Pyrrole‐Driven Structural Phase Engineering of Prussian Blue Analogs for Ultrastable and Highly Efficient Na‐Ion Storage

ABSTRACT Iron‐based Prussian blue analogs (PBAs) represent promising, facile‐to‐prepare, and low‐cost positive electrode materials for sodium‐ion batteries. However, their practical application is hindered by the markedly irreversible three‐phase transitions and severe lattice distortion that occur during sodium ion storage, leading to capacity limitations and diminished cycling stability. Herein, a simple pyrrole‐induced phase transition engineering strategy is proposed to successfully transform monoclinic PBAs into cubic polypyrrole‐PBAs (PPy‐PBAs). In situ X‐ray diffraction (XRD) testing and density functional theory (DFT) calculations reveal that the phase transition mechanism transforms from an unfavorable three‐phase process to a highly reversible two‐phase transition. Compared to complex three‐phase transition (PBAs), the efficient two‐phase transition (PPy‐PBAs) exhibits smaller lattice volume contraction/expansion and less Fe‐C/Fe‐N bond length stretching/shrinking, demonstrating remarkable structural stability. Moreover, this strategy effectively reduced the energy barrier for sodium‐ion (Na + ) migration, with the density of states crossing the Fermi level, significantly enhancing electronic conductivity, and thereby facilitating redox reactions and Na + transport kinetics within the material. The reversible two‐phase transition enables sustainable sodium‐ion storage through phase‐transition engineering. Compared with PBAs that undergo structural distortion and significant lattice strain, the optimized positive electrode material demonstrates a discharge capacity of 136 mAh/g and an ultralong stable cycling lifespan of 1700 cycles, establishing new possibilities for advanced sodium‐ion batteries.