Mechanical stress accumulation during roll-to-roll electrode fabrication is a critical factor influencing the quality and performance of lithium-ion batteries. The process involves handling thin, flexible electrode materials under controlled tension, where improper management can lead to defects such as wrinkles, cracks, or delamination. Understanding the sources of stress and implementing precise tension control models are essential for minimizing these defects while maintaining production efficiency.

The roll-to-roll process subjects electrode materials to multiple stress-inducing stages, including unwinding, coating, drying, and rewinding. Each stage introduces mechanical forces that, if unbalanced, contribute to cumulative stress. Electrode foils, typically made of aluminum for cathodes and copper for anodes, have thicknesses ranging from 8 to 20 micrometers, making them susceptible to deformation. The substrate’s elastic modulus, yield strength, and elongation properties determine its response to applied tension. Excessive stress can cause permanent deformation, while insufficient tension leads to misalignment or wrinkling.

Tension control models are employed to regulate the forces acting on the electrode web. A common approach is the decentralized tension control system, where individual rollers adjust tension based on feedback from load cells or dancer rollers. The relationship between web tension (T), roller radius (R), and torque (τ) is given by T = τ/R. Maintaining uniform tension across the web requires real-time adjustments to account for speed variations, material elasticity, and friction. PID (Proportional-Integral-Derivative) controllers are widely used to stabilize tension by minimizing deviations from setpoints. Advanced models incorporate predictive algorithms to anticipate disturbances, such as sudden speed changes or material property variations.

Wrinkle formation is a major defect arising from non-uniform stress distribution. Wrinkles can be categorized into two types: shear wrinkles and buckling wrinkles. Shear wrinkles occur due to misalignment between rollers, causing lateral displacement of the web. Buckling wrinkles result from compressive forces exceeding the critical buckling stress of the material. The critical buckling stress (σ_cr) for a thin film under tension can be approximated by σ_cr = (π²E)/(12(1-ν²))(t/w)², where E is Young’s modulus, ν is Poisson’s ratio, t is thickness, and w is the width of the web. Preventing wrinkles requires precise alignment of rollers, uniform tension distribution, and minimizing lateral forces.

Dry room conditions, as referenced in G10, play a significant role in stress management. Electrode materials are hygroscopic, and moisture absorption can alter their mechanical properties. For instance, increased humidity reduces the elastic modulus of polymer-coated electrodes, making them more prone to stretching or wrinkling. Maintaining a dew point below -40°C in the dry room ensures consistent material behavior, reducing variability in stress responses. Additionally, temperature stability prevents thermal expansion-induced stress, which can exacerbate wrinkle formation.

Empirical studies have quantified the impact of tension variations on defect rates. Research indicates that tension deviations exceeding 10% of the target value increase wrinkle occurrence by over 50%. Optimal tension ranges vary by material; for example, copper anodes typically require 50-100 N/m, while aluminum cathodes tolerate slightly higher tensions of 70-120 N/m. These values are derived from tensile testing and in-line monitoring during production.

Residual stress, accumulated during winding, also affects electrode quality. The winding process imposes radial and hoop stresses on the coiled material, which can lead to layer-to-layer adhesion or deformation. Residual stress is influenced by winding tension, core hardness, and lap alignment. Finite element modeling has shown that residual stress can be minimized by optimizing winding tension profiles, such as taper tension control, where tension decreases linearly from the inner to outer layers.

In summary, stress accumulation in roll-to-roll electrode fabrication is governed by tension control, material properties, and environmental conditions. Effective management requires a combination of real-time control systems, predictive modeling, and stringent dry room standards. By addressing these factors, manufacturers can reduce defects and enhance the mechanical integrity of battery electrodes. Future advancements may focus on adaptive control algorithms and in-situ stress measurement techniques to further improve process reliability.

The interplay between mechanical stress and electrode performance underscores the need for interdisciplinary approaches, integrating materials science, mechanical engineering, and process control. As battery technologies evolve, optimizing roll-to-roll fabrication will remain a key enabler for scalable, high-quality production.