Patent Publication Number: US-7897279-B2

Title: Jelly-roll type electrode assembly and secondary battery including the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of application Ser. No. 10/911,661, filed on Aug. 5, 2004, now U.S. Pat. No. 7,666,545, which claims the benefit of Korean Application No. 10-2003-0057277, filed on Aug. 19, 2003, in the Korean Patent Office, the disclosures of which are incorporated herein, by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a jelly-roll type electrode assembly and a secondary battery including the same, and particularly to, a jelly-roll type electrode assembly in which short-circuiting between electrodes can be prevented, and a secondary battery including the same 
     2. Description of the Related Art 
     In general, secondary batteries, which are distinguished from primary batteries by their ability to be repeatedly charged and discharged, have been widely used in cellular phones, notebook computers, camcorders, and other portable electronic devices. Lithium secondary batteries having an operating voltage of 3.6V or greater, which is three times higher than nickel-cadmium (Ni—Cd) batteries, have become popular as a power source for various kinds of electronic equipment and nickel-hydrogen batteries and, are frequently used because of their high energy density per unit of weight. 
     Such lithium secondary batteries mostly use a lithium oxide as a positive active material and a carbonaceous material as a negative active material. Lithium secondary batteries can be classified into liquid electrolyte batteries, also known as lithium ion batteries, and polymer electrolyte batteries, also known as lithium polymer batteries, according to the type of electrolyte used. Lithium secondary batteries are manufactured in various shapes, typically, in cylindrical, rectangular, or pouch forms. 
     A second battery uses a jelly-roll type electrode assembly manufactured by applying an active material onto a substrate, drying the substrate, pressing it using a roller, and severing it into a positive electrode strip and a negative electrode strip, interposing a separator between the positive and negative electrode strips to make a multi-layer structure, and rolling the multi-layer structure in a jelly-roll form. A cylindrical battery is made by placing such an electrode assembly into a cylindrical can, injecting an electrolyte into the cylindrical can, and sealing the cylindrical can. A rectangular battery is made by applying a pressure to such an electrode assembly so that it has plane surfaces, and then, placing the electrode assembly into a rectangular can. 
     In such an electrode assembly, a positive electrode tab and a negative electrode tap are drawn out from the positive electrode strip and the negative electrode strip, respectively. In particular, one of the positive and negative electrode tabs is drawn out upward from its related electrode strip and electrically connected to a cap assembly for sealing the can. The other electrode strip is drawn out downward from its related electrode strip and electrically connected to a projection formed at the bottom of the can. In the case of the rectangular secondary battery, both the positive and negative electrode tabs may be drawn out upward from their electrode strips. 
     As described above, a conventional cylindrical battery includes an electrode assembly made by interposing only a separator between a major electrode strip and an auxiliary electrode strip to make a multi-layer structure and rolling the multi-layer structure. The separator is, however, prone to be broken at portions of the positive and negative electrode strips welded together with electrode tabs, thus causing electrical short-circuiting. An electrode tab enables movement of electric charges from a battery to the outside, and therefore, when electric charges are concentrated on the electrode tab, the electrode tab is heated. As a result, an electrode strip attached with the heated electrode tab is also heated, thus resulting in breaking of the separator. Then, although the electrode strip attached with the electrode tab is separated via the separator from the other electrode strip of different polarity, the breaking of the separator causes electrical short-circuiting between the heated electrode strip and the other electrode strip. 
     Also, when a defect, such as a burr, forms at an end of an electrode tab during manufacture of the electrode tab, the burr damages the separator, especially a thin separator, thus causing an electrical short-circuiting via a damaged portion of the separator. 
     Japanese Patent Publication No. 11-273660 has suggested a battery electrode assembly in which a portion of an electrode strip welded together with an electrode tab is coated with a polymer material, thereby preventing a separator from being damaged due to the formation of a burr or other defect of the electrode tab. However, in this case, since a conventional battery manufacturing process further requires a polymer coating process, the manufacture process is more complicated and manufacturing costs are increased. 
     Japanese Patent Publication No. 4-109551 discloses a jelly-roll type battery electrode assembly in which an electrode tab is bent inward more than the rolled battery electrode assembly and a bent portion of the electrode tab is located at a center portion of an inner diameter of the rolled electrode assembly. However, it is very difficult to position the electrode tab at the center of the rolled electrode assembly. Even if the electrode is positioned at the center, it is difficult to insert a welding rod into the electrode assembly due to the presence of the electrode tab during a subsequent process of welding an electrode tab together with a base plane of a can. 
     U.S. Pat. No. 5,508,122 discloses a secondary battery with a spiral electrode unit. In the spiral electrode unit, same-polarity regions of an electrode strip attached with a lead toward a center portion of the spiral electrode unit are positioned via a separator on both sides of exposed regions of electrode core material, and a lead attached to the other electrode strip is positioned at the outmost winding of the spiral electrode unit. Accordingly, even if the separator is damaged, electrical short-circuiting can be prevented. 
     However, in the spiral electrode unit, the exposed regions must be long enough to position the same-polarity regions of the electrode strip via a separator on both sides of the exposed regions, and a starting point of rolling the other electrode strip is later than that of rolling the electrode strip. Thus, the secondary battery includes many portions of electrode strips unnecessary for battery reaction, thereby increasing manufacturing costs. If the secondary battery is rectangular shaped, that is, when a length of the battery in circumference is long, this problem becomes more serious. 
     SUMMARY OF THE INVENTION 
     The invention provides a jelly-roll type electrode assembly designed to reduce electrical short-circuiting at a portion of an electrode strip welded together with an electrode tab, and a secondary battery including the same. 
     The invention also provides a jelly-roll type electrode assembly in which electrical short-circuiting can be prevented even if an electrode tab has a surface defect such as a burr, and a secondary battery including the same. 
     The invention also provides a jelly-roll type electrode assembly fabricated using conventional equipment but electrical short-circuiting between electrode strips can be prevented, and a secondary battery including the same. 
     According to an aspect of the invention, there is provided an electrode assembly comprising a first electrode strip including a first electrode collector coated with at least a first electrode active material, an exposed portion of the first electrode collector attached with a first electrode tab; a second electrode strip which includes a second electrode collector coated with at least a second electrode active material and is rolled together with the first electrode strip, an exposed portion of the second electrode collector attached with a second electrode tab; and at least one inter-electrode strip separator is sandwiched between the first and second electrode strips. Here, at least a sheet of protective separator, which is extended from the inter-electrode strip separator, is further positioned on a side of the first electrode strip attached with the first electrode tab. 
