Patent Publication Number: US-2013230752-A1

Title: Sealed battery

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims a priority from Japanese Patent Application No. 2012-45765, filed Mar. 1, 2012, which is incorporated herein by reference in its entirety. 
     BACKGROUND 
     1. Technical Field 
     The present invention relates to a sealed battery having a cleavage groove formed on a side of a battery case encapsulating an electrode assembly and electrolyte, the cleavage groove configured to cleave up when the pressure in the battery case exceeds a threshold. 
     2. Description of the Background Art 
     Sealed batteries with a cleavage groove formed on a side of the battery case that is configured to cleave up when the pressure in the battery case exceeds a threshold are known. As disclosed in Japanese Patent No. 4166028, for example, such a sealed battery includes a cleavage groove located on a side of the battery case to intersect a raised ridge (i.e. a ridge line), which is formed when the battery case swells up due to an increase in internal pressure. When the pressure in the battery case exceeds a threshold, the battery case is deformed to cause the cleavage groove to cleave up. This releases gas or the like in the battery case to the outside. 
     SUMMARY 
     If a cleavage groove is provided on a side of the battery case, as disclosed in Japanese Patent No. 4166028, the cleavage groove may cleave up from an impact to the battery case if the battery falls, for example. In such a case, electrolyte in the battery case may leak out. 
     In view of this, the cleavage line formed by the cleavage groove may be shaped such that the groove is unlikely to cleave up when the battery falls, for example. However, if a cleavage line is thus shaped, the cleavage groove may not cleave up even when the pressure in the battery case exceeds a threshold. 
     Further, the cleavage line is preferably shaped such that the cleavage groove opens up as widely as possible when cleaved in order to release gas effectively from within the battery case. However, if the area where a cleavage is generated is increased to form a relatively large opening, some cleaved portions may get in contact with the electrode assembly in the battery case to cause a short circuit, or may damage an exterior cladding film that covers the battery case. 
     In view of this, the cleavage line formed by the cleavage groove may be formed exclusively of a curved line made up of a first curved segment curved to protrude in one direction and a second curved segment curved to protrude in a direction forming an angle of 90 degrees or larger with the direction in which the first curved segment protrudes, the first and second curved segments being connected with each other. A cleavage line thus shaped allows the cleavage groove to cleave up safely and easily in response to a certain pressure in the battery case and, when cleaved, leaves a relatively large opening. Moreover, a cleavage groove forming a cleavage line in such a shape is not likely to cleave up from an impact if the battery falls. 
     However, batteries of different types have battery cases in different sizes and with different plate thicknesses. Consequently, even a cleavage line in the above described shape may not always allow the cleavage groove to cleave up in response to a certain pressure in the battery case. 
     In view of the above, an object of the present invention is to provide a sealed battery having a cleavage groove on a side of the battery case, the cleavage groove formed exclusively of a curved line made up of a first curved segment curved to protrude in one direction and a second curved segment curved to protrude in a direction forming an angle of 90 degrees or larger with the direction in which the first curved segment protrudes, the first and second curved segments being connected with each other, where the cleavage groove is configured to cleave up more reliably in response to a certain pressure in the battery case even with varying size and plate thickness of the battery case. 
     A sealed battery according to an embodiment includes a hollow and cylindrical battery case configured to encapsulate an electrode assembly and electrolyte. A side of the battery case includes a cleavage groove forming a cleavage line intersecting a ridge line which is formed on the side of the battery case when the battery case swells up due to an increase in internal pressure. The cleavage line is formed exclusively of a curved line and formed of a first curved segment curved to protrude in one direction as viewed in a direction normal to the side of the battery case and a second curved segment curved to protrude in a direction forming an angle of 90 degrees or larger with the direction in which the first curved segment protrudes, the first and second curved segments being connected with each other. An end of the first curved segment is connected with an end of the second curved segment. At least one of the first curved segment and the second curved segment intersects the ridge line. The cleavage groove has a depth that produces a ratio of a remaining thickness relative to a plate thickness of the battery case of 75% or smaller. 
     In the sealed battery according to an embodiment, a cleavage groove is formed on a side of the battery case, the cleavage groove formed exclusively of a curved line made up of a first curved segment curved to protrude in one direction and a second curved segment curved to protrude in a direction forming an angle of 90 degrees or larger with the direction in which the first curved segment protrudes, the first and second curved segments being connected with each other, where the depth of the cleavage groove is such that the remaining thickness ratio is 75% or smaller. This will allow the cleavage groove to cleave up in a more reliable manner in response to a certain pressure in the battery case even with varying plate thickness of the battery case. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic perspective view of a sealed battery of an embodiment. 
