Secondary battery

Abstract

To provide a battery that does not cause an internal short circuit.SOLUTION: In a secondary battery in which an electrode wound body having a structure in which a strip-shaped first electrode and a second electrode having conductivity different from that of the strip-shaped first electrode are laminated and wound with a separator interposed therebetween, and a first electrode current collector plate are housed in a battery can, the first electrode has a first conductive active material coating part coated with a first conductive active material and a first conductive active material non-coating part on a strip-shaped first electrode foil. The first conductive active material non-covering part is joined to the first electrode current collector plate at one end part of the electrode wound body. The electrode wound body is bent toward a central axis of the structure in which the first conductive active material non-covering portion is wound. The first electrode has a first electrode notch part at one end in a short direction of the first electrode on a winding start side of the electrode wound body. An end of the first electrode notch part is present in the first conductive active material non-covered part, and a length of the first electrode notch part along a longitudinal direction of the first electrode is equal to or longer than one circumference of an inner peripheral part of the electrode wound body.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to a secondary battery. [Background technology]

[0002] Lithium-ion batteries are also being developed for applications requiring high power output, such as power tools and automobiles. One method of achieving high power output is high-rate discharge, which involves passing a relatively large current through the battery. High-rate discharge batteries have a structure in which the uncoated portion of the active material on the end face of the electrode winding is folded and a current collector is welded to it.

[0003] For example, Patent Document 1 describes a technique in which an active material non-coated portion is formed on a portion of a strip-shaped positive electrode or negative electrode, the active material non-coated portion is bent by pressing to make the end portion flat, and a current collector plate is welded to the portion. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-294222 Summary of the Invention [Problem to be solved by the invention]

[0005] In the technology of Patent Document 1, since the non-covered portions of the active material are densely packed in the inner periphery of the electrode winding body, there is a problem in that the stress increases due to pressing, and the positive electrode or negative electrode in the inner periphery of the electrode winding body deforms, causing the active material to peel off and potentially causing an internal short circuit.

[0006] Therefore, one of the objects of the present invention is to provide a battery that does not cause internal short circuits. [Means for solving the problem]

[0007] In order to solve the above-described problems, the present invention provides a secondary battery including an electrode winding body having a structure in which a strip-shaped first electrode and a strip-shaped second electrode having a conductivity different from that of the first electrode are stacked and wound with a separator interposed therebetween, and a first electrode current collector plate housed in a battery can, the first electrode has a first conductive active material-coated portion coated with a first conductive active material and a first conductive active material-uncoated portion on a strip-shaped first electrode foil, the first conductive active material uncoated portion is joined to the first electrode current collector at one end of the electrode winding body, The electrode wound body has a first conductive active material uncoated portion bent toward a central axis of the wound structure, the first electrode has a first electrode notch at one end in a lateral direction of the first electrode, which is on the winding start side of the electrode winding body; an edge of the first electrode cutout portion is present in the first conductive active material uncoated portion, In this secondary battery, the length of the first electrode cutout along the longitudinal direction of the first electrode is equal to or longer than one turn around the inner periphery of the electrode winding body. [Effects of the Invention]

[0008] At least one embodiment of the present invention provides a battery capable of high-power discharge without causing an internal short circuit. However, the effects exemplified in this specification should not be construed as limiting the scope of the present invention. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a cross-sectional view of a battery according to one embodiment. [Figure 2] FIG. 2 is a diagram illustrating an example of the relative positions of the positive electrode, negative electrode, and separator in the wound electrode body. [Figure 3] FIG. 3A is a plan view of a positive electrode current collector plate, and FIG. 3B is a plan view of a negative electrode current collector plate. [Figure 4] 4A to 4F are diagrams illustrating the assembly process of a battery according to one embodiment. [Figure 5] 5A and 5B are diagrams for explaining the cutout portion of the positive electrode and the cutout portion of the negative electrode. [Figure 6] FIG. 6A is a diagram for explaining Examples 1 to 5, and FIG. 6B is a diagram for explaining Comparative Examples 2 and 3. As shown in FIG. [Figure 7] FIG. 7A is a diagram for explaining Examples 1 to 5, and FIG. 7B is a diagram for explaining Comparative Example 1. [Figure 8] FIG. 8A is a diagram for explaining Examples 11 to 15, and FIG. 8B is a diagram for explaining Comparative Examples 12 and 13. [Figure 9] 9A to 9E are diagrams for explaining modified examples. [Figure 10] FIG. 10A is an enlarged cross-sectional view of a battery with an insulating plate on the negative electrode side, and FIG. 10B is an enlarged cross-sectional view of a battery with no insulating plate on the negative electrode side. [Figure 11] FIG. 11 is a graph showing the temperature change of the battery during the high current load test. [Figure 12] FIG. 12 is a connection diagram used to explain a battery pack as an application example of the present invention. [Figure 13] FIG. 13 is a connection diagram used to explain a power tool as an application example of the present invention. [Figure 14] FIG. 14 is a connection diagram used to explain an electric vehicle as an application example of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of the present invention will be described with reference to the drawings. The description will be made in the following order. <1. One embodiment> <2. Modifications> <3. Application Examples> The following embodiments are preferred examples of the present invention, and the present invention is not limited to these embodiments.

[0011] In the embodiments of the present invention, a cylindrical lithium ion battery will be described as an example of a secondary battery.

[0012] <1. One embodiment> First, the overall configuration of a lithium-ion battery will be described. Fig. 1 is a schematic cross-sectional view of a lithium-ion battery 1. The lithium-ion battery 1 is, for example, a cylindrical lithium-ion battery 1 in which an electrode winding body 20 is housed inside a battery can 11, as shown in Fig. 1.

[0013] Specifically, the lithium ion battery 1 includes, for example, a pair of insulating plates 12 and 13 and an electrode winding body 20 inside a cylindrical battery can 11. However, the lithium ion battery 1 may further include, for example, one or more of a thermosensitive resistor (PTC) element and a reinforcing member inside the battery can 11.

[0014] [Battery can] The battery can 11 is a member that mainly houses the electrode winding body 20. The battery can 11 is, for example, a cylindrical container with one end face open and the other end face closed. That is, the battery can 11 has one end face (open end face 11N) that is open. The battery can 11 contains, for example, one or more types of metal materials such as iron, aluminum, and alloys thereof. However, the surface of the battery can 11 may be plated with, for example, one or more types of metal materials such as nickel.

