Laser processing method

Abstract

This invention provides a laser processing method that can ensure both high welding strength and low current density while suppressing the welding area when welding the current collector foil and tab lead of a battery. [Solution] A laser processing method comprising the step of laser welding a current collector foil 2 and a tab lead 3 at the battery end, wherein the laser welding is performed by irradiating with laser light selected from a blue laser and a green laser. When the direction in which the tab lead extends from the battery end is defined as the first direction, it is preferable to perform the laser welding so that the welding locations are arranged in a plurality of linear rows perpendicular to the first direction.

Description

Technical Field

[0001] The present invention relates to a laser processing method.

Background Art

[0002] In recent years, in order to enable more people to have accessible, reliable, sustainable, and advanced energy at hand, research and development on secondary batteries that contribute to energy efficiency have been carried out. In a laminated lithium-ion secondary battery, the joining of the current collector foil and the tab lead is generally performed by an ultrasonic joining method. However, in the case of the ultrasonic joining method, there are problems such as a large amount of metal powder scattering, the conditions changing due to the aging of the horn and anvil, the vibration application conditions changing greatly depending on the surface properties of the foil, and the need to secure a large joining area due to the thickness attenuation of the load application part.

[0003] In addition to ultrasonic joining, joining by resistance welding or laser welding is also similarly carried out. However, in resistance welding, there is a problem that the spatter scattering level is greatly affected by the maintenance situation such as the adhesion of the oxide film on the tool. Regarding laser welding, IR (around wavelength 1000 nm) laser welding is known. In IR laser welding, generally, in order to maintain the joining strength, in addition to continuous welding (CW) with a beam having a wide spot diameter, percussion, and intermittent irradiation called trepanning, the joining area is increased by wobbling with a beam having a narrow spot diameter (such as a fiber laser). Furthermore, Patent Document 1 discloses a laser processing method for processing a workpiece (object to be processed) by emitting laser light, wherein the workpiece has at least a first processing target part and a second processing target part located away from the first processing target part, and the laser light includes a first laser light (for example, a blue laser light with a wavelength of 600 nm or less) and a second laser light with a longer wavelength than the first laser light (for example, an infrared laser light with a wavelength of 800 nm or more), and the laser processing method comprises a first step of emitting the first laser light to the first processing target part, a second step of emitting the second laser light to the first processing target part from which the first laser light was emitted, a third step of emitting the first laser light to the second processing target part, and a fourth step of emitting the second laser light to the second processing target part from which the first laser light was emitted. Furthermore, Patent Document 2 discloses a laser welding method in which a scanning means that scans a laser beam guided from a laser oscillator is moved, and the laser beam scanned by the scanning means is irradiated onto a plurality of metal plates that are to be welded, thereby welding a plurality of metal plates. The laser welding method is performed within the scanning range of the scanning means while moving the scanning means, in which a step of tack welding predetermined locations in the welding direction of the workpiece and a step of main welding corresponding to the tack-welded locations of the workpiece are performed. In this document, it is disclosed that the shape of the welded area is a spot shape with a diameter of about 2 mm. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2023-112734 [Patent Document 2] Japanese Patent Publication No. 2016-150363 [Overview of the project] [Problems that the invention aims to solve]

[0005] However, in IR laser welding, particularly when the target material is a metal such as copper, there is a problem in that spatter scattering occurs significantly due to the low absorption rate of laser light by the metal. Furthermore, when laser welding the current collector foil and tab lead of a battery, there was no known method that could ensure both high welding strength and low current density (low electrical resistance) while suppressing the welding area. Patent documents 1 and 2 do not disclose welding of the current collector foil and tab lead of a battery, nor do they disclose the electrical characteristics of the welded area. The present invention has been made in view of the above, and aims to provide a laser processing method that can ensure both high welding strength and low current density while suppressing the welding area when welding the current collector foil and tab lead of a battery. This will ultimately contribute to energy efficiency. [Means for solving the problem]

[0006] To solve the aforementioned problems, the present invention proposes the following means. [1] The process includes a step of laser welding the current collector foil and the tab lead at the battery terminal, The laser welding is characterized by being performed by irradiation with laser light selected from a blue laser and a green laser.

