Patent Publication Number: US-2022238954-A1

Title: Venting Device for Secondary Batteries and Pouch-Shaped Secondary Battery Including the Same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority from Korean Patent Application No. 10-2021-0010426 filed Jan. 25, 2021, the disclosure of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to a venting device for secondary batteries and a pouch-shaped secondary battery including the same, and more particularly to a venting device for secondary batteries configured such that at least one venting device is inserted into a sealed portion of a battery case of a pouch-shaped secondary battery, whereby, when the pressure in the battery case is increased, it is possible to rapidly discharge only gas in the battery case to the outside and at the same time to prevent deformation of the battery case, and a pouch-shaped secondary battery including the same. 
     BACKGROUND ART 
     With an increase in demand for mobile devices, such as smartphones, demand for secondary batteries used as energy sources thereof has increased. In addition, secondary batteries are used in electric vehicles (EV), hybrid electric vehicles (HEV), plug-in hybrid electric vehicles (P-HEV), energy storage systems (ESS), etc. 
     Meanwhile, secondary batteries may be classified as a cylindrical battery, a prismatic battery, and a pouch-shaped battery, depending on the shape thereof. Among these batteries, the pouch-shaped battery, which can be stacked with high integration, has high energy density per unit weight, and can be easily modified, has attracted considerable attention. The pouch-shaped battery is a battery including a battery case made of a laminate sheet, and has a structure in which an electrode assembly is mounted in the battery case. 
     As shown in  FIGS. 1A and 1B , a conventional pouch-shaped secondary battery  10  is a battery including a battery case  12  made of a laminate sheet, and has a structure in which an electrode assembly is mounted in the battery case  12 . The electrode assembly is received in a receiving portion of the battery case, upper and lower edges of the battery case are brought into tight contact with each other, a portion of the tight contact region is thermally fused, an electrolytic solution is injected into the battery case, and the remaining portion of the tight contact region is sealed in vacuum to form a sealed portion  13 , whereby a pouch-shaped secondary battery is completed. 
     For a secondary battery, a process of activating a positive electrode active material and an activation process for creating a stable solid electrolyte interface (SEI) at a negative electrode must be essentially performed first during a first cycle due to characteristics thereof. A large amount of gas is generated in a battery cell as the result of such activation processes. The generated gas is collected in a separate gas pocket  14 , a battery case is cut along a cutting portion C, and thermal fusion is performed again for sealing. A process of collecting in the pocket gas that is generated in the battery cell, removing the pocket by cutting, and thermally fusing a cut region of the battery case  12  again is generally referred to as a degassing process. This process has problems in that a separate gas pocket  14  is provided; the process is complicated due to gas collection, cutting, and thermal fusion of a cut region; and cost is increased. In addition, if gas generated in the battery cell during the activation processes is not efficiently removed, the gas may occupy space in the battery cell. As a result, a central region of the battery case may swell, whereby the battery may become deformed, and the performance of the battery (such as capacity and output) and the lifespan of the battery may be adversely affected. 
     In addition, the pouch-shaped secondary battery, the outer cover of which is soft, may swell due to a high temperature phenomenon that may be caused by an abnormal operation of the battery, such as inner short circuit, an overcharged state exceeding allowable current and voltage, exposure to a high-temperature environment, dropping, or deformation due to external impact. 
     In particular, a pouch-shaped lithium secondary battery may swell due to gas generated when an electrolyte in the battery is decomposed, which is referred to as electrolytic solution swelling. Decomposition of the electrolyte is accelerated at high temperature. When the battery remains exposed to high temperature, therefore, such a swelling phenomenon is accelerated. When the temperature of the battery is increased, the electrolytic solution is decomposed or gas, such as carbon dioxide or carbon monoxide, is generated as a side reaction, whereby the pressure in the battery can further increase. When the pressure in the pouch-shaped secondary battery exceeds a specific pressure, the pouch-shaped secondary battery can explode. A danger of explosion due to an increase in inner pressure is the greatest shortcoming of pouch-shaped secondary batteries. 
     In the development of a pouch-shaped secondary battery, therefore, it is essential that safety be essentially considered. 
