JP2000124081A - Electric double-layer capacitor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the withstand voltage by forming a positive electrode of an active C layer and a negative electrode of an active C layer previously doped with Li on a collector and comprising an org. electrolytic soln. SOLUTION: A positive electrode is made by activating coconut husks with steam as an active C, a negative electrode is made by activating a phenol material with steam into an active C, mixing carboxymethylcellulose and acetylene black with the active C and dispersing it in a mixed soln. of methanol with water to prepare a slurry. As the collector, an aluminum-etched foil is coated on the positive electrode 1, and one surface of a Ti-etched foil for the negative electric 3 is coated with an electrode slurry, dried with air at the ordinary temp., and heated and cut in a prescribed size to obtain electrodes. Only the negative electrode 3 is doped. As the electrolytic soln., a liq. made by dissolving tetraethyl ammonium tetrafluoroborate and lithium fluoroborate in propylene carbonate is impregnated.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an electric double layer capacitor having a high withstand voltage.

[0002]

2. Description of the Related Art In recent years, an electric vehicle driven by a battery instead of an internal combustion engine has been regarded as important from the viewpoint of environmental concerns and the viewpoint of suppressing carbon dioxide gas. In particular, a hybrid vehicle that converts driving energy from an internal combustion engine into electric energy via a generator and stores the electric energy in a power storage device to improve efficiency has attracted attention. Secondary batteries and electric double layer capacitors have been proposed as power storage devices for hybrid vehicles.

[0003] As batteries for use in hybrid vehicles, batteries that are currently under study include sealed lead-acid batteries, nickel-metal hydride batteries, and lithium-ion secondary batteries. In this case, since the vehicle travels mainly by the power of the internal combustion engine during steady traveling, the required energy amount is much smaller than that for an electric vehicle. However, instead, a large amount of power is required when starting, accelerating, and climbing a hill, and a high power density is required. In addition, the charging and discharging frequency is much higher than that for electric vehicles, and the cycle life is required to be 100,000 cycles or more.

In the case of a secondary battery, the cycle life at the time of full charge and full discharge is currently about 1000 times, and in order to solve this, the cycle life is extended by making the depth of discharge shallow. Therefore, the effective capacity density is greatly reduced, and it is necessary to mount more battery capacity. Further, the capacity density of the secondary battery tends to decrease at a low environmental temperature.

On the other hand, an electric double layer capacitor is a type of capacitor and has the highest energy density among capacitors. Since the electric double layer capacitor stores energy by the adsorption and desorption of ions at the interface between the electrode and the electrolyte, an electrochemical reaction involving oxidation and reduction does not occur. Therefore, it has the advantages of a high output density, a long service life of 10 years or more, and a high resistance to low temperatures. Further, since the remaining energy is proportional to the square root of the terminal voltage, it is easy to grasp and fine control is possible. However, at present, the energy density is low, which is one order of magnitude smaller than that of a general secondary battery.
Therefore, in order to apply the electric double layer capacitor to a hybrid vehicle, it is desirable to increase the energy density.

On the other hand, in recent years, a device having an intermediate structure has been devised and appeared on the market in order to take advantage of both the secondary battery and the electric double layer capacitor. In this method, graphite in which activated carbon is doped into the positive electrode and lithium is doped into the negative electrode is used as an electrode material, and an organic electrolytic solution containing lithium ions is used as an electrolytic solution. The feature of this device is that it has a high withstand voltage of about 3.3 V due to the use of lithium ions, but has a lower resistance than a lithium secondary battery. However, this device stores energy by absorbing and desorbing the same ions as the electric double layer for the positive electrode, but uses the same chemical reaction as the secondary battery for the negative electrode. However, it is difficult to apply this method to applications requiring high output.

[0007]

The electric double layer capacitor is roughly classified into the following two types. That is, a solution using an aqueous solution such as a sulfuric acid aqueous solution and a solution using an organic solution based solution obtained by adding an electrolyte to an organic solvent such as propylene carbonate. Capacitors using an organic electrolyte have the advantage of a higher energy density than those using an aqueous solution, because they have a higher withstand voltage. Energy E of the electric double layer capacitor is expressed as E = 0.5 × C × V × V (where C is the capacitance and V is the withstand voltage).

