JPH1154384A - Electric double layer capacitor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce deterioration of a negative pole by mixing non-graphite baked material and graphite based material to form the negative pole. SOLUTION: An electric double layer capacitor is provided with the following; a positive pole composed of a polarizable electrode whose main body is active carbon, a negative pole whose main body is carbon material capable of occluding and desorbing lithium ions, and organic electrolyte using lithium salt as salute. In the carbon material, lithium ions are desorbed in the mixture of graphite based material and non-graphite based material wherein the spacing of a [002] face is 0.34-0.41 nm, by a chemical method or an electrochemical method. As the non-graphite based material, carbon of easy graphitization, carbon of difficult graphitization, etc., are used. As the graphite based material, natural graphite, artificial graphite, etc., are used. The weight ratio of graphite based material of the negative pole to the non-graphite based material is 0.1-1. The ratio C<+> /C<-> of the electric double layer capacitor is 0,001-0.9. The specific surface area of active carbon used in the positive pole is 800-3000 m<2> /g.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electric double layer capacitor having excellent charge / discharge cycles, high withstand voltage, and 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 a polarizable electrode mainly composed of activated carbon. In this case, the withstand voltage is 1.2 V when an aqueous electrolyte is used, and is 2.5 to 3.3 V when an organic electrolyte is used.

Since the 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 and a carbon material in which lithium ions are previously absorbed by a chemical method or an electrochemical method is used for a negative electrode. I have. Japanese Patent Application Laid-Open No. 9-55342 discloses that the maximum voltage of a negative electrode having a carbon material capable of absorbing and desorbing lithium ions supported on a porous current collector that does not form an alloy with lithium is 4 V
Has been proposed.

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. The withstand voltage of the electric double layer capacitor in which the positive electrode is combined is higher than the withstand voltage of the electric double layer capacitor mainly composed of activated carbon for both the positive electrode and the negative electrode.

[0006]

The discharge characteristics of a three-electrode system using a carbon material capable of absorbing and desorbing lithium ions as a working electrode and lithium as a counter electrode and a reference electrode show that the potential is constant from the beginning of discharge. There are two types, one in which the potential rises sharply at the end of discharge and the other in which the potential rises gently throughout.

The former is a graphite-based material, and the latter is a non-graphite-based material such as graphitizable carbon, non-graphitizable carbon, and low-temperature fired carbon material. Here, the non-graphitizable carbon is obtained by firing a furfuryl alcohol resin or a phenol resin.
The crystallite size is several nm or less, and the density is 1.5 to 1.8 g /
cm ³ carbon material. The easily graphitizable carbon is coke, mesocarbon microbeads, mesophase pitch-based carbon fiber, pyrolytic vapor growth carbon fiber, etc., having a crystallite size of 1.5 to 5 nm and a density of 1.8 to 2. .1g
/ Cm ³ carbon material. Graphite-based materials have high initial Coulomb efficiency, and non-graphite-based materials have low initial Coulomb efficiency.

[0008] Therefore, when an electric double layer capacitor element is manufactured by combining a negative electrode mainly composed of a non-graphite material with a positive electrode mainly composed of activated carbon, and the discharge capacity is measured, the discharge time is increased because the potential of the negative electrode increases. There was a problem that became shorter. Also, when a negative electrode mainly composed of a graphite material is combined with a positive electrode mainly composed of activated carbon to produce an electric double layer capacitor element and the discharge capacity is measured, the discharge time is longer than that of the non-graphite carbon material. At the end of discharge, the potential changes abruptly with respect to the lithium reference electrode.

[0009]

Means for Solving the Problems As a result of repeated studies to solve the above-mentioned problems, the present inventors have found that when a non-graphite material and a graphite material are mixed to form a negative electrode, graphite It was found that the characteristics of the material appeared, the potential was flat and a high capacity could be developed, and in the latter half of the discharge, the characteristics of the non-graphite material appeared, and a gradual change in the potential was observed, and the deterioration of the negative electrode was reduced.