     The first electrode tab may be formed at a center of the rolled electrode assembly. At least one of the inter-electrode strip separator and the protective separator is positioned on an opposite side of the side of the first electrode strip attached with the first electrode tab. 
     The at least one protective separator may be an end of the inter-electrode strip separator. The at least one protective separator, which is the end of the inter-electrode strip separator, is bent more than one time, for example twice, and sandwiched between the first and second electrode strips. 
     The first electrode tab may be positioned at a side of the first electrode collector a center of the electrode assembly. 
     The at least one protective separator may be extended from the inter-electrode strip separator while being rolled in the opposite direction in which the electrode assembly is rolled. 
     The electrode assembly is a rolled structure in which a sheet of the inter-electrode strip separator, the first electrode strip, a sheet of the inter-electrode strip separator, and the second electrode strip are sequentially multi-layer, wherein the at least one protective separator is extended from at least one of the two sheets of inter-electrode strip separators. 
     Starting points of applying the first and second active materials may be the same. 
     The second electrode strip may be positioned via the protective separator on a side of the first electrode strip attached with the first electrode tab. 
     The at least one protective separator may be extended to contact the opposite side of a side of the inter-electrode strip separator, which resides between the side of the first electrode strip attached with the first electrode tab and the second electrode strip, toward the center of the electrode assembly, the at least one protective separator positioned on at least the side of the first electrode strip attached with the first electrode tab. 
     According to another aspect of the invention, there is provided an electrode assembly comprising a first electrode strip including a first electrode collector covered with at least a first electrode active material, an exposed portion of the first electrode collector attached with a first electrode tab; and a second electrode strip including a second electrode collector covered with at least a second electrode active material, an exposed portion of the second electrode collector attached with a second electrode tab. The respective first and second electrode strips are sandwiched between two sheets of separators and positioned on one of the two sheets of separators so as to make a multi-layer structure, the multi-layer structure is inserted into a mandrel and rolled using the mandrel, and at least one of the two sheets of separators is inserted into the mandrel and its portion is drawn out from the mandrel by at least half a length the mandrel in conference, and is rolled using the mandrel. 
     The drawn portion may be extended to the side of the first electrode strip attached with the first electrode tab. 
     The second electrode strip may be positioned via the drawn portion of the separator and the other separator on the side of the first electrode strip attached with the first electrode tab. 
     Only the separators may be present on the opposite side of the side of the first electrode strip attached with the first electrode tab. 
     The length of the drawn portion of the separator is preferably longer than a sum of half the length of the mandrel in circumference and a distance between the mandrel and the side of the first electrode strip attached with the first electrode tab. 
     The drawn portion of the separator may be rolled at least half a round of the mandrel, and then the first electrode strip is rolled. 
     The first electrode tab may be positioned at a side of the first electrode collector toward a center portion of the electrode assembly. 
     The drawn portion of the separator may be extended while being wound in the opposite direction in which the electrode assembly is rolled. 
     According to another aspect of the invention, there is provided an electrode assembly comprising a first electrode strip including a first electrode collector coated with at least a first electrode active material, an exposed portion of the first electrode collector attached with a first electrode tab; a second electrode strip which includes a second electrode collector coated with at least a second electrode active material and is rolled together with the first electrode strip, an exposed portion of the second electrode collector attached with a second electrode tab; and at least an inter-electrode strip separator positioned between the first and second electrode strips. At least a sheet of protective separator, which is formed to be united with the at least one inter-electrode strip separator, and the exposed portion of the first electrode collector are further positioned at a side of the first electrode strip attached with the first electrode tab. 
     The first electrode tab may be positioned at a center of the electrode assembly. At least one of the at least one inter-electrode strip separator and the at least one protective separator may be positioned at the opposite side of the side of the first electrode strip attached with the first electrode tab. 
     The at least one protective separator may be an end of the at least one inter-electrode strip separator. The at least one protective separator, which is the end of the at least one inter-electrode strip separator, may be bent twice and sandwiched between the first and second electrode strips. 
     The exposed portion of the first electrode collector may be extended to the side of the first electrode strip attached with the first electrode tab and bent twice and interposed between the first and second electrode strips. 
     The at least one protective separator may be extended from the inter-electrode strip separator while being wound in the opposite direction in which the electrode assembly is rolled. 
     The exposed portion of the first electrode collector may be extended to the side of the first electrode strip attached with the first electrode tab while the exposed portion being wound in the opposite direction in which the electrode assembly is rolled. 
     The electrode assembly may be a rolled structure in which a sheet of the inter-electrode strip separator, the first electrode strip, a sheet of separator, and the second electrode strip are sequentially positioned, wherein the protective separator is extended from at least one of the two sheets of separators. 
     Starting points of applying the first and second electrode active material may be the same. 
     The second electrode strip may be positioned via the protective separator and the exposed portion of the first electrode collector on the side of the first electrode strip attached with the first electrode tab. 
     According to another aspect of the invention, there is provided an electrode assembly comprising a first electrode strip including a first electrode collector coated with at least a first electrode active material, an exposed portion of the first electrode collector attached with a first electrode tab; and a second electrode strip including a second electrode collector coated with at least a second electrode active material, an exposed portion of the second electrode collector attached with a second electrode tab. The respective first and second electrode strips are sandwiched between two sheets of separators or positioned on one of the two sheets of separators so as to make a multi-layer structure, the multi-layer structure is inserted into a mandrel and rolled using the mandrel, and at least one of the two sheets of separators and the exposed portion of the first electrode collector are inserted into the mandrel and their portions are drawn out from the mandrel by a length equal to at least half a circumference of the mandrel, and are rolled. 
