Abstract:
An electric storage cell has an electric storage element and a covering body. The covering body houses the electric storage element and has a metal layer with a first principal face on the electric storage element side and a second principal face on the opposite side, an internal resin layer made of resin laminated onto the first principal face, and an external resin layer made of resin laminated onto the second principal face, with a groove part formed on the second principal face side of the metal layer. The above structures provides a highly reliable electric storage cell, covering film, and electric storage module capable of releasing the rising internal pressure safely in the event of abnormality.

Description:
BACKGROUND 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an electric storage cell comprising an electric storage element sealed by a covering film, to a covering film, and also to an electric storage module comprising a stack of such electric storage cells. 
         [0003]    2. Description of the Related Art 
         [0004]    Film-covered electric storage cells, each comprising an electric storage element sealed by a covering film, are widely used in recent years. Film-covered electric storage cells are subject to rising pressure in the electric storage cell due to generation of gaseous species as a result of electrolysis of the electrolytic medium during use, if the control circuit of the electric storage cell fails for some reason and abnormal voltage is applied or the ambient temperature rises to an abnormally high level for some reason. As the internal pressure of the film-covered electric storage cell rises, eventually its covering material ruptures and gas erupts from the ruptured location; since it is impossible to predict where the covering material will rupture, however, the surrounding equipment, etc., may be negatively affected depending on the ruptured location. 
         [0005]    To solve this problem, Patent Literature 1, for example, discloses a constitution where a peninsula-shaped projecting fusion part is provided in the seal part of the covering film and a through hole is formed in the projecting fusion part in order to release the pressure in case the covering film expands and its separation progresses. This way, the tear-off stress generated by the expansion of the film can be concentrated to the projecting fusion part to facilitate the progression of separation, so that the pressure is easily released as the film expands. 
       BACKGROUND ART LITERATURES 
       [0006]    [Patent Literature 1] Japanese Patent Laid-open No. 2005-203262 
       SUMMARY 
       [0007]    With the constitution of Patent Literature 1, however, the through hole and the narrow seal width of the projecting fusion part may result in moisture content permeating through the fusion resin layer to affect long-term reliability. Also, there are constitutions other than the one described in Patent Literature 1, where the expanded covering film is penetrated and broken by a projecting object to release the internal pressure; however, these constitutions add to cost because a projecting part must be attached for each device, for example. Handling of the device also requires caution because of the projecting part. 
         [0008]    In light of the aforementioned circumstance, an object of the present invention is to provide a highly reliable electric storage cell, covering film, and electric storage module capable of safely releasing the rising internal pressure in the event of abnormality. 
         [0009]    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. 
         [0010]    To achieve the above object, an electric storage cell pertaining to an embodiment of the present invention has an electric storage element and a covering body. 
         [0011]    The covering body houses the electric storage element and has a metal layer with a first principal face on the electric storage element side and a second principal face on the opposite side, an internal resin layer made of resin laminated onto the first principal face, and an external resin layer made of resin laminated onto the second principal face, with a groove part formed on the second principal face side of the metal layer. The groove part may be formed in the second principal face of the metal layer through the external resin layer which may be the outermost layer. 
         [0012]    According to this constitution, rise in the internal pressure of the electric storage cell due to abnormality of the electric storage cell causes the metal layer to rupture at the location where the groove is formed, and the internal resin layer expands through the resulting opening. As the internal pressure rises further, the internal resin layer ruptures and the internal pressure is released. In other words, the internal pressure is released at the location where the groove part is formed and therefore pressure release from parts other than the groove part can be prevented. Also, in a normal state (when no abnormality is present in the electric storage cell), permeation of moisture content into the housing space is prevented by the metal layer, which allows for ensuring of reliability of the electric storage cell. 
         [0013]    The covering body may have a first region where the internal resin layer has a first thickness and a second region where the internal resin layer has a second thickness smaller than the first thickness, and the groove part may be formed in the second region. The first region and the second region are defined in along a thickness direction. 
         [0014]    According to this constitution, the second region of the smaller thickness is formed at the location where the groove part is present, and therefore the internal pressure can be released at low pressure regardless of the first thickness of the internal resin layer. 
         [0015]    The internal resin layer may contain resin whose melting point is lower than that of the resin constituting the external resin layer. 
         [0016]    According to this constitution, the internal resin layer alone can be thermally processed by pressing the covering film with a heater whose temperature is set higher than the melting point of the resin constituting the internal resin layer but lower than the melting point of the resin constituting the external resin layer, which allows for easy formation of the second region of the internal resin layer. 
         [0017]    The internal resin layer may be made of non-oriented polypropylene, while the external resin layer may include a first external resin layer made of nylon and laminated onto the second principal face, as well as a second external resin layer made of polyethylene terephthalate and laminated onto the first external resin layer. 
