Solvent-Dependent Oxidizing Power of LiI Redox Couples for Li-O2 Batteries

Abstract

Li-O₂ batteries offer higher gravimetric energy density than commercial Li-ion batteries. Despite this promise, catalyzing oxidation of discharge products, Li₂O₂ and LiOH, during charging remains an obstacle to improved cycle life and round-trip efficiency. In this work, reactions between LiI, a soluble redox mediator added to catalyze the charging process, and Li₂O₂ and LiOH are systematically investigated. We show that stronger solvation of Li⁺ and I⁻ ions led to an increase in the oxidizing power of I₃⁻, which allowed I₃⁻ to oxidize Li₂O₂ and LiOH in DMA, DMSO, and Me-Im, whereas in weaker solvents (G4, DME), the more oxidizing I₂ was needed before a reaction could occur. We observed that Li₂O₂ was oxidized to O₂, whereas LiOH reacts to form IO⁻, which could either disproportionate to LiIO₃ or attack solvent molecules. This work clarifies significant misconceptions in these reactions and provides a thermodynamic and selectivity framework for understanding the role of LiI in Li-O₂ batteries.