Patent Publication Number: US-2021167425-A1

Title: Energy storage device

Description:
TECHNICAL FIELD 
     The present invention relates to an energy storage device including an electrode assembly having an electrode plate and a separator wound therearound. 
     BACKGROUND ART 
     Patent Document 1 discloses a battery that has a foil-shaped positive electrode plate and a foil-shaped negative electrode plate wound around a winding core with a separator sandwiched between them, and has a power generating element formed in a flat shape. In this battery, the winding core is made of a porous member, and the foil-shaped positive electrode plate and the foil-shaped negative electrode plate are arranged in a state where their respective surfaces on which active material layers are formed face each other while sandwiching the winding core therebetween, on the innermost circumference of the winding. With this configuration, the foil-shaped positive electrode plate and the foil-shaped negative electrode plate function as a battery also on the innermost circumference of the power generating element. 
     PRIOR ART DOCUMENT 
     Patent Document 
     Patent Document 1: WO 2011/148866 A1 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     The core material included in the power generating element in the conventional battery has flexibility. Therefore, there is a possibility that a gap may be generated between the edge of the electrode plate and the separator, and if a conductive foreign substance (contamination) such as a metal piece or metal powder enters the gap, a problem such as a slight short circuit may occur. 
     The present invention, in consideration of the above-mentioned conventional problem, has an object to provide a highly reliable energy storage device, which is an energy storage device including an electrode assembly having an electrode plate and a separator wound therearound. 
     Means for Solving the Problems 
     The energy storage device according to one aspect of the present invention is an energy storage device including an electrode assembly having a tubular core material and an electrode plate and a separator wound around the core material, in which when viewed from a direction of a winding axis of the electrode assembly, the core material has a first line part and a second line part extending along a first imaginary line and a second imaginary line parallel to a long side surface of a case of the energy storage device, respectively, at least one of the first line part and the second line part has a curved portion that protrudes toward the other beyond the first imaginary line or the second imaginary line, and an inner peripheral edge of the electrode plate at a winding start position is located at a position other than the curved portion in at least one of the first line part and the second line part. 
     Advantages of the Invention 
     According to the present invention, it is possible to provide a highly reliable energy storage device, which is an energy storage device including an electrode assembly having an electrode plate and a separator wound therearound. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view showing an external appearance of an energy storage device according to an embodiment. 
         FIG. 2  is a perspective view showing components arranged in a case of the energy storage device according to the embodiment. 
         FIG. 3  is a perspective view showing an external appearance of a current collector according to the embodiment. 
         FIG. 4  is a perspective view showing an outline of a configuration of an electrode assembly according to the embodiment. 
         FIG. 5  is a diagram showing an outline of a configuration when the electrode assembly according to the embodiment is viewed from a direction of a winding axis. 
         FIG. 6  is a diagram simply showing a method of manufacturing the electrode assembly according to the embodiment. 
         FIG. 7  is a diagram showing an outline of a configuration of a core material of the electrode assembly and its surroundings according to the embodiment. 
         FIG. 8  is a diagram showing an outline of a configuration of an electrode assembly according to a comparative example. 
         FIG. 9  is a diagram showing an outline of a configuration of a core material of an electrode assembly and its surroundings according to a first modification example of the embodiment. 
         FIG. 10  is a diagram showing an outline of a configuration of a core material of an electrode assembly and its surroundings according to a second modification example of the embodiment. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     The inventors of the present application have found that the following problems arise with respect to the battery in Patent Document 1. In an electrode assembly having an electrode plate and a separator wound around a core material as in the power generating element in Patent Document 1, the core material is formed in a tubular shape with a highly flexible material such as a resin film. Therefore, when the electrode assembly is formed into a flat shape by pressing from a direction orthogonal to the winding axis, for example, the core material does not damage the electrode plate and the separator. However, since the core material has flexibility, it tends to be partially curved inward by receiving the tightening force from the separator and the electrode plate. When a part of the core material is curved inward in this way, for example, the edge on the inner peripheral side of the electrode plate may float (lift inward) from the adjacent separator on the outside of the curved part. That is, there is a possibility that a gap may be generated between the edge of the electrode plate and the separator, and if a conductive foreign substance (contamination) such as a metal piece or metal powder enters the gap, a problem such as a slight short circuit may occur. 
     The energy storage device according to one aspect of the present invention is an energy storage device including an electrode assembly having a tubular core material and an electrode plate and a separator wound around the core material, in which when viewed from a direction of a winding axis of the electrode assembly, the core material has a first line part and a second line part extending along a first imaginary line and a second imaginary line parallel to a long side surface of a case of the energy storage device, respectively, at least one of the first line part and the second line part has a curved portion that protrudes toward the other beyond the first imaginary line or the second imaginary line, and an inner peripheral edge of the electrode plate at a winding start position is located at a position other than the curved portion in at least one of the first line part and the second line part. 
     As described above, in the energy storage device according to the present aspect, in the electrode assembly having the electrode plate and the separator wound around the tubular core material, the inner peripheral edge of the electrode plate (the edge at the start of winding) is located at a position other than the curved portion of the core material. As a result, the inner peripheral edge of the electrode plate is pressed outward by the separator on the inner side (core material side), and as a result, the floating of the inner peripheral edge is suppressed. Therefore, the problem such as a slight short circuit due to contamination entering from the end portion of the electrode assembly is unlikely to occur. As described above, the energy storage device according to the present aspect is a highly reliable energy storage device. 
