Electric double layer capacitor

Details Electric double layer capacitor Electric double layer capacitor

2001-09-07

2002-03-05

Tsai, Jey (Department: 2812)

Metal working

Barrier layer or semiconductor device making

Barrier layer device making

C361S502000, C162S091000

Reexamination Certificate

active

06352565

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a non-aqueous type electric double layer capacitor which has a high power output and a high energy density.
2. Discussion of Background
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 high power applications. However, with a separator made of organic fibers e.g. a polyethylene, 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 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, it is desired to lower the resistance and to increase the capacitance per unit volume, and it is accordingly required to make the separator as thin as possible. However, if a separator made of paper is made thin, the insulating property between the positive and negative electrodes tends to be inadequate, thus leading to micro-short circuiting, serious self-discharge.
SUMMARY OF THE INVENTION
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, even if it is thin, and has high strength and excellent electronic 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 a paper prepared to contain at least 50 wt % of fibers obtained by beating regenerated cellulose fibers.
Now, the present invention will be described in further detail with reference to the preferred embodiments.
In the present invention, the starting material of the separator for the electric double layer capacitor using a non-aqueous electrolyte, is regenerated cellulose fibers which can be beaten with a beating machine installed in a usual paper-making process, such as regenerated cellulose fibers having a high polymerization degree, or solvent-spun rayon fibers. They are used after beaten for the material of the separator.
If the paper constituting the separator is made of less than 50 wt % of fibers obtained by beating regenerated cellulose fibers, the separator obtained therefrom tend to have high electric resistance and poor strength. In order to maintain the strength of the separator, it is conceivable to increase its thickness. However, it is no good since such method increases high electric resistance of the separator. In the present invention, the paper constituting the separator is made of preferably at least 65 wt %, particularly preferably at least 80 wt %, of fibers obtained by beating regenerated cellulose fibers.
Regenerated cellulose fibers will be uniformly fibrillated and will have increased flexibility with the beating treatment. Accordingly, the separator made of paper prepared to contain at least 50 wt % of such fibers, is excellent in tensile strength. Further, the fibers fibrillated by beating, are extremely dense, and the cross sections of fibrils are substantially circular. Accordingly, the separator made of paper prepared to contain at least 50 wt % of such fibers, has a low resistance.
The regenerated cellulose fibers of the present invention are preferably beaten until from 0 to 600 ml of Canadian Standard Freeness (hereafter referred as CSF value) stipulated in JIS P8121. The regenerated cellulose fibers from solvent-spun rayon etc. have a CSF value of about 800 ml in the unbeaten state. Unless such fibers are adequately beaten, they will not be sufficiently fibrillated and will have low strength due to inadequate bonding site obtained by fibrils. Accordingly, such fibers are preferably beaten until the CSF value become 600 ml or less. The regenerated cellulose fibers may be beaten until the CSF value become 0 ml because they are expected to have a high strength as their density increases in proportion to the degree of the beating.
Other material to be incorporated to the beaten regenerated cellulose fibers, is not particularly limited, and any fibers such as Manila hemp, sisal or kraft pulp, may be used. Such a material is preferably beaten depending upon the degree of beating of the regenerated cellulose fibers.
The separator in the present invention can be prepared, for example, as follows. Firstly, regenerated cellulose fibers cut into a few mm are beaten to have proper CSU value by a beating machine. On the other hand, fibers to be used as a blend material are likewise beaten to a proper degree, followed by mixing so that the regenerated cellulose fibers are contained in an amount of at least 50 wt %, whereupon a paper having predetermined thickness is prepared. The paper thus obtained is used as the separator which is interposed between positive and negative electrodes.
In the present invention, the separator has preferably a thickness of from 20 to 60 &mgr;m and a density of from 0.30 to 0.60 g/cm
3
. If the thickness is less than 20 &mgr;m, inadequate insulation between the positive and negative electrodes is likely to occur. If the thickness is more than 60 &mgr;m, the energy density tends to be low. Further, if the density is less than 0.3 g/cm
3
, the strength of the separator becomes inadequate. If the density is more than 0.60 g/cm
3
, the ion permeability become insufficient. It is particularly preferred that the thickness is from 30 to 50 &mgr;m and the density is from 0.35 g/cm
3
to 0.50 g/cm
3
.
In the present invention, the separator may be made by overlaying sheets of the paper prepared to contain at least 50 wt % of fibers obtained by beating regenerated cellulose fibers. Further, the separator may also be composed by overlaying said paper and paper made of different material from said paper or non-woven fabric. When the separator is composed of sheets of the paper overlaid, a multilayered paper prepared at a procedure of paper making can be used. The separator prepared by overlaying a plurality of said paper or by overlaying the other paper and said paper is preferred in order to avoid short-circuit between the positive and negative electrodes. However, its power output per unit volume tends to be low because the thickness of the separator become large.
The separator composed of the paper prepared to contain at least 50 wt % of fibers obtained by beating regenerated cellulose fibers usually contains from 3 to 10 wt % of moisture. It is preferred to remove such a moisture in order to reduce the leakage of current and to secure a high withstand voltage in the non-aqueous type e

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