Electrode body, electrolytic capacitor including the same, and manufacturing method for electrode body

Abstract

To provide an electrode body that produces excellent cathode side capacitance and an electrolytic capacitor including the same.SOLUTION: The electrode body used for a cathode of the electrolytic capacitor has a cathode foil and a carbon layer. The cathode foil is made of a valve action metal and has a spreading layer formed on its surface. The carbon layer is formed on the spreading layer. An interface between the spreading layer and the carbon layer has an irregular shape.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an electrode body, an electrolytic capacitor including the electrode body, and a method for manufacturing the electrode body.

Background Art

[0002] An electrolytic capacitor includes a valve metal such as tantalum or aluminum as an anode foil and a cathode foil. The anode foil is surface-expanded by forming the valve metal into a sintered body or an etched foil, etc., and has a dielectric oxide film layer on the surface-expanded surface. An electrolytic solution is interposed between the anode foil and the cathode foil. The electrolytic solution is in close contact with the uneven surface of the anode foil and functions as a true cathode. This electrolytic capacitor obtains an anode-side capacitance by the dielectric polarization action of the dielectric oxide film layer.

[0003] An electrolytic capacitor can be regarded as a series capacitor in which capacitances are generated on the anode side and the cathode side. Therefore, the cathode-side capacitance is very important for efficiently utilizing the anode-side capacitance. Thus, although the surface area of the cathode foil is also increased by, for example, an etching treatment, there is a limit to the surface expansion of the cathode foil from the viewpoint of the thickness of the cathode foil.

[0004] Therefore, an electrolytic capacitor in which a film of a metal nitride such as titanium nitride is formed on a cathode foil has been proposed (see Patent Document 1). In a nitrogen gas environment, titanium is evaporated by a vacuum arc evaporation method, which is a kind of ion plating method, and titanium nitride is deposited on the surface of the cathode foil. The metal nitride is inert and it is difficult to form a natural oxide film. Further, fine irregularities are formed on the deposited film, and the surface area of the cathode is enlarged.

[0005] An electrolytic capacitor in which a porous carbon layer containing activated carbon is formed on a cathode foil is also known (see Patent Document 2). The capacitance on the cathode side of this electrolytic capacitor is manifested by the charge storage action of the electric double layer formed at the interface between the polarizable electrode and the electrolyte. The cations of the electrolyte are aligned at the interface with the porous carbon layer, pair with the electrons in the porous carbon layer at a very short distance, and a potential barrier is formed at the cathode. The cathode foil formed with this porous carbon layer is produced by kneading a water-soluble binder solution in which porous carbon is dispersed into a paste, applying the paste to the surface of the cathode foil, and drying it by exposing it to high temperature.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0007] The vapor deposition process of metal nitride is complex and difficult to introduce industrially, leading to high costs and poor yields of electrolytic capacitors. An electrolytic capacitor in which a porous carbon layer containing activated carbon is formed on a cathode foil by paste coating has a lower capacitance than an electrolytic capacitor in which metal nitride is vapor deposited on the cathode foil under normal temperature environment and high temperature environment. Therefore, a satisfactory capacitance has not yet been obtained in an electrolytic capacitor in which a porous carbon layer is formed on a cathode foil by paste coating.

[0008] The present invention has been proposed to solve the above problems, and an object thereof is to provide an electrode body that exhibits good capacitance, an electrolytic capacitor including this electrode body, and a method for manufacturing the electrode body.

Means for Solving the Problems

[0009] In order to solve the above problems, the electrode body according to the present invention is an electrode body used for the cathode of an electrolytic capacitor, comprising a cathode foil made of a valve metal and having a surface layer formed on the surface thereof, and a carbon layer formed on the surface layer. The interface between the surface layer and the carbon layer has an uneven shape.

[0010] The uneven depth of the uneven shape may be 0.5 μm or more.

[0011] The surface layer is formed by digging a plurality of etching pits. The diameter of the etching pits is 0.12 μm or more and 0.43 μm or less near the surface layer, the depth of the etching pits is 1.5 μm or more and 5.0 μm or less, the uneven shape is such that the distance between both ends of a cross section obtained by cutting the convex region along the height direction is 1.5 μm or more and 8.0 μm or less, and the uneven shape may be such that the height of the convex region is 0.15 μm or more and 0.80 μm or less.

[0012] The surface layer is formed by digging a plurality of etching pits, and the carbon layer may enter further into the etching pits from the interface having the uneven shape.

[0013] The carbon layer that has entered further into the etching pits from the concave region may enter up to a position that is recessed by an average of 0.5 μm or more in the depth direction from the apex of the convex region adjacent to the concave region.

[0014] The carbon layer that has entered further into the etching pits from the concave region may enter up to a position that is recessed by an average of 0.7 μm or more in the depth direction from the apex of the convex region adjacent to the concave region.

[0015] The surface layer is formed by digging a plurality of etching pits, and the ratio of the interface length Y to the range length X (Y / X×100) may be 110% or more. However, the interface length Y is the length along the interface between the surface layer and the carbon layer from an arbitrary starting point to an arbitrary ending point, and includes the length of the carbon layer that has entered the etching pit. The range length X is the length of the component in the direction orthogonal to the height direction of the uneven shape among the vectors connecting the starting point and the ending point at which the interface length Y is measured.

[0016] The uneven shape may be compressed and deformed by pressing.

[0017] The carbon layer may include a flaky carbon material and a spherical carbon material.

[0018] An electrolytic capacitor having this electrode body as a cathode is also one aspect of the present invention.

[0019] Moreover, a method for manufacturing an electrode body used for the cathode of an electrolytic capacitor according to the present invention is a step of forming a carbon layer on a cathode foil made of a valve action metal and having a surface layer formed thereon, and a step of pressing the cathode foil on which the carbon layer is formed, wherein the interface between the surface layer and the carbon layer has an uneven shape.

[0020] The cathode foil on which the carbon layer is formed may be pressed at 1.54 kNcm -1 or more.

[0021] The carbon layer may be formed by applying a slurry containing flaky carbon and spherical carbon to the cathode foil and drying it.

Advantages of the Invention

[0022] According to the present invention, even when a carbon layer is used for the cathode body, a good capacitance can be exhibited.

