Electricity storage device

Abstract

The present invention is characterized by comprising: a cylindrical case (20) having an opening and a bottom (50) located below the opening; an electrode body (14) disposed inside the case (20); and a metal sheet (60) fixed to the bottom (50) of the case (20) and disposed inside the case (20) between the electrode body (14) and the bottom (50), wherein a first electrode plate (12) includes a core material section (40) and a composite section (41) disposed on the surface of the core material section (40), the core material section (40) includes a core material exposure section (42) fixed to the metal sheet (60), the bottom (50) includes a protrusion (51) projecting upward, and the protrusion (51) and the core material exposure section (42) are welded with the metal sheet (60) interposed therebetween.

Description

Power storage device

[0001] The present disclosure relates to a power storage device.

[0002] Conventionally, as a power storage device, a cylindrical secondary battery including an electrode body including a positive electrode and a negative electrode and a case that houses the electrode body is known. Generally, the positive electrode and the negative electrode each have a core material portion and a mixture portion that contains an active material or the like and is disposed on the surface of the core material portion. Patent Document 1 discloses a cylindrical secondary battery in which a plate-like current collector member welded to the bottom of the case is provided between the electrode body and the bottom of the case, and the core material portion of the negative electrode is welded to the current collector member. By electrically connecting the core material portion of the negative electrode and the bottom of the case via the current collector member, the resistance of the battery can be reduced, and the input / output characteristics of the battery can be improved.

[0003] Japanese Patent Application Laid-Open No. HEI 16-233707

[0004] From the viewpoint of further reducing resistance and increasing capacity, a method of welding the core material portion of the electrode plate and the bottom of the case without using a plate-like current collector member is conceivable. On the other hand, when welding the core material portion of the electrode plate and the bottom of the case, it has been found that the separator constituting the electrode body may thermally contract due to the heat during welding. When the separator thermally contracts, for example, when charging and discharging are repeated and the electrode plate expands, insulation between the positive electrode and the negative electrode cannot be ensured, and an internal short circuit may occur. Therefore, when welding the core material portion of the electrode plate and the bottom of the case, it is required to suppress thermal contraction of the separator while ensuring the welding area.

[0005] A power storage device according to one aspect of the present disclosure includes a cylindrical case having an opening and a bottom portion located below the opening, an electrode body disposed in the case and having a first electrode plate, a second electrode plate, and a separator disposed between the first electrode plate and the second electrode plate, the first electrode plate and the second electrode plate being wound with the separator interposed therebetween, and a metal sheet disposed in the case and between the electrode body and the bottom of the case and fixed to the bottom. The first electrode plate has a core material portion and a mixture portion disposed on the surface of the core material portion. The core material portion includes a core material exposed portion fixed to the metal sheet. The bottom portion includes a convex portion protruding upward. The convex portion and the core material exposed portion are welded via the metal sheet.

[0006] According to one aspect of the present disclosure, when welding the core material portion of the electrode plate to the bottom of the case, it is possible to suppress thermal shrinkage of the separator while ensuring sufficient welding area. As a result, it is possible to provide a highly reliable energy storage device while achieving low resistance.

[0007] This is an axial cross-sectional view of a cylindrical secondary battery, which is one example of an embodiment. This is a perspective view of the electrode body constituting a cylindrical secondary battery, which is one example of an embodiment. This is an axial cross-sectional view of a cylindrical secondary battery, which is one example of an embodiment, showing an enlarged view of the vicinity of the bottom of the case. This is a cross-sectional perspective view of the case of a cylindrical secondary battery, which is one example of an embodiment. This is a plan view of the bottom of the case of a cylindrical secondary battery, which is one example of an embodiment, as seen from below. This is a cross-sectional perspective view of the case of a cylindrical secondary battery, which is another example of an embodiment. This is a plan view of the bottom of the case of a cylindrical secondary battery, which is another example of an embodiment, as seen from below. This is a plan view of the metal sheet as seen from above, with the metal sheet placed at the bottom of the case of a cylindrical secondary battery, which is another example of an embodiment. This is a plan view of the metal sheet as seen from above, with the metal sheet placed at the bottom of the case of a cylindrical secondary battery, which is another example of an embodiment. This is a plan view of the metal sheet as seen from above, with the metal sheet placed at the bottom of the case of a cylindrical secondary battery, which is another example of an embodiment. This is a plan view of a cylindrical secondary battery case, as seen from above, with a metal sheet placed at the bottom of the case, which is another example of the embodiment.

[0008] The embodiments of the energy storage device according to this disclosure will be described in detail below with reference to the drawings. In the following, a cylindrical secondary battery using a non-aqueous electrolyte, more specifically a lithium-ion cylindrical secondary battery, will be given as an example of the energy storage device, but the energy storage device of this disclosure is not limited to this. The energy storage device of this disclosure is not limited to a battery using a non-aqueous electrolyte, but may also be a battery using an aqueous electrolyte. Furthermore, the energy storage device of this disclosure is not limited to a secondary battery, but may also be a primary battery. Furthermore, the energy storage device of this disclosure may be a battery other than a cylindrical battery, for example, a prismatic battery, etc. Furthermore, the energy storage device of this disclosure may be a capacitor (capacitor) instead of a battery.

[0009] A cylindrical secondary battery 10, which is an example of an embodiment, will be described in detail. In the following description, the specific shape, material, numerical values, direction, etc., are examples to facilitate understanding of the present invention and can be appropriately modified according to the specifications of the cylindrical secondary battery 10. Furthermore, if the following description includes multiple embodiments and modifications, it is intended from the outset that their characteristic parts may be used in appropriate combinations.

[0010] First, the configuration of a cylindrical secondary battery 10, which is an example of an embodiment, will be described with reference to Figures 1 and 2. Figure 1 is an axial cross-sectional view of the cylindrical secondary battery 10, and Figure 2 is a perspective view of the electrode body 14 that constitutes the cylindrical secondary battery 10.

[0011] As shown in Figure 1, the cylindrical secondary battery 10 comprises an electrode body 14 in which a first electrode plate and a second electrode plate with opposite polarities are wound around a separator 13, a non-aqueous electrolyte (not shown), and a case 20 that houses the electrode body 14 and the non-aqueous electrolyte. The case 20 has a cylindrical shape with a bottom 50 and an opening 23, and the opening 23 of the case 20 is sealed by a sealing plate 15. The opening 23 of the case 20 may also be sealed by a sealing body including a gasket 17 or other multiple members in addition to the sealing plate 15. The cylindrical secondary battery 10 further comprises a positive electrode current collector plate 16 disposed between the sealing plate 15 and the electrode body 14, and a metal sheet 60 disposed between the electrode body 14 and the bottom 50 of the case 20. In this specification, the side with the sealing plate 15 of the cylindrical secondary battery 10 is referred to as "upper," and the side with the bottom 50 of the case 20 is referred to as "lower." Furthermore, the following section will describe the case where the first electrode plate is the negative electrode 12 and the second electrode plate is the positive electrode 11.

