Mechanical Modelling of Positive Electrode in All-Solid-State Battery Cells

All-solid-state lithium-based batteries represent a critical evolution in energy storage, offering enhanced safety, higher energy density, and superior fast-charging capabilities. However, the integration of solid-state electrolytes introduces complex mechanical interactions at the electrode-electrolyte interface that significantly impact performance and longevity. This study introduces a cyclic plastic hardening model for ceramic electrolytes, moving beyond traditional brittle or linear-elastic assumptions. It presents a Finite Element Method (FEM) analysis of a positive electrode representative volume element (RVE), consisting of spherical Nickel-Manganese-Cobalt (NMC811) active material particles embedded in an Li7La3Zr2O12 (LLZO) solid-state electrolyte matrix, with Gaussian-distribution of particle sizes aimed to capture the stochastic heterogeneity of electrode microstructures. The simulation results illustrate the volumetric expansion and contraction of active material during lithiation and de-lithiation (electrochemical loading) cycles using a thermal expansion analogy. Due to the scarcity of cyclic plasticity data for ceramic electrolytes, the plastic hardening behavior of LLZO is approximated using a proxy material model (SiMo5 steel at 700°C) to qualitatively capture strain hardening effects. The study analyzes stress distribution, volumetric deformation, and contact evolution over five charge-discharge cycles under a uniform assembly external pressure of 10 MPa. Results indicate progressive strain hardening in the solid electrolyte, characterized by increasing tensile stresses and stabilizing volumetric deformation. Furthermore, the analysis reveals cyclic contact loss during de-lithiation, which poses risks for increased interfacial resistance. These findings provide theoretical insights into the mechanical degradation mechanisms of ASSBs, emphasizing the key role of stack pressure and material hardening in their long-term stability.