JP2007103540A - Separator for capacitor and capacitor using same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To ensure breakdown voltage characteristics of a separator without increasing the thickness of separator, to improve impedance characteristics and to improve capacitor service life by improvement of an electrolyte storing quantity. <P>SOLUTION: The separator for a capacitor in which a spherical cellulose is deposited on one surface or both surfaces, the cellulose has a diameter of 2-15 &mu;m, an alkali metal content is 1 wt.% or less, and at least one kind of zeolite or hydroxyapatite preferably is deposited on the spherical cellulose. Also, a layer including the spherical cellulose deposited on the separator has a thickness of 2-40 &mu;m. <P>COPYRIGHT: (C)2007,JPO&amp;INPIT

Description

  The present invention is applied to a capacitor and its separator, and is excellent in impedance characteristics without degrading its withstand voltage characteristics, and further improves the life of the capacitor by increasing the amount of electrolyte retained.

  In general, capacitors, particularly electrolytic capacitors, have the following configuration. That is, a high-purity aluminum foil formed in a band shape is chemically or electrochemically etched to expand the surface, and the expanded aluminum foil is subjected to chemical conversion in a chemical conversion solution such as an aqueous ammonium borate solution. By doing so, the anode foil in which the oxide film layer is formed on the surface of the aluminum foil and the cathode foil obtained by enlarging the same high-purity aluminum foil are wound through a separator to form a capacitor element. The capacitor element is impregnated with a driving electrolyte solution and stored in a metal bottomed cylindrical outer case. Further, a sealing body made of elastic rubber is accommodated in the opening end portion of the outer case, and the opening end portion of the outer case is sealed by drawing to constitute an electrolytic capacitor.

  In addition, as an electrolytic solution impregnated in a capacitor element of a small-sized, low-pressure electrolytic capacitor, conventionally, an ethylene glycol as a main solvent and an ammonium salt such as adipic acid or benzoic acid as a solute, or γ Known are those having butyrolactone as a main solvent and quaternized cyclic amidinium salts such as phthalic acid and maleic acid as solutes.

  Since these conventional electrolytic capacitors are impregnated with an electrolytic solution in the separator, impedance characteristics as a capacitor, in particular, equivalent series resistance (hereinafter abbreviated as ESR) are likely to be high, and there is a risk of deterioration with time even during use. is there. For this reason, it is conceivable to reduce the resistance of the electrolytic solution in order to improve impedance characteristics, or to make the separator thinner or lower in density. However, lowering the resistance value of the electrolyte causes corrosiveness to the aluminum foil, while reducing the separator thickness and density increases the short-circuit defect rate when aging the capacitor element, Even if there is no short circuit, there is a problem that the short defect rate after the product is commercialized and put on the market increases.

Therefore, in order to improve the impedance characteristics, the shape and orientation of the fibers constituting the separator have been studied. In the case of the same thickness and the same density, it is desirable that the voids in the electrolytic paper penetrate efficiently from the front surface to the back surface of the separator. Therefore, the raw material of the separator is required to have two points that the fiber diameter is as small as possible and that it is close to a circle.
Further, in order to have the same thickness and withstand voltage characteristics, it is desirable that the separator fibers have as high a density as possible. However, when the density is increased, the ESR is extremely lowered. In particular, in order to secure a withstand voltage, a medium- and high-voltage capacitor has coped with the fact that the separator fiber is made dense, and the impedance characteristic has been lowered.

