Improved welding method for laser welding of battery foil stacks, welding method, laser processing system, related computer program product

Abstract

The present invention relates to an improved method for improving a weld seam formed by welding a multi-layer aluminum foil stack of a battery current collector and a battery component, which includes at least a remelting step in which the weld seam edge of the weld seam provided on the surface of the multi-layer aluminum foil stack is remelted at least once by a laser beam with a lower heat input than the heat input for forming the weld seam. The present invention further relates to a corresponding welding method, a laser processing system, a control device and a computer program product. The advantages of the present invention are: by remelting the molten edge of the initial weld seam with a lower power, the internal stresses in the molten edge area of ​​the newly formed weld seam during the solidification process can be reduced, thereby reducing cracks in the molten edge area; by repeated remelting, the length of the crack can be reduced, and the crack becomes discontinuous due to the successive reduction of the welding energy.

Description

Technical Field

[0001] The present invention relates to an improved method for improving a welding seam, a welding method for welding a multilayer aluminum foil stack of a current collector of a battery and a battery component, a laser processing system, a control device for the laser processing system, and a computer program product. The present invention relates particularly to the fields of lithium-ion batteries and laser welding.

[0002] Background Art Lithium-ion batteries are widely used in various fields because they are superior in various aspects compared to other types of batteries.

[0003] In a lithium-ion battery, the positive electrode includes a current collecting electrode composed of lithium cobaltate (or lithium nickel cobalt manganate, lithium manganate, lithium iron phosphate, etc.) and aluminum foil, and the negative electrode includes a current collecting electrode composed of a graphitized carbon material and copper foil. The aluminum foil is also used as a current collector of the positive electrode. A multilayer aluminum foil stack is used in a lithium-ion battery to obtain a larger aluminum foil surface that can be coated with more active material. To manufacture a battery, the multilayer aluminum foil stack needs to be effectively connected to the electrode. Since the aluminum foil is extremely thin, ultrasonic welding is usually suitable for connection. Usually, since the shape of the electrode is irregular, a sheet-shaped connector is usually added between the aluminum foil stack and the electrode. In this case, first, the aluminum foil stack and the connector are connected by ultrasonic welding, and then a connector with less likelihood of welding cracks is connected to the electrode by laser welding.

[0004] However, for the weight reduction and ultra-rapid charging of battery cells, eliminating the connector is the goal. In this case, it is required to directly weld the multi-layer aluminum foil stack to the electrode. However, the thin aluminum foil is extremely prone to cracking, especially at the melting interface edge during welding. This is mainly due to: the surface of the aluminum foil is often covered with an Al2O3 oxide layer, and the melting point and hardness of this oxide layer are significantly higher than those of the aluminum foil as the base material. As a result, the oxide layer part cannot be completely melted during the welding process, gathers at the welding seam edge, which significantly increases the hardness of the welding seam edge and is prone to cracking; due to the thin thickness of the aluminum foil, the aluminum foil near the heat-affected zone of the molten pool is extremely prone to significant deformation under the action of high temperature, and the tensile stress generated by this process also increases the risk of molten pool cracking; the temperature unevenness and material deformation caused by the laser energy input during welding also increase the risk of cracking.

[0005] Furthermore, since the welding seam is elongated in the feeding direction, the molten pool of the welding seam is U-shaped when viewed in a cross-section perpendicular to the feeding direction, that is, the edge contour of the molten pool is extremely steep. As a result, the aluminum foil is significantly deformed at the edge of the molten pool, and continuous cracking is likely to occur at the molten edge of the molten pool due to the resulting tensile stress.

[0006] Summary of the Disclosure The object of the present invention is to provide an improved method for improving the welding seam that can reduce cracking when welding a multi-layer aluminum foil stack and can provide a high-strength and high-quality welding seam.