[0015] [Insulating plate] The insulating plates 12 and 13 are dish-shaped plates having surfaces that are approximately perpendicular to the winding axis (Z-axis in FIG. 1) of the electrode winding body 20. The insulating plates 12 and 13 are arranged, for example, to sandwich the electrode winding body 20 between them.

[0016] [Crimped structure] A battery lid 14 and a safety valve mechanism 30 are crimped to an open end surface 11N of the battery can 11 via a gasket 15, forming a crimp structure 11R. This keeps the battery can 11 airtight when the electrode wound body 20 and other components are housed inside the battery can 11.

[0017] [Battery cover] The battery lid 14 is a member that mainly closes the open end surface 11N of the battery can 11 when the electrode winding body 20 and the like are housed inside the battery can 11. The battery lid 14 contains, for example, the same material as the material from which the battery can 11 is formed. The central region of the battery lid 14 protrudes, for example, in the +Z direction. As a result, the region of the battery lid 14 other than the central region (peripheral region) comes into contact with, for example, the safety valve mechanism 30.

[0018] [gasket] The gasket 15 is a member that is mainly interposed between the battery can 11 (folded portion 11P) and the battery lid 14, thereby sealing the gap between the folded portion 11P and the battery lid 14. However, the surface of the gasket 15 may be coated with, for example, asphalt.

[0019] The gasket 15 includes, for example, one or more types of insulating materials. The type of insulating material is not particularly limited, but examples include polymeric materials such as polybutylene terephthalate (PBT) and polypropylene (PP). Among these, polybutylene terephthalate is preferable as the insulating material. This is because the gap between the folded portion 11P and the battery lid 14 is sufficiently sealed while electrically isolating the battery can 11 and the battery lid 14 from each other.

[0020] [Safety valve mechanism] The safety valve mechanism 30 mainly releases the internal pressure of the battery can 11 when the pressure inside the battery can 11 (internal pressure) increases, by releasing the sealed state of the battery can 11 as necessary. The increase in the internal pressure of the battery can 11 is caused, for example, by gas generated due to a decomposition reaction of the electrolyte solution during charging and discharging.

[0021] [Electrode winding body] In a cylindrical lithium-ion battery, a strip-shaped positive electrode 21 and a strip-shaped negative electrode 22 are spirally wound with a separator 23 sandwiched between them, and are impregnated with an electrolyte and housed in a battery can 11. The positive electrode 21 is a positive electrode foil 21A with a positive electrode active material layer formed on one or both sides thereof, and the positive electrode foil 21A is made of a metal foil made of, for example, aluminum or an aluminum alloy. The negative electrode 22 is a negative electrode foil 22A with a negative electrode active material layer formed on one or both sides thereof, and the negative electrode foil 22A is made of a metal foil made of, for example, nickel, a nickel alloy, copper, or a copper alloy. The separator 23 is a porous, insulating film that electrically insulates the positive electrode 21 from the negative electrode 22 while allowing the movement of substances such as ions and the electrolyte.

[0022] The positive electrode active material layer and the negative electrode active material layer cover a large portion of the positive electrode foil 21A and the negative electrode foil 22A, respectively, but intentionally leave one end periphery in the band direction uncoated. Hereinafter, the portions not covered with the active material layer will be referred to as active material uncoated portions 21C and 22C, and the portions covered with the active material layer will be referred to as active material coated portions 21B and 22B, as appropriate. In a cylindrical battery, the electrode winding body 20 is wound by stacking the positive electrode active material uncoated portion 21C and the negative electrode active material uncoated portion 22C with the separator 23 interposed between them, so that they face in opposite directions.

[0023] FIG. 2 shows an example of a structure in which the positive electrode 21, the negative electrode 22, and the separator 23 are stacked before being wound. The width of the active material uncoated portion 21C of the positive electrode (the upper dotted portion in FIG. 2) is A, and the width of the active material uncoated portion 22C of the negative electrode (the lower dotted portion in FIG. 2) is B. In one embodiment, it is preferable that A>B, for example, A=7 mm and B=4 mm. The length of the portion of the active material uncoated portion 21C of the positive electrode that protrudes from one end of the separator 23 in the width direction is C, and the length of the portion of the active material uncoated portion 22C of the negative electrode that protrudes from the other end of the separator 23 in the width direction is D. In one embodiment, it is preferable that C>D, for example, C=4.5 mm and D=3 mm.

[0024] The positive electrode active material uncoated portion 21C is made of, for example, aluminum, and the negative electrode active material uncoated portion 22C is made of, for example, copper. Therefore, the positive electrode active material uncoated portion 21C is generally softer (has a lower Young's modulus) than the negative electrode active material uncoated portion 22C. Therefore, in one embodiment, A > B and C > D are more preferable. In this case, when the positive electrode active material uncoated portion 21C and the negative electrode active material uncoated portion 22C are bent simultaneously with the same pressure from both electrode sides, the heights of the bent portions measured from the tip of the separator 23 may be approximately the same for the positive electrode 21 and the negative electrode 22. In this case, the active material uncoated portions 21C, 22C are bent to a moderate overlap, facilitating laser welding of the active material uncoated portions 21C, 22C to the current collector plates 24, 25. In one embodiment, "joining" refers to joining by laser welding, but the joining method is not limited to laser welding.

[0025] The positive electrode 21 is coated with an insulating layer 101 (the gray area in FIG. 2 ) in a 3 mm-wide section including the boundary between the active material uncoated portion 21C and the active material coated portion 21B. The entire area of ​​the positive electrode active material uncoated portion 21C facing the negative electrode active material coated portion 22B via the separator is covered with the insulating layer 101. The insulating layer 101 has the effect of reliably preventing an internal short circuit in the battery 1 when a foreign object enters between the negative electrode active material coated portion 22B and the positive electrode active material uncoated portion 21C. Furthermore, when an impact is applied to the battery 1, the insulating layer 101 has the effect of absorbing the impact and reliably preventing the positive electrode active material uncoated portion 21C from bending or from short-circuiting with the negative electrode 22.

