Patent Publication Number: US-2018047960-A1

Title: Electric storage cell, covering film and electric storage module

Description:
BACKGROUND 
     Field of the Invention 
     The present invention relates to an electric storage cell constituted by an electric storage element sealed with covering film, a covering film, and an electric storage module comprising a stack of such electric storage cells. 
     Description of the Related Art 
     Film-sealed batteries, which are electric storage elements sealed with covering film, are widely used in recent years. Film-sealed batteries are subject to rising pressure inside the battery due to generation of gaseous species as a result of electrolysis of the electrolyte medium, if the battery control circuit fails for some reason and abnormal voltage is applied as a result, or if the ambient temperature becomes abnormally high for some reason, while the battery is in use. As their internal pressure rises, film-sealed batteries will eventually experience a rupture of exterior material and gas will erupt from the ruptured area; however, it is not predictable where the rupture may occur, and depending on the location of rupture, surrounding equipment, etc., may be negatively affected. 
     To solve this problem, a configuration is disclosed in Patent Literature 1, for example, which involves a covering film whose seal part has a peninsula-shaped projected fusing part, so that when the covering film package expands and the projected fusing part peels, a through hole will be formed to serve as a pressure release part. This way, the peel-off stress generated by expansion of the film can be concentrated onto the projected fusing part to facilitate the progress of its peeling, thereby facilitating the release of the pressure caused by expansion of the package. 
     Background Art Literatures 
     [Patent Literature 1] Japanese Patent Laid-open No. 2005-203262 
     SUMMARY 
     According to the configuration in Patent Literature  1 , however, the narrow seal width at the through hole and projected fusing part may affect long-term reliability, in that moisture may permeate in through the fusing resin layer. Besides the configuration described in Patent Literature 1, there are other configurations designed to release the internal pressure by causing a projection to break through the expanded covering film; however, they are costly because, among other reasons, a component that serves as this projection must be provided on each device. Also, handling of such device requires attention because a projection is permanently present on it. 
     In light of the aforementioned situations, an object of the present invention is to provide a reliable electric storage cell, covering film, and electric storage module, which would allow for safe release of rising internal pressure in the event of abnormality. 
     Any discussion of problems and solutions involved in the related art has been included in this disclosure solely for the purposes of providing a context for the present invention, and should not be taken as an admission that any or all of the discussion were known at the time the invention was made. 
     To achieve the aforementioned object, an electric storage cell pertaining to one mode of the present invention has an electric storage element and a covering film package. 
     The covering film package houses the electric storage element and comprises: a metal layer having a first principle face on the electric storage element side and a second principle face on the opposite side of the first principle face, an internal resin layer made of synthetic resin and laminated to the first principle face, and an external resin layer made of synthetic resin and laminated to the second principle face, with a slit formed in the external resin layer; wherein a seal area formed by the internal resin layers thermally fused to each other around the periphery of the electric storage element, and a non-seal area where the internal resin layers are contacting each other between the seal area and the electric storage element, are provided, the seal area has a projecting area that projects toward the electric storage element, and the slit intersects with the boundary between the projecting part and the non-seal area. 
     According to this configuration, a rise in the internal pressure due to an abnormality of the electric storage cell generates a stress that tries to separate the covering film package, and this stress concentrates at the apex of the projecting area in the seal area. Then, as the internal pressure continues to rise further, the stress continues to concentrate at the projecting area and the separation of the projecting area starting from the apex of the projecting area progresses, and consequently the stress propagates to the slit. Then, this stress that has propagated to the slit causes the covering film package to break open through the slit, and the internal pressure is released as a result. 
     This means that, since the internal pressure is released where the slit is formed, safety is ensured as pressure release from a part other than the slit can be prevented. Also, in a normal state (when no abnormality is present in the electric storage cell), moisture permeation into the housing space is prevented by the metal layer, and consequently reliability of the electric storage cell can be ensured. 
     The slit starts from the non-seal area and traverses the projecting area to reach the non-seal area, while the projecting area has a triangle shape formed by a first point of intersection and a second point of intersection, respectively positioned where the slit intersects with the aforementioned boundary, and the apex of the projecting area positioned on the electric storage element side of the first point of intersection and second point of intersection. 
