Abstract:
Provided is a lithium secondary battery which is capable of preventing high-temperature short circuit by incorporation of a clad negative electrode tab having a nickel/copper bilayer structure. 
     For this purpose, the present invention provides a lithium secondary battery comprising an electrode assembly including a positive electrode plate, a separator, a negative electrode plate, a positive electrode tab drawn from the positive electrode plate and a clad negative electrode tab drawn from the negative electrode plate and formed of a Ni/Cu bilayer; a can having an open upper part to house the electrode assembly; and a cap assembly for sealing the open upper part of the can, wherein the positive electrode plate, the separator and the negative electrode plate are sequentially wound into a jelly roll configuration.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a lithium secondary battery. More specifically, the present invention relates to a lithium secondary battery which is capable of preventing a high-temperature short circuit by using a clad negative electrode tab having a nickel/copper bilayer structure. 
         [0003]    2. Description of the Related Art 
         [0004]    Generally, a secondary battery is fabricated by housing an electrode assembly and an electrolyte in a can and hermetically sealing an open upper part of the can with a cap assembly. 
         [0005]    In order to increase electrical capacity of the cap assembly, the electrode assembly may be prepared to have a jelly roll structure by stacking a positive electrode plate, a negative electrode plate and a separator disposed therebetween to insulate the electrode plates and winding the resulting stacked structure into a jelly roll shape. Even though there may be some differences depending upon kinds of secondary batteries, the positive and negative electrode plates are formed conventionally by applying an electrode active material to a metal substrate, followed by drying, roll pressing and cutting. In the case of a lithium secondary battery, the positive electrode plate employs a lithium transition metal oxide as an electrode active material, and aluminum (Al) as a current collector. On the other hand, the negative electrode plate employs a carbon or carbon composite as an electrode active material, and copper (Cu) as a current collector. The separator serves to electrically isolate the positive electrode plate from the negative electrode plate so as to avoid the occurrence of a short circuit due to direct contact between two electrode plates. The separator is formed of a microporous film of a polyolefin resin, such as polyethylene, polypropylene, or the like. 
         [0006]    For electrical connection of the electrode assembly to the cap assembly, a positive electrode tab and a negative electrode tab is formed to protrude from an upper part of the electrode assembly. The positive and negative electrode tabs may be formed of aluminum (Al) or nickel (Ni). Conventionally, the positive electrode tab may be formed of aluminum (Al) or an aluminum alloy, whereas the negative electrode tab may be formed of nickel (Ni) or a nickel alloy. 
         [0007]    However, the negative electrode tab made of nickel or nickel alloy suffers from problems associated with production of a large amount of heat upon charging/discharging of the secondary battery, arising from high resistance of Ni per se. Further, since the welding portions between the negative electrode plate and the negative electrode tab and between the cap assembly and the negative electrode tab are joining regions of heterogeneous metal components, internal resistance (IR) increases to result in localization of heat generation. Local concentration of heat may, in tun, cause a high-temperature short circuit, thus causing the danger of explosion of the secondary battery. 
       SUMMARY OF THE INVENTION 
       [0008]    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 lithium secondary battery which is capable of preventing a high-temperature short circuit by provision of a clad negative electrode tab having a nickel/copper bilayer structure. 
         [0009]    It is another object of the present invention to provide a lithium secondary battery with reduced internal resistance and heat generation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is an exploded perspective view of a lithium secondary battery in accordance with an embodiment of the present invention; 
           [0011]      FIG. 2   a  is a perspective view of an electrode assembly in accordance with an embodiment of the present invention, before winding of electrode components; 
           [0012]      FIG. 2   b  is a perspective view of an electrode assembly in accordance with an embodiment of the present invention, after winding of electrode components; 
           [0013]      FIG. 2   c  is a plan view of an electrode assembly in accordance with an embodiment of the present invention; 
           [0014]      FIG. 3   a  is a sectional view of a negative electrode tab in accordance with an embodiment of the present invention; 
           [0015]      FIG. 3   b  is a side plan view of a negative electrode tab in accordance with an embodiment of the present invention; 
           [0016]      FIG. 4   a  is a graph showing the relationship between kinds of negative electrode tabs and a heat generation temperature; and 
           [0017]      FIG. 4   b  is a graph showing the relationship between kinds of negative electrode tabs and a depth of thermal oxidation. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0018]    Now, exemplary embodiments of the present invention will be described in more detail with reference to the accompanying drawings. 
