Abstract:
A secondary battery includes an electrode assembly having first and second electrode plates and a separator interposed between them, a metallic case adapted to contain the electrode assembly, the case provided with an insulation layer, and a cap assembly adapted to complete the case. The insulation layer helps prevent a short circuit between the electrode plates and the case.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority to and the benefit of Korean Patent Application No. 2004-0086889, filed Oct. 28, 2004, the entire content of which is hereby incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the invention  
         [0003]     The present invention relates to a secondary battery, and more particularly to a secondary battery having an electrode assembly and a case insulated from each other.  
         [0004]     2. Description of the Prior Art  
         [0005]     Recently, compact and light electronic apparatus including cellular phones, laptop computers, and camcorders have been actively developed and produced. Such portable electronic apparatus is typically equipped with a battery pack so that they can be operated even in a place where no separate power supply is provided. The battery pack has at least one battery positioned therein to output a predetermined level of electric power necessary for driving the portable electronic apparatus for a period of time.  
         [0006]     Current battery packs use a secondary battery which is rechargeable. Secondary batteries include, for example, nickel-cadmium (Ni—Cd) batteries, nickel-hydrogen (Ni-MH) batteries, lithium (Li) batteries, and lithium ion batteries.  
         [0007]     In particular, lithium secondary batteries have an operating voltage of 3.6 V, which is three times larger than nickel-hydrogen batteries or nickel-cadmium batteries widely used as the power supply of portable electronic appliances, as well as a high energy density per unit weight. For these reasons, the lithium secondary batteries are increasingly used in the industry.  
         [0008]     Lithium secondary batteries generally use lithium-based oxide as the positive electrode active material and carbon material as the negative electrode active material. According to the type of electrode, lithium secondary batteries are classified into lithium ion batteries using a liquid electrolyte and lithium polymer batteries using a polymer electrolyte. Lithium secondary batteries are manufactured in various types including a cylinder type, a square type, and a pouch type.  
         [0009]     In general, a secondary battery includes an electrode assembly formed by winding first and second electrode plates coated with active materials and a separator positioned between the first and second electrode plates to avoid a short circuit and allowing only lithium ions to move. The secondary battery also usually includes a case for containing the electrode assembly, and an electrolyte injected into the secondary battery case to enable lithium ions to move.  
         [0010]     However, in conventional secondary batteries constructed as above, the electrode plates have a polarity different from the case. Thus, if the electrode plates contact the case, a short circuit can occur.  
         [0011]     More specifically, the separator constituting the electrode assembly is larger than the electrode plates to prevent them from contacting each other. When the electrode assembly is contained in the case, the separator is bent and during this process the electrode plates having a polarity different from the case often contact the case.  
         [0012]     In the case of a cylinder-type case, a sheath made of an insulation material is wound around the outer surface of the case to prevent it from being damaged and to avoid a short circuit between the case and a conductor.  
         [0013]     In general, the sheath is fabricated by inserting a cylinder-type case into a flexible PVC tube and heating it at about 200° C. or higher so that the tube thermally contracts. Then, the tube is laminated on the outer surface of the cylinder-type case. As a result, a sheath for protecting the outer surface of the case is provided.  
         [0014]     However, the electrode assembly or electrolyte contained in the secondary battery deforms due to the heat used to contract the PVC tube. This degrades the performance of the secondary battery.  
         [0015]     In addition, the use of PVC is restricted because it generates harmful substances including phthalate ester, which acts as an environment hormone and as a carcinogenic, and silicon chloride, which when heated causes environmental pollution.  
       SUMMARY OF THE INVENTION  
       [0016]     Accordingly, a secondary battery is provided having an insulation layer between a case and electrode plates having a polarity different from the case so that direct contact may be avoided between the case and the electrode plates.  
         [0017]     The secondary battery includes an electrode assembly having first and second electrode plates and a separator interposed between them; a metallic case adapted to contain the electrode assembly and provided with a surface insulation layer; and a cap assembly adapted to complete the case.  
         [0018]     The insulation layer may be formed on the interior surface of the case, on the exterior surface thereof, or on both the interior surface and the exterior surface of the case.  
         [0019]     The insulation layer may be provided as a coating and the coating may be formed by dipping, spraying, laminating or painting. The insulation layer may be made of epoxy, nylon, or acryl. The insulation layer may be formed on the lower side to cover the bottom surface of the case. The case may be a square type or a cylinder type. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]      FIG. 1  is an exploded perspective view showing a square-type secondary battery according to the present invention.  
