Spatially Selective Substitution for Structural Stabilization of Sodium Layered Oxide Cathodes

O3-type layered transition metal oxides are considered promising cathode materials for sodium-ion batteries (SIBs) due to their high capacity and favorable Na⁺ storage characteristics. However, their practical application is severely hindered by structural instability associated with multiphase transitions during electrochemical cycling. Herein, we propose a spatially selective multi-element substitution strategy that induces spatially differentiated distributions of Mg, Cu, Ti, and B, thereby enhancing structural robustness. This spatially differentiated substitution architecture synergistically improves structural stability by concurrently inhibiting interfacial degradation and strengthening the lattice framework. The optimized composition (NaNi_0.4Mg_0.05Cu_0.05Mn_0.3Ti_0.2B_0.05O₂) enables a stabilized O3 → P3 phase transition, which relieves lattice distortion and suppresses structural collapse upon cycling. Density functional theory (DFT) analysis reveals that the strong covalency of B─O bonds is crucial for anchoring the P3 framework. Hence, it delivers superior high-temperature performance in half cells and durable cycling in full cells (85% capacity retention after 300 cycles at 0.5 C within 1.9-3.9 V). By elucidating the role of spatially selective substitution in structural stabilization, this work provides fundamental insights and paves the way for the design of advanced SIB cathodes.