Abstract:
Disclosed is a cylindrical secondary battery that can improve pressing force of a gasket pressing a cap up and a safety vent. The cylindrical secondary battery prevents leakage of electrolyte by improving pressing force of the gasket by including a cap up whose terminal portion is formed thicker than the peripheral portion. By using the cap up of the above structure, a curled portion is formed at the edge of the cap up or safety vent so as to surround the end of the safety vent or cap up. Thus, internal resistance can be reduced when the vibration or external impact is applied to the battery, and assembling error can be prevented.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on, and claims priority to, Korean Patent Application No. 2008-24424, filed on Mar. 17, 2008, in the Korean Intellectual Property Office (KIPO), the disclosure of which is incorporated herein, by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Aspects of the present invention relate to a cylindrical secondary battery, and more particularly, to a cylindrical secondary battery including a gasket that improves the coupling of a cap up and a safety vent. 
     2. Description of the Related Art 
     Generally, a secondary battery can be classified as cylindrical, rectangular, or pouch-type, according to a shape of a case of the battery. 
     In a cylindrical secondary battery, an electrode assembly is inserted into a cylindrical can, and then a cap assembly is assembled to an upper opening of the cylindrical can. The cap assembly is electrically coupled to the electrode assembly, to transmit current to an external device. 
     The cap assembly improves the stability of the battery, by reducing an internal pressure of the can, or by interrupting current flow. The internal pressure can be increased by overcharging, or an abnormal operation of, the battery, which can cause an explosion. 
     The cap assembly includes a cap up (cathode terminal), and a safety vent that is provided under the cap up. The safety vent is deformed to interrupt current, or to discharge gas to relieve the increased internal pressure. A cap down and a sub-plate are sequentially provided under the safety vent, and are connected to a cathode of the electrode assembly. 
     Vibrations or external impacts applied to a cylindrical battery can result in an unwanted increase in the resistance of contact surfaces between components of the cap assembly. Particularly, battery performance is degraded, when the contact resistance, of an interfacial surface between the safety vent and cap up, is increased. 
     To solve the above problem, the contact surfaces of the safety vent and cap up can be welded, or the safety vent can be bent, to surround an outer surface of the cap up. However, the welding complicates an assembling process of a battery, and defects can occur, when the welding is not properly performed. 
     In addition, it is difficult to bend the safety vent, so as to surround the outer circumference surface of the cap up, because thickness of the cap up is increased in high-capacity secondary batteries. In addition, in high capacity secondary batteries, the pressing force of a gasket is reduced, because of the larger thickness of the stacked portions of the cap up and the safety vent. 
     SUMMARY OF THE INVENTION 
     An aspect of the present invention is to provide a cylindrical secondary battery that can prevent the leakage of an electrolyte, by improving the pressing force applied by a gasket to the outer surface of a cap assembly, by providing a cap up and a safety vent that have reduced thicknesses. 
     Another aspect of the present invention is to provide a cylindrical secondary battery that can prevent internal resistances of the cap up and safety vent from being increased, by vibrations or external impacts. 
     Another aspect of the present invention is to provide a cylindrical secondary battery that can prevent assembly errors, by precisely assembling the cap up and safety vent. 
     A further still another aspect of the present invention is to provide a cylindrical secondary battery that can improve assembling workability, by facilitating a curling process to combine the cap up and the safety vent. 
     According to one aspect of the present invention, there is provided a cylindrical secondary battery that comprises: a cylindrical can; an electrode assembly received in the cylindrical can; and a cap assembly including a safety vent, electrically coupled to the electrode assembly, and a cap up provided over the safety vent. The safety vent interrupts current, and discharges internal gas from the battery when internal pressure of the battery is increased. A thickness of the terminal portion of the cap up is different from that of a peripheral portion thereof. 
     According to aspects of the present invention, the peripheral portion of the cap up may be thinner than the terminal portion thereof. 
     According to aspects of the present invention, the terminal portion of the cap up may be outwardly projected, and the peripheral portion may be flat and extend outward from the terminal portion. 
