Laser welding of metal foil stack to metal base

Abstract

A method for laser welding a stack of metal foil to a metal substrate includes: clamping the foil stack against a support surface of the substrate; and irradiating the stack with a beam of pulsed laser light to weld the foil to the substrate. The beam is a composite beam comprising a central beam and surrounding annular beams. An initial series of laser pulses is incident on the stack at mutually different locations on its top surface, and subsequent series of laser pulses are incident on the stack at mutually different locations on its sides. The resulting weld nuggets penetrate deeply into the stack, with an average penetration depth exceeding the average spacing between the weld nuggets. The method is capable of welding more than 100 layers of foil to the substrate. The welded assembly has been shown to withstand large shear forces.

Description

[0001] Cross-references to related applications

[0002] This application claims the benefit of U.S. Non-Provisional Application No. 18 / 492,391, filed October 23, 2023, the entire contents of which are incorporated herein by reference. Technical Field

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

[0004] There is currently a strong push for carbon-free transportation. This effort involves phasing out a large number of existing diesel and gasoline-powered vehicles and replacing them with electric vehicles. Highly efficient lithium-ion battery technology is crucial to the success of this effort. Battery capacity must be matched to the rate of energy consumption to achieve a considerable driving range before recharging.

[0005] A basic unit of a lithium-ion battery cell consists of a positive electrode, a negative electrode, and a separator between them. The separator is filled with an electrolyte containing lithium salts. Each of the positive and negative electrodes includes a current collector in the form of a metal foil. Typically, the metal foil for the negative electrode is made of copper, and the metal foil for the positive electrode is made of aluminum. Some types of lithium-ion battery cells contain only a single basic unit, while others contain multiple basic units connected in parallel. In applications requiring high storage capacity, multiple lithium-ion battery cells are typically connected in series and / or 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 density and volumetric energy storage capacity, i.e., the highest possible energy density, while keeping manufacturing costs at an acceptable level.

[0006] Lithium-ion battery cells are manufactured in three different cell forms: cylindrical, prismatic, and pouch. In a cylindrical cell, a single basic unit (i.e., the positive electrode, negative electrode, and separator) is wound and arranged in a rigid metal cylinder. Cylindrical cells were the original form used for lithium-ion batteries and are still widely used, but the cylindrical shape hinders the efficient packaging of multiple battery cells within a battery module. A large portion of the total module volume remains unutilized. Prismatic shapes are better suited for applications requiring a large number of battery cells and high energy density, such as electric vehicles. Therefore, prismatic lithium-ion battery cells are currently the most widely used form in electric vehicles. Pouch cells offer further improvements in energy density achievable per unit volume and per unit weight. However, while prismatic cells have a rigid metal casing similar to cylindrical cells, pouch 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 cells contain a single basic lithium-ion battery cell that is wound or folded in a flatter shape than cylindrical cells. Other prismatic cells and most pouch cells contain multiple basic cells stacked on top of each other and electrically connected in parallel. These stacked cell structures contain many layers organized in a general configuration such as positive electrode, separator, negative electrode, separator, positive electrode, separator, etc. Instead of using multiple separate separator layers, a single separator strip can be folded in a Z-shape between multiple positive and negative electrode layers. Compared to winding a single basic cell, the winding process is simpler and faster than the stacking process. 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 stacked battery cell, all current collector foils for both positive and negative electrodes protrude from the sides of the multilayer structure. All the current collector foils for the positive electrodes form a stack of foils soldered to metal tabs, and all the current collector foils for the negative electrodes form another stack of foils soldered to another metal tab. Each stack typically contains 20-40 layers of foil. The thickness of each individual foil is typically between approximately 5 micrometers (µm) and 20 micrometers (µm). The thickness of the tabs 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 cell designs. However, bonding many thin metal foils to thicker metal tabs is challenging. The finished joint must be strong, durable, and have low resistance. Precision resistance welding is used, but it relies on interfacial resistance, and the high thermal conductivity of the metals being welded means that high currents must be applied. Ultrasonic welding is the most widely used technique, but contaminants between the foils in the welding area are a problem. If not removed, these contaminants can weaken the weld joint. Laser welding has emerged as an attractive alternative, providing precise energy delivery to minimize overall heat buildup, while the high laser intensity vaporizes contaminants. Summary of the Invention

[0010] The lithium-ion battery industry is pushing towards manufacturing battery cells with more layers in their stacked structures. This push is part of an ongoing effort to increase the energy density of lithium-ion battery packs for electric vehicles. If the number of layers in a single battery cell can be increased, the number of battery cells can be reduced accordingly while maintaining the same total storage capacity. This increases the energy density of the battery pack for at least two reasons: the ratio of the storage capacity of a single battery cell to the packaging material increases; and fewer electrical connections (typically in the form of buses) are needed to electrically connect the battery cells within the pack, further increasing the pack's energy density. However, increasing the number of layers in a battery cell necessarily corresponds to an increase in the number of current collector foils that need to be stacked and welded to the corresponding metal tabs. Welding stacks of more than about 40 foil layers to tabs using conventional techniques has proven difficult. For example, in ultrasonic welding, the energy required to weld higher foil stacks can damage the welding equipment. Some of the most advanced laser welding methods are capable of welding up to about 60 foil layers to tabs.

