Electrode foil for electrolytic capacitors and electrolytic capacitors

Abstract

This electrode foil for electrolytic capacitors comprises a porous part and a core part that is continuous with the porous part. In the thickness direction of the porous part, when the porous part is divided into three equal parts, namely a first region, a second region, and a third region, in order from the outer surface side of the porous part, the relationships 1.1 ≤ L2 / L1 and 1.1 ≤ L2 / L3a hold, where L1 is the circumferential length of the pit of the first region, L2 is the circumferential length of the pit of the second region, and a L3 is the circumferential length of the pit of the third region.

Description

[Technical Field]

[0001] This disclosure relates to electrode foil for electrolytic capacitors and electrolytic capacitors. [Background technology]

[0002] For the anode of an electrolytic capacitor, for example, a metal foil containing a valve-acting metal is used. To increase the capacitance of the electrolytic capacitor, the main surface of the metal foil is etched to form a porous region. Subsequently, the metal foil is subjected to a chemical conversion treatment to form a layer of metal oxide (dielectric) on the surface of the metal framework that constitutes the porous region.

[0003] Patent Document 1 proposes an electrolytic capacitor in which an anode foil made of etched foil with a porosity of 51% or less contains a compound in which a phosphate ion is bonded to a water-soluble metal complex and a solvent mainly composed of water within the capacitor element. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2002-359165 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] Further improvements in the performance of electrolytic capacitors are needed. [Means for solving the problem]

[0006] One aspect of this disclosure relates to an electrode foil for an electrolytic capacitor, comprising a porous portion and a core portion continuous with the porous portion, wherein when the porous portion is divided into three equal parts in the thickness direction of the porous portion, starting from the outer surface side of the porous portion, the perimeter length L1 of the pit in the first region, the perimeter length L2 of the pit in the second region, and the perimeter length L3 of the pit in the third region satisfy the relationship 1.1 ≤ L2 / L1 and 1.1 ≤ L2 / L3.

[0007] Another aspect of the present disclosure relates to an electrolytic capacitor comprising the above-described electrode foil for an electrolytic capacitor having a dielectric layer covering at least a portion of the surface of a metal skeleton constituting the porous portion, and a cathode portion covering at least a portion of the dielectric layer. [Effects of the Invention]

[0008] According to this disclosure, highly reliable and high-performance electrolytic capacitors can be obtained.

[0009] Novel features of the present invention are described in the appended claims, but the present invention, both in terms of structure and content, and in conjunction with other objects and features of the present invention, will be better understood by the following detailed description in conjunction with the drawings. [Brief explanation of the drawing]

[0010] [Figure 1] This is a schematic cross-sectional view of an electrode foil (anode) according to one embodiment of the present disclosure. [Figure 2] This is a schematic cross-sectional view showing an enlarged portion of the porous area of ​​an electrode foil having a dielectric layer according to one embodiment of the present disclosure. [Figure 3] This is a schematic cross-sectional view of an electrolytic capacitor according to one embodiment of the present disclosure. [Figure 4] Figure 3 is a schematic perspective view showing the structure of the wound body. [Modes for carrying out the invention]

[0011] Hereinafter, embodiments of the present disclosure will be described by way of 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 can be obtained. In this specification, the description "numerical value A to numerical value B" includes numerical value A and numerical value B, and can be re-read as "not less than numerical value A and not more than numerical value B". In the following description, when lower limits and upper limits of numerical values for specific physical properties and conditions are exemplified, any one of the exemplified lower limits and any one of the exemplified upper limits can be arbitrarily combined as long as the lower limit does not exceed the upper limit. When a plurality of materials are exemplified, one type may be selected from them and used alone, or two or more types may be combined and used.

[0012] In addition, the present disclosure encompasses combinations of matters described in two or more claims arbitrarily selected from the plurality of claims set forth in the appended claims. That is, as long as no technical contradiction arises, matters described in two or more claims arbitrarily selected from the plurality of claims set forth in the appended claims can be combined.

[0013] In the following description, the term "comprising" or "containing" is an expression that encompasses "comprising (or containing)", "consisting essentially of" and "consisting of".

[0014] Electrolytic capacitors include electrolytic capacitors having an electrolytic solution, solid electrolytic capacitors having a solid electrolyte, and electrolytic capacitors having both an electrolytic solution or a liquid component and a solid electrolyte. A capacitor may also be rephrased as "capacitor".

[0015] Hereinafter, the electrode foil for an electrolytic capacitor before forming a dielectric layer is also referred to as "first electrode foil" or "anode body", and the electrode foil for an electrolytic capacitor having a dielectric layer is also referred to as "second electrode foil".

[0016] (First Electrode Foil) The electrode foil (first electrode foil) for an electrolytic capacitor according to this embodiment comprises a porous portion and a core portion continuous with the porous portion. That is, the first electrode foil is an integrated product of the core portion and the porous portion. The first electrode foil can be used as the anode body of an electrolytic capacitor. The metal portion constituting the porous portion and the core portion contain a first metal.

[0017] The first electrode foil is obtained, for example, by roughening a metal foil formed from a first metal contained in the metal portion constituting the porous part by etching or the like. The porous part is the surface (outer) portion of the metal foil that has been porousized by etching, and the remaining portion, which is the inner part of the metal foil, is the core. The porous part has pits (or pores) surrounded by the metal portion containing the first metal.

