Lithium-oxygen (Li–O2) batteries with ionic liquid-based electrolytes have attracted much attention because of their superior battery performance and high safety. However, the practical energy capacities achieved to date are markedly lower than those of Li-ion batteries. A dominant factor that limits the energy capacities of Li–O2 batteries is the insulating characteristics and insolubility of lithium peroxide (Li2O2), which gradually accumulates on the positive electrode as a discharge product. Herein, we report that ammonium ions function as a promoter for the solution-route formation of Li2O2, which results in a significant improvement of the energy capacity of Li–O2 cells. Scanning electron microscopy analyses revealed that the structure of Li2O2 changed from spherical particles to toroidal particles with an increase in the ammonium ions concentration. Ionic additives that have been reported to function as promoters the solution-route formation of Li2O2 in ether-based electrolyte systems do not exhibit a promoting effect in ionic liquid-based electrolytes.