Patent Publication Number: US-2023137973-A1

Title: Electrolytic capacitor and manufacturing method therefor

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to an electrolytic capacitor and a method for manufacturing an electrolytic capacitor. 
     2. Description of the Related Art 
     An electrolytic capacitor includes an electrode foil that a first principal surface and a second principal surface opposite to the first principal surface, and a lead member that is connected to the electrode foil. The electrode foil and the lead member are connected by steps (a) and (b). In step (a), the lead member is overlapped on the first principal surface of the electrode foil, and a predetermined position of an overlapping part is drilled from the lead member side by using a needle-shaped member to form a through-hole. In step (b), a portion of the lead member is led out from a peripheral edge portion of the through-hole above the second principal surface of the electrode foil by drilling, and then folded back onto the second principal surface and tightened. 
     Unexamined Japanese Patent Publication No. 2021-97163 proposes an electrolytic capacitor that includes a pair of electrode bodies connected to a lead-out terminal and an electrolyte interposed between the electrode bodies. In the electrolytic capacitor, one or both of the electrode bodies are graphite exposed electrode bodies and have a carbon layer containing graphite exposed on an outer surface. The lead-out terminal and the graphite exposed electrode body have a stitch connection structure, and the stitch connection structure has a through-hole penetrating the graphite exposed electrode body, a burr generated only in the lead-out terminal and led out from the through-hole to a back side surface of the graphite exposed electrode body, and a folded part of the burr folded back onto the back surface side of the graphite exposed electrode body. 
     SUMMARY 
     An electrolytic capacitor according to one aspect of the present disclosure includes an electrode foil and a lead member connected to the electrode foil. The electrode foil has a first principal surface and a second principal surface opposite to the first principal surface. The electrode foil and the lead member are connected by a caulking part in an overlapping part in which the first principal surface of the electrode foil and the lead member overlap each other. The caulking part has a through-hole penetrating the electrode foil and the lead member. The electrode foil in the caulking part includes a first folded part that is folded back at a peripheral edge portion of the through-hole to be disposed on the second principal surface. The lead member in the caulking part includes (i) a penetrating part that penetrates the electrode foil and (ii) a second folded part that is folded back at an end portion of the penetrating part to be disposed on the second principal surface. The penetrating part includes an inner wall of the through-hole. The second folded part covers the first folded part. 
     A method for manufacturing an electrolytic capacitor according to another aspect of the present disclosure includes a first step of preparing an electrode foil and a lead member, the electrode foil having a first principal surface and a second principal surface opposite to the first principal surface, and a second step of connecting the electrode foil and the lead member. The second step includes a second A step, a second B step, and a second C step to be described below. In the second A step, a preliminary through-hole is formed in the electrode foil and then the lead member is overlapped in a predetermined region of the electrode foil that includes the preliminary through-hole in the first principal surface. In the second B step, after the second A step, a hole is formed by piercing a needle-shaped member in a position of the lead member corresponding to the preliminary through-hole from a surface of the lead member to lead out a peripheral edge portion of the hole of the lead member above the second principal surface of the electrode foil. And then the needle-shaped member is allowed to pass through the preliminary through-hole to push and widen the preliminary through-hole so that a peripheral edge portion of the preliminary through-hole of the electrode foil protrudes from the second principal surface of the electrode foil. In the second C step, portion P 1  of the electrode foil and a portion P 2  of the lead member onto the second principal surface to tighten a folded part of the portion P 1  and a folded part of the portion P 2  to the second principal surface. The portion P 1  protrudes from the second principal surface of the electrode foil, The portion P 2  is led out above the second principal surface. In the second B step, diameter D 1  of the preliminary through-hole is smaller than a diameter D 2  of the needle-shaped member. The diameter D 1  corresponds to the diameter D 2  of the needle-shaped member when the needle-shaped member is allowed to pass through the preliminary through-hole. In the second C step, portion P 2  is folded back to cover the folded part of portion P 1  on the second principal surface. 
     According to the present disclosure, the electrolytic capacitor, contact resistance between the lead member and the electrode foil can be reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a front view illustrating main parts of an electrode foil and a lead member in an electrolytic capacitor according to an exemplary embodiment of the present disclosure as viewed from a lead member side; 
         FIG.  2    is a front view illustrating main parts of the electrode foil and the lead member in the electrolytic capacitor according to the exemplary embodiment of the present disclosure as viewed from an electrode foil side; 
         FIG.  3    is an enlarged front view illustrating a first caulking part in  FIG.  2   ; 
         FIG.  4    is a sectional view taken along line X-X of  FIG.  2   ; 
         FIG.  5    is a sectional view schematically illustrating main parts of the electrode foil and the lead member after a second A step (before drilling) in the method for manufacturing an electrolytic capacitor according to the exemplary embodiment of the present disclosure; 
         FIG.  6    is a sectional view schematically illustrating main parts of the electrode foil and the lead member after a second B step (after drilling) in the method for manufacturing an electrolytic capacitor according to the exemplary embodiment of the present disclosure; 
         FIG.  7    is a sectional view schematically illustrating the electrolytic capacitor according to the exemplary embodiment of the present disclosure; and 
         FIG.  8    is a perspective view schematically illustrating a configuration of a wound body included in the electrolytic capacitor according to the exemplary embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTIONS OF EMBODIMENTS 
     In an electrolytic capacitor, it is required to reduce contact resistance between a lead member and an electrode foil. 
     [Electrolytic Capacitor] 
     An electrolytic capacitor according to an exemplary embodiment of the present disclosure includes an electrode foil that has a first principal surface and a second principal surface opposite to the first principal surface, and a lead member that is connected to the electrode foil. The electrode foil and the lead member are connected by a caulking part in an overlapping part in which the first principal surface of the electrode foil and the lead member overlap each other. 
