Patent Publication Number: US-11660704-B2

Title: Secondary battery

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a National Phase Patent Application of International Patent Application Number PCT/KR2018/011760, filed on Oct. 5, 2018, which claims priority of Korean Patent Application No. 10-2017-0166108, filed Dec. 5, 2017. The entire contents of both of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to a secondary battery. 
     BACKGROUND ART 
     Lithium ion secondary batteries are being widely used in portable electronic devices and as power sources of hybrid automobiles and electric vehicles because of various characteristics, including a relatively high operation voltage, relatively high energy density per unit weight, and so forth. The lithium ion secondary battery can be generally classified as a cylinder-type secondary battery, a prismatic-type secondary battery, or 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. 
     The above information disclosed in this Background section is only for enhancement of understanding of the background of the described technology and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. 
     DESCRIPTION OF EMBODIMENTS 
     Technical Problem 
     Provided is a secondary battery capable of improving the coupling force between a safety vent and a cap-up. 
     Solution to Problem 
     According to an aspect of the present invention, provided is a secondary battery including: an electrode assembly; a case for accommodating the electrode assembly; a cap assembly coupled to the upper part of the case; and a gasket interposed between the cap assembly and the case, wherein the cap assembly includes a cap-up and a safety vent, which is provided at the lower part of the cap-up and has a vent extension part extending to the upper part of the cap-up so as to encompass the edge of the cap-up, a welding region, in which the safety vent and the cap-up are welded and coupled by laser welding, is formed in the vent extension part, and the welding region is formed in a line shape. 
     The welding region may be spaced apart from an end of the vent extension part. 
     A distance between the welding region and the end of the vent extension part may be greater than 1% of a length of the vent extension part. 
     The welding region may be covered by the gasket. 
     A length of the welding region may be greater than 5% of the overall circumference of the vent extension part. 
     The welding region may be formed by moving a laser beam sideways while rotating the laser beam in one direction. 
     The upper part of the welding region may be concavely formed from a surface of the vent extension part. 
     The welding region may include two or more welding regions. 
     The cap-up may include a first metal layer made of iron and a second metal layer formed by plating nickel on a surface of the first metal layer, and the welding region may be formed such that the vent extension part, the first metal layer and the second metal layer are fused together and then cured. 
     Advantageous Effects of Disclosure 
     As described above, in the secondary battery according to an embodiment of the present invention, a safety vent and a cap-up are laser-welded in a line shape, thereby improving a coupling force therebetween. 
     In addition, in the secondary battery according to an embodiment of the present invention, a welding region between a safety vent and a cap-up is formed to be spaced a predetermined distance apart from an end of the safety vent to cover the welding region using a gasket, thereby improving quality by preventing the cap-up from being oxidized. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a cross-sectional view illustrating a secondary battery according to an embodiment of the present invention. 
         FIG.  2    is a cross-sectional view illustrating a cap assembly in the secondary battery according to an embodiment of the present invention. 
         FIG.  3    is a cross-sectional view illustrating a welding region formed between a cap-up and a safety vent in the secondary battery according to an embodiment of the present invention. 
         FIGS.  4 A and  4 B  are plan views illustrating welding regions in the secondary battery according to an embodiment of the present invention. 
         FIG.  5    is a photo showing a welding region formed between a cap-up and a safety vent in the secondary battery according to an embodiment of the present invention. 
         FIG.  6    is an enlarged photo of the welding region shown in  FIG.  5   . 
         FIG.  7    is a graphical representation for comparing welding heights of a point-shape welding region and a line-shape welding region. 
         FIG.  8    is a graphical representation for comparing sealing capacities of a point-shape welding region and a line-shape welding region. 
     
    
    
     MODE OF DISCLOSURE 
     Hereinafter, example embodiments of the present invention will be described in detail. 
     Various embodiments of the present invention may be embodied in many different forms and should not be construed as being limited to the example embodiments set forth herein. Rather, these example embodiments of the invention are provided so that this invention will be thorough and complete and will convey inventive concepts of the 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. 
     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 “comprise” and/or “comprising” 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. 
       FIG.  1    is a cross-sectional view illustrating a secondary battery according to an embodiment of the present invention.  FIG.  2    is a cross-sectional view illustrating a cap assembly in the secondary battery according to an embodiment of the present invention.  FIG.  3    is a cross-sectional view illustrating a welding region formed between a cap-up and a safety vent in the secondary battery according to an embodiment of the present invention. 
