Patent Publication Number: US-2022238947-A1

Title: Sealing plate including gas release vent and secondary battery using sealing plate

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority on the basis of Japanese Patent Application No. 2021-9448 filed in Japan on Jan. 25, 2021, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a sealing plate including a gas release vent and to a secondary battery using the sealing plate. 
     2. Description of the Related Art 
     Secondary batteries such as lithium-ion secondary batteries include, for example, an electrode body and a battery case housing the electrode body. The battery case includes an outer package which is a container of which one face is an opening and a sealing plate which closes the opening of the outer package. In a secondary battery of this type, the battery case (typically, the sealing plate) may be provided with a gas release vent in order to improve safety. The gas release vent refers to a vent designed to discharge gas inside the battery case. The gas release vent is constructed to open at pressure determined in advance when a large amount of gas is suddenly generated inside the battery case. For example, a square storage battery described in JP2012-252809 has a lid member (a sealing plate) in which a base portion forming an upper surface, a peripheral wall portion forming a recessed portion which is recessed from the base portion, and a safety valve (a gas release vent) which is connected to and supported by an inner peripheral wall surface of the peripheral wall portion. In JP2012-252809, an aluminum flat plate is pressed to form the recessed portion and the safety valve with a thin film shape is formed in a bottom portion of the recessed portion. 
     SUMMARY 
     In recent years, there is an increasing demand for safety in secondary batteries. As a result of a study conducted by the present inventors to meet the demand, it was found that with a conventional gas release vent configured as described above, there is a possibility that a metallic fragment may scatter in all directions when the gas release vent is activated (opens) due to a rise in internal pressure of the case. In addition, a scattered metallic fragment coming into contact with an electrode terminal or the like may cause an external short circuit or the like to occur. The technique disclosed herein has been devised in consideration of the circumstances described above and an object thereof is to provide a sealing plate including a gas release vent and a secondary battery capable of suppressing scattering of a metallic fragment upon activation. 
     In order to achieve the object described above, the technique disclosed herein provides a sealing plate configured as described below. 
     The sealing plate disclosed herein is a sealing plate for a secondary battery, the sealing plate being a plate-shaped member with an approximately rectangular shape in a plan view and which closes an opening of an outer package and which is provided with a gas release vent. The gas release vent of the sealing plate includes: a base portion with a flat plate shape; a thin portion of which a thickness is thinner than a thickness of the base portion; a groove portion with an approximately annular shape formed on a surface of the thin portion; and a valve element formed inside the groove portion with the approximately annular shape. In addition, in the sealing plate disclosed herein, the groove portion has a remaining portion which is a region with a larger remaining thickness as compared to another region of the groove portion. Furthermore, the remaining portion is formed in a region including one intersection point among two intersection points where a straight line which passes a center of the gas release vent and which extends in a short-side direction of the sealing plate intersects the groove portion. 
     With a gas release vent in which a groove portion with the approximately annular shape is formed on a surface of the thin portion as described above, when internal pressure of a case reaches prescribed pressure, a fracture of the thin portion occurs along the groove portion with the approximately annular shape. At this point, when the internal pressure of the case rises suddenly, the thin portion fractures at once along an entire periphery of the groove portion and the valve element present inside the groove portion is completely detached from the base portion. In addition, there is a possibility that gas ejected from inside the battery case may cause the detached valve element (a metallic fragment) to scatter in all directions. By contrast, in the sealing plate disclosed herein, the groove portion is provided with the remaining portion which has a larger remaining thickness as compared to another region of the groove portion. Accordingly, the thin portion can be prevented from fracturing at once along the entire periphery of the groove portion and a state where the valve element is connected to the base portion via the remaining portion can be maintained. As a result, scattering of the valve element of the gas release vent as a metallic fragment can be suppressed and a contribution can be made toward improving safety of a secondary battery. 
     When the remaining portion is formed in a part of the groove portion, since a continuous fracture of the thin portion along the groove portion stops at the remaining portion, there is a possibility that the gas release vent will fail to open sufficiently depending on a fracture start position. By contrast, in the sealing plate disclosed herein, the remaining portion is formed in a region including one intersection point among two intersection points where a straight line which passes a center of the gas release vent and which extends in the short-side direction of the sealing plate intersects the groove portion with the approximately annular shape. Accordingly, a fracture of the thin portion can be started at a position that is farthest from the remaining portion in a peripheral direction of the groove portion. As a result, a fracture of the thin portion along the groove portion can be created so that only the remaining portion is connected to the base portion in the gas release vent after activation. As described above, according to the sealing plate disclosed herein, not only can scattering of the valve element upon activation of the gas release vent be suppressed but the remaining portion formed as an anti-scattering measure of the valve element can also be prevented from inhibiting activation of the gas release vent. 
     In addition, in a preferable aspect of the sealing plate disclosed herein, a protective tape is affixed so as to cover the gas release vent. Accordingly, damage and deterioration of the gas release vent due to corrosive foreign objects or the like can be prevented. 
     In addition, in a preferable aspect of the sealing plate disclosed herein, a length L 1  of the protective tape affixed to a region outside of the gas release vent in the short-side direction of the sealing plate is shorter than a length L 2  of the protective tape affixed to a region outside of the gas release vent in a long-side direction of the sealing plate. By adjusting an affixing margin of the protective tape so as to satisfy the dimensional relationship described above, the protective tape can be peeled off in an appropriate manner when the gas release vent is activated. 
     In a preferable aspect of the sealing plate disclosed herein, a gap with a height of 1 mm or more is formed between the thin portion and the protective tape. Accordingly, the protective tape can be prevented from being peeling off by an expansive deformation of the gas release vent during normal use before desired internal pressure of the case is reached. 
