The capacity limits, local formation of Li2O2, passivation of active surfaces, and depletion of oxygen by mass transport characteristics in a composite cathode are modeled, numerically simulated, and experimentally evaluated for non-aqueous Li-O2 batteries employing composites of a solid polymer electrolyte and carbon particles as the cathode, Li metal as the anode, and an ion conductive oxide membrane as the separator. Although the theoretical maximum specific energy of the Li-O2 battery is known to be 3458 Wh kg−1 cathode, our simulation predicts a maximum specific energy of 1840 Wh kg−1 cathode with an optimized weight ratio of all essential components as well as cathode thickness. A specific energy of 1713 Wh kg−1 cathode is experimentally demonstrated in a cell with a composite cathode of poly(ethylene oxide) electrolyte and Printex carbon nanoparticles with 48% carbon volume and 30 μm thickness. The model also predicts that the incorporation of voids in the cathode can significantly improve the specific energy.