Patent Publication Number: US-2021184308-A1

Title: Cylindrical lithium ion secondary battery

Description:
TECHNICAL FIELD 
     Various embodiments of the present invention relate to a cylindrical lithium ion secondary battery. 
     BACKGROUND ART 
     Lithium ion secondary batteries are being widely used in portable electronic devices and power sources of hybrid automobiles or electric vehicles because of various advantages, including a high operation voltage, a high energy density per unit weight, and so forth. 
     The lithium ion secondary battery can be largely classified as a cylinder type secondary battery, a prismatic type secondary battery, a pouch type secondary battery. Specifically, the cylindrical lithium ion secondary battery generally includes a cylindrical electrode assembly, a cylindrical can coupled to the electrode assembly, an electrolyte injected into the can to allow movement of lithium ions, and a cap assembly coupled to one side of the can to prevent leakage of the electrolyte and separation of the electrode assembly. 
     DESCRIPTION OF EMBODIMENTS 
     Technical Problems 
     Various embodiments of the present invention provide a cylindrical lithium ion secondary battery which, when an internal gas pressure is larger than a predetermined first reference pressure (operating pressure) and is smaller than a predetermined second reference pressure (breaking pressure) during overcharging, can maintain an internal sealing while a current path is blocked by a cap assembly. 
     Various embodiments of the present invention provide a cylindrical lithium ion secondary battery which can rapidly release the internal gas without any obstructions by allowing the cap assembly to be broken or ruptured (opened) when an internal gas pressure is larger than a predetermined second reference pressure (breaking pressure). 
     Various embodiments of the present invention provide a cylindrical lithium ion secondary battery which can arbitrarily determine the breaking pressure of the cap assembly according to the location of a welding region formed. 
     Solutions to Problems 
     According to various embodiments of the present invention, provided is a cylindrical lithium ion secondary battery comprising: a cylindrical can; an electrode assembly received in the cylindrical can; and a cap assembly for sealing the cylindrical can, wherein the cap assembly comprises a top plate having a flat surface on which a notch is formed, a middle plate coupled to the top plate and including a first through-hole formed through the center thereof, and a bottom plate electrically connected with the electrode assembly, attached to the middle plate with an insulating plate interposed therebetween, and connected to the top plate through the first through-hole of the middle plate. 
     The top plate may include a flat top surface and a flat bottom surface opposite to the top surface, and the notch is formed on the bottom surface. 
     The top plate may include a flat upper region positioned on the middle plate, a side region downwardly bent from the upper region and positioned at a side portion of the middle plate, and a lower region bent from the side region and positioned at a bottom portion of the middle plate. 
     The notch may be formed at an exterior side of a region corresponding to the first through-hole of the middle plate. 
     The middle plate may further include a plurality of second through-holes formed around the first through-hole. 
     When the internal gas pressure of the cylindrical can is larger than a predetermined first pressure and smaller than a predetermined second pressure, the top plate may be upwardly convexly deformed by the internal gas pressure, and the top plate may be electrically disconnected from the bottom plate. 
     When the internal gas pressure of the cylindrical can is larger than the predetermined second pressure, the notch may be broken, and the internal gas of the cylindrical can may then be released to the outside. 
     To determine the breaking pressure of the top plate, one or more welding regions may further be formed between the top plate and the middle plate. 
     As the welding regions formed are located far away from the edge of the top plate, the breaking pressure of the top plate may be relatively small. 
     Advantageous Effects of Invention 
     As described above, in the cylindrical lithium ion secondary battery according to various embodiments, when an internal gas pressure is larger than a predetermined first reference pressure (operating pressure) and is smaller than a predetermined second reference pressure (breaking pressure), an internal sealing can be maintained while a current path is blocked by a cap assembly. 
     In the cylindrical lithium ion secondary battery according to various embodiments, the internal gas can be released to the outside without any obstructions by allowing the cap assembly to be broken or ruptured (opened) when the internal gas pressure, after a current path is blocked by the cap assembly, is larger than a predetermined second reference pressure (breaking pressure). 
     In the cylindrical lithium ion secondary battery according to various embodiments, the breaking pressure of the cap assembly can be arbitrarily determined according to the location of a welding region formed. 
     In the cylindrical lithium ion secondary battery according to various embodiments, a relatively large battery capacity can be achieved by making an upper end height of the cap assembly equal to or smaller than that of a cylindrical can. 
