Patent Publication Number: US-11380965-B2

Title: Cell structure for secondary battery and secondary battery having the cell structure

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 14/922,682, filed on Oct. 26, 2015, which claims priority to Korean Patent Application No. 10-2015-0063222, filed on May 6, 2015, and all the benefits accruing therefrom under 35 U.S.C. § 119, the content of which in its entirety is herein incorporated by reference. 
    
    
     BACKGROUND 
     1. Field 
     The disclosure relates to a cell structure for a second battery and a secondary battery including the cell structure. 
     2. Description of the Related Art 
     Secondary batteries have increasingly received attention with development of technology for electronic apparatuses. Recently, electronic apparatuses that are variously deformable in shapes thereof, e.g., flexible or wearable electronic apparatuses, are being developed. Accordingly, a research for a structure of a secondary battery for supplying electric power to such electronic apparatuses is performed. 
     SUMMARY 
     When a secondary battery having no flexibility is used for a flexible electronic apparatus, stress generated due to an external environment concentrates on an electrode or a connection portion in a secondary battery such that the electrode may be cut off or the connection portion is short-circuited, and thus durability of a secondary battery may be degraded. 
     Exemplary embodiments of the invention relate to a cell structure for a second battery having flexibility and a secondary battery including the cell structure. 
     According to an exemplary embodiment of the invention, a cell structure for a secondary battery includes an electrode assembly including a plurality of electrodes, a plurality of electrode tabs extending from the electrodes to the outside of the electrode assembly, and a plurality of lead tabs electrically connected to the electrode tabs and contacting the electrode assembly. 
     In an exemplary embodiment, the cell structure for a secondary battery may further include a fixing unit which fixes the lead tabs to a contact surface of the electrode assembly contacting the lead tabs. 
     In an exemplary embodiment, a part of each of the lead tabs may be folded and the electrode tabs may be inserted into the folded part of each of the lead tabs. 
     In an exemplary embodiment, each of the electrode tabs may be folded at least once. 
     In an exemplary embodiment, the electrode assembly may include a flexible material. 
     In an exemplary embodiment, the cell structure may further include an insulation layer disposed on a contact surface of the electrode assembly contacting the lead tabs. 
     In an exemplary embodiment, each of the electrode tabs may have a width less than about 50% of a width of the electrode assembly, and each of the lead tabs may have a width equal to or less than about 25% of the width of the electrode assembly. 
     In an exemplary embodiment, the electrodes may include a first electrode and a second electrode, which are alternately stacked one on another, and the electrode assembly may further include a separation film disposed between the first electrode and the second electrode. 
     In an exemplary embodiment, the electrode tabs may include a first electrode tab extending from the first electrode and a second electrode tab extending from the second electrode, and the lead tabs may include a first lead tab electrically connected to the first electrode tab and a second lead tab electrically connected to the second electrode tab. 
     In an exemplary embodiment, the first lead tab and the second lead tab may be in contact with a same surface of the electrode assembly. 
     In an exemplary embodiment, the first lead tab and the second lead tab may be in contact with different surfaces of the electrode assembly, respectively. 
     In an exemplary embodiment, at least one of the first electrode, the second electrode and the separation film may be partially bound by a binding member. 
     According to another exemplary embodiment of the invention, a cell structure for a secondary battery includes an electrode assembly including a plurality of electrodes, a plurality of electrode tabs extending from the electrodes to an outside of the electrode assembly, and a plurality of lead tabs electrically connected to the electrode tabs, in which a part of each of the lead tabs is folded and the electrode tabs are inserted into the folded part of each of the lead tabs. 
     In an exemplary embodiment, the lead tabs may be in contact with the electrode assembly. 
     In an exemplary embodiment, the cell structure for a secondary battery may further include a fixing unit which fixes the lead tabs to a contact surface of the electrode assembly contacting the lead tabs. 
     In an exemplary embodiment, each of the electrode tabs may be folded at least once. 
     According to another exemplary embodiment of the invention, a secondary battery includes an exterior member and a cell structure disposed in the exterior member, in which the cell structure includes an electrode assembly including a plurality of electrodes, a plurality of electrode tabs extending from the electrodes to an outside of the electrode assembly, and a plurality of lead tabs electrically connected to the electrode tabs and contacting the electrode assembly. 
     In an exemplary embodiment, the secondary battery may further include a fixing unit which fixes the lead tabs to a contact surface of the electrode assembly contacting the lead tabs. 
     In an exemplary embodiment, a part of each of the lead tabs may be folded and the electrode tabs may be inserted into the folded part of each of the lead tabs. 
     In an exemplary embodiment, each of the electrode tabs may be folded at least once. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other features of the invention will become apparent and more readily appreciated from the following description of the exemplary embodiments, taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a plan view of a cell structure for a secondary battery according to an exemplary embodiment of the invention; 
         FIG. 2  is a cross-sectional view taken along line I-I′ of  FIG. 1 ; 
         FIG. 3  is an enlarged view of a portion A of  FIG. 2 ; 
         FIG. 4  is a cross-sectional view taken along line II-II′ of  FIG. 1 ; 
         FIG. 5  illustrates an alternative exemplary embodiment of electrode tabs in the cell structure for a secondary battery illustrated in  FIG. 1 ; 
         FIG. 6  illustrates another alternative exemplary embodiment of the electrode tabs in the cell structure for a secondary battery illustrated in  FIG. 1 ; 
         FIG. 7  illustrates an alternative exemplary embodiment of a tab connection portion in the cell structure for a secondary battery illustrated in  FIG. 1 ; 
         FIG. 8  illustrates another alternative exemplary embodiment of the tab connection portion in the cell structure for a secondary battery illustrated in  FIG. 1 ; 
         FIG. 9  illustrates another alternative exemplary embodiment of the tab connection portion in the cell structure for a secondary battery illustrated in  FIG. 1 ; 
         FIG. 10  is a plan view of a cell structure for a secondary battery according to another exemplary embodiment of the invention; 
         FIG. 11  is a plan view of a cell structure for a secondary battery according to another exemplary embodiment of the invention; 
         FIG. 12  is a plan view of a cell structure for a secondary battery according to another exemplary embodiment of the invention; 
         FIG. 13  is a cross-sectional view taken along line III-III′ of  FIG. 12 ; 
         FIG. 14  is a plan view of a cell structure for a secondary battery according to another exemplary embodiment of the invention; 
         FIG. 15  is a cross-sectional view taken along line IV-IV′ of  FIG. 14 ; 
         FIG. 16  is a plan view of a secondary battery according to another exemplary embodiment of the invention; 
         FIG. 17A  is a plan view of a cell structure for a secondary battery according to an exemplary embodiment of the invention; 
         FIG. 17B  is a plan view of a cell structure for a secondary battery according to another exemplary embodiment of the invention; 
         FIG. 17C  is a plan view of a cell structure for a secondary battery according to another exemplary embodiment of the invention; 
         FIG. 17D  is a plan view of a cell structure for a secondary battery according to another exemplary embodiment of the invention; 
         FIG. 17E  is a plan view of a cell structure for a secondary battery according to a comparative embodiment; 
         FIG. 17F  is a plan view of a cell structure for a secondary battery according to another comparative embodiment; 
         FIG. 18  is a graph showing a relationship between a capacity retention rate and a bending number with respect to secondary batteries including the cell structures illustrated in  FIGS. 17A to 17F ; and 
         FIG. 19  is a graph showing bending numbers having a capacity retention rate of about 90% or high, with respect to secondary batteries including the cell structures illustrated in  FIGS. 17A to 17F . 
     
