Uncovering the Electrolyte-Dependent Transport Mechanism of LiO2 in Lithium-Oxygen Batteries.

Lithium-oxygen batteries (LOBs) offer extremely high theoretical energy density and are therefore strong contenders for bringing conventional batteries into the next generation. To avoid deactivation and passivation of the electrode due to the gradual covering of the surface by discharge products, electrolytes with high donor number (DN) are becoming increasingly popular in LOBs. However, the mechanism of this electrolyte-assisted discharge process remains unclear in many aspects, including the lithium superoxide (LiO2) intermediate transportation mechanism and stability at both electrode/electrolyte interfaces and in bulk electrolytes. Here, we performed a systematic Born-Oppenheimer molecular dynamics (BOMD)-level investigation of the LiO2 solvation reactions at two interfaces with high- or low-DN electrolytes (dimethyl sulfoxide (DMSO) or acetonitrile (CH3CN), respectively), followed by examinations of stability and condensation once the LiO2 monomers are solvated. Release of partial discharge product LiO2 is found to be energetically favorable into DMSO from the Co3O4 cathode with a small energy barrier. However, in the presence of CH3CN electrolyte, the release of LiO2 from the electrode surface is found to be energetically unfavorable. Dissolved LiO2(sol) clusters in bulk DMSO solvents are found to be more favorable to dimerize and agglomerate into a toroidal shape rather than to decompose, which avoids the emergence of strong oxidant ions (O2-) and preserves the system stability. This study provides two complete molecular-level pathways (solution and surface) from first-principles understanding of LOBs, offering guidance for future selection and design of electrode catalysts and solvents.