As described above, the withstand voltage of the capacitor is determined by the decomposition voltage of the electrolytic solution. That is, in an aqueous electrolyte solution,
It is about 0.9 to 1.2 V, and about 2.3 to 3.0 V for an organic electrolyte. In order to further improve the energy density, it is necessary to increase the withstand voltage, but there is a limit even with an organic electrolyte. Further, since the voltage of the electric double layer capacitor is proportional to the square root of the remaining energy from the above equation, it is necessary to extract until the voltage becomes 0 V in order to extract all the energy. Generally, the load connected to the energy storage device does not operate at a certain voltage or lower, so that the available energy is smaller than E represented by the above equation.

[0009]

In order to solve the above-mentioned problems, an electric double layer capacitor according to the present invention comprises a positive electrode having an activated carbon layer formed on a current collector and an activated carbon having lithium previously doped on the current collector. A negative electrode having a layer formed thereon, and an organic electrolyte solution.

At this time, it is effective that copper or nickel is used for the current collector constituting the negative electrode.

It is effective that titanium or aluminum is used as a current collector constituting the positive electrode.

Further, the electrolyte salt constituting the electrolytic solution desirably has at least one salt selected from tetraalkylammonium tetrafluoroborate or tetraalkylammonium pentafluorophosphate and at least a lithium salt.

At this time, the concentration of tetraethylammonium tetrafluoroborate and tetraalkylammonium pentafluorophosphate is 0.1 mol / L or more and 1 mol.
/ L or less, the concentration of the lithium salt is 0.01mol / L or more and 0.2mo
It is desirable to be less than l / L.

The activated carbon contained in the polarizable electrodes of the positive electrode and the negative electrode preferably has a particle size of 1 μm or more and 20 μm or less.

The thickness of the positive electrode current collector and the negative electrode current collector is
The thickness is preferably 10 μm or more and 80 μm or less, and the thickness of the polarizable electrode formed on one or both surfaces thereof is preferably 20 μm or more and 200 μm.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The object of the present invention is to improve the withstand voltage without significantly increasing the resistance of the electric double layer capacitor. An electric double layer capacitor is an electric double layer formed between an electrolytic solution and a polarizable electrode,
That is, electricity is stored by ions facing the electrolyte side and electrons or holes facing the polarizable electrode side.
In the case of a flat plate capacitor, before charging, there are no electrons or holes in the electrodes, and no potential difference occurs in any part of the capacitor. However, in the case of an electric double capacitor, when the chemical potential of the electrolytic solution differs from that of the polarizable electrode, a potential difference called a spontaneous potential occurs, which is determined by the reaction between the electrolytic solution and the surface of the polarizable electrode.

Normally, a high specific surface area activated carbon is used for the polarizable electrode of the electric double layer capacitor, so that it is determined by the reaction between the activated carbon surface and the ions present in the electrolyte. However, if the same type of activated carbon is used for both electrodes of the capacitor,
Since the natural potentials of both electrodes are the same, there is no potential difference between the two electrodes as in the case of the plate capacitor. However, if the polarizable electrode of the negative electrode is doped with lithium and a small amount of lithium ions are added to the electrolytic solution, the natural potential of the negative electrode will become an equilibrium state between lithium in the polarizable electrode and lithium ions in the electrolytic solution. Will be determined by Since the natural potential of the positive electrode does not change, a potential difference is finally generated between the positive electrode and the negative electrode.

It should be noted here that lithium ions are hardly intercalated with respect to the activated carbon during charging and discharging of the capacitor. When intercalated, the energy density is improved, but lithium ions enter and exit between carbon layers, so that the resistance increases and the output density decreases. Therefore,
It is necessary to select a carbon material such that lithium ions are fixed to some extent near the surface of carbon but do not enter between layers. For this purpose, graphite, which is usually used for lithium ion batteries, is not suitable, and phenol resin is 900
It is preferable that the carbonized material is carbonized at about ℃ and the surface of the benzene ring has a basal plane oriented.