[0018] In the thickness direction of the porous portion of the first electrode foil, when the porous portion is divided into three equal regions, a first region, a second region, and a third region, in order from the outer surface side (opposite the core), the perimeter length L1 of the pit in the first region, the perimeter length L2 of the pit in the second region, and the perimeter length L3 of the pit in the third region satisfy the relationship 1.1 ≤ L2 / L1 and 1.1 ≤ L2 / L3. In this specification, when L2 is said to be greater than L1(L3), it means that L2 is 1.1 times or more than L1(L3). When L1(L3) is said to be less than L2, it means that L1(L3) is 1 / 1.1 times or less than L2.

[0019] The "perimeter of the pit" refers to the length of the contour of the area occupied by the pit in the cross-section in the thickness direction of the electrode foil (porous portion), and is expressed as the total length of the contour contained per unit area of ​​that cross-section. The larger the perimeter of the pit, the larger the surface area of ​​the electrode foil tends to be. The more small-diameter pits there are, the larger the perimeter of the pit becomes.

[0020] When L1 to L3 satisfy the above relationship, a high-performance electrode foil with high strength can be obtained. In other words, by making L2 sufficiently large, the surface area of ​​the porous part can be effectively increased, thereby increasing the capacity.

[0021] When L1 is smaller than L2, a good dielectric layer can be formed even in the deeper parts of the porous region (second to third regions), resulting in a high-performance second electrode foil. Furthermore, in electrolytic capacitors using electrolytes, solid electrolytes, etc., as cathode materials, the permeability of the electrolyte into the porous region and the packing of the solid electrolyte (e.g., conductive polymer) are improved. This increases the capacitance achievement rate of the electrolytic capacitor and is advantageous for reducing ESR and suppressing leakage current.

[0022] When L1 and L3 are smaller than L2, the bending strength of the electrode foil improves. In this case, the resistance to bending stress improves in the second region, and it is presumed that this stress concentrates in the second region and is relieved in the first and third regions. Furthermore, when L2 is larger than L1 and L3, the overall hardness of the porous portion improves, and the tensile strength of the electrode foil improves. In other words, high tensile strength and bending strength can be achieved simultaneously for the electrode foil as a whole. As a result, foil breakage during the manufacturing process of electrolytic capacitors (e.g., the winding process) is suppressed, and the reliability of electrolytic capacitors is improved.

[0023] The perimeter lengths L1 to L3 of the pits in the first to third regions of the porous portion of the first electrode foil can be determined as follows.

[0024] (i) Obtain a cross-sectional image of the first electrode foil (anode) in the thickness direction using an electron microscope. A scanning electron microscope (SEM) or a transmission electron microscope (TEM) can be used.

[0025] (ii) Image processing is performed. First, filtering is performed to remove noise. Furthermore, binarization is performed to distinguish between pits (voids) and the metal framework that constitutes the porous part, and the edges of the metal framework (contours of the areas occupied by pits) are extracted.

[0026] (iii) In the cross-section of the porous portion with thickness T (μm), an area with a width T / 9 extending in the thickness direction of the porous portion is arbitrarily selected. This area is divided into 9 equal parts in the thickness direction of the porous portion, and these are designated as the 1st section to the 9th section, starting from the outer surface side of the porous portion (first electrode foil). That is, the 1st section to the 9th section are arranged in a line in the thickness direction of the porous portion, with a T / 9 × T / 9 square area (for example, area: 9 μm). 2 ~64μm 2 Sections 1 to 3 correspond to the first region, sections 4 to 6 correspond to the second region, and sections 7 to 9 correspond to the third region.

[0027] (iv) Determine the total contour length of the area occupied by the pits contained within Section 1, and set this as the pit perimeter length L9-1 of Section 1. Similarly, determine the pit perimeter lengths L9-2 to L9-9 of Sections 2 to 9 in the same manner. Calculate the average of the pit perimeter lengths L9-1 to L9-3 of Sections 1 to 3, and set this as the pit perimeter length L1 of Area 1. Calculate the average of the pit perimeter lengths L9-4 to L9-6 of Sections 4 to 6, and set this as the pit perimeter length L2 of Area 2. Calculate the average of the pit perimeter lengths L9-7 to L9-9 of Sections 7 to 9, and set this as the pit perimeter length L3 of Area 3.

[0028] L2 / L1 may be 1.2 or higher, 1.3 or higher, 1.1 or higher and 2.0 or lower, or 1.2 or higher (or 1.3 or higher) and 2.0 or lower. L2 / L3 may be 1.2 or higher, 1.3 or higher, 1.1 or higher and 2.0 or lower, or 1.2 or higher (or 1.3 or higher) and 2.0 or lower.

[0029] 2×L2 / (L1+L3) is preferably 1.2 or greater, but may be 1.2 or greater and 1.4 or less. In this case, it is easier to obtain high bending strength and tensile strength along with increased capacity.

[0030] From the viewpoint of improving the strength of the surface layer of the porous portion, 1 < L1 / L3 may be satisfied. Further, from the viewpoint of forming a dielectric layer by a chemical conversion solution or a vapor phase method, L1 / L3 < 1 may be satisfied. That is, the pit diameter may be relatively smaller and the surface area may be larger in the third region than in the first region. Even if the surface area near the surface of the anode body (for example, in the first region) is relatively small, it becomes easy to secure a sufficiently large capacitance. L1 / L3 may be, for example, 0.75 or more and 1.25 or less.

