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 to control Li2O2 growth as surface film or as large particles. Here we show that Li2O2 forms across a wide range of electrolytes, carbons, and current densities as particles via solution mediated LiO2 disproportionation, questioning 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. Deciding for 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 are high, high capacities are also achieved with weakly solvating electrolytes, previously considered prototypical for low capacity via surface growth.