JP2003142155A - Energy storage element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an energy storage element having a high energy density like a secondary battery and an excellent output characteristic and charge- discharge cycle characteristic like a capacitor. SOLUTION: The energy storage element is composed of a positive electrode consisting of metal oxide admitting electrochemical insertion and separation of lithium ions and containing carbon having a specific surface area of no less than 1000 m<2> /g, a negative electrode containing lithium, and an electrolytic solution containing Degree 4 ammonium salt or Degree 4 phosphonium salt, wherein the metal oxide is an oxide of at least one of the metals belonging to the extent between the 4th and 6th period and between Group 3 and 12 in the Periodic Table.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an energy storage element, and more particularly to an organic electrolyte type energy storage element having both a redox capacity using lithium ion as an active material medium and an electrostatic capacity due to an electric double layer.

[0002]

2. Description of the Related Art At present, secondary batteries such as lithium-ion batteries, nickel-hydrogen batteries and lead batteries, and capacitors and capacitors are mainly used as reversible stores of electric energy. In particular, a secondary battery typified by a lithium-ion battery has a high energy density and is widely used as a power source for various mobile devices.

[0003]

The secondary battery is for converting an electrochemical redox reaction of a chemical substance into electric energy, and the ionized reaction medium is a positive electrode active material and a negative electrode active material via an electrolytic solution. The reaction that moves into the crystal lattice and layer of is used. Therefore, there is a problem in that the mobility of the ions is limited and it is difficult to perform a rapid charge / discharge reaction, and as a result, the output characteristics are not always satisfactory.

In order to solve this problem, various measures have been taken, such as thinning the electrodes, which certainly improves the output characteristics of the battery. However, a lithium ion battery using lithium as a reaction medium is not desirable from a safety point of view because the thinning of the electrodes increases the risk of smoke and ignition due to overcharge and internal short circuit.

On the other hand, the capacitor is a conventional capacitor having a higher capacity, and uses an electric double layer generated at the interface between the electrode and the electrolytic solution as an energy source. Therefore, unlike the secondary battery, the active material medium does not move three-dimensionally into the bulk, energy can be taken in and out by rapid charge and discharge, and the cycle does not occur because an oxidation-reduction reaction does not occur. It has the advantage of almost no deterioration.

However, the capacitor cannot achieve the high energy density of the secondary battery because the redox reaction does not occur, and the energy density of the lithium ion battery is 100.
While it is Wh / kg or more, there is a problem that the energy density of the electric double layer capacitor is about 1 to 2 Wh / kg.

Therefore, the present invention solves the above-mentioned conventional problems, and is a novel energy storage device having a high energy density like a secondary battery and excellent output characteristics and charge / discharge cycle characteristics like a capacitor. The purpose is to provide.

[0008]

Means for Solving the Problems As a result of intensive studies by the present inventors, for example, in addition to a redox capacity due to an electrochemical redox reaction using lithium as a reaction medium, static electricity due to an electric double layer at an interface between an electrode and an electrolyte solution is obtained. It was thought that the above-mentioned conventional problems could be solved if the capacitance could be utilized in one element. And, an electrode in which carbon having a high specific surface area is added to a metal oxide capable of electrochemical insertion and desorption of lithium ions and a salt not containing a lithium salt such as a quaternary ammonium salt or a quaternary phosphonium salt are mainly used. It has been found that it is possible to provide an energy storage element having both redox capacity and electrostatic capacity by combining with an organic electrolyte solution containing the supporting electrolyte described above.

That is, to achieve the above object, the energy storage device of the present invention comprises a metal oxide capable of electrochemically inserting and releasing lithium ions and has a specific surface area of at least 1000 m ² / g. It is characterized by comprising a positive electrode containing carbon, a negative electrode containing lithium, and an electrolytic solution containing a quaternary ammonium salt or a quaternary phosphonium salt as an electrolyte.

Further, in the energy storage device of the present invention, the metal oxide is at least one kind selected from metals belonging to Groups 4 to 6 and Groups 3 to 12 of the periodic table. It is preferable that the metal oxide is.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the energy storage element of the present invention will be described below. In the present invention, as the metal oxide that is the active material that is the basis of the redox capacity, oxides of metals that belong to the fourth to sixth periods and the groups 3 to 12 of the periodic table can be used. . Specifically, for example, Sc, Ti, V, Cr, Mn, Fe, C
o, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc,
Ru, Pd, Ag, Cd, lanthanoid, Hf, Ta,
Examples thereof include oxides of metals such as W, Re, Os, Ir, Pt, Au, and Hg, and particularly V, Cr, Mn, Fe, Co,
An oxide of a metal, such as Ni, belonging to the range of the fifth to tenth groups of the fourth period of the periodic table, or at least one of these metals
Composite metal oxides containing more than one kind are preferred, and those metal oxides are more preferably amorphous.