    
    
     DETAILED DESCRIPTION 
     The invention now will be described more fully hereinafter with reference to the accompanying drawings, in which various embodiments are shown. This invention may, however, be embodied in many different forms, and should not be construed as 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 scope of the invention to those skilled in the art. Reference will now be made in detail to exemplary embodiments, examples of which are illustrated in the accompanying drawings, in which like reference numerals refer to like elements throughout. The thickness or size of each layer illustrated in the drawings may be exaggerated for convenience of explanation and clarity. 
     It will be understood that when an element is referred to as being “on” another element, it can be directly on the other element or intervening elements may be present therebetween. In contrast, when an element is referred to as being “directly on” another element, there are no intervening elements present. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms, including “at least one,” unless the content clearly indicates otherwise. “Or” means “and/or.” As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. It will be further understood that the terms “comprises” and/or “comprising,” or “includes” and/or “including” when used in this specification, specify the presence of stated features, regions, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, regions, integers, steps, operations, elements, components, and/or groups thereof 
     “About” or “approximately” as used herein is inclusive of the stated value and means within an acceptable range of deviation for the particular value as determined by one of ordinary skill in the art, considering the measurement in question and the error associated with measurement of the particular quantity (i.e., the limitations of the measurement system). For example, “about” can mean within one or more standard deviations, or within ±30%, 20%, 10%, 5% of the stated value. 
     Since a material forming each layer in the following exemplary embodiments is exemplary, other materials may be used therefor. As used herein, 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. Exemplary embodiments are described herein with reference to cross section illustrations that are schematic illustrations of idealized embodiments. As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, embodiments described herein should not be construed as limited to the particular shapes of regions as illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. For example, a region illustrated or described as flat may, typically, have rough and/or nonlinear features. Moreover, sharp angles that are illustrated may be rounded. Thus, the regions illustrated in the figures are schematic in nature and their shapes are not intended to illustrate the precise shape of a region and are not intended to limit the scope of the present claims. 
     Hereinafter, exemplary embodiments of the invention will be described in detail with reference to the accompanying drawings. 
       FIG. 1  is a plan view of a cell structure for a secondary battery according to an exemplary embodiment of the invention.  FIG. 2  is a cross-sectional view taken along line I-I′ of  FIG. 1 .  FIG. 3  is an enlarged view of a portion A of  FIG. 2 .  FIG. 4  is a cross-sectional view taken along line II-II′ of  FIG. 1 . 
     Referring to  FIGS. 1 to 4 , an exemplary embodiment of a cell structure  101  for a secondary battery may include an electrode assembly  110 , electrode tabs  121  and  122  extending to outside of the electrode assembly  110 , and lead tabs  131  and  132  connected to the electrode tabs  121  and  122 . 
     The electrode assembly  110  may include a plurality of first electrodes  111  and a plurality of second electrodes  112 , which are alternately disposed one on another, and a plurality of separation films  113  disposed between the first electrodes  111  and the second electrodes  112 . Although  FIGS. 1 to 4  exemplarily illustrate that the number of each of the first and second electrodes  111  and  112  is three, the invention is not limited thereto. In one alternative exemplary embodiment, for example, the number of each of the first and second electrodes  111  and  112  may be one or more. The first and second electrodes  111  and  112  and the separation films  113 , which collectively define the electrode assembly  110 , may be flexible or include a flexible material to be bendable. Accordingly, the electrode assembly  110  may have a feature of flexibility. However, the invention is not limited thereto, and the electrode assembly  110  may have a feature of rigidity. Generally, the flexibility of a material may be defined by a Young&#39;s modulus (i.e., a tensile strength) and the flexibility of a sheet, an electrode or a film may be defined by a Specific Flexure Rigidity (=Et 3 /12). E is a Young&#39;s Modulus and t is the thickness of a sheet. Herein, a material having flexibility means that the material may each independently have a Young&#39;s modulus (i.e., a tensile strength) of about 0.01 gigapascal (GPa) to about 300 GPa, e.g., about 0.05 GPa to about 220 GPa. Herein, a sheet, an electrode or a film having flexibility means that the sheet may each independently have a Specific Flexure Rigidity of about 1.04×10 −10  to about 1.2×10 −1  Nm, e.g., about 8.33×10 −10  to about 9.75×10 −3  Nm, or about 1.15×10 −9  to 2.6×10 −3  Nm. 
     Any one of the first electrode  111  and the second electrode  112  may be a positive electrode and the other one thereof may be a negative electrode. In an exemplary embodiment, when the first electrode  111  is a positive electrode, the second electrode  112  may be a negative electrode. Alternatively, when the first electrode  111  is a negative electrode, the second electrode  112  may be a positive electrode. 
     The first electrode  111  may include a first electrode collector (not shown) and a first electrode active material layer (not shown) disposed or formed on at least one surface of the first electrode collector. The second electrode  112  may include a second electrode collector (not shown) and a second electrode active material layer (not shown) disposed or formed on at least one surface of the second electrode collector. In an exemplary embodiment, where the first electrode  111  is a positive electrode and the second electrode  112  is a negative electrode, the first electrode collector becomes a positive collector and the first electrode active material layer may be a positive active material layer. In such an embodiment, the second electrode collector may be a negative collector and the second electrode active material layer may be a negative active material layer. 
     The positive collector may include or formed of a metal, for example, aluminum, stainless steel, titan, copper, silver or a combination thereof. The positive active material layer may include a positive active material, a binder, and a conductive material. In an exemplary embodiment, where the secondary battery is a lithium secondary battery, the positive active material layer may include a material capable of reversibly occluding and discharging lithium ions. 
