(637f) Theoretical Evidence of Water Serving As a Promoter for the Cathode Reaction in Aprotic Li-O2 Battery

The Li-O₂ battery is regarded as a promising alternative to the rechargeable Li–ion battery owing to its high theoretical specific energy (~3500 Wh kg^-1). The fundamental challenge faced by Li-O₂ battery lies in the efficiency of O₂ reduction reaction (ORR) to Li₂O₂ on cathode materials during discharge. Intensive efforts have been made to facilitate ORR during discharge. It has been reported that a trace amount of water in the aprotic electrolyte was able to promote Li₂O₂ formation from ORR, resulting in an increased discharge capacity. However, the reaction mechanism involving the activated water effect on ORR is still under debate.

In this work, the first principles calculations were performed to investigate water’s promoting role for Li₂O₂ production on an Au(100) electrode immersed in the explicit dimethoxyethane (DME) electrolyte. Two reaction mechanisms have been evaluated. The first reaction mechanism proposed that water was able to promote the reaction of Li ion and O₂ to form Li₂O₂ via first activating O₂ . The second reaction mechanism assumed that water is capable of enhancing the disproportionation reaction from LiO₂ dimer to Li₂O₂. The Ab initio Molecular Dynamics (AIMD) calculations under experimental conditions indicated that the water failed to activate O₂ molecule, but succeeded in promoting the disproportionation reaction for Li₂O₂ formation. The density functional theory (DFT) calculations based on the G4MP2 level of theory further proved the disproportionation reaction to Li₂O₂ becomes more thermodynamically and kinetically favorable when water exists in the DME electrolyte. This theoretical work advances the fundamental understandings of complex interface reactions on cathode and the efficiency improvement for the Li-O₂ battery.