     When viewed from the direction of the winding axis, the core material has a fixed portion which is a portion where the separator is fixed in the first line part, and the curved portion may be formed adjacent to the fixed portion in the first line part. 
     The fixed portion in the core material is a portion that is directly pulled in the winding direction by the separator in the winding step of winding the separator and the electrode plate around the core material. Therefore, the core material tends to have a curved portion formed at a position adjacent to the fixed portion. For example, a relatively large curved portion is likely to be formed on the side of the fixed portion in the winding direction of the electrode plate and the separator. The portion other than the curved portion tends to protrude outward. Therefore, by arranging the inner peripheral edge of the electrode plate at a position avoiding the outer region of the curved portion, the suppression of the floating of the inner peripheral edge is further ensured. Before winding the separator and the electrode plate around the core material, the position of the curved portion formed after that can be easily specified with reference to the fixed portion. Therefore, it is easy to manufacture an electrode assembly in which the inner peripheral edge of the electrode plate is arranged at a position other than the curved portion in the first line part. 
     In the electrode assembly, the inner peripheral edge may be located in the second line part. 
     According to this configuration, the fixed portion is arranged in the first line part, and the inner peripheral edge of the electrode plate is arranged in the second line part. In other words, the fixed portion is arranged in one of regions divided into two by the long axis, and the inner peripheral edge of the electrode plate is arranged in the other of the regions divided into the two. Therefore, the inner peripheral edge of the electrode plate is not easily affected by the curved portion located on the side of the fixed portion. As a result, the floating of the inner peripheral edge is more reliably suppressed. 
     In the electrode assembly, the fixed portion and the inner peripheral edge may be arranged at positions facing each other with the winding axis interposed therebetween. 
     According to this configuration, the inner peripheral edge of the electrode plate exists at or near the position farthest from the fixed portion in the circumferential direction of the core material, and is therefore not easily affected by the curved portion located on the side of the fixed portion. As a result, the floating of the inner peripheral edge is more reliably suppressed. 
     When viewed from the direction of the winding axis, the core material may have a shape that is elongated in a predetermined direction and has a pair of curve parts facing each other in the predetermined direction, the fixed portion may be arranged on one of the pair of curve parts, and the inner peripheral edge may be arranged outside the other of the pair of curve parts. 
     The outside of the curve part of the core material is a portion where tension of the electrode plate and the separator (laminated elements) is easily applied at the time of winding, that is, the density of the laminated elements is high. By locating the inner peripheral edge of the electrode plate in this portion, the inner peripheral edge is firmly pressed inward by the outer separator, and the position is far also from the fixed portion, so that it is also not easily affected by the curved portion on the side of the fixed portion. As a result, the floating of the inner peripheral edge is more reliably suppressed. 
     The internal space of the core material is divided into two spaces by a partition wall part that crosses the internal space when viewed from the direction of the winding axis, and the fixed portion may be located outside one of the two spaces, and the inner peripheral edge may be located outside the other of the two spaces. 
     The curved portion formed due to the fixed portion being pulled in the winding direction is hard to be formed in the portion on the side opposite to the fixed portion with the partition wall part sandwiched by the partition wall part being pulled. Therefore, in the energy storage device according to the present aspect, the inner peripheral edge of the electrode plate is arranged at a position where the separator inside the electrode plate is hard to float, whereby the floating of the inner peripheral edge is more reliably suppressed. 
     The core material may be formed by winding a part of the separator. In this way, when the core material is formed by winding a part of the separator, the core material has relatively high flexibility, so that a curved portion is formed. Therefore, by arranging the inner peripheral edge of the electrode plate at a position other than the curved portion, the floating of the inner peripheral edge is suppressed. As a result, the occurrence of a problem such as a slight short circuit due to contamination is suppressed. 
     Hereinafter, an energy storage device according to an embodiment and a modification example of the present invention is described with reference to the drawings. Each drawing is a schematic view and is not necessarily an exact illustration. 
     Each of embodiments and modification examples described below shows a specific example of the present invention. However, shapes, materials, components, arrangement positions and connection modes of the components, order of manufacturing steps, and the like described in the embodiments and modification examples hereinafter are only examples and are not intended to limit the present invention. Among the components in the embodiments and modification examples described hereinafter, the components which are not described in independent claims are described as arbitrary components. 
     In the following embodiments, modification examples, and claims, expressions indicating relative directions or postures, such as parallel and orthogonal, may be used. Strictly speaking, these expressions also include cases where they are not in that direction or posture. For example, the fact that two directions are parallel not only means that the two directions are completely parallel, but also means that they are substantially parallel, that is, that they include a difference of, for example, about several percent. 
     Embodiment 
     1. General Description of Energy Storage Device 
     First, a general description of an energy storage device  10  according to an embodiment will be described with reference to  FIGS. 1 to 3 .  FIG. 1  is a perspective view showing an external appearance of the energy storage device  10  according to the embodiment.  FIG. 2  is a perspective view showing components arranged in a case  100  of the energy storage device  10  according to the embodiment.  FIG. 3  is a perspective view showing an external appearance of a current collector  120  according to the embodiment. 
