Understanding Reaction Pathways in High Dielectric Electrolytes Using β-Mo₂C as a Catalyst for Li-CO₂ Batteries

The rechargeable Li-CO₂ battery has attracted considerable attention in recent years because of its carbon dioxide (CO₂) utilization and because it represents a practical Li-air battery. As with other battery systems such as the Li-ion, Li-O₂, and Li-S battery systems, understanding the reaction pathway is the first step to achieving high battery performance because the performance is strongly affected by reaction intermediates. Despite intensive efforts in this area, the effect of material parameters (e.g., the electrolyte, the cathode, and the catalyst) on the reaction pathway in Li-CO₂ batteries is not yet fully understood. Here, we show for the first time that the discharge reaction pathway of a Li-CO₂ battery composed of graphene nanoplatelets/beta phase of molybdenum carbide (GNPs/β-Mo₂C) is strongly influenced by the dielectric constant of its electrolyte. Calculations using the continuum solvents model show that the energy of adsorption of oxalate (C₂O₄^2-) onto Mo₂C under the low-dielectric electrolyte tetraethylene glycol dimethyl ether is lower than that under the high-dielectric electrolyte N,N-dimethylacetamide (DMA), indicating that the electrolyte plays a critical role in determining the reaction pathway. The experimental results show that under the high-dielectric DMA electrolyte, the formation of lithium carbonate (Li₂CO₃) as a discharge product is favorable because of the instability of the oxalate species, confirming that the dielectric properties of the electrolyte play an important role in the formation of the discharge product. The resulting Li-CO₂ battery exhibits improved battery performance, including a reduced overpotential and a remarkable discharge capacity as high as 14,000 mA h g^-1 because of its lower internal resistance. We believe that this work provides insights for the design of Li-CO₂ batteries with enhanced performance for practical Li-air battery applications.