     The drawn portions may be extended to a side of the first electrode strip attached with the first electrode tab. 
     The second electrode strip may be positioned on the side of the first electrode strip attached with the first electrode tab, via the drawn portion of the separator and at least a separator between the first and second electrode strips. 
     The drawn portion of the extended portion of the first electrode collector may be extended to the side of the first electrode strip attached with the first electrode tab, wherein the second electrode strip is positioned via the drawn portion of the separator on a side of the drawn portion of the extended portion of the first electrode collector. 
     A length of the drawn portion of the exposed portion of the first electrode collector may be longer than a sum of half a length of the mandrel in circumference and a distance between the mandrel and the side of the first electrode strip attached with the first electrode tab, and a length of the drawn portion of the separator may be longer than that of the drawn portion of the exposed portion of the first electrode collector. 
     The drawn portion of the exposed portion of the first electrode collector may be wound half a round of the mandrel, and, then, the first electrode strip is rolled. 
     The first electrode tab may be positioned on a side of the first electrode collector toward a center portion of the electrode assembly. 
     The drawn portions of the separator and the exposed portion of the first electrode collector may be drawn out from the mandrel while the drawn portions are wound in the opposite direction in which the electrode assembly is rolled. 
     The separators may include a material selected from a group including polyethylene, polypropylene, and a copolymer of polyethylene and polypropylene, have a multi-layered structure, or include a polyolefin-based polymer with a molecular weight of 350 thousand or greater. 
     An active material may not be applied onto a portion of the first or second electrode strip which resides on the other adjacent second or first electrode strip of different polarity via an active material of the other electrode strip and the separators. 
     The first active material may include a lithium-based oxide and the second electrode active material comprises a carbon-based material. The first electrode collector may be formed of aluminum or an aluminum alloy and the second electrode collector may be formed of copper or a copper alloy. 
     According to another aspect of the invention, there is provided a secondary battery including an electrode assembly placed in a cylindrical or rectangular can or a pouch case. 
     Additional aspects and/or advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the embodiments taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a cross-sectional view of a cylindrical secondary battery including an electrode assembly according to an embodiment of the invention; 
         FIG. 2  is a cross-sectional view of a center portion of an electrode assembly according to an embodiment of the invention; 
         FIG. 3  illustrates a method of rolling the electrode assembly of  FIG. 2  according to an embodiment of the invention; 
         FIG. 4  is a cross-sectional view of the center portion of the electrode assembly of  FIG. 1 ; 
         FIG. 5  is a cross-sectional view of the center portion of the electrode assembly of  FIG. 2 ; 
         FIG. 6  is a cross-sectional view of a center portion of an electrode assembly according to another embodiment of the invention; 
         FIG. 7  illustrates a method of rolling of the electrode assembly of  FIG. 6  according to an embodiment of the invention; 
         FIG. 8  is a cross-sectional view of the center portion of the electrode assembly of  FIG. 6 ; 
         FIG. 9  is another cross-sectional view of the center portion of the electrode assembly of  FIG. 6 ; and 
         FIG. 10  is another cross-sectional view of the center portion of the electrode assembly of  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below to explain the present invention by referring to the figures. 
     The invention is described with respect to a cylindrical secondary battery with a jelly-roll type electrode assembly; however, the invention is not limited to this disclosure. For example, if a battery can include a jelly-roll type electrode assembly according to the present invention, a shape of the battery is not limited to any particular shape. Further, the present invention is also applicable to a primary battery. 
       FIG. 1  is a cross-sectional view of a cylindrical secondary battery with a jelly-roll type electrode assembly  20  according to an aspect of the invention. Referring to  FIG. 1 , the electrode assembly  20  is placed in a cylindrical can  1 , and the electrode assembly  20  is a rolled multi-layer structure in which a first electrode strip  21  and a second electrode strip  31  are positioned and a separator  40  is interposed between the first and second electrode strips  21  and  31  so as to insulate them from each other. The cylindrical can  1  is formed of a metal material so that it acts as a connector to the first or second electrode strip  21  or  31 . The can  1  is preferably formed of aluminum or an aluminum alloy and electrically connected to the second electrode strip  31  to form a second electrode connector. 
     In the electrode assembly  20 , a first electrode tab  25  and a second electrode tab  35  are drawn out from the first and second electrode strips  21  and  31 , respectively. More specifically, the first electrode tab  25  is drawn out from a center portion of the electrode assembly  20  and the second electrode tab  35  is drawn out from an edge of the electrode assembly  20 . The first electrode tab  25  is drawn out upward to be electrically connected to a cab assembly  2  for sealing the can  1 . The second electrode tab  35  is drawn out downward to be electrically connected to a bottom  3  of the can  1 . The cab assembly  2  connected to the first electrode tab  25  acts as a first electrode connector and the can  1  connected to the second electrode tab  35  acts as the second electrode connector. 
     Referring to  FIG. 2 , an electrode assembly  20  according to an aspect of the invention includes a first electrode strip  21  and a second electrode strip  31 . In this embodiment, the first electrode strip  21  and the second electrode strip  31  act as a positive electrode strip and a negative electrode strip, respectively. However, the invention is not limited to the above description. That is, the first electrode strip  21  and the second electrode strip  31  may act as a negative electrode strip and a positive electrode strip, respectively. 
     As shown in  FIG. 2 , the first electrode strip  21  includes a first electrode collector  22  and an electrode mixture  23  containing an electrode active material, and the second electrode strip  31  includes a second electrode collector  32  and an electrode mixture  33  containing an electrode active material.  FIG. 2  illustrates a center portion of the rolled electrode assembly  20  according to an aspect of the invention.  FIG. 3  illustrates a method of rolling the electrode assembly  20  of  FIG. 2 , according to an aspect of the invention. 
     Hereinafter, the electrode assembly  20  will be described with reference to  FIGS. 2 and 3  in a greater detail. 
