Patent Publication Number: US-2013230745-A1

Title: Current interrupting device and secondary battery using the same

Description:
CLAIM OF PRIORITY 
     This application claims the benefit of Korean Patent Application No. 10-2012-0022410, filed on Mar. 5, 2012 in the Korean Intellectual Property Office, the disclosures of which are incorporated herein in their entirety by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     One or more embodiments of the present invention relate to a current interrupting device and a secondary battery using the same, the current interrupting device being designed to withstand external impact. 
     2. Description of the Related Art 
     Along with technical developments and increased production of mobile devices, such as mobile phones and laptop computers, demand for secondary batteries as power sources is rapidly increasing. For safety, such secondary batteries may include safety devices for detecting malfunctions thereof, such as overheating and overcurrent, and taking appropriate action for protecting the secondary battery, such as current interruption. 
     SUMMARY OF THE INVENTION 
     One or more embodiments of the present invention include a current interrupting device that is resilient to an external force and a secondary battery including the same. 
     Additional aspects will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of the presented embodiments. 
     According to one or more embodiments of the present invention, there is provided a current interrupting device that includes a thermal fuse, a pair of conductive plates respectively connected to two opposite ends of the thermal fuse and a sealing member that surrounds and seals the thermal fuse, wherein each of the conductive plates comprises a connecting unit that is connected to the thermal fuse, and wherein at least one of the pair of conductive plates comprises a deformation inducing unit that is arranged adjacent to the connecting unit and has a smaller cross-sectional area than a cross-sectional area of the connecting unit. The cross-sectional area of the deformation inducing unit may be from about 30% to about 50% of the cross-sectional area of the connecting unit. The deformation inducing unit may include a notch arranged in a widthwise direction of the current interrupting device. The deformation inducing unit comprises a notch arranged in a thickness-wise direction of the current interrupting device. A thickness of the connecting unit and a thickness of the deformation inducing unit may be smaller than a thickness of a body unit arranged opposite from the connecting unit of the conductive plate. The connecting units may be surrounded and sealed by the sealing member, and each of the at least one deformation inducing unit may be external and adjacent to the sealing member. 
     According to another aspect of the present invention, there is provided a secondary battery that includes an electrode assembly including a positive electrode plate, a separator, and a negative electrode plate, a can including an opening and a space to accommodate both the electrode assembly and an electrolyte, a cap plate to seal the opening of the can and a current interrupting device arranged inside the can, wherein the current interrupting device includes a thermal fuse, a pair of conductive plates that are respectively connected to two opposite ends of the thermal fuse and a sealing member that surrounds and seals the thermal fuse and the connections between the conductive plates and the thermal fuse, each of the conductive plates includes a connecting unit that is connected to the thermal fuse and a deformation inducing unit arranged adjacent to the connecting unit, wherein a cross-sectional area of the deformation inducing unit is smaller than a cross-sectional area of the connecting unit. 
     The cross-sectional area of the deformation inducing unit may be from about 30% to about 50% of the cross-sectional area of the connecting unit. The cross-sectional area of the connecting unit and the cross-sectional area of the deformation inducing unit may be smaller than a cross-sectional area of a body unit arranged opposite from the connecting unit of the conductive plate. The deformation inducing unit may include a notch arranged in a widthwise direction of the current interrupting device. The deformation inducing unit may also or instead include a notch arranged in a thickness-wise direction of the current interrupting device. A width of the deformation inducing unit may be smaller than a width of the connecting unit. A thickness of the deformation inducing unit may be smaller than a thickness of the connecting unit. The secondary battery may also include an electrode terminal including a first end exposed to an outside via a top surface of the cap plate and a second end penetrating through the cap plate and being combined with the current interrupting device and an electrode tab extending from the electrode assembly to the current interrupting device. The pair of conductive plates may include a first conductive plate perforated by an aperture, the electrode terminal extending through the aperture, and a second conductive plate including a groove that combines with a bottom surface of the cap plate. A thickness of the connecting unit and a thickness of the deformation inducing unit may be smaller than the thickness of a body unit arranged opposite from the connecting unit of the conductive plate. A width of the connecting unit and a width of the deformation inducing unit may be smaller than a width of a body unit arranged opposite from the connecting unit of the conductive plate. Each connecting unit may be surrounded and sealed by the sealing member while each deformation inducing unit may be arranged adjacent and external to the sealing member. The sealing member may include a lower film arranged at bottom surfaces of the pair of the conductive plates, an upper film arranged at top surfaces of the pair of the conductive plates; and a flux arranged on the thermal fuse to prevent corrosion of the thermal fuse, the flux and the thermal fuse being arranged in between the upper and lower films. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  is a schematic exploded perspective view of a secondary battery according to an embodiment of the present invention; 
