CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to and the benefit of U.S. Provisional Application No. 61/305,906 filed on Feb. 18, 2010 in the U.S. Patent and Trademark Office, the entire content of which is incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Field 
     Embodiments of the present invention relate to a rechargeable battery, more particularly, to a rechargeable battery with an improved vent member. 
     2. Description of the Related Technology 
     A rechargeable battery is a battery that is chargeable and dischargeable, unlike primary batteries that cannot be recharged. A low-capacity rechargeable battery is typically been used for small portable electronic devices, such as mobile phones, laptop computers, and camcorders, and a large-capacity rechargeable battery is typically used as a power supply for driving motors, such as in hybrid vehicles, etc., or a large-capacity power storage device. 
     Recently, high output rechargeable batteries using non-aqueous electrolytes with high energy density have been developed and such high power rechargeable batteries are configured of large-capacity battery modules by connecting a plurality of rechargeable batteries in series so that they can be used, for example, to drive motors for electric vehicles, etc. The rechargeable battery may be formed of a cylindrical type, a square type, etc. 
     While the rechargeable battery repeats charging and discharging, gas can be generated in the rechargeable battery to increase the pressure therein. When the increase in pressure of the rechargeable battery is not handled properly, there is a risk that the rechargeable battery may explode. 
     The above information is only presented for enhancement of understanding of the background of the invention and may contain information that does not form the prior art. 
     SUMMARY 
     An embodiment of the present invention provides a rechargeable battery including: an electrode assembly; a battery case accommodating the electrode assembly; and a cap assembly comprising: a cap plate comprising a top portion and at least one opening; and a vent member comprising two or more notches and a supporting portion, wherein the vent member is configured to break at the two or more notches and bend along two or more lines adjacent to the supporting portion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cut perspective view showing a rechargeable battery according to a first embodiment of the present invention; 
         FIG. 2A  is a perspective view showing a vent member of a rechargeable battery according to the first embodiment of the present invention; 
         FIG. 2B  is a perspective view showing a state where a notch of a vent member shown in  FIG. 2A  is fractured and opened; 
         FIG. 2C  is a plan view showing the relationship between a vent member and a cap plate of a rechargeable battery according to the first embodiment of the present invention; 
         FIG. 3  is a partial cross-sectional view of a rechargeable battery according to the first embodiment of the present invention; 
         FIG. 4  is a partial cross-sectional view showing a state where the notch of the vent member of  FIG. 3  is fractured and opened; 
         FIG. 5A  is a partial cross-sectional view showing a rechargeable battery according to the second exemplary embodiment of the present invention; 
         FIG. 5B  is a partial cross-sectional view showing a state where the notch of the vent member of  FIG. 5A  is fractured and opened; 
         FIG. 6A  is a perspective view showing a vent member of a rechargeable battery according to a third exemplary embodiment of the present invention; 
         FIG. 6B  is a perspective view showing a state where the notch of the vent member shown in  FIG. 6A  is fractured and opened; and 
         FIG. 7  is a partial cross-sectional view showing a rechargeable battery according to the third exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention. Like reference numerals designate like elements in the specification and drawings. 
       FIG. 1  is a cut perspective view showing a rechargeable battery according to an embodiment of the present invention. 
     Referring to  FIG. 1 , a rechargeable battery  100  according to an embodiment includes an electrode assembly  110  where an anode  112  and a cathode  113  are positioned, a separator  114  therebetween and a case  120  whose one end is opened so that it can receive the electrode assembly  110  with an electrolyte. The opening of the case  120  is installed with a cap assembly  140  that seals the case  120 . 
     In detail, case  120  may be made of a conductive metal, such as aluminum, aluminum alloy, or nickel-plated steel. A shape of the case  120  according to the present embodiment may be configured to be of a cylindrical type having an inner space in which the electrode assembly  110  is positioned. The cap assembly  140  may be inserted into the case  120  and then be clamped to fix the cap assembly  140  to the case  120 . In this process, the case  120  may be formed with a beading portion  123  and a clamping portion  125 . 
     The electrode assembly  110  according to the present embodiment may be configured of a cylindrical type wound like a vortex after the anode  112 , the separator  114 , and the cathode  113  are stacked, but the structure of the electrode assembly  110  is not necessarily limited thereto but can be formed to have a different structure. The anode  112 , the cathode  113 , and the separator  114  may be formed in a strip shape that is connected in one direction. 
