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.