         FIG. 2  is a partial cross-sectional view of the secondary battery of  FIG. 1 ; 
         FIG. 3  is a schematic cross-sectional view of a current interrupting device according to a first embodiment of the present invention; 
         FIG. 4  is an exploded perspective view of first and second conductive plates, a lower film, and a thermal fuse of the current interrupting device shown in  FIG. 3 ; 
         FIG. 5  is a plan view of the first and second conductive plates and a lower film of the current interrupting device shown in  FIG. 3 ; 
         FIG. 6  is a schematic cross-sectional view of a current interrupting device according to a second embodiment of the present invention; 
         FIG. 7  is an exploded perspective view of first and second conductive plates and a thermal fuse of the current interrupting device shown in  FIG. 6 ; 
         FIG. 8  is a plan view of the first and second conductive plates and a lower film of the current interrupting device shown in  FIG. 6 ; and 
         FIG. 9  is an exploded perspective view of first and second conductive plates and a thermal fuse of a current interrupting device according to a third embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. In this regard, the present embodiments may have different forms and should not be construed as being limited to the descriptions set forth herein. Accordingly, the embodiments are merely described below, by referring to the figures, to explain aspects of the present description. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. Expressions such as “at least one of,” when preceding a list of elements, modify the entire list of elements and do not modify the individual elements of the list. 
     The present invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. The invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to those skilled in the art. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. It will be understood that although the terms first and second are used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another element. 
     Turning now to  FIGS. 1 and 2 ,  FIG. 1  is a schematic exploded perspective view of a secondary battery  100  according to an embodiment of the present invention, and  FIG. 2  is a partial cross-sectional view of the secondary battery  100 . Referring to  FIGS. 1 and 2 , the secondary battery  100  includes an electrode assembly  112 , a can  111  in which the electrode assembly  112  is accommodated, a cap plate  121  for sealing an opening of the can  111 , and a current interrupting device  140  arranged inside the can  111 . 
     The electrode assembly  112  may include a negative electrode plate  112   a  and a positive electrode plate  112   b,  to which electrode active materials are applied, and a separator  112   c  interposed therebetween. The electrode assembly  112  may be formed by forming a stacked structure in which the negative electrode plate  112   a,  the separator  112   c,  and the positive electrode plate  112   b  are stacked in the order stated and winding the stacked structure to produce a jelly-roll configuration. The negative electrode plate  112   a  and the positive electrode plate  112   b  may be electrically connected to first and second electrode tabs  113  and  114 , which are arranged for transferring charges formed in chemical reactions to outside, respectively. 
     The electrode assembly  112  may be accommodated inside the can  111  while being impregnated with an electrolyte (not shown). The opening of the can  111  may be sealed by the cap plate  121  after the electrode assembly  112  is accommodated inside the can  111 . The cap plate  121  and the can  111  may be laser-welded to maintain the internal space airtight. 
     An electrolyte inlet  124  may be formed at the cap plate  121 . After the cap plate  121  and the can  111  are combined, the electrolyte is injected via the electrolyte inlet  124 , and the electrolyte inlet  124  may be sealed by a cap  125 . 
     An electrode terminal  123  may be arranged on the cap plate  121 . A first end of the electrode terminal  123  is exposed to an outside via a top surface of the cap plate  121 , whereas a second end of the electrode terminal  123  penetrates through the cap plate  121  into the can  111 . 
     The cap plate  121  and the can  111  may include electrically conductive materials. The electrode terminal  123  may be electrically connected to the first electrode tab  113  of the electrode assembly  112  and may have a first polarity, whereas the cap plate  121  may be electrically connected to the second electrode tab  114  of the electrode assembly  112  and may have a second polarity. 