[0007] According to a first aspect of the present invention, there is provided an improved method for improving a weld seam formed by welding a multilayer aluminum foil stack of a current collector of a battery and a component of the battery, the method comprising at least a remelting step of remelting at least once, by a laser beam, a weld seam edge of the weld seam provided on the surface of the multilayer aluminum foil stack with an input heat lower than the input heat for forming the weld seam.

[0008] In this context, the "component" should be understood as any component of the battery that is welded together with the multi-layer aluminum foil stack in particular. The "weld seam" should be understood as any form of weld seam formed by welding the multi-layer aluminum foil stack to the component by a laser beam, in particular the first weld seam formed in the conventional manner. The weld seam can be formed by a linear trajectory, a curved trajectory or a similar trajectory of the laser beam on the multi-layer aluminum foil stack. The "weld seam edge of the weld seam provided on the surface of the multi-layer aluminum foil stack" should be understood as the contour line of the weld seam provided on the surface of the multi-layer aluminum foil stack, or the boundary line between the weld seam and the aluminum foil as the base material of this weld seam or the weld termination of the weld seam. The weld seam edge defines the width of the weld seam in particular. The "width of the weld seam" should be understood as the width of the weld seam in the lateral direction with respect to the feed direction of the laser beam in particular. After each remelting, a new weld seam edge is formed on the surface of the multi-layer aluminum foil stack in particular. In this specification, the term "weld seam edge" always refers to the boundary line between the weld seam and the aluminum foil as the base material, rather than the internal line of the weld seam generated based on remelting. The feature of "remelting the weld seam edge of the weld seam provided on the surface of the multi-layer aluminum foil stack" encompasses, in particular, remelting at least a part of the two weld seam edges of the weld seam, in particular completely remelting the two weld seam edges. Naturally, when the weld seam edge is remelted, the molten pool formed by the laser beam is, for example, wider than the weld seam edge, whereby the area near the weld seam edge is also melted simultaneously. Furthermore, naturally, after remelting, the melted area solidifies, and thus a new weld seam edge is formed.

[0009] According to an optional embodiment of the present invention, in the remelting step, the welding seam edges of the welding seam provided on the surface of the multilayer aluminum foil stack are remelted multiple times by a laser beam. The heat input used for the first remelting is lower than the heat input used to form the welding seam. The heat input used for each remelting after the first remelting is reduced compared to the previous remelting. In each remelting, the welding seam edge formed by the previous remelting is remelted. The expression "multiple times" is particularly understood as "at least two times". Each remelting is particularly related to the two welding seam edges of the welding seam provided on the surface of the multilayer aluminum foil stack.

[0010] According to an optional embodiment of the present invention, in the remelting step, the laser beam is moved relative to the multilayer aluminum foil stack parallel to the welding seam edge along or in the vicinity of the welding seam edge. Obviously, it is not essential that the focus of the laser beam must be along the welding seam edge, but it is sufficient if the welding seam edge is inside the melting pool of the laser beam.

[0011] According to an optional embodiment of the present invention, in the remelting step, in order to achieve a lower heat input, the laser output of the laser beam is reduced compared to the laser output for forming the welding seam, and / or the moving speed of the laser beam relative to the multilayer aluminum foil stack is increased compared to the moving speed for forming the welding seam.

[0012] According to an optional embodiment of the present invention, the welding seam edge formed by each remelting is offset in a direction away from the welding seam starting from the welding seam edge formed by the previous remelting.

[0013] According to an optional embodiment of the present invention, the lengths of the welded seam edges formed by individual remelting are equal to each other. Here, "equal" should be understood to encompass, in particular, "absolutely equal" and "substantially equal", and "substantially equal" means, in particular, including a deviation within ±10%, particularly within ±5%.

[0014] According to an optional embodiment of the present invention, the depth of the molten pool in each remelting is decreased as the number of remelting increases and is smaller than the thickness of the multilayer aluminum foil stack.

[0015] According to an optional embodiment of the present invention, the total number of remelting is selected so that the welded seam satisfies the requirements regarding cracking.

