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

When lithium-oxygen batteries discharge, O-2 is reduced at the cathode to form solid Li2O2. Understanding the fundamental mechanism of O-2 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 O-2 reduction through its effect on the solubility of LiO2, or, more precisely, the free energy of the reaction LiO2* reversible arrow Li (+)((sol)) +O-2((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.