Metal foil for electrolytic capacitor and electrolytic capacitor

Abstract

The present invention provides a metal foil for an electrolytic capacitor, said metal foil including a valve metal and having two main surface regions. At least one of the main surface regions has a plurality of recesses extending in a line shape, and the depth of the recesses is between 4 μm and 58 μm inclusive, or the projected area ratio of the recesses in the main surface regions is between 8% and 45% inclusive.

Description

Metal foil for electrolytic capacitors and electrolytic capacitors CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This disclosure claims the benefit of priority to Japanese Patent Application No. 2024-011369, filed on January 29, 2024, in the Japan Patent Office, the entire contents of which are incorporated herein by reference.

[0002] The present disclosure relates to a metal foil for an electrolytic capacitor and an electrolytic capacitor.

[0003] Patent Document 1 discloses an electrode foil comprising "a strip-shaped foil, comprising an enlarged surface portion formed on the surface of the foil, a core portion which is the remainder of the foil excluding the enlarged surface portion, and a plurality of dividing portions which extend in the width direction of the strip on the enlarged surface portion and divide the enlarged surface portion, wherein the dividing portions have a groove width of 50 μm or less (including 0 μm) when the foil is flattened."

[0004] JP 2017-224844 A

[0005] The surface of metal foil (raw foil) for electrolytic capacitors has a small number of unavoidable irregularities (or scratches). The raw foil may have rolling marks formed during its manufacturing process. If the rolling marks cause uneven etching pits to form in the electrode foil for electrolytic capacitors, the foil may break or crack during the manufacturing process of the electrode foil or electrolytic capacitor.

[0006] Inevitable irregularities in metal foils can also be formed by contact with rollers other than rolling rollers. Furthermore, during the processing of raw foil, unavoidable irregularities can be formed not only by contact with rollers but also by contact between the raw foil (or electrode foil) and various processing solutions (e.g., etching solutions, chemical conversion solutions). Concentration of stress on a small number of irregularities can cause foil breaks or cracks in the metal foil during the manufacturing process of an electrolytic capacitor. Foil breaks can also occur during use of an electrolytic capacitor due to vibration or impact. Furthermore, electrode foils formed from raw foils have porous portions. Increasing the porosity of the porous portions or thickening the porous portions to meet the demand for higher capacity electrode foils tends to reduce the strength of the electrode foil. There is a trade-off between capacity and strength, and there is a great demand for higher strength electrode foils.

[0007] One aspect of the present disclosure relates to a metal foil for an electrolytic capacitor (raw foil), the metal foil including a valve metal and having two main surface regions, at least one of the main surface regions having a plurality of linearly extending recesses, the depth of the recesses being 4 μm or more and 58 μm or less.

[0008] Another aspect of the present disclosure relates to a metal foil for an electrolytic capacitor (raw foil), the metal foil including a valve metal and having two main surface regions, at least one of the main surface regions having a plurality of linearly extending recesses, and a projected area ratio of the recesses to the main surface region being 8% or more and 45% or less.

[0009] Yet another aspect of the present disclosure relates to a metal foil for an electrolytic capacitor (electrode foil (etched foil) for an electrolytic capacitor), wherein the metal foil includes a valve metal and has two main surface regions, at least one of the main surface regions has a plurality of recesses extending linearly, the recesses having a depth of 4 μm or more and 58 μm or less, the metal foil has a porous portion having pores and a core portion continuous with the porous portion, and the two main surface regions are each the main surface regions of the porous portion.

[0010] Yet another aspect of the present disclosure relates to a metal foil for an electrolytic capacitor (electrode foil (etched foil) for an electrolytic capacitor), wherein the metal foil includes a valve metal and has two main surface regions, at least one of the main surface regions has a plurality of linearly extending recesses, and a projected area ratio of the recesses to the main surface region is 8% or more and 45% or less, the metal foil has a porous portion having pores and a core portion continuous with the porous portion, and the two main surface regions are each the main surface regions of the porous portion.

[0011] Yet another aspect of the present disclosure relates to the metal foil for an electrolytic capacitor (electrode foil for an electrolytic capacitor) described above, in which the porous portion further has a dielectric layer.

[0012] Yet another aspect of the present disclosure relates to an electrolytic capacitor including the above-mentioned metal foil for an electrolytic capacitor.

[0013] According to the present disclosure, foil breakage and cracking of metal foil for electrolytic capacitors can be suppressed.

[0014] The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the drawings.

[0015] 4 is a schematic diagram of a cross section parallel to the thickness direction and the TD direction of an electrode foil corresponding to the raw foil shown in FIG. 4. FIG. 5 is an enlarged schematic diagram of a cross section of the electrode foil 2 of FIG. 5. FIG. 6 is an SEM image of a portion of the main surface region of an electrode foil according to an embodiment of the present disclosure, viewed from the normal direction. FIG. 7 is an SEM image of a cross section parallel to the thickness direction and the TD direction of an electrode foil according to an embodiment of the present disclosure. FIG. 8 is a schematic cross-sectional view of an electrolytic capacitor according to an embodiment of the present disclosure. FIG. 9 is a schematic perspective view of the configuration of a wound body. FIG. 10 is a schematic diagram of an example of a wound body viewed from the end face side. FIG. 11 is an enlarged schematic diagram of another example of the main surface region of a raw foil, viewed from the normal direction of the main surface region, having a group of recesses arranged in a grid pattern.

[0016] The following describes embodiments of the present disclosure using examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be exemplified, but other numerical values ​​and materials may be applied as long as the effects of the present disclosure are obtained. In this specification, the term "numerical value A to numerical value B" includes numerical value A and numerical value B and can be read as "numerical value A or more and numerical value B or less." In the following description, when lower and upper limits are exemplified for numerical values ​​of specific physical properties or conditions, any of the exemplified lower limits and any of the exemplified upper limits can be arbitrarily combined, as long as the lower limit is not equal to or greater than the upper limit. When multiple materials are exemplified, one may be selected from them and used alone, or two or more may be used in combination.

[0017] The present disclosure encompasses any combination of two or more claims arbitrarily selected from the appended claims, i.e., any combination of two or more claims arbitrarily selected from the appended claims may be combined unless a technical contradiction arises.

[0018] In this disclosure, the type of electrolytic capacitor is not particularly limited, and the present disclosure can be applied to all electrolytic capacitors, including electrolytic capacitors containing an electrolytic solution, solid electrolytic capacitors containing a solid electrolyte, and solid-liquid hybrid electrolytic capacitors containing a liquid component and a solid electrolyte. In all electrolytic capacitors, phenomena caused by foil tearing and cracking in metal foils (electrode foils) made of valve metals (such as aluminum, tantalum, niobium, and titanium) can be suppressed.

[0019] In the present disclosure, the electrode foil for an electrolytic capacitor may be used as at least one of the anode foil and cathode foil of a wound electrolytic capacitor, or as an anode body of a laminated electrolytic capacitor, or as an anode body of other electrolytic capacitors.

[0020] In particular, when the metal foil for an electrolytic capacitor according to the present disclosure is used in a solid electrolytic capacitor having a conductive polymer as a solid electrolyte, the retention of the conductive polymer is significantly improved.Furthermore, when the metal foil for an electrolytic capacitor according to the present disclosure is used in an electrolytic capacitor containing an electrolytic solution or a liquid component, the effects of suppressing dry-up and reducing ESR can be expected.

[0021] "Metal foil for electrolytic capacitors (hereinafter also simply referred to as "metal foil")" is a general concept that includes "raw material foil for electrolytic capacitors (hereinafter also simply referred to as "raw material foil")" and "electrode foil for electrolytic capacitors (hereinafter also simply referred to as "electrode foil")." The raw material foil is the raw material for electrode foil. The raw material foil may be metal foil before etching. In other words, the raw material foil may be metal foil for electrolytic capacitors that does not have a porous portion. The raw material foil may be metal foil that does not have a dielectric layer formed by chemical conversion treatment, atomic layer deposition, or the like. An electrode foil that has been etched is also called an "etched foil." An electrode foil that has been chemically treated is also called a "chemically treated foil." The raw material foil and electrode foil can be used for both anode foil and cathode foil.

[0022] (Raw Foil) The raw foil according to an embodiment of the present disclosure contains a valve metal and has two main surface regions. The raw foil is, for example, strip-shaped with a width of 1.5 mm or more and 520 mm or less. At least one of the main surface regions of the raw foil has a plurality of recesses extending in a line (hereinafter also referred to as a "group of recesses"). The shape of the "lines (lines or line segments)" is not limited to straight lines, and may be curved or other lines. The recesses may be spaced apart from one another, may intersect with one another, or may be in a mesh pattern. Each of the plurality of recesses is open to the main surface region. The arrangement of the recesses may be uniform or non-uniform, regular or irregular.

[0023] The two main surface regions are at least a portion of the two main surfaces of the raw foil, and each main surface region may have an area of ​​50% or more (e.g., 90% to 100%) of the area of ​​the corresponding main surface of the raw foil.

[0024] The raw foil satisfies at least one of the following conditions (A) and (B): (A) the depth H of the recesses is 4 μm or more and 58 μm or less (preferably 5 μm or more and 40 μm or less, and more preferably 6 μm or more and 30 μm or less), and (B) the projected area ratio of the recesses to the main surface region is 8% or more and 45% or less (preferably 10% or more and 40% or less, and more preferably 12% or more and 35% or less).

[0025] Condition (A) may be satisfied by only one main surface region, but is preferably satisfied by both of the two main surface regions. Condition (B) may be satisfied by only one main surface region, but is preferably satisfied by both of the two main surface regions. It is preferable that each main surface region satisfies both conditions (A) and (B).

[0026] A first group of recesses may be provided in one main surface region of the raw foil, and a second group of recesses may be provided in the other main surface region of the raw foil. In this case, the depth H of the recesses of the first group of recesses and the proportion of the projected area of ​​the recesses in the main surface region may be the same as or different from those of the second group of recesses. When viewed from the normal direction of the main surface region, the recesses constituting the first group of recesses and the recesses constituting the second group of recesses may or may not overlap each other. From the viewpoint of improving tensile strength, non-overlapping is preferable.

