Suppressing Dissolution of Solid Electrolyte Interphase Enables Highly Stable Sodium‐Ion Batteries

ABSTRACT Maintaining the integrity of solid electrolyte interphase (SEI) is pivotal for ensuring the cycling stability of sodium‐ion batteries (SIBs). However, conventional SEIs exhibit subtle yet persistent dissolution during electrochemical cycling, accelerating battery performance degradation. Meanwhile, the correlation between SEI solubility, composition evolution, and cycling reversibility remains ambiguous. Herein, we construct a robust, inorganic‐rich SEI layer via boron‐containing ion‐induced interfacial chemistry, which effectively suppresses SEI dissolution by minimizing electrode–electrolyte contact and inhibiting the repeated generating‐dissolving process of organic components. We further elucidate the correlation between SEI components and its solubility, and quantify that the average capacity loss caused by the dissolution of organic‐rich SEI is 1.36 times that of inorganic‐rich SEI. Benefiting from the stabilized SEI, the hard carbon (HC) anode delivers excellent cycling stability, retaining 81.9% capacity after 400 cycles even at 60°C. Moreover, the HC||O3‐NaNi 1/3 Fe 1/3 Mn 1/3 O 2 (NNFMO) full cell achieves 78.6% capacity retention over 300 cycles. This study provides fundamental insights into SEI dissolution mechanisms and presents an effective strategy to enhance the durability of SIBs.