[0007] According to [1] above, when laser welding the current collector foil and tab lead of a battery, it is possible to ensure both high welding strength and low current density (low electrical resistance) while suppressing the area of ​​the welding region.

[0008] [2] The laser processing method according to [1], wherein the laser welding is performed such that, when the direction in which the tab lead extends from the battery end is defined as the first direction, the welding locations are arranged in a plurality of linear rows perpendicular to the first direction.

[0009] According to [2] above, the welding strength can be further improved and the current density can be further reduced.

[0010] [3] The laser processing method according to [2], wherein the number of rows of welding points is 2 to 5 or 3. According to [3] above, the welding strength can be further improved and the current density can be further reduced.

[0011] [4] The laser processing method according to [1], wherein the laser welding is performed such that the welding locations are arranged in a plurality of linear rows extending in the first direction, when the direction in which the tab lead extends from the battery end is defined as the first direction. According to [4] above, since the rows of multiple welds are aligned along the direction of the current, smoother current flow is possible.

[0012] [5] The laser processing method according to any one of [1] to [4], wherein the current collector foil is made up of multiple layers.

[0013] According to [5] above, multiple current collector foils of a stacked battery can be efficiently welded together to a tab lead. [Effects of the Invention]

[0014] This laser processing method can be provided for welding the current collector foil and tab lead of a battery, which can suppress the area of ​​the welding region while ensuring not only high welding strength but also low current density (low electrical resistance) by suppressing the gate effect near the weld upstream of the current flow. [Brief explanation of the drawing]

[0015] [Figure 1] This is a plan view showing a part of a lithium-ion secondary battery with the tab leads and current collector foil welded together. [Figure 2] Figure 1 is an enlarged view showing an example of a current collector foil and tab lead welding location. [Figure 3] Figure 1 is an enlarged view showing an example of a current collector foil and tab lead welding location. [Figure 4] Figure 1 is an enlarged view showing an example of a current collector foil and tab lead welding location. [Figure 5]It is an enlarged view showing an example of a current collector foil and a tab lead welding location in FIG. 1. [Figure 6] It is a contour diagram of the current density at the negative electrode current collector foil and the tab lead welding location during charging of the battery of Example 1 and an arrow plot diagram showing the flow of current in a part thereof. [Figure 7] It is a contour diagram of the current density at the negative electrode current collector foil and the tab lead welding location during charging of the battery of Example 2 and an arrow plot diagram showing the flow of current in a part thereof. [Figure 8] It is a contour diagram of the current density at the negative electrode current collector foil and the tab lead welding location during charging of the battery of Example 3 and an arrow plot diagram showing the flow of current in a part thereof. [Figure 9] It is a contour diagram of the current density at the negative electrode current collector foil and the tab lead welding location during charging of the battery of Example 4 and an arrow plot diagram showing the flow of current in a part thereof. [Figure 10] It is a contour diagram of the current density at the negative electrode current collector foil and the tab lead welding location during charging of the battery of Example 5 and an arrow plot diagram showing the flow of current in a part thereof. [Figure 11] It is a contour diagram of the current density at the negative electrode current collector foil and the tab lead welding location during charging of the battery of Example 6 and an arrow plot diagram showing the flow of current in a part thereof.

Mode for Carrying Out the Invention

[0016] Hereinafter, with reference to the drawings, a laser processing method according to an embodiment of the present invention will be described. The method of the present embodiment includes a step of laser welding a current collector foil and a tab lead by irradiating laser light at the end of the battery.