As the electrolyte ions used in the electrolytic solution, the lithium salt usually used in lithium ion batteries does not occupy most of the medium, but only to the extent that it can maintain an equilibrium state with lithium doped in the negative electrode.
The concentration may be about 0.01 to 0.2 mol / L. Charging and discharging are performed by an electric double layer formed by solvent ions, so it is not a carbon material with a relatively small specific surface area used in lithium secondary batteries, etc., but both positive and negative electrodes are 1000 m ² / g
A material such as activated carbon having the above specific surface area is desirable. Also, the electrolyte is preferably a quaternary ammonium salt such as tetraethylammonium tetrafluoroborate and tetraalkylammonium pentafluorophosphate. Therefore, it is considered that a mixed solute system of the electrolyte used for charging and discharging and the electrolyte for defining the natural potential is most suitable. The anions of both electrolytes may be the same or different.

In the above-mentioned system, in order to increase the output density, it is necessary that the activated carbon is thinly applied to the metal foil current collector together with the conductive agent, the binder and the like, and the active carbon is wound. Normally, aluminum is used for the metal foil current collector for the electric double layer capacitor. However, if this is used for the negative electrode, it is corroded, so it is necessary to use a metal having a low ionization tendency, such as copper or nickel. An object of the present invention is to improve the withstand voltage without significantly increasing the resistance of the electric double layer capacitor. With the above configuration, the withstand voltage of the electric double layer capacitor can be improved.

Hereinafter, preferred embodiments of the present invention will be specifically described.

[0022]

EXAMPLES (Example 1) A coconut shell material was used at 900 ° C for the positive electrode.
10 g of activated carbon activated with steam in the negative electrode, 10 g of activated carbon in which the phenolic material was activated with steam at 900 ° C., 2 g of carboxymethyl cellulose as a binder, and 4 g of acetylene black as a conductive material were mixed with 40 ml of methanol and 40 ml of water, respectively.
A slurry dispersed in 60 ml of the mixed solution was prepared. As a current collector, the positive electrode has aluminum etched foil with a thickness of 40 μm,
On the negative electrode, one side of a titanium-etched foil having a thickness of 20 μm was coated with an electrode slurry to a thickness of 50 μm using a doctor blade, air-dried at room temperature for 15 minutes, and then dried at 140 ° C. for 15 minutes.
Heating for a minute and cutting into a size of 15 mm square gave an electrode body.

Next, only the negative electrode was subjected to doping treatment.
First, in a 0.5 mol / L lithium fluoroborate propylene carbonate solution, a cathode was opposed to lithium metal, and constant current charging was performed with lithium being positive and the negative electrode being negative. At this time, the total current was adjusted to 50 mAh / g of activated carbon. The doping-finished negative electrode is
After washing with propylene carbonate, an evaluation cell was prepared together with the positive electrode. Impregnated with a solution of tetraethylammonium tetrafluoroborate at a concentration of 0.5 mol / L in propylene carbonate as an electrolytic solution, and lithium fluoroborate of 0.05 mol / L, and opposed to each other via a polypropylene separator, an electric double layer capacitor An evaluation cell was created. FIG. 1 schematically shows the structure of this device. In FIG. 1, a positive electrode 1 made of a polarizable electrode containing activated carbon powder is formed on both or one side of a positive electrode current collector 2 made of aluminum or titanium metal, and is made of a polarizable electrode containing activated carbon doped with lithium in advance. The negative electrode 3 is formed on both surfaces or one surface of a negative electrode current collector 4 made of copper or nickel. And are integrally connected. A positive electrode lead 5 and a negative electrode lead 6 made of the same type of metal as the respective current collectors are connected to the positive electrode current collector 2 and the negative electrode current collector 4 in advance. A positive electrode current collector on which a positive electrode is formed, a negative electrode current collector on which a negative electrode is formed,
The separator 7 was wound into a cylindrical shape, placed in a case 8, filled with an electrolytic solution, and then sealed to obtain an electric double layer capacitor as shown in FIG.

The following devices were prepared as comparative examples. 900 ° C coconut shell material at positive and negative electrodes
Was mixed with 2 g of carboxymethylcellulose as a binder and 4 g of acetylene black as a conductive material, and dispersed in a mixed solution of 40 ml of methanol and 60 ml of water. As a current collector, one side of a 40 μm-thick aluminum-etched foil was coated on the positive electrode and the negative electrode with a slurry for an electrode to a thickness of 50 μm using a doctor blade, air-dried for 15 minutes at room temperature, and dried at 140 ° C.
Heating for a minute and cutting into a size of 15 mm square gave an electrode body.
An electrolyte was impregnated with a solution of tetraethylammonium tetrafluoroborate dissolved in propylene carbonate at a concentration of 0.5 mol / L and opposed to each other with a polypropylene separator interposed therebetween to prepare an electric double layer capacitor evaluation cell of a comparative example.

