Solvent- and Anion-Dependent Li⁺–O₂– Coupling Strength and Implications on the Thermodynamics and Kinetics of Li–O₂ Batteries

Lithium–oxygen (Li–O₂) batteries offer considerably higher gravimetric energy density than commercial Li-ion batteries (up to three times) but suffer from poor power, cycle life, and round-trip efficiency. Tuning the thermodynamics and pathway of the oxygen reduction reaction (ORR) in aprotic electrolytes can be used to enhance the Li–O₂ battery rate and discharge capacity. In this work, we present a systematic study on the role of the solvent and anion on the thermodynamics and kinetics of Li⁺-ORR, from which we propose a unified descriptor for its pathway and kinetics. First, by thoroughly characterizing the solvation environment of Li⁺ ions using Raman spectroscopy, ⁷Li NMR, ionic conductivity, and viscosity measurements, we observe increasing Li⁺–anion interactions with increasing anion DN in low DN solvents such as 1,2-dimethoxyethane and acetonitrile but minimal Li⁺–anion interactions in the higher DN dimethyl sulfoxide. Next, by determining the electrolyte-dependent Li⁺/Li, TBA⁺,O₂/TBA⁺–O₂–, and Li⁺,O₂/Li⁺–O₂– redox potentials versus the solvent-invariant Me₁₀Fc reference potential, we show that stronger combined solvation of Li⁺ and O₂– ions leads to weaker Li⁺–O₂⁻ coupling. Finally, using rotating ring disk electrode measurements, we show that weaker Li⁺–O₂– coupling in electrolytes with strong combined solvation leads to an increased generation of soluble Li⁺–O₂–-type species and faster overall kinetics during Li⁺-ORR.