[0027] The raw foil may be a rolled foil. When the raw foil is wound, the winding direction may be parallel to the rolling direction (MD direction) of the rolled foil, or may intersect with the rolling direction at an angle of -20° to 20°. This reduces the influence of rolling marks during winding.

[0028] Furthermore, the angle formed between the multiple linear recesses and the rolling direction may be within a range of -20° to 20°. The direction in which the multiple recesses extend means an average direction. The average direction is determined depending on each individual linear recess. When the multiple recesses are not in a mesh-like pattern, the average direction is a direction from one end of the linear recesses to the other end. When the multiple recesses are in a mesh-like pattern, the average direction is the direction of the line segments that make up the mesh, and there are multiple average directions. It is preferable that the angle formed between at least one direction and the rolling direction is within a range of -20° to 20°. Such an angle is determined as the average value of the angles between at least five linear recesses and the rolling direction.

[0029] The depth H of the recesses under condition (A) can be measured in a cross section of the raw foil (hereinafter simply referred to as the "cross section of the raw foil") that is parallel to the thickness direction and intersects the direction in which the linear recesses extend at an angle of 80° to 100° (for example, 90°). The cross section of the raw foil is formed so that the deepest parts of at least two mutually adjacent recesses can be observed. The cross section of the raw foil can be observed with a scanning electron microscope (SEM). The depth H of the recesses is the average of two distances (distances parallel to the thickness direction of the raw foil) between the highest point of a non-recessed portion between the two mutually adjacent recesses and the deepest parts of the two mutually adjacent recesses sandwiching the non-recessed portion. The depth H of the recesses is determined by measuring at least five points on the raw foil and averaging these distances.

[0030] In the cross section of the raw foil, the width of the recess on the surface of the raw foil may be greater on the opening side than on the deep side of the recess. At least a part of the recess in the cross section of the raw foil may have an arc shape.

[0031] The "proportion of the projected area of ​​the recesses in the main surface region" in condition (B) is the ratio of the total projected area of ​​the recesses onto the main surface region (i.e., the projected area of ​​the recess group onto the main surface region) to the projected area of ​​the main surface region as viewed from the normal direction of the main surface region (hereinafter also referred to as "recess projected area ratio Ras"). The area of ​​the main surface region for measuring the recess projected area ratio Ras must be at least 2 mm ２ The area of ​​the main surface region for measuring the recess projected area ratio Ras is preferably as large as possible, for example, 3 mm ２ When the main surface region is observed with an SEM from the normal direction of the main surface region, recesses and non-recesses can be distinguished, for example, from color tones. Recesses and non-recesses may also be distinguished from each other by binarizing the SEM image based on common technical knowledge.

[0032] The explanation regarding conditions (A) and (B) applies not only to the "raw foil" but also to the "electrode foil" described below.

[0033] The raw foil according to this embodiment has particularly enhanced tensile strength and folding endurance. Specifically, the presence of the recesses alleviates stress caused by winding or bending. For example, large cracks (i.e., foil breaks) extending linearly from one end of the raw foil to the other in the width direction are suppressed. The winding or bending of the raw foil is performed, for example, by winding the raw foil with a roller, slitting the raw foil, or etching or chemical conversion treating the raw foil. Hereinafter, when the term "strength" is used simply for the raw foil (and the electrode foil described below), it refers to tensile strength, folding endurance, or both.

[0034] The ratio (H / F) of the depth H (μm) of the recess to the thickness F (μm) of the raw material foil (electrode foil) is preferably 0.05 or more and 0.55 or less, more preferably 0.26 or more and 0.47 or less, and even more preferably 0.3 or more and 0.43 or less. When the H / F ratio is within the above range, the strength of the raw material foil and the strength of the electrode foil formed from that raw material foil are easily ensured. The thickness F (μm) of the raw material foil is measured at at least five non-recessed locations and calculated as the average value.

[0035] When viewed from the normal direction of the main surface region, the locus of each recess constituting the plurality of recesses extending linearly is not particularly limited. Below, some parameters that define the recesses are explained. A recess (group of recesses) that meets any of the following conditions is highly effective in increasing the strength of the raw foil. As a result, the electrode foil or etched foil formed from the raw foil also has excellent strength. Furthermore, when used in a capacitor, filling the recesses with electrolyte also has the effect of reducing ESR. Therefore, the explanation of the following parameters applies not only to the "raw foil" but also to the "electrode foil" described below.

[0036] The linear recesses may be arranged regularly or irregularly on the main surface region.

[0037] Each linearly extending recess may be formed continuously in the extending direction of the recess. For example, each recess may be continuous without interruption over a length L of at least 20 mm or more.

[0038] Each linearly extending recess may be formed discontinuously (for example, in a broken line) in the extending direction of the recess. For example, each recess may be formed intermittently.

[0039] The length of each linear recess is, for example, 20 μm or more, or may be 50 μm or more, or 100 μm or more. The length of the recess is calculated as the average length of at least five arbitrarily selected recesses. When the first recess group and the second recess group are provided on one and the other main surface regions of the raw foil (electrode foil), respectively, the length L of the first recess group and the second recess group may be the same or different.

[0040] The width Lmin of the recesses as viewed from the normal direction of the main surface regions (the dimension perpendicular to the length direction of the recesses) is, for example, 15 μm or more and 110 μm or less, and may be 30 μm or more and 100 μm or less (preferably 33 to 37 μm in the case of a diagonal grid / diagonal pattern), or 35 μm or more and 90 μm or less (preferably 70 μm or less in the case of a mesh pattern). The width Lmin of the recesses is determined as the average value of the widths of the recesses at at least five arbitrarily selected locations. When a first group of recesses and a second group of recesses are provided on one and the other main surface regions of the raw material foil (electrode foil), respectively, the widths of the first group of recesses and the second group of recesses may be the same or different.

[0041] The ratio (L / Lmin) of the recess length L to the recess width Lmin is, for example, 1.01 or more, preferably 4.0 or more, and more preferably 40 or more, and the recesses may be continuous so that the ratio is even larger. For example, the recesses may be continuous over a length of 10 m or more (e.g., 3000 m or 5000 m) in the longitudinal direction of the raw foil (electrode foil). When the first recess group and the second recess group are provided in one and the other main surface regions of the raw foil (electrode foil), respectively, the L / Lmin ratio of the first recess group and the second recess group may be the same or different.

[0042] The ratio (Lmin / F) of the width Lmin (μm) of the recess to the thickness F (μm) of the raw foil is preferably less than 0.6. The Lmin / F ratio may be 0.1 or more.

[0043] The ratio (Lmin / H) of the width Lmin (μm) of the recess to the depth H (μm) of the recess is preferably equal to or less than 7. The Lmin / H ratio may be equal to or greater than 0.5.

[0044] The shortest distance dmin between the edges of the recesses closest to each other may be 10 μm or more and 120 μm or less. The shortest distance dmin may be 15 μm or more and 110 μm or less, or 13 μm or more and 105 μm or less. The shortest distance dmin is calculated as the average value of the shortest distances between the edges of at least five arbitrarily selected pairs of recesses closest to each other.

[0045] The "edge of a recess" means a point on the contour line that defines the shape of the opening of the recess. When the first and second recess groups are provided on one and the other of the main surface regions of the raw foil (electrode foil), respectively, the shortest distance dmin between the edges of the recesses in the first and second recess groups that are closest to each other may be the same or different.

[0046] Here, dmin is the distance between adjacent horizontal lines facing in the same direction. The same direction also includes directions that intersect with each other at a small angle of 20° or less. The same direction is not a strict term.

[0047] For example, when a plurality of recesses are arranged in a stripe pattern, the shortest distance dmin between the edges of the nearest recesses in the width direction of the recesses may be 10 μm or more and 120 μm or less. Furthermore, when a plurality of recesses are arranged in a mesh or lattice pattern, the shortest distance dmin between the opposing sides of the squares surrounded by the recesses may be 10 μm or more and 120 μm or less, or may be 13 μm to 110 μm, or may be 15 μm to 105 μm.

[0048] When the plurality of recesses are in a mesh or lattice pattern, the density Nd of the squares in the main surface region is 6 squares / mm ２ Above, 86 pieces / mm ２ The area of ​​the main surface region for measuring the number density Nd may be at least 2 mm ２ (For example, 3 mm ２ ) The "number density" refers to the number of squares formed per unit area of ​​the main surface region.

[0049] When the plurality of recesses are in a mesh or lattice pattern, the perimeter density Ld of the squares in the main surface region is 8 mm / mm ２ Above, 40mm / mm ２ The area of ​​the main surface region for measuring the perimeter density Ld may be at least 2 mm ２ (For example, 3 mm ２ The "perimeter density" is the sum of the lengths of the contour lines defining the shapes of all the squares formed per unit area of ​​the main surface region.

[0050] When the recesses are in a mesh or lattice pattern, the perimeter Ln of each square (i.e., the length of the outline defining the shape of the square) may be 0.08 mm or more and 4 mm or less. The perimeter Ln of each square is calculated as the average value of the perimeters of at least five arbitrarily selected squares.

[0051] The projected area Sn per square (the projected area of ​​the square onto the main surface area as viewed from the normal direction of the main surface area) is 0.001 mm ２ More than 0.07 mm ２ The projected area Sn per square is calculated as the average value of the projected areas of at least five arbitrarily selected squares.

[0052] The raw foil may have a shape having long sides and short sides. The shape having long sides and short sides may be a strip. When the strip-shaped raw foil is a rolled foil, the rolling direction (MD direction) usually substantially coincides with the length direction (long side direction) of the raw foil. However, the angle formed by the length direction (long side direction) and the MD direction may be within the range of -20° to 20°.

[0053] The extending directions of the plurality of recesses may be independently parallel, perpendicular, or oblique to a first direction parallel to the long side. For example, the extending directions of the plurality of recesses may be independently at an angle between −20° and 20° with the first direction. The plurality of recesses may be arranged in a diagonal line pattern oblique to the first direction, or in a diagonal lattice pattern (mesh pattern) made up of diagonal lines oblique to the first direction.

[0054] The raw foil contains a valve metal, such as aluminum (Al), tantalum (Ta), niobium (Nb), titanium (Ti), or zirconium (Zr). The raw foil may contain an alloy or compound containing the valve metal.

