Unlock CO 2 Reduction Reaction Pathways in Aprotic Li-CO 2 Batteries with In Situ Isotope-Labeled Spectroscopy and Theoretical Calculations.

The CO 2 reduction reaction (CO 2 RR) pathway significantly dictates the reversibility and overpotential of aprotic Li-CO 2 batteries; however, it has remained incompletely understood due to the lack of direct in situ spectroscopic evidence. Herein, the Li-CO 2 RR 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 CO 2 RR proceeds through the CO 2 -to-CO pathway (i.e., 2Li + + 2CO 2 + 2e - → CO + Li 2 CO 3 ) initiated at a low overpotential (ca. 2.1 V vs Li/Li + ), and the CO 2 -to-Li 2 C 2 O 4 pathway (i.e., 2Li + + 2CO 2 + 2e - → Li 2 C 2 O 4 ) initiated at a high overpotential (ca. 1.7 V vs Li/Li + ), where the potential-dependent pathways critically depend on the coverage of LiCO 2 intermediates. Simultaneously, the entire Li-CO 2 RR process is also accompanied by parasitic reactions to form gaseous C 2 H 4 with COOH* as the crucial intermediate, which is induced by the H + -abstraction reaction between the reactive LiCO 2 intermediate and the DMSO solvent. These fundamental insights enable us to establish a molecular picture for Li-CO 2 RR pathways in aprotic media and will serve as a crucial guideline for reversible Li-CO 2 electrochemistry.