Exclusive Solution Discharge in Li-O₂ Batteries?

Capacity, rate performance, and cycle life of aprotic Li-O₂ batteries critically depend on reversible electrodeposition of Li₂O₂. Current understanding states surface-adsorbed versus solvated LiO₂ controls Li₂O₂ growth as surface film or as large particles. Herein, we show that Li₂O₂ forms across a wide range of electrolytes, carbons, and current densities as particles via solution-mediated LiO₂ disproportionation, bringing into question the prevalence of any surface growth under practical conditions. We describe a unified O₂ reduction mechanism, which can explain all found capacity relations and Li₂O₂ morphologies with exclusive solution discharge. Determining particle morphology and achievable capacities are species mobilities, true areal rate, and the degree of LiO₂ association in solution. Capacity is conclusively limited by mass transport through the tortuous Li₂O₂ rather than electron transport through a passivating Li₂O₂ 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.