Tunable Redox Mediators for Li-O₂ Batteries Based on Interhalide Complexes

Li-O₂ batteries can provide significantly higher gravimetric energy density than Li-ion batteries, but their practical use is limited by a number of fundamental issues associated with oxidizing discharge products such as Li₂O₂ and LiOH during charging. Soluble inorganic redox mediators (RMs) like LiI and LiBr have been shown to enhance round-trip efficiency where different solvents can greatly shift the redox potential of the RMs, significantly altering the overpotential during charging, as well as their oxidizing power against the discharge product. Unfortunately, other design requirements like (electro)chemical stability with the electrode as well as reactive discharge products greatly constrain the selection of solvent, making it impractical to additionally design the solvent to provide optimal RM performance. In this work, we demonstrate that interhalide RMs based on LiI/LiBr and LiI/LiCl mixtures can enable tuning of the oxidizing power of the RM in a given solvent. I-Br interhalides I₂Br^- to IBr₂^- showed increasing chemical oxidizing power toward Li₂O₂ and LiOH with increasing Br, and DEMS measurements during charging of Li-O₂ cells demonstrated that these I-Br interhalide RMs led to increased O₂ evolution with respect to LiI and reduced charging potential and CO₂ evolution with respect to LiBr.