[0026] A through-hole 26 is formed in the central axis of the electrode winding body 20. The through-hole 26 is a hole for inserting a winding core for assembling the electrode winding body 20 and an electrode rod for welding. The electrode winding body 20 is wound such that the positive electrode active material uncoated portion 21C and the negative electrode active material uncoated portion 22C face in opposite directions. Therefore, the positive electrode active material uncoated portion 21C is gathered at one end face (end face 41) of the electrode winding body, and the negative electrode active material uncoated portion 22C is gathered at the other end face (end face 42) of the electrode winding body 20. To improve contact with current collector plates 24 and 25 for extracting current, the active material uncoated portions 21C and 22C are bent, and the end faces 41 and 42 are flat. The bending direction is from outer edge portions 27, 28 of end faces 41, 42 toward through hole 26, and when wound, the active material uncoated portions of adjacent circumferences overlap and bend. In this specification, the term "flat surface" does not only refer to a completely flat surface, but also includes a surface that has some unevenness or surface roughness to the extent that the active material uncoated portions and the current collector plate can be joined.

[0027] At first glance, folding the active material-uncovered portions 21C and 22C so that they overlap each other may appear to flatten the end faces 41 and 42. However, without any processing prior to folding, wrinkles and voids (gaps or spaces) will form on the end faces 41 and 42, preventing the end faces 41 and 42 from becoming flat. Here, "wrinkles" and "voids" refer to unevenness in the folded active material-uncovered portions 21C and 22C, preventing the end faces 41 and 42 from becoming flat. To prevent wrinkles and voids from forming, grooves 43 (see, for example, FIG. 4B ) are formed in advance in the radial direction from the through-hole 26. The grooves 43 extend from the outer edges 27 and 28 of the end faces 41 and 42 to the through-hole 26. The electrode winding 20 has a through-hole 26 at its center, which is used as a hole for inserting a welding tool during the assembly process of the lithium-ion battery 1. The active material uncoated portions 21C, 22C at the start of winding the positive electrode 21 and the negative electrode 22 near the through-hole 26 have notches. This is to prevent the through-hole 26 from being blocked when the positive electrode 21 and the negative electrode 22 are bent toward the through-hole 26. The grooves 43 remain in the flat surfaces even after the active material uncoated portions 21C, 22C are bent, and the portions without the grooves 43 are joined (by welding or the like) to the positive electrode current collector 24 or the negative electrode current collector 25. Note that the grooves 43 may be joined to parts of the current collectors 24, 25, in addition to the flat surfaces. The detailed configuration of the electrode winding body 20, that is, the detailed configurations of the positive electrode 21, the negative electrode 22, the separator 23, and the electrolyte will be described later.

[0028] [Current collector plate] In a typical lithium-ion battery, for example, a lead for current extraction is welded to one point on each of the positive and negative electrodes. However, this increases the battery's internal resistance, and the lithium-ion battery generates heat and reaches high temperatures during discharge, making it unsuitable for high-rate discharge. Therefore, in one embodiment of the lithium-ion battery, a positive electrode current collector 24 and a negative electrode current collector 25 are disposed on end faces 41 and 42, and are welded at multiple points to the positive and negative electrode active material-uncoated portions 21C and 22C present on end faces 41 and 42, thereby keeping the battery's internal resistance low. The flat, curved surfaces of end faces 41 and 42 also contribute to the low resistance.

[0029] 3A and 3B show examples of current collector plates. FIG. 3A shows positive current collector plate 24, and FIG. 3B shows negative current collector plate 25. Positive current collector plate 24 is made of, for example, a metal plate made of aluminum or an aluminum alloy, or a composite material, and negative current collector plate 25 is made of, for example, nickel, a nickel alloy, copper, or a copper alloy, or a composite material. As shown in FIG. 3A, positive current collector 24 has a shape in which a rectangular strip portion 32 is attached to a flat, fan-shaped plate portion 31. A hole 35 is formed near the center of plate portion 31, and the position of hole 35 corresponds to that of through-hole 26.

[0030] The dotted areas in Figure 3A are insulating areas 32A, where insulating tape is attached to strip-shaped portion 32 or an insulating material is applied, and the area below the dotted areas in the drawing is connecting portion 32B to the sealing plate, which also serves as an external terminal. Note that in the case of a battery structure in which through-hole 26 does not have a metal center pin (not shown), strip-shaped portion 32 is unlikely to come into contact with a portion at the negative electrode potential, and therefore insulating portion 32A may not be necessary. In that case, the width between positive electrode 21 and negative electrode 22 can be increased by an amount corresponding to the thickness of insulating portion 32A to increase the charge / discharge capacity.

[0031] The shape of the negative current collector 25 is almost the same as that of the positive current collector 24, but the strip portion is different. The strip portion 34 of the negative current collector in FIG. 3B is shorter than the strip portion 32 of the positive current collector and does not have a portion corresponding to the insulating portion 32A. The strip portion 34 has a round projection 37 indicated by multiple circles. During resistance welding, current concentrates at the projection, melting the projection and welding the strip portion 34 to the bottom of the battery can 11. Like the positive current collector 24, the negative current collector 25 has a hole 36 near the center of the plate portion 33, which is located at a position corresponding to the through-hole 26. The plate portion 31 of the positive current collector 24 and the plate portion 33 of the negative current collector 25 are fan-shaped, so that they cover portions of the end faces 41 and 42. The reason for not covering the entire electrode is to allow the electrolyte to smoothly penetrate into the electrode winding when assembling the battery, and to make it easier for gas generated when the battery is in an abnormally high temperature state or overcharged state to be released outside the battery.

[0032] [Positive electrode] The positive electrode active material layer contains at least a positive electrode material (positive electrode active material) capable of absorbing and releasing lithium, and may further contain a positive electrode binder, a positive electrode conductive agent, etc. The positive electrode material is preferably a lithium-containing composite oxide or a lithium-containing phosphate compound. The lithium-containing composite oxide has, for example, a layered rock salt type or a spinel type crystal structure. The lithium-containing phosphate compound has, for example, an olivine type crystal structure.

[0033] The positive electrode binder contains synthetic rubber or a polymer compound. The synthetic rubber is styrene butadiene rubber, fluorine-based rubber, ethylene propylene diene, etc. The polymer compound is polyvinylidene fluoride (PVdF), polyimide, etc.

[0034] The positive electrode conductive agent is a carbon material such as graphite, carbon black, acetylene black, or ketjen black, but may also be a metal material or a conductive polymer.