     The anode  112  may include an anode active material layer that is formed on an anode current collector and both sides of the anode current collector and a cathode  113  may include a cathode current collector and a cathode active material layer that is formed on both sides of the cathode current collector. The separator  114  may be inserted between the anode  112  and the cathode  113  to insulate the anode  112  and the cathode  113  and to provide a passage through which ions move. 
     An upper end of the anode  112  may be formed with an anode non-coated portion  112   a  on which the anode active material is not formed, and a lower end of the cathode  113  may be formed with a cathode non-coated portion  113   a  on which the cathode active material layer is not formed. The present embodiment illustrates a case in which the anode non-coated portion  112   a  and the cathode non-coated portion  113   a  are formed on the upper end and the lower end of the electrode assembly  110 , respectively, but embodiments of the present invention are not limited thereto. The anode non-coated portion  112   a  may be formed at one end of the anode  112  in the length direction and the cathode non-coated portion  113   a  may be formed at the other end of the cathode  113  in the length direction. 
     The cap assembly  140  may include the cap plate  143 , a gasket  144  that is adjacently disposed to the case  120  and winds the vent member  160  installed below the cap plate  143  and the cap plate  143 , and a safety plate  141  disposed between the cap assembly  140  and the vent member  160 . 
     The cap plate  143  may be formed with an upwardly protruded protrusion  143   a  and a penetrated exhaust port  143   b , and the vent member  160  may be formed with notches  161  and  163  fractured at the set pressure condition. A safety plate  141  that electrically connects the cap plate  143  and the vent member  160  may be installed between the cap plate  143  and the vent member  160 . The safety plate  141  may be configured of a positive temperature coefficient element and an element that increases the electric resistance approximately up to infinity when the positive temperature coefficient element exceeds a predetermined temperature. The safety plate  141  may perform a role of blocking the flow of charging and discharging current when the rechargeable battery  100  exceeds a temperature of the predetermined value. 
     The gasket  144  may be installed to the circumference of the cap plate  143 , the safety plate  141 , the vent member  160  and insulate the cap assembly  140  from the case  120 . 
     The anode  112  may be electrically connected to the vent member  160  via the anode current collecting tap  135  and the cathode  113  may be electrically connected to the bottom of the case  120  via the cathode current collecting tap  136 . The present embodiment illustrates the case in which the anode current collecting tap  135  is directly connected to the vent member  160 , but embodiments of the present invention are not limited thereto. The intermediate member of the vent member  160  may be installed such that the vent member  160  and the anode current collecting tap  135  may be connected via the intermediate member. 
       FIG. 2A  is a perspective view showing the vent member according to the first illustrated embodiment of the present invention and  FIG. 2B  is a perspective view showing the state where the notch of the vent member shown in  FIG. 2A  is fractured and opened, and  FIG. 2C  is a plan view showing the relationship between the vent member and a cap plate of the rechargeable battery according to the first embodiment of the present invention. 
     Referring to  FIGS. 2A ,  2 B and  2 C, the vent member  160  according to the present illustrated embodiment is formed with the first notch  161  and the second notch  163 , and each of the notches  161  and  163  may be formed of a line segment having an arc shape. The present illustrated embodiment illustrates the case where the notches  161  and  163  are formed to have an arc shape but embodiments of the present invention are not limited thereto. The notch can be formed in various shapes, such as a triangle, a quadrangle, etc. The first notch  161  and the second notch  163  according to the present embodiment is formed of a semicircular type, and the first notch  161  and the second notch  163  are disposed to be spaced from each other. A supporting portion  165  may be formed between the bent lines  164  that connect both ends of each notch  161  and  163 . The supporting portion  165  may perform a role of separating an opening  162  where the notches  161  and  163  may be formed to be fractured. The first notch  161  and the second notch  163  may be symmetrical such that the supporting portion  165  is provided therebetween. 
     The notches  161  and  163  may be formed to be in a line that is convexly protruded toward the outside of the vent member  160  from the bent line  164 . A first fracture piece  167  that is bent and raised at the time of fracturing the first notch  161  may be positioned between the first notch  161  and the supporting portion  165 , and a second fracture piece  168  that is bent and raised at the time of fracturing the second notch  163  may be positioned between the second notch  163  and the supporting portion  165 . 
     The second notch may be formed of the same as the first notch and therefore, the description of the second notch will be described as the description of the first notch. 
     Referring to  FIG. 2C , the supporting portion  165  is under the protrusion  143   a  of the cap plate  143  and does not extend beyond the protrusion  143   a  of the cap plate  143 . That is, when viewed in the plan view, the bent lines  164  and the supporting portion  165  are inside the protrusion  143   a.    
