Deciphering CO2 Reduction Reaction Mechanism in Aprotic Li-CO2 Batteries using In Situ Vibrational Spectroscopy Coupled with Theoretical Calculations

The aprotic Li-CO2 battery represents a sustainable technology by virtue of energy storage capability and CO2 recyclability. However, the CO2 reduction reaction (CO2RR) mechanism underpinning the operation of Li-CO2 batteries is not yet completely understood. Herein, using in situ surface-enhanced Raman spectroscopy coupled with density functional theory calculations, we obtain direct spectroscopic evidence of the CO2RR (i.e., CO2-, CO, and Li2CO3) and propose a surface-mediated discharge pathway (i.e., 2Li(+) + 2CO(2) + 2e(-) -> CO + Li2CO3) in Li-CO2 batteries. We also highlight the significant effect of the electrocatalysts' near-Fermi-level d-orbital states on the CO2RR activity through a systematic comparative study of model electrocatalysts. Moreover, deep CO2RR via "4Li(+) + 3CO(2) + 4e(-) -> 2Li(2)CO(3) + C" may be difficult to proceed because of the sluggish chemical steps involved (e.g., dimerization of two CO2- intermediates). This work provides molecular insights into the CO2RR mechanism in a Li+-aprotic medium and will be beneficial for nextgeneration Li-CO2 batteries.