[0031] L2 may be, for example, 145 μm / μm 2 or more, and may be 145 μm / μm 2 or more and 165 μm / μm 2 or less. In this case, L1 may be less than 140 μm / μm 2 and may be 130 μm / μm 2 or less, and may be 110 μm / μm 2 or more and less than 140 μm / μm 2 In this case, L3 may be 130 μm / μm 2 or less, and may be 125 μm / μm 2 or less, and may be 105 μm / μm 2 or more and 130 μm / μm 2 or less.

[0032] From the viewpoints of strength and electrolyte impregnation property, in the first region of the first electrode foil, the closer to the second region, the larger the perimeter of the pit may be. That is, the pit perimeters L9-1 to L9-3 of the first section to the third section may satisfy the relationship of L9-1 < L9-2 < L9-3. In this case, the pit perimeter L9-1 of the first section is 90 μm / μm 2 or more and 125 μm / μm 2 or less is preferable. In this case, the dielectric layer is easily formed up to the deepest part of the porous portion, and the impregnation property of the electrolyte into the electrode foil is easily improved.

[0033] From the viewpoint of strength and electrolyte impregnation property, in the third region of the first electrode foil, the closer the position is to the second region, the larger the perimeter of the pits may be. That is, the pit perimeters L9-7 to L9-9 of the 7th to 9th sections may satisfy the relationship of L9-9 < L9-8 < L9-7.

[0034] Any one of the pit perimeters L9-4 to L9-6 of the 4th to 6th sections in the second region may be the maximum value (local maximum) Lmax of the pit perimeters of the 1st to 9th sections. The maximum pit perimeter Lmax is, for example, 150 μm / μm 2 or more, or 150 μm / μm 2 or more and 170 μm / μm 2 or less.

[0035] The ratio of the maximum pit perimeter Lmax to the pit perimeter L9-1 of the first section: Lmax / L9-1 is, for example, 1.25 or more (or 1.5 or more) and 2.0 or less. The ratio of the maximum pit perimeter Lmax to the pit perimeter L9-9 of the ninth section: Lmax / L9-9 is, for example, 1.35 or more (or 1.5 or more) and 2.0 or less.

[0036] FIG. 1 is a schematic cross-sectional view of an anode body (first electrode foil) according to an embodiment of the present invention. The anode body 110 is an integrated body of a core portion 111 and a porous portion 112, and the porous portion 112 has a thickness T. The porous portion 112 can be equally divided into three regions in order from the side opposite to the core portion 111: a first region R1, a second region R2, and a third region R3 each having a thickness of T / 3. The pit perimeter L1 of the first region R1, the pit perimeter L2 of the second region R2, and the pit perimeter L3 of the third region R3 satisfy the relationships of 1.1 ≤ L2 / L1 and 1.1 ≤ L2 / L3.

[0037] (Second Electrode Foil) The electrode foil for an electrolytic capacitor according to the present embodiment may be a second electrode foil comprising a first electrode foil (or an anode body) and a dielectric layer covering at least a part of the surface of a metal portion (metal skeleton) constituting the porous portion of the first electrode foil. The configuration of the dielectric layer is not particularly limited.

[0038] In the second electrode foil, the thickness of the dielectric layer varies depending on the rated voltage of the electrolytic capacitor, but it has a thickness of 4 nm to 300 nm and is formed relatively thinly along the surface shape of the metal part. Therefore, when the porous part of the second electrode foil is divided into three equal parts in the thickness direction, starting from the outer surface side (opposite the core), the ratio of the perimeter length D2 of the pits in the second region to the perimeter length D1 of the pits in the first region: D2 / D1 is approximately equal to L2 / L1. The ratio of the perimeter length D2 of the pits in the second region to the perimeter length D3 of the pits in the third region: D2 / D3 is approximately equal to L2 / L3. Therefore, if L2 / L1 and L2 / L3 are 1.1 or greater in the first electrode foil, then D2 / D1 and D2 / D3 can be 1.1 or greater in the second electrode foil. Furthermore, D1 to D3 above can be determined in the same way as in the case of L1 to L3 above.

[0039] The dielectric layer is provided so as to cover at least a portion of the surface of the metal portion surrounding the pit (or pore). The dielectric layer may contain an oxide of the first metal contained in the metal portion. The dielectric layer containing the oxide of the first metal is formed, for example, by a chemical conversion treatment using a chemical conversion solution. When L2 is greater than L1, the chemical conversion solution is more easily impregnated into the depths of the porous portion, making it easier to form the dielectric layer.

[0040] Furthermore, the dielectric layer may have a first layer with a thickness T1 containing an oxide of a second metal different from the first metal contained in the metal portion. When an oxide of a second metal different from the first metal is included in the dielectric layer, for example, a second metal with a high dielectric constant can be selected without being limited by the first metal. Therefore, it becomes easier to improve the capacitance of the electrolytic capacitor. In addition, since the range of choices for the second metal is broadened, it becomes possible to impart various properties to the dielectric layer without being limited by the first metal.

