Patent Publication Number: US-2016240883-A1

Title: Secondary battery

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Korean Patent Application No. 10-2015-0023319 filed on Feb. 16, 2015 in the Korean Intellectual Property Office, and all the benefits accruing therefrom under 35 U.S.C. 119, the entire contents of which are herein incorporated by reference. 
     BACKGROUND 
     1. Field 
     The present invention relates to a secondary battery. 
     2. Description of the Related Art 
     A secondary battery is rechargeable to be reused and is generally used as a power supply for a mobile device, a hybrid vehicle or an electric vehicle. A secondary battery typically includes an electrode assembly and an outer case accommodating the electrode assembly. Secondary batteries may be classified into a pouch type secondary battery, a prismatic secondary battery, and a cylindrical secondary battery depending on the kind of outer case used. In addition, the electrode assembly accommodated in the outer case may be classified into a wound electrode assembly and a stacked electrode assembly depending on the configuration of the electrode assembly. 
     The wound electrode assembly is configured such that a separator is located between a positive electrode plate and a negative electrode plate, which may be elongated sheets and the resultant product is wound. The stacked electrode assembly is configured such that a positive electrode plate and a negative electrode plate are sequentially stacked with a separator therebetween. 
     SUMMARY 
     Embodiments of the present invention provide a secondary battery which can provide a two-fold increase in the capacity while reducing the number of electrode plates stacked by about half. 
     In embodiments, the secondary battery can be provided in various forms depending on the types of electrode plates folded. Further, embodiments also provide a secondary battery which can allow a process of folding an electrode plate and a process subsequent to the folding process by forming a coupling part on a folding part of the electrode plate to be simplified. 
     These and other aspects of the present invention will be described in or be apparent from the following description of the embodiments. 
     According to an aspect of the present invention, there is provided a secondary battery including an electrode assembly having a first electrode plate, a separator and a second electrode plate stacked and folded about a folding part, an outer case surrounding the electrode assembly, and an electrode lead electrically connected to the electrode assembly and extending to the outside of the outer case. 
     The electrode assembly may be is defined by first and second regions about the folding part and the first and second regions may be symmetrically formed. 
     The electrode assembly may be defined by first and second regions about the folding part and the first and second regions may be asymmetrically formed. 
     The electrode assembly may be defined by first and second regions about the folding part and the first region or the second region may have an inclined surface formed at its end. 
     In addition, the electrode assembly may be defined by first and second regions about the folding part and areas of the first and second regions may be equal to each other. 
     Further, the electrode assembly may be defined by first and second regions about the folding part and areas of the first and second regions may be different from to each other. 
     The electrode assembly may further include a coupling part formed along the folding part. 
     The coupling part may be formed on an inner surface of the electrode assembly. 
     The coupling part may be formed between each of the first electrode plate, the separator and the second electrode plate. 
     The coupling part may be formed on an outer surface of the electrode assembly. 
     The coupling part may be made of polypropylene (PP), polyethylene (PE) or ethylene propylene diene M-class (EPDM) rubber. 
     The electrode assembly may further include an additional coupling part formed at a position spaced apart from the folding part to surround the electrode assembly. 
     As described above, the secondary battery according to embodiments of the present invention can provide a two-fold increase in the capacity while reducing the number of electrode plates stacked to half. In an exemplary embodiment, the present invention provides a secondary battery, which can provide a two-fold increase in the capacity while reducing the number of electrode plates stacked to half by stacking a predetermined number of electrode plates and then folding to provide an electrode assembly. 
     In addition, the secondary battery according to embodiments of the present invention can be provided in various forms according to types of electrode plates folded. In an exemplary embodiment, an electrode assembly folded in various forms can be attained by differently designing a lower electrode plate and an upper electrode plate folded with respect to each other. Accordingly, embodiments of the present invention provide a secondary battery formed in various forms so as to comply with requirements of external sets. 
