Abstract:
A lithium secondary battery that can improve safety of the battery. The lithium secondary battery comprises a jelly-roll type electrode assembly including first and second electrode plates having different polarities, and a separator interposed between the electrode plates. A porous ceramic film is coated on an active material of the first electrode plate, and the porous ceramic film is coated on one surface of an active material uncoated part of the first an outmost electrode plate and the porous ceramic film is not coated on another surface thereof.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on and claims priority to Korean Patent Application No. 10-2007-0052616, filed on May 30, 2007, in the Korean Intellectual Property Office (KIPO), the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Aspects of the present invention relate to a lithium secondary battery, and more particularly, to a lithium secondary battery having improved safety characteristics. 
     2. Description of the Related Art 
     Generally, a secondary battery can be rechargeable, miniaturized, and have large capacity. Recently, secondary batteries have been used as main power supplies of portable electronic devices, such as a camcorder, a portable computer, and a cellular phone. Typically, nickel-hydrogen (Ni-MH) batteries, lithium ion (Li-ion) batteries, and lithium ion polymer batteries have been actively developed. 
     Lithium, which is commonly used as an active material of a secondary battery, has a low atomic weight, and thus, is suitable for manufacturing a battery having a large electrical capacity per unit mass. Further, lithium intensively reacts with water, and thus, a non-aqueous electrolyte is used in a lithium battery. Lithium batteries are not affected by an electrolysis voltage of water. Therefore, there is an advantage in that lithium batteries can generate an electromotive force of 3 to 4 volts. 
     The non-aqueous electrolytes used in the lithium ion secondary batteries include a liquid electrolyte and a solid electrolyte. The liquid electrolyte is formed by dissociating lithium salts in an organic solvent. Ethylene carbonate, propylene carbonate, carbonate containing alkyl groups, or similar organic compounds, are commonly used as the organic solvent. 
     The electrolytes have low ion conductivity. The low ion conductivity of the electrolytes can be supplemented, by increasing an area of an electrode active material, and a facing area between two electrodes. However, there are several limitations related to increasing the facing area between two electrodes. As a result, the low ion conductivity of the electrolyte increases an internal impedance of the battery, resulting in a large internal voltage drop, and limiting output, by restricting a current of the battery when a high current discharge is required. 
     A separator, interposed between two electrodes, restricts the movement of lithium ions. In the case where the separator does not have sufficient permeability and wettability, the separator restricts the movement of lithium ions between the two electrodes, thereby degrading electrical properties of the battery. Accordingly, important properties of the separator, which relate to the performance of the battery, include heat-resistance, chemical resistance, mechanical strength, void content, and wettability by an electrolyte. The void content is an area of vacant space at a random sectional surface. 
     The separator of the lithium ion battery also functions as a safety device, which prevents overheating of the battery. A polyolefin-type, micro-porous film, which is commonly used as material of the separator, is softened and partially melted when heated above a predetermined temperature. Accordingly, micro-holes of the micro-porous film, which are passages for lithium ions and a connecting passages for the electrolyte, become closed. As a result, the movement of the lithium ions is stopped, and a current flow of the interior/exterior of the battery is interrupted, and a temperature increase of the battery is stopped. 
     However, in the case where temperature of the battery is increased, the separator may be damaged, even if the micro-holes of the separator are closed. The separator is partially melted, and two electrodes of the battery directly contact each other at the melted point, thereby allowing producing an internal electrical short. The separator can also shrink, thereby allowing two electrodes to contact each other, and be electrically shorted. 
     When an over-current flows in the battery, due to a high capacity of the battery, a large amount of heat can be generated. The heat can damage the separator, which can increase the probability of an internal electrical short, to a level that is higher than in the case where the battery temperature causes the micro-holes of the separator to close, because the separator is continuously melted by over-current generated heat. 
     Accordingly, it is more important to solve the problems of melting or shrinking of the separator, at the time of over-heating of the battery, rather than the current shutdown produced by closing the micro-holes of the separator. 
     To solve the heat-related problems, a ceramic film is used to prevent internal electrical shorts between electrodes, even at high temperatures. The ceramic film is usually manufactured by forming a film solution, which includes uniformly dispersed ceramic particles, a binder, and a solvent. The film solution can be applied by dipping an electrode plate, coated with an active material, in the film solution. The ceramic film is coated on surfaces where cathode and anode plates face each other, thereby preventing an electrical short between the electrode plates, while allowing lithium ions to pass there through. 
     In the secondary battery having the coated ceramic film (as an additional separator), it is possible to effectively prevent the internal electrical short, by coating the ceramic film on the electrode active material, as well as on an uncoated part of an electrode, where the electrode active material is not present. 
