, conductive material, binder, and a solvent capable of uniformly dissolving or dispersing the binder are kneaded to obtain a paste. The obtained paste is then applied to the surface of a current collector foil, dried, and compressed to form a positive electrode plate.

Next, the detailed configuration of the negative electrode (negative electrode plate 20) will be described with reference to FIG. 2. FIG. 2 is a perspective view showing a part of the electrode body of the secondary battery shown in FIG. 1. In FIG. 2, only the negative electrode plate 20 of the electrode body 15 is shown, and the positive electrode plate and separator are not shown.

In the following description, the width direction of the current collecting foil 21 is the X direction, a direction perpendicular to the X direction, the length direction of the current collecting foil 21 is the Y direction, a direction perpendicular to the X and Y directions, and the Z direction is a direction perpendicular to the surface 21s (XY plane) of the current collecting foil 21.

In addition, in the electrode body 15, the X direction coincides with the axial direction of the electrode body 15, and the Y direction is along the winding direction in which the electrode body 15 is wound. The electrode body 15 is placed in the battery case so that the axial direction approximately coincides with the length direction of the battery case 10, and the height direction perpendicular to the axial direction of the electrode body 15 approximately coincides with the height direction of the battery case 10.

The negative electrode plate 20 has a current collector foil 21 and an active material layer 22. The current collector foil 21 is a metal foil made of a long strip-shaped conductive material. The current collector foil 21 can be made of, for example, copper or an alloy thereof. The active material layer 22 has at least a carbon nanostructure 30, a catalyst layer 31, and an active material 40. The active material layer 22 may contain a conductive material, a binder, etc.

As the conductive material, for example, the same carbon material as that of the positive electrode plate can be used. As the binder, for example, the same material as that of the positive electrode plate can be used. In addition, additives such as thickeners can be used as appropriate. As the thickener, for example, carboxymethylcellulose (CMC), methylcellulose (MC), etc. can be used.

The carbon nanostructure 30 has a microstructure in which carbon atoms are geometrically bonded. The carbon nanostructure 30 is a carbon material in which graphene sheets, consisting of hexagonal plate-shaped carbon crystals linked together in a single layer, grow vertically to the surface 21s of the current collecting foil 21 and spread out two-dimensionally. A known example of such a two-dimensional structure is the carbon nanowall, in which multiple graphene sheets are stacked and grown in a wall shape.

In this embodiment, the carbon nanostructure 30 is a carbon nanotube structure composed of an aggregate of multiple carbon nanotubes (hereinafter, CNTs). CNTs have a tube shape in which graphene sheets are rolled into a cylindrical shape, and are classified as one-dimensional structures. CNTs grow vertically from the surface 21s of the current collector foil 21 with their axial direction aligned vertically. The dimensions of the CNTs can be, for example, a diameter of several nm to several hundred nm, and a length of several μm or more. The CNTs may be single-layered or multi-layered. The carbon nanostructure 30 constructed from CNTs contributes to improving the electronic conductivity of the negative electrode plate 20.

The carbon nanostructures 30, which are aggregates of CNTs, are vertically oriented on the surface 21s of the current collector foil 21, starting from the catalyst layer 31 described below. The base ends of the carbon nanostructures 30 are in contact with the catalyst layer 31 or the current collector foil 21. Multiple carbon nanostructures 30 are arranged in a regular pattern on the surface 21s of the current collector foil 21.

The catalyst layer 31, which is the starting point of the carbon nanostructure 30, is composed of metal particles such as iron (Fe), nickel (Ni), cobalt (Co), molybdenum (Mo), or gold (Au). The catalyst layer 31 is formed in a regular pattern with multiple catalyst layers 31 corresponding to the number of carbon nanostructures 30. The metal particles that make up the catalyst layer 31 serve as nuclei for the vapor phase growth of the CNTs that make up the carbon nanostructure 30. Each catalyst layer 31 is located on the surface 21s of the current collecting foil 21, on the base end side of the corresponding carbon nanostructure 30.

On the surface 21s of the current collector foil 21, active material 40 is held between the opposing carbon nanostructures 30. The active material 40 is a material capable of absorbing and releasing lithium ions, and may be, for example, a carbon material such as natural graphite, artificial graphite, hard carbon, soft carbon, or activated carbon. There are no particular limitations on the shape or dimensions of the active material 40, but in this embodiment, flake, scale, or chunky natural graphite is preferably used as the active material 40 because it has excellent charge/discharge characteristics. Such graphite may also be used after being subjected to a granulation process.