         [0018]    According to this constitution, the second region can be formed easily on the internal resin layer made of non-oriented polypropylene, by means of pressing with the heater, because the melting point of non-oriented polypropylene is lower than the melting point of polyethylene terephthalate or that of nylon. 
         [0019]    The groove part may continue to the external resin layer. 
         [0020]    According to this constitution, the metal layer can be ruptured quickly in the event of abnormality and the rising internal pressure of the electric storage cell can be released more efficiently. 
         [0021]    The groove part may be partially or entirely filled with an insulating material. 
         [0022]    This way, the strength of the external resin layer improves to prevent the covering film from rupturing in a normal state. 
         [0023]    The insulating material may be resin. 
         [0024]    For the insulating material to be filled in the groove part, any of the various insulating materials such as resin, paper, and glass can be used, but resin is preferred from the viewpoint of sealing property. 
         [0025]    The difference between the second thickness and the first thickness may be 20 μm or more to 50 μm or less. 
         [0026]    By making the second thickness of the internal resin layer smaller than the first thickness by 20 μm or more to 50 μm or less, rupturing of the internal resin layer can be easily facilitated in the second region while still maintaining the strength of the internal resin layer in the first region. 
         [0027]    The metal layer may be an aluminum foil having a thickness of 25 μm or more to 80 μm or less, while the groove part may have a depth, in the metal layer, corresponding to one-tenth or more to two-thirds or less of the thickness of the metal layer. 
         [0028]    According to this constitution, the metal layer is kept from rupturing when the electric storage cell is in a normal state and permeation of moisture content from the outside can be prevented as a result, by keeping the depth of the groove part in the metal layer to one-tenth or more to two-thirds or less of the thickness of the metal layer. In addition, the release pressure at the groove part can be reduced because the metal layer ruptures quickly in the event of abnormality. 
         [0029]    The groove part may have a depth, in the metal layer, corresponding to one-third or more to one-half or less of the thickness of the metal layer. 
         [0030]    This way, not only is the metal layer kept from rupturing when the electric storage cell is in a normal state and permeation of moisture content from the outside can be prevented as a result, but the metal layer can also be ruptured quickly in the event of abnormality to release the rising internal pressure of the electric storage cell in a safer manner. 
         [0031]    The covering body may have an element housing part that forms a flat surface facing the electric storage element, a seal part formed by internal resin layers thermally fusing with each other along the peripheral edge of the element housing part, and an intermediate part between the element housing part and seal part, and the groove part may be formed in the intermediate part. 
         [0032]    When these electric storage cells are stacked to form an electric storage module, each element housing part contacts the adjacent electric storage cell and other members. For this reason, if the groove part is formed in the element housing part, expansion of the internal resin layer at the groove part is prevented by other members. According to the above constitution, however, the groove part is provided at a position where it does not contact other members and therefore expansion of the internal resin layer at the groove part is not prevented by other members and the release pressure at the groove part can be reduced. 
         [0033]    The element housing part may be formed by means of embossing. 
         [0034]    The covering body may be such that a flexible covering film is used to clad the electric storage element and form the element housing part in the shape of the electric storage element. In addition, the covering body may be such that the element housing part is formed beforehand by means of embossing. 
         [0035]    The groove part may be formed in parallel with the peripheral edge of the seal part. 
         [0036]    According to this constitution, expansion and rupturing of the internal resin layer at the groove part can be facilitated to reduce the release pressure at the groove part. 
         [0037]    To achieve the aforementioned object, a covering film pertaining to an embodiment of the present invention represents a covering film for forming a housing space in which to house an electric storage element, wherein such covering film has a metal layer with a first principal face on the electric storage element side and a second principal face on the opposite side, an internal resin layer made of resin laminated onto the first principal face, and an external resin layer made of resin laminated onto the second principal face, with a groove part formed on the second principal face side of the metal layer. 
         [0038]    By cladding the electric storage element with the covering film having the above constitution, a highly reliable electric storage cell can be produced, which is capable of releasing the rising internal pressure safely in the event of abnormality. 
         [0039]    The groove part may continue to the external resin layer. 
         [0040]    According to this constitution, the metal layer can be ruptured quickly in the event of abnormality and the rising internal pressure of the electric storage cell can be released more efficiently. 
         [0041]    The groove part may be partially or entirely filled with an insulating material. 
         [0042]    This way, the strength of the external resin layer improves to prevent the covering film from rupturing in a normal state. 
         [0043]    The insulating material may be resin. 
         [0044]    For the insulating material to be filled in the groove part, any of the various insulating materials such as resin, paper, and glass can be used, but resin is preferred from the viewpoint of sealing property. 