[0011] This paper discloses a method for laser welding metal foil stacks to metal substrates using deep-penetration welding. This method reliably welds relatively large quantities of foil to the substrate. For stacks containing more than 100 foil layers, this method has demonstrated successful foil stack-to-substrate welding, and the welded assembly has been shown to withstand shear forces greater than 700 Newtons without foil breakage. This method represents an enabling technology for increasing the energy density of stacked battery cells in the electric vehicle industry.

[0012] This method leverages the unique ability of a laser beam to rapidly deliver a large amount of highly localized energy to the material while minimizing undesirable nonlocalized heating. Unlike melting a large, continuous volume of foil stacks, this method is tailored to melt deeply penetrating localized volumes to form relatively separate deep-penetration weld nuggets (although some overlap near the surface is acceptable). These deep-penetration weld nuggets anchor the foil stack to the substrate in a nail-like manner. This method locally and rapidly deposits a large amount of energy to form these deep-penetration weld nuggets. This approach minimizes undesirable damage to the foil, such as thinning of individual foils near the weld area, which could be caused by more sustained and less localized laser irradiation. Therefore, deep-penetration welding anchors the foil stack to the substrate with high strength while maintaining the integrity of the weld area and the individual foils nearby.

[0013] Deep penetration welding is achieved by scanning a pulsed laser beam through a stack of metal foils to deliver a series of laser pulses. Each laser pulse produces a corresponding narrow deep penetration weld. The laser beam is a composite beam, consisting of a narrow central beam surrounded by a larger annular beam. The deep penetration weld is primarily formed by the central beam, while the annular beam provides slightly less localized heating, thus assisting the welding process performed by the central beam.

[0014] In one aspect of the invention, a method for laser welding a stack of metal foil to a metal substrate includes the steps of: clamping the stack of metal foil against a support surface of the metal substrate, and irradiating the stack of metal foil with a beam of laser pulses to weld the stack of metal foil to the metal substrate. The beam is a composite beam comprising a central beam and an annular beam surrounding the central beam. For each laser pulse, the peak power of the central beam is at least 0.5 kW. For each laser pulse, the duration of the central beam being on is between 20 microseconds and 500 microseconds. The step of irradiating the stack includes: scanning the composite beam such that (a) an initial series of laser pulses are incident on the stack at corresponding series of mutually different locations on the top surface of the topmost metal foil of the stack, away from the support surface, and (b) subsequent series of laser pulses are incident on the stack at corresponding series of mutually different locations on a side surface of the stack, located between the support surface and the top surface. The irradiation step further includes focusing the composite beam such that the maximum lateral 1 / e² dimension of the central beam at the stack is less than 150 µm.

[0015] In another aspect of the invention, a battery includes: a metal substrate having a supporting surface; and a stack of metal foil disposed on the supporting surface. The stack of metal foil is welded to the metal substrate by a plurality of weld nuggets extending from the surface of the stack into the stack. The weld nuggets include deep-penetrating weld nuggets. At least some of the deep-penetrating weld nuggets further extend into the metal substrate. The deep-penetrating weld nuggets are oriented at an angle relative to the supporting surface, and the average penetration depth, measured from the surface of the stack of metal foil, exceeds the average spacing between the nearest adjacent deep-penetrating weld nuggets. Attached Figure Description

[0016] 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 preferred embodiments given below, serve to explain the principles of the invention.

[0017] Figure 1 An example of a method for laser welding a stack of metal foil to a metal substrate is illustrated according to one embodiment. Figure 1 A cross-sectional view of the foil stack and the substrate is provided, in an exemplary scenario the foil is the current collector foil of a lithium-ion battery cell.

[0018] Figure 2 It shows Figure 1 An exemplary cross-sectional profile of the composite laser beam used in the method.

[0019] Figure 3 Instructions Figure 1 An exemplary incident location on the foil stack is shown in the method where a pulse of the composite laser beam is incident on the composite laser beam.

[0020] Figure 4 A more detailed illustration is provided in one implementation method. Figure 1 The method involves a laser irradiation process in which a composite laser beam is incident on a foil stack at an angle relative to the supporting surface of the substrate. Figure 4 The deep-penetrating weld nugget formed as a result is also illustrated.

[0021] Figure 5 It shows Figure 1 An exemplary time-power curve of a single laser pulse of the composite laser beam of the method.

[0022] Figure 6A and Figure 6B An example of a method for in one embodiment is illustrated. Figure 1 The method involves scanning a composite laser beam.

[0023] Figure 7 Examples of using Figure 6A and Figure 6B An exemplary pattern of the incident position illuminated on a foil stack by a scanning technique.

[0024] Figure 8 It is based on Figure 1 Method (implementing the scanning technique shown in Figure 6) Figure 7 Microscopic image of a cross-section of an exemplary foil stack-substrate assembly (with pattern) welded together.

[0025] Figure 9 Examples Figure 1 A modification to the method, wherein, according to one embodiment, the foils are offset from one another such that the illuminated sides of the foil stack are tilted. Detailed Implementation

[0026] Referring now to the accompanying drawings, the same parts are designated by the same numbers. Figure 1 An example of a method 100 for laser welding a stack 122 of metal foil 120 to a metal substrate 130 is illustrated. Figure 1 A cross-sectional view of foil stack 122 and substrate 130 is provided, in which foil 120 is a current collector foil for lithium-ion battery cell 102 in an exemplary scenario. Method 100 is not limited to this scenario. Rather, method 100 is generally applicable to soldering metal foil stacks to metal substrates.

