Ultralong‐Life Aqueous Ammonium‐Ion Batteries Enabled by Unlocking Inert‐Site of Medium‐Entropy Prussian Blue Analogs

Abstract Prussian blue analogs (PBAs) have been heralded as promising alternative cathodes for aqueous ammonium‐ion batteries (AAIBs) owing to their chemical flexibility at the molecular level and eco‐friendliness. However, the low capacity, irreversible phase, and structure transition are the enormous challenges toward practical application. Herein, an entropy‐regulating strategy is proposed to boost both specific capacity and structural stability by introducing Cu, Ni, Co, Mn, and Fe at the 4b sites in PBAs (CNCMF‐PBAs). The synergistic effect of randomly dispersed metal elements creates abundant redox centers and enhances structural durability. This inhibits the dissolution of transition metal elements and facilitates a highly reversible phase transition between cubic and tetragonal structures with minimal lattice strain (only 0.8%) for NH 4 + (de)intercalation. Moreover, it is interesting to find that this gradually growing cathode capacity roots from the activation of Cu 2+ /Cu + , Mn 3+ /Mn 2+ , and Ni 3+ /Ni 2+ pairs by entropy induction at low voltage region. As a result, the CNCMF‐PBAs cathode achieves a high reversible specific capacity of 101.2 mAh g −1 without attenuation over 45 000 cycles (lasting over 180 days) at 20 C. This study provides a substantial advance on PBAs cathode materials with excellent NH 4 + storage and rapid multi‐electron transfer kinetics.