JPH1154385A - Electric double layer capacitor - Google Patents

Abstract

[PROBLEMS] To provide an electric double layer capacitor having excellent charge / discharge cycle characteristics, high withstand voltage, large capacity, low resistance and excellent rapid charge / discharge characteristics. A positive electrode comprising a polarizable electrode mainly composed of activated carbon, and a negative electrode containing a carbon material capable of absorbing and releasing lithium ions and copper or nickel and absorbing lithium ions by a chemical method or an electrochemical method. And an organic electrolytic solution containing a lithium salt as a solute.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an electric double layer capacitor having a low resistance, an excellent charge / discharge cycle, a high withstand voltage and a large capacity.

[0002]

2. Description of the Related Art The electrodes of a conventional electric double layer capacitor are:
Both the positive and negative electrodes consisted of polarizable electrodes mainly composed of activated carbon. The withstand voltage in this case is 1.2 V when an aqueous electrolyte is used, and 2.5 to 3.3 V when an organic electrolyte is used.
It is.

Since the electrostatic energy of an electric double layer capacitor is proportional to the square of the withstand voltage, the use of an organic electrolyte having a high withstand voltage is higher than the use of an aqueous electrolyte. However, the energy density of an electric double layer capacitor using an organic electrolyte and having a positive electrode and a negative electrode both of which are polarizable electrodes mainly composed of activated carbon is 10 minutes less than that of a secondary battery such as a lead storage battery or a lithium ion secondary battery. 1 or less, and further improvement in energy density is required.

Japanese Patent Application Laid-Open No. 64-14882 discloses a method in which an electrode mainly composed of activated carbon is used as a positive electrode and measured by X-ray diffraction.
2] A secondary battery with an upper limit voltage of 3 V has been proposed, in which an electrode in which lithium ions are previously absorbed in a carbon material having a plane spacing of 0.338 to 0.356 nm is used as a negative electrode. JP-A-8-107048 proposes an electric double layer capacitor in which a carbon material capable of absorbing and desorbing lithium ions is used as a negative electrode with a carbon material in which lithium ions are previously absorbed by a chemical method or an electrochemical method. Have been. Japanese Patent Application Laid-Open No. 9-55342 discloses that the maximum voltage of a negative electrode having a negative electrode in which a carbon material capable of absorbing and desorbing lithium ions is supported on a porous current collector that does not form an alloy with lithium is disclosed.
V electric double layer capacitors have been proposed.

[0005]

A negative electrode in which lithium ions are previously stored in a carbon material capable of storing and releasing lithium ions as described above has a lower potential than a negative electrode mainly composed of activated carbon. Occludes ions,
The withstand voltage of an electric double layer capacitor that combines a negative electrode in which lithium ions are occluded in a removable carbon material in advance and a positive electrode mainly composed of activated carbon is the withstand voltage of an electric double layer capacitor mainly composed of activated carbon for both the positive electrode and the negative electrode. Higher.

On the other hand, the resistance of the electric double layer capacitor is determined by the resistance of the positive electrode, the resistance of the electrolytic solution, and the resistance of the negative electrode. When the resistance is low, the electric double layer capacitor has excellent rapid charge / discharge characteristics. However, in the case of an electric double layer capacitor using activated carbon for the positive electrode and a carbon material capable of occluding and releasing lithium ions for the negative electrode, the resistance of the negative electrode is particularly large, making it difficult to charge and discharge a large current.

[0007]

Means for Solving the Problems The present invention has been made to solve the above problems, and comprises a positive electrode comprising a polarizable electrode mainly composed of activated carbon, a negative electrode, and an organic electrolyte containing a lithium salt as a solute. In an electric double layer capacitor having
Provided is an electric double layer capacitor, wherein the negative electrode contains a carbon material capable of occluding and desorbing lithium ions and copper or nickel, and occluding lithium ions by a chemical method or an electrochemical method.

[0008] Carbon materials capable of occluding and releasing lithium ions include natural graphite, artificial graphite, non-graphitizable carbon, easily graphitizable carbon, and low-temperature fired carbon. Here, the non-graphitizable carbon is a carbon material obtained by firing a furfuryl alcohol resin or a phenol resin, and has a crystallite size of several n.
m and a carbon material having a density of 1.5 to 1.8 g / cm ³ . The graphitizable carbon is coke, mesocarbon microbeads, mesophase pitch-based carbon fiber, pyrolytic vapor growth carbon fiber, etc., and has a crystallite size of 1.8.
Refers to a carbon material of 炭素 2.1 g / cm ³ .

