JPH07296799A - Carbon negative electrode and li secondary battery - Google Patents

Abstract

PURPOSE:To provide a carbon negative electrode excellent in storing performance of lithium while keeping the characteristic in which a dendrite is difficult to grow, and provide a Li secondary battery excellent in battery characteristics such as energy density, discharge capacity, and cycle life. CONSTITUTION:A carbon negative electrode 1 for Li secondary battery has a layer consisting of the graphite structure of fullerene molecule at least on the surface, and a Li secondary battery has such a carbon negative electrode. Thus, the limitation to electrolyte is minimized, and electrolytes having various compositions can be widely used.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Li secondary battery having an excellent lithium storage capacity and an excellent energy density, discharge capacity and cycle life, and a carbon-based negative electrode thereof.

[0002]

2. Description of the Related Art Conventionally, as a negative electrode that has overcome the problem of short circuit between positive and negative electrodes due to dendrite growth of Li-based negative electrode in a Li secondary battery, Li is intercalated and retained in carbonaceous state in an ionic state. Carbon-based negative electrodes have been known. However, the Li intercalation-type carbon-based negative electrode has a problem that the discharge capacity and energy density are inferior to those of the Li-based negative electrode.

[0003]

DISCLOSURE OF THE INVENTION The present invention provides a carbon-based negative electrode having an excellent lithium storage capacity while maintaining the property that the dendrites of the carbon-based negative electrode are hard to grow, thereby obtaining energy density, discharge capacity, cycle life, etc. Li with excellent battery characteristics
An object is to obtain a secondary battery.

[0004]

The present invention has a carbon-based negative electrode for a Li secondary battery characterized by having a layer composed of a graphite type structure of fullerene molecules on at least the surface thereof, and such a carbon-based negative electrode. The present invention provides a Li secondary battery characterized by the above.

[0005]

[Exemplary Embodiments] A carbon-based negative electrode is a powder molded product of a graphite-type structure of fullerene molecules, or a layer formed of a graphite-type structure of fullerene molecules on a conductive support substrate by a slurry coating method, a vapor deposition method, a CVD method. It is formed as a device attached by a method, a PVD method, an LB method, or the like. As the conductive support base material, a sheet or net made of a metal such as copper, nickel, stainless steel, aluminum or silver is used. The Li secondary battery is formed as one in which a positive electrode and a carbon-based negative electrode are arranged via a porous insulating film containing an electrolyte.

[0006]

[Function] The graphite-type structure of fullerene molecules has a structure similar to that of carbon in graphite substituted with fullerene molecules, and the layers formed by the network plane composed of the benzene ring-type 6-membered rings are superposed in multiple stages. The inter-layer distance in the layered structure is larger than about 3.354Å in the case of graphite. Incidentally, a graphite type structure having such an interlayer distance of 10 Å or more, especially 15 to 20 Å can also be used.

Based on the above, that is, based on the large interlayer distance of the fullerene molecule in the graphite type structure, such a graphite type structure can store a large amount of lithium and has a characteristic that dendrites hardly grow. maintain. As a result, by using a negative electrode composed of a graphite-type structure of fullerene molecules, the Li secondary having excellent energy density and charge / discharge capacity due to the large amount of lithium occlusion and excellent cycle life due to the difficulty of dendrite growth. A battery can be formed.

[0008]

EXAMPLE A carbon-based negative electrode of the present invention has a layer of a graphite type structure of fullerene molecules on at least the surface thereof and is used for forming a Li secondary battery. Therefore, the carbon-based negative electrode of the present invention is, for example, an appropriate form such as a powder molded product of a graphite-type structure of fullerene molecules, or a conductive support substrate provided with a layer of a graphite-type structure of fullerene molecules. Can be formed as. Examples thereof are shown in FIGS. 1 and 2.

The carbon-based negative electrode 1 illustrated in FIG. 1 is formed by powder molding a powder of a graphite type structure of fullerene molecules. The carbon-based negative electrode 2 illustrated in FIG. 2 is formed by attaching a layer 21 made of the graphite type structure on the surface of a conductive support base material 22. The graphite structure layer may be provided on both sides of the conductive support substrate, or may be partially provided on one side or both sides.