     If the bent lines  164  are not inside the protrusion  143   a , a portion of the supporting portion  165  positioned outside the protrusion  143   a  blocks the passage to the exhaust port  143   b  when the vent member  160  is opened. Since the bent lines  164  in the present embodiment are inside the protrusion  143   a , the supporting portion  165  does not block the passage to the exhaust port  143   b  and the first and second fracture pieces  167  and  168  guide the gas discharged through the opening  162  to the exhaust port  143   b.    
     As shown in  FIGS. 3 and 4 , when the maximum distance from the bent line  164  to the first notch  161  is referred to as a long width L 2 , the long width L 2  of the first notch  161  according to the present illustrated embodiment may be formed of the same distance as the distance L 3  from the bent line  164  to the upper end of the exhaust port  143   b  that is formed in the cap plate  143 . Therefore, as shown in  FIG. 4 , when the internal pressure of the rechargeable battery is raised to break the first notch  161 , the upper end of the first fracture piece  167  may contact the inside end of the exhaust port  143   b  to sufficiently secure the passage through which gas moves. However, embodiments of the present invention are not limited thereto and the long width L 2  may be formed to be smaller than the distance L 3  from the bent line  164  to the upper end of the exhaust port  143   b.    
     In addition, since the notches  161  and  163  may be formed to have a predetermined space therebetween while positioning the supporting portion  165  therebetween, each notch  161  and  163  can be easily fractured without each of the notches  161  and  163  interfering with each other when being fractured, such that the opening  162  is formed at two places. 
     In addition, since the first notch  161  is convexly formed toward the outside of the vent member  160  with respect to the bent line  164 , when the first notch  161  is fractured such that the first fracture piece  167  is raised, the first fracture piece  167  is not positioned between the exhaust port  143   b  and the opening  162  that is formed by the fracture of the first notch  161  and is positioned near the opening  162  and the exhaust port  143   b . In other words, in reference to the drawing, it can be appreciated that the lower end of the first fracture piece  167  may contact the inner side end of the opening  162  and the upper end of the first fracture piece  167  may also contact the inner side end of the exhaust port  143   b . At this time, the inner side may be inwardly positioned based on the center of the vent member  160 . Therefore, since gas discharged through the opening  162  can move the exhaust port  143   b  using the guide of the first fracture piece  167 , gas can be discharged more rapidly. However, when the first notch  161  is positioned more inwardly than the bent line  164  and the exhaust port  143   b  is spaced from the center of the cap plate  143  and is disposed in a circumferential direction, since the first fracture piece  167  is positioned between the opening  162  and the exhaust port  143   b , there may be a problem in that first fracture piece  167  prevents the progress of gas. 
     When the first notch  161  is determined by the rise of internal pressure, the first fracture piece  167  may be bent at an inclined angle of 30° to 90° with respect to the supporting portion  165  in the bent line  164 . When the bending inclination of the first fracture piece  167  is smaller than 30°, there may be a problem in that the gas inside the rechargeable battery cannot be discharged rapidly, and when the bending inclination of the first fracture piece  167  is larger than 90°, a vortex may be generated at an area adjacent to the upper end of the first fracture piece  167  such that the discharge speed of gas is deteriorated. In other words, when the bending inclination of the first fracture piece  167  is larger than 90°, a reflowing phenomenon can occur by the introduction of gas in a reverse direction than the direction that discharges gas. Therefore, the discharge of gas may be delayed due to the reflowing gas. 
     The discharge speed of gas is very important in terms of safety of the rechargeable battery, and when the discharge speed of gas is delayed, there may be a serious problem in that the rechargeable battery explodes according to the conditions. Since the discharge of gas can be performed within a short time in a sudden situation, safety can be improved only in the case where gas needs to be rapidly discharged in a short time. 
     An experiment in which the battery module that includes 10 rechargeable batteries exploded under a situation using fire was tested. In the rechargeable battery of a comparative example in which one notch having a large semicircle is formed, four batteries exploded and 6 batteries became inflamed, while in the rechargeable battery according to the present embodiment, all 10 rechargeable batteries became inflamed but they did not explode. Through the above experiment, even in the scene of a fire or an abnormal situation where the temperature was very high, it could be appreciated that the rechargeable battery according to the present embodiment did not explode. The reason appears to be that before the rechargeable battery was able to explode, the vent member  160  was rapidly opened, thereby making it possible to rapidly discharge the internal gas to the ambient. 