Brief Description of the Drawings

[0023]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Mode for Carrying Out the Invention

[0024] The cathode body and the electrolytic capacitor including the cathode body according to the embodiment of the present invention will be described. In this embodiment, an electrolytic capacitor having an electrolytic solution will be exemplified and described, but it is not limited thereto. It can be applied to any of electrolytic capacitors having an electrolyte such as an electrolytic solution, a solid electrolyte layer such as a conductive polymer, a gel electrolyte, or an electrolyte in which an electrolytic solution is used in combination with a solid electrolyte layer and a gel electrolyte.

[0025] (Electrolytic Capacitor) An electrolytic capacitor is a passive element that stores and discharges charges according to the capacitance. This electrolytic capacitor has a wound type or a stacked type capacitor element. The capacitor element has an anode foil and a cathode body opposed to each other via a separator and is impregnated with an electrolytic solution. This electrolytic capacitor generates a cathode side capacitance by the electric double layer action occurring at the interface between the electrolytic solution and the cathode body, and also generates an anode side capacitance by the dielectric polarization action.

[0026] That is, a dielectric oxide film layer that causes dielectric polarization is formed on the surface of the anode foil. A carbon layer that causes an electric double layer effect at the interface with the electrolytic solution is formed on the surface of the cathode body. The electrolytic solution is interposed between the anode foil and the cathode body and is in close contact with the dielectric oxide film layer of the anode foil and the carbon layer of the cathode body. The separator is interposed between the anode foil and the cathode body to prevent short circuits between the anode foil and the cathode body and holds the electrolytic solution.

[0027] In addition, when a solid electrolyte is used, it will be electrically connected to the solid electrolyte by the carbon layer in contact with the current collector, and the electrostatic capacitance of the electrolytic capacitor is constituted by the anode-side capacitance due to the dielectric polarization effect.

[0028] (Cathode body) This cathode body has a two-layer structure of a cathode foil and a carbon layer. The cathode foil serves as a current collector, and an enlarged surface layer is formed on its surface. The carbon layer contains a carbon material and adheres to the enlarged surface layer of the cathode foil.

[0029] The cathode foil is a long foil made of a valve-acting metal. The valve-acting metal is aluminum, tantalum, niobium, niobium oxide, titanium, hafnium, zirconium, zinc, tungsten, bismuth, antimony, etc. A purity of about 99% or more is desirable, but it may contain impurities such as silicon, iron, copper, magnesium, and zinc. As the cathode foil, for example, an aluminum material with a temper symbol of H as defined in JIS standard H0001, so-called H material, or an aluminum material with a temper symbol of O as defined in JIS standard H0001, so-called O material may be used. When using a highly rigid metal foil made of H material, deformation of the cathode foil due to press working described later can be suppressed.

[0030] This cathode foil has a surface expansion treatment applied to a metal foil in which a valve-acting metal has been drawn. The expanded layer is formed by electrolytic etching, chemical etching, sandblasting, etc., or by vapor depositing or sintering metal particles, etc. on the metal foil. Examples of electrolytic etching include methods such as direct current etching or alternating current etching. In chemical etching, the metal foil is immersed in an acid solution or an alkaline solution. The formed expanded layer is a layer region having tunnel-shaped etching pits or sponge-shaped etching pits dug from the foil surface toward the foil core. Note that the etching pits may be formed so as to penetrate the cathode foil.

[0031] In the carbon layer, the carbon material is fibrous carbon, carbon powder, or a mixture thereof. The fibrous carbon is carbon nanotubes, carbon nanofibers, etc. The carbon nanotubes may be single-walled carbon nanotubes with a single graphene sheet, or multi-walled carbon nanotubes (MWCNT) in which two or more graphene sheets are coiled coaxially and the tube wall forms multiple layers. The carbon powder is activated carbon, ketjen black, acetylene black, channel black, etc. derived from natural plant tissues such as coconut husks, synthetic resins such as phenol, fossil fuels such as coal, coke, and pitch, carbon nanohorns, amorphous carbon, natural graphite, artificial graphite, graphitized ketjen black, mesoporous carbon, etc. In an electrolytic capacitor using an electrolytic solution as the electrolyte, the carbon material preferably exhibits an electric double layer effect.

[0032] Fibrous carbon and carbon powder may be subjected to a porosity treatment such as an activation treatment or an opening treatment for forming pores. Although the activation method of carbon powder varies depending on the raw material used, conventionally known activation treatments such as a gas activation method and a chemical activation method can usually be used. Examples of the gas used in the gas activation method include water vapor, air, carbon monoxide, carbon dioxide, hydrogen chloride, oxygen, or a gas composed of a mixture thereof. Examples of the chemical used in the chemical activation method include hydroxides of alkali metals such as sodium hydroxide and potassium hydroxide, hydroxides of alkaline earth metals such as calcium hydroxide, inorganic acids such as boric acid, phosphoric acid, sulfuric acid, and hydrochloric acid, or inorganic salts such as zinc chloride. Heat treatment is performed as necessary during this activation treatment.

[0033] Figure 1(a) is a schematic diagram showing the surface where the cathode foil and the carbon layer are in contact, that is, the interface structure between the extended surface layer and the carbon layer. The carbon layer (reference numeral 2 in the figure) enters the etching pit (reference numeral 1 in the figure), so that a part of the interface between the extended surface layer (reference numeral 3 in the figure) and the carbon layer has a fine uneven shape along the etching pit. Hereinafter, the uneven shape in which the carbon layer enters the etching pit is referred to as an etching unevenness (reference numeral 4 in the figure).

[0034] Separate from the etching unevenness, undulating unevenness, that is, unevenness with a long wavelength, is generated at the interface between the extended surface layer and the carbon layer. Hereinafter, this unevenness is referred to as an external appearance unevenness (reference numeral 5 in the figure). In other words, assuming that there is no etching pit, the surface of the extended surface layer forms undulations in which a wide convex region and a concave region are connected, and the carbon layer adheres along this undulation. The etching unevenness continuously extends deep from the convex region and the concave region of the external appearance unevenness. The external appearance unevenness provides a three-dimensional structure to the surface of the extended surface layer and increases the adhesion area with the carbon layer.