     Referring to  FIGS.  1  to  3   , the secondary battery according to an embodiment of the present invention includes an electrode assembly  110 , a case  120 , a cap assembly  130 , and a gasket  190 . 
     The electrode assembly  110  includes a first electrode  111 , a second electrode  112 , and a separator  113  interposed between the first electrode  111  and the second electrode  112 . The electrode assembly  110  may be formed by winding a stacked structure of the first electrode  111 , the separator  113  and the second electrode  112  in a jelly-roll configuration. Here, the first electrode  111  may function as a positive electrode, and the second electrode  112  may function as a negative electrode. A first electrode tab  114  is connected to the cap assembly  130  on the electrode assembly  110 , and a second electrode tab  115  is connected to a bottom plate  122  of the case  120  under the electrode assembly  110 . 
     The first electrode  111  is formed by coating a first electrode active material such as a transition metal oxide on a first electrode current collector formed of a metal foil made of aluminum. A first electrode uncoated portion without a first electrode active material coated thereon is formed on the first electrode  111 , and a first electrode tab  114  is attached to the first electrode uncoated portion. One end of the first electrode tab  114  is electrically connected to the first electrode  111 , and the other end of the first electrode tab  114  upwardly protrudes from the electrode assembly  110  and is electrically connected to the cap assembly  130 . 
     The second electrode  112  is formed by coating a second electrode active material such as graphite or carbon on a second electrode current collector formed of a metal foil made of copper or nickel. A second electrode uncoated portion without a second electrode active material coated thereon is formed on the second electrode  112 , and a second electrode tab  115  is attached to the second electrode uncoated portion. One end of the second electrode tab  115  is electrically connected to the second electrode  112 , and the other end of the second electrode tab  115  downwardly protrudes from the electrode assembly  110  and is electrically connected to the bottom plate  122  of the case  120 . 
     The separator  113  is positioned between the first electrode  111  and the second electrode  112  and prevents a short circuit while allowing lithium ions to move. The separator  113  may include polyethylene, polypropylene, or a composite film of polyethylene, polypropylene. 
     The case  120  includes a cylindrical side surface plate  121  having a predetermined diameter to form a space for accommodating the electrode assembly  110 , and the bottom plate  122  sealing the bottom portion of the side surface plate  121 . A top opening of the case  120  is opened to be sealed after the electrode assembly  110  is inserted into the case  120 . In addition, a beading part  123  for preventing movement of the electrode assembly  110  is formed at the upper part of the case  120 . A crimping part  124  for fixing the cap assembly  130  is formed at the topmost part of the case  120 . 
     The cap assembly  130  includes a cap-up  140 , a safety vent  150 , an insulator  160 , a cap-down  170 , and a sub-plate  180 . 
     The cap-up  140  has a top portion that is convexly formed and is configured to be electrically connected to an external circuit. In addition, the cap-up  140  has a gas discharge hole  141  formed to provide for a path through which the internal gas generated in the case  120  can be discharged. The cap-up  140  is electrically connected to the electrode assembly  110  to transmit current generated in the electrode assembly  110  to an external circuit. In addition, as illustrated in  FIG.  3   , the cap-up  140  includes a first metal layer  140   a  made of iron (Fe), and a second metal layer  140   b  formed by plating nickel (Ni) on a surface of the first metal layer  140 a. 
     The safety vent  150  is formed to have a circular panel shaped to correspond to the cap-up  140 . A downwardly protruding protrusion part  151  is formed at the center of the safety vent  150 . The safety vent  150  is electrically connected to the sub-plate  180  fixed to a bottom surface of the cap-down  170  by the protrusion part  151 , which passes through a through-hole  171  of the cap-down  170 . In some embodiments, the protrusion part  151  of the safety vent  150  and the sub-plate  180  may be welded to each other by laser welding, ultrasonic welding, resistance welding or an equivalent thereof. In addition, a notch  152  for guiding a rupture of the safety vent  150  is formed on or along the outer peripheral edge of the protrusion part  151 . 