[0012] As shown in Figures 1 and 2, the electrode body 14 has a positive electrode 11, a negative electrode 12, and a separator 13, and the positive electrode 11 and the negative electrode 12 are wound in a spiral shape via the separator 13. The positive electrode 11, the negative electrode 12, and the separator 13 that make up the electrode body 14 are all elongated strip-shaped bodies, and are alternately stacked in the radial direction of the electrode body 14 by being wound in a spiral shape. The negative electrode 12 is formed to be slightly larger in dimensions than the positive electrode 11 in order to prevent lithium deposition. That is, the negative electrode 12 is formed to be longer in the longitudinal and width directions than the positive electrode 11.

[0013] The positive electrode 11 has a long positive electrode core material portion 30 and a positive electrode mixture portion 31 formed on the positive electrode core material portion 30. The positive electrode core material portion 30 can be made of a metal foil that is stable in the potential range of the positive electrode 11, such as aluminum or an aluminum alloy, or a film on which such metal is arranged on the surface. The positive electrode mixture portion 31 contains a positive electrode active material, a conductive agent, and a binder, and is preferably formed on both sides of the positive electrode core material portion 30. The positive electrode 11 can be manufactured, for example, by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, and a binder onto the positive electrode core material portion 30, drying the coating film, and then compressing it to form the positive electrode mixture portion 31 on both sides of the positive electrode core material portion 30.

[0014] The thickness of the positive electrode core material portion 30 is, for example, 10 μm or more and 30 μm or less. The thickness of the positive electrode mixture portion 31 is, for example, 50 μm or more and 120 μm or less on one side of the positive electrode core material portion 30. The thickness here refers to the length of the electrode body 14 as seen in the radial direction.

[0015] The positive electrode mixture portion 31 contains particulate lithium metal composite oxide as the positive electrode active material. The lithium metal composite oxide is a composite oxide containing metal elements such as Co, Mn, Ni, and Al in addition to Li. The metal elements constituting the lithium metal composite oxide are, for example, at least one selected from Mg, Al, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Y, Zr, Sn, Sb, W, Pb, and Bi. Among these, it is preferable to contain at least one selected from Co, Ni, and Mn. Examples of suitable composite oxides include lithium metal composite oxides containing Ni, Co, and Mn, or lithium metal composite oxides containing Ni, Co, and Al.

[0016] Examples of conductive agents included in the positive electrode mixture 31 include carbon black such as acetylene black and Ketjenblack, graphite, carbon nanotubes (CNTs), carbon nanofibers, and graphene. Examples of binders included in the positive electrode mixture 31 include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide, acrylic resins, and polyolefins. In addition, these resins may be used in combination with carboxymethylcellulose (CMC) or its salts, polyethylene oxide (PEO), etc.

[0017] In this embodiment, the positive electrode core material portion 30 does not have a positive electrode mixture portion 31 formed on its surface, and has a positive electrode core material exposed portion 32 at its upper end where the positive electrode core material portion 30 is exposed. The positive electrode core material exposed portion 32 is provided over a range from the beginning end to the end end of the winding in the longitudinal direction of the elongated positive electrode 11. The width of the positive electrode core material exposed portion 32 is, for example, 2 mm or more and 20 mm or less.

[0018] The negative electrode 12 has a long negative electrode core material portion 40 and a negative electrode mixture portion 41 formed on the negative electrode core material portion 40. The negative electrode core material portion 40 can be made of a metal foil that is stable in the potential range of the negative electrode 12, such as copper or a copper alloy, or a film on which such metal is arranged on the surface. The negative electrode mixture portion 41 contains a negative electrode active material, a binder, and optionally a conductive agent, and is preferably formed on both sides of the negative electrode core material portion 40. The negative electrode 12 can be manufactured by applying a negative electrode mixture slurry containing a negative electrode active material and a binder to the surface of the negative electrode core material portion 40, drying the coating film, and then compressing it to form the negative electrode mixture portion 41 on both sides of the negative electrode core material portion 40.

[0019] The thickness of the negative electrode core material portion 40 is, for example, 5 μm or more and 15 μm or less. The thickness of the negative electrode mixture portion 41 is, for example, 50 μm or more and 120 μm or less on one side of the negative electrode core material portion 40.

[0020] The negative electrode mixture 41 generally contains a carbon material that reversibly intercepts and releases lithium ions as the negative electrode active material. A suitable example of the carbon material is graphite such as natural graphite like flake graphite, lump graphite, or clay graphite, or artificial graphite such as lump graphite (MAG) or graphitized mesophase carbon microbeads (MCMB). In addition, a material containing at least one of an element that alloys with Li, such as Si or Sn, and a material containing such an element may be used as the negative electrode active material. Among these, composite materials containing Si are preferred.

[0021] A preferred example of a composite material containing Si is SiO ２ Examples include materials in which Si fine particles are dispersed in a phase or silicate phase such as lithium silicate, or materials in which Si fine particles are dispersed in an amorphous carbon phase. A conductive layer, such as a carbon film, is formed on the particle surface of the composite material. Using a carbon material and a Si-containing composite material in combination as a negative electrode active material is preferable from the viewpoint of achieving both high capacity and high durability of the battery.

[0022] The binder in the negative electrode mixture 41 may be a fluororesin, PAN, polyimide, acrylic resin, polyolefin, etc., as in the positive electrode mixture 31, but styrene-butadiene rubber (SBR) is preferred. Furthermore, the negative electrode mixture 41 preferably contains CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), etc. In particular, it is preferable to use SBR in combination with CMC or a salt thereof, PAA or a salt thereof. The negative electrode mixture 41 may also contain a conductive agent such as CNT.

[0023] The negative electrode core material portion 40 has no negative electrode mixture portion 41 formed on its surface, and has a negative electrode core material exposed portion 42 at its lower end where the negative electrode core material portion 40 is exposed. The negative electrode core material exposed portion 42 is provided over the range from the beginning end to the end end of the winding in the longitudinal direction of the elongated negative electrode 12. The width of the negative electrode core material exposed portion 42 is, for example, 2 mm or more and 20 mm or less.