         [0021]      FIG. 2  is a partial sectional view of the square-type secondary battery shown in  FIG. 1  taken along lines A′-A according to one embodiment of the present invention.  
         [0022]      FIG. 3  is a partial sectional view of the square-type secondary battery shown in  FIG. 1 , taken along lines A′-A according to another embodiment of the present invention.  
         [0023]      FIG. 4  is a partial sectional view of the square-type secondary battery according to yet another embodiment of the present invention.  
         [0024]      FIG. 5  is a perspective view showing a cylinder-type secondary battery according to still another embodiment of the present invention.  
         [0025]      FIG. 6  is a sectional view of the cylinder-type secondary battery shown in  FIG. 5 , taken along lines B′-B according to a further embodiment of the present invention.  
         [0026]      FIG. 7  is a sectional view of the cylinder-type secondary battery shown in  FIG. 5 , taken along lines B′-B according to yet another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0027]     As shown in the drawings, a secondary battery  100  according to the present invention includes a secondary battery case  110 , a jelly-roll type electrode assembly  220  contained in the case  110 , and a cap assembly  300  for completing the case  110 .  
         [0028]     The case  110  is made of a metallic material and has an approximately cylindrical or square shape. The case  110  has an opening formed on an end thereof and may act as a terminal on its own.  
         [0029]     The case  110  may be coated with an insulation layer  250 . More specifically, the case  110  may have an insulation layer  250   a  formed on its interior and/or an insulation layer  250   b  on its exterior. The insulation layer  250   b  may also be formed on the bottom side if the case  110  has a square shape.  
         [0030]     The insulation layer  250  may be formed by dipping the case  110  into an insulation material or spraying an insulation material on the interior of the case  110 . In addition, the insulation layer  250  may be formed by painting the case  110  with an insulation material or laminating a film-type insulation material on the case  110 .  
         [0031]     The insulation material constituting the insulation layer  250  may be epoxy resin, which is a thermosetting resin formed by polymerization of at least two epoxy groups and which has excellent viscosity, adherence, resistance to drugs, electrical properties, and mechanical properties (for example, bending strength and hardness).  
         [0032]     Nylon is a chain-type polymer having amide bond —CONH—. Particularly, it is a polyamide-based synthetic polymer. Nylon has good tenacity and excellent resistance to oil and drugs and it may be used at both low and high temperatures in a stable state. Nylon plastic (non-spun nylon) is a typical example of engineering plastic having a small friction coefficient and excellent resistance to wear. Nylon 6 has tenacity, oxygen interruption properties, and resistance to oil as a film and is used to wrap oily foods or frozen foods. Nylon 6, 6 has excellent resistance to heat. Nylon fibers have high tensile strength, low elastic modulus, low absorption ratio, and high resistance to wear. They can be permanently set and are widely used as knitted or woven fabrics. Due to high tensile strength, nylon fibers are especially suitable for very thin woven structures, for instance, stockings for women. They are also used extensively mixed with wool, cotton, or rayon. Recently, aromatic polyamide (aramide) is drawing attention as a new type of polyamide. It includes benzene rings in the molecular structure and has better high-temperature properties (higher melting point and elastic modulus) than aliphatic polyamide. Therefore, it is used as heat-resistant fibers, tire cords, or other industrial materials. Nylon retains excellent strength, flexibility, and elasticity even when it has the minimum volume. For this reason, nylon may be processed into a thin film.  
         [0033]     When the case  110  is a cylinder type, as shown in  FIGS. 5-7 , the insulation layer  250   b  on the outer surface of the case  110  has a hole at the bottom of the case  110  for electric coupling to outer electric or electronic devices. The insulation layer  250   b  may be a thin film of nylon. The film-type insulation layer  250   b  may be laminated on the outer surface of the case  110  by a small amount of heat. The insulation material may be acryl material, but the material is not limited herein.  
         [0034]     The electrode assembly  200  is formed by winding first and second electrode plates  210 ,  220  having first and second electrode tabs  215 ,  225  attached thereto, respectively, with a separator  230  interposed between the first and second electrode plates  210 , 220 .  
         [0035]     In order to insulate the first and second electrode plates  210 ,  220  from each other, the separator  230  is larger than the first and second electrode plates  210 ,  220  so that they do not contact each other.  
         [0036]     The insulation layer  250  formed on the case  110  may cover either the interior of the case  110  as shown in  FIG. 2  or the exterior thereof as shown in  FIG. 3 . Alternatively, it may cover both the interior and exterior of the case  110  as shown in  FIG. 4 .  