     Because of this configuration, the location where the stress that generates as the internal pressure rises in the event of abnormality of the electric storage cell (force that tries to separate the covering film package) concentrates first, is limited to the apex of the projecting area. As a result, the release pressure at which the rising internal pressure of the electric storage cell is released can be controlled to a desired level by adjusting the distance between the apex of the projecting area and the slit. 
     The slit may have a depth of 0 μm or more but no more than 15 μm, measured as the distance from the bottom of the slit in the external resin layer to the second principle face. 
     Under the present invention, it is sufficient for the slit to have the aforementioned depth in the external resin layer and the slit need not reach the metal layer. This way, corrosion of the metal layer is prevented even when the electric storage cell is used in a corrosive environment. 
     The internal resin layer may be made of non-oriented cast polypropylene or polyethylene, and the external resin layer may be made of at least one of polyethylene terephthalate and nylon. 
     To achieve the aforementioned object, a covering film pertaining to one mode of the present invention forms a housing space in which an electric storage element is housed, wherein the covering film houses the electric storage element and comprises: a metal layer having a first principle face on the electric storage element side and a second principle face on the opposite side of the first principle face, an internal resin layer made of synthetic resin and laminated to the first principle face, and an external resin layer made of synthetic resin and laminated to the second principle face, with a slit formed in the external resin layer; wherein a seal area formed by the internal resin layers thermally fused to each other around the periphery of the electric storage element, and a non-seal area where the internal resin layers are contacting each other between the seal area and the electric storage element are provided, the seal area has a projecting area that projects toward the electric storage element, and the slit intersects with the boundary between the projecting area and the non-seal area. 
     By covering the storage element with the covering film having the aforementioned configuration, a reliable electric storage cell can be produced which would allow its rising internal pressure to be released safely in the event of abnormality. 
     To achieve the aforementioned object, an electric storage module pertaining to one mode of the present invention represents an electric storage module constituted by multiple electric storage cells stacked on top of each other. 
     Each of the electric storage cells has an electric storage element and an covering film package. 
     The covering film package houses the electric storage element and comprises: a metal layer having a first principle face on the electric storage element side and a second principle face on the opposite side of the first principle face, an internal resin layer made of synthetic resin and laminated to the first principle face, and an external resin layer made of synthetic resin and laminated to the second principle face, with a slit formed in the external resin layer; wherein a seal area formed by the internal resin layers thermally fused to each other around the periphery of the electric storage element, and a non-seal area where the internal resin layers are contacting each other between the seal area and the electric storage element, are provided, the seal area has a projecting area that projects toward the electric storage element, and the slit intersects with the boundary between the projecting area and the non-seal area. 
     The covering film package may have contact areas where the internal resin layers are contacting each other around the periphery of the electric storage element, and the slit is formed in a location corresponding to, of the contact areas of each electric storage cell, the one facing a contact area of the adjacent electric storage cell. 
     According to this configuration, by providing a leakage-countermeasure component (sponge or other absorbent member) in the aforementioned location, this leakage-countermeasure component applied commonly to the adjacent electric storage cells can be used, in the event that the rising internal pressure of the electric storage cell due to its abnormality causes the electrolyte to leak from the slit, to absorb the electrolyte. 
     If slits are formed near a back-to-back connection part of the adjacent electric storage cells, a leakage-countermeasure for leakage of the electrolyte must be provided for each cell; if the slits are formed to face in the same direction, a leakage-countermeasure component must be provided for each cell based on a different structure. 
     This means that, by providing slits in the aforementioned locations, an electric storage module that can address leakage of electrolyte from the slits, should it occur, without complicating the apparatus configuration and also at low cost, can be provided. 
     As described above, a reliable electric storage cell, covering film, and electric storage module, which would allow for safe release of rising internal pressure in the event of abnormality, can be provided according to the present invention. 
     For purposes of summarizing aspects of the invention and the advantages achieved over the related art, certain objects and advantages of the invention are described in this disclosure. Of course, it is to be understood that not necessarily all such objects or advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein. 
     Further aspects, features and advantages of this invention will become apparent from the detailed description which follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features of this invention will now be described with reference to the drawings of preferred embodiments which are intended to illustrate and not to limit the invention. The drawings are greatly simplified for illustrative purposes and are not necessarily to scale. 
         FIG. 1  is a perspective view of an electric storage cell pertaining to an embodiment of the present invention. 