     When the electrode assembly  20  is used in conjunction with a lithium secondary battery, the first electrode strip  21  may be used as a positive electrode strip. In this case, the first electrode collector  22  is formed using a film type thin metal plate, preferably, a thin aluminum film. The first electrode mixture  23 , which is formed at least one surface of the first electrode collector  22 , may be a mixture of a first electrode active material (a lithium-based oxide), a binder, a plasticizer, and/or a conductive material. The first electrode collector  22  has at least one surface covered with the first electrode mixture  23  and an exposed portion  24 , which is not coated with the first electrode mixture  23 . The exposed portion is preferably a front end. 
     The second electrode strip  31 , i.e., a negative electrode strip, may include the second electrode collector  32  made of a thin metal film, preferably copper. The second electrode mixture  33 , which is formed at least one surface of the second electrode collector  32 , may be made of a mixture of a carbon-based second electrode active material, a binder, a plasticizer, and/or a conductive material. The second electrode collector  32  includes at least one surface covered with the second electrode mixture  33  and includes an exposed portion (not shown), which is not covered with the second electrode mixture  33 , at the electrode assembly  20 . As shown in  FIG. 1 , the exposed portion  34  may be welded together with a second electrode tab  35 . Also, as shown in  FIG. 2 , the second electrode collector  32  includes an exposed portion  34 , which is not covered with the second electrode mixture  33 , at its front end, the exposed portion  34  extending beyond a center portion of the electrode assembly  20 , thus allowing the electrode assembly  20  to be more easily inserted into and rolled by a mandrel. 
     A first electrode tab  25  attached to the exposed portion  24  may be formed of aluminum material and a second electrode tab  35  attached to the outermost winding of the electrode assembly  20  may be formed of a nickel material. 
     The electrode assembly  20  is a rolled multi-layer structure in which the first electrode strip  21 , at least a separator  40 , and the second electrode strip  31  are sequentially positioned. The separator  40  insulates the first electrode strip  21  and the second electrode strip  31  from each other and allows for an exchange of active material ions between the first and second electrode strips  21  and  31 , thus causing a battery reaction. The separator  40  is preferably long enough to completely insulate the first and second electrode strips  21  and  31  from each other during a contraction or expansion phase. 
     If the separator  40  is applicable to a lithium secondary battery, the type of the separator  40  is not limited. For example, the separator  40  may be formed of a material selected from a group including polyethylene, polypropylene, and a copolymer of polyethylene and polypropylene. Otherwise, the separator  40  may be formed of a polyolefin-based polymer with a molecular weight of at least 350 thousand or be a multi-layered polymer. 
     The separator  40  may be made by biaxially orienting a gel-type sheet and removing a solvent from the sheet. Here, the gel-type sheet is formed using a composition (solution) with 18 percentage by weight of UHMWPE with an average modular weight from 200 thousand to 250 thousand and a composition (solution) with 82 percentage by weight of HDPE with an average modular weight from 40 thousand to 50 thousand. Alternatively, the separator  40  may be a three-layered structure of polypropylene, polyethylene, and polypropylene, wherein a diameter of a micro hole of each element ranges from 0.01 μm to 0.05 μm. Otherwise, the separator  40  may be made by fusing two sheets of polyethylene, or be made of a sheet of polyethylene. The fusing may be performed using heat. 
     The electrode assembly  20  is placed into the can  1 , wherein the can  1  is filled with an electrolyte solution. The electrolyte solution may be a mixture (solution) of lithium salt and a carbonate-based organic solvent. Further, a halogenized aromatic compound such as fluorinated benzene may be added to the electrolyte solution. 
     According to the invention, the separator  40  includes two sheets of the first and second separators  41  and  42 . The first electrode strip  21  and the second electrode strip  31  are formed between the first and second separators  41  and  42  and on sides of the first or second separator  41  or  42 , respectively. As shown in  FIGS. 2 and 3 , the first separator  41  is sandwiched between the first and second electrode strips  21  and  31  and the second separator  42  is formed at a side of the first electrode strip  21 . Accordingly, since the electrode assembly  20  is rolled using the two sheets of first and second separators  41  and  42 , the first and second electrode strips  21  and  31  are insulated from each other. However, because the first and second electrode strips  21  and  31  are insulated from each other, arrangement of the first and second separators  41  and  42  is not limited to the above description. For instance, it is possible to manufacture the electrode assembly  20  to include a sheet of separator between the first and second electrode strips  21  and  31  or on a side of one of the first and second electrode strips  21  and  31 . 
     The first separator  41  includes a first inter-electrode strip separator  41   a  and a first protective separator  41   b  which is extended from the first inter-electrode strip separator  41   a . The second separator  42  includes a second inter-electrode strip separator  42   a  and a second protective separator  42   b , which is extended from the second inter-electrode strip separator  42   a . The first and second inter-electrode strip separators  41   a  and  42   a  are used as separation films of the secondary battery, allowing for an exchange of active material ions between the first and second electrode strips  21  and  31  coated with the first and second electrode mixtures  23  and  33 , respectively. As shown in  FIG. 2 , the first and second protective separators  41   b  and  42   b  are extended from the first and second inter-electrode strip separators  41   a  and  42   a , respectively. At least one of the first and second protective separators  41   b  and  42   b  is further positioned on a side of the first electrode strip  21  attached with the first electrode tab  25 , thereby preventing electrical short-circuiting between the first and second electrode strips  21  and  31 . 
     It is preferable that the first and second protective separators  41   b  and  42   b  are ends of the first and second inter-electrode strip separators  41   a  and  42   a  and further positioned on the opposite side of the side of the first electrode strip  21  attached with the first electrode tab  25 . Referring to  FIG. 2 , the second electrode strip  31  may be positioned on the opposite side of the side of the first electrode strip  21  attached with the first electrode tab  25 . In this way, it is possible to prevent electrical short-circuiting between the first and second electrode strips  21  and  31  resulting from damage to the separator caused by heating of the first electrode tab  25  attached to the first electrode strip  21  due to a concentration of electric charges. 
     In other words, as shown in  FIG. 2 , the first and second protective separators  41   b  and  42   b , which are the ends of the first and second inter-electrode strip separators  41   a  and  42   a , are further interposed between the side of the first electrode strip  21  attached with the first electrode tab  25  and the second electrode strip  31 , thus increasing insulation between the first and second electrode strips  21  and  31 . Thus, even if the inter-electrode strip separators  41   a  and  42   a  are damaged due to heat, the electrical short-circuiting may be avoided between the first and second electrode strips  21  and  31  because of the first and second protective separators  41   b  and  42   b.    
