Patent Publication Number: US-2021175566-A1

Title: Battery

Description:
TECHNICAL FIELD 
     The present invention relates to a battery including an electrode body and a battery can housing the electrode body. 
     BACKGROUND ART 
     With regard to the shape of the battery can (outer case), Patent Literature 1 discloses an outer case having an outer sidewall whose cross-sectional shape is made hexagonal for improving the heat dissipation efficiency, and an inner sidewall whose cross-sectional shape is made circular for suppressing the deformation of a battery unit due to an increase in internal pressure, as well as for bringing an electrode group into surface contact with the outer case. 
     CITATION LIST 
     Patent Literature 
     [PTL 1] Japanese Laid-Open Patent Publication No. 2010-257733 
     SUMMARY OF INVENTION 
     Technical Problem 
     In order to increase the battery energy density, improvements have been made to increase the space occupied by the electrode body in the battery can. In some cases, however, simply increasing the space occupied by the electrode body fails to ensure the amount of electrolyte sufficiently relative to the amount of positive and negative electrode active materials, failing to realize a higher energy density. 
     Solution to Problem 
     One aspect of the present invention relates to a battery including: an electrode body having a circular-cylindrical shape; and a battery can housing the electrode body, the battery can including a cylindrical portion, a bottom wall closing one end of the cylindrical portion, and an open rim continuing to the other end of the cylindrical portion, the cylindrical portion having a shape of inner circumference including a corner portion. 
     Advantageous Effects of Invention 
     According to the present invention, a higher battery energy density can be easily achieved. 
     While the novel features of the invention are set forth particularly in the appended claims, the invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description taken in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  A schematic vertical cross-sectional view of an essential part of a battery according to an embodiment of the present invention. 
         FIG. 2  An oblique view showing an appearance of a battery can in the battery. 
         FIG. 3  A view showing a cross-sectional shape of a cylindrical portion of the battery can, taken along a plane perpendicular to the axis of the cylindrical portion, in the battery. 
         FIG. 4A  A schematic vertical cross-sectional view of an essential part of the battery including a cap. 
         FIG. 4B  An oblique view (a) of the cap and a rear view (b) thereof. 
         FIG. 5  Explanatory diagrams of an example of a production method of the battery including a preparation step (A), a sealing body placing step (B), and a sealing step (C). 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A battery according to the present invention includes: an electrode body having a circular-cylindrical shape; and a battery can housing the electrode body. The battery can includes a cylindrical portion, a bottom wall closing one end of the cylindrical portion, and an open rim continuing to the other end of the cylindrical portion. The cylindrical portion has a shape of inner circumference including a corner portion. 
     Here, the corner portion means, in a cross-sectional shape of the inner sidewall of the cylindrical portion taken along a plane perpendicular to the axis of the cylindrical portion (that is, shape of inner circumference), a space formed by at least two linear sides adjacent to each other, in the vicinity of the intersection of two sides (the vertex). Each side does not need to be a mathematically exact straight line, and may have some curvature. When the shape of inner circumference is composed only of substantially straight sides, the shape of inner circumference is polygonal, and a region in the vicinity of each vertex of the polygon constitutes the corner portion. The shape of inner circumference may include a curved-line portion (e.g., an arc). The shape of inner circumference may include a curved line portion and a straight-line portion in combination. 
     When the electrode body having a circular-cylindrical shape is housed in the cylindrical portion, there is left a space at the corner portion, the space unoccupied by the electrode body. In the battery of the present embodiment, this space can serve to retain electrolyte, and thus, to prevent electrolyte depletion. Therefore, even when the electrode body is highly densely placed in the battery can, it is possible to increase the volume of the electrode body and ensure a sufficient amount of electrolyte, which can realize a battery excellent in resistance to the lifetime deterioration associated with electrolyte depletion. 
     When the shape of inner circumference is a polygonal shape, the polygonal shape is not limited to a regular polygon. However, in view of preventing non-uniform distribution of electrolyte in the inner circumferential direction, the corner portions in which the electrolyte can be retained are preferably arranged uniformly in the inner circumferential direction. Therefore, the shape of inner circumference of the cylindrical portion is preferably a regular polygon. 
