Abstract:
Disclosed herein is an electrode assembly including two or more unit cells, each of which includes a cathode, an anode, and a separator disposed between the cathode and the anode, electrode tabs protruding from the respective electrodes, wherein the unit cells are stacked in a height direction on the basis of a plane, at least two of the unit cells having different planer sizes, and one or more corners of each of the unit cells, which do not tangent to one side of each of the unit cells at which the electrode tabs are formed, are round.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an electrode assembly including two or more unit cells, each of which includes a cathode, an anode, and a separator disposed between the cathode and the anode, electrode tabs protruding from the respective electrodes, wherein the unit cells are stacked in a height direction on the basis of a plane, at least two of the unit cells having different planer sizes, and one or more corners of each of the unit cells, which do not tangent to one side of each of the unit cells at which the electrode tabs are formed, are round. 
       BACKGROUND ART 
       [0002]    As mobile devices have been increasingly developed, and the demand for such mobile devices has increased, the demand for secondary batteries has also sharply increased. Among such secondary batteries is a lithium secondary battery exhibiting high energy density and operating voltage and excellent charge retention and service-life characteristics, which has been widely used as an energy source for various electronic products as well as mobile devices. 
         [0003]    Based on the appearance thereof, a lithium secondary battery may be classified as a cylindrical battery, a prismatic battery or a pouch-shaped battery. Based on the kind of an electrolyte, a lithium secondary battery may also be classified as a lithium-ion battery, a lithium-ion polymer battery or a lithium polymer battery. 
         [0004]    A recent trend in the miniaturization of mobile devices has increased the demand for a prismatic battery or a pouch-shaped battery, which has a small thickness. In particular, much interest is currently focused on such a pouch-shaped battery because it is easy to modify the shape of the pouch-shaped battery, the manufacturing cost of the pouch-shaped battery is low, and the pouch-shaped battery is lightweight. 
         [0005]    Generally, a pouch-shaped battery is a battery having an electrode assembly and an electrolyte in a pouch-shaped battery case, formed of a laminate sheet including a resin layer and a metal layer, in a sealed state. The electrode assembly mounted in the battery case is configured in a jelly-roll (wound) type structure, a stacked type structure or a combination (stacked/folded) type structure. 
         [0006]      FIG. 1  is a view typically showing the structure of a pouch-shaped secondary battery including a stacked type electrode assembly. 
         [0007]    Referring to  FIG. 1 , a pouch-shaped secondary battery  10  is configured to have a structure in which an electrode assembly  30 , including cathodes, anodes, and separators, coated with a solid electrolyte, disposed respectively between the cathodes and the anodes, is mounted in a pouch-shaped battery case  20  in a sealed state such that two electrode leads  40  and  41  electrically connected to cathode tabs  31  and anode tabs  32  of the electrode assembly  30  are exposed to the outside. 
         [0008]    The battery case  20  includes a case body  21  having a depressed receiving part  23 , in which the electrode assembly  30  is located, and a cover  22  integrally connected to the case body  21 . 
         [0009]    The battery case  20  is formed of a laminate sheet including an outer resin layer  20 A constituting the outermost portion of the laminate sheet, an isolation metal layer  20 B to prevent penetration of materials, and an inner resin layer  20 C for sealing. 
         [0010]    The cathode tabs  31  and the anode tabs  32  of the stacked type electrode assembly  30  are respectively coupled to the electrode leads  40  and  41  by welding. In addition, insulative films  50  are attached to the top and bottom of each of the electrode leads  40  and  41  to prevent the occurrence of a short circuit between a thermal welding device (not shown) and the electrode leads  40  and  41  and to achieve sealing between the electrode leads  40  and  41  and the battery case  20  when the upper end  24  of the case body  21  and the upper end of the cover  22  are thermally welded to each other using the thermal welding device. 
         [0011]    In recent years, however, a new type of battery cell is required in accordance with a slim type design trend or various other design trends. 
         [0012]    In addition, the above-mentioned battery cells are configured to include electrode assemblies having the same size or the same capacity. For this reason, in order to manufacture a battery cell having a novel structure in consideration of the design of a device, to which the battery cell is applied, it is necessary to reduce the capacity of the battery cell or modify the design of the device so that the size of the device is increased. 
