JP2008159355A - Coin type lithium battery - Google Patents

Abstract

In a cylindrical and flat rectangular lithium battery, an active material is applied on a metal foil serving as a current collector to increase a reaction area and improve load characteristics by forming a spiral structure. . However, it is very difficult to improve load characteristics with a coin-type lithium battery.
In a coin-type lithium battery including a positive electrode, a negative electrode, an electrolyte, and a gasket, the negative electrode is a metal metal foam having a mean particle size of 2 μm or less and a foamed metal current collector with a carbon layer formed on the surface. A coin-type lithium battery composed of a body.
[Selection] Figure 1

Description

  The present invention is intended to improve the heavy load characteristics of a coin-type lithium battery.

  In cordless and portable electronic devices represented by mobile communication, lithium batteries having high energy density are widely used as the size and weight are reduced. In recent years, various additional functions have been provided in electronic devices, and there is a demand for improvement in load characteristics to a power source.

  In a cylindrical and flat rectangular lithium battery, an active material is applied onto a metal foil serving as a current collector to form a spiral structure, thereby increasing a reaction area and improving load characteristics. However, it is very difficult to improve load characteristics with a coin-type lithium battery.

  The reason for this is that, in general, a coin-type lithium battery uses a metal oxide that is an active material and carbon powder that assists conductivity as a binder and is pressure-molded into a pellet shape. This is because it is difficult to increase the reaction area like a mold or a flat square.

As a means for improving the load characteristics, as seen in Patent Document 1, a method using an active material having a small particle size and a large specific surface area, a method using a pellet added with a large amount of a conductive auxiliary agent, and a patent There is a method of providing a wound structure in a coin shape as seen in Document 2.
JP 2003-137547 A JP 2000-164259 A

  However, an active material having a small particle size and a large specific surface area as shown in Patent Document 1 has a potential to improve load characteristics. However, in a coin-type lithium battery in which an electrode is formed into a pellet shape, the active material is wetted with an electrolyte. Because it is difficult, the surface of all active materials does not become a reaction field. Moreover, since the impedance of the pellet is high, the electron conduction after the reaction is not smooth. For the above reasons, the potential cannot be extracted, and therefore the load characteristics cannot be improved as a coin-type lithium battery.

  In addition, the method of using pellets to which a large amount of conductive aid is added has a problem that the capacity of the battery is reduced because the filling amount of the active material is reduced.

  The method of providing a wound structure in a coin shape as seen in Patent Document 2 has problems that it is difficult to reduce the size, and that it is difficult to configure a battery and is poor in mass productivity.

  The present invention solves such problems. In a coin-type lithium battery including a positive electrode, a negative electrode, an electrolyte, and a gasket, the negative electrode has a metal oxide having a particle size of 2 μm or less and a carbon layer formed on the surface. It is characterized by comprising a foamed metal current collector.

According to the present invention, on the active material having a small particle size and a large specific surface area, the electrode reaction is promoted and the electron conduction after the electrode reaction can smoothly proceed. Can be obtained.

  The present invention relates to a coin-type lithium battery comprising a positive electrode, a negative electrode, an electrolytic solution, and a gasket, wherein the negative electrode has a metal oxide having an average particle size of 2 μm or less and a foamed metal current collector having a carbon layer formed on the surface. It is a coin-type lithium battery composed of a body and can easily supply a coin-type battery excellent in load characteristics.

  An active material having a smaller particle size is superior in load characteristics. In order to obtain the same battery capacity, the filling mass of the active material must be the same. In the case of the same filling mass, the smaller the particle size, the larger the reaction area, so that the high load characteristics and High capacity can be realized.

  A metal foam is used as the current collector. Since the metal foam has a three-dimensional network of electron conduction, it is superior in electron conductivity to pellets using graphite powder, which is a conductive auxiliary agent usually used when constructing a coin-type battery. In addition, since the distance between the active material of the battery and the current collector is short, the electron conduction with the current collector proceeds smoothly after the active material undergoes electrode reaction. Since the contact between the current collector and the outer can is a metal-metal contact, the impedance is smaller than the carbon-carbon contact in the conventional coin type. As described above, high load characteristics can be obtained as battery characteristics.

