Soft-Acid-Triggered Water Speciation Engineering: Reconfiguring the Aqueous Microenvironment in Prussian Blue Cathodes for High-Performance Sodium-Ion Batteries

Abstract

Prussian blue analogs (PBAs) are premier candidates for sodium-ion batteries, yet their electrochemical fate is inextricably linked to the heterogeneous nature of lattice water. Herein, we propose a ”Water Speciation Engineering” strategy guided by the Hard and Soft Acids and Bases (HSAB) theory to fundamentally resolve the water induced instability. We demonstrate that minor substitution of the soft-acid Cd2+ triggers a strategic reconfiguration of the aqueous microenvironment. Unlike conventional dehydration, the distinct coordination of Cd2+ with cyanide ligands induces localized lattice distortion and creates [Fe(CN)6] vacancies as thermodynamic anchoring sites. This process drives the topotactic migration of detrimental interstitial water into these vacancies, transforming it into chemically integrated coordinated water. The enriched coordinated water acts as a molecular dielectric, homogenizing the electrostatic potential to facilitate frictionless Na+ transport while simultaneously functioning as a mechanical buffer to dissipate lattice strain. Consequently, the engineered FeCdPBA cathode achieves an exceptional rate capability (106.7 mAh g–1 at 2 A g–1) and cycling longevity (88.0 % retention over 1000 cycles). This work provides a transformative paradigm for addressing the "aqueous dilemma" in open framework energy materials.