     The energy storage device  10  is a secondary battery which can charge electricity or discharge electricity, more specifically, is a nonaqueous electrolyte secondary battery such as a lithium ion secondary battery. The energy storage device  10  is applied to a power source for an automobile (or a mobile body) such as an electric vehicle (EV), a hybrid electric vehicle (HEV), or a plug-in hybrid electric vehicle (PHEV), a power source for an electronic device, or a power source for power storage. The energy storage device  10  may be mounted on a vehicle such as a gasoline vehicle and a diesel vehicle as a battery for starting the engine. The energy storage device  10  is not limited to a nonaqueous electrolyte secondary battery, and may be a secondary battery other than the nonaqueous electrolyte secondary battery or may be a capacitor. The energy storage device  10  may be a primary battery that can use electricity which is stored without the user having to charge the battery. 
     As shown in  FIG. 1 , the energy storage device  10  includes a case  100 , a negative electrode terminal  200 , and a positive electrode terminal  300 . As shown in  FIG. 2 , a negative electrode-side current collector  120 , a positive electrode-side current collector  130 , and an electrode assembly  400  are housed inside the case  100 . 
     In addition to the above components, the energy storage device  10  may include spacers arranged on the sides of the current collectors  120  and  130 , a gas discharge valve for releasing the pressure when the pressure in the case  100  rises, or an insulating film or the like that wraps the electrode assembly  400  or the like. A liquid such as an electrolyte solution (nonaqueous electrolyte) is sealed inside the case  100  of the energy storage device  10 , but the illustration of the liquid is omitted. The electrolyte solution sealed in the case  100  is not particularly limited in type as long as it does not impair the performance of the energy storage device  10 , and various electrolyte solutions can be selected. 
     The case  100  includes a main body  111  having a rectangular tubular shape and a bottom, and a lid  110  which is a plate-shaped member that closes an opening of the main body  111 . As shown in  FIG. 2 , the rectangular tubular main body  111  has a pair of long side surfaces  111   a  and a pair of short side surfaces  111   b.  The case  100  has a structure in which the electrode assembly  400  and the like are housed inside, and then the lid  110  and the main body  111  are, for example, welded to seal the inside. The electrode assembly  400  includes a positive electrode plate, a negative electrode plate, and a separator, and is a member capable of storing electricity. The detailed configuration of the electrode assembly  400  will be described later with reference to  FIG. 4  and the like. 
     The negative electrode terminal  200  is an electrode terminal electrically connected to the negative electrode of the electrode assembly  400  via the current collector  120 . The positive electrode terminal  300  is an electrode terminal electrically connected to the positive electrode of the electrode assembly  400  via the current collector  130 . The negative electrode terminal  200  and the positive electrode terminal  300  are attached to the lid  110  arranged above the electrode assembly  400  via an insulating gasket (not shown). 
     The current collector  120  is arranged between the negative electrode of the electrode assembly  400  and the wall surface of the main body  111  of the case  100 , and is a member having conductivity and rigidity that is electrically connected to the negative electrode terminal  200  and the negative electrode of the electrode assembly  400 . 
     The current collector  130  is arranged between the positive electrode of the electrode assembly  400  and the wall surface of the main body  111  of the case  100 , and is a member having conductivity and rigidity that is electrically connected to the positive electrode terminal  300  and the positive electrode of the electrode assembly  400 . 
     Specifically, the current collectors  120  and  130  are fixed to the lid  110 . The current collector  120  is joined to the negative electrode side end of the electrode assembly  400 , and the current collector  130  is joined to the positive electrode side end of the electrode assembly  400 . In the present embodiment, each of the current collectors  120  and  130  is bonded to the electrode assembly  400  by ultrasonic bonding. In the present embodiment, the shapes and mounting structures of the current collectors  120  and  130  are substantially the same. Therefore, the configuration of the current collector  120  on the negative electrode side will be described with reference to  FIG. 3 , and the description of the configuration of the current collector  130  on the positive electrode side will be omitted. 
     As shown in  FIG. 3 , the current collector  120  in the present embodiment has a pair of legs  122  arranged so as to sandwich the negative electrode side ends of the electrode assembly  400  from both sides. The pair of legs  122  are elongated portions extending from the end portion of a terminal connection portion  121  of the current collector  120 . The terminal connection portion  121  is a portion connected to the negative electrode terminal  200 . For example, the negative electrode terminal  200  and the current collector  120  are connected by caulking the rivet provided in the negative electrode terminal  200  in a state of penetrating a through hole  121   a  of the terminal connection portion  121 . The pair of legs  122  is bonded to the negative electrode side end of the electrode assembly  400  by ultrasonic bonding. As a result, the current collector  120  is electrically connected to the negative electrode of the electrode assembly  400 . As a method for bonding the electrode assembly  400  and the current collectors  120  and  130 , a method such as resistance welding or clinch bonding may be adopted in addition to ultrasonic bonding. 