     In addition, it is possible to prevent electrical short-circuiting between the first and second electrode strips  21  and  31  caused by a breaking of the separator due to a surface defect, such as a burr, of the first electrode tab  25 . As shown in  FIG. 2 , only the separators are located on the opposite side of the side of the first electrode strip  21  attached with the first electrode tab  25  so as to completely prevent electrical short-circuiting at the side of the first electrode strip  21  with the first electrode tab  25 . 
     In this embodiment, as shown in  FIG. 2 , both or one of the first and second protective separators  41   b  and  42   b  may be positioned on the side of the first electrode strip  21  with the first electrode tab  25 . 
     As shown in  FIG. 2 , the first and second protective separators  41   b  and  42   b  are bent twice while being extended from the first and second inter-electrode separators  41   a  and  42   a , respectively, extended while being wound in the opposite direction of rolling the electrode assembly  20 , and then are positioned on the opposite side of a side of the first inter-electrode strip separator  41   a , toward the center portion of the electrode assembly, between the first and second electrode strips  21  and  31 . Although not shown in the drawings, when only the second protective separator  42   b  is extended and positioned on the opposite side of the first inter-electrode strip separator  41   a  between the first and second electrode strips  21  and  31 , the second protective separator  42   b  may be extended to the first electrode tab  25  while being inserted between the first inter-electrode strip separator  41   a  and the first electrode strip  21 . 
     Also, although not shown in the drawings, both the first and second inter-electrode strip separators  41   a  and  42   a  may be positioned on the opposite side of the side of the first electrode collector  22  attached with the first electrode tab  25 . Also, a portion of the first electrode collector  22  may be exposed while at least two separators, e.g., the first and second inter-electrode strip separators  41   a  and  42   a , are positioned on the opposite side of the side of the first electrode strip  21  attached with the first electrode tab  25 . 
     General equipment used for rolling the electrode assembly  20  may be used for the insertion of the first and second protective separators  41   b  and  42   b . For instance, when rolling the electrode assembly  20 , using a device such as the mandrel  50  shown in  FIG. 3 , the electrode assembly  20  is fabricated by inserting two sheets of the first and second separators  41  and  42  into the mandrel  50 , sandwiching the first electrode strip  21  between the first and second separators  41  and  42 , placing the second electrode strip  31  on the other side of the first separator  41 , and rotating the mandrel  50  in a direction as shown by the arrow in  FIG. 3 . 
     More specifically, the first and second separators  41  and  42  are inserted into the mandrel  50 , a portion of each of these separators  41  and  42  is extended out from an end of the mandrel  50  by a length L 1 , and the mandrel  50  is rotated. Then, the portions of the lengths L 1  of the first and second separators  41  and  42 , which pass through the mandrel  50 , are rotated in the opposite direction in which the mandrel  50  is rolled, as shown in  FIG. 3 . The rotated portions of the first and second separators  41  and  42  are extended to the portion of the first electrode strip  21  attached with the first electrode tab  25 , so that an upper portion of the first separator  41  is covered with these portions. Next, the mandrel  50  is kept rolled to obtain the first and second protective separators  41   b  and  42   b  of  FIG. 3 . 
     The lengths L 1  of the portions of the first and second separators  41  and  42 , which are drawn out from the mandrel  50 , must be at least half a length of the mandrel  50  in circumference. In this case, as shown in  FIG. 3 , the first electrode tab  25  is welded at a front end of the exposed portion  24  and the electrode assembly  20  is rolled while a front end of the first electrode tab  21  is placed adjacent to a mouth of the mandrel  50 . More specifically, the lengths L 1  of the portions of the first and second separators  41  and  42  must be longer than a sum of half the length of the mandrel  50  in circumference and a distance between the mandrel  50  and the side of the first separator  41  attached with the first electrode tab  25 , so that the upper portion of the first separator  41  attached with the first electrode tab  25  is entirely covered with and protected by the portions of the first and second separators  41  and  42 . 
     If the lengths L 1  of the portions of the first and second separators  41  and  42  are extremely long, it unnecessarily increases the thickness of the electrode assembly  20 . As shown in  FIG. 3 , the lengths L 1  are preferably determined such that the first and second protective separators  41   b  and  42   b , of the first and second separators  41  and  42 , which are drawn out from the mandrel  50 , are one-half a circumference of the mandrel  50  and are extended to a beginning of the second electrode strip  31 . In other words, the lengths L 1  are preferably equal to or less than a sum of half the circumference of the mandrel  50  and the distance between the mandrel  50  and an end of the second electrode strip  31  nearest to the mandrel  50 . 
     After rolling the electrode assembly  20 , the mandrel  50  is removed to obtain the electrode assembly  20  shown in  FIG. 2 . Otherwise, the mandrel  50  may be kept to be inserted into the electrode assembly  20 . 
     Meanwhile, as shown in  FIG. 2 , it is possible to further prevent electrical short-circuiting between the first and second electrode strips  21  and  31  by welding the first electrode tab  25  to a side of the first electrode collector  22  toward the center of the electrode assembly  20 . 
     In detail, when the first electrode tab  25  is welded to the side of the first electrode collector  22  toward the center of the electrode assembly  20 , the first electrode tab  25  confronts the center of the electrode assembly  20  and only the first or second separator  41  or  42  contacts the first electrode tab  25 . Therefore, even if the first electrode tab  25  includes a surface defect such as a burr, electrical short-circuiting is not caused. 
     Further, even if the burr of the first electrode tab  25  projects outside the electrode assembly  20 , electrical short-circuiting does not occur unless all the first electrode collector  22  and the first and second protective separators  41   b  and  42   b  are damaged. 
     In addition, installation of the first and second protective separators  41   b  and  42   b  at the opposite side of the side of the first electrode strip  21  attached with the first electrode tab  25  prevents electrical short-circuiting between the first and second electrode strips  21  and  31  caused by heating of the first electrode tab  25 . 
     As described above with reference to  FIG. 2 , the invention is described with respect to a case where the first electrode tab  25  is welded to the side of the first electrode collector  22  facing the center portion of the electrode assembly  20 . Alternatively, as shown in  FIG. 4 , electrical short-circuiting may be prevented by welding the first electrode tab  25  to a side of the first electrode collector  22  facing an outer side of the electrode assembly  20 . 
     When protecting the side of the first electrode strip  21  attached with the first electrode tab  25  using the first and second protective separators  41   b  and  42   b , starting points of the electrode mixtures  23  and  33  of the electrode strips  21  and  31  are constructed as shown in  FIGS. 2 through 4 . In this case, a battery reaction area is increased by rolling the electrode assembly  20 , even to a small degree, thereby maximizing battery capacity. Although not shown in the drawings, an area available for battery reaction may be increased by forming a starting point of the first electrode strip  21  where the first electrode mixture  23  starts to be adjacent to the first electrode tab  25 . 