     When an abnormal internal pressure is generated in the case  120 , the safety vent  150  is configured to discharge the internal gas while interrupting the flow of current. When an internal pressure of the case  120  exceeds an operating pressure of the safety vent  150 , the protrusion part  151  upwardly rises due to the gas discharged through a gas discharge hole  172  in the cap-down  170  and is electrically disconnected from the sub-plate  180 . In some embodiments, the welded portion of the protrusion part  151  is broken and the safety vent  150  and the sub-plate  180  are electrically disconnected from each other. If the internal pressure of the case  120  exceeds a rupture pressure that is higher than the operating pressure of the safety vent  150 , the notch  152  of the safety vent  150  ruptures, thereby preventing the secondary battery  100  from exploding. The safety vent  150  may be made of aluminum (Al). 
     An outer peripheral edge of the safety vent  150  is installed to tightly contact portions of the cap-up  140  other than the upwardly protruding portion of the cap-up  140 . That is to say, the outer peripheral edge of the safety vent  150  and an outer peripheral edge of the cap-up  140  contact each other. In addition, the outer peripheral edge of the safety vent  150  extends to the upper part of the cap-up while encompassing the edge of the cap-up  140 . Here, the part upwardly extending from the cap-up  140  is defined as the vent extension part  153 . In addition, the upper part of the vent extension part  153  is welded by laser welding to fix the safety vent  150  to the cap-up  140 . Some portions of the safety vent  150  and the cap-up  140  are fused by the laser welding to form welding regions  155 . The welding regions  155  will later be described in greater detail. 
     The insulator  160 , which is interposed between the safety vent  150  and the cap-down  170 , insulates the safety vent  150  and the cap-down  170  from each other. Specifically, the insulator  160  is interposed between the outer peripheral edge of the safety vent  150  and the outer peripheral edge of the cap-down  170 . The insulator  160  may be made of a resin material, such as polyethylene (PE), polypropylene (PP), or polyethylene terephthalate (PET). 
     The cap-down  170  is formed of a circular panel body. The through-hole  171  is formed at the center of the cap-down  170 , and the protrusion part  151  of the safety vent  150  passes through the through-hole  171 . In addition, the gas discharge hole  172  is formed at one side of the cap-down  170 , and the sub-plate  180  is coupled to the lower part of the cap-down  170 . The gas discharge hole  172  allows the internal gas to be discharged through the gas discharge hole  172  when an excessive internal pressure is generated in the case  120 . In some embodiments, the protrusion part  151  of the safety vent  150  rises due to the gas that is discharged through the gas discharge hole  172  such that the protrusion part  151  may be separated from the sub-plate  180 . 
     The sub-plate  180  is welded between the first electrode tab  114  and the protrusion part  151  of the safety vent  150 , which passes through the opening  171  in the cap-down  170 . Accordingly, the sub-plate  180  electrically connects the first electrode tab  114  and the safety vent  150  to each other. 
     The gasket  190  is installed in the top opening of the case  120 . That is to say, the gasket  190  is tightly adhered to regions between the outer peripheral edges of the cap-up  140  and the safety vent  150  and the top opening of the case  120 . The gasket  190  may be formed to cover the welding regions  155  between the safety vent  150  and the cap-up  140 . The gasket  190  may be made of a resin material, such as polyethylene (PE), polypropylene (PP), or polyethylene terephthalate (PET). The gasket  190  may prevent the cap assembly  130  from being separated from the case  120 . 
       FIGS.  4 A and  4 B  are plan views illustrating welding regions in the secondary battery according to an embodiment of the present invention.  FIG.  5    is a photo showing a welding region formed between a cap-up and a safety vent in the secondary battery according to an embodiment of the present invention.  FIG.  6    is an enlarged photo of the welding region shown in  FIG.  5   . 
     As illustrated in  FIG.  3   , the welding region  155  is formed such that the vent extension part  153  and the first metal layer  140   a  and the second metal layer  142   b  of the cap-up  140  are fused together and then cured. A top surface of the welding region  155  is concavely formed by laser beam applied during laser welding. In addition, the welding region  155  is formed at a portion spaced a predetermined distance apart from an end  154  of the vent extension part  153  and the top portion of the welding region  155  is covered by the gasket  190 . Therefore, the first metal layer  140   a  of the cap-up  140  is not exposed to the outside, thereby preventing the first metal layer  140   a  from being oxidized. That is to say, the welding region  155  is outwardly spaced a predetermined distance apart from the end  154  of the vent extension part  153 . Here, a distance L 1  between the welding region  155  and the end  154  of the vent extension part  153  is greater than 1% of a length L 2  of the vent extension part  153 . For example, the distance L 1  may be formed to be at least 0.03 mm or greater. If the distance L 1  is smaller than 1% of the length L 2  of the vent extension part  153 , the welding region  155  extends over the end  154  of the vent extension part  153 , so that the first metal layer  140   a  of the cap-up  140  may be exposed to the outside. 
