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 NaNi1/3Fe1/3Mn1/3O2 (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.