Oxygen Radical Anion Substituted Iron Phthalocyanine as an Effective Redox Mediator for Li-O₂ Batteries

Transition metal phthalocyanines are potential soluble redox mediators for Li-O₂ batteries. In this work, effective strategies to control the redox potentials and activities of iron phthalocyanine (FePc) based redox mediators are designed by the introduction of electron-withdrawing or electron-donating groups. Substituted electron-donating groups can shift the oxidation potential of FePc to a higher energy level, consequently reducing the charging voltage of Li-O₂ batteries. Especially, oxygen radical anion (-O^-) modified FePc (FePc-O^-) shows the most significant improvement to the oxygen reduction and evolution reactions of Li-O₂ batteries. Electronic analysis indicates that -O^- substitution can break the symmetry of electronic structures of FePc which further tunes the reduction of O₂ and the oxidation of Li₂O₂. Detailed reaction mechanisms of (FePc-O^-)-mediated Li-O₂ batteries are proposed based on first-principles molecular dynamics simulations and thermodynamic free energy calculations.