Redox Chemistry Enables Excellent Capacity and Ultra Long Life Aqueous Ammonium Ion Batteries

Ammonium-ion batteries are promising for energy storage with low cost, high ionic conductivity, and excellent safety. However, their long-term cycling stability is far from the industrial application expectation owing the lack of suitable electrode materials. Here, we report the redox chemistry enables excellent capacity and ultra long life of Prussian blue analogues as performant electrode materials by regulating their t_2ge_g occupancy of M' d-orbitals. This work shows that the electronic configuration of M' is crucial for redox reversibility, structural robustness, and ultimately tolerance for NH₄ ⁺ storage stability. For (NH₄)₂NiFe(CN)₆, the metallic center with fully filled t_2g orbitals ensures the stable structure, while the half-filled e_g orbitals in the high-spin states enhance the axial transport of electrons. As a result, the key metric for the best one (NH₄)₂NiFe(CN)₆ delivers a reversible capacity of ∼70 mAh g^-1, superior rate performance, and outstanding long-life performance, sustaining stable output for over 10 000 h at 100 mA g^-1 and 30 000 cycles at 1000 mA g^-1. In situ XRD and first-principles calculations further confirm highly reversible redox processes. This study presents a novel and effective strategy for improving the performance and stability of electrode materials, offering valuable insights for the development of next-generation energy storage systems.