[0024] A porous sheet having ion permeability and insulating properties is used for the separator 13. Specific examples of porous sheets include microporous thin films, woven fabrics, and nonwoven fabrics. Suitable materials for the separator 13 include polyethylene, polyolefins such as polypropylene, and cellulose. The separator 13 may have a single-layer structure or a multi-layer structure. A heat-resistant resin layer, such as aramid resin, may be formed on the surface of the separator 13. A filler layer containing an inorganic filler may be formed at the interface between the separator 13 and at least one of the positive electrode 11 and the negative electrode 12.

[0025] Non-aqueous electrolytes are lithium ion conductive. Non-aqueous electrolytes may be liquid electrolytes (electrolytes) or solid electrolytes.

[0026] A liquid electrolyte (electrolyte solution) comprises a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of non-aqueous solvents include esters, ethers, nitriles, amides, and mixtures of two or more of these. Examples of non-aqueous solvents include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and mixtures thereof. The non-aqueous solvent may also contain halogen-substituted solvents (e.g., fluoroethylene carbonate) in which at least some of the hydrogen atoms of the solvent are replaced with halogen atoms such as fluorine. Examples of electrolyte salts include LiPF4. ６ Lithium salts such as these are used.

[0027] As the solid electrolyte, for example, a solid or gel-like polymer electrolyte, an inorganic solid electrolyte, etc., can be used. As the inorganic solid electrolyte, materials known for all-solid-state lithium-ion secondary batteries, etc. (for example, oxide-based solid electrolytes, sulfide-based solid electrolytes, halogen-based solid electrolytes, etc.) can be used. The polymer electrolyte includes, for example, a lithium salt and a matrix polymer, or a non-aqueous solvent, a lithium salt and a matrix polymer. As the matrix polymer, for example, a polymer material that absorbs a non-aqueous solvent and gels is used. Examples of polymer materials include fluororesins, acrylic resins, polyether resins, etc.

[0028] As shown in Figure 1, the case 20 is a bottomed cylindrical metal container with an open top. Examples of metal materials that make up the case 20 include carbon steel, stainless steel, aluminum, and aluminum alloys, with carbon steel being preferred. The surface of the case 20 may be plated with a known metal such as nickel.

[0029] The case 20 has a bottom portion 50 and a side portion 21 that forms the side surface of the cylindrical secondary battery 10. The side portion 21 is the part of the case 20 excluding the bottom portion 50 and includes a grooved portion 22 and an opening 23, which will be described later. The side portion 21 and the bottom portion 50 of the case 20 are integrally formed by drawing a single metal plate. In this embodiment, the thickness of the case 20 is substantially constant throughout. The thickness of the case 20 is, for example, 0.2 mm or more and 1 mm or less.

[0030] The bottom portion 50 has a circular shape in plan view. The inner surface of the bottom portion 50 is flat except for the area where the convex portion 51, described later, is formed. The bottom portion 50 may also have a thin-walled portion that is locally formed to be thinner. The thin-walled portion is formed in an annular or arc shape, and when the internal pressure of the battery reaches a predetermined pressure, the thin-walled portion preferentially ruptures, and the area surrounded by the thin-walled portion functions as a discharge valve. If the internal pressure of the battery rises due to abnormal heat generation or the like, the battery components such as the electrode body 14 are discharged to the outside of the battery through the discharge valve.

[0031] Furthermore, the bottom portion 50 has a protrusion 51 that projects upward. The protrusion 51 can be formed, for example, by press-forming a part of the bottom portion 50 to create an indentation on the upward side. The protrusion 51 includes a top portion 52 that projects most upward, and a connecting portion 53 that connects the top portion 52 to the portion of the bottom portion 50 other than the protrusion 51 (hereinafter sometimes referred to as the "base portion 54"). The inner surface of the top portion 52 is formed to be flat throughout its plane. In this embodiment, the connecting portion 53 extends upward along a direction that is inclined radially inward with respect to the vertical direction. As will be described in detail later, in the battery manufacturing process, the negative electrode core material exposed portion 42 and the metal sheet 60 are melted by irradiating the outer surface of the protrusion 51 with a laser and heating it, and the protrusion 51 and the negative electrode core material exposed portion 42 are welded together via the metal sheet 60. The weld marks at this time are provided across the protrusion 51, the metal sheet 60, and the negative electrode core material exposed portion 42. As a result, the negative electrode 12 and the case 20 are electrically connected, and the case 20 becomes the negative electrode terminal.

[0032] The grooved portion 22 is a part of the side surface 21 that is recessed radially inward, and is provided in an annular shape along the circumferential direction of the case 20. The grooved portion 22 supports the sealing plate 15 via the gasket 17 on its upper surface. The grooved portion 22 can be formed, for example, by recessing a part of the side surface 21 radially inward in an annular shape by pressing, spinning, or the like.

[0033] The opening 23 forms the opening of the case 20 and is bent radially inward when the sealing plate 15 is crimped and fixed to the case 20. The side portion 21 is provided integrally with the bottom portion 50, and the opening 23 is located at the top of the side portion 21.

[0034] The sealing plate 15 is a disc-shaped metal member. The sealing plate 15 is crimped and fixed to the opening 23 of the case 20 via a gasket 17. The sealing plate 15 is made of a metal, for example, mainly composed of aluminum. However, the configuration of the sealing plate 15 is not limited to this, as long as it can close the opening 23 of the case 20. The sealing plate 15 may be made of multiple members stacked together, for example. The sealing plate 15 is a separate member from the case 20 (side portion 21, bottom portion 50). The sealing plate 15 is integrated with the case 20 by crimping or welding in order to close the opening 23.

[0035] The positive electrode current collector plate 16 is a disc-shaped metal member having an outer diameter smaller than the outer diameter of the sealing plate 15. The positive electrode current collector plate 16 is joined to the sealing plate 15 at its outer circumference 16B by laser welding or the like, and is electrically connected to the sealing plate 15. The positive electrode current collector plate 16 is made of a metal mainly composed of aluminum, similar to the sealing plate 15. In this embodiment, the positive electrode current collector plate 16 has a substantially uniform thickness in its plane. The thickness of the positive electrode current collector plate 16 is, for example, 0.15 mm or more and 1.0 mm or less. The positive electrode current collector plate 16 may have an outer diameter similar to that of the sealing plate 15 and be crimped and fixed to the opening 23 of the case 20 via a gasket 17.

[0036] The positive electrode current collector plate 16 has a recess 16A in a region radially inward from the outer periphery 16B that is joined to the sealing plate 15, and the recess is recessed downward relative to the outer periphery 16B. The exposed portion 32 of the positive electrode core material of the positive electrode 11 is welded to the lower surface of the recess 16A by laser welding or the like. As a result, the sealing plate 15, which is electrically connected to the positive electrode current collector plate 16, becomes the positive electrode terminal.

