Patent Publication Number: US-2023140593-A1

Title: Cylindrical secondary battery

Description:
TECHNICAL FIELD 
     An embodiment of the present invention relates to a cylindrical secondary battery capable of preventing component deformation during assembling. 
     BACKGROUND ART 
     In general, a cylindrical secondary battery includes a cylindrical electrode assembly, a cylindrical can for accommodating the electrode assembly and electrolyte, and a cap assembly that is coupled to the upper opening of the can to seal the can and allows current generated from the electrode assembly to flow to an external device. 
     In order to prevent the electrode assembly from moving, a beading part is formed by applying pressure along the upper outer circumferential surface of the can. At this time, as the pressure is applied from the outside to the inside of the can, the pressure is also applied to the cap assembly, which creates stress and causes deformation of components of the cap assembly. 
     In particular, when the safety vent is deformed, vent fracture pressures may change, so that the pressure is not properly discharged during overcharging, leading to defects and damages to the secondary battery, which is problematic. 
     The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not constitute prior art. 
     DESCRIPTION OF EMBODIMENTS 
     Technical Problem 
     It is an object of the present invention to provide a cylindrical secondary battery having a structure in which the stress caused by the pressure generated during can processing can be dispersed and deformation of a safety vent can be prevented. 
     Solution to Problem 
     A cylindrical secondary battery according to an embodiment of the present invention may include: a cylindrical can; an electrode assembly accommodated in the cylindrical can; and a cap assembly comprising a cap-up, a cap-down arranged under the cap-up, a vent plate which is arranged between the cap-up and the cap-down; is separated from the cap-down, and has at least one notch formed thereon, and an insulating member inserted between the vent plate and the cap-down so as to insulate the vent plate and the cap-down from each other, wherein the cap assembly may further include a support member inserted between the vent plate and the cap-down, separated from the insulating member, and arranged on the inner side of the insulating member. 
     The support member may be arranged on the inner region facing the longitudinal central axis of the can, on the basis of the notch. 
     The insulating member and the support member may have a circular ring shape, and the vent plate may further include a contact portion penetrating the insulating member and the support member to be in contact with the cap-down. 
     The support member may be arranged between the notch and the contact portion. 
     In the support member, the width in a direction perpendicular to the longitudinal central axis of the can is equal to or greater than the length between the notch and the contact portion. 
     The support member may be welded to the insulating member. 
     In addition, the present invention provides a secondary battery including: a cylindrical can; an electrode assembly accommodated in the cylindrical can; and a cap assembly comprising a top plate having at least one notch formed thereon, a bottom plate arranged under the top plate and having a contact portion in which a portion of the plate surface protrudes toward the top plate and comes into contact with the top plate, and an insulation plate arranged between the top plate and the bottom plate to insulate the top plate and the bottom plate from each other, except for the contact portion, wherein the cap assembly may further comprise a support member inserted between the top plate and the insulation plate to support the top plate. 
     The support member may be arranged on the inner region facing the longitudinal central axis of the can, on the basis of the notch. 
     The insulating member and the support member may have a circular ring shape, and the contact portion of the bottom plate may penetrate the insulating member and the support member to be in contact with the top plate. 
     The support member may be arranged between the notch and the contact portion, and the top plate may be shaped to surround the edge of the support member. 
     The support member may be welded to the insulating member. 
     In the support member, a portion of the surface thereof being in contact with the notch may be spaced apart from the notch. 
     The support member has a plurality of through-holes formed therethrough at positions corresponding to the positions of the notch. 
     ADVANTAGEOUS EFFECTS OF DISCLOSURE 
     According to an embodiment of the present invention, a deformation portion of a safety vent is supported to prevent deformation of the safety vent if stress occurs, and thus vent fracture can be stabilized. 
     In addition, according to an embodiment of the present invention, since the vent fracture pressure is maintained, it is possible to stably respond to changes in pressure inside the can during overcharging, thereby preventing defects and damage to secondary batteries and ensuring reliability. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a longitudinal cross-sectional view showing a cylindrical secondary battery according to a comparative embodiment of the present invention. 
         FIG.  2    is a longitudinal cross-sectional view illustrating a cylindrical secondary battery according to a first embodiment of the present invention. 
         FIG.  3    is an enlarged longitudinal cross-sectional view of a cap assembly according to  FIG.  2   . 
         FIG.  4    is a table showing the experimental results of deformation prevention for various numerical values of major parts of  FIG.  3   . 
