Abstract:
A cap assembly for a battery and a battery incorporating the cap assembly. The cap assembly includes a plate that connects the electrodes of the bare cell of the battery. The cap assembly includes a vent member that includes a protrusion that is physically connected to the plate. Excess pressure within the bare cell results in the protrusion disconnecting from the plate thereby interrupting current. The vent further includes two circular notches, an inner notch and an outer notch. The vent further includes a cross notch that extends radially outward so as to intersect and extend beyond both the first and second notches. The notches are configured to release when the pressure within the bare cell exceeds a burst pressure.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application Ser. No. 61/118,996, entitled Cap Assembly and Secondary Battery Using the Same, filed Dec. 1, 2008 which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a cap assembly and a secondary battery using the same, and more particularly, to a cap assembly having a vent which is capable of operating even when a low pressure is generated in a battery to thereby improve safety of the battery, and a secondary battery using the same. 
     2. Description of the Related Art 
     In recent years, active development and production of compact, lightweight portable electronic/electrical devices, such as cellular phones, notebook computers, and camcorders has been accompanied by widespread use of secondary batteries to power the portable electronic/electrical devices even in places without additional power sources. While dry cell batteries cannot be reused after they are electrically discharged once, the secondary batteries may be repeatedly used because they are capable of being charged and discharged. 
     Also, since it becomes possible to embody high-capacity secondary batteries, the secondary batteries may be applied not only to portable electronic devices but also to high-output products, such as hybrid automobiles and electric-powered tools. 
     Secondary batteries may include, for example, nickel-cadmium (Ni—Cd) batteries, nickel-hydrogen (Ni—H) batteries, nickel-zinc (Ni—Zn) batteries, lithium ion secondary batteries, and lithium polymer secondary batteries. Among these secondary batteries, lithium ion secondary batteries are extensively employed because they operate at high voltages and have high energy density per unit weight. 
     Formation of lithium ion secondary batteries includes putting an electrode assembly and an electrolyte in an exterior material and sealing the exterior material. The lithium ion secondary batteries may be classified into can-type batteries and pouch-type batteries depending on the type of exterior material. Can-type lithium ion secondary batteries may be divided into cylinder-type batteries and prismatic-type batteries. 
     Formation of a cylindrical secondary battery includes forming an electrode assembly by stacking a positive electrode plate, a separator, and a negative electrode plate and winding the stack structure, putting the electrode assembly and an electrolyte into a cylindrical can, and sealing the cylindrical can using a cap assembly. 
     When a secondary battery is overcharged, the electrolyte may evaporate so that the resistance of the secondary battery can increase and heat can be generated. As a result, an internal temperature of the secondary battery may be raised, and an internal pressure of the secondary battery may also increase due to gas generated by an electrode assembly. This may result in accidents, such as fires and explosions. 
     In order to address the foregoing problems, a cylindrical secondary battery includes a safety device for interrupting current flow when an internal pressure of the battery is at a predetermined level or higher. 
     When the internal pressure of the secondary battery reaches the predetermined level or higher due to generation of a predetermined amount of gas in overcharge, a current breaker, such as a vent, may be enabled to prevent current flow, thereby providing battery safety. 
     However, a conventional vent operates under a relatively high pressure of about 9 kgf/cm 2 , meaning that current flow may be cut off only when an internal pressure of a secondary battery is about 9 kgf/cm 2  or higher. Therefore, it is more difficult to ensure battery safety when the internal pressure of the secondary battery is lower. 
     SUMMARY OF THE INVENTION 
     The present invention provides a cap assembly having a vent which is capable of operating under a low burst pressure to improve battery safety, and a secondary battery using the same. 
     In one embodiment, the present invention comprises a cap assembly for a battery that includes a bare cell with an electrode. In this embodiment the cap assembly comprises a plate that is adapted to be electrically connected to the electrode of the bare cell. The cap assembly further comprises a vent member that includes a protrusion that is physically connected to the plate so as to be electrically connected to the electrode assembly and wherein the protrusion is adapted to physically disconnect from the plate at a first operating pressure of the battery. The vent member further defines first and second circular notches and a third cross notch that intersects the first and second circular notches. The first and second circular notches extend around the protrusion and the cross notch extends outward from the first circular notch through the second circular notch so as to intersect the second circular notch and further extend outward form the second circular notch. 
     In this embodiment, the vent is capable of operating under a lower battery pressure which thereby improves the safety of the battery. These and other objects and advantages of the present invention will become more apparent from the following description taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features of the present invention will become more apparent by describing certain exemplary embodiments thereof with reference to the attached drawings in which: 
         FIG. 1A  is an exploded perspective view of a secondary battery according to an exemplary embodiment of the present invention; 
         FIG. 1B  is a cross-sectional view of the secondary battery of  FIG. 1A  assembled; 
         FIG. 2A  is a plan view showing the shape of a vent according to an exemplary embodiment of the present invention; 
         FIG. 2B  is a cross-sectional view taken along line I-I of  FIG. 2A ; 
         FIG. 2C  is a plan view showing the shape of a vent according to another exemplary embodiment of the present invention; 
         FIG. 3A  is a plan view showing the shape of a vent according to a comparative example; 
         FIG. 3B  is a plan view showing the shape of a vent according to another comparative example; 
         FIG. 3C  is a plan view showing the shape of a vent according to another comparative example; 
         FIG. 4  is a plan view showing another shape of a vent according to an exemplary embodiment of the present invention; and 
         FIGS. 5 and 6  are graphs showing a swelling height according to an operating pressure of a vent according to an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention will be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. In the drawings, the thicknesses of layers and regions are exaggerated for clarity. Like reference numerals designate like elements throughout the specification. 
       FIG. 1A  is an exploded perspective view of a secondary battery according to an exemplary embodiment of the present invention, and  FIG. 1B  is a cross-sectional view of the secondary battery of  FIG. 1A  assembled. 
     Referring to  FIGS. 1A and 1B , a secondary battery  1  includes an electrode assembly  10 , a can  20  containing the electrode assembly  10 , and a cap assembly  70  for sealing the can  20 . 
     Also, the secondary battery  1  may further include a bottom insulating plate  30 , a top insulating plate  40 , a center pin  50 , and an insulating gasket  60 . 
     The electrode assembly  10  may be formed as a jelly-roll type by stacking and winding first and second electrode plates  11  and  13 , each of which is formed as a rectangular plate type. 
     Thus, the electrode assembly  10  may have a hollow cylindrical shape. 
     The first and second electrode plates  11  and  13  may have different polarities, and a separator  15  may be interposed between the first and second electrode plates  11  and  13  in order to prevent short-circuiting between the first and second electrode plates  11  and  13 . 
     Each of the first and second electrode plates  11  and  13  may be formed by coating positive electrode active material slurry or negative electrode active material slurry on a collection plate formed of aluminum (Al) or copper (Cu). 
     Each of the first and second electrode plates  11  and  13  may include a non-coating portion that is not coated with slurry, and first and second electrode tabs  17  and  19  may be respectively adhered to non-coating portions of the first and second electrode plates  11  and  13  so as to form an electrical conduction path between the first and second electrode plates  11  and  13 . 
     That is, the first electrode tab  17  may be adhered to the non-coating portion of the first electrode plate  11 , while the second electrode tab  19  may be adhered to the non-coating portion of the second electrode plate  13 . 
     Thus, the first and second electrode tabs  17  and  19  may have the same polarities as the first and second electrode plates  11  and  13 , respectively. 
