Abstract:
A battery sheath having a ferrite stainless steel (SUS) layer is provided. The ferrite SUS layer has a first surface and a second surface. A first insulation layer, such as a cast polypropylene layer, is formed on the first surface of the ferrite SUS layer. A second insulation layer, such as a nylon layer or a polyethylene terephthalate layer, is formed on the second surface of ferrite SUS layer.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority to and the benefit of Korean Patent Application No. 10-2005-0034726, filed on Apr. 26, 2005, the entire content of which is incorporated herein by reference.  
       FIELD OF THE INVENTION  
       [0002]     The present invention relates to a battery sheath and rechargeable battery using the same. More particularly, the invention relates to a battery sheath having enhanced mechanical strength, excellent workability and reduced thickness.  
       BACKGROUND OF THE INVENTION  
       [0003]     As is generally known in the art, rechargeable batteries, for example lithium polymer batteries, include electrode assemblies, each of which typically includes a separator positioned between positive and negative electrode collectors. The separator acts as an electrolyte, serving as a medium for ion conduction. The separator also serves as a medium for separation, a function similar to their role in lithium ion batteries. The separator includes a gel-type polymer electrolyte, which is manufactured by impregnating a polymer with an electrolyte, thereby improving ion conductivity.  
         [0004]     Unlike lithium ion batteries, lithium polymer batteries can have plate structures and do not require winding process. Therefore, the electrode assembly in a lithium polymer battery can include a number of plates laminated together and can have a square shaped structure. In addition, the electrolyte in a lithium polymer battery is injected into a completely integrated cell, and rarely leaks. Also, the plate structure of the lithium polymer battery makes it unnecessary to apply pressure when making the square shaped structure. Therefore, a thin flexible pouch may be used as the battery sheath, instead of a hard square or cylindrical can.  
         [0005]     When a flexible pouch is used as the battery sheath, the thickness of the battery is substantially less than that of a can, enabling more electrode assemblies to be formed within the same volume allowing an increase in battery capacity. The flexible battery sheath allows the battery to take a desired shape and enables the easy mounting of the battery on various electronic appliances.  
         [0006]     However, although pouch-type battery sheaths have increased battery capacity and can be processed into various shapes, they have low mechanical strength and are very vulnerable to external impact. For example, a hole can be easily formed in the battery sheath when the battery sheath is pierced by a sharp object (e.g., a needle or nail), and the sheath can be easily torn if, for example, it is bitten by a pet. Furthermore, when a sharp object penetrates the sheath and contacts the internal electrode assembly, a short circuit can occur between the positive and negative electrode collectors, and may cause the battery to catch fire or explode.  
         [0007]     In addition, lithium polymer batteries using such a sheath can swell severely at high temperatures. Because the sheath surrounding the electrode assembly is flexible and has a low mechanical strength, the thickness and shape of the battery are easily deformed by gas generated from the internal polymer electrolyte.  
       SUMMARY OF THE INVENTION  
       [0008]     In accordance with one embodiment of the present invention a battery sheath having a ferrite stainless steel (SUS) layer is provided. The battery sheath has enough mechanical strength to stably protect the battery from external impact. The battery sheath having a ferrite SUS layer also suppresses the battery swelling phenomenon, preventing deformation of the thickness and shape of the battery.  
         [0009]     According to another embodiment of the present invention, a battery sheath having a ferrite SUS layer has a reduced thickness and increased mechanical strength, thereby improving battery capacity.  
         [0010]     According to another embodiment of the present invention, a battery sheath having a ferrite SUS layer has excellent workability so that there is no blowout or no rupture when forming a cavity for containing an electrode assembly.  
         [0011]     One exemplary battery sheath includes a ferrite SUS layer having a first surface and a second surface. A first insulation layer such as a cast polypropylene (CPP) layer is then attached to the first surface of the ferrite SUS layer. A second insulation layer such as a nylon layer or a polyethylene terephthalate (PET) layer is attached to the second surface of ferrite SUS layer.  
