Homogeneous nucleation of Li2O2 under Li–O2 battery discharge

Abstract

The development of high-energy lithium–oxygen batteries has significantly slowed by numerous challenges including capacity limitations due to electrode surface passivation by the discharge product Li₂O₂. Since the passivation rate and intensity are dependent on the deposit morphology, herein, we focus on the mechanisms governing Li₂O₂ formation within the porous cathode. We report evidence of homogeneous nucleation of Li₂O₂ crystallites and their further assembly in bulk of the electrolyte solution in DMSO, which possesses a high donor number. After careful estimation of the superoxide ion concentration distribution within a phenomenological model, it was found that the high stability of superoxide ions formed during the ORR towards disproportionation and sufficient diffusivity of (0.5–1.2) × 10⁻⁶ cm² s⁻¹ enabled Li₂O₂ nucleation and crystallization not only at the surface but also in the electrolyte, and the reaction zone spread throughout the internal space of the porous electrode. High initial supersaturation promoted the homogeneous nucleation of Li₂O₂ nanoplates, which instantly assembled into mesocrystals also in the solution bulk. These results were supported by operando SAXS/WAXS and morphology observations. Thus, although homogeneous nucleation is not dominant, it is important for achieving a high capacity in Li–O₂ batteries.