Abstract:
Disclosed herein are a method for welding electrode taps of a secondary battery which improves insulation defects generated during welding between the electrode taps and lead parts and an electrode assembly manufactured thereby. The method for welding electrode taps includes performing main welding to connect electrode taps protruded from electrode plates provided in a battery and lead parts electrically connecting the electrode taps to external terminals and having a lead film on the surfaces thereof and, in main welding, a welding part is formed in a partial section of an overlapping region between the electrode tap and the lead part, and a non-welding section is formed between the welding part and the lead film.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a method for welding electrode taps of a secondary battery which improves insulation defects generated during welding between the electrode taps and lead parts of the secondary battery and an electrode assembly manufactured thereby. 
         [0003]    2. Description of the Related Art 
         [0004]    As demand for mobile devices rapidly increases, demand for secondary batteries applied to mobile devices increases and thus technical research on secondary batteries is vigorously carried out. 
         [0005]    In general, secondary batteries are rechargeable and may be small-sized and large-sized, and a lithium (Li) battery is representatively used as a secondary battery. 
         [0006]    However, although secondary batteries are rechargeable and used as an eco-friendly energy source, as described above, they have a problem regarding stability thereof. 
         [0007]    That is, various chemical reactions in a secondary battery are caused by temperature rise due to abnormal energy conversion in the secondary battery. Thereby, lowering of stability of the secondary battery, i.e., ignition and explosion, may occur. 
         [0008]    In more detail, if the secondary battery is exposed to high temperature or a high intensity of current flows in the secondary battery within a short time due to external short circuit, the secondary battery is heated by IR heating and a dangerous state, such as heating, ignition, explosion, etc., may be caused. Therefore, durability of secondary batteries needs to be improved. 
         [0009]    A conventional secondary battery includes an electrode assembly  2  in which anode plates, separator films, and cathode plates are alternately stacked, installed within a battery case  1 , as exemplarily shown in  FIG. 1 . Among the electrode assembly  2 , the anode plate is formed of aluminum or an aluminum alloy as a base material, and the cathode plate is formed of copper as a base material. 
         [0010]    Further, electrode taps  3  protruding one side of the electrode assembly  2  for connection to lead parts  4  are provided at the respective plates. The lead parts  4  serve to connect the respective plates to external terminals, the lead parts  4  and the electrode taps  3  are mainly connected by welding, and welding parts W are formed through welding. 
         [0011]    However, during a resistance welding process, a plurality of nuggets is generated between the electrode taps  3  and the lead parts  4  due to fusion of the welding parts W by welding rods. During such a process, foreign substances, such as fine metal particles, are dispersed and move to the lead parts  4  and thus destroy a lead film  4 ′ during a subsequent process, i.e., sealing of the battery case  1 , thereby causing insulation defects. 
       SUMMARY OF THE INVENTION 
       [0012]    Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a welding method in which dispersion of metal particles caused by welding is prevented and thus an insulation defect rate is greatly reduced, and an electrode assembly manufactured thereby. 
         [0013]    In accordance with an aspect of the present invention, the above and other objects can be accomplished by the provision of a method for welding electrode taps including performing main welding to connect electrode taps protruded from electrode plates provided in a battery and lead parts electrically connecting the electrode taps to external terminals and having a lead film on the surfaces thereof, wherein, in main welding, a welding part is formed in a partial section of an overlapping region between the electrode tap and the lead part, and a non-welding section is formed between the welding part and the lead film. 
         [0014]    The method may further include performing tack welding to perform welding for interconnection between a plurality of stacked electrode taps, prior to main welding. 
         [0015]    The method may further include cutting the ends of the electrode taps on which tack welding is not performed, after tack welding. 
         [0016]    Main welding may be performed using a welding guide member to measure the position of the non-welding section. 
         [0017]    The electrode plate may be a cathode plate. 
         [0018]    The electrode plate may be an anode plate. 
         [0019]    In accordance with another aspect of the present invention, there is provided an electrode assembly including electrode taps protruded from electrode plates provided in a battery, lead parts electrically connecting the electrode taps to external terminals, having a lead film on the surfaces thereof, and overlapping with the electrode taps in a designated section, welding parts, each of which is formed in a partial section of an overlapping region between the electrode tap and the lead part, and non-welding sections formed between the welding parts and the lead films. 
         [0020]    The electrode plate may be a cathode plate. 