[0041] When L2 is greater than L1, when forming a dielectric layer using a vapor phase method such as atomic layer deposition, the raw material gas for the dielectric layer diffuses more easily to the depths of the porous region, allowing for the formation of a good dielectric layer even in the depths of the porous region (second to third regions). For example, even if the oxide of the second metal is preferentially deposited on the surface of the porous region (i.e., the first region) in the initial stage of dielectric layer deposition, L1 is smaller than L2, and the pit diameter is larger in the first region than in the second region, so the pit entrances are less likely to be blocked by the dielectric layer. Therefore, the deposition of the dielectric layer proceeds smoothly. This allows for higher capacitance of the electrode foil, as well as improved penetration of the electrolyte into the porous region and better filling of the solid electrolyte (e.g., conductive polymer), resulting in a higher capacitance achievement rate for electrolytic capacitors and advantages in reducing ESR and suppressing leakage current.

[0042] From the viewpoint of improving the ease of forming the dielectric layer in the third region (deepest part) and the impregnation of the electrolyte, L2 is set to 160 μm / μm 2 (or 155 μm / μm) 2 The following may also apply: The maximum value Lmax of the pit circumference in sections 4 to 6 is 170 μm / μm. 2 (or 160 μm / μm) 2 The following may also be acceptable.

[0043] Furthermore, in the vapor phase method, the raw material gas is consumed first in the surface layer (first region) of the etching pit, so the amount of raw material gas reaching the deepest part (third region) is small. On the other hand, L1 is smaller than L2, making it easier for the raw material gas to penetrate the etching pit. Also, L3 is smaller than L2, and the third region has a smaller surface area than the second region, so a small amount of raw material gas is required to form the dielectric layer. Therefore, a good dielectric layer can be efficiently formed even to the deepest part of the etching pit. For example, even in a sponge-like etching pit with a specific surface area 50 times larger, a dielectric layer can be easily formed to its deepest part.

[0044] The pit perimeter lengths L1 to L3 in the first to third regions and the pit perimeter length L9-1 in the first section may be within the ranges exemplified above. In this case, when forming the dielectric layer in a liquid phase, such as by chemical conversion (anodic oxidation), the conversion liquid penetrates more easily to the depths of the porous region. Furthermore, when forming the dielectric layer by a gas phase method, such as atomic layer deposition, the diffusivity of the raw material gas for the dielectric layer to the depths of the porous region is further improved.

[0045] The thickness of the porous portion is not particularly limited and can be appropriately selected depending on the application of the electrolytic capacitor, the required voltage withstand voltage, etc. For example, the thickness of the porous portion can be selected from a range of 10 μm to 160 μm. Alternatively, the thickness of the porous portion may be, for example, 1 / 10 or more and 5 / 10 or less of the thickness of the first electrode foil or the second electrode foil.

[0046] The thickness T of the porous portion can be determined by cutting the first or second electrode foil so that a cross-section in the thickness direction of the core and the porous portion is obtained, taking an electron microscope image of the cross-section, and taking the average value of the thickness at any 10 points in the porous portion.

[0047] The pit diameter peak (or pore diameter peak) of the pits in the porous portion is not particularly limited, but from the viewpoint of increasing the surface area and forming the dielectric layer deep into the porous portion, it may be set to, for example, 50 nm to 2000 nm, or it may be set to 100 nm to 300 nm. The pit diameter (pore diameter) peak is the most frequent pore diameter in the volume-based pore diameter distribution measured, for example, with a mercury porosimeter.

[0048] The withstand voltage of an electrolytic capacitor is not particularly limited and may have a relatively small withstand voltage of, for example, 1V or more and less than 4V, or a relatively large withstand voltage of 4V or more, 15V or more, or 100V or more. When obtaining an electrolytic capacitor with a withstand voltage of 4V or more, it is preferable that the thickness of the dielectric layer be 4nm or more. Furthermore, when obtaining an electrolytic capacitor with a withstand voltage of 15V or more, it is preferable that the thickness of the dielectric layer be 21nm or more.

[0049] More specifically, when obtaining an electrolytic capacitor with a high withstand voltage of 60V or more, for example, the pore size peak of the porous portion may be, for example, 50 to 300 nm, the thickness of the porous portion may be, for example, 30 to 160 μm, and the thickness of the dielectric layer may be, for example, 30 to 100 nm.

[0050] When obtaining an electrolytic capacitor with a relatively low withstand voltage, for example, with a withstand voltage of 10V or less, the pore diameter peak of the porous portion may be, for example, 20 to 200 nm, the thickness of the porous portion may be, for example, 30 to 160 μm, and the thickness of the dielectric layer may be, for example, 4 to 30 nm.

[0051] The first metal may include, for example, Al. In this case, the second metal may include, for example, at least one selected from the group consisting of Ta, Nb, Ti, Si, Zr, and Hf.

[0052] In the dielectric layer, an oxide of the first metal may be provided between the metal portion containing the first metal and the oxide of the second metal. Hereinafter, the portion of the dielectric layer formed by the oxide containing the second metal will be referred to as the first layer, and the portion formed by the oxide containing the first metal will be referred to as the second layer.

[0053] For example, an oxide containing a second metal (first layer) may be formed on a native oxide film of a first metal formed on the surface of a metal part. Alternatively, after forming the first layer on the native oxide film, the metal part may be anodized to form an oxide of the first metal (second layer) of any thickness between the metal part and the oxide containing the second metal (first layer).

[0054] The second layer may contain a composite oxide of an oxide of the first metal and an oxide of the second metal. By forming the second layer, defects in the first layer can be repaired. Therefore, the performance of the dielectric layer is further improved.