     Korean Patent Application Publication No. 2016-0014828 (“Patent Document 1”) discloses a venting system of a pouch-shaped lithium secondary battery including a wing-shaped body portion having a hollow portion configured to allow the inside and the outside of a pouch to communicate with each other. The body portion is inserted into the pouch, with opposite surfaces of the body portion being attached to the pouch, and a gas discharge adjustment portion is installed in a passage communicating with the hollow portion of the body portion. The gas discharge adjustment portion is configured to open or close the passage depending on a change in the inner pressure of the pouch. However, Patent Document 1 does not suggest technology capable of discharging gas in a battery case to the outside and at the same time preventing leakage of an electrolytic solution, as well as technology capable of maximally guiding inner gas while minimizing face-to-face area with a receiving portion of the battery case. 
     Japanese Patent Application Publication No. 2020-050439 (“Patent Document 2”) discloses a valve mechanism communicating with the interior of a battery case, the valve mechanism being configured to discharge gas generated in the battery case, wherein the valve mechanism includes a valve seat, a ball, a spring, and a membrane. The membrane transmits only gas in the battery case, and the gas presses the ball located above the O-ring type valve seat, whereby the spring is contracted, and thus the gas in the battery case is discharged to the outside through a gap between the ball and the valve seat. However, Patent Document 2 does not disclose technology capable of forming a stable flow channel for discharging high-pressure gas generated in the battery case. 
     Effective technology capable of effectively discharging gas in a battery case and at the same time preventing deformation of the battery case when the pressure in the battery case is increased has not yet been suggested. 
     DISCLOSURE 
     Technical Problem 
     The present invention has been made in view of the above problems. The present disclosure provides, among other things, a venting device for secondary batteries configured such that a degassing process is unnecessary, as well as a pouch-shaped secondary battery including the same. 
     The present disclosure also provides a venting device for secondary batteries capable of rapidly discharging high-pressure gas in a battery case while preventing deformation of the battery case when pressure in the battery case is increased, as well as a pouch-shaped secondary battery including the same. 
     The present disclosure also provides a venting device for secondary batteries configured to be easily coupled to a sealed portion of a battery case while minimizing deformation of the sealed portion of the battery case, as well as a pouch-shaped secondary battery including the same. 
     The present disclosure also provides a venting device for secondary batteries capable of stably changing a gas discharge channel depending on the pressure in a battery case, as well as a pouch-shaped secondary battery including the same. 
     Technical Solution 
     The present invention may address some of the above problems. For example, a venting device for a secondary battery according to an aspect of the present invention includes a lower vent portion, an upper vent portion, a wing portion, a discharge portion, and a shaft unit. The lower vent portion desirably defines a cavity therein and is positionable within a receiving portion of a battery case. The upper vent portion is desirably positionable outside of the battery case and communicates with the cavity of the lower vent portion. The wing portion is desirably positionable along a sealed portion of the battery case and encircles an outer perimeter of the upper vent portion. The discharge portion desirably communicates with the upper vent portion to allow gas to pass from the upper vent portion through the discharge portion. The shaft unit is desirably located in the cavity of the lower vent portion, and the shaft unit preferably includes an elastic portion. 
     In accordance with some aspects of the present invention, the shaft unit may further include a lower disk, a shaft body, a guide passage, and an upper disk. The lower disk is preferably located at the lower end of the shaft unit. The shaft body preferably has a flange, with the shaft body extending upwardly from the flange, and the shaft body having a main passage defined therein. The guide passage preferably extends horizontally and communicates with the main passage of the shaft body. The upper disk is preferably located at an upper end of the guide passage. The elastic portion may extend around an outer perimeter of the shaft body. 
     In accordance with some aspects of the present invention, the lower disk may be configured to allow a gas component generated in the battery case to pass therethrough without allowing an electrolytic solution to pass therethrough. 
     In accordance with some aspects of the present invention, an inner fixing portion having an opening defined therethrough may be located in the cavity of the lower vent portion, and the elastic portion may be located between the flange and the inner fixing portion. 
     In accordance with some aspects of the present invention, the elastic portion is configured to bias the shaft unit such that: (i) when the pressure in the battery case is normal, the guide passage of the shaft unit is sealed by a surface of the inner fixing portion, and (ii) when the pressure in the battery case is higher than a reference pressure, the shaft unit moves upwards such that the guide passage may communicate with an upper passage of the upper vent portion to form an exhaust gas flow channel through the venting device. 
     In accordance with some aspects of the present invention, the lower disk may be located outside the cavity of the lower vent portion. 
     In accordance with other aspects of the present invention, the lower disk may be located in the cavity of the lower vent portion. 