         FIG. 2  is a cross-sectional view of the battery taken on line II-II of  FIG. 1 . 
         FIG. 3  is a schematic side view of the sealed battery of the embodiment. 
         FIG. 4  is a perspective view illustrating the sealed battery during venting. 
         FIG. 5  is a cross-sectional view of the battery case taken on line V-V of  FIG. 4 . 
         FIG. 6  illustrates part of a calculation model of an S-shaped cleavage groove. 
         FIG. 7  is a cross-sectional view of the battery case taken on line VII-VII of  FIG. 6 . 
         FIG. 8  is a graph showing the remaining thickness of the cleavage groove versus the venting pressure as obtained by calculation and by experiment. 
         FIG. 9  illustrates the remaining thickness ratio versus the venting pressure of the cleavage groove in different implementations where a cleavage groove is provided on a side of battery cases with different sizes. 
         FIG. 10  illustrates the location of a cleavage groove provided on a flat section of the battery case. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     A sealed battery according to an embodiment includes a hollow and cylindrical battery case configured to encapsulate an electrode assembly and electrolyte. A side of the battery case includes a cleavage groove forming a cleavage line intersecting a ridge line which is formed on the side of the battery case when the battery case swells up due to an increase in internal pressure. The cleavage line is formed exclusively of a curved line and formed of a first curved segment curved to protrude in one direction as viewed in a direction normal to the side of the battery case and a second curved segment curved to protrude in a direction forming an angle of 90 degrees or larger with the direction in which the first curved segment protrudes, the first and second curved segments being connected with each other. An end of the first curved segment is connected with an end of the second curved segment. At least one of the first curved segment and the second curved segment intersects the ridge line. The cleavage groove has a depth that produces a ratio of a remaining thickness relative to a plate thickness of the battery case of 75% or smaller (first arrangement). 
     In the above arrangement, the cleavage groove formed on a side of the battery case forms, as viewed in a direction normal to the side of the battery case, a cleavage line formed exclusively of a curved line made up of a first curved segment curved to protrude in one direction and a second curved segment curved to protrude in a direction forming an angle of 90 degrees or larger with the direction in which the first curved segment protrudes, the first and second curved segments being connected with each other. A cleavage groove forming a cleavage line thus shaped can cleave up safely and easily in response to a certain pressure in the battery case and, when cleaved, leaves a relatively large opening. Moreover, the cleavage groove is unlikely to cleave up from an impact if the battery falls, for example. 
     Further, the cleavage groove has a depth that produces a ratio of the remaining thickness relative to the plate thickness of the battery case (hereinafter referred to as “remaining thickness ratio”) of 75% or smaller. Thus, the cleavage groove can cleave up in response to a certain pressure in the battery case even with varying plate thickness of battery case. More specifically, as shown in  FIG. 9 , the amount of change in the venting pressure for the cleavage groove (i.e. the pressure threshold where the cleavage groove cleaves up) in a given range of remaining thickness ratio is smaller for remaining thickness ratios of 75% than for remaining thickness ratios of larger than 75%; thus, for remaining thickness ratios of 75% and smaller, the cleavage groove cleaves up when the pressure is near the design value of venting pressure even if the remaining thickness ratio is somewhat incorrect due to an error during machining or the like. As discussed above, as the depth of the cleavage groove is defined by the remaining thickness ratio, the cleavage groove can cleave up in a more reliable manner in response to a certain pressure in the battery case if the cleavage groove has a depth that produces a remaining thickness ratio of 75% or smaller, even with varying plate thickness of the battery case. 
     In the first arrangement above, the cleavage groove has a depth that produces a ratio of the remaining thickness relative to the plate thickness of the battery case of 70% or smaller (second arrangement). 
     As shown in  FIG. 9 , the amount of change in the venting pressure for the cleavage groove in a given range of remaining thickness ratio is smaller for remaining thickness ratios of 70% and smaller than for remaining thickness ratios in the range of 70% to 75%. Thus, for remaining thickness ratios of 70% and smaller, the cleavage groove cleaves up in a still more reliable manner when the pressure is near the design value of venting pressure than for remaining thickness ratios in the range of 70% to 75%, even if the remaining thickness ratio is somewhat incorrect. 
     In the first or second arrangement above, the cleavage line is formed of a combination of a single first curved segment and a single second curved segment (third arrangement). Thus, a cleavage groove forming a cleavage line in a simple shape (S-shape, for example) can cleave up more easily when the battery case is deformed and, after the cleavage groove cleaves up, a relatively large opening can be easily created. 
