Thermal runaway propagation in cylindrical sodium-ion batteries via radial and axial in-situ high-speed synchrotron radiography

The increasing relevance of sodium-ion batteries (SIBs) and the limited knowledge of the thermal runaway (TR) characteristics requires an improvement in safety understanding. This study presents a novel approach for understanding the three-dimensional TR propagation in SIB cells during mechanical abuse caused by nail penetration using complementary radial and axial projection geometries during high-speed synchrotron X-ray radiography. Structural progression of three SIB cells and a lithium-ion battery (LIB) reference cell across the full TR sequence were characterized and compared with pre- and post-CT-Analysis and quantitative frame-to-frame analysis. The results show a longer TR duration within nickel iron manganese oxide (NFM) SIB cells (1.47 s - 8.80 s) compared to the nickel manganese cobalt oxide (NMC) LIB cells (1.15 s - 1.33 s) and an absence of early-stage jelly roll delamination in the SIB cells. Terminal design could be identified as a critical factor to the failure outcome, with the two-stage current interrupt device (CID) designs leading to catastrophic explosive failure, while the simplified CID managed a controlled venting. These findings demonstrate that safety behavior is strongly influenced by cell design rather than chemistry alone and highlights the importance of design optimization in SIB safety development.