     In a preferable aspect of the sealing plate disclosed herein, a length of the remaining portion in a peripheral direction is ⅛ or more and ⅜ or less of a length of an entire periphery of the groove portion. Accordingly, scattering of the valve element can be appropriately suppressed without significantly inhibiting the activation of the gas release vent. 
     In a preferable aspect of the sealing plate disclosed herein, a remaining thickness of the remaining portion is thicker than the thickness of the thin portion that is adjacent to the remaining portion. Accordingly, scattering of the valve element can be more appropriately suppressed. 
     In a preferable aspect of the sealing plate disclosed herein, a thickness of the thin portion that is adjacent to the remaining portion is thicker than the thickness of the thin portion that is adjacent to another region of the groove portion. Accordingly, scattering of the valve element can be more appropriately suppressed. 
     In addition, in a preferable aspect of the sealing plate disclosed herein, a planar shape of the thin portion is approximately annular, a thickness of the valve element is equal to or greater than the thickness of the thin portion, and a second moment of area of the valve element is larger than a second moment of area of the thin portion. Accordingly, since stress applied to the gas release vent when internal pressure of the case rises concentrates on the thin portion around the groove portion, a fracture of the thin portion along the groove portion more readily occurs. 
     Furthermore, as another aspect of the technique disclosed herein, a secondary battery is provided. The secondary battery disclosed herein is a secondary battery including an electrode body including a positive electrode and a negative electrode and a battery case housing the electrode body. The battery case of the secondary battery includes an outer package which is a flat square container of which one face is an opening and a sealing plate with a rectangular planar shape which closes the opening of the outer package. In addition, the sealing plate is the sealing plate configured as described above. According to the secondary battery disclosed herein, not only can scattering of the valve element be suppressed but the remaining portion formed as an anti-scattering measure of the valve element can also be prevented from inhibiting the operation of the gas release vent. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view schematically showing a sealing plate according to a first embodiment; 
         FIG. 2  is a plan view schematically showing the sealing plate according to the first embodiment; 
         FIG. 3  is a longitudinal sectional view schematically showing the sealing plate according to the first embodiment; 
         FIG. 4  is a perspective view schematically showing a secondary battery according to the first embodiment; 
         FIG. 5  is a plan view schematically showing a sealing plate according to a second embodiment; 
         FIG. 6  is a longitudinal sectional view schematically showing the sealing plate according to the second embodiment; 
         FIG. 7  is a longitudinal sectional view schematically showing a sealing plate according to a third embodiment; 
         FIG. 8  is a longitudinal sectional view schematically showing a sealing plate according to a fourth embodiment; 
         FIG. 9  is a longitudinal sectional view schematically showing a sealing plate according to a fifth embodiment; and 
         FIG. 10  is a plan view schematically showing the sealing plate according to the fifth embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, some preferable embodiments of the technique disclosed herein will be described with reference to the drawings. It should be noted that, with the exception of matters specifically mentioned in the present specification, matters required to carry out the technique disclosed herein (for example, materials of an electrode body, an electrolyte solution, and the like) can be understood to be design matters of a person with ordinary skill in the art based on the prior art in the relevant technical field. In other words, the technique disclosed herein can be implemented based on the contents disclosed in the present specification and common general technical knowledge in the relevant field. 
     It should be noted that, in the drawings referred to in the following description, members and portions that produce same effects will be denoted by same reference signs. It should also be noted that dimensional relationships (a length, a width, a thickness, and the like) shown in the respective drawings do not reflect actual dimensional relationships. In addition, in the drawings, it is assumed that a reference sign X denotes a “width direction”, a reference sign Y denotes a “depth direction”, and a reference sign Z denotes a “height direction”. However, it should be noted that such directions have merely been determined for the sake of illustration and are not intended to limit aspects of installation when a secondary battery is in use or when the secondary battery is being manufactured. Furthermore, a notation of “A to B” representing a numerical value range in the present specification is to mean “A or more and B or less” but also includes the meanings of “favorably more than A” and “favorably less than B”. 
     Sealing Plate 
     Hereinafter, an embodiment of the sealing plate disclosed herein will be described with reference to  FIGS. 1 to 3 .  FIG. 1  is a perspective view schematically showing a sealing plate according to a first embodiment.  FIG. 2  is a plan view schematically showing the sealing plate according to the first embodiment.  FIG. 3  is a longitudinal sectional view schematically showing the sealing plate according to the first embodiment. In the present specification, for explanatory convenience, a surface opposing an electrode body inside a battery case when the sealing plate is attached to a secondary battery will be referred to as a “first surface” and a direction toward the electrode body will be referred to as a “first direction”. On the other hand, a surface to be exposed to the outside of the battery case when the sealing plate is attached to the secondary battery will be referred to as a “second surface” and a direction toward the outside of the electrode body will be referred to as a “second direction”. In addition, the “first direction” refers to downward in a height direction Z in  FIG. 3  and the “second direction” refers to upward in the height direction Z in  FIG. 3 . 
     A sealing plate  1  according to the present embodiment is a component for a secondary battery (a secondary battery component) which constitutes one side wall of a battery case of the secondary battery. In the present specification, a “secondary battery” is a term that refers to repetitively chargeable and dischargeable power storage devices in general and is a concept that encompasses so-called storage batteries (chemical batteries) such as a lithium-ion secondary battery and a nickel hydride battery as well as capacitors (physical batteries) such as an electrical double layer capacitor. In other words, the sealing plate  1  according to the present embodiment is not limited to a secondary battery of a specific type and the sealing plate  1  can be used without any particular limitations in secondary batteries in general in which gas may be created when a failure such as an overcharge occurs. 
