Mechanism of internal thermal runaway propagation in blade batteries

Blade batteries are extensively used in electric vehicles, but unavoidable thermal runaway is an inherent threat to their safe use. This study experimentally investigated the mechanism underlying thermal runaway propagation within a blade battery by using a nail to trigger thermal runaway and thermocouples to track its propagation inside a cell. The results showed that the internal thermal runaway could propagate for up to 272 s, which is comparable to that of a traditional battery module. The velocity of the thermal runaway propagation fluctuated between 1 and 8 mm s−1, depending on both the electrolyte content and high-temperature gas diffusion. In the early stages of thermal runaway, the electrolyte participated in the reaction, which intensified the thermal runaway and accelerated its propagation. As the battery temperature increased, the electrolyte evaporated, which attenuated the acceleration effect. Gas diffusion affected thermal runaway propagation through both heat transfer and mass transfer. The experimental results indicated that gas diffusion accelerated the velocity of thermal runaway propagation by 36.84%. We used a 1D mathematical model and confirmed that convective heat transfer induced by gas diffusion increased the velocity of thermal runaway propagation by 5.46%–17.06%. Finally, the temperature rate curve was analyzed, and a three-stage mechanism for internal thermal runaway propagation was proposed. In Stage I, convective heat transfer from electrolyte evaporation locally increased the temperature to 100 °C. In Stage II, solid heat transfer locally increases the temperature to trigger thermal runaway. In Stage III, thermal runaway sharply increases the local temperature. The proposed mechanism sheds light on the internal thermal runaway propagation of blade batteries and offers valuable insights into safety considerations for future design.