Abstract:
A battery unit with improved safety measures, a lithium polymer battery using the battery unit, and a method for manufacturing the lithium polymer battery are provided. The lithium polymer battery has a battery unit and a case accommodating the battery unit, wherein the battery unit includes: a cathode plate having a cathode collector and a cathode active material layer coated on at least one surface of the cathode collector; a cathode lead electrically connected to the cathode collector; an anode plate having an anode collector and an anode active material layer coated on at least one surface of the anode collector; an anode lead electrically connected to the anode collector; a separator interposed between the cathode plate and the anode plate, which insulates the cathode plate and the anode plate from each other; and an insulating member formed on at least one of the cathode lead and the anode lead, which prevents a short circuit between the cathode lead and the anode plate or between the anode lead and the cathode plate. In the battery unit, the cathode plate, the separator, and the anode plate are sequentially and repeatedly stacked upon one another. Therefore, the insulating member incorporated in the lithium polymer battery can enhance the safety of the battery.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of prior application Ser. No. 10/347,251, filed on Jan. 21, 2003 now U.S. Pat. No. 7,169,505 issued Jan. 30, 2007, which claims the benefit of Korean Patent Application No.: 2002-0006737, filed Feb. 6, 2002, which are both hereby incorporated by reference for all purposes as if fully set forth herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a lithium polymer battery, and more particularly, to a battery unit configured to prevent a short circuit due to contact between the electrode leads and the electrode plates, a lithium polymer battery using the battery unit, and a method for manufacturing the lithium polymer battery. 
     2. Description of the Related Art 
     Lithium secondary batteries have a high energy density per unit weight and an operating voltage of 3.6V or greater, which is three times higher than nickel-cadmium (Ni—Cd) batteries, nickel-metal hydride (Ni-MH) batteries, and nickel-hydrogen batteries. For these reasons, their use has become widespread. Lithium secondary batteries can be classified into lithium-ion batteries that use a liquid organic electrolyte and lithium polymer batteries that use a solid polymeric electrolyte. 
     Specifically, lithium polymer batteries are rather safe and can accommodate a variety of shapes, especially, in thin film form to comply with the need for small, lightweight, portable electronic products. These properties could not typically be achieved in lithium-ion batteries. Due to these advantages, the lithium polymer battery has recently attracted attention. 
     Referring to  FIGS. 1A and 1B , a conventional lithium polymer battery  10  is shown. The conventional lithium polymer battery  10  includes a battery unit  11 , an electrode lead  12  drawn out from the battery unit  11 , an electrode terminal  13  welded to the plurality of electrode leads  12 , and a case  14  for accommodating the battery unit  11 . 
     The battery unit  11  has a structure in which a cathode plate, an insulating separator, and an anode plate are sequentially and repeatedly stacked upon one another. The electrode lead  12  is drawn out from each of the cathode and anode plates of the battery unit  11 . A plurality of electrode leads  12  drawn out from the electrode plates of the battery unit  11  are electrically connected to electrode terminals  13 , wherein a portion of the electrode terminal  13  is exposed to the outside of the case  14 . The case  14 , shown in  FIG. 1A  as a pouch shape provides space for accommodating the battery unit  11 . 
     The case  14  has a sealing portion  14 a to be coupled to another element to seal the battery unit  11  placed in the case  14 . A sealing tape  15  is wound around a portion of the electrode terminals  13  which contacts the sealing portion  14 a. Accordingly, as the sealing portion  14 a is thermally fused to seal the case  14 , the sealing tape  15  is also fused and bound to internal layers of the case  14 , thereby enhancing the hermetic containment of the battery. 
     However, the conventional lithium polymer battery  10  has the following problems. A group of electrode leads  12  extending from electrode plates, which have the same polarity as the electrode leads  12 , is bent in a U-shape and then welded to one electrode terminal  13 . When the insulating separator shrinks due to overheating during the manufacturing process or during operation of the lithium polymer battery  10 , the electrode leads  12  may contact the electrode plates of the battery unit  11  having the opposite polarity to the electrode leads  12 , thereby causing one or more short circuits. 