     1-1. Basic Configuration of Electrode Assembly 
     Next, the basic configuration of the electrode assembly  400  included in the energy storage device  10  configured as described above will be described with reference to  FIG. 4 .  FIG. 4  is a perspective view showing an outline of the configuration of the electrode assembly  400  according to the embodiment. In  FIG. 4 , elements (laminated elements) such as electrode plates that are laminated and wound are partially developed and shown. The alternate long and short dash line with a symbol W in  FIG. 4  represents the winding axis of the electrode assembly  400 . The winding axis W is a imaginary axis serving as a central axis when winding the electrode plate or the like, and in the present embodiment, is a straight line parallel to the X axis passing through the center of the electrode assembly  400 . That is, in the present embodiment, the “direction of the winding axis W” is synonymous with the “X-axis direction”. 
     The electrode assembly  400  is an example of an electrode assembly formed by winding an electrode plate and a separator around a core material described later. As shown in  FIG. 4 , the electrode assembly  400  has a flat shape in the direction orthogonal to the winding axis W (in the present embodiment, the Z-axis direction). That is, the electrode assembly  400  has an oval shape as a whole when viewed from the direction of the winding axis W, the straight portion of the oval shape has a flat shape, and the curved portion of the oval shape has a curved shape. Therefore, the electrode assembly  400  has a pair of curved end portions facing each other (portions facing each other in the Y-axis direction with the winding axis W sandwiched) and a pair of intermediate portions (portions facing each other in the Z-axis direction with the winding axis W sandwiched) that are portions between the pair of curved end portions. 
     In the present embodiment, the positive electrode plate  410  includes a long strip-shaped metal foil made of aluminum (positive electrode substrate layer  411 ) and a positive electrode mixture layer  414  formed on the surface of the metal foil and containing a positive active material. The negative electrode plate  420  has a long strip-shaped metal foil made of copper (negative electrode substrate layer  421 ) and a negative electrode mixture layer  424  formed on the surface of the metal foil and containing a negative active material. In the present embodiment, the separators  430  and  450  each have a microporous sheet made of resin as a substrate. 
     More specifically, in the electrode assembly  400  configured as described above, the positive electrode plate  410  and the negative electrode plate  420  are wound so as to be shifted from each other in the direction of the winding axis W via the separator  430  or  450 . The positive electrode plate  410  and the negative electrode plate  420  each have a mixture layer non-forming portion, which is a portion of the substrate layer in which the mixture layer is not formed, at the respective end portions in the shifted directions. 
     Specifically, the positive electrode plate  410  has a mixture layer non-forming portion  411   a  in which a positive electrode mixture layer is not formed at one end in the direction of the winding axis W (the end portion on the plus side in the X-axis direction in  FIG. 4 ). The negative electrode plate  420  has a mixture layer non-forming portion  421   a  in which a negative electrode mixture layer is not formed at the other end in the direction of the winding axis W (the end portion on the minus side in the X-axis direction in  FIG. 4 ). 
     That is, the positive electrode side end is formed by the layer of the exposed metal foil (mixture layer non-forming portion  411   a ) of the positive electrode plate  410 , and the negative electrode side end is formed by the layer of the exposed metal foil (mixture layer non-forming portion  421   a ) of the negative electrode plate  420 . The positive electrode side end is joined to the current collector  130 , and the negative electrode side end is joined to the current collector  120 . 
     1-2. Core Material and Surrounding Structure 
     In the electrode assembly  400  configured as described above, the edge at the start of winding of the electrode plate (in the present embodiment, the inner peripheral edge of the negative electrode plate  420 ) is arranged at a position different from the outer region of the curved portion of the core material. As a result, the floating of the inner peripheral edge is suppressed. This structure will be described below with reference to  FIGS. 5 to 8 . 
       FIG. 5  is a diagram showing an outline of the configuration when the electrode assembly  400  according to the embodiment is viewed from the direction of the winding axis W.  FIG. 6  is a diagram simply showing a method of manufacturing the electrode assembly  400  according to the embodiment.  FIG. 7  is a diagram showing an outline of the configuration of a core material  500  of the electrode assembly  400  and its surroundings according to the embodiment.  FIG. 8  is a diagram showing an outline of a configuration of an electrode assembly  490  according to a comparative example. 
     In  FIG. 6 , the separators  430  and  450  that are first wound around the core material  500  are shown, and the illustration of the negative electrode plate  420  and the positive electrode plate  410  that are wound while being sandwiched between the separators  430  and  450  is omitted. In  FIGS. 7 and 8 , only a part of the winding start of each of the negative electrode plate  420  and, the separators  430  and  450  is shown, and the illustration of the positive electrode plate  410  is omitted. In  FIGS. 6 to 8 , the separator  430  is represented by a solid line and the separator  450  is represented by a dotted line so that the separator  430  and the separator  450  can be easily distinguished. These supplementary items regarding  FIGS. 6 to 8  also apply to  FIGS. 9 and 10  described later. 
     As shown in  FIG. 5 , the electrode assembly  400  according to the present embodiment has an oval shape that is flat in the Z-axis direction when viewed from the direction of the winding axis W. Such a shape is formed by winding an element constituting the electrode assembly  400  (hereinafter, also referred to as a “laminated element”) such as a negative electrode plate  420  and then compressing the element in the Z-axis direction. 