     Further, the secondary battery according to embodiments of the present invention can facilitate a process of folding an electrode plate and a process subsequent to the folding process by forming a coupling part on a folding part of the electrode plate. In an exemplary embodiment, the present invention provides a secondary battery, which can facilitate folding of an electrode assembly by forming beforehand a coupling part on a folding part of an electrode plate, like a rubber band that does not react with an electrode plate, and which can easily perform storage and transportation of the electrode assembly and encasing of the electrode assembly 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and advantages of the present invention will become more apparent by describing in detail embodiments thereof with reference to the attached drawings in which: 
         FIGS. 1A and 1B  are perspective views of a secondary battery and an electrode assembly, respectively, according to an embodiment of the present invention, and  FIG. 1C  is a perspective view of an unfolded electrode assembly; 
         FIG. 2  is a cross-sectional view of the secondary battery illustrated in  FIG. 1A ; 
         FIGS. 3A and 3B  are perspective views of a secondary battery and an electrode assembly, respectively, according to another embodiment of the present invention; 
         FIG. 4  is a perspective view of a secondary battery according to still another embodiment of the present invention; 
         FIG. 5A  is a perspective view of a secondary battery according to yet another embodiment of the present invention and  FIG. 5B  is a perspective view depicting a state in which an electrode assembly is yet to be folded; 
         FIG. 6  is a perspective view of a secondary battery according to still another embodiment of the present invention; 
         FIG. 7  is a perspective view of a secondary battery according to yet another embodiment of the present invention; 
         FIGS. 8A and 8B  are a perspective view and a partly enlarged perspective view of an electrode assembly, respectively, according to still another embodiment of the present invention; 
         FIGS. 9A, 9B and 9C  are a perspective view and two cross-sectional views, respectively, of an unfolded electrode assembly; and 
         FIG. 10  is a perspective view of an electrode assembly of a secondary battery according to still another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Example embodiments of the present invention will now be described in more detail with reference to accompanying drawings, such that those skilled in the art can easily practice the present invention. 
     The present 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, and the present invention will only be defined by the appended claims. 
     In the following drawings, the thickness of layers and regions are exaggerated for clarity. Like numbers refer to like elements throughout. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     In addition, it will be understood that when an element A is referred to as being “connected to” an element B, it can be directly connected to the element B or an intervening element C may be present between the element A and the element B, so that the element A and the element B can be indirectly connected to each other. 
     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 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, second, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer and/or section from another element, component, region, layer and/or section. Thus, for example, a first element, a first component, a first region, a first layer and/or a first section discussed below could be termed a second element, a second component, a second region, a second layer and/or a second section without departing from the teachings of the present invention. 
     In addition, it will be understood that the term “secondary battery” used herein is intended to encompass a rechargeable secondary battery, such as a lithium ion battery, a lithium polymer battery, or a lithium ion polymer battery, and to mean a low-capacity battery employed to a smart phone, a cellular phone, a tablet, a notebook computer, or a digital camera, and/or a high-capacity battery employed to an electric vehicle, a hybrid electric vehicle, an electric bicycle, or an electric motorcycle, but aspects of the present invention are not limited thereto. 
     Referring to  FIGS. 1A and 1B , perspective views of a secondary battery ( 100 ) and an electrode assembly ( 110 ), respectively, according to an embodiment of the present invention are illustrated and referring to  FIG. 1C , a perspective view illustrating a state an unfolded electrode assembly  110  is illustrated. Referring to  FIG. 2 , a cross-sectional view of the secondary battery  100  illustrated in  FIG. 1A  is illustrated. 
     As illustrated in  FIGS. 1A to 1C  and  FIG. 2 , the secondary battery  100  according to the embodiment of the present invention includes a stacked and folded electrode assembly  110 , an outer case  120  surrounding the electrode assembly  110 , and electrode leads  111   b  and  112   b  extending from the electrode assembly  110  to the outside of the outer case  120 . 
     The electrode assembly  110  may include a first electrode plate  111 , a second electrode plate  112 , and a separator  113  located therebetween, which are stacked and then wound approximately one time using a roughly central portion as a winding shaft. In the following description, the central portion may also be referred to as a folding part  118 . 
     In more detail, the electrode assembly  110  is stacked in a first direction (Z-axis) and is folded approximately one time in a second direction (Y- or X-axis) in view of a centrally positioned folding part  118 . In such a manner, the electrode assembly  110  is configured such that a region surrounding the folding part  118  defines a substantially semicircular section and regions  114  and  115  opposite to the folding part  118  define substantially rectangular sections. Accordingly, a first side  128  of the outer case  120  surrounding the electrode assembly  110  is defines a substantially semicircular section and second sides  124  and  125  of the outer case  120  also define substantially rectangular sections. 