[0035] Here, in the aging process of the electrolytic capacitor, an oxide film is formed on the exposed portion of the surface of the enlarged layer. Also, an increase in the adhesion area between the enlarged layer and the carbon layer leads to a decrease in the exposed portion of the surface of the enlarged layer. Then, the external surface unevenness that increases the adhesion area between the enlarged layer and the carbon layer reduces the area where the oxide film is formed as a whole for the cathode foil in the aging process. Therefore, an electrolytic capacitor using a cathode body having external surface unevenness at the interface between the enlarged layer and the carbon layer increases the capacitance due to charge and discharge in a high-frequency region such as 10 kHz compared to an electrolytic capacitor using a cathode foil vapor-deposited with titanium nitride, and also suppresses the reduction rate of the capacitance in a high-temperature environment such as 125°C.

[0036] In particular, an appropriate entanglement pattern of the external surface unevenness enhances the effect of increasing the adhesion area between the enlarged layer and the carbon layer. First, naturally or due to the electrolytic solution, an oxide film is formed on the surface of the enlarged layer. However, when the carbon layer is formed on the enlarged layer and pressed, the carbon material of the carbon layer is pushed against the surface of the enlarged layer, breaking the oxide film, and the unoxidized enlarged layer and the carbon layer come into direct contact. Second, the force for further pushing the carbon layer in the concave region into the etching pits is easily transmitted, and the carbon layer penetrates into the enlarged layer by the depth of the concave region and the etching pits added, further increasing the adhesion area between the carbon layer and the enlarged layer and enabling the cathode body to have a lower resistance.

[0037] On the other hand, when the entanglement pattern becomes somewhat complicated to a certain extent, it becomes difficult to transmit the force for further pushing the carbon layer that has entered the concave region into the etching pits. Therefore, when the entanglement pattern becomes somewhat complicated to a certain extent, although the capacitance improves, the degree of improvement is limited.

[0038] The external surface unevenness having an appropriate entanglement pattern is as follows. First, the unevenness depth Hf of the external surface unevenness is preferably 0.5 μm or more. This unevenness depth Hf of the external surface indicates the distance from the flat line L, which is a line connecting two vertices of the external surface unevenness, to the deepest concave portion of the external surface unevenness. The cross-section of the cathode body may be photographed with a scanning electron microscope, and the distance to the deepest concave portion may be measured based on the flat line connecting the two highest convex portions among the external surface unevenness in the SEM image. By setting the unevenness depth Hf of the external surface to 0.5 μm or more, the adhesion to the carbon layer is enhanced.

[0039] Further, the length Lc of the convex region of the surface irregularities is in the range of 1.5 μm or more and 8.0 μm or less, and on average is about 2.9 μm to 4.0 μm. The height Hc of the convex region of the surface irregularities is in the range of 0.15 μm or more and 0.80 μm or less, and on average is about 0.3 μm to 0.6 μm. The length Lc of the convex region is the distance between both ends in a cross-section obtained by cutting the convex region along the height direction. The length Ld of the concave region is in the range of 2.0 μm or more and 7.7 μm or less, and on average is about 3.8 μm. For each numerical value representing the surface irregularities, a cross-section of the cathode body may be photographed with a scanning electron microscope and measured in a SEM image. The average may be measured by extracting 5 points.

[0040] However, even within this range, if the undulation of the surface irregularities is severe, that is, if the length Lc of the convex region of the surface irregularities is in the range of 3.77 μm or less, the superiority regarding the deterioration of the capacitance during use in a high-temperature environment such as 125°C and in a high-frequency region such as 10 kHz for the cathode body with titanium nitride vapor-deposited becomes small. On the other hand, when the undulation of the surface irregularities becomes gentle, that is, although there are surface irregularities, if the length Lc of the convex region of the surface irregularities is 3.78 μm or more, the superiority regarding the deterioration of the capacitance is great for the cathode body with titanium nitride vapor-deposited during use in a high-temperature environment such as 125°C and in a high-frequency region such as 10 kHz.

[0041] Due to these surface irregularities, the deepest distance Hd of the carbon layer pushed further into the etching pits from the interface in the concave region is 0.42 μm or more and 1.40 μm or less in the depth direction from the apex of the convex region adjacent to the concave region, and on average is about 0.54 μm to 0.96 μm. When the surface irregularities are gentle, it is also 0.7 μm or more on average. If it is 0.5 μm or less, the effect of improving the capacitance is small. If it is 0.7 μm or more, it has a good capacitance and also high thermal stability. Thermal stability means that the deterioration of the capacitance in a high-temperature environment is small.

[0042] The etching pits still have a diameter De in the range of 0.12 μm or more and 0.43 μm or less near the surface, and an average diameter of about 0.25 μm. Further, the etching pits formed by the etching process have a depth He in the range of 1.5 μm or more and 5.0 μm or less, and an average depth of about 3.3 μm.

[0043] Furthermore, another index is shown for the external surface unevenness having a suitable degree of intricacy. Fig. 1(b) is a cross-sectional view cut along the height direction of the unevenness, in other words, a cross-sectional view cut along the thickness direction of the cathode body from the surface of the cathode body toward the deep part. In this cross-section, the length along the interface between the diffusion layer and the carbon layer from an arbitrary starting point to an arbitrary ending point is defined as the interface length Y. This interface includes the carbon layer that has entered the etching pits. Also, among the vectors connecting the starting point and the ending point where the interface length Y is measured in a straight line, the length of the direction component where the height of the unevenness is constant, that is, the direction component orthogonal to the height direction of the unevenness, is defined as the range length X.

[0044] At this time, the ratio of the interface length Y to the range length X is preferably 110% or more. Less than 110% indicates that almost no external surface unevenness is formed, and when there is no load at 20 °C and after continuously applying a load of direct current 2.4 V for 250 hours in an environment of 125 °C, the respective capacitances during charge and discharge at 120 Hz and 10 kHz will be inferior compared to the electrolytic capacitor manufactured with a cathode body on which titanium nitride is deposited.

