High-capacity π-conjugated organic anode with multielectron redox for zinc-metal-free zinc-ion batteries

Aqueous zinc-ion batteries (ZIBs) features prominently in virtue of their high theoretical capacity, reliable security, eco-friendliness and excellent rate capability. However, conventional zinc anodes suffer from the poor reversibility and unsatisfactory cycle durability due to the serious dendrite growth and parasitic side reactions, becoming a huge technological challenge to their commercial applications. Herein, a redox-active π-conjugated molecule, 2,2′,2''-(benzene-1,3,5-triyl)tris (1H-naphtho [2,3-d]imidazole-4,9-dione) (BTHI), is designed as substitute for Zn-metal anode. The BTHI electrode shows a high reversible capacity of 342 mAh g−1 at 0.1 A g−1 (approaching its theoretical capacity of 362 mAh g−1, 9 electrons transfer), along with rapid kinetics for charge storage. Both ex-situ spectroscopy characterizations and electrochemical experiments demonstrate reversible H+/Zn2+ co-insertion/extraction behaviors in the BTHI electrode. The density-functional theory (DFT) simulation results further indicate that H+ insertion is thermodynamically favored over Zn2+ uptake and reveal that protons are first inserted into six carbonyl groups, followed by the CN bonds of three imidazole subunits. By pairing the BTHI anode with copper hexacyanoferrate (CuHCF) cathode, a Zn-metal-free BTHI//CuHCF full cell is assembled, which delivers a record high capacity (315 mAh g−1 at 1 A g−1), outstanding rate performance (207 mAh g−1 at 10 A g−1), and remarkable cycling durability (71.9 % capacity retention rate after 1000 cycles at 10 A g−1).