     In the present embodiment, the electrode assembly  400  has the core material  500 , and the core material  500  also has a roughly flat shape in the Z-axis direction. As shown in  FIG. 7 , the core material  500  has a first line part  501  extending along a first imaginary line VL 1  and a second line part  502  extending along a second imaginary line VL 2 . The first imaginary line VL 1  and the second imaginary line VL 2  are imaginary lines parallel to the long side surface  111   a  (see  FIG. 2 ) of the case  100  of the energy storage device  10 . More specifically, the first imaginary line VL 1  and the second imaginary line VL 2  are parallel to the long side surface  111   a  and, when viewed from the direction of the winding axis W, are a pair of imaginary straight lines passing through both end portions of the core material  500  in the thickness direction (Z-axis direction). 
     In the present embodiment, as the core material  500 , a core material  500  formed in a tubular shape by winding a resin sheet  600  made of polypropylene, polyethylene or the like is adopted. That is, the core material  500  is a member having relatively high flexibility. Therefore, when the electrode assembly  400  is pressed in the Z-axis direction as described above, the core material  500  is flattened according to the pressing force without damaging the laminated elements such as the separator  430  around the core material  500 . That is, the core material  500  is formed in a shape that is elongated in the Y-axis direction and has a pair of curve parts  531  and  532  that face each other in the Y-axis direction when viewed from the direction of the winding axis W. 
     More specifically, when the core material  500  according to the present embodiment is manufactured, the winding start portion of the resin sheet  600  is formed into an S shape, and as shown in  FIG. 5 , for example, two places P 1  and P 2  are welded. Further, the resin sheet  600  is wound around the S-shaped portion. As a result, the core material  500  is formed with a partition wall part  520  that crosses the internal space when viewed from the direction of the winding axis W. That is, as shown in  FIG. 5 , the internal space of the tubular core material  500  is divided into a first hollow portion  521  and a second hollow portion  522  by the partition wall part  520 . 
     In the drawings after  FIG. 5 , the core material  500  is formed by winding the resin sheet  600  about one and a half turns, but the number of turns of the resin sheet  600  forming the core material  500  is not particularly limited. For example, the core material  500  may be formed by winding the resin sheet  600  around the S-shaped portion for one or more turns. It is not essential that the core material  500  is formed by winding the resin sheet  600  one or more turns. For example, a cylinder is manufactured by resin molding using a mold, and the cylinder may be adopted as the core material  500 . 
     When a laminated element such as a separator  430  is wound around the core material  500  formed in this way, a part of the highly flexible core material  500  is curved inward by the tension of the laminated element. As a result, the core material  500  is formed with a curved portion  510  as shown in  FIG. 5 . 
     Specifically, in the step of winding the laminated element such as the separator  430  around the core material  500  (winding step), a winding device  700  for rotating the core material  500  is used. As shown in  FIG. 6 , the winding device  700  has a pair of support members  710  for rotating the core material  500 . That is, the core material  500  is supported by the support member  710  inserted into the first hollow portion  521  and the support member  710  inserted into the second hollow portion  522 , and is rotated around the winding axis W in that state. As shown in  FIG. 6 , the end portions of the separators  430  and  450  are fixed to the core material  500  by a predetermined method such as welding, and the rotation of the core material  500  is started in this state. After that, the negative electrode plate  420  is sandwiched outside the separator  430  and inside the separator  450 , and the positive electrode plate  410  is sandwiched outside the separator  450  and inside the separator  430 . As a result, the electrode assembly  400  in which the separator  430 , the negative electrode plate  420 , the separator  450 , and the positive electrode plate  410  are wound around the core material  500  can be obtained. 
     In  FIG. 7 , only one layer of the separator  430  is arranged inside the negative electrode plate  420  (the side closer to the core material  500 ), but inside the negative electrode plate  420 , a multi-layer separator  430  wound around the core material  500  may be arranged. Inside the negative electrode plate  420 , the separators  430  and  450  may be wound around the core material  500  in a state of being overlapped with each other. 
     In the above winding step, the core material  500  is supported by the support members  710  at both ends in the longitudinal direction (horizontal direction in  FIG. 6 ), and at this point, the curved portion  510  does not exist clearly in the core material  500 . However, when the pair of support members  710  are subsequently removed from the core material  500  around which the separator  430  and the like are wound, the core material  500  is partially curved inward due to the force received from the laminated elements such as the separator  430 . As a result, the curved portion  510  is formed. 
     More specifically, as shown in  FIG. 7 , the core material  500  has a fixed portion  560  to which the separators  430  and  450  are fixed, and in the winding step, the fixed portion  560  is in a state of being pulled in the winding direction (rightward in  FIG. 7 ). When the pair of support members  710  is removed from the core material  500  in that state, the fixed portion  560  is in a state in which the side of the fixed portion  560  in the winding direction is easy to curve due to the tension of the separators  430  and  450  acting on the fixed portion  560 . Further, a tightening force due to a laminated element such as a separator  430  wound around the core material  500  acts on the core material  500 , and as a result, a curved portion  510  is formed on the side of the fixed portion  560  in the winding direction. In the present embodiment, the first line part  501  of the core material  500  is formed with the curved portion  510  that protrudes toward the second imaginary line VL 2  beyond the first imaginary line VL 1 . The curved portion  510  is formed adjacent to the fixed portion  560  in the first line part  501 . Even when the electrode assembly  400  on which the curved portion  510  is formed is pressed in the Z-axis direction, the curved portion  510  is not straightened flat, and the curved portion  510  remains on the core material  500 , and the current collectors  120  and  130  (see  FIGS. 1 and 2 ) are joined to the electrode assembly  400  and housed in the case  100  (see  FIG. 2 ). 