The metal oxide prepared by any method is preferably used, but the one prepared by heating the colloidal solution of the metal oxide is preferable. Further, when a heat treatment is performed after adding a conductive substance to the colloidal solution of the metal oxide, a structure in which the metal oxide is coated on the surface of the conductive substance is obtained, and the utilization rate of the metal oxide as the active material is increased. It is more preferable because it becomes higher and the capacity can be easily increased.

The metal oxide colloidal solution is prepared by
Usually, it is difficult to directly convert the metal oxide itself into a colloidal solution, so either mix the metal powder with a solution containing an oxidizing agent such as hydrogen peroxide, or use metal acetate, nitrate, carbonate, etc. It is preferable to mix and adjust with a solution containing an oxidizing substance.

In addition, as the conductive substance, a material,
The shape and the like are not limited, but a carbonaceous material is preferably used substantially. Specific examples include carbon black, amorphous carbon, carbon fiber, natural graphite and artificial graphite. Further, the mixing ratio of the metal oxide and the conductive substance is 70:30 to 10:90 in terms of mass ratio.
Is preferred, and particularly 50:50 to 25:75 is more preferred. This is because if it is within this range, good discharge capacity and conductivity can be secured.

When mixing and dispersing the colloidal solution of the metal oxide and the conductive substance, a stirrer,
Any mixing means such as a ball mill and ultrasonic dispersion can be adopted. Further, the temperature and time during the mixing are not particularly limited, but are, for example, 0 to 40.
It is preferable to mix and disperse at a temperature of about 1 to 12 hours.

The heat treatment after mixing and dispersing may be performed after the mixture of the metal oxide and the conductive substance is separated from the dispersion liquid to some extent by filtration, centrifugation, or the like, or the mixed dispersion liquid may be used as it is. You can go. The conditions for this heat treatment are not particularly limited, but the temperature is preferably 50 ° C. or higher, more preferably 80 ° C. or higher, and 45
The temperature is preferably 0 ° C or lower, more preferably 300 ° C or lower. Especially when a carbonaceous material is used as the conductive substance, 4
When the temperature exceeds 50 ° C., an oxidative decomposition reaction of carbon occurs. Therefore, the heat treatment is preferably performed at a lower temperature than that when metal powder is used as the conductive substance, and more preferably 300 ° C. or less. The heating time is preferably 1 hour or longer, more preferably 3 hours or longer, and
It is preferably 24 hours or less, more preferably 10 hours or less.

The positive electrode of the energy storage device of the present invention must contain carbon having a specific surface area of at least 1000 m ² / g in addition to the metal oxide or the complex of the metal oxide and the conductive material. is there. This makes it possible to secure a large capacitance due to the electric double layer. If the specific surface area of the carbon is less than 1000 m ² / g, the capacitance due to the electric double layer becomes small, so that the specific surface area is 10
It is necessary to be at least 00 m ² / g. Particularly, the specific surface area of the carbon is preferably 1500 m ² / g or more. The upper limit value of the specific surface area of the carbon is not particularly limited, but if the specific surface area is too large, the particle size is inevitably too small and the dispersibility is reduced, which deteriorates the homogeneity of the electrode. Therefore, the preferable upper limit of the specific surface area is 3000 m ² / g.

The carbon is, for example, coconut shell,
Examples include phenolic resin, activated carbon made of PAN and the like, activated carbon fiber and the like. The mixing ratio of the metal oxide and the carbon is 90:10 to 10: 9 by mass ratio.
0 is preferable, and 70:30 to 25:75 is particularly preferable.

To prepare an electrode using these, a binder such as polytetrafluoroethylene or polyvinylidene fluoride is added to the metal oxide or the composite of the metal oxide and the conductive material and the carbon. The electrode mixture obtained by mixing may be molded by an appropriate means. For example, the electrode mixture is pressure-molded, or the electrode mixture is dispersed in a solvent to prepare an electrode mixture-containing paste.