     According to aspects of the present invention, the cap up may include a curled portion that extends from an edge of the peripheral portion, to surround an end of the safety vent. 
     According to aspects of the present invention, a thickness (T) of the curled portion is not more than the sum of a thickness (t 1 ) of the terminal portion of the cap up, and a thickness (t 3 ) of the safety vent. 
     According to aspects of the present invention, the thickness (t 1 ) of the terminal portion of the cap up may be more than twice of the thickness (t 2 ) of the peripheral portion. 
     According to aspects of the present invention, the thickness (t 3 ) of the safety vent may be the same as the thickness (t 2 ) of the peripheral portion. 
     According to aspects of the present invention, the thickness (t 1 ) of the terminal portion of the cap up may be more than 0.6 mm, and the thickness (t 3 ) of the safety vent may be 0.3 mm. 
     According to aspects of the present invention, the radial thickness (r 2 ) of the peripheral portion of the cap up may be 10 to 40% of the radial thickness (r 1 ) of the cap up. 
     According to aspects of the present invention, the radial thickness (r 3 ) of the curled portion of the cap up may be less than the radial thickness (r 2 ) of the peripheral portion. 
     According to aspects of the present invention, the safety vent may include a curled portion that is extended from an edge thereof, to surround an end of the cap up. 
     According to aspects of the present invention, a thickness (T′) is not more than the sum of a thickness (t 1 ′) of the terminal portion of the cap up and a thickness (t 3 ′) of the safety vent, where the thickness (T′) is a thickness of a portion where the end of the cap up is surrounded by the curled portion of the safety vent. 
     According to aspects of the present invention, the thickness (t 1 ′) of the terminal portion of the cap up may be more than twice of the thickness (t 2 ′) of the peripheral portion thereof. 
     According to aspects of the present invention, the thickness (t 3 ′) of the safety vent may be the same as the thickness (t 2 ′) of the peripheral portion of the cap up. 
     According to aspects of the present invention, the thickness (t 1 ′) of the terminal portion of the cap up may be more than 0.6 mm, and the thickness (t 3 ′) of the safety vent may be 0.3 mm. 
     According to aspects of the present invention, the radial thickness (r 2 ) of the peripheral portion of the cap up may be 10 to 40% of the radial thickness (r 1 ) of the cap up. 
     According to aspects of the present invention, the radial thickness (r 3 ′) of the curled portion of the safety vent may be less than the radial thickness (r 2 ) of the peripheral portion. 
     Additional aspects and/or advantages of the invention will be set forth in part in the description which follows, and in part, will be obvious from the description, or may be learned by practice of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the exemplary embodiments, taken in conjunction with the accompanying drawings, of which: 
         FIG. 1  is a vertical sectional view illustrating a cylindrical secondary battery, according to one exemplary embodiment of the present invention; 
         FIG. 2  is a magnified sectional view illustrating a cap assembly of  FIG. 1 ; 
         FIG. 3  is a magnified view illustrating part ‘III’ of  FIG. 2 ; 
         FIG. 4  is a plan view illustrating a cap up of the cylindrical secondary battery; 
         FIG. 5  is a sectional view illustrating a cap assembly of a cylindrical secondary battery, according to another exemplary embodiment of the present invention; and 
         FIG. 6  is a magnified view illustrating part ‘VI’ of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Reference will now be made in detail to the exemplary embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The exemplary embodiments are described below, in order to explain the aspects of the present invention, by referring to the figures. 
       FIG. 1  is a vertical sectional view illustrating a cylindrical secondary battery  100 , according to one exemplary embodiment of the present invention, and  FIG. 2  is a magnified sectional view illustrating a cap assembly of  FIG. 1 .  FIG. 3  is a magnified view illustrating part ‘III’ of  FIG. 2 , and  FIG. 4  is a plan view illustrating a cap up of the cylindrical secondary battery  100 . 
