Abstract:
A method of manufacturing a secondary battery, which is capable of simplifying a processing method. The method includes preparing a cap plate that closes an opening of a case, the case accommodating an electrode assembly therein and having the opening at an end thereof, forming a short-circuit portion and a vent portion on the cap plate, performing a first heat treatment on the short-circuit portion, and performing a second heat treatment on the vent portion. The secondary battery manufactured by this method is economical because the processing method is simplified and a manufacturing cost is reduced.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Korean Patent Application No. 10-2014-0058295, filed on May 15, 2014, in the Korean Intellectual Property Office, the entire content of which is incorporated herein by reference in its entirety. 
     BACKGROUND 
     1. Field 
     Aspects of embodiments of the present invention relate to a secondary battery and a method of manufacturing the secondary battery. 
     2. Description of the Related Art 
     in recent years, as electronics and communication industries rapidly grow, a portable electronic device (such as a camcorder, a cellular phone or a notebook computer) is coming into wide use. This leads to the increased use of a secondary battery. The secondary battery is being used (utilized) for the portable electronic device as well as medium and large sized equipment, such as an electric tool requiring high output and high power, a vehicle, a boat, a space transportation system, a motorbike, a scooter or an air transportation vehicle. 
     Recently, a high-output secondary battery with high-energy density using (utilizing) a non-aqueous electrolyte is being developed. The above-mentioned high-output secondary battery forms a high-capacity secondary battery by connecting a plurality of secondary batteries to each other so as to be used in driving a motor of a device requiring high power, for example, an electric vehicle. 
     If a short circuit occurs in the secondary battery, an overcurrent flows in the secondary battery. The continuous flow of the overcurrent generates an excessive amount of heat, thus causing the bursting and/or ignition of the secondary battery. In order to solve the problem, a short-circuit portion and a vent portion may be provided on a cap plate. Thereby, if an internal pressure of the secondary battery exceeds a preset pressure, the short-circuit portion induces the short circuit, thus interrupting the flow of a current, and the vent portion is broken, thus discharging gas generated by the excessive amount of heat. 
     However, the related art is problematic in that the short-circuit portion and the vent portion are separately processed and then are welded to the cap plate, so that a manufacturing cost is high and productivity is low. Therefore, there are needs to conduct various types (kinds) of suitable research into a secondary battery that is capable of reducing a manufacturing cost and improving productivity. 
     SUMMARY 
     Accordingly, aspects of embodiments of the present invention have been made keeping in mind the above problems occurring in the related art, and an aspect of an embodiment of the present invention is to provide a secondary battery, which is configured such that a short-circuit portion and a vent portion are integrated with a cap plate. 
     An aspect of an embodiment of the present invention is to provide a secondary battery, including a short-circuit portion and a vent portion that are lower in hardness than another portion (e.g., remaining portion) of a cap plate. 
     An aspect of an embodiment of the present invention is to provide a secondary battery, which makes it easy to induce a short circuit and a breakage when an internal pressure of the battery increases to be higher than a preset pressure. 
     An aspect of an embodiment of the present invention is to provide a method of manufacturing a secondary battery, which renders the grain size of a short circuit portion and a vent portion to be smaller than that of another portion (e.g., peripheral portion or remaining portion) of a cap plate. 
     An aspect of an embodiment of the present invention is to provide a secondary battery which is capable of simplifying a manufacturing (processing) method. 
     According to an embodiment of the present invention, there is provided a secondary battery, including an electrode assembly having a first electrode plate, a second electrode plate, and a separator interposed between the first electrode plate and the second electrode plate, a case accommodating the electrode assembly therein, the case having an opening at an end thereof (e.g., being open at a surface thereof), and a cap plate closing the opening of the case, with a vent portion and a short-circuit portion formed in set or predetermined portions of the cap plate. The vent portion and the short-circuit portion are integrated with the cap plate. 
     A grain size of each of the vent portion and the short-circuit portion may be larger than that of a remaining portion of the cap plate except the vent portion and the short-circuit portion. 