In the present invention, lithium ions are occluded,
The desorbable carbon material is determined by X-ray diffraction measurement [00
2] It is preferable that the plane interval between the planes is 0.335 to 0.410 nm. A carbon material having an interplanar spacing of more than 0.410 nm is not preferable because deterioration is large in a charge / discharge cycle. More preferably 0.356 to 0.390 nm
It is. Specifically, a non-graphitizable carbon material or a non-graphitizable carbon material heat-treated at 1000 to 2000 ° C. can be preferably used. In addition, natural graphite, artificial graphite, a carbon material obtained by heat-treating a graphitizable carbon material at 2500 ° C. or more has a [002] plane spacing of 0.335 to 0.338 n.
m, and these can also be preferably used.

The resistance of a negative electrode obtained by storing lithium ions in a carbon material capable of storing and releasing lithium ions by a chemical method or an electrochemical method is the resistance caused by the storage and release of lithium ions and the resistance of the negative electrode itself. Consists of The resistance of the negative electrode itself is based on the resistance of the carbon material particles constituting the negative electrode and the contact resistance between the carbon material particles. In the negative electrode according to the present invention, since copper or nickel is mixed with a carbon material capable of absorbing and desorbing lithium ions, the contact resistance between the carbon materials becomes negligibly small, and the resistance of the electrode becomes small.

In particular, the [002] plane spacing is 0.34 to
When using graphitizable carbon, non-graphitizable carbon, or low-temperature fired carbon, which is 0.410 nm and not highly graphitic, the effect of reducing resistance is large. In these carbon materials, the resistance of the material itself may not be low, and when a large current is discharged, the resistance loss increases by the electrode layer resistance, and the discharge voltage decreases,
Characteristics deteriorate. Therefore, the effect of reducing the resistance of the negative electrode by adding a low-resistance metal such as copper or nickel to the negative electrode is large.

In the present invention, copper or nickel is preferably contained in the negative electrode in an amount of 5 to 50% by weight. If it is less than 5% by weight, there is almost no effect of reducing the resistance.
If the content is more than 10% by weight, the amount of the carbon material in the negative electrode decreases, so that the capacity of the negative electrode decreases. More preferably, it is 8 to 20% by weight.

Further, copper or nickel is 0.1 to 50 μm.
m are preferred. If it is less than 0.1 μm, it is difficult to handle the electrode when producing it. On the other hand, if it exceeds 50 μm, it is difficult to reduce the resistance while maintaining a high capacity. More preferably, it is 1 to 10 μm.

The capacity of the electric double layer capacitor is given by equation 1.
Can be Where C is the cell capacity, C ⁺ Is the positive electrode capacity, C^- 
Is the negative electrode capacity. 1 / C = 1 / C⁺ + 1 / C^- ... Equation 1

The electric double layer capacitor mainly composed of activated carbon for both the positive electrode and the negative electrode has almost the same capacity of the positive electrode and the negative electrode. ^{1 / C = 1 / C +} + 1 / C - ≒ 2 / C + ··· type 2

That is, the capacity of the electric double layer capacitor cell is half the capacity of the positive electrode or the negative electrode. However, when the capacity of the positive electrode is constant, as is apparent from Equation 3 obtained by rewriting Equation 1, the capacity of the electric double layer capacitor cell increases as the capacity of the negative electrode becomes larger than the capacity of the positive electrode. ^{C = C + {1 / (} 1 + C + / C -)} ··· Equation 3

When C ^- ≫C ⁺ , C ⁺ / C
^- ≒ 0, the capacity of the cell is almost equal to the capacity of the positive electrode, and the capacity of both the positive electrode and the negative electrode is twice as large as that of an electric double layer capacitor mainly composed of activated carbon.

C ⁺ / C of the electric double layer capacitor of the present invention
^- is at a current 1mA in the organic electrolytic solution 0.0
It is preferably from 01 to 0.9. Since the positive electrode capacity must be reduced in order to make it less than 0.001, the cell capacity is reduced as a result. On the other hand, if the value exceeds 0.9, the capacity of the positive electrode and that of the negative electrode are almost equal, and the cell capacity cannot be increased. In such a carbon material, the potential of the negative electrode is not so low as compared with the positive electrode. , The deterioration due to charge / discharge cycles is remarkable, and rapid charge / discharge becomes difficult. More preferably, C ⁺ / C ⁻ is from 0.01 to 0.2.