     In one exemplary embodiment, for example, the positive active material may include at least one material selected from lithium transition metal oxide, such as lithium cobalt oxide, lithium nickel oxide, lithium nickel cobaltate, lithium nickel cobalt aluminate, lithium nickel cobalt manganese oxide, lithium manganese oxide and lithium iron phosphate, nickel sulfide, copper sulfide, sulfur, iron oxide, and vanadium oxide. 
     In one exemplary embodiment, for example, the binder may include at least one material selected from a polyvinylidene fluoride-based binder such as polyvinylidene fluoride, vinylidene fluoride/hexafluoropropylene copolymer, vinylidene fluoride/tetrafluoroethylene copolymer, etc., a carboxymethyl cellulose-based binder such as sodium-carboxymethylcellulose, lithium carboxymethylcellulose cellulose, etc., an acrylate-based binder such as polyacrylate, lithium polyacrylate, acrylic, polyacrylonitrile, polymethyl methacrylate, polybutyl acrylate, etc., polyamide-imide, polytetrafluoroethylene, polyethylene oxide, polypyrrole, lithium-Nafion, and a styrene butadiene rubber-based polymer. 
     In one exemplary embodiment, for example, the conductive material may include at least one material selected from a carbon-based conductive material, such as carbon black, carbon fibers and graphite, conductive fibers such as metallic fibers, metallic powder such as carbon fluoride powder, aluminum powder and nickel powder, conductive whiskers such as zinc oxide and potassium titanate, conductive metal oxide such as titan oxide, and a conductive polymer such as polyphenylene derivatives. 
     In one exemplary embodiment, for example, the negative collector may include at least one selected from copper, stainless steel, nickel, aluminum, and titan. The negative active material layer may include a negative active material, a binder, and a conductive material. In an exemplary embodiment, where the secondary battery is a lithium secondary battery, the active material layer may include a material capable of alloying with lithium or reversibly occluding and discharging lithium ions. 
     In one exemplary embodiment, for example, the negative active material may include at least one material selected from a metal, a carbon-based material, a metal oxide, and a lithium metal nitride. The metal may include at least one material selected from lithium, silicon, magnesium, calcium, aluminum, germanium, tin, lead, arsenic, antimony, bismuth, silver, gold, zinc, cadmium, mercury, copper, iron, nickel, cobalt, and indium. The carbon-based material may include at least one material selected from graphite, graphitized carbon fiber, coke, mesocarbon microbeads (“MCMB”), polyacene, pitch-based carbon fiber, and hard carbon. The metal oxide may include at least one material selected from lithium titanium oxide, titanium oxide, molybdenum oxide, niobium oxide, iron oxide, tungsten oxide, tin oxide, amorphous tin oxide compound, silicon mono-oxide, cobalt oxide, and nickel oxide. Alternatively, the binder and the conductive material included in the positive active material layer may be respectively and identically used as the binder and conductive material included in the negative active material layer. 
     The separation films  113  are disposed between the first electrodes  111  and the second electrodes  112 . The separation films  113  electrically separate the first electrodes  111  and the second electrodes  112 . The separation films  113  may include, for example, a porous polymer film including polyethylene or polypropylene, woven fabrics or non-woven fabrics including polymer fibers, ceramic particles, or polymer solid electrolyte. However, the invention is not limited thereto. 
     The electrode tabs  121  and  122  may include a plurality of first electrode tabs  121  extending from the first electrodes  111  to the outside of the electrode assembly  110  and a plurality of second electrode tabs  122  extending from the second electrodes  112  to the outside of the electrode assembly  110 . In an exemplary embodiment, as shown in  FIG. 1 , the first and second electrode tabs  121  and  122  may be arranged at opposite sides of the electrode assembly  110  in a width direction of the electrode assembly  110 . The first electrode tabs  121  are electrically connected to the first electrodes  111 , e.g., the first electrode collectors, and the second electrode tabs  122  are electrically connected to the second electrodes  112 , e.g., the second electrode collectors. The first and second electrode tabs  121  and  122  may include, for example, a metal exhibiting a high conductivity, such as, Cu or Al, but the invention is not limited thereto. 
     Each of the first and second electrode tabs  121  and  122  may have, for example, a width W 1  that is less than about 50% of the width W of the electrode assembly  110 . In one exemplary embodiment, for example, each of the first and second electrode tabs  121  and  122  may have a relatively wide width W 1  that is equal to or greater than about 25% and less than about 50% of the width W of the electrode assembly  110 . In one alternative exemplary embodiment, for example, each of the first and second electrode tabs  121  and  122  may have a relatively narrow width W 1  that is less than about 25% of the width W of the electrode assembly  110 . However, the invention is not limited thereto, and the width of each of the first and second electrode tabs  121  and  122  may be variously modified. 
     The lead tabs  131  and  132  may include a first lead tab  131  connected to the first electrode tabs  121  and a second lead tab  132  connected to the second electrode tabs  122 . The first and second lead tabs  131  and  132  may include, for example, a metal exhibiting a high conductivity, such as, Ni or Al, but the invention is not limited thereto. 
     Each of the first and second lead tabs  131  and  132  may have, for example, a width W 2  that is equal to or less than about 25% of the width W of the lead assembly  110 . In one exemplary embodiment, for example, each of the first and second lead tabs  131  and  132  may have a relatively wide width W 2  that is greater than about 15% and equal to or less than about 25% of the width W of the lead assembly  110 . In one alternative exemplary embodiment, for example, each of the first and second lead tabs  131  and  132  may have a relatively narrow width W 1  that is equal to or less than about 15% of the width W of the lead assembly  110 . However, the invention is not limited thereto, and the width of each of the first and second lead tabs  131  and  132  may be variously modified. 
