A laser welding method for removing charred coating from the separator of a cylindrical battery

Abstract

This invention discloses a laser welding method for removing charring from the separator of a cylindrical battery, belonging to the field of battery welding technology. In this invention, a composite annular laser spot is used. The high-power beam in the outer ring forms the boundary of the annular molten pool at the interface between the electrode tab and the current collector, achieving a reliable annular deep-melt connection. The precisely set low-power beam in the inner ring heats the central region of the annular molten pool, creating a specific temperature gradient on the surface of the molten pool. This induces high-intensity Marangoni convection, flowing from the high-temperature outer ring edge to the relatively cooler central region. This strong inward convection guides most of the heat and molten metal input from the outer ring towards the central region, physically dissipating the heat through radial flow and circulation, rather than the traditional downward conduction.

Description

Technical Field

[0001] This invention belongs to the field of battery welding technology, and particularly relates to a laser welding method for removing charred residue from the separator of a cylindrical battery. Background Technology

[0002] In the manufacturing process of cylindrical lithium-ion batteries, the typical welding sequence is as follows: first, the positive electrode tabs of the cell core are welded to the current collector via the first internal weld; then, the current collector is welded to the battery cap; and finally, the cap is sealed to the steel casing. This sequence is the industry standard process.

[0003] However, a core defect that has long remained unresolved exists in the first weld seam—the welding of the positive electrode tab to the current collector. When welding the positive electrode tab (usually aluminum foil to aluminum or aluminum alloy current collectors), a very thin polymer separator, such as polyethylene (PE) or polypropylene (PP), is directly attached beneath it, separating the positive and negative electrodes. Traditional laser welding technology, whether using a concentrated single-mode Gaussian beam or a more uniformly distributed multimode or flat-top beam, requires a very short input of high energy to form a molten pool and achieve connection. This process generates intense, top-down axial heat conduction. A large amount of heat rapidly penetrates the extremely thin tab and current collector, directly impacting the separator below. This easily causes the separator to overheat beyond its tolerance limit, resulting in scorching, melting shrinkage, or micro-perforation. This pre-weld damage is an inherent defect introduced before battery packaging is complete, posing a significant safety hazard that leads to abnormal battery self-discharge, internal short circuits, and even thermal runaway.

[0004] Existing technologies for addressing the heat-affected zone problem in welding primarily focus on subsequent processes, such as optimizing the welding of the current collector and the cap, or attempting to design a hollow structure on the current collector to increase thermal resistance. However, they do not address the mechanism of the laser heat source itself in the first weld. To ensure full penetration, increasing energy can exacerbate damage, while reducing energy can lead to incomplete welds, leaving room for improvement. Summary of the Invention

[0005] The purpose of this invention is to provide a laser welding method for removing charring from the separator of a cylindrical battery in order to solve the problem of damage to the polymer separator inside the battery cell caused by downward heat conduction during the welding process.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A laser welding method for removing charring from the separator of a cylindrical battery uses a single-mode composite ring laser with independently programmable power control for the inner and outer rings as a heat source to weld the interface between the positive electrode tab and the current collector. Specifically, the method includes: S1: Workpiece preparation and positioning, fixing the current collector to the tooling, precisely aligning the core positive electrode tab with the surface of the current collector and applying constant contact pressure; S2: Laser parameter settings, configuring the core welding parameters for the composite annular spot, specifically including: Outer ring laser power The setting range is 700W to 900W; Inner ring laser power The setting range is 200W to 350W; The power ratio of the inner loop to the outer loop is: Between 0.28 and 0.45; Welding speed V: The setting range is 25mm / s to 45mm / s; Based on the above parameters, calculate and control the composite line energy density. To satisfy ; S3: The welding process is executed. The laser and motion system are started, so that the composite annular laser spot scans and welds the contact area between the positive electrode tab and the current collector along the planned path. At the same time, high-purity inert protective gas is coaxially delivered in the welding area.

[0007] As a further description of the above technical solution: The output beam of the single-mode composite ring spot laser is formed by combining a central Gaussian beam with a hollow ring beam that coaxially surrounds the Gaussian beam.

[0008] As a further description of the above technical solution: In the single-mode composite ring spot laser, the fiber core diameter of the inner ring used to generate the central Gaussian beam is 10-20 μm, and the fiber core diameter of the outer ring used to generate the peripheral hollow ring beam is 80-120 μm.

[0009] As a further description of the above technical solution: The outer ring beam is preferably transmitted and shaped into a hollow ring by a single-mode fiber with a core diameter of 100 μm, and the inner ring beam is preferably transmitted directly by a single-mode fiber with a core diameter of 14 μm.

[0010] As a further description of the above technical solution: The outer ring laser power is =800W, the inner ring laser power is =280W.

[0011] As a further description of the above technical solution: The S3 welding uses negative defocusing, with the laser focus located 0.5-1.5mm below the upper surface of the current collector workpiece to expand the effective area of ​​the annular spot.

[0012] As a further description of the above technical solution: The protective gas is argon with a purity of not less than 99.99%, and the gas flow direction forms an angle of 15-30° with the laser beam axis, with the flow rate controlled at 10-20 L / min.

