Unraveling the Catalytic Mechanism of Rutile RuO₂ for the Oxygen Reduction Reaction and Oxygen Evolution Reaction in Li–O₂ Batteries

Abstract

Because of the involvement of solid-state discharge product Li₂O₂, how a catalyst works in nonaqueous lithium-oxygen batteries is yet to be determined, although the question has undergone fierce debate. In this work, we take an effective and widely used catalyst, rutile RuO₂, as a representative and studied its catalytic mechanism in lithium-oxygen batteries via ab initio calculations. For the oxygen reduction reaction (ORR), it is found that rutile RuO₂ can provide large adsorption energies toward LiO₂ and Li₂O₂, thus resulting in high initial discharge voltages. Moreover, the normalized degree of unsaturation of surface oxygen is identified as a descriptor for the ORR catalytic activity. For the oxygen evolution reaction (OER), we propose that, in addition to the three-phase interface, the OER may also occur at the two-phase interface of Li₂O₂/RuO₂, where rutile RuO₂ provides pathways for the lithium ions while oxygen evolves from the exposed surfaces of Li₂O₂. Calculation results show that our proposed catalytic scenario is both thermodynamically and kinetically viable. Along with the charge process, the remaining Li₂O₂ can be attracted to the catalytic surfaces spontaneously, which can effectively preserve the reaction interface.