[0045] Also, most of the voids in the carbon layer preferably disappear and the carbon layer becomes dense. The porosity of the carbon layer is preferably less than 18%. The small porosity of the carbon layer enhances the adhesion between the carbon layer and the etched surface. The porosity of the carbon layer is calculated by (area of the void part of the carbon layer / total area of the carbon layer) × 100. The porosity can be calculated by using the image analysis software ImageJ (NIH, National Institutes of Health, USA) for each cross-sectional SEM image (observation magnification: 25,000 times) and performing analysis.

[0046] An example of a method for forming an interface structure having such surface irregularities and etching irregularities is illustrated. However, the method is not limited to the illustrated method as long as the interface structure is constituted by the surface irregularities and the etching irregularities. For example, uneven irregularities may be formed on the surface of the enlarged layer by another method such as shot peening, except for the etching pits.

[0047] First, an enlarged layer is formed on the cathode foil. Typically, the enlarged layer is formed by DC etching or AC etching in which DC or AC is applied in an acidic aqueous solution such as nitric acid, sulfuric acid, or hydrochloric acid.

[0048] Regarding the carbon layer, powder of a carbon material is dispersed in a solvent, and a binder is added to prepare a slurry. The solvent is alcohol such as methanol, ethanol, or 2-propanol, a hydrocarbon-based solvent, an aromatic-based solvent, an amide-based solvent such as N-methyl-2-pyrrolidone (NMP) or N,N-dimethylformamide (DMF), water, and a mixture thereof. As the dispersion method, a mixer, jet mixing (jet impingement), or ultracentrifugation, other ultrasonic treatment, etc. is used. In the dispersion step, the powder of the carbon material and the binder in the mixed solution are subdivided and homogenized and dispersed in the solution. Examples of the binder include styrene-butadiene rubber.

[0049] Next, the slurry is applied to the enlarged layer, dried, and then pressed at a predetermined pressure to bring the cathode foil and the carbon layer into close contact and integrate them. In the pressing step of bringing the cathode foil and the carbon layer into close contact, it is considered that the interface structure between the enlarged layer and the carbon layer is formed.

[0050] In this pressing process, the carbon material of the carbon layer is pressed against the extended surface layer, and the entire surface of the cathode foil undergoes compressive deformation. Furthermore, the dense region of the etching pits undergoes significant compressive deformation. As a result, the smooth surface state of the cathode foil is deformed, and undulating irregularities are formed at the interface between the extended surface layer and the carbon layer. At the same time as the appearance of the irregularities, the extended surface layer and the carbon layer adhere closely. Also, the carbon material penetrates the natural oxide film present on the surface of the cathode foil and comes into direct contact with the cathode foil. Thus, when passing through the pressing process with a predetermined pressure, the formation of external irregularities and the close adhesion between the extended surface layer and the carbon layer are achieved simultaneously, and it becomes difficult for a gap to occur between the undulating extended surface layer and the carbon layer.

[0051] This pressing process is, for example, applying a press line pressure to the cathode foil coated with the carbon layer using a press roller. At this time, a press roller heated to a temperature equal to or higher than the softening temperature of the binder contained in the carbon layer may be used. By doing so, the fluidity of the carbon layer is increased. Therefore, the carbon layer easily enters into the etching pits, the contact area between the carbon layer and the cathode foil increases, and the interface resistance becomes lower.

[0052] Also, by the pressing process with a predetermined pressure, gentle external irregularities are formed, and due to these gentle external irregularities, the press pressure is easily transmitted to the carbon layer within the concave region, and the carbon layer within the concave region more easily enters into the etching pits. Furthermore, the carbon layer is compressed by the pressing process, and it becomes easier to eliminate the voids within the carbon layer. The line pressure by pressing is desirably such that the carbon material penetrates the natural oxide film present on the surface of the cathode foil and comes into direct contact with the cathode foil, specifically 1.54 kN / cm -1 or more, and more desirably 3.85 kN / cm -1 or more. When the press line pressure is 1.54 kN / cm -1 or more, external irregularities are formed, and when the press line pressure is 3.85 kN / cm -1 or more, the external irregularities become gentle.

[0053] As a carbon material that helps the change in the interface structure, it is preferably a mixture of flaky carbon material and spherical carbon material. The flaky carbon material is easy to press against the expanded surface layer, easy to neatly spread on the interface with the expanded surface layer, and easy to push against the natural oxide film on the foil surface by press bonding and easy to break through the natural oxide film. Therefore, the flaky carbon material is easy to form the interface structure into a wavy concave-convex shape and is easy to adhere to the cathode foil at the interface. Also, the flaky carbon material is easy to neatly stack on the interface of the expanded surface layer. Therefore, the flaky carbon material reduces the voids in the carbon layer. The spherical carbon material fills the voids in the carbon layer and is easy to enter the etching pits during the bonding process with the cathode foil.

[0054] Examples of the flaky carbon material include natural graphite, artificial graphite, or graphitized Ketjen black. It is desirable to use those having an aspect ratio of the minor axis to the major axis in the range of 1:5 to 1:100. Examples of the spherical carbon material include carbon blacks such as Ketjen black (hereinafter referred to as KB), acetylene black, and channel black. The primary particle size of the carbon black is desirably 100 nm or less and it is easy to enter the deep part of the etching pit. Generally, these flaky carbon materials and spherical carbon materials have a smaller specific surface area and a smaller electric double layer capacitance compared with activated carbon and carbon nanotubes, and thus generally play a role as a conductive assistant rather than an active material. However, in this electrolytic capacitor, sufficient cathode-side capacitance is extracted due to the interface structure between the expanded surface layer and the carbon layer, that is, the occurrence of external surface unevenness or further etching unevenness.

[0055] Of course, in addition to these flaky carbon materials and spherical carbon materials, activated carbon, carbon nanotubes, etc. can also be contained, or only these flaky carbon materials and spherical carbon materials can be contained as the active material in the carbon layer. Activated carbon and carbon nanotubes have delocalized π electrons and a large specific surface area. In this electrolytic capacitor, the interface between the extended layer and the carbon layer has external surface irregularities or further etching irregularities, and sufficient capacitance on the cathode side can be extracted. Therefore, the flaky carbon material and the spherical carbon material may not be subjected to a porosity treatment. Of course, the flaky carbon material and the spherical carbon material can also be used after being subjected to a porosity treatment.