     In the case where the battery unit  11  has a stacked structure where the cathode and anode plates are stacked upon one another, sharp burrs may result at the edges of the electrode leads  12  in cutting a stack of the electrode plates to form the battery unit  11 . The electrode leads  12  with such burrs may penetrate one of the insulating separators and directly contact electrode plates having the opposite polarity to the electrode leads  12 , resulting in the occurrence of one or more short circuits in the battery. 
     Heat generated when a short circuit occurs in the battery unit  11  is transferred to a thin polymeric layer coated on the inner surface of the case  14 . Accordingly, the thin polymeric film melts, and the electrode leads  12  electrically contact metal foil which is included as an intermediate structural layer of the case  14 . As a result, the case  14  gradually corrodes depending on a difference in ionization with respect to the electrode plates of the battery unit  11 . 
     SUMMARY OF THE INVENTION 
     Accordingly, the invention provides a battery unit with improved safety measures, in which electrical contact between electrode plates and electrode leads that have opposite polarities with respect to each other is blocked. The invention also includes a lithium polymer battery with the battery unit, and a method for manufacturing the lithium polymer battery. 
     In one embodiment of the present invention, a battery unit includes a cathode plate having a cathode collector and a cathode active material layer coated on at least one surface of the cathode collector; a cathode lead electrically connected to the cathode collector; an anode plate having an anode collector and an anode active material layer coated on at least one surface of the anode collector; an anode lead electrically connected to the anode collector; a separator interposed between the cathode plate and the anode plate, which insulates the cathode plate and the anode plate from each other; and an insulating member formed on at least one of the cathode lead and the anode lead, which prevents a short circuit between the cathode lead and the anode plate or between the anode lead and the cathode plate. 
     In another embodiment, the present invention provides a lithium polymer battery comprising a battery unit and a case accommodating the battery unit, wherein the battery unit includes: a cathode plate having a cathode collector and a cathode active material layer coated on at least one surface of the cathode collector; a cathode lead electrically connected to the cathode collector; an anode plate having an anode collector and an anode active material layer coated on at least one surface of the anode collector; an anode lead electrically connected to the anode collector; a separator interposed between the cathode plate and the anode plate, which insulates the cathode plate and the anode plate from each other; and an insulating member formed on at least one of the cathode lead and the anode lead, which prevents a short circuit between the cathode lead and the anode plate or between the anode lead and the cathode plate, and in the battery unit the cathode plate, the separator, and the anode plate are sequentially repeatedly stacked upon one another. 
     In yet another embodiment, the present invention provides a method for manufacturing a lithium polymer battery, the method comprising: mixing source materials for an electrode active material layer; coating at least one surface of a collector substrate with the mixture of the source materials in a pattern corresponding to the electrode active material layer; cutting the collector substrate with the pattern of the electrode active material layer into individual collectors; pre-attaching an insulating member to an electrode lead extending from each of the collectors; thermally fusing the insulating member to the electrode lead; and completing formation of an electrode plate including the collector which has the electrode active material layer and the electrode lead which extends from the collector and to which the insulating member is fused. 
     In another embodiment, the present invention provides a method for manufacturing a lithium polymer battery, the method comprising: mixing source materials for an electrode active material layer; coating at least one surface of a collector substrate with the mixture of the source materials in a pattern corresponding to the electrode active material layer; cutting the collector substrate with the pattern of the electrode active material layer into individual collectors; dropping a composition for an insulating member onto an electrode lead extending from each of the collectors; fixing the composition dropped onto the electrode lead by compression molding to form the insulating member; completing formation of an electrode plate including the collector which has the electrode active material layer and the electrode lead which extends from the collector and on which the insulating member is formed by compression molding. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above objects and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings. 
         FIG. 1A  is a sectional view of a conventional lithium polymer battery. 
         FIG. 1B  is an enlarged view of portion A of  FIG. 1A . 
         FIG. 2A  is an exploded perspective view of a lithium polymer battery according to an embodiment of the present invention. 
         FIG. 2B  is an enlarged view of portion B of  FIG. 2A . 
         FIG. 3  is an exploded perspective view of a battery unit of  FIG. 2 . 
         FIG. 4  is a flowchart for illustrating a method for manufacturing electrode plates for the battery unit of  FIG. 2  according to an embodiment of the present invention. 