[0013] As a further description of the above technical solution: The welding speed V is 30 mm / s to 40 mm / s.

[0014] As a further description of the above technical solution: The composite annular laser spot in S3 is a circle or a continuous spiral along the planned path.

[0015] As a further description of the above technical solution: It also includes: quality monitoring, which uses coaxially integrated process monitoring sensors to monitor the stability of the molten pool in real time and enables closed-loop adaptive fine-tuning of welding parameters.

[0016] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are: 1. In this invention, the traditional laser's "penetrating" heating is transformed into a "ring-shaped" heating mode combined with internal control by using a composite annular beam. The high-power (800W) beam in the outer ring is responsible for forming the annular molten pool boundary at the interface between the electrode and the collector, achieving a reliable annular deep-melting connection. The precisely set low-power (280W) beam in the inner ring heats the central region of the annular molten pool, creating a specific temperature gradient on the surface of the molten pool. This induces high-intensity Marangoni convection, flowing from the high-temperature outer ring edge to the relatively low-temperature central region. This strong inward convection guides most of the heat and molten metal input from the outer ring towards the central region, dissipating the heat in radial flow and circulation rather than the traditional downward conduction.

[0017] 2. Through the heat flow redirection mechanism, the heat conducted downward to the core and acting on the diaphragm is reduced to an extremely low level. The diaphragm directly below the positive electrode tab welding area retains its original color, shape and mechanical integrity, completely eliminating "burnt" or melting marks and reducing thermal damage to the first weld.

[0018] 3. The weld formed by this method has uniform penetration depth and reliable connection, effectively avoiding tab embrittlement or incomplete welding caused by improper heat input. The outer ring energy ensures the reliability of the connection, while the inner ring energy improves the stability of the molten pool. This makes the process more adaptable to the surface condition of the tab (such as slight undulations) and assembly gaps, eliminating the inherent safety defects of the battery from the very beginning of the manufacturing chain and significantly improving the safety level and consistency of the final product. Attached Figure Description

[0019] Figure 1This is a schematic diagram showing the assembly relationship of the cylindrical battery core, current collector, and cap in a laser welding method for removing scorch from the separator of a cylindrical battery proposed in this invention. Figure 2 This is a schematic diagram of the single-mode composite annular spot laser welding system used in this invention; Figure 3 A schematic diagram of the heat conduction path during the first weld seam in traditional single-mode laser welding and the resulting scorching of the diaphragm. Figure 4 This is a schematic diagram of the heat conduction and molten pool convection path when welding the first weld using the method of the present invention; Figure 5 This is a schematic diagram of Marangoni convection in the molten pool under the action of the composite annular light spot of the present invention; Figure 6 A micrograph of the diaphragm below the positive electrode tab under conventional welding. Figure 7 This is a micrograph of the corresponding area of ​​the separator in the laser welding method for removing char from the separator of a cylindrical battery proposed in this invention.

[0020] Legend: 1. Cap; 2. First weld; 3. Core; 4. Second weld; 5. Single-mode composite annular spot laser welding system; 501. Single-mode internal optical path laser; 502. Single-mode external optical path laser; 503. First collimating lens; 504. Semi-transparent lens; 505. Focusing lens; 506. Second collimating lens; 507. Reflecting lens; 508. Welded workpiece. Detailed Implementation

[0021] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] Please see Figure 1-7 This embodiment takes the welding of the aluminum positive electrode tab and the aluminum alloy current collector (thickness 0.3mm) of the 32140 cylindrical battery as an example; Equipment: A customized dual-output fiber laser welding system is adopted. The inner ring source is a 300W single-mode fiber laser (14μm core). The outer ring source is a 1000W single-mode fiber laser (100μm core). The beam is shaped into a hollow ring by a diffractive optical element (DOE). The two beams are output coaxially through a polarization combiner and then collimated and focused by a welding head with a focal length of 200mm.

[0023] Workpiece: 23140 battery cell core (top is positive aluminum foil tab), aluminum alloy current collector.

[0024] Welding parameters: Outer ring laser power 800W Inner ring laser power 280W Welding speed V: 35mm / s (laser spiral scanning) Defocusing amount Δf: -0.8mm (focal point is located 0.8mm below the surface of the collector plate) Protective gas: 99.999% high-purity argon, flow rate 15L / min, coaxial delivery.

[0025] The light spot characteristics are as follows: the outer ring light spot diameter is about 0.45 mm and the inner ring light spot diameter is about 0.15 mm.

[0026] During the welding process, the vision system guides the current collector to be precisely aligned with the positive electrode tab of the core. The pressure head applies gentle pressure to ensure contact, and the welding program is started. The laser spot scans the entire welding surface along a predetermined spiral path.

[0027] Results detection and analysis: 1. Electrical performance testing: After welding, the connection resistance between the electrode tab and the current collector was measured. The resistance value was low and stable, meeting the requirements.

[0028] 2. Destructive testing (core verification): Metallographic analysis of the weld, cutting open the weld area, preparing metallographic samples, and observation shows that the weld achieves good metallurgical bonding between the electrode lug and the current collector, with appropriate penetration depth and no overheating or lack of fusion.