         [0045]    To achieve the aforementioned object, an electric storage module pertaining to an embodiment of the present invention represents an electric storage module comprising multiple electric storage cells stacked together. 
         [0046]    The electric storage cells each have an electric storage element and a covering body. 
         [0047]    The covering body houses the electric storage element and has a metal layer with a first principal face on the electric storage element side and a second principal face on the opposite side, an internal resin layer made of resin laminated onto the first principal face, and an external resin layer made of resin laminated onto the second principal face, as well as an element housing part that houses the electric storage element with the internal resin layer on the electric storage element side and forms a flat surface facing the electric storage element, a seal part formed by internal resin layers thermally fusing with each other along the peripheral edge of the element housing part, and an intermediate part between the housing part and seal part, with a groove part formed on the second principal face side of the metal layer. 
         [0048]    According to this constitution, the groove part is provided in the intermediate part of the covering body, or specifically at a position where it does not contact other members, and therefore expansion of the internal resin layer at the groove part is not prevented by other members and the release pressure at the groove part can be reduced. 
         [0049]    As described above, according to the present invention a highly reliable electric storage cell, covering film, and electric storage module can be provided, which are capable of releasing the rising internal pressure safely in the event of abnormality. 
         [0050]    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. 
         [0051]    Further aspects, features and advantages of this invention will become apparent from the detailed description which follows. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0052]    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. 
           [0053]      FIG. 1  is a perspective view of an electric storage cell pertaining to an embodiment of the present invention. 
           [0054]      FIG. 2  is a section view of the same electric storage cell. 
           [0055]      FIG. 3  is a plan view of the same electric storage cell. 
           [0056]      FIG. 4  is a section view of the covering film provided in the electric storage cell. 
           [0057]      FIG. 5  is a section view of the covering film provided in the electric storage cell. 
           [0058]      FIG. 6  is a section view of the covering film provided in the electric storage cell. 
           [0059]      FIG. 7  is a section view of the same electric storage cell. 
           [0060]      FIG. 8  is a plan view of the same electric storage cell. 
           [0061]      FIG. 9  is a plan view of the same electric storage cell. 
           [0062]      FIG. 10  is a plan view of the same electric storage cell. 
           [0063]      FIG. 11  shows a schematic drawing of an electric storage module pertaining to an embodiment of the present invention. 
           [0064]      FIG. 12  is a section view of an electric storage cell pertaining to an example of variation of the present invention. 
           [0065]      FIG. 13  is a section view of a covering film pertaining to another example of variation of the present invention. 
           [0066]      FIG. 14  shows a schematic drawing of a groove-part processing machine that can be utilized in manufacturing the covering film provided in an electric storage cell pertaining to an embodiment of the present invention. 
           [0067]      FIGS. 15A, 15B, and 15C  show schematic drawings showing the forming process of the groove part of the covering film provided in the electric storage cell. 
           [0068]      FIGS. 16A and 16B  each show schematic drawings of a sealer that can be utilized in manufacturing the covering film provided in the electric storage cell. 
           [0069]      FIGS. 17A and 17B  show schematic drawings showing the forming process of the thin part of the covering film provided in the electric storage cell. 
           [0070]      FIG. 18  shows a schematic drawing showing the inspection process of the groove part of the covering film provided in the electric storage cell. 
           [0071]      FIG. 19  shows a schematic drawing of an electric storage cell pertaining to an example of the present invention. 
           [0072]      FIG. 20  is a table showing the results of the electric storage cell pressurization test in the example of the present invention. 
       
    
    
     DESCRIPTION OF THE SYMBOLS 
       [0000]    
       
         
           
               10  - - - Electric storage cell 
               20  - - - Covering film 
               20   a  - - - Seal part 
               20   b  - - - Element housing part 
               20   c  - - - Intermediate part 
               25  - - - Metal layer 
               25   a  - - - First principal face 
               25   b  - - - Second principal face 
               26  - - - Internal resin layer 
               27  - - - External resin layer 
               30  - - - Electric storage element 
               100  - - - Electric storage module 
             S - - - Groove part 
           
         
       
     
       DETAILED DESCRIPTION OF EMBODIMENTS 
       [0086]    An embodiment of the present invention is explained below by referring to the drawings. 
         [0087]    [Structure of Electric Storage Cell] 
         [0088]      FIG. 1  is a perspective view of an electric storage cell  10  pertaining to this embodiment, while  FIG. 2  is a section view of the electric storage cell  10  cut along line D-D in  FIG. 1 . In the drawings mentioned below, the X direction, Y direction, and Z direction represent the three directions that intersect at right angles. 
         [0089]    As shown in  FIG. 1  and  FIG. 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 . 
         [0090]    In the electric storage cell  10 , the covering body comprising 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 along the peripheral edge of the electric storage element  30 , and the covering body has a seal part  20   a . The seal part  20   a  is described later. 
