Tailoring Unconventional Cyanogen Defect in High-Entropy Prussian Blue Cathode Material for Advanced Sodium-Ion Batteries.

High-entropy Prussian blue analogues (PBAs) are promising cathodes for sodium-ion batteries (SIBs). However, inherent [Fe(CN)6]4- defects deteriorate electrochemical kinetics and phase stability. Herein, an unusual cyanide (CN-) vacancy is tailored in high-entropy PBA (HE-Cu-PA, Na1.58Mn0.191Fe0.2Co0.195Ni0.2Cu0.19[Fe(CN)5.85]), constructed via a phytic acid (PA) assisted coprecipitation method, as cathode material for SIBs. The precisely designed high-entropy composition with CN- defects create adaptive coordination flexible sites and local electronic delocalization regions, synergistically enhancing structural stability, electrochemical dynamics, and redox reversibility. The large-sized [Fe(CN)6]4- vacancy in PA-free high-entropy PBA (HE-Cu) exhibits poor electronic transfer capability and accumulated lattice strain, while static local lattice distortion generated by Ti3+N6 octahedron in high-entropy composition (HE-Ti-PA) causes large lattice stress and Na-ion diffusion barrier. The complex structural evolution (monoclinic ↔ cubic ↔ tetragonal) originated from Jahn-Teller effect and octahedron instability can be completely restrained in HE-Cu-PA, achieving a zero-strain solid-solution Na-ion storage mechanism, where Mn, Fe, Co, and Cu-ions act as redox sites for charge compensation. Therefore, HE-Cu-PA delivers high initial capacity of 117.6 mAh·g-1, superior rate capability and ultra-long lifespan over 6000 cycles with ultra-low decay-rate of 0.0085% per cycle. And ultra-long cycling lifetime over 4000 cycles can be acquired for high-energy-density (338.0 Wh·kg-1) quasi-solid-state Na-ion full batteries.