[0055] When the raw foil contains aluminum (Al) as a valve metal, the Al content in the raw foil may be 98% by mass or more, 99% by mass or more, or 99.5% by mass or more. When the valve metal is mainly aluminum, from the viewpoint of increasing the capacity of the electrode foil, the Al content in the raw foil is preferably 99% by mass or more. In particular, from the viewpoint of further increasing the capacity, the Al content in the raw foil is preferably 99.8% by mass or more. From the viewpoint of improving folding endurance, it is preferable that the raw foil be a soft foil.

[0056] The raw foil may contain trace amounts of elements other than the valve metals, such as silicon (Si), copper (Cu), and iron (Fe).

[0057] The Si content in the raw material foil is, for example, preferably 1 ppm by mass or more and 100 ppm by mass or less, and more preferably 5 ppm by mass or more and 80 ppm by mass or less.

[0058] The Cu content in the raw material foil is, for example, preferably 5 ppm by mass or more and 100 ppm by mass or less, and more preferably 5 ppm by mass or more and 80 ppm by mass or less.

[0059] The Fe content in the raw material foil is, for example, preferably 5 ppm by mass or more and 200 ppm by mass or less, and more preferably 5 ppm by mass or more and 100 ppm by mass or less.

[0060] In the raw material foil, the total content of Si, Cu, and Fe is preferably 0.1 mass % or less, and more preferably 0.05 mass % or less.

[0061] The tensile strength of the raw foil having the recessed portions is 25 N / mm ２ or more, 40 N / mm ２ or more, 60 N / mm ２ The tensile strength may be, for example, 120 N / mm ２ The following is also acceptable.

[0062] The elongation of the raw foil having the recessed portions is preferably 16% or less, more preferably 3% or less, and even more preferably 1% or less. The smaller the elongation, the easier it is to suppress the decrease in tensile strength and capacity.

[0063] In the raw foil having the recessed portions, the tensile strength is 25 N / mm ２ or more (or 40N / mm ２ When the elongation is 16% or more (or 3% or more), foil breakage in the subsequent process is likely to be suppressed. This is because the reduction in foil strength due to elongation in the subsequent process is suppressed. In other words, the strength of the electrode foil (etched foil) and chemical foil is sufficiently ensured in the subsequent process.

[0064] Post-processing steps include etching the raw foil (and further chemical conversion treatment if necessary), transporting and winding the raw foil or electrode foil with rollers, slitting the electrode foil, and forming a roll containing the electrode foil.

[0065] A plain foil containing a valve metal (e.g., having an arithmetic mean roughness Ra of 3 μm or less) is used as the metal foil before forming the recessed portions. The arithmetic mean roughness Ra is determined in accordance with JIS B 0601:2001. In the recessed portion forming process, the recessed portions may be formed by pressing a plurality of protrusions or a jig having protrusions onto the plain foil. Recesses (and protrusions between recesses) may be formed in the plain foil by so-called embossing. The recessed portions may be formed by pressing the plain foil with a pair of rollers having multiple protrusions. The pair of rollers may have the same shape or different rollers. Pressing with the pair of rollers may be performed multiple times. For example, after forming recessed portions on both sides, the thickness of the metal foil, the depth of the recesses, the shape of the protrusions, etc. may be adjusted using a flat roller. Recesses may be formed on one side of the raw foil, and then recesses may be formed on the other side. The recessed portions may be formed by laser processing, blasting, etching, etc. While it is easy to form recesses using laser processing, it is difficult to form protrusions between recesses. Embossing or other methods are preferred for forming convex portions. From the perspective of producing high-quality, high-performance capacitors, laser processing is advantageous because it is easy to control the accuracy of the shape and position of the concave portions. On the other hand, laser processing has a low tact time and increases equipment and production costs. Embossing is preferred for general capacitor production.

[0066] A rolled foil is usually used as the plain foil. The tensile strength and elongation refer to the tensile strength and elongation in the rolling direction, respectively. The rolling direction of the strip-shaped raw foil usually roughly coincides with the length direction of the raw foil. The tensile strength and elongation of the raw foil are determined, for example, in accordance with JIS Z 2241 (tensile test for metal materials).

[0067] In the cross section of the raw material foil, the number N of crystal grains of the raw material foil exposed on the inner wall surface of the recess is preferably 2 or more (or 3 or more), and may be 2 or more (or 3 or more) and 10 or less. In this case, the strength (tensile strength, folding endurance) of the electrode foil (etched foil, chemically processed foil) is likely to be ensured. It is desirable that the number N is within the above range for 40% or more (or 60% or more) of the multiple recesses constituting the recess group.

[0068] The number N of crystal grains can also be said to be the number of crystal grains exposed on the inner wall surface of the recess. The number N of crystal grains is determined in the cross section of the raw foil by the following method. The cross section of the raw foil is formed so that the deepest parts of at least two mutually closest recesses can be observed, and is observed with an SEM. At this time, one outline of the inner wall surface of the recess is observed for each recess. The number N of grain boundaries extending from the outline of one inner wall surface into the interior of the raw foil is counted. The average value of (N + 1) for at least five recesses selected at random is determined as the number N of crystal grains.

[0069] The average crystal grain size of the raw material foil is preferably 18 μm or more (or 20 μm or more) and 60 μm or less. In this case, the raw material foil (electrode foil) has appropriate hardness, and sufficient strength is easily ensured. Therefore, damage to the raw material foil (electrode foil) is suppressed during slitting, winding, and connecting the electrode foil to a lead member. When the average crystal grain size is 18 μm or more, grain boundaries are present in an appropriate amount. When there are many grain boundaries, the distribution of etching pits in the porous portion is less likely to be uneven between the vicinity of the grain boundaries and other areas.

[0070] The average crystal grain size is determined by arbitrarily selecting five regions in a cross-sectional SEM image of a raw material foil (electrode foil) having a thickness F, each having a length of F in the thickness direction and a length of 2×F in the planar direction, measuring the maximum diameters of the multiple crystals contained in each of the selected regions, and averaging these values.

[0071] The thickness F of the raw material foil (electrode foil) may be 50 μm or more and 200 μm or less, and is preferably 90 μm or more (or 100 μm or more) and 200 μm or less from the viewpoint of increasing capacity. When the thickness F of the raw material foil (electrode foil) is 80 μm or more (or 100 μm or more), the stress generated during winding is large, and the effect of alleviating this stress by crack formation between the recesses is significantly obtained. The thickness F of the raw material foil (electrode foil) is calculated by measuring the thickness at any 10 non-recessed points in an SEM image of the cross section of the raw material foil (electrode foil) and averaging the measured values.

[0072] The recesses are preferably arranged regularly in the surface direction of the raw material foil. The recesses are preferably arranged at equal intervals in the surface direction of the raw material foil. The recesses may be arranged in a mesh pattern (e.g., a lattice pattern) in the surface direction of the raw material foil.

[0073] When viewed from the normal direction of the main surface region, the plurality of recesses are arranged in the main surface region of the raw foil in, for example, straight lines, curved lines, wavy lines, zigzag lines, or a collection of line segments pointing in different directions, or in a regular or irregular pattern composed of various line segments and / or curves. Typical linear recesses have a stripe or lattice pattern, etc.

[0074] In the cross section of the raw material foil, the shape of the recesses may be columnar, conical, frustum, hemispherical, arc-shaped, cone-shaped, etc. From the viewpoint of ease of processing, cone-shaped, hemispherical, cone-shaped, etc. are also acceptable.

[0075] (Electrode Foil) An electrode foil (etched foil) according to an embodiment of the present disclosure includes a valve metal and has two main surface regions. At least one of the main surface regions has a plurality of recesses (group of recesses) extending in a line shape. The form of the "lines (lines or line segments)" is not limited to straight lines, and may be curved or other lines. The recesses may be spaced apart from one another, may intersect with one another, or may be in a mesh or lattice shape. Each of the plurality of recesses is open to the main surface region. The arrangement of the recesses may be uniform or non-uniform, regular or irregular.

[0076] The electrode foil has a porous portion having pores and a core portion continuous with the porous portion. The two main surface regions are the main surface regions of the porous portion. That is, the porous portion is the outer portion of the raw foil that has been made porous, and the remaining portion, which is the inner portion of the raw foil, is the core portion. The porous portion is formed in the main surface region of the raw foil that has multiple recesses.

[0077] Some of the pores in the porous portion are also open at the surface of the porous portion, but the opening diameter of such pores is usually, for example, less than 2 μm, which is much smaller than the dimension of the short side of the recess.

[0078] In a cross section of the electrode foil parallel to the thickness direction, the ratio (TPmin / TCmin) of the minimum thickness of the porous portion (the thickness of the porous portion at the deepest part) TPmin to the minimum thickness of the core portion TCmin may be 1.1 or more. TPmin and TCmin are each calculated as the average value of measurements taken at least five arbitrarily selected points.

[0079] The two main surface regions are at least partial regions on the two main surfaces of the porous portion of the electrode foil, and each main surface region may have an area of ​​50% or more (e.g., 90% to 100%) of the area of ​​the respective main surfaces of the porous portion of the electrode foil.

[0080] The electrode foil satisfies at least one of the following conditions (A) and (B): (A) the depth of the recesses is 4 μm or more and 58 μm or less (preferably 5 μm or more and 40 μm or less, and more preferably 6 μm or more and 30 μm or less), and (B) the ratio of the projected area of ​​the recesses to the main surface region is 8% or more and 45% or less (preferably 10% or more and 40% or less, and more preferably 12% or more and 35% or less).

[0081] Condition (A) may be satisfied by only one main surface region, but is preferably satisfied by both of the two main surface regions. Condition (B) may be satisfied by only one main surface region, but is preferably satisfied by both of the two main surface regions. It is preferable that each main surface region satisfies both conditions (A) and (B).

[0082] The characteristics of the recesses in the raw foil are substantially identical to the characteristics of the recesses in the electrode foil, and therefore the above-mentioned explanations regarding the conditions (A) and (B) for the "raw foil" also apply to the conditions (A) and (B) for the "electrode foil."

[0083] The characteristics of the recesses in the raw foil are substantially identical to the characteristics of the recesses in the electrode foil, and therefore the above-mentioned explanations regarding the conditions (A) and (B) for the "raw foil" also apply to the conditions (A) and (B) for the "electrode foil."