         [0091]    The electric storage element  30 , as shown in  FIG. 2 , has a positive electrode  31 , negative electrode  32 , and separator  33 . The positive electrode  31  and negative electrode  32  are housed in the housing space R in a manner facing each other via the separator  33 . 
         [0092]    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 containing a positive electrode active material, binder, etc. The positive electrode active material may be active carbon, for example. Any desired positive electrode active material is selected as deemed appropriate according to the type of the electric storage cell  10 . 
         [0093]    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 containing a negative electrode active material, binder, etc. The negative electrode active material may be graphite, hard carbon, or other carbon-group material, for example. A desired negative electrode active material is selected as deemed appropriate according to the type of the electric storage cell  10 . 
         [0094]    The separator  33  is placed between the positive electrode  31  and negative electrode  32  and allows the electrolytic solution to pass through it, while also preventing the contact between (insulating) the positive electrode  31  and negative electrode  32 . The separator  33  may be made of woven fabric, non-woven fabric, resin microporous film, etc. 
         [0095]    In  FIG. 2 , one positive electrode  31  and one negative electrode  32  are provided; however, multiple sets of positive and negative electrodes can also be provided. In this case, the multiple positive electrodes  31  and negative electrodes  32  may be stacked together alternately via the separators  33 . Also, the electric storage element  30  may be a laminate of a positive electrode  31 , negative electrode  32 , and separator  33  that has been rolled up. 
         [0096]    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. Electrolytic solution is housed in the housing space R together with the electric storage element  30 . This electrolytic solution may be a propylene carbonate (PC) solution of lithium hexafluorophosphate (LiPF 6 ), etc., for example, but any desired solution can be selected according to the type of the electric storage element  30 . 
         [0097]    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  and led out, in the seal part  20   a , from the inside to the outside of the housing space R between the two covering films  20 . The positive electrode terminal  40  may be foil, wiring material, etc., made of conductive material. 
         [0098]    The negative electrode terminal  50  is an external terminal of the negative electrode  32 . As shown in  FIG. 2 , the negative electrode terminal  50  is electrically connected to the negative electrode  32  via negative electrode wiring  51  and led out, in the seal part  20   a , from the inside to the outside of the housing space R between the two covering films  20 . The negative electrode terminal  50  may be foil, wiring material, etc., made of conductive material. 
         [0099]      FIG. 3  is a schematic drawing of the electric storage cell  10  as viewed from the Z direction. As shown in this figure, the seal part  20   a  is formed along the peripheral edge of the electric storage element  30 . The seal part  20   a  is formed by the covering films  20  thermally fused to each other, and seals the housing space R. The width of the seal part  20   a  may be around several millimeters to several tens of millimeters, for example. 
         [0100]    [Constitution of Covering Film] 
         [0101]      FIG. 4  is a section view of the covering film  20 . As shown in this figure, the covering film  20  comprises a metal layer  25 , internal resin layer  26 , and external resin layer  27 . 
         [0102]    The metal layer  25  is a layer made of foil-shaped metal, and functions to prevent the moisture content in air from permeating through. The metal layer  25 , as shown in  FIG. 4 , has a first principal face  25   a  and a second principal face  25   b  on the opposite side. 
         [0103]    The metal layer  25  may be metal foil made of aluminum, for example. In addition, the metal layer  25  may be foil of copper, nickel, stainless steel, etc. Preferably the thickness of the metal layer  25  is 25 μm or more to 80 μm or less. 
         [0104]    The internal resin layer  26  is laminated onto the first principal face  25   a  to constitute the inner periphery face of the housing space R and clad and insulate the metal layer  25 . 
         [0105]    The internal resin layer  26  is made of resin, such as non-oriented polypropylene (CPP), for example. In addition, the internal resin layer  26  may be made of polyethylene or acid-modified product thereof, polyphenylene sulfide, polyethylene terephthalate, polyamide, ethylene-vinyl acetate copolymer, etc. The internal resin layer  26  may also comprise multiple resin layers that are stacked together. It should be noted that preferably the material constituting the internal resin layer  26  is one whose melting point is lower than that of the material constituting the external resin layer  27 . 
         [0106]    The external resin layer  27  is laminated onto the second principal face  25   b  to constitute the surface of the electric storage cell  10  and clad and protect the metal layer  25 . 
         [0107]    The external resin layer  27  may be made of two resin layers. As shown in  FIG. 4 , the external resin layer  27  comprises a first external resin layer  271  and second external resin layer  272  that are stacked together. The external resin layer  27  may also comprise one resin layer. 
