Metal-Organic Open-Framework Molecular Magnet Based on Vanadium Hexacyanoferrate Prussian Blue Analogs as Cathode Material for Advanced Potassium Ion Aqueous Battery

The development of high-performance electrode materials is vital for advancing next-generation energy storage systems. In this study, we successfully synthesized an open-framework vanadium hexacyanoferrate (VHCF) compound and explored its application as a cathode for aqueous potassium-ion batteries (AKIBs). Infrared and Raman studies confirmed VHCF formation through characteristic –C≡N– stretching (≈1900–2200 cm⁻¹), while structural analysis revealed a stable face-centered cubic (Fm3m) framework with open tunnels enabling efficient K⁺ diffusion. Magnetic measurements and neutron depolarization confirmed the compound's paramagnetic nature. X-ray photoelectron, Mössbauer, and synchrotron X-ray absorption studies revealed Fe²⁺ (low spin) and V⁴⁺ oxidation states, with evidence of ligand-to-metal and metal-to-ligand charge transfer. Electrochemical analysis showed high specific capacity (≈121 mAh g⁻¹ at 0.5 A g⁻¹) with ≈99% coulombic efficiency, and ≈96% efficiency at 2 A g⁻¹ with 45% capacity retention after 620 cycles. The open-tunnel-like network of V(O)-Fe(CN)₆ is responsible for the compound's higher cyclic stability and reversibility. However, ex-situ X-ray diffraction showed slight amorphization and lattice contraction from 10.22 Å to 10.13 Å after 350 cycles. The K⁺ diffusion coefficient (D_k⁺) obtained from the galvanostatic intermittent titration technique displayed a V-shaped charging trend (9.46 × 10⁻¹¹ cm²·s⁻¹), increasing to 1.96 × 10⁻⁸ cm²·s⁻¹ during discharge. Density functional theory calculations indicated a low K⁺ migration barrier energy (≈0.45 eV). With cost-effective synthesis, a robust metal-organic framework, and excellent structural, magnetic, and electrochemical properties, VHCF is a promising cathode material for future AKIBs.