     The first electrode tab  17  may be a top electrode tab projecting upward from a top surface of the electrode assembly  10  toward an opening of the can  20 , while the second electrode tab  19  may be a bottom electrode tab projecting downward a bottom surface of the electrode assembly  10 . 
     Conversely, the first electrode tab  17  may project downward the bottom surface of the electrode assembly  10 , while the second electrode tab  19  may project upward from the top surface thereof. 
     Of course, the first and second electrode tabs  17  and  19  may project in the same direction according to a method of forming a battery. 
     The can  20  may be formed of a metal, such as Al or stainless steel (SUS), and have various shapes, such as a cylindrical shape. 
     In addition, the can  20  may have an opening in one surface thereof. 
     The electrode assembly  10  may be inserted into the can  20  through the opening of the can  20 , and the bottom insulating plate  30  may be disposed on the bottom surface of the electrode assembly  10 . 
     Before the electrode assembly  10  is inserted into the can  20 , the second electrode tab  19  may be bent toward the center of the electrode assembly  10  to be parallel to the bottom surface of the electrode assembly  10 . 
     A portion of the bent second electrode tab  19  may run across a hollow of the electrode assembly  10 . 
     When the secondary battery  1  includes the bottom insulating plate  30 , the bottom insulating plate  30  may have a through hole corresponding to the hollow of the electrode assembly  10 . 
     Accordingly, the portion of the bent second electrode tab  19  may also run across the through hole of the bottom insulating plate  30 . 
     The bottom insulating plate  30  may have a plurality of holes  31  to minimize the reduction of an injected electrolyte due to a space reduced by the bottom insulating plate  30 . 
     When components of the secondary battery  1  are prepared as described above, a welding rod may be inserted into the hollow of the electrode assembly  10  and the through hole of the bottom insulating plate  30  so that the second electrode tab  19  can be welded to a bottom surface of the can  20 . 
     As a result, the can  20  may have the same polarity as the second electrode tab  19  and function as an electrode terminal. 
     The top insulating plate  40  may be located on the electrode assembly  10  inserted into the can  20 , and the center pin  50  may be inserted into the hollow formed in the center of the electrode assembly  10 . 
     The top insulating plate  40  may include a plurality of first holes  41  to facilitate permeation of the electrolyte into the electrode assembly  10 . 
     Also, the top insulating plate  40  may include a second hole  43  to externally project the first electrode tab  17 . 
     The center pin  50  may prevent deformation of the electrode assembly  10  due to an external force. When the center pin  50  has a central hollow, it may function as a path through which gas generated by the electrode assembly  10  passes. 
     The center pin  50  may include a plurality of holes  51  formed in a lateral surface thereof to facilitate penetration of the electrolyte and emission of gas generated by the electrode assembly  10 . 
     The can  20  may include a bead  21  which is formed by bending a lateral surface of the can  20  inward on the same level with the top of the top insulating plate  40 . 
     The bead  21  may prevent the electrode assembly  10  inserted into the can  20  from moving freely up and down. 
     The insulating gasket  60  may be inserted into the opening of the can  20 , and the cap assembly  70  may be inserted into and combined with the insulating gasket  60  to seal the can  20 . 
     The insulating gasket  60  may be formed of an insulating elastic material and wrap an outer surface of the cap assembly  70 . 
     The insulating gasket  60  may insulate the can  20  and cap assembly  70  having different polarities from each other and seal the can  20 . 
     The cap assembly  70  may include a cap-up  71 , which functions as an electrode terminal, and a lower component disposed under the cap-up  71 . 
     Respective components of the cap assembly  70  may be assembled and then installed in the insulating gasket  60  at the same time, or sequentially stacked in the insulating gasket  60 . 
     The lower component may include a positive temperature coefficient (PTC) thermistor  72 , a vent  73 , a cap-down  74 , and a sub-plate  75 , which are sequentially located under the cap-up  71 . 
     Specifically, the vent  73  may be located under the PTC thermistor  72 , and the cap-down  74  having a hollow may be located under the vent  73  by interposing an insulator  76  therebetween. Thus, the vent  73  may be insulated from the cap-down  74  by the insulator  76 . 
     The cap-down  74  may further include a through hole functioning as a path through which a pressure is applied to a bottom surface of the vent  73  when an internal pressure of the secondary battery  1  increases. 
     The sub-plate  75  may be located under the cap-down  74  and run across the hollow of the cap-down  74 . 
     The sub-plate  75  may be connected to a protrusion  737  of the vent  73 , which is exposed by the hollow of the cap-down  74 , using a welding process. 
     Accordingly, the protrusion  737  may be convex toward the bottom of the cap assembly  70 , that is, the electrode assembly  10  contained in the can  20 . 
     The first electrode tab  17 , which projects upward from the electrode assembly  10 , may be connected to a bottom surface of the cap-down  74  or a bottom surface of the sub-plate  75  using a welding process. 
     The cap-down  74  and the sub-plate  75  may be connected to each other using a laser welding process, and a protrusion  73   a  of the vent  73  and the sub-plate  75  may be connected to each other using an ultrasonic welding process. 
     Hereinafter, the characteristics of the secondary battery  1  according to the present invention will now be described. 
     According to the present invention, the vent  73  may include a notch portion. 
     The notch portion may be formed to facilitate the bending or burst of a portion of the vent  73  in order to prevent current flow when an internal pressure of the secondary battery  1  increases. The notch portion may include a first notch  734  which is formed as a circular type along a circumference of the protrusion  737 , a second notch  736  which extends as a cross (+) type from the first notch  734 , and a third notch  735  which intersects the cross-shaped second notch  736  and is formed as a circular type outside the first notch  734 . 
     That is, according to the present invention, the notch portion includes the first and third notches  734  and  735  each having a circular shape, and the second notch  736  having a cross shape. Also, the notch portion includes an intersection portion  738   a  at which the second notch  736  intersects the third notch  735 . After intersecting the third notch  735 , the second notch  736  may further extend to have an extension  738   b.    
     As shown in  FIG. 2A , the second notch  736  extends from the first notch  734  and does not form a clear cross shape. However, when the second notch  736  further extends along a virtual line inward from the first notch  734 , it may form a clear cross (+) shape. Thus, in the present invention, the shape of the second notch  736  shown in  FIG. 2A  will be defined as a cross (+) shape. 
       FIG. 2A  is a plan view showing the shape of a vent according to an exemplary embodiment of the present invention, and  FIG. 2B  is a cross-sectional view taken along line I-I of  FIG. 2A . 
     Referring to  FIGS. 2A and 2B , a vent  73  of a secondary battery according to the present invention may include a body portion  731 , a flange portion  733 , a connection portion  732  for connecting the body  731  and the flange  733 , and a protrusion  737 . The body portion  731  includes a notch portion. 
     As described above, the notch portion may include a first notch  734  which is formed as a circular type along a circumference of the protrusion  737 , a second notch  736  which extends as a cross type from the first notch  734 , and a third notch  735  which intersects the cross-shaped second notch  736  and is formed as a circular type outside the first notch  734 . Also, the notch portion includes an intersection portion  738   a  at which the second notch  736  intersects the third notch  735 . After intersecting the third notch  735 , the second notch  736  may further extend to have an extension  738   b.    
     In this case, the first notch  734  having a circular shape may have a diameter of about 2.5 to 3 mm, and the third notch  735  having a circular shape may have a diameter of about 5.0 to 7.0 mm. 
     Also, the first notch  734  may have a width W 3  of about 0.1 to 0.2 mm, the third notch  735  may have a width W 2  of about 0.1 to 0.2 mm, and the second notch  736  may have a width W 1  of about 0.05 to 0.15 mm. 
     The second notch  736 , which extends from the first notch  734  and has the cross shape, may have an entire length or width L 1  of about 8.08 mm to 8.18 mm. Thus, a length L 2  of the extension  738   b  of the second notch  736  may depend on the diameter of the third notch  735 . 
     In addition, the first notch  734  may have a depth d 3  of about 0.14 to 0.20 mm, and the third notch  735  may have a depth d 2  of about 0.16 mm or more, for example, about 0.18 to 0.19 mm. The second notch  736  may have a depth d 1  of about 0.17 mm or more, which is greater than the depth d 2  of the third notch  735 . 