[0056] (Anode foil) Next, the anode foil is a long foil body made of a valve metal. The purity is desirably about 99.9% or more with respect to the anode foil. This anode foil is formed by subjecting a stretched foil to an etching treatment, by sintering valve metal powder, or by depositing a film such as metal particles on the foil to form a film. The anode foil has an extended layer or a porous structure layer on the surface.

[0057] The dielectric oxide film layer formed on the anode foil is typically an oxide film formed on the surface layer of the anode foil. If the anode foil is made of aluminum, it is an aluminum oxide layer obtained by oxidizing a porous structure region. This dielectric oxide film layer is formed by a forming treatment in which a voltage is applied in a solution free of halogen ions such as an acid such as ammonium borate, ammonium phosphate, ammonium adipate, or an aqueous solution of these acids. Note that a natural oxide film layer can be formed on the cathode foil, or a dielectric oxide film layer may be intentionally provided.

[0058] (Separator) The separator includes cellulose such as kraft, manila hemp, esparto, hemp, rayon, and mixed papers thereof, polyester resins such as polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, and their derivatives, polytetrafluoroethylene resins, polyvinylidene fluoride resins, vinylon resins, polyamide resins such as aliphatic polyamides, semi-aromatic polyamides, and wholly aromatic polyamides, polyimide resins, polyethylene resins, polypropylene resins, trimethylpentene resins, polyphenylene sulfide resins, acrylic resins, etc., and these resins can be used alone or in combination.

[0059] (Electrolyte solution) The electrolyte solution is a mixed solution in which a solute is dissolved in a solvent and additives are added as required. The solvent can be any of water, protic organic polar solvents, or aprotic organic polar solvents. Representative protic organic polar solvents include monohydric alcohols, polyhydric alcohols, and oxyalcohol compounds. Representative aprotic organic polar solvents include sulfone-based, amide-based, lactones, cyclic amide-based, nitrile-based, and oxide-based solvents.

[0060] Examples of monohydric alcohols include ethanol, propanol, butanol, pentanol, hexanol, cyclobutanol, cyclopentanol, cyclohexanol, benzyl alcohol, etc. Examples of polyhydric alcohols and oxyalcohol compounds include ethylene glycol, propylene glycol, glycerin, methyl cellosolve, ethyl cellosolve, methoxypropylene glycol, dimethoxypropanol, etc. Examples of sulfones include dimethyl sulfone, ethyl methyl sulfone, diethyl sulfone, sulfolane, 3-methyl sulfolane, 2,4-dimethyl sulfolane, etc. Examples of amides include N-methylformamide, N,N-dimethylformamide, N-ethylformamide, N,N-diethylformamide, N-methylacetamide, N,N-dimethylacetamide, N-ethylacetamide, N,N-diethylacetamide, hexamethylphosphoric amide, etc. Examples of lactones and cyclic amides include γ-butyrolactone, γ-valerolactone, δ-valerolactone, N-methyl-2-pyrrolidone, ethylene carbonate, propylene carbonate, butylene carbonate, isobutylene carbonate, isobutylene carbonate, etc. Examples of nitriles include acetonitrile, 3-methoxypropionitrile, glutaronitrile, etc. Examples of oxides include dimethyl sulfoxide, etc. These may be used alone as the solvent, or in combination of two or more kinds.

[0061] The solute contained in the electrolyte contains anionic and cationic components. Typically, it is an organic acid or its salt, an inorganic acid or its salt, or a composite compound of an organic acid and an inorganic acid or its ion-dissociable salt, and is used alone or in combination of two or more kinds. The acid serving as the anion and the base serving as the cation may be separately added to the electrolyte as solute components.

[0062] Examples of organic acids that serve as anion components in the electrolyte include carboxylic acids such as oxalic acid, succinic acid, glutaric acid, pimelic acid, suberic acid, sebacic acid, phthalic acid, isophthalic acid, terephthalic acid, maleic acid, adipic acid, benzoic acid, toluic acid, enanthic acid, malonic acid, 1,6-decanedicarboxylic acid, 1,7-octanedicarboxylic acid, azelaic acid, undecanedioic acid, dodecanedioic acid, tridecanedioic acid, etc., phenols, and sulfonic acids. Examples of inorganic acids include boric acid, phosphoric acid, phosphorous acid, hypophosphorous acid, carbonic acid, silicic acid, etc. Examples of composite compounds of organic acids and inorganic acids include borodisalicylate, boromalonate, borodiglycolate, etc.

[0063] In addition, examples of salts of at least one of organic acids, inorganic acids, and composite compounds of organic acids and inorganic acids include ammonium salts, quaternary ammonium salts, quaternized amidinium salts, amine salts, sodium salts, potassium salts, etc. Examples of quaternary ammonium ions of quaternary ammonium salts include tetramethylammonium, triethylmethylammonium, tetraethylammonium, etc. Examples of quaternized amidinium include ethyldimethylimidazolinium, tetramethylimidazolinium, etc. Examples of amines of amine salts include primary amines, secondary amines, and tertiary amines. Examples of primary amines include methylamine, ethylamine, propylamine, etc., examples of secondary amines include dimethylamine, diethylamine, ethylmethylamine, dibutylamine, etc., and examples of tertiary amines include trimethylamine, triethylamine, tributylamine, ethyldimethylamine, ethyldiisopropylamine, etc.

[0064] Furthermore, other additives can also be added to the electrolyte. Examples of additives include polyethylene glycol, complex compounds of boric acid and polysaccharides (such as mannitol, sorbitol, etc.), complex compounds of boric acid and polyhydric alcohols, boric acid esters, nitro compounds (such as o-nitrobenzoic acid, m-nitrobenzoic acid, p-nitrobenzoic acid, o-nitrophenol, m-nitrophenol, p-nitrophenol, etc.), phosphate esters, etc. These can be used alone or in combination of two or more.

[0065] When a solid electrolyte is used as the electrolyte, examples include polythiophenes such as polyethylene dioxythiophene, and conductive polymers such as polypyrrole and polyaniline.

Examples

[0066] Hereinafter, the present invention will be described in more detail based on examples. Note that the present invention is not limited to the following examples.

