Single-step laser cutting and pre-welding of metal foil stacks

Abstract

A method of laser cutting and laser pre-welding a stack of metal foils in a single laser process. The method includes clamping the stack of metal foils together and irradiating the clamped stack with a laser beam to complete cutting through the entire stack and to form a weld joint that joins the metal foils together at the cut at the same time the stack is cut. Subsequently, the cut and pre-welded stack of metal foils can be laser welded to a metal substrate that is significantly thicker than the individual foils. The method can be applied to a stack of anode and cathode foils in an electrochemical cell, such as a lithium-ion battery.

Description

[0001] Cross-reference to related applications

[0002] This application claims the benefit of U.S. Provisional Application No. 63 / 538,553, filed September 15, 2023, the entire contents of which are incorporated herein by reference. Technical Field

[0003] This invention generally relates to the preparation of metal foil stacks for laser welding to metal substrates, particularly for use in the production of electrochemical batteries such as lithium-ion batteries. Background Technology

[0004] Vigorously promote carbon-free transportation. This work involves phasing out the existing fleet of large diesel and gasoline-powered vehicles and replacing them with electric vehicles. Efficient and reliable lithium-ion batteries, along with their cost-effective manufacturing, are crucial to the success of this effort.

[0005] A basic unit of a lithium-ion battery cell consists of an anode, a cathode, and a separator between them. An electrolyte containing lithium salt is injected into the separator. Each of the anode and cathode includes a current collector in the form of a metal foil. Typically, the anode foil is made of copper, and the cathode foil is made of aluminum. Some types of lithium-ion battery cells contain only a single basic unit, while others contain multiple basic units coupled in parallel. In applications requiring high storage capacity, multiple lithium-ion battery cells are typically coupled together in series and / or in parallel. For example, battery packs for electric vehicles contain multiple battery modules, each containing multiple lithium-ion battery cells. Especially for electric vehicles, the goal is to achieve the highest possible energy storage capacity per volume and per weight, while ensuring reliability and keeping manufacturing costs at an acceptable level.

[0006] Lithium-ion battery cells are manufactured in three different cell formats: cylindrical, prismatic, and pouch-shaped. In cylindrical cells, a single basic cell (anode, cathode, and separator) is wound and arranged in a rigid metal cylinder in a jelly roll form. The cylindrical cell is the original format for lithium-ion batteries, but the cylindrical shape hinders the efficient packaging of multiple cells in a battery module. The prismatic cell shape is better suited for applications requiring many cells and high energy density, such as electric vehicles. Pouch-shaped batteries offer further improvements in achievable energy density per volume and per weight. Prismatic cells have a rigid metal casing similar to cylindrical cells, while pouch-shaped cells have a flexible polymer-coated aluminum foil casing that is thinner and lighter than the rigid metal casings of prismatic and cylindrical cells.

[0007] Some prismatic battery cells contain a single, basic lithium-ion battery cell that is wound or folded into a flatter shape than that used in cylindrical battery cells. Other prismatic batteries and most pouch cells contain multiple basic cells stacked on top of each other and electrically coupled in parallel. These stacked battery structures include numerous layers organized in a general anode structure, separators, cathodes, anodes, separators, etc. Instead of using multiple separate separator layers, a single separator can be folded in a Z-shape between multiple anode and cathode layers. The winding process is simpler and faster than the stacking process compared to the winding of a single basic cell. However, stacked structures have several advantages, including higher energy density, faster charging and discharging, and greater flexibility in the overall shape of the battery cell.

[0008] In a tandem solar cell, the current collector foils for both the anode and cathode protrude from the sides of the multilayer structure. The current collector foils for all the cathodes form a foil stack, which is soldered to a metal contact plate, and the current collector foils for all the anodes form another foil stack, which is soldered to yet another metal contact plate. Typically, each stack contains 20-40 foils. The thickness of each individual foil is typically between approximately 5 and 30 micrometers (μm). The contact plate thickness is typically about ten times or more than the foil thickness.

