JP3473221B2 - Negative electrode for alkaline storage battery and battery using the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to increasing the capacity of a negative electrode of an alkaline storage battery and improving the performance of a battery using the same, in particular to improving the capacity density.

[0002]

2. Description of the Related Art As a storage battery using an alkaline electrolyte, a nickel-cadmium storage battery using cadmium as a negative electrode active material and a nickel-hydrogen storage battery using a hydrogen storage alloy as an alternative substance to the environmental problem of cadmium have been put into practical use. Development has been advanced for increasing capacity density. However, with the development and commercialization of lithium-ion storage batteries and the portable and cordless use of various electronic devices, storage batteries are expected to be smaller and lighter, and development is urgent.

[0003]

In the conventional negative electrode, when cadmium is used, it is about 250 mAh / g, and when a typical AB ₅ type hydrogen storage alloy is used, it is 300 mAh / g.
The battery is designed by estimating the capacity of about g. However, in order to obtain a battery having a higher energy density than these, it is necessary to obtain a novel negative electrode material capable of electrochemically absorbing and desorbing hydrogen.

The present invention solves such a problem, paying attention to the characteristics of fullerenes capable of occluding and releasing hydrogen, and by modifying them into a compound containing a metal, it is useful for a battery. The object of the present invention is to find a novel negative electrode material, and to provide a negative electrode having a high capacity density and an alkaline storage battery having a high energy density using the same.

[0005]

The present invention provides Rb, Cs,
Provided is a negative electrode for a high capacity density alkaline storage battery using a metal-encapsulated fullerene compound containing at least one kind of a metal group consisting of Sr and Ba, and further, using this negative electrode, a positive electrode using nickel hydroxide, and an alkaline electrolyte. A high-performance alkaline storage battery is provided by constructing a battery provided with.

Further, as the metal-encapsulated fullerene compound, the general formula MxCn (M is Rb, Cs, Sr, Ba
At least one of the metals, x represents a stoichiometric composition ratio, x = 1, n represents the number of carbon atoms, and n =
82) is used.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION Fullerene having a spherical shell-like molecular structure is described in H.H. W. Kroto et al. (Nature, 318, 1
62 (1985)) and C ₆₀ is a typical one, but other fullerenes are C ₇₀ ,
It is formed of C ₈₂ or more carbon atoms, and is composed of a 5-membered ring and a 6-membered ring. Regarding the metal-encapsulated fullerene compound in which a metal such as LaC ₈₂ is incorporated in the spherical shell, Y. Chai et al. (J. Phys, Che
m. , 95, 7564 (1991)). Since this fullerene is spherical, it has an electronic structure different from that of semi-metallic graphite, which has a normal planar structure, and exhibits semiconductor-like properties. The reason is the electronic structure, different electron density states of pi of electrons in the outer and inner sphere, tended to be lower in the high inside the outside, to further sigma orbital is mixed, in a simple SP ² hybrid orbital This is because it cannot be specified and a band gap of about 1.5 eV is generated in the conduction band and the valence band.

Although the electron conductivity and charge transfer of fullerene are inferior to those of graphite, if there is an electron donation from another atom in the conduction band in advance, hydrogen having a large electron affinity is easily transferred to the fullerene, resulting in hydrogen ion. Are easily occluded. This is because, for example, in the material of C ₈ K in which potassium is pre-intercalated into graphite, electrons from potassium are easily contained in the conduction band of graphite, so that it easily donates electrons to hydrogen (charge transfer), and occludes. It is similar to the fact that the above is possible (Enoki et al., Carbon 143, 136, (1990)).

The storage of hydrogen is possible by forming C--H bonds mainly outside the spherical shell-shaped fullerene. Hydrogen ions that have almost no ionic radius can occlude such hydrogen, but due to the 2S orbital closure structure, such as Li ⁺ ions, it is difficult for occluded species to absorb due to steric factors. Is.

