Comprehensive analysis of degradation mechanisms in 18650 Li-Ion cells under prolonged cycling conditions

Li-ion batteries are essential for applications like electromobility and stationary energy storage, where long-term performance and safety are critical. While previous studies have explored degradation mechanisms in Li-ion cells, detailed insights into structural changes during extended cycling remain limited. This study aims to investigate the degradation mechanisms in a Samsung 18650 cylindrical Li-ion cell over 800 cycles (10 % to 90 % State of Charge) to understand performance fade and structural changes. Utilizing periodic micro-CT scans, we observed significant geometric alterations in the electrode stack, including delamination and bending towards the cell axis, correlating with capacity loss. Post-mortem analyses using broad ion beam (BIB), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and synchrotron CT confirmed voids in the cathode active material, copper deposition on the anode surface, and elevated levels of phosphorus and fluorine, likely from electrolyte decomposition and SEI layer formation. These insights shed light on the structural changes and failure modes in cylindrical Li-ion cells during prolonged cycling and highlight the need for design improvements and optimized manufacturing processes to enhance mechanical robustness and reduce defects, thereby extending battery cycle life and safety.