JP2010061941A - Coin type lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a coin type lithium secondary battery which eliminates the need for a high level processing control in manufacturing a battery and is superior in safety. <P>SOLUTION: The coin type lithium secondary battery houses a power generating element having a separator 3 interposed between a positive electrode 4 and a negative electrode 2 in a battery case 5 and a sealing plate 1 together with an electrolyte, and seals the battery case 5 and the sealing plate 1 by a gasket 6. The positive electrode 4 and the negative electrode 2 are conducted to the battery case 5 and the sealing plate 1 through a conductive layer 7 which is made of a conductive material having a resin melting at a low temperature as a nucleus and covering its surface with a conductive material. Thereby, when the temperature has risen, the resin melts and forms an insulating layer to suppress a thermorunaway reaction of the coin type lithium secondary battery. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a coin-type lithium secondary battery, and particularly provides a coin-type lithium secondary battery with improved safety.

  In recent years, in cordless and portable electronic devices typified by mobile communication, lithium batteries having high energy density are widely used as the size and weight are reduced. Batteries have become widely used as power sources for mobile phones. In addition, with the spread, there is a demand from the market to further increase the energy density, and intense development competition is taking place. Under such circumstances, as the energy density of the battery increases, the risk increases, and it has been reported that the battery is ignited or ruptured due to a discrepancy in matching with an electronic device or a malfunction.

  Therefore, various studies have been made to add a safety mechanism to the lithium secondary battery, and improvements have been made so that the lithium secondary battery does not ignite or rupture even if it is used unexpectedly. In particular, rectangular and cylindrical lithium secondary batteries have a relatively high degree of freedom in design, and various efforts have been made, such as attaching a safety circuit. The degree of freedom to provide functions is low and has not been studied much.

As a precedent example of safety improvement being studied in the coin-type shape, as shown in Patent Document 1, an additive is added to the system, and when the lithium secondary battery is overcharged, it reacts proactively, The reaction product forms a resistive film on the electrode and suppresses side reactions that accompany heat generation and gas generation that occur in the lithium secondary battery during overcharging, such as active material peroxidation and electrolyte decomposition. Examples of preventing ignition and rupture, and as seen in Patent Document 2, when a part of the outer can is processed thinly by etching or the like, gas generation occurs due to overcharge or the like, and the pressure inside the battery rises. An example is an example in which the thin part of the outer can processed before the internal pressure becomes extremely large and causes dangerous rupture breaks to release the pressure.
Japanese Patent No. 3344424 JP 2002-134072 A

  However, the problem with the technique of Patent Document 1 is that the reaction of the additive occurs not only in an abnormal state but also in a normal use environment, which leads to deterioration of battery characteristics. Further, in the technique of Patent Document 2, since it is very difficult to control the processing, the accuracy cannot be maintained, and it is difficult to obtain the reproducibility of the battery internal pressure leading to the rupture. In addition, the production of lithium secondary batteries also causes problems in mass production, such as the production conditions being limited due to problems such as the strength of the outer can.

  Therefore, the present invention is different from the conventional methods such as the above-described prior examples, and does not affect battery characteristics, and does not require advanced processing control in battery manufacture, and is a coin type that is excellent in safety. An object is to provide a lithium secondary battery.

In order to solve the above problems, the present invention stores a power generation element configured by interposing a separator between a positive electrode and a negative electrode together with an electrolytic solution in a battery case and a sealing plate. In a coin-type lithium secondary battery sealed with a gasket, the positive electrode and the negative electrode are placed through a conductive layer made of a conductive material having a resin melted at a low temperature as a core and a surface coated with a conductive material. It is characterized by being electrically connected to the case and the sealing plate.

  According to the present invention, it is possible to easily produce a lithium secondary battery with improved safety even in a general simple coin-type shape.

  The present invention relates to a coin formed by storing a power generation element configured by interposing a separator between a positive electrode and a negative electrode together with an electrolyte in a battery case and a sealing plate, and sealing the battery case and the sealing plate with a gasket. In a lithium secondary battery, the positive electrode and the negative electrode are electrically connected to the battery case and the sealing plate through a conductive layer made of a conductive material having a resin melted at a low temperature as a core and a surface coated with a conductive material. It is characterized by having.

  Generally, in a coin-shaped battery, a conductive layer is interposed between the positive electrode and negative electrode in the form of pellets and the battery case and sealing plate constituting the outer can. Is composed of graphite and an adhesive that fixes the graphite. In the present invention, instead of graphite, the surface of a resin that melts at a low temperature is coated with a conductive material such as graphite.

  The side reaction and temperature greatly contribute to the ignition and rupture of the battery. When a large current flows due to external overcharge or internal short circuit, a side reaction other than the battery reaction occurs, and the temperature rises at that time. As the temperature rises, the reaction is further accelerated, and the accelerated reaction causes a further increase in temperature. Such a thermal runaway reaction cycle is repeated, causing the battery to ignite and rupture.