Graphite is a laminated structure of multiple graphene sheets, each of which is a single layer of hexagonal carbon platelet crystals. The reversible insertion and desorption of lithium ions stored between the layers occurs through edge faces 40e (layered faces) where the edges of multiple overlapping graphite layers are exposed. On the other hand, the basal faces 40b (planes of the graphene sheets, benzene condensation faces) that occupy a relatively large area in graphite particles are generally not considered to contribute to the insertion and desorption of lithium ions. Therefore, when graphite particles are used as the active material 40, the reaction speed of the insertion and desorption of lithium ions from the edge faces 40e is one factor that limits the input/output characteristics of the secondary battery.

In this embodiment, a case where a scaly graphite particle having a flat disk shape among natural graphite is used as the active material 40 constituting the negative electrode plate 20 will be described. In this embodiment, the active material 40 made of graphite particles has an arbitrary aspect ratio, its major axis is 100 μm, its minor axis is a length equal to or shorter than the major axis, and its thickness perpendicular to the major axis and minor axis is 1.0 μm will be described as an example. In addition, the surface of the active material 40 that extends in the major axis direction is the basal surface 40b, and the edge surface 40e is formed in the direction perpendicular to the basal surface 40b (see FIG. 10). Note that in the drawings from FIG. 2 onwards, the active material 40 may be hatched to ensure visibility, but the hatching does not indicate a cross section.

Here, the arrangement pattern of the carbon nanostructures 30 will be described in detail with reference to Figures 3 to 5. Figure 3 is a diagram showing an example in which the carbon nanostructures are oriented in a first direction in a secondary battery according to the first embodiment. Figure 4 is a diagram showing another example in which the carbon nanostructures are oriented in a first direction in a secondary battery according to the first embodiment. Figure 5 is a diagram showing an example in which the carbon nanostructures are oriented in a first direction and a second direction in a secondary battery according to the first embodiment.

In addition, in the upper part of each of Figures 3 to 5, a plan view of the appearance of the current collector foil 21 with carbon nanostructures 30 formed on the surface 21s before the active material layer 22 is formed is shown, as viewed from above. Also, in the lower part of each of Figures 3 to 5, a perspective view of the appearance of the negative electrode plate 20 constituting the wound electrode body 15 after the active material layer 22 is formed is shown, as viewed from an oblique direction. And in Figures 3 to 5, the outlined arrows indicate the flow of the electrolyte.

First, the orientation of the carbon nanostructures 30 in the Z direction will be described. As shown in Figures 3 to 5, each carbon nanostructure 30 is vertically oriented on the surface 21s of the current collecting foil 21. That is, each carbon nanostructure 30 is oriented perpendicular to the axial direction (X direction) and winding direction (Y direction) of the electrode body 15. Below, the orientation of the carbon nanostructures 30 in the X direction and Y direction will be described in detail using three arrangement patterns as examples.

Figure 3 shows a first pattern as an example of an arrangement pattern of carbon nanostructures 30. In the first pattern, carbon nanostructures 30 oriented in the X direction are arranged two-dimensionally in a staggered pattern on the surface 21s of the current collecting foil 21. That is, in the first pattern, the X direction corresponds to the first direction.

Specifically, each carbon nanostructure 30 is oriented to extend in the X direction and has a flake shape extending in the XZ plane. A plurality of carbon nanostructures 30 oriented in the X direction are spaced apart from each other to form a row 30a arranged in parallel in the X direction at a predetermined interval. Furthermore, the rows 30a are spaced apart from each other in the Y direction, and a plurality of rows 30a are formed at a predetermined interval, and each row 30a extends in the X direction. The carbon nanostructures 30 adjacent and facing each other in the Y direction are arranged alternately so that one carbon nanostructure 30 constituting the adjacent row 30a is arranged at the midpoint between two carbon nanostructures 30 adjacent in the X direction. In other words, the three carbon nanostructures 30, namely the two carbon nanostructures 30 adjacent in the X direction and one carbon nanostructure 30 constituting the adjacent row 30a arranged at the midpoint between the two carbon nanostructures 30, are arranged at the vertices of an equilateral triangle or an isosceles triangle.

In a secondary battery 1 housing an electrode body 15 including a negative electrode plate 20 to which the first pattern is applied, the electrolyte flows through the gaps between adjacent rows 30a of the carbon nanostructures 30. In the first pattern, these gaps extend in the axial direction, so that the supply of electrolyte can be promoted from both ends, which are the periphery of the active material layer 22 in the axial direction, toward the center.

Next, FIG. 4 shows a second pattern as another example of the arrangement pattern of the carbon nanostructures 30. In the second pattern, the carbon nanostructures 30 oriented in the Y direction are arranged two-dimensionally in a staggered manner on the surface 21s of the current collecting foil 21. That is, in the second pattern, the Y direction corresponds to the first direction.