     The vent portion and the short-circuit portion may be lower in hardness than a remaining portion of the cap plate except the vent portion and the short-circuit portion. 
     The short-circuit portion may have a hardness of 3 to 5 kgf/cm 2 . 
     The vent portion may have a hardness of 6 to 8 kgf/cm 2 . 
     The vent portion and the short-circuit portion may be formed by machining the cap plate and then performing a heat treatment. 
     The heat treatment may be performed by an induction heating apparatus. 
     According to another embodiment of the present invention, there is provided a method of manufacturing a secondary battery, including preparing a cap plate that closes an opening of a case, the case accommodating an electrode assembly therein and having an opening at an end thereof (e.g., being open at a surface thereof), forming a short-circuit portion and a vent portion on the cap plate, performing a first heat treatment on the short-circuit portion, and performing a second heat treatment on the vent portion. 
     The short-circuit portion and the vent portion may be formed by a process selected out from a group consisting of casting, forging, rolling and pressing. 
     The first heat treatment and the second heat treatment may be performed by the induction heating apparatus. 
     The first heat treatment may be performed for 70 minutes or less (e.g., within 70 minutes) at a temperature of 400° C. to 500° C. 
     The second heat treatment may be performed for 70 minutes or less (e.g., within 70 minutes) at a temperature of 200° C. to 300° C. 
     As is apparent from the above description, the secondary battery according to an embodiment of the present invention is advantageous in that the short-circuit portion and the vent portion are integrated with the cap plate, so that a processing method can be simplified and a manufacturing cost can be reduced, and thereby the secondary battery is economical. 
     Further, the secondary battery according to an embodiment of the present invention is advantageous in that the short-circuit portion and the vent portion are lower in hardness than a portion of the cap plate, so that, when the internal pressure of the battery exceeds a preset pressure, the short-circuit portion and the vent portion are broken, thus interrupting the flow of a current and discharging the generated gas and thereby improving the safety of the battery. 
     Furthermore, the method of manufacturing the secondary battery according to an embodiment of the present invention is advantageous in that the short-circuit portion and the vent portion are subjected to heat treatment, respectively, so that the grain size and hardness thereof are smaller than those of the peripheral portion of the cap plate, and thereby the short-circuit portion and the vent portion can be rapidly and easily broken when the internal pressure of the battery exceeds the preset pressure, thus improving the safety of the battery. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Example embodiments will now be described more fully hereinafter with reference to the accompanying drawings; however, they may be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the example embodiments to those skilled in the art. 
       In the drawing figures, dimensions may be exaggerated for clarity of illustration. It will be understood that when an element is referred to as being “between” two elements, it can be the only element between the two elements, or one or more intervening elements may also be present. Like reference numerals refer to like elements throughout. 
         FIG. 1  is a perspective view showing a secondary battery according to an embodiment of the present invention; 
         FIG. 2  is a sectional view taken along line I-I′ of  FIG. 1 ; 
         FIG. 3  is an enlarged view of portion A marked in  FIG. 2 ; 
         FIG. 4  is a flowchart showing a method of manufacturing a secondary battery according to an embodiment of the present invention; 
         FIG. 5A  is a photograph showing a vent portion that is subjected to heat treatment; 
         FIG. 5B  is a photograph showing the vent portion that is not subjected to heat treatment; 
         FIG. 6A  is a photograph showing a short-circuit portion that is subjected to heat treatment; and 
         FIG. 6B  is a photograph showing the short-circuit portion that is not subjected to heat treatment. 
     
    
    
     DETAILED DESCRIPTION 
     The invention will be described more fully hereinafter with reference to the accompanying drawings, in which example embodiments of the inventions are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. 
     Other and further objects and features of the invention will be apparent from the following description taken in connection with the accompanying drawings. 
     Hereinafter, the configuration of the present invention will be described with reference to the accompanying drawings. 
