JP2000030745A - Secondary power source - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize a high withstand voltage and a high capacity, and enhance fast charge/discharge cycle reliability, by making up a secondary power source out of a positive electrode mainly composed of activated carbon and having a thickness of a specific value, a negative electrode mainly composed of a carbon material capable of storing/desorbing lithium ions and having a thickness of a specific percentage of the thickness of the positive electrode, and an organic electrolyte including a lithium salt. SOLUTION: A positive electrode mainly composed of activated carbon and a negative electrode mainly composed of a carbon material capable of storing/desorbing lithium ions are disposed opposite to each other interposing a separator therebetween. The thickness of the positive electrode is 80 to 250 μm (preferably, 100 to 220 μm), and the thickness of the negative electrode, oppositely disposed thereto interposing the separator therebetween, is 7 to 60% of the thickness of the positive electrode. As the active carbon included in the positive electrode, a coconut-husk activated carbon activated by steam, or a phenol-resin activated carbon activated by steam, is used, and the carbon material used for the negative electrode has a [002] surface spacing of 0.335 to 0.410 nm by X-ray diffraction measurement. An electrolyte solvent including propylene carbonate of 70 wt.% or more is used.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a secondary power supply having a high withstand voltage, a large capacity, and a high rapid charge / discharge cycle reliability.

[0002]

2. Description of the Related Art Polarizable electrodes mainly composed of activated carbon are used for both positive and negative electrodes of conventional electric double layer capacitors. The withstand voltage of the electric double layer capacitor is 1.2 V when an aqueous electrolyte is used, and 2.5 to 3.3 V when an organic electrolyte is used. Since the energy of the electric double layer capacitor is proportional to the square of the withstand voltage, the organic electrolyte having a higher withstand voltage has higher energy than the aqueous electrolyte. However, even an electric double layer capacitor using an organic electrolyte has an energy density of 1/10 or less of a secondary battery such as a lead storage battery, and further improvement in energy density is required.

On the other hand, Japanese Patent Application Laid-Open No. 64-14882 discloses that an electrode mainly composed of activated carbon is used as a positive electrode, and the [002] plane spacing by X-ray diffraction is 0.338 to 0.356 nm.
A secondary power supply with an upper limit voltage of 3 V using an electrode in which lithium ions are previously absorbed in a carbon material as a negative electrode,
JP-A-8-107048 describes that lithium ions are occluded,
A battery using a carbon material in which lithium ions are previously absorbed by a desorbable carbon material by a chemical method or an electrochemical method as a negative electrode is disclosed in Japanese Patent Application Laid-Open No. 9-55342. A secondary power supply having an upper limit voltage of 4 V and having a negative electrode in which a material is supported on a porous current collector that does not form an alloy with lithium has been proposed. However, these secondary power supplies have a problem that a step of previously storing lithium ions in the carbon material of the negative electrode is required.

In addition to the electric double layer capacitor, a power source capable of charging and discharging a large current is a lithium ion secondary battery.
Lithium-ion secondary batteries have higher voltage and higher capacity than electric double-layer capacitors, but have the problems of high resistance and a significantly shorter life due to rapid charge / discharge cycles than electric double-layer capacitors.

[0005]

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a secondary power supply capable of rapid charge / discharge, high withstand voltage, high capacity, high energy density, and high charge / discharge cycle reliability. .

[0006]

According to the present invention, there is provided a positive electrode mainly composed of activated carbon having a thickness of 80 to 250 μm, and a carbon material mainly composed of a carbon material capable of occluding and releasing lithium ions having a thickness of 7 to 7 times the thickness of the positive electrode. A secondary power supply, comprising: a 60% negative electrode; and an organic electrolyte containing a lithium salt.

[0007] In the present specification, a negative electrode body is formed by joining and integrating a negative electrode mainly composed of a carbon material capable of inserting and extracting lithium ions and a current collector. The same definition applies to the positive electrode body. Further, both the secondary battery and the electric double layer capacitor are one type of secondary power supply, but in this specification,
A secondary power supply having a specific configuration in which a positive electrode contains activated carbon and a negative electrode contains a carbon material capable of inserting and extracting lithium ions is simply referred to as a secondary power supply.

In a secondary power supply in which a positive electrode mainly composed of activated carbon and a negative electrode mainly composed of a carbon material capable of inserting and extracting lithium ions are opposed to each other with a separator interposed therebetween, the thickness of the positive electrode and that of the negative electrode are almost the same. Then, the negative electrode capacity becomes larger than the positive electrode capacity. Therefore, when the secondary power supply is sufficiently impregnated with an electrolyte and charged, the potential of the negative electrode does not become low, and the secondary power supply cannot be a high-voltage secondary power supply. However, if the thickness of the negative electrode is made sufficiently smaller than the thickness of the positive electrode to make the capacity of the positive electrode equal to that of the negative electrode, the potential of the negative electrode becomes low when charging by impregnating the electrolyte sufficiently, and a high-voltage secondary power supply is required. Can be configured.

