Heavy Lanthanide‐Induced Epitaxial Rock‐Salt Shells Enable Chemo‐Mechanically Robust Sodium‐Ion Cathodes

ABSTRACT Sodium‐ion batteries represent a promising technology for large‐scale cost‐effective energy storage. However, the deployment of O3‐type layered cathodes for high‐voltage operation is critically impeded by coupled chemo‐mechanical failure, manifesting as intergranular microcracking and irreversible lattice oxygen release. Here, we construct a coherent, epitaxial interface that mechanically arrests crack propagation and chemically locks surface oxygen against high‐voltage anionic redox. Guided by the Goldschmidt tolerance factor, we exploit the ionic radius mismatch of heavy late lanthanides to drive the formation of a dense, cation‐disordered rock‐salt shell. Advanced diagnostics, including O K ‐edge mRIXS and 3D electron diffraction, provide unambiguous evidence that this architecture effectively suppresses gas evolution and crack propagation. Consequently, the stabilized architecture limits voltage decay to 0.74 mV per cycle and retains 84.3% of the initial capacity after 200 cycles at 1C, nearly doubling the retention of the pristine counterpart (45.0%). This reconstruction strategy mitigates the chemo‐mechanical degradation of O3‐type cathodes, offering a practical pathway toward stable, long‐life sodium‐ion batteries.