Abstract:
A lithium ion secondary battery has a shape memory safety vent adapted to discharge internal compressed gas by temporarily being opened when the temperature reaches a predetermined level to avoid a swelling phenomenon of the battery and improve safety. The lithium ion secondary battery includes an electrode assembly having first and second electrode plates wound a number of times with a separator interposed between them; a can having an opening formed on a side thereof to contain the electrode assembly; and a cap plate adapted to cover the can and provided with a vent hole on a side thereof, to which a safety vent adapted to deform at a predetermined temperature and discharge gas from inside the can to the exterior is coupled.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Korean Patent Application No. 10-2004-0071415 filed on Sep. 7, 2004 at 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 shape memory safety vent adapted to discharge internal compressed gas. 
     2. Description of Related Art 
     A typical lithium ion secondary battery includes: an electrode assembly formed by winding a positive electrode plate having a positive electrode active material attached thereto, a negative electrode plate having a negative electrode active material attached thereto, and a separator positioned between the positive and negative electrode plates to avoid a short circuit and allow only lithium ions to move, into a commonly known “jelly roll” structure; an electrolyte for enabling lithium ions to move; a can for containing and sealing the electrode assembly and the electrolyte; and a cap assembly for covering the can and preventing the electrode assembly from escaping. 
     Such a lithium ion secondary battery is manufactured as follows. A positive electrode plate having a positive electrode active material attached thereto, a negative electrode plate having a negative electrode active material attached thereto, and a separator are laminated and wound into a jelly roll. They are placed into a square-type can and a cap assembly is welded to the top thereof to seal it; an electrolyte is injected. Charging and inspection are performed to complete a bare cell. Various protective devices are attached to the bare cell. Assembly and inspection are then preformed to complete a conventional battery pack. 
     The lithium ion secondary battery is charged in a static voltage/static current condition and no overcharging occurs as long as the charging voltage is correctly controlled by the charger. When the charger is damaged or erroneously operated, however, abnormal charging occurs and the voltage and temperature of the battery abruptly increase. Such an increase decomposes the positive electrode active material or the electrolyte inside the battery. As a result, gas is generated and the battery swells. Such gas generation and swelling phenomenon may also result from heat supplied from outside the battery. The generated gas increases the internal pressure of the battery and causes the electrolyte to leak out. The battery may then explode or catch fire. 
     Safety measures to prevent such problems include a positive temperature coefficient (PTC) thermistor and a separator incorporating a shutdown function, as well as a safety vent actuated by gas generation as mentioned above. 
     Particularly, the safety vent of a conventional square-type lithium ion secondary battery refers to a thinner region formed on the bottom surface of the can or on the cap assembly. The safety vent fractures when the internal pressure of the can reaches a reference level and discharges gas to the exterior. Once actuated, the safety vent cannot return to the original state (i.e., it is irreversible) and must be disposed of. 
     As mentioned above, conventional safety vents are actuated only when the pressure reaches a reference level, regardless of the temperature of the battery, in an irreversible manner. During overcharging, however, voltage rise is generally preceded by temperature rise (or the battery temperature rises due to heat supplied from the exterior), which is then followed by gas generation. Although actuation in response to temperature is ideal, conventional safety vents respond only to pressure. This is an obstacle to improving the safety of the battery. 
     A large deviation exists in the battery pressure which fractures a safety vent formed as a thinner region. In other words, a safety vent may be unnecessarily actuated below a reference temperature or pressure or may fail to be actuated even above a reference temperature or pressure. This may result in explosion or fire since the safety vent is not designed to be actuated in proportion to temperature, but is designed to be physically actuated within a predetermined pressure range. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention a lithium ion secondary battery is provided having a shape memory safety vent adapted to discharge internal compressed gas by temporarily being opened when the temperature reaches a predetermined level to avoid a swelling phenomenon of the battery and improve safety. 
     In one aspect of the present invention a lithium ion secondary battery includes an electrode assembly having first and second electrode plates wound a number of times with a separator interposed between them, a can to contain the electrode assembly and having an opening formed at an end thereof; and a cap plate adapted to cover the opening and be provided with a vent hole through the cap plate, the vent hole mounting a safety vent adapted to deform at a predetermined temperature and discharge gas from inside the can to the exterior. 
     The safety vent may be adapted to deform in a temperature range of 70-150° C. and opens the vent hole. 
     The safety vent may include a cylindrical body having the same diameter as that of the vent hole and a disk-shaped latching plate positioned on top of the cylindrical body and having a diameter larger than that of the cylindrical body. 
     The cap plate may have a retaining plate attached to the top surface thereof to cover the vent hole and the safety vent. The retaining plate may comprise an edge plate welded to the cap plate on both opposite sides of the safety vent and a center plate connected to the edge plate in a position corresponding to the safety vent. 
     The center plate may have a curved portion formed on the interior thereof while being curved from the central top thereof toward the edge plate on the outer periphery thereof with a predetermined curvature so that, when the safety vent contracts at a predetermined temperature, it is not released to the exterior and a space portion formed among the curved portion, the safety vent, and the cap plate so that gas can be easily discharged to the exterior. 