[0037] The gasket 17 is a flexible insulating member that electrically insulates the sealing plate 15, which is the positive terminal, from the case 20, which is the negative terminal, while ensuring airtightness inside the case 20 when compressed. The material of the gasket 17 is not particularly limited as long as it is a compressible insulating material, and for example, polypropylene (PP), polyphenylene sulfide (PPS), polyethylene (PE), polybutylene terephthalate (PBT), perfluoroalkoxyalkane (PFA), polytetrafluoroethylene (PTFE), polyamide (PA), etc. can be used.

[0038] Next, the configuration of the metal sheet 60 and the lower part of the cylindrical secondary battery 10 will be described in detail with reference to Figures 3 to 5. Figure 3 is an enlarged view of the vicinity of the bottom 50 of the case 20 in Figure 1, Figure 4 is a cross-sectional perspective view of the case 20, and Figure 5 is a plan view of the bottom 50 as seen from below.

[0039] As shown in Figure 3, a bendable metal sheet 60 is provided inside the case 20 between the electrode body 14 and the bottom 50 of the case 20. In this embodiment, the metal sheet 60 has a substantially circular shape in plan view. The metal sheet 60 has a diameter that is about the same as, or smaller than, the inner diameter of the bottom 50, and is welded to the inner surface of the bottom 50. Furthermore, the negative electrode core material exposed portion 42 is welded to the upper surface of the metal sheet 60, and it is positioned in a state where it is pressed downward by the electrode body 14. The bendability of the metal sheet 60 is preferably such that when one end is grasped, the other end bends under its own weight.

[0040] By providing the metal sheet 60, when a laser is irradiated from the outer surface of the protrusion 51 and the exposed negative electrode core material portion 42 and the bottom portion 50 of the case 20 are welded, the heat generated by the laser irradiation is less likely to be transmitted to the area where the separator 13 of the electrode body 14 is placed. As a result, thermal shrinkage of the separator 13 due to the heat of the laser irradiation is suppressed. In other words, if the exposed negative electrode core material portion 42 and the bottom portion 50 of the case 20 are welded without providing the metal sheet 60, the heat generated by the laser irradiation may spread through the exposed negative electrode core material portion 42 to the area where the separator 13 is placed, causing thermal shrinkage of the separator 13. If the separator 13 shrinks due to thermal shrinkage, for example, when the electrode plates are stretched due to repeated charging and discharging, the positive electrode 11 and the negative electrode 12 may come into contact, potentially causing an internal short circuit.

[0041] Furthermore, by providing the metal sheet 60, when a laser is irradiated from the outer surface of the protrusion 51 and the exposed negative electrode core material portion 42 and the bottom portion 50 of the case 20 are welded, the heat generated by the laser irradiation is more easily transmitted through the metal sheet 60 and spreads horizontally. As a result, the exposed negative electrode core material portion 42 is heated and melted over a wide area. Consequently, the welding area between the exposed negative electrode core material portion 42 and the metal sheet 60 increases, making it easier to achieve lower resistance in the battery.

[0042] Preferably, the metal sheet 60 is mainly composed of the metal (for example, copper) that constitutes the negative electrode core material portion 40. By making the metal sheet 60 mainly composed of the metal that constitutes the negative electrode core material portion 40, the exposed portion 42 of the negative electrode core material and the metal sheet 60 are more easily welded together when the laser is irradiated. Also, by making the metal sheet 60 mainly composed of the metal that constitutes the negative electrode core material portion 40, it is easier to make the rigidity of the metal sheet 60 and the rigidity of the negative electrode core material portion 40 roughly the same. As a result, the metal sheet 60 is more easily deformed to conform to the shape of the bottom portion 50, and the metal sheet 60 is more easily brought into contact with the inner surface 54A of the base portion 54, which is the part of the bottom portion 50 other than the convex portion 51. As a result, the contact area between the metal sheet 60 and the inner surface of the bottom portion 50 increases, making it easier to achieve low resistance of the battery. In this specification, "main component" means the component that has the largest mass percentage among the materials that make up the metal sheet 60.

[0043] The thickness of the metal sheet 60 is, for example, 50 μm or less, preferably less than 50 μm, and more preferably 30 μm or less. By setting the thickness of the metal sheet 60 to less than 50 μm, the metal sheet 60 becomes more easily deformable along the shape of the bottom portion 50, and the metal sheet 60 becomes more likely to contact the inner surface 54A of the base portion 54 of the bottom portion 50. As a result, it becomes easier to achieve a lower resistance of the battery. Further, by setting the thickness of the metal sheet 60 to less than 50 μm, the volume of the electrode body 14 can be ensured, and it becomes easier to achieve a higher capacity. Further, by setting the thickness of the metal sheet 60 to less than 50 μm, the production cost can be reduced. In the present specification, the thickness of the metal sheet 60 means the thickness of the metal sheet 60 in a cross-sectional view in the vertical direction of the cylindrical secondary battery 10.

[0044] Further, the thickness of the metal sheet 60 is preferably equal to or greater than the thickness of the negative electrode core material portion 40. By setting the thickness of the metal sheet 60 to be equal to or greater than the thickness of the negative electrode core material portion 40, while ensuring the welding area between the negative electrode core material exposed portion 42 and the metal sheet 60, the thermal shrinkage of the separator 13 can be more suppressed. Therefore, the thickness of the metal sheet 60 is, for example, 5 μm or more and 50 μm or less, preferably 8 μm or more and less than 50 μm. More specifically, it is more preferably 8 μm or more and 30 μm or less. As a result, the amount of energy required during welding of the bottom portion 50 and the electrode body 14 is reduced, and thus deformation of the convex portion 51 due to heat can be suppressed.

[0045] The metal sheet 60 is in contact with the inner surface 51A of the convex portion 51 and the inner surface 54A of the base portion 54, which is the portion of the bottom portion 除凸部51以外的部分50. Here, among the metal sheet 60, the region in contact with the inner surface 51A of the convex portion 51 is defined as the first region 61, and the region in contact with the inner surface 54A of the base portion 54 is defined as the second region 62. The second region 62 of the metal sheet 60 is pressed downward by the electrode body 14. As a result, the contact area between the metal sheet 60 and the inner surface of the bottom portion 50 increases, and it becomes easier to achieve a lower resistance of the battery.

