High-speed synchrotron radiography of nail penetration-induced thermal runaway: Understanding the explosive behavior of commercial sodium-ion batteries with NFM cathode

The dynamics of mechanically initiated thermal runaway (TR) events in cylindrical 18650 cells with NFM (Na(Ni_1/3Fe_1/3Mn_1/3)O₂), LFP (LiFePO₄), and NMC532 (LiNi_1/2Mn_1/3Co_1/5O₂) cathode chemistries were investigated using high-speed synchrotron X-ray imaging. Structural similarity index measures (SSIM) were employed to identify and track rapid structural changes. In this manner, thermal decompositions and internal propagation dynamics, influencing the safety mechanisms of the cells, were studied. This lead to two major findings: (I) Among NFM, LFP, and NMC532 cells, the TR-characteristics differ significantly in temperature and internal propagation speed. Internal safety mechanisms appear, however, visually similar. Among all samples, LFP cells exhibit higher safety performance concerning the initiation of TR by nail penetration and the progression of TR. (II) The NFM cells used in this study displayed an almost explosive TR. This finding appears counterintuitive on a first glance, since sodium-ion batteries are usually considered safe. High-speed imaging revealed that the explosive TR is not necessarily caused by the thermochemical decomposition reactions, but rather by a failure of the venting mechanism. This results in a significant pressure buildup within the cell upon TR initiation and eventually a severely violent TR. These results underline that battery safety depends on many factors and not solely on optimized cell chemistries or materials.