         FIG. 2  is a cross sectional view of the electric storage cell pertaining to the embodiment of the present invention. 
         FIG. 3  is a plan view of the electric storage cell pertaining to the embodiment of the present invention. 
         FIG. 4  is a cross sectional view of a covering film provided in an electric storage cell pertaining to an embodiment of the present invention. 
         FIG. 5  is a plan view of the electric storage cell illustrated in  FIG. 3 . 
         FIG. 6  is an enlarged view, from one direction, of the projecting area provided in the electric storage cell illustrated in  FIG. 5 . 
         FIG. 7  is a cross sectional view of a covering film provided in an electric storage cell pertaining to an embodiment of the present invention. 
         FIG. 8  is a plan view of an electric storage cell pertaining to another embodiment of the present invention. 
         FIG. 9  is a plan view of an electric storage cell pertaining to still another embodiment of the present invention. 
         FIG. 10  is a schematic view of an electric storage module pertaining to an embodiment of the present invention. 
         FIG. 11  is a cross sectional view of an electric storage cell pertaining to a variation example of the present invention. 
         FIG. 12  is an enlarged view, from one direction, of the projecting area provided in an electric storage cell pertaining to another embodiment of the present invention. 
         FIG. 13  is an enlarged view, from one direction, of the projecting area provided in an electric storage cell pertaining to still another embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE SYMBOLS 
       10 —Electric storage cell 
       20 —Covering film 
       20   a —Contact area 
       20   b —Element housing part 
       25 —Metal layer 
       25   a —First principle face 
       25   b —Second principle face 
       26 —Internal resin layer 
       27 —External resin layer 
       30 —Electric storage element 
       100 —Electric storage module 
     E 1 —Seal area 
     E 2  Non-seal area 
     E 3 —Projecting area 
     P 1 —Bottom of the slit 
     P 2 —Apex of the projecting area 
     P 3 —First point of intersection 
     P 4 —Second point of intersection 
     S—Slit 
     DETAILED DESCRIPTION OF EMBODIMENTS 
     An embodiment of the present invention is explained below by referring to the drawings. 
     [Structure of Electric Storage Cell] 
       FIG. 1  is a perspective view of an electric storage cell  10  pertaining to this embodiment, while  FIG. 2  is a cross sectional view of the electric storage cell  10  in  FIG. 1  along line A-A. In the figures below, the X direction, Y direction and Z direction represent three directions that are orthogonal to each other. 
     As shown in  FIGS. 1 and 2 , the electric storage cell  10  has covering films  20 , an electric storage element  30 , a positive electrode terminal  40 , and a negative electrode terminal  50 . 
     In the electric storage cell  10 , the covering film package constituted by the two covering films  20  forms a housing space R, and the electric storage element  30  is housed in the housing space R. The two covering films  20  are sealed around the periphery of the electric storage element  30 , and the covering film package has contact areas  20   a  where the two covering films  20  contact each other, and an element housing part  20   b  where the electric storage element  30  is housed. The contact areas  20   a  and element housing part  20   b  will be described later. 
     The thickness of the electric storage cell  10  in this embodiment is not limited in any way, but it may be 12 mm or less, for example. This way, the operations and effects achieved by the formation of the slit S and projecting area E 3  in the electric storage cell  10 , as explained later, will become more significant. 
     As shown in  FIG. 2 , the electric storage element  30  has a positive electrode  31 , a negative electrode  32 , and a separator  33 . The positive electrode  31  and negative electrode  32  face each other with the separator  33  in between, and are housed in the housing space R. 
     The positive electrode  31  functions as the positive electrode of the electric storage element  30 . The positive electrode  31  may be made of a positive electrode material that contains positive electrode active material, binder, etc. The positive electrode active material may be activated carbon, for example. The positive electrode active material may be changed as deemed appropriate according to the type of the electric storage cell  10 . 
     The negative electrode  32  functions as the negative electrode of the electric storage element  30 . The negative electrode  32  may be made of a negative electrode material that contains negative electrode active material, binder, etc. The negative electrode active material may be carbon material, for example. The negative electrode active material may be changed as deemed appropriate according to the type of the electric storage cell  10 . 
     The separator  33  is provided between the positive electrode  31  and the negative electrode  32 , to allow the electrolyte to pass through it and also prevent (insulate) the positive electrode  31  and the negative electrode  32  from contacting each other. The separator  33  may be a woven fabric, non-woven fabric, synthetic microporous resin membrane, etc. 