     The sealing plate  1  according to the present embodiment is a plate-shaped member with an approximately rectangular planar shape. Although details will be provided later, the sealing plate  1  is a plate-shaped member which closes an opening of an outer package that is a component of a battery case of the secondary battery. Materials with a prescribed strength can be used without any particular limitations as the sealing plate  1 . Examples of a raw material of the sealing plate  1  include a metal material of which a main component is aluminum and a metal material of which a main component is iron. As an example, from the perspective of ease of design, moldability, and the like in consideration of working pressure of a gas release vent  10 , the sealing plate  1  is favorably constituted of a metal material of which a main component is aluminum. “A metal material of which a main component is aluminum” according to the present specification is a metal material which contains 90 weight percent or more of aluminum and which includes aluminum and aluminum alloys. 
     As shown in  FIG. 1 , the sealing plate  1  according to the present embodiment includes the gas release vent  10 . The gas release vent  10  according to the present embodiment includes a base portion  12 , a thin portion  16 , a groove portion  17  with the approximately annular shape, and a valve element  14 . In addition, as shown in  FIG. 3 , the groove portion  17  has a remaining portion  17   b  which is a region with a larger remaining thickness as compared to another region (a fracture portion  17   a ) of the groove portion  17 . Furthermore, as shown in  FIG. 2 , the remaining portion  17   b  of the groove portion  17  is formed in a region including one intersection point IP 1  among two intersection points IP 1  and IP 2  where a straight line L which passes a center C of the gas release vent  10  and which extends in a short-side direction of the sealing plate  1  (a depth direction Y) intersects the groove portion  17  with the approximately annular shape. According to the sealing plate  1  configured as described above, not only can scattering of a metallic fragment (the valve element  14 ) during activation of the gas release vent  10  be suppressed but inhibition of the operation of the gas release vent  10  can be prevented. Hereinafter, a specific configuration of the gas release vent  10  of the sealing plate  1  according to the present embodiment will be described. 
     (1) Base Portion 
     The base portion  12  is a region molded in a flat plate shape. The gas release vent  10  according to the present embodiment is molded by pressing a flat plate-shaped metal member. At this point, a region where the valve element  14  and the thin portion  16  to be described later are not formed becomes the base portion  12 . In addition, the base portion  12  becomes a junction between the gas release vent  10  and another region of the sealing plate  1 . Specifically, in the present embodiment, by directly subjecting the sealing plate  1  to pressing, the sealing plate  1  in which the base portion  12  of the gas release vent  10  and another region of the sealing plate  1  are seamlessly integrated is molded. Accordingly, an electrolyte solution can be reliably prevented from leaking from a joining portion of the gas release vent  10 . In addition, since a step of joining the gas release vent  10  to the sealing plate  1  can be omitted, a contribution toward improving work efficiency can also be made. However, the technique disclosed herein is not limited to an aspect in which the base portion  12  of the gas release vent  10  and the sealing plate  1  are integrated. In other words, a base portion of a gas release vent having been separately molded and another region of a sealing plate may be joined to each other. In this case, means can be considered in which an opening is provided on the sealing plate and, after fitting the gas release vent into the opening, the base portion of the gas release vent and the sealing plate are welded to each other. An aspect in which a gas release vent is separately molded in this manner is advantageous in that the gas release vent can be relatively readily molded. In addition, another advantage is that sales and distribution of the gas release vent after molding can be readily performed. 
     Moreover, a thickness T B  of the base portion  12  can be set to 1 mm to 10 mm and to 1 mm to 5 mm. As the thickness T B  of the base portion  12  increases, durability of the base portion  12  with respect to a rise in internal pressure of the case tends to improve. On the other hand, as the thickness T B  of the base portion  12  decreases, a processing load during molding of the valve element  14  and the thin portion  16  tends to decrease. However, the thickness T B  of the base portion  12  is not particularly limited and can be adjusted as appropriate in consideration of a thickness of the sealing plate  1  or the like. 
     (2) Thin Portion 
     The thin portion  16  is a region of which a thickness is thinner than the thickness T B  of the base portion  12  (T T &lt;T B ). As shown in  FIG. 3 , the gas release vent  10  according to the present embodiment is provided with a recessed portion  18  which is recessed from a second surface  12   b  of the base portion  12 , and the thin portion  16  and the valve element  14  are formed on a bottom surface of the recessed portion  18 . In addition, an annular peripheral wall  18   a  rises approximately vertically from an outer peripheral edge of the thin portion  16 . In other words, the gas release vent  10  according to the present embodiment has the recessed portion  18  which is enclosed by the thin portion  16 , the valve element  14 , and the peripheral wall  18   a . Furthermore, as shown in  FIG. 2 , a planar shape of the thin portion  16  in the present embodiment is an annular shape. A thickness T T  of the annular thin portion  16  is approximately the same in a peripheral direction. While an outer peripheral edge of the annular thin portion  16  may be an elliptical shape, the outer peripheral edge is more favorably an approximately circular shape. Accordingly, a variation in pressure (working pressure) at which the gas release vent  10  opens can be suppressed. It should be noted that an “approximately circular shape” according to the present specification refers to a circular shape of which a ratio between a long diameter and a short diameter is 90% or higher (favorably 95% or higher and more favorably 98% or higher). In addition, in consideration of an inner volume, an environment of usage, and the like of the battery case of the secondary battery, the thin portion  16  is favorably designed so as to stably fracture along a groove portion  17  to be described later. As an example, the thickness T T  of the thin portion  16  favorably ranges from 0.1 mm to 0.6 mm and more favorably ranges from 0.3 mm to 0.5 mm. Accordingly, when internal pressure of the case reaches desired pressure, the thin portion  16  can be stably fractured along the groove portion  17 . 
