N-Heteroaromatic fused-ring cyanides extended as redox polymers for high rate capability aqueous zinc-ion battery

Abstract

Reversible zinc-ion storage in aqueous electrolytes is emerging as a promising grid-scale energy storage technology with desirable operational safety and environmental friendliness. Polymer electrodes with robust structure and good redox-activities are in high demand for the development of devices with superior rate capability. We report herein two redox polymers (PHATN-t and PHAT-t) with extended π-conjugated structure via multi-site trimerization of diquinoxalino[2,3-a:2′,3′-c]phenazine and dipyrazino[2,3-f:2′,3′-h]quinoxaline cyanides. The facile synthesis and structural features of the two polymers and their potential as high rate capability electrodes in zinc-ion supercapatteries (ZIS) have been systematically studied with both experimental tests and theoretical calculation. PHATN-t and PHAT-t exhibit maximum specific capacities of 190 and 171 mA h g⁻¹ at 0.3 A g⁻¹, which remain at 132 and 121 mA h g⁻¹ at 10 A g⁻¹, respectively. Outstanding capacity retentions of 58% and 56% are even observed at 20 A g⁻¹. Importantly, PHATN-t and PHAT-t display 86.8% and 77.6% capacity retention after 1000 cycles. Mechanistic studies show that PHATN-t possesses higher pore volume and pore size for cation hosting due to it having more extensive conjugation planes. Theoretical simulation reveals stronger electron delocalization and more undisturbed redox sites to afford higher internal charge transfer, faster reaction kinetics and thus higher capacitance output for PHATN-t.