     The cylindrical lithium ion secondary battery according to various embodiments includes a cap assembly including relatively soft pure aluminum or an aluminum alloy, so that the cap assembly is easily broken (opened) when the internal gas pressure reaches a predetermined reference pressure, thereby improving the safety of battery. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIGS. 1A and 1B  are a perspective view and a cross-sectional view of a secondary battery according to various embodiments of the present invention, and 
         FIG. 1C  is an exploded perspective view illustrating only a cap assembly. 
         FIGS. 2A and 2B  are cross-sectional views illustrating states in which the cap assembly operates and ruptures in the cylindrical secondary battery according to an embodiment of the present invention. 
         FIGS. 3A and 3B  are cross-sectional views illustrating cap assemblies of the secondary battery according to various embodiments of the present invention. 
         FIGS. 4A and 4B  are a cross-sectional view and a graph illustrating the relationship between breaking pressures/operating pressures and welding regions of the cap assembly in the cylindrical secondary battery according to various embodiments of the present invention. 
     
    
    
     MODE OF INVENTION 
     Hereinafter, embodiments of the present invention will be described in detail. 
     The embodiments of the present invention, however, may be modified in many different forms and should not be construed as being limited to the example (or exemplary) embodiments set forth herein. Rather, these example embodiments are provided so that this invention will be thorough and complete and will convey the aspects and features of the present invention to those 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 invention. 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. 
       FIGS. 1A and 1B  are a perspective view and a cross-sectional view of a cylindrical lithium ion secondary battery  100  according to various embodiments of the present invention, and  FIG. 1C  is an exploded perspective view illustrating only a cap assembly  140 . 
     As illustrated in  FIGS. 1A, 1B and 1C , the cylindrical lithium ion secondary battery  100  according to various embodiments may include a cylindrical can  110 , an electrode assembly  120  and a cap assembly  140 . In some cases, the cylindrical lithium ion secondary battery  100  may further include a center pin  130 . In addition, in the cylindrical lithium ion secondary battery  100  according to various embodiments, the cap assembly  140  performs a current blocking operation, and thus may be referred to as a current interrupt device in some cases. 
     The cylindrical can  110  includes a circular bottom portion  111  and a side wall  112  upwardly extending a predetermined length from the periphery of the bottom portion  111 . In the process of manufacturing the secondary battery, a top portion or top end of the cylindrical can  110  is left open. Therefore, in the process of assembling the secondary battery  100 , the electrode assembly  120  and the center pin  130  may be inserted into the cylindrical can  110  together with an electrolyte. The cylindrical can  110  may be made of, for example, steel, a steel alloy, aluminum, an aluminum alloy, or an equivalent thereof, but embodiments of the present invention are not limited thereto. 
     In addition, the cylindrical can  110  may include an inwardly recessed beading part  113  formed below the cap assembly  140  so as to prevent the electrode assembly  120  from being separated from the cap assembly  140  and an inwardly bent crimping part  114  formed on or above the beading part  113 . 
     The electrode assembly  120  is received in the cylindrical can  110 . The electrode assembly  120  includes a negative electrode plate  121  coated with a negative electrode active material (e.g., graphite or carbon), a positive electrode plate  122  coated with a positive electrode active material (e.g., a transition metal oxide, such as LiCoO 2 , LiNiO 2 , or LiMn 2 O 4 ), and a separator  123  interposed between the negative electrode plate  121  and the positive electrode plate  122  to prevent a short circuit between the negative electrode plate  121  and the positive electrode plate  122  while allowing only movement of lithium ions. The negative electrode plate  121 , the positive electrode plate  122 , and the separator  123  are wound in a substantially cylindrical shape or configuration. Here, the negative electrode plate  121  may be formed of a copper (Cu) or nickel (Ni) foil, the positive electrode plate  122  may be formed of an aluminum (Al) foil, and the separator  123  may be made of polyethylene (PE) or polypropylene (PP), but embodiments of the present invention are not limited thereto. 
     In addition, a negative electrode tab  124  may be welded to the negative electrode plate  121  to downwardly protrude and extend a predetermined length therefrom, and a positive electrode tab  125  may be welded to the positive electrode plate  122  to upwardly protrude and extend a predetermined length therefrom, or vice versa. In addition, the negative electrode tab  124  may be made of copper or nickel, and the positive electrode tab  125  may be made of aluminum, but embodiments of the present invention are not limited thereto. 
