N-Methyl-N-propyl pyrrolidine bromide (MPPBr) as a bi-functional redox mediator for rechargeable Li–O2 batteries

Abstract

Rechargeable lithium–oxygen (Li–O₂) batteries have received great attention due to their high theoretical specific density. However, their high over-potential during the oxygen reduction reaction (ORR) or oxygen evolution reaction (OER) might cause the degradation of the electrolyte and carbon electrodes and result in lower energy efficiency. To solve this problem, N-methyl-N-propyl pyrrolidine bromide (MPPBr), an ionic liquid (IL), was introduced into the electrolyte as a redox mediator (RM). With the presence of MPPBr, the oxidation over-potential was reduced down to about 3.6 V, in which the bromine redox couple (Br⁻/Br₃⁻) plays the dominant role and is akin to the LiBr RM. Additionally, the MPP⁺ cation is beneficial for the formation of a stable solid state electrolyte interface (SEI) layer on the surface of Li foil. Having these two virtues, a Li–O₂ battery with 0.2 M MPPBr in 1.0 M LiTFSI-TEGDME delivered a good cycling stability.