         FIG.  5    is a longitudinal cross-sectional view illustrating a cap assembly of a cylindrical secondary battery according to a second embodiment of the present invention. 
         FIG.  6    is a longitudinal cross-sectional view illustrating a cap assembly of a cylindrical secondary battery according to a third embodiment of the present invention. 
         FIG.  7    is a plan view illustrating a partial configuration of the cap assembly according to  FIG.  6   . 
     
    
    
     Best Mode 
     Example embodiments of the present invention are provided to more completely explain the present invention to a person skilled in the art. The following embodiments may be modified in various different forms, but the scope of the present invention is not limited to the following embodiments. Rather, these embodiments are provided so that this disclosure will be more thorough and complete, and will fully convey the spirit of the invention to a person skilled in the art. 
     In addition, in the accompanying drawings, sizes or thicknesses of various components are exaggerated for brevity and clarity. Like numbers refer to like elements throughout. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. In addition, it will be understood that when an element A is referred to as being “connected to” an element B, the element A can be directly connected to the element B or an intervening element C may be present therebetween such that the element A and the element B are indirectly connected to each other. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used herein, the singular forms are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms that the terms “comprise or include” and/or “comprising or including,” when used in this specification, specify the presence of stated features, numbers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, numbers, steps, operations, elements, components, and/or groups thereof. 
     It will be understood that, although the terms first, second, etc. may be used herein to describe various members, elements, regions, layers and/or sections, these members, elements, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one member, element, region, layer and/or section from another. Thus, for example, a first member, a first element, a first region, a first layer and/or a first section discussed below could be termed a second member, a second element, a second region, a second layer and/or a second section without departing from the teachings of the present invention. 
     Spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper,” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the element or feature in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “on” or “above” the other elements or features. Thus, the exemplary term “below” can encompass both an orientation of above and below. 
       FIG.  1    is a longitudinal cross-sectional view showing a cylindrical secondary battery according to a comparative embodiment of the present invention. 
     As shown in  FIG.  1   , a general cylindrical secondary battery  1  includes a cylindrical can  10  having an opening formed at one end in the longitudinal direction, an electrode assembly  30  accommodated in the can, and a cap assembly  50  inserted into the opening. The cap assembly  50  includes a safety vent  51 , a cap-up  52  disposed above the safety vent  51 , and a cap-down  53  disposed below the safety vent  51 . The cap assembly  50  may further include an insulating member  54  inserted between the safety vent  51  and the cap-down  53  to insulate a portion of the safety vent  51  other than the central portion thereof from contacting the cap-down  53 , and an insulation gasket  55  that insulates the cap assembly  50  and the can  10  from each other. In the safety vent  51 , a portion near the longitudinal central axis A of the can  10  is in contact with the cap-down  53 , and a portion supported by the insulating member  54  is spaced apart from the cap-down  53 . 
     In the opened side of the can  10 , the lower portion of the cap assembly  50  is depressed inwardly in a ring shape to form a beading part  12 , and the upper portion of the cap assembly  50  is bent inwardly to form a crimping part  14 . In order to form the beading part  14  and the crimping part  14 , a process of applying a force from the outside to the inside of the can  10  is performed, and stress is also generated in the cap assembly  50  by the external force. 
     The direction in which the stress is applied is a direction from the outside to the inside of the can  10 , as indicated by the arrow shown in  FIG.  1   . Stress acts on the cap assembly  50  as a whole, but since the notch  51   a  is formed in the safety vent  51 , the safety vent  51  may be deformed because it is relatively vulnerable to stress, compared to other components of the cap assembly  50 . A portion of the safety vent  51  where deformation occurs is a portion spaced apart from the cap-down  53  (a dotted line region in  FIG.  1   , to be referred to as a deformed portion, hereinafter). The deformed portion of the safety vent  51  is vulnerable to be deformed toward the cap-down  53  when stress occurs, so that when the safety vent  51  is deformed, the fracture pressure of the notch  51   a  may vary and thus, when abnormal gas is generated inside the secondary battery  1 , the gas may not be smoothly discharged, which is problematic. Accordingly, there exists a need for a structure in which the safety vent  51  does not deform even if stress occurs during assembling of the cylindrical secondary battery  1 . To this end, in the present invention, proposed is a cap assembly having a novel structure. 
     Hereinafter, a cylindrical secondary battery according to an embodiment of the present invention will be described in detail with reference to the accompanying drawings. 