     In one of the first to third arrangements above, it is preferable that the first curved segment is curved to protrude toward a corner of the battery case located at a base end of the ridge line, and the cleavage groove is formed on the side of the battery case such that the first curved segment is located on the ridge line (fourth arrangement). 
     Thus, the protrusion of the first curved segment is located closer to the end of the battery case as measured on the ridge line. Thus, the first curved segment, which is located on the ridge line, can easily cleave up as the battery case is deformed. More specifically, as the battery case is deformed, a ridge line is generated beginning from the area near the end of the battery case. In view of this, having the first curved segment curved to protrude toward that end will allow the first curved segment to cleave up early during the deformation of the battery case. Thus, the cleavage groove can cleave up in a more reliable manner as the battery case is deformed. 
     In one of the first to fourth arrangements above, the cleavage groove is formed on the side of the battery case to be located in an area with a side of one half of a vertical dimension and a side of one half of a horizontal dimension of the battery case with a corner at a corner of the battery case located at a base end of the ridge line, as viewed in a direction normal to the side of the battery case (fifth arrangement). 
     This will allow the cleavage groove to be provided closer to the base end of a ridge line formed on the side of the battery case. Thus, the cleavage groove can cleave up in a more reliable manner as the side of the battery case is deformed due to a change in pressure in the battery case. 
     In one of the first to fourth arrangements above, the cleavage groove is formed on the side of the battery case to be located in an area with a side of one third of a vertical dimension and a side of one third of a horizontal dimension of the battery case with a corner at a corner of the battery case located at a base end of the ridge line, as viewed in a direction normal to the side of the battery case (sixth arrangement). 
     This will allow the cleavage groove to be provided still closer to the base end of a ridge line formed on the side of the battery case. Thus, the cleavage groove can cleave up in a still more reliable manner as the side of the battery case is deformed due to a change in pressure in the battery case. 
     Now, embodiments of the present invention will be described in detail with reference to the drawings. The dimensions of the components in the drawings do not exactly represent the dimensions of the actual components or the dimension ratios of the components. 
     (Overall Arrangement) 
       FIG. 1  is a schematic perspective view of a sealed battery  1  according to an embodiment. The sealed battery  1  includes: an exterior can  10  in the form of a cylinder with a bottom; a cap  20  that covers the opening of the exterior can  10 ; and an electrode assembly  30  contained in the exterior can  10 . The exterior can  10  together with the attached cap  20  forms a hollow cylindrical battery case  2  with a space inside. It should be noted that, in addition to the electrode assembly  30 , non-aqueous electrolyte (hereinafter referred to as “electrolyte”), is enclosed in the battery case  2 . 
     As shown in  FIG. 2 , the electrode assembly  30  is a jellyroll electrode assembly formed of a stacked and spirally wound sheet-shaped positive electrode  31  and negative electrode  32 , where a separator  33  is placed between the two electrodes and one below the negative electrode  32 , for example. The positive electrode  31 , negative electrode  32  and separator  33  are all stacked upon one another and spirally wound before being pressed to form a flattened electrode assembly  30 . 
       FIG. 2  only shows a few outer layers of the electrode assembly  30 . An illustration of an inner portion of the electrode assembly  30  is omitted in  FIG. 2 ; of course, the positive electrode  31 , negative electrode  32  and separator  33  exist in the inner portion of the electrode assembly  30 . Also, an illustration of an insulator or the like located in a space within the battery and near the cap  20  is omitted in  FIG. 2 . 
     The positive electrode  31  includes a positive current collector made of metal foil, such as aluminum foil, and a positive electrode active material layer containing positive electrode active material provided on both sides of the positive current collector. Specifically, the positive electrode  31  is fabricated by applying a positive electrode mixture containing a positive electrode active material, a conductive aid, a binder and the like to the positive current collector of aluminum foil or the like, the positive electrode active material being a lithium-containing oxide that can occlude and discharge lithium ions, and drying the applied materials. Preferably, lithium-containing oxides used as a positive electrode active material may include, for example, a lithium cobalt oxide such as LiCoO 2 , a lithium manganese oxide such as LiMn 2 O 4 , or a lithium composite oxide including a lithium nickel oxide, such as LiNiO 2 . It should be noted that just one positive electrode active material may be used, or two or more materials may be combined. Moreover, the positive electrode active materials are not limited to those mentioned above. 
