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

Abstract

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.