     In this case, a thickness t 1  of the second notch  736 , a thickness t 2  of the third notch  735 , and a thickness t 3  of the first notch  734  may respectively correspond to values obtained by subtracting the depth d 1  of the second notch  736 , the depth d 2  of the third notch  735 , and the depth d 3  of the first notch  734  from a thickness of the body portion  731  of the vent  73 . 
     Furthermore, the connection portion  732  may have a thickness t 4  of about 0.18 mm or more. When the connection portion  732  has a thickness t 4  of less than about 0.18 mm, cracks are likely to be generated in the connection portion  732 . 
     Meanwhile, a predetermined amount of gas may be generated in the secondary battery due to various factors, for example, overcharging, thus resulting in a rise in an internal pressure of the secondary battery. 
     In this case, the protrusion  737  which protrudes downward in the center of the vent  73  may be electrically connected to a top surface of a sub-plate  75  by welding. Thus, the protrusion  737  may move upward due to the rise in the internal pressure of the secondary battery. 
     Due to the motion of the protrusion  737 , welded portions of the protrusion  737  and the sub-plate  75  may be detached from each other or a predetermined region of the sub-plate  75  may be cut, thereby blocking electrical flow of the secondary battery. 
     In the present invention, an internal pressure of the secondary battery measured when the welded portions of the protrusion  737  and the sub-plate  75  are detached from each other or the predetermined region of the sub-plate  75  is cut is referred to as an operating pressure for current interruption. 
     Also, in the present invention, the cross-shaped second notch  736  may be deformed due to the internal pressure of the secondary battery and mainly control the operating pressure. 
     Specifically, the second notch  736  may be deformed due to the increased internal pressure of the secondary battery so that the protrusion  737  can move upward. As a result, the welded portions of the protrusion  737  and the sub-plate  75  are separated from each other or the predetermined region of the sub-plate  75  is cut, thereby interrupting the electrical flow of the secondary battery. 
     Also, when the electrical flow of the secondary battery is interrupted by separating the welding portions of the protrusion  737  and the sub-plate  75  or cutting the predetermined region of the sub-plate  75 , even if the charging of the secondary battery is interrupted, gas may be continuously generated due to internal factors so as to increase the internal pressure of the secondary battery, or the internal pressure of the secondary battery may further increase due to external factors. 
     The increased internal pressure may lead to a burst in the vent  73 . 
     Specifically, the secondary battery may burst due to a continuous increase in the internal pressure of the secondary battery. In order to prevent bursting of the secondary battery, when the internal pressure of the secondary battery is a predetermined pressure or higher, the vent  73  may be allowed to burst so that gas can be emitted to lower the internal pressure of the secondary battery. 
     In the present invention, an internal pressure of the secondary battery measured during the burst in the vent  73  is referred to as a burst pressure. 
     Also, in the present invention, a portion of the vent  73  which bursts due to the internal pressure of the secondary battery may correspond to the third notch  735  that intersects the cross-shaped second notch  736  and is formed as the circular type outside the first notch  734 . 
     In other words, the circular third notch  735  may burst due to the increased internal pressure of the secondary battery so that gas can be emitted through the third notch  735  to reduce the internal pressure of the secondary battery. 
     Meanwhile, as described above, the depth d 1  of the second notch  736  may be greater than the depth d 2  of the third notch  735 . 
     That is, according to the present invention as explained above, the cross-shaped second notch  736  may be deformed due to the internal pressure and control the operating pressure, while the circular third notch  735  may burst due to the increased internal pressure. In most cases, since a burst pressure is higher than the operating pressure, by controlling the depth d 1  of the second notch  736  to be greater than the depth d 2  of the third notch  735 , only the second notch  736  may be affected under the operating pressure, while the third notch  735  having a relatively great thickness is not affected. Afterwards, the third notch  735  may burst due to the burst pressure that is higher than the operating pressure. 
     In addition, the burst pressure may be controlled by adjusting the thickness t 2  of the circular third notch  735 . For example, as the thickness t 2  of the third notch  735  increases, the burst pressure also increases. 
       FIG. 2C  is a plan view showing the shape of a vent according to another exemplary embodiment of the present invention. 
     The vent shown in  FIG. 2C  may have the same shape as the vent  73  shown in  FIG. 2A  except for particulars mentioned below. 
     Referring to  FIG. 2C , a vent  73  according to the present embodiment may include a body portion  731 , a flange portion  733 , a connection portion (not shown) for connecting the body portion  731  and the flange portion  733 , and a protrusion  737 . The body portion  731  may include a notch portion. 
     As described above, the notch portion may include a first notch  734  which is formed as a circular type along a circumference of the protrusion  737 , a second notch  736  which extends as a cross type from the first notch  734 , and a third notch  735 ′ which intersects the cross-shaped second notch  736  and is formed as a circular type outside the first notch  734 . Also, the notch portion includes an intersection portion  738   a  at which the second notch  736  intersects the third notch  735 ′. After intersecting the third notch  735 ′, the second notch  736  may further extend to have an extension  738   b.    
     In this case, the third notch  735 ′ may have a notch region  735   a  and a disconnected region  735   b  corresponding to a predetermined unnotched region. 
     As described above, a burst may occur in the circular third notch  735 ′ so that gas can be emitted through the third notch  735 ′ to lower an internal pressure of a secondary battery. In this case, when the third notch  735 ′ does not have the disconnected region  735   b , the entire third notch  735 ′ may burst. As a result, a portion of the vent  73  which includes the protrusion  737  formed within the third notch  735 ′ may collide with a cap-up or move inside the secondary battery to resume current flow. 
     Accordingly, in order to prevent the entire third notch  735 ′ from bursting, the third notch  735 ′ may have the disconnected region  735   b  so that the disconnection region  735   b  can inhibit the portion of the vent  73  including the protrusion  737  formed within the third notch  735 ′ from being separated from the vent  73 . 
     Since the third notch  735 ′ of the vent  73  according to another exemplary embodiment includes the notch region  735   a  and the disconnected region  735   b  corresponding to the unnotched region, the third notch  735 ′ does not form a clear circular shape. However, when the notch region  735   a  of the third notch  735 ′ further extends along a virtual line illustrated with a dotted line in  FIG. 2C  to the disconnected region  735   b , the third notch  735 ′ may form a clear circular shape. Thus, in the present invention, the shape of the third notch  735 ′ shown in  FIG. 2C  will be defined as a circular shape. 
     Hereinafter, an operating pressure and burst pressure of a secondary battery having a vent according to exemplary embodiments of the present invention will be described. 
     Embodiment 1 
     To begin with, as shown in  FIG. 2B , a vent of a secondary battery according to the present embodiment included a body portion, a flange portion, a connection portion for connecting the body portion and the flange portion, and a protrusion, and the body portion included a notch portion. 
     In this case, as shown in  FIG. 2A , the notch portion included a first notch formed as a circular type along a circumference of the protrusion, a second notch which extended as a cross type from the first notch, and a third notch which intersected the cross-shaped second notch and was formed as a circular type outside the first notch. Also, the notch portion included an intersection portion at which the second notch intersected the third notch. After intersecting the third notch, the second notch further extended to have an extension. 
     The body portion had a thickness of about 0.30 mm, and the connection portion had a thickness t 4  of about 0.20 mm. Also, the first notch had a diameter of about 2.5 mm, and the third notch had a diameter of about 5 mm. The first notch had a width W 3  of about 0.15 mm, the third notch had a width W 2  of about 0.15 mm, and the second notch had a width W 1  of about 0.1 mm. Also, the first notch had a depth d 3  of about 0.17 mm, the third notch had a depth d 2  of about 0.18 mm, and the second notch had a depth d 1  of about 0.20 mm. In addition, the cross-shaped second notch had an entire length or width L 1  of about 8.13 mm. 
     In this case, when the notch portion was formed, forming the second notch using a first mold was followed by forming the third notch using a second mold. 
     The operating pressure and burst pressure of the secondary battery according to the present embodiment were measured as shown in Table 1. 
     
