Microscale analysis on cycling aging effect in mechanical behavior of graphite anode coating: Morphological statistics and nanoindentation

Throughout the full life cycle of lithium-ion batteries (LIBs), capacity degradation is inevitable and directly influences the battery response under mechanical loading. Understanding how cycling aging affects the mechanical response is essential for developing swelling and mechanical abuse models of aged LIBs. For LiFePO₄/graphite LIBs, changes in mechanical response primarily originate from the anode coating. This study first employed image recognition technology to analyze and reconstruct the geometrical structure of anode coating at different states of health (SOHs). We characterized graphite particles as ellipses and quantified the representative particle-assembly structure using probabilistic and statistical methods. Significant structural changes were identified in the anode coating along with capacity fading, including increased porosity and reduced particle size. Employing nanoindentation technique, we measured the Young's modulus and hardness, and identified the stress-strain relationship via the nanoindentation force-depth curves for the graphite particles at multiple SOH levels. Only slight variations in the mechanical properties of graphite particles can be detected across different SOH levels, suggesting that cycling aging has a negligible impact on mechanical performance of graphite particles. Considering the microscale reconstruction of the coating structure in conjunction with the stress-strain behavior identified for graphite particles, we developed a representative volume element (RVE) model for the anode coating at different SOHs. The simulation results aligned with the test curves, where the mechanical response of anode coating shifts rightward along with capacity degradation. This further verified that the structural changes induced by cycling aging is the major mechanism affecting the mechanical response of anode coating.