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 CO2 reduction reactions (CO2RR) in aprotic Li-CO2 batteries, the electrolyte's role as an equally critical factor remains inadequately understood. Herein, we present a mechanistic study of the solvent-dependent CO2RR 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 CO2-to-Li2C2O4 pathway (i.e., 2Li+ + 2CO2 + 2e– → Li2C2O4) initiated at 2.2 V vs Li/Li+. Conversely, high-DN solvents (e.g., DMSO) enable two reaction pathways: CO2-to-CO (i.e., 2Li+ + 2CO2 + 2e– → CO + Li2CO3) initiated at 2.4 V vs Li/Li+ and CO2-to-Li2C2O4 pathway initiated at 2.2 V vs Li/Li+. The discrepancy originates from the solubility of CO2– intermediates: high-DN solvents enhance the CO2– solvation and enable C–O coupling with CO2 to form CO/Li2CO3, while the Li+-CO2– interaction drives LiCO2 dimerization into Li2C2O4 as a common pathway. These findings establish solvent coordination chemistry as a fundamental design parameter for next-generation metal-CO2 batteries.