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

Medium-entropy Prussian blue analogues (ME-PBAs) are synthesized through an entropy-regulated strategy. The ME-PBAs can not only endow highly reversible phase transitions, but also enable the activation of Cu²⁺/Cu⁺, Mn³⁺/Mn²⁺, and Ni³⁺/Ni²⁺ redox pairs by entropy induction at low-voltage. Upon activation, it delivers a high reversible capacity of 101.2 mAh g⁻¹ at 20 C rate, demonstrating its exceptional stability.

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 NH₄ ⁺ (de)intercalation. Moreover, it is interesting to find that this gradually growing cathode capacity roots from the activation of Cu²⁺/Cu⁺, Mn³⁺/Mn²⁺, and Ni³⁺/Ni²⁺ 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 NH₄ ⁺ storage and rapid multi-electron transfer kinetics.