Promoting Li2O2 oxidation via solvent-assisted redox shuttle process for low overpotential Li-O2 battery

Abstract

Non-aqueous lithium-oxygen battery is considered as the next-generation battery because it has the highest theoretic energy density among all rechargeable batteries. But its discharge product Li2O2 is difficult to be oxidized back in the charge process, which causes high charge overpotential, serious electrolyte decomposition and poor cyclability. An effective way to solve this problem is adding redox mediator (RM) in the electrolyte to catalyze the charge reaction via redox shuttle process. However, the reaction kinetics between the oxidized RM and Li2O2 is seldom reported. Here, taking iodine species as the model RM, we find that the efficiency of the solution phase catalysis is limited by the reaction rate between the oxidized RM and Li2O2, which is largely dependent on the electrolyte solvent. Protic solvents, such as water and alcohols lead to high reaction rate because of the hydrogen bond-assisted solvation of Li2O2. However, too high proton activity may also bring unwanted peroxide disproportionation. Among a series of alcohols, n-butanol is found to show proper proton activity and significantly promote the efficiency of the redox shuttle process. To prove the electrochemical mechanisms, this paper also introduces a Li2O2-graphite cell to evaluate the reversibility of cathodic reaction. Comparing with conventional cycling tests with excess lithium metal anode, the Li2O2-graphite configuration can eliminate the “fake reversible capacity” caused by parasitic cathodic reactions, and thus is more reliable.