[0035] The thickness of the positive electrode foil 21A is preferably 5 μm or more and 20 μm or less. This is because, by making the thickness of the positive electrode foil 21A 5 μm or more, the positive electrode 21 can be manufactured without breaking when the positive electrode 21, the negative electrode 22, and the separator 23 are stacked and wound. By making the thickness of the positive electrode foil 21A 20 μm or less, a decrease in the energy density of the battery 1 can be prevented, and the opposing area between the positive electrode 21 and the negative electrode 22 can be increased, resulting in a battery 1 with high output.

[0036] [Negative electrode] The surface of the negative electrode foil 22A is preferably roughened to improve adhesion to the negative electrode active material layer. The negative electrode active material layer contains at least a negative electrode material (negative electrode active material) capable of absorbing and releasing lithium, and may further contain a negative electrode binder, a negative electrode conductive agent, etc.

[0037] The negative electrode material includes, for example, a carbon material. The carbon material is graphitizable carbon, non-graphitizable carbon, graphite, low-crystalline carbon, or amorphous carbon. The shape of the carbon material has a fibrous, spherical, granular or flaky shape.

[0038] Also, the negative electrode material includes, for example, a metal-based material. Examples of the metal-based material include Li (lithium), Si (silicon), Sn (tin), Al (aluminum), Zr (zinc), Ti (titanium). The metal-based element forms a compound, mixture or alloy with other elements, and examples thereof include silicon oxide (SiOx (0 < x ≤ 2)), silicon carbide (SiC) or an alloy of carbon and silicon, and lithium titanate (LTO).

[0039] The thickness of the negative electrode foil 22A is preferably 5 μm or more and 20 μm or less. By setting the thickness of the negative electrode foil 22A to 5 μm or more, it becomes possible to manufacture the negative electrode 22 without breakage when the positive electrode 21, the negative electrode 22 and the separator 23 are overlapped and wound. By setting the thickness of the negative electrode foil 22A to 20 μm or less, it is possible to prevent a decrease in the energy density of the battery 1, and the facing area between the positive electrode 21 and the negative electrode 22 becomes large, making it possible to obtain a battery 1 with a high output.

[0040] [Separator] The separator 23 is a porous film containing a resin, and may be a laminated film of two or more porous films. The resin is, for example, polypropylene and polyethylene. The separator 23 may include a resin layer on one or both sides with the porous film as a base layer. This is because the adhesion of the separator 23 to each of the positive electrode 21 and the negative electrode 22 is improved, suppressing the distortion of the electrode wound body 20.

[0041] The resin layer contains a resin such as PVdF. To form this resin layer, a solution in which the resin is dissolved in an organic solvent is applied to the substrate layer, and the substrate layer is then dried. Alternatively, the substrate layer may be immersed in the solution and then dried. It is preferable that the resin layer contains inorganic or organic particles from the viewpoint of improving heat resistance and battery safety. Examples of inorganic particles include aluminum oxide, aluminum nitride, aluminum hydroxide, magnesium hydroxide, boehmite, talc, silica, and mica. Alternatively, instead of the resin layer, a surface layer mainly composed of inorganic particles formed by a sputtering method, an ALD (atomic layer deposition) method, or the like may be used.

[0042] The thickness of the separator 23 is preferably 4 μm or more and 30 μm or less. By making the thickness of the separator 23 4 μm or more, it is possible to prevent an internal short circuit due to contact between the positive electrode 21 and the negative electrode 22, which face each other via the separator 23. By making the thickness of the separator 23 30 μm or less, it is possible to facilitate passage of lithium ions and the electrolyte solution through the separator 23, and also to increase the electrode density of the positive electrode 21 and the negative electrode 22 when wound.

[0043] [Electrolyte] The electrolyte solution contains a solvent and an electrolyte salt, and may further contain additives as necessary. The solvent is a non-aqueous solvent such as an organic solvent, or water. An electrolyte solution containing a non-aqueous solvent is called a non-aqueous electrolyte solution. The non-aqueous solvent is a cyclic carbonate ester, a chain carbonate ester, a lactone, a chain carboxylic acid ester, a nitrile (mononitrile), or the like.

[0044] A typical example of the electrolyte salt is a lithium salt, but salts other than lithium salts may also be included. Examples of lithium salts include lithium hexafluorophosphate (LiPF), lithium tetrafluoroborate (LiBF), lithium perchlorate (LiClO), lithium methanesulfonate (LiCHSO), lithium trifluoromethanesulfonate (LiCFSO), and dilithium hexafluorosilicate (LiSF). Mixtures of these salts can also be used, and a mixture of LiPF and LiBF is particularly preferred from the perspective of improving battery performance. The content of the electrolyte salt is not particularly limited, but is preferably 0.3 mol / kg to 3 mol / kg relative to the solvent.

[0045] [How to make a lithium-ion battery] A method for fabricating the lithium-ion battery 1 according to one embodiment will be described with reference to FIGS. 4A to 4F. First, a positive electrode active material was applied to the surface of a strip-shaped positive electrode foil 21A, forming the coated portion of the positive electrode 21. A negative electrode active material was applied to the surface of a strip-shaped negative electrode foil 22A, forming the coated portion of the negative electrode 22. Active material-uncoated portions 21C and 22C, to which the positive electrode active material and the negative electrode active material were not applied, were fabricated at one end of the short side of the positive electrode 21 and one end of the short side of the negative electrode 22. Notches were formed in portions of the active material-uncoated portions 21C and 22C, corresponding to the start of winding. The positive electrode 21 and the negative electrode 22 were then subjected to processes such as drying. Then, the positive electrode active material uncoated portion 21C and the negative electrode active material uncoated portion 22C were stacked with the separator 23 interposed between them so that they were facing in opposite directions, and were wound in a spiral shape so that a through-hole 26 was formed in the central axis and the produced notch was positioned near the central axis, thereby producing an electrode wound body 20 as shown in FIG. 4A.

[0046] Next, as shown in FIG. 4B , the edge of a thin flat plate (e.g., 0.5 mm thick) was pressed perpendicularly against the end faces 41 and 42 to locally bend the end faces 41 and 42, thereby creating grooves 43. In this manner, grooves 43 extending radially from the through holes 26 toward the central axis were created. The number and arrangement of grooves 43 shown in FIG. 4B are merely an example. Then, as shown in FIG. 4C , the same pressure was simultaneously applied from both electrode sides in a direction substantially perpendicular to the end faces 41 and 42, bending the positive electrode active material uncoated portion 21C and the negative electrode active material uncoated portion 22C, so that the end faces 41 and 42 became flat. At this time, a load was applied using the flat plate or the like so that the active material uncoated portions on the end faces 41 and 42 overlap and bend toward the through holes 26. Thereafter, the plate-shaped portion 31 of the positive current collector plate 24 was laser-welded to the end face 41, and the plate-shaped portion 33 of the negative current collector plate 25 was laser-welded to the end face 42.

