Sulfur Heterocyclic Quinone Cathodes for Rechargeable Magnesium Batteries: Discharge Voltage, Cycling Stability, and Reaction Reversibility Improved by Sulfur Substitution

Rechargeable magnesium batteries have the potential for large-scale energy-storage applications, but traditional inorganic cathodes suffer from inferior performance and insufficient selections. Organic conjugated carbonyl compounds are promising cathode materials with a delocalized negative charge, reversible carbonyl enolization, and wide designability. Herein, sulfur heterocyclic quinones of dibenzo[b,i]thianthrene-5,7,12,14-tetraone (DTT) and benzo[b]naphtho[2′,3′:5,6][1,4]dithiino[2,3-i]thianthrene-5,7,9,14,16,18-hexone (BNDTH) are prepared and investigated as cathode materials for rechargeable magnesium batteries and compared with normal quinone of 5,7,12,14-pentacenetetrone (PT). DTT and BNDTH show higher redox potentials, higher magnesium storage capacities, and better cycling stabilities than PT. Mechanism study and theoretical computation reveal that the adjacent carbonyls connected with sulfur atoms in DTT and BNDTH coordinate with bivalent magnesium cations better than rigid PT via conformation change, leading to better reaction reversibility. DTT and BNDTH have lower LUMO energy levels and thus higher redox potentials than PT. DTT and BNDTH show lower solubilities in the electrolytes than PT and result in a higher cycling stability. The comparative study herein would provide scientific insights into the rational design of organic cathode materials suitable for reversible and stable storage reactions of bivalent magnesium cations.