[0016] According to an optional embodiment of the present invention, the battery is a lithium-ion battery.

[0017] According to an optional embodiment of the present invention, the component is the positive electrode of the battery.

[0018] According to a second aspect of the present invention, there is provided a welding method for welding a multilayer aluminum foil stack of a current collector of a battery and a component of the battery, the method including: a first welding step of welding the multilayer aluminum foil stack and the component with a laser beam to form a welded seam; and an improvement step of performing the above-described improvement method on the welded seam.

[0019] According to an optional embodiment of the present invention, in the first welding step, the welded seam is formed on the surface of the multilayer aluminum foil stack by a linear or curved trajectory of the laser beam.

[0020] According to an optional embodiment of the present invention, the welding method is performed by a scanning optical system or a fixed welding head. The fixed welding head particularly refers to a welding head in which the laser beam does not move relative to the welding head.

[0021] According to an optional embodiment of the present invention, prior to the welding method, the aluminum foils in the multilayer aluminum foil stack were pre-welded to each other by ultrasonic waves.

[0022] According to a third aspect of the present invention, there is provided a laser processing system comprising at least a laser device for generating a laser beam and a control device for controlling at least the laser device, the laser processing system being configured to perform the improved method or the welding method described above.

[0023] According to a fourth aspect of the present invention, there is provided a control device for a laser processing system, the control device being configured to perform the improved method or the welding method described above.

[0024] According to a fifth aspect of the present invention, there is provided a computer program product comprising computer program instructions which, when executed by a processor, perform the improved method or the welding method described above.

[0025] The positive effects of the present invention are: by remelting the melting edge of the first weld seam at a low power, the internal stress in the melting edge region of the newly formed weld seam during the solidification process can be reduced, thereby reducing cracks in the melting edge region; by repeating the remelting, the crack length can be reduced and the cracks become discontinuous due to the continuous decrease in welding energy.

[0026] The principle, features and advantages of the present invention can be better understood by explaining the present invention in more detail below with reference to the accompanying drawings.

Brief Description of the Drawings

[0027]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

[0028] Detailed description of the preferred embodiment In order to more clearly clarify the technical problems to be solved, the technical solutions, and the beneficial technical effects according to the present invention, the present invention will be described in more detail below with reference to the accompanying drawings and a plurality of exemplary embodiments. Of course, the specific embodiments described in this specification are not intended to limit the protection scope of the present invention, but are only used to explain the present invention.

[0029] First, for the purpose of clarity and conciseness of the drawings, it should be pointed out that only one reference numeral is assigned to only one welding seam edge, only one crack, or only one welding seam edge formed by remelting of the welding seam and the like.

[0030] FIG. 1 schematically shows an example of a battery in a partial cross-sectional view. In this specification, the battery is a lithium-ion battery as an example. However, it is obvious that the idea of the present invention is not limited to lithium-ion batteries only, and may be applied to other types of batteries, such as sodium-ion batteries. Furthermore, the idea of the present invention is not limited to batteries with a rectangular shell, and can also be applied to pouch-type batteries, cylindrical batteries, and other forms of batteries with similar welding seams. The cells of the lithium-ion battery are formed, for example, by a multilayer stack structure. In this multilayer stack structure, separators are provided between an aluminum foil and a copper foil, respectively. Further substances required for manufacturing the battery are interposed between these layers. This is well known to those skilled in the art and will not be described in detail. Such an aluminum foil layer protrudes at one end of the cell and is pre-welded by ultrasonic waves to form a current collector of the positive electrode. The multilayer aluminum foil stack 10 of the current collector of the positive electrode includes, for example, 20 to 130 layers of aluminum foil layers. The positive electrode of the battery also includes, for example, a positive electrode, a positive tab, and the like. The current collector of the positive electrode generally needs to be connected to the positive electrode, and in some cases, it also needs to be connected to the positive tab or another component of the battery. Such a connection is usually achieved by welding. For example, the multilayer aluminum foil stack 10 and a component 20 below the lowermost aluminum foil layer of the multilayer aluminum foil stack 10, such as a positive electrode, need to be welded by a laser beam 430 to form a welding seam 30.

