Bistate-type ion storage of azo polymer for aqueous zinc ion battery

Azo organic molecules are emerging as promising electrode candidates for energy storage systems due to their high theoretical specific capacity and excellent kinetic performance. In general perception, conventional azo organic small molecules used as cathode material in aqueous batteries remain a challenge because of the poor cycling stability in aqueous electrolytes. Herein, azo organic polymers (AOPs) were synthesized using a diazo-coupling reaction under mild conditions and developed as cathode materials for aqueous zinc-ion batteries. Owing to the long conjugate structure, the AOPs afford extra-long stability with outstanding cyclability of >1000 cycles at 2 A g−1, a comparable capacity (170 mAh g−1 at 0.5 A g−1), and excellent rate performance from 0.5 g−1 to 10 A g−1 in aqueous electrolytes. Significantly, the hydroxyl oxygen and azo functional groups on AOPs can spontaneously form metal heterocyclic complexes with zinc ions in 1 M Zinc trifluoromethanesulfonate (Zn(OTF)2) electrolyte, leading to a notable increase in voltage (by 0.2 V) and capacity (by 43 mAh g−1 at a current density of 2 A g−1) in aqueous zinc-ion battery. Additionally, mechanistic analysis reveals that the AOPs are an active material for bistate storage of zinc ions during the charging and discharging process. The AOPs not only store zinc ions in fully charged state by forming metal heterocyclic complexes but also store zinc ions by a redox reaction between azo functional groups and zinc ions at the discharge process. The results put forward a novel storage mechanism for zinc ions and will pave the way for AOPs' development in aqueous batteries.