# An Advanced High‐Performance Ultrafast Ammonium‐Ion Aqueous Battery Based on Dual‐Metal Redox Open Framework Molecular Magnet

## TL;DR

A new battery cathode made from a molecular magnet offers fast performance, long life, and low cost for ammonium-ion batteries.

## Contribution

A dual-metal redox Prussian blue analog cathode enables ultrafast ammonium-ion aqueous batteries with high capacity and stability.

## Key findings

- KMnFeHCF delivers ~145 mAh/g at 3 A/g and retains 50% capacity after 1850 cycles.
- Ammonium insertion causes lattice expansion and fast ion transport with a 1.29 eV migration barrier.
- A full cell with graphite anode achieves 71 mAh/g at 1.25 A/g and 51 mAh/g at 2.2 A/g.

## Abstract

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.

The Prussian Blue Analogue molecular magnet KMnFeHCF is demonstrated as a high‐performance cathode for ultra‐fast aqueous ammonium‐ion batteries. A full cell using KMnFeHCF and graphite delivers ~71 mAh g−1 at 1.25 A g−1 and ~51 mAh g−1 at 2.2 A g−1, retaining 50% capacity after 1850 cycles. Its scalability, cycling stability, and low cost offer strong application potential.