     The inventive lithium ion secondary battery has greatly improved safety because, when the internal temperature rises above a predetermined level due to overcharging or heat supplied from the exterior, the safety vent temporarily contracts and discharges internal gas. Instead of being fractured and actuated in a pressure range having a large deviation as in the prior art, the inventive safety vent temporarily contracts and functions at a predetermined temperature and is actuated. As such, the operating condition of the safety vent becomes more precise and the safety of the battery improves further. 
     When the battery temperature drops below the predetermined range, the safety vent regains the original size and suppresses the discharge of internal gas. The battery is then ready for use again. The internal pressure of the battery decreases or the interior is in a substantially vacuum state as the temperature drops to the normal range. This further 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 shape memory safety vent 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 sectional view taken along line  2 - 2  of  FIG. 1  wherein the temperature is below a predetermined level and the shape memory safety vent has not yet been actuated. 
         FIG. 5A  is a sectional view taken along line  2 - 2  of  FIG. 1  wherein the temperature is above a predetermined level and the shape memory safety vent has been actuated. 
         FIG. 5B  is a sectional view taken along line  3 - 3  of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     In the following description and drawings, the same reference numerals are used to designate the same or similar components, and so repetition of the description on the same or similar components will be omitted. 
     Referring now to  FIGS. 1 ,  2  and  3 , a lithium ion secondary battery  100  includes an electrode assembly  110 , a can (or a sheath)  120  for containing the electrode assembly  110 , an electrolyte (not shown) injected into the can  120  to enable 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 deform at a predetermined temperature. 
     The electrode assembly  110  includes a first electrode plate  111  having a first active material (not shown) attached thereto, a second electrode plate  112  having a second active material (not shown) attached thereto, and a separator  113  positioned between the first and second electrode plates  111 ,  112  to avoid a short circuit and allow only lithium ions to move. The first and second electrode plates  111 ,  112  and the separator  113  are wound a number of times into a jelly roll and are contained in the can  120 . The first and second electrode plates  111 ,  112  have first and second leads  114 ,  115  welded thereto, respectively, which protrude upward a predetermined distance. 
     The first active material may be a positive electrode active material (for example, lithium cobalt oxide (LiCoO 2 )) and the first electrode plate  111  may be a positive electrode plate made up of aluminum (Al). The second active material may be a negative electrode active material (for example, graphite) and the second electrode plate  112  may be a negative electrode plate made up of copper (Cu). The first and second leads  114 ,  115  may be positive and negative electrode leads made up of aluminum and nickel, respectively. The separator  113  may be made up of polyethylene (PE) or polypropylene (PP), but the material is not limited in the present invention. 
     The can  120  includes at least one first surface  121 , at least one second surface  122  connected to the first surface  121  and having an area smaller than that of the first surface  121 , and a third surface  123  connected to both of the first and second surfaces  121  and  122 . The can  120  has an opening  124  formed on the top thereof facing the third surface  123 . Particularly, the can  120  has an approximately cuboid shape having an opening  124  formed on the top thereof. The can  120  may be made up of aluminum (Al), iron (Fe), an alloy, or an equivalent thereof, but the material is not limited herein. 
     The electrolyte (not shown) is injected into the can  120  and is positioned between the first and second electrode plates  111 ,  112  of the electrode assembly  110 . The electrolyte acts as a medium for movement of lithium ions created by an electrochemical reaction near the first and second electrode plates  111 ,  112  inside the battery during charging/discharging and 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. 
     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 , but these components are not always necessary in the present invention. The insulation case  131 , the terminal plate  132 , and the insulation plate  133  have through-holes  131   a ,  132   a , and  133   a  formed thereon, respectively, so that the second d  115  extends through in the upward direction. The insulation case  131  has an electrolyte through-hole  131   b  formed thereon so that, when the electrolyte is injected through the cap plate  141  (described later), it can easily flow toward the electrode assembly  110 . 
     The cap assembly  140  is welded to the opening  124  of the can  120  by a laser welding and includes a 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 electrolyte injection, and a vent hole  147  formed on the other side thereof to be coupled to the safety vent  148 . An insulation gasket  143  is coupled to the through-hole  142  of the cap plate  141  and an electrode terminal  144  is coupled to the insulation gasket  143 . The electrode terminal  144  is welded to the second lead  115  to act as a negative or positive electrode during discharging or charging. The first lead  114  is welded between the electrolyte injection hole  145  of the cap plate  141  and the electrode terminal  144 , so that the cap plate  141  and the can  120  act as a positive or negative electrode as a whole. A plug  146  is coupled and welded to the electrolyte injection hole  145  of the cap plate  141  so that, after the electrolyte is injected, it is prevented from leaking out. 
     The safety vent  148  having an approximately cylindrical shape is coupled to the vent hole  147  formed on the cap plate  141  and a retaining plate  149  is welded to the cap plate  141  to cover the vent hole  147  and the safety vent  148 . 