As described above, in the present invention, the carbon-based negative electrode may be in any form, and its formation may be carried out, for example, by adding the powder of the graphite type structure to polyvinylidene fluoride or ethylene / propylene / diene copolymer as needed. Coalescing, powder molding using a suitable binder such as polytetrafluoroethylene or polyethylene, casting of a slurry prepared using a binder and a dispersion medium, or coating on a conductive support substrate. Or a method of coating the surface of the conductive support substrate with a layer of the graphite-type structure, or the like.

As the above-mentioned coating method, for example, an evaporation method such as a vacuum evaporation method, a thermal CVD method or a plasma CV method is used.
Appropriate methods such as CVD method such as D method and MOCVD method, PVD method such as MBE method and ICB method, sputtering method, and LB method can be adopted.

The formation of a graphite type structure composed of fullerene molecules is carried out, for example, with Pb, Si, Al, Pd, Co, Fe, N.
It can be carried out by crystallization treatment of a fullerene molecule by a method of heat treatment, vapor deposition treatment, CVD treatment, PVD treatment or the like in combination with an atom having a large radius such as i or a cation thereof.

By the above crystallization treatment, the fullerene molecule a forms a benzene ring type six-membered ring around the combined atom or its cation b as shown in FIG. A hexagonal crystal is grown to form a graphite type structure. In this case, it is preferable that the amount of the above atom or its cation used in combination is about 1 mol per 2 mols of the fullerene molecule in order to form a network plane composed of a 6-membered ring, and thus to form a graphite structure. As the fullerene molecule, an appropriate one such as C ₆₀ type or C ₇₀ type in which a large number of carbon atoms are spherically bonded can be used, and there is no particular limitation.

In the above, as the powder of the graphite type structure of fullerene molecules used for forming the powder compact or the like,
A particle having an appropriate particle size may be used depending on the form of the carbon-based negative electrode to be formed. The powder that can be preferably used from the viewpoint of negative electrode characteristics and the like is 1 to 500 μm or less based on the average particle diameter, and 5 to
The thickness is 300 μm, particularly 10 to 100 μm. The amount of the binder used may be appropriately determined depending on the strength and the like, and generally 0.1 to 30% by weight of the graphite-type structure powder in terms of mechanical strength and electrode characteristics of the formed negative electrode, Above all 1-20
%, Especially 2 to 15% by weight is preferred.

Also, as the conductive supporting base material, an appropriate one may be used depending on the form of the negative electrode to be formed. Examples include copper, nickel, stainless steel, aluminum,
Examples include sheets and nets made of metal such as silver.
When the purpose is to form a sheet-like carbon-based negative electrode, the conductive supporting base material is generally 1 to 500 μm, especially 5
The thickness of ˜300 μm, especially 10 to 100 μm is used. In that case, the thickness of the graphite-type structure layer provided on the conductive supporting substrate is arbitrary and may be appropriately determined according to the purpose of use of the electrode, etc., and is generally 5 to 800 μm, especially 1
It is set to 0 to 500 μm, and particularly 20 to 300 μm.

The carbon-based negative electrode of the present invention is for forming a Li secondary battery, but the formation of the Li secondary battery is not particularly limited except that the carbon-based negative electrode is used. It can be carried out in a conventional manner using an electrolyte and a positive electrode. Therefore, the form of the Li secondary battery can be appropriately determined according to the purpose of use and the like. For example, like a coin type, a button type, or a wound type, a positive electrode is provided via a porous insulating film containing an electrolyte. It is possible to adopt an appropriate form such as a structural form in which the carbon-based negative electrode is arranged and an external form such as a cylindrical type or a rectangular type.

Incidentally, FIG. 4 illustrates a coin type Li secondary battery. 3 is a negative electrode can, 4, 8 is a nickel plate for collecting current, 5 is a carbon-based negative electrode, 6 is an electrolyte layer (separator made of a porous insulating film), 7 is a positive electrode, 9 is a positive electrode can, and 10 is insulation sealing. It is a material. The above-mentioned wound body type is one in which a tape-shaped or sheet-shaped positive electrode and a carbon-based negative electrode are housed in a can body which forms a positive and negative electrode part by winding a separator made of a porous insulating film. The above-mentioned sheet-shaped positive electrode and carbon-based negative electrode may have any thickness, but may have a thickness of several to several hundreds of μm.

