Storage Failure Mechanism of Sodium‐Ion Full Cells: Unsteady Structural Transformation and Interfacial Chemical Reconstruction

ABSTRACT Resource‐rich sodium‐ion batteries (SIBs) stand as a highly promising energy storage alternative for large‐scale energy storage. Although research on their failure mechanisms and optimization under ideal conditions is relatively sufficient, studies on failure mechanisms under practical/harsh operating conditions (e.g., high‐temperature storage) remain severely insufficient, limiting safety improvement and commercialization. Combining multiple characterization techniques and electrochemical tests, this study reveals SIBs’ storage failure mechanism involving unsteady structural transformation and interfacial chemical evolution. The results show that the hard carbon anode exhibits significant extraction of Na⁺ and quasi‐sodium metal clusters—this greatly depletes the limited sodium source and is the primary cause of shortened service life; the anode interfacial layer undergoes repeated dissolution‐reformation, which disrupts ion transport channels and increases charge transfer impedance. The NaNi 1/3 Fe 1/3 Mn 1/3 O 2 cathode undergoes spontaneous Na⁺ intercalation (preferring bulk surfaces), and the stress induced by uneven Na⁺ distribution easily causes electrode particle cracking; increased oxygen vacancies and Mn reduction thicken the disordered rock‐salt phase, boosting Na⁺ diffusion resistance. Electrolyte decomposition forms a low‐modulus, low‐conductivity, organic‐rich cathode interfacial layer, exacerbating charge transfer impedance. This study clarifies the core mechanism of storage degradation in SIBs and provides key guidance for the selection and design of electrode materials for SIBs.