Patent Publication Number: US-2011076532-A1

Title: Secondary battery

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Korean Application No. 10-2009-0093149, filed in the Korean Intellectual Property Office on Sep. 30, 2009, the disclosure of which is incorporated herein by reference. 
     BACKGROUND 
     1. Field 
     Aspects of the present invention relate to a secondary battery, and more particularly, to a secondary battery capable of improving weldability of an electrode tab to a cap assembly and current collection efficiency. 
     2. Description of the Related Art 
     Recently, as portable electronic apparatuses are rapidly being made small and light, research on the secondary battery used as a driving power source have been performed. A nickel-cadmium battery, a nickel-hydrogen battery, a nickel-zinc battery, and a lithium secondary battery are generally used as the secondary battery. 
     The lithium secondary battery is rechargeable, compact, and large in capacity, and thus is widely applied to high-tech electronic devices because of its high operating voltage and high energy density per unit weight. Such a lithium secondary battery may be manufactured by accommodating a jelly-roll type electrode assembly in a can, injecting an electrolyte into the can, and then sealing the top of the can with a cap assembly. The jelly-roll type electrode assembly is formed by respectively coating active materials on a positive electrode collector and a negative electrode collector and then winding the positive electrode collector and the negative electrode collector and a separator interposed between the positive and negative electrode collectors. A cylindrical battery having an electrode assembly accommodated in a cylindrical can or a prismatic battery having an electrode assembly accommodated in a rectangular can is frequently used as the lithium secondary battery. 
     The electrode assembly is formed by winding the positive electrode plate, the negative electrode plate, and the separator. A positive electrode tab is connected to the positive electrode plate to protrude upward from a top portion of the electrode assembly. A negative electrode tab is connected to the negative electrode plate to protrude upward from a top portion of the electrode assembly. In the electrode assembly, the positive and negative electrode tabs are spaced apart from each other at a predetermined interval so as to be electrically isolated from each other. 
     The cap assembly includes a cap plate, an insulation plate, a terminal plate, and an electrode terminal. The cap assembly is coupled to a separate insulation case and seals the can while being coupled to the top opening of the can. Holes for positive and negative electrode tabs are formed at the insulation case. The positive and negative electrode tabs are respectively inserted into the holes so as to prevent a short circuit between an upper portion of the electrode assembly inserted into the can and a lower portion of the cap assembly. The insulation case functions to prevent a short circuit that may be caused by the contact of the bent negative and positive electrode tabs with an inner wall of the can. 
     In the electrode tabs respectively connected to the electrode plates, the positive electrode tab is attached to the cap plate or the can to be electrically connected to the electrode terminal, and the negative electrode tab is attached to the terminal plate to be electrically connected to the electrode terminal. The polarities of the positive and negative electrode tabs may be reversed. 
     However, an end portion of the electrode tab connected to the electrode plate to protrude upward from the electrode plate is attached to the cap assembly or the can through welding while being slightly bent. Therefore, when the secondary battery receives an impact, a welding defect may occur when a welded portion is broken off due to its weakness, and it is difficult to select a position of the electrode tab. 
     SUMMARY 
     According to aspects of the present invention, a secondary battery is provided, wherein an electrode tab is formed long, so that the area at which the electrode tab is attached to a cap assembly can be broadened, thereby improving weldability of the electrode tab to the cap assembly. 
     According to an aspect of the present invention, a secondary battery is provided. The secondary battery includes an electrode assembly formed by winding a first electrode plate, a second electrode plate, and a separator, the first electrode plate being connected to a first electrode tab at one side of the electrode assembly, the second electrode plate being connected to a second electrode tab at another side of the electrode assembly, and the separator being interposed between the first and second electrode plates; a can having one opened portion to accommodate the electrode assembly; and a cap assembly to seal the opened portion of the can, wherein at least one of the first and second electrode tabs includes a drawing portion protruding upward from the first or second electrode plate and a bonding portion formed by bending one region of the drawing portion to be attached to the cap assembly, and a length of the bonding portion is formed longer than a width of the drawing portion. 
     According to another aspect of the present invention, the at least one of the first and second electrode tabs, of which the length of the bonding portion is formed longer than the width of the drawing portion, may be formed of aluminum. 
     According to another aspect of the present invention, the at least one of the first and second electrode tabs, of which the length of the bonding portion is formed longer than the width of the drawing portion, may be a positive electrode tab. 
     According to another aspect of the present invention, when the length of the bonding portion of both of the first and second electrode tabs is longer than the width of the drawing portion, end portions of the first and second electrode tabs may be bent in opposite directions to each other. 
     According to another aspect of the present invention, the first electrode tab may be attached to a cap plate of the cap assembly or to the can, and the second electrode tab may be attached to a terminal plate of the cap assembly. 