     (3) Valve Element 
     The valve element  14  is a region formed inside the groove portion  17  formed in an approximately annular. As shown in  FIGS. 1 and 2 , in this case, a shape of the valve element  14  in a plan view is an approximately circular shape. In other words, the approximately circular valve element  14  is formed on an inner side in a radial direction of the thin portion  16  which has an annular shape in a plan view. In addition, a thickness T V  of the valve element  14  is not particularly limited and can be appropriately adjusted. However, from the perspective of causing a fracture in the thin portion  16  to readily occur, the thickness T V  of the valve element  14  is favorably equal to or thicker than the thickness T T  of the thin portion  16 . For example, in the sealing plate  1  according to the present embodiment, the gas release vent  10  is formed in which the thickness T T  of the thin portion  16  and the thickness T V  of the valve element  14  are approximately the same. When the thickness T T  of the thin portion  16  and the thickness T V  of the valve element  14  are set approximately the same in this manner, since the gas release vent  10  can be relatively readily molded, a contribution can be made toward improving manufacturing efficiency. 
     (4) Groove Portion 
     In the present embodiment, the groove portion  17  having approximately annular shape is formed on a surface of the thin portion  16 . A portion in which the groove portion  17  is formed becomes a fragile portion of which strength is particularly low among the thin portion  16 . Therefore, in the present embodiment, when the internal pressure of the case reaches prescribed pressure, the thin portion  16  fractures in an approximately annular shape along the groove portion  17 . Accordingly, since the base portion  12  and the valve element  14  are separated from each other and the gas release vent  10  opens, gas inside the battery case can be discharged to the outside. However, in the opening of the gas release vent  10 , when the thin portion  16  fractures at once along an entire periphery of the groove portion  17 , since the valve element  14  is completely detached from the base portion  12 , there is a possibility that the valve element  14  may scatter in all directions due to gas erupting from inside the battery case. By contrast, in the sealing plate  1  according to the present embodiment, in the groove portion  17 , the remaining portion  17   b  is formed which is a region with a larger remaining thickness as compared to another region (hereinafter, the fracture portion  17   a ) of the groove portion  17 . Accordingly, the thin portion  16  can be prevented from fracturing along the entire periphery of the groove portion  17  and a state where the valve element  14  is connected to the base portion  12  via the remaining portion  17   b  can be maintained. As a result, the valve element  14  can be prevented from being completely detached from the base portion  12  and scattered in all directions as a metallic fragment. In addition, since the thin portion  16  can be prevented from fracturing at once along the entire periphery, even if the valve element  14  is separated from the base portion  12 , scattering energy (a scattering speed) thereof can be suppressed. 
     On the other hand, when the remaining portion  17   b  is formed in a part of the groove portion  17 , a continuous fracture of the thin portion  16  along the groove portion  17  stops at the remaining portion  17   b . Therefore, depending on a fracture start position, there is a possibility that the fracture of the thin portion is inhibited by the remaining portion and the gas release vent will fail to open sufficiently. By comparison, in the present embodiment, the remaining portion  17   b  is formed in a region including one intersection point IP 1  among two intersection points IP 1  and IP 2  where a straight line L which passes a center C of the gas release vent  10  and which extends in the short-side direction of the sealing plate  1  (a depth direction Y in  FIG. 2 ) intersects the groove portion  17  with the approximately annular shape. Accordingly, inhibition of the operation of the gas release vent  10  due to forming the remaining portion  17   b  can be prevented. Specifically, when the internal pressure of the battery case to which the approximately rectangular sealing plate  1  is attached rises, the possibility of an occurrence on the sealing plate  1  of a bending deformation causing a ridge extending along the short-side direction Y to be formed increases. In addition, when stress generated by the bending deformation of the sealing plate  1  is applied to the gas release vent  10 , a vicinity of any of the two intersection points IP 1  and IP 2  between the straight line L along the short-side direction Y and the groove portion  17  is likely to become a fracture start position. Furthermore, when the remaining portion  17   b  is formed in a region including one intersection point IP 1  among the two intersection points IP 1  and IP 2 , the possibility that a vicinity of the other intersection point IP 2  becomes a fracture start position increases significantly. In other words, according to the present embodiment, a fracture of the thin portion  16  can be started from a position (the intersection point IP 2 ) which opposes the remaining portion  17   b  across a center C of the gas release vent  10 . In this manner, by causing a fracture start point to be created at a farthest position from the remaining portion  17   b  in the peripheral direction, the groove portion  17  can be appropriately fractured so that only the remaining portion  17   b  is connected to the base portion  12  in the gas release vent  10  after activation. 
     As described above, with the sealing plate  1  according to the present embodiment, not only can scattering of a metallic fragment (the valve element  14 ) upon activation of the gas release vent  10  be suppressed but the remaining portion  17   b  formed as an anti-scattering measure of the valve element  14  can also be prevented from inhibiting the operation of the gas release vent  10 . 