       FIG. 1  is a perspective view showing the appearance of a secondary battery  100  according to an embodiment of the present invention,  FIG. 2  is a sectional view taken along line I-I′ of  FIG. 1 , and  FIG. 3  is an enlarged view of portion A marked in  FIG. 2 . 
     As shown in  FIGS. 1 and 2 , the secondary battery  100  includes an electrode assembly  110 , a case  140 , and a cap plate  151 . The electrode assembly  110  includes a first electrode plate  111 , a second electrode plate  112 , and a separator  113  interposed between the first electrode plate  111  and the second electrode plate  112 . The case  140  accommodates the electrode assembly  110  therein and has an opening at an end thereof (e.g., is open at a surface or surface end thereof). The cap plate  151  closes the opening of the case  140 , with a vent portion  154  and a short-circuit portion  155  provided in set or predetermined portions of the cap plate  151 . Here, the vent portion  154  and the short-circuit portion  155  are integrated with the cap plate  151 . 
     According to the embodiment of the present invention, the vent portion  154  and the short-circuit portion  155  are not welded to the cap plate  151  after being separately formed, but are integrally formed on the cap plate  151  by performing a heat treatment subsequent to machining such as casting, forging, rolling or pressing, as shown in  FIG. 3 . An induction heating apparatus may be used (utilized) for the heat treatment. 
     In one embodiment, the heat treatment is performed at the set or predetermined portions of the cap plate  151 . 
     The cap plate  151  may be made of aluminum and configured to have a set or predetermined hardness to allow a high current generated in the electrode assembly  110  to stably flow. That is, the cap plate  151  is configured to entirely have a robust structure of a set or predetermined hardness, but the vent portion  154  and the short-circuit portion  155  provided on a portion of the cap plate  151  are lower in hardness than another portion of the cap plate  151 . For example, when the internal pressure of the battery increases due to overcharge, the vent portion  154  releases internal gas to ensure the safety of the battery. If the internal pressure of the secondary battery becomes higher than a preset pressure, the short-circuit portion  155  may be deformed by the pressure and come into contact with a second terminal plate  133 , thus inducing a short circuit. In order to guarantee the stable short circuit, the second terminal plate  133  and the short-circuit portion  155  of the cap plate  151  may have different polarities. For instance, the second terminal plate  133  may be a cathode, and the short-circuit portion  155  may be an anode. A notch  154   a  may be further formed in a set or predetermined portion of the vent portion  154  to allow the vent portion  154  to be easily opened at a preset pressure. 
     In view of the above, the vent portion  154  and the short-circuit portion  155  formed in a portion of the cap plate  151  are lower in hardness than another portion (e.g., remaining portion) of the cap plate  151 . Thus, if the internal pressure of the battery increases, the short circuit and breakage occur first, thus interrupting the flow of a current or discharging the internal gas. The vent portion  154  and the short-circuit portion  155  may have the strength of 6 to 8 kgf/cm 2  and 3 to 5 kgf/cm 2 , respectively. 
     An electrolyte inlet port  153  and first and second terminal holes  152   a  and  152   b  may be further formed in the cap plate  151 . The electrolyte inlet port  153  is used (utilized) to inject an electrolyte into the case  140 . First and second connection terminals  120  and  130  are inserted into the first and second terminal holes  152   a  and  152   b , respectively. 
     The secondary battery will be described in brief with reference to  FIG. 2 . 
     The electrode assembly  110  may be manufactured in the form of a Jelly-Roll by winding the first electrode plate  111 , the second electrode plate  112  and the separator  113  that are stacked on one another, or be manufactured in the form of a stack by stacking up a plurality of first electrode plates  111 , second electrode plates  112  and separators  113 , or be manufactured by both winding and stacking. 
     The first electrode plate  111  includes a first active-material coating portion which is formed by intermittently coating a first active material on a first base material that is a sheet-shaped conductive material, and a first non-coating portion  111   a  which is not coated with the first active material so that the first base material is exposed. The first non-coating portion  111   a  may protrude to a side of the first electrode plate  111 . For example, the first electrode plate  111  includes an anode plate, and the first active material may comprise an active positive polar material containing lithium, such as LiCoO 2 , LiNiO 2 , LiMnO 2 , LiMn 2 O 4  or LiNi 1-x-y Co x Mn y O 2 . 
