Role of Crystal Water in Prussian Blue Analogs: Calcium Ions Stabilizers Enable High-Performance Sodium-Ion Batteries

Prussian blue analogs (PBAs) have emerged as highly promising cathode materials for large-scale applications of sodium-ion batteries. PBAs usually contain the crystal water, which is thought to cause a series of negative effects and shall be removed as much as possible. However, the role of crystal water on the structure and the mechanism of its evolution remains poorly understood. In this study, the electrochemical in situ doping of PBAs during battery cycling is achieved by adding Ca²⁺ stabilizers into the electrolyte. These findings reveal that crystal water molecules crucially compensate for [Fe(CN)₆]^4- vacancies, serving as structural pillars that maintain lattice integrity. The optimized PBAs cathode demonstrates a discharge specific capacity of 121.0 mAh g^-1 at 0.2 C with a capacity retention of 91.38% after 500 cycles at 1 C, and even 98.27% after 2000 cycles at a high current density of 10 C, which provides new insights into the design of high-performance PBAs cathode materials.