         [0108]    The first external resin layer  271  may be made of nylon, while the second external resin layer  272  may be made of polyethylene terephthalate. In addition, the first external resin layer  271  and second external resin layer  272  may be made of other materials such as polyethylene naphthalate, biaxially-oriented polypropylene, polyimide, and polycarbonate. It should be noted that preferably the material for the external resin layer  27  (first external resin layer  271  and second external resin layer  272 ) is one whose melting point is higher than the material constituting the internal resin layer  26 . 
         [0109]    The covering body comprising the two covering films  20  having the aforementioned constitution, which are facing each other via the electric storage element  30  and sealed at the seal part  20   a , forms the housing space R. The internal resin layers  26  of the two covering films  20  are thermally fused to each other at the seal part  20   a . The covering films  20  are placed in such a way that their internal resin layer  26  comes to the housing space R side (inside) and the external resin layer  27  comes to the surface side (outside). 
         [0110]    The covering films  20  may be used in a manner exhibiting flexibility and forming a shape with curved peripheral edges, as shown in  FIG. 2 , according to the shape of the electric storage element  30 . The covering films  20  may also be used in a manner forming the same shape by means of prior embossing. A groove part is formed in one of the two covering films  20 . 
         [0111]    [Groove Part] 
         [0112]      FIG. 5  is a section view of the covering film  20  including the groove part S. The groove part S, as shown in this figure, is formed from the surface of the external resin layer  27  to a middle of the metal layer  25 . The external resin layer  27 , as shown in  FIG. 5 , is completely separated at the location where the groove part S is formed, while the metal layer  25 , as shown in the same figure, is partially separated by the groove part S. 
         [0113]    To be specific, the groove part S may have, in the metal layer  25 , a depth between the first principal face  25   a  and second principal face  25   b . As shown in  FIG. 5 , the thickness F of the groove part S in the metal layer  25  is preferably one-tenth or more of the thickness T of the metal layer  25  (T/10) to two-thirds or less of the T (2T/3), or more preferably one-third or more of the T (T/3) to one-half or less of the T (T/2). 
         [0114]    In addition, the covering film  20  may have a region on the internal resin layer  26  where a thin part is formed, and the groove part S may be formed in this region.  FIG. 6  is a section view of a covering film  20  whose internal resin layer  26  has a thin part. 
         [0115]    As shown in this figure, the covering film  20  may have a first region A 1  where the internal resin layer  26  has a first thickness L 1 , and a second region A 2  where the internal resin layer  26  has a second thickness L 2  smaller than the first thickness L 1 . Of the internal resin layer  26 , the part having the second thickness L 2  is hereinafter referred to as “thin part  26   a .” It should be noted that, although the first thickness L 1  and second thickness L 2  are not limited in any way, preferably the first thickness L 1  and second thickness L 2  are such that their difference becomes 20 μm or more to 50 μm or less. 
         [0116]    The groove part S, as shown in  FIG. 6 , may be formed in the second region A 2 . To be specific, the groove part S is formed from the external resin layer  27  in a manner facing the thin part  26   a  via the metal layer  25 . The groove part S may be facing the thin part  26   a  only partially. 
         [0117]    [Effects of Groove Part] 
         [0118]    While the electric storage cell  10  is in use, the covering film  20  maintains the condition shown in  FIG. 5  or  FIG. 6  in a normal state (no abnormality is present in the electric storage element  30 ) or specifically when the internal pressure of the housing space R is within an allowable range. In this condition, the groove part S does not separate the metal layer  25  completely and therefore permeation of moisture content through the covering film  20  is prevented by the metal layer  25 . 
         [0119]    If abnormality occurs in the electric storage element  30  while the electric storage cell  10  is in use and the internal pressure rises as a result, the covering film  20  expands. This causes the metal layer  25  to rupture where the groove part S is formed. Next, the internal resin layer  26  projects partially to the outside of the covering film  20  through the opening of the ruptured metal layer  25 , and expands. Then, when the internal pressure reaches or exceeds a certain level, the internal resin layer  26  that has projected to the outside ruptures and the internal pressure of the housing space R is released. 
         [0120]    As the groove part S is formed this way, the position where the internal resin layer  26  will rupture can be specified beforehand. If the groove part S is not provided, the seal part  20   a , which is the weakest part of the covering body, will break open and the internal pressure will be released. In this case, a break-open part of the seal part  20   a  formed all around the electric storage element  30  along its peripheral edge cannot be specified beforehand. 
         [0121]    Preferably the depth of the groove part S is such that the metal layer  25  prevents permeation of moisture content in a normal state, while the metal layer  25  ruptures quickly in the event of abnormality. To be specific, this can be achieved by setting the depth F of the groove part S of the metal layer  25  as one-tenth or more of the thickness T of the metal layer  25  (T/10) to two-thirds or less of the T (2T/3). 
