Chemistry: electrical current producing apparatus – product – and – Means externally releasing internal gas pressure from closed...
Reexamination Certificate
1998-07-23
2001-06-05
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Means externally releasing internal gas pressure from closed...
C429S054000, C429S055000, C429S056000
Reexamination Certificate
active
06242126
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates to explosion-proof top assemblies for sealed cells to be used in sealing sealed cells, especially high energy density cells such as lithium secondary cells, and their method of producing.
BACKGROUND OF THE TECHNOLOGY
In recent years, there has been rapid progress in portable and/or cordless designs of audiovisual equipment, personal computers and other electronic equipment. As the power supply for these electronic equipment, high capacity type of non-aqueous electrolyte secondary batteries such as represented by various alkaline storage batteries and lithium secondary batteries are suitable. These non-aqueous electrolyte secondary batteries are desired to be implemented as sealed batteries with a high energy density and with superior load characteristics.
On the other hand, sealed batteries with a high energy density tend to generate abnormal gas inside the cells due to chemical reaction resulting from troubles in associated equipment including the charger or from over charge or erroneous use, thus causing an excessive pressure inside cells, explosion of cells, or damage to the electronic equipment using the cells as the power supply.
In order to prevent these accidents, these types of cells have hitherto been equipped with a safety device against explosion to allow gas to exhaust by opening a vent when the cell internal pressure exceeds a preset value. Furthermore, as non-aqueous electrolyte secondary batteries have a danger of ignition due to a rapid temperature rise, they are equipped with a reliable safety mechanism against explosion which will completely cut off electric current prior to exhausting gas by detecting the internal pressure.
As an example, in Japanese Laid-Open Patent No. Hei 6-196150, a mechanism is disclosed wherein a vent on top of a cell and a terminal plate having a vent hole are made electrically conductive through their central welded section, and when the internal pressure rises to a predetermined value, the vent which is exerted with the pressure through the vent hole of the terminal plate will be detached from the section welded with the terminal plate by an outwardly deforming stress due to the pressure through the vent hole of the terminal plate, thereby cutting off an electric current.
In the above-described explosion-proof safety mechanism, ultrasonic welding which is capable of welding to a low weld strength is employed in welding the vent and the terminal plate because of the necessity of welding necessary portions of the vent and the terminal plate to a weld strength low enough to allow detachment at a certain internal pressure. However, since ultrasonic welding causes fusion by vibration heating only on the surface of the work piece, there is a possibility of causing a large dispersion in the weld strength.
Consequently, in the above-described explosion-proof safety mechanism, as the pressure of cutting off an electric current is determined dependent on the weld strength of the welded portion, the pressure of cutting off an electric current varies with the variability of the weld strength, exhibiting a drawback of not being able of setting to a fixed value. As a result, it suffers problems of cutting off an electric current before the cell internal pressure rises to a predetermined value, or not cutting off an electric current even when the cell internal pressure has risen to a predetermined value.
Therefore, a higher-accuracy method of cutting off an electric current has become necessary which is not affected by the weld strength in cutting off an electric current.
DISCLOSURE OF THE INVENTION
It is the object of this invention to provide an explosion-proof seal plate for a sealed cell and a production method thereof which can prevent with a high accuracy accidents such as ignition by cutting off an electric current without fail in the event of an increase in cell internal pressure and can limit the increase in resistance due to permeation of an electrolyte.
In order to attain the object, this invention comprises an upper metallic foil and a lower metallic foil disposed one on top of the other, wherein the two metallic foils are so constructed as to be electrically connected in sections encircled by concentric rings of thin portions, the breaking strength of the thin portion of the lower metallic foil is smaller than the breaking strength of the thin portion of the upper metallic foil, and the diameter of the concentric thin portion of the upper metallic foil is made larger than the diameter of the concentric thin portion of the lower metallic foil. The invention also comprises two elastic metallic foils, upper and lower, disposed one on top of the other, wherein the upper metallic foil is provided with a central concave portion swelling downward and the lower metallic foil is provided with a central convex portion swelling upward and an easy-to-break portion of which the breaking strength is set at a value at which it breaks when the cell internal pressure rises to a predetermined value, the periphery of each of the two metallic foils is fixed with an insulating gasket interposed and the two metallic foils are electrically connected by welding the tip of the concave portion and the tip of the convex portion. Consequently, with this invention, when the cell internal pressure rises to a value predetermined by the breaking strength of the easy-to-break portion of the lower metallic foil, the easy-to-break portion will break thereby causing the lower metallic foil and the upper metallic foil to separate from each other thus cutting off an electric current flowing through the connecting section of the two metallic foils.
As the pressure at which an electric current is to be cut off is determined by the breaking strength of the easy-to-break portion, it is not necessary to cause the connecting section of the two metallic foils to be detached by cell internal pressure as has heretofore been practiced, making it possible to firmly weld the connecting section by laser welding and the like.
Also, in welding the two metallic foils, when the peripheries of both metallic foils are placed one on top of the other with an insulating gasket interposed, the central concave portion of the upper metallic foil swelling downward and the central convex portion of the lower metallic foil swelling upward are elastically brought into contact, thus leaving no gap between the concave portion and the convex portion without using any jig or other auxiliary means, and allowing a defect-free laser welding at all times.
The method of producing an explosion-proof seal plate (hereinafter called “seal plate”) for a sealed cell of the present invention comprises a process of disposing one on top of the other and opposite to each other an upper elastic metallic foil provided with a central concave portion swelling downward and a lower elastic metallic foil provided with an easy-to-break portion provided on a central convex portion swelling upward, a process of pressing with each other the tips of the concave portion and the convex portion by disposing the peripheries of the two metallic foils one on top of the other with an insulating gasket interposed which gasket having a thickness smaller than the sum of the thicknesses of the swelling of each of the metallic foils from respective periphery, a process of fixing peripheries of the two metallic foils by holding them from the top and the bottom with a fixing jig, and a process of welding by laser welding the tips of the concave portion and the convex portion which are in contact with each other, thereby forming a connecting section. With this invention, as the periphery of each of the two metallic foils is placed one on top of the other with an insulating gasket interposed having a thickness smaller than the sum of the thicknesses of the concave swelling and the convex swelling from the respective peripheries, the tips of the concave portion and the convex portion are brought into contact with each other, and as the metallic foils are slightly bent causing the portion in
Hirakawa Yasushi
Kashihara Yoshihiro
Mori Katsuhiko
Saito Kazuo
Watanabe Kazuhiko
Chaney Carol
Matsushita Electric - Industrial Co., Ltd.
Ratner & Prestia
Ruthkosky Mark
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