[0009] The mechanical attachment and electrical connection of each foil to its corresponding tab are crucial for the integrity, reliability, and performance of batteries based on stacked structure battery designs. However, coating many thin metal foils onto a thicker metal sheet is challenging. The finished joint must be strong, durable, and have low resistance. Ultrasonic welding is the most widely used welding technique. In one prior art, ultrasonic welding is first used to weld the foils together to strengthen the structure of the foil stack. Then, in a second ultrasonic welding step, the pre-welded foil stack is welded to the tab. This second ultrasonic welding step benefits from the reinforced structure provided by the pre-welding. Summary of the Invention

[0010] Laser welding is an attractive alternative to ultrasonic welding for joining foil stacks to tabs. Compared to ultrasonic welding, laser welding offers more precise power delivery and minimizes overall heat buildup. Furthermore, the high laser intensity evaporates contaminants, unlike in ultrasonic welding where such contaminants can jeopardize the integrity of the welded structure.

[0011] In applications such as the production of lithium-ion battery cells, laser welding of metal foil stacks to metal tabs may require irradiating the edges of the foil stack with a laser beam, where the foil terminates. The laser irradiation process forms a weld joint that welds the edges of the foil stack to the tab. The foil is cut before welding the stack to the tab to form straight stack edges, with each individual foil extending to the edge and thus included in the stack-to-tab welding process. Due to the considerable thickness of the tabs, the stack-to-tab welding process requires the deposition of a relatively large amount of laser energy onto the assembly, including onto the metal foil. Potential exists for individual foils that respond to the laser energy in an undesirable manner. For example, a significant amount of material may be lost through spatter (e.g., jetting of material droplets), or the foil may curl. The risk of such problems occurring can be mitigated by pre-welding the foils together at the stack edges before initiating the stack-to-tab welding process. Unlike the stack-to-tab welding process, the pre-welding process can be specifically optimized for the characteristics of the foils, particularly their relatively small thickness of, for example, 5–30 μm. Therefore, fully and entirely laser-based techniques for joining foils and tabs can include laser cutting of the stack to form regular stack edges, laser pre-welding of the stack edges, and subsequent laser welding of the foil stack to the tab.

[0012] This paper discloses a laser-based method for cutting and pre-welding foil stacks in a single, common process step. By irradiating the foil stack with a laser beam, the laser beam cuts the stack to form regular stack edges, while simultaneously welding the ends of the foil together at the stack edges. By combining the cutting and pre-welding processes, the disclosed method facilitates joining foil stacks and tabs in only two process steps in total, instead of three or more. This method is useful in lithium-ion battery production and is more generally applicable to the cutting and edge welding of metal foil stacks.

[0013] In one aspect of the invention, a method for laser cutting and laser pre-welding of metal foil stacks includes: (a) clamping the metal foil stacks together, and (b) irradiating the clamped stacks with a laser beam to complete the cutting of the entire stack and forming a welded joint at the cut to join the metal foils together.

[0014] Brief description of the attached figures

[0015] The accompanying drawings, which are incorporated in and constitute a part of this specification, schematically illustrate preferred embodiments of the invention and, together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention.

[0016] Figure 1 A cross-sectional view illustrates a method for laser cutting and laser pre-welding metal foil stacks according to an embodiment.

[0017] Figure 2 Is Figure 1 A top view of the metal foil stack held during the method.

[0018] Figure 3 It shows Figure 1 The method involves laser irradiation of metal foil stacks.

[0019] Figure 4 It shows Figure 1 The result of the method.

[0020] Figure 5 An embodiment is shown for completing according to Figure 1 The method involves cutting and pre-welding the foil stack, followed by laser welding the metal foil stack to a metal substrate. Detailed Implementation

[0021] Referring now to the accompanying drawings, where the same parts are indicated by the same numbers. Figure 1 A method 100 is shown for laser cutting and laser pre-welding of a stack 122 of a metal foil 120 in a single step. Figure 1 A cross-sectional view of foil stack 122 is provided in an exemplary case where foil 120 is the current collector anode or cathode foil of lithium-ion battery cell 102. Method 100 is not limited to this scenario. Rather, method 100 can generally be applied to cutting and edge welding of metal foil stacks.

