Facet‐Engineered Na ₂ FeSiO ₄ Cathodes Realizing Zero‐Strain Operation and Ultra‐Stable Sodium Storage

ABSTRACT Designing polyanionic cathodes that simultaneously deliver structural stability and fast Na + transport remains a key challenge for sodium‐ion batteries. Here, we present a facet‐directed lattice engineering strategy that enables controlled crystallographic orientation, defect chemistry, and ion‐transport kinetics in Na 2 FeSiO 4 (NFS). Solvent coordination coupled with thermodynamically guided calcination yields a cubic P 2 1 3 phase with preferential exposure of the (210)/(211) facets. Structural analyses and density functional theory identify these planes as dominant Na + migration highways with ultralow barriers (0.21–0.25 eV) and reversible elastic lattice breathing, supporting near‐zero‐strain operation during cycling. The optimized cathode delivers a high reversible capacity of 173.6 mAh g − 1 at 0.2 C with 92.3% retention, minimal voltage hysteresis, and excellent rate capability. Beyond facet effects, the engineered lattice environment stabilizes the Fe 2 + /Fe 3 + redox balance and an optimized oxygen‐vacancy landscape, while the nanosheet architecture shortens diffusion lengths and buffers interfacial strain; concurrently, in situ–derived carbon forms a percolating conductive network that promotes electron transport and stabilizes the cathode–electrolyte interface (CEI). In pouch‐cell configuration, the material retains >80% capacity with the lowest charge‐transfer resistance among all variants, underscoring scalability and durability. This facet‐governed design concept links atomic‐scale crystal engineering with practical electrodes, offering a general route to high‐rate, long‐life sodium‐ion batteries.