[0029] The thermal impact assessment of the separator involved longitudinally dissecting the welded battery cell core. Under optical and scanning electron microscopy, the separator region directly below the positive electrode tab welding point was observed. The results showed that the microstructure, surface morphology, and color of the separator in this region were completely identical to the original separator region located several millimeters away from the welding point, with no observable signs of thermal damage (see comparative example). Figure 6 The results of this invention are shown in Figure 7 ).

[0030] 1. Reliability testing: The welded samples are subjected to mechanical reliability tests such as vibration and drop tests to ensure that the weld joints are stable.

[0031] Comparative Example: Using a traditional single-mode fiber laser with a total power of 1080W (M²≈1.05) and a spot diameter of 0.03mm, spot welding or scanning welding of the same specifications of workpieces was performed with the same total energy input (power and speed adjusted). After dissection, obvious circular scorched yellow discoloration areas were observed on the diaphragm directly below all weld points. Microscopic examination revealed that the diaphragm polymer had melted and recrystallized, resulting in a thinner thickness, confirming thermal damage.

[0032] In conclusion, this invention successfully achieved proactive and precise control over the direction of welding heat conduction by applying the original dual-ring independent controllable laser heat source technology to the first critical weld in cylindrical battery manufacturing—the connection between the core positive electrode tab and the current collector. This technology solves the industry-wide problem of thermal damage during separator welding from the source of the problem, providing crucial process assurance for the production of high-safety and high-consistency power batteries.

[0033] In this invention, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance; the term "multiple" refers to two or more unless otherwise explicitly defined. The terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, "connection" can be a fixed connection, a detachable connection, or an integral connection; "linking" can be a direct connection or an indirect connection through an intermediate medium. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0034] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A laser welding method for removing charred coating from the separator of a cylindrical battery, characterized in that, A single-mode composite ring laser with independently programmable power control for the inner and outer rings of the output beam is used as a heat source to weld the interface between the positive electrode tab and the current collector. Specifically, this includes: S1: Workpiece preparation and positioning, fixing the current collector to the tooling, precisely aligning the core positive electrode tab with the surface of the current collector and applying constant contact pressure; S2: Laser parameter settings, configuring the core welding parameters for the composite annular spot, specifically including: Outer ring laser power The setting range is 700W to 900W; Inner ring laser power The setting range is 200W to 350W; The power ratio of the inner loop to the outer loop is: Between 0.28 and 0.45; Welding speed V: The setting range is 25mm / s to 45mm / s; Based on the above parameters, calculate and control the composite line energy density. To satisfy ; S3: The welding process is executed. The laser and motion system are started, so that the composite annular laser spot scans and welds the contact area between the positive electrode tab and the current collector along the planned path. At the same time, high-purity inert protective gas is coaxially delivered in the welding area.

2. The laser welding method for removing charred coating from the separator of a cylindrical battery according to claim 1, characterized in that, The output beam of the single-mode composite ring spot laser is formed by combining a central Gaussian beam with a hollow ring beam that coaxially surrounds the Gaussian beam.

3. The laser welding method for removing charred coating from the separator of a cylindrical battery according to claim 2, characterized in that, In the single-mode composite ring spot laser, the fiber core diameter of the inner ring used to generate the central Gaussian beam is 10-20 μm, and the fiber core diameter of the outer ring used to generate the peripheral hollow ring beam is 80-120 μm.

4. The laser welding method for removing charred coating from the separator of a cylindrical battery according to claim 3, characterized in that, The outer ring beam is preferably transmitted and shaped into a hollow ring by a single-mode fiber with a core diameter of 100 μm, and the inner ring beam is preferably transmitted directly by a single-mode fiber with a core diameter of 14 μm.

5. The laser welding method for removing charred coating from the separator of a cylindrical battery according to claim 1, characterized in that, The outer ring laser power is 800W, the inner ring laser power is 280W.

6. The laser welding method for removing charred coating from the separator of a cylindrical battery according to claim 1, characterized in that, The S3 welding uses negative defocusing, with the laser focus located 0.5-1.5mm below the upper surface of the current collector workpiece to expand the effective area of ​​the annular spot.

7. The laser welding method for removing charred coating from the separator of a cylindrical battery according to claim 1, characterized in that, The protective gas is argon with a purity of not less than 99.99%, and the gas flow direction forms an angle of 15-30° with the laser beam axis, with the flow rate controlled at 10-20 L / min.

8. The laser welding method for removing charred coating from the separator of a cylindrical battery according to claim 1, characterized in that, The welding speed V is 30 mm / s to 40 mm / s.

9. The laser welding method for removing charred coating from the separator of a cylindrical battery according to claim 1, characterized in that, The composite annular laser spot in S3 is a circle or a continuous spiral along the planned path.

10. A laser welding method for removing charred coating from the separator of a cylindrical battery according to claim 1, characterized in that, Also includes: Quality monitoring utilizes coaxially integrated process monitoring sensors to monitor the stability of the molten pool in real time and enable closed-loop adaptive fine-tuning of welding parameters.

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