Specifically, each carbon nanostructure 30 is oriented to extend in the Y direction and has a flake shape extending in the YZ plane. A plurality of carbon nanostructures 30 oriented in the Y direction are spaced apart from each other to form a row 30b arranged in parallel in the Y direction at a predetermined interval. Furthermore, the rows 30b are spaced apart from each other in the X direction, and a plurality of rows 30b are formed at a predetermined interval, and each row 30b extends in the Y direction. The carbon nanostructures 30 adjacent and facing each other in the X direction are arranged alternately so that one carbon nanostructure 30 constituting the adjacent row 30b is arranged at the midpoint between two carbon nanostructures 30 adjacent in the Y direction. In other words, the three carbon nanostructures 30, namely the two carbon nanostructures 30 adjacent in the Y direction and the one carbon nanostructure 30 constituting the adjacent row 30b arranged at the midpoint between the two carbon nanostructures 30, are arranged at the vertices of an equilateral triangle or an isosceles triangle.

In a secondary battery 1 housing an electrode assembly 15 including a negative electrode plate 20 to which the second pattern is applied, the electrolyte flows through the gap between adjacent rows 30b of the carbon nanostructures 30. In the second pattern, this gap extends in the winding direction, so that the supply of electrolyte can be promoted from the lower end to the upper end of the active material layer 22 (electrode assembly 15) in the winding direction.

Next, FIG. 5 shows a third pattern as another example of the arrangement pattern of the carbon nanostructures 30. The third pattern is an arrangement pattern that combines the first pattern and the second pattern. In the third pattern, a region in which the carbon nanostructures 30 oriented in the X direction are two-dimensionally arranged in a staggered pattern and a different orientation region in which the carbon nanostructures 30 oriented in the Y direction are two-dimensionally arranged in a staggered pattern are formed on the surface 21s of the current collecting foil 21. That is, in the third pattern, one of the X direction and the Y direction corresponds to the first direction, and the other corresponds to the second direction.

In the third pattern, multiple rows 30b extending in the Y direction are arranged in the central portion 22a of the active material layer 22, and multiple rows 30a extending in the X direction are arranged in the remaining portion (non-central portion 22b) of the active material layer 22 excluding the central portion 22a. The central portion 22a is the central region of the active material layer 22 that includes the center of the active material layer 22 in the X direction, has a predetermined width in the X direction, and extends along the Y direction. In other words, the different orientation region is arranged in the central portion 22a.

In the central portion 22a, the rows 30b are spaced apart from each other in the X direction and are formed at a predetermined interval. The rows 30b adjacent in the X direction are staggered so that one carbon nanostructure 30 constituting the row 30b adjacent in the X direction is located at the midpoint between two carbon nanostructures 30 adjacent in the Y direction. In the non-central portion 22b, the rows 30a are spaced apart from each other in the Y direction and are formed at a predetermined interval. The rows 30a adjacent in the Y direction are staggered so that one carbon nanostructure 30 constituting the row 30a adjacent in the Y direction is located at the midpoint between two carbon nanostructures 30 adjacent in the X direction.

The rows 30a and 30b are spaced apart from each other. In order to ensure a gap between the rows 30a or between the rows 30b that serves as a flow path for the electrolyte to flow, it is preferable to provide at least two rows 30a and two rows 30b.

In the third pattern, the gaps between the rows 30a in the non-central portion 22b extend in the axial direction. Also, the gaps between the rows 30b in the central portion 22a extend in the winding direction. Therefore, in the non-central portion 22b, the electrolyte flows from both ends that are the periphery of the active material layer 22 in the axial direction toward the center, and in the central portion 22a, the electrolyte flows from the lower end to the upper end of the active material layer 22 (electrode body 15) in the winding direction. This promotes the supply of electrolyte to the central portion 22a and the upper end of the electrode body 15 (active material layer 22), improving the permeability of the electrolyte to the region of the central portion 22a and the upper end.

The first direction and the second direction are not limited to the X direction and the Y direction. The first direction and the second direction may be different directions along the surface 21s of the current collecting foil 21, and do not necessarily have to be perpendicular to each other, such as the X direction and the Y direction.

The orientation of the carbon nanostructures 30 has been described above using the above three arrangement patterns as examples. Next, the configuration of the carbon nanostructures 30, such as their dimensions, will be described with reference to FIG. 6. FIG. 6 is a diagram illustrating the configuration of the carbon nanostructures possessed by the secondary battery according to the first embodiment.

