Molecular Unravelling\nof the Structural Effect of\nQuinone Redox Mediators on Oxygen Reduction Reaction in Aprotic Lithium–Oxygen\nBatteries

Redox mediators (RMs) provide tantalizing solutions to\nunlock the\nenergy capabilities of aprotic lithium–oxygen (Li–O<sub>2</sub>) batteries by driving solution-mediated Li<sub>2</sub>O<sub>2</sub> growth. However, the structural effect of RMs on the catalytic\nefficiency of the oxygen reduction reaction remains incompletely understood.\nHerein, we present the interplay between the structure of RMs and\ntheir discharge capabilities by a comparative study of model quinone\n(Q)-based RMs. Specifically, at low current densities, incorporating\nelectron-withdrawing groups onto the Q ring can positively move the\ndischarge potential and deliver larger discharge capacity by extending\nthe lifespan of the LiQO<sub>2</sub> intermediate and allowing for\nLi<sub>2</sub>O<sub>2</sub> growth into deeper electrolyte regions.\nConversely, at high current densities, the absence of electron-withdrawing\ngroups facilitates homogeneous reaction kinetics from LiQ to regenerate\nQ (i.e., decreased lifespan of LiQO<sub>2</sub>), mitigating electrode\npotential polarization and preserving catalytic activity of Q for\nhigher discharge capacity. The work establishes structure–property\nrelationships that guide the rational design of RMs toward next-generation\nLi–O<sub>2</sub> batteries.