[0022] In battery cell 102, foils 120 are separated by material layers 110. In one example, the foils 120 are current collectors for the cathode of battery cell 102, and each layer 110 includes two separators and an anode. In this example, each foil 120 may be made of aluminum. In another example, the foils 120 are current collectors for the anode of battery cell 102, and each layer 110 includes two separators and a cathode. In this example, each foil 120 may be made of copper. Although Figure 1 Only one stack 122 of foil 120 to be soldered to substrate 130 is shown, but battery cell 102 may include two stacks 122 of foil 120, each foil being soldered to the corresponding substrate 130 using method 100. One of these stacks includes a current collector foil for the anode of battery cell 102, and the other stack includes a current collector foil for the cathode of battery cell 102. Battery cell 102 may be a prismatic cell or a pouch cell.

[0023] More generally, the material of foil 120 can be selected from copper, aluminum, nickel, lithium, sodium, calcium, magnesium, iron, zinc, and their bimetallic forms. The thickness of each foil 120 can be less than 50 micrometers (μm), for example, in the range of 1 to 50 μm or in the range of 5 to 30 μm. In one case, the number of foils 120 in the foil stack 122 is in the range of 10 to 200.

[0024] Method 100 clamps the foil stack 122 to restrict the movement of the individual foils 120. In the described embodiment, the foil stack 122 is clamped between (a) an upper clamp 130U that contacts the top surface 122T of the foil stack 122 and (b) a lower clamp 130L that contacts the bottom surface 122B of the foil stack 122. The foil stack 122 and its edges 122E protrude from the clamps 130U and 130L to allow the laser beam 190 to approach the top surface 122T. In some embodiments, other techniques for physically securing the foils 120 together may be used in addition to clamping.

[0025] Method 100 scans a laser beam 190 along the top surface 122T to form a notch 140 across the entire height of the foil stack 122 from the top surface 122T to the bottom surface 122B. The laser beam 190 can be infrared, visible, or ultraviolet. The laser beam 190 can be continuous wave, pulsed, or otherwise time-modulated. The location of the notch 140 is... Figure 1 The dashed lines are used to indicate this, and for clarity, these lines extend beyond the top surface 122T and the bottom surface 122B. Figure 1 In an alternative embodiment not shown, the lower clamp 130L extends beyond the laminate edge 122E or at least beyond the cutout 140. In this alternative embodiment, care must be taken to avoid unintentionally soldering the foil laminate 122 to the lower clamp 130L. Hereinafter, it is assumed that the foil laminate 122 and edge 122E protrude from both clamps 130U and 130L.

[0026] Figure 2 This is a top view of the foil stack 122 being held during method 100. Figure 1 and 2 It's best to watch them together. Figure 2 In the diagram, the location of the notch 140 is indicated by two opposing arrows. The path 242 on the top surface 122T coincides with the notch 140. The notch 140 and path 242 span the entire width 260W of the foil stack 122. In one example, the width 260W is between 20 and 100 mm.

[0027] like Figure 1 and Figure 2As shown in both examples, the edge 122E of the foil stack 122 is initially uneven. A notch 140, when performed by the laser beam 190, trims the foil 120 to form a straight stack edge. In the depicted example, the notch 140 is planar and orthogonal to the foil 120, and the corresponding path 242 on the top surface 122T is linear. In another example, the notch 140 is at an angle to the foil 120 and / or curved in a dimension parallel to the foil 120. Method 100 can produce the notch 140 by drawing a path 242 on the top surface 122T, for example, as... Figure 2 As indicated by arrow 280 in the diagram. Optionally, method 100 utilizes an auxiliary gas, such as nitrogen.

[0028] Figure 3 and Figure 4 This is a cross-sectional view of the clamped foil stack 122, which further illustrates various aspects of method 100 in detail. (See attached image.) Figure 3 As shown, a laser beam 190 penetrates from the top surface 122T into the foil stack 122 and eventually passes through the foil stack 122 to complete the cut 140. The foil stack 122 can be positioned at the focal point of the laser beam 190. The Rayleigh range of the laser beam 190 is typically significantly greater than the height of the foil stack 122. The laser beam 190 can trace a path 242 on the top surface 122T along the intended location of the cut 140, for example as... Figure 2 As indicated by arrow 280. In one embodiment, the laser beam 190 completes the cut 140 in a single pass along path 242. In another embodiment, the laser beam 190 makes multiple passes along path 242 before completing the cut 140, for example, between 2 and 15 passes or up to 100 passes. In this multiple-pass embodiment, one or more initial passes may only cut a portion of the height of the foil stack 122.