First, the distance between adjacent carbon nanostructures will be described. As shown in FIG. 6, multiple carbon nanostructures 30 oriented in the same direction are arranged in parallel in the same direction at a predetermined distance from each other. In carbon nanostructures 30 oriented in the same direction, the distance C between adjacent carbon nanostructures 30 in the same direction is preferably 20 μm or more and less than the major axis of the active material 40. If the distance C is less than 20 μm, it may hinder the flow of the electrolyte. If the distance C is greater than the major axis of the active material 40 (100 μm in this embodiment), it becomes difficult to hold the active material 40 between the carbon nanostructures 30.

In addition, the carbon nanostructures 30 oriented in the same direction and the carbon nanostructures 30 facing each other in a direction perpendicular to the carbon nanostructures 30 facing each other in the thickness T direction are opposed to each other. It is preferable that the distance L between the carbon nanostructures 30 facing each other in the thickness T direction is equal to or greater than the thickness of the active material 40. In this embodiment, the distance L is equal to or greater than 10 μm. If the distance L is smaller than the thickness of the active material 40, it becomes difficult to hold the active material 40 between the carbon nanostructures 30.

Furthermore, for example, when the carbon nanostructures 30 are oriented in two different directions, as in the third pattern, the spacing between adjacent carbon nanostructures 30 that are perpendicular to each other is preferably 20 μm or more and less than the major axis of the active material 40, similar to spacing C.

Next, the preferred dimensions of the individual carbon nanostructures 30 will be described. The individual carbon nanostructures 30 are formed in a thin flake shape and have a thickness T, a width W, and a height H. Note that the thickness T and the width W are the lengths of the carbon nanostructures 30 along the XY plane, and the height H is the length of the carbon nanostructures 30 along the Z direction.

The thickness T of the carbon nanostructure 30 is preferably 3 μm or more and 10 μm or less. If the thickness T is less than 3 μm, manufacturing may be difficult. On the other hand, if the thickness T is 3 μm or more, the arrangement pattern can be freely created using the inkjet printing method described below. If the thickness T is more than 10 μm, the volume occupancy rate of the carbon nanostructure 30 in the negative electrode plate 20 increases, which may result in a decrease in energy density.

The width W of the carbon nanostructure 30 is preferably equal to or greater than the major axis of the active material 40 and equal to or less than 200 μm. If the width W of the carbon nanostructure 30 is equal to or greater than the major axis of the active material 40 (100 μm in this embodiment), the active material 40 can be reliably held between the carbon nanostructures 30 facing each other in the thickness direction. If the width W exceeds 200 μm, the flow of the electrolyte in a direction perpendicular to the width direction may be impeded.

The height H of the carbon nanostructure 30 is preferably equal to or less than the major axis of the active material 40. In this embodiment, the height H is equal to or less than 100 μm. If the height H is higher than necessary, it may take a long time to produce the carbon nanostructure 30, resulting in reduced productivity. Furthermore, if the height H is higher than necessary, the thickness of the active material layer 22 may increase, which may result in an increased size of the secondary battery 1.

Furthermore, it is preferable that the interval L and the height H are both 1/√2 times or less of the major axis of the active material 40. In this embodiment, the interval L and the height H are 70 μm or less. When the interval L and the height H are 1/√2 times or more of the major axis of the active material 40, the angle between the basal surface 40b of the active material 40 held between the carbon nanostructures 30 and the carbon nanostructure 30 (Z direction) is 45° or less. By reducing the inclination of the basal surface 40b of the active material 40 with respect to the Z direction, the edge surface 40e of the active material 40 is exposed more to the electrolyte surface. When the edge surface 40e of the active material 40 is exposed more to the electrolyte surface, the lithium ion acceptance in the negative electrode is improved. This increases the reaction rate of the insertion and desorption of lithium ions from the edge surface 40e, improving the input/output characteristics of the secondary battery 1.

The density of the carbon nanostructure 30 is preferably 50 mg/cm ³ or more and 200 μg/cm ³ or less. If it is outside this range, manufacturing may become difficult.

A method for manufacturing an electrode for a secondary battery including the carbon nanostructure 30 having the above configuration will be described with reference to FIG. 7. FIG. 7 is a flowchart showing an example of a method for manufacturing an electrode for a secondary battery according to the first embodiment. In the following description, the method for manufacturing an electrode for a secondary battery according to this embodiment will be specifically described as a method for manufacturing the negative electrode plate 20 described above, but the method for manufacturing an electrode for a secondary battery according to this embodiment can also be applied to a positive electrode plate.

As shown in FIG. 7, the method for manufacturing an electrode for a secondary battery according to this embodiment includes the following steps S1 to S4. In the catalyst layer formation step of step S1, a catalyst layer 31 is formed on the surface 21s of the current collector foil 21. In the vapor phase growth step of step S2, vertically oriented flaky carbon nanostructures 30 are vapor-grown on the surface 21s of the current collector foil 21 starting from the catalyst layer 31. In the active material layer formation step of step S3, an active material layer 22 containing at least an active material 40 is formed on the surface 21s of the current collector foil 21.

