Thermal runaway front in failure propagation of long-shape lithium-ion battery

•A new concept, the thermal runaway front, is proposed for characterizing the thermal runaway propagation within a battery.•The thermal runaway front is symmetric alongside the centerline of the battery.•Based on testing, the average velocity of thermal runaway front is found to be approximately 24.14 mm•s−1.•The speed of the thermal runaway front has a square root correlation with the thermal conductivity and heat generation rate. Long, large-format lithium-ion batteries have become prominent in recent years in high-power application scenarios, such as in electrochemical energy storage stations, electric vehicles, and electric ships. In these batteries, failure is always initiated from a local point and then propagates to the full cell, requiring countermeasures to quench the in-cell thermal runaway propagation. This study investigates the thermal runaway propagation behaviors of long, large-format lithium-ion batteries. A thermal runaway front exists during the propagation of thermal runaway; it separates the failure zone and normal zone and carries significant information regarding the thermal runaway reactions. The characteristics of the thermal runaway front are investigated through experiments and simulations. The thermal runaway front moves forward with an average velocity of approximately 24.14 mm·s−1, driven by the large temperature gradient between the failure and intact zones. The velocity of the thermal runaway front is correlated with the thermophysical properties of the battery. A modeling analysis indicates that the velocity of the thermal runaway front in propagation has a square root correlation with the thermal conductivity and heat generation rate. This square root correlation links the failure process and the thermophysical properties of the battery and can contribute to the future safety design of large-format batteries.