     In addition, the negative electrode tab  124  of the electrode assembly  120  may be welded to the bottom portion  111  of the cylindrical can  110 . Therefore, the cylindrical can  110  may function as a negative electrode. In other embodiments, the positive electrode tab  125  may be welded to the bottom portion  111  of the cylindrical can  110 . In these embodiments, the cylindrical can  110  may function as a positive electrode. 
     Additionally, a first insulating plate  126 , which is coupled to the cylindrical can  110  and has a first hole  126   a  formed at its center and a second hole  126   b  formed around the first hole  126   a,  may be interposed between the electrode assembly  120  and the bottom portion  111  of the cylindrical can  110 . The first insulating plate  126  may prevent the electrode assembly  120  from electrically contacting the bottom portion  111  of the cylindrical can  110 . Specifically, the first insulating plate  126  prevents the positive electrode plate  122  of the electrode assembly  120  from electrically contacting the bottom portion  111 . Here, when a relatively large amount of gas is generated due to an abnormality in the secondary battery, the first hole  126   a  allows the gas to rapidly move upwardly through the center pin  130 , and the second hole  126   b  allows the negative electrode tab  124  to pass therethrough to be welded to the bottom portion  111 . 
     In addition, a second insulating plate  127 , which is coupled to the cylindrical can  110  and has a first hole  127   a  formed at its center and a plurality of second holes  127   b  formed around the first hole  127   a,  may be interposed between the electrode assembly  120  and the bottom portion  111  of the cylindrical can  110 . The second insulating plate  127  may prevent the electrode assembly  120  from electrically contacting the bottom portion  111  of the cylindrical can  110 . Specifically, the second insulating plate  127  prevents the negative electrode plate  121  of the electrode assembly  120  from electrically contacting the cap assembly  140 . Here, when a relatively large amount of gas is generated due to an abnormality in the secondary battery, the first hole  127   a  allows the gas to rapidly move to the cap assembly  140 , and the second hole  127   b  allows the positive electrode tab  125  to pass therethrough to be welded to the cap assembly  140 . In addition, during injection of an electrolyte, the other second hole  127   b  allows the electrolyte to rapidly flow into the electrode assembly  120 . 
     In addition, since diameters of the first holes  126   a  and  127   a  of the first and second insulating plates  126  and  127  are smaller than a diameter of the center pin  130 , it is possible to prevent the center pin  130  from electrically contacting the bottom portion  111  of the cylindrical can  110  or the cap assembly  140  due to an external shock. 
     The center pin  130  is shaped of a hollow cylindrical pipe and is coupled to a substantially central portion of the electrode assembly  120 . The center pin  130  may be made of steel, stainless steel, aluminum, an aluminum alloy, or polybutylene terephthalate, but embodiments of the present invention are not limited to the above materials. The center pin  130  prevents the electrode assembly  120  from being deformed during charging or discharging of the secondary battery, and may serve as a gas movement path. Of course, in some embodiments, the center pin  130  may not be provided. 
     The cap assembly  140  may include a top plate  141 , a middle plate  142 , an insulating plate  143  and a bottom plate  144 . 
     The top plate  141  includes a substantially flat top surface  141   a  and a substantially flat bottom surface  141   b  opposite to the top surface  141   a.  Particularly, the top plate  141  may further at least one notch  141   c  formed on the bottom surface  141   b.  Here, the notch  141   c  may have a substantially inverted V (“∧”) shaped cross section. In addition, when viewed from below, the notch  141   c  may have, for example, a substantially circular, elliptical or “C” shape, but embodiments of the present invention are not limited to the above shapes. The notch  141   c  is broken or ruptured when the internal gas pressure of the secondary battery is larger than a predetermined reference pressure, thereby rapidly releasing the internal gas of the battery to the outside and ultimately securing the safety of battery. 
     In addition, the top plate  141  may include an upper region  141   d,  a side region  141   e,  and a lower region  141   f.  The upper region  141   d  may be positioned on the middle plate  142  and may be substantially flat. The upper region  141   d  may serve as a terminal of the secondary battery, and thus may be electrically connected to an external device (e.g., a load or a charger). The side region  141   e  may be downwardly bent from the upper region  141   d  to substantially encompass a side portion of the middle plate  142 . The lower region  141   f  is horizontally inwardly bent from the side region  141   e  to then be positioned at a bottom portion of the middle plate  142 . In such a manner, the top plate  141  may be combined with the middle plate  142  by the upper region  141   d,  the side region  141   e,  and the lower region  141   f.    