[0055] The thickness T1 of the first layer and the thickness T2 of the second layer may satisfy T1≧2×T2 and T1≧3×T2 in at least the third region. By relatively increasing the thickness of the first layer in this way, for example, when a second metal with a high dielectric constant is selected, the capacitance of the electrolytic capacitor can be significantly improved. Furthermore, with the above porous structure, it is easier for the raw material gas to reach deeper parts, so T1>T2 can be made even in the first region.

[0056] The thicknesses of the first and second layers can be determined by cutting the anode body so that a cross-section in the thickness direction of the porous portion is obtained, taking electron microscope images of the cross-section, and taking the average of the thicknesses at any 10 points in the first or second layer.

[0057] The first layer preferably contains at least one additive element selected from the group consisting of C, P, B, and N. The additive element is preferably distributed from the surface of the first layer to a depth of at least 0.05 × T1 (thickness of the first layer). This allows the dielectric layer to be given sufficient acid resistance and to sufficiently reduce leakage current. The first layer is formed of a dielectric containing an oxide of a second metal different from the first metal. While the second metal can form a dielectric with a high dielectric constant, its formation process is prone to generating defects in the dielectric layer that cause an increase in leakage current. The additive element penetrates these defects, imparting acid resistance to the dielectric layer and suppressing the increase in leakage current. In the electrolytic capacitor according to this embodiment, the above elements can be efficiently added to the dielectric layer.

[0058] A method for manufacturing electrode foil for electrolytic capacitors includes, for example, a step of preparing a metal foil containing a first metal, and a step of roughening the metal foil. The roughening step includes an etching step of etching the metal foil. Roughening forms a porous portion having a plurality of pits (or pores) on the surface side of the metal foil. At the same time, a core portion integrated with the porous portion is formed on the inner part of the metal foil. Etching can be performed, for example, by DC etching with a DC current or AC etching with an AC current.

[0059] The etching process may include multiple steps. For example, multiple etching tanks for holding the etching solution may be arranged, or the process may be carried out in a roll-to-roll manner. For example, by changing the etching current (current density, frequency), etching solution temperature, etc., according to the step, the perimeter length of the pits in the first to third regions (first to ninth sections) can be controlled. The changes in etching current, etc., may be made continuously or in stages. For example, the frequency may be reduced as the etching process progresses. This allows the starting point for pit formation to be moved to deeper parts of the electrode foil. Also, the current density may be reduced and the etching temperature may be lowered as the etching process progresses. This allows for the densification of the pit shape. Furthermore, the etching solution (concentration, main component) and the application time of the etching current may be changed according to the step. The etching solution may, for example, contain hydrochloric acid as its main component.

[0060] Furthermore, the etching process (multiple steps) may be temporarily interrupted, and an intermediate treatment step may be performed in which the material is immersed in an acid treatment solution containing a phosphorus compound (for example, an aqueous potassium phosphate solution). In the intermediate treatment step, a protective layer is partially formed on the inner wall surface of the pits in the porous section, and pit growth is suppressed in the areas where the protective layer is formed. The pit circumference (pit diameter) may be controlled by the intermediate treatment step. The area in which the protective layer is formed may be controlled by the timing and number of times the intermediate treatment step is performed, the immersion time, etc.

[0061] For example, if the etching process is carried out under constant conditions without changing the etching current, the pit circumference (pit diameter) will be almost uniform in the thickness direction of the porous portion, and L2 / L1 and L2 / L3 will each be less than 1.1.

[0062] The type of first metal is not particularly limited, but valve metals such as aluminum (Al), tantalum (Ta), and niobium (Nb), or alloys containing valve metals, can be used because they facilitate the formation of a dielectric layer or second layer by chemical conversion. The thickness of the metal foil is not particularly limited, but for example, it is 15 μm or more and 300 μm or less.

[0063] A method for manufacturing electrode foil for electrolytic capacitors may further include a step of forming a dielectric layer that covers the metal portion constituting the porous part of the electrode foil.

[0064] The step of forming the dielectric layer may be, for example, a step of chemical conversion (anodic oxidation) of the anode (first electrode foil). For example, by applying a voltage to the first electrode foil while it is immersed in a chemical conversion solution such as ammonium adipate solution, ammonium phosphate solution, or ammonium borate solution, a second electrode foil is obtained in which a dielectric layer is formed on the surface of the metal portion.

[0065] Furthermore, the process of forming the dielectric layer may be, for example, a process of depositing an oxide of a second metal different from the first metal contained in the metal portion onto the surface of the metal portion by a vapor phase method to form a first layer with a thickness T1. This results in a second electrode foil having a dielectric layer formed on the surface of the metal portion.

[0066] Examples of secondary metals include Al, Ta, Nb, silicon (Si), titanium (Ti), zirconium (Zr), and hafnium (Hf). These may be used individually or in combination of two or more. That is, the first layer may contain Al2O3, Ta2O5, Nb2O5, SiO2, TiO2, ZrO2, HfO2, etc., individually or in combination of two or more. When the first layer contains oxides of two or more secondary metals, the two or more oxides may be mixed together, or each may be arranged in layers. From the viewpoint of increasing the capacitance of the electrolytic capacitor, it is preferable that the oxide of the secondary metal has a higher dielectric constant than the oxide of the first metal. Furthermore, from the viewpoint of improving the dielectric strength of the electrolytic capacitor, it is preferable that the secondary metal is Ta, Ti, Si, etc.

