Structural–redox regulation suppresses dual degradation for air-stable layered sodium oxides

Layered sodium transition-metal oxides are promising cathode materials for sodium-ion batteries but suffer from rapid air-induced degradation, particularly in Ni-rich compositions. Here, P2-type Na_2/3Ni_1/2Mn_1/2O₂ (NaNM) is employed as a model system to elucidate the coupled surface–bulk degradation pathway at the particle level. Air exposure triggers spontaneous surface Na⁺ loss, predominantly compensated by Ni oxidation, which propagates inward to generate a radial Ni valence gradient. Guided by this mechanistic insight, low-electronegativity Ti⁴⁺ incorporation into Na_2/3Ni_3/10Mn_1/2Ti_1/5O₂ (NaNMT) limits air-induced surface Na⁺ depletion and mitigates the resulting bulk redox heterogeneity. Consequently, the Ti-modified NaNMT exhibits only 4.0% first-discharge capacity loss after air exposure (vs. 18.5% for NaNM) and retains 75.9% of its initial capacity after 300 cycles at 1C (vs. 58.2% for NaNM). This work establishes a particle-resolved framework for understanding air-induced degradation in layered sodium oxides and provides a rational strategy for air-stable, high-performance cathode design.