Abstract:
A secondary battery that includes a cylindrical can, an electrode assembly arranged in a jelly-role configuration within the cylindrical can and having a core extending about an axis thereof and a hollow center pin arranged within the core of the electrode assembly and having an inner diameter and an outer diameter, the outer diameter forming ones of a pair of radial lengths diametrically opposite from each other, each of said pair of radial lengths extending from the outer diameter of the center pin to an external surface of the core, wherein the sum of the pair of radial lengths is in the range of 5% to 54% of the inner diameter of the center pin.

Description:
CLAIM OF PRIORITY 
       [0001]    This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. §119 from an application earlier filed in the Korean Intellectual Property Office on 24 Aug. 2009 and there duly assigned Serial No. 10-2009-0078159. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a cylindrical secondary battery, and more particularly, to a cylindrical secondary battery having a center pin capable of improved exhaustion of heat and gas resulting in improved safety by preventing combustion and explosion. 
         [0004]    2. Description of the Related Art 
         [0005]    Generally, as handheld wireless devices including video cameras, mobile phones and notebook computers are getting compact and multi-functionalized, research on secondary batteries used as a driving power source for these devices is being conducted. 
         [0006]    Secondary batteries can be embodied as a nickel-cadmium (Ni—Cd) battery, a nickel-metal hydride (Ni-MH) battery, a nickel-zinc (Ni—Zn) battery, or a lithium (Li) secondary battery. Among them, the Li secondary battery is rechargeable and compact, has a high capacity, a high operating voltage and a high energy density per unit weight, resulting in it being widely used in high technology electronic devices. 
         [0007]    The secondary batteries can be classified into cylindrical, prismatic and pouch types depending on the shape of a battery case. Here, cylindrical or prismatic type includes an electrode assembly arranged in a cylindrical or prismatic metal can, and the pouch type includes an electrode assembly arranged in a pouch-type case made out of an aluminum-laminated sheet. 
         [0008]    In addition, the electrode assembly in the battery case is a rechargeable power-generating device arranged in a stacked structure of a positive electrode, a separator and a negative electrode. The electrode assembly is classified into a jelly-roll type in which a separator is interposed between a positive electrode and a negative electrode, which are formed in long sheets on which an active material is applied, and a stack type in which a plurality of positive and negative electrodes having a predetermined size are sequentially stacked at opposite sides of a separator. The jelly-roll type electrode assembly is easily manufactured and has a high energy density per unit weight, and thus is mainly used in cylindrical and prismatic batteries. 
         [0009]    The cylindrical Li secondary battery includes an electrode assembly wound in a cylindrical shape, a cylindrical can housing the electrode assembly, an electrolyte to allow ions to move, and a cap assembly connected to one side of the cylindrical can to prevent leakage of the electrolyte and prevent separation of the electrode assembly from the can. 
         [0010]    Meanwhile, during overcharge or overcurrent, the secondary batteries increase in inner temperature, thereby generating gas. The gas flows toward the cap assembly through a core of the wound electrode assembly and a center pin arranged in the core prevents deformation of the electrode assembly during the charging and discharging of the battery. 
         [0011]    A safety vent of the cap assembly is deformed due to the flowing gas, and a circuit in the battery is broken due to the safety vent, thereby preventing the combustion and explosion of the secondary battery. 
         [0012]    However, in recent times, as the number of windings in the electrode assembly increases to enlarge the capacity of the cylindrical secondary battery, the diameter of the center of the electrode assembly is reduced. 
         [0013]    Accordingly, when the cylindrical secondary battery is combusted, the electrode assembly is deformed due to heat. Through the narrow core, heat and gas generated in the secondary battery find it difficult to be exhausted, resulting in deterioration of safety and reliability of the secondary battery. 
       SUMMARY OF THE INVENTION 
       [0014]    Aspects of the present invention provide a cylindrical secondary battery in which a center pin is formed to have an optimized inner diameter with respect to a diameter of a core to prevent combustion and explosion, resulting in improved safety and reliability. 
