Electricity: electrical systems and devices – Electrolytic systems or devices – Double layer electrolytic capacitor
Reexamination Certificate
2000-10-26
2002-07-23
Reichard, Dean A. (Department: 2831)
Electricity: electrical systems and devices
Electrolytic systems or devices
Double layer electrolytic capacitor
C361S512000, C361S503000
Reexamination Certificate
active
06424517
ABSTRACT:
The present invention relates to a non-aqueous electric double layer capacitor which has a high power output and a high energy density; and which is excellent in the voltage-retention property.
Heretofore, as a separator to be disposed between positive and negative electrodes of an electric double layer capacitor, a polyethylene nonwoven fabric, a polypropylene nonwoven fabric, a polyester nonwoven fabric, kraft paper, a rayon/sisal mixed sheet, a manila hemp sheet, a polyester fiber sheet or a glass fiber sheet is, for example, known (e.g. JP-A-9-45586, JP-A-1-304719). The role of a separator is to electrically insulate the positive electrode from the negative electrode on one hand and to facilitate transfer of ions in the electrolyte, which takes place accompanying charge and discharge, on the other hand.
In recent years, an attention has been drawn to an electric double layer capacitor for high power applications. However, with a separator made of organic fibers of e.g. a polyethylene, the ion conductivity is low, and the internal resistance of the electric double layer capacitor is high, since the electrolyte absorbing-property and the electrolyte-holding property are low. Accordingly, if instantaneous large current discharge is carried out as one of main characteristics of an electric double layer capacitor, the voltage drop is substantial, such being not practical.
Further, a conventional separator made of paper is excellent in heat resistance and tensile strength and is sometimes effective for an electric double layer capacitor to be used for a power source wherein no large current discharge is carried out, like a hybrid power source used with a solar cell. However, when a conventional separator made of paper is used for an electric double layer capacitor for high power applications, wherein the electrolyte is non-aqueous, the ion permeability tends to be inadequate.
With an electric double layer capacitor for high power applications, it is desired to lower the resistance and to increase the capacitance per unit volume (hereinafter referred to as capacitance density), and it is accordingly required to make the separator as thin as possible. At the same time, thee separator is required to have a high porosity from the viewpoint of the electrolyte-absorbing property and the electrolyte-holding property. However, if a separator made of paper having a high porosity is made thin, the insulating property between the positive and negative electrodes tends to be inadequate, thus leading to micro-short circuiting, self-discharge or a decrease in the yield in the production of the capacitor.
Further, when an electrolyte is impregnated to an element formed by laminating a plurality of electrodes with separators interposed or winding two sheets of electrodes with a separator interposed, so that the electrodes and the separator are laminated in close contact to one another, for example, with a separator made of a paper having a thickness of not more than 100 &mgr;m. there is a limit in increasing the porosity, and it takes much time to inject the electrolyte, thus leading to a problem in the productivity.
Accordingly, it is an object of the present invention to provide an electric double layer capacitor having a low resistance and a high capacitance density with good productivity by employing a separator which is excellent in heat resistance, ion permeability, electrolyte-absorbing property and electrolyte-holding property and which has high strength even when it is thin, and is excellent in the electrical insulating property.
The present invention provides an electric double layer capacitor comprising a positive electrode and a negative electrode, each comprising a carbonaceous material as the main component, a separator interposed between the positive and negative electrodes, and a non-aqueous electrolyte, wherein said separator comprises a sheet having a thickness of from 10 to 80 &mgr;m and a porosity of from 50 to 85%, and a nonwoven fabric having a thickness of from 10 to 80 &mgr;m, a maximum pore diameter of from 10 to 60 &mgr;m and an air permeability of from 10 to 200 cm
3
/(cm
2
·sec), laminated one on the other.
In the present invention, the thickness of the sheet is from 10 to 80 &mgr;m. If the thickness is less than 10 &mgr;m, the strength tends to be weak, and the handling tends to be difficult. If the thickness exceeds 80 &mgr;m, when it is laminated with a nonwoven fabric to form a separator, the proportion of the volume occupied by the separator in the electric double layer capacitor element tends to be high, whereby the capacitance density of the electric double layer capacitor can not be made sufficiently high. The thickness of the sheet is particularly preferably from 20 to 60 &mgr;m.
In the present invention, the porosity of the sheet is from 50 to 85%. If it is less than 50%, the electrolyte-absorbing property or the electrolyte-holding property tends to be inadequate, and the ion permeability tends to be inadequate. If it exceeds 85%, the strength of the sheet tends to be weak, whereby the handling tends to be difficult. It is particularly preferably from 60 to 80%. Further, in the present invention, the sheet preferably has substantially no opening. If the sheet has pinholes, micro-short circuiting is likely to take place, and accordingly, it should preferably have no such pinholes.
With a high porosity sheet with a porosity of at least 50% having adequate electrolyte-absorbing property and electrolyte-holding property, if the thickness is for example at most 100 &mgr;m, it tends to have pinholes although the capacitance density of the electric double layer capacitor can be made high. Accordingly, if one such high-porosity sheet is used as a separator for the electric double layer capacitor, short circuiting is likely to take place due to the pinholes, and self-discharge is likely to result.
On the other hand, in the present invention, the separator comprises a nonwoven fabric and a sheet having a porosity of from 50 to 85%, laminated one on the other, whereby the electrolyte can be held in both the above sheet and the nonwoven fabric, whereby electrolyte-holding can adequately be carried out even when a high density sheet is used, rather than the separator made solely of the above mentioned one high-porosity sheet. If the density is high, the probability of the presence of pinholes in the sheet will be low. Further, even if the above sheet having a porosity of from 50 to 85% has pinholes, by the presence of the nonwoven fabric, the distance between the electrodes facing each other with the separator interposed, can be maintained, whereby no substantial self-discharge will take place, and it is possible to increase the yield in the production of the electric double layer capacitor.
Further, with the nonwoven fabric of the present invention having a large maximum pore diameter and a high air permeability, the production efficiency is high in production of the electric double layer capacity having high capacitance. Namely, when an electrolyte is impregnated to an element formed by laminating electrodes in a close contact with a separator, such as a laminated structure wherein a plurality of, electrodes are laminated with separators interposed therebetween or a wound structure wherein two sheets of electrodes facing each other with a separator interposed therebetween are wound, the nonwoven fabric has a function to form paths for the electrolyte to permeate through the entire capacitor element, i.e. a function as a spacer, whereby the injection of the electrolyte can be carried out in a short period of time, and the productivity of the electric double layer capacitor can be increased.
Namely, in the case of a separator having the same thickness, a separator comprising the sheet of the present invention and the nonwoven fabric scarcely undergoes self-discharge and is excellent in the electrolyte-absorbing property and the electrolyte-holding property, as compared with the separator made solely of the above-mentioned one sheet, whereby the productivity of the ele
Hiratsuka Kazuya
Ikeda Katsuji
Kawasato Takeshi
Asahi Glass Company Limited
Reichard Dean A.
Thomas Eric W.
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