Rechargeable nonaqueous Li-air batteries are attracting much attention due to their far higher theoretical energy density than lithium-ion batteries. However, Li-air batteries suffers from poor round-trip efficiency, low rate capability and poor cycle life. To reduce charge overpotentials by understanding reaction mechanism and to operate in ambient air instead of pure oxygen are prerequisites to realization of practical Li-air batteries. Here, we demonstrate a practical Li-air battery using Mo2C/CNT as a potential promoter with high round-trip efficiency (∼80%) and improved cycling performance (40 cycles) because Mo2C stabilizes the intermediate species from reduction of both O2 and CO2. The stabilization via formation of Mo-O bonds prevents further reduction and disproportionation of intermediate species to generate crystalline Li2O2 and Li2CO3, thus reducing the charge overpotentials normally caused by the decomposition of crystalline Li2O2 and Li2CO3. In all, this work provides improved understanding of the general role of solid promoters and enables rational design of promoters towards practical Li-air batteries.