Power storage device and method for manufacturing power storage device

Abstract

This power storage device includes a welded part (60) in which an edge section of a metal foil (40) constituting an electrode and a current collector plate (17) made of a metal different from that of the metal foil (40) are welded in a linear shape. The linear welded part (60) includes an intermediate melted / solidified section (63) having a composition different from the composition of the current collector plate (17) and formed in a single layer in the longitudinal direction of the welded part (60) so as to be positioned between the metal foil (40) and a current collector plate melted / solidified section (61) formed by melting and solidifying a portion of the current collector plate (17).

Description

Electric energy storage device and method for manufacturing the same

[0001] The present disclosure relates to an electric energy storage device and a method for manufacturing the same.

[0002] Conventionally, as described in Patent Document 1, in a secondary battery which is an electric energy storage device, it is described that a metal foil constituting a negative electrode and a plate-shaped current collector terminal made of the same material as the metal foil are joined by resistance welding. Resistance welding is a method of welding by Joule heat by sandwiching a member to be joined with a pair of electrodes and passing an electric current between the electrodes.

[0003] Japanese Patent No. 5949535

[0004] By the way, there are cases where an end portion of a metal foil constituting an electrode of an electric energy storage device and a current collector plate made of a metal different from the metal foil are joined by welding. In the welding in this case, from the viewpoint of improving the welding quality, it is conceivable to use laser welding for scanning the irradiation portion of a spot-shaped laser beam.

[0005] However, when performing spot-shaped laser welding, there are cases where stable welding strength and stable electrical resistance cannot be obtained at the connection portion between the metal foil and the current collector plate. Also, from the viewpoints of improving the welding quality and shortening the welding time, it is also conceivable to perform line laser welding in which a line-shaped laser beam is irradiated to weld the metal foil and the current collector plate. However, even when performing line laser welding, there are cases where stable welding strength and stable internal electrical resistance cannot be obtained at the connection portion between the metal foil and the current collector plate.

[0006] An object of the present disclosure is to realize an electric energy storage device capable of obtaining stable welding strength and stable electrical resistance at a connection portion between a metal foil constituting an electrode and a current collector plate in an electric energy storage device and a method for manufacturing the same.

[0007] The electric energy storage device according to the present disclosure has a welded portion in which an end portion of a metal foil constituting an electrode and a current collector plate made of a metal different from the metal foil are welded in a line shape, and the line-shaped welded portion is formed in a single layer in the longitudinal direction of the welded portion so as to be positioned between a current collector plate melting and solidifying portion formed by melting and solidifying a part of the current collector plate and the metal foil, and has an intermediate melting and solidifying portion having a composition different from that of the current collector plate, and is characterized by being an electric energy storage device.

[0008] The present disclosure relates to a method for manufacturing an energy storage device, comprising the steps of: arranging the ends of metal foils constituting electrodes in an axial position; bending the tips of the ends; and contacting the bent metal foils with a current collector plate having a higher melting point than the metal foils. The method involves irradiating the surface of the current collector plate with a line-shaped laser beam having a uniform beam profile in the longitudinal direction, melting the current collector plate to its back surface by heat conduction, then melting the metal foil with the heat of the molten current collector plate, and forming a single-layer molten solidified portion mainly composed of metal foil on the entire surface of the metal foil side of the molten solidified portion of the current collector plate.

[0009] According to this disclosure, in a power storage device and a method for manufacturing a power storage device, it is possible to realize a power storage device that can obtain stable welding strength and stable electrical resistance at the connection portion between the metal foil constituting the electrode and the current collector plate.

[0010] This is an axial cross-sectional view of a power storage device according to an embodiment of this disclosure. This is a perspective view showing a portion of the electrode body of the power storage device of the embodiment unfolded before bending at both axial ends. This is an enlarged view corresponding to part A in Figure 1 of the electrode body of the power storage device of the embodiment before joining the negative electrode metal foil and the negative electrode current collector plate. This is a view from below, with the electrode body and negative electrode current collector plate removed from Figure 1. This is an enlarged view of part B in Figure 4. This is a cross-sectional view taken along line C-C in Figure 5. This is a flowchart showing the manufacturing method of the power storage device of the embodiment. This is a schematic diagram showing a part of the laser welding apparatus used in the manufacturing method of the power storage device of the embodiment. This is a diagram showing an example of the diffraction pattern of a branched DOE. This is a schematic diagram showing a cross-section of the state in which the negative electrode current collector plate and the negative electrode metal foil are welded by a line-shaped welding beam of the laser welding apparatus in the embodiment. This is a diagram showing the profile of the laser beam irradiated from the laser welding apparatus in the embodiment. This is a diagram corresponding to Figure 6 in a comparative example secondary battery.