     For example, the cap plate  121  may function as a positive electrode of the secondary battery  100 , whereas the electrode terminal  123  may function as a negative electrode of the secondary battery  100 . Here, a gasket  122  including an insulating material may be arranged between the cap plate  121  and the electrode terminal  123  to prevent short-circuits therebetween. 
     Inside the can  111 , an insulating case  134  may be arranged above the electrode assembly  112 . The insulating case  134  may insulate the electrode assembly  112  from the cap plate  121 . The insulating case  134  may include via holes through which the first and second electrode tabs  113  and  114  may be withdrawn. 
     The current interrupting device  140  is arranged inside the can  111  and is fused when the surrounding temperature exceeds a reference temperature. By using the current interrupting device  140 , ignition or explosion of the secondary battery  100  due to overcurrent may be prevented. 
     The current interrupting device  140  is arranged inside the can  111  and may be electrically connected to the electrode terminal  123  and the first electrode tab  113 . The current interrupting device  140  may be arranged below the cap plate  121 . 
     A hole  141  that may be formed at a first end of the current interrupting device  140  such that the second end of the electrode terminal  123  may penetrate the hole  141 , whereas a groove  142  is formed at a second end of the current interrupting device  140  to be combined with a protrusion  126  formed on a bottom surface of the cap plate  121 . 
     As the second end of the electrode terminal  123  penetrates the hole  141  formed at the first end of the current interrupting device  140 , a position of the current interrupting device  140  may be fixed. At the same time, the current interrupting device  140  may be electrically connected to the electrode terminal  123 . An insulator  132  may be arranged between the cap plate  121  and the current interrupting device  140  to prevent a short-circuit between the current interrupting device  140  and the cap plate  121 . 
     Turning now to  FIGS. 3-5 ,  FIG. 3  is a schematic cross-sectional view of a current interrupting device  300  according to a first embodiment of the present invention,  FIG. 4  is an exploded perspective view of first and second conductive plates, a lower film, and a thermal fuse of the current interrupting device  300  shown in  FIG. 3 , and  FIG. 5  is a plan view of the first and second conductive plates and a lower film of the current interrupting device  300  shown in  FIG. 3 . For convenience of explanation, a sealing member is omitted from  FIG. 4 , and only the first and second conductive plates and the lower film are shown in  FIG. 5 . 
     Referring now to  FIG. 3 , the current interrupting device  300  according to the present embodiment may include a thermal fuse  310 , first and second conductive plates  320  and  330 , and a sealing member  340 . The thermal fuse  310  is arranged on the first and second conductive plates  320  and  330 , which are a predetermined distance apart from each other, and the thermal fuse  310  may be surrounded by the sealing member  340 . 
     The thermal fuse  310  may include a conductive material via which current flows. For example, the thermal fuse  310  may include tin (Sn), bismuth (Bi), indium (In), lead (Pb), zinc (Zn), or an alloy thereof. 
     The thermal fuse  310  may block current according to a temperature of surroundings of the current interrupting device  300 . For example, if the surrounding temperature exceeds a reference temperature, the thermal fuse  310  may block the current by being blown. A first end of the thermal fuse  310  may be connected to the first conductive plate  320 , whereas a second end of the thermal fuse  310  may be connected to the second conductive plate  330 . 
     The first conductive plate  320  and the second conductive plate  330  may include a metal. For example, the first and second conductive plates  320  and  330  may include nickel, copper, iron, or an alloy thereof such as invar, which is an alloy of nickel and iron. 
     The sealing member  340  may seal the thermal fuse  310  and the first and second connecting units  320   c  and  330   c  of the first and second conductive plates  320  and  330  that are combined with the thermal fuse  310 . As described above with reference to  FIGS. 1 and 2 , since the current interrupting device  140  is arranged inside the secondary battery  100 , the current interrupting device  140  may be directly exposed to the electrolyte. Since the secondary battery  100  is used for an extended period of time and is repeatedly charged and discharged, the current interrupting device  140 , and more particularly, the thermal fuse  310 , may be corroded by the electrolyte. Once the thermal fuse  310  is corroded, it may be difficult for the thermal fuse  310  to function normally, that is, to block current at a temperature exceeding the reference temperature. Therefore, the thermal fuse  310  may be prevented from being corroded by appropriately including and arranging the sealing member  340  which seals the thermal fuse  310  and the connection between the thermal fuse  310  and the first and second conductive plates  320  and  330 . 