The present invention provides an electric power comprising a positive electrode composed of a polarizable electrode mainly composed of activated carbon, a negative electrode mainly composed of a carbon material capable of occluding and releasing lithium ions, and an organic electrolyte containing a lithium salt as a solute. In the double-layer capacitor, the carbon material is formed by mixing a graphite-based material with a non-graphite-based material having a [002] plane spacing of 0.34 to 0.41 nm by a chemical method or an electrochemical method. The present invention provides an electric double layer capacitor characterized by being a carbon material having occluded therein.

In the present invention, the spacing between [002] planes of the non-graphite material used is 0.34 to 0.41 nm. If it is less than 0.34 nm, the degree of graphitization is large, so that the potential does not change smoothly in the latter half of the discharge. 0.41 nm
When the charge / discharge cycle is repeated, the electric double layer capacitor deteriorates greatly when the charge / discharge cycle is repeated. Preferably, 0.
34 to 0.37 nm.

[002] X-ray diffraction in the present invention
Examples of the non-graphite-based material having a plane spacing of 0.34 to 0.41 nm include graphitizable carbon, non-graphitizable carbon, and low-temperature fired carbon materials. Here, as the graphitizable carbon, for example, pitch, coke or the like is used at 900 to 2000 ° C.
And carbon materials in the process of graphitization. Examples of the non-graphitizable carbon include those obtained by firing polyparaphenylene at 700 to 900 ° C. As the low-temperature calcined carbon material, for example, furfural resin, phenol resin, pitch,
Heat treated at １０００1000 ° C. is exemplified.

Examples of the graphite-based material in the present invention include natural graphite, artificial graphite, graphitized mesocarbon microbeads, and graphitized pitch-based carbon fiber.

In the present invention, the weight ratio of the graphite material to the non-graphite material (weight of the graphite material / weight of the non-graphite material) of the negative electrode is preferably 0.1 to 1. If less than 0.1,
The properties of the non-graphite material dominate the performance of the negative electrode, and the potential does not become flat during discharge. On the other hand, if it exceeds 1, the potential change of the negative electrode at the end of discharge becomes large, and the deterioration becomes large by repeating the charge / discharge cycle.

The capacity of the electric double layer capacitor is given by equation (1). Where C is the cell capacity, C ⁺ is the positive electrode capacity, C ⁻
Is the negative electrode capacity. ^{1 / C = 1 / C +} + 1 / C - ··· Formula 1

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

That is, the capacity of the electric double layer capacitor cell is half of 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. With such a carbon material, the potential of the negative electrode is not so low as compared with the positive electrode, so the withstand voltage as a cell Is not high, and deterioration due to charge / discharge cycles is remarkable,
Furthermore, rapid charging and discharging also becomes difficult. More preferably C ⁺
/ C ^- is 0.01 to 0.2.

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

For example, a negative electrode carbon material is prepared by adding a fluoropolymer resin as a binder, adding ethanol, kneading the mixture, forming a sheet, and adhering to a current collector such as copper or nickel using a conductive adhesive. High capacity is obtained and charge / discharge cycle characteristics are also good. In this case, the fluoropolymer resin is 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 1
5% by weight.

Activated carbon used for the positive electrode is not particularly limited, but one obtained by activating steam, molten KOH, or the like of coconut, phenolic 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 solute lithium salt 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.

[0027]

The following examples (Examples 1 to 7) and comparative examples (Example 8)
The present invention will be described more specifically with reference to Examples 10 to 10, but the present invention is not limited to the following Examples.

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
0 parts by weight, kneaded with ethanol, rolled, and vacuum-dried at 200 ° C. for 2 hours, and then adhered to an aluminum foil with a conductive adhesive to be integrated with the current collector. A positive electrode was obtained. The effective electrode area of this positive electrode is 1
cm ² , the thickness of the electrode layer mainly composed of activated carbon is 250 μm
Met.