     Referring to  FIGS. 1 to 4 , the secondary battery  100  includes an electrode assembly  110 , a cylindrical can  120  to house the electrode assembly  110 , a cap assembly  130  combined to an upper opening of the cylindrical can  120 , and a gasket  140  provided on an inner circumference of the upper opening of the cylindrical can  120 . 
     Middle and peripheral portions of a cap up  131  of the cap assembly  130  have different thicknesses. In other words, the peripheral portion of the cap up  131  is thinner than the terminal portion. 
     The electrode assembly  110  is formed by winding a cathode plate  111 , an anode plate  112 , and a separator  113  in a jelly-roll shape. The cathode plate  111  includes a cathode active material layer coated on a cathode collector, and the anode plate  112  includes an anode active material layer coated on an anode collector. The separator  113  is interposed between the cathode and anode plates  111  and  112 , to insulate them electrically from each other. At an upper part of the electrode assembly  110 , a cathode tab  114  is connected to the cap assembly  130 , and at a lower part of the electrode assembly  110 , an anode tab  115  is connected to a bottom surface of the cylindrical can  120 . 
     The cathode collector of the cathode plate  111  is made of a conductive metal, to collect electrons from the cathode active material layer, and to transfer the electrons to an external circuit. The cathode active material layer is formed by mixing a cathode active material, a conductive material, and a binder, and coating the mixture on the cathode collector. Cathode uncoated parts are formed on both ends of the cathode plate, where the cathode active material layer is not coated. The cathode tab  114  is welded to one of the cathode uncoated parts. 
     The anode collector of the anode plate  112  is made of a conductive metal, to collect electrons from the anode active material layer, and to transfer the electrons to the external circuit. The anode active material layer is formed by mixing an anode active material, a conductive material, and a binder, and coating the mixture on the anode collector. Anode uncoated parts are formed on both ends of the anode plate, where the anode active material layer is not coated. The anode tab  115  is welded to one of the anode uncoated parts. 
     The separator  113  is interposed between the cathode and anode plates  111  and  112 , and may be extended to surround an outer circumference surface of the electrode assembly  110 . The separator  113  prevents a short between the cathode and anode plates  111  and  112 . The separator  113  is made of porous polymer that lithium ions can pass through. 
     The cylindrical can  120  includes a side plate  121  to form a space to receive the electrode assembly  110 , and a lower plate  122  to seal a lower part of the side plate  121 . The cylindrical can  120  is usually made of a lightweight conductive metal, such as aluminum, or an aluminum alloy, and is generally formed by a deep drawing method. The upper opening of the cylindrical can  120  is sealed, after the electrode assembly  110  is inserted. In addition, a beading part  123  is formed at an upper part of the cylindrical can  120 , to secure the electrode assembly  110 . A crimping part  124  is formed at the uppermost part of the cylindrical can  120 , to fix the cap assembly  130 . 
     The cap assembly  130  includes a cap up  131 , a safety vent  132 , a cap down  134 , an insulator  133 , and a sub-plate  135 . The cap up  131  is electrically coupled to the electrode assembly  110 , and transmits current generated in the electrode assembly  110  to an external device. The safety vent  132  contacts a lower surface of the cap up  131 , interrupts current, and discharges internal gas when abnormal internal pressure is generated in the cylindrical can  120 . The cap down  134  is provided below the safety vent  132 , to seal the cylindrical can  120 . The insulator  133  is interposed between the safety vent  132  and cap down  134 . The sub-plate  135  is fixed to a lower surface of the cap down  134 , and the cathode tab  114  is attached to the sub-plate  135 . 
     The cap up  131  can be a circular plate. The cap up  131  includes: a terminal portion  131 A that is electrically coupled to the external device, and which projects from the center of the cap up  131 ; a peripheral portion  131 B that extends radially from the terminal portion  131 A; and a curled portion  131 C that is bent from the peripheral portion  131 B, and extends around an edge of the safety vent  132 . Several gas discharge holes  137  are formed on an outer edge of the terminal portion  131 A, to discharge gas generated inside the cylindrical can  120 . 