     The sealing member  340  may include a lower film  341  and an upper film  342 . The lower film  341  is attached to the bottom of the first and second conductive plates  320  and  330 , and more particularly, bottom surfaces of the first and second connecting units  320   c  and  330   c.  The upper film  342  is attached to top surfaces of the first and second conductive plates  320  and  330 , and more particularly, top surfaces of the first and second connecting units  320   c  and  330   c.    
     The lower film  341  and the upper film  342  may include resin materials. For example, the lower film  341  and the upper film  342  may each include at least one of polyethylene naphthalate, polyethylene terephthalate, polyamide, polyimide, polybutyleneterephthalate, polyphenyleneoxide, polyethylene sulfide, and polysulfone. When the upper film  342  is attached to the top surfaces of the first and second connecting units  320   c  and  330   c,  an intermediate film  343  may be interposed therebetween. 
     The sealing member  340  may be filled with flux  345 . The flux  345  is a polymer-based material and helps melting and breaking of the thermal fuse  310  by improving wettability. The flux  345  may also prevent corrosion of the thermal fuse  310 . 
     The first conductive plate  320  may include a first body unit  320   a,  the first connecting unit  320   c  connected to a first end of the thermal fuse  310 , and a first deformation inducing unit  320   b  interposed between the first body unit  320   a  and the first connecting unit  320   c.  The first body unit  320   a  may include a hole  321  which may be penetrated by an end of an electrode terminal  123 , to connect the first body unit  320   a  to the first electrode tab  113 . 
     While the first connecting unit  320   c  is sealed by the sealing member  340 , the first deformation inducing unit  320   b  is not sealed by the sealing member  340  and is exposed to the outside, so that the first deformation inducing unit  320   b  is external to the sealing member  340 . 
     The second conductive plate  330  may include a second body unit  330   a,  the second connecting unit  330   c  connected to a second end of the thermal fuse  310 , and a second deformation inducing unit  330   b  interposed between the second body unit  330   a  and the second connecting unit  330   c.  The second body unit  330   a  may include a groove  331  with which a protrusion  126  arranged at the bottom of a cap plate  121  may be combined. 
     While the second connecting unit  330   c  is sealed by the sealing member  340 , the second deformation inducing unit  330   b  is not sealed by the sealing member  340  and is exposed to the outside, so that the second deformation inducing unit  330   b  is arranged outside the sealing member  340 . 
     Referring now to  FIG. 4 , the first connecting unit  320   c  is arranged at an end of the first conductive plate  320  and is connected to the thermal fuse  310 . For example, the first connecting unit  320   c  may be welded to the thermal fuse  310 . Here, a thickness t 1  of the first connecting unit  320   c  may be smaller than a thickness t 3  of the first body unit  320   a  to improve a combining strength between the first connecting unit  320   c  and the thermal fuse  310 . In the current interrupting device  300  according to the first embodiment of the present invention, a thickness of the first deformation inducing unit  320   b  may be the same as the thickness t 1  of the first connecting unit  320   c.  For example, the thickness of the first deformation inducing unit  320   b  and the thickness t 1  of the first connecting unit  320   c  may be about 0.23 mm, whereas the thickness t 3  of the first body unit  320   a  may be about 0.3 mm. 
     Similarly, the second connecting unit  330   c  is arranged at an end of the second conductive plate  330  and is connected to the thermal fuse  310 . For example, the second connecting unit  330   c  may be welded to the thermal fuse  310 . Here, a thickness t 1  of the second connecting unit  330   c  may be smaller than a thickness t 3  of the second body unit  330   a  to improve a combining strength between the second connecting unit  330   c  and the thermal fuse  310 . In the current interrupting device  300  according to the first embodiment of the present invention, a thickness of the second deformation inducing unit  330   b  may be the same as the thickness t 1  of the second connecting unit  330   c.  For example, the thickness of the second deformation inducing unit  330   b  and the thickness t 1  of the second connecting unit  330   c  may be about 0.23 mm, whereas the thickness t 3  of the second body unit  330   a  may be about 0.3 mm. 
