JPH10247523A - Lithium secondary battery - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To ensure the safety of a lithium secondary battery that has been heated at a high temperature in a charged state. A battery contains at least one of a material that reduces a positive electrode active material that is charged at a temperature exceeding 60 ° C. and a material that oxidizes a negative electrode active material that is charged at a temperature exceeding 60 ° C. Let it.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in a lithium secondary battery, and more particularly to an improvement in safety.

[0002]

2. Description of the Related Art With the advance of electronic technology, the performance of electronic equipment has been improved, and its size and portability have been advanced. Conventional secondary batteries include a lead storage battery and a nickel-cadmium battery, but they are still insufficient in terms of high energy density. Therefore, as an alternative to these batteries, lithium secondary batteries with high energy density have been developed,
Spreading rapidly. However, lithium secondary batteries usually use flammable organic solvents for the electrolyte. Therefore, when an operation for raising the battery temperature is performed, for example, when the battery is short-circuited or when the battery is overcharged or dropped into a fire, the electrolyte may burn and the battery may explode violently. For this reason, Japanese Patent Application Laid-Open No. 4-184870 discloses a technique for incorporating a phosphate ester having a self-digesting property into an electrolytic solution. Techniques for containing an acid ester compound have been proposed. Further, Japanese Unexamined Patent Publication No.
Japanese Patent No. 45544 proposes a technique in which a chlorine-substituted ester compound is used in an electrolytic solution itself to increase the flash point and ensure safety.

[0003]

However, if the above-mentioned conventional self-digestible substance or a substance having a high flash point is contained in the electrolytic solution, the ionic conductivity of the electrolytic solution is reduced. Discharge characteristics and low-temperature characteristics are reduced.
In addition, even when such a method was used, it was found that the battery exploded when dropped into a fire in a charged state (particularly an overcharged state). That is, even if a technique for making the electrolyte solution nonflammable is employed, it is difficult to prevent a violent explosion when the battery is extremely heated in a charged state. An object of the present invention is to provide a lithium battery that does not reduce the ionic conductivity of the electrolyte and that suppresses a violent explosion even when the battery is exposed to an extremely high temperature in a substantially fully charged state. It is to provide a secondary battery.

[0004]

In order to solve the above-mentioned problems, the present invention provides a material for reducing a positive electrode active material which is charged at a temperature exceeding 60 ° C. and a negative electrode active material which is charged at a temperature exceeding 60 ° C. The battery is characterized in that at least one of materials that oxidize a substance is contained in the battery. The reason that the safety can be ensured by incorporating the above materials into the battery is that when the positive electrode active material in the charged state is reduced with a reducing agent, the potential advances in a negative direction (discharges) and the negative electrode active in the charged state is discharged. This is because, when a substance is oxidized with an oxidizing agent, the potential advances (discharges) in a noble direction, so that the energy of the positive and negative electrode active materials decreases. When the battery is heated, a violent explosion does not occur because the active material has no energy in advance as described above. In addition, since the oxidizing agent and the reducing agent can be appropriately selected from substances that do not dissolve in the electrolytic solution, they do not lower the ionic conductivity of the electrolytic solution unlike the conventional flame retardant. Since the general operating temperature of the battery is −20 ° C. to 60 ° C., the temperature at which oxidation and reduction reaction of the positive electrode active material are started is 6 ° C.
It is necessary that the temperature be appropriately higher than 0 ° C.

[0005]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS. 1 and 2 using a cylindrical lithium ion secondary battery (18650 type) as an example. First, the reducing agent and the oxidizing agent added to the electrode plate will be described. Using tin powder as the reducing agent and iron chloride powder as the oxidizing agent, a coating layer of low-density polyethylene is formed on these surfaces by the following method. A tin powder having a particle size of 5 μm and a low-density polyethylene powder having a particle size of 0.5 μm are charged into a hybridization system manufactured by Nara Machinery. As shown in FIG. 1, in the step of coating the polyethylene, the polyethylene powder 1 is applied to the surface of the tin powder 2, and then a film forming process is performed to form the low-density polyethylene layer 3 on the surface of the tin powder 2. The iron chloride powder can also form the low-density polyethylene layer 3 on the surface of the powder by the above method.

The melting point of this polyethylene is about 115 ° C.
The molecular weight is about 4800. Naturally, the melting point of polyethylene can be changed within a certain range depending on crystallinity, molecular weight, branching, density and the like. The reducing agent and the oxidizing agent added to the electrode plate start to work only when the battery is heated and the low-density polyethylene layer is melted to expose the surfaces of the reducing agent and the oxidizing agent. Therefore, polyethylene having a melting point more suitable for the battery use conditions is properly used.

Next, the fabrication of the positive and negative electrodes will be described. The positive electrode was made of a lithium cobaltate having a diameter of 20 μm, a carbon powder having a diameter of 3 μm, polyvinylidene fluoride, and a reducing agent having the above-described tin powder coated with polyethylene at 80: 5:
A slurry is prepared by adding and mixing N-methylpyrrolidone at a ratio of 5:10 wt%. Then, this slurry is applied to both sides of a 20 μm aluminum foil, dried and rolled,
A positive electrode mixture electrode is prepared, and then cut into a width of 54 mm to form a strip. The negative electrode was composed of a carbon material having a particle diameter of 20 μm, polyvinylidene fluoride, and the above-described iron chloride powder at a ratio of 80: 1.
A slurry is prepared by charging and mixing with N-methylpyrrolidone at a ratio of 0:10 wt%. This slurry is applied to both sides of a 10 μm copper foil, dried and rolled to produce a negative electrode mixture electrode, and then cut into a width of 56 mm to form a strip.

