Why Do Pouch Battery Tabs Crack?

The tabs of a pouch battery cell may seem like just conductive leads, but they are actually one of the weakest links in the overall cell's reliability. Tab cracking can lead to a surge in internal resistance and a drop in capacity, or even trigger thermal runaway. From production line defects to customer-side cycle failures, the causes of tab breakage span multiple dimensions, including manufacturing, electrochemistry, and mechanics. This article will provide a brief analysis.

The vast majority of tab cracks can be traced back to microscopic damage left by the welding process. Currently, the mainstream pouch battery cell uses ultrasonic welding to fuse multiple layers of current collector foil to the external tabs. Even slight deviations in process parameters can create hidden dangers. When the welding energy is too high or the amplitude is too great, the copper and aluminum foil can be crushed by the welding head, creating microcracks. The stress concentration effect is most pronounced at the transition between the weld edge and the coating area. According to the principles of fracture mechanics, these initial microcracks will propagate perpendicular to the tensile force under subsequent stress, eventually leading to a through-crack.

ACEY-USW-1000 ultrasonic welding machine is mainly composed of ultrasonic generator, transducer, upper and lower ultrasonic pole, pneumatic components, frame and so on.The circuit mode adopts full-bridge high-power mode, stable performance and strong output. The upper and lower ultrasonic pole materials are made of new alloy materials with good wear resistance.

Besides these, incomplete soldering, burn-through, and hot cracking are also common welding defects. Incomplete soldering fails to form an effective metallurgical bond, making it prone to delamination under stress; burn-through directly damages the structural integrity of the foil; and low-melting-point eutectic formed during welding cooling can leave potential hot cracks in the weld center. Many cells that initially pass inspection on the production line crack from the solder joint edge after a few hundred cycles, essentially due to the continuous propagation of micro-cracks embedded during welding under stress.

If welding defects are the internal cause, the volume deformation during charging and discharging is the primary source of external force. When lithium ions are inserted and extracted at the positive and negative electrodes, the active materials undergo periodic expansion and contraction-graphite negative electrodes change volume by about 10%, while silicon-based negative electrodes experience changes as high as 300%. This volume change is transmitted through the electrode sheet to the root of the tab, equivalent to repeatedly subjecting the welded joint to tensile fatigue testing.

Soft-pack cells lack the constraint of a rigid shell, allowing for greater freedom of expansion in the thickness direction of the stacked or wound structure, resulting in more direct tensile stress on the tabs. When fully charged, the battery cell expands in thickness. A significant stress concentration point forms at the connection between the tab and the current collector. With increasing cycle count, the plastic deformation of the metal foil accumulates, hindering grain slip and forming fatigue bands. When the damage exceeds the material's fatigue limit, cracks propagate rapidly. This explains why the probability of tab breakage in high-silicon anode, high-energy-density battery cells increases significantly-the greater the expansion, the more severe the alternating stress the tab endures.

Many people easily overlook that tab cracking is not just a purely mechanical problem; electrochemical corrosion also plays a significant role. The electrolyte system inside the pouch cell contains lithium salts and organic solvents, which decompose during charging and discharging, producing trace amounts of corrosive substances such as HF. The tab welding area, due to its coarse grains and residual stress, is inherently a corrosion-sensitive zone. Acidic substances like HF can penetrate along grain boundaries and cracks, causing intergranular corrosion and stress corrosion cracking.

Corrosion accelerates failure in two dimensions: firstly, corrosion products crowd out crack space, creating a wedging effect and propelling crack propagation; secondly, corrosion reduces the fracture toughness of materials, allowing stress levels that would otherwise be insufficient to cause fracture to lead to failure. Especially under high-temperature storage or high SOC conditions, electrolyte decomposition intensifies, and the corrosion rate increases exponentially. Many battery cells experience tab breakage after high-temperature storage, largely a result of the coupling effect of corrosion and stress.

The actual service environment of the customer further amplifies the risk of tab failure. In automotive scenarios, battery cells are subjected to long-term random vibration and impact loads, causing fatigue damage to the tab weld joints under alternating vibration stress. When the vibration frequency is close to the cell's natural frequency, a resonance effect occurs, increasing the stress level at the tab root several times over. Simultaneously, diurnal temperature variations and the heat generated during charging and discharging create thermal cycling. Differences in the thermal expansion coefficients of different materials generate thermal stress at the welding interface, which, under repeated action, can also lead to interface cracking.

The impact of the packaging process should also be mentioned here. During the top sealing of pouch cells, the tab adhesive is hot-pressed onto the aluminum-plastic film. If the tab length is insufficient or the bending angle is unreasonable, additional bending stress will be generated on the tab during the sealing process. After repeated bending, the metal material undergoes work hardening, reducing its plasticity, making it more prone to fracture under subsequent cyclic stress.

Finally, returning to the design aspect, the redundancy in tab size and structure directly determines its failure margin. Many high-energy-density designs, in order to reduce volume, shorten the tab length and thin the tab as much as possible, resulting in a significant decrease in the tab's ability to absorb deformation. A reasonable tab redundancy design can effectively disperse expansion stress.

Material selection is also crucial. Positive aluminum tabs and negative copper tabs have different mechanical properties, corrosion resistance, and failure modes. Aluminum tabs have poor plasticity and are more prone to brittle fracture; copper tabs have good ductility but weak corrosion resistance and are more prone to corrosion fatigue. If the material thickness and structure are not matched to specific operating conditions during the design phase, tab cracking is almost inevitable.

In summary, tab cracking is caused by a combination of factors. Welding introduces initial cracks; cyclic expansion increases the probability of cracking; and long-term wear and tear at the application end accelerates cracking.

As pouch lithium batteries continue to gain popularity in applications such as drones, consumer electronics, medical devices and electric mobility, manufacturers are placing greater emphasis on cell quality, tab welding reliability, and production consistency. Preventing defects such as tab cracking is essential for improving battery safety, cycle life, and overall product performance.

ACEY New Energy specializes in providing intelligent equipment and complete assembly solutions for lithium battery manufacturing. From laboratory R&D to automated mass production, we offer complete pouch cell and battery pack assembly lines, including dedicated solutions for UAV (drone) battery pack production. Our integrated systems cover cell sorting, tab welding, testing, assembly, aging, and final inspection, helping customers build efficient, reliable, and high-quality battery manufacturing processes.

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