     The caulking part has a through-hole penetrating the electrode foil and the lead member. The electrode foil in the caulking part includes a first folded part that is folded back at a peripheral edge portion of the through-hole to be disposed on the second principal surface. The lead member in the caulking part includes (i) a penetrating part that penetrates the electrode foil and (ii) a second folded part that is folded back at an end portion of the penetrating part to be disposed on the second principal surface. The penetrating part includes an inner wall of the through-hole. The second folded part covers the first folded part. 
     Note that “the second folded part covers the first folded part” means that the following conditions (i) and (ii) are satisfied. 
     Condition (i): When the caulking part is viewed from a normal direction of the second principal surface of the electrode foil, area S 1  of a region where the second folded part covers the first folded part is smaller than area S 2  of the second folded part. 
     Condition (ii): When the caulking part is viewed from the normal direction of the second principal surface of the electrode foil, area S 3   a  of a region where the first folded part is covered with the second folded part is larger than area S 3   b  of a region where the first folded part is not covered with the second folded part, or the entire first folded part is covered with the second folded part. 
     A ratio S 1 /S 2  of area S 1  to area S 2  may be, for example, greater than or equal to ⅓ (or greater than or equal to ½) and less than 1, may be greater than or equal to ⅓ (or greater than or equal to ½) and less than or equal to 9/10, may be greater than or equal to ⅓ (or greater than or equal to ½) and less than or equal to ⅘. 
     When the second folded part is constituted by a plurality of caulking pieces which are formed to be separated with each other, ½ or more of the plurality of caulking pieces may satisfy S1/S2&lt;1, and ¾ or more (or all) of the plurality of caulking pieces preferably satisfy S1/S2&lt;1. 
     In the case of caulking part  710  illustrated in  FIG.  3   , second folded part  412  is constituted by four caulking pieces (four triangular portions) which are separated with each other. And each of the four caulking pieces has a region (four hatched portions in  FIG.  3   ) which covers first folded part  312 . In this case, area S 2  in the above condition (i) is an area of one caulking piece (one triangular portion). And area S 1  is an area of one hatched portion in one caulking piece. In caulking part  710  illustrated in  FIG.  3   , S1/S2&lt;1 is satisfied in all the four caulking pieces. 
     A ratio S 3   b /S 3   a  of area S 3   b  to area S 3   a  may range, for example, from 0 to ⅓ (or to ½), inclusive, or from 0 to ¼, inclusive. 
     In the case of the caulking part  710  illustrated in  FIG.  3   , area S 3   a  of the above condition (ii) is an area of a region where first folded part  312  is covered with second folded part  412  (four caulking pieces) (an area obtained by adding four hatched portions in  FIG.  3   ). And area S 3   b  is a value obtained by subtracting area S 3   a  from the entire area of first folded part  312 . In caulking part  710  illustrated in  FIG.  3   , S 3   b /S 3   a&lt; 1 is satisfied. 
     The caulking part that includes the first folded part and the second folded part is formed by a manufacturing method to be described later. When the second folded part is formed to cover the first folded part, the first folded part is pressed together with the second folded part in a state of being wrapped by the second folded part on the second principal surface when the caulking part is formed (when the overlapping part is pressed). Thus, the first folded part and the second folded part firmly adhere to each other, and a region where the first folded part and the second folded part adhere to each other is also sufficiently secured. As a result, contact resistance between the lead member and the electrode foil is reduced. Connection strength between the lead member and the electrode foil is increased, and even when a load is applied to the connecting part (caulking part) between the lead member and the electrode foil, low contact resistance can be maintained. 
     When the second folded part does not cover the first folded part, the first folded part is less likely to be fixed by the second folded part during pressing, and is easily deviated in a direction protruding from the second folded part on the second principal surface. Thus, adhesiveness between the first folded part and the second folded part is not sufficiently secured, and the region where the first folded part and the second folded part adhere to each other is also reduced. 
     The electrode foil includes a metal foil containing a first metal. The first metal includes a valve metal such as aluminum, tantalum, or niobium. The metal foil may be a plain foil or a metal foil whose surface is roughened by an etching treatment or the like. 
     The metal foil whose surface is roughened includes a porous part and a core part continuous with the porous part. A thickness of the porous part (thickness per one surface) ranges, for example, from 1/10 to 3/10, inclusive, of a total thickness of the metal foil. The porous part has a large number of pits (or pores) surrounded by the metal portion. A peak of a pit diameter (or pore diameter) ranges, for example, from 50 nm to 2000 nm, inclusive, (or from 100 nm to 300 nm, inclusive). The peak of the pit diameter (or pore diameter) is a most frequent pore diameter of a volume-based pore diameter distribution measured by, for example, a mercury porosimeter. 
     The surface of the metal foil is usually covered with a natural oxide film or a coating layer to be described later. When the caulking part is formed by a manufacturing method to be described later by using the electrode foil in which the surface of the metal foil is covered with the coating layer (or the natural oxide film), the first folded part may have a region not covered with the coating layer (or the natural oxide film), that is, a region (region C to be described later) where a metal base of the electrode foil is exposed on the surface covered with the second folded part. Even when the conductivity of the coating layer is low, the contact resistance between the lead member and the electrode foil can be reduced due to the presence of region C. The metal base of the electrode foil can also be said to be a metal structure of the metal foil. However, when the electrode foil is a metal foil whose surface is roughened, the metal base of the electrode foil means a metal structure of the core part of the metal foil. 
     When the second folded part does not cover the first folded part, region C is likely to be covered with an end portion of the second folded part, and adhesiveness between region C and the second folded part may be low. Alternatively, region C may be exposed from the second folded part. 