[0046] In a plan view of the metal sheet 60, it is preferable that the first region 61 and the second region 62 are not adjacent to each other and are formed with a gap therebetween. That is, the metal sheet 60 does not completely cover the connecting portion 53 of the convex portion 51 in close contact, and it is preferable that a gap is formed between the bottom portion 50 and the metal sheet 60 in the vicinity of the connecting portion 53.

[0047] As shown in FIGS. 4 and 5, in the present embodiment, the bottom portion 50 has one convex portion 51. In a plan view of the bottom portion 50, the convex portion 51 is formed linearly along the diameter direction of the bottom portion 50. The length of the convex portion 51 in the longitudinal direction is, for example, 50% or more and 95% or less of the diameter of the bottom portion 50, and may be 60% or more and 90% or less. Also, the length (width) of the convex portion 51 in the short direction can be set according to the spot diameter of the laser during welding, etc., and is, for example, 1 mm or more and 5 mm or less. Further, the convex portion 51 may be provided in a curved shape. By providing the convex portion 51 so as to have a curved portion, the welding length becomes larger compared to a straight line, so that the resistance can be reduced.

[0048] In a plan view of the bottom portion 50, the ratio of the area of the region where the convex portion 51 is formed to the total area of the bottom portion 50 is, for example, 3% or more and 40% or less, and may be 5% or more and 30% or less. When the ratio of the area of the region where the convex portion 51 is formed to the total area of the bottom portion 50 is 3% or more and 40% or less, it becomes easier to achieve both ensuring the welding area and suppressing the thermal shrinkage of the separator 13.

[0049] The height of the convex portion 51 is, for example, 0.2 mm or more and 1.0 mm or less, and may be 0.25 mm or more and 0.7 mm or less. Note that the height of the convex portion 51 means the length along the vertical direction from the inner surface of the base portion 54 to the inner surface of the top portion 52.

[0050] Also, when a thin portion that breaks when the internal pressure of the battery reaches a predetermined pressure and functions as a discharge valve is formed in the bottom portion 50, it is preferable that the convex portion 51 is provided in a region radially inside the thin portion. In this case, while ensuring the welding area, it becomes easier to operate the discharge valve when the internal pressure of the battery rises.

[0051] As shown in Figure 5, a weld mark 55 is formed on the bottom 50 after welding. Since the laser is irradiated along the longitudinal direction of the protrusion 51, the weld mark 55 is formed along the longitudinal direction of the protrusion 51. Here, it is preferable that the bead width, which is the length of the weld mark 55 in the short direction, is greater than the thickness of the bottom 50 (case 20). In this case, the welding area increases, making it easier to achieve low resistance in the battery.

[0052] Furthermore, the bead width of the weld mark 55 may be greater than the length (width) of the protrusion 51 in the shorter direction. In this case, the welding area increases further, making it easier to achieve lower resistance in the battery. When welding by irradiating the outer surface of the protrusion 51 with a laser, instead of performing keyhole welding, heat conduction welding is performed to weld the exposed negative electrode core material 42 by heat conduction, which increases the bead width of the weld mark 55.

[0053] Next, the manufacturing method of the cylindrical secondary battery 10 of this embodiment will be described. However, the manufacturing method of the cylindrical secondary battery 10 is not limited to the method described below.

[0054] First, with the metal sheet 60 placed on the inner surface of the bottom 50 of the case 20, the electrode body 14 having the negative electrode core material exposed portion 42 is inserted. At this time, the electrode body 14 may be inserted into the case 20 with the positive electrode core material exposed portion 32 and the positive electrode current collector plate 16 welded together beforehand, or after inserting the electrode body 14 into the case 20, a laser may be irradiated from the upper surface of the positive electrode current collector plate 16 to weld the positive electrode core material exposed portion 32 and the positive electrode current collector plate 16 together.

[0055] Then, a laser is irradiated from the outer surface of the bottom portion 50 along the protrusion 51 to heat it. The heat from the laser irradiation is then transferred to the metal sheet 60, and the heat spreads horizontally through the metal sheet 60. As a result, the exposed portion 42 of the negative electrode core material is heated over a wide area and melted. Consequently, the welding area between the exposed portion 42 of the negative electrode core material and the metal sheet 60 increases, making it easier to achieve lower resistance in the battery.

[0056] Furthermore, the metal sheet 60 makes it difficult for the heat generated by laser irradiation to reach the region where the separator 13 of the electrode body 14 is located. As a result, thermal contraction of the separator 13 due to the heat of laser irradiation is suppressed.

[0057] Furthermore, if, for example, a non-bendable plate-shaped current collector is provided instead of the film-like metal sheet 60, as described in Patent Document 1 (Japanese Patent Application Publication No. 2004-139898), the exposed portion 42 of the negative electrode core material cannot be sufficiently heated even when a laser is irradiated from the bottom 50 of the case 20. For this reason, it is necessary to pre-weld the exposed portion 42 of the negative electrode core material to the current collector before housing the electrode body 14 in the case 20, then insert the electrode body 14 into the case 20, irradiate the case 20 with a laser from the bottom 50 of the case 20, and weld the case 20 and the current collector. In other words, if a plate-shaped current collector is provided, the number of welding operations for current collection on the negative electrode 12 side will be multiple. An increase in the number of welding operations may lead to voltage defects due to spatter generation during welding, which is undesirable from the viewpoint of reliability of the cylindrical secondary battery 10. In addition, if a plate-shaped current collector is provided, the volume of the electrode body 14 decreases by the thickness of the current collector, which leads to a decrease in battery capacity.

[0058] In this embodiment, the electrode body 14 is inserted with the metal sheet 60 placed on the inner surface of the bottom portion 50, and the protrusion 51 and the exposed negative electrode core material portion 42 are welded together via the metal sheet 60 by irradiating the protrusion 51 along the protrusion 51 from the outer surface of the bottom portion 50. In other words, the number of welding operations required for current collection on the negative electrode 12 side can be reduced to one. As a result, spatter generation during welding is suppressed, and a highly reliable cylindrical secondary battery 10 can be provided. Furthermore, because the metal sheet 60 is a thin film, the volume of the electrode body 14 can be secured, making it easier to achieve a higher battery capacity.

[0059] The laser output during laser irradiation can be adjusted as appropriate by the thickness of the base 50 and the metal sheet 60. Generally, it is preferable to increase the laser output as the thickness of the base 50 and the metal sheet 60 increases.

[0060] Then, the protrusion 51 and the exposed negative electrode core material portion 42 are welded together via a metal sheet 60, and a groove 22 is formed on the side portion 21 of the case 20 by pressing, spinning, or the like. The positive electrode current collector plate 16 is then connected to the electrode body 14, and a non-aqueous electrolyte is poured into the inside of the case 20. After that, a sealing plate 15 is placed on the upper surface of the groove 22 via a gasket 17, and the opening end of the case 20 is crimped to fix the sealing plate 15 to the opening 23 of the case 20 and the positive electrode current collector plate 16.