When preparing the electrode mixture-containing paste, the binder may be dissolved in a solvent in advance and then mixed with the electrode mixture. Then, the obtained electrode mixture-containing paste is applied to a current collector made of a metal foil, a metal net, or the like, and dried to form a thin film electrode mixture layer, whereby an electrode is produced. However, the manufacturing method of the electrode is not limited to the above-exemplified method, and another method may be used.

Further, the energy storage element of the present invention is characterized in that the electrolytic solution contains a quaternary ammonium salt or a quaternary phosphonium salt. As a result, it becomes possible to oxidize and reduce lithium even in an electrolytic solution containing no lithium salt.

The quaternary ammonium salt is, for example,
(CH ₃ ) ₄ NPF ₆ , (C ₂ H ₅ ) ₄ NPF ₆ , (CH ₃ ) ₄
Examples of NBF ₄ , (C ₂ H ₅ ) ₄ NBF ₄ , and quaternary phosphonium salts include (CH ₃ ) ₄ PPF ₆ and (C
₂ H ₅ ) ₄ PPF ₆ , (CH ₃ ) ₄ PBF ₄ , (C ₂ H ₅ ) ₄ PB
Such as F _4, and the like. These are added to an organic solvent of a cyclic carbonic acid ester such as ethylene carbonate or propylene carbonate or a chain carbonic acid ester such as methyl carbonate or ethyl carbonate to prepare an electrolytic solution. The amount of quaternary ammonium salt or quaternary phosphonium salt added is
0.5 to 2.0 mol / dm ³ is preferable, and 1 to 1.5
More preferred is mol / dm ³ . The electrolytic solution may contain at least one of the quaternary ammonium salt and the quaternary phosphonium salt, and may contain a lithium salt if necessary.

The negative electrode used in the present invention may be any electrode containing lithium, and examples thereof include those containing lithium from the sources such as metallic lithium and lithium-containing transition metal nitrides, graphite and hard carbon. It is possible to use a material such as carbon containing lithium.

As described above, according to the present invention, in addition to having the redox capacity by the doping / dedoping function of lithium ions in the metal oxide, carbon having a specific surface area of at least 1000 m ² / g is used. It is an energy storage element that simultaneously develops the electrostatic capacity due to the electric double layer generated at the interface with the electrolytic solution. That is, it is an element having both the properties of a lithium ion battery and an electric double layer capacitor. INDUSTRIAL APPLICABILITY The energy storage element of the present invention is suitable for a power source of a device that requires a momentary large current and energy capacity and a power source of an apparatus that requires a long-term charge / discharge cycle life.

[0025]

EXAMPLES Next, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to only those examples.

Example 1 1 g of metallic vanadium and 30 substances
100% of hydrogen peroxide water of volume%³And mix in an ice bath for 3
Stir and mix for hours. Leave this for 24 hours at room temperature
Vanadium iodide (V ₂O_Five) Was obtained as a colloidal solution
It was

Obtained vanadium pentoxide (metal oxide)
0.1 g of acetylene black (conductive material) having a specific surface area by nitrogen adsorption of 58 m ² / g determined by the BET 5-point method, ² g of water and 1 g of acetone were added to 5 g of the colloidal solution of, and stirred with a stirrer for 3 hours. The mixture was dispersed to obtain a dispersion liquid. This dispersion was heat-treated at 120 ° C. for 5 hours to obtain a composite powder of vanadium pentoxide and acetylene black. The mass ratio of vanadium pentoxide to acetylene black in this composite was 7:10.

Further, activated carbon having a specific surface area of 2000 m ² / g by nitrogen adsorption determined by the BET 5-point method and an average particle size of 20 μm was added to the complex: the complex: activated carbon = 1: 1.
5% by mass of the dispersion obtained by mixing in a ratio of (mass ratio) and further dispersing polytetrafluoroethylene in water.
In addition, the mixture was kneaded in a mortar. This kneaded product was applied to an aluminum foil having a thickness of 20 μm and pressed at a pressure of 294 MPa to obtain an electrode.

The obtained electrode was punched into a circle with a diameter of 10 mm.
Remove it and use it as the working electrode, with a thickness of 1 mm and a diameter of 17
mm disc-shaped lithium is used, ethylene carbonate
Volume ratio of (EC) and diethyl carbonate (DEC)
1: 1 mixed solvent (C ₂H_Five)_FourNBF_Four(Fourth grade Anne
Monium salt) 1 mol / dm³Dissolve to the concentration of
Using the prepared electrolyte, diameter 20mm, height
A 1.6 mm coin type lithium battery was produced.

