Abstract: The safety of lithium batteries is a core requirement for the airworthiness certification of electric aircraft. Developing an effective thermal runaway early warning system is of great significance for ensuring the safety of lithium battery systems in aviation. This study analyzes the changes in the impedance phase angle of lithium battery electrochemical impedance spectroscopy (EIS) with temperature and state of charge (SOC) through experiments. A thermal runaway experimental platform is used to monitor the changes in AC impedance during the thermal triggering process of lithium batteries at characteristic frequencies in real time. At the same time, the Raman spectrum changes of organic solvent gases discharged in the early stage of thermal runaway after the battery safety valve is opened are monitored. The experimental results show that the impedance phase angle at a frequency of 200 Hz is strongly correlated with battery temperature and is less affected by the state of charge of the battery. The minimum value of battery impedance amplitude and the phase angle at this time are determined as the first-level early warning characteristic points, the sudden increase in amplitude as the second-level early warning characteristic points, the sharp drop in voltage as the third-level early warning characteristic points, and the minimum peak height difference of organic solvent of 358 a.u and the peak height difference of N₂ of 131 a.u selected through gas analysis as the fourth-level early warning characteristic points for gas. An integrated four-level early warning strategy for lithium battery thermal runaway is proposed based on the multi-parameter characteristic values of battery impedance, voltage, and early thermal runaway gases, which provides a reference for the design of early warning systems for thermal runaway in aviation lithium batteries.

Key words: aerospace lithium battery, thermal runaway, tiered early warning, alternating current impedance, Raman spectros copy