     In an exemplary embodiment, a first tab connection portion  141  may be defined by portions of the first lead tab  131  and the first electrode tabs  121 , where are connected to each other. In the first tab connection portion  141 , a part, for example, one end, of the first lead tab  131  is folded, and the first electrode tabs  121  may be inserted into the folded part of the first lead tab  131 . In the first tab connection portion  141 , the first electrode tabs  121  are electrically connected to each other, and the first electrode tabs  121  and the first lead tab  131  are electrically connected to each other. In an exemplary embodiment, a second tab connection portion  142  may be defined by portions of the second lead tab  132 , and the second electrode tabs  122 , which connected to each other. In the second tab connection portion  142 , a part, for example, one end, of the second lead tab  132  is folded, and the second electrode tabs  122  may be inserted into the folded part of the second lead tab  132 . In the second tab connection portion  142 , the second electrode tabs  122  are electrically connected to each other, and the second electrode tabs  122  and the second lead tab  132  are electrically connected to each other. The first and second tab connection portions  141  and  142  may be provided or formed by, for example, welding, pressing, or adhesion, but the invention is not limited thereto. 
     The first and second lead tabs  131  and  132  may contact the electrode assembly  110 . In one exemplary embodiment, for example, each of the first and second lead tabs  131  and  132  may be in contact with one surface of the electrode assembly  110 , that is, an upper surface of the electrode assembly  110  in  FIGS. 1 to 4 . In such an embodiment, the first and second lead tabs  131  and  132  contacting the electrode assembly  110  may be effectively fixed on a contact surface of the electrode assembly  110  by a fixing portion  160  of  FIG. 14  that is described later. 
     An insulation layer  114  may be disposed or formed on a contact surface, for example, the upper surface, of the electrode assembly  110 , and in contact with the first and second lead tabs  131  and  132 . The insulation layer  114  provides insulation between the first and second lead tabs  131  and  132  and the first electrode  111 , and may include at least one material selected from various insulation materials. In an exemplary embodiment, the insulation layer  114  may include a separation film material such as a porous polymer film. The insulation layer  114  may be disposed to cover or overlap an entire upper surface of the electrode assembly  110  or only on a contact surface, contacting the first and second lead tabs  131  and  132 , of the upper surface of the electrode assembly  110 . 
     In an exemplary embodiment, as described above, in the first tab connection portion  141 , a part of the first lead tab  131  is folded and the first electrode tabs  121  are inserted into the folded part of the first lead tab  131 . The first tab connection portion  141  may be disposed on the contact surface of the electrode assembly  110  which the first lead tab  131  is in contact with. In such an embodiment, the first lead tab  131  in the first tab connection portion  141  is in contact with the upper surface of the electrode assembly  110 . In an exemplary embodiment, in the second tab connection portion  142 , a part of the second lead tab  132  is folded and the second electrode tabs  122  are inserted into the folded part of the second lead tab  132 . The second tab connection portion  142  may be provided on the contact surface of the electrode assembly  110  that the second lead tab  132  is in contact with. In such an embodiment, the second lead tab  132  in the second tab connection portion  142  is in contact with the upper surface of the electrode assembly  110 . 
     Each of the first and second electrode tabs  121  and  122  may be provided to be folded. In an exemplary embodiment, the first electrode tabs  121  may be folded at least twice between the electrode assembly  110  and the first tab connection portion  141 . In an exemplary embodiment the second electrode tabs  122  may be folded at least twice between the electrode assembly  110  and the second tab connection portion  142 .  FIG. 2  illustrates an exemplary embodiment, in which the first electrode tabs  121  include tri-folded parts  121   a .  FIG. 4  illustrates an exemplary embodiment in which the second electrode tabs  122  include tri-folded portions  122   a . However, the invention is not limited thereto or thereby. In an alternative exemplary embodiment, the first and second electrode tabs  121  and  122  may be folded four or more times. 
     In an exemplary embodiment, as described above, the first and second lead tabs  131  and  132  are in contact with the upper surface of the electrode assembly  110 , but not being limited thereto. In an alternative exemplary embodiment, the first and second lead tabs  131  and  132  may contact a lower surface of the electrode assembly  110 . In such an embodiment, an insulation layer  114  provided for insulation between the first and second lead tabs  131  and  132  and the second electrode  112  may be disposed on the lower surface of the electrode assembly  110 .  FIGS. 2 and 4  illustrate exemplary embodiments in which the first and second electrode tabs  121  and  122  are folded in a direction parallel to the first and second electrodes  111  and  112 , but the invention is not limited thereto. In an alternative exemplary embodiment, the first and second electrode tabs  121  and  122  may be folded in a direction perpendicular to the first and second electrodes  111  and  112 , e.g., a thickness direction. 
     In such an embodiment of the cell structure  101  for a secondary battery, as described above, the first and second lead tabs  131  and  132  that are connected to the first and second electrode tabs  121  and  122  are in contact with one surface of the electrode assembly  110 , and the first and second electrode tabs  121  and  122  are folded at least twice. Accordingly, the first and second lead tabs  131  and  132  contacting with the electrode assembly  110  may support or absorb stress generated due to bending deformation of the cell structure  101 , and thus bending durability may be improved. In such an embodiment, since the folded parts of the first and second electrode tabs  121  and  122  may additionally support or absorb the stress, bending durability may be further improved. 
       FIG. 5  illustrates an alternative exemplary embodiment of the first electrode tabs  121  in the cell structure  101  for a secondary battery illustrated in  FIG. 1 . Referring to  FIG. 5 , in an exemplary embodiment, the first electrode tabs  121  include a single-folded part  121   a .  FIG. 6  illustrates another alternative exemplary embodiment of the first electrode tabs  121  in the cell structure  101  for a secondary battery illustrated in  FIG. 1 . Referring to  FIG. 6 , the first electrode tabs  121  may include no folded part unlike the above-described exemplary embodiments. 
       FIG. 7  illustrates an alternative exemplary embodiment of the first tab connection portion  141  in the cell structure  101  for a secondary battery illustrated in  FIG. 1 . Referring to  FIG. 7 , in an exemplary embodiment, the first lead tab  131  may have a structure in which one end portion of the first lead tab  131  is folded, and may be in contact with the upper surface of the electrode assembly  110 . In such an embodiment, the first electrode tabs  121  are inserted into the folded part of the first lead tab  131 , thereby defining or forming the first tab connection portion  141 . In an alternative exemplary embodiment, the first tab connection portion  141  may be spaced apart from the contact surface of the electrode assembly  110 , which the first lead tab  131  is in contact with. 
