d-Band Center Engineering of Ru-Based Oxygen Electrode Catalysts for Li–O ₂ Batteries

Lithium–oxygen (Li–O2) batteries (LOBs) exhibit the highest theoretical specific energy density (∼3500 Ah kg–1), making them an ideal next-generation energy storage system with the potential to surpass the limits of conventional Li–ion batteries in both energy capacity and gravimetric performance. However, LOBs suffer from large discharge–charge polarization and limited durability because of insulating Li2O2 formation and excessively strong or weak oxygenated intermediate binding with the cathode. Ruthenium-based catalysts play an important role as cathode electrocatalysts in LOBs, owing to their outstanding bifunctional activity for oxygen reduction and evolution reactions, high electrical conductivity, and tunable surface electronic structure, which enable efficient Li2O2 formation and decomposition. This review proposes a unifying electronic structure framework for Ru-based electrocatalysts in which the d-band center (εd) governs oxygen intermediate (O2–/LiO2) adsorption and Li2O2 nucleation, morphology, and reversibility. We present d-band center engineering strategies for enhancing the oxygen electrode catalysis performance of LOBs across five strategies: oxygen vacancy engineering, alloying, heteroatom doping, coordination environment control, and heterostructure formation. We next discuss how εd upshift and downshift influence the oxygen intermediate binding strength and Li2O2 formation. Finally, we provide our perspective on the direction of Ru-based oxygen electrocatalysts for LOBs.