This battery was subjected to a constant current charge / discharge test at a charge cut voltage of 4.2 V and a discharge cut voltage of 2.0 V at a current density of 1 mA / cm ² . The test was conducted at 25 ° C. FIG. 1 shows the discharge curve at the fifth cycle. In FIG. 1, the vertical axis represents the battery voltage and the horizontal axis represents the discharge capacity per mass of the electrode active material excluding the mass of the aluminum foil. Further, FIG. 3 shows the capacity transition when charging and discharging are repeated 1000 times under the same conditions as above. In FIG. 3, the vertical axis represents the capacity retention rate obtained by dividing the discharge capacity of each cycle by the discharge capacity of the first cycle, and the horizontal axis represents the number of charge / discharge cycles.

Example 2 Ethylene carbonate (E
C) and diethyl carbonate (DEC) in a volume ratio of 1:
Same as Example 1 except that an electrolytic solution prepared by dissolving (CH ₃ ) ₄ PBF ₄ (quaternary phosphonium salt) in the solvent mixed in 1 to a concentration of 1.5 mol / dm ³ was used. A coin-type lithium battery was manufactured and a charge / discharge test was performed under the same conditions as in Example 1. FIG. 2 shows the discharge curve at the fifth cycle.

(Comparative Example 1) A coin type lithium battery was prepared in the same manner as in Example 1 except that the composite prepared in Example 1 was not mixed with activated carbon, and charged under the same conditions as in Example 1. A discharge test was conducted. FIG. 1 shows the discharge curve at the fifth cycle.

Comparative Example 2 Ethylene carbonate (E
C) and diethyl carbonate (DEC) in a volume ratio of 1:
A coin-type lithium battery was produced in the same manner as in Comparative Example 1 except that an electrolytic solution prepared by dissolving LiPF ₆ in the solvent mixed in 1 to have a concentration of 1 mol / dm ³ was used. A charge / discharge test was conducted under the same conditions as described above. FIG. 2 shows the discharge curve at the fifth cycle. Further, FIG. 3 shows the capacity transition when charging and discharging are repeated 1000 times as in the case of Example 1.

As can be seen from FIGS. 1 and 2, the discharge curves of Examples 1 and 2 are 4 V higher than those of Comparative Examples 1 and 2.
The new discharge capacity is shown in the vicinity. Since a linear potential change is shown in the vicinity of 4 V, it is presumed that the discharge capacity in the vicinity of this is due to the electrostatic capacity of the electric double layer. Further, it can be seen that in the region of 3.5 V or less, the redox capacity due to the reduction of lithium is shown, as can be seen by comparing with the discharge curves of Comparative Examples 1 and 2.

Further, as is apparent from FIG.
It can be seen that is superior to Comparative Example 2 in output characteristics and cycle characteristics.

[0036]

As described above, according to the present invention, it is possible to provide an energy storage element having a high energy density like a secondary battery and excellent output characteristics and charge / discharge cycle characteristics like a capacitor.

[Brief description of drawings]

FIG. 1 is a diagram showing a discharge curve at a fifth cycle of Example 1 and Comparative Example 1.

FIG. 2 is a diagram showing discharge curves at the 5th cycle of Example 2 and Comparative Example 2.

FIG. 3 is a diagram showing changes in discharge capacity with respect to charge / discharge cycles of Example 1 and Comparative Example 2.

   ─────────────────────────────────────────────────── ─── Continued front page    F term (reference) 5H029 AJ02 AJ05 AK02 AK06 AK18                       AL01 AL06 AL07 AL08 AL12                       AM03 AM05 AM07 HJ07                 5H050 AA02 AA07 BA16 BA17 CA02                       CA14 CA29 CB01 CB07 CB08                       CB09 HA07

Claims (2)

[Claims] 1. A positive electrode containing carbon having a specific surface area of at least 1000 m ² / g, which is composed of a metal oxide capable of electrochemically inserting and releasing lithium ions, a negative electrode containing lithium, and an electrolyte as an electrolyte. An energy storage element comprising an electrolytic solution containing a quaternary ammonium salt or a quaternary phosphonium salt. 2. The metal oxide is an oxide of at least one kind of metal selected from metals belonging to groups 4 to 6 and groups 3 to 12 of the periodic table. The energy storage element according to claim 1.