[0084] Similarly, the above-mentioned explanation of the parameters that define the recesses of the "raw foil" also applies to the parameters that define the recesses of the "electrode foil" (i.e., the width Lmin of the recesses, the shortest distance dmin between the edges of the nearest recesses, the L / Lmin ratio, the number density Nd of the squares in the main surface region, the perimeter density Ld of the squares in the main surface region, the perimeter Ln per square, the projected area Sn per square, the angle between the extension direction of the plurality of recesses and the first direction, etc.). The explanation of the shape, dimensions, etc. of the plurality of recesses that the raw foil had also applies to the shape of the recesses of the electrode foil.

[0085] The electrode foil according to this embodiment can be manufactured by forming porous portions in the raw foil described above. One method for forming porous portions in the raw foil is etching. When a raw foil having a group of recesses is etched, pores (etching pits) opening onto the main surface region of the raw foil having the group of recesses are formed, forming porous portions. This increases the surface area of ​​the raw foil, enabling the electrolytic capacitor to have a higher capacity. The pores (etching pits) opening onto the main surface region are also formed on the inner wall surfaces of the recesses.

[0086] The etching process may be electrolytic etching or chemical etching. Electrolytic etching allows for mass production of electrode foils having porous portions with pore diameters of less than 2 μm. AC etching is suitable for forming porous portions with spongy etching pits with pore diameters of 1 μm or less. When the pore diameter is 0.5 μm or less, further increases in capacity are possible. DC etching is suitable for forming porous portions with tunnel-shaped etching pits with pore diameters of less than 2 μm.

[0087] The multiple recesses are sufficiently large compared to the openings of the pores (etching pits) in the porous portion, and the characteristics of the multiple recesses provided in the raw foil do not change substantially even after the etching process, and the group of recesses is maintained.

[0088] A dielectric layer may be formed on the surface of the etched foil after etching. The dielectric layer may be formed by a liquid phase method or a gas phase method. As a liquid phase method, chemical conversion treatment (anodic oxidation) is preferred. As a gas phase method, atomic layer deposition or vapor deposition is preferred.

[0089] By chemical conversion treatment of the etched foil, an oxide film containing a valve metal is formed as a dielectric layer on the surface of the porous portion. When an electrode foil containing Al is subjected to chemical conversion treatment, the chemical conversion voltage may be, for example, 4 V or more, or 40 V or more. When the electrode foil is produced by AC etching, the chemical conversion voltage is preferably 200 V or less.

[0090] The electrode foil (etched foil) or chemical foil according to this embodiment has enhanced tensile strength and folding endurance. Specifically, the presence of the recesses alleviates stress caused by winding. For example, when the electrode foil is wound, high-quality cracks can form in the porous portions between the recesses. High-quality cracks tend to form extending from the inner walls of the recesses when viewed from the normal direction of the main surface region of the electrode foil, and can be formed so that the cracks connect to each other. High-quality cracks suppress large cracks (i.e., foil tears) that extend linearly from one end to the other end in the width direction of the electrode foil.

[0091] The formation of recesses is particularly effective in suppressing foil breakage in etched foils and chemically processed foils, which are prone to foil breakage during winding. In particular, the recesses provide significant stress relief for chemically processed foils with high chemical conversion voltages. Suppressing foil breakage also improves the vibration resistance required for, for example, automotive electrolytic capacitors. When manufacturing a laminated capacitor using a chemically processed foil with a low chemical conversion voltage (e.g., 4 to 80 V), thicker foils (increasing the porosity of the porous layer and increasing the thickness of the porous layer) may be required to achieve higher capacitance. In this case, foil breakage and cracks are likely to occur during transport and stacking of the chemically processed foil, but the recesses provide a significant stress relief effect.

[0092] The winding of the electrode foil is carried out by chemically treating the electrode foil, transporting and winding the electrode foil by rollers, slitting the electrode foil, and forming a wound body including the electrode foil.

[0093] The shape of the recess group can also control the length, shape, and direction of the crack. Depending on the arrangement of the recess group, the crack may be formed to curve slightly between the recesses.

[0094] On the other hand, consider the case where a raw foil without recesses is etched and then recesses are formed on the etched foil. In this case, when the recesses are formed, at least the inner wall surfaces of the recesses and the pores (etching pits) in their vicinity are likely to be blocked. For example, when recesses are formed by pressing a jig with protrusions against the electrode foil, some of the etching pits are blocked due to deformation of the porous portion. In this case, the surface area of ​​the electrode foil is reduced. Furthermore, when recesses are formed by laser processing, the etching pits are blocked due to melting of the laser-irradiated portion. The deeper the recesses, the greater the energy required for processing, making the etching pits more likely to be blocked due to deformation or melting. Blockage of the etching pits can reduce the capacity and the strength of the electrode foil.

[0095] In contrast, when etching is performed after forming the recess group, it is possible to avoid a decrease in the capacitance of the electrolytic capacitor and a decrease in the strength of the electrode foil due to blockage of the etching pits. In particular, this effect is particularly pronounced in electrode foils that satisfy condition (A) (a recess depth of 4 μm or more and 58 μm or less). Furthermore, in solid electrolytic capacitors that use a conductive polymer as the solid electrolyte, the retention of the conductive polymer present in the recesses is improved. Furthermore, the conductive polymer present in the recesses significantly improves ESR. Furthermore, the effect of suppressing dry-up and reducing ESR can be expected in electrolytic capacitors that contain an electrolyte solution or liquid components.

[0096] The electrode foil (etched foil) obtained by etching the raw foil described above has various structural features due to the multiple recesses that existed in the raw foil.

[0097] For example, in a cross section of the electrode foil (hereinafter simply referred to as the "cross section of the electrode foil") parallel to the thickness direction and intersecting the direction in which the linear recesses extend at an angle of 80° to 100° (e.g., 90°), the non-recessed surface where no recesses are formed in the porous portion may have an outwardly convex curved shape. The non-recessed surface between two adjacent recesses may have an outwardly convex curved portion (opposite the core portion side). This shape is generally considered to be formed from the multiple recesses that the raw foil had. When multiple recesses are formed in the raw foil by plastic deformation, at least a portion of the surface of the transition portion between the recesses and the non-recessed portion may become a smoothly curved surface that deforms continuously. As a result, the non-recessed portion may have an outwardly convex curved shape.

[0098] In the cross section of the electrode foil, when the midpoint P1 on the surface of the transition portion between the concave portion and the non-concave portion of the porous portion is identified and a line segment A is drawn connecting the pair of closest midpoints P1, the non-concave portion is divided into an area a outside the line segment A and an area inside (towards the core portion) the line segment A. Similarly, when a line segment B is drawn connecting the deepest portions of the two closest concave portions that sandwich the non-concave portion, the non-concave portion is divided into an area b outside the line segment B and an area inside (towards the core portion) the line segment B. The ratio of the area of ​​the area a to the area of ​​the area b (protrusion ratio) can be 5% or more. The protrusion ratio is calculated as the average value of measurements taken at least at five arbitrarily selected locations.

[0099] The ratio (P1 / P2) of the porosity Po1 of the thin portion between the bottom surface of the recess and the core to the porosity Po2 of the thick portion between the non-recessed surface where no recesses are formed and the core may be in the range of 0.8 or more and 1.2 or less. When a porous portion is formed in a raw foil with multiple recesses pre-formed, as in the electrode foil of this embodiment, the thin portion between the bottom surface of the recess and the core and the thick portion between the non-recessed surface and the core are etched under approximately the same conditions. Therefore, there is no significant difference between the porosity Po1 and the porosity Po2. Therefore, the P1 / P2 ratio can be in the range of 0.8 or more and 1.2 or less. However, from a microscopic perspective, structural differences may occur between the thin and thick portions, as described below.

[0100] In the cross section of the electrode foil, the midpoint P1 on the surface of the transition portion between the recessed and non-recessed portions of the porous portion is identified, and three straight lines L3 parallel to the thickness direction passing through the three closest midpoints P1 are drawn. Any point in the porous portion can be divided into a thin portion R1 and a thick portion R2 by the three straight lines. The porosity Po1 of the thin portion R1 and the porosity Po2 of the thick portion R2 can be determined by image processing of the respective SEM images. Based on common technical knowledge, the SEM images may be binarized to distinguish between voids and non-void portions. Po1, Po2, and the Po1 / Po2 ratio are each calculated as the average of measurements at at least five arbitrarily selected locations.

[0101] As an example of a microscopic structural difference between the thin and thick portions, the porosity Po3 of the surface layer of the transition portion between the thin and thick portions may be greater than the porosity Po1 and the porosity Po2.

[0102] The surface layer of the transition portion is a corner portion of a non-convex portion (in other words, a convex portion). Because current for the etching process flows easily through such corner portions, etching proceeds selectively. In this case, the porosity Po3 is greater than the porosities Po1 and Po2. The voids in such corner portions with a large porosity Po3 have a significant effect of alleviating compressive stress, etc., and are thought to contribute greatly to improving the strength of the electrode foil. Furthermore, selective application of current to the corner portions also suppresses local current concentration in areas where current concentration is not desired.

[0103] The porosity Po3 is the porosity of a portion R3 surrounded by a circle of radius r (μm) centered at the midpoint P1 of the surface of the transition portion in the cross section of the electrode foil. The radius r is preferably 50% of the depth of the recess. When embossing a plain foil with a roller having a convex surface, if the processing pressure is increased or a deep recess is formed, the bent portion of the portion R3 surrounded by a circle of radius r (μm) centered at the midpoint P1 tends to bulge, increasing the porosity of the portion R3.

[0104] As a result of the microstructural differences between the thin and thick portions, in a given cross section of the core, the thickness of the core can decrease periodically from 5 μm to 30 μm along the surface direction of the core. In both the thin and thick portions, the formation of porous portions progresses toward the core, ensuring a larger core thickness in the thick portions. Such a large-period wavy shape is characteristic of an example of the electrode foil according to this embodiment. The period can be measured by dividing the core into thin and thick portions. Such division can be performed in a manner similar to the method used to divide porous portions into thin and thick portions. The thick portion ensures a larger core thickness, and the periodic thickness change distributes stress, further improving the strength of the electrode foil.

