The rechargeable Li-CO2 battery is a novel and promising energy storage system with the capability of CO2 capture due to the reversible reaction between lithium ions and carbon dioxide. Carbon materials as the cathode, however, limit both the cycling performance and the energy efficiency of the rechargeable Li-CO2 battery, due to the insulating Li2CO3 formed in the discharge process, which is difficult to decompose in the charge process. Here, a Mo2C/carbon nanotube composite material is developed as the cathode for the rechargeable Li-CO2 battery and can achieve high energy efficiency (77%) and improved cycling performance (40 cycles). A related mechanism is proposed that Mo2C can stabilize the intermediate reduction product of CO2 on discharge, thus preventing the formation of insulating Li2CO3. In contrast to insulating Li2CO3, this amorphous Li2C2O4-Mo2C discharge product can be decomposed below 3.5 V on charge. The introduction of Mo2C provides an effective solution to the problem of low round-trip efficiency in the Li-CO2 battery.