Abstract:
A secondary battery includes an electrode assembly; a laminated container laminated with flexible films, which encloses the electrode assembly while the electrode tabs are drawn out of the laminated container and is sealed by fusing resulting structure within a sealed portion; and a safety unit in the sealed portion, capable of being melted by excess heat generated from the secondary battery.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a secondary battery, and more particularly, to a lithium-ion secondary battery. 
     2. Description of the Related Art 
     Lithium secondary batteries have high energy densities per unit weight and operating voltages, which are typically three times higher than other secondary batteries, such as nickel-cadmium (Ni—Cd) batteries and nickel-metal hydride (Ni—MH) batteries. For these reasons, research and development thereof have constantly increased. 
     Lithium secondary batteries can be classified into liquid electrolyte batteries and solid polymeric electrolyte batteries according to the electrolyte used. In general, batteries using a liquid electrolyte are called as lithium-ion batteries, and batteries using a polymeric electrolyte are called as lithium polymer batteries. 
     Lithium secondary batteries can be manufactured in various shapes. Cylindrical and rectangular shapes are frequently used for lithium-ion batteries. Since lithium polymer batteries are rather safe, light, and can be molded in various shapes, they can be more advantageously used for slim, lightweight portable electronic devices, compared to other secondary batteries. 
     However, when a lithium secondary battery is overcharged or short-circuited between the anode and cathode plates while assembling electrodes, an electrolyte containing lithium salts in an organic solvent decomposes at the cathode plate and the lithium metal is separated at the anode plate. This degrades battery properties and causes internal short circuits in a battery. Furthermore, heats and gases generated when the lithium secondary battery is overcharged increase the internal pressure of the battery, risking explosions or fire. 
     To eliminate these problems of overcharging and short circuits in the lithium secondary battery, a general canned lithium-ion battery has an internal safety device, such as a shut-down separator, a positive temperature coefficient (PTC) device for cutting off the supplied electric current when the internal temperature rises above its limit, or a safety vent for ventilation when the internal pressure rises above its limit. 
     The typical safety device adapted for a canned lithium-ion battery can endure an internal pressure of up to 20 kg /cm 2  when overcharged. However, it has a complicated structure and is difficult to manufacture. 
     While canned lithium-ion batteries use a nickel-plated metal as a packaging material, plastic lithium-ion batteries are enclosed within an aluminum pouch formed by coating aluminum foil with nylon, polyethylene-co-acrylic acid (EAA), or polyethylene (PE) films. Although applications of such a metal container for secondary batteries are limited due to its rigidity, since the aluminum pouch as a packaging material for batteries is flexible, the aluminum pouch can be more easily adjusted to accommodate various sizes. 
     Also, the aluminum pouch is less resistant to the internal pressure increases due to the gas generation as described above than metal canning materials, so it can easily burst even with a relatively small increase in the internal pressure. Furthermore, since the gases generated within the aluminum pouch are flammable, the risk of explosion is even higher. 
     An example of an aluminum pouch for secondary lithium polymer batteries, capable of eliminating those problems, is disclosed in Japanese Patent Laid-open Publication No. hei 2000-100399. In the disclosure, the aluminum pouch includes thermally fusible layers capable of sealing the accommodation space of an assembly of electrode plates bound together when fused, and a region of the aluminum pouch is formed to rupture easily. The rupture-susceptible region is melted at a lower temperature than other regions and thus has a small binding strength to rupture when the internal pressure of the battery rises above a predetermined level. 
     However, it is difficult to form the rupture-susceptible region by selectively melting it at a lower temperature and to control the binding strength at the rupture-susceptible region according to rupture pressures. Accordingly, it is more likely to fail thereby lowering productivity. 
     Other examples of secondary lithium batteries are disclosed in U.S. Pat. Nos. 6,040,081 and 6,145,280. 
     SUMMARY OF THE INVENTION 
     Accordingly, the invention provides a secondary battery having reduced risk of explosion due to the gases and heats generated by thermal decomposition of the electrolyte in the battery. 
     The invention also provides a secondary battery with improved hermeticity between electrode tabs of an electrode assembly and sealed portions. 
     In one embodiment, the invention provides a secondary battery comprising: an electrode assembly having two electrode tabs; a laminated container laminated with flexible films, which encloses the electrode assembly while the electrode tabs are drawn out of the laminated container and is sealed by fusing the resulting structure within a sealed portion; and a safety unit in the sealed portion, capable of being melted by excessive heat generated from the secondary battery. 
     In the secondary battery according to the present invention, the safety unit may be a tape formed of a low melting point synthetic resin. The tape is formed of a single or dual-layered structure including at least one selected from the group consisting of ethylene vinylacetates, linear low-density polyethylenes, and polypropylenes. 
     In another embodiment, the invention provides a secondary battery comprising: an electrode assembly having two electrode tabs; a laminated container having a main body including a receiving portion in which the electrode assembly is placed and a first binding portion around the receiving portion, and an upper cover including a second binding portion along its edge, wherein the first and second binding portions are bound together to form a sealed portion while the electrode tabs protrude from one edge of the sealed portion; and a safety unit interposed between the first and second binding portions forming the sealed portion, capable of being melted by excess heat generated from the electrode assembly. 
     In the secondary battery, the safety unit may be a tape formed of at least one selected from the group consisting of ethylene vinylacetates, linear low-density polyethylenes, and polypropylenes. 
     The secondary battery may further comprise a sealing member between each of the electrode tabs and the first and second binding portions. In this case, the sealing member is a tape formed of at least one selected from the group consisting of ethylene vinylacetates, linear low-density polyethylenes, and polypropylenes. The tape can be formed of a single or dual-layered structure including the forgoing materials. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings. 