[0105] The difference between the maximum thickness TCmax and the minimum thickness TCmin of the core may be 3% or more of the maximum thickness TCmax, may be 3% to 57%, or may be 5% to 54%. For convenience, TCmin is measured at the midpoint of the thin-walled portion of the core. For convenience, TCmax is measured at the midpoint of the thick-walled portion of the core. TCmax and TCmin are each calculated as the average of measurements taken at at least five arbitrarily selected locations.

[0106] The thickness T of the porous portion is not particularly limited and may be appropriately selected depending on the application of the electrolytic capacitor, the required capacitance, the withstand voltage, etc. The thickness T of the porous portion may be, for example, 1 / 10 or more and less than 5 / 10 (or 2 / 5 or less) of the thickness F of the raw material foil per side. In the case of an anode foil, the thickness T of the porous portion is, for example, 10 μm or more and 160 μm or less. When used in a wound capacitor, the thickness T of the porous portion is preferably 30 μm or more and 100 μm or less. When used in a high-capacity multilayer capacitor, the thickness T of the porous portion is preferably 40 μm or more and 160 μm or less.

[0107] The thickness T of the porous portion means the thickness of the porous portion in the non-recessed portion of the electrode foil, and is determined by measuring the thickness at any 10 points in the non-recessed portion using an SEM image of the cross section of the electrode foil and averaging the measured values.

[0108] The porous portion may be formed on one main surface of the electrode foil, or may be formed on both main surfaces of the metal foil.

[0109] The electrode foil (porous portion) includes a metal skeleton that constitutes the porous portion. The metal skeleton refers to a metal portion in the porous portion that has a microstructure. The porous portion has a large number of pores (etching pits) surrounded by the metal skeleton. From the viewpoint of increasing the surface area and forming the dielectric layer deep within the porous portion, the pore diameter of the porous portion may be less than 2 μm, and may be in the range of 100 nm to 1500 nm.

[0110] The shape of the pores (etching pits) may be sponge-like or tunnel-like. Tunnel-like etching pits include pits that extend from the surface side of the porous portion toward the core side.

[0111] In the case of spongy etching pits, the pore diameter range is preferably, for example, 600 nm or less, more preferably 50 nm or more and 500 nm or less. In the case of spongy etching pits, the average pore diameter Dp is preferably 80 nm or more and 400 nm or less, more preferably 100 nm or more and 300 nm or less. Electrode foils having spongy etching pits are suitable for use in low-voltage electrolytic capacitors (e.g., electrolytic capacitors using chemically formed foils formed at a chemically formed voltage of 200 V or less).

[0112] In the case of tunnel-shaped etching pits, the pore diameter range is, for example, 1900 nm or less, and may be 100 nm or more and 1800 nm or less. In the case of tunnel-shaped etching pits, the average pore diameter Dp is preferably 200 nm or more and 1700 nm, more preferably 400 nm or more and 1400 nm or less. Electrode foils having tunnel-shaped etching pits are suitable for use in medium- to high-voltage electrolytic capacitors that use chemically formed foils formed at a chemically formed voltage of 180 V or more.

[0113] The average pore diameter Dp of the porous portion is determined by measuring the pore diameter distribution of the electrode foil (porous portion) using a mercury porosimeter. Specifically, the pore diameter (mode diameter) corresponding to the apex of the peak (the largest peak if multiple peaks exist) that appears on the pore diameter distribution curve (vertical axis: log differential pore volume, horizontal axis: pore diameter) obtained by the measurement is determined as the average pore diameter Dp. For example, an AutoPore V series manufactured by Micromeritics is used as the measuring device. From such a pore diameter distribution curve, a pore diameter distribution in the pore diameter range of less than 2 μm can be obtained. Typically, the dimensions of the short sides of the recesses are much larger than the pores in the porous portion, so the dimensions of the short sides of the recesses are not reflected in the pore diameter distribution of the porous portion.

[0114] The porous portion of the electrode foil may have a dielectric layer. The dielectric layer is formed so as to cover at least a portion of the metal skeleton constituting the porous portion of the electrode foil. In this case, the electrode foil can be used mainly as an anode foil. The dielectric layer covers at least a portion of the inner wall surfaces of the pores of the porous portion.

[0115] The thickness of the dielectric layer may be 2 nm or more, 4 nm or more, 12 nm or more, or 24 nm or more. Electrode foils having a dielectric layer thickness of 24 nm or more are suitable for use as anode foils for electrolytic capacitors with a rated voltage of 20 V or more. In particular, electrode foils used in solid-liquid hybrid electrolytic capacitors preferably have a dielectric layer of 50 nm or more, and the chemical conversion voltage during chemical conversion is preferably 30 V or more. When the chemical conversion voltage is as high as 30 V or more, the dielectric layer also becomes thick, which can easily lead to issues with the strength of the electrode foil. Therefore, the stress relaxation effect of forming cracks between recesses is significant. The thickness of the dielectric layer is determined by measuring the thickness of any 10 points on the dielectric layer using SEM or TEM images of a cross section of the electrode foil in the thickness direction, and averaging these measurements.

[0116] The etched foil on which the dielectric layer has been formed may then be slit. For example, a strip-shaped electrode foil with a width of 500 mm may be slit to a width of 1.5 mm or more and 40 mm or less. The slit electrode foil may then be wound around a roller. By providing a group of recesses, even when the slit width is as small as 10 mm or less, foil tearing during winding is suppressed.

[0117] The following description will be made with reference to the drawings. Note that the raw material foil, electrode foil, and electrolytic capacitor according to the present disclosure are not limited to the embodiments shown in the drawings. Each drawing is a schematic diagram, and the shape or characteristics of each component in each drawing do not necessarily reflect the actual dimensions, and are not necessarily depicted to the same scale.

[0118] FIG. 1 is a perspective view showing an example of a rolled raw foil according to an embodiment of the present disclosure. In FIG. 1, the MD direction and the TD direction are the length direction and width direction, respectively, of the strip-shaped raw foil 1. When the raw foil 1 is a rolled foil, the length direction MD is the rolling direction of the raw foil 1 and also the winding direction. The raw foil 1 has a first main surface S1 and a second main surface S2, each of which is provided with a main surface region 100. Each main surface region 100 has a plurality of recesses (recess groups). FIG. 2 is an enlarged view of a portion of an example of the main surface region 100 as viewed from the normal direction of the main surface region 100.

[0119] As shown in FIG. 2 , the main surface region 100 of the raw foil 1 is provided with a plurality of linear recesses 110. The region other than the recesses is a non-recess 120. In the example of FIG. 2 , the linear recesses 110 are arranged in a regular stripe pattern, but the arrangement is not particularly limited and may be randomly arranged. The opening of each recess 110 when viewed from the normal direction of the main surface region 100 is a simple stripe shape, but the shape of the recess opening is not particularly limited. Various linear recesses may also be arranged randomly. The recesses may intersect with each other and may have different shapes and / or dimensions. The recesses may be arranged in a mesh or lattice pattern.

[0120] 3 shows various shapes of the opening of the linearly extending recesses as viewed from the normal direction of the main surface region 100. The linear recesses may be in a beaded shape as shown in FIGS. 3(a) to 3(c), a stripe shape diagonal to the MD direction, a square lattice shape, or the like. They may also be in a lattice shape (diagonal lattice shape) as shown in FIG. 12.

[0121] The width Lmin of the recessed portion may be, for example, 2 μm or more, but is preferably 15 μm or more and 110 μm or less. The shortest distance dmin between the edges of the nearest recessed portions 110 is, for example, 10 μm or more and 120 μm or less. The dmin / Lmin ratio is, for example, 0.2 or more and 5 or less.

[0122] In the example of FIG. 2 , from the viewpoint of significantly improving the strength of the raw foil, the length of each recess is, for example, 20 μm or more. It is preferable that each recess is continuous and uninterrupted over a length of at least 20 μm or more. The width Lmin of each recess as viewed from the normal direction of the main surface region 100 may be, for example, 15 μm or more and 110 μm or less. The L / Lmin ratio is, for example, 40 or more. The Lmin / F ratio is, for example, less than 0.5. The Lmin / H ratio is, for example, 7 or less. The shortest distance dmin between the edges of the nearest recesses is, for example, 10 μm or more and 120 μm or less. When the band-shaped raw foil is a rolled foil, the angle formed between the length direction (long side direction) and the MD direction is, for example, within a range of −20° to 20°. The extending directions of the plurality of recesses each independently form an angle within a range of −20° to 20° with a first direction parallel to the long side.

[0123] Fig. 4 is a cross-sectional view taken along line IV-IV in Fig. 2. Fig. 4 is a schematic diagram of a cross section of the raw foil 1 in Fig. 2. In the cross section of the raw foil 1, the shape of the recesses 110 is hemispherical, arc-shaped, or cone-shaped. The H / F ratio of the raw foil is, for example, 0.05 or more and 0.55 or less.

[0124] Fig. 5 is a schematic diagram of a cross section of an electrode foil corresponding to the raw foil 1 shown in Fig. 4. The electrode foil 2 in Fig. 5 is obtained by etching the raw foil 1 to form a porous portion. The electrode foil 2 has a porous portion 101 having pores and a core portion 102 continuous with the porous portion 101. The two main surface regions 100 are each the main surface region of the porous portion 101. The porous portion 101 has pores that open to the main surface region 100, but the pore diameter is less than 2 µm.

[0125] The electrode foil 2 may be a chemically processed foil having a dielectric layer in the porous portion 101. In Figures 4 and 5, no particular distinction is made between etched foil and chemically processed foil.

[0126] The thickness T of the porous portion is, for example, 1 / 10 or more and less than 5 / 10 (or 2 / 5 or less) of the thickness F of the raw material foil per side. In the case of an anode foil, the thickness T of the porous portion is, for example, 10 μm or more and 160 μm or less.