         FIG. 5  is a flowchart for illustrating a method for manufacturing electrode plates for the battery unit of  FIG. 2  according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIGS. 2A and 2B , a lithium polymer battery  20  according to one embodiment of the present invention is shown. The lithium polymer battery  20  includes a battery unit  21  and a case  22  for accommodating the battery unit  21 . The battery unit  21  includes a cathode plate  23 , an anode plate  24 , and a separator  25  interposed between the cathode plate  23  and the anode plate  24  for insulating the cathode plate  23  and the anode plate  24  from each other. In the battery unit  21 , the cathode plate  23 , the separator  25 , and the anode plate  24  are sequentially and repeatedly stacked upon one another. 
     The cathode plate  23  consists of a cathode collector and a cathode active material layer coated on at least one surface of the cathode collector. The anode plate  24  consists of an anode collector and an anode active material layer coated on at least one surface of the anode collector. 
     A cathode lead  26  and an anode lead  27  are drawn out from the respective cathode and anode plates  23  and  24  the ends of the cathode and anode leads  26  and  27  are welded to respective cathode and anode terminals  28  and  29 . 
     The battery unit  21  including the cathode plate  23 , the anode plate  24 , and the separator  25  is mounted in a space  22 a of the case  22 . A portion of the cathode and anode terminals  28  and  29  extend out of the case  22 . 
     The case  22  has a sealing portion  22 b at which the case  22  containing the battery unit  21  is sealed. A sealing tape  200  is wound around a portion of the cathode and anode terminals  28  and  29  which contacts the sealing portion  22 b. The sealing tape  200  is fused and bound to the sealing portion  22 b of the pouch type case  22  during a sealing process by thermal fusion, thereby enhancing the hermitic containment of the battery. An insulating member  210  is formed around each of the cathode leads  26  to prevent direct contact with the anode plate  24 . 
     The structure of the secondary battery according to the present invention will be described in detail with reference to  FIG. 3 . 
       FIG. 3  shows one unit cell of the battery unit  21  of  FIG. 2 . Referring to  FIG. 3 , the cathode plate  23  includes a cathode collector  23 a formed of expanded metal or punched metal using, for example, aluminum. Front and rear cathode active material layers  23 b and  23 c, which include a lithium oxide, a binder, a plasticizer, and a conductive material, are formed on both surfaces of the cathode collector  23 a. 
     A cathode lead  26  is drawn out from one corner of the cathode collector  23 a to a predetermined length. It is preferable that the cathode lead  26  be integrally formed with the cathode collector  23 a for manufacturing efficiency. 
     The anode plate  24  is disposed opposite to the cathode plate  23  with the separator  25  therebetween. The anode plate  24  includes an anode collector  24 a formed of, for example, copper foil. Front and rear anode active material layers  24 b and  24 c, which include a carbonic material, a binder, a plasticizer, and a conductive material, are formed on both surfaces of the anode collector  24 a. 
     An anode lead  27  is drawn out to a predetermined length from the diagonally opposite corner of the anode collector  24 a with respect to that corner of the cathode plate  23  from which the anode plate  24  extends. It is preferable that the anode lead  27  be integrally formed with the anode collector  24 a. 
     The insulating member  210  is formed around the cathode lead  26 , which is bent in a U-shape to be mounted in the case  22 , as shown in  FIG. 2 . When the cathode lead  26  and the cathode collector  23 a are cut together, burrs may result at the edge of the cathode lead  26 . It is highly likely that the burrs of the cathode lead  26  penetrate the separator  25  and contact the anode plate  24 , thereby causing an electrical short. 
     To prevent these electrical short circuits, a polymeric insulating member  210  is formed around the portion of the cathode lead  26  that seems to likely contact the anode plate  24  having the opposite polarity to the cathode lead  26 . 
     Heat-resistant insulating tapes formed of, for example, polyethylenes or polypropylenes, can be used as the insulating member  210 . Such an insulating tape may be thermally fused to that portion of the cathode lead  26 . 
     Alternatively, the insulating member  210  can be formed using a polymeric resin composition containing, for example, polyethylenes, polypropylenes, or amorphous polyamides. In this case, a predetermined amount of the polymeric resin composition is dropped onto the cathode lead  26  and set by compression molding. 
     The insulating member  210  can be attached to the anode lead  27 , instead of the cathode lead  26 , or to both the cathode and anode leads  26  and  27 . 
     A method for manufacturing electrode plates for the lithium polymer battery having the structure as described above will be described. 
