Beyond Non-Propagation: Residual Safety Qualification of Lithium-Ion Cells Surviving a Neighboring Thermal Runaway Event

Preventing cell-to-cell thermal runaway propagation is a fundamental objective of electric-vehicle battery safety. However, successful non-propagation does not necessarily demonstrate that thermally exposed neighboring cells remain safe for continued operation. This preprint proposes a physics-based framework for the residual safety qualification of lithium-ion cells surviving a neighboring thermal runaway event. It distinguishes immediate event survival from post-event safe operability and characterizes thermal exposure through peak temperature, heating rate, spatial temperature gradients, exposure duration and transferred thermal energy. A controlled experimental methodology is proposed to compare cells exposed to an actual neighboring thermal runaway with cells subjected to controlled heating at approximately the same peak temperature. Post-event assessment combines electrical and electrochemical diagnostics, self-discharge measurements, mechanical indicators and subsequent thermal-stability testing. The central hypothesis is that non-propagation is necessary but not sufficient to establish safe continued operation, and that peak temperature alone may be insufficient to characterize residual safety when thermal histories differ. The proposed framework aims to establish a residual-safety envelope supporting post-event decisions such as continued operation, isolation or controlled handling, with potential implications for EV battery validation, BMS diagnostics and service strategies. This preprint presents a research framework and proposed experimental methodology; it does not report completed experimental results.