[0047] 4D, strip-shaped portions 32 and 34 of current collectors 24 and 25 were folded, insulating plates 12 and 13 (or insulating tape) were attached to positive current collector 24 and negative current collector 25, and the electrode winding body 20 assembled as described above was inserted into battery can 11 shown in Fig. 4E, followed by welding the bottom of battery can 11. After the electrolyte was poured into battery can 11, it was sealed with gasket 15 and battery lid 14 as shown in Fig. 4F. [Example]

[0048] The present invention will be specifically described below based on examples in which the open circuit voltage defect rates were compared using the lithium ion batteries 1 fabricated as described above. However, the present invention is not limited to the examples described below.

[0049] In all of the following Examples and Comparative Examples, the battery size was 21700 mm, and the separator 23 was stacked so as to cover the entire area of ​​the positive electrode active material coating portion 21B and the negative electrode active material coating portion 22B. The number of grooves 43 was eight. FIG. 5A is a partially enlarged view (a corner view on the winding start side) of the positive electrode active material uncoated portion 21C located on the winding start side of the electrode wound body 20. The positive electrode has a positive electrode cutout portion 51 at one end in the short direction of the positive electrode 21 located on the winding start side of the electrode wound body 20. As shown in FIG. 5A, the width of the positive electrode active material uncoated portion 21C located on one end in the short direction of the positive electrode 21 is Hc1, and the width of the positive electrode cutout portion 51 is Hc2. Hc1 is, for example, 7 mm. FIG. 5B is a partially enlarged view (a corner view on the winding start side) of the negative electrode active material uncoated portion 22C located on the winding start side of the electrode wound body 20. The negative electrode has a negative electrode cutout portion 52 at one end in the short direction of the negative electrode 22 located on the winding start side of the electrode wound body 20. 5B, the width of the active material uncovered portion 22C of the negative electrode at one end in the short direction of the negative electrode 22 is Ha1, and the width of the cutout portion 52 of the negative electrode is Ha2. Ha1 is, for example, 4 mm.

[0050] The value of E shown in Fig. 5A is the length of the cutout portion 51 of the positive electrode along the longitudinal direction of the positive electrode 21, and the value of F shown in Fig. 5B is the length of the cutout portion 52 of the negative electrode along the longitudinal direction of the negative electrode 22. In one embodiment, the inner periphery refers to the innermost periphery and the vicinity of the innermost periphery of the positive electrode 21 and the negative electrode 22 of the electrode winding body 20. The values ​​of E and F each preferably correspond to one or more and five or less peripheries of the inner periphery of the electrode winding body 20.

[0051] 6A and 6B are partial cross-sectional views of the positive electrode 21 or the negative electrode 22 after the active material uncoated portions 21C, 22C have been folded (see FIG. 4C ), and show either the positive electrode 21 or the negative electrode 22. The left side of the drawings shows the inner periphery of the electrode wound body 20, and the right side of the drawings shows the outer periphery of the electrode wound body 20. If the values ​​of E and F are equal to or greater than one turn of the inner periphery of the electrode wound body 20, the active material uncoated portions 21C, 22C can be folded without damaging the positive electrode 21 or the negative electrode 22, as shown in FIG. 6A . However, if the values ​​of E and F are less than one turn, as shown in FIG. 6B , when the active material uncoated portions 21C, 22C are folded, the active material coated portion 21B of the positive electrode or the active material coated portion 22B of the negative electrode is folded, as shown in FIG. 6B . This causes the active material to peel off from the positive electrode 21 or the negative electrode 22, resulting in an internal short circuit. If the value of E or F is greater than five turns of the inner circumference of the electrode winding body 20, the number of weldable points between the end faces 41, 42 and the current collector plates 24, 25 will decrease, possibly increasing the internal resistance.

[0052] 7A, 7B, 8A, and 8B are diagrams showing the negative electrode 22 superimposed on the positive electrode 21 before winding, with the right side of the diagram being the winding start side of the electrode winding body 20 and the left side of the diagram being the winding end side of the electrode winding body 20. Hereinafter, the term "end 53 of the cutout portion of the positive electrode" refers to the end of the cutout portion 51 of the positive electrode that is along the longitudinal direction of the positive electrode 21, and the term "end 54 of the cutout portion of the negative electrode" refers to the end of the cutout portion 52 of the negative electrode that is along the longitudinal direction of the negative electrode 22.

[0053] [Example 1] 7A , the position of edge 53 of the cutout portion of the positive electrode was determined as active material uncoated portion 21C of the positive electrode, the position of edge 54 of the cutout portion of the negative electrode was determined as active material uncoated portion 22C of the negative electrode, Hc2 / Hc1×100=15, Ha2 / Ha1×100=15, and the values ​​of E and F were set to one turn of the electrode wound body 20. Active material uncoated portions 22C with a width of 1 mm (W=1 mm) were arranged at both ends of the negative electrode 22 in the longitudinal direction (the ends on the winding start side and winding end side).

[0054] [Example 2] The same procedures as in Example 1 were carried out except that Hc2 / Hc1×100=20 and Ha2 / Ha1×100=20.

[0055] [Example 3] The same procedures as in Example 1 were carried out except that Hc2 / Hc1×100=30 and Ha2 / Ha1×100=30.

[0056] [Example 4] The same procedures as in Example 1 were carried out except that Hc2 / Hc1×100=90 and Ha2 / Ha1×100=90.

[0057] [Example 5] The same procedures as in Example 1 were carried out except that Hc2 / Hc1×100=95 and Ha2 / Ha1×100=95.

[0058] [Comparative Example 1] No notches 51, 52 were formed in the positive electrode 21 and the negative electrode 22, and Hc2 / Hc1×100=0 and Ha2 / Ha1×100=0. Active material uncovered portions 22C each having a width of 1 mm were arranged on both ends of the negative electrode 22 in the longitudinal direction (the ends on the winding start side and winding end side).