     When the electrode body having a circular-cylindrical shape is housed in the cylindrical portion having a shape of inner circumference being a polygonal shape, a maximum diameter of the electrode body that can be housed therein is equal to or larger than the length of the diameter of the inscribed circle of a polygon defining the polygonal shape. Therefore, by making the outer diameter of the electrode body substantially equal to the length of the diameter of the inscribed circle, the energy density of the battery can be increased to a maximum extent. On the other hand, in the rest portions (corner portions) in the polygon excluding the inscribed circle, the electrolyte can be retained. 
     Here, that the outer diameter of the electrode body is substantially equal to the length of the diameter of the inscribed circle means that, with taken into account a tolerance in battery production and the like, the outer diameter of the electrode body is equal to the length of the diameter of the inscribed circle within a range of ±100 nm. 
     In the height direction (axial direction) of the cylindrical portion (hereinafter, sometimes referred to as Z direction) of the battery can, the position at which the shape of inner circumference includes the corner portion may be within any range of height of the cylindrical portion, but preferably, throughout its height direction, the cylindrical portion has a shape of inner circumference including the corner portion. Moreover, in the inner circumferential direction, the positions at which the corner portion is provided may be different along the height direction of the cylindrical portion, but are preferably the same regardless of the height direction of the cylindrical portion. 
     Furthermore, when a cross-sectional shape of the outer sidewall (shape of outer circumference) of the cylindrical portion has a corner portion as in the case of the shape of inner circumference, the heat dissipation characteristics is improved by the increase of the surface area. Moreover, when the corner portion is provided at the same position in the outer circumferential direction within a certain height range of the cylindrical portion, the corner portion can be used for positioning of the battery and other purposes. 
     When the shape of inner circumference has a shape of a regular polygon, the smaller the number of the sides constituting the regular polygon is, the more the electrolyte can be retained in the corner portions. In view of allowing the corner portions to retain electrolyte sufficiently, the shape of inner circumference may be a regular polygon having, for example, 24 or less sides, or 20 or less, or 16 or less. 
     On the other hand, however, the smaller the number of the sides constituting the regular polygon is, the more difficult it is to apply a processing to the battery can for sealing the open rim of the battery can. For ease of processing of the battery can, the shape of inner circumference is preferably a regular polygon having 12 or more sides. 
     In the battery can, while the shape of inner circumference of the cylindrical portion includes the corner portion, the open rim preferably has a cross-sectional shape of the inner sidewall (shape of inner circumference) being a circular shape, for ease of sealing. 
     Conventionally, for sealing the open rim of the battery can, a constricted portion has been provided between the open rim and the cylindrical portion, such that the constricted portion has an inner diameter smaller than both the inner diameter of a circle defined by the open rim and the inner diameter (minimum inner diameter) of the cylindrical portion. On the constricted portion, a sealing plate is placed with a gasket interposed therebetween. Subsequently, the open rim of the metal can is pressed in the Z direction so as to curl over the gasket and the sealing plate, and crimped. In this case, the constricted portion is present more inward than the sealing plate in the Z direction. In this method, however, the inner space surrounded by the constricted portion has been a dead space where the electrode body cannot occupy, which has been an obstacle in obtaining a battery with a high energy density. 
     On the other hand, the electrolyte can be held in this dead space. Therefore, the electrolyte depletion has not been surfaced. 
     In contrast, by modifying the sealing method of the battery can and employing a sealing method without providing the constricted portion, the height of the electrode body having a circular-cylindrical shape can be increased, and thus, the energy density can be increased. However, with increasing the space occupied by the electrode body in the battery can, the space that can hold the electrolyte is reduced and becomes insufficient with respect to the amount of the positive and negative electrode active materials. As a result, the electrolyte depletion issue has been surfaced. 