     In this way, when the curved portion  510  exists in the core material  500 , the separators  430  and  450  can move inward in the region immediately outside the curved portion  510  (outer region  550 ), whereby a gap is likely to be formed between the separators  430  and  450 . 
     Therefore, when the inner peripheral edge  420   a  of the negative electrode plate  420  is arranged in the outer region  550  as in the electrode assembly  490  according to the comparative example shown in  FIG. 8 , the inner peripheral edge  420   a  tends to float from the adjacent separator  450 . A gap is likely to be generated also between the inner peripheral edge  420   a  and the adjacent separator  430 . In this case, for example, there is a high possibility that the following problem occurs. That is, for example, fine metal powder (contamination) generated at the time of joining the current collector  120  and the electrode assembly  400  enters from the end portion of the electrode assembly  400  and is ionized, and comes into contact with the negative electrode plate  420 . As a result, a dendrite is formed on the negative electrode plate  420 , and the dendrite penetrates the separator  450  to cause a slight short circuit between the positive electrode plate  410  and the negative electrode plate  420 . 
     Therefore, in the present embodiment, as shown in  FIG. 7 , a configuration is adopted in which the inner peripheral edge  420   a  at the winding start position of the negative electrode plate  420  is arranged at a position other than the curved portion  510  of the core material  500 . Specifically, a configuration is adopted in which the inner peripheral edge  420   a  is not arranged in the outer region  550  of the curved portion  510  of the core material  500 . 
     That is, the energy storage device  10  according to the present embodiment includes a tubular core material  500 , and an electrode assembly  400  having an electrode plate and a separator wound around the core material  500 . When viewed from the direction of the winding axis W of the electrode assembly  400 , the core material  500  has a first line part  501  and a second line part  502  extending along a first imaginary line VL 1  and a second imaginary line VL 2  parallel to the long side surface  111   a  of the case  100 , respectively. At least one of the first line part  501  and the second line part  502  has a curved portion  510  that protrudes toward the other beyond the first imaginary line VL 1  or the second imaginary line VL 2 . The inner peripheral edge  420   a  of the negative electrode plate  420  at the winding start position is located at a position other than the curved portion  510  in at least one of the first line part  501  and the second line part  502 . In the present embodiment, the first line part  501  has a curved portion  510  that protrudes toward the second imaginary line VL 2  beyond the first imaginary line VL 1 . The inner peripheral edge  420   a  is arranged at a position other than the curved portion  510  in the first line part  501 . 
     Specifically, in the electrode assembly  400  according to the present embodiment, as illustrated in  FIG. 7 , the negative electrode plate  420  and the separator  430  are wound around the tubular core material  500  with the separator  430  inside. In this electrode assembly  400 , the inner peripheral edge  420   a  of the negative electrode plate  420  is not located in the outer region  550  of the curved portion  510  of the core material  500 . That is, the inner peripheral edge  420   a  is arranged at a position other than the curved portion  510 , whereby the inner peripheral edge  420   a  of the negative electrode plate  420  is pressed outward by the separator  430  on the inner side (core material  500  side). As a result, the floating of the inner peripheral edge  420   a  is suppressed. That is, the end portion of the negative electrode plate  420  including the inner peripheral edge  420   a  is sandwiched between the separators  430  and  450  from both sides, and the problem such as a slight short circuit due to contamination entering from the end portion of the electrode assembly  400  is hard to occur. Therefore, the energy storage device  10  according to the present embodiment is a highly reliable energy storage device. 
     In the present embodiment, when viewed from the direction of the winding axis W, the core material  500  has a fixed portion  560 , which is a portion to which the separator  430  is fixed, in the first line part  501 . The curved portion  510  is formed adjacent to the fixed portion  560  in the first line part  501 . In the present embodiment, the end portions of the separators  430  and  450  are fixed to the fixed portion  560  by heat welding or the like. 
     As described above, the fixed portion  560  in the core material  500  is a portion that is directly pulled in the winding direction by the separators  430  and  450  in the winding step. Therefore, in the core material  500 , the curved portion  510  is likely to be formed at a position adjacent to the fixed portion  560 . For example, a relatively large curved portion  510  is likely to be formed on the side of the fixed portion  560  in the winding direction of the negative electrode plate  420  and the separator  430 . The portion other than the curved portion  510  tends to be in a state of protruding outward. Therefore, by arranging the inner peripheral edge  420   a  of the negative electrode plate  420  at a position avoiding the outer region  550  of the curved portion  510 , the suppression of the floating of the inner peripheral edge  420   a  is further ensured. For example, before starting the winding step, the position of the curved portion  510  formed after that can be easily specified with reference to the fixed portion  560 . Therefore, it is easy to manufacture the electrode assembly  400  in which the inner peripheral edge  420   a  of the negative electrode plate  420  is arranged at a position other than the curved portion  510 . 