         [0013]    In addition, electrical connection is complicated during modification of the design of the device, and therefore, it is difficult to manufacture a battery cell satisfying desired conditions. 
         [0014]    Furthermore, it is also necessary to manufacture the battery case based on the shape of the electrode assembly. 
         [0015]    Therefore, there is a high necessity for an electrode assembly and a battery case that can be used depending upon the shape of a device, to which a battery cell is applied, and a battery cell including the same. 
       DISCLOSURE 
     Technical Problem 
       [0016]    Therefore, the present invention has been made to solve the above problems, and other technical problems that have yet to be resolved. 
         [0017]    Specifically, it is an object of the present invention to provide an electrode assembly configured to have a structure in which the electrode assembly can be mounted in various spaces of a device, thereby maximizing utilization of the inner space of the device and also configured to have a structure in which the electrode assembly can be efficiently mounted in devices having various external appearances in addition to a rectangular external appearance. 
       Technical Solution 
       [0018]    In accordance with one aspect of the present invention, the above and other objects can be accomplished by the provision of an electrode assembly including two or more unit cells, each of which includes a cathode, an anode, and a separator disposed between the cathode and the anode, electrode tabs protruding from the respective electrodes, wherein the unit cells are stacked in a height direction on the basis of a plane, at least two of the unit cells having different planer sizes, and one or more corners of each of the unit cells, which do not tangent to one side of each of the unit cells at which the electrode tabs are formed, are round. 
         [0019]    The electrode assembly according to the present invention, formed by stacking unit cells having different sizes, includes round corners. Consequently, it is possible for the electrode assembly according to the present invention to more accurately correspond to shapes, including a curved shape, of a device in which a battery cell including the electrode assembly is mounted and thus to further decrease the size of the device. 
         [0020]    The number of unit cells having different planar sizes included in the electrode assembly may be flexibly adjusted based on the shape or required capacity of a device in which the battery cell is installed. Specifically, the electrode assembly may include two or three unit cells. Alternatively, the electrode assembly may include four or more unit cells. 
         [0021]    In a concrete example, the maximum value of the radius of curvature of each of the round corners may be equivalent to 50% the length of each of the unit cells. The length of each of the unit cells may be the size of one side of each of the unit cells in the direction in which the electrode tabs protrude from each of the unit cells. In addition, the width of each of the unit cells, which will hereinafter be described, may be the size of one side of each of the unit cells in the direction perpendicular to the direction in which the electrode tabs protrude from each of the unit cells. 
         [0022]    If the radius of curvature of each of the round corners is greater than 50% the length of each of the unit cells, the electrode sheet may be broken during the manufacture of the electrode assembly. Generally, a unit cell is manufactured by (i) notching a portion of an electrode sheet to form electrode tabs and round corners at the electrode sheet, (ii) winding the notched electrode sheet, (iii) unwinding the electrode sheet, applying active materials, disposing a separator between the electrode sheet and another electrode sheet, and cutting the stack constituted by the separator and the electrode sheets. If the electrode sheet is deeply notched in the longitudinal direction of the electrode sheet to increase the radius of curvature of a round corner of the electrode sheet during the manufacture of the unit cell, stress may concentrate at the notched portions of the electrode sheet, which have relatively small areas, when the electrode sheet is wound with the result that the electrode sheet may be broken. In a case in which round corners are formed at the electrode sheet to manufacture electrode assemblies having various shapes, therefore, the radius of curvature of each of the round corners to prevent the electrode sheet from being broken during the manufacture of the electrode assemblies may be equivalent to maximum 50% the length of each of the unit cells. Specifically, the radius of curvature of each of the round corners may be equivalent to 5 to 45% the length of each of the unit cells. 
         [0023]    In a case in which the electrode assembly includes n unit cells, the size of an n-th unit cell located at the uppermost end of the electrode assembly may be equivalent to more than 1% and less than 100% that of an (n−1)-th unit cell adjacent to the n-th unit cell. 
