Promoting Solution Discharge of Li-O2 Batteries with Immobilized Redox Mediators.

For years, the aprotic Li-O2 battery suffered from a severe capacity-current trade-off that would be unacceptable for a beyond Li-ion battery. Recent fundamental study of Li-O2 electrochemistry revealed that this dilemma is caused by the growth of Li2O2 on the cathode surface and can be solved by discharging Li2O2 in the electrolyte solution. Among the strategies that can promote solution growth of Li2O2, redox mediators (i.e., soluble catalysts) demonstrate prominent performance. However, soluble redox mediators may shuttle from the cathode to the lithium anode and decompose thereon, causing severe deterioration of the lithium anode and degradation of the mediators' functionality. Here, we report that immobilized redox mediators (e.g., anthraquinone, AQ) in the form of a thin conductive polymer film (PAQ) on the cathode can effectively promote solution growth of Li2O2 even in weakly solvating electrolyte solutions that would otherwise lead to surface film growth and early cell death. The PAQ-catalyzed Li-O2 battery can deliver a discharge capacity that is up to ∼50 times what its pristine counterpart does at the same current densities and is comparable to the capacity realized by soluble AQ-catalyzed Li-O2 batteries. Most importantly, the adverse "cross-talk" between the lithium anode and the redox mediators immobilized on the cathode has been completely eliminated.