     The electrode foil may include a metal foil and a coating layer covering a surface of the metal foil. The coating layer is formed, for example, for the purpose of improving corrosion resistance of a cathode foil, improving conductivity of the cathode foil, and the like. In this case, the coating layer may include at least one selected from the group consisting of a metal oxide layer, a metal nitride layer, a metal carbide layer, and a conductive layer. The coating layer may contain a second metal. Examples of the second metal include titanium, nickel, tantalum, and niobium. The coating layer may contain two or more kinds of second metals. The second metal may be the same as or different from the first metal. The coating layer may contain carbon, and the conductive layer may be a carbon layer. When the surface of the metal foil is roughened, the electrode foil may include a coating layer covering a metal skeleton constituting the porous part. 
     When the electrode foil is used as the cathode foil, a thickness of the electrode foil ranges, for example, from 20 μm to 60 μm, inclusive, and a thickness of the coating layer ranges, for example, from 0.1 μm to 5 μm, inclusive. In the case of the metal foil whose surface is roughened, the thickness of the coating layer refers to a thickness of the coating layer covering an outer surface of the porous part. 
     The electrode foil may include a metal foil having a porous part and a core part continuous with the porous part, and a metal oxide layer covering a metal skeleton constituting the porous part. In this case, the electrode foil can be used as an anode foil. The metal oxide layer may function as a dielectric layer. The metal oxide layer may be an anodization coating film (first metal oxide layer) formed by an anodizing treatment. 
     In a cross section of the caulking part in a plane parallel to a thickness direction of the electrode foil, length L 1  from a center of the through-hole to an edge of the first folded part in a plane direction of the electrode foil is preferably smaller than length L 2  from the center of the through-hole to an edge of the second folded part in the plane direction of the electrode foil. L 1 /L 2  may be less than or equal to 0.9, and may be less than or equal to 0.8. L 1 /L 2  is preferably greater than ½ times diameter D 2  of the needle-shaped member to be described later, and may be greater than or equal to 0.4 or greater than or equal to 0.5. L 1 /L 2  may be in a range (for example, more than D/2 and less than 1) obtained by arbitrarily combining the above upper limit and lower limit. When L 1 /L 2  is within the above range, the first folded part (region C) is easily covered with the second folded part, and good adhesiveness between the first folded part and the second folded part is easily secured. 
     The “edge of the second folded part” is point P of the second folded part farthest from the center of the through-hole. L 2  is a shortest distance from the center to point P. The “edge of the first folded part” is point Q of the first folded part farthest from the center of the through-hole in the cross section including point P of the caulking part (cross section of the electrode foil in a plane parallel to the thickness direction). L 1  is a shortest distance from the center to point Q. When the second folded part is constituted by a plurality of caulking pieces which are separated with each other, a relationship of L 1 &lt;L 2  may be satisfied in at least one caulking piece, and the relationship of L 1 &lt;L 2  is preferably satisfied in ½ or more (or all) of the plurality of caulking pieces. In a case that a shape of the through-hole is a regular polygon when the caulking part is viewed from the normal direction of one principal surface of the electrode foil, the center of the through-hole coincides with the center of a circumscribed circle of the regular polygon. 
       FIG.  1    is a front view illustrating main parts of an electrode foil and a lead member in an electrolytic capacitor according to an exemplary embodiment of the present disclosure as viewed from a lead member side. In  FIG.  1   , overlapping part  600  is viewed from lead member  400  side (first principal surface S 1  side of electrode foil  300 ).  FIG.  2    is a front view illustrating main parts of the electrode foil and the lead member in the electrolytic capacitor according to the exemplary embodiment of the present disclosure as viewed from an electrode foil side. In  FIG.  2   , overlapping part  600  is viewed from electrode foil  300  side (second principal surface S 2  side). In  FIGS.  1  and  2   , overlapping part  600  is hatched for the sake of convenience. 
     Electrode foil  300  has first principal surface S 1  and second principal surface S 2  opposite to first principal surface S 1 . Electrode foil  300  includes metal foil  301 , first coating layer  302   a  (on first principal surface S 1  side) that covers one principal surface of metal foil  301 , and second coating layer  302   b  (on second principal surface S 2  side) that covers the other principal surface of metal foil  301 . Metal foil  301  is a plain foil, and coating layers  302   a  and  302   b  are formed on both surfaces of the plain foil. The metal foil is not limited thereto, and may be a metal foil whose surface is roughened. The metal foil may include a first porous part on first principal surface S 1  side, a second porous part on second principal surface S 2  side, and a core part continuous with the first porous part and the second porous part. 
     Lead member  400  includes lead wire  427 , flat tab part  425 , and lead wire connecting part  426  to which lead wire  427  is connected. Lead member  400  is not particularly limited as long as the lead member is a conductive member having tab part  425 , lead wire connecting part  426 , and lead wire  427 , and for example, can be prepared as follows. A metal bar-shaped member is prepared, and one end thereof is extended flat by pressing or the like to form tab part  425 . The other end is remained in the bar shape to form lead wire connecting part  426 . 
     Lead wire connecting part  426  and lead wire  427  are connected by welding or the like. 
     Overlapping part  600  is formed by overlapping electrode foil  300  and tab part  425 . Electrode foil  300  and tab part  425  of lead member  400  are connected to each other by four caulking parts  700  (first caulking part  710 , second caulking part  720 , third caulking part  730 , and fourth caulking part  740 ) in overlapping part  600 . Four caulking parts  700  are each formed by a manufacturing method to be described later. 
     Each of the plurality of caulking parts  700  has one through-hole  701  (first through-hole  711 , second through-hole  721 , third through-hole  731 , or fourth through-hole  741 ) penetrating electrode foil  300  and lead member  400 . Although an inner wall of each through-hole  701  is mainly formed by lead member  400  partially led out to second principal surface S 2  of electrode foil  300 , a part of the inner wall may be formed by exposing electrode foil  300 . Lead member  400  forms the inner wall of through-hole  701 , and has a penetrating part penetrating electrode foil  300  from first principal surface S 1  to second principal surface S 2 . When caulking part  700  is viewed from a normal direction of one principal surface of electrode foil  300 , a region where neither lead member  400  nor electrode foil  300  is present is through-hole  701 . An outer periphery of through-hole  701  is an annular line formed by projecting through-hole  701  onto one principal surface of the electrode foil. 
