Modeling Volume and Porosity Change of Lithium-Ion Cells Due to Lithium Intercalation and External Pressure

Volume and subsequent porosity changes of lithium-ion cells during charge-discharge cycles are major challenges in the development of high-performance and long-lasting batteries. This work presents a novel coupled electrochemical-mechanical modeling approach to describe porosity changes due to intercalation and external compression. Based on the Doyle-Fuller-Newman model, the electrochemical model is extended by incorporating a mechanical model grounded in solid mechanics. The elasticity of the anode, separator, and cathode is modeled as poroelastic to account for the non-linear stress-strain behavior of these porous materials. A porosity model to calculate the local changes in porosity is introduced and discussed. The coupled model is validated against a 70.2A h cell regarding cell voltage, reversible cell swelling, and pressure increase for C-rates between C/20 and 1C, achieving a root-mean-squared error ≤12 mV in cell voltage and <7% in reversible cell swelling. Simulation studies reveal that mechanical aspects significantly impact electrochemical performance, potentially leading to earlier lithium plating. Furthermore, the mathematical formulation shows that porosity changes alone have minimal impact on performance, and that effective mass transport should be updated based on porosity changes. The model prediction regarding internal states like porosity and volume changes aligns with the literature.