Beyond Electrocatalysis: Solvent-Dependent CO₂ Reduction Reaction Mechanisms in Aprotic Li-CO₂ Batteries Revealed by In Situ Spectroscopies

Although significant progress has been made in the electrocatalyst design for regulating CO 2 reduction reactions (CO 2 RR) in aprotic Li-CO 2 batteries, the electrolyte’s role as an equally critical factor remains inadequately understood. Herein, we present a mechanistic study of the solvent-dependent CO 2 RR pathway on model Cu | Li + -dimethoxyethane (DME) and Cu | Li + -dimethyl sulfoxide (DMSO) interfaces by using in situ spectroscopic techniques and theoretical calculations. It is revealed that low-Donor Number (DN) solvents (e.g., DME) exclusively promote 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 2.2 V vs Li/Li + . Conversely, high-DN solvents (e.g., DMSO) enable two reaction pathways: CO 2 -to-CO (i.e., 2Li + + 2CO 2 + 2e – → CO + Li 2 CO 3 ) initiated at 2.4 V vs Li/Li + and CO 2 -to-Li 2 C 2 O 4 pathway initiated at 2.2 V vs Li/Li + . The discrepancy originates from the solubility of CO 2 – intermediates: high-DN solvents enhance the CO 2 – solvation and enable C–O coupling with CO 2 to form CO/Li 2 CO 3, while the Li + -CO 2 – interaction drives LiCO 2 dimerization into Li 2 C 2 O 4 as a common pathway. These findings establish solvent coordination chemistry as a fundamental design parameter for next-generation metal-CO 2 batteries.