         [0019]      FIG. 1  is an exploded perspective view of a lithium secondary battery in accordance with an embodiment of the present invention.  FIG. 2   a  is a perspective view of an electrode assembly in accordance with an embodiment of the present invention, before winding of electrode components,  FIG. 2   b  is a perspective view of an electrode assembly in accordance with an embodiment of the present invention, after winding of electrode components, and FIG.  2   c  is a plan view of an electrode assembly in accordance with an embodiment of the present invention.  FIG. 3   a  is a side view of a negative electrode tab in accordance with an embodiment of the present invention, and  FIG. 3   b  is a side plan view of a negative electrode tab in accordance with an embodiment of the present invention. Finally,  FIG. 4   a  is a graph showing the relationship between kinds of negative electrode tabs and a heat generation temperature, and  FIG. 4   b  is a graph showing the relationship between kinds of negative electrode tabs and a depth of thermal oxidation. 
         [0020]    Referring to  FIGS. 1 to 3   b , a lithium secondary battery  10  in accordance with an embodiment of the present invention includes an electrode assembly  100 , a can  200  and a cap assembly  300 . The electrode assembly  100  further includes a clad negative electrode tab  127  having a bilayer structure of nickel (Ni)  127   a  and copper (Cu)  127   b . The clad negative electrode tab  127  is a negative electrode tab with improved electrical properties, as compared to a conventional art negative electrode tab. That is, an embodiment of the present invention provides a lithium secondary battery  10  having improved short-circuit characteristics at high temperatures, by using the clad negative electrode tab  127  as a negative electrode tab of the lithium secondary battery  10 . 
         [0021]    The electrode assembly  100  includes a positive electrode plate  110 , a negative electrode plate  120  and a separator  130 . In order to increase electrical capacity, the electrode assembly  100  is conventionally fabricated into a jelly roll structure by stacking the positive electrode plate  110 , the negative electrode plate  120  and the separator  130  disposed therebetween to provide electrical isolation between the electrode plates  110  and  120 , and winding the resulting stacked structure into a jelly roll. 
         [0022]    The positive electrode plate  110  includes a positive electrode current collector  111 , a positive electrode active material layer  113 , a positive electrode non-coating portion  115  and a positive electrode tab  117 . The positive electrode current collector  111  is formed of thin aluminum (Al) foil. The positive electrode active material layer  113  is coated on both sides of the positive electrode current collector  111 . The positive electrode active material layer  113  may be made of a lithium manganese oxide having high stability. The positive electrode non-coating portion  115  is defined as a region of the positive electrode current collector  111  which was not coated with the positive electrode active material layer  113 . The positive electrode non-coating portion  115  may be formed on both ends of the positive electrode current collector  111 . The positive electrode tab  117  is formed to be fixed to the positive electrode non-coating portion  115 . For electrical connection with the cap assembly  300 , one end of the positive electrode tab  117  is formed to protrude upward above the positive electrode current collector  111 , and is formed to protrude upward from an outer periphery of the electrode jelly roll structure. The positive electrode tab  117  may be made of aluminum (Al) or nickel (Ni). The portion with a protrusion of the positive electrode tab  117  is wound with an insulating tape  140  for prevention of an electrode-to-electrode short circuit. 
         [0023]    The negative electrode plate  120  includes a negative electrode current collector  121 , a negative electrode active material layer  123 , a negative electrode non-coating portion  125  and a clad negative electrode tab  127 . The negative electrode current collector  121  is formed of thin copper (Cu) foil. The negative electrode active material layer  123  is coated on both sides of the negative electrode current collector  121 . The negative electrode active material layer  123  may be made of a carbon material. The negative electrode non-coating portion  125  is defined as a region of the negative electrode current collector  121  which was not coated with the negative electrode active material layer  123 . The negative electrode non-coating portion  125  may be formed on both ends of the negative electrode current collector  121 . The clad negative electrode tab  127  is formed to be fixed to the negative electrode non-coating portion  125 . For electrical connection with the cap assembly  300 , one end of the clad negative electrode tab  127  is formed to protrude upward above the negative electrode current collector  121 . The portion with a protrusion of the clad negative electrode tab  127  is wound with an insulating tape  140  for prevention of a short circuit between the electrodes. Further, the clad negative electrode tab  127  is formed to protrude upward from an inner periphery of the electrode jelly roll structure. 
         [0024]    Hereinafter, a clad negative electrode tab in accordance with an embodiment of the present invention will be described in more detail. 
