The Li-CO2/O-2 battery, owing to the sluggish decomposition kinetics of the discharge product Li2CO3, is still far from practical applications. Herein Ru/N-doped carbon nanotube (Ru/NC) catalyst is prepared by solvothermal method and Ru nanoparticles are uniformly distributed on the surface of NC. Due to the superior catalytic activity of Ru nanoparticles and N-doped feature, the decomposition kinetics of the Li2CO3 could be accelerated. Therefore the overpotential of Li-CO2/O-2 (V-CO2:V-O2 = 4:1) battery is decreased from 1.45 V to 1.06 V and thus-assembled battery could operate smoothly for 90 cycles with a capacity of 1000 mAh g(-1) or 184 cycles with 500 mAh g(-1). XRD and SEM characterizations after cycles further reveal that the as-generated Li2CO3 could be effectively decomposed by the Ru/NC. Besides, with Ru/NC catalyst, Li-CO2/O-2 (V-CO2:V-O2 = 0:1, 2:1 and 1:0) batteries also show enhanced cycling stability of 200, 190 and 150 cycles, respectively, with 500 mAh g(-1). Interestingly, the discharge potential increases from 2.35 V to 2.90 V when introducing 20% O-2 in Li-CO2 battery. Last but not least, our investigation on the failure mechanism puts an emphasis on the consumption of electrolyte and the degradation of Li anode as the major causes for the decay of the Li-CO2/O-2 batteries.