Unlock CO2 Reduction Reaction Pathways in Aprotic Li-CO2 Batteries with In Situ Isotope-Labeled Spectroscopy and Theoretical Calculations

The CO2 reduction reaction (CO2RR) pathway significantly dictates the reversibility and overpotential of aprotic Li-CO2 batteries; however, it has remained incompletely understood due to the lack of direct in situ spectroscopic evidence. Herein, the Li-CO2RR pathways at the model Au | dimethyl sulfoxide (DMSO) interface are interrogated using a combination of in situ isotope-labeled spectroscopy techniques and theoretical calculations. This obtained direct spectroscopic evidence presents that the primary CO2RR proceeds through the CO2-to-CO pathway (i.e., 2Li+ + 2CO2 + 2e– → CO + Li2CO3) initiated at a low overpotential (ca. 2.1 V vs Li/Li+), and the CO2-to-Li2C2O4 pathway (i.e., 2Li+ + 2CO2 + 2e– → Li2C2O4) initiated at a high overpotential (ca. 1.7 V vs Li/Li+), where the potential-dependent pathways critically depend on the coverage of LiCO2 intermediates. Simultaneously, the entire Li-CO2RR process is also accompanied by parasitic reactions to form gaseous C2H4 with COOH* as the crucial intermediate, which is induced by the H+-abstraction reaction between the reactive LiCO2 intermediate and the DMSO solvent. These fundamental insights enable us to establish a molecular picture for Li-CO2RR pathways in aprotic media and will serve as a crucial guideline for reversible Li-CO2 electrochemistry.