[0017] (Battery) There are no particular restrictions on the batteries processed by the laser processing method of this embodiment, and the laser processing method of this embodiment can be applied to various known batteries. Examples of batteries include lead-acid batteries, lithium-ion secondary batteries, lithium-ion polymer secondary batteries, nickel-metal hydride batteries, nickel-cadmium batteries, nickel-iron batteries, nickel-zinc batteries, silver oxide-zinc batteries, and cobalt-titanium lithium secondary batteries. In particular, the laser processing method of this embodiment can be suitably used for processing laminated batteries (such as laminated lithium-ion secondary batteries). Furthermore, the current collector foil may be either a positive electrode or a negative electrode current collector foil, but it is preferable that it be a negative electrode current collector foil. There are no particular restrictions on the material of the current collector foil, but for a positive electrode current collector, for example, aluminum foil or SUS can be used, and for a negative electrode current collector, nickel foil, copper foil, or iron foil with nickel plating can be used. From the viewpoint of more reliably obtaining the effects of the present invention, nickel foil and copper foil are preferred as negative electrode current collector foils, and copper foil is more preferred. Furthermore, the current collector foil may be made up of multiple layers. That is, the processing method of this embodiment allows for the efficient welding of multiple current collector foils of a stacked battery to a tab lead. There are no particular restrictions on the number of layers of current collector foil, but it depends on the required electrical characteristics. Furthermore, it also depends on the configuration of the battery cell, called the electrode group. In the case of negative electrode-positive electrode-negative electrode, the number of layers of the negative electrode is twice that of the positive electrode. For example, when considering installation in an automobile, 15 to 100 layers is preferable, 24 to 80 layers is more preferable, and 27 to 54 layers is even more preferable.

[0018] (Tabreed) As tab leads, metal plates made of aluminum, aluminum alloys, copper, copper alloys, nickel, or nickel alloys can be used. From the viewpoint of more reliably obtaining the effects of the present invention, the tab lead is preferably a metal plate made of copper, copper alloys, nickel, or nickel alloys as a negative electrode tab lead, and a metal plate made of copper or a copper alloy is more preferable. From the viewpoint of bonding, it is desirable that the tab lead be a metal plate made of the same material as the current collector foil. That is, for example, in the case of a negative electrode, if the current collector foil is copper foil, it is desirable that the tab lead also be a copper plate. (Laser welding) The laser light used in laser welding is selected from blue lasers and green lasers. Here, "laser light selected from blue and green lasers" refers to laser light with a wavelength of 400 nm to approximately 580 nm. The blue laser preferably has a wavelength of 480 nm to 400 nm, and more preferably a wavelength of 400 nm to 465 nm. The green laser is preferably 580 nm to 500 nm in wavelength, and more preferably 560 nm to 500 nm in wavelength. In this embodiment, the laser light is preferably a blue laser. When the wavelength of the laser light is within the above range, the absorption rate by copper and nickel is high. Therefore, especially when the current collector foil and / or tab lead are made of copper or nickel, it becomes possible to narrow the bead width and lengthen the joint ridge line, as described later, by using a focused laser source. In this embodiment, laser light of different wavelengths within the above range may be multiplexed and irradiated, or a blue laser and / or a green laser may be multiplexed and irradiated with an infrared (IR) laser.

[0019] As the laser processing apparatus (laser welding apparatus) used for laser welding, a known apparatus can be used, for example, the apparatus described in Patent Document 1 and Patent Document 2 can be used. Furthermore, the specific conditions for laser welding can be determined as appropriate, taking into consideration factors such as the wavelength of the laser light, the thickness and number of layers of the tab lead, and the intended welding pattern.