[0011] Hereinafter, embodiments of the energy storage device and the method for manufacturing the energy storage device according to this disclosure will be described in detail with reference to the drawings. In this disclosure, a cylindrical non-aqueous electrolyte secondary battery will be described as the energy storage device, but the energy storage device according to this disclosure is not limited to this, and any device having a configuration in which metal foil constituting electrodes and a current collector plate are welded together may be used, and the energy storage device may be a capacitor.

[0012] In the following embodiments, the same components are denoted by the same reference numerals in the drawings, and redundant explanations are omitted. Furthermore, multiple drawings include schematic diagrams, and the dimensional ratios such as length, width, and height of each component do not necessarily match between different drawings. In this specification, the axial opening side of the outer casing 15 of the cylindrical secondary battery 10, which is an energy storage device, is referred to as "upper," and the axial bottom 15a side is referred to as "lower." That is, the bottom 15a of the outer casing 15 is described as the lower end. The energy storage devices of this disclosure are not necessarily limited to those in which the bottom of the outer casing is located vertically below the opening in the operating state. For example, the bottom of the outer casing may be configured to be vertically above the opening of the outer casing in the operating state.

[0013] Figure 1 is an axial cross-sectional view of the secondary battery 10 of the embodiment. Figure 2 is a perspective view showing a portion of the electrode body 14 of the secondary battery 10 of the embodiment unfolded before bending at both axial ends.

[0014] As shown in Figures 1 to 3, the secondary battery 10 comprises a wound electrode body 14, a non-aqueous electrolyte (not shown), and an outer casing 15 and a sealing body 16, which are metal cans. The wound electrode body 14 has a positive electrode 11, a negative electrode 12, and a separator 13, with the positive electrode 11 and the negative electrode 12 wound in a spiral shape via the separator 13. The positive electrode 11, the negative electrode 12, and the separator 13 are all in the shape of a roughly rectangular, elongated strip. The non-aqueous electrolyte contains a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent.

[0015] In the electrode body 14, as shown in Figure 2, the positive electrode 11 protrudes above the negative electrode 12 and the separator 13, and the negative electrode 12 protrudes below the positive electrode 11 and the separator 13.

[0016] As shown in Figure 2, the positive electrode 11 has an uncoated positive electrode portion 34 in which the positive electrode metal foil 30 is exposed without a positive electrode mixture layer 32. The uncoated positive electrode portion 34 is located at the upper end, which is one end in the winding axis direction (hereinafter sometimes referred to as the axis direction) from the winding start end to the winding end in the longitudinal direction of the electrode plate of the positive electrode 11. The longitudinal direction of the electrode plate is the direction corresponding to the winding direction in the wound state of the positive electrode 11 or negative electrode 12, and is the longitudinal direction of the elongated rectangle when the positive electrode 11 or negative electrode 12 is viewed in the thickness direction when the positive electrode 11 or negative electrode 12 is unfolded along a plane.

[0017] The negative electrode 12 has an uncoated negative electrode portion 44 in which the negative electrode metal foil 40 (Figure 2) is exposed without a negative electrode mixture layer 42 (Figure 2). The uncoated negative electrode portion 44 is located at the lower end, which is the other end in the axial direction, from the beginning end to the end end in the longitudinal direction of the electrode plate of the negative electrode 12. Therefore, the upper end in the axial direction of the electrode body 14 is composed of the uncoated positive electrode portion 34, and the lower end in the axial direction of the electrode body 14 is composed of the uncoated negative electrode portion 44.

[0018] The non-aqueous electrolyte has ionic conductivity (e.g., lithium ion conductivity). The non-aqueous electrolyte comprises a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. The non-aqueous electrolyte is not limited to a liquid electrolyte (non-aqueous electrolyte solution), but may also be a solid electrolyte using a gel-like polymer or the like. The secondary battery 10 is preferably a lithium-ion battery. The electrolyte salt may be, for example, LiBF ４ LiPF ６ Lithium salts such as the above are used. Non-aqueous solvents include, for example, esters such as ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), and methyl propionate (MP), as well as ethers, nitriles, amides, and mixed solvents of two or more of these. The non-aqueous solvent may contain halogen-substituted products in which at least some of the hydrogen atoms of these solvents are replaced with halogen atoms such as fluorine.

[0019] Examples of halogen-substituted compounds include fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), fluorinated linear carbonates, and fluorinated linear carboxylic acid esters such as methyl fluoropropionate (FMP). In terms of suppressing the deterioration of the charge-discharge cycle characteristics of non-aqueous electrolyte secondary batteries or improving the input characteristics, the non-aqueous electrolyte preferably contains 5% by mass or more of FEC relative to the mass of the non-aqueous electrolyte, and more preferably contains 5% to 15% by mass of FEC.