       FIG.  2    is a longitudinal cross-sectional view illustrating a cylindrical secondary battery according to a first embodiment of the present invention.  FIG.  3    is an enlarged longitudinal cross-sectional view of a cap assembly according to  FIG.  2   . 
     As shown in  FIGS.  2  and  3   , the cylindrical secondary battery  1000  according to a first embodiment of the present invention may include a cylindrical can  100 , an electrode assembly  300  inserted into the can  100 , a cap assembly  500  inserted into one end of the can  100 , and an insulation gasket  700  inserted between the can  100  and the cap assembly  500 . A center pin  380  may be coupled to the electrode assembly  300 . 
     The can  100  includes a circular bottom portion  110  and a side portion  130  extending upwardly from the bottom portion  110 , and an upper portion of the side portion  130  ids shaped to be opened (hereinafter referred to as an opening). In the manufacturing process of the secondary battery  1000 , the electrode assembly  300  is inserted into the can  100  together with an electrolyte through the opening of the can  100 , The can  100  may be formed of steel, a steel alloy, nickel-plated steel, nickel-plated steel alloy, aluminum, an aluminum alloy, or an equivalent thereof, but the material is not limited thereto. 
     The cap assembly  500  is inserted into the opening of the can  100 . A beading part  132  and a crimping part  134  may be formed on the side portion  130  so as to prevent the inserted cap assembly  500  from escaping to the outside through the opening of the can  100 . 
     The beading part  132  is formed under the cap assembly  500  and is shaped to be depressed in the inward direction of the can  100 . The crimping part  134  is formed on the cap assembly  500  and is shaped to be bent in the inward direction of the can  100 . Since the beading part  132  and the crimping part  134  hold the cap assembly  500  in the vertical direction, the cap assembly  500  is not separated from the can  100 . The electrode assembly  300  is disposed under the cap assembly  500  inside the can  100 . 
     The electrode assembly  300  includes a negative electrode plate  310  coated with a negative electrode active material (e.g., graphite, carbon, etc.), a positive electrode plate  320  coated with a positive active material (e.g., transition metal oxide (LiCoO2, LiNiO2, LiMn2O4, etc.)), and a separator  330  disposed between the negative electrode plate  310  and the positive electrode plate  320  to prevent a short circuit and enable only movement of lithium ions. The negative electrode plate  310 , the positive electrode plate  320 , and the separator  330  may be wound in a substantially cylindrical shape and accommodated in the can  100 . The negative electrode plate  310  may be copper (Cu) or nickel (Ni) foil, the positive electrode plate  320  may be an aluminum (Al) foil, and the separator  330  may be polyethylene (PE) or polypropylene (PP), but the present invention is not limited thereto. A negative electrode tab  340  that downwardly protrudes and extends a predetermined length may be welded to the negative electrode plate  310 , and a positive electrode tab  350  that upwardly protrudes a predetermined length may be welded to the positive electrode plate  320 , but vice versa. The negative electrode tab  340  may be made of copper or nickel, and the positive electrode tab  350  may be made of aluminum, but the present invention is not limited thereto. The negative electrode tab  340  may be welded to the bottom portion  110  of the can  100 , and in this case, the can  100  may operate as a negative electrode. Conversely, the positive electrode tab  350  may be welded to the bottom portion  111  of the can  100 , and in this case, the can  100  may operate as a positive electrode. 
     In addition, a first insulation plate  360  and a second insulation plate  370  may be interposed on and under the electrode assembly  300 , respectively. The first insulation plate  360  prevents the positive electrode plate  320  from electrically contacting the bottom portion  110  of the can  100 , and the second insulation plate  370  prevents the negative electrode plate  310  from electrically contacting the cap assembly  500 . 
     In the first insulation plate  360 , a first hole  362  and a second hole  364  may be formed therethrough, the first hole  362  communicating with the center pin  380  so that the gas can move upward through the cylindrical center pin  380  when a large amount of gas is generated due to an abnormality of a secondary battery, and the second hole  364  allowing the negative electrode tab  340  to pass therethrough. The negative electrode tab  340  may be welded to the bottom portion  110  through the second hole  364 . 