         [0024]    For example, in a case in which the electrode assembly includes three unit cells, the size of a third unit cell located at the uppermost end of the electrode assembly may be equivalent to about 75% that of a second unit cell under the third unit cell. In addition, the size of the second unit cell may be equivalent to about 75% that of a first unit cell located at the lowermost end of the electrode assembly. Consequently, the sizes of the third unit cell, the second unit cell, and the first unit cell are gradually increased. 
         [0025]    Alternatively, in a case in which the electrode assembly includes two unit cells, the size of a second unit cell located at the upper end of the electrode assembly may be equivalent to more than 1% and less than 100% that of a first unit cell under the second unit cell. For example, the size of the second unit cell may be equivalent to about 75% that of the first unit cell. 
         [0026]    The minimum difference between the width or the length of the n-th unit cell and the width or the length of the (n−1)-th unit cell may be 0.5 mm. The widths and the lengths of the unit cells are decided through notching and cutting steps. If the difference between the width or the length of the n-th unit cell and the width or the length of the (n−1)-th unit cell is less than 0.5 mm due to tolerances generated during the notching and cutting steps, it is not possible to manufacture unit cells having significantly different planar sizes. In order to manufacture unit cells having significantly different planar sizes, therefore, the minimum difference between the width or the length of the n-th unit cell and the width or the length of the (n−1)-th unit cell may be 0.5 mm, preferably 1 mm. 
         [0027]    For example, in a case in which a third unit cell having a width of 100 mm and a second unit cell having a width of 103 mm are stacked to manufacture an electrode assembly including unit cells having different planar sizes based on a difference in width between the third unit cell and the second unit cell of 3 mm, the difference between the width of the third unit cell and the width of the second unit cell may be slight due to a tolerance generated during a cutting step with the result that an electrode assembly including the unit cells having substantially the same planar size may be manufactured. Consequently, the electrode assembly manufactured as described above may not be different in structure from a conventional electrode assembly having a uniform width and length. 
         [0028]    In the same manner, in order to secure a significant difference between the radii of curvature of round corners of adjacent unit cells, the minimum difference between the radius of curvature of each round corner of the n-th unit cell and the radius of curvature of each round corner of the (n−1)-th unit cell may be 0.5 mm, specifically 1 mm. 
         [0029]    In addition, the minimum difference between the width or the length of the first unit cell located at the lowermost end of the electrode assembly and the width or the length of the second unit cell adjacent to the first unit cell may be 2 mm. The electrode assembly according to the present invention is mounted in a battery case having the internal structure corresponding to the external structure of the electrode assembly in a sealed state to manufacture a battery cell. The battery case may be formed of a laminate sheet. The laminate sheet includes a resin layer and a metal layer. Generally, a short circuit may occur at the metal layer of the laminate sheet. For this reason, the end of the laminate sheet, from which the metal layer is exposed outward, is bent and disposed on the battery case in tight contact and an insulative tape is attached to the exposed metal layer to prevent the occurrence of a short circuit at the metal layer. 
         [0030]    In a case in which the first unit cell located at the lowermost end of the electrode assembly and the second unit cell adjacent to the first unit cell have different sizes as in the electrode assembly according to the present invention, the insulative tape is attached to the outside of the battery case corresponding to a portion of the top of the first unit cell, i.e. a portion of the first unit cell which does not overlap with the second unit cell. If the difference between the width or the length of the first unit cell and the width or the length of the second unit cell 2 is less than 2 mm, the area of the battery case to which the insulative tape is attached is decreased with the result that the insulative tape may not provide sufficient adhesive strength. Consequently, the insulative tape may be separated from the battery case, and therefore, a short circuit may occur due to the exposed metal layer of the laminate sheet. 
         [0031]    In order to secure the adhesive strength of the insulative tape with respect to the first unit cell and thus to prevent the occurrence of short circuit due to the exposed metal layer of the laminate sheet, therefore, the electrode assembly according to the present invention may be configured such that the minimum difference between the width or the length of the first unit cell located at the lowermost end of the electrode assembly and the width or the length of the second unit cell is 2 mm, specifically 3 mm. 