[0067] As for vapor phase methods, for example, vacuum deposition, chemical deposition, mist deposition, sputtering, pulsed laser deposition, and atomic layer deposition (ALD) can be selected. Among these, the ALD method is superior in that it can form a dense dielectric layer even deep within the porous part. The thickness of the first layer is not particularly limited, but for example, it may be 0.5 nm or more and 200 nm or less, or 5 nm or more and 200 nm or less.

[0068] Figure 2 shows an example of an anode foil 10 comprising an anode body 110 which is an integrated core portion 111 and a porous portion 112, and a dielectric layer 120 which covers the surface of the metal portion constituting the porous portion 112. Figure 2 is an enlarged schematic cross-sectional view showing a part of the porous portion 112 which has only a first layer 121 as the dielectric layer 120.

[0069] The porous portion 112 has numerous pits (or pores) P surrounded by the metal portion. The dielectric layer 120 (first layer 121) is provided so as to cover at least a portion of the surface of the metal portion. The first layer 121 contains an oxide of a second metal different from the first metal contained in the metal portion, and its thickness is indicated by T1. The dielectric layer 120 is composed of the first layer 121, but it may also be a layer (conversion film) containing an oxide of the first metal.

[0070] (Electrolytic capacitor) The electrolytic capacitor according to this embodiment comprises a second electrode foil and a cathode portion that covers at least a part of the dielectric layer of the second electrode foil.

[0071] The cathode portion may contain an electrolyte. The electrolyte covers at least a portion of the dielectric layer. When L2 / L1 and L2 / L3 (D2 / D1 and D2 / D3) are each 1.1 or greater, it becomes easier to impregnate the electrolyte into the third region.

[0072] The electrolyte comprises at least one of a solid electrolyte and an electrolyte solution. The cathode portion may contain a solid electrolyte and an electrolyte solution, or a solid electrolyte and a non-aqueous solvent. Hereinafter, the electrolyte solution and non-aqueous solvent will be collectively referred to as the "liquid component." Coating of the dielectric layer with a solid electrolyte (or electrolyte solution) is performed, for example, by impregnating the second electrode foil (or winding) with a processing solution (or electrolyte solution) containing a conductive polymer. The processing solution may contain a non-aqueous solvent. When L2 / L1 and L2 / L3 (D2 / D1 and D2 / D3) are each 1.1 or greater, the impregnation of the processing solution (or electrolyte solution) into the porous portion becomes good, and the retention of the processing solution (or electrolyte solution) after impregnation into the porous portion also becomes good.

[0073] The solid electrolyte contains a conductive polymer. Examples of conductive polymers include π-conjugated polymers. Examples of conductive polymers include polypyrrole, polythiophene, polyfuran, and polyaniline. The conductive polymer may be used alone, in combination of two or more types, or as a copolymer of two or more monomers. The weight-average molecular weight of the conductive polymer is, for example, 1,000 to 1,000,000.

[0074] In this specification, polypyrrole, polythiophene, polyfuran, polyaniline, etc., refer to polymers that have polypyrrole, polythiophene, polyfuran, polyaniline, etc. as their basic skeletons. Therefore, polypyrrole, polythiophene, polyfuran, polyaniline, etc., may also include their respective derivatives. For example, polythiophene includes poly(3,4-ethylenedioxythiophene) (PEDOT), etc.

[0075] Conductive polymers can be doped with dopants. Solid electrolytes may contain dopants together with the conductive polymer. Examples of dopants include polystyrene sulfonic acid (PSS). Solid electrolytes may further contain additives as needed.

[0076] The liquid component is in direct contact with the dielectric layer or via a conductive polymer. The liquid component may be a non-aqueous solvent or an electrolyte. The electrolyte contains a non-aqueous solvent and an ionic substance (solute (e.g., an organic salt)) dissolved therein. The non-aqueous solvent may be an organic solvent or an ionic liquid.

[0077] As the non-aqueous solvent, a high-boiling point solvent is preferred. For example, polyol compounds such as ethylene glycol, sulfone compounds such as sulfolane, lactone compounds such as γ-butyrolactone, ester compounds such as methyl acetate, carbonate compounds such as propylene carbonate, ether compounds such as 1,4-dioxane, and ketone compounds such as methyl ethyl ketone can be used.

[0078] The liquid component may contain an acid component (anion) and a base component (cation). A salt (solute) may be formed by the acid component and the base component. The acid component contributes to the film repair function. Examples of acid components include organic carboxylic acids and inorganic acids. Examples of inorganic acids include phosphoric acid, boric acid, and sulfuric acid. Examples of base components include primary to tertiary amine compounds.

[0079] An organic salt is a salt in which at least one of the anion and cation is an organic substance. Examples of organic salts include trimethylamine maleate, triethylamine borodisalicylate, ethyldimethylamine phthalate, mono-1,2,3,4-tetramethylimidazolinium phthalate, and mono-1,3-dimethyl-2-ethylimidazolinium phthalate.

[0080] From the viewpoint of suppressing dopant dedoping (degradation of solid electrolyte) from conductive polymers, it is preferable that the liquid component contains more acidic components than basic components. Furthermore, since the acidic components contribute to the film repair function of the liquid component, it is also preferable that the liquid component contains more acidic components than basic components. The molar ratio of acidic components to basic components (acidic component / basic component) is, for example, 1.1 or higher. From the viewpoint of suppressing dopant dedoping from conductive polymers, the pH of the liquid component may be 6 or less, or it may be 1 or more, or 5 or less.