[0020] (Welding pattern) In this embodiment, it is preferable to perform laser welding so that the welds form multiple rows. More specifically, when the direction in which the tab lead extends from the battery end (the length direction of the tab lead) is taken as the first direction, it is preferable to perform the laser welding so that the welds are arranged in multiple linear rows (weld beads) perpendicular to the first direction (hereinafter sometimes referred to as "width direction welding"), or to perform the laser welding so that the welds are arranged in multiple linear rows extending in the first direction (hereinafter sometimes referred to as "length direction welding"). Of these, length direction welding is more preferable because the multiple welds (weld beads) are arranged in a direction parallel to the direction of the current, allowing for smoother current flow. In the length direction welding, the rows of welds (weld beads) do not necessarily have to be perfectly parallel to the length direction of the tab lead, and it is preferable that they are inclined with respect to the length direction from the viewpoint of improving the mechanical strength against load in the direction in which the tab lead extends. In this case, the inclination angle is preferably, for example, 10° to 60°, and more preferably 20° to 45°. By irradiating with the aforementioned specific laser light, welding becomes easier, allowing for an increase in the ridge length of the weld interface (the interface with molten and re-solidified material) within a limited area. Here, the ridge of the weld interface is, in other words, the outline of the welded area, the outer circumference of the convex-shaped raised portion. In this embodiment, it is possible to increase the ridge length of the weld interface within a limited area, thereby reducing the current density at the weld while ensuring sufficient welding strength. The ridge length is approximately twice the bead length in the case of a thin bead, but welding to the same width as the current collector foil is difficult. Also, when forming multiple beads, the ridge length is preferably 80mm to 120mm, more preferably 190mm to 290mm, and even more preferably 120mm to 190mm, depending on the size of the battery, etc. If the bead length is very large, even if it is good from the viewpoint of current density, it will worsen the cycle time and affect the cell and heat-welded resin, so it should not be increased unnecessarily. Furthermore, the ratio of the ridge length (L1) to the welding area (A1) (L1 / A1) is preferably 3 / 100 to 28 / 100, more preferably 3 / 100 to 21 / 100, and even more preferably 13 / 100 to 5 / 100.

[0021] In this embodiment, there are no particular restrictions on the dimensions, number, arrangement, etc. of the welded parts, as long as the effects of the present invention are achieved. For example, when considering a laminated battery for automotive use, taking into account the layout constraints of the welding area, the laser spot diameter is preferably 0.08 mm to 0.6 mm, more preferably 0.08 mm to 0.25 mm, and even more preferably 0.08 mm to 0.15 mm. Furthermore, the welded area may be a continuous straight line, or multiple welding points may be scattered at a constant pitch. When multiple welding points are scattered at a constant pitch, the pitch (distance between the centers of adjacent welding points) depends on the diameter and width of the welding area, but for example, the pitch of the melting points (P1) must be greater than the diameter of the welding point (D1), and the ratio of P1 / D1 is more preferably 2.5 to 15, more preferably 2.5 to 10, and even more preferably 2.5 to 6. If the ratio of P1 / D1 is above the lower limit, sufficient conductive performance at the welded area can be ensured. If the ratio of P1 / D1 is below the lower limit, it becomes easier to ensure sufficient weld strength. For example, if the diameter of the welding point (D1) is 0.4 mm, it is preferable that the pitch of the melting points (P1) be 1.0 mm or more. When P1 is 1.0 mm, the distance of the unwelded portion between adjacent melting points is 0.6 mm.

[0022] The specific welding patterns will be explained below with reference to Figures 1 to 5. Figure 1 is a plan view showing a lithium-ion secondary battery in which the current collector foil is welded to a tab lead, with the current collector foil 2 protruding from the end of the laminated lithium-ion secondary battery 1. This current collector foil 2 and the tab lead 3 are welded together in the welding area 4. Figures 2 and 3 show enlarged views of the welded area. The welding pattern shown in Figure 2 is an example of the widthwise welding described above. When the direction in which the tab lead 3 extends from the end of the battery 1 (i.e., the lengthwise direction of the tab lead 3) is considered the first direction, multiple rows of linear weld beads extend in a direction perpendicular to the first direction (i.e., the widthwise direction of the tab lead 3). In this welding pattern, if the length (number) of the rows (weld beads) at the welded area increases too much, the joining resistance at the welded area increases, and it may not be possible to sufficiently reduce the current density to the electrode (the electrical resistance becomes high). In Figure 3, the weld bead is formed in a grid pattern. In this case, compared to the welding pattern in Figure 2, longer ridges are secured, resulting in higher weld strength. However, the current flow is significantly obstructed compared to the welding pattern in Figure 2, resulting in an increase in current density upstream of the current inflow, making it difficult to secure a low current density for the electrode (resulting in high electrical resistance). This is because large blowholes remain inside the molten and re-solidified area, which is the current passage, and numerous fine bubbles appear near the outer edge of the ridges, and this phenomenon cannot be eliminated. Furthermore, especially in aluminum, cracks called necking appear near the ridges, and suppressing them and eliminating them by secondary melting is difficult. Therefore, to satisfy the required weld strength and electrical properties, practical considerations should be taken into account. Furthermore, instead of a continuous linear weld bead, patterns in which multiple welding points are arranged in multiple parallel linear rows as shown in Figure 2, or patterns in which multiple welding points are arranged in a grid as shown in Figure 3, are also within the scope of the present invention. In either case, the multiple welding points not only increase the ridge length of the weld interface and ensure welding strength, but the current density near the weld can also be reduced by allowing current to flow between the welding points.

