Oscillating laser scanning welding method for welding multi-layer aluminum foil to an electrode body of a battery cell

Abstract

The present invention relates to an oscillating laser scanning welding method for welding a multilayer aluminum foil, in particular a positive electrode current collector of a lithium battery, to an electrode body of an electric cell, wherein the laser scanning welding method is capable of directly welding the multilayer aluminum foil to the electrode body of the electric cell, the laser scanning welding method comprising at least the following steps: oscillating scanning the laser beam over the area of the aluminum foil to be welded for welding the multilayer aluminum foil to the electrode body of the electric cell, wherein the power of the laser beam is modulated in a wave-like oscillation in the area of the inflection points of the trajectory of the oscillating scanning. The electrode body of the electric cell is in particular a columnar electrode body of a positive electrode current collector. The present invention also relates to a device for carrying out the above-mentioned laser scanning welding method.

Description

Technical Field

[0001] This invention relates to an oscillating laser scanning welding method for welding multilayer aluminum foil to battery cell electrodes. Background Technology

[0002] In lithium batteries, aluminum foil is typically used as the positive electrode current collector (or cathode current collector), and copper foil is used as the negative electrode current collector (or anode current collector). A very fine and highly permeable insulating material, such as an insulating membrane, is used between the positive and negative electrodes for insulation. The aluminum foil is often pre-welded using ultrasonic welding, serving a pre-pressing function. Due to the fragility of the aluminum foil in the positive electrode current collector, the connection between the aluminum foil and the battery cell electrode is usually indirect. This involves using an adapter plate for indirect connection between the aluminum foil and the battery cell electrode. First, ultrasonic welding is used to connect the aluminum foil to the adapter plate, and then laser welding is used to weld the adapter plate to the battery cell electrode.

[0003] With the continuous development of the battery industry, lightweighting of battery cells is becoming increasingly important. Therefore, omitting the adapter plate and directly welding multiple layers of aluminum foil to the cell electrode has become a trend in the industry. Furthermore, eliminating the adapter plate reduces the number of solder joints, which lowers resistance and consequently reduces heat generation during supercharging. However, the electrode is typically a columnar structure. Directly stacking multiple layers of aluminum foil onto a columnar electrode makes it impossible for ultrasonic welding heads to hold the components, meaning ultrasonic welding cannot be used; only non-contact laser welding can be employed.

[0004] Because the thickness of aluminum foil is typically only 8-13 micrometers, traditional laser welding is prone to cracking in the molten pool, especially at the interface edges of the molten zone. This is mainly because the surface of the aluminum foil is often covered with an oxide layer (alumina), which has a melting point and hardness much higher than pure aluminum. During welding, some of this oxide layer does not have enough time to completely melt and accumulates at the weld edge, significantly increasing the hardness and leading to cracking. Additionally, due to the thickness of the aluminum foil, the foil near the heat-affected zone of the molten pool is prone to relatively large deformation under high temperatures, resulting in tensile stress, which also increases the risk of cracking in the molten pool.

[0005] Furthermore, traditional laser welding employs unidirectional feed, with heat conducted perpendicular to the feed direction. This results in significant uneven heating and cooling at the weld pool edges, leading to substantial welding stress. In traditional laser welding, the laser beam energy is distributed in a near-Gaussian pattern, meaning the energy at the center of the beam is much higher than at the edges. This causes a solid-liquid state at the weld edges, resulting in the accumulation of a hard oxide film and increasing the likelihood of cracking. These cracks not only significantly reduce the weld's current carrying capacity but also drastically decrease its strength. Summary of the Invention

[0006] To address the aforementioned problems, this invention proposes a novel laser welding method that can effectively control the welding process and reduce the generation of cracks in the molten pool when welding multilayer aluminum foil, even when the aluminum foil is only a very thin layer and covered with a high-hardness oxide layer, ultimately achieving the effect of improving weld strength.

