Abstract:
A lithium ion secondary battery having a safety vent responsive to temperature and pressure. The lithium ion secondary battery includes an electrode assembly having a positive electrode plate, a separator, and a negative electrode plate which are simultaneously wound and laminated, and positive and negative electrode leads extending outward from the positive and negative electrode plates, respectively. A can containing the electrode assembly and having an opening; and a cap plate coupled to the opening of the can, wherein an electrode terminal extends through and is coupled to the center of the cap plate with a gasket interposed therein, the negative electrode lead being connected to the electrode terminal, the positive electrode lead being connected to the cap plate, a coupling hole being formed on the cap plate, and a safety vent adapted to soften at a predetermined temperature is coupled to the coupling hole.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Korea Patent Application No. 2004-0046671 filed on Jun. 22, 2004, in the Korean Intellectual Property Office, the entire content of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a lithium ion secondary battery, and more particularly to a lithium ion secondary battery having a safety vent responsive to temperature and pressure for improved safety. 
     2. Description of the Prior Art 
     As generally known in the art, a lithium ion secondary battery includes an electrode assembly having a positive electrode plate with positive electrode active materials attached thereto, a negative electrode plate with negative electrode active materials attached thereto, and a separator positioned between the positive and negative electrode plates to prevent a short circuit and to allow movement of lithium ions. The positive and negative electrode plates and the separator may be wound into a jelly roll configuration. The secondary battery may also include an electrolyte for enabling lithium ions to move; a can which contains the electrode assembly and the electrolyte and which is then sealed; and a cap assembly for covering the can and preventing the electrode assembly from escaping. 
     Such a lithium ion secondary battery may be manufactured as follows: a positive electrode plate having positive electrode active materials attached thereto, a negative electrode plate having negative electrode active materials attached thereto, and a separator are laminated and wound into a jelly roll configuration and placed into a square type can. Then, a cap assembly is welded to the top of the can to seal it and an electrolyte is injected into the can. A bare cell then may be charged and inspected and various safety devices may be attached to the bare cell to complete a conventional battery pack. 
     A constant voltage/current charging method is used for lithium ion secondary batteries and overcharging does not occur as long as the charging voltage is correctly controlled in chargers. However, abnormal charging sometimes occurs as the chargers are damaged or erroneously operated. When this happens, the electrical potential of positive electrode active materials, e.g., lithium cobalt oxide (LiCoO 2 ), continuously rises causing unceasing rise of the battery voltage and an abnormal heating phenomenon. 
     Safety measures against such overcharging include a positive temperature coefficient (PTC) thermistor, a separator having a shutdown function, and a safety vent actuated by gas generation. As used herein, a safety vent of a square-type lithium ion secondary battery generally refers to a relatively thin region formed on the bottom surface of the can or on the cap assembly which is adapted to fracture during severe swelling caused by gas generation and allows gas to be discharged to outside the battery. 
     The gas generation occurs when the amount of lithium carbonate (Li 2 CO 3 ) added to form positive electrode active materials, such as LiCoO 2 , exceeds the stoichiometry. Particularly, the extra lithium carbonate remains in a non-reacted state within the positive electrode active materials (lithium cobalt oxide) and decomposes to produce carbonate gas when abnormal charging increases the battery voltage and generates heat. Such production of carbonate gas generally causes the can to swell excessively. The safety vent is actuated when the can swells severely and prevents the explosion and/or firing of the battery. 
     The swelling of the can may be avoided by reducing the amount of lithium carbonate added. However, cobalt oxide (CoO 2 ) then remains in the positive electrode active materials and corrodes the positive electrode, which dissolves into the electrolyte during charging. This causes cobalt precipitation to the negative electrode, which increases the possibility of an internal short circuit. As such, the excessive addition of Li 2 CO 3  is inevitable. 
     As mentioned above, the safety vent is not actuated until the battery pressure reaches a predetermined level. However, temperature, as well as battery pressure, generally increase during overcharging. Therefore, safety can be additionally improved if the safety vent is actuated in response not only to pressure, but also to temperature. 
     However, the conventional safety vent formed on the cap assembly or the can with a reduced thickness, as mentioned above, is actuated in response only to the battery pressure, and not to the battery temperature. Thus , there is a need for a safety vent that may be actuated in response to battery pressure as well as battery temperature. 
     SUMMARY OF THE INVENTION 
     A lithium ion secondary battery is provided having a safety vent responsive to temperature and pressure. The battery includes an electrode assembly having a positive electrode plate, a separator, and a negative electrode plate which are wound a number of times while being laminated. Additionally, positive and negative electrode leads are provided extending outward a predetermined length from the positive and negative electrode plates, respectively; as well as a can containing the electrode assembly and having an opening formed on an end thereof. A cap plate is coupled to the opening of the can, wherein an electrode terminal extends through and is coupled to the center of the cap plate with a gasket interposed therein. Further, the negative electrode lead is connected to the electrode terminal, the positive electrode lead is connected to a side of the cap plate, a coupling hole is formed on the other side of the cap plate, and a safety vent adapted to soften at a predetermined temperature is coupled to the coupling hole. 
