Dual-Site Spin State Modulation in Iron-Based Prussian Blue Cathode Enabling Improved Sodium Storage

Prussian blue (PB) and its analogues are promising cathode materials for sodium-ion batteries (SIBs), yet their practical application is hindered by uncontrolled [Fe(CN)₆] vacancies and interstitial water, which lead to unsatisfactory cycling stability and capacity utilization. Herein, we report a ligand engineering strategy to simultaneously modulate the spin states of both low-spin (LS) and high-spin (HS) Fe sites in PB. By introducing ─CN vacancies and terminal ─NH₂ groups via a pentacyanide precursor, a modified PB (denoted as PPB) is achieved. Spectroscopic and computational analyses confirm a dual-site spin transition in which both LS and HS Fe sites predominantly convert into an intermediate-spin (IS) state. As a result, the PPB cathode exhibits impressive rate capability (129 and 92 mAh g^-1 at 0.05 and 2 A g^-1, respectively) and cycling stability (70% retention after 4000 cycles at 2 A g^-1), significantly outperforming the routine hexacyanide-based PB. Density functional theory calculations reveal that the ─NH₂ groups serve as preferential adsorption sites for Na⁺, while ─CN vacancies lower the migration barrier, which collectively enhance the Na⁺ storage kinetics. This work presents a prudent ligand engineering strategy for designing high-performance cathode materials through deliberate spin state manipulation.