Electric double layer capacitor

Details Electric double layer capacitor Electric double layer capacitor

2000-02-07

2001-10-09

Koslow, C. Melissa (Department: 1755)

Compositions

Electrolytes for electrical devices

C361S502000, C361S525000

Reexamination Certificate

active

06299790

ABSTRACT:

TECHNICAL FIELD
This invention relates to an electric double layer capacitor utilizing a solid electrolyte.
BACKGROUND OF THE INVENTION
PRIOR ART
The electrolyte used in the conventional electric double layer capacitor is a liquid electrolyte, that is a solution of an electrolyte salt in a polar medium. The electric double layer capacitor has gained in size in recent years as epitomized by the one used as the auxiliary power source of an electric vehicle and is required to have safety and reliability, among others.
However, an electric double layer capacitor utilizing a liquid electrolyte is liable to develop electrolyte leaks so that its safety and reliability are not necessarily satisfactory. This capacitor further has the disadvantage of a high rate of self discharge.
To overcome those disadvantages, much research has been undertaken into polymer type solid electrolytes in recent years. As such polymer solid electrolytes, a complex between poly(ethylene oxide) and an electrolyte salt and a complex between an ethylene oxide-propylene oxide random copolymer and an electrolyte salt (JP-A-62249361), among others, are known.
However, despite improvements in the incidence of electrolyte leakage and self discharge, such polymer solid electrolytes are seriously deficient in ionic conductivity and ion diffusion rate at low temperatures below room temperature so that when any of them is used in an electric double layer capacitor, only a very low capacitance is obtained at such a low temperature and a high discharge current cannot be obtained even In the ambient temperature.
OBJECT OF THE INVENTION
This invention has for its object to provide an electric double layer capacitor overcoming the above disadvantages, namely one capable of functioning well over a broad range of temperature with a reduced incidence of electrolyte leaks and a low rate of self discharge.
SUMMARY OF THE INVENTION
The electric double layer capacitor of this invention is characterized in that it comprises a solid electrolyte composed of (A) a crosslinking product of a polyoxyalkylene having a crosslinkable double bond terminally and/or in a side chain thereof, (B) an electrolyte salt and (c) a low molecular weight polar solvent.
The electric double layer capacitor utilizing the solid electrolyte of this invention has a discharge capacity comparable to that of the conventional double layer capacitor even at room temperature and, in addition, a low rate of self discharge. Moreover, because the electrolyte is a solid electrolyte, this capacitor has substantially no risk for electrolyte leakage, assuring long-term reliability.
DETAILED DESCRIPTION OF THE INVENTION
(A) Polyoxyalkylene
The polyoxyalkylene having a polymerizable double bond, such as an acryloyl or methacryloyl group, terminally and/or in a side chain for use in this invention is not particularly restricted. Preferred, however, is a polymer of the following chemical formula (1) or a polymer of the following chemical formula (2), or a mixture of them, which is capable of holding the low molecular weight polar solvent stably in a large quantity. Polyoxyalkylenes of these respective formulas can be used each independently or as a mixture of two or more species.
The average molecular weight of each polymer is not particularly restricted but is preferably 100 to 3,000,000 and more preferably 200 to 2,000,000.
Formula (1) is as follows:
wherein Z represents an active hydrogen compound residue; k represents an integer of 1 to 6; R
1
represents an alkyl group having 1 to 8 carbon atoms; Y
1
represents an acryloyl or methacryloyl group; and m represents an integer from 0 to 460 and n represents an integer from 0 to 350, excluding the case in which both m and n are simultaneously equal to 0. Formula (2) is as follows:
wherein Z represents an active hydrogen compound residue; k represents an integer from 1 to 6; R
1
represents an alkyl group having from 1 to 8 carbon atoms; R