         [0032]    Similarly, in order to secure the adhesive strength of the insulative tape with respect to the first unit cell and thus to prevent the occurrence of short circuit due to the exposed metal layer of the laminate sheet, the minimum difference between the radius of curvature of each round corner of the first unit cell and the radius of curvature of each round corner of the second unit cell may be 2 mm, specifically 3 mm. 
         [0033]    In a case in which the electrode assembly includes n unit cells, the minimum distance between each of the electrode tabs and a corresponding end of the n-th unit cell located at the uppermost end of the electrode assembly in the lateral direction may be 4 mm, specifically 4.4 mm. 
         [0034]    The electrode sheet is notched to form the electrode tabs at the respective unit cells. In order to prevent the electrode tabs from being warped due to stress, wedge-shaped or arc-shaped grooves are further formed on a cutting line along which the unit cells are cut, i.e. at the upper ends of opposite sides of the unit cells, by notching. If distances between the electrode tabs and corresponding ends of each unit cell are too small when the grooves are formed by notching, interference between the electrode tabs and the grooves occurs with the result that the notching process may not be easily carried out. For this reason, the minimum distance between each of the electrode tabs and a corresponding end of the unit cell located at the uppermost end of the electrode assembly in the lateral direction may be 4 mm, specifically 4.4 mm. 
         [0035]    Meanwhile, the electrode tabs of the first unit cell located at the lowermost end of the electrode assembly to the (n−1)-th unit cell adjacent to the n-th unit cell may be formed at the same positions in plane as the electrode tabs of the n-th unit cell. The unit cells have different planar sizes. If the electrode tabs are formed at the unit cells at the same intervals from opposite ends of the unit cells, therefore, it may be difficult to connect the electrode tabs of the unit cells such that the electrode tabs are connected to electrode leads. In order to easily achieve connection between the electrode tabs of the unit cells, therefore, the electrode tabs of the first unit cell to the (n−1)-th unit cell adjacent to the n-th unit cell may be formed at the same positions in plane as the electrode tabs of the n-th unit cell. For example, the electrode tabs of the first unit cell and the second unit cell may be formed at the same positions in plane as the electrode tabs of the third unit cell. 
         [0036]    The minimum distance between the cathode tab and the anode tab may be 10 mm. In order to couple the electrode tabs to a charging and discharging apparatus during a charging process to manufacture the electrode assembly, the distance between the cathode tab and the anode tab may be 10 mm or more. The distance between the cathode tab and the anode tab may be adjusted according to the widths of the electrode tabs and the distance between each of the electrode tabs and a corresponding end of each unit cell. 
         [0037]    In accordance with another aspect of the present invention, there is provided an electrode sheet having an electrode active material applied to one major surface or opposite major surfaces of a current collector sheet, the electrode sheet being cut to manufacture a plurality of unit cells, wherein first notch portions are formed at one side, selected from between an upper side and a lower side, of the electrode sheet such that the first notch portions are arranged at intervals corresponding to a width of each of the unit cells and second notch portions corresponding to the first notch portions are formed at the other side of the electrode sheet, and wherein an upper end cut side for a cutting margin is formed at each of the second notch portions, the upper end cut side being smaller in size than a lower end cut side. 
         [0038]    That is, in the electrode sheet including the notch portions according to the present invention, the upper end cut side is formed at each of the second notch portions unlike conventional notch portions. Even when a cutting tolerance is generated, therefore, it is possible to manufacture a unit cell conforming with an initial design shape and having no protrusion by cutting the electrode sheet along the cutting line on the upper end cut side. 
         [0039]    In an embodiment, each of the first notch portions may be configured to have a wedge-shaped or arc-shaped structure having a width decreased in the longitudinal direction of the electrode sheet. 
         [0040]    Each of the second notch portions may have a larger area than each of the first notch portions. Each of the second notch portions may be larger than each of the first notch portions such that various structures may be formed at one corner of each of the unit cells. 