  As the material of the foam metal used for the current collector, nickel or stainless steel is suitable for the negative electrode of the lithium secondary battery system. This is because it is necessary to consider the electrodeposition of lithium when an overvoltage is applied to the negative electrode, and it is better to use a material that is difficult to alloy with the electrodeposited lithium. The reason is that when alloyed with lithium, volume expansion occurs, so that the three-dimensional network of electron conduction is destroyed, and when alloying with lithium is canceled, volume shrinkage occurs and electron conduction This is because the three-dimensional network is disconnected.

  However, when nickel is used as the material of the foam metal used as the negative electrode current collector, if the negative electrode itself shifts to the potential on the oxidation side, nickel as the current collector metal is oxidized and dissolved, and the elution of nickel proceeds to cause electrons. The three-dimensional network of conduction collapses and inhibits the battery reaction. Therefore, it is effective to form a protective layer on the foam metal. For example, by forming a carbon layer on nickel foam metal, elution due to oxidation dissolution of nickel can be suppressed, and the battery characteristics can improve the overdischarge resistance in which the negative electrode potential increases in the oxidation direction. it can.

  In addition, since the metal oxide that is the active material and the nickel foam that is the current collector have poor electrical contact, the impedance of the interface can be lowered and the load characteristics can be improved by configuring the carbon layer. be able to.

  When such an electrode is used for the negative electrode, the negative electrode active material is preferably a metal oxide that does not alloy with lithium. For example, in the case of an active material such as lithium titanate, there is no volume expansion due to charge / discharge reaction of the battery, and even if the electrode reaction is repeated, the same electrode reaction as in the initial stage can be performed stably. In the case of alloying such as silicon and aluminum, the volume expansion during the reaction is large, so the negative electrode itself also expands greatly, destroying the three-dimensional network structure of the foamed electrode and inhibiting the battery reaction, or repeating the charge / discharge cycle The active material peels off from the surface and does not contribute to the reaction.

In the electrode having excellent current collecting properties as described above, reducing the particle size of the active material is very useful for improving the load characteristics of a battery formed using the active material. It is because the size of the specific surface area of the active material can be effectively utilized for the battery reaction, and the electron transfer after the reaction can be performed smoothly, so that a large current can be obtained in the coin-shaped battery, Load characteristics can be improved.

  The negative electrode can be easily manufactured by filling a foamed electrode with a paste in which an active material and graphite powder of a conductive agent are mixed and punching it into a circular pellet shape.

  The structure of the battery can also be easily manufactured using the same method as the conventional one, and is excellent in mass productivity.

  Hereinafter, the shape of the coin-type battery according to the present invention will be described in detail with reference to FIG. In FIG. 1, a negative electrode 2 is arranged on the sealing plate 1 side, and a positive electrode 4 is arranged on the positive electrode case 5 side through a separator 3. The negative electrode 2 is composed of a metal oxide having an average particle size of 2 μm or less as an active material and a foamed metal current collector having a carbon layer formed on the surface. The sealing plate 1 and the negative electrode 2 are electrically connected by contact of the foam metal, which is the current collector of the negative electrode, with the sealing plate 1 and the metal-metal, and the positive electrode case 5 and the positive electrode 4 are carbon-based collectors. Both are electrically connected by pressure contact via the electric layer 7. Sealing agents 8 and 9 are interposed between the sealing plate 1 and the gasket 6 and between the positive electrode case 5 and the gasket 6, and the positive electrode case 5 is caulked inward.

  Examples of the present invention will be described below.

(Example 1)
The positive electrode is a mixture in which lithium cobaltate as an active material, ketjen black as a conductive agent, and PTFE (polytetrafluoroethylene) as a binder are mixed at a weight ratio of 90: 5: 5. Was pressure-molded and used as pellets.

The foam metal used was a component ratio of 98.5% nickel, 1.5% iron, 0.77 mm thick and 400 g / m ² weight per unit. As pretreatment, 30% natural graphite and 3% carboxymethylcellulose were used. The carbon coating was immersed in the contained carbon coating, kept in an atmosphere of 200 mmHg pressure for 2 seconds to degas the air in the pores, applied to the carbon coating, removed, and dried to form a carbon layer on the nickel surface. .

