Abstract:
An electrode assembly includes a positive plate, a negative plate and a separating plate interposed between those two plates, a container receiving the electrode assembly inside thereof, a cap plate fixed onto an opening of the container to seal the container, and a safety valve formed on the region where the container and the cap plate are joined.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims priority to and benefit of Korean Patent Application No. 2003-39959 filed on Jun. 19, 2003 in the Korean Intellectual Property Office, the entire disclosure of which is incorporated herein by reference. 
   BACKGROUND 
   (a) Field 
   The present invention relates to a secondary battery, and more particularly, to a structure of a safety valve in a secondary battery. 
   (b) Description of the Related Art 
   A secondary battery is a rechargeable battery that can be made into a smaller size or a larger size. Common types of secondary batteries include the nickel-hydrogen battery, the lithium battery and the lithium-ion battery. Depending on the external shapes, they may be classified into cylindrical type batteries and square type batteries (e.g., square shaped or rectangular shaped). 
   Generally, the secondary battery includes a power generating element, that is, an electrode assembly having a positive plate, a negative plate and a separating plate, and a metal container receiving the electrode assembly and electrolyte, and a cap assembly fixed onto the opening of the container to seal the container. 
   Depending on the structure of the secondary battery, the cap assembly can be made in various forms, one of which has a cap plate welded onto the opening of the container to seal the container, a terminal pin, and a gasket insulating the cap plate from the terminal pin. 
   The terminal pin is welded onto a negative tap that is drawn out from the negative plate so that the terminal pin functions as a negative terminal. 
   Furthermore, a positive tap drawn out from the positive plate is electrically connected directly to the bottom of the cap plate or to the inner wall of the container to make the whole outside of the battery except the terminal pin function as a positive terminal. 
   Meanwhile, a secondary battery has a safety valve in order to prevent the battery from exploding by reducing the internal pressure when gases are generated in the battery and the internal pressure increases to a level higher than a prescribed level. 
   A conventional safety valve is either formed as an integrated structure of the cap plate or the container through mechanical, etching or electroforming process, or formed as a separate structure attached on the cap plate or the container. 
   Therefore, conventionally a safety valve must be provided separately to a secondary battery for its safety, and consequently it is necessary to add more process and equipment, which reduces the productivity (increase of complexity of process and manufacturing costs) of a secondary battery. 
   SUMMARY 
   In one embodiment of the present invention, a secondary battery is provided having a safety valve that is formed without a separate additional process. In another embodiment, a method of manufacturing a secondary battery is provided having a safety valve that is formed without a separate additional process. 
   In another embodiment, a secondary battery having an electrode assembly includes a positive plate, a negative plate and a separating plate interposed between those two plates, a container receiving the electrode assembly inside thereof, a cap plate fixed to an opening of the container to seal the container, and a safety valve formed on the region where the container and the cap plate are joined. 
   In this embodiment, the container and the cap plate are joined by welding, and the safety valve is formed on a sealing part where the container and the cap plate are sealed. The sealing part includes a first sealing part formed with a certain welding strength, and a second sealing part formed with a weaker welding strength than the first sealing part. The second sealing part forms the safety valve. 
   The sealing part includes a pair of long edges and a pair of short edges, and the safety valve can be formed on at least one of the long edges, and is formed with a length that is equal to or less than 30% of L, where L is the whole length of the long edge. Also, a length (l) can be formed to be 5 mm and 20 mm, where the length (l) is the length of the safety valve. 
   In one embodiment, the sealing part includes a pair of long edges and a pair of short edges, and the safety valve can be formed on at least one of the short edges. In another embodiment, the safety valve can be formed on the corners that are formed between the long edges and the short edges. 
   The container may be formed in a square or a cylindrical shape. 
   Also, a method of manufacturing a secondary battery according to one embodiment of the present invention includes a method of manufacturing a square secondary battery having a safety valve that is to be exploded to reduce the internal pressure when the internal pressure of the battery reaches a level higher than a predetermined level. A low sealing part functions as the safety valve by forming the low sealing part on a portion of a sealing part with a weaker welding strength than the other portion. The welding strength can be controlled when the sealing part is formed by welding a cap plate onto an opening of a square shaped container. 
