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Electrodepositing insulating and insoluble Li2O2 is the key process during discharge of aprotic Li-O2 batteries and determines rate, capacity, and reversibility. Current understanding states that the partition between surface adsorbed and solvated LiO2 governs whether Li2O2 grows as surface film or particles, leading to low or high capacities, respectively. Here we show that Li2O2 forms exclusively as particles via solution mediated LiO2 disproportionation. We describe a unified O2 reduction mechanism that conclusively explains capacity limitations across the whole range of electrolytes. Low-donor-number electrolytes are shown to accelerate disproportionation rather than slowing it down. Deciding for particle morphology and achievable capacities are species mobilities, true areal rate and the rate at which associated LiO2 forms in solution. Provided that species mobilities and surface are high, this allows for high capacities even with low-donor-number electrolytes, previously considered prototypical for low capacity via surface growth. The tools for these insights include microscopy, hydrodynamic voltammetry, a numerical reaction model, and in situ small/wide angle X-ray scattering (SAXS/WAXS). Combined with sophisticated data analysis, SAXS allows retrieving rich quantitative information from complex multi-phase systems. On a wider perspective, this SAXS method is a powerful in situ metrology with atomic to sub-micron resolution to study mechanisms in complex electrochemical systems and beyond.