Aging matters: How degradation pathways govern thermal runaway in lithium-ion batteries

Understanding how aging influences the thermal hazards of lithium-ion batteries (LIBs) is critical for enhancing their safety across a wide range of applications. This study systematically investigates the thermal runaway (TR) behavior of LIBs, with particular emphasis on combined-pathway aging, evaluated in terms of normalized usable capacity (UE). Key thermal safety parameters, i.e., TR triggering temperature, mass loss, and heat generation under diverse aging conditions, are quantified. To enable a fair comparison, thermal hazards are evaluated based on equivalent usable capacity, revealing that aged cells exhibit lower TR triggering temperatures and higher heat generation than fresh cells under thermal abuse with elevated thermal risks. Mechanistic analysis identifies lithium plating, solid electrolyte interphase (SEI) formation, and lithium depletion, particularly under high-temperature charging, as the dominant contributors to increased hazard. Using an aging-stressor matrix, a trade-off between high-C-rate-induced thermal instability and high-temperature-induced thermal stability is discovered and quantified, underscoring the strong dependence of thermal hazards on specific aging pathways. This work advances the fundamental understanding of aging-induced safety risks in LIBs and offers practical guidance for the development of safer battery systems, optimized charging protocols, and improved battery management strategies across applications in electric vehicles, consumer electronics, and grid-scale energy storage.