     The cross-sectional area A 1  of the first deformation inducing unit  320   b  may be smaller than the cross-sectional area A 2  of the first connecting unit  320   c.  For example, the cross-sectional area A 1  of the first deformation inducing unit  320   b  may be from about 30% to about 50% of the cross-sectional area A 2  of the first connecting unit  320   c.  Meanwhile, the cross-sectional areas A 1  and A 2  of the first deformation inducing unit  320   b  and the first connecting unit  320   c  may be smaller than the cross-sectional area of the first body unit  320   a.    
     If the cross-sectional area A 1  of the first deformation inducing unit  320   b  is less than 30% of the cross-sectional area A 2  of the first connecting unit  320   c,  a resistance with respect to a current flowing through the current interrupting device  300  increases, and thus it is difficult for the thermal fuse  310  to function properly and durability of the first deformation inducing unit  320   b  may be reduced. If the cross-sectional area A 1  of the first deformation inducing unit  320   b  is more than 50% of the cross-sectional area A 2  of the first connecting unit  320   c,  the first deformation inducing unit  320   b  may be insufficiently deformed by an external force. 
     In the same regard, the cross-sectional area A 1  of the second deformation inducing unit  330   b  may be smaller than the cross-sectional area A 2  of the second connecting unit  330   c.  For example, the cross-sectional area A 1  of the second deformation inducing unit  330   b  may be from about 30% to about 50% of the cross-sectional area A 2  of the second connecting unit  330   c.  Meanwhile, the cross-sectional areas A 1  and A 2  of the second deformation inducing unit  330   b  and the second connecting unit  330   c  may be smaller than the cross-sectional area of the second body unit  330   a.    
     If the cross-sectional area A 1  of the second deformation inducing unit  330   b  is less than 30% of the cross-sectional area A 2  of the second connecting unit  330   c,  resistance with respect to a current flowing through the current interrupting device  300  increases, and thus it is difficult for the thermal fuse  310  to function properly and durability of the second deformation inducing unit  330   b  may be reduced. If the cross-sectional area A 1  of the second deformation inducing unit  330   b  is more than 50% of the cross-sectional area A 2  of the second connecting unit  330   c,  the second deformation inducing unit  330   b  may be insufficiently deformed by an external force. 
     Referring now to  FIG. 5 , the first deformation inducing unit  320   b  may include a notch N that is formed in a widthwise direction of the current interrupting device  300 . Due to the notch N formed in the first deformation inducing unit  320   b,  the width w 1  of the first deformation inducing unit  320   b  may be smaller than the width w 2  of the first connecting unit  320   c.  Meanwhile, the widths w 1  and w 2  of the first deformation inducing unit  320   b  and the first connecting unit  320   c  may be smaller than the width of the first body unit  320   a.  For example, the width w 1  of the first deformation inducing unit  320   b  may be about 0.9 mm, the width w 2  of the first connecting unit  320   c  may be about 1.4 mm, and the width of the first body unit  320   a  may be about 2.3 mm. 
     The second deformation inducing unit  330   b  may include a notch N that is formed in the widthwise direction of the current interrupting device  300 . Due to the notch N formed in the second deformation inducing unit  330   b,  the width w 1  of the second deformation inducing unit  330   b  may be smaller than the width w 2  of the second connecting unit  330   c.  Meanwhile, the widths w 1  and w 2  of the second deformation inducing unit  330   b  and the second connecting unit  330   c  may be smaller than the width of the second body unit  330   a.  For example, the width w 1  of the second deformation inducing unit  330   b  may be about 0.9 mm, the width w 2  of the second connecting unit  330   c  may be about 1.4 mm, and the width of the second body unit  330   a  may be about 2.3 mm. 
     The notches N formed in the first deformation inducing unit  320   b  and the second deformation inducing unit  330   b  may be rounded grooves for uniform weight distribution. For example, the notches N may be U-shaped grooves with about 0.35 mm curvatures. 
     Due to the first deformation inducing unit  320   b  and the second deformation inducing unit  330   b  as described above, when an external force are applied to the first conductive plate  320  and the second conductive plate  330 , the first and second conductive plates  320  and  330  may be elastically bent-deformed around the first deformation inducing unit  320   b  and the second deformation inducing unit  330   b,  respectively. 