         [0025]    The clad negative electrode tab  127  is made of a bilayer structure of nickel (Ni)  127   a  and copper (Cu)  127   b . Further, the clad negative electrode tab  127  is formed by pressure welding of Ni  127   a  and Cu  127   b . Ni  127   a  is a metal material having a resistance/unit sectional area which is about 4 times higher than that of Cu  127   b . Therefore, when a clad is formed of Ni  127   a  and Cu  127   b , the presence of Cu  127   b  results in lowering of resistance of the electrode tab, so resistance of the electrode tab can be reduced to a half of a conventional negative electrode tab formed of Ni or Ni-containing alloy. According to an embodiment of the present invention, the clad negative electrode tab  127  may exhibit a resistance value of 2.0 to 5.0 mΩ which corresponds to a half reduction of the tab resistance, as compared to when a negative electrode tab of the Ni  127   a  monolayer having the same sectional area exhibits a resistance value of about 7.5 mΩ. That is, the clad negative electrode tab  127  provides reduced heat generation due to having decreased resistance, as compared to a conventional art negative electrode tab. As a result, it is possible to improve high-temperature short circuit characteristics of the lithium secondary battery  10 . The reason why the negative electrode tab is not formed only of low-resistance Cu  127   b  is as follows. When the electrode assembly  100  or the cap assembly  300  is welded with the negative electrode tab, the Cu component is melted by heat. If a large amount of Cu  127   b  is present, spattering of Cu particles may occur upon melting of Cu, which consequently results in a micro short circuit of the lithium secondary battery  10  by fine particles. 
         [0026]    The clad negative electrode tab  127  is preferably formed to have a length (L) of 10 to 50 mm. If a length (L) of the clad negative electrode tab  127  is shorter than 10 mm, it may be difficult to secure a welding space when the negative electrode tab  127  is welded with a negative electrode non-coating portion  125  of the negative electrode plate  120  or is welded with a terminal plate  350  of the cap assembly  300 . On the other hand, if a length (L) of the clad negative electrode tab  127  is longer than 50 mm, it may be likely to result in a short circuit due to potential contact of the electrode tab  127  with the cap plate  310  or the positive electrode tab  117 . Further, since the resistance of an ohmic conductor is proportional to its length, it is meaningless that the clad negative electrode tab  127  has a length (L) larger than a desired size. 
         [0027]    The clad negative electrode tab  127  is preferably formed to have a thickness (T) of 0.05 to 0.15 mm. If a thickness (T) of the clad negative electrode tab  127  is thinner than 0.05 mm, the tab  127  may be broken when it is welded or bent several times in the process of housing the electrode assembly into the can. On the other hand, if a thickness (T) of the clad negative electrode tab  127  is thicker than 0.15 mm, it may result in a prolonged process time when the clad negative electrode tab  127  is welded with the negative electrode non-coating portion  125  of the negative electrode plate  120  or with the terminal plate  350  of the cap assembly  300 . As described above, the clad negative electrode tab  127  is inevitably bent several times in the process of housing the electrode assembly into the can. Therefore, when the clad negative electrode tab  127  is formed to have a thickness (T) of more than 0.15 mm, such a large thickness (T) results in decreased flexibility, which may, in turn, lead to difficulty of installation. 
         [0028]    Further, the clad negative electrode tab  127  is preferably formed to have a width (W) of 2.0 to 5.0 mm. Upon welding with the negative electrode non-coating portion  125  of the negative electrode plate  120  or with the terminal plate  350  of the cap assembly  300 , the clad negative electrode tab  127  is welded through two or more weld points. Therefore, if a width (W) of the clad negative electrode tab  127  is narrower than 2.0 mm, it may be difficult to secure a welding space. On the other hand, if a width (W) of the clad negative electrode tab  127  is wider than 5.0 mm, a welding process requires larger numbers of weld points for firm welding, which results in increased numbers of additional processes, thus lowering the productivity. 
         [0029]    Meanwhile, it is preferred that each layer of Ni  127   a  and Cu  127   b  is formed to have a 5 to 95% thickness of a counterpart layer of the clad negative electrode tab  127 . That is, for example, when the Ni layer  127   a  is formed to have a 5% thickness proportion based on the total thickness of the clad negative electrode tab  127 , the Cu layer  127   b  may have a 95% thickness proportion. On the other hand, when the Ni layer  127   a  is formed to have a 95% thickness proportion of the clad negative electrode tab  127 , the Cu layer  127   b  may be formed to have a 5% thickness proportion of the clad negative electrode tab  127 . If the Ni layer  127   a  has a thickness proportion of less than 5%, an excessive amount of Cu  127   b  may cause a problem associated with spattering of Cu  127   b  during a welding process. On the other hand, if Cu  127   b  is formed to have a thickness proportion of less than 5%, it is difficult to achieve desired reduction of resistance. If Ni  127   a  accounts for a thickness proportion of more than 95%, it is difficult to achieve desired reduction of resistance. On the other hand, if Cu  127   b  is formed to have a thickness proportion of more than 95%, spattering of Cu  127   b  may occur during a welding process. Therefore, a proportion of the as-formed thickness (t 1 , t 2 ) of Ni  127   a  and Cu  127   b  should be set taking into consideration the resistance and spattering of the clad negative electrode tab  127 . It is preferred that Ni  127   a  and Cu  127   b  have the same layer thickness. 