     In one exemplary embodiment of the lithium ion secondary battery of the present invention, the safety vent plugs the coupling hole formed through the cap plate and itself softens and opens the through-hole to discharge internal gas when the temperature of the battery rises above a predetermined level due to overheating. 
     As such, the safety vent is actuated in response to both temperature and pressure of the battery and additionally improves the safety of the battery. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view showing a lithium ion secondary battery having a safety vent responsive to temperature and pressure according to the present invention. 
         FIG. 2  is an exploded perspective view of the lithium ion secondary battery shown in  FIG. 1 . 
         FIG. 3  is a sectional view taken along line  1 - 1  of  FIG. 1 . 
         FIG. 4  is a magnified view of area  3  shown in  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
     As shown in  FIGS. 1 to 3 , a lithium ion secondary battery  100  according to an exemplary embodiment of the present invention includes an electrode assembly  110 , a can  120  containing the electrode assembly  110 , an electrolyte (not shown) injected into the can  120  to allow lithium ions to move, and a cap assembly  140  which covers the can  120  and prevents the electrode assembly  110  and the electrolyte from escaping to the exterior and which has a safety vent  148  adapted to soften at a predetermined temperature. 
     The electrode assembly  110  includes a positive electrode plate  111  having positive electrode active materials (not shown), for example LICoO 2 , attached thereto, a negative electrode plate  112  having negative electrode active materials (not shown), for example, graphite, attached thereto, and a separator  113  positioned between the positive and negative electrode plates  111  and  112  to prevent a short circuit and to allow only lithium ions to move. The positive and negative electrode plates  111  and  112  and the separator  113  are wound a number of times into a jelly roll configuration while being laminated and are placed in the can  120 . The positive electrode plate  111  may be made of aluminum (Al) foil, the negative electrode plate  112  may be made of copper (Cu) foil, and the separator  113  may be made of polyethylene (PE) or polypropylene (PP), but the materials are not limited to those mentioned in the present invention. The positive electrode plate  111  has a positive electrode lead  114  welded thereto while protruding upward a predetermined length and the negative electrode plate  112  has a negative electrode lead  115  welded thereto while protruding upward a predetermined length. The positive electrode lead  114  may be made of Al and the negative electrode lead  115  may be made of nickel (Ni), but the materials are not limited to those described herein. 
     The can  120  includes at least one first surface  121 , at least one second surface  122  connected to the first surface  121  and having a smaller area than the first surface  121  and a third surface  123  connected to both first and second surfaces  121  and  122 . The can  120 , which in one exemplary embodiment is a hexahedron, has an opening  124  formed at the top thereof which faces the third surface  123 . The can  120  may be made of Al, an iron (Fe) alloy, or an equivalent thereof, but the material is not limited to those described herein. 
     An electrolyte (not shown) is injected into the can  120  and is positioned between the positive and negative electrode plates  111  and  112  of the electrode assembly  110 . The electrolyte acts as a medium for movement of lithium ions created by electrochemical reactions at the positive and negative electrode plates  111  and  112  inside the battery during charging and discharging. The electrolyte may be a non-aqueous organic electrolyte which is a mixture of a lithium salt and a high-purity organic solution. The electrolyte may also be a polymer using a high-molecular electrolyte. 
     In one exemplary embodiment, an insulation case  131 , a terminal plate  132 , and an insulation plate  133  may be successively coupled to the opening  124  of the can  120  on top of the electrode assembly  110 . The insulation case  131 , the terminal plate  132 , and the insulation plate  133  have through-holes  131   a,    132   a,  and  133   a  formed therein so that the negative electrode lead  115  can extend upward through them. The insulation plate  133  has an electrolyte through-hole  131   b  formed therein so that when an electrolyte is injected through a cap plate  141  (described later), the electrolyte can easily flow to the electrode assembly  110 . 
     The cap assembly  140  is laser-welded to the opening  124  of the can  120  and includes an approximately rectangular plate-shaped cap plate  141 . The cap plate  141  has a through-hole  142  formed at the center thereof with a predetermined size, an electrolyte injection hole  145  formed on a side thereof for injecting an electrolyte, and a coupling hole  147  formed on the other side thereof for coupling the safety vent  148  thereto. An insulation gasket  143  is coupled to the through-hole  142  of the cap plate  141  and an electrode (negative electrode) terminal  144  is coupled to the insulation gasket  143 . The electrode terminal  144  is welded to the negative electrode lead  115  to act as a negative electrode during charging or discharging of the battery. The positive electrode lead  114  is welded between the electrode injection hole  145  of the cap plate  141  and the electrode terminal  144 , so that the cap plate  141  and the can  120  as a whole play the role of a positive electrode. After an electrolyte is injected through the electrolyte injection hole  145  of the cap plate  141 , a plug  146  is coupled and welded thereto to prevent the electrode from leaking out. 