     The thickness (t 2 ), of a peripheral portion  131 B of the cap up  131 , is less than the thickness (t 1 ) of the terminal portion  131 A. The thickness (t 1 ) of the terminal portion  131 A may be at least twice the thickness (t 2 ) of the peripheral portion  131 B (t 1 ≧2t 2 ). 
     The safety vent  132  can be a circular plate corresponding to the cap up  131 . The safety vent  132  includes a terminal portion  132 A that extends toward the electrode assembly  120 . The thickness (t 3 ) of the safety vent  132  is generally the same as the thickness (t 2 ) of the peripheral portion  131 B of the cap up  131  (t 2 =t 3 ). The cap up  131  and safety vent  132  are combined prior to the combination of other components of the cap assembly  130 . 
     The thickness (T) of the curled portion  131 C is equal to twice the thickness (t 2 ) of the peripheral portion  131 B, and to the thickness (t 3 ) of the safety vent  132  (T=2t 2 +t 3 ). In other words, the thickness (T) is not more than the sum of the thickness (t 1 ) of the terminal portion  131 A of the cap up  131 , and the thickness (t 3 ) of the safety vent  132  (T≦t 1 +t 3 ). 
     Once combined, the cap up  131  and safety vent  132  are seated inside of a gasket  140 , at the upper opening of the cylindrical can  120 , and assembled by pressing the gasket  140  against the curled portion  131 C of the cap up  131 . The insulator  133  is interposed between the safety vent  132  and the cap down  134 . The insulator  133  insulates the safety vent  132  from the cap down  134 . 
     The cap down  134  can be a circular plate. A through-hole  134 A is formed in the middle of the cap down  134 . The terminal portion  132 A of the safety vent  132  passes through the through-hole  134 A. A gas discharge hole  134 B is formed at one side of the cap down  134 . When the internal pressure is excessively increased, gas is discharged through the gas discharge hole  134 B, past the terminal portion  132 A. 
     The sub-plate  135  is welded to the terminal portion  132 A of the safety vent  132 , to electrically couple the cathode tab  114  to the safety vent  132 . A PTC (Positive Temperature Coefficient) device may be further provided between the cap up  131  and safety vent  132 , as a secondary protective device. 
     Generally, in a high capacity cylindrical secondary battery, the cap up has a thickness of more than 0.6 mm, so as to endure a vibration safety test. Accordingly, in the cylindrical secondary battery  100 , the thickness (t 1 ) of the terminal part  131 A of the cap up  131  is 0.6 mm, and the thickness (t 2 ) of the peripheral portion  131 B of the cap up  131  is 0.3 mm, for example. 
     The peripheral portion  131 B of the cap up  131  is bent to form the curled portion  131 C. The thickness (t 2 ) of the peripheral portion  131 B is 0.3 mm. The thickness (t 3 ) of the safety vent  132  is 0.3 mm. Thus, the thickness (T) of the curled portion is 0.9 mm (twice the thickness of the peripheral portion  131 B, plus the thickness of the safety vent  132 ). In comparison, in the conventional secondary battery, the combined thickness of the cap up (0.6 mm) and the of the safety vent (0.3 mm) is also 0.9 mm. Thus, pressing force exerted by the gasket  140  onto the outer circumference surfaces of the cap up  131  and safety vent  132 , is not decreased. 
     When the thickness (t 2 ) of the peripheral portion  131 B of the cap up  131  is not reduced, as compared to the conventional art, and the curled portion  131 C is curled around the safety vent  132 , the thickness (T) of the curled portion  131 C becomes 1.5 mm, because twice the thickness of the curled portion  131 C is 0.6 mm, and the thickness of the safety vent is 0.3 mm. Accordingly, the pressing force exerted by the gasket to the curled portion  131 C is decreased. 
