Abstract:
A secondary cell capable of ensuring thermal and mechanical stabilities required for a high-capacity cell with a simple structure is provided. The secondary cell includes: a can; and an electrode jelly-roll wound with two different electrodes and a separator interposed between the electrodes therein and accommodated in the can, the outer surface of the electrode jelly-roll being wound around one more turn with the separator. Only the separator is wound at the core of the electrode jelly-roll to form a rod-like stability member which is cured by absorbing heat generated from the cell. The separator wound at the core of the electrode jelly-roll is continuous from a portion of the separator which is stacked with the two different electrodes.

Description:
CLAIM OF PRIORITY 
   This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. §119 from my application entitled ELECTRODE JELLY-ROLL OF SECONDARY CELL filed with the Korean Industrial Property Office on 16 Oct. 2001 and there duly assigned Ser. No. 2001-63715. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a secondary cell with an electrode jelly roll in which a cathode, separator, and anode are wound together, and more particularly, a secondary cell with an electrode jelly-roll, which is improved in structural and thermal stabilities. 
   2. Description of the Related Art 
   Secondary cells are rechargeable and can be made into a smaller size with high capacity. Typical examples of secondary cells include nickel-metal hydride (Ni-MH) cells, lithium cells, lithium ion cells (Li-ion), and polymer lithium cells. Secondary cells are classified into cylindrical cells, rectangular cells, pouch type cells containing electrode jelly-rolls in pouches. 
   A secondary cell includes an electrode jelly-roll wound to be circular or elliptical. The electrode jelly-roll is attained by coating a substrate with an active material, and drying, roll-pressing and cutting the substrate to form a cathode and anode, and by winding the cathode and anode with a separator therebetween. A circular cell is formed by winding the electrode jelly-roll to have a circular cross-section, accommodating it in a cylindrical can, filling the can with an electrolyte solution, and sealing the can. A rectangular cell is formed by flattening the electrode jelly-roll under pressure and accommodating it in a rectangular can. 
   For example, a cylindrical secondary cell having a structure of an electrode jelly-roll, includes an electrode such as a cathode and anode can be manufactured by different methods according to the type of electrode. In general a cathode and anode are formed by coating a slurry containing a cathode active material and a slurry containing an anode active material on both sides of respective substrates, and drying, roll-pressing and cutting the substrates to a predetermined size. A separator is interposed between the cathode and the anode to prevent the cathode and anode from being electrically connected, and then wound in a roll. 
   The resulting electrode jelly-roll is placed in a can, a cap assembly is mounted on the top of the can to be connected with the cathode of the electrode jelly-roll, and the can is filled with an electrolyte solution and sealed, resulting in a cylindrical cell. 
   In such a cylindrical cell, the anode substrate contacts the inner wall of the can at the outer-side of the electrode jelly-roll, or an anode tap welded to the anode substrate contacts the bottom of the can. A tap at the core of the electrode jelly-roll, extending from the cathode substrate, is connected to the cap assembly. On the top and bottom surfaces of the electrode jelly-roll, insulating plates are placed to prevent short-circuiting between the cap assembly and the can. 
   Many efforts have been made to manufacture a high-capacity cell by tightly winding thin, long electrodes such that a large amount of active material can be incorporated into the cylindrical cell having the structure described above. 
   Another consideration to be taken into account in manufacturing a high-capacity cell is the mechanical and thermal stabilities of the cell. 
   A problem of thermal stability in a cell is caused as a result of the heat generated from the reaction in the cell cannot be effectively dissipated. This problem occurs when the electrodes of the cell are tightly wound so that the heat generated from the inside cannot be effectively dissipated to the outside. As a result, the temperature of the cell continues to rise and a thermal runaway phenomenon occurs, thereby degrading the stability of the cell. 
   A problem of mechanical stability in a cell refers to a reduction in cell stability when the electrodes are damaged due to external environments such as physical impacts. 
   An effort to improve the heat-dissipating structure of a cell has been made to prevent a reduction in thermal stability of the cell. In particular, U.S. Pat. No. 5,571,632 for  Nonaqueous Electrolyte Solution Secondary Cell and Method for Producing the Same  by Teramoto discloses a “non-aqueous electrolyte secondary cell” having a structure in which the cathode is welded to an inner Al tube and the anode is welded to a Ni foil on the outer side of the electrode jelly-roll. 