In view of the above problems, as disclosed in JP-A-5-291080, it has been proposed to internally add cellulose fine powder as an organic fine powder to a separator. Further, as disclosed in JP-A-5-251275, it has been proposed to attach hydroxyethyl (alkyl) cellulose to at least one of an anode foil, a cathode foil, and a separator. (Patent Documents 1 and 2)
Japanese Patent Laid-Open No. 2003-100558 discloses a method for improving impedance characteristics and a short-circuit defect rate in an electrolyte solution in order to prevent localization and uneven distribution of cellulose fibers in a capacitor element. A method for fixing cellulose fibers with a soluble binder has been proposed. (Patent Document 3)
Japanese Patent Laid-Open No. 5-291080 JP-A-5-251275 Japanese Patent Publication No. 2003-100558

However, in the method proposed in Patent Document 1, since the cellulose fine powder is easily dropped when the capacitor element is wound, the dropped gap portion causes a decrease in the capacitor withstand voltage. Moreover, about apply | coating the powder of the hydroxyethyl (alkyl) cellulose proposed by patent document 2 to a separator, adhesiveness with a cellulose is bad, workability | operativity of application | coating is bad, and the magnitude | size of powder is also Due to the fine powder that passes through a 42 mesh (355 μm or less) wire mesh, the thickness of the separator of the medium- and high-voltage capacitor is greatly increased with respect to 15 to 90 μm. Since it is difficult to form a coating layer having a large thickness, the variation in the electrode spacing between the anode foil and the cathode foil increases, and the withstand voltage characteristics of the capacitor are degraded.
Further, the method proposed in Patent Document 3 is still insufficient for ensuring the withstand voltage and improving the impedance characteristics of the electrolytic capacitor.

  The present invention improves these problems, and can withstand the voltage without increasing the thickness of the separator, has excellent impedance characteristics, and improves the life of the capacitor by improving the amount of electrolyte retained. It is intended to do.

  In order to achieve the above object, the present invention is a capacitor separator in which spherical cellulose is attached to one side or both sides.

  In the form of the capacitor element using the separator as described above, the spherical cellulose layer can form a layer having a uniform withstand voltage on the separator. The defective rate can be reduced.

  In addition, spherical cellulose has a shape close to a spherical shape, a small particle size, and a uniform particle size as compared with fibrous cellulose such as hydroxyethyl (alkyl) cellulose. For this reason, when spherical cellulose is used for a low-density separator, when ions pass through the separator, the outer side of the spherical cellulose is easily passed in a short distance and linearly. As a result, compared to a separator having a high density of fibers, since the hindrance is reduced, ions can easily pass through, good ion permeability can be maintained, and deterioration of impedance characteristics is reduced.

  In addition, spherical cellulose has good affinity with an electrolyte solution as well as general cellulose, and is porous and has many voids therein, so that the electrolyte solution can easily enter, and the retention property of the electrolyte solution is good. For this reason, transpiration of the electrolytic solution held inside the spherical cellulose is reduced even during heat generation when an overvoltage is applied, so that the capacitor life can be improved by increasing the amount of electrolytic solution retained in the capacitor.

  According to the capacitor separator of the present invention, it is possible to realize a capacitor that has excellent impedance characteristics and good high-temperature life characteristics by improving the amount of electrolyte retained without lowering the withstand voltage of the capacitor.

The capacitor separator of the present invention is a capacitor separator in which spherical cellulose is adhered on one or both sides, and the spherical cellulose has a diameter of 2 to 15 μm and an alkali metal content of 1% by weight or less, preferably At least one of zeolite or hydroxyapatite is adhered to the spherical cellulose.
The layer containing spherical cellulose deposited on the separator has a thickness of 2 to 40 μm.

  Further, the capacitor element formed by winding the capacitor separator, the anode foil, and the cathode foil of the present invention is impregnated with a driving electrolyte, and the capacitor element is housed in an exterior case, and the opening of the case is sealed with a sealing member. Use an electrolytic capacitor.

The layer thickness for adhering the spherical cellulose to the separator may be determined according to the required characteristics of the capacitor product. However, if the thickness is too thin, the withstand voltage characteristic of the capacitor product is lowered, and an improvement in the amount of electrolyte retained cannot be expected. . On the other hand, if the thickness of the deposited layer is too thick, the capacitor element size that enters the outer case of the capacitor is determined, so the electrode area of the capacitor element must be reduced, and the capacitance of the capacitor decreases. Therefore, the layer thickness to which the spherical cellulose is attached is optimally in the range of 2 to 40 μm.
In addition, about the thickness of the separator used for this invention, it is 15-70 micrometers normally.