     A remaining thickness T C  (refer to  FIG. 3 ) in the fracture portion  17   a  of the groove portion  17  is favorably appropriately adjusted in consideration of operational stability of the gas release vent  10 . For example, a ratio (T C /T T ) of the remaining thickness T C  of the fracture portion  17   a  with respect to the thickness T T  of the thin portion  16  is favorably 10% to 50% and more favorably 20% to 40%. As T C /T T  decreases (a groove in the fracture portion  17   a  becomes deeper), the likelihood that a fracture of the thin portion  16  along the fracture portion  17   a  is to occur increases and operational stability of the gas release vent  10  tends to improve. On the other hand, as T C /T T  increases (a groove in the fracture portion  17   a  becomes shallower), the likelihood that a malfunction of the gas release vent  10  is to occur decreases. 
     On the other hand, from the perspective of more appropriately suppressing scattering of the valve element  14 , the ratio (T R /T T ) of a remaining thickness T R  of the remaining portion  17   b  with respect to the thickness T T  of the thin portion  16  is favorably 50% or higher and more favorably 65% or higher. On the other hand, the remaining thickness T R  of the remaining portion  17   b  is not particularly limited as long as the remaining thickness T R  is thicker than the remaining thickness T C  of the fracture portion  17   a . For example, from the perspective of improving an outgassing property after activation of the gas release vent  10 , a groove with a certain depth is favorably formed in the remaining portion  17   b . Accordingly, when the gas release vent  10  is activated, since the valve element  14  is readily rotated upward with the remaining portion  17   b  as a fulcrum point, an opening area of the gas release vent  10  after activation can be sufficiently secured. In consideration of the above, an upper limit value of T R /T T  described above is favorably 99% or lower, more favorably 95% or lower, and particularly favorably 90% or lower. 
     In addition, a length of the remaining portion  17   b  in the peripheral direction is favorably ⅛ or more and ⅜ or less of a length of an entire periphery of the groove portion  17  with the approximately annular shape. Accordingly, scattering of the valve element  14  can be appropriately suppressed without significantly inhibiting the activation of the gas release vent  10 . Specifically, there is a tendency that when the ratio of the remaining portion  17   b  with respect to the entire periphery of the groove portion  17  having the approximately annular shape increases, scattering of the valve element  14  is more readily suppressed. On the other hand, there is a tendency that when the ratio of the remaining portion  17   b  with respect to the entire periphery of the groove portion  17  decreases, since the gas release vent  10  more readily opens, a gas discharging ability after the gas release vent  10  is activated improves. 
     As shown in  FIG. 3 , in the present embodiment, the groove portion  17  is formed on a second surface  16   b  (an upper surface in  FIG. 3 ) of the thin portion  16 . However, the surface on which the groove portion may be a first surface  16   a  (a lower surface in  FIG. 3 ) of the thin portion  16 . Even in this case, the thin portion can be caused to fracture in an approximately annular shape along the groove portion. However, when using the sealing plate  1  according to the present embodiment in a secondary battery, the side of the first surface  16   a  of the thin portion  16  is to be arranged inside a case (a positive pressure side during a rise in internal pressure of the case). In consideration thereof, the annular groove portion  17  is more favorably formed on the second surface  16   b  to be arranged outside of the case. Accordingly, when the internal pressure of the case rises and the first surface  16   a  of the thin portion  16  is pressurized in a second direction (upward in the height direction Z in  FIG. 3 ), the thin portion  16  can be caused to fracture so as to expand the groove portion  17 . As a result, operational stability of the gas release vent  10  can be further improved. 
     In addition, the gas release vent  10  is favorably formed in a central region of the sealing plate  1  in a long-side direction X. Accordingly, since stress due to a bending deformation of the sealing plate  1  is efficiently applied to the gas release vent  10 , a vicinity of the intersection point IP 2  between the annular groove portion  17  and the straight line L is likely to become a fracture start point of the thin portion  16 . It should be noted that, in the present specification, a “central region of the sealing plate” refers to a region including a center point of the sealing plate in the long-side direction (a width direction X in  FIG. 2 ). In other words, when a formation region of the gas release vent in a plan view includes the center point of the sealing plate, a description of “the gas release vent is formed in a central region of the sealing plate” can be used. Furthermore, the gas release vent of the sealing plate disclosed herein need not necessarily be formed in the central region of the sealing plate. For example, depending on various components (an electrode terminal, a sealing plug of an electrolyte injection hole, and the like) which can be attached to the sealing plate, a position where a bending deformation of the sealing plate occurs may deviate from the central region of the sealing plate. Therefore, favorably, after specifying a position where a bending deformation of the sealing plate occurs by conducting a preliminary test or the like, the position where a bending deformation of the sealing plate occurs is to be included in a formation region of the gas release vent. Accordingly, since the likelihood of the vicinity of the intersection point IP 2  between the annular groove portion  17  and the straight line L being a fracture start point of the thin portion  16  increases, inhibition of the operation of the gas release vent  10  due to providing the remaining portion  17   b  can be reliably prevented. 
     Secondary Battery 
     The sealing plate  1  configured as described above is a component for a secondary battery which constitutes one side wall of a battery case of the secondary battery. Hereinafter, a secondary battery using the sealing plate  1  configured as described above will be described.  FIG. 4  is a perspective view schematically showing a secondary battery according to the first embodiment. 