       FIG. 5A  is a partial cross-sectional view showing a rechargeable battery according to a second embodiment of the present invention and  FIG. 5B  is a partial cross-sectional view showing a state where the notch of the vent member of  FIG. 5A  is fractured and opened. Referring to  FIGS. 5A and 5B , the rechargeable battery according to the present embodiment is configured to have the same structure as the rechargeable battery of to the first described embodiment except for the structure of the vent member  170 , and thus, certain common descriptions thereof will not be repeated. 
     The vent member  170  may be formed with the first notch  171  and the second notch  172  having an arc type formed to be spaced, putting the supporting portion  175  therebetween, similar to the first described embodiment. The first notch  171  and the second notch  172  may be formed to be symmetrical with respect to the position of the supporting portion  175 , and the line segment that connects both ends of the first notch  171 , and the line segment that connects both ends of the second notch  172 , can become the bent line  174 . When the internal pressure of the rechargeable battery is raised, the first notch  171  may be fractured such that the first fracture piece  177  is bent and raised at the bent line  174 , and the second notch  172  may also be fractured such that the second fracture piece  178  is bent and raised at the bent line  174 . 
     The vent member  170  according to the present embodiment may be formed so that a long width L 4 , that is the maximum distance from the bent line  174  to the first notch, may be smaller than a distance L 5  from the bent line  174  to the inner side end of the exhaust port  143   b , and may be larger than a distance h 1  from the bent line  174  to the upper end of the cap plate  143 . Thereby, the upper ends of the fracture pieces  177  and  178  may be positioned between the inner side end of the exhaust port  143   b  and the place just above the bent line  174 , such that gas discharged through the opening  173  can be rapidly guided to the exhaust port  143   b.    
       FIG. 6A  is a perspective view showing a vent member of a rechargeable battery according to a third embodiment of the present invention and  FIG. 6B  is a perspective view showing a state where the notch of the vent member shown in  FIG. 6A  is fractured and opened. 
     Referring to  FIGS. 6A and 6B , the rechargeable battery according to the present embodiment is configured to have the same structure as the rechargeable battery according to the first described embodiment, except for a vent member  180  and thus, certain common description thereof will not be repeated. 
     The vent member  180  may be formed with a first notch  181  and a second notch  182 , and a third notch  183 . The notches  181 ,  182 , and  183  may be disposed along the circumferential direction of the vent member  180  in equidistance and may be formed in a curved line that is convexly curved toward the outside of the vent member  180 . A line segment that connects both ends of the notches  181 ,  182 , and  183  may become a bent line  184 . The cap plate  143  may be formed with three exhaust ports  143   b  and may be formed with three notches  181 ,  182 , and  183  to correspond to the number of exhaust ports  143   b . At this time, the notches  181 ,  182 , and  183  may be disposed at the corresponding positions below each exhaust port. When the notches  181 ,  182 , and  183  are fractured due to the increase in the internal pressure of the battery, a first fracture piece  186 , a second fracture piece  187 , and a third fracture piece  188  may be bent and raised at the bent line  184 . A supporting portion  185  that supports the fracture pieces  186 ,  187 , and  188  may be formed at the center of the vent member  180  among the notches  181 ,  182 , and  183 . 
     When the fracture pieces  186 ,  187 , and  188  are raised, since the vent member  180  is formed with an opening  189 , the opening may be positioned at the corresponding positions below the exhaust port. Thereby, gas discharged through each opening  189  can be rapidly discharged through the corresponding exhaust port  143   b.    
     Since the second notch  182  and the third notch  183  are configured to have the same structure as the first notch  181  according to an embodiment, the description of the second notch  182  and the third notch  183  will not be repeated since the description of the first notch  181  is described above. 
     A width L 6  that is the maximum distance from the bent line  184  to the first notch  181  may be formed to be smaller than the distance h 1  from the bent line  184  to the upper end of the cap plate  143 . Therefore, the fracture pieces  186 ,  187 , and  188  can freely bend at any angle according to the fracture pressure but interfere with the adjacent fracture pieces  186 ,  187 , and  188 , thereby making it possible to prevent the fracture piece  186 ,  187 , and  188  from excessively bending. 
     While this invention has been described in connection with what is presently considered to be practical embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.
A rechargeable battery according to an embodiment of the present invention includes: an electrode assembly; a battery case accommodating the electrode assembly; and a cap assembly comprising: a cap plate comprising a top portion and at least one opening; and a vent member comprising two or more notches and a supporting portion, wherein the vent member is configured to break at the two or more notches and bend along two or more lines adjacent to the supporting portion.