Given their high theoretical specific energy, lithium-oxygen batteries have received enormous attention as possible alternatives to current state-of-the-art rechargeable Li-ion batteries. However, the maximum discharge capacity in nonaqueous lithium-oxygen batteries is limited to a small fraction of its theoretical value due to the build-up of insulating lithium peroxide (Li2O2), the battery's primary discharge product. The discharge capacity can be increased if Li2O2 forms as large toroidal particles rather than as a thin conformal layer. Here, we show that trace amounts of electrolyte additives, such as H2O, enhance the formation of Li2O2 toroids and result in significant improvements in capacity. Our experimental observations and a growth model show that the solvating properties of the additives prompt a solution-based mechanism that is responsible for the growth of Li2O2 toroids.We present a general formalism describing an additive's tendency to trigger the solution process, providing a rational design route for electrolytes that afford larger lithium-oxygen battery capacities.