[0127] The characteristics described for the multiple recesses in the raw foil 1 substantially coincide with the characteristics for the multiple recesses in the electrode foil 2. The characteristics of the recesses 110 in the raw foil 1 are mirrored by the characteristics of the recesses 110 in the porous portion 101 of the electrode foil 2. Therefore, if the recesses 110 in the raw foil 1 are striped, the recesses 110 in the porous portion 101 of the electrode foil 2 are also striped. The width Lmin of the recesses 110 in the raw foil 1 is mirrored by the width Lmin of the recesses 110 in the porous portion 101 of the electrode foil 2, and is preferably 15 μm or more and 110 μm or less. The shortest distance dmin between the edges of the closest recesses 110 (lines) in the porous portion 101 of the electrode foil 2 is, for example, 10 μm or more and 120 μm or less, and the dmin / Lmin ratio is, for example, 0.2 or more and 5 or less. Here, dmin is the distance between horizontally adjacent lines facing the same direction. The same direction also includes directions that intersect with each other at a small angle of 20° or less.

[0128] The ratio (Lmin / F) of the width Lmin (μm) of the recess 110 to the thickness F (μm) of the electrode foil 2 is preferably less than 0.6. The Lmin / F ratio may be 0.1 or more. The ratio (Lmin / H) of the width Lmin (μm) of the recess in the porous portion 101 of the electrode foil 2 to the depth H (μm) of the recess is preferably 7 or less. The Lmin / H ratio may be 0.5 or more.

[0129] 6 is an enlarged schematic diagram of the cross section of the electrode foil 2 in FIG. 5. The surface of the non-recessed portion 120 where no recessed portion 110 of the porous portion 101 is formed has an outwardly convex curved shape. In the cross section of the porous portion 101, the surface of the non-recessed portion 120 between two adjacent recessed portions 110 has an outwardly convex curved shape. Furthermore, the width of the recessed portion 110 gradually increases on average from the deepest portion of the recessed portion 110 toward the opening. The shape of the recessed portion is a convex curved shape toward the core material side.

[0130] As shown in FIG. 6 , in the cross section of the electrode foil 2, a midpoint P1 can be identified on the surface of the transition portion between the recessed portion 110 and the non-recessed portion 120 of the porous portion 101. When the transition portion is approximated by a curve, an inflection point is usually observed. Such an inflection point may be identified as the midpoint P1. When a line segment A is drawn connecting a pair of closest midpoints P1, the non-recessed portion 120 is divided into a region a outside the line segment A and a region inside the line segment A (toward the core). When a line segment B is drawn connecting the deepest portions of the two closest recessed portions 110 sandwiching the non-recessed portion 120, the non-recessed portion 120 is divided into a region b outside the line segment B and a region inside the line segment B (toward the core). Therefore, the ratio of the area of ​​region a to the area of ​​region b (protrusion ratio) can be calculated.

[0131] 6, when three straight lines L3 are drawn that pass through the three closest midpoints P1 and are parallel to the thickness direction of the electrode foil 2, the porous portion 101 is divided into a thin portion R1 and a thick portion R2 by the three straight lines L3. Therefore, the porosity Po1 of the thin portion R1 and the porosity Po2 of the thick portion R2 can be calculated.

[0132] FIG. 6 shows a circle of radius r (= H / 2 (μm)) centered at the midpoint P1 of the surface of the transition portion. The porosity of the portion R3 surrounded by the circle is porosity Po3. Po3 is greater than porosity Po1 and porosity Po2. For example, at least one of 1.1≦Po3 / Po1 and 1.1≦Po3 / Po2 is satisfied. At least one of 1.2≦Po3 / Po1 and 1.2≦Po3 / Po2 may also be satisfied.

[0133] 5 and 6 , the thickness of the core portion 102 decreases periodically along the surface of the core portion, at intervals of 5 μm to 30 μm. The thin and thick portions of the core portion can be separated in the same manner as the porous portion 101 is separated into thin and thick portions R1 and R2. That is, in the cross section of the electrode foil 2, a midpoint P2 of the transition portion from the thin portion to the thick portion of the core portion 102 can be identified on the surface. When the transition portion is approximated by a curve, an inflection point is usually observed. Such an inflection point may be identified as the midpoint P2. The midpoint P2 can be used to separate the core portion 102 into thin and thick portions. The thickness of the core portion 102 can be said to decrease periodically, at a distance between the pair of closest midpoints P2 that sandwich the thick portion of the core portion. The distance between these P2s is measured at five arbitrary locations and calculated as an average. The difference between the maximum thickness TCmax and the minimum thickness TCmin of the core portion 102 is, for example, 3% or more and 57% or less of the maximum thickness TCmax.

[0134] 7 is an SEM image of an electrode foil according to an embodiment of the present disclosure, viewed from the normal direction, showing a portion of the main surface region of the electrode foil. The vertical direction of the SEM image in Fig. 7 is the TM direction (the length direction (winding direction) of the strip-shaped electrode foil) and is also the rolling direction of the raw material foil.

[0135] 8 is an SEM image of a cross section of an electrode foil according to an embodiment of the present disclosure. The electrode foil has a recess 110 with a cone-shaped cross section that opens to the main surface region 100. The electrode foil 2 is an etched foil, and porous portions 101 are formed in the recess 110 and non-recess 120 of the main surface region 100. The porous portions 101 have pores (etching pits) that open to the recess 110 and non-recess 120.

[0136] [Electrolytic Capacitor] An electrolytic capacitor according to an embodiment of the present disclosure includes an electrode foil (etched foil or chemically formed foil) according to the present disclosure. The type of such electrolytic capacitor is not limited, and the electrode foil according to the present disclosure can be widely applied to electrolytic capacitors that include electrode foil. The electrode foil according to the present disclosure may be used, for example, as at least one of the anode foil and cathode foil of a wound electrolytic capacitor, or as an anode body of a laminated electrolytic capacitor.

[0137] The following describes the configuration of a wound electrolytic capacitor as an example. The wound electrolytic capacitor includes a wound body and an electrolyte. The wound body may be formed by winding an anode foil and a cathode foil with a separator interposed therebetween. The wound body and the electrolyte are also collectively referred to as a capacitor element. At least one of the anode foil and the cathode foil includes the above-mentioned electrode foil. The electrolyte may be an electrolytic solution, a solid electrolyte, or a combination of a liquid component and a solid electrolyte.

[0138] In the wound body, good quality cracks may exist between the recesses of the electrode foil. The cracks are formed by winding the electrode foil having a group of recesses, and are formed so as to connect the recesses. It is preferable that the cracks extend so as to connect at least two or more recesses in a direction (width direction) intersecting the winding direction of the electrode foil. For example, the cracks are formed so as to connect 2 to 100 recesses.

[0139] Fig. 9 is a cross-sectional view schematically illustrating an electrolytic capacitor according to an embodiment of the present disclosure. Fig. 10 is a perspective view schematically illustrating the configuration of a wound body. In Fig. 10, the MD direction indicates the length direction of the strip-shaped anode foil 10 and cathode foil 20, and the TD direction indicates the width direction of the anode foil 10 and cathode foil 20.

[0140] Electrolytic capacitor 3 includes a capacitor element. The capacitor element includes a wound body 4 and an electrolyte. Wound body 4 is formed by winding anode foil 10 and cathode foil 20 with separator 30 interposed therebetween. Height Lc of wound body 4 is approximately equal to the dimension of anode foil 10 and cathode foil 20 in the width direction (TD direction).

[0141] One end of each of lead tabs 50A and 50B is connected to anode foil 10 and cathode foil 20, respectively, and lead tabs 50A and 50B are wound to form wound body 4. Lead wires 60A and 60B are connected to the other ends of lead tabs 50A and 50B, respectively.

[0142] The outermost layer of the wound body 4 is the cathode foil 20, and a stop tape 40 is disposed on the outer surface of the cathode foil 20. The ends of the cathode foil 20 are fixed by the stop tape 40. When the anode foil 10 is prepared by cutting it from a large foil, the wound body 4 may be further subjected to a chemical conversion treatment in order to provide a dielectric layer on the cut surface.

[0143] An electrolyte is interposed between the anode foil 10 (dielectric layer) and the cathode foil 20 of the wound body 4. The capacitor element is obtained, for example, by impregnating the wound body 4 with a treatment liquid containing the electrolyte. The impregnation may be performed under reduced pressure (for example, in an atmosphere of 10 kPa to 100 kPa).

[0144] The wound body 4 is housed in the bottomed case 211 so that the lead wires 60A and 60B are located on the opening side of the bottomed case 211. The material of the bottomed case 211 is a metal, and aluminum, stainless steel, copper, iron, brass, or an alloy thereof can be used.

[0145] A sealing member 212 is placed at the opening of the bottomed case 211 in which the wound body 4 is housed, the open end of the bottomed case 211 is crimped to the sealing member 212 and curled, and a seat plate 213 is placed at the curled portion, thereby sealing the wound body 4 inside the bottomed case 211.

[0146] The sealing member 212 is formed so as to allow the lead wires 60A and 60B to pass therethrough. The sealing member 212 may be made of any insulating material, but is preferably made of an elastic material. Among these, highly heat-resistant materials such as silicone rubber, fluororubber, ethylene propylene rubber, hypalon rubber, butyl rubber, and isoprene rubber are preferred.

[0147] The height Lc of the wound body (Lc in FIG. 9) is, for example, 50 mm or less, or may be 20 mm or less, or 15 mm or less, and is approximately equal to or slightly longer than the width dimension of the electrode foil.

[0148] When the product size is large (for example, when the height Lc of the wound body is 30 mm or more), an electrode foil with a large width (for example, 30 mm or less) is used. Electrode foil with a large width is prone to breakage due to twisting. By providing a group of recesses, foil breakage caused by twisting of electrode foil with a large width is significantly suppressed. This improves the reliability of large-sized products (for example, large capacitors with screw terminals or lead terminals).

[0149] On the other hand, when the product size is small (e.g., when the height Lc of the wound body is 20 mm or less), an electrode foil with a small width (e.g., 20 mm or less) is used. A small-width electrode foil can be obtained by slitting a large-width electrode foil (e.g., 125 mm or more and 500 mm or less) to the desired small width (20 mm or less). The presence of the recesses distributes the stress applied to the electrode foil during slitting, suppressing the occurrence of cracks caused by the stress and the resulting foil breakage. This significantly improves the quality of electrode foil with small width dimensions. Furthermore, in the capacitor manufacturing process, the presence of the recesses significantly suppresses foil breakage due to tension generated during roller transport. As a result, aging (repair chemical formation) during the manufacturing of electrolytic capacitors is performed stably. This stabilizes the leakage current of the capacitor and improves the reliability of small-sized products.