     On the other hand, in the case of providing the constricted portion, it is not easy to apply a constricting processing to the battery can which has a polygonal shape including the corner portions. Moreover, the strain stress is likely to accumulate at the corner portions during the constricting processing, and the strength of the battery can tends to be reduced at the corner portions. In this respect, when sealing is performed with providing the constricted portion, it is not easy to employ a battery can whose shape of inner circumference includes the corner portions (esp., whose shape of inner circumference is a polygonal shape of less than 12 sides), for the purpose of ensuring a large possible amount of electrolyte. 
     However, in the case of employing a sealing method without providing the constricted portion, the reduction in strength of the battery can as mentioned above can be suppressed. According to one embodiment of the present invention, in which a sealing method without providing the conventional constricted portion is combined with a battery can provided with a corner portion for retaining electrolyte, it is possible to realize a battery having a high energy density and being excellent in resistance to the lifetime deterioration due to electrolyte depletion. 
     Specifically, in a non-limiting embodiment, the battery includes a sealing body fixed to the open rim so as to seal the opening defined by the open rim. 
     The sealing body has a sealing plate and a gasket disposed at the peripheral portion of the sealing plate. The gasket is compressed in the radial direction of the opening, between the end surface of the peripheral portion of the sealing plate and the open rim. Specifically, the open rim has a pressing portion for pressing the gasket against the end surface of the peripheral portion of the sealing plate. The gasket is compressed via the pressing portion in the radial direction of the opening, and the repulsive force of the gasket acts to ensure the airtightness between the sealing body and the open rim. 
     In other words, the open rim of the battery can presses the gasket not in the axial direction (Z direction) of the battery can but in the direction perpendicular to the Z direction (hereinafter, sometimes referred to as XY direction). In this case, the pressing force of the open rim exerted on the gasket has a larger scalar component in the XY direction than that in the Y direction. 
     The open rim of the battery can may have a projection as at least part of the pressing portion, the projection protruding inward in the radial direction. In this case, the gasket is compressed in the radial direction at least via the projection. Such a projection can be formed by constricting the open rim inward. The projection may be formed intermittently in a plurality of numbers along the circumferential direction of the opening, or may be formed continuously along the circumferential direction of the opening. The continuously formed projection can form an annular groove along the circumferential direction of the opening. The projection(s) can press the gasket or its side wall portion more strongly toward the end surface of the peripheral portion of the sealing plate. In this way, the airtightness between the sealing body and the open rim can be more reliably ensured. 
     The gasket may be of any shape, and has: for example, an inner ring portion disposed on the side facing the electrode body (the inner side) of the peripheral portion of the sealing plate; and a side wall portion covering the end surface of the peripheral portion of the sealing plate. In this case, the side wall portion is compressed in the radial direction. The gasket preferably further has an outer ring portion disposed on the outer side of the peripheral portion of the sealing plate. More specifically, the gasket preferably has an outer ring portion and an inner ring portion sandwiching the peripheral portion of the sealing plate therebetween, and a side wall portion covering the end surface of the peripheral portion of the sealing plate so as to connect the outer ring portion with the inner ring portion. 
     When the projection is formed intermittently in a plurality of numbers, the projections (at least two, preferably four or more projections) are provided preferably at equi-angular intervals with respect to the center of the opening. 
     In the height direction of the battery can, the projection is preferably substantially equal in position to the center of the end surface of the peripheral portion of the sealing plate. By aligning the position of the projection and the center position of the end surface flush with each other, in forming the projection on the open rim of the battery, the deformation of the sealing plate can be suppressed. Moreover, the pressure applied to the gasket or its side wall portion is unlikely to be uneven. Accordingly, the deformation of the gasket tends to be suppressed, and the gasket can be compressed to a higher degree. This increases the airtightness inside the can. 
     Here, that the projection is substantially equal in position to the center of the end surface of the peripheral portion of the sealing plate means that, in the height direction of the battery can, the difference between the position of the projection and the center position of the end surface of the sealing plate is 2% or less of a height L of the battery can. 