         [0015]    According to one aspect of the present invention, there is provided a secondary battery that includes a cylindrical can, an electrode assembly arranged in a jelly-role configuration within the cylindrical can and having a core extending about an axis thereof and a hollow center pin arranged within the core of the electrode assembly and having an inner diameter and an outer diameter, the outer diameter forming ones of a pair of radial lengths diametrically opposite from each other, each of said pair of radial lengths extending from the outer diameter of the center pin to an external surface of the core, wherein the sum of the pair of radial lengths is in the range of 5% to 54% of the inner diameter of the center pin. 
         [0016]    The electrode assembly can include a first electrode plate, a second electrode plate and a separator arranged between the first and second electrode plates. The inner diameter of the center pin can be inversely proportional to the sum of the pair of radial lengths. The core can have a diameter in the range of 3.8 to 4.2 mm. When the core has a diameter of approximately 4.2 mm, and the center pin has a thickness of approximately 0.1 mm, the center pin can have an inner diameter in the range of 2.6 to 3.8 mm, and the sum of the radial lengths from the external surface of the core to the outer diameter of the center pin can be in the range of 0.2 to 1.4 mm. The center pin can have a vertical length in the range of 57 to 59 mm. 
         [0017]    According to another aspect of the present invention, there is provided a secondary battery that includes a cylindrical can, an electrode assembly arranged in a jelly-role configuration within the cylindrical can and in snug contact with an inner surface of said cylindrical can and having a core extending about an axis thereof and a hollow center pin having a closed circular cross section arranged concentrically within the core of the electrode assembly and having an inner diameter I and an outer diameter, a first radial distance D being defined as a distance from the outer diameter of the center pin to a nearest point on an external surface of the core, wherein the radial distance D and the inner diameter I satisfy the inequality 0.05 I≦2D≦0.54 I. 
         [0018]    The electrode assembly can include a first electrode plate, a second electrode plate and a separator arranged between the first and second electrode plates. The inner diameter of the center pin can be inversely proportional to the radial distance D. The core can have a diameter in the range of 3.8 to 4.2 mm When the core has a diameter of approximately 4.2 mm, and when the center pin has a thickness of approximately 0.1 mm, the center pin can have an inner diameter in the range of 2.6 to 3.8 mm, and 2D is in the range of 0.2 to 1.4 mm The center pin can have a vertical length in the range of 57 to 59 mm. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    A more complete appreciation of the invention and many of the attendant advantages thereof, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein: 
           [0020]      FIG. 1  is a cross-sectional view of a cylindrical secondary battery according to an exemplary embodiment of the present invention; 
           [0021]      FIG. 2  is a schematic cross-sectional view illustrating a core and a center pin of  FIG. 1 ; and 
           [0022]      FIG. 3  is a schematic plan view illustrating the core and the center pin of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    Reference will now be made in detail to the present embodiments of the present invention, examples of which are shown in the accompanying drawings. Further, in the drawings, the length and thickness of a layer and a region can be exaggerated for convenience. In addition, like reference numerals refer to the like elements throughout the specification. 
         [0024]      FIG. 1  is a cross-sectional view of a cylindrical secondary battery according to an exemplary embodiment of the present invention,  FIG. 2  is a schematic cross-sectional view illustrating a core and a center pin of  FIG. 1 , and  FIG. 3  is a schematic plan view illustrating the core and the center pin of  FIG. 1 . 
         [0025]    Referring to  FIG. 1 , a cylindrical secondary battery  100  according to an exemplary embodiment of the present invention includes a cylindrical can  110 , an electrode assembly  120 , a cap assembly  130 , and a center pin  150 . The cylindrical can  110  houses the electrode assembly  120  and an electrolyte, and the electrode assembly  120  serves as a rechargeable power-generating device. The cap assembly  130  is assembled to an upper portion of the cylindrical can  110 , thereby sealing the cylindrical can  110  to prevent separation of the electrode assembly  120  from the cylindrical can  110 . 
         [0026]    The center pin  150  is arranged at a core  152  of the electrode assembly  120  to prevent deformation of the electrode assembly  120  during charging or discharging of the secondary battery, and serves as a path to allow gases generated in the cylindrical can  110  to escape. 
