Mechanical Stress Modeling in Jellyroll-Wound Cells
Mechanical stress modeling is fundamental for assessing the structural integrity, performance degradation, and safety of jellyroll-wound lithium-ion batteries. Finite element analysis (FEA) in cylindrical coordinates offers a robust framework to analyze stress distribution within the complex spiral-wound electrode geometry. The anisotropic nature of layered components—current collectors, active materials, and separators—requires detailed modeling to capture radial, circumferential, and axial stress gradients.
Stress Components and Their Origins
The jellyroll structure comprises concentric layers wound under tension, creating inherent residual stresses. During electrochemical cycling, lithiation-induced expansion in anode materials like graphite generates additional mechanical loads. These stresses exhibit non-uniformity due to edge effects at electrode layer terminations.
Radial Stress Distribution
Radial stress is governed by contact pressure between adjacent windings. Expansion during charging increases radial compression, particularly near the core where curvature is highest. In high-energy-density cells, radial stresses can exceed 10 MPa, potentially causing separator deformation and internal short circuits. The stress gradient from core to outer layers follows a logarithmic decay.
Circumferential (Hoop) Stresses
Hoop stresses result from winding tension and differential expansion between electrodes. Anode expansion typically exceeds cathode contraction, creating tensile stresses in the cathode and compressive stresses in the anode. Edge effects amplify these stresses near terminations, with concentrations reaching 1.5 times bulk values. Incomplete electrode coverage exacerbates these effects.
Axial Stresses and Constraints
Axial stresses are influenced by constraint conditions at the cell’s ends. Constrained swelling induces through-thickness compression, while unconstrained designs may exhibit buckling. The central mandrel in cylindrical cells provides axial stiffness, creating non-linear z-axis stress distribution. Thermal gradients during operation further modulate these stresses.
Material Anisotropy and Residual Stresses
Material anisotropy significantly influences stress patterns:
- Copper and aluminum current collectors exhibit orthotropic elasticity
- Separators act as stress buffers but undergo plastic deformation
- Residual hoop stresses of 5-20 MPa persist in as-manufactured cells
These pre-stresses interact with operational loads, affecting long-term fatigue behavior.
Cyclic Loading Effects
Repeated cycling induces stress evolution through multiple mechanisms:
- Plastic deformation causes stress relaxation in metallic components
- Viscoelastic behavior introduces time-dependent effects in polymers
- Progressive particle cracking modifies global stress distribution
Accurate FEA simulations incorporating winding history and material properties are essential for predicting deformation modes and ensuring battery reliability.