     At the center position of the end surface of the peripheral portion of the sealing plate, a recessed groove may be formed so as to correspond to the projection provided on the open rim of the battery can. By providing the recessed groove, in forming the projection on the open rim of the battery, the deformation of the sealing plate can be more effectively suppressed, and the pressure applied to the gasket or its side wall portion is less likely to be uneven. The difference between the center position of the recessed groove and the position of the projection in the height direction of the battery can is 2% or less of the height L of the battery can. 
     In the height direction of the battery can, the open rim is made smaller in outer diameter at its lowermost position (the innermost position in the Z direction) in contact with the gasket or its inner ring portion, than the cylindrical portion. In this case, it is preferable to provide an annular cap which covers the gasket or its outer ring portion in the Z direction and covers the outer peripheral surface of the open rim of the battery can in the XY direction. The cap serves to protect the peripheral portion of the sealing plate and the open rim of the battery can. At this time, by joining the cap to the open rim, the sealing body can be more securely fixed to the battery can. The cap may be designed in such a thickness that the outer diameter of the cap becomes almost equal to the outer diameter of the cylindrical portion. 
     It is desirable that the sealing plate and the gasket are integrally molded by an insert molding technique or the like. According to the integral molding, the sealing plate and the gasket are easily welded to each other. By integrally molding the sealing plate and the gasket, the sealing body can be handled as one component, which can simplify the production of the battery. 
     According to the above configuration, it is not necessary to press the gasket in the Z direction for hermetically sealing the battery can. This eliminates the necessity of providing the battery can with a constricted portion interposed between the gasket or its inner ring portion and the electrode body. Therefore, the shortest distance between the sealing body and the electrode body can be decreased, and the energy density inside the can tends to be increased. Specifically, the shortest distance between the sealing body and the electrode body can be set to, for example, 2 mm or less, and preferably 1.5 mm or less. 
     A description will be given below of a battery according to an embodiment of the present invention with reference to the drawings. It is to be noted, however, the present invention is not limited thereto. 
       FIG. 1  is a schematic vertical cross-sectional view of an essential part of a battery  10  according to the present embodiment. The battery  10  is cylindrical in shape, and includes a cylindrical bottomed battery can  100 , a circular-cylindrical electrode body  200  housed in the can, and a sealing body  300  sealing the opening of the battery can  100 . 
     The battery can  100  includes: a cylindrical portion  120  housing the electrode body  200 ; a bottom wall  130  closing one end of the cylindrical portion  120 ; and an open rim  110  continuing to the other end of the cylindrical portion  120 . The opening defined by the open rim  110  is closed by the sealing body  300 . 
       FIG. 2  is an oblique view showing an appearance of the battery can  100  in the battery  10 . In the battery can  100 , the shape of inner circumference of the cylindrical portion  120  is a polygon (regular hexadecagon), and the shape of outer circumference thereof is also a polygon (regular hexadecagon). On the other hand, the open rim  110  is processed such that the shapes of inner and outer circumferences are both circular. 
       FIG. 3  is a view showing a cross-sectional shape of the cylindrical portion  120 , taken along a plane perpendicular to its axis, in the battery  10 . As illustrated in  FIG. 3 , the volume of the electrode body  200  is maximized when the electrode body  200  has an outer diameter substantially equal to the diameter of the inscribed circle of the regular hexadecagon defining the shape of inner circumference of the cylindrical portion  120 . Even in this case, a region S where the space cannot be not occupied by the electrode body  200  is present in the vicinity of each vertex (corner portion) of the regular hexadecagon. The region S can serve to retain electrolyte. 
     Referring again to  FIG. 1 , the sealing body  300  has a sealing plate  310  and a gasket  320  disposed at a peripheral portion  311  of the sealing plate  310 . The sealing plate  310  is disk-shaped and has an explosion-proof function. Specifically, the sealing plate  310  includes the peripheral portion  311  and a center region  312 , both having a thick wall thickness and serving to provide structural strength, and a thin-walled portion  313  configured to exhibit an explosion-proof function. The thin-walled portion  313  is provided between the peripheral portion  311  and the center region  312 . To the inner surface of the center region  312 , one end of a lead wire  210  extended from a positive electrode or a negative electrode constituting the electrode body  200  is connected. Thus, the sealing plate  310  functions as a terminal of one of the electrodes. 
