JP2001015115A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance the capacity and improve the cycle characteristic under high temperature by coating the surface of active material powder with amorphous silica. SOLUTION: Among active materials used for a positive and a negative electrode are transition metal oxides such as lithium manganese composite oxides. The active material coated with amorphous silica can be obtained by the following method. While the active material powder is stirred under the presence of organic solvent such as alcohol, tetraalkoxysilane is dripped therein and stirred for a prescribed time, and then water and acid catalyst are dripped therein. The organic solvent is thus dried and removed under a prescribed temperature. The quantity of the amorphous silica coating the active material is preferably 1.4 mg or more per m2 of an active material surface area. This constitution can prevent the contact between the electrolyte solution molecules and the active material powder surface and prevent the dissolution of the electrolyte solution molecules so as to improve the capacity and the cycle characteristic under high temperatures. If the quantity of the amorphous silica is 14 mg or more, the initial discharge capacity is drastically improved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium secondary battery, and more particularly to a lithium secondary battery in which an electrolyte is sandwiched between electrodes made of an active material and sealed in an outer package.

[0002]

2. Description of the Related Art In recent years, there has been a remarkable performance and miniaturization of portable electronic devices such as notebook personal computers and mobile phones. Further higher energy density and smaller size are required.

[0003] To meet such demands, lithium secondary batteries utilizing lithium ion insertion / removal have been actively studied.

Although this lithium secondary battery has a high average operating voltage of 3.6 V and a high energy density, it uses a high voltage and a non-aqueous electrolyte and an electrolyte which are easily decomposed at a high voltage. Therefore, precipitation on the surface of the active material such as an organic substance which is a decomposition product of the electrolytic solution and a lithium oxide or lithium carbonate which is a decomposition product of the electrolyte is inevitable, and the precipitate impedes lithium ion deintercalation. This hinders an increase in capacity. Also, for the above reasons, it is difficult to say that the high-temperature storage characteristics and the cycle characteristics are sufficient,
This is an obstacle to further market expansion.

In order to solve such a problem, development of a method of coating the surface of an active material with a lithium ion conductive solid electrolyte layer has been actively promoted.

For example, in Japanese Patent Application Laid-Open No. 9-82360, the surface of a lithium composite oxide powder serving as a positive electrode active material is coated with polyethylene oxide, polypropylene oxide, polyester, polyimine, polyether, polyacrylonitrile, polyvinyl sulfone, polyvinyl chloride, or the like. Lithium ion conductive polymer compound, or a material obtained by adding an electrolyte such as LiClO ₄ to the lithium ion conductive polymer compound, or LiI, Li ₄ I, Li ₅ AlO
₄ , Li ₅ FeO ₄ , Li-Na-β-alumina, Li
AlSiO ₄ , Li ₄ Zn (GeO ₄ ) ₄ , Li11N
It has been proposed to coat with a lithium ion conductive inorganic compound such as ₃ Cl ₂ and Li ₆ NBr ₃ . However, in all of the examples of JP-A-9-82360,
The initial capacity is rather reduced, the decrease in the initial capacity must be suppressed by reducing the thickness of the coating material, and the cycle characteristics in a high-temperature atmosphere are improved when the coating material thickness is 0.06 μm or less. However, since the polymer itself is gradually decomposed by repeated charge and discharge, it is still insufficient to improve the high-temperature storage characteristics and the cycle characteristics while increasing the capacity.

Japanese Patent Application Laid-Open No. 9-171813 proposes that the surface of an active material powder is coated with an inorganic ion conductive film composed of a lithium / aluminum hydroxide composite. The inorganic ion conductive membrane composed of the lithium / aluminum hydroxide composite is described as being formed from a hydroxide or formed from an alkoxide, which is formed by a so-called sol-gel method. In this method, since only lithium ions diffuse in the bulk of the inorganic ion conductive membrane and the electrolyte molecules do not enter the inorganic ion conductive membrane, the surface of the active material powder does not come into contact with the electrolyte. The molecule does not decompose. Therefore, the initial capacity is also improved. The thickness of the coating material is as thin as 50 to 500 Å.