         [0030]    One end of the clad negative electrode tab  127  is welded with the negative electrode plate  120 , whereas the other end of the clad negative electrode tab  127  is welded with the cap assembly  300 . More specifically, the negative electrode non-coating portion  125  of the negative electrode plate  120  is welded in contact with one end of the Cu layer  127   b  of the clad negative electrode tab  127 , and a welding rod is in contact with the Ni layer  127   a . Further, the terminal plate  350  of the cap assembly  300  is welded in contact with the other end of the Cu layer  127   b  of the clad negative electrode tab  127 , and a welding rod is in contact with the Ni layer  127   a . As described above, welding of the clad negative electrode tab  127  with the negative electrode plate  120  or the cap assembly  300  may be carried out using any conventional method selected from ultrasonic welding, laser welding, and resistance welding. 
         [0031]    In order to improve the bonding strength upon welding with the negative electrode plate  120  or the cap assembly  300 , the clad negative electrode tab  127  may be welded in at least two weld points (a 1 , a 2 ). When the spacing between two weld points a 1  and a 2  is narrow, there is no significant difference when compared with single-point welding. Therefore, it is preferred that the weld points (a 1 , a 2 ) are formed spaced apart on the clad negative electrode tab  127 . Of course, the weld points (a 1 , a 2 ) may also be additionally formed to further improve the bonding strength between the clad negative electrode tab  127  and the negative electrode plate  120  or the cap assembly  300 . 
         [0032]    The separator  130  prevents a short circuit between the positive electrode plate  10  and the negative electrode plate  120 , and serves as a migration path of lithium ions. The separator  130  is formed of polyethylene or polypropylene, even though there is no particular limit to the material for the separator  130 . 
         [0033]    In the polygonal secondary battery, the can  200  has a generally rectangular parallelepiped shape made of metal, which has an open-end part and is formed by a processing method such as deep drawing. The can  200  may be formed of an aluminum alloy or aluminum that is a light-weight conductive metal. Therefore, the can  200  can also serve as a terminal. The can  200  serves as a container of the electrode assembly  100  and the electrolyte, and has an open upper part to allow insertion of the electrode assembly  100  and is hermetically sealed by the cap assembly  300 . 
         [0034]    The cap assembly  300  includes a cap plate  310 , a gasket  320 , an electrode terminal  330 , an insulation plate  340 , a terminal plate  350 , an insulating case  360  and a plug  370 . 
         [0035]    The cap plate  310  includes a terminal through-hole  311  and an electrolyte injection hole  313 . The terminal through-hole  311  provides a path through which the electrode terminal  330  is inserted. For insulation of the metallic cap plate  310  from the electrode terminal  330 , the electrode terminal  330  is inserted into the terminal through-hole  311  after the gasket  320  made of an insulating material is positioned around an exterior surface of the electrode terminal  330 . One side of the cap plate  310  is provided with an electrolyte injection hole  313  for injection of an electrolyte into the can  200 . After injection of the electrolyte is complete, the electrolyte injection hole  313  is sealed with a plug  370  to prevent leakage of the electrolyte. 
         [0036]    The insulating plate  340  is installed below the cap plate  310 . Below the insulating plate  340  is provided a terminal plate  350 . Therefore, the insulating plate  340  provides insulation between the cap plate  310  and the terminal plate  350 . Meanwhile, the terminal plate  350  is formed to be coupled with a lower end of the electrode terminal  330 . Therefore, the negative electrode plate  120  of the electrode assembly  100  is electrically connected to the electrode terminal  330  through the clad negative electrode tab  127  and the terminal plate  350 . The positive electrode plate  110  of the electrode assembly  100  is electrically connected to the cap plate  310  or the can  200  through the positive electrode tab  117 . 
         [0037]    The insulating case  360  is installed below the terminal plate  350 . The insulating case  360  includes a negative electrode tab pass-through portion  361 , a positive electrode tab pass-through portion  363  and an electrolyte inlet  365 . 
         [0038]    The plug  370  is used to hermetically seal the electrolyte injection hole  313  after injection of the electrolyte into the hole  313  formed on the cap plate  310 . As an alternative to the plug  370 , a ball may be press-fitted to seal the electrolyte injection hole  313 . 
         [0039]    As described above, the lithium secondary battery  10  in accordance with an embodiment of the present invention is provided with the clad negative electrode tab  127  having a bilayer structure of Ni  127   a  and Cu  127   b . The clad negative electrode tab  127  exhibits lower resistance as compared to that of a conventional art. Therefore, according to the embodiment of the present invention, it is possible to improve high-temperature short circuit characteristics of the lithium secondary battery  10 . That is, according to the embodiment of the present invention, resistance of the lithium secondary battery  10  can be decreased to thereby result in reduction of heat generation in the lithium secondary battery  10 , ultimately by which the lithium secondary battery  10  can be protected against the risk of explosion and malfunction. 
         [0040]    Table 1 shows the resistance, resistivity, heat generation temperature and thermal oxidation depth measured for individual metals used as an electrode tab material.  FIGS. 4   a  and  4   b  graphically show the measured values of Table 1. Hereinafter, an explanation will be given with reference to Table 1 and  FIGS. 4   a  and  4   b.    
         [0000]    
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                   
                   
