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+ + 2CO2 + 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+ + 3CO2 + 4e– → 2Li2CO3 + 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 next-generation Li–CO2 batteries.