         [0021]    The electrode plate may be an anode plate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
           [0023]      FIG. 1  is a view illustrating the connection structure between an electrode tap and a lead part in a general secondary battery; 
           [0024]      FIG. 2  is a flowchart illustrating a method for welding electrode taps in which an insulation defect rate of a secondary battery in accordance with one embodiment of the present invention is reduced; 
           [0025]      FIG. 3  is a view schematically illustrating the structures of an electrode tap and a battery cell before a welding operation is performed; 
           [0026]      FIG. 4  is a view illustrating tack welding performed on the electrode tap shown in  FIG. 3 ; 
           [0027]      FIG. 5  is a view illustrating cutting of the end of the tack-welded part of the electrode tap shown in  FIG. 4 ; 
           [0028]      FIG. 6  is a view illustrating connection of the cut electrode tap shown in  FIG. 5  and a lead part; 
           [0029]      FIG. 7  is a view illustrating the connected electrode tap and lead part shown in  FIG. 6 ; 
           [0030]      FIG. 8  is a view schematically illustrating a main welding process in accordance with one embodiment of the present invention; 
           [0031]      FIG. 9  is a photograph illustrating a welding resultant product in which a non-welded section is not formed between an electrode tap and a lead part; 
           [0032]      FIG. 10  is a photograph illustrating a welding resultant product in which a non-welded section is formed between an electrode tap and a lead part in accordance with one embodiment of the present invention; 
           [0033]      FIG. 11  is a graph illustrating insulation defect rates according to whether or not a non-welding section is formed; and 
           [0034]      FIG. 12  is a view schematically illustrating welding using a welding guide member during the main welding process in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0035]    Now, preferred embodiments in accordance with the present invention will be described in detail with reference to the annexed drawings. 
         [0036]    A method for welding electrode taps in which an insulation defect rate of a secondary battery in accordance with one embodiment of the present invention is reduced includes performing tack welding (Operation S 100 ), cutting taps (Operation S 200 ), and performing main welding (Operation S 300 ). 
         [0037]      FIGS. 3 to 7  are views schematically illustrating the structures of a battery cell  10  and an electrode tap  100  which are changed according to performance of the welding method in accordance with the present invention. Herein, for conceptual description, only one electrode tap  100  is illustrated. 
         [0038]    Prior to welding, the electrode tap  100  in which a plurality of electrode plates is stacked protrudes upward from the upper surface of the battery cell  10 , as exemplarily shown in  FIG. 3 . If tack welding (Operation S 100 ) is performed in such a state, a tack welding part  101  is formed on the electrode tap  100 , as exemplarily shown in  FIG. 4 . 
         [0039]    In more detail, tack welding (Operation  100 ) serves to interconnect a plurality of stacked electrode taps  100  and is carried out through resistance welding. Resistance welding is a method in which heat is generated by internal electrical resistance to current flowing in a conductor, heat is acquired by contact resistance, generated from metal or generated on a contact surface between different kinds of metal by supplying a high intensity of current under the condition that pressure is applied through welding rods  400 , and inherent resistance of the metal, and thus junction between the different kinds of metal is carried out by the applied pressure while the different kinds of metal are heated or melted. 
         [0040]    That is, when current flows under the condition that pressure is applied to the upper and lower surfaces of the stacked electrode taps  100  through the welding rods  400 , the electrode taps  100  are melted and connected by pressure and, during such a process, depressed nuggets N are formed on the tack welding part  101 . 
         [0041]    Tack welding (Operation S 100 ) may be performed after a plurality of electrode taps  100  is collected in a bundle so that a subsequent operation, i.e., cutting of the taps (Operation S 200 ), may be easily performed, and the number of times of welding in main welding (Operation S 300 ) may be reduced through tack welding (Operation S 100 ). 
         [0042]    After tack welding (Operation S 100 ), cutting of the taps (Operation S 200 ) is performed. 
         [0043]    Cutting of the taps (Operation S 200 ) is an operation of cutting an end of the tap  100  which is not welded during tack welding (Operation S 100 ). The boundary ends of both sides of the tack welding part  101  are cut so that a non-welded portion is not left at the cut end of the tack welding part  101  and thereby, a non-welded portion is effectively removed (With reference to  FIG. 5 ). Therefore, the end of the tack welding part  101  on which cutting of the taps (Operation S 200 ) has been performed forms a welded end. 
         [0044]    After cutting of the taps (Operation S 200 ), main welding (Operation S 300 ) is performed. 
         [0045]    Main welding (Operation S 300 ) is an operation of electrically connecting the electrode tap  100  and a lead part  200 . 
         [0046]    The lead part  200  has a function of electrically connecting the electrode tap  100  to an external terminal, and a lead film  201  is attached to the lead part  200 . 