The negative electrode was composed of 80 parts by weight of a carbon material obtained by heat-treating petroleum coke at 1000 ° C., 10 parts by weight of natural graphite, and PT.
A negative electrode integrated with a current collector by kneading 10 parts by weight of FE using ethanol, vacuum-drying a sheet obtained by rolling at 200 ° C. for 2 hours, and adhering to a copper foil using a conductive adhesive. I got The effective electrode area is 1 cm ² and the thickness of the electrode layer is 2
It was 00 μm.

The positive electrode and the negative electrode were each a single electrode, and a current of 1 mA was used using a lithium reference electrode in a solution in which 1 mol / L of LiBF ₄ was dissolved in a mixed solvent of ethylene carbonate and ethyl methyl carbonate having a volume ratio of 1: 1. As a result of the evaluation, the positive electrode capacity was 0.583 mAh in the range of 4.25 V to 2.75 V, the negative electrode capacity was 8.684 mAh in the range of 0.005 V to 2 V, and the capacity ratio of the positive electrode to the negative electrode was 0.0677.

Next, a lithium electrode was previously used as a counter electrode, and a constant current of 1 mA and a current of 5 mAh were obtained by an electrochemical method.
Then, lithium ions were occluded by charging the battery until it reached, and a model cell was fabricated by facing the positive electrode via a separator. The initial capacity was measured in the range of 4 V to 2.5 V. Thereafter, this was subjected to a charge / discharge cycle at a charge / discharge current of 10 mA, and the capacity after 2000 cycles was measured.

Example 2 A negative electrode integrated with a current collector was obtained in the same manner as in Example 1 except that artificial graphite was used instead of natural graphite. When the capacity of this negative electrode was measured in the same manner as in Example 1, it was 8.219 mAh, and the capacity ratio of the positive electrode to the negative electrode was 0.0709. A negative electrode was charged and a cell was fabricated in the same manner as in Example 1 except that the above-described negative electrode was used.
The initial capacity and the capacity after 2,000 cycles were measured in the same manner as described above.

Example 3 A negative electrode integrated with a current collector was obtained in the same manner as in Example 1 except that a carbon material heat-treated at 3000 ° C. in mesophase pitch was used instead of natural graphite.
7. The capacity of the negative electrode was measured in the same manner as in Example 1.
210 mAh, and the capacity ratio of the positive electrode to the negative electrode was 0.1 mAh.
0710. A negative electrode was charged and a cell was manufactured 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 The negative electrode was made of petroleum coke of 1000
A negative electrode integrated with a current collector was obtained in the same manner as in Example 1, except that 50 parts by weight of the carbon material heat-treated at 0 ° C. and 40 parts by weight of natural graphite instead of 10 parts by weight of conductive carbon black were used. When the capacity of this negative electrode was measured in the same manner as in Example 1, it was 6.540 mAh, and the capacity ratio of the positive electrode to the negative electrode was 0.0891. A negative electrode was charged and a cell was manufactured 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 5 A negative electrode integrated with a current collector was obtained in the same manner as in Example 1, except that the thickness of the positive electrode was changed to 500 μm and the thickness of the negative electrode was changed to 130 μm. When the capacities of the positive electrode and the negative electrode were measured in the same manner as in Example 1, each was 1.1.
01 mAh and 5.52 mAh, and the capacity ratio of the positive electrode to the negative electrode was 0.199. A negative electrode was charged and a cell was manufactured in the same manner as in Example 1 except that the above-described positive electrode and negative electrode were used, and the initial capacity and the capacity after 2,000 cycles were measured in the same manner as in Example 1.