     In accordance with some aspects of the present invention, a lower plate may be located at a lower end of the lower vent portion, and the lower plate may be configured to allow a gas component generated in the battery case to pass therethrough without allowing an electrolytic solution to pass therethrough. 
     In accordance with some aspects of the present invention, a partition may be located in the cavity of the lower vent portion, and the elastic portion may be configured to bias the shaft unit such that, when a pressure in the battery case is higher than a reference pressure, the shaft unit is permitted to move upwards until the comes into contact with the partition. 
     The present invention desirably also provides a pouch-shaped secondary battery including the venting device for a secondary battery in accordance with any of the identified aspects of the invention, as well as a battery module including such pouch-shaped secondary battery. 
     In accordance with some aspects of such invention, the venting device of such pouch-shaped secondary battery may be positioned through the sealed portion of the battery case, and the venting device may be positioned adjacent to an electrode lead extending outwardly from the battery case. 
     In addition, the present invention may provide all possible combinations of the above solving means. 
     Advantageous Effects 
     A venting device for secondary batteries in accordance with aspects of the present invention, as well as a pouch-shaped secondary battery including the same, may not require a degassing process, whereby a process of producing the pouch-shaped secondary battery may be simplified and the cost of producing the pouch-shaped secondary battery may be reduced. 
     A venting device for secondary batteries according to some aspects of the present invention may be capable of transmitting only gas from the battery case when the pressure in the battery case is increased, whereby it may be possible to prevent deterioration in quality of the pouch-shaped secondary battery. 
     A venting device for secondary batteries according to some aspects of the present invention may include a conical lower vent portion, whereby it is possible to minimize the size of the portion of the venting device located in the battery case, and therefore it is possible to minimize the impact on the electrode assembly and the electrolytic solution in the battery case. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1A  is a schematic view of a conventional pouch-shaped secondary battery having a gas pocket formed therein, and  FIG. 1B  is a schematic view of the pouch-shaped secondary battery after the gas pocket has been removed. 
         FIG. 2  is a schematic view of a pouch-shaped secondary battery according to a first embodiment of the present invention. 
         FIG. 3  is a schematic view of a venting device according to a first embodiment of the present invention. 
         FIG. 4  is a vertical sectional view of the venting device of  FIG. 3 . 
         FIG. 5  is a vertical sectional view of a lower vent portion according to a first embodiment of the present invention. 
         FIG. 6  is a vertical sectional view of a shaft unit according to a first embodiment of the present invention. 
         FIG. 7  is a vertical sectional view showing the construction of the lower vent portion, the shaft unit, and an elastic portion in a normal state according to a first embodiment of the present invention. 
         FIG. 8  is a vertical sectional view showing a change in position of the lower vent portion, the shaft unit, and the elastic portion in a pressurized state according to a first embodiment of the present invention. 
         FIG. 9  is a vertical sectional view showing the construction of a lower vent portion having a vertical partition, a shaft unit, and an elastic portion according to a second embodiment of the present invention. 
         FIG. 10  is a vertical sectional view showing the construction of a lower vent portion having a horizontal partition, a shaft unit, and an elastic portion according to a third embodiment of the present invention. 
     
    
    
     BEST MODE 
     In the present application, it should be understood that the terms “comprises,” “has,” “includes,” etc. specify the presence of stated features, numbers, steps, operations, elements, components, or combinations thereof, but do not preclude the presence or addition of one or more other features, numbers, steps, operations, elements, components, or combinations thereof. 
     In addition, the same reference numbers will be used throughout the drawings to refer to parts that perform similar functions or operations. In the case in which one part is said to be connected to another part in the specification, not only may the one part be directly connected to the other part, but the one part also may be indirectly connected to the other part via a further part. In addition, the fact that a certain element is included does not mean that other elements are excluded, but rather means that such elements may be further included unless mentioned otherwise. 
     Hereinafter, a venting device for secondary batteries according to the present invention will be described with reference to the accompanying drawings. 
       FIG. 2  is a schematic view of a pouch-shaped secondary battery according to a first embodiment of the present invention. 
     Referring to  FIG. 2 , in the pouch-shaped secondary battery  100  according to the first embodiment of the present invention, a battery case  120  performs functions of protecting internal components (such as an electrode assembly and an electrolytic solution), complementing electrochemical properties by the electrode assembly (not shown) and the electrolytic solution (not shown), and dissipating heat. 