     In the second insulation plate  370 , a first hole  372  and a second hole  374  may be formed therethrough, the first hole  372  allowing the gas to move to the cap assembly  500  when a large amount of gas is generated due to an abnormality of a secondary battery, and the second hole  374  allowing the positive electrode tab  350  to pass therethrough. The positive electrode tab  350  may be welded to a cap-down  550  to be described later through the second hole  374 . The second hole  374  may include a plurality of second holes to serve as an inlet through which the electrolyte is injected into the electrode assembly  300  in an electrolyte injection process. 
     The center pin  380  has a hollow circular pipe shape, and may be coupled to the center of the electrode assembly  300 . The center pin  380  may be formed of steel, a steel alloy, nickel-plated steel, a nickel-plated steel alloy, aluminum, an aluminum alloy, or polybutylene terephthalate, but the material is not limited thereto. The center pin  380  serves to suppress deformation of the electrode assembly  300  during charging and discharging of the secondary battery, and serves as a passage for gas generated inside the secondary battery. In some cases, the center pin  380  may be omitted. 
     Meanwhile, as shown in  FIG.  2   , the cap assembly  500  includes a cap-up  510  exposed to the outside of the can  100 , a cap-down  550  disposed below the cap-up  510 , a vent plate  530  disposed between the cap-up  510  and the cap-down  550 , and an insulating member  570  and a support member  590 , disposed between the vent plate  530  and the cap-down  550 . In  FIGS.  2  and  3   , on the basis of the imaginary central axis B along the longitudinal direction of the can  100 , the direction toward the central axis B is defined as the inward direction, and the direction away from the central axis B is defined as the outward direction. 
     As shown in  FIG.  3   , the cap-up  510  is disposed at the uppermost portion of the cap assembly  500  and may include a through-hole  512  for discharging gas generated inside the can  100  to the outside. The cap-up  510  may have a substantially disk shape, and may have a predetermined region that convexly protrudes upward about the central axis B, and a through-hole  512  may be formed in the protruding portion. A vent plate  530  is disposed under the cap-up  510 , and the vent plate  530  may be coupled to surround the edge of the cap-up  510 . 
     The vent plate  530  has a substantially disk shape, the edge of which is bent toward the edge of the cap-up  510  to be in contact with the lower edge of the cap-up  510 , and may be bent back toward the inside of the can  100  around the contact portion to then be in contact with the upper edge of the cap-up  510 . In the vent plate  530 , a disk portion that is not bent is defined as a vent bottom portion  532 , a portion that is bent from the vent bottom portion  532  toward the cap-up  510  is defined as a first support portion  534 , a portion that is inwardly bent from the first support portion  534  is defined as a second support portion  536 , and a portion that convexly protrude downward from the vent bottom portion  532  to be in contact with the cap-down  550  is defined as a contact portion  538 . The vent plate  530  is formed so that all regions except the contact portion  538  do not come into contact with the cap-down  550 . At least one notch  532   a  may be formed on the vent bottom portion  532  of the vent plate  530 . 
     For example, the notch  532   a  may be formed in a circular ring shape. Alternatively, a plurality of notches  532   a  may be formed in a streamlined shape and disposed so as to have a substantially circular shape. When the gas pressure inside the can  100  is greater than the predetermined fracture pressure due to overcharging, etc., the vent plate  530  may be inverted upward, and the notch  532   a  may be broken so that the gas inside the can  100  can be rapidly discharged to the outside through the through-hole  512  of the cap-up  510 . 
     The cap-down  550  is disposed under the vent plate  530  and has a substantially disk shape. For example, the cap-down  550  may be formed of aluminum, an aluminum alloy, or an equivalent thereof, but the material thereof is not limited thereto. The cap-down  550  serves to support the cap-up  510  and prevent deformation of the cap-up  510  against an external force. The edge of the cap-down  550  may be bent toward the vent plate  530 , and the insulating member  570  is disposed on the bent portion. The portion that is bent toward the vent plate  530  is defined as a third support portion  552 . A disk portion of the cap-down  550 , which is not bent, is defined as a cap-down bottom portion  554 , and the cap-down bottom portion  554  may be formed to have a predetermined spacing from the vent bottom portion  532  of the vent plate  530 . However, the approximately central portion of the cap-down bottom portion  554  is in contact with the contact portion  538  of the vent plate  530 . The cap-down  550  may also have a through-hole  554   a  formed on the cap-down bottom  554 , and when an abnormal internal pressure occurs, the internal gas passes through the through-hole  554   a  of the cap-down  550  to then be discharged to the outside of the can  100  through the notch  532   a  of the vent plate  530  and the through-hole  512  of the cap-up  510 . 