         [0041]    The lower end cut side of each of the second notch portions may have a length equivalent to 10 to 80% the width of each of the unit cells. The length of the lower end cut side of each of the second notch portions may be adjusted based on a desired shape of each of the unit cells. 
         [0042]    The upper end cut side of each of the second notch portions may have a length of 0.1 to 1 mm. If the length of the upper end cut side of each of the second notch portions is too small, the electrode sheet is not cut on the upper end cut side of each of the second notch portions when a cutting tolerance is generated with the result that a protrusion may be formed at each of the unit cells. If the length of the upper end cut side of each of the second notch portions is too large, on the other hand, a ratio in size of the upper end cut side to each of the unit cells is increased with the result that it may be necessary to further cut the upper end cut side of each of the second notch portions in order to manufacture an electrode assembly having a desired shape. 
         [0043]    Specifically, when the upper end cut side of each of the second notch portions has a length of 0.2 to 0.8 mm, it is possible to prevent generation of a protrusion due to a cutting tolerance and thus to manufacture an electrode assembly having a desired shape without further cutting of the upper end cut side of each of the second notch portions. 
         [0044]    In addition, the distance between the upper end cut side and the lower end cut side of each of the second notch portions may be equivalent to 30 to 150% a length of lower end cut side. The distance between the upper end cut side and the lower end cut side of each of the second notch portions may be adjusted based on a desired shape of each of the unit cells. 
         [0045]    Opposite lateral cut sides of each of the second notch portions may be symmetrical. Alternatively, the opposite lateral cut sides of each of the second notch portions may be asymmetrical. 
         [0046]    In a case in which the opposite lateral cut sides of each of the second notch portions are symmetrical, one corner and the other corner of each of the unit cells manufactured from the electrode sheet may also be symmetrical. 
         [0047]    In a case in which the opposite lateral cut sides of each of the second notch portions are asymmetrical, on the other hand, one corner and the other corner of each of the unit cells manufactured from the electrode sheet may also be asymmetrical. 
         [0048]    At least one lateral cut side of each of the second notch portions may be formed in an arc shape protruding toward each of the second notch portions. As a result, one corner of each of the unit cells may be rounded. 
         [0049]    According to the present invention, each unit cell is formed by cutting the electrode sheet with the above-stated construction. At least one lateral cut side tangent to an upper side of the unit cell may be provided with an upper end cut side of a second notch portion based on a cutting margin. According to circumstances, a portion of the upper end cut side of the second notch portion based on the cutting margin may not be provided at opposite lateral cut sides of the unit cell. 
         [0050]    In accordance with another aspect of the present invention, there is provided a battery cell including the electrode assembly with the above-stated construction, wherein the electrode assembly is mounted in a battery case in a sealed state such that the electrode assembly is impregnated with an electrolyte. 
         [0051]    As previously described, the battery case may be formed of a laminate sheet comprising a resin layer and a metal layer. Alternatively, the battery case may be a metal container used in a prismatic battery. 
         [0052]    The battery case formed of the laminate sheet or the metal container may be provided with a receiving part, in which the electrode assembly according to the present invention is mounted. The receiving part may have an internal structure corresponding to the shape of the electrode assembly according to the present invention. 
         [0053]    The battery case may include a lower case having the receiving part and an upper case to cover the lower case such that the electrode assembly is sealed in the battery case. The upper case may be connected to one side of the lower case in the longitudinal direction of the lower case. Alternatively, the upper case may be connected to one side of the lower case in the lateral direction of the lower case. In a case in which the upper case is connected to one side of the lower case in the longitudinal direction of the lower case, the upper case may be folded in the lateral direction of the lower case in a state in which the electrode assembly is mounted in the receiving part such that that the electrode assembly is sealed in the battery case. On the other hand, in a case in which the upper case is connected to one side of the lower case in the lateral direction of the lower case, the upper case may be folded in the longitudinal direction of the lower case in a state in which the electrode assembly is mounted in the receiving part such that that the electrode assembly is sealed in the battery case. 
         [0054]    The electrode tabs of the unit cells constituting the electrode assembly of the battery cell may be coupled to an electrode lead to form an electrode terminal. That is, even in a case in which a plurality of electrode tabs protrudes from unit cells having different electrode structures, the electrode tabs may be coupled to an electrode lead to form an electrode terminal of the battery cell. 