                 Oxidation 
               
               
                   
                   
                 Tab IR 
                 Resistivity 
                 Temp. 
                 depth 
               
               
                   
                 Spec. 
                 [mmΩ] 
                 [Ω · m] 
                 [° C.] 
                 [mm] 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Embod- 
                 Ni/Cu 
                 L: 3 mm 
                 3.3 
                 2.52E−8 
                 52.0 
                 0.0 
               
               
                 iment 1 
                 clad 
                 T: 0.1t 
               
               
                 Comp. 
                 Cu tab 
                 L: 4 mm 
                 1.6 
                 1.72E−8 
                 45.7 
                 0.0 
               
               
                 Ex. 1 
                   
                 T: 0.1t 
               
               
                 Comp. 
                 Ni tab 
                 L: 4 mm 
                 7.5 
                 9.13E−8 
                 108.7 
                 11.3 
               
               
                 Ex. 2 
                   
                 T: 0.1t 
               
               
                 Comp. 
                 Ni tab 
                 L: 3 mm 
                 11.5 
                 8.86E−8 
                 124.3 
                 12.0 
               
               
                 Ex. 3 
                   
                 T: 0.1t 
               
               
                 Comp. 
                 Ni tab 
                 L: 4 mm 
                 14.3 
                 11.1E−8 
                 134.0 
                 14.7 
               
               
                 Ex. 4 
                   
                 T: 0.05t 
               
               
                 Comp. 
                 Ni tab 
                 L: 4 mm 
                 16.8 
                 13.0E−8 
                 35.3 
                 0.0 
               
               
                 Ex. 5 
                   
                 T: 0.05t 
               
               
                   
                   
                 (notch) 
               
               
                   
               
             
          
         
       
     
         [0041]    In Table 1 above, Embodiment 1 shows the internal resistance, resistivity, heat generation temperature and oxidation depth measured for the clad negative electrode tab  127  having a bilayer structure of Ni  127   a  and Cu  127   b . Comparative Example 1 shows the internal resistance, resistivity, heat generation temperature and oxidation depth measured for the Cu electrode tab, whereas Comparative Examples 2 to 5 show the internal resistance, heat generation temperature and oxidation depth of the Ni electrode tab with respect to length (L) and thickness (T) thereof, in conjunction with resistivity of tab materials. 
         [0042]    The clad negative electrode tab  127  of Embodiment 1 exhibited lower resistance and resistivity, as compared to the Ni electrode tabs of Comparative Examples 2 to 4. Further, the clad negative electrode tab  127  of Embodiment 1 exhibited a relatively low heat generation temperature, as compared to the Ni electrode tabs of Comparative Examples 2 to 4. Further, it can be seen that the clad negative electrode tab  127  of Embodiment 1 exhibits substantially no formation of a thermal oxide film. That is, as shown in Table 1, it can be seen that the heat generation temperature increases as the resistance is higher, whereby an insulating thermal oxide film is formed on the electrode plate surface. 
         [0043]    The Cu electrode tab of Comparative Example 1 exhibited low resistance and resistivity values, whereby the heat generation temperature is low and a thermal oxide is not substantially formed. However, as discussed hereinbefore, the electrode tab made only of Cu was not employed due to the potential problem of copper scattering. 
         [0044]    On the other hand, Comparative Example 5 shows the internal resistance, heat generation temperature and oxide depth measured for the Ni electrode tab with formation of a notch. The Ni electrode tab of Comparative Example 5 exhibited a relatively low heat generation temperature and no formation of a thermal oxide, but had a disadvantage of high resistance. 
         [0045]    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.