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.

The addition of HO, even trace amount, in aprotic Li-O batteries has a remarkable impact on achieving high capacity by triggering solution mechanism, and even reducing charge overpotential. However, the critical role of HO in promoting solution mechanism still lacks persuasive spectroscopic evidence, moreover, the origin of low polarization remains incompletely understood. Herein, by in situ spectroscopic identification of reaction intermediates, we directly verify that HO additive is able to alter oxygen reduction reaction (ORR) pathway subjected to solution-mediated growth mechanism of LiO. In addition, ingress of HO also induces to form partial LiOH, resulting in reduced charging polarization due to its higher conductivity; however, LiOH could not contribute to O evolution upon recharge. These original results unveil the complex effects of HO on cycling the aprotic Li-O batteries, which are instructive for the mechanism study of aprotic Li-O batteries with protic additives or soluble catalysts.

Surface‐enhanced Raman spectroscopy (SERS), as a nondestructive and ultra‐sensitive single molecular level characterization technique, is a powerful tool to deeply understand the interfacial electrochemistry reaction mechanism involved in energy conversion and storage, especially for oxygen electrochemistry in Li‐O2 batteries with unrivaled theoretical energy density. SERS can provide precise spectroscopic identification of the reactants, intermediates and products at the electrode|electrolyte interfaces, independent of their physical states (solid and/or liquid) and crystallinity level. Furthermore, SERS's power to resolve different isotopes can be exploited to identify the mass transport limitation and reactive sites of the passivated interface. In this review, the application of in situ SERS in studying the oxygen electrochemistry, specifically in aprotic Li‐O2 batteries, is summarized. The ideas and concepts covered in this review are also extended to the perspectives of the spectroelectrochemistry in general aprotic metal‐gas batteries.