     The radial thickness (r 2 ) of the peripheral portion  131 B of the cap up  131  is generally within 10% to 40% of the radial thickness (r 1 ) of the entire cap up  131 . It is difficult to secure the thickness (T) of the curled portion  131 C, when the radial thickness (r 2 ) of the peripheral portion  131 B is less than 10% of the radial thickness (r 1 ) of the cap up  131 . When the radial thickness (r 2 ) of the peripheral portion  131 B of the cap up  131  is more than 40% of the radial thickness (r 1 ) of the cap up  131 , the cap up  131  may not be strong enough to pass the vibration safety test. 
     The radial thickness (r 3 ) of the curled portion  131 C is less than the radial thickness (r 2 ) of the peripheral portion  131 B. The curled portion  131 C should have a length sufficient to surround the end of the safety vent  132 , and to prevent leakage of an electrolyte. 
     The secondary battery  100  is assembled by inserting the electrode assembly  110  into the cylindrical can  120 , injecting an electrolyte, and sealing the upper opening of the cylindrical can  120  with the cap assembly  130 . The cap up  131  and the safety vent  132  are assembled prior to disposing the cap assembly in the can  120 . During the assembly, the curled portion  131 C is curled around the edge of the safety vent  132 , through a curling process. 
     The curling process involves bending a plate material. By the curling process, the end of the safety vent  132  is surrounded by the peripheral edge (curled portion  131 C) of the cap up  131 , such that the curled portion  131 C is pressed against the outer edge of the safety vent  132 . 
     The assembled cap up  131  and safety vent  132  are positioned on the cap down  134 , with the insulator  133  interposed therebetween. The terminal portion  132 A of the safety vent  132  is positioned to extend through the through-hole  134 A. A sub-plate  135  is welded to the terminal portion  132 A of the safety vent  132 . The cap up  131  and safety vent  132  are always precisely positioned, because the cap up  131  and safety vent  132  are previously combined, and then positioned on the cap down  134 , as described above. Thus, assembling errors can be prevented. 
     The beading part  123  is formed at the upper part of the cylindrical can  120 , to prevent movement of the electrode assembly  110 , after the electrode assembly  110  is inserted into the cylindrical can  120  and the electrolyte is injected. After the gasket  140  is inserted into the cylindrical can  120 , and the cap assembly  130  is seated thereon, the upper end of the cylindrical can  120  is crimped. 
     During the crimping, the thickness (T) the curled portion  131 C is 0.9 mm. Thus, the gasket  140  is pressed against the outer circumference surface of the cap up  131 , with a predetermined pressing force. 
       FIG. 5  is a sectional view illustrating a cap assembly  230  of a cylindrical secondary battery  200 , according to another exemplary embodiment of the present invention, and  FIG. 6  is a magnified view illustrating portion ‘VI’ of  FIG. 5 . Referring to  FIGS. 5 and 6 , the secondary battery  200  includes an electrode assembly  110 , a cylindrical can  120  to house the electrode assembly  110 , and the cap assembly  230 , which is combined to an upper opening of the cylindrical can  120 . The cap assembly  230  includes a cap up  231  and a safety vent  232 . The cap up  231  is electrically coupled to the electrode assembly  110 , and transmits current generated in the electrode assembly  110 , to an external device. The safety vent  232  interrupts current, and discharges an internal gas, when abnormal internal pressure is generated in the cylindrical can  120 . 
     The thickness (t 2 ′) of a peripheral portion  231 B of the cap up  231  is less than the thickness (t 1 ′) of a terminal portion  231 A of the cap up  231 . In addition, the edge of the safety vent  232  is bent, to form a curled portion  232 A that surrounds an edge of the peripheral portion  231 B of the cap up  231 . 
     Other elements of the electrode assembly  110 , the cylindrical can  120 , and the cap assembly  230  are similar to those described above. Thus, a detailed explanation thereof, is omitted. 
     The thickness (t 1 ′) of the terminal portion  231 A of the cap up  231  is 0.6 mm, and the thickness (t 2 ′) of the peripheral portion  231 B of the cap up  231  is 0.3 mm. The thickness of the safety vent  232  is 0.3 mm, and the thickness (t 2 ′) of the peripheral portion  231 B of the cap up  231  is 0.3 mm. 