       FIG. 8  illustrates another alternative exemplary embodiment of the first tab connection portion  141  in the cell structure  101  for a secondary battery illustrated in  FIG. 1 . Referring to  FIG. 8 , the first lead tab  131  has a structure in which an end portion of the first lead tab  131  is folded twice. The first electrode tabs  121  are inserted into one of the folded parts of the first lead tab  131 , thereby defining or forming the first tab connection portion  141 . The other portion of the folded parts of the first lead tab  131  is in contact with the electrode assembly  110 . The first tab connection portion  141  where the first lead tab  131  and the first electrode tabs  121  are connected to each other may be spaced apart from the contact surface of the electrode assembly  110 , which the first lead tab  131  is in contact with. 
       FIG. 9  illustrates another alternative exemplary embodiment of the first tab connection portion  141  in the cell structure  101  for a secondary battery illustrated in  FIG. 1 . Referring to  FIG. 9 , in an exemplary embodiment, the first lead tab  131  has a structure in which an end portion of the first lead tab  131  is folded. The folded part of the first lead tab  131  is in contact with the upper surface of the electrode assembly  110 . In such an embodiment, the first electrode tabs  121  are in contact with a surface (e.g., an outer surface of the folded end portion) of the first lead tab  131 , thereby defining or forming the first tab connection portion  141 . The first tab connection portion  141  where the first lead tab  131  and the first electrode tabs  121  are connected to each other may be spaced apart from the contact surface of the electrode assembly  110 , which the first lead tab  131  is in contact with. 
       FIG. 10  is a plan view of a cell structure  102  for a secondary battery according to another exemplary embodiment of the invention. Referring to  FIG. 10 , in an exemplary embodiment, the first and second electrode tabs  121  and  122  may be arranged at opposite sides of the electrode assembly  110  in a width direction of the electrode assembly  110 . In an exemplary embodiment, the first lead tab  131  and the second lead tab  132  may be in contact with opposing surface of the electrode assembly  110 . In an embodiment, as shown in  FIG. 10 , the first lead tab  131  connected to the first electrode tabs  121  is in contact with the upper surface of the electrode assembly  110 , and the second lead tab  132  connected to the second electrode tabs  122  is in contact with the lower surface of the electrode assembly  110 . In such an embodiment, the insulation layer  114  of  FIG. 2  may be disposed on the upper and lower surfaces of the electrode assembly  110 . Alternatively, the first lead tab  131  may be in contact with the lower surface of the electrode assembly  110 , and the second lead tab  132  may be in contact with the upper surface of the electrode assembly  110 . 
       FIG. 11  is a plan view of a cell structure  103  for a secondary battery according to another exemplary embodiment of the invention. Referring to  FIG. 11 , in an exemplary embodiment of the cell structure  103 , the first and second electrode tabs  121  and  122  may be arranged at opposite sides of the electrode assembly  110  in a length direction of the electrode assembly  110 . In such an embodiment, the first lead tab  131  connected to the first electrode tabs  121  and the second lead tab  132  connected to the second electrode tabs  122  are in contact with the upper surface of the electrode assembly  110 . Alternatively, the first and second lead tabs  131  and  132  may be in contact with the lower surface of the electrode assembly  110 . In another alternative exemplary embodiment, any one of the first and second lead tabs  131  and  132  may be in contact with the upper surface of the electrode assembly  110 , and the other one thereof may be in contact with the lower surface of the electrode assembly  110 . 
       FIG. 12  is a plan view of a cell structure  104  for a secondary battery according to another exemplary embodiment of the invention.  FIG. 13  is a cross-sectional view taken along line III-III′ of  FIG. 12 . 
     Referring to  FIGS. 12 and 13 , the cell structure  104  according to an exemplary embodiment of the invention may include the electrode assembly  110 , the first and second electrode tabs  121  and  122  extending to the outside of the electrode assembly  110 , the first and second lead tabs  131  and  132  connected to the first and second electrode tabs  121  and  122 , respectively, and a binding member  150  that fixes one end portion of the electrode assembly  110 . In such an embodiment, the electrode assembly  110 , the first and second electrode tabs  121  and  122 , and the first and second lead tabs  131  and  132  are substantially the same as those in the exemplary embodiments described above, and any repetitive detailed description thereof will be omitted. 
     In such an embodiment, the binding member  150  may fix an end portion of the electrode assembly  110 . A portion of the electrode assembly  110  may protrude between the first and second electrode tabs  121  and  122 . The protruding portion of the electrode assembly  110  may be fixed by the binding member  150 . In an exemplary embodiment, as shown in  FIGS. 12 and 13 , the first electrode  111 , the second electrode  112  and the separation films  113  are fixed by the binding member  150 , but the invention is not limited thereto. In alternative exemplary embodiment, at least one of the first electrode  111 , the second electrode  112  and the separation films  113  may be fixed by the binding member  150 . 
     When the electrode assembly  110  is not fixed, relative positions between individual layers defining the electrode assembly  110  are changed during repetitive bending motions of the electrode assembly  110  such that alignment therebetween may be lost, and thus stability may be deteriorated. In an exemplary embodiment, where a part of the electrode assembly  110  is fixed by the binding member  150 , even when the electrode assembly  110  repeatedly performs bending deformation, misalignment between individual layers of the electrode assembly  110  may be substantially reduced. In an exemplary embodiment, as described above, one end portion of the electrode assembly  110  is fixed by the binding member  150 , but the invention is not limited thereto. In one alternative exemplary embodiment, for example, opposing end portions of the electrode assembly  110 , or a center portion of the electrode assembly  110 , may be fixed by the binding member  150 . 
       FIG. 14  is a plan view of a cell structure  105  for a secondary battery according to another exemplary embodiment of the invention.  FIG. 15  is a cross-sectional view taken along line IV-IV′ of  FIG. 14 . 
     Referring to  FIGS. 14 and 15 , an exemplary embodiment of the cell structure  105  may include the electrode assembly  110 , the first and second electrode tabs  121  and  122  extending to the outside of the electrode assembly  110 , the first and second lead tabs  131  and  132  respectively connected to the first and second electrode tabs  121  and  122 , and a fixing unit  160  that fixes the first and second lead tabs  131  and  132  to the electrode assembly  110 . 