[0150] 11 is a diagram schematically illustrating an example of a wound body as viewed from the end face side. The wound body 4 is configured by winding an anode foil and a cathode foil around a winding core 410 with a separator interposed therebetween. When the radial thickness from the innermost periphery E1 to the outermost periphery E2 of the wound body 4 is defined as t, the "region P" refers to a region of the wound body 4 whose radial distance from the innermost periphery E1 is equal to or less than (¼)t.

[0151] Good quality cracks can be present at least in region P of the wound body. There may be more of these cracks in region P than in regions other than region P. Breakage of the electrode foil during winding tends to occur in region P, where the stress caused by winding is likely to be large. On the other hand, good quality cracks that relieve stress are likely to be formed in region P due to the winding of the electrode foil. By having these cracks present in region P, it is possible to efficiently suppress breakage of the electrode foil during winding.

[0152] The recessed portions may be provided in one main surface region of the electrode foil, or may be provided in both main surface regions of the electrode foil. The recessed portions may have a higher number density in the recessed portions provided in one main surface region of the electrode foil than in the recessed portions provided in the other main surface region of the electrode foil. The stress generated by winding the electrode foil is likely to be greater on the outer main surface of the wound body than on the inner main surface. Therefore, it is preferable that the electrode foil is wound so that the one main surface of the electrode foil on which the recessed portions are provided faces the outer periphery of the wound body.

[0153] In the case where the stress caused by winding is large, the recessed portions may be provided on both main surfaces of the electrode foil. In the region P of the wound body, the stress caused by winding is large, and it is desirable to provide the recessed portions on both main surfaces of the electrode foil.

[0154] (Anode Foil) The anode foil may be the electrode foil described above. The thickness of the anode foil may be, for example, 60 μm or more and 200 μm or less, or 80 μm or more (or 100 μm or more) and 200 μm or less. The larger the capacity, the thicker the anode foil tends to be. For example, in the case of a high-capacity anode foil with a thickness of 100 μm or more, the stress relaxation effect due to crack formation between recesses is significantly obtained.

[0155] (Cathode foil) The cathode foil may be the electrode foil described above. In the case of a cathode foil, a coating layer may be further formed on the surface of the raw foil or electrode foil. Examples of the coating layer include a carbon layer, a metal oxide layer, a metal nitride layer, a metal carbide layer, and other conductive layers (e.g., a layer containing at least one of a metal and carbon). The thickness of the cathode foil is, for example, 10 μm or more and 70 μm or less.

[0156] (Separator) The separator is not particularly limited, and may be, for example, a nonwoven fabric containing fibers of cellulose, polyethylene terephthalate, vinylon, or polyamide (for example, aliphatic polyamide, aromatic polyamide such as aramid).

[0157] (Electrolyte) The electrolyte covers at least a portion of the anode foil (dielectric layer) and is interposed between the anode foil (dielectric layer) and the cathode foil. The electrolyte may be a solid or liquid electrolyte. The electrolytic capacitor may contain a liquid component (electrolytic solution or non-aqueous solvent) in addition to the solid electrolyte.

[0158] The solid electrolyte preferably contains a conductive polymer. Examples of the conductive polymer include π-conjugated polymers. Examples of the conductive polymer include polypyrrole, polythiophene, polyfuran, and polyaniline. The conductive polymer may be used alone or in combination of two or more types, or may be a copolymer of two or more types of monomers. The weight-average molecular weight of the conductive polymer is, for example, 1,000 to 100,000.

[0159] In this specification, polypyrrole, polythiophene, polyfuran, polyaniline, etc. refer to polymers having polypyrrole, polythiophene, polyfuran, polyaniline, etc. as their basic skeletons, respectively. Therefore, polypyrrole, polythiophene, polyfuran, polyaniline, etc. may also include their respective derivatives. For example, polythiophene includes poly(3,4-ethylenedioxythiophene), etc.

[0160] The conductive polymer may be doped with a dopant. The solid electrolyte may contain a dopant together with the conductive polymer. Examples of the dopant include polystyrene sulfonic acid. The solid electrolyte may further contain an additive, if necessary.

[0161] The liquid component is in contact with the dielectric layer directly or via a conductive polymer. The liquid component may be a non-aqueous solvent or an electrolyte solution. The electrolyte solution contains a non-aqueous solvent and an ionic substance (solute (e.g., organic salt)) dissolved therein. The non-aqueous solvent may be an organic solvent or an ionic liquid.

[0162] The non-aqueous solvent is preferably a high-boiling solvent, for example, a polyol compound such as ethylene glycol, a sulfone compound such as sulfolane, a lactone compound such as γ-butyrolactone, an ester compound such as methyl acetate, a carbonate compound such as propylene carbonate, an ether compound such as 1,4-dioxane, or a ketone compound such as methyl ethyl ketone.

[0163] The solute may contain an acid component (anion) and a base component (cation). The acid component and the base component may form a salt. The acid component contributes to the repair function of the dielectric layer. Examples of the acid component include organic carboxylic acids and inorganic acids. Examples of the inorganic acid include phosphoric acid, boric acid, and sulfuric acid. Examples of the base component include primary to tertiary amine compounds.

[0164] The organic salt is a salt in which at least one of the anion and cation contains an organic substance. Examples of the organic salt include trimethylamine maleate, triethylamine borodisalicylate, ethyldimethylamine phthalate, mono-1,2,3,4-tetramethylimidazolinium phthalate, and mono-1,3-dimethyl-2-ethylimidazolinium phthalate.

[0165] From the viewpoint of suppressing dedoping of the dopant from the conductive polymer (deterioration of the solid electrolyte), the liquid component preferably contains more acid components than base components. Furthermore, since the acid components contribute to the film repair function of the liquid component, it is also preferable that the liquid component contains more acid components than base components. The molar ratio of the acid component to the base component (acid component / base component) is, for example, 1.1 or more. From the viewpoint of suppressing dedoping of the dopant from the conductive polymer, the pH of the liquid component may be 6 or less, or may be 1 or more and 5 or less.

[0166] <<Supplementary Notes>> The above embodiments disclose the following technologies. (Technology 1) A metal foil for an electrolytic capacitor, wherein the metal foil contains a valve metal and has two main surface regions, at least one of which has a plurality of linearly extending recesses, and the depth of the recesses is 4 μm or more and 58 μm or less. (Technology 2) A metal foil for an electrolytic capacitor, wherein the metal foil contains a valve metal and has two main surface regions, at least one of which has a plurality of linearly extending recesses, and the proportion of the projected area of ​​the recesses in the main surface region is 8% or more and 45% or less. (Technology 3) A metal foil for an electrolytic capacitor according to Technology 1 or 2, wherein the length of the recesses is 20 μm or more. (Technology 4) A metal foil for an electrolytic capacitor according to any one of Technology 1 to 3, wherein the width of the recesses is 15 μm or more and 110 μm or less. (Technology 5) The metal foil for electrolytic capacitors according to any one of Technologies 1 to 4, wherein the ratio of the length of the recess to the width of the recess is 1.01 or more. (Technology 6) The metal foil for electrolytic capacitors according to any one of Technologies 1 to 5, wherein the plurality of recesses are striped, and the shortest distance between edges of the recesses closest to each other in the width direction of the recesses is 10 μm or more and 120 μm or less, or the plurality of recesses are mesh-shaped, and the shortest distance between opposing sides of squares surrounded by the recesses is 10 μm or more and 120 μm or less. (Technology 7) The metal foil for electrolytic capacitors according to any one of Technologies 1 to 6, wherein the metal foil has a shape having long sides and short sides, and the extension directions of the plurality of recesses are each independently parallel, perpendicular, or oblique to a first direction parallel to the long sides. (Technology 8) The metal foil for an electrolytic capacitor according to any one of Techniques 1 to 7, wherein the metal foil has a porous portion having pores and a core portion continuous with the porous portion, and the two main surface regions are each a main surface region of the porous portion. (Technology 9) The metal foil for an electrolytic capacitor according to Technique 8, wherein a non-recessed surface of the porous portion where the recesses are not formed has an outwardly convex curved shape.(Technology 10) The metal foil for an electrolytic capacitor according to Technology 8 or 9, wherein the ratio of the porosity Po1 of a thin portion between the bottom surface of the recess and the core to the porosity Po2 of a thick portion between the core and a non-recessed surface where the recess is not formed, Po1 / Po2, is in the range of 0.8 to 1.2. (Technology 11) The metal foil for an electrolytic capacitor according to Technology 10, wherein the porosity Po3 of a surface layer portion of the porous portion in a transition portion between the thin portion and the thick portion is greater than the porosity Po1 and the porosity Po2. (Technology 12) The metal foil for an electrolytic capacitor according to any one of Technology 8 to 11, wherein, in a predetermined cross section of the core, the thickness of the core decreases in a period of 5 μm to 30 μm along the surface direction of the core. (Technology 13) The metal foil for electrolytic capacitors according to Technology 12, wherein the difference between the maximum thickness TCmax and the minimum thickness TCmin of the core portion is 3% or more of the maximum thickness TCmax. (Technology 14) The metal foil for electrolytic capacitors according to any one of Technology 8 to 13, wherein the porous portion further has a dielectric layer. (Technology 15) An electrolytic capacitor comprising the metal foil for electrolytic capacitors according to Technology 14.

[0167] [Examples] Hereinafter, the present disclosure will be specifically described based on examples and comparative examples, but the present disclosure is not limited to the examples.

[0168] Examples 1 to 30 and Comparative Example 1 (Preparation of Raw Foil) First, a strip of aluminum foil (plain foil, thickness F: 150 μm, average crystal grain size: 40 μm) was prepared. The aluminum foil was a rolled foil in which the rolling direction (MD direction) was parallel to the length direction. The aluminum foil had an aluminum content of 99.98 mass%, a silicon content of 40 mass ppm, an iron content of 40 mass ppm, and a copper content of 30 mass ppm.

[0169] A predetermined jig was used to form a plurality of linear recesses on both sides of the Al foil, to obtain a raw foil (thickness F: 150 μm) having a group of recesses arranged in a stripe pattern as shown in FIG. 2. The type of jig was adjusted appropriately to obtain the various parameters of the recesses shown in Table 1. The raw foil has two main surface regions. A plurality of linear recesses were formed in each of the main surface regions. Various other recesses were also formed. The parameters of the recesses in both main surface regions are summarized in Table 1.