     However, there is a problem that the battery, including the electrode plate coated with the ceramic film, is not effectively electrically shorted by an external impact or stimulus, as determined using a nail penetration test, and thus, the safety of the battery is reduced. 
     SUMMARY OF THE INVENTION 
     Aspects of the present invention provide a lithium secondary battery having improved safety characteristics, and that includes a porous ceramic film coated on a first surface of an uncoated part of an electrode plate, which is located adjacent to an outer surface of an electrode assembly. A second surface of the uncoated part is not coated with the ceramic film, and can be used to generate an electrical short in an outer electrode layer of the electrode assembly, if the electrode assembly experiences an external impact. 
     According to an aspect of the present invention, there is provided a lithium secondary battery, which comprises: a jelly-roll type electrode assembly, including first and second electrode plates having different polarities, and a separator interposed between the electrode plates; and an electrolytic solution. A porous ceramic film is coated on an active material of the first electrode plate. The porous ceramic film is coated on a first surface of an active material uncoated part of the second electrode plate. The uncoated part is disposed adjacent to an outer surface of the electrode assembly, and the porous ceramic film is not coated on a second surface of the uncoated part. 
     According to aspects of the present invention, the porous ceramic film is not coated on one surface of an active material uncoated part of a first electrode, which faces an uncoated part of a second electrode. 
     According to aspects of the present invention, the separator may be a resin separator. The resin separator may be formed of a multi-layer film of polyethylene, polypropylene, or combination thereof, each of which has fine porous structure. 
     According to aspects of the present invention, a porous ceramic film may be coated on an inner surface of the active material uncoated part of an electrode plate, and not be coated on an outer surface in wound state. 
     According to aspects of the present invention, an electrode plate coated with the porous ceramic film may be an anode plate. 
     According to aspects of the present invention, the porous ceramic film may be formed by combining a binder with a ceramic material. 
     According to aspects of the present invention, the porous ceramic film may be formed by: dipping, spraying, or printing an electrode with a mixture of a ceramic material, a binder, and a solvent. 
     According to aspects of the present invention, the ceramic material may include at least one of silica (SiO 2 ), alumina (Al 2 O 3 ), zirconium oxide (ZrO 2 ), and titanium oxide (TiO 2 ). 
     According to aspects of the present invention, the ceramic material may include at least one of an insulating nitride, hydrate, alkoxide, or ketonide, of silicon (Si), aluminum (Al), zirconium (Zr), titanium (Ti), or a combination thereof. 
     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 
       These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the exemplary embodiments, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is a perspective view illustrating a lithium secondary battery, according to an exemplary embodiment of the present invention; 
         FIG. 2  is a construction view illustrating an electrode plate and a separator of an electrode assembly, of the lithium secondary battery according to the exemplary embodiment of the present invention; 
         FIG. 3  is a schematic plan view illustrating the electrode assembly of the lithium secondary battery disposed in a can; and 
         FIG. 4  is a magnified view illustrating section “IV” of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Reference will now be made in detail to the exemplary 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 exemplary embodiments are described below, in order to explain the aspects of present invention, by referring to the figures. 
       FIG. 1  is a perspective view illustrating a lithium secondary battery, according to an exemplary embodiment of the present invention, and  FIG. 2  is a construction view illustrating an electrode plate and a separator of an electrode assembly of the lithium secondary battery, and  FIG. 3  is a schematic plan view illustrating the electrode assembly of the lithium secondary battery is received in a can, and  FIG. 4  is a magnified view illustrating “IV” part of  FIG. 3 . 
     Referring to  FIGS. 1 to 4 , a lithium secondary battery  10  includes an electrode assembly  100 , a can  200  to house the electrode assembly  100 , and a cap assembly  300  to seal an upper end of the can  200 . The cap assembly  300  includes an electrode terminal  310  to electrically couple the electrode assembly  100  to an external terminal. 
     The electrode assembly  100  includes: a cathode plate  110  provided with a cathode active material layer  114  formed on a predetermined region of a cathode collector  112 , which is a base material; an anode plate  120  provided with an anode active material layer  124  formed on a predetermined region of an anode collector  122 ; and a resin separator  130  interposed between the cathode plate  110  and the anode plate  120 . Cathode uncoated parts  116  and  118  are disposed at both ends of the cathode plate  110 , where the cathode active material layer  112  is not formed. Anode uncoated parts  126  and  128  are regions at both ends of the anode plate  120 , on which the anode active material layer  122  is not formed. 