         [0012]     In other embodiments, the present invention is directed to rechargeable batteries using the battery sheaths. A rechargeable battery may include an electrode assembly having at least one positive electrode collector, at least one negative electrode collector, and at least one separator between the positive and negative electrode collectors. The battery further includes positive and negative electrode tabs coupled to the electrode assembly and extended with a predetermined length from the positive and negative electrode collectors. A sheath includes a first region having a cavity with a predetermined depth for containing the electrode assembly, and a second region adapted to cover the cavity of first region. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  is a perspective view of a battery sheath before formation of a cavity, according to one embodiment of the present invention.  
         [0014]      FIG. 2  is a cross-sectional view of a battery sheath taken along line  1 - 1  in  FIG. 1 .  
         [0015]      FIG. 3   a  is a perspective view of a battery sheath having a cavity for containing an electrode assembly according to one embodiment of the present invention.  
         [0016]      FIG. 3   b  is a magnified view of the region  3   b  of  FIG. 3   a.    
         [0017]      FIG. 4  is a perspective view of a rechargeable battery according to one embodiment of the present invention.  
         [0018]      FIG. 5  is a cross-sectional view of the rechargeable battery taken along line  4 - 4  in  FIG. 4 . 
     
    
     DETAILED DESCRIPTION  
       [0019]     Referring to  FIGS. 1 and 2 , a battery sheath  10  includes a ferrite SUS layer  11 , a first insulation layer  12  formed on a surface of the ferrite SUS layer  11  and a second insulation layer  13  formed on the other surface of the ferrite SUS layer  11 .  
         [0020]     The ferrite SUS layer  11  has an approximately planar or a completely planar first surface  11   a  and an approximately planar or a completely planar second surface  11   b  opposite the first surface  11   a . In addition, the thickness of the ferrite SUS layer  11  between the first and second surfaces  11   a ,  11   b  ranges from about 10 μm to about 60 μm, which is less than the thickness of prior art sheaths by several microns to tens of microns. Namely, since the ferrite SUS layer  11  has increased mechanical strength because of the material characteristics, it may have more reduced thickness than that of the prior art sheaths. Furthermore, the ferrite SUS layer  11  doesn&#39;t need to increase the thickness in order to enhance the elongation ratio related to workability because of the material characteristics. The ferrite SUS layer  11 , therefore, may not only reduce the thickness thereof but also keep up the high mechanical strength. For a comparative reference, in the case of using an austenite SUS layer, it needs to increase its thickness in order to enhance the elongation ratio, thus it is difficult for its thickness to stay less than 60 μm. Therefore, in accordance with the present invention more electrode assemblies (not shown) can be contained within the same volume. That is, the capacity of the battery increases.  
         [0021]     The ferrite SUS layer  11  may include an alloy having from about 84% to about 88.2% iron, about 0.5% or less carbon, from about 11% to about 18% chromium, and from about 0.3% to about 0.5% manganese. Furthermore, the ferrite SUS layer  11  may include a material selected from the group consisting of Korean Industrial Standard (KS) STS430 and Japanese Industrial Standard (JIS) SUS430. However, it is understood that any suitable material may be used for the ferrite SUS layer  11 . Since the ferrite SUS layer has high mechanical strength and high resistance to chemical corrosion, it increases the mechanical strength of the battery sheath  10  and increases the resistance to the electrolyte. The ferrite SUS layer  11 , of course, prevents moisture from penetrating the battery. The ferrite SUS layer  11  has an elongation ratio of about 10% to about 60%, enabling easy formation of a cavity (not shown). This elongation ratio prevents the ferrite SUS layer  11  from being damaged during formation of the cavity. The cavity is formed to a predetermined depth by a die, and contains the electrode assembly. For example, the ferrite SUS layer  11  may be annealed in an inactive gas atmosphere at a temperature of hundreds of degrees Celsius to maintain the elongation ratio at about 10% to about 60%. Of course, since the ferrite SUS layer  11  has excellent workability, it has high elongation ratio by itself and there may be no need to be annealing.  