         [0055]    The width of the electrode lead may be equivalent to 50 to 90% that of each of the electrode tabs. If the width of the electrode lead is greater than that of each of the electrode tabs, a portion of another electrode lead which is not coupled to the electrode tabs may come into contact with the electrode assembly when external impact is applied to the battery cell with the result that a short circuit may occur. 
         [0056]    The battery cell may be a lithium ion battery cell or a lithium ion polymer battery cell; however, the present invention is not limited thereto. 
         [0057]    In accordance with another aspect of the present invention, there is provided a device including the battery cell with the above-stated construction as a power source. The device may be selected from among a mobile phone, a portable computer, a smartphone, a tablet personal computer (PC), a smart pad, a netbook computer, a light electric vehicle (LEV), an electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, and a power storage device. 
         [0058]    In accordance with a further aspect of the present invention, there is provided a battery pack including the battery cell with the above-stated construction as a unit battery, wherein the battery cell includes two or more battery cells. That is, there is provided a battery pack configured to have a structure in which two or more battery cells, as unit batteries, are connected in series and/or parallel to each other. The battery pack may be used in a device selected from among a mobile phone, a portable computer, a smartphone, a tablet PC, a smart pad, a netbook computer, an LEV, an electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, and a power storage device. 
         [0059]    The structure of the device and a method of manufacturing the device are well known in the art to which the present invention pertains, and therefore, a detailed description thereof will be omitted. 
       Advantageous Effects 
       [0060]    As is apparent from the above description, an electrode assembly according to the present invention, formed by stacking unit cells having different sizes, includes round corners. Consequently, it is possible for the electrode assembly according to the present invention to more accurately correspond to shapes, including a curved shape, of a device in which a battery cell including the electrode assembly is mounted and thus to further decrease the size of the device. 
         [0061]    In addition, it is possible to prevent an electrode sheet from being broken and to prevent the occurrence of a short circuit through secure attachment of an insulative tape during the manufacture of electrode assemblies having various shapes. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0062]    The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
           [0063]      FIG. 1  is a perspective view showing a conventional battery cell; 
           [0064]      FIG. 2  is a plan view showing an electrode assembly according to an embodiment of the present invention; 
           [0065]      FIG. 3  is a plane view showing an electrode sheet notched to manufacture the electrode assembly of  FIG. 2 ; 
           [0066]      FIG. 4  is a plan view showing an electrode assembly according to another embodiment of the present invention; and 
           [0067]      FIG. 5  is an exploded perspective view showing a battery cell according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0068]    Now, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. It should be noted, however, that the scope of the present invention is not limited by the illustrated embodiments. 
         [0069]      FIG. 2  is a plan view typically showing an electrode assembly according to an embodiment of the present invention and  FIG. 3  is a plane view typically showing an electrode sheet notched to manufacture the electrode assembly according to the present invention. 
         [0070]    Referring to  FIGS. 2 and 3 , an electrode assembly  100  includes a first unit cell  110 , a second unit cell  120 , a third unit cell  130 , and electrode tabs  141  and  142 . The second unit cell  120  is stacked on the upper end of the first unit cell  110 . The third unit cell  130  is stacked on the upper end of the second unit cell  120 . The second unit cell  120  has a smaller planar size than the first unit cell  110 . The third unit cell  130  has a smaller planar size than the second unit cell  120 . In addition, a radius R2 of curvature of a round corner of the second unit cell  120  is smaller than a radius R1 of curvature of a round corner of the first unit cell  110 . A radius R3 of curvature of a round corner of the third unit cell  130  is smaller than the radius R2 of curvature of the round corner of the second unit cell  120 . 