[0007] The oscillating laser scanning welding method according to the present invention is used to weld multilayer aluminum foil to a battery cell electrode, wherein the multilayer aluminum foil is particularly used as the positive current collector or cathode current collector of a battery. The aluminum foil particularly has a thickness of 8-13 micrometers, and the number of layers in the multilayer aluminum foil is particularly 20-130 layers. The laser scanning welding method enables direct welding of the multilayer aluminum foil to the battery cell electrode, and the laser scanning welding method includes at least the following steps:

[0008] The laser beam is oscillated and scanned over the area of ​​the aluminum foil to be welded to perform welding, thereby welding the multilayer aluminum foil to the battery cell electrode.

[0009] Specifically, within the region of the inflection point of the oscillating scan trajectory, the power of the laser beam is modulated in a waveform oscillation manner.

[0010] According to the method of the invention, preferably, a laser beam is incident on the aluminum foil substantially perpendicular to the plane of the aluminum foil, and the spot of the laser beam oscillates and scans within the weld seam, for example, in a direction substantially perpendicular to the welding feed direction, within the plane of the aluminum foil. "Substantially perpendicular" means that the angle between the laser beam and the plane of the aluminum foil is around 90°, for example, between 80° and 100°, particularly between 82° and 98°, advantageously between 85° and 95°, and especially advantageously between 88° and 92°.

[0011] According to the laser scanning welding method of the present invention, in the region of the trajectory inflection point, the power of the laser beam can preferably be modulated in the form of a sine wave, a triangular wave, or a trapezoidal wave. By modulating the power of the laser beam in a waveform oscillation manner, the energy received in the region of the trajectory inflection point can be reduced, thereby optimizing the abrupt energy change at the weld edge, avoiding excessive heating and cooling unevenness at the weld edge, and thus reducing the generation of cracks.

[0012] Naturally, besides sine, triangular, or trapezoidal waves, those skilled in the art can also conceive of modulating the power of the laser beam with other waveforms, as long as it achieves the goal of reducing the received energy in the region of trajectory inflection points. It should be understood here that the energy received in the region of trajectory inflection points is not necessarily as low as possible, but rather that a certain balance is achieved with the high heat in the weld center region within this region to avoid sudden energy changes.

[0013] According to a preferred embodiment of the present invention, in the region of the trajectory inflection point, the power of the laser beam undergoes at least one cycle of the waveform oscillation. Preferably, the power of the laser beam can also undergo multiple cycles of the waveform oscillation in the region of the trajectory inflection point.

[0014] According to a preferred embodiment of the invention, the power of the laser beam is modulated with the same or different waveforms within a region of multiple successive trajectory inflection points. It should be understood that the same waveform means a waveform consistent in frequency and amplitude, while different waveforms mean waveforms different in frequency and / or amplitude.

[0015] According to a preferred embodiment of the present invention, in the welding feed direction, the amplitude of the waveform oscillation is smaller in the region of the subsequent trajectory inflection point compared to the region of the preceding trajectory inflection point, resulting in less energy received at the subsequent trajectory inflection point than at the preceding trajectory inflection point. This preferred embodiment is particularly advantageous because the successive trajectory inflection points have a small gap. When the laser beam scans to the subsequent inflection point, the energy received at the preceding inflection point may not have had time to fully diffuse. Using the same waveform to modulate the laser beam power at both inflection points may lead to significant heat accumulation. Therefore, reducing the energy received in the region of the subsequent trajectory inflection point relative to the energy received in the region of the preceding trajectory inflection point effectively avoids excessive heating and cooling unevenness.

[0016] In traditional laser welding, Gaussian beams are typically used. Within the cross-section of the laser beam (i.e., the spot incident on the welding area), with the point of maximum energy density as the origin (usually the center of the spot), the radius of the cross-section as the horizontal axis, and the magnitude of the energy density as the vertical axis, the energy density distribution follows a Gaussian function, hence the name Gaussian beam. The energy at the center of a Gaussian beam is much higher than at the edges; that is, the energy drops too rapidly at the edges of the laser beam. This leads to a solid-liquid state at the weld edge, causing the accumulation of a harder oxide film, thus increasing the likelihood of crack formation.