As the electrolyte, it is possible to use an appropriate electrolyte capable of moving Li ions. Examples thereof include a solid electrolyte such as a salt electrolytic polymer mixed with a lithium salt, and a non-aqueous electrolytic solution type in which a lithium salt is dissolved in an organic solvent such as ester or ether.

Typical examples of the above salt-electrolytic polymers include polyethylene oxide, polyphosphazene, polyaziridine, polyethylene sulfide, polyvinyl alcohol, their derivatives, mixtures and complexes. The solid electrolyte has the advantage that it can also serve as a separator between the positive and negative electrodes.

Typical examples of the organic solvent include propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, tetrahydrofuran, 2-methyltetrahydrofuran, dimethoxyethane, dimethylsulfoxide, sulfolane, γ-butyrolactone and 1,2-dimethoxyethane. , Diethyl ether, 1,3-dioxolane, methyl formate, methyl acetate,
Examples thereof include N, N-dimethylformamide, acetonitrile, a mixture thereof and the like. In the conventional graphite-based negative electrode, it was necessary to use an ethylene carbonate-based electrolytic solution because it has a catalytic action for electrolytic solution decomposition, but in the negative electrode of the present invention, the electrolytic solution is difficult to decompose, and the electrolytic solution is not particularly limited.

Typical examples of lithium salts include LiI and Li
CF ₃ SO ₃ , Li (CF ₂ SO ₂ ) ₂ , LiBF ₄ , LiClO
₄ , LiAlCl ₄ , LiPF ₄ , LiPF ₆ , LiAsF ₃ , Li
AsF _{6 and the} like. The concentration of lithium salt in the electrolytic solution is generally 0.1 to 3 mol / liter, but is not limited to this. When forming the above-mentioned non-aqueous electrolyte, organic additives such as 2-methylfuran, thiophene, pyrrole and crown ether are added as necessary for the purpose of improving battery characteristics such as life, discharge capacity and electromotive force. It is also possible to add substances.

As the positive electrode, a suitable one such as a metallic one and an organic conductive substance such as a conjugated polymer can be used. Examples of the metal-based positive electrode include L
Ti, Mo, Cu, Nb, V, Mn, C containing i
Examples thereof include complex oxides of metals such as r, Ni, Co, P, sulfides, selenides, and the like.
MnO ₂ , TiS ₂ , MoO ₂ , MoO ₃ , V ₂ O ₅ , V ₂ O ₅
-P ₂ O ₅ (amorphous), LiCoO ₂ , LiMn
₂ O ₄ , LiNiO ₂ , LiNi _0.5 Co _0.5 O ₂ , Li _w Co
_1-xy M _x P _y O _{2 + z} (where M is one or more transition metals, w is 0 <w ≦ 2, x is 0 ≦ x <1, y is 0 <y
<1, z is −1 ≦ z ≦ 4. ), Or a phosphate containing Li to Li · Co and / or an oxide of Co to Li · Co and containing 0.1 mol or more of Co and 0.2 mol or more of P per 1 mol of Li. The active material is, for example.

The sheet-like positive electrode may be formed, for example, by using an active material together with a conductive material such as acetylene black or Ketjen black and a binder, if necessary, a casting method, a compression molding method, a roll molding method, a doctor blade. It can be carried out by a method such as a method of molding by an appropriate method according to the above-described method of forming a carbon-based negative electrode. Therefore, the positive electrode may be a reinforced form in which the positive electrode material is adhered to the conductive supporting base material by an appropriate method such as soldering, brazing, ultrasonic welding, spot welding, or coating with a binder resin.

On the other hand, as the porous insulating film (separator) interposed between the positive electrode and the carbon-based negative electrode, an appropriate porous material such as a porous polymer film made of polypropylene or the like, a glass filter, a nonwoven fabric or the like is used. You can The electrolyte-containing porous insulating film may be formed by an appropriate method such as a method of impregnating or filling the porous insulating film with an electrolyte or an electrolytic solution, or a method of filling the battery can with the electrolytic solution or the like. it can.