     When the internal pressure of the battery can  100  rises, the sealing plate  310  bulges outward, and the stress due to tension is concentrated, for example, on the boundary between the peripheral portion  311  and the thin-walled portion  313 , causing a break to occur from the boundary. As a result, the internal pressure of the battery can  100  is released, and the safety of the battery  10  can be ensured. 
     The sealing plate  310  may be of any shape. In the illustrated example, the peripheral portion  311  is made thicker than the center region  312 . The thick peripheral portion  311  can receive over a large area the pressure applied thereto from the open rim  110  of the battery can  100  in the radial direction of the opening, allowing the stress to be easily dispersed. 
     A recessed groove  3111  is formed at the center position of an end surface  311 T of the peripheral portion  311  so as to correspond to a projection  111  of the open rim  110 . 
     The gasket  320  has an outer ring portion  321  and an inner ring portion  322 , and a side wall portion  323  connecting the outer ring portion  321  with the inner ring portion  322 . The end surface  311 T of the peripheral portion  311  of the sealing plate  310  is covered with the side wall portion  323 . The outer ring portion  321  and the inner ring portion  322  sandwich the peripheral portion  311  of the sealing plate  310  therebetween, and thereby the gasket  320  is secured to the sealing plate  310 . 
     The outer ring portion  321 , the inner ring portion  322 , and the side wall portion  323  are formed as an integrally molded product. The gasket  320  can be integrally molded with the sealing plate  310 , for example, by an insert molding technique. 
     To ensure the airtightness between the open rim  110  of the battery can  100  and the sealing body  300 , it is necessary that at least part of the open rim  110  presses the side wall portion  323  of the gasket  320  against the end surface  311 T of the peripheral portion  311  of the sealing plate  310 , compressing the side wall portion  323  in the radial direction of the opening. Here, the projection  111  protruding inward is formed on the open rim  110  along the circumference of the opening, pressing the side wall portion  323  against the end surface  311 T. The open rim  110  of the battery can  100  has an end surface  110 T. 
     The side wall portion  323  of the gasket  320  may be provided with a recessed portion  3231  in advance at a position corresponding to the projection  111 . Providing the recessed portion  3231  on the gasket  320  can prevent the gasket  320  from being excessively deformed when the side wall  323  is compressed. 
     In the height direction of the battery can  100 , the projection  111  is substantially equal in position to the center of the end surface  311 T of the peripheral portion  311  of the sealing plate  310 . 
     By aligning the positions as above, the deformation of the sealing plate  310  and the gasket  320  can be suppressed, and the side wall portion  323  tends to be compressed to a higher degree. This can more reliably ensure the airtightness between the sealing body  300  and the open rim  110 . 
     In the open rim  110  of the battery can  100 , the endmost portion having the end surface  110 T extends in the direction forming an angle of less than  5 ° with the axial direction (Z direction) of the battery can  100 . This prevents the gasket  320  from being subjected to excessive stress, making it possible for the gasket  320  to ensure the airtightness more easily and reliably. 
     The open rim  110  of the battery can  100  is made smaller in outer diameter than the cylindrical portion  120 , at the lowest position of the open rim in contact with the inner ring portion  322  of the gasket  320 , in the height direction of the battery can  100  of the battery  10 . The outer ring portion  321  protrudes beyond the end surface  110 T of the open rim  110  in the axial direction (Z direction) of the battery can  100 . In such a case, it is preferable to provide a protective member so as to cover the open rim  110  of the battery can  100  and the outer ring portion  321  of the gasket  320 . 
       FIG. 4A  is a schematic vertical cross-sectional view of an essential part of the battery  10  including a cap  400  serving as the protective member.  FIG. 4B  is an oblique view (a) of the cap  400  and a rear view (b) thereof. 