As described above, in both the embodiment and the conventional example, the platinum electrode was brought into contact with the current collector using a spring for taking out the electric charge, and was fixed with a Teflon holder. Since the pressure resistance deteriorates when water is mixed with the electrolyte, the assembly was performed in a dry nitrogen atmosphere, and after the assembly was completed, the assembly was taken out in a normal atmosphere. The characteristics of the evaluation cell prepared by the above method were evaluated by the following test method.

1. The cell for capacity evaluation is charged at a constant voltage for 30 minutes at the set voltage,
Thereafter, constant current discharge was performed at 10 mA. At that time, the terminal voltage
From the time tV required for the voltage to drop from 1.0 V to 0.5 V, the capacity was calculated by C (capacity) = 0.01 × tV, and divided by the activated carbon weight of both electrodes to obtain the capacity density. In addition, the IR immediately after the start of discharge
From the drop, we asked for resistance.

2. Life Put the cells for evaluation together in a thermostat at 65 ° C and keep applying the set voltage. The capacity was measured at regular intervals in accordance with the above-described method, and the life was defined as the time 30% lower than the capacity immediately after the cell was assembled.

Table 1 shows the characteristics of the conventional device when the electrolyte concentration and the set voltage were changed by the above-described measuring methods.
Table 2 shows the characteristics of the devices of the examples.

[0029]

[Table 1]

[0030]

[Table 2]

From the above results, the following has been found. (1) Compared with the conventional example, the embodiment can secure the life even at a higher voltage. (2) When the concentration of tetraethylammonium tetrafluoroborate is constant, as the concentration of lithium fluoroborate increases, the capacity increases but the resistance also increases.
0.05mol /
It is about L. (3) When the concentration of lithium fluoroborate is constant, the capacity is high and the life is long when the concentration of tetraethylammonium tetrafluoroborate is around 0.5 mol / L.

In this embodiment, tetraethylammonium tetrafluoroborate is used as the electrolyte salt constituting the electrolyte. However, similar effects can be obtained by using an asymmetric tetraalkyltetrafluoroborate such as dimethyldiethylammonium tetrafluoroborate. I was able to. Further, the same effect was obtained by using an electrolyte salt having pentafluorophosphate as an anion instead of tetrafluoroborate.

[0033]

As described above, according to the present invention, the withstand voltage of the electric double layer capacitor can be improved.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of an electric double layer capacitor according to an embodiment of the present invention.

[Explanation of symbols]

 Reference Signs List 1 positive electrode 2 positive electrode current collector 3 negative electrode 4 negative electrode current collector 5 positive electrode lead 6 negative electrode lead 7 separator 8 case

Claims (7)

[Claims] A positive electrode having an activated carbon layer formed on a current collector;
An electric double layer capacitor comprising: a negative electrode in which a layer made of activated carbon doped with lithium in advance is formed on a current collector; and an organic electrolyte. 2. The electric double layer capacitor according to claim 1, wherein the current collector constituting the negative electrode is made of copper or nickel. 3. The current collector constituting the positive electrode is made of titanium or aluminum.
The electric double layer capacitor as described in the above. 4. The electrolyte according to claim 1, wherein the electrolyte comprises at least one salt selected from tetraalkylammonium tetrafluoroborate or tetraalkylammonium pentafluorophosphate and a lithium salt. Multilayer capacitor. 5. The concentration of tetraethylammonium tetrafluoroborate and tetraalkylammonium pentafluorophosphate is 0.1 mol / L or more and 1 mol / L or less, and the concentration of lithium salt is 0.01 mol / L or more and 0.2 mol / L or less. 5. The electric double layer capacitor according to claim 4, wherein: 6. The electric double layer according to claim 1, wherein the activated carbon contained in the polarizable electrodes of the positive electrode and the negative electrode has a particle size of 1 μm or more and 20 μm or less. Capacitors. 7. The thickness of the positive electrode current collector and the negative electrode current collector is 10 μm.
7. The electric power according to claim 1, wherein the thickness of the polarizable electrode formed on one or both sides thereof is not less than 20 μm and not more than 80 μm and not more than 80 μm. Double layer capacitor.