[0031] FIG. 2 schematically shows an example of a laser processing system 40 in a perspective view. For example, the laser processing system 40 is used for welding and / or improving the weld seam 30 of FIG. 1. For example, the laser processing system 40 includes a laser device 410 for generating a laser beam 430 and a control device 420 for controlling at least the laser device 410. The laser processing system 40 may further include a support base (schematically shown as a plane in FIG. 2) for supporting the object to be welded (i.e., the multilayer aluminum foil stack 10 and the component 20) and / or a fixture for holding the object to be welded and the like. The support base and / or the fixture may be fixed or movable. The control device 420 may control the movement of the support base and / or the fixture as necessary. The laser device 410 may include, for example, a scanning optical system or a fixed welding head.

[0032] When the multilayer aluminum foil stack 10 of the current collector of the battery shown in FIG. 1 is welded, the welding seam 30 shown in FIG. 3 is typically used. FIG. 3 schematically shows this welding seam 30 in a plan view. The length-to-width ratio is only for the purpose of clarity in FIG. 3, and in reality, the width may be significantly shorter than the length. The length of the welding seam 30 is, for example, 20 mm to 60 mm, and the width is, for example, 2 mm to 6 mm. The welding seam 30 is, for example, elongated in FIG. 3. To form such a welding seam 30, the laser beam 430 generally travels along a relatively linear path relative to the multilayer aluminum foil stack 10. However, in this specification, it is also possible to use another path depending on the shape and size of the weldable area. For example, the laser beam 430 may perform a vibratory movement with a narrow amplitude in a direction transverse to the feed direction in addition to the linear feed movement in the feed direction. The feed direction of the laser beam 430 is indicated by the arrow and the symbol V in FIG. 3. FIG. 3 shows the area 340 (i.e., the area near the first welding seam edge 300) where cracks are likely to occur in the welding seam 30 with hatched diagonal lines. Although the welding seam 30 has two first welding seam edges 300, only one of them is labeled with a reference numeral for the purpose of clarity and conciseness of the drawing. FIG. 4 schematically shows a cross-sectional view of the molten pool of the welding seam 30 shown in FIG. 3. FIG. 4 also shows the area 340 where cracks are likely to occur with hatched diagonal lines. The cross-section of FIG. 4 is schematically shown by the dashed-dotted line in FIG. 3.

[0033] Figure 5 schematically shows in plan view a welding seam 30 and an example of improving this welding seam 30 according to the present invention. The improvement method for improving the welding seam 30 is implemented, for example, by the laser processing system 40 of FIG. 2. The improvement method for improving the welding seam 30 includes at least a remelting step. In this remelting step, the welding seam edge of the welding seam 30 provided on the surface of the multilayer aluminum foil stack 10 is remelted at least once by the laser beam 430 with an input heat lower than the input heat for forming the welding seam 30. The first welding seam 30 may be formed by the laser processing system 40 that implements the improvement method. In this case, the laser processing system 40 implements a welding method for welding the multilayer aluminum foil stack 10 of the current collector of the battery to the battery component 20. The welding method includes an initial welding step and an improvement step. First, in the initial welding step, the multilayer aluminum foil stack 10 and the component 20 are welded by the laser beam 430, thereby forming the welding seam 30. For the first welding seam 30 shown in FIG. 5, in particular, refer to the description of the first welding seam 30 in FIGS. 3 and 4. Thereafter, in the improvement step, the above-described improvement method for improving the welding seam 30 is implemented. In the improvement step, the welding method is implemented, for example, by a scanning optical system or a fixed welding head. However, the initial welding step and the improvement step may be implemented by different laser processing systems 40. In this case, the two laser processing systems 40 may be located in different factories.