However, in the method disclosed in Japanese Patent Application Laid-Open No. Hei 9-171813, since a strong alkali is present on the surface of the active material powder, a polymer adhesive such as polyvinylidene fluoride rapidly gels, and molding is extremely difficult. It becomes difficult.
Moreover, even if it can be molded, it tends to become a very porous molded body.

[0009]

According to the present invention, there is provided a lithium secondary battery according to the present invention, wherein an electrolyte is sandwiched between electrodes made of an active material and sealed in an outer package. In the battery, the surface of the active material powder is coated with amorphous silica.

In the above lithium secondary battery, the surface of the active material powder is 1.4 mg per 1 m ^{2 of the} active material surface area.
It is desirable to coat with the above amorphous silica.

In the above-mentioned lithium secondary battery, it is desirable that the surface of the active material powder is coated with amorphous silica formed by a sol-gel method.

[0012]

The surface of the active material powder is coated with at least 1.4 mg of amorphous silica per 1 m ² of active material surface area, so that contact between the electrolyte solution molecules and the surface of the active material powder can be prevented. The liquid molecules are not decomposed, so that the capacity can be improved, and the cycle characteristics at high temperatures are also improved. Also, unlike the method of JP-A-9-171813, since no lithium alcohol is added, no strong alkali exists on the surface of the active material powder, and for example, a polymer adhesive such as polyvinylidene fluoride is used. It does not gel rapidly, is easy to mold, and hardly becomes a porous molded body.

[0013]

Embodiments of the present invention will be described below. FIG. 1 is a cross-sectional view showing a configuration example of a lithium secondary battery of the present invention, wherein 1 is a positive electrode can, 2 is a positive electrode current collecting layer, 3 is a positive electrode,
4 is an insulating packing, 5 is a solid electrolyte or a separator containing an electrolyte, 6 is a negative electrode, 7 is a negative electrode current collecting layer, and 8 is a negative electrode can.

The active materials used for the positive electrode 3 and the negative electrode 6 include the following transition metal oxides. For example,
Lithium manganese composite oxide, manganese dioxide, lithium nickel composite oxide, lithium cobalt composite oxide,
Examples thereof include lithium nickel cobalt composite oxide, lithium vanadium composite oxide, lithium titanium composite oxide, titanium oxide, niobium oxide, vanadium oxide, tungsten oxide, and derivatives thereof. Here, the active materials used for the positive electrode 3 and the negative electrode 6 are not clearly distinguished, and those showing a more noble potential by comparing the charge and discharge potentials of the two types of transition metal oxides are given to the positive electrode 3 and the more negative ones. A battery having an arbitrary voltage can be formed by using each of the negative electrodes 6 having a high potential. A transition metal oxide may be used only for the positive electrode 3, and a carbon material or metallic lithium may be used for the negative electrode 6. These can remove and insert lithium ions accompanying the electrochemical redox reaction.

The active material coated with amorphous silica in the present invention is prepared by the following method. That is,
(1) Tetraalkoxysilane is added dropwise while stirring the active material powder in the presence of an organic solvent such as alcohol, and (2)
After stirring for a predetermined time, (3) a predetermined amount of water and an acid catalyst are dropped, and (4) the organic solvent is dried and removed at a predetermined temperature for a predetermined time.

The amount of the amorphous material added is 1 m ^{2 of} the active material surface area.
1.4 mg or more, preferably 14 mg or more. When the amount is 1.4 mg or more, the initial charge capacity is already improved, but the initial discharge capacity is only slightly increased, whereas when the amount is 14 mg or more, the initial discharge capacity is greatly improved.