     Each of the plurality of caulking parts  700  has one caulking piece  702  (first caulking piece  712 , second caulking piece  722 , third caulking piece  732 , or fourth caulking piece  742 ) formed in a peripheral edge portion of one through-hole  701  (first through-hole  711 , second through-hole  721 , third through-hole  731 , or fourth through-hole  741 ). 
     Here,  FIG.  3    is an enlarged front view of first caulking part  710  of  FIG.  2   .  FIG.  4    is a sectional view taken along line X-X in  FIG.  2   , and illustrates a cross section in which a length (corresponding to lengths L 1  and L 2  to be described later) of caulking piece  712  (folded parts  312  and  412 ) wraparound toward second principal surface S 2  side becomes maximum.  FIG.  4    is a sectional view schematically illustrating first caulking part  710 . Second caulking part  720  to fourth caulking part  740  have the same configuration as the configuration of first caulking part  710 , and the description thereof will be omitted. 
     In first caulking part  710 , electrode foil  300  has first folded part  312  that is folded back at the peripheral edge portion of first through-hole  711  to be disposed on second principal surface S 2 . Lead member  400  includes penetrating part  430  that constitutes the inner wall of through-hole  711  and penetrates electrode foil  300 , and second folded part  412  that is folded back at an end portion of penetrating part  430  to be disposed on second principal surface S 2 . Second folded part  412  covers first folded part  312  on second principal surface S 2  side. First caulking piece  712  is constituted by caulking pieces of first folded part  312  and second folded part  412 . Second folded part  412  is constituted by four caulking pieces (triangular portions in  FIG.  3   ) which are formed to be separated with each other. First folded part  312  is constituted by one caulking piece. Meanwhile, first folded part  312  may be constituted by a plurality of (for example, four) caulking pieces which are formed to be separated with each other. Lead member  400  (penetrating part  430 ) forms the inner wall of first through-hole  711 . A surface of first folded part  312  which is covered with second folded part  412  has region C where a metal structure of metal foil  301  is exposed. Region C is present near an edge of first folded part  312 . 
     In a cross section of first caulking part  710  in a plane parallel to a thickness direction of electrode foil  300  illustrated in  FIG.  4    (cross section taken along line X-X in  FIG.  2   ), a length from a center of first through-hole  711  to an edge of first folded part  312  in a plane direction of electrode foil  300  is defined as L 1 . A length from the center of first through-hole  711  to an edge of second folded part  412  in the plane direction of electrode foil  300  is defined as L 2 . At this time, L 1 /L 2  is preferably within the above range. In this case, the first folded part is easily covered with the second folded part, and good adhesiveness between the first folded part and the second folded part is easily secured. 
     A sectional shape of caulking piece  712  varies depending on which cross section of caulking piece  712  is viewed. The same applies to sectional shapes of the other caulking pieces.  FIG.  4    is a cross section taken along line X-X in  FIG.  2   , and illustrates a cross section in which a length of caulking piece  712  (folded parts  312  and  412 ) wraparound toward second principal surface S 2  side becomes maximum. 
     Although  FIGS.  1  and  2    illustrate an example in which four through-holes  701  are arranged in a line in a width direction of electrode foil  300 , the arrangement of the through-holes is not limited thereto. From the viewpoint of securing the caulking part and connection strength, for example, the four through-holes may be provided to be spaced apart by 0.5 mm or more, or may be provided to be spaced apart by 0.5 mm or more and 3.0 mm or less. 
     Although the number of caulking parts  700  (through-holes  701 ) formed in the overlapping part in  FIGS.  1  and  2    is four, the number of caulking parts (through-holes) is not limited thereto, and is usually greater than or equal to 2. For example, the number of caulking parts ranges preferably from 2 to 4 (or from 3 to 4) from the viewpoint of contact resistance and connection strength. 
     Although a size of through-hole  701  is not particularly limited, a maximum diameter ranges preferably from 0.5 mm to 1.2 mm, inclusive (or from 0.7 mm to 1 mm, inclusive). When the maximum diameter of the through-hole is greater than or equal to 0.5 mm, electrical connection is easily reliable. When the maximum diameter of the through-hole is less than or equal to 1.2 mm, mechanical strength of the lead member and the electrode foil is easily maintained. The sizes of the plurality of through-holes may be different from each other. 
     Although some of the four caulking parts may have a structure other than the structure illustrated in  FIG.  4   , it is preferable that any of the caulking parts has the structure illustrated in  FIG.  4    from the viewpoint of reducing the contact resistance between the electrode foil and the lead member. 
     [Method for Manufacturing Electrolytic Capacitor] 
     A method for manufacturing an electrolytic capacitor according to an exemplary embodiment of the present disclosure includes a first step of preparing a lead member and an electrode foil having a first principal surface and a second principal surface opposite to the first principal surface, and a second step of connecting the electrode foil and the lead member. The second step includes a second A step to a second C step. 
     In the second A step, a preliminary through-hole is formed in the electrode foil and then the lead member is overlapped in a predetermined region of the electrode foil which includes the preliminary through-hole in the first principal surface. 
     In the second B step, a hole is formed by piercing a needle-shaped member in a position of the lead member corresponding to the preliminary through-hole from the lead member side to lead out a peripheral edge portion of the hole of the lead member above the second principal surface of the electrode foil (that is, the peripheral edge portion of the hole of the lead member is allowed to penetrate through the electrode foil to be led out from the first principal surface side of the electrode foil to the second principal surface). And then the needle-shaped member is allowed to pass through the preliminary through-hole to push and widen the preliminary through-hole so that a peripheral edge portion of the preliminary through-hole of the electrode foil protrudes from the second principal surface of the electrode foil. 