[0023] Figure 4 is an enlarged view showing an example of a tab lead welding location using the laser processing method of this embodiment. In Figure 4, the multiple welding points are arranged at a predetermined pitch in multiple linear rows A extending in the width direction of the tab lead 3, and the centers of the welding points in adjacent rows A are located on the same straight line extending in the length direction of the tab lead 3. Even with such an arrangement, the length of the ridge of the welding interface can be increased by the multiple welding points and the welding strength can be ensured. However, from the viewpoint of reducing the current density, it is preferable to arrange the welding points so that the centers of the welding points in adjacent rows A are not located on the same straight line extending in the length direction of the tab lead 3. There are no particular restrictions on the number of rows A, but from the viewpoint of ensuring a balance between welding strength and current density, 2 to 5 rows are preferred, and from the viewpoint of layout, 3 rows are particularly preferred.

[0024] Figure 5 is an enlarged view showing another example of a tab lead weld using the laser processing method of this embodiment. The welding pattern shown in Figure 5 is an example of the longitudinal welding described above. In Figure 5, the welding points are arranged in multiple linear rows B extending in the first direction (i.e., the length direction of the tab lead 3). In this case, since the multiple welding points (weld beads) are arranged parallel to the direction of the current, smoother current flow is possible. There are no particular restrictions on the number of rows B, but from the viewpoint of ensuring a balance between welding strength and current density, 6 to 20 rows are preferred, and 10 to 16 rows are more preferred. However, since the welding length should also be kept down, it is important from a layout perspective to keep the length within approximately 3 mm, so the number of rows will naturally be on the higher side.

[0025] <Effects and Effects> According to the present invention, by using a welding method with a spatter-reducing laser source and performing multiple bead welding in areas where the layout pattern is limited by small-diameter spots, the length of the molten area ridge can be secured to be several times longer compared to the molten range equivalent to that of an ultrasonic welding area. This ensures sufficient electrical performance even if there are some joint defects, and solves the aforementioned problems of joint resistance and strength. In other words, by obtaining many weld ridges, the welding area can be narrowed, allowing for a smaller layout and enabling the area to function as a heat dissipation area. Furthermore, sputter-free laser welding suppresses spatter and metal powder scattering, eliminating the root cause of contamination. It also frees users from the high maintenance requirements associated with resistance welding and ultrasonic welding.

[0026] It should be noted that the technical scope of the present invention is not limited to the embodiments described above, and various modifications can be made without departing from the spirit of the invention. Furthermore, it is possible to replace the components in the embodiments with well-known components as appropriate, without departing from the spirit of the invention. [Examples]

[0027] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0028] <Experimental Example 1> [Manufacturing of lithium-ion secondary batteries] A lithium-nickel-cobalt-manganese composite oxide was mixed with acetylene black as a lithium composite oxide, styrene-butadiene rubber as a conductive additive, and SBR (styrene-butadiene rubber) as a binder to obtain a coating solution for the positive electrode composite layer.

[0029] As a positive electrode current collector, the area is 100 cm². 2 A coating solution for the positive electrode composite layer was applied to a 12 μm thick aluminum foil and dried, and then 20 mg / cm³ was added.2 After forming the positive electrode composite layer, it was rolled to obtain the positive electrode.

[0030] The negative electrode current collector and separator each have an area of ​​12 cm². 2 A 6 μm thick copper foil and a 20 μm thick porous polyethylene (PE) film were used.