     In the energy storage device  10  according to the present embodiment, the inner peripheral edge  420   a  is located at the second line part  502 . 
     As described above, in the present embodiment, the fixed portion  560  is arranged in the first line part  501 , and the inner peripheral edge  420   a  of the negative electrode plate  420  is arranged in the second line part  502 . In other words, the fixed portion  560  is arranged in one of the regions divided into two by the long axis L, and the inner peripheral edge  420   a  of the negative electrode plate  420  is arranged in the other of the regions divided into the two. (See  FIG. 7 ). Therefore, the inner peripheral edge  420   a  of the negative electrode plate  420  is not easily affected by the curved portion  510  located on the side of the fixed portion  560 . As a result, the floating of the inner peripheral edge  420   a  is more reliably suppressed. 
     In the energy storage device  10  according to the present embodiment, in the electrode assembly  400 , the fixed portion  560  and the inner peripheral edge  420   a  are arranged at positions facing each other with the winding axis W interposed therebetween. That is, when the electrode assembly  400  is viewed from the direction of the winding axis W, the inner peripheral edge  420   a  of the negative electrode plate  420  is located on or near the straight line passing through the fixed portion  560  and the winding axis W 
     That is, in the present embodiment, the inner peripheral edge  420   a  of the negative electrode plate  420  exists at a position farthest from the fixed portion  560  or its vicinity in the circumferential direction of the core material  500 , and is therefore not easily affected by the curved portion  510  located on the side of the fixed portion  560 . As a result, the floating of the inner peripheral edge  420   a  is more reliably suppressed. 
     In the energy storage device  10  according to the present embodiment, the internal space of the core material  500  is divided into two spaces by a partition wall part  520  that crosses the internal space when viewed from the direction of the winding axis W. The fixed portion  560  is located outside one of the two spaces, and the inner peripheral edge  420   a  is located outside the other of the two spaces. More specifically, as shown in  FIG. 7 , for example, the internal space of the core material  500  is divided into a first hollow portion  521  and a second hollow portion  522  by the partition wall part  520 . The fixed portion  560  is located outside the first hollow portion  521 , and the inner peripheral edge  420   a  is located outside the second hollow portion  522 . 
     The curved portion  510  formed due to the fixed portion  560  being pulled in the winding direction (see  FIG. 7 ) is hard to be formed at a portion opposite to the fixed portion  560  with the partition wall part  520  sandwiched by the partition wall part  520  being pulled. Therefore, according to the energy storage device  10  according to the present embodiment, the inner peripheral edge  420   a  of the negative electrode plate  420  is arranged at a position where the separator  430  inside the negative electrode plate  420  is hard to float. As a result, the floating of the inner peripheral edge  420   a  is more reliably suppressed. 
     Although the energy storage device  10  according to the embodiment has been described above, the configuration of the electrode assembly  400  included in the energy storage device  10  may be different from the configuration shown in  FIGS. 5 to 7 . Therefore, a modification example of the configuration of the electrode assembly  400  will be described below, focusing on the difference from the above embodiment. 
     First Modification Example 
       FIG. 9  is a diagram showing an outline of a configuration of a core material  500  of an electrode assembly  400   a  and its surroundings according to the first modification of the embodiment. In the electrode assembly  400   a  shown in  FIG. 9 , a fixed portion  560  and an inner peripheral edge  420   a  of a negative electrode plate  420  are arranged at positions facing each other in the longitudinal direction (Y-axis direction) of the core material  500  when viewed from the direction of the winding axis W. 
     That is, in the energy storage device  10  according to this modification example, the core material  500  is elongated in a predetermined direction (Y-axis direction in this modification example) when viewed from the direction of the winding axis W, and has a shape having a pair of curve parts  531  and  532  facing each other in the Y-axis direction. The fixed portion  560  is arranged on one of the pair of curve parts  531  and  532 , and the inner peripheral edge  420   a  is arranged outside the other of the pair of curve parts  531  and  532 . In the example shown in  FIG. 9 , the fixed portion  560  is arranged on the curve part  531  of the core material  500 , and the inner peripheral edge  420   a  is arranged outside the curve part  532 . 
     In the core material  500  formed in an oval shape when viewed from the direction of the winding axis W, tension of laminated elements such as the separator  430  is likely to be applied to the outside of each of the curve parts  531  and  532  during winding. That is, the outside of each of the curve parts  531  and  532  is a portion where the density of the laminated elements is high. Therefore, since the inner peripheral edge  420   a  of the negative electrode plate  420  is located in this portion, the inner peripheral edge  420   a  is firmly pressed inward by the outer separator  450 , and since it is also located far from the fixed portion  560 , it is not easily affected by the curved portion  510  on the side of the fixed portion  560 . As a result, the floating of the inner peripheral edge  420   a  is more reliably suppressed. As a result, the occurrence of a problem such as a slight short circuit due to contamination is suppressed. 
     Second Modification Example 
       FIG. 10  is a diagram showing an outline of a configuration of an electrode assembly  400   b  according to the second modification example of the embodiment. In the electrode assembly  400   b  according to the present modification example, unlike the electrode assembly  400  according to the above embodiment, a core material  580  is formed by a part of the laminated elements. Specifically, as shown in  FIG. 10 , the core material  580  is formed by winding a part of a separator  430 . That is, the electrode assembly  400   b  is formed by winding a negative electrode plate  420 , a separator  450 , a positive electrode plate  410 , and a remaining portion of the separator  430  around an outer circumference of the core material  580  formed by a part of the separator  430 . 
