Patent Publication Number: US-2015068029-A1

Title: Cathode for lithium battery with excelent output properties, method of manufacturing the cathode and lithium battery using the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit under 35 U.S.C. §119 of Korean Patent Application No. 10-2012-0019905, filed on Feb. 27, 2012 in the Korean Intellectual Property Office, the entirety of which disclosure is incorporated herein by reference. 
     BACKGROUND 
     1. Technical Field 
     The present invention relates to lithium batteries, and more particularly, to a cathode for lithium batteries with a structure having excellent output properties through an enlarged surface area of a cathode active material, and a lithium battery using the same. 
     2. Description of the Related Art 
     A lithium battery refers to a battery that uses a lithium component such as lithium or a lithium alloy as an anode material. 
     Such lithium batteries have higher power and higher capacity than existing manganese batteries and are widely used as power sources for various electronic appliances. 
       FIG. 1  schematically shows a general lithium battery. 
     Referring to  FIG. 1 , the lithium battery includes a case  110  open at top thereof, an anode  120 , a separator  130 , a cathode  140  and a terminal part  150   a,    150   b,    150   c.    
     The anode  120  is disposed on an inner wall of the case  110  and containing a lithium component. 
     The separator  130  is disposed inside the anode  120  and separates the anode  120  from the cathode  140 . 
     The cathode  140  is disposed inside the separator  130  and includes a current collector  145  and a cathode active material. 
     The terminal part includes an anode terminal  150   a  and a cathode terminal  150   b  which are insulated from each other by an insulator  150   c.  Generally, the anode terminal  150   a  is connected to the anode through the case  110 . The cathode terminal  150   b  is connected to the current collector  145  of the cathode  140  through a lead wire  160 . 
     In the lithium battery shown in  FIG. 1 , the terminal part serves as a cap for covering the top of the case. In this case, an insulating plate  170  is provided to insulate the terminal part from elements inside the case. The insulating plate  170  may be made of fluorinated ethylene propylene (FEP) or the like. 
       FIG. 2  is a schematic view of one example of a conventional cathode for lithium batteries. 
     Referring to  FIG. 2 , the cathode  200  for lithium batteries includes a current collector  210 , and a cathode active material  220  coupled to the current collector  210  to surround the current collector  210 . 
     However, in a cathode structure as shown in  FIG. 2 , the cathode active material  220  has a small surface area as compared with its volume. Thus, electrochemical reaction occurs only on a half region  225  of the surface area of the cathode active material  220  in operation of the lithium battery. As a result, the cathode structure shown in  FIG. 2  has limited output power. 
     Various attempts have been made to increase the surface area of the cathode active material  220 . 
       FIG. 3  is a schematic view of another example of a conventional cathode for lithium batteries. 
     In a cathode  300  for lithium batteries shown in  FIG. 3 , cathode active materials  310   a,    310   b  are separated from each other while being coupled to a current collector  320 . 
     However, the cathode  300  does not improve output characteristics. 
     Korean Patent Publication No. 10-2011-0106506 (published on Sep. 29, 2011) discloses such a conventional lithium battery. 
     BRIEF SUMMARY 
     The present invention provides a cathode for lithium batteries having a structure capable of outputting high power. 
     In addition, the present invention provides a method of manufacturing a cathode for lithium batteries. 
     Further, the present invention provides a lithium battery including such a cathode. 
     In accordance with one aspect of the present invention, a cathode for lithium batteries has a stack structure including a first cathode member, a separating member and a second cathode member stacked in sequence, wherein each of the first and second cathode members includes a current collector and a cathode active material formed on either side of the current collector. 
     The first cathode member may have a pipe shape; the separating member may be formed on an outer periphery of the first cathode member; and the second cathode member may be formed on an outer periphery of the separating member. 
     The first cathode member may include a first cathode sheet including a current collector and a cathode active material formed on either side of the current collector; the separating member may include a separator sheet; and the second cathode member may include a second cathode sheet including a current collector and a cathode active material formed on either side of the current collector. Both ends of the stack structure may be joined to each other to form a pipe shape. 
     The stack structure may be folded so that both ends of the stack structure are spaced from each other. 
     The stack structure may be folded in a “⊂” or “C” shape. 
     In accordance with another aspect of the present invention, there is provided a method of manufacturing a cathode for lithium batteries, wherein the cathode has a stack structure including a first cathode member, a separating member and a second cathode member stacked in sequence, and each of the first and second cathode members includes a current collector and a cathode active material formed on either side of the current collector. 