         [0122]    Additionally, as mentioned above, the release of internal pressure in the event of abnormality occurs as the internal resin layer  26  ruptures. In other words, the internal pressure at which this release occurs (release pressure) can be adjusted based on the strength of the internal resin layer  26 . The strength of the internal resin layer  26  can be adjusted based on the thickness of the internal resin layer  26  and therefore, as shown in  FIG. 6 , the strength of the internal resin layer  26  can be adjusted by providing the thin part  26   a.    
         [0123]    Also, as shown in  FIG. 5 , the internal resin layer  26  may have a constant thickness, in which case the strength of the internal resin layer  26  can be adjusted based on the overall thickness of the internal resin layer  26 . Either way, it suffices that the internal pressure that causes the internal resin layer  26  to rupture at the groove part S is lower than the internal pressure that causes the seal part  20   a  to break open. 
         [0124]    [Forming Position of Groove Part] 
         [0125]    The groove part S may be provided on either of the covering films  20  other than in the seal part  20   a .  FIG. 7  through  FIG. 10  are schematic drawings showing the forming position of the groove part S, where  FIG. 7  is a section view of the electric storage cell  10 , while  FIG. 8  through  FIG. 10  are plan views of the electric storage cell  10 . 
         [0126]    As shown in  FIG. 7 , the part of the covering body that forms a flat surface facing the electric storage element  30  is defined as an “element housing part  20   b ,” while the part between the element housing part  20   b  and seal part  20   a  is defined as an “intermediate part  20   c.”   
         [0127]    As shown in  FIG. 8 , the groove part S may be formed in the intermediate part  20   c . To be specific, the groove part S, as shown in this figure, may be formed in parallel with the length direction of the closest seal part  20   a  for a length of around several tens of millimeters. 
         [0128]    Also, the groove part S need not be provided in the intermediate part  20   c , but it may be provided on the element housing part  20   b , as shown in  FIG. 9  and  FIG. 10 . The extending direction of the groove part S is not limited in any way, and it may be vertical to the length direction of the seal part  20   a  where the positive electrode terminal  40  and negative electrode terminal  50  are provided, as shown in  FIG. 9 , or parallel with such length direction, as shown in  FIG. 10 . 
         [0129]    [Electric Storage Module] 
         [0130]    An electric storage module may be constituted by stacking together multiple units of electric storage cells  10  according to this embodiment.  FIG. 11  is a schematic drawing of an electric storage module  100 . The electric storage module  100 , as shown in this figure, has multiple electric storage cells  10 , a thermal conduction sheet  101 , plates  102 , and supporting members  103 . 
         [0131]    The multiple electric storage cells  10  are stacked together via the thermal conduction sheet  101  and supported by the supporting 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 interconnected between the electric storage cells  10  by wiring or terminals that are not illustrated. The plates  102  are stacked as the topmost face and bottommost face of the multiple electric storage cells  10 . 
         [0132]    As shown in this figure, stacking the electric storage cells  10  together causes their element housing part  20   b  to contact the thermal conduction sheet  101  or plate  102 . This means that, if the groove part S is formed in the element housing part  20   b , these members prevent the internal resin layer  26  from expanding. If the groove part S is formed in the intermediate part  20   c , on the other hand, expansion of the internal resin layer  26  is not prevented and the internal pressure can be released at the specified pressure. 
         [0133]    [Example of Variation] 
         [0134]      FIG. 12  is a section view showing an electric storage cell  10  pertaining to an example of variation, while  FIG. 13  is a section view of a covering film  20  pertaining to another example of variation. In the aforementioned embodiment, the electric storage cell  10  was such that its covering body comprising two covering films  20  sealed the housing space R, but this is not the only embodiment. As shown in  FIG. 12 , the electric storage cell  10  may be constituted in such a way that one covering film  20  is bent via the electric storage element  30  and its three sides are sealed to form the covering body, which then seals the housing space R. The groove part S, as shown in  FIG. 12 , may be provided in any part of the covering film  20  other than the seal part  20   a.    
         [0135]    In addition, the covering film  20 , as shown in  FIG. 13 , may be constituted in such a way that the groove part S is filled with an insulating material R. In this case, the groove part S is partially or entirely filled with the insulating material R. The insulating material R is not limited in any way and any of the various insulating materials such as resin, paper and glass can be used, but resin is preferred from the viewpoint of sealing property. 
         [0136]    [Manufacturing Method of Covering Film] 
         [0137]    The manufacturing method of the covering film  20  pertaining to this embodiment is explained. It should be noted that the manufacturing method described below is an example and the covering film  20  may be manufactured using a method different from the one shown below. 
         [0138]    &lt;Groove Part Forming Process&gt; 
         [0139]      FIG. 14  is a schematic drawing of a groove-part processing machine  80  used for forming the groove part S. The groove-part processing machine  80 , as shown in  FIG. 14 , has a hone  81  and blade  82 . 
