Berlin Green with tunable iron content as ultra-high rate host for efficient aqueous ammonium ion storage

Prussian blue analogues (PBAs) are regarded as promising cathode materials for ammonium-ion batteries (AIBs) because of their low cost and superb theoretical capacity. However, its inherently poor conductivity and structural collapse can significantly limit the enhancement of rate property and cycling stability. In this work, Berlin Green (BG) electrode materials with similar wool-like clusters were constructed by direct precipitation method to accelerate the kinetic, which realizes outstanding cycling stability. Berlin Green with the appropriate amount of iron (BG-2) has a fast ion transport channel, enhanced structure stability, highly reversible insertion/extraction of NH₄⁺, and fine electrochemical reaction activity. Benefiting from the unique architecture and component, the BG-2 electrode shows an excellent rate performance with a discharge/charge specific capacity of 60.1/59.3 mAh g^-1 at 5 A g^-1. Even at 5 A g^-1, BG-2 exhibits remarkable cycling stability with an initial discharge capacity of 59.5 mAh g^-1 and a capacity retention rate of approximately 76% after 30,000 cycles. The BG-2 reveals exceedingly good electrochemical reversibility during the process of NH₄⁺ (de)insertion. BG materials indicate huge potential as a cathode material for the next generation of high-performance aqueous batteries.