     While one positive electrode  31  and one negative electrode  32  are provided in  FIG. 2 , multiple positive electrodes and multiple negative electrodes can be provided. In this case, the multiple positive electrodes  31  and multiple negative electrodes  32  may be stacked together alternately, with separators  33  in between. Also, the electric storage element  30  may be constituted by rolling a laminate comprising a positive electrode  31 , a negative electrode  32 , and a separator  33 , into a roll. 
     The type of the electric storage element  30  is not limited in any way, and it may be a lithium ion capacitor, lithium ion battery, electrical double-layer capacitor, etc. Together with the electric storage element  30 , electrolyte is housed in the housing space R. This electrolyte is a solution that contains SBP-BF 4  (spirobipyrrolidinium tetrafluoroborate) or the like, for example, as a solute, and any electrolyte may be selected according to the type of the electric storage element  30 . 
     The positive electrode terminal  40  is an external terminal of the positive electrode  31 . As shown in  FIG. 2 , the positive electrode terminal  40  is electrically connected to the positive electrode  31  via positive electrode wiring  41 , being routed between the two covering films  20  in the contact area  20   a  and led out from the interior to the exterior of the housing space R. The positive electrode terminal  40  may be a foil or wire made of conductive material. 
     The negative electrode terminal  50  is an external terminal of the negative electrode  32 . The negative electrode terminal  50  is electrically connected to the negative electrode  32  via negative electrode wiring  51 , being routed between the two covering films  20  in the contact area  20   a  and led out from the interior to the exterior of the housing space R. The negative electrode terminal  50  may be a foil or wire made of conductive material. 
     As described above, the electric storage cell  10  has the contact areas  20   a  and the element housing part  20   b . The contact areas  20   a  are where the two covering films  20  contact each other, while the element housing part  20   b , enclosed by the contact areas  20   a , is where the electric storage element  30  is housed. 
       FIG. 3  is a schematic view of the electric storage cell  10  as viewed from the Z direction. As shown in this figure, the contact areas  20   a  each have a seal area E 1  and a non-seal area E 2 . The width of the contact area  20   a  may be anywhere from around several millimeters to several tens of millimeters, for example. The seal area E 1  pertaining to this embodiment has a projecting area E 3  that projects toward the electric storage element  30 , as shown in  FIG. 3 , and the projecting area E 3  will be explained later in relation to  FIG. 5 . 
     The seal area E 1  is an area formed by the covering films  20  thermally fused to each other, and provided around the periphery of the covering films  20 . 
     The non-seal area E 2  is an area contacted by the covering films  20  as a result of the thermal fusion in the seal area E 1 , and provided between the seal area E 1  and the element housing part  20   b . The width of the seal area E 1  and non-seal area E 2  may be anywhere from around several millimeters to several tens of millimeters, for example. 
     [Configuration of Covering film] 
       FIG. 4  is a cross sectional view of each covering film  20 . As shown in this figure, the covering film  20  is constituted by a metal layer  25 , an internal resin layer  26 , and an external resin layer  27 . 
     The metal layer  25  is a layer made of foil-like metal, and has a function to prevent moisture in air from permeating through it. As shown in  FIG. 4 , the metal layer  25  has a first principle face  25   a , and a second principle face  25   b  on the opposite side thereof. 
     The metal layer  25  may be a metal foil made of aluminum, for example. Besides the foregoing, the metal layer  25  may also be a foil of copper, nickel, stainless steel, etc. Preferably the thickness of the metal layer  25  pertaining to this embodiment is around several tens of micrometers. 
     The internal resin layer  26  is laminated to the first principle face  25   a  to constitute the inner periphery face of the housing space R, covering and insulating the metal layer  25 . 
     The internal resin layer  26  is made of synthetic resin, such as non-oriented cast polypropylene (CPP) or polyethylene, for example. Besides the foregoing, the internal resin layer  26  may be made of acid-modified polyethylene, polyphenylene sulfide, polyethylene terephthalate, polyamide, ethylene-vinyl acetate copolymer, or the like. Also, the internal resin layer  26  may be constituted by multiple synthetic resin layers laminated together. 
     The external resin layer  27  is laminated to the second principle face  25   b  to constitute the surface  27   a  of the electric storage cell  10 , covering and protecting the metal layer  25 . 