     In the second B step, diameter D 1  of the preliminary through-hole is smaller than diameter D 2  of the needle-shaped member. Here, the diameter D 1  of the preliminary through-hole corresponds to diameter D 2  of the needle-shaped member when the needle-shaped member is allowed to pass through the preliminary through-hole. When D 1 /D 2 &lt;1 is satisfied, the protrusion is easily formed together with the led part. A protrusion having region B to be described later can be formed. D 1 /D 2  may be less than or equal to 0.9, and may be less than or equal to 0.8. In the second B step, the needle-shaped member is allowed to pass through the preliminary through-hole such that the diameter indicating diameter D 2  of the needle-shaped member coincides with the diameter indicating diameter D 1  of the preliminary through-hole when viewed from the normal direction of the first principal surface of the electrode foil. 
     Diameter D 2  of the needle-shaped member means, in a cross section corresponding to a bottom surface of a pyramidal distal end portion of the needle-shaped member, a length of a line segment that passes through a center of the cross section (an axial center of the needle-shaped member) and has a minimum distance between the center and at least one end of the line segment among any line segments crossing the cross section. When a shape of the cross section is a regular polygon, it can be said that diameter D 2  corresponds to a height dimension when any side of the regular polygon is a base. When the shape of the cross section is a regular square, diameter D 2  corresponds to a length of one side of the regular square. When the cross section is a regular polygon, the center of the cross section is a center of a circumscribed circle of the regular polygon. 
     In the second C step, portion P 2  led out to the second principal surface of the lead member together with portion P 1  protruding from the second principal surface of the electrode foil is folded back onto the second principal surface, and the folded parts of portion P 1  and portion P 2  are tightened to the second principal surface. The caulking part is formed by the second C step. In the second C step, the overlapping part between the electrode foil and the lead member is pressed in a thickness direction of the electrode foil and the lead member. Hereinafter, portion P 1  and portion P 2  are also referred to as a “protrusion” and a “led part”, respectively. The folded parts of portion P 1  and portion P 2  are also referred to as a “first folded part” and a “second folded part”, respectively. 
     In the second C step, portion P 2  is folded back to cover the folded part of portion P 1  on the second principal surface. In this case, when the caulking part is formed in the second C step (when the overlapping part is pressed), the first folded part is pressed in a state of being wrapped by the second folded part on the second principal surface. Thus, the first folded part and the second folded part firmly adhere to each other, and a region where the first folded part and the second folded part adhere to each other is also sufficiently secured. As a result, contact resistance between the lead member and the electrode foil is reduced. The connection strength between the lead member and the electrode foil is also improved, and low contact resistance is maintained even when a load is applied to the connecting part (caulking part) between the lead member and the electrode foil. 
     In the second A step, region A where the metal base of the electrode foil is exposed is formed on an inner wall surface of the preliminary through-hole of the electrode foil along with the formation of the preliminary through-hole. In the second B step, along with the formation of the protrusion, region B which is derived from region A and from which the metal base of the electrode foil is exposed is formed on an upper end face of the protrusion. In the second C step, along with the formation and pressing of the first folded part and the second folded part, region C which is derived from region B and from which the metal base of the electrode foil is exposed is formed on a surface of the first folded part covered with the second folded part (surface opposite to the second principal surface). Region C is formed around a distal end of the first folded part, and is formed firmly adhere to the second folded part. Since region A is stretched to the second principal surface side when the protrusion is formed, and region B is pushed and widened when the first folded part is formed, region C is easily secured to be large. Region C and the second folded part are easily brought into planar contact, and a metal bond may be generated between region C (first metal) and the second folded part. As a result, contact resistance between the lead member and the electrode foil can be reduced. 
     Region C can be sufficiently formed even when region A is small, and an effect of reducing the contact resistance can be remarkably obtained in the case of a cathode foil having a small thickness. A thickness of the cathode foil ranges, for example, from 20 μm to 60 inclusive. 
     When the second folded part is formed not to cover the first folded part, the first folded part is less likely to be fixed by the second folded part when the folded part is pressed, and is likely to be deviated in a direction protruding from the second folded part on the second principal surface. Thus, adhesiveness between the first folded part (region C) and the second folded part is not sufficiently secured, and the adhering region is also reduced. Region C may not be covered with the second folded part. 
     When the preliminary through-hole is not provided in the second A step, the first folded part is less likely to be covered by the second folded part, and region C may not be covered with the second folded part. 
     When 1≤D 1 /D 2 , in the second B step, the periphery of the preliminary through-hole is less likely to be led out, the protrusion (region B) is less likely to be formed, and the first folded part firmly adhering to the second folded part is less likely to be formed. Thus, the strength of the connecting part (caulking part) between the lead member and the electrode foil may decrease, and the contact resistance may increase when a load is applied to the connecting part. The protrusion (region B) is hardly formed, and in the second B step, the penetrating part of the lead part is formed to cover an inner wall surface of the preliminary through-hole, and in the second C step, the overlapping part is pressed in a state where the penetrating part of the lead part covers the inner wall surface of the preliminary through-hole. In this case, adhesiveness between region A and the penetrating part is low (the adhesion region is small), and the contact between the metal base of the electrode foil and the lead member may be insufficient in the caulking part. 
     Height H 1  (maximum height) from the second principal surface of portion P 1  before being folded back onto the second principal surface is preferably smaller than height H 2  (maximum height) from the second principal surface of portion P 2  before being folded back onto the second principal surface. H 1 /H 2  may be greater than 0 and less than 1, may range from 0.1 to 0.9, inclusive, or may range from 0.2 (or from 0.4) to 0.8, inclusive. In this case, in the second C step, the second folded part enclosing the first folded part is easily formed. 
     Diameter D 1  (mm) of the preliminary through-hole, diameter D 2  (mm) of the needle-shaped member, and thickness T (mm) of the lead member (tab part) preferably satisfy the relationship of the following Expression (1). 
       0&lt;( D 2− D 1)/ T≤ ½  (1)
 
