Copper hexacyanoferrate as ultra-high rate host for aqueous ammonium ion storage

Rechargeable aqueous batteries have aroused increasing attention as an effective energy strategy. In this research, non-metallic ammonium ions are reported as a highly efficient charge carrier to reversible insert into NH4+-rich copper hexacyanoferrate (N-CuHCF) in aqueous NH4NO3 electrolyte. It is worth noting that N-CuHCF displays remarkable rate performance with no obvious capacity loss (60.0 mAh g−1 at 1 C, 59.0 mAh g−1 at 50 C) and negligible voltage polarization (∼0.1 V) from 1 to 50 C. Even at ultra-high rate (≥100 C), N-CuHCF still demonstrates favorable long-life cycling performance. For example, capacity retentions of 91.5% and 86.5% are achieved over 17,000 and 20,000 cycles at 100 and 180 C, respectively. Furthermore, XRD, FTIR, and XPS reveal the essence of reversible redox reaction and stable structure of N-CuHCF based on Fe3+/Fe2+ and Cu2+/Cu+ couples. Hence, the rechargeable aqueous batteries based on the infinite ammonium ion and the stable Prussian blue analogues provide a novel imagine for large-scale and fast energy applications.