Asymmetric charge distribution of active centers in small molecule quinone cathode boosts high-energy and high-rate aqueous zinc-organic batteries

Abstract

Small molecule quinone compounds are attractive cathode materials for rechargeable aqueous zinc-organic batteries (AZOBs) because of their structural diversity and low-cost merits. Among them, nonpolar quinones are dominant given the relatively low solubilities in aqueous electrolytes. However, their poor electronic conductivity and accumulated Coulombic repulsion lead to underutilized active sites and sluggish redox kinetics. Here, polar 2,6-dimethoxy-1,4-benzoquinone (m-DMBQ) works as an advanced AZOB cathode with unexpectedly superior performance over the nonpolar isomer of 2,5-dimethoxy-1,4-benzoquinone (p-DMBQ). The asymmetric charge distribution of active centers in the p−π conjugated backbone of m-DMBQ induces reduced bandgap with improved electronic conductivity and redox activity, thus achieving a high specific capacity of 312 mAh g−1 approaching the theoretical limit. Additionally, the lowest unoccupied molecular orbital energy level is lowered for an increased average discharge voltage of 0.88 V. Characterizations and computational studies revealed boosted competitiveness of H+ relative to Zn2+ for significantly enhanced charge transfer kinetics and reversibility. As a result, the as-fabricated AZOB achieves a high energy density of 275 Wh kg−1 based on m-DMBQ along with high-rate capability and long-term cycling stability. This work provides a new molecular engineering strategy through regulating charge distribution symmetry for boosting charge storage in organic cathodes.