Relationship between interior temperature and exterior parameters for thermal runaway warning of large-format LiFePO₄ energy storage cells with various heating patterns and heating powers

With the widespread adoption of lithium-ion cell-based energy storage systems and the increasing prevalence of larger-format cells, the safety challenges and limitations of traditional thermal runaway warning technologies in large-format energy storage cells warrant greater attention. Therefore, this study conducts an experimental investigation to explore the thermal runaway characteristics of large-format LiFePO₄ (LFP) energy storage cells under typical heating patterns, to uncover the relationship between interior and exterior parameters for more effective thermal runaway warning, while also evaluating the impact of heating power. Heating patterns significantly impact the thermal runaway behavior of large-format LFP cells. When heated using an electric plate, thermal runaway propagation appears within the cell, with only smoke/gas being released throughout the process. In contrast, under oven heating, which provides uniform heating, both jelly rolls within the cell undergo thermal runaway simultaneously, accompanied by intense fire, combustion, and the release of smoke/gas. A notable correlation is identified between the cell's interior temperature and key exterior parameters (exterior temperature, expanding force, and open-circuit voltage) especially for the linear relation between the interior and exterior temperatures, suggesting the potential to predict the internal conditions of cells by monitoring external parameters, thereby enhancing the thermal runaway warning effectiveness. Ultimately, a three-level thermal runaway warning is proposed before the occurrence of safety valve opening based on the variation in open-circuit voltage, expanding force, and estimated interior temperature to offer a margin over 1000 and 6000 s before thermal runaway under the two heat patterns separately.