     The electrode assembly  110  may include a plurality of first electrodes  111  and plurality of second electrodes  112 , which are alternately stacked one on another, and a plurality of separation films  113  disposed between the first electrodes  111  and the second electrodes  112 . Any one of the first electrode  111  and the second electrode  112  may be a positive electrode and the other one thereof may be a negative electrode. The first and second electrodes  111  and  112  and the separation films  113  of the electrode assembly  110  may include a flexible material that allows bending deformation of the electrode assembly  110 . However, the invention is not limited thereto. 
     The first and second electrode tabs  121  and  122  may be arranged at opposite sides of the electrode assembly  110  in a width direction of the electrode assembly  110 . The first electrode tabs  121  are electrically connected to the first electrodes  111  and the second electrode tabs  122  are electrically connected to the second electrodes  112 . Alternatively, the first and second electrode tabs  121  and  122  may be arranged at opposite sides of the electrode assembly  110  in a length direction of the electrode assembly  110 . Each of the first and second electrode tabs  121  and  122  may have, for example, the width W 1  that is less than about 50% of the width W of the electrode assembly  110 . 
     The first lead tab  131  is electrically connected to the first electrode tabs  121 , and the second lead tab  132  is electrically connected to the second electrode tabs  122 . Each of the first and second lead tabs  131  and  132  may have, for example, the width W 2  that is equal to or less than about 25% of the width W of the electrode assembly  110 . 
     In such an embodiment, the first tab connection portion may be defined by portions of the first lead tab  131  and the first electrode tabs  121 , which are connected to each other. In the first tab connection portion, one end portion of the first lead tab  131  is folded. The first electrode tabs  121  may be inserted into the folded part of the first lead tab  131 . In an exemplary embodiment, the second tab connection portion  142  may be defined by portions of the second lead tab  132  and the second electrode tabs  122 , which are connected to each other. In the second tab connection portion, one end portion of the second lead tab  132  is folded. The second electrode tabs  122  may be inserted into the folded part of the second lead tab  132 . 
     The first and second lead tabs  131  and  132  may be in contact with the electrode assembly  110 . In one exemplary embodiment, for example, each of the first and second lead tabs  131  and  132  is in contact with the upper surface of the electrode assembly  110 . An insulation layer may be disposed on the contact surface of the electrode assembly  110 , which the first and second lead tabs  131  and  132  contact. In an alternative exemplary embodiment, the first and second lead tabs  131  and  132  may be in contact with the lower surface of the electrode assembly  110 . In another alternative exemplary embodiment, any one of the first and second lead tabs  131  and  132  may be in contact with the upper surface of the electrode assembly  110 , and the other one thereof may be in contact with the lower surface of the electrode assembly  110 . 
     The first tab connection portion  141  may be disposed on the contact surface of the electrode assembly  110 , which the first lead tab  131  contacts, and the second tab connection portion  142  may be disposed on the contact surface of the electrode assembly  110 , which the second lead tab  132  contacts. Alternatively, the first and second tab connection portions may be spaced apart from the contact surface of the electrode assembly  110 . 
     The fixing unit  160  may fix the first and second lead tabs  131  and  132  contacting the electrode assembly  110  to the electrode assembly  110 . The fixing unit  160  may fix a portion of the first and second lead tabs  131  and  132 , which contacts the electrode assembly  110 , to the electrode assembly  110 . The fixing unit  160  may include, for example, a tape or an adhesive, but not being limited thereto. In an alternative exemplary embodiment, the fixing unit  160  may include at least one of a variety of materials that enables the first and second lead tabs  131  and  132  to be fixed to the electrode assembly  110 . The fixing unit  160  may fix the first and second lead tabs  131  and  132  to the electrode assembly  110  while maintaining a contact state with the electrode assembly  110 . 
     Each of the first and second electrode tabs  121  and  122  may be folded.  FIG. 15  illustrates an exemplary embodiment in which each of the first and second electrode tabs  121  and  122  includes tri-folded parts  121   a  and  122   a . However, the invention is not limited thereto. In an alternative exemplary embodiment, each of the first and second electrode tabs  121  and  122  may be folded once, twice, or four or more times. In another alternative exemplary embodiment, the first and second electrode tabs  121  and  122  may have a flat shape without being folded. 
     In an exemplary embodiment of the cell structure  105  according to the invention, since the first and second lead tabs  131  and  132  are fixed by the fixing unit  160  and maintain a contact state with the electrode assembly  110 , the first and second lead tabs  131  and  132  may effectively support or absorb the stress generated due to bending of the cell structure  105 . 
       FIG. 16  is a plan view of a secondary battery  200  according to another exemplary embodiment of the invention. 
     Referring to  FIG. 16 , an exemplary embodiment of the secondary battery  200  may include an exterior member  210  and a cell structure  100  that is packaged by the exterior member  210 . The cell structure  100  may include an exemplary embodiment of the cell structure  101 ,  102 ,  103 ,  104  or  105  described above. The cell structure  100  is packaged and sealed by the exterior member  210 , and the inside of the exterior member  210  may be filled with an electrolyte. The first and second lead tabs  131  and  132  of the cell structure  100  may be partially exposed outside the exterior member  210 . Meanwhile, when the cell structure  100  is packaged with the exterior member  210 , sealing members  171  and  172  may be further disposed on the first and second lead tabs  131  and  132  to effectively or completely seal the first and second lead tabs  131  and  132  and a periphery thereof. 
