Increasing the capacity of Li-O2 batteries requires a detailed understanding of the growth mechanism of the primary discharge product, lithium peroxide. Growth of lithium peroxide via direct reduction and chemical disproportionation pathways is investigated using an advanced electrochemical interface model. The electrochemical interface model is based on density functional theory (DFT) calculations that account for electrolyte solvation and surface potential effects. The solvation effects are incorporated using an implicit solvent described by the linearized Poisson-Boltzmann model, while the surface potential is set by adjusting the Fermi level through electron population. Growth through direct reduction is found to have a low overpotential that agrees with experimental measurements. Growth through chemical disproportionation is found to be favorable near the equilibrium potential. Both mechanisms, then, are favorable during battery discharge. These findings are shown to corroborate the recent experimental observation that electrolyte donor number mediates conformal and solution-based growth of Li2O2 and offer a possible means of improving the capacity of Li-O2 batteries.