Exclusive Solution Discharge in Li-O 2 Batteries?

Capacity, rate performance, and cycle life of aprotic Li-O 2 batteries critically depend on reversible electrodeposition of Li 2 O 2 . Current understanding states surface-adsorbed versus solvated LiO 2 controls Li 2 O 2 growth as surface film or as large particles. Herein, we show that Li 2 O 2 forms across a wide range of electrolytes, carbons, and current densities as particles via solution-mediated LiO 2 disproportionation, bringing into question the prevalence of any surface growth under practical conditions. We describe a unified O 2 reduction mechanism, which can explain all found capacity relations and Li 2 O 2 morphologies with exclusive solution discharge. Determining particle morphology and achievable capacities are species mobilities, true areal rate, and the degree of LiO 2 association in solution. Capacity is conclusively limited by mass transport through the tortuous Li 2 O 2 rather than electron transport through a passivating Li 2 O 2 film. Provided that species mobilities and surface growth are high, high capacities are also achieved with weakly solvating electrolytes, which were previously considered prototypical for low capacity via surface growth.