     Here, the battery case  120  may be made of a laminate sheet including a resin layer and a metal layer, and the edge of the battery case may be thermally bonded in the state in which the electrode assembly and the electrolytic solution are received in the battery case, whereby a sealed portion  130  may be formed. More specifically, the battery case  120  may be constituted by two cases, i.e., an upper case (not shown) and a lower case (not shown), at least one of which is provided with a concave inner space. The edges of the upper case and the lower case may be thermally bonded, whereby the inner space in which the electrode assembly is received may be hermetically sealed. 
     In addition, the laminate sheet generally includes a gas barrier layer, a surface protection layer, and a sealant layer. The gas barrier layer provides mechanical strength to the battery case, prevents introduction of gas or moisture into the secondary battery from the outside, and prevents leakage of the electrolytic solution. 
     In general, the gas barrier layer includes a metal, and aluminum (Al) foil is mainly used. The reason for this is that aluminum foil exhibits more than a predetermined level of mechanical strength, is lightweight, complements electrochemical properties of the electrode assembly and the electrolytic solution, and provides heat dissipation. 
     The surface protection layer is made of a polymer, and is located at the outermost layer to electrically insulate the electrode assembly from the outside while protecting the secondary battery from friction and collision with the outside. Here, the outermost layer is a layer oriented outwardly in a direction opposite to an inward direction (towards the electrode assembly). 
     The surface protection layer is mainly made of a polymer that exhibits wear resistance and heat resistance, such as a nylon resin or polyethylene terephthalate (PET). In addition, the surface protection layer may have a single film structure constituted by any one material or a composite film structure including a layer formed by each of two or more materials. 
     The sealant layer is made of a polymer, and is located at the innermost layer so as to directly contact the electrode assembly. When the laminate sheet having the above-described stacked structure is drawn using a punch, a portion of the laminate sheet is stretched, whereby an upper receiving portion or a lower receiving portion is formed in the pouch-shaped battery case  120 . 
     After the electrode assembly is received in the receiving portion, the electrolytic solution is injected into the receiving portion. Afterwards, the upper case and the lower case are brought into contact with each other, and the edges of the upper case and the lower case are pressed against each other while heat is applied thereto, whereby the sealant layers are bonded to each other, and thus the battery case  120  becomes hermetically sealed. 
     Since the sealant layer directly contacts the electrode assembly, the sealant layer must have an insulating property, and since the sealant layer also contacts the electrolytic solution, the sealant layer must have corrosion resistance. In addition, since the battery case must be completely hermetically sealed in order to prevent movement of a material between the inside and the outside of the battery case, the sealant must have high sealability. That is, the sealed portion formed as the result of bonding the sealant layers must have excellent thermal bonding strength. 
     In general, the sealant layer is mainly made of a polyolefin-based resin, such as polypropylene (PP) or polyethylene (PE). Since polypropylene (PP) exhibits excellent mechanical-physical properties, such as tensile strength, rigidity, surface hardness, wear resistance, and heat resistance, as well as excellent chemical properties, such as corrosion resistance, polypropylene is mainly used to manufacture the sealant layer. Furthermore, the sealant layer may be made of cast polypropylene or a polypropylene-butylene-ethylene terpolymer. In addition, the sealant layer may have a single film structure constituted by any one material or a composite film structure including a layer formed by each of two or more materials. 
     Next, the electrode assembly will be described. Although not shown, the electrode assembly may be configured to have a structure in which a positive electrode plate and a negative electrode plate are disposed so as to face each other. Each of the positive electrode plate and the negative electrode plate is formed so as to have a structure in which an active material slurry is applied to a current collector. In general, the slurry may be formed by stirring active material particles, an auxiliary conductor, a binder, and a plasticizer to which a solvent has been added. 
     The positive electrode plate and the negative electrode plate are formed so as to be flat, and they are spaced apart from each other so as to face each other. A separator is interposed between the positive electrode plate and the negative electrode plate in order to prevent direct contact between the positive electrode plate and the negative electrode plate. The separator isolates the positive electrode plate and the negative electrode plate from each other to prevent the occurrence of short circuit therebetween; however, the separator may be configured to have a porous structure such that movement of electric charge is possible during charging or discharging. 