In a lithium ion secondary battery, the positive electrode is an electrode mainly composed of a transition metal oxide containing lithium, and the negative electrode is an electrode mainly composed of a carbon material capable of absorbing and desorbing lithium ions. Is released from the lithium-containing transition metal oxide, and is stored in a carbon material capable of storing and releasing lithium ions of the negative electrode. The lithium ions are released from the negative electrode by discharge, and the lithium ions are stored in the positive electrode. Therefore, lithium ions in the electrolyte do not essentially participate in charging and discharging of the battery.

On the other hand, in the secondary power supply of the present invention, the anion in the electrolytic solution is adsorbed on the activated carbon of the positive electrode by charging, and the lithium ions in the electrolytic solution are occluded in the carbon material capable of absorbing and desorbing the lithium ions of the negative electrode. You. Then, lithium ions are desorbed from the negative electrode by discharging, and the anions are desorbed in the positive electrode. That is, in the secondary power supply of the present invention, the solute of the electrolytic solution is essentially involved in the charging and discharging, and the charging and discharging mechanism is different from that of the lithium ion battery. Further, unlike the lithium ion secondary battery, since the lithium ion is not inserted or extracted from the positive electrode active material itself, the secondary power supply of the present invention has excellent charge / discharge cycle reliability.

In the present invention, the thickness of the positive electrode is 80 to 25.
0 μm, and preferably 100 to 220 μm.
If it is less than 80 μm, the capacity of the secondary power supply cannot be increased. On the other hand, when the thickness exceeds 250 μm, the resistance increases during charging and discharging, so that it is not practical for rapid charging and discharging. In the present invention, the thickness of the negative electrode opposed to the thickness of the positive electrode via the separator is 7 to 60%, preferably 10 to 40%.
It is. By setting the ratio of the thickness of the positive electrode to the thickness of the negative electrode in this range, the capacity of the positive electrode and the capacity of the negative electrode can be balanced, and a secondary power supply with high withstand voltage can be configured.

The thickness of the negative electrode is specifically 10 to 150 μm.
m is preferred. A negative electrode having a thickness of less than 10 μm is difficult to form. Particularly preferably, the thickness of the positive electrode is 100 to 20.
0 μm and the thickness of the negative electrode is 10 to 50 μm. In addition,
In the present invention, the positive electrode and the negative electrode may be formed on one side or both sides of the current collector. It indicates the thickness of the electrode layer formed on one side of the body.

In the present invention, the activated carbon contained in the positive electrode preferably has a specific surface area of 800 to 3000 m ² / g. Activated carbon raw materials and activation conditions are not limited,
For example, as raw materials, coconut, phenolic resin, petroleum coke, and the like can be mentioned. As the activation method, a steam activation method,
A molten alkali activation method and the like can be mentioned. In the present invention,
Steam activated charcoal activated carbon or steam activated phenolic resin activated carbon is preferred. In addition, in order to lower the resistance of the positive electrode, it is preferable to include conductive carbon black or graphite as a conductive material in the positive electrode. In this case, the conductive material is 0.1 to 20% by weight in the positive electrode. Is preferred.

As a method for producing a positive electrode body, for example, polytetrafluoroethylene as a binder is mixed with activated carbon powder, kneaded, and then molded into a sheet to form a positive electrode, which is used as a current collector with a conductive adhesive. There is a way to fix. Further, a dispersion of activated carbon powder and lithium-containing transition metal oxide powder in a varnish in which polyvinylidene fluoride, polyamideimide, polyimide, etc. are dissolved as a binder, and applying this liquid on a current collector by a doctor blade method or the like, It may be obtained by drying. The amount of binder contained in the positive electrode is
1-20 from the balance between the strength of the positive electrode body and characteristics such as capacity
% By weight.

The carbon material capable of inserting and extracting lithium ions according to the present invention is obtained by X-ray diffraction measurement [002].
It is preferable that the spacing between the surfaces is 0.335 to 0.410 nm. A carbon material having a plane spacing of more than 0.410 nm is liable to be deteriorated in a charge / discharge cycle. Specific examples include petroleum coke, a material obtained by heat-treating mesophase pitch-based carbon material or vapor-grown carbon fiber at 800 to 3000 ° C., natural graphite, artificial graphite, and non-graphitizable carbon material. In the present invention, any of these materials can be preferably used.

