Residual stresses in battery cells originate from manufacturing processes such as calendering and welding, which induce mechanical deformations in materials. These stresses influence the structural integrity and electrochemical performance of cells, contributing to degradation mechanisms like particle cracking, delamination, and accelerated aging. Understanding and mitigating residual stresses through modeling and measurement techniques is critical for improving cell longevity and reliability.
Calendering, a key step in electrode manufacturing, compresses the coated electrode to enhance density and adhesion. The high-pressure rolling process generates non-uniform stress distributions within the electrode layers, particularly in the active material, conductive additives, and current collector. Residual stresses from calendering can lead to microcracks in brittle materials like silicon anodes or high-nickel cathodes, reducing mechanical stability and increasing charge-transfer resistance. Finite element modeling (FEM) has been employed to simulate stress distributions during calendering, revealing that excessive compression forces exacerbate stress concentrations near particle boundaries. Optimizing parameters such as roller pressure, speed, and temperature can minimize these stresses while maintaining electrode density.
Welding processes, including laser and ultrasonic welding, introduce localized thermal stresses in battery tabs and interconnects. Rapid heating and cooling cycles during welding create residual stresses that may propagate into adjacent regions, weakening joints and increasing electrical resistance. Studies using thermomechanical simulations demonstrate that improper welding parameters—such as excessive laser power or insufficient cooling time—result in higher residual stresses, raising the risk of joint failure during cycling. Process optimization, such as pulsed laser welding with controlled energy input, reduces stress accumulation while ensuring robust electrical connections.
X-ray diffraction (XRD) is a widely used experimental technique for quantifying residual stresses in battery materials. By measuring lattice strain through Bragg angle shifts, XRD provides non-destructive insights into stress magnitudes and distributions. For example, studies on nickel-manganese-cobalt (NMC) cathodes have shown that calendering-induced stresses can exceed 200 MPa, leading to anisotropic strain that affects lithium-ion diffusion. Synchrotron XRD offers higher resolution for mapping stress gradients across large areas, aiding in the validation of simulation models.
Mechanical modeling approaches, including elastic-plastic and viscoelastic models, are applied to predict residual stress evolution during manufacturing. Multiscale simulations combine macroscopic process models with microscale material behavior to capture interactions between particles and binders. For instance, cohesive zone modeling has been used to analyze delamination risks at electrode-separator interfaces caused by residual stresses. These models inform process adjustments, such as graded calendering pressures or post-welding annealing, to relieve stresses without compromising performance.
Process optimization strategies focus on reducing residual stresses while maintaining manufacturing efficiency. Adaptive control systems in calendering machines adjust roller forces in real-time based on in-line stress measurements, ensuring uniform compression. In welding, preheating and post-weld heat treatments mitigate thermal gradients, lowering residual stress levels by up to 30% in some cases. Statistical design of experiments (DoE) methods identify optimal parameter combinations that balance stress reduction with production throughput.
The impact of residual stresses on cell performance is evident in cycling tests and post-mortem analyses. Cells with higher initial stresses exhibit faster capacity fade and increased impedance growth due to mechanical degradation. For example, lithium-ion cells with poorly optimized welding processes show higher rates of tab fracture after 500 cycles compared to those with stress-relieved joints. Mechanical modeling coupled with electrochemical performance data enables root-cause analysis, linking specific stress profiles to observed failure modes.
Emerging techniques like digital image correlation (DIC) and neutron diffraction complement XRD for stress measurement, providing full-field strain maps and deeper material penetration. These methods validate simulation results and guide further refinements in manufacturing processes. Additionally, machine learning algorithms are being integrated into stress prediction workflows, enabling rapid parameter optimization based on historical process data.
Residual stress management is increasingly important for next-generation batteries, such as solid-state cells, where interfacial stresses between rigid electrolytes and electrodes pose unique challenges. Advanced modeling tools that account for chemo-mechanical coupling effects are essential for designing stress-tolerant architectures.
In summary, residual stresses from calendering and welding significantly influence battery performance and durability. Combining experimental measurements with sophisticated modeling techniques allows for precise stress characterization and process optimization. Reducing these stresses through controlled manufacturing parameters enhances cell reliability, supporting the development of longer-lasting and safer energy storage systems. Continued advancements in measurement technologies and simulation methods will further improve the understanding and mitigation of residual stresses in battery production.