     Hereinafter, the cell structures for a secondary battery according to the exemplary embodiments and cell structures for a secondary battery according to comparative embodiments will be described in detail.  FIGS. 17A to 17D  illustrate cell structures  301 ,  302 ,  303 , and  304  for a secondary battery according to exemplary embodiments of the invention.  FIGS. 17E and 17F  illustrate cell structures  401  and  402  for a secondary battery according to comparative embodiments. In the cell structures  301 ,  302 ,  303 ,  304 ,  401  and  402  for a secondary battery illustrated in  FIGS. 17A to 17F , the electrode assembly  110  includes four first electrodes and four second electrodes, which are alternately stacked one on another. In  FIGS. 17A to 17F , when the cell structures  301 ,  302 ,  303 ,  304 ,  401  and  402  are packaged with the exterior member  210  of  FIG. 16 , sealing members  171  and  172  seal the peripheries of the first and second lead tabs  131  and  132 . 
     The cell structures  301 ,  302 ,  303  and  304  for a secondary battery according to the exemplary embodiments illustrated in  FIGS. 17A to 17D  have substantially the same structure as the cell structure  105  for a secondary battery illustrated in  FIG. 14 . 
       FIG. 17A  is a plan view of the cell structure  301  for a secondary battery according to an exemplary embodiment (hereinafter, will be referred to as a first exemplary embodiment). Referring to  FIG. 17A , in the first exemplary embodiment, the first and second electrode tabs  121  and  122  include tri-folded parts  121   a  and tri-folded parts  122   a , respectively, and the first and second lead tabs  131  and  132  respectively connected to the first and second electrode tabs  121  and  122  are in contact with the upper surface of the electrode assembly  110  and fixed by the fixing unit  160 . Each of the first and second electrode tabs  121  and  122  may have a wide width W 1 ′ that is equal to or greater than about 25% and less than about 50% of the width W of the electrode assembly  110 . Each of the first and second lead tabs  131  and  132  has a narrow width W 2 ″ that is equal to or less than about 15% of the width W of the electrode assembly  110 . 
       FIG. 17B  is a plan view of the cell structure  302  for a secondary battery according to another exemplary embodiment (hereinafter, will be referred to as a second exemplary embodiment). Referring to  FIG. 17B , in the second exemplary embodiment, the first and second electrode tabs  121  and  122  include tri-folded parts  121   a  and tri-folded parts  122   a , respectively, and the first and second lead tabs  131  and  132  respectively connected to the first and second electrode tabs  121  and  122  are in contact with the upper surface of the electrode assembly  110  and fixed by the fixing unit  160 . Each of the first and second electrode tabs  121  and  122  may have a wide width W 1 ′ that is equal to or greater than about 25% and less than about 50% of the width W of the electrode assembly  110 . Each of the first and second lead tabs  131  and  132  has a wide width W 2 ′ that is greater than about 15% and equal to or less than about 25% of the width W of the electrode assembly  110 . 
       FIG. 17C  is a plan view of the cell structure  303  for a secondary battery according to another exemplary embodiment (hereinafter, will be referred to as a third exemplary embodiment). Referring to  FIG. 17C , in the third exemplary embodiment, the first and second electrode tabs  121  and  122  include one folded part  121   a  and one folded part  122   a , respectively, and the first and second lead tabs  131  and  132  respectively connected to the first and second electrode tabs  121  and  122  are in contact with the upper surface of the electrode assembly  110  and fixed by the fixing unit  160 . Each of the first and second electrode tabs  121  and  122  may have a wide width W 1 ′ that is equal to or greater than about 25% and less than about 50% of the width W of the electrode assembly  110 . Each of the first and second lead tabs  131  and  132  has a wide width W 2 ′ that is greater than about 15% and equal to or less than about 25% of the width W of the electrode assembly  110 . 
       FIG. 17D  is a plan view of the cell structure  304  for a secondary battery according to another exemplary embodiment (hereinafter, will be referred to as a fourth exemplary embodiment). Referring to  FIG. 17D , in the fourth exemplary embodiment, the first and second electrode tabs  121  and  122  include tri-folded parts  121   a  and tri-folded parts  122   a , respectively, and the first and second lead tabs  131  and  132  respectively connected to the first and second electrode tabs  121  and  122  are in contact with the upper surface of the electrode assembly  110  and fixed by the fixing unit  160 . Each of the first and second electrode tabs  121  and  122  may have a narrow width W 1 ″ that is less than about 25% of the width W of the lead assembly  110 . Each of the first and second lead tabs  131  and  132  has a narrow width W 2 ″ that is equal to or less than about 15% of the width W of the electrode assembly  110 . 
       FIG. 17E  is a plan view of the cell structure  401  for a secondary battery according to a comparative embodiment (hereinafter, will be referred to as a first comparative embodiment). Referring to  FIG. 17E , in the first comparative embodiment, the first and second electrode tabs  121  and  122  include tri-folded parts  121   a  and tri-folded parts  122   a , respectively, and the first and second lead tabs  131  and  132  respectively connected to the first and second electrode tabs  121  and  122  are not in contact with the electrode assembly  110  and are not fixed by the fixing unit  160 . Each of the first and second electrode tabs  121  and  122  may have a narrow width W 1 ″ that is less than about 25% of the width W of the electrode assembly  110 . Each of the first and second lead tabs  131  and  132  has a narrow width W 2 ″ that is equal to or less than about 15% of the width W of the electrode assembly  110 . 
       FIG. 17F  is a plan view of the cell structure  402  for a secondary battery according to another comparative embodiment (hereinafter, will be referred to as a second comparative embodiment). Referring to  FIG. 17F , in the second comparative embodiment, the first and second electrode tabs  121  and  122  include tri-folded parts  121   a  and tri-folded parts  122   a , respectively, and the first and second lead tabs  131  and  132  respectively connected to the first and second electrode tabs  121  and  122  are not in contact with the electrode assembly  110  and are not fixed by the fixing unit  160 . In the second comparative embodiment, one end portion of the electrode assembly  110  is fixed by the binding member  150 . Each of the first and second electrode tabs  121  and  122  may have a narrow width W 1 ″ that is less than about 25% of the width W of the electrode assembly  110 . Each of the first and second lead tabs  131  and  132  has a narrow width W 2 ″ that is equal to or less than about 15% of the width W of the electrode assembly  110 . 
     Table 1 below shows charge/discharge properties before bending of secondary batteries including the embodiments of the cell structure  301 ,  302 ,  303 ,  304 ,  401  and  402  illustrated in  FIGS. 17A to 17F , respectively. A solution of 1.15M LiPF6 in EC/EMC/DEC (3:5:2)+0.2% LiBF4+5.0% FEC+0.5% VEC+3.0% SN is used as an electrolyte. 0.1C (1 cycle), 0.2C (1 cycle), and 0.5C (6 cycles) are used as conditions for measuring charge/discharge properties. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Initial 
                 Coulombic 
                   