[0067] (Cathode body) First, various cathode bodies were fabricated. Aluminum foils shown in Table 1 below were prepared as cathode foils. In Table 1, the etching magnification indicates the magnification of the surface area after etching treatment with respect to the surface area of the plane, that is, the surface area in the state before etching treatment. The etched layer thickness indicates the average depth from the surface to the deepest part of the etching pits. The remaining core thickness indicates the thickness of the layer where the etching pits have not reached. The oxide film is indicated by the nominal formation voltage. That is, for the cathode foils of foil type 1, foil type 2, and foil type 4, a voltage was applied with an aqueous solution of ammonium dihydrogen phosphate to intentionally form an oxide film layer. For the cathode foil of foil type 3, no formation treatment was performed.

[0068]

Table 1

[0069] Slurries were prepared under common conditions for the carbon layers to be formed on the cathode foils of each foil type in Table 1. 16.3 g of graphite was selected as the flaky carbon material, 5 g of carbon black was selected as the spherical carbon material, and 3.7 g of styrene-butadiene rubber was selected as the binder. Then, these were added to 75 milliliters of pure water adjusted to pH 8 with ammonia and dispersed with a stirrer. This slurry was applied to each cathode foil and dried at 100°C. The particle size of the graphite is 4 μm, the primary particle size of the carbon black is 35 nm, and both are untreated with activation treatment and opening treatment.

[0070] Then, as shown in Table 2 below, various cathode bodies with different etching treatments and press line pressures were produced by applying each press line pressure to each cathode foil coated with a carbon layer. The press line pressure was applied using a press machine manufactured by Takumi Giken Co., Ltd. In this press process, the diameter of the press roller was 180 mm, the press treatment width was 130 mm, and the cathode body was conveyed once at 3 m / min.

[0071]

Table 2

[0072] As shown in Table 2, an electrolytic capacitor using unetched foil type 1 is called Comparative Example 1 regardless of the press line pressure. In an electrolytic capacitor using foil type 2 with an etching magnification of 7 times, the un-pressed one is called Comparative Example 2, and the ones with low press line pressure in order are called Examples 1 to 3, and the one with the press line pressure between Example 2 and Example 3 is called Example 10. In an electrolytic capacitor using foil type 3 with an etching magnification of 17 times and no oxide film formed, the un-pressed one is called Comparative Example 3, and the ones with low press line pressure in order are called Examples 4 to 6. An electrolytic capacitor using foil type 4 with an etching magnification of 22 times, the un-pressed one is called Comparative Example 4, and the ones with low press line pressure in order are called Examples 7 to 9.

[0073] (Observation of Cathode Body Cross-Section) The cross-sections of the cathode bodies of Comparative Example 1, Comparative Example 2, Examples 1 to 3, Example 5, and Example 8 were photographed at 5,000 times magnification with a scanning electron microscope to obtain SEM images. The results are shown in Figures 2 to 8. Figure 2 is the SEM image of Comparative Example 1, where (a) is the cathode body with a press line pressure of 0 kN / cm -1 and (b) is the cathode body with a press line pressure of 3.85 kN / cm -1 . Figure 3 is the SEM image of Comparative Example 2. Figure 4 is the SEM image of Example 1, Figure 5 is the SEM image of Example 2, Figure 6 is the SEM image of Example 3, Figure 7 is the SEM image of Example 5, and Figure 8 is the SEM image of Example 8. In Figures 4 to 8, (a) and (b) are the same image, but (a) represents the convex length Lc and (b) represents the uneven depth Hf.

[0074] As shown in FIGS. 2 and 3, for the cathode bodies of Comparative Example 1 and Comparative Example 2, the interface between the current collector layer and the carbon layer substantially follows the flat line L. On the other hand, when comparing FIGS. 4 to 8 with FIGS. 2 and 3, it can be seen that for the cathode bodies of Examples 1 to 3, Example 5, and Example 8, the interface between the current collector layer and the carbon layer does not follow the flat line L, and it can be confirmed that there are external surface irregularities, and etching irregularities can also be confirmed.

[0075] The lengths Lc and heights Hc of the convex regions numbered 1 to 6 at most were measured from the SEM images of FIGS. 4 to 8, and the porosity of the carbon layer was measured, and the results shown in Table 3 below were obtained. In the table, the units of the numerical values of the length and height are μm.

[0076]

Table 3

[0077] As shown in Table 3, specifically, for the cathode body of Example 1, external surface irregularities with severe undulations are formed within an appropriate range. The average length Lc of the convex regions of the external surface irregularities of Example 1 is as short as 2.91 μm, and the irregularities are steep. In this Example 1, although the current collector layer is overall compressed, the degree of compression deformation is sparse, and it can be said that regions with large compression deformation and regions with small compression deformation are prominent. Also, gaps remain in some places between the carbon layer and the current collector layer. Although etching irregularities are formed, there are still many unfilled voids in the etching pits with carbon materials remaining, and the porosity of the carbon layer is also high.

[0078] On the other hand, in the cathode bodies of Examples 2, 3, 5, and 8, the undulations of the outer surface unevenness become gentle within an appropriate range. The length LC of the convex regions of the outer surface unevenness is as long as 3.99 μm, 3.86 μm, 3.78 μm, and 3.82 μm. In Examples 2, 3, 5, and 8, the extended layer is overall compressed, and the degree of compression deformation is approaching uniformity, but it can be visually recognized that it is sparse, and regions with large compression deformation and regions with small compression deformation remain. The gap between the carbon layer and the extended layer is hardly visible. Also, the etching pits are filled with a large amount of carbon material, and the voids are considerably reduced, and the porosity of the carbon layer is also small. The higher the etching magnification, as in Examples 5 and 8, the lower the porosity of the carbon. Also, the higher the etching magnification, the greater the average height of the convex portions, and the interface with the carbon layer is greatly undulated, and it is considered that good adhesion is exhibited.