         [0027]    The electrode assembly  120  can include a first electrode plate  122  on which a positive electrode active material is coated, a second electrode plate  126  on which a negative electrode active material is coated, and a separator  124  interposed between the first and second electrode plates  122  and  126 . 
         [0028]    The first electrode plate  122  serves as a positive electrode plate and can be made out of a conductive metal foil such as an aluminum (Al) foil, and can include positive electrode non-coating portions at both ends of the first electrode plate  122  on which the positive electrode active materials are not coated. 
         [0029]    The second electrode plate  126  serves as a negative electrode plate and can be made out of a conductive metal foil such as copper (Cu) or nickel (Ni) foil and can include negative electrode non-coating portions at both ends of the second electrode plate  126  on which the negative electrode active materials are not coated. 
         [0030]    The electrode assembly  120  can include a first electrode tab  128  made out of Al and extending a predetermined length from the positive electrode non-coating portions of the first electrode plate  122 . In addition, the electrode assembly  120  can include a second electrode tab  129  made out of Ni and extending a predetermined length from the negative electrode non-coating portion of the second electrode plate  126 . 
         [0031]    The separator  124  prevents an electrical short circuit between the first and second electrode plates  122  and  126 , and allows ions to move. 
         [0032]    The electrode assembly  120  is wound in a circular direction, which means that the first electrode plate  122 , the separator  124  and the second electrode plate  126  are wound in a jelly-roll configuration, and then housed within the cylindrical can  110 . 
         [0033]    The core  152  formed by the electrode assembly  120  can have a larger diameter than a conventional core in order to facilitate exhaustion of heat and gas which can be generated by the secondary battery. For example, the diameter of the core can be about 3.8 to 4.2 mm. 
         [0034]    The electrode assembly  120  can be arranged to be tightly fitted to the cylindrical can  110  to reduce a void between the cylindrical can  110  and the electrode assembly  120 . By preventing the formation of a void between the cylindrical can  110  and an exterior of the electrode assembly, heat and gas generated by the secondary battery can be easily exhausted through the core  152  without occupying and stagnating in a space between the electrode assembly  120  and the cylindrical can  110 . 
         [0035]    Upper and lower insulating plates  142  and  144  can be arranged at upper and lower portions of the electrode assembly  120  to prevent direct contact between the cap assembly  130  and the cylindrical can  110 . 
         [0036]    The cylindrical can  110  can include a cylindrical side  112  having a predetermined diameter and a bottom  114  covering a lower portion of the side  112  to have a predetermined space capable of housing the electrode assembly  120 , and an upper portion of the cylindrical can  110  that is open to allow for insertion of the electrode assembly  120  therein. 
         [0037]    The second electrode tab  129  connected to the negative electrode non-coating portion of the electrode assembly  120  can be joined to a center of the bottom  114  of the cylindrical can  110 , and thus the cylindrical can  110  can be in electrical contact with the second electrode plate  126  to serve as a negative electrode terminal. 
         [0038]    The cylindrical can  110  can have a crimped portion  118  formed by bending an upper portion of the cylindrical can  110  inward to press an upper portion of the cap assembly  130 . Here, the cap assembly  130  is connected to an opening, which is arranged at an upper portion of the cylindrical can  110 . 
         [0039]    The cylindrical can  110  can also have a beaded portion  116  formed by bending the cylindrical can  110  inward to fix the cap assembly  130  in place between the beaded portion  116  and the crimped portion  118 . The location of the beaded portion  116  is spaced from the crimping portion by a distance that corresponds to the thickness of the cap assembly  130 . 
         [0040]    The cap assembly  130  can include a safety vent  132 , a current breaker  134 , a positive temperature coefficient (PTC) element  136 , an electrode cap  138  and an insulating gasket  140 . 
         [0041]    The safety vent  132  can be made out of metal, joined to the first electrode tab  128  and thus electrically connected to the positive first electrode plate  122 . The safety vent  132  can have a protrusion in the middle, which projects toward the bottom  114  of the cylindrical can  110 . The protrusion can be deformed so that it projects in an upward direction of the cylindrical can  110  or be broken when pressure within the cylindrical secondary battery  100  builds. 