       
         
               
               
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                   
                 Embodiment 1 
                   
               
             
          
           
               
                   
                 Operating 
                 Burst 
               
               
                   
                 pressure (Kgf/cm 2 ) 
                 pressure (Kgf/cm 2 ) 
               
               
                 Specification 
                 8 Kgf/cm 2   
                 20 Kgf/cm 2   
               
               
                   
               
             
          
           
               
                 1 
                 7.70 
                 19.30 
               
               
                 2 
                 7.00 
                 19.10 
               
               
                 3 
                 7.60 
                 19.70 
               
               
                 4 
                 7.40 
                 19.60 
               
               
                 5 
                 7.40 
                 19.50 
               
               
                 6 
                 7.50 
                 19.40 
               
               
                 7 
                 7.60 
                 19.50 
               
               
                 8 
                 7.70 
                 19.60 
               
               
                 9 
                 7.70 
                 19.40 
               
               
                 10 
                 7.20 
                 19.70 
               
               
                 11 
                 7.50 
                 19.40 
               
               
                 12 
                 7.40 
                 19.40 
               
               
                 13 
                 7.10 
                 19.70 
               
               
                 14 
                 7.10 
                 19.40 
               
               
                 15 
                 7.70 
                 19.50 
               
               
                 16 
                 7.70 
                 19.80 
               
               
                 17 
                 7.50 
                 19.80 
               
               
                 18 
                 7.50 
                 19.50 
               
               
                 19 
                 7.50 
                 19.40 
               
               
                 20 
                 7.70 
                 19.30 
               
               
                 21 
                 7.20 
                 19.60 
               
               
                 22 
                 7.90 
                 19.60 
               
               
                 23 
                 7.70 
                 19.60 
               
               
                 24 
                 7.70 
                 19.50 
               
               
                 25 
                 7.40 
                 19.60 
               
               
                 26 
                 7.60 
                 19.80 
               
               
                 27 
                 7.50 
                 19.40 
               
               
                 28 
                 7.90 
                 19.60 
               
               
                 29 
                 7.50 
                 19.10 
               
               
                 30 
                 7.60 
                 19.80 
               
               
                 31 
                 7.70 
                 19.60 
               
               
                 32 
                 7.50 
                 19.40 
               
               
                 33 
                 7.60 
                 19.10 
               
               
                 34 
                 7.60 
                 19.30 
               
               
                 35 
                 7.50 
                 19.80 
               
               
                 36 
                 7.80 
                 19.30 
               
               
                 37 
                 7.80 
                 19.50 
               
               
                 38 
                 7.80 
                 19.70 
               
               
                 39 
                 7.90 
                 19.60 
               
               
                 40 
                 7.70 
                 19.40 
               
               
                 41 
                 7.50 
                 19.50 
               
               
                 Average 
                 7.56 
                 19.51 
               
               
                 Short-term 
                 2.31 
                 4.48 
               
               
                 process capability 
                   
                   
               
               
                 (Cpk) 
               
               
                   
               
             
          
         
       
     
     Referring to Table 1, it can be seen that the vent of the secondary battery according to the present embodiment had an average operating pressure of about 7.56 Kgf/cm 2 , which was lower than an operating pressure (about 9 Kgf/cm 2  or higher) of a conventional vent. Accordingly, current flow may be blocked even when the internal pressure of the secondary battery is low, thereby improving battery safety. 
     Furthermore, the secondary battery had a short-term process capability Cpk of 2.31, which facilitates mass production of the secondary battery. In general, a short-term process capability Cpk of 1.33 or higher is satisfactory in terms of mass production. 
     Embodiment 2 
     The present embodiment was the same as Embodiment 1 except that a first notch had a diameter of about 3.0 mm and a third notch had a diameter of about 6 mm. 
     An operating pressure and burst pressure of a secondary battery according to the present embodiment were measured as shown in Table 2. 
     
