Unravelling the Formation and Evolution Mechanism of Solid Electrolyte Interphase Toward Stable and Rapid Sodium Storage

ABSTRACT The development of sodium‐ion battery (SIB) anode materials is limited by the instability of the solid electrolyte interphase (SEI) and unclear degradation mechanisms, which severely restrict their rate performance and cycle life. To address these challenges, we synthesize Co 1.29 Ni 1.71 O 4 /NiFe 2 O 4 heterostructures as SIB anodes through high‐temperature calcination of a tri‐metallic layered double hydroxide. Density functional theory and molecular dynamics (MD) simulations reveal that this design can rationally modulate the electronic structure of anodes, as well as the absorption and diffusion properties of ions, which promote an efficient desolvation process and induce the directional formation of a rich inorganic SEI layer, thereby enabling rapid and stable sodium storage. Notably, by investigating the variation in SEI components during cycling, we have, for the first time, established a direct correlation between the evolution of the key component NaF in the SEI layer and the characteristic four‑stage (increase‑drop‑recovery‑decay) capacity fluctuations in SIBs. Consequently, a new perspective on battery degradation mechanisms involving temporal and spatial heterogeneity at the electrode/electrolyte interface is proposed. These groundbreaking insights offer crucial theoretical foundations and new perspectives for a deeper understanding of interface formation and evolution mechanisms. They also provide valuable guidance for designing high‐performance SIBs through electrode interface engineering strategies.