Surface Engineering through In Situ Construction of CoxB‐Spinel Dual Coating Layers for High‐Voltage Stable Sodium‐Ion Batteries

Abstract Transition metal layered oxides (Na x TMO 2 ), boasting a high theoretical specific capacity and affordability, have emerged as prominent cathodes for sodium‐ion batteries (SIBs). Their potential, however, is hindered when operating at higher voltage range (4.0–4.3 V) due to irreversible phase transition, heterogeneous surface reconstruction, and side reaction. Herein, using a straightforward room‐temperature liquid‐phase reductive method, a dual conformal protective layer is in situ constructed on the surface of NaNi 1/3 Fe 1/3 Mn 1/3 O 2 (NFM). This layer comprises both a spinel structure and an amorphous Co x B coating, thereby achieving a layered‐spinel‐Co x B configuration. The spinel structure provides 3D Na + transport channels and works as a pillar to anchor the intrinsic layered structure. Simultaneously, the external Co x B layer effectively mitigates O loss, transition metal ion dissolution, and undesired side reactions on the surface. Benefiting from the synergistic effects on both the material's bulk and surface, the 1wt% Co x B coated cathode displays superior stability. After 300 cycles, the capacity retention is 79.6% between 2 and 4 V, significantly outperforming pristine‐NFM's(p‐NFM) 51.4%. When charged to 4.3 V, its capacity retention stands at 70%, much higher than that of p‐NFM (51.2%). This work provides new insights into exploiting high‐voltage stable cathode through constructing a dual conformal protective layer for high energy density SIBs.