Electrochemical capacitor

Details Electrochemical capacitor Electrochemical capacitor

2000-10-13

2002-07-02

Reichard, Dean A. (Department: 2831)

Electricity: electrical systems and devices

Electrolytic systems or devices

Liquid electrolytic capacitor

C361S503000, C361S508000, C361S529000, C361S504000, C029S025350

Reexamination Certificate

active

06414837

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electro-chemical capacitor.
2. Description of the Related Art
Electrochemical capacitors are a device for storing energy in an interface between an electrode as an electron conductor and an electrolyte as an ion conductor, and are classified into electric double-layer capacitors and pseudo capacitors (redox capacitors).
An electric double-layer capacitor comprises electrode elements as an anode and a cathode that are disposed in confronting relationship to each other with a separator interposed therebetween. Both the anode and the cathode comprise polarized electrodes made of a formed body of fibers or particles of activated carbon and a coated film of particles of activated carbon, the electrode elements being impregnated with an electrolyte. The electric double-layer capacitor stores electric charges in electric double layers produced in the interfaces between the polarized electrodes and the electrolyte.
A pseudo capacitor comprises electrode elements as an anode and a cathode that are disposed in confronting relationship to each other with a separator interposed therebetween. One of the anode and the cathode comprises a polarized electrode made of a formed body of fibers or particles of activated carbon and a coated film of particles of activated carbon, and the other of a non-polarized electrode made of a metal oxide or an electrically conductive polymer, the electrode elements being impregnated with an electrolyte. When a potential difference across the interface between the non-polarized electrode and the electrolyte is changed, electric charges move in the non-polarized electrode. The metal oxide may comprise ruthenium oxide, iridium oxide, nickel oxide, lead oxide, or the like, and the electrically conductive polymer may comprise polypyr-role, polythiophene, or the like. The pseudo capacitor of the above structure utilizes the phenomenon in which electric charges are stored in an electric double layer produced in the interface between the polarized electrode and the electrolyte, and also utilizes a pseudo capacitance due to the movement of electric charges caused by an oxidation and reduction in the vicinity of the interface between the non-polarized electrode and the electrolyte (see B. E. Conway, J. Electrochem. Soc., 138, 1539 (1991)).
The electrolytes used in the electrochemical capacitors are roughly classified into liquid electrolytes (electrolytic liquids) and solid electrolytes from their, state, and the electrolytic liquids are also classified aqueous and nonaqueous ones from a kind of solvent. The nonaqueous electrolytic liquids comprise electrolytic solutions prepared by dissolving quaternary ammonium salt, quaternary phosphonium salt, etc. into an organic solvent of propylene carbonate, etc. The solid electrolytes include a polyethylene oxide—alkali metal salt complex, RbAg
4
I
5
, etc. (Makoto Ue, “Electrochemistry”, 66, 904 (1998)).
If an electrochemical capacitor uses a nonaqueous electrolytic liquid, then it has an increased withstand voltage and also has a higher energy density than if it uses an aqueous electrolytic liquid. Electrochemical capacitors using nonaqueous electrolytic liquids find use as backup power supplies for electronic devices that need to be smaller in size and lower in profile, and are also suitable for use in power applications such as electric vehicles, hybrid vehicles, and power storage devices which have been drawing attention in recent years.
When the electrochemical capacitor utilizes the phenomenon in which electric charges are stored in an electric double layer produced in the interface between the polarized electrode and the electrolyte, the energy W accumulated in the electric double layer at the time the electro-chemical capacitor is discharged at a constant current I from a voltage V
i
to a voltage V
f
is expressed by the following equation (1):
W
=
1
/
2
·
C
·
(
V
i
2
-
V
f
2
)
=
1
/
2
·
C
·
[
(
V
0
-
IR
)
2
-
V
f
2
]
(
1
)
Therefore, in order to increase the energy density of the electrochemical capacitor which utilizes the phenomenon in which electric charges are stored in the electric double layer, it is necessary to increase the capacitance C (F) or the open-circuit voltage V
0
(V) or to reduce the internal resistance R (&OHgr;). The capacitance C increases in proportion to the effective surface area of contact between the polarized electrode and the electrolyte, and is determined by a withstand voltage that is determined by the reactivity between the polarized electrode and the electrolyte. The internal resistance R includes the electric resistance of the electrode itself, and also a diffusion resistance for ions to move in pores of the electrode and a diffusion resistance for ions to move in the electrolyte. The diffusion resistance for ions to move in the electrolyte is in inverse proportion to the electric conductivity of the electrolyte. Consequently, the electrolyte is generally desired to have a high electric conductivity.
Japanese laid-open patent publication No. 63-173312 discloses, as the above electrolyte, a nonaqueous electrolytic liquid prepared by dissolving an asymmetric quaternary ammonium salt as an electrolytic salt into a nonaqueous solvent. The electrolyte is used in an electric double-layer capacitor which uses a polarized electrode of activated carbon as each of an anode and a cathode. The above publication states that an electric double-layer capacitor having a low internal resistance, a low capacitance degradation ratio under high temperature conditions, and excellent long-term reliability is produced by using an electrolytic liquid which employs the above electrolytic salt.
However, in view of stricter performance demands for electrochemical capacitors in recent years, there has been desired the development of electrochemical capacitors of lower internal resistance, greater capacitance, and higher energy density.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an electrochemical capacitor of high energy density.
To achieve the above object, an electrochemical capacitor comprises electrode elements as an anode and a cathode that are disposed in confronting relationship to each other with a separator interposed therebetween, and a nonaqueous electrolytic liquid impregnated in the electrode elements, the nonaqueous electrolytic liquid being prepared by dissolving an electrolytic salt Q
+
A
−
including cations Q
+
whose van der Waals volume ranges from 0.10 to 0.125 nm
3
into a nonaqueous solvent, the cathode comprising a polarized electrode made of activated carbon having pore diameters whose mode is at most 1.5 nm.
Since the van der Waals volume of the cations Q
+
ranges from 0.10 to 0.125 nm
3
, the cations Q
+
can be diffused relatively freely in the pores of the activated carbon of the polarized electrode. If the van der Waals volume of the cations Q
+
were smaller than 0.10 nm
3
, then they strongly interact with the molecules of the nonaqueous solvent, producing large solvation ions which prevent the cations Q
+
from being diffused in the pores of the activated carbon. If the van der Waals volume of the cations Q
+
were greater than 0.125 nm
3
, then the cations Q
+
are prevented from being diffused in the pores of the activated carbon due to the size of the cations Q
+
themselves.
The van der Waals volume of the cations Q
+
can be calculated from a model in which spherical atoms of the cations Q
+
are bonded and superposed at a given bonding distance and angle (see M. Ue, J. Electrochem. Soc., 141, 3336 (1994)). For the radii of the spherical atoms, numerical values proposed by A. Bondi, J. Phys. Chem., 68, 441 (1964) (see “Chemical Manual, Basics II”, edited by Chemical Society of Japan, H: 0.120 nm, C: 0.170 nm, N: 0.155 nm, P: 0.180 nm, As: 0.185 nm, Sb: 0.210 nm) are employed. Values measured by an X-ray diffraction analysis or a neutron di

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