     The electrode assembly having the above construction may be configured as a stacked type electrode assembly, a wound type electrode assembly, or a stacked and folded type electrode assembly. A stacked type electrode assembly is one in which a plurality of positive electrode plates and a plurality of negative electrode plates are alternately stacked. A wound type electrode assembly is one in which one positive electrode plate and one negative electrode plate are wound with a separator interposed therebetween. 
     Meanwhile, a solid electrolyte, a gel type quasi-solid electrolyte (in which an additive is added to a solid electrolyte so as to have an intermediate state between liquid and solid), or a liquid electrolyte (as is commonly used) may be used as an electrolyte. 
     An electrode tab may be formed at each of the positive electrode plate and the negative electrode plate so as to protrude therefrom, and an electrode lead  110 , which is a portion that is electrically connected to the electrode tab and that is exposed outwards from the battery case  120 , serves as an electrode terminal configured to be electrically connected to another secondary battery or an external device. Of course, the electrode tab alternatively may be directly connected to the external device with the omission of the electrode lead  110 . Although the electrode leads  110  are oriented in the same direction in  FIG. 2 , the electrode leads may alternatively be oriented in opposite directions. 
     A venting device  200  according to a first embodiment of the present invention may be located adjacent to the electrode lead  110 , specifically between a pair of electrode leads  110  extending from the battery case  120 , so as to be oriented in the same direction as the electrode lead(s)  110 . The reason for disposing the venting device  200  in the vicinity of the electrode leads  110 , the temperature of which may be relatively rapidly increased and which may be easily expanded during repeated charging and discharging, is that it may facilitate the rapid discharge of gas from the battery case  120 . 
     Here, the sealed portion  130  of the battery case  120  in which the venting device  200  is located may extend obliquely from the venting device  200 . The reason for this is that it may facilitate moving gas generated in the battery case  120  toward the venting device  200 . 
       FIG. 3  is a schematic view of a venting device according to a first embodiment of the present invention, and  FIG. 4  is a vertical sectional view of the venting device of  FIG. 3 . 
     Referring to  FIGS. 3 and 4 , the venting device  200  according to the first embodiment of the present invention may mainly include a lower vent portion  230 , an upper vent portion  250 , a discharge portion  260 , and a wing portion  270 . 
     The discharge portion  260  is horizontally located at the uppermost end of the venting device  200 , and the discharge portion  260  has a horizontal hollow passage formed therein. Gas discharge ports may be formed in opposite ends of the discharge portion  260 , and each of the gas discharge ports may be provided with a cover configured to cover the respective gas discharge port in a normal state. The cover may be opened by gas discharged from the interior of the battery case  120 , whereby the gas may be discharged to the outside. 
     The upper vent portion  250  may be located vertically (in a Z-axis direction) below the discharge portion  260  while communicating with a middle portion of the lower end of the discharge portion  260 . The upper vent portion  250  may be formed in a cylindrical shape, and the upper vent portion  250  may have an upper passage  252  defined by an upper body  251  surrounding the upper passage  252 . The upper passage  252  may communicate with the hollow passage of the discharge portion  260  to form a gas flow channel. 
     The lower vent portion  230  may be formed by extending the upper body  251  of the upper vent portion  250 . Alternatively, the upper vent portion  250  and the lower vent portion  230  may be separately manufactured and may then be coupled to each other. Here, the lower vent portion  230 , which may have a hollow structure, may have a wide top portion and a narrow bottom portion, i.e., it may be conical. 
     The wing portion  270  may be formed so as to have a width gradually decreasing toward opposite ends thereof while extending around the outer circumference of the upper vent portion  250 . The reason for this is that it may facilitate joining the wing portion to the sealed portion  130  of the battery case  120  while minimizing deformation of the sealed portion after the wing portion is inserted into the sealed portion. The venting device  200  and the battery case  120  are coupled to each other along the outer circumferential surface of the wing portion  270 . 
       FIG. 5  is a vertical sectional view of a lower vent portion according to a first embodiment of the present invention,  FIG. 6  is a vertical sectional view of a shaft unit according to the first embodiment of the present invention,  FIG. 7  is a vertical sectional view showing the construction of the lower vent portion, the shaft unit, and an elastic portion in a normal state according to the first embodiment of the present invention, and  FIG. 8  is a vertical sectional view showing a change in position of the lower vent portion, the shaft unit, and the elastic portion in a pressurized state according to the first embodiment of the present invention. 