The above steps will be described in more detail with reference to Figures 8 to 10. Figure 8 is a perspective view showing the catalyst layer formation step in the flowchart shown in Figure 7. Figure 9 is a perspective view and a plan view showing the vapor phase growth step in the flowchart shown in Figure 7. Figure 10 is a perspective view and a plan view showing the active material layer formation step in the flowchart shown in Figure 7. Note that in Figures 8 to 10, the perspective view shows the external appearance of the current collector foil 21 as seen from an oblique direction, and the plan view shows the external appearance of the current collector foil 21 as seen from above. Also, the active material 40 is omitted from the perspective view of Figure 10.

As shown in FIG. 8, first, in step S1, metal particles constituting the catalyst layer 31 are applied to at least one surface 21s of the current collecting foil 21. The application pattern for applying the metal particles is designed based on the regular arrangement pattern described above. For example, a printing method such as inkjet printing can be used to apply the metal particles.

When forming the catalyst layer 31 using the inkjet printing method, first, a catalyst ink is prepared. The catalyst ink is a dispersion liquid having the required viscosity that contains metal particles, an appropriate binder, and a solvent. The prepared catalyst ink is sprayed from the nozzle of an inkjet printer and adhered to the surface 21s of the current collector foil 21 based on a coating pattern that has been designed in advance on a computer. The catalyst ink adhered to the surface 21s of the current collector foil 21 can be dried to remove the solvent, thereby forming the catalyst layer 31 on the surface 21s of the current collector foil 21.

In this way, inkjet printing can be used to easily apply metal particles to the current collector foil 21, similar to the process of creating and printing an image using a computer and a printer. Therefore, inkjet printing can be used to precisely control the shape of the catalyst layer 31, making it possible to produce a very thin and uniform catalyst layer 31.

In addition, with the inkjet printing method, the application pattern can be freely designed on a computer, so that the catalyst layer 31 can be easily produced based on a desired arrangement pattern, such as the first pattern, the second pattern, or the third pattern described above.

Next, as shown in FIG. 9, in step S2, a carbon nanostructure 30, which is an aggregate of CNTs, is produced using a chemical vapor deposition (CVD) method. First, the current collector foil 21 having the catalyst layer 31 obtained in step S1 is placed in a reactor of a CVD device and heated to a required temperature. Gas containing a hydrocarbon gas as a carbon source is introduced into this reactor, and the reaction temperature is heated and maintained at several hundred to several thousand degrees Celsius. The heating causes benzene ring condensation of the hydrocarbon gas, and multiple CNTs grow in the vapor phase with the metal fine particles that constitute the catalyst layer 31 as nuclei. At this time, each CNT grows from the surface 21s of the current collector foil 21 toward the upper side in the Z direction. In each catalyst layer 31, a large number of CNTs grow in an aggregated state, forming a flaky carbon nanostructure 30.

The carbon nanostructures 30 thus formed are vertically oriented on the surface 21s of the current collector foil 21. The arrangement pattern of the carbon nanostructures 30 corresponds to the application pattern of the catalyst layer 31. The carbon nanostructures 30 thus formed are preferably arranged in parallel in a first direction with their widths oriented in a first direction along the surface 21s of the current collector foil 21, and spaced apart from each other. Furthermore, among the multiple carbon nanostructures 30 formed, there may be a different orientation region in which the carbon nanostructures 30 are arranged in parallel in a second direction with their widths oriented in a second direction different from the first direction, and spaced apart from each other. The carbon nanostructures are formed in a band-like range extending in the length direction of the current collector foil 21, excluding the exposed portion 23.

The shape (thickness T, width W, height H) of the carbon nanostructure 30 can be freely controlled by adjusting the shape of the catalyst layer 31 formed in step S1 and the reaction time of the vapor phase growth in step S2. In addition, by appropriately changing the application conditions of the metal microparticles and the CVD reaction conditions, it is possible to form carbon nanostructures 30 with any density. Furthermore, the arrangement pattern of the carbon nanostructures 30 corresponds to the application pattern of the catalyst layer 31 in step S1, and therefore can be freely controlled by patterning the catalyst layer 31 using an inkjet printing method.

Next, in step S3, an active material layer 22 containing at least an active material 40 is formed on the surface 21s of the current collector foil 21 to produce the negative electrode plate 20. The method for producing the negative electrode plate 20 in step S3 is not particularly limited, but for example, as shown in FIG. 10, a paste 41 containing at least the active material 40 and a solvent capable of uniformly dissolving or dispersing the binder is prepared. The paste 41 contains the active material 40 and a solvent, and is obtained by adding a conductive material, a binder, etc. as necessary and kneading them.