     Accordingly, the thickness (T′) of the curled portion  232 A is 0.9 mm. The thickness (T′) of the curled portion  232 A is the thickness of the safety vent  232 , added to twice the thickness of the safety vent  232 . In other words, the thickness (T′) does not exceed the sum of the thickness (t 1 ′) of the terminal portion  231 A of the cap up, and the thickness (t 3 ′) of the safety vent  232 . 
     The thickness (T′) is the same as the total thickness (0.9 mm) of the cap up and safety vent, in a conventional high capacity secondary battery. Thus, the pressing force applied by the gasket to the outer edge of the safety vent, is not decreased. 
     When the thickness of the peripheral portion of the cap up is not reduced, as compared to the conventional art, if the safety vent is bent around the cap up, the total thickness becomes 1.2 mm, because thickness of the cap up is 0.6 mm, and the thicknesses of the safety vent is 0.3 mm. Accordingly, a pressing force of the gasket  140  is decreased. In other words, if the sum of the thicknesses (T) and (T′), of the cap up  231  and safety vent  232 , increases, the relative pressing force of the gasket  140  decreases. 
     The radial thickness (r 2 ) of the peripheral portion  231 B of the cap up  231  is within 10% to 40%, of the radial thickness (r 1 ) of the entire safety vent  232 . It is difficult to secure the thickness (T 3 ′) of the portion pressed by the gasket  140 , after the edge of the peripheral portion  231 B is surrounded by the curled portion  232 A, when the radial thickness (r 2 ) of the peripheral portion  231 B is less than 10% of the radial thickness (r 1 ) of the entire cap up  231 . In addition, the strength of the cap up may not be sufficient to pass the vibration safety test, when the radial thickness (r 2 ) of the peripheral portion  231 B of the cap up  231  is more than 40% of the radial thickness (r 1 ) of the entire cap up  231 . 
     The radial thickness (r 3 ′) of the curled portion  232 A of the safety vent  232  is generally less than the radial thickness (r 2 ) of the peripheral portion  231 B of the cap up  231 . The radial thickness (r 3 ′) of the curled portion  232 A is generally long enough for the curled portion  232 A to surround the end of the peripheral portion  231 B, so as to prevent leakage of the electrolyte. 
     An assembling process of the secondary battery  200  is substantially the same as described above, except that the curling process is performed, so as to surround the end of the cap up  231  with the safety vent  232 , at the time of assembling the cap assembly  230 . As described above, the cap up  231  and safety vent  232  are first integrated with each other, and the safety vent  232  is assembled over the cap down  134 . Thus, the safety vent  232  is always precisely arranged, and assembling errors can be prevented. 
     The thickness (T′) does not exceed the sum of the thickness (t 1 ′) of the terminal portion  231 A, and the thickness (t 3 ′) of the safety vent  232 . Thus, the gasket  140  is pressed against the outer circumference surface of the safety vent  232 , with a predetermined pressing force. 
     As described above, a cylindrical secondary battery, according to aspects of the present invention, produces the following effects. First, the cap up of the cap assembly is thin, except for the area where the strength can be maintained. When the cap up surrounds the end of the safety vent, or the safety vent surrounds the end of the cap up, the sum of thicknesses of the cap up and safety vent does not exceed a predetermined thickness. Accordingly, the pressing force of the gasket against the cap up and safety vent is not decreased. Thus, leakage of the electrolyte is prevented between the cap assembly and gasket. In addition, the thickness of the stacked cap up and safety vent is decreased, and the curling process can be easily performed. 
     Second, the cap up and safety vent of the cap assembly are combined with each other, such that one part surrounds the end of the other part. Thus, battery performance is improved, by preventing an increase of internal resistance, due to vibrations or external impacts. Third, the cap cup and safety vent are preassembled. Thus, stable battery performance can be obtained, by assembling components at exact positions. 
     Although a few embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in this embodiment without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.