       
         
               
               
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                   
                 Embodiment 2 
                   
               
             
          
           
               
                   
                 Operating 
                 Burst 
               
               
                   
                 pressure (Kgf/cm 2 ) 
                 pressure (Kgf/cm 2 ) 
               
               
                 Specification 
                 8 Kgf/cm 2   
                 20 Kgf/cm 2   
               
               
                   
               
             
          
           
               
                 1 
                 7.95 
                 19.05 
               
               
                 2 
                 8.22 
                 19.17 
               
               
                 3 
                 7.92 
                 19.10 
               
               
                 4 
                 8.50 
                 19.07 
               
               
                 5 
                 8.32 
                 19.20 
               
               
                 6 
                 7.87 
                 19.17 
               
               
                 7 
                 8.32 
                 19.32 
               
               
                 8 
                 8.05 
                 18.92 
               
               
                 9 
                 8.60 
                 19.45 
               
               
                 10 
                 8.10 
                 19.20 
               
               
                 11 
                 8.17 
                 19.40 
               
               
                 12 
                 7.87 
                 18.97 
               
               
                 13 
                 8.02 
                 19.22 
               
               
                 Average 
                 8.15 
                 19.17 
               
               
                 Short-term 
                 2.62 
                 4.66 
               
               
                 process capability 
                   
                   
               
               
                 (Cpk) 
               
               
                   
               
             
          
         
       
     
     Referring to Table 2, it can be seen that a vent of the secondary battery according to the present embodiment had an average operating pressure of about 8.15 Kgf/cm 2 , which is lower than an operating pressure (about 9 Kgf/cm 2  or higher) of a conventional vent. Accordingly, current flow may be blocked even when the internal pressure of the secondary battery is low, thereby improving battery safety. 
     Furthermore, the secondary battery had a short-term process capability Cpk of 2.62, which facilitates mass production of the secondary battery. 
     Embodiment 3 
     The present embodiment was the same as Embodiment 1 except that a third notch had a diameter of about 6 mm. 
     An operating pressure and burst pressure of a secondary battery according to the present embodiment were measured as shown in Table 3. 
     
       
         
               
               
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                   
                 Embodiment 3 
                   
               
             
          
           
               
                   
                 Operating 
                 Burst 
               
               
                   
                 pressure (Kgf/cm 2 ) 
                 pressure (Kgf/cm 2 ) 
               
               
                 Specification 
                 8 Kgf/cm 2   
                 20 Kgf/cm 2   
               
               
                   
               
             
          
           
               
                 1 
                 8.05 
                 19.17 
               
               
                 2 
                 8.30 
                 19.55 
               
               
                 3 
                 8.00 
                 19.40 
               
               
                 4 
                 7.87 
                 19.50 
               
               
                 5 
                 7.97 
                 19.57 
               
               
                 6 
                 8.40 
                 19.30 
               
               
                 7 
                 8.30 
                 19.30 
               
               
                 8 
                 7.95 
                 19.47 
               
               
                 9 
                 8.32 
                 19.60 
               
               
                 10 
                 8.37 
                 19.40 
               
               
                 11 
                 8.15 
                 19.43 
               
               
                 Average 
                 8.15 
                 19.43 
               
               
                 Short-term 
                 3.21 
                 6.16 
               
               
                 process capability 
                   
                   
               
               
                 (Cpk) 
               
               
                   
               
             
          
         
       
     
     Referring to Table 3, it can be seen that a vent of the secondary battery according to the present embodiment had an average operating pressure of about 8.15 Kgf/cm 2 , which is lower than an operating pressure (about 9 Kgf/cm 2  or higher) of a conventional vent. Accordingly, current flow may be blocked even when the internal pressure of the secondary battery is low, thereby improving battery safety. 
     Furthermore, the secondary battery had a short-term process capability Cpk of 3.21, which facilitates mass production of the secondary battery. 
     Embodiment 4 
     The present embodiment was the same as Embodiment 1 except that a third notch had a diameter of about 7 mm. 
     An operating pressure and burst pressure of a secondary battery according to the present embodiment were measured as shown in Table 4. 
     
       
         
               
               
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                   
                 Embodiment 4 
                   
               
             
          
           
               
                   
                 Operating 
                 Burst 
               
               
                   
                 pressure (Kgf/cm 2 ) 
                 pressure (Kgf/cm 2 ) 
               
               
                 Specification 
                 8 Kgf/cm 2   
                 20 Kgf/cm 2   
               
               
                   
               
             
          
           
               
                 1 
                 7.95 
                 19.17 
               
               
                 2 
                 7.75 
                 19.17 
               
               
                 3 
                 7.85 
                 19.37 
               
               
                 4 
                 8.06 
                 19.27 
               
               
                 5 
                 7.95 
                 19.27 
               
               
                 6 
                 8.16 
                 19.07 
               
               
                 7 
                 7.95 
                 19.37 
               
               
                 8 
                 7.95 
                 18.97 
               
               
                 9 
                 7.95 
                 19.27 
               
               
                 10 
                 8.16 
                 19.17 
               
               
                 11 
                 7.95 
                 19.17 
               
               
                 12 
                 8.16 
                 18.97 
               
               
                 13 
                 8.36 
                 18.76 
               
               
                 14 
                 8.06 
                 18.86 
               
               
                 15 
                 7.65 
                 19.07 
               
               
                 16 
                 7.95 
                 18.76 
               
               
                 17 
                 7.85 
                 19.07 
               
               
                 18 
                 8.16 
                 18.97 
               
               
                 19 
                 8.16 
                 19.07 
               
               
                 20 
                 8.26 
                 19.07 
               
               
                 21 
                 8.46 
                 19.17 
               
               
                 22 
                 8.26 
                 18.86 
               
               
                 23 
                 8.16 
                 18.97 
               
               
                 24 
                 8.06 
                 19.07 
               
               
                 25 
                 7.85 
                 19.17 
               
               
                 26 
                 7.75 
                 18.76 
               
               
                 27 
                 8.06 
                 18.76 
               
               
                 28 
                 8.06 
                 19.37 
               
               
                 29 
                 8.06 
                 18.97 
               
               
                 30 
                 7.95 
                 19.37 
               
               
                 Average 
                 8.03 
                 19.08 
               
               
                 Short-term 
                 3.62 
                 3.61 
               
               
                 process capability 
                   
                   
               
               
                 (Cpk) 
               
               
                   
               
             
          
         
       
     
     Referring to Table 4, it can be seen that a vent of the secondary battery according to the present embodiment had an average operating pressure of about 8.03 Kgf/cm 2 , which is lower than an operating pressure (about 9 Kgf/cm 2  or higher) of a conventional vent. Accordingly, current flow may be blocked even when the internal pressure of the secondary battery is low, thereby improving battery safety. 
     Furthermore, the secondary battery had a short-term process capability Cpk of 3.62, which facilitates mass production of the secondary battery. 
     Embodiment 5 
     The present embodiment was the same as Embodiment 1 except that a first notch had a diameter of about 3.0 mm and a third notch had a diameter of about 6 mm. 
     Also, unlike Embodiment 1 in which forming the second notch using the first mold was followed by forming the third notch using the second mold, in the present embodiment, a second notch and the third notch were formed at the same time using a single mold. 
     An operating pressure and burst pressure of a secondary battery according to the present embodiment were measured as shown in Table 5. 
     