[0027] In battery cell 102, foils 120 are separated by material layers 110. In one example, foils 120 are current collectors for the positive electrode of battery cell 102, and each layer 110 includes two separators and a negative electrode. In this example, each foil 120 may be made of aluminum. In another example, foils 120 are current collectors for the negative electrode of battery cell 102, and each layer 110 includes two separators and a positive electrode. In this example, each foil 120 may be made of copper. Although Figure 1 Only one foil 120 stack 122 to be soldered to substrate 130 is shown, but battery cell 102 may include two foil 120 stacks 122, each foil stack being soldered to the corresponding substrate 130 using method 100. One of these stacks includes a current collector foil for the negative electrode of battery cell 102, and the other stack includes a current collector foil for the positive electrode of battery cell 102.

[0028] From a laser welding perspective, it is generally preferred that foil 120 and substrate 130 be made of the same material. Therefore, in one embodiment, substrate 130 is made of the same material as foil 120. For example, when foil 120 is made of aluminum, substrate 130 is made of aluminum, and when foil 120 is made of copper, substrate 130 is made of copper. However, when foil 120 and substrate 130 are part of battery cell 102, weight considerations may be more important. In this scenario, substrate 130 may be made of a different material than foil 120. For example, even if foil 120 is made of another material (e.g., copper), substrate 130 may be made of aluminum.

[0029] The thickness of each foil 120 can be less than 20 µm, for example, in the range of 5 µm to 20 µm. The number of foils 120 in the foil stack 122 can exceed 40, or 60, or even 100. The foil stack 122 can have a height 122H of 250 µm or greater, for example, in the range of 250 µm to 1000 µm. The substrate 130 is significantly thicker than a single foil 120. The substrate 130 can be one or several orders of magnitude thicker than a single foil 120. In one example, the thickness of the substrate 130 is at least 250 µm.

[0030] Method 100 clamps the foil stack 122 against the support surface 130S of the substrate 130 and irradiates the foil stack 122 with a laser beam 190. The support surface 130S may be planar, wherein the foil 120 is substantially parallel to the support surface 130S. Figure 1 In the depicted embodiment, the foil stack 122 is clamped between the upper clamp 140 and the base 130. The base 130 can be used as a lower clamp, or the foil stack 122 and the base 130 can be clamped between the upper clamp 140 and the lower clamp 150. The clamps 140 and 150 are tooling fixtures that can be removed after method 100 is completed.

[0031] When the foil stack 122 is clamped against the substrate 130, the top surface of the topmost foil 120 of the foil stack 122 forms the top surface 122T of the foil stack 122 facing away from the support surface 130S. The foil stack 122 has side surfaces 122S where the foil 120 terminates. The side surfaces 122S can be formed by cutting through the foil stack 122 before clamping it against the substrate 130. When the side surfaces 122S are formed by cutting the foil stack 122, the side surfaces 122S are generally orthogonal to the support surface 130S. In the following, unless otherwise stated, it is assumed that the side surfaces 122S are orthogonal to the support surface 130S at least within 10 degrees. That is, unless otherwise stated, the angle 122A between the side surfaces 122S and the support surface 130S is between 80 degrees and 100 degrees. The clamp 140 retracts a non-zero distance 142 from the side 122S, making the top surface 122T near a portion of the side 122S accessible to the laser beam 190. The non-zero distance 142 can be between 1 mm and 3 mm to provide a path for the laser beam 190 to the top surface 122T while securing the protruding end of the foil 120. In the depicted embodiment, the substrate 130 extends beyond the side 122S, such that the end 130E of the substrate 130 is at a distance 132 from the side 122S. The distance 132 can be several millimeters or greater. Alternatively, the end 130E can be aligned with the side 122S, corresponding to a distance of zero at 132. Although the foil 120 is clamped between the clamp 140 and the substrate 130, gaps may exist between at least some of the foils in the foil 120 (particularly outside the area covered by the clamp 140 on the foil stack 122). Such gaps may be caused by unevenness of the foil 120.

[0032] When the foil stack 122 is clamped against the support surface 130S, method 100 irradiates the foil stack 122 with a laser beam 190. Depending on the materials of the foil 120 and the substrate 130, the wavelength of the laser beam 190 can be in the near-infrared or visible spectrum. In one scenario, the foil 120 is made of copper or aluminum, and the laser beam 190 is near-infrared. For example, the wavelength of the laser beam 190 can be in the range between 900 nanometers (nm) and 1200 nanometers (nm), or in the range between 1030 nm and 1085 nm when using a ytterbium-doped laser gain medium. In another scenario, a laser beam 190 with a visible wavelength is used to weld the copper foil stack 122.

[0033] Figure 2 The cross-sectional profile of laser beam 190 is shown. Laser beam 190 is a composite laser beam comprising a central beam 210C and a surrounding annular beam 210A. Laser beam 190 can be delivered by an optical fiber having a central core and a surrounding annular core. The depicted cross-sectional profile and the following discussion relate to the location of the focal point of laser beam 190 or within the Rayleigh range of the focal point of laser beam 190. Figure 2In the depicted embodiment, the central beam 210C and the annular beam 210A are circular. The following discussion assumes a circular beam, but can be easily extended to an elliptical beam.