[0081] Here, Figure 3 is a schematic cross-sectional view showing an electrolytic capacitor according to one embodiment of the present invention. Figure 3 shows an example of an electrolytic capacitor equipped with a wound capacitor element. Figure 4 is a schematic perspective view showing the configuration of the wound body in Figure 3.

[0082] The electrolytic capacitor 200 comprises a winding body 100. The winding body 100 is constructed by winding an anode foil 10 and a cathode foil 20 with a separator 30 in between. The separator 30 is not particularly limited and may be made of, for example, a nonwoven fabric containing fibers of cellulose, polyethylene terephthalate, vinylon, or polyamide (e.g., aliphatic polyamide, aromatic polyamide such as aramid).

[0083] One end of lead tabs 50A and 50B are connected to the anode foil 10 and cathode foil 20, respectively, and the winding body 100 is formed by winding the lead tabs 50A and 50B. Lead wires 60A and 60B are connected to the other ends of lead tabs 50A and 50B, respectively.

[0084] A winding stopper tape 40 is placed on the outer surface of the cathode foil 20, which is located in the outermost layer of the winding body 100, and the end of the cathode foil 20 is fixed by the winding stopper tape 40. If the anode foil 10 is prepared by cutting from a large sheet of foil, the winding body 100 may be further treated with a chemical conversion process to provide a dielectric layer on the cut surface.

[0085] The winding body 100 contains an electrolyte, with the electrolyte interposed between the anode foil 10 (dielectric layer) and the cathode foil. The winding body 100 containing the electrolyte is produced, for example, by impregnating the winding body 100 with a processing solution (or electrolyte) containing a conductive polymer. The impregnation may be carried out under reduced pressure, for example, in an atmosphere of 10 kPa to 100 kPa.

[0086] The winding body 100 is housed in the closed-bottom case 211 such that the lead wires 60A and 60B are located on the opening side of the closed-bottom case 211. The material of the closed-bottom case 211 can be a metal such as aluminum, stainless steel, copper, iron, or brass, or an alloy thereof.

[0087] The winding body 100 is sealed inside the bottomed case 211 by placing a sealing member 212 at the opening of the bottomed case 211, crimping the open end of the bottomed case 211 to the sealing member 212 to create a curl, and placing a seat plate 213 on the curled portion.

[0088] The sealing member 212 is formed so that the lead wires 60A and 60B pass through it. The sealing member 212 can be made of any insulating material, and an elastic material is preferred. Among these, silicone rubber, fluororubber, ethylene propylene rubber, Hypalon rubber, butyl rubber, isoprene rubber, etc., which have high heat resistance are preferred.

[0089] Although the above embodiments describe a wound electrolytic capacitor, the scope of application of the present invention is not limited to the above and can be applied to other electrolytic capacitors, such as multilayer electrolytic capacitors. A multilayer electrolytic capacitor comprises, for example, a multilayer capacitor element and an outer casing that seals the capacitor element. The multilayer capacitor element comprises an anode, a solid electrolyte layer, and a cathode layer covering the solid electrolyte layer. The anode comprises an electrode foil (first electrode foil) with a porous portion formed on a part of its surface, and a dielectric layer covering the metal skeleton constituting the porous portion of the electrode foil. The solid electrolyte layer is formed to cover the dielectric layer. The cathode layer comprises, for example, a silver paste layer and a carbon layer. An anode lead is connected to the region of the anode not covered by the dielectric layer, and a cathode lead is connected to the cathode layer. Parts of the anode lead and cathode lead are exposed from the outer casing. Multiple capacitor elements may be stacked to form a laminate.

[0090] The present disclosure will be described in more detail below based on examples, but the present disclosure is not limited to these examples.

[0091] Examples 1-2 and Comparative Example 1 (Fabrication of the first electrode foil) Al foil (130 μm thick) was prepared as a metal foil. The Al foil was pre-treated with an aqueous hydrochloric acid solution, and then an etching process was carried out by applying an alternating current in an etching solution mainly composed of hydrochloric acid. The etching process consisted of multiple steps, and the etching current (current density, frequency) and the temperature of the etching solution were adjusted as appropriate according to the step. In Examples 1 and 2, the etching current (current density, frequency) was reduced in three stages and the temperature of the etching solution was lowered in three stages as the etching process progressed. On the other hand, in Comparative Example 1, the etching process was carried out with the etching current (current density, frequency) and the temperature of the etching solution kept constant.

[0092] Furthermore, in Examples 1 and 2 and Comparative Example 1, an intermediate treatment step was performed in which the Al foil was immersed in an aqueous potassium phosphate solution during the etching process (multiple steps). The timing and number of times the intermediate treatment step was performed were adjusted as appropriate.

[0093] In this way, an etched foil (first electrode foil) having porous regions (thickness 45 μm) on both surfaces of the Al foil was obtained. The pit perimeter lengths L9-1 to L9-9 in the first to ninth sections (5 μm × 5 μm square) of the porous region were set to the values ​​shown in Table 1, and the pit perimeter lengths L1 to L3 in the first to third regions were set to the values ​​shown in Table 2. In Tables 1 and 2, A1-1 to A2-1 are the first electrode foils of Examples 1 and 2, and B1-1 is the first electrode foil of Comparative Example 1. The pit perimeter lengths L9-1 to L9-9 in the first to ninth sections, and the pit perimeter lengths L1 to L3 in the first to third regions were determined by the method described above.