   Also, a method of manufacturing a secondary battery according to another embodiment of the present invention includes a method of manufacturing a cylindrical secondary battery having a safety valve that is to be exploded to reduce the internal pressure when the internal pressure of the battery reaches a level higher than a predetermined level. A low sealing part functions as the safety valve by forming the low sealing part on a portion of the sealing part with a weaker crimping strength than the other portion. The crimping strength can be controlled when the sealing part is formed by crimping a cap plate onto an opening of a cylinder shaped container. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a partial exploded perspective view of a secondary battery according to the first embodiment of the present invention. 
       FIG. 2  is a front view of a secondary battery according to the first embodiment of the present invention. 
       FIG. 3  is a partial perspective view illustrating a sealing part of a secondary battery according to the first embodiment of the present invention. 
       FIG. 4  is a partial perspective view illustrating a sealing part according to another embodiment of the present invention. 
       FIG. 5  is a partial perspective view illustrating a searling part according to yet another embodiment of the present invention. 
       FIG. 6  is a partial perspective view illustrating a secondary battery according to still yet another embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   Exemplary embodiments of the present invention will now be described in detail with reference to the accompanying drawings. 
     FIG. 1  is a partial exploded perspective view of a secondary battery according to the first embodiment of the present invention, and  FIG. 2  is a side view of the  FIG. 1  in an assembled form. 
   Referring to the drawings, a secondary battery according to an embodiment of the present invention includes a container  10  with an opening  12  on one side, an electrode assembly  20  that is inserted into the container  10  through the opening  12 , and a cap assembly  30  fixed to the opening  12  of the container  10  to seal the container  10 . 
   The electrode assembly  20  is formed with a positive plate  22 , a negative plate  24  and a separating plate  26 . According to an exemplary embodiment of the present invention, as shown in  FIG. 1 , the electrode assembly  20  in a form of a jelly roll can be used that is formed by stacking and winding the positive plate  22 , the negative plate  24  and the separating plate  26 . 
   The negative plate  24  includes a negative collector of a thin metal plate in a strip form, and a thin copper plate can be used as the negative collector. On at least one side of the negative collector is formed a negative coating portion that is coated with a negative metal composite comprised of a negative active material. 
   Also, the positive plate  22  includes a positive collector of a thin metal plate in a strip form, e.g., a thin aluminum plate. On at least one side of the positive collector is formed a positive coating portion that is coated with a positive metal composite with a positive active material. 
   On the upper part of the electrode assembly  20  are drawn out a positive tap  28  and a negative tap  28 ′ that are electrically connected to the positive plate  22  and the negative plate  24  respectively. A thin nickel plate can be used as a negative tap  28 ′, and a thin aluminum plate can be used as a positive tap  28 , but are not limited thereto. The positive tap  28  and the negative plate  28 ′ can be arranged in an opposite way to that in  FIG. 1 . 
   Meanwhile, the container  10  can be made of a metallic material with a shape of an approximate hexahedron, and so the container, itself, can function as a terminal. According to an embodiment of the present invention the container  10  can be made of aluminum or aluminum alloy that is light and conductive metal. Also, the container  10  has an opening  12  on one side, through which the opening the electrode assembly  20  can be inserted into the container  10 . 
   The container  10  can be formed in a square shape the corners of which are angled. Although it is not shown in the drawing, the angled corners can also be formed in a rounded shape. 
   The cap assembly  30  is placed on the opening  12  of the container  10  to seal the container  10 , and the cap assembly  30  has a cap plate  32  that is welded directly onto the opening  12  of the container  10 . The container  10  and the cap plate  32  can be made of the same metallic material to make the welding easier. 
   The cap assembly  30  has a terminal pin  36  which goes through the cap plate  32  while insulated by the gasket  34 . An insulating plate and a terminal plate (not shown in the drawing) are added on the lower part of the terminal pin  36  to insulate the terminal pin  36  from the cap plate  32 . The negative tap  28 ′ is welded onto the lower part of the terminal pin  36  so that the terminal pin  36  can function as a negative terminal. 
   Meanwhile, the positive tap  28  that is drawn out of the positive plate  22  is connected electrically directly to the bottom of the cap plate  32  or to the inside of the container  10 , and thereby, the outside of the whole battery except the terminal pin  36  can function as a positive terminal. 
   However, the structure of the positive terminal and negative terminal is not limited only to that explained above, and the structure of the positive terminal, like the structure of the negative terminal, can be formed through a separate terminal pin. Also, other structures can be applied thereto. 
