Thermal runaway and fire behaviors of lithium iron phosphate battery induced by over heating

Lithium ion batteries (LIBs) have been widely used in various electronic devices, but numerous accidents related to LIBs frequently occur due to its flammable materials. In this work, the thermal runaway (TR) process and the fire behaviors of 22 Ah LiFePO4/graphite batteries are investigated using an in situ calorimeter. The cells are over heated using a heating plate. The heating plate is utilized to simulate the abuse process triggered by TR of the adjacent battery in modules. The fire behaviors, heat release rate (HRR), temperature characteristics of cell surface and flame, mass loss, voltage variation and gas release are obtained. For 100% state of charge (SOC) cells, the fire is blown out at the TR process. Jet fire is only observed for 50% and 100% SOC cells and the average trigger temperatures of TR are 198.6 and 184.8 °C for 50 and 100% SOC batteries, respectively. The state of charge of the cell has a significant influence on the peak HRR, total heat release (THR) and mass loss. HF and H2 are detected during combustion process. The productions of CO and HF increase as the increasing SOCs, which have much greater toxicity. And that of 100% SOC cells is much larger due to the flame being blown out. The relationship between TR and fire behaviors is revealed. Two failure modes are proposed to describe the fire behaviors at different phases. Based on the energy balance of the cell, the mechanism and phenomenon related to SOCs are discussed. From the safety perspective, several proposals are advanced for application and fire protection.