In situ small-angle X-ray scattering reveals solution phase discharge of Li-O batteries with weakly solvating electrolytes.

Electrodepositing insulating lithium peroxide (LiO) is the key process during discharge of aprotic Li-O batteries and determines rate, capacity, and reversibility. Current understanding states that the partition between surface adsorbed and dissolved lithium superoxide governs whether LiO grows as a conformal surface film or larger particles, leading to low or high capacities, respectively. However, better understanding governing factors for LiO packing density and capacity requires structural sensitive in situ metrologies. Here, we establish in situ small- and wide-angle X-ray scattering (SAXS/WAXS) as a suitable method to record the LiO phase evolution with atomic to submicrometer resolution during cycling a custom-built in situ Li-O cell. Combined with sophisticated data analysis, SAXS allows retrieving rich quantitative structural information from complex multiphase systems. Surprisingly, we find that features are absent that would point at a LiO surface film formed via two consecutive electron transfers, even in poorly solvating electrolytes thought to be prototypical for surface growth. All scattering data can be modeled by stacks of thin LiO platelets potentially forming large toroidal particles. LiO solution growth is further justified by rotating ring-disk electrode measurements and electron microscopy. Higher discharge overpotentials lead to smaller LiO particles, but there is no transition to an electronically passivating, conformal LiO coating. Hence, mass transport of reactive species rather than electronic transport through a LiO film limits the discharge capacity. Provided that species mobilities and carbon surface areas are high, this allows for high discharge capacities even in weakly solvating electrolytes. The currently accepted Li-O reaction mechanism ought to be reconsidered.