A volatile redox mediator boosts the long-cycle performance of lithium-oxygen batteries

Abstract To improve the performance of lithium-oxygen (Li-O2) batteries with an extremely high theoretical energy density, redox mediators (RMs) are usually added to liquid electrolytes to assist with the charge process and reduce the overpotential. However, the shuttle effect and the instability of RMs towards a Li metal anode degrade the cycle performance of Li-O2 batteries. Herein, we report a volatilization-dissolution strategy to supply RMs by introducing 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) into the O2 atmosphere (TEMPO-O2) outside an assembled cell. Due to not directly adding RMs to liquid electrolytes, the parasitic reactions between the Li metal anode and TEMPO, including the TEMPO's shuttle effect, are alleviated. TEMPO-O2 mediates the formation-decomposition of lithium peroxide (Li2O2) in Li-O2 batteries and shows a uniform catalytic effect towards commercial Li2O2. Thanks to the continuous redox mediation of volatile TEMPO and the suppression of the RM's shuttle effect, the Li-O2 battery demonstrates an ultra-long cycle life of 400 cycles (1600 h) at 250 mA g−1. Our strategy to supply volatile RMs shows a universal adaptability towards different kinds of cathodes and electrolytes, which may trigger broad applications in various gas-involved Li metal batteries.