     The method may include: preparing a first cathode sheet, a separator sheet, and a second cathode sheet; preparing a first cathode sheet, a separator sheet, and a second cathode sheet; joining both ends of the first cathode sheet to each other to form a first cathode member having a pipe shape; attaching the separator sheet to an outer periphery of the first cathode member to form a separating member; and attaching the second cathode sheet to an outer periphery of the separating member to form a second cathode member, wherein each of the first and second cathode sheets comprises a current collector and a cathode active material formed on either side of the current collector. 
     Alternatively, the method may include: preparing a first cathode sheet, a separator sheet, and a second cathode sheet; sequentially stacking the separator sheet and the second cathode sheet on the first cathode sheet to form the stack structure; and joining both ends of the stack structure to form a pipe shape, wherein each of the first and second cathode sheets includes a current collector and a cathode active material formed on either side of the current collector. 
     In accordance with a further aspect of the present invention, a lithium battery includes: a case; an anode disposed on an inner wall of the case and containing a lithium component; a separator disposed inside the anode; a cathode disposed inside the separator; an anode terminal electrically connected to the anode; a cathode terminal electrically connected to the cathode; and an electrolyte filling the case, wherein the cathode has a stack structure including two cathode members stacked on a separating member interposed between the two cathode members, and each of the cathode members comprises a current collector and a cathode active material formed on either side of the current collector. 
     The cathode may have a pipe shape with both ends thereof joined to each other or a folded shape with both ends thereof spaced from each other. 
     As described above, the cathode for lithium batteries according to the present invention has a stack structure wherein two cathode members are stacked on a separating member interposed therebetween. Also, each of the two cathode members has a structure where cathode active materials are formed on both sides of the current collector. 
     With these features, the cathode for lithium batteries according to the present invention has an effect of substantially increasing the surface area, such that electrochemical reaction may occur in all sides of the cathode active material. 
     Therefore, in a lithium battery using the cathode according to the present invention, electrochemical reaction may more actively occur than in the lithium battery using the conventional cathode structure, thereby providing high output properties in operation of the lithium battery. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features, and advantages of the invention will become apparent from the detailed description of the following embodiments in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a schematic view of a general lithium battery; 
         FIG. 2  is a schematic view of one example of a conventional cathode for lithium batteries; 
         FIG. 3  is a schematic view of another example of the conventional cathode for lithium batteries; 
         FIG. 4  is a schematic plan view of a cathode for lithium batteries in accordance with one embodiment of the present invention; and 
         FIG. 5  is a schematic view of a stack structure in which a first cathode sheet, a separator sheet and a second cathode sheet are stacked. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the invention will now be described in detail with reference to the accompanying drawings. It should be understood that the present invention is not limited to the following embodiments and may be embodied in different ways, and that the embodiments are given to provide complete disclosure of the invention and to provide thorough understanding of the invention to those skilled in the art. The scope of the invention is limited only by the accompanying claims and equivalents thereof. Like components will be denoted by like reference numerals throughout the specification. 
     Now, a cathode for lithium batteries with a structure having excellent output properties according to the present invention, and a lithium battery using the same will be described with reference to the accompanying drawings. 
       FIG. 4  is a schematic plan view of a cathode for lithium batteries in accordance with one embodiment of the present invention. 
     Referring to  FIG. 4 , a cathode  400  includes a first cathode member  410 , a second cathode member  420 , and a separating member  430 . 
     The cathode for lithium batteries according to the present invention has various features. Particularly, two cathode members  410  and  420  are stacked with the separating member  430  interposed therebetween. In addition, each of two cathode members includes a current collector and cathode active materials formed on both sides of the current collector. 
     Such features of the cathode increase a surface area of the cathode active material on which electrochemical reaction may occur, thereby providing excellent output properties. 
     In addition, when the separating member  430  is formed as a porous insulator through which lithium ions can pass, electrochemical reaction can occur on all of four sides of the cathode active material, thereby providing excellent output properties. 
     Referring to  FIG. 4 , the first cathode member  410  has a pipe shape. Further, the separating member  430  is disposed on an outer periphery of the first cathode member  410 . The second cathode member  420  is disposed on an outer periphery of the separating member  430 . 
     Here, each of the first and second cathode members  410 ,  420  includes a current collector and a cathode active material formed on either side of the current collector. 
     The cathode structure as shown in  FIG. 4  may be embodied by sequentially stacking the separating member and the second cathode member on the outer periphery of the first cathode member. 
     In addition, the cathode structure as shown in  FIG. 4  may be embodied by a stack structure having a cross section as shown in  FIG. 5 . 
     Referring to  FIG. 5 , the stack structure includes a first cathode sheet  510 , a separator sheet  520 , and a second cathode sheet  530  which are sequentially stacked. Further, each of the first and second cathode sheets  510 ,  530  includes cathode active materials  510   a,    510   c;    530   a,    530   c  formed on both sides of each current collector  510   b,    530   b  by coating or the like. 