The negative electrode in the present invention is obtained by a method of mixing a carbon material capable of occluding and releasing lithium ions, copper or nickel, and a binder, adding ethanol, kneading the mixture, and forming a sheet. Also, lithium ions can be occluded, a carbon material capable of desorbing, a metal powder that does not form an alloy with lithium, and a binder dispersed in an organic solvent,
It may be applied to a metal foil serving as a current collector by a method such as a doctor blade and dried. In the present invention, any of the methods is effective for reducing the electrode resistance.

In the present invention, lithium ions are chemically or electrochemically occluded in the carbon material of the negative electrode. As a chemical method, for example, there is a method in which a carbon material of a negative electrode is immersed in an electrolytic solution in a state of being in contact with lithium metal to ionize lithium and occlude the lithium carbon material. As an electrochemical method, there is a method in which a carbon material of a negative electrode is opposed to a lithium metal via a separator, and the carbon material of the negative electrode is charged at a constant current or a constant voltage in an electrolytic solution.

In this case, the binder is preferably a fluoropolymer resin, and particularly preferably polytetrafluoroethylene (hereinafter referred to as PTFE) from the viewpoint of heat resistance and solvent resistance. The amount of the binder is preferably 1 to 20% by weight based on the weight of the negative electrode carbon material. If it is less than 1% by weight, it is difficult to form a sheet, and if it exceeds 20% by weight, the liquid absorbing property of the electrolytic solution is poor. More preferably, 3 to 15
% By weight.

The activated carbon used for the positive electrode is not particularly limited, but those obtained by activating steam, molten KOH, or the like of coconut, phenol resin, petroleum coke, or the like can be suitably used. The activated carbon has a specific surface area of 800 to 3000 m ^2.
/ G is preferable because the capacity is large.

It is preferable that the positive electrode contains a conductive material and a binder. As the binder, a fluoropolymer resin can be used in the same manner as the binder used for the negative electrode. A method for producing a positive electrode is to knead activated carbon, carbon black as a conductive material, and a binder using a solvent such as ethanol, form a sheet, and then bond the sheet to a current collector using, for example, a conductive adhesive. Is preferred.

The lithium salt of the solute of the organic electrolyte in the present invention includes LiPF ₆ , LiBF ₄ , LiClO
₄ , LiN (CF ₃ SO ₂ ) ₂ , CF ₃ SO ₃ Li, L
iC (SO ₂ CF ₃ ) ₃ , LiAsF ₆ and LiSbF
_{6 and the} like. As a solvent, ethylene carbonate, propylene carbonate, butylene carbonate,
It is preferable to include at least one selected from dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, sulfolane, and dimethoxyethane.

The organic electrolyte comprising the above-mentioned solute and solvent has a high withstand voltage and a high electric conductivity. The concentration of the lithium salt in the organic electrolyte of the present invention is 0.1 to 2.5 m.
ol / l is preferable, and more preferably 0.5 to 2 mol
/ L.

[0026]

Next, the present invention will be described more specifically with reference to Examples (Examples 1 to 4) and Comparative Examples (Example 5), but the present invention is not limited to these.

Example 1 80 parts by weight of activated carbon having a specific surface area of 2000 m ² / g obtained by a steam activation method, 10 parts by weight of conductive carbon black, and PTFE1 as a binder
A sheet obtained by mixing 0 parts by weight, kneading and rolling using ethanol is vacuum-dried at 200 ° C. for 2 hours, and then adhered to an aluminum foil using a conductive adhesive to be integrated with the current collector. The obtained positive electrode was obtained. The effective electrode area is 1 cm ² , and the thickness of the electrode layer is 250 μm.

Carbon material obtained by heat-treating petroleum coke at 1000 ° C. (interval of [002] plane is 0.345 nm) 80
Parts by weight, 10 parts by weight of copper powder having a particle size of 1 μm and PTFE10
Parts by weight, kneaded with ethanol, rolled, and vacuum-dried at 200 ° C for 2 hours, and then adhered to a copper foil with a conductive adhesive to integrate with the current collector The obtained negative electrode was obtained. The effective electrode area is 1 cm ² , and the thickness of the electrode layer is 200 μm.

A positive electrode and a negative electrode were each a single electrode, and a current of 1 mA was used in a solution in which 1 mol / L of LiBF ₄ was dissolved in a mixed solvent of ethylene carbonate and ethyl methyl carbonate in a volume ratio of 1: 1 using a lithium reference electrode. Upon evaluation, the positive electrode capacity was 0.583 mAh in the range from 4.25 V to 2.75 V, and the negative electrode capacity was 0.005 V to 2
It was 9.12 mAh in the range up to V. The capacity ratio of the positive electrode to the negative electrode was 0.0639.