[0094] [Table 1]

[0095] [Table 2]

[0096] (Fabrication of the second electrode foil) The first electrode foils A1-1, A2-1, and B1-1 were subjected to a chemical conversion treatment to form a dielectric layer containing Al2O3, thereby obtaining the second electrode foils A1-2, A2-2, and B1-2.

[0097] [evaluation] (Folding strength and tensile strength) For the first electrode foil, the bending strength and tensile strength were measured in accordance with the test method for electrode foils for aluminum electrolytic capacitors of the Japan Electronic Machinery Industry Standard (EIAJ RC-2364A).

[0098] (Capacitance) The capacitance (frequency 120 Hz) of the second electrode foil was measured under conditions of 20°C.

[0099] (Electrolyte impregnation) The second electrode foil was impregnated with a treatment solution containing a conductive polymer, dried, and a solid electrolyte layer was formed covering the inner walls of the pits in the porous region. The impregnation was performed by immersing the second electrode in the treatment solution for 90 seconds under atmospheric pressure. A PEDOT / PSS aqueous dispersion (concentration 4.5 mass%) was used as the treatment solution. A cross-sectional image in the thickness direction of the second electrode foil after the formation of the solid electrolyte layer was obtained by SEM. Using this image, a region (5 μm × 5 μm square) was arbitrarily selected within the third region R3 of the porous region, and the total length E0 of the inner walls of the pits contained within that region (the contour of the region occupied by the pits) and the length E1 of the portion of the inner wall of the pits covered by the solid electrolyte layer were determined. (E1 / E0) × 100 was calculated as the electrolyte impregnation rate (%).

[0100] The evaluation results are shown in Tables 3 and 4. Note that the folding strength and tensile strength in Table 3 are expressed as relative values, with the measured value of B1-1 (first electrode foil) of Comparative Example 1 set to 100. The capacitance in Table 4 is expressed as a relative value, with the measured value of B1-2 (second electrode foil) of Comparative Example 1 set to 100.

[0101] [Table 3]

[0102] [Table 4]

[0103] As shown in Table 3, A1-1 and A2-1 showed higher tensile strength and bending strength than B1-1. A1-1 showed even higher tensile strength and bending strength.

[0104] As shown in Table 4, A1-2 and A2-2 showed higher capacitance and electrolyte impregnation rates than B1-2. [Industrial applicability]

[0105] The electrode foil for electrolytic capacitors according to this disclosure is suitably used in electrolytic capacitors where high reliability and high capacitance are required.

[0106] Although the present invention has been described in relation to preferred embodiments at present, such disclosure should not be interpreted restrictively. Various modifications and alterations will undoubtedly become apparent to those skilled in the art in the field to which the invention pertains by reading the above disclosure. Accordingly, the appended claims should be interpreted as encompassing all modifications and alterations without departing from the true spirit and scope of the invention. [Explanation of Symbols]

[0107] 10: Anode foil, 20: Cathode foil, 30: Separator, 40: Winding stopper tape, 50A, 50B: Lead tab, 60A, 60B: Lead wire, 100: Winding body, 110: Anode body, 111: Core, 112: Porous part, 120: Dielectric layer, 121: First layer, 200: Electrolytic capacitor, 211: Bottomed case, 212: Sealing material, 213: Base plate

Claims

1. Electrode foil for electrolytic capacitor, It comprises a porous portion and a core portion continuous with the porous portion, In the thickness direction of the porous portion, when the porous portion is divided into three equal parts, a first region, a second region, and a third region, in order from the outer surface side of the porous portion, the perimeter length L1 of the pit in the first region, the perimeter length L2 of the pit in the second region, and the perimeter length L3 of the pit in the third region satisfy the relationship 1.1 ≤ L2 / L1 and 1.1 ≤ L2 / L3. The electrode foil for an electrolytic capacitor has a porous portion thickness of 10 μm to 160 μm, or is 1 / 10 or more and 5 / 10 or less of the thickness of the electrode foil for the electrolytic capacitor.

2. The electrode foil for an electrolytic capacitor according to claim 1, wherein L2 / L1 is 1.2 or greater.

3. The electrode foil for an electrolytic capacitor according to claim 1, wherein L2 / L3 is 1.2 or greater.

4. The electrode foil for electrolytic capacitor according to any one of claims 1 to 3, wherein 2 × L2 / (L1 + L3) is 1.2 or greater.

5. L2 is 145μm / μm ２ Above, 165μm / μm ２ The electrode foil for an electrolytic capacitor according to any one of claims 1 to 3, which is as follows:

6. Furthermore, the electrode foil for an electrolytic capacitor according to any one of claims 1 to 3, further comprising a dielectric layer covering at least a portion of the surface of the metal skeleton constituting the porous portion.

7. An electrolytic capacitor comprising an electrode foil for an electrolytic capacitor as described in claim 6, and a cathode portion covering at least a part of the dielectric layer.

8. The electrolytic capacitor according to claim 7, wherein the cathode portion includes an electrolyte.

9. The electrolytic capacitor according to claim 7, wherein the cathode portion includes a solid electrolyte.

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