Shuttling Induced Starvation of Redox Mediators in High Areal Capacity Rechargeable Lithium-Oxygen Batteries

Redox mediators (RMs) for lithium-oxygen batteries (LOBs) are electrochemically active soluble molecules or ions that can chemically oxidize insoluble discharge products to improve the battery rechargeability. Most LOBs investigated so far in research laboratories were of low cyclable areal capacities (0.5 mAh/cm2 or less). In future practical LOBs, ultrahigh areal capacities beyond 10 mAh/cm2 may be needed. It is known that RMs may be subjected to various side reactions in LOBs, such as shuttling toward the Li anode. However, because the amount of RMs in the electrolyte solution in the low areal capacity research batteries were often in large excess, the implications of inevitable RM depletion during cycling toward practical LOBs have rarely been discussed. Here we report the use of a RM in LOBs of high areal capacities up to 10 mAh/cm2 enabled by using our recently developed high mass loading air cathode system based on holey graphene scaffold. With lithium iodide (LiI) as a model RM, the cycling performance, the catalytic capacity, and the parasitic shuttling kinetics were studied in detail. The understanding of these phenomena is a crucial step toward effective solutions with regard to RM applications for practical LOBs with ultrahigh areal capacities.