[0020] As solid electrolytes, for example, solid or gel-like polymer electrolytes, inorganic solid electrolytes, etc., are used. Polymer electrolytes include, for example, a lithium salt and a matrix polymer, or a non-aqueous solvent, a lithium salt and a matrix polymer. As matrix polymers, for example, polymer materials that absorb non-aqueous solvents and gel are used. As polymer materials, for example, fluororesins, acrylic resins, polyether resins, etc., are used. As inorganic solid electrolytes, for example, materials known for all-solid-state lithium-ion secondary batteries, etc. (for example, oxide-based solid electrolytes, sulfide-based solid electrolytes, halide-based solid electrolytes, etc.) are used.

[0021] The positive electrode 11 has a positive electrode metal foil 30 and a positive electrode mixture layer 32 formed on both sides of the positive electrode metal foil 30. The positive electrode metal foil 30 is a metal foil that is stable in the potential range of the positive electrode 11, such as aluminum or an aluminum alloy, but a film with the metal arranged on its surface may be used instead of the positive electrode metal foil 30. The thickness of the positive electrode metal foil 30 is, for example, 10 μm or more and 30 μm or less. The positive electrode mixture layer 32 contains a positive electrode active material, a conductive agent, and a binder. 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 metal foil 30, drying the coating film, and then compressing it to form the positive electrode mixture layer 32 on both sides of the positive electrode metal foil 30. The positive electrode mixture layer 32 may be formed on only one side of the positive electrode metal foil 30. The thickness of the positive electrode mixture layer 32 is, for example, 10 μm to 150 μm on one side of the positive electrode metal foil 30.

[0022] The positive electrode active material is mainly composed of a lithium-containing metal composite oxide. Examples of metal elements contained in the lithium-containing metal composite oxide include Ni, Co, Mn, Al, B, Mg, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Zr, Nb, In, Sn, Ta, and W. A preferred example of a lithium-containing metal composite oxide is a composite oxide containing at least one of Ni, Co, Mn, and Al.

[0023] Examples of conductive agents included in the positive electrode mixture layer 32 include carbon materials such as carbon black, acetylene black, Ketjen black, and graphite. Examples of binders included in the positive electrode mixture layer 32 include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide resin, acrylic resin, and polyolefin resin. These resins may be used in combination with cellulose derivatives such as carboxymethylcellulose (CMC) or its salts, polyethylene oxide (PEO), etc.

[0024] The negative electrode 12 has a negative electrode metal foil 40 and a negative electrode mixture layer 42 formed on both sides of the negative electrode metal foil 40. The negative electrode metal foil 40 is made of a metal foil that is stable in the potential range of the negative electrode 12, such as copper or a copper alloy. The thickness of the negative electrode metal foil 40 is, for example, 5 μm to 30 μm. The negative electrode mixture layer 42 contains a negative electrode active material and a binder. The negative electrode 12 can be manufactured, for example, by applying a negative electrode mixture slurry containing a negative electrode active material and a binder onto the negative electrode metal foil 40, drying the coating film, and then compressing it to form the negative electrode mixture layer 42 on both sides of the negative electrode metal foil 40. The negative electrode mixture layer 42 may also be formed on only one side of the negative electrode metal foil 40. The thickness of the negative electrode mixture layer 42 is, for example, 10 μm to 150 μm on one side of the negative electrode metal foil 40.

[0025] Generally, carbon materials that reversibly intercalate and release lithium ions are used as the negative electrode active material. Preferred carbon materials are graphite such as natural graphite such as flake graphite, lump graphite, and earthy graphite, and artificial graphite such as lump graphite and graphitized mesophase carbon microbeads. The negative electrode mixture layer 42 may contain a silicon (Si) material as the negative electrode active material. In addition, metals other than Si that alloy with lithium, alloys containing such metals, compounds containing such metals, etc., may be used as the negative electrode active material.

[0026] The binder contained in the negative electrode mixture layer 42 may be fluororesin, PAN, polyimide resin, acrylic resin, polyolefin resin, etc., as in the case of the positive electrode 11, but preferably styrene-butadiene rubber (SBR) or a modified version thereof is used. In addition to SBR, the negative electrode mixture layer 42 may also contain CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol, etc.

[0027] 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. The material of the separator 13 is preferably polyethylene, polyolefin resins such as polypropylene, or cellulose. The separator 13 may have either a single-layer structure or a laminated structure. A heat-resistant layer or the like may be formed on the surface of the separator 13.

[0028] As shown in Figure 1, the secondary battery 10 has a metal negative electrode current collector plate 17 on the axially lower side of the electrode body 14. The negative electrode current collector plate 17 is made of, for example, nickel-plated iron, nickel, or a nickel alloy. In this example, the melting point of the negative electrode current collector plate 17 is higher than the melting point of the negative electrode metal foil 40.

[0029] The negative electrode current collector plate 17 can be radial, such as in a cross shape, or disc shape, but the following description will explain the case where it is radial with multiple arms extending in the radial direction. The unpainted negative electrode portion 44 protruding from the electrode body 14 is joined to the negative electrode current collector plate 17, and the negative electrode current collector plate 17 is joined to the inner surface of the bottom plate of the outer casing 15. The outer casing 15 to which the unpainted negative electrode portion 44 is electrically connected via the negative electrode current collector plate 17 becomes the negative electrode terminal.