[0034] In this specification, the improvement method or the welding method of the present invention is implemented, for example, by the control device 420 of FIG. 2. In this control device 420, there is, for example, a corresponding computer program product. When this computer program product is executed by a processor, it includes computer program instructions for controlling the laser processing system 40 of FIG. 2 to implement the improvement method or the welding method.

[0035] According to an optional embodiment of the present invention, in the remelting step, in order to achieve a lower heat input, the laser output of the laser beam 430 is reduced compared to the laser output for forming the weld seam 30, and / or the moving speed of the laser beam 430 relative to the multilayer aluminum foil stack 10 is increased compared to the moving speed for forming the weld seam 30.

[0036] According to an exemplary embodiment of the present invention, the laser output used for remelting is 5% to 50% of the laser output used for welding. Other parameters of the laser beam 430 other than the output used for remelting, such as the focus diameter, feed rate, etc., may remain unchanged compared to the first welding, but may also be changed. Furthermore, the parameters of the laser beam 430 may be changed during each remelting. For example, since the section behind the weld seam 30 along the length direction may have more cracks due to the heat accumulation effect, different laser beam parameters may be used for the section behind the weld seam edge compared to the front section in each remelting.

[0037] According to an exemplary embodiment of the present invention, in the remelting step, the weld seam edge of the weld seam 30 provided on the surface of the multilayer aluminum foil stack 10 is remelted a plurality of times by the laser beam 430. In this case, the heat input used for the first remelting is lower than the heat input used to form the weld seam 30, and the heat input used for each remelting after the first remelting is reduced compared to the immediately previous remelting. In each remelting, the weld seam edge formed by the immediately previous remelting is remelted. In particular, each remelting is performed at least after the molten pool of the immediately previous remelting has solidified. FIG. 5 shows that the weld seam edge has been remelted twice. In FIG. 5, the first weld seam 30 is represented by a thick black solid line, the first remelting is represented by a thin dashed line, and the second remelting is represented by an even thinner dashed line. In FIG. 5, reference numerals are assigned to the first weld seam edge 300, the weld seam edge 311 formed by the first remelting, and the weld seam edge 322 formed by the second remelting. FIG. 6 schematically shows a cross-sectional view of the molten pool of the weld seam 30 shown in FIG. 5. The cross-section is schematically shown by a dashed-dotted line in FIG. 5. In FIG. 6, reference numerals are assigned to the first weld seam 30, the molten pool 310 during the first remelting, and the molten pool 320 during the second remelting. Of course, these overlap each other.

[0038] According to an exemplary embodiment of the present invention, in the remelting step, the laser beam 430 is moved relative to the multilayer aluminum foil stack 10 parallel to the weld seam edge along or in the vicinity of the weld seam edge. However, other trajectories of the laser beam 430 relative to the surface of the multilayer aluminum foil stack 10 for remelting are possible, for example, trajectories involving additional lateral oscillatory movement.

[0039] According to an exemplary embodiment of the present invention, the weld seam edge formed by each remelting is offset in a direction away from the weld seam 30 starting from the weld seam edge formed by the immediately preceding remelting. As shown in FIG. 5, the weld seam edge 322 formed by the second remelting is offset in a direction away from the weld seam 30 starting from the weld seam edge 311 formed by the first remelting. This can also be regarded as a continuous spread of the weld seam 30. However, it is also possible that the weld seam edge formed by remelting is the same as the first weld seam edge 300, that is, the width of the weld seam 30 remains constant.

[0040] According to an exemplary embodiment of the present invention, the lengths of the weld seam edges formed by individual remeltings are equal to each other. However, it is also possible to produce a weld seam edge that is slightly shorter (see FIG. 5) or slightly longer than that of the immediately preceding remelting by each remelting.

[0041] According to an exemplary embodiment of the present invention, as shown in FIG. 6, the depth of the molten pool in each remelting is decreased as the number of remeltings increases and is smaller than the thickness of the multilayer aluminum foil stack 10.