     Additionally, a height of the upper region  141   d  of the top plate  141  may be made to be equal to or smaller than that of the crimping part  114  of the cylindrical can  110 , which increases the internal volume of the cylindrical can  110 , thereby increasing the capacity of the secondary battery. Here, the height means a height ranging from the bottom portion  111  of the cylindrical can  110 . 
     The top plate  141  may be made of, for example, aluminum, aluminum, an aluminum alloy or equivalents thereof, but embodiments of the present invention are not limited to the above materials. Accordingly, a bus bar, an external lead or an external device, made of aluminum, may be easily connected (or welded) to the top plate  141 . 
     Here, the top plate  141  may be made of one selected from the group consisting of 1XXX series alloys, that is, pure aluminum of 99.0% or greater purity, 2XXX series alloys, that is, Al—Cu alloys, 3XXX series alloys, that is, Al—Mn alloys, 4XXX series alloys, that is, Al—Si alloys, 5XXX series alloys, that is, Al—Mg alloys, 6XXX series alloys, that is, Al—Mg—Si alloys, and 7XXX series alloys, that is, Al—Zn—(Mg, Cu) alloys. 
     Specifically, the top plate  141  is preferably made of soft aluminum among the above-mentioned series alloys. For example, the top plate  141  may be made of, but not limited to, a 5XXX series (e.g., 5052, 5056, 5083, or 5454) Al—Mg alloy having a high strength, excellent corrosion resistance and good weldability. Additionally, a 1XXX, 3XXX or 4XXX series alloy, which is a non-heat treatable alloy, may be used as a material of the top plate  141 . 
     In some cases, the top plate  141  may further include a bent region  141   g  formed on the upper region  141   d.  When viewed from below, the bent region  141   g  may be shaped of a substantially circular ring. As an example, the upper region  141   d  located inside the bent region  141   g  may be positioned higher than the upper region  141   d  located outside the bent region  141   g.  In addition, the notch  141   c  may be formed on the upper region  141   d  located inside the bent region  141   g.    
     The middle plate  142  may be positioned under the top plate  141  and may be substantially flat. In addition, the middle plate  142  may include a first through-hole  142   a  formed at a roughly central portion. Moreover, the middle plate  142  may include a plurality of second through-holes  142   b  formed around the first through-hole  142   a.    
     Here, a bottom plate  144 , which will later be described, may pass through the first through-hole  142   a  to then be electrically connected to the top plate  141 , and may allow the internal gas pressure to be directly applied to the top plate  141 . In addition, the second through-holes  142   b  may also allow the internal gas pressure to be directly applied to the top plate  141 . 
     The notch  141   c  formed on the bottom surface  141   b  of the top plate  141  may be located to correspond to, for example, a region between the first through-hole  142   a  and each of the second through-holes  142   b  of the middle plate  142 . 
     Additionally, the middle plate  142  may also include a bent region  142   c  formed on a region corresponding to the bent region  141   g  of the top plate  141 . In addition, the second through-holes  142   b  may be formed in the bent region  142   c.  Therefore, the middle plate  142  may be generally configured such that it makes a close contact with the bottom surface  141   b  of the top plate  141 . 
     The middle plate  142  may be made of, for example, aluminum, aluminum, an aluminum alloy or equivalents thereof, but embodiments of the present invention are not limited thereto. 
     The insulating plate  143  may be positioned under (attached to a bottom portion of) the middle plate  142  and may include a through-hole  143   a  located to correspond to the first through-hole  142   a.  When viewed from below, the insulating plate  143  may be shaped of a substantially circular ring having a predetermined width. As an example, the insulating plate  143  may be located to correspond to a region between the first through-hole  142   a  and each of the second through-holes  142   b  of the middle plate  142 . In addition, the insulating plate  143  serves to insulate the middle plate  142  and the bottom plate  144  from each other. For example, the insulating plate  143  may be positioned between the middle plate  142  and the bottom plate  144  and may be subjected to ultrasonic welding, but embodiments of the present invention are not limited thereto. 
     The insulating plate  143  may be made of, for example, polyethylene (PE), polypropylene (PP), ethylene propylene diene monomer (M-class) rubber (EPDM rubber), or equivalents thereof, but embodiments of the present invention are not limited to the above materials. These insulating materials do not react with an electrolyte, and thus the insulating plate  143  may not be deformed even after the long-period use of the secondary battery  100 . 
