Resolving the Kinetics-Stability Trade-Off in Prussian Blue Analogues for Aqueous Ca-Ion Batteries: A High-Entropy Vacancy Caging Strategy

The development of Prussian blue analogues (PBAs) as cathode materials for aqueous Ca2+ batteries (CIBs) faces a critical trade-off between kinetics and stability: intrinsic vacancies that enhance ionic storage kinetics concurrently destabilize the crystal framework. This study addresses this issue by deliberately inducing vacancies at the low-spin Fe(CN)6 sites while engineering a high-entropy configuration at the high-spin transition metal (TM) sites. This high-entropy environment promotes d–d orbital coupling among the various TM centers, generating a delocalized electronic network. This network effectively shields local Coulombic repulsion through charge compensation, thereby suppressing the detrimental vacancy migration and aggregation that are the primary drivers of TM dissolution. The “vacancy caging” mechanism allows for the full realization of the kinetic benefits afforded by vacancies while neutralizing their associated destabilizing effects. The resultant high-entropy PBA (HEPBA) cathode achieves the highest reversible capacity (112 mAh g–1 at 50 mA g–1) and exceptional cycling stability (88.24% capacity retention over 24,000 cycles at 2000 mA g–1) reported to date for PBAs in aqueous CIBs. Our findings suggest a landscape to resolve the intrinsic kinetics-stability conflict that has long constrained the development of vacancy-rich cathode materials.