     A secondary battery  100  shown in  FIG. 4  includes an electrode body (not illustrated) and a battery case  20  which houses the electrode body. Although a detailed illustration will be omitted, the electrode body includes a positive electrode, a negative electrode, and a separator. For example, the electrode body can be a wound electrode body in which a band-like positive electrode and a band-like negative electrode are laminated via two band-like separators and wound around a winding axis. Other examples of a structure of the electrode body include a laminated electrode body in which a plurality of square-shaped (typically, rectangular-shaped) positive electrodes and a plurality of square-shaped (typically, rectangular-shaped) negative electrodes are stacked up in an insulated state. Since materials and structures which can be adopted in a general secondary battery (for example, a lithium-ion secondary battery) can be adopted without any particular limitations as a material and a structure of each member (such as a positive electrode, a negative electrode, and a separator) which constitutes the electrode body and since the material and the structure of each member do not limit the technique disclosed herein, a detailed description thereof will be omitted. In addition, although not illustrated, an electrolyte solution is also housed in the battery case  20 . As the electrolyte solution, electrolyte solutions that can be adopted in a general secondary battery can be adopted without any particular limitations. 
     The battery case  20  is a casing which houses the electrode body described above. A material of the battery case  20  may be similar to those conventional used and is not particularly limited. For example, the battery case  20  is favorably a metallic battery case with prescribed strength. Examples of the material of the battery case  20  include aluminum, an aluminum alloy, iron, and an iron alloy. 
     As shown in  FIG. 4 , the battery case  20  has an external shape that is a flat and bottomed rectangular parallelopiped shape (square shape). The battery case  20  includes an outer package  22  having an opening on an upper surface thereof and the sealing plate  1  which closes the opening of the outer package  22 . The outer package  22  is a box-like member including a bottom wall (not illustrated) with a rectangular planar shape, a pair of long-side walls  22   a  which extend along the height direction Z from long sides of the rectangular bottom wall and which oppose each other, and a pair of short-side walls  22   b  which extend along the height direction Z from short sides of the rectangular bottom wall and which oppose each other. In addition, an approximately rectangular-shaped opening (not illustrated) enclosed by respective upper sides of the pair of long-side walls  22   a  and the pair of short-side walls  22   b  is formed on the upper surface of the outer package  22 . Furthermore, the sealing plate  1  having the gas release vent  10  configured as described above is attached to the outer package  22  so as to close the opening of the upper surface of the outer package  22  and opposes the bottom wall of the outer package  22 . Moreover, the battery case  20  with a sealed (hermetically sealed) interior is constructed by joining (for example, welding) a peripheral edge of the opening of the outer package  22  and an outer peripheral edge of the sealing plate  1  to each other. For example, laser welding can be used to join the sealing plate  1  and the outer package  22  to each other. 
     In addition, a positive electrode terminal  30  and a negative electrode terminal  40  are attached to the sealing plate  1  of the secondary battery  100 . The positive electrode terminal  30  is an elongated conductive member that extends in the height direction Z. A lower end of the positive electrode terminal  30  is connected inside the battery case  20  to the positive electrode of the electrode body. On the other hand, an upper end of the positive electrode terminal  30  is exposed to the outside of the battery case  20 . The positive electrode terminal  30  is favorably constituted of aluminum, an aluminum alloy, or the like. On the other hand, the negative electrode terminal  40  has a structure that is approximately the same as that of the positive electrode terminal  30 . Specifically, a lower end of the negative electrode terminal  40  is connected inside the battery case  20  to the negative electrode and an upper end of the negative electrode terminal  40  is exposed to the outside of the battery case  20 . The negative electrode terminal  40  is favorably constituted by copper, a copper alloy, or the like. In addition, attachment positions of the positive electrode terminal and the negative electrode terminal are not particularly limited and the positive electrode terminal and the negative electrode terminal may be provided on a side wall of the battery case (a side wall of the outer package) other than the sealing plate. Furthermore, although not illustrated, the sealing plate  1  may be provided with an electrolyte injection hole for injecting an electrolyte solution during a manufacturing process of the secondary battery  100 . Normally, the electrolyte injection hole is sealed by a prescribed sealing plug. As the sealing plug of the electrolyte injection hole, a blind rivet or the like is used. 
     In addition, the sealing plate  1  of the secondary battery  100  according to the present embodiment is provided with the gas release vent  10 . In this case, the sealing plate  1  is arranged so that the first surface (the lower surface in  FIG. 3 ) opposes the electrode body. In other words, in the present embodiment, the sealing plate  1  is attached so that the recessed portion  18  is arranged outside of the battery case  20 . Furthermore, in the sealing plate  1  configured as described above, the remaining portion  17   b  is formed in a part of the groove portion  17  of the gas release vent  10 . Accordingly, since a state where the valve element  14  and the base portion  12  are connected to each other via the remaining portion  17   b  can be maintained even after the gas release vent  10  is activated, scattering of a metallic fragment (the valve element  14 ) in all directions can be preferably suppressed. Moreover, by causing the remaining portion  17   b  and a fracture start position (the intersection point IP 2 ) to oppose each other across the center C of the gas release vent  10 , inhibition of activation of the gas release vent  10  due to providing the remaining portion  17   b  can be prevented. 
     Other Embodiments 
     An embodiment (the first embodiment) of the technique disclosed herein has been described above. However, the technique disclosed herein is not limited to the embodiment described above and encompasses various embodiments. Hereinafter, other embodiments of the sealing plate disclosed herein will be described. 