2
represents —CH
2
—O—Re
1
—Re
2
wherein Re
1
represents —(CH
2
CH
2
O)p
1
— (p
1
is equal to 0 or an integer of 1 or more) and R
2
represents an alkenyl group; Y
2
represents an alkyl group, an aralkyl group, an aryl group, acryloyl or methacryloyl; q represents an integer from 0 to 100,000; r represents an integer from 0 to 50,000; and s represents an integer from 1 to 22,000.
The above polyoxyalkylene can be prepared by subjecting an active hydrogen compound, such as ethylene glycol monomethyl ether, ethylene glycol, glycerol, diglycerol, pentaerythritol or the like, and a monomer, such as ethylene oxide, propylene oxide, allyl glycidyl ether or the like, to addition polymerization reaction and then carrying out an esterification reaction with an unsaturated organic acid, such as acrylic acid or methacrylic acid, a dehydrochlorination reaction using an acid chloride, such as acryloyl chloride, methacryloyl chloride or the like, or a condensation reaction with an alkyl halide or the like.
Crosslinking of said polyoxyalkylene can be effected by ultraviolet irradiation, electron beam treatment or thermal crosslinking, for instance. In this procedure, a polymerization initiator or a sensitizer may be employed where necessary.
(B) Electrolyte Salt
The electrolyte salt for use in this invention is not particularly restricted but includes LiClO
4
, LiSCN, LiBF
4
, LiCF
3
SO
3
, LiPF
6
, LiN (CF
3
SO
2
)
2
, LiC(CF
3
SO
2
)
3
, LiF, LiCl, LiBr, LiI, NaClO
4
, NsSCN, NaBF
4
, NaPF
6
, NaCF
3
SO
3
, NaN(CF
3
SO
2
)
2
, NaC(CF
3
SO
2
)
3
, alkali metal salts such as NaF, NaCl, NaBr, NaI, etc., quaternary ammonium salts such as (CH
3
)
4
NBF
4
, (CH
3
)
4
NBr, (CH
3
)
4
NClO
4
, (C
2
H
5
)
4
NBF
4
, (C
2
H
5
)
4
NClO
4
, etc., quaternary phosphonium salts such as (C
2
H
5
)
4
PBF
4
etc.; and protonic acids such as sulfuric acid, perchloric acid, p-toluenesulfonic acid, etc., to mention preferred examples. Those electrolyte salts may be used in a combination of two or more different species.
The formulating amount of said electrolyte salt, as its concentration in the low molecular weight polar solvent described below, is preferably 0.01 to 3 mol/l, more preferably 0.1 to 1.5 mol/l. If the concentration of the electrolyte salt is less than 0.01 mol/l, the ionic conductivity will be so low that the capacitor may not fully express the desired performance characteristics. Conversely if the electrolyte salt concentration exceeds 3 mol/l, a perturbation will occur in the electric double layer to prevent attainment of a high capacitance.
(C) Low Molecular Weight Polar Solvent
The low molecular weight polar solvent is not restricted to any particular type but includes cyclic esters, cyclic carbonic esters, cyclic ethers, nitriles, lactones, acyclic carboxylic esters, acyclic carbonic esters, sulfolane, sulfolane derivatives, dimethyl sulfoxide and N,N-dimethylformamide, to mention but a few preferred examples.
The molecular weight of such a low molecular weight polar solvent is not particularly restricted but is preferably within the range of 40 to 1,000. If the molecular weight is less than 40, the solvent will be so volatile that it may not be easy to handle. On the other hand, if the molecular weight of the solvent exceeds 1,000, the deceased ionic conductivity of the solid electrolyte will be a problem.
The proportion of the low molecular weight polar solvent in the solid electrolyte should be somewhere between 220 and 1900 weight % based on the weight of the polyoxyalkylene or the sum of the weights of the polyoxyalkylene and polyacrylonitrile. If the proportion of the low molecular weight polar solvent is too small, the ionic conductivity will not be as high as required, with the result that the capacitance of the capacitor at temperatures below room temperature will be insufficient. If the proportion of the low molecular weight polar solvent is too large, the mechanical strength of the solid electrolyte will not be as high as necessary so that the dependability will be sacrificed as, for example, short-circuits occur between the electrodes.
(D) Polyacrylonitrile
Polyacrylonitrile for use is

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