Hydrogen bond chemistry in Fe4[Fe(CN)6]3 host for aqueous NH4+ batteries

Low-cost and high-safety aqueous batteries can provide high-value opportunities for grid-scale energy storage. In this article, the electrochemical intercalation of non-metallic charge carrier (NH4+) into iron hexacyanoferrate (Fe4[Fe(CN)6]3) has been systematically analyzed in aqueous NH4+ batteries. Comprehensive discussions are made based on the experimental and calculation results. Multiple testing methods are applied to illustrate the intercalation mechanism, and novel hydrogen bonds formed between NH4+ and N species of Fe4[Fe(CN)6]3 is initiatively proposed for the stabilization of the system, based on the density functional theory (DFT) calculations. Meanwhile, the density of states (DOSs) data of Fe4[Fe(CN)6]3 with and without HN4+ intercalation further indicates the formation of the strong covalent bonds, which is responsible for the good structural stability of the compound. Additionally, a low-cost rocking-chair aqueous NH4+ full cell is assembled based on NH4·Fe4[Fe(CN)6]3 cathode and 3,4,9,10-Perylene-bis(dicarboximide) (PTCDI) anode, which exhibits durable cycling stability (capacity retention of nearly 90% over 300 cycles) as expected for great potential applications in the sustainable batteries.