[0061] Next, a modified example of Case 20 will be described with reference to Figures 6 to 8. Figure 6 is a cross-sectional perspective view of the modified example of Case 20, and Figure 7 is a plan view of the bottom 50 as seen from below. Note that the welding marks 55 (see Figure 5) are not shown in Figure 7.

[0062] The case 20 shown in Figures 6 and 7 differs from the embodiments shown in Figures 1 to 4 in the shape of the protrusions 51 formed on the bottom 50. Specifically, the protrusions 51 of the case 20 include a first protrusion 56, a second protrusion 57, a third protrusion 58, and a fourth protrusion 59, each extending radially from near the center of the bottom 50. The first protrusion 56, the second protrusion 57, the third protrusion 58, and the fourth protrusion 59 have the same shape and are provided at 90° intervals in the circumferential direction.

[0063] The longitudinal lengths of the first protrusion 56, second protrusion 57, third protrusion 58, and fourth protrusion 59 are, for example, 50% to 95% of the radius of the base 50, and may be 60% to 90%. The transverse lengths (widths) of the first protrusion 56, second protrusion 57, third protrusion 58, and fourth protrusion 59 can be set according to the laser spot diameter during welding, for example, 1 mm to 5 mm.

[0064] When welding the protrusions 51 and the exposed negative electrode core material portion 42 via the metal sheet 60, a laser is irradiated along each protrusion 51 from its outer surface. This heats the exposed negative electrode core material portion 42 over a wide area, increasing the welding area between the exposed negative electrode core material portion 42 and the metal sheet 60, making it easier to achieve low resistance in the battery. The number of protrusions 51 is not limited to four; it can be two to three or five or more. Also, each protrusion 51 may have a different shape from the others. Separately, welding may be performed between the metal sheet 60 and the base portion 54 of the bottom portion 50 before welding the protrusions 51 and the exposed negative electrode core material portion 42 via the metal sheet 60. This makes it less likely for the metal sheet 60 to shift position when welding the protrusions 51 and the exposed negative electrode core material portion 42 via the metal sheet 60.

[0065] Furthermore, a thin-walled portion 50A is formed at the bottom 50 of the case 20 shown in Figure 7. In the example shown in Figure 7, the thin-walled portion 50A is formed in an annular shape, and when the internal pressure of the battery reaches a predetermined pressure, the thin-walled portion preferentially ruptures, and the area surrounded by the thin-walled portion 50A functions as a discharge valve. The thin-walled portion 50A may also be formed in an arc shape. In addition, each protrusion 51 is provided in a region radially inward from the thin-walled portion 50A. This ensures a welding area while making it easier to activate the discharge valve when the internal pressure of the battery rises.

[0066] Figure 8 is a plan view of the metal sheet 60 as seen from above, with the metal sheet 60 placed on the inner surface of the bottom 50 of the case 20 shown in Figure 7. In Figure 8, the metal sheet 60 is shown using hatching.

[0067] As shown in Figure 8, the metal sheet 60 has a first region 61 (area enclosed by a solid line) that contacts the inner surface 51A of the protrusion 51, and a second region (area enclosed by a dotted line) that contacts the inner surface 54A of the base portion 54, which is the part of the bottom portion 50 other than the protrusion 51. In a plan view of the metal sheet 60, the first region 61 and the second region 62 are not adjacent and are formed with a gap between them.

[0068] Next, a modified example of the metal sheet 60 will be described with reference to Figures 9 to 11. Figures 9 to 11 are plan views of the metal sheet 60 as seen from above, with the metal sheet 60 placed on the inner surface of the bottom 50 of the case 20 shown in Figure 7. In Figures 9 to 11, the metal sheet 60 is illustrated using dot hatching.

[0069] As shown in Figure 9, the metal sheet 60 may have a plurality of through holes 63. The through holes 63 are formed, for example, in a circular or rectangular shape in plan view. In the example shown in Figure 9, the metal sheet 60 has 12 circular through holes 63 in plan view. The diameter of the through holes 63 is not particularly limited, and is, for example, 0.1 mm or more and 10 mm or less.

[0070] Furthermore, as shown in Figure 10, the metal sheet 60 may have a plurality of notches 64 on its outer edge. The notches 64 are formed in a semicircular or rectangular shape in plan view. In the example shown in Figure 10, the metal sheet 60 has four triangular notches 64 in plan view.

[0071] By providing at least one of the through-holes 63 and notches 64 in the metal sheet 60, the metal sheet 60 becomes more easily deformable along the inner surface of the bottom portion 50, and the metal sheet 60 also becomes more easily in contact with the inner surface 54A of the base portion 54, which is the part of the bottom portion 50 other than the convex portion 51. As a result, the contact area between the metal sheet 60 and the inner surface of the bottom portion 50 increases, making it easier to achieve lower resistance in the battery.

[0072] Preferably, at least one of the through-hole 63 and the notch 64 is provided in a position that does not overlap with the protrusion 51 in a plan view. In this case, when a laser is irradiated from the outer surface of the protrusion 51, the heat generated by the laser irradiation is less likely to be transmitted to the area where the separator 13 of the electrode body 14 is placed, and thermal contraction of the separator 13 is suppressed. In addition, by providing the through-hole 63 in a position that does not overlap with the protrusion 51 in a plan view, the welding area between the exposed negative electrode core material portion 42 and the metal sheet 60 is increased, making it easier to achieve low resistance of the battery.

[0073] Furthermore, as shown in Figure 11, the metal sheet 60 may be composed of multiple sheets. In the example shown in Figure 11, the metal sheet 60 includes a first metal sheet 65 and a second metal sheet 66, both having a semicircular shape in plan view. The first metal sheet 65 is fixed to a first protrusion 56 and a second protrusion 57, and the second metal sheet 66 is fixed to a third protrusion 58 and a fourth protrusion 59. The metal sheet 60 may be composed of three or more sheets. Also, the multiple metal sheets 60 may have regions that overlap each other in the thickness direction.

[0074] By composing the metal sheet 60 with multiple sheets, each sheet becomes more easily deformable when the metal sheet 60 is pressed downward by the electrode body 14, and the metal sheet 60 also comes into contact with the inner surface 54A of the base portion 54, which is the part of the bottom portion 50 other than the convex portion 51. As a result, the contact area between the metal sheet 60 and the inner surface of the bottom portion 50 increases, making it easier to achieve lower resistance in the battery.