     The cathode active material can be a lithium oxide, such as, LiCoO 2 , LiMn 2 O 4 , LiNiO 2 , and LiMnO 2 . The anode active material can be a carbonic material, silica (Si), tin (Sn), tin oxide, composite tin alloys, and a transition metal oxide. 
     The cathode collector  112  of the cathode plate  110  may be made of aluminum (Al), for example. The anode collector  122  of the anode plate  120  may be made of copper (Cu), for example. The resin separator  130  is formed of a multi-layer film of polyethylene, polypropylene, or combination thereof, each of which has fine porous structure. 
     The lithium secondary battery  10  includes a porous ceramic film  140 , formed by coating a ceramic material on the anode plate  120 . The porous ceramic film  140  prevents an electrical short between the cathode plate  110  and the anode plate  120 , and is permeable to lithium ions. 
     The porous ceramic film  140  is formed by dipping the anode plate  102  in a film solution. The film solution includes a ceramic material (particles) uniformly dispersed in a binder and a solvent. The porous ceramic film  140  is coated on at least one of the two electrode plates  110  and  120 . The porous ceramic film  140  may be formed by spraying, dipping, or printing the film solution on the anode plate  120 . 
     The ceramic material may include at least one of silica (SiO 2 ), alumina (Al 2 O 3 ), zirconium oxide (ZrO 2 ), and titanium oxide (TiO 2 ). The ceramic material can include an insulating nitride, hydrate, alkoxide, or ketonide, of silicon (Si), aluminum (Al), zirconium (Zr), titanium (Ti), or a combination thereof, but is not limited thereto. For example, the ceramic material can be a titanium hydrate, an aluminum alkoxide, a silicon nitride, a zirconium ketonide, etc. 
     Referring to  FIG. 2 , the porous ceramic film  140  is coated on the active material layer  122  of the anode plate  120 . The porous ceramic film  140  is coated on both surfaces of the anode uncoated part  126 . The anode uncoated part  126  is arranged at a winding center of the electrode assembly  100 . The porous ceramic film  140  is coated on an inner surface  128 A of the anode uncoated part  128 , and is not coated on an outer surface  128 B of the anode uncoated part  128 . The anode uncoated part  128  is disposed at an outer surface of the electrode assembly  100  (adjacent to the can  200 ). 
     Referring to  FIGS. 3 and 4 , the electrode assembly  100  includes the cathode and anode plates  110  and  120 , which are wound together. The electrode assembly  100  is received in the can  200 . The porous ceramic film  140  is not coated on the outer surface  128 B, which faces the can  200 , but is coated on the inner surface  128 a, which faces the winding center of the electrode assembly  100 . 
     As described above, when the porous ceramic film  140  is coated only on the inner surface  128 A of the anode uncoated part  128 . Only the resin separator  130  is interposed between the cathode uncoated part  118  of the cathode plate  110 , and the anode uncoated part  128  of the anode plate  120 . Accordingly, the cathode uncoated part  118 , and the anode uncoated part  128 , are not separated from each other by the porous ceramic film  140 . 
     When an external impact occurs, as exemplified by a nail penetration test, an electrical short may occur between the cathode uncoated part  118  and the anode uncoated part  128 . The safety characteristics of the battery  10  are improved, by a current-dispersing effect of the electrical short. 
     As described above, the ceramic film  140  is not coated on both surfaces of the anode uncoated part  128 , which is disposed adjacent to the periphery of the electrode assembly  100 . The ceramic film  140  is coated on the inner surface  128 A of the anode uncoated part  128 . Accordingly, an electrical short can occur between the outer surface  128 B and the cathode uncoated part  118 . Further, an electrical short is prevented from occurring between the inner surface  128 A and the cathode plate  110 . 
     In the exemplary embodiment, the porous ceramic film  140  is coated on one surface of the anode uncoated part  128 , however, if the orientation of the electrode plates  110  and  120  is reversed in the electrode assembly  100 , the cathode plate  110  may be coated instead of the anode plate  120 . In other words, the polarity of the electrode plates  110  and  120  is not critical. 
     As described above, a lithium secondary battery, according to aspects of the present invention, produces the following effects. First, an internal electrical short can be prevented at internal portion of an electrode assembly, by a porous ceramic film coated on an electrode plate. Second, an electrical short can occur between an uncoated portion of an electrode plate and an adjacent uncoated portion of a second electrode plate, adjacent to an outer surface of the electrode assembly, due to an external impact applied to the battery, thereby allowing the safety of the battery to be assured. 
     Although an exemplary embodiment of the present invention has been shown and described, it would be appreciated by those skilled in the art that changes may be made in this exemplary embodiment, without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.