     The negative electrode  32  includes a negative current collector made of metal foil, such as copper, and a negative electrode active material layer containing a negative electrode active material provided on both sides of the negative current collector. Specifically, the negative electrode  32  is fabricated by applying a negative electrode mixture containing a negative electrode active material, a conductive aid, a binder and the like to the negative current collector of copper foil or the like, the negative electrode active material being capable of occluding and discharging lithium ions, and drying the applied materials. Preferably, negative electrode active materials may include, for example, a carbon material that is capable of occluding and discharging lithium ions (graphites, pyrolytic carbons, cokes, glass-like carbons or the like). The negative electrode active materials are not limited to those mentioned above. 
     The positive electrode  31  of the electrode assembly  30  is connected with a positive lead  34 , while the negative electrode  32  is connected with a negative lead  35 . The positive and negative leads  34  and  35  extend to the outside of the electrode assembly  30 . An end of the positive lead  34  is connected to the cap  20 . An end of the negative lead  35  is connected to the negative terminal  22  via a lead plate  27 , as described later. 
     The exterior can  10  is in the form of a cylinder with a bottom made of an aluminum alloy. The exterior can  10 , together with the cap  20 , forms the battery case  2 . As shown in  FIG. 1 , the exterior can  10  is in the form of a cylinder with a bottom having a rectangular bottom  11  with arc-like short sides. More specifically, the exterior can  10  includes a bottom  11  and a flattened and cylindrical side wall  12  having a smooth and rounded surface. The side wall  12  includes a pair of opposite flat sections  13  (sides) and a pair of semi-cylindrical sections  14  connecting the flat sections  13 . The exterior can  10  is in a flattened shape where the thickness, which corresponds to the dimension of the short sides of the bottom  11 , is smaller than the width, which corresponds to the dimension of the long sides of the bottom  11  (for example, the thickness may be about one tenth of the width). Moreover, the exterior can  10  is joined to the cap  20  which is in turn connected to the positive lead  34 , as described later. Thus, the external can  10  also serves as a positive electrode of the sealed battery  1 . 
     As shown in  FIG. 2 , on the inside of the bottom of the exterior can  10  is placed an insulator  15  made of a polyethylene sheet for preventing a short circuit between the positive electrode  31  and the negative electrode  32  of the electrode assembly  30  via the exterior can  10 . The electrode assembly  30  described above is positioned in such a way that one of its ends is on the insulator  15 . 
     The cap  20  is joined to the opening of the exterior can  10  with welding to cover the opening of the exterior can  10 . The cap  20  is made of an aluminum alloy, similar to the exterior can  10 . The cap  20  has arc-like short sides of the rectangle such that it can fit with the inside of the opening of the exterior can  10 . Further, the cap  20  has a through-hole in the center in its longitudinal direction. Through this through-hole pass an insulating packing  21  made of polypropylene and a negative terminal  22  made of stainless steel. Specifically, a generally cylindrical insulating packing  21  penetrated by a generally cylindrical negative terminal  22  fits with the periphery of the through-hole. The negative terminal  22  has flat portions integrally formed with the respective ends of the cylindrical axle. The negative terminal  22  is positioned relative to the insulating packing  21  such that a flat portion is exposed to the outside while the axle is inside the insulating packing  21 . The negative terminal  22  is connected with a lead plate  27  made of stainless steel. Thus, the negative terminal  22  is electrically connected with the negative electrode  32  of the electrode assembly  30  via the lead plate  27  and the negative lead  35 . An insulator  26  is placed between the lead plate  27  and the cap  20 . 
     A fill port  24  for electrolyte is formed on the cap  20  next to the negative terminal  22 . The fill port  24  is generally in the form of a circle in a plan view. The fill port  24  has a portion with a small radius and a portion with a large radius, where the radius changes in two steps as it goes in a thickness direction of the cap  20 . The fill port  24  is sealed with a seal plug  25  formed in steps corresponding to the different radii of the fill port  24 . The outer perimeter of the portion with a large radius of the seal plug  25  is laser-welded to the perimeter of the fill port  24  to prevent a gap from being produced between the seal plug  25  and the perimeter of the fill port  24 . 
     (Vent) 
     As shown in  FIGS. 1 and 3 , a cleavage groove  41  that constitutes a vent  23  is formed on a side of the exterior can  10 . More particularly, a cleavage groove  41  that forms a generally S-shaped cleavage line is formed on a flat section  13 , i.e. a portion of the side wall  12  of the exterior can  10  that extends in a width direction of the sealed battery  1 . This cleavage groove  41  is configured to cleave up when the pressure in the battery case  2  exceeds a threshold. 
     As shown in  FIG. 3 , the cleavage groove  41  has a first curved segment  42  curved to protrude outward along the side (i.e. in one direction) as in a side view of the exterior can  10 , and a second curved segment  43  curved to protrude inward along the side, i.e. in a direction opposite the outward direction. In this embodiment, the direction in which the first curved segment  42  protrudes (i.e. the direction in which the projection protrudes; the same shall apply hereinafter) and the direction in which the second curved segment  43  protrudes are at an angle of 180 degrees. The cleavage groove  41  forms a generally S-shaped cleavage line, as discussed above, where one end of the first curved segment  42  is connected with one end of the second curved segment  43 . In other words, the cleavage line formed by the cleavage groove  41  is made up exclusively of a curved line with an inflexion point. 