     The external resin layer  27  is made of synthetic resin, and it may be made of at least one of polyethylene terephthalate and nylon, for example. Also, the external resin layer  27  may have a two-layer structure consisting of a nylon layer made of oriented nylon, etc., and a polyethylene terephthalate layer laminated to it. Besides the foregoing, the external resin layer  27  may be made of bi-axially oriented polypropylene, polyimide, polycarbonate, or the like. 
     In this embodiment, the housing space R is formed by the covering film package constituted as above, where the two covering films  20  are facing each other with the electric storage element  30  in between and sealed in a seal area E 1  of the contact areas  20   a  which also include a non-seal area E 2 . In the seal area E 1 , the internal resin layers  26  of the two covering films  20  are thermally fused to each other. The covering films  20  are each positioned in such a way that the internal resin layer  26  faces the housing space R side (inside) and the external resin layer  27  constitutes the surface  27   a  side (outside). 
     The covering films  20  are used in a condition where they maintain flexibility, and may be formed in a manner being curved at the peripheries of the electric storage element  30  according to the shape of the electric storage element  30 , as shown in  FIG. 2 . Also, the covering films  20  may be used in a condition where they have been pre-formed to such shape by means of embossing. A slit S is formed in one of the two covering films  20 . 
     [Configuration of Projecting Area] 
       FIG. 5  is a schematic view of the electric storage cell  10  as viewed from the Z direction. The seal area E 1  pertaining to this embodiment has a projecting area E 3  that projects toward the electric storage element  30 , as shown in  FIG. 5 . This provides a configuration where the seal area E 1  penetrates into the non-seal area E 2 , and the projecting area E 3  becomes the closest part of the seal area E 1  to the electric storage element  30 . 
     Because the seal area E 1  has the projecting area E 3 , the boundary B between the seal area E 1  and the non-seal area E 2  comprises a boundary B 1  and a boundary B 2 , as shown in  FIG. 5 . The boundary B 1  represents a boundary between the projecting area E 3  and the non-seal area E 2 , and the boundary B 2 , which surrounds the non-seal area E 2 , represents a boundary between the seal area E 1  and the non-seal area E 2 , excluding the boundary B 1 . 
       FIG. 6  is an enlarged view of the projecting area E 3  as viewed from the Z direction. The projecting area E 3  pertaining to this embodiment has a triangle shape, as shown in this figure. As shown in  FIG. 6 , the triangle shape is formed by a first point of intersection P 3  and a second point of intersection P 4 , respectively positioned where the slit S intersects with the boundary B 1  between the non-seal area E 2  and the projecting area E 3 , and the apex P 2  of the projecting area E 3  positioned on the electric storage element  30  side of the first and second points of intersection P 3 , P 4 . 
     Also, with respect to the projecting area E 3 , the distance D 1  between the slit S and its apex P 2 , and its maximum width D 3  in the X direction, may each independently be set to, for example, anywhere between around several millimeters and several tens of millimeters (e.g., 3 mm to 30 mm, in some embodiments, D 3 &gt;D 1 ), depending on the size of the electric storage cell  10  (e.g., the dimensional ratios illustrated in  FIG. 5  ±50%). 
     The shape of the projecting area E 3  pertaining to this embodiment is not limited to triangle as shown in  FIG. 6 , and it may be rectangle, semi-circle, or the like. 
     The position where the projecting area E 3  is formed is not limited to the position shown in  FIG. 5 . For example, the projecting area E 3  may project toward the electric storage element  30  from a part of the seal area E 1  running orthogonal to the longitudinal direction of the part of the seal area E 1  where the positive electrode terminal  40  and negative electrode terminal  50  are provided (refer to  FIG. 8 ). Or, it may project toward the electric storage element  30  from a part of the seal area E 1  running parallel with the longitudinal direction of the part of the seal area E 1  where the positive electrode terminal  40  and negative electrode terminal  50  are provided (refer to  FIG. 9 ). 
     [Configuration of Slit] 
       FIG. 7  is a cross sectional view of the covering film  20  including the slit S. As shown in  FIG. 7 , the slit S is formed from the surface  27   a  of the external resin layer  27  to midway through the layer. This way, the external resin layer  27  is partially separated by the slit S. 