[0030] The secondary battery 10 has a positive electrode current collector plate 18 located inside the outer casing 15. The positive electrode current collector plate 18 is a metal disc made of aluminum or an aluminum alloy, etc., located above the electrode body 14 in the axial direction. When the positive electrode current collector plate 18 is disc-shaped, a through hole is formed to allow gas generated in the electrode body 14 to escape upward. The unpainted positive electrode portion 34 protruding from the electrode body 14 is joined to the positive electrode current collector plate 18 in a state where it is bent inward in at least a part of its circumferential direction. As a result, the positive electrode current collector plate 18 is fixed to the upper end of the electrode body 14 in the axial direction and electrically connected. Alternatively, it may be joined to multiple unpainted positive electrode portions provided at multiple locations on the upper part of the positive electrode 11, and the electrode body 14 may be connected to the positive electrode current collector plate via multiple positive electrode leads protruding from the upper end of the positive electrode 11. The secondary battery 10 has an annular insulating plate 19 on the above the positive electrode current collector plate 18 in the axial direction.

[0031] One end of the positive electrode connection lead 20 is joined to the upper surface of the positive electrode current collector plate 18 by welding or the like. The positive electrode connection lead 20 extends through a through-hole in the insulating plate 19 towards the sealing body 16, and the other end of the positive electrode connection lead 20 is connected to the lower surface of the internal terminal plate 22 of the sealing body 16 by welding or the like. The cap 26 that forms the top plate of the sealing body 16 is electrically connected to the internal terminal plate 22. As a result, the positive electrode current collector plate 18 is electrically connected to the cap 26, and the cap 26 becomes the positive electrode terminal. The positive electrode connection lead 20 is a conductive member made of a metal mainly composed of aluminum.

[0032] The secondary battery 10 further includes a resin gasket 27 positioned between the outer casing 15 and the sealing body 16. The gasket 27 is sandwiched between the outer casing 15 and the sealing body 16, insulating the sealing body 16 from the outer casing 15. The gasket 27 serves as a sealing material to maintain airtightness inside the battery and as an insulating material to insulate the outer casing 15 from the sealing body 16. The outer casing 15 has an annular groove 21 in a part of its axial direction.

[0033] The grooved portion 21 can be formed, for example, by spinning a part of the side surface radially inward to create a recess in the radial direction. The outer casing 15 has a bottomed cylindrical portion including the grooved portion 21 and an annular shoulder portion. The bottomed cylindrical portion houses the electrode body 14 and the non-aqueous electrolyte, and the shoulder portion is bent radially inward from the opening end of the bottomed cylindrical portion and extends inward. The shoulder portion is formed when the upper end of the outer casing 15 is bent inward and crimped to the periphery of the sealing body 16. The sealing body 16 is crimped and fixed to the outer casing 15 via a gasket 27 between the shoulder portion and the grooved portion 21. In this way, the internal space of the secondary battery 10 is sealed.

[0034] The sealing body 16 has a structure in which an internal terminal plate 22, a lower valve body 23, an insulating member 24, an upper valve body 25, and a cap 26 are stacked in order from the electrode body 14 side. Each component constituting the sealing body 16 has, for example, a disc shape or a ring shape, and each component except the insulating member 24 is electrically connected to one another. The internal terminal plate 22 has at least one through hole. The lower valve body 23 and the upper valve body 25 are connected at their respective centers, and the insulating member 24 is interposed between their respective peripheral edges.

[0035] When the secondary battery 10 overheats abnormally and its internal pressure rises to a predetermined value, the lower valve body 23 deforms and ruptures, pushing the upper valve body 25 towards the cap 26, thereby interrupting the current path between the lower valve body 23 and the upper valve body 25. If the internal pressure rises further and reaches a predetermined value, the upper valve body 25 ruptures, and gas is discharged from the through-hole 26a of the cap 26. This gas discharge prevents the secondary battery 10 from deforming or rupturing due to an excessive rise in internal pressure, thereby improving the safety of the secondary battery 10. Furthermore, it can suppress the impact on adjacent components (not shown) due to deformation or rupture of the secondary battery 10.

[0036] Next, with reference to Figures 1 to 4, the surrounding structure of the joint between the negative electrode metal foil 40 and the negative electrode current collector plate 17 at the bottom of the secondary battery 10 will be described. Figure 3 is an enlarged view corresponding to part A in Figure 1, before the joining of the negative electrode metal foil 40 and the negative electrode current collector plate 17 in the electrode body 14. Figure 4 is a view from below, with the electrode body 14 and the negative electrode current collector plate 17 removed from Figure 1.

[0037] As shown in Figure 3, the negative electrode 12 has a laminated portion 45 in which a negative electrode mixture layer 42 is laminated on both sides of the negative electrode metal foil 40, and an uncoated negative electrode portion 44 provided at the end located on the lower side (one axial side) of the laminated portion 45, on the negative electrode current collector plate 17 side. The uncoated negative electrode portion 44 has an axial extension portion 47 extending substantially parallel to the axial direction of the electrode body 14, and a bent portion 48 bent inward from the lower end of the axial extension portion 47. The bent portion 48 is welded to the upper surface of the negative electrode current collector plate 17. As a result, the uncoated negative electrode portion 44 is provided at the lower axial end of the negative electrode 12 on the negative electrode current collector plate 17 side, and the negative electrode mixture layer is not laminated therein.

[0038] In this example, the bent portion 48 of the unpainted negative electrode portion 44 is welded to the negative electrode current collector plate 17 by a linear weld 60. As a result, stable welding strength and stable electrical resistance can be obtained at the connection point between the negative electrode metal foil 40 and the negative electrode current collector plate 17, as will be described later.

[0039] More specifically, the negative electrode 12 has a welded portion 60 formed by welding the end of the negative electrode metal foil 40 and the negative electrode current collector plate 17 in a linear fashion. In this example, the negative electrode current collector plate 17 is welded to multiple different portions of the lower end of the negative electrode metal foil that extend from different positions in the winding direction of the electrode body 14. The negative electrode current collector plate 17 is made of a different metal than the negative electrode metal foil 40. In the following description, we will explain the case where the negative electrode metal foil 40 is copper foil and the negative electrode current collector plate 17 is made of nickel-plated iron plate, but the negative electrode current collector plate 17 can be made of a different metal than the negative electrode metal foil 40.

[0040] As shown in FIG. 4, the negative electrode current collector 17 has a plate shape with a plurality of arm portions 17a whose shape when viewed from below extends outward along the radial direction from the center. In the illustrated example, the negative electrode current collector 17 is formed in a cross shape having four arm portions 17a, but the number of arm portions 17a can be three or five or more. The plurality of arm portions 17a are connected by a flat plate-shaped central plate portion 17b provided at the center portion. Further, on each arm portion 17a, a protruding portion 17c that protrudes toward the electrode body 14 side and extends in the radial direction is formed from the base end to the tip of the arm portion 17a.

[0041] For example, the protruding portion 17c has a rectangular cross-sectional shape orthogonal to the longitudinal direction. The protruding portion 17c is joined to the negative electrode non-coated portion 44 in a state of being pressed against the negative electrode non-coated portion 44 bent to the inner peripheral side of the electrode body 14. For example, in a state where the protruding portion 17c is pressed against the negative electrode non-coated portion 44, a laser beam, which is a laser light, is irradiated from the side opposite to the negative electrode non-coated portion 44 of the arm portion 17a toward the bottom of the groove forming the protruding portion 17c. Thereby, the negative electrode non-coated portion 44 and the protruding portion 17c are joined by laser welding.

[0042] At the radially outer end portion of each arm portion 17a, a tip plate portion 17d having a circumferential width larger than that of the intermediate portion of the arm portion 17a is provided. In the illustrated example, the tip plate portion 17d has a trapezoidal shape in which the circumferential length of the tip is larger than the circumferential length of the base end side, but is not limited thereto, and various shapes can be adopted. The central plate portion 17b is joined to the bottom portion 15a of the exterior can 15.

[0043] When joining the central plate portion 17b and the bottom portion 15b, for example, in a state where the lower surface of the central plate portion 17b is brought into contact with the inner surface of the bottom portion 15b, the central plate portion 17b and the bottom portion 15b are joined by laser welding by irradiating laser light from the outside of the bottom portion 15b.

[0044] The negative electrode current collector 17 and the negative electrode metal foil 40 are welded by the linear weld portion 60 as described above. FIG. 5 is an enlarged view of part B of FIG. 4. FIG. 6 is a cross-sectional view taken along line C - C of FIG. 5.

[0045] The linear weld portion 60 substantially follows the longitudinal direction of the arm portion 17a, but may follow a direction slightly inclined with respect to the longitudinal direction of the arm portion 17a (the direction of arrow α in FIG. 5) as shown in FIG. 5.

[0046] As shown in FIG. 6, the welding part 60 has an intermediate molten and solidified part 63 formed in a single layer along the longitudinal direction (the direction of arrow α in FIG. 5) of the welding part 60 so as to be located between a current collector molten and solidified part 61 formed by melting and solidifying a part of the negative electrode current collector plate 17 and the negative electrode metal foil 40. Here, "single layer" means a state in which two parts overlap without a gap, are not divided, and are continuous.

[0047] The intermediate molten and solidified part 63 has a composition different from that of the negative electrode current collector plate 17. For example, in this example, the negative electrode current collector plate 17 is an iron plate with nickel plating, and the intermediate molten and solidified part 63 is an alloy containing 80% or more of copper.

[0048] The thickness in the vertical direction of the intermediate molten and solidified part 63 is substantially constant over at least a predetermined range in the longitudinal direction of the welding part 60. The length in the longitudinal direction of the welding part 60 of the part where the thickness of the intermediate molten and solidified part 63 is substantially constant is preferably 7 mm or more, more preferably 10 mm or more, and still more preferably 15 mm or more. When the length of the part where the thickness of the intermediate molten and solidified part 63 is substantially constant is 7 mm or more, a more stable bonding strength between the negative electrode current collector plate 17 and the negative electrode metal foil 40 can be obtained.

[0049] As described above, the composition of the single-layer intermediate molten and solidified part 63 is an alloy mainly containing 80% or more of copper. In this specification, a component whose proportion in the whole occupies 80% or more is referred to as the main component. More preferably, the intermediate molten and solidified part 63 contains 90% or more of copper. Also, the thickness of the negative electrode current collector plate 17 is 0.1 mm or more and 0.4 mm or less.

[0050] According to the secondary battery 10 described above, the linear welded portion 60 has an intermediate molten and solidified portion 63, which is formed in a single layer in the longitudinal direction of the welded portion 60 and has a different composition from that of the negative electrode current collector plate 17, so as to be located between the current collector plate molten and solidified portion 61, which is formed by the melting and solidification of a part of the negative electrode current collector plate 17, and the negative electrode metal foil 40. This makes it possible to obtain stable welding strength and stable electrical resistance at the connection portion between the negative electrode metal foil 40 and the negative electrode current collector plate 17, unlike when the welded portion between the negative electrode current collector plate 17 and the negative electrode metal foil 40 is located between the current collector plate molten and solidified portion 61 and the negative electrode metal foil 40 and is composed of multiple intermediate molten and solidified portions divided in the longitudinal direction of the welded portion.

[0051] Next, a method for manufacturing the secondary battery 10 will be described. Figure 7 is a flowchart showing the method for manufacturing the secondary battery 10 according to an embodiment. As shown in Figure 7, this manufacturing method includes a negative electrode end placement step (S10), a bending step (S12), a negative electrode current collector contact step (S14), and a melted and solidified part formation step (S16).

[0052] The negative electrode end placement step involves aligning the end of the negative electrode metal foil 40, which will be the lower end when in use, in the axial direction so that the lower end when in use coincides with the axial position. For example, with the positive electrode 11 and the negative electrode 12 wound around a separator 13 to form an electrode body 14, the end of the wound negative electrode metal foil 40 is placed at one axial end of the electrode body 14 so as to extend in the axial direction, and its axial end is aligned.

[0053] Next, in the bending process, as shown in Figure 3, the tip of the end of the negative electrode metal foil 40 is bent inward. Next, in the negative electrode current collector contact process, as shown in Figure 4, the negative electrode current collector 17, which has a higher melting point than the negative electrode metal foil 40, is brought into contact with the negative electrode metal foil 40 that was bent in the bending process.

[0054] Next, in the melting and solidification section formation process, a line-shaped laser beam with a uniform beam profile in the longitudinal direction is irradiated onto the surface of each arm portion 17a of the negative electrode current collector plate 17. Then, after the negative electrode current collector plate 17 is melted to the back surface by heat conduction, the heat from the molten negative electrode current collector plate 17 melts the negative electrode metal foil 40. As shown in Figure 6, a single-layer intermediate melting and solidification section 63, mainly composed of copper (the negative electrode metal foil 40), is formed on the entire surface of the current collector plate melting and solidification section 61 of the melted and solidified negative electrode current collector plate 17 on the negative electrode metal foil 40 side. Therefore, both the intermediate melting and solidification section 63 and the current collector plate melting and solidification section 61 are line-shaped.

[0055] In this example, in the molten solidification section formation process described above, a line-shaped laser beam is formed using a branched DOE, and the negative electrode current collector plate 17 and the negative electrode metal foil 40 are welded by irradiation with this laser beam. The branched DOE forms a line-shaped laser beam 93 at the processing position, with a beam profile uniform in the longitudinal direction, as shown in Figure 10 described later. The length of one line of the laser beam 93 at the processing position is preferably 7 mm or more, more preferably 10 mm or more, and even more preferably 15 mm or more. In this case, the number of branches of the branched DOE is preferably 31 or more.

[0056] Figure 8A is a schematic diagram showing a part of a laser welding apparatus 80 used in the manufacturing method of a secondary battery. Figure 8B is a diagram showing an example of a diffraction pattern of a branched DOE.

[0057] As shown in Figure 8A, the laser welding apparatus 80 is composed of a laser oscillator (not shown), a branched DOE 81, and a focusing lens 82. The laser oscillator outputs a laser beam 90. The laser beam 90 output from the laser oscillator is irradiated onto the branched DOE 81 in a parallel state via an optical fiber, collimator, etc.

[0058] The branched DOE 81 is a diffractive optical component that has the function of branching a single incident laser beam 90 into multiple laser beams 91. After the laser beam 90 is branched into multiple laser beams 91 by the branched DOE 81, it is incident on the focusing lens 82. At the focusing lens 82, the laser beams 92 are irradiated in a linear arrangement at the processing point, which is the processing position. At this time, by bringing the focusing lens 82 closer to the branched DOE 81, the spacing of the laser beams 92 at the processing point can be reduced, and as shown in Figure 9, a linear laser beam 93 can be irradiated onto the surface of the negative electrode current collector plate 17. As a result, the negative electrode current collector plate 17 and the negative electrode metal foil 40 are welded together at a linear weld 60.

[0059] As shown in Figure 8B, multiple unit cells 84 are arranged side by side at the branching point of the laser beam in the branched DOE 81. A common diffraction pattern is formed in each unit cell 84. Therefore, in the branched DOE 81, the branching characteristics are constant regardless of the incident position of the laser beam, and a line-shaped laser beam can be stably formed at the processing position.

[0060] Figure 10 shows the profile of the laser beam 93 irradiated from the laser welding apparatus 80. The profile of the laser beam 93 is uniform in the longitudinal direction, as shown by the line-shaped portion in black on the inside of Figure 10. Such a uniform profile can be achieved by appropriately adjusting the pitch, which is the distance between the irradiation positions of the branched laser beam, and the number of branches. For example, a uniform profile can be obtained by setting the number of branches to 31 when the longitudinal length of the laser beam profile is 13.5 mm. For this reason, it is preferable to set the number of branches to 31 or more. It is preferable that the longitudinal length of the beam profile of the laser beam be 7 mm or more, and that the number of branches of the branch DOE be 31 or more.

[0061] As described above, when the laser welding apparatus 80 uses a configuration that forms a line-shaped laser beam using a branched DOE 81, it becomes easier to ensure the uniformity of the laser beam and to easily perform optical adjustments. In this case, variations in the penetration depth at multiple positions in the longitudinal direction of the welded area 60 can be suppressed to a small extent.

[0062] In the secondary battery manufacturing method described in this example, instead of a branched DOE, a beam shaper DOE or a laser welding device that forms a line-shaped laser beam using a homogenizer optical system can also be used. In a configuration using a beam shaper DOE, the incident beam is divided into multiple blocks provided in the beam shaper DOE, and the incident beam is rearranged at the processing position. Therefore, if the incident position of the laser beam in the beam shaper DOE shifts from the initial position when the laser beam profile is adjusted, the laser beam profile will tilt at the processing position, which may necessitate optical adjustment. For this reason, using a branched DOE 81 is more advantageous.

[0063] Furthermore, using a homogenizer optical system requires a microlens array, making it expensive and less tolerant of misalignment. Additionally, changing the laser beam profile length is difficult. From this perspective, using a branched DOE 81 is more advantageous.

[0064] Figure 11 is a diagram corresponding to Figure 6 in the comparative example of a secondary battery. In the comparative example, a welded portion 60a is formed in a line shape by scanning with a spot-shaped laser. When a welded portion 60a is formed in this way, the state of the back side of the welded portion 60a (lower side in Figure 11) in the cross section of the joint between the negative electrode current collector plate 17 and the negative electrode metal foil 40 along the scanning direction after welding is easily affected by whether or not the negative electrode metal foil 40 and the negative electrode current collector plate 17 are in contact. For example, in the welded portion 60a of Figure 11, an intermediate melted and solidified portion 63a is not formed in the part where the negative electrode metal foil 40 is separated from the negative electrode current collector plate 17 before welding, resulting in a void 70. As a result, it is difficult to obtain stable welding strength and stable electrical resistance in the comparative example. This leads to a decrease in yield and an increase in the manufacturing cost of the secondary battery. According to the above embodiment, such problems can be prevented.

[0065] In the above description, the secondary battery has a welded portion 60 formed by linearly welding the end of the negative electrode metal foil 40 constituting the negative electrode 12 to the negative electrode current collector plate 17, and the linear welded portion 60 has a single-layer intermediate molten and solidified portion 63. On the other hand, the secondary battery may also have a welded portion formed by linearly welding the end of the positive electrode metal foil 30 constituting the positive electrode 11 to the positive electrode current collector plate 18, and this linear welded portion may have a single-layer intermediate molten and solidified portion, similar to the welded portion 60 on the negative electrode side.