     In addition, the electrode assembly  110  may be defined as a first region  110   a  and a second region  110   b  in view of the folding part  118 . In one embodiment, the first region  110   a  and the second region  110   b  may be symmetrical with each other. In other words, areas or volumes of the first and second regions  110   a  and  110   b  of the electrode assembly  110  may be substantially equal to each other in view of the folding part  118 . In addition, surfaces of the first and second regions  110   a  and  110   b  and opposing ends  114  and  115  thereof may be substantially perpendicular to each other. Further, electrode tabs to be described later may outwardly extend from the first region  110   a  and/or the second region  110   b.    
     The first electrode plate  111  may be a positive electrode plate, and the second electrode plate  112  may be a negative electrode plate, or vice versa. The first electrode plate  111  may include a first current collector and a first active material, and the second electrode plate  112  may include a second current collector and a second active material. 
     The first electrode plate  111  may be formed by coating a first active material made of, for example, a metal oxide, a metal sulfide, or a specific polymer, on the first current collector. 
     The first current collector may include, for example, aluminum, titanium, or an alloy thereof. The first current collector may be in the form of, for example, a thin film, a lath, a punched metal, or a net. In order to provide a thin film type secondary battery, a thickness of the first current collector may be smaller than, for example, 20 μm. 
     In addition, a first tab  111   a  may be formed in the first current collector to be connected to a first lead  111   b , and the first lead  111   b  extends to the outside of the outer case  120 . The first tab  111   a  and the first lead  111   b  may include, for example, aluminum, titanium, or an alloy thereof. A partial region of the periphery of the first tab  111   a  may be covered by a first insulation film  111   c  so as to not be shorted by the metal layer of the outer case  120 . 
     The first active material used may vary depending on the type of secondary battery manufactured but is not particularly limited. For example, in a case of manufacturing a lithium battery or a lithium ion battery, any suitable material that is capable of intercalating and deintercalating lithium ions may be used as the first active material. For example, the first active material may include a metal sulfide or oxide not containing lithium, such as Ti S2 , Mo S2 , NbS e2 , or  V205 , or a lithium composite oxide represented by a general formula LixM O2 , where M is one or more transition metals, and generally 0.05≦x≦1.10 according to the charged or discharged state of battery. In one embodiment, the transition metal M may be Co, Ni, or Mn. Specific examples of the lithium composite oxide may include LiC O2 , LiNi O2 , Li Ny C o1-yO2  (0&lt;y&lt;1), or LiM n2O4 . The lithium composite oxide may generate a high voltage and may have a superior density. 
     In particular, lithium cobalt oxide or lithium nickel oxide may be used as the first active material to attain a high voltage, a high volume density, and good cycle life characteristics. The lithium composite oxide may be prepared by pulverizing and mixing a carbonate, acetate, oxide, or hydride of lithium, and a carbonate, acetate, oxide, or hydride of cobalt, manganese, or nickel according to a desired composition ratio and sintering the mixture at a temperature in a range of 600 to 1,000° C. in an oxygen atmosphere. In addition, when an electrode is formed using one of the aforementioned first active materials, a suitable conductive agent or a binder may be further added. 
     The second electrode plate  112  may be formed by coating a second active material onto the second current collector. 
     The second current collector may include, for example, copper, nickel, or an alloy thereof. The second current collector may be in the form of, for example, a thin film, a lath, a punched metal, or a net. In order to provide a thin film type secondary battery, a thickness of the second current collector may be smaller than, for example, 20 μm. 
     In addition, a second tab  112   a  may be formed in the second current collector to be connected to a second lead  112   b , and the second lead  112   b  extends to the outside of the outer case  120 . The second tab  112   a  and the second lead  112   b  may include, for example, copper, nickel, or an alloy thereof. A partial region of the periphery of the second tab  112   a  may be covered by a second insulation film  112   c  so as to not be shorted by the metal layer of the outer case  120 . 