  In order to suppress the ignition and rupture of the battery, it is important to stop the cycle of the thermal runaway reaction, and it is effective to prevent the current from flowing so as not to cause a side reaction.

  In the present invention, since the resin in the conductive layer is melted by the temperature rise to form the insulating layer, the internal resistance of the battery is increased and no current flows. By suppressing the thermal runaway reaction in this way, battery ignition and rupture can be suppressed.

  Here, it is not necessary to simply include a resin that melts at a low temperature in the conductive layer. The conductive layer has an important function of maintaining electrical contact between the positive electrode and negative electrode in the form of pellets and the battery case and sealing plate constituting the outer can. If this function does not work, the internal resistance of the battery increases and the battery characteristics Cause a drop in Simply mixing the resin and the conductive material causes the resin to become a resistance component, reducing the number of electrical contacts, and thus increasing the internal resistance of the battery and degrading the battery characteristics.

  As a method for solving the above problems, there is a method of coating a conductive material on the surface of a resin. By coating the surface of the insulating resin with a conductive material, the resin has the same function as that of a conductive material such as graphite, while maintaining the electrical contact between the pellet-like positive electrode and negative electrode, Electron transfer can be made smooth using a sex substance as a medium.

  As a method for coating the surface of the resin with a conductive material, there are a mechanochemical method such as Ongmill and a heat treatment method in which only the resin surface is melted to fuse the conductive material.

  As a selection of the resin material, a material having a low melting point and excellent chemical resistance is preferable, and examples thereof include polyethylene, polypropylene, a styrene copolymer, and a polyester resin. An example of the melting point of the resin is shown in (Table 1). The melting point of polyethylene varies depending on the molecular weight, and the lower the molecular weight, the lower the melting point, which is desirable for the present invention. However, depending on the intended use of the battery, it may be exposed to a high temperature environment, and in that case, it is necessary to select a material having an appropriate melting point.

  The shape of the resin is preferably a shape that is easily melted, and in the case of a particle shape, a fine particle size is preferable. It is also possible to use a fiber having a small diameter. This is because the resin in the conductive layer easily takes a three-dimensional structure, and when the resin melts at a high temperature, the resin can easily form an insulating layer by breaking the three-dimensional structure.

  As the conductive substance, a material that is conductive and easily adheres to a resin is desirable, and examples thereof include artificial graphite, natural graphite, acetylene black, ketjen black, and noble metals. Finer particles than the resin particle size are preferred so that the resin surface can be easily coated with a high coverage.

  Hereinafter, the shape of the coin-type lithium secondary 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 made of stainless steel, and a positive electrode 4 is arranged on the battery case 5 side made of stainless steel. A separator 3 made of a polypropylene nonwoven fabric is interposed between the negative electrode 2 and the positive electrode 4. Electrical connection is established between the sealing plate 1 and the negative electrode 2 and between the battery case 5 and the positive electrode 4 through a conductive layer 7 by pressure contact. A gasket 6 is disposed between the sealing plate 1 and the battery case 5, and the battery case 5 is crimped inward to be sealed.

  FIG. 2 illustrates the structure of the conductive layer in the present invention. As shown in FIG. 2A, the conductive layer 7 is formed of a resin 9 whose surface is covered with a conductive material 8 such as graphite. Usually, the electroconductive substance 8 mediates the electron transfer between the negative electrode 2 and the sealing plate 1 and between the positive electrode 4 and the battery case 5, so that the battery reaction can be smoothly promoted. However, when the coin-type lithium secondary battery reaches a high temperature environment, as shown in FIG. 2B, the resin 9 melts to form the insulating layer 10 that breaks the connection of the conductive material 8, so that the negative electrode 2-sealing Electron transfer between the plates 1 and between the positive electrode 4 and the battery case 5 is inhibited, all reactions including the battery reaction can be stopped, and rupture and ignition of the coin-type lithium secondary battery can be suppressed.

  Examples of the present invention will be described below.

Example 1
A paste for forming a conductive layer was prepared.

  35 g of powdered polyethylene powder having an average particle diameter of 20 μm and 45 g of artificial graphite having an average particle diameter of 4 μm were mixed with a mixer, and the mixture was put into an ong mill chamber (2 l) and subjected to mechanochemical treatment at a rotation speed of 1500 rpm for 5 minutes. . The treated powder, sodium salt powder of carboxymethylcellulose (CMC) and ion-exchanged water were mixed at a mass ratio of 26: 1: 73 and stirred to prepare a paste.

  5 μl of the paste was applied to the center of the battery case so as to have a diameter of about 8 mm, and dried at 80 ° C. for 3 minutes to remove moisture, thereby forming a carbon layer. The battery case in which the carbon layer was formed was stored for 12 hours under a reduced pressure of 400 mmHg or less before the battery was produced, and water was further removed.

  The coin-type lithium secondary battery was manufactured as follows.

