Unlocking Prussian Blue Analogues Inert‐Site to Achieve High‐Capacity Ammonium Storage

Abstract Aqueous ammonium‐ion batteries (AIBs) are considered a promising alternative for large‐scale energy storage due to their cost‐effectiveness and high safety. Prussian blue analogues (PBAs) are widely regarded as potential cathode materials for AIBs because of their high working potential and stable 3D framework. However, the low capacity of PBAs (≈60 mAh g⁻¹ in the existing reported works) hinders their further development. Herein, the ion insertion is first proposed to double the capacity of PBAs by unlocking the inert‐site. Using NH₄⁺‐rich copper hexacyanoferrate as a representative, trace amounts of induced ion in the electrolyte can change the electronic states of atoms at the inert site to achieve much higher capacity. What's more, the construction of NH₄⁺‐rich high‐entropy Prussian blue (N‐HEPBA) further facilitates structural stability. N‐HEPBA unlocked by corresponding induced ions can exhibit an impressive specific capacity (129 mAh g⁻¹ at 0.1 A g⁻¹), which is the highest and approximately twice that of reported PBAs for aqueous AIBs. Meanwhile, it delivers excellent cycling stability with nearly 100% capacity retention achieved over 1000 cycles at 2 A g⁻¹. This innovative unlocking method can provide an effective way to obtain high‐capacity PBAs in AIBs, thus promoting the development of large‐scale energy storage.