Then, the prepared paste 41 is applied to the surface 21s of the current collector foil 21 having the carbon nanostructures 30 obtained in step S2. The paste 41 is applied to a band-like area extending in the longitudinal direction of the current collector foil 21 excluding the exposed portion 23. On the surface 21s of the current collector foil 21 on which the paste 41 is applied, the active material 40 is filled in the gaps between the carbon nanostructures 30. Since the carbon nanostructures 30 are formed using a predetermined arrangement pattern, the active material 40 is filled in the gaps between the carbon nanostructures 30 so that the angle between the basal surface 40b of the active material 40 and the carbon nanostructures 30 (Z direction) is 45° or less.

Next, in step S4, the paste 41 applied on the surface 21s of the current collector foil 21 is dried to remove the solvent, and then compressed. In this manner, a negative electrode plate 20 is obtained in which an active material layer 22 is formed on the surface 21s of the current collector foil 21. The wound negative electrode plate 20 is, for example, as shown in FIG. 2. The active material 40 in the active material layer 22 is held between the carbon nanostructures 30 so that the angle between the basal surface 40b of the active material 40 and the carbon nanostructures 30 (Z direction) is 45° or less.

In this way, the carbon nanostructures 30 are formed in a regular arrangement pattern on the surface 21s of the current collecting foil 21, and active material 40 is filled between the formed carbon nanostructures 30, thereby producing a negative electrode plate 20 having an active material layer 22 in which active material 40 is held between the carbon nanostructures 30.

Next, secondary batteries for evaluation (Examples and Comparative Examples 1 and 2) were fabricated with the following negative electrode plates, and their performance was evaluated. Note that the Examples do not limit the present invention.

(Example)
A secondary battery (cell) according to the embodiment was fabricated using a negative electrode plate 20, a positive electrode plate, and a separator, which were manufactured according to the manufacturing method shown in the flow of Fig. 3. The secondary battery 1 of the embodiment includes the negative electrode plate 20 shown in Fig. 5, in which the third pattern is applied as the arrangement pattern of the carbon nanostructures 30.

In this negative electrode plate 20, the distance between adjacent carbon nanostructures 30 is 40 μm (C) and 50 μm (L), and the distance between adjacent carbon nanostructures 30 that are perpendicular to each other is 40 μm. The dimensions of each individual carbon nanostructure 30 are a thickness T of 5 μm, a width W of 150 μm, a height H of 80 μm, and a density of 100 mg/ ^cm3 .

(Comparative Example 1)
The secondary battery 100 of Comparative Example 1 includes a negative electrode plate in which no carbon nanostructures 30 are formed on the surface 21s of the current collecting foil 21. The secondary battery 100 of Comparative Example 1 has the same configuration as that of the Example, except that it does not include the carbon nanostructures 30.

(Comparative Example 2)
The secondary battery of Comparative Example 2 includes a negative electrode plate in which carbon nanostructures 30 are randomly formed on the surface 21s of the current collector foil 21. In this negative electrode plate, the carbon nanostructures 30 are vertically oriented on the surface 21s of the current collector foil 21 starting from the catalyst layer 31, but their orientation along the surface 21s of the current collector foil 21 is irregular. In addition, the dimensions of the individual carbon nanostructures 30 are the same as in the Example, but the distance between adjacent carbon nanostructures 30 is not specified. The other configurations of the secondary battery of Comparative Example 2 are the same as in the Example.

The results of measuring the IV resistance for each of the secondary batteries of the above Example and Comparative Examples 1 and 2 are shown in Figure 11. Figure 11 is a graph showing the results of comparing the IV resistance of the secondary batteries of the Example and Comparative Examples 1 and 2. The vertical axis of Figure 11 shows the IV resistance as a relative value, with the resistance value of the secondary battery 100 of Comparative Example 1 set to 100%. The IV resistance was measured by dividing the voltage drop amount when discharging under specified conditions by the discharge current value.

As shown in FIG. 11, when the IV resistance of the secondary battery 100 of Comparative Example 1 is taken as 100%, the IV resistance of Comparative Example 2 was reduced by 5%, and that of the Example was reduced by 9%. As can be seen from these results, it was confirmed that the IV resistance of the secondary battery (Example, Comparative Example 2) having a negative electrode plate in which vertically oriented carbon nanostructures 30 are formed on the surface 21s of the current collector foil 21 is lower than that of the secondary battery 100 (Comparative Example 1) having a negative electrode plate without carbon nanostructures 30. This is thought to be because, in the secondary batteries of the Example and Comparative Example 2, the carbon nanostructures 30 oriented in the Z direction are formed, which improves the flow of the electrolyte in the Z direction and improves the permeability of the electrolyte from the surface of the active material layer 22 to the inside.