Using the battery case prepared above, lithium cobaltate as an active material for the positive electrode, ketjen black as a conductive agent, and PTFE (polytetrafluoroethylene) as a fluororesin as a binder in a mass ratio of 90: 5: 5 and mixed with pressure to form pellets having a diameter of 10 mm and a thickness of 0.5 mm. The negative electrode is lithium titanate as an active material, ketjen black as a conductive agent, and styrene-butadiene as a binder. The polymer was mixed at a mass ratio of 90: 5: 5, pressure-molded and used as pellets with a diameter of 11 mm and a thickness of 0.5 mm. Propylene carbonate (PC) and diethyl carbonate (DEC) were used as the electrolyte. ) were mixed at a volume ratio of 45:55, and the LiPF ₆ was used after dissolving 1 mol / l, also, the gasket material with PPS 16mm diameter having a structure as shown in 1, to prepare a coin-type lithium secondary battery having a thickness of 1.6mm size. This is Example 1.

(Example 2)
A coin-type lithium secondary battery manufactured in the same manner as in Example 1 except that polypropylene is used as a resin that melts at a low temperature is referred to as Example 2.

(Example 3)
A coin-type lithium secondary battery manufactured in the same manner as in Example 1 except that a styrene copolymer is used as a resin that melts at a low temperature is referred to as Example 3.

(Comparative Example 1)
A coin-type lithium secondary battery manufactured in the same manner as in Example 1 is used as Comparative Example 1 except that when a paste for forming a conductive layer is prepared, a paste is prepared by simply mixing powdered polyethylene and artificial graphite.

(Comparative Example 2)
A coin-type lithium secondary battery produced in the same manner as in Example 1 except that the paste was produced only with artificial graphite when producing the paste for forming the conductive layer is referred to as Comparative Example 2.

  The coin type lithium secondary batteries of Examples 1 to 3 and Comparative Examples 1 and 2 were initialized by applying a constant voltage of 2.6 V for 48 hours.

  After initialization, the internal resistance of the coin-type lithium secondary battery was measured. Next, each coin-type lithium secondary battery was stored in an atmosphere of 120 ° C. for 1 hour, then stored at room temperature for 1 day, and then the internal resistance of the coin-type lithium secondary battery was measured again. The results are shown in (Table 2).

  In Example 1, the initial internal resistance is not different from that in Comparative Example 2, but the internal resistance value is increased after storage at 120 ° C. This is because the polyethylene particles contained in the conductive layer were dissolved to form an insulating film.

  In Example 2, the same tendency as in Example 1 was observed, but the amount of increase in internal resistance was smaller than in Example 1. This is because there is a difference in melting point between polyethylene and polypropylene of the resin contained in the conductive layer, and the melting point is low because polypropylene has a higher melting point.

  Example 3 was not significantly different from Example 1.

  In Comparative Example 1, the internal resistance of the coin-type lithium secondary battery after initialization is high. This is because the insulating portion of the resin is present on the particle surface, so that the area that is in electrical contact with the pellets as electrodes is reduced. After storage at 120 ° C., as in Example 1, the resin in the conductive layer melted to form an insulating layer, so that the internal resistance value increased.

  In Comparative Example 2, the initial internal resistance of the coin-type lithium secondary battery was low, and no significant increase in internal resistance was observed even after storage at 120 ° C.

  As described above, in the examples, without deteriorating the initial battery characteristics, when the temperature rose, the battery resistance increased and no current flowed. Thereby, the ignition and rupture of the coin-type lithium secondary battery can be suppressed.

  The coin-type lithium secondary battery according to the present invention is particularly useful in improving the safety of the coin-type lithium secondary battery.

Sectional drawing of the coin-type lithium secondary battery concerning the Example of this invention (A) Schematic diagram of a conductive layer according to an embodiment of the present invention (state without high temperature load), (b) Schematic diagram of a conductive layer according to an embodiment of the present invention (state with high temperature load)

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Sealing plate 2 Negative electrode 3 Separator 4 Positive electrode 5 Battery case 6 Gasket 7 Conductive layer 8 Conductive substance 9 Resin 10 Insulating layer

Claims (3)

A coin-type lithium secondary battery in which a power generation element configured by interposing a separator between a positive electrode and a negative electrode is housed in a battery case and a sealing plate together with an electrolyte, and the battery case and the sealing plate are sealed with a gasket. In the battery, the positive electrode and the negative electrode are electrically connected to the battery case and the sealing plate through a conductive layer made of a conductive material having a resin melted at a low temperature as a core and a surface coated with a conductive material. A coin-type lithium secondary battery. The coin-type lithium secondary battery according to claim 1, wherein the resin that melts at a low temperature is one of polyethylene, polypropylene, styrene copolymer, polyester resin, or a mixture thereof. . 2. The coin-type lithium secondary battery according to claim 1, wherein any one of artificial graphite, natural graphite, acetylene black, ketjen black, or a mixture thereof is used as the conductive material.

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