It is well known that an element having a low ionization potential is generally an alkali metal or an alkaline earth metal.

The present inventors have paid attention to these matters, and an element effective for giving an electron to the conduction band of fullerene is an electron (5s) having a high ionization potential and a high energy level capable of obtaining a high battery voltage. And 6s orbital) are considered to be alkali metals or alkaline earth metals involved in electron donation. By encapsulating these, it was expected that the battery voltage could be increased and the electron conductivity could be improved, thereby facilitating the absorption and desorption of hydrogen in fullerenes with increased electron density. In the present invention, it was noted that Rb, Cs, Sr, and Ba as an alkali metal or alkaline earth metal are particularly suitable as an element to be included.

The inventors of the present invention have formed a conduction band t _1u orbital (regular icosahedron symmetry year, triple degenerate LUMO (minimum LUMO (minimum)) even if Rb, Cs, Sr, and Ba are included. Electrons enter the orbits that form the empty orbits), and the electron density and electron conductivity are improved. As a result, hydrogen ions, which have a high electron affinity, easily exchange electrons (charge transfer) from these orbits and transfer negative charges. We thought that it would be occluded by the fullerene in the charged electronic state. The electrochemical action when the metal-encapsulated fullerene compound thus obtained is used as the negative electrode of an alkaline storage battery can be explained as follows. First, at the time of charging, hydrogen ions in the electrolytic solution form C—H bonds on the metal-encapsulated fullerene compound having an increased charge density by charge transfer, and the hydrogen ions are occluded. During discharge, the C—H bond is broken due to charge transfer due to a deelectron reaction from the metal-encapsulated fullerene compound, and hydrogen ions diffuse into the electrolytic solution. In this case, up to 82 hydrogen atoms can be occluded in one C ₈₂ molecule in the state where the metal is encapsulated, and when converted into a discharge capacity density, about 2000 mAh / g becomes possible, which is more than that of the hydrogen occluding alloy negative electrode. Theoretically, high capacity density can be achieved.

As described above, it becomes possible to provide a high capacity alkaline storage battery by using the metal-containing fullerene compound for the negative electrode.

[0014]

EXAMPLES FIG. 1 is a schematic view showing the structure of a metal-encapsulated fullerene compound.

FIG. 2 is a schematic configuration diagram of a nickel-carbon storage battery using a metal-encapsulated fullerene compound in one embodiment of the present invention. In FIG. 2, 1 is a negative electrode plate using the metal-encapsulated fullerene compound of the present invention, 2 is a nickel hydroxide positive electrode plate, 3 is a separator, 4 is a case, 5 and 6 are insulating plates, 7 is a safety valve, and 8 is a sealing plate. , 9 is a positive electrode terminal, and 10 is a positive electrode lead.

The method for producing the metal-encapsulated fullerene compound used for the negative electrode in the examples of the present invention will be described. In advance, carbonates of various metals and graphite powder were mixed, solidified on a pitch, and carbonized at 400 to 1200 ° C., and then 1200
It is produced by irradiating a YAG laser of 532 nm and evaporating the mixed rod heat-treated at ℃ under the condition of argon gas flow. This was fractionated by solvent extraction to a purity of about 90%, and the obtained metal-encapsulated fullerene compound was used. For the negative electrode plate 1, 5% by weight of a fluororesin dispersion was added so that the weight of the resin was 3 times the weight of the metal-encapsulated fullerene compound to form a paste.
Then paste this paste to a thickness of 0.17 mm and a hole diameter of 1.8 m.
m, opening ratio 53%, made of iron and applied to a nickel-plated punching metal plate, smoothed through a slit of 0.6 mm, and then dried at 120 ° C. for 1 hour, and this electrode was passed through a roller press to a thickness of It was adjusted to 0.5 mm and configured. The negative electrode lead plate was attached by spot welding. As the positive electrode plate 2, a porous foamed nickel plate substrate filled with nickel hydroxide was used, and as the separator, a hydrophilic nonwoven fabric made of polypropylene was used. An electrolytic solution was prepared by dissolving 25 g / l of lithium hydroxide in a caustic potash aqueous solution having a specific gravity of 1.25.