     The insulating member  570  allows the vent plate  530  and the cap-down  550  to maintain a spaced state except for the contact portion  538 , and serves to insulate the vent plate  530  and the cap-down  550  from each other. Therefore, the insulating member  570  may be shaped of a circular ring having a predetermined width when viewed from the top. For example, the insulating member  570  may be formed of polyethylene (PE), polypropylene (PP), ethylene propylene diene monomer (EPDM) (M-class) rubber, or an equivalent thereof, but is not limited thereto. The insulating member  570  may be welded to the vent plate  530  and the cap-down  550  by ultrasonic welding or laser welding. 
     When an external force is applied when forming the beading part  132  on the can  100  in the manufacturing process of the secondary battery  1000 , stress may be generated in the cap assembly  500 . The vent plate  530  may be deformed due to the stress generated, and deformation may easily occur in a region where a spacing exists between the vent plate  530  and the cap-down  550  (see  FIG.  1   ). The reason of the foregoing is that the region where a spacing exists does not have a structure capable of supporting the vent plate  530 , and since the notch  532   a  is formed in the corresponding region, the region may be vulnerable to deformation due to the stress generated. Therefore, in order to prevent the deformation of the vent plate  530 , the support member  590  may be interposed in the region spaced from the cap-down  550  (hereinafter, to be referred to as a deformed portion C, the dotted line region of  FIG.  3   ). 
     The support member  590 , when viewed from above, may be shaped of a substantially circular ring having a predetermined width, and may be made of the same or similar material as the insulating member  570 . Alternatively, the cap-down  550  may be made of the same or similar material as the cap-down  550 . When the vent plate  530  is fractured, the support member  590  is located under the vent plate  530 , and the vent plate  530  swells toward the cap-up  510 , and thus the vent plate  530  and the support member  590  are not electrified. Accordingly, the support member  590  may be formed of aluminum, an aluminum alloy, or an equivalent thereof. 
     In addition, the support member  590  is disposed between the notch  532   a  and the contact portion  538  to fill the space between the vent plate  530  and the cap-down  550 . Since the support member  590  supports the vent plate  530  from the lower portion, the vent plate  530  may be supported so as not to be deformed in the downward direction due to the stress. In addition, since the support member  590  is in contact with the bent plate  530 , the stress can be dispersed. 
     The support member  590  is not fixed to the vent plate  530  so as not to interfere with the movement of gas and fracture of the notch  532   a  when abnormal internal pressure occurs, but may be fixed with the cap-down  550  by welding, etc. To this end, the support member  590  may be disposed inside the notch  532   a  on the basis of  FIG.  3   . 
     Referring to  FIG.  3   , an effective value at which the support member  590  can prevent deformation of the vent plate  530  will be described. 
     As shown in  FIG.  3   , the diameter of the vent bottom portion  532  of the vent plate  530  is defined as R 1 , the diameter of the contact portion  538  is defined as R 2 , and the width of the support member  590  is defined as L 1 . A total length of a portion where the bent plate  530  and the cap-down  550  indirectly contact through the support member  590  is twice the width L 1  of the support member  590 . In addition, the length of the portion where the vent plate  530  and the cap-down  550  directly contact it corresponds to the diameter R 2  of the contact portion  538 . Accordingly, the total length of the portion where the vent plate  530  and the cap-down  550  directly or indirectly contact corresponds to (L 1 * 2 )+R 2  (hereinafter, defined as the total contact length R 3 ). Whether or not the deformed portion C of the vent plate  530  is deformed depends on how much of the total contact length R 3  occupies, in percent (%), the length of the vent bottom portion  532  of the vent plate  530 . The experiments therefor were conducted and summarized in  FIG.  4   . 
       FIG.  4    is a table showing the experimental results of deformation prevention for various numerical values of major parts of  FIG.  3   . 
     As shown in  FIG.  4   , for example, in the case in which the diameter R 1  of the vent bottom portion  532  of the vent plate  530  is 11.4 mm, the diameter R 2  of the contact portion  538  is 3.28 mm, and the width L 1  of the support member  590  is 1.2 mm, the total contact length R 3  is 5.68 mm, and, on the basis of the diameter of the vent bottom portion  532 , the occupancy (R 3 /R 1 ) occupied by the total contact length R 3  is 51.4%. As a result of experiments conducted whether the vent plate  530  is deformed during the forming process of the beading part  132  by using the cap assembly  500  having these values, it can be seen that deformation prevention is not achieved, as shown in  FIG.  4   . 