Furthermore, when comparing the secondary battery 1 of the embodiment with the secondary battery of the comparative example 2, it was found that the effect of reducing the IV resistance was greater in the case of the secondary battery 1 of the embodiment, in which the carbon nanostructures 30 were regularly oriented in the direction along the surface 21s of the current collecting foil 21. The IV resistance of the secondary battery 1 of the embodiment was 4% lower than the IV resistance of the secondary battery of the comparative example 2. Therefore, it was confirmed that, of these three types of secondary batteries, the secondary battery 1 of the embodiment, which has carbon nanostructures 30 regularly oriented in the X, Y, and Z directions, has the most excellent battery performance.

Here, with reference to FIG. 12, the distribution of internal resistance of a secondary battery having a wound electrode body will be described using a secondary battery 100 of Comparative Example 1 as an example. FIG. 12 is a diagram showing the distribution of internal resistance in the secondary battery of Comparative Example 1. The secondary battery 100 shown in FIG. 12 is the secondary battery 100 of Comparative Example 1 in which an electrode body including a negative electrode plate without carbon nanostructures 30 is housed in a battery case formed inside a battery case 10 consisting of a case body 11 and a lid 12.

In such a secondary battery 100, the electrolyte impregnated inside the electrode body is discharged from both ends of the electrode body in the axial direction during discharge due to the expansion and contraction of the active material layer (active material) when charging and discharging at a high rate. If the electrolyte discharged to the outside of the electrode body does not impregnate (permeate) again into the electrode body during the next charge, there is a possibility that the electrolyte will become non-uniform inside the electrode body or that some parts will be depleted of electrolyte.

If the electrolyte becomes non-uniform, the distribution of salt concentration becomes non-uniform, and the current density flowing between the positive and negative plates becomes uneven. Also, if there are areas where the electrolyte is depleted, lithium ions, which are charge carriers, cannot be supplied to the charge-discharge reaction inside the electrode body. Due to these factors, lithium may be precipitated in the negative plate where the current is concentrated, causing capacity degradation. On the other hand, in areas where the current does not flow easily, the internal resistance of the battery increases. As a result, the battery performance of the secondary battery decreases. Also, if the electrolyte does not penetrate sufficiently into the active material layer during the injection process when manufacturing such a secondary battery, the charge-discharge reaction becomes more non-uniform. For this reason, the permeability of the electrolyte affects the battery performance.

The electrolyte permeates from the surface and periphery of the active material layer toward the center of the active material layer. In a secondary battery equipped with a flat wound electrode body, the electrolyte does not easily permeate the central part of the active material layer, which is downstream of the electrolyte permeation path in the axial direction. In addition, the electrolyte is likely to decrease at the upper end of the upper end side of the electrode body due to the influence of gravity. Therefore, as shown in FIG. 12, in the secondary battery 100 of Comparative Example 1, the electrolyte permeability is particularly poor in the central part and upper end of the secondary battery 100 (battery case), and the electrolyte is easily depleted in these parts, causing the internal resistance of the battery to increase.

In contrast, in the secondary battery 1 of the embodiment, the negative electrode plate 20 has a carbon nanostructure 30 formed by applying the third pattern. This ensures a permeation path along which the electrolyte flows from the surface of the active material layer 22 toward the inside along the Z direction. In addition, a permeation path along which the electrolyte flows from both ends of the active material layer 22 toward the central portion 22a along the X direction, and a permeation path along which the electrolyte flows from the lower end to the upper end of the active material layer 22 (electrode body 15) along the Y direction are ensured.

In this way, in the secondary battery 1 of the embodiment, the permeation path of the electrolyte is controlled, and the permeability of the electrolyte to the central portion 22a and the upper end portion of the secondary battery 1 (battery case), where the electrolyte is likely to run out, is improved. As a result, in the secondary battery 1 of the embodiment, the electrolyte permeates throughout the entire electrode body 15, and the uniformity of the charge and discharge reaction is increased. As a result, compared to the secondary batteries of Comparative Examples 1 and 2, the increase in the internal resistance of the battery is suppressed, especially in the central portion 22a and the upper end portion of the battery case.

In addition, in the secondary battery 1 of the embodiment, the active material 40 contained in the negative electrode plate 20 is held between the carbon nanostructures 30 with the edge surface 40e more exposed to the electrolyte surface. This improves the lithium ion acceptance at the edge surface 40e, and increases the reaction rate of the insertion and desorption of lithium ions, thereby improving the input/output characteristics of the secondary battery 1. Furthermore, since the carbon nanostructure 30 is composed of CNTs with high electronic conductivity, in the secondary battery 1 of the embodiment, electrons move smoothly between the active materials 40 and/or between the active material 40 and the current collector (current collector foil 21).