     The annular cap  400  covers the outer ring portion  321  of the gasket  320  in the Z direction, and covers the outer peripheral surface of the open rim  110  of the battery can  100  in the XY direction. The cap  400  may have any thickness, and may be designed, for example, in such a thickness that the outer diameter of the cap  400  becomes substantially equal to the outer diameter of the cylindrical portion  120 . A joining material  410  may be interposed between the cap  400  and the outer peripheral surface of the open rim  110 . The difference between the outer diameter or the maximum outer diameter of the cap  400  and the outer diameter or the maximum outer diameter of the cylindrical portion  120  is, for example, 20% or less of an outer diameter D of the cylindrical portion  120 . The difference may be 10% or less, and may be 5% or less or 2% or less. 
     When the cap  400  has electrical conductivity, the cap  400  can be configured to function as a terminal having a polarity different from that of the sealing plate  310 . When the cap  400  is made function as a terminal, the cap  400  is preferably joined to the open rim  110  by welding or the like. The cap  400  is an accessory, the shape of which can be flexibly designed according to use. 
     In the battery  10  illustrated in  FIGS. 1 and 4 , the battery can  100  does not have a constricted portion interposed between the gasket  320  or the inner ring portion  322  and the electrode body  200 . Therefore, the shortest distance between the sealing body  300  and the electrode body  200  can be reduced to, for example, 1 mm or less. 
     Next, a description will be given of an example of a production method of the battery  10 , with reference to  FIG. 5 . 
     (1) Preparation Step 
     As illustrated in  FIG. 5(A) , the battery can  100  including an electrode body  200  housed in the cylindrical portion  120 , and the sealing body  300  are prepared first. The battery can  100  used here has a cylindrical portion whose shape of inner circumference is polygonal, as illustrated in  FIG. 2 . Note that before inserting the electrode body  200  into the can, unlike in  FIG. 2 , the battery can  100  has the open rim  110  formed sufficiently larger in diameter than the electrode body  200 . Also, the projection  111  is not yet formed. After the electrode body  200  is housed in the can, the open rim  110  is constricted radially, so that the outer diameter of the open rim  110  becomes smaller than that of the cylindrical portion  120 . 
     The sealing body  300  can be prepared by insert-molding a gasket  320  together with the sealing plate  310 . The thickness of the sealing plate  310  at the peripheral portion  311  is larger than that at the center region  312 , and the peripheral portion  311  is provided with the recessed groove  3111  at the center position of an end surface  311 T. Likewise, the gasket  320  is provided with the recessed portion  3231  at a position corresponding to the recessed groove  3111 . 
     The gasket  320  may be made of any material Examples of the material include polypropylene (PP), polyphenylene sulfide (PPS), polyether ether ketone (PEEK), polyethylene (PE), polybutylene terephthalate (PBT), perfluoroalkoxyalkane (PFA), polytetrafluoroethylene (PTFE), and polyamide (PA). 
     (2) Sealing Body Placing Step 
     Next, as illustrated in  FIG. 5(B) , the sealing body  300  is placed inside the open rim  110  of the battery can  100 . The sealing body  300  may be positioned in any way, but can be positioned by, for example, as illustrated in  FIG. 4 , using a protruding portion provided at the upper end of the gasket  320  so as to protrude outward in the radial direction of the opening. The protruding portion may be provided in a flange-like shape, or may be provided intermittently in a projection-like shape along the circumferential direction of the opening. Alternatively, a step portion may be provided on the inner side of the open rim  110  of the battery can  100 , so that the sealing body  300  can be positioned at the step portion. 
     (3) Sealing Step 
     Next, as illustrated in  FIG. 5(C) , a grooving processing is applied to the open rim  110  of the battery can  100  so as to be recessed inward at a position corresponding to the recessed groove  3111  and the recessed portion  3231 . The projection  111  protruding inward with a constriction is thus formed on the open rim  110 , so that the projection  111  presses the side wall portion  323  of the gasket  320  against the end surface  311 T of the peripheral portion  311  of the sealing plate  310 . As a result, the side wall portion  323  of the gasket  320  is compressed in the radial direction of the opening, and due to the repulsive force of the gasket, the airtightness between the sealing body  300  and the open rim  110  is ensured. 
