Dehydration-induced CN^- defects enable adaptive water regulation in Prussian blue cathodes for high-performance sodium-ion batteries

Prussian blue analogues have emerged as a promising cathode material owing to their open‑framework structure and high theoretical capacities; however, the presence and dynamic evolution of crystalline water severely constrain their electrochemical performance. Despite extensive studies, a systematic understanding of dehydration-induced structural reconstruction and the role of water during electrochemical cycling remains elusive. Herein, we elucidate the structural evolution and water-regulated electrochemical behavior of Prussian blue (PB) through controlled dehydration-rehydration process. It is found that dehydration irreversibly triggers the fractures of the one-dimensional C ≡ N framework, forming novel crystal defects, that give rise to defect-associated water with lower binding energy. Density functional theory calculations corroborate the weakened water-framework interactions and reveal defect-mediated energetics. Upon rehydration, the optimized PB‑3 exhibits adaptive dehydration behavior during electrochemical cycling, in which defect-associated water is preferentially extracted upon charging, while coordinated and interstitial water persist as structural pillars, enabling high reversible capacity and long-term cycling stability. By precisely tuning the rehydration degree, PB‑3(H/L) achieves excellent long‑cycle life and rate performance. This work uncovers the mechanistic origin of dehydration-induced CN⁻ defect formation and establishes adaptive water regulation as a governing principle for achieving high-capacity and high-stability Prussian blue cathodes.