An Advanced High-Performance Ultrafast Ammonium-Ion Aqueous Battery Based on Dual-Metal Redox Open Framework...

We report a potassium manganese-iron hexacyanoferrate (KMnFeHCF) Prussian blue analog molecular magnet as a promising, low-cost, environmentally friendly cathode for metal-free ammonium-ion aqueous batteries. KMnFeHCF crystallizes in a face-centered cubic structure (Fm3m, lattice constant ≈ 10.19 Å) and exhibits a weak ferromagnetism, with Mössbauer spectroscopy confirming a mixed-valence Fe⁺³/Fe⁺² states. The material delivers a high specific capacity of ~145 mAh/g at 3 A/g, and ~130 mAh/g at 5 A/g along with excellent coulombic efficiency of 97%. Electrochemical performance is governed by reversible Fe²⁺/Fe³⁺ and Mn²⁺/Mn³⁺ redox transitions supported by the open-framework tunnel-like crystal structure which effectively accommodates structural distortions during ammoniation/de-ammoniation. X-ray photoelectron spectroscopy confirms mixed +2/+3 oxidation states for Fe and Mn. Density functional theory calculations show ammonium insertion induces tensile strain along Fe-C≡N-Mn linkages, expanding the lattice. The calculated migration barrier for NH₄⁺ transport between 8c sites via the 24d site is 1.29 eV, reflecting favourable ion mobility. A full cell with a graphite anode achieves 71 mAh/g at 1.25 A/g and 51 mAh/g at 2.2 A/g, operating efficiently up to 1.8 V. It retains 50% capacity after 1850 cycles. Galvanostatic intermittent titration technique reveals a diffusion coefficient of 8.28 × 10^-8 cm²/s, confirming fast transport kinetics.