Tailoring coordinated water in sodium-rich phase for high-performance Prussian blue cathodes

Sodium-ion batteries (SIBs) are promising low-cost alternatives to lithium-ion batteries due to their similar operating principles. Prussian blue analogues (PBAs) have emerged as attractive cathode materials because of their simple synthesis and scalability. However, the presence of interstitial and coordinated water in PBAs limits their practical application by compromising both capacity and cycling stability. To address this challenge, we report a sodium-rich PBA phase (PBA-200) with partial coordinated water removal, achieving a balance between electrochemical performance and structural stability. Partial removal of coordinated water alters the Fe^LS (Fe-C) coordination environment, activating additional redox activity, while also modifying the Fe^HS coordination environment, lowering its redox potential and enhancing stability. Concurrently, sodium enrichment mitigates cycling degradation caused by lattice contraction from water removal. The synergistic regulation of sodium ions and coordinated water in PBA-200 thus allows optimized trade-offs between capacity, rate capability, and cycling stability. Consequently, PBA-200 delivers an initial specific capacity of 105.5 mAh g^-1 at 5 C, exceeding PBA-120 with coordinated water (83.8 mAh g^-1) by 21.7 mAh g^-1. It maintains 72.1% capacity retention after 500 cycles and achieves a specific capacity of 82.3 mAh g^-1 even at 10 C, demonstrating simultaneous enhancement in capacity and cycling stability. This work presents a route for structural optimization of PBAs via combined control of coordinated water and sodium content.