[0034] The central beam 210C has a diameter 220C of 1 / e². The annular beam 210A has an outer diameter 222A of 1 / e² and an inner diameter 224A of 1 / e². The inner diameter 224A of the annular beam 210A exceeds the diameter 220C of the central beam 210C. The combined intensity distribution of the central beam 210C and the annular beam 210A reaches a minimum along a circle 230 located outside the diameter 220C of the central beam 210C and within the inner diameter 224A of the annular beam 210A. In one example, the diameter 220C is less than 150 µm or less than 100 µm, for example, in the range between 15 µm and 50 µm, and the outer diameter 222A is in the range of four to ten times the diameter 220C.

[0035] In most embodiments of method 100, the laser beam 190 is focused at the foil stack 122, i.e., within or on the surface of the foil stack 122. The Rayleigh range is typically much greater than the height 122H. Thus, the cross-sectional profile of the laser beam 190 presents a distinct central beam and annular beam throughout the foil stack 122.

[0036] The laser beam 190 is pulsed. The duration of a single pulse of the laser beam 190 can be a fraction of a millisecond. The laser beam 190 can be generated by on / off modulation of the output of a continuous wave (cw) laser source, such as a fiber laser. The center beam 210C and the ring beam 210A can be obtained from the same laser source or from different corresponding laser sources. In one embodiment, the respective powers of the center beam 210C and the ring beam 210A are controlled independently of each other, and the two beams can be turned on and off at slightly different times to optimize the laser welding process.

[0037] Method 100 scans a laser beam 190 along the surface of the foil stack 122. (The central beam 210C and the ring beam 210A are scanned together.) Performing the scan causes the laser beam 190 to deliver a series of laser pulses to corresponding series of mutually different locations on the surface of the foil stack 122.

[0038] Figure 3 This is a perspective view of the foil stack 122 and the substrate 130 (and the fixture), schematically indicating exemplary incident positions 392 where pulses of the laser beam 190 are incident on the foil stack 122. Each incident position is... Figure 3 The center is indicated by a circle. For clarity, only one incident position is marked. Each incident position 392 is the position where the center beam 210C is incident on the foil stack 122.

[0039] Now combine Figure 1 and Figure 3 Referring to method 100, the laser beam 190 is scanned such that (a) an initial series of laser pulses are incident on the top surface 122T at corresponding series of mutually different incident positions 392, and subsequent series of laser pulses are incident on the side surface 122S at another corresponding series of mutually different incident positions 392. The incident positions 392 can be distributed along the full width 322W of the foil stack 122. In one example, the width 322W ranges from 25 mm to 60 mm. In one embodiment, the laser beam 190 is substantially stationary during each laser pulse, wherein scanning between incident positions 392 is performed between laser pulses. In another embodiment, wherein the pulse duration is negligible compared to the scanning time between incident positions 392, the laser beam 190 scans continuously without pausing at each incident position 392.

[0040] Method 100 focuses the laser beam 190 to an intensity such that, for each laser pulse, the central beam 210C penetrates deeply into the foil stack 122, and for at least some of the laser pulses, also penetrates into the substrate 130. The optimal density of incident positions 392 is a trade-off between (a) securing the foil stack 122 to the substrate 130 with the desired intensity and (b) maintaining the integrity of the foil 120. The foil 120 may be damaged by continuous heating in the same area and / or by excessive overlap between weld nuggets formed at adjacent incident positions 392. Preferably, the incident positions 392 are spaced apart by a distance exceeding the maximum lateral 1 / e² dimension of the central beam 210C at the foil stack 122. This corresponds to the central beam 210C having a 1 / e² width in any lateral dimension that is smaller than the spacing between the incident positions 392. In one example, the incident positions 392 are spaced at least 100 µm apart. When the central beam 210 is circular, the maximum lateral dimension is 1 / e² of the diameter of the central beam 210. For example, if the central beam 210 is elliptical rather than circular, the maximum lateral 1 / e² dimension is the width of the central beam 210 along the major axis of the elliptical cross-sectional profile, which is 1 / e². The spacing between the incident positions 392 is the same as the center-to-center distance between corresponding pairs of pulses of the laser beam 190 on the foil stack 122, which is measured as orthogonal to the propagation direction of the laser beam 190. More preferably, the distance between the incident positions 392 is at least twice the maximum lateral 1 / e² dimension of the central beam 210C. This corresponds to the central beam 210C having a 1 / e² width less than half the spacing between the incident positions 392 in any lateral dimension.

[0041] Optionally, in method 100, a protective gas (not depicted) is introduced over the foil stack-substrate assembly during laser beam irradiation 190. The protective gas may include nitrogen, an inert gas, or clean, dry air.