[Example 6] Polyparaphenylene was used instead of carbon material obtained by heat-treating petroleum coke at 1000 ° C.
A negative electrode integrated with a current collector was obtained in the same manner as in Example 1 except that a carbon material heat-treated at 0 ° C. was used. When the capacity of the negative electrode was measured in the same manner as in Example 1, it was 9.812 mAh, and the capacity ratio of the positive electrode to the negative electrode was 0.0594. Using the above negative electrode, 1
The battery was charged at a constant current of mA until the current reached 6 mAh to absorb lithium ions, and the initial capacity and the capacity after 2,000 cycles were measured in the same manner as in Example 1.

Example 7 A negative electrode integrated with a current collector was obtained in the same manner as in Example 1 except that a carbon material heat-treated at 700 ° C. was used instead of heat-treating petroleum coke at 1000 ° C. When the capacity of this negative electrode was measured in the same manner as in Example 1, it was 9.268 mAh, and the capacity ratio of the positive electrode to the negative electrode was 0.0629. Using the above negative electrode, lithium ions were occluded by being charged in advance with an electrochemical method at a constant current of 1 mA until the current reached 5 mAh.
The initial capacity and the capacity after 2,000 cycles were measured in the same manner as described above.

[Example 8] 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 was used and natural graphite was not added. When the capacity of the 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 fabricated in the same manner as in Example 1 except that the above-described negative electrode was used.
The initial capacity and the capacity after 2,000 cycles were measured in the same manner as described above.

Example 9 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 natural graphite was used, and no carbon material obtained by heat-treating petroleum coke at 1000 ° C. was added. When the capacity of this negative electrode was measured in the same manner as in Example 1, it was 8.740 mAh, and the capacity ratio of the positive electrode to the negative electrode was 0.0667. A negative electrode was charged and a cell was fabricated in the same manner as in Example 1 except that the above-described negative electrode was used.
The initial capacity and the capacity after 2,000 cycles were measured in the same manner as described above.

Example 10 A positive electrode was obtained in the same manner as in Example 1 except that the thickness was changed to 250 μm. Then, the thickness is 70 μm
A negative electrode was obtained in the same manner as in Example 1, except that When the capacities of the positive electrode and the negative electrode were measured in the same manner as in Example 1, they were 2.531 mAh and 2.62 mAh, respectively, and the capacity ratio of the positive electrode to the negative electrode was 0.966. Charge of the negative electrode in the same manner as in Example 1 except that the above positive electrode and negative electrode were used,
A cell was manufactured, and the initial capacity and 200 were obtained in the same manner as in Example 1.
The capacity after 0 cycles was measured.

Table 1 shows the initial capacity of the electric double layer capacitors of Examples 1 to 10 and the rate of change in capacity after 2000 cycles.

[0042]

[Table 1]

[0043]

According to the present invention, it is possible to obtain an electric double layer capacitor having a large initial capacity and a small rate of change in capacity even when charge / discharge cycles are repeated.

Claims (4)

[Claims] An electric double layer comprising a positive electrode comprising a polarizable electrode mainly composed of activated carbon, a negative electrode mainly composed of a carbon material capable of absorbing and desorbing lithium ions, and an organic electrolyte containing a lithium salt as a solute. In the capacitor, the carbon material has a surface distance between the graphite-based material and the [002] plane of 0.34 to 0.34.
An electric double layer capacitor comprising a carbon material in which a mixture with a non-graphite material having a thickness of 0.41 nm is made to occlude lithium ions by a chemical method or an electrochemical method. 2. The electric double layer capacitor according to claim 1, wherein the weight ratio of the graphite-based material to the non-graphite-based material is 1/10 to 1/1. 3. The ratio of the capacity of the positive electrode to the capacity of the negative electrode is:
The electric double layer capacitor according to claim 1, wherein the value is 0.001 to 0.9. 4. The positive electrode has a specific surface area of 800 to 3000 m ^2.
4. The electric double layer capacitor according to claim 1, wherein the electric double layer capacitor comprises activated carbon, conductive carbon black and a binder.