[0031] Twenty-seven positive electrodes (positive electrode composite layer, positive electrode current collector) and fifty-four negative electrode current collectors were laminated with a separator in between. An aluminum positive electrode tab lead (0.8 mm thick) was welded to the aluminum foil, which is the positive electrode current collector. Furthermore, a negative electrode tab lead (0.6 mm thick) was laser-welded to the Cu foil, which serves as the negative electrode current collector, under the following conditions. Laser light: Blue laser (wavelength 450nm) Melt resolidification width: 0.2~0.4mm Laser output: 1-3kW Welding spot diameter: 0.2~0.8mm Scanning speed: 4-40 mm / sec Scanning trajectory: Linear patterns are AL: pulsed irradiation, Cu: linear (fixed point) movement due to CW and wobbling. SPOT patterns are fixed point rotations in both cases. Ar gas flow rate: 20 L / min Thermal conductivity λ of the jig: 80 w / m·k Applied waveform: CW (Continuous Wave)

[0032] Sample cells 1-8 were created with different welding patterns for the negative electrode tab lead. The welding patterns for sample cells 1-8 are as follows: Sample cell 1: A welding pattern with a single row of straight weld beads extending in the width direction of the tab lead 3, as shown in Figure 2 (Pattern name: "Single straight line of current flow"). Sample cell 2: A welding pattern with two rows of straight weld beads extending in the width direction of the tab lead 3, as shown in Figure 2 (Pattern name: "Two straight lines of current flow"). Sample cell 3: A welding pattern with three rows of straight weld beads extending in the width direction of the tab lead 3, as shown in Figure 2 (Pattern name: "Current flow straight three lines"). Sample cell 4: A welding pattern with four rows of straight weld beads extending in the width direction of the tab lead 3, as shown in Figure 2 (Pattern name: "Four straight lines of current flow"). Sample cell 5: A welding pattern (pattern name: "3-row spot") in which three linear rows of welding points extending in the width direction of the tab lead 3 are arranged, as shown in Figure 4. Sample cell 6: A welding pattern (pattern name: "4-row spot") in which four linear rows of welding points extending in the width direction of the tab lead 3 are arranged, as shown in Figure 4. Sample cell 7: A welding pattern (pattern name: "5-row staggered") in which five linear rows of welding points are arranged in the width direction of the tab lead 3, as shown in Figure 4, and the centers of the welding points in adjacent rows are not located on the same straight line extending in the length direction of the tab lead 3. Sample cell 8: A welding pattern (pattern name: "Current Flow Parallel Lines") in which five rows of welding spot lines (rows of welding points arranged in a straight line) are arranged along the length of the tab lead 3, as shown in Figure 5. Table 1 shows the characteristics of various dimensions and other features of sample cells 1 to 8.

[0033] [Assessment of resistance tendency] For the obtained sample cells 1-8, the trend in resistance values ​​was evaluated based on the length of the ridges, according to the following criteria. (The longer the ridge, the lower the resistance tends to be.) A: The length of the ridge is 400mm or more. B: Ridge length is 150mm or more and less than 400mm C: Ridge length is less than 150mm The results are shown in Table 1.

[0034] [Table 1]

[0035] <Experimental Example 2> A lithium-ion secondary battery was fabricated in the same manner as in Experimental Example 1, except that the welding pattern of the negative electrode tab lead was as shown in Figure 6. In Figure 6, five points (circles) converge to form a single petal shape; this cluster of five points is a schematic blowhole, and the region surrounded by these blowholes is the melted and resolidified area. In this analysis, the diameter of the blowholes was 0.75 mm, which is 10 times the normal size. The pitch of the blowholes was set to 3.8 mm. The current density distribution at the negative electrode tab lead weld of the obtained lithium-ion secondary battery was analyzed under the following conditions. Analysis application: J-MAG Designer Analysis conditions: Set appropriately according to the model dimensions. The current was set to 108A.

[0036] The results are shown in Figure 6. The negative electrode tab lead is 0.6 mm thick, while the copper foil is 0.8 mm thick. As a result, the current density on the copper foil side is lower than on the negative electrode tab lead side. Although the diameter of the blowhole was relatively large, it was confirmed that current flowed in up to the third row from the negative electrode tab lead.