     An active material may not be applied on a side of the first or second electrode strip  21  or  31  that does not contact an active material of the first or second electrode strip  21  or  31  of the opposite polarity via the first or second separator  41  or  42 . More specifically, as shown in  FIG. 5 , an active material layer  33   a  and another active material layer  33   b  are positioned on a side of a second electrode collector  32  facing a center portion of the electrode assembly  20  and the opposite side thereof, respectively. Starting points of the active material layers  33   a  and  33   b , at a front end of the second electrode strip  31 , that is provided on the first electrode strip  21  via the separators  41  and  42  toward the center of the rolled electrode assembly  20 , are different from each other. A side of the front end of the second electrode strip  31 , which is unavailable for battery reaction, is not covered with an electrode mixture. Accordingly, increasing the volume of the electrode assembly  20  is not necessary. Such asymmetrical application of an active material is applicable to not only the first and second electrode strips  21  and  31  but also an outermost surface of a battery. 
     According to the invention, starting points of the first and second electrode strips  21  and  31 , which are covered with an electrode mixture or not covered with the electrode mixture, may be set in various positions or locations. 
       FIG. 6  is a cross-sectional view of a center portion of an electrode assembly  20  according to an aspect of the invention. Referring to  FIG. 6 , in addition to the first and second protective separators  41   b  and  42   b , an exposed portion  24  of the first electrode strip  21  of a first electrode collector  22  may be positioned on the side of a first electrode strip  21  attached with a first electrode tab  25  in order to prevent electrical short-circuiting at the side of the first electrode strip  21  attached with the first electrode tab  25 . 
     More specifically, a portion  24   a  extended from the exposed portion  24  is positioned between the first and second protective separators  41   b  and  42   b  on the other side of a side of a first inter-electrode strip separator  41   a  facing the opposite side of the side of the first electrode strip  21  attached with the first electrode tab  25 . Accordingly, even if first and second separators  41  and  42  located on the side of the electrode strip  21  attached with the first electrode tab  25  are removed or torn-out, the portion  24   a  of the same polarity is positioned on the other side of the first electrode strip  21 , thus preventing of electrical short-circuiting between the first and second electrode strips  21  and  31 . 
       FIG. 7  illustrates a method of rolling of the electrode assembly of  FIG. 6  according to an aspect of the invention. As described above with reference to  FIG. 3 , the first and second protective separators  41   b  and  42   b  are inserted into a mandrel  50  and their respective portions are drawn out from the mandrel  50  by a length L 1 . Next, a portion  24   a  of the exposed portion  24  of the first electrode strip  21  between the first and second separators  41  and  42 , is inserted into the mandrel  50  and the portion  24   a  is drawn out from the mandrel  50  by a length L 2 . The drawn portions of the first and second protective separators  41   b  and  42   b  and the drawn portion of the portion  24   a  are then rolled by rotating the mandrel  50  in a predetermined direction. For example, the mandrel  50  may be rolled in a direction as shown by the arrow as shown in  FIG. 7 . The length L 2  of the portion  24   a  passing through the mandrel  50  must be at least half the circumference of the mandrel  50 . More particularly, the length L 2  must be longer than a sum of half the length of the mandrel  50  in circumference and a distance between the mandrel  50  and the first electrode tab  25 . Also, the length L 2  of the portion  24   a  must be shorter than the lengths L 1  of the first and second protective separators  41   b  and  42   b  passing through the mandrel  50 . If the length L 2  is longer than the length L 1 , the portion  24   a  may contact the second electrode strip  31  when the portion  24   a  is rolled back by the mandrel  50 . 
     Similar to the rolled portions of the first and second protective separators  41   b  and  42   b , the rolled portion of the portion  24   a  is bent twice by the mandrel  50  and positioned on the opposite side of the side of the first electrode strip  21  attached with the first electrode tab  25 . That is, the portion  24   a  is extended to the side of the first electrode strip  21  attached with the first electrode tab  25  while being wound in the opposite direction in which the electrode assembly  20  is rolled. 
     The method of  FIG. 7  includes a situation where the first electrode tab  25  is welded to a side of the first electrode collector  22  toward a center portion of the first electrode assembly  20 . However, the method of  FIG. 7  is also applicable to a situation where the first electrode tab  25  may be welded to the other side of the first electrode collector  22 , as shown in  FIG. 8 . 
     Alternatively, as shown in  FIG. 9 , only the first protective separator  41   a  may be extended to the side of the first electrode strip  21  attached with the first electrode tab  25  while the portion  24   a  is not positioned between the first and second protective separators  41   b  and  42   b . In this case, the portion  24   a  of the exposed portion  24  protects the opposite side of the side of the first electrode strip  21  attached with the first electrode tab  25 . 
     As describe above, in the electrode assembly  20 , an electrode mixture may not be applied onto a side of the first or second electrode strip  21  or  31  that does not contact an active material layer of the other adjacent electrode strip  21  or  31  of different polarity and the separators  41  and  42 . That is, referring to  FIG. 10 , the active material layer  33   a  is positioned on a side of the second electrode collector  32  facing the center of the electrode assembly  20  and the active material layer  33   b  is positioned on the other side. Starting points of the active material layers  33   a  and  33   b  are different from each other at an end of the second electrode strip  31 , which resides on the first electrode strip  21  via the separators  41  and  42 . An electrode mixture is not applied onto a front side of the first or second electrode strip  21  or  31  that is not available for battery reaction. Accordingly, volume of the electrode assembly  20  is not unnecessarily increased. Such asymmetrical application of the active material is also applicable to the outermost surface of a battery. 
     The invention is applicable to manufacturing various shapes, sizes, and types of electrode assembly, including the cylindrical batteries shown in  FIGS. 1 through 10 . For example, a rectangular or pouch-type battery may be fabricated by applying pressure onto an electrode assembly according to the present invention and placing the flattened electrode assembly into a rectangular can or a pouch case. 
     In addition to the method described above of protecting the side of the first electrode strip  21  attached with the first electrode tab  25 , it is also possible to protect a side of the second electrode strip  31  attached with the second electrode tab  35 . 
     Table 1 shows results obtained when performing several tests to evaluate the effectiveness of a secondary battery with a structure shown in  FIG. 2  compared to that of a conventional secondary battery in which a sheet of separator is positioned on a side of an electrode strip attached with an electrode tab. Five secondary batteries according to the present invention and five conventional secondary batteries were used in the tests. 
     