     In the same way, while increasing the width L 1  of the support member  590  by 0.2 mm, the occupancy of the total contact length R 3  relative to the diameter of the vent bottom portion  532  can be obtained, and through the same experiment as described above, the experiment can be conducted as to whether or not the vent plate  530  is deformed. 
     As a result, as shown in  FIG.  4   , it can be seen that no deformation occurred in the deformed portion C of the vent plate  530  when the width L 1  of the support member  590  is 1.8 mm, and the occupancy of the total contact length R 3  relative to the diameter of the vent bottom portion  532  becomes 62% or more. In addition, when the width L 1  of the support member  590  is at least 1.8 mm, most of the space between the notch  532   a  of the vent plate  530  and the contact portion  538  is occupied. Therefore, it can be seen that deformation does not occur in the deformed portion C of the vent plate  530  only when the width L 1  of the support member  590  at least occupies the inward direction of the notch  532   a  (a minimum deformation prevention condition), and that being more than that will be okay. 
     Based on the experimental results, the diameter of the contact portion  538  and the length of the support member  590  relative to the diameter of the vent bottom portion  532  of the vent plate  530  in which the vent plate  530  is not deformed may be calculated. Therefore, even when an external force is applied, the support member  590  supports the deformed portion C to prevent deformation of the vent plate  530 , and has an effect of smoothly discharging gas when abnormal pressure is generated. 
     In the above-described embodiment, a structure for preventing deformation of the vent plate, by providing a support member to the cap assembly in which the cap-up of the upward protruding form and the vent plate surround the edge of the cap-up, has been described. However, the support member of the present invention can also be applied to other types of cylindrical secondary batteries. Hereinafter, another embodiment of the present invention will be described (as to the same configuration as that of the above-described first embodiment, a detailed description will be omitted, and the direction toward the central axis D is defined as the inward direction, and the opposite direction is defined as the outward direction). 
       FIG.  5    is a longitudinal cross-sectional view illustrating a cap assembly of a cylindrical secondary battery according to a second embodiment of the present invention. 
     As shown in  FIG.  5   , the cap assembly  900  of the cylindrical secondary battery according to the second embodiment of the present invention may include a top plate  910 , a bottom plate  920 , an insulation plate  930  disposed between the top plate  910  and the bottom plate  920 , and a support member  940  disposed between the top plate  910  and the insulation plate  930 . Although not shown in the drawing, a beading part and a crimping part may be formed on the upper portion of a can of the secondary battery, and the edge of the cap assembly  900  may be inserted and fixed between the beading part and the crimping part with an insulating gasket interposed therebetween (see the cap assembly structure of  FIG.  2   ). 
     The top plate  910  has a substantially disk shape, and may be configured such that the edge thereof is bent downward and then bent again in an inward direction (hereinafter, a bent portion). For example, the top plate  910  may be formed of aluminum, an aluminum alloy, or an equivalent thereof. A terminal portion  912  in which at least one notch  912   a  is formed may be formed in a predetermined region, on the basis of the central axis D of the top plate  910 . 
     The terminal portion  912  serves as a terminal of the secondary battery and may be electrically connected to an external device. The terminal portion  912  may convexly protrude upwardly to a predetermined height, and a notch  912   a  is formed on the terminal portion  912 . 
     The notch  912   a  is fractured when the internal gas pressure of the secondary battery is greater than a predetermined fracture pressure, and serves to rapidly discharge the internal gas of the secondary battery to the outside. The bottom plate  920  is disposed under the top plate  910 . 
     The bottom plate  920  may include a base portion  922  positioned below the insulating plate  930  and a contact portion  924  that convexly protrude upwardly from the base portion  922  to be in contact with the bottom surface of the terminal portion  912  of the top plate  910 . That is, the contact portion  924  passes through a hole in the inward direction of the ring-shaped insulating plate  930  and the support member  940  to be in contact with the terminal portion  912 . The base portion  922  and the contact portion  924  are integrally formed, and the contact portion  924  may be welded and fixed to the lower surface of the terminal portion  912  of the top plate  910 . At least one notch  924   a  may be formed on the contact portion  924 . 