Next, a lithium electrode was previously used as a counter electrode on the negative electrode, and a constant current of 1 mA and a current of 5 mAh were obtained by an electrochemical method.
By charging until the lithium ion is absorbed, the lithium ion is occluded, the model cell is made to face the positive electrode via the separator, and the initial capacity is measured in a range from 4 V to 3 V.
DC resistance was measured with a 10 mA discharge. Thereafter, the battery was subjected to a charge / discharge cycle with a charge / discharge current of 10 mA, and the capacity after 2000 cycles and the DC resistance at the time of 10 mA discharge were measured.

Example 2 A negative electrode integrated with a current collector was obtained in the same manner as in Example 1 except that nickel powder having a particle size of 1 μm was used instead of copper powder. When the capacity of this negative electrode was measured in the same manner as in Example 1, it was 9.08 mAh, and the capacity ratio of the positive electrode to the negative electrode was 0.0642. A negative electrode was charged and a cell was prepared in the same manner as in Example 1 except that the above-described negative electrode was used, and the initial capacity and the capacity after 2,000 cycles were measured in the same manner as in Example 1.

Example 3 A current collector was prepared in the same manner as in Example 1 except that 60 parts by weight of a carbon material obtained by heat-treating petroleum coke at 1000 ° C., 30 parts by weight of copper having a particle size of 1 μm, and 10 parts by weight of PTFE were used. A negative electrode integrated with the above was obtained. When the capacity of the negative electrode was measured in the same manner as in Example 1, it was 8.421 mAh, and the capacity ratio of the positive electrode to the negative electrode was 0.0692. A negative electrode was charged and a cell was prepared in the same manner as in Example 1 except that the above-described negative electrode was used, and the initial capacity and the capacity after 2,000 cycles were measured in the same manner as in Example 1.

[Example 4] In place of copper having a particle size of 1 µm, a particle size of 1 was used.
A negative electrode integrated with a current collector was obtained in the same manner as in Example 1 except that 0 μm copper was used. When the capacity of this negative electrode was measured in the same manner as in Example 1, it was 9.049 mAh, and the capacity ratio of the positive electrode to the negative electrode was 0.0644.

A negative electrode was charged and a cell was prepared in the same manner as in Example 1 except that the above-mentioned negative electrode was used, and the initial capacity and the capacity after 2,000 cycles were measured in the same manner as in Example 1.

Example 5 A negative electrode integrated with a current collector was obtained in the same manner as in Example 1 except that 90 parts by weight of a carbon material obtained by heat-treating petroleum coke at 1000 ° C. without using copper powder was used. When the capacity of this negative electrode was measured in the same manner as in Example 1, it was 6.871 mAh, and the capacity ratio of the positive electrode to the negative electrode was 0.0848. A negative electrode was charged and a cell was prepared in the same manner as in Example 1 except that the above negative electrode was used.
The initial capacity and the capacity after 2,000 cycles were measured in the same manner as described above.

Table 1 shows the initial capacity of the electric double layer capacitors of Examples 1 to 5 and the rate of change in capacity after 2000 cycles.

[0037]

[Table 1]

[0038]

According to the present invention, it is possible to obtain an electric double layer capacitor which has a large capacity, a high withstand voltage, a small deterioration of characteristics due to charge / discharge cycles, a small resistance and a rapid charge / discharge.

Claims (5)

[Claims] 1. An electric double layer capacitor having a positive electrode comprising a polarizable electrode mainly composed of activated carbon, a negative electrode, and an organic electrolytic solution containing a lithium salt as a solute, wherein the negative electrode can occlude and desorb lithium ions. An electric double layer capacitor comprising a carbon material and copper or nickel, and absorbing lithium ions by a chemical method or an electrochemical method. 2. The method according to claim 1, wherein copper or nickel is contained in the negative electrode in an amount of 5 to 50% by weight.
The electric double layer capacitor according to claim 1, which is included. 3. The method according to claim 1, wherein copper or nickel has an average particle size of 0.1 to 50.
3. The electric double layer capacitor according to claim 1, which is a powder of μm. 4. A carbon material capable of occluding and desorbing lithium ions has a [002] plane spacing of 0.335 to 0.35 in X-ray diffraction.
4. The electric double layer capacitor according to claim 1, wherein the thickness is 0.410 nm. 5. A positive electrode / negative electrode having a capacity ratio of 0.001 to 0.9.
The electric double layer capacitor according to claim 1, 2, 3, or 4.