       
         
           
               
               
               
            
               
                   
                   
               
               
                   
                 Secondary Batteries 
                 Conventional 
               
               
                   
                 According to 
                 Secondary 
               
               
                   
                 the Invention 
                 Batteries 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Item 
                 L0 
                 L1 
                 L2 
                 L3 
                 L4 
                 L5 
                 L0 
                 L1 
                 L2 
                 L3 
                 L4 
                 L5 
               
               
                   
               
               
                 Overcharge Test 
                 5 
                   
                   
                   
                   
                   
                 5 
                   
                   
                   
                   
                   
               
               
                 Penetration Test 
                   
                 5 
                   
                   
                   
                   
                   
                 5 
               
               
                 Crush Test 
                   
                 5 
                   
                   
                   
                   
                   
                 2 
                 3 
               
               
                 Impact Test 
                 5 
                   
                   
                   
                   
                   
                 3 
                   
                   
                 2 
               
               
                 Drop Test 
                 5 
                   
                   
                   
                   
                   
                 5 
               
               
                 (1.9 M) 
               
               
                 Drop Test (10 M) 
                 5 
                   
                   
                   
                   
                   
                 5 
               
               
                 Thermal 
                   
                   
                   
                   
                   
                 13 min. or 
                   
                   
                   
                   
                   
                 10 min. 
               