     Next, an illustrative description will be given of a configuration of the electrode body  200 , with a lithium ion secondary battery taken as an example. 
     The circular-cylindrical electrode body  200  is of a wound type, and is formed by spirally winding a positive electrode and a negative electrode with a separator interposed therebetween. To one of the positive and negative electrodes, the lead wire  210  is connected. The lead wire  210  is connected to the inner surface of the center region  312  of the sealing plate  310  by welding or the like. To the other one of the positive and negative electrodes, another lead wire is connected. The another lead wire is connected to the inner surface of the battery can  100  by welding or the like. 
     (Negative Electrode) 
     The negative electrode has a belt-like negative electrode current collector and a negative electrode active material layer formed on both sides of the negative electrode current collector. The negative electrode current collector is, for example, a metal film, a metal foil, or the like. The material of the negative electrode current collector is preferably at least one selected from the group consisting of copper, nickel, titanium, alloys thereof, and stainless steel. The negative electrode current collector preferably has a thickness of, for example, 5 to 30 μm. 
     The negative electrode active material layer contains a negative electrode active material, and optionally contains a binder and an electrically conductive material. The negative electrode active material layer may be a deposition film formed by a gas phase method (e.g., vapor deposition). Examples of the negative electrode active material include Li metal, a metal or an alloy that electrochemically reacts with Li, a carbon material (e g graphite), a silicon alloy, a silicon oxide, and a metal oxide (e.g., lithium titanate). The negative electrode active material layer preferably has a thickness of, for example, 1 to 300 μm. 
     (Positive Electrode) 
     The positive electrode has a belt-like positive electrode current collector and a positive electrode active material layer formed on both sides of the positive electrode current collector. The positive electrode current collector is, for example, a metal film, a metal foil (stainless steel foil, aluminum foil, or aluminum alloy foil), or the like. 
     The positive electrode active material layer contains a positive electrode active material and a binder, and optionally contains an electrically conductive material. The positive electrode active material is not limited, but may be a lithium-containing composite oxide, such as LiCoO 2  or LiNiO 2 . The positive electrode active material layer preferably has a thickness of, for example, 1 to 300 μm. 
     Examples of the conductive material contained in each active material layer include graphite and carbon black. The conductive material is contained in an amount of, for example, 0 to 20 parts by mass per 100 parts by mass of the active material. Examples of the binder contained in the active material layer include fluorocarbon resin, acrylic resin, and rubber particles. The binder is contained in an amount of, for example, 0.5 to 15 parts by mass per 100 parts by mass of the active material. 
     (Separator) 
     The separator is preferably a microporous resin film or a nonwoven resin fabric. Examples of the material (resin) of the separator include polyolefin, polyamide, and polyamide imide. The separator has a thickness of, for example, 8 to 30 μm. 
     (Electrolyte) 
     The electrolyte may be a non-aqueous solvent in which a lithium salt is dissolved. Examples of the lithium salt include LiClO 4 , LiBF 4 , LiPF 6 , LiCF 3 SO 3 , LiCF 3 CO 2 , and imide salts. Examples of the non-aqueous solvent include: cyclic carbonic esters, such as propylene carbonate, ethylene carbonate, and butylene carbonate; chain carbonic esters, such as diethyl carbonate, ethyl methyl carbonate, and dimethyl carbonate; and cyclic carboxylic acid esters, such as γ-butyrolactone and γ-valerolactone. 
     Although a description is given above with a lithium ion secondary battery taken as an example, the present invention is applicable to a battery employing a liquid form of any electrolyte regardless of whether the battery is a primary or secondary battery, and can realize a high energy density, while suppressing electrolyte depletion by ensuring a sufficient amount of electrolyte in the corner portion. 