     When Expression (1) is satisfied, height H 1  can be easily adjusted to be smaller than height H 2 , and H 1 /H 2  can be easily adjusted to fall within the above range. Length L 1  can be easily adjusted to be smaller than length L 2 , and L 1 /L 2  can be easily adjusted to fall within the above range. In the second C step, the second folded part enclosing the first folded part is easily formed. 
     Hereinafter, an example of a second step in a method for manufacturing an electrolytic capacitor according to the exemplary embodiment of the present disclosure will be described.  FIG.  5    is a sectional view schematically illustrating main parts of the electrode foil and the lead member after a second A step.  FIG.  6    is a sectional view schematically illustrating main parts of the electrode foil and the lead member after a second B step.  FIG.  4    is a sectional view schematically illustrating main parts of the electrode foil and the lead member after the second C step. 
     (Second A Step) 
     A cylindrical preliminary through-hole  330  is provided in electrode foil  300  by using a predetermined jig such as a punch. Electrode foil  300  has region A where the metal structure of metal foil  301  is exposed on an inner wall surface of preliminary through-hole  330 . When electrode foil  300  is viewed from the normal direction of the principal surface, preliminary through-hole  330  has a circular shape. The shape of the preliminary through-hole is not limited thereto, and may be a polygon or a star polygon. When the shape of the preliminary through-hole is a (star) regular polygon, a center of the preliminary through-hole is a center of a circumscribed circle of the (star) regular polygon. 
     Overlapping part  600  between electrode foil  300  and lead member  400  is formed by overlapping lead member  400  in a predetermined region including preliminary through-hole  330  on first principal surface S 1  of electrode foil  300 . 
     (Second B Step) 
     A predetermined position of overlapping part  600  is drilled by using needle-shaped member  500 . Specifically, in overlapping part  600 , needle-shaped member  500  is pierced from lead member  400  side to a position of lead member  400  corresponding to preliminary through-hole  330 . Through-hole  711  penetrating both electrode foil  300  and lead member  400  is formed at a location drilled by needle-shaped member  500 . 
     With formation of the through-hole  711 , a part of lead member  400  is led out to second principal surface S 2  and a part of electrode foil  300  protrudes from second principal surface S 2  around through-hole  711 . That is, with the formation of through-hole  711 , a part of lead member  400  is led out to second principal surface S 2  of electrode foil  300  to form led part  411 . Needle-shaped member  500  passes through preliminary through-hole  330 , and thus, preliminary through-hole  330  is pushed and widened. At this time, a peripheral edge portion of preliminary through-hole  330  of electrode foil  300  protrudes from second principal surface S 2  of electrode foil  300  to form protrusion  311 . 
     A distal end shape of needle-shaped member  500  is a quadrangular pyramid shape, and a cross section of the distal end shape is a quadrangle. Electrode foil  300  (around preliminary through-hole  330 ) and lead member  400  are pierced along a corner of the cross section of the distal end of needle-shaped member  500 . Protrusion  311  and led part  411  protruding from second principal surface S 2  along with drilling by needle-shaped member  500  have a shape pushed and widened in a petal shape in four directions. The distal end shape of the needle-shaped member is not limited thereto, and may be a pyramid shape other than the quadrangular pyramid shape. In this case, the shapes of the through-hole and the protrusion (led part) may be different from the shapes in the examples of  FIGS.  1  and  2   . 
     As illustrated in  FIGS.  5  and  6   , diameter D 1  of the preliminary through-hole corresponding to diameter D 2  of needle-shaped member  500  when needle-shaped member  500  is allowed to pass through preliminary through-hole  330  is smaller than diameter D 2  of needle-shaped member  500 . Diameter D 2  of needle-shaped member  500  corresponds to a length of one side of a quadrangle in a cross section corresponding to a bottom surface of a distal end portion of the quadrangular pyramid shape, and can also be said to correspond to a length of through-hole  711  in an X-X direction in  FIG.  3   . An end face of protrusion  311  has region B which is derived from region A and from which the metal structure of metal foil  301  is exposed. 
     It is preferable that diameter D 1  (mm) of preliminary through-hole  330 , diameter D 2  (mm) of needle-shaped member  500 , and thickness T (mm) of lead member  400  (tab part  425 ) illustrated in  FIG.  5    satisfy a relationship of the above Expression (1). When Expression (1) is satisfied, H 1  and H 2  in  FIG.  6    are easily adjusted such that H 1 /H 2  is less than 1 (or less than or equal to 0.9). L 1  and L 2  in  FIG.  4    are easily adjusted such that L 1 /L 2  is less than 1 (or less than or equal to 0.9). In  FIG.  5   , H 1  represents a maximum height of protrusion  311  from the second principal surface, and H 2  represents a maximum height of led part  411  from the second principal surface. In  FIG.  5   , the heights of the protrusion and the led part become maximum in the same cross section of the overlapping part, but may become maximum in different cross sections of the overlapping part. 
     (Second C Step) 
     Caulking part  710  is formed by pressing overlapping part  600  in which protrusion  311  and led part  411  are formed. Overlapping part  600  is pressed, for example, at a pressure of 8 MPa to 12 MPa. A pressing time is not particularly limited, and is, for example, about 0.3 seconds to 1 second. By pressing overlapping part  600 , led part  411  is folded back onto the second principal surface together with protrusion  311 , and first folded part  312  and second folded part  412  are formed. Overlapping part  600  (first folded part  312  and second folded part  412 ) is pressed, and first folded part  312  and second folded part  412  are tightened to the second principal surface. 
     led part  411  is folded back to cover first folded part  312  on second principal surface S 2 . That is, second folded part  412  enclosing first folded part  312  is formed in caulking part  710 . A surface of first folded part  312  covered with second folded part  412  (surface opposite to second principal surface S 2 ) has region C which is derived from region B and from which the metal structure of metal foil  301  is exposed. 
     Overlapping part  600  is pressed in the thickness direction of electrode foil  300 , and through-hole  711  and electrode foil  300  and lead member  400  around through-hole  711  are deformed together with protrusion  311  and led part  411 . Due to the deformation of protrusion  311  and led part  411 , caulking piece  712  having first folded part  312  and second folded part  412  is formed on the outer periphery of through-hole  711 . Caulking piece  712  is formed in a petal shape in four directions from through-hole  711 . Electrode foil  300  is strongly pressed against caulking piece  712  and is pressure-bonded to lead member  400 . By this pressure-bonding, electrode foil  300  and lead member  400  are electrically connected to each other. 