     Preferably the depth D 4  of the slit S is such that the metal layer  25  prevents moisture permeation in a normal state, but in the event of abnormality, the metal layer  25  would rupture quickly. To be specific, the depth may be 0 μm or more but no more than 5 μm, for example, measured as the distance D 5  from the bottom P 1  of the slit S in the external resin layer  27  to the second principle face  25   b . It should be noted that the distance D 5  is not limited to 0 μm or more but no more than 5 μm, and it may be 0 μm or more but no more than 15 μm, for example. 
     The slit S intersects with the boundary B 1  between the projecting area E 3  and the non-seal area E 2 , as shown in  FIGS. 5 and 6 . To be specific, the slit S is formed in a manner starting from the non-seal area E 2  and traversing the projecting area E 3  to reach the non-seal area E 2 , as shown in these figures. This way, the projecting area E 3  is divided by the slit S. The longitudinal-direction distance (length) of the slit S may be set to around several tens of millimeters, for example. 
     In this embodiment, as long as the slit S intersects with the boundary B 1  between the projecting area E 3  and the non-seal area E 2 , its extending direction is not limited in any way; however, preferably it is formed in parallel with the periphery of the seal area E 1 . This way, the internal resin layer  26  can expand and rupture easily from the slit S in the event of abnormality of the electric storage cell  10 , which in turn allows for lowering of the release pressure at which the internal pressure of the electric storage cell  10  is released. 
       FIGS. 8 and 9  are each a schematic view showing the position where the slit S and projecting area E 3  are formed. The slit S in this embodiment may be orthogonal to the longitudinal direction of the seal area E 1  where the positive electrode terminal  40  and negative electrode terminal  50  are formed, as shown in  FIG. 8 , or it may be parallel with this longitudinal direction, as shown in  FIG. 9 . 
     [Operations of Slit and Projecting Area] 
     While the electric storage cell  10  is in use, the covering film  20  remains in the state shown in  FIGS. 4 and 5  in a normal state (when no abnormality is present in the electric storage cell  30 ), or specifically when the internal pressure of the housing space R is within the allowable range. In this state, the slit S does not separate the metal layer  25 , and therefore the metal layer  25  prevents moisture from permeating through the covering film  20 . 
     On the other hand, if an abnormality occurs in the electric storage cell  10  while the electric storage cell  10  is in use and its internal pressure rises as a result, the covering film  20  expands. And, once the internal pressure reaches or exceeds a certain level, the covering film  20  breaks open at the part where the slit S is formed. As a result, the internal pressure of the housing space R is released. 
     This means that, in this embodiment, the formation of the slit S in the covering film  20  allows the position at which the covering film  20  would break open, to be specified beforehand. If no slit S is provided, the weakest part of the covering film package, or specifically the seal area E 1 , will break open and the internal pressure will be released. In this case, there is no way of knowing which part of the seal area E 1  formed over the entire periphery of the electric storage element  30 , will break open. 
     Also, the release of the internal pressure in the event of abnormality occurs as a result of the covering film  20  breaking open, as described above. In other words, the release pressure at which the internal pressure of the electric storage cell  10  is released can be adjusted by the strength of the covering film  20 . 
     The strength of the covering film  20  can be adjusted by the thickness of the covering film  20 , for example. In this case, the strength of the covering film  20  can be adjusted by the overall thickness of the covering film  20  including its metal layer  25 , internal resin layer  26 , and external resin layer  27 . At any rate, it is sufficient that the internal pressure at which the covering film  20  breaks open at the slit S is lower than the internal pressure at which the seal area E 1  breaks open. 
     In addition, it is also possible, in this embodiment, to adjust the release pressure at which the internal pressure of the housing space R is released after having risen due to an abnormality of the electric storage cell  10 , by the position where the slit S is formed. 
     To be more specific, the electric storage cell  10  in this embodiment has a projecting area E 3  that corresponds to where the seal area E 1  penetrates into the non-seal area E 2 , as shown in  FIGS. 5 and 6 . This means that, with the electric storage cell  10 , when the internal pressure rises due to an abnormality, the covering film  20  expands, which causes the stress that tries to separate the covering films  20  from each other (hereinafter referred to as “stress”) to concentrate at the apex P 2  of the projecting area E 3  before it concentrates at the boundary B 2 . 