         [0071]    The radii R1, R2, and R3 of curvature of the round corners of the respective unit cells  110 ,  120 , and  130  are equivalent to about 15% lengths L1, L2, and L3 of the respective unit cells  110 ,  120 , and  130 . In order to form the round corners at the respective unit cells  110 ,  120 , and  130 , the lower end of an electrode sheet  200  is notched to form second notch portions  210 . If the radii R1, R2, and R3 of curvature of the round corners of the respective unit cells  110 ,  120 , and  130  are increased such that the radii R1, R2, and R3 of curvature of the round corners of the respective unit cells  110 ,  120 , and  130  exceed 50% the length of the electrode sheet  200 , i.e. the lengths L1, L2, and L3 of the respective unit cells  110 ,  120 , and  130 , to form large round corners, stress concentrates at cutting lines  20  of the electrode sheet  200  when the electrode sheet  200  is wound after the notching process is performed with the result that the electrode sheet  200  may be broken. For this reason, it is preferable to set the radii R1, R2, and R3 of curvature of the round corners of the respective unit cells  110 ,  120 , and  130  such that the radii R1, R2, and R3 of curvature of the round corners of the respective unit cells  110 ,  120 , and  130  are equivalent to 5 to 45% the lengths L1, L2, and L3 of the respective unit cells  110 ,  120 , and  130 . 
         [0072]    The size of the second unit cell  120  is equivalent to about 75% that of the first unit cell  110  adjacent to the second unit cell  120 . The size of the third unit cell  130  is equivalent to about 75% that of the second unit cell  120  adjacent to the third unit cell  130 . Consequently, the sizes of the unit cells  110 ,  120 , and  130  are gradually decreased from the first unit cell  110  to the third unit cell  130 . 
         [0073]    A difference D1 between a width W2 of the second unit cell  120  and a width W3 of the third unit cell  130  is set to have a minimum value. In a case in which an electrode assembly having a small difference in width between adjacent unit cells is manufactured, the difference in width between the unit cells is slight due to a tolerance generated during cutting with the result that it is possible to manufacture an electrode assembly configured such that the unit cells have a uniform width and length. Consequently, it is preferable to set the difference in width between the adjacent unit cells such that the difference in width between the adjacent unit cells is at least 1 mm. 
         [0074]    Similarly, a difference D3 between the radius R2 of curvature of the round corner of the second unit cell  120  and the radius R3 of curvature of the round corner of the third unit cell  130  is set to have a minimum value equal to the minimum value of the difference D1 between the width W2 of the second unit cell  120  and the width W3 of the third unit cell  130 . 
         [0075]    In addition, a difference D2 between a width W1 of the first unit cell  110  and the width W2 of the second unit cell  120  is set to have a minimum value. In a case in which the difference D2 between the width W1 of the first unit cell  110  and the width W2 of the second unit cell  120  is too small, an area of a battery case to which an insulative tape is attached is decreased with the result that the insulative tape may not provide sufficient adhesive strength. Consequently, the insulative tape may be separated from the battery case. For this reason, it is preferable to set the difference D2 between the width W1 of the first unit cell  110  and the width W2 of the second unit cell  120  such that the difference D2 between the width W1 of the first unit cell  110  and the width W2 of the second unit cell  120  is at least 3 mm. 
         [0076]    Similarly, a difference D4 between the radius R1 of curvature of the round corner of the first unit cell  110  and the radius R2 of curvature of the round corner of the second unit cell  120  is set to have a minimum value equal to the minimum value of the difference D2 between the width W1 of the first unit cell  110  and the width W2 of the second unit cell  120 . 
         [0077]    A distance D5 between the electrode tab  141  and one end of the third unit cell  130  or between the electrode tab  142  and the other end of the third unit cell  130  is at least 4.4 mm. If distances between electrode tabs  140  and corresponding ends of a unit cell are too small when the upper end of the electrode sheet  200  is notched to form a first notch portion  230  configured to have a groove shape, interference between the electrode tabs  140  and the first notch portion  230  occurs with the result that the notching process may not be easily carried out. For this reason, it is preferable to set the distance D5 between the electrode tab  141  and one end of the third unit cell  130  or between the electrode tab  142  and the other end of the third unit cell  130  such that the distance D5 between the electrode tab  141  and one end of the third unit cell  130  or between the electrode tab  142  and the other end of the third unit cell  130  is 4.4 mm. 