     The second electrode plate  112  has a polarity that is different from that of the first electrode plate  111 , and includes a second active-material coating portion which is formed by intermittently coating a second active material on a second base material that is a sheet-shaped conductive material, and a second non-coating portion  112   a  which is not coated with the second active material so that the second base material is exposed. The second non-coating portion  112   a  may protrude to a side of the second electrode plate  112 . For example, the second electrode plate  112  may be a cathode plate, and the second active material may be an active negative polar material containing a carbon material such as crystalline carbon, amorphous carbon, carbon composite, carbon fiber, lithium metal or lithium alloy. 
     The separator  113  is positioned between the first electrode plate  111  and the second electrode plate  112 , thus insulating the first electrode plate  111  and the second electrode plate  112  from each other. Further, the separator  113  allows the first electrode plate  111  and the second electrode plate  112  to exchange lithium ions. Such a separator  113  in one embodiment has a length sufficient to completely insulate the first and second electrode plates  111  and  112  from each other, even if the electrode assembly  110  contracts and expands. 
     The first or second base material may include metal in the form of a thin film. For example, the first base material may include aluminum, while the second base material may include copper. The first and second electrode plates  111  and  112  discharge ions into the electrolyte to cause the flow of current or electrons, and the current or electrons are transmitted through the first and second non-coating portions  111   a  and  112   a  to the outside. The first non-coating portion  111   a  may be the anode, while the second non-coating portion  112   a  may be the cathode. 
     The case  140  may be formed in the shape of a box, which has an opening at an end thereof (e.g., is open at a surface or surface end thereof), to accommodate the electrode assembly  110  and the electrolyte therein, the opening being closed by the cap assembly  150 . Although  FIG. 1  shows that the case  140  has the shape of the box, the case  140  may be manufactured to have the shape of a cylinder, a pouch or a coin, and embodiments of the present invention should not be limited to the shape of the box. 
     The cap assembly  150  may include the cap plate  151  that is configured to close the opening of the case  140 , and the first and second terminal plates  123  and  133  disposed on the top of the cap plate  151 . 
     The first and second terminal plates  123  and  133  may be electrically connected to the first and second non-coating portions  111   a  and  112   a  via the first and second connection terminals  120  and  130  coupled to first and second current collectors  121  and  131 . Sealing gaskets, provided on the first and second connection terminals  120  and  130  and the cap plate  151  to seal gaps between the first and second connection terminals  120  and  130  or the cap plate  151  and insulation plates  124  and  124 , may be further provided in the terminal holes  152   a  and  152   b  through which the first and second connection terminals  120  and  130  pass. 
     Hereinafter, the method of manufacturing the above-described secondary battery will be described. 
       FIG. 4  is a flowchart illustrating the method of manufacturing the secondary battery according to an embodiment of the present invention. 
     As shown in  FIG. 4 , the cap plate  151  is first prepared, which closes an opening of the case  140  that accommodates the electrode assembly  110  and has the opening at an end thereof (e.g., is open at a surface thereof), at act S 10 . 
     A plate made of an electric conductor is cut to have a shape corresponding to that of the opening of the case  140 , thus providing the cap plate  151 . 
     The cap plate  151  is formed of aluminum, and allows a high current to stably flow. 
     Next, the short-circuit portion  155  and the vent portion  154  are formed in a portion of the cap plate  151  prepared as such, at act S 20 . 
     The cap plate  151  prepared at act S 10  undergoes any one process of casting, forging, rolling and pressing, thus forming the short-circuit portion  155  and the vent portion  154 . 
     The short-circuit portion  155  and the vent portion  154  may be formed to be relatively thinner than the peripheral portion of the cap plate  151 . 
     Subsequently, the short-circuit portion  155  (e.g., only the short-circuit portion  155 ) is subjected to the first heat treatment at act S 30 . 
