Bidirectional electron transfer boosts Li-CO₂ electrochemistry

The activation mechanism for the CO₂ reactant remains a crucial and controversial question in the field of Li-CO₂ batteries, which is primarily attributed to the catalyst injecting electrons into CO₂ while ignoring the reverse electron transfer. Here, extending classical unidirectional activation theory, a novel “bidirectional electron transfer (BET)” mechanism was unraveled to well establish a fundamental understanding of the interplay between the metal active sites and CO₂ redox. By means of spin-polarized density functional theory (DFT) calculations, the number of bidirectional transferred electrons was confirmed to be a good indicator to characterize the adsorption strength of CO₂ in the presence of TM/Ti₁₇B₁₈ (TM = V, Cr, Mn, Fe, Co, Ni or Cu) as aprotic Li-CO₂ battery catalysts. Of particular interest is that the selective nucleation of Li₂CO₃ and reverse dissociation with carbon products were achieved on these catalysts with abundant metal active sites. The results obtained from the constant charge method and constant potential implicit solvent method together demonstrate that Fe/Ti₁₇B₁₈ delivers dramatically boosted bifunctional activity towards Li-CO₂ redox benefiting from its appropriate BET behavior with CO₂, in sharp contrast to pristine TiB monolayers. The current findings provide intriguing insights into the CO₂ activation mechanisms and potential-dependent Li-CO₂ electrochemical performance.