[0079] Next, based on the photographs shown in FIGS. 2 to 8, in the cathode bodies of Comparative Example 1, Examples 1 to 3, Example 5, and Example 8 with a press line pressure of 3.85 kN / cm -1 the penetration depth Hd of the carbon layer at five arbitrary locations was measured. That is, the height difference in the height direction from the deepest part of the carbon layer that further penetrated into the etching pit from the concave region to the apex of the convex region adjacent to the concave region was measured at five locations. Also, based on the photographs shown in FIGS. 2 to 8 as well, in the cathode bodies of Comparative Example 1, Examples 1 to 3, Example 5, and Example 8 with a press line pressure of 3.85 kN / cm -1 the unevenness depth of the uneven shape (outer surface unevenness) was measured. The results are shown in Table 4 below.

[0080]

Table 4

[0081] As shown in Table 4, with a press line pressure of 3.85 kN / cm -1In Comparative Example 1, the penetration depth Hd was 0.28 μm on average. Also, it was found that the uneven depth Hf of the external surface unevenness was as small as 0.36 μm. This is because Comparative Example 1 was unetched, so there was no room to penetrate into the etching pits, and also because it was unetched, it was difficult to undergo compressive deformation and there was no concave region. Also, it shows the degree of unevenness of the flat cathode foil.

[0082] On the other hand, in Example 1, the penetration depth was 0.54 μm or more on average. Also, the uneven depths of the external surface unevenness were all 0.5 μm or more. In Examples 2, 3, 5, and 8, a concave region corresponding to the density of the etching pits occurred, and furthermore, the carbon material entered the etching pits from this concave region, so that the penetration depth was 0.7 μm or more on average. From this, it can be seen that the external surface unevenness with an appropriate penetration pattern promotes the generation of etching unevenness.

[0083] In addition, for the cathode bodies of Examples 4 and 7, although the etching magnification of the cathode foil is higher than that of Example 1, the same pressing line pressure of 1.54 kN / cm -1 is applied, so that external surface unevenness with intense undulations is formed within an appropriate range. Also, for the cathode bodies of Examples 6 and 9, since the etching magnification of the cathode foil is higher than that of Example 1 and a pressing line pressure of 3.85 kN / cm -1 or more is applied, external surface unevenness with gentle undulations is formed within an appropriate range, and etching unevenness also occurs.

[0084] Also, from the SEM images in FIGS. 2 to 8, for Comparative Example 1 with a pressing line pressure of 0 kN / cm -1 and for Comparative Example 1, Comparative Example 2, and the cathode bodies of Examples 1 to 3, 5, and 8 with a pressing line pressure of 3.85 kN / cm -1 , the ratio of the interface length Y to the range length X was measured. Also, for the cathode body of Example 10, the ratio of the interface length Y to the range length X was measured. The results are shown in Table 5.

[0085]

Table 5

[0086] As shown by the ratio of the interface length Y to the range length X in Table 5, in Comparative Example 1 and Comparative Example 2, the range length X and the interface length Y hardly changed, and no external surface irregularities occurred. On the other hand, as can be seen by comparing Comparative Example 2 with Examples 1, 2, 3, and 10, in Examples 1, 2, and 10, the ratio is 110% or more, external surface irregularities occurred, and it can be seen that the surface area of the cathode body is improved. Also, as can be seen by comparing Example 5 and Example 8, it can be seen that external surface irregularities also occurred in Example 5 and Example 8, and the surface area of the cathode body is improved.

[0087] In addition, for the cathode bodies of Comparative Example 1 (press line pressure 0 kN / cm -1 ) and Comparative Example 2 where pressing was not performed, the uneven depth of the external surface irregularities was almost non-existent, and the external surface shape was substantially flat. Also, FIG. 9 is an SEM photograph of a cathode foil of the same foil type as Comparative Example 2 without a carbon layer when pressed. (a) in FIG. 9 is without pressing, (b) is with a press line pressure of 3.85 kN / cm -1 , and (c) is with a press line pressure of 7.69 kN / cm -1 . As shown in FIG. 9, even when the cathode foil was pressed, if there was no carbon layer, no external surface irregularities occurred even when a press line pressure of 1.54 kN / cm -1 or more was applied.

[0088] (Electrolytic capacitor) Next, in addition to the cathode bodies of Comparative Examples 1 to 4 and Examples 1 to 9, a cathode body was produced using an untreated etched plain foil as a base current collector and forming a titanium nitride layer by an electron beam evaporation method, and this was set as Comparative Example 5. Electrolytic capacitors were produced using these cathode bodies of Comparative Examples 1 to 5 and Examples 1 to 9.

[0089] For each electrolytic capacitor, an anodic foil produced under common conditions and various cathode bodies were opposed to each other through the same separator, impregnated with an electrolytic solution produced under common conditions, made into a laminated cell, and subjected to a common reformation treatment. Specifically, for all electrolytic capacitors, the aluminum foil was subjected to an etching treatment, and the nominal formation voltage was 4 V fsA dielectric oxide film was formed so as to obtain an aluminum foil with a projected area of 2.1 cm 2 in size, and this was used as the anode foil. As the separator, rayon was used in all electrolytic capacitors. Also, as the electrolytic solution common to all electrolytic capacitors, it was prepared with tetramethylimidazolium phthalate as the solute and γ-butyrolactone as the solvent. During reformation, a voltage of 3.35 V was applied for 60 minutes to all electrolytic capacitors in an environment of 105°C.

[0090] (Capacitance Test) Then, for the electrolytic capacitors of Comparative Examples 1 to 5 and Examples 1 to 9, the capacitance (Cap) during charge and discharge at 120 Hz and 10 kHz was measured at 20°C with no load (initial), and after continuously applying a DC load of 2.4 V for 250 hours in an environment of 125°C (during high-temperature environment load). The results are shown in Table 6.

[0091]

Table 6

[0092] As can be seen from the comparison between Comparative Example 1 and Comparative Example 5 in Table 6, an electrolytic capacitor composed of a carbon layer and a cathode foil using an unetched plane foil was inferior in capacitance in all combinations of initial characteristics, characteristics during high-temperature environment load, and low-frequency and high-frequency regions compared to an electrolytic capacitor fabricated with a cathode body on which titanium nitride was deposited, and the capacitance change rate was also not good. Also, when comparing Comparative Example 5 with Comparative Examples 2 to 4, even if the etching treatment was performed, when no external surface unevenness was formed at the interface between the enlarged layer and the carbon layer, it was generally inferior in all measurement items compared to an electrolytic capacitor fabricated with a cathode body on which titanium nitride was deposited.

