Thermal runaway behavior and features of LiFePO 4 /graphite aged batteries under overcharge

In this paper, the overcharge tests of 25 Ah LiFePO4/graphite batteries are conducted in an open environment and the overcharge-to-thermal-runaway characteristics are studied. The effects of current rates (C-rates: 2C, 1C, 0.5C, and 0.3C) and states of health (SOHs: 100%, 80%, 70%, and 60%) on thermal runaway features are discussed in detail. The overcharge process can be summarized into five stages based on the experimental phenomena (C-rate ≥ 1 and SOH ≥ 80%): expansion, fast venting after safety valve rupture, slow venting, intense jet smoke, and explosion, while the battery cannot explode at lower C-rates and SOHs. The maximum pressure increases with the increase in C-rate or SOH. There are five obvious inflection points in the voltage curve during overcharge process. The V1 (point B) of aged battery, corresponding to lithium plating on the anode, changes little with C-rates. It is slightly lower than that of the new battery. A sharp drop in voltage (point E) is probably due to the internal short circuit (ISC), caused by the local melting and rupture of the separator. It takes more than 2 minutes from the moment of ISC to thermal runaway regardless of the SOH, indicating that there are a few minutes to take safety measures if the voltage is an indication parameter. The onset temperature of thermal runaway decreases first and then increases as the SOH decreases from 100% to 60% during 1C constant overcharge tests. These results can provide guidance for the thermal management of the whole battery life cycle and the reuse of retired batteries.