     For example, if an external force (e.g., a weight) is applied to the first conductive plate  320  and/or the second conductive plate  330 , the first conductive plate  320  and/or the second conductive plate  330  are/is bent around the first deformation inducing unit  320   b  and/or the second deformation inducing unit  330   b,  respectively. When the external force is removed, the first conductive plate  320  and/or second conductive plate  330  are/is restored to their original shape(s). During fabrication or assembly of the current interrupting device  300 , even if an external force is applied to the current interrupting device  300 , the first deformation inducing unit  320   b  and/or the second deformation inducing unit  330   b  are/is deformed, and thus force applied to the first connecting unit  320   c  and/or the second deformation inducing unit  330   c  are/is reduced. As a result, sealing between the sealing member  340  and the first connecting unit  320   c  and/or the second connecting unit  330   c  may be maintained. 
     Turning now to  FIGS. 6-8 ,  FIG. 6  is a schematic cross-sectional view of a current interrupting device  600  according to a second embodiment of the present invention,  FIG. 7  is an exploded perspective view of first and second conductive plates and a thermal fuse of the current interrupting device  600  shown in  FIG. 6 , and  FIG. 8  is a plan view of the first and second conductive plates and a lower film of the current interrupting device  600  shown in  FIG. 6 . For convenience of explanation, a sealing member is omitted from  FIG. 7 , and only the first and second conductive plates and a lower film are shown in  FIG. 8 . 
     Referring now to  FIG. 6 , the current interrupting device  600  according to the second embodiment may include a thermal fuse  610 , first and second conductive plates  620  and  630 , and a sealing member  640 . The thermal fuse  610  is arranged on the first and second conductive plates  620  and  630  that are a predetermined distance apart from each other, and the thermal fuse  610  may be surrounded by the sealing member  640 . The sealing member  640  may include upper and lower films  641  and  642  and an intermediate film  643  and may be filled with a flux  645 . The detailed configurations of the thermal fuse  610 , the first and second conductive plates  620  and  630 , and the sealing member  640  constituting the current interrupting device  600  according to the second embodiment are the same as those of the equivalent components described above. 
     However, although the notches N are formed in the first and second deformation inducing units  320   b  and  330   b  of the current interrupting device  300  described previously in the first embodiment with reference to  FIGS. 3 through 5  in the widthwise direction, notches N are formed in the first and second deformation inducing units  620   b  and  630   b  of the current interrupting device  600  according to the second embodiment in the thickness-wise direction. Detailed descriptions of components that are the same as those described above in relation to the current interrupting device  300  will be omitted, and the descriptions given below will focus on differences between the current interrupting device  600  of the second embodiment as compared to the current interrupting device  300  of the first embodiment. 
     Referring now to  FIG. 7 , the cross-sectional area A 1  of the first deformation inducing unit  620   b  may be smaller than the cross-sectional area A 2  of the first connecting unit  620   c,  whereas the cross-sectional area A 1  of the second deformation inducing unit  630   b  may be smaller than the cross-sectional area A 2  of the second connecting unit  630   c.  For example, the cross-sectional areas A 1  of the first and second deformation inducing units  620   b  and  630   b  may be from about 30% to about 50% of the cross-sectional area A 2  of the first and second connecting units  620   c  and  630   c,  respectively. Meanwhile, the cross-sectional areas A 1  and A 2  of the first and second deformation inducing units  620   b  and  630   b  and the first and second connecting units  620   c  and  630   c  may be smaller than the cross-sectional areas of the first and second body units  620   a  and  630   a,  respectively. 
     If the cross-sectional areas A 1  of the first and second deformation inducing units  620   b  and  630   b  are less than 30% of the cross-sectional areas A 2  of the first and second connecting units  620   c  and  630   c,  respectively, resistance with respect to a current flowing through the current interrupting device  600  increases, and thus resistances of the first and second deformation inducing units  620   b  and  630   b  increase. Therefore, when a current flows through the first and second deformation inducing units  620   b  and  630   b,  the first and second deformation inducing units  620   b  and  630   b  are overheated, so that the thermal fuse  610  is blown at too low a temperature, and thus it is difficult for the thermal fuse  610  to function properly by blocking current only when an excessive temperature leading to an unsafe condition is present. Furthermore, by having the cross-sectional area A 1  of the deformation inducing units  620   b  and  630   b  too small, durability of the first and second deformation inducing units  620   b  and  630   b  may be reduced. 