     (Relationship Between Shape of Battery can and Amount of Retained Electrolyte) 
     Given that the shape of inner circumference of the cylindrical portion  120  of the battery can is a regular n-sided polygon. By employing the sealing method without providing the constricted portion as described in the present embodiment, for example, in a lithium ion battery having a diameter of 21 mm and a height of 70 mm, belt-like positive and negative electrodes which are each 0.5 mm or more wider than those used in the case of providing the constricted portion can be used, and thereby the width of the electrode body  200  (the height of the cylindrical portion) can be made 0.5 mm or more longer than that in the case of providing the constricted portion. When belt-like positive and negative electrodes each 0.9 mm or more wider than those used in the case of providing the constricted portion are used, the width of the electrode body  200  (the height of the cylindrical portion) can be made 0.9 mm or more longer than that in the case of providing the constricted portion. However, by increasing the volume of the electrode body  200 , the space that can retain the electrolyte within the battery can is reduced. 
     Given that the minimum inner diameter of the cylindrical portion  120  of the battery can is denoted by 2r. r corresponds to the radius of the inscribed circle of a regular n-sided polygon defining the shape of inner circumference of the cylindrical portion  120 . 
     When the width of the electrode body  200  (the height of the cylindrical portion) is made longer by Δh in order to realize a higher energy density, a reduced space C 1  for retaining the electrolyte can be expressed by πr 2 ·Δh. 
     Whereas, by making the shape of inner circumference of the cylindrical portion  120  into a regular n-sided polygon, the space that can retain the electrolyte is increased. Given that the width of the electrode body  200  (the height of the cylindrical portion) is denoted by H. The area S n  of the regular n-sided polygon can be expressed by S n =nr 2  tan(π/n). Therefore, an increased space C 2  for retaining the electrolyte can be expressed at maximum by 
         C   2   ={nr   2  tan(π/ n )−π r   2   }·H.  
 
     If C 2 ≥C 1 , the electrolyte can be retained in an amount equal to or higher than that before, despite the increase Δh in the width of the electrode body  200  (the height of the cylindrical portion). Modifying the above equation gives the condition for ensuring a sufficient amount of electrolyte, which can be expressed by the following formula: 
       (n/π)tan(n/π)−1≥Δh/H.   [Math Formula 1]
 
     In the above exemplified lithium ion battery, supposing that H=66 mm and the width is to be increased by Δh=0.5 mm, these values are substituted into the above formula 1. This gives n≤20, that is, the shape of inner circumference of the cylindrical portion  120  is to be a regular icosagon. Likewise, for example, for increasing the width by Δh=0.9 mm, from the above formula 1, when the shape of inner circumference of the cylindrical portion  120  is a regular hexadecagon (n≤16), the amount of electrolyte can be substantially maintained as before. 
     INDUSTRIAL APPLICABILITY 
     The battery according to the present invention is useful for non-aqueous electrolyte secondary batteries (esp., lithium ion secondary batteries) required to have a high energy density, and is suitably applicable as a power source for, for example, portable devices, hybrid vehicles, electric vehicles, and the like. 
     Although the present invention has been described in terms of the presently preferred embodiments, it is to be understood that such disclosure is not to be interpreted as limiting. Various alterations and modifications will no doubt become apparent to those skilled in the art to which the present invention pertains, after having read the above disclosure. Accordingly, it is intended that the appended claims be interpreted as covering all alterations and modifications as fall within the true spirit and scope of the invention. 
     REFERENCE SIGNS LIST 
       10 : battery,  100 : battery can,  110 : open rim,  110 T: end surface,  111 : projection,  120 : cylindrical portion,  130 : bottom wall,  200 : electrode body,  210 : lead wire,  300 : sealing body,  310 : sealing plate,  311 : peripheral portion,  311 T: end surface,  3111 : recessed groove,  312 : center region,  313 : thin-walled portion,  320 : gasket,  321 : outer ring portion,  322 : inner ring portion,  323 : side wall portion,  3231 : recessed portion,  324 : protruding portion,  400 : cap,  410 : joining material