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

Abstract

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 NH₄⁺ 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 NH₄⁺ 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.