         [0042]    The current breaker  134  can be electrically and mechanically connected to the safety vent  132  at an upper portion of the safety vent  132 , and thus can be broken when the safety vent  132  is deformed or broken. The PTC element  136  is electrically and mechanically connected to the current breaker  134  at an upper portion of the current breaker  134 . The electrode cap  138  can be electrically and physically connected to the PTC element  136  at an upper portion of the PTC element  136  to serve as a terminal to apply a current to the outside. 
         [0043]    The insulating gasket  140  can surround sides of the safety vent  132 , the current breaker  134 , the PTC element  136  and the electrode cap  138  to fix the safety vent  132 , the current breaker  134 , the PTC element  136  and the electrode cap  138  these elements in place. In addition, the insulating gasket  140  can insulate the cylindrical can  110  which serves as the negative electrode from the electrode cap  138  serves as the positive electrode. 
         [0044]    Referring to  FIGS. 2 and 3 , the center pin  150  has a center hole  154 , and can be made out of a metal such as stainless steel which has excellent mechanical strength and chemical resistance. The center pin  150  is arranged in the core  152  to prevent deformation or movement of the electrode assembly  120 . Moreover, the center pin  150  can serve as a path to allow gas generated in the cylindrical can  110  to escape toward the cap assembly  130 . 
         [0045]    The center pin  150  can be arranged to create an external space  156  between an external surface of the center pin  150  and an internal boundary of the electrode assembly  120  in order to prevent damage to the separator  124  of electrode assembly  120  that forms an outer boundary of the core  152  and the inner boundary of the electrode assembly  120 . 
         [0046]    The core  152  can be formed to have a larger diameter than the conventional core to provide for easy exhaust of heat and gas which can be generated within the secondary battery. The diameter of the core can be about 3.8 to 4.2 mm. 
         [0047]    The center pin  150  can be formed to have a greater vertical length L 5  than a conventional center pin, for example, about 57 to 59 mm, and have a thickness L 2  of about 0.1 mm so that the gas and heat produced by the electrode assembly  120  can easily escape. 
         [0048]    When the diameter L 1  of the core  152  and the thickness L 2  of the center pin  150  are fixed, an inner diameter L 3  of the center pin  150  can be inversely proportional to a length L 4  of the external space  156 . That is, in the cross-sectional view, when the inner diameter L 3  (or I) of the center pin  150  increases, the lengths L 4  (or D) of the external spaces  156  arranged at both sides of the center pin  150  decreases. 
         [0049]    In the cross-sectional view, the sum of the lengths L 4  of both external spaces  156  extending from the external surface  158  of the core  152  to the outer diameter of the center pin  150  can be 5 to 54% of the inner diameter L 3  of the center pin  150  when the diameter L 1  of the core  152  and the thickness L 2  of the center pin  150  are fixed. This can be expressed by the following inequality. 
         [0050]    Inequality 1: 0.05L 3 ≦2L 4 ≦0.54L 3    
         [0051]    Hereinafter, the following examples are provided to explain the present invention in further detail, but the scope of the present invention is not limited to these examples. 
       Example 1 
       [0052]    As shown in  FIGS. 1 to 3 , a center pin  150  made out of metal was manufactured to have a thickness L 2  of 0.1 mm, a height L 5  of 59 mm, and an inner diameter L 3  of 2.6 to 3.8 mm. The center pin  150  was inserted into a core  152  of an electrode assembly  120  having a diameter L 1  of 4.2 mm, and thus a cylindrical secondary battery was completed. 
       Comparative Example 1 
       [0053]    As shown in  FIGS. 1 to 3 , a center pin  150  made out of metal was manufactured to have a thickness L 2  of 0.1 mm, a height L 5  of 59 mm, and an inner diameter L 3  of 0.5 to 2.3 mm The center pin  150  was inserted into a core  152  of an electrode assembly  120  having a diameter L 1  of 4.2 mm, and thus a cylindrical secondary battery was completed. 
       Experimental Example 1 
       [0054]    As shown in Table 1, combustion tests were performed on 10 samples of each of 7 kinds of cylindrical secondary batteries manufactured in Example 1 and 6 kinds of cylindrical secondary batteries manufactured in Comparative example 1. The combustion tests were performed by heating a horizontal middle portion of the cylindrical secondary battery for a predetermined time using a torch, and confirming whether the cylindrical secondary battery ignites or explodes due to the heat applied thereto. 
         [0000]    
       