     Then, as the internal pressure rises further, the stress continues to concentrate at the projecting area E 3  and the separation of the projecting area E 3  starting from the apex P 2  of the projecting area E 3  progresses, and consequently the stress propagates to the slit S. It should be noted that, in this embodiment, “separation of the projecting area E 3 ” refers to a condition where, with respect to the covering films  20  that are thermally fused with each other and thus constituting the projecting area E 3 , one covering film  20  separates from the other covering film  20 , and the same applies in the following explanations. 
     Then, this stress that has propagated to the slit S causes the covering film  20  to break open through the slit S formed at the projecting area E 3  and the internal pressure of the housing space R is released. Next, as the releasing of the internal pressure of the housing space R progresses, the covering film  20  breaks open through the slit S formed in the non-seal area E 2 . In other words, the covering film  20  breaks open over the entire location where the slit S is formed. As a result, the rising internal pressure of the electric storage cell  10  is released over a short period of time. 
     In this embodiment, the stress that generated due to an abnormality of the electric storage cell  10  propagates to the projecting area E 3  first, and this stress becomes a stress that tries to separate the projecting area E 3 . Then, as this stress propagates to the slit S, the covering film  20  breaks open through the slit S and the internal pressure of the housing space R is released. This means that, in this embodiment, the release pressure of the electric storage cell  10  can be adjusted by means of adjusting the forming position of the slit S within the range where it intersects with the projecting area E 3 . 
     In particular, the projecting area E 3  pertaining to this embodiment has a triangle shape formed by the first and second points of intersection P 3 , P 4  and the apex P 2 , as shown in  FIG. 6 . This way, the location where the stress concentrates first, before it does at the boundary B 2 , is limited to the apex P 2  of the projecting area E 3 . 
     Accordingly, it is possible, in this embodiment, to control the release pressure of the electric storage cell  10  to a desired level by adjusting the distance D 1  between the apex P 2  of the projecting area E 3  and the slit S and/or the distance D 2  between the boundary B 2  and the slit S. 
     For example, making the distance D 1  longer than the distance D 2  increases the separation area of the projecting area E 3  needed before the stress propagates to the slit S. This means that the stress needed before it propagates to the slit S becomes higher, and the release pressure of the electric storage cell  10  becomes higher as a result. 
     On the other hand, making the distance D 1  shorter than the distance D 2  decreases the separation area of the projecting area E 3  needed before the stress propagates to the slit S. This means that the stress needed before it propagates to the slit S becomes lower, and the release pressure of the electric storage cell  10  becomes lower as a result. 
     Also, with the electric storage cell  10  pertaining to this embodiment, the internal pressure of the housing space R is released through the slit S before the stress concentrates at the boundary B 2 , as described above. This way, the release pressure at which the internal pressure is released after having risen due to an abnormality can be reduced to levels lower than heretofore possible with conventional electric storage cells. To be specific, the release pressure can be reduced to approx. 0.05 MPa with the electric storage cell  10 . 
     Also, with the electric storage cell  10  pertaining to this embodiment, the adjustability of the release pressure in the event of abnormality prevents the release pressure from becoming higher than a desired level even when the thickness of the electric storage cell  10  is relatively small. This way, release of internal pressure from a location other than the location where the slit S is formed, can be prevented. 
     Also, in this embodiment, the release pressure of the electric storage cell  10  can be adjusted to a desired level by adjusting the distance D 1  between the apex P 2  and the slit S and/or the distance D 2  between the boundary B 2  and the slit S. This means that, in this embodiment, the depth D 4  of the slit S does not have much bearing on the setting of the release pressure of the electric storage cell  10 . As a result, the processing accuracy of the slit S can be relaxed more than what has been heretofore permitted and the productivity of electric storage cells  10  can be improved. 
     To be specific, it is sufficient for the depth D 4  of the slit S in this embodiment to be such that the distance D 5  between the bottom P 1  of the slit S and the second principle face  25   b  of the metal layer  25  becomes 0 μm or more but no more than 15 μm, and it need not reach the metal layer  25 . This way, corrosion of the metal layer  25  is prevented even when the electric storage cell  10  is used in a corrosive environment. 
     [Electric Storage Module] 
     An electric storage module can be constituted by stacking multiple electric storage cells  10  per this embodiment on top of each other.  FIG. 10  is a schematic view of an electric storage module  100 . The electric storage module  100  has multiple electric storage cells  10 , heat conductive sheets  101 , plates  102 , and support members  103 , as shown in this figure. 
