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

Transition metal phthalocyanines are potential solubleredox mediatorsfor Li-O-2 batteries. In this work, effective strategiesto control the redox potentials and activities of iron phthalocyanine(FePc) based redox mediators are designed by the introduction of electron-withdrawingor electron-donating groups. Substituted electron-donating groupscan shift the oxidation potential of FePc to a higher energy level,consequently reducing the charging voltage of Li-O-2 batteries. Especially, oxygen radical anion (-O-) modified FePc (FePc-O-) shows the most significantimprovement to the oxygen reduction and evolution reactions of Li-O-2 batteries. Electronic analysis indicates that -O- substitution can break the symmetry of electronicstructures of FePc which further tunes the reduction of O-2 and the oxidation of Li2O2. Detailed reactionmechanisms of (FePc-O-)-mediated Li-O-2 batteries are proposed based on first-principles moleculardynamics simulations and thermodynamic free energy calculations.