     According to another aspect of the present invention, the drawing portion of the at least one of the first and second electrode tabs may be bent twice to form a “T” shape. 
     According to another aspect of the present invention, the first and second electrode tabs may be welded to the cap assembly. 
     According to another aspect of the present invention, the welding may be an ultrasonic or resistance welding. 
     According to another aspect of the present invention, the number of welding points at the bonding portion may be from 3 to 10. 
     According to another aspect of the present invention, the welding points at the bonding portion may be sequentially formed in the length direction of the bonding portion. 
     According to another aspect of the present invention, a welding portion at the bonding portion may be formed longer in the length direction of the bonding portion than in the width direction of the bonding portion. 
     According to another aspect of the present invention, the secondary battery may further include an insulation case positioned between the electrode assembly and the cap assembly. 
     As described above, according to aspects of the present invention, an electrode tab is formed long, so that the area at which the electrode tab is attached to a cap assembly can be broadened. Accordingly, it is possible to improve weldability of the electrode tab to the cap assembly and current collection efficiency. Further, the position of the electrode tab can be easily selected by considering alignment when winding an electrode assembly. 
     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 embodiments, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is an exploded perspective view of a secondary battery according to an embodiment of the present invention; 
         FIG. 2  is an assembled sectional view of the secondary battery of  FIG. 1 ; 
         FIG. 3  is a perspective view of an electrode tab according to an embodiment of the present invention; 
         FIG. 4A  is a perspective view illustrating a welding state of the electrode tab according to an embodiment of the present invention; 
         FIG. 4B  is a perspective view illustrating another welding state of the electrode tab according to an embodiment of the present invention; and 
         FIG. 5  is a perspective view of an electrode tab according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to the present 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 embodiments are described below in order to explain the present invention by referring to the figures. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention. Accordingly, the drawings and description are to be regarded as illustrative in nature and not restrictive. In addition, when an element is referred to as being “on” another element, it can be directly on the another element or be indirectly on the another element with one or more intervening elements interposed therebetween. Also, when an element is referred to as being “connected to” another element, it can be directly connected to the another element or be indirectly connected to the another element with one or more intervening elements interposed therebetween. 
       FIG. 1  is an exploded perspective view of a secondary battery  100  according to an embodiment of the present invention.  FIG. 2  is an assembled sectional view of the secondary battery  100  of  FIG. 1 . Referring to  FIGS. 1 and 2 , the secondary battery  100  includes an electrode assembly  12 , a can  10 , a cap assembly  20 , and an insulation case  70  positioned between the electrode assembly  12  and the cap assembly  20 . 
     The can  10  accommodates the electrode assembly  12  through an opened portion thereof. The horizontal section of the can  10  is formed in a rectangular shape with rounded corners and includes a pair of short side portions  10   a  and a pair of long side portions  10   b . The horizontal section of the can  10  is not limited thereto. Although not shown in the figures, according to other aspects of the present invention, the horizontal section of the can  10  may be formed in a rectangular or elliptical shape. The can  10  may be formed of a metallic material that is light and flexible, such as aluminum or aluminum alloy. The can  10  may be easily manufactured using a deep drawing method. 
     The electrode assembly  12  inserted into the can  10  is formed by winding a first electrode plate (not shown), a second electrode plate (not shown), and a separator (not shown) interposed between the first and second electrode plates. A first electrode tab  16  is connected to the first electrode plate to protrude upward from a top portion of the electrode assembly  12 , and a second electrode tab  17  is connected to the second electrode plate to protrude upward from a top portion of the electrode assembly  12 . For convenience of illustration, the first and second electrode tabs  16  and  17  are hereinafter referred to as positive and negative electrode tabs, respectively. However, according to other aspects of the present invention, the second electrode tab  17  may be a positive electrode tab and the first electrode tab  16  may be a negative electrode tab. 
     In the electrode assembly  12 , the positive and negative electrode tabs  16  and  17  are spaced apart from each other at a predetermined interval so as to be electrically isolated from each other. A lamination tape  18  is wound at a portion of each of the positive and negative electrode tabs  16  and  17 , drawn out from the electrode assembly  12 . The lamination tape  18  blocks heat generated from the positive or negative electrode tab  16  or  17  and prevents the electrode assembly  12  from being compressed by an edge of the positive or negative electrode tab  16  or  17 . 
     The positive and negative electrode tabs  16  and  17  are electrically connected to the positive and negative electrode plates of the electrode assembly  12 , respectively. The positive and negative electrode tabs  16  and  17  are drawn out in the opened direction of the can  10 . To this end, the positive and negative electrode tabs  16  and  17  are electrically connected to the cap plate  40  or can  10  and the terminal plate  60  by passing through lid through-holes  72  of the insulation case  70  fixed to the top of the electrode assembly  12  in the can  10 , respectively. 