     The lower vent portion  230 , the shaft unit  210 , and the elastic portion  220  will be described in detail with reference to  FIGS. 5 to 8 . 
     An inner fixing portion  240  may be located along an inner side wall of the upper part of the lower vent portion  230 , and the inner fixing portion  240  may be a plate that is horizontally located in an inner space of the lower vent portion  230  and that has a central hole formed therein. The section (xy plane) of the hollow portion of the lower vent portion  230  and the section (xy plane) of the central hole of the inner fixing portion  240  have the same center. Here, an upper end surface  241  of the inner fixing portion  240  may be located in the upper vent portion  250 , where it supports an upper disk  216  of the shaft unit  210 , a description of which will follow. 
     The shaft unit  210  may include a cylindrical shaft body  213 , a lower disk  211  located at the lower end of the shaft body  213  (the lower disk being configured to cover a lower-end inlet of the shaft body  213 ), a main passage  214  vertically formed in the shaft body  213 , a guide passage  215  configured to communicate with the main passage  214  (the guide passage being horizontally oriented), and an upper disk  216  horizontally arranged at the upper end of the guide passage  215 . The guide passage  215  is formed so as to vertically communicate with the main passage  214 , and the guide passage  215  guides gas introduced through the main passage  214  in opposite directions. Gas is discharged through discharge ports formed in opposite ends of the guide passage  215 . 
     The lower disk  211  may be made of a polytetrafluoroethylene (PTFE) membrane or Gore-Tex, which does not allow the transmission of the electrolytic solution in the battery case  120  but instead transmits only gas. The lower disk  211  may alternatively be configured by attaching the PTFE membrane or the Gore-Tex to a rigid member having predetermined porosity. 
     The lower disk  211  may be directly coupled to the shaft body  213 , or the lower disk  211  may be coupled to a lower flange  212  extending outwards at the lower end of the shaft body  213 . In the case in which the lower flange  212  is located at the lower end of the shaft body  213 , the PTFE membrane or the Gore-Tex may be directly attached to the lower flange  212  without first attaching the PTFE membrane or the Gore-Tex to a rigid member. Desirably, attaching the PTFE membrane or the Gore-Tex directly to the lower flange  212  without the rigid member may make it easier to discharge the high pressure gas G in the battery case  120  to the outside than when the rigid member is present. 
     The elastic portion  220  is located about the outer circumferential surface of the shaft body  213 , specifically between an upper end surface of the lower flange  212  and a lower end surface  242  of the inner fixing portion. Here, the elastic portion  220  may be constituted by a spring, e.g., one made of stainless steel. The modulus of elasticity of the elastic portion  220  may be set based on reference pressure of gas venting from the battery case  120 . 
     When the pressure in the battery case  120  reaches the reference pressure, the elastic portion  220  contracts, whereby the shaft unit  210  moves upwardly toward the upper vent portion  250 . Thus, the guide passage  215 , which was initially sealed by tight contact with the vertical section (xz plane) (i.e., the radially inwardly facing surface) of the inner fixing portion  240 , moves upwardly toward the upper passage  252 . As a result, the main passage  214  and the guide passage  215  of the shaft unit  210  come into communication with the upper passage  252  of the upper vent portion  250  to form a gas flow channel. 
     In addition, a lateral outer circumferential surface of the upper disk  216  of the shaft unit  210  and an inner circumferential side surface of the upper passage  252  of the upper body  251  are spaced apart from each other by a predetermined distance to form a channel configured to allow the gas G introduced from the battery case  120  to be discharged to the outside. That is, an integrated gas flow channel is defined by the main passage  214  and the guide passage  215  of the shaft unit  210 , the upper passage  252  of the upper vent portion  250 , the gap between the lateral outer circumferential surface of the upper disk  216  and the inner circumferential side surface of the upper passage  252  of the upper body  251 , and the hollow passage of the discharge portion  260 . 
     In a normal state, the lower flange  212  and the lower end of a conical body of the lower vent portion  230  are spaced apart from each other. The elastic portion  220  then contracts when the pressure in the battery case  120  is increased, and, as a result, the shaft unit  210  is moved upwardly toward the upper vent portion  250  by the reduced length of the elastic portion  220 . In addition, the lower flange  212  and the lower disk  211  move toward the conical body of the lower vent portion  230 , and the elastic portion is prevented from contracting any further when the lower flange  212  is brought into tight contact with the lower end of the conical body of the lower vent portion  230 . At such point the entirety of a lateral section (yz plane) of the guide passage  215  of the shaft unit  210  is located in the upper passage  252 , whereby the gas flow channel may reach its maximum cross-sectional area. At the same time, the length of the lower vent portion  230  along the z axis may be minimized, due to the contraction of the elastic portion  220  and the shaft unit  210  into the venting device  200 . As a result, the dimension of the venting device  200  extending into the pouch-shaped secondary battery  100  may be reduced, and therefore it is possible to minimize any impact on the pouch-shaped secondary battery  100 . 