     (1) Second Embodiment 
       FIG. 5  is a plan view schematically showing a sealing plate according to a second embodiment.  FIG. 6  is a longitudinal sectional view schematically showing the sealing plate according to the second embodiment. As shown in  FIGS. 5 and 6 , in the sealing plate  1  according to the present embodiment, a protective tape  50  is affixed so as to cover the gas release vent  10 . Accordingly, damage and deterioration of the gas release vent  10  due to corrosive foreign objects or the like adhering to the gas release vent  10  can be prevented. Specifically, corrosion of a metallic material (for example, aluminum) which constitutes the sealing plate  1  can be promoted by bringing the metallic material into contact with a dissimilar metal (bimetallic corrosion). In addition, as described above, since the negative electrode terminal  40  constituted of copper or a copper alloy is exposed to the outside of the battery case  20  in the secondary battery  100 , there is a possibility that a copper fragment detached from the negative electrode terminal  40  may adhere to the gas release vent  10 . When adherence of the copper fragment causes the valve element  14  or the thin portion  16  of the gas release vent  10  to corrode and results in forming a hole, there is a risk that an electrolyte solution may flow out from inside the battery case  20  or water may penetrate into the battery case  20 . Conversely, by affixing the protective tape  50  so as to cover the gas release vent  10 , corrosion of the gas release vent  10  due to adherence of a copper fragment or the like can be prevented. 
     As the protective tape  50 , a configuration in which a pressure-sensitive adhesive is applied to a surface of a film-like base material can be adopted. Conventional and known materials can be used without any particular limitations as the base material of the protective tape  50  as long as corrosion of the sealing plate  1  and the gas release vent  10  is not promoted. For example, the base material of the protective tape  50  is favorably constituted of a resin material such as polypropylene (PP) or polyethylene terephthalate (PET), a same metallic material (such as aluminum) as the sealing plate  1 , or the like. In a similar manner, as the pressure-sensitive adhesive, various pressure-sensitive adhesives can be used without any particular limitations as long as corrosion of the sealing plate  1  and the gas release vent  10  is not promoted. As examples of the pressure-sensitive adhesive, a rubber pressure sensitive adhesive, an acrylic pressure sensitive adhesive, a silicon pressure sensitive adhesive, and the like can be used. 
     In addition, in the present embodiment, as shown in  FIG. 5 , a length L 1  of the protective tape  50  affixed to a region outside of the gas release vent  10  in the short-side direction Y of the sealing plate  1  is favorably shorter than a length L 2  of the protective tape  50  affixed to a region outside of the gas release vent  10  in the long-side direction X of the sealing plate  1 . Accordingly, the protective tape  50  can be peeled off in an efficient manner when the gas release vent  10  is activated. Specifically, when the internal pressure of the case rises significantly in the present embodiment, the thin portion  16  fractures along the fracture portion  17   a  of the groove portion  17  and, at the same time, the valve element  14  is pushed upward by gas discharged from inside the battery case. In addition, since the valve element  14  is connected to the base portion  12  via the remaining portion  17   b , the valve element  14  rotates upward with the remaining portion  17   b  as a fulcrum point. At this point, when the affixing length L 1  of the protective tape  50  in the short-side direction Y is set shorter than the affixing length L 2  of the protective tape  50  in the long-side direction X, since the protective tape  50  can be pushed upward and readily peeled off by the valve element  14  having rotated upward, a decline in gas discharging ability due to the protective tape  50  can be prevented. 
     It should be noted that the length L 1  in the configuration described above is favorably a “length of the protective tape  50  affixed to a region opposing the remaining portion  17   b  across the center C of the gas release vent  10  (in other words, a region in proximity of the fracture start position (the intersection point IP 2 )) among regions on both outer sides of the gas release vent  10  in the short-side direction Y of the sealing plate  1 ”. In addition, the length L 2  in the configuration described above is favorably a “length of the protective tape  50  in a region with a short affixing margin of the protective tape  50  among regions on both outer sides of the gas release vent  10  in the long-side direction X of the sealing plate  1 ”. Setting the lengths L 1  and L 2  of the protective tape  50  described above enables the protective tape  50  to be peeled off in an efficient manner when the gas release vent  10  is activated. 
     In addition, as shown in  FIG. 6 , a gap S with a height of 1 mm or more is favorably formed between the thin portion  16  and the protective tape  50 . Accordingly, when the gas release vent  10  expands during normal use before desired internal pressure of the case is reached, the protective tape  50  can be prevented from being pushed upward and peeled off by the expanded gas release vent  10 . On the other hand, an upper limit of the height of the gap S is not particularly limited and may be 5 mm or less, 3 mm or less, or 2 mm or less. 
     Furthermore, a slit may be formed in the protective tape  50 . Accordingly, when the gas release vent  10  is activated, the protective tape  50  can be fractured by the valve element  14  which rotates upward. As a result, a decline in gas discharging ability due to the protective tape  50  can be appropriately prevented. In addition, a shape of the slit in a plan view is not particularly limited and various shapes can be adopted depending on an object of the slit. For example, the slit of the protective tape  50  favorably has a dashed-line shape formed approximately parallel to a short side surface. Accordingly, the protective tape  50  can be readily fractured when the gas release vent  10  is activated. 
     (2) Third Embodiment 
       FIG. 7  is a sectional view schematically showing a sealing plate according to a third embodiment. In the first embodiment, the gas release vent  10  is formed in which the thickness T T  of the thin portion  16  and the thickness T V  of the valve element  14  are approximately the same (refer to  FIG. 3 ). However, as shown in  FIG. 7 , the thickness T V  of the valve element  14  may be equal to or thicker than the thickness T T  of the thin portion  16 . Accordingly, since a second moment of area of the valve element  14  becomes larger than a second moment of area of the thin portion  16  and stress applied to the gas release vent  10  when internal pressure of the case rises concentrates on the thin portion  16  around the groove portion  17 , a fracture of the thin portion  16  along the groove portion  17  more readily occurs. A ratio (T V /T T ) of the thickness T V  of the valve element  14  with respect to the thickness T T  of the thin portion  16  in the present embodiment is favorably 100% or higher, more favorably 500% or higher, and particularly favorably 900% or higher. Accordingly, stress can be more readily concentrated on the thin portion  16 . On the other hand, an upper limit value of T V /T T  is not particularly limited and may be 3000% or lower or 1500% or lower. 
