Evolution of safety risks during battery degradation: mechanisms and implications

Degradation and safety are the fundamental imperatives of batteries, tightly intertwined and inseparable. Only by understanding the coupled mechanisms of degradation and thermal runaway (TR) can we gain control over battery safety across the entire lifecycle. In this work, we investigate commercial 20 Ah NCM/graphite pouch cells subjected to low-temperature and fast-charging coupled protocols, and track how their degradation from the beginning of life (BOL) to the end of life (EOL) reshapes TR behavior. By combining electrochemical diagnostics with multi-scale post-mortem characterization, adiabatic TR tests on full cells, electrode-level differential scanning calorimetry and post-TR gas chromatography, we build a stage-resolved framework that links interfacial composition, lithium plating and structural damage to changes in TR onset, heat release and gas composition. The results reveal a clear degradation critical point at around 90% remaining capacity, at which the dominant degradation mode shifts from mild loss of lithium inventory to lithium-plating-driven accelerated aging with pronounced interface reconstruction, leading to reduced T1/T2, stronger low-temperature anode exotherms and a transition from CO₂-dominated to H₂– and hydrocarbon-rich vented gases. Notably, loss of active material already initiates in the BOL → CP stage, although capacity fade remains predominantly governed by loss of lithium inventory. With further degradation towards EOL, superposed interfacial thickening, cathode cracking and loss of active material further narrow the safety margin and increase TR severity relative to the remaining energy. This work provides a concise mechanistic link between specific degradation signatures and TR metrics, offering practical guidance for defining aging-aware safety envelopes and designing low-temperature fast-charging/discharging strategies for energy storage systems.