         [0140]    The hone  81  is constituted in such a way that it can be vibrated ultrasonically and is movable in the direction facing the blade  82  (Z direction). The vibrating direction of the hone  81  may be set to the Z direction. The blade  82  is supported in such a way that its tip faces the hone  81 . The shape and size of the blade  82  are not limited in any way, and any shape such as straight line, broken line, wavy line, round, square, etc., and any size matching the size of the covering film  20  may be adopted. 
         [0141]    Desirably the hone  81  and blade  82  are such that any gap formed between the hone  81  and blade  82  becomes 10 μm or less. It should be noted that the groove-part processing machine  80  may be constituted in such a way that the blade  82  is vibrated ultrasonically instead of the hone  81 . 
         [0142]      FIGS. 15A, 15B, and 15C  illustrate schematic drawings showing the forming process of the groove part. As shown in  FIG. 15A , the covering film  20  is set between the blade  82  and hone  81  of the groove-part processing machine  80 . Next, as shown in  FIG. 15B , the covering film  20  is sandwiched between the hone  81  and blade  82  while vibrating the hone  81  ultrasonically in the Z direction. 
         [0143]    At this time, the vibration of the hone  81  in the Z direction propagates to the covering film  20  and, as shown in this figure, the tip of the blade  82  reaches between the first principal face  25   a  and second principal face  25   b  of the metal layer  25 . As a result, the groove part S is formed on the covering film  20 , as shown in  FIG. 15C . 
         [0144]    The groove part S can also be formed by sandwiching the covering film  20  between the blade  82  and hone  81  while vibrating the blade  82  ultrasonically. 
         [0145]    Also, pressing the hone  81  against the covering film  20  without applying ultrasonic vibration results in varying depths of the groove part S depending on the condition of the tip of the blade  82  and the surface condition of the covering film  20 . However, processing without ultrasonic vibration is possible so long as the condition of the tip of the blade  82  and the surface condition of the covering film  20  are controlled. Since the groove part S is processed by only around several tens of μm, inconsistent depths of the groove part S will affect the release pressure at the groove part S. 
         [0146]    &lt;Forming Process of Thin Part&gt; 
         [0147]      FIGS. 16A and 16B  each illustrate a schematic drawing of a sealer  90  used for forming the thin part  26   a , where  FIG. 16A  is a front view and  FIG. 16B  is a side view. The sealer  90  has a heater  91  and another heater  92 . 
         [0148]    The heater  91  heats the covering film  20  by pressing it from the internal resin layer  26  side. This heater  91  is temperature-controllable and has a function to provide a variable heater-pressing thrust force with respect to the covering film  20 . 
         [0149]    The heater  92  heats the covering film  20  by pressing it from the external resin layer  27  side. This heater  92  is temperature-controllable and has a function to provide a variable heater-pressing thrust force with respect to the covering film  20 . 
         [0150]    The heaters  91 ,  92  are controlled at the same temperature. Also, desirably the parallelism of the heaters  91 ,  92  is adjusted to a range of ±0.02 mm in order to ensure a uniform thickness of the thin part  26   a.    
         [0151]      FIGS. 17A and 17B  illustrate schematic drawings showing the forming process of the thin part  26   a . The covering film  20  is set to the sealer  90  as shown in  FIG. 17A . At this time, the film is set in such a way that the internal resin layer  26  comes to the heater  91  side and the external resin layer  27  comes to the heater  92  side. Next, the heaters  91 ,  92  are pressed against the covering film  20  to heat the film. 
         [0152]    Here, the internal resin layer  26  of the covering film  20  pertaining to this embodiment may be constituted by a material whose melting point is lower than the material constituting the external resin layer  27 . This way, the thin part  26   a  is formed only on the internal resin layer  26  side without crushing the groove part S, as shown in  FIG. 17B , by setting the temperature of the heaters  91 ,  92  higher than the melting point of the material constituting the internal resin layer  26  but lower than the melting point of the material constituting the external resin layer  27 . 
         [0153]    The crush amount of the thin part  26   a  can be controlled by adjusting certain parameters such as the temperature, thrust force, and pressing time of the heaters  91 ,  92 . The thin part  26   a  is ideally formed in a manner facing the entire groove part S, but it may be formed in a manner facing a part of the groove part S so long as the release pressure can be ensured. 
         [0154]    &lt;Groove Part Inspection Process&gt; 
         [0155]      FIG. 18  is a schematic drawing of the inspection process of the covering film  20 . The covering film  20  is bent in a cylindrical shape, as shown in  FIG. 18 , after the thin part  26   a  and groove part S have been formed, to inspect the depth of the groove part S. 