[0059] Comparative Example 2 7B, the position of edge 53 of the cutout portion of the positive electrode was set to active material coated portion 21B of the positive electrode, the position of edge 54 of the cutout portion of the negative electrode was set to active material coated portion 22B of the negative electrode, Hc2 / Hc1×100=100, Ha2 / Ha1×100=100, and the values ​​of E and F were set to one turn of the electrode wound body. Active material uncoated portion 22C with a width of 1 mm was arranged at both ends of the negative electrode 22 in the longitudinal direction (the ends on the winding start side and winding end side).

[0060] Comparative Example 3 The same procedure as in Comparative Example 2 was carried out except that Hc2 / Hc1×100=110 and Ha2 / Ha1×100=110.

[0061] [evaluation] The open-circuit voltage failure rate was determined for the batteries 1 of Examples 1 to 5 and Comparative Examples 1 to 3. The open-circuit voltage failure rate was determined by charging the batteries at a constant current and constant voltage of 500 mA at an ambient temperature of 25°C, determining the voltage of the batteries 1 immediately (within one hour) after the voltage reached 4.2 V as V1, and then leaving the batteries to stand for two weeks as V2. Batteries 1 for which V1 - V2 ≥ 50 mV were determined to be open-circuit voltage failures, and the number of such batteries was counted to determine the percentage relative to the total. One hundred batteries were tested for each example. The results are shown in Table 1.

[0062] [Table 1]

[0063] The open-circuit voltage defect rates of Examples 1 to 5 were relatively low, at 3% or less, whereas the open-circuit voltage defect rates of Comparative Examples 1 to 3 were relatively high, at 6% or more. It is believed that, in the electrode wound bodies 20 of Examples 1 to 5, the active material uncoated portions 21C and 22C could be bent without damaging the positive electrode 21 and the negative electrode 22, as shown in FIG. 6A . In Comparative Example 1, as shown in FIG. 6B , bending the active material uncoated portions 21C and 22C caused the active material coated portion 21B of the positive electrode and the active material coated portion 22B of the negative electrode to bend, causing the active material to peel off from the positive electrode 21 and the negative electrode 22, resulting in an internal short circuit. In Comparative Examples 2 and 3, the innermost active material uncoated portions 21C and 22C, which act as lids for the end faces 41 and 42 of the electrode wound body 20, were not present, and therefore it is believed that metal powder or the like was mixed into the inside of the electrode wound body 20 from the outside, causing an internal short circuit.

[0064] Table 1 shows that when the edge 53 of the cutout portion of the positive electrode is located in the active material uncoated portion 21C of the positive electrode and the edge 54 of the cutout portion of the negative electrode is located in the active material uncoated portion 22C of the negative electrode, or when 15≦Hc2 / Hc1×100≦95 and 15≦Ha2 / Ha1×100≦95, the battery 1 can be charged and discharged with almost no internal short circuit. In Examples 2 to 4, the open circuit voltage failure rate was even lower, at 1%. Table 1 also shows that when 20≦Hc2 / Hc1×100≦90 and 20≦Ha2 / Ha1×100≦90, the battery 1 can be charged and discharged without any internal short circuit.

[0065] Next, a case was investigated in which the negative electrode 22 did not have a 1 mm-wide (W=1 mm) active material-uncoated portion 22C at both longitudinal ends thereof, as shown in FIGS. 7A and 7B . In Examples 11 to 15 and Comparative Example 11, the negative electrode active material-coated portion 22B and the negative electrode active material-uncoated portion 22C were arranged so as to be aligned at both longitudinal ends (the ends at the winding start and end) of the negative electrode 22, as shown in FIG. 8A . In Comparative Examples 12 and 13, the negative electrode active material-coated portion 22B and the negative electrode active material-uncoated portion 22C were arranged so as to be aligned at the ends at the winding end in the longitudinal direction of the negative electrode 22, as shown in FIG. 8B , and the negative electrode active material-coated portion 22B was arranged at the ends at the winding start side.

[0066] [Example 11] 8A , the position of edge 53 of the cutout portion of the positive electrode was determined as active material uncoated portion 21C of the positive electrode, the position of edge 54 of the cutout portion of the negative electrode was determined as active material uncoated portion 22C of the negative electrode, Hc2 / Hc1×100=15, Ha2 / Ha1×100=15, and the values ​​of E and F were set to one turn of the electrode wound body 20. The active material coated portion 22B of the negative electrode and the active material uncoated portion 22C of the negative electrode were arranged so as to be aligned at both longitudinal ends of the negative electrode 22 (the ends on the winding start side and winding end side).

[0067] [Example 12] The same procedures as in Example 11 were carried out except that Hc2 / Hc1×100=20 and Ha2 / Ha1×100=20.

[0068] [Example 13] The same procedures as in Example 11 were carried out except that Hc2 / Hc1×100=30 and Ha2 / Ha1×100=30.

[0069] [Example 14] The same procedures as in Example 11 were carried out except that Hc2 / Hc1×100=90 and Ha2 / Ha1×100=90.

[0070] [Example 15] The same procedures as in Example 11 were carried out except that Hc2 / Hc1×100=95 and Ha2 / Ha1×100=95.

[0071] [Comparative Example 11] No notches were formed in the positive electrode 21 and the negative electrode 22, and Hc2 / Hc1×100=0 and Ha2 / Ha1×100=0. The negative electrode active material coated portion 22B and the negative electrode active material uncoated portion 22C were arranged so as to be aligned at both longitudinal ends of the negative electrode 22 (the ends at the winding start side and the winding end side).

[0072] [Comparative Example 12] 8B , the position of edge 53 of the cutout portion of the positive electrode was determined as positive electrode active material covered portion 21B, the position of edge 54 of the cutout portion of the negative electrode was determined as negative electrode active material covered portion 22B, Hc2 / Hc1×100=100, Ha2 / Ha1×100=100, and the values ​​of E and F were set to one turn of electrode wound body 20. Negative electrode active material covered portion 22B and negative electrode active material uncovered portion 22C were arranged so as to be aligned at the end of negative electrode 22 on the longitudinal winding end side, and negative electrode active material covered portion 22B was arranged at the end of negative electrode 22 on the winding start side.

[0073] [Comparative Example 13] The same procedure as in Comparative Example 12 was carried out except that Hc2 / Hc1×100=110 and Ha2 / Ha1×100=110.