     When the pressure is restored to a normal state after the gas is discharged from the battery case  120 , the elastic portion  220  is returned to the initial state, whereby the shaft unit  210  is moved downwardly within the lower vent portion  230 , and the upper disk  216  is brought into tight contact with the upper end surface  241  of the inner fixing portion  240 . 
     The venting device  200  according to the first embodiment of the present invention may be made of a superplastic material, excluding the lower disk  211  and the elastic portion  220 . The reason for this is that it may minimize an increase in weight of the secondary battery while maintaining rigidity of the venting device  200 . 
     Also, in an example in which the inner diameters of the main passage  214  and the guide passage  215  of the shaft unit  210  according to the first embodiment of the present invention are equal to each other and have a diameter d, the outer diameter of the shaft body  213  may be 2d, the outer diameter of the lower flange  212  may be 8/3d, the inner diameter of the hollow portion of the lower vent portion  230  may be 5/2d, the diameter of the central hole of the inner fixing portion  240  may be 2d, and the inner diameter of the upper passage  252  may be 4d. The inner diameter d of the main passage  214  is not particularly restricted as long as deformation of the battery case  120  is not caused and the performance of the secondary battery  100  is not lowered. In the present invention, d may be 1.5 mm or more. If d is less than 1.5 mm, it may not be possible to smoothly discharge high-pressure gas from the battery case. 
       FIG. 9  is a vertical sectional view showing the construction of a lower vent portion having a vertical partition, a shaft unit, and an elastic portion according to a second embodiment of the present invention. 
     Referring to  FIG. 9 , a venting device according to a second embodiment of the present invention is identical to the venting device  200  described with reference to  FIGS. 4 to 8 , except that the lower vent portion  1230  according to the second embodiment of the present invention includes a vertical partition  1233  formed therein, a lower body  1232  (which is a conical body), and a lower plate  1231  provided at the lower end of lower body. Therefore, only the lower vent portion  1230  will be described hereinafter. 
     A lower disk  1211  of the shaft unit  1210  according to the second embodiment of the present invention is located in an inner space defined by the lower plate  1231  and the lower body  1232 , and the shaft unit  1210  is located in an inner space defined by the upper body  1251 , the lower body  1232 , and the lower plate  1231 . Consequently, contact between the elastic portion  1220  and the electrolytic solution of the secondary battery is prevented, whereby introduction of the electrolytic solution into the upper passage is fundamentally prevented during contraction and restoration of the elastic portion  1220  (and upward and downward movement of the shaft unit  1210 ), and therefore only high-pressure gas in the battery case may be discharged to the outside. 
     The lower plate  1231  according to the second embodiment of the present invention may be made of the same material as the lower disk  1211 . That is, the lower plate may be made of a polytetrafluoroethylene (PTFE) membrane or Gore-Tex, which does not allow the transmission of the electrolytic solution in the battery case but instead transmits only gas. The lower disk  1211  may alternatively be configured by attaching the PTFE membrane or the Gore-Tex to a rigid member having predetermined porosity. In addition, although not shown in  FIG. 9 , a flange (not shown) may be provided at the lower end of the lower body  1232  so as to extend outwardly in the inner space of the lower vent portion  1230 , such that a sheet made of the PTFE membrane or the Gore-Tex may be joined to the flange (not shown). As noted above, attaching the PTFE membrane or the Gore-Tex directly to the lower flange  212  without first attaching the PTFE membrane or the Gore-Tex to a rigid member may make it easier to discharge the high pressure gas G to the outside than when a rigid member is present. 
