Reversible and irreversible reaction mechanisms of Li-CO₂ batteries

Li-CO₂ batteries are considered a versatile solution for CO₂ utilization. However, their development, including reversibility and efficiency, is impeded by an inadequate understanding of Li-CO₂ electrochemistry, particularly the decomposition of carbon and the generation of by-product O₂. Here, using typical Ru(0001) (reversible) and Ir(111) (irreversible) as model catalysts and employing state-of-the-art first-principles calculations, the rechargeable/reversible reaction mechanisms of Li-CO₂ batteries are disclosed. We find that electrolyte, often neglected or oversimplified in Li-CO₂ modelling, plays an essential role in CO₂ activation and C-C coupling affects the generation pathways of discharge intermediates due to the sluggish kinetics. The results rationalize experimental observations, which are also examined by constant-potential modelling. Specifically, by exploring the kinetics of the charging process, we discover that the reversibility of Ru(0001) is attributed to its ability to suppress O-O coupling while co-oxidizing Li₂CO₃ and carbon. In contrast, Li₂CO₃ decomposition on Ir(111) preferentially produces O₂, during which carbon can only be partially decomposed. These findings solve long-standing questions and highlight the necessity of describing the explicit solvent effect in modelling, which can promote further studies on Li-CO₂ batteries.