The positive electrode plate 2 is constructed so that the filling capacity of the positive electrode active material is in excess of the filling capacity of the negative electrode.
The battery was constructed so that the battery characteristics were controlled by the characteristics of the negative electrode. As a comparative example, a nickel-hydrogen storage battery was manufactured by the same construction method as that of the example using a negative electrode constructed in the same manner as above using an AB ₅ type hydrogen storage alloy containing Misch metal and nickel as main components. did. The batteries of these Examples and the batteries of Comparative Examples were subjected to constant current charging at 0.17 A for 11 hours, and then constant current discharging to 0.5 V at 0.5 A. The results are shown in (Table 1).

Regarding the capacity, the discharge capacity density per weight of the hydrogen storage alloy used for the negative electrode of the comparative sample was 1
The relative value of the discharge capacity density per weight of the metal-encapsulated fullerene compound when 00 was shown. The average voltage of the comparative sample was 1.24V.

[0019]

[Table 1]

As can be seen from Table 1, Rb, Cs,
A battery using a fullerene compound containing Sr and Ba in the negative electrode is superior in both voltage and capacity to a fullerene containing no metal, and has a higher capacity density than the case using a hydrogen storage alloy in the negative electrode. Has been. First, the reason why the voltage is low in the fullerene containing no metal element is that its negative electrode potential is nobler than that of the metal hydride.
This is because the Fermi level exists below the band gap of eV, so that the electrode potential is placed at a noble potential. However, when a metal is included, electrons are donated to the t _1u orbit, so the Fermi level rises, the electrode potential is placed at a position with a high energy level, and the potential shifts to the base direction, which increases the battery voltage. It is believed that Also,
In fullerenes not containing metal elements, sufficient capacity cannot be obtained because the above-mentioned band gap exists, so that overvoltage is large due to charge transfer or decrease in electron conductivity, and the negative electrode potential operated at a noble potential. It is considered that this is because the discharge end voltage was reached early due to the above.

[0021]

As described above, according to the present invention, Rb, Cs, S
By using the fullerene compound containing at least one of the metals r and Ba in the negative electrode for alkaline storage batteries, a negative electrode having a high capacity and the battery can be provided.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing the structure of a metal-encapsulated fullerene compound.

FIG. 2 is a schematic configuration diagram of a nickel-carbon storage battery using a metal-encapsulated fullerene compound negative electrode in an example of the present invention.

[Explanation of symbols]

1 Negative electrode plate using metal-encapsulated fullerene compound 2 Nickel hydroxide positive electrode plate 3 separator 4 cases 5 insulating plate 6 insulating plate 7 Safety valve 8 sealing plate 9 Positive terminal 10 Positive electrode lead

─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-5-101850 (JP, A) JP-A-5-270801 (JP, A) JP-A-8-7887 (JP, A) (58) Field (Int.Cl. ⁷ , DB name) H01M 4/00-4/62 H01M 10/24-10/30 JISST file (JOIS)

Claims (3)

(57) [Claims] 1. A negative electrode for an alkaline storage battery, comprising a fullerene compound containing at least one metal group consisting of Rb, Cs, Sr and Ba. 2. A fullerene compound is represented by the general formula MxCn (M
Is at least one kind of metal selected from Rb, Cs, Sr, and Ba, x is a stoichiometric composition ratio, and x = 1,
The negative electrode for an alkaline storage battery according to claim 1, wherein n represents the number of carbon atoms and is a metal-encapsulated fullerene compound represented by n = 82). 3. A positive electrode using nickel hydroxide and Rb, C
An alkaline storage battery comprising: a negative electrode using a metal-encapsulated fullerene compound containing at least one of a metal group consisting of s, Sr, and Ba, and an alkaline electrolyte.