Strain-Regulated Lattice Integrated with a Self-Purifying Interface for Ultra-Stable O3-Type Sodium Cathodes

The commercialization of O3-type layered cathodes is obstructed by two intertwined degradation pathways: anisotropic lattice strain accumulation causing mechanical failure and a chemically unstable surface triggering relentless parasitic reactions. Herein, we devise a dual-aspect strategy to concurrently address these coupled challenges through synergistic bulk and interface engineering. We employ Cu2+ as a near-surface lattice strain buffer to deliberately expand Na layers and mitigate Jahn–Teller distortion, which uniformizes internal strain evolution and suppresses crack formation, as directly visualized by in situ XRD and post cycling strain mapping. Simultaneously, an in situ formed phosphate coating functions beyond a passive barrier as an active interfacial purifier, scavenging residual carbonates to construct an electrochemically pristine and stable cathode electrolyte interface. This mutually reinforcing, inside-out synergy enables the modified cathode to deliver a capacity retention of 76% after 300 cycles (vs 53% for the pristine) and exceptional air stability (93% capacity retention after 30 days). This work transcends the conventional additive approach of substitution and coating, establishing a codesign principle that integrates physical strain regulation with interfacial chemistry control for developing ultra-stable layered oxide cathodes.