Nanoporous Iron-Based Prussian Blue Analogues for High-Performance Sodium-Ion Batteries

Iron-based Prussian blue analogues (Fe-PBAs) have garnered significant attention as cathode materials for sodium-ion batteries due to their high specific capacity (∼170 mAh g–1), environmental compatibility, and cost effectiveness. However, their performance is hindered by substantial crystalline water and structural defects, which result in the insufficient electrochemical activity of FeLS(C). The low contribution of FeLS(C) to the overall capacity, compared to FeHS(N), results in diminished battery performance and rapid cycling degradation. This study presents an innovative synthesis strategy for low-defect, high-sodium-content nanoporous Prussian blue using an oxalic acid-assisted single-iron-source method. Subsequent heat treatment effectively removes crystalline water and introduces a controlled number of defects, further modulating the nanoporous architecture and activating the FeLS(C) capacity. The resulting thermally treated nanoporous material (PBA-HT) exhibits a high stable discharge capacity of 120.2 mAh g–1, an initial Coulombic efficiency of 95.4%, and an outstanding cycling stability (70.3% capacity retention after 1000 cycles at 5 C). Density functional theory calculations reveal that heat treatment reduces the crystal field energy, thereby activating FeLS(C). In situ electrochemical impedance spectroscopy and galvanostatic intermittent titration technique analyses confirm a significant enhancement in diffusion kinetics, facilitated by the optimized nanoporous structure, following thermal treatment. Moreover, PBA-HT demonstrates stable operation at extreme temperatures (−20 and 50 °C), highlighting its practical potential and offering a synthesis strategy for high-performance nanoporous Prussian blue analogues.