Understanding the fundamentals of redox mediators in Li-O2 batteries: a case study on nitroxides

The development of aprotic lithium-oxygen (Li-O2) batteries suffers from high charging overvoltages. Dissolved redox mediators, like nitroxides, providing increased energy efficiency and longer lifetime are promising tools to overcome this challenge. Since this auspicious concept is still in its infancy, the underlying chemical reactions as well as the impact of the different (electro)chemical parameters are poorly understood. Herein, we derive an electrochemical model for the charging reactions, which is validated by potentiostatic measurements. The model elucidates the impact of the major factors including basic cell parameters and the chemical properties of the redox mediator. The model is applied to the promising class of nitroxides, which is systematically investigated by using derivatives of TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy), AZADO (2-azaadamantane-N-oxyl), and an azaphenalene based nitroxide. The nitroxides are electrochemically characterized by cyclic voltammetry and their performance as redox mediators is studied in Li-O2 batteries with an ether-based electrolyte. Based on the presented model, the charging profiles of the different nitroxide redox mediators are correlated with their molecular structures.