   To manufacture the secondary cell having the structure described above, an inner tube is formed through the can, the electrode is welded to the inner tube, and a separate member is welded to the outer side of the electrode jelly-roll. This structure of the secondary cell adds complexity to the manufacturing process and therefore is burdensome. Also, the structure of the secondary cell differs from that of a conventional cylindrical cell, so equipment commonly used to manufacture the secondary cell should be replaced. In addition, incorporation of such a large-volume inner tube limits to wind the electrode tightly for a high-capacity, compact cell. 
   SUMMARY OF THE INVENTION 
   To solve the above-described and other problems, it is an object of the present invention to provide a secondary cell capable of ensuring thermal and mechanical stabilities required for a high-capacity cell with a simple structure. 
   To achieve the above and other objects of the present invention, there is provided a secondary cell including: a can; and an electrode jelly-roll wound with two different electrodes and a separator interposed between the electrodes therein and accommodated in the can, the outer surface of the electrode jelly-roll being wound around one more turn with the separator. 
   It is preferable that only the separator is wound at the core of the electrode jelly-roll to form a rod-like stability member which is cured by absorbing heat generated from the cell. In this case, the separator wound at the core of the electrode jelly-roll is continuous from a portion of the separator which is stacked with the two different electrodes. 
   It is preferable that a substrate of one of the electrodes which is more towards the exterior than the other, the substrate being not coated with an active material, surrounds the outer surface of the electrode jelly-roll in contact with the inner wall of the can. In this case, a polyolefin-based thin film may be additionally formed to coat the substrate surrounding the outer surface of the electrode jelly-roll. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the invention, and many of the attendant advantages thereof, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein: 
       FIG. 1  is a cross-sectional view of the electrode jelly-roll of a conventional secondary cell; 
       FIG. 2  is a cross-sectional view showing the electrode jelly-roll of a secondary cell according to a preferred embodiment of the present invention; 
       FIG. 3  is a longitudinal sectional view showing the structure of the secondary cell of  FIG. 2 ; 
       FIG. 4  is a partially cutaway perspective view showing a stability member in the electrode jelly-roll of the secondary cell according to the preferred embodiment of the present invention; 
       FIG. 5  is a cross-sectional view showing the electrode jelly-roll of a secondary cell according to another preferred embodiment of the present invention; 
       FIG. 6  is a longitudinal sectional view showing the structure of the secondary cell of  FIG. 5 ; 
       FIG. 7  is a cross-sectional view showing the electrode jelly-roll of a secondary cell according to still another preferred embodiment of the present invention; and 
       FIG. 8  is a cross-sectional view showing the electrode jelly-roll of a secondary cell according to another preferred embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  is a cross-sectional view of a cylindrical secondary cell showing the structure of its electrode jelly-roll. 
   An electrode such as a cathode and anode can be manufactured by different methods according to the type of electrode. In general, as shown in  FIG. 1 , a cathode  3  and anode  4  are formed by coating a slurry containing a cathode active material  3   b  and a slurry containing an anode active material  4   b  on both sides of respective substrates  3   a  and  4   a , and drying, roll-pressing and cutting the substrates  3   a  and  3   b  to a predetermined size. A separator  5  is interposed between the cathode  3  and the anode  4  to prevent the cathode  3  and anode  4  from being electrically connected, and then wound in a roll. 
   The resulting electrode jelly-roll  2  is placed in a can  1 , a cap assembly (not shown) is mounted on the top of the can  1  to be connected with the cathode  3  of the electrode jelly-roll  2 , and the can  1  is filled with an electrolyte solution and sealed, resulting in a cylindrical cell. 
   In such a cylindrical cell, the anode substrate  4   a  contacts the inner wall of the can  1  at the outer-side of the electrode jelly-roll  2 , or an anode tap welded to the anode substrate  4   a  contacts the bottom of the can  1 . A tap at the core of the electrode jelly-roll  2 , extending from the cathode substrate  3   a , is connected to the cap assembly. On the top and bottom surfaces of the electrode jelly-roll  2 , insulating plates are placed to prevent short-circuiting between the cap assembly and the can  1 . 
   Preferred embodiments of the present invention will be described with reference to the appended drawings. Although the preferred embodiment is described with reference to a cylindrical cell having an electrode jelly-roll, the present invention is not limited to the cylindrical cell and can be applicable to any type of cell as long as it includes the electrode jelly-roll described in the preferred embodiment. 