         [0078]    In addition, electrode tabs of the first unit cell  110  and electrode tabs of the second unit cell  120  are formed at the same positions as the electrode tabs of the third unit cell  130 . 
         [0079]    A distance D6 between the cathode tab  141  and the anode tab  142  is 10 mm. In order to couple the electrode tabs  141  and  142  to a charging and discharging apparatus during a charging process to manufacture the electrode assembly  100 , it is preferable to set the distance D6 between the cathode tab  141  and the anode tab  142  such that the distance D6 between the cathode tab  141  and the anode tab  142  is at least 10 mm. The distance D6 between the cathode tab  141  and the anode tab  142  may be adjusted according to the widths of the electrode tabs  141  and  142  and the distance D5 between the electrode tab  141  and one end of the third unit cell  130  or between the electrode tab  142  and the other end of the third unit cell  130 . 
         [0080]    Electrode tabs  140  and first notch portions  230  are formed at the upper side of an electrode sheet  200  such that the first notch portions  230  are arranged at intervals corresponding to the width of a unit cell. Second notch portions  210  are formed at the lower side of the electrode sheet  200  such that the second notch portions  210  are arranged at the intervals corresponding to the width of the unit cell. 
         [0081]    An upper end cut side  310  for a cutting margin is formed at each of the second notch portions  210 . The upper end cut side  310  is shorter than a lower end cut side  320 . 
         [0082]    In addition, a left side  330  of each of the second notch portions  210  is formed in an arc shape, by which the right lower end corner of the unit cell is rounded after the electrode sheet  200  is cut. 
         [0083]    In a normal process, the electrode sheet  200  is cut along a cutting line  220  interconnecting the middle point of each of the first notch portions  230  and the middle point of the upper end cut side  310  of each of the second notch portions  210 . If a cutting tolerance is generated, however, the electrode sheet  200  is cut at a position deviating from the cutting line  200  by a predetermined width W5. 
         [0084]    The unit cell manufactured with the cutting tolerance may be slightly different from a unit cell based on an initially designed standard. However, one corner of the unit cell is rounded, which conforms to a design concept of the unit cell. Consequently, the unit cell may be used as an element necessary to manufacture an electrode assembly without an additional process. 
         [0085]      FIG. 4  is a plan view typically showing an electrode assembly according to another embodiment of the present invention. 
         [0086]    Referring to  FIG. 4 , an electrode assembly  400  includes a first unit cell  410  and a second unit cell  420 . The electrode assembly  400  is identical in construction to the electrode assembly  100  of  FIGS. 2 and 3  except that the number of unit cells constituting the electrode assembly  400  is different from that of the unit cells constituting the electrode assembly  100 , and therefore, a detailed description thereof will be omitted. 
         [0087]      FIG. 5  is an exploded perspective view typically showing a battery cell according to an embodiment of the present invention. 
         [0088]    Referring to  FIG. 5 , a battery cell  500  includes an electrode assembly  510  and a battery case  520  to receive the electrode assembly in a sealed state. The battery case  510  includes a lower case  522  having a depressed receiving part  521 , in which the electrode assembly  510  is located, and an upper case  523  to cover the lower case  522  such that the electrode assembly  510  is sealed in the battery case  510 . 
         [0089]    The upper case  523  is connected to one side of the lower case  522 . The upper case  523  is folded in the lateral direction of the lower case  522  in a state in which the electrode assembly  510  is mounted in the lower case  522  such that the upper case  523  covers the lower case  522 . Subsequently, the upper case  523  is coupled to the lower case  522  in a sealed state by thermal welding to form an upper end sealed part  544 , a side sealed part  525 , and a lower end sealed part  526 . 
         [0090]    Although corners of the electrode assembly and the battery case are shown as being formed in a quadrangular shape, the electrode assembly shown in  FIG. 5  may include a plurality of unit cells having different planar sizes and have a round corner in the same manner as shown in  FIGS. 2 and 4 . In addition, the battery case may have an internal structure corresponding to the external structure of the electrode assembly. 
         [0091]    Although the exemplary embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.