     When the core material is formed by a part of the separator, for example, a portion from the edge at the start of winding of the separator to the lateral position (electrode plate start position) of the inner peripheral edge may be defined as “core material”. That is, in the case of the present modification example, as shown in  FIG. 10 , a portion around which the separator  430  is wound, which is a portion from the innermost peripheral edge Pa to an electrode plate start position Pb, can be treated as the core material  580 . 
     In this way, even when the core material  580  is formed by winding the separator  430 , the curved portion  510  is formed because the core material  580  has relatively high flexibility. That is, when a pair of support members  710  is removed from the core material  580  after the winding step using the winding device  700  (see  FIG. 6 ), the core material  580  is partially curved inside due to the force received from the laminated elements. As a result, the curved portion  510  is formed. Therefore, by arranging the inner peripheral edge  420   a  of the negative electrode plate  420  at a position other than the curved portion  510  (in  FIG. 10 , a position different from the outer region  550  of the curved portion  510  in the radial direction of the core material  580 ), the floating of the inner peripheral edge  420   a  is suppressed. As a result, the occurrence of a problem such as a slight short circuit due to contamination is suppressed. 
     In the example shown in  FIG. 10 , the core material  580  is formed only by the separator  430 , but for example, the core material  580  may be formed by winding the separators  430  and  450  in layers. The winding start end of the separator  450  may be fixed to the core material  580  formed of the separator  430  having one or more layers inside the separator  450  by welding or the like. In this case, the portion of the core material  580  to which the separator  450  is fixed is the fixed portion of the core material  580 , and the curved portion  510  is formed adjacent to the fixed portion. 
     Other Embodiments 
     The energy storage device according to the present invention has been described above based on the embodiments and modification examples. However, the present invention is not limited to the above-mentioned embodiments and modification examples. As long as the gist of the present invention is not deviated, various modification examples that can be conceived by those skilled in the art are applied to the above-described embodiment or modification example, or a form constructed by combining the plurality of components described above is also included within the range of the present invention. 
     For example, in the embodiments and modification examples, focusing on the negative electrode plate  420  arranged on the inner peripheral side of the negative electrode plate  420  and the positive electrode plate  410 , that the inner peripheral edge  420   a  of the negative electrode plate  420  is arranged at a portion other than the curved portion  510  (for example, the position different from the outer region  550 , the same applies hereinafter) and its effect have been described. However, in addition to or in place of the inner peripheral edge  420   a  of the negative electrode plate  420 , the inner peripheral edge of the positive electrode plate  410  may be arranged at a position other than the curved portion  510 . As a result, a gap is hard to be generated in the vicinity of the inner peripheral edge of the positive electrode plate  410 , and as a result, the occurrence of the problem such as a slight short circuit due to contamination is suppressed. 
     In the core material  500  according to the embodiment, the partition wall part  520  is not essential. For example, by simply winding the resin sheet  600 , a tubular core material  500  having no wall that crosses the internal space may be formed. 
     For example, when the position of the fixed portion  560  on the core material  500  is the position shown in  FIG. 7 , the position of the inner peripheral edge  420   a  of the negative electrode plate  420  does not have to be the position shown in  FIG. 7 . For example, in the core material  500 , the position of the fixed portion  560  and the position of the inner peripheral edge  420   a  may exist in the same side region when the long axis L is referred to (for example, the region below the long axis L in  FIG. 7 ). Even in this case, since the inner peripheral edge  420   a  is arranged at a position other than the curved portion  510 , the floating of the inner peripheral edge  420   a  is suppressed. 
     In the present embodiment, the energy storage device  10  includes only one electrode assembly  400 , but the number of electrode assemblies  400  included in the energy storage device  10  may be two or more. For example, when the energy storage device  10  includes two electrode assemblies  400 , the current collector  120  may have four legs  122  joined to the two electrode assemblies  400 . 
     Forms which are constructed by arbitrarily combining the configurations described in the above-mentioned embodiments are also included in the scope of the present invention. 
     The present invention can be realized not only as the energy storage device described above, but also as the electrode assembly  400  included in the energy storage device. The present invention can also be realized as an energy storage apparatus including a plurality of the energy storage devices. 
     INDUSTRIAL APPLICABILITY 
     The present invention can be applied to an energy storage device or the like such as a lithium ion secondary battery. 
     DESCRIPTION OF REFERENCE SIGNS 
     
         
           10 : energy storage device 
           100 : case 
           111   a:  long side surface 
           400 ,  400   a,    400   b:  electrode assembly 
           410 : positive electrode plate 
           420 : negative electrode plate 
           420   a:  inner peripheral edge 
           430 ,  450 : separator 
           500 ,  580 : core material 
           501 : first line part 
           502 : second line part 
           510 : curved portion 
           520 : partition wall part 
           521 : first hollow portion 
           522 : second hollow portion 
           531 ,  532 : curve part 
           550 : outer region 
           560 : fixed portion