A series of Co3O4/Ketjen Black cathodes are fabricated by electrostatic spray deposition technique for Li-O2 batteries. A sluggish kinetics of oxygen reduction reaction and oxygen evolution reaction processes is noted either when Co3O4 is lacked or Ketjen Black is insufficient, which leads to much higher overpotentials between charge and discharge profiles. By contrast, with the optimal design in terms of electric conduction and catalytic activity, the Co3O4/Ketjen Black (80 wt%) composite achieves enhanced electrochemical performance with an initial discharge capacity of 2044 mAh g-1 and maintaining 33 cycles at a fixed capacity of 500 mAh g-1. The electrochemical characterization indicates that the improved Li-O2 battery performance may benefit from the highest oxygen reduction reaction and oxygen evolution reaction activity under this electro-chemically optimized composite. This work may shed light on the design principle of future cathode materials for Li-O2 batteries.