The negative electrode is lithium titanate (Li _4/3 Ti _5/3 O ₄ ) as an active material, ketjen black as a conductive agent, and carboxymethyl cellulose as a thickener in a weight ratio of 94: 5: 1, Pure water was added to a solid content ratio of 25% to prepare a negative electrode paste, the paste was filled in pretreated foamed nickel, dried and rolled to a thickness of 0.77 mm, and punched to a diameter of 12 mm to obtain a negative electrode. .

  Here, the particle size distribution of the negative electrode active material was measured by a laser diffraction scattering method, and the average particle size was 0.8 μm.

As the electrolytic solution, an ethylene carbonate (EC) and ethyl methyl carbonate (EMC) mixed at a volume ratio of 1: 3 and LiPF ₆ dissolved at 1 mol / L was used.

Using these, a coin-type lithium secondary battery having a structure as shown in FIG. 1 and having a diameter of 16 mm and a thickness of 1.6 mm was produced.
(Comparative Example 1)
A battery of Comparative Example 1 having the same configuration as that of the battery of Example 1 was produced except that the carbon layer was not formed on the foamed metal made of nickel as the negative electrode current collector.
(Comparative Example 2)
A battery of Comparative Example 2 having the same configuration as that of the battery of Example 1 except that pellets that maintain the conductivity of the negative electrode only with carbon powder as a conductive auxiliary agent was used. The negative electrode pellet was prepared by mixing lithium titanate as an active material, ketjen black as a conductive agent, and styrene-butadiene copolymer as a binder in a weight ratio of 90: 5: 5. 100 mg was pressure-molded at 2 ton / cm ² to prepare a pellet having a diameter of 12 mm, and used as a negative electrode.

  The batteries of Example 1, Comparative Example 1, and Comparative Example 2 were initialized by applying a constant voltage of 2.6 V for 48 hours.

  After the battery voltage reached 2.6V by initialization, discharging was performed with a resistance value of 200Ω.

  FIG. 2 shows the discharge result. In Example 1 and Comparative Example 1, the discharge time until the battery voltage reaches 2.0 V is 0.8 hour and 0.7 hour, respectively, which is significantly larger than the 0.1 hour of Comparative Example 2. Improved. From this, it can be seen that even if an active material having a small particle size is used, the heavy load characteristics cannot be improved in the coin-type battery. In Comparative Example 2, the characteristics were poor from the beginning of the discharge test.

  Table 1 shows the results of measuring the changes in the number of days and capacity retention rate when a 2 kΩ discharge resistor is connected and stored in a high temperature environment of 60 ° C.

ここで放電容量の確認は２．６Ｖの定電圧で２４時間充電した後、５分間の休止時間をおき、抵抗値２０ｋΩで放電を行うことで実施し、放電容量は電池電圧が２．０Ｖに達するまでに流れた電気量とした。

Here, the discharge capacity is confirmed by charging for 24 hours at a constant voltage of 2.6 V, and then performing a rest period of 5 minutes and discharging at a resistance value of 20 kΩ. The amount of electricity that flowed to reach it.

  The battery capacity recovery rates in Example 1 and Comparative Example 1 were 100% and 62%, respectively, after 10 days, and 98% and 14%, respectively, after 20 days. In Example 1 in which a protective layer was formed of a carbon layer on a nickel foam metal current collector, elution of nickel was suppressed, and battery characteristics were improved.

  The coin-type lithium battery according to the present invention is particularly useful in a device that requires a heavy load.

Sectional drawing of the coin-type lithium battery concerning the Example of this invention The figure which shows the discharge time of the coin-type lithium secondary battery in a present Example

Explanation of symbols

1 Sealing plate 2 Negative electrode 3 Separator 4 Positive electrode 5 Positive electrode case 6 Gasket 7 Carbon-based conductive layer 8 Sealant layer (minus side)
9 Sealant layer (positive side)

Claims (3)

  In a coin-type lithium battery including a positive electrode, a negative electrode, an electrolyte, and a gasket, the negative electrode includes a metal oxide having an average particle size of 2 μm or less and a foamed metal current collector having a carbon layer formed on the surface. Coin type lithium battery.   The coin-type lithium battery according to claim 1, wherein the foamed metal current collector is made of nickel.   The coin-type lithium battery according to claim 1, wherein the metal oxide is made of lithium titanate.

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