       
         
               
               
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                   
                 Embodiment 5 
                   
               
             
          
           
               
                   
                 Operating 
                 Burst 
               
               
                   
                 pressure (Kgf/cm 2 ) 
                 pressure (Kgf/cm 2 ) 
               
               
                 Specification 
                 8 Kgf/cm 2   
                 20 Kgf/cm 2   
               
               
                   
               
             
          
           
               
                 1 
                 7.86 
                 19.90 
               
               
                 2 
                 7.51 
                 19.92 
               
               
                 3 
                 7.48 
                 19.98 
               
               
                 4 
                 7.99 
                 19.97 
               
               
                 5 
                 7.33 
                 19.77 
               
               
                 6 
                 7.39 
                 19.69 
               
               
                 7 
                 7.66 
                 20.08 
               
               
                 8 
                 7.62 
                 19.79 
               
               
                 9 
                 7.84 
                 19.82 
               
               
                 10 
                 7.51 
                 20.20 
               
               
                 11 
                 7.62 
                 19.95 
               
               
                 12 
                 8.07 
                 19.79 
               
               
                 13 
                 7.79 
                 19.92 
               
               
                 14 
                 7.75 
                 19.85 
               
               
                 15 
                 7.46 
                 20.21 
               
               
                 16 
                 7.95 
                 20.18 
               
               
                 17 
                 7.45 
                 20.01 
               
               
                 18 
                 7.24 
                 19.79 
               
               
                 19 
                 7.30 
                 19.74 
               
               
                 20 
                 7.34 
                 19.83 
               
               
                 21 
                 7.39 
                 19.95 
               
               
                 22 
                 7.51 
                 20.11 
               
               
                 23 
                 7.52 
                 19.62 
               
               
                 24 
                 7.48 
                 19.85 
               
               
                 25 
                 7.79 
                 19.95 
               
               
                 26 
                 7.59 
                 19.92 
               
               
                 27 
                 7.31 
                 19.82 
               
               
                 28 
                 7.36 
                 19.78 
               
               
                 29 
                 7.72 
                 19.81 
               
               
                 30 
                 7.48 
                 20.01 
               
               
                 31 
                 7.51 
                 19.84 
               
               
                 32 
                 7.57 
                 19.79 
               
               
                 33 
                 7.57 
                 19.81 
               
               
                 34 
                 7.60 
                 19.78 
               
               
                 35 
                 7.57 
                 19.79 
               
               
                 36 
                 7.37 
                 19.64 
               
               
                 37 
                 7.45 
                 19.82 
               
               
                 38 
                 7.34 
                 19.92 
               
               
                 39 
                 7.66 
                 19.89 
               
               
                 40 
                 7.71 
                 19.89 
               
               
                 41 
                 7.72 
                 19.75 
               
               
                 42 
                 7.44 
                 19.85 
               
               
                 43 
                 7.67 
                 19.89 
               
               
                 44 
                 7.54 
                 19.49 
               
               
                 45 
                 7.72 
                 19.75 
               
               
                 46 
                 7.57 
                 19.65 
               
               
                 47 
                 7.88 
                 19.70 
               
               
                 48 
                 7.61 
                 19.55 
               
               
                 49 
                 7.45 
                 19.61 
               
               
                 50 
                 7.87 
                 19.71 
               
               
                 Average 
                 7.58 
                 19.85 
               
               
                 Short-term 
                 2.73 
                 6.13 
               
               
                 process capability 
                   
                   
               
               
                 (Cpk) 
               
               
                   
               
             
          
         
       
     
     Referring to Table 5, it can be seen that a vent of the secondary battery according to the present embodiment had an average operating pressure of about 7.58 Kgf/cm 2 , which was lower than an operating pressure (about 9 Kgf/cm 2  or higher) of a conventional vent. Accordingly, current flow may be blocked even when the internal pressure of the secondary battery is low, thereby improving battery safety. 
     Furthermore, the secondary battery had a short-term process capability Cpk of 2.73, which facilitates mass production of the secondary battery. 
     In addition, even though the second and third notches were formed at the same time, it was possible to control the operating pressure of the vent to be less than about 9 Kgf/cm 2 . Moreover, a process time could be shortened compared with Embodiment 2 in which the second and third notches were formed separately. 
     Hereinafter, an operating pressure and burst pressure of a secondary battery having a vent according to comparative examples will be described. 
     Comparative Example 1 
       FIG. 3A  is a plan view showing the shape of a vent according to Comparative example 1. 
     To begin with, referring to  FIG. 3A , a vent  73  of a secondary battery according to Comparative example 1 included a body portion  731 , a flange portion  733 , a connection portion (not shown) for connecting the body portion  731  and the flange portion  733 , and a protrusion  737 , and the body portion included a notch portion. 
     In this case, the notch portion of the vent  73  included a first notch  734  formed as a circular type along a circumference of the protrusion  737 , and a second notch  836  extending from the first notch  735  and formed as a cross type. 
     The body portion  731  had a thickness of about 0.30 mm, and the connection portion had a thickness t 4  of about 0.20 mm. Also, the first notch  734  had a diameter of about 2.5 mm and a width of about 0.15 mm. The first notch  734  had a depth of about 0.17 mm, and the second notch  836  had a depth d 1  of about 0.20 mm. In addition, the cross-shaped second notch  836  had an entire length or width of about 8.13 mm. 
     An operating pressure and burst pressure of the secondary battery according to the present comparative example were measured as shown in Table 6. 
     
       
         
               
               
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 6 
               
             
             
               
                   
               
               
                   
                 Comparative example 1 
                   
               
             
          
           
               
                   
                 Operating 
                 Burst 
               
               
                   
                 pressure (Kgf/cm 2 ) 
                 pressure (Kgf/cm 2 ) 
               
               
                 Specification 
                 8 Kgf/cm 2   
                 20 Kgf/cm 2   
               
               
                   
               
             
          
           
               
                 1 
                 9.22 
                 18.37 
               
               
                 2 
                 9.05 
                 18.40 
               
               
                 3 
                 9.07 
                 18.32 
               
               
                 4 
                 8.95 
                 18.52 
               
               
                 5 
                 8.90 
                 18.25 
               
               
                 6 
                 8.90 
                 18.32 
               
               
                 7 
                 8.92 
                 18.00 
               
               
                 8 
                 9.02 
                 18.20 
               
               
                 9 
                 8.90 
                 18.57 
               
               
                 10 
                 9.12 
                 18.52 
               
               
                 Average 
                 9.01 
                 18.35 
               
               
                 Short-term 
                 3.01 
                 2.61 
               
               
                 process capability 
                   
                   
               