[0075] The above embodiments can be modified as appropriate within the scope of the purpose of this disclosure. For example, in the above embodiments, the case in which the protrusion 51 and the exposed negative electrode core material portion 42 are welded via a metal sheet 60 was described, but the protrusion 51 and the exposed positive electrode core material portion 32 may be welded via a metal sheet 60. That is, the first electrode plate may be the positive electrode 11 and the second electrode plate may be the negative electrode 12.

[0076] The present disclosure will be further illustrated by the following examples, but the present disclosure is not limited to these examples.

[0077] <Example 1> [Preparation of positive electrode] A lithium metal composite oxide containing nickel was used as the positive electrode active material. 100 parts by mass of this positive electrode active material was mixed with 1 part by mass of acetylene black (AB) as a conductive agent and 1 part by mass of polyvinylidene fluoride (PVDF) as a binder. An appropriate amount of N-methyl-2-pyrrolidone (NMP) was then added to prepare a positive electrode mixture slurry. Next, this positive electrode mixture slurry was applied to both sides of a positive electrode core made of aluminum foil, dried, cut to a predetermined electrode size, and rolled using a roller to obtain a strip-shaped positive electrode. In addition, a positive electrode core exposed portion was formed at one end in the width direction of the positive electrode, where no positive electrode mixture portion was formed.

[0078] [Preparation of the negative electrode] A mixture of natural graphite and SiO was used as the negative electrode active material. 100 parts by mass of this negative electrode active material was mixed with 1 part by mass of styrene-butadiene rubber (SBR) as a binder and 1 part by mass of carboxymethylcellulose (CMC) as a thickener, and an appropriate amount of water was added to prepare a negative electrode mixture slurry. Next, this negative electrode mixture slurry was applied to both sides of the negative electrode core material made of copper foil (thickness: 8 μm), dried, cut to a predetermined electrode size, and rolled using a roller to obtain a strip-shaped negative electrode. In addition, a negative electrode core material exposed portion without the negative electrode mixture was formed at one end in the width direction of the negative electrode.

[0079] [Electrode Fabrication] The fabricated positive and negative electrodes were wound in a spiral shape via a polyolefin separator to create a wound electrode body.

[0080] [Preparation of Non-Aqueous Electrolyte] Mix ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) in a volume ratio of EC:EMC:DMC = 3:3:4 to a mixed solvent, and add LiPF ６ A non-aqueous electrolyte was prepared by adding 1 mol / L of .

[0081] [Fabrication of Test Cell (Cylindrical Secondary Battery)] A positive electrode current collector plate was placed on top of the fabricated electrode body, and the exposed portion of the positive electrode core material and the positive electrode current collector plate were welded by laser welding. The positive electrode current collector plate and the sealing plate were also joined by laser welding. A bottomed cylindrical container made of carbon steel with a thickness of 0.4 mm was used as the case. A protrusion (height: 0.4 mm) shown in Figure 5 was formed on the bottom of the case by press working. Then, with a copper foil with a thickness of 8 μm placed as a metal sheet on the bottom of the case, the electrode body was inserted. A laser was then irradiated from the outer surface of the bottom along the protrusion, and the protrusion and the exposed portion of the negative electrode core material were welded through the metal sheet. After that, grooves were formed on the side of the case by press working, etc., then a non-aqueous electrolyte was poured into the inside of the case, and the sealing plate was crimped and fixed to the opening of the case.

[0082] <Example 2> In the preparation of the test cell, the thickness of the copper foil used as the metal sheet was changed to 30 μm, and the output of the laser irradiated from the outer surface of the bottom was increased, except that the test cell was prepared in the same manner as in Example 1.

[0083] <Example 3> In the preparation of the test cell, the thickness of the copper foil used as the metal sheet was changed to 50 μm, and the output of the laser irradiated from the outer surface of the bottom was increased, except that the test cell was prepared in the same manner as in Example 1.

[0084] <Comparative Example 1> In the preparation of the test cell, the electrode body was inserted without placing a metal sheet at the bottom of the case, and the protruding portion and the exposed portion of the negative electrode core material were welded together.

[0085] <Comparative Example 2> The test cells were prepared in the same manner as in Example 1, except that the process for preparing the test cells was carried out using the following method.

[0086] [Fabrication of Test Cell (Cylindrical Secondary Battery)] A positive electrode current collector plate was placed on top of the fabricated electrode body, and the exposed portion of the positive electrode core material and the positive electrode current collector plate were welded together by laser welding. The positive electrode current collector plate and the sealing plate were also joined by laser welding. A negative electrode current collector plate (thickness: 300 μm) made of steel was placed on the bottom of the electrode body, and the exposed portion of the negative electrode core material and the negative electrode current collector plate were welded together by laser welding.

[0087] Furthermore, a bottomed cylindrical container made of carbon steel with a thickness of 0.4 mm was used as the case. The bottom of the case had no protrusions and was flat throughout its surface. The electrode body was inserted into the bottom of the case without placing a metal sheet therein. A laser was then irradiated from the outer surface of the bottom near the center of the bottom to weld the bottom to the negative electrode current collector plate. After that, grooves were formed on the side of the case by press working or the like, a non-aqueous electrolyte was poured into the inside of the case, and a sealing plate was crimped and fixed to the opening of the case.

[0088] The test cells prepared in Examples 1-3 and Comparative Examples 1 and 2 were disassembled, and the degree of thermal shrinkage of the separator, the welding range of the exposed negative electrode core material, and the contact area between the metal sheet (negative electrode current collector plate) and the bottom in areas other than the welded areas (hereinafter simply referred to as "contact area in areas other than the welded areas") were visually inspected and evaluated. Each evaluation item was evaluated with ○, △, or × based on the following criteria. <Degree of thermal shrinkage of the separator> ○: Almost no thermal shrinkage (bending) of the separator is observed throughout ×: Thermal shrinkage (bending) of the separator is observed throughout <Welding range of the exposed negative electrode core material> ○: Welded areas are uniformly formed over a wide area ×: There is a large variation in the welded areas, and there are areas that are not welded <Contact area in areas other than the welded areas> ○: Almost complete contact over a wide area △: Generally in contact, but there are areas that are not in contact ×: Almost no contact

[0089] Table 1 shows the evaluation results of the test cells for Examples 1-3 and Comparative Examples 1 and 2.