When a non-graphitizable carbon material or a carbon material obtained by treating petroleum coke at a low temperature is used, the resistance can be reduced by using a vapor-grown carbon mixed with a graphitic carbon material such as a graphitized material. It is preferred. In this case, the weight ratio of the non-graphitizable carbon material or the like to the graphitic carbon material is 9%.
The ratio is preferably 5: 5 to 70:30. If the graphite carbon material is less than 5%, the effect of reducing the resistance cannot be exhibited, and
If it exceeds 0%, the capacity of the negative electrode decreases.

The negative electrode body of the present invention can be obtained by kneading polytetrafluoroethylene as a binder, forming the same into a sheet, and adhering it to a current collector using a conductive adhesive, similarly to the positive electrode body. Further, polyvinylidene fluoride, polyamideimide or polyimide as a binder, the carbon material is dispersed in a solution in which a resin serving as a binder or a precursor thereof is dissolved in an organic solvent, applied to a current collector, and dried. There are ways. All of these methods are preferred.

In the method obtained by applying the negative electrode layer to the current collector, the solvent for dissolving the resin serving as the binder or the precursor thereof is not limited, but the resin constituting the binder or the precursor thereof can be easily dissolved. N-methyl-2-pyrrolidone (hereinafter referred to as NMP) because it is easily available
Is preferred. Here, a precursor of polyvinylidene fluoride,
The term “polyamideimide precursor” or “polyimide precursor” refers to those which are polymerized by heating to become polyvinylidene fluoride, polyamideimide or polyimide, respectively.

The binder obtained as described above is cured by heating and is excellent in chemical resistance, mechanical properties and dimensional stability. The temperature of the heat treatment is preferably 200 ° C. or higher. If the temperature is 200 ° C. or higher, even if it is a polyamideimide precursor or a polyimide precursor, it is usually polymerized to be a polyamideimide or a polyimide, respectively. The atmosphere for the heat treatment is preferably an inert atmosphere such as nitrogen or argon, or a reduced pressure of 1 torr or less. Polyamide imide or polyimide has resistance to the organic electrolyte used in the present invention, and is sufficiently resistant to high temperature heating of about 300 ° C. or heating under reduced pressure to remove water from the negative electrode.

In the present invention, if an adhesive layer made of polyamideimide or polyimide is interposed between the negative electrode and the current collector, the adhesive force between the negative electrode and the current collector becomes stronger. In this case, a varnish obtained by previously dissolving polyamideimide, polyimide or their precursors in a solvent on a current collector is applied by a coating method such as a doctor blade method, and dried to form an adhesive layer. A negative electrode is formed. Also, if a conductive material such as copper and graphite is dispersed in a varnish forming an adhesive layer,
This is preferable because the contact resistance between the negative electrode and the current collector can be reduced.
The varnish containing this conductive material can also be used as a conductive adhesive interposed between the positive electrode and the current collector when the positive electrode is formed into a sheet.

The lithium salt contained in the organic electrolyte according to the present invention is LiPF ₆ , LiBF ₄ , LiClO ₄ , L
iN (SO ₂ CF ₃ ) ₂ , CF ₃ SO ₃ Li, LiC
_{(SO 2 CF 3) 3,} LiAsF 6 and one or more selected from the group consisting of LiSbF ₆ are preferred. The solvent preferably contains at least one selected from the group consisting of ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, sulfolane, and dimethoxyethane. Electrolyte solutions composed of these lithium salts and solvents have high withstand voltage and high electric conductivity. The concentration of the lithium salt in the electrolyte is preferably 0.1 to 2.5 mol / L, more preferably 0.5 to 2 mol / L.

Among the above-mentioned solvents, a solvent containing propylene carbonate is particularly preferable. Activated carbon is particularly stable with respect to propylene carbonate among the above-mentioned solvents, and it is preferable that propylene carbonate be contained in an electrolyte solution solvent in an amount of 70% by weight or more. A particularly preferred electrolytic solution is a propylene carbonate solution containing LiBF ₄ which is well combined with activated carbon of the positive electrode, and has excellent charge / discharge cycle characteristics and voltage application characteristics.

[0023]

Next, the present invention will be described more specifically with reference to Examples (Examples 1 to 3) and Comparative Examples (Example 4), but the present invention is not limited thereto. The production and measurement of the cells in Examples 1 to 4 were all performed in an argon glove box having a dew point of −60 ° C. or less.