[0093] On the one hand, as can be seen by comparing Comparative Example 5 with Examples 1 to 9, in terms of initial characteristics, for an electrolytic capacitor in which surface irregularities are formed at the interface between the cathode foil and the carbon layer, the capacitance due to charge and discharge at 10 kHz is improved compared to an electrolytic capacitor using a cathode body with titanium nitride vapor-deposited thereon. Also, as can be seen by comparing Comparative Example 5 with Examples 8 and 9, due to the gently undulating surface irregularities and the etching irregularities that reach a sufficient depth to promote this surface irregularity and a high etching magnification, in terms of initial characteristics, the capacitance due to charge and discharge at 120 Hz is larger compared to an electrolytic capacitor using a cathode body with titanium nitride vapor-deposited thereon.

[0094] Furthermore, when comparing Comparative Example 5 with Examples 1 to 9, all electrolytic capacitors with surface irregularities formed at the interface between the carbon layer and the extended surface layer have a higher capacitance compared to an electrolytic capacitor using a cathode body with titanium nitride vapor-deposited thereon when the charge and discharge is at 120 Hz in terms of the load characteristics in a high-temperature environment. In particular, as can be seen by comparing Comparative Example 5 with Examples 1 to 9, regarding the rate of increase and decrease of capacitance when a load is applied in a high-temperature environment with respect to the initial capacitance, at 120 Hz of charge and discharge, when surface irregularities are formed, the degree of capacitance decrease is dramatically suppressed.

[0095] Furthermore, as can be seen by comparing Comparative Example 5 with Examples 2, 3, 5, 6, 8, and 9, due to the gently undulating surface irregularities within an appropriate range and the etching irregularities that reach a sufficient depth to promote this surface irregularity, the rate of increase and decrease of capacitance at 10 kHz of charge and discharge is dramatically suppressed.

[0096] From the above, it was confirmed that the adhesion between the carbon layer and the extended surface layer is improved by the surface irregularities, and the capacitance is improved and the thermal stability is also improved regardless of whether it is in the low-frequency region or the high-frequency region, and regardless of whether it is the initial characteristics or the load characteristics in a high-temperature environment. In particular, the presence of surface irregularities has a thermal stability superior to the case where titanium nitride is vapor-deposited on the cathode foil in the combination of the low-frequency region and the high-temperature thermal environment, and exhibits a high capacitance.

[0097] Furthermore, if the undulation of the external surface unevenness is gentle, it has higher thermal stability than the case where titanium nitride is deposited on the cathode foil even in the high-frequency region and high-temperature thermal environment, and exhibits a high capacitance. Also, if the external surface unevenness has gentle undulation and a high etching magnification, it exhibits a capacitance exceeding that in the case where titanium nitride is deposited on the cathode foil in all combinations in the low-frequency region, high-frequency region, and furthermore, the initial characteristics and high-temperature thermal environment.

Claims

1. An electrode body used for the cathode of an electrolytic capacitor, comprising a cathode foil made of a valve-acting metal and having a roughened layer formed on its surface, and a carbon layer formed on the roughened layer, wherein the interface between the roughened layer and the carbon layer has an uneven shape, characterizing the electrode body.

2. The uneven depth of the uneven shape is 0.5 μm or more, characterizing the electrode body according to Claim 1.

3. The roughened layer is formed by digging a plurality of etching pits, the diameter of the etching pits is 0.12 μm or more and 0.43 μm or less near the surface layer, the depth of the etching pits is 1.5 μm or more and 5.0 μm or less, the uneven shape is such that the distance between both ends of a cross-section obtained by cutting the convex region along the height direction is 1.5 μm or more and 8.0 μm or less, the uneven shape is such that the height of the convex region is 0.15 μm or more and 0.80 μm or less, characterizing the electrode body according to Claim 1 or 2.

4. The roughened layer is formed by digging a plurality of etching pits, the carbon layer enters further into the etching pits from the interface of the uneven shape, characterizing the electrode body according to any one of Claims 1 to 3.

5. The carbon layer that enters further into the etching pits from the concave region enters up to a position that is, on average, 0.5 μm or more sunken in the depth direction from the apex of the convex region adjacent to the concave region, characterizing the electrode body according to Claim 4.

6. The carbon layer that enters further into the etching pits from the concave region enters up to a position that is, on average, 0.7 μm or more sunken in the depth direction from the apex of the convex region adjacent to the concave region, characterizing the electrode body according to Claim 4.

7. The roughened layer is formed by digging a plurality of etching pits, the ratio (Y / X×100) of the interface length Y to the range length X is 110% or more, characterizing the electrode body according to Claim 1. Interface length Y: The length along the interface between the roughened layer and the carbon layer from an arbitrary starting point to an arbitrary ending point, including the length of the carbon layer that has entered the etching pits Range length X: The length of the component in the direction orthogonal to the height direction of the uneven shape among the vectors connecting the starting point and the ending point for measuring the interface length Y

8. The uneven shape is compressed and deformed by pressing, characterizing the electrode body according to any one of Claims 1 to 7.

9. The carbon layer contains a flaky carbon material and a spherical carbon material. The electrode body according to any one of claims 1 to 8, characterized in that.

10. The electrode body according to any one of claims 1 to 9 is provided as a cathode. An electrolytic capacitor characterized by that.

11. A method for manufacturing an electrode body used for the cathode of an electrolytic capacitor, a step of forming a carbon layer on a cathode foil made of a valve action metal and having a roughened layer formed on its surface; a step of pressing the cathode foil on which the carbon layer is formed; comprising the interface between the roughened layer and the carbon layer has an uneven shape; A method for manufacturing an electrode body, characterized in that.

12. The cathode foil on which the carbon layer is formed is pressed at a press line pressure of 1.54 kN / cm －１ or more. The method for manufacturing an electrode body according to claim 11, characterized in that.

13. The carbon layer is formed by applying a slurry containing flaky carbon and spherical carbon to the cathode foil and drying it. The method for manufacturing an electrode body according to claim 11 or 12, characterized in that.

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