     The bottom plate  144  is electrically connected to the top plate  141  through the through-hole  143   a  of the insulating plate  143  and the first through-hole  142   a  of the middle plate  142  to then be attached to the insulating plate  143 . That is to say, the bottom plate  144  may include a first area  144   a  connected (welded) to the upper region  141   d  of the top plate  141 , a second area  144   b  bent from the first area  144   a  and passing through the through-hole  142   a  of the middle plate  142  and the through-hole  143   a  of the insulating plate  143 , and a third area  144   c  substantially outwardly bent from the second area  144   b  and attached to the insulating plate  143 . In  FIG. 1C , undefined reference numeral  144   e  refers to a welding region in which the first area  144   a  of the bottom plate  144  is welded to the bottom surface  141   b  of the upper region  141   d  of the top plate  141 . 
     Here, the positive electrode tab  125  may be electrically connected to the third area  144   c  of the bottom plate  144 . In addition, the third area  144   c  is spaced apart from the middle plate  143  and is also spaced apart from the third region  141   f  of the top plate  141 . In addition, the first area  144   a  of the bottom plate  144  may further include one or more concavely recessed grooves  144   d.  When the internal gas pressure of the battery is larger than a predetermined pressure, the top plate  141  is upwardly convexly deformed. In this case, the grooves  144   d  may serve to make the first area  144   a  of the bottom plate  144  easily separated from the second area  144   b.  Consequently, a current path between the top plate  141  and the bottom plate  144  may be blocked. 
     The bottom plate  144  may be made of, for example, aluminum, aluminum, an aluminum alloy or equivalents thereof, and thus the positive electrode tab  125  made of aluminum may be easily welded thereto. 
     The cap assembly  140  may further include an insulating gasket  145  insulating the top plate  141  and the sidewall  111  of the cylindrical can  110  from each other. Here, the insulating gasket  145  is configured to be substantially compressed between the beading part  113  and the crimping part  114  formed on the sidewall  111  of the cylindrical can  110 . In addition, the insulating gasket  145  may substantially encompass the side region  141   e  of the top plate  141 , and the top region  141   d  and the lower region  141   g  located therearound, thereby sealing the interior of the battery. 
     Additionally, an electrolyte (not shown) is injected into the cylindrical can  110 , and lithium ions generated by an electrochemical reaction in the negative electrode plate  121  and the positive electrode plate  122  in the secondary battery during charging and discharging are allowed to move. The electrolyte may be a non-aqueous, organic electrolyte including a mixture of a lithium salt and a high-purity organic solvent. In addition, the electrolyte may be a polymer using a polymer electrolyte or a solid electrolyte. However, embodiments of the present invention are not limited to the above electrolytes. 
     With such features of the cap assembly  140 , the cylindrical lithium ion secondary battery  100  according to the embodiment may have a relatively large capacity by making the upper end height of the cap assembly  140  equal to or smaller than that of the cylindrical can  110 . In addition, the cylindrical lithium ion secondary battery  100  includes the cap assembly  140  including relatively soft pure aluminum or an aluminum alloy, so that the cap assembly  140  is easily broken or ruptured (opened) when the internal gas pressure reaches a predetermined reference pressure, thereby improving the safety of battery. 
       FIGS. 2A and 2B  are cross-sectional views illustrating states in which the cap assembly  140  operates and ruptures in the cylindrical lithium ion secondary battery  100  according to an embodiment of the present invention. 
     As illustrated in  FIG. 2A , in the cylindrical lithium ion secondary battery  100  according to various embodiments, when the internal gas pressure of the cylindrical can  110  is larger than a predetermined first reference pressure (operating pressure) and is smaller than a predetermined second reference pressure (breaking pressure), the top plate  141  is upwardly convexly deformed (inverted), and the top plate  141  may be electrically disconnected from the bottom plate  144 . That is to say, the first area  144   a  of the bottom plate  144  is broken to then be separated from the second area  144   b.  In other words, the grooves  144   d  of the first area  144   a  are ruptured, and some regions of the first area  144   a  upwardly move in a state in which they are still connected to the top plate  141 . Consequently, a current path between the top plate  141  and the bottom plate  144  may be blocked. 
     However, when the internal gas pressure of the secondary battery is smaller than the predetermined second reference pressure (breaking pressure), a sealing of the secondary battery may be maintained, thereby preventing the internal gas from being released to the outside. 