Next, as shown in FIG. 2, the positive electrode 4 and the negative electrode 5 are wound through a separator 6 made of a polyethylene microporous film having a thickness of 25 μm and a width of 58 mm to form a spiral electrode group. The wound electrode group is inserted into a battery container, and a nickel tab terminal that has been welded to the copper foil of the negative electrode current collector in advance is welded to the bottom of the container. Next, 3.7 ml of an electrolyte obtained by dissolving LiPF ₆ at a concentration of 1 mol / l in a solution in which propylene carbonate and dimethyl carbonate were mixed at a volume ratio of 1: 1 was injected into the battery container. Next, the aluminum tab terminal, which was previously welded to the aluminum foil of the positive electrode current collector, was welded to the upper lid, and the upper lid was placed at the top of the battery container via an insulating gasket. did. The initial capacity of the manufactured battery is 1400 mAh.

The reducing agent may be any material that causes an oxidation reaction, such as tin chloride, iron powder, iron sulfate, magnesium, aluminum amalgam, aluminum nickel alloy, lithium aluminum hydride, etc., in addition to the above-mentioned materials.
The oxidizing agent may be any material that causes a reduction reaction, such as silver oxide, copper acetate, copper oxide, manganese dioxide, and lead dioxide, in addition to the above-described materials. In particular, a material that causes a mild reaction is desirable. A material that does not affect the battery reaction and does not cause an oxidation or reduction reaction at 60 ° C. or lower is most preferable. On the other hand, the temperature at which the oxidation and reduction reactions take place is determined by the battery operating temperature. Usually, the reaction may proceed at a temperature exceeding 60 ° C., but when the battery is used in a high-temperature atmosphere, the reaction proceeds at a temperature exceeding 100 ° C. The reaction should be better.

[0010]

EXAMPLE A lithium ion secondary battery (Example 1), a battery containing only a reducing agent in the positive electrode (Example 2), and a oxidizing agent containing only the oxidizing agent in the negative electrode according to the above-described embodiment of the present invention. Battery (Example 3) and a battery (conventional example) in which the positive and negative electrodes contained neither a reducing agent nor an oxidizing agent were manufactured in the same manner as in the manufacturing method described in the embodiment. And charge 1400
mA, 4.2V, 2.5h, discharge 1400mA, final voltage 2.5V, check the capacity, then charge 1400m
A, 4.2 V, 3 h and 1250 mA, 4.4 V, 3 h were performed, and a projection test (battery drop-in into the fire) according to UL standard was performed. In each case, the test was performed with 10 batteries, and the results are shown in Table 1. The passed number is shown in the table.

[0011]

[Table 1]

As can be seen from Table 1, the battery according to the present invention has a projection test after charging with a normal voltage (4.2 V) and a projection test after overcharging with a voltage (4.4 V) with a high voltage. In most cases, it has passed, suppressing severe explosions and improving safety. In particular, in this battery configuration, the safety was enhanced by adding a reducing agent to the positive electrode. Further, with respect to other battery characteristics such as charge / discharge characteristics up to 60 ° C. and cycle characteristics, similar results were obtained between the product of the present invention and the conventional product. The reason why the product of the present invention has improved safety is considered to be the result described below. In a normal battery use range, both the reducing agent and the oxidizing agent contained in the positive and negative electrode mixture electrodes are covered with the polyethylene layer, so that the battery reaction is not affected at all. However, when the temperature of the battery rises to 115 ° C. or higher, the polyethylene layer melts, and the reducing agent and the oxidizing agent wrapped in the polyethylene layer come into contact with the active material of the battery, thereby oxidizing the battery. Causes a reduction reaction. For this reason, both the positive and negative electrode active materials approach a discharged state and the energy is reduced, so that a violent explosion at a high temperature can be suppressed.

In the above embodiment, an example in which the reducing agent and the oxidizing agent are covered with polyethylene is shown. However, even if the material is not polyethylene, it is insoluble in the electrolytic solution and can be melted at an appropriate temperature exceeding 60 ° C. It may be coated with any material,
Even if no coating is applied, the reducing agent and the oxidizing agent may be used as long as they have the property of acting as the reducing agent and the oxidizing agent only when the temperature of the reducing agent and the oxidizing agent itself becomes high. It is more preferable that one material has the above characteristics.

[0014]

The present invention relates to a material for reducing a charged positive electrode active material, a material for oxidizing a charged negative electrode active material at an appropriate temperature where the battery temperature exceeds 60.degree. Since it is characterized in that it is contained in the battery, it is possible to suppress a violent explosion of a fully charged battery at a high temperature. Even if the battery is not in a fully charged state, the active material in a charged state can be brought into a discharged state in advance at a certain high temperature. Can be.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating an example of preparation of a reducing agent according to the present invention.

FIG. 2 is an explanatory diagram showing a configuration of a battery according to an example of the present invention.

[Explanation of symbols]

 1. 1. polyethylene powder 2. Tin powder 3. polyethylene layer Positive electrode 5. Negative electrode 6. Separator

Claims (2)

[Claims] 1. A lithium secondary battery comprising a positive electrode, a negative electrode active material and an organic electrolyte which can be repeatedly charged and discharged,
The battery contains at least one of a material that reduces a positive electrode active material that is charged at a temperature higher than 60 ° C. and a material that oxidizes a negative electrode active material that is charged at a temperature higher than 60 ° C. Lithium secondary battery. 2. A reducing agent having a coating layer in which a material for reducing a positive electrode active material melts at a predetermined temperature exceeding 60 ° C., and a coating layer in which a material for oxidizing a negative electrode active material melts at a predetermined temperature exceeding 60 ° C. The lithium secondary battery according to claim 1, which is an oxidant having the formula: