Secondary battery and method for manufacturing secondary battery

Abstract

A secondary battery according to the present invention comprises: an electrode plate group in which a strip-shaped first electrode, a strip-shaped second electrode, and a separator inserted between the first electrode and the second electrode are wound in a spiral shape; a metal case in which the electrode plate group is housed; and a sealing portion that seals an upper opening of the metal case. The metal case has a bottom surface provided with a protrusion on the inner side, and a side surface integrally molded with the bottom surface. The first electrode has a metal foil containing a first metal, and an end of the metal foil is exposed on one end surface of the electrode plate group. The end of the metal foil and the protrusion on the bottom surface of the metal case are welded.

Description

Secondary battery and method for manufacturing the same Cross-reference to related applications

[0001] This disclosure claims the benefit of priority with respect to Japanese Patent Application No. 2024-165522, filed on September 24, 2024, with the Japan Patent Office, and the entire contents of the said patent application are incorporated herein by reference.

[0002] This disclosure relates to a secondary battery and a method for manufacturing the same.

[0003] Conventionally, a cylindrical storage battery having a wound electrode plate group wound in a spiral shape has been known (for example, Patent Document 1). The cylindrical storage battery of Patent Document 1 includes a strip-shaped positive electrode plate, a negative electrode plate, and a separator. The tip portion of the current collector of one electrode plate protrudes upward, and the tip portion of the current collector of the other electrode plate protrudes downward and is wound in a spiral shape to form an electrode plate group. A metal bottomed case that houses an electrolyte and positive and negative current collectors welded to the tip portions of the current collectors of the respective electrode plates protruding from above and below the electrode plate group. It also includes a sealing plate that seals the opening of the metal bottomed case. The positive current collector welded to the tip portion of the current collector of the electrode plate protruding upward from the electrode plate group has a terminal portion that is raised by one step at its approximate center, and this terminal portion and the sealing plate are electrically connected.

[0004] Japanese Patent Application Laid-Open No. 2007-280743

[0005] In a so-called end-face current collection type secondary battery, after welding a current collection plate to the electrode plate group, it is necessary to weld the current collection plate and the metal bottomed case (metal case) to establish electrical continuity between the electrode plate group and the metal case, resulting in an increase in the number of parts. Also, when attempting to directly weld the current collection foil of the metal case and the electrode plate group through the metal case, the gap between the current collection foil and the metal case is unstable, making it difficult to weld without causing sputtering or perforation.

[0006] One aspect of the present disclosure relates to a secondary battery. The secondary battery comprises a group of electrode plates wound in a spiral shape, each containing a strip-shaped first electrode, a strip-shaped second electrode, and a separator inserted between the first and second electrodes; a metal case housing the group of electrode plates; and a sealing portion that seals the upper opening of the metal case. The metal case has a bottom surface with an inwardly protruding portion and a side surface integrally molded with the bottom surface. The first electrode has a metal foil containing a first metal, and the end of the metal foil is exposed on one end face of the group of electrode plates. The end of the metal foil and the protruding portion on the bottom surface of the metal case are welded together.

[0007] Another aspect of this disclosure relates to a method for manufacturing a secondary battery. The manufacturing method comprises the steps of: preparing an electrode plate group in which a strip-shaped first electrode having a metal foil, a strip-shaped second electrode, and a separator inserted between the first electrode and the second electrode are wound in a spiral shape, with the end of the metal foil exposed on one end face; preparing a metal case having an upper opening, a bottom surface with an inwardly protruding portion, and a side surface integrally molded with the bottom surface; housing the electrode plate group inside the metal case and bringing the end of the metal foil into contact with the bottom surface of the metal case; and welding the end of the metal foil to the protruding portion of the metal case by applying a first alternating current between the first electrode and the second electrode.

[0008] According to this disclosure, it is possible to provide a secondary battery and a method for manufacturing a secondary battery that do not require a current collector plate to connect the electrode plate group and the metal case, and that are easy to manufacture. Novel features of the present invention are described in the appended claims, but the present invention, in conjunction with other objects and features of the present invention, will be better understood by the following detailed description in reference to the drawings, with respect to both the structure and content.

[0009] This is a schematic cross-sectional view showing an example of the configuration of a secondary battery 10 according to one embodiment of the present disclosure. This is a perspective view showing the appearance of the electrode plate group 20 included in the secondary battery 10 shown in Figure 2. This is a schematic explanatory diagram of resistance welding in the manufacturing method of the secondary battery 10 according to one embodiment of the present disclosure. This is another schematic explanatory diagram of resistance welding shown in Figure 3. This is a schematic explanatory diagram showing an example of scanning of the welding electrode 80. This is an explanatory diagram showing an example of the arrangement of a convex portion 40c with a linearly formed top.

[0010] The secondary battery and method for manufacturing the secondary battery described in this disclosure will be explained below with examples. However, this disclosure is not limited to the examples described below. In the following explanation, specific numerical values ​​and materials may be given as examples, but other numerical values ​​and materials may be applied as long as the effects of this disclosure are achieved.

[0011] 1. Secondary Battery The secondary battery according to this disclosure comprises an electrode plate group, a metal case having an upper opening, and a sealing portion that seals the upper opening. The secondary battery may be an aqueous electrolyte secondary battery or a non-aqueous electrolyte secondary battery. Non-aqueous electrolyte secondary batteries include lithium-ion secondary batteries that use a material that reversibly intercepts and releases lithium ions as the negative electrode active material, lithium metal secondary batteries in which lithium metal is deposited at the negative electrode during charging and dissolves during discharge, solid batteries containing a gel electrolyte, and all-solid-state batteries that have a solid electrolyte.

[0012] Non-aqueous electrolytes can be prepared by dissolving an electrolyte salt, such as a lithium salt, in a non-aqueous solvent. Examples of non-aqueous solvents include cyclic carbonates such as ethylene carbonate, propylene carbonate, and butylene carbonate; linear carbonates such as dimethyl carbonate and diethyl carbonate; and lactones such as γ-butyrolactone. Non-aqueous solvents can be used alone or in combination of two or more.

[0013] 1.1 Electrode Group The electrode group is formed by winding a first electrode, a second electrode, and a separator inserted between the first and second electrodes in a spiral shape. That is, the electrode group is a columnar wound body composed of the first electrode, the second electrode, and the separator.

[0014] Both the first and second electrodes are formed in a strip shape, but are not limited to this shape. The first electrode may be a negative electrode or a positive electrode. For example, if the first electrode is a negative electrode, the second electrode will be a positive electrode, and if the first electrode is a positive electrode, the second electrode will be a negative electrode.

[0015] The first electrode has a metal foil containing at least a first metal as a current collector, and may have an electrode mixture layer coated on the surface of the metal foil. The end of the metal foil is exposed at one end face of the electrode group.

[0016] The separator is formed in a strip shape, but is not limited to this shape. The separator material can be a porous membrane made of polyolefin-based material or a combination of polyolefin-based material and heat-resistant material. Examples of porous membranes made of polyolefin-based material include polyethylene, polypropylene, and ethylene-propylene copolymer. These resins can be used individually or in combination of two or more. Other thermoplastic polymers may be used in combination with polyolefins as needed. Examples of heat-resistant materials include heat-resistant resins and inorganic fillers. The porous membrane may be a single-layer membrane or a multilayer membrane. The multilayer membrane may include a membrane of a mixture of heat-resistant resin and inorganic filler.

[0017] 1.2 Metal Case The metal case houses the electrode plate group through an opening at the top. The metal case contains the electrode plate group. The non-aqueous electrolyte is housed in the metal case after welding the end of the metal foil exposed on one end face of the electrode plate group to a protrusion on the bottom surface of the metal case.

[0018] The metal case has a bottom (base) with an inward protrusion and side walls (sides) integrally molded with the bottom. The external shape of the metal case may be, for example, a bottomed cylindrical shape or a bottomed rectangular tubular shape, but is not limited to these shapes. The metal case having side walls integrally molded with the bottom can be formed by pressing, drawing, ironing, or other processes on a metal sheet.

[0019] 1.3 The metal foil of the first metal in the electrode plate group is welded to the protrusion on the bottom surface of the metal case. This welding can be achieved by resistance welding by applying alternating current between the first electrode and the second electrode. Since the electrode plate group has basically the same configuration as a capacitor, it can conduct alternating current, making resistance welding possible. In addition, since a protrusion is formed on the bottom surface of the metal case, the exposed ends of the metal foil from the end faces of the electrode plate group can be reliably brought into contact with the metal case, making it possible to weld without generating spatter or holes. As a result, the metal foil of the first metal in the electrode plate group and the bottom surface of the metal case can be directly welded, eliminating the need for a current collector plate to connect the electrode plate group and the metal case, thus realizing a secondary battery that is easy to manufacture.

[0020] A second metal layer may be formed at the edges of the metal foil. This allows for better welding. The second metal layer may be formed by, for example, plating, but is not limited to this method.

[0021] The second metal layer may be formed on only one side of the edge of the metal foil. This eliminates waste compared to forming the second metal layer on both sides of the edge of the metal foil, as the second metal layer is formed only on the side actually used in resistance welding.

[0022] The second metal layer preferably contains a metal with a higher electrical resistance than the main component metal of the metal foil. This increases the amount of heat generated when alternating current is applied, enabling good resistance welding.

[0023] The second metal layer may, specifically, contain at least one of nickel and tin, but is not limited to these metals.

[0024] The edges of the metal foil may be bent. This ensures stable contact between the metal foil and the protrusions on the bottom surface of the metal case, enabling good resistance welding.

[0025] The edges of the metal foil may be folded, and a second layer of metal may be formed on the surface facing the bottom of the metal case.

[0026] 1.4 Shape and arrangement of the protrusions on the bottom surface of the metal case The protrusions include a plurality of projections arranged on the bottom surface. The plurality of projections may be arranged regularly on the bottom surface, or randomly. Examples of regular arrangements include, but are not limited to, a square grid, a triangular grid, or a hexagonal grid.

[0027] The top of the protrusion may be formed as a point. This reduces the contact area and increases the electrical resistance, thereby increasing the amount of heat generated and enabling good resistance welding.

[0028] Alternatively, the top of the protrusion may be formed linearly. This increases the area welded per protrusion, thereby reducing the electrical resistance of the welded area and improving the current output characteristics of the secondary battery.

[0029] If the top of the projection is formed linearly, the projection may be positioned such that, for example, one end of the linear top points toward the center of the base. In other words, the linear top may be positioned radially from the center of the base. However, the arrangement is not limited to this.

[0030] 2. Method for manufacturing a secondary battery The method for manufacturing a secondary battery according to this disclosure comprises the following steps (a) to (d).

[0031] In step (a), a group of electrode plates is prepared in which a strip-shaped first electrode having a metal foil, a strip-shaped second electrode, and a separator inserted between the first and second electrodes are wound in a spiral shape, with the end of the metal foil exposed on one end face.

[0032] In step (b), a metal case is prepared having an upper opening, a bottom (bottom surface) with an inward protrusion, and side walls (sides) integrally molded with the bottom.

[0033] In step (c), the electrode plate group is housed inside the metal case, and the edges of the metal foil are brought into contact with the bottom surface of the metal case.

[0034] In step (d), the edge of the metal foil and the protrusion of the metal case are welded by applying a first alternating current between the first electrode and the second electrode. The first alternating current is a relatively large current necessary for resistance welding.

[0035] According to these processes (a) to (d), the metal foil of the first metal of the electrode plate group and the bottom surface of the metal case can be directly welded, making it possible to easily manufacture a secondary battery that does not require a current collector plate to connect the electrode plate group and the metal case.

[0036] The method for manufacturing a secondary battery according to this disclosure may further include the following steps (d1) and (d2).

[0037] In step (d1), a second alternating current smaller than the first alternating current is applied between the welding electrode and the second electrode while scanning the welding electrode on the bottom surface of a metal case that is electrically connected to the first electrode, in order to search for the minimum point of electrical resistance.

[0038] In step (d2), a first alternating current is applied between the welding electrode and the second electrode at the minimum point. However, during the search in step (d1), it is not actually possible to know that the electrical resistance is at a minimum point until it has been passed. Here, "at the minimum point" includes at least the case where the process returns to the minimum point after it has been determined to be the minimum point, and the case where the stopping position is immediately after passing the minimum point and the distance difference from the minimum point is within an acceptable range (near the minimum point). In the latter case, "at the minimum point" may be rephrased as, for example, "immediately after detecting the minimum point" or "at or near the minimum point."

[0039] By following these steps (d1) and (d2), the welding electrode can be precisely stopped at a position corresponding to the protrusion on the bottom surface of the metal case, thereby properly welding the edge of the metal foil to the protrusion on the metal case.

[0040] As described above, this disclosure provides a secondary battery and a method for manufacturing a secondary battery that do not require a current collector plate to connect the electrode plate group and the metal case, and is easy to manufacture.

[0041] 3. Specific Examples of Secondary Battery 10 and Method for Manufacturing Secondary Battery Hereinafter, examples of the secondary battery and the method for manufacturing the secondary battery according to the present disclosure will be specifically described with reference to the drawings. For the components and steps of the secondary battery and the method for manufacturing the secondary battery in the examples described below, the above-described components and steps can be applied and can be modified based on the above-described description. Further, the matters described below may be applied to the above-described embodiments. Among the components and steps of the secondary battery and the method for manufacturing the secondary battery in the examples described below, the components and steps that are not essential for the secondary battery and the method for manufacturing the secondary battery according to the present disclosure may be omitted. Note that the drawings shown below are schematic and do not accurately reflect the actual shape, number, etc. of the members.

[0042] First, after explaining the configuration of the secondary battery 10 of the present embodiment, the principle of resistance welding in the secondary battery 10 will be explained, and further, the method for manufacturing the secondary battery 10 of the present embodiment will be explained.

[0043] 3.1 Secondary Battery 10 The secondary battery 10 of the present embodiment is configured as a lithium-ion secondary battery, but is not limited thereto. As shown in FIG. 1, the secondary battery 10 includes a wound electrode plate group 20, a current collector plate 31, a metal case 40, and a sealing plate 51.

[0044] As shown in FIG. 2, the electrode plate group 20 is formed by winding a strip-shaped first electrode 21 and a second electrode 22 with a strip-shaped separator 23 interposed therebetween. In the present embodiment, the first electrode 21 is a negative electrode plate, and the second electrode 22 is a positive electrode plate. Although the separator 23 is exposed at the outermost peripheral portion of the electrode plate group 20, the configuration is not limited to this.

[0045] As shown in FIG. 3, the first electrode 21 has a metal foil 21a which is a strip-shaped core material and contains a first metal. Although not shown, an electrode mixture layer may be formed in a region of the surface of the metal foil 21a other than the end portion 21x of the metal foil 21a. The end portion 21x of the metal foil 21a is exposed on one end surface (the lower side in FIG. 1) of the electrode plate group 20. The exposed end portion 21x is gently bent so that its tip faces in the horizontal direction. This configuration will be described later with reference to FIG. 4.

[0046] The second electrode 22 is formed of a strip-shaped core material, a metal foil.

[0047] The current collector plate 31 is disposed above the electrode plate group 20 and connected to the second electrode 22 by welding. The current collector plate 31 is made of, for example, stainless steel.

[0048] The metal case 40 is made of metal and formed in a bottomed cylindrical shape having an upper opening 40d at one end (the upper end in FIG. 1). The metal case 40 houses the electrode plate group 20 and the current collector plate 31 through the upper opening 40d. The metal case 40 has a bottom surface 40b provided with a plurality of convex portions 40c on the inside and a side surface 40a integrally formed with the bottom surface 40b. The end portion 21x of the metal foil 21a of the first electrode 21 and the top of the convex portion 40c provided on the bottom surface 40b of the metal case 40 are welded together. Therefore, the metal case 40 of the present embodiment functions as an external negative electrode terminal.

[0049] As the convex portion 40c, as shown in FIG. 5, a plurality of protrusions randomly arranged on the bottom surface 40b and having a dot-shaped top may be used. Specifically, a conical protrusion may be used, but it is not limited to such a shape and arrangement. The shape of the protrusion may be, for example, a pyramid, or the top of the protrusion may be rounded. The plurality of protrusions may be regularly arranged. Examples of the regular arrangement include, but are not limited to, a square lattice, a triangular lattice, or a hexagonal lattice arrangement.

[0050] The top of the protrusion may be formed in a linear shape. For example, as shown in FIG. 6, a linear top slightly shorter than the radius of the bottom surface 40b may be arranged radially from the center of the bottom surface 40b. However, the linear top is not limited to such a shape, length, and arrangement.

[0051] As shown in Figure 1, the sealing plate 51 seals the upper opening 40d of the metal case 40. A gasket 52 is placed around the periphery of the sealing plate 51, and the inside of the metal case 40 is sealed by crimping the end of the upper opening 40d of the metal case 40 to the gasket 52. The sealing plate 51 is electrically connected to the current collector plate 31 via a metal tab 53. Therefore, the sealing plate 51 in this embodiment functions as a positive electrode external terminal. Note that the sealing plate 51 is an example of the "sealing portion" in this disclosure.

[0052] 3.2 Principle of resistance welding in secondary battery 10 As shown in Figure 3, the first electrode 21 and the second electrode 22, which are insulated by the separator 23, are not conductive, but they constitute a capacitor, so when AC is applied from an AC power source, current flows.

[0053] Here, R1 is defined as the electrical resistance value (contact resistance value) when the end of the first electrode 21 contacts a protrusion 40c provided on the bottom surface 40b of the metal case 40, and R2 is defined as the electrical resistance value between adjacent protrusions 40c on the bottom surface 40b. Note that the electrical resistance values ​​of the bottom surface 40b and the protrusions 40c of the metal case 40 are sufficiently small to be negligible compared to R1 and R2.

[0054] At the protrusion 40c (the central protrusion 40c on the left side of Figure 3) that the welding electrode 80 contacts from below, the electrical resistance value when alternating current is applied is R1. However, at the adjacent protrusions 40c to the right and left, the electrical resistance value when alternating current is applied is R1 + R2. This difference in electrical resistance causes the current to concentrate at the protrusion 40c that the welding electrode 80 contacts, generating significant heat, which in turn enables resistance welding.

[0055] As shown in Figure 3, if the top of the protrusion 40c is formed as a point, it may be difficult to precisely position the end 21x of the first electrode 21 so that it makes contact and to maintain that position during welding. Therefore, for example, by forming the top of the protrusion 40c as a line, the required positioning accuracy can be relaxed. The line-shaped top may be formed to be shorter than the radius of the bottom surface 40b of the metal case 40, but is not limited to this length. As shown in Figure 6, the line-shaped top may be positioned so that one end faces the center of the bottom surface 40b. This ensures that the top of the protrusion 40c and the end 21x of the metal foil 21a of the first electrode 21 make contact.

[0056] Alternatively, as shown in Figure 4, the end portion 21x of the metal foil 21a of the first electrode 21 may be bent. This stabilizes the contact between the metal foil 21a and the protrusion 40c on the bottom surface 40b of the metal case 40. In this configuration, it is preferable to form a plating layer 21m of a second metal (for example, nickel or tin) containing a metal with a higher electrical resistance than the main component metal of the metal foil 21a only on one side (the bottom surface in Figure 4) of the bent end portion 21x.

[0057] 3.3 Method for Manufacturing a Secondary Battery 10 The method for manufacturing a secondary battery 10 according to this disclosure comprises the following steps (a) to (d).

[0058] In step (a), as shown in Figures 1 and 2, an electrode plate group 20 is prepared in which a strip-shaped first electrode 21 having a metal foil 21a, a strip-shaped second electrode 22, and a separator 23 inserted between the first electrode 21 and the second electrode 22 are wound in a spiral shape, with the end portion 21x of the metal foil 21a exposed on one end face.

[0059] In step (b), a metal case 40 is prepared, as shown in Figure 1, having an upper opening 40d, a bottom surface 40b with an inward protrusion 40c, and a side surface 40a integrally molded with the bottom surface 40b.

[0060] In step (c), as shown in Figure 1, the electrode plate group 20 is housed inside the metal case 40, and the end portion 21x of the metal foil 21a is brought into contact with the bottom surface 40b of the metal case 40. However, since the bottom surface 40b of the metal case 40 is provided with a protrusion 40c, in reality, the end portion 21x of the metal foil 21a will be brought into contact with the top portion of this protrusion 40c.

[0061] In step (d), the first alternating current is applied between the first electrode 21 and the second electrode 22 to weld the end portion 21x of the metal foil 21a to the protrusion 40c of the metal case 40.

[0062] Step (d) may further comprise steps (d1) and (d2) described below.

[0063] In step (d1), as shown in Figures 3 to 5, the welding electrode 80 is brought into contact with the bottom surface 40b of the metal case 40, which is electrically connected to the first electrode 21, from the outside and scanned while a second alternating current smaller than the first alternating current is applied between the welding electrode 80 and the second electrode 22 to search for the minimum point of electrical resistance. The second alternating current is for measuring electrical resistance, so it can be a much smaller current than the first alternating current.

[0064] While applying the second AC current, the change in electrical resistance is monitored, and when a minimum point in electrical resistance is found, it is determined that this corresponds to the position of the protrusion 40c with appropriate contact resistance between the metal foil 21a of the first electrode 21 and the electrode. Then, the first AC current for resistance welding is applied for the required time to perform resistance welding. At this time, if the current value and phase are monitored, it is also possible to detect whether or not a spark has occurred.

[0065] Furthermore, if a plating layer 21m (see Figure 4), for example nickel, is formed on the end 21x of the metal foil 21a by plating or sputtering, the electrical resistance of the welded area will increase during resistance welding, increasing the amount of heat generated and enabling good welding. Also, even with a lower current value, the amount of heat generated will be sufficient for resistance welding, thus suppressing the generation of sputter. In Figure 4, an example is shown where the end 21x of the metal foil 21a is bent and the plating layer 21m (an example of a second metal layer) is provided on the surface facing the bottom surface of the metal case 40, but the position of providing the second metal layer is not limited to this example. For example, the position of providing the second metal may be the tip of the protrusion 40c, or it may be on the inner surface of the metal case 40.

[0066] This disclosure is not limited to the embodiments described above, and can be implemented in various forms without departing from its essence. Furthermore, various disclosures can be formed by appropriately combining the multiple components disclosed in the embodiments described above. For example, some components may be removed from all the components shown in the embodiments. The drawings schematically show each component for ease of understanding, and the number of each component shown may differ from the actual number due to the convenience of drawing creation. In addition, each component shown in the embodiments described above is an example and is not particularly limiting, and various modifications are possible without substantially departing from the effects of this disclosure.

[0067] Although the present invention has been described in relation to preferred embodiments at present, such disclosure should not be interpreted restrictively. Various modifications and alterations will undoubtedly become apparent to those skilled in the art in the field to which the invention pertains by reading the above disclosure. Accordingly, the appended claims should be interpreted as encompassing all modifications and alterations without departing from the true spirit and scope of the invention.

[0068] 10: Secondary battery, 20: Electrode plate group, 21: First electrode, 21a: Metal foil, 21x: End of metal foil, 21m: Plating layer, 22: Second electrode, 23: Separator, 31: Current collector plate, 40: Metal case, 40a: Side, 40b: Bottom, 40c: Protrusion, 40d: Top opening, 51: Sealing plate, 52: Gasket, 53: Metal tab, 80: Welding electrode

Claims

1. A secondary battery comprising: a group of electrode plates wound in a spiral shape, comprising a strip-shaped first electrode, a strip-shaped second electrode, and a separator inserted between the first electrode and the second electrode; a metal case housing the group of electrode plates; and a sealing portion that seals the upper opening of the metal case, wherein the metal case has a bottom surface with an inwardly protruding portion and a side surface integrally molded with the bottom surface; the first electrode has a metal foil containing a first metal, the end of the metal foil is exposed on one end face of the group of electrode plates, and the end of the metal foil and the protruding portion on the bottom surface of the metal case are welded together.

2. The secondary battery according to claim 1, wherein a second metal layer is formed on the end of the metal foil.

3. The secondary battery according to claim 2, wherein the second metal layer is formed only on one side of the end of the metal foil.

4. The secondary battery according to claim 2, wherein the second metal layer contains a metal with a higher electrical resistance than the main component metal of the metal foil.

5. The secondary battery according to claim 4, wherein the second metal layer comprises at least one of nickel and tin.

6. The end of the metal foil is bent, as described in claim 1.

7. The secondary battery according to claim 1, wherein the end of the metal foil is bent and a second metal layer is formed on the surface of the metal case facing the bottom surface.

8. The secondary battery according to claim 1, wherein the convex portion includes a plurality of protrusions arranged on the bottom surface.

9. The secondary battery according to claim 8, wherein the top of the projection is formed in the shape of a dot.

10. The secondary battery according to claim 8, wherein the top of the projection is formed in a linear shape.

11. The secondary battery according to claim 10, wherein the projection is arranged such that one end of the linear top of the projection points toward the center of the bottom surface.

12. A method for manufacturing a secondary battery, comprising the steps of: preparing an electrode plate group in which a strip-shaped first electrode having a metal foil, a strip-shaped second electrode, and a separator inserted between the first electrode and the second electrode are wound in a spiral shape, with the end of the metal foil exposed on one end face; preparing a metal case having an upper opening, a bottom surface with an inwardly protruding portion, and a side surface integrally molded with the bottom surface; housing the electrode plate group inside the metal case and bringing the end of the metal foil into contact with the bottom surface of the metal case; and welding the end of the metal foil to the protruding portion of the metal case by applying a first alternating current between the first electrode and the second electrode.

13. A method for manufacturing a secondary battery according to claim 12, further comprising the steps of: scanning a welding electrode on the bottom surface of the metal case which is electrically connected to the first electrode, applying a second alternating current smaller than the first alternating current between the welding electrode and the second electrode to search for a minimum point of electrical resistance; and applying the first alternating current between the welding electrode and the second electrode at the minimum point.

Images