Lithium-ion cells are well-established on the market for portable electronic devices and electric vehicles and their safety has been extensively studied. In contrast, a potential alternative to lithium-ion technology, sodium-ion cells, has yet to be thoroughly investigated, particularly concerning their safety and thermal behaviour during thermal runaway incidents. For this reason, one commercial nickel-rich lithium-ion cell type and one commercial sodium-ion cell type, both in the 18650 format, were examined and compared regarding their thermal runaway behaviour.

The primary focus of this study was on the comprehensive venting behaviour exhibited by nickel-rich lithium-ion and sodium-ion cells during thermal runaway. Experiments were conducted using a developed test bench to measure the recoil force during venting, allowing gas characterisation. A high-speed camera with a resolution of 10,000 fps was also employed to capture the thermal runaway process in detail.

The results show that investigated sodium-ion cells exhibit different behaviour during thermal runaway compared to nickel-rich lithium-ion cells. The reproducibility of these results may suggest a higher thermal stability of commercial sodium-ion cells compared to nickel-rich lithium-ion cells.

The high-speed camera measurements reveal that the investigated sodium-ion cells do not exhibit gas ignition during thermal runaway. Investigated sodium-ion cells do not produce any sparks during thermal runaway. Furthermore, the vent gas temperature of the investigated sodium-ion cells is lower compared to that of nickel-rich lithium-ion cells, and they demonstrate lower total mass loss during thermal runaway than the investigated lithium-ion cells. However, the measured recoil force of the investigated sodium-ion cells does not significantly differ from that of the investigated lithium-ion cells.

The measurement results suggest that the investigated sodium-ion cells in cylindrical format appear to be safer than investigated lithium-ion cells, owing to the absence of gas ignition and the lower temperatures of vented gas during thermal runaway.

Two high-speed camera videos are available on the TUM-website: https://mediatum.ub.tum.de/1737912 for lithium-ion cell and https://mediatum.ub.tum.de/1737942 for sodium-ion cell.