[0017] To address the aforementioned drawbacks of traditional laser welding, according to a preferred embodiment of the present invention, the energy density distribution within the cross-section of the laser beam is modulated into a Gaussian-flat-top mode before the laser beam exits the optical fiber. Particularly preferably, the Gaussian-flat-top mode laser beam can be generated using the applicant's "BrightLine Welding" technology, employing a "two-in-one" optical fiber. The beam source simultaneously guides the laser into both the inner core fiber and the ring fiber, with the two laser beams overlapping in the processing area. This allows for adjustment of the energy density within the cross-section of the laser beam, for example, adjusting the energy density of the center and outer ring (e.g., a circular ring) within the cross-section of the adjustable laser beam. The "Gaussian-flat-top mode" optimizes the energy density at the center and edges of the laser beam, optimizes the heat input structure, reduces cracks caused by welding stress to a certain extent, and can reduce welding spatter.

[0018] It should be particularly understood here that the modulation of the energy density distribution within the cross-section of the laser beam does not involve the aforementioned modulation of the overall output power of the laser beam in a waveform oscillation manner, but rather the adjustment of the energy distribution within the cross-section of the laser beam or the spot, so that the laser beam cross-section has a Gaussian-flat-top light mode energy density distribution throughout the entire welding process.

[0019] According to a preferred embodiment of the invention, the power of the Gaussian-flat-top mode laser beam has a Gaussian distribution in the central region and a more gradually changing annular region, for example, having a substantially uniform power distribution or a substantially uniform energy density distribution within the annular region.

[0020] According to a preferred embodiment of the invention, the linear velocity of the laser beam oscillating and scanning on the aluminum foil is approximately 300 mm / s, and the welding feed rate is, for example, 30 mm / s. Naturally, other linear velocities that are meaningful to those skilled in the art can also be used without departing from the framework of the invention.

[0021] According to the laser scanning welding method of the present invention, multilayer aluminum foil can be directly welded to, in particular, cylindrical battery cell electrodes. For cylindrical battery cell electrodes (i.e., battery cell posts), where multilayer aluminum foil is directly stacked on top of the posts, the ultrasonic welding head cannot hold the parts, meaning ultrasonic welding cannot be used. Therefore, non-contact laser welding is particularly advantageous. However, this is not always the case; for other configurations of battery cell electrodes, the laser scanning welding method according to the present invention can also be used to directly weld multilayer aluminum foil to the electrode with particularly good results.

[0022] The present invention also relates to a laser scanning welding apparatus, wherein the laser scanning welding apparatus includes a laser welding beam generating system configured to implement any of the embodiments of the laser scanning welding method described above.

[0023] This specification provides the method steps described above, but may include more or fewer steps based on conventional or non-inventive labor. The order of steps listed in the embodiments is merely one possible order of execution among many steps and does not represent the only possible order. Therefore, the specific order of steps set forth in the specification should not be construed as a limitation on the claims. Furthermore, the claims concerning the method should not be limited to performing the steps in the written order; those skilled in the art will readily understand that these orders can be varied while still remaining within the spirit and scope of this application. Attached Figure Description

[0024] The following diagram schematically illustrates the oscillating laser scanning welding method according to the invention for the positive electrode current collector of lithium batteries, particularly for the direct connection of multilayer aluminum foil to the cell electrode. It shows:

[0025] Figure 1 A partial schematic diagram of aluminum foil and battery cell electrode during laser welding;

[0026] Figure 2 A schematic diagram of the trajectory of the laser beam within the weld seam.

[0027] Figure 3 The power of the laser beam varies in a sinusoidal pattern within the region of the inflection point of the oscillating scanning trajectory. Detailed Implementation

[0028] It should be understood that the following embodiments of the present invention are exemplary and not limiting, serving to illustrate typical implementations of the oscillating laser scanning welding method according to the present invention. More precisely, in addition to the embodiments described below, numerous variations that are meaningful to those skilled in the art can be derived by combining various features of the present invention.

[0029] In this document, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified. In the accompanying drawings, the size of each component, the thickness of a layer, or the area is sometimes exaggerated for clarity. Therefore, the dimensions and shapes in the drawings do not reflect true scale.