     The notch  924   a  is a portion that is fractured when the internal gas pressure of the secondary battery is greater than a predetermined fracture pressure. When the internal gas pressure of the secondary battery is greater than a predetermined pressure, the top plate  910  is convexly deformed upward before the notch  924   a  is fractured. Here, since the contact portion  924  is welded to the lower surface of the terminal portion  912 , the contact portion  924  may be separated from the bottom plate  920  while the notch  924   a  of the bottom plate  920  is fractured. Accordingly, a current path between the top plate  910  and the bottom plate  920  is blocked, and thus the gas inside the secondary battery can be quickly discharged to the outside. 
     The insulation plate  930  is disposed between the top plate  910  and the bottom plate  920 , and the lower surface thereof may be fixed to the upper surface of the base portion  922  of the bottom plate  920  by ultrasonic welding, etc. The insulation plate  930  may be shaped of a circular ring having a predetermined width when viewed from the top. For example, the insulation plate  930  may be formed of polyethylene (FE), polypropylene (PP), ethylene propylene diene monomer (EPDM) (M-class) rubber, or an equivalent thereof, but is not limited thereto. The support member  940  is inserted between the insulation plate  930  and the top plate  910 . 
     The support member  940  may be shaped of a substantially ring having a greater width than the insulation plate  930 , and may be made of the same or similar material as the insulation plate  930 . Alternatively, the support member  940  may be made of the same or similar material as the top plate  910 . Even if the support member  940  is electrified with the top plate  910 , since it can be insulated from the bottom plate  920  by means of the insulation plate  930 , the material of the support member  940  can be applied in various ways. 
     The support member  940  may be formed to have a greater thickness in the longitudinal direction of the central axis 0 than that of the insulation plate  930 . The thickness of the bent portion  914  of the top plate  910  may support the support member  940  so as to surround the edge of the support member  940 . In addition, part of the lower portion of the support member  940  may be supported while being in contact with the insulation plate  930 . Contact portions of the support member  940  and the insulation plate  930  may be combined by ultrasonic welding, etc. 
     However, it is preferable that the support member  940  is not fixed by welding, etc. to the top plate  910  having the notch  912   a  formed therein so as not to impede gas movement inside the secondary battery. In addition, in the top plate  910  and the support member  940 , portions adjacent to the notch  912   a  may be spaced apart from each other by a predetermined interval. To this end, the support member  940  may be formed to have a step difference between a surface in contact with the lower surface of the terminal portion  912  of the top plate  910  and a surface in contact with the lower surface of the bent portion  914 . The stepped region may be a partial surface of the support member  940 . Alternatively, the support member  940  may have a through-hole  942  located to correspond to the position of the notch  912   a  of the top plate  910 . 
       FIG.  6    is a longitudinal cross-sectional view illustrating a cap assembly of a cylindrical secondary battery according to a third embodiment of the present invention.  FIG.  7    is a plan view illustrating a partial configuration of the cap assembly according to  FIG.  6   . 
     As shown in  FIG.  6   , a cap assembly  900 ′ of a cylindrical secondary battery according to a third embodiment of the present invention has the same structure as the cap assembly  900  of  FIG.  5   , and may include a support member  940 ′ having a through-hole  942 ′ formed in a portion located to correspond to of a notch  912   a ′ of a top plate  910 ′. 
     As shown in  FIG.  7   , a plurality of through-holes  942 ′ may be formed in the support member  940 ′. The through-holes  942 ′ may be located to correspond to the positions of notches  912   a ′ of the top plate  910 ′, and a plurality of through-holes may be formed through the plate surface of the support member  940 ′ with a predetermined spacing therebetween. Therefore, when an abnormal pressure occurs inside the secondary battery, the internal gas can quickly move through the through-holes  942 ′ of the support member  940 ′. 
     In the case of the support member  940 ′ shown in  FIGS.  5  to  7   , the support member  940 ′ has a structure of supporting both the inward and outward directions from the lower side around the notches  912   a ′ of the top plate  910 ′. That is, the support member  940 ′ shown in  FIGS.  5  to  7    has a structure that sufficiently satisfies the minimum deformation prevention condition of the vent plate  530  shown in  FIGS.  2  to  4   . Accordingly, even when pressure is applied to form a beading part in a can during a secondary battery assembling process, it is possible to prevent the top plate  910 ′ having the notches  912   a ′ from being deformed. 
     While the foregoing embodiment has been provided for carrying out the present invention, it should be understood that the embodiment described herein should be considered in a descriptive sense only and not for purposes of limitation, and various changes in form and details may be made therein without departing from the spirit and scope of the disclosure as defined by the following claims.