     The second active material used may vary depending on the type of battery manufactured but may not be particularly limited. For example, in a case of manufacturing a lithium secondary battery, any suitable material that is capable of doping and undoping lithium ions, such as a minimally graphitizable carbon-based material or a graphite-like carbon material, can be used as the second active material. For example, the second active material may be a carbonaceous material, such as an organic polymer compound sintered product, carbon fiber, or activated carbon, prepared by sintering pyrolyzed carbon, cokes such as pitch cokes, needle cokes or petroleum cokes, graphite, glass-like carbon, phenol resin, furan resin, etc. at an appropriate temperature, and carbonizing the same. Examples of the material capable of doping and undoping lithium ions may also include a polymer such as polyacetylene or polypyrrole, or an oxide such as SnO 2 . When an electrode is formed using one of the aforementioned second active materials, a conductive agent or a binder that is widely used in the art may be further added. 
     The separator  113 , in non-limiting examples, a porous polyolefin based separator or a ceramic separator. The polyolefin based separator may have a three-layered cylindrical pore structure of polypropylene (PP)/polyethylene (PE)/PP, or a single-layered net pore structure of PP or PE. The ceramic separator may be obtained, for example, by coating a ceramic onto a surface of the polyolefin based separator or by coating ceramic onto a surface of a non-woven fabric. The ceramic may be typically alumina. 
     In addition, a polymer electrolyte layer may be used as the separator  113 . In this case, the polymer electrolyte layer may completely surround only the second electrode plate (negative electrode plate)  112 . The polymer electrolyte layer may include, for example, a polymer solid electrolyte having a film separating characteristic, or a gel electrolyte having a plasticizer added thereto. 
     In addition, although not shown, if the separator does not include a polymer electrolyte layer, a separate electrolyte may be used. The electrolyte may include a lithium salt dissolved in an aprotonic solvent, or a mixed solvent having two or more kinds of these solvents. Examples of the aprotonic solvent may include propylene carbonate, ethylene carbonate, butylenes carbonate, benzonitrile, acetonitrile, tetrahydrofuran, 2-methyl tetrahydrofuran, y-butyrolactone, dioxolane, 4-methyl dioxolane, N,N-dimethyl formamide, dimethyl acetamide, dimethyl sulfoxide, dioxane, 1,2-dimethoxy ethane, sulfolane, dichloroethane, chlorobenzene, nitrobenzene, dimethyl carbonate, methylethyl carbonate, diethyl carbonate, methylpropyl carbonate, methylisopropyl carbonate, ethylbutyl carbonate, dipropyl carbonate, diisopropyl carbonate, dibutyl carbonate, diethylene glycol, dimethyl ether, or a mixture thereof. Examples of the lithium salt may include, but not limited to, LiPF 6 , LiBF 4 , LiSbF 6 , LiAsF 6 , LiClO 4 , LiCF 3 SO 3 , Li(CF 3 SO 2 ) 2 N, LiC 4 F 9 SO 3 , LiSbF 6 , LiAlO 4 , LiAlCl 4 , LiN(CxF 2 x+1SO 2 )(CyF 2 y+1SO 2 ) (where x and y are natural numbers), LiCl, LiI, or mixtures thereof. 
     In one embodiment, in order to increase flexibility of the electrode assembly  110 , the first current collector and/or the second current collector may be a mesh type. In other implementations, the first current collector and/or the second current collector may be a foam type. For example, the first current collector may be made of foamed aluminum, and the second current collector may be made of foamed copper and/or foamed nickel. The second current collector (negative electrode plate) may be made of a carbon fiber. In this case, the second current collector itself may be capable of doping and undoping lithium ions, which may be advantageous in view of battery capacity. 
     The outer case  120  may surround the electrode assembly  110  so as to protect the electrode assembly  110  from the external environment. In other words, the outer case  120  may be in the form of a pouch or an envelope. In addition, the outer case  120  may include a first outer case  121  surrounding the first region  110   a  (e.g., approximately a top portion) of the electrode assembly  110  and a second outer case  122  surrounding the second region  110   b  (e.g., approximately a bottom portion) of the electrode assembly  110 . The outer case  120  may further include a fused region  123  formed at an outer portion of the electrode assembly  110 . While  FIG. 1A  illustrates a receiving area of the electrode assembly  110  as being formed in both the first outer case  121  and the second outer case  122  and the fused region  123  as being formed at roughly a central portion of the outer case  120 , in other implementations, the receiving area of the electrode assembly  110  may be formed only in the second outer case  122  and the fused region  123  may be formed at approximately a bottom portion of the outer case  120 . 