[0066] This disclosure is further illustrated by the following embodiments. Configuration 1: A power storage device having a welded portion formed by linearly welding the end of a metal foil constituting an electrode to a current collector plate made of a different metal from the metal foil, wherein the linear welded portion has an intermediate molten and solidified portion having a different composition from the current collector plate, formed in a single layer in the longitudinal direction of the welded portion, located between the metal foil and a molten and solidified portion of the current collector plate formed by the melting and solidification of a part of the current collector plate. Configuration 2: The power storage device according to Configuration 1, wherein the metal foil is copper foil and the current collector plate is a nickel-plated iron plate. Configuration 3: The power storage device according to Configuration 1 or Configuration 2, wherein the composition of the intermediate molten and solidified portion is an alloy containing 80% or more copper. Configuration 4: The power storage device according to any one of Configurations 1 to 3, wherein the thickness of the current collector plate is 0.1 mm or more and 0.4 mm or less. Configuration 5: An energy storage device according to any one of Configurations 1 to 4, wherein the length in the longitudinal direction of the portion where the thickness of the intermediate melted and solidified portion is substantially constant is 7 mm or more. Configuration 6: A method for manufacturing an energy storage device, comprising the steps of: arranging the ends of metal foil constituting electrodes in an axial position; bending the tips of the ends; and contacting the bent metal foil with a current collector plate having a higher melting point than the metal foil, wherein a line-shaped laser beam with a uniform beam profile in the longitudinal direction is irradiated onto the surface of the current collector plate, the current collector plate is melted to the back surface by heat conduction, the metal foil is melted by the heat of the molten current collector plate, and a single-layer melted and solidified portion mainly composed of the metal foil is formed on the entire surface of the current collector plate melted and solidified portion on the metal foil side of the solidified current collector plate. Configuration 7: A method for manufacturing an energy storage device according to Configuration 6, wherein a line-shaped laser beam with a uniform beam profile in the longitudinal direction is formed at the processing position by branched DOE. Configuration 8: A method for manufacturing an energy storage device according to Configuration 7, wherein the length of one line at the processing position of the laser beam is 7 mm or more, and the number of branches of the branch DOE is 31 or more.

[0067] 10 Secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14, 14a Electrode body for secondary battery (electrode body), 15 Outer can, 16 Sealing body, 17 Negative electrode current collector plate, 18 Positive electrode current collector plate, 19 Insulating plate, 20 Positive electrode connection lead, 21 Grooved section, 22 Internal terminal plate, 23 Lower valve body, 24 Insulating member, 25 Insulating member, 25 Upper valve body, 26 Cap, 27 Gasket, 30 Positive electrode metal foil, 32 Positive electrode mixture layer, 34 Positive electrode uncoated section, 40 Negative electrode metal foil, 42 Negative electrode mixture layer, 44 Negative electrode uncoated section, 45 Laminated section, 47 Axial extension section, 48 Bent section, 60, 60a Welded section, 61 Current collector plate melted and solidified section, 63 Intermediate melted and solidified section, 70 80. Void, 81. Laser welding device, 82. Branching DOE, 84. Focusing lens, 85. Unit cell, 96, 91, 92, 93. Laser beam.

Claims

1. An energy storage device having a welded portion formed in a line by welding the end of a metal foil constituting an electrode to a current collector plate made of a different metal from the metal foil, wherein the line-shaped welded portion has an intermediate molten and solidified portion having a different composition from the current collector plate, formed in a single layer in the longitudinal direction of the welded portion, located between the molten and solidified portion of the current collector plate, which is formed by the melting and solidification of a part of the current collector plate, and the metal foil.

2. The energy storage device according to claim 1, wherein the metal foil is copper foil and the current collector plate is a nickel-plated iron plate.

3. The energy storage device according to claim 1, wherein the composition of the intermediate molten and solidified portion is an alloy containing 80% or more copper.

4. The energy storage device according to claim 1, wherein the thickness of the current collector plate is 0.1 mm or more and 0.4 mm or less.

5. The energy storage device according to claim 1, wherein the length in the longitudinal direction of the portion where the thickness of the intermediate melted and solidified portion is substantially constant is 7 mm or more.

6. A method for manufacturing an energy storage device, comprising the steps of: arranging the ends of metal foils constituting electrodes in an axial position; bending the tips of the ends; and contacting the bent metal foils with a current collector plate having a higher melting point than the metal foils, wherein a line-shaped laser beam with a uniform beam profile in the longitudinal direction is irradiated onto the surface of the current collector plate, the current collector plate is melted to the back surface by thermal conduction, the metal foil is melted by the heat of the molten current collector plate, and a single layer of molten solidified material mainly composed of the metal foil is formed on the entire surface of the metal foil side of the molten solidified portion of the current collector plate.

7. The method for manufacturing an energy storage device according to claim 6, wherein a line-shaped laser beam with a beam profile uniform in the longitudinal direction is formed at the processing position by branching DOE.

8. The method for manufacturing an energy storage device according to claim 7, wherein the length of one line at the processing position of the laser beam is 7 mm or more, and the number of branches of the branched DOE is 31 or more.

Images