A Nature-Inspired Molecular Catalyst for Sustainable and Efficient Lithium-Oxygen Batteries

Abstract

Lithium-oxygen (Li-O2) batteries employing a lightweight and gaseous oxygen cathode have been spotlighted because of their exceptional high-energy density (practically 2~3 times higher than lithiumion cells).[1-2] However, their poor efficiency and cyclability originating from the sluggish kinetics for the formation and decomposition of lithium oxide products (i.e., LiO2, Li2O2) necessitate utilization of efficient catalysts. Although loading of the solid catalysts on the electrode effectively facilitates the Li-O2 cell reactions, they are quickly deactivated upon cycling due to accumulating product residues.[3-4] Incorporation of soluble redox molecules into an electrolyte has been considered an effective alternative strategy to address the deactivation issues.[5] Their redox properties enable rapid electron transfer from/to the electrode through their self-diffusion and subsequent redox reactions instead of a slow electron movement through the insulating discharge products. Development of potential redox catalysts for Li-O2 cells should meet the following requirements: (i) high mobility in electrolyte; (ii) reversible redox properties for fast electron transfer; (iii) robust stability; and (iv) environmental friendliness. In this work, we report a class of nature-inspired molecules as an efficient and stable redox catalyst for Li-O2 batteries. Interestingly, the catalyst molecules exhibits catalytic activity for both oxygen evolution and reduction reactions under a certain condition when it forms a stable dispersion of molecular aggregates, which can be controlled by types of electrolyte solvents and exposure to light. As a result of the optimized catalytic activity, the Li-O2 cells facilitated by redox reactions successfully achieve improved efficiency and a longer cycle life compared to reference cells. The reversibility of the Li-O2 reactions in the presence of the molecular catalysts is confirmed by ex-situ characterizations.