         FIG. 1  is an exploded perspective view of a secondary battery according to the present invention. 
         FIGS. 2A and 2B  show examples of a tape as a safety unit. 
         FIG. 3  is a plan view of a secondary battery according to the present invention. 
         FIG. 4  is a side view of a sealed portion of the secondary battery according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1 and 3  show an embodiment of a secondary battery according to the present invention. Referring to  FIGS. 1 and 3 , the secondary battery includes an electrode assembly  30  having a cathode plate  31 , an anode plate  32 , and a separator  33  interposed between the cathode plate  31  and the anode plate  32  for electrically insulating the same, wherein each of the cathode plate  31  and the anode plate  32  has an active material layer of an active material and an additive on a respective current collector. The cathode plate  31 , the anode plate  32 , and the separator can be cut to a predetermined size and stacked upon one another or can be wound together to form a roll. A cathode tab  34  is electrically connected to the cathode plate  31  of the electrode assembly  30 , and an anode tab  35  is electrically connected to the anode plate  31  of the electrode assembly  30 . 
     The electrode assembly  30  is sealed in a laminated container  40  that is formed of a flexible material. The cathode tab  34  and the anode tab  35  are drawn out of the laminated container  40  and act as terminals of the battery. The laminated container  40  is formed by coating aluminum foil with nylon, polyethylene-co-acrylic acid (EAA), or polyethylene (PE) films. However, materials for the laminated container  40  are not limited to these materials, and any flexible material having a predetermined tensile strength and insulating the electrode assembly  30  can be used. 
     The laminated container  40  for enclosing the electrode assembly  30  includes a main body  42  having a receiving portion  42   a  in which the electrode assembly  30  is placed and a first binding portion  41  around the receiving portion  42   a , and a cover  44  extending from the main body  42  and having a second binding portion  43  along its edge. When the first binding portion  41  and the second binding portion  42  are fused together, the electrode assembly  30  in the receiving portion  42   a  is sealed. The first binding portion  41  and the second biding portion  42  are fused to bind together, and forms a sealed portion  45 . 
     As a safety unit, a tape  50  formed to a predetermined length using a material having a lower melting point than the material for the laminated container  40  is fused between the first binding portion  41  of the main body  42  and the second binding portion  43  of the upper cover  44 , as shown in  FIG. 4 . The tape  50  is formed of a low melting point synthetic resin. Suitable synthetic resins include polyethylene vinylacetates having a melting point of 60–100° C., linear-low density polyethylenes having a melting point of 90–120° C., and polypropylenes having a melting point of 100–180° C. 
     The tape  50  may be formed of a single layer using the above-listed materials. Alternatively, the tape  50  can be formed of at least two or three layers using those materials, as shown in  FIGS. 2A and 2B . When the tape  50  is formed of three layers, a linear-low density polyethylene layer  51  can be interposed between two polypropylene layers  52 . The tape  50  can be formed in various ways using low melting point materials without limitation to the above embodiment. However, it is preferable that a middle portion of the tape  50  have a lower melting point than an upper surface portion and a lower surface portion of the tape  50 . 
     The cathode tab  34  and the anode tab  35  of the electrode assembly are located in the sealed portion  45 , i.e., between the first binding portion  41  and the second binding portions  43 , wherein a sealing member  60  is interposed between the cathode tab  34  and the anode tab  35 , and each of the first and second binding portions  41  and  43 . The tape  50  as described above can be used as the sealing member  60 . In this case, it is unnecessary for a tape for the sealing member  60  to have a lower melting point in the middle portion than in the upper and lower surface portions. 
     Any safety structure can be applied in the present invention without limitation to the above-described embodiment as long as it can allow a portion of the sealed portion formed by fusing the first and second binding portions  41  and  43  to melt earlier than the laminated container  40  at a temperature lower than the melting point of the laminated container  40  by the heat generated when the electrode assembly  30  is abnormally charged or discharged. 
     In the secondary battery according to the present invention having the above-described structure, when thermal decomposition of the electrolyte occurs at the cathode due to overcharging of the battery. This generates heat and gases. The heat is transferred to the tape  50  disposed in the sealed portion  45  of the laminated container  40  as the safety unit. Consequently, the tape  50  melts and the internal gases leak to the outside, thereby preventing explosion of the laminated container  40 . 
     The tape  50  interposed between the first binding portion  41  and the second binding portion  43  forming the sealed portion  45  has a relatively low melting point in its middle portion than in its upper surface portion and lower surface portion. Accordingly, while the upper surface portion and the lower surface portion of the tape  50  are fused to the first and second binding portions  41  and  43 , the middle portion of the tape  50  can be easily melted by heat. 
     According to the present invention, to enhance hermeticity of the secondary battery near the cathode tab  34  and the anode tab  35  disposed between the first and second binding portions  41  and  43 , the sealing member  60  is used. The sealing member  60  is formed of a synthetic resin having a relatively low melting point, such as ethylene vinylacetate, linear-low density polyethylene, or polypropylene, thereby ensuring hermeticity in binding the first and second binding portions  41  and  43 . 
     As described above, in the secondary battery according to the present invention, the first and second sealing portions are fused to form the sealed portion of the secondary battery while the tape having a low melting point is interposed between the first and second sealing portions as a safety unit. The tape melts earlier than the laminated container by heat generated when the battery is overcharged, and internal gases leak to the outside, thereby preventing explosion of the battery. 
     While the present invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the appended claims.