     Here,  FIG.  7    is a sectional view schematically illustrating the electrolytic capacitor according to the exemplary embodiment of the present disclosure.  FIG.  7    illustrates an example of an electrolytic capacitor including a wound-type capacitor element.  FIG.  8    is a perspective view schematically illustrating a configuration of a wound body of  FIG.  7   . 
     Electrolytic capacitor  200  includes wound body  100 . Wound body  100  is formed by winding anode foil  10  and cathode foil  20  with separator  30  interposed therebetween. 
     End portions on one side of lead tabs  50 A and  50 B are connected to anode foil  10  and cathode foil  20 , respectively, and wound body  100  is formed while lead tabs  50 A and  50 B are wound. Lead wires  60 A and  60 B are connected to end portions on the other side of lead tabs  50 A and  50 B, respectively. 
     Connection between anode foil  10  and lead tab  50 A and/or connection between cathode foil  20  and lead tab  50 B are performed by the method for manufacturing an electrolytic capacitor according to the present disclosure (second step). In particular, since cathode foil  20  has a small thickness, an effect of reducing the contact resistance between the cathode foil and lead tab  50 B by using the manufacturing method can be remarkably obtained. 
     Winding stop tape  40  is arranged on an outer surface of cathode foil  20  positioned at an outermost layer of wound body  100 , and an end portion of cathode foil  20  is fixed by winding stop tape  40 . When anode foil  10  is prepared by cutting a large foil, an anodizing treatment may further be performed on wound body  100  in order to provide a dielectric layer on a cut surface. 
     Wound body  100  contains an electrolyte, and the electrolyte is interposed between anode foil  10  (dielectric layer) and the cathode foil. Wound body  100  containing the electrolyte is formed, for example, by impregnating wound body  100  with a treatment solution containing a conductive polymer. The impregnation may be performed under a reduced pressure, for example, in an atmosphere of 10 kPa to 100 kPa. An electrolytic solution may be contained in wound body  100 . 
     Wound body  100  is housed in bottomed case  211  such that lead wires  60 A and  60 B are positioned on an opening side of bottomed case  211 . As a material of bottomed case  211 , a metal such as aluminum, stainless steel, copper, iron, or brass, or an alloy thereof can be used. 
     Sealing member  212  is arranged at an opening portion of bottomed case  211  in which wound body  100  is housed, an opening end of bottomed case  211  is caulked to sealing member  212  to be curled, and base plate  213  is disposed at a curled portion. Thus, wound body  100  is sealed in bottomed case  211 . Separator  30  is not particularly limited. For example, an unwoven fabric including fibers of cellulose, polyethylene terephthalate, vinylon, or polyamide (for example, aliphatic polyamide or aromatic polyamide such as aramid). 
     Sealing member  212  is formed such that lead wires  60 A and  60 B penetrate therethrough. Sealing member  212  may be an insulating substance, and is preferably an elastic body. Among these materials, silicone rubber, fluororubber, ethylene propylene rubber, Hypalon rubber, butyl rubber, isoprene rubber, and the like, having high heat resistance, are preferable. 
     The electrolyte contains at least one of a solid electrolyte and an electrolytic solution. The electrolytic solution contains a non-aqueous solvent and a solute (for example, an organic salt) dissolved in the non-aqueous solvent. The non-aqueous solvent may be used together with the solid electrolyte. The solid electrolyte contains a conductive polymer such as polythiophene. The solid electrolyte may contain a dopant such as polystyrenesulfonic acid together with the conductive polymer. 
     EXAMPLES 
     Hereinafter, the present disclosure will be described in more detail based on examples, but the present disclosure is not limited to the examples. 
     Example 1 
     The caulking part illustrated in  FIGS.  1  to  4    was formed, and the cathode foil and the lead member were connected. 
     As the electrode foil, a cathode foil in which a titanium nitride layer (thickness: 1 μm) was formed on both surfaces of an aluminum foil (plane foil) having a thickness of 28 μm was prepared. The cathode foil was cut into a strip shape having a length of 300 mm and a width of 10 mm. Four cylindrical preliminary through-holes (diameter D 1 : 0.5 mm) were provided at predetermined positions of the cathode foil by using a predetermined punch. The four preliminary through-holes were provided at regular intervals along a width direction of the cathode foil. The lead member (thickness T: 0.3 mm) was overlaid on the cathode foil at a position where the preliminary through-hole was provided to form the overlapping part. Drilling was performed at a position of the overlapping part corresponding to the preliminary through-hole by using the needle-shaped member (diameter D 2 : 0.6 mm) of which the distal end shape is the quadrangular pyramid shape. Drilling was performed from the lead member side. Subsequently, pressing was performed to connect the cathode foil and the lead member by four caulking parts. As stated above, sample A in which the lead member and the electrode foil were connected was obtained. In sample A, the second folded part enclosing the first folded part was formed. L 1 /L 2  was a value represented in Table 1. 
     [Evaluation 1: Measurement of Contact Resistance] 
     The contact resistance between the cathode foil and the lead member was measured by a four-terminal method. Fifteen samples A were prepared, and an average value of contact resistances of fifteen samples A was obtained. 
     [Evaluation 2: Measurement of Contact Resistance after Load] 
     The cathode foil was fixed, and a load of 100 gf was applied in a direction parallel to a length direction of the cathode foil from a side end face at a location close to the overlapping part of the lead member (position at a distance of 2 mm from the end portion of the overlapping part). A time during which the load was applied is 3 seconds. Thereafter, the contact resistance after the load was measured in the same manner as in Evaluation 1. Ten samples A were prepared, and an average value of contact resistances of ten samples A after the load was obtained. 
     Comparative Example 1 
     Sample B1 was obtained and evaluated in the same manner as in Example 1 except that the preliminary through-hole was not provided in the cathode foil. In sample B1, the first folded part was not covered by the second folded part. 
     Comparative Example 2 
     Sample B2 was obtained and evaluated in the same manner as in Example 1 except that diameter D 1  of the preliminary through-hole was set to 0.7 mm. In sample B2, the first folded part was hardly formed. 
     Evaluation results are represented in Table 1. The contact resistance in Table 1 is represented as a relative value when the contact resistance of sample B1 obtained in Evaluation 1 is 100. 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Diameter D1 of 
                 Diameter D2 of 
                 Thickness T 
                   