         [0022]     Furthermore, the characteristics of the ferrite SUS layer  11  enable suppression of swelling which may occur at higher temperatures after battery assembly. Therefore, deformation of the thickness and shape of the battery is sufficiently prevented. More particularly, massive gas may be generated by decomposition of the electrolyte at high temperature after assembling the battery. And then, the swelling phenomenon, wherein the battery sheath swells outwardly, may occur because of the massive gas. However, since the battery sheath in accordance with the present invention uses the ferrite SUS layer  11  having high mechanical strength, the swelling phenomenon is sufficiently prevented from deforming the battery.  
         [0023]     The first insulation layer  12  which is applied to the first surface  11   a  of the ferrite SUS layer  11  may be a CPP layer. A CPP layer with a thickness of about 30 μm to about 40 μm may be applied to the first surface  11   a  of the ferrite SUS layer  11 . The CPP layer may have a thickness slightly greater than that of the ferrite SUS layer  11  because the CPP layer directly contacts to the electrode assembly and is thermally bonded to each other.  
         [0024]     The second insulation layer  13  which is applied to the second surface  11   b  of the ferrite SUS layer  11  may be one selected from a nylon layer and a PET layer. For example, the nylon layer or the PET layer is applied to the second surface  11   b  of the ferrite SUS layer  11  by lamination at high temperature. The nylon or the PET layer with a thickness of about 5 μm to about 10 μm is applied to the second surface  11   b . The PET layer as the second insulation layer  13  may include an alloy film. More particularly, the PET layer may further include rubber particles for enhancing resistance to impact, a solubilizer surrounding the rubber particles for enhancing adherence, and an adhesive. The rubber particles increase the elongation ratio and the resistance to impact. The solubilizer improves adherence to the ferrite SUS layer  11 , and particularly to the second surface  11   b  of the ferrite SUS layer  11 . The adhesive, previously applied to the PET layer enables direct lamination of the PET layer at high temperature without applying any special adhesive to the ferrite SUS layer  11 . This further simplifies the manufacturing process of the battery sheath  10 . The PET layer may not include an adhesive. In that case, an adhesive is previously formed on the second surface  11   b  of the ferrite SUS layer  11 . The PET layer is then applied to the ferrite SUS layer  11 .  
         [0025]      FIG. 3   a  is a perspective view of a battery sheath  110  according to one embodiment of the present invention. The sheath  110  includes a cavity  116  for containing an electrode assembly.  FIG. 3   b  is a magnified view of region  3   b  in  FIG. 3   a . Referring to  FIGS. 3   a  and  3   b , the battery sheath  110  includes a first region  117   a  and a second region  117   b  which are folded together such that their edges are thermally bonded. The first region  117   a  may include a cavity  116  having a predetermined width and depth for containing an electrode assembly (not shown). The electrode assembly includes at least one positive electrode collector, at least one negative electrode collector and at least one separator between the positive and negative electrode collectors. The second region  117   b  may also include a cavity (not shown). A ferrite SUS layer  111 , which is the main material of the sheath  110 , has an elongation ratio of about 10% to about 60% for preventing the sheath  110  from being damaged during formation of the cavity  116 .  
         [0026]     The thickness of the first layer  112  is greater than the thickness of the ferrite SUS layer  111 , and the thickness of the ferrite SUS layer  111  is greater than the thickness of a second insulation layer  113 , such as a PET layer. The first insulation layer  112  is the thickest because the portion of the first insulation layer  112  on the outer peripheral edges of the first and second regions  117   a ,  117   b , respectively, are thermally bonded to each other.  
         [0027]      FIG. 4  is a perspective view of a rechargeable battery  200  according to another embodiment of the present invention.  FIG. 5  is a cross-sectional view of the rechargeable battery taken along line  4 - 4  in  FIG. 5 . As shown, the rechargeable battery  200  includes an electrode assembly  221 , a sheath  210 , and a protective circuit module  223 .  
