Versatile Co₉S₈ Nanocoating Stabilizes Prussian Blue Analogue Cathodes for Aqueous Sodium-Ion Batteries

Prussian blue analogues (PBAs) show great promise as cathodes for aqueous sodium-ion batteries but suffer from severe structural degradation and sluggish kinetics. Herein, we develop an atomic layer deposition-driven interfacial engineering strategy to construct conformal Co₉S₈ nanocoatings (3-6 nm) on cobalt hexacyanoferrate (CoHCF) cathodes. Multiscale characterizations and theoretical calculations reveal that this artificial cathode-electrolyte interphase enhances electrochemical performance through a tripartite synergistic mechanism: (1) thermodynamic stabilization, which suppresses Fe dissolution by 85% through elevating Fe vacancy formation energy; (2) mechanical buffering, which alleviates mechanical strain by reducing lattice deformation by 59%; and (3) kinetic acceleration, which triggers a semiconductor-to-metal transition to boost electronic conductivity while accelerating Na⁺ diffusion, evidenced by a 1.5-order-of-magnitude increase in the minimum diffusion coefficient and a 38% reduction in the migration energy barrier. Consequently, the CoHCF@Co₉S₈-40//NaTi₂(PO₄)₃@C full batteries achieve exceptional capacity retention of 80.2% over 500 cycles (vs 3.8% for pristine counterparts), along with superior rate capabilities (72.0 mAh g^-1 vs 42.7 mAh g^-1 at 40 C, 1 C = 120 mA g^-1), outperforming most PBA-based cathodes. This work establishes a universal protocol for precision interfacial design in high-performance aqueous energy storage systems.