     As the cleavage groove  41  is generally in a S-shape with the first curved segment  42  and second curved segment  43 , as discussed above, the cleavage groove  41  can cleave up in response to a certain pressure in the battery case  2  more easily than a straight or arc-shaped cleavage line, as discussed below in more detail. 
     Further, since the cleavage groove  41  is generally S-shaped, the cleavage groove  41  may be formed in a smaller area than a straight or arc-shaped cleavage groove with the same length. Particularly, if the cleavage groove forms a straight line, the cleavage groove may cleave up in one stroke if there is an impact in a direction of an extension of this straight line. The above configuration will prevent the groove from rupturing from an impact in a particular direction. Thus, the cleavage groove  41  is unlikely to cleave up even when there is an impact on the battery case  2  during a fall or the like. 
     Further, in the present embodiment, portions of the flat section have a smaller thickness than other portions of the flat section  13  and thus form the cleavage groove  41 . For example, the cleavage groove  41  is formed by pressing together with the exterior can  10  when the exterior can  10  is press-formed. Pressing causes work hardening in the portions of the flat section that surround the cleavage groove  41 . This will improve the strength of the portions of the flat section surrounding the cleavage groove  41 . Thus, even when there is an impact on the sealed battery  1  during a fall or the like, the cleavage groove  41  may be prevented from rupturing from the impact. 
     The cleavage groove  41  has a cross section in the shape of an inverted trapezoid, for example. More particularly, the cross section of the cleavage groove  41  is shaped as an inverted trapezoid with decreasing groove width as it goes toward the groove bottom. The cross section of the cleavage groove  41  may be shaped as a quadrangle other than a trapezoid, or may take other shapes such as triangles or ellipses. 
     Preferably, the cleavage groove  41  has a depth that produces a ratio of the remaining thickness of portions of the flat section  13  that have the groove relative to the plate thickness of the flat section (hereinafter referred to as “remaining thickness ratio”) of 75% or smaller, as discussed below. More preferably, the cleavage groove  41  has a depth that produces a remaining thickness ratio of 70% or smaller. Having a depth of the cleavage groove  41  that produces such a remaining thickness ratio will allow the cleavage groove  41  to cleave up in a more reliable manner in response to a certain pressure in the battery case  2  even with varying plate thickness of the flat section  13 . 
     As shown in  FIG. 3 , the cleavage groove  41  is provided on one of the ridge lines L (indicated by broken lines in  FIG. 3 ) formed on the exterior can  10  when the battery case  2  swells up due to an increase in interior pressure caused by an interior short circuit, for example, of the sealed battery  1 . More specifically, in the present embodiment, the cleavage groove  41  is provided on the flat section  13  of the exterior can  10  such that the first curved segment  42  intersects the ridge line L. In addition, the cleavage groove  41  is provided on the flat section  13  such that the first curved segment  42  is curved to protrude toward a corner of the battery case  2  located at the base end of the ridge line L. 
     A ridge line L is formed as the battery case  2  swells up, causing portions of the flat section  13  of the exterior can  10  to bulge, drawn by peripheral portions of the battery case  2  (i.e. the four corners in a battery case  2  shaped as in the present embodiment). Thus, as indicated by one-dot chain lines in  FIG. 3 , ridge lines L extend inwardly from the four corners of the battery case  2  as in a side view of the battery case  2 . In  FIG. 3 , straight ridge lines L extending inwardly from the four corners of the battery case  2  are formed. However, since the ridge lines are formed of bulging portions of the flat section  13  of the exterior can  10  formed when the battery case  2  swells up, as discussed above, the ridge lines L may be curved in shape, and some ridge lines L may be connected with each other. 
     A ridge line L is a portion of the exterior can  10  that receives large stresses when the battery case  2  swells up. As such, as discussed above, a cleavage groove  41  may be provided to intersect a ridge line L such that the cleavage groove  41  may easily cleave up as the exterior can  10  is deformed. More specifically, as the battery case  2  swells up, the flat section  13  of the exterior can  10  is drawn along ridge lines L such that the cleavage groove  41 , which is a portion of the flat section  13  that has a smaller strength, cleaves up. 
     Particularly, as discussed above, the cleavage groove  41  may be provided on the flat section  13  in such a way that the first curved segment  42  is curved to protrude toward a corner of the battery case  2  located at the base end of a ridge line L such that the protrusion of the first curved segment  42  is located closer to the corner of the battery case  2 . A ridge line L is generated beginning from a portion of the battery case  2  near a corner as the battery case  2  is deformed. Thus, the first curved segment  42  located on a ridge line L can cleave up relatively early during deformation of the battery case  2 . 