         
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                   
                   
                 Distance between 
                   
                   
               
               
                   
                   
                 Thickness 
                   
                 Inner 
                 Outer Diameter of 
               
               
                   
                 Diameter 
                 of Center 
                 Height of 
                 diameter of 
                 Center Pin and 
                   
                 Success Rate for 10 
               
               
                   
                 of Core 
                 Pin 
                 Center Pin 
                 Center Pin 
                 External Surface of 
                 (L4 × 2)/L3 
                 Samples 
               
               
                   
                 L1(mm) 
                 L2(mm) 
                 L5(mm) 
                 L3(mm) 
                 Core L4(mm) 
                 (%) 
                 (%) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Example 1 
                 S1 
                 4.2 
                 0.1 
                 59 
                 3.8 
                 0.1 
                 5 
                 100 
               
               
                   
                 S2 
                   
                   
                   
                 3.6 
                 0.2 
                 11 
                 100 
               
               
                   
                 S3 
                   
                   
                   
                 3.4 
                 0.3 
                 18 
                 100 
               
               
                   
                 S4 
                   
                   
                   
                 3.2 
                 0.4 
                 25 
                 100 
               
               
                   
                 S5 
                   
                   
                   
                 3.0 
                 0.5 
                 33 
                 100 
               
               
                   
                 S6 
                   
                   
                   
                 2.8 
                 0.6 
                 43 
                 100 
               
               
                   
                 S7 
                   
                   
                   
                 2.6 
                 0.7 
                 54 
                 100 
               
               
                 C. example 
                 S1 
                   
                   
                   
                 2.3 
                 0.9 
                 78 
                 60 
               
               
                 1 
                 S2 
                   
                   
                   
                 2.1 
                 1.0 
                 95 
                 50 
               
               
                   
                 S3 
                   
                   
                   
                 1.8 
                 1.1 
                 122 
                 50 
               
               
                   
                 S4 
                   
                   
                   
                 1.5 
                 1.3 
                 173 
                 30 
               
               
                   
                 S5 
                   
                   
                   
                 1.0 
                 1.5 
                 300 
                 40 
               
               
                   
                 S6 
                   
                   
                   
                 0.5 
                 1.8 
                 720 
                 10 
               
               
                   
               
             
          
         
       
     
         [0055]    As a result, among the samples of the cylindrical secondary batteries according to Example 1, when the diameter L 1  of the core  152  and the thickness L 2  of the center pin  150  are fixed, as seen from the cross-sectional view, it can be concluded that samples having the sum of the both lengths L 4  from the external surface  158  of the core  152  to the outer diameter of the center pin  150 , that amounts from 5 to 54% of the inner diameter L 3  of the center pin  150 , was neither combusted nor exploded. That is, it can be concluded that although the shape of the electrode assembly in the secondary battery is deformed due to heat, the center pin  150  can easily exhaust the gas and heat generated inside, thereby preventing the combustion or explosion of the battery. 
         [0056]    On the other hand, it can be concluded that 4 to 9 out of 10 samples among each of the cylindrical secondary batteries according to Comparative example 1 combusted or exploded when the sum of the both lengths L 4  from the external surface  158  of the core  152  to the outer diameter of the center pin  150  exceeded 54% of the inner diameter L 3  of the center pin  150 . 
         [0057]    As described above, in the cylindrical secondary battery in which the cylindrical can is tightly fitted to the electrode assembly to increase the diameter of the core of the electrode assembly and reduce an outside void volume, the inner diameter of the center pin is controlled to have a predetermined distance ratio with respect to a distance between the outer diameter of the center pin arranged in the core and the external surface of the core. This is done for a center pin having an increased vertical length. 
         [0058]    That is, when the diameter of the core and the thickness and the vertical length (height) of the center pin are fixed as seen from the cross-sectional view, the cylindrical to secondary battery can have the center pin, in which the sum of both lengths from the external surface of the core to the outer diameter of the center pin is 5 to 54% of the inner diameter of the center pin. 
         [0059]    Accordingly, the heat and gas generated in the secondary battery does not remain in a space between the electrode assembly and the cylindrical can, and can be exhausted through the core. Although the shape of the electrode assembly in the secondary battery may deform, the heat and gas generated inside the can be easily exhausted through the center pin. 
         [0060]    In addition, as the vertical length of the center pin increases, the gas and heat generated inside can easily escape, thereby reducing an amount of the gas remaining at an upper portion of the center pin. Moreover, as the vertical length of the center pin increases, noise generated in the secondary battery can also be reduced. 
         [0061]    Therefore, due to the easy escape the heat and gas which can be generated in the secondary battery, the combustion or explosion of the cylindrical secondary battery can be prevented, resulting in an improvement in safety and reliability of the cylindrical secondary battery. 
         [0062]    According to the present invention, it is possible to manufacture a cylindrical secondary battery where a cylindrical can is tightly fitted to an electrode assembly to increase a diameter of a core of the electrode assembly, and where an inner diameter of the center pin is controlled such that a distance between an outer diameter of the center pin arranged in the core and an external surface of the core is from 5 to 54% of the inner diameter of the center pin, and where a vertical length of the center pin is increased. 
         [0063]    Thus, heat and gas generated in the secondary battery can be easily exhausted through the core and the center pin without remaining in a space between the electrode assembly and the cylindrical can. Even though the shape of the electrode assembly in the secondary battery may deform, the gas and heat inside can easily escape through the center pin. 
         [0064]    Consequently, due to easy exhaustion of heat and gas generated in the secondary battery, combustion or explosion of the cylindrical secondary battery can be prevented, resulting in an improvement in safety and reliability of the cylindrical secondary battery. 
         [0065]    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 can be made in this embodiment without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.