[0170] <Parameters of recesses> H: Depth of recess (μm) Ras: Proportion of recess projected area (%) Lmin: Width of recess (μm) (or short side of island-shaped recess (μm)) dmin: Shortest distance between edges of nearest recesses (μm) F: Thickness of raw foil (electrode foil) (μm) T: Thickness of porous portion (μm) Tensile strength of raw foil: 65 N / mm ２ Elongation of raw foil: 10% or more Number of crystal grains of Al foil exposed on the inner wall surface of the recess: 3 or more

[0171] Ld: perimeter density of recesses (μm / mm ２ ) (total length of contour lines defining the shapes of the openings of all recesses formed per unit area of ​​the main surface region) Nd: number density of recesses (number / mm ２ ) (number of recesses formed per unit area of ​​the main surface region) Ln: perimeter per recess (μm) (length of the outline defining the shape of the opening of one recess) Sn: projected area per recess (μm ２ ) (projection area of ​​one recess onto the main surface region when the recess is viewed from the normal direction of the main surface region)

[0172] Recess groups a1 to a6 and recess groups b1 to b6 have linear recesses. Recess groups c1 to c6 have island-shaped recesses, but Ln is an average value due to the random shape achieved by blasting or the like. Recess groups c1 and c2 have recesses with an Ln of at least 0.1 to 2.6 μm, and recess groups c3 and c4 have recesses with an Ln of at least 0.1 to 3.2 μm. Recess groups c5 and c6 have recesses with an Ln of at least 0.1 to 3.4 μm. Recess groups d1 to d6 are generally as shown in FIG. 2, and have recesses with rectangular openings arranged in a matrix. FIG. 14 shows an SEM photograph of an example of the uneven regions of recess groups d1 to d6 viewed from the normal direction. Recess group e has random needle-shaped recesses, recess group f has random island-shaped recesses achieved by blasting, recess group g has lattice-shaped recesses, and recess group h has intermittently formed oblique recesses.

[0173]

[0174] (Etching Treatment (Formation of Porous Portions)) The raw foil was etched to form porous portions (thickness T: 60 μm, average pore diameter Dp: 0.2 μm) with spongy etching pits on both main surfaces of the Al foil. Etching was performed under predetermined conditions by applying an alternating current in an etching solution containing hydrochloric acid. In this way, an etched foil was obtained.

[0175] (Chemical Conversion Treatment) The etched foil was further subjected to a chemical conversion treatment to form a dielectric layer having a withstand voltage equivalent to 10 V on the surface of the metal portion constituting the porous portion, thereby obtaining a chemically converted foil. In this manner, electrode foils a1 to h of Examples 1 to 30 were obtained, respectively.

[0176] Comparative Example 1 An electrode foil r1 was produced in the same manner as in Example 1(a1), except that the recessed portion group was not formed in the Al foil (plain foil).

[0177] [Evaluation] The electrode foils (chemically formed foils) obtained in the examples and comparative examples were evaluated as follows.

[0178] [Evaluation 1: Measurement of folding endurance] The folding endurance of the electrode foil in the width direction (TD) was measured. The measurement was performed in accordance with the test method for electrode foil for aluminum electrolytic capacitors (EIAJ RC-2364A) of the Electronic Industry Standards of Japan. The measurement was performed using a test piece obtained by cutting the electrode foil to a length of 100 mm and a width of 10 mm.

[0179] [Evaluation 2: Measurement of Capacity] The capacity of the electrode foil was measured in accordance with the test method for electrode foil for aluminum electrolytic capacitors (EIAJ RC-2364A) of the Electronic Industrial Standards of Japan.

[0180] The folding endurance and capacity were expressed as relative values ​​when the folding endurance and capacity of the electrode foil r1 of Comparative Example 1 were set to 100. The larger the value, the better. The evaluation results are shown in Table 2.

[0181]

[0182] The electrode foils a1 to h had higher folding strength and capacitance than the electrode foil r1.

[0183] Comparative Example 2 (Preparation of Etched Foil) An Al foil was etched to form a porous portion (thickness T: 45 μm, average pore diameter Dp: 0.2 μm) with spongy pits on both main surfaces of the Al foil. In this way, an etched foil was obtained. The Al foil used was the same as that used in Example 1 (a1).

[0184] (Formation of Recess Groups on Etched Foil) Recess groups were formed on both main surfaces of the etched foil using a predetermined jig. The recess groups were formed in the same manner as in Example 1.

[0185] Except for the above, electrode foil r2 was produced and evaluated in the same manner as electrode foil a1 in Example 1. The evaluation results of electrode foils a1 and r1 as well as electrode foil r2 are shown in Table 3. The folding strength and capacity were expressed as relative values ​​when the folding strength and capacity of electrode foil r1 in Comparative Example 1 were set to 100, respectively.

[0186]

[0187] The electrode foil r2 had a higher folding strength than the electrode foil r1, but the capacity was lower.

[0188] When etching a raw foil and then forming a group of recesses on the etched foil, etching pits located near the inner wall surfaces of the recesses are blocked during the formation of the group of recesses, making it difficult to form etching pits that open on the inner wall surfaces of the recesses, thereby reducing the surface area of ​​the metal foil and decreasing the capacitance. In this case, the etching pits near the inner wall surfaces of the recesses are blocked during the formation of the recesses, preventing the conductive polymer dispersion (or electrolyte) from penetrating deep into the etching pits, and this portion may not contribute to the capacitance.

[0189] In contrast, the electrode foil a1 had a higher folding strength and a higher capacity than the electrode foil r1.

[0190] When a metal foil (raw foil) having a group of recesses is etched, etching pits are formed on the main surface of the metal foil, opening onto the inner wall surfaces of the recesses. This increases the surface area of ​​the metal foil and enables higher capacitance. Furthermore, the reduction in strength and capacitance due to deformation (melting) of the porous portion during the formation of the group of recesses can be avoided. Furthermore, because the etching pits near the inner wall surfaces are not blocked, the conductive polymer dispersion (or electrolyte) penetrates deep into the etching pits, allowing the deep portions of the etching pits to also contribute to capacitance. In hybrid electrolytic capacitors that use both solid electrolytes and electrolytes, a small amount of electrolyte can efficiently penetrate deep into the pits, making it easier to achieve the ESR reduction effect of the electrolyte.

[0191] The electrode foil according to the present disclosure is suitable for use in electrolytic capacitors that require high reliability.

[0192] While the present invention has been described in terms of presently preferred embodiments, such disclosure is not to be interpreted as limiting. Various changes and modifications will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. It is therefore intended that the appended claims be interpreted to cover all changes and modifications that do not depart from the true spirit and scope of the invention.

[0193] 1: raw foil, 2: electrode foil, 3: electrolytic capacitor, 4: wound body, 100: main surface region, 101: porous portion, 102: core portion, 110: recessed portion, 120: non-recessed portion, 10: anode foil, 20: cathode foil, 30: separator, 40: winding stop tape, 50A, 50B: lead tabs, 60A, 60B: lead wires, 211: bottomed case, 212: sealing member, 213: seat plate, 410: winding core

Claims

1. A metal foil for an electrolytic capacitor, the metal foil comprising a valve metal and having two main surface regions, at least one of the main surface regions having a plurality of linearly extending recesses, the depth of the recesses being 4 μm or more and 58 μm or less.

2. A metal foil for an electrolytic capacitor, the metal foil comprising a valve metal and having two main surface regions, at least one of the main surface regions having a plurality of linearly extending recesses, the proportion of the projected area of the recesses in the main surface region being 8% or more and 45% or less.

3. The metal foil for electrolytic capacitors according to claim 1 or 2, wherein the length of the recess is 20 μm or more.

4. The metal foil for electrolytic capacitors according to claim 1 or 2, wherein the width of the recess is 15 μm or more and 110 μm or less.

5. The metal foil for electrolytic capacitors according to claim 4, wherein the ratio of the length of said recess to the width of said recess is 1.01 or more.

6. The metal foil for electrolytic capacitors according to claim 1 or 2, wherein the plurality of recesses are in a stripe pattern, and the shortest distance between the edges of the recesses that are closest to each other in the width direction of the recesses is 10 μm or more and 120 μm or less, or the plurality of recesses are in a mesh pattern, and the shortest distance between opposing sides of a square surrounded by the recesses is 10 μm or more and 120 μm or less.

7. The metal foil for electrolytic capacitors according to claim 1 or 2, wherein the metal foil has a shape having long sides and short sides, and the directions in which the plurality of recesses extend are each independently parallel, perpendicular, or oblique to a first direction parallel to the long sides.

8. The metal foil for electrolytic capacitors according to claim 1 or 2, wherein the metal foil has a porous portion having pores and a core portion continuous with the porous portion, and the two main surface regions are each the main surface regions of the porous portion.

9. The metal foil for an electrolytic capacitor according to claim 8, wherein the non-recessed surface of the porous portion where the recesses are not formed is curved and convex outward.

10. The metal foil for electrolytic capacitors according to claim 8, wherein the ratio (Po1 / Po2) of the porosity Po1 of the thin portion between the bottom surface of the recess and the core to the porosity Po2 of the thick portion between the core and the non-recess surface where the recess is not formed is in the range of 0.8 or more and 1.2 or less.

11. The metal foil for an electrolytic capacitor according to claim 10, wherein the porosity P3 of the surface layer of the porous portion in the transition portion between the thin portion and the thick portion is greater than the porosity Po1 and the porosity Po2.

12. The metal foil for electrolytic capacitors according to claim 8, wherein the thickness of the core decreases in a predetermined cross section of the core at intervals of 5 μm or more and 30 μm or less along the surface direction of the core.

13. The metal foil for an electrolytic capacitor according to claim 12, wherein the difference between the maximum thickness TCmax and the minimum thickness TCmin of the core portion is 3% or more of the maximum thickness TCmax.

14. The metal foil for an electrolytic capacitor according to claim 8, wherein the porous portion further comprises a dielectric layer.

15. An electrolytic capacitor comprising the metal foil for electrolytic capacitors according to claim 14.

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