     Thus, once a cleavage is generated at a portion of the cleavage groove  41  where the groove intersects a ridge line L, the cleavage advances along the cleavage groove  41 . Thus, the entire cleavage groove  41  cleaves up. As the cleavage groove  41  cleaves up, generally semicircular tongues  44  and  45  are formed on the battery case  2 , as shown in  FIG. 4 . 
     More specifically, when the pressure in the battery case  2  exceeds a threshold and the battery case  2  is deformed to cause the cleavage groove  41  to cleave up, the first curved segment  42  and second curved segment  43  of the cleavage groove  41  form tongues  44  and  45 , respectively, on the battery case  2 , as shown in  FIG. 4 . In other words, the tongues  44  and  45  are shaped according to the shape of the first curved segment  42  and second curved segment  43  of the cleavage groove  41  (i.e. generally semicircular in the present embodiment). 
     At this time, as shown in  FIG. 5 , the tongues  44  and  45  of the flat section  13  of the exterior can  10  are floating above other portions of the flat section  13  as the cleavage groove  41  cleaves up, thereby forming a gap  46 . More specifically, when the cleavage groove  41  cleaves up and the flat section  13  of the exterior can  10  is slit up, portions of the flat section  13  along the ridge line L are drawn toward the corner of the exterior can  10  in such a way that portions closer to the corner are drawn outwardly to raise the tongues  44  and  45  relative to other portions of the side wall  12  (indicated by the hollow arrow in the drawing). The gap  46 , formed between these tongues  44  and  45  and other portions of the flat section  13 , releases gas or the like accumulated in the battery case  2  to the outside. In other words, portions of the flat section  13  that have the cleavage groove  41  serve as a vent  23 . 
     In such a configuration, as the tongues  44  and  45  are raised, a cleavage forms an opening area larger than in implementations with a straight cleavage line. Thus, gas or the like in the battery case  2  can be effectively released to the outside. 
     Moreover, the tongues  44  and  45  formed by the cleavage of the cleavage groove  41  protrude in a thickness direction of the battery case  2 , to the outside. This will prevent the tongues  44  and  45  from getting in contact with the electrode assembly  30  in the battery case  2 , which would cause a short circuit. 
     In addition, in such a configuration, the size of the tongues formed by a cleavage is smaller than in implementations with a cleavage groove in a semicircular cleavage line with the same length as the cleavage groove  41 . This will prevent the tongues  44  and  45  from contacting an exterior cladding film (not shown) covering the side wall  12  of the battery case  2 . This will prevent tongues  44  and  45  in contact with the exterior cladding film from preventing a cleavage of the cleavage groove  41 . 
     As shown in  FIG. 3 , the cleavage groove  41  is located in an area with a side of one half of a vertical dimension and a side of one half of a horizontal dimension of the flat section  13  with a corner at a corner of the battery case  2  located at the base end of the ridge line L, as viewed in a direction normal to the flat section  13 . Thus, the cleavage groove  41  is provided closer to the base end of the ridge line L on the flat section  13 . Thus, the cleavage groove  41  may cleave up in a more reliable manner as the flat section  13  is deformed. 
     More preferably, the cleavage groove  41  is located in an area with a side of one third of a vertical dimension and a side of one third of a horizontal dimension of the flat section  13  with a corner at a corner of the battery case  2  located at the base end of the ridge line L, as viewed in a direction normal to the flat section  13 . Thus, the cleavage groove  41  is provided still closer to the base end of the ridge line L on the flat section  13 . Thus, the cleavage groove  41  may cleave up in a still more reliable manner as the flat section  13  is deformed. 
     (Effects of Differences in Remaining Thickness Ratio of Cleavage Groove) 
     Next, the relationship between the ratio of the remaining thickness of the cleavage groove  41  (i.e. the remaining thickness of the flat section at the groove, see  FIG. 7 ) relative to the plate thickness of the flat section  13  (see  FIG. 7 ) (hereinafter referred to as “remaining thickness ratio) and the pressure at which the cleavage groove  41  cleaves up (i.e. the venting pressure) will be discussed using calculation results and other data. 
       FIG. 6  schematically illustrates a portion of a calculation model used in the calculations described below.  FIG. 6  shows a calculation model of a battery case  2  with a cleavage groove  41  forming a generally S-shaped cleavage line. As shown in  FIG. 6 , in the calculations below, the cleavage groove  41  is spaced apart (by X and Y in the drawing) from the semi-cylindrical section  14  side and the bottom  11  side of the flat section  13  of the battery case  2 . In the calculations below, X=5 mm and Y=6 mm and the curvatures of the first curved segment  42  and second curved segment  43  of the cleavage groove  41  are represented as R=5 mm and 6 mm, respectively. The cross section of the cleavage groove  41  forms an inverted trapezoid where the width at the bottom is 0.03 mm and the angle formed by the sides of the groove is 20 degrees. 
     The calculations below used structural analysis software called LS-DYNA (Registered Trademark). In the calculations, the following equation for determining ductile fracture was used to determine whether the cleavage groove cleaved up (i.e. whether the battery was vented): 
     
