Unraveling Factors Affecting Performance of Quinone-based Polymer Cathode in Aqueous Zinc-Ion Battery

Organic cathode materials for aqueous rechargeable Zinc-ion batteries (ZIBs) include a large variety of aromatic molecules with redox-activity, and such polymer molecules vary largely in terms of specific capacity, discharge voltage plateau, and cycling stability. Here, three different quinone polymers are prepared by electropolymerizing dihydroxynaphthalene (DHN) isomers of 1,5-DHN, 1,6-DHN, and 1,7-DHN, respectively, and their energy storage performances in aqueous ZIBs are compared. Among the three cathode materials, the poly(1,6-DHN) cathode shows the best performance in specific capacity and cycling stability. The Zn||poly(1,6-DHN) cell shows a high areal capacity of 1.4 mAh cm⁻² and a high capacity retention of 90 % after 5000 cycles. By combing experiment with computation, the HOMO level of the polymer molecule is found to play a key role behind the specific capacity. Using the poly(1,7-DHN) cathode that suffers from faster capacity decay in the cycling process, our investigation suggests that residual ions stuck between molecular chains in the insertion/deinsertion process account for the capacity loss. This study provides a further understanding of organic cathode materials for aqueous ZIBs.