     On the other hand, if the cross-sectional areas A 1  of the first and second deformation inducing units  620   b  and  630   b  exceed 50% of the cross-sectional areas A 2  of the first and second connecting units  620   c  and  630   c,  respectively, the first and second deformation inducing units  620   b  and  630   b  may be insufficiently deformed. Therefore, when an external force is applied, an electrolyte may be introduced through a gap formed between the sealing member  640  and the first and second connecting units  620   c  and  630   c,  and thus the thermal fuse  610  may corrode. 
     Referring now to  FIGS. 6 and 7 , the first and second deformation inducing units  620   b  and  630   b  may include notches N formed in a thickness-wise direction of the current interrupting device  600 . The notches N may be rounded grooves for uniform weight distribution. 
     Due to the notches N, the thickness t 1  of the first and second deformation inducing units  620   b  and  630   b  may be smaller than the thickness t 2  of the first and second connecting units  620   c  and  630   c,  respectively. Meanwhile, the thicknesses t 1  and t 2  of the first and second deformation inducing units  620   b  and  630   b  and the first and second connecting units  620   c  and  630   c  may be smaller than the thickness t 3  of the first and second body units  620   a  and  630   a,  respectively. For example, the thickness t 1  of the first and second deformation inducing units  620   b  and  630   b  may be about 0.16 mm, the thickness t 2  of the first and second connecting units  620   c  and  630   c  may be about 0.23 mm, and the thickness t 3  of the first and second body units  620   a  and  630   a  may be about 0.3 mm. 
     Referring now to  FIG. 8 , the width w 1  of the first and second deformation inducing units  620   b  and  630   b  may be the same as the width w 2  of the first and second connecting units  620   c  and  630   c  and may be smaller than the width w 3  of the first and second body units  620   a  and  630   a,  respectively. For example, the widths w 1  and w 2  of the first and second deformation inducing units  620   b  and  630   b  and the first and second connecting units  620   c  and  630   c  may be 1.4 mm, whereas the width w 3  of the first and second body units  620   a  and  630   a  may be about 2.3 mm. 
     Due to the structure described above, when an external force is applied to the current interrupting device  600 , the first and second conductive plates  620  and  630  may be elastically bent-deformed around the first and second deformation inducing units  620   b  and  630   b  that are interposed between the first and second body units  620   a  and  630   a  and the first and second connecting units  620   c  and  630   c,  respectively. 
     For example, when an external force (e.g., a weight) is applied to the first conductive plate  620  and/or the second conductive plate  630 , the first conductive plate  620  and/or the second conductive plate  630  is/are bent around the first deformation inducing unit  620   b  and/or the second deformation inducing unit  630   b,  and, when the external force is removed, the first conductive plate  620  and/or the second conductive plate  630  is/are restored to its/their original shape(s). Therefore, even if an external force is applied, sealing between the sealing member  640  and the first and second connecting units  620   c  and  630   c  is not destroyed, and thus corrosion of the thermal fuse  610  due to the introduction of an electrolyte may be prevented. 
     Turning now to  FIG. 9 ,  FIG. 9  is an exploded perspective view of first and second conductive plates and a thermal fuse of a current interrupting device  900  according to a third embodiment of the present invention. For convenience of explanation, a sealing member is omitted from  FIG. 9 . 
     The current interrupting device  900  according to the third embodiment may include a thermal fuse  910 , first and second conductive plates  920  and  930 , and a sealing member (not shown). The thermal fuse  910  is arranged on the first and second conductive plates  920  and  930  that are a predetermined distance apart from each other, and the thermal fuse  910  may be surrounded by the sealing member. The detailed configurations of the thermal fuse  910 , the first and second conductive plates  920  and  930 , and the sealing member constituting the current interrupting device  900  according to the third embodiment are the same as those of the equivalent components described above, 
     However, although the notches N are formed in the first and second deformation inducing units  320   b,    330   b,    620   b,  and  630   b  of the current interrupting devices  300  and  600  of the first and second embodiments as described above, in either the widthwise direction or in the thickness-wise direction, notches N are formed in the first and second deformation inducing units  920   b  and  930   b  of the current interrupting device  900  according to the third embodiment in both the widthwise direction and the thickness-wise direction. 
