Electric double layer capacitor and separator therefor

Details Electric double layer capacitor and separator therefor Electric double layer capacitor and separator therefor

1999-07-22

2002-06-25

Reichard, Dean A. (Department: 2831)

Electricity: electrical systems and devices

Electrolytic systems or devices

Double layer electrolytic capacitor

C361S503000, C361S504000, C361S512000

Reexamination Certificate

active

06411496

ABSTRACT:

The present invention relates to a non-aqueous type electric double layer capacitor which has a high power output and a high energy density and which is excellent in a voltage-holding property.
Heretofore, as a separator to be disposed between positive and negative electrodes of an electric double layer capacitor, a polyethylene non-woven fabric, a polypropylene non-woven fabric, a polyester non-woven fabric, kraft paper, a rayon/sisal mixed sheet, a Manila hemp 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 large current charge and discharge. However, with a separator of e.g. a polyethylene non-woven fabric, the ion conductivity is low, and the internal resistance of the electric double layer capacitor is high, since the liquid absorbing property and the liquid-holding property for the electrolyte are low. Accordingly, if instantaneous large current discharge was carried out as one of main characteristics of an electric double layer capacitor, the voltage drop was 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 whereby no large current discharge is carried out, like a hybrid power source with a solar cell. However, when a conventional separator made of paper is used for an electric double layer capacitor for large current charge and discharge, wherein the electrolyte is non-aqueous, the ion permeability tends to be inadequate.
With an electric double layer capacitor for large current charge and discharge, it is desired to lower the resistance and to increase the capacitance per unit volume (hereinafter referred to as a capacitance density), and it is accordingly required to make the separator as thin as possible. From the viewpoint of the liquid-absorbing property and the liquid-holding property for the electrolyte, the separator is required to have a high porosity. 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 microshortcircuiting, self-discharge or decrease in the yield for the production of capacitors.
Further, when an electric double layer capacitor having a large capacitance is to be produced, a plurality of positive and negative electrodes may alternately be laminated with a separator interposed therebetween, or elongated positive and negative electrodes may be wound with a separator interposed therebetween, to form an element having the electrodes and the separator intimately laminated, and usually, an electrolyte is impregnated to such an element. Accordingly, especially when the viscosity of the electrolyte is relatively high, the separator is required to be excellent not only in the liquid-absorbing property and the liquid-holding property for the electrolyte, but also in the impregnation property for the electrolyte. Otherwise, there will be a problem that the productivity tends to be poor.
Under these circumstances, it is an object of the present invention to realize reduction of the resistance and increase of the capacitance for an electric double layer capacitor by employing a separator which is excellent in heat resistance and ion permeability and which, at the same time, is excellent in the liquid-absorbing property, the liquid-holding property and the impregnation property for an electrolyte even if it is thin, and has high strength and excellent insulating properties.
The present invention provides an electric double layer capacitor having an element impregnated with a non-aqueous electrolyte, said element comprising positive and negative electrodes made of carbonaceous electrodes, and a separator interposed between the electrodes, wherein said separator is made of at least two paper layers laminated to form one sheet, each of the paper layers has a thickness of from 20 to 60 &mgr;m, and at least one of the paper layers is a cellulose paper layer having a density of from 0.37 to 0.60 g/cm
3
; and the separator for such a capacitor.
Now, the present invention will be described in further detail with reference to the preferred embodiments.
The separator in the present invention is a paper (hereinafter referred to as a multilayered paper) prepared by laminating at least two paper layers into one sheet on a paper-making machine, and each of the paper layers has a thickness of from 20 to 60 &mgr;m. With paper layers having a thickness of less than 20 &mgr;m, the strength tends to be weak, and the handling tends to be difficult when they are laminated by a paper-making machine. On the other hand, if the thickness exceeds 60 &mgr;m, when a separator is made by laminating a plurality of such paper layers, the volume of the separator occupying per unit volume of the electric double layer capacitor, tends to be large, whereby the capacitance density of the electric double layer capacitor can not be made adequately high. The paper-making machine to be used for making the multilayered paper, is preferably combination machine.
Further, in the present invention, at least one of the above-mentioned paper layers has a density of from 0.37 to 0.60 g/cm
3
. If a separator is made solely of paper layers having a density of less than 0.37 g/cm
3
, the positive and negative electrodes can not adequately be insulated. On the other hand, if the separator contains a paper layer having a density exceeding 0.60 g/cm
3
, the ion permeability tends to be inadequate, and the resistance of the separator tends to be high. Preferably, the density is within a range of from 0.40 to 0.50 g/cm
3
. Here, the porosity of the cellulose paper layer is determined solely by the starting material and the density. The separator made of the multilayered paper preferably has a porosity of from 60 to 90%, more preferably from 65 to 85%.
With a low density cellulose paper having an adequate porosity to provide the liquid-absorbing property and the liquid-holding property for an electrolyte, if the thickness is, for example, at most 100 &mgr;m, the capacitance density of an electric double layer capacitor can be made high, but pinholes are likely to be present. Accordingly, if a single sheet of cellulose paper having a thickness of at most 100 &mgr;m is used alone as a separator for an electric double layer capacitor, short circuiting may occur due to the pinholes, and self discharge is likely to take place.
Whereas, in the present invention, the separator is made of a plurality of paper layers laminated one on another, whereby the probability of formation of pinholes is very low by the lamination of a plurality of paper layers even though each paper layer may have pinholes, whereby the electric double layer capacitor will be scarcely susceptible to self discharge, and the yield in the production can also be increased. Namely, even with a separator having the same thickness, the separator made of a multilayered paper has a low probability of having pinholes and is less susceptible to self discharge, as compared with a usual separator made of a single sheet of cellulose paper.
In the present invention, the separator is preferably a multilayered paper or multi ply made of multilayers prepared by laminating two paper layers i.e. a double layered paper or double ply made of two layers. If the number of paper layers is too much, the separator tends to be thick, whereby the capacitance density of the electric double layer capacitor tends to be low. The thickness of the separator is preferably less than 100 &mgr;m, more preferably from 40 to 80 &mgr;m.
Further, the plurality of paper layers constituting the multilayered paper fo

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