Relationship between interior temperature and exterior parameters for thermal runaway warning of large-format LiFePO₄ energy storage cells under overcharge scenarios

With the fast development of the energy storage industry and the growing adoption of large-format lithium-ion cells, the safety issues and deficiencies of traditional thermal runaway warning protocols in these high-capacity cells demand greater attention. This work investigates the thermal runaway properties of large-format LiFePO₄ (LFP) energy storage cells at overcharge scenarios, aiming to establish the correlation between internal temperature and external parameters, further contributing to the development of more effective thermal runaway warning strategies. The cell's voltage exhibits two distinct plateaus during the overcharge process, which likely correspond to the formation of lithium plating and the full lithiation of graphite, respectively. Internal short-circuit occurs prior to thermal runaway, leading to a rapid voltage drop and a dramatic rise in interior temperature. The cell's surface temperature shows a clear relationship with its interior temperature, increasing linearly with the latter before thermal runaway. Notably, the interior temperature is about 1.5 times of the surface temperature. This relationship remains consistent regardless of overcharge rate and preload force, suggesting the potential for inferring interior cell conditions from exterior temperature measurements. The overcharge rate significantly shortens the thermal runaway process, worsens its severity and limits the heat release of the cell. Increasing the preload force enhances the cell's resistance to overcharge by suppressing lithium plating, which in turn delays the onset of thermal runaway and mitigates its severity. Based on the profile of multiple parameters, a three-level thermal runaway warning strategy for overcharge scenarios is proposed to provide a safety margin of more than 400 s before thermal runaway occurs.