[0042] Figure 4 The laser irradiation process of method 100 in one embodiment is illustrated in more detail by a cross-sectional view, wherein a laser beam 190 is incident on a foil stack 122 at an angle 492 relative to a support surface 130S. The angle 492 allows for irradiation of both the top surface 122T and the side surface 122S while maintaining a fixed spatial relationship between the foil stack-substrate assembly and the laser processing head that directs the laser beam 190 toward the foil stack 122. Deflection control of the laser beam 190 to approach different incident positions 392 on the foil stack 122 can be performed by one or more movable optics of the laser processing head. The angle 492 can be in the range of 30 degrees and 60 degrees. Figure 4 The diagram shows a laser beam 190 incident at five different incident positions 392. Controlling the deflection of the laser beam 190 may result in a different tilt angle 492 for each incident position 392. In most scenarios, such differences are negligible.

[0043] The laser beam 190 can be focused on or within the foil stack 122 to achieve the intensity required for deep-penetration welding with the central beam 210C. In one embodiment, the laser beam 190 is focused at a focal plane 494 coinciding with the foil stack 122. In this embodiment, the focusing of the laser beam 190 does not need to be adjusted when deflected to different incident positions 392. The focal plane 494 can be slightly curved. Depending on the incident position 392, the focal plane 494 can be at different depths from the surface where the laser beam 190 enters the foil stack 122. The Rayleigh length of the laser beam 190 is typically significantly greater than the height 122H of the foil stack 122. Therefore, the welding process is generally not adversely affected by differences in focusing characteristics between different incident positions 392. In another embodiment, the focusing of the laser beam 190 is adjusted so that it is at the same or similar depth from the surface where the laser beam 190 enters the foil stack 122 at each incident position 392. For example, the laser beam 190 can be focused at each incident position 392. Alternatively, during irradiation at each incident position 392, the focal point of the laser beam 190 can be maintained near the support surface 130S.

[0044] Figure 4 A deep penetration weld nugget 480 formed by method 100 is further illustrated. Figure 4 The cross section was selected to coincide with the five deep-penetrating weld nuggets 480. Figure 4The depiction is slightly idealized, showing a very regular weld nugget 480, a flat top surface 122T and side surface 122S, and a 90-degree angle between the top surface 122T and side surface 122S. In practice, welded foil stack-substrate assemblies may exhibit irregularities and variations in the shape of the weld nugget 480, flatness deviations in the top surface 122T and side surface 122S, and rounding of the angle between the top surface 122T and side surface 122S.

[0045] Each weld nugget 480 is generated when a corresponding pulse of the laser beam 190 is incident on the foil stack 122 at a corresponding incident position 392. Each weld nugget 480 extends into the foil stack 122 along a longitudinal axis that at least approximately corresponds to the propagation direction of the laser beam 190. Each weld nugget 480 has a penetration depth 482 measured from the surface of the foil stack 122 after welding. The penetration depth 482 is measured along the propagation direction of the laser beam 190 or substantially equivalently along the longitudinal axis of the resulting weld nugget 480. The weld nugget 480 is characterized by a relatively large penetration depth 482, for example, greater than 0.25 mm or greater than 0.5 mm. In one embodiment, the average penetration depth 482 of the weld nugget 480 exceeds the average spacing 484 between the nearest adjacent weld nuggets 480. The average penetration depth 482 may be at least twice the average spacing 484 between the nearest adjacent weld nuggets 480. The spacing 484 is measured orthogonally to the propagation direction of the laser beam 190 or orthogonally to the longitudinal axis of the weld nugget 480. The spacing 484 is at least approximately equal to the distance between the corresponding incident positions 392, which is measured orthogonally to the propagation direction of the laser beam 190.

[0046] Some weld nuggets 480 may be more than Figure 4 The penetration depicted is shallower into the foil stack-substrate assembly. For example, while weld nuggets 480 preferably penetrate into the substrate 130, some weld nuggets 480 may instead terminate within the foil stack 122. In one example, most, or at least 75%, of the weld nuggets 480 penetrate into the substrate 130. In another example, most weld nuggets 480 are confined to the foil stack 122, and in most cases, only weld nuggets 480 incident on the lower portion of the side 122S relatively close to the substrate 130 penetrate into the substrate 130. Ultimately, the purpose of the weld nuggets 480 is to bond the foil 120 and secure the foil stack 122 to the substrate 130 with sufficient strength to withstand the mechanical forces experienced by the foil stack-substrate assembly in the intended application. For example, in lithium-ion batteries for electric vehicles, the foil stack-substrate assembly may need to withstand some shear or tensile force applied to the foil stack 122 relative to the substrate 130. The magnitude of the relevant force depends on the application.

[0047] Figure 5An exemplary time-power profile of a single laser pulse of a composite laser beam 190 in one embodiment of method 100 is shown, wherein the annular beam 210A has a longer on-time than the central beam 210C during each laser pulse. For each pulse of the laser beam 190, the central beam 210C is characterized by time profile 520C, and the annular beam 210A is characterized by time profile 520A. The central beam 210C has a pulse duration Δ. C Furthermore, the annular beam 210A has a value exceeding Δ C Pulse duration Δ A .

[0048] More specifically, for each pulse of laser beam 190, the annular beam 210A is turned on before the central beam 210C, and the central beam 210C is turned off before the annular beam 210A. Therefore, during the initial duration δ1 within each laser pulse, the foil stack 122 is irradiated only by the annular beam 210A. This initial exposure to the annular beam 210A preheats the corresponding local area of ​​the foil stack 122 in a relatively gentle manner, thus preparing the foil stack 122 for the more intense irradiation by the central beam 210C. During the irradiation by the central beam 210C, the annular beam 210A remains on. After the central beam 210C is turned off, the annular beam 210A is further kept on for a duration δ2. The extended irradiation of the foil stack 122 by the annular beam 210A after exposure to the central beam 210C helps control the cooling of the metal that was molten during exposure to the central beam 210C. For example, this extended irradiation by the annular beam 210A can prevent the formation of cracks in the corresponding weld nugget 480.