Furthermore, the carbon nanostructures 30 constructed from CNTs are formed as discontinuous flake-like structures, compared to, for example, carbon nanowalls, which have a continuous wall-like structure. With this configuration, it is possible to form flow paths through the gaps between the carbon nanostructures 30 for the electrolyte to flow smoothly. This improves the flow of electrolyte in the active material layer 22, and suppresses a decrease in the permeability of the electrolyte. Therefore, in the secondary battery 1 of the embodiment, ion diffusion between the active materials 40 and/or at the interface between the active material 40 and the electrolyte is smoothly performed.

The present invention is not limited to the above embodiment, and can be modified as appropriate without departing from the spirit of the invention. For example, the various materials constituting the secondary battery 1 are not limited to those listed in the above embodiment.

In addition, in the above embodiment, the carbon nanostructures 30 are oriented in the X and Y directions along the surface 21s of the current collecting foil 21, but the orientation and arrangement pattern of the carbon nanostructures 30 are not limited to this. By appropriately combining carbon nanostructures 30 oriented in the desired direction, it is possible to control the permeation path of the electrolyte and locally improve the deteriorated flow of the electrolyte.

In the above embodiment, a case has been described in which flake-shaped graphite particles are used as the active material 40. However, the type, shape, and dimensions of the active material 40 are not limited to this, and the arrangement pattern and dimensions of the carbon nanostructures 30 can be appropriately designed according to the active material 40 used.

Furthermore, in the above embodiment, the secondary battery 1 having a flat wound electrode body has been described, but the secondary battery 1 can also be configured as a secondary battery having a stacked electrode body in which multiple plate-shaped positive and negative electrode plates 20 are stacked with separators between them.

1, 100 Secondary battery 10 Battery case 11 Case body 12 Lid 13 Negative electrode terminal 14 Positive electrode terminal 15 Electrode body 20 Negative electrode plate 21 Current collector foil 21s Surface 22 Active material layer 22a Central portion 22b Non-central portion 23 Exposed portion 30 Carbon nanostructures 30a, 30b Row 31 Catalyst layer 40 Active material 40b Basal surface 40e Edge surface 41 Paste C Spacing H Height L Spacing T Thickness W Width

Claims (9)

A current collecting foil;
A flaky carbon nanostructure vertically oriented on a surface of the current collecting foil;
an active material held between carbon nanostructures facing each other on a surface of the current collector foil;
having
the carbon nanostructures are arranged in parallel in a first direction along a surface of the current collecting foil while being spaced apart from each other and oriented in the first direction;
a different orientation region in which the carbon nanostructures are oriented in a second direction different from the first direction and are spaced apart from each other and arranged in parallel in the second direction;
Secondary battery. The first direction is one of the width direction and the length direction of the current collecting foil.
The secondary battery according to claim 1 . The second direction is the other of the width direction and the length direction of the current collecting foil.
The secondary battery according to claim 1 or 2 . When the first direction is the width direction of the current collecting foil and the second direction is the length direction of the current collecting foil,
the different orientation region is disposed in a central portion including a center of an active material layer containing the active material in a width direction of the current collector foil.
The secondary battery according to claim 1 . When the direction perpendicular to the surface of the current collector foil is defined as the vertical direction and the active material has a disk shape,
The vertical height of the carbon nanostructure is 1/√2 times or more the major axis of the active material,
The distance between the opposing carbon nanostructures on the surface of the current collecting foil is 1/√2 times or less of the major axis of the active material.
The secondary battery according to claim 1 . The distance between adjacent carbon nanostructures oriented in the same direction is equal to or less than the major axis of the active material having a disk shape.
The secondary battery according to claim 1 . The carbon nanostructure is a carbon nanotube structure.
The secondary battery according to claim 1 . forming a catalyst layer on a surface of a current collector foil;
A step of vapor-growing vertically oriented flake-like carbon nanostructures on the surface of the current collector foil starting from the catalyst layer;
forming an active material layer containing at least an active material on a surface of the current collector foil;
having
the carbon nanostructures are arranged in parallel in a first direction along a surface of the current collecting foil while being spaced apart from each other and oriented in the first direction;
The carbon nanostructures have different orientation regions in which the carbon nanostructures are oriented in a second direction different from the first direction and are spaced apart from each other and arranged in parallel in the second direction.
A method for manufacturing an electrode for a secondary battery. The formation of the catalyst layer is
This is done using an inkjet printing method.
The method for producing the secondary battery electrode according to claim 8 .

Images