Defect‐Regulated Cubic Prussian Blue Analog Cathodes With Exceptional Stability for Potassium‐Ion Batteries

Prussian blue analogs (PBAs) have attracted particular attention as promising high‐energy‐density cathodes for potassium‐ion batteries (PIBs). Yet their development is hindered by crystal‐degradation‐driven capacity fading. Here, a structure‐defect‐dual‐regulation strategy is developed for Fe‐based PBAs as a generalizable approach to advance scalable, high‐energy PIB cathodes, in which deliberately introduced cation vacancies at Fe 3+ sites in cubic PB (Iron(III) hexacyanoferrate) lattices are charge‐balanced by inserted potassium ions at neighboring octahedral interstitial sites. This approach, in turn, yields a low‐anion‐defect cubic structure, thereby increasing K + storage capacity and stabilizing the Fe─C octahedral framework. In turn, this design effectively suppresses K + insertion/extraction distortion and preserves low‐spin Fe redox activity, while enhancing conductivity and ion diffusion. Cation vacancies mitigate lattice strain and facilitate cooperative coupling between low‐spin and high‐spin Fe centers, thereby enabling a durable dual‐site redox process. The optimized cathode delivers a high reversible capacity of 124.5 mAh g −1 at 50 mA g −1 with an initial Coulombic efficiency of 92.21%, and exceptional long‐term cycling stability with a capacity retention ratio of 84.9% after 800 cycles at 100 mA g −1 .