Thermal runaway in lithium-ion batteries: experimental insights from mechanical, thermal, and electrical stress tests

The increasing use of lithium-ion batteries (LIBs) in electric vehicles and energy storage systems raises new safety concerns, especially in confined environments such as road tunnels. In the event of thermal runaway (TR), cells can release flammable gases, toxic substances, and thermal loads capable of compromising structural integrity. This study aims to provide more realistic experimental results compared to conventional tests in the literature, which often focus on ideal conditions and single-event scenarios. Several critical aspects of LIB cell behaviour under mechanical, thermal, and electrical abuse were investigated. Specifically, the thermal effects on concrete samples, representative of tunnel linings, were evaluated under direct exposure conditions during TR events. Importantly, TR events in proximity to cement mortar samples revealed their thermal damage, leading to significant microstructural degradation (mainly the development of a network of microcracks) after repeated exposure, as illustrated by computed tomography and scanning electron microscopy (SEM) analyses. Nail penetration tests were performed at different ambient temperatures and with cells at various states of health (SOH), simulating real-world operating conditions. Results show that cells triggered at higher ambient temperatures are more reactive, reaching higher peak temperatures and pressures during TR. Results also show that degraded cells produce less intense but still hazardous reactions. In overheating tests, a critical delay window (~350 s) was observed in safety valve activation, which plays a key role in TR mitigation. For the external short circuit, no thermal runaway occurred at different SOC states. Finally, post-mortem tests by micro-CT scan analyses of the cells revealed increased reactivity in TR events triggered by overheating. Unlike conventional studies limited to idealized conditions, this work provides the first comprehensive experimental evidence of LIB thermal runaway under realistic abuse conditions, including effects on concrete tunnel-lining materials and cells with reduced SOH values. The results will be useful for developing safety strategies and engineering models aimed at improving the resilience of underground infrastructure.