     Referring now to  FIG. 9 , the cross-sectional area A 1  of the first deformation inducing unit  920   b  may be smaller than the cross-sectional area A 2  of the first connecting unit  920   c,  whereas the cross-sectional area A 1  of the second deformation inducing unit  930   b  may be smaller than the cross-sectional area A 2  of the second connecting unit  930   c.  For example, the cross-sectional area A 1  of the first and second deformation inducing units  920   b  and  930   b  may be from about 30% to about 50% of the cross-sectional area A 2  of the first and second connecting units  920   c  and  930   c.  Meanwhile, the cross-sectional areas A 1  and A 2  of the first and second deformation inducing units  920   b  and  930   b  and the first and second connecting units  920   c  and  930   c  may be smaller than the cross-sectional areas of the first and second body units  920   a  and  930   a,  respectively. 
     If the cross-sectional area A 1  of the first and second deformation inducing units  920   b  and  930   b  is less than 30% of the cross-sectional area A 2  of the first and second connecting units  920   c  and  930   c,  when currents flow through the first and second deformation inducing units  920   b  and  930   b,  the first and second deformation inducing units  920   b  and  930   b  overheat, so that the thermal fuse  910  is blown at too low a temperature, and thus it is difficult for the thermal fuse  910  to function properly by blocking a current only when the temperature rises too high to unsafe levels. Furthermore, durability of the first and second deformation inducing units  920   b  and  930   b  may be reduced. On the other hand, if the cross-sectional areas A 1  of the first and second deformation inducing units  920   b  and  930   b  exceed 50% of the cross-sectional areas A 2  of the first and second connecting units  920   c  and  930   c,  respectively, the first and second deformation inducing units  920   b  and  930   b  may be insufficiently deformed. Therefore, when an external force is applied, an electrolyte may be introduced through a gap formed between the sealing member  940  and the first and second connecting units  920   c  and  930   c,  and thus the thermal fuse  910  may corrode. 
     The first and second deformation inducing units  920   b  and  930   b  may include notches N that are formed in the widthwise direction and in the thickness-wise direction of the current interrupting device  900 . The notches N may be rounded grooves for uniform weight distribution. 
     Due to the notches N formed in the widthwise direction, the width of the first and second deformation inducing units  920   b  and  930   b  may be smaller than the width of the first and second connecting units  920   c  and  930   c.  Furthermore, the widths of the first and second deformation inducing units  920   b  and  930   b  and the first and second connecting units  920   c  and  930   c  may be smaller than the width of the first and second body units  920   a  and  930   a.    
     Due to the notches N formed in the thickness-wise direction, the thickness of the first and second deformation inducing units  920   b  and  930   b  may be smaller than the thickness of the first and second connecting units  920   c  and  930   c.  Furthermore, the thicknesses of the first and second deformation inducing units  920   b  and  930   b  and the first and second connecting units  920   c  and  930   c  may be smaller than the thickness of the first and second body units  920   a  and  930   a.    
     Due to the structure as described above, when an external force is applied to the current interrupting device  900 , the first and second conductive plates  920  and  930  may be elastically bent-deformed around the first and second deformation inducing units  920   b  and  930   b,  respectively. Therefore, sealing of the sealing member may be maintained. 
     As described above, according to the one or more of the above embodiments of the present invention, current interrupting devices having deformation inducing units which include notches in the widthwise direction and/or the thickness-wise direction and have cross-sectional area smaller than that of connecting units, are provided. Consequently, upon an application of an external force, the deformation inducing units allow the current interruption device to flex, thereby preserving the integrity of a seal between a sealing member and a thermal fuse, so that the thermal fuse will not be exposed to the electrolyte and corrode. Therefore, even if an external force is applied to the current interrupting device, sealing of a sealing member may be maintained, and thus a thermal fuse may be prevented from being corroded by an electrolyte. Therefore, safety of a secondary battery that is to be used for an extended period of time may be improved. 
     It should be understood that the exemplary embodiments described herein should be considered in a descriptive sense only and not for purposes of limitation. Descriptions of features or aspects within each embodiment should typically be considered as available for other similar features or aspects in other embodiments.