Doubled Capacity and Cyclability of Berlin Green Cathodes with Low‐Spin Fe Activation and NH₄⁺ Co‐Insertion for Rocking‐Chair Fe‐Ion Batteries

Abstract Rechargeable Fe‐ion batteries with overwhelming cost/environmental benign superiorities hold great promise in large‐scale charge‐storage applications. However, there is still a lack of competitive cathodes with prominent capacity and long‐term cyclability. To overcome these constraints, a unique Fe²⁺/NH₄⁺ co‐insertion strategy is proposed, enabling both doubled reversible capacity and cyclic life of Berlin green (FeHCF) cathodes. The Fe²⁺ insertion proves to induce FeHCF lattice contractions, alter ligand environment/activation energies of low‐spin Fe^III, and hence boost the redox conversion depth, while NH₄⁺ co‐insertions help to elevate the cathode capacities by utilizing Fe²⁺‐unoccupied reaction sites. Besides, they generate hydrogen bonds between inserted NH₄⁺ ions and N atoms in FeHCF, capable of vastly strengthening the cathode's cyclic durability. Cathodes in such collaborative dual‐cation electrolytes exhibit a remarkable capacity of 162 mAh g⁻¹ (4 A g⁻¹) and far prolonged operation lifespan (over 1000 cycles) when compared to single‐cation cases. The assembled full cells demonstrate impressive energy densities up to 70.4 Wh kg⁻¹, and far superior capacity retention of 88% after all the cycling. This work may offer a simple/feasible, and effective route to render high‐capacity and long‐lasting cathodes for future/practical Fe‐ion battery systems.