Hierarchical Surface‐to‐Bulk Architecture for High‐Performance O3‐Type Sodium Cathodes

O3‐type layered oxide cathodes suffer from surface chemical instability and sluggish Na + transport within the O‐type framework, limitations that are aggravated by humid‐air exposure and fast‐charging operation, leading to severe interfacial degradation and rapid capacity decay. Herein, we propose a mild ethylene‐glycol–assisted treatment that in situ constructs a coherent surface‐to‐bulk architecture within NaNi 1/3 Fe 1/3 Mn 1/3 O 2 (NFM) particles, comprising a nanoscale surface rock‐salt layer, a depth‐dependent Na‐vacancy gradient, and a Na‐deficient bulk. This hierarchical configuration locks the surface chemistry while opening continuous Na + percolation pathways across the surface–bulk junction, thereby flattening radial (de)sodiation heterogeneity and steering a more uniform, highly reversible phase evolution during prolonged cycling. As a result, the modified sample exhibits outstanding fast‐charging performance, delivering 107.6 mAh g −1 at 5C (600 mA g −1 ) with 81.6% capacity retention after 400 cycles. This work highlights gradient interphase coupled with Na‐vacancy engineering as an effective strategy to develop high‐performance layered oxide cathodes for sodium‐ion batteries.