NH₄⁺ Topotactic Insertion in Berlin Green: An Exceptionally Long-Cycling Cathode in Aqueous Ammonium-Ion Batteries

Aqueous batteries represent promising solutions for large-scale energy storage considering the cost, safety, and performance. Despite the tremendous efforts devoted to the metal cations as charge carriers for batteries, scarce attention has been paid to the non-metal cations such as proton or ammonium. In this study, we report that a Berlin green framework exhibits much greater structural compatibility for NH4+ (de)insertion than Na+ and K+. Ex situ structural studies reveal that the topochemistry of NH4+ in Berlin green is of nearly zero strain. The NH4+ topotactic performance gives rise to a higher operation potential and an ultralong cycling performance of 50,000 cycles with 78% capacity retention, far superior to Na+ and K+ (de)insertion. Furthermore, we propose a double-ion battery, where the Berlin green cathode hosts NH4+ and sodium titanium phosphate NaTi2(PO4)3 accommodates Na+ during operation. Such a new system exhibits promising results in capacity and cycling life. Our results point to a new direction of expanding the battery chemistry with NH4+ as a charge carrier.