     Referring now to  FIG. 4 , a magnified view of region  3  of  FIG. 3  is illustrated, wherein the cap plate  141  includes an approximately or completely planar first surface  141   a , an approximately or completely planar second surface  141   b  opposing the first surface  141   a , and a vent hole  147  formed between the first and second surfaces  141   a  and  141   b  with a predetermined diameter to be coupled to the safety vent  148 . The safety vent  148  coupled to the vent hole  147  has a cylindrical body  148   a  having the same diameter as the vent hole  147  at a normal operating temperature and a disk-shaped latching plate  148   b  positioned on top of the cylindrical body  148   a  to contact the first surface  141   a  of the cap plate  141  and having a diameter larger than that of the cylindrical body  148   a.    
     In an exemplary embodiment the safety vent  148  may be made up of such a material that, when the battery temperature rises above a predetermined level, it temporarily contracts and open the vent hole  147 . For example, the safety vent  148  may be made up of such a material that is actuated at a temperature range of 70-150° C., in which gas is generally generated in the battery, and returns to the original shape when the temperature drops. 
     The safety vent  148  may be made up of a shape memory alloy which contracts in a predetermined temperature range and regains the original volume below the temperature range. The shape memory alloy may be any one chosen from Ni—Ti alloy, Cu—Zn—Al alloy, Cu—Al—Ni alloy, and an equivalent thereof, but the material is not limited thereto. The shape memory alloy may be any one chosen from Ni—Ti alloy, Cu—Zn—Al alloy, Cu—Al—Ni alloy, and an equivalent thereof and having a predetermined amount of Mn added thereto. The shape memory alloy may be any one chosen from Cu—Zn—Al alloy, Cu—Al—Ni alloy, and an equivalent thereof and having a predetermined amount of Ti added thereto. 
     When the safety vent  148  contracts in a predetermined temperature range and discharges internal gas, it may be completely released out of the cap plate  141  by the gas pressure. Therefore, a retaining plate  149  may be attached to the first surface  141   a  of the cap plate  141  outside the safety vent  148  to cover it. The retaining plate  149  may include an edge plate  149   a  welded to the cap plate  141  on both opposite sides of the safety vent  148  and a center plate  149   b  connected to the edge plate  149   a  in a position corresponding to the safety vent  148 . The retaining plate  149  may further include a curved portion  149   c  curved from the central top of the center plate  149   b  toward the edge plate  149   a  on the outer periphery thereof with a predetermined curvature and a space portion  149   d  formed between the curved portion  149   c  and the first surface  141   a  of the cap plate  141  while being in communication with the exterior so that the gas inside the battery can be easily discharged to the exterior. The retaining plate  149  may be made up of aluminum, iron, an alloy, or an equivalent thereof, as in the case of the cap plate  141 , but the material is not limited herein. 
     Referring now to  FIG. 5A , a sectional view taken along line  2 - 2  of  FIG. 1  wherein the temperature is above a predetermined level and the shape memory safety vent has been actuated is illustrated. Also referring to  FIG. 5B , a sectional view taken along line  3 - 3  of  FIG. 1  wherein the temperature is above a predetermined level and the shape memory safety vent has been actuated is illustrated. The safety vent  148  contracts when the temperature inside the battery is approximately 70-150° C. (this temperature range give as only an example and can be modified) and opens the vent hole  147 . Particularly, the diameter of the cylindrical body  148   a  of the safety vent  148  becomes smaller than that of the vent hole  147  and the gas pressure  150  pushes the cylindrical body  148   a  and the disk-shaped latching plate  148   b  in the outward direction, so that internal gas is discharged to the exterior through the space  147   a  between the safety vent  148  and the vent hole  147  and the space portion  149   d  between the center plate  149   b  of the retaining plate  149  and the cap plate  141 . The center plate  149   b  of the retaining plate  149  limits the traveling distance of the latching plate  148   b  of the safety vent  148  such that the safety vent  148  is not completely released to the exterior by the gas pressure. When the battery temperature returns to the normal range during such gas discharge, the safety vent  148  regains the original volume or size and again blocks the vent hole  147 . Specifically, the diameter of the cylindrical body  148   a  of the safety vent  148  becomes equal to that of the vent hole  147  and completely blocks the vent hole  147 . The gas discharge is then interrupted and the battery is again ready for use. As such, the battery does not need to be discarded once the safety vent  148  is actuated as in the prior art. 
     As mentioned above, the inventive lithium ion secondary battery has greatly improved safety because, when the internal temperature rises above a predetermined level due to overcharging or heat supplied from the exterior, the safety vent temporarily contracts and discharges internal gas. Instead of being fractured and actuated in a pressure range having a large deviation as in the prior art, the inventive safety vent temporarily contracts and functions at a predetermined temperature and is actuated. As such, the operating condition of the safety vent becomes more precise and the safety of the battery improves further. 
     When the battery temperature drops below the predetermined range, the safety vent regains the original size and suppresses the discharge of internal gas. The battery is then ready for use again. The internal pressure of the battery decreases or the interior is in a substantially vacuum state as the temperature drops to the normal range. This further 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.