[0042] According to an exemplary embodiment of the present invention, the total number of remeltings is selected such that the weld seam 30 satisfies the requirements regarding cracking.

[0043] Although specific embodiments of the present invention have been described in detail herein, these embodiments are presented for illustrative purposes only and should not be considered as limiting the scope of the present invention. Various substitutions, modifications, and changes may be contemplated without departing from the spirit and scope of the present invention.

Claims

1. An improved method for improving a welding seam (30) formed by welding a multilayer aluminum foil stack (10) of a current collector of a battery and a component (20) of the battery, comprising: At least including a remelting step of remelting at least once, by a laser beam (430), a welding seam edge of the welding seam (30) provided on the surface of the multilayer aluminum foil stack (10), with a heat input lower than the heat input for forming the welding seam (30).

2. In the remelting step, the welding seam edge of the welding seam (30) provided on the surface of the multilayer aluminum foil stack (10) is remelted a plurality of times by the laser beam (430). The heat input used for the first remelting is lower than the heat input used for forming the welding seam (30). The heat input used for each remelting after the first remelting is reduced compared to the immediately previous remelting. In each remelting, the welding seam edge formed by the immediately previous remelting is remelted, and / or In the remelting step, the laser beam (430) is moved relative to the multilayer aluminum foil stack (10) along or in the vicinity of the welding seam edge and parallel to the welding seam edge, and / or In the remelting step, in order to achieve a lower heat input, the laser output of the laser beam (430) is reduced compared to the laser output for forming the welding seam (30), and / or the moving speed of the laser beam (430) relative to the multilayer aluminum foil stack (10) is increased compared to the moving speed for forming the welding seam (30), and / or The surface is the upper surface of the multilayer aluminum foil stack (10) facing the laser beam (430), and the component (20) is below the lower surface of the multilayer aluminum foil stack (10) facing away from the laser beam (430). The improved method according to Claim 1.

3. The welding seam edge formed by each remelting is offset in a direction away from the welding seam (30) starting from the welding seam edge formed by the immediately previous remelting, and / or The lengths of the welding seam edges formed by individual remeltings are equal to each other, and / or The depth of the melt pool in each remelting is decreased as the number of remelting increases, is smaller than the thickness of the multilayer aluminum foil stack (10), and / or select the total number of remelting so that the welding seam (30) meets the requirements regarding cracking, The improved method according to claim 2.

4. The battery is a lithium ion battery, and / or The component (20) is the positive electrode of the battery, The improved method according to any one of claims 1 to 3.

5. A welding method for welding a multilayer aluminum foil stack (10) of a current collector of a battery and a component (20) of the battery, a first welding step of welding the multilayer aluminum foil stack (10) and the component (20) with a laser beam (430) to thereby form a welding seam (30), and an improvement step of performing the improvement method according to claim 1 on the welding seam (30) including, a welding method.

6. The welding method according to claim 5, wherein in the first welding step, the welding seam (30) is formed on the surface of the multilayer aluminum foil stack (10) by a linear or curved trajectory of the laser beam (430).

7. The welding method is performed by a scanning optical system or a fixed welding head, and / or prior to the welding method, the aluminum foils of the multilayer aluminum foil stack (10) are pre-welded to each other by ultrasonic waves, The welding method according to claim 5 or 6.

8. A laser processing system (40), a laser device (410) for generating a laser beam (430), and at least a control device (420) for controlling at least the laser device (410) at least provided with, configured to perform the improvement method according to claim 1 or the welding method according to claim 5, a laser processing system (40).

9. A control device (420) for a laser processing system (40), configured to perform the improvement method according to claim 1 or the welding method according to claim 5, a control device (420).

10. A computer program product, including computer program instructions that, when executed by a processor, perform the improvement method according to claim 1 or the welding method according to claim 5, a computer program product.

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