     Further, the first outer case  121  and the second outer case  122  may be directly connected to each other at a region corresponding to the folding part  118  of the electrode assembly  110 . Therefore, a region at which the first outer case  121  and the second outer case  122  are folded may be defined as a folding part  128 . In one embodiment, the first outer case  121  and the second outer case  122  may be integrally formed and may be folded at a region corresponding to the folding part  118  of the electrode assembly  110  to then be fused at the fused region  123 . In addition, a region of the first outer case  121 , corresponding to the end (right-angle surface)  114  of the electrode assembly  110 , may be defined as a right-angle surface  124  and a region of the second outer case  122 , corresponding to the end (right-angle surface)  115  of the electrode assembly  110 , may be defined as another right-angle surface  125 . 
     In other words, a surface of the first region  110   a  of the electrode assembly  110  and the end  114  of the electrode assembly  110  are perpendicular to each other, and a surface of the second region  110   b  of the electrode assembly  110  and the end  115  of the electrode assembly  110  are perpendicular to each other. Similarly, a surface of the first outer case  121  and the end  124  of the first outer case  121  are perpendicular to each other, and a surface of the second outer case  122  and the end  125  of the first outer case  121  are perpendicular to each other. 
     The outer case  120  may include a metal layer  121   a  having a first surface and a second surface in a form of a thin film, a first insulating layer  121   b  formed on the first surface of the metal layer  121   a , and a second insulating layer  121   c  formed on a second surface of the metal layer  121   a . In one embodiment, the metal layer  121   a  may be made of one selected from the group consisting of aluminum, copper, nickel and stainless steel. In addition, the first insulating layer  121   b  may be made, for example, of polyethylene terephthalate (PET) or nylon. The second insulating layer  121   c  may be a thermally adhesive layer, and may be made, for example, of a denatured polyolefin resin such as casted polypropylene (CPP) or a tercopolymer of propylene, butylene and ethylene. 
     The fused region  123  may be formed on the outer portion of the assembly  110  in such a manner that the second insulating layer  121   c  of the first outer case  121  and a second insulation layer of the second outer case  122  are thermally adhered to each other. 
     In such a manner, a highly productive secondary battery may be provided by attaining an electrode assembly having a thickness or capacity of approximately two times by folding a roughly central portion approximately one time, instead of reducing the number of electrode plates stacked to approximately half. 
     Referring to  FIGS. 3A and 3B , perspective views of a secondary battery ( 200 ) and an electrode assembly ( 210 ) according to another embodiment of the present invention are illustrated. 
     As illustrated in  FIGS. 3A and 3B , the secondary battery  200  according to another embodiment of the present invention includes an inclined surface  224  formed at an upper portion of one side of the secondary battery  200 . In other words, the inclined surface  224  may be formed in a first outer case  121  of the outer case  220 . 
     In more detail, the electrode assembly  210  may be defined by first and second regions  110   a  and  110   b  about a folding part  118 . An inclined surface  214  is formed at an end of the first region  110   a , positioned at an upper portion of the electrode assembly  210  and opposite to the folding part  118 , and a right-angle surface  115  is formed at an end of the second region  110   b , positioned at a lower portion of the electrode assembly  210  and opposite to the folding part  118 . 
     In other words, the electrode assembly  210  is defined by the first and second regions  110   a  and  110   b  about the folding part  118  and the first and second regions  110   a  and  110   b  are asymmetrically formed. In other words, the electrode assembly  210 , which is defined by the first and second regions  110   a  and  110   b  about the folding part  118 , and areas and/or volumes of the first and second regions  110   a  and  110   b  are different from each other. In an exemplary implementation, the area and/or the volume of the upper, first region  110   a  may be smaller than the area and/or the volume of the lower, second region  110   b.    