               
                 Exposure Test 
                   
                   
                   
                   
                   
                 more 
                   
                   
                   
                   
                   
                 or more 
               
               
                   
               
            
           
         
       
     
     In Table 1, the secondary batteries according to the present invention and the conventional secondary batteries include the same type of separators. In particular, a sheet of separator is installed at a side of an electrode strip attached with an electrode tab of each of the conventional secondary batteries, the side facing a center of a rolled electrode assembly. Three sheets of separators are installed at a side of an electrode strip attached with an electrode tab of each of the secondary batteries according to the present invention, the side facing a center of an electrode assembly. 
     In the overcharge test of Table 1, the secondary batteries and the conventional batteries were charged to 250 percent of the rated capacity and exteriors of the batteries were observed. 
     In the penetration test, after standard charging of the secondary batteries and the conventional secondary batteries, the charging stopped within a range from 10 minutes to 72 hours. Next, the batteries were placed on a plane surface, a conductive tool, such as a nail or a chisel, was penetrated into the batteries, and the penetration was maintained until their surface temperatures reached 40° C. or less. The conductive tool was then removed from the batteries and the battery surface temperatures and exteriors were observed for 10 minutes. 
     In the impact test, after standard charging of the secondary batteries and the conventional secondary batteries, the charging stopped within a range from 10 minutes to 72 hours. Next, each battery was placed on a plane surface and fixed with a jig, a bar with a diameter of 15.8 mm was laid on the battery approximately perpendicular to the battery in a lengthwise direction. An object of 9.1 kg was then dropped on the battery from a height of 61 cm. Thereafter, the battery was left alone until the battery surface temperature reached 40° C. or less, the jig was removed, and the battery surface temperature and exterior were observed for 10 minutes. 
     In the crush test, after standard charging of the secondary batteries and the conventional secondary batteries, the charging stopped within a range from 10 minutes to 72 hours. Each battery was placed between two sheets of steel plates while being fixed with a jig, and a force of 13 KN was applied onto the battery or a pressure was applied onto the battery to press it to two-thirds of the original thickness. Thereafter, the battery was left alone until its surface temperature reaches 40° C. or less, the jig was removed, and then, the battery surface temperature and exterior were observed for 10 minutes. 
     In the drop test, the secondary batteries and the conventional batteries were dropped from heights of 1.9 m and 10 m, and their exteriors were observed. 
     In the thermal exposure test, after standard charging of the secondary batteries and the conventional ones, the charging was stopped within a range from 10 minutes to 72 hours. Each battery was then placed in a chamber, battery surface temperature was increased by 5° C. per minute, and the battery surface temperature and exterior were observed while maintaining current battery state for 1 hour when the battery surface temperature reached 150° C. 
     In Table 1, L 0  includes a situation where there is no change regarding the exteriors of the batteries. In addition, L 0  includes cases where there is no flash of light even when battery leakage is detected, and the battery exteriors are changed due to impacts applied during the tests while the batteries are remain hermetic. 
     In Table 1, L 1  includes situations where the original battery weight decreases by at least 0.1 percent due to loss of an inner part of battery such as an electrolyte, a ruptured vent, and/or electrolyte leakage is visually detected. In general, electrolyte leakage is determined by measuring the weight of a battery after a test and comparing the measured weight with the original weight. However, such a process may be omitted from a battery safety test. Further, L 1  includes leakage of electrolyte from the inside of battery due to battery components breakage caused by impacts applied during a test without causing smoke, emission of gas, or fire. 
     L 2  includes a situation where smoke, such as vapor, emits from the inside of a battery without causing sudden heating and battery surface temperature is less than 200° C. 
     L 3  includes a situation where smoke emits from the inside of a battery causing sudden heating, battery surface temperature is more than 200° C., gas emits intensely, and a battery surface temperature is greater than 200° C. due to sudden heating of the battery, even if no smoke is generated from the side of battery. 
     L 4  includes a situation where battery contents are spontaneously ignited and burned causing violent flames or of the exterior of the battery is burned. 
     L 5  includes a situation where battery contents come out from a battery due to a pressure applied from the inside of the battery, and portions of a battery case are broken into pieces, the portions do not include a vent of a battery. 
     In the thermal exposure test, a time required for each battery to enter the situation L 5  is measured, starting from when the temperature of a chamber reaches 150° C. 
     As is apparent from Table 1, the secondary batteries and the conventional batteries showed the same results in the overcharge test, the penetration test, and the drop test. However, in the crush test, battery leakage was detected from all five secondary batteries, but battery leakage was detected from two of the conventional batteries and flashes of light were generated in the other three conventional batteries. In the impact test, none of the five secondary batteries showed any sign of burning but two of the conventional batteries ignited causing flames. In the thermal exposure test, the five secondary batteries exploded at least 13 minutes after the temperature of a chamber reached 150° C., but the conventional batteries exploded only 10 minutes after the temperature of a chamber reached 150° C. 
     The test results shown in Table 1 revealed that including an electrode assembly, according to the invention, into a secondary battery improves battery safety. In particular, the thermal exposure test showed that the explosion of a secondary battery including the electrode assembly according to the invention may be delayed by at least 3 minutes compared to that of a conventional battery. 
     As described above, a secondary battery according to the invention has the following advantages. First, the secondary battery does not under go much electrical short-circuiting between electrodes as a result of a separator breaking caused by heating a side of an electrode strip attached with an electrode tab. Second, electrical short-circuiting between electrodes may be prevented even if an electrode tab includes a surface defect, such as a burr. Third, battery safety is improved. Four, electrical short-circuiting may be prevented at a side of an electrode strip attached with an electrode tab even if a secondary battery is manufactured using conventional equipment. Fifth, the volume of an electrode assembly is not unnecessarily increased by having starting points of electrode mixtures positioned on first and second electrode strips the same. 
     Although a few embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in this embodiment without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.