         [0037]     When the electrode assembly  200  is contained in the case  110 , the separator  230 , larger than the first and second electrode plates  210 ,  220 , does not lean to one side and prevents the bottom of the case  110  from contacting the first or second electrode plate  210 ,  220 . In addition, when a conductor contacts the exterior of the case  110 , a short circuit is avoided.  
         [0038]     When the case  110  is a square type as shown in  FIGS. 1-4 , the cap assembly  300  includes a cap plate  310 , an insulation plate  340 , a terminal plate  350 , and an electrode terminal  320 . The cap assembly  300  is coupled to the top opening of the case  110  while being coupled to a separate insulation case  360  and seals the case  110 .  
         [0039]     The cap plate  310  is made of a metal plate and has a size and a shape corresponding to the top opening of the case  110 . The cap plate  310  has a centrally located first terminal through-hole  311  into which the electrode terminal  320  is inserted.  
         [0040]     When the electrode terminal  320  is inserted into the first terminal through-hole  311 , a tubular gasket  330  is coupled to the outer surface of the electrode terminal  320  and inserted together to insulate the electrode terminal  320  from the cap plate  310 .  
         [0041]     The cap plate  310  has an electrolyte injection hole  312  formed on a side thereof with a predetermined size. After the cap assembly  300  is assembled to the top opening of the case  110 , an electrolyte is injected through the electrolyte injection hole  312  and the injection hole is then sealed by a plug  315 .  
         [0042]     The electrode terminal  320  is connected to the second electrode tab  225  of the second electrode plate  220  or to the first electrode tab  215  of the first electrode plate  210  and acts as a negative or positive electrode terminal. When the case  110  is a square type, the electrode terminal  320  may act as a negative electrode terminal.  
         [0043]     The insulation plate  340  is made of an insulation material, such as a gasket, and is coupled to the lower surface of the cap plate  310 . The insulation plate  340  has a second terminal through-hole  341  formed in a position corresponding to the first terminal through-hole  311  of the cap plate  310  so that the electrode terminal  320  may be inserted therein.  
         [0044]     The terminal plate  350  is made of Ni metal or its alloy and is coupled to the lower surface of the insulation plate  340 . The terminal plate  350  has a third terminal through-hole  351  formed in a position corresponding to the first terminal through-hole  311  of the cap plate  310  so that the electrode terminal  320  may be inserted therein. Since the electrode terminal  320  is coupled to the terminal plate  350  via the first terminal through-hole  311  of the cap plate  310  while being insulated by the gasket  330 , the terminal plate  350  is electrically connected to the electrode terminal  320  while being electrically insulated from the cap plate  310 .  
         [0045]     In order to couple the electrode terminal  320  to the cap plate  310 , insulation plate  340 , and terminal plate  350 , the electrode terminal  320  is inserted into the first terminal through-hole  311  of the cap plate  310  with a uniform rotational force.  
         [0046]     After passing through the first terminal through-hole  311 , the electrode terminal  320  passes through the second and third terminal through-holes  341 ,  351  formed on the insulation plate  340  and terminal plate  350 , respectively, which are coupled to the lower surface of the cap plate  310 . The inner diameter of the second terminal through-hole  341  formed on the insulation plate  340  is equal to or slightly larger than the outer diameter of the electrode terminal  320  so that when the electrode terminal  320  is inserted into the second terminal through-hole, the outer surface of the electrode terminal is forced against it.  
         [0047]     The electrode assembly  200  has an insulation case  360  positioned on the upper surface thereof to electrically insulate the electrode assembly  200  from the cap assembly  300  and to cover the top of the electrode assembly  200 .  
         [0048]     The insulation case  360  has an electrolyte injection through-hole  316  formed in a position corresponding to the electrolyte injection hole  312  of the cap plate  310  through which an electrolyte may be injected. The insulation case  360  is made of an insulating polymer resin, for example, polypropylene, but the material is not limited in the present invention.  
         [0049]     Since the insulation case  360  is seated on the upper side of the electrode assembly  200  and insulates the case  110  from the first and second electrode plates  210 ,  220 , the upper side of the case  110  may be insulated without any additional insulation layer.  
         [0050]     When the case  110  is a cylinder type, the opening  110   a  of the case  110  is completed by a cap-up constituting the cap assembly  300 .  
         [0051]     As mentioned above, the secondary battery according to the present invention has insulation layers formed on the interior and exterior of the case to avoid a short circuit caused by contact between the electrode plates and the case.  
         [0052]     In the case of a cylinder-type secondary battery, the insulation layer for covering the exterior is made of a material which does not violate environmental regulations.  
         [0053]     Although exemplary embodiments of the present invention have been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.