JP2001266872A - Secondary power source and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a secondary power source that has a high voltage resistance, a high capacity and is excellent in rapid charge and discharge cycle characteristics. SOLUTION: The secondary power source comprises a positive electrode composed mainly of activated charcoal, a negative electrode composed mainly of a graphite which has a surface space of 0.335-0.336 nm at (002) face as measured by X-rays diffraction and a crystallite size of Lc of 10-100 nm, La of 10-100 nm and which stores and separates lithium ion, and an organic electrolyte containing lithium salt.

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 having 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 is described. Japanese Patent Application Laid-Open No. 8-107048 describes a battery in which a carbon material which can occlude and desorb lithium ions by absorbing lithium ions in advance by a chemical method or an electrochemical method is used as a negative electrode. Also,
Japanese Patent Application Laid-Open No. 9-55342 proposes a secondary power supply having an upper limit voltage of 4 V and having a negative electrode in which a carbon material capable of absorbing and releasing lithium ions is supported on a porous current collector that does not form an alloy with lithium. 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.
A lithium ion secondary battery has a higher voltage and a higher capacity than an electric double layer capacitor, but has a problem that it has a high resistance and has a significantly shorter life due to a rapid charge / discharge cycle than an electric double layer capacitor.

[0005]

SUMMARY OF THE INVENTION Accordingly, the present invention is directed to a secondary power supply capable of rapid charging and discharging, having a high withstand voltage, a high capacity and a high energy density. The purpose is to provide.

[0006]

According to the present invention, there is provided a positive electrode mainly composed of activated carbon, a negative electrode mainly composed of a carbon material capable of inserting and extracting lithium ions, and an organic electrolyte containing a lithium salt. in the next power, wherein the carbon material surface separation d _{00 2} of [002] plane obtained by X-ray diffraction is 0.
Is a 335~0.336nm, crystallite size L _c is 1
Is 0 to 100 nm, and the crystallite size L _a is 10
A secondary power source is provided, which is made of 100 nm graphite.

Further, the present invention provides a method of manufacturing a secondary power supply including a positive 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. In the method, the carbon material has a [002] plane spacing d ₀₀₂ obtained by X-ray diffraction of 0.335 to 0.336 nm and a crystallite size L.
_c and L _a is reacted with concentrated sulfuric acid graphite all at 200nm greater, washed with water, dried, provide a method of manufacturing a secondary power source, characterized in that obtained by rapidly heating the 200 to 800 ° C. I do.

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 occluding and releasing lithium ions with 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 the positive electrode is mainly made of activated carbon and the negative electrode is mainly made of a carbon material capable of absorbing and desorbing lithium ions is simply called a secondary power supply.

Generally, graphite powder is concentrated at 100 to 150 ° C. with concentrated sulfuric acid (mass concentration 95 to 98%) and concentrated nitric acid (specific gravity 1.33).
When 3) is reacted with a mixed acid mixed in a molar ratio of 95: 5 to 70:30, sulfuric acid is absorbed between graphite layers.
When this is sufficiently washed with water and dried, most of the sulfuric acid occluded between the graphite layers is desorbed.
It is known that when heated rapidly to a temperature of 800 ° C., sulfuric acid remaining between the graphite layers slightly decomposes, and it is known that expanded graphite can be obtained by the pressure of the gas generated at that time.

However, in the treatment of the graphite powder,
If the expansion of the graphite is gently caused by a method such as using only concentrated sulfuric acid instead of the mixed acid, the [002] plane spacing d obtained by X-ray diffraction does not completely expand.
₀₀₂ 0.335～0.336Nm, crystallite size L _c is 10 to 100 nm, L _a is 10 to 100 nm of the graphite is obtained. What usually referred to as graphite, if the surface spacing d ₀₀₂ of [002] plane is 0.335～0.336Nm, crystallite size L _c and L _a is greater than 200 nm. That is, the crystallite size can be reduced by the above-described processing without changing the surface spacing of graphite.

In the present invention, for example, the [002] plane spacing d ₀₀₂ obtained by performing the above processing is 0.33.
A 5～0.336Nm, crystallite size L _c and L _a is a negative electrode of the main graphite secondary power is both a 10 to 100 nm. By using the graphite, the secondary power supply of the present invention has less deterioration due to charge / discharge cycles. Generally, when a secondary power supply is charged and discharged, the crystal structure of graphite changes as lithium ions are absorbed and desorbed. Most of the lithium ion occlusion and desorption sites of graphite are on the edge surface, and it is considered that the solid electrolyte interface on the edge surface is damaged by repeated charging and discharging.

In the secondary power supply of the present invention, it is considered that the apparent lithium ion occlusion and desorption sites are reduced due to the small crystallite size of graphite of the negative electrode. Therefore, it is considered that the change in the crystal structure due to occlusion and desorption of lithium ions is relatively small, and the deterioration of the negative electrode due to the charge / discharge cycle is reduced. as a result,
It is considered that the charge / discharge cycle durability of the secondary power supply is improved, and the reliability is improved in the long term.

