Modeling the Coupled Stress Relaxation and SEI Evolution in Preload-Constrained Lithium-Ion Cells

This work investigates the role of preload pressure in governing the electro-chemo-mechanical (ECM) behavior of lithium iron phosphate (LFP)/graphite pouch cells during calendar aging. Cells aged at 60 °C under different preload levels were systematically evaluated through in situ monitoring of force evolution and capacity retention. To interpret these behaviors, a coupled model was developed that integrates solid electrolyte interphase (SEI)-induced electrode expansion, viscoelastic relaxation, and stiffness evolution, and it was validated against multi-rate discharge experiments, showing excellent agreement with measured voltage and force responses. The results reveal that higher preload amplifies internal pressure fluctuations, prolongs viscoelastic relaxation, and delays irreversible force recovery, while the overall capacity fade remains largely unaffected. A slight mitigation in capacity loss is observed at high preload, primarily due to suppressed SEI growth resulting from reduced electrode porosity and a decrease in active surface area available for interfacial reactions. Fitting parameters for stiffness correction, relaxation amplitude, and relaxation time exhibited systematic preload dependence. By decoupling irreversible and relaxation forces, the framework enables quantitative analysis of aging-induced pressure accumulation. Overall, this study underscores the critical role of mechanical constraints in long-term battery degradation and demonstrates the predictive capability of the proposed ECM model for guiding preload design in practical modules.