     The positive and negative electrode tabs  16  and  17  have drawing portions  16   a  and  17   a  and bonding portions  16   b  and  17   b , respectively. The drawing portions  16   a  and  17   a  are respectively protruded upward from the positive and negative electrode plates, and the bonding portions  16   b  and  17   b  are respectively formed by bending one region of each of the drawing portions  16   a  and  17   a  to be attached to the cap assembly  20 . The length of each of the bonding portions  16   b  and  17   b  is formed to be longer than the width of each of the drawing portions  16   a  and  17   a . End portions of the positive and negative electrode tabs  16  and  17  are bent in directions opposite to each other. Accordingly, a welding portion formed at each of the electrode tabs  16  and  17  is formed broader than that formed at each of the conventional electrode tabs, so that welding defects of the electrode tabs  16  and  17  can be reduced. Further, the positions of the electrode tabs  16  and  17  can be easily selected by considering alignment when winding the electrode assembly  12 . 
     As described above, the drawing portions  16   a  and  17   a  and the bonding portions  16   b  and  17   b  are formed at both of the positive and negative electrode tabs  16  and  17 , respectively. However, the drawing portion  16   a  and the bonding portion  16   b  may be formed only at the positive electrode tab  16  formed of aluminum. Since aluminum is weak to welding, it is difficult to control its weld strength. For example, aluminum is broken if its weld strength is intensified, and its weldability is lowered if its weld strength is weakened. Accordingly, the welding portion is broadened, so that the attachment of the positive electrode tab  16  to the cap plate  40  or can  10  can be maintained by the welding at one region of the welding portion even though the welding at another region of the welding portion has failed. 
     The positive electrode plate and the negative electrode plate are manufactured by dry-coating an aluminum metal foil and a copper metal foil with respective slurries. The slurries include active materials of the positive and negative electrode plates and a fixing agent to attach the respective active materials to the metal foils. In the case of a lithium secondary battery, an oxide containing lithium may be used as a positive electrode active material, and any one of hard carbon, soft carbon, graphite and a carbon material may be used as a negative electrode active material. However, aspects of the present invention are not limited to the lithium secondary battery. For example, according to other aspects of the present invention, the secondary battery  100  may be a nickel-cadmium battery, nickel-hydrogen battery, nickel-zinc battery, or other type of secondary battery. 
     The cap assembly  20  includes a cap plate  40 , an electrode terminal  30 , an insulation plate  50 , a terminal plate  60  and a gasket  35 . When the cap assembly  20  is coupled to the can  10 , the cap plate  40  allows the opened portion of the can  10  to be sealed tightly, thereby forming one surface of the can  10 . To this end, the cap plate  40  may be coupled to the opened portion of the can using a method such as welding. 
     Hereinafter, the components of the cap assembly  20  will be described in detail. The cap plate  40  is electrically connected to either the positive electrode tab  16  or the negative electrode tab  17  protruded by respectively passing through the lid through-holes  72  of the insulation case  70 . A first terminal hole  41  for coupling the gasket  35  to the cap plate  40  therethrough and an electrolyte injection hole  42  for injecting an electrolyte are formed at the cap plate  40 . The electrolyte injection hole  42  is formed at the cap plate  40  and used as a passage through which the electrolyte is injected into the can  10 . After the electrolyte is injected into the can  10 , the electrolyte injection hole  42  is tightly sealed with a stopper  43 . 
     The gasket  35  provides insulation between the electrode terminal  30  and the cap plate  40 . A terminal hole to couple the electrode terminal  30  to the gasket  35  therethrough is formed at the gasket  35 , and the electrode terminal  30  is coupled to the gasket  35  by passing through the terminal hole. 
     The terminal plate  60  is electrically connected to the electrode terminal  30  through a third terminal hole  61 . The terminal plate  60  is electrically connected to whichever of the positive and negative electrode tabs  16  and  17  is not connected to the cap plate  40 . For example, when the cap plate  40  is electrically connected to the positive electrode tab  16 , the terminal plate  60  is connected to the negative electrode tab  17 . Accordingly, the terminal plate  60  can provide electrical connection of the electrode terminal  30  and the negative electrode tab  17 . 
     The insulation case  70  is positioned between the electrode assembly  12  and the cap assembly  20  so that they are electrically isolated from each other. The insulation case  70  is positioned such that it is inserted into the top portion of the can  10 . Corners of the insulation case  70  are formed in a rounded rectangular shape identical to the horizontal sectional shape of the can  10  so that the insulation case  70  can be forcibly fitted into the can  10 . 
