Modeling the Effect of Lithium Superoxide Solvation and Surface Reduction Kinetics on Discharge Capacity in Lithium-Oxygen Batteries

Li-O-2 batteries could potentially yield significantly higher capacities than Li-ion batteries. Achieving high capacity requires efficient void-filling of the cathode by the insoluble insulating discharge product Li2O2, which forms by two competing mechanistic pathways. One is a surface-mediated pathway that leads to formation of thin films of Li2O2 on the electrode. The other is a solvent-mediated pathway, involving the solvation of a Li+-O-2(-) intermediate that disproportionates and leads to the formation of large toroidal particles. As the solvent pathway produces large particles that are more efficient for void-filling than thin films produced by the surface pathway, there has been an active search for modifications that can promote the solvent pathway. We construct a model that demonstrates how discharge parameters influence each pathway and can be optimized to yield high capacity. We test the model with rotating ring-disk electrode experiments, which allow for the direct measurement of the relative contributions of solution and solvent pathways as a function of rotation rate, water content, voltage, and choice of solvent. We show that favorability of solvation of Li+-O-2(-) has a weak effect on the solvation rate and a large effect on the surface pathway rate. This insight can help guide strategies to optimize capacity in Li-O-2 batteries.