       
         
           
             I 
             = 
             
               
                 1 
                 b 
               
                
               
                 
                   ∫ 
                   0 
                   ɛ 
                 
                  
                 
                   
                     ( 
                     
                       
                         
                           σ 
                           m 
                         
                         σ 
                       
                       + 
                       a 
                     
                     ) 
                   
                    
                   
                      
                     ɛ 
                   
                 
               
             
           
         
       
     
     Here, a and b are material parameters calculated from results of material tests. m represents average stress, equivalent stress, equivalent strain, and d increment in equivalent strain. 
     It will be assumed that rupturing begins at the cleavage groove when I exceeds 1 in the above equation, and the pressure in the battery case at that time will be referred to as venting pressure. In the present calculations, a is 0.3 and b is 0.14. 
     First, to determine whether the above-described calculation method used herein is appropriate, comparisons were made, in an arrangement with a cleavage groove  41  formed in a flat section  13  with a plate thickness of 0.25 mm, between values of venting pressure obtained by the above calculation method (i.e. calculation results) and values of venting pressure measured when a cleavage groove with the same shape at the same location as in the calculation model actually cleaved up (i.e. measurement results). The results of the comparisons are shown in  FIG. 8 .  FIG. 8  shows measurement results for venting pressure of the cleavage groove with varying remaining thickness of the cleavage groove (indicated by white circles in the drawing) and calculation results (indicated by the solid line in the drawing). The battery case had a width of 44 mm, a height of 61 mm and a case thickness of 4.6 mm. To actually cause the cleavage groove to cleave up, air was injected into the battery case until the cleavage groove cleaved up, and the pressure in the battery case measured when rupturing occurred will be referred to as venting pressure. 
     As shown in  FIG. 8 , the measurement results and calculation results for venting pressure substantially match, and the calculations simulate the tendency of the measurement results for venting pressure to rapidly increase between the remaining thicknesses of the cleavage groove of 0.16 mm to 0.2 mm. Therefore, the calculation method used is capable of simulating actual situations. In the description below, the venting pressure that would be measured when a cleavage groove provided in battery cases with different sizes cleaves up will be obtained by calculation and, based on the results of the calculations, various remaining thickness ratios (remaining thickness ratio (%)=remaining thickness/plate thickness 100) will be evaluated. 
       FIG. 9  shows calculation examples for five battery cases  2  of different sizes.  FIG. 9  illustrates the remaining thickness ratio versus the venting pressure. In  FIG. 9 , for Calculation Example 1, the battery case  2  has a width of 51 mm, a height of 56 mm and a case thickness of 4.6 mm, and the flat section  13  has a plate thickness of 0.25 mm. For Calculation Example 2, the battery case  2  has a width of 50 mm, a height of 59 mm and a case thickness of 5.3 mm, and the flat section  13  has a plate thickness of 0.27 mm. For Calculation Example 3, the battery case  2  has a width of 44 mm, a height of 61 mm and a case thickness of 4.6 mm, and the flat section  13  has a plate thickness of 0.25 mm. For Calculation Example 4, the battery case  2  has a width of 43 mm, a height of 50 mm and a case thickness of 4.8 mm, and the flat section  13  has a plate thickness of 0.25 mm. For Calculation Example 5, the battery case  2  has a width of 44 mm, a height of 61 mm and a case thickness of 4.8 mm, and the flat section  13  has a plate thickness of 0.28 mm. 
       FIG. 9  shows the calculation results of the five calculation examples for battery cases  2  with different sizes, using the ratio of the remaining thickness relative to the plate thickness of the flat section  13  of the battery case  2  (i.e. remaining thickness ratio) which would be measured at the cleavage groove  41 . As shown in  FIG. 9 , even when the battery case has different sizes and the flat section  13  has different plate thicknesses, the venting pressure relative to the remaining thickness ratio exhibits a similar tendency. More particularly, the venting pressure increases as the remaining thickness ratio increases. Further, the amount of change in the venting pressure relative to the remaining thickness ratio (represented as the inclination of the lines in  FIG. 9 ) for remaining thickness ratios of 75% and smaller (see the hatched arrow in the drawing) is significantly different from that for remaining thickness ratios of 75% and larger. More particularly, for remaining thickness ratios of 75% and smaller, the venting pressure does not change significantly as the remaining thickness ratio changes, while, for remaining thickness ratios of 75% and larger, the venting pressure changes significantly as the remaining thickness ratio changes. If the venting pressure changes significantly as the remaining thickness ratio changes, the venting pressure changes significantly if the remaining thickness ratio is slightly incorrect due to an error during machining or the like, meaning that the cleavage groove  41  may not cleave up in some cases. 
     Consequently, it is preferable that the cleavage groove  41  on the flat section  13  of the battery case  2  has a depth that produces a remaining thickness ratio of 75% or smaller, in which case the venting pressure will not change significantly even when the remaining thickness ratio is somewhat incorrect. 