     The insulation case  70  is generally manufactured using a light plastic resin with excellent electric insulation, although other materials with similar properties may also be employed. Therefore, when the insulation case  70  is inserted into the can  10 , the deformation of the insulation case  70  is not caused by the electrolyte, and it is easy to provide insulation between the electrode assembly  12  and the cap assembly  20 . However, when a light plastic resin is used as the insulation case  70 , it may be difficult to couple the insulation case  70  to the can  10  due to its weak elasticity. To solve such a problem, the insulation case  70  includes a base portion  74  and a support portion  73 . Accordingly, the insulation case  70  can be stably coupled to the can  10 . An electrolyte injection hole (not shown) and lid through holes  72  are formed at the base portion  74 . 
       FIG. 3  is a perspective view of an electrode tab according to an embodiment of the present invention. Referring to  FIG. 3 , the positive electrode tab  16  is provided with a drawing portion  16   a  and a bonding portion  16   b  formed by bending one region of the drawing portion  16   a  once. The positive electrode tab  16  may be formed so that the drawing portion  16   a  is bent at 90 degrees to the horizontal direction. Accordingly, a wide and flat surface of the bonding portion  16   b  of the positive electrode tab  16  can be attached to the cap plate  40  or the can  10 . 
     If the width of the drawing portion  16   a  of the positive electrode tab  16  is “A” and the length of the bonding portion  16   b  formed by bending the drawing portion  16   a  of the positive electrode tab  16  is “B”, “B” is formed longer than “A”. Thus, the area of the bonding portion  16   b  attached to the cap plate  40  or the can  10  is increased, so that the weldable region of the positive electrode tab  16  can be broadened. 
       FIG. 4A  is a perspective view illustrating a welding state of the electrode tab according to an embodiment of the present invention.  FIG. 4B  is a perspective view illustrating another welding state of the electrode tab according to an embodiment of the present invention. 
     The bonding portion  16   b  is attached to the can  10  or the cap plate  40  by welding. The welding may be an ultrasonic or resistance welding. Referring to  FIG. 4A , three welding points  19  are formed at the bonding portion  16   b . The welding points  19  may be sequentially formed in the length direction of the bonding portion  16   b . However, aspects of the present invention are not limited thereto. The number of welding points  19  may be from 3 to 10. If the number of welding points  19  is below 3, the electrode tab  16  may be separated from the can  10  or the cap plate  40  due to its weak welding portion. If the number of welding points  19  is over 10, the effect may not be superior to that when the number of welding points is 10 or less, and productivity may be lowered. Referring to  FIG. 4B , the welding point  19  formed at the bonding portion  16   b  may be formed long in the length direction of the bonding portion  16   b.    
     The ultrasonic welding is performed while removing foreign matters (a pollutant, an oxide and the like) formed between pressurized objects to be welded with high-frequency vibration energy generated by ultrasonic waves of 20 kHz and narrowing the gap between the objects to be welded by an interatomic distance. Since heat is not applied in the ultrasonic welding, the ultrasonic welding is classified as a cold welding. Accordingly, it is possible to prevent thermal damage caused by welding. 
     In the resistance welding, a large amount of current is transmitted through a welding base metal so that heat is generated by contact resistance of its welding portion. Then, the welding base metal is heated with the generated heat to be in a melted state, and mechanical pressure is applied to the welding base metal. The resistance welding is classified into a butt welding, a point welding, and the like. The butt welding is used when welding the ends of metal rods, pipes, or the like. If current is transmitted while their ends are connected to each other, a bonding portion is heated red hot. The butt welding is performed by applying pressure to the bonding portion. 
     In the point welding, welding base metals are stacked, and rod-shaped electrodes formed of a copper alloy are connected to upper and lower sides of the stacked welding base metals. Then, a strong current is transmitted into the welding base metals through the electrodes under a pressure atmosphere. When the temperature of contact portions between the electrodes and the welding base metal is increased, the point welding is performed by re-applying pressure to the contact portions. 
       FIG. 5  is a perspective view of an electrode tab according to another embodiment of the present invention. Referring to  FIG. 5 , the positive electrode tab  26  according to the embodiment of the present invention may be formed in a “T” shape by bending a drawing portion  26   a  twice. The positive electrode tab  26  may be formed by first bending one region  26   b  of the drawing portion  26   a  at 90 degrees to the horizontal direction and then bending another region  26   c  of the drawing portion  26   a  in the opposite direction to that of the first bent region  26   b  of the drawing portion  26   a  so as to partially overlap with the first bent region  26   b . Accordingly, the shape of the positive electrode tab  26  formed by bending the drawing portion  26   a  twice is a “T” shape, and a wide and flat surface of the positive electrode tab  26  can be attached to the cap plate  40  or the can  10 . 
     If the width of the drawing portion  26   a  of the positive electrode tab  26  is “C” and the length of the bonding portion  26   c  formed by bending the drawing portion  26   a  twice is “D”, “D” may be formed longer than “C”. Accordingly, the area of the bonding portion  26   c  attached to the cap plate  40  or the can  10  is increased, so that the weldable region of the positive electrode tab  26  can be broadened. 
     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 these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.