                   
                   
                   
                 Contact 
               
               
                   
                 preliminary 
                 needle-shaped 
                 of lead 
                   
                   
                   
                 Contact 
                 resistance 
               
               
                   
                 through-hole 
                 member 
                 member 
                   
                   
                   
                 resistance 
                 after load 
               
               
                   
                 (mm) 
                 (mm) 
                 (mm) 
                 D1/D2 
                 (D2 − D1)/T 
                 L1/L2 
                 (index) 
                 (index) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 A 
                 0.5 
                 0.6 
                 0.3 
                 0.83 
                  0.33 
                 0.7 
                 70 
                 80 
               
               
                 B1 
                 — 
                 0.6 
                 0.3 
                 — 
                 — 
                 1.2 
                 100 
                 130 
               
               
                 B2 
                 0.7 
                 0.6 
                 0.3 
                 1.17 
                 −0.33 
                 — 
                 60 
                 100 
               
               
                   
               
            
           
         
       
     
     In sample A, the contact resistance was reduced, the connection strength was high, and the contact resistance after the load was also suppressed to be small. In sample B1, the contact resistance was high, the connection strength was low, and the contact resistance after the load also increased. In sample B2, the connection strength was low, and the contact resistance after the load increased. 
     The method for manufacturing an electrolytic capacitor according to the present disclosure can be suitably used for manufacturing an electrolytic capacitor having a small contact resistance between a lead member and an electrode foil.