     (3) Fourth Embodiment 
       FIG. 8  is a sectional view schematically showing a sealing plate according to a fourth embodiment. In the first embodiment, the thin portion  16  of which the thickness T T  is approximately the same in a peripheral direction is formed (refer to  FIG. 3 ). However, as shown in  FIG. 8 , the thickness of the thin portion  16  in the peripheral direction need not be constant. Specifically, the gas release vent  10  according to the present embodiment is formed so that a thickness T T1  of a thin portion  16   b  adjacent to the remaining portion  17   b  is thicker than a thickness T T2  of the thin portion  16   b  adjacent to another region (the fracture portion  17   a ) of the groove portion  17 . Accordingly, the thin portion  16   b  along the fracture portion  17   a  of the groove portion  17  fractures more readily and, at the same time, the thin portion  16   b  in the vicinity of the remaining portion  17   b  fractures less readily. As a result, both operational stability of the gas release vent  10  and suppression of scattering of the valve element  14  can be realized at higher levels. 
     A ratio (T T2 /T T1 ) of a thickness T T2  of the thin portion  16   b  adjacent to the fracture portion  17   a  with respect to a thickness T T1  of the thin portion  16   b  adjacent to the remaining portion  17   b  is favorably 20% to 100% and more favorably 25% to 50%. Accordingly, both operational stability of the gas release vent  10  and suppression of scattering of the valve element  14  can be realized at even higher levels. In addition, a shape of a boundary between the thin portion  16   b  adjacent to the remaining portion  17   b  which is relatively thick and the thin portion  16   b  adjacent to the fracture portion  17   a  which is relatively thin is not particularly limited. For example, a step or an inclined surface may be formed at the boundary between the thin portions  16   b  with different thicknesses. 
     (4) Fifth Embodiment 
       FIG. 9  is a sectional view schematically showing a sealing plate according to a fifth embodiment. In addition,  FIG. 10  is a plan view schematically showing the sealing plate according to the fifth embodiment. The sealing plate  1  according to the first embodiment is configured so that the remaining thickness T R  of the remaining portion  17   b  is thinner than the thickness T T  of the thin portion  16  adjacent to the remaining portion  17   b  (refer to  FIG. 2 ). However, the remaining thickness T R  of the remaining portion  17   b  is not particularly limited as long as the remaining thickness T R  is thicker than the remaining thickness T C  of the fracture portion  17   a . For example, as shown in  FIG. 9 , the remaining thickness T R  of the remaining portion  17   b  may be the same as the thickness T T  of the thin portion  16  adjacent to the remaining portion  17   b  (T R /T T =100%). Accordingly, a fracture of the thin portion  16  along the entire periphery and scattering of the valve element  14  can be more appropriately suppressed. In addition, from the perspective of more appropriately suppressing scattering of the valve element  14 , the remaining thickness T R  of the remaining portion  17   b  may be made thicker than the thickness T T  of the thin portion  16 . In other words, an upper limit value of T R /T T  may be 120% or lower, 110% or lower, or 100% or lower. 
     It should be noted that when the thickness T R  of the remaining portion  17   b  is made equal to or thicker than the thickness T T  of the thin portion  16  as in the present embodiment, as shown in  FIG. 10 , the gas release vent  10  is formed in which the groove portion  17  is broken in a region where the remaining portion  17   b  is formed. In the present specification, an outer shape of the partially-broken groove portion  17  is to be determined by an auxiliary line LA (a dotted line in  FIG. 9 ) having been drawn so as to complement the broken region in accordance with a shape of a remaining portion (the fracture portion  17   a ) of the groove portion  17 . In other words, as shown in  FIG. 10 , when an upper part is broken and the remaining fracture portion  17   a  has an approximately annular shape, the external shape of the groove portion  17  is determined by drawing a circular auxiliary line LA along the approximately annular fracture portion  17   a . Furthermore, an intersection point IP 1  between the auxiliary line LA and the straight line L extending in the short-side direction of the sealing plate can be adopted as one of the two intersection points which can be fracture start points. 
     (5) Other Modes 
     A planar shape of the gas release vent  10  in each of the first to fifth embodiments described above is an approximately circular shape. However, the planar shape of the gas release vent  10  is not particularly limited and various shapes can be adopted without any particular limitations. For example, the planar shape of the gas release vent  10  may be an elliptical shape or a polygonal shape (for example, a quadrangle or a pentagon). In addition, a planar shape of each component that forms the gas release vent  10  is also not particularly limited. For example, in each of the embodiments described above, the approximately annular groove portion  17  is formed on a surface of the annular thin portion  16  and the approximately circular valve element  14  is formed inside the approximately annular groove portion  17 . However, a groove portion may be formed on a surface of a rectangular annular thin portion  16  and an approximately circular valve element may be formed inside the groove portion with the approximately annular shape. Alternatively, a rectangular annular groove portion may be formed on a surface of an annular thin portion and an approximately square valve element may be formed inside the rectangular annular groove portion. However, as described above, from the perspective of suppressing a variation in working pressure of the gas release vent, the planar shape of each component is favorably an approximately circular shape as in each of the embodiments described above. 
     Embodiments of the technique disclosed herein have been described above. However, it should be understood that the description presented above is merely illustrative and is not intended to limit the scope of claims. Techniques described in the scope of claims include various modifications and changes made to the specific examples exemplified in the description presented above.