[0029] When the laser beam 190 cuts through the foil stack 122, the laser beam 190 also melts the end of the foil 120 at the newly formed stack edge 422E to form a weld joint 470, such as Figure 4 As shown. The weld joint 470 connects all foils 120 at the stack edge 422E. The completion of the cut 140 results in a straight newly formed stack edge 422E for the foil stack 122. The cut 140 removes a portion 424 with an uneven edge 122E. Due to the welding of the laser beam 190, the removed portion 424 can be a single piece or multiple pieces.

[0030] The number of passes required (or used) to complete the cut 140 and pre-weld the newly formed straight edge 422E may depend on several parameters, including: (a) the material, thickness, and / or quantity of foil 120; and (b) laser beam parameters, such as power, beam size, and / or the rate of movement along path 242. Laser beam parameters can be selected to ensure the formation of the weld joint 470 without compromising the integrity of foil 120 (or at least simultaneously minimizing deformation / damage to foil 120). In cases where the cut and pre-welded foil stack 122 is subsequently welded to a substrate, such as a lithium-ion battery tab, it is also preferable that the weld joint 470 does not form a substantial weld bead. Depending on its size and location, a substantial weld bead may prevent good contact between the foil stack 122 and the substrate. In one example, the thickness of the weld joint 470, orthogonal to the stack edge 422E, is less than 100 μm. In the dimension along the stack edge 422E, the maximum extent of the weld joint 470 exceeding at least one of the top surface 122T and the bottom surface 122B can be less than twice the thickness of a single foil 120.

[0031] Many applications also impose caps on the time required to complete method 100. For example, to meet the requirements of efficient lithium-ion battery production, some embodiments of method 100 are tailored to complete the cutting and pre-welding process in less than two seconds.

[0032] Successful cutting and welding results have been demonstrated with and without the aiding gas. Embodiments of method 100, utilizing multiple passes along path 242 without the aiding gas, have proven particularly advantageous for preparing foil stacks for subsequent welding to a substrate. In one example of method 100, the power of the laser beam 190 is in the range of 0.5 to 2.5 kW. The diameter of the laser beam 190 at the foil stack 122 can be between 10 and 100 μm. The travel rate of the laser beam 190 along path 242 can be in the range of 100 to 8000 mm / s.

[0033] Some embodiments of method 100 include oscillating the laser beam 190 while tracking the path 242. For example, as the laser beam 190 typically moves along the path 242, the exact incident position of the laser beam 190 on the top surface 122T is redirected to an orbit around the center point of the tracking path 242. The oscillation frequency can be up to 4 kHz, and the width of the area irradiated by the laser beam 190 along the path 242 can be up to 500 μm.

[0034] Figure 5A method 500 is shown for laser welding the foil stack 122 to a metal substrate 550 after cutting and pre-welding according to method 100. The substrate 550 is, for example, a metal tab of an electrochemical cell. The electrochemical cell may be a lithium-ion cell. However, method 500 is also applicable to electrochemical cells based on ions other than lithium, such as sodium, magnesium, calcium, aluminum, iron, and / or zinc. The substrate 550 is much thicker than a single foil 120, for example, by one or several orders of magnitude. In one example, the thickness of the substrate 550 is at least 300 μm.

[0035] From a laser-welding perspective, it is preferable that the foil 120 and the substrate 550 are made of the same material. Therefore, in one embodiment, the substrate 550 has the same material as the foil 120. For example, when the foil 120 is made of aluminum, the substrate 550 can be made of aluminum, and when the foil 120 is made of copper, the substrate 550 can be made of copper. However, for example, when the foil 120 and the substrate 550 are part of the battery cell 102, weight considerations may be more important. Therefore, the substrate 550 can be made of a different material than the foil 120. For example, even if the foil 120 is made of another material such as copper, the substrate 550 can still be made of aluminum.