     Therefore, the inclined surface  224  is naturally formed in the outer case  220  surrounding the electrode assembly  210 . In one embodiment, the inclined surface  224  is formed to range from the fused region  123  to a top surface of the first outer case  121 . The right-angle surface  225  is formed to range from the fused region  123  to a bottom surface of the second outer case  122 . 
     In such a manner, the present invention provides the secondary battery  200  including the electrode assembly  210  and the outer case  220  having a top portion and a bottom portion, which are substantially asymmetrical with respect to each other, using shapes of electrode plates folded. Therefore, the secondary battery  200  can be easily received in various forms of receiving spaces of external sets. 
     Referring to  FIG. 4 , a perspective view of a secondary battery ( 300 ) according to still another embodiment of the present invention is illustrated. 
     As illustrated in  FIG. 4 , the secondary battery  300  according to still another embodiment of the present invention includes inclined surfaces  324  and  325  formed at upper and lower portions of its one side. In other words, the inclined surfaces  324  and  325  may be formed in the first outer case  121  and the second outer case  122  of the outer case  120 . 
     Similarly, an electrode assembly is defined by first and second regions about a folding part, and inclined surfaces are formed at ends of a first region and a second region, respectively positioned at upper and lower portions of the electrode assembly and opposite to the folding part. 
     Therefore, the inclined surfaces  324  and  325  are naturally formed at the upper and lower portions of the outer case  320  surrounding the electrode assembly. In one embodiment, the inclined surface  324  is formed to range from the fused region  123  to the top surface of the first outer case  121  and the inclined surface  325  is formed to range from the fused region  123  to the bottom surface of the second outer case  122 . 
     In such a manner, the present invention provides the secondary battery  300  including the electrode assembly and the outer case  320  having a top portion and a bottom portion, which are substantially asymmetrical with respect to each other, using shapes of electrode plates folded. Therefore, the secondary battery  300  can be easily received in various forms of receiving spaces of external sets. 
     Referring to  FIGS. 5A and 5B , a perspective view of a secondary battery ( 400 ) according to still another embodiment of the present invention and a perspective view illustrating an unfolded electrode assembly ( 410 ) are illustrated. 
     As illustrated in FIGS.  FIGS. 5A and 5B , the secondary battery  400  according to still another embodiment of the present invention has top and bottom regions substantially asymmetrically formed. In other words, a first outer case  421  and a second outer case  422 , respectively positioned at upper and lower portions of the secondary battery  400 , are asymmetrically formed about a folding part  128 . 
     In more detail, the electrode assembly  410  is defined by first and second regions  410   a  and  410   b  about a folding part  418  and an area or a volume of the first region  410   a , positioned at an upper portion of the electrode assembly  410 , is smaller than an area or a volume of the second region  410   b , positioned at a lower portion of the electrode assembly  410 . 
     Therefore, in the outer case  420  surrounding the electrode assembly  410 , the upper, first outer case  421  and the lower, second outer case  422  may have different areas and/or volumes. In other words, an area or a volume of the upper, first outer case  421 , is smaller than an area or a volume of the lower, second outer case  422 . 
     In one embodiment, a region of the first outer case  421 , corresponding to the end (right-angle surface)  414  of the electrode assembly  410 , may be defined as a right-angle surface  424  and a region of the second outer case  422 , corresponding to the end (right-angle surface)  415  of the electrode assembly  410 , may be defined as another right-angle surface  425 . 
     In such a manner, the embodiment provides the secondary battery  400  including the electrode assembly  410  and the outer case  420  having a top portion and a bottom portion, which are substantially asymmetrical with respect to each other, using shapes of electrode plates folded. Therefore, the secondary battery  400  can be easily received in various forms of receiving spaces of external sets. 
     Referring to  FIG. 6  is a perspective view of a secondary battery ( 500 ) according to still another embodiment of the present invention is illustrated. Referring to  FIG. 7 , a perspective view of a secondary battery ( 600 ) according to still another embodiment of the present invention is illustrated. 
     As illustrated in  FIG. 6 , the secondary battery  500  according to still another embodiment of the present invention includes an inclined surface  524  formed at an end of a first region of an upper, first outer case  421  of an outer case  420  and/or an electrode assembly. As illustrated in  FIG. 7 , the secondary battery  600  according to still another embodiment of the present invention includes an inclined surface  625  formed at an end of an upper, first outer case  421  of an outer case  420  or/and a first region of an electrode assembly, and an inclined surface  625  formed at an end of a lower, second outer case  422  of the outer case  420  or/and a second region of the electrode assembly. 