The separator used in the present invention is a low-density separator that requires low ESR, and spherical cellulose is adhered to one or both sides thereof. Such a separator has sparse fiber spacing, a large space, and a large variation in fiber spacing. When spherical cellulose is attached to the surface of the separator by coating or the like, when the particle size is smaller than the fiber spacing of the separator, a part of the spherical cellulose is interposed in the gap between the fibers of the separator. When the particle size is large, the spherical cellulose adheres to the surface of the separator, or a part of the cellulose stays in the recesses between the fibers on the separator surface.
As a result, it enters into the spherical cellulose so as to fill the space between the fibers existing in the separator, and the surface of the separator is efficiently covered with the spherical cellulose. From this, since the layer of spherical cellulose can form a layer having a uniform withstand voltage on the separator, a separator having a withstand voltage characteristic can be obtained without increasing the density per unit volume of the separator. it can.

  As for the material of the separator used in the present invention, cellulose fiber, a continuous porous substrate made of synthetic polymer, non-wood pulp fiber and the like are preferable.

Moreover, about the density of a separator, it is preferable to use 0.3-0.6 g / cm < ³ > of center density regions. If it is less than 0.3 g / cm ³ , the number of fibers constituting the separator decreases, and the strength becomes weak because it becomes thin, so that it cannot withstand winding of the capacitor.
If it exceeds 0.6 g / cm ³ , the separator fibers are dense and the ion permeability is poor, and if spherical cellulose is further deposited on the separator, the ESR of the capacitor is rapidly deteriorated.

  About the particle size of spherical cellulose, 2-15 micrometers is preferable from the relationship between the fiber space | interval of the separator to be used, and its foil thickness. When the thickness is less than 2 μm, the particularly sparse space between the fibers of the separator cannot be filled, so that the effect of improving the withstand voltage of the capacitor product is reduced. On the other hand, when the particle size is small, the retained amount of the electrolytic solution per unit volume is lowered and the life extending effect is also lowered. If the thickness exceeds 15 μm, the thickness of the adhesion layer tends to increase, so that ESR decreases. For this reason, the optimal particle size of the spherical cellulose is in the range of 2 to 15 μm, and good capacitor characteristics cannot be obtained outside this range.

  Spherical cellulose does not contain impurities that elute in the electrolyte, is stable to the electrolyte, and does not change over time due to stress such as heat or pressure when incorporated in a capacitor. There is a need. When the alkali metal content exceeds 1% by weight, the withstand voltage of the capacitor may be reduced, and short-circuits during aging frequently occur.

  Further, the present invention can further enhance the affinity of the spherical cellulose with the electrolyte by attaching at least one of zeolite or hydroxyapatite to the spherical cellulose. Capacitor life can be extended by improving the amount of electrolyte retained.

As a method of attaching spherical cellulose to the separator in the present invention, there is a method of attaching using a binder soluble in an electrolytic solution. In this case, various types of ionic and nonionic binders are used, and in particular, those using water-soluble polymers such as polyvinyl alcohol, polyacrylamide, and polyacrylic acid have excellent spherical cellulose adhesion. In addition, the capacitor can have good characteristics.
Further, when spherical cellulose is kneaded with isopropyl alcohol or water and adhered to the separator, it can be adhered to the separator by the hydrogen bonding force of cellulose itself, but in this case, the adhesion is weak.
A binder that is insoluble in the electrolytic solution is not suitable because it forms a film that does not penetrate the electrolytic solution on the surface of the separator, which increases the ESR of the capacitor.

  The anode foil, the cathode foil, and the electrolyte used in the present invention may be any electrode foil that is usually used for capacitors. There is no limitation except that the electrolytic solution is an electrolytic solution containing any one or more of a polyhydric alcohol solvent, an amide solvent, an organic solvent and water.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited to this.