[0074] [evaluation] The open circuit voltage defect rate was determined in the same manner as above for the batteries 1 of Examples 11 to 15 and Comparative Examples 11 to 13. Similarly, 100 batteries were tested for each example. The results are shown in Table 2.

[0075] [Table 2]

[0076] Even when the negative electrode 22 does not have active material-uncoated portions 22C at both longitudinal ends, as in Examples 11 to 15 and Comparative Examples 11 to 13, it was found that similar results were obtained as when the negative electrode 22 has active material-uncoated portions 22C at both longitudinal ends (Examples 1 to 5 and Comparative Examples 1 to 3). Table 2 shows that when the edge 53 of the positive electrode cutout is located on the active material-uncoated portion 21C of the positive electrode and the edge 54 of the negative electrode cutout is located on the active material-uncoated portion 22C of the negative electrode, or when 15≦Hc2 / Hc1×100≦95 and 15≦Ha2 / Ha1×100≦95, the battery 1 can be charged and discharged with little internal short circuiting. Table 1 also shows that when 20≦Hc2 / Hc1×100≦90 and 20≦Ha2 / Ha1×100≦90, the battery 1 can be charged and discharged with almost no internal short circuiting.

[0077] <2. Modifications> Although one embodiment of the present invention has been specifically described above, the content of the present invention is not limited to the above-described embodiment, and various modifications based on the technical concept of the present invention are possible.

[0078] The shape of the cutout portions 51, 52 may be such that the width of the cutout portions 51, 52 is wider at the winding start side, as shown in Figures 9A to 9D, or such that the width is widest at the middle part of the cutout portions 51, 52, as shown in Figure 9E, or may be any other shape.

[0079] In the examples and comparative examples, the number of grooves 43 was eight, but any other number may be used. The battery size was 21700, but it may be 18650 or any other size. The positive current collector plate 24 and the negative current collector plate 25 have the plate-like portions 31 and 33 in a sector shape, but may have other shapes.

[0080] In the examples and comparative examples, as shown in FIG. 10A, insulating plates 12 and 13 are arranged to sandwich electrode winding 20. However, as shown in FIG. 10B, a structure without insulating plate 13 is also possible. In a structure without insulating plate 13, heat generated from the electrode winding during battery discharge is more easily transferred to the bottom plate of battery can 11 via current collector plate 25. This improves the battery's heat dissipation. This is due to the effect of reducing the battery surface temperature during discharge. As shown in FIG. 11, for a 18650 battery (diameter 18 mm, height 65 mm), the maximum battery temperature during a high-load discharge test was confirmed to be reduced by 6°C. In devices that discharge large currents, the device has a function to stop discharging at, for example, 75°C to protect the battery. Reducing the battery temperature during discharge means that the use time of devices that discharge large currents can be extended and deterioration of battery performance can be suppressed.

[0081] The high-load discharge test involves charging the battery at a constant voltage and current of 4.2V and 5A for 2.5 hours in a room temperature environment of 23±2°C, then leaving it until the battery temperature drops below 27°C, and then discharging it at a current of 50A and leaving it until the battery surface temperature drops below 30°C. At this time, the surface temperature of the side of the battery (32.5mm above the bottom of the can) was measured for an 18650 battery (diameter 18mm, height 65mm).

[0082] Without departing from the spirit of the present invention, the present invention can be applied to batteries other than lithium ion batteries and batteries other than cylindrical batteries (for example, laminated batteries, prismatic batteries, coin batteries, and button batteries). In this case, the shape of the "end surface of the electrode wound body" can be not only cylindrical, but also elliptical or flat.

[0083] <3. Application Examples> (1) Battery pack 12 is a block diagram showing an example of a circuit configuration when battery 1 according to an embodiment or example of the present invention is applied to a battery pack 300. Battery pack 300 includes a battery pack 301, a switch unit 304 including a charge control switch 302a and a discharge control switch 303a, a current detection resistor 307, a temperature detection element 308, and a control unit 310. Control unit 310 controls each device, and can also control charging and discharging in the event of abnormal heat generation, and calculate and correct the remaining capacity of battery pack 300. A positive terminal 321 and a negative terminal 322 of battery pack 300 are connected to a charger or electronic device for charging and discharging.

[0084] The battery pack 301 is made up of a plurality of secondary batteries 301a connected in series and / or parallel. Fig. 12 shows an example in which six secondary batteries 301a are connected in two parallel and three series (2P3S).

[0085] The temperature detection unit 318 is connected to the temperature detection element 308 (e.g., a thermistor), measures the temperature of the battery assembly 301 or the battery pack 300, and supplies the measured temperature to the control unit 310. The voltage detection unit 311 measures the voltage of the battery assembly 301 and each secondary battery 301a that constitutes it, A / D converts the measured voltage, and supplies it to the control unit 310. The current measurement unit 313 measures the current using the current detection resistor 307, and supplies the measured current to the control unit 310.

[0086] The switch control unit 314 controls the charge control switch 302a and the discharge control switch 303a of the switch unit 304 based on the voltage and current input from the voltage detection unit 311 and the current measurement unit 313. When the voltage of the secondary battery 301a becomes equal to or higher than the overcharge detection voltage (for example, 4.20 V±0.05 V) or equal to or lower than the overdischarge detection voltage (2.4 V±0.1 V), the switch control unit 314 sends an OFF control signal to the switch unit 304, thereby preventing overcharging or overdischarging.

[0087] After the charge control switch 302a or the discharge control switch 303a is turned OFF, charging or discharging is possible only via the diode 302b or the diode 303b. These charge / discharge switches can be semiconductor switches such as MOSFETs. Although the switch unit 304 is provided on the positive side in FIG. 12, it may also be provided on the negative side.

[0088] The memory 317 is made up of RAM and ROM, and stores and rewrites values ​​of battery characteristics calculated by the control unit 310, full charge capacity, remaining capacity, and the like.

[0089] (2)Electronic equipment The battery 1 according to the above-described embodiment or example of the present invention can be mounted in devices such as electronic devices, electric transport devices, and power storage devices, and can be used to supply power.

[0090] Examples of electronic devices include laptop computers, smartphones, tablet devices, PDAs (personal digital assistants), mobile phones, wearable devices, digital still cameras, e-books, music players, game consoles, hearing aids, power tools, televisions, lighting equipment, toys, medical equipment, and robots. Furthermore, electric transport equipment, power storage devices, power tools, and electric unmanned aerial vehicles, which will be described later, can also be included in the category of electronic devices in a broad sense.