       FIG. 4  is a flowchart for illustrating an embodiment of a method for manufacturing electrode plates for the lithium polymer battery according to one embodiment of the present invention. As an example, the formation of cathode plates will be described below with reference to  FIG. 4 . As shown in  FIG. 3 , the cathode plate  23  includes the cathode collector  23 a and the front and rear cathode active material layers  23 b and  23 c. Initially, to form the front and rear cathode active material layers  23 b and  23 c, source materials for the cathode active material layers are mixed together. In particular, a lithium oxide as a cathode active material, a conductive material, and a plasticizer are mixed with a binder solution to prepare a slurry (S 10 ). 
     For efficient large-scale production, a plurality of cathode collectors  23 a are simultaneously formed using a single, large aluminum foil (hereinafter, referred to as a “cathode collector substrate”). Both surfaces of the cathode collector substrate are coated with the slurry for the cathode active material layers  23 b and  23 c in a pattern corresponding to the shape of the cathode active material layers  23 b and  23 c (S 20 ). Coating both surfaces of the cathode collector substrate with the slurry for the front and rear cathode active material layers  23 b and  23 c may be performed by casting. 
     To improve the adhesion of the active material layers to the cathode collector substrate, which is preferably made of expanded or punched aluminum, and to reduce the interfacial resistance therebetween for the extended lifespan and enhanced charging/discharging properties of the battery, foreign materials on the surfaces of the cathode collector substrate are removed prior to coating the cathode collector substrate with the slurry. After coating the slurry for the front and rear cathode active material layers  23 b and  23 c on both surfaces of the cathode collector substrate, a calendaring process is performed in order to enhance the adhesion of the front and rear cathode active material layers  23 b and  23 c to the cathode collector substrate and to correct thickness deviations that may be present in the front and rear cathode active material layers  23 b and  23 c. The calendaring process is preferably performed by passing the cathode collector substrate coated with the slurry between heating rollers (S 30 ). 
     Next, the cathode collector substrate with the pattern of the front and rear cathode active material layers  23 b and  23 c is cut into individual cathode collectors  23 a having a predetermined shape using a mold. Each of the resulting cathode collectors  23 a has a cathode lead  26  extending from one edge of the cathode collector (S 40 ). 
     Next, the insulating member  210 , for example, an insulating tape, is pre-attached to the cathode lead  26  (S 50 ). The insulating tape for the insulating member  210  can be formed of polypropylenes or polyethylenes having a low melting point of about 150° C. or less. The insulating tape includes a tape layer having a thickness of about 20-70 μm and an adhesive layer coated on the tape layer and having a thickness of about 5-20 μm. The insulating member  210  is pre-attached around a portion of the cathode lead  26  and preferably has a width of about 2-4 mm. 
     After the insulating member  210  is pre-attached to the cathode lead  26 , the cathode lead  26  is preferably passed between rollers pre-heated to a temperature of about 140-180° C. so that the insulating member  210  is thermally fused to the cathode lead  26  (S 60 ). At this time, there is a need to control the thickness of the adhesive layer of the insulating member  210  so as to prevent agglomeration of the adhesive composition in the adhesive layer coated on the tape layer as the tape layer is fused to the cathode lead  26 . The temperature of and the space between the rollers needs to be uniformly maintained in order to allow easier thermal fusion of the insulating member  210  and so that no foreign materials remain on the surfaces of the rollers after fusing. 
     In step S 60 , another consideration is to maintain the surface evenness of an insulating member applying roller (not shown) in order to prevent the melted tape layer of the insulating member  210  from spreading toward the cathode collector  23 a. In addition, the rotating rate of the insulating member applying roller needs to be substantially the same as the rate at which the cathode collector  23 a is moved on a conveyer belt, and the insulating member applying roller needs to apply the insulating member  210  in a tensioned state to the cathode lead  26  in order to prevent sagging of the insulating member  210 . 
     Steps S 10  through S 60  provide a complete cathode plate  23  including the cathode collector  23 a which has the front and rear cathode active material layers  23 b and  23 c and the cathode lead  26  which extends from one side of the cathode collector  23 a and to which the insulating member  210  for preventing electrical contact between the cathode lead  26  and the anode plate  24  is thermally fused (S 70 ). 