     FIG. 2  is a cross-sectional view of a preferred embodiment of a cylindrical secondary cell having an electrode jelly-roll according to the present invention.  FIG. 3  is a longitudinal sectional view of the cylindrical secondary cell of  FIG. 2 . As shown in  FIGS. 2 and 3 , an electrode jelly-roll  20  wound with a cathode  30 , anode  40 , and separator  50  therein is accommodated in a can  10 . The electrode jelly-roll  20  is attained by sequentially stacking a separator  50 , an anode  40  (or cathode.  30 ), a separator  50 , and a cathode  30  (or anode  40 ), and rolling the stack. 
   The cathode  30  and the anode  40  are formed by depositing active materials  34  and  44  of lithium metal oxide, carbon, or carbon composite on one side or both sides of substrates  32  and  42 , respectively. The substrate  32  of the cathode  30  is positioned at the core of the electrode jelly-roll  20  and is connected to a cap assembly  12  via a cathode tap  16 , which is welded to the substrate  32 . An insulating plate  14  is provided on the top and bottom of the electrode jelly-roll  20  to prevent short-circuiting between the cap assembly  12  and the can  10 . 
   In the electrode jelly-roll of the cell having the structure described above according to the present invention, the separator  50  rolled together with the cathode  30  and the anode  40  is used to dissipate heat from the cell and to provide structural stability. 
   In particular, the outer surface of the electrode jelly-roll  20  is wrapped with the separator  50  around one more turn to result in an outermost separator  52 . Also, at the core of the electrode jelly-roll  20 , an innermost separator  54  is additionally interposed as the innermost turn of the electrode jelly-roll  20 . It is preferable that the innermost separator  54  is tightly wound at the core of the electrode jelly-roll  20 . 
   The present invention is intended to improve the thermal and mechanical stabilities of the cell by forming additional turns of the separator  50  (i.e., the innermost separator  54  and the outermost separator  52 ) at the core of the electrode jelly-roll  20  and between the electrode jelly-roll  20  and the can  10 . 
   Suitable materials for the separator include polyethylene (PE), polypropylene (PP), and a composite of PP/PE/PP. The present invention is based on the fact that these materials for the separator easily absorb heat and are thermally cured. 
   In particular, as the temperature of the cell rises, the outermost separator  52  and the innermost separator  54 , which are additionally wound around the outer surface and at the core of the electrode jelly-roll  20 , respectively, absorb the heat, providing a heat-dissipating effect. At the same time, the outermost separator  52  and the innermost separator  54  are cured by absorbing the heat, so they act as external and internal protectors for the electrode jelly-roll  20 . 
   As the innermost separator  54 , which is additionally wound at the core of the electrode jelly-roll  20 , is cured by absorbing the heat generated inside the cell, a rod-like stability member  56  with a center cavity  56   a  is formed, as shown in  FIG. 4 . The rod-like shape of the stability member  56  is more advantageous for mechanical stability than other shapes. The stability member  56  can be used as a mandrel. Unlike the stability member  56 , the mandrel is commonly a separate member formed at the center of an electrode jelly-roll. The stability member  56 , which is integrally formed from the cured innermost separator  54 , has equivalent or better effects than the separately formed mandrel. Furthermore, since the separate mandrel is not necessary, the cell can be tightly rolled with reduced volume. 
   An appropriate physical stability against external impacts as well as the thermal stability capable of effectively dissipating the heat generated in a cell are important considerations in manufacturing cells. Damage to a cell caused by an external impact significantly affects the cell stability. As a cell is increasingly charged, the volume of the electrode expands so that an edge current flows. This localized current flow increases the likelihood of localized heat generation. In addition, as the thickness of the electrode increases by charging, it is more likely that the edge of the coated electrode is broken. 
   Therefore, it is necessary to protect the cell from external impacts as well as to effectively dissipate heat from the cell. 
   The present invention can meet these two requirements by using the outermost separator  52  and the innermost separator  54 , which are additionally wound around the outer surface and at the core of the electrode jelly-roll  20 , respectively, and are thermally curable. 