               
                 (Cpk) 
               
               
                   
               
             
          
         
       
     
     Referring to Table 6, it can be seen that the vent  73  of the secondary battery according to Comparative example 1 had an average operating pressure of about 9.01 Kgf/cm 2 , which is about the same as an operating pressure (about 9 Kgf/cm 2  or higher) of a conventional vent. Thus, current flow may be blocked only when the internal pressure of the secondary battery is sufficiently high. Accordingly, it is difficult to ensure battery safety. Although an operating pressure of the vent  73  was 9.0 Kgf/cm 2  or lower in some cases, the operating pressure of the vent  73  substantially approximated 9.0 Kgf/cm 2 , and thus it is difficult to say that the vent  73  of the secondary battery according to the present comparative example had a low operating pressure. 
     Comparative Example 2 
       FIG. 3B  is a plan view showing the shape of a vent according to Comparative example 2. 
     Referring to  FIG. 3B , a vent  73  of a secondary battery according to Comparative example 2 included a body portion  731 , a flange portion  733 , a connection portion (not shown) for connecting the body portion  731  and the flange portion  733 , and a protrusion  737 , and the body portion included a notch portion. 
     In this case, the notch portion of the vent  73  included a first notch  734  formed as a circular type along a circumference of the protrusion  737 , and a second notch  835  formed as a circular type outside the first notch  734 . 
     The body portion  731  had a thickness of about 0.30 mm, and the connection portion had a thickness of about 0.20 mm. Also, the first notch  734  had a diameter of about 3 mm, and the second notch  835  had a diameter of about 7 mm. Each of the first and second notches  734  and  835  had a width of about 0.15 mm. The first notch  734  had a depth of about 0.17 mm, and the second notch  835  had a depth d 1  of about 0.18 mm. 
     An operating pressure and burst pressure of the secondary battery according to the present comparative example were measured as shown in Table 7. 
     
       
         
               
               
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 7 
               
             
             
               
                   
               
               
                   
                 Comparative example 2 
                   
               
             
          
           
               
                   
                 Operating 
                 Burst 
               
               
                   
                 pressure (Kgf/cm 2 ) 
                 pressure (Kgf/cm 2 ) 
               
               
                 Specification 
                 8 Kgf/cm 2   
                 20 Kgf/cm 2   
               
               
                   
               
             
          
           
               
                 1 
                 9.30 
                 19.70 
               
               
                 2 
                 9.60 
                 19.60 
               
               
                 3 
                 9.60 
                 19.60 
               
               
                 4 
                 9.60 
                 19.50 
               
               
                 5 
                 9.80 
                 19.50 
               
               
                 6 
                 9.80 
                 19.80 
               
               
                 7 
                 9.90 
                 19.70 
               
               
                 8 
                 9.60 
                 19.50 
               
               
                 9 
                 9.70 
                 19.60 
               
               
                 10 
                 9.50 
                 19.50 
               
               
                 11 
                 9.70 
                 19.60 
               
               
                 12 
                 9.80 
                 19.70 
               
               
                 13 
                 9.70 
                 19.70 
               
               
                 14 
                 9.90 
                 19.70 
               
               
                 15 
                 9.70 
                 19.70 
               
               
                 Average 
                 9.68 
                 19.63 
               
               
                 Short-term 
                 0.68 
                 9.11 
               
               
                 process capability 
                   
                   
               
               
                 (Cpk) 
               
               
                   
               
             
          
         
       
     
     Referring to Table 7, it can be seen that the vent  73  of the secondary battery according to Comparative example 2 had an average operating pressure of about 9.68 Kgf/cm 2 , which is higher than an operating pressure (about 9 Kgf/cm 2  or higher) of a conventional vent. Thus, current flow may be blocked only when the internal pressure of the secondary battery is sufficiently high. Accordingly, it is difficult to ensure battery safety. 
     Furthermore, the secondary battery had a short-term process capability Cpk of 0.68, which precludes mass production of the secondary battery. In general, a short-term process capability Cpk of less than 1.33 is disadvantageous in terms of mass production. 
     Comparative Example 3 
       FIG. 3C  is a plan view showing the shape of a vent according to Comparative example 3. 
     Referring to  FIG. 3C , a vent  73  of a secondary battery according to Comparative example 3 included a body portion  731 , a flange portion  733 , a connection portion (not shown) for connecting the body portion  731  and the flange portion  733 , and a protrusion  737 , and the body portion included a notch portion. 
     In this case, the notch portion of the vent  73  included a first notch  734  formed as a circular type along a circumference of the protrusion  737 , a second notch  836 ′ extending from the first notch  735  and formed as a cross type, and a third notch  835  formed as a circular type outside the first notch  734 . However, unlike the notch portion according to the present invention, although the third notch  835  and the second notch  836 ′ were connected to each other, the notch portion according to Comparative example 3 included neither an intersection portion at which the second notch  836 ′ intersected the third notch  83  nor an extension of the second notch  836 ′ intersecting the third notch  835 . 
     The body portion  731  had a thickness of about 0.30 mm, and the connection portion had a thickness of about 0.20 mm. Also, the first notch  734  had a diameter of about 3.0 mm, and the third notch  835  had a diameter of about 7 mm. Each of the first and third notches  734  and  835  had a width of about 0.15 mm, and the second notch  836 ′ had a width W 1  of 0.5 mm. The first notch  734  had a depth of about 0.17 mm, the third notch  835  had a depth of about 0.18 mm, and the second notch  836 ′ had a depth of about 0.20 mm. In this case, forming the second notch  836 ′ using a first mold was followed by forming the third notch  835  using a second mold. 
     An operating pressure and burst pressure of the secondary battery according to the present comparative example were measured as shown in Table 8. 
     
       
         
               
               
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 8 
               
             
             
               
                   
               
               
                   
                 Comparative example 3 
                   
               
             
          
           
               
                   
                 Operating 
                 Burst 
               
               
                   
                 pressure (Kgf/cm 2 ) 
                 pressure (Kgf/cm 2 ) 
               
               
                 Specification 
                 8 Kgf/cm 2   
                 20 Kgf/cm 2   
               
               
                   
               
             
          
           
               
                 1 
                 9.27 
                 20.24 
               
               
                 2 
                 9.27 
                 20.15 
               
               
                 3 
                 9.15 
                 20.27 
               
               
                 4 
                 9.25 
                 19.97 
               
               
                 5 
                 9.35 
                 20.00 
               
               
                 6 
                 9.27 
                 20.55 
               
               
                 7 
                 9.30 
                 20.22 
               
               
                 8 
                 9.42 
                 20.72 
               
               
                 9 
                 9.30 
                 20.20 
               
               
                 10 
                 9.37 
                 20.37 
               
               
                 11 
                 9.07 
                 20.12 
               
               
                 12 
                 9.32 
                 20.27 
               
               
                 Average 
                 9.28 
                 20.26 
               
               
                 Short-term 
                 2.56 
                 4.30 
               
               
                 process capability 
                   
                   
               