     The short-circuit portion  155  formed at step S 20  is subjected to the first heat treatment for 70 minutes or less (within 70 minutes) at the temperature of 400° C. to 500° C., in one embodiment at the temperature of 430° C. to 470° C., using (utilizing or by) the induction heating apparatus. In this context, if the temperature is less than 400° C., internal stress is not reliably removed, so that it is difficult to obtain a desired hardness. In contrast, if the temperature is more than 500° C., a crystal grain may be large, roughness may be high, and scale may be produced due to oxidation. Further, if the heat treatment time exceeds 70 minutes, aluminum may be lost due to oxidation. Therefore, in order to ensure the strength of 3 to 5 kgf/cm 2  (e.g., the strength of 4 kgf/cm 2 ), it is preferable to perform the first heat treatment within 70 minutes at the temperature of 400° C. to 500° C. 
     Finally, the vent portion  154  (e.g., only the vent portion  154 ) is subjected to a second heat treatment at act S 40 . 
     After the first heat treatment has been completed, the vent portion  154  formed at act S 20  is subjected to the second heat treatment for 70 minutes or less (within 70 minutes) at the temperature of 200° C. to 300° C., in one embodiment, at the temperature of 270° C., using (utilizing) the induction heating apparatus. Here, if the temperature is less than 200° C., the internal stress is not reliably removed, so that it is difficult to obtain a desired hardness. In contrast, if the temperature is more than 300° C., a crystal grain may be large, roughness may be high, and scale may be produced due to oxidation. Further, if the heat treatment time exceeds 70 minutes, aluminum may be lost due to oxidation. Therefore, in order to ensure the strength of 6 to 8 kgf/cm 2 , (e.g., the strength of 7 kgf/cm 2 ), it is preferable to perform the second heat treatment within 70 minutes at the temperature of 200° C. to 300° C. 
     The second heat treatment may be performed prior to the first heat treatment, unlike the above-described embodiment. Alternatively, in one embodiment, the first heat treatment is performed currently (e.g., simultaneously) with the second heat treatment. 
       FIG. 5A  is a photograph showing the vent portion that is, subjected to the heat treatment,  FIG. 5B  is a photograph showing the vent portion that is not subjected to the heat treatment,  FIG. 6A  is a photograph showing the short-circuit portion that is subjected to the heat treatment, and  FIG. 6B  is a photograph showing the short-circuit portion that is not subjected to the heat treatment. 
     If the first heat treatment and the second heat treatment have been completed as such, as shown in  FIGS. 5A, 5B, 6A and 6B , the grain size of each of the short-circuit portion and the vent portion is increased as compared to the grain size of each of the short-circuit portion and the vent portion before the heat treatment is performed. That is, the grain size of each of the short-circuit portion and the vent portion is about 25 μm before the heat treatment, while the grain sizes of the short-circuit portion and the vent portion are about 70 μm and 50 μm, respectively, after the heat treatment. Further, a grain boundary may be reduced and internal energy may also be reduced, thus providing a more stable structure. Therefore, the short-circuit portion and the vent portion which are subjected to the heat treatment are relatively lower in hardness than the short-circuit portion and the vent portion which are not subjected to the heat treatment. For example, if the internal pressure of the battery becomes higher than the preset pressure, the short-circuit portion having relatively low hardness on the cap plate is deformed, thus causing the short circuit, and then the vent portion is broken, thus discharging the internal gas. 
     Example embodiments have been disclosed herein, and although specific terms are employed, they are used and are to be interpreted in a generic and descriptive sense only and not for purpose of limitation. In some instances, as would be apparent to one of ordinary skill in the art as of the filing of the present application, features, characteristics, and/or elements described in connection with a particular embodiment may be used singly or in combination with features, characteristics, and/or elements described in connection with other embodiments unless otherwise specifically indicated. Accordingly, it will be understood by those of skill in the art that various changes in form and details may be made without departing from the spirit and scope of the present invention as set forth in the following claims, and equivalents thereof.