[0049] exist Figure 5 In the depicted example, each of the central beam 210C and the annular beam 210A has only two states: on and off. The transition between the on and off states is instantaneous, or relative to a duration Δ. C Δ A δ1 and δ2 are at least negligible. For example, the transition time can be less than the duration Δ. C Δ A 10% of each of δ1 and δ2. The power levels of the center beam 210C and the ring beam 210A in their on state are P C and P A In the example depicted, P A More than P C However, due to its smaller size, the central beam 210C is more intense than the annular beam 210A.

[0050] Power P A and P Cand duration Δ C Δ A The optimal values ​​of δ1 and δ2 are interrelated and also depend on other parameters, such as the dimensions and materials of the foil stack 122 and the substrate 130, as well as the lateral dimensions of the center beam 210C and the ring beam 210A. In one example, the power P C At least 0.5 kilowatts (kW), for example, in the range between 0.5 kW and 5 kW. Power P A They can be within the same range. Figure 5 In the example depicted, power P A More than P C This relationship has been found to be advantageous in certain scenarios. Duration Δ C It can be within the range of 20 microseconds (µs) to 500 microseconds (µs). Duration Δ A It can exceed Δ C 5% to 50%, and the duration δ1 can exceed the duration δ2. In another example, the maximum lateral 1 / e² dimension of the center beam 210C at foil stack 122 is at most 50 µm, and the power P C At least 1 kW, and duration Δ C The repetition frequency is between 50 and 200 microseconds. In most scenarios, the pulse repetition frequency of the laser beam 190 is limited by the time required to move the laser beam 190 from one incident position 392 to the next. The pulse repetition frequency can be on the order of kilohertz.

[0051] Although not depicted, one or both of the center beam 210C and the ring beam 210A can be ramped up to open and / or ramped down to close, rather than instantaneously on and off. Furthermore, the power of one or both of the center beam 210C and the ring beam 210A can be varied during their respective on states.

[0052] Furthermore, method 100 can adjust the characteristics of the laser beam 190 during scanning. Such adjustment may include changing the power, pulse duration, and / or size of one or both of the central beam 210C and the annular beam 210A. In one embodiment, method 100 adjusts the peak power of the central beam 210C and optionally the annular beam 210A based on the distance from the incident position 392 to the substrate 130 along the propagation direction of the laser beam 190. For example, refer to... Figure 4As illustrated in the schematic diagram, to ensure that all weld nuggets 480 penetrate into the substrate 130, it may be advantageous to adjust the peak power of the central beam 210C according to the propagation distance from the incident position 392 to the substrate 130. Therefore, the peak power of the central beam 210C (and optionally the annular beam 210A) may be highest for the incident position on the top surface 122T, and lowest for the incident position 392 on the side surface 122S closest to the substrate 130.

[0053] Figure 6A and Figure 6B An example of a technique 600 for scanning laser beam 190 in method 100 is illustrated. Figure 6A It is a perspective view showing the path followed by the laser beam 190 in the scanning technology 600. Figure 6B This is a cross-sectional view indicating certain geometric features. In scanning technique 600, the laser beam 190 is controlled to sequentially trace multiple linear paths on the surface of the foil stack 122. Each of these paths is substantially parallel (e.g., within 10 degrees) to the interface corner 672 between the side 122S and the substrate 130 (see...). Figure 6B The laser beam 190 first traces at least one path 610 on the top surface 122T. Next, the laser beam 190 traces multiple paths 612 on the side surface 122S. In the depicted example, the laser beam 190 traces four paths 612 (1-4) on the side surface 122S. More generally, the laser beam 190 traces one or more paths 610 on the top surface 122T and one or more paths 612 on the side surface 122S.

[0054] Regardless of the number of paths traced on each of the top surface 122T and the side surface 122S, the laser beam 190 is controlled to trace these paths in a specific order. In the example of tracing two or more paths 610 on the top surface 122T, the laser beam 190 first traces the edge 670 between the top surface 122T and the side surface 122S (see [reference]). Figure 6B The laser beam 190 first follows the path 610 furthest from the edge 670. Then the laser beam 190 continues to follow the path 610 second furthest from the edge 670, and so on. Similarly, for the path 612 on the side 122S, the laser beam 190 first follows the path 612 furthest from the interface corner 672 (path 612(1) in the depicted example), and then moves towards the interface corner 672 one path 612 at a time (paths 612(2), 612(3), and 612(4) in the listed order in the depicted example). In summary, each path 610 / 612 followed by the laser beam 190 is closer to the interface corner 672 than any of the preceding paths.

[0055] like Figure 6AAs depicted, the laser beam 190 can trace each path 610 / 612 in the same direction. This maximizes the cooling time between irradiating adjacent portions of adjacent paths. Alternatively, the laser beam 190 can trace at least one of the paths in opposite directions. For example, the laser beam 190 can alternate its tracing direction between paths to achieve optimal scanning speed.

