Less is more: Retaining uncyclized o-quinone group in quinone/pyrazine hybrid molecules for high-energy zinc–organic batteries

TAHPHO is studied as a novel organic cathode for aqueous rechargeable Zn batteries. • Its high theoretical capacity of 567 mAh g −1 can be fully utilized within 0.2–1.6 V. • It delivers 371 mAh g −1 at an average voltage of 0.78 V, with high cycling stability. • Electrode mass gain and Zn/S atomic ratio analysis are proposed for mechanism study. • The Zn 2+ –H + co-insertion mechanism of TAHPHO is quantitatively revealed. Small-molecule organic cathode materials (SMOCMs) hold great promise for aqueous rechargeable zinc batteries (ARZBs) but face challenges of insufficient energy density and ambiguous Zn 2+ –H + co-insertion mechanisms. Herein, we propose a “less is more” molecular design, retaining an uncyclized o-quinone group in quinone/pyrazine hybrids, and synthesize 6,9,16,17-tetraza-5,7,8,10,15,18-heptaphenehexone (TAHPHO) as a proof-of-concept. It delivers a higher theoretical capacity (567 mAh g −1 , 10-electron reaction) and higher redox potential than the conventional fully cyclized analogue. In an optimized window of 0.4–1.6  V , TAHPHO delivers a reversible capacity of 371 mAh g −1 at an average discharge voltage of 0.78  V with negligible fade over 100 cycles, and retains 70% of its reversible capacity (317 mAh g −1 ) after 2000 cycles at 500 mA g −1 , ranking among the best SMOCMs for ARZBs. Furthermore, we introduce two quantitative methods, i.e., electrode mass gain and Zn/S atomic ratio analysis, to reveal Zn 2+ and H + ion contributions during discharge. Both confirm Zn 2+ governs the initial 4-electron reduction, while H + dominates the subsequent 6-electron process. This work advances both rational molecular design of high-energy, high-stability SMOCMs and quantitative mechanistic understanding of Zn 2+ –H + co-insertion, paving a more scientific and practical path for ARZB development.