Monodispersed MnO nanoparticles in graphene-an interconnected N-doped 3D carbon framework as a highly efficient gas cathode in Li-CO2 batteries

Li-CO2 batteries have been developed in recent years, aiming to utilize CO2, a major cause of the greenhouse effect, as an effective energy storage medium. However, current Li-CO2 batteries still suffer from low energy efficiency, poor rate capability and short cycle life, demanding the design of more efficient CO2 cathodes. Herein, we synthesized ultrafine MnO nanoparticles dispersed in a graphene-interconnected N-doped 3D carbon framework, MnO@NC-G, by pyrolyzing a composite of a GO-wrapped metal-organic framework (MOF) containing Mn(ii) active sites as the cathode material for Li-CO2 batteries. This material can enable low voltage hysteresis (0.88 V at 50 mA g(-1)), high rate capability (up to 1 A g(-1)) and long cycle life (more than 200 cycles) in cells. By comparing MnO@NC-G with four other Mn(ii)-based cathodes, MnO@NC, parent Mn-MOF, MnO@KB and bulk MnO, we propose three key aspects for designing CO2 cathodes: (1) dispersed catalytic species, (2) fast electron transport, and (3) a robust interconnected network. We also found that the performance of a cycled MnO@NC-G cathode can be replenished simply by replacing the anode, indicating that the cell cycle life can be further extended with effective anode protection. Our findings here provide useful guidelines for improving the performance of Li-CO2 batteries, thus shedding light on the development of practical Li battery systems based on gaseous cathodes.