In general, in the case of 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. Lithium ions are desorbed from the lithium-containing transition metal oxide of the positive electrode, are stored in a carbon material capable of occluding and desorbing lithium ions of the negative electrode, lithium ions are desorbed from the negative electrode by discharging, and 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 anions in the electrolytic solution are adsorbed on the activated carbon of the positive electrode by charging, and the lithium ions in the electrolytic solution are stored in a carbon material capable of storing and releasing lithium ions in 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 a secondary power supply mainly using activated carbon for the positive electrode and a carbon material capable of occluding and releasing lithium ions for the negative electrode, ions dissolved in the electrolyte participate in charging and discharging. Therefore, when the solute concentration of the electrolytic solution is low, there is a possibility that the battery cannot be sufficiently charged. The solute concentration of the electrolyte is preferably 0.5 to 2.0 mol / L, particularly preferably 0.75 to 1.5 mol / L.

In the secondary power supply of the present invention, the cycle efficiency of the negative electrode in the first charge / discharge cycle is not necessarily 100%.
Instead, some occluded lithium ions do not desorb. In this case, since the lithium ion concentration in the electrolyte decreases and charging may not be sufficiently performed from the next charging, it is preferable to add a lithium-containing transition metal oxide to the positive electrode to prevent characteristic deterioration. By this method, lithium ions that cannot be eliminated from the negative electrode can be supplemented. In this case, the content of the lithium-containing transition metal oxide contained in the positive electrode is preferably 0.1 to 20%, more preferably 3 to 15% of the total weight of the positive electrode. If it is less than 0.1%, the effect is small.
If it exceeds 0%, the capacity of the lithium-containing transition metal oxide is large, so that the secondary power supply performance of high output and high reliability, which is a characteristic of the activated carbon electrode, cannot be obtained.

The lithium-containing transition metal oxide is preferably a composite oxide of lithium and at least one transition metal selected from the group consisting of V, Mn, Fe, Co, Ni, Zn and W. In particular, a composite oxide of lithium and at least one selected from the group consisting of Mn, Co, and Ni is preferable. Among them, Li _x Co _y Ni _(1-y) O ₂ or Li _z M
n ₂ O ₄ (however, 0 <x <2, 0 ≦ y ≦ 1, 0 <z <
2. ) Is preferred.

In the present invention, the activated carbon used as the main component of the positive electrode preferably has a specific surface area of 800 to 3000 m ² / g. The raw material and activation conditions of the activated carbon are not limited. For example, the raw material includes bean, phenol resin, petroleum coke and the like, and the activation method includes a steam activation method and a molten alkali activation method. In the present invention, steam activated activated carbon or phenolic resin activated carbon activated with steam is particularly preferred. In addition, in order to reduce the resistance of the positive electrode, it is preferable to include conductive carbon black or graphite as a conductive material in the positive electrode,
At this time, it is preferable that the conductive material is contained in an amount of 0.1 to 20% in the total mass of the positive electrode.

As a method of manufacturing a positive electrode body, for example, polytetrafluoroethylene as a binder is mixed with activated carbon powder, kneaded, and then formed into a sheet to obtain a positive electrode, which is used as a current collector with a conductive adhesive. There is a way to fix. Further, using polyvinylidene fluoride, polyamide imide, polyimide or the like as a binder, the activated carbon powder is dispersed in a solution in which these are dissolved in a solvent, and this solution is coated on a current collector by a doctor blade method or the like, and dried. You may get it. The amount of the binder contained in the positive electrode may be 1 to 20 times the total weight of the positive electrode, based on the balance between the strength of the positive electrode body and characteristics such as capacity.
%.

The negative electrode body of the present invention is obtained by dispersing the carbon material in a solution obtained by dissolving a resin serving as a binder or a precursor thereof in an organic solvent, using polyvinylidene fluoride, polyamideimide or polyimide as a binder. It is preferable to obtain by coating and drying. In the above method, the solvent for dissolving the resin serving as the binder or the precursor thereof is not limited. However, the resin constituting the binder or the precursor thereof can be easily dissolved and is easily available.
-Methyl-2-pyrrolidone (hereinafter referred to as NMP) is preferred. Here, the term “polyamide imide precursor” or “polyimide precursor” refers to those which are polymerized by heating to form polyamide imide or polyimide, respectively.

The above-mentioned binders are cured by heating and are 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 133 Pa or less. Polyamide imide or polyimide is resistant to the organic electrolyte used in the present invention, and is used to remove water from the negative electrode.
It is sufficiently resistant to high temperature heating of about 00 ° C. or heating under reduced pressure.

The lithium contained in the organic electrolyte according to the present invention
Um salt is LiPF₆, LiBF_Four, LiClO_Four, Li
N (SO_TwoCF_Three)_Two, CF_ThreeSO_ThreeLi, LiC (SO_TwoC
F_Three) _Three, LiAsF₆And LiSbF₆Selected from the group consisting of
One or more types are preferred. Solvent is ethylene carbonate
G, propylene carbonate, butylene carbonate,
Dimethyl carbonate, ethyl methyl carbonate, di
Ethyl carbonate, sulfolane and 1,2-dimethoxy
One or more selected from the group consisting of cyethanes is preferred.
The electrolyte consisting of these lithium salts and solvents has a withstand voltage.
High electrical conductivity.

