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

ABSTRACT

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, Na _1.58 Mn _0.191 Fe _0.2 Co _0.195 Ni _0.2 Cu _0.19 [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 Ti ³⁺ N ₆ 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.