   After the electrode assembly  20  is inserted into a container  10 , a protection case  38  made of an insulating material can be placed between the electrode assembly  20  and the cap assembly  30  to fix the electrode assembly  20  more firmly. 
   Furthermore, after welding the cap assembly  30  onto the opening  12  of the container  10 , electrolyte is injected through an electrolyte injection hole  40  on the cap plate  32 , and then it is sealed with a plug (not shown in the drawing). 
   For the square secondary battery with the above described structure, the cap plate  32  is welded onto a portion of the container  10  around the opening  12  by the seam welding method, and in an embodiment of the present invention, a sealing part (SP), i.e., the welded portion between the cap plate  32  and the container  10 , that comprises a first sealing part of a high sealing part (HSP) with a certain predetermined welding strength, and a second sealing part of a low sealing part (LSP) with a less welding strength than the first sealing part. 
   In order to differentiate clearly the high sealing part (HSP) and the low sealing part (LSP) in  FIG. 2 , the HSP is marked with a solid line and the LSP is marked with a dotted line. 
   The low sealing part (LSP) functions as a safety valve (SV) that is to explode to reduce the internal pressure in the container  10  when the internal pressure increases at a level higher than a prescribed level. 
   Accordingly, the low sealing part (LSP) of this embodiment of the present invention is formed with such a welding strength that the low sealing part explodes when the gas pressure reaches a level higher than 12 kgf/cm 2 . 
   Also, the high sealing part (HSP) is formed with such welding strength that the high sealing part withstands a pressure of around 20 kgf/cm 2  as an ordinary situation. 
   Formation of such a sealing part (SP) can be achieved by welding the cap plate  32  onto the opening  12  of the container  10  with a lower welding strength by controlling the welding strength on a certain region (the region of the low sealing part). 
   The safety valve (SV) of this embodiment of the present invention, as shown in  FIG. 3 , can be formed on at least one of the long edges when the sealing part (SP) is formed with a combination of a pair of long edges and a pair of short edges. 
   In this case, when the length of a long edge is L, a length (l) of the safety valve is to be less than 30% of L. In detail, the length (l) is to be less than 1 cm, preferably to be formed within the range of 0.1 to 10 mm. 
     FIG. 4  and  FIG. 5  are modified examples of the embodiment previously described.  FIG. 4  is an example where the safety valve is formed on at least one of the short edges of the sealing part (SP), and  FIG. 5  is an example where the safety valve is formed on the corners of the sealing part (SP). 
   Up to now, the examples have been explained where the safety valve (SV) is formed with the low sealing part (LSP) by a welding process. However, in another embodiment of a secondary battery in a cylindrical shape that is made by a crimping process to seal the cap plate and the container, the low sealing part can be formed by controlling the crimping strength process. 
     FIG. 6  is a partial perspective view of a secondary battery in a cylindrical shape according to another embodiment of the present invention. 
   In the secondary battery in a cylindrical shape, a cap plate  32 ′ and a r container  10 ′ in a cylindrical shape is normally sealed by a crimping process. The secondary battery of the embodiment of the present invention has a low sealing part (LSP) that is crimped with a lower crimping strength on a certain region than the other region of the sealing part where the cap plate  32 ′ and the container  10 ′ are sealed. The HSP is a high sealing part that is crimped with a higher crimping strength than the low sealing part (LSP). 
   Consequently, when the internal pressure increases at a level higher than a prescribed level, for example 12 kgf/cm 2 , the low sealing part (LSP) functioning as a safety valve explodes, and thereby the internal pressure is reduced to prevent the battery from exploding. 
   Also, according to another embodiment of the present invention, by controlling the welding strength or the crimping strength on the low sealing part that functions as a safety valve at a desired level, an effect can be attained to set the operating pressure more easily and more precisely. 
   As explained above, because the present invention can form a safety valve without an additional separate step for the sealing process of a cap plate and a container, a simpler manufacturing process and manufacturing cost saving can be realized. 
   Although embodiments of the present invention have been described in detail hereinabove in connection with certain exemplary embodiments, it should be understood that the invention is not limited to the disclosed exemplary embodiment, but, on the contrary is intended to cover various modifications and/or equivalent arrangements included within the spirit and scope of the present invention, as defined in the appended claims.