         [0156]    To be specific, as shown in  FIG. 18 , the covering film  20  is bent in a cylindrical shape to widen the groove part S, and a backlight is illuminated from the internal resin layer  26  side in a dark room, to check for leakage of light from the external resin layer  27  side. At this time, leakage of light from the groove part S is observed if the metal layer  25  is penetrated or cracked. If the metal layer  25  is free from abnormality, no leakage of light from the groove part S is observed. By deforming the covering film  20  in a cylindrical shape, the groove part S is forcibly opened and observation of leakage of light becomes easy. 
         [0157]    This inspection method not only allows for checking whether or not the metal layer  25  was penetrated when the groove part S was formed, but it also allows for checking whether or not the metal layer  25  cracked due to the effect of ultrasonic vibration when the groove part S was formed. In addition, this inspection method is a non-destructive inspection and thus supports 100% inspection of covering films  20 . 
         [0158]    If penetrant solution is permitted to be contacted, the groove part S of the covering film  20  can also be inspected using penetrant solution. 
         [0159]    [Manufacturing Method of Electric Storage Cell] 
         [0160]    The electric storage cell  10  pertaining to this embodiment can be manufactured by enclosing the electric storage element  30  in the covering film  20  produced according to the aforementioned manufacturing method, filling electrolytic solution, and then sealing the electric storage element with the seal part  20   a . To seal the housing space R with two covering films  20 , a covering film  20  with a groove part S and covering film  20  without a groove part S can be used. To seal the housing space R with one covering film  20 , a covering film  20  with a groove part S can be used. 
       Example 
       [0161]    An example of the present invention is explained below. The electric storage cell explained in the aforementioned embodiment was produced and evaluated.  FIG. 19  is a schematic drawing of the electric storage cell pertaining to the example of the present invention, as viewed from the Z direction. 
         [0162]    [Production of Electric Storage Cell] 
         [0163]    First, an 80-μm thick internal resin layer made of non-oriented polypropylene (CPP) was laminated onto a 40-μm thick metal layer made of aluminum using adhesive. Next, a 15-μm thick first external resin layer made of oriented nylon was laminated onto the side of the metal layer opposite the one on which the internal resin layer was stacked, using adhesive. Then, a 12-μm thick second external resin layer made of polyethylene terephthalate (PET) was laminated on top of the first external resin layer using adhesive, to produce a laminate film of 156 μm in thickness. 
         [0164]    Next, a 50-mm long groove part was formed on the laminate film. The depth of the groove part was such that it penetrated the first external resin layer and second external resin layer and penetrated around one half of the metal layer (20 μm). Next, a thin part was formed on the internal resin layer, to produce a covering film. The thickness of the thin part was set to 30 μm. 
         [0165]    Next, the electric storage element was enclosed by the covering film on which the groove part was formed and the covering film on which no groove part was formed, after which the internal resin layers of both covering films were thermally fused to form a seal part (refer to  FIG. 2 ) and thereby seal the electric storage element, to obtain an electric storage cell. At this time, as shown in  FIG. 19 , the width of the seal part was set to 10 mm on the positive electrode terminal and negative electrode terminal side, and elsewhere the width of the seal part was set to 6 mm. Also, as shown in the same figure, the groove part was formed at a position 13 mm from the edge of the seal part on the positive electrode terminal and negative electrode terminal side. 
         [0166]    Two types of electric storage cells of different sizes were produced. Electric storage cell A had an X-direction width of 122 mm. Electric storage cell B had an X-direction width of 84 mm. Also, a version without thin part on the internal resin layer was produced for both electric storage cell A and electric storage cell B. 
         [0167]    [Electric Storage Cell Pressurization Test] 
         [0168]    The release pressure of each electric storage cell was measured. Measurement was performed with the electric storage cell being sandwiched between plates and not being sandwiched between plates. The plate-sandwiched condition recreates the condition of the electric storage cell in the electric storage module. 
         [0169]    The measured results are shown in  FIG. 20 . “Free” indicates a result of measuring the electric storage cell not being sandwiched between plates, while “Pressed” indicates a result of measuring it while sandwiched between plates. As shown in this figure, the electric storage cells in which a thin part was formed had lower release pressure regardless of the size of the electric storage cell or the condition of the electric storage element. It should be noted that, with either structure, pressure release occurred from the groove part. 
         [0170]    Accordingly, these results confirmed that, by forming a thin part on the internal resin layer, the release pressure by the groove part could be reduced and the safety of the electric storage cell could be improved. 
         [0171]    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. 
         [0172]    The present application claims priority to Japanese Patent Application No. 2015-42848, filed Mar. 4, 2015, and No. 2015-225920, filed Nov. 18, 2015, each disclosure of which is incorporated herein by reference in its entirety including any and all particular combinations of the features disclosed therein. 
         [0173]    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.