[0030] Figure 1 A partial schematic diagram of aluminum foil and battery cell electrode 120 in laser welding is shown as an example. Here, the positive current collector 100 is composed of multiple layers of aluminum foil 110. The positive current collector 100 and the corresponding battery cell electrode 120 are stacked on top of each other. Figure 1As shown, during laser welding, a high-energy laser beam L irradiates the areas to be welded of the positive current collector 100 and the cell electrode 120, causing the multilayer aluminum foil 110 in the positive current collector 100 and the material in the cell electrode 120 to melt into a molten pool M. Here, the aluminum foil in the heat-affected zone near the molten pool M deforms due to heat, and tensile stress F is applied to the molten pool M, especially the material at the edge of the molten pool, particularly during the solidification process after welding.

[0031] In an exemplary embodiment, the laser beam in oscillating laser scanning welding is modulated with a sinusoidal power pattern. However, this is not mandatory, and those skilled in the art can conceive of modulating the laser beam power with other waveforms, such as triangular waves, trapezoidal waves, etc., as long as it achieves the goal of reducing the received energy in the region of trajectory inflection points.

[0032] like Figure 2 As shown, in an exemplary embodiment, the welding feed is performed at a speed v along the direction of the arrow. The laser beam is preferably incident on the aluminum foil substantially perpendicular to the plane of the aluminum foil (the plane shown in the figure in this embodiment).

[0033] like Figure 2 As shown, the laser beam oscillates and scans within the weld seam width 240 along path 210. The direction of movement of the laser beam is substantially perpendicular to, for example, the direction of welding feed, i.e., substantially perpendicular to... Figure 2 The direction indicated by the middle arrow. Thus, the laser beam moves back and forth along the weld edge, i.e., on both sides of the weld width 240, forming strip-shaped modulation power regions 230 near the laser beam trajectory inflection points 221, 222, 223 (three trajectory inflection points are shown exemplarily, but not exclusively), i.e., at the two edges of the weld. Each modulation power region 230 has a plurality of trajectory inflection points 221, 222, 223 successively along the welding feed direction. Within the regions of the laser beam trajectory inflection points 221, 222, 223, the power of the laser beam is modulated in a waveform oscillating manner overall. In this embodiment, the laser beam power is modulated in a sinusoidal mode.

[0034] In an exemplary embodiment, when the laser beam approaches trajectory inflection points 221, 222, and 223, the power of the laser beam is adjusted according to... Figure 3 The sine wave shown is modulated. Figure 3 The diagram illustrates a sinusoidal wave period, where the dashed horizontal axis represents time, and P1 to P9 represent different power values, with greater distance from the horizontal axis indicating higher power. P1 to P9 show the change in laser beam power over time throughout a sinusoidal wave period.

[0035] According to this embodiment, the power of the laser beam is sinusoidally modulated within the modulation power region 230. This reduces the energy received within the modulation power region 230, optimizes the abrupt energy changes on both sides of the weld width 240, avoids excessive heating and cooling unevenness at the weld edge, and thus reduces the generation of cracks.

[0036] In an exemplary embodiment, within the region of each trajectory inflection point 221, 222, 223, the power of the laser beam undergoes the following... Figure 3 The power of the laser beam must experience at least one cycle of a sine wave, meaning the power of the laser beam must experience the power values ​​represented by at least nine points from P1 to P9. Preferably, the power of the laser beam can also experience the power values ​​represented by nine points from P1 to P9 within the region of each trajectory inflection point 221, 222, 223. Figure 3 The sine wave shown has multiple periods.

[0037] In an exemplary embodiment, within the region of multiple successive trajectory inflection points, the power of the laser beam is modulated with the same waveform, i.e. Figure 3 The sine wave does not change in frequency or amplitude.

[0038] However, it should be noted that within the framework of this invention, those skilled in the art can also conceive of embodiments different from this one, such as setting different sine waves for each different trajectory inflection point 221, 222, 223. For example, the laser beam power in the region of the later trajectory inflection point 22 along the welding feed direction has a smaller amplitude of waveform oscillation compared to the laser beam power in the region of the earlier trajectory inflection point 221, resulting in less energy received at the later trajectory inflection point 222 than at the earlier trajectory inflection point 221. This effectively avoids excessive heating and cooling unevenness within the modulation power region 230, i.e., at the weld edge.

