Catalytic redox mediators for non-aqueous Li-O2 battery

Limited by the energy density and stability of currently developed Li-ion battery, there are eager demands for high energy density of electrochemical energy storage devices. Among various candidates, Li-O2 battery has been recognized as one type of the next generation lithium battery to achieve the energy density goal of 350–500 Wh kg−1 due to its extremely high theoretical energy density. However, there are gaps in terms of achievable energy density, rate-capability and cycling performance with respect to practical applications. One of the main reasons is the sluggish reaction kinetics of oxygen electrochemistry at the cathode caused by the insolubility and insulation of the discharge product Li2O2. Considering the advantage of mobility in Li-O2 battery over general solid catalysts, soluble catalytic redox mediators have been introduced into aprotic Li-O2 battery to facilitate the kinetics of both oxygen evolution reaction and oxygen reduction reaction on the cathode and extensively investigated. In this paper, recent progress on the application of soluble catalytic redox mediators in non-aqueous Li-O2 battery is systematically reviewed with focus on the underpinning chemical and electrochemical reaction mechanisms, after a brief discussion on the electrolyte solvent which plays a crucial role on the battery performance. The challenges and future opportunities of Li-O2 battery containing soluble catalytic redox mediators are also summarized and analyzed in this paper.