[0056] Scanning technique 600 can be extended to irradiate nonlinear paths. In one extension, scanning technique 600 processes a set of regions on the surface of foil stack 122 with laser beam 190. Laser beam 190 first irradiates the incident position 392 in the region furthest from the interface corner 672 (see...). Figure 3 And then perform the same operation in the area that gradually gets closer to the corner of the interface 672.

[0057] The incident positions 392 illuminated by the scanning technique 600 can be uniformly or non-uniformly distributed. It has been found advantageous to utilize incident positions 392 with a higher density near the interface corner 672 than elsewhere. For example, in the illustrated embodiment of the scanning technique 600, it is advantageous to utilize incident positions 392 with a higher density in path 612 (4) than in paths 610 and 612 (1-3).

[0058] Figure 7 An exemplary pattern 700 is shown at the incident position 392 that can be illuminated using scanning technology 600. Figure 7 Best combination Figure 6A and Figure 6B Let's examine them together. Pattern 700 depicts and discusses an embodiment of scanning technology 600, which follows a single path 610 on the top surface 122T and four paths 612 (1-4) on the side surface 122S, as shown below. Figure 6A The pattern 700 is readily generalizable to different numbers of paths on one or both of the top surface 122T and the side surface 122S. Figure 7 The incident position 392 is shown projected onto a common plane orthogonal to the laser beam 190. Figure 6B The example public plane 696 is indicated in the diagram. Each path 610 / 612 can be compared to... Figure 7 The one depicted is longer, so as to at least almost span the full width of the foil stack 122, 322W (see [reference]). Figure 3 ).

[0059] In pattern 700, each of paths 610 and 612 (1-4) includes a row of equally spaced incident positions 392. Except for the path closest to the interface corner 672 (i.e., path 612 (4)), the distance 770 between adjacent incident positions 392 is the same for all paths. The distance 772 between adjacent incident positions 392 in path 612 (4) is only half the distance 770. To prevent continuous heating of the same local area, it is advantageous to illuminate the denser distribution of incident positions 392 along path 612 (4) in two or more staggered scan passes, rather than in a single scan pass. For example, the first scan pass along path 612 (4) can illuminate every other incident position 392. The remaining incident positions 392 can then be illuminated in the second scan pass along path 612 (4). In other words, the path 612 (4) can be repeated to illuminate a higher density of incident positions 392.

[0060] exist Figure 7 In the depicted embodiment, paths 610 and 612 (1-3) are arranged such that the associated incident positions 392 form a hexagonal grid (see hexagon 750). Other grid configurations are possible, such as square or rectangular grids, or even irregular grids. In addition to the increased density of incident positions 392 along path 612 (4), the hexagonal grid corresponds to a uniform distribution of incident positions 392. A uniform distribution is generally expected to provide an optimal trade-off between securing the foil stack 122 to the substrate 130 with the desired strength and maintaining the integrity of the foil 120. Reference Figure 1 and Figure 3 The discussion of the preferred distance between the incident positions 392 also applies to scanning technology 600 and pattern 700.

[0061] Figure 8 This is a microscopic image of a cross-section of a foil stack-substrate assembly welded according to method 100 (scanning technique 600 implementing a total of ten tracing paths and pattern 700). In this example, the foil stack 122 consists of 64 layers of aluminum foil, each with a thickness of 13 µm, and the substrate 130 is an aluminum substrate with a thickness of 800 µm. The foil stack-substrate assembly was sheared to expose individual deep-penetrating weld nuggets 880. The shearing process caused some damage to the welded assembly. However, the image shows four weld nuggets 880 (1-4). Line 810 indicates the interface between the foil stack 122 and the substrate 130. At least some of the weld nuggets 880 are deviated from the plane of the imaging cross-section, and therefore... Figure 8 Its full penetration depth is not shown. However, it is clear that at least the weld nugget 880(1) penetrates into the substrate 130.

[0062] In the region closest to the outer surface 828 of the post-weld foil stack 122, the weld nuggets 880 merge and form a larger, continuous weld. However, the discrete weld nuggets 880 extend further into the material. At least for the weld nuggets 880 (1-3), the longitudinal axes 886 of the weld nuggets 880 are identifiable from the images. In the depicted example, the average penetration depth of the weld nuggets 880 exceeds the spacing between them (i.e., the spacing between the longitudinal axes 886). Figure 8 The spacing values ​​extracted from the cross-section may not represent the spacing between adjacent weld nuggets. The relevant nearest neighbor weld nugget may be another weld nugget outside the plane of the depicted cross-section. However, the spacing between the nearest neighbor weld nuggets is no greater than [missing value]. Figure 8 The visible spacing. Assuming... Figure 8 The weld nugget visible in the cross-section is similar to other weld nuggets outside the cross-section, therefore it can be seen from... Figure 8 The conclusion is that the average penetration depth of the weld nugget exceeds the average spacing between the nearest adjacent weld nuggets.

[0063] and Figure 8 Similar foil stack-based components have been shown to withstand shear forces exceeding 700 Newtons relative to the substrate 130 applied to the foil stack 122.