[0037] <Experimental Example 3> A lithium-ion secondary battery was fabricated in the same manner as in Experimental Example 1, except that the welding pattern of the negative electrode tab lead was as shown in Figure 7. Note that the multiple points (circles) in Figure 7 are blowholes, and the area surrounded by these blowholes is the melted and re-solidified region. In Figure 7, there is only one row of welds (weld bead). The current density distribution at the negative electrode tab lead weld of the obtained lithium-ion secondary battery was analyzed in the same manner as in Experimental Example 2.

[0038] The results are shown in Figure 7. Because the weld re-solidification width was set to a wide 0.7 mm, there was a relatively good flow in one row, but areas with high current density were evident in places where the flow was not at the ends, indicating that a smooth current flow could not be formed. The ideal melt-re-solidification width is about 0.3 mm, in which case the effect would be even more pronounced.

[0039] <Experimental Example 4> A lithium-ion secondary battery was fabricated in the same manner as in Experimental Example 1, except that the welding pattern of the negative electrode tab lead was as shown in Figure 8. Note that the multiple points (circles) in Figure 8 are blowholes, and the area surrounded by these blowholes is the melted and re-solidified region. In Figure 8, there are three rows of welds (weld beads). The current density distribution at the negative electrode tab lead weld of the obtained lithium-ion secondary battery was analyzed in the same manner as in Experimental Example 2.

[0040] The results are shown in Figure 8. Because the weld re-solidification width is 0.7 mm, which is wider than the total thickness t of the current collector foil (the sum of the thicknesses of all current collector foils), there is relatively good current flow in the first row when viewed from the negative electrode tab lead, but areas with high current density are evident where the flow is not at the ends. Current flow was also confirmed in the second and third rows.

[0041] <Experimental Example 5> A lithium-ion secondary battery was fabricated in the same manner as in Experimental Example 1, except that the welding pattern of the negative electrode tab lead was as shown in Figure 9. The current density distribution was measured at the welded area of ​​the negative electrode tab lead of the obtained lithium-ion secondary battery in the same manner as in Experimental Example 2.

[0042] The results are shown in Figure 9. A significant bias was observed in the distribution of current density. At both ends of the linear weld (weld bead), it was observed that current was leaking out on the side opposite to the direction of current flow.

[0043] <Experimental Example 6> A lithium-ion secondary battery was fabricated in the same manner as in Experimental Example 1, except that the welding pattern of the negative electrode tab lead was as shown in Figure 10. The current density distribution at the negative electrode tab lead weld of the obtained lithium-ion secondary battery was analyzed in the same manner as in Experimental Example 2.

[0044] The results are shown in Figure 10. Similar to Experiment Example 5, a large bias in current density was observed, and furthermore, the leaked current from both ends of the linear weld (weld bead) flowed towards the tab lead in the second and third rows as viewed from the tab lead.

[0045] <Experimental Example 7> A lithium-ion secondary battery was fabricated in the same manner as in Experimental Example 1, except that the welding pattern of the negative electrode tab lead was as shown in Figure 11. The current density distribution at the negative electrode tab lead weld of the obtained lithium-ion secondary battery was analyzed in the same manner as in Experimental Example 2.

[0046] The results are shown in Figure 11. In the weld area in the direction of current flow, there is a gradual inflow of current from the periphery, and a contour gradient can be seen at both the weld area and both ends, with a smooth vector. [Explanation of Symbols]

[0047] 1. Laminated lithium-ion secondary battery 2 Current collector foil 3 Tabread 4. Welding area 5. Sealant

Claims

1. The process includes a step of laser welding the current collector foil and the tab lead at the battery terminal, The laser welding is characterized by being performed by irradiation with laser light selected from a blue laser and a green laser.

2. The laser processing method according to claim 1, wherein, when the direction in which the tab lead extends from the battery end is defined as the first direction, the laser welding is performed such that the welding locations are arranged in a plurality of linear rows perpendicular to the first direction.

3. The laser processing method according to claim 2, wherein there are two or more rows of welding locations.

4. The laser processing method according to claim 1, wherein, when the direction in which the tab lead extends from the battery end is defined as the first direction, the laser welding is performed such that the welding locations are arranged in a plurality of linear rows extending in the first direction.

5. The laser processing method according to claim 1, wherein the current collector foil is made up of multiple layers.

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