     The safety vent  148  having, for example, an approximately cylindrical shape is coupled to the coupling hole  147  formed on the cap plate  141 . Particularly, the safety vent  148  includes an approximately cylindrical body  148   a  having the same diameter with the coupling hole  147  and an approximately disk-shaped catching plate  148   b  having a larger diameter than the body  148   a  and positioned on the bottom of the body  148   a.    
     The safety vent  148  has a softening point of 70-150° C., and in one exemplary embodiment, a softening point of 90-100° C. If the softening point is below 70° C., a melting problem may occur during an aging process when the secondary battery is manufactured. If the softening point is above 150° C., the secondary battery may explode due to overheating. According to this configuration, the safety vent  148  softens and opens the coupling hole  147  when the temperature of the secondary battery  100  rises above a reference level due to overcharging. As a result, gas in the interior of the secondary battery  100  under high pressure is easily discharged to the exterior and the explosion or firing of the battery is prevented. 
     The safety vent  148  may be made of plastic. As is widely known in the art, plastic may be classified into thermosetting resin and thermoplastic resin and the latter may be used in an exemplary embodiment of the present invention. 
     More particularly, the safety vent  148  may be made up of any one chosen from vinyl polymerized polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), polyvinylidene dichloride (PVDC), fluorine resin, acryl resin, and polyacetate vinyl resin. 
     The safety vent  148  may also be made up of any one chosen from polycondensation ring-opening polymerized polyamide resin, acetal resin, polycarbonate (PC), polyphenylene oxide, polyester, polysulphone, and polyimide. 
     Instead of a plastic-based material, the safety vent  148  may be made of a metal-based material. Particularly, the safety vent  148  may be made up of an alloy of fin (Sn), zinc (Zn), and lead (Pb), an alloy of fin (Sn), lead (Pb), and bismuth (Bi), or an equivalent thereof. More particularly, the safety vent  148  may be made up of an alloy including 70-90% of tin (Sn), 5-10% of zinc (Zn), 1-4% of lead (Pb), and balance of other metal or an alloy including 22% of tin (Sn), 28% of lead (Pb), and 50% of bismuth (Bi). 
     Referring to  FIG. 4 , a magnified view of area  3  shown in  FIG. 3  is illustrated. 
     As shown, the cap plate  141  has a substantially planar first surface  141   a,  a substantially planar second surface  141   b  opposite to the first surface  141   a,  and a coupling hole  147  formed with a predetermined diameter between the first and second surfaces  141   a  and  141   b  so that the safety vent  148  can be coupled thereto as mentioned above. The safety vent  148  having a body  148   a  and a catching plate  148   b  is coupled to the coupling hole  147 . The diameter of the catching plate  148   b  in one exemplary embodiment corresponds to about 1.1-2 times that of the body  148   a . If the diameter of the catching plate  148   b  is smaller than 1.1 times that of the body  148   a,  the safety vent  148  may be actuated below the reference temperature and pressure and degrade the credibility of the battery  100 . If the diameter of the catching plate  148   b  is larger than 2 times that of the body  148   a,  the safety vent  148  may fail to be actuated even at the reference temperature and pressure or above and degrade the safety of the battery  100 . 
     In one exemplary embodiment, the thickness of the body  148   a  of the safety vent  148  is equal to the distance between the first and second surfaces  141   a  and  141   b  of the cap plate  141  and the thickness of the catching plate  148   b  corresponds to 0.1-0.9 times that of the body  148   a . If the thickness of the catching plate  148   b  is less than 0.1 times that of the body  148   a,  the safety vent  148  may be actuated below reference temperature and pressure and degrade the credibility of the battery  100 . If the thickness of the catching plate  148   b  is greater than 0.9 times that of the body  148   a,  the safety vent  148  may fail to be actuated even at the reference temperature and pressure or above and degrade the safety of the battery  100 . 
     Although the safety vent  148  has been described with reference to an example having a cylindrical body  148   a  and a disk-shaped catching plate  148   b,  the configuration of the safety vent  148  is not limited to the above-mentioned example. For example, the safety vent  148  may include a body having the shape of a triangular post, a square post, a pentagonal post, or any other shape and a catching plate having the shape of a triangular plate, a square plate, a pentagonal post, or any other shape. 
     The safety vent is released from the through-hole when internal pressure, as well as temperature, rises above a predetermined level and discharges internal gas. As such, the safety vent is actuated in response to both temperature and pressure of the battery and thus improves the safety of the battery. 
     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.