Control of discharge products with respect to composition, size and morphology is of importance to reduce charge potential, suppress side reactions and improve reversibility of Li-O2 batteries, but faces significant challenge due to complicated electrochemical reactions. Here, we report a cathode architecture composed of ZnO nanoparticles anchored on vertically aligned carbon nanotubes (ZnO/VACNTs) that significantly suppresses LiO2 disproportionation, forming the composites of LiO2, Li3O4 and Li2O2 in nanometer size as the final discharge products. The exposed surface of VACNTs provides the electrochemical reaction sites for reducing O2 to O2 −. Transmission electron microscopy measurements in combination with first-principles calculations reveal that the Li2−xO2 compounds nucleated at the interfaces between the ZnO and the VACNTs are stabilized by the nanosize effect (in the scale of 6nm), which is related to the semiconductive behavior of ZnO. This discharge chemistry leads to the reduced charge overpotential and extended cycle life. The results here demonstrate that the electrochemical reaction tuned by the cathode architecture is a powerful tool to improve Li-O2 cell performance.