Vacancy‐Induced Activation of Low‐Spin Iron in Prussian Blue for High‐Capacity Aqueous Ammonium‐Ion Storage

Aqueous ammonium‐ion batteries (AAIBs) represent a safe, sustainable, and cost‐effective energy storage solution, yet their development hinges on identifying high‐capacity, stable cathode materials. Prussian blue analogues (PBAs), with their open framework and multi‐electron redox capability, are promising candidates for NH 4 + storage. However, their capacity is fundamentally limited by the electrochemically inert low‐spin Fe (Fe LS ) sites, which contribute minimally to redox activity. In this work, it is demonstrated that the kinetic inactivity of Fe LS is not intrinsic but can be partially unlocked through targeted defect engineering. The introduction of [Fe(CN) 6 ] 4− vacancies and coordinated water molecules significantly improves NH 4 + diffusion and charge transfer, enabling the participation of Fe LS in the electrochemical reaction. The resulting defect‐rich FeHCF material (PB‐0) achieves a high reversible capacity of 120 mAh g −1 , exceeding conventional PBA benchmarks. Furthermore, by optimizing the electrolyte composition, the system delivers outstanding long‐term cycling stability (96.4% capacity retention after 500 cycles) and excellent rate performance (82.4% capacity retention at 500 mA g −1 ). These results demonstrate how defect engineering can improve the performance of PBA‐based electrodes for AAIBs.