Decoupling of O3‐Layered Cathode Stability from System‐Driven Thermal Runaway in Sodium‐Ion Batteries

Although next‐generation sodium‐ion batteries (SIBs) possess more stable cathode materials than lithium‐ion batteries (LIBs), thermal runaway (TR) remains a critical barrier to SIB applications. To resolve this safety paradox, atomic‐scale investigations are conducted on the O3‐NaNi 1/3 Fe 1/3 Mn 1/3 O 2 (NFM) cathode. Combining accelerating rate calorimetry (ARC) and transmission electron microscopy (TEM), the material‐intrinsic resilience is decoupled from cell‐level failure mechanisms. The ARC analysis revealed high safety metrics of the NFM/hard carbon pouch cells; specifically, the maximum TR temperature ( T 3 ) stabilizes at ≈310 °C (vs. >800 °C in Ni‐rich LIBs) and the TR onset time extends to ≈40 h. As demonstrated in the TEM analysis, the NFM cathode maintains its structural integrity at 310 °C under inert conditions, although post‐TR cathodes undergo catastrophic “brush‐like” fragmentation with rock‐salt/spinel phase transformation. This degradation is mechanistically attributed to reductive attack by electrolyte decomposition products and anode‐derived gases (H 2 /CO), which overwhelm the inherent stability of the cathode. To guarantee the inherent safety of SIBs, SIB design based on cathode thermochemistry alone must shift to the co‐optimization of flame‐retardant electrolytes, gas scavengers, and anode passivation.