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

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.