To produce the positive electrode 3 and the negative electrode 6,
(1) An active material coated with amorphous silica, an electron conductivity imparting agent, and a molding aid are dispersed in water or an organic solvent to prepare a slurry, and the slurry is used as a current collecting layer. After coating and drying on an aluminum foil or a copper foil, it is cut, or (2) an active material coated with an amorphous coating material and an electron conductivity-imparting agent directly or by adding a molding aid. After being granulated and put into a mold, and pressed by a press machine,
A method such as pressure bonding to the current collecting layer is used.

Examples of the molding aid usable here include polytetrafluoroethylene, polyacrylic acid, carboxymethylcellulose, polyvinylidene fluoride, polyvinyl alcohol, diacetylcellulose, hydroxypropylcellulose, polybutyral, polyvinyl chloride, polyvinylpyrrolidone. One or two such as
Mixtures of more than one species.

The electrolyte 5 may be an organic electrolyte obtained by dissolving a required electrolyte salt in an organic solvent, a solid polymer electrolyte obtained by dissolving an electrolyte salt in an ion-conductive polymer material, or a gel electrolyte obtained by combining them. An inorganic solid electrolyte made of an inorganic material can be used. When an organic electrolyte is used as an electrolyte, a separator for separating the positive electrode 3 and the negative electrode 6 is required.

Examples of the organic solvent used for the organic electrolyte include ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyrolactone, sulfolane,
There is one or a mixture of two or more solvents selected from 1,2-dimethoxyethane, 1,3-dimethoxypropane, dimethyl ether, tetrahydrofuran, 2-methyltetrahydrofuran, and methylethyl carbonate.

As the electrolyte salt, for example, LiCl
O ₄ , LiBF ₄ , LiPF ₆ , LiCF ₃ SO ₃ , L
A lithium salt such as iN (CF ₃ SO ₂ ) ₂ can be given.

As the separator, for example, a nonwoven fabric made of polyolefin fiber or a microporous film made of polyolefin can be used. Here, examples of the polyolefin include polyethylene and polypropylene.

Examples of the ion conductive polymer material include polyethylene oxide, polyacrylonitrile, and mixtures and copolymers thereof.

As the inorganic solid electrolyte, for example, Li _1.3
Al _0.3 Ti _1.7 (PO ₄ ) ₃ , Li _3.6 Ge _0.6 V
Oxide crystalline solid electrolyte such as _0.4 O ₄ , 40 Li _{2
O-35B 2 O 3 -25LiNbO 3} , 30LiI-4
1Li ₂ O-29P ₂ O ₅ oxide-based amorphous solid electrolytes such as, mention may be made of sulfide-based amorphous solid electrolytes such as _{1Li 3 PO 4 -63Li 2 S-} 36SiS 2.

The positive electrode current collecting layer 2 and the negative electrode current collecting layer 7 are arranged for contact and current collection between the positive electrode can 1 or the negative electrode can 8 and the positive electrode 3 or the negative electrode 6, and are made of, for example, aluminum foil or copper foil.

[0026]

Example 1 Li [Li _0.1 M having a specific surface area of 2.1 m ² / g measured by the BET method was used as the active material powder.
n _1.9 ] O ₄ , a predetermined amount of Si (OE _t ) ₄ was added in the presence of isopropyl alcohol, and the mixture was stirred for a predetermined time, and then water and an acid catalyst were added in a predetermined amount to carry out hydrolysis polymerization. . Thereafter, drying and heat treatment were performed at a predetermined temperature to obtain an active material powder coated with amorphous silica.

The active material powder thus obtained, acetylene black and a Teflon-based polymer adhesive were mixed in an amount of 82: 11: 7 wt.
%, Using N-methylpyrrolidinone, applying the mixture on an aluminum foil, and then using the dried product as a positive electrode, using graphite and a Teflon-based polymer adhesive material at 97: 3 wt%.
Of N-methylpyrrolidinone, applied on an aluminum foil, dried and used as a negative electrode, and propylene carbonate: dimethyl carbonate = 1: 1 with LiClO ₄ dissolved at 1 mol / l as an electrolyte. The solution,
A microporous polypropylene membrane was used as a separator, and a coin cell for capacity evaluation was assembled.

