Ligand Field Induced Electronic Structure Modulation Enables Triple Redox Activity in Prussian Blue Analogue...

Ligand field induced electronic structure modulation provides an effective pathway to tune the electrochemical functionality of Prussian Blue Analogue molecular magnets. Partial substitution of Fe by Co²⁺ in Fe-C≡N-Fe framework forms cobalt iron hexacyanoferrate (KCoFeHCF), significantly modifying the local ligand field and metal-cyanide bonding through strong Co/Fe-N≡C-Fe linkages. It alters d-orbital splitting and spin configuration, producing an additional low-spin Fe³⁺ state alongside high-spin Fe³⁺ and low-spin Fe²⁺ states. Enhanced charge redistribution and electron delocalization across the framework stabilizes these states and increase the crystal field stabilization energy to -5.2 Δ₀ + 6P, compared with -2.4 Δ₀ + 3P for KFeHCF and -2.8 Δ₀ + 4P for KCoHCF, improving structural stability. Density functional theory confirms preferential electron redistribution toward low-spin Fe sites. Importantly, ligand field modulation activates the otherwise inaccessible Co²⁺/Co³⁺ redox couple by lowering its operating potential from ∼1.92 to ∼0.6 V, enabling a triple-redox mechanism involving Co-N, Fe-N, and Fe-C environments. KCoFeHCF delivers ∼148 mAh/g at 2.5 A/g with 88% capacity retention after 180 cycles. Full cell achieves ∼58 Wh/ kg at ∼1505 W/kg and retains ∼60% capacity after 5000 cycles, demonstrating excellent electrochemical durability and promising potential for aqueous potassium-ion batteries.