     Further, as can be understood from  FIG. 9 , for remaining thickness ratios of 70% and smaller (see the hollow arrow in the drawing), the amount of change in the venting pressure relative to the remaining thickness ratio is even smaller than that for remaining thickness ratios in the range of 70% to 75%. Consequently, it is more preferable that the cleavage groove  41  provided on the flat section  13  of the battery case  2  has a depth that produces a remaining thickness ratio of 70% or smaller. 
     The above-described range of remaining thickness ratio (75% or smaller) is yet more preferable if, as shown in  FIG. 10 , the cleavage groove  41  is located in an area with a side of one half of a vertical dimension T and a side of one half of a horizontal dimension W of the flat section  13  with a corner at a corner of the battery case  2  located at the base end of the ridge line L (in an area defined by thin broken lines in  FIG. 10 ), as viewed in a direction normal to the flat section  13 . If the cleavage groove  41  is located in this area, the cleavage groove  41  can cleave up in a more reliable manner as the flat section  13  is deformed. 
     The above-described range of remaining thickness ratio (75% or smaller) is still more preferable if the cleavage groove  41  is located in an area with a side of one third of a vertical dimension T and a side of one third of a horizontal dimension W of the flat section  13  with a corner at a corner of the battery case  2  located at the base end of the ridge line L (in an area defined by thick broken lines in  FIG. 10 ), as viewed in a direction normal to the flat section  13 . If the cleavage groove  41  is located in this area, the cleavage groove  41  can cleave up in a still more reliable manner as the flat section  13  is deformed. 
     Effects of Embodiment 
     Thus, in the present embodiment, a cleavage groove  41  is provided on a flat section  13  of a battery case  2  of a sealed battery  1 , having a first curved segment  42  curved to protrude in one direction as in a side view and a second curved segment  43  curved to protrude in a direction opposite that one direction. This cleavage groove  41  has a depth that produces a ratio of the remaining thickness relative to the plate thickness of the flat section  13  (i.e. remaining thickness ratio) of 75% or smaller. Thus, the cleavage groove  41  can cleave up when the pressure is near the design value of venting pressure even if the depth of the cleavage groove  41  is somewhat different from its design value. Thus, the cleavage groove  41  can work in a more reliable manner. 
     Moreover, using the above-described remaining thickness ratio as a parameter, the relationship between remaining thickness ratio and venting pressure can be depicted by a graph as shown in  FIG. 9  even when the battery case  2  has different sizes and the flat section  13  has different plate thicknesses. Thus, using the remaining thickness ratio as a parameter will make it possible to establish a depth of the cleavage groove  41  that can cleave up more reliably even when the battery case  2  has different sizes and the flat section  13  has different plate thicknesses. 
     Further, the cleavage groove  41  having a depth that produces a remaining thickness ratio of 70% or smaller will allow the cleavage groove  41  to cleave up still more reliably even when the groove depth is somewhat different from its design value. 
     Other Embodiments 
     While an embodiment of the present invention has been illustrated, the above embodiment is merely an example that may be used to carry out the present invention. Thus, the present invention is not limited to the above embodiment, and the above embodiment may be modified as appropriate without departing from the spirit of the invention. 
     In the above embodiment, a cleavage groove  41  is provided such that the first curved segment  42  is located on a ridge line L. Alternatively, a cleavage groove  41  may be provided such that the second curved segment  43  is located on a ridge line L. 
     Further, the present invention is not limited to the configuration of the above embodiment, and the cleavage groove  41  may be located anywhere on the flat section  13  of the exterior can  10  as long as a portion of the cleavage groove  41  is located on a ridge line L, and the direction of the cleavage line formed by the cleavage groove  41  is not limited to the direction in the above embodiment. 
     In the above embodiment, the cleavage groove  41  has two curved segments  42  and  43 . Alternatively, the cleavage groove may have three or more curved segments. In such implementations, the cleavage groove is suitably provided on the battery case  2  so as to form a cleavage line having curved segments being connected with each other, the curved segments each curved to protrude in a direction opposite that of the adjacent one(s). 
     In the above embodiment, the cleavage groove  41  is formed by pressing. Alternatively, the cleavage groove  41  may be formed by laser machining, cutting or the like. 
     In the above embodiment, the cleavage groove  41  is formed of a continuous groove. Alternatively, the cleavage groove may be divided into a plurality of segments, where several separate grooves constitute the cleavage groove  41 . 
     In the above embodiment, the cleavage groove  41  has a first curved segment  42  curved to protrude outward along the side as in a side view of the exterior can  10  and a second curved segment  43  curved to protrude inward along the side, i.e. in a direction opposite the outward direction. However, the cleavage groove on the flat section  13  of the battery case  2  may be shaped such that the direction in which the first curved segment protrudes and the direction in which the second curved segment protrudes form an angle of about 90 degrees or larger. In other words, the cleavage groove may be in any shape as long as the direction in which the first curved segment protrudes and the direction in which the second curved segment protrudes form an angle of 90 degrees or larger. 
     In the above embodiment, the battery case  2  of the sealed battery  1  is shaped as a cylinder having a rectangular bottom surface with arc-shaped short sides. Alternatively, the battery case may be in another shape, such as a hexahedron. 
     In the above embodiment, the sealed battery  1  is a lithium-ion battery. Alternatively, the sealed battery  1  may be a battery other than a lithium-ion battery.