[0036] In method 500, a foil stack 122 is disposed on surface 552 of substrate 550. Substrate 550, or at least surface 552, may be planar, and foil 120 is generally parallel to surface 552. The foil stack 122 may be arranged such that bottom surface 122B contacts surface 552, as... Figure 5 As shown. Alternatively, the orientation of the foil stack 122 is reversed such that the top surface 122T contacts the surface 552. In one embodiment, the foil stack 122 is clamped onto the substrate 550 using an upper clamp 530. The substrate 550 can be used as a lower clamp, or the foil stack 122 and the substrate 550 can be clamped together on the upper clamp 530 and... Figure 5 Between the lower clamps (not shown).

[0037] Laser beam 590 bonds foil stack 122 to substrate 550. Typically, laser beam 590 is primarily incident on the stack edge 422E. However, laser beam 590 may also irradiate the surface of foil stack 122 facing away from substrate 550 (top surface 122T in the depicted embodiment) near the stack edge 422E.

[0038] The invention has been described above with reference to a preferred embodiment and other embodiments. However, the invention is not limited to the embodiments described and depicted herein. Rather, the invention is limited only by the appended claims. Claims (as amended under Article 19 of the Treaty) 1. A method for laser cutting and laser pre-welding metal foil stacks, comprising the steps of: clamping the metal foil stacks together; and A laser beam is used to irradiate the clamped stack to complete the cutting of the entire stack and to form a weld joint at the cut point that binds the metal foils together. 2. The method of claim 1, wherein the welding joint joins all the metal foils of the stack together. 3. The method according to any one of claims 1-2, wherein the irradiation step comprises tracing a path on the topmost metal foil of the stack using the laser beam. 4. The method of claim 3, wherein the irradiation step cuts the stack and forms the weld joint in a single pass of the laser beam along the path. 5. The method of claim 3, wherein the irradiation step comprises performing multiple passes of the laser beam along the path. 6. The method according to any one of claims 1-5, wherein: The clamping step compresses the stack between a first clamp contacting the top surface of the stack and a second clamp contacting the bottom surface of the stack, causing a portion of the stack to protrude from the first clamp and the second clamp; and The cut passes through the portion of the stack that protrudes from the first clamp and the second clamp. 7. The method according to any one of claims 1-6, wherein the method does not use an auxiliary gas. 8. A method for bonding a metal foil stack to a metal substrate, comprising the following steps: Perform the clamping and irradiation steps according to any one of claims 1-7; and After performing the clamping and irradiation steps, the stacked layers are laser welded to the metal substrate. 9. The method of claim 8, further comprising implementing the metal foil stack and the metal substrate in an electrochemical cell. 10. The method according to claim 9, wherein the electrochemical battery is a lithium-ion battery.

Claims

1. A method for laser cutting and laser pre-welding of metal foil stacks, comprising the following steps: The metal foil layers are clamped together; as well as A laser beam is used to irradiate the clamped stack to complete the cutting of the entire stack and to form a weld joint at the cut point that binds the metal foils together.

2. The method according to claim 1, wherein, The weld joint joins all the metal foils of the stack together.

3. The method according to any one of claims 1-2, wherein, The irradiation step includes drawing a path on the topmost metal foil of the stack using the laser beam.

4. The method according to claim 3, wherein, The irradiation step cuts through the stack and forms the weld joint in a single pass of the laser beam along the path.

5. The method according to claim 3, wherein, The irradiation step includes performing multiple passes of the laser beam along the path.

6. The method according to any one of claims 1-5, wherein: The clamping step compresses the stack between a first clamp contacting the top surface of the stack and a second clamp contacting the bottom surface of the stack, causing a portion of the stack to protrude from the first clamp and the second clamp; and The cut passes through the portion of the stack that protrudes from the first clamp and the second clamp.

7. The method according to any one of claims 1-6, wherein, The irradiation step completes the cutting and forms the weld joint in less than two seconds.

8. A method for bonding a metal foil stack to a metal substrate, comprising the following steps: Perform the clamping and irradiation steps as described in any one of claims 1-7; as well as After performing the clamping and irradiation steps, the stacked laser is laser welded to the metal substrate.

9. The method of claim 8, further comprising implementing the metal foil stack and the metal substrate in an electrochemical cell.

10. The method according to claim 9, wherein, The electrochemical battery is a lithium-ion battery.

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