Achieving a high-performance Prussian blue analogue cathode with an ultra-stable redox reaction for ammonium ion storage

Aqueous rechargeable batteries with advantages of safety, low cost, and environmental kindness have displayed high feasibility of practical applications for large-scale energy storage. Developing high-performance electrode materials is a necessary gateway to commercially available batteries. Here, we demonstrate the controlled synthesis of sodium iron hexacyanoferrates, NaFe^IIIFe^II(CN)₆ (Na-FeHCFs). Ball-cutting Na-FeHCF nanocubes are first synthesized and used as a cathode material for aqueous ammonium-ion batteries. Due to fast charge transfer and diffusion, the ball-cutting Na-FeHCF nanocubes exhibit a high discharge capacity of 62 mA h g^-1 at 0.25 A g^-1 and 77.4% capacity retention at 2 A g^-1. Such excellent capacity and rate performance are superior to those of other Na-FeHCFs and the reported ammonium-ion intercalation cathodes. Furthermore, they present unparalleled cycling stability with no capacity loss over 50 000 cycles, thanks to the highly stable redox reaction of the high-spin nitrogen-coordinated Fe^II/Fe^III (Fe^H) couple. This work supplies a new view to design high-performance cathode materials for ammonium ion storage.