   In the embodiment of the electrode jelly-roll  20  shown in  FIGS. 2 and 3 , the substrate  42  of the anode  40  does not contact the can  10 . Therefore, it is preferable that an additional tap  17  is extended from the anode  40  to contact a projection  18  formed on the inner bottom of the can  10 . In  FIGS. 2 and 3 , the cathode  30  and the anode  40  may be interchangeable. 
     FIG. 5  is a cross-sectional view showing the electrode jelly-roll of a secondary cell according to another preferred embodiment of the present invention.  FIG. 6  is a longitudinal sectional view showing the structure of the secondary cell of  FIG. 5 . 
   As shown in  FIGS. 5 and 6 , an additional outermost separator  52  and innermost separator  54  are wound around one more turn, respectively, around the outer surface and at the core of the electrode jelly-roll  20 . As described above, as the outermost separator  52  and the innermost separator  54  are cured, they act as a stability enhancer, especially the innermost separator  54  wound at the core of the electrode jelly-roll  20  forms a rod-like stability member  56  after being cured, as shown in  FIG. 4 . 
   In the electrode jelly-roll  20  shown in  FIGS. 5 and 6 , the outermost separator  52  is wrapped around one more turn with the substrate  42  of the anode  40 , which is disposed more towards the outside (more towards the exterior such as away from the center of the cell and towards the outside and the can  10  or, wound or stacked more towards the outside) than the cathode  30  rolled together with the separator  50  therebetween. The height of the separator  50  is always over (greater than) the height of the cathode and anode  30  and  40  rolled together, so that the electrode jelly-roll  20  is surrounded by the outermost separator  52  as it is cured. In addition, as the outermost separator  52  is surrounded by the substrate  42  of the anode  40 , the heat-dissipating effect is enhanced. 
   In this structure, the substrate  46  of the anode  40  covering the outside of the electrode jelly-roll  20  contacts the inner wall of the can  10 , so that an additional tap, as described in the previous embodiment with reference to  FIGS. 2 and 3 , is unnecessary. In the present embodiment, the cathode and anode  30  and  40  may be interchangeable. 
     FIG. 7  is a cross-sectional view showing the electrode jelly-roll of a secondary cell according to still another preferred embodiment of the present invention. In this embodiment, an innermost separator  54  is additionally wound at the core of the electrode jelly-roll  20  so that it forms a rod-like stability member  56 , as shown in  FIG. 4 , when cured by the heat generated from the cell. An additional polyolefin-based thin film  60  is formed between the outer surface of the electrode jelly-roll  20  and the inner wall of the can  10 . An outer substrate  46  may be additionally interposed inside the polyolefin-based thin film  60 . The outer substrate  46  is formed by winding the polyolefin-based thin film  60  around one more turn with the substrate  42  of the anode  40 , which is disposed more towards the outside (more towards the exterior such as away from the center of the cell and towards the outside and the can  10  or, wound or stacked more towards the outside) than the cathode  30  rolled together with the separator  50  therebetween. 
   Like the separator  50 , the polyolefin-based thin film  60  formed to surround the outer surface of the electrode jelly-roll  20  may be formed of polypropylene (PP), polyethylene (PE), or a composite of PE/PP/PE/PP. An adhesive film  62 , instead of the polyolefin-based thin film  60 , can be attached to the outer surface of the electrode jelly-roll  20  after its formation is complete, as seen in  FIG. 8 . 
   The polyolefin-based thin film  60  acts as the outermost separator  52  described with reference to  FIGS. 3 and 5  and is thermally cured. 
   In the present embodiment, the anode  40  is made into contact with the can  10  via a tap  10 . The anode  40  and the cathode  30  may be interchangeable. 
   The electrode jelly-roll for a secondary cell according to the present invention having any structure described above provides the following effects. 
   First, additional turns of separator formed at the core and around the outer surface of the electrode jelly-roll for a cell form a protective structure for the cell so that it has mechanical stability against external impacts. 
   Second, in addition to the improvement in mechanical stability, the additional innermost and outermost turns of separator can improve the heat-dissipating capability by absorbing the heat generated from the cell to increase thereby the thermal stability of the cell. 
   Third, since an additional member such as a mandrel is unnecessary, the cell can be tightly rolled with reduced volume and equivalent to or better effects than when using a mandrel. 
   Fourth, the mechanical and thermal stabilities of the cell can be attained by simply winding an additional turn of separator or substrate without welding or incorporation of an additional member. Therefore, productivity can be increased with reduced processing steps and expenses. 
   While this invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.