Relationship between interior temperature and exterior parameters for thermal runaway warning of large-format LiFePO₄ cells with various states of charge, preload forces and capacities

With the wide application of energy storage based on lithium-ion cells and the popularity of cells with a larger format, the safety challenge and the limitation of traditional thermal runaway warning technologies faced by large-format energy storage cells deserve of more focus. Thus, an experimental investigation is conducted here to reveal the thermal runaway characteristics of large-format LiFePO₄ (LFP) energy storage cells at overheating scenarios and unveil the relationship between the interior temperature and exterior parameters for effective thermal runaway warning. Although the thermal runaway of large-format LFP energy storage cells is gentler than traditional ternary cells, where only large quantities of gas/smoke instead of violent combustion/explosion are observed, the comitant toxic hazard and the hindrance on personnel evacuation cannot be neglected. There is a noticeable relationship between the cell interior temperature and several critical exterior parameters, e.g., exterior temperature, expanding force, and open-circuit before the safety valve opens, which might be utilized to achieve the goal of using exterior parameters to predict the internal state of cells, and finally improve the warning accuracy and timeliness. A thermal runaway warning assumption based on multiple parameters (open-circuit voltage, expanding force, estimated interior temperature, and dynamic resistance) is proposed, composed of four levels of warning that successfully offers a margin over 1000 s before the occurrence of thermal runaway. This work might contribute to deepen the understanding on safety characteristics of large-format energy storage cells and promote the development of efficient thermal runaway warning technology.