         [0028]     The electrode assembly  221  is formed by laminating at least one positive electrode collector  221   a , at least one negative electrode collector  221   b , and at least one separator  221   c  between the positive and negative electrode collectors  221   a ,  221   b , respectively. The positive electrode collector  221   a  includes lithium cobalt oxide (LiCoLO 2 ) on aluminum (Al) foil. The negative electrode collector  221   b  includes graphite on copper (Cu) foil. The separator  221   c  includes a gel-type polymer electrolyte. At least one positive electrode tab  222   a  of aluminum is bonded to the aluminum foil of the positive electrode collector  221   a , and at least one negative electrode tab  222   b  of nickel is bonded to the copper foil of the negative electrode collector  221   b . The positive and negative electrode tabs  222   a ,  222   b  extend a predetermined length from the exterior of the sheath  210 .  
         [0029]     The sheath  210  includes a first region  217   a  having a cavity  216  of a predetermined depth for containing the electrode assembly  221 , and a second region  217   b  for covering the cavity  216  of the first region  217   a.    
         [0030]     The sheath  210  includes a ferrite SUS layer  211 . A first insulation layer  212 , such as a CPP layer, is applied to a surface of the ferrite SUS layer  211  and a second insulation layer  213 , such as a PET layer, is laminated at high temperature on the other surface of the ferrite SUS layer  211 . An adhesive (not shown) may optionally be applied between the ferrite SUS layer  211  and the first insulation layer  212 . The other adhesive (not shown) may also be optionally applied between the ferrite SUS layer  211  and the second insulation layer  213 . The first insulation layer  212  surrounds the electrode assembly  221 , and the second insulation layer  213  is positioned on the outermost surface of the sheath  210 . The first insulation layers  211  on the outer peripheral edges  217   c  of the first and second regions  217   a ,  217   b , respectively, of the sheath  210 , are thermally bonded to each other and can be folded such that the volume of the sheath  210  is minimized. The remaining features of the sheath  210  are similar to those described above with reference to  FIGS. 1 through 3   b.    
         [0031]     The protective circuit module  223  is attached to a front side of the sheath  210  to protect the battery  200  from voltage or current generated during overcharging or over-discharging. The protective circuit module  223  is electrically connected to the positive and negative electrode tabs  222   a ,  222   b , respectively.  
         [0032]     As shown in  FIG. 5 , the positive electrode  221   a  is positioned on the outer surface of the electrode assembly  221 . Therefore, although the first insulation layer  212  is formed so that the positive electrode  221   a  contacts the ferrite SUS layer  211 , the ferrite SUS layer isn&#39;t corroded. Namely, since the ionization tendency of the positive electrode  221   a  is greater than that of the ferrite SUS layer  211 , the positive electrode may be corroded but the ferrite SUS layer  211  is not corroded. Therefore, the electrolyte doesn&#39;t leak through the ferrite SUS layer  211 .  
         [0033]     As described above, the battery sheath includes a ferrite SUS layer having high mechanical strength such that the sheath stably protects the battery from external impact. The high mechanical strength of the sheath enables to have a reduced battery thickness and an increased volume of the electrode assembly. This increases battery capacity. The high mechanical strength of the sheath also suppresses a swelling phenomenon and prevents a deformation of the thickness and shape of the battery. The excellent workability of the battery sheath makes it possible to easily form the cavity for containing the electrode assembly. The high resistance to chemical corrosion of the battery sheath enables the battery to stably prevent from the resistance to the electrolyte and an external acid solution.  
         [0034]     Exemplary embodiments of the present invention have been described for illustrative purposes. However, those skilled in the art will appreciate that various modifications, additions and substitutions may be made to the described embodiments without departing from the scope and spirit of the invention as disclosed in the accompanying claims.