[0091] Examples of electric transportation devices include electric vehicles (including hybrid vehicles), electric motorcycles, electrically assisted bicycles, electric buses, electric carts, automated guided vehicles (AGVs), and railroad cars. Electric passenger aircraft and unmanned electric aircraft for transportation are also included. The secondary battery according to the present invention can be used not only as a driving power source for these devices, but also as an auxiliary power source and a power source for energy regeneration.

[0092] Examples of the power storage device include commercial or home power storage modules, and power storage power sources for buildings such as houses, buildings, and offices, or for power generation facilities.

[0093] (3) Power tools Referring to Figure 13, an example of an electric screwdriver as a power tool to which the present invention can be applied will be described in brief. An electric screwdriver 431 is provided with a motor 433 that transmits rotational power to a shaft 434, and a trigger switch 432 that is operated by the user. A battery pack 430 and a motor control unit 435 according to the present invention are housed in a housing below the handle of the electric screwdriver 431. The battery pack 430 is either built into the electric screwdriver 431 or is detachable. The battery 1 of the present invention can be applied to the battery that constitutes the battery pack 430.

[0094] The battery pack 430 and the motor control unit 435 may each be provided with a microcomputer (not shown) so that they can communicate with each other regarding charging and discharging of the battery pack 430. The motor control unit 435 controls the operation of the motor 433 and can cut off the power supply to the motor 433 in the event of an abnormality such as over-discharge.

[0095] (4) Energy storage system for electric vehicles As an example of applying the present invention to a power storage system for an electrically powered vehicle, a configuration example of a hybrid vehicle (HV) employing a series hybrid system is shown schematically in Fig. 14. A series hybrid system is a vehicle that runs on an electric power driving force conversion device using electric power generated by a generator powered by an engine, or electric power that is temporarily stored in a battery.

[0096] This hybrid vehicle 600 is equipped with an engine 601, a generator 602, an electric power driving force conversion device 603 (a DC motor or an AC motor, hereinafter simply referred to as "motor 603"), driving wheels 604a, 604b, wheels 605a, 605b, a battery 608, a vehicle control device 609, various sensors 610, and a charging port 611. As the battery 608, the battery pack 300 of the present invention or a power storage module equipped with a plurality of batteries 1 of the present invention can be applied.

[0097] The motor 603 is operated by power from the battery 608, and the rotational force of the motor 603 is transmitted to the drive wheels 604a and 604b. The rotational force produced by the engine 601 can be used to generate power in the generator 602, which can be stored in the battery 608. Various sensors 610 control the engine speed via the vehicle control device 609 and the opening of a throttle valve (not shown).

[0098] When hybrid vehicle 600 is decelerated by a braking mechanism (not shown), the resistance force generated during deceleration is applied to motor 603 as a rotational force, and regenerative power generated by this rotational force is stored in battery 608. Battery 608 can be charged by connecting to an external power source via a charging port 611 of hybrid vehicle 600. Such an HV vehicle is called a plug-in hybrid vehicle (PHV or PHEV).

[0099] The secondary battery according to the present invention can also be applied to a miniaturized primary battery and used as a power source for a tire pressure monitoring system (TPMS) built into the wheels 604 and 605.

[0100] Although the above description has been given using a series hybrid vehicle as an example, the present invention can also be applied to a parallel hybrid vehicle that uses both an engine and a motor, or a hybrid vehicle that combines a series and parallel hybrid system.Furthermore, the present invention can also be applied to an electric vehicle (EV or BEV) that runs only on a drive motor without an engine, and a fuel cell vehicle (FCV). [Explanation of symbols]

[0101] 1 Lithium-ion battery, 12 Insulating plate, 21 Positive electrode, 21A Positive electrode foil, 21B Positive electrode active material coated portion, 21C Positive electrode active material uncoated portion, 22 Negative electrode, 22A Negative electrode foil, 22B Negative electrode active material coated portion, 22C Negative electrode active material uncoated portion, 23 Separator, 24 Positive electrode current collector, 25 Negative electrode current collector, 26 Through hole, 27, 28 Outer edge portion, 41, 42 End surface, 43 Groove, 51 Positive electrode cutout portion, 52 Negative electrode cutout portion, 53 Edge of positive electrode cutout portion, 54 Edge of negative electrode cutout portion

Claims

1. A secondary battery in which a battery can accommodates a first electrode current collector and an electrode winding body having a structure in which a strip-shaped first electrode and a second electrode having a conductivity different from that of the strip-shaped first electrode are stacked and wound with a separator interposed therebetween, the first electrode has a first conductive active material-coated portion coated with a first conductive active material and a first conductive active material-uncoated portion on a strip-shaped first electrode foil, the first conductive active material uncovered portion is joined to the first electrode current collector at one end of the electrode winding body, The electrode winding body is configured such that the first conductive active material uncoated portion is bent toward a central axis of the wound structure, the first electrode has a first electrode notch at one end in a lateral direction of the first electrode, the end being on the winding start side of the electrode winding body; an end of the first electrode cutout portion is present in the first conductive active material uncoated portion, A secondary battery, wherein the length of the first electrode cutout portion along the longitudinal direction of the first electrode is equal to or greater than one turn around the inner periphery of the electrode winding body.

2. 2. The secondary battery according to claim 1, wherein a width of a first electrode active material uncovered portion located at one end of the first electrode in the short side direction is Hc1, a width of the first electrode cutout portion is Hc2, a width of a second electrode active material uncovered portion located at one end of the second electrode in the short side direction is Ha1, and a width of the second electrode cutout portion is Ha2, and the relationship 15≦Hc2 / Hc1×100≦95 is satisfied.

3. 2. The secondary battery according to claim 1, wherein a width of a first electrode active material uncovered portion located at one end of the first electrode in the short side direction is Hc1, a width of the first electrode cutout portion is Hc2, a width of a second electrode active material uncovered portion located at one end of the second electrode in the short side direction is Ha1, and a width of the second electrode cutout portion is Ha2, and the relationship 20≦Hc2 / Hc1×100≦90 is satisfied.

4. 4. The secondary battery according to claim 1, wherein the first electrode foil has a thickness of 5 μm or more and 20 μm or less, and the second electrode foil has a thickness of 5 μm or more and 20 μm or less.

Images