[0090]

[0091] As shown in Table 1, in the test cells of Examples 1 to 3, where copper foil as a metal sheet was placed between the electrode body and the bottom of the case, thermal shrinkage of the separator during welding was suppressed. Furthermore, in the test cells of Examples 1 to 3, the welded area of ​​the exposed negative electrode core material was uniformly formed over a wide area, and the metal sheet and the bottom were in contact over a wide area even outside the welded area. From this, it can be said that the test cells of Examples 1 to 3 are low-resistance batteries with suppressed thermal shrinkage of the separator during welding compared to the test cells of Comparative Examples 1 and 2.

[0092] The present disclosure will be further described by the following embodiments. Configuration 1: A power storage device comprising: a cylindrical case having an opening and a bottom located below the opening; an electrode body disposed inside the case and having a first electrode plate, a second electrode plate, and a separator disposed between the first electrode plate and the second electrode plate, with the first electrode plate and the second electrode plate wound around the separator; and a metal sheet disposed inside the case and between the electrode body and the bottom of the case, and fixed to the bottom, wherein the first electrode plate has a core material portion and a composite portion disposed on the surface of the core material portion, the core material portion includes a core material exposed portion fixed to the metal sheet, the bottom includes a convex portion projecting upward, and the convex portion and the core material exposed portion are welded together via the metal sheet. Configuration 2: The power storage device according to Configuration 1, wherein the thickness of the metal sheet is less than 50 μm. Configuration 3: The power storage device according to Configuration 2, wherein the thickness of the metal sheet is equal to or greater than the thickness of the core material portion. Configuration 4: The energy storage device according to any one of Configurations 1 to 3, wherein the metal sheet is mainly composed of the metal that constitutes the core material. Configuration 5: The energy storage device according to any one of Configurations 1 to 4, wherein the metal sheet includes a first metal sheet and a second metal sheet, and the protrusion includes a first protrusion and a second protrusion provided at a different position from the first protrusion, the first protrusion is fixed to the first metal sheet, and the second protrusion is fixed to the second metal sheet. Configuration 6: The energy storage device according to any one of Configurations 1 to 5, wherein the metal sheet has at least one of a through hole and a notch, and the through hole and the notch are provided at positions that do not overlap with the protrusion when viewed from above. Configuration 7: The energy storage device according to any one of Configurations 1 to 6, wherein the bottom of the case has an annular or arc-shaped thin-walled portion, and when viewed from below, the protrusion is located radially inward of the case than the thin-walled portion. Configuration 8: The energy storage device according to any one of Configurations 1 to 7, wherein the protrusion and the core material exposed portion are welded by thermal conduction welding, and the weld mark between the protrusion and the core material exposed portion has a bead width greater than the thickness of the bottom portion.Configuration 9: The energy storage device according to any one of Configurations 1 to 8, wherein the metal sheet has a first region that contacts the inner surface of the protrusion and a second region that contacts the inner surface of the base portion, which is the part of the bottom other than the protrusion, and in a plan view of the metal sheet, the first region and the second region are not adjacent and are formed with a gap between them. Configuration 10: The energy storage device according to Configuration 9, wherein the second region of the metal sheet is pressed towards the bottom by the electrode body. Configuration 11: The energy storage device according to any one of Claims 1 to 10, wherein the metal sheet is fixed to the inner surface of the protrusion and to the inner surface of the base portion, which is the part of the bottom other than the protrusion.

[0093] 10 Cylindrical battery (energy storage device), 11 Positive electrode (second electrode plate), 12 Negative electrode (first electrode plate), 13 Separator, 14 Electrode body, 15 Sealing plate, 16 Positive electrode current collector plate, 16A Recess, 17 Gasket, 20 Case, 21 Side part, 22 Grooved part, 23 Opening, 30 Positive electrode core material part, 31 Positive electrode mixture part, 40 Negative electrode core material part (core material part), 41 Negative electrode mixture part (mixture part), 42 Negative electrode core material exposed part (core material exposed part), 50 Bottom part, 50A Thin-walled part, 51 Protrusion, 51A Inner surface, 52 Top part, 53 Connection part, 54 Base part, 54A Inner surface, 55 Weld mark, 56 First protrusion, 57 Second protrusion, 58 Third protrusion, 59 Fourth protrusion, 60 Metal sheet, 61 First region, 62 Second region, 63 Through hole, 64 Notch, 65 First metal sheet, 66 Second metal sheet

Claims

1. An energy storage device comprising: a cylindrical case having an opening and a bottom located below the opening; an electrode body disposed inside the case and having a first electrode plate, a second electrode plate, and a separator disposed between the first electrode plate and the second electrode plate, with the first electrode plate and the second electrode plate wound around the separator; and a metal sheet disposed inside the case and between the electrode body and the bottom of the case, and fixed to the bottom, wherein the first electrode plate has a core material portion and a composite portion disposed on the surface of the core material portion, the core material portion includes a core material exposed portion fixed to the metal sheet, the bottom includes a convex portion projecting upward, and the convex portion and the core material exposed portion are welded together via the metal sheet.

2. The energy storage device according to claim 1, wherein the thickness of the metal sheet is less than 50 μm.

3. The energy storage device according to claim 2, wherein the thickness of the metal sheet is equal to or greater than the thickness of the core material.

4. The energy storage device according to claim 1, wherein the metal sheet is mainly composed of the metal that constitutes the core material.

5. The energy storage device according to claim 1, wherein the metal sheet includes a first metal sheet and a second metal sheet, the protrusion includes a first protrusion and a second protrusion provided at a different position from the first protrusion, the first protrusion is fixed to the first metal sheet, and the second protrusion is fixed to the second metal sheet.

6. The energy storage device according to claim 1, wherein the metal sheet has at least one of a through hole and a notch, and the through hole and the notch are positioned so as not to overlap with the protrusion when viewed from above.

7. The bottom of the case has an annular or arc-shaped thin-walled portion, and when viewed from below, the convex portion is located radially inward of the case than the thin-walled portion, as described in claim 1.

8. The convex portion and the core material exposed portion are welded by thermal conduction welding, and the weld mark between the convex portion and the core material exposed portion has a bead width greater than the thickness of the bottom portion, as described in claim 1.

9. The energy storage device according to claim 1, wherein the metal sheet has a first region that contacts the inner surface of the protrusion and a second region that contacts the inner surface of the base portion, which is the part of the bottom other than the protrusion, and in a plan view of the metal sheet, the first region and the second region are not adjacent and are formed with a gap between them.

10. The energy storage device according to claim 9, wherein the second region of the metal sheet is pressed toward the bottom by the electrode body.

11. The energy storage device according to claim 1 or 5, wherein the metal sheet is fixed to the inner surface of the protrusion and to the inner surface of the base portion, which is the part of the bottom other than the protrusion.

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