         [0047]    The lead film  201  acts as an adhesive member between the battery case (not shown) and the lead part  200  during sealing of the lead part  200  and serves to prevent damage to the battery case (not shown) due to the lead part  200 . 
         [0048]    As exemplarily shown in  FIGS. 5 and 9 , the lead part  200  having such a structure is stacked on the electrode tap  100  so that the ends of the lead part  200  and the electrode tap  100  may contact each other, a pair of welding rods  400  contacts the upper surface of the electrode tap  100  and the lower surface of the lead part  200 , and then pressure is applied to the upper surface of the electrode tap  100  and the lower surface of the lead part  200 . 
         [0049]    Thereafter, when current is applied to the welding rods  400 , the electrode tap  100  and the lead part  200  are connected by welding, and nuggets N are formed on the upper surface of the electrode tap  100  due to high temperature and high pressure of the welding rods  400 . 
         [0050]    During such a process, metal particles, such as copper, used as a base material of the electrode tap  100 , are dispersed from the nuggets N, and the dispersed particles are separated from the welding part W and move to the lead film  201 . 
         [0051]    Further, metal particles remaining at the end of the electrode tap  100  in cutting of the taps (Operation S 200 ) actively float in main welding (Operation S 300 ). 
         [0052]    Therefore, in the method for welding electrode taps in which an insulation defect rate of the secondary battery is reduced in accordance with the present invention, a non-welding section A is formed at a connection region between the electrode tap  100  and the lead part  200  in the main welding operation (Operation S 300 ) and thus, movement of dispersed metal particles to the lead film  201  may be prevented. 
         [0053]    Through experimentation, it is confirmed that the method for welding electrode taps in accordance with the present invention in which the non-welding section A is formed greatly reduces an insulation defect rate. 
         [0054]    That is,  FIG. 9  is a photograph illustrating a case that welding is carried out up to the end of the electrode tap  100  (i.e., a non-welding section is not formed) in the main welding operation (Operation S 300 ), and  FIG. 10  is a photograph illustrating a case that welding is carried out while forming a non-welding section A at the end of the electrode tap  100 . 
         [0055]      FIG. 11  is a graph illustrating insulation defect rates according to whether or not a non-welding section A is formed during main welding (Operation S 300 ). In  FIG. 11 , the horizontal axis means target objects (samples) of an insulation defect experiment, and the vertical axis means insulation defect rates. It may be understood that samples D 06  to D 20  represent cases that a non-welding section A is not formed and insulation defect rates thereof are in a comparatively wide range of 0.15 to 0.8%, and samples E 07  to E 15  represent cases that a non-welding section A is formed and insulation defect rates thereof are greatly reduced and have a small deviation. 
         [0056]    It may be interpreted that, in cases that a non-welding section A is formed, insulation defect rates are regularly reduced regardless of proficiency of a worker and environments. 
         [0057]    Further, in order to set an accurate position of the non-welding section A, main welding (Operation S 300 ) may be performed using a welding guide member  300  which may measure the length of the non-welding section A, as exemplarily shown in  FIG. 12 . 
         [0058]    The welding guide member  300  may include a guide plate  310 , a guide rail bar  320 , and a connection member  330 . 
         [0059]    The guide plate  310  is formed in the shape of a rod having a designated length, has a function of guiding the position of the non-welding section A on the electrode tap  100 , and moves vertically on the guide rail bar  320  having a slit. Further, the guide rail bar  320  may be connected to the battery cell  320  through the connection member  330 . 
         [0060]    Therefore, a worker locates the guide plate  310  at a position separated from the electrode tap  100  by a designated distance and then performs welding only up to this position during main welding (Operation S 300 ), thus forming a non-welding section A having a regular length and being capable of effectively preventing dispersion of metal particles. That is, since the non-welding section A may prevent foreign substances, such as metal, from dispersing and moving to the lead film  201  but, if the non-welding section A is excessively wide, electrical connection between the electrode tap  100  and the lead part  200  may be lowered, the welding guide member  300  may both prevent foreign substances from dispersing and achieve electrical connection between the electrode tap  100  and the lead part  200 . 
         [0061]    As apparent from the above description, in a method for welding electrode taps of a secondary battery in accordance with one embodiment of the present invention, a non-welding section is formed so that a distance between a welding part, electrically connecting the electrode tap to a lead part, and a lead film may be regularly maintained and thus, dispersion and movement of metal particles generated during a welding process toward the lead film may be prevented and insulation defects may be greatly reduced. 
         [0062]    Further, in the method for welding electrode taps in accordance with one embodiment of the present invention, main welding is performed using a welding guide member and thus, a secondary battery having uniform insulation effects may be manufactured. 
         [0063]    Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.