     The above-described method for manufacturing the cathode plates for the secondary battery according to the present invention can be applied to the formation of anode plates having the opposite polarity to the cathode plates. In manufacturing anode plates, copper foil is preferably used as an anode collector substrate. Front and rear anode active material layers  24 b and  24 c are formed on both surfaces of the anode collector  24 a, with the anode lead  27  extending from one edge of the anode collector  24 a. The insulating member  210  may also be attached to the anode lead  27 . Alternatively, the insulating member  210  may be attached only to the anode lead  27 , not to the cathode lead  26 . 
       FIG. 5  is a flowchart for illustrating another embodiment of a method for manufacturing electrode plates for the lithium polymer battery according to the pre invention. As an example, the formation of cathode plates will be described below with reference to  FIG. 5 . 
     Initially, source materials for the front and rear cathode active material layers  23 b and  23 c are mixed together. In particular, a lithium oxide, a plasticizer, and a conductive material are mixed with a binder solution to prepare a slurry (S 110 ). 
     The prepared slurry for the front and rear cathode active material layers  23 b and  23 c is coated on the front and rear surfaces of a cathode collector substrate from which foreign materials have been removed through a pre-treatment (S 120 ). Both surfaces of the cathode collector substrate are coated with the slurry for the front and rear cathode active material layers  23 b and  23 c. This step is preferably performed by casting. 
     After coating the slurry for the front and rear cathode active material layers  23 b and  23 c on both surfaces of the cathode collector substrate, a calendaring process is performed in order to enhance the adhesion of the front and rear cathode active material layers  23 b and  23 c to the cathode collector substrate (S 130 ). Next, the cathode collector substrate with the front and rear cathode active material layers  23 b and  23 c is cut into individual cathode collectors  23 a having a predetermined shape using a mold. Each of the resulting cathode collectors  23 a has a cathode lead  26  extending from its one edge (S 140 ). In other words, the cathode lead  26  is integrally formed with the cathode collector  23 a. 
     Next, a composition for the insulating member  210  is dropped onto at least a portion of the cathode lead  26 . The composition for the insulating member  210  may be a polymeric emulsion containing, for example, polypropylenes, polyethyelenes, or amorphous polyamides. 
     Suitable polypropylenes include stereospecific polymers, such as atactic polymers, syndiotactic polymers, and isotatic polymers, having a melting point of about 120-160° C. and a melt flow index of about 15 g/10 min. Suitable polyethylenes have a degree of crystallinity of about 20-50% and a melt flow index of 5 g/min or greater. Suitable polyethylenes also include high density polyethylenes having a melting point of about 100-160° C., linear low density polyethylenes having a melting point of about 100-140° C., and linear low density polyethylenes having a melting point of about 90-120° C. 
     Preferably, a composition containing 1-5% acrylic acid by weight and the balance of a polypropylene or polyethylene matrix polymer is coated on the cathode lead  26  for adhesion enhancement. 
     Suitable amorphous polyamides may have a melting point of about 120-160.degree. C. 
     The composition for the insulating member is preferably dropped onto the cathode lead  26  through a single or a plurality of screw extruders and then a temperature regulator (S 150 ). At this time, it is preferable that a precise pressure gauge be attached to the extruder so as to accurately control the pressure at which the composition for the insulating member is dispensed. 
     After the composition for the insulating member  210  is dropped onto the cathode lead  26 , compression molding is performed using a mold in order to shape an insulating member having a preferable width of about 1.5-4.5 mm and a preferable length of 2.0-2.5 mm (S 160 ). 
     In order to easily release the insulating member while keeping it intact and in the desired shape from the mold and onto the cathode lead  26 , it is preferable that a release agent be coated on the inside of the mold for compression molding. 
     Steps S 110  through S 160  provide a complete cathode plate  23  including the cathode collector  23 a which has the front and rear cathode active material layers  23 b and  23 c and the cathode lead  26  which extends from one edge of the cathode collector  23 a and on which the insulating member  210  for preventing electrical contact between the cathode lead  26  and the anode plate  24  is formed by compression molding (S 170 ). 
     A battery unit according to the present invention and a lithium polymer battery using the battery unit according to the present invention, which is manufactured by the above-described method according to the present invention, improves the safety and reliability of the battery. It also prevents a voltage drop. 
     While the present invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the appended claims.