     When the battery is overcharged, when an internal short-circuit occurs to the cylindrical secondary battery due to penetration and/or collapse, or when an external short-circuit occurs to the battery, internal gas may be generated due to decomposition of an electrolyte and/or decomposition of an active material, resulting in an increase in the internal gas pressure of the secondary battery. Here, the secondary battery is designed such that the second reference pressure (breaking pressure) is larger than the first reference pressure (operating pressure). Such an increase in the internal gas pressure of the secondary battery may suggest that the secondary battery is at an abnormal state, and thus the current path is first blocked by the above-mentioned mechanical mechanism (charge current, discharge current, short-circuit current, or overcurrent), thereby improving the safety of the secondary battery. 
     As illustrated in  FIG. 2B , in the cylindrical lithium ion secondary battery  100  according to various embodiments, when the internal gas pressure of the cylindrical can  110  is larger than the predetermined second pressure (breaking pressure), the top plate  141  is ruptured to thus rapidly release the internal gas without any obstructions. That is to say, as the notch  141   c  formed on the bottom surface  141   b  of the top plate  141  is ruptured, the gas existing within the secondary battery  100  is rapidly released to the outside, thereby preventing explosion of the secondary battery  100  and ultimately increasing the safety of the secondary battery  100 . From the viewpoint of safety, releasing the internal gas to the outside in advance is more advantageous than letting the secondary battery  100  explode under a high pressure as described above. 
     In addition, the breaking pressure (or the second pressure) of the top plate  141  may be adjusted by the depth of the notch  141   c  formed. For example, the breaking pressure may be increased by forming the notch  141   c  so as to have a relatively small depth, and the breaking pressure may be reduced by forming the notch  141   c  so as to have a relatively large depth. 
     As described above, when the internal gas pressure is larger than the predetermined first reference pressure (operating pressure) and is smaller than the predetermined second reference pressure (breaking pressure), the cylindrical lithium ion secondary battery  100  according to the embodiments may primarily block the current path by the cap assembly  140 . Here, the internal sealing of the battery is still maintained. In addition, when the internal gas pressure, after the current path is blocked by the cap assembly  140 , is larger than the predetermined second reference pressure (breaking pressure), the cap assembly  140  is broken or ruptured (opened), and thus the cylindrical lithium ion secondary battery  100  according to the embodiments may secondly release the internal gas to the outside without any obstructions. 
     That is to say, the cylindrical lithium ion secondary battery  100  according to the embodiments may perform a safety-related operation in two steps by primarily blocking the current path when the internal gas pressure is larger than the predetermined first reference pressure, and secondly releasing the internal gas to the outside when the internal gas pressure is larger than the predetermined second reference pressure. 
     Meanwhile, the breaking pressure of the top plate  141  may be determined by a hinge point formed during operation (inversion) of the top plate  141  as well as by the depth of the notch  141   c.  That is to say, when the top plate  141  is inverted, there may be a hinge point at which the inversion is initiated. For example, the hinge point may be a boundary region between the insulating gasket  145  and the top plate  141 . That is to say, the hinge point may be a region of the upper region  141   d  of the top plate  141 , which corresponds to an end of the insulating gasket  145 . Therefore, the breaking pressure of the top plate  141  may vary according to the location of the hinge point, which will be described below. 
       FIGS. 3A and 3B  are cross-sectional views illustrating cap assemblies  140 A and  140 B of the cylindrical lithium ion secondary battery  100  according to various embodiments of the present invention. 
     As illustrated in  FIGS. 3A and 3B , in the cylindrical lithium ion secondary battery  100  according to various embodiments, in order to determine (adjust) the breaking pressure of the top plate  141 , the cap assemblies  140 A and  140 B may further include one or more welding regions  146 A and  146 B formed between the top plate  141  and the middle plate  142 , respectively. The welding regions  146 A and  146 B may be formed by, for example, laser welding, resistance welding or ultrasonic welding, but embodiments of the present invention are not limited thereto. When viewed from above, for example, the welding regions  146 A and  146 B may be shaped such that substantially continuously circular rings or points are arranged, but embodiments of the present invention are not limited thereto. In addition, the welding regions  146 A and  146 B may be formed at regions near the edges of the top plate  141  and the middle plate  142 , for example, but embodiments of the present invention are not limited thereto. That is to say, the welding regions  146 A and  146 B may be formed at regions of the top plate  141  and the middle plate  142 , which correspond to a location between each of the second through-holes  142   b  of the middle plate  142  and the periphery of the middle plate  142 , but embodiments of the present invention are not limited thereto. 
