Li-O2 Battery Discharge Redox Mediation by Triarylmethyl Cations

Abstract

Low discharge capacities resulting from electronically insulating Li2O2 film growth on carbon electrodes is a major impediment to Li-O2 battery commercialization. Redox mediation is an effective strategy to drive oxygen chemistry into solution, avoiding surface-mediated Li2O2 film growth and extending discharge lifetimes. However, to continue improving upon prior research, exploration of new classes of redox mediators and discovery of novel selection criteria is required. Herein, we report a new class of triarylmethyl cations which are effective at enhancing discharge capacities up to 26-fold. Surprisingly, we observe that redox mediators with more positive redox mediator reduction potentials, and thus more sluggish kinetics for reaction with oxygen, lead to larger discharge capacities because of their improved ability to suppress the surface-mediated reduction pathway. This result provides important structure- property relationships for future improvements in redox-mediated O2/Li2O2 discharge capacities. To aid future redox mediator discovery, we applied a chronopotentiometry model to investigate the zones of redox mediator standard reduction potentials and concentrations needed to achieve efficient redox mediation at a given current density. This analysis is expected to guide future redox mediator exploration.