     The multiple electric storage cells  10  are stacked together with the heat conductive sheets  101  in between, and supported by the support members  103 . The number of electric storage cells  10  may be two or more. The positive electrode terminals  40  and negative electrode terminals  50  of the electric storage cells  10  may be connected between the electric storage cells  10  via wiring or terminals that are not illustrated. Also, plates  102  are stacked at the top face and bottom face of the multiple electric storage cells  10 . 
     As shown in  FIG. 10 , the electric storage module  100  has each slit S formed in each non-seal area E 2 , and therefore expansion of each internal resin layer  26  is not disrupted by each plate  102  and consequently the internal pressure can be released at the specified pressure. 
     Also, as shown in  FIG. 10 , the electric storage module  100  pertaining to this embodiment is such that each slit S is formed in a location of the contact areas  20   a  of each electric storage cell  10 , where the contact areas  20   a  of the adjacent electric storage cells  10  face each other. 
     This means that, by providing a leakage-countermeasure component (sponge or other absorbent member) in the aforementioned location, this leakage-countermeasure component applied commonly to the adjacent electric storage cells  10  can be used (e.g., by shortening a part  101   a  of the heat conductive sheet in  FIG. 10  to make a common space for accommodating the common leakage-coping component), in the event that the rising internal pressure of the electric storage cell  10  due to its abnormality causes the electrolyte to leak from the slit S, to absorb the electrolyte. 
     If slits S are formed near the back-to-back connection part of the adjacent electric storage cells  10 , a leakage-countermeasure must be provided for each cell; if the slits S are formed in the same direction, a leakage-countermeasure component must be provided for each cell based on a different structure. 
     This means that, by providing slits S in the aforementioned locations, an electric storage module  100  that can address leakage of electrolyte from the slits S, should it occur, without complicating the apparatus configuration and also at low cost, can be provided. 
     [Variation Examples] 
       FIG. 11  is a cross sectional view of the covering films  20  pertaining to a variation example, while  FIGS. 12 and 13  are each an enlarged view, from the Z direction, of the projecting area E 3  pertaining to a variation example. Although the electric storage cell  10  in the aforementioned embodiment is such that the housing space R is sealed by the covering film package constituted by the two covering films  20 , this is not always the case. As shown in  FIG. 11 , the electric storage cell  10  may be constituted in such a way that the housing space R is sealed by a covering film package which is formed by bending one covering film  20  around an electric storage element  30  and then sealing the three sides. 
     Also, while the slit S in the aforementioned embodiment has two points of intersection with the boundary B 1 , this is not always the case and, as shown in  FIG. 12 , it may have only one point of intersection with the boundary B 1 . 
     Furthermore, while there is one slit S in the aforementioned embodiment, this is not always the case and, as shown in  FIG. 13 , multiple slits S may be provided in a manner intersecting with multiple boundaries B 1 . This improves the certainty that the internal pressure of the electric storage cell  10  that has risen due to its abnormality will be released through the slit S. 
     In the present disclosure where conditions and/or structures are not specified, a skilled artisan in the art can readily provide such conditions and/or structures, in view of the present disclosure, as a matter of routine experimentation. Also, in the present disclosure including the examples described above, any ranges applied in some embodiments may include or exclude the lower and/or upper endpoints, and any values of variables indicated may refer to precise values or approximate values and include equivalents, and may refer to average, median, representative, majority, etc. in some embodiments. Further, in this disclosure, “a” may refer to a species or a genus including multiple species, and “the invention” or “the present invention” may refer to at least one of the embodiments or aspects explicitly, necessarily, or inherently disclosed herein. The terms “constituted by” and “having” refer independently to “typically or broadly comprising”, “comprising”, “consisting essentially of”, or “consisting of” in some embodiments. In this disclosure, any defined meanings do not necessarily exclude ordinary and customary meanings in some embodiments. 
     The present application claims priority to Japanese Patent Application No. 2016-159304, filed Aug. 15, 2016, the disclosure of which is incorporated herein by reference in its entirety including any and all particular combinations of the features disclosed therein. 
     It will be understood by those of skill in the art that numerous and various modifications can be made without departing from the spirit of the present invention. Therefore, it should be clearly understood that the forms of the present invention are illustrative only and are not intended to limit the scope of the present invention.