     When the sheets are wound such that both ends of the stack structure are joined to each other, the cathode of a pipe shape may be formed. Here, although the pipe shape of the cathode may be formed by winding the stack structure such that both ends of the stack structure exactly meet each other, the stack structure may be wound such that an upper surface of one end of the stack structure may be joined to a lower surface of the other end of the stack structure. 
     Both ends of the stack structure may be joined to each other in various ways, such as bonding agents, pressure application, pins or other coupling members, and the like. 
     Besides the pipe shape as shown in  FIG. 4 , the cathode for lithium batteries according to the present invention may have a structure in which both ends of the stack structure are spaced from each other by folding the stack structure of  FIG. 5  in a certain form. 
     The folded shape may be determined depending on the shape of the battery case. Advantageously, the cathode active material may have an increased surface area when the stack structure is folded in a “⊂” or “C” shape. 
     Meanwhile, each current collector of the first and second cathode members  410 ,  420  may be made of a metal, such as nickel (Ni), copper (Cu), aluminum (Al), and alloys thereof, which exhibit excellent electrical conductivity. The current collector may be formed as a grid or the like. 
     In addition, the cathode active material formed on either side of the current collector may contain a carbon-based material such as active carbon or amorphous carbon, without being limited thereto. 
     In  FIG. 4 , the separating member  430  prevents a cathode active material surface  420   b  inside the second cathode member  420  from contacting a cathode active material surface  410   a  outside the first cathode member  410  in order to prevent a reduction in the surface area of the cathode active material. Further, the separating member  430  is configured to allow lithium ions to pass therethrough such that electrochemical reaction may occur not only on the exposed cathode active material surfaces  420   a,    410   b,  but also on the cathode active material surfaces  420   b,    410   a  contacting the separating member  430 . 
     To this end, the separating member may be made of a porous insulator, which has electrically insulating properties and allows lithium ions to pass therethrough. In some embodiments, the separating member may be composed of at least one material selected from among micro glass fibers and long glass fibers. 
     Although  FIG. 4  shows the cathode structure having a circular cross-section suited to a cylindrical lithium battery, the present invention is not limited thereto. In other words, the cathode according to the present invention may have various transverse sections, such as a triangular shape or the like, depending on the shape of the lithium battery. 
     The cathode for lithium batteries according to the present invention may be manufactured by various methods using the first cathode sheet, the separator sheet and the second cathode sheet. 
     First, one end of the first cathode sheet is joined to the other end thereof to form a first cathode member having a pipe shape, and the separator sheet is coupled to the outer periphery of the first cathode member to form a separating member having a pipe shape. Then, the second cathode sheet is coupled to the outer periphery of the separating member to form a second cathode member having a pipe shape. 
     In another embodiment, the separator sheet and the second cathode sheet are sequentially stacked on the first cathode sheet to form a stack structure having a longitudinal cross-section as shown in  FIG. 5 , and the stack structure is wound such that both ends of the stack structure are joined to each other to form a pipe shape. 
     In a further embodiment, the stack structure is formed to have a longitudinal cross-section as shown in  FIG. 5 , and then folded in a “⊂” or “C” shape such that both ends of the stack structure are spaced from each other. 
     The cathode according to the present invention may be used for the lithium battery as shown in  FIG. 1 . 
     In this case, the lithium battery according to the present invention includes a case  110 , an anode  120  disposed on an inner wall of the case  110  and containing a lithium component, a separator  130  disposed inside the anode  120 , a cathode  140  disposed inside the separator  130 , an anode terminal  150   a  electrically connected to the anode  120 , a cathode terminal  150   b  electrically connected to the cathode  140 , and an electrolyte (not shown) filling the case  110 . The lithium battery according to the present invention may further include an insulating plate  170  as shown in  FIG. 1 . 
     At this time, the lithium battery according to the present invention has a structure wherein two cathode members are stacked on the separating member interposed therebetween. Here, each of the two cathode members may include a current collector and a cathode active material formed on either side of the current collector. 
     In operation of the battery, electrons generated by electrochemical reaction of the cathode active materials on the two cathode members are collected in the current collectors, and move to the cathode terminal  150   b  through the lead wire  160 . The two current collectors and the cathode terminals may be connected in various ways. By way of example, the two current collectors and the cathode terminals may be connected via a lead wire and a lead wire connector  440  as shown in  FIG. 4 . 
     In the lithium battery having the cathode structure according to the present invention, electrochemical reaction occurs on all four sides of the cathode active material during operation of the battery, thereby providing excellent output properties. 
     Although some embodiments have been described herein, it should be understood by those skilled in the art that these embodiments are given by way of illustration only, and that various modifications, variations, and alterations can be made without departing from the spirit and scope of the invention. Therefore, the scope of the invention should be limited only by the accompanying claims and equivalents thereof.