               
                 (Cpk) 
               
               
                   
               
             
          
         
       
     
     Referring to Table 8, it can be seen that the vent  73  of the secondary battery according to Comparative example 3 had an average operating pressure of about 9.28 Kgf/cm 2 , which is about the same as an operating pressure (about 9 Kgf/cm 2  or higher) of a conventional vent. Thus, current flow may be blocked only when the internal pressure of the secondary battery is sufficiently high. Accordingly, it is difficult to ensure battery safety. 
     As described above, in Comparative example 1, the notch portion of the vent  73  included the circular first notch  734  formed along the circumference of the protrusion  737  and the cross-shaped second notch  836  extending from the first notch  735 . In Comparative example 2, the notch portion of the vent  73  included the circular first notch  734  formed along the circumference of the protrusion  737  and the circular second notch  835  formed outside the first notch  734 . Also, in Comparative example 3, the notch portion of the vent  73  included the circular first notch  734  formed along the circumference of the protrusion  737 , the cross-shaped second notch  836 ′ extending from the first notch  735 , and the circular third notch  835  formed outside the first notch  734 , but it included neither the intersection portion at which the second notch  836 ′ intersected the third notch  83  nor the extension of the second notch  836 ′ intersecting the third notch  835 . In Comparative examples 1 through 3, since it is difficult to substantially lower the operating pressure of the vent  73  to less than about 9 Kgf/cm 2 , battery safety cannot be ensured when the internal pressure of the secondary battery is low. 
     On the other hand, the notch portion of the vent according to the exemplary embodiments of the present invention included the circular first notch formed along the circumference of the protrusion, the cross-shaped second notch extending from the first notch, and the circular third notch intersecting the cross-shaped second notch and formed outside the first notch. Also, the notch portion further included the intersection portion at which the second notch intersected the third notch and the extension of the second notch intersecting the third notch. In this case, it was possible to lower the operating pressure of the vent to less than 9 Kgf/cm 2 , and specifically, to about 8 Kgf/cm 2 . Therefore, current flow may be interrupted even when the internal pressure of the secondary battery is low, thereby improving battery safety. 
     Accordingly, a secondary battery according to the present invention can improve battery safety even when internal pressure is low. 
     In addition, the secondary battery according to the present invention lowers an operating pressure of a vent so that current flow can be blocked even when internal pressure is low. 
       FIG. 4  is a plan view showing another shape of a vent according to an exemplary embodiment of the present invention. 
     In  FIG. 4 , the shape of the vent may be the same as that shown in  FIG. 2A , except for the following. 
     To begin with, referring to  FIG. 4 , a vent  73  of a secondary battery according to an exemplary embodiment of the present invention includes a body portion  731 , a flange portion  733 , a connection portion (not shown) for connecting the body portion  731  and the flange portion  733 , and a protrusion  737 , and the body portion  731  includes in a notch portion. 
     The notch portion, as described above, includes a first notch  734  which is formed in a circular shape along the circumference of the protrusion  737 , a second notch  736  which is formed in a cross (+) shape, and a third notch  735  which intersects the cross-shaped second notch  736  and is formed in a circular shape outside the first notch  734 . Also, the notch portion includes an intersection portion  738   a  at which the second notch  736  intersects the third notch  735 . After intersecting the third notch  735 , the second notch  736  may further extend to have an extension  738   b.    
     In this case, unlike the shape of the vent shown in  FIG. 2A , the vent of  FIG. 4  has the cross-shaped second notch  736  intersecting the circular first notch  734 . 
     That is, at the intersection portion  739   a , the first notch intersects the second notch. After intersecting the first notch, the second notch  736  may further extend to have an extension  739   b.    
       FIGS. 5 and 6  are graphs showing a swelling height according to an operating pressure of a vent according to the present invention. 
     To begin with,  FIG. 5  is a graph of a swelling height according to an operating pressure of a vent according to the third exemplary embodiment as described above. 
     In this case, a line X E  shows test results obtained using a vent manufactured by molding, and a line X C  shows simulation results. 
     As described above, in the third exemplary embodiment, a vent has an average operating pressure of 8.15 Kgf/cm 2 , which is lower than an operating pressure (about 9 Kgf/cm 2  or higher) of a conventional vent. 
     Referring to the lines X E  and X C , it can be seen that there is no difference between the test results obtained using the vent manufactured by molding and the simulation results. 
     Meanwhile,  FIG. 5  is provided to show that there is no difference between the test results obtained using the vent manufactured by molding and the simulation results, but the results may be differently interpreted by changing a reference value of an operating pressure. For example, when the reference value of the operating pressure is set to 8 Kgf/cm 2 , the vent has to have a swelling height of 0.05 mm or less at an operating pressure of 5 Kgf/cm 2 , and a swelling height has to be 0.45 mm or more at an operating pressure of 8 Kgf/cm 2 . 
     That is, when the vent has a swelling height of more than 0.05 mm at an operating pressure of 5 Kgf/cm 2 , the operating pressure of the vent is formed at too low an internal pressure, resulting in blocking current flow. When the vent has a swelling height of less than 0.45 mm at an operating pressure of 8 Kgf/cm 2 , it is difficult for the vent to maintain an operating pressure of 8 Kgf/cm 2 . 
     As described above, these conditions are dependant on the reference value of the operating pressure. In the third exemplary embodiment of the present invention, it can be seen that the lines X E  and X C  both show that the vents have a swelling height of 0.05 mm or less at an operating pressure of 5 Kgf/cm 2 , and a swelling height of 0.45 mm or more at an operating pressure of 8 Kgf/cm 2 . 
       FIG. 6  is a graph showing the effect of a vent according to  FIG. 4  as described above. 
     In this case, a line X E  shows test results obtained using the vent manufactured by molding according to the third exemplary embodiment, which is the same as the line X E  of  FIG. 5 . A line Y C  shows simulation results obtained using the vent according to  FIG. 4 , which was not actually manufactured, but only evaluated in effect through simulation. 
     Referring to  FIG. 6 , in the third exemplary embodiment as described above, an average operating pressure of a vent is 8.15 Kgf/cm 2 , which is lower than an operation pressure (about 9 Kgf/cm 2  or more) of a conventional vent. Also, referring to the lines X E  and Y C , it can be seen that there is no difference between their results. 
     That is, like other exemplary embodiments of the present invention, it is possible to lower the operating pressure of the vent according to  FIG. 4  to less than 9 Kgf/cm 2 , and specifically, to about 8 Kgf/cm 2 . Therefore, current flow may be blocked even when an internal pressure of the secondary battery is low, thereby further improving battery safety. 
     Although the present invention has been described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that a variety of modifications and variations may be made to the present invention without departing from the spirit or scope of the present invention defined in the appended claims, and their equivalents.