[0023]

Next, the present invention will be described more specifically with reference to Examples (Example 1) and Comparative Examples (Example 2), but the present invention is not limited thereto. The production and measurement of the cells in Examples 1 to 3 were all performed in an argon glove box having a dew point of −60 ° C. or less.

Example 1 Activated carbon having a specific surface area of 2000 m ² / g, conductive carbon black, and polytetrafluoroethylene as a binder obtained by a steam activation method using a phenol resin as a raw material, and polytetrafluoroethylene as a binder were 8: 1: 1 by mass ratio. Was added to the mixture contained in the above, kneaded by adding ethanol, rolled, and then vacuum dried at 200 ° C. for 2 hours to obtain an electrode sheet. From this electrode sheet, size 6cm x 3cm, thickness 1
A 50 μm electrode was obtained, joined to an aluminum foil using a conductive adhesive having polyamideimide as a binder, and heat-treated at 300 ° C. for 2 hours under reduced pressure to obtain a positive electrode body.

Next, occluding lithium ions, as a carbon material capable of desorbed surface spacing d ₀₀₂ of the X-ray diffraction [002] plane is 0.3357nm, L _a is 350 nm, L _c is 3
Using 40 nm graphite, it was mixed with concentrated sulfuric acid having a mass concentration of 98%, and kept at 120 ° C. for 5 hours. Then wash thoroughly with water,
Heated rapidly to 300 ° C. and analyzed by X-ray diffraction [002]
Plane spacing d ₀₀₂ of face 0.3357nm, L _a is 70 nm,
L _c is obtained of 70 nm, it absorbs lithium ions, a graphite capable eliminated.

Next, the obtained graphite and d ₀₀₂ were 0.33
A 7 nm graphitized vapor grown carbon fiber (hereinafter referred to as VGCF) is dispersed in a solution of polyvinylidene fluoride in NMP, applied to a current collector made of copper, and dried. Thus, a negative electrode body having a negative electrode layer formed on a current collector was obtained. The mass ratio of graphite: graphitized VGCF: polyvinylidene fluoride in the negative electrode layer was 8: 1: 1. This negative electrode body was further pressed by a roll press. The obtained negative electrode layer had a size of 6 cm × 3 cm and a thickness of 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, and LiBF ₄ is mixed with ethylene carbonate and diethyl carbonate in a mixed solvent (volume ratio of 1 mol / L) so as to be 1 mol / L. 1: 1)
The battery was impregnated with the electrolytic solution dissolved in a sufficient amount of time to obtain a secondary power source. After measuring the initial capacity of this secondary power supply, charge and discharge current 1
A charge / discharge cycle test was performed at a range of 4.2 V to 2.75 V at 80 mA, the capacity after 2,000 cycles was measured, and the capacity change rate was calculated. Table 1 shows the results.

[Example 2] As a carbon material capable of inserting and extracting lithium ions, graphite-based carbon having _a [002] plane spacing d ₀₀₂ of 0.3357 nm, La of 350 nm, and L _c of 340 nm as determined by X-ray diffraction. A negative electrode layer having a size of 6 cm × 3 was obtained in the same manner as in Example 1 except that the material was not treated with sulfuric acid.
cm and a thickness of 26 μm. A secondary power supply was prepared in the same manner as in Example 1 except that the above-mentioned negative electrode body was used, and evaluated in the same manner as in Example 1. Table 1 shows the results.

[0029]

[Table 1]

[0030]

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

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Claims (3)

[Claims] 1. A secondary power supply comprising: a positive electrode mainly composed of activated carbon; a negative electrode mainly composed of a carbon material capable of inserting and extracting lithium ions; and an organic electrolyte containing a lithium salt, wherein the carbon material is [00] obtained by X-ray diffraction
Plane spacing d ₀₀₂ of 2] surface is 0.335～0.336Nm, and wherein the crystallite size L _c is 10 to 100 nm, and the crystallite size L _a is made of graphite is 10 to 100 nm Secondary power supply. 2. A positive electrode mainly composed of activated carbon, and a lithium ion
A negative electrode mainly composed of a carbon material capable of occluding and releasing ions;
Manufacture of a secondary power supply comprising: an organic electrolytic solution containing a titanium salt
In the method, the carbon material is obtained by X-ray diffraction.
[002] plane spacing d ₀₀₂Is 0.335-0.3
36 nm and crystallite size L_cAnd L_aBut both
React graphite with over 200 nm with concentrated sulfuric acid, wash with water, dry
After drying, heat rapidly to 200-800 ° C.
A method for manufacturing a secondary power supply, comprising: 3. The carbon material has a [002] plane spacing d ₀₀₂ obtained by X-ray diffraction of 0.335 to 0.336.
is nm, the crystallite size L _c is 10 to 100 nm, and the secondary power supply method according to claim 2 crystallite size L _a is graphite is 10 to 100 nm.