The contents of the spherical cellulose used in the examples are shown in Table 1.
The spherical cellulose used in this example is a water / oil type viscose emulsion-based spherical cellulose.
As for the attachment condition of zeolite, artificial zeolite having a particle size of 0.1 μm or less was directly crystallized on the surface of spherical cellulose at a weight ratio of 1/40.

  With respect to specific examples and comparative examples shown in Table 2, electrolytic capacitors were prepared and product tests were conducted. The rating of the capacitor is 50 WV-100 μF.

[High temperature life test]
A rated voltage was applied to these electrolytic capacitors, and a high-temperature life test was conducted at 125 ° C. for 1000 hours. The test results are shown in Table 2. The number of tests is 20, and the characteristics are shown as average values of 20 capacitors.
The separator used has a foil thickness of 30 μm. The weight ratio of spherical cellulose and polyvinyl alcohol as a binder was mixed at a ratio of 1/20, and was rubbed so as to have a constant thickness on one side of the separator (however, in Example 5) using the doctor blade method. After drying by the reduced pressure drying method, it was slit to a predetermined width and wound around a capacitor element to prepare an electrolytic capacitor.
In addition, the separator foil thickness of a comparative example was 30 micrometers, and the same separator foil thickness of a conventional example was used as the Example, and the thing of 40 micrometers was used. The layer thickness of Comparative Example 1 was set to a predetermined thickness by attaching only the binder.

  As is clear from Table 2, the change in ESR after the high-temperature life test is significantly reduced in Example 1 using the spherical cellulose of the present invention as compared with Comparative Example 1. Moreover, the aging short-circuit rate is also low, and favorable withstand voltage characteristics can be maintained by adhering spherical cellulose.

In Example 2 using zeolite, ΔCAP after the high-temperature life test is low.
Further, in Examples 2 and 4 in which zeolite was adhered to spherical cellulose, the ESR after the high-temperature life test was equal to or lower than the initial value, and it can be seen that there is an effect in improving the life characteristics.
The same result was obtained even under conditions using hydroxyapatite instead of zeolite.

In the comparison between Example 3 and the conventional example, when the foil thickness of the separator is the same, it can be seen that the withstand voltage characteristic is improved by using the spherical separator because the aging short rate of the conventional example is high.
In addition, after the high-temperature life test, the ESR is also lower than that of the conventional example, indicating that the impedance characteristics are excellent.

  Example 5 in which spherical cellulose was adhered to both sides of the separator showed almost the same capacitor characteristics as Example 1 that was adhered to one side.

  In Comparative Example 2 in which the adhesion layer thickness of the spherical cellulose is thin, the aging short rate is high. Therefore, it is understood that the effect on the withstand voltage characteristic is small when the adhesion amount is small. In Comparative Example 3 where the adhesion layer thickness is thick, the initial ESR is higher than that in Example 2 although it is the same spherical cellulose.

  In Comparative Example 4, when the alkali metal content exceeds 1% by weight, the aging short-circuit rate is increased, and the effect of using spherical cellulose having an alkali metal content of 1% by weight or less is apparent.

In Comparative Example 5 in which the particle size of the spherical cellulose is small, the ESR change rate after the high-temperature life test is larger than that in Example 1. Also, the aging short rate is high.
In Comparative Example 6 in which the spherical cellulose has a large particle diameter, the initial ESR is higher than that in Example 1.

Claims (6)

Capacitor separator with spherical cellulose attached on one or both sides. 2. The capacitor separator according to claim 1, wherein the spherical cellulose has a diameter of 2 to 15 μm. 3. The separator for a capacitor according to claim 1, wherein the alkali metal content in the spherical cellulose is 1% by weight or less. The capacitor separator according to claim 3, wherein at least one of zeolite or hydroxyapatite is adhered to the spherical cellulose. The capacitor separator according to claim 4, wherein the layer containing the spherical cellulose has a thickness of 2 to 40 μm. A capacitor formed by interposing the separator according to claim 1.