     Here, the breaking pressure of the top plate  141  may be gradually decreased as the welding regions  146 A and  146 B formed are getting far from the edge of the top plate  141 . In other words, the breaking pressure of the top plate  141  may be gradually increased as the welding regions  146 A and  146 B formed are getting close to the edge of the top plate  141 . Here, the breaking pressure means a pressure at which the notch  141   c  formed in the top plate  141  is broken or ruptured. 
     As an example, as illustrated in  FIG. 3A , when the welding region  146 A is located relatively far from the edge of the top plate  141  (that is, when the welding region  146 A is located relatively close to the center of the top plate  141 ), the breaking pressure of the top plate  141  may be relatively small. That is to say, the notch  141   c  of the top plate  141  may be broken or ruptured by a relatively small internal pressure of the battery. 
     As another example, as illustrated in  FIG. 3B , when the welding region  146 B is located relatively close to the edge of the top plate  141  (that is, when the welding region  146 B is located relatively far from the center of the top plate  141 ), the breaking pressure of the top plate  141  may be relatively large. That is to say, the notch  141   c  of the top plate  141  may be broken or ruptured by a relatively large internal pressure of the battery. 
     Such a change in the breaking pressure is attributable to a volumetric change for breaking. 
     In other words, the welding regions  146 A and  146 B may be considered to be hinge points, and the upper region  141   d  of the top plate  141  may be considered to be an inverted region. 
     As the welding regions  146 A and  146 B are getting close to the periphery of the top plate  141  (that is, getting far from the center), the volume for inverting the top plate  141  may be increased, and thus a relatively large internal pressure may be required for operating or breaking the top plate  141 . Accordingly, the breaking pressure/operating pressure may be increased. 
     However, as the welding regions  146 A and  146 B are getting far from the periphery of the top plate  141  (that is, getting close to the center), the volume for inverting the top plate  141  may be decreased, and thus a relatively small internal pressure may be required for operating or breaking the top plate  141 . Accordingly, the breaking pressure/operating pressure may be reduced. 
     That is, according to the present invention, the welding regions  146 A and  146 B, instead of contact boundary regions of the insulating gasket  145  and the top plate  141 , are arbitrarily determined as the hinge points of the top plate  141 , thereby allowing the secondary battery  100  to arbitrarily adjust the operating pressure and/or the breaking pressure of the top plate  141 . 
     In addition, the operating pressure and/or the breaking pressure of the top plate  141  in any type of secondary battery may be uniformly controlled by adjusting the operating pressure and/or the breaking pressure of the top plate  141  using the welding regions  146 A and  146 B. 
     Practically, the breaking pressure and the operating pressure may be increased or decreased together. 
       FIGS. 4A and 4B  are a cross-sectional view and a graph illustrating the relationship between rupture pressures/operating pressures and welding regions of the cap assembly  140  in the cylindrical lithium ion secondary battery  100  according to various embodiments of the present invention. In  FIG. 4B , the X-axis indicates the distance of a welding region from the center, and the Y-axis indicates the rupture pressure/operating pressure. 
     As illustrated in  FIGS. 4A and 4B , the breaking pressure of a top plate is gradually increased away from the center of welding regions of the top plate toward the periphery of the top plate. In other words, as a welding region is formed to be closer to the periphery of the top plate, the pressure required for operating or breaking the top plate is increased. In still other words, as a welding region is formed to be closer to the center of the top plate, the pressure required for operating or breaking the top plate is decreased. 
     In addition, as illustrated in  FIGS. 4A and 4B , the breaking pressure of the top plate is gradually decreased away from the center of welding regions of the top plate toward the center of the top plate. In other words, as the welding region is formed to be closer to the center of the top plate, the pressure required for operating or breaking the top plate is decreased. In still other words, as the welding region is formed to be closer to the periphery of the top plate, the pressure required for operating or breaking the top plate is increased. 
     Although the foregoing embodiments have been described to practice the cylindrical lithium ion secondary battery of the present invention, these embodiments are set forth for illustrative purposes and do not serve to limit the invention. Those skilled in the art will readily appreciate that many modifications and variations can be made, without departing from the spirit and scope of the invention as defined in the appended claims, and such modifications and variations are encompassed within the scope and spirit of the present invention.