The role of LiO₂ solubility in O₂ reduction in aprotic solvents and its consequences for Li-O₂ batteries

When lithium–oxygen batteries discharge, ​O₂ is reduced at the cathode to form solid ​Li₂O₂. Understanding the fundamental mechanism of ​O₂ reduction in aprotic solvents is therefore essential to realizing their technological potential. Two different models have been proposed for ​Li₂O₂ formation, involving either solution or electrode surface routes. Here, we describe a single unified mechanism, which, unlike previous models, can explain ​O₂ reduction across the whole range of solvents and for which the two previous models are limiting cases. We observe that the solvent influences ​O₂ reduction through its effect on the solubility of LiO₂, or, more precisely, the free energy of the reaction LiO₂^* ⇌ Li_(sol)⁺ + O₂⁻_(sol) + ion pairs + higher aggregates (clusters). The unified mechanism shows that low-donor-number solvents are likely to lead to premature cell death, and that the future direction of research for lithium–oxygen batteries should focus on the search for new, stable, high-donor-number electrolytes, because they can support higher capacities and can better sustain discharge.