                   
               
               
                   
                 Efficiency 
                 Efficiency 
                 Capacity 
                 Energy 
               
               
                   
                 (%) 
                 (%) 
                 (mAh) 
                 Density(Wh/L) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 1 st  exemplary 
                 87.7 
                 99.7 
                 450 
                 330 
               
               
                 embodiment 
               
               
                 2 nd  exemplary 
                 88.6 
                 99.6 
                 449 
                 313 
               
               
                 embodiment 
               
               
                 3 rd  exemplary 
                 87.8 
                 99.7 
                 450 
                 314 
               
               
                 embodiment 
               
               
                 4 th  exemplary 
                 88.3 
                 99.7 
                 452 
                 331 
               
               
                 embodiment 
               
               
                 1 st  comparative 
                 88.4 
                 99.6 
                 449 
                 330 
               
               
                 embodiment 
               
               
                 2 nd  comparative 
                 88.0 
                 99.8 
                 450 
                 330 
               
               
                 embodiment 
               
               
                   
               
            
           
         
       
     
     Referring to Table 1, the secondary batteries including the cell structures  301 ,  302 ,  303  and  304  according to the first to fourth exemplary embodiments, respectively, have initial efficiency, Coulombic efficiency, capacity, and energy density that are similar to those of the secondary batteries including the cell structures  401  and  402  according to the first and second comparative embodiment, respectively. Accordingly, the secondary batteries including the cell structures  301 ,  302 ,  303  and  304  according to the first to fourth exemplary embodiments, respectively, have normal charge/discharge properties like the secondary batteries including the cell structures  401  and  402  according to the first and second comparative embodiments. 
       FIG. 18  is a graph showing a relationship between a capacity retention rate and a bending number with respect to the secondary batteries respectively including the cell structures  301 ,  302 ,  303 ,  304 ,  401  and  402  illustrated in  FIGS. 17A to 17F .  FIG. 19  is a graph showing bending numbers having a capacity retention rate of about 90% or high, with respect to the secondary batteries respectively including the cell structures  301 ,  302 ,  303 ,  304 ,  401  and  402  illustrated in  FIGS. 17A to 17F . In  FIGS. 18 and 19 , the capacity retention rate signifies a rate of capacity according to a bending number with respect to an initial capacity. A solution of 1.15M LiPF6 in EC/EMC/DEC (3:5:2)+0.2% LiBF4+5.0% FEC+0.5% VEC+3.0% SN is used as an electrolyte. 0.5C (2 cycles) is used as a condition for measuring charge/discharge properties. Bending durability is evaluated by a pressure-type bending evaluation method. 
     Referring to  FIGS. 18 and 19 , the secondary batteries including the cell structures  301 ,  302 ,  303  and  304  according to the first to fourth exemplary embodiments, respectively, have bending durability greater than that of the secondary battery including the cell structure  401  according to the first comparative embodiment. Accordingly, when the first and second lead tabs  131  and  132  are in contact with the electrode assembly  110  and fixed by the fixing unit  160 , bending durability may be improved. Also, the secondary battery including the cell structure  304  according to the fourth exemplary embodiment has bending durability similar to that of the secondary battery including the cell structure  402  according to the second comparative embodiment. However, since the cell structure  402  according to the second comparative embodiment additionally includes the binding member  150  to bind a part of the electrode assembly  110 , an additional process may be used in a manufacturing process of the cell structure  402 . The cell structure  304  according to the fourth exemplary embodiment does not include the binding member, such that a manufacturing process of the cell structure  304  may be simplified compared to the cell structure  402  according to the second comparative embodiment. 
     When the secondary battery including the cell structure  302  according to the second exemplary embodiment and the secondary battery including the cell structure  303  according to the third exemplary embodiment are compared with each other, it may be seen that bending durability is superior in the structure in which each of the first and second electrode tabs  121  and  122  has a structure of being folded twice or more, than in the structure in which each of the first and second electrode tabs  121  and  122  has a structure of being folded once. When the secondary battery including the cell structure  301  according to the first exemplary embodiment and the secondary battery including the cell structure  302  according to the second exemplary embodiment are compared with each other, it may be seen that the secondary battery including the cell structure  301  according to the first exemplary embodiment, in which the first and second electrode tabs  121  and  122  have a relatively wide width and the first and second lead tabs  131  and  132  have a relatively narrow width, has a superior bending durability. 
     As described above, in an exemplary embodiment, where the first and second lead tabs  131  and  132  are in contact with the electrode assembly  110  and fixed by the fixing unit  160 , the first and second lead tabs  131  and  132  contacting the electrode assembly  110  may support or absorb stress generated when the secondary battery is bending-deformed. Accordingly, in such an embodiment, bending durability of the secondary battery may be improved. In such an embodiment, as the first and second electrode tabs  121  and  122  are folded one or more times, the stress may be additionally supported or absorbed and thus bending durability of the secondary battery may be further improved. 
     According to the exemplary embodiments set forth herein, the lead tab connected to the electrode tabs is in contact with the electrode assembly, and the electrode tabs are folded a plurality of times. Accordingly, in such an embodiment, the lead tab contacting the electrode assembly may support or absorb stress generated when the cell structure is bending-deformed and thus bending durability of the secondary battery may be improved. In such an embodiment, the folded parts of the electrode tabs additionally support or absorb stress, such that bending durability of the secondary battery may be further improved. 
     It should be understood that 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 exemplary embodiment should typically be considered as available for other similar features or aspects in other exemplary embodiments. 
     While one or more exemplary embodiments have been described with reference to the figures, 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 as defined by the following claims.