Example 1 80% by weight of activated carbon having a specific surface area of 2000 m ² / g, obtained by a steam activation method using a phenolic resin as a raw material, 10% by weight of conductive carbon black, and 10% by weight of polytetrafluoroethylene as a binder Is kneaded by adding ethanol, rolled, and then vacuum-dried at 200 ° C. for 2 hours to form a mixture having a thickness of 1 μm.
An electrode sheet of 50 μm was obtained. 4c from this electrode sheet
Obtain an electrode of mx 6 cm, joined to aluminum foil using a conductive adhesive with polyamideimide as a binder,
Heat treatment was performed at 300 ° C. for 2 hours under reduced pressure to obtain a positive electrode body.

Next, a petroleum coke-based carbon material was
A carbon material capable of occluding and desorbing lithium ions was obtained by heat treatment at ℃. The plane interval of the [002] plane of this carbon material by X-ray diffraction was 0.341 nm. In a solution in which polyvinylidene fluoride is dissolved in NMP, a carbon material capable of absorbing and desorbing lithium ions and a material obtained by graphitizing vapor-grown carbon at 3000 ° C. are dispersed and applied to a current collector made of copper. After drying, a negative electrode body was obtained. The weight ratio of the carbon material capable of occluding and releasing lithium ions in the negative electrode, the material obtained by graphitizing vapor-grown carbon, and polyvinylidene fluoride was 8: 1: 1. This negative electrode body was further pressed by a roll press. The obtained negative electrode had an area of 6 cm × 4.
cm, and the thickness was 15 μm.

The positive electrode body and the negative electrode body obtained as described above are opposed to each other with a polypropylene separator interposed therebetween.
A secondary power source was obtained by impregnating a propylene carbonate solution containing 1 / L LiBF ₄ for a sufficient time. After measuring the initial capacity of this secondary power supply, 4.2 V at a charging / discharging current of 240 mA.
Charge-discharge cycle test in the range from
The capacity after 00 cycles was measured, and the capacity change rate was calculated. Table 1 shows the results.

Example 2 A positive electrode was obtained in the same manner as in Example 1 except that coconut was used instead of the phenol resin as a raw material for the activated carbon of the positive electrode. A secondary power supply was prepared in the same manner as in Example 1 except that this positive electrode body was used, and the measurement was performed in the same manner as in Example 1. Table 1 shows the results.

Example 3 A mixture of polyvinylidene fluoride and polyamide imide (weight ratio of 1:
Using 1), the composition of the negative electrode was the same as in Example 1 such that the carbon material capable of absorbing and desorbing lithium ions, the material obtained by graphitizing the vapor-grown carbon, and the binder were in a weight ratio of 7: 1: 1. A negative electrode body having a negative electrode having an area of 6 cm × 4 cm and a thickness of 15 μm was obtained in the same manner as in Example 1 except for performing the above. A secondary power supply was manufactured in the same manner as in Example 1 except that this negative electrode body was used, and the measurement was performed in the same manner as in Example 1. Table 1 shows the results.

Example 4 A secondary power supply was prepared in the same manner as in Example 1 except that the thickness of the positive electrode was set to 300 μm, and the thickness of the negative electrode was set to 200 μm, and measurement was performed in the same manner as in Example 1. Table 1 shows the results.

[0030]

[Table 1]

[0031]

According to the present invention, since the capacity of the positive electrode and the capacity of the negative electrode can be balanced, it is possible to provide a secondary power supply having a high withstand voltage, a large capacity, and a high rapid charge / discharge cycle reliability.

Continuation of the front page (51) Int.Cl. ⁷ Identification code FI Theme coat II (reference) // C07D 317/36 C07D 317/36 F term (reference) 5H003 AA01 AA02 AA04 BA00 BB02 BD00 BD02 BD03 BD04 5H014 AA02 BB00 EE08 HH01 HH06 5H029 AJ02 AJ03 AJ05 AK08 AL06 AM03 AM07 CJ11 DJ09 EJ11 HJ01 HJ04 HJ13

Claims (4)

[Claims] 1. A thickness mainly of activated carbon of 80 to 250 μm.
A positive electrode, a negative electrode mainly composed of a carbon material capable of inserting and extracting lithium ions and having a thickness of 7 to 60% of the thickness of the positive electrode, and an organic electrolyte solution containing a lithium salt. Secondary power supply. 2. The secondary power supply according to claim 1, wherein the solvent of the organic electrolyte contains propylene carbonate in an amount of 70% by weight or more. 3. The secondary power source according to claim 1, wherein the carbon material has a [002] plane spacing of 0.335 to 0.410 nm. 4. The secondary power source according to claim 1, wherein the activated carbon of the positive electrode is steam activated activated carbon or phenolic resin activated carbon.