     In addition, the lower vent portion  1230  according to the second embodiment of the present invention may include a vertical partition  1233  vertically extending from an inner fixing portion  1240  in a direction toward the lower disk  1211 . The elastic portion  1220  may be located between the vertical partition  1233  and an outer wall of a shaft body of the shaft unit  1210 , and the elastic portion  1220  may be contracted until a lower flange is brought into tight contact with the lower end of the vertical partition  1233 . In  FIG. 9 , the vertical partition  1233  is shown as having a wide top and narrow bottom shape. However, the shape of the vertical partition is not limited thereto as long as the volume of the lower vent portion  1230  is not increased. Also, in  FIG. 9 , the vertical partition  1233  is shown as being spaced apart from an inner wall of the lower body  1232 . However, the vertical partition may be disposed in tight contact with the inner wall of the lower body  1232 , or the vertical partition may be integrally formed with the inner wall of the lower body  1232 . In addition, the vertical partition  1233  may be an integrated partition configured to extend around the circumference of the elastic portion  1220 , or the vertical partition may be constituted by two or more partitions spaced apart from each other by a predetermined distance. 
     When the pressure in the battery case is increased, the elastic portion  1220  contracts, and, as a result, the shaft unit  1210  is moved upwardly toward the upper vent portion. That allows high-pressure gas to be discharged to the outside via an integrated gas flow channel defined by the main passage and the guide passage of the shaft unit  1210 , a space between the upper disk of the shaft unit  1210  and an inner circumferential side surface of the upper body  1251 , the upper passage (not shown), and a hollow passage (not shown) of a discharge portion. 
     In the second embodiment of the present invention, the lower body  1232  may be formed by applying a PTFE membrane or Gore-Tex to a rigid member. In this case, it is possible to rapidly transmit high-pressure gas generated in the battery case to the interior of the venting device and to rapidly discharge the high-pressure gas to the outside. 
       FIG. 10  is a vertical sectional view showing the construction of a lower vent portion having a horizontal partition, a shaft unit, and an elastic portion according to a third embodiment of the present invention. 
     A venting device according to a third embodiment of the present invention is identical to the venting device according to the second embodiment of the present invention except for the horizontal partition  2234 , and therefore only the horizontal partition  2234  will be described hereinafter. 
     The horizontal partition  2234  may be a ring-shaped plate formed along an inner side wall of the lower body  2232  so as to protrude therefrom. When the pressure in the battery case is increased, the elastic portion  2220  contracts, and, as a result, the shaft unit  2210  is moved upwardly toward the upper vent portion. That allows high-pressure gas in the battery case to be discharged to the outside. The elastic portion  2220  may be contracted until the lower flange is brought into tight contact with the horizontal partition  2234 . 
     Here, the horizontal partition  2234  may be constituted by two or more partitions spaced apart from each other by a predetermined distance. 
     The venting device for pouch-shaped secondary batteries according to the present invention described above may be included in a pouch-shaped secondary battery, and pouch-shaped secondary batteries may be assembled to constitute a battery module. 
     Although the specific details of the present invention have been described in detail, those skilled in the art will appreciate that the detailed description thereof discloses only preferred embodiments of the present invention and thus does not limit the scope of the present invention. Accordingly, those skilled in the art will appreciate that various changes and modifications are possible, without departing from the category and the technical idea of the present invention, and it will be obvious that such changes and modifications fall within the scope of the appended claims. 
     DESCRIPTION OF REFERENCE SYMBOLS 
     
         
         
           
               10 ,  100 : Pouch-shaped secondary batteries 
               11 ,  110 : Electrode terminals 
               12 ,  120 : Battery cases 
               13 ,  130 : Sealed portions 
               14 : Degassing portion 
               200 : Venting device 
               210 ,  1210 ,  2210 : Shaft units 
               211 ,  1211 ,  2211 : Lower disks 
               212 : Lower flange 
               213 : Shaft body 
               214 : Main passage 
               215 : Guide passage 
               216 : Upper disk 
               220 ,  1220 ,  2220 : Elastic portions 
               230 ,  1230 ,  2230 : Lower vent portions 
               232 ,  1232 ,  2232 : Lower bodies 
               240 ,  1240 : Inner fixing portions 
               241 : Upper end surface of inner fixing portion 
               242 : Lower end surface of inner fixing portion 
               250 : Upper vent portion 
               251 ,  1251 ,  2251 : Upper bodies 
               252 : Upper passage 
               260 : Discharge portion 
               270 : Wing portion 
               1231 ,  2231 : Lower plates 
               1233 : Vertical partition 
               2234 : Horizontal partition 
             C: Cutting portion 
             G: High-pressure gas