Monodispersed Ru Nanoparticles Functionalized Graphene Nanosheets as Efficient Cathode Catalysts for O₂-Assisted...

In Li-CO₂ battery, due to the highly insulating nature of the discharge product of Li₂CO₃, the battery needs to be charged at a high charge overpotential, leading to severe cathode and electrolyte instability and hence poor battery cycle performance. Developing efficient cathode catalysts to effectively reduce the charge overpotential represents one of key challenges to realize practical Li-CO₂ batteries. Here, we report the use of monodispersed Ru nanoparticles functionalized graphene nanosheets as cathode catalysts in Li-CO₂ battery to significantly lower the charge overpotential for the electrochemical decomposition of Li₂CO₃. In our battery, a low charge voltage of 4.02 V, a high Coulomb efficiency of 89.2%, and a good cycle stability (67 cycles at a 500 mA h/g limited capacity) are achieved. It is also found that O₂ plays an essential role in the discharge process of the rechargeable Li-CO₂ battery. Under the pure CO₂ environment, Li-CO₂ battery exhibits negligible discharge capacity; however, after introducing 2% O₂ (volume ratio) into CO₂, the O₂-assisted Li-CO₂ battery can deliver a high capacity of 4742 mA h/g. Through an in situ quantitative differential electrochemical mass spectrometry investigation, the final discharge product Li₂CO₃ is proposed to form via the reaction 4Li⁺ + 2CO₂ + O₂ + 4e^- → 2Li₂CO₃. Our results validate the essential role of O₂ and can help deepen the understanding of the discharge and charge reaction mechanisms of the Li-CO₂ battery.