     In such a manner, the present invention provides the secondary battery  500 ,  600  including the electrode assembly and the outer case  420  having a top portion and a bottom portion, which are substantially asymmetrical with respect to each other, using shapes of electrode plates folded. Therefore, the secondary batteries  500  and  600  can be easily received in various forms of receiving spaces of external sets. 
     Referring to  FIGS. 8A and 8B , a perspective view and a partly enlarged perspective view of an electrode assembly ( 810 ) according to still another embodiment of the present invention are illustrated. In addition, referring to  FIGS. 9A, 9B and 9C , perspective views and a cross-sectional view illustrating a state in which the electrode assembly ( 810 ) according to the present invention is yet to be folded are illustrated. 
     As illustrated in  FIGS. 8A and 8B , the secondary battery according to the present invention may further include a coupling part  811  formed along a folding part  118  of an electrode assembly  810 . In other words, the electrode assembly  810  may be defined by a first region  110   a  and a second region  110   b  about the folding part  118 , and the coupling part  811 , similar to a bookbinding line, is further formed along the folding part  118 . The coupling part  811  fixes beforehand central regions of a first electrode plate  111 , a separator  113  and a second electrode plate  112  constituting the electrode assembly  810 , thereby facilitating folding of the electrode assembly  810  about the coupling part  811 , that is, the folding part  118 . In other words, when the first region  110   a  and the second region  110   b  of the electrode assembly  810  are folded, the coupling part  811  fixes a central region of the electrode assembly  810  beforehand, thereby easily folding the first region  810   a  of the electrode assembly  810  from the second region  810   b  or easily folding the second region  810   b  of the electrode assembly  810  from the first region  810   a.    
     As illustrated in  FIG. 9B , the coupling part  811  may be formed on an inner surface of the electrode assembly  810 . In other words, the coupling part  811  is formed between each of the first electrode plate  111 , the separator  113  and the second electrode plate  112 , and may coupling to each other. 
     In addition, as illustrated in  FIG. 9C , the coupling part  812  may be formed on an outer surface of the electrode assembly  810 . In other words, the coupling part  812  binds the surface of the electrode assembly  810 , as a rubber band does. 
     The coupling parts  811  and  812  may be made of polypropylene (PP), polyethylene (PE) or ethylene propylene diene M-class (EPDM) rubber, but not limited thereto. Additionally, various materials that do not react with an electrolyte may be used as the coupling parts  811  and  812 . 
     As described above, since the coupling parts  811  and  812  are formed at roughly at a central region of the electrode assembly  810 , folding of the electrode assembly  810  can be easily performed. 
     Referring to  FIG. 10 , a perspective view of an electrode assembly ( 910 ) of a secondary battery according to still another embodiment of the present invention is illustrated. 
     As illustrated in  FIG. 10 , the secondary battery according to still another embodiment of the present invention may further include a second coupling part  911  formed at a position spaced from a folding part  118  to surround the electrode assembly  910 . The second coupling part  911  is substantially the same as the coupling parts  811  and  812  in view of materials and/or configurations, except for the position of the coupling part formed. 
     The second coupling part  911  may prevent the electrode assembly  910  folded about the coupling parts  811  and  812  or the folding part  118  from restoring to its original position. Therefore, after the coupling parts  811  and  812  are formed roughly at the central region of the electrode assembly  910  and are then folded about the coupling parts  811  and  812  or the folding part  118 , the second coupling part  911  is formed in the electrode assembly  910  opposed to the coupling parts  811  and  812  or the folding part  118 , thereby maintaining the shape of the electrode assembly  910  without deformation during a fabrication process of the secondary battery. Therefore, processes of connecting an electrode lead to the electrode assembly  910  and encasing the electrode assembly  910  can be easily performed. 
     While the secondary battery of the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims. It is therefore desired that the present embodiments be considered in all respects as illustrative and not restrictive, reference being made to the appended claims rather than the foregoing description to indicate the scope of the invention.