Using this coin cell, 3.0 to 4.3
Between V, the charge / discharge capacity was measured at a current value of 10 mA / g.

[0029]

Comparative Example 1 A coin cell was prepared in the same manner as in Example 1 except that the active material was not coated with silica, and the charge / discharge capacity was evaluated. Table 1 shows the results.

[0030]

[Table 1]

As can be seen from the results, the active material surface area 1
When an active material coated with 1.4 mg or more of amorphous silica with respect to m ² is used, the initial charge capacity and the 60 ° cycle characteristic are improved, and further, the active material surface area is 1 m ^{2 and} 14 m ^2.
When the active material coated with amorphous silica of g or more is used, the initial discharge capacity is greatly improved.

This is because the surface of the active material is coated with amorphous silica, so that the contact between the electrolyte solution molecules and the surface of the active material powder can be prevented, so that the electrolyte solution molecules do not decompose.

However, unlike the method disclosed in Japanese Patent Application Laid-Open No. 9-171813, it is not clear why lithium ion conduction is not hindered despite the fact that silica is coated with no lithium ion alone. Even though silica is porous, it is not clear why lithium ions do not pass through the electrolyte solution and carry only lithium ions. However, it is considered that the mechanism of ionic conduction is other than bulk diffusion.

[0034]

As described above, by coating the surface of the active material powder with amorphous silica, contact between the electrolyte solution molecules and the surface of the active material powder can be prevented, so that the electrolyte solution molecules are not decomposed. Therefore, the capacity can be improved, and the cycle characteristics at high temperatures are also improved. Also, unlike the method of JP-A-9-171813, since a strong alkali does not exist on the surface of the active material powder, for example, a polymer adhesive such as polyvinylidene fluoride does not rapidly gel, Molding is easy, and it is difficult to produce a porous molded body.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view illustrating a configuration example of a coin-type lithium-ion battery using an electrochemical device according to the present invention.

[Explanation of symbols]

1 ... Positive electrode can, 2 ... Positive electrode current collecting layer, 3 ... Positive electrode, 4 ...
Insulation packing, 5 ... solid electrolyte or separator containing electrolyte, 6 ... negative electrode, 7 ... negative electrode current collecting layer, 8 ...
Negative electrode can

Continuing from the front page (72) Inventor Yoko Mishima 3-5 Koikodai, Seika-cho, Soraku-gun, Kyoto Prefecture Inside the Central Research Laboratory, Sera Corporation (72) Inventor Shinji Magome 3-5 Koikodai, Seika-cho, Soraku-gun, Kyoto Kyoto Inside the Central Research Laboratory of Sera Corporation (72) Inventor Makoto Osaki 3-5 Koukodai, Seika-cho, Soraku-gun, Kyoto Prefecture Inside of Central Research Laboratory Kyoto Sera Corporation (72) Inventor Ei Higuchi 3-5-2 Kodaidai, Seika-cho, Kyoto Prefecture Address Kyocera Co., Ltd. Central Research Laboratory F-term (reference) EJ05 HJ01 HJ07

Claims (3)

[Claims] 1. A lithium secondary battery in which an electrolyte is sandwiched between electrodes made of an active material and sealed in an outer package, wherein the surface of the active material powder is coated with amorphous silica. Rechargeable battery. 2. The lithium secondary battery according to claim 1, wherein the surface of the active material powder is coated with 1.4 mg or more of amorphous silica per 1 m ^{2 of the} active material surface area. . 3. The lithium secondary battery according to claim 1, wherein the surface of the active material powder is coated with amorphous silica formed by a sol-gel method.