[0039] In a preferred embodiment of the invention, the energy density distribution within the cross-section of the laser beam can be modulated into a Gaussian-flat-top mode before the laser beam exits the optical fiber. Particularly preferably, the Gaussian-flat-top mode laser beam can be generated using the applicant's "BrightLine Welding" technology, employing a "two-in-one" optical fiber. The beam source simultaneously guides the laser into both the inner core fiber and the ring fiber, with the two laser beams overlapping in the processing area. This allows for adjustment of the energy density within the cross-section of the laser beam, for example, adjusting the energy density of the center and outer ring (e.g., a circular ring) within the cross-section of the laser beam.

[0040] It should be particularly understood here that modulating the energy density distribution within the cross-section of the laser beam does not involve changing the overall output power of the laser beam, but rather adjusting the energy distribution within the cross-section or the spot of the laser beam, so that the laser beam has a Gaussian-flat-top light mode energy density distribution throughout the entire welding process.

[0041] According to the laser scanning welding method of this embodiment, multilayer aluminum foil can be directly welded to, in particular, cylindrical battery cell electrodes. For cylindrical battery cell electrodes, non-contact laser welding is therefore particularly advantageous. However, this is not always the case; for other battery cell electrode configurations, the laser scanning welding method according to this embodiment can also effectively weld multilayer aluminum foil directly to the electrode.

Claims

1. An oscillating laser scanning welding method for welding multilayer aluminum foil to battery cell electrodes, wherein, The laser scanning welding method can directly weld multilayer aluminum foil to the battery cell electrode, and the laser scanning welding method includes at least the following steps: The laser beam is oscillating and scanning over the area to be welded on the aluminum foil to weld the multilayer aluminum foil to the battery cell electrode, wherein the direction of movement of the laser beam is substantially perpendicular to the welding feed direction; Specifically, within the region of the inflection point of the oscillating scan trajectory, the power of the laser beam is modulated in a waveform oscillation manner, thereby reducing the energy received within the trajectory inflection point region. In the welding feed direction, the laser beam power in the region of the later trajectory inflection point is smaller than that in the region of the earlier trajectory inflection point, resulting in a smaller amplitude of waveform oscillation. This means that the energy received at the later trajectory inflection point is less than that received at the earlier trajectory inflection point.

2. The laser scanning welding method according to claim 1, wherein, Within the region of the trajectory inflection point, the power of the laser beam is modulated in the form of a sine wave, a triangular wave, or a trapezoidal wave.

3. The laser scanning welding method according to claim 1 or 2, wherein, Within the region of the trajectory inflection point, the power of the laser beam undergoes at least one cycle of the waveform oscillation.

4. The laser scanning welding method according to claim 1 or 2, wherein, Within a region of multiple successive trajectory inflection points, the power of the laser beam is modulated with different waveforms.

5. The laser scanning welding method according to claim 3, wherein, Within a region of multiple successive trajectory inflection points, the power of the laser beam is modulated with different waveforms.

6. The laser scanning welding method according to any one of claims 1-2 and 5, wherein, Before the laser beam exits the optical fiber, the energy density distribution within the cross-section of the laser beam is modulated into a Gaussian-flat-top light mode.

7. The laser scanning welding method according to claim 3, wherein, Before the laser beam exits the optical fiber, the energy density distribution within the cross-section of the laser beam is modulated into a Gaussian-flat-top light mode.

8. The laser scanning welding method according to claim 4, wherein, Before the laser beam exits the optical fiber, the energy density distribution within the cross-section of the laser beam is modulated into a Gaussian-flat-top light mode.

9. The laser scanning welding method according to claim 6, wherein, The power of the laser beam cross-section in the Gaussian-flat-top mode has a central region with a Gaussian distribution and a ring region with a more gradual change relative to the central region.

10. The laser scanning welding method according to any one of claims 1-2, 5, 7-9, wherein, The linear velocity of the oscillating scan is 300 mm / s.

11. The laser scanning welding method according to any one of claims 1-2, 5, 7-9, wherein, The laser scanning welding method is used to directly weld multilayer aluminum foil to columnar battery cell electrodes.

12. The laser scanning welding method according to any one of claims 1-2, 5, 7-9, wherein, The multilayer aluminum foil is the positive electrode current collector of the lithium battery.

13. A laser scanning welding apparatus, wherein, The laser scanning welding apparatus includes a laser welding beam generation system configured to perform the method according to any one of claims 1-12.

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