For charging the Li secondary battery, a constant current is continuously applied, a constant voltage method is used to regulate the current so as not to exceed a constant voltage, and a pulse current is supplied using an appropriate pulse power source. It can also be carried out by a method of doing. The charging method using a pulse current has the advantage that dendrites are less likely to grow because the concentration change of the electrolyte is suppressed because energization / stopping is repeated.

[0026] Example 1 diameter 20 mm, the thickness of 0.5mm nickel substrate, fullerene C ₆₀ fullerene C ₆₀ and Pb atom by a vacuum deposition apparatus
A hexagonal crystal structure (Fig. 3) having a thickness of about 80 µm, which is vapor-deposited at a ratio of 2 mol and 1 mol of Pb atom and has one Pb atom at the center of the six-membered ring of the hexagonal plane network of fullerene C ₆₀ A structural layer was formed to obtain a carbon-based negative electrode.

Next, as shown in FIG. 5, the carbon-based negative electrode 11,
Positive electrode 13 made of a Li sheet having a thickness of 100 μm, and L
Electrolyte solution 14 (ethylene carbonate (water content 20 pp
LiPF ₆ was added at a concentration of 1 mol / liter) to a tripolar Li secondary battery.

Comparative Example 30 parts by weight of natural graphite and 10 parts of polytetrafluoroethylene
A weight ratio of the homogeneous mixture was press-molded on a nickel substrate having a diameter of 20 mm and a thickness of 0.5 mm to a thickness of about 100 μm to obtain a carbon-based negative electrode. I got a battery.

Evaluation Test Regarding the tripolar Li secondary battery obtained in Example 1 or Comparative Example, a constant current of 0.5 mA was used using a galvanostat, and the upper limit (charging) potential was 2 based on the potential with respect to the reference electrode.
Charge and discharge are repeated through the positive and negative electrodes under the conditions of V and lower limit (discharge) potential of 0V (30 ° C), and the charge capacity, discharge capacity, and Coulombic efficiency at the 1st, 25th, and 50th cycles are examined. It was

The above results are shown in the table below.

Graphite forms an intercalation compound to form 6 carbon atoms.
A maximum of one lithium atom per number since the intercalated (charging: Li + C ₆ ⇒LiC _6, discharge: LiC ₆ ⇒
Li + C ₆ ), whose theoretical capacity is 372 mAh / g. From the table, the comparative example is close to the theoretical capacity of graphite, but Example 1 shows a larger charging capacity and discharging capacity than the theoretical capacity of graphite. From this, the graphite type structure based on the size of the fullerene molecule is shown. It is speculated that more lithium atoms are occluded. In Example 1, almost 100%
It shows that Coulomb efficiency is shown, and that it has battery characteristics with little cycle deterioration with almost no decomposition of the electrolytic solution.

[0032]

EFFECTS OF THE INVENTION According to the present invention, a carbon-based negative electrode can be obtained which maintains the characteristics that dendrites do not easily grow, has an excellent lithium occlusion capacity, and does not easily decompose the electrolytic solution. It is possible to obtain various types of Li secondary batteries having excellent battery characteristics such as cycle life and excellent safety and reliability. Further, there are few restrictions on the electrolytic solution, and a wide range of electrolytic solutions having various compositions can be used.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of an example of a carbon-based negative electrode.

FIG. 2 is a cross-sectional view of another carbon-based negative electrode example.

FIG. 3 is an explanatory diagram of a hexagonal plane network of fullerene molecules.

FIG. 4 is an explanatory diagram of a battery example.

FIG. 5 is an explanatory diagram of another battery example.

[Explanation of symbols]

 1, 2, 5: carbon-based negative electrode 21: layer made of graphite type structure of fullerene molecule 22: conductive support base material 3: negative electrode can 6: electrolyte layer (separator made of porous insulating film) 7: positive electrode

Claims (3)

[Claims] 1. A carbon-based negative electrode for a Li secondary battery, which has a layer composed of a graphite type structure of fullerene molecules on at least the surface thereof. 2. A layer comprising a graphite type structure of fullerene molecules is provided on a sheet-shaped conductive supporting substrate.
The carbon-based negative electrode described in. 3. A Li secondary battery comprising the carbon-based negative electrode according to claim 1.