[0064] Figure 9 A modification of method 100 is illustrated, wherein the foils 120 are offset from each other such that the side surfaces 122S are tilted away from the end 130E. In other words, at the interface corner 672, the side surfaces 122S form an obtuse angle 922 with the support surface 130S. The tilted side surfaces 122S allow the laser beam 190 to reach both the top surface 122T and the side surfaces 122S while propagating orthogonally to the support surface 130S. Orthogonal propagation of the laser beam 190 relative to the support surface 130S may be preferred in some scenarios. More generally, the tilted side surfaces 122S can allow a wider range of propagation angles 992 of the laser beam 190 relative to the support surface 130S. Angle 992 can be an acute angle, a normal angle, or an obtuse angle.

[0065] The present invention has been described above with reference to preferred embodiments 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

1. A method for laser bonding a stack of metal foil to a metal substrate, the method comprising the following steps: The stack of metal foils is clamped against the support surface of the metal substrate; as well as The metal foil stack is irradiated with a laser pulse beam to weld the metal foil stack to the metal substrate. The laser pulse beam is a composite beam comprising a central beam and a ring beam surrounding the central beam. For each laser pulse, the peak power of the central beam is at least 0.5 kilowatts. For each laser pulse, the duration of the central beam being on is between 20 microseconds and 500 microseconds. The irradiation process includes: The composite beam is scanned such that (a) an initial series of laser pulses are incident on the stack at corresponding, mutually different positions on the top surface of the topmost metal foil of the stack, away from the support surface; and (b) subsequent series of laser pulses are incident on the stack at corresponding, mutually different positions on the side surface of the stack, located between the support surface and the top surface. The composite beam is focused such that the maximum lateral 1 / e² dimension of the central beam at the stack is less than 150 micrometers.

2. The method of claim 1, wherein each of at least some of the laser pulses forms a weld nugget that penetrates into the metal substrate.

3. The method according to any one of claims 1 to 2, the method further comprising arranging the metal foil such that the side of the stack is orthogonal to the support surface within 10 degrees, and wherein the irradiation step comprises guiding the composite beam onto the stack at an angle of inclination relative to the support surface.

4. The method of claim 3, wherein the tilt angle is between 30 degrees and 60 degrees.

5. The method according to any one of claims 1 to 4, wherein the support surface extends beyond the side of the stack.

6. The method according to any one of claims 1 to 5, wherein for each pair of laser pulses, the corresponding incident position of the central beam on the stack is characterized by a center distance of at least 100 micrometers as measured orthogonal to the incident direction of the composite beam.

7. The method according to any one of claims 1 to 6, wherein for each pair of laser pulses, the corresponding incident position of the central beam on the stack is characterized by a center distance as measured orthogonal to the incident angle of the composite beam, the center distance exceeding the maximum lateral 1 / e² dimension of the central beam on the stack.

8. The method according to any one of claims 1 to 7, wherein the irradiation step comprises sequentially tracing a plurality of paths parallel to the interface corner between the side surface of the stack and the support surface with the composite beam, each path being closer to the interface corner than each preceding path, at least one path being traced during irradiation by the initial series of laser pulses, and at least another path being traced during irradiation by the subsequent series of laser pulses.

9. The method according to claim 8, wherein: The path includes the final path closest to the interface; and The method further includes, after the sequential tracing step, repeating the tracing of the final path using a different set of incident positions of the central beam on the stack than those used during the sequential tracing step.

10. The method of claim 9, wherein the incident position of the laser pulse delivered along the final path during the repeated tracking step is alternately arranged with the incident position of the laser pulse delivered along the final path during the sequential tracking step.

11. The method according to any one of claims 1 to 10, wherein the maximum lateral 1 / e² dimension of the central beam at the metal foil stack is at most 50 micrometers, the peak power of the central beam is at least 1 kilowatt, and the duration of the central beam for each laser pulse in the laser pulse is between 50 microseconds and 200 microseconds.

12. The method according to any one of claims 1 to 11, further comprising, for each of the laser pulses, activating the annular beam before the central beam.

13. The method of claim 12, further comprising, for each of the laser pulses, turning off the central beam before the annular beam.

14. The method according to any one of claims 1 to 13, wherein the stack comprises at least 100 layers of metal foil.

15. The method of claim 14, wherein each layer of metal foil has a thickness of up to 20 micrometers.

16. A battery, the battery comprising: A metal substrate having a supporting surface; and A metal foil stack is disposed on the support surface and welded to the metal substrate by a plurality of weld nuggets extending from the surface of the metal foil stack into the metal foil stack, the weld nuggets including deep penetration weld nuggets, at least some of the deep penetration weld nuggets further extending into the metal substrate, the deep penetration weld nuggets being oriented at an angle relative to the support surface, and the average penetration depth measured from the surface of the metal foil stack exceeding the average spacing between the nearest adjacent deep penetration weld nuggets.

17. The battery of claim 16, wherein the average penetration depth is at least twice the average spacing.

18. The battery according to any one of claims 16 to 17, wherein the average penetration depth exceeds 0.25 mm.

19. The battery according to any one of claims 16 to 18, wherein the tilt angle is between 30 degrees and 60 degrees.

20. The battery according to any one of claims 16 to 19, wherein each metal foil is an aluminum foil, and the metal substrate is an aluminum substrate.

21. The battery according to any one of claims 16 to 19, wherein each metal foil is a copper foil, and the metal substrate is a copper substrate.

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