Synergetic Effect of Liquid and Solid Catalysts on the Energy Efficiency of Li-O2 Batteries: Cell Performances and Operando STEM Observations.

The sluggish cathodic kinetics and lower energy efficiency, associated with solid and insulating discharge products of Li2O2, are the key factors that prevent the practical implementation of Li-O2 batteries (LOBs). Here we demonstrate that the combination of the solid catalyst (RuO2) and soluble redox mediator tetrathiafulvalene (TTF) exhibits a synergetic effect in improving the cathodic kinetics and energy efficiency of LOBs by reducing both charge and discharge overpotentials. Operando electron microscopy observations and electrochemical measurements reveal that RuO2 not only exhibits bifunctional catalysis for Li-O2 reactions but also benefits the catalytic efficiency of TTF. Meanwhile, TTF plays an important role in activating the Li2O2 passivated RuO2 catalysts and in helping RuO2 effectively oxidize the discharge products during charging. The synergetic effect of solid and liquid catalysts, beyond traditional bifunctional catalysis, obviously increases the cathodic kinetics and round-trip energy efficiency of LOBs.