     When conventionally welding a safety vent and a cap-up to each other, spot welding has been performed on an end of the safety vent, that is, a boundary surface of the safety vent to be welded to the cap-up, thereby fixing the safety vent to the cap-up. In this case, however, a second metal layer of the cap-up may be stripped off, and a first metal layer may be exposed to the outside, resulting in rust generation on the exposed first metal layer. By contrast, according to the present invention, the welding region  155  between the safety vent  150  and the cap-up  140  is formed to be spaced a predetermined distance apart from the end  154  of the safety vent  150 , so that the welding region  155  is covered by the gasket  190 , thereby preventing the first metal layer  140   a  from being oxidized by preventing the first metal layer  140   a  of the cap-up  140  from being exposed to the outside. 
     In addition, as illustrated in  FIGS.  4 A and  4 B , laser welding between the safety vent  150  and the cap-up  140  is performed in a line shape. Additionally, the welding region  155  may be provided at two points (see  FIG.  4 A ) or at four points (see  FIG.  4 B ). In addition, as illustrated in  FIGS.  5  and  6   , the line-shape welding regions  155  may be formed by slowly moving a laser beam sideways while rotating the laser beam in one direction. If the secondary battery  100  has a relatively small diameter or if a length of the welding region  155  is increased, the welding region  155  may be formed in an arch or arc shape. 
     Unlike in the conventional welding between the safety vent and the cap-up, in which spot welding is performed in a point shape, in the present invention, the welding region  155  is formed in a line shape, thereby improving coupling strength between the safety vent  150  and the cap-up  140 . A length D 1  of the welding region  155  is greater than 5% of the overall circumference D 2  of the vent extension part  153 . If the length D 1  of the welding region  155  is smaller than 5% of the overall circumference D 2  of the vent extension part  153 , the welding performed in the line shape is almost like the spot welding, and thus improvement in the coupling strength between the safety vent  150  and the cap-up  140  is insufficient. 
     As such, the secondary battery  100  according to an embodiment of the present invention may have an improved coupling force between the safety vent  150  and the cap-up  140  by coupling the safety vent  150  and the cap-up  140  by laser welding in a line shape. 
     In addition, in the secondary battery  100  according to an embodiment of the present invention, the welding region  155  between the safety vent  150  and the cap-up  140  is spaced a predetermined distance apart from the end  154  of the safety vent  150  so as to cover the welding region  155  using the gasket  190 , thereby improving quality by preventing the cap-up  140  from being oxidized. 
       FIG.  7    is a graphical representation for comparing welding heights of a point-shape welding region and a line-shape welding region. 
     Referring to  FIG.  7   , a point-shape welding region has a height in a range of about 1.66 mm to about 1.68 mm, while a line-shape welding region has a height of about 1.58 mm. That is to say, the conventional point-shape welding may provide a reduced sealing force between the gasket and the case due to the protruding welding region. In the present invention, however, the welding region between the safety vent and the cap-up is welded in a line shape, and thus the welding height is reduced, as illustrated in  FIG.  7   , thereby improving the coupling force between the gasket and the case. 
       FIG.  8    is a graphical representation for comparing sealing capacities of a point-shape welding region and a line-shape welding region. 
     Referring to  FIG.  8   , when the safety vent and the cap-up are welded to each other at four points in a point shape, a sealing pressure applied between the safety vent and the cap-up is in a range of about 13 kg/cm 2  to about 19 kg/cm 2 . When the safety vent and the cap-up are welded to each other at two points